Methanol Oxidation on PtRu Electrodes. Influence of Surface Structure

Bing-Joe Hwang , Loka Subramanyam Sarma , Guo-Rung Wang , Ching-Hsiang Chen , Din-Goa Liu , Hwo-Shuenn Sheu , Jyh-Fu Lee. Chemistry - A European ...
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Langmuir 2000, 16, 522-529

Methanol Oxidation on PtRu Electrodes. Influence of Surface Structure and Pt-Ru Atom Distribution T. Iwasita,*,† H. Hoster, A. John-Anacker, W. F. Lin, and W. Vielstich† Institut fu¨ r Physik, Universita¨ t der Bundeswehr Mu¨ nchen, Werner-Heisenberg Weg 39, 85577 Neubiberg, Germany Received May 17, 1999. In Final Form: August 10, 1999 The activities of different types of PtRu catalysts for methanol oxidation are compared. Materials used were: UHV-cleaned PtRu alloys, UHV-evaporated Ru onto Pt(111) as well as adsorbed Ru on Pt(111) prepared with and without additional reduction by hydrogen. Differences in the catalytic activity are observed to depend on the preparation procedure of the catalysts. The dependence of the respective catalytic activities upon the surface composition is reported. UHV-STM data for Pt(111)/Ru show the formation of two- and three-dimensional structures depending on surface coverage. A molecular insight on the electrochemical reaction is given via in situ infrared spectroscopy. Analysis of the data indicates that the most probable rate-determining step is the reaction of adsorbed CO with Ru oxide.

Introduction 1.1. Electrooxidation of Methanol via Binary PtRu Catalysts. It is well known that a binary PtRu catalyst can produce a noticeable enhancement of current for methanol electrooxidation at low potentials.1-4 Although the importance of this finding was not ignored in the field of fuel cell research,5 it took more than three decades until the possibility of technically using the PtRu/CH3OH systems was recognized.6,7 Presently, binary PtRu catalysts for methanol oxidation are studied in diverse forms, PtRu alloys,8-11 Ru electrodeposits on Pt,12,13 PtRu codeposits,14-16 and Ru adsorbed on Pt.17 It is noteworthy that despite the diversity of methods for catalyst prepara* Corresponding author. E-mail: [email protected]. † Present address: Instituto de Quimica de Sa ˜ o Carlos, Universidade de Sa˜o Paulo, C.P. 780, CEP 13560-970 Sa˜o Carlos (SP), Brazil. (1) Entina, V. S.; Petry, O. A. Elektrokimiya 1968, 4, 678-681. (2) Binder, H.; Ko¨hling, A.; Sandstede, G. In From Electrocatalysis to Fuel Cells; Sandstede, G., Ed.; University of Washington Press: Seattle, WA, 1972; p 43. (3) Watanabe, M.; Motoo, S. J. Electroanal. Chem. 1975, 60, 267273 (4) Hamnett, A. In Interfacial Electrochemistry: Accomplishments and Challenges; Wieckowski, A., Ed.; Marcel Dekker: New York, 1999. (5) Niedrach, L. W.; McKee, D. W.; Paynter, J.; Danzig, I. F. Electrochem. Technol. 1967, 5, 318-327. (6) Surampudi, S.; Narayanan, S. R., Vamos, E.; Frank, H.; Halpert, G.; LaConti, A.; Kosek, J.; Surja Prakash, G. K.; Olah, G. A. J. Power Sources 1994, 47, 377-385. (7) Garche, J.; Vielstich, W.; Waidhas, M. Proc. Erste Ulmer Tage; Universita¨tsverlag Ulm: Ulm, Germany, 1994, pp 178-190. (8) Iwasita, T.; Nart , F. C.; Vielstich, W. Ber. Bunsen-Ges. Phys. Chem. 1990, 94, 1030-1034. (9) Gasteiger, H. A.; Markovic, N.; Ross, P. N.; Cairns, E. J. J. Phys. Chem. 1993, 97, 12020-12029. (10) Gasteiger, H. A.; Markovic, N.; Ross, P. N.; Cairns, E. J. J. Electrochem. Soc. 1994, 141, 1795-1803. (11) Markovic N.; Gasteiger, H. A.; Ross, P. N.; Villegas, I.; Weaver, M. J. Electrochim. Acta 1995, 40, 91-98. (12) Friedrich, K. A.; Geyzers, K. P.; Linke, U.; Stimming, U.; Stumper, J. J. Electroanal. Chem. 1996, 402, 123-128. (13) Chrzanowski, W.; Wieckowski, A. Langmuir 1998, 14, 19671970. (14) Krausa, M.; Vielstich, W. J. Electroanal. Chem. 1994, 379, 307314. (15) Hogarth, M. P.; Munk, J.; Shukla, A. K.; Hamnett, A. J. Appl. Electrochem. 1994, 24, 85. (16) Iudice de Souza, J. P.; Iwasita, T.; Nart, F. C.; Vielstich, W. J. Appl. Electrochem., in press. (17) Chrzanowski, W.; Kim, H.; Wieckowski, A. Catal. Lett. 1998, 50, 69-75.

tion, all these materials present an enhanced activity toward methanol oxidation. 1.2. Characterization of PtRu Surfaces. The determination of the surface composition of PtRu electrodes has been a matter of several investigations. For this purpose, the use of cyclic voltammetry has been handicapped by the lack of definition of Ru features associated with some surface processes having a defined stoichiometry. Using spontaneously adsorbed Ru on polycrystalline Pt, Watanabe and Motoo3 suggested the formation of RuOH in the potential range between 0.34 and 0.9 V, during a cyclic voltammogram in H2SO4. They calculated the ratio between the charge for RuOH formation and the saturation charge for H desorption as a measure for the degree of coverage with Ru atoms. On the basis of microbalance data, Frelink et al.18 suggested RuO to be formed in the range from -0.3 to 0.6 V vs Hg,Hg2SO4 and calculated the percentages of electrodeposited Ru on the basis of a 2-electron process for RuO formation. Besides the differences in the suggested oxide stoichiometry, one must note some facts concerning the nature of the data used in both these procedures. Thus, for example, within the potential limits in the method of Watanabe and Motoo, the form of the voltammogram suggests at least two different surface processes (see the voltammograms of Figure 3 in ref 18). On the other hand, in the microbalance experiments, adsorbed sulfate from the supporting electrolyte may contribute to the observed changes in weight. For clean (unmodified) single-crystal Pt surfaces as model catalysts, the electrooxidation of methanol in acid solutions was characterized via in situ IR spectroscopy.19-21 Since methanol electrooxidation is a surface sensitive process, measuring infrared spectroscopy using singlecrystal surfaces covered with submonolayers of ruthenium is an obvious approach toward the possibility of modeling the electrocatalytic processes. It has been pointed out that electrodeposition of noble metals on noble metal substrates usually has a three(18) Frelink, T.; Visscher, W.; van Veen, J. A. R. Surf. Sci. 1995, 335, 353-360. (19) Chang S.-C.; Leung, L.-W. H.; Weaver, M. J. J. Phys. Chem. 1990, 94, 6013-6421. (20) Xia, X. H.; Iwasita , T.; Ge, F.; Vielstich, W. Electrochim. Acta 1996, 41, 711-718. (21) Clavilier, J.; Lamy, C.; Leger, J. M. J. Electroanal. Chem. 1981, 125, 249.

10.1021/la990594n CCC: $19.00 © 2000 American Chemical Society Published on Web 11/16/1999

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dimensional character, where the long-range order, typical of underpotential deposition (upd) metal lattices, is absent.13 Nevertheless, it has been observed that low index Pt single-crystal surfaces partially covered with Ru exhibit very different behavior toward methanol oxidation. The Ru-modified Pt(111) surface is the one presenting the highest catalytic activity.13 1.3. Ru Island Formation by Spontaneous Adsorption. Chrzanowski et al.17 used Auger spectroscopy to analyze Pt(111) surfaces covered with so-called spontaneously adsorbed Ru from solutions containing 0.1 M HClO4 with 5 × 10-5 M or 5 × 10-4 M RuCl3. Surface compositions, expressed as degree of coverage, were 0.1 and ca. 0.18 respectively.17 The spontaneous adsorption of Ru atoms was interpreted in terms of a reaction involving the aqua-complex RuO[(H2O)4]2+, which is the species present in the aged RuCl3 solution

RuO[(H2O)4] 2+ f RuO2ads + 3H2O + 2H+

(1)

Reduction of the adsorbed oxide follows adsorption. It was suggested that the surface chemistry of Ru oxides, obtained either by spontaneous adsorption or electrochemical deposition, should be responsible for the specific catalytic activity.17 On account of results presented later on in this paper, we suggest that spontaneous adsorption of Ru out of freshly prepared RuCl3 solutions, as used here, is an indirect process where the primarily adsorbed ion is Cl-, Ru3+ being coadsorbed as a counterion. The reaction can be formally written as

3Pt + RuCl3 f 3Pt(Cl-)ads + (Ru3+)coads

(2)

Due to the strong Cl- adsorption, the adlayer remains on the surface even after rinsing the electrode with pure water. Reduction of Ru3+ to Ru(0) and desorption of Cltakes place in the electrochemical cell as low potentials (e.g., 0.05 V) are applied. In favor of this interpretation is the fact that in order to reach high Ru coverages, solutions containing only RuCl3 dissolved in H2O (without HClO4) had to be used. The reason for this is that in the absence of other positive ions in solution, only the Ru3+ species can be adsorbed as a counterion. In the present work we study the electrooxidation of methanol on Pt(111)/Ru electrodes with different Ru coverage and different methods of preparation, namely, (i) Open circuit adsorption of Ru ions on Pt(111) from RuCl3 solutions, (ii) ibid. with simultaneous reduction by H2, (iii) Ru evaporation onto Pt(111) in UHV, and (iv) Polycrystalline PtRu alloy surfaces cleaned in UHV. This is an attempt to find out how size and structure of Ru islands, as prepared via the different procedures, influence the current for methanol oxidation. The catalytic activity is measured in form of current-time curves at constant potential. The surface compositions are characterized by cyclic voltammetry and Auger spectroscopy. The topography of the UHV-prepared deposits is observed by Scanning Tunneling Microscopy (STM). In addition, a molecular insight into surface processes is achieved by the use of in situ infrared spectroscopy. 2. Experimental Section 2.1. Solutions, Electrodes. Solutions were prepared with Millipore water (>18 MΩ) and analytical grade (Merck) chemicals; 0.1 M HClO4 was used as supporting electrolyte. Nitrogen 5.0 and Argon 5.0 (Linde) were used to deaerate the solutions and to keep an air-free atmosphere over the solution during the measurements.

A reversible hydrogen electrode (RHE) in the supporting electrolyte was used as a reference electrode. A glassy carbon rod was used as counter electrode. All experiments were performed at room temperature (ca. 22 °C). Two Pt(111) electrodes, having 3.46 mm2 and 0.67 cm2 of surface area, were used. The former was prepared at the Department of Physical Chemistry, University of Alicante and was used for voltammetric and UHV measurements as well. The second, purchased by Ma-Teck GmbH, was used for voltammetry and infrared spectroscopy. Single crystals were pretreated by flame annealing and cooling in Ar/H2 as usual. Following the procedure described by Gasteiger et al.,22 discshaped PtRu alloys (Johnson-Matthey) having 5 mm diameter and 2 mm thickness were polished to a mirror finish and treated in UHV by Ar sputtering and heating. 2.2. Ru Adsorption at Pt(111). After a cyclic voltammogram was measured between 0.05 and 1.15 V vs RHE in pure supporting electrolyte, the potential scan was stopped at potentials below 0.35 V. The electrode was taken out of the cell and adsorption of ruthenium was carried out by contacting the electrode surface with RuCl3 solutions of varied concentrations in the range from 10-5 to 1.0 M, with or without 0.1 M HClO4. After different adsorption times (10 s-10 min) the electrode was thoroughly rinsed with water. High Ru coverage could be obtained by repeating the procedure of adsorbing plus rinsing with water in the absence of HClO4 several times. Only freshly prepared Rusolutions were used for adsorption. Aging effects occurred in a time ranging between minutes and hours depending on the solution concentration. This affected the reproducibility of the Ru deposits. In a second procedure, after controlling the state of the Pt(111) surface via CV, Ru was adsorbed while gently bubbling hydrogen in the solution. This caused the direct reduction of Ru ions on the surface. In a third procedure, adlayers were formed in UHV by evaporation of Ru onto Pt(111). Before evaporation the Pt(111) sample was cleaned by several cycles of Ar-sputtering and heating at 800 K in 5 × 10-6 bar O2. This procedure reduced carbon contamination of the surface to a negligible level, as controlled by Auger. The surface structure was investigated before and after evaporation by STM. After preparation, the modified electrodes were characterized by means of a cyclic voltammogram in 0.1 M HClO4 solution. Potentials higher than 0.90 V were avoided in order to ensure the stability of the Ru layer. It was observed that repetitive cycling produced some rearrangements of the adlayer, probably favoring the growth of Ru clusters. Since we were interested in a good distribution of Ru atoms on the surface, normally only two potential cycles were applied for characterization. 2.3. Instrumentation. The Pt(111)Ru surfaces prepared in UHV were examined by scanning tunneling microscopy and Leed/ Auger spectroscopy, using an Omicron system. The STM tip was fabricated by constant current etching in 2 M NaOH. The base material was a 0.2 mm tungsten wire. In situ FTIR experiments were performed with a Bruker IFS 66 FTIR spectrometer equipped with a MCT detector. Parallel polarized light was obtained from a BaF2 supported Al grid polarizer. The spectroelectrochemical cell, fitted with a 60° prismatic CaF2 window, was designed to allow electrolyte exchange under potential control.8 Reflectance spectra were calculated as the ratio (R/Ro) of a sample (R) and a reference (Ro) spectrum. Positive- and negative-going bands represent respectively the loss and gain of species at the sampling potential.23

3. Results and Discussion 3.1. Evaluation of Ru Coverage at Pt(111). In Figure 1 we show characteristic cyclic voltammograms for the electrodes used in this work. For the clean Pt(111) surface (Figure 1a), the characteristic features for hydrogen adsorption-desorption and the so-called anomalous states (22) Gasteiger, H. A.; Ross, P. N.; Cairns, E. J. Surf. Sci. 1993, 293, 67-80. (23) Iwasita T.; Nart F. C. In Advances in Electrochemical Science and Engineering; Gerischer, H., Tobias, Ch., Eds.; Verlag Chemie: Weinheim, 1995; Vol. 4.

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Figure 1. Cyclic voltammograms at 50 mV/s for different PtRu surfaces in 0.1 M HClO4; temperature ca. 25 °C. (a) Pt(111) single crystal; (b-d) Ru modified Pt(111) using spotaneous adsorption; (e) Ru modified Pt(111) via Ru adsorption and simultaneous reduction by hydrogen; (f) PtRu alloy.

are observed. As the surface becomes covered with increasing amounts of Ru (Figure 1b-d), the sharp peak at 0.78 V decreases and the charge related to the formation of Ru oxide, in the double layer region of Pt(111), increases. The electrodes prepared via Ru reduction with H2 (Figure 1e), exhibit a sharp peak at ca. 0.1 V. This feature has been related to hydrogen desorption from Ru islands in the case of electrodes prepared by electrodeposition.14 We have not observed this peak for electrodes covered with spontaneously adsorbed Ru. From this fact we infer that the spontaneous adsorption does produce a Ru adlayer exhibiting a behavior different from electrodeposited materials. In any case, we expect from the open circuit adsorption, the formation of a thin (monatomic) Ru layer. The form of the voltammograms for Pt(111) partially covered by adsorbed Ru, Figures 1b-d, confirms our statement on the lack of clear features indicating welldefined surface processes which could be used for coverage determinations. However, a closer examination of the current in the 0.05-0.9 V potential range allows us to distinguish at least three potential regions. One overlaps the Pt-H region up to 0.35 V, a second covers the double layer region of Pt up to ca. 0.6 V, and a third is above this potential. These limits are not well defined but one can observe that when the amount of adsorbed Ru increases, the charge above 0.35 V grows. In fact, in the CVs with a high Ru percentage (Figure 1c,d) the slope of the i/E curve slightly changes at ca. 0.7 V. This could be an indication that a second process is occurring. The existence of more than one surface process has been clearly observed

in the work of Chrzawnoski and Wieckowski24 at Ru modified Pt(100) electrodes. It was demonstrated in a previous paper25 that the Ru: Pt ratios for Ru-modified Pt(111) electrodes, determined by Auger spectroscopy, can be linearly correlated with the anodic charge between 0.35 and 0.6 V in voltammograms measured in 0.1 M HClO4. For the Pt(111) electrodes with adsorbed Ru we have determined the coverage using this correlation. The composition of the alloys has been determined by EDXS and, after the cleaning procedure in UHV, confirmed by Auger spectroscopy. Because of the cross sensitivity between the Ru(273) and C(272) peaks and the overlapping of the remaining Pt and Ru peaks, we used the factor analysis technique.26,27 Instead of single peaks, the whole spectrum is analyzed in the range between 50 and 300 eV. 3.2. STM Data on the Structure Formed by UHV Ru-Evaporation onto Pt(111). Figure 2 shows STM data for a Pt(111) probe without (a) and with (b-f) Ru layers formed by vapor deposition. Several remarks concerning the surface topography can be made. Ru is deposited in the form of islands having a height of 2.1-2.2 Å. Given (24) Chrzanowski, W.; Wieckowski, A. Langmuir 1997, 13, 59745978. (25) Lin, W. F.; Zei, M. S.; Eiswirth, M.; Ertl, G.; Iwasita, T.; Vielstich, W. J. Phys. Chem. 1999, 103, 6968-6977. (26) Powell, C. J.; Seah, M. P. J. Vac. Sci. Technol. A 1990, 8, 735763. (27) Turner, N. H.; Murday, J. S.; Ramaker, D. E. Anal. Chem. 1980, 52, 84-92.

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Figure 2. STM images (93 nm × 93 nm) showing (a) a clean Pt(111) surface and (b-f) island formation after Ru evaporation on Pt(111) (see text). (ML) amount of Ru expressed as fractions of a monolayer, percentage of covered surface as indicated. Note the 3D formation for 0.4 ML. All images have been obtained with +0,4 V sample bias and 1 nA tunneling current.

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Figure 3. Plot of the Ru island density vs Ru coverage. Data obtained from STM images in Figure 2. Estimated error: (10 islands, (20 for the last image. Uncertainties result from island coalescence, which may in some cases be a tip artifact. For the last image the average space between islands is so small that a larger error has to be assumed. Uncertainty of Ru coverage, as measured by height statistics in the STM images: (10%.

a Ru-Ru distance of 2.14 Å in the (001) direction, we can state that under these conditions the islands are monatomic. With increasing coverage the diameter increases and a second layer is formed on top (e.g., see Figure 2d, for a coverage of 25% Ru, the Ru amount is 0.4 ML or fraction of a monolayer). Their average diameter is about 1-6 nm, depending on the coverage. The island sizes have been determined using line profiles, assuming a broadening of about 1 nm caused by the tip. For the catalytic properties we are investigating, only the order of magnitude of the island width is of interest. In the whole range of Ru coverage, the density of islands remains nearly constant as can be seen in Figure 3, where the island density is plotted against the amount of deposited Ru. Pt(111)/Ru samples prepared by deposition of Ru-vapor have been tested as catalysts for methanol oxidation in the form of i vs t curves at constant potential (see 3.3). STM studies on spontaneously adsorbed Ru islands and on Ru layers produced via reduction by hydrogen are in preparation. 3.3. Reactivity of Different PtRu Materials for Methanol Oxidation. The cyclic voltammograms of Figure 4 show the effect of adsorbed Ru (39%) on Pt(111) upon methanol oxidation in 0.5 M CH3OH + 0.1 M HClO4 solution. The current densities refer to the geometric surface area of the Pt(111) electrode. During the positive-going scan, the modified electrode exhibits a higher activity at potentials below 0.7 V. However, since the rise in current is faster at clean platinum, both materials do not differ very much in the value for the maximum current during the positive-going scan. After reversing the potential, the reactivation is more pronounced for the Pt(111) surface than for the modified electrode and the Pt(111) surface remains remarkably more active up to ca. 0.68 V. Considering that the reactivation current is specifically due to the competition between methanol and water for adsorption on Pt sites,28 the differences in the reactivation current can be ratio(28) Iwasita, T.; Xia, X. H.; Liess H. D.; Vielstich, W. J. Phys. Chem. B 1997, 101, 7542-7547.

Figure 4. Cyclic voltammograms at 50 mV/s for Pt(111) and Ru modified Pt(111) in 0.5 M CH3OH + 0.1 M HClO4; temperature ca. 25 °C.

Figure 5. Current-time curves for Pt(111) and different PtRu electrodes at 0.5 V vs RHE in 0.5 M CH3OH + 0.1 M HClO4; temperature ca. 25 °C. (a) PtRu alloy (85:15); (b) Pt(111)/Ruads. (39%); (c) Pt(111)/Ruevap. (30%); (d) clean Pt(111).

nalized in terms of the difference in the amount of Pt sites available for this process. Stationary i-t curves were measured at 0.5 V on Pt(111) and on Ru-modified Pt(111) obtained by different procedures. Some examples of these are shown in Figure 5. The current densities after 300 s as a function of the Ru percentages are plotted in Figure 6, together with data for PtRu alloys that are discussed below. This plot is logarithmic for a better comparison with literature data.10

Methanol Oxidation on PtRu Electrodes

Figure 6. Plot of the current density for methanol oxidation from current-time curves at 0.5 V as in Figure 4 as a function of Ru coverage. Data obtained at 300 s: (B) Pt(111)/Ru formed by spontaneous adsorption, (0) Ru modified Pt(111) via Ru adsorption and simultaneous reduction by hydrogen, (×) Ru coverage by UHV vapor deposition. Data obtained at 20 min: (n) UHV-prepared PtRu alloys (see text), (2) data of ref 10 for comparison.

For Pt(111)/Ru prepared by spontaneous adsorption of Ru, a pronounced growth of activity is observed at low coverage, which is followed by a broad maximum with an average value of 31 ( 2.4 µA cm2 for a Ru coverage ranging between 0.15 and 0.5. The clean Pt(111) surface exhibits the lowest current for methanol oxidation (0.65 µA cm2 after 300 s). Related to pure platinum, the maximum activity for spontaneously adsorbed Ru represents an increase in current by a factor of 47. The plot in Figure 6 clearly shows the influence of the preparation procedure for the Ru adlayer. Ru deposits obtained via reduction of Ru with H2 or via evaporation in UHV present a lower activity than spontaneously adsorbed Ru. Tests of activity for methanol oxidation were also performed with alloy electrodes prepared in UHV by argon sputtering. To compare our values with those of Gasteiger et al.,10 the oxidation currents were measured at ca. 25 °C, after 20 min of application of a potential step from 0.05 to 0.5 V. The values obtained are plotted in Figure 6, together with the values of Gasteiger et al. We observe that both sets of data are in reasonable agreement. Moreover, our data behave complementarily to those of Gasteiger et al.10 and both sets of results convey a definition of the form of the dependence of methanol oxidation on the alloy surface composition. A maximum of high catalytic activity, 120 ( 12 µA cm2, is observed between, say, 10% and 40% Ru. It is noteworthy that Watanabe and Motoo,3 using PtRu-Raney alloy electrodes, observed a broad maximum centered at 50% for methanol oxidation at 40 °C. According to a model suggested for the catalytic effect of Ru on methanol oxidation, a surface structure having one Ru atom neighboring three Pt sites represents the optimum geometry for methanol reaction.29 On account of (29) Ross, P. N. In Electrocatalysis; Lipkowski, J., Ross, P. N., Eds.; Wiley-VCH: New York, 1998.

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Figure 7. In situ FTIR spectra for Pt(111), Pt(111)/Ru 39%, and PtRu alloy (85:15) in 0.5 M CH3OH + 0.1 M HClO4. Potentials as indicated on each spectrum; reference spectrum taken at 0.05 V.

this, it was predicted that activity vs composition plots such as in Figure 6 should have a maximum for a Ru composition near 10%.29 The experimental results in Figure 6, however, do not support such a prediction. This is not surprising since an ordered surface structure as ideally proposed29 does not exist in real PtRu systems even for ordered substrates such as Pt(111). In this respect, it is noteworthy that Pt deposited onto Ru(001) under UHV conditions, segregates into clusters also.30 In addition, it must be noted that the mentioned model calculation giving a sharp peak near 10% Ru, only holds if the dissociative adsorption of methanol is the rate-determining step. We will return to discuss this point in Section 3.5. It must be emphasized that highlighting the benefit of a homogeneous distribution of Pt-Ru sites in contrast to an island structure, the alloys present a factor of 3 higher currents than spontaneously adsorbed Pt(111)/Ru electrodes. 3.4. In Situ FTIR Spectroscopy Studies. A molecular insight in the catalytic process was afforded by in situ FTIR spectroscopy, as described in this section. In Figure 7 we show in situ FTIR spectra obtained during oxidation of methanol on three different materials: Pt(111), Pt(111)/Ru prepared by spontaneous adsorption, and a PtRu alloy (85:15). For Pt(111), the main features are due to solution species, at 2341 cm-1 (CO2), and 1710 cm-1 (formic acid c/o methyl formate20). The bands at ca. 2050 and 1820 cm-1 are due respectively to linearly and bridge adsorbed CO.31 In the presence of Ru, no band for bridge bonded CO is observed. From all three materials, the alloy shows up the largest production of CO2. This electrode also presents larger intensities for linear bonded CO at low potentials, (30) Bautier de Mongeot, F.; Scherer, M.; Gleich, B.; Kopatzki, E.; Behm, R. J. Surf. Sci. 1998, 411, 249-262. (31) Beden, B.; Lamy, C.; Bewick, A.; Kunimatsu, K. J. Electroanal. Chem. 1981, 121, 343-347.

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the voltammogram in Figure 1e corresponding to this material shows a peak at 0.1 V, which is assigned to H-desorption from large Ru domains. 3.5. Discussion of Reaction Mechanism. In the range of potentials where methanol oxidation occurs, the surface of a platinum electrode (either clean or modified by ruthenium) undergoes several processes that play a deciding role on the reaction. Thus, below 0.35 V at pure Pt(111), CH3OH adsorption is partially inhibited by adsorbed hydrogen. At higher potentials (the so-called double-layer region), CH3OH must displace H2O molecules in order to become physisorbed, a step which is previous to chemisorption.28 Above 0.6 V, dissociation of water produces another inhibitor of methanol adsorption, namely, [OH]ads. It has been pointed out that the latter facilitates oxidation of the intermediate CO but inhibits methanol adsorption. Thus, it plays a dual role, which causes the observed maximum near 0.7 V in the voltammogram of methanol.28 In the presence of Ru, we must consider that oxide is formed at potentials above 0.2 V.34 In addition, the intermediate [CO]ads can move from the Pt place where it is formed to a Ru place. All this indicates that when discussing the mechanisms of methanol oxidation one should specify the potential region considered. Thus, between 0.35-0.6 V and in the presence of ruthenium we propose to write a bifunctional mechanism3 in the form of a sequence of (not elementary) steps as follows: Figure 8. Comparison of the CO features in FTIR spectra for methanol obtained as those in Figure 7, for Pt(111), Pt(111) with spontaneously adsorbed Ru (39%), Pt(111) with Ru reduced by hydrogen, and PtRu alloy (85:15).

CH3OH + Pt(H2O) f Pt(CH3OH)ads + H2O

(i)

Pt(CH3OH)ads f Pt(CO)ad + 4 H+ + 4 e-

(ii)

due to the ease of dissociative adsorption of this material. It is noteworthy that despite a strong production of CO2 at 0.5V, the band intensity for CO does not diminish. This can only be possible if either (a) CO is not oxidized at all, the production of CO2 taking place over a parallel reaction pathway, or (b) CO is being oxidized to CO2 but the rate of readsorption of methanol (to form CO) is high enough as to keep its concentration at a given level. Possibility (a) can be discarded since it is well established that carbon monoxide can be oxidized at 0.5 V on the PtRu alloy.32 Moreover a parallel reaction pathway can be neglected under the present experimental conditions.4 Possibility (b) is more likely to happen and the constant value of adsorbed CO is congruent with its role as a reaction intermediate. The influence of the procedure for Ru adsorption is demonstrated by the spectra in Figure 8. There, one electrode prepared by spontaneous adsorption is compared with another prepared by bubbling H2, the Ru composition being ca. 20% for both electrodes. For the latter electrode we clearly observe a band at 1965 cm-1 characteristic of CO adsorbed Ru. This band appears only as a very weak feature on the electrode prepared by spontaneous adsorption and it is absent in the alloy. Recent results in the literature33 show that for PtRu materials, Ru segregation into islands with a minimum size of ca. 8 atoms is necessary for the CO feature at Ru sites to appear. The results in Figure 8 can thus be rationalized in terms of the formation of islands of different size depending on the experimental procedure. Larger islands seem to be formed via the H2-reduction procedure. This justifies the low catalytic activity of the latter material (Figure 6). Also,

Ru(H2O) f RuOH + H+ + e-

(iii)

(32) Gasteiger, H. A.; Markovic, N.; Ross, P. N.; Cairns, E. J. J. Phys. Chem. 1994, 98, 617-625. (33) Zou, S.; Villegas I.; Stuhlmann, C.; Weaver, M. J. Electrochim. Acta 1998, 43, 2811-2824.

Pt(CO)ads + RuOH f Pt + Ru + CO2 + H+ + e(iv) Ru(CO)ads + RuOH f 2 Ru + CO2 + H+ + e-

(v)

Here we have emphasized the adsorption of methanol (i) as a H2O-replacement reaction.28 We assume in steps iv and v the stoichiometry RuOH for the Ru oxide. The real nature of this oxide is meaningless for the present discussion. Moreover, reaction ii must occur in several elementary steps,20,35 which might require multiple neighboring Pt sites in order to facilitate methanol dissociation. We can now search for the possible factors affecting the rate of reaction. However, as pointed out above, we are not writing the elementary steps of the reaction, and thus we shall simply look for the process, within i-v where the rate determining step could be involved. For better clarity, we shall enumerate the facts in the discussion. (a) In the range of potentials between 0.35 and 0.6 V, reaction i is not rate determining if sufficient neighboring Pt sites are available. Above ca. 0.7 V water adsorption starts being stronger than methanol physisorption. This leads to the formation of Pt(OH)ads and to a decrease in the rate of reaction.28,36 (b) The second reaction, namely the dissociation of methanol to form CO (and to a lesser extent, other adsorbed particles20,37), can already occur at relatively low poten(34) Ticianelli, E.; Berry, J. G.; Paffet, M. T.; Gottesfeld, S. J. Electroanal. Chem. 1989, 258, 61-77. (35) Bagotzky, V. S.; Vassiliev, Y. B.; Khazova, O. J. Electroanal. Chem. 1977, 81, 229. (36) Iwasita, T.; Xia, X. H. J. Electroanal. Chem. 1996, 411, 95-102. (37) Iwasita, T.; Nart, F. C. J. Electroanal. Chem. 1991, 317, 291298.

Methanol Oxidation on PtRu Electrodes

tials. For platinum, it sets in at 0.15 V and gives a welldefined maximum at 0.25 V as demonstrated e.g., by Krausa et al. using DEMS.14 In the presence of electrodeposited Ru, the same authors demonstrate that the dissociative adsorption is shifted by 50 mV toward more negative potentials. It is therefore difficult to think of this reaction as being the rate determining one at 0.35-0.6 V, unless we have only a reduced number of Pt sites. In other words, methanol dissociation immediately takes place producing the well-known blocking of the surface with CO and other species partially dehydrogenated. (c) The data in Figure 6 show that in the range of alloy surface compositions between say 10 and 40% Ru, the reaction is more or less independent of the surface composition of the catalyst. Within this range of Ru concentration, neither the number of Pt sites (necessary for CH3OH dissociation) nor of Ru sites (necessary for H2O dissociation) can be regarded as being rate-limiting factors. This result is at variance with the earlier suggestion29 that the Ru:Pt ratio of 7:93 represents the optimum surface composition for methanol oxidation. This statement was based on geometric considerations of an optimum Pt sitedistribution assuming that, at room temperature, methanol dissociation is rate determining.29 In view of the present results, this assumption is not sustainable. Correctly, kinetic data for methanol oxidation on alloy electrodes should be collected within the range of Ru composition between 10 and 40%. Under these conditions the surface composition of the catalyst is not a limiting factor for the reaction. (d) In view of the facts above (a-c), we have to conclude that the reaction between adsorbed CO and adsorbed OH must be responsible for the rate of the process, at least for potentials between 0.35 and 0.6 V. (e) As observed in Figure 6, the maximum of alloy activity is displaced toward a composition with higher amounts of Pt than of Ru. This fact can be related to the need of enough neighboring Pt sites to release the H atoms during the adsorption step (ii). This does not mean, however, that methanol dissociation is the rate-determining step.

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As a practical consequence of the foregoing discussion we can state that a catalyst composition of Ru:Pt between, say, 10% and 40%, provides enough Pt and Ru sites for the reaction of methanol. The kinetic limitation is caused by the reaction between adsorbed CO and RuOH and, in this respect, a homogeneous distribution of Pt and Ru atoms must have a strong influence on the reaction rate. Therefore, the optimum distribution is that of the alloys. For the other materials forming Ru islands, the method of preparation must be chosen so as to diminish the diameter of the islands as much as possible. These findings are in complete agreement with the results shown above. Infrared spectra suggest that Ru islands formed via spontaneous adsorption are much smaller than those obtained by using H2 as a reducing agent. 4. Conclusions The reaction rate of methanol on Ru modified Pt(111) electrodes depends on the preparation procedure. Using Pt(111) as a substrate, spontaneously adsorbed Ru presents higher catalytic activity than Ru reduced by H2 or deposited via UHV evaporation. This result can be rationalized in terms of the formation of smaller islands in the case of spontaneously adsorbed Ru. On the other hand, the activity of PtRu alloys in the range of Ru concentration between 10% and 40% is up to a factor of 10 higher than the activity of some of the modified Pt(111) surfaces. Surface structure and Pt-Ru distribution determine the catalyst behavior. Within the range of surface composition Ru:Pt of 10-40%, the rate of methanol oxidation is controlled by the reaction between adsorbed CO and Ru oxide. Acknowledgment. Fellowships from the Fonds der Chemischen Industrie (H.H.) and the von HumboldtStiftung (W.L.) are gratefully acknowledged. The authors are indebted to Birgit Kohnen, Forschungszentrum Ju¨lich, for the determination of alloy compositions. The present work was financially supported by the Deutsche Forschungsgemeinschaft. LA990594N