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All rights reserved. A series of carbon-supported bimetallic catalysts with different metallic loadings was synthesized, using platinum as the principal active phase and molybdenum or tungsten as promoting phases. The materials were prepared by organometallic precursor thermolysis and characterized by direct current electrochemical methods, transmission electron microscopy, scanning electron microscopy and x-ray diffraction. Electrodes were elaborated with each catalyst and their electrochemical performances were studied by cyclic voltammetry. These results show an increased activity of the catalysts with small amounts of Mo or W, towards oxidation of methanol with respect to the catalyst containing only platinum. XRD results reveal the presence of molybdenum or tungsten bronzes (H xMoO 3, H xWO 3) that are responsible for the increase in activity. It is believed that the bronzes participate in a spillover effect by promoting the removal of protons from the platinum surface. It was found that the presence of molybdenum in this type of catalyst prevents the platinum phase from sintering during the thermal treatment and allows them to keep platinum particles with mean sizes between 2 and 8 nm. The proposed catalysts are adequate for methanol oxidation in liquid-fuel alcohol fuel cell systems, since it was found that oxidation potentials are lower than those observed with platinum catalysts.
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INTERNATIONAL JOURNAL OF CHEMICAL
REACTOR ENGINEERING
Volume 5 2007 Article A99
Synthesis and Characterization of
Carbon-Supported Platinum-Molybdenum
and Platinum-Tungsten Catalysts for
Methanol Oxidation in Direct Alcohol Fuel
Cells
Pedro RoqueroLuis Carlos Ord´
o˜
nezOmar Herrera
Orlando Ugalde∗∗ Jorge Ram´
ırez††
Universidad Nacional Aut´
onoma de M´
exico, roquero@servidor.unam.mx
Centro de Investigaci´
on Cient´
ıfica de Yucat´
an, lcol@cicy.mx
University of British Columbia, omarhh@interchange.ubc.ca
∗∗Universidad Nacional Aut´
onoma de M´
exico, iqorlandougalde@hotmail.com
††Universidad Nacional Aut´
onoma de M´
exico, jrs@servidor.unam.mx
ISSN 1542-6580
Copyright c
2007 The Berkeley Electronic Press. All rights reserved.
Synthesis and Characterization of Carbon-Supported
Platinum-Molybdenum and Platinum-Tungsten
Catalysts for Methanol Oxidation in Direct Alcohol
Fuel Cells
Pedro Roquero, Luis Carlos Ord´
o˜
nez, Omar Herrera, Orlando Ugalde, and Jorge
Ram´
ırez
Abstract
A series of carbon-supported bimetallic catalysts with different metallic load-
ings was synthesized, using platinum as the principal active phase and molybde-
num or tungsten as promoting phases. The materials were prepared by organometal-
lic precursor thermolysis and characterized by direct current electrochemical meth-
ods, transmission electron microscopy, scanning electron microscopy and x-ray
diffraction. Electrodes were elaborated with each catalyst and their electrochem-
ical performances were studied by cyclic voltammetry. These results show an in-
creased activity of the catalysts with small amounts of Mo or W, towards oxidation
of methanol with respect to the catalyst containing only platinum. XRD results
reveal the presence of molybdenum or tungsten bronzes (HxMoO3, HxWO3) that
are responsible for the increase in activity. It is believed that the bronzes partici-
pate in a spillover effect by promoting the removal of protons from the platinum
surface. It was found that the presence of molybdenum in this type of catalyst pre-
vents the platinum phase from sintering during the thermal treatment and allows
them to keep platinum particles with mean sizes between 2 and 8 nm. The pro-
posed catalysts are adequate for methanol oxidation in liquid-fuel alcohol fuel cell
systems, since it was found that oxidation potentials are lower than those observed
with platinum catalysts.
KEYWORDS: methanol oxidation, electrocatalyst, direct alcohol fuel cell
1.INTRODUCTION
Direct alcohol fuel cells are electrochemical reactors that allow conversion of chemical energy to electrical current
with efficiencies sometimes larger than those of heat machines and internal combustion engines (Burstein et. al.,
1997; Wasmus, 1999; Acres, 2001). Methanol is an ideal fuel for this kind of device, because it has a higher energy
density than hydrogen, it can be stored in liquid phase and it can be obtained from biomass (Hamelinck and Faaij,
2002).
The transfer of six electrons in the complete oxidation of methanol, occurs with the formation of adsorbed
intermediates (CO, CHxOH, –COH, –COOH) (Haile, 2003), which are bonded on the Pt surface, resulting in a poor
activity of most catalysts in this reaction. It is important to develop electro-catalysts tolerant to intermediate species.
Good performance results have been obtained by combining Pt with other metallic elements (Watanabe and Motoo,
1976; Nakajima and Kita, 1990; Götz and Wendt, 1998; Frelik et. al., 1998).
Several studies have been devoted to Pt-Ru catalysts, while only a few have dealt with promising materials
as Pt-Mo (Grgur et. al., 1999; Samjeské et. al., 2002; Oliviera et. al., 2003; Pinheiro et. al., 2003) or Pt-W catalysts
(Tseung and Chen, 1997; Shijun et. al., 2002; Park et. al., 2003; Liu et. al., 2003; Yang et. al., 2004; Pereira et. al.,
2006).
Dffierent theories have been put forward to explain the behaviour of bimetallic electro-catalysts for
methanol oxidation. The bifunctional mechanism hypothesis (Watanabe and Motoo, 1975) states that while the
organic species adsorbs at Pt surfaces, water dissociation occurs on the adjacent surface of the second metal, at low
electric potentials, thus promoting the complete oxidation of the organic to CO2. This mechanism is usually accepted
for Pt-Ru catalysts. In the case of Pt-W it has also been proposed that a proton spillover effect, that cleans the Pt
surface, is the main reason for its enhanced activity (Tseung and Chen, 1997). It has also been recently proposed that
Mo and MoOx block the CO adsorption sites on Pt (110) surfaces (Zhiquan et. al., 2007). No conclusive evidence
has been reported for any of these effects in the case of electrochemical methanol oxidation on Pt-Mo or Pt-W
supported catalysts. In the first study on catalytic electrochemical oxidation promoted by molybdates, a 300 mV
decrease in methanol oxidation potential was found (Shropshire, 1965) these results, however, have never been
reproduced and most recent studies report decreases in oxidation potential, not as large as that found by Shropshire
(Ordóñez et. al., 2005; Kita et. al., 1988).
In this work the results from the synthesis and characterization of different Pt-Mo and Pt-W catalysts are
presented. Results indicate that the performances of these materials on the methanol oxidation reaction are higher
than those of Pt catalysts. This higher performance is observed as a decrease in methanol oxidation electrical
potential.
2. EXPERIMENTAL PROCEDURE
2.1 Catalyst synthesis
Platinum carbonyl complex was synthesized by bubbling CO during 24 h through 50 cm3 of an aqueous
hexachloroplatinic acid solution (10 mg/cm3) (Longoni and Chini, 1976; Dickinson, et. al., 2002). At the end of the
reaction time the Pt carbonyl precipitate was filtered and dried under CO atmosphere.
The synthesis of the catalysts was carried out by placing the appropriate quantities of Pt carbonyl, vulcan
XC72R carbon and commercial molybdenum or tungsten hexacarbonyl complex in a reflux system using o-xylene
as solvent. The system temperature was set at 140 °C and reflux was maintained for 24 h, after what the solvent was
distilled. Compositions of the nine prepared materials are presented in table 1.
1Roquero et al.: Carbon-Supported Pt-Mo and Pt-W Catalysts
Published by The Berkeley Electronic Press, 2007
Table 1. Catalysts compositions.
Catalyst W or Mo (M) content in
active phase
Weight percent
M /
(M+Pt)
Atomic ratio
Pt:M Pt Mo or W C
Pt-C 0.0 1:0 20.0 0.0 80
Pt-Mo2080, Pt-W2080 0.2 4:1 17.8 2.2 80
Pt-Mo5050, Pt-W5050 0.5 1:1 13.4 6.6 80
Pt-Mo2080, Pt-W2080 0.8 1:4 6.7 13.3 80
Mo-C, W-C 1.0 0:1 0.0 20.0 80
2.2 Characterization methods
Electrochemical tests were carried out at 25 °C using a Radiometer Voltalab 50 potentiostat-galvanostat. A three-
electrode cell was used with a saturated calomel electrode (SCE:Hg/Hg2Cl2 /sat. KCl) as reference and a platinum
wire as counter electrode. All potentials reported in this work are referred to the normal hydrogen electrode (NHE).
The working electrode was prepared by mixing graphite paste with 5 mg of each catalyst and placing it on
the surface of a 0.5 cm diameter graphite disk. The geometric surface area of the disk was used for the calculation of
current density. The different working solutions consisted of 1.0 M methanol and 0.5 M H2SO4 as supporting
electrolyte. All solutions were purged with nitrogen for 40 minutes previous to each experiment.
During cyclic voltammetry measurements, the system was kept without stirring. All the potential sweeps in
CV were first carried out towards positive potentials, and then reversed towards negative potentials.
X-Ray Diffractograms were obtained at room temperature using Cu Ka (λ = 1.5406 Å) radiation on a
Siemens D-500 diffractometer, with a 28 min-1 rate.
For the obtention of Transmission Electron Micrographs the catalysts samples were dispersed in heptane
and a few drops of the supernatant liquid were deposited on copper grids covered with a carbon film. Images were
obtained with a JEOL 2010 microscope.
3. RESULTS AND DISCUSSION
3.1. X-Ray diffraction
The XRD patterns obtained with the Pt-Mo series is presented in figure 1. In this figure are marked the diffraction
lines of the (110), (200), (220) (311) and (222) Pt planes. On the Pt-Mo catalysts can also be observed the lines
corresponding to MoO2 at 2θ = 26 and 37 ° (card 32-0671), as well as those attributed to Mo9O26 at 2θ = 12, 21 and
24 ° (card 01-1194). These species are able to undergo protonation and thus to form bronzes (HxMoO3) (Adams,
2000). These bronzes are supposed to be capable of removing protons from adjacent Pt surfaces, cleaning thus the
active sites of the main component of the active phase. In tungsten materials similar results were obtained (Tseung
and Chen, 1997). This element is also able to form this kind of bronze (HxWO3).
2International Journal of Chemical Reactor Engineering Vol. 5 [2007], Article A99
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Figure 1. XRD patterns of the Pt-Mo series.
3.2 Electrochemical tests
The electrical currents observed in W-C and Mo-C materials (figure 2) can be attributed to changes in the oxidation
state of tungsten or molybdenum species present in the electrode. In the case of tungsten, a small oxidation peak is
found between 0.5 and 0.78 V vs. NHE in the forward scan, and a reduction peak at 0.59 V vs NHE in the reverse
scan. The material formulated with molybdenum presents an oxidation peak at 0.82 V vs. NHE and two reduction
peaks, in the reverse scan, at 0.69 and 0.5 V vs. NHE. After one hundred potential cycles these currents remain
stable, indicating that these materials are not undergoing corrosion (Bard and Faulkner, 2001). It can also be stated
that neither tungsten nor molybdenum are efficient catalysts for methanol electrochemical oxidation, since no
electrical currents are observed in the presence of this molecule. The current – potential behaviour of these two
materials reveals an important capacitance of the electrode – electrolyte interphase, that can be seen in the overall
slope of both curves and in the separation of the electrical current in the forward and reverse scans. Besides the
capacitive and hydrophobic characteristics inherent to vulcan carbon, this is due to the fact that no reaction is
occuring at a considerable extent, and potential changes lead to accumulation of electrical charge at the interphase.
This is not observed in catalysts formulated with Pt because the surface reaction leads to depolarization of the
electrode.
The cyclic voltammetry response of different catalysts in sulfuric acid and methanol media, obtained with a 50 mV/s
sweep rate, are shown in figure 3. The voltammetric response of Pt and Pt-Mo based materials towards anodic oxidation
of methanol are presented in figures 3-I and 3-II, respectively. Here, a first oxidation peak of methanol is found
above 0.9 V vs. NHE in the forward sweep. A second peak appears in the reverse sweep between 0.55 and 0.8 V vs.
3Roquero et al.: Carbon-Supported Pt-Mo and Pt-W Catalysts
Published by The Berkeley Electronic Press, 2007
NHE and is due to oxidation of reaction intermediates adsorbed on Pt sites (Ordóñez et. al., 2005). From these plots
it can be seen that the oxidation potential of methanol considerably decreases in the Pt-Mo catalysts, with respect to
the one containing only platinum (Figure 3-I). This is a desirable situation in the operation of a direct alcohol fuel
cell, because an oxidation reaction carried out at low potentials allows larger cell voltages.
Figure 2. Cyclic voltammetry results in (I) W-C,
(II) Mo-C in H2SO4 Figure 3. Cyclic voltammetry results in (I) Pt-C,
(II) different Pt-Mo catalysts in CH3OH
In table 2 are shown the resulting electrical charges (Q) from the forward and reverse peaks attributed to
organics oxidation, calculated from the curves in figure 3 and from the corresponding plots obtained with Pt-W
catalysts. These charges can be roughly correlated to the extent of reaction, if we consider that the electrical currents
are mainly faradic. An optimum amount of W or Mo is found to be present in the best catalysts for the overall
oxidation reaction. Materials with samll amounts of molybdenum or tungsten (Pt-Mo8020 and Pt-W8020) are better
catalysts than carbon-supported platinum and are also better than materials containing higher quantities of Mo or W.
In Pt-Mo8020 or Pt-W8020, electrical charges in the corresponding forward peaks are almost two times that of Pt-C.
In the reverse peak, these two materials present charges almost three times that of Pt-C, indicating that this optimum
composition has an important influence in the oxidation of adsorbed intermediates. The difference between W and
Mo in the charge is not remarkable. These two elements might be working according to the same mechanism.
Table 2. Electrical charges from methanol oxidation peaks
Catalyst Pt/C Pt-Mo2080 Pt-Mo5050 Pt-Mo8020 Pt-W2080 Pt-W5050 Pt-W8020
Q forward
(μCcm-2) 9.5 2.85 4.48 16.75 2.81 5.01 13.95
Q reverse
(μCcm-2) 3.35 1.13 2.02 9.22 1.15 2.14 9.51
4International Journal of Chemical Reactor Engineering Vol. 5 [2007], Article A99
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3.3. Electron microscopy
Catalysts were pressed into pellets and analyzed by means of Scanning Electron Microscopy (SEM) coupled with
Electron Dispersive X-ray Element analysis (EDX). This technique was applied to map the surface of the catalysts
and determine the distribution of active phases in a 1000 μm line across the pellet. As an example of the obtained
information, results from these measurements in the Pt-W5050 catalyst are presented in figure 4. It can be seen that
both metals are homogeneously distributed on the surface and that Pt and W occupy almost the same points in the
scanned line, showing that bimetallic materials are formed and that tungsten sites are adjacent to the platinum ones.
Figure 4. SEM-EDX elementary analysis by a line scan of Pt-W5050 catalyst
Transmission Electron Micrographs of different Pt-W catalysts are presented in figure 5. TEM images of
Pt-Mo catalysts are presented in figure 6. Vulcan carbon presents mean particle sizes around 40 nm. On this support
the active phases (platinum and tungsten are distributed in smaller particles).
Tungsten forms irregular spots (figure 5-a), while platinum particles are semispherical with mean particle
sizes between 2 and 8 nm (figure 5-b). Combination of both metals results in samller Pt particle size but some
degree of accumulation of these particles, as can be appreciated from figure 5-c.
In the case of Pt-Mo catalysts, the presence of the promoting phase was found to avoid accumulation, or
sintering of the platinum particles (figures 6 a and b).
Relief
Carbon
Oxygen
Tungsten
Platinum
5Roquero et al.: Carbon-Supported Pt-Mo and Pt-W Catalysts
Published by The Berkeley Electronic Press, 2007
Figure 5. Transmission Electron Microscopy images of (a) W/C, (b) Pt/C, (c) Pt-W5050
Figure 6. Transmission Electron Microscopy images of (a) Pt-Mo2080, (b) Pt-Mo5050
4. CONCLUSION
Nine carbon-supported catalysts were synthesized with different contents of active phases, where platinum sites are
responsible for the methanol oxidation currents observed. Molybdenum and tungsten phases, consisting mainly of
MoO3 and WO3, have only a promoting effect and do not participate directly as catalyst in the methanol electro-
oxidation. The promoting effect is traduced in a change in the oxidation onset potential to lower values than in Pt-C.
6International Journal of Chemical Reactor Engineering Vol. 5 [2007], Article A99
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The synthesis method based on the thermolysis of metal carbonyl complexes produces a high dispersion of
the metallic particles over the support surface, providing small particle sizes between 2 and 3 nm.
The catalyst active phase is stable since no corrosion currents are observed during potential sweeps or
potential steps.
X-Ray Diffraction revealed the existence of tungsten and molybdenum bronzes that are supposed to be
responsible for the enhancement in catalyst activity, by a mechanism of proton removal and cleaning of the adjacent
platinum particles. It might also be possible that these bronzes provide the active sites with hydroxil groups that
facilitate the complete oxidation of adsorbed intermediates. However, there is no concluisve evidence to confirm or
discard this hypothesis.
Electron microscopy showed the existence of bimetallic particles with different sizes and distribution on the
carbon surface. Although some of the effects of tungsten and molybdenum on particles sizes and distribution are not
well understood, it is clear that both elements influence the way in which platinum is deposited on the catalyst.
Tungsten seems to result in smaller platinum particle sizes but these particles become somewhat agglomerated. On
the other hand, molybdenum prevents sintering of the platinum phase and produces good dispersion of the active
phase.
Further research is required to improve the activity of methanol oxidation electrocatalysts in direct alcohol
fuel cells. The tolerance of the catalyst towards poisoning by reaction intermediates is still a major concern in order
to have commercially efficient methanol fuel cell technologies. This is certainly the first step towards direct ethanol
fuel cells. Ethanol can be considered as an ideal biomass-obtained fuel, however, the cleavage of carbon-carbon
bonds in the anode of these devices can only be achieved by the application of high anodic overpotentials. The
materials here presented can also be considered as promising electrode catalysts in this reaction.
ACKNOWLEDGEMENTS
The authors thank Mr. Iván Puente Lee, and Mr. Manuel Aguilar Franco for their assistance in microscopy and XRD
measurements, respectively. Financial support from UNAM project La Ciudad Universitaria y la Energía is
gratefully acknowledged.
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9Roquero et al.: Carbon-Supported Pt-Mo and Pt-W Catalysts
Published by The Berkeley Electronic Press, 2007
... As compared to vapor-phase synthesis, thermolysis in condensed matters generally proceeds at a lower temperate, e.g., at high pressures [90,91], in a solid polymer matrix [92,93] or in a liquid solvent [94][95][96][97][98][99][100][101][102][103][104] with an optional sonochemical [105,106] or photochemical [107,108] impact. ...
... These solvothermal methods [94][95][96][97][98][99][100][101][102][103][104][105][106][107][108] allow one to obtain both films and unsupported dispersed particles (a substrate-less synthesis) from tungsten hexacarbonyl dissolved in high-boiling solvents (typical examples: hexadecane, diphenylmethane, diphenyl ether, dichlorobenzenes, xylenes, etc.) at temperatures of about or somewhat below 200°C. As a rule, synthesis becomes possible at considerably lower temperatures as compared to the above-discussed CVD approach, i.e., in vapor phase. ...
... As deposited, they both demonstrate some electrochemical response during CV testing. The shape of the CV curves is rather typical for WO x species in general [13,72,73,100,106,116,139]. ...
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