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J. Phys. Chem. C 2007, 111, 7038-7048

DEMS Study on Methanol Oxidation at Poly- and Monocrystalline Platinum Electrodes: The Effect of Anion, Temperature, Surface Structure, Ru Adatom, and Potential Hongsen Wang* and Helmut Baltruschat† Institute of Physical and Theoretical Chemistry, UniVersity of Bonn, D-53117 Bonn, Germany ReceiVed: December 4, 2006; In Final Form: March 18, 2007

Methanol oxidation on Ru-unmodified and modified polycrystalline platinum, Pt(111), and Pt(332) was studied by on-line differential electrochemical mass spectrometry (DEMS) in combination with the dual thin-layer flow-through cell. The effects of anion, temperature, surface structure of Pt electrodes, Ru adatom, and potential on methanol oxidation were discussed. Methanol oxidation on smooth polycrystalline platinum, Pt(111), and Pt(332) electrodes proceeds via the parallel pathway mechanism, and mainly forms the soluble intermediates, i.e., formaldehyde and formic acid. Anion, temperature, and surface structure of Pt electrodes markedly influence the apparent rate of methanol oxidation; however, they do not significantly alter the current efficiency of CO2, and therefore do not change the mechanism of methanol oxidation. On pure platinum electrodes current efficiency of CO2 for methanol oxidation increases from ca. 20% to 32% with the increase of potential from 0.6 to 0.8V, accompanied by the increasing Faradaic current. Ru adatoms can promote the methanol oxidation via COad to form CO2 in the low potential region (e0.65 V) on three platinum electrodes, leading to higher current efficiency of CO2, even ca. 100% for Pt(111)/Ru. At potentials of >0.65V, Ru adatoms lose their cocatalytic activity for methanol oxidation.

1. Introduction Direct methanol fuel cells (DMFCs) have attracted great attention due to their potential application as power sources for vehicles and portable devices.1,2 At present one of major problems in the development of low-temperature DMFCs is the low catalytic activity of anodic catalyst toward methanol oxidation at low temperatures. For these fuel cells to be economically viable, the turnover rate for complete oxidation of methanol must be higher than 0.1 s-1 between 0.2 and 0.4 V versus reversible hydrogen electrode (RHE).3 To improve the anodic performance platinum-based binary catalysts such as PtRu,4-14 PtSn,12,15-17 and PtMo18-20 have been examined toward methanol oxidation, and especially PtRu binary catalyst has been proven to possess stable catalytic activity toward methanol electrooxidation. Nevertheless, the optimal alloy composition and the mechanistic role of the cocatalysts are still a subject of ongoing research and discussion. At present the action of the second metal as an alloy or adlayer element on platinum toward CO or methanol oxidation has been ascribed to the following reasons: (a) bifunctional mechanism (for example, Ru sites adsorb oxygen-containing species at 0.20.3 V lower potentials than the pure platinum surface, and the carbonaceous species adsorbed on platinum sites are preferentially oxidized by oxygen-containing species formed on neighboring Ru atoms4,21) and (b) electronic effect or ligand effect (the second metal modifies the electronic nature of the surface of base metal;22 such an effect was observed, for example, where the Pt-CO bond strength was found to be weakened in the presence of Ru or Sn23). The oxidation of adsorbed CO in two * Address correspondence to this author. Phone: 1-607-255-7568. Fax: 1-607-255-9864. E-mail: [email protected]. Present address: Department of Chemistry and Chemical Biology, Baker Laboratory, Cornell University, Ithaca, New York 14853-1301. † E-mail: [email protected].

well-separated waves on surfaces modified by small amounts of Ru has been taken as an indication that both mechanisms are active.18 To achieve the higher conversion efficiency or higher power density with a low Pt catalyst loading, DMFCs should be operated at the higher temperature. To obtain the kinetic information on methanol electrooxidation at elevated temperatures, the temperature dependence of methanol oxidation has been studied by some groups.6,8,11,24-28 The activation energy of methanol oxidation in the literature varies from 20 to 95 kJ mol-1. This could be due to the applied potential, electrolyte concentration, catalyst type, and even the synthetic method. For example, Wakabayashi et al.24 reported that the apparent activation energy for methanol electrooxidation on a thin film platinum deposited on gold in 1 M CH3OH + HClO4 solution increases with increasing potential, i.e., 15 kJ mol-1 at 0.6 V, 20 kJ mol-1 at 0.65 V, and 24 kJ mol-1 at 0.7 V in the temperature region from 20 to 120 °C. In contrast, measuring methanol oxidation on GC supported Pt/Vulcan catalyst with a loading of 50-100 µg cm-2 in 0.015 M CH3OH + 0.5 M H2SO4 solution at elevated temperature, Madden et al.25 found that increasing the temperature from 50 to 100 °C at 0.35 VPdH results in a substantial increase in methanol electrooxidation rates with an activation energy of 70 kJ mol-1. Methanol electrooxidation was found to be a surface structure sensitive reaction, and the activity of various monocrystalline platinum electrodes such as low-index basal planes and stepped platinum surfaces29-34 as well as platinum nanoparticles8,35-37 has been examined. Of the three basal planes of platinum, Pt(110) was found to be the most active. Pt(111) was proven to be the least reactive toward methanol decomposition, and the rate of methanol oxidation increases with the increasing step density of Pt(111). At present, there is still a contradiction on how the step density influences the reaction of methanol

10.1021/jp068328n CCC: $37.00 © 2007 American Chemical Society Published on Web 04/24/2007

DEMS Study on Methanol Oxidation oxidation. Shin et al.38 found that methanolic CO formation is inhibited on Pt(111) at the potential of the classic hydrogen adsorption region, and the Pt(335) (Pt[4(111)×(110)]) surface plane promotes methanol dissociative chemisorption, therefore suggesting that the defects catalyze methanol decomposition. Studying the voltammetric behavior of methanol oxidation on Pt(111), Pt(554) (Pt[9(111)×(110)]), and Pt(553) (Pt[4(111)×(110)]), Housmans et al.31 also found that the overall oxidation rate of methanol increases with increasing step density. However, Tripkovic et al.39 reported that the initial surface activity for methanol oxidation on stepped electrodes decreases with the increasing step density in the sequence of Pt(755) > Pt(211) > Pt(311) (Pt[n(111)×(100)]-type electrodes with n ) 6, 3, and 2, respectively). Recently Park et al.35,40 studied methanol oxidation on carbon-supported platinum nanoparticle catalysts, and observed that the methanol oxidation rate decreases with a decreasing particle size. They explained that methanol decomposition requires terrace sites rather than defect or step sites. As a model binary catalyst Ru-modified monocrystalline platinum toward methanol oxidation has also been extensively studied.7,41-43 It was found that Pt(111)/Ru exhibits the highest catalytic activity among the three modified basal planes of platinum.11 In most studies of methanol electrooxidation, sulfuric acid or perchloric acid was used as the supporting electrolyte. Sulfate anion adsorbs more strongly than perchlorate anion, leading to the lower oxidation rate of methanol in sulfuric acid solution than perchloric acid solution, especially for Pt(111).30,44 In addition, the effect of potentials on methanol oxidation has also often been studied.14,24,31,45 Although much effort has been made to study the effects of various factors, i.e., anion, temperature, surface structure of electrode, platinum based alloys, and potentials on methanol oxidation, these studies have mainly focused on the oxidation current, and did little to consider the effects of these factors on the distribution of products. Early Ota et al. reported that the yield of CO2 increases when the roughness factor of the platinized platinum electrode and the reaction temperature increase during the initial 50 min of eletrolysis.46 The yield of products for methanol oxidation on different platinum electrodes was later studied by Belgsir et al.,47 and more recently by Korzeniewski et al.,48 Wang et al.,8,42,49 and Batista et al.45 Recently, Housmans et al.30 investigated the selectivity and structure sensitivity of the methanol oxidation pathways on the basal planes Pt(111), Pt(110), and Pt(100) and the stepped Pt electrodes Pt(554) and Pt(553) in sulfuric and perchloric acid electrolytes with DEMS. However, due to the pinhole inlet system in their setup, they only could detect the relative ratio of CO2 (m/z 44) and methyl formate (m/z 60), but could not get any quantitative information on products. Our previous results8,42,49 showed that large amounts of intermediates such as formaldehyde and formic acid are formed during methanol oxidation on the smooth platinum electrode. The formation of these intermediate products could cause air pollution and corrosion, and also reduce the efficiency of fuels; the current efficiency of CO2, therefore, is one of the most important considerations in evaluating the anodic performance. In previous works,8,42,49 our DEMS results proved that methanol oxidation on the platinum electrode proceeds via a parallel pathway mechanism, i.e., in one pathway methanol is oxidized to CO2 via COad, while soluble intermediates formaldehyde and formic acid are also formed in other parallel pathways. Here, we continue to report the effects of various factors on the methanol oxidation mechanism and the current

J. Phys. Chem. C, Vol. 111, No. 19, 2007 7039 efficiency of CO2. The effects of the surface structure of electrodes, particularly single-crystal platinum, anion, elevated temperature, and potential, on the current efficiency of CO2 have been studied. A submonolayer amount of Ru was deposited on polycrystalline platinum, Pt(111), and Pt(332) (Pt[6(111)×(111)]) to generate the model bimetallic catalyst surface. The effect of PtRu cocatalysts on methanol oxidation has also been investigated. 2. Experimental Section A dual thin-layer flow-through cell made of titanium was used for DEMS in combination with a Balzers quadrupole mass spectrometer QMG 511. The construction of this cell was described in the refs 42 and 50. The area of the working electrode exposed to the electrolyte in the DEMS cell was 0.28 cm2. Smooth polycrystalline platinum has a diameter of 1 cm and a roughness factor of ca. 1.8, as determined according to hydrogen adsorption charge. Pt(111) single crystal was purchased from MaTeck, Juelich, Germany. Pt(332) single crystal was purchased from Goodfellow, Cambridge, UK. Both electrodes (diameter 1 cm) used in these experiments were prepared according to Clavilier’s method by annealing them in a hydrogen (99.999%) flame for about 30 s to a slightly red color and then transferring them into a glass cell, where they were cooled down to room temperature over 4 min in a highly pure argon (99.999%) atmosphere. Afterward, the surface was immersed in the supporting electrolyte (0.5 M H2SO4). The quality of each preparation was checked by recording a cyclic voltammogram in the supporting electrolyte (0.5 M H2SO4). All solutions were prepared with Millipore water. Either 0.5 M H2SO4 p.a. or 1 M HClO4 p.a. was used as the supporting electrolyte. Methanol was spectropure grade (Merck). The solutions were deaerated with highly pure argon (99.999%). All measurements were carried out at room temperature (25 ( 1°C), except for the elevated temperature experiment. Small coverage of Ru on polycrystalline platinum, Pt(111), and Pt(332) was achieved as follows: Ru was deposited at 0.6 V vs RHE for 5 min from freshly prepared 5 × 10-3 M RuCl3 + 0.1 M H2SO4 solution. Details of the deposition procedure are given in ref 51. A coverage of 25-35% should be obtained, according to ref 52, which was also checked with the method of Motoo.53 Before DEMS measurement, the cleanliness of all electrodes (and in the case of single-crystal electrodes also surface structure) was again checked by cyclic voltammetry in the DEMS cell between 0.05 and 1.5 V for polycrystalline platinum, and 0.05 and 0.85 V for single-crystal and Ru-modified electrodes. For all potential step measurements, first the potential was stopped at 0.05 V, where methanol does not yet adsorb at Pt and Pt-Ru,9 and the supporting electrolyte was replaced by the solution containing methanol. All subsequent measurements were carried out under the condition of flowing electrolyte (5 µL/s). An average value for the current efficiency of CO2 was determined as follows: CV and MSCV of m/z 44 were integrated from 0.3 to 1.5 V for smooth polycrystalline platinum, or 0.85 V for Pt(111), Pt(332), and Ru modified platinum electrodes, and then back to 0.3 V to obtain the Faradaic charge, Qf, and the integrated ion current, Qi(44). The part of the Faradaic charge that corresponds to the formation of CO2 was obtained by eq 1

Qf* ) 6Qi(44)/K*(44)

(1)

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Figure 1. Cyclic voltammograms of polycrystalline platinum (a), Pt(111) (b), and Pt(332) (c) electrodes in the hanging Meniscus arrangement in a H-cell (glass-cell) after deposition of Ru (solid lines). The CVs of the bare electrodes are also shown for comparison (dotted lines). Scan rate 50 mV s-1; solution 0.5 M H2SO4.

where Qf* is the Faradaic charge corresponding to the formation of CO2 in a cycle of potential sweep, Qi(44) is the integrated ion current for CO2 in a cycle of potential sweep, 6 is the number of electrons needed for methanol oxidation to CO2, and K*(44) is the calibration constant, which is determined from a calibration experiment involving adsorbed CO oxidation, as described in ref 54. Then the current efficiency of CO2 was determined by eq 2:

Aq ) Qf*/Qf

(2)

where Aq is the average current efficiency of CO2 in a cycle of potential sweep, and Qf is the Faradaic charge in a cycle of potential sweep. Similarly, for the potential step experiments, the current efficiency of CO2 can also be determined from the Faradaic current, If, and ion current of m/z 44, Ii(44):

If* ) 6Ii(44)/K*(44)

(3)

Ai ) If*/If

(4)

where If* is the Faradaic current corresponding to the formation of CO2, If is the Faradaic current, Ii(44) is ion current of CO2, and Ai is the current efficiency for CO2. The ratio of the amount of methyl formate to that of CO2 formed during methanol oxidation was obtained by carefully calibrating the DEMS setup. The details of the calibration were given in ref 42. 3. Results 3.1. Surface Quality of Single Crystals and Ru Modification. Prior to each experiment of methanol oxidation, Pt(111) and Pt(332) surfaces were prepared by Clavilier’s method. Then the surface quality was checked by measuring a blank cyclic voltammogram (CV) in 0.5 M H2SO4 in the traditional electrochemical cell (Figure 1). The resulting CVs agree well with those previously published in the literature.51,55,56 After-

Figure 2. Simultaneously recorded CVs (a) and MSCVs of m/z 44 (b) and m/z 60 (c) on smooth polycrystalline platinum in 0.1 M methanol + 0.5 M H2SO4 solution (‚‚‚) and in 0.1 M methanol + 1 M HClO4 solution (s) at ambient temperature (25 °C). The inset shows the expanded view of first and second positive-going scans in the low potential region. Scan rate 10 mV s-1; electrolyte flow rate 5 µL s-1. Arrows indicate the direction of potential sweep.

ward, these monocrystalline platinum electrodes, which were protected with a drop of supporting electrolyte, were transferred into the DEMS cell, and then were checked again by measuring a blank cyclic voltammogram in 0.5 M H2SO4. After ascertaining the CVs are same as those obtained in the traditional electrochemical cell, the experiments of methanol oxidation were performed. The influence of small amounts of Ru on the cyclic voltammograms of polycrystalline platinum, Pt(111), and Pt(332) is shown in Figure 1. For Ru-modified polycrystalline platinum, the double layer region becomes broader, due to OH or anion adsorption/desorption on Ru adatoms, and the hydrogen adsorption/desorption peaks are inhibited, compared to polycrystalline platinum. According to ref 52 a Ru coverage of 2535% should be obtained, which was also checked with the method of Motoo.53 For Pt(111)/Ru, Ru adatoms form 2D islands on Pt(111) with a diameter of 3-5 nm, and under our deposition condition a Ru coverage of 0.25 should be obtained, as found by STM.57 As for Pt(332)/Ru, Ru adatoms preferably deposit at the step sites, as indicated by the peak at 0.12 V corresponding to hydrogen adsorption at step sites being nearly completely inhibited, and the resulting Ru coverage is ca. 0.25.51 3.2. The Effect of Anion on Methanol Oxidation on Smooth Polycrystalline Platinum. Cyclic voltammograms (CVs) for methanol oxidation on the smooth polycrystalline platinum in 0.1 M methanol + 1 M HClO4 and in 0.1 M methanol + 0.5 M H2SO4 solutions and the corresponding mass spectrometric cyclic voltammograms (MSCVs) of m/z 44 (CO2) and m/z 60 (methylformate) at room temperature (25 ( 1°C) are shown in Figure 2. The CVs of methanol oxidation on polycrystalline platinum are similar to those published before. It is well-known that methanol is irreversibly adsorbed only at potentials above 0.2V giving rise to the shoulder around 0.5 V. Therefore, in the first sweep, the hydrogen desorption peaks are still visible (the inset of Figure 2). However, whereas at

DEMS Study on Methanol Oxidation

Figure 3. Simultaneously recorded transients of Faradaic currents (a) and ion currents of m/z 44 (b) and m/z 60 (c) on smooth polycrystalline platinum in 0.1 M methanol + 0.5 M H2SO4 solution (‚‚‚) and in 0.1 M methanol + 1 M HClO4 solution (s) at ambient temperature (25 °C) after the potential step from 0.05 to 0.6 V. Electrolyte flow rate 5 µL s-1.

lower methanol concentrations (e0.01 M) the first sweep overlaps completely with a sweep in pure sulfuric acid in the hydrogen region,21,42 at this concentration of 0.1 M CH3OH, the first hydrogen adsorption peak is somewhat surpressed. To our knowledge, this was never discussed before. In a control experiment involving an additional electrolyte exchange with sulfuric acid before starting the potential sweep, we checked that the suppression of the first hydrogen peak is not due to an irreversibly formed adsorbate. This rather indicates that at this potential methanol adsorbs weakly and reversibly. It is probably a weakly adsorbed methoxy species that in UHV is known to be formed as the first step of methanol adsorption.58,59 The oxidation of methanol in 0.1 M methanol + 1 M HClO4 solution (solid line) exhibits a similar behavior to that in 0.1 M methanol + 0.5 M H2SO4 solution (dotted line). In the positivegoing scan the oxidation of methanol in both solutions starts from ca. 0.5 V, and the oxidative current increases with the increasing potentials, reaching a maximum around 0.83 V, and then decreases due to the formation of Pt oxide. As the potential increases above 1.1 V, the Pt surface becomes active again for methanol oxidation. In the negative-going scan an oxidative peak is observed around 0.76 V, caused by the freshly reduced Pt surface. There are also some different features for two solutions: (i) for perchloric acid solution one shoulder on the negative side of the main peak is observed around 0.75 V in the positive-going scan, which is mainly paralleled by the peak of methyl formate formation (Figure 2c), while the main peak coincides with the peak of CO2 formation (Figure 2b); (ii) the peak current of methanol oxidation in perchloric acid solution is more than 2 times as high as that in sulfuric acid solution, which can be explained by the fact that sulfate/bisulfate anions adsorb more strongly than perchlorate anions,60,61 therefore inhibiting the oxidation of methanol.

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Figure 4. Simultaneously recorded CV (a) and MSCVs of m/z 44 (b) and m/z 60 (c) on smooth polycrystalline platinum in 0.1 M methanol + 0.5 M H2SO4 solution at 50 °C. Scan rate 10 mV s-1; electrolyte flow rate 5 µL s-1. Arrows indicate the direction of potential sweep.

During the oxidation of methanol, CO2 and methyl formate were on-line detected by mass spectrometry (Figure 2b,c). Methyl formate is formed through the chemical reaction between methanol and electrochemically formed formic acid. The formation of CO2 and methyl formate also starts from ca. 0.5 V, consistent with the onset potential of methanol oxidation. After calibrating the DEMS setup, the amount or current efficiency of formed CO2 and methyl formate can be determined. The average current efficiency of CO2 in a cycle of potential scan is only (28 ( 3)% for sulfuric acid and ca. (30 ( 3)% for perchloric acid solution, respectively. The average ratio of CO2 to methyl formate is 40 for sulfuric acid and 27 for perchloric acid solution. At the potential corresponding to maximum Faradaic current, the current efficiency of CO2 is ca. 39% for sulfuric acid and 43% for perchloric acid solution. At the low potentials current efficiency of CO2 and the ratio of CO2 to methyl formate decrease (Figure 2), as is also shown in potential step experiments of Figure 3. Figure 3 shows the typical potential step experiments of smooth polycrystalline platinum in 0.1 M methanol + 1 M HClO4 and in 0.1 M methanol + 0.5 M H2SO4 solutions. The potential was stepped from 0.05 to 0.6 V, and the Faradaic current and the corresponding mass spectrometric currents of m/z 44 (CO2) and m/z 60 (methyl formate) were simultaneously recorded. As for the case of cyclic voltammetry, the Faradaic current and the formation of CO2 and methyl formate are much higher in perchloric acid solution compared to that in sulfuric acid solution. At the second minute after potential step the Faradaic current and the mass spectrometric current of m/z 44 are used to calculate the current efficiency of CO2. At 0.6 V the current efficiency of CO2 is (18 ( 2)% for sulfuric acid and (22 ( 2)% for perchloric acid solution, respectively. A quantitative analysis of ion currents of m/z 44 and 60 showed that the amount of methyl formate formed on smooth polycrystalline platinum is up to 10% of that of CO2 for sulfuric acid solution and 13% that for perchloric acid, respectively. These

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Figure 5. Simultaneously recorded CVs (a, d) and MSCVs of m/z 44 (b, e) and m/z 60 (c, f) on Pt(111), Pt(332), Pt(111)/Ru, and Pt(332)/Ru electrodes in 0.1 M methanol + 0.5 M H2SO4 solution at ambient temperature (25 °C). Scan rate 10 mV s-1; electrolyte flow rate 5 µL s-1. Arrows indicate the direction of potential sweep.

results suggest that at low potentials the current efficiency of CO2 is lower than the average current efficiency, and a larger relative amount of methyl formate to CO2 is formed, i.e., the lower current efficiency of CO2 at low potentials is paralleled by a higher relative rate of methyl formate formation. Methyl formate is presumably formed directly in a surface reaction, rather than in a subsequent reaction between formed formic acid and methanol in bulk solution.42 In general, the oxidation rate of methanol in perchloric acid solution is much higher than that in sulfuric acid solution. However, the current efficiency of CO2 increases only slightly when changing solution from sulfuric acid to perchloric acid. This increase can be understood on the basis of anion adsorption. In perchloric acid solution, more free platinum sites are present due to less strong adsorption of the perchloric anion, thus the formed intermediates (acetaldehyde and formic acid) have more chance to be further oxidized to CO2 via the COad or non-CO pathway. Analyzing by HPLC the product yield for 1000 s electrolysis of methanol at 0.6 V on polycrystalline platinum, Iwasita et al.45 also found that the product distribution only slightly depends on the supporting electrolyte, though the Faradaic current of methanol oxidation is much higher in perchloric acid than that in sulfuric acid. 3.3. The Effect of Temperature on Methanol Oxidation on Smooth Polycrystalline Platinum. Under atmospheric pressure we can measure methanol oxidation at elevated temperature up to 50 °C without appreciable variation of methanol concentration due to the low boiling point of methanol. Figure 4 shows CVs and the corresponding MSCVs of m/z 44 (CO2) and m/z 60 (methyl formate) for methanol oxidation on smooth polycrystalline platinum in 0.1 M methanol + 0.5 M H2SO4 solution at 50 °C. Compared to methanol oxidation at 25 °C, the activity of smooth polycrystalline platinum for methanol oxidation at 50 °C is markedly enhanced, as indicated by an ca. 20 mV negative shift of onset potential and the significant increase of Faradaic current and amount of CO2 and methyl formate. The maximum Faradaic currents in the positive-

and negative-going scans at 50 °C are 3.5 and 4.3 times as high as that at 25 °C, respectively. However, the average current efficiency of CO2 in one cycle of potential scan at 50 °C is ca. (28 ( 3)%, the same as that at 25 °C. The average ratio of CO2 to methyl formate is ca. 50, compared to 40 at 25 °C. This clearly suggests that at elevated temperature the formation of CO2 and intermediates (formaldehyde and formic acid) is simultaneously enhanced, and moreover, the activation energy for CO2 formation is very close to the apparent activation energy for overall methanol oxidation. Otherwise, the current efficiency of CO2 would change with the temperature increase. It should be noted that the ratio of the intermediates (formaldehyde and formic acid) could be changed a little, as indicated by the variation of current efficiency for methyl formate. On the basis of oxidative charge at 25 and 50 °C in one cycle of potential scan, the average apparent activation energy for overall methanol oxidation is estimated to be ca. 40 kJ mol-2, which is close to that obtained by Ota et al.46 Ota et al. reported that the activation energies are 55, 38, and 34 kJmol-1 for CO2, HCOOH, and HCHO formation at 0.6 V over platinized platinum (roughness factor 700), respectively. 3.4. The Effect of Pt Surface Structure on Methanol Oxidation. To disclose the effect of steps of monocrystalline Pt on the product distribution during methanol oxidation, Pt(111) and Pt(332) single crystals were studied by DEMS. Figure 5 (dotted lines) shows CVs and the corresponding MSCVs of m/z 44 (CO2) and m/z 60 (methyl formate) for methanol oxidation on Pt(111) and Pt(332) in 0.1 M methanol + 0.5 M H2SO4 solution at room temperature (25 ( 1 °C). The hydrogen adsorption/desorption process on Pt(111) is less suppressed in the methanol-containing solution, thus Pt(111) is less covered by strongly adsorbed species (COad) formed from the decomposition of methanol, compared to smooth polycrystalline platinum and Pt(332). In the positive-going scan the oxidation of methanol on Pt(111) and Pt(332) also starts at ca. 0.5 V, as

DEMS Study on Methanol Oxidation

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Figure 6. Simultaneously recorded transients of Faradaic currents (a, d) and ion currents of m/z 44 (b, e) and m/z 60 (c, f) on Pt(111), Pt(332), Pt(111)/Ru, and Pt(332)/Ru electrodes in 0.1 M methanol + 0.5 M H2SO4 solution at ambient temperature (25 °C) after the step of potential from 0.05 to 0.65 V. Electrolyte flow rate 5 µL s-1.

indicated by the onset of CO2 (Figure 5b,e) and methyl formate formation (Figure 5c,f). The oxidative peaks are observed at 0.79 V for Pt(111) and 0.76 V for Pt(332), and the peak currents are 0.027 mA for Pt(111) and 0.16 mA for Pt(332), respectively. This clearly indicates the step sites are much more active for methanol oxidation, as also reported by other groups.31,38 After calibrating the DEMS setup, the average current efficiency of CO2 and the ratio of the average amount of methyl formate to CO2 on Pt(111) and Pt(332) in one cycle of potential scan are also calculated. On Pt(111) the average current efficiency of CO2 is only (20 ( 2)%, and the ratio of CO2 to methyl formate is ca. 30, while the average current efficiency of CO2 on Pt(332) is only (27 ( 3)%, and the ratio of CO2 to methyl formate is ca. 37. Therefore, the presence of steps on the Pt(111) vicinal surface can only slightly enhance the complete oxidation of methanol to CO2. To study the oxidation of methanol at constant potential, the typical potential step experiments of Pt(111) and Pt(332) in 0.1 M methanol + 0.5 M H2SO4 solution have been performed, which are shown in Figure 6 (dotted line). The potential was stepped from 0.05 to 0.65 V, and the Faradaic current and the corresponding mass spectrometric currents of m/z 44 (CO2) and m/z 60 (methyl formate) were simultaneously recorded. Similarly to the results from cyclic voltammetry, Pt(332) exhibits a higher oxidative current than Pt(111). At the first minute after potential step the Faradaic current and mass spectrometric current of m/z 44 are used to calculate the current efficiency of CO2. At 0.65 V the current efficiencies of CO2 are (21 ( 2)% for Pt(111) and (22 ( 2)% for Pt(332), respectively. The ratio of CO2 to methyl formate is ca. 25 for Pt(111) and Pt(332). In general, the Faradaic current and the ion current of CO2 for methanol oxidation increase with an increase of the step density, but the current efficiency of CO2 seems to be only slightly dependent on the surface structure. This demonstrates the increase of step density promotes the oxidation of methanol not only via COad, but also via soluble intermediates, i.e., formal-

dehyde and formic acid,8,42 therefore, on different pure platinum electrodes, the current efficiency for CO2 formation is approximately the same, though the Faradaic current increases with increasing step density. 3.5. The Effect of Ru Submonolayer on Methanol Oxidation. In the previous paper,8 we have studied methanol oxidation on Ru-modified polycrystalline platinum in 0.1 M methanol + 0.5 M H2SO4 solution. Here we will show the effect of Ru deposits on methanol oxidation on single-crystal Pt(111) and Pt(332). CVs and the corresponding MSCVs of m/z 44 (CO2) and m/z 60 (methyl formate) on Ru-modified Pt(111) and Pt(332) in 0.1 M methanol + 0.5 M H2SO4 solution are shown in Figure 5 (solid line). In the positive-going scan the formation of CO2 on Ru-modified Pt(111) starts at ca. 0.4 V (Figure 5b), i.e., over 100 mV more negative than that on Pt(111). Two oxidation peaks are observed in CV of Pt(111)/Ru (Figure 5a). The small peak around 0.66 V is paralleled by the formation of CO2, as indicated by the ion peak of m/z 44 (Figure 5b). The main peak at 0.8 V, which is followed by the ion peak of m/z 60 (Figure 5c), should mainly correspond to the formation of intermediates (formaldehyde and formic acid). The maximum Faradaic current and corresponding potential are nearly the same for Pt(111)/ Ru and Pt(111); however, at the low potentials negative of 0.65 V the Faradaic current and the formation of CO2 increase markedly on Pt(111)/Ru. The amount of CO2 formed on Pt(111)/Ru in one sweep cycle and at 0.66 V is ca. 3.5 and 6 times as much as that on Pt(111), respectively; however, the amount of formed methyl formate is nearly the same for both electrodes. In one cycle of potential sweep the average current efficiency of CO2 reaches 61% on Pt(111)/Ru, and the ratio of formation amount of CO2 to methyl formate is up to 80. At the maximum Faradaic current peak, the current efficiency of CO2 is only 35%. However, at the maximum ion peak of m/z 44, a current efficiency for CO2 up to 150% is calculated, because a part of the current is due to the oxidation of adsorbed methanol (COad) formed at lower potentials.

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Figure 7. Simultaneously recorded transients of Faradaic currents (a, c) and ion current of m/z 44 (b, d) on smooth polycrystalline platinum (pc-Pt) (a, b) and pc-Pt/Ru (c, d) in 0.1 M methanol + 0.5 M H2SO4 solution at ambient temperature (25 °C) after the step of potential from 0.05 V to different higher potentials: (9) 0.5, (b) 0.6, (1) 0.65, (2) 0.7, and ([) 0.8 V. Electrolyte flow rate 5 µL s-1.

In the positive-going scan the formation of CO2 on Rumodified Pt(332) also starts at ca. 0.4 V (Figure 5e), i.e., over 100 mV more negative than that on Pt(332). With the increasing potentials the Faradaic current and ion intensity for CO2 increase in the low potential region (e0.65 V). At potentials above 0.65 V the ion intensity of m/z 44 for CO2 formation does not further increase, while the Faradaic current further increases until 0.77 V, and then decreases. One oxidative peak and one shoulder on the negative side are observed in CV of Pt(332)/Ru (Figure 5d). The shoulder around 0.65 V is paralleled by the formation of CO2, as indicated by the ion peak of m/z 44 (Figure 5e). The main peak at 0.77 V, which is followed by the ion peak of m/z 60 (Figure 5f), should mainly correspond to the formation of intermediates (formaldehyde and formic acid). Compared to Pt(332), the maximum Faradaic current on Pt(332)/Ru is significantly reduced; however, the Faradaic current and the rate of CO2 formation increase significantly on Pt(332)/Ru at low potentials negative of 0.65 V. The formation of methyl formate starts at 0.5 V, i.e., the same as that on Pt(332), but its amount is less than that on Pt(332). In a whole potential cycle the average current efficiency of CO2 reaches 46%, and the average ratio of the amount of CO2 to methyl formate is up to 71. At the maximum Faradaic current peak, the current efficiency of CO2 is only 37%, whereas the current efficiency of CO2 at the maximum ion peak of m/z 44 is up to 96%, part of which should be due to the oxidation of methanol adsorbate formed in the low potential region. To eliminate the effect of adsorbed CO formed from methanol dissociation on the bulk oxidation of methanol, typical potential step experiments on Pt(111)/Ru and Pt(332)/Ru have also been carried out, which are shown in Figure 6 (solid line). At the first minute after the potential step the Faradaic current and mass spectrometric current of m/z 44 are used to calculate the current efficiency of CO2. At 0.65 V the Faradaic current is a little smaller on Pt(111)/Ru than that on Pt(111); however, the formation of CO2 increases markedly and the current efficiency

of CO2 reaches about 100%, compared to (21 ( 2)% on Pt(111). In addition, the ratio of CO2 to methyl formate (ratio at the mole numbers) is ca. 140, compared to 30 on Pt(111). Compared to Pt(332), the Faradaic current is remarkably reduced on Pt(332)/Ru at 0.65 V due to the blockage of step sites by Ru adatoms; however, the formation of CO2 increases a little and the current efficiency of CO2 reaches about 61%, compared to (22 ( 2)% on Pt(332). Additionally, the ratio of CO2 to methyl formate is ca. 90, compared to ca. 37 on Pt(332). 3.6. Dependence of Current Efficiency of CO2 on Potential. From cyclic voltammetry only the average current efficiency of CO2 in a potential cycle can be determined. To investigate the influence of potential on the current efficiency of CO2, a series of potential step experiments was carried out on smooth polycrystalline platinum and Ru-modified smooth polycrystalline platinum. The potential was stepped from 0.05 V to different more positive potentials, as shown in Figure 7. On smooth polycrystalline platinum the Faradaic current of methanol oxidation and the formation of CO2 increase with increasing potential in the low potential region (e0.8 V). On Ru-modified smooth polycrystalline platinum the Faradaic current also increases with the increase of potential, and the formation rate of CO2 increases as the potential increases from 0.5 to 0.6 V; however, at 0.7 V the rate of CO2 formation is nearly the same as that at 0.6 V. In addition, at 0.7 V the Faradaic current on polycrystalline platinum/Ru is lower than that on polycrystalline platinum due to the blockage of some platinum sites by Ru adatoms. At the second minute after potential step the Faradaic current and mass spectrometric current of m/z 44 are used to calculate the current efficiencies of CO2, which are shown in Figure 8. The current efficiencies of CO2 are plotted vs potentials in Figure 8. For comparison, the current efficiencies of CO2 on Pt(111), Pt(111)/Ru, Pt(332), and Pt(332)/Ru are also shown. The current efficiency of CO2 for methanol oxidation on smooth polycrystalline platinum increases from ca. (18 ( 2)% to (32

DEMS Study on Methanol Oxidation

J. Phys. Chem. C, Vol. 111, No. 19, 2007 7045 SCHEME 1: Simple Scheme of the Pathways for Methanol Oxidation on Platinum Electrodesa

a x ) 1-3. “sur” indicates the electrode surface, “bulk” refers to solution bulk, “diff” marks the diffusion of species, and “ad” denotes adsorbate.

Figure 8. The dependence of current efficiency for CO2 formation during methanol oxidation on the potentials in 0.1 M methanol + 0.5 M H2SO4 solution at smooth polycrystalline platinum (9) and Ru modified smooth polycrystalline platinum (b). For comparison, the current efficiency for CO2 formation on Pt(111) (2), Pt(111)/Ru (1), Pt(332) (X), and Pt(332)/Ru (x) at 0.65 V is also shown.

( 3)% as potential increases from 0.6 to 0.8 V, while on Rumodified polycrystalline platinum the current efficiency of CO2 is as high as 52% at the low potential below 0.65 V, but decreases steeply to (24 ( 2)% at 0.7 V, i.e., nearly the same as (26 ( 2)% on polycrystalline platinum. This demonstrates that Ru loses its cocatalytic properties at the high potentials (g0.7 V), perhaps due to the transformation of an active Ru hydrous oxide to an inactive anhydrous oxide at high potentials.62 4. Discussion As we have previously shown,8,42,49 methanol electrooxidation on platinum proceeds via a parallel pathway mechanism, which is simply described in Scheme 1. In one pathway methanol is oxidized via COad to form CO2, meanwhile in other pathways methanol oxidation proceeds via soluble intermediates, i.e., formaldehyde and formic acid. The extent to which these soluble intermediates are further oxidized to CO2 depends on the diffusion conditions. At the high flow rate of solution and high concentration of methanol, and on the smooth electrode surface, most soluble intermediates are transported away from the electrode without further oxidation to CO2. On smooth polycrystalline platinum and monocrystalline Pt(111) and Pt(332) in 0.1 M methanol solution the oxidation of methanol proceeds mainly to form the soluble intermediates, i.e., formaldehyde and formic acid, since the low current efficiency of CO2 such as 20-30% was observed. It is wellknown that on a pure platinum electrode the oxidative removal of adsorbed CO formed from methanol decomposition is the rate-determining step for the first consecutive reaction at the low potentials. Since the adsorption of oxygen-like species on pure platinum does not occur to any appreciable extent below ∼0.7 V, the oxidative removal of adsorbed CO takes place very slowly in the low potential range, resulting in methanol oxidation mainly via soluble intermediates. As mentioned before,8,42 on a very rough Pt surface, e.g., platinized platinum, porous platinum, and high-loading platinum nanoparticle catalyst, soluble intermediates have a greater chance to be further oxidized to CO2 before they are transferred away from the electrode surface, therefore, a higher current efficiency for CO2 is obtained.

Adsorbed anions can affect the reactivity of practically all important processes involved in electrocatalysis, electrosynthesis, and deposition, and also are effective in corrosion inhibition or promotion. Perchloric acid is considered a nonadsorbing electrolyte. In contrast sulfate/bisulfate anions in sulfuric acid solution can be adsorbed strongly on the platinum surface, thus inhibiting methanol adsorption, leading to the reduction of the oxidation rate of methanol. In the perchloric acid solution the Faradaic current of methanol oxidation and the rate of CO2 and methyl formate formation are over 2 times as high as that in the sulfuric acid solution; however, the current efficiency of CO2 remains nearly the same, i.e., ca. 30% for the average current efficiency in one sweep cycle and ca. 20% for current efficiency at 0.6 V. This result indicates that anion adsorption simultaneously hinders two parallel reaction pathways, and does not remarkably alter the mechanism of methanol oxidation on the platinum electrode. This finding is in agreement with that obtained by Iwasita et al. on the basis of analysis by HPLC.45 Iwasita et al.45 found that on polycrystalline platinum the yield of CO2, HCOOH, and HCHO varies slightly for sulfuric acid and perchloric acid solution. At elevated temperature, the reaction rate of methanol oxidation is enhanced, and the amount of formed CO2 and methyl formate also increases. However, the current efficiency of CO2 does not significantly change with the elevated temperature. It can be deduced that the elevated temperature simultaneously enhances the reaction rate of two parallel pathways with the same or close activation energy, and the mechanism of methanol oxidation on the platinum electrode does not change with temperature. Recently, two groups24,25 have studied methanol electrooxidation on the platinum electrode in a wide temperature range from ambient temperature to higher than 100 °C at a pressurized condition; however, they obtained a different apparent activation energy of methanol oxidation, i.e., about 20 kJ mol-1 vs 70 kJ mol-1. To explain the difference of apparent activation energy between the two groups, we should consider the effect of some factors such as the roughness or catalyst loading of the Pt electrode, methanol concentration, supporting electrolyte, and pH value. The prior experiment on the anion effect suggests that the supporting electrolyte does not seem to have a significant influence on the mechanism of methanol oxidation and the product distribution. It was also found early that methanol oxidation on platinized Pt in 0.05 M H2SO4 at 25 °C reveals similar current efficiency for CO2 production as that in 0.5 M H2SO4, and the Tafel slope for methanol oxidation on smooth platinum does not change with the pH value.63 Therefore, we should focus on the other two factors. Watanabe’s group used a thin-film Pt electrode with a low roughness factor of 1.4 prepared by Ar-sputtering and a high methanol concentration (1 M). In contrast, Stuve’s group adopted the carbon-supported Pt nanoparticle electrode with a high Pt loading of 50-100 µg cm-2, i.e., a roughness factor of 40-80 and a low methanol concentration (0.015 M). As previously mentioned,8,42 methanol electrooxidation mainly

7046 J. Phys. Chem. C, Vol. 111, No. 19, 2007

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TABLE 1: The Coverage of Methanol Adsorbate Formed at 0.35 V on Different Electrodesa electrodes coverage

Pt(111) 0.09

Pt(111)-Ru 0.30

Pt(332) 0.15

Pt(332)-Ru 0.30

a The coverage of methanol adsorbate means the number of methanol adsorbate molecules per surface platinum atom.

produces intermediates (formaldehyde and formic acid) at the high concentration of methanol and on smooth polycrystalline platinum; inversely at a low concentration of methanol and on highly rough Pt/Vulcan electrode methanol electooxidation mainly generates CO2, since the intermediates have a greater chance to be oxidized to CO2 before they are transported away from the electrode surface. In the case of Watanabe’s experiment, the low apparent activation energy should mainly correspond to methanol oxidation to intermediates. In the case of Stuve’s experiment, the high apparent activation energy seems to mainly be associated with methanol oxidation to CO2 via COad, and further oxidation of soluble intermediates to CO2. The Faradaic current of methanol oxidation and the amount of CO2 formation increases with an increase in the step density of the platinum electrode surface, i.e., Pt(111) < Pt(332) < polycrystalline platinum, which is consistent with the results of other groups;31,38 however, the current efficiency of CO2 is not appreciably dependent on the surface structure. The methanol decomposition on platinum has been proven to be sensitive to the surface structure. Gibson et al.64 found that under ultrahighvacuum condition, as a small amount of defects exist on the surface of Pt(111), the thermal decomposition of methanol can form CO at 200 K, whereas on a defect-free surface, no carbon monoxide could be detected. On the basis of density functional calculations, Desai et al.65 reported that defect-free Pt(111) is unreactive toward methanol decomposition to form CO. Under electrochemical conditions, methanol oxidation also seems to occur at the defect sites of Pt(111), which usually possesses terraces 200-500 atoms wide, and on well-prepared Pt(111) surface methanol oxidation will be significantly inhibited. The finding that the current efficiency of CO2 is independent of step density implies that the increase of step density only leads to the increase of active sites, but does not influence the product distribution, similarly to the increase in the apparent area of the electrode surface. However, Iwasita et al.45 found that on the Pt(111) electrode the yield of CO2 is much lower than that on polycrystalline platinum, i.e., 6% vs 24%. This inconsistency should be further examined. Note that they did not directly detect the amount of CO2, just calculated it from the charge and mass balance on the total Faradaic charge and the amount of HCOOH and HCHO; however, in our case the amount of CO2 is directly detected by DEMS. The enhancement in catalytic activity of platinum modified by Ru toward methanol oxidation in acidic medium is mainly attributed to the oxidation of adsorbed CO, the main poisoning species, at less positive potentials. The promotion of CO oxidation has been ascribed to the bifunctional mechanism,4,21 and/or the modification of the electronic properties of Pt.23,66-68 Figures 5 and 6 show that Ru adatoms not only promote the methanol oxidation on Pt(111) and Pt(332) at low potentials, but also increase the current efficiency of formed CO2. As previously mentioned, the effect of Ru adatoms is twofold: they lower the oxidation potential of COad on Pt (due to an electronic and the bifunctional effect), and they promote methanol decomposition into COad on Pt at the low potentials;8,14,49 therefore, Ru adatoms induce a shift of methanol oxidation from the reaction path via soluble intermediates to that via adsorbed CO, leading to higher current efficiency of CO2. The following

experiments of mathanol adsorption and subsequent oxidative stripping of strongly adsorbed species (COad) on Pt(111)/Ru and Pt(332)/Ru also support this assumption. Methanol adsorption proceeds in the DEMS cell by holding the potential at 0.35 V in 0.5 M H2SO4 solution and then injecting 0.1 M CH3OH + 0.5 M H2SO4 solution at a rate of 5 µL s-1 for 5 min. Afterward 0.1 M CH3OH + 0.5 M H2SO4 solution was replaced by supporting electrolyte to remove methanol from the bulk solution. Finally, methanol adsorbate was oxidized in the positive-going scan, and the amount of formed CO2 was detected by monitoring m/z 44. The calculated coverage of methanol adsorbate is listed in Table 1. Table 1 shows that small coverage of Ru can promote the methanol decomposition adsorption to form strongly adsorbed species (COad, as indentified by FTIR) on Pt(111) and Pt(332) at 0.35 V, as found on Ru-modified polycrystalline platinum.49 In addition, the coverage of methanol adsorbate on Pt(332) is higher than that on Pt(111), indicating that the step can promote methanol decomposition adsorption. Similarly to pure platinum electrodes, on Ru-modified platinum electrodes the Faradaic current of methanol oxidation also increases in the following sequence: Pt(111)/Ru < Pt(332)/ Ru < polycrystalline platinum/Ru. This is in agreement with the finding obtained by Kolb et al.43 However, the current efficiency of CO2 decreases in the sequence Pt(111)/Ru > Pt(332)/Ru > polycrystalline platinum/Ru (Figure 8). In the low potential region (e0.65 V), the current efficiency of CO2 even approaches 100% on Pt(111)/Ru. It can be deduced that methanol oxidation via COad to form CO2 mainly takes place near Ru islands, while methanol oxidation via soluble intermediates occurs at the steps or defects. On the Pt(111)/Ru electrode very few steps or defects lead to a very small amount of intermediates, while methanol oxidation via COad to form CO2 is markedly promoted by Ru adatoms, therefore, approximately 100% current efficiency of CO2 is obtained. With the increasing step and defect sites the contribution of the formation of soluble intermediates to the overall methanol oxidation increases, and although Faradaic current increases, the current efficiency of CO2 decreases. As for Pt(332)/Ru, the oxidative current and the amount of formed methyl formate at 0.65 V are significantly reduced (Figures 5 and 6), due to the blockage of step sites by Ru adatoms, though the amount of formed CO2 increases. This further proves the above assumption. On pure platinum electrodes the oxidation rate of methanol increases with the increasing potentials in the potential region of e0.8 V, and the current efficiency of CO2 also increases from ca. 20% to 32% with the increase of potential from 0.6 to 0.8 V. This suggests that at higher potentials the formation of CO2 is more enhanced than intermediate formation through either promotion of the pathway via COad or acceleration of the further oxidation of intermediates, or both. Although on Pt/ Ru electrodes the Faradaic current for methanol oxidation also increases with the rising potentials in the potential region of e0.8 V, at potentials >0.65 V the current efficiency of CO2 during methanol oxidation decreases and is the same as that on Pt at 0.7 V. This implies that Ru loses its cocatalytic activity for methanol oxidation at high potentials, perhaps due to the transformation of an active Ru hydrous oxide to an inactive anhydrous oxide at high potentials. In the high potential region, Ru adatoms seem to act mainly by blocking the electrode surface. 5. Conclusion Methanol oxidation on Ru-unmodified and -modified smooth polycrystalline platinum, Pt(111), and Pt(332) electrodes pro-

DEMS Study on Methanol Oxidation ceeds via the parallel pathway mechanism. Under the studied condition (0.1 M for methanol concentration and 5 µL s-1 for flow rate of solution), methanol oxidation on smooth polycrystalline platinum, Pt(111), and Pt(332) mainly forms the soluble intermediates, i.e., formaldehyde and formic acid, as indicated by the low current efficiency of CO2 such as 20-30%. In perchloric acid solution, the rate of methanol oxidation is higher than that in sulfuric acid solution due to the stronger adsorption of sulfate anion; however, the mechanism of methanol oxidation does not change and the current efficiency of CO2 does not significantly change. Similarly, at elevated temperature methanol oxidation is markedly enhanced; however, the reaction mechanism does not change, and the current efficiency of CO2 remains nearly unchanged. The increase in step density of the Pt electrode surface can enhance methanol oxidation; however, the current efficiencies of CO2 do not have an appreciable difference for smooth polycrystalline platinum, Pt(111), and Pt(332) electrodes. The step or defect sites seem to simultaneously enhance the two parallel pathways of methanol oxidation. On a pure smooth platinum electrode the current efficiency of CO2 for methanol oxidation increases from ca. 20% to 32% with the increase of potential from 0.6 to 0.8 V, accompanied by the increasing Faradaic current. From this we may conclude that both reaction paths involve a common first reaction step that is rate limiting, and which depends on the surface structure and the anion. The ratio of the reaction rates of reaction steps 1 and 3 determines the experimental current efficiency and depends on the potential. Ru adatoms can promote methanol oxidation via COad to form CO2 in the low potential region (e0.65 V), therefore the higher current efficiency of CO2 is observed on Ru-modified platinum electrodes than on pure platinum electrodes. On the Pt(111)/ Ru electrode the current efficiency of CO2 in the low potential region (e0.65V) even approaches 100%. Methanol oxidation via COad to form CO2 mainly takes place near Ru islands, while methanol oxidation via soluble intermediates occurs at the steps or defects. At potentials of >0.65 V, Ru adatoms lose their cocatalytic activity toward methanol oxidation possibly due to the formation of inactive anhydrous Ru oxide, as indicated by the same current efficiency of CO2 as on pure Pt. Acknowledgment. Thanks are due to the DFG for financial support. H.W. is grateful to the Hanns-Seidel-Stiftung for a stipend. References and Notes (1) Gottesfeld, S. Electrochim. Acta 1995, 40, 283. (2) Lamy, C.; Belgsir, E. M.; Srinivasan, S. Direct Methanol Fuel Cells-From a 20th Century Electrochemist’s Dream to a 21th Century Emerging Technology. In Modem Aspects of Electrochemistry; Bockris, J. O. M., Ed.; Plenum: New York, 2001; p 68. (3) Lipkowski, J.; Ross, P. N. Electrocatalysis: Frontiers in Electrochemistry; Wiley: New York, 1998. (4) Watanabe, M.; Motoo, S. J. Electroanal. Chem. 1975, 60, 267. (5) Gasteiger, H. A.; Markovic, N.; Ross, P. N.; Cairns, E. J. J. Phys. Chem. 1993, 97, 12020. (6) Gasteiger, H. A.; Markovic, N.; Ross, P. N.; Cairns, E. J. J. Electrochem. Soc. 1994, 141, 1795. (7) Iwasita, T.; Hoster, H.; John-Anacker, A.; Lin, W. F.; Vielstich, W. Langmuir 2000, 16, 522. (8) Wang, H.; Wingender, C.; Baltruschat, H.; Lopez, M.; Reetz, M. T. J. Electroanal. Chem. 2001, 509, 163. (9) Krausa, M.; Vielstich, W. J. Electroanal. Chem. 1994, 379, 307. (10) Tremiliosi-Filho, G.; Kim, H.; Chrzanowski, W.; Wieckowski, A.; Grzybowska, B.; Kulesza, P. J. Electroanal. Chem. 1999, 467, 143. (11) Chrzanowski, W.; Wieckowski, A. Langmuir 1998, 14, 1967. (12) Morimoto, Y.; Yeager, E. B. J. Electroanal. Chem. 1998, 444, 95.

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