Methanol Electrochemistry at Carbon-Supported Pt and PtRu Fuel Cell

Publication Date (Web): February 1, 2003 .... (Top) C/PtRu, 30% Pt, 15% Ru catalyst in 0.1 M HClO4 containing 0.050 M CH3OH at 23 and 60 °C, as indic...
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Methanol Electrochemistry at Carbon-Supported Pt and PtRu Fuel Cell Catalysts: Voltammetric and in Situ Infrared Spectroscopic Measurements at 23 and 60 °C Ganesh Vijayaraghavan, Lin Gao, and Carol Korzeniewski* Department of Chemistry and Biochemistry, Texas Tech University, Lubbock, Texas 79409-1061 Received August 27, 2002. In Final Form: November 25, 2002 The electrochemistry of methanol (50-200 mM) in 0.1 M HClO4 at Pt (10 wt %) and PtRu (30 wt % Pt, 15 wt % Ru) catalysts supported on Vulcan XC-72R carbon was investigated with voltammetry and in situ infrared spectroscopy at ambient temperature and 60 °C. The catalyst materials were each prepared as a thin film on a polished gold electrode according to a recently developed procedure [Weaver, M. J.; et al. J. Phys. Chem. B 2001, 105, 9719]. Methanol electrochemical oxidation was more sluggish on the Pt catalyst than on either bulk polycrystalline Pt or the PtRu catalyst material. The PtRu catalyst showed responses for methanol that were similar to those for a bulk PtRu alloy containing 46 wt % Ru. However, the PtRu catalyst appeared more resistant than bulk PtRu to CO adsorption. Differences are discussed in terms of the surface properties of bulk versus nanometer-scale Pt and PtRu.

Introduction External reflection infrared spectroscopy is an important technique for probing molecular-level processes at electrode-solution interfaces.1-4 High sensitivity at midinfrared spectral wavelengths toward carbon monoxide adsorbed on transition metal surfaces, in particular, has stimulated its application in the study of fuel cell related surface electrochemistry.5-13 Bulk electrodes from the platinum group and coinage metals display high reflectivity across the mid-infrared spectral region. However, fuel cell catalysts consist of transition metal particles that have diameters on the order of a few nanometers, often supported on carbon powders. At mid-infrared spectral wavelengths, these materials have reduced reflectivity, which can cause anomalous features to appear in infrared spectra.14-18 * Corresponding author: Phone: 806-742-4181. Fax: 806-7421289. E-mail: [email protected]. (1) Iwasita, T.; Nart, F. C. In Advances in Electrochemical Science and Engineering; Gerischer, H., Tobias, C., Eds.; VCH: New York, 1995; Vol. 4, p 123. (2) Weaver, M. J.; Zou, S. In Advances in Spectroscopy; Clark, R. J. H., Hester, R. E., Eds.; Wiley: Chichester, U.K., 1997; Vol. 26, p 219. (3) Korzeniewski, C. In Handbook of Vibrational Spectroscopy; Chalmers, J. M., Griffiths, P. R., Eds.; Wiley: New York, 2002; Vol. 4, pp 2699. (4) Korzeniewski, C. Crit. Rev. Anal. Chem. 1997, 27, 81. (5) Beden, B.; Leger, J.-M.; Lamy, C. In Modern Aspects of Electrochemistry; Bockris, J. O. M., Conway, B. E., White, R. E., Eds.; Plenum: New York, 1992; Vol. 22, p 97. (6) Sun, S.-G. In Electrocatalysis; Lipkowski, J., Ross, P. N., Eds.; Wiley-VCH: New York, 1998; Chapter 6, p 243. (7) Korzeniewski, C. In Interfacial Electrochemistry: Theory, Experiment and Applications; Wieckowski, A., Ed.; Dekker: New York, 1999; p 345. (8) Hammett, A. In Interfacial Electrochemistry. Theory, Experiment, and Applications; Wieckowski, A., Ed.; Dekker: New York, 1999; p 843. (9) Lin, W. F.; Iwasita, T.; Vielstich, W. J. Phys. Chem. B 1999, 103, 3250. (10) Liu, R.; Iddir, H.; Fan, Q.; Hou, G.; Bo, A.; Ley, K. L.; Smotkin, E. S.; Sung, Y.-E.; Kim, H.; Thomas, S.; Wieckowski, A. J. Phys. Chem. B 2000, 104, 3518. (11) Park, S.; Wasileski, S. A.; Weaver, M. J. J. Phys. Chem. B 2001, 105, 9719. (12) Park, S.; Tong, Y. Y.; Wieckowski, A.; Weaver, M. J. Langmuir 2002, 18, 3233. (13) Park, S.; Xie, Y.; Weaver, M. J. Langmuir 2002, 18, 5792.

Recently, a method has been developed for adsorbing carbon-supported fuel cell catalysts onto the surface of polished, bulk Au.11-14 Catalyst films a few monolayers in thickness can be prepared with the method.11-14 The modified substrate maintains the high reflectivity of the bulk Au, and upon reflection of infrared radiation, strong electric fields are sustained at the metal surface.18-20 Highquality infrared spectra of molecules interacting with adsorbed catalyst on the Au can be obtained under these conditions.11-14 In our group, the focus of recent work involving the use of external reflection infrared spectroscopy has been to elucidate effects of temperature on methanol electrochemical oxidation pathways.21-23 On bulk Pt, the onset potential for surface oxide formation shifts negative by about 50 mV as temperature is raised from ambient to 70 °C.22,24 This thermal activation of water increases the rate of methanol derived CO2 formation at low potentials.22 On bulk PtRu alloy electrodes, temperature effects are even stronger.23-25 In addition to enhanced activation of water, Ru passivation toward methanol dissociative chemisorption lifts above about 60 °C.23,24,26 The present study investigates methanol electrochemical oxidation at Pt and PtRu catalysts supported on Vulcan (14) Park, S.; Tong, Y. Y.; Wieckowski, A.; Weaver, M. J. Electrochem. Commun. 2001, 3, 509. (15) Lu, G.-Q.; Sun, S.-G.; Cai, L.-R.; Chem, S.-P.; Tian, Z.-W. Langmuir 2000, 16, 778. (16) Bo, A.; Sanicharane, S.; Sompalli, B.; Fan, Q.; Gurau, B.; Liu, R.; Smotkin, E. S. J. Phys. Chem. B 2000, 104, 7377. (17) Bjerke, A. E.; Griffiths, P. R. Anal. Chem. 1999, 71, 1967. (18) Porter, M. D. Anal. Chem. 1988, 60, 1143A. (19) Seki, H.; Kunimatsu, K.; Golden, W. G. Appl. Spectrosc. 1985, 39, 437. (20) Popenoe, D. D.; Stole, S. M.; Porter, M. D. Appl. Spectrosc. 1992, 46, 79. (21) Kardash, D.; Huang, J.; Korzeniewski, C. Langmuir 2000, 16, 2019. (22) Kardash, D.; Korzeniewski, C. Langmuir 2000, 16, 8419. (23) Kardash, D.; Korzeniewski, C.; Markovic, N. J. Electroanal. Chem. 2001, 500, 518. (24) Gasteiger, H. A.; Markovic, N.; Ross, P. N., Jr.; Cairns, E. J. J. Electrochem. Soc. 1994, 141, 1795. (25) Gasteiger, H. A.; Markovic, N.; Ross, P. N., Jr.; Cairns, E. J. J. Phys. Chem. 1993, 97, 12020. (26) Long, J. W.; Stroud, R. M.; Swider, K. E.; Rolison, D. R. J. Phys. Chem. B 2000, 104, 9772.

10.1021/la0207466 CCC: $25.00 © 2003 American Chemical Society Published on Web 02/01/2003

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XC-72R carbon at temperatures between ambient and 60 °C. For Pt, the focus is on a low metal loading (10 wt % Pt) catalyst, as the small diameter (2-4 nm) particles typical of the material can display CO adsorption characteristics that contrast those of bulk Pt.11,13 The carbonsupported catalysts were cast as films a few monolayers in thickness on optically flat Au electrodes according to procedures developed by the Weaver group.11-14 The electrochemistry of methanol on the modified Au electrodes was probed with voltammetry and in situ reflectance infrared spectroscopy. On Pt catalysts, the potential for methanol oxidation shifted negative with temperature by an amount similar to that for polycrystalline Pt. The rate of CO2 formation was slower than expected, but was consistent with earlier findings reported for methanol oxidation on carbon-supported, 2-3 nm diameter Pt particles.13 PtRu catalysts showed strong activity for methanol oxidation, and the onset potential for CO2 formation at PtRu shifted negative by 200 mV in stepping from ambient temperature to 60 °C. Experimental Section The catalysts Pt at 10 wt % metal loading on Vulcan XC-72R carbon (C/Pt, 10%) and PtRu at 30% Pt, 15 wt % Ru loading on Vulcan XC-72R carbon (C/PtRu, 30 wt % Pt, 15% Ru) were from Alfa Aesar/Johnson-Matthey (Ward Hill, MA). All solutions were prepared with deionized water (18 MΩ‚cm) from a four-cartridge Nanopure Infinity System (Barnstead, Dubuque, IA). Perchloric acid (99.999% purity) was obtained from Aldrich. Methanol (Burdick & Jackson (Muskegon, MI) GC grade, or EM Science (Gibbstown, NJ) 99.8%) was washed over alumina, filtered, distilled, and stored refrigerated. The working electrode consisted of a disk of polycrystalline Au (8 mm diameter by 2 mm thickness) pressure sealed into a shallow well that had been milled into the end of a Kel-F rod (Boedeker Plastics, Shiner, TX). A copper wire made electrical contact to the backside of the disk through a drop of silver epoxy. Prior to experiments, the electrode was polished mechanically with alumina from 1.0 µm down to 0.05 µm followed by sonication to remove debris. A KCl saturated silver-silver chloride electrode was employed as a reference in all experiments. The electrode was fitted with a Vycor glass frit and isolated behind a wetted stopcock to prevent leakage of chloride into the working electrode compartment. For reporting purposes, the measured potentials were converted to the reversible hydrogen electrode (RHE) scale and expressed as volts versus RHE, VRHE. A loop of Pt wire served as the counter electrode in all experiments. Cyclic voltammograms were recorded with the use of a Princeton Applied Research (PAR) model 273 potentiostat/galvanostat controlled by a computer running PAR research electrochemistry software version 4.10. Thin films of C/Pt, 10% catalyst and C/PtRu, 30% Pt, 15% Ru catalyst were prepared according to the procedure reported in refs 11-14. A suspension of catalyst (ca. 3 mg/mL) was prepared in ultrapure water, sonicated for 3 min, and dispensed with a micropipet onto the surface of a polished Au electrode. The electrode surface was allowed to dry in a stream of nitrogen for about 15 min. The surface was then rinsed in a jet of ultrapure water to remove loosely held particles. The resulting electrode surface supported a thin catalyst layer while maintaining the high reflectivity of Au. Just prior to experiments, the catalyst modified Au electrode was cycled briefly between -0.02 VRHE and +1.05 VRHE (for C/Pt, 10%), or 0.0 VRHE and 0.8 VRHE (for C/PtRu, 30% Pt, 15% Ru), in 0.1 M HClO4 to ensure waves characteristic of Pt were identifiable. Then, the electrode was held at 0.0 V while sufficient methanol was added to bring the cell to the desired concentration. Infrared spectral measurements were performed with a Mattson Instruments R/S-1 Fourier transform infrared spectrometer system running WinFirst (version 2.10) software (Thermo Electron Corp., Madison, WI). The optics used for external reflection measurements have been described.27 Spectra were obtained at 4 cm-1 resolution. Other spectral acquisition

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Figure 1. Cyclic voltammograms of Vulcan XC-72R carbonsupported Pt and PtRu catalysts adsorbed as a thin layer on a polycrystalline Au electrode. (A) C/Pt, 10% loading catalyst in 0.1 M HClO4 (bottom) and in 0.1 M HClO4 containing 0.1 M CH3OH (top) at 23 °C and a scan rate of 50 mV/s. (B) C/PtRu, 30% Pt, 15% Ru catalyst in 0.1 M HClO4 at 23 °C (bottom) and in 0.1 M HClO4 containing 0.050 M CH3OH at 23 °C (middle) and 60 °C (top). The scan rate was 20 mV/s. conditions are specified in the figure legends. The spectroelectrochemical cell consisted of a glass chamber that was jacketed to maintain constant temperature.28 Elements for heating and temperature control were built into the working electrode.28,29 Temperature was monitored at the backside of the Au disk working electrode by a thermocouple, and cell heating was enabled by a thermofoil heater (Mincoproducts, Minneapolis, MN) inserted into the electrode body.28 A PID temperature controller (LFI-3551, Wavelength Electronics, Bozeman, MT) was used to maintain the cell temperature. During spectral data collection, the cell potential was applied with a Pine Instruments (Grove City, PA) potentiostat.

Results Cyclic Voltammetry. Figure 1A shows cyclic voltammograms of a Au electrode after modification with an ultrathin layer of C/Pt, 10% loading catalyst. The current and peak potentials of the general features are in good agreement with voltammograms reported earlier for the Au/C/Pt, 10% loading system in 0.05 M H2SO412,14 and in 0.05 M H2SO4 containing 0.1 M methanol.13 The faradaic current relative to the charging current in each case suggests the catalyst film coverage is about 1 monolayer.13,14 In clean 0.1 M HClO4 (Figure 1A, bottom), waves appear at 0.0-0.2 VRHE characteristic of hydrogen adsorption and hydrogen desorption processes at polycrystalline Pt.13,14 Weak features associated with oxide formation and stripping on Pt can also be discerned at 0.50.8 VRHE. Addition of methanol to the solution (Figure 1A, top) gives the response typical for the oxidation of methanol and methanol dissociative chemisorption fragments on Pt electrodes.13,30 Voltammograms of a C/PtRu, 30% Pt, 15% Ru loading film on Au are shown in Figure 1B. The voltammograms were recorded in the temperature controlled in situ infrared cell with the working electrode pulled a few millimeters away from the window. The positive potential was limited to 0.8 VRHE to minimize oxidation-induced changes in the Ru composition at the catalyst surface.31 The film was a few monolayers in thickness.13,14 The broad, (27) Korzeniewski, C.; Huang, J. Anal. Chim. Acta 1999, 397, 53. (28) Kardash, D.; Huang, J.; Korzeniewski, C. J. Electroanal. Chem. 1999, 476, 95. (29) Huang, J.; Korzeniewski, C. J. Electroanal. Chem. 1999, 471, 146. (30) Herrero, E.; Franaszczuk, K.; Wieckowski, A. J. Phys. Chem. 1994, 98, 5074. (31) Markovic, N. M.; Gasteiger, H. A.; Ross, P. N., Jr.; Jiang, X.; Villegas, I.; Weaver, M. J. Electrochim. Acta 1994, 40, 91.

Methanol Electrochemistry at Fuel Cell Catalysts

Figure 2. Linear sweep voltammograms of Vulcan XC-72R carbon-supported Pt and PtRu catalysts adsorbed as a thin layer on a polycrystalline Au electrode. The scan rates were 20 mV/s. (Bottom) C/Pt, 10% loading catalyst in 0.1 M HClO4 containing 0.1 M CH3OH at 23 and 60 °C, as indicated. (Top) C/PtRu, 30% Pt, 15% Ru catalyst in 0.1 M HClO4 containing 0.050 M CH3OH at 23 and 60 °C, as indicated.

featureless voltammogram recorded in 0.1 M HClO4 (Figure 1B, bottom) is similar to responses that have been observed for high Ru content bulk PtRu alloys24,25 and related materials that contain nanometer-scale Ru.26,32,33 With the addition of methanol, the current begins to increase rapidly just past 0.4 VRHE on a positive going sweep from 0.0 VRHE (Figure 1B, middle). On the return sweep, the methanol oxidation current remains at a steady-state, consistent with reports of methanol voltammetry on PtRu bulk alloys over the limited potential range.24,25 The topmost cyclic voltammogram in Figure 1B was recorded immediately afterward with the same electrode and catalyst layer, but with the cell thermostated at 60 °C. The responses for methanol oxidation at 23 and 60 °C on C/PtRu, 30% Pt, 15% Ru catalysts are similar, but at the higher temperature the current is greater above 0.4 VRHE. These differences are shown more clearly in Figure 2 (top), where the forward scans of the voltammograms recorded at 23 and 60 °C are overlaid. The two scans begin to diverge at about 0.35 VRHE. At a potential of 0.6 VRHE, the current for the scan recorded at 60 °C is twice as large as the current on the ambient temperature scan. These trends have been observed in voltammograms recorded with bulk PtRu alloy electrodes under similar conditions.23-25 Figure 2 also includes voltammograms for methanol oxidation at a C/Pt, 10% loading catalyst recorded at 23 and 60 °C (Figure 2, bottom). Again, the responses were recorded with the same electrode and catalyst layer without visible loss of catalyst between scans. Methanol oxidation does not appear to be as sensitive to temperature on the Pt catalyst as on PtRu. The two scans begin to diverge at about 0.4 VRHE, and at 0.6 VRHE, the current for the scan recorded at 60 °C is only 1.4 times as great as the current on the ambient temperature scan. The onset potential for methanol oxidation on the carbon-supported Pt is 36 mV more negative at 60 °C than at ambient temperature, which is consistent with the effects of water (32) Long, J. W.; Swider, K. E.; Merzbacher, C. I.; Rolison, D. R. Langmuir 1999, 15, 780. (33) Waszczuk, P.; Solla-Gullon, J.; Kim, H. S.; Tong, Y. Y.; Montiel, V.; Aldaz, A.; Wieckowski, A. J. Catal. 2001, 203, 1.

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Figure 3. Infrared spectra of C/Pt, 10% loading catalyst adsorbed as a thin layer on a polycrystalline Au electrode in 0.1 M HClO4 containing 0.2 M 13CH3OH at 23 °C. The initial spectrum was recorded at 0.0 VRHE and used as the background. Then, the potential was stepped positive in 100 mV increments, and after each step, a single beam spectrum was recorded from the average of 122 interferograms. Data acquisition at each potential required approximately 1.6 min.

thermal activation that have been observed for bulk Pt electrodes.22,24 Infrared Spectroscopy. Infrared spectra recorded during methanol oxidation on carbon-supported Pt (10% loading) are shown in Figure 3. The bands at 2277 cm-1 are from 13CO2 formed upon the oxidation of 13CH3OH at the indicated potentials. The carbon-13 isotope of methanol was used as reactant to enable CO2 from methanol oxidation to be detected with minimal interference from CO2 in the atmosphere. The methanol concentration was 0.2 M instead of 0.1 M to provide for improved sensitivity toward adsorbed CO and CO2 during reactions on the low Pt metal loading material. In the experiments, the band for 13CO2 became evident initially at 0.8 VRHE. The 13CO2 band is weak, and its shape is broadened by baseline drift and a small feature that extends downward at 2343 cm-1 due to excess atmospheric CO2 in the background spectrum. The evolution of 13CO2 continued to be slow until 1.2 VRHE was reached. At 1.2 VRHE, the rate of 13CO2 formation was more rapid and a downward extending feature assignable to adsorbed 13CO (1969 cm-1) appeared (Figure 3). The 1969 cm-1 band arises from adsorbed 13CO present at the background potential of 0.0 VRHE. The band was detectable only after the 13CO coverage was lowered upon its oxidation at the high positive potential. The rapid increase in 13CO2 band intensity at 1.2 VRHE likely results from faster adsorbed 13CO oxidation and decreased surface poisoning of 13CH3OH dissociative chemisorption due to a lower steady-state 13CO coverage. The curved spectral baselines in Figure 3 result from compositional changes that take place in the thin layer cavity in stepping the electrode from the background up to the sample potential. In earlier studies of methanol oxidation on carbon-supported Pt catalysts, baseline fluctuations were reduced by maintaining a small interval between the reference and sample potentials and keeping the interval constant by shifting in equal amounts the potentials at which both spectra were recorded.12 Using the approach in the present studies did not reveal bands from CO2 or adsorbed CO below 0.8 VRHE. The low Pt content of the thin films probably sets limits on the ability to detect these species. In earlier studies of oxidation reactions involving dissolved CO11 or methanol13 at C/Pt, 10% loading catalysts, spectral bands of adsorbed CO were

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toward adsorbed CO and CO2 in experiments with C/Pt, 10% loading catalysts. Discussion

Figure 4. Infrared spectra of C/PtRu, 30% Pt, 15% Ru catalyst adsorbed as a thin layer on a polycrystalline Au electrode in 0.1 M HClO4 containing 0.1 M 13CH3OH at 23 °C. The initial spectrum was recorded at 0.0 VRHE and used as the background. Then, the potential was stepped positive in 50 mV increments, and after each step, a single beam spectrum was recorded from the average of 75 interferograms. Data acquisition at each potential required approximately 0.9 min.

Figure 5. Same as in Figure 4, except for 0.05 M 13CH3OH and a cell temperature of 60 °C.

barely apparent. We are further investigating the factors that make it difficult to observe bands of CO2 and adsorbed CO in experiments with the low Pt loading catalysts. Methanol oxidation to CO2 occurred readily at thin films of C/PtRu, 30% Pt, 15% Ru catalysts on Au. Figure 4 shows 13CO2 reaches detectable levels as early as 0.5 VRHE in the presence of 0.1 M 13CH3OH, and the 13CO2 band grows steadily with increasing positive potential. At 60 °C (Figure 5), the catalyst film becomes more active. The concentration of 13CH3OH was reduced to 0.050 M to enable CO2 formation to be followed without saturating the CO2 concentration in the thin layer cavity. In Figure 5, 13CO2 becomes detectable at 0.3 VRHE, a shift of 200 mV toward more negative potentials compared to the C/PtRu catalyst at ambient temperature. Investigations of temperature effects on methanol oxidation at C/Pt, 10% loading catalysts were also initiated. In preliminary work, raising the cell from 23 to 60 °C shifted the onset potential for the appearance of CO2 bands negative by 50-100 mV, as expected from experiments with bulk Pt electrodes.22 More detailed studies of the system above ambient temperatures are continuing in connection with efforts to improve spectral sensitivity

It is useful to compare responses observed in the present study for the reaction of methanol at carbon supported Pt and PtRu catalysts to results of earlier experiments involving methanol electrochemical oxidation at bulk Pt and PtRu.22-25 One striking difference occurs in the voltammetry of methanol at Pt materials. At bulk Pt (as well as low Ru content bulk PtRu), the current density for methanol oxidation at 0.6 VRHE can change by almost 1 order of magnitude between ambient and 60 °C.22-25 In contrast, for C/Pt, 10% loading catalyst the current at 0.6 VRHE increased by only a factor of 1.4 over the same temperature range (Figure 2, bottom). Also, near 0.35 VRHE bulk Pt and C/Pt, 10% loading catalyst show expected temperature-dependent shifts in the onset potential for rapid methanol oxidation, but the shift is lower (by ∼14 mV) for bulk Pt. We believe the responses reflect differences in the surface properties and preparation procedures of the bulk materials and nanometer-scale catalyst particles. Just prior to electrochemical measurements, the bulk metals referred to were either sputter cleaned in ultrahigh vacuum24,25 or cleaned by oxidative chemical and electrochemical treatments.22,23 In comparison, the thin films of Pt and PtRu supported on Vulcan XC-72R carbon were treated only briefly by voltammetric cycling in 0.1 M HClO4 to avoid inducing structural changes in the nanometerscale catalysts through excessive oxidation. It is likely the clean surface characteristics of the bulk materials after processing contribute to their higher activity for methanol oxidation. In continuing experiments, we are examining the surface electrochemistry of methanol at Pt catalysts following thermal34,35 and electrochemical26 pretreatment protocols that aim to remove surface contamination. For PtRu materials, the temperature controlled voltammetry of bulk alloys and C/PtRu, 30% Pt, 15% Ru catalysts is similar at 0.6 VRHE. For the oxidation of 0.5 M methanol at a bulk PtRu alloy containing 46 atomic % Ru (XRu ) 0.46), the current at 0.6 VRHE during a potential sweep was almost twice as large at 60 °C than at ambient temperature,24 which compares well to results for C/PtRu, 30% Pt, 15% Ru catalyst in Figure 2 (top), where the current for the 60 °C scan is 2.1 times greater than the current on the ambient temperature scan. However, voltammograms obtained with the PtRu materials show a disparity in the potential at which methanol oxidation begins. On bulk PtRu with XRu ) 0.46, the onset potential for methanol oxidation at 60 °C is about 0.1 VRHE lower than the C/PtRu, 30% Pt, 15% Ru catalyst displayed under the conditions of the present studies.24,25 In situ infrared spectroscopy measurements show PtRu materials are especially active for CO2 formation.23,31,36 On bulk PtRu with XRu ) 0.10, CO2 was detected at potentials as low as 0.2 VRHE in the presence of 1.0 M methanol for temperatures of 25 and 70 °C.23 The potential at which CO2 evolution was first detected was approximately 100 mV more negative on the bulk alloy than on bulk polycrystalline Pt under the same conditions.22 The C/PtRu, 30% Pt, 15% Ru catalyst shows related behavior in that it readily transforms methanol to CO2 (34) Schmidt, T. J.; Gasteiger, H. A.; Stab, G. D.; Urban, P. M.; Kolb, D. M.; Behm, R. J. J. Electrochem. Soc. 1998, 145, 2354. (35) Jusys, Z.; Behm, R. J. J. Phys. Chem. B 2001, 105, 10874. (36) Iwasita, T.; Hoster, H.; John-Anacker, A.; Lin, W. F.; Vielstich, W. Langmuir 2000, 16, 522.

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(Figures 4 and 5) and it is much more active than the C/Pt, 10% loading catalyst. However, the onset potential for CO2 formation appears to be lower for bulk PtRu materials than the C/PtRu, 30% Pt, 15% Ru catalyst investigated. In situ infrared spectroscopy measurements also suggest there may be differences in the rate of steps that control the steady-state coverage of adsorbed CO during methanol oxidation at bulk versus nanometer-scale PtRu. On bulk alloys, compositions of XRu ) 0.10 and XRu ) 0.90 in 1.0 M methanol were observed to support near-saturation coverages of adsorbed CO in parallel with rapid CO2 formation.23 The adsorbed CO detected appeared to be the steady-state coverage that resulted from the balance between CO formation during methanol dissociative chemisorption and its conversion to CO2 through reaction with surface oxides.23 The same behavior has been reported for a PtRu bulk alloy with XRu ) 0.15 in 0.1 M HClO4 containing 0.5 M methanol.36 On C/PtRu, 30% Pt, 15% Ru catalyst in 0.1 M methanol, adsorbed CO was not detected. Attempts to raise the methanol concentration to 1.0 M caused gas bubbles of CO2 to accumulate in the thin layer cavity between the working electrode and the cell’s infrared transparent window. It appears the transformation of adsorbed CO to CO2 may be faster on C/PtRu, 30% Pt, 15% Ru catalyst than on bulk PtRu materials, but effects of increasing methanol concentration on adsorbed CO coverage at the catalyst was difficult to probe. As already discussed for Pt, pretreatment procedures are expected to affect electrochemical responses for methanol oxidation at bulk metal versus nanometer-scale particles. In the case of PtRu materials, inherent surface properties may also account for differences. For materials that contain Ru, there has been interest in identifying the phases that provide the greatest activity for the electrochemical oxidation of methanol and adsorbed CO (cf. refs 26 and 37-41). Experiments with ultrahigh vacuum characterized Ru single crystals have demonstrated thin RuO2 films enable rapid CO electrochemical oxidation.40,41 (37) Dinh, H. N. J. Electroanal. Chem. 2000, 491, 222. (38) Stroud, R. M.; Long, J. W.; Swider-Lyons, K. E.; Rolison, D. R. Microsc. Microanal. 2002, 8, 50.

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These findings are consistent with those that show the highest activity for methanol oxidation at nanometer-scale PtRu is achieved when a hydrated phase consisting of RuOH and RuO2 is present.26,38 In our studies reported herein pertaining to methanol oxidation at carbon supported, nanometer-scale PtRu catalysts, a hydrated oxide phase may be active in maintaining the observed high rates of CO2 formation and low steady-state coverages of adsorbed CO. To probe this important question further, CO2 evolution rates and steady-state adsorbed CO coverages are being measured during methanol oxidation at bulk and nanometer-scale PtRu materials over a range of temperature and methanol concentrations. Concluding Remarks Thin films of PtRu (30 wt % Pt, 15 wt % Ru) Vulcan XC-72R carbon catalyst on bulk Au are active for the electrochemical oxidation of methanol to CO2 at ambient temperature as well as at 60 °C. Methanol oxidation was more sluggish at the low metal loading Pt (10 wt %) Vulcan XC-72R carbon catalyst than at either bulk Pt or PtRu catalysts. However, just as for bulk metal electrodes, the responses of nanometer-scale catalysts are expected to be sensitive to conditions of pretreatment and preparation. These factors are being investigated in continuing experiments that employ quantitative methods for the characterization of catalyst cleanliness, structure, size distribution and film thickness.26,33-35,42 Acknowledgment. Support for this work from the Office of Naval Research and the Robert A. Welch Foundation is gratefully acknowledged. LA0207466 (39) Tong, Y. Y.; Kim, H. S.; Babu, P. K.; Waszczuk, P.; Wieckowski, A.; Oldfiled, E. J. Am. Chem. Soc. 2002, 124, 468. (40) Lin, W. F.; Zei, M. S.; Kim, Y. D.; Over, H.; Ertl, E. J. Phys. Chem. B 2002, 104, 6040. (41) Wang, W. B.; Zei, M. S.; Ertl, G. Chem. Phys. Lett. 2002, 355, 301. (42) Long, J. W.; Ayers, K. E.; Rolison, D. R. J. Electroanal. Chem. 2002, 522, 58.