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Introduction
Main topic Focus on first general information about Ru application then second, Ru nanoparticles as catalysis in different application(Ru
Nanoparticles
: Application in
Catalysis)
Please, use one example from each article if possible, or if there is one example summarize it.
General Application of Ru
Application 2
Ruthenium Nanoparticles Decorated Tungsten Oxide as a
Bifunctional Catalyst for Electro catalytic and Catalytic Applications
Application 3
Catalysis with Colloidal Ruthenium Nanoparticles
Application 4
Sensitive Colorimetric Assay of H2S Depending on the High-Efficient
Inhibition of Catalytic Performance of Ru Nanoparticles
Application 5
Synthesis of PtRu Nanoparticles from the Hydrosilylation Reaction and Application as
Catalyst for Direct Methanol Fuel Cell
Application 6
Role of Ru Oxidation Degree for Catalytic Activity in Bimetallic Pt/Ru
Nanoparticles
Conclusion
(6 References) page
Ruthenium Nanoparticles Decorated Tungsten Oxide as a
Bifunctional Catalyst for Electrocatalytic and Catalytic Applications
Chellakannu Rajkumar,†,⊥ Balamurugan Thirumalraj,†,‡,⊥ Shen-Ming Chen,*,†
Pitchaimani Veerakumar,*,§,¶ and Shang-Bin Liu§
†Department of Chemical Engineering and Biotechnology, National Taipei University of Technology, Taipei 10608, Taiwan
‡Department of Chemical Engineering, National Taiwan University of Science and Technology, Taipei 10607, Taiwan
§Institute of Atomic and Molecular Sciences, Academia Sinica, Taipei 10617, Taiwan
¶Department of Chemistry, National Taiwan University, Taipei 10617, Taiwan
*S Supporting Information
ABSTRACT: The syntheses of highly stable ruthenium
nanoparticles supported on tungsten oxides (Ru-WO3)
bifunctional nanocomposites by means of a facial microwave-
assisted route are reported
.
The physicochemical properties of
these Ru-WO3 catalysts with varied Ru contents were
characterized by a variety of analytical and spectroscopic
methods such as XRD, SEM/TEM, EDX, XPS, N2
physisorption, TGA, UV−vis, and FT-IR. The Ru-WO3
nanocomposite catalysts so prepared were utilized for
electrocatalytic of hydrazine (N2H4) and catalytic oxidation
of diphenyl sulfide (DPS). The Ru-WO3-modified electrodes
were found to show extraordinary electrochemical performances for sensitive and selective detection of N2H4 with a desirable
wide linear range of 0.7−709.2 μM and a detection limit and sensitivity of 0.3625 μM and 4.357 μA μM−1 cm−2, respectively,
surpassing other modified electrodes. The modified GCEs were also found to have desirable selectivity, stability, and
reproducibility as N2H4 sensors, even for analyses of real samples. This is ascribed to the well-dispersed metallic Ru NPs on the
WO3 support, as revealed by UV−vis and photoluminescence studies. Moreover, these Ru-WO3 bifunctional catalysts were also
found to exhibit excellent catalytic activities for oxidation of DPS in the presence of H2O2 oxidant with desirable sulfoxide yields.
Tungstate-based nanostructured materials have received
considerable research and development (R&D) attention
recently due to their remarkable properties for perspective
applications such as optical, photo, and electrochemical
catalyses.1−4 For example, while metal nanoparticles (MNPs)
supported tungstate nanocomposites have been used as
electrode materials for high-performance supercapacitors,5,6
they have also been exploited as fast and highly sensitive
electrochemical sensors for detection of volatile organic
compounds (VOCs), biomolecules, and hazardous substan-
ces.7,8
Hydrazine (N2H4), a transparent oily liquid that has been
widely employed as chemical corrosion inhibitor or reducing
agent in chemical, pharmaceutical, agricultural industries as well
as in bioimaging, and military and aerospace industries.9−11
Nevertheless, N2H4 is not only extremely unstable (flammable
and highly explosive), unless handled in solution, but also
highly toxic to humans and animals even at trace levels.12,13
Moreover, N2H4 may cause serious adverse effects to our
digestion, kidney, liver, and neurological systems, when exposed
by inhalation, oral, or dermal routes.14 United States Environ-
mental Protection Agency (EPA) has classified N2H4 as a
human carcinogen with a low threshold limit value (TLV) as
low as 10 ppb in drinking water. As such, several analytical and
spectroscopic techniques have been developed for the detection
of N2H4, including high-performance liquid chromatography
(HPLC),15 spectrophotometry,16 chemiluminescence,17 and
electrochemical methods.18−21 Among them, electrochemical
detection of N2H4 is recognized as a desirable technique not
only because of its high sensitivity and selectivity but also
because of its characteristics as eco-friendly, facile operation,
and low cost.
Over the past few years, tungsten trioxide (WO3) plays an
important role, rendering a wide range of applications in
materials sciences and chemistry.22 Because of these potential
features and the unique property of the WO3, it has been
studied as a promising material for electrodes in the electro-
oxidation reactions of N2H4.
23 However, WO3 has suffered in
the acidic as well as basic environments and exhibits poor
Received: May 30, 2017
Accepted: August 29, 2017
Published: August 29, 2017
electrocatalytic activity.24 Interestingly, the metal-supported
WO3-based catalysts exhibit an excellent conductive substrate,
when compared to bare WO3 working electrode (i.e., catalytic
activity through metal−support interaction).25 Thus, the
application of a WO3 matrix should increase the electrochemi-
cally active surface area and facilitate charge (electron, proton)
distribution as well as diffusion of analysts.
Herein, we report the microwave (MW)-assisted synthesis of
a bifunctional Ru-WO3 catalysts and their application as
electrochemical sensors for efficient detection of N2H4 and as
oxidation catalysts. At the current time, to the best of our
knowledge, there is no report in the literature of the use of Ru-
WO3 composite in the electrochemical determination of N2H4.
As will be shown subsequently, the Ru-WO3-modified glassy
carbon electrodes (GCEs) exhibit excellent electrocatalytic
activity, sensitivity, and selectivity for detection of N2H4, and
hence, they are most suitable for applications as practical and
cost-effective N2H4 sensors. Moreover, the Ru-WO3 catalyst
also shows excellent activity for catalytic oxidation of diphenyl
sulfide (DPS) to diphenyl sulfoxide (DPSO) in the presence of
hydrogen peroxide (H2O2) under MW heating.
2. EXPERIMENTAL SECTION
2.1. Chemicals. Research grade ruthenium(III) chloride (RuCl3,
98%), sodium tungstate dihydrate (Na2WO4·2H2O), oxalic acid
(H2C2O4, 98%), cetyltrimethylammonium bromide (CTAB,
C19H42NBr), polyvinylpyrrolidone (PVP, Mw ∼ 40 000), hydrazine
(N2H4, 98%), 1,2-propanediol (C3H8O2), and hydrogen peroxide
(H2O2, 30 wt % in water) were purchased from Sigma-Aldrich. The
supporting electrolytes (pH = 3−11) were prepared by using 0.05 M
Na2HPO4 and NaH2PO4 solutions. All other chemicals were of
analytical grade and used without further purification. All solutions
were prepared using Millipore DI water.
2.2. Preparation of WO3 and Ru-WO3 Catalysts. As illustrated
in Scheme 1, the preparation of Ru-WO3 catalysts invoked a two-step
synthesis procedure: Step I, first, 2.7 g of Na2WO4·2H2O and 0.895 g
of CTAB were dissolved in 15 mL of distilled water while under
magnetic stirring. The pH (from basic to acidic; 11 to 3) of the
solution was adjusted by using hydrochloric acid (HCl; 2.0 M).
Subsequently, ca. 1.0 g of H2C2O4 was added into the reaction system.
The reaction mixture was then heated by means of microwave
irradiation (power 300 W; Milestone’ START) at 150 °C for 1 h while
under rigorous stirring condition (at 1200 rpm). The yellow
precipitate was recovered by centrifugation, then subjected to washing
(with DI water) and drying (at 110 °C overnight), followed by a
calcination treatment in air at different temperatures (T = 200−500
°C) for 3 h. The resultant yellow powder was labeled as WO3-T, where
T represents calcination temperature in °C. In Step II, the as-
synthesized WO3 (200 mg), RuCl3 (5−15 mg), and PVP (0.583 g)
were dissolved in 1,2-propanediol (C3H8O2) (50 mL) under
continuous stirring condition to form a dark red solution. Note that,
here, the 1,2-propanediol was employed as a solvent as well as a
reducing agent. After stirring for an additional 1 h, the mixture was
subjected to microwave heating (power 300 W) at 180 °C for 2 h,
during which the solution changed from dark brown color to black. As
revealed by ultraviolet−visible (UV−vis) spectra shown in Figure S1 of
the Supporting Information (hereafter denoted as SI), the Ru3+ ions
were effectively reduced to metal Ru(0) state on the surfaces of the
WO3 during the microwave irradiation. Finally, the precipitate was
collected by centrifugation (at 8000 rpm), followed by washing
consecutively with acetone and ethanol, then dried in vacuum at 60 °C
for 6 h. The materials so obtained were denoted as Rux-WO3, were x =
0.5, 1.0, and 1.5 wt % represents the Ru loading.
2.3. Fabrication of the Ru-WO3-Modified Electrodes. To
prepare the working electrode, first the bare GCE was polished by
using alumina and were cleaned by ultrasonication in distilled water,
ethanol, and subsequently dried in a hot air oven. Typically, 5.0 mg of
Ru-WO3 composite was first dispersed in 1.0 mL of DI water and
sonicated for 2 h; then 6.0 μL of Ru-WO3 catalyst was dropped onto
the precleaned GCE and dried overnight for further measurements.
2.4. Catalyst Characterization and Electrochemical Measure-
ments. The X-ray diffraction (XRD) patterns were recorded on a
diffractometer (PANalytical X’Pert PRO) using Cu Kα radiation (λ =
0.1541 nm). Surface morphological studies of various samples were
conducted using a scanning electron microscope (SEM; Hitachi S-
3000 H). Elemental compositions of the samples were carried out with
an energy-dispersive X-ray (EDX) analyzer, which was an accessory of
the SEM instrument. The structural morphology of various samples
were studied by field-emission transmission electron microscopy (FE-
TEM; JEOL JEM-2100F) at room temperature (25 °C) operating at
200 kV. X-ray photoelectron spectroscopy (XPS) measurements were
performed using an ULVAC-PHI 5000 VersaProb apparatus. Nitrogen
adsorption/desorption isotherm measurements were carried out on a
Quantachrome Autosorb-1 volumetric adsorption analyzer at −196
°C. Prior to measurement, the sample was purged with flowing N2 at
150 °C for 12 h. Moreover, UV−vis and photoluminescence (PL)
spectroscopies performed by using PerkinElmer LS-45 spectropho-
tometer instruments, respectively, were also employed to investigate
the optical properties of various catalyst samples. Electrochemical
measurements, including cyclic voltammetry (CV) and chronoamper-
ometry (CA), were conducted on a CHI 1205b analyzer (CH
Instruments). A conventional three-electrode cell system was utilized
using the modified glassy carbon electrode (GCE) as the working
electrode, Ag/AgCl (saturated KCl) as the reference electrode, and a
platinum (Pt) wire as the counter electrode.
3. RESULTS AND DISCUSSION
3.1. Structural and Physicochemical Properties of
WO3 and Ru-WO3 Nanocomposites. The powder XRD
patterns of the as-synthesized WO3 samples calcined at
different temperatures are depicted in Figure S2 (SI). For
WO3-T samples calcined at lower temperatures T ≤ 300 °C,
Scheme 1. Schematic Illustrations of the Procedures Used for the Syntheses of WO3 and Ru-WO3 Catalysts
ACS Applied Materials & Interfaces Research Article
several broad diffraction peaks centering at 2θ angles of 23.6,
34.2, 47.5, and 55.7° were observed, revealing the character-
istics of monoclinic WO3. This is confirmed by the sharp
features at 2θ = 23.1, 23.6, 24.4, 33.3, and 34.2°, which
corresponds to (002), (020), (200), (120), and (112)
crystallographic planes of WO3, respectively (JCPDS card no.
00-043-1035),2,7 observed for the WO3-500 sample calcined at
500 °C, as shown in Figure 1A(a). On the other hand, the XRD
patterns of the as-prepared Rux-WO3 (x = 0.5, 1.0, and 1.5 wt
%) samples, especially the Ru1.5-WO3 with the highest Ru
loading, exhibited weak characteristic diffraction peaks at 2θ =
38.3, 42.2, 44.0, 58.3, 69.4, and 78.4°, which may be assigned to
the (100), (002), (101), (102), (110), and (103) planes of the
hexagonal close-packed (hcp) Ru metal (ICDD JCPDS card
no. 06-0663).26 The presences of weak diffraction peaks on top
of the intense characteristic peaks accountable for the WO3
support reveal a well-dispersed Ru NPs on the surfaces of the
WO3. By means of the FULLPROF software, the Rietveld
refinement powder XRD spectrum of the Ru1.0-WO3 sample
was obtained (see Figure S3; SI) with a reasonable goodness of
fit of χ2 = 5.12. All diffraction peaks were well-fitted with the
monoclinic structure with the P21/c space group and refined
lattice parameters of a = 7.3099(4) Å, b = 7.5433(5) Å, c =
7.6989(6) Å, β = 90.7691(3), which are in good agreement
with previous reports.27,28 Strong reflections accountable for
RuO2 (peak at 28.1°) and hcp Ru metals (peaks at 42.2 and
69.4°) were found. However, no evidence accountable for
RuO2 structure was found in XRD patterns observed for the
Ru-WO3 samples.
To gain information on textural properties of the WO3 and
Ru-WO3 samples, N2 adsorption/desorption isotherm measure-
ments were performed, as shown in Figure 1B. All samples
showed typical type IV isotherm (IUPAC classification) with a
type H3 hysteresis loop, indicating the presence of
mesoporosities.29 Further textural analyses revealed that the
bare WO3 exhibited only low BET surface area (SBET), total
pore volume (VTot), and BJH pore size (dBJH) of materials are
about 12.8 m2 g−1, 0.062 cm3 g−1, and 6.4 nm, respectively (see
Table S1; SI). Upon incorporating Ru NPs onto the WO3
support, progressive decreases in SBET, VTot, and dBJH with
increasing Ru metal loading were observed, indicating the
successful loading of Ru NPs in the pore walls of the WO3
support.
The thermal stabilities of the bare WO3 and Ru-WO3
samples were studied by TGA, as shown in Figures S4 and
S5 (SI). Typically, the calcined WO3 samples showed multiple
weight-loss peaks. The initial weight-loss of ca. 12% occurred in
the temperature range of RT−220 °C was attributed to the loss
of crystal water, whereas the peaks at ca. 286 °C was due to
decompositions of CTAB and organic moiety.30 However, the
weight-loss above 600 °C is due to desorption of oxygen-
containing groups. Overall, a net weight-loss of ca. 18.8 wt % at
900 °C was obtained, indicating the formations of crystallized
Ru−W−O inorganic phase.
As revealed by the SEM image shown in Figure S6A (SI), the
nanosized Ru1.0-WO3 composite showed random aggregates in
surface morphology. Further analysis by EDX and element
mapping clearly indicate a homogeneous distribution of O
(56%), Ru (0.9%), and W (44%) elements throughout the
Ru1.0-WO3 substrate, confirming the uniform dispersion of Ru
NPs on the WO3 support (see Figures S6B−E; SI).
31
Moreover, both the bare WO3 and the Ru1.0-WO3 samples
may be homogeneously suspended in water after a brief
sonication treatment (2 min) at room temperature (Figure S7;
SI). The structural morphology of the WO3 and Ru-WO3
catalysts were also confirmed by FE-TEM study. As shown in
Figure 2A−F, the corresponding FE-TEM images revealed that
the bare WO3 exhibited crystalline platelet morphology with
particle sizes in the range of 40−60 nm. Moreover, a well-
dispersed Ru NPs with sizes in the range of 3−6 nm on the
surfaces of WO3 was also observed.
32 By comparison, the
uncalcined WO3 material showed micron-size crystalline
aggregates with irregular shapes (Figure S8; SI).
The XPS survey spectrum in Figure 3A clearly indicates the
presences of Ru, W, and O elements on the surfaces of the
Ru1.0-WO3 catalyst sample. The strong peak with binding
energy (BE) centering at ca. 282 eV should be due to
overlapping contributions from C 1s (ca. 289 eV), Ru 3d3/2
(284.3 eV), and Ru 3d5/2 (280.7 eV),
31 as shown in Figure 3B.
Moreover, as revealed earlier by UV−vis study (Figure S1; SI),
a complete reduction of Ru3+ ions to Ru0 metal state on the
surfaces of the WO3 support during the microwave irradiation
may be inferred. The spectrum observed for the W 4f core-level
(Figure 3C) revealed XPS peaks corresponding to W 4f7/2
(35.9 eV) and W 4f5/2 (38.2 eV), indicating the presence of
W6+ oxidation state.27,28 Whereas, the peaks with BEs of ca.
530.5 and 531.8 eV in the O 1s core-level spectrum (Figure
3D) may be assigned to the oxygen atoms O2− in the lattice and
the W−O bands in the WO3, respectively.
33
Moreover, by comparing the FT-IR spectra obtained from
the bare WO3 and the Ru1.0-WO3 materials with key synthesis
ingredients such as CTAB and PVP, as shown in Figure 4A,
various absorption bands may be assigned. The FT-IR bands at
812 and 1062 cm−1 observed for the structure-directing agent
CTAB in Figure 4A(b) may be attributed to the C−N
+
stretching modes, whereas the bands at 1378 and 1462 cm−1
were due to symmetric vibrational mode of the methylene
(N+−CH3) moiety and CH2 scissoring mode, respectively.
34
However, the bands in the range of 1600−3000 cm−1 are due
to CH2 symmetric and asymmetric stretching vibrations. These
characteristic bands observed for CTAB were also visible in the
FT-IR spectrum of the uncalcined WO3 substrate, in which
additional bands at 793 and 3418 cm−1 corresponding to
stretching vibrations of O−W−O bonds and O−H stretching,35
respectively, were also observed. Nonetheless, the characteristic
bands responsible for CTAB diminished in the FT-IR spectrum
of WO3 after it was calcined at 500 °C in air, as can be seen in
Figure 4A(c), indicating a complete removal of embedded
CTAB moieties. Likewise, by comparing the IR spectra
obtained from PVP in Figure 4A(d) with the Ru1.0-WO3
Figure 1. (A) XRD patterns and (B) N2 adsorption/desorption
isotherms of (a) the as-prepared WO3, (b) Ru0.5-WO3, (c) Ru1.0-WO3,
and (d) Ru1.5-WO3 catalysts.
composite in Figure 4A(e), successful capping of the binder
onto the catalyst. In brief, the prominent IR bands at 1463 and
1424 cm−1 may be attributed to the characteristic absorptions
of the pyrrolidinyl group, while the bands at 1661, 1018, and
3485 cm−1 may be ascribed due to CO, C−N, and −OH
stretching vibrations in PVP, respectively.36
Furthermore, the optical properties of the bare WO3 sample
were monitored by additional UV−vis and photoluminescence
(PL) spectroscopic techniques. The UV−vis absorbance peak
located at ca. 340 nm in Figure 4B(a) may be assigned to
ligand-to-metal charge transition (O2p → W5d-O2p) of the WO3
for which the energy required for the transition depends
strongly on concentration of W and oxidation temperature.37
Figure 2. (A,B) FE-TEM images of the bare WO3 and (C−F) Ru1.0-WO3 catalysts at different magnifications; Insets in (A) and (F) are the
corresponding SAED patterns.
On the other hand, the as-prepared WO3 showed emission
peaks in the visible light region at 440, 481, and 527 nm due to
due to the nanosized particles and quantum confinement effect
of the semiconductor material.38 As expected, the yellowish
WO3 NPs suspension exhibited a strong blue luminescence
emission under UV light (365 nm) excitation, as illustrated in
Figure 4B (inset). The above results revel the presences of size-
dependent and charge transition effects in the WO3 crystalline
NPs.
3.2. Electrocatalytic Activity of Ru-WO3-Modified
Electrodes for Oxidation of N2H4. The electrocatalytic
activity of the Ru-WO3-modified electrodes for oxidation of
N2H4 was investigated by cyclic voltammetry (CV) and
chronoamperometry (CA) methods. To avoid impairing of
catalytic activity by the binder,39 the Ru-WO3 catalyst was
coated onto a polished glassy carbon electrode (GCE) substrate
in the absence of a polymeric binder. Figure 5A shows the CV
curves obtained from the bare GCE before and after
modification by the WO3, Ru NPs, and Ru1.0-WO3 catalysts
in 10 μM N2H4 containing N2-saturated phosphate buffer
solution (PBS; pH = 7) at a scan rate of 50 mV s−1. It is
obvious that the bare GCE and WO3-modified GCE showed
nearly null response for oxidation of N2H4 within the potential
range of −0.8 to 0.7 V.
By comparison, the Ru NPs-modified GCE showed a weak
oxidation peak at −0.3 V. On the other hand, the Ru1.0-WO3-
modified GCE had a pronounced oxidation peak with an
anodic peak potential (Epa) of 0.257 V and highest oxidation
peak current (Ipa; ca. 100 μA) for oxidation of N2H4. Compared
to the bare GCE, and Ru NPs-and WO3-modified electrodes,
the excellent catalytic activity observed over the Ru1.0-WO3-
modified electrode during oxidation of N2H4 is due to their
unique structural and physicochemical properties of the
nanocomposite as well as the synergistic effect of the WO3
support and well-dispersed Ru NPs, which provoke formations
of surface active sites favorable for enhancing the reversibility of
the electron transfer process. It is notable that in the absence of
the N2H4 analyte, the Ru1.0-WO3-modified GCE alone also
exhibited a redox behavior, as shown in Figure 5B. In the
presence of N2H4, the Ru1.0-WO3-modified GCE showed
enhanced oxidation peak current at a lower potential (0.261 V),
revealing an effective oxidation reaction, which is desirable as a
binder-free electrochemical sensor for N2H4 detection. The
electrooxidation of N2H4 over the Ru1.0-WO3-modified GCE
could be established by the four-electron transfer process,
which may be expressed as40,41
+ → + ++ −N H H O N H H O e (slow)2 4 2 2 3 3 (1)
+ → + ++ −N H 3H O N 3H O 3e (fast)2 3 2 2 3 (2)
Here, eqn 1 represents the rate-determining step invoking a
single-electron transfer, followed by a fast step involving three-
electron transfer processes to give N2 as a final product (eq 2).
Thus, the overall mechanism for N2H4 oxidation can be
expressed as
+ → + ++ −N H 4H O N 4H O 4e2 4 2 2 3 (3)
The effects of Ru loading (x) on the electrocatalytic activities
of Rux-WO3-modified GCEs in the presence of N2H4 were also
investigated, the oxidation peak currents obtained for various
sensors with x = 0.5, 1.0, and 1.5 wt % are summarized in
Figure S9 (SI). Based on the oxidation peak currents obtained
from CV measurements (scan rate 50 mV s−1), it is obvious
that the Ru1.0-WO3-modified GCE showed slightly higher Ipa
value than its counterparts with x = 0.5 and 1.5 wt %. Clearly,
the growth and crystalline natures of Ru NPs and the WO3
support tend to change with the amount of Ru loading. Thus,
Figure 3. XPS (A) survey spectrum, and corresponding (B) Ru 3d,
(C) W 4f, and (D) and O 1s core-level spectra of the Ru1.0-WO3
catalyst.
Figure 4. (A) FTIR of (a) CTAB, (b) uncalcined, and (c) calcined WO3, (d) PVP, and (e) Ru1.0-WO3 samples, and (B) UV−vis (a) absorption and
(b) emission spectra of WO3. Inset: photoluminescence photographs of water suspended WO3 under sunlight and UV (365 nm) excitations.
we have chosen the Ru1.0-WO3-modified GCE for the
subsequent electrochemical studies.
3.3. Effect of Electrolyte pH and Scan Rate on
Electrocatalytic Activity during Oxidation of N2H4. The
pH of the electrolyte solution normally plays a major role
during the electrooxidation process, and thus, its influence on
electrocatalytic activity of N2H4 oxidation was also investigated.
As shown in Figure S10A (SI), CV curves for the Ru1.0-WO3-
modified GCE with 10 μM N2H4 in controlled N2-saturated
PBS solutions at different pH values (3−11) were recorded at a
scan rate of 50 mV s−1. It can be seen that a maximum peak
current was observed at a pH of 7, as shown in Figure S10B
(SI). Further increasing the electrolyte pH beyond 7 resulted in
a notable decrease in the observed peak current due to the
protonation of N2H4. As such, an electrolyte pH of 7 was
chosen for the subsequent experiments. Moreover, a linear
correlation between the anodic peak potential (Epa) with pH of
the electrolyte solution may be inferred with a correlation
coefficient of 0.992. The slope observed for the Epa vs pH plot,
− 54.8 mV, was in good agreement with that reported for other
N2H4 sensors.
42 The above results further verify that the
electrooxidation of N2H4 over the Ru1.0-WO3-modified GCE
indeed invoked a four-electron transfer process. Previously, it
has been shown43,44 that metal−supported tungsten oxide (M-
WO3) catalysts showed inferior electrooxidation activity under
either strong acid or strong base conditions. Although
formations of several possible stable phases of tungsten oxides
such as hydrogen tungsten bronzes (H0.18WO3 and H0.35WO3)
and substoichiometric WO3−y (0 < y ≤ 1) have been proposed
in acidic electrolyte systems.45 Experimental results reported
herein reveal that the best electrooxidation activity of N2H4
over the Ru-WO3-modified GCE is under a neutral electrolyte
solution with pH = 7. Because no catalyst aggregation was
observed, it is indicative that the Ru1.0-WO3 catalyst remained
stable during oxidation of N2H4 at an electrolyte pH of 7. As
such, the formation of hydrogen tungsten bronzes compound
(HxWO3) may also be ruled out.
Thus, it is conclusive that the oxidation of N2H4 over the Ru-
WO3-modified GCE in the neutral PBS electrolyte solution
readily invoked a four-electron process (eq 3) through the
formations of reaction intermediates such as N2H3 and H3O
+
(eqs 1 and 2) to result in N2 and 4H3O
+ products. In this
context, the enhanced catalytic activity may be due to the
dispersed Ru NPs on the surfaces of the WO3 support, which
tend to promote formations of active W5+ sites during the
reaction. Such synergistic effect between the Ru NPs and the
WO3 support was accountable for the superior performances
during catalytic reactions.46,47
The effect of scan rate on electrochemical performances of
the N2H4 sensor based on Ru1.0-WO3-modified GCE electrode
was also investigated, as shown in Figure 5C. It is clear that,
upon gradually increasing the scan rates from 10 to 200 mV s−1,
a progressive increase in the oxidation peaks current (Ipa) and
shifting of the corresponding peak potential toward positive
direction were observed. Besides, the oxidation peak currents
(Ipa) are linear over the square root of scan rates from 10−200
mV s−1 (Figure 5D), indicating the electro-oxidation of N2H4
was diffusion controlled kinetic process over the Ru1.0-WO3-
modified GCE.48
3.4. Reaction Kinetics and Proposed Mechanism for
Electrooxidation of N2H4. The kinetics of the electro-
chemical sensor system were further studied by chronoamper-
ometry (CA). Figure 6 displays the CA profiles observed for
the Ru1.0-WO3-modified GCE without and with the presence of
N2H4 in N2-saturated PBS solution (pH = 7). Compared to the
bare modified GCE, which exhibited only very low response
current (Ib, the limiting current without N2H4 analyte), a
notable increase in CA response current (Ip) over the Ru1.0-
Figure 5. (A) CV curves of bare GCE, WO3, Ru NPs, and Ru1.0-WO3-modified GCE with N2H4; (B) Ru1.0-WO3-modified GCE with and without
the presence of N2H4; and (C) Ru1.0-WO3-modified GCE with N2H4 recorded at different scan rates (10−200 mV s
−1). (D) The corresponding
calibration plot of peak current (Ipa) vs square scan rate (10−200 mV s
−1). All CV measurements were carried out using 10 μM N2H4 in N2-
saturated PBS solution (pH 7) at a scan rate of 50 mV s−1.
WO3-modified GCE was observed when in the presence of
N2H4. The rate equation of response current may be expressed
as49
π=
I
I
KCt( )
p
b
1 / 2
(4)
where Ip and Ib represents the response and limiting current
with and without the presence of N2H4, respectively. C is the
concentration (in mol cm−3) of the N2H4 analyte, t is the time
(in second), and K denotes the reaction rate constant. Thus, on
the basis of the Ip/Ib vs t
1/2 plot shown in Figure 6 (inset), a K
value of 2.81× 104 M−1 s−1 may be derived. In addition, the
diffusion coefficient (D) of N2H4 may also be estimated by the
Cottrell equation:50,51
π
=I
nFAC D
tp (5)
where Ip denotes the peak current (in A), n is the number of
electron, F = 96,485 C mol−1 is the Faraday constant, A is the
surface area of the electrode (cm2), C represents bulk
concentration of the analyte (mol cm−3), and t is time (s).
Accordingly, by taking the eq 5, the value of D = 3.16 × 10−6
cm2 s−1 for N2H4. Moreover, the surface coverage of the
electroactive species (Γ) on the Ru1.0-WO3-modified working
electrode may be calculated from the equation:52
υ
=
Γ
I
n F A
RT4p
2 2
(6)
where υ is the scan rate (mV s−1), R is the gas constant (8.314 J
mol−1 K−1) and T is the temperature (in °C). By using eq 6, the
Γ value was calculated to be 1.46 × 10−9 mol cm−2. The above
values of K, D, and Γ so obtained are in close agreements with
those reported earlier for the other N2H4 sensors.
53 In addition,
the plot of peak potential (Ep) showed a linear relationship over
the log of scan rates. Based on the literature, the linear
relationship can be expressed as the following eq 7:54
α
υ= +
α
⎡
⎣⎢
⎤
⎦⎥E K
RT
n F
2.303
2
logp
(7)
where, Ep is the peak potential of N2H4 oxidation, α is s the
transfer coefficient for N2H4 oxidation, nα is s the electron
transfer number involved in the rate-determining step of N2H4
oxidation, K is a constant; R, T, and F have their usual
significance (R = 8.314 JK−1 mol−1, T = 298 K, F = 96485 C
mol−1). Assuming that one-electron transfer is the rate-
determining step (n = 1), the values of α and n involved
during the electron transfer process were calculated as 0.34 and
3.72, respectively.
On the basis of the above results, which are in accordance
with previous literature reports,43−47,55 a plausible reaction
pathway for electrooxidation of N2H4 is proposed, as illustrated
in Scheme 2. In brief, during the oxidation reaction over the
Ru-WO3 catalyst, the N2H4 molecules tend to adsorb on the
surfaces of the catalyst at first, followed by interfacial electron
transfers between the Ru NPs and the WO3 support, which
result in formations of hydronium ions (H3O
+) in aqueous
solution and partial reduction of W6+ to W5+. This W5+ active
site tends to accelerate the oxidation of N2H4, leading to an
enhanced electrochemical activity. Thus, the rate-determining
step for electrooxidation of H2N4 involved an one-electron
transfer process, followed by a three-electron process to give N2
as the final product, as specified in (eqs 1−3).
3.5. Electrochemical Performances of the N2H4
Sensor. To further assess the electrochemical performances
of the Ru1.0-WO3-modified GCE during detection of N2H4,
additional measurements by the amperometric (i−t) method
were performed. Figure 7A shows the amperometric i−t
response of different additions of N2H4 at Ru1.0-WO3-modified
rotating disk electrode (RDE) in constantly stirred N2-saturated
PBS (pH 7) at an applied potential of 0.261 V and rotation
speed of 1200 rpm. The amperometric response of N2H4 shows
a sharp oxidation peak current with the addition of 0.7 μM
N2H4 into the constantly stirred N2-saturated PBS. The steady-
state current of N2H4 oxidation was reached within 3 s, which
indicated fast electro-oxidation of N2H4 on the electrode
surface. As expected, it can be clearly seen that the oxidation
peak current was gradually increased with the successive
addition of N2H4 from the concentrations of 0.7−1129.9 μM,
which indicated the rapid electro-oxidation of N2H4 at Ru1.0-
WO3-modified electrode. In addition, the anodic peak current
of N2H4 oxidation had a linear relationship over the N2H4
concentrations from 0.7−709.2 μM with the correlation
coefficient of 0.9903, as shown in Figure 7B. The calculated
sensitivity of the sensor was 4.357 μA μM−1 cm−2. The limit of
detection (LOD) was estimated to be 0.3625 μM based on the
standard formula as mentioned below (5)20,48
Figure 6. CA profiles of the Ru1.0-WO3-modified electrode with (blue
curve) and without (red curve) the presence of N2H4 (10 μM) in PBS
electrolyte solution (pH 7). Inset: variations of Ip/Ib with square root
of time (t1/2).
Scheme 2. Schematic Illustration of Electrooxidation of
N2H4 over the Ru-WO3 Catalyst
where Sb is the standard deviation of the blank signal and q is
the slope value (obtained from calibration plot). The analytical
performance (sensitivity, LOD, and linear range) of the
proposed sensor was compared with previously reported
N2H4 sensor, and the results are summarized in Table
1.22−24,56−61 These findings demonstrate that the Ru1.0-WO3-
modified electrode showed an excellent electrocatalytic activity,
good linear range, and lower LOD toward the oxidation of
N2H4.
3.6. Selectivity, Stability, and Reproducibility of the
N2H4 Sensor. Selectivity of the N2H4 sensor was normally
affected by common metal ions and biological molecules such
as dopamine (DA), uric acid (UA), ascorbic acid (AA) and
glucose (Glu). Hence, we have investigated the selectivity of
the sensor in the presence of common metal ions and biological
molecule by amperometry, as shown in Figure 8A. It can be
seen that a sharp peak was observed with the addition of 10 μM
of N2H4 (a) in N2-saturated constantly stirred PBS. There is no
change in the peak current even in the presence of 200-fold
excess concentrations of Ni2+, Co2+, Zn2+, Ca2+, Br−, Cl−, I−, F−,
SO3
2− and 50-fold higher concentration of DA, UA, AA, and
Glu in N2-saturated PBS. These results further conclude that
the proposed sensor exhibits an excellent anti-interference
ability toward the detection of N2H4. In addition, the
operational stability of the sensor was exhibited up to 93.6%
of its initial response current in the presence of 10 μM of N2H4
containing PBS constantly run up to 2000 s as shown in Figure
8B. This result suggested good operational stability of the Ru-
WO3-modified electrode.
The storage stability of the sensor was important for
evaluating the material stability. Hence, we have also
Figure 7. (A) Amperometric responses of the Ru1.0-WO3-modified GCE under consecutive injection of N2H4 within a total dosage range of 0.7−
1129.9 μM and (B) the corresponding calibration plot of response current vs N2H4 concentration. All measurements were conducted in N2-saturated
PBS (pH = 7) at a rotation speed of 1200 rpm and an anodic potential (Epa) of +0.261 V.
Table 1. Comparison of Analytical Parameters at Ru-WO3-modified Electrode with Previously Reported N2H4 Sensors
modified electrode method pH linear range (μM) detection limit (μM) sensitivity (μA μM−1 cm−2) ref
Figure 8. (A) Amperometric response of Ru1.0-WO3-modified RDE
containing 10 μM N2H4 (a), in the presence of a 200-fold excess
concentration of metal ions (Ni2+, Co2+, Zn2+, Ca2+); anions (Br−, Cl−,
I−, F−, SO3
2−) and 50-fold excess concentration of DA, UA, AA, and
Glu; and (B) Stability of Ru1.0-WO3-modified electrode in the
presence of 10 μM of N2H4 containing PBS constantly run up to
2000 s.
investigated the storage stability of the sensor by CV. The Ru-
WO3-modified electrode was performed toward the oxidation
of N2H4 in N2-saturated PBS (pH 7). The response current was
carefully checked for every 2 days. The sensor retains 91.3% of
its initial response current after 10 days. The electrode was
stored at 4 °C, when not in use. This result authenticates the
excellent storage stability of the sensor. In order to evaluate the
reproducibility of the sensor, it was examined by CV toward the
detection of 10 μM of N2H4 in N2-saturated PBS. Prior to
analysis, five different modified electrodes were prepared and
investigated by CV in the presence of 10 μM of N2H4-
containing PBS buffer. The relative standard deviation (RSD)
was estimated to be 3.8%. The repeatability of the sensor was
examined using a Ru1.0-WO3-modified electrode by CV. The 10
successive measurements were performed in PBS (pH 7)
containing 10 μM of N2H4. The RSD value of the sensor
retains 3.85%. Hence, the proposed Ru1.0-WO3-modified
electrode shows excellent storage stability, good repeatability,
and reproducibility toward the detection of N2H4.
3.7. Real Sample Test. The Environmental Protection
Agency (EPA) declares that N2H4 is present in cigarette sample
at a level of 50 ± 5 μg/gram.62,63 On this basis, we can use the
cigarette sample for determination of N2H4 content for
practical application. In order to determine the N2H4 level in
a cigarette sample, first we need to prepare the sample. Briefly,
the commercially available cigarette was purchased from the
local market. Then, the cigarette sample was prepared in PBS.
The unknown concentration of the N2H4-containing cigarette
sample was studied by amperometry using the standard
addition method. The cigarette sample was diluted 10 times
before the experiment. After that, the known concentration of
N2H4 was spiked into the PBS containing the cigarette sample.
The recovery values of the sensor were ranging from 94.5% to
99.5%, suggesting accuracy of the proposed sensor. In addition,
we have also performed the quantitative analysis N2H4 using
the high-performance liquid chromatography (HPLC) method.
The obtained recovery values are compared with our
electrochemical method. The experimental results are summar-
ized in Table 2.
Compared with HPLC method, our proposed sensor has
almost reached the same recovery values for N2H4. This result
confirms that the developed sensor is reliable and accurate
determination of N2H4 in cigarette sample and can be
employed for the determination of N2H4 for practical
applications.
3.8. Catalytic Oxidation of Diphenyl Sulfide in H2O2.
For catalysis applications, WO3 is a very promising material
regarding its low cost and ease to synthesis, high thermal
stability, good morphological, and structural properties.64 In
addition, WO3 is a well-studied wide band gap semiconductor
(∼2.75 eV) used for several applications including pH
sensors,65 biosensors,66 catalysis,67,68 and so on. In the past
few years, various types of tungsten-based heterogeneous
catalysts have been receiving much more attention in the
selective oxidation of sulfides to sulfoxides using H2O2 as
oxidant.69,70 Recently, our group reported the use of
heterogeneous Ru/Al2O3 catalyst for the direct oxidation of
sulfides by H2O2 in the acetonitrile (CH3CN) and water mixed
solvent.70 In our report, for the first time, we used Ru-WO3 as a
catalyst and H2O2 as an oxidant for the oxidation of diphenyl
sulfide (DPS) to diphenyl sulfoxide (DPSO) using CH3CN as a
solvent at 60 °C in 5 min under MW irradiation (Scheme 3)
sulfoxides or sulfones
In the catalysis reaction, the mixture of catalyst (0.5 mol %),
1 mmol of DPS, and 30% H2O2 (1.5 mmol) in 3 mL of
CH3CN were heated under MW irradiation at 60 °C for 5 min.
After completion of the reaction, as indicated by thin-layer
chromatography (TLC), the product was extracted with ethyl
acetate (10 mL). The combined organic extracts were
concentrated in vacuum and the resulting product was purified
by column chromatography on silica gel with ethyl acetate and
n-hexane as eluent to afford the product (yield 98%; colorless
solid). However, when the reaction was conducted in the
presence of the WO3 as a catalyst, the main product of DPSO
was efficiently formed with yield (94%), compared to Ru-WO3
catalyst, indicating Ru1.0-WO3 was the active catalyst. Excess the
amount of oxidant (H2O2), causes the formation of sulfones as
a final product with highest yield (99%). The obtained products
were confirmed with authentic sample. We have compiled the
catalytic performance of our catalyst system with other catalysts
in Table S2 (SI). Notably, our catalysts also show excellent
catalytic performance for the oxidation of DPS in to DPSO was
obtained in good to excellent yields. Even in the case of other
tungstate−based catalysts afforded in moderate yields 52 and
55%, respectively (Table S2, SI).
As can be clearly seen, all the catalysts were required long
time, but our catalyst protocol needed only 5 min, which
superior to those of the other catalysts. Since, the advantage of
microwave-assisted oxidation reaction route is more energy
efficient, cost-effective, and time-saving and so on. Moreover,
considering these initial promising results and the selective
oxidations of various challenging sulfides were explored under
identical reaction conditions in future.
4. CONCLUSIONS
In summary, Ru1.0-WO3 catalyst was synthesized via a facile
microwave method have been developed and exploited as
electrode supports for both electrocatalytic oxidation of N2H4
as well as catalytic oxidation of aromatic sulfides. The fabricated
carbon-free nanostructured Ru-WO3 catalysts were character-
ized by a variety of analytical and spectroscopy techniques. The
performance of bare WO3 was found to be poor compared with
the Ru1.0/WO3 in terms of electro-oxidation of N2H4. A
possible electrocatalytic reaction mechanism for the N2H4 over
the Ru1.0/WO3 catalyst is proposed. However, it is a more
stable phase during the reaction in the presence of H2O2 and is
therefore a prospective material for catalytic applica-
Table 2. Determination of N2H4 in Cigarette Sample at Ru-
WO3-Modified Electrode by Amperometry
sample spiked (μM) found (μM) recovery (%) RSD (%)
tions.64,67−69 Moreover, these results clearly demonstrate that
the WO3-supported Ru catalyst possesses desirable electro
catalytic and catalytic properties, which should facilitate
prospective applications. Further investigations on other useful
applications of this catalyst are in progress.
■ ASSOCIATED CONTENT
*S Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acsami.7b07645.
Experimental results from UV−vis, XRD, TGA, SEM,
TEM, EDX, and CV studies and textural property data of
assorted WO3 and Ru-WO3 samples (PDF)
■ AUTHOR INFORMATION
Corresponding Authors
*E-mail for S.-M.C.: smchen78@ms15.hinet.net.
*E-mail for P.V.: spveerakumar@gmail.com.
ORCID
Shen-Ming Chen: 0000-0002-8605-643X
Pitchaimani Veerakumar: 0000-0002-6899-9856
Author Contributions
⊥C.R. and B.T. contributed equally.
Notes
The authors declare no competing financial interest.
■ ACKNOWLEDGMENTS
The authors are grateful for the financial support (NSC 101-
2113-M-027-001-MY3 to S.M.C; NSC 104-2113-M-001-020-
MY3 to S.B.L.) from the Ministry of Science and Technology
(MOST), Taiwan.
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Synthesis of PtRu Nanoparticles from the Hydrosilylation Reaction and Application as
Catalyst for Direct Methanol Fuel Cell
Junchao Huang,* Zhaolin Liu, Chaobin He, and Leong Ming Gan
Institute of Materials Research and Engineering (IMRE), 3 Research Link, Singapore 117602
ReceiVed: May 21, 2005; In Final Form: July 20, 2005
Nanosized Pt, PtRu, and Ru particles were prepared by a novel process, the hydrosilylation reaction
.
The
hydrosilylation reaction is an effective method of preparation not only for Pt particles but also for other metal
colloids, such as Ru. Vulcan XC-72 was selected as catalyst support for Pt, PtRu, and Ru colloids, and TEM
investigations showed nanoscale particles and narrow size distribution for both supported and unsupported
metals. All Pt and Pt-rich catalysts showed the X-ray diffraction pattern of a face-centered cubic (fcc) crystal
structure, whereas the Ru and Ru-rich alloys were more typical of a hexagonal close-packed (hcp) structure.
As evidenced by XPS, most Pt and Ru atoms in the nanoparticles were zerovalent, except a trace of oxidation-
state metals. The electrooxidation of liquid methanol on these catalysts was investigated at room temperature
by cyclic voltammetry and chronoamperometry. The results concluded that some alloy catalysts showed higher
catalytic activities and better CO tolerance than the Pt-only catalyst; Pt56Ru44/C have displayed the best
electrocatalytic performance among all carbon-supported catalysts.
Introduction
Metal and semiconductor nanoparticles have been extensively
explored for many years due to their wide application in the
fields of catalysis, photography, optics, electronics, optoelec-
tronics, data storage, and biological and chemical sensor.1-6 Pt
and Pt alloy nanoparticles are catalytically active in room
temperature electrooxidation reactions of interest to direct
methanol fuel cell (DMFC) applications. However, the perfor-
mance of DMFC is significantly affected by CO concentrations
in the fuel cell.7 This is because of the strong adsorption of
carbon monoxide on the Pt anode, which inhibits the hydrogen
oxidation reaction. It has been reported that electrocatalytic
activities of the anode is significantly enhanced as Pt is alloyed
with Ru, Sn, and Mo, etc. So far, the incorporation of Ru into
the Pt catalyst has yielded the best results. To address the
improved Pt catalytic activities toward methanol oxidation by
Ru, two mechanisms have been proposed. One is the bifunc-
tional mechanism: In the presence of Ru surface atoms,
adsorbed CO is oxidized at potentials more negative than that
on Pt. Thus, the Pt surface sites become more available for
hydrogen adsorption and oxidation;8 the other mechanism is the
ligand-effect mechanism: the modification of electronic proper-
ties of Pt via a Pt-Ru orbital overlap.9
It is well-known that the properties of metal nanoparticles,
such as catalytic activity, photoluminance, and optical properties,
are strongly dependent on the particle shape, size, and size
distribution.10-16 Conventional preparation techniques based on
wet impregnation and chemical reduction of the metal precursors
often do not provide adequate control of particle shape and
size.17 There are continuing efforts to develop alternative
synthesis methods based on microemulsions,18 sonochemis-
try,19,20 microwave irradiation,21-25 and catalytic organic reac-
tion,26,27 which are more conducive to generating nanoscale
colloids or clusters with better uniformity.
The hydrosilylation reaction, an addition of a hydrosilane unit
(Si-H) to a double bond (CdC) to form an alkylsilane (Scheme
1), is widely utilized in the production of silicon polymers, liquid
injection molding products, paper release coatings, and pressure-
sensitive adhesives.28 The hydrosilylation reaction can be
initiated in numerous ways, and one of the most commonly used
platinum-based catalysts is the Karstedt catalyst (platinum
divinyltetramethyldisiloxane complex).29-31 During the course
of the Pt-catalyzed hydrosilylation reaction, the formation of
colloidal Pt species was previously regarded as an undesired
side reaction, which resulted in coloration of the final reaction
solution.32 In contrast, this “side reaction” can be exploited to
synthesize Pt or Ru nanoparticles.
In the previous papers, we reported the synthesis of Pt
nanoparticles from the hydrosilylation reaction,26,27 and micro-
wave-assisted synthesis of carbon-supported PtRu nano-
partcles,24,25,33,34which could be applied as catalysts for direct
methanol fuel cell. In the current paper, the electrooxidation of
liquid methanol on Pt and PtRu alloy nanoparticles, synthesized
from the hydrosilylation reaction, was investigated. Pt and Pt
alloys show catalytic activities in room temperature electrooxi-
dation reactions that are of interest to fuel cell applications. To
the best of our knowledge, we are not aware of any other
investigation into electrochemical properties of Pt and PtRu
alloys synthesized in the hydrosilylation reaction. Moreover,
we sought to extend this method to prepare other metal
nanoparticles; it was found for the first time that Ru nano-
particles were successfully synthesized in the hydrosilylation
reaction, and further studies on other metal nanoparticles are
in progress.
Chemicals. 1,1,3,3-Tetramethyldisiloxane (T2H) and di-
chlorotris(triphenylphosphine)ruthenium (Ru(PPh3)3Cl2) were
purchased from Aldrich and used as received. 1-Decene was
purchased from Lancaster. Toluene was distilled over sodium/
benzophenone under nitrogen immediately prior to use. Platinum
divinyltetramethyldisiloxane complex (Pt(dvs)) was obtained
from Aldrich and diluted to a 10 mM solution in anhydrous
toluene before use. Other chemicals were used as received
without further purification.
Synthesis of Pt, Ru, and PtRu Alloy Nanoparticles.T2H
(2 mmol, 0.269 g), 1-decene (8 mmol, 1.124 g), and the
predetermined amount (Table 1) of Ru(PPh3)3Cl2 were placed
in a 50 mL of Schlenk flask with a magnetic stirrer. The reaction
flask was charged by anhydrous toluene and stirred at room
temperature until all chemical dissolved in toluene. The flask
was evacuated and refilled with nitrogen three times. After that,
Pt(dvs) (10 mM solution) was added by a syringe, and then the
reaction was stirred under nitrogen at 100°C for several days.
The reaction solutions were centrifuged; the black powders were
obtained after decanting off the solvent. And, the samples were
washed by toluene and centrifuged twice to remove the
uncoordinated molecules and dried under vacuum. The formula-
tion and yield of the hydrosilylation reaction were shown in
Table 1. In the hydrosilylation reaction, two reaction catalysts,
Pt(dvs) and Ru(PPh3)3Cl2, were used, which induced the
formation of Pt, PtRu, or Ru nanoparticles in reaction solutions.
As the concentration of Pt(dvs) in the reaction solution increased,
the reaction rate also increased, indicating catalytic activity of
Pt(dvs) is higher than that of Ru(PPh3)3Cl2.
Characterization. Transmission electron microscopy (TEM)
images were acquired on a Philip CM300 TEM operating at an
acceleration voltage of 300 kV. TEM samples were prepared
by depositing several drops of diluted colloidal solution onto
standard carbon-coated copper grids, followed by drying under
ambient condition for 1 h. X-ray diffraction (XRD) patterns were
recorded by a Bruker GADDS diffractometer with area detector
using a Cu KR source (λ ) 0.1542 nm) operating at 40 kV and
40 mA. XPS spectra were obtained using a VG Scientific
EscaLab 220 IXL with a monochromator Al KR X-ray source
(hν ) 1486.6 eV), and narrow scan photoelectron spectra were
recorded for Ru 3p and Pt 4f. Fourier transform infrared (FTIR)
spectra were measured with a Bio-Rad 165 FTIR spectropho-
tometer. 1H NMR spectra were collected on a Bruker 400
spectrometer using chloroform-d as solvent and tetramethylsilane
as internal standard. UV-vis spectra were collected using a
SHIMADZU UV-2501PC UV-vis recording spectrophotom-
eter.
Electrochemical Measurement.The Pt or PtRu nanoparticles
were washed by toluene to get rid of uncoordinated molecules
that formed in the hydrosilylation reaction. The Pt or PtRu
nanoparticles were supported on high surface area Vulcan XC-
72 carbon (20 wt % metal content) by combining a toluene
dispersion of Pt nanoparticles with a suspension of Vulcan
carbon in toluene. The solution was vigorously stirred for 2 h.
Solvent was evaporated and the powder was dried at 60°C in
a vacuum. To remove the stabilizing shell on the Pt nanopar-
ticles, as-synthesized Pt/C catalysts were heat-treated in argon
at 360°C for 10 h. The furnace was purged with argon gas for
at least 15 min prior to the heat treatment. The prepared Pt/C
catalysts for electrochemical measurement had a nominal metal
loading of 20 wt % on the Vulcan carbon black support.
An EG&G Model 273 potentiostat/galvanostat and a con-
ventional three-electrode test cell were used for electrochemical
measurements. The working electrode was a thin layer of
Nafion-impregnated catalyst cast on a vitreous carbon disk held
in a Teflon cylinder. The catalyst layer was prepared as reported
previously.33 Pt gauze and a saturated calomel electrode (SCE)
were used as the counter and reference electrodes, respectively.
All potentials quoted in this paper were referred to the SCE.
All electrolyte solutions were deaerated by high-purity argon
for 2 h prior to any measurement. For cyclic voltammetry and
chronoamperometry of methanol oxidation, the electrolyte
solution was 2 M CH3OH in 1 M H2SO4, which was prepared
from high-purity sulfuric acid, high-purity grade methanol, and
distilled water.
Results and Discussion
Nanoparticles Preparation. In our previous paper,27 when
1-decene and T2H were selected as starting materials, it was
easy to prepare the carbon-supported Pt catalyst for the direct
methanol fuel cell. Furthermore, when the hydrosilylation
reaction was carried out at the excess olefin (1-decene)
concentration, the byproducts containing the Si compound were
easy to remove. Without a Si-contained shell, the catalytic
activity of the PtRu nanoparticles was higher than those from
other methods.
Inductively coupled plasma spectroscopy (ICP) was used to
determine the actual platinum and ruthenium contents in PtRu
alloy nanoparticles. The measured compositions of PtRu alloy
nanoparticles were obtained as Pt26Ru74, Pt56Ru44, Pt77Ru23, and
Pt89Ru11, where the numerical subscripts denote the weight
percentage of the alloying metal. As compared to the theoretical
compositions of Pt25Ru75, Pt50Ru50, Pt75Ru25, and Pt87Ru13, the
measured ruthenium contents in the PtRu alloy nanoparticles
were lower, likely due to the lower catalytic activity of Ru-
(PPh3)3Cl2 in the hydrosilylation reaction.
Narrow-distributed Ru nanoparticles were successfully syn-
thesized by the hydrosilylation reaction. Figures 1 and S1
(Supporting Information) present TEM images and high-
resolution TEM (HRTEM) images of Ru nanoparticles from
the hydrosilylation reaction. In the TEM images (Figure 1) Ru
nanoparticles with a diameter of 3.5 nm were observed. Careful
inspection (Figure S1 in Supporting Information) revealed that
most Ru nanoparticles could be discernible as single crystals
of hexagonal close-packed (hcp) lattice, because clear{101}
lattice planes are observed to cover the whole particles if the
particles were viewed in a proper direction. The lattice spacing,
TABLE 1: Formulation of the Hydrosilylation Reaction for Nanoparticle Synthesis
system Ru(PPh3)3Cl2 Pt(dvs) (10mM) toluene (mL) Reaction time (days) Yield (%)a
a The yield of the hydrosilylation reaction was calculated based on the amount of the metal starting materials.
PtRu Nanoparticles for Direct Methanol Fuel Cell J. Phys. Chem. B, Vol. 109, No. 35, 200516645
0.21 nm, is consistent with that of Ru metal35 and the XRD
results (Figure 1). The XRD patterns show several diffraction
peaks that were indexed to{100}, {101}, {102}, {110}, {103},
{112}, and {201} planes of Ru, respectively. From the XRD
patterns the mean particle sizes, 4.5 nm, were calculated by the
Scherrer equation:36
wherek is a coefficient (0.9),λ is the wavelength of X-ray used
(1.540 56 Å), â is the full width at half-maximum of the
respective diffraction peak (rad), andθ is the angle at the
position of the peak maximum (rad).
Morphology. The morphology of Pt nanoparticles from the
hydrosilylation reaction has been fully characterized in our
previous work.27 In this paper, parts a and b of Figure 2 exhibit
two typical TEM images of the as-synthesized nanoparticles
Pt89Ru11 (Pt-rich alloy) and Pt26Ru74 (Ru-rich alloy); uniform
and well-dispersed alloy particles were observed. As shown in
Figure 2e,f, the average diameters of 2.4 (for Pt89Ru11) and 3.4
nm (for Pt26Ru74) were obtained by direct measurement of TEM
images, as well as relatively narrow particle size distributions
((0.4 nm). Careful investigation of TEM images (Figures S2
and S3 in the Supporting Information) revealed that clear lattice
planes were observed to cover the whole particles if the particles
are viewed in a proper direction; most PtRu nanoparticles could
therefore be discernible as a single-crystal lattice, which
indicated the formation of Pt-rich or Ru-rich alloys. All of the
TEM images of other as-synthesized nanoparticles were shown
in Figure S4 for reference (Supporting Information). Adsorption
of the colloidal particles on Vulcan carbon followed by thermal
treatment (in an argon gas at 360°C for 10 h) to remove the
stabilizing capping agents did not cause significant morphologi-
cal changes (Figure 2c,d). The Pt and alloy nanoparticles were
in a state of high dispersion over the carbon surface, and the
size of the particles was nearly unchanged.
X-ray diffraction patterns provide a bulk analysis of the crystal
structure, lattice constant, and crystal orientation of the as-
synthesized PtRu nanoparticles and their supported catalysts for
the fuel cell. Figure 3 shows the XRD pattern of the as-
synthesized Pt, PtRu, and Ru nanoparticles. For Pt or Pt-rich
alloy nanoparticles, several broad diffraction peaks could be
indexed to the [111], [200], [220], and [311] planes of a Pt
face-centered cubic (fcc) crystal structure. For Ru or Ru-rich
alloy nanoparticles, the diffraction peaks could be indexed to
the [100], [101], [110], [103], and [201] planes of a Ru
hexagonal closed-packed (hcp) lattice. Similarly, the XRD
patterns of the supported catalyst (Pt/C, PtRu/C, and Ru/C) were
exhibited in Figure 4. The diffraction peaks in XRD patterns
could be accordingly indexed to the planes of Pt (fcc) or Ru
(hcp) lattices, respectively. It was also noted that after thermal
treatment (in an argon gas at 360°C for 10 h) the diffraction
peaks increased in intensity and sharpness for Pt and Pt-rich
alloy catalysts, an indication of increase in crystallinity of metals.
The lattice constant of 3.924 Å (for Pt/C catalysts) was in good
agreement with 3.923 Å for pure Pt. The strong diffraction at
2θ < 35° was observed in the Figure 4 due to the X-ray
diffraction of the carbon black support.
As seen in the XRD patterns of the as-synthesized nano-
particles (Figure 3), usually Ru alone would display the feature
reflections of a hcp lattice, and Pt would display the charac-
teristic fcc reflections as described previously. The diffraction
patterns of Pt-rich nanoparticles (Pt77Ru23 and Pt89Ru11) dis-
played mostly the reflection characteristics of the Pt fcc
Figure 1. XRD pattern of Ru nanoparticles from the hydrosilylation
reaction. Inserts show the TEM image and size distribution of Ru
nanoparticles.
d (Å) )
kλ
â cos(θ)
Figure 2. TEM images of the as-synthesized Pt89Ru11 (a) and Pt26-
Ru74 (b) colloids and the heat-treated Pt89Ru11/C (c) and Pt26Ru74/C (d)
catalysts. Histograms of particle size distributions for the as-synthesized
Pt89Ru11 (e) and Pt26Ru74 (f).
Figure 3. X-ray diffraction patterns of the as-synthesized Pt, PtRu, or
Ru nanoparticles.
16646 J. Phys. Chem. B, Vol. 109, No. 35, 2005 Huang et al.
structure, indicating an alloy formation based on the substitution
of the Pt lattice sites.37 However, the hcp-featured pattern of
Ru-rich alloy nanoparticles (Pt26Ru74) could be clearly identified,
suggesting the formation of Ru-rich alloys. Likewise, the
presence of Pt-rich or Ru-rich alloys was evidenced by the XRD
patterns (Figure 4) of PtRu/C nanoparticles after heat treatment.
The lattice structures observed in TEM images also agreed well
with the XRD results.
The particle size in Figure 5 was a volume-average value
calculated by the Scherrer equation. It was found that the as-
synthesized and heat-treated Pt/C have particle sizes (<6 nm)
of nanometer scale, which will lead to the heat-treated alloy
nanoparticles of high catalytic activities in the application of
fuel cell. Although there was a thermally induced limited particle
growth observed in the heat treatment, the effect on catalytic
activities of the alloy particles can be negligible. As seen in
Figure 5, the size of both as-synthesized and heat-treated
nanoparticles increases with increasing Ru concentration, from
2.3 to 4.5 nm for the as-synthesized samples and from 3.4 to
5.5 nm for the heat-treated samples. This trend of increasing
particle size was also confirmed by TEM images (Figure S4 in
Supporting Information). In addition, the particle size of
Pt56Ru44 could not be calculated correctly from the Scherrer
equation, because Pt-rich alloy and Ru-rich alloy possibly
coexisted in this sample, and the diffraction peaks of both alloys
were obscured by each other.
Careful investigation of Figure 4 reveals that all diffraction
peaks were shifted synchronously to higher 2θ values with
increasing Ru concentration in the Pt-rich alloys (Pt, Pt89Ru11,
and Pt77Ru23). The shift was an indication of the reduction in
lattice constant. The lattice constants (a0) of Pt, Pt89Ru11, and
Pt77Ru23 were 3.924, 3.908, and 3.895 Å, respectively. Accord-
ing to Vegard’s law, the lattice constant was usually used to
measure the extent of alloying. The lattice constant for heat-
treated Pt-rich alloy displayed a decrease monotonically with
the Ru concentration. The reduction of lattice constant primarily
arose from substitution of platinum atoms with Ru atoms,
resulting in contraction of the fcc lattice, which indicated the
formation of the PtRu alloy.
XPS. The surface oxidation states of the PtRu catalysts were
investigated by X-ray photoelectron spectroscopy (XPS). Be-
cause the Ru3d binding energy (EB) of zerovalent ruthenium at
284.3 eV38 is very close to the C1sEB resulting from
adventitious carbonaceous species, the Ru3p line was used
instead for the analysis of the Ru oxidation state. Figure 6 shows
Pt4f, and Ru3p regions of the XPS spectrum of the as-
sythesized, and heat-treated the Pt56Ru44/Vulcan carbon catalyst,
respectively. After the thermal treatment (360°C for 10 h), the
slight shift in the Pt4f and Ru3d peaks to lower binding energy
was likely caused by the removal of the capping agents on the
nanoparticles and the change in the surface oxidation state.
Before the thermal treatment of the PtRu catalyst, the Pt4f signal
(Figure 6) could be deconvoluted into one pair of peaks atEB
) 72.2 and 75.6 eV, this could be assigned to the complex state
of Pt-olefin, which was in good agreement with our previous
results.27 After the thermal treatment, the Pt4f signal consisted
of two pairs of doublets. The most intense doublet (71.2 and
74.6 eV) was due to metallic Pt(0). The second set of doublets
(72.3 and 76.3 eV) could be assigned to the Pt(II) chemical
state.39 Likewise, before the thermal treatment, the Ru3p3/2 signal
of the PtRu nanoparticles could be convoluted into one peak at
EB ) 461.9 eV, and after the thermal treatment, the Ru3p3/2
signal could be deconvoluted into two distinguishable peaks of
different intensities atEB ) 461.0 and 462.9 eV, which
corresponded well with Ru(0) and RuO2,40 respectively. It was
Figure 4. X-ray diffraction patterns of the heat-treated Pt/C, PtRu/C,
or Ru/C catalysts for the fuel cell.
Figure 5. Dependence of particle size on Ru content of PtRu
nanoparticles.
Figure 6. X-ray photoelectron spectra of the as-synthesized and heat-
treated PtRu catalysts for fuel cell.
PtRu Nanoparticles for Direct Methanol Fuel Cell J. Phys. Chem. B, Vol. 109, No. 35, 200516647
also noted that the slight shift of the Ru3p3/2 binding energy
was likely attributed to the removal of the capping agent on
the alloy nanoparticles.
Electrochemical Performances. In the previous experi-
ments,33 both the as-synthesized and heat-treated Pt/C catalysts
were characterized by cyclic voltammetry (0-1 V, 50 mV/s)
in an electrolyte of 1 M H2SO4 and 2 M CH3OH; the
as-synthesized Pt/C catalyst displayed an almost featureless
voltammogram with low current density values, indicating the
poor catalytic activities in methanol electrooxidation. Therefore,
in this report all electrical measurements were carried out on
the heat-treated catalyst (Pt/C or PtRu/C at 360°C for 10 h).
The voltammograms of methanol oxidation on all heat-treated
Pt/C catalysts consisted of two parts, i.e., the forward scan and
the reverse scan. In the forward scan, methanol oxidation
produced a prominent symmetric anodic peak around 0.65 V.
In the reverse scan, an anodic peak current density was detected
at around 0.44 V. Manohara and Goodenough attributed this
anodic peak in the reverse scan to the removal of the
incompletely oxidized carbonaceous species formed in the
forward scan.41 These carbonaceous species are mostly in the
form of linearly bonded PtdCdO, the accumulation of inter-
mediate carbonaceous species on the catalysts surface leading
to “catalyst poisoning”. Hence the ratio of the forward anodic
peak current density (If) to the reverse anodic peak current
density (Ib), If/Ib, can be used to describe the catalyst tolerance
to carbonaceous species accumulation. LowIf/Ib ratio indicates
poor oxidation of methanol to carbon dioxide during the anodic
scan and excessive accumulation of carbonaceous residues on
the catalyst surface. HighIf/Ib ratio shows the converse case.
The effect of the potential scan limit on the backward scan
current is shown in Figure 7. Since the backward scan peak
current decreased with increasing the anodic limit in the forward
scan, it appeared that the backward scan peak was primarily
associated with residual carbon species on the surface rather
than the oxidation of freshly chemisorbed species. The reaction
of the backward scan peak as mentioned by Manohara and
Goodenough41 would be written as PtOHad + PtdCdO f CO2
+ 2Pt + H+ + e-, so theIf/Ib ratio increased with the anodic
limit.
As shown in Table 2, the catalytic performance of the heat-
treated Pt/C (10 h) and PtRu/C (10 h) catalysts with different
Ru contents was analyzed and compared in the following
attributes: the onset potential of methanol oxidation (the
potential whereI g 0.025 A/(mg of Pt)), the anodic peak
potential, the ratio of the forward anodic peak current density
(If) to the reverse anodic peak (Ib), and chronoamperometry.
As seen from Figure 8, there was no significant feature
difference between the voltammograms of carbon-supported Pt
catalyst and carbon-supported PtRu alloy catalysts. Anodic peaks
appeared in both the forward and reverse scans. The forward
anodic peak current density of methanol oxidation over heat-
treated Pt/C and PtRu/C catalysts decreased in the order Pt>
Pt89Ru11 > Pt77Ru23 > Pt56Ru44 > Pt26Ru74. This was under-
standable, as alloying by Ru would cause a dilution of the
platinum concentration on the catalyst surface. Comparing with
the voltammograms of other Pt/C or PtRu/C catalysts, it was
noticeable that the forward anodic peak of the catalyst
Pt56Ru44/C was observed to shift cathodically to 0.55 V, this
would likely arise from its exceptional heat-treated crystalline
structure as shown in its XRD pattern (Figure 4).
In accordance with Goodenough’s report, the anodic peak in
the backward scan, which indicates the removal of carbonaceous
species not completely oxidized in the anodic scan,41 the ratio
of If/Ib can be used as an indicator of the catalyst tolerance to
carbonaceous species. The heat-treated Pt/C catalyst had the
lowest If/Ib ratio of 1.10 (Table 2), confirming the known low
CO tolerance of Pt catalysts. The catalyst, Pt56Ru44/C, presented
the anodic current density ratio of 6.73, suggesting the least
carbonaceous accumulation and the most “tolerance” toward
CO poisoning. This could be attributed to the presence of Pt-
Ru pair sites on the catalysts surface, and Ru is known to adsorb
carbonaceous species more favorably than pure Pt. However,
in Ru-rich catalyst (Pt26Ru74/C), the electrochemical activity
became virtually inactive mainly because ruthenium played a
role in the dissociation of carbonaceous species, not in the
promotion of the methanol oxidation reaction.
Table 2 shows that comparison between the different carbon-
supported catalysts, the onset potential for heat-treated Pt/C was
detected at 0.40 V, when the weight percent of ruthenium
increased to 44%, i.e., heat-treated Pt56Ru44/C; the onset potential
was lowered to 0.21 V. However, the weight percent of
ruthenium continuously increased to 75%; the onset potential
shifted up to 0.36V.
Figure 7. Cyclic voltammograms of room-temperature methanol
oxidation on the heat-treated Pt/C catalysts in 1 M H2SO4 and 2 M
CH3OH at 50 mV/s for different potential scan limits.
Figure 8. Cyclic voltammograms of room-temperature methanol
oxidation on the heat-treated Pt/C and PtRu/C catalysts in 1 M H2SO4
and 2 M CH3OH at 50 mV/s.
TABLE 2: Performance of the Heat-treated Pt/C and
PtRu/C Catalysts
16648 J. Phys. Chem. B, Vol. 109, No. 35, 2005 Huang et al.
Figure 9 shows the different curves of current decay for each
carbon-supported catalyst. For heat-treated Pt/C and Pt89Ru11,
the rate of current decay was higher than others even after 1 h,
supposedly because of catalyst poisoning by the chemisorbed
carbonaceous species. The heat-treated Pt56Ru44/C was able to
maintain the highest current density and the low rate of current
decay for over 1 h among all the catalysts. The catalytic activity
of Pt26Ru74 catalysts was worse than that of pure Pt, as a result
of ruthenium dissolution over long electrochemistry time. In
conclusion, the Pt56Ru44/C catalyst displayed the best electro-
catalytic performance among all carbon-supported Pt-based
catalysts prepared in this paper.
Conclusion
Pt and PtRu nanoparticles supported on Vulcan XC-72 carbon
were prepared by a unique approach, the hydrosilylation
reaction. Pt and its alloy particles were nanoscopic-sized and
had narrow particle size distributions. XRD analysis revealed
that the as-synthesized nanoparticles already had considerable
crystallinity, as well as the heat-treated nanoparticles. All Pt-
rich catalysts displayed the characteristic diffraction peaks of
the Pt fcc structure, but the 2θ values were all shifted to slightly
higher values, while the Ru-rich catalysts displayed the feature
peaks of the Ru hcp structure. XPS results showed that the
catalysts mainly composed of Pt(0) and Ru(0), with traces of
oxidation states Pt and Ru. The Pt and PtRu catalysts, especially
the bimetallic system of Pt56Ru44, showed excellent catalytic
activities in room-temperature electrooxidation of methanol.
Some alloy catalysts were more active than the Pt-only catalyst
and more tolerant toward CO poisoning, as expected from the
bifunctional mechanism of alloy catalysts.
Supporting Information Available: High-resolution TEM
images of Ru and PtRu alloy nanoparticles and TEM images
of the as-synthesized Pt, PtRu, and Ru nanoparticles. This
material is available free of charge via the Internet at http://
pubs.acs.org.
References and Notes
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(9) Frelink, T.; Visscher, W.; Van Veen, J. A. R.Langmuir1996, 12,
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G.; Whetten, R. L.J. Phys. Chem. B2002, 106, 3410.
(11) Woehrle, G. H.; Warner, M. G.; Hutchison, J. E.J. Phys. Chem. B
2002, 106, 9979.
(12) Yang, Y.; Chen, S.Nano Lett.2003, 3, 75.
(13) Negishi, Y.; Tsukuda, T.J. Am. Chem. Soc.2003, 125, 4063.
(14) Wilcoxon, J. P.; Provencio, P.J. Phys. Chem. B 2003, 107, 12949.
(15) Buining, P. A.; Humbel, B. M.; Philipse, A. P.; Verkleij, A. J.
Langmuir 1997, 13, 3921.
(16) Liu, S. M.; Gan, L. M.; Liu, L. H.; Zhang, W. D.; Zeng, H. C.
Chem. Mater.2002, 14, 1391.
(17) Armadi, I. S.; Wang, Z. L.; Green, T. C.; Henglein, A.; El-Sayed,
M. A. Science1996, 272, 1924.
(18) Liu, Z. L.; Lee, J. Y.; Han, M.; Chen, W. X.; Gan, L. M.J. Mater.
W. Chem. Mater.2001, 13, 1057.
(21) Yu, W. Y.; Tu, W. X.; Liu, H. F.Langmuir 1999, 15, 6.
(22) Tu, W. X.; Liu, H. F.Chem. Mater.2000, 12, 564.
(23) Komarneni, S.; Li, D. S.; Newalkar, B.; Katsuki, H.; Bhalla, A. S.
Langmuir 2002, 18, 5959.
(24) Chen, W. X.; Lee, J. Y.; Liu, Z. L.Chem. Commun.2002, 2588.
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Figure 9. Polarization current vs time plots for the room-temperature
electrooxidation of methanol on the heat-treated Pt and PtRu catalysts
in 1 M H2SO4 and 2 M CH3OH electrolyte at 0.4 V (vs SCE).
PtRu Nanoparticles for Direct Methanol Fuel Cell J. Phys. Chem. B, Vol. 109, No. 35, 200516649
Chapter 17
Properties and Applications of Ruthenium
Anil K. Sahu, Deepak K. Dash, Koushlesh Mishra,
Saraswati P. Mishra, Rajni Yadav and
Pankaj Kashyap
Additional information is available at the end of the chapter
http://dx.doi.org/10.5772/intechopen.76393
Provisional chapter
Properties and Applications of Ruthenium
Anil K. Sahu, Deepak K. Dash, Koushlesh Mishra,
Saraswati P. Mishra, Rajni Yadav and
Pankaj Kashyap
Additional information is available at the end of the chapter
Abstract
Ruthenium (Ru) with atomic number of 44 is one of the platinum group metals, the others
being Rh, Pd, Os, Ir and Pt. In earth’s crust, it is quite rare, found in parts per billion
quantities, in ores containing some of the other platinum group metals. Ruthenium is silvery
whitish, lustrous hard metal with a shiny surface. It has seven stable isotopes. Recently,
coordination and organometallic chemistry of Ru has shown remarkable growth. In this
chapter, we review the application of Ru in diverse fields along with its physical and
chemical properties. In the applications part of Ru we have primarily focused on the
biomedical applications. The biomedical applications are broadly divided into diagnostic
and treatment aspects. Ru and their complexes are mainly used in determination of ferritin,
calcitonin and cyclosporine and folate level in human body for diagnosis of diseases. Treat-
ment aspects focuses on immunosuppressant, antimicrobial and anticancer activity.
Ruthenium is one of the 118 chemical elements given in the periodic table. Out of these 118
elements, 92 elements originated from natural sources and remaining 26 elements have been
synthesized in laboratories [1, 2]. The last naturally occurring element to be discovered was
Uranium in 1789 [1, 3]. Technetium was the first man-made element to be synthesized in the
year 1937 [2]. Recently in the year 2016, four of the man-made elements were included in
periodic table. The four newly added elements goes by the name nihonium (Nh), moscovium
(Mc), tennessine (Ts), and oganesson (Og), respectively for element 113, 115, 117 and 118 [4].
Discovery of Ruthenium had many twist and turns. A polish Chemist Jedrzej Sniadecki
(1768–1838) in 1808 was first to announce the discovery of an element which he named
Vestium after an asteroid called Vesta [3]. However, none of the contemporary Chemists
were able to confirm his discovery. Later he again reported discovery of element 44 while
working on the platinum ores from South America and published his results but again none
of the fellow chemist were able to confirm the element 44 [4]. Due to repeated failures of his
claim, Sniadecki got depressed and dropped the idea of further research on this element [1,
5]. After 20 years, a Russian chemist, Gottfried W. Osann, claimed the discovery of element
44. His discovery had the same fate as that of Sniadecki as none of his fellow chemist could
repeat his results [5].
At last in the year 1844, another Russian chemist Carl Ernst Claus [also known in Russian as
Karl Karlovich Klaus (1796–1864)] tried his luck on discovery of element 44. He succeeded in it
as he gave positive proof about the new element extracted from platinum ores obtained from
the Ural Mountains in Russia [6]. Claus had suggested the name of newly discovered element
as Ruthenium after the name Ruthenia which was the ancient name of Russia. Earlier Osann
had also suggested the same name for the element 44 [2, 5]. Ruthenium with atomic number 44
was given the symbol Ru. It is included in group 8, period 5 and block d in modern periodic
table and it is a member of the platinum group metals [5].
2. Occurrence in nature
Like other platinum group metals, Ruthenium is also one of the rare metals in the earth’s crust.
It is quite rare in that it is found as about 0.0004 parts per million of earth crust [6]. This fraction
of abundance makes it sixth rarest metal in earth crust. As other platinum group metals, it is
obtained from platinum ores [7]. For instance, it is also obtained by purification process of a
mineral called osmiridium [5].
3. Electronic configuration of Ru
In the modern periodic table, group 8 consists of four chemical elements. These elements are
Iron (Fe), Ruthenium (Ru), Osmium (Os) and Hassium (Hs) [7]. Ruthenium has atomic
number of 44, that is, it contains 44 electrons distributed in atomic orbitals and its nucleus
has 44 protons and 57 neutrons (Figure 1). Electron distribution in atomic or molecular
orbitals is called electron configuration which for Ru and the other group 8 chemical ele-
ments is shown in Table 1. Except for Ru, the electron configuration of group 8 elements
shows two electrons in their outer most shell; Ruthenium has only one electron in its
outermost shell. This tendency is quite similar to its neighboring metals such as niobium
(Nb), molybdenum (Mo) and rhodium (Rh) [8].
Noble and Precious Metals – Properties, Nanoscale Effects and Applications378
4. Isotopes of Ru
Any atom having same number of protons, but different number of neutrons is termed as an
Isotope. Isotopes can be differentiated on the basis of mass number as each isotope consists of
different mass number which is being written on the right of the element name [1, 7]. Mass
number indicates sum total of proton and neutron present in the nucleus of atom [9]. Ruthe-
nium has many isotopes although only seven of them are stable. Apart from seven stable
isotopes, 34 radioactive isotopes of Ruthenium are also found [8]. The most stable radioactive
isotopes are 106Ru, 103Ru, 97Ru having a half-life of 373.59, 39.26, 2.9 days, respectively. Other
characteristics of the main isotopes are listed in Table 2 [8].
Figure 1. Schematic of the electron configuration and nucleus of an atom of Ruthenium.
Atomic
number
Element Electron configuration Number of
electrons per shell
Table 1. Electron configuration of group 8 chemical elements.
Properties and Applications of Ruthenium
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5. Physical and chemical properties of Ru
Ruthenium (Ru), Rhodium (Rh), Palladium (Pd), Osmium (Os), Iridium (Ir) and Platinum (Pt)
form the Platinum group metals. Some of the fundamental properties of platinum group metals
are summarized in Table 3 [8]. Ruthenium is silvery whitish, lustrous hard metal with a shiny
surface. At room temperature, Ru does not lose its luster because it is unreactive in that condition
but shows paramagnetic behavior [7]. At the higher temperature of around 800�C, Ru reacts with
oxygen and gets oxidized [11]. It also reacts with halogens at higher temperature. As far as
dissolution is concerned, Ruthenium does not dissolve in most of the acid or mixture of acids such
as aqua regia which is a mixture of hydrochloric acid and nitric acid [7, 10]. When it is reacted with
alkali it forms ruthenate ion which leads to dissolution of Ruthenium in alkalies (Eq. 1) [6].
Main isotopes of Ruthenium
S. No. Isotopes Abundance Half-life
1 96Ru 5.54% Stable with 52 neutrons
2 97Ru Synthetic 2.9 days
3 98Ru 1.87% Stable with 54 neutrons
4 99Ru 12.76% Stable with 55 neutrons
5 100Ru 12.60% Stable with 56 neutrons
6 101Ru 17.06% Stable with 57 neutrons
7 102Ru 31.55% Stable with 58 neutrons
8 103Ru Synthetic 39.26 days
9 104Ru 18.62% Stable with 60 neutrons
10 106Ru Synthetic 373.59 days
Table 2. Physical properties of platinum group elements.
Ru Rh Pd Os Ir Pt
Atomic number 44 45 46 76 77 78
Atomic weight 101.07
u � 0.02 u
102.9055
u � 0.00002 u
106.42
u � 0.01 u
190.23
u � 0.03 u
192.217
u � 0.003 u
195.084 u
Electronic
configuration
Kr 4d7 5 s1 Kr 4d8 5 s1 Kr 4d10 Xe 4f14 5d6
6 s2
Xe 4f14 5d7 6 s2 Xe 4f14 5d9
6 s1
Density(g/cc) 12.2 12.41 11.9 22.59 22.56 21.45
Melting point(�C) 2334 1963 1555 3033 2447 1768
Boiling point(�C) 4150 3697 2963 5027 4130 3825
Electronegativity 2.2 2.28 2.2 2.2 2.2 2.28
Table 3. Characteristics of main isotopes of ruthenium.
Noble and Precious Metals – Properties, Nanoscale Effects and Applications380
ð1Þ
6. Chemical reactivity of ruthenium
6.1. Oxidation reaction of ruthenium
As noted above, Ruthenium undergoes oxidation reaction to form Ruthenium oxide [11]. When
Ruthenium oxide undergoes further oxidation in the presence of sodium metaperiodate, Ruthe-
nium tetraoxide (RuO4) is formed (Eq. 2), with properties somewhat similar to those of OsO4, in
that both are strong oxidizing agents. However, RuO4 differs from OsO4 since it can easily
oxidize diluted form of hydrochloric acid as well as ethanol at normal room temperature [12].
At temperatures above 100�C, RuO4 get reduced to its dioxide. RuO4 also has specific stain
property which is utilized in electron microscopy to investigate organic polymer samples [11, 13].
ð2Þ
At lower oxidation states such as +2 or +3, Ru does not undergo oxidation reaction. Ruthenium
reacts with hydroxide ions to attain higher coordination number [13]. Ruthenium does not form
oxoanion readily as seen with iron. Ruthenium attains +7 oxidation states when it reacts with cold
and diluted potassium hydroxide to form potassium perruthenate [14]. Ruthenium can also attain
same oxidation state when potassium ruthenate gets oxidize in the presence of chlorine gas [9].
6.2. Coordination complexes of ruthenium
Coordination complex is the process where a center molecule makes bond with surrounding
atoms or ions which are also known as ligands. Ruthenium readily forms coordinate com-
plexes with different derivatives. It reacts with pentaamines to form different coordination
complex. Ruthenium reacts with pyridine derivatives to form tris (bipyridine) ruthenium (II)
chloride (Eq. 3) [15]. Ruthenium also reacts with carbon containing compounds. Ruthenium
forms Roper’s complex when trichloride form of Ruthenium reacts with carbon monoxide [10,
15]. Ruthenium makes hydride complex when Ruthenium trichloride is heated in presence of
alcohol which then reacts with triphenylphosphine to form chlorohydridotris (triphenyl-
phosphine) ruthenium (II) (Eq. 4) [10].
ð3Þ
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ð4Þ
6.3. Catalytic activity of ruthenium
Ruthenium acts as a catalyst in many reactions. In the olefin metathesis, the carbene and
alkylidene complex of Ruthenium act as a catalyst. In Fischer Tropsch reaction (Eq. 5),
Ruthenium also acts as a catalyst [16]. Fischer Tropsch reaction is a reaction in which liquid
hydrocarbons are formed as a product of reaction between hydrogen and carbon monoxide.
Decomposition process of ammonia also employs Ru as catalyst [17]. Ru also catalyzes
group of reactions called “borrowing hydrogen reactions”. Borrowing hydrogen reaction
is a reaction where two atoms of hydrogen are transferred to the catalyst to covert alcohol to
carbonyl. The same reaction occurs in the conversion of alcohol to alkenes [5, 17].
Ruthenium carbonyl complex catalyzes the conversion of primary alcohol to aldehydes and
secondary alcohol to aldehydes and ketones in the presence of a co-oxidant N-methylmor-
pholine-N-oxide (NMO) [8]. Ruthenium acts as a unique catalyst in oxidation reaction because
of its varying oxidation state that ranges from �2 to +8 [6].
ð5Þ
7. Ruthenium complexes
In recent years, there has been remarkable growth and evaluation in the field of coordination and
organometallic chemistry of Ru. Many publications have appeared recently on the formation of
Ru-based complexes and their applications in such areas as medicine, catalysis, biology,
nanoscience, redox and photoactive materials. These developments can be related to the fact that
Ru has the unique ability to exist in multiple oxidation states. Examples of these complexes and
various applications of Ru are reviewed in the following sections.
7.1. Development of half-sandwich para-cymene ruthenium (II) naphthylazophenolato
complexes
Ruthenium (II)-arene complex has a structure of three-legged piano stool with a metal at
the center in a quasi-octahedral geometry which is occupied by byan arene complex.
2-(naphthylazo)phenolate ligands reacts with chloro-bridged (g6-p-cymene) ruthenium com-
plex [{(g6-pcymene)RuCl}2(l-Cl)2] in methanol having molar ratio 1:1 at room temperature
leads to formation of monomeric ruthenium(II) complexes. The formed complexes (Figure 2)
Noble and Precious Metals – Properties, Nanoscale Effects and Applications382
show the solubility in polar solvents (dichloromethane and acetone) and are insoluble in non-
polar solvents (aspentane and hexane). It is stable in air and shows diamagnetic nature with
the +2 oxidation state [6, 10].
Figure 2. Structure of (p-cymene) ruthenium (II) 2-(naphthylazo)phenolate complexes.
Figure 3. Structure of Tris (bipyridine) ruthenium (II) chloride.
Properties and Applications of Ruthenium
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7.2. Development of functionalized polypyridine ligands for ruthenium complexes
Polypyridine are coordination complexes containing polypyridine ligands such as 2,20-
bipyridine, 1,10-phenanthroline and 2,20,60200-terpyridine. Polypyridines are multi-denated
ligands which are responsible for characteristics property of metal complex they formed.
Some of complexes show the characteristics of absorption of light by a process called
metal-to-ligands charge transfer (MLCT). This said property of metal complex is due to
the change in substituent to the polypyridine moiety. Among the polypyridine ligands for
ruthenium complexes the mostly studied complex is Tris (bipyridine) ruthenium (II) chlo-
ride (Figure 3). It is a red crystalline salt having a hexahydrate form. Tris (bipyridine)
ruthenium (II) chloride salt is prepared when aqueous solution of ruthenium trichloride
reacts with 2,20-bipyridine in the presence of reducing agent hypo-phosphorus acid. In
this reaction Ru(III) gets reduced to Ru(II) [18].
8. Applications of ruthenium
Ruthenium has a wide variety of application in diverse fields. Few of the applications of
Ruthenium are listed below.
8.1. General applications
Ruthenium finds application both in electronic industry and chemical industry. In electrical
industry it is used in manufacturing of electronic chips [19]. Chemically it is used in the form of
anodes for chlorine production in electrochemical cells [20]. Ruthenium is used as a hardener
when it is mixed with other metals to form alloy. This characteristic of ruthenium is used in the
preparation of jewelry of palladium [18, 20]. When Ruthenium forms alloy with titanium it
improves its corrosion resistant property. Ruthenium alloys also find application in manufa-
cturing of turbines of jet engines [17]. Fountain pen nibs also contain Ru tips. Ruthenium has
also application in therapy. For instance 106 isotope of Ru has application in radiotherapy of
malignant cells of eye [11]. RuO4 is used in criminal investigations as it reacts with any fat or
fatty substance having sebaceous pigments to give black or brown coloration due to formation
of ruthenium dioxide pigments [12].
Ruthenium complexes tend to absorb light rays of visible spectrum. This property of ruthe-
nium finds application in manufacturing solar cells for production of solar energy. [16] Ruthe-
nium vapor get deposited on the surface of substrate and has magneto-resistive property. This
property of Ru is used in making a layer or film on hard disk drives [12].
8.2. Biomedical applications
8.2.1. Applications in diagnosis
• Ruthenium is used for determination of calcitonin level in blood. This determination
is helpful in diagnosis and treatment of diseases related to thyroid and parathyroid
Noble and Precious Metals – Properties, Nanoscale Effects and Applications384
glands. In treatment of medullary thyroid carcinoma (MTC), determination of calcito-
nin level plays an important role. The process of determination of calcitonin level
involves one step sandwich assay method. This method is carried out in two incuba-
tion steps. Each incubation process takes 9 min each. In first incubation, 50 micro-
liters of sample of biotinylated monoclonal human calcitonin specific antibody and
monoclonal human calcitonin specific antibody labeled with ruthenium complex are
incubated. This incubation leads to formation of sandwich like complex where human
calcitonin is carrying both biotinylated and ruthenylated complex. After the first step,
second incubation step is done where streptavidin-coated microparticles is added.
Streptavidin-coated microparticle makes complex with biotin. After the incubation
step, measurement is done. For measurement, the mixture of incubation is aspirated
into measuring cells and micro particles of mixture are magnetically attracted to the
surface of electrode. After that the unbound particles are removed. Voltage is applied
on to the electrode and induction of chemi-lumiscent emission is done and after that
the response is studied with photomultiplier [12].
• Folate is the main constituent of synthesis of DNA. It is also essential for formation of red
blood cells. Deficiency of folate leads to megalobalstic anemia. Deficiency of folate is esti-
mated by determination of folate level in erythrocytes as well as serum. Ruthenium plays an
important role in Elecys folate RBC assay in estimating folate deficiency in RBC. The process
involved in folate determination is competition principle. This process involves three steps
incubation method. In first incubation step folate pretreatment reagent is added which leads
to release of folate from its binding sites (erythrocytes). In the second incubation step, Ru-
labeled folate binding protein is added which makes complex with the sample. In the third
incubation step streptavidin bounded microparticles are added which get attached to
unbound sites of ruthenium-labeled folate binding protein. The whole complex is bound to
solid phase via streptavidin and biotin. For measurement, the mixture of incubation is
aspirated into measuring cells and microparticles of mixture are magnetically attracted to
the surface of electrode. After that the unbound particles are removed. Voltage is applied on
to the electrode and induction of chemi-lumiscent emission is done and after that the
response is studied with photomultiplier [12].
• Ruthenium is also employed in detection of cyclosporine by Elecsys cyclosporine assay.
Determination of cyclosporine is an important aspect for management of liver, kidney,
heart lungs and bone marrow transplant patients receiving cyclosporine therapy [12].
8.2.2. Applications in treatment
History of medical science shows metals like gold has always been used for medicinal purpose.
Though it is known that metals may have beneficial effect for health, but the exact mode of
activity remains unknown. Ruthenium also has been applied in treatment [21].
• Immunosuppressant: Immunosuppressant is drug used to suppress hyperactivity of
body’s immune system. An immunosuppressant Cyclosporin A which has wide applica-
tion in treatment of disease like anemia and psoriasis eczema has shown side effects
such as nausea, renal diseases, and hypertension. To modify the action of Cyclosporin A,
Properties and Applications of Ruthenium
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385
complex is made with Ru(III). Ruthenium cyclosporin complex gives a stable compound
which results in an inhibitory effect on T lymphocyte proliferation [22].
• Antimicrobial action: antimicrobial drugs are drugs that inhibit microbial growth in
human body. Ruthenium complex has its effectiveness against wide range of parasitic
diseases. Microbial strains which are exposed to a certain kind of antimicrobial therapy
become resistant to that drug. The resistance develops because the microbes mutate
themselves against the organic compound of the drug. But with the formation of complex
with certain metals the effectiveness of the drug increases as the microbes are unable to
deal with the metal part of the organometallic complex of drug. In case of Chloroquine,
Plasmodium species develops résistance against it, whereas when Chloroquine is
complexed with ruthenium, resistance does not develop [23].
• Antibiotic action: antibiotics are drugs which are made from one particular microorgan-
ism and act on the other microorganism. Synthetic antibiotics are also nowadays made in
laboratory. Antibiotic exhibit their action by entering the cell of microbes and targeting
any vital biosynthetic pathway. Ruthenium has upper edge if it gets complexes with
synthetic antibiotics. Ruthenium being a metal has better tendency to bind to the cellular
component similar to Iron. When an organic moiety gets bind to a metal ion, at that time
sharing or delocalization of cations between the two moieties occurs. The change in
charges among the component of drug increases the permeability of cellular component
in favor of drug. For example, Thiosemicarbazone shows a remarkable increase in its
activity due to formation of complex of Ru [24].
• Inhibitory effect on nitric oxides: nitric oxide is a cellular component which is produced
by many cells. The main physiological role of nitric oxide is to produce vasodilation.
Nitric oxide does this action my increasing cellular level of cyclic-guanosine 30,50-
monophosphate (CGMP) which is a secondary messenger in the physiological system.
Over production of nitric acid can cause many disorders associated with respiratory
system such as tumor of respiratory system. It also causes severe hypotension on over
production. It also causes gastric inflammatory disorders. Ruthenium has beneficial effect
in treatment of over production of nitric oxides. When ruthenium is administered in
complex form such as ruthenium poly amino carboxylates, excess nitric oxide present in
blood binds to this complex readily and reduces ruthenium to form an unabsorbable
complex there by inhibiting its unwanted effects [25].
8.2.3. Applications of Ruthenium in cancer research
• Anti-carcinogenic activity: cancer or carcinoma is a stage where body cells undergo
uncontrolled proliferation and having invasiveness and metastatic property. To treat carci-
noma, drug therapy aims at inhibiting synthesis of cancerous protein as well as inhibiting
DNA replication. In market there are drugs such as Cisplatin which uses platinum as
anticancer agent. Though platinum has shown better results in treatment of cancer but in
some cancers, platinum is unable to show positive results. This shortcoming of Platinum
made way for use of Ruthenium as a new entrant in treatment of cancer. Ruthenium shows
Noble and Precious Metals – Properties, Nanoscale Effects and Applications386
the ability to bind to the DNA and inhibits its replication as well as protein synthesis.
Ruthenium has low aqueous solubility which was the only drawback of it. This drawback
was countered by using dialkyl sulfoxide derivative of ruthenium. The mechanism of action
of ruthenium as an anticancer agent is that it causes apoptosis of tumor cells by acting at
DNA level. Apoptosis is a controlled destruction of cells [17, 18].
• Radiation therapy: in cancer treatment radiotherapy has also been used. Radiation ther-
apy becomes beneficial only when it is proximal to the cancerous cell. The agents used in
radiation therapy are called radio sensitizers. To increase the proximity to cancerous cells
radio sensitizers’ complexes with ruthenium are used as Ru has the affinity to bind to
DNA easily [18, 19].
• Photodynamic therapy: it is a therapy where chemicals and electromagnetic radiations
are used. In this therapy chemicals are targeted on the cancerous cell, these chemicals
become cytotoxic when they interact with electromagnetic radiation. In this therapy
Ruthenium find its application as it increases the access of these chemicals to the cancer-
ous cells [20, 21].
• Action on cancerous mitochondria: mitochondria are the power house of any cell. This
makes it a potential target for anticancer therapy. Ruthenium red is a type of ruthenium
which is used to stain mitochondria. Mitochondrial surface has some calcium entity on it.
When ruthenium red is added, it reacts with this calcium and stains the mitochondria.
Ruthenium red also has tumor inhibiting activity. However, ruthenium red is not prefer-
ably used clinically as it has major side effects [20, 22].
• Effect on metastasis: metastasis is the ability of cancerous cell to spread in the body by
lymphatic or circulatory system. A tumor cell more than 1 mm in size requires additional
blood supply to spread in the body. Formations of new blood vessels are called angiogen-
esis. Drugs which act as anti-metastasis many inhibit this action. Ruthenium complexes
anti-metastatsis drug namely NAMI-A does the same action by binding to the mRNA and
production of denatured protein which gets accumulated on the surface of tumor making
a hard film and prevents any blood supply to the tumor cell. This action inhibits the
metastasis. Ruthenium has additional benefit that it easily crosses any cell so the reach of
the drug increases [23, 26].
9. Summary and conclusions
Ruthenium with atomic number of 44 and symbol Ru was discovered by Russian chemist
Karl Klaus (1796–1864). In earth’s crust, it is quite rare, found in parts per billion quantities,
in ores containing some of the other platinum group metals. It is silvery whitish, lustrous
hard metal with a shiny surface. The ability of Ru to exist in many oxidation states is an
important property of this rare element which plays an important part in its applications.
Ruthenium readily forms coordinate complexes and these complexes have their applications
in diverse fields such as medicine, catalysis, biology, nanoscience, redox and photoactive
Properties and Applications of Ruthenium
http://dx.doi.org/10.5772/intechopen.76393
387
materials. In biomedical fields Ru is used for diagnosis and treatment purpose. For example,
Ru is used for determination of calcitonin level in blood which is helpful in diagnosis and
treatment of diseases related to thyroid and parathyroid glands. Also, Ru plays an important
role in Elecys folate RBC assay in estimating folate deficiency in RBC. Ruthenium cyclo-
sporin complex gives a stable compound which results in an inhibitory effect on T lympho-
cyte proliferation which shows its immune-suppressant action. Ruthenium complex has its
effectiveness against wide range of parasitic diseases. Ruthenium shows the ability to bind to
the DNA and inhibits its replication as well as protein synthesis. This property helps in the
treatment of cancer. This chapter gives a brief account of the various properties of Ru which
are exploited for applications in the medical field. It is likely that in the coming years, further
research will lead to even more useful applications of this miraculous element.
Author details
Anil K. Sahu1, Deepak K. Dash1, Koushlesh Mishra1, Saraswati P. Mishra1, Rajni Yadav2 and
Pankaj Kashyap1*
*Address all correspondence to: pankajkashyap333@gmail.com
1 Royal College of Pharmacy, Chhattisgarh Swami Vivekanand Technical University, Raipur,
Chhattisgarh, India
2 Columbia Institute of Pharmacy, Raipur, Chhattisgarh, India
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Noble and Precious Metals – Properties, Nanoscale Effects and Applications390
Sensitive Colorimetric Assay of H2S Depending on the High-Efficient
Inhibition of Catalytic Performance of Ru Nanoparticles
Yuan Zhao,† Yaodong Luo,† Yingyue Zhu,‡ Yali Sun,† Linyan Cui,† and Qijun Song*,†
†Key Lab of Synthetic and Biological Colloids, Ministry of Education, School of Chemical and Material Engineering, Jiangnan
University, 1800 Lihu Road, Wuxi, Jiangsu 214122, China
‡School of Biotechnology and Food Engineering, Changshu Institute of Technology, No
.
99 3dr South Ring Road, Changshu, Jiangsu
215500, China
*S Supporting Information
ABSTRACT: Nanocatalysts depended colorimetric assay possesses the
advantage of fast detection and provides a novel avenue for the detection of
hydrogen sulfide (H2S). The exploration of nanocatalysts with superior catalytic
activity is challenging to achieve ultrasensitive colorimetric assay of H2S. Herein,
1.7 ± 0.2 nm ruthenium nanoparticles (Ru NPs) were prepared and exhibited
outstanding catalytic hydrogenation activity. The degradation rate constants of
orange I in the presence of Ru NPs were 4-, 47- and 165-fold higher than those
of platinum (Pt) NPs, iridium (Ir) NPs and control groups without catalysts.
H2S-induced deactivation of Ru NP catalysts was designed for the sensitive
colorimetric assay of H2S, attributing to the poor thiotolerance of Ru NPs. A
standard linear curve between the rate constants and the concentration of H2S
was established. The limit of detection (LOD) was as low as 0.6 nM. A Ru NPs
based colorimetric principle was also used to fabricate colorimetric paper strips
for the on-site visual analysis of H2S. The proposed approach shows potential
prospective for the preparation of highly selective colorimetric NP sensors for specific purposes.
■ INTRODUCTION
H2S along with nitric oxide and carbon monoxide are well-
known environmental pollutants and the endogenous gaso-
transmitter.1,2 H2S as one of the most important exhaled
gaseous signaling molecules plays a significant role in a variety
of physiological and pathological processes.3 Its level is not only
an important environmental index but also is linked to various
diseases (e.g., Alzheimer’s disease, Down’s syndrome, diabetes
and liver cirrhosis).4−6 It is necessary to propose a powerful
monitoring sensor for the precise investigation of H2S.
Currently, the most common analysis for H2S detection
mainly focuses on the instrumental analysis (such as gas
chromatography, gas chromatography−mass spectrometry),
fluorescence methods and colorimetric sensors, etc.1,5,7,8
However, instrumental analysis often requires tedious sample
preparation or sophisticated equipment, and is not suitable for
routine laboratory and on-site analyses.1,9 Fluorescence
methods mainly depend on the fluorescence of probes, which
are easily interrupted by the quenching effects due to the
oxygen, humidity and foreign species.5,10 Alternatively,
colorimetric assay gains increasing attention, attributing to
the simple detection by naked eyes, short assay time, relatively
low cost and no requirements for skillful technicians.3 Due to
the unique fluorescence properties, localized surface plasmon
resonance and catalytic performances of NPs,2,5,6,11−13 NPs
based colorimetric methods have been widely exploited for the
detection of H2S (Table 1).
Nanocatalysts depended colorimetric assay, by contrast,
possesses the advantages of simple operation, fast responses
and high sensitivity, and is convenient to achieve on-site visual
analysis of H2S. However, the conventional and reported
catalysts are mainly limited to Au NPs, Ag NPs, Au@Pt NPs
and graphene, etc.3,6,14−16 The detection sensitivity of
colorimetric assay is still far from satisfying, and its performance
is still restricted due to the limited catalytic property of the used
NPs. With the rapid development of nanocatalysts, Ru NPs as a
transition metal show superior catalytic hydrogenation
activities, and have been investigated and employed in the
reduction of nitroaromatic compounds and azo dyes.1
7
Nevertheless, studies on Ru NPs are limited to the exploration
of novel synthetic methods and the investigation of shape-
determined catalytic properties,18−22 but Ru NP catalysts as a
signal amplifier for the colorimetric assay are not explored. The
mechanism of H2S induced Ru NP catalysts deactivation is not
fully understood, and it is imperative and challenging to
evaluate the deactivation degrees using Ru NPs-triggered
catalytic system.
Received: May 8, 2017
Revised: July 15, 2017
Published: August 14, 2017
In this paper, uniform Ru NPs were synthesized and showed
superior catalytic hydrogenation activities for the degradation of
orange I. Orange I−Ru NPs as an amplifier system was first
designed for the sensitive and selective colorimetric monitoring
of H2S, depending on H2S-induced poisoning of the catalytic
active sites of Ru NPs. The degradation kinetic curves of orange
I−Ru NPs amplifier were investigated in the presence of
different concentrations of H2S, and the color fading process of
orange I was monitored. The relationship between H2S
concentration and the degradation rate constants of orange I
was established, and the LOD was as low as 0.6 nM. The
proposed Ru NPs based colorimetric assay can be served as an
innovative signal transduction and amplification method for the
sensitive detection of H2S.
■ EXPERIMENTAL SECTION
Materials and Reagents. Ruthenium chloride hydrate (RuCl3·
nH2O) was purchased from J&K Chemical CO., Ltd. Poly-
(vinylpyrrolidone) (PVP), ethylene glycol, hydrazine hydrate (N2H4,
85%), orange I, anhydrous acetone, histidine (His), alanine (Als),
threonine (Thr), arginine (Arg), aspartic acid (Asp), glutamic acid
(Glu), tyrosine (Tyr), phenylalanine (Phe), cysteine (Cys) and
glutathione (GSH), NaCO3, NaHCO3, NaNO2, NaNO3, NH4Cl,
NaSO4, NaSO3, Na2S2O8 and Na2S were all purchased from
Sinopharm Chemical Reagent Co., Ltd. All reagents were of
analytical-reagent grade and were used without further purification.
Synthesis of Ru NPs. 12.3 mg of RuCl3 and 55.5 mg of PVP were
dissolved in 10 mL of ethylene glycol at room temperature. The
mixture was heated at 170 °C for 6 h. The color of the solution
changed from dark red to dark brown and finally to dark brown. An
aliquot of 10 mL Ru NPs solution was purified by anhydrous acetone
for three times and then dispersed to 625 μL of ultrapure water. The
concentration of Ru NPs was calculated to be about 1.6 μM according
to the previous reported procedures.14 PVP stabilized Pt NPs and Ir
NPs were respectively prepared according to the previous
methods.17,23 The average sizes of Pt NPs and Ir NPs were 3.8 ±
1.3 nm and 1.9 ± 0.5 nm (Figure S1, Supporting Information).
Catalytic Hydrogenation Performance of Ru NPs. An aliquot
of 4 μL 10 mM orange I was mixed with 2 mL of 0.8 M N2H4 solution.
And then, an amount of 10 μL Ru NPs, Pt NPs, Ir NPs was added into
the above solution, respectively. The final concentration of Ru NPs in
the system was about 8 nM. The catalytic performances of Ru NPs, Pt
NPs and Ir NPs at the same concentration were compared by
measuring the degradation kinetic curves at 512 nm in the reduction of
orange I.
Colorimetric Sensor for the Detection of H2S. Na2S generally
exists in the form of HS− under alkaline condition, and is widely used
as the source of H2S in solution.
2,4,11,24 An amount of 20 μL different
concentrated stock solution of Na2S (0, 5, 10, 20, 40, 60, 80, 100, 200,
400, 600 and 800 nM) was mixed with 10 μL of Ru NPs, respectively.
The Na2S−Ru NPs solution was added into the mixtures of 4 μL of 10
mM orange I and 2 mL of 0.8 M N2H4. UV−vis absorption spectrum
of orange I was measured at 512 nm by monitoring the degradation
kinetic curves in the presence of different concentration of Na2S
donors.
Specificity and Reproducibility. The specificity of the developed
method was explored for the detection of other sulfhydryl compounds,
such as Cys and GSH. An amount of 20 μL of 2 μM Na2S donors and
amino acids (His, Als, Thr, Arg, Asp, Glu, Tyr, Phe, Cys and GSH)
were added to the mixture of Ru NPs, orange I and N2H4, respectively.
The degradation kinetic curves of orange I were monitored. The
selectivity of the proposed colorimetric assay was assessed in the
presence of other interfering substances, including NaCO3, NaHCO3,
NaNO2, NaNO3, NH4Cl, NaSO4, Na2S2O8 and NaSO3. An amount of
20 μL of 200 nM Na2S donors and 20 μL of 2 μM different interfering
substances were added to the mixtures of Ru NPs, orange I and N2H4,
respectively. The mixtures were applied to evaluate the selectivity in
the monitoring of H2S.
The reproducibility of the developed colorimetric sensor was
investigated for the detection of H2S in Tai lake water. An aliquot of 1
mL of negative Tai lake water was filtrated three times to remove other
substances. An amount of Na2S donors was spiked into the mentioned
1 mL of negative Tai lake water with the final concentration of 30, 50,
70, 90, 300 and 500 nM. The concentration of Na2S was measured by
the developed colorimetric sensors at the same detection procedures.
Fabrication of Paper Strip for H2S Gas Detection. A paper
strip was fabricated for the visual detection of H2S gas. Generally, an
aliquot of 10 μL of 1 M NaOH solution was added into 1 mL of 4 mM
orange I, and the color of orange I was red under alkaline conditions.
Filter papers (1 cm × 1 cm) were soaked with the above solution.
After 1 min, filter papers were got out, and then 5 μL of Ru NPs was
injected onto the filter papers. The prepared filter papers were dried at
40 °C oven for 10 min, and then were placed in a clear glass container
(500 mL in volume).
H2S gas is prepared by a stoichiometric reaction between Na2S and
diluted H2SO4. An amount of 0.5 mmol Na2S was added into a sealed
flask (500 mL), and then 0.4 mL of H2SO4 (0.1 mmol) was slowly
injected. Different amounts of H2S gas were obtained by a micro
syringe and separately injected into the above container with the
prepared filter papers. The final concentration of H2S gas was 0, 1, 10
and 100 μM. After incubatiion for 5 min, an aliquot of 5 μL of 0.8 M
N2H4 solution was added onto the surface of orange I−Ru NPs
modified filter papers. The color changes of filter papers were recorded
at 2 min for visual detection of H2S gas. The fabricated paper strips
were also applied to study the effect of the interference gases using
their dissolved forms, involving CO3
2−, HCO3
−, NO2
−, NO3
−, NH4
+,
SO4
2−, S2O8
2−, SO3
2−. To explore the efficacy of colorimetric paper
strips, the concentration was designed to 2 μM for interfering
substances and 200 nM for Na2S.
Instrumentation and Measurements. The UV−vis spectra were
recorded in the range of 200−900 nm using a double beam UV−vis
spectrophotometer with a 1 cm quartz cuvette (Model TU-1901). XPS
analysis was performed on a PHI5000 Versa Probe high-performance
electron spectrometer (Japan), using monochromatic Al Kα radiation
(1486.6 eV), operating at accelerating voltage of 15 kV. Phase
identification of the Ru NPs were conducted with X-ray diffraction
(XRD, D8, Bruker AXS Co., Ltd.) using Cu Kα radiation source (λ =
1.54051 Å) over the 2θ range of 3−90°. High-resolution transmission
electron microscopy (HRTEM, JEM-2100, Japan Electron Optics
Laboratory Co., Ltd.) was performed at 200 kV to characterize the
structure of NPs. The ζ-potential of Ru NPs was surveyed by using ζ-
Table 1. Comparison of LODs of NPs Based Colorimetric
Sensors for H2S Detection
Signals NPs
Linear
range LODs refs
Luminescence Upconversion
NPs
0−100 μM / 4
Fluorescence Carbon
nanodots
5−100 μM 0.7 μM 37
Fluorescence Au nanoclusters 7−100 μM 0.73
μM
7
UV−vis absorption Au NPs 3−45 μM 2.4 μM 3
UV−vis absorption Ag NPs 0.8−6.4
μM
0.35
μM
38
UV−vis absorption Ag NPs 0.7−10 μM 0.2 μM 11
UV−vis absorption Au nanorods 0.5−5 μM 0.2 μM 3
6
Localized resonance
scattering
Au@Ag NPs 0.05−100
μM
50 nM 5
Catalytic properties Graphene 40−400
μM
25.3
μM
6
Catalytic properties Au@Pt NPs 10−100
nM
7.5 nM 14
Catalytic properties Au NPs 0.5−10 μM 80 nM 15
Catalytic properties Ru NPs 5−100 nM 0.6 nM this
work
ACS Sustainable Chemistry & Engineering Research Article
■ RESULTS AND DISCUSSION
Ru NPs Based Colorimetric Principle for H2S Assay. A
schematic diagram illustrated the mechanism of the proposed
Ru NPs based colorimetric assay of H2S (Scheme 1). Ru NPs
exhibited superior catalytic hydrogenation performance in the
degradation of azo dyes. Ru NPs were applied to attack the azo
bonds of orange I, leading to the rapidly degradation of orange
I to aromatic amines or hydrazine derivatives through the
hydrogenation reduction (Scheme 1a). Red colored orange I
could be rapidly degraded to colorless using Ru NPs as catalysis
and N2H4 as reducing agents. When H2S existed, the poor
thiotolerance of Ru NPs induced the poisoning of the catalytic
active sites of Ru NPs and deactivated the catalytic perform-
ances of Ru NPs. With the increasing concentration of H2S, the
degradation kinetic curves of orange I became slow and the
degradation rate constants decreased (Scheme 1b). There was a
linear relationship between the concentration of H2S and the
degradation rate constants. The color of orange I gradually
faded under the H2S triggered Ru NPs catalytic system, and a
paper strip sensor was fabricated for successful detection of H2S
using the optimized sensor solutions.
Preparation and Characterization of Ru NPs. Ru NPs
stabilized by PVP were synthesized by the reduction of RuCl3
in the presence of ethylene glycol at 170 °C for 6 h. As
illustrated in TEM images (Figure 1a), Ru NPs showed good
monodispersity and uniform morphology. The average
diameter of Ru NPs was 1.7 ± 0.2 nm, which was statistically
analyzed from about 85 Ru NPs (Figure 1b). Representative
HR-TEM images revealed that the lattice fringes of Ru NPs
were separated by 0.236 nm. Ru NPs exhibited hexagonalclose-
packed (hcp) crystal structures, which was in accordance with
the XRD patterns (Figure 1c,d).17,18
The oxidation state of Ru NPs was characterized by XPS
spectra (Figure 2a,b). Two peaks at 280.2 and 285.3 eV were
attributed to the binding energies of 3d5/2 for Ru NPs in the
zero oxidation state, and the binding energy at 281.1 and 287.1
eV was assigned to the high valence state of RuO2 3d5/2, owing
to surface oxidized of Ru(0) during the XPS sampling
procedure (Figure 2a).25,26 C 1s exhibited a peak located at
284.8 eV in the XPS spectra. Figure 2b shows two peaks at
462.0 and 463.5 eV, corresponding to the binding energies of
Ru(0) 3p3/2 and RuO2 3p3/2, respectively.
25−27 Additionally,
when Ru3+ was reduced to Ru0, the absorption peak at 308 nm
for Ru3+ generally decreased and finally disappeared, and the
color of the solution changed from dark red to dark brown,
indicating the formation of Ru NPs (Figure 2c). The ζ-
potential of Ru NPs solution was measured to be −22.0 mV
(Figure 2d), indicating the excellent stability of Ru NPs.23 The
hydroxyl from PVP endowed Ru NPs with negatively charge,
which further stabilized them against agglomeration by
electrostatic repulsion.
Catalytic Hydrogenation Performances of Ru NPs.
Orange I as an azo dye could be quickly degraded to aromatic
amines in the presence of Ru NPs and N2H4, ascribing to the
breakage of the −NN− bonds (Figure S1, Supporting
Information).17 The catalytic performances of Ru NPs in the
reduction of orange I were compared with Pt NPs, Ir NPs and
the control group without catalysts. Under alkaline conditions,
the color of orange I was red with the maximum absorbance of
512 nm (Figure S2, Supporting Information). The changes in
the absorption at 512 nm as a function of time were monitored
in the presence of different catalysts. As demonstrated in Figure
3a, the absorption at 512 nm showed no obvious changes for
the control groups and Ir NPs within 2.0 min, and the
degradation process generally took around 12 h. However, the
absorption at 512 nm exhibited a sharp decline under the
catalysis of Ru NPs. Even though orange I could also be
degraded using Pt NPs as catalysts, the degradation kinetics
curve was much slower than that of Ru NPs (Figure S3,
Supporting Information). The degradation rate constants of
orange I for Ru NPs were 4-, 47- and 165-fold higher than that
of Pt NPs, Ir NPs and control groups (Figure 3a).
The superior catalytic hydrogenation performances of Ru
NPs can be ascribed to the vacant orbitals and the strong
coordination effect with N2H4. Ru NPs acted as an electron
mediator transferred the electron and hydrogen from N2H4 to
azo bonds, leading to the degradation and decolorization of
orange I.17,28 Meanwhile, the catalytic degradation reaction
could also be inhibited after the addition of H2S, due to H2S-
triggered catalytic poisoning and the deactivation efficiency of
Ru NP catalysts.3,29,30 Therefore, the degradation kinetics curve
became slower after the addition of H2S, and the color of
orange I did not change to colorless but became lighter when
Ru NPs and H2S both existed (Figure 3b).
Ru NPs Based Colorimetric Assay of H2S. The catalytic
hydrogenation reaction of orange I using Ru NPs as catalysts
could be applied to detect H2S. The logarithm plot of the
absorbance at 512 nm with reaction time in the presence of
different concentrations of Na2S donors was investigated. As
demonstrated in Figure 4a, with the increasing concentration of
Scheme 1. Schematic Illustration of Colorimetric Assay of
H2S Depending on the Catalytic Hydrogenation Activity of
Ru NPs
Na2S donors, the degradation kinetics curve of orange I became
slower, and the color of orange I became deepened. As
illustrated in Figure 4b, the kinetic rate constants decreased
with the increasing concentration of Na2S donors.
31 The
standard linear curves between rate constants and the
concentration of Na2S donors was established with a good
correlation in the range of 5−100 nM (R2 = 0.9923) and 200
−
800 nM (R2 = 0.9981) (Figure 4c,d). The LOD was calculated
to be 0.6 nM based on 3σ criterion (Supporting Information),
which was much sensitive than those of previous reported
approaches (Table 1).
The sensitivity for the specific H2S detection was determined
by the superior catalytic activity of Ru NPs and H2S-triggered
catalytic deactivation efficiency of Ru NPs. The single Ru NPs
Figure 1. (a) TEM images of synthesized Ru NPs. (b) Statistic analysis of the size of Ru NPs. (c) Respective HR-TEM images of Ru NPs. (d) XRD
patterns of Ru NPs.
Figure 2. (a,b) XPS spectra of Ru NPs. (c) UV−vis spectra of RuCl3 and Ru NPs. Inset: photograph of Ru NPs solution. (d) ζ-potential of Ru NPs.
without the utilization of an acidic support or the addition of a
second metal showed poor thiotolerance, which weakened the
thioresistance of Ru NPs in the catalytic hydrogenation of
orange I.30 Na2S donor generally exists in the form of HS
−
under alkaline condition.24 A number of HS− absorbed on the
surfaces of Ru NPs, and the catalytic active sites on Ru NPs
were reduced, resulting in the formation HS−-induced catalytic
deactivation of Ru NPs.3,29 To validate this, other biological
thiols, such as GSH and Cys, were employed to discuss the
responses of the absorption of orange I at the same conditions.
As shown in Figure 5a, a significant decreased absorption
occurred for the control groups and other amino acids without
sulfhydryl groups. An obvious absorption at 512 nm for orange
I was observed for GSH, Cys and H2S, convincingly suggesting
the interaction between Ru NPs and HS−. The different
absorption at 512 nm under the same concentration of GSH,
Cys and H2S was due to the spatial effect and steric hindrance
from various molecules. H2S molecules were easy to expose
HS−, and thus could directly contact Ru NP catalysts to
deactivate the catalytic active sites on the surface.
Selectivity Evaluation. The developed colorimetric assay
was planned to achieve ultrasensitive detection of H2S in the
atmosphere, and thus the existing biological thiols in bio-
logicalsystem could not interfere the detection results. The
selectivity of the developed colorimetric assay was further
assessed by challenging the system with interfering gases using
their dissolved forms, involving CO3
2−, HCO3
−, NO2
−, NO3
−,
NH4
+, SO4
2−, S2O8
2−, SO3
2−. As illustrated in Figure 5b, there
were no obvious changes in the absorbance except for H2S,
revealing that the present sensing system exhibited excellent
Figure 3. (a) Time-dependent absorbance of orange I at 512 nm in the presence of Ru NPs, Pt NPs and Ir NPs. (b) Time-dependent absorbance of
orange I at 512 nm under the catalysis of Ru NPs before and after the addition of H2S. Inset: corresponding photographs of orange I at different
conditions.
Figure 4. (a) Time-dependent absorbance of orange I at 512 nm in the presence of Ru NPs and different concentration of Na2S donors. Inset:
photographs of orange I within 2 min after the addition of Ru NPs and different concentration of Na2S. (b) Linear fit plots of ln(A0/At) vs time at
different concentration of Na2S. (c) Rate constants as a function of Na2S concentration ranging from 5 to 100 nM. (d) The rate constants as a
function of Na2S concentration ranging from 200 to 800 nM.
selectivity and antijamming capability for the monitoring of
H2S in the atmosphere.
Analysis of Real Samples and Evaluation of Method
Accuracy. The application of the developed colorimetric assay
was investigated by detecting H2S in negative Tai lake water.
Different amounts of Na2S donors were spiked into negative
Tai lake water, and the H2S level was calculated referring to the
regression equation in Figure 4c,d. It was reported that the
heavy metal ions existed in water could deactivate the catalytic
activity of metal catalysts,32−34 but the heavy metal ions would
be precipitated by the formation of hydroxide under alkaline
conditions, which systematically indicated the accuracy and
precision of the developed colorimetric sensor for H2S
detection in the polluted water. As demonstrated in Table S1
of the Supporting Information, the recovery for the samples
was in the range of 97.5%−102.3%, and the RSD was within
1.9%.
Colorimetric Assay of H2S Using Fabricated Paper
Strip Sensor. It was clearly seen that Ru NPs depended
colorimetric assay of H2S showed laudable advantages against
the literature procedures, in terms of response times, sensitivity
and selectivity. The proposed colorimetric principle was
devoted to fabricate a colorimetric paper strip for H2S gas
assay. H2S gas was generated by a stoichiometric reaction
between Na2S and diluted H2SO4 (Na2S + 2H
+ = H2S↑ +
2Na+).35 The sensing pH was controlled at acidic con-
ditions.2,11,35,36 As demonstrated in Figure 6a, an obvious red
color was observed for the paper strips when just orange I was
existed under alkaline conditions. However, the red colored
paper strip rapidly faded to colorless in the presence of Ru NPs
and N2H4 (Figure 6e), attributed to the superior catalytic
hydrogenation performances of Ru NPs. Interestingly, with
increasing amounts of H2S gas (from Figure 6d to 6b), more
catalytic active sites on Ru NPs were deactivated, introducing
the varying degrees of color fading. The fabricated paper strips
were also applied to study the effect of gases using their
dissolved forms. As illustrated in Figure 6f−n, no color changes
were observed for ions other than H2S. The favorable selectivity
for H2S was well suitable for processing complex sample
matrixes for the environmental samples. The fabricated paper
strip sensor was appropriate for the specific and reliable
colorimetric monitoring H2S with the concentration of above 1
μM in the atmosphere, and has the potential to be a convenient
and portable detection kit without the need of sophisticated
instrumentation.
■ CONCLUSION
In summary, a simple Ru NPs depended colorimetric principle
was proposed for the specific and ultrasensitive detection of
H2S. Ru NPs were synthesized and exhibited superior catalytic
performances, which were 4- and 47-fold higher than that of Pt
NPs, Ir NPs. Red-colored orange I could be rapidly degraded to
colorless by Ru NPs, but slowly degraded to pink by the
introduction of H2S to Ru NPs solution, due to the weak
thioresistance of Ru NPs and the poisoning of the catalytic
active sites of Ru NPs. The deactivation degrees were evaluated
by kinetic rate constants of H2S−Ru NPs triggered catalytic
system. Attributing to the superior catalytic activity of Ru NPs
Figure 5. (a) Absorbance intensities of orange I at 512 nm toward the same concentration of biological thiols and other amino acids without
sulfhydryl groups. Inset: corresponding photographs of orange I within 2 min in the presences of Ru NPs and biological thiols/amino acids. (b)
Selectivity of the proposed colorimetric assay against Na2S donors and the interfering substances. Inset: the corresponding photographs of orange I
within 2 min in the presence of Ru NPs and interfering substances.
Figure 6. (a−e) Visual responses of different concentration of H2S toward fabricated paper strip sensors. (a) Control group of orange under alkaline
condition; (b−e) addition of Ru NPs and 100, 10, 1, 0 μM Na2S donors. (f−n) Visual responses of different interfering substances toward fabricated
paper strip sensors. f−n, CO3
and the rapid H2S-induced specific response, the developed
assay for H2S detection displayed a high sensitivity with a wide
linear range of 5−100 nM and a low LOD of 0.6 nM. The
proposed principle for colorimetric assay enabled the visual
readout with the naked eyes, and showed potential as a novel
detection paper strip for point-of-care testing of H2S.
■ ASSOCIATED CONTENT
*S Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acssusche-
meng.7b01448.
Photographs of orange I before and after the addition of
Ru NPs, UV−vis spectra of orange I at different pH,
TEM images of Pt NPs and Ir NPs, table of colorimetric
assay of H2S spiked in Tai lake water (PDF)
■ AUTHOR INFORMATION
Corresponding Author
*Q. Song. E-mail: qsong@jiangnan.edu.cn.
ORCID
Qijun Song: 0000-0002-7579-885X
Notes
The authors declare no competing financial interest.
■ ACKNOWLEDGMENTS
This work is financially supported by the National Natural
Science Foundation of China (21403090), China Postdoctoral
Science Foundation (2015M570405, 2016T90417), the
foundation of Key Lab of Synthetic and Biological Colloids,
Ministry of Education, Jiangnan University (No. JDSJ2015-08
and JDSJ2016-01) and the 111 Project (B13025).
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Role of Ru Oxidation Degree for Catalytic Activity in Bimetallic Pt/Ru
Nanoparticles
Huanhuan Wang,† Shuangming Chen,*,† Changda Wang,† Ke Zhang,† Daobin Liu,† Yasir A
.
Haleem,†
Xusheng Zheng,† Binghui Ge,‡ and Li Song*,†
†National Synchrotron Radiation Laboratory, CAS Center for Excellence in Nanoscience, University of Science and Technology of
China, Hefei 230029, China
‡Beijing National Laboratory for Condensed Mater Physics, Institute of Physics, Chinese Academy of Sciences, Beijing 100190, China
*S Supporting Information
ABSTRACT: Understanding the intrinsic relationship between the catalytic activity of bimetallic
nanoparticles and their composition and structure is very critical to further modulate their
properties and specific applications in catalysts, clean energy, and other related fields. Here we
prepared new bimetallic Pt−Ru nanoparticles with different Pt/Ru molar ratios via a solvothermal
method. In combination with X-ray diffraction (XRD), transmission electron microscopy (TEM)
coupled with energy-dispersive X-ray spectroscopy (EDX), X-ray photoelectron spectroscopy
(XPS), and synchrotron X-ray absorption spectroscopy (XAS) techniques, we systematically
investigated the dependence of the methanol electro-oxidation activity from the obtained Pt/Ru
nanoparticles with different compositions under annealing treatment. Our observations revealed
that the Pt−Ru bimetallic nanoparticles have a Pt-rich core and a Ru-rich shell structure. After
annealment at 500 °C, the alloying extent of the Pt−Ru nanoparticles increased, and more Pt atoms
appeared on the surface. Notably, subsequent evaluations of the catalytic activity for the methanol
oxidation reaction proved that the electrocatalytic performance of Pt/Ru bimetals was increased
with the oxidation degree of superficial Ru atoms.
■ INTRODUCTION
Among various kinds of fuel cells, direct methanol fuel cells
(DMFCs) have been considered to be promising power sources
for future energy needs due to their high energy densities, low
emissions, and facile fuel distribution and storage.1−3 Pt-based
catalysts are the most efficient anode catalysts for the methanol
oxidation reaction (MOR) in DMFCs.4 Nevertheless, challeng-
ing issues of Pt-based catalysts such as the high cost, low
abundance, and poison formation are the main obstacles to the
commercialization of DMFCs.5 This has led to the develop-
ment of Pt-based binary metallic systems, such as PtRu, PtMo,
and PtSn, and ternary compounds, such as PtRuW, PtRuMo,
and PtRuSn.6−8 PtRu alloy nanocrystals have been recognized
as being greatly efficient electrocatalysts for methanol oxidation
reaction.9 The effect of PtRu structural characteristics, such as
composition, degree of alloying and Ru oxidation state, on the
electrocatalytic activity for methanol oxidation has been
reviewed.10 Guo et al. stated that the Pt−Ru (1:1) catalyst
exhibited a highest methanol oxidation current and a lower
poisoning rate.11 But Selda et al. found that a 0.25 Ru/Pt ratio
is optimum at room temperature.12 An optimum ratio of 10−
30% Ru at room temperature for methanol oxidation has also
been reported.13 There is also a debate on whether a PtRu
bimetallic alloy or a Pt and Ru oxide mixture is the most
effective methanol oxidation catalyst. Gasteiger et al. concluded
that the high catalytic activity of Pt−Ru alloys for the
electrooxidation of methanol is described very well by
bifunctional action of the alloy surface.14 Huang et al. suggested
that the presence of crystalline RuO2 is essential to have a
better methanol oxidation from Pt nanoparticles.15 On the
other hand, Rolison et al. found that a commercial Pt−Ru
catalyst composed of oxides of Pt and Ru could deliberately
control the chemical state of Ru to form RuOxHy rather than
Ru metal or particularly anhydrous RuO2 because of poor
proton conduction.16 However, no unanimous conclusion has
been reached until now. Therefore, understanding the intrinsic
relationship between the catalytic activity of bimetallic
nanoparticles and their composition and structure is very
critical for further modulating their properties and specific
applications in catalysts, clean energy, etc.
The primary goal of the present work is to conclusively
establish the relative methanol oxidation activity of bimetallic
Pt−Ru nanoparticles with different compositions and annealing
treatments, using a consistent experimental approach. The
catalyst samples were thoroughly characterized by physical and
electrochemical technologies. Our detailed analysis of the
bimetal’s catalytic activity for methanol oxidation reaction
revealed that Pt/Ru nanoparticles with a Pt-rich core and Ru-
rich shell structure promote increased electro-oxidation of
methanol with the oxidation state of Ru atoms. This study
provides useful insight for understanding the intrinsic relation-
ship between catalytic property and structure/composition,
Received: December 15, 2015
Revised: February 26, 2016
Published: February 29, 2016
DOI: 10.1021/acs.jpcc.5b12267
J. Phys. Chem. C 2016, 120, 6569−
6576
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■ EXPERIMENTAL SECTION
Sample Preparation. In a typical procedure for PtRu,
0.0889 g of poly(vinylpyrrolidone) (PVP), 400 μL of 0.2 M
RuCl3(aq), and 800 μL of 0.1 M H2PtCl6(aq) were dissolved in
38.8 mL of ethylene glycol (EG) under constant magnetic
stirring for 30 min. Then the mixed solution was transferred
into a stainless autoclave having a 50 mL Teflon liner and
heated in an oven at 200 °C for 12 h. After the autoclave was
naturally cooled to room temperature, 23.68 mg of acetylene
black was added to the resulting black solution and
continuously stirred for 30 min. The final product was obtained
by centrifugation, washed several times with deionized water
and absolute ethanol, and dried in a vacuum oven at 60 °C for
12 h. The procedure for Pt2Ru and PtRu2 was the same as that
for PtRu except that the molar ratio of RuCl3 and H2PtCl6 was
changed to 1:2 and 2:1. To investigate the influence of
annealing process, the resulting PtRu powder was calcined at
500 °C under 100 sccm H2/Ar flow for 4 h.
Sample Characterization and XAFS Data Analysis. X-
ray diffraction was performed on a TTR-III high-power X-ray
powder diffractometer employing a scanning rate of 0.02 s−1 in
a 2θ range from 30° to 90° with Cu Kα radiation. The
morphology of samples was characterized by transmission
electron microscopy (TEM, JEM-2100F), equipped with
energy-dispersive X-ray spectroscopy (EDX). The sample for
TEM was prepared by placing a drop of ultrasonically dispersed
ethanol solution onto a carbon-coated copper grid and allowing
the solvent to be evaporated in air at room temperature. Metal
concentrations were measured by inductively coupled plasma
(ICP) atomic emission spectroscopy (AES) using an Atomscan
Advantage Spectrometer. HAADF-STEM and EDX elemental
mapping analysis were carried out in a JEOL ARM-200
microscope at 200 kV. X-ray photoelectron spectroscopy (XPS)
experiments were performed at the Photoemission Endstation
at the BL10B beamline in the National Synchrotron Radiation
Laboratory (NSRL) in Hefei, China. This beamline is
connected to a bending magnet and equipped with three
gratings that cover photon energies from 100 to 1000 eV with a
typical photon flux of 1 × 1010 photons/s and a resolution (E/
ΔE) better than 1000 at 244 eV. The Pt L3-edge and Ru K-edge
XAFS measurements were made in transmission mode at the
beamline 14W1 in Shanghai Synchrotron Radiation Facility
(SSRF) and 1W1B station in Beijing Synchrotron Radiation
Facility (BSRF). The X-ray was monochromatized by a double-
crystal Si(311) monochromator, and the energy was calibrated
using a platinum metal foil for the Pt L3-edge and a ruthenium
metal foil for the Ru K-edge. The monochromator was detuned
to reject higher harmonics. XAFS data were analyzed with
WinXAS3.1 program.17 The energy thresholds were deter-
mined as the maxima of the first derivative. Absorption curves
were normalized to 1, and the EXAFS signals χ(k) were
obtained after the removal of pre-edge and postedge back-
ground. The Fourier transform (FT) spectra were obtained as
k3χ(k) with a Bessel window in the range 3−12.5 Å−1 for the Pt
L3-edge and 3.2−13.2 Å
−1 for the Ru K-edge. Theoretical
amplitudes and phase-shift functions of Pt−Pt, Ru−Ru, Pt−O,
and Ru−O were calculated with the FEFF8.2 code18 using the
crystal structural parameters of the Pt foil, Ru foil, PtO2, and
RuO2.
19−21 On the basis of a face-centered cubic (fcc) model,
the Pt−Ru bond was modeled. The S0
2 values were found to be
1.06 and 0.93 for Pt and Ru, respectively.
Electrochemical Measurements. Electrochemical meas-
urements were taken using a conventional three-electrode
system, with a Pt mesh electrode as counter electrode, a silver/
silver chloride electrode (Ag/AgCl) as the reference electrode,
and a 3 mm diameter glassy carbon electrode as working
electrode. The working electrode was prepared by coating a
small amount of catalyst ink on glassy carbon electrode.
Carbon-supported PtRu catalyst (2.0 mg) was dispersed into a
solution containing 1 mL of ethanol and 10 μL of Nafion
solution (5 wt %), followed by ultrasonic treatment for 30 min,
and then the resultant suspension (ca. 10 μL) was pipetted
onto glassy carbon electrode and dried at room temperature for
20 min. Prior to coating with catalyst ink, the glassy carbon
electrode was polished with alumina paste and washed with
deionized water. Cyclic voltammetry was carried out to study
the methanol oxidation reaction (MOR) at room temperature
in an electrolyte containing 1.0 M KOH and 1.0 M CH3OH
between −0.8 and 0.3 V (vs Ag/AgCl) at a scan rate of 50 mV/
s. Prior to each cyclic voltammetry measurement, the
electrolytic solution was purged with pure N2 for 30 min to
remove dissolved oxygen.
■ RESULTS AND DISCUSSION
XRD and TEM Characterization. Figure 1 shows the
comparison of XRD patterns for different samples. The
characteristic peaks for a face-centered cubic phase (fcc) were
clearly observed in all samples. No additional peaks, such as
those attributed to Pt or Ru oxides, can be detected.
Interestingly, the characteristic peaks shifted to a higher angle
with increasing Ru percentage, indicating the contraction of the
lattice parameter due to formation of the Pt−Ru alloy. In
addition, the diffraction peaks shifted to higher angles and
became slightly sharper after annealing. This suggests that the
annealing process can reduce the lattice parameter and slightly
increase the grain size and the alloying extent of the Pt/Ru
nanocrystals.
The particle size and corresponding histograms of size
distribution of different samples are shown in Figure 2. Most
particles of PtRu, Pt2Ru, and PtRu2 are monodisperse with an
average size about 3−4 nm. After annealing, the particles
became slightly larger in size (Figure 2d). The compositions of
the catalyst were measured by ICP-AES and EDX and are
Figure 1. XRD patterns of PtRu, PtRu-annealed, Pt2Ru, and PtRu2.
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shown in Figure S1 and Table S1 (Supporting Information), in
which the overall chemical compositions for PtRu, Pt2Ru, and
PtRu2 alloy nanoparticle electrocatalysts are well confirmed
with 1:1, 2:1, and 1:2 Pt:Ru atomic ratios. The high angle
annular dark field scanning transmission electron microscopy
(HAADF-STEM) image and the corresponding EDX elemental
mapping image of PtRu are shown in Figure 2e and Figure 2f.
These observations reveal that the prepared PtRu particles are
formed by Ru and Pt elements. The EDX elemental mapping
image indicates that Ru atoms have a degree of dispersion
higher than that of Pt atoms.
XANES and XPS Analysis. To identify the microstructure
of Pt/Ru bimetals, we performed synchrotron-based X-ray
absorption spectroscopy (XAS) of the samples. The X-ray
absorption near-edge structure (XANES) spectra of the Pt L3-
edge and Ru K-edge are shown in Figure 3. In the Pt L3-edge of
Figure 3a, all samples exhibited more intense white line peaks
than that of Pt foil. It is known that the Pt L3-edge white line
corresponds to the excitation of 2p3/2 electrons to empty 5d
orbitals,22 which means more unoccupied 5d states of Pt atoms
in these Pt/Ru alloy nanoparticles in contrast to Pt foil. In
general, this explanation can be ascribed to three effects: size
effect, alloying effect, and surface oxidation effect. However, as
the Pt atoms in pure Pt nanoparticles have more d electrons
than that in bulk,23 the influence of the size effect can be
eliminated.
To clarify the alloying effect, we investigated the Pt L3-edge
XANES spectrum of Pt−Ru alloy and compared it with the
spectrum of pure Pt. In the calculations, we modeled the Pt L3-
edge XANES spectra of Pt−Ru alloy by replacing some of the
12 nearest-neighbored Pt atoms around the central Pt atom
with Ru atoms. As shown in Figure 3b, Pt/Ru alloy has a
slightly weaker white line peak compared to pure Pt. That
means the alloying effect cannot cause the increase in white line
peak intensity. Finally, we suggest that the increase can be
attributed to a surface oxidation effect. More precisely, it
originates from the oxidation of some surface Pt atoms. Besides,
it is worth noting that the white line intensity for PtRu, Pt2Ru,
and PtRu2 was almost constant while PtRu-annealed exhibited a
distinct increase, which can be explained by the increased
oxidized Pt atoms after annealing. However, strong oxidation of
Pt in these Pt−Ru alloy nanoparticles should be ruled out based
on the direct comparison with bulk Pt and PtO2. For the Ru K-
edge XANES spectra in Figure 3c, the Ru atoms in sample
PtRu, Pt2Ru, and PtRu2 were partially oxidized where the order
of oxidation degree is Pt2Ru > PtRu > PtRu2. Similarly, strong
oxidation of Ru should also be eliminated. Notably, there is
almost no oxidation of Ru in PtRu after annealing. This means
that oxidized Ru atoms in PtRu were reduced by the annealing
process.
To further investigate the electronic structure of these Pt−Ru
nanoparticles, XPS spectra for the Pt 4f and Ru 3d core level
region for all samples were measured as shown in Figure 4. As
shown in Figure 4a, the binding energies (BE) of Pt 4f7/2 for all
PtRu, Pt2Ru, and PtRu2 are almost the same while a slight right
shift to higher BE can be observed for PtPu-annealed,
Figure 2. TEM images and histograms of particle-size distributions of (a) PtRu, (b) Pt2Ru, (c) PtRu2, and (d) PtRu after annealing treatment. (e)
HAADF-STEM image. (f) The corresponding EDX elemental mapping image of PtRu.
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suggesting an increase in the d-vacancy of the Pt atoms.24 The
Ru 3d core level region was deconvoluted as shown in Figure
4b−e, as described by Roblison et al.25 The corresponding
deconvoluted results are summarized in Table 2. The XPS data
suggest that there are three Ru species (Ru metal, RuO2, and
RuO2·xH2O) present on the surface of the Pt−Ru catalyst. The
percentages of Ru−OH species (RuO2·xH2O) and Ru-oxide
increase in the following trend: Pt2Ru > PtRu > PtRu2 > PtRu-
annealed, consistent with the XANES analysis.
EXAFS Analysis. To further study the structure, the
corresponding extended X-ray absorption fine structure
(EXAFS) of the samples was analyzed. The k3-weighted
EXAFS signals of the Pt L3-edge and Ru K-edge are shown
in Figure S2 (Supporting Information). It has been noted that
EXAFS oscillations of all samples were lower in amplitude
compared to that of bulk Pt and bulk Ru in both the Pt L3-edge
and Ru K-edge, which can be attributed to the size effect of the
nanoparticles. In contrast to amplitude, the phase of EXAFS
oscillations of PtRu, Pt2Ru, and PtRu2 were similar to that of
bulk sample, which indicates that these nanoparticles are more
likely to be a core−shell structure rather than an alloying
structure. For the control sample, the EXAFS oscillations of
PtRu-annealed were slightly phase-shifted at each edge,
indicating the increased alloying extent after the annealing
process. Particularly, the comparison with the EXAFS signals of
standard Pt and Ru oxides further confirmed that strong
oxidation of Pt and Ru could be eliminated in our samples.
Figure 5a and 5b shows the corresponding Fourier-
transformed EXAFS spectra of the Pt L3-edge and Ru K-
edge. It is observed that the Pt L3-edge for PtRu, Pt2Ru, and
PtRu2 exhibit similar local structure around Pt. However, there
is a significant change in local structure around Pt in PtRu after
annealing. On the basis of the Ru K-edge, we can conclude that
Ru atoms in Pt2Ru have the highest oxidation degree. EXAFS
data analysis was carried out by simultaneously fitting both the
Pt L3-edge and the Ru K-edge. The comparisons of
experimental and fitting data for the Pt L3-edge and Ru K-
edge are shown in Figures S3 and S4 (Supporting Information),
and corresponding fitting parameters are summarized in Table
S2 (Supporting Information).
According to previously reported literature,26 we can
determine atomic distribution and alloying extent in bimetallic
nanoparticles based on four parameters: Pobserved(NPt−Ru/NPt‑i),
Robserved(NRu−Pt/NRu‑i), Prandom, and Rrandom. For PtRu and PtRu-
annealed samples, Prandom and Rrandom can be taken as 0.5, as the
atomic ratio of Pt and Ru is 1:1. For the Pt2Ru sample, Prandom
and Rrandom can be taken as 0.33 and 0.67, respectively, as the
atomic ratio of Pt and Ru is 2:1. Conversely, Prandom and Rrandom
can be taken as 0.67 and 0.33 for PtRu2. Then alloying extents
of Pt (JPt) and Ru (JRu) can be calculated using the following
equations:
= ×J P P( / ) 100%Pt observed random (1)
= ×J P P( / ) 100%Ru observed random (2)
All the calculated results based on this method are
summarized in Table 1. The observed parameter relationships
∑NPt−M > ∑NRu−M and JRu, JPt < 100% indicate that all of the
as-synthesized Pt−Ru nanoparticles adopt a Pt-rich core and
Ru-rich shell structure. The larger JPt and JRu values in PtRu-
annealed indicate the increased extent of atomic dispersion and
alloying extent after the annealing process, which is consistent
with the above analysis. The higher values of Robserved and JRu
suggest a higher alloying extent of Ru atoms compared with Pt.
Figure 3. XANES spectra at the (a) Pt L3-edge and (c) Ru K-edge for Pt foil, Ru foil, PtO2, RuO2, and all samples. (b) The comparison of the
calculated Pt-L3 edge XANES spectra of pure Pt and Pt−Ru alloy with some Pt atoms substituted by Ru atoms in the first shell.
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This means that most of the Ru atoms were reduced and
involved in alloying after the annealing process. Meanwhile,
some Pt atoms migrated to the surface and were then oxidized
by air, according to the XANES and XPS analysis. Here we can
summarize that as-grown Pt/Ru nanoparticles have a Pt-rich
core and Ru-rich shell structure. After the annealing process,
the alloying extent of Pt/Ru nanoparticles had been increased,
and more Pt atoms appeared on the surface. The structures of
Pt/Ru nanoparticles are schematically shown in Figure 5c.
Catalytic Performance in the Methanol Electro-
oxidation. Cyclic voltammetry experiments were performed
in N2-saturated freshly prepared 1 M KOH solution by
sweeping the electrode potential from −0.8 to 0.3 V vs Ag/
AgCl at a scan rate of 50 mV/s, to measure the electrochemical
active surface area (ECSA) of the catalysts, as shown in Figure
S5 (Supporting Information). The integrated charge in the
hydrogen adsorption/desorption peak area in the CV curves
represents the total charge concerning H+ adsorption, QH, and
has been used to determine ECSA by employing the following
equation:27
μ
μ
=
×
Q
ECSA [m /g of Pt]
charge [ , C/cm ]
210 [ C/cm ] electrode loading [g of Pt/m ]
2
H
2
2 2
The trend in ECSA values varied in the following order: Pt2Ru
(80.71 m2/g) > PtRu (64.01 m2/g) > PtRu2 (52.08 m
2/g) >
PtRu-annealed (27.63 m2/g). Among these electrocatalysts,
Pt2Ru was ascertained to possess the greatest electrochemical
activity. Accordingly, it is rational to assume that the higher
ECSA value may signify the better electrocatalyst that has more
catalyst sites available for electrochemical reaction.
To investigate the effect of Pt/Ru bimetal structure on the
catalytic property, a methanol electro-oxidation experiment was
carried out. Figure 6a displays cyclic voltammograms (CVs) of
methanol oxidation on Pt2Ru, PtRu, PtRu2, and PtRu-annealed
in 1.0 M KOH containing 1.0 M CH3OH solution. Two well-
defined oxidation peaks can be clearly observed: one in the
forward scan is produced because of oxidation of freshly
chemisorbed species coming from methanol adsorption, and
the other in the reverse scan is primarily ascribed to removal of
incompletely oxidized carbonaceous species formed during the
forward scan. As known, the oxidation peak during the forward
Figure 4. XPS spectra of (a) Pt 4f and C 1s + Ru 3d for (b) PtRu, (c) PtRu-annealed, (d) Pt2Ru, and (e) PtRu2. The entire Ru 3d + C 1s envelope
was deconvolved for all spectra, but for clarity, only the fits for Ru 3d5/2 lines are shown. The envelopes are fitted with three Ru 3d5/2 peaks.
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scan can be used to evaluate the catalytic activity of the catalyst.
It is estimated that the values of current density increase in the
following trend: Pt2Ru > PtRu > PtRu2. This phenomenon is
attributed to two probable reasons: one is increasing oxidation
degree of surface Ru atoms in these samples (Pt2Ru > PtRu >
PtRu2), which is consistent with the order of catalytic activity of
the catalysts, while the other is increasing Pt concentration in
these Pt/Ru catalysts. However, with the same composition, the
PtRu-annealed sample with the lowest oxidation degree of Ru
atoms and more Pt atoms on the surface exhibits the worst
catalytic activity. Thus, we can suggest that the higher methanol
oxidation catalytic activity originates from the increasing
oxidation degree of surface Ru atoms in Pt/Ru bimetals. This
is probable due to the content of Ru−OH increasing with the
oxidation degree of surface Ru atoms, as Ru−OH is a critical
component of the MOR of the Pt−Ru catalyst which
determines the electrocatalytic activity of Pt−Ru.25 Further-
more, the ratio of the forward anodic peak current density (If)
to the reverse anodic peak current density (Ib), If/Ib, can be
used as an important index to evaluate the catalyst tolerance to
CO accumulation.28,29 Our calculation indicates that Pt2Ru,
PtRu, and PtRu2 have almost the same If/Ib value, while the If/
Ib value of PtRu-annealed is obviously larger. This may be
attributed to the increasing alloying extent after the annealing
process, as it has been proved that the tolerance to CO
accumulation by the Pt−Ru alloying catalyst will increase with
the alloying degree.30 Thus, the best catalyst for oxidation of
accumulated CO is not necessarily the best one for methanol
oxidation.10
Moreover, chronoamperometry (CA) was also performed to
investigate the long-term stability of those catalysts under the
same conditions. Figure 6b shows CA curves performed in 1.0
M KOH + 1.0 M CH3OH at −0.2 V (vs Ag/AgCl) for 2500 s.
After a sharp drop in the initial period of around 300 s, the
currents decay at a much slower speed and then approach a flat
line. During the whole time, it was clear that current density
produced on the Pt2Ru catalyst was higher than the current
density produced on the PtRu, PtRu2, and PtRu-annealed
catalysts. These results are in agreement with those of the cyclic
voltammetry measurements, indicating that Pt/Ru bimetals
Figure 5. Fourier-transformed EXAFS spectra of the (a) Pt L3-edge and (b) Ru K-edge for Pt foil, Ru foil, PtO2, RuO2, and all samples. (c)
Schematic representation of the structure of the Pt−Ru nanoparticles having different molar ratios synthesized by EG reduction and after annealing.
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with higher Ru oxidation degree can pose better methanol
oxidation catalytic activity.
■ CONCLUSIONS
Bimetallic Pt−Ru nanoparticles with different Pt/Ru molar
ratios were synthesized by a solvothermal method and
characterized by various methods. Our observations revealed
that these Pt−Ru nanoparticles have a Pt-rich core and a Ru-
rich shell structure. After annealing at 500 °C, the alloying
extent of Pt/Ru nanoparticles increased, a portion of Pt atoms
migrated to surface, and most of the surficial oxidized Ru atoms
were reduced and involved in alloying. The evaluations of
methanol electro-oxidation activity elucidated that electro-
catalytic performance improved with the increasing oxidation
degree of superficial Ru atoms. This study provides useful
information and deep insight for understanding the relationship
of electrocatalytic performance of bimetallic nanoparticles with
their structure, which may help us to further tune the bimetal
structure, composition, and catalytic activity for specific
applications.
■ ASSOCIATED CONTENT
*S Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.jpcc.5b12267.
EDX analyses of Pt2Ru, PtRu, and PtRu2. Comparison of
compositions determined from EDX and ICP. Compar-
ison of k3-weighted EXAFS signals, experimental data,
and the fitting curves for Pt L3-edge and Ru K-edge.
Cyclic voltammograms (CVs) of all samples in 1 M
KOH. Best fit parameters of the Pt L3-edge and Ru K-
edge EXAFS spectra (PDF)
■ AUTHOR INFORMATION
Corresponding Authors
*E-mail: csmp@ustc.edu.cn.
*E-mail: song2012@ustc.edu.cn.
Notes
The authors declare no competing financial interest.
■ ACKNOWLEDGMENTS
Financial support comes from 973 program (2014CB848900),
NSF (U1232131, U1532112, 11375198, 11574280), the
Postdoctoral Science Foundation of China (2015M581990),
the Fundamental Research Funds for the Central Universities
(WK2310000053), and User with Potential from CAS Hefei
Science Center (CX2310000080). We also thank the SSRF
(BL14W1), BSRF (1W1B), MCD, and Photoemission
Endstations in NSRL for help with synchrotron-based
measurements and the USTC Center for Micro and Nanoscale
Research and Fabrication.
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Catalysis with Colloidal Ruthenium Nanoparticle
s
M. Rosa Axet and Karine Philippot*
UPR8241, Universite ́ de Toulouse, UPS, INPT, CNRS, LCC (Laboratoire de Chimie de Coordination), 205 Route de
NarbonneF-31077 Toulouse cedex 4, France
ABSTRACT: This review provides a synthetic overview of the recent researc
h
advancements addressing the topic of catalysis with colloidal ruthenium metal
nanoparticles through the last five year
s.
The aim is to enlighten the interest of
ruthenium metal at the nanoscale for a selection of catalytic reactions performed in
solution condition. The recent progress in nanochemistry allowed providing well-
controlled ruthenium nanoparticles which served as models and allowed study of how
their characteristics influence their catalytic properties. Although this parameter is not
enough often taken into consideration the surface chemistry of ruthenium nanoparticles
starts to be better understood. This offers thus a strong basis to better apprehend
catalytic processes on the metal surface and also explore how these can be affected by
the stabilizing molecules as well as the ruthenium crystallographic structure. Ruthenium
nanoparticles have been reported for their application as catalysts in solution for diverse
reactions. The main ones are reduction, oxidation, Fischer−Tropsch, C−H activation
,
CO2 transformation, and hydrogen production through amine borane dehydrogenation
or water-splitting reactions, which will be reviewed here. Results obtained showed that ruthenium nanoparticles can be highly
performant in these reactions, but efforts are still required in order to be able to rationalize the results. Beside their catalytic
performance, ruthenium nanocatalysts are very good models in order to investigate key parameters for a better controlled
nanocatalysis. This is a challenging but fundamental task in order to develop more efficient catalytic systems, namely more
active and more selective catalysts able to work in mild conditions.
CONTENTS
1. Introduction
1086
2. Interests of Ruthenium and Metal Nanoparticles
1087
2.1. Physicochemical Properties and Interests of
Ruthenium 1087
2.2. Interests of Metal Nanoparticles in Catalysis 1087
2.3. Present Challenges in Nanocatalysis and
Place of Ruthenium Nanocatalysts
1088
3. Synthesis Methods of Ruthenium Nanoparticles 1088
3.1. Reduction of Ruthenium(III) Chloride Hy-
drate
1089
3.2. Polyol Method
1090
3.3. Use of an Organometallic Precursor 1090
3.4. Supported Nanoparticles
1092
4. Ruthenium Nanoparticles As Catalysts 109
2
4.1. Reduction Reactions 1092
4.1.1. Reduction of CC and CO Bonds
1096
4.1.2. Reduction of Nitro Compounds
1097
4.1.3. Hydrodeoxygenation
1100
4.1.4. Reductive Amination of Carbonyl Com-
pounds, Amination of Alcohols, and
Other Miscellaneous Reduction Reac-
tions
1105
4.2. Oxidation Reactions
1106
4.3. Fischer−Tropsch Reaction
1111
4.4. C−H Activation and Other Reactions
1113
4.5. Transformation of CO2 1113
4.5.1. Transformation of CO2 into HCOOH 1113
4.5.2. Transformation of CO2 into CO, CH4, or
C2+ Hydrocarbons
1119
4.5.3. Conclusions on CO2 Transformation
1123
4.6. Dehydrogenation of Amine Boranes 112
4
4.6.1. Dehydrogenation of Amine Boranes by
Dehydrocoupling
1125
4.6.2. Dehydrogenation of Amine Boranes by
Methanolysis
1127
4.6.3. Dehydrogenation of Amine Boranes by
Hydrolysis
1128
4.6.4. Dehydrogenation of Amine Boranes by
Supported Ruthenium Nanocatalysts
1130
4.6.5. Conclusions on Amineborane Dehydro-
genation 1130
4.7. Water Splitting 1130
4.7.1. Ru NPs as Electrocatalysts for HER
1131
4.7.2. Ru NPs as (Photo)catalysts for HER
1133
4.7.3. Conclusions on Water Splitting 1133
5. Concluding Remarks and Outlook 1133
Author Information
With symbol Ru and the 44th position in the periodic table of
elements, ruthenium is part of the transition metals group. It is
considered as a scarce metal with limited availability. This may
be hindering wider commercial applications involving
ruthenium due to its high price (even if still the least expensive
precious metal) and wide fluctuations in the market. The
applications of ruthenium mainly concern technological
devices and catalysis sectors. In 2018, ruthenium consumption
has achieved 42 tons for industrial applications concerning
electronics (33%), electrochemistry (17%), and chemistry
(37%).1 For instance, ruthenium is commonly added at a small
quantity in alloys given its ability to harden them. This is the
case of super alloys used for the manufacture of turbine blades
of jet engines. It reinforces the rhodium, palladium, and
platinum-based alloys used for wear-resistant electrical contacts
(high-end spark plugs have electrodes coated with a Pt−Ru
alloy; pen tips are made with alloys containing ruthenium).
Ruthenium dioxide, RuO2, and ruthenates of lead and bismuth
are involved in resistive chips. In electronics, ruthenium is used
in the manufacture of hard disks as a coating between two
magnetic layers.
Regarding catalysis, ruthenium is a polyvalent metal because
it can easily adopt formal oxidation states in a wide range
(from II to VIII), leading to a multitude of complexes that
display interesting and often unique properties. These
properties can be tuned by an appropriate choice of the
ligands because these latter strongly affect the reactivity as well
as stability of ruthenium complexes. A molecular level
understanding of structure−activity relationships in complexes
is a key parameter for the development of better catalysts. For
instance bipyridines- and terpyridine-containing ruthenium
complexes are known for their luminescent and photoredox
properties. Such properties are at the basis of the photo-
dissociation of water into O2 and H2 (water splitting
)
2 and of
the development of new generation photovoltaic cells.3
Another important application of ruthenium is the catalytic
production of added-value chemicals like acetic acid.4
Carbene-based ruthenium complexes are well-known for
their central role in olefin metathesis that provides active
molecules or functionalized polymers among others. Ru
complexes with phosphorus-containing ligands (for example
phosphines, diphosphines as the so-called BINAP, or
phosphites) are active for hydrogenation reactions such as
hydrogenation of CC and CO double bonds among
others, including the enantioselective version.5 Ru complexes
are also known for their catalytic performance in the synthesis
of formic acid and its decomposition into H2 and CO2 or also
the dehydrogenation of alcohols, two important reactions
regarding hydrogen storage.6 Finally Ru species are also
catalysts of oxidation reactions.7 In heterogeneous conditions,
ruthenium is the most active catalyst for the production of
ammonia.8 It is also active in the hydrogenation of diverse
substrates. As ligands in molecular catalysis, supports play a
key-role in the properties of supported ruthenium catalysts due
to metal−support interactions. The fine understanding of
microscopic properties of the heterogeneous catalysts, in
particular, the nature of surface active sites and their chemical
or sterical environment is of utmost importance in order to
improve catalytic performances. Finally, the oxidized form of
ruthenium, RuO2, is known for its performance in heteroge-
neous oxidation catalysis and in electrocatalysis.
The exaltation of properties at the nanoscale regime can
increase the relevance of ruthenium for catalysis. The recent
progress in nanochemistry allowed having at disposal better
controlled Ru NPs in terms of size, dispersion, shape,
composition, and surface state, etc. All these characteristics
may influence strongly their surface properties and con-
sequently their catalytic performance (both reactivity and
selectivity), and numerous efforts are presently made in this
sense. Using a molecular approach, namely studying the
interface between surface atoms and stabilizers (ligands) by a
combination of techniques from molecular chemistry (like
nuclear magnetic resonance) to theoretical studies allows a
better understanding of the surface chemistry of ruthenium
nanoparticles. As will be seen in the next sections, these
findings give thus a strong basis to better apprehend catalytic
processes on the metal surface as well as how these can be
affected by the presence of stabilizing molecules or by the
crystallographic structure of the ruthenium cores, eventually by
taking benefit of these parameters.
This review will start by summarizing the physicochemical
properties and interests of ruthenium together with those of
metal nanoparticles (section 2) and following, the main
synthesis methods to produce ruthenium metal nanoparticles
in solution (section 3). Then, the purpose is to provide a
synthetic overview of the recent advancements in research that
address the investigation of ruthenium metal nanoparticles (Ru
NPs) in catalysis in solution (or suspension) conditions in the
period 2014−2019 (section 4). The aim is to highlight the
potential of ruthenium metal when it is divided at the
nanoscale in a controlled manner, namely under the form of
well-defined Ru NPs, in colloidal catalysis. Ru NPs have been
reported for their application as catalysts in diverse reactions.
The reactions reviewed here include reduction, oxidation,
Fischer−Tropsch, C−H activation and amine borane dehy-
drogenation reactions where Ru NPs show to be very
performant. Even if at a lesser extent, Ru NPs have been
also investigated for the reduction of carbon dioxide and water
splitting process. Relevant works involving Ru NPs in these
catalytic reactions will be described. Selection of examples was
governed by the degree of control of the characteristics of the
described Ru NPs that was made possible by solution synthesis
methods, thus allowing precise catalytic investigations.
Heterogeneous catalysts are not considered due to the fact
the metal nanoparticles they contain are generally poorly
controlled due to drastic conditions applied for their
preparation. However, a few examples of supported Ru NP-
based catalysts are presented. This is justified either by their
initial preparation method, which enabled to obtain well-
controlled nanostructures, thus providing complementary
information to the discussed subjects or by the relevant or
pioneering character of the contribution to the field of
catalysis. Also, a few papers from earlier years are included
due to their high input. Finally concluding remarks and
perspectives will be given for each type of reaction treated.
2.1. Physicochemical Properties and Interests of
Ruthenium
Identified and isolated by Karl Karlovich Klaus in 1844,9
ruthenium has as its symbol Ru and the 44th position in the
periodic table of elements. Ruthenium is the 74th most
abundant metal, a rare element, and is part of the precious
metals, being the first of the series beside rhodium, palladium,
osmium, iridium, and platinum. With a current price of ca.
7000 €/kg,10 ruthenium is still the least expensive precious
metal.
Ruthenium is a hard, silvery white metal which is unalterable
in the ambiant air and does not tarnish at room temperature
(rt). Ruthenium is a transition metal with electronic
configuration [Kr]4d75s1 for the isolated atom in ground
state. The oxidation states of ruthenium range from II to VIII,
the most common ones being II, III, and IV. These different
oxidation states provide a large number of stable ruthenium
catalysts (at 16 or 18 electrons). Ruthenium is not easily
oxidized at atmospheric condition but RuO2, a stable oxide,
may be formed under oxygen pressure. Ruthenium tetroxide
(RuO4), a volatile compound, is a powerful oxidizing and very
toxic.9 The dissolution of ruthenium is not easy and requires
use of aqua regia in heating conditions. Crystalline structure of
bulk ruthenium is hexagonal closed-packed (hcp) but at the
nanoscale, face-centered cubic (fcc) structure is also
known.11−13 Ruthenium is the only noble metal that can
crystallize in the nanometer scale with the hcp structure or the
fcc one. The anisotropy of the hexagonal system is expected to
lead more easily to anisotropic crystals, but there are only a few
papers reporting anisotropic Ru NPs, and none with a high
aspect ratio.14
The applications of ruthenium mainly concern technological
devices and catalysis sectors.15 In catalysis, ruthenium is a
polyvalent metal which proved to be active in both
homogeneous and heterogeneous conditions. RuCl3·3H2O is
often the starting point of a rich coordination and organo-
metallic chemistry, thus leading to a wide variety of ruthenium
complexes of high interest for homogeneous catalysis.
Ruthenium complexes are able to activate unique and multiple
bonds and make possible selective C−C, C−H, or C-
heteroatom bond formation and cleavage.16 Ruthenium
catalysts are thus involved in a great variety of organic
reactions, such as alkylation, allylation, arylation, cyclization,
cyclopropanation, hydrogenation, hydroformylation, hydro-
silylation, hydroxylation, isomerization, olefin metathesis,
oxidation, transfer hydrogenation, tandem reactions, water
splitting, etc. Ru-catalysis is effectively exploited in the
synthesis of natural and biologically active organic compounds,
to access recognized chemotherapeutic agents, supramolecular
assemblies, smart materials, specialty polymers, biopolymers,
agrochemicals, and, increasingly, in valorization of renewable
resources as platform chemicals for polymers. Presently,
intensive research efforts are devoted in C−H and C−X
bond activation, olefin metathesis, and newest trends of green
chemistry, such as water oxidation and hydrogen production,
reduction of CO2 to CO, oleochemistry, and reactions in eco-
friendly media.17 Because of their matter state, heterogeneous
transition metal catalysts are also of high interest in catalysis
and largely exploited at the industrial level. Heterogeneous
catalysts are extended inorganic solids where the d orbitals play
a key role in the adsorption and transformation of substrates.
The catalytic activity of transition metals shows a strong
periodic effect with a maximum of reactivity for group-VIII
transition metals among which ruthenium. Ruthenium is able
to chemisorb diverse small molecules such as O2, C2H2, CO,
H2, N2, and CO2. In heterogeneous and colloidal conditions,
ruthenium is reputed to be active in hydrogenation of nitrogen
for ammonia synthesis, hydrogenation of diverse substrates like
olefins, and carbonylated molecules but also of aromatics for
which molecular ruthenium is not known, as well as for
dehydrogenation of amine boranes and hydrogen evolution
reactions. Interestingly, it is not very known for hydrogenation
of CO2 and dehydrogenation of formic acid. RuO2 turned out
to be an excellent oxidation catalyst in heterogeneous catalysis
(mainly oxidation of CO) and electrocatalysis (oxidation of
water).18
2.2. Interests of Metal Nanoparticles in Catalysis
Heterogeneous transition metal catalysts are extended
inorganic solids where the d orbitals play a key role in the
adsorption of substrates due to their ability to donate and
accept electron density to and from the substrates. This is
particularly true for the degenerate states in band structures.
The electronic flexibility provided by the d electrons of the
metal surface has to be such that the bond with the substrate
atoms is intermediate between weak and strong. The metal
surface must be able to bind the substrate atoms strongly
enough to provoke their dissociation in the chemisorption
process. But the surface-atom bond created has to be not too
strong, for the bonded substrate atom to be able to further
react with other surface-bonded atoms and form the products
that can rapidly desorb. If the surface-atom bond is too strong,
further reaction will be precluded. The catalytic activity of
transition metals shows a strong periodic effect with a
maximum of reactivity for group-8 transition metals where
ruthenium is located.19
Being part of heterogeneous catalysts, metal nanoparticles
(MNPs) have been known for a long time, but a renewed
interest emerged in the last three decades for the design of
better defined systems.20 Numerous research efforts are
devoted to the design of well-controlled MNPs and even at
an atomic precision level.21,22 This keen interest for MNPs
derives from the particular matter state (finely divided metal
s)
and exalted electronic properties, influencing physical and
chemical properties that they present in comparison to bulk
metals and molecular complexes. Besides fundamental aspects
of research, this interest is also governed by the specific
properties and the potential applications that MNPs may find
in various domains including optoelectronics, sensing,
biomedicine, catalysis, energy conversion, and storage, as
nonexhaustive examples.23−26 Several books focus specifically
on nanocatalysis.27−37 For catalysis, MNPs are attractive
species due to the high surface to volume ratio they display.
This ratio is even more pronounced when MNPs are at a size
as close as one nanometer, or even below, because the number
of surface atoms can be >90%, thus providing a vast number of
potential active sites. It is thus of prime importance to have
synthesis tools that enable obtaining ultrasmall NPs in order to
promote high surface area. Besides the size, other key
parameters need also to be controlled. The crystalline structure
is important because depending on it, different types of
crystalline plans can be exposed at the nanoparticle surface,
which can lead to different catalytic properties. Controlling the
shape of MNPs is another way to orientate the crystalline plans
exposed.38−40 The last key parameter but not the least is the
composition of MNPs. The composition has to be adjusted
depending on the catalysis target. Apart from the nature of the
metallic core that may govern the reactivity (some metals are
well-known for certain catalyzes but not for others), the
surrounding stabilizer for colloidal catalysis (ionic liquids (IL),
polymers, surfactants, polyols, ligands, etc.) or the support for
supported catalysis (metal oxides, metal organic frameworks
(MOFs), carbon derivatives, etc.) may also influence or even
orient the catalytic performance. If calcination is usually
applied in heterogeneous catalysis in order to suppress any
organics and liberate the active sites, such treatment on small
nanoparticles can be critical because of sintering. Moreover,
naked MNPs are not always optimal catalysts. In modern
nanocatalysis, the presence of organic ligands at the NP surface
is not seen as detrimental but instead is a way to improve or
even modify the chemoselectivity.41 Using ligands as stabilizers
allows to make a parallel with molecular catalysis; the ligand
interaction with surface metal atoms of the nanoparticles can
be compared to ligand interactions with the metal centers in
homogeneous catalysts, which is of paramount importance for
stability and catalytic properties (activity and selectivity).
Ligands can be chosen in order to tune the surface properties
of MNPs through steric or electronic effects.42,43 The challenge
is to find ligands able to stabilize well-defined MNPs while
controlling accessibility at the metal surface and reactivity.41,44
Strongly bound capping ligands (like thiols or phosphines) can
result in the poisoning of a nanocatalyst at high surface
coverage. But a limited amount of ligand can be beneficial. The
coordination of a ligand at a metal surface can also be a way to
block selectively some active sites in order to orientate the
catalysis evolution. Compared to the investigation of facet
dependency,40,45 the ligand influence on the catalytic activity
has been less intensively studied but recent results illustrate
well the interest to do so.46−50 Ligand-stabilized MNPs can be
applied to catalysis as stable colloidal suspensions but also in
heterogeneous conditions when deposited on the surface or
confined in the pores of a solid support.51 Ionic liquids52 are
also very efficient to stabilize metal NPs, and colloidal
suspensions in ionic liquids can even be deposited onto
inorganic supports.53
2.3. Present Challenges in Nanocatalysis and Place of
Ruthenium Nanocatalysts
Having at disposal synthesis strategies that allow access, in a
reproducible manner, to well-defined MNPs in terms of size,
crystalline structure, composition (metal cores and stabilizing
agents), chemical order (bimetallic or multimetallic systems),
shape, and dispersion is a beneficial condition to investigate
finely their catalytic properties and define structure/properties
relationships. Taking advantage of recent developments in
nanochemistry in solution, and in particular of the use of
molecular chemistry tools, nanocatalysis is now well-
established as a borderline domain between homogeneous
and heterogeneous catalysis. Nanocatalysts can be seen as
assemblies of individual active sites where metal−metal and
metal−stabilizer bonds will both have influence.54 Precisely
designed MNPs are expected to present benefits from both
homogeneous and heterogeneous catalysts, namely high
reactivity and better selectivity together with high stability.55
The understanding of structure−properties relationships is
required for the design of more performant nanocatalysts in
order to develop more efficient and eco-compatible chemical
production.56 If a certain progress has been done in the past
decade, this topic remains very challenging. Model nano-
catalysts are needed in order to better understand the link
between the characteristics of MNPs and their catalytic
performance and thus bridge the gap between model surfaces
and real catalysts. Each progress that contributes to reduce the
gap of knowledge between nanocatalysts and homogeneous
catalysts constitutes a step forward the development of more
efficient and selective catalytic systems. Intensive efforts in this
direction are needed in order to one day be able to anticipate
the design of suitable catalysts for a given reaction.
Various metals are investigated in nanocatalysis toward these
principles, with a huge number of studies dedicated to gold
which is highly reputed for CO oxidation and emerges now in
hydrogenation catalysis,57,58 or palladium which intervenes in
various C−C coupling reactions and also in hydrogenation
catalysis.59,60 Other metals like rhodium, platinum, iridium,
nickel, cobalt, and iron, among others, are also the object of
numerous studies. Compared to all these metals, the number
of works focusing on the use of Ru metal NPs in nanocatalysis
may appear to be lower. This may be quite surprising given the
large and successful application of this metal in homogeneous
catalysis but can be explained by the fact it is an expensive
metal. However, as it will be seen hereafter, ruthenium proved
to be an interesting metal to carry out precise studies in order
to establish structure−properties relationships in diverse
catalytic reactions, mainly hydrogenation, hydrodeoxygenation,
Fischer−Tropsch, C−H activation, amine borane dehydrogen-
ation, water splitting, and carbon dioxide reduction.
3. SYNTHESIS METHODS OF RUTHENIUM
NANOPARTICLES
Being part of heterogeneous catalysts, metal NPs have been
known for a long time, but a renewed interest emerged in the
last three decades for the design of better defined systems,
studies in which Ru NPs stand at a good place.33 This arises
from fundamental hurdles met in scientific research with badly
defined NPs such as the common issue of size dispersity (e.g.,
5% in even highly monodispersed samples), the unascertained
surfaces of NPs, the unknown core−ligand interfaces, the
defects and elusive edge structures in 2D materials, and the still
missing information on alloy patterns in bi- and multimetallic
NPs. Such imprecisions preclude deep understanding of many
fundamental aspects of NPs, including the atomic-level
mechanism of surface catalysis.22 Developing synthesis
strategies that allow preparing, in a reproducible manner,
well-defined MNPs in terms of size, crystalline structure,
composition (metal cores and stabilizing agents), chemical
order, shape, and dispersion is a prerequisite in order to
investigate finely their catalytic properties and determine the
links between structural features and catalytic properties. For
this purpose, bottom-up liquid-phase techniques are very
attractive because they are versatile and easy to use,
necessitating straightforward equipment than physic routes.
Recent developments in nanochemistry offer efficient tools to
reach these objectives and make nanocatalysis to be a
recognized domain at the frontier between homogeneous
and heterogeneous catalyzes, thanks to better-controlled NPs
that allow progressively to take benefit of advantages of both
types of catalysts.33 Metal NPs stabilized by ligands allow
performing fine surface studies as done with homogeneous
catalysts. Indeed such NPs display a metal surface with an
interface close to that of molecular complexes (isolated surface
atoms can be seen like metal centers with their coordination
sphere) while benefiting from the influence of neighboring
metal atoms. It is also worth to mention that recent
developments of theoretical tools allow to bring computational
chemistry applied to small NPs to the same level of accuracy
and relevance as in molecular chemistry.61 All together
nanochemistry and computational chemistry enable to have
precise mapping of the surface properties of MNPs.
At the nanoscale level, ruthenium showed to be of interest in
diverse catalytic reactions and different synthesis tools have
been developed to access well-defined Ru NPs. The synthesis
of ruthenium NPs62 is often performed by chemical reduction
of ruthenium(III) chloride hydrate because of its availability,
using various reagents such as amines, carbon monoxide,
hydride salts (NaBH4, LiAlH4), hydrazine, alcohols, citrate
salts, or hydrogen. The drawback of these methods is the
presence of surface contaminants resulting from the reaction
conditions, such as water, salts, organic residues, or even an
oxide shell, which can alter the NP properties and limit access
to their surface. An elegant approach to circumvent these
difficulties is the use of organometallic (or metal−organic)
complexes as metal sources which are generally decomposed
under hydrogen atmosphere in mild conditions (low temper-
ature and pressure) in organic solution.63 The main
disadvantages of this approach is the access to the metal
precursors and the need to handle them in inert conditions and
in degassed organic solvents in order to preserve their initial
properties. The gain is the high quality of the obtained NPs
which display well-controlled characteristics and allow precise
surface studies. In between, the polyol method allows the
access to MNPs starting from metal complexes, similarly to the
organometallic approach, but usually using harsher synthesis
conditions.14 Whatever the preparation method followed, the
particles are generally stabilized by a polymer, an ionic liquid, a
surfactant, or a ligand added to the reaction mixture for
preventing undesired metal agglomeration and precipitation. A
large interest is presently devoted to ligand-protected particles
due to the intrinsic physicochemical properties of these ligan
ds
which can contribute to tune those of the particles.41 Before
describing the catalytic applications of Ru NPs, we will
summarize in the next subparts the main strategies developed
in order to access Ru NPs in colloidal solutions, namely the
reduction of ruthenium trichloride, polyol method, and the use
of an organometallic precursor. It is important to note that
apart from these very often used methods, others are reported
in the literature, such as the usage of ultrasounds or
microwaves, microemulsion systems, coprecipitation techni-
ques, sol−gel method, and hydrothermal/solvothermal pro-
cessing. These synthesis approaches will not be here described
because they are not applied for the preparation of the Ru
nanocatalysts cited in the following parts of this review.
3.1. Reduction of Ruthenium(III) Chloride Hydrate
The reduction of ruthenium(III) chloride hydrate in water is
the most used method to prepare Ru NPs because of its low
cost, ease of implementation, and scalability. This method
(Figure 1) consists in treating an aqueous solution of
commercial RuCl3·xH2O (with x = 3 depending on purity;
hereafter referred as RuCl3) by a reducing agent in the
presence of a stabilizer, at ambient conditions (room
temperature; rt) and without taking specific cautions.64 Diverse
reductants can be used among which alcohols (EtOH),65
hydrides (NaBH4, KBH4, or other amine boranes, LiAlH4),
66
as well as hydrogen at low pressure (1−3 bar)67 are very
common. Concerning the stabilizers whose role is to avoid the
agglomeration of Ru NPs and to control their growth (size,
shape), they need to be water soluble. It can be an organic
polymer like polyvinylpyrrolidone (PVP), a sugar derivative
like cyclodextrins or chitosans, a surfactant like quaternary
ammoniums, an ionic liquid (like imidazolium salts) or organic
ligands (sulfonated phosphines, phosphonates, etc.), among
others. By this way, stable aqueous colloidal suspensions of Ru
NPs are fastly obtained that can be directly used for in catalysis
in neat water or biphasic media without any purification.
Figure 1. Synthesis of Ru nanocatalysts by reduction of ruthenium(III) chloride. Adapted with permission from ref 64. Copyright 2016 Wiley.
However, if no purification, one drawback can be the presence
of byproducts resulting from the reactants which can act as
pollutants at the metal surface. Another inconvenient can be
the (partial) oxidation of the metal surface, which is often
circumvented by treating the colloidal suspension under
hydrogen pressure (low pressure: 1−20 bar) before catalysis.
Nevertheless, the so-obtained Ru NPs can be isolated and
purified, in particular to have a characterization reference
before involving them in catalytic reactions for comparison
purposes.
3.2. Polyol Method
In a recent review, Fiev́et, Piquemal, and co-workers recently
described into detail the polyol process and its interests
(Figure 2) to prepare MNPs with tailored sizes, shapes,
compositions, and architectures.14
It is also a low cost and facile process, where a polyol
(including 1,2-diols and ether glycols) is used as the liquid
organic compound, acting as both as a solvent of the metal
precursor and reducing agent as well as sometimes as colloidal
stabilizer. The high boiling point of the polyols allows working
at high temperature that assures the formation of well-
crystallized NPs and enlarges the possibilities of syntheses. The
polyol coordination ability to metal precursors and to NP
surface via −OH groups both facilitates the dissolution of the
metal sources and minimizes the NP coalescence. The high
viscosity of polyols favors a diffusion-controlled regime for the
NP growth resulting in controlled structures and morpholo-
gies. Despite the intrinsic properties of polyols, reducing agents
(like acetates or hydrogen), and stabilizers (like polymers or
surfactants) are often added to improve the characteristics of
the NPs. Concerning Ru NPs, only a few papers describe their
formation by the polyol process, mostly from RuCl3.
14 But
ruthenium complexes like [Ru(acac)3] have been also
described. In the presence of a protecting agent (PVP,68,69
thiol,70 or NaOH71) the formation of isotropic NPs in a size
range 1−6 nm has been reported. An example of anisotropic
Ru NPs69 and others of fcc Ru NPs (active in CO oxidation,72
reduction of nitrophenol and dehydrogenation of amino-
boranes,73 nitrogen reduction for ammonia synthesis,74 or
oxygen evolution reaction11) prepared in a polyol have also
been reported.
3.3. Use of an Organometallic Precursor
First inspired by Bradley and co-workers,75−78 and then mainly
developed by Chaudret and collaborators,79 the use of an
organometallic complex is nowadays a well-established method
to access model nanocatalysts. It allows getting well-defined
soluble MNPs and exploring their surface properties. The key
point of this strategy is the use of an organometallic complex
(and in some extent metal−organic complex) as the source of
metal atoms together with adequate stabilizers. It allows
building diverse nano-objects with modulable sizes including
ultrasmall size (ca. 1−10 nm) and a metallic surface free of
contaminants, which can be tuned at will. An advantageous
benefit from organometallic or metal−organic complexes is
their easy decomposition in mild conditions (1−3 bar H2, rt, or
T ≤ 423 K) through reduction or ligand displacement from the
metal coordination sphere in an organic solvent and in the
presence of a stabilizer.63 When accessible, olefinic complexes
are preferred as they provide clean metal surfaces as treatment
by H2 releases alkanes that are inert toward the NP surface and
easily eliminated. Using this method, monodisperse assemblies
of NPs with an efficient control of size, shape and surface state
can be synthesized and then isolated and purified for a fine
determination of their characteristics before application in
catalysis. [Ru(COD)(2-methylallyl)2] and [Ru(COD)(COT)]
(where 1,5-cyclooctadiene (COD) and 1,3,5-cyclooctatriene
(COT)) are particularly relevant precursors to access well-
defined Ru NPs (Figure 3). [Ru(acac)3] and [Ru3(CO)12] can
Figure 2. General view of the advantages of the polyol process. Reproduced with permission from ref 14. Copyright 2016 Royal Society of
Chemistry.
also be used but their decomposition requires higher
temperatures and in the case of the latter, CO can remain at
the metal surface. However, [Ru3(CO)12] complex allowed to
access shape-controlled Ru NPs which is uncommon.80
The choice of the stabilizer is also fundamental as it governs
the growth, stability, solubility properties, and catalytic
performance of the NPs. Besides organic polymers, like PVP,
that provide steric stabilization and weak interaction with the
metal surface, a plethora of organic ligands coordinating via N,
S, Si, P, or C atoms to the metal surface have been used leading
to fine-tuned surface properties.82 Ionic liquids can also be
used.83,84 The employment of water-soluble stabilizers, namely
polymers like PVP,85 ligands like 1,3,5-triaza-7-phosphaada-
mantane (PTA),86 or sulfonated phosphines87 and also
cyclodextrins88 allowed production of aqueous suspensions of
Ru NPs that are stable and active in C−H activation89 or
hydrogenation catalysis,90 thus offering other opportunities in
catalysis.
If a major inconvenient of this synthesis process is the access
to the metal precursors which are costly, in some cases difficult
to prepare, and most often need to be handled under inert
atmosphere, the quality of the obtained MNPs is a real plus for
fundamental studies. Indeed, a good control over the particle
formation process is achieved, due to the mild reaction
conditions. Moreover, except the stabilizer voluntary added or
traces of solvent, no contaminant, such as halides or other ions,
is introduced. This makes this method powerful to have
suitable NP models for performing fundamental studies on
surface properties and also for following catalytic reactions, and
numerous studies have been done with ruthenium (Figure
4).81,91
The use of H2 as reducing agent to synthesize MNPs leads
to hydrogen atoms at the metal surface, a clear advantage for
reduction catalysis (vide infra). Computational chemistry
performed onto ethanoic acid-stabilized Ru NPs indicated
that ruthenium atoms present a positive charge density and
hydrogen atoms a negative one, thus showing that hydrogen
atoms are likely hydrides.92 The presence of hydrides has been
experimentally supported by 1H MAS NMR on PTA-stabilized
Ru NPs which presented a signal at −14 ppm, a typical value
for hydrides on ruthenium complexes.90 The surface hydrides
content has been shown to vary depending on the surface state
of Ru NPs but is generally high (>1/surface Ru atom) even
after Np transfer into water.81 Although this value can be also
modulated with the species present on the surface; in Ru NPs
stabilized by carboxylates, the number of hydrides per surface
Figure 3. Synthesis of Ru NPs from an organometallic complex. Adapted with permission from ref 81. Copyright 2014 Springer.
Figure 4. Schematic representation of some surface studies performed with Ru NPs prepared from an organometallic complex. Reproduced with
permission from ref 91. Copyright 2018 American Chemical Society.
Chemical Reviews Review
DOI: 10.1021/acs.chemrev.9b00434
Chem. Rev. 2020, 120, 1085−1145
1091
Ru atom was found to be significantly lower (ca. 0.4 H/surface
Ru atom) by both experimental and theoretical techniques, as
the result of the coordination mode of the stabilizer.92 The
surface hydrides can be displaced by coordination of CO at the
surface. NMR methods, in particular solid-state 2H NMR,
evidenced H−D exchange between Ru NPs surface and ligand
sites: incorporation of 2H atoms in the alkylchains of HDA
used as capping ligand was observed, as the result of a C−H
activation phenomenon.93,94 This was further exploited in
order to perform the deuteration of different substrates (vide
infra). Using 13CO as a probe molecule and IR (Infrared) and
MAS NMR (magic angle spinning nuclear magnetic
resonance) techniques provided indirect information on
location and mobility of ligands at metal surface and helped
to understand the surface properties and catalytic reactivity of
NPs.95 For instance, it has been demonstrated that the strong
coordination of phosphine ligands at a Ru NP surface blocks
CO mobility contrarily to the few, weak bonds involved when a
polymer is used as stabilizer. Similar strategies allowed
localizing carbene96 or betaine adduct of NHC−carbene and
carbodiimide95 ligands at Ru NP surface. CO oxidation was
used to compare the reactivity of phosphine- and PVP-
stabilized Ru NPs by Fourier transform infrared spectroscopy
(FTIR), nuclear magnetic resonance (NMR), and wide-angle
X-ray scattering (WAXS): CO oxidation proceeds at rt in each
case, but a rapid deactivation occurred for PVP-stabilized NPs
due to the formation of RuO2, while phosphine effectively
protects the NPs against bulk oxidation. Reduction of 13CO2
by H2 was studied on PVP- and phosphine-stabilized Ru NPs
by solid-state MAS NMR spectroscopy. Formation of 13CO
was observed in mild conditions (3 bar H2, 393 K),
97 which
was reduced upon heating into CH4 or hydrocarbons in a
Fischer−Tropsch process as observed also when studying
reduction of CO at the surface of the same Ru NPs.98
3.4. Supported Nanoparticles
As the main purpose of this review is to discuss on the
application of Ru NPs into colloidal (or suspension)
conditions, the synthesis of supported Ru NPs is here only
briefly discussed. From the synthesis methods described above,
it is quite easy to access supported Ru NPs using different
types of supports (most often oxide-type and carbon-based
supports). The most simple strategy is certainly the
immobilization of preformed Ru NPs following an impregna-
tion method, meaning mix a chosen support (eventually
previously treated by treatment in temperature or vacuum)
with a colloidal suspension of Ru NPs. If any, the porosity of
the support will enable the NPs to diffuse inside the pores of
the matrix and thus to be dispersed. An important point in this
approach is the size of the pores which needs to be compatible
with the NP size in order to get a high dispersion level. A
favorable advantage deals with the presence of anchoring
groups at the surface or in the pores of the support. The
anchoring groups are generally chosen in order to provide
interaction with the metal NPs and thus retain them more
firmly than with simple physical adsorption. This interaction
can be electrostatic, π−π stacking, or even covalent in nature.
For example, immobilization of Ru NPs, previously prepared
from an organometallic precursor, into alumina, silica, or
carbon materials99 was carried out by this way in order to
improve stability and recovery of the nanomaterials and also
take advantage of the support properties during catalysis.
Aqueous suspensions of Ru NPs prepared by reduction of
RuCl3
64 as well as by polyol suspensions14 can also be used to
disperse NPs onto a support. The main advantage of this route
is that the control of the NP growth is previously performed in
solution and is generally kept after their deposition on the
support. This makes possible to carry out comparison catalytic
studies from NPs displaying similar characteristics in terms of
size, shape, and stabilizer nature, either being in suspension or
supported conditions, but it is a two-step synthesis process.
Another strategy consists in the direct synthesis of NPs in the
presence of the chosen support, keeping all the reaction
conditions equal otherwise. Functionalized supports bearing
chemical groups similar to those present in the stabilizers can
improve the grafting of the NPs and their stability. Ionic liquids
can be used also as stabilizing layer in the presence of an extra
ligand or not.83 If this strategy is a one-step process, the
structural characteristics of the growing NPs can be strongly
influenced by the support properties which make comparison
studies more complicated or even impossible.
4. RUTHENIUM NANOPARTICLES AS CATALYSTS
In the next sections, the use of Ru colloidal NPs as catalysts is
described. Reduction reactions are mainly focused on arene
hydrogenations, which have been extensively studied using Ru
NPs as catalysts. Other reduction reactions like of nitro-
benzene and azo compounds with NaBH4 are reported as well.
Ru-based catalysts are outstanding for this kind of reductions,
but the intensive work in these reactions is also due to the fact
that the properties of Ru NPs can be easily evaluated, namely
electronic and steric effects of the surface ligands, the
crystalline structure, or the addition of a second metal,
among others. Similarly, CO oxidation with O2 can be used as
a model reaction to evaluate such parameters. Hydro-
deoxygenation, a valuable procedure to upgrade biomass, is
also studied with Ru NPs catalysts. Remarkably, bimetallic
systems such as RuNi and RuFe NPs gave interesting results
which pave the way to new applications of hydrodeoxygena-
tion. An objective beyond is its application directly to biomass
compounds and not only limited to oxygen containing model
compounds. More recently, C−H activation has been
described with Ru NPs, allowing to selectively deuterate
organic compounds in mild conditions. Colloidal Ru NPs have
found less application in other types of catalytic reactions such
as oxidations or Fischer−Tropsch and a few others, which are
also described thereafter. Contrarily to Ru complexes, Ru NPs
are not commonly reported for the transformation of CO2, but
recent papers provide promising results. In the opposite, Ru
NPs are largely studied in the dehydrogenation of amine
boranes. If often in supported conditions, but Ru NPs in
colloidal suspensions are also highly performant and ruthenium
is among the best catalysts for this reaction. Ru-based NPs are
presently the object of a renewed interest in water-splitting
catalysis, with some catalysts showing a performance
approaching that of Pt in the hydrogen evolution reaction.
4.1. Reduction Reactions
Rh, Ir, and Ru compounds are very well-known as effective
homogeneous catalysts.100 Similarly, the emerging single atom
catalysts for reduction reactions are based in these metals.101 It
is not surprising that Ru NPs have found applications as
catalysts for a large panel of reduction reactions, mainly CC
and CO bonds, in a broad range of reaction conditions. Ru
NPs used as catalysts in reduction reactions are synthesized by
one of the methodologies described above using a large variety
re
du
ct
io
n
of
[R
u(
C
O
D
)(
C
O
T
)]
w
ith
H
2
T
EM
,
W
A
X
S,
SS
N
M
R
,I
R
hy
dr
og
en
at
io
n
of
ar
en
es
an
d
al
ke
ne
s
by
H
2
se
le
ct
iv
ity
m
od
ul
at
ed
w
ith
su
rf
ac
e
lig
an
d
13
5
[R
u(
C
O
D
)(
C
O
T
)]
(1
00
m
g)
,N
H
C
(0
.1
−
0.
3
eq
ui
v)
,H
2
(3
ba
r)
,T
H
F
(5
0
m
L)
,2
98
K
,2
0
h
ca
ta
ly
st
(1
m
g)
,s
ub
st
ra
te
(0
.2
m
m
ol
),
so
lv
en
t
(1
m
L)
,H
2
(3
.5
−
5
ba
r)
,2
98
−
30
3
K
ch
ir
al
N
H
C
re
du
ct
io
n
of
[R
u(
C
O
D
)(
C
O
T
)]
w
ith
H
2
T
EM
,I
C
P,
EA
,I
R
,
SS
N
M
R
hy
dr
og
en
at
io
n
of
ar
en
es
an
d
al
ke
ne
s
by
H
2
ne
gl
ig
ib
le
en
an
tio
m
er
ic
ex
ce
ss
ob
se
rv
ed
;
no
re
cy
cl
in
g
te
st
or
ch
ar
ac
te
ri
za
tio
n
of
th
e
sp
en
t
ca
ta
ly
st
s
13
4
[R
u(
C
O
D
)(
C
O
T
)]
(3
95
.6
m
g)
,N
H
C
(0
.2
−
0.
5
eq
ui
v)
,H
2
(3
ba
r)
,p
en
ta
ne
(1
50
m
L)
,2
98
K
,2
0
h
ca
ta
ly
st
(2
m
g)
,s
ub
st
ra
te
(0
.1
5
m
m
ol
),
so
lv
en
t
(2
m
L)
,H
2
(5
−
60
ba
r)
,2
98
−
35
3
K
,1
5
h
ch
ir
al
N
H
C
re
du
ct
io
n
of
[R
u(
C
O
D
)(
C
O
T
)]
w
ith
H
2
T
EM
,W
A
X
S,
IC
P,
A
E,
IR
,S
N
M
R
hy
dr
og
en
at
io
n
of
se
ve
ra
ls
ub
st
ra
te
s
by
H
2
ne
gl
ig
ib
le
en
an
tio
m
er
ic
ex
ce
ss
ob
se
rv
ed
;
no
re
cy
cl
in
g
te
st
;
T
EM
of
th
e
sp
en
t
ca
ta
ly
st
s
13
3
[R
u(
C
O
D
)(
C
O
T
)]
(3
95
.1
6
m
g,
1.
26
m
m
ol
),
N
H
C
(0
.5
eq
ui
v)
,H
2
(3
ba
r)
,p
en
ta
ne
(1
50
m
L)
,2
98
K
,
20
h;
[R
u(
C
O
D
)(
C
O
T
)]
(1
20
m
g)
,N
H
C
(0
.2
eq
ui
v)
,H
2
(3
ba
r)
,p
en
ta
ne
(4
5
m
L)
,2
98
K
,2
0
h
ca
ta
ly
st
(1
m
g)
,s
ub
st
ra
te
(0
.1
m
m
ol
),
so
lv
en
t
(1
m
L)
,H
2
(1
0−
25
ba
r)
,2
98
−
31
3
K
PP
h 3
re
du
ct
io
n
of
[R
u(
C
O
D
)(
C
O
T
)]
w
ith
H
2
T
EM
,
X
R
D
,X
PS
,E
A
,
W
A
X
S,
T
G
A
hy
dr
og
en
at
io
n
of
se
ve
ra
lp
ol
yc
yc
lic
ar
om
at
ic
hy
dr
oc
ar
bo
ns
by
H
2
go
od
ac
tiv
iti
es
an
d
se
le
ct
iv
iti
es
un
de
r
m
ild
re
ac
tio
n
co
nd
iti
on
s;
ra
te
an
d
se
le
ct
iv
ity
de
pe
nd
on
nu
m
be
r
of
cy
cl
es
on
th
e
su
bs
tr
at
e;
se
le
ct
iv
ity
de
pe
nd
on
th
e
nu
m
be
r
an
d
na
tu
re
of
su
bs
tr
at
e
su
bs
tit
ue
nt
s;
no
re
cy
cl
in
g
te
st
;
no
ch
ar
ac
te
ri
za
tio
n
of
th
e
sp
en
t
ca
ta
ly
st
11
2
[R
u(
C
O
D
)(
C
O
T
)]
(9
40
0
m
g)
,P
Ph
3
(0
.4
eq
ui
v)
H
2
(3
ba
r)
,T
H
F
(4
00
m
L)
,2
98
K
ca
ta
ly
st
(3
m
g)
,s
ub
st
ra
te
(0
.6
2
m
m
ol
),
so
lv
en
t
(1
0
m
L)
,H
2
(3
−
20
ba
r)
,3
03
−
35
3
K
ph
os
ph
in
es
re
du
ct
io
n
of
[R
u(
C
O
D
)(
C
O
T
)]
w
ith
H
2
T
EM
,X
R
D
,X
PS
,E
A
,
W
A
X
S,
T
G
A
hy
dr
og
en
at
io
n
of
ar
om
at
ic
ke
to
ne
s
by
H
2
re
du
ct
io
n
of
th
e
ar
en
e
fa
vo
re
d
ag
ai
ns
t
ke
to
ne
gr
ou
p;
se
le
ct
iv
ity
m
od
ul
at
ed
by
th
e
su
rf
ac
e
lig
an
d;
ca
ta
ly
tic
re
ac
tio
n
pr
of
ile
11
6
[R
u(
C
O
D
)(
C
O
T
)]
(4
00
m
g)
,P
Ph
3
or
dp
pb
(0
.4
eq
ui
v)
H
2
(3
ba
r)
,T
H
F
(4
00
m
L)
,2
98
K
ca
ta
ly
st
(2
m
ol
%
),
su
bs
tr
at
e
(1
.2
4
m
m
ol
),
so
lv
en
t
(1
0
m
L)
,H
2
(3
−
20
ba
r)
,3
03
K
ch
iti
n
re
du
ct
io
n
of
R
uC
l 3
w
ith
N
aB
H
4
T
EM
,X
R
D
,I
C
P
hy
dr
og
en
at
io
n
of
be
nz
yl
gl
yc
id
yl
et
he
r
an
d
ot
he
r
ar
en
es
by
H
2
no
hy
dr
og
en
ol
ys
is
si
de
pr
od
uc
ts
;
no
R
u
le
ac
hi
ng
as
as
ce
rt
ai
ne
d
by
IC
P;
T
EM
af
te
r
ca
ta
ly
si
s
sh
ow
a
sl
ig
ht
ly
in
cr
ea
se
of
N
P
si
ze
14
1
R
uC
l 3
(7
1.
6
m
g)
,c
hi
tin
(2
.9
7
g)
,N
aB
H
4
(3
0.
6
m
g)
,
H
2O
(9
m
L)
,3
03
K
,3
.5
h
ca
ta
ly
st
(0
.8
m
ol
%
R
u)
,s
ub
st
ra
te
(1
m
m
ol
),
H
2O
(5
m
L)
,H
2
(2
0
ba
r)
,3
23
K
,1
.5
h
Chemical Reviews Review
DOI: 10.1021/acs.chemrev.9b00434
Chem. Rev. 2020, 120, 1085−1145
1093
T
ab
le
1.
co
nt
in
ue
d
st
ab
ili
zi
ng
ag
en
t
sy
nt
he
tic
m
et
ho
do
lo
gy
ch
ar
ac
te
ri
za
tio
n
ca
ta
ly
tic
re
ac
tio
n
co
nd
iti
on
s
co
m
m
en
ts
re
f
fu
lle
re
ne
C
60
re
du
ct
io
n
of
[R
u(
C
O
D
)(
C
O
T
)]
w
ith
H
2
T
EM
,I
R
,N
M
R
,
W
A
X
S,
R
am
an
,
EX
A
FS
,X
PS
hy
dr
og
en
at
io
n
of
tr
an
s-
ci
nn
am
al
de
hy
de
by
H
2
se
le
ct
iv
ity
to
tr
an
s-
ci
nn
am
yl
al
co
ho
l
of
77
%
T
O
F
=
12
8
h−
1
14
3,
15
6
[R
u(
C
O
D
)(
C
O
T
)]
,C
60
(0
.0
3−
1
eq
ui
v)
,H
2
(3
ba
r)
,
C
H
2C
l 2,
29
8
K
ca
ta
ly
st
(5
m
g)
,t
ra
ns
-c
in
na
m
al
de
hy
de
(4
m
m
ol
),
IP
rO
H
(3
0
m
L)
,p
yr
id
in
e
(4
.5
eq
ui
v)
,H
2
(2
0
ba
r)
,3
43
K
,2
0
h,
10
00
rp
m
po
ly
si
lo
xa
ne
m
at
ri
x
re
du
ct
io
n
of
[R
u(
C
O
D
)(
2-
m
et
hy
la
lly
l)
2]
w
ith
H
2
in
a
fu
se
d-
si
lic
a
co
at
ed
co
lu
m
n
T
EM
hy
dr
og
en
at
io
n
of
va
ri
ou
s
ca
rb
on
yl
co
m
po
un
ds
on
–
co
lu
m
n
re
ac
tio
n
by
H
2
ch
ro
m
at
og
ra
ph
y
re
cy
cl
in
g
te
st
s
15
7
[R
u(
C
O
D
)(
2-
m
et
hy
la
lly
l)
2]
(0
.1
m
g)
,H
2
(0
.1
ba
r)
,
31
3−
46
3
K
(0
.5
K
/m
in
),
10
h
ca
ta
ly
st
(0
.3
m
ol
%
),
su
bs
tr
at
e,
H
2
(0
.5
ba
r)
,3
63
K
,r
et
en
tio
n
tim
e
(5
.2
s)
ph
os
ph
in
e-
fu
nc
tio
na
liz
ed
IL
re
du
ct
io
n
of
R
uO
2
or
[R
u(
C
O
D
)(
2-
m
et
hy
la
lly
l)
2]
w
ith
H
2
T
EM
,
X
R
D
,X
PS
,
N
M
R
,I
R
hy
dr
og
en
at
io
n
of
va
ri
ou
s
su
bs
tr
at
es
by
H
2
se
le
ct
iv
ity
tu
ne
d
w
ith
re
ac
tio
n
co
nd
iti
on
s;
po
is
on
te
st
w
ith
H
g;
re
cy
cl
in
g
te
st
an
d
le
ac
hi
ng
of
9
pp
m
of
R
u
in
th
e
hy
dr
og
en
at
io
n
of
st
yr
en
e
12
2
R
uO
2
or
[R
u(
C
O
D
)(
2-
m
et
hy
la
lly
l)
2]
(0
.0
18
m
m
ol
),
ph
os
ph
in
e-
fu
nc
tio
na
liz
ed
io
ni
c
liq
ui
ds
(0
.0
18
m
m
ol
),
[B
M
IM
]B
F 4
(
1
m
L)
,H
2
(1
0
ba
r)
,3
48
K
,
4
h
ca
ta
ly
st
(s
ub
st
ra
te
/R
u
=
10
0)
,s
ub
st
ra
te
(1
m
L
so
lu
tio
n
at
1.
8
M
),
H
2
(5
0
ba
r)
,3
03
K
,1
5
h
cy
cl
od
ex
tr
in
po
ly
m
er
re
du
ct
io
n
of
R
uC
l 3
w
ith
N
aB
H
4
T
EM
,I
R
,X
R
D
,T
G
A
,
U
V
−
vi
s,
N
M
R
hy
dr
og
en
at
io
n
of
ce
llu
lo
se
-d
er
iv
ed
pl
at
fo
rm
m
ol
–
ec
ul
es
by
H
2
re
cy
cl
ed
5
co
ns
ec
ut
iv
e
ru
ns
;
T
EM
af
te
r
ca
ta
ly
si
s
sh
ow
ed
a
sl
ig
ht
R
u
N
P
ag
gr
eg
at
io
n
an
d
a
sl
ig
ht
in
cr
ea
se
of
N
P
si
ze
14
4
R
uC
l 3
(3
.6
×
10
−
3
m
m
ol
),
cy
cl
od
ex
tr
in
po
ly
m
er
(0
.5
g)
,N
aB
H
4
(0
.5
m
L,
0.
1
M
),
H
2O
(1
.5
m
L)
,2
73
K
ca
ta
ly
st
(3
.6
×
10
−
3
m
m
ol
),
su
bs
tr
at
e
(5
m
m
ol
),
H
2O
(1
m
L)
,H
2
(4
0
ba
r)
,3
53
−
40
3
K
,2
−
4
h
N
H
C
re
du
ct
io
n
in
si
tu
of
R
u−
N
H
C
co
m
pl
ex
du
ri
ng
hy
dr
og
en
at
io
n
re
ac
tio
n
us
in
g
H
2
T
EM
hy
dr
og
en
at
io
n
of
le
vu
lin
ic
ac
id
by
H
2
R
u
N
P
fo
rm
ed
du
ri
ng
R
u
ho
m
og
en
eo
us
ca
ta
ly
ze
d
hy
dr
o-
ge
na
tio
n
re
ac
tio
n
13
6
ca
ta
ly
st
(0
.1
m
ol
%
),
su
bs
tr
at
e
(4
.3
1
m
m
ol
),
H
2O
(1
0
m
L)
,H
2
(1
2
ba
r)
,4
33
K
,1
60
m
in
ch
ir
al
N
-d
on
or
lig
an
ds
re
du
ct
io
n
of
[R
u(
C
O
D
)(
C
O
T
)]
w
ith
H
2
T
EM
hy
dr
og
en
at
io
n
of
ac
et
op
he
no
ne
de
ri
va
tiv
es
by
H
2
lo
w
en
an
tio
m
er
ic
ex
ce
ss
es
;
no
re
cy
cl
in
g
te
st
;
no
ch
ar
ac
te
r-
iz
at
io
n
of
th
e
sp
en
t
ca
ta
ly
st
15
8
[R
u(
C
O
D
)(
C
O
T
)]
(3
0
m
g,
0.
1
m
m
ol
),
ch
ir
al
lig
an
d
(0
.0
2
m
m
ol
),
H
2
(3
ba
r)
,T
H
F
(8
0
m
L)
,2
98
K
ca
ta
ly
st
(0
.0
1
m
m
ol
),
su
bs
tr
at
e
(1
m
m
ol
),
he
pt
an
e
(2
5
m
L)
,H
2
(4
0
ba
r)
,3
23
K
,1
6
h
IL
th
er
m
al
de
co
m
po
si
tio
n
of
[R
u(
C
O
D
)(
2-
m
et
hy
la
lly
l)
2]
T
EM
hy
dr
og
en
at
io
n
of
th
e
al
de
hy
de
in
te
rm
ed
ia
te
or
ig
in
at
ed
fr
om
th
e
ac
id
-c
at
al
yz
ed
cl
ea
va
ge
of
lig
ni
n
β-
O
-4
m
od
el
by
H
2
R
u
N
Ps
on
IL
ar
e
ac
tiv
e
in
ar
en
e
an
d
ke
to
ne
hy
dr
og
en
at
io
n;
bi
fu
nc
tio
na
lr
ea
ct
io
n
m
ed
ia
co
nt
ai
ni
ng
bo
th
a
B
rø
ns
te
d
ac
id
ca
ta
ly
st
an
d
R
u
N
Ps
le
ad
to
go
od
yi
el
ds
of
2-
ph
en
yl
al
co
ho
l
12
0
[R
u(
C
O
D
)(
2-
m
et
hy
la
lly
l)
2]
(1
0−
20
m
g)
,I
L
(1
.4
−
2.
8
g)
,3
48
K
,1
8
h
ca
ta
ly
st
(5
−
30
m
ol
%
),
su
bs
tr
at
e
(0
.0
5
m
m
ol
,0
.1
M
),
H
2
(1
0
ba
r)
,4
53
K
,5
−
20
m
in
or
37
3
K
,
1−
3
h
R
uP
t/
PP
P
st
ep
w
is
e
re
du
ct
io
n
of
[R
u(
C
O
D
)(
C
O
T
)]
an
d
[P
t
(C
H
3)
2(
C
O
D
)]
)
or
[P
t 2
(d
ba
) 3
]
w
ith
H
2
T
EM
,W
A
X
S,
IR
hy
dr
og
en
at
io
n
of
tr
an
s-
ci
nn
am
al
de
hy
de
by
H
2
co
re
−
sh
el
l
st
ru
ct
ur
e,
se
le
ct
iv
ity
tu
ne
d
by
st
ru
ct
ur
e
an
d
co
m
po
si
tio
n
of
th
e
ca
ta
ly
st
;
sy
ne
rg
is
tic
ef
fe
ct
s
ob
se
rv
ed
11
7
[R
u(
C
O
D
)(
C
O
T
)]
(5
7−
14
2
m
g)
,[
Pt
(C
H
3)
2(
C
O
D
)]
)
(1
50
−
24
0
m
g)
,P
PP
(0
.2
4
m
m
ol
),
T
H
F,
H
2
(3
ba
r)
,3
43
K
,1
8
h;
[P
t 2
(d
ba
) 3
]
(9
8−
24
6
m
g)
,[
R
u(
C
O
D
)(
C
O
T
)]
(1
42
−
22
7
m
g)
,
PP
P
(0
.2
2
m
m
ol
),
T
H
F,
H
2
(3
ba
r)
,r
t,
18
h
ca
ta
ly
st
(2
.5
m
g)
,t
ra
ns
-c
in
na
m
al
de
hy
de
(7
.5
m
m
ol
),
no
na
ne
(3
.5
m
m
ol
),
2-
Pr
O
H
(5
0
m
L)
,
H
2
(2
0
ba
r)
,3
43
K
T
EM
af
te
r
ca
ta
ly
si
s
sh
ow
ed
th
at
sh
el
lr
ic
h
N
Ps
ag
gl
om
er
at
ed
an
d
co
al
es
ce
d
af
te
r
ca
ta
ly
si
s
w
hi
le
ri
ch
R
u
or
sh
el
l-R
u
N
Ps
w
er
e
st
ab
le
Chemical Reviews Review
DOI: 10.1021/acs.chemrev.9b00434
Chem. Rev. 2020, 120, 1085−1145
1094
T
ab
le
1.
co
nt
in
ue
d
st
ab
ili
zi
ng
ag
en
t
sy
nt
he
tic
m
et
ho
do
lo
gy
ch
ar
ac
te
ri
za
tio
n
ca
ta
ly
tic
re
ac
tio
n
co
nd
iti
on
s
co
m
m
en
ts
re
f
R
uF
e/
SI
LP
re
du
ct
io
n
of
[F
e[
N
(S
i(
C
H
3)
3)
2]
2]
2
an
d
[R
u(
C
O
D
)
(C
O
T
)]
w
ith
H
2
T
EM
,
X
A
FS
hy
dr
og
en
at
io
n
of
su
bs
tit
ut
ed
ar
om
at
ic
su
bs
tr
at
es
Fe
25
R
u 7
5/
SI
LP
hi
gh
ly
se
le
ct
iv
e
fo
r
ke
to
ne
hy
dr
og
en
at
io
n,
w
hi
le
R
u/
SI
LP
pr
oc
ee
d
to
th
e
fu
ll
hy
dr
og
en
at
io
n
of
th
e
fu
rf
ur
al
ac
et
on
e
m
ol
ec
ul
e;
ho
t
fil
tr
at
io
n
te
st
re
cy
cl
ed
tw
ic
e
w
ith
ou
t
lo
ss
of
ac
tiv
ity
13
1
Fe
[N
(S
i(
C
H
3)
3)
2]
2]
2
(1
5.
1−
75
.3
m
g)
,[
R
u(
C
O
D
)
(C
O
T
)]
(2
5.
2−
63
.1
m
g)
,S
IL
P
(5
00
m
g)
,
m
es
itl
ye
ne
(5
m
L)
,H
2
(3
ba
r)
,4
23
K
,1
8
h
ca
ta
ly
st
(0
.0
16
m
m
ol
of
m
et
al
),
fu
rf
ur
al
ac
et
on
e
(0
.4
m
m
ol
),
B
M
I·P
F 6
(1
m
L)
,m
es
ity
le
ne
(0
.5
m
L)
,H
2
(2
0
ba
r)
,3
73
K
,1
8
h
R
uF
e/
H
D
A
re
du
ct
io
n
of
[F
e[
N
(S
i(
C
H
3)
3)
2]
2]
2
an
d
[R
u(
C
O
D
)
(C
O
T
)]
w
ith
H
2
T
EM
,I
C
P,
W
A
X
S,
IR
,m
ag
ne
tic
m
e-
su
ar
em
en
ts
hy
dr
og
en
at
io
n
of
st
yr
en
e
an
d
2-
bu
ta
no
ne
se
le
ct
iv
ity
tu
ne
d
by
R
u/
Fe
ra
tio
;
no
re
cy
cl
in
g
te
st
;
no
ch
ar
ac
te
ri
za
tio
n
of
th
e
sp
en
t
ca
ta
ly
st
11
8
Fe
[N
(S
i(
C
H
3)
3)
2]
2]
2
(0
.5
m
m
ol
,1
88
.3
m
g)
,
[R
u(
C
O
D
)(
C
O
T
)]
(0
.5
m
m
ol
,1
57
.7
m
g)
,H
D
A
(1
.5
m
m
ol
,3
62
.2
m
g)
,m
es
itl
ye
ne
(1
0
m
L)
,H
2
(3
ba
r)
,4
23
K
,1
8
h
ca
ta
ly
st
(5
m
ol
%
),
su
bs
tr
at
e
(2
m
m
ol
),
B
M
I·P
F 6
(1
m
L)
,m
es
ity
le
ne
(0
.5
m
L)
,H
2
(3
ba
r)
,r
t,
24
h
R
uS
n/
ph
os
ph
in
e
re
du
ct
io
n
of
[R
u(
C
O
D
)(
C
O
T
)]
w
ith
H
2
fo
l
lo
w
ed
by
re
ac
tio
n
w
ith
tr
ib
ut
yl
tin
hy
dr
id
e
T
EM
,H
R
T
EM
,
W
A
X
S,
IR
,N
M
R
hy
dr
og
en
at
io
n
of
st
yr
en
e
by
H
2
sy
nt
he
si
s
of
tin
-d
ec
or
at
ed
na
no
pa
rt
ic
le
s;
re
ac
tiv
ity
tu
ne
d
by
Sn
su
rf
ac
e
sp
ec
ie
s
11
3
[R
u(
C
O
D
)(
C
O
T
)]
(1
57
m
g,
0.
50
m
m
ol
),
PV
P
(1
g)
or
dp
pb
(2
0.
8
m
g,
0.
04
9
m
m
ol
,0
.1
eq
ui
v)
,T
H
F
(6
0
m
L)
,H
2
(3
ba
r)
,r
t,
68
h;
tr
i-n
-b
ut
yl
tin
hy
dr
id
e
(1
3.
5
μL
,0
.0
5
m
m
ol
,0
.1
eq
ui
v)
,T
H
F
(1
0
m
L)
,r
t,
18
h
ca
ta
ly
st
(0
.0
3
m
m
ol
R
u)
,s
ty
re
ne
(1
m
L)
,T
H
F
(5
m
L)
,H
2
(3
ba
r)
,r
t
R
u-
Pd
C
u
yo
lk
−
sh
el
l
na
no
–
cr
ys
ta
ls
st
ep
w
is
e
re
du
ct
io
n
of
[P
d(
ac
ac
) 2
]/
C
uC
l 2·
2H
2O
an
d
R
uC
l 3
T
EM
,X
R
D
,I
C
P
hy
dr
og
en
at
io
n
of
st
yr
en
e,
di
ph
en
yl
ac
et
yl
en
e,
4-
ni
tr
oc
hl
or
ob
en
ze
ne
fc
c
ch
ar
ac
te
r
of
R
u
de
pe
nd
s
on
%
Pd
;
th
e
re
du
ct
io
n
of
ni
tr
o
gr
ou
p
w
as
m
or
e
pe
rf
or
m
an
t
w
he
n
us
in
g
fc
c
N
Ps
co
m
pa
re
d
to
hc
p
N
Ps
;
th
e
op
po
si
te
tr
en
d
w
as
ob
se
rv
ed
in
st
yr
en
e
hy
dr
og
en
at
io
n;
no
re
cy
cl
in
g
te
st
s;
no
ch
ar
ac
te
ri
za
tio
n
of
th
e
sp
en
t
ca
ta
ly
st
;
no
re
cy
cl
in
g
te
st
15
4
[P
d(
ac
ac
) 2
]
(7
.5
m
g)
,C
uC
l 2·
2H
2O
(0
−
40
m
g)
,
ol
ey
la
m
in
e
(3
m
L)
,1
-o
ct
ad
ec
yl
en
e
(3
m
L)
,E
tO
H
,
(1
m
L)
,3
93
K
,1
0
m
in
;R
uC
l 3
(1
5.
6
m
g)
,E
tO
H
(1
m
L)
,4
73
K
,1
2
h
ca
ta
ly
st
(0
.0
05
m
m
ol
),
st
yr
en
e
(0
.1
7
m
m
ol
)
or
di
ph
en
yl
ac
et
yl
en
e
(0
.0
56
m
m
ol
),
to
lu
en
e
(1
.5
m
L)
,H
2,
35
3
K
;
ca
ta
ly
st
(0
.0
05
m
m
ol
),
4-
ni
tr
oc
hl
or
ob
en
ze
ne
(6
m
g)
,t
ol
ue
ne
(0
.5
m
L)
,
D
M
F
(1
.5
m
L)
H
2
(b
al
lo
n)
,3
68
K
Chemical Reviews Review
DOI: 10.1021/acs.chemrev.9b00434
Chem. Rev. 2020, 120, 1085−1145
1095
of stabilizing agents, such as polymers,73,74,102−111 phos-
phines,112−116 N-donor ligands,50,117,118 ILs,83,84,119−131
NHC,96,132−139 alkynes,140 chitin,141 fullerene,50,142,143 cyclo-
dextrins,144,145 dendrimers,146−148 and others. The stabilizing
agents not only allow to synthesize and maintain the Ru NPs
stable but also modulate their surface chemistry in a way which
can be beneficial to obtain more efficient catalysts. The
modulation of the surface properties is of major interest for
catalysis, as the presence, or lack, of surface stabilizers can
improve both the activity and selectivity on a given reaction.
The noninnocent role of surface compounds in catalysis is
nowadays well accepted, and therefore, more efforts are
devoted to this topic.91 Because of the accessibility and the
surface sensitivity toward the metal surface of reduction
reactions, they have also been used as an indirect character-
ization method to understand the surface of Ru-based NPs.113
Also, the addition of a second metal has been successfully used
to improve the catalytic performances of Ru NPs cata-
lysts.26,41,149−152 In this case, not only the nature of the
second metal, but also the composition, the crystalline
structure, or the chemical order of the associated metals
(alloy, core−shell, among others), play an important role in the
results. More complex systems, based in the combination of
three153,154 or four155 different metals, have been also
described as catalysts for reduction reactions.
4.1.1. Reduction of CC and CO Bonds. Reduction
of CC and CO double bonds have been extensively
studied using Ru colloidal NPs as catalysts. Tables 1 and 3
summarize Ru catalyzed hydrogenation reactions of substrates
containing these double bonds. By far, styrene has been the
most studied substrate, but also a plethora of other arene-type
compounds, ketones, aldehydes, among others, is also
described.
Selective hydrogenation reactions can provide useful
information about the surface chemistry of the nanoparticles.
For example, in the case of the hydrogenation of styrene, as Ru
is very active in the hydrogenation of the arene moiety, the
obtention of the partially hydrogenated product (ethyl-
benzene) is challenging and can give information about the
role of the stabilizing surface compounds, such as their steric
hindrance or electronic properties or the potential blockage of
active sites. Ru NPs capped with terminal and internal alkynes
showed different activity and selectivity in the selective
hydrogenation of styrene; NPs capped with internal alkynes
were highly selective toward the hydrogenation of the vinyl
group.140 The characterization of the Ru NPs combined with
theoretical calculations suggested that internal and terminal
alkynes coordinate differently to the Ru surface; η2 side-on
configuration and RuCCH−, respectively; which could
explain the different reactivity of both systems. Likewise, the
deposition of Sn atoms onto the surface or Ru/PVP or Ru/
dppb NPs modulated the reactivity of these systems when used
as catalysts in the styrene hydrogenation.113 Indeed, the
amount of Sn able to be accommodated onto the Ru NPs
surface was dependent on the capping agent; Ru/PVP was able
to integrate more Sn on the surface, when compared to Ru/
dppb, in which the reaction with tin precursor is limited due to
the presence of the bulky ligand. Then, the nature of the
stabilizing agent together with the amount of Sn deposited on
the ruthenium surface tuned the catalytic activity of the Ru
NPs (Table 2). Introducing 0.2 equiv of Sn onto the Ru/PVP
catalyst led to a highly selective catalyst, as 95% of styrene was
obtained at 100% of conversion. The same selectivity was
reached by only introducing 0.05 equiv of Sn onto the Ru/
dppb surface. Both the presence of a bulky ligand and of a
small amount of tin onto the surface led to a highly selective
catalyst. The increase of the amount of tin on the NP surface
was detrimental to the activity in both catalysts used, Ru/PVP
and Ru/dppb, indicating that the control of the selectivity is
more likely due to a decrease on the reaction rate, than to a
specific reactivity. This later has not being checked for instance
by following the reaction over time.
Similarly, styrene and 2-butanone hydrogenation selectivity
was modulated by the Fe content in RuFe NPs stabilized with
HDA.118 The same synthesis procedure allowed preparation of
a series of RuFe NPs displaying several Ru/Fe ratios, in this
case using a supported ionic liquid phase (SILP)131 as a
stabilizer. Fe25Ru75/SILP was highly selective for ketone
hydrogenation in furan-based substrates, while Ru/SILP
promoted the full hydrogenation of the substrates. The
reduction of furfuralacetone was found highly sensitive to the
amount of iron in the catalyst. Best compromise in terms of
activity and selectivity was obtained for a Fe25Ru75
composition. Reaction rates for the CO hydrogenation of
intermediates in furfuralacetone reduction were calculated to
be 0.107 and 0.025 M/h for Fe25Ru75 and Ru100, respectively.
These data and also reaction profiles over time supported that
by adding a second metal to the ruthenium catalyst the
hydrogenation of the heteroarene can be suppressed but also
that the hydrogenation of the ketone group can be enhanced,
leading to a highly selective catalyst.
The crystalline structure of the metal cores has been found
to also influence the reactivity of Ru nanocatalysts in
hydrogenation reactions. The crystalline structure of Ru NPs
synthesized by epitaxial growth on PdCu alloyed NPs could be
controlled in a way to obtain Ru NPs presenting a fcc or a hcp
structure.154 The crystal structure of the nanoparticles affected
the catalytic activity of the hydrogenation of 4-chloronitro-
benzene; fcc Ru NPs had a superior activity when compared to
the hcp ones. In opposition, fcc Ru NPs were less efficient in
the hydrogenation of styrene. The reported conversion of
styrene toward ethylbenzene at 4 h of reaction was over 98%
catalyzed by hcp Ru NPs compared with 53% conversion with
fcc Ru NPs catalyst. The different reactivity toward the
reduction of the two different functional groups was attributed
to a different adsorption of the substrates over Ru surface, but
no further evidence is reported.
Styrene hydrogenation activity and selectivity were also
tuned with Ru NPs bearing two different rigid and bulky NHC
ligands derived from cholesterol.138 The different perform-
ances observed were related to the flexibility of the NHC
backbones; while ligands with higher steric hindrance lower
Table 2. Hydrogenation of Styrene with Ru/PVP/Sn or Ru/
dppb/Sn NPs as Catalystsa
aConversion determined by GC. (A = styrene; B = ethylbenzene; C =
ethylcyclohexane. Reproduced with permission from ref 113.
Copyright 2014 The Royal Society of Chemistry.
the amount of ligand on the NP surface, higher quantities of
free faces are accessible at the metallic surface, which are
needed for the hydrogenation of aromatic rings, and therefore
reduces the selectivity toward partially hydrogenated product.
Differences on activity were also reported for substrates like
acetophenone, biphenyl, and naphthalene. Other NHC ligands
displaying different backbones and substituents at the N atoms
have been also used as stabilizers for Ru NPs.133−135,138 The
reactivity of these species in catalyzed hydrogenation reactions
was governed by the bulkiness of the ligand, nevertheless, the
use of slightly different synthetic and catalytic reaction
conditions make the comparison difficult among them.
In Ru NPs stabilized with phosphines, PPh3 or dppb, both
arene and carbonyl group of the acetophenone coordinate to
the NPs surface competitively, giving predominantly the fully
hydrogenated product. It was pointed out that the steric
hindrance of the phosphine ligand governed the selectivity in
several reduction reactions.116 The reported TON for Ru/
PPh3 are superior to those for Ru/dppb system in the
hydrogenation of acetophenone, but not being a general rule,
which indicates that the activity and selectivity depend on the
reaction conditions too. In contrast to ruthenium systems, for
Rh NPs stabilized by the same phosphine ligands, no ligand
effect was observed.
Polycyclic aromatic hydrocarbons were also hydrogenated
with Ru/PPh3 NPs under mild reaction conditions.
112 The
selectivity in the hydrogenation reaction of naphthalene,
phenanthrene, triphenylene, and pyrene was mainly governed
by experimental conditions, and the nature and number of
substituents of the substrates (Figure 5).
Ru NPs are able to hydrogenate nonconjugated CC
double bonds in very mild reaction conditions. In the case of
α-pinene (Figure 6), Ru NPs have proven to be very efficient
among other metals, such as Pd or Ni. Also, the reaction is
more selective when performed in water.159 This explains than
mainly Ru NPs stabilized with water-soluble polymers are
described for this application and also that water-soluble Ru
salts are the preferred starting precursors to synthesize them
(Table 3).107,109,159−162 Usually high selectivities toward cis-
pinane are reported, and the catalytic systems can be recycled
several times without significative loss of activity. Interesting
enough, Ru NPs synthesized in the presence of a β-
cyclodextrin polymer145 were able to selectively convert
phenylacethylene to styrene in water under mild conditions
(1 bar H2, 323 K).
4.1.2. Reduction of Nitro Compounds. Besides the
reduction of CC and CO bonds by molecular hydrogen,
Ru NPs are also active in the reduction of nitro derivatives,
using H2
50,110,121,142,164 or NaBH4
73,74,103,104,106,148 as reducing
agents (Tables 4 and 5, respectively). Similarly, azo
compounds were reduced in related conditions, by using Ru
NPs as catalyst and NaBH4
106 or N2H4
111,165 as reductants
(Table 5).
The catalytic hydrogenation of nitrobenzene may lead to
aniline, by hydrogenation of the nitro group, and/or to
cyclohexylamine, by reduction of both the nitro and arene
moieties, but other byproducts can be produced during the
hydrogenation reaction, including azoxy, azo, and hydrazo
derivatives, among others.166−168 Reactions performed in the
liquid phase have used a variety of metal catalysts (Ni, Pt, and
Pd), but Ru, due to its excellent ability to hydrogenate
aromatic rings, is an interesting alternative to obtain selectively
cyclohexylamine, or if modified conveniently, aniline.169,170
Ru/C60 system has demonstrated to be highly selective for the
reduction of nitrobenzene, being able to hydrogenate the nitro
group in first place and successively after the aromatic ring
(Figure 7).164 This behavior is in contrast with that of other
Ru-based heterogeneous catalysts.171 Theoretical calculations
have shown that the coordination of the arene on Ru/C60 NPs
is favored over the nitro group in terms of adsorption energy,
but the addition of hydrides onto the Ru NP surface, which are
likely to be present on the surface during the hydrogenation
reaction, favors the coordination through the nitro group
(Figure 7).
Ligand effects on the selective hydrogenation of nitro-
benzene to cyclohexylamine were further studied by
introducing several stabilizing ligands onto the surface of the
Ru NPs.50 Ru/C60, Ru/PVP, and Ru/NHC proceeded in a
stepwise manner (Figure 8), producing aniline first and then
cyclohexylamine. This agrees with the fact that the reaction
selectivity is mainly governed by surface hydrides present onto
the Ru NPs surface. Ru/HDA showed a slightly different
behavior which can be explained by the lability of the ligand.
Even if the selectivity was mainly dominated by the intrinsic
nature of the small Ru NPs, a clear influence of the ligands was
also noticed. Less donor ligands promoted the hydrogenation
of the N-phenylhydroxylamine intermediate, leading to more
active and selective catalysts. Reported TOFs at 1 h of reaction
were 136.9, 129.2, 82.8, 64.8 h−1 Ru/C60, Ru/HDA, Ru/PVP,
and Ru/NHC, respectively.
The evaluation of the catalytic properties and the reaction
kinetics in the reduction of nitroarenes or azo dyes with
NaBH4 is widely used to obtain information about the
performances of a catalyst because it can be easily
implemented and conveniently measured by UV−vis spec-
trophotometry (Table 5). These reactions have been reported
to be sensitive to the size and structure of Ru NPs. Ru
nanocages or nanoframes displaying a fcc structure have been
synthesized through the chemical etching of a sacrificial
seed,73,74 and tested as catalysts in the reduction of 4-
Figure 5. Conversion and selectivity of reduction using Ru/PPh3 NPs.
Reproduced with permission from ref 112. Copyright 2015 The Royal
Society of Chemistry.
st
ab
ili
zi
ng
ag
en
t
sy
nt
he
tic
m
et
ho
do
lo
gy
ch
ar
ac
te
ri
za
tio
n
ca
ta
ly
tic
re
ac
tio
n
co
nd
iti
on
s
co
m
m
en
ts
re
f
po
ly
vi
ny
l
al
co
ho
l
(P
V
A
)
re
du
ct
io
n
of
R
uC
l 3
w
ith
H
2
T
EM
,X
PS
,I
C
P,
co
nf
oc
al
la
se
r
sc
an
ni
ng
m
ic
ro
–
sc
op
e
(C
LS
M
)
hy
dr
og
en
at
io
n
of
α
-p
in
en
e
an
d
ot
he
r
al
ke
ne
s
by
H
2
re
cy
cl
ed
ei
gh
t
tim
es
w
ith
ou
t
lo
ss
in
th
e
ca
ta
ly
tic
ac
tiv
ity
an
d
se
le
ct
iv
ity
15
9
R
uC
l
3
(2
.1
m
g,
0.
01
m
m
ol
),
PV
A
(M
w
:7
8,
00
0,
15
m
g)
,H
2O
(m
L)
,H
2
(5
0
ba
r)
,3
23
K
ca
ta
ly
st
/α
-p
in
en
e
10
00
/1
,α
–
pi
ne
ne
(1
0
m
m
ol
),
w
at
er
,H
2
(2
0
ba
r)
,3
43
K
m
et
hy
l
la
ur
at
e-
m
od
ifi
ed
ca
rb
ox
ym
et
hy
lc
el
lu
lo
se
(H
M
-C
M
C
)
re
du
ct
io
n
of
R
uC
l 3
w
ith
H
2
T
EM
,X
R
D
,C
LS
M
,D
LS
,
IR
hy
dr
og
en
at
io
n
of
α
-p
in
en
e
by
H
2
96
.6
%
co
nv
w
ith
98
.4
%
co
nv
;
re
cy
cl
ed
20
tim
es
w
ith
lo
ss
of
ac
tiv
ity
du
e
to
ca
ta
ly
st
s
ag
gl
om
er
at
io
n
an
d
R
u
le
ac
hi
ng
(
m
ea
su
re
d
by
IC
P)
16
0
R
uC
l 3
(0
.0
08
m
m
ol
),
H
M
-C
M
C
(2
m
g)
,H
2O
(2
m
L)
,H
2
(2
0
ba
r)
,
33
3
K
ca
ta
ly
st
(2
m
g)
,α
-p
in
en
e
(5
m
m
ol
),
2
m
g
N
a 2
C
O
3,
w
at
er
,H
2
(1
5
ba
r)
,3
48
K
,5
h
T
PG
S-
10
00
re
du
ct
io
n
of
R
uC
l 3
w
ith
H
2
T
EM
,X
PS
,X
R
D
,D
LS
,
IC
P
hy
dr
og
en
at
io
n
of
α
-p
in
en
e
by
H
2
re
cy
cl
ed
at
10
0%
co
nv
er
si
on
up
to
1
4
tim
es
,t
he
n
ab
ru
pt
de
cr
ea
se
of
co
nv
er
si
on
;
T
EM
of
th
e
sp
en
t
ca
ta
ly
st
s
in
di
ca
te
s
N
P
ag
gl
om
er
at
io
n
10
7
R
uC
l 3
(2
m
g)
,T
PG
S-
10
00
(2
m
L
0.
5%
in
H
2O
),
(2
m
L)
,H
2
(5
ba
r)
,
32
3
K
ca
ta
ly
st
(0
.0
1
m
m
ol
),
α
-p
in
en
e
(2
m
m
ol
),
N
a 2
C
O
3
(2
m
g)
,H
2
(1
5
ba
r)
,r
t,
32
3
K
tr
ib
lo
ck
co
po
ly
m
er
re
du
ct
io
n
of
R
uC
l 3
w
ith
H
2
T
EM
,X
R
D
,X
PS
,U
V
−
vi
s
hy
dr
og
en
at
io
n
of
α
-p
in
en
e
by
H
2
re
cy
cl
ed
at
10
0%
co
nv
er
si
on
up
to
5
tim
es
,t
he
n
ab
ru
pt
de
cr
ea
se
of
co
nv
er
si
on
;
T
EM
of
th
e
sp
en
t
ca
ta
ly
st
s
in
di
ca
te
s
N
P
ag
gl
om
er
at
io
n
10
9
R
uC
l 3
(2
m
g)
,T
PG
S-
10
00
(2
m
L
0.
5%
in
H
2O
),
(2
m
L)
,H
2
(5
ba
r)
,
32
3
K
ca
ta
ly
st
(0
.0
1
m
m
ol
),
α
-p
in
en
e
(2
73
m
g)
,H
20
(2
m
L)
,H
2
(3
ba
r)
,3
13
K
,2
h
β-
cy
cl
od
ex
tr
in
po
ly
m
er
re
du
ct
io
n
of
[R
u(
N
O
)(
N
O
3)
3]
w
ith
N
aB
H
4
T
EM
,
D
LS
,N
M
R
,I
R
,
X
PS
,T
G
A
hy
dr
og
en
at
io
n
of
te
tr
ad
ec
en
e
an
d
ot
he
r
lo
ng
-c
ha
in
al
ke
ne
s
by
H
2
R
u
N
Ps
or
ga
ni
ze
d
in
to
sm
al
l
w
or
m
-li
ke
m
ic
ro
do
m
ai
ns
of
si
ze
-c
on
tr
ol
le
d
na
no
pa
rt
ic
le
s;
ca
ta
ly
st
re
cy
cl
ed
an
d
re
us
ed
10
tim
es
w
ith
ou
t
lo
ss
of
ac
tiv
ity
14
5
[R
u(
N
O
)(
N
O
3)
3]
(2
69
m
g,
40
μm
ol
,)
,(
7.
8
m
g,
0.
03
m
m
ol
),
C
T
A
B
(2
35
m
g
of
th
e
po
ly
m
er
(0
.4
m
m
ol
of
am
m
on
iu
m
gr
ou
p)
,N
aB
H
4
(4
m
L,
0.
1
M
),
H
2O
(8
m
L)
,2
98
K
ca
ta
ly
st
(4
0
μm
ol
),
su
bs
tr
at
e
(2
m
m
ol
),
w
at
er
(1
2
m
L)
,H
2
(1
0
ba
r)
,3
03
K
,1
.5
h
se
m
ih
yd
ro
ge
na
tio
n
of
ph
en
yl
ac
et
yl
en
e
w
ith
10
0%
se
le
ct
iv
ity
to
w
ar
d
st
yr
en
e
hy
dr
og
en
at
io
n
of
ph
en
yl
ac
et
yl
en
e
ca
ta
ly
st
(4
0
μm
ol
),
su
bs
tr
at
e
(2
m
m
ol
),
w
at
er
(1
2
m
L)
,H
2
(1
ba
r)
,3
23
K
,2
0
h
m
on
tm
or
ill
on
ite
cl
ay
re
du
ct
io
n
of
[R
u(
N
H
3)
6]
C
l 3
w
ith
N
aB
H
4
T
EM
,S
A
X
S,
IC
P,
B
ET
hy
dr
og
en
at
io
n
of
al
ke
ne
s
by
H
2
re
cy
cl
ed
9
tim
es
w
ith
a
sl
ig
ht
ly
lo
ss
of
ac
tiv
ity
16
3
[R
u(
N
H
3)
6]
C
l 3,
m
on
tm
or
ill
on
ite
cl
ay
,N
aB
H
4
(4
m
L,
0.
1
M
),
H
2O
(4
0
m
L)
,r
t
ca
ta
ly
st
(0
.1
g)
,s
ub
st
ra
te
(2
m
L)
,
w
at
er
(1
2
m
L)
,H
2
(5
−
20
ba
r)
,
31
3−
37
3
K
Chemical Reviews Review
DOI: 10.1021/acs.chemrev.9b00434
Chem. Rev. 2020, 120, 1085−1145
1098
T
ab
le
4.
R
u
N
P
s
as
H
yd
ro
ge
na
ti
on
C
at
al
ys
ts
of
N
it
ro
be
nz
en
e
D
er
iv
at
iv
es
st
ab
ili
zi
ng
ag
en
t
sy
nt
he
tic
m
et
ho
do
lo
gy
ch
ar
ac
te
ri
za
tio
n
ca
ta
ly
tic
re
ac
tio
n
co
nd
iti
on
s
co
m
m
en
ts
re
f
fu
lle
re
ne
C
60
re
du
ct
io
n
of
[R
u(
C
O
D
)(
C
O
T
)]
w
ith
H
2
T
EM
,W
A
X
S,
IC
P,
IR
,
R
am
an
,
EX
A
FS
,X
PS
,
D
FT
re
du
ct
io
n
of
ni
tr
ob
en
ze
ne
by
H
2C
at
al
ys
t
(5
m
g)
ni
tr
o-
be
nz
en
e
(4
m
m
ol
),
do
de
ca
ne
(1
m
m
ol
),
H
2,
(3
0
ba
r)
,
Et
O
H
(3
0
m
L)
,3
53
K
ch
em
os
el
ec
tiv
e
an
d
st
ep
w
is
e
hy
dr
og
en
at
io
n;
D
FT
16
4
[R
u(
C
O
D
)(
C
O
T
)]
(3
0−
25
0
m
g)
,C
60
(0
.1
0−
0.
16
o
r
0.
18
m
m
ol
)
H
2
(3
ba
r)
,C
H
2C
l 2
(5
0−
40
0
m
L)
,2
98
K
ca
lc
ul
at
io
ns
sh
ow
th
at
th
e
co
or
di
na
tio
n
m
od
e
of
ni
tr
ob
en
ze
ne
ch
an
ge
s
w
ith
th
e
hy
dr
id
e
co
ve
ra
ge
;
re
cy
cl
in
g
te
st
w
ith
sl
ig
ht
ly
de
cr
ea
se
of
ac
tiv
ity
;
T
EM
ch
ar
ac
te
ri
za
tio
n
of
th
e
sp
en
t
ca
ta
ly
st
P
V
P,
H
D
A
,f
ul
le
r-
en
e
C
60
,N
H
C
re
du
ct
io
n
of
[R
u(
C
O
D
)(
C
O
T
)]
w
ith
H
2
T
EM
,I
C
P,
IR
,
D
FT
re
du
ct
io
n
of
ni
tr
ob
en
ze
ne
by
H
2
ch
em
os
el
ec
tiv
e
an
d
st
ep
w
is
e
hy
dr
og
en
at
io
n;
D
FT
50
[R
u(
C
O
D
)(
C
O
T
)]
(9
0−
25
0
m
g)
,s
ta
bi
liz
er
(0
.0
4
m
m
ol
C
60
,o
r
0.
18
m
m
ol
H
D
A
,o
r
0.
38
m
m
ol
N
H
C
,1
00
m
g,
or
10
00
m
g
of
PV
P)
)
H
2
(3
ba
r)
,T
H
F,
29
8
K
ca
ta
ly
st
(0
.0
25
m
m
ol
of
R
u)
ni
tr
ob
en
ze
ne
(4
m
m
ol
),
do
de
ca
ne
(1
m
m
ol
),
H
2,
(3
0
ba
r)
,E
tO
H
(3
0
m
L)
,3
53
K
ca
lc
ul
at
io
ns
po
in
t
ou
t
th
at
hy
dr
id
e
co
ve
ra
ge
is
cr
uc
ia
l
fo
r
ad
so
rp
tio
n
of
th
e
ph
en
yl
hi
dr
ox
yl
am
in
e
in
te
rm
ed
ia
te
;
su
rf
ac
e
lig
an
ds
m
od
ul
at
e
th
e
ac
tiv
ity
an
d
se
le
ct
iv
ity
C
66
(C
O
O
H
) 1
2
re
du
ct
io
n
of
[R
u(
C
O
D
)(
C
O
T
)]
w
ith
H
2
T
EM
,I
C
P,
IR
,
SS
N
M
R
,
SA
X
S,
W
A
X
S,
X
PS
,t
om
og
ra
–
ph
y
re
du
ct
io
n
of
ni
tr
ob
en
ze
ne
by
H
2
as
se
m
bl
ie
s
of
R
u
N
P;
se
le
ct
iv
ity
to
w
ar
d
an
ili
ne
up
to
90
%
;n
o
si
gn
ifi
ca
tiv
e
ch
an
ge
on
th
e
si
ze
of
N
P
af
te
r
ca
ta
ly
si
s
(b
y
T
EM
);
no
re
cy
cl
in
g
te
st
s
14
2
[R
u(
C
O
D
)(
C
O
T
)]
(0
.1
3−
0.
36
m
m
ol
),
C
66
(C
O
O
H
) 1
2
(0
.0
2−
0.
2
eq
ui
v)
,H
2
(3
ba
r)
,
T
H
F
(1
0−
15
0
m
L)
,2
98
K
ca
ta
ly
st
(5
m
g)
ni
tr
ob
en
ze
ne
(4
m
m
ol
),
do
de
ca
ne
(1
m
m
ol
),
H
2,
(3
0
ba
r)
,E
tO
H
(3
0
m
L)
,3
53
K
ph
os
ph
in
e-
fu
nc
–
tio
na
liz
ed
[B
M
M
IM
] 3
[t
pp
t]
re
du
ct
io
n
of
R
uO
2
w
ith
H
2
T
EM
,
X
R
D
,
X
PS
re
du
ct
io
n
of
ni
tr
ob
en
ze
ne
by
H
2
be
tt
er
ac
tiv
ity
th
an
co
m
m
er
ci
al
R
u/
C
;
th
e
ad
di
tio
n
of
[B
M
M
IM
] 3
[t
pp
t]
is
in
de
tr
im
en
t
of
th
e
ac
tiv
ity
;
no
ch
ar
ac
te
ri
za
tio
n
of
th
e
sp
en
t
ca
ta
ly
st
;
no
re
cy
cl
in
g
te
st
s
12
1
R
uO
2
(3
m
g,
0.
02
25
m
m
ol
),
[B
M
M
IM
] 3
[t
pp
t]
(1
6.
3
m
g,
0.
02
25
m
m
ol
),
IL
(1
m
L)
,H
2
(4
ba
r)
,3
43
K
ca
ta
ly
st
(1
7.
75
.1
0−
3
m
m
ol
),
IL
(1
m
L)
,n
itr
ob
en
ze
ne
de
ri
va
tiv
e
(s
ub
st
ra
te
/R
u
=
20
0)
,d
od
ec
an
e
(1
m
m
ol
),
H
2,
(5
0
ba
r)
,E
tO
H
(3
0
m
L)
,3
33
K
R
uR
uO
2/
PV
P
st
ep
w
is
e
re
ac
tio
n;
re
du
ct
io
n
of
[R
u(
ac
ac
) 3
]
ov
er
pr
ef
or
m
ed
ir
on
ox
id
e
N
Ps
T
EM
,X
R
D
,
X
PS
,X
R
F,
D
LS
,I
C
P,
IR
re
du
ct
io
n
of
ni
tr
ob
en
ze
ne
by
H
2
ca
ta
ly
st
s
di
sp
la
ys
a
R
u4
+ /
R
u0
m
ix
tu
re
;s
om
e
sy
nt
he
si
s
le
ad
to
a
m
ix
tu
re
of
m
on
om
et
al
lic
N
Ps
;
se
le
ct
iv
e
hy
dr
og
en
at
io
n
to
w
ar
d
an
ili
ne
;
re
cy
cl
in
g
te
st
11
0
ir
on
ox
id
e
N
Ps
(1
5
m
g)
,d
io
ct
yl
et
he
r
(7
m
L)
,
1,
2-
he
xa
de
ca
ne
di
ol
(0
.0
5
g)
,O
A
(1
0
μL
),
[R
u(
ac
ac
) 3
]
(0
.0
25
g)
,5
58
K
,4
5
m
in
ca
ta
ly
st
(3
g·
L−
1 )
ni
tr
ob
en
ze
ne
(0
.0
6
μM
),
H
2,
(3
0
ba
r)
,
42
3
K
R
uC
o/
O
A
re
du
ct
io
n
of
[R
u 3
(C
O
) 1
2]
an
d
[C
o(
ac
ac
) 2
]
in
he
pt
an
ol
T
EM
,X
A
FS
,
X
R
D
,I
C
P,
X
A
N
ES
,
EX
A
FS
,
hy
dr
og
en
at
io
n
of
4-
ni
tr
os
ty
re
ne
du
m
bb
el
l-s
ha
pe
d
C
o−
R
u
na
no
st
ru
ct
ur
e
co
m
po
se
d
of
a
C
o
na
no
ro
d
w
ith
tw
o
en
ds
ca
pp
ed
w
ith
R
u
na
no
pl
at
es
;
tu
ni
ng
m
et
al
la
tt
ic
e
st
ra
in
11
0
[R
u 3
(C
O
) 1
2]
(8
m
g)
,[
C
o(
ac
ac
)2
]
(6
.6
m
g)
,
gl
uc
os
e
(1
0
m
g)
,h
ep
ta
no
l
(2
m
L)
,O
A
m
(4
m
L)
,4
23
K
,2
h
ca
ta
ly
st
(0
.3
m
ol
%
4-
ni
tr
os
ty
re
ne
(0
.5
m
m
ol
),
C
M
eO
H
(3
m
L)
,H
2
(b
al
lo
n)
,2
98
K
R
u
w
ith
3%
la
tt
ic
e
co
m
pr
es
si
on
ex
hi
bi
ts
hi
gh
se
le
ct
iv
ity
fo
r
hy
dr
og
en
at
io
n
of
4-
ni
tr
os
ty
re
ne
to
4-
am
in
os
ty
re
ne
;r
ec
yc
le
d
4
tim
es
;D
FT
ca
lc
ul
at
io
ns
R
u−
Pd
C
u
yo
lk
−
sh
el
l
na
no
–
cr
ys
ta
ls
st
ep
w
is
e
re
du
ct
io
n
of
[P
d(
ac
ac
) 2
]/
C
uC
l 2·
2H
2O
an
d
R
uC
l 3
T
EM
,X
R
D
,I
C
P
hy
dr
og
en
at
io
n
of
st
yr
en
e,
di
ph
en
yl
ac
et
yl
en
e,
4-
ni
tr
oc
hl
or
ob
en
ze
ne
fc
c
ch
ar
ac
te
r
of
R
u
de
pe
nd
s
on
%
Pd
;
th
e
re
du
ct
io
n
of
ni
tr
o
gr
ou
p
w
as
m
or
e
pe
rf
or
m
an
t
w
he
n
us
in
g
fc
c
N
Ps
co
m
pa
re
d
to
hc
p
N
Ps
;
th
e
op
po
si
te
tr
en
d
w
as
ob
se
rv
ed
in
st
yr
en
e
hy
dr
og
en
at
io
n;
no
re
cy
cl
in
g
te
st
s;
no
ch
ar
ac
te
ri
za
tio
n
of
th
e
sp
en
t
ca
ta
ly
st
;
no
re
cy
cl
in
g
te
st
15
4
Chemical Reviews Review
DOI: 10.1021/acs.chemrev.9b00434
Chem. Rev. 2020, 120, 1085−1145
1099
nitrophenol, in order to demonstrate the higher reactivity of
this crystallographic structure. Ru fcc icosahedral nanocages,
which are very stable against temperature retaining their
structure up to 573 K, were active in this reaction and
displayed higher activities than Ru hcp NPs.74 Ru cubic,
octahedral, and icosahedral nanocages were tested as catalysts
displaying rate constants of 17.62, 20.64, and 41.21 s−1 mg−1,
respectively. Likewise, Ru fcc nanoframes, synthesized as well
by chemical etching of a nanosized template, performed better
in this reaction than Ru nanowires displaying a hcp structure.73
In this case, the rate constants of Ru fcc nanoframes were
reported to be 0.022 min−1 in opposition to 0.005 min−1
displayed by the hcp Ru nanowires. Nevertheless, no recycling
test or characterization of the spent catalysts are reported. The
reaction is also sensitive to the size of the Ru NPs.106 Ru NPs
ranging from 2.6 to 51.5 nm were synthesized by a polyol
reduction (using RuCl3 as Ru source and PVP as capping
agent) where the size of the as-synthesized NPs was controlled
mainly by the reaction temperature but also with the pH of the
solution. Catalytic activity of the different sized Ru NPs was
compared with that of other reported noble metal NPs. Ru-
based catalysts were more active for the nitrophenol reduction
than other nanosized metals (Ag, Au, Ir, and Pt). The reactivity
of Ru NPs was dependent on their size and displayed a volcano
trend, where 8 nm sized NPs were observed to be the most
performant. The degradation of azo dyes was also successfully
achieved using this Ru-based catalytic system. A multidentate
bulky ligand with weak interactions with the metal NPs but
strong enough to stabilize them has been described.104 The
amphiphilic tripodal ligand tris(1,2,3-triazolyl)-polyethylene
glycol (tristrz-PEG) (Figure 9), allowed to stabilize several
metal NPs (Fe, Co, Ni, Cu, Ru, Ag, Pt, Pd, and Au). Ru NPs
displayed a high catalytic activity in the reduction of
nitrophenol and was recycled three times.
Lattice strain can modify the electronic structure of catalysts
and therefore affect the adsorption of reactants. The reduction
of [Ru3(CO)12] and [Co(acac)2] in heptanol using oleylamine
as stabilizer allowed preparing dumbbell-shaped CoRu
nanostructures, where a Co nanorod is capped with a Ru
plate. NPs of several Ru/Co ratios were synthesized, and
Co0.23−Ru0.77 catalyst was shown to be highly selective toward
−NO2 hydrogenation (99%) in the hydrogenation of 4-
nitrostyrene to 4-aminostyrene. The selectivity of RuCo NPs
follows a volcano-type curve with increasing the Ru
compressive lattice strain.172
4.1.3. Hydrodeoxygenation. To produce basic chemicals
and renewable fuels from biomass feedstocks, it is necessary to
remove oxygen from these materials due to the high amount of
oxygenated moieties present in their structure. Hydrodeoxyge-
nation is a metal catalyzed reaction, which allows removal of
oxygen from oxygen-containing compounds in the presence of
H2.
174−176 Ni, Co, Mo, Pt, Rh, Ru, among other supported
metals have been used to upgrade biomass model com-
pounds.175 Lignin, one of the components of biomass, requires
depolymerization through C−O cleavage followed by hydro-
deoxygenation. Likewise, cellulose requires the same procedure
to produce polyols. Also, hydrodeoxygenation of vegetable oils
can produce long-chain alkanes, a renewable fuel from
biomass.174 Unsupported Ru NPs have found applications in
hydrodeoxygenation of long-chain fatty acids177 and lignin
monomeric and dimeric model substrates,130 including
bimetallic RuNi NPs,102,178 eucalyptol,179 and carbonylT
ab
le
4.
co
nt
in
ue
d
st
ab
ili
zi
ng
ag
en
t
sy
nt
he
tic
m
et
ho
do
lo
gy
ch
ar
ac
te
ri
za
tio
n
ca
ta
ly
tic
re
ac
tio
n
co
nd
iti
on
s
co
m
m
en
ts
re
f
[P
d(
ac
ac
) 2
]
(7
.5
m
g)
,C
uC
l 2·
2H
2O
(0
−
40
m
g)
,
ol
ey
la
m
in
e
(3
m
L)
,1
-o
ct
ad
ec
yl
en
e
(3
m
L)
,
Et
O
H
,(
1
m
L)
,3
93
K
,1
0
m
in
;
R
uC
l 3
(1
5.
6
m
g)
,E
tO
H
(1
m
L)
,4
73
K
,1
2
h
ca
ta
ly
st
(0
.0
05
m
m
ol
),
st
yr
en
e
(0
.1
7
m
m
ol
)
or
di
ph
en
yl
a-
ce
ty
le
ne
(0
.0
56
m
m
ol
),
to
lu
en
e
(1
.5
m
L)
,H
2,
35
3
K
;
ca
ta
ly
st
(0
.0
05
m
m
ol
),
4-
ni
tr
oc
hl
or
ob
en
ze
ne
(6
m
g)
,
to
lu
en
e
(0
.5
m
L)
,D
M
F
(1
.5
m
L)
H
2
(b
al
lo
n)
,3
68
K
Chemical Reviews Review
DOI: 10.1021/acs.chemrev.9b00434
Chem. Rev. 2020, 120, 1085−1145
1100
T
ab
le
5.
R
u
N
P
s
as
R
ed
uc
ti
on
C
at
al
ys
ts
of
N
it
ro
be
nz
en
e
an
d
A
zo
D
er
iv
at
iv
es
U
si
ng
N
aB
H
4
or
N
2H
4
as
R
ed
uc
in
g
A
ge
nt
s
st
ab
ili
zi
ng
ag
en
t
sy
nt
he
tic
m
et
ho
do
lo
gy
ch
ar
ac
te
ri
za
tio
n
ca
ta
ly
tic
re
ac
tio
n
co
nd
iti
on
s
co
m
m
en
ts
re
f
P
V
P
R
u
fc
c
ic
os
ah
ed
ra
ln
an
oc
ag
es
;
ch
em
ic
al
et
ch
in
g
of
Pd
R
u
co
re
−
sh
el
l
N
Ps
.R
uP
d
N
P
(0
.1
m
g)
,F
eC
l 3
(3
0
m
g)
,K
B
r
(3
00
m
g)
,P
V
P
T
EM
,X
R
D
,t
he
r-
m
al
st
ab
ili
ty
fo
l-
lo
w
ed
by
us
in
g
in
si
tu
X
R
D
re
du
ct
io
n
of
4-
ni
tr
op
he
no
l
by
N
aB
H
4
R
u
fc
c
st
ru
ct
ur
e
en
ha
nc
es
ca
ta
ly
tic
pr
op
er
tie
s;
R
u
cu
bi
c,
oc
ta
he
dr
al
,a
nd
ic
os
ah
ed
ra
l
na
no
ca
ge
s
ra
te
co
ns
ta
nt
s:
17
.6
2,
20
.6
4,
an
d
41
.2
1
s−
1
m
g−
1 ,
re
sp
ec
tiv
el
y
74
(5
0
m
g)
,H
C
l
(0
.1
8
m
L)
,H
2O
(4
.8
2
m
L)
ca
ta
ly
st
(0
.2
m
M
,0
.5
m
L)
,N
aB
H
4
(2
0
m
M
,1
m
L)
4-
ni
tr
op
he
no
l(
0.
2
m
M
,
1
m
L)
,H
2O
,2
98
K
PV
P
R
u
fc
c
na
no
fr
am
es
;c
he
m
ic
al
et
ch
in
g
of
Pd
R
u
co
re
−
sh
el
lN
Ps
.R
uP
d
N
P,
Fe
C
l 3
(2
5
m
g)
,K
B
r
(1
50
m
g)
T
EM
,X
R
D
,I
C
P
re
du
ct
io
n
of
4-
ni
tr
op
he
no
l
by
N
aB
H
4
R
u
fc
c
na
no
fr
am
es
ac
tiv
e
in
th
is
re
ac
tio
n;
no
re
cy
cl
ab
ili
ty
or
st
ab
ili
ty
te
st
s
af
te
r
ca
ta
ly
si
s
73
PV
P
(2
5
m
g)
,H
C
l
(0
.1
5
m
L)
,H
2O
(2
.8
5
m
L)
ca
ta
ly
st
(1
0
μL
0.
21
8
m
M
),
N
aB
H
4
(5
μL
,2
M
))
4-
ni
tr
op
he
no
l
(2
9.
5
μL
,
0.
5
m
M
),
H
2O
(0
.6
9
m
L)
,2
98
K
PV
P
re
du
ct
io
n
of
R
uC
l 3
in
n-
pr
op
an
ol
,
T
EM
,X
R
D
,U
V
−
vi
s
,
D
LS
,X
PS
re
du
ct
io
n
of
4-
ni
tr
op
he
no
l
an
d
ot
he
r
ni
tr
ob
en
ze
ne
de
ri
va
tiv
es
by
N
aB
H
4
R
u
N
Ps
si
ze
s
fr
om
2.
6
to
51
.5
nm
by
ad
ju
st
in
g
th
e
pH
an
d
te
m
pe
ra
tu
re
;
si
ze
de
pe
nd
en
t
ca
ta
ly
tic
ac
tiv
ity
;
be
tt
er
pe
rf
or
m
an
ce
s
th
an
Pt
an
d
Ir
N
Ps
;
lo
ss
of
ac
tiv
ity
af
te
r
se
ve
n
re
cy
cl
in
g
cy
cl
es
;
fe
w
in
fo
rm
at
io
n
ab
ou
t
th
e
sp
en
t
ca
ta
ly
st
10
6
R
uC
l 3
(5
00
μL
,1
00
m
M
),
PV
P
(5
0
m
M
),
n-
pr
op
an
ol
(1
0
m
L)
,
30
3−
37
1
K
,1
0
h
C
at
al
ys
t(
4
μL
,1
0
nM
),
N
aB
H
4
(2
m
L
m
M
,0
.1
M
),
ni
tr
oa
re
ne
(2
0
μL
,1
0
m
M
),
29
8
K
am
ph
ip
hi
lic
tr
ip
od
al
lig
an
d
tr
is
(1
,2
,3
-t
ri
–
az
ol
yl
)-
po
ly
et
hy
–
le
ne
gl
yc
ol
re
du
ct
io
n
of
R
uC
l 3
w
ith
N
aB
H
4
T
EM
,X
PS
,U
V
−
vi
s
re
du
ct
io
n
of
ni
tr
ob
en
ze
ne
by
N
aB
H
4
an
d
tr
an
sf
er
hy
dr
og
en
at
io
n
R
u
N
P
ac
tiv
e
in
re
du
ct
io
n
re
ac
tio
ns
in
w
at
er
;
R
u
N
P
re
cy
cl
ed
3
tim
es
w
ith
ou
t
si
gn
ifi
ca
nt
lo
ss
of
ac
tiv
ity
;
T
EM
of
th
e
sp
en
t
ca
ta
ly
si
s
in
di
ca
te
s
a
sl
ig
ht
ly
in
cr
ea
se
of
th
e
N
P
si
ze
10
4
ca
ta
ly
st
(0
.2
−
2
m
ol
%
),
N
aB
H
4
(1
0
eq
ui
v)
,n
itr
oa
re
ne
(1
eq
ui
v)
,2
98
K
R
uC
l 3
(1
eq
ui
v)
,s
ta
bi
liz
er
(1
eq
ui
v)
,N
aB
H
4
(1
0
eq
ui
v)
,H
2O
(6
m
L)
,2
98
K
ca
ta
ly
st
(0
.2
−
2
m
ol
%
),
N
aO
H
(0
.2
m
m
ol
),
ni
tr
oa
re
ne
(0
.1
m
m
ol
),
H
2O
/i
-p
ro
pa
no
l
(1
/4
,5
m
L)
,3
53
K
,2
4
h
de
nd
ri
m
er
re
du
ct
io
n
of
R
uC
l 3
w
ith
N
aB
H
4
T
EM
,X
R
D
,X
PS
,
U
V
−
vi
s,
IR
,c
y-
cl
ic
vo
lta
m
m
o-
gr
am
s
re
du
ct
io
n
of
p-
ni
tr
op
he
no
l
by
N
aB
H
4
no
re
cy
cl
in
g
te
st
;
no
ch
ar
ac
te
ri
za
tio
n
of
th
e
sp
en
t
ca
ta
ly
st
14
7
R
uC
l 3·
3H
2O
(1
0
m
L,
1.
63
×
10
−
3
M
),
de
nd
ri
m
er
(4
.2
×
10
−
5
M
),
N
aB
H
4
(5
m
L,
1
M
),
M
eO
H
(6
5
m
L)
,H
2O
(1
00
m
L)
,r
t,
24
h
ca
ta
ly
st
(1
00
μL
),
N
aB
H
4
(0
.2
5
m
L,
10
0
m
M
),
p-
ni
tr
op
he
no
l(
0.
25
m
L,
1
m
M
),
29
8
K
po
ro
us
po
ly
m
er
re
du
ct
io
n
of
R
uC
l 3
w
ith
N
aB
H
4
or
et
hy
le
ne
gl
yc
ol
T
EM
,D
R
X
,I
C
P,
X
PS
,B
ET
,N
M
R
re
du
ct
io
n
of
ni
tr
oa
re
ne
s
R
u
N
Ps
m
or
e
ef
fic
ie
nt
w
he
n
st
ab
ili
ze
d
w
ith
th
e
po
ly
m
er
co
m
pa
re
d
to
ot
he
r
st
an
da
rd
su
pp
or
ts
;
be
st
ca
ta
ly
st
re
cy
cl
ed
11
tim
es
w
ith
lo
ss
of
ac
tiv
ity
;
T
EM
,N
M
R
,X
PS
,I
C
P
of
th
e
sp
en
t
ca
ta
ly
st
s
in
di
ca
te
th
at
is
st
ab
le
10
3
R
uC
l 3·
3H
2O
(1
5
m
g)
,p
ol
ym
er
(5
0
m
g)
,N
aB
H
4
(4
m
L,
1.
63
×
10
−
2
M
),
M
eO
H
(2
0
m
L)
,r
t,
24
;
R
uC
l 3·
3H
2O
(1
5
m
g)
,p
ol
ym
er
(5
0
m
g)
,e
th
yl
en
e
gl
yc
ol
(5
0
m
L)
,4
53
K
,3
or
4
h
ca
ta
ly
st
(5
m
g)
,N
aB
H
4
(2
.5
m
m
ol
),
ni
tr
oa
re
ne
(0
.5
m
m
ol
),
T
H
F/
H
20
(1
/3
,m
L)
,2
98
K
PV
P
re
du
ct
io
n
of
R
uC
l 3
in
et
hy
le
ne
gl
yc
ol
at
44
3
K
,
T
EM
,X
R
D
,U
V
−
vi
s,
X
PS
hy
dr
og
en
at
io
n
of
or
an
ge
I
(a
zo
dy
e)
by
N
2H
4
de
gr
ad
at
io
n
ki
ne
tic
cu
rv
es
m
ea
su
re
d
by
ab
so
rb
an
ce
in
te
ns
iti
es
of
or
an
ge
I
at
51
2
nm
;R
u
N
Ps
sh
ow
ed
be
tt
er
pe
rf
or
m
an
ce
s
th
an
Pt
an
d
11
1
Chemical Reviews Review
DOI: 10.1021/acs.chemrev.9b00434
Chem. Rev. 2020, 120, 1085−1145
1101
T
ab
le
5.
co
nt
in
ue
d
st
ab
ili
zi
ng
ag
en
t
sy
nt
he
tic
m
et
ho
do
lo
gy
ch
ar
ac
te
ri
za
tio
n
ca
ta
ly
tic
re
ac
tio
n
co
nd
iti
on
s
co
m
m
en
ts
re
f
Ir
N
Ps
;
R
u
N
ps
ar
e
po
is
on
ed
w
ith
H
2S
,t
hi
s
pa
rt
ic
ul
ar
ity
is
ex
pl
oi
te
d
to
pr
ep
ar
e
pa
pe
r
st
ri
ps
fo
r
H
2S
ga
s
de
te
ct
io
n
R
uC
l 3
(1
2.
3
m
g)
,P
V
P
(5
5.
5
m
g)
,e
th
yl
en
e
gl
yc
ol
(1
0
m
L)
,4
43
K
,
6
h
C
at
al
ys
t
(8
nM
),
or
an
ge
I
(4
μL
,1
0
m
M
),
N
2H
4
(2
m
L,
0.
8
M
)
PV
P
re
du
ct
io
n
of
R
uC
l 3
in
n-
pr
op
an
ol
,
T
EM
,X
R
D
,U
V
−
vi
s,
D
LS
,X
PS
hy
dr
og
en
at
io
n
of
az
o
dy
es
by
N
aB
H
4
R
u
N
Ps
de
co
m
po
se
s
az
o
dy
es
in
se
co
nd
s;
no
re
cy
cl
ab
ili
ty
te
st
;
no
in
fo
rm
at
io
n
ab
ou
t
th
e
sp
en
t
ca
ta
ly
st
10
6
R
uC
l 3
(5
00
μL
,1
00
m
M
),
PV
P
(5
0
m
M
),
n-
pr
op
an
ol
(1
0
m
L)
,
30
3−
37
1
K
,1
0
h
ca
ta
ly
st
(4
μL
,1
0
m
M
),
az
o
dy
e
(2
0
μL
,1
0
m
M
),
N
aB
H
4
(2
m
L
m
M
,
0.
1
M
)
4-
su
lfo
ca
lix
[4
]a
re
ne
re
du
ct
io
n
of
R
uC
l 3
w
ith
N
aB
H
4.
T
EM
,S
EM
,X
R
D
,
T
G
A
,I
R
,D
LS
hy
dr
og
en
at
io
n
of
az
o
dy
e
by
N
2H
4
re
cy
cl
ed
9
tim
es
;l
ea
ch
in
g
te
st
;s
pe
nt
ca
ta
ly
st
ch
ar
ac
te
ri
ze
d
by
SE
M
,I
R
,
X
R
D
16
5
R
uC
l 3
(0
.4
02
m
m
ol
),
st
ab
ili
ze
r
(0
.2
01
m
m
ol
),
N
aB
H
4
(2
.4
m
m
ol
),
H
2O
(1
00
m
L)
,2
98
K
,1
2
h
ca
ta
ly
st
(0
.5
m
g)
,a
zo
dy
e
(0
.0
5
m
M
),
N
2H
4
(1
5
μL
),
H
20
(3
m
L)
R
uP
d
na
no
sh
ee
ts
st
ep
w
is
e
re
du
ct
io
n
of
[P
d(
ac
ac
) 2
]
an
d
[R
u(
ac
ac
) 3
]
T
EM
,X
R
D
,X
PS
,
IC
P
re
du
ct
io
n
of
4-
ni
tr
op
he
no
l
by
N
aB
H
4
su
bm
on
ol
ay
er
ed
R
u
de
po
si
te
d
on
ul
tr
at
hi
n
Pd
na
no
sh
ee
ts
;
be
tt
er
pe
rf
or
m
an
ce
s
in
te
rm
s
of
ac
tiv
ity
th
an
m
on
om
et
al
lic
R
u
an
d
Pd
N
Ps
in
bo
th
re
ac
tio
ns
17
3
[P
d(
ac
ac
) 2
]
(1
6
m
g)
,P
V
P
(3
0
m
g)
,c
itr
ic
ac
id
(1
70
m
g)
,C
T
A
B
(6
0
m
g)
,[
W
(C
O
) 6
]
(1
00
m
g)
,D
M
F,
(1
0
m
L)
,3
53
K
,1
h;
[R
u
(a
ca
c)
3]
(4
m
g)
,P
V
P
(5
0
m
g)
,a
sc
or
bi
c
ac
id
(5
0
m
g)
,e
th
yl
en
e
gl
yc
ol
(1
0
m
L)
,4
33
K
,1
h
ca
ta
ly
st
(P
d:
7.
6
m
M
;
R
u:
1.
0
m
M
),
N
aB
H
4
(2
5
μL
,2
M
),
4-
ni
tr
op
he
no
l
(4
.9
5
m
L,
0.
15
m
M
),
H
2O
,2
98
K
re
du
ct
io
n
of
1-
oc
ty
ne
ca
ta
ly
st
(P
d:
7.
6
m
M
;
R
u:
1.
0
m
M
),
1-
oc
ty
ne
(7
3.
5
μL
,0
.0
5
m
m
ol
),
n-
de
ca
ne
(1
0
μL
,0
.0
5
m
m
ol
),
Et
O
H
(6
m
L)
,H
2
(1
ba
r)
,2
98
K
A
uP
dR
u
st
ep
w
is
e
pr
oc
ed
ur
e
us
in
g
ga
lv
an
ic
re
pl
ac
em
en
t
T
EM
,
U
V
−
vi
s
re
du
ct
io
n
of
4-
ni
tr
op
he
no
l
an
d
az
o
dy
e
by
N
aB
H
4
no
re
cy
cl
in
g
te
st
;
no
ch
ar
ac
te
ri
za
tio
n
of
th
e
sp
en
t
ca
ta
ly
st
15
3
C
oC
l 2
(1
m
L,
0.
4
M
),
ca
ta
ly
st
(1
00
μL
,5
pM
),
N
aB
H
4
(1
00
μL
,1
00
m
M
M
),
4-
ni
tr
op
he
no
l(
10
0
μL
,1
m
M
),
bu
ffe
r
(7
00
μL
),
29
8
K
N
aB
H
4
(1
00
m
L,
8
m
M
),
so
di
um
ci
tr
at
e
(1
m
M
),
H
A
uC
l 4
(6
0
m
L,
0.
44
m
M
),
PV
P
(1
%
),
32
3
K
2
h;
N
aB
H
4
(0
.4
m
L,
0.
5
M
),
0.
31
m
L,
20
m
M
),
32
3
K
,2
h;
R
uC
l 3
(0
.1
66
m
M
)
Chemical Reviews Review
DOI: 10.1021/acs.chemrev.9b00434
Chem. Rev. 2020, 120, 1085−1145
1102
compounds by a bimetallic RuFe bifunctional catalyst119
(Table 6).
Lignin monomeric and dimeric model compounds, such as
phenol, guaiacol, diphenyl ether (4-O-5), benzyl phenyl ether
(α-O-4), 2-phenylethyl phenyl ether (β-O-4), and benzofuran
(β-5), have been hydrodeoxygenated using several metallic
NPs (Pt, Rh, Ru, and Pd) stabilized in different ILs.130 In
general, Pt/IL systems were more active and selective with all
substrates, while Rh and Ru displayed similar behavior, the
nature of the IL slightly modifying the selectivity. Ru NPs
synthesized over a porous organic network exhibited high
catalytic performance in stearic acid hydrogenation reaction
with 95.6% conversion of stearic acid.177 The alcohol-
hydrogenated product was then hydrodeoxygenated to
produce C18 alkane or decarbonylated to C17 alkane. The
ratio between C17/C18 could be modulated by the temper-
ature and pressure of the catalytic reaction. The Ru NPs
stabilized with the porous organic network were better
performing than other Ru-supported heterogeneous catalysts.
Bifunctional Ru120,179 or RuFe119 NPs stabilized in IL or
SILP have been used as catalysts in the hydrodeoxygenation of
eucalyptol, hydrogenation of the aldehyde intermediate
originated from the acid-catalyzed cleavage of lignin β-O-4
model, and the hydrodeoxygenation of carbonyl-substituted
aromatic substrates. Hydrodeoxygenation is often carried out
with bifunctional catalysts that contain both metal and acid
sites and are generally prepared by dispersing the metal NPs in
a solid acidic support.174 Ru/SILP NPs were highly active and
selective to the formation of p-menthane from eucalyptol, and
the reaction selectivity was dependent on the acidity of the
SILP.179 Acid cleavage of lignin β-O-4 model in the presence
of Ru NPs allowed hydrogenation of the aldehyde intermediate
product into 2-phenylalcohol in good yields.120 Bimetallic
RuFe/SILP+IL-SO3H
119 was shown to be a very efficient
system in the hydrodeoxygenation of carbonyl groups
contained in aromatic substrates, the presence of Fe in small
amounts (25%), preventing the hydrogenation of the aromatic
ring131 and leading to the production of the aromatic
dehydrodeoxygenated product in a very selective manner.
The catalyst had a large substrate scope and could be easily
recycled four times without loss of activity.
NixRu100−x catalysts (x = 0, 75, 80, 85, 90, 95, and 100,
where x represents the molar percentage of Ni), were prepared
Figure 7. (a) π-mode coordination of a nitrobenzene molecule on a facet of a naked 2C60−Ru13 molecular complex. (b) NO2-mode coordination of
a nitrobenzene molecule on the edge of a naked 2C60−Ru13 molecular complex. (c) Evolution of the energy difference between the two adsorption
modes with respect to the ratio of H per Ru surface atoms present on the metallic cluster. Reproduced with permission from ref 164. Copyright
2016 American Chemical Society.
Figure 8.Most stable states after N-phenylhydroxylamine adsorption on (a) Ru13−(C60)2 and (b) Ru13H18−(C60)2. (c) Time−concentration curve
for nitrobenzene hydrogenation with Ru−C60. Reproduced with permission from ref 50. Copyright 2018 American Chemical Society.
Chemical Reviews Review
DOI: 10.1021/acs.chemrev.9b00434
Chem. Rev. 2020, 120, 1085−1145
1103
T
ab
le
6.
R
u
N
P
s
as
H
yd
ro
de
ox
yg
en
at
io
n
C
at
al
ys
ts
st
ab
ili
zi
ng
ag
en
t
m
et
ho
do
lo
gy
ch
ar
ac
te
ri
za
tio
n
re
ac
tio
n
co
nd
iti
on
s
co
m
m
en
ts
re
f
PV
P
re
du
ct
io
n
of
R
uC
l 3
in
et
ha
no
l/
H
2O
,
T
EM
re
hy
dr
og
en
at
io
n
of
ce
llo
bi
os
e
se
le
ct
iv
ity
to
w
ar
d
so
rb
ito
ld
ep
en
ds
on
re
ac
tio
n
pH
an
d
m
et
al
us
ed
;
no
re
cy
cl
ab
ili
ty
te
st
;
no
in
fo
rm
at
io
n
ab
ou
t
th
e
sp
en
t
ca
ta
ly
st
18
0
R
uC
l 3
(0
.1
0
g,
0.
5
m
m
ol
),
PV
P
(0
.5
5
g,
5
m
m
ol
),
et
ha
no
l
(1
00
m
L)
,H
20
(1
00
m
L)
,3
53
,2
h
ca
ta
ly
st
(1
.6
7
×
10
−
3
m
ol
R
u/
L)
,c
el
lo
bi
os
e
(7
.3
1
m
m
ol
),
H
20
(3
0
m
L)
,H
2
(4
0
ba
r)
,3
93
K
,1
2
h
or
ou
s
or
ga
ni
c
ne
tw
or
k
re
du
ct
io
n
of
R
uC
l 3
w
ith
N
aB
H
4
T
EM
,X
R
D
,T
G
A
,
N
M
R
,I
R
,X
PS
,
N
2
so
rp
tio
n,
D
FT
,I
C
P
N
H
3-
T
PD
an
al
–
ys
is
de
hy
dr
og
en
at
io
n
of
lo
ng
-c
ha
in
fa
tt
y
ac
id
s
be
tt
er
ac
tiv
ity
an
d
se
le
ct
iv
ity
th
an
R
u
ov
er
in
or
ga
ni
c
su
pp
or
ts
;
re
cy
cl
ed
6
tim
es
w
ith
ou
t
lo
ss
of
ac
tiv
ity
;
sp
en
t
ca
ta
ly
st
an
al
yz
ed
by
T
EM
,X
R
D
an
d
X
PS
sh
ow
in
g
no
ch
an
ge
re
sp
ec
t
th
e
as
-s
yn
th
es
iz
ed
m
at
er
ia
l
17
7
R
uC
l 3
(6
0
m
g)
,p
ol
ym
er
(2
00
m
g)
,M
eO
H
(1
30
m
L)
,N
aB
H
4
(1
0
m
L,
1
M
),
29
8
K
ca
ta
ly
st
(2
0
m
g)
,s
ub
st
ra
te
(0
.3
50
m
m
ol
),
w
at
er
(7
0
m
L)
,H
2
(3
0
ba
r)
,4
53
K
IL
re
du
ct
io
n
in
si
tu
of
m
et
al
sa
lts
du
ri
ng
hy
dr
og
en
at
io
n
re
ac
tio
n
us
in
g
H
2
T
EM
,X
PS
,X
R
D
C
−
O
cl
ea
va
ge
an
d
hy
dr
od
eo
xy
ge
na
tio
n
lig
ni
n
m
on
om
er
ic
an
d
di
m
er
ic
m
od
el
co
m
po
un
ds
by
H
2
ca
ta
ly
st
re
cy
cl
in
g
fo
r
di
ph
en
yl
et
he
r
us
in
g
Pt
ba
se
d
ca
ta
ly
st
;
lo
ss
of
ca
ta
ly
tic
ac
tiv
ity
af
te
r
3
ru
ns
13
0
ca
ta
ly
st
(0
.0
1
m
m
ol
m
et
al
),
IL
(2
g,
su
bs
tr
at
e
(1
m
m
ol
),
H
3P
O
4
(0
.1
5
g)
,H
2
(5
ba
r)
,4
03
K
,1
0
h
SI
LP
de
co
m
po
si
tio
n
of
[R
u(
co
d(
m
et
hy
la
lly
l)
2]
by
H
2
T
EM
,I
C
P
hy
dr
od
eo
xy
ge
na
tio
n
of
eu
ca
ly
pt
ol
in
te
gr
at
io
n
of
bo
th
a
m
et
al
an
d
ac
id
ca
ta
ly
st
on
to
a
si
ng
le
su
pp
or
t;
se
le
ct
iv
e
ca
ta
ly
st
s
fo
r
th
e
hy
dr
o-
de
ox
yg
en
at
io
n
of
eu
ca
ly
pt
ol
to
p-
m
en
th
an
e;
se
le
c-
tiv
ity
de
pe
nd
s
on
th
e
ac
id
ity
of
th
e
SI
LP
17
9
[R
u(
co
d(
m
et
hy
la
lly
l)
2]
(4
0.
8
m
g)
,S
IL
P
(4
g)
,
C
H
2C
l 2
(4
0
m
L)
,H
2
(1
20
ba
r)
,3
73
K
,1
6
h
ba
tc
h:
ca
ta
ly
st
(7
5
m
g)
,e
uc
al
yp
to
l(
2.
4
m
m
ol
),
H
2
(1
20
ba
r)
,4
23
K
flo
w
:
ca
ta
ly
st
(5
47
m
g)
,e
uc
al
yp
to
l
(0
.0
5
M
eu
ca
ly
pt
ol
in
he
pt
an
e,
0.
3−
0.
9
m
L/
m
in
),
H
2
(8
0
ba
r,
flo
w
=
3−
37
N
m
L/
m
in
),
38
6−
42
0
K
R
uF
e/
SI
LP
+I
L-
SO
3H
re
du
ct
io
n
of
[F
e[
N
(S
i(
C
H
3)
3)
2]
2]
2
an
d
[R
u(
C
O
D
)(
C
O
T
)]
w
ith
H
2
T
EM
,S
EM
,B
ET
hy
dr
od
eo
xy
ge
na
tio
n
of
ca
rb
on
yl
-s
ub
st
itu
te
d
ar
om
at
ic
su
bs
tr
at
es
hi
gh
ly
se
le
ct
iv
e;
no
hy
dr
og
en
at
io
n
of
ar
om
at
ic
m
oi
et
ie
s;
ca
ta
ly
st
re
cy
cl
ed
4
tim
es
w
ith
ou
t
lo
ss
of
ac
tiv
ity
;
no
le
ac
hi
ng
11
9
Fe
[N
(S
i(
C
H
3)
3)
2]
2]
2
(1
8.
8
m
g)
,R
u(
C
O
D
)(
C
O
T
)]
(4
7.
0
m
g)
,S
IL
P
(5
00
m
g)
,m
es
itl
ye
ne
(5
m
L)
,H
2
(3
ba
r)
,4
23
K
,1
8h
;
Fe
R
u/
SI
LP
(3
75
.0
m
g,
0.
15
m
m
ol
),
ac
et
on
e
(5
m
L)
,I
L-
SO
iH
(2
04
.0
m
g,
0.
37
5
m
m
ol
),
rt
,1
h
ca
ta
ly
st
(5
8
m
g,
co
nt
ai
ni
ng
0.
01
5
m
m
ol
m
et
al
an
d
0.
03
8
m
m
ol
(2
.5
0
eq
ui
v)
IL
-S
O
3H
),
su
bs
tr
at
e
(0
.3
8
m
m
ol
),
m
es
ity
le
ne
(0
.5
m
L)
,H
2
(5
0
ba
r)
,4
48
K
,1
0
h
R
uN
i/
C
T
A
B
re
du
ct
io
n
of
R
uC
l 3
an
d
N
iC
l 2
w
ith
N
aB
H
4
T
EM
,X
R
D
,X
PS
hy
dr
og
en
ol
ys
is
of
th
re
e
lig
ni
n
m
od
el
su
bs
tr
at
es
N
i
re
sp
on
si
bl
e
fo
r
th
e
hy
dr
og
en
ol
ys
is
;
R
u
an
d
R
h
ar
e
pr
ed
om
in
an
tly
ac
tiv
e
in
th
e
hy
dr
og
en
at
io
n
of
th
e
ar
om
at
ic
ri
ng
s;
hy
dr
og
en
at
io
n
re
te
de
pe
nd
s
on
R
H
an
d
R
u
lo
ad
in
g;
hy
dr
og
en
ol
ys
is
of
C
(s
p3
)−
O
bo
nd
s
is
pr
ef
er
re
d
ov
er
C
(s
p2
)−
O
bo
nd
s
17
8
N
iC
l 2·
6H
2O
(4
0.
4
m
g,
0.
17
m
m
ol
),
R
uC
l 3·
3H
2O
(7
.8
m
g,
0.
03
m
m
ol
),
C
T
A
B
(1
00
m
g,
0.
27
4
m
m
ol
),
N
aB
H
4
(2
0
m
g,
0.
52
9
m
m
ol
),
H
2O
(3
m
L)
,2
73
K
ca
ta
ly
st
(9
.4
5
×
10
−
3
m
m
ol
),
ar
om
at
ic
et
he
r
(0
.1
89
m
m
ol
),
H
2O
(1
m
L)
,H
2
(1
ba
r)
,3
68
K
,1
6
h
R
uN
i/
PV
P
re
du
ct
io
n
of
R
uC
l 3
an
d
N
iC
l 2
w
ith
N
aB
H
4
T
EM
,X
R
D
,X
A
S,
X
A
N
ES
,E
X
A
FS
,
U
V
−
vi
s
hy
dr
og
en
ol
ys
is
of
th
re
e
lig
ni
n
m
od
el
su
bs
tr
at
es
N
iR
u
(8
5%
N
ia
nd
15
%
R
u,
N
is
ur
fa
ce
en
ri
ch
ed
)
be
st
ca
ta
ly
st
;
bi
m
et
al
lic
sy
st
em
s
be
tt
er
pe
rf
or
m
an
ce
s
10
2
Chemical Reviews Review
DOI: 10.1021/acs.chemrev.9b00434
Chem. Rev. 2020, 120, 1085−1145
1104
by reduction of RuCl3 and NiCl2 with NaBH4,
102 and later,
tested as catalysts in the dehydrodeoxygenation of β-O-4
model compound. The yield and selectivity were correlated to
the Ru/Ni ratio following a volcano-type curve (Figure 10). Ru
NPs were able to hydrogenate the aromatic ring, while the
increasing amount of Ni enhanced the C−O cleavage, Ni85Ru15
being the catalyst giving higher amounts of monomeric species.
In addition, under the catalytic conditions studied, fully
hydrogenated dimeric compounds did not undergo further C−
O hydrogenolysis (Figure 11).
More recently, it has been reported the application of the
same procedure to synthesize RuNi NPs but in the presence of
the surfactant cetyltrimethylammonium bromide (CTAB)
instead of PVP.178 Similar results were found, i.e., NixRu100−x
catalysts were efficient toward C−O cleavage, while Ru NPs
were mainly active in the arene hydrogenation (Figure 12).
Cellulose can be converted to polyols through hydro-
deoxygenation reaction catalyzed by Ru-based nanocata-
lysts.180−185 In a pioneering work,180 water-soluble Ru NPs
were used to conduct hydrogenation and hydrogenolysis
reactions of cellobiose into monomeric polyols, thus opening a
new route for the valorization of cellulose, the world’s most
abundant biopolymer. In this work, 2.4 nm of Ru NPs were
obtained by reduction of RuCl3 in the presence of PVP in an
ethanol/water mixture at 353 K. The catalytic reduction of
cellobiose was conducted at 393 K at 40 bar of H2. Ru over
performed other metals such as Pd, Pt, and Rh, in terms of
selectivity to produce sorbitol (100% conversion and
selectivity). Subsequently, Ru supported catalysts have been
used to upgrade cellulose, mainly using carbonaceous
supports.181,182,184,185 Interestingly enough, support effects
were reported for this reaction by using the transfer
hydrogenation methodology instead of molecular H2.
182 Ru
over several carbon supports was reported to be active using 2-
propanol as reduction agent, but Ru over alumina was not
active to produce sugar alcohols from cellulose.
4.1.4. Reductive Amination of Carbonyl Compounds,
Amination of Alcohols, and Other Miscellaneous
Reduction Reactions. To obtain primary amines several
methodologies have been developed, including hydroamino-
methylation/hydroamination,186,187 alcohol amination,188,189
and reductive amination of carbonyl compounds.189,190
Colloidal Ru-based catalysts have found applications in these
later reactions for the production of primary amines from
ammonia.191 This could open new opportunities, for instance,
to upgrade biomass-derived oxygen-rich materials.191−193
Heterogeneous catalysts for alcohol amination are
scarce191,193−202 but include the use of Ru-based materi-
als.191,193,194,199,201 These later are mainly supported catalysts,
which often display better performances than other metals for
this reaction,193,194 although Ni-based catalysts were also
displaying high performances.197,198 Amino acids were
obtained from α-hydroxyl acids derived from biomass and
ammonia in high yields in the presence of Ru/CNT through
the amination reaction.193 Ru/CNT catalyst surpassed other
metal-based catalysts, including Pd, Pt, Rh, and Ir over CNT
and Ni Raney, in terms of activity, and also other Ru-based
catalysts supported in oxides such as SiO2, Al2O3, ZrO2, CeO2,
and MgO. As mentioned before, colloidal-based catalysts allow
a fine-tuning of their properties, if compared to supported
catalysts, which permits access to more detailed information
about the impact of certain characteristics in a given reaction.
Recently, nonsupported Ru NPs stabilized with CTAB (ca. 2−Ta
bl
e
6.
co
nt
in
ue
d
st
ab
ili
zi
ng
ag
en
t
m
et
ho
do
lo
gy
ch
ar
ac
te
ri
za
tio
n
re
ac
tio
n
co
nd
iti
on
s
co
m
m
en
ts
re
f
th
an
m
on
om
et
al
lic
co
un
te
rp
ar
ts
;
lo
w
H
2
pr
es
su
re
en
an
hc
es
hy
dr
og
en
ol
ys
is
ov
er
hy
dr
og
en
at
io
n
N
iC
l 2·
6H
2O
(4
.4
m
g,
0.
01
87
m
m
ol
),
R
uC
l 3·
3H
2O
(0
.9
m
g,
0.
00
33
m
m
ol
),
C
T
A
B
(4
8.
8
m
g,
0.
44
m
m
ol
),
N
aB
H
4
(4
m
g,
0.
11
m
m
ol
),
H
2O
(3
m
L)
,
27
3
K
ca
ta
ly
st
(0
.0
22
m
m
ol
m
et
al
an
d
0.
44
m
m
ol
PV
P
in
3
m
L
H
2O
),
su
bs
tr
at
e
(0
.2
2
m
m
ol
),
H
2O
(1
m
L)
,H
2
(1
0
ba
r)
,3
68
K
,1
6
h
de
po
ly
m
er
iz
at
io
n
of
or
ga
no
so
lv
lig
ni
n-
ca
ta
ly
st
(0
.0
22
m
m
ol
m
et
al
an
d
0.
44
m
m
ol
PV
P
in
3
m
L
H
2O
),
su
bs
tr
at
e
(5
0
m
g)
,H
2O
(1
m
L)
,H
2
(1
0
ba
r)
,3
68
K
,1
6
h
Chemical Reviews Review
DOI: 10.1021/acs.chemrev.9b00434
Chem. Rev. 2020, 120, 1085−1145
1105
9 nm) were investigated in direct amination of octanol and
other alcohols into primary amines in the presence of
ammonia.191 This work revealed that the amination of alcohol
toward octylamine is insensitive to the size of the nano-
particles, but the selectivity is not at high conversions. The self-
coupling of the amine, leading to less selective systems because
of the formation of secondary and tertiary amines, is almost
suppressed for small NPs, therefore leading to highly selective
catalyst (89% conversion, 90% selectivity). Electronic and
steric properties of the NPs and the substrates are claimed to
be plausible explanations of the size sensitive of this reactions
but without any further evidence.
Ru-based catalysts have found applications in the reductive
amination of carbonyl compounds in order to obtain amines
selectively.189,192,199,203−208 Special focus is given to primary
amines using NH3 and H2. Similarly to the amination of
alcohols, the reductive amination of carbonyl compounds
allows efficient upgrading of oxygen-rich biomass deriva-
tives.189,192,199 Other metal-based heterogeneous catalysts have
been successfully used in this catalytic reaction,205,209−217 but
Ru seems to be highly efficient to produce primary
amines.189,205 Up to now, Ru-based catalysts used in this
reaction consist mainly in supported materials. It has been
evidenced that a support effect on the performances of
supported Ru catalysts.199,204 Ru/Nb2O5, Ru/TiO2 and Ru/
SiO2 catalysts displayed a different behavior in the reductive
amination of furfural.199 Ru/Nb2O5 was very efficient for this
reaction, and this fact was attributed to the lower electron
density of Ru NPs deposited on Nb2O5 when compared to
those of Ru/TiO2 and Ru/SiO2, which gave more electron-rich
Ru surfaces. Support effects were also evidenced elsewhere,189
but in this case the control of the reactivity was related to the
mixture of Ru and RuO2 on the surface. Recently, unsupported
Ru NPs displaying a fcc structure proved to be an extremely
efficient catalyst for the reductive amination of furfural and
other substrates.192 The fcc Ru NPs (TOF = 1850 h−1, at 363
K) outperformed Ru/Nb2O5 (TOF = 520 h
−1, at 363 K) and
Rh/Al2O3 (TOF = 990 h
−1, at 353 K) catalysts in terms of
activity but displaying similar selectivity toward the primary
amine (99%, 99%, and 92%, respectively). This catalyst was
reused four times and was highly active and selective for other
substrates.
Other reduction reactions have been studied using Ru NPs
as catalysts, such as transfer hydrogenation reactions,155,218 or
reduction of NOx105 which are summarized in Table 7.
4.2. Oxidation Reactions
Ru NPs have been successfully used as catalysts in oxidation
reactions. Thus, the oxidation of several substrates with
oxidation agents such as tert-butyl hydroperoxide
(TBHP),219 H2O2,
220 or aerobic conditions135 is described in
the literature (Table 8). Water-soluble Ru NPs were used in
the allylic oxidation of α-pinene by TBHP to produce
verbenone with 39% yield.219 Also, Ru NPs catalyzed the
Figure 10. (a) Thirteen products identified after β-O-4 hydrogenolysis. (b) Yields of monomers and dimers over Ni, Ru, and NiRu with varying
Ni/Ru ratio. Reaction conditions: 0.22 mmol β-O-4, 3 mL of freshly prepared aqueous solution containing 0.022 mmol of metal and 0.44 mmol of
PVP, 10 bar H2, 403 K, 1 h. Adapted with permission from ref 102.
Copyright 2014 American Chemical Society.
Figure 11. Kinetic study on hydrogenolysis of β-O-4 over (a)
Ni85Ru15 and (b) Ru. Reaction conditions: 0.22 mmol of β-O-4, 3 mL
of freshly prepared aqueous solution containing 0.022 mmol of metal
and 0.44 mmol of PVP, 10 bar H2, 403 K. Adapted with permission
from ref 102. Copyright 2014 American Chemical Society.
oxidation of substrates such as 3,3,5,5-tetramethylbenzidine, o-
phenylenediamine, and dopamine hydrochloride by H2O2. Ru/
PVP NPs converted ethanol to acetaldehyde with molecular
O2 (30 bar).
221 Milder conditions (1 bar O2) were applied in
the oxidation of alcohol and amine derivatives, using aerobic
conditions by Ru/NHC NPs.135 The oxidation with Ru/NHC
NPs proceeded smoothly, and it was also possible to perform
consecutive oxidation/hydrogenation reactions. WAXS anal-
yses of the Ru/NHC catalysts exposed to air showed that
amorphous ruthenium oxide was formed only at the surface of
the nanoparticles providing an unoxidazed Ru core, thus
indicating the stability of the Ru nanosystem in the applied
conditions.
Because of the importance of CO removal from car exhaust
or fuel-cell systems, CO oxidation has been studied
thoughtfully, both theoretically and experimentally.222 CO
oxidation can be seen also as a model reaction, similar to the
case of styrene hydrogenation as previously mentioned, which
can bring further information about metal NPs nature and
characteristics.223 Mono- and bimetallic Ru-based catalysts
synthesized by wet procedures have been investigated for CO
oxidation (Table 9). The influence of parameters such as Ru
crystal structure, size, and in bimetallic systems, the ratio of the
two metals, on the activity of the reaction has been underlined.
Ru NPs displaying fcc or hcp crystalline structures were
prepared selectively from [Ru(acac)3] and RuCl3, respectively,
with controllable sizes ranging from 2 to 5.5 nm.13,224 The
crystalline structure was controlled by the choice of the Ru
source and the solvent, ethylene glycol or triethylene glycol,
and the size was adjusted by varying the concentration of
reagents and the stabilizer (PVP). TEM and XRD analyses
pointed out the fcc character of the Ru NPs. In situ XRD
probed the high thermal stability of the Ru fcc NPs, which
were stable up to 723 K. The CO oxidation was dependent on
both crystalline phase and size; small Ru fcc NPs outperformed
hcp ones when displaying small sizes, while hcp Ru NPs were
more performant at larger sizes (Figure 13). Ru nanochains
were synthesized in water from Ru seeds with cetyl
trimethylammonium bromide as capping agent. The self-
assembled nanochains were more efficient as CO oxidation
catalysts than Ru nanoseeds (3.5 nm) and Ru spheres (6
nm).225
Bimetallic RuPd,226 RuCu,227,228 and RuCo3O4
229 catalysts
have been described as well. Ru deposited onto Co-rods and
further thermally treated gave RuCo3O4 species, which were
active toward the CO-oxidation reaction and outperformed the
corresponding monometallic NPs (Figure 14).229 DFT
calculations attributed the enhancement of the catalytic activity
of RuCo3O4 species to the charge transfer from ruthenium to
Co3O4, which activated more efficiently O2 and lowered the
activation energy.
A series of RuPd NPs have been synthesized from RuCl3 and
K2[PdCl4] by tunning the Ru/Pd ratio.
226 The crystallographic
structure of the bimetallic NPs changed from fcc to hcp when
increasing the Ru content. Surface characterization was
performed using solid-state 2H NMR; 2H NMR spectra after
2H adsorption showed that the chemical shift of the hydrides
on the surface of the NPs depends on their composition
(Figure 15). Ru0.5Pd0.5 was the most active catalyst, performing
better than other RuPd mixtures and also than monometallic
Ru, Pd, and Rh based catalysts (Figure 15).
Following a similar procedure, nanosized RuxCu1−x alloys
were synthesized, which is remarkable because Ru and Cu are
completely immiscible in bulk phase.227,228 XRD, TEM, and
EDX suggest that Cu and Ru atoms are randomly mixed to
form alloy structures. As observed with the close RuPd NPs
system described above, the catalytic activity of RuxCu1−x
alloys in the CO oxidation reaction depends on the Ru/Cu
ratio; Cu0.2Ru0.8 nanoparticles demonstrated the best catalytic
activity. IR studies provided better insights on the catalytic
system. CO adsorbed onto the NPs surface was observed by
IR; pure Ru NPs, displayed a CO band at 1986 cm−1 along
with those of free CO gas at 2200−2050 cm−1. A blue-shift was
observed when increasing the Cu content in the samples. After
Figure 12. Hydrogenolysis/hydrogenation of (left) 1-phenoxy-2-phenylethane (β-O-4 linkage), (middle) benzyl phenyl ether (α-O-4 linkage),
(right) diphenyl ether (4-O-5 linkage), and product yield for selected metal combinations catalyzed by Ru100−xNix NCs. The black arrows refer to
the M15Ni85 NCs, and the corresponding yields are in black. The blue arrows refer to the M60Ni40 NCs, and the corresponding yields are in blue in
parentheses. The fractions comprise partially/fully hydrogenated dimers (orange), nonhydrogenated monomers (darker green), and hydrogenated
monomers (lighter green). Adapted with permission from ref 178. Copyright 2018 The Royal Society of Chemistry.
Chemical Reviews Review
DOI: 10.1021/acs.chemrev.9b00434
Chem. Rev. 2020, 120, 1085−1145
1107
T
ab
le
7.
R
u
N
P
s
as
C
at
al
ys
ts
in
M
is
ce
lla
ne
ou
s
R
ed
uc
ti
on
R
ea
ct
io
ns
st
ab
ili
zi
ng
ag
en
t
m
et
ho
do
lo
gy
ch
ar
ac
te
ri
za
tio
n
re
ac
tio
n
co
nd
iti
on
s
co
m
m
en
ts
re
f
C
T
A
B
re
du
ct
io
n
of
R
uC
l 3
w
ith
N
aB
H
4
T
EM
,
O
2
tit
ra
tio
n,
X
R
D
,
X
PS
am
in
at
io
n
of
oc
ta
no
l
N
Ps
si
ze
eff
ec
t;
th
re
e
re
cy
cl
in
g
te
st
s;
no
le
ac
hi
ng
19
1
R
uC
l 3
(0
.2
2
g)
,C
T
A
B
(2
.9
−
5
eq
ui
v)
,N
aB
H
4
(0
.1
3
g)
,h
ex
an
ol
(2
.6
−
4.
5
eq
ui
v)
,H
2O
(0
.5
−
4.
5
eq
ui
v)
,
27
3
K
ca
ta
ly
st
(1
0−
20
0
m
g)
,s
ub
st
ra
te
(1
m
L)
,d
ec
an
e
(1
m
m
ol
),
N
H
3
ga
s,
H
2
(2
ba
r)
,4
53
K
,1
−
24
h
no
ne
ac
id
ic
tr
ea
tm
en
t
of
R
u/
C
a(
N
H
2)
2
T
EM
,X
R
D
,N
2
ad
so
rp
–
tio
n−
de
so
rp
tio
n
is
o-
th
er
m
s,
C
O
ch
em
is
or
p-
tio
n,
X
PS
,I
R
re
du
ct
iv
e
am
in
at
io
n
re
cy
cl
in
g
te
st
,n
o
fu
rt
he
r
ch
ar
ac
te
ri
za
tio
n
of
th
e
sp
en
t
ca
ta
ly
st
19
2
R
u/
C
a(
N
H
2)
2
(2
g)
,2
-p
ro
pa
no
l(
15
m
L)
,H
N
O
3
(2
M
un
til
pH
=
4)
,H
2O
(2
0
m
L)
,3
33
K
,2
−
4
h
ca
ta
ly
st
(0
.2
m
g)
,s
ub
st
ra
te
(0
.5
m
m
ol
),
N
H
3-
m
et
ha
no
l
(4
m
L,
8
m
m
ol
),
H
2
(2
0
ba
r)
,3
63
K
,0
−
6
h
R
uF
e
st
ep
w
is
e
re
ac
tio
n;
Fe
SO
4
re
du
ct
io
n
w
ith
N
aB
H
4
fo
llo
w
ed
by
ga
lv
an
ic
re
du
ct
io
n
T
EM
,I
C
P,
X
PS
tr
an
sf
er
hy
dr
og
en
at
io
n
ho
t
fi
ltr
at
io
n
te
st
;m
et
al
le
ac
hi
ng
(R
u
(1
2
pp
m
),
Fe
(4
pp
m
);
re
cy
cl
ed
5
tim
es
w
ith
a
sl
ig
ht
ly
lo
ss
of
ac
tiv
ity
21
8
Fe
SO
4
(4
.5
g)
,N
aB
H
4
(0
.8
g)
,M
eO
H
(6
0
m
L)
,
H
2O
(3
60
m
L)
;
ca
ta
ly
st
(5
0
m
g,
1.
3
m
ol
%
),
su
bs
tr
at
e
(1
m
m
ol
),
de
ca
ne
(1
m
m
ol
),
K
O
H
(1
5
m
ol
%
),
2-
Pr
O
H
(5
m
L)
,3
73
K
R
uC
l 3
(1
0
m
g)
,F
e
N
Ps
(1
00
m
g)
,M
eO
H
N
i/
R
u/
Pt
/A
u
re
du
ct
io
n
of
m
et
al
pr
ec
ur
so
rs
w
ith
lit
hi
um
tr
ie
th
yl
–
bo
ro
hy
dr
id
e
T
EM
,I
C
P
tr
an
sf
er
hy
dr
og
en
at
io
n
te
tr
am
et
al
lic
ca
ta
ly
st
di
sp
la
ye
d
hi
gh
er
co
nv
er
si
on
to
th
e
de
si
re
d
pr
od
uc
t
th
an
m
on
o-
,
bi
-,
or
tr
im
et
al
lic
co
un
te
rp
ar
ts
;
no
re
cy
cl
in
g
te
st
s;
no
ch
ar
ac
te
ri
za
tio
n
of
th
e
sp
en
t
ca
ta
ly
st
s
15
5
N
iC
l 2,
ca
ta
ly
st
(0
.3
−
0.
7
m
ol
%
),
4-
ph
en
yl
-1
-b
ut
en
e
(1
m
m
ol
),
H
2O
/2
-P
rO
H
(3
/1
0,
3.
3
m
L)
,3
73
K
,2
4
h
R
uC
l 3,
K
A
uC
l 4)
H
2P
tC
l 6,
(0
.5
0
m
m
ol
in
to
ta
l)
,
tr
io
ct
yl
ph
os
ph
in
e
ox
id
e
(0
.5
0
m
m
ol
),
T
H
F
(1
0
m
L)
lit
hi
um
tr
ie
th
yl
bo
ro
hy
dr
id
e
(7
.5
m
L,
1
M
),
rt
,
2
h
R
uP
d/
PV
P
re
du
ct
io
n
of
K
2[
Pd
C
l 4]
an
d
R
uC
l 3
in
tr
ie
th
yl
en
e
gl
yc
ol
T
EM
,X
R
D
,X
PS
,S
SN
M
R
re
du
ct
io
n
of
N
O
x
R
uP
d
N
P
di
sp
la
ys
be
tt
er
N
O
x
re
du
ct
io
n
ac
tiv
ity
th
an
R
h;
th
eo
re
tic
al
ca
lc
ul
at
io
ns
sh
ow
th
at
th
e
el
ec
tr
on
ic
st
ru
ct
ur
e
of
Pd
0.
5
R
u 0
.5
is
si
m
ila
r
to
th
at
of
R
h
in
ve
rs
e
vo
lc
an
o-
ty
pe
be
ha
vi
or
in
re
du
ct
io
n
ac
tiv
ity
w
ith
re
sp
ec
t
th
e
at
om
ic
ra
tio
of
Pd
an
d
R
u
10
5
K
2[
Pd
C
l 4]
(1
63
.4
m
g)
,R
uC
l 3
(1
31
.1
),
PV
P
(4
44
m
g)
,t
ri
et
hy
le
ne
gl
yc
ol
(1
00
m
L)
,H
20
(4
0
m
L)
,
47
3
K
tu
bu
la
r
qu
ar
tz
re
ac
to
r
w
ith
ca
ta
ly
st
,
m
ix
tu
re
si
m
ul
at
in
g
au
to
m
ot
iv
e
ex
–
ha
us
t,
29
3−
87
3
K
Chemical Reviews Review
DOI: 10.1021/acs.chemrev.9b00434
Chem. Rev. 2020, 120, 1085−1145
1108
T
ab
le
8.
R
u
N
P
s
as
O
xi
da
ti
on
C
at
al
ys
ts
st
ab
ili
zi
ng
ag
en
t
m
et
ho
do
lo
gy
ch
ar
ac
te
ri
za
tio
n
re
ac
tio
n
co
nd
iti
on
s
co
m
m
en
ts
re
f
am
m
on
iu
m
su
rf
ac
ta
nt
s
(
H
EA
16
C
l,
H
EA
16
B
r,
H
EA
16
B
F 4
,T
H
E-
A
16
C
l)
re
du
ct
io
n
of
R
uC
l 3
an
d
N
iC
l 2
w
ith
N
aB
H
4
T
EM
,D
LS
ox
id
at
io
n
of
α
-p
in
en
e
39
%
yi
el
d
of
ve
rb
en
on
e
fr
om
α
-p
in
en
e;
R
u
N
Ps
w
ith
am
m
on
iu
m
su
rf
ac
ta
nt
s
H
EA
pe
rf
or
m
be
tt
er
th
an
ot
he
r
R
u
N
Ps
;c
ou
nt
er
io
n
(X
=
C
l,
B
r,
B
F 4
)
sl
ig
ht
ly
in
fl
ue
nc
es
th
e
ke
to
ne
se
le
ct
iv
ity
;
re
cy
cl
in
g
te
st
;
T
EM
af
te
r
ca
ta
ly
si
s
21
9
R
uC
l 3·
3H
2O
(1
0
m
g,
3.
8
×
10
−
5
m
ol
,1
eq
ui
v)
,
am
m
on
iu
m
su
rf
ac
ta
nt
(7
.6
×
10
−
5
m
ol
,2
eq
ui
v)
,
N
aB
H
4
(3
.6
m
g,
2.
5
eq
ui
v)
,H
2O
(1
0
m
L)
,2
73
K
ca
ta
ly
st
(1
.9
×
10
−
5
m
ol
),
α
-p
in
en
e
(1
.9
×
10
−
3
m
ol
),
t-
B
H
P
(5
.7
×
10
−
3
m
ol
),
w
at
er
(5
m
L)
,3
h,
29
3
K
−
co
m
m
er
ci
al
T
EM
,S
EM
,
D
LS
,z
et
a
po
te
nt
ia
l,
U
V
−
vi
s
ox
id
at
io
n
of
se
ve
ra
l
so
m
e
te
st
us
in
g
O
2
as
ox
id
iz
in
g
ag
en
t;
no
re
cy
cl
in
g
te
st
or
ch
ar
ac
te
ri
za
tio
n
of
th
e
ca
ta
ly
st
s
af
te
r
re
ac
tio
n
22
0
3,
3,
5,
5-
te
tr
am
et
hy
lb
en
zi
di
ne
,
o-
ph
en
yl
en
ed
ia
m
in
e,
an
d
do
pa
m
in
e
hy
dr
o-
ch
lo
ri
de
)
ca
ta
ly
st
(2
.5
−
20
μg
/m
L)
,s
ub
st
ra
te
(0
.1
m
M
),
H
20
2
(0
.1
m
M
)
lo
ng
-c
ha
in
N
H
C
re
du
ct
io
n
of
[R
u(
C
O
D
)(
C
O
T
)]
w
ith
H
2
T
EM
,W
A
X
S,
SS
N
M
R
,I
R
ox
id
at
io
n
of
se
ve
ra
ls
ub
st
ra
te
s
w
ith
O
2
se
le
ct
iv
ity
m
od
ul
at
ed
w
ith
su
rf
ac
e
lig
an
d;
ox
id
iz
ed
N
P
ch
ar
ac
te
ri
ze
d
by
T
EM
an
d
W
A
X
S;
no
re
cy
cl
in
g
te
st
13
5
[R
u(
C
O
D
)(
C
O
T
)]
(1
00
m
g)
,N
H
C
(0
.1
−
0.
3
eq
ui
v)
,H
2
(3
ba
r)
,T
H
F
(5
0
m
L)
,2
98
K
,2
0
h
ca
ta
ly
st
(1
m
g)
,s
ub
st
ra
te
(0
.2
m
m
ol
),
tr
ifl
uo
ro
to
lu
en
e
(1
m
L)
,O
2
(1
ba
r)
,2
98
K
,
16
h
ox
id
at
io
n/
hy
dr
og
en
at
io
n
of
se
ve
ra
l
su
bs
tr
at
es
w
ith
O
2,
th
en
H
2
ca
ta
ly
st
(1
−
1.
5
m
g)
,s
ub
st
ra
te
(0
.2
m
m
ol
),
tr
ifl
uo
ro
to
lu
en
e
(1
m
L)
,O
2
(1
ba
r)
,2
98
K
,
16
h;
H
2
(5
ba
r)
,r
t
or
31
8
K
,4
or
16
h
Chemical Reviews Review
DOI: 10.1021/acs.chemrev.9b00434
Chem. Rev. 2020, 120, 1085−1145
1109
T
ab
le
9.
R
u
N
P
s
as
O
xi
da
ti
on
C
at
al
ys
ts
of
C
O
st
ab
ili
zi
ng
ag
en
t
m
et
ho
do
lo
gy
ch
ar
ac
te
ri
za
tio
n
re
ac
tio
n
co
nd
iti
on
s
co
m
m
en
ts
re
f
PV
P
re
du
ct
io
n
of
[R
u(
ac
ac
) 3
]
or
R
uC
l 3
in
et
hy
le
ne
gl
yc
ol
or
tr
ie
th
yl
en
e
gl
yc
ol
T
EM
,X
R
D
C
O
ox
id
at
io
n
sy
nt
he
si
s
of
R
u
fc
c
([
R
u(
ac
ac
) 3
])
or
hc
p
(R
uC
l 3)
de
pe
nd
in
g
on
th
e
m
et
al
pr
ec
ur
so
r
us
ed
;
C
O
ox
id
at
io
n
si
ze
–
an
d
st
ru
ct
ur
e-
de
pe
nd
en
t;
hi
gh
er
C
O
ox
id
at
io
n
ac
tiv
ity
of
fc
c
R
u
N
P
co
m
pa
re
d
w
ith
th
at
of
hc
p
R
u
N
Ps
,f
or
si
ze
s
la
rg
er
th
an
3
nm
13
,2
24
[R
u(
ac
ac
) 3
]
or
R
uC
l 3
(2
.1
m
m
ol
),
PV
P
(1
−
10
m
m
ol
),
so
lv
en
t
(2
5−
50
0
m
L)
,4
73
K
,3
h
tu
bu
la
r
qu
ar
tz
re
ac
to
r
w
ith
qu
ar
tz
w
oo
l,
ca
ta
ly
st
(1
50
m
g)
,g
as
m
ix
tu
re
of
C
O
/O
2/
H
e
(C
O
/O
2/
H
e:
0.
5/
0.
5/
49
m
L·
m
in
−
1 )
,3
73
K
C
T
A
B
re
du
ct
io
n
of
[R
u(
N
O
)(
N
O
) 3
]
w
ith
N
aB
H
4
T
EM
,X
R
D
,U
V
−
vi
s,
D
LS
C
O
ox
id
at
io
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R
u
na
no
ch
ai
ns
sy
nt
he
si
ze
d
in
a
tw
o
st
es
pr
oc
ed
ur
e
ar
e
m
or
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pe
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or
m
an
t
in
C
O
ox
id
at
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th
an
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u
sp
he
ri
ca
lN
PS
;c
at
al
yt
ic
ac
tiv
ity
de
pe
nd
s
al
so
on
th
e
su
pp
or
tu
se
d
R
u
na
no
ch
ai
ns
ca
n
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re
cy
cl
ed
w
hi
le
R
u
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Ps
te
nd
to
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cr
ea
se
th
e
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ze
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d
lo
ss
so
m
e
ac
tiv
ity
22
5
[R
u(
N
O
)(
N
O
) 3
]
(1
25
μL
,1
.5
w
t
%
),
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aB
H
4
(5
0
μL
,0
.2
5
M
)
fi
xe
d
be
d
re
ac
to
r,
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ta
ly
st
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00
m
g)
,g
as
m
ix
tu
re
of
C
O
/O
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N
2
(C
O
/O
2/
N
2:
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19
,5
0
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L·
m
in
−
1 )
,3
23
−
57
3
K
C
T
A
B
(4
m
L
of
22
m
M
),
as
co
rb
ic
ac
id
(3
00
m
L,
0.
1M
)
an
d
[R
u(
N
O
)(
N
O
) 3
]
(5
0
m
L,
1.
5
w
t
%
w
),
34
3
K
,0
.5
h,
rt
,1
2
h
R
u−
C
o 3
O
4
an
ne
al
in
g
of
R
u
T
EM
,X
R
D
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pe
ci
fi
c
su
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ac
e
ar
ea
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d
po
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lu
m
e,
T
G
A
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PS
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D
FT
C
O
ox
id
at
io
n
ca
ta
ly
st
st
ab
le
an
d
ac
tiv
e
af
te
r
30
h
of
us
e;
T
EM
an
d
X
R
D
an
al
ys
es
af
te
r
ca
ta
ly
si
s
sh
ow
in
g
no
ap
pr
ec
ia
bl
e
ch
an
ge
22
9
in
co
rp
or
at
ed
C
o-
M
O
Fs
in
N
2
(8
73
K
)
an
d
th
en
in
ai
r
(5
23
K
)
fi
xe
d-
be
d
fl
ow
re
ac
to
r;
ca
ta
ly
st
(5
0
m
g)
,
fe
ed
ga
s
(1
%
C
O
,9
9%
ai
r,
fl
ow
ra
te
30
m
L/
m
in
),
32
3
K
R
u x
C
u 1
−
x
po
ly
ol
sy
nt
he
si
s
T
EM
,X
R
D
,X
R
F,
in
si
tu
IR
,
th
er
m
al
st
ab
ili
ty
in
ve
st
ig
at
ed
by
in
si
tu
sy
nc
hr
ot
ro
n
X
R
D
m
ea
su
re
m
en
ts
C
O
ox
id
at
io
n
R
u 0
.8
C
u 0
.2
di
sp
la
ye
d
hi
gh
er
ca
ta
ly
tic
ac
tiv
ity
th
an
ot
he
r
bi
m
et
al
lic
m
ix
tu
re
s
an
d
m
on
om
et
al
lic
R
u
an
d
C
u
N
Ps
22
8
[R
u(
ac
ac
) 3
],
(3
18
.7
m
g,
0.
8
m
m
ol
)
[C
u(
O
A
c)
2·H
2O
],
(2
39
.6
m
g,
1.
2
m
m
ol
),
di
et
hy
le
ne
gl
yc
ol
(2
00
m
L)
,P
V
P
(4
40
m
g,
4
m
m
ol
),
49
3
K
tu
bu
la
r
qu
ar
tz
re
ac
to
r
w
ith
qu
ar
tz
w
oo
l,
ca
ta
ly
st
(1
50
m
g)
,g
as
m
ix
tu
re
of
C
O
/O
2/
N
2
(C
O
/O
2/
N
2:
0.
5/
0.
5/
49
m
L·
m
in
−
1 )
,4
33
K
C
u 0
.5
R
u 0
.5
po
ly
ol
sy
nt
he
si
s
T
EM
,X
R
D
,X
R
F,
in
si
tu
IR
,
th
er
m
al
st
ab
ili
ty
in
ve
st
ig
at
ed
by
in
si
tu
sy
nc
hr
ot
ro
n
X
R
D
m
ea
su
re
m
en
ts
C
O
ox
id
at
io
n
fc
c
st
ru
ct
ur
e,
al
lo
y
N
P
C
u 0
.5
R
u 0
.5
be
tt
er
ca
ta
ly
tic
pe
rf
or
m
an
ce
s
in
C
O
ox
id
at
io
n
th
at
fc
c
R
u
N
P
22
7
[R
u(
ac
ac
) 3
],
(7
96
.8
m
g,
2.
0
m
m
ol
))
[C
u(
O
A
c)
2·H
2O
],
(3
99
.4
m
g,
2.
0
m
m
ol
))
,
di
et
hy
le
ne
gl
yc
ol
(3
30
m
L)
,P
V
P
(8
80
m
g,
4
m
m
ol
),
49
3
K
tu
bu
la
r
qu
ar
tz
re
ac
to
r
w
ith
qu
ar
tz
w
oo
l,
ca
ta
ly
st
(1
50
m
g)
,g
as
m
ix
tu
re
of
C
O
/O
2/
H
e
(C
O
/O
2/
H
e:
0.
5/
0.
5/
49
m
L·
m
in
−
1 )
,4
33
K
R
u x
Pd
1−
x
po
ly
ol
sy
nt
he
si
s
T
EM
,X
R
D
,h
yd
ro
ge
n
ab
so
rp
–
tio
n
by
pr
es
su
re
−
co
m
po
si
tio
n
is
ot
he
rm
s,
SS
N
M
R
,X
PS
C
O
ox
id
at
io
n
in
cr
ea
si
ng
th
e
R
u
co
nt
en
t
ch
an
ge
s
th
e
cr
ys
ta
llo
gr
ap
hi
c
st
ru
ct
ur
e
fr
om
fc
c
to
hc
p;
R
u 0
.5
Pd
0.
5
be
st
ca
ta
ly
st
22
6
R
uC
l 3,
(2
5.
9−
23
5.
6
m
g)
K
2[
Pd
C
l 4]
,
(3
2.
6−
29
3.
8
m
g)
,t
ri
et
hy
le
ne
gl
yc
ol
(1
00
m
L)
,H
2O
(4
0
m
L)
,P
V
P
(4
44
m
g,
4
m
m
ol
),
47
3
K
tu
bu
la
r
qu
ar
tz
re
ac
to
r
w
ith
qu
ar
tz
w
oo
l,
ca
ta
ly
st
(1
50
m
g)
,g
as
m
ix
tu
re
of
C
O
/O
2/
H
e
(C
O
/O
2/
H
e:
0.
5/
0.
5/
49
m
L·
m
in
−
1 )
Chemical Reviews Review
DOI: 10.1021/acs.chemrev.9b00434
Chem. Rev. 2020, 120, 1085−1145
1110
further exposure to O2, only CO bands corresponding to the
CO adsorbed onto Ru atoms remained, indicating that
probably CO is activated on this metal. DFT calculations
suggested that the Ru atoms are responsible for the CO
activation as well and that the presence of Cu lowers the CO
adsorption energy. The decrease of the CO adsorption energy
was originated by a site exchange from Ru hollow sites to Ru
top sites.228
4.3. Fischer−Tropsch Reaction
Ru-based compounds are very active catalysts for Fischer−
Tropsch reaction, but the limitation of their use in industry
probably arises from their price even if they are active under
milder temperatures and are less sensitive to H2O in
comparison to Fe and Co based catalysts, which are greatly
exploited.230 This reaction is largely studied in gas phase, but it
can be achieved in liquid phase by using Ru NPs.231,232
Fischer−Tropsch reaction with Ru catalysts is a size233−235 and
structure236 sensitive reaction (Table 11). Fischer−Tropsch
reaction catalyzed by fcc and hcp Ru NPs was studied
experimentally and theoretically.236 The main conclusion of
the DFT study points out that fcc Ru displays some open
facets with low CO dissociation barriers, which is in contrast
with the fact that only few edges with low CO dissociation
barriers are available in hcp Ru catalyst. Experimentally,
synthesized Ru NPS with fcc structure and a size of 6.8 nm
showed a high mass specificity toward the reaction, as
predicted, and superior to hcp Ru NPs (Figure 16).
To obtain better insights of the size effect in Ru NPs-
catalyzed Fischer−Tropsch catalysis, theoretical calculations
on the electronic structure of CO adsorbed in Ru step-edge
Figure 13. Size dependence of the temperature for 50% conversion of
CO to CO2 (T50) for fcc (blue) and hcp (red) Ru NPs. Adapted with
permission from ref 13. Copyright 2013 American Chemical Society.
Figure 14. (a) FESEM and (b) TEM images of the as-prepared Co-MOF precursor. (c) TEM, (d) HRTEM, and (e) SAED images of the Ru−
Co3O4 interfacial structure. (f−i) EDS mapping images of the Ru−Co3O4 interfacial structure. Adapted with permission from ref 229. Copyright
2018 The Royal Society of Chemistry.
sites have been carried out (Figure 17).234 It has been
demonstrated that step-edge sites are more reactive toward CO
activation than flat surfaces by using theoretical Ru NPs
models of 1 and 2 nm diameter in size. The CO cleavage is
easier in step-edge sites in larger NPs; this is due to the smaller
extent of the Ru−O interaction in the η2 adsorption mode on
smaller NPs, which destabilizes the transition state for direct
CO cleavage.
Experimentally, the size effect was investigated by using Ru
NPs synthesized from RuCl3 and [Ru(acac)3], which allowed
the obtaining of Ru NPs ranging from 1.2 to 5.2 nm. Ru NPs
catalysts showed a maximum of activity around 2.3 nm for
nanoparticles between 1.2 and 3.7 nm. With a further increase
of the Ru NPs size, the conversion rate increased strongly.
Also, it was observed that the nanoparticle size affected the
selectivity; by increasing the size a decrease on the oxygenate
products, selectivity was observed.237 Later on, the study was
extended in order to understand the size effect observed235 by
combining high-energy XRD with theoretical calculations. By
using The high-energy XRD technique, the core and surface
atomic-scale structure of real Ru NPs smaller than 6 nm was
determined in good detail, allowing identification and
quantification of step-edge and terrace sites on the surface of
Ru NPs. DFT calculations confirmed that CO dissociation
proceeds easily on these surface atoms, and it has been
observed that CO hydrogenation correlates with Ru surface
atoms with coordination numbers of 10−11. In previous
studies by the same authors,238−240 stepped Ru (
1121
)
surfaces, which display low barrier for CO activation and
bind reaction intermediates strongly, were compared to Ru
(0001) dense surfaces, with a high barrier for CO activation
and a high selectivity for methane production. It was pointed
out that the sites with low barrier for CO dissociation were
responsible for the Fischer−Tropsch reaction with low
production of methane; on the other hand, the dense surfaces
were the preferred sites for CO hydrogenation to produce
methane.
Size and surface ligands effects on the Fischer−Tropsch
reaction were also investigated.98 Ru/PVP (1.3 nm size) and
Figure 15. (a) The solid-state 2H NMR spectra for PdxRu1−x nanoparticles and
2H2 gas. All of the samples were measured under 101.3 kPa of
2H2
gas at 303 K. (b) The chemical shift position of the broad absorption lines in PdxRu1−x. (c) Temperature dependence of CO conversion in
PdxRu1−x nanoparticles supported on γ-Al2O3; x = 0 (red downward focusing triangles), 0.1 (orange open squares), 0.3 (yellow open triangles), 0.5
(green solid circles), 0.7 (blue-green solid triangles), 0.9 (light-blue solid squares), and 1.0 (blue solid downward facing triangles). Inset: metal
composition dependence of T50. Adapted with permission from ref 226. Copyright 2014 American Chemical Society.
Figure 16. Reaction performance of Ru catalysts. (A) Activity of fcc
NCs (6.8 nm) and hcp NCs (6.8 and 1.9 nm) at 413 and 433 K. (B)
The Arrhenius plot and the extracted apparent FTS barriers are
indicated. The reaction was conducted at 3.0 MPa syngas (CO/H2 =
1:2 mol ratio), 0.2 mmol catalyst, 800 rpm stirring. Adapted with
permission from ref 236. Copyright 2017 American Chemical Society.
Figure 17. (a) Blyholder model for CO adsorption on Ru surface
sites. (b) Different types of terrace and step-edge sites on metal NPs
(marked in yellow) of different sizes and experimental NP size effect
on reactivity. Adapted with permission from ref 234. Copyright 2016
American Chemical Society.
Chemical Reviews Review
DOI: 10.1021/acs.chemrev.9b00434
Chem. Rev. 2020, 120, 1085−1145
1112
Ru/dppb (1.9 and 3.1 nm size) were synthesized from
[Ru(COD)(COT)] in the presence of the respective
stabilizing agents. This study did not evidence a clear effect
of the Ru NPs size on catalysis performance in terms of activity
or selectivity. Nevertheless, the dppb ligands on the surface
were shown to play a key role on the activity. Ru/PVP NPs
were not active at 393 K and slightly active at 423 K, while the
Ru/dppb NPs were active in both cases, with high selectivity
toward alkenes and alkanes (Table 10).
4.4. C−H Activation and Other Reactions
H/D (or T) exchange through C−H activation has been
achieved with Ru NPs for several compounds in mild reaction
conditions. Nitrogen,85,89,242,243 phosphorus,163,244 and sul-
fur245 containing compounds, or alkanes,163,246 have been
selectively deuterated using Ru NPs as catalysts, stabilized with
PVP, phosphines or NHC ligands, and in some cases by
supported Ru catalysts (Table 12). The first study by Chaudret
and co-workers on deuteration247 demonstrated that Ru/PVP
NPs were able to deuterate pyridines, quinolones, indoles, and
alkyl amines with D2 with high chemo- and regioselectivity;
this methodology was also successful for the enantiospecific
C−H activation/deuteration of amino acids and peptides.
Experimental evidence and theoretical calculations showed that
the labeling is governed by the coordination of the substrate to
the ruthenium surface and that the surface ligands modulate
the efficiency of the labeling procedure.
Unsupported Ru NPs have been applied as catalysts to other
reactions such as Wittig olefination,163,249 selenylation,245 or
isomerization.250 The synthetic procedure and the catalytic
reaction conditions, together with the main features of the
catalytic system, are summarized in Table 13.
4.5. Transformation of CO2
Because it is a cheap, nontoxic, abundant, renewable feedstock,
CO2 appears as an attractive building block in order to produce
fuels and value-added products that are currently issued from
nonrenewable resources (see Figure 18 for chemicals that may
be obtained from CO2),
252,253 but intensive efforts are still
required in order to develop technologies for its valorization as
a “raw material”.254
Chemical production based on CO2 is not a facile task due
to several technical challenges. It requires major scientific
breakthroughs because only highly efficient technologies can
make it economically viable while aiming at more sustainable
chemical production. The main difficulty to transform CO2
derives from its high thermodynamic stability. Large-scale CO2
transformation requires to develop very effective and selective
catalytic systems,255 which present a good balance between the
energy needed and the gain obtained (Figure 19).
Chemical transformation of CO2 has been largely inves-
tigated with homogeneous catalysts.6,257,258 Heterogeneous
(bulk) catalysts are also explored,259 with good performance
toward the formation of formic acid, methanol, and dimethyl
ether260 or methane.261 More recently, encouraging results
were achieved with metal catalysts at the nanoscale prepared
by a molecular approach, thus evidencing the relevance of this
class of materials for this catalysis.260 As it will be seen
hereafter, to our best knowledge, only a few papers describe
ruthenium catalysts based on well-defined Ru NPs or
bimetallic RuM NPs for the challenging chemical trans-
formation of CO2. Products obtained are mainly HCOOH,
CO, and CH4 but also C2+ hydrocarbons.
4.5.1. Transformation of CO2 into HCOOH. Formic acid
(FA; HCOOH) is a valuable basic chemical with different uses
(preservative agent, antibacterial, insecticide, or deicing) and
plays also a major role in synthetic chemistry (as an acid,
reductant, and precursor) for syntheses.262 Despite a relatively
small hydrogen content (4.4 wt %; 53 g·L−1 hydrogen at rt and
ambient pressure), FA also provides an alternative for chemical
energy storage, being one of the best among liquid storage and
transport media for H2.
263 If the chemical reduction of CO2 by
using hydrogen is a highly attractive route to produce FA, it
remains a significant challenge. This process is thermodynami-
cally unfavorable, due to the strong entropic contribution
(ΔG0298 = 32.9 kJ mol−1) and thus necessitates appropriate
catalysts.
Direct hydrogenation of CO2 into FA has been extensively
studied using homogeneous catalysts (mainly based on Ru, Rh,
and Ir but also on non-noble metals like Fe, Co, Ni, and Cu)
using various conditions and temperatures in the range rt to
393 K).6,257,262,264,265 Efficient complexes display electron-rich
metal centers by using electron-donating ligands and are able
to activate H2 under the form of hydrides and to transfer these
hydrides to CO2 for some of them under mild conditions, but
despite excellent catalytic performances (both in terms of
activity and selectivity) and heterogenization (mainly on silica-
and polymer-based materials or porous organic polymers) to
Table 10. Fischer−Tropsch Activitiesa and Selectivitiesb of Ru NPs as a Function of the Stabilizer, Size, and Reaction
Temperaturec
aActivity evaluated from the consumption of H2. TOFs normalized per number of Ru surface atoms.
bSelectivity calculated only for methane,
alkanes, and alkenes as products (water and remaining H2 and CO omitted for the sake of clarity).
cAdapted with permission from ref 98.
Copyright 2014 American Chemical Society.
Chemical Reviews Review
DOI: 10.1021/acs.chemrev.9b00434
Chem. Rev. 2020, 120, 1085−1145
1113
T
ab
le
11
.
R
u
N
P
s
as
Fi
sc
he
r−
T
ro
ps
ch
C
at
al
ys
ts
st
ab
ili
zi
ng
ag
en
t
m
et
ho
do
lo
gy
ch
ar
ac
te
ri
za
tio
n
re
ac
tio
n
co
nd
iti
on
s
co
m
m
en
ts
re
f
ol
ei
c
ac
id
th
er
m
al
de
co
m
po
si
tio
n
(5
08
K
)
of
[C
o 2
(C
O
) 8
]
an
d
[R
u 3
(C
O
) 1
2]
in
di
ph
en
yl
et
he
r
T
EM
,A
P-
X
PS
,X
A
S
(u
nd
er
ox
id
iz
in
g,
re
–
du
ci
ng
,a
nd
re
ac
tiv
e
ga
s
en
vi
ro
nm
en
ts
)
ca
ta
ly
st
(7
0
m
g)
fe
ed
ga
s
m
ix
tu
re
of
H
2/
C
O
/A
r
(2
0
ba
r,
H
2/
C
O
/A
r:
2/
1/
0.
08
)
sy
nt
he
si
s
of
a
va
ri
et
y
of
C
o−
M
bi
m
et
al
lic
ca
ta
ly
st
s;
sl
ig
ht
di
ff
er
en
ce
s
to
th
at
of
pu
re
C
o
24
1
PV
P
hy
dr
ot
he
rm
al
sy
nt
he
si
s
K
2P
tC
l 4
(0
.0
24
m
m
ol
),
R
uC
l 3·
xH
2O
(0
.2
16
m
m
ol
),
PV
P
(1
00
m
g)
,H
C
H
O
(0
.1
m
L)
,H
C
l
(0
.0
62
m
L,
1M
),
H
2O
(1
5
m
L)
,f
or
m
al
de
hy
de
(0
.1
m
L,
40
w
t
%
),
43
3
K
,8
h
T
EM
,I
C
P,
X
R
D
,
X
A
N
ES
,E
X
A
FS
ca
ta
ly
st
(0
.2
m
m
ol
),
sy
n-
ga
s
(C
O
:H
2
=
1:
2
30
ba
r)
,4
23
K
R
u
fc
c
hi
gh
er
ac
tiv
ity
in
FT
S
hi
gh
er
se
le
ct
iv
ity
to
w
ar
d
C
5+
co
m
po
un
ds
th
an
hc
p
N
P;
re
cy
cl
in
g
ex
pe
ri
m
en
ts
at
42
3
K
sh
ow
sl
ig
ht
ly
de
cr
ea
se
of
ac
tiv
ity
in
fi
rs
t
ru
ns
an
d
re
m
ai
ne
d
co
nt
an
t
af
te
r
10
cy
cl
es
;
D
FT
ca
lc
ul
at
io
n
po
in
ts
ou
t
th
at
C
O
di
ss
oc
ia
tio
n
is
m
or
e
fa
vo
ra
bl
e
is
fc
c
R
u
N
P
23
6
PV
P
re
du
ct
io
n
of
[R
u(
ac
ac
) 3
]
in
1,
4-
bu
ta
ne
di
ol
T
EM
,I
C
P,
IR
,E
X
A
FS
R
u
(5
0
μm
ol
),
H
2O
(3
m
L)
,C
O
/H
2
(3
0
ba
r,
H
2/
C
O
=
2)
,
40
3−
50
3
K
,3
−
24
h
R
u
N
Ps
ra
ng
in
g
fr
om
1.
2
to
5.
2
nm
;
se
le
ct
iv
ity
an
d
ac
tiv
ity
de
pe
nd
on
R
u
N
Ps
si
ze
23
5,
23
7
[R
u(
ac
ac
) 3
]
(3
0
m
g)
,P
V
P
(1
70
m
g)
,T
H
F
(2
m
L)
,1
,4
–
bu
ta
ne
di
ol
(3
0
m
L)
,(
25
−
50
0
m
L)
,4
98
K
,2
h
re
du
ct
io
n
of
R
uC
l 3
w
ith
H
2
R
uC
l 3
(4
0
m
g)
,P
V
P
(2
20
m
g)
,H
2O
(1
m
L)
,H
2
(2
0
ba
r)
,
42
3
K
,2
h
PV
P; dp
pb
re
du
ct
io
n
of
[R
u(
C
O
D
)(
C
O
T
)]
w
ith
H
2
T
EM
,S
SN
M
R
,i
n
si
tu
am
bi
en
t-
pr
es
su
re
X
PS
re
ac
tio
n
do
ne
on
a
qu
ic
k
pr
es
su
re
va
lv
e
N
M
R
tu
be
no
si
ze
eff
ec
t;
lig
an
d
eff
ec
t
on
th
e
ac
tiv
ity
of
th
e
re
ac
tio
n
98
[R
u(
C
O
D
)(
C
O
T
)]
,P
V
P
or
dp
pb
,H
2
(3
ba
r)
,2
98
K
R
u
(0
.0
2−
0.
05
m
m
ol
R
u)
,
13
C
O
/H
2
(3
ba
r,
13
C
O
/
H
2
1/
1)
,3
93
−
42
3
K
1−
5
da
ys
Chemical Reviews Review
DOI: 10.1021/acs.chemrev.9b00434
Chem. Rev. 2020, 120, 1085−1145
1114
T
ab
le
12
.
R
u
N
P
s
as
C
−
H
A
ct
iv
at
io
n
C
at
al
ys
ts
fo
r
La
be
lli
ng
A
pp
lic
at
io
ns
st
ab
ili
zi
ng
ag
en
t
sy
nt
he
tic
m
et
ho
do
lo
gy
ch
ar
ac
te
ri
za
tio
n
ca
ta
ly
tic
re
ac
tio
n
co
nd
iti
on
s
co
m
m
en
ts
re
f
R
u/
dp
pb
,
R
uP
t/
dp
pb
,
Pt
/d
pp
b
re
du
ct
io
n
of
[R
u(
C
O
D
)(
C
O
T
)]
;
[P
t(
C
H
3)
2(
C
O
D
)]
;
[P
t(
db
a)
2]
w
ith
H
2
de
ut
er
at
io
n
of
al
ka
ne
s
D
2
(6
ba
r)
,3
33
K
,
24
h
is
ot
op
e
ex
ch
an
ge
an
d
P−
C
bo
nd
cl
ea
va
ge
94
PV
P;
N
H
C
re
du
ct
io
n
of
[R
u(
C
O
D
)(
C
O
T
)]
w
ith
H
2
de
ut
er
iu
m
an
d
tr
iti
um
la
be
lin
g
of
pu
ri
ne
de
ri
va
tiv
es
an
d
ph
ar
m
ac
eu
tic
al
s
D
2
(2
ba
r)
,3
28
−
35
3
K
,3
6
h
hy
dr
og
en
-is
ot
op
e
la
be
lin
g
of
nu
cl
eo
ba
se
de
ri
va
tiv
es
in
m
ild
co
nd
iti
on
s;
br
oa
d
sc
op
e;
m
od
ifi
ca
tio
n
of
th
e
su
rf
ac
e
st
ab
ili
ze
r
co
ul
d
in
cr
ea
se
th
e
effi
ci
en
cy
of
th
e
la
be
lin
g
24
2
[R
u(
C
O
D
)(
C
O
T
)]
,P
V
P
or
N
H
C
,H
2
(3
ba
r)
,T
H
F,
29
8
K
dp
pb
re
du
ct
io
n
of
[R
u(
C
O
D
)(
C
O
T
)]
w
ith
H
2
de
ut
er
at
io
n
of
al
ka
ne
s
R
u/
dp
pb
,s
ub
st
ra
te
(1
m
L)
,T
H
F
(1
m
L)
,D
2
(6
ba
r)
,3
33
K
,
24
h
C
−
H
ac
tiv
at
io
n
of
al
ka
ne
s
w
as
st
ru
ct
ur
e
de
pe
nd
en
t;
on
ly
cy
cl
op
en
ta
ne
w
as
sm
oo
th
ly
de
ut
er
at
ed
24
4
[R
u(
C
O
D
)(
C
O
T
)]
,d
pp
b,
H
2
(3
ba
r)
,T
H
F,
29
8
K
R
u/
C
co
m
m
er
ci
al
ca
ta
ly
st
de
ut
er
iu
m
an
d
tr
iti
um
la
be
lin
g
of
th
io
et
he
r
su
bs
tr
uc
tu
re
s
in
co
m
pl
ex
m
ol
ec
ul
es
C
(s
p3
)−
H
ac
tiv
at
io
n
di
re
ct
ed
24
7
R
u/
C
(5
w
t
%
,1
21
.2
m
g,
30
m
ol
%
),
su
bs
tr
at
e
(0
.2
m
m
ol
),
D
2
(2
ba
r)
,s
ol
ve
nt
(2
m
L)
,3
33
K
,2
or
72
h
by
a
su
lfu
r
at
om
;
la
be
lin
g
of
co
m
pl
ex
st
ru
ct
ur
es
in
m
ild
co
nd
iti
on
s
su
lfo
na
te
d
N
H
C
re
du
ct
io
n
of
[R
u(
C
O
D
)(
C
O
T
)]
w
ith
H
2
T
EM
,W
A
X
S,
IR
,T
G
A
,
N
M
R
de
ut
er
at
io
n
of
L-
ly
si
ne
en
an
tio
sp
ec
ifi
c
H
/D
ex
ch
an
ge
of
th
e
am
in
o
ac
id
L-
ly
si
ne
;
in
fl
ue
nc
e
of
pH
on
th
e
ac
tiv
ity
an
d
se
le
ct
iv
ity
:
lo
w
pH
H
/D
is
re
du
ce
d
or
ne
gl
ig
ib
le
;
hi
gh
pH
in
cr
ea
se
s
ac
tiv
ity
an
d
ch
an
ge
s
se
le
ct
iv
ity
89
[R
u(
C
O
D
)(
C
O
T
)]
(2
50
m
g,
0.
8
m
m
ol
),
su
lfo
na
te
d
N
H
C
(0
.2
eq
ui
v)
,K
O
tB
u
(1
9.
7
m
g,
0.
17
6
m
m
ol
,0
.2
2
eq
ui
v)
,H
2
(3
ba
r)
,
T
H
F
(3
0
m
L)
,2
98
K
,2
0
h
[R
u(
C
O
D
)(
C
O
T
)]
,P
V
P,
H
2
(3
ba
r)
,T
H
F,
29
8
K
ca
ta
ly
st
(2
m
g,
8%
),
L-
ly
si
ne
(2
1.
92
m
g,
0.
15
m
m
ol
),
D
2
(2
ba
r)
,D
2O
(2
m
L)
,
32
8
K
,4
2
h
PV
P
re
du
ct
io
n
of
[R
u(
C
O
D
)(
C
O
T
)]
w
ith
H
2
de
ut
er
at
io
n
of
ch
ir
al
am
in
es
de
ut
er
iu
m
in
co
rp
or
at
io
n
at
st
er
eo
ge
ni
c
ce
nt
er
s;
hi
gh
se
le
ct
iv
ity
to
w
ar
d
he
te
ro
at
om
α
-p
os
iti
on
;m
ec
ha
ni
st
ic
st
ud
ie
s
su
gg
es
t
th
at
a
di
m
et
al
la
cy
cl
e
is
th
e
ke
y
in
te
rm
ed
ia
te
85
[R
u(
C
O
D
)(
C
O
T
)]
,P
V
P,
H
2
(3
ba
r)
,T
H
F,
29
8
K
ca
ta
ly
st
(8
m
g,
3.
3%
),
su
bs
tr
at
e
(0
.1
5
m
m
ol
),
D
2
(2
ba
r)
,T
H
F
or
D
2O
(2
m
L)
,3
28
K
,3
6
h
PV
P
re
du
ct
io
n
of
[R
u(
C
O
D
)(
C
O
T
)]
w
ith
H
2
de
ut
er
at
io
n
of
ph
os
ph
in
e,
ph
os
ph
in
e
ox
id
e
an
d
ph
os
ph
ite
ph
en
yl
ri
ng
s
in
ph
en
yl
-o
r
ph
en
yl
-a
lk
yl
ph
os
ph
in
es
ar
e
se
le
ct
iv
el
y
de
ut
er
at
ed
at
th
e
or
th
o
po
si
tio
n;
in
di
ca
tio
n
of
lig
an
d
co
or
di
na
tio
n
tr
ho
ug
th
th
e
P
at
om
;
no
de
ut
er
at
io
n
of
tr
ip
he
ny
lp
ho
sp
hi
te
24
8
[R
u(
C
O
D
)(
C
O
T
)]
,P
V
P,
H
2
(3
ba
r)
,T
H
F,
29
8
K
ca
ta
ly
st
(8
m
g,
3.
3%
),
su
bs
tr
at
e
(0
.1
5
m
m
ol
),
D
2
(2
ba
r)
,T
H
F
(1
m
L)
PV
P
re
du
ct
io
n
of
[R
u(
C
O
D
)(
C
O
T
)]
w
ith
H
2
de
ut
er
at
io
n
of
az
a
co
m
po
un
ds
m
ild
re
ac
tio
n
co
nd
iti
on
s;
go
od
la
be
lin
g
yi
el
ds
w
ith
hi
gh
ch
em
o-
an
d
re
gi
os
el
ec
tiv
iti
es
24
3
[R
u(
C
O
D
)(
C
O
T
)]
,P
V
P,
H
2
(3
ba
r)
,T
H
F,
29
8
K
ca
ta
ly
st
(3
%
),
D
2
(1
or
2
ba
r)
,T
H
F,
rt
or
32
8
K
,3
6
h
Chemical Reviews Review
DOI: 10.1021/acs.chemrev.9b00434
Chem. Rev. 2020, 120, 1085−1145
1115
T
ab
le
13
.
R
u
N
P
s
as
C
at
al
ys
ts
in
O
th
er
R
ea
ct
io
ns
st
ab
ili
zi
ng
ag
en
t
m
et
ho
do
lo
gy
ch
ar
ac
te
ri
za
tio
n
re
ac
tio
n
co
nd
iti
on
s
co
m
m
en
ts
re
f
PV
P
hy
dr
ot
he
rm
al
sy
nt
he
si
s
T
EM
,I
C
P,
X
PS
,E
X
A
FS
,
EP
R
ae
ro
bi
c
cr
os
s-
de
hy
dr
og
en
at
iv
e
co
up
lin
g
(C
−
H
)
ac
tiv
a-
tio
n
R
u
na
no
ca
ta
ly
st
w
ith
a
di
ff
er
en
t
ox
id
at
io
n
le
ve
l;
lo
ss
of
ac
tiv
ity
af
te
r
6
ca
ta
ly
tic
cy
cl
es
24
6
R
uC
l 3·
xH
2O
(0
.2
4
m
m
ol
),
PV
P
(1
00
m
g)
,
N
a 2
C
3H
2O
4·H
2O
(8
0
m
g)
,H
C
l(
0.
06
2
m
L,
1M
),
H
2O
(2
5
m
L)
,f
or
m
al
de
hy
de
(0
.1
m
L,
40
w
t
%
),
43
3
K
,8
h,
1
h,
or
24
h
ca
ta
ly
st
(8
m
ol
%
R
u)
,t
et
ra
hy
dr
oi
so
qu
in
ol
in
e
de
ri
va
–
tiv
es
(0
.1
m
m
ol
),
in
do
le
s
(4
eq
ui
v)
,H
2O
/M
eO
H
(1
/1
),
A
cO
H
(1
0−
48
m
L)
,2
98
K
IL
re
du
ct
io
n
of
se
ve
ra
l
R
u
co
m
pl
ex
es
w
ith
H
2;
R
u
co
m
pl
ex
(1
.1
6
w
t
%
R
u)
,I
L
(0
.8
5
m
L)
,H
2
(4
ba
r)
,3
h,
32
3
K
T
EM
,I
C
P,
X
R
D
,X
PS
W
itt
ig
ol
efi
na
tio
n
go
od
yi
el
ds
in
st
ilb
en
e
pr
od
uc
ts
,b
ut
lo
w
E/
Z
se
le
ct
iv
ity
;
[R
uC
l 2(
C
6H
6)
] 2
pr
ec
ur
so
r
pr
od
uc
ed
th
e
m
os
t
ac
tiv
e
ca
ta
ly
st
;
re
cy
cl
ed
5
tim
es
w
ith
ou
t
ap
pr
ec
ia
bl
e
lo
ss
of
ac
tiv
ity
24
9
ca
ta
ly
st
(2
50
m
g)
,a
lc
oh
ol
(0
.1
m
ol
),
ph
os
ph
or
us
yl
id
e
(0
.1
1
m
ol
),
w
at
er
(5
m
L)
,1
h,
34
3
K
m
on
tm
or
ill
on
ite
cl
ay
re
du
ct
io
n
of
[R
u(
N
H
3)
6]
C
l 3
w
ith
N
aB
H
4
T
EM
,S
A
X
S,
IC
P,
B
ET
W
itt
ig
-t
yp
e
re
ac
tio
n
of
be
nz
yl
al
co
ho
ls
an
d
ph
os
ph
or
us
yl
id
es
m
od
er
at
e
yi
el
d
an
d
lo
w
di
as
te
re
os
el
ec
tiv
ity
;
no
re
cy
cl
in
g
te
st
;
no
ch
ar
ac
te
ri
za
tio
n
of
th
e
sp
en
t
ca
ta
ly
st
16
3
[R
u(
N
H
3)
6]
C
l 3,
m
on
tm
or
ill
on
ite
cl
ay
,N
aB
H
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solve separation and recovery concerns, homogeneous catalysts
are still far from the industrial expectation.260,266
In the opposite, despite the early works involving Pd
black267 and Ni-Raney268 and their advantages for continuous
operation and product separation, the development of
heterogeneous catalysts for this reaction lags signifi-
cantly,260,266 but presently, the number of supported nano-
particulate metal catalysts tends to increase, mainly based on
Pd or Au.253 Very surprisingly, only a few examples of Ru-
based heterogeneous catalysts or nanocatalysts are reported
although Ru complexes (including heterogenized and isolated
single-atomic systems269) are known to be efficient for the
synthesis of FA.270 If low to moderate catalytic performances
are observed in comparison to the TON or TOF values
achieved by ruthenium molecular catalysts encouraging results
are reported, as it will be described hereafter.
An interesting bridge between homogeneous Ru catalysts
and nanocatalysts has been made by Dupont and co-workers
who reported excellent results in the hydrogenation of CO2
using a ruthenium cluster. It is worth to mention that
“nanocluster” is usually used for metal NPs that are very small
and well-controlled. They studied the behavior of
[Ru3(CO)12] dispersed in ionic liquids (ILs).
271 They
observed remarkable activity and selectivity for the formation
of HCOOH with high TON (17000) and TOF values at mild
pressure (total pressure 40 bar; H2/CO2 = 1/1) and
temperature (333 K). Among the ILs tested, they observed
that the imidazolium-based IL associated with the acetate
anion acts as a precursor for the formation of the catalytically
active Ru−H species, as a catalyst stabilizer, and as an acid
buffer, shifting the equilibrium toward free formic acid.
Moreover, the immobilization of this catalytic system onto a
solid support facilitated the separation of FA. What is
important to note here is the multiple role of the IL that
enhances the catalytic activity of the [Ru3(CO)12] cluster.
Second, even if it contains only three ruthenium atoms, the
catalytic performance of this Ru cluster strongly encourages
studying of more Ru NPs because higher activity can be
expected due to the multiple active sites they expose.
As a first example of Ru NPs, Kojima and co-workers
reported on the use of metallic RuNPs (primary particles of ca.
3−5 nm and agregates of ca. 200−240 nm) prepared by
reduction of RuCl3 in a methyl alcohol solution under
solvothermal conditions for the hydrogenation of supercritical
CO2 to formic acid in the presence of triethylamine as a base
(total pressure 13 MPa ; H2/CO2 = 5/8 ; T = 353 K).
272 The
activity was drastically improved by using a prereduction
procedure and adding an appropriate quantity of water to the
colloidal suspension in methyl alcohol. The most active
nanocatalyst was obtained with 4 mL of water, providing a
TON (expressed as the number of moles of FA produced per
mole of Ru) of 6351 in 3 h. When adding PPh3, a negligible
activity was observed, indicating the presence of a negligible
amount of Ru ions in solution and discarding the role of
molecular species in the catalytic act. Describing the first
performance of pure ruthenium colloidal catalyst, this work
opened the door toward the use of solution Ru NPs for the
hydrogenation of CO2.
Srivastava and co-workers published a comparative study on
the reactivity of nanocatalysts made of Ru NPs (ca. 6−22 nm
from TEM analysis depending on the Ru loading in the range
1−6 wt %) dispersed onto TiO2 as a support for the
hydrogenation of CO2 to FA in the presence or not of an ionic
liquid (IL).273 Ru-TiO2 nanocatalysts were prepared by a
microemulsion protocol from a suspension of TiO2 (ca. 30
nm) and a suspension of RuCl3 and citric acid followed by a
Figure 18. Potential chemicals from CO2 transformation. Reproduced with permission from ref 253. Copyright 2018 Elsevier.
Figure 19. Reaction pathways for the CO2 hydrogenation.
Reproduced with permission from ref 256. Copyright 2018 American
Chemical Society.
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reduction treatment of the final solids at 573 K for 2 h. The
effects of pressure (total pressure of 30−60 bar with H2/CO2 =
1/1), temperature (313−353 K), reaction time, and presence
of water in the absence of IL were first studied. This allowed to
determine the best Ru-TiO2 nanocatalyst to be that with the
smallest size of Ru NPs (ca. 6.0 nm as determined by TEM for
a Ru loading of 3 wt %) with a TOF (expressed as the number
of moles of FA produced per mole of Ru per hour) of ca. 28
h−1 at 353 K and a total pressure of 40 bar (H2/CO2 = 1/1).
Then, the influence of the addition of an IL on the catalytic
conversion was studied from the most promising Ru-TiO2
system just cited (Figure 20). ILs are known to absorb gases
and can be expected to improve catalysis involving gaseous
reactants.274 Catalytic experiments were performed in 1,3-
di(N,N-dimethylaminoethyl)-2-methylimidazolium bis-
( t r i f u o r o m e t h y l s u l f o n y l ) i m i d e ( [ D AM I ] –
[CF3CF2CF2CF2SO3]) at different pressures, temperatures,
and water contents. TOF values up to ca. 47 h−1 evidenced the
IL positive effect on the CO2 hydrogenation into FA.
Recyclability studies led to a slight loss of catalytic activity
after 10 runs attributed to a Ru leaching into the product phase
(ICP analysis of the filtrates). Thus, the use of an IL was
clearly beneficial to the catalytic transformation of CO2 into
FA by small Ru NPs deposited onto TiO2, this being attributed
to the fact IL can act as both as a solvent for the reaction and
enabled to capture CO2. But, ILs are also known to be suitable
media to stabilize Ru NPs,83,129 being excellent alternatives to
surfactants or solid supports. Thus, the IL probably increased
the stability of the Ru NPs while favoring exchange at the
metal surface.
Then, the same group reported data on the solubility of CO2
into various ILs.275 The previously cited IL, [DAMI]-
[CF3CF2CF2CF2SO3], provided the best solubility thus
confirming the high potential of this compound. For
comparison purpose, three other ILs ([DAMI][TfO] where
T fO = t r i f uo rome th ane su l f on a t e ; [mammim] –
[CF3CF2CF2CF2SO3] with mammim = 1-(N,N-dimethylami-
noethyl)-2,3-dimethylimidazolium and [DAMI][TfO]) were
employed to prepare nanocatalysts from four different
ruthenium precursors ([RuCl2(C6H6)]2, [Ru(COD)(2-meth-
ylallyl)2], [trans-RuCl2(DMSO)4], [Ru(COD)Cl2], [Ru-
(COD)(COT)]) by decomposing them under H2 (5 bar) at
323 K, which led to Ru NPs in a size range of 7−14 nm. XPS
data (from samples introduced under argon atmosphere)
evidenced no RuO2 contamination. Small-angle-X-ray scatter-
ing (SAXS) and TEM data revealed that ionic interaction
between cations and anions of the ILs plays an important role
in the structural features of Ru NPs (stability, size, dispersion,
a n d a g g l ome r a t i o n ) . L e s s c o o r d i n a t i n g i o n s
[CF3CF2CF2CF2SO3
−] prevent the separation of Ru NPs
from IL better than [TfO−], and this effect was dropped while
lowering the carbon chain ([mammim][CF3CF2CF2CF2SO3]).
These IL-immobilized Ru NPs were then investigated in CO2
hydrogenation in different reaction conditions (temperature:
303−373 K; CO2/H2 total pressure: 20−50 bar, absence or
presence of water, nature of IL, etc.). Although their results are
not very clear, the authors claimed that the highest activity was
observed with the Ru NPs immobilized into [DAMI][TfO].
They also claimed higher catalytic efficiency when using in situ
formed [DAMI][TfO]-Ru NPs with TOF up to 3300 h−1 of
FA obtained at 323 K and 50 bar in 8 h. Finally a slow decrease
in stability was observed after successive recycling.
Dupont and co-workers reported on the selective hydro-
genation of CO2 either to FA or to hydrocarbons catalyzed by
a colloidal catalytic system prepared by a single-step organo-
metallic approach (hydrogen codecomposition of [Fe(CO)5]
Figure 20. TEM images of the Ru-TiO2 nanocatalysts for different Ru contents, catalytic scheme, and recycling studies. Reproduced with
permission from ref 273. Copyright 2016 Royal Society of Chemistry.
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and [Ru(COD)(2-methylallyl)2] into small RuFe NPs (ca. 1.7
nm) dispersed in ILs (1-butyl-3-methyl-1H-imidazol-3-ium
acetate, BMi·OAc, or 1-butyl-3-methyl-1H-imidazol-3-ium
bis((trifluoromethyl)sulfonyl)amide, BMi·NTf2) under mild
reaction conditions (DMSO/H2O; 333 K; 30 bar H2/CO2 =
2/1). The selectivity was observed to depend on the nature of
the IL anion (Figure 21).256
FA was more produced with ILs containing basic anions
(BMi·OAc) with a TOF value of 23.5 h−1, whereas heavy
hydrocarbons (up to C21) were more produced with nonbasic
anions (BMi·NTf2). The composition of the metal alloy and
the basicity/hydrophobicity of the IL ion pair (mainly imposed
by the anion) appeared to be the key points for the selective
transformation of CO2. First, the IL forms a cage around the
NPs that controls the diffusion/residence time of the
substrates, intermediates, and products. Second, compared to
Ru and Fe monometallic NPs, the presence of Fe in RuFe NPs
showed a dual effect: a positive metal dilution effect toward the
formation of FA through the formation of bicarbonate species
(Figure 22, route (I)) and a synergetic one for the formation of
hydrocarbons through the conversion of CO2 to CO followed
by chain propagation via FTS pathway (Figure 22, route (II)).
This work clearly evidences that the precise design of a
nanocatalyst (here a combination between metal alloy as active
phase and IL as stabilizer) can lead to chemoselectivity in CO2
hydrogenation. Not only the ILs act as stabilizers for the NPs,
but also their chemical properties lead to a different interface
between the metallic phase, the reactants, the intermediates,
and products that orient the catalytic selectivity.
4.5.2. Transformation of CO2 into CO, CH4, or C2+
Hydrocarbons. Catalytic transformation of CO2 into hydro-
carbons (like methane and superior alkanes (C2+) or carbon
monoxide) is a very attractive alternative to fossil fuels. The
hydrogenation of CO2 to methane (CO2 + 4 H2 → CH4 +
H2O; −114 kJ mol −1) is well-known as CO2 methanation
reaction or Sabatier’s process. This reaction is usually
performed at temperature 423−773 K and pressure 1−100
bar.264 Methane is more advantageous because it can be
injected directly into already existing natural gas pipelines and
it can be used as a fuel or raw material for the production of
other chemicals. In addition, CO2 methanation is a more
simple reaction which can generate CH4 under atmospheric
pressure (production of methanol and dimethyl ether from
CO2 requires high pressures ∼5 MPa and conversion is low in
the case of MeOH). Thus, the thermochemical conversion of
CO2 to CH4 at low temperature has become an important
breakthrough in the use of CO2 despite a low conversion. CO2
methanation remains an advantageous reaction with respect to
thermodynamics because it is faster than reactions leading to
hydrocarbons or alcohols.
Both homogeneous and heterogeneous catalysts have been
investigated to hydrogenate CO2 to methane.
259 In heteroge-
Figure 21. (left) Schematic representation of the chemoselectivity observed in CO2 hydrogenation depending on the nature of the IL. (right) (a,b)
TEM image of RuFe NPs and size distibution, (c) EDS map, overlay of Ru-L and Fe-K of RuFe NPs in BMI-NTf2. Adapted with permission from
ref 256. Copyright 2018 American Chemical Society.
Figure 22. Representation of mechanistic route for the chemoselective hydrogenation of CO2 by RuFe NPs in ILs. Reproduced with permission
from ref 256. Copyright 2018 American Chemical Society.
neous conditions, metals such as Ru, Rh, Ni, Co, Fe, and so
forth on various supports are recognized to be effective
catalysts for this reaction. Noble metals proved to be efficient
catalysts as the result of their high ability to dissociate H2, a
required step in CO2 methanation. Note that for most catalysts
in use, CO2 methanation is considered to be a linear
combination of the reverse water−gas shift reaction (rWGS ;
CO2 + H2 → CO + H2O), after which CO can lead to
hydrocarbons via FTs pathways and the direct hydrogenation
of CO2 into methane (CO2 + 3 H2 → CH4 + H2O). Given
that, the choice of the catalyst is essential to get high
conversion and selectivity, both varying with the active metal
species, support, promoters, and synthesis strategies. For the
most significant catalysts, the trends of activity and selectivity
can be summarized as follows: activity, Ru > Fe > Ni > Co >
Mo; selectivity Ni > Co > Fe > Ru. Ruthenium is renowned as
being the most active metal for the methanation of both CO
and CO2 and to be quite stable when operating in a wide
temperature range. However, Ru is less selective while being
more costly in comparison to non-noble metals.264 The
catalytic activity can be greatly promoted at the metal/support
interface due to synergistic interactions, which can tune the
reaction mechanism and in turn the selectivity of CO2
hydrogenation. Thus, when deposited onto oxide supports
(such as MgO, SiO2, TiO2, Al2O3, ZrO2, and CeO2), particles
of Ni or Ru were reported to promote the formation of
CH4.
276 CO2 methanation via better defined heterogeneous
Ru-based catalysts received more attention in recent
years.277−279 The main objective is to obtain the best catalytic
performance in terms of stability, selectivity, CO2 conversion,
and CH4 production, especially aimed at mild reaction
conditions (i.e., low reaction temperature). In these works,
the structure−performance relationships appeared to be a key
for the development of highly performant catalysts.
A relevant example by Zeng and co-workers280 provides an
elegant alternative to pure heterogeneous catalysts, by
combining a solution synthesis approach and a sol−gel
approach in order to get a nanomaterial of Ru into a silica
matrix. Selective hydrogenation of CO2 into CO was catalyzed
by small Ru NPs (ca. 1−3 nm) encapsulated into silica
nanowires (denoted as Ru/mSiO2).
239 Combining colloidal
and heterogeneous approaches made this catalytic system
closer to a nanocatalyst than to pure heterogeneous ones given
the presence of better controlled Ru NPs. As shown in Figure
23, a colloidal suspension of Ru NPs was first prepared by
following a polyol-assisted method (decomposition of RuCl3
into ethylene glycol at 353 K in the presence of NaOH), and
then silica was grown around the Ru NPs by hydrolysis/
condensation of TEOS (tetraethylorthosilicate) using ethylene
glycol as a solvent instead of usual ethanol and an organic
template (hexadecyltrimethylammonium chloride; CTACl). A
calcination step at 573 K allowed elimination of the organic
template. Calcination in N2 led to SiO2-encapsulated Ru NPs
of almost unchanged size (1−3 nm depending on the Ru
content introduced), while calcination in air conditions led to
Ru NPs of larger sizes (5−30 nm) due to sintering.
Comparatives studies in flow conditions inside a fix bed
reactor (temperature: 473−673 K; 25 mL·min−1 of H2/CO2 at
ratio 4:1) revealed a selective transformation of CO2 into CO
with Ru/mSiO2 calcined in inert conditions and that contained
small Ru NPs while the catalyst obtained in air condition and
displaying large Ru NPs led preferentially to CH4. Fine surface
studies (including temperature-programmed reduction (TPR)
and temperature-programmed desorption (TPD), XPS, and in
situ diffuse reflectance infrared Fourier transform spectroscopy
(DRIFTS)), performed on the two catalysts (1−3 nm Ru/
mSiO2 and 5−20 nm Ru/mSiO2) after adsorption of H2 and
CO2 revealed the formation of different reaction intermediates
on catalyst surface: CO-Run+ on 1−3 nm Ru/mSiO2 and
formate species on 5−20 nm Ru/mSiO2, thus explaining the
different selectivity observed as the result of different reaction
pathways. The high selectivity of CO over CH4 is attributed to
low affinity and hence coverage of atomic hydrogen on the
surface of the 1−3 nm Ru NPs. DRIFTS, TPR, and TPD
experiments supported a surface redox mechanism for CO2
hydrogenation on 1−3 nm Ru/mSiO2, where carbonyl species
formed by dissociative adsorption of CO2 and desorbed
directly to generate CO. A formate route is established for 5−
20 nm Ru/mSiO2 catalysts, where adsorbed atomic hydrogen
associates with adsorbed CO2 to form formate species, which
are further hydrogenated to CH4 with sufficient supply of
surface hydrogen atoms due to the large metal surface. In
addition, 1−3 nm Ru/mSiO2 nanocatalyst demonstrated to be
stable in terms of activity and selectivity in extended reaction
time up to 50 h. This work provides an elegant way to maintain
the advantage of small-sized Ru NPs while having them
encapsulated into the pores of a silica support for a selective
catalytic transformation of CO2 into CO.
Another relevant example by Chaudret and co-workers
describes the use of nickel-coated iron carbide nanoparticles
Figure 23. Schematic representation of the synthesis and TEM/
HREM images of Ru/mSiO2nanocatalysts for selective reduction of
CO2 to either CO (top) or CH4 (bottom). Adapted with permission
from ref 280. Copyright 2017 Elsevier.
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1120
(ICNPs) prepared by the organometallic approach for the
catalytic transformation of CO2 into CO and CH4 in a
continuous-flow reactor under atmospheric pressure.281
Interestingly, with this ICNP-based catalytic system, the
heating arises from the magnetic properties of the iron cores
that are induced after applying a magnetic field. This catalytic
system was optimized by deposition onto an inorganic support
previously impregnated with 1 wt % Ru (also from an
organometallic precursor). CO2 methanation with total
selectivity and 93% yield was achieved in a model flow reactor.
The presence of small Ru NPs in the alumina support (1 wt %)
greatly enhanced the catalytic performance of the system and
allowed a highly efficient conversion of CO2 to CH4 in
continuous flow (Figure 24). If not a pure Ru catalytic system,
however, this work has the merit to show the synergy afforded
by the proximity of Ru NPs onto the catalytic performance of a
Ni-based nanocatalyst.
Apart from these supported catalysts, Ru NPs dispersed into
ILs also allowed the formation of CO, CH4, or C2x. A
previously cited work by Dupont and co-workers,256 described
the influence of the nature of the IL used as a stabilizer on the
catalytic properties of bimetallic RuFe NPs during hydro-
genation of CO2, more precisely on the selectivty (HCOOH vs
C2+). In a very recent paper, the same group reported on the
conversion of CO2 into CO or light hydrocarbons (C2−C6)
under very mild conditions (H2/CO2 = 4:1, 8.5 bar, 423 K) by
using bimetallic RuNi NPs deposited into ionic liquids.241 This
nanocatalyst was easily prepared by codecomposition of
Figure 24.Magnetically induced Sabatier reaction in continuous-flow reactor using ICNPs-RuSiRAlOx catalyst (ratio H2/CO2 = 4/1, 25 mL min@
1, 18.3 Lh@1 g(Fe+Ru)@1, or 214.3 L h@1 gRu@1, residence time t = 0.00067 h, P atm). (a) Schematic representation of the reactor, (b) TEM
of ICNPs and Ru NPs supported on a SiRAlOx particle, (c) Zoom on small Ru NPs, scale bar = 100 nm, (d) schematic representation of the
catalytic system, (e) gas chromatogram obtained for m0Hrms = 28 mT, and (f) catalytic results as a function of m0Hrms. Because the selectivity is
total, X (CO2) and Y (CH4) are overlapping. Reproduced with permission from ref 281. Copyright 2016 Wiley.
Table 14. Catalytic Systems for the Hydrogenation of CO2 to Hydrocarbons in ILs
a
aReactions conditions: Catalyst 20 mg, IL (0.5 mL), CO2/H2 gas (1:4,8.5 bar), 60 h and 423 K. Reprinted with permission from ref 282. Copyright
2019 Elsevier. bSelectivity of the products was calculated as equivalent amount of desired hydrocarbon with respect to the total number of
hydrocarbons produced. cReaction was performed in BMI-BF4 hydrophilic IL. dWithout IL
organometallic precursors [Ni(COD)] and [Ru(COD)(2-
methylallyl)2] under H2 atmosphere in an ionic liquid acting
both as solvent and stabilizer. The so-obtained RuNi NPs
presented a size of ca. 2−3 nm and a Ni-rich core with a Ru-
rich shell whatever the synthetic conditions studied, but after
the catalytic reactions, an enrichment of Ni in the shell was
observed as the result of migration of Ni atoms toward the NP
surface under catalytic conditions. In terms of catalytic
performance, among the different RuNi compositions tested,
Ru3Ni2 NPs dispersed into an hydrophobic IL (BMI·NTf 2 (l-
butyl-3-methyl-lH-imidazol-3-ium bis((tri-fluoromethyl)
sulfonyl)amide)) offered the highest conversion (up to 30%)
and promoted the direct hydrogenation of CO2 into light
hydrocarbons. The same Ru3Ni2 NPs gave rise to 22%
conversion into hydrophilic IL (1-n-butyl-3-methy l-lH-
imidazol-3-ium tetrafluoborate) with CO as the main product
(see Table 14).
Given the bimetallic RuNi NPs afforded higher efficiencies
(up to 30% of conversion) than their monometallic counter-
parts (17% and 5% of conversion with Ru and Ni NPs,
respectively), there is a strong synergy effect between Ru and
Ni in this catalytic system. The presence of Ni yielded a more
active rWGS catalyst, while Ru increased the FTS toward the
heavier hydrocarbons. In addition, as in their previous study
with RuFe NPs,256 the obtained results showed that the nature
of the IL (mainly, the choice of IL cations and anions) may
orient the selectivity of the reaction due to different geometric
and electronic properties of the IL-supported metal NPs.
Diffusion of reactants, intermediates, or products across the
interface between ILs and the catalyst surface plays an
important role in the final chemoselectivity. The hydrophobic
IL (BMI·NTf2) influenced the hydrogenation of CO2 to
heavier hydrocarbons by repelling the formed water from the
active catalytic phase of RuNi NPs, hence increasing the water
gas shift reaction and increasing the FTS reaction pathways. In
the opposite, the dominance of CO pathway into hydrophilic
IL (BMI·BF4) results from a higher solubility of the formed
water which causes the reduction of FT catalytic active surface
species (Figure 25).
As a last example, Branco and co-workers described the
hydrogenation of CO2 into methane using in situ formed IL-
supported Ru NPs (Figure 26).283 The nanocatalyst was
Figure 25. (top) Schematic representation of the chemoselectivity observed in CO2 hydrogenation by RuNi NPs depending on the nature of the
IL. (bottom) (a) Surface composition of Ni in RuNi NPs vs methane selectivity and (b) STEM-HAADF analysis of Ru1Ni2 NPs after catalysis.
Adapted with permission from ref 282. Copyright 2019 Elsevier.
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prepared in situ by mixing in an autoclave the [Ru(COD)(2-
methylallyl)2] complex and an IL (1-butyl-3-methylimidazo-
lium bistrifluoromethanesulfonylimide, [bmim][NTf2], or 1-
octyl-3-methylimidazolium bistrifluoromethanesulfonylimide,
[omim][NTf2]), followed by the application of hydrogen
pressure and temperature (313 K) before introduction of CO2
(up to a total pressure of 80 bar) and increasing temperature
(up to 423 K) to perform the catalysis. TEM analysis of the
black colloidal solution obtained after catalysis in [omim]-
[NTf2] revealed the presence of Ru NPs of ca. 2.5 nm.
It is worth mentioning that the presence of PPh3 in the
reaction medium led to no substantial results, whereas
methane and water were formed in its absence. Several
reaction conditions were first tested using [bmim][NTf2]
(amount of catalyst, hydrogen, and CO2 pressures and ratio,
reaction time, and temperature). No methane was produced at
20 bar H2, whereas 40 or 60 bar led to the same quantity of
methane (up to 4.7% yield, with TON (expressed as mol
CH4/mol cat) of 12, after 24 h at 413 K with 60 bar H2). The
change of the IL to [omim][NTf2], which is reported to better
stabilize NPs, led in general to better performance for CH4
production. The best yield of methane (69%) was achieved
with 0.24 mol % ruthenium catalyst, at 40 bar of H2 plus 40 bar
of CO2 and at 423 K (see Table 15, entry 8).
This work highlights that CO2 can be selectively hydro-
genated to CH4 using a simply prepared nanocatalyst made of
Ru NPs dispersed into an IL in reasonable reaction conditions.
It also shows that the choice of the IL may change the catalytic
performance. The better conversions reached with [omim]-
[NTf2] compared to those observed in [bmim][NTf2] are
attributed to a better solubility of CO2 (which contributes to a
reduced viscosity of the IL and increases both miscibility of
reagents in the IL) and also to a better stability of the Ru NPs
given the longer alkylchain (C8 against C4) beared by the of
[omim][NTf2] IL. Catalysis investigation performed with
preformed and isolated Ru NPs led to a reduction in CH4
production of (5% of yield and 25% of TON), thus these
comparative results point out that the conditions applied are
the key point to achieve higher methane production perform-
ance. Moreover this catalytic system is very simple to
implement.
4.5.3. Conclusions on CO2 Transformation. The
literature provides only a few works showing the potential of
Ru-based nanocatalysts (both as monometallic and bimetallic
systems) for the thermochemical hydrogenation of CO2. This
probably derives from the present (and necessary) trend to use
more abundant and less expensive metals for application in
catalysis of industrial interest, which is not the case of
ruthenium. Even if quite low values have been achieved in
terms of activity compared to those reported for homogeneous
ruthenium complexes, and even if not numerous today, the
obtained results evidenced that the control of size and the
nature of chemical environment around the particles are key
factors. These findings thus open some ways that merit being
more deeply explored in order to get more active Ru-based
nanocatalysts, but apart from ILs that were shown to orient the
catalytic results by providing adequate chemical environment,
to our knowledge, effects of capping ligands have not been
studied. Moreover, when considering the needs in terms of
mechanistic studies in order to better understand what occurs
at the surface of metal NPs, ruthenium may provide nice
opportunities because it allows the access to NMR
spectroscopic techniques, tools that are usually applied for
mechanistic studies with homogeneous catalysts. This
possibility needs to be better exploited. Indeed, such an
Figure 26. Hydrogenation of CO2 into CH4 catalyzed by Ru NPs dispersed into ionic liquids. Reproduced with permission from ref 283. Copyright
2016 Wiley.
Table 15. Hydrogenation of CO2 into Methane with Ru NPs
Stabilized into [omim][NTf2]
a
entry precursor [μmol] pH2 [bar] t [h] T [K] yield [%] TON
b
aReaction conditions: [Ru(COD)(2-methylallyl)2] as precursor, 1
mL of IL, total pressure = 80 bar at 313 K. Reprinted with permission
from ref 283. Copyright 2016 Wiley. bmol CH4/mol Ru catalyst.
approach is expected to provide insights at the atomic level on
the surface state of metal nanoparticles as well as on
intermediates formed, and thus it could greatly complement
usual surface studies coming from heterogeneous catalysis. We
believe it is a necessary step in order to define structure−
activity relationships to, in turn, design better appropriate
nanocatalysts for more efficient and more selective CO2
hydrogenation. Dupont and co-workers reported some data
in this direction using ionic liquids. They observed by high-
pressure NMR (40 bar H2/CO2 at ratio 1/1) the presence of
HCO3
− species on the surface of Ru NPs dispersed in ILs.271
4.6. Dehydrogenation of Amine Boranes
Hydrogen is considered as a clean energy carrier because it can
be produced in a renewable way from various nonfossil
feedstocks. Hydrogen has a much higher gravimetric energy
density than petroleum (120 vs 44 kJ g−1 for hydrogen and
petroleum, respectively) and can be readily used to operate
high-energy efficient fuel cells that produce water as the only
waste, which makes it an ideal alternative energy vector.284
However, a main challenge relies with its storage in secure
conditions while having an easy and fast release for an “on
demand” usage. When employed as an energy carrier in
portable electronic devices and vehicles, hydrogen fuel cells
should have the highest possible energy content combined
with the smallest possible volume and mass. As a consequence,
numerous works focus on the development of strategies for
efficient hydrogen storage with the objective to fulfill this
criterion. Physical (compressed hydrogen gas, cryocompressed
hydrogen storage, and hydrogen adsorbents) and chemical
storage systems are studied (e.g., sorbent materials, metal
hydrides, organic hydrides, borane−nitrogen (B−N) com-
pounds, and aqueous solution of hydrazine), but no hydrogen
storage methods are mature enough for industrial applications
under mild conditions.284
Covalently bound hydrogen-containing materials, in either
liquid or solid form, are very attractive for chemical hydrogen
storage because of their generally high gravimetric hydrogen
densities. Among them, amine boranes (B−N), with protic N−
H and hydridic B−H, have attracted much attention due to
their high hydrogen contents and favorable kinetics of
hydrogen release.285 Ammonia borane (NH3−BH3; AB),
which is the simplest B−N compound represents a leading
material given its high hydrogen density (19.6 wt %), low
molecular weight (30.7 g mol−1), solubility in polar solvents
like water and methanol (vide infra), high stability under
ambient conditions, and environmental friendliness.286 Methyl-
amine borane (CH3NH2-BH3; hydrogen content of 17.86 wt
%) and dimethylamine borane ((CH3)2NH-BH3; hydrogen
content of 17.1 wt %) or also hydrazine borane (N2H4-BH3;
hydrogen content of 15.28 wt %) are other substrates of
interest but they are less widely investigated, probably due to
their lower hydrogen content and necessary conditions for the
release. Hydrogen formation is generally quantified by
volumetry and the reaction monitored by 11B NMR to analyze
the byproducts formed
A convenient method to release hydrogen from ammonia
borane consists in its solvolysis using a protic solvent like water
or methanol, namely hydrolysis (eq 1) and methanolysis (eq
2), respectively.
The use of a catalyst (homogeneous, heterogeneous, or
nanoparticulate) allows to make the solvolysis to occur at rt,
leading to the stoichiometric production of 3 equiv of H2.
Dehydrocoupling (eqs 3 and 4) is another way to liberate
hydrogen from ammonia borane, using this time a nonprotic
solvent like tetrahydrofuran (THF) or toluene. Catalytic
activation allows to drive this reaction at rt, mainly using
homogeneous species but heterogeneous species and nano-
particles are also developed.
· → +n nNH BH (sol) (NH BH ) (s or sol) H (g)n3 3 2 2 2 (3)
· → +n nNH BH (sol) (NHBH) (s or sol) 2 H (g)n3 3 2 (4)
As it will be seen hereafter, kinetic studies allow to quantify
catalyst performance (in terms of turnover frequency (TOF),
Figure 27. (a) Dehydrogenation of dimethylamine−borane catalyzed by Ru/APTS NPs in THF at rt, (b) mol H2/mol DMAB vs time graph ([Ru]
= 2.24 mM; [DMAB] = 54 mM, 240 equiv of Hg(0) after ∼50% conversion of DMAB), and (c) TEM image of Ru/APTS NPs (∼2.9 nm) after the
third catalytic run. Adapted with permission from ref 288. Copyright 2012 Royal Society of Chemistry.
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activation energy (Ea), activation enthalpy (ΔH*), and
entropy (ΔS*) values). Other important parameters are the
stability and the reusability/recycling of the catalysts, both
being key parameters for cost decrease and technology transfer.
Numerous metals are used for the dehydrogenation of amine
boranes like noble metals and non-noble ones, among which
ruthenium is in the top list, either under the form of molecular
complexes, heterogeneous catalysts, or nanoparticles. The next
parts of this section will provide recent results in the use of
well-defined Ru NPs (mono-, bi-, or trimetallic systems) for
dehydrocoupling or solvolysis of amine boranes. Among the
reported Ru-based catalytic systems many involve supported
nanocatalysts, while only a few articles describe Ru NPs in
solution. The examples here presented correspond to catalysts
made of Ru NPs prepared in mild conditions of wet chemistry,
allowing thus a good control of their characteristics.
4.6.1. Dehydrogenation of Amine Boranes by
Dehydrocoupling. Not a lot of examples describe the use
of Ru nanocatalysts for the dehydrogenation of amine boranes
in a nonprotic solvent. It mainly concerns dimethylamine
borane (DMAB) and THF as solvent as well as supported
nanocatalysts. Nevertheless, a few unsupported systems have
been reported, as follows.
The catalytic performance of 3-aminopropyltriethoxysilane-
stabilized Ru nanoclusters (Ru/APTS) synthesized from the
organometallic [Ru(COD)(COT)] complex (Figure 27) has
been evaluated in the dehydrogenation of DMAB.287,288 A size
control operated by varying the Ru/ligand ratio allowed
studying of the influence of this parameter in catalysis (THF,
298 K). Hydrogen evolution started immediately with an initial
turnover frequency (TOF) of 53 h−1 for the best system (ca.
2.9 nm) and continued until completion (1 equiv H2 per mol
DMAB released). Adding Hg(0) in the catalytic mixture led to
suppression of the activity, thus evidencing heterogeneous
catalysis (Figure 28). The initial TOF value of 53 h−1 attained
with this system was comparable to that of the best
heterogeneous rhodium catalyst known at that time (TOF =
60 h−1). Moreover, it was the first example of an isolable,
bottleable, and reusable transition metal nanocatalyst for the
dehydrogenation of DMAB. APTS concentration increase
significantly decreased the catalytic activity as a result of a
higher coverage of metallic surface. This evidenced the
necessary compromise between the NP mean size and the
surface accessibility to get efficient catalytic behavior.
The in situ generation of Ru NPs was also studied taking
benefit of the catalysis reaction conditions for their formation,
without adding extra stabilizer.289 [Ru(COD(COT)] easily
decomposed during the dehydrogenation of DMAB in THF at
298 K, forming Ru NPs as seen by TEM. NMR studies on the
obtained Ru NPs showed their stabilization by B−N polymers
resulting from DMAB decomposition. It was the first example
of Ru nanocatalyst prepared in situ, displaying a TOF value of
35 h−1 together with a H2 production superior than 1.0 equiv
at the complete conversion of DMAB.
Oleylamine-stabilized Ru NPs were also used in the
dehydrocoupling of DMAB by S. Özkar’s group.290 The
nanocatalyst was generated in situ by reduction of RuCl3 at rt
in toluene and in the presence of oleylamine as stabilizer and of
DMAB as both reducing agent and catalysis substrate. This led
to Ru/oleylamine NPs of ca. 1.8 nm that were reproducibly
isolated and fully characterized. These Ru/oleylamine NPs
proved to be a highly active catalyst in the dehydrogenation of
DMAB, providing a release of 1 mol H2 per mole of DMAB
and an initial TOF value of 137 h−1 at 298 K and Ea value of
29 ± 2 kJ mol−1. The optimum ligand/Ru ratio to have active
and stable NPs was found to be 3. At this ratio, Ru/oleylamine
NPs were shown stable and reusable, giving rise to 20,660 total
turnovers and preserving 75% of their initial activity after the
fifth catalytic run with the complete conversion of DMAB and
the release of 1 equiv of H2. Although these oleylamine-
stabilized Ru NPs have a mean size similar to that of APTS-
stabilized Ru NPs previously described, their activity is almost
the double. This can be explained by a difference in terms of
available of active sites: oleylamine being less voluminous than
APTS, it probably leads to less-crowded metal surface and
consequently to more accessible ruthenium atoms compared to
APTS.
In the very recent years, the group of F. Ṣen studied several
ruthenium-containing catalytic systems in the dehydrocoupling
of DMAB, including colloidal NPs. For example, well-dispersed
Figure 28. (a) Rate of dehydrogenation of 55 mM DMAB vs [APTS]/[Ru] ratio, using Ru/APTS NPs. (b) Plot of mol H2/mol DMAB vs time for
the dehydrogenation rxn (55 mM DMAB; 0.25 mM Ru/APTS 3). Adapted with permission from ref 288. Copyright 2012 Royal Society of
Chemistry.
PVP-stabilized RuNi NPs were prepared by a facile sodium
hydroxide-assisted aqueous reduction method under inert
atmosphere that consists in treating an aqueous solution of
RuCl3 and NiCl2 by NaBH4 in the presence of NaOH and PVP
as stabilizing agent.291 Optimum conditions in terms of Ru/Ni
and PVP/metal ratios were found to be 1/1 and 5/1,
respectively. In these conditions, ca. 3.5 ± 0.4 nm in size
RuNi NPs, well-dispersed in the polymer matrix, stable, easily
isolable, and redispersible have been obtained and charac-
terized as RuNi alloy. These NPs were investigated in the
dehydrogenation of DMAB (eq 5), an easy catalytic reaction to
implement, just consisting in adding the DMAB substrate into
a THF colloidal suspension of RuNi/PVP NPs.
RuNi/PVP nanocatalyst allowed a complete release of H2 (1
mol H2 per mol of DMAB) at 298 K in a short time with no
induction period. A comparative study performed with Ru/
PVP NPs, Ni/PVP NPs, and a physical mixture of both
evidenced the superior performance of the RuNi/PVP
nanocatalyst, attributed to its alloy character that provided a
synergistic effect. A TOF value of 458.57 h−1 makes it be
among the best catalysts reported in the literature for
dehydrocoupling of DMAB. This catalytic system also showed
a low Ea value of 36.52 ± 3 kJ mol−1, an activation enthalpy
(ΔH* = 34.02 ± 2 kJ mol−1), and activation entropy (ΔS*) =
−84.47 J·mol−1). High negative values of activation entropy
and small activation enthalpy value refer to an associate
mechanism in the dehydrocoupling of DMAB. These RuNi/
PVP NPs also appeared to be a reusable catalyst with 78% of
initial activity preserved after four catalytic runs and no
leaching observed.
The same group also published the catalytic performances of
alloyed PdRu/PVP NPs (ca. 3.8 ± 1 nm) in the
dehydrocoupling of DMAB (THF, 298 K).292 The synthesis
of the NPs was performed by an ultrasonic double reduction
technique (reduction of aqueous solution of RuCl3 and
K2PdCl4 under ultrasounds at 363 K in the presence of
PVP). Their catalytic behavior was compared to those of Pd/
PVP NPs, Ru/PVP NPs, and a physical mixture of both in
similar conditions. No induction time and complete DMAB
conversion were observed. Kinetic parameters were found tobe
TOF = 803.03 h−1, Ea = 60.49 ± 2 kJ mol−1, ΔH* = 57.99 ± 2
kJ mol−1, and ΔS* = −53.17 J·mol−1. Reusability tests
indicated ca. 80% of initial activity kept after four runs.
Theoretical calculations by DFT using Pd/PVP, Ru/PVP, and
PdRu/PVP model clusters in optimized geometries were
performed in order to determine adsorption energy of DMAB.
The obtained theoretical results supported well the exper-
imental results. The PdRu/PVP cluster presented a markedly
lower chemical potential, adsorption energy, and enthalpy
values than those of Pd/PVP and Ru/PVP clusters. Also,
higher chemical hardness and electronegativity values were
observed for PdRu/PVP cluster compared to those of
monometallic counterparts. All these differences may explain
the outstanding efficiency of the PdRu/PVP NPs.
A summary of the catalytic properties of the previously
described soluble Ru nanocatalysts is given in Table 16. The
obtained results clearly evidence that colloidal ruthenium is a
good metal for the dehydrogenation of DMAB. Interestingly,
even if only a few ligands were tested, variation of capping
ligand led to a variation in reactivity. Also, these results show
the progress attained in terms of performance when associating
a second metal like Ni or Pd to Ru. Similar studies with ligand-
stabilized alloys could be of interest to perform.
F. Ṣen and co-workers also reported on the application of
monometallic, bimetallic, and even trimetallic Ru-based
supported NPs in dehydrocoupling of DMAB (THF, 298
K).293−298 If these data are here cited, it is because these
catalysts were prepared in mild reaction conditions by
reduction of the metal source(s) in the presence of the chosen
support sometimes together with a polymer (PVP) or ligand
(oleylamine), thus leading to controlled NPs. Graphite,293
graphene,298 functionnalized multiwalled carbon nanotubes (f-
MWCNT),294,256 or graphene oxide (GO) were used as a
support.295−297 The kinetic parameters measured for these
catalysts are summarized in Table 17. It can be seen that
different values are obtained depending on the composition of
the nanocatalyst both in terms of metal composition and
nature of metal−support interaction. Also, in Ṣen’s group’s
papers, comparisons with other catalysts reported in the
literature are described, highlighting the interest of Ru-based
nanocatalysts for DMAB dehydrocoupling. However, it is
difficult to rationalize the observed effects because several
Table 16. Comparison of Kinetic Data in Dehydrocoupling of DMAB by Soluble Ru NPs (298 K; THF Except for Ru/
Oleylamine, Toluene; Total Conversion)
Table 17. Comparison of Kinetic Data in Dehydrocoupling of DMAB (THF, 298 K, Total Conversion) for Diverse Supported
Nanocatalysts Studied by F. Ṣen and Coworkers
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parameters are different. In fact the works performed
correspond more to trial−error works than real systematic
comparison. More rationalization is thus required in order to
define precisely the important parameters to master in order to
get the best performance.
4.6.2. Dehydrogenation of Amine Boranes by
Methanolysis. Only a few papers report on the dehydrogen-
ation of amine boranes with nanoscale ruthenium using
methanol as a solvent. Compared to hydrolysis (vide infra), the
methanolysis presents a few merits such as a single gaseous
product (H2), recoverable byproducts, and the possibility of
handling at low temperatures (<273 K).299
H.-C. Zhou and co-workers described the synthesis of
ultrasmall fcc Ru NPs confined into the pores of a soluble and
negatively charged porous coordination cage (PCC) of 4.2 nm
in size that presents three different cavity diameters (ca. 2.5
and 1.4 nm for inner and intermolecular cavities, respec-
tively).299 The preparation of this nanocatalyst consisted in the
addition of RuCl3 to a DMF solution of PCC-2
(Na24(Et3NH)6[[Co4(μ4-OH)V]6L8]30
−5·MeOH·10H2O
with V = phenolate groups and L = carboxylates) using a Ru/
PCC-2 molar ratio of 2/1, followed by addition of NaBH4 also
in DMF solution (Figure 29). This protocol led to a
homogeneous colloidal dispersion containing PCC-2-stabilized
Ru NPs with a narrow size distribution and a mean size of ca.
2.5 nm that corresponds well to the mean diameter of the inner
cavities of the host. HREM analysis clearly showed that Ru/
PCC-2 NPs have a truncated octahedral fcc structure, not
usual for Ru NPs, which was also confirmed by powder XRD.
XPS analysis indicated a major content of metallic Ru. No
precipitation from the colloidal suspension was observed up to
6 months in ambient air. The isolation of the NPs could be
performed by addition of acetonitrile to the DMF suspension,
which allowed the precipitation of a black solid redissolvable in
DMF. These Ru/PCC-2 NPs were investigated in the
dehydrogenation of ammonia borane by simply adding the
DMF/MeOH colloidal suspension to solid AB. The catalysis
was carried out at 298 K. Reaction was completed after 4.5
min, showing a TOF value of 304.4 mol H2 per mol Ru per
min, which appeared to be higher than the TOF value of 205
min−1 reported by Xu and co-workers using a PCC-stabilized
Rh nanocatalyst for catalyzing the same reaction.300 The
catalytic performance of the Ru/PCC-2 nanocatalyst, being the
best catalytic activity ever reported for the methanolysis of AB,
was attributed to the small size and the fcc structure of the
particles. Furthermore, the anionic and soluble PCC-2 played a
critical role in encapsulating, stabilizing, homogenizing, and
distributing the metal nanoclusters by regulating the size and
the atomic arrangement of the encapsulated NPs. The soluble
catalyst Ru/PCC-2 was also five times without a significant loss
of activity.
The results of F. Wang and co-workers are also among the
best ones in methanolysis of AB. Their nanocatalyst was
prepared by direct deposition of ultrafine Ru NPs onto
tetrabutylammonium hydroxide-intercalated graphene as a
support by the reduction of RuCl3 in water with KBH4 at
303 K.301 The obtained Ru/graphene nanomaterial displayed
well-dispersed Ru NPs onto the support with a mean size of ca.
1.6 nm. This nanomaterial was investigated in the methanolysis
of AB. Up to 35,600 total turnovers over a period of 300 h and
Figure 29. (left) Representation of the PCC-2 cage. (right) Scheme of the synthesis of Ru/PCC-2 NPs and their investigation as catalyst in
dehydrogenation of AB. Adapted with permission from ref 299. Copyright 2018 Elsevier.
Figure 30. Synthesis of metastable Ru NPs. Reproduced with permission from ref 302. Copyright 2014 Wiley.
a TOF value of 99.4 min−1 were obtained at 293 K before
deactivation and a Ea value of 54.1 ± 2 2 kJ mol−1.
Additionally this nanocatalyst showed a satisfactory stability
and retained 73.2% of its initial activity at the 15th run.
4.6.3. Dehydrogenation of Amine Boranes by
Hydrolysis. Regarding hydrolytic dehydrogenation of AB,
numerous studies are conducted on diverse monometallic and
heterometallic nanocatalysts (mainly Pt, Ru, Rh, Ag, Pd) that
display high catalytic activity, among which numerous ones are
Ru-based catalysts. Various stabilizers and supports are used in
order to control the size, morphology, and stability of the NPs.
The addition of a second metal to ruthenium appeared to be
positive to boost the catalysis.
As first example of soluble Ru NPs, one can cite the
metastable Ru NPs reported by O.A. Scherman and co-
workers.302 This work relates on a very facile catalytic system
made of highly stable Ru NPs (up to 8 months) despite the
absence of any extra stabilizer. In fact, the authors simply
decomposed RuCl3 by NaBH4 in a H2O/EtOH mixture (1/1)
at rt (Figure 30).
The presence of monodisperse Ru NPs of ca. 2.2 nm was
evidenced by HREM. The initial concentration of RuCl3
played a crucial role in the control of the NP size. A fcc
structure was determined by HAADF-STEM and XPS showed
a signal corresponding to Ru(0) for ca. 19.4% together with a
signal attributed to remaining RuCl3, which probably acts as
stabilizer for the Ru NPs. The % of Ru(0) species increased to
over 75% after a complete catalytic cycle. These Ru NPs
allowed the hydrolysis of AB yielding hydrogen gas with a
TON of 21.8 min−1 at 298 K. The high surface area available at
Ru surface translated an Ea value of 27.5 kJ mol−1, which was
notably lower than that of other Ru NPs based catalysts.
As another example of AB hydrolysis with monometallic Ru
NPs, by S. Özkar, M. Zahmakiran, and co-workers reported on
the use of dihydrogenophosphate-stabilized Ru NPs.303 This
catalytic system was prepared by reduction of an aqueous
solution of RuCl3 and ((C4H9)4N[OP(OH)2O] with DMAB
at rt, leading to a stable colloidal dispersion of Ru NPs (ca. 2.9
nm) with no precipitation after 2 days of storage. When
investigated in the hydrolysis of AB at rt, this catalytic system
presented an initial TOF value of 80 min−1. Moreover, the high
stability of these Ru NPs made them long-lived and reusable
nanocatalysts for the hydrolysis of AB, providing 56,800 total
turnovers over 36 h before deactivation, an initial TOF value of
31.6 min−1 (at 283 K), an Ea value of 69 ± 2 kJ mol−1 and
retaining 80% of their initial activity at the fifth catalytic run.
These authors also published on the hydrolytic dehydrogen-
ation of DMAB catalyzed by similar ((C4H9)4N[OP(OH)2O]-
stabilized Ru NPs but synthesized in situ, i.e., in the catalysis
medium (Ru/stabilizer ratio = 1/20).304 RuCl3 was reduced by
addition of DMAB, being also the catalysis substrate, leading to
the formation of Ru NPs of ca. 2.9 nm mean size. Kinetic
studies revealed an initial TOF value of 500 h−1 at 298 K and
stability studies an exceptional catalytic lifetime with 11600
total turnovers.
M. Rakap published the use of PVP-protected PtRu NPs for
the hydrolysis of AB.305 This catalyst was synthesized by
alcoholic reduction of RuCl3 and PtCl6 in the presence of PVP
under mild conditions (EtOH; 363 K; 2 h). The obtained
colloidal suspension was found stable for months at rt.
Isolation of the particles was performed by simple solvent
evaporation. Characterization techniques (TEM-EDX, ICP,
XRD, XPS) revealed ca. 3.2 nm in size alloyed PtRu NPs with
a Pt/Ru composition of 1/1 as well as the presence of Pt(0)
and Ru(0) species but no higher oxidation states. The catalytic
activity of PtRu/PVP NPs in the hydrolytic dehydrogenation
of AB (at 298 K) was much higher than that reached with a
physical mixture of monometallic Ru/PVP NPs (ca. 4.6 nm)
and Pt/PVP NPs (ca. 4.2 nm) prepared in the same
conditions, thus indicating a synergistic effect attributed to
Pt−Ru interaction in the alloy although the reduced mean size
of the PtRu/PVP NPs may also have an effect. It is worth
noting that PtRu/PVP nanocatalyst led to complete hydrogen
release (3 mol H2·mol AB
−1) for the hydrolysis of 0.100 M AB
solutions in 195 s, corresponding to a record average TOF of
308 min−1 with a low Ea value of 53.3 kJ mol−1. Recyclability
tests showed a remaining activity of 72% after the fifth catalytic
cycle. The same author also reported on the hydrolysis of AB
using RuRh/PVP NPs.306 The nanocatalyst was prepared
following the same synthesis method as described for RuPt/
PVP one and also the catalysis performed in the same
conditions. This catalyst was shown to be efficient and durable
providing an average TOF value of 386 min−1 and Ea value of
47.4 kJ mol−1, thus reflecting a higher efficiency than the
previous PtRu system just by changing Pt by Rh in the alloy.
Similarly, F. Ṣen’s group reported on the use of RuRh/PVP
nanocatalyst for the hydrolysis of methylamineborane (MAB)
at rt307 The NPs were also prepared by alcoholic reduction of a
mixture of RuCl3 and RhCl3 in mild conditions (H2O/EtOH;
363 K; 2 h) in the presence of PVP as stabilizer. HREM, XRD,
and EELS data indicated alloyed RuRh NPs of ca. 3.4 mean
size and XPS data the presence of Ru(0) and Rh(0) species.
Then this nanocatalyst was evaluated in hydrolysis of MAB at
rt, showing a high efficiency with an initial TOF value of 206.2
min−1, EA value of 43.5 kJ mol−1, as well as ΔH* = 41.18 kJ
mol−1 and ΔS* = −104.25 ± 2 J·mol−1. Reusability tests
indicated a 67% retention of the initial catalytic activity after
five cycles. All together, these characteristics place this
nanocatalyst among the best for hydrolysis of MAB, a storage
material which may lead to less volatile byproducts than AB.
Indeed, the decomposition of AB results in a distinct
contamination of released H2 by NH3 and borazine, which is
a major problem for application in fuel cells.
All the results described above are summarized in Table 18.
Here again it is difficult to rationalize these results given the
different parameters used. Nevertheless, they prove the leader
Table 18. Comparison of Kinetic Data in Hydrolysis of Amineboranes at 298 K
nanocatalyst (substrate) NP mean size (nm) TOF (min−1) Ea (kJ·mol−1) ΔH* (kJ·mol−1) ΔS* (±2 J·mol−1) ref
position of ruthenium for the hydrolysis of amine borane and
the positive effect of the addition of a second metal like Rh or
Pt.
As it will be described hereafter, a few papers describe the
use of supported nanocatalysts that were preformed in solution
in mild conditions. The preparation of these catalysts generally
consists in two steps: (1) reduction of the Ru source to get the
colloidal suspension and (2) impregnation of a given support
from the colloidal suspension in order to deposit the NPs at
the surface or in the pores of the material, followed by
evaporation of the solvent. Control of Ru NPs is thus operated
in solution before deposition on the support, and the influence
of the support can be studied independently.
U. B. Demirci and co-workers studied the catalytic
performance of RuCo NPs and RuCu NPs with metal ratio
1/1 prepared by the polyol process ([Ru(acac)3], [Co(acac)2],
and [Cu(acac)2] with acetylacetonate (acac); ethylene glycol;
458 K) in the absence of added stabilizer and then deposited
onto γ-Al2O3 as a support in the hydrolysis of AB (323−338
K). A higher activity was observed for RuCo than for RuCu
NPs, with activation energies of 47 and 52 kJ mol−1,
respectively. Moreover, the RuCu NPs presented a similar
activity as Ru NPs prepared in the same conditions. The
addition of Co thus had a positive effect on the catalytic
behavior of Ru that may result from synergistic interactions
between Ru and Co atoms in the RuCo NPs.
G. Chen, D. Ma, and co-workers prepared a series of NiRu/
ligands alloy NPs at different metal ratios and deposited them
onto a carbon black support for their evaluation in the
hydrolysis of AB.308 The NPs were prepared by reduction of a
diphenylether solution of [Ru(acac)3] and [Ni(acac)2]
complexes by triethyborohydride (LiBEt3H) in the presence
of oleic acid and oleylamine as stabilizers at 523 K. As
confirmed by full characterization (HREM, XRD, XPS), such
reaction conditions (strong reducing agent, high temperature)
allowed alloying NiRu NPs of ca. 9 nm mean size while Ru and
Ni are immiscible in bulk form. The NPs were purified by
precipitation with ethanol and redispersed in hexane for their
further deposition onto the carbon support followed by solvent
evaporation in order to get the final nanocatalysts. Catalysis
was done in water at ca. 303 K. A comparison with
monometallic Ru NPs and Ni NPs as well as core−shell Ni/
Ru309 stabilized by the same ligand evidenced the superior
catalytic activity of the NiRu alloy nanocatalysts (Figure 31).
With a complete dehydrogenation of AB in 12 min, the best
activity was obtained with the Ni richest nanocatalyst, namely
Ni0.74Ru0.26, while Ni NPs were almost inactive and Ru NPs
showed an intermediate activity. Moreover, the Ni@Ru NPs
needed almost 3 times as long for a total conversion, thus
showing the strong influence of the Ru−Ni interaction in the
alloy. The determination of the activation energies, revealed a
lower value for NiRu alloy nanocatalyst than for Ni/Ru one.
Thus, alloying Ru with Ni decreased the reaction activation
energy and significantly enhanced the catalytic activity of Ru. A
reusability test showed that the Ni0.74Ru0.26 still exhibited high
catalytic activity after five catalytic cycles.
Recently, G. Chen and co-workers studied the effect of the
size and of Ru crystal phase on the catalytic activity of Ru/
PVP/γ-Al2O3 nanocatalyst in hydrolysis of AB.
310 For this
purpose, they prepared hcp Ru NPs and fcc Ru NPs exhibiting
narrow size distributions and similar sizes (ca. 2.4 nm). These
Ru NPs were synthesized by decomposing [Ru(acac)3] or
RuCl3 at 473 K in triethylene glycol (TEG) in the presence of
PVP as a stabilizing agent. Ru NPs of different size/crystal
phase were synthesized by adjusting the amount/nature of
metal precursors, type of solvents, and the amount of PVP. As
demonstrated by characterization results, [Ru(acac)3] led to
fcc Ru NPs, while RuCl3 provided hcp Ru NPs. The so-
obtained Ru NPs were further deposited onto γ-Al2O3 by wet
impregnation method before evaluating their catalytic perform-
ance in the hydrolysis of AB at rt The hcp Ru NPs exhibited a
higher activity than fcc Ru NPs at similar sizes. Also, with the
size increase, the gap of activity became narrow. More
interestingly, with the particle size change, an opposite
variation of the activity trend for fcc and hcp structured Ru/
γ-Al2O3 was observed (Figure 32).
With the size increase, fcc Ru NPs presented an increased
catalytic performance while hcp Ru NPs displayed a converse
trend with a decreased performance at higher sizes. A
reusability test showed that the fcc Ru NPs still exhibited
high catalytic activity after four runs, although fcc Ru has a
thermodynamically unstable structure. DFT calculations
evidenced that fcc Ru NPs were easier to oxidize than hcp
ones (values of adsorption energy of O2 onto (001) crystal
plane of fcc and hcp Ru were found to be −2.17 and −1.81 eV,
respectively). This difference in oxidation state could explain
why hcp Ru NPs were more performant than fcc Ru NPs,
without taking into account other parameters. Considering that
the surface-to-volume ratio increases with the size decrease
(so-called “size effect”) and that smaller NPs are more
Figure 31. Comparison of catalytic activities (left) of activation energies (right) of monometallic Ni, monometallic Ru, Ni/Ru core−shell, and
NiRu alloy NPs for AB hydrolysis at 30 ± 1 °C. Adapted with permission from ref 308. Copyright 2012 Wiley.
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1129
subjected to a higher oxidability than larger ones, the surface
oxidation may play a dominant role on the fcc Ru NPs catalytic
activity while the size effect may be responsible for the activity
trend for hcp Ru NPs. Nevertheless other factors like the
difference in step edges/step density between fcc and hcp Ru
cannot be ruled out, but this requires more mechanism
investigations. The results of this work are of particular interest
because the influence of Ru crystal structure in Ru NPs is only
recently studied, while this parameter could have an important
effect in the various possible catalytic applications of Ru NPs.
4.6.4. Dehydrogenation of Amine Boranes by
Supported Ruthenium Nanocatalysts. Despite the objec-
tive of this review is to highlight the interests of solution Ru
NPs in catalysis, the high number of papers describing the use
of supported-Ru NPs (mono- or heterometallic) for the
dehydrogenation of amine boranes in the past decade makes
that we cannot not mention it.284,311
The preparation of the nanocatalysts is generally done by
decomposition of the Ru source (most often RuCl3 or
[Ru(acac)3]) in the presence of a reducing agent (NaBH4,
polyol) and a chosen support (oxides, Al2O3, SiO2, CeO2,
TiO2; carbon derivatives, CNTs, GO; MOFs, etc.) Among the
recent papers, one can mention different works by S. Özkar
and co-workers who used nanohafnia,312 nanozirconia,313 and
silica coated Fe3O4
314 as novel supports of Ru NPs and also
that of L. Zhou and co-workers315 with a MOF support for the
dehydrolytic dehydrogenation of AB at rt.
A second common preparation method is an in situ
synthesis of the Ru NPs directly in the catalytic medium.
The synthesis of the Ru NPs is carried out in the presence of a
given support and using an amine borane as both reducing
agent and catalysis substrate. In these conditions, the NP
growth happens in parallel of the dehydrogenation of the
amine borane and the catalysis is then pursued. For example G.
Fan and co-workers investigated Ru NPs supported onto TiO2
nanotubes316 as well as RuNi317 and RuCo318 NPs deposited
onto a graphene-like transition metal carbide (Ti3C2X2; with X
= OH and/or F). With Ti3C2X2 supporting material (hydro-
philic surface), they observed a very good size control and
dispersion of the NPs all over the support, and a good
dispersion of the catalyst in the reaction medium, probably
enhancing the contact between the metal surface and the
substrate (AB). The so-obtained RuNi and RuCo nano-
catalysts provided close catalytic performances, namely TOF/
Ea values of 824.7 mol H2·(mol metal·min
−1)/25.7 kJ mol−1
and 896.0 mol H2·(mol metal·min
−1)/31.1 kJ mol−1,
respectively. Moreover, these two catalysts showed a good
stability reaching 100% conversion of AB after four catalytic
cycles even if a decrease of velocity was observed. These
catalytic performances are among the best ones claimed today
for Ru-based nanocatalysts as the result of enhanced contact
between the metal surface and the substrate.
4.6.5. Conclusions on Amineborane Dehydrogen-
ation. Ru is one of the most attractive catalysts in the
dehydrogenation of amine boranes and most particularly of
ammonia borane due to its high efficiency in accelerating the
release of hydrogen from these substrates (either by
dehydrocoupling or solvolysis). A high number of papers
concern the hydrolytic dehydrogenation of ammonia borane
because of its simplicity and green approach given it avoids the
use of organic solvents as well as of its high efficiency. The
preparation of better defined Ru NPs for this reaction has been
extensively investigated using different stabilizers (mainly PVP
as polymer and amines as ligands) to get stable colloidal
solutions, which were proven to be very active in this catalysis.
But the influence of the stabilizing ligand is not studied yet in a
systematic way, thus limiting the understanding of the ligand−
activity relationships. Also a large panel of supports were tested
to deposit the Ru NPs (either by wet impregnation or by direct
synthesis of the nanoparticles in the presence of the support)
and thus increase the stability of the catalysts as well as getting
easier their separation from the reaction media for recycling
concerns. Here also the support−activity relationships are not
well-studied. For economic purposes, some works provided
promising results for the improvement of Ru activity and
simultaneously minimize its use/cost by forming Ru-based
bimetallic structures (RuCo, RuNi, PtRu, RuRh). If com-
petitive results have been already obtained compared to those
reached with nanocatalysts of other metals (in particular Rh
ones), further research is still needed to improve synthesis
methodologies to access more performant catalyst in terms of
activity, lifetime, and reusability. More rationalization works
are also needed because up to now it is very difficult to
compare the numerous results described.
4.7. Water Splitting
Fitting the green chemistry principles and known as the water
splitting process, the production of hydrogen from water (eq
6) is a very attractive route toward a clean energy vector and
even more if envisaging its activation by sunlight. Besides the
requirement in active, stable, and if possible low-cost catalysts,
the photoactivated water splitting needs to associate a light-
harvester, also called photosensitizer (PS) (organic, molecular
complex or inorganic semiconductor material), for allowing the
electron transfers.
The splitting of water is a redox process consisting in two
successive half reactions, namely oxygen evolution reaction
(OER) and hydrogen evolution reaction (HER). It starts by
the oxidation of water to molecular oxygen at the anode (eq 7a
and (7b) at neutral/acidic and basic pH, respectively). Then
the released electrons and protons produce molecular
hydrogen at the cathode (eq 8a and (8b) at neutral/acidic
and basic pH, respectively).
→ +2H O O 2H2 2 2 (6)
Figure 32. Schematic representation of the effect of Ru crystal
structure (fcc vs hcp) on the hydrolysis of AB by Ru/PVP/Al2O3
nanocatalysts. Reproduced with permission from ref 310. Copyright
2018 Elsevier.
These two key steps are generally conducted into two
different compartments separated by a proton exchange
membrane of a (photo)electrochemical cell. They are kineti-
cally slow because of their mechanistic complexity, especially
for the oxidation half reaction, and the difficulty of evolving
gases from a liquid phase. It is therefore of upmost importance
to find suitable catalysts able to accelerate them. A main
difficulty is having efficient catalysts with compatible kinetics in
order to enable the complete splitting process to occur and so
the total conversion of H2O into O2 and H2. Another issue is
the stability of the catalysts given the harsh necessary
conditions (acidic or basic pH). For these reasons, numerous
studies aim at evaluating the catalyst performances by studying
only one part of the splitting process (either OER or HER).
Intensive research activity has been devoted to the use of
molecular catalysts, among which polypyridyl ruthenium
complexes showed to be very active for OER.2,319 Among
heterogeneous catalysts, iridium oxide (IrO2) anodes display
excellent electroactivity for the OER.320 However, heteroge-
neous RuO2 also showed significant activity in the OER.
321
Concerning the HER, in the solid phase, the most active metal
in reducing protons and especially in acidic conditions is
platinum. Nanomaterials have also received high attention
among which Ru-based nanocatalysts emerged as true
potential substitutes for the state-of-the-art platinum and
iridium oxide catalysts for OER and HER, under the form of
oxide Ru or metal Ru species, respectively. As the application
of RuO2 NPs and Ru NPs as (photo)electrocatalysts for the
water-splitting process has been reviewed very recently,322 we
will not provide here a complete description of these
nanocatalysts. Interestingly, among the Ru-based nanocatalysts
evaluated in water splitting, only scarce examples describe
controlled Ru NPs synthesized in mild conditions of wet
chemistry for the HER. Because they correspond well to the
objectives of the present review, these works will be hereafter
briefly presented.
4.7.1. Ru NPs as Electrocatalysts for HER. The use of
Ru-based nanocatalysts for the HER is recent but fast evolving
(most of the relevant literature was published in the period
2016−2019). This derives from advantageous characteristics of
Ru compared to Pt, the state-of-the-art metal for this reaction.
First, in HER the M−H bond energy strongly affects proton
reduction catalysis given that a high M−H binding energy
favors the adsorption of protons (but hardens the H2
desorption), while a low M−H binding energy results in a
contrary effect. With an optimum M−H binding energy
(neither too low nor too high), platinum stands at the center of
the volcano plot for proton reduction catalysts.323,324 In
comparison to Pt, Ru displays a slightly weaker M−H bond
which hardly decreases the HER catalytic efficiency, both
according to experimental results and DFT calculations.12
Furthermore, Ru showed to be stable both under acidic and
basic conditions while Pt is not optimally stable at basic pH.
Finally, the Ru cost is lower than that of Pt. All together these
Figure 33. (left) TEM/HREM images and powder-XRD diagram of MeOH/THF stabilized Ru NPs. (right) (a) LSV curves of the Ru/MeOH/
THF nanomaterial (red), Ru powder (blue), and Pt/C (green) in 0.5 M H2SO4 solution at 10 mV·s−1. The LSV curve of a bare GC electrode
(orange). (b) Galvanostatic experiment of the Ru/MeOH/THF nanomaterial at a current density of 10 mA·cm−2 in 0.5 M H2SO4, without ohmic
drop compensation. (c) LSV curves of the initial Ru/MeOH/THF nanomaterial (red) and after 12 h of galvanostatic experiment (blue) in 0.5 M
H2SO4 solution at 10 mV·s
−1. (d) Tafel plots of the Ru/MeOH/THF nanomaterial (red), Ru powder (blue), and Pt/C (green) in 0.5 MH2SO4
solution. Adapted with permission from ref 325. Copyright 2017 Royal Society of Chemistry.
characteristics have boosted the attractivity of Ru metal as
HER electrocatalyst in the last three years.
Even if some photocatalytic examples exist, most of the
described systems consist in Ru NPs deposited or supported/
embedded onto conductive C-based (or even metallic)
materials that are electrochemically triggered. However, a few
papers report on nonsupported systems prepared ex situ
through various methods (thermal decomposition/calcination
of anhydrous RuO2, Ru salt, or a Ru complex; electroreduction
of a Ru salt, Ru perovskite-type precursor, or Ru complex) and
then deposited onto the electrode for catalytic evaluation, but
the tailored synthesis and rational catalytic fine-tuning of
nonsupported Ru-based NPs for water splitting is not a simple
matter. First, the use of a stabilizer, typically a coordinating
solvent, ligand, or the surface of a material, is mandatory to
maintain nanoscale systems, preventing the formation of
thermodynamically favored bulk species. Also, the metal
oxidation state at the NP surface may evolve and even
reversibly switch (typically between metallic Ru and Ru (IV) in
RuO2) in contact with air and/or under (electro)catalytic
turnover conditions. So, as for all catalysis, disposing of an
effective way to have model Ru-based NPs (with controlled
size, shape, oxidation state, and surface composition) for the
splitting of water is of utmost interest for performing
fundamental studies in order to develop more efficient
catalysts. In this regard, the use of an organometallic complex
as precursor recently allowed getting interesting results. The
decomposition of the [Ru(COD)COT)] complex under
hydrogen, in a MeOH/THF mixture without any stabilizer,
allowed obtaining significantly active Ru NPs when deposited
onto glassy carbon (GC) electrodes (Figure 33).325 Thus, the
21.4 nm porous Ru NPs in 0.5 M H2SO4 led to values of η0 ≈
0 mV, η10 = 83 mV, b = 46 mVdec
−1, TOF100 mV = 0.87 s
−1, a
Faradaic efficiency of 97%, and excellent durability for up to 12
h (Figure 33).
Also, the electrochemical analysis of 4-phenylpyridine (PP)-
capped Ru NPs (mean size ca. 1.5 nm) synthesized from the
same complex and then drop-casted onto a GC electrode (PP-
Ru-GC) together with their thoroughly characterization in air
and under HER turnover conditions (in both acidic and basic
electrolytes), evidenced the influence of the coordinated PP
ligand on the catalytic performance. The surface of these Ru
NPs spontaneously oxidized to RuO2 upon exposure to air,
yielding a mixed Ru/RuO2 system in which the PP ligand was
still present. Although this mixed Ru/RuO2 system was less
active toward the HER compared with that of pure Ru NPs, it
was converted into the metallic Ru form under reductive
conditions (20 min bulk electrolysis at −10 mA·cm−2) at acidic
pH (Figure 34).326,327 Thus, the recovered PP-Ru-GC system
exhibited values of η0 ≈ 0 mV, η10 = 20 mV, b = 29 mV dec−1,
and a TOF as high as 17.4 s−1 at η = 100 mV in 1 M H2SO4,
with complete stability after 12 h of continuous operation. In
contrast, in 1 M NaOH, the only stable form of the PP-Ru-GC
system was a Ru/RuO2 mixture, yielding a slightly less active
and stable catalytic system, although still outperforming the
performance and stability of commercial Pt/C. The presence
of the PP capping agent is believed to induce a good
mechanical stability, thus allowing the nanostructured
character of the material to be maintained, even after a long
run. This hypothesis is supported by DFT calculations, which
showed the coordination of 11 PP molecules onto the surface
of a Ru55H53 NP both through N-σ and π-coordination modes;
the latter was more stable and preferentially took place on the
edges of the NP. Furthermore, the d-band energy levels of the
surface Ru atoms were significantly modified by the presence
of hydride ligands, which have a stabilizing effect, whereas
these energy levels were not significantly altered by the PP
capping ligands, thus indicating a moderate adsorption
strength of the latter onto the NP surface. As a consequence,
a larger number of hydride ligands were present on the NP
surface compared with those of PP (53 vs 11), thus accounting
for the enhanced H2 evolution behavior. These results clearly
show that a capping ligand like a phenylpyridine can tune the
properties of a Ru nanocatalyst for the HER. Nevertheless, the
real effect of the ligand needs to be deeper studied and
comparative studies with other ligands need to be performed.
Supported Ru-based nanomaterials prepared in more drastic
conditions have been also reported as active species for the
Figure 34. (left) TEM images of Ru-PP NPs at low (a) and high (b) magnification and size histogram. DFT model of PP-protected 1 nm RuNP
(Ru55H53σPP9πPP2). (right) polarization curves in 1 M H2SO4 at 10 mVs1 and XPS data for metallic PP-Ru NPs and their Ru/RuO2 surface-
passivated counterpart, which formed upon exposure to air. Adapted with permission from refs 326 and 322. Copyright 2018 American Chemical
Society and Copyright 2019 Wiley.
Chemical Reviews Review
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HER showing high influence of the nature of the support on
the catalytic performance. The effect of the crystal structure of
the Ru phase has also been demonstrated. Moreover, the
interaction of metallic Ru with other metal/semimetal-based
nanostructures in mixed catalysts was shown to increase the
HER catalytic activity compared with that of the respective
separated systems, as a result of the synergistic effect between
metals, which improves the electron conductivity and lowers
the H adsorption energy.
4.7.2. Ru NPs as (Photo)catalysts for HER. Concerning
the inclusion of Ru NPs in HER photocatalytic systems, it is
not an easy task given the inherent difficulties in properly
transferring electrons from a photosensitive molecule or
material to the nanocatalyst, while avoiding undesired back-
electron transfer processes. Indeed, the electron-transfer
process between Ru NPs and the widely employed molecular
PS [Ru(bpy)3]
2+ is generally not optimal.328 Thus, together
with a sacrificial electron-donor (SED; e.g., reduced
nicotinamide adenine dinucleotide (NADH)) supplying the
necessary electrons in half-cell systems, the use of an electron
mediator (e.g., methyl viologen) is generally required. Only
PSs with sufficient and long-lived charge-separated states after
photoexcitation are able to inject electrons into the HER
electrocatalyst without the need to use an electron mediator,
thus making the systems less complex and more efficient. In
this regard, Fukuzumi and co-workers reported on the use of a
molecular dyad that acts both as a PS and as an efficient
electron supplier for Ru NPs, namely the 2-phenyl-4-(1-
naphthyl)quinolinium ion (QuPh+-NA; Figure 35).329 Using
PVP-stabilized Ru NPs with QuPh+-NA PS in alkaline
solution, they found optimal conditions for the photocatalytic
HER. No increase in the photocatalytic activity above a certain
optimal catalyst concentration (presumably due to light
dispersion and opacity if more nanomaterial present in the
reaction medium), and an activity-size dependency of the
tested NPs were observed. Small NPs displayed a higher
negative charge density, which eased the proton reduction
process but hindered the hydrogen-atom association step
because of low density of hydrogen atoms on a single particle.
Larger NPs eased the hydrogen-atom association step due to
the presence of more hydrogen atoms on their surface but
hindered the previous proton reduction process because the
negative charge density of the surface was initially lower. As a
consequence, the best results were obtained with NPs of
intermediate size, namely 4.1 nm.329 Finally, the deposition of
the Ru NPs and QuPh+-NA onto oxide-based materials (SiO2,
TiO2, CeO2, etc.) led to less agglomeration under HER
turnover conditions and enhanced photocatalytic stability with
regard to the corresponding nonsupported systems.328 Apart
from the QuPh+-NA ion, only the dye Eosin Y330 and the
combination of [Ru(bpy)3]
2+ with 9-phenyl-10-methylacridi-
nium derivatives as electron mediators331 have led to relative
success in the photocatalytic HER with Ru-based NPs.
4.7.3. Conclusions on Water Splitting. Very recently, Ru
NPs have received a renewed interest for their application as
catalysts in the water splitting process. Available data on
nonsupported systems indicate amorphous RuO2-based NPs
and highly crystalline Ru NPs as the species of choice for
attaining high-performance HER NP electrocatalysts. Remark-
ably, the mild conditions of solution chemistry provided
interesting catalytic systems to conduct fundamental studies
where an effect of capping ligand was observed. The catalytic
performances achieved evidenced that Ru NPs may be a
potential substitute of Pt which is still the most active metal for
this reaction. Concerning the photoactivated version of the
HER, even if still in their infancy in terms of development,
tandem particle-based photocatalysts proved to be promising
candidates.
5. CONCLUDING REMARKS AND OUTLOOK
In this review, we gathered main recent advances in the use of
Ru-based NPs as catalysts in relevant catalytic reactions such as
reduction, oxidation, Fischer−Tropsch, C−H activation, CO2
Figure 35. Electron-transfer processes involved in photocatalytic HER promoted by Ru NPs in the presence of the QuPh+-NA organic donor−
acceptor photoabsorber described by Fukuzumi and co-workers. Adapted with permission from ref 322. Copyright 2019 Wiley.
transformation, dehydrogenation of amine boranes, and water
splitting. All together, the research data here assembled clearly
evidence the significance of Ru metal at the nanoscale for these
reactions. If from the point of view of industrial applications
and thus at large scale and for a long-term, the use of noble
metals like Ru in catalytic conversions is certainly not realistic
due to economic reasons, Ru systems can allow developing
fundamental researches in order to better apprehend the
prerequisites for rendering a given catalysis more effective.
Recent progress in solution nanochemistry allowed having at
disposal better controlled Ru NPs (in terms of size, dispersion,
shape, composition, and surface state, etc.), all these
characteristics influencing strongly their surface properties.
Even if not always satisfying, this led to progress in the
understanding of the relationships between their structure and
their potential in catalysis (in terms of both reactivity and
selectivity). Most particularly, the surface chemistry of Ru NPs
starts to be better understood, which gives a strong basis to
better apprehend catalytic processes on the metal surface as
well as how these can be affected by the presence of stabilizing
molecules or by the crystallographic structure of the ruthenium
cores, eventually by taking benefit of these parameters.
However, this is only in its infancy and numerous studies are
trial−error or screening works and the rationalization of the
catalysis findings with the NP structural features is not often
done. Such a rationalization is not possible from published
works given synthesis conditions and parameters change from
one study to another one. More efforts are thus required in
order to bridge this gap. This is fundamental if one want to be
able one day to anticipate about the needed Ru NP structure
for making a target catalysis highly performant and also highly
selective, but this is not true only for ruthenium because such
studies are generally missing in nanocatalysis whatever the
metals used. For instance, regarding the influence of ligand,
this is not an easy task because this requires having preformed
NPs that enable a complete ligand exchange or have a synthesis
method that provides always the same size of particles
whatever the stabilizing ligand added in the reaction mixture.
To our best knowledge, such means are not accessible yet.
Concluding remarks and perspectives are hereafter given more
specifically for the catalytic reactions described above.
Ru NPs are very versatile catalysts for reduction reactions.
As reviewed above, this versatility is illustrated with the large
range of reduction reactions, including the hydrogenation of
CC, CO, and −NO2 motives using several reducing
agents. Because of the straightforward implementation of some
of these reactions, for instance, reduction of styrene by H2 or
reduction of −NO2 groups by NaBH4, and the facility to
compare the obtained results to other reported works, these
reactions are often used as an indirect characterization way to
get information on the surface properties of the nanocatalysts.
Ru-based nanocatalysts for reduction reactions underwent
important evolution in the last years. If first they were only
stabilized with simple molecules, ruthenium nanocatalysts are
now more complex because their design has strongly benefited
from the development of nanochemistry tools. Such evolution
is visible either by the use of new and sometimes sophisticated
ligands that have been deliberately designed to obtain a desired
property or by introducing a second (or more) metal or by
using a more reactive fcc structure. Water-soluble ligands or
polymers, stabilizers containing long carbon chains, and ligands
with a specific electronic property are among examples that
have been successfully explored. It is important to note that Ru
NPs systems able to induce chirality are only elusive, even if
some efforts have been done in this topic. All the knowledge
obtained in these model reactions is currently been used to
explore the applicability of Ru NPs in challenging reduction
reactions such as hydrodeoxygenation together with C−O
cleavage of biomass derivatives. Bimetallic Ru-based systems
proved to be very efficient catalysts as the result of the subtle
balance of the properties of the metals used, their combination
leading to synergistic effects. In contrast, unsupported Ru NPs
as catalysts for oxidation reactions are scarce and are essentially
devoted to the oxidation of CO. The catalysts of this reaction
are principally bimetallic systems with a specific tuning of the
NPs, or the metal ratio, or the Ru structure, or both. Ru NPs
with a fcc structure have proven to be highly reactive for this
reaction. Fischer−Tropsch reaction was demonstrated to be
also sensitive to the crystalline structure of the Ru NPs, giving
highly active catalysts when adopting the fcc structure. Also,
the reaction is sensitive to the size of the Ru NPs which can be
related to the CO energy adsorption in different surface
positions. The ability of Ru NPs to activate C−H bonds
reported in the past has been recently exploited to produce
labeled organic compounds in a highly selective manner. A
general weakness of the Ru NPs colloidal-based catalysis is the
lack of knowledge on the catalytic active species that is
operating. Characterization of the spent catalyst, recycling test,
hot filtration, among other procedures, are far to be
systematically performed, and when carried out they are not
always done in the appropriate manner (a typical example is to
carry out recycling tests at 100% conversion). In situ or in
operando characterization techniques are, by now, scarce for
these catalysts.
The chemical transformation of CO2 has not been
investigated into detail over well-defined heterogeneous
catalysts including nanoparticle-containing ones. This topic
remains a challenging but of high interest task given the
advantages provided by heterogeneous catalysts compared to
homogeneous ones for industrial applications. State-of-the-art
data revealed substantial limitations, but no clear insights at the
molecular level have been reported, hindering concrete
progress. In particular, there is a clear lack of understanding
of structure−reactivity correlations and of catalyst designing
principles for this catalysis. As described above, recent results
involving Ru-based nanocatalysts have shown that efforts
performed for the precise design of solution and supported
nanocatalysts can lead to the chemoselective CO2 hydro-
genation into HCOOH, CO, CH4, or other hydrocarbons.
Such studies make a parallel with those reported on Ru
molecular complexes. Recent knowledge and know-how in
nanotechnology and nanocatalysis should lead to novel
strategies in the design of efficient and more stable nano-
catalysts for CO2 transformation. Prospective studies with a
molecular approach may allow tuning more finely the catalytic
properties of nanocatalysts. Mechanistic details being critical to
the development of improved nanocatalysts, more investiga-
tions in this direction are also required in order to achieve
higher catalytic performances. Even if catalytic activities are not
elevated with this metal, Ru-based nanocatalysts may offer the
possibility to access spectroscopic NMR studies which can be
very complementary to infrared studies in order to get
mechanistic insights. Associate another metal (Pd, Ni, or Fe)
to Ru is certainly a strategy to explore more in order to
increase the catalytic performance (both reactivity and
selectivity). Also separately optimizing the metal active sites
Chemical Reviews Review
DOI: 10.1021/acs.chemrev.9b00434
Chem. Rev. 2020, 120, 1085−1145
1134
and the support may provide benefit. Finally, it is also needed
to keep in mind that harmonization is necessary to ensure a
constant and dynamic balance of all things to be considered,
for a sustainable and green chemistry. The use of H2 issued
from green sources like water also appears as a great solution to
reach a closed nature’s carbon cycle.
Concerning the H2 production by dehydrogenation of amine
boranes, Ru-based nanocatalysts are highly efficient and stand
at the top list. If extensive research efforts focused on the
dehydrogenation of ammonia borane by hydrolysis (due to its
simplicity and green character as well as efficiency), interesting
results were also obtained by methanolysis or dehydrocou-
pling. These last approaches merit more efforts, at least at the
fundamental level, in order to get mechanism insights, enable
the development of more performant catalytic systems and
improve hydrogen productivity. If numerous kinetics param-
eters are available and allow comparing the efficiency of the Ru
nanocatalysts reported for the dehydrogenation of amine
boranes in water, there is no clear insight explaining the high
activity generally observed. What about the real effect of
particle size, Ru crystal structure, surface area, stabilizer, and/
or support nature on the catalytic performances? Answers to
these questions remain to be found in most cases. Moreover,
the catalytic lifetime parameter has received a quite low
attention until now, whereas NPs are not thermodynamically
stable entities and can be readily deactivated, which may harm
their long-term performance. If AB solvolytic dehydrogenation
is a promising hydrogen generation system (in particular, for
cases that require a convenient and reliable hydrogen source),
the decomposition of AB results in a contamination of released
H2 by NH3 and borazine, which is a major problem for
application in fuel cells. Thus, further efforts are required in
order to solve pending issues like breaking the strong B−O
bonds in byproducts of AB solvolysis and reducing NH3
release. Other important issues are the storage irreversibility
and cost factor. Regeneration of AB from byproducts of
solvolysis, especially hydrolysis, is cost-ineffective, as undesired
byproducts of the recycling process cannot be converted to the
main reactants.243 So, other hydrogen storage materials need
to be studied in order to have less volatile byproducts than
with AB. Only a few papers deal with AMB and DMAB that
are alternative substrates, thus showing that more efforts have
to be done in this direction.
Ru-based NPs have clearly emerged as promising (electro)-
catalytic systems for the two half-cell reactions in water
splitting and potential substitutes of standard Pt and IrOx
species used for catalyzing the HER and OER, respectively, in
commercial electrolyzers. Most particularly, the development
of Ru-based NPs as catalysts for the HER was highly dynamic
in the last three years. Reports on nonsupported catalytic
systems showed that the active sites of the Ru NPs can be
tuned with ease and the surface chemistry resembles that of
molecular complexes. In this regard, the organometallic
synthesis of nanostructures opens up numerous possibilities
through the inexhaustible ligand pool of NP stabilizers. The
combination of electrochemical analysis, detailed structural
and surface characterization, and DFT modeling of the
reaction pathways involved can lead to structure−activity/
stability relationships, thus allowing the subsequent rational
improvement of the electrocatalytic HER systems.
To conclude, even if less expensive than other noble metals,
the high price and limited abundance of Ru probably hinder
the practical applications of Ru NPs-based catalysts for
industrial purposes, but studied systems are of high interest
at the fundamental level because they allow doing nice
breakthroughs and getting precious insights on the catalytic
properties of Ru NPs. As a nonexhaustive example, Ru is a 4d
transition metal that in the bulk adopts an hcp structure at all
temperatures, but thanks to the development of effective tools,
Ru NPs with a crystallographic fcc structure could be prepared
although they are thermodynamically unstable, thus high-
lighting the interest of modern nanochemistry. In this way, the
crystal phase effect of Ru could be explored toward a few
catalytic reactions (like CO oxidation, nitrophenol reduction,
hydrolysis of ammoniaborane, oxygen evolution reaction),
allowing observation of differences compared to hcp Ru NPs.
These advances underline that not only the size of the NPs is
of paramount importance if one wants to tune finely their
catalytic performance but also how important is the control of
their other characteristics such as their crystalline structure and
their composition/surface state. Indeed, catalytic properties are
closely correlated with the catalyst surface geometric and
electronic structures and an optimal compromise among
reactant adsorption rate, adsorbate−surface interaction, and
product desorption is necessary to promote catalytic activity.
This is true whatever the target catalytic reaction. It thus
requires development of effective synthesis tools in order to
have at disposal model NPs with an atomic precision level to
be able to conduct precise comparative studies. Besides the
synthesis aspects, in operando techniques could bring very
useful information on the surface state of the NPs in catalysis
conditions (IRFT, NMR, XPS, environmental-HREM, EXAFS,
etc.). Such approaches are still rare in the papers describing the
interests of well-defined Ru NPs in catalysis. Interestingly, Ru
is a metal which permits to take benefit of NMR techniques to
access a fine mapping of the surface state of the NPs, as it is
generally done for metal active centers in molecular catalysts.
Moreover, in parallel of experimental techniques, theoretical
studies can afford a better understanding of the influencing
parameters of a given catalysis within the aim to develop more
performant nanocatalysts in terms of activity and selectivity.
Efficient theoretical tools are now accessible that allow
obtaining an overview of a nanoscale surface with a resolution
close to that usually got for molecular catalysts or extended
metal surfaces. As a final message, we thus do believe that
future developments crossing experimentally well-defined
model metal nanoparticles together with theoretically close
simulated nanoclusters will enable nice breakthroughs for the
development of more performant nanocatalysts and that Ru is
a highly interesting metal to do so.
postdoctoral fellowship in Trieste (Prof. Milani), Toulouse (Dr.
Chaudret and Dr. Philippot), and Paris (Dr. Amouri), she joined
CNRSFrance as an associate researcher at the Laboratoire de
Chimie de Coordination in Toulouse, where she started her research
activities focusing on nanocatalysis. Her current research activities
include organometallic and nanomaterials chemistry areas, mainly for
applications in catalysis. She is interested in the study of the
structure−properties relationships in several nanomaterials including
bimetallic, supported, or shape-controlled nano-objects, with special
attention to the effects of the stabilizing ligands of the nanoparticles
on their properties.
Karine Philippot is research director at CNRS, at the Laboratory of
Coordination Chemistry of Toulouse, where she is the head of the
team “Engineering of Metal Nanoparticles”. Being involved in
different projects, her current research interests cover the design of
metal nanoparticles and composite nanomaterials by using molecular
chemistry concepts and their applications, mainly in colloidal or
supported catalysis and for energy production (CO2 valorization,
water-splitting, fuel cells). She is coauthor of 175 peer reviewed
papers (including 7 reviews, 9 book chapters, and 6 patents) and over
200 presentations at national and international conferences. She also
coedited a special issue devoted to “Catalysis in Solution by Defined
Nanoparticles” (Topics in Catalysis, 2013) and the book “Nanoma-
terials in Catalysis” (Wiley, 2013).
ACKNOWLEDGMENTS
We acknowledge the Laboratory of Coordination Chemistry
(LCC-UPR8241), the Centre National de la Recherche
(CNRS), and the University de ToulouseUniversite ́ Paul
Sabatier for financial support.
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Please, summarize the all 6 articles (5 articles and part of chapter of book) in two pages, with the third page of references. Then complete the power point. Example from each article (one article in each slide).
Please, no space between lines.
Write the whole two pages.
Please, take the information of ( general application of Ru) from this chapter of this book(Properties and Applications of Ruthenium)
Please, note that I will ask you to use those 6 articles with other new 6 articles and 2 parts of chapters of books) to write short research (about 15 to 20 pages) of this topic( Ru element: application catalysis with other information).
Ru Metal application
Final presentation
Here is the name of the authors
Represented by my name
Introduction
Main topic Focus on first general information about Ru application then second, Ru nanoparticles as catalysis in different application(Ru
Nanoparticles
: Application in
Catalysis)
Please, use one example from each article if possible, or if there is one example summarize it.
General Application of Ru
Application 2
Ruthenium Nanoparticles Decorated Tungsten Oxide as a
Bifunctional Catalyst for Electro catalytic and Catalytic Applications
Application 3
Catalysis with Colloidal Ruthenium Nanoparticles
Application 4
Sensitive Colorimetric Assay of H2S Depending on the High-Efficient
Inhibition of Catalytic Performance of Ru Nanoparticles
Application 5
Synthesis of PtRu Nanoparticles from the Hydrosilylation Reaction and Application as
Catalyst for Direct Methanol Fuel Cell
Application 6
Role of Ru Oxidation Degree for Catalytic Activity in Bimetallic Pt/Ru
Nanoparticles
Conclusion
(6 References) page
Photochemistry Assignment #
2
This assignment covers material from Chapter 2 section 1 to Chapter 2 Section 14.
1) According to the principles of quantum mechanics, what is the wave function?
2) What is the Born-Oppenheimer approximation?
3) Under what two types of interactions does the approximation given in equation 2.4
break down?
4) (a) What is the four-letter abbreviation for the highest energy occupied molecular
orbital?
(b) What is the four-letter abbreviation for the lowest energy unoccupied molecular
orbital?
5) To what does the square of a wave function relate?
6) What is an expectation value?
7) What is meant by an electronic configuration?
8) An alkene like ethylene will usually have only one low-energy electronic transition.
What is it?
9) An organic molecule which contains a carbonyl functional group, like formaldehyde,
typically has two relatively low-energy electronic transitions. What are they?
10) What are the two possible spin states for the configurations shown for ethylene and
formaldehyde in Figure 2.1-b?
11) A single electron configuration is often adequate to approximate the electronic
characteristics of an electronic state. In some cases, however, a combination of two or
more configurations are required to achieve a good approximation of a single state.
When does this occur?
12) (a) What is Hund’s rule for organic photochemistry?
(b) What is the physical basis for this rule?
13) (a) What is the symbol for the Coulomb integral?
(b) What is the symbol for the electron exchange integral?
(c) Are both of these integrals positive quantities? Why?
(d) Which integral is purely a quantum mechanical phenomenon?
14) According to equation 2.18, the difference in the electronic energy between singlet
and triplet states derived from the same electron orbital configuration (ΔEST) is equal to
what?
15) In Section 2.14, the text says that the
2
12
e
r term can be factored out of the electron
exchange integral leaving the overlap integral. The text then states that the quantum
mechanical mathematical overlap integral corresponds to the degree of physical overlap
of the orbitals in space. Thus, the smaller the overlap, the smaller the value of the
overlap integral; and the larger the overlap, the larger the value of the overlap integral.
(a) So, in a carbonyl containing compound, which is larger: , *nJ which is proportional to
, *n or , *J which is proportional to , * ?
(b) Since the value of ΔEST is related to the value of the electron exchange integral J,
which is larger: ΔEST from a , * configuration in ethylene or ΔEST from a , *n
configuration in formaldehyde?
16) Does increasing conjugation increase or lower ΔEST from the , * configuration in
alkenes? (Refer to Table 2.3.)
Annotated Bibliography
Your annotated bibliography should be prepared according to ACS format, using The ACS Style Guide: Effective Communication of Scientific Information, 3rd edition. One or more copies of this book are held on reserve in the library.
offers further information regarding annotated bibliographies.
This annotated bibliography, along with a correctly formatted citation, should include a summary of the content of the source and a two-pronged critical analysis of the source. The first part of the critical analysis will be your objective evaluation of the source and the second part will be your subjective evaluation. Even if a source is found to be credible, if it does not contribute to your research question, it should not be included.
Prepare your annotated bibliographic entry according to the following guidelines:
1. Bibliography Entry: Include the complete bibliographic information correctly formatted according to the ACS style guidelines
1. Summary of Content: Include a descriptive paragraph summarizing the source. Include key concepts and quotations when appropriate.
Objective Evaluation: Objectively evaluate the credibility of the source using the criteria that are most relevant. Use the questions presented in the TRAAP criteria found under “Evaluate Sources” at
1. to stimulate your ideas, but don’t feel as if you need to address each criteria as a checklist. Use the criteria that are appropriate for your source. When relevant, address such things as bias or lack of bias, outdated material or current material, author’s point of view, and author’s credentials and qualifications to write on the topic. What is the author’s purpose in writing the information? Is the information presented without prejudice? Or does the author, publisher, or research funding organization have a stake in the outcome or the controversy you are investigating?
1. Subjective Evaluation: Include a summary of the relevance of the source to your research topic or question. How will the source contribute to your research, and how useful will it be? Does it offer a unique perspective? Does it offer a contradictory viewpoint to another source?
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