Ru Decoration of Stepped Pt Single Crystals and the Role of the

The influence of monatomic steps on the oxidation of adsorbed CO is examined using potentiodynamic and galvanostatic experiments. Galvanostatic experi...
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Langmuir 2002, 18, 4659-4666

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Ru Decoration of Stepped Pt Single Crystals and the Role of the Terrace Width on the Electrocatalytic CO Oxidation Gabor Samjeske´, Xiao-Yin Xiao, and Helmut Baltruschat* Institut fu¨ r Physikalische und Theoretische Chemie, Universita¨ t Bonn, Ro¨ merstrasse 164, D-53117 Bonn, Germany Received August 16, 2001. In Final Form: February 21, 2002 The influence of monatomic steps on the oxidation of adsorbed CO is examined using potentiodynamic and galvanostatic experiments. Galvanostatic experiments allow a better determination of the lowest possible oxidation potential than cyclic voltammetry. By comparing results on Pt(111), Pt(665), Pt(332), and Pt(755), we found that steps with a local (110) geometry are more active than those with a (100) geometry. Steps can be decorated by Ru, thus leading to surfaces with a known atomic arrangement of the components. During galvanostatic oxidation of adsorbed CO on such surfaces a second potential plateau is observed, the value of which depends on the density of steps. These transients suggest the existence of an electronic effect in addition to the widely accepted bifunctional mechanism: Due to the electronic effect, only CO in the neighborhood of Ru is destabilized, whereas CO adsorbed at more distant sites is not influenced and has to diffuse “uphill” to the reactive sites, leading to an apparently slow diffusion. In addition, the spillover effect according to the bifunctional mechanism lowers the height of the activation barrier for CO oxidation and therefore has an influence on all adsorbed CO.

Introduction It is generally accepted that the action of Ru as a cocatalyst for the oxidation of adsorbed CO is by adsorbing OH at lower potentials than Pt itself.1 A Ru submonolayer on Pt acts in a manner similar to that of Ru in a PtRu alloy.2 Since Ru is also active if deposited in islands of 2-3 nm on Pt(111),3-6 it was concluded that surface diffusion of adsorbed CO to the active PtRu sites is possible.5 The surface diffusion coefficient of CO was estimated to be between 10-13 and 10-12 cm2 s-1. Koper et al.7 performed a Monte Carlo simulation of the CO oxidation on PtRu electrocatalysts, including voltammetry. They found that the appearance of a second oxidation peak depends on the ratio of Ru island diameter (or distance between the islands) to the diffusion coefficient. To achieve a fundamental understanding of electrocatalytic processes including bimetallic surfaces, the atomic arrangement of the components has to be known. Whereas in studies using ultrahigh vacuum (UHV) this can be achieved in some cases by using ordered alloys, this is not possible for most electrochemical applications. We therefore used an alternative method: conceptually, the second component, Ru in this case, is deposited solely at the monatomic steps of stepped Pt single crystal surfaces. Many secondary metals (such as Cu, Sn, Mo, Bi, Tl, and also Ru) are indeed preferentially deposited at such step sites.8-12 The preferential deposition can be easily monitored by the suppression of the hydrogen peaks corresponding to the step sites in cyclic voltammetry. * Corresponding author. E-mail: [email protected]. (1) Watanabe, M.; Motoo, S. J. Electroanal. Chem. 1975, 60, 275283. (2) Herrero, E.; Franaszczuk, K.; Wieckowski, A. J. Electroanal. Chem. 1993, 361, 269-273. (3) Chrzanowski, W.; Wieckowski, A. Langmuir 1997, 13, 59745978. (4) Friedrich, K. A.; Geyzers, K.-P.; Linke, U.; Stimming, U.; Stumper, J. J. Electroanal. Chem. 1996, 402, 123-128. (5) Friedrich, K. A.; Geyzers, K.-P.; Marmann, A.; Stimming, U.; Vogel, R. Z. Phys. Chem. 1999, 208, 137-150. (6) Herrero, E.; Feliu, J. M.; Wieckowski, A. Langmuir 1999, 15, 4944-4948.

As we have outlined before,11 for a fast discrimination between reaction mechanisms, e.g., of the Eley-Rideal and the Langmuir-Hinshelwood types, galvanostatic experiments are much better suited than cyclic voltammetry. Characteristic of both the “classical” LangmuirHinshelwood kinetics (corresponding to a fast diffusion of adsorbed CO) and a nucleation and growth mechanism (corresponding to a Langmuir-Hinshelwood mechanism with slow or no diffusion13,14) are minima in the potentialtime curve at intermediate coverages; such minima are observed for the oxidation of adsorbed CO at Pt electrodes, but not on electrodes partially covered by Sn. (Hereafter, the case of fast diffusion (“mean field limit”7) will be called “simple” Langmuir-Hinshelwood mechanism because it is the one treated in textbooks, with the rate proportional to ΘCO(1 - ΘCO).) In the case of processes involving nucleation, galvanostatic experiments also offer the advantage that the growth can be monitored at low rates (given by the preset current value), whereas in potentiostatic or potentiodynamic experiments, the growth process occurs at a much larger rate than the initial nucleation. We will show here for the oxidation of adsorbed CO that processes are revealed in this way which otherwise cannot unequivocally be separated from each other. It is the aim of this study to gain information on the importance of surface diffusion of adsorbed CO during the oxidation process by using the step sites as prede(7) Koper, M. T. M.; Lukien, J. J.; Jansen, A. P. J.; Santen, R. A. v. J. Phys. Chem. B 1999, 103, 5522-5529. (8) Berenz, P.; Tillmann, S.; Massong, H.; Baltruschat, H. Electrochim. Acta 1998, 43, 3035-3043. (9) Massong, H.; Tillmann, S.; Langkau, T.; El Meguid, E. A. A.; Baltruschat, H. Electrochim. Acta 1998, 44, 1379-1388. (10) Abd El Meguid, E. A.; Berenz, P.; Baltruschat, H. J. Electroanal. Chem. 1999, 467, 50-59. (11) Massong, H.; Wang, H. S.; Samjeske, G.; Baltruschat, H. Electrochim. Acta 2000, 46, 701-707. (12) Macia, M. D.; Herrero, E.; Feliu, J. M.; Aldaz, A. J. Electroanal. Chem. 2001, 500, 498-509. (13) Koper, M. T. M.; Jansen, A. P. J.; Santen, R. A. v.; Lukien, J. J.; Hilbers, P. A. J. J. Chem. Phys. 1998, 109, 6051-6062. (14) Koper, M. T. M.; Jansen, A. P. J.; Lukkien, J. J. Electrochim. Acta 1999, 45, 645-651.

10.1021/la011308m CCC: $22.00 © 2002 American Chemical Society Published on Web 05/15/2002

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Figure 1. In situ STM image and section analysis of (a) Pt(332) cooled in pure Ar and (b) Pt(665) cooled in Ar/H2. (a) Section analysis at 0.25 V vs Cu/Cu2+, It ) 1 nA. (b) Section analysis at 0.2 V vs Cu/Cu2+, It ) 5 nA. Both: 0.05 M H2SO4 + 4 × 10-4 CuSO4 solution, Vb ) 20 mV.

termined reaction sites. By decoration with Ru, oxidation at these sites at much lower potentials than that at the Pt terraces is achieved. Experimental Section Disk-shaped platinum single crystals with a diameter of 1 cm and a thickness of 3 mm with the orientations (111), (332) ()[6(111)×(111)]), (665) ()[12(111)×(111)]), and (755) ()[6(111)× (100)]) were obtained from Metal Crystals & Oxides, UK. All single crystals were oriented to an accuracy of 0.5° and finally polished to mirror-grade smoothness. Preparation was done by flame annealing according to the method of Clavilier15 by heating the crystal to red color in a hydrogen flame or Bunsen flame and afterward cooling for 4 min in a mixture consisting of Ar and H2 (purity 99.999%; Air Products, Hattingen, Germany) in a ratio of 4:1 above Millipore water (Millipore Q system). The mixture was monitored by counting bubbles in a wash flask filled with Millipore water. We had previously prepared our stepped surfaces by cooling them in a pure Ar stream. In ref 16 it was shown that not only does cooling in iodine vapor lead to faceting of stepped surfaces as already demonstrated in ref 17, but also cooling in air leads to some faceting, with diatomic steps predominating, in accordance with ref 18. Cooling in a Ar/H2 mixture led to surfaces with monatomic steps.16 Figure 1a shows for the case of a Pt(332) surface that also cooling in pure Ar leads to terraces with a width of 2.48 nm which is nearly twice that of the expected 1.32 nm. This is probably due to traces of O2, which when using an Ar/H2 mixture are reduced to water. We therefore performed also experiments with a Pt(332) cooled in pure Ar (hereafter designated as “Pt(332)(fac)”), to have a link to our previous experiments. For comparison, Figure 1b shows for Pt(665) that cooling in the Ar/H2 mixture indeed leads to monatomic steps with the nominal terrace width (after subtraction of 10% as correction for thermal drift). (In these experiments, Cu2+ ions were present in solution because this usually results in a better (15) Clavilier, J.; Faure, R.; Guinet, G.; Durand, R. J. Electroanal. Chem. 1980, 107, 205-209. (16) Herrero, E.; Orts, J. M.; Aldaz, A.; Feliu, J. M. Surf. Sci. 1999, 440, 259-270. (17) Vogel, R.; Kamphausen, I.; Baltruschat, H. Ber. Bunsen-Ges. Phys. Chem. 1992, 96, 525. (18) Lang, B.; Joyner, R. W.; Somorjai, G. A. Surf. Sci. 1972, 30, 440-453.

resolution. The Cu UPD (under potential deposition) layer has no influence on the step-terrace topology; the fine structure on the terraces in Figure 1b shows the x3 × x7 sulfate adlattice.) All experiments were carried out in a glass cell in the hanging meniscus arrangement. As reference electrode an RHE (reversible hydrogen electrode) was used; its compartment was separated from the working electrode compartment by a glass valve. Solutions were freshly prepared from Millipore water and sulfuric acid (Merck, p.a., 95-97%) and deaerated with Ar for about 15 min prior to each experiment. Cyclic voltammograms (CV) were recorded in 0.5 M H2SO4 with a scan rate of 50 or 10 mV/s, respectively. CO adsorption was achieved under potential control at 70 mV by bubbling the electrolyte with CO (99.998%; Messer Griessheim, Germany) for about 10 min. It was followed by change of the electrolyte three times at the same potential and subsequent bubbling with a strong Ar gas flow through the electrolyte for another 10 min. In the galvanostatic experiments, adsorption of CO was also performed under potential control at 50 mV; then the cell was disconnected from the potentiostat and connected to the galvanostat. The selected current was applied until the potential reached 800 mV and then switched back to potentiodynamic conditions to avoid roughening of the crystal. Before and after each galvanostatic experiment a CV was recorded to control the condition of the surface. Ru deposition was achieved by bringing the single crystal surface into contact with a deaerated solution of 0.005 M ruthenium in 0.5 M H2SO4 for 5 min at a potential of 570 mV, following the procedure of Geyzers; deposition at this potential should lead to a Ru coverage of around 20%.19 The Ru-containing solution had to be freshly prepared before every experiment by diluting a stock solution of 0.1 M RuCl3/0.1 M HClO4 with 0.5 M H2SO4 (which was not older than 3 weeks, but may have contained some RuO(H2O)42+ according to ref 3). The CV and potential transients were recorded using a EG&G 273 potentiostat/galvanostat and a PC equipped with a data acquisition card (National Instruments) together with software based on LabView (National Instruments).

Results and Discussion Cyclic voltammograms of the Pt(665) and the Pt(755) surfaces are shown in Figure 2 together with those of the (19) Cramm, S.; Friedrich, K. A.; Geyzers, K. P.; Stimming, U.; Vogel, R. Fresenius J. Anal. Chem. 1997, 358, 189-192.

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Figure 2. (a) CV of Pt(665) in 0.5 M H2SO4 before (---) and after (s) Ru deposition. dE/dt 50 mV/s. (b) CV of Pt(755) in 0.5 M H2SO4 before (---) and after (s) Ru deposition. dE/dt 50 mV/s. Table 1. Experimental and Theoretical Charges for the Hydrogen Regiona

Pt(665) Pt(755) Pt(332) Pt(332)(fac)

Qpeakexp/ µC cm-2

Qpeaktheor/ µC cm-2

Q50-750 mV/ µC cm-2

Qexp(Clavilier)/ µC cm-2

20.2 52.7 39 44

21.3 42.5 45.2 45.2

302 333 265 280

292 296 264 264

a Q 50-750 mV is the charge obtained for one-half cycle in the potential region from 50 to 750 mV. Qexp(Clavilier) is the experimental value for one sweep reportet by Clavilier.34,35 Pt(332)(fac) is Pt(332) cooled in pure Ar.

same surfaces after decoration of the steps by Ru. Experimental charges for the peaks due to hydrogen adsorption at step sites are close to those expected theoretically20 for Pt(665); the charge of the peak at 125 mV, which is due to hydrogen adsorption at the steps of local 110 geometry, is 20 µC cm-2, as compared to a theoretical value of 21 µC cm-2. For Pt(755) the charge is 53 µC cm-2 for the peak at 267 mV due to hydrogen adsorption at steps with local (100) geometry as compared to a theoretical value of 43 µC cm-2. Also the total charges between 50 and 750 mV of 302 (for Pt(665)) and 333 µC cm-2 (for Pt(755)) are close to the values of 292 and 296 µC cm-2 respectively reported by Clavilier.20 The peak at 267 mV for Pt(665) is due to a slight misorientation. The hydrogen charge values are summarized in Table 1. Cyclic voltammograms after deposition of Ru show that the hydrogen adsorption peaks at the steps are suppressed. We can therefore safely assume that Ru is decorating the (20) Clavilier, J.; Achi, K. E.; Rhodes, A. J. Electroanal. Chem. 1989, 272, 253-161.

steps, probably by a monatomic row of Ru atoms. Scanning tunneling microscopic (STM) measurements of Pt(111) electrodes covered by Ru,5,21 on the other hand, showed the formation of single islands homogeneously distributed over the large terraces and no indication of step decoration. This, however, is not an argument against the decoration of steps with a monatomic row of Ru, which would hardly be resolved by STM; the atomic structure of the substrate steps was not resolved in the above references. Oxidation of adsorbed CO on the Ru-free surfaces starts between 0.4 and 0.5 V. This small shoulder (or “prepeak”) corresponds to the transition from a compressed CO adlayer to a decompressed adsorbate.22,23 The main oxidation peak is much broader for Pt(755) than for Pt(665) (and Pt(332)) and also much more asymmetric (cf. Figure 3a and 3b, dashed line). On the Ru-covered Pt(665) electrode, this prepeak is not visible, but the peak potential of the main peak is shifted downward to 520 mV. The slow rise of the current above 0.4 V overlaps with the prepeak on the Ru-free surface and is due to CO oxidation indeed, as was revealed by differential electrochemical mass spectrometry (DEMS) for Pt(332)(fac).11 The peak is much narrower than without Ru, but a second peak is visible around 0.6 V. This has been observed before for Pt(111) partially covered by Ru.11 Also for the Pt(775), the oxidation peak is much narrower after step decoration by Ru; however, no clear shoulder is observable at its high potential side. We did not observe this shoulder for the Ru-modified Pt(332) surface cooled in Ar/H2, either. We did, however, observe such a shoulder when the Pt(332) was cooled in pure Ar (Pt(332)(fac)): the CV looked very similar to that of the Pt(665) cooled in Ar/H2, in accordance with the identical step width. In ref 11, we already reported on the oxidation of adsorbed CO on Pt(332)(fac) partially covered by Ru using electrochemical mass spectrometry. There, the oxidation peak was broader and occurred at a somewhat lower potential. The reason cannot be a higher iR drop in the thin layer cell used there because this should lead to a shift to more positive potentials (if it has an influence at all). Rather, the CO coverage achieved in the thin layer cell is slightly lower because some of the dissolved CO is lost by evaporation, leading to a lower concentration in solution. A higher coverage probably leads to a higher nucleation overpotential and subsequently also to a higher oxidation rate at this potential, resulting in a narrower peak. Such a dependence of the CO oxidation peaks on the CO coverage has been reported in ref 24. Oxidation charges are listed in Table 2. They are much larger on the surfaces partially covered by Ru. By measurements with an electrochemical quartz crystal microbalance combined with DEMS, we have shown before that a PtRu (50:50) alloy electrode is covered by an anionic species even at 100 mV.25 This species, probably oxygen or hydroxide, is displaced upon adsorption of CO. It is, however, readsorbed when the CO is oxidized, leading to an additional charge for oxidation even after background subtraction. For a lower integration limit of 0.28 mV, an additional charge of 40% was thus obtained for the PtRu alloy electrode, whereas for a pure polycrystalline electrode (21) Crown, A.; Moraes, I. R.; Wieckowski, A. J. Electroanal. Chem. 2001, 500, 333-343. (22) Villegas, I.; Weaver, M. J. J. Chem. Phys. 1994, 101, 16481660. (23) Villegas, I.; Gao, X.; Weaver, M. Electrochim. Acta 1995, 40, 1267-1275. (24) Lebedeva, N. P.; Koper, M. T. M.; Herrero, E.; Feliu, J. M.; van Santen, R. A. J. Electroanal. Chem. 2000, 487, 37-44. (25) Jusys, Z.; Massong, H.; Baltruschat, H. J. Electrochem. Soc. 1999, 146, 1093-1098.

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Figure 3. (a) Pt(665): (s) CV of the oxidation of adsorbed CO in CO-free electrolyte at the Ru step-decorated electrode. CV of the Ru-free electrode for comparison: (---) anodic stripping of CO (‚‚‚) second cycle. 0.5 M H2SO4 dE/dt 10 mV/s. Inset: First cycle for CO oxidation at the Ru-free electrode. (b): Pt(755): (s) CV of the oxidation of adsorbed CO in CO-free electrolyte from the Ru step-decorated electrode. CV of the Ru-free electrode for comparison: (---) anodic stripping of CO (‚‚‚) second cycle. 0.5 M H2SO4 dE/dt 10 mV/s. Inset: First cycle for CO oxidation at the Ru-free electrode. Table 2. Tabulated Overview of Some Experimental Data Obtained for the Different Pt Single Crystal Electrodesa Pt(111) Pt(111)/Ru Pt(665) Pt(665)/Ru Pt(332)(fac) Pt(332)(fac)/Ru Pt(332) Pt(332)/Ru Pt(755) Pt(755)/Ru Qox(CO)/µC/cm-2 Eonset(CO-ox)/mV Epeak(CO-ox)/mV Egalv(CO-ox)/mV

273 640 790 673

347 455 558 461

279 630 723 620

398 445 517 438

334 628 695 602

482 452 486 421

348 624 697 605

616 353 494 433

328 615 735 636

473 450 537 440

a Q (CO) is the charge for the anodic stripping of the CO obtained from the CV and corrected with respect to the double layer charge ox by subtracting the second cycle. Eonset is the potential at which the oxidation of the COad in the main peak begins. Epeak is the potential of the main oxidation peak. Egalv is the minimum obtained from the potential transients of the galvanostatic experiments for a current of 5.27 µA. All potentials are referred to RHE.

Figure 4. Galvanostatic oxidation of adsorbed CO on bare electrodes in CO-free electrolyte. I(appl) 5.27 µA, 0.5 M H2SO4. Q is the charge consumed and therefore the product of I(appl) and time.

only 20% of the oxidation charge (after background subtraction) is due to this double layer charging. Galvanostatic potential transients for the oxidation of adsorbed CO on the bare Pt surfaces are shown in Figure 4. Here, the x-axis corresponds both to the time and to the partial oxidation charge and therefore the complement of the (residual) coverage. The potential maximum at the

beginning of the transients corresponds to a nucleation overpotential, but can also be explained by a “simple” Langmuir-Hinshelwood mechanism, (involving fast diffusion): Due to the Θ(1 - Θ) term in the rate equation, at a constant rate the overpotential has to be high at high and low coverages and is low at medium coverages (Θ ) 1 /2). The slow increase at the beginning in particular for Pt(332) and Pt(755) is due to the transition from the compressed CO adlayer to the decompressed adlayer and corresponds to the prepeak. In the case of Pt(665), the applied constant current is higher than the current in the prepeak (cf. Figure 3a); therefore, this process does not show up in the potential transient. The same is true for Pt(111), where due to the low number of steps or defects the “prepeak” often is not visible even in cyclic voltammetry. (It should be noted that the oxidation of adsorbed CO in the prepeak probably occurs only at defect sites, to which it is transported simply by expansion of the CO adlayer.) The values of the potential maximum correspond qualitatively to the (however less defined) onset potentials of the main peak in cyclic voltammetry (cf. Table 2). All transients show a clear minimum at intermediate coverages. This minimum, which in the case of a “simple” Langmuir-Hinshelwood mechanism should occur exactly at a (relative) coverage of Θ ) 0.5, is observed at much higher coverages on Pt(332), but at much lower coverages on Pt(111) and in particular on Pt(755). This is an argument against a pure “simple” Langmuir-Hinshelwood mechanism. The corresponding potential value, which increases in the order from Pt(332) < Pt(665) < Pt(755) < Pt(111), is notably lower than the main peak potentials obtained from cyclic voltammetry (cf. Table 2).

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This demonstrates the role of the steps for the continuous oxidation in a more reliable way than does the peak potential, because the latter may refer to different current densities (peak currents). Obviously, the steps with local (100) geometry are less efficient than the step sites with local (110) geometries. This is in accordance with the onset and peak potentials observed during potentiodynamic CO oxidation which are higher on Pt(100) than on Pt(110).26 Since the adsorption enthalpy of CO on the terraces should not vary much with the step orientation of local (100) or (110) geometry, it follows that the different oxidation potentials are of pure kinetic origin. Therefore, also the different oxidation potentials found for Pt(100) and Pt(110)26 are not due to different adsorption enthalpies, but rather due to different kinetics. According to nucleation and growth models, the maximum rate of a desorption process at constant potential occurs at coverages above 0.5 (Θ ) 0.51 for progressive and 0.61 in the case of instantaneous nucleation) as can be calculated from the corresponding rate equations.27 (Here, Θ refers to the relative coverage, the maximum value of which is 1.0.) Under galvanostatic conditions, when the rate is forced to be constant, the potential necessary to maintain that rate should be minimal at those coverages. Only in the case of the stepped surfaces with 110 step sites is the minimum observed at coverages close to 0.5 (Pt(665) or higher Pt(332)). In the latter case, the potential minimum at short times (high coverages) therefore may be taken as an indication for an instantaneous nucleation, with the steps acting as nucleation centers due to their high catalytic activity. Due to their lower catalytic activity, the 100 sites on the Pt(755) surface do not or less effectively act as nucleation sites. The reason for this is that the oxidation rates at (100) sites and (111) terrace sites do not differ very much, as indicated by the corresponding peak potentials.26 Nevertheless, one would expect the potential minimum at times before Θ ) 0.5 is reached for both Pt(111) and Pt(755). Possible reasons for a potential minimum (or a rate maximum in potentiostatic experiments) at low coverages are the following: 1. In an “ideal” case, where the nucleation takes place only at the steps (also the (111) face has a certain step density due to a small misorientation) and the onedimensional (1D) reaction zone only propagates vertical to the steps, the reaction zone is always parallel to the steps and its length is constant. At constant potential, therefore, the current is constant and only drops when the coverage approaches zero. Such a case has been observed for the dissolution of a Cu UPD monolayer from the Pt(332)(fac) surface.10 In a galvanostatic experiment, this should lead to a constant potential. In a less ideal situation, the 1D reaction zone gets “rough” in time and the rate increases slowly (or the potential decreases, respectively) with decreasing coverage. It is, however, not easily understandable why this is not observed for the (332) and the (665) orientations, and therefore this possibility will not be further discussed. 2. According to a “simple” Langmuir-Hinshelwood mechanism (fast diffusion), the site requirement for the activated complex is higher and therefore the Θ(1 - Θ) term has to be replaced by Θ(1 - Θ)x with x > 1. An electronic repulsion between adsorbed CO and OH would

lead to a similar effect. Such an effect might only be operative on (111) and possibly on (100) sites. Even if it is only operative on (111) sites, surfaces with local (100) step geometry behave similarly to the (111) surface because the oxidation potential is similar and therefore oxidation also occurs on the terraces. On surfaces with (110) steps, oxidation at the steps is predominant. The role of steps may be 3-fold: First, they act as nucleation centers. (Also, in the case of a “simple” Langmuir-Hinshelwood mechanism, the reaction will start at “defects” for an initial coverage of 1.0.) This effect leads to a decrease of the initial potential maximum in the potential transients. Second, steps may offer sites for a reaction path with a lower activation barrier. Third, steps offer sites for preferential OH adsorption (acting in the same way as Ru). Whereas in the second case the reaction rate is still determined by ΘCOΘOH, with ΘOH ∝ (1 - ΘCO), in the third case the rate is given by the product of CO coverage at the terraces and the OH coverage at the steps: ΘtCOΘsOH, where ΘsOH ∝ (1 - ΘsCO). The relations between these coverages are schematically shown in Figure 5. At the beginning of the oxidation, also the steps are completely covered by CO, but this coverage decreases faster than that at the terraces, and therefore the coverage with OH increases faster than at the terraces. The term ΘtCOΘsOH therefore has its maximum (and the potential its minimum) at higher coverages, as observed for Pt(332) and Pt(665). Using cyclic voltammetry and IR spectroscopy,28 we have shown that, using low overall coverages, on Pt(755) CO adsorption at low potentials occurs preferentially at step sites, whereas at high potentials the band for CO at step sites disappears first. Because of the lower oxidation potential, this effect should be larger at steps with local (110) geometry. Also, at Pt(755) we would therefore expect the potential minimum at high coverages. The effect of a larger site requirement or repulsive interaction of OH is also schematically shown in Figure 5b. Here, OH adsorption only increases slowly with

(26) Gomez, R.; Feliu, J. M.; Aldaz, A.; Weaver, M. J. Surf. Sci. 1998, 410, 48-61. (27) Schmickler, W. Interfacial Electrochemistry; Oxford University Press: New York, 1996.

(28) Tillmann, S. Zinn-UPD auf Pt(111), Pt(332) und Pt(755). Elektrochemische und IR-spektroskopische Untersuchung des Einflusses auf die CO-Adsorption und -Oxidation. Dissertation, University of Bonn, 1999.

Figure 5. Schematic of (a) dependence of OH coverage at step sites on CO coverage and (b) influence of repulsive interactions or higher site requirement. The arrows indicate the position of the rate maximum (or potential minimum in galvanostatic experiments).

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decreasing CO coverage. On Pt(755), this effect seems to override the effect of preferred OH adsorption at steps. In this context, also the role of competing sulfate adsorption has to be discussed. On Pt(111) the peak and the spike at 0.45 V of a cyclic voltammogram in sulfuric acid is due to a phase transition to an ordered x3 × x7 adlayer.29 Whereas the phase transition is only observed for large domains and in the case of stepped surfaces only for terrace widths larger than 10 atoms, also for smaller terraces the adsorption peak (without spike) is the more prominent the larger the terraces are. This peak and the spike have also been observed for surfaces with partial coverages of CO, which shows that CO and sulfate form separate domains. Whereas this points to a nucleation and growth mechanism for the oxidation of adsorbed CO, this is not an argument against surface diffusion of CO: the CO in the domains and in particular at the domain boundaries may be mobile as well as the CO domain as a whole. Furthermore, these domains might only form below a critical coverage of CO. Due to this dependence of the sulfate adsorption strength on the size of the domains, the sulfate adsorption strength increases with decreasing CO coverage. On the other hand, the stronger the sulfate adsorption, the more destabilized does the CO get. This effect is less strong on the (332) and the (665) surfaces because of the lower oxidation potential, at which sulfate adsorption is less strong. In control experiments, we also recorded current transients during potentiostatic oxidation of the adsorbed CO on Pt(332)(fac), after cooling in pure Ar. The transients at low oxidation potentials could roughly be fitted by a “simple” Langmuir-Hinshelwood mechanism as in the case of polycrystalline Pt.30 In particular at high oxidation potentials, however, the transients are asymmetric (with the current maximum at times shorter than corresponding to half-coverage) and the fits are not convincing. Therefore, both the galvanostatic and potentiostatic experiments indicate at least a large contribution of a nucleation and growth process or another deviation from the “simple” Langmuir-Hinshelwood description. By decoration of the steps with Ru, they all should become catalytically active. The corresponding transients are shown in Figure 6 for three different current densities. After a short nucleation period, the potential drops to values which are not too different for the surfaces studied. However, a second potential plateau appears after oxidation of approximately half of the adsorbate. In the case of Pt(332) and for low currents, it is even lower than the first plateau. On the contrary, oxidation of the second part of the adsorbate on Pt(665) and Pt(111) requires a much higher overpotential. Obviously, not the kind of step site is important here, but rather the step density and therefore the terrace width. (Of course, on Pt(111) the steps are only introduced by deposition of Ru and the distance between the islands is approximately 4 nm according to ref 5.) The reason for this higher overpotential is the same as that giving rise to a second peak in cyclic voltammetry (cf. Figure 3): the diffusion of CO to the reactive PtRu sites is too slow. In the case of galvanostatic experiments, therefore, the potential shifts to a value where COad at some distance of Ru is also oxidized. From the galvanostatic experiments, it should be possible to evaluate an upper limit of the diffusion (29) Funtikov, A. M.; Stimming, U.; Vogel, R. J. Electroanal. Chem. 1997, 428, 147-153. (30) Richarz, F. Elektrochemisch erzeugte Pt-, Ru- und PtRuElektroden: Kinetik der Elektrooxidation von Kohlenmonoxid. Dissertation, Universita¨t Bonn, 1995.

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Figure 6. Galvanostatic oxidation of adsorbed CO on Rudecorated electrodes in CO-free electrolyte. 0.5 M H2SO4. I(appl): (a) 2.7, (b) 5.27, and (c) 10.5 µA.

coefficient. In order for the second potential plateau to appear, the applied current density has to be larger than the surface diffusion rate. In the case of the Pt(665) surface (with 10 “free” Pt rows in the terraces), this is the case for all applied current densities, even the lowest one. In the case of Pt (332) and Pt(755), a clear second plateau at higher potentials is not visible, despite the distinct shape of the transients. Here, the step density is nearly twice that of the Pt(665); the current density per step is therefore half that of Pt(665). The highest current per step used is 5 times larger than the lowest one applied to the Pt(665) for which the second plateau was still observed. Therefore, one would expect a higher second plateau for the higher current for Pt(332) as well. However, only four rows of Pt atoms are uncovered by Ru, one of which is a step atom. The maximum distance to a Ru atom is only 2 Pt atoms. Therefore, it is not unlikely that these sites are all electronically influenced, leading to a destabilization of CO and a higher diffusion rate at short distances of the Ru atoms. When, as previously done in ref 11, the Pt(332) crystal was cooled in Ar (instead of an H2/Ar mixture), terraces that are twice as large16 and therefore as large as those of the Pt(665) cooled in H2/Ar are formed. The galvanostatic potential transient for such an electrode is shown in Figure 7. The Ru-covered Pt(332)(fac) crystal with such large terraces shows two potential plateaus indeed, in accordance with the two peaks observed in a cyclic voltammogram of that surface.11 Assuming that the diffusion is not fast enough even to maintain the lowest current at the lower potential on Pt(665), from Fick’s first law of diffusion the surface diffusion coefficient can be estimated to be lower than 3 × 10-17 cm2 s-1. This value is much smaller than that estimated by Friedrich et al.5 (10-13-10-12 cm2 s-1) from potentiostatic oxidation of adsorbed CO at 0.5 V at a Rumodified Pt(111) electrode in perchloric acid. As we have shown before for Pt(111) and Pt(332)(fac) (cooled in Ar)

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Figure 7. Galvanostatic oxidation of adsorbed CO on Ru-covered Pt(332)(fac) (s) in CO-free electrolyte. Cooling was done in a pure Ar atmosphere. The dashed line (---) was obtained for the Ru-free Pt(332)(fac). 0.5 M H2SO4 I(appl) 5.27 µA.

covered by approximately 20% Ru11 (albeit in sulfuric acid), around 30% of the adsorbate is not oxidized at the potential of the onset of the main peak in cyclic voltammetry. Our galvanostatic experiments corraborate these results: approximately two-thirds of the adsorbate is oxidized at a lower potential than the residual one-third on a Ru-modified Pt(111) surface, if the distance between the Ru sites on Ru-decorated steps is larger than approximately 2 nm. The galvanostatic experiments reveal these two oxidation potentials in a more reliable manner than cyclic voltammetry, because in the latter the onset potential is mainly determined by the nucleation overpotential. Two explanations can be given for this behavior: According to the first one, only CO in close vicinity of Ru sites is oxidized at low potentials due to the bifunctional mechanism. An electronic effect of Ru extends over a larger distance from the Ru sites, and therefore also the residual adsorbed CO is oxidized at a lower potential compared to a pure Pt surface. Since the amount of CO oxidized at the low potential is higher than one would expect from the number of CO molecules directly or adjacent to Ru sites (in the case of the Pt(665)/Ru surface approximately 30%), one would have to assume a high diffusion rate in the vicinity of the Ru, due to an electronic effect only in the neighborhood of Ru. This is somewhat inconsistent with the electronic effect also for the residual adsorbate, decreasing its oxidation potential. Therefore, we favor the second explanation: here, an apparently site dependent diffusion rate originates from a site dependent adsorption enthalpy; cf. Figure 8. Due to the electronic influence of Ru, the adsorption enthalpy in the neighborhood of Ru is increased (i.e., its absolute value is decreased) also as suggested by thermodesorption experiments in UHV31 and recently by density functional theory calculations.32 This influence extends over all the terrace of the Pt(332) crystal, i.e., over at least four rows of atoms. In the case of the Pt(665) (and the Pt(332)(fac) cooled in Ar), nearly one-third of the adsorption sites are not influenced. Their diffusion to sites in the neighborhood of the steps is then just slowed by the higher adsorption (31) de Mongeot, F. B.; Scherer, M.; Gleich, B.; Kopatzki, E.; Behm, R. J. Surf. Sci. 1998, 411, 249-262. (32) Koper, M. T. M.; Shubina, T. E.; van Santen, R. A. J. Phys. Chem. B 2002, 106, 686-692.

Figure 8. Influence of Ru step decoration on adsorption enthalpy of COad with respect to the reaction coordinate (schematically). (---) Ru-decorated step; (s) Ru-free surface.

enthalpy of these sites. The two peaks in the cyclic voltammogram and the two plateaus in the galvanostatic experiments correspond to these different adsorption sites. Oxidation of those CO molecules adsorbed at sites not influenced by Ru still occurs at somewhat lower potentials than at Ru-free electrodes because the activation barrier is decreased due to the spillover effect of OH from Ru according to the bifunctional mechanism. In a quite recent publication, Hayden and co-workers33 performed an experiment similar to ours11 described above, i.e., the separate oxidation of CO in the first and second oxidation peaks. Their interpretation is that diffusion of CO to the vicinity of Ru is too slow because of sulfate adsorption, and therefore they assume a slow diffusion of adsorbed OH. However, it is not clear why OHad diffusion should not be hindered by adsorbed sulfate. The action of Ru has to be compared to the influence of adsorbed tin as a cocatalyst for CO oxidation:9 there, a minor part of the adsorbate is oxidized at appreciably lower potentials, whereas the main part is oxidized at a nearly unchanged potential. This is not due to a desorption of tin at the higher potentials (suggested by one of the referees): it was confirmed by both galvanostatic experiments and potential steps in cyclic voltammograms. (33) Davies, J. C.; Hayden, B. E.; Pegg, D. J.; Rendall, M. E. Surf. Sci. 2002, 496, 110-120. (34) Clavilier, J.; Rodes, A.; El Achi, K.; Zamakchardi, M. A. J. Electroanal. Chem. 1990, 284, 245-253. (35) Rhodes, A.; Achi, K. E.; Zamakhchari, M. A.; Clavilier, J. J. Electroanal. Chem. 1990, 284, 245-253.

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Although tin does slowly desorb at potentials above 700 mV (from the terraces of the Pt(332)(fac)28), it was still present on the surface after these sweeps. Therefore, an oxygen spillover effect according to the bifunctional mechanism is not operative there, the activation barrier is not lowered (cf. Figure 8), and the only effect is a substantial decrease in the energy of the adsorbed state for adsorption sites close to tin atoms. A more detailed discussion on the role of tin will be given in a forthcoming paper. Conclusion On stepped Pt(111) vicinal surfaces, steps with a local (110) geometry are catalytically more active than those with a local (100) geometry. Since this effect is observed not only in potentiodynamic experiments but also in galvanostatic experiments, the steps do not only influence the nucleation behavior, they also influence the oxidation of all adsorbed CO. This points toward the possibility of surface diffusion of CO. On the other hand, the shape of the galvanostatic potential transients is different from what would be expected from a “simple” LangmuirHinshelwood mechanism. Therefore, rate-limiting surface diffusion and adsorption isotherms including repulsive terms have to be taken into account for a full description of the oxidation rate.

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Ru decoration of the steps shows that Ru acts in a 2-fold manner: (1) An electronic effect leads to an increase in the enthalpy of the adsorbate for adsorption sites in close proximity of the Ru (extending around two atomic rows in both directions of the Ru-decorated step). This CO is therefore oxidized at lower potentials than that adsorbed farther away from Ru. This different adsorption enthalpy also leads to a slow apparent “uphill” diffusion of the CO from the more distant adsorption sites to those sites in the vicinity of Ru. Actually, this is not really a slow diffusion but a low population of the adsorption sites in the vicinity of Ru atoms. (2) The well-known spillover effect of OH adsorbed on Ru according to the bifunctional mechanism leads to a decrease of the main activation barrier for oxidation and thus to a decrease of the oxidation potentials for CO adsorbed on both kinds of sites. This is different from the case of adsorbed tin: there, only a small part of the CO is oxidized at very low potentials due to a repulsive (electronic) interaction. The larger part is oxidized at unchanged potential: we suggest that for tin the oxygen spillover effect is not operative. Acknowledgment. Thanks are due to the DFG for financial support. Stimulating discussions with M. Koper and J. Feliu are gratefully acknowledged. LA011308M