Synthesis of PtRu nanoparticles from the hydrosilylation reaction and application as catalyst for direct methanol fuel cell

Article abstract of DOI:10.1021/jp052667j

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 (hep) 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; Pt ₅₆Ru₄₄/C have displayed the best electrocatalytic performance among all carbon-supported catalysts.

Full text of DOI:10.1021/jp052667j

16644
J. Phys. Chem. B 2005, 109, 16644-16649
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
SCHEME 1: Hydrosilylation Reaction
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,24,25,33,34 which 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.
* Corresponding author. Telephone: 65-68741972. Fax: 65-68727528.
E-mail: jc-huang@imre.a-star.edu.sg.
10.1021/jp052667j CCC: $30.25 © 2005 American Chemical Society
Published on Web 08/13/2005

PtRu Nanoparticles for Direct Methanol Fuel Cell
J. Phys. Chem. B, Vol. 109, No. 35, 2005 16645
TABLE 1: Formulation of the Hydrosilylation Reaction for Nanoparticle Synthesis
system
Pt
Pt89Ru11
Pt77Ru23
Pt56Ru44
Pt26Ru74
Ru
Ru(PPh3)3Cl2
0 mmol, 0 mg
0.01 mmol, 9.5 mg
0.0198 mmol, 19.0 mg
0.0395 mmol, 37.9 mg
0.0593 mmol, 56.9 mg
0.0792 mmol, 75.9 mg
Pt(dvs) (10mM)
toluene (mL)
Reaction time (days)
Yield (%)a
0.041 mmol, 4.1 mL
0.0358 mmol, 3.58 mL
0.0307 mmol, 3.07 mL
0.0205 mmol, 2.05 mL
0.0103 mmol, 1.03 mL
0 mmol, 0 mL
14.0
15.0
15.5
16.5
17.0
18.0
2
4
4
4
4
5
96
92
95
93
92
90
a The yield of the hydrosilylation reaction was calculated based on the amount of the metal starting materials.
Experimental Section
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.
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.
H
Synthesis of Pt, Ru, and PtRu Alloy Nanoparticles. T2
(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.
Results and Discussion
Nanoparticles Preparation. In our previous paper,27 when
H
1-decene and T2 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,
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

16646 J. Phys. Chem. B, Vol. 109, No. 35, 2005
Huang et al.
Figure 1. XRD pattern of Ru nanoparticles from the hydrosilylation
reaction. Inserts show the TEM image and size distribution of Ru
nanoparticles.
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
kλ
d (Å) )
â cos(θ)
where k 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), and θ 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.
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.
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

PtRu Nanoparticles for Direct Methanol Fuel Cell
J. Phys. Chem. B, Vol. 109, No. 35, 2005 16647
Figure 4. X-ray diffraction patterns of the heat-treated Pt/C, PtRu/C,
or Ru/C catalysts for the fuel cell.
Figure 6. X-ray photoelectron spectra of the as-synthesized and heat-
treated PtRu catalysts for fuel cell.
Figure 5. Dependence of particle size on Ru content of PtRu
nanoparticles.
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.
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.
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 C1s EB 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 at EB
) 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 at EB ) 461.0 and 462.9 eV, which
corresponded well with Ru(0) and RuO2,40 respectively. It was
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-

16648 J. Phys. Chem. B, Vol. 109, No. 35, 2005
Huang et al.
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.
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.
TABLE 2: Performance of the Heat-treated Pt/C and
PtRu/C Catalysts
potentials
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. Low If/Ib ratio indicates
poor oxidation of methanol to carbon dioxide during the anodic
scan and excessive accumulation of carbonaceous residues on
the catalyst surface. High If/Ib ratio shows the converse case.
catalyst
Pt/C
Pt89Ru11/C
Pt77Ru23/C
Pt56Ru44/C
Pt26Ru74/C
Ru/C
onset
anodic peak
If/Ib ratio
0.40
0.39
0.28
0.20
0.36
0.48
0.69
0.70
0.74
0.55
0.71
1.10
1.21
1.54
6.73
1.60
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.
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 the If/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 where I 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.
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.

PtRu Nanoparticles for Direct Methanol Fuel Cell
J. Phys. Chem. B, Vol. 109, No. 35, 2005 16649
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Figure 9 shows the different curves of current decay for each
carbon-supported catalyst. For heat-treated Pt/C and Pt89Ru11,
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supposedly because of catalyst poisoning by the chemisorbed
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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.
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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.
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