A PtAu nanoparticle electrocatalyst for methanol electro-oxidation in direct methanol fuel cells

Article abstract of DOI:10.1149/1.2224055

PtAu alloy nanoparticle catalysts for use in direct methanol fuel cells were synthesized by reduction with NaBH₄ and freeze-drying and their electrocatalytic activities were examined for reactions of methanol, formaldehyde, and formic acid. The extent of alloy formation and average particle size were characterized by X-ray diffraction and transmission electron microscopy. X-ray photoelectron spectra confirmed that the surface state of Au in the PtAu alloy was exclusively metallic, while the Ru in the alloy has not only metallic characteristics but is also present in oxidized form. Based on electrochemical measurements, the PtAu catalyst showed a more enhanced activity than pure Pt for the oxidation of methanol, having a lower onset potential and a larger current density. The electrocatalytic activity of the PtAu catalyst was also enhanced in the oxidation of formic acid but not formaldehyde. This provides evidence for differences in the catalytic activity of PtAu in the oxidation of low molecular weight organic compounds. The origin of the enhanced catalytic activity of the PtAu catalysts is discussed from the standpoint of a modified methanol oxidation pathway in which formaldehyde, formic acid, and CO are produced as putative intermediates.

Full text of DOI:10.1149/1.2224055

A1812
Journal of The Electrochemical Society, 153 ͑10͒ A1812-A1817 ͑2006͒
0
013-4651/2006/153͑10͒/A1812/6/$20.00 © The Electrochemical Society
A PtAu Nanoparticle Electrocatalyst for Methanol Electro-
oxidation in Direct Methanol Fuel Cells
a
b,
c
d
a
Jong-Ho Choi, Kyung-Won Park, * In-Su Park, Keon Kim, Jae-Suk Lee, and
c, ,z
Yung-Eun Sung *
a
Department of Materials Science and Engineering, Gwangju Institute of Science and Technology,
Gwangju 500-712, South Korea
b
Department of Chemical and Environmental Engineering, Soongsil University, Seoul 156-743, Korea
c
School of Chemical and Biological Engineering and Research Center for Energy Conversion and Storage,
Seoul National University, Seoul 151-744, Korea
d
Division of Chemistry and Molecular Engineering, Korea University, Sungbuk-Ku, Seoul, 136-701, Korea
PtAu alloy nanoparticle catalysts for use in direct methanol fuel cells were synthesized by reduction with NaBH and freeze-drying
4
and their electrocatalytic activities were examined for reactions of methanol, formaldehyde, and formic acid. The extent of alloy
formation and average particle size were characterized by X-ray diffraction and transmission electron microscopy. X-ray photo-
electron spectra confirmed that the surface state of Au in the PtAu alloy was exclusively metallic, while the Ru in the alloy has not
only metallic characteristics but is also present in oxidized form. Based on electrochemical measurements, the PtAu catalyst
showed a more enhanced activity than pure Pt for the oxidation of methanol, having a lower onset potential and a larger current
density. The electrocatalytic activity of the PtAu catalyst was also enhanced in the oxidation of formic acid but not formaldehyde.
This provides evidence for differences in the catalytic activity of PtAu in the oxidation of low molecular weight organic com-
pounds. The origin of the enhanced catalytic activity of the PtAu catalysts is discussed from the standpoint of a modified methanol
oxidation pathway in which formaldehyde, formic acid, and CO are produced as putative intermediates.
©
2006 The Electrochemical Society. ͓DOI: 10.1149/1.2224055͔ All rights reserved.
Manuscript submitted May 4, 2005; revised manuscript received December 12, 2005. Available electronically July 26, 2006.
Direct methanol fuel cells ͑DMFCs͒ have attracted considerable
effectively employed in fuel cells, such as hydrogen purification and
39-42
interest for use as a power source in portable electronic devices
because they have a variety of merits, including low operating tem-
peratures, ease of handling liquid fuel, and the high-energy density
electrochemical oxidation reactions of organic species.
How-
ever, the science of Au catalysis is still quite new and is currently
undeveloped.
1
-5
of methanol. The excellent catalytic activity of Pt for methanol
oxidation, especially at low temperatures, makes this metal electro-
catalyst attractive for use as an anode in DMFCs. However, pure Pt
is readily poisoned by carbon monoxide ͑CO͒, an intermediate that
is produced during the electro-oxidation of methanol at low tem-
peratures. Based on the bifunctional mechanism, the CO-poisoned
Pt can be regenerated via the reaction of surface CO with oxygen
In this article, PtAu alloy nanoparticle catalysts for enhancing the
electro-oxidation of methanol were synthesized by a borohydride
reduction method combined with freeze-drying. The structural prop-
erties of the alloy catalysts were characterized using X-ray diffrac-
tion ͑XRD͒, transmission electron microscopy ͑TEM͒, and X-ray
photoelectron spectroscopy ͑XPS͒, and the electrochemical charac-
teristics of the catalysts with respect to methanol oxidation were
evaluated. In order to identify the origin of the catalytic activity
enhancement by PtAu nanoparticle catalysts in the electro-oxidation
of methanol, their catalytic activities were also investigated assum-
ing formaldehyde and formic acid are produced as reaction interme-
diates.
species associated with an element such as Ru to yield CO . In
2
addition to the applicability range of this mechanism, electronic ef-
fects ͑ligand effects͒ might also be involved in the enhancement by
6
-16
Ru.
Other species such as Bi, Os, Rh, W, Sn, Ni, and Mo have
been reported to have a positive effect on catalytic activity with
17-27
respect to methanol oxidation.
Although Ru and other elements
Experimental
are added to Pt to enhance the oxidation of CO, the rates of metha-
nol oxidation are still too low to guarantee the commercialization of
a DMFC. In order to design a new anode catalyst with an acceptable
activity, it is essential to clarify the mechanism involved in the oxi-
dation of methanol. Although the formation of various adsorbates,
such as ͑CH OH͒ , -͑COH͒ , ͑-HCO͒ , ͑-COOH͒ , linear
Unsupported Pt-based alloy nanoparticles were synthesized by a
conventional borohydride reduction method using NaBH ͑Aldrich
4
Chemical Co.͒ combined with freeze-drying. H PtCl ·xH O,
2
6
2
RuCl ·xH O, and HAuCl ·3H O ͑all from Aldrich Chemical Co.͒ in
3
2
4
2
x
ad
ad
ad
ad
the desired stoichiometry were completely dissolved in Millipore
bonded CO, and bridged CO, have been proposed as reactive inter-
mediates in methanol oxidation from in situ infrared spectroscopy
data, it is well known that formic acid ͑HCOOH͒ and formaldehyde
water ͑18 M⍀ cm͒. After several hours, the solutions were reduced
by the addition of 0.2 M NaBH . A threefold excess of reducing
4
agent over the valences of the metal salts was used, which was
sufficient for the complete reduction of the salts to the elemental
state. After precipitation, the resulting materials were washed with
Millipore water several times and dried by freeze-drying using liq-
͑
HCHO͒ are formed as intermediates and are dissolved in the
28-33
solution.
The goal of this study was to design Pt-based alloy catalysts
containing Au for use in methanol electro-oxidation. Au has long
been known as being catalytically less active than other noble met-
als. Since the discovery of the high catalytic activity of nanosized
Au depending on cluster size and types of supports used, many
recent studies have focused on the development of Au-based cata-
lysts for use in low-temperature catalytic combustion, the partial
oxidation of hydrocarbons, the hydrogenation of unsaturated hydro-
uid N without the use of any heat-treatment.
2
Structural analyses of pure metal and Pt-based alloy catalysts
were carried out using a Rigaku X-ray diffractometer equipped with
a Cu K␣ source. To estimate the extent of alloy formation of the
Pt-based alloy catalysts, the ͑111͒ peak was fitted using the
Lorentzian/Gaussian function. The composition of the alloy cata-
2
4
lysts was determined by Vegard’s law. The size of the alloy cata-
lysts was investigated by TEM on a JEOL instrument ͑JEM-
2000FXII͒ at a 200 kV accelerating potential. A specimen was
prepared by ultrasonically suspending the particles in deionised wa-
ter. A drop of the resulting suspension was deposited on a standard
Cu grid covered with a carbon film ͑200 mesh͒ and allowed to dry
before being inserted into the microscope. In order to analyze and
34-38
carbons, and the reduction of nitrogen oxides.
It also has been
reported that Au-based catalysts are potentially capable of being
*
Electrochemical Society Active Member.
E-mail: ysung@snu.ac.kr
z
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Journal of The Electrochemical Society, 153 ͑10͒ A1812-A1817 ͑2006͒
A1813
Figure 1. XRD patterns of PtRu ͑1:1͒, PtRuAu ͑5:4:1͒, pure Pt, PtAu ͑1:1͒,
and pure Au.
compare the surface chemical states of the samples, XPS was
carried out using a VG Scientific ͑ESCALAB 250͒ photoelectron
spectrometer. The X-ray source was Al K␣ with 1486.6 eV operat-
ing at 150 kV and 150 W. The base pressure of the system was
−
9
2
ϫ 10 Torr. The Brunauer, Emmett, and Teller ͑BET͒ specific
surface areas of the Pt-based alloy catalysts powders were deter-
mined using an accelerated surface area and porosimetry system
͑
ASAP͒ 2010 ͑Micrometrics Instrument Corporation͒ at the Korea
Basic Science Institute ͑KBSI͒, Chonju Branch.
Electrochemical measurements were made using a three-
electrode cell. A Pt gauze and Ag/AgCl ͑in saturated KCl͒ were
used as a counter and a reference electrode, respectively. The elec-
trode potentials in this paper are given with reference to a reversible
hydrogen electrode ͑RHE͒ in a 0.5 M H SO solution. The glassy
Figure 2. ͑a͒ TEM ͑inset: TED pattern͒ and ͑b͒ high resolution TEM image
images of PtAu ͑1:1͒.
2
4
carbon electrode, working electrode, was polished with 1, 0.3, and
0
.05 ␮m Al O paste and then washed in deionized water. The cata-
2
3
lyst ink was prepared by ultrasonically dispersing catalyst powders
in an appropriate amount of Millipore water, 2-propanol, and 5%
Nafion solution ͑1100 equivalent weight͒. A 10 ␮L portion of the
catalyst ink was pipetted onto a glassy carbon working electrode.
After drying in a vacuum oven, the total catalyst loadings were
consistently 3 mg cm−2 in which the geometric area of the working
electrode was used for the calculation. The solution of 0.5 M H SO
synthesized by the borohydride reduction method were located at
3
9.79 and 38.20, respectively, having the original peak positions
corresponded to their fcc metallic states. The 2␪ of the ͑111͒ for
PtAu ͑1:1͒ and PtRu ͑1:1͒ are 39.10 and 40.48, respectively. The
͑
111͒ peak of PtAu ͑1:1͒ was shifted to lower angles while that of
PtRu ͑1:1͒ was shifted to higher angles compared to the original
peak for pure Pt. Assuming alloy formation among Pt, Au, and Ru
based on a substitutional solid solution, such a shift can be attributed
to the difference in atomic size. For example, a Pt atom is larger
than a Ru atom but smaller than a Au atom. Such types of peak
shifts indicate good alloy formation between Pt and Au and Ru.
2
4
and 2 M CH OH + 0.5 M H SO was continuously stirred and
3
2
4
purged with nitrogen. In addition, 0.05 M HCHO and 0.05 and
.5 M HCOOH in 0.5 M H SO solutions were also prepared in a
0
2
4
manner similar to the intermediates of methanol electro-oxidation.
All chemicals used were of analytical grade. Electrochemical ex-
periments were performed using an Eco Chemie AUTOLAB instru-
ment. In order to identify the properties of Pt-based alloy catalysts
in sulfuric acid and to measure the catalytic activities for methanol
and its reaction intermediates, cyclic voltammetry ͑CV͒ was per-
formed in the potential between −0.05 and 1.45 V and chrono-
amperometry ͑CA͒ measurements were made at 30 min intervals,
while a potential of 0.55 V was maintained. For electrochemical
44
PtRu ͑1:1͒ is known to alloy well. The ͑111͒ peak for PtRuAu
͑
5:4:1͒ follows this trend depending on the amounts of the second
and third metal involved and is located between that of pure Pt and
PtRu ͑1:1͒. By fitting the ͑111͒ peaks, it is clear that the d spacings
of ͑111͒ in the PtRu ͑1:1͒, PtRuAu ͑5:4:1͒, and PtAu ͑1:1͒ alloy
catalysts are 0.223, 0.224, and 0.230 nm, whereas those of pure Pt
and Au are 0.226 and 0.235 nm. In addition, the intended atomic
percent of binary alloy catalysts can also be identified on the basis of
Vegard’s law.
Figure 2 shows a TEM image of the PtAu ͑1:1͒ catalyst, which
confirms the presence of a nanosize alloy as evidenced by the ring
pattern of transmission electron diffraction ͑TED͒ in the insert of
Fig. 2a. No major agglomeration was observed in the absence of a
carbon support. The particle size of the PtAu ͑1:1͒ alloy catalyst is
approximately 4–5 nm, in good agreement with XRD data calcu-
lated using the Debye–Scherrer equation, and the particles are
largely spherical in shape. This size of PtAu ͑1:1͒ is slightly larger
43
CO stripping voltammetry,
methanol was chemisorbed in
2
0
M CH OH + 0.5 M H SO as the surface CO on alloy catalysts at
3
2
4
.2 V for 30 min. After rinsing with 0.5 M H SO , the adsorbed CO
2
4
−
1
was monitored by CV at a scan rate of 20 mV s .
Results and Discussion
Structural analysis.— XRD patterns clearly demonstrate all of
the Pt-based alloy catalysts have a face-centered cubic ͑fcc͒ struc-
ture as shown in Fig. 1. The ͑111͒ peaks for Pt and Au nanoparticles
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A1814
Journal of The Electrochemical Society, 153 ͑10͒ A1812-A1817 ͑2006͒
Figure 4. ͑a͒ Potential-current plot for PtAu ͑1:1͒ in 0.5 M H SO , and
2
4
PtAu ͑1:1͒ and pure Pt with respect to methanol oxidation in
.0 M CH OH + 0.5 M H SO , and ͑b͒ time-current plot for PtAu ͑1:1͒ and
2
3
2
4
Figure 3. ͑a͒ Ru 3d for PtRu ͑1:1͒ and ͑b͒ Au 4f for PtAu ͑1:1͒ X-ray
photoelectron spectra.
pure Pt in 2.0 M CH OH + 0.5 M H SO under constant applied potential of
3 2 4
0.55 V ͑vs RHE͒.
than that of PtRu ͑1:1͒, and this result can also be attributed to the
theoretical ratio of 4 to 3, indicating that Au is present only in the
metallic state in the alloy without any surface oxide being evident.
This property of PtAu catalysts is associated with the catalytic ac-
tivity for methanol oxidation.
difference in atomic size between them. In the same manner, the
2
BET surface area of the PtAu catalyst was 15–25 m /g, while that
2
of PtRu was 65–75 m /g. Figure 2b shows a high-resolution TEM
lattice image of PtAu ͑1:1͒. The interplanar lattice parameter of
͑
111͒ plane was observed to be 0.230 nm, in good agreement with
Electro-oxidation of methanol
.
— The Pt and PtAu ͑1:1͒ catalyst
were evaluated in 0.5 M H SO with and without a 2 M CH OH
the results obtained from a d spacing calculation using the XRD
data. The fact that the interplanar distance of PtAu ͑1:1͒ increased
compared to that of pure Pt also proves that Au is well alloyed with
Pt by interposition. Although the atomic ratios of Pt and Au in the
alloy catalysts were found to be varied, ranging from 8:1 to 1:1,
alloy formation was good, as evidenced by XRD and TEM data.
This indicates that Pt-based binary and ternary alloy catalysts con-
taining Au can be prepared using the borohydride reduction method.
Figure 3 shows Ru 3d in PtRu ͑1:1͒ and Au 4f in PtAu ͑1:1͒
X-ray photoelectron spectra. Metallic ruthenium appears as a spin-
orbit at 279.8 ͑Ru 3d5/2͒ and 284.0 eV ͑Ru 3d3/2͒ with an area ratio
of 1.5 and the additional Ru peaks indicate the formation of surface
oxides, such as RuO ͑280.9 and 285.1 eV͒ and RuO ͑282.7 and
2
4
3
solution using voltammetry and chronoamperometry. All current
densities in the voltammograms were normalized to the electro-
chemically active surface area. This electrochemically active surface
area for a pure Pt catalyst could be calculated by utilizing the hy-
drogen desorption integral on the catalysts in the positive scan be-
cause platinum has well separated H desorption and metal oxidation
regions and it is well known that the stoichiometry of H to Pt is
4
5-47
1:1.
The active surface area of PtAu catalyst can be also ob-
tained using the same procedure, assuming that only Pt is active for
hydrogen desorption. Assuming that the surface composition of a
PtAu alloy is identical in composition to that of the bulk, the Pt
density at the surface of the alloy should be only half that for Pt at
the surface of pure Pt. Therefore, the electrochemically active sur-
face area calculated from the experimentally measured current for
hydrogen desorption on PtAu was multiplied by 2 and used for the
normalization of current density to have the same number of Pt
active sites for two catalysts. The CV for PtAu ͑1:1͒ in 0.5 M
2
3
2
86.9 eV͒. According to the bifunctional mechanism, oxophilic Ru
sites covered by various ruthenium oxides constitute the source of
oxygen required for the removal of surface CO, and as a result, PtRu
has excellent catalytic activity compared to Pt. In contrast, Au 4f7/2
and Au 4f5/2 lines appear at 84.0 and 87.7 eV, respectively, with a
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Journal of The Electrochemical Society, 153 ͑10͒ A1812-A1817 ͑2006͒
A1815
Figure 6. Potential-current plot for PtAu ͑1:1͒ and pure Pt in
.5 M HCOOH + 0.5 M H SO .
0
2
4
has been reported that Ru is the most promising component in Pt-
based binary catalysts for methanol oxidation, by virtue of either a
bifunctional mechanism or an electronic effect ͑ligand effect͒. In
contrast, it is generally understood that the enhanced catalytic activ-
ity of PtAu does not follow a bifunctional mechanism because Au
does not exist in an oxide form and cannot easily adsorb OH to
provide an oxygen source for the removal of adsorbed CO. In order
to decipher the origin of the enhanced methanol oxidation, the
electro-oxidation of formic acid and formaldehyde, putative reaction
intermediates in methanol oxidation, was investigated using the
PtAu catalyst.
Electro-oxidation of formic acid and formaldehyde.— In order
to investigate the catalytic activity with respect to the above reaction
intermediates, 0.05 M HCOOH and 0.05 M HCHO in 0.05 M
H SO were substituted for methanol. The reason for using such low
Figure 5. Potential-current plot for PtAu ͑1:1͒ and pure Pt ͑a͒ in
.05 M HCOOH + 0.5 M H SO4 and ͑b͒ in 0.05 M HCHO ϩ 0.05 M
2
4
0
concentrations, compared to a 2 M CH OH solution, is that the re-
2
3
H SO .
2
4
action kinetics for HCOOH and HCHO are much faster than that of
methanol. Figure 5a shows voltammograms of PtAu ͑1:1͒ and pure
Pt in 0.05 M HCOOH + 0.5 M H SO solutions. The findings show
2
4
that PtAu ͑1:1͒ is a superior catalyst for HCOOH oxidation than
pure Pt, showing lower onset potentials and larger current densities.
The onset potential and current density at 0.45 V for the PtAu ͑1:1͒
H SO is represented in Fig. 4a as a thick, solid line. Peaks corre-
2
4
sponding to hydrogen adsorption and desorption and a preoxidation
peak on the catalyst surface can clearly be seen in Fig. 4a, indicating
the characteristics of Pt in nanoparticles, and are consistent with
−
2
catalyst are 0.35 V and 41 ␮A cm , respectively, while those of
24,48
−2
pure Pt are 0.41 V and 17 ␮A cm . Unlike HCOOH, there is no
literature involved.
An alloying effect by Au in the presence of
distinct difference in the activity for HCHO, as shown in Fig. 5b.
This supports the view that both catalysts have similar onset poten-
tials and current densities for HCHO oxidation. In spite of the simi-
lar reaction intermediates, PtAu ͑1:1͒ shows a difference in catalytic
activity for HCOOH and HCHO. In other words, PtAu ͑1:1͒ oxi-
dizes HCOOH more readily than HCHO. When the molar concen-
tration of HCOOH was increased, clear differences between the two
catalysts were evident. Figure 6 shows voltammograms of PtAu
͑1:1͒ and pure Pt in 0.5 M HCOOH + 0.5 M H2SO4. Compared to
Fig. 5a, the current density values in 0.5 M HCOOH were more than
double for both catalysts and the difference in current density was
also increased. For example, the current densities of PtAu ͑1:1͒ and
methanol shows a lower onset potential and a larger current density
compared to pure Pt. For example, the current densities at 0.45 V
−
2
for PtAu ͑1:1͒ and pure Pt are 9.7 and 5.5 ␮A cm , respectively.
The lower onset potential indicates that methanol oxidation starts at
a lower potential on PtAu ͑1:1͒ than on pure Pt. In addition to the
PtAu ͑1:1͒ catalyst, we investigated other PtAu alloy catalysts con-
taining different compositions of Pt and Au from 8:1 to 1:1 for
methanol oxidation ͑not shown here͒. As a result, only one catalyst,
PtAu ͑8:1͒, showed almost similar catalytic activity with pure Pt and
the others were even better than pure Pt. This indicates that alloying
with a second metal, such as Au, enhances the electrocatalytic ac-
tivity for methanol oxidation. Using these catalysts, chronoamper-
ometry data were measured in 2 M CH OH + 0.5 M H SO at
−
2
pure Pt at 0.45 V in 0.5 M HCOOH are 94 and 27 ␮A cm , re-
3
2
4
−
2
0
.55 V for 30 min. Figure 4b shows a plot of the oxidation current
spectively, while those in 0.05 M HCOOH are 41 and 17 ␮A cm .
Summarizing the above results, PtAu ͑1:1͒ is superior to pure Pt for
the electro-oxidation of HCOOH but not HCHO. The reason for this
density vs time. As expected from the voltammetry results, the cur-
rent density of PtAu ͑1:1͒ for the oxidation of methanol is larger
than that of pure Pt over all time intervals measured, indicating that
a better activity for methanol oxidation is maintained over time. By
combining voltammetry and CA, we conclude that the activity of
PtAu ͑1:1͒ for methanol oxidation is superior to that of pure Pt. It
is that electro-oxidation of HCOOH produces CO , a final product,
2
and is not required to adsorb oxygen species such as OH compared
3
0,49-53
to HCHO oxidation.
It was also proved that Au adsorbs OH
groups in water with great difficulty, based on a voltammogram
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A1816
Journal of The Electrochemical Society, 153 ͑10͒ A1812-A1817 ͑2006͒
Figure 8. Pt 4f X-ray photoelectron spectra for PtRu ͑1:1͒ and PtAu ͑1:1͒.
pure Pt. This indicates that less CO is adsorbed to PtAu catalyst
surfaces after oxidizing more methanol than for pure Pt. These phe-
nomena were definitely different from those by PtRu catalysts. In
order to examine the possibility of an electronic effect by Au, the
Pt 4f spectra for PtAu and PtRu are shown in Fig. 8. No shift in the
binding energy of Pt 4f in the PtAu ͑1:1͒ alloy nanoparticles is
evident in spite of the suggestion by Shubina et al. and Hammer et
al. that, indeed, an electronic interaction occurs between Pt and
5
6,57
Au.
This indicates that the enhanced activity of PtAu for metha-
nol oxidation is not related to either the modification of the elec-
tronic structure or to a bifunctional mechanism.
In summary, we propose that there is another effect for catalytic
enhancement by a second element, such as Au, for methanol oxida-
tion. A PtAu catalyst is able to oxidize more methanol than pure Pt
and, at the same time, less CO accumulates on the PtAu catalyst
surfaces compared to pure Pt. In addition, the findings indicate that
the PtAu catalyst was preferable for formic acid oxidation but not
for formaldehyde oxidation. Therefore, the overall reaction might be
explained by the alteration of the major methanol oxidation pathway
by alloying Au with Pt. Thus, PtAu could open an alternative route
for methanol oxidation and solve the CO poisoning problem by
suppressing the formation of CO, which is distinctly different from
the PtRu catalyst. More fundamental studies will be required to
verify this hypothesis by analyzing reaction products using in situ
infrared spectroscopy and chromatography techniques.
Figure 7. CO stripping voltammetry for ͑a͒ pure Pt and ͑b͒ PtAu ͑1:1͒ in
0
2
.5 M H SO at a scan rate of 20 mV s−1. Methanol was adsorbed in
2 4
.0 M CH OH + 0.5 M H SO at 0.2 V ͑vs RHE͒ for a period of 30 min.
3
2
4
obtained in a 0.5 M H SO solution. Although Au is known to oxi-
2
4
dize HCOOH, but not CH OH and HCHO, the main oxidation re-
3
action of HCOOH on a Au electrode could occur, particularly in the
potential region of Au oxide formation ͓Ͼ1.2 V vs normal hydrogen
electrode ͑NHE͔͒ and the direct oxidation of HCOOH in the double-
54,55
layer region was very small compared to pure Pt.
As a result, it
Conclusions
does not appear plausible that the increased activity of the PtAu
alloy might simply be due to an increased amount of Au in the
catalyst. Combining the results for the electro-oxidation of HCOOH
and HCHO, we found that one possibility for the enhancement in
catalytic activity for methanol electro-oxidation by alloying Pt with
Au is the accelerated rate of HCOOH electro-oxidation. This indi-
cates that Au in PtAu alloy aids in increasing the probability of
inducing a major reaction pathway for methanol oxidation to avoid
the formation of adsorbed CO. This conclusion is supported by the
CO stripping voltammetry results shown in Fig. 7. As described, CO
was not adsorbed from CO-containing gas but was produced by
The catalytic activities with respect to methanol oxidation for
unsupported Pt alloy nanoparticle catalysts containing Au, synthe-
sized by reduction of the metal salts with NaBH and freeze-drying,
4
were investigated using 0.5 M H SO + 2 M CH OH solutions.
2
4
3
The catalytic activity of PtAu ͑1:1͒ was found to be superior to that
of pure Pt, showing a lower onset potential and large current density.
The enhanced activity of the PtAu catalyst for methanol oxidation
was investigated using formaldehyde and formic acid, known inter-
mediates in the methanol oxidation pathway. PtAu ͑1:1͒ shows a
better catalytic activity for the oxidation of formic acid, but not
formaldehyde, compared to pure Pt. In addition, it was found that
Au, when added to Pt, formed a metallic phase that was similar to Pt
based on XPS data and the CO coverage on the PtAu surface after
methanol oxidation was decreased compared to that for pure Pt.
Given the present results, we propose that the alteration in the major
reaction pathway by Au may contribute to the enhanced methanol
oxidation, that is, the methanol oxidation pathway is modified when
Au is present and the major route is the formation of formic acid and
being adsorbed from CH OH. The most obvious feature in Fig. 7 is
3
the slight positive shift in the stripping peak potential for PtAu at
ϳ0.80 V compared to pure Pt at ϳ0.76 V. This indicates that the
reason for the enhanced methanol oxidation by PtAu is not due to
the effective removal of adsorbed CO, unlike PtRu. In addition, the
CO coverage of the PtAu catalyst, calculated from the difference in
1
0
hydrogen desorption integral between the first and second scan,
was decreased by as much as 10%, from 0.87 to 0.78, compared to
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Journal of The Electrochemical Society, 153 ͑10͒ A1812-A1817 ͑2006͒
A1817
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the production of CO and not CO. Although it was not possible to
2
provide more sufficient supporting evidence for this effect, we be-
lieve that the findings herein will be helpful in attempts to under-
stand methanol oxidation mechanism. Furthermore, we also confirm
the possibility that formic acid could be used as a fuel in conjunction
with PtAu catalysts, which is a topic for a future study.
2
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Acknowledgments
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This work was supported by KOSEF through the Research Cen-
ter for Energy Conversion and Storage and Korea Research Foun-
dation ͑grant no. KRF-2004-005-D00064͒. We are grateful to the
Korea Basic Science Institute, Busan Branch, for XPS analyses.
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