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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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Volume 5 2007 Article A99
Synthesis and Characterization of
Carbon-Supported Platinum-Molybdenum
and Platinum-Tungsten Catalysts for
Methanol Oxidation in Direct Alcohol Fuel
Pedro RoqueroLuis Carlos Ord´
nezOmar Herrera
Orlando Ugalde∗∗ Jorge Ram´
Universidad Nacional Aut´
onoma de M´
Centro de Investigaci´
on Cient´
ıfica de Yucat´
University of British Columbia,
∗∗Universidad Nacional Aut´
onoma de M´
††Universidad Nacional Aut´
onoma de M´
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´
nez, Omar Herrera, Orlando Ugalde, and Jorge
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
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,
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.,
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
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 /
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.
3.1. 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
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
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
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).
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
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
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
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.
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]. ...
Full-text available
Electrochemically active nanocrystalline tungsten oxide was synthesized in supercritical carbon dioxide from tungsten hexacarbonyl at 150 °C and 400 bar in the presence of oxygen (partial pressure of 15 bar). The supercritical fluid is a solvent for the precursor (i.e., this is a sc solvothermal synthesis route), whereas the admixed gaseous oxygen serves as an oxidizer, promoting thermal decomposition of the precursor. During the substrate-free synthesis, 200–500 nm aggregates are formed. They consist of smaller grains having the size of about 100 nm. Therefore, a certain structural hierarchy is detected. The electrochemical activity of the as-synthesized particulate material is pronouncedly increasing during both potential cycling and exposure in an aqueous aerated electrolyte. After such a hydration/oxidation process, the electrochemical response of the material shows rather fast and reversible recharging of the entire tungsten-containing phase. This is an indication of facilitated proton transport in bulk of the tungsten oxide phase synthesized in the supercritical carbon dioxide with subsequent hydration/oxidation. Quite differently, the material synthesized at the same temperature only in compressed oxygen (partial pressure of 15 bar) without any presence of supercritical carbon dioxide is highly crystalline one. It does not demonstrate any significant electrochemical rechargeability; neither is the response improving with hydration/oxidation.
The influence of thermal treatment under different environments of PtRuMo/C catalyst has been investigated for CO and methanol electrooxidation in a half cell and in a DMFC single cell. The PtRuMo/C catalysts were synthesized following two step procedure while the thermal treatments consisted of heating at 300 °C in H2 or He atmosphere for 1 h. Structural characteristics of the electrocatalysts have been studied employing a wide range of instrumental methods, including physicochemical techniques like X-ray diffraction, TEM, TPR, XPS, and electrochemical techniques like single cell studies and Fourier Transform Infrared Spectroscopy adapted to the electrochemical system for in situ studies. These electrocatalysts exhibited good dispersion and small particle size, which increased upon increasing thermal treatment. Moreover, thermal treatment, mainly under H2 is responsible for the decrease of the lattice parameter and the increase of the spill over effect to Mo sites. These effects were also accompanied by increasing the proportion of the more reduced Ru species in this catalyst. The electrochemical characterization revealed that although all ternary catalysts were more active towards CO and methanol oxidation than the binary catalyst, the catalyst treated with H2 improves its performance by ca. 15% higher with respect to the ternary catalysts treated either in He treatment or with no treatment. The enhancement in activity is associated with a change in the reaction path, which promotes the direct oxidation of CHO species to CO2 without the production of the CO poisoning species. The synergistic effect of the three metals seems to be improved and the Mo–Pt and Mo–Ru interaction strengthened.
Full-text available
The synthesis and characterization of bimetallic catalysts with Pt and W as active phases, supported on carbon, were carried out. A synthesis method based on the thermolysis of metal precursors was developed with systems in which an aqueous phase, an organic one and solid carbon are present. The Pt loading in the synthesized catalysts was kept constant and the amounts of W and carbon changed. It was found that the Pt phase forms hemispherical particles with average diameter of 3 nm. The W is predominantly in the form of hexagonal WO3 crystals with average dimensions 35 by 15 nm. No evidence of alloy formation is found. The electrochemical characterization of ethanol oxidation includes cyclic voltammetry and current sampled voltammetry techniques. Tungsten exerts a promoting role in the activity of the catalysts, revealed by considerably increased current densities with respect to carbon supported Pt. Details of this enhanced activity are revealed by cyclic voltammetry with different operating conditions and electrolyte formulations. These experiments allowed to identify the oxidation of adsorbed organic intermediates influenced by the tungsten phase. WO3 appears as well to cause the decrease in the rate at which Pt oxides reduce, leading to higher surface coverage of OH center dot species.
Full-text available
Electro-oxidation kinetics of Hâ and Hâ/CO mixture were studied on bimetallic Pt-Mo catalysts supported on a high-surface-area carbon black. The Pt:Mo atomic ratios in the catalysts were 3:1 and 4:1. Characterization of these catalysts by X-ray diffraction indicated the existence of a face-centered cubic metallic phase with an average particle size of ca. 4 nm. Because the lattice constants for the Pt-Mo solid solutions are so close to those of pure Pt, the composition of the nanocrystalline phase could not be determined. The kinetic results with the supported catalysts were compared quantitatively with results from bulk alloy electrodes having well-characterized surface compositions varying from 15 to 33 atom % Mo. The kinetic properties of the supported catalysts were comparable to those of bulk alloys having somewhat higher Mo concentrations than the atomic ratios in the catalysts. This suggests that either the surface segregation phenomena in the alloy nanocrystals are different from those in the bulk or that the alloying by Pt is incomplete, and the alloy nanocrystals are rich in Mo relative to the atomic ratios in the catalysts. The authors prefer the latter interpretation. These Pt-Mo alloy catalysts are predicted to have significantly better CO tolerance in polymer electrolyte membrane fuel cells than Pt-Ru alloy catalysts, consistent with previous predictions based on studies of bulk alloy electrodes.
The previous conclusion that the redox couple of Mo(III)/Mo(IV) plays a key role in the enhancement of the catalytic activity of the Pt electrode was confirmed even at a high temperature of 80°C. An extended analysis of voltammograms to the electrode coadsorbed with molybdenum and methanol species showed that the highest oxidation current is obtained at an equal coverage of the molybdenum and methanol species. The bifunctional mechanism was discussed.
The details of the preparation and characterization of co-deposited Pt/WO[sub 3] electrodes are presented. X-ray, scanning, and transmission electron microscopic studies revealed that WO[sub 3] is amorphous and that [approximately]40 [angstrom] Pt crystallites are uniformly dispersed in the deposit. The influence of the deposition conditions and the effect of the solution acidity on the activity of such electrodes for methanol oxidation have been studied. The results demonstrated that the Pt/WO[sub 3] electrodes are much more active and more resistant to poisoning than Pt or Pt/Ru alloy catalysts. The reaction mechanism was studied by various electrochemical and surface analysis techniques. A reaction scheme that involves successive stepwise dehydrogenation of methanol, formation/oxidation of hydrogen tungsten bronze, and oxidation of organic intermediates and CO at the Pt/WO[sub 3] interface is proposed.
Hydrogen molybdenum bronzes HxMoO3 (0< x< 2) are consistently described as low-dimensional mixed conductors, whose properties under ambient conditions are controlled by charge density wave modulations. Proton conduction pathways in the bronzes are modeled by a bond valence approach. The redistribution of hydrogen during the intercalation process between two types of potential proton sites is simulated in a molecular mechanics study. Therefrom a structure model for the bronze phase II (0.85 < x < 1.04) is derived, which permits a Rietveld refinement of its previously unknown structure from powder X-ray data (space group I12/m1; a=14.5191(6) Å, b=3.7944(1) Å, c=7.7248(3) Å, β=93.743(2)° for x≈0.9). Both the doubling of the host cell along the c-axis in phase II and the 6×c superstructure found for phase I with x≈1/3 meet the expectations for quasi-one-dimensional Peierls distorted systems. Modifications in the structure, proton ordering, and properties of the bronzes are studied as a function of temperature. A time-resolved powder XRD investigation on the oxidation of phase II indicates the existence of a intermediate phase H0.6MoO3. The powder structure determination of this metastable phase (space group C2/m, a=14.543(2) Å, b=3.8520(4) Å, c=3.7691(4) Å, β= 90.73(1)°) indicates a redistribution of the protons during this oxidation step.
This paper describes some aspects of recent investigations into the anodic oxidation of methanol. Methanol has long been proposed as an anode fuel for a fuel cell, chiefly because of its ease of carriage, distribution and manipulation. However, methanol is very much more difficult to oxidise anodically than hydrogen, the more conventional anode fuel, and this has hampered development of commercial direct methanol fuel cells. Platinum-ruthenium catalysts are the most active discovered to date. Some advances in electrocatalysis of the methanol reaction by non-noble materials are discussed.
The Pt(110) model surfaces modified by the metallic molybdenum and by the MoOx (molybdenum oxide species) were fabricated via thermal decomposition of Mo(CO)6 and subsequent oxidation on a clean Pt(110) substrate, by means of Auger electron spectroscopy (AES) and X-ray photoelectron spectroscopy (XPS), as well as high-resolution electron-energy-loss spectroscopy (HREELS). CO chemisorption on these model surfaces was investigated using thermal desorption spectroscopy (TDS) and HREELS. The presence of the metallic molybdenum and MoOx species suppresses CO chemisorption on the Pt(110) substrate, indicating a physically site-blocking effect. The electron-deficiency of the Pt(110), due to the modification of metallic molybdenum, causes the low-temperature peak of CO desorption to shift downward in temperature by ∼30K. The interaction between the MoOx and the underlying Pt(110) substrate has no influence on CO desorption temperature.
A new type of electrode, in which porous ceramic membranes are used as substrate, has been prepared by electrodepositing the active components on a carbonized or Au sputtered membrane. The electrocatalytic oxidation of methanol on the electrodes was investigated. The membrane-based electrode showed very good activity, which was much better than a platinum-based electrode. Tungsten and molybdenum were very effective promoters; the activity of a tungsten-containing electrode was three times higher than that of an electrode without tungsten. However, ruthenium showed a negative effect on the activity of electrodes whilst tungsten was used as a promoter.
Enhancement of carbon monoxide oxidation on platinum by ruthenium ad-atoms was found to take place in two θRu regions; one by the increase of (nPt-Ru)av and another by the increase of (nRu-Ru)av, as expected from the standpoint of the bi-functional theory proposed by the authors.