Microwave-assisted microemulsion synthesis of carbon supported Pt-WO 3 nanoparticles as an electrocatalyst for methanol oxidation

Article abstract of DOI:10.1016/j.electacta.2012.04.107

A microwave-assisted microemulsion synthesis of carbon supported Pt-WO ₃ nanoparticles is presented. Amorphous WO₃ nanoparticles of 1.0 nm in size were first deposited onto carbon from an alkaline tungstate containing microemulsion via mixing with an acid containing microemulsion under controlled microwave exposure. Platinum was subsequently deposited onto the carbon supported WO₃ nanoparticles by reducing H₂PtCl ₆ in a microemulsion under controlled microwave exposure forming nanoparticles with an average size of 2.5 nm. This method produced a homogeneous distribution of nanoparticles on the carbon support with a uniform Pt:W ratio. The Pt:W can be well controlled by varying the metal precursors' Pt:W ratio. Hydrogen adsorption, CO-stripping and Cu-UPD stripping method were used to estimate the electrochemical surface area of Pt in the Pt-WO₃ mixed catalyst system. Cyclic voltammetry and chronopotentiometry experiments demonstrate that a 1:1 ratio of Pt:W has the highest electrocatalytic activity based on Pt mass for methanol oxidation in sulfuric acid. The enhanced electrocatalytic activity is attributed to both physical and chemical influence of the WO₃ component.

Full text of DOI:10.1016/j.electacta.2012.04.107

Electrochimica Acta 75 (2012) 262–272
Contents lists available at SciVerse ScienceDirect
Electrochimica Acta
journal homepage: www.elsevier.com/locate/electacta
Microwave-assisted microemulsion synthesis of carbon supported Pt-WO3
nanoparticles as an electrocatalyst for methanol oxidation
a
a,b,∗
a,∗∗
, Xin Zhangc
Chunzhen Yang , Nicole K. van der Laak
, Kwong-Yu Chan
a
Department of Chemistry, The University of Hong Kong, Pokfulam Road, Hong Kong
Department of Chemistry, Centre for Advanced Energy Storage and Recovery (CAESAR), Herriot-Watt University, Edinburgh EH14 4AS, United Kingdom
Department of Chemistry, Shantou University, China
b
c
a r t i c l e i n f o
a b s t r a c t
Article history:
A microwave-assisted microemulsion synthesis of carbon supported Pt-WO3 nanoparticles is presented.
Amorphous WO3 nanoparticles of 1.0 nm in size were first deposited onto carbon from an alkaline
tungstate containing microemulsion via mixing with an acid containing microemulsion under controlled
microwave exposure. Platinum was subsequently deposited onto the carbon supported WO3 nanoparti-
cles by reducing H2PtCl6 in a microemulsion under controlled microwave exposure forming nanoparticles
with an average size of 2.5 nm. This method produced a homogeneous distribution of nanoparticles on
the carbon support with a uniform Pt:W ratio. The Pt:W can be well controlled by varying the metal
precursors’ Pt:W ratio. Hydrogen adsorption, CO-stripping and Cu-UPD stripping method were used to
estimate the electrochemical surface area of Pt in the Pt-WO3 mixed catalyst system. Cyclic voltammetry
and chronopotentiometry experiments demonstrate that a 1:1 ratio of Pt:W has the highest electrocat-
alytic activity based on Pt mass for methanol oxidation in sulfuric acid. The enhanced electrocatalytic
activity is attributed to both physical and chemical influence of the WO3 component.
Received 9 November 2011
Received in revised form 25 April 2012
Accepted 28 April 2012
Available online 6 May 2012
Keywords:
Pt-WO3 nanoparticles
Microwave
Microemulsion
Methanol oxidation
Cu-UPD
©
2012 Elsevier Ltd. All rights reserved.
1
. Introduction
for methanol electro-oxidation. Among these, Pt-Ru bimetallic cat-
alysts dispersed on carbon supports show the best performance. As
a precious metal, however, ruthenium is as expensive as Pt and does
not help to reduce production costs in scaled commercial applica-
The interest in developing fuel cells for high energy density
applications, in particular in long-range transportation continues
to grow. Platinum has been the most effective electrocatalyst for
anodic oxidation of fuels at low temperature, but the commonly
generated CO intermediate seriously undermines the catalytic
activity of platinum. The development of CO-tolerant electrocata-
lysts and the lowering of Pt loading for cost-effectiveness are some
of the critical issues facing commercialization of fuel cells [1–3].
The addition of a second or third metal has demonstrated to be
effective in increasing performance as a function of Pt loading, as
well as improving the resistance to CO poisoning. According to the
bi-functional mechanism, the CO intermediate formed on the Pt
sites can be oxidized by an oxygen-containing species on the neigh-
bouring co-catalyst metal [4,5]. Binary and ternary platinum-based
alloys such as Pt–Ru [6,7], Pt–Mo [8,9], Pt-Bi [10], Pt-Co [11,12], Pt-
Mn [12], and Pt–Ru–M [13–15], and metal oxides, including RuO2
tions. Apromisingcost-effective alternativetoRuis WO , whichhas
3
been extensively studied for promoting the electrocatalytic activity
of Pt. The costs saving effectiveness, however, should be compared
in terms of both current and voltage output. Tseung’s group demon-
strated the hydrogen spill-over effect [29–32] for typical fuel cell
anode reactions on Pt/WO electrocatalyst when WO forms a con-
3
3
ducting, blue hydrogen tungsten bronze, HxWO , when receiving
3
protons in acidic media at the potential region of 0–0.55 V vs. RHE,
as shown in the reaction.
HxWO3 ↔ WO3 + xH+ + xe−
(1)
The formation of HxWO3 effectively enhances the dehydrogena-
tion step of in a fuel cell anode reaction such as methanol oxidation
[29–32]. In addition, OH groups adsorbed on the WO3 surface may
help to oxidize the CO intermediate formed on the Pt site [29],
giving the bi-functional catalyst characteristics [30,33–38].
[
[
16], TiO2 [17,18], CeO2 [19,20], MoOx [21,22], SnO2 [23] and WO3
24–28] have been studied to improve stability and CO tolerance
Understanding the structural and compositional effects of Pt-
WO3 on electrocatalysis has been hindered by our ability to
synthesize nanoparticles with good control over particle size and
the Pt:W ratio. Typically Pt nanoparticles are supported on a
WO3 support by, for example, reducing a platinum precursor
∗
Corresponding author.
Corresponding author at: Department of Chemistry, The University of Hong
∗
∗
Kong, Pokfulam Road, Hong Kong.
E-mail address: n.vanderlaak@gatesscholar.org (N.K. van der Laak).
0
013-4686/$ – see front matter © 2012 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.electacta.2012.04.107

C. Yang et al. / Electrochimica Acta 75 (2012) 262–272
263
Fig. 1. The preparation process of making Pt-WO3/C electrocatalysts through microwave-assisted microemulsion.
onto commercial monoclinic tungsten oxide powder [33,39,40] or
by precipitating Pt nanoparticles onto mesoporous WO3 [25,41].
Other morphologies of tungsten oxide such as nanotubes [42] and
nanorods [43,44], have also been prepared and used as supports for
present a new approach to synthesizing nanosized Pt-WO3 using a
microwave-assisted microemulsion methodology. Microwave
heating is a good alternative to traditional approaches offering
faster, more uniform control of heating and has already been used
to successfully synthesize nanosized metal and metal oxide parti-
cles including gold [51,52], palladium [53,54], copper oxide [55],
tungsten oxide [49], and zinc oxide [56]. In our case, the Pt-WO3
nanoparticles synthesized in this approach show a narrow size dis-
tribution and are well dispersed on a carbon support. We discuss
here the details of the synthesis, characterization of the Pt-WO3
nanoparticles, and the electrocatalytic activities of the nanoparticle
system towards methanol oxidation.
platinum. While these Pt/WO composites show high CO tolerance
3
and electrochemical activity comparable to commercial PtRu/C as
an anode catalyst, the Pt:W ratio is not controlled and tungsten
oxide is normally in excess. The effects of Pt:W ratio, size and struc-
ture of WO on the electroactivity cannot be clarified in studies that
3
contain excess WO3.
While syntheses of Pt nanoparticles have been well reported,
the synthesis of Pt-WO3 with control of the Pt:W ratio at the
nanoparticle level has been difficult. Platinum is usually deposited
by chemical reduction while WO3 is typically deposited from
tungstate, the deposition of which is induced by a pH change
according to the reaction WO42− + 2H → WO ·H O. Controlling the
2. Experimental
+
3
2
size of the deposited WO nanoparticles in this reaction is problem-
2.1. Synthesis
3
atic. Different methods have been used to prepare WO3 powders,
including electrochemical deposition [45], chemical vapor deposi-
tion [46], wet chemical deposition [26], a microemulsion-mediated
method [47], a sol–gel method [48], hydrothermal reactions [49],
and impregnation [50]. The size of WO3 particle highly depends
on the preparation technique and few of these techniques have
reported nanosized WO3 particles less than 2 nm in diameter.
Moreover, the crystallinity and particle morphology changes with
increasing particle size. As tungsten oxide has limited electrical
conductivity the electroactivity can be adversely affected when
WO3 is present as larger particles. This in turn may affect the
promotion of Pt and the ability to remove CO. Some control of
All chemicals were of analytical grade and used as received.
They include Triton X-100 (t-octylphenoxypolyethoxyethanol)
from Sigma Chemical Company, dihydrogen hexachloroplatinate
(IV) hydrate (H PtCl ·H O) from Chempure Ltd., sodium tungstate
2
6
2
(Na WO ) from Aldrich Chemical Company, Inc, butan-1-ol and
2
4
cyclohexane from BDH Chemical Ltd., methanol and sulfuric acid
from Aldrich Chemicals. A 10 wt% Pt/Vulcan carbon was supplied
by E-TEK Co., and a 5 wt% Nafion solution was used as received from
DuPont. All the solutions were prepared using deionized water
(18.2 Mꢀ cm) produced from a Milli-Q ultrapure system of Milli-
pore Ltd., USA.
WO nanoparticle sizes has been achieved by hydrolyzing tungsten
A typical Pt-WO /carbon catalyst was prepared in two steps
3
3
alkoxide in a reverse microemulsion, forming nanoparticles whose
as depicted in Fig. 1. Firstly, WO3 was deposited onto the Vulcan
XC-72 carbon by a two-emulsion technique. The two microemul-
sions had identical fractions of aqueous, surfactant, co-surfactant,
and oil phases, but contained different compositions in their aque-
ous phases, as shown in Table 1. Two 10 ml portions were taken,
each from the two microemulsions and mixed together with Vul-
can XC-72 carbon by sonicating the solutions for 30 min. The mixed
microemulsions were sealed in a HP-500 PTFE reactor and heated
in a microwave oven (CEM MARS) at 300 W for 6 min. We observe
at 300 W irradiation power, small WO3 nanoparticles formed only
after a minimum of 6 min heating time according to the reaction
size was controlled between 1.5 nm and 50 nm [47]. WO nanopar-
3
ticles approximately 1.4 nm in diameter have also been successfully
formed in the mesopores of a silica matrix [50].
Recently Pt-WO3 nanoparticles were synthesized with good
control of Pt:W ratio at the nanoparticle level using ethylene
glycol as a medium for depositing colloidal Pt and WO42
−
precursors [26]. The H PtCl6 precursor was first reduced in alka-
2
line glycol to a colloidal form and following a decrease in the
pH, subsequent deposition of Pt and WO3 (from WO42 ) was
induced. Due to the different and somewhat slow deposition
rates of Pt and WO42−, good particle size control was difficult
to achieve.To control both the composition and particle size we
−
2−
+
WO4 + 2H → WO ·H O. After cooling to room temperature, the
3
2
suspension was centrifuged and the solid fraction was washed

2
64
C. Yang et al. / Electrochimica Acta 75 (2012) 262–272
Table 1
Compositions of the microemulsion systems used for the synthesis of WO3 nanoparticles.
Microemulsion I
Microemulsion II
Volume (ml)
Aqueous phase
Surfactant
Co-surfactant
Oil phase
20 mM Na2WO4
Triton X-100
Butan-1-ol
0.1 M HCl aq. solution
Triton X-100
Butan-1-ol
6
12
12
60
Cyclohexane
Cyclohexane
Table 2
Attempts to simultaneously react Pt and W together in a
single step microwave exposure did not result in a uniform
composition, probably due to the two deposition reactions having
different reaction rates. Moreover, reversing the order of prepara-
tion by deposition Pt onto carbon first, did not yield a uniform Pt:W
ratio. By precipitating WO3 first, a uniform distribution on the car-
bon support can be achieved, which can then act as nucleation sites
for subsequent uniform deposition of Pt.
Compositions of the microemulsion system used for the synthesis of Pt nanoparticles
deposited onto WO3/C.
Microemulsion
Volume (ml)
Aqueous phase
Surfactant
Co-surfactant
Oil phase
10 mM H2PtCl6 aq. solution
Triton X-100
Butan-1-ol
6
12
12
60
Cyclohexane
2.2. Characterization
with ethanol and acetone to remove surfactant and any unreacted
WO42 , and then dried at 60 C in a vacuum oven.
−
◦
2.2.1. Transmission electron microscopy (TEM)
Pt nanoparticles were then deposited on to the carbon sup-
ported WO by reducing H PtCl in butan-1-ol. An aqueous solution
Transmission electron microscopy analysis of the Pt-WO3/C cat-
alyst was performed on a Philips Tecnai G2 20 Scanning TEM
operating at 200 kV. High-resolution TEM (HRTEM) images were
obtained on a JEOL 2010F TEM. Composition of the nanoparticles
was analyzed by an energy-dispersive X-ray (EDX) spectroscope
(EDX Microanalysis System, Oxford Instrument) attached to the
transmission electron microscope (Philips Tecnai G2 20 Scan-
ning TEM). The samples for analysis were prepared by dispersing
nanoparticles in ethanol and drop-casting onto a carbon coated
3
2
6
of H PtCl was used to prepare the microemulsion (Table 2).
2
6
This Pt containing microemulsion was thoroughly mixed with
WO /carbon by sonicating for 30 min. The mixture was sealed in
3
a HP-500 PTFE reactor and heated in the microwave oven at 900 W
for 2 min.
We observed that Pt nanoparticles began to form after 1.5 min
and that a wider distribution of particle sizes was obtained for
lower power and longer exposure times. After heating, the HP-500
PTFE reactor was quickly cooled to room temperature by flow-
ing compressed air over the tube to suppress further nucleation
and/or growth of the Pt nanoparticles. The product catalyst was
◦
copper grid followed by drying in air at room temperature (22 C).
2.2.2. Thermal gravity analysis (TGA)
The Pt and WO3 loading on the Vulcan XC-72 carbon support
was determined by TGA. Thermal analyses were performed on a
PERKIN ELMER TGA-7 Thermogravimetric Analyzer. The samples
◦
then washed by ethanol and acetone, then dried at 60 C in a vac-
uum oven.
◦
◦
As with the recent report on the synthesis of PtWO3 nanopar-
ticles [26], we used carbon as a support to provide sites for the
deposition of the metal from the microemulsions. We propose that
the carbon support acts to as a medium for controlling the size of the
nanoparticles; for a deposition reaction occurring outside the pores
of carbon material, the metal nanoparticles nucleate directly on
the carbon and further growth is prevented, while for metal depo-
sition within the pores of carbon, the reaction zones are isolated
and small pores limit the growth of the nanoparticles even when
surfactant protection is absent. In the earlier reported work [26],
a Pt glycol colloidal solution was mixed with a pre-acidified car-
bon pre-deposited with WO3 nanoparticles. Since the acidic sites
on the carbon are different from the WO3 sites, pure Pt nanopar-
were hold at 50 C for 10 min then heated to 800 C at a rate of
◦
10 C/min under flowing air.
2.3. Electrochemical measurements
The working electrodes were prepared by dispersing 5 mg of
catalyst in a mixed solution of 0.9 mL ethanol/water (1:1, volume
scale) and 0.1 mL Nafion (5 wt%) and sonicating the suspension for
20 min. 20 ␮L of the suspension was dropped onto on to a glassy
carbon electrode (5 mm in diameter) previously polished to mirror-
like finish using an alumina suspension. The electrode was then
dried at room temperature before use. Table 3 shows the catalyst
loading on each of the working electrodes.
ticles may be formed away from WO . In the present synthesis,
All electrochemical tests were performed on a Solatron SI1287
in a three-electrode cell at room temperature (22 C). A Pt plate
3
◦
we suggest that the platinum precursor is uniformly impregnated
into carbon pores when microwave irradiation is triggered and the
Pt nanoparticles preferentially nucleate at the WO3 sites. As the
dielectric constant of carbon is lower than water, the reactions
zones can also be confined to individual regions of water within
the carbon pores.
and an Hg/Hg SO4 electrode were used as the counter and the ref-
2
erence electrodes, respectively. A solution of 1 M methanol in 0.5 M
H SO was used to study the methanol oxidation activity. The elec-
2
4
trolytes were degassed with nitrogen before each electrochemical
measurement. Cyclic voltammetry (CV) and chronopotentiometry
Table 3
The composition of the Pt-WO3/C catalysts analyzed by EDX.
Elements (wt%)a
Pt
−2
Pt loading on GC electrode (mg cm )
Pt-WO3/C samples
WO3
WO3/C
–
12.1%
–
0
1
2
.5:1
:1
:1
4.73%
9.36%
16.57%
11.47%
10.91%
10.05%
0.024
0.047
0.083
a
Pt and WO3 loading on Vulcan carbon support were determined by thermal gravimetric analysis (TGA).

C. Yang et al. / Electrochimica Acta 75 (2012) 262–272
265
◦
CP) were performed at room temperature (22 C) to investigate the
(
electrochemical properties of the catalysts.
The electrochemical active surface area (ECSA) of Pt in Pt-WO3
mixed catalyst system was determined by hydrogen adsorption,
CO-stripping method and Cu underpotential deposition/stripping
method. In the CO-stripping experiment, the pre-adsorption of
CO was achieved by bubbling CO gas into 0.5 M H SO4 solution,
2
while the potential of working electrode was kept at −0.54 V vs.
Hg/Hg SO reference electrode for 20 min. The solution was later
2
4
purged with N2 gas for another 30 min to remove the dissolved
CO in the solution. The Cu-UPD experiment was conducted accord-
ing to previous reports [27,57]. In a solution containing 0.002 M
CuSO /0.1 M H SO , the working electrode was held at −0.34 V vs.
4
2
4
Hg/Hg SO reference electrode for 60 s before the stripping of Cu
2
4
with a linear scan from −0.34 to 0.46 V vs. Hg/Hg SO .
2
4
3
. Results and discussion
3.1. Characterization
Fig. 2(a) shows a HRTEM micrograph of WO nanoparticles sup-
3
ported on Vulcan XC-72 carbon. The sizes of around 100 WO3
particles were measured to construct a histogram of the par-
ticle size distribution (Fig. 2(b)). These nanoparticles show an
average diameter of 1.0 nm. For all batches of the WO /carbon
3
hybrid catalysts prepared by this procedure, a narrow particle size
distribution is observed, indicating that the microwave-assisted
microemulsion method offers a reproducible route to synthesize
WO3 nanoparticles with a small average size and a narrow parti-
cle size distribution. The size of WO3 nanoparticles can be tuned
by varying the composition of microemulsion, the concentration
of W and HCl in the precursor solutions, and the microwave irra-
diation power and heating length. No lattice-fringe images were
obtained in the HRTEM analysis and the diffract pattern in the inset
of Fig. 2(a) suggests that the WO3 nanoparticles are amorphous.
Longer microwave exposure or higher precursor concentrations
leads to the formation of larger WO nanoparticles. Fig. 2(c) shows a
3
TEM micrograph of WO nanoparticles supported on Vulcan XC-72
3
carbon with an average particle size of 6–10 nm in diameter. This
sample was prepared by mixing a 200 mM Na WO microemulsion
2
4
with a 2 M HCl-containing microemulsion while keeping the other
reaction conditions the same as in the sample shown in Fig. 2(a).
We observe that high precursor concentration can accelerate the
growth of WO3 nanoparticles. Fig. 2(d) shows size distribution of
the nanoparticles.
Fig. 2. (a) HRTEM image of the as-synthesized WO3 nanoparticles supported on VC
X-72 carbon. (b) Histogram of particle size distribution for 100 WO3 particles. (c)
TEM images of the WO3 nanoparticles supported on VC X-72 carbon synthesized
with high precursor concentration. (d) Histogram of particle size distribution for
Batches of Pt-WO nanoparticles with different Pt:W ratios were
3
prepared using the same WO /C starting materials which contain
3
amorphous WO3 nanoparticles of 1 nm in diameter as shown in
100 WO3 particles.
Fig. 2(a). The HRTEM images of typical Pt-WO nanoparticles on car-
3
bon with 1:1 Pt:W ratio are shown in Fig. 3(a–c). Fig. 3(d) shows that
the Pt-WO nanoparticles have an average diameter of 2.5 nm and a
3
narrow size distribution. In HRTEM images of Fig. 3(a–c), crystalline
lattice regions are observed. The high intensity lattice spacings of
a standard face centered cubic (fcc) Pt crystal are 0.227 nm cor-
responding to {111} planes and 0.196 nm corresponding to {200}
planes. From the HRTEM images and the FFT analyses shown in
the insets, only fcc lattices were observed correspond to that of
platinum crystals while the rhombic or monoclinic lattices of WO3
with large spacings were not observed. This, however, cannot rule
domains are therefore most likely to be covered or overlapped with
Pt fcc nanoparticles. The lattice spacings in Fig. 3(a), (b), and (c)
are 0.224, 0.223, 0.224, 0.197, 0.195, 0.198 nm, which are slightly
different from the standard values of fcc Pt. It is possible that
the lattice spacing is affected by the presence of W and O atoms.
Many nanoparticles show clear flat surfaces and edges with def-
inite angles, which is uncommon for Pt nanoparticles as small as
2.5 nm. This suggests some influence of WO3 domains on the lat-
tice structure of adjacent Pt nanocrystals. It is unclear whether and
out the presence of WO domains adjacent to Pt nanoparticles. The
3
small WO3 nanoparticles, if present, as in Fig. 2(a) would be amor-
phous. In the HRTEM images of PtWO3 samples, we cannot not
observe individual small amorphous nanoparticles as in Fig. 2(a)
while the corresponding EDX analysis (as shown in Fig. 5) of the
same TEM sample indicates the presence of tungsten. The WO3
how the interfaces of adjacent domains of Pt and WO enhance the
electrocatalytic activity for methanol oxidation.
From electrochemical surface area measurements, as discussed
in later section and shown in Table 4, the Pt surface area increase
3
with the presence of WO , as compared to the case of pure Pt
3

2
66
C. Yang et al. / Electrochimica Acta 75 (2012) 262–272
Fig. 3. (a)–(c) HRTEM images of Pt-WO3 nanoparticles supported on VC X-72 carbon with 1:1 Pt:W ratio with inserted FFT analysis of the region marked by the white
rectangular. (d)–(f) TEM images with inserted histograms of particle size distribution for Pt-WO3 nanoparticles (d) Pt:W = 1:1, (e) Pt:W = 0.5:1 and (f) Pt:W = 2:1.
Table 4
The electrochemical active surface area (ECSA) of Pt sites in Pt-WO3/C catalysts determined by H-adsorption, CO-stripping, and Cu-UPD method. (*Note: Pt surface area is
calculated based on the average Pt particle sizes estimated by TEM, assuming that all Pt particles are spherical and with the same diameter. The estimated average particle
sizes for Pt-WO3/C catalysts with Pt:W ratio = 0.5:1, 1:1, and 2:1 are 1.8, 2.5, and 2.7 nm, respectively.).
ECSA of Pt (m2
g
−1
)
Pt surface area
estimated from
Pt:W ratio in samples
TEM* (m2
g
−1
)
By H adsorption
By CO-stripping
By Cu-UPD
0
1
2
1
.5:1
:1
:1
163
174
116
89
149
133
96
129
108
97
155
112
104
93
:0 (E-TEK Pt/C)
94
86

C. Yang et al. / Electrochimica Acta 75 (2012) 262–272
267
1
00
2
1
1
0
.0
.5
.0
.5
8
6
4
2
0
0
0
0
0
Pt-WO /C 2:1
26.2 %
20.3 %
16.2 %
3
Pt-WO /C 1:1
3
Pt-WO /C 0.5:1
3
1
2.1 %
WO /C
3
0.0
0
.0
0.2
0.4
0.6
0.8
1.0
0
200
400
600
800 1000
o
Pt:W atomic ratio in precursors
Temperature / C
Fig. 6. Correlation of Pt:W atomic ratio in the synthesized Pt-WO3/C catalysts to
Pt:W atomic ratio in precursors. (Note: Pt:W atomic ratios in the synthesized Pt-
WO3/C catalysts are obtained from EDX analysis, as described in Section 2.)
Fig. 4. Thermal gravimetric analysis (TGA) of as-synthesized WO3/C and Pt-WO3/C
electrocatalysts with different compositions.
nanoparticles. This also suggests deposition of platinum onto the
small amorphous WO3 nanoparticles.
ratio across the different areas analyzed are in close agreement. A
higher W content in the initial precursor indicates that the pre-
cipitation of tungsten oxide or the adsorption onto carbon support
3.2. Control of Pt:W ratio
may not be completed in the first WO synthesis step. However, this
3
incomplete reaction, does not affect the control of the Pt:W ratio in
the final product and we are able to control the Pt:W ratio simply
by changing the amount of Pt containing microemulsion added in
the second step while using the same reaction conditions in the
first WO3 precipitation step.
The WO and Pt weight percentage of the WO /C and Pt-WO /C
3
3
3
catalysts were summarized in Table 3. They were determined by
TGA results in Fig. 4 and Pt:W ratio from EDX analyses. It is known
that ionic surfactants are difficult to remove from the surface of
nanoparticles. In our synthesis, non-ionic surfactant Trition X-100
was used. TGA analysis showed that Triton X-100 was removed
successfully after washing by ethanol and acetone for 4–6 times.
The chemical compositions of the as-synthesized catalysts were
3.3. Measurement of Pt active surface area
The measurement of electrochemical active surface area of the
confirmed by EDX measurements. Different regions with analyti-
cal areas of approximately 100 nm2 were randomly selected and
catalysts is important to evaluate electrocatalytic activity based on
accessible surface. We have separately evaluated ECSA based on
three methods, viz. electrosorption of hydrogen, CO stripping and
Cu-UPD stripping.
analyzed. Fig. 5 shows a typical EDX spectrum of the Pt-WO /C cat-
3
alysts with a 1:1 Pt:W ratio. In different analytical regions the Pt:W
ratio was found to be consistently 1:1. The EDX spectrum shows,
in addition to Pt and W, C and O. The loading of Pt and W cannot
be determined by EDX analysis as a carbon TEM grid was used. The
absence of a chlorine and sodium peaks indicates that the chlorine
and sodium ions were completely removed in the washing steps.
3.3.1. ECSA by hydrogen adsorption
The active surface area of a Pt catalyst is usually determined
by hydrogen adsorption/desorption assuming that a monolayer of
adsorbed H formed on the surface Pt sites [58,59]. The charge asso-
Since the same starting WO /C materials was used to prepare var-
3
+
ious electrocatalyst of different Pt:W ratios, the Pt:W ratios in the
synthesized products can also be calculated from the TGA results
of different samples. The results of TGA are consistent with those
analyzed by EDX.
Fig. 6 compares the Pt:W ratio in the each of the synthesized
catalyst with the initial Pt:W ratio used in the precursors. The Pt:W
ciated with the H adsorption–desorption reaction on Pt sites is
−
2
210 ␮C cm , related to the number of platinum atoms exposed
[59].
+
+ Ptsurf + e− → Pt-H 210 ␮C cm−2
H
(2)
For Pt-WO /C catalyst, the hydrogen desorption method (in the
3
potential region 0–0.3 V vs. RHE) faces uncertainty because of the
interaction between WO and protons forming tungsten bronzes in
3
the potential region of 0–0.55 V vs. RHE [29,36,37,60–62]. Fig. 7(a)
shows the cyclic voltammograms in sulfuric acid medium of
1
7
2.8 wt% WO3 on Vulcan carbon in contrast to blank Vulcan XC-
2 carbon. It can be observed that the interaction of protons and
WO3 is taking place forming tungsten bronze in the potential
region of −0.64 V to −0.2 V vs. Hg/Hg SO . This overlaps with H
2
4
2
adsorption–desorption region on Pt. A possible method to evaluate
the electrochemical surface area due to Pt alone is to subtract the H
adsorption peaks due to pure WO in Fig. 7(a). The ECSA thus deter-
3
mined are tabulated in Table 4. This decoupling of H sorption due to
+
WO , however, is problematic since H sorption on WO can be cat-
3
3
alyzed by the presence of Pt, as pointed out by Kulesza et al. [64] and
Tseung et al. [29,32,36,37]. The H-sorption determined ECSA values
in Table 4 are therefore systematically higher than corresponding
values determined by CO-stripping and Cu-UPD methods which are
described below.
Fig. 5. EDX spectrum of the Pt-WO3/C catalyst (Pt:W = 1:1).

2
68
C. Yang et al. / Electrochimica Acta 75 (2012) 262–272
1
0
0
.0
.5
.0
3.3.3. ECSA by Cu-UPD stripping
(
a)
Using CO-stripping voltammetry to determine the electrochem-
ical surface area of Pt has the ambiguity in the adsorption mode of
CO on Pt site in the presence of alloyed metal/metal oxide, whether
CO is linearly bonded or bridge-bonded [57]. Green and Kucernak
reported the determination Pt and Ru surface areas in Pt-Ru alloys
by the Cu-UPD method [57]. In this method, a monolayer of Cu is
underpotential deposited, and from the stripping voltammograms,
the surface area of Pt and Ru can be estimated separately. Jayaraman
-
-
-
0.5
1.0
and McFarland later applied this method on Pt-WO mixed catalyst
system to estimate the Pt surface area [27]. The copper stripping
reaction is a two-electron-transfer reaction in the potential region
3
Vulcan XC-72
WO /C
3
1.5
4
of 0.3–0.8 V vs. RHE, and the charge associated with this reaction is
-
0.6 -0.4 -0.2
0.0
0.2
0.4
−2
4
20 ␮C cm [27,54].
(
b)
Cuupd → Cu2+ + 2e− 420 ␮C cm−2
(5)
2
0
The Cu-UPD stripping method proves to be a viable method for
Pt surface area determination in Pt-WO catalysts [27]. Fig. 9 shows
the Cu underpotential deposition/stripping experiments on WO /C,
3
3
Pt/C (E-TEK) and Pt-WO /C composite electrodes. In Fig. 9(a), the
3
curve 1 is the cyclic voltammogram of the WO /C electrode in 0.1 M
-
-
-
2
4
6
3
WO /C
3
H2SO4. Curves 2 and 3 are the cyclic voltammograms of the WO3/C
electrode in 0.002 M CuSO /0.1 M H SO with different potential
Pt-WO /C 0.5:1
3
4
2
4
scan windows. It can be observed that curve 3 overlapped with
curve 1, indicating no underpotential deposition or stripping of Cu
occurs on WO3/C electrode in the potential region of −0.34 V to
Pt-WO /C
1:1
2:1
3
Pt-WO /C
3
-
0.6 -0.4 -0.2 0.0
0.2
0.4
0
.46 V vs. Hg/Hg SO . Notably, Fig. 9(b) shows the Cu-UPD on Pt/C
2 4
Potential vs. (Hg/Hg SO ) /V
electrode. The anodic peaks indicate that underpotential deposi-
tion of Cu has occurred on Pt during the initial holding potential at
−0.34 V vs. Hg/Hg SO . These results show that Cu-UPD method can
2
4
Fig. 7. Cyclic voltammograms of (a) WO3/C and (b) Pt-WO3/C electrocatalysts in
2
4
−1
0
.5 M H2SO4 at a scan rate of 20 mV s . The current scale is normalized by the
be used to determine the Pt surface area in Pt-WO3 mixed catalyst
system.
geometrical electrode area (0.196 cm−2).
The electrochemical surface areas of Pt in Pt-WO /C catalysts
3
3
.3.2. ECSA by CO stripping
CO-stripping voltammetry is an alternative method for mea-
determined by hydrogen adsorption, CO-stripping, and Cu-UPD
stripping methods are summarized in Table 4. ECSA was calculated
suring the electrochemical surface area of Pt [61,62,24,63]. It is
generally agreed that CO adsorbs on Pt in a 1:1 linearly bonded
fashion, assuming the charge associated in the CO monolayer
0
by Q/[Pt]Q , where Q is the charge involved in the adsorption and
desorption reaction occurred on the surface of Pt while [Pt] repre-
sented the Pt mass loading on the electrode. Q0 is the monolayer
−
2
adsorption on Pt site is 420 ␮C cm
.
adsorption charge. It is notable that the surface areas of Pt esti-
mated by hydrogen adsorption method were larger than by using
CO-stripping and Cu-UPD method, possibly due to the involvement
Pt-(CO)ads + H O → Pt + CO + 2H + 2e− 420 ␮C cm
+
−2
(3)
2
2
Fig. 8 shows the CV curves for CO stripping on WO /C, Pt/C and
of H WO3 in the sensitivity to potential limits of adsorption.
3
x
Pt-WO /C composite electrodes with various Pt:W ratio. The oxi-
3
dation peak for CO on Pt-WO /C (1:1) electrode is 46 mV compared
3.3.4. Estimation of ECSA from TEM images
3
to 96 mV (vs. Hg/Hg SO ) for Pt/C from E-TEK. For the Pt-WO /C
Pt surface area can also be estimated from TEM images shown
in Fig. 3. The nanoparticles within each sample are assumed to be
pure Pt, perfect spheres, and uniform in size as determined by the
peak of the particle size distributions of Fig. 3(d), (e) and (f). The sur-
face areas thus determined are listed in the last column of Table 4.
For a 3 nm diameter pure Pt nanoparticle, the estimated Pt surface
2
4
3
(
0.5:1) and (2:1) electrodes, the CO oxidation peaks were also neg-
atively shifted, but to a smaller extent of about 30 mV. These results
suggest that the WO3 nanoparticles promote CO oxidation on Pt
[
33,39,40]. The mechanism can be described in the following reac-
tion:
2
−1
area is 93 m g Pt. The ECSA of the E-TEK Pt/C sample measured
Pt-(CO)ads + Pt-(OH)ads + WO3 → 2Pt + CO + HxWO3
2
(4)
2
−1
electrochemically has values between 89 and 94 m g Pt which is
close to this TEM estimation. In addition to the fully exposed perfect
spheres of uniform size, other uncertainties exist in this approach.
It is not clear whether only Pt regions are visible in TEM images of
Jayaraman et al. [27] speculated that CO can also oxidize on the
surface of WO . They found that tungsten changes oxidation states
3
between +6 and +5, providing the oxygen species required for the
removal of CO [27]. From Fig. 8(a), no CO stripping peak is observed
in pure WO3 sample. This may be due to little CO adsorption on
WO3 during the CO pretreatment step, rather than inability of CO
oxidation on WO3 surface. The absence of CO stripping current in
Fig. 3, and whether Pt forms its own particle on top of WO domain;
3
or WO3 are integrated into PtWO3 nanoparticles. There is absence
of any characteristic feature of WO3 lattice in the TEM images and
the corresponding FFT patterns in Fig. 3. The WO however, may be
3
WO enables a distinct evaluation of Pt surface area available to CO
present in amorphous form as in Fig. 2(a). The TEM particle size can
be larger than the size of Pt regions and the corresponding calcu-
lated surface area will be an underestimation since Pt can be more
dispersed. On the other hand, the assumption of a fully exposed
sphere is an over-estimation since a good part of the particle can
be embedded in the support. The Pt surface area estimated by TEM
which is a non-electrochemical measurement, will not be the same
3
sorption. The complication of WO on amount of CO stripping from
3
Pt still exists, however, since CO adsorption at E = 0.1 V vs. RHE on
Pt-WO3 nanoparticles yields production of CO2 at E = 0.1 V vs. RHE,
as nicely demonstrated by Micoud et al. with FTIR [39]. The method
of stripping under-potential deposited copper is therefore applied
as described below.

C. Yang et al. / Electrochimica Acta 75 (2012) 262–272
269
9
6
3
0
3
6
9
(
a) WO /C
(b) E-TEK 10% Pt/C
3
6
3
0
-
-
-3
-6
-
0.6 -0.4 -0.2 0.0 0.2 0.4
Potential vs. (Hg/Hg SO ) /V
-0.6 -0.4 -0.2 0.0 0.2 0.4
Potential vs. (Hg/Hg SO ) /V
2
4
2
4
9
9
(
c) Pt-WO /C 0.5:1
(d) Pt-WO /C 1:1
3
3
6
3
0
3
6
6
3
0
-
-
-3
-6
-
0.6 -0.4 -0.2 0.0 0.2 0.4
Potential vs. (Hg/Hg SO ) /V
-0.6 -0.4 -0.2 0.0 0.2 0.4
Potential vs. (Hg/Hg SO ) /V
2
4
2
4
9
6
3
0
3
6
(
e) Pt-WO /C 2:1
3
-
-
-
0.6 -0.4 -0.2 0.0 0.2 0.4
Potential vs. (Hg/Hg SO ) /V
2
4
−
1
Fig. 8. COads stripping voltammograms recorded at 20 mV s in 0.5 M H2SO4 for (a) WO3/C and (b) 10 wt% Pt/C from E-TEK, and (c)–Pt-WO3/C electrocatalysts with different
Pt:W ratios. Solid black lines show the first scan (i.e., the CO stripping peak) while the dotted lines show the second scan (i.e., the background CV curves of the electrode in
−
2
0
.5 M H2SO4). The current scale is normalized by the geometrical electrode area (0.196 cm ).
as ECSA area, but serving as a reference. The Cu-UPD values are
taken as more reliable representations of ECSA. The deviations in
ECSA values determined by different methods are small in the case
of pure Pt nanoparticles and increase with W content.
the Pt-WO3 catalysts, oxidation of methanol starts from −0.1 V vs.
Hg/Hg SO4 and reaches a peak at about 0.2 V vs. Hg/Hg SO . The
2
2
4
highest activity is observed in the Pt-WO3 electrocatalyst with 1:1
Pt:W atomic ratio, whether based on Pt-mass or Pt ECSA. The anodic
−
1
peak current density of 1:1 Pt-WO /C was 271 mA mg Pt which
3
is 1.4 times that of E-TEK Pt/C catalyst.
3.4. Methanol oxidation measurements
A constant current experiment was conducted to evaluate
steady-state methanol oxidation by the best performing 1:1 Pt:W
ratio catalyst. The chronopotentiogram in Fig. 11 was performed
at a 110 mA mg Pt and a very steady oxidizing potential can be
achieved for over 6 min.
Performance of the electrocatalysts was evaluated for oxidation
of methanol by performing cyclic voltammetry and chronopoten-
−
1
tiometry experiments. The electrochemical behavior of Pt-WO /C
3
composite catalysts with three different Pt:W ratios (0.5:1, 1:1,
and 2:1) were compared to a commercial E-TEK 10 wt% Pt/C cat-
alyst. Fig. 10 shows the cyclic voltammograms of Pt-WO /C and
3.5. Pt:W ratio and role of WO3 in Pt activity
3
E-TEK Pt/C composite catalysts for methanol oxidation in 1.0 M
CH OH + 0.5 M H SO electrolyte and the current reported was nor-
For the composite Pt-WO3 catalyst synthesized by the
microwave-assisted microemulsion method, the presence of WO3
can physically and chemically affects the catalytic performance
of Pt nanoparticles. As shown in Table 4, the ECSA of various Pt-
3
2
4
malized by Pt mass and ECSA of Pt estimated by Cu-UPD method.
Each voltammogram was recorded after 10 initial cycles. Some
characteristic parameters of the CVs are summarized in Table 5,
in which E1/2 is the anodic half-peak potential, jmass and jECSA repre-
sent activities based on Pt-mass and Pt-surface-area, respectively.
WO , determined by different methods, all show higher areas with
3
higher W content. As discussed earlier, the ECSA determined by
Cu-UPD stripping is more reliable and least chemically affected.
No current peak was observed in the CV curve for WO /C indi-
3
2
−1
Pt for the Pt:W = 0.5:1 sample,
cating the absence of catalytic activity for methanol oxidation. For
The Cu-UPD value is 129 m g

2
70
C. Yang et al. / Electrochimica Acta 75 (2012) 262–272
9
6
3
0
3
6
9
6
(
a) WO /C
(b) E-TEK 10% Pt/C
3
2
3
0
3
6
1
3
-
-
-
1
: in H SO
2 4
-
-
2: in CuSO4
: in CuSO4
in H SO
Cu-UPD
2
4
3
-
0.6 -0.4 -0.2 0.0 0.2 0.4
Potential vs. (Hg/Hg SO ) /V
-0.6 -0.4 -0.2 0.0 0.2 0.4
Potential vs. (Hg/Hg SO ) /V
2
4
2
4
6
3
0
6
3
0
(
c) Pt-WO /C 0.5:1
(d) Pt-WO /C 1:1
3
3
-
-
3
6
-3
-6
in H SO
Cu-UPD
in H SO
Cu-UPD
2
4
2
4
-
0.6 -0.4 -0.2 0.0 0.2 0.4
Potential vs. (Hg/Hg SO ) /V
-0.6 -0.4 -0.2 0.0 0.2 0.4
Potential vs. (Hg/Hg SO ) /V
2
4
2
4
6
3
0
3
6
(
e) Pt-WO /C 2:1
3
-
-
in H SO
Cu-UPD
2
4
-
0.6 -0.4 -0.2 0.0 0.2 0.4
Potential vs. (Hg/Hg SO ) /V
2
4
−
1
Fig. 9. (a) Cu deposition experiments on WO3/C electrode: (1) Cyclic voltammograms of WO3/C in 0.1 M H2SO4 at a scan rate of 20 mV s and (2) (3) Cyclic voltammograms
−
1
of WO3/C in 0.002 M CuSO4/0.1 M H2SO4 at a scan rate of 20 mV s with different potential scan windows. (b)–(e) Cu-UPD experiments on Pt-WO3/C composite electrodes
−
1
with different Pt:W ratios. Cu-stripping voltammogram (Solid line) was recorded in 0.002 M CuSO4/0.1 M H2SO4 at a scan rate of 20 mV s with potential held at −0.34 V vs.
Hg/Hg2SO4 for 60 s before the start of stripping to 0.46 V vs. Hg/Hg2SO4. The dotted lines show the control experiments of CV in 0.1 M H2SO4 with a larger potential window
−
2
showing no stripping current. The current scale is normalized by the geometrical electrode area (0.196 cm ).
2
−1
decreased to 97 m g Pt for the Pt:W = 2:1 sample and a value
After normalization to ECSA of Pt, the specific activities of the
Pt-WO3 samples are similar, as shown in the anodic forward scans
of Fig. 10(b). The peak current densities per Pt area jECSA are com-
pared in Table 5. The case of Pt:W = 1:1 still gives the best specific
activity compared to 0.5:1 and 2:1, indicating an optimum ratio
of Pt to WO3. The onset potentials were similar among the Pt-
WO3 samples, but lowest for Pt:W = 0.5:1 with value of −0.17 V (vs.
Hg/Hg2SO4). This compares well to the onset potential of Pt/C (E-
TEK) at −0.1 V (vs. Hg/Hg2SO4). The onset potentials of the sample
catalysts can be ranked in the order of 0.5:1 < 1:1 < 2:1 < Pt/C. These
2
−1
of 86 m g Pt in for pure Pt on carbon. This enhancement of the
Pt-WO /C catalysts is due to better dispersion of Pt with smaller
particle size (3.1 nm of pure Pt particles vs. 1.8 of 0.5:1 Pt:W cat-
alyst as estimated by TEM). The chemical enhancement effect is
evident since there is enhanced H sorption on WO , negative shifts
in CO oxidation, and the ECSA values determined by other meth-
ods are higher than the Cu-UPD value of 129 m g Pt in the case of
Pt:W = 0.5:1 while more or less in agreement for pure Pt nanopar-
ticles.
3
3
2
−1
Table 5
Electrocatalytical performance data from cyclic voltammograms (Fig. 10) of electrodes with different Pt:W ratios. *Note that E1/2 is the anodic half-peak potential, jmass is the
specific current density normalized by Pt mass, jECSA is the specific current density normalized by the electrochemical active surface area (ECSA) of Pt obtained by Cu-UPD
method.
Pt:W ratio in samples
Forward anodic peak
jmass (mA mg−1)
jECSA (mA cm -Pt)
−2
E1/2 vs. (Hg/Hg2SO4) (V)
0
1
2
.5:1
:1
:1
0.017
0.013
0.047
0.028
256
271
206
187
0.198
0.251
0.213
0.217
ETEK 10% Pt

C. Yang et al. / Electrochimica Acta 75 (2012) 262–272
271
Pt-(CHO)ads + Pt-(OH)ads → 2Pt + CO + 2H + 2e−
+
(7)
4
3
2
1
00 (a)
Pt-WO /C 0.5:1
2
3
Pt-WO /C 1:1
3
Chemically, the abundant interfaces between Pt and WO3
nanoparticles enhance the hydrogen spillover effects. Hydrogen
spill over onto the surface of WO3 and form hydrogen tungsten
Pt-WO /C 2:1
3
00
00
00
0
E-TEK 10% Pt/C
bronze (HxWO ), free the active Pt sites for further chemisorption
3
of methanol molecules and make the dehydrogenation of methanol
molecules adsorbed on Pt sites more effective [29,30,41]. In addi-
tion, WO3 also takes part in removal of CO. The oxophilic nature of
the oxide helps in removing the adsorbed intermediates during the
methanol oxidation [30]. Since CO removal is the rate-limiting step
of the methanol oxidation [27], this intrinsic improvement in CO
oxidation will lead to an overall improved performance of Pt-WO3
catalysts for methanol oxidation.
-0.6
-0.4
-0.2
0.0
0.2
0.4
Potential vs. (Hg/Hg SO )/V
2
4
4. Conclusions
(b)
Pt-WO /C 0.5:1
3
Nanoparticle size and composition control of a Pt-WO /C
3
Pt-WO /C 1:1
3
0
.3
.2
.1
.0
catalyst was achieved through the reaction of
a water-oil
Pt-WO /C 2:1
3
microemulsion using microwave irradiation in a two-step pro-
cess: WO3 nanoparticles were first deposited on Vulcan XC-72
carbon by mixing of two microemulsion systems containing HCl
E-TEK 10% Pt/C
0
0
0
and WO42 , to yield WO /C, followed by the subsequent addition of
−
3
Pt by microwave irradiation of a Pt-containing microemulsion. TEM
images showed homogenous distribution of WO nanoparticles on
3
the carbon support with an average size of 1 nm. Under microwave
irradiation, butan-1-ol, the co-surfactant of the microemulsion sys-
tem, played a role of reducing agent and successfully reduced
hexachloroplatinic acid to Pt nanoparticles. The PtWO3 nanopar-
ticles produced in this step had an average size of 2.5 nm. EDX
analysis at different locations confirmed the Pt:W ratio of the
-0.6
-0.4
-0.2
0.0
0.2
0.4
Potential vs. (Hg/Hg SO )/V
2
4
as-synthesized Pt-WO /C catalyst was homogeneous. The electro-
3
Fig. 10. Cyclic voltammograms of the Pt-WO3/C electrocatalysts with different Pt:W
−
1
chemical experiments showed that these Pt-WO3/C nanoparticles
have high catalytic activity toward methanol oxidation at room
ratios in 1.0 M CH3OH + 0.5 M H2SO4 at room temperature. Scan rate is 20 mV s
.
Current scale is normalized by the Pt-mass (a) and by the Pt surface area estimated
from Cu-UPD stripping experiments (b).
◦
temperature (22 C). Hydrogen adsorption, CO-stripping and Cu-
UPD stripping methods were used to estimate electrochemical
active surface area of Pt in the Pt-WO /C catalysts, in which Cu-
UPD method is relative more reliable due to the involvement
3
negative shifts suggest some chemical effects of WO in promoting
methanol oxidation on Pt.
Methanol oxidation reaction is a six-electron-transfer reaction.
It is generally thought that methanol oxidation over Pt occurs when
3
of WO3 component. Pt-WO /C electrocatalysts with large WO3
3
component present higher Pt surface area, suggesting that WO3
nanoparticlessupportedoncarbongreatlyimprovethe distribution
of Pt nanoparticles. CO-stripping experiments show an intrinsic
methanol molecules (CH OH) adsorbed on Pt and subsequently
3
dissociate to form a Pt-(CHO)ads intermediate. This intermediate
enhancement of CO oxidation for Pt-WO /C catalysts by comparing
+
3
further reacts with Pt-(OH)ads to give CO2 and H . The reac-
the onset potential of CO stripping [27,39,40]. The methanol oxida-
tion measurement shows that a 1:1 Pt:W ratio catalyst exhibits the
tion mechanism is proposed by several researchers in literatures
[
36,37,64], as shown in the following chemical reactions:
Normal Pt catalysis mechanisms:
−
1
highest Pt-mass current density of 271 mA mg , 1.4 times higher
than that of commercial E-TEK catalyst. However, the specific cat-
alytic activities based on Pt-surface-area appear less difference.
Therefore, the enhancement in catalytic activity mainly comes from
Pt + H O → Pt-(OH)ads + H+ + e−
(6)
2
two parts: the physical modification of Pt from WO nanoparticles,
3
0
0
.1
.0
such as surface area, and the chemical effects, such as improved
performance on hydrogen spillover and CO oxidation.
Applied specific current:
-
1
1
10 mA mg
Acknowledgments
This work was supported by the Hong Kong Research Grants
Council (GRF HKU 700209P), and the HKU Development Fund for
the Initiative of Clean Energy and Environment. We thank the Uni-
versity of Hong Kong Electron Microscope Unit for the use of TEM
and EDX facilities. The HRTEM work was performed in MCPF of
HKUST.
-0.1
-0.2
-0.3
Pt-WO /C 1:1
3
0
100
200
300
Time /s
400
500
600
References
[
1] L. Carrette, K.A. Friedrich, U. Stimming, ChemPhysChem 1 (2000) 162.
Fig. 11. Chronopotentiogram of methanol oxidation on Pt-WO3/C (1:1 Pt:W ratio)
at 110 mA mg Pt. Electrolyte is 0.5 M H2SO4 containing 1.0 M CH3OH.
[2] M. Winter, R.J. Brodd, Chemical Reviews 104 (2004) 4245.
[3] N. Kizhakevariam, E.M. Stuve, Surface Science 286 (1993) 246.
−1

2
72
C. Yang et al. / Electrochimica Acta 75 (2012) 262–272
[
[
[
4] S. Mukerjee, R.C. Urian, Electrochimica Acta 47 (2002) 3219.
5] L.G.S. Pereira, V.A. Paganin, E.A. Ticianelli, Electrochimica Acta 54 (2009) 1992.
6] C. Bock, C. Paquet, M. Couillard, G.A. Botton, B.R. MacDougall, Journal of Amer-
ican Chemical Society 126 (2004) 8028.
[35] P.J. Barczuk, H. Tsuchiya, J.M. Macak, P. Schmuki, D. Szymanska, O. Makowski,
K. Miecznikowski, P.J. Kulesza, Electrochemical and Solid-State Letters 9 (2006)
E13.
[36] B.S. Hobbs, A.C.C. Tseung, Journal of Electrochemical Society 119 (1972) 580.
[37] B.S. Hobbs, A.C.C. Tseung, Journal of Electrochemical Society. 120 (1973) 766.
[38] A.C.C. Tseung, P.K. Shen, K.Y. Chen, Journal of Power Sources 61 (1996) 223.
[39] F. Micoud, F. Maillard, A. Bonnefont, N. Job, M. Chatenet, Physical Chemistry
Chemical Physics 12 (2010) 1182.
[
7] W.M. Chen, G.Q. Sun, Z.X. Liang, Q. Mao, H.Q. Li, G.X. Wang, Q. Xin, H. Chang,
C.H. Pak, D.Y. Seung, Journal of Power Sources 160 (2006) 933.
8] N.P. Lebedeva, G.J.M. Janssen, Electrochimica Acta 21 (2005) 29.
9] L. Li, B.Q. Xu, Acta Physico-Chimica Sinica 21 (2005) 1132.
[
[
[
10] X. Li, G. Chen, J. Xie, L.J. Zhang, D.G. Xia, Z.Y. Wu, Journal of Electrochemical
[40] F. Maillard, E. Peyrelade, Y. Soldo-Olivier, M. Chatenet, E. Chainet, R. Faure,
Electrochimica Acta 52 (2007) 1958.
Society 157 (2010) B580–B584.
[
[
11] X.Z. Cui, J.L. Shi, L.X. Zhang, M.L. Ruan, J.H. Gao, Carbon 47 (2009) 186.
12] C.W. Xu, Y.Z. Su, L.L. Tan, Z.L. Liu, J.H. Zhang, S.A. Chen, S.P. Jiang, Electrochimica
Acta 54 (2009) 6322.
13] K.R. Lee, M.K. Jeon, S.I. Woo, Applied Catalysis B-Environmental 91 (2009) 428.
14] L.X. Yang, R.G. Allen, K. Scott, P. Christenson, S. Roy, Journal of Power Sources
[41] X.Z. Cui, J.L. Shi, H.R. Chen, L.X. Zhang, L.M. Guo, J.H. Gao, J.B. Li, The Journal of
Physical Chemistry B 112 (2008) 12024.
[42] Z.G. Zhao, M. Miyauchi, Angewandte Chemie International Edition 47 (2008)
7051.
[43] J. Rajeswari, B. Viswanathan, T.K. Varadarajan, Material Chemistry and Physics
106 (2007) 168.
[
[
1
37 (2004) 257.
[
[
[
15] T. Kawaguchi, Y. Rachi, W. Sugimoto, Y. Murakami, Y. Takasu, Journal of Applied
Electrochemistry 36 (2006) 1117.
16] F. Peng, C.M. Zhou, H.J. Wang, H. Yu, J.H. Liang, J.A. Yang, Catalysis Communi-
cations 10 (2009) 533.
17] K. Drew, G. Girishkumar, K. Vinodgopal, P.V. Kamat, Journal of Physical Chem-
istry B 109 (2005) 11851.
[44] X. Zhang, K.Y. Chan, J.K. You, Z.G. Lin, A.C.C. Tseung, Journal of Electroanalytical
Chemistry 430 (1997) 147.
[45] S.H. Lee, R. Deshpande, P.A. Parilla, K.M. Jones, B. To, A.H. Mahan, A.C. Dillon,
Advanced Materials 18 (2006) 763.
[46] C.M. White, J.S. Jang, S.H. Lee, J. Pankow, A.C. Dillon, Electrochemical and Solid-
State Letters 13 (2010) B120.
[
[
18] K.W. Park, S.B. Han, J.M. Lee, Electrochemistry Communications 9 (2007) 1578.
19] C.L. Campos, C. Roldan, M. Aponte, Y. Ishikawa, C.R. Cabrera, Journal of Electro-
analytical Chemistry 581 (2005) 206.
[47] L.F. Xiong, T. He, Chemistry of Materials 18 (2006) 2211.
[48] S. Fardindoost, A.I. Zad, F. Rahimi, R. Ghasempour, International Journal of
Hydrogen Energy 35 (2010) 854.
[
[
[
20] J.W. Guo, T.S. Zhao, J. Prabhuram, R. Chen, C.W. Wong, Journal of Power Sources
[49] A. Phuruangrat, D.J. Ham, S.J. Hong, S. Thongtem, J.S. Lee, Journal of Materials
Chemistry 20 (2010) 1683.
[50] D. Tanaka, Y. Oaki, H. Imai, Chemical Communications 46 (2010) 5286.
[51] M.B. Mohamed, K.M. AbouZeid, V. Abdelsayed, A.A. Aljarash, M.S. El-Shall, ACS
Nano 4 (2010) 2766.
[52] M. Tsuji, N. Miyamae, K. Matsumoto, S. Hikino, T. Tsuji, Chemistry Letters 34
(2005) 1518.
[53] P.S.S. Prasad, N. Lingaiah, S. Chandrasekhar, K.S.R. Rao, P.K. Rao, K.V. Raghavan,
F.J. Berry, L.E. Smart, Catalysis Letters 66 (2000) 201.
[54] D.L. Boxall, C.M. Lukehart, Chemistry of Materials 13 (2001) 806.
[55] Z.H. Liang, Y.J. Zhu, Chemistry Letters 33 (2004) 1314.
[56] S.C. Padmanabhan, D. Ledwith, S.C. Pillai, D.E. McCormack, J.M. Kelly, Journal of
Materials Chemistry 19 (2009) 9250.
[57] C.L. Green, A. Kucernak, The Journal of Physical Chemistry B 106 (2002) 11446.
[58] M. DeBlois, J. Lessard, G. Jerkiewicz, Electrochimica Acta 50 (2005) 3517.
[59] A. Essalik, K. Amouzegar, O. Savadogo, Journal of Applied Electrochemistry 25
(1995) 404.
1
56 (2006) 345.
21] N. Tsiouvaras, M.V. Martinez-Huerta, R. Moliner, M.J. Lazaro, J.L. Rodriguez, E.
Pastor, M.A. Pena, J.L.G. Fierro, Journal of Power Sources 186 (2009) 299.
22] Z. Jusys, T.J. Schmidt, L. Dubau, K. Lasch, L. Jorissen, J. Garche, R.J. Behm, Journal
of Power Sources 105 (2002) 297.
[
[
23] X.Z. Cui, F.M. Cui, Q.J. He, L.M. Guo, M.L. Ruan, J.L. Shi, Fuel 89 (2010) 372.
24] J.L. Ye, J.G. Liu, Z.G. Zou, J. Gu, T. Yu, Journal of Power Sources 195 (2010)
2
633.
25] X.Z. Cui, L.M. Guo, F.M. Cui, Q.J. He, J.L. Shi, The Journal of Physical Chemistry C
13 (2009) 4134.
26] K.Y. Tsang, T.C. Lee, J.W. Ren, K.Y. Chan, H.Z. Wang, H.T. Wang, Journal of Exper-
imental Nanoscience 1 (2006) 113.
27] S. Jayaraman, T.F. Jaramillo, S.H. Baeck, E.W. McFarland, The Journal of Physical
Chemistry B 109 (2005) 22958.
28] B. Rajesh, V. Karthik, S. Karthikeyan, K.R. Thampi, J.M. Bonard, B. Viswanathan,
Fuel 81 (2002) 2177.
[
[
[
[
1
[
[
[
[
[
29] B.S.T. Hobbs, A.C.C. Tseung, Nature 222 (1969) 556.
[60] R. Ganesan, J.S. Lee, Journal of Power Sources 157 (2006) 217.
[61] K.Y. Chen, Z. Sun, A.C.C. Tseung, Electrochemical and Solid-State Letters 3 (2000)
10.
[62] Z. Sun, H.C. Chiu, A.C.C. Tseung, Electrochemical and Solid-State Letters 4 (2001)
E9.
30] A.C.C. Tseung, K.Y. Chen, Catalysis Today 38 (1997) 439.
31] A.C.C. Tseung, B.S. Hobbs, Journal of Electrochemical Society 117 (1970) C241.
32] B.S. Hobbs, A.C.C. Tseung, Journal of Electrochemical Society 122 (1975) 1174.
33] F. Micoud, F. Maillard, A. Gourgaud, M. Chatenet, Electrochemistry Communi-
cations 11 (2009) 651.
[63] L.X. Yang, C. Bock, B. MacDougall, J. Park, Journal of Applied Electrochemistry
34 (2004) 427.
[34] K.W. Park, J.H. Choi, K.S. Ahn, Y.E. Sung, The Journal of Physical Chemistry B 108
(
2004), 108, 5989.
[64] P.J. Kulesza, L.R. Faulkner, Journal of Electroanalytical Chemistry 259 (1989) 81.