Langmuir 2002, 18, 6969-6975
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Surface Characterization and Electrochemical Behavior of Well-Defined Pt-Pd{111} Single-Crystal Surfaces: A Comparative Study Using Pt{111} and Palladium-Modified Pt{111} Electrodes T. J. Schmidt, N. M. Markovic, V. Stamenkovic, and P. N. Ross, Jr. Lawrence Berkeley National Laboratory, Materials Sciences Division, the University of California, Berkeley, California 94720
G. A. Attard* and D. J. Watson Department of Chemistry, Cardiff University, P.O. Box 912, Cardiff CF10 3TB, United Kingdom Received January 11, 2002. In Final Form: May 3, 2002 Two Pd-Pt{111} single-crystal alloy surfaces have been prepared with 6.25% and 25% Pd bulk composition. Their surface structure and composition have been characterized both in ultrahigh vacuum (UHV) using low-energy electron diffraction (LEED), Auger electron spectroscopy (AES), and low-energy ion scattering (LEIS) and electrochemically using cyclic voltammetry (CV). Comparison has been made with the voltammetric response of Pt{111} and Pt{111} modified with a monolayer of palladium [Pd-Pt{111}]. The alloy crystals when clean and thermally annealed in UHV gave rise to sharp (1 × 1) LEED patterns. AES and LEIS revealed that a surface enrichment in palladium took place in agreement with an earlier study using polycrystalline Pd-Pt alloy samples. Gentle ion etching of the thermally equilibrated surface (to remove the selvedge region) followed by AES and LEIS analysis established that the bulk composition was in precise agreement with the nominal bulk composition expected on the basis of the amount of palladium introduced into the melt for the 25% alloy, but a slightly higher value was determined for the 6.25% alloy. Nonetheless, the veracity of the Clavilier method in producing high-quality single-crystal Pt-Pd alloy bead electrodes is confirmed. Electrochemical studies of the alloy surfaces in perchloric acid, perchloric acid containing chloride ions, and aqueous acidic copper chloride revealed a systematic and smooth gradation of voltammetric response when compared with results for Pt{111} and Pd-Pt{111}. For example, CV features associated with “Pt-like” and “Pd-like” regions can be identified readily and a scaling in their intensity can be related rather straightforwardly to the concentration of palladium in the surface.
Introduction The need to understand the key structural parameters governing the electrocatalytic behavior of metal surfaces continues to provide a strong impetus toward fundamental studies of electrosorption.1 To this end, systematic variation of surface crystallography2 and controlled modification of surface composition by adatoms3,4 have often been employed to delineate these electrocatalytic trends. However, very few electrocatalytic studies of bimetallic alloy single crystals have been reported.5,6 Since electronic perturbations associated with alloy phases are almost certainly connected with several high-performance catalysts,7 this relative lack of fundamental experimental data * To whom correspondence should be addressed. E-mail: attard@ cardiff.ac.uk. (1) Wieckowski, A. Interfacial Electrochemistry. Theory, Experiment, and Applications; Marcel Dekker: New York, 1999. (2) Clavilier, J.; El-Achi, K.; Rodes, A.; Zamakhchari, M. A. J. Electroanal. Chem. 1990, 245, 284. (3) Adzic, R. R. In Advances in Electrochemistry and Electrochemical Engineering; Gerischer, H., Tobias, C. W., Eds.; Wiley: New York, 1984; p 159. (4) Clavilier, J.; Feliu, J. M.; Aldaz, A. J. Electroanal. Chem. 1988, 243, 419. (5) Gasteiger, H. A.; Markovic, N. M.; Ross, P. N. J. Phys. Chem. 1995, 99, 8945. (6) Gasteiger, H. A.; Markovic, N. M.; Ross, P. N., Jr. Catal. Lett. 1996, 36, 1. (7) Sinfelt, J. M. Bimetallic Catalysts: Discoveries, Concepts and Applications; Wiley: New York, 1983.
constitutes a significant weakness on the part of the surface science approach to electrochemistry. To improve this situation, we have embarked upon a research program, which utilizes a simple modification of the Clavilier method of single-crystal manufacture8 to prepare a series of well-defined bimetallic bead electrodes. In the first of these studies, the enantioselective response of a Pd-Pt{643} alloy electrode was measured using the electro-oxidation of D- and L-glucose as the chiral probe reaction.9 Although a positive outcome to these experiments was found, the work lacked a fundamental appraisal of the bulk and surface compositions of the electrodes used. Since such data are crucial to any theoretical interpretation of the electrocatalytic activity of an electrode surface, the present study is designed to remedy this situation by providing a detailed surface structural and compositional analysis of two Pt-Pd{111} alloy bead electrodes. Future work will then focus on the electrocatalytic properties of these electrodes, for example, in their propensity toward methanol oxidation. A combination of low-energy electron diffraction (LEED), Auger electron spectroscopy (AES), and low-energy ion scattering (LEIS) in conjunction with a dedicated electrochemical-ultrahigh vacuum (UHV) transfer system has been utilized in this context. In addition, the voltammetric behavior of the alloy bead (8) Clavilier, J.; Faure, R.; Guinet, G.; Durand, R. J. Electroanal. Chem. 1980, 107, 205. (9) Watson, D. J.; Attard, G. A. Electrochim. Acta 2001, 46, 3157.
10.1021/la025521+ CCC: $22.00 © 2002 American Chemical Society Published on Web 07/30/2002
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electrodes in the presence and the absence of specifically adsorbed anions is reported together with the first copper underpotential deposition (UPD) results on such a welldefined bulk bimetallic system. Numerous electrochemical and surface science studies of thin palladium films deposited on platinum single crystals10-19 are reported in the literature due to the fact that the growth mode of palladium on Pt{111} is pseudomorphic and high-quality palladium films may be prepared. As pointed out in refs 17, 20, and 21, the use of epitaxial thin palladium films also avoids the complications that may arise when studying hydrogen chemisorption in that bulk absorption of hydrogen may be avoided. Palladium is also catalytically active like platinum and is closely related chemically and physically, being in the same group of the periodic table and exhibiting an almost identical crystal lattice constant. Some fundamental questions arising from these previous studies of palladium adsorption on platinum in relation to bulk alloys would include the following: (i) Does hydrogen absorb into bulk Pt-Pd alloys? (ii) Does palladium form a continuous solution phase in the surface of the alloy or does it segregate into palladium islands? (iii) Are palladium monolayer islands, which are deposited on Pt{111}, electronically similar to palladium atoms in the alloy phase? (iv) Is surface segregation of the alloy components occurring, and if so, which metal segregates and to what extent? The present study will attempt to answer all of these questions and hopefully illustrate that it is quite feasible to electronically “tune” the electrocatalytic behavior of a platinum electrode by systematic introduction of a second metal into the bulk phase. Experimental Section The preparation of the Pt-Pd alloy crystals has been described previously.9 Briefly, a platinum single-crystal Clavilier bead is prepared by careful melting and cooling of the end of a 0.5 mm diameter platinum wire (99.999%, Goodfellow Metals). After a check for crystal imperfections using a laser mounted on an optical bench, the lower half of the bead crystal is remelted and an amount of pure palladium wire (99.999%, Goodfellow Metals) is introduced into the melt sufficient to provide the nominal bulk composition required. Melting and careful cooling of the whole of the Pt-Pd mixture then leads to the formation of a facecentered cubic (fcc) Pt-Pd single crystal as measured by diffraction of He-Ne laser light from the edges of the {111} and {100} microfacets formed on the surface of the bead. After alignment, grinding, and polishing with diamond pastes (down to 1/4 micron), the mirror surface of the hemispherical crystal is flame-annealed for 1 h to remove surface imperfections and then allowed to cool. The Pt{111} (a disk 4 mm thick and 6 mm in diameter) and Pt-Pd{111} bead electrodes were then flame(10) Clavilier, J.; Llorca, M. J.; Feliu, J. M.; Aldaz, A. J. Electroanal. Chem. 1991, 310, 429. (11) Feliu, J. M.; Clavilier, J.; Llorca, M. J.; Aldaz, A. J. Electroanal. Chem. 1993, 351, 29. (12) Inukai, J.; Ito, M. J. Electroanal. Chem. 1993, 358, 307. (13) Han, M.; Mrozek, P.; Wieckowski, A. Phys. Rev. B 1993, 48, 8329. (14) Attard, G. A.; Price, R.; Al-Akl, A. Electrochim. Acta 1994, 39, 152. (15) Attard, G. A.; Price, R. Surf. Sci. 1995, 335, 63. (16) Baldauf, M.; Kolb, D. M. J. Phys. Chem. 1996, 100, 11375. (17) Markovic, N. M.; Lucas, C. A.; Climent, V.; Stamenkovic, V.; Ross, P. N. Surf. Sci. 2000, 465, 103. (18) Alvarez, B.; Climent, V.; Rodes, A.; Feliu, J. M. J. Electroanal. Chem. 2001, 497, 125. (19) Climent, V.; Markovic, N. M.; Ross, P. N. J. Phys. Chem. B 2000, 104, 3116. (20) Attard, G. A.; Bannister, A. J. Electroanal. Chem. 1991, 300, 467. (21) Baldauf, M.; Kolb, D. M. Electrochim. Acta 1993, 38, 2145.
Schmidt et al. annealed and cooled in a H2 + Ar atmosphere. After that, the surface was protected with a droplet of ultrapure water and transferred to the electrochemical cell forming a meniscus contact with the electrolyte. Palladium films were deposited via continuous potential cycling from a ca. 10-5 M PdO solution in 0.05 M sulfuric acid at 50 mV/s. The amount deposited was controlled by the continuous change of the voltammetric features from those characteristic of Pt{111} to those of a pseudomorphic monolayer of palladium.17,19 After completion of a monolayer of palladium, the electrode was rinsed with water and transferred to a second electrochemical cell (thermostated at 293 K) containing a solution free of Pd2+ ions and immersed at -0.1 V. Solutions were prepared from sulfuric acid and perchloric acid (Baker Ultrex) and PdO (Alfa Products) employing pyrolytically triply distilled water. The voltammetry was recorded in a solution deaerated by Ar (Bay Air Gas, research grade). Potentials were measured against a saturated calomel electrode (SCE) separated by a bridge from the working electrode compartment. The Cl-- and Cu2+-containing perchloric acid was prepared from HCl (EM Science, suprapure) and CuSO4 (EM Science). For the copper UPD measurements, the electrodes were immersed at 0.6 V. The rapid transfer UHV-electrochemistry system has been described previously.22 After flame-annealing, cooling in hydrogen, and protection of the active electrode surface with a droplet of ultrapure water, the alloy beads were placed on a specially engineered molybdenum holder and transferred rapidly to the UHV chamber (base pressure ) 10-10 mbar) equipped with a Phi Instruments double-pass cylindrical mirror analyzer (CMA) detector for subsequent surface analysis. Negligible contamination from carbonaceous material could be detected after transfer (see later for AES data), attesting to the cleanliness of the transfer procedure. The surface coverage of Pt was estimated from lowenergy ion scattering data using a 1 keV helium ion beam energy with sample currents from 5 to 100 nA. The scattering angle was 127°, and the incidence angle was 45°. The ion beam was rastered over a 3 × 3 mm area, and the time for recording a spectrum was 1 min. Coverages were calculated using the equation23
xPt,surface )
sPt/PdI exp,Pt sPt/PdI exp,Pt + I exp,Pd
(1)
with sPt/Pd equal to the ratio of the LEIS peak height from pure Pt to the LEIS peak height from pure Pd, Iexp,Pt equal to the LEIS peak height of Pt in the alloy, and Iexp,Pd equal to the LEIS peak height of Pd in the alloy. All subsequent AES data were calibrated using the LEIS results.
Results and Discussion (a) LEED, AES, and LEIS. Figure 1 shows an example of the quality of the LEED patterns obtained from one of the clean, thermally annealed Pd-Pt{111} alloy surfaces. They demonstrate that both surfaces were well-ordered, giving rise to sharp diffraction spots and negligible diffuse scattering. This confirms that the experimental procedure, based on a modified version of the Clavilier method of crystal manufacture, is robust and produces high-quality crystal surfaces. A (1 × 1) hexagonal pattern was obtained for all temperatures between 293 and 900 K. Hence, formation of ordered surface alloy superstructures was not observed in agreement with predictions based on the bulk phase diagram of the Pd-Pt system24 whereby both metals are reported to form a continuous series of solid solutions. Low-energy ion scattering data, however, indicated that the selvedge was enriched in palladium after both thermal equilibration in UHV and cooling in (22) Gasteiger, H. A.; Ross, P. N.; Cairns, E. J. Surf. Sci. 1993, 293, 67. (23) Niehus, H.; Heiland, W.; Taglauer, E. Surf. Sci. Rep. 1993, 17, 213. (24) Pearson, W. B. Handbook of Lattice Spacings and Structures of Metals and Alloys; Pergamon Press: Oxford, 1958.
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Figure 1. LEED pattern obtained for the clean and thermally annealed Pd-Pt{111}-6.25% Pd surface (55 eV). The 25% Pd alloy gave rise to an identical (1 × 1) LEED pattern.
Figure 3. (a) LEIS data from the sputtered Pd-Pt{111}-25% Pd crystal (constant Pd and Pd peak intensity obtained). (b) The corresponding Auger spectrum.
Figure 2. (a) LEIS data from the clean and thermally annealed Pd-Pt{111}-25% surface. (b) Auger spectrum from the same sample.
a hydrogen atmosphere, in agreement with earlier results for polycrystalline Pt-Pd alloy samples by Kuijers et al.25 Figure 2a shows the LEIS spectrum obtained from Pt-Pd(111)-25% Pd after flame annealing and cooling in hydrogen. Using the experimental peak heights determined from Figure 2a in eq 1, a surface concentration of 44% Pd is evaluated. The Auger spectrum from the same sample is reported in Figure 2b. No other Auger transitions other than those of Pd and Pt could be discerned from the spectrum. To confirm that surface segregation had occurred, gentle sputtering of the surface was continued (25) Kuijers, F. J.; Tieman, B. M.; Ponec, V. Surf. Sci. 1978, 75, 657.
until the LEIS signals from Pt and Pd surface atoms stabilized. When this stage was reached, a surface concentration of palladium of 25% was determined in agreement with the nominal bulk composition (Figure 3a). Figure 3b shows the Auger spectrum collected after sputtering, and it is evident that here, also, the palladium 330 eV peak has diminished in size relative to that shown in Figure 2b. We emphasize that this effect is not due to preferential sputtering of palladium relative to platinum since if this were the case, one would expect a continuous decrease in palladium signal. This did not occur. Therefore, surface enrichment by palladium in the thermally annealed surface is confirmed. Whether the second layer of the alloy was enriched in platinum in order to balance the surface concentration of palladium in the top layer could not be determined at this juncture. However, in a future publication, it will be demonstrated that electrochemical stripping of first-layer palladium from a Pt-Pd{110} alloy clearly reveals an almost pure Pt{110} surface underneath.26 In addition, it will be shown later from cyclic voltammetry (CV) measurements of palladium adatoms on Pt{111} that palladium-palladium interactions are quite distinct from those for platinum-palladium and that the likelihood of the second layer of the alloy also being enriched in palladium is low. In fact, recent work by Radosavkic using surface core level shifts on Pt-Pd{111} alloys27 confirms not only the findings of the present study in relation to palladium surface segregation but also that the second layer is indeed enriched in platinum. This oscillatory surface compositional behavior has already (26) Watson, D. J.; Attard, G. A. In preparation. (27) Radosavkic, D. Private communication, 2001.
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Figure 4. LEIS data (a) from the clean and thermally annealed Pd-Pt{111}-6.25% Pd surface and (b) after sputtering to constant Pt and Pd peak intensity.
been reported in several other metal alloy systems,28 for example, for the Pt-Ni and Pt-Co alloy systems.29,30 Since palladium and platinum atoms are practically the same size and so lattice strain should be minimal, we speculate that the lower surface energy of palladium compared to platinum31 is the driver for the segregation response observed. Figure 4a shows LEIS data for the Pt-Pd{111}-6.25% alloy. In this case, a surface layer concentration of 15% Pd is determined consistent with expectations based on the previous analysis of the Pt-Pd{111}-25% palladium data. Again, gentle sputtering reveals a gradual decrease in palladium surface concentration relative to that of platinum until a steady state is reached at 10% Pd (Figure 4b). That this figure is somewhat higher than the nominal bulk composition of 6.25% Pd may be a reflection of the difficulty in deconvoluting the small Pd peak from the “tail” of the much larger Pt signal. However, consideration of the Auger data (Figure 5) for the sputtered and unsputtered surface also indicates that the surface coverage after sputtering is slightly greater than 6.25%. The ratio of the Auger peak intensities for the Pt 237 eV and Pd 330 eV peak intensities in Figure 3b is given by
IPt,237eV ) 1.53 IPd,330eV For the thermally annealed 6.25% alloy, the corresponding (28) Polak, M.; Rubinovich, L. Surf. Sci. Rep. 2000, 38, 127. (29) Gauthier, Y. Surf. Rev. Lett. 1996, 3, 1663. (30) Vasiliev, M. A. J. Phys. D: Appl. Phys. 1997, 30, 3027.
Schmidt et al.
Figure 5. Auger spectra of the Pd-Pt{111}-6.25% alloy for (a) thermally annealed and (b) sputtered samples.
ratio is
IPt,237eV ) 2.20 IPd,330eV Therefore, if the thermally annealed surface corresponds to a palladium surface coverage of 15%, then the sputtered surface, according to the Auger, should give rise to a surface palladium coverage of
1.53 × 15% ) 10.4% 2.20 Hence, a bulk palladium concentration of about 10% is indicated for this crystal. The reason for the discrepancy between this value and the nominal bulk composition is as yet unknown, although some error is expected using eq 1 due to discrepancies when using sensitivity factors based on clean surface data from the pure metals. Nonetheless, this effect should be small. (b) Cyclic Voltammetric Data. Figure 6 shows cyclic voltammograms of the two alloy {111} surfaces in 0.1 M perchloric acid together with those of Pt{111} and Pt{111} modified by a monolayer (ML) of palladium. The systematic addition of palladium to form the alloy has the effect of gradually modifying the voltammetric response of Pt{111} such that intensity is lost from the so-called “butterfly” peak in the double layer leading to broadening (31) de Boer, F. R.; Boom, R.; Mattens, W. C. M.; Miedema, A. R.; Niessen, A. K. Cohesion and Structure; North-Holland: Amsterdam, 1988.
Well-Defined Pt-Pd{111} Single-Crystal Surfaces
Figure 6. Base voltammetry of (a) Pt{111}, (b) Pd-Pt{111}6.25%, (c) Pd-Pt{111}-25%, and (d) Pt{111}-1 ML Pd. 50 mV/ s, 0.1 M HClO4, 293 K. The vertical dotted lines highlight the shifts in potential alluded to in the text.
of this feature, attenuation of the sharp “spike” at 0.49 V, and a slight shift in the spike potential to more negative values. For the palladium adlayers on Pt{111}, both the H UPD peak multiplet and the peak in the double layer at 0.38V appear at more negative potentials relative to the corresponding peaks in either of the alloys. Hence, increasing quantities of palladium in the surface lead to systematic changes in the voltammetry and the appearance of sharp, intense H UPD peaks at increasingly more negative potentials. That many of these electrosorption features are the result of competitive coadsorption of anions/oxygenated species/hydrogen has often been reported in the literature (see ref 32 and references therein). In terms of the general theme of the present paper, one may suggest that the slightly more negative value of potential of zero total charge (pztc) of palladium relative to platinum37 may be controlling the onset potential of many of these features and that as a consequence, the (32) Markovic, N. M.; Ross, P. N., Jr. Surf. Sci. Rep., in press.
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Figure 7. Voltammetry (solid lines) in 0.1 M HClO4 + 10-3 M Cl- of (a) Pt{111}, (b) Pd-Pt{111}-6.25%, (c) Pd-Pt{111}-25%, and (d) Pt{111}-1 ML Pd. The base cyclic voltammograms in pure 0.1 M HClO4 (dashed lines) are shown as a reference. 50 mV/s, 293 K. The vertical dotted lines highlight the potential shifts alluded to in the text.
balance of competition between double-layer species is modified strongly. The systematic nature of these changes may also be gleaned from inspection of the oxide peak at 0.75 V (SCE) which, although known not to introduce defects into the Pt{111} surface, does give rise to substantial irreversibility with the majority of desorption taking place in the butterfly peak potential region. As palladium alloy composition is increased, this feature becomes much more reversible. Again, continuous potential cycling between 0.8 and -0.25 V did not lead to changes in the alloy cyclic voltammograms, indicating a lack of structural transformation in the surface of the electrode, as found for Pt{111}. (c) Specific Adsorption of Chloride Anions. Since perchlorate anions are believed to be weakly adsorbed on metal surfaces, it was thought interesting to examine the H UPD and double-layer region of the two alloy electrodes in the presence of a specifically adsorbing anion such as chloride. Figure 7 depicts the changes observed when chloride is added to the perchloric acid electrolyte. For Pt{111}, the butterfly peak is immediately quenched,
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Figure 8. (a) Plot of palladium peak potential versus surface coverage obtained using LEIS. (b) Plot of surface coverage obtained using LEIS versus palladium CV peak area in chloridecontaining solution.
consistent with the much stronger interaction of chloride with the electrode surface relative to water/hydroxide/ perchlorate in this potential range. The hydrogen UPD region is hardly altered by chloride, although a small peak at 0.0 V has appeared together with a very broad feature in the double layer between 0.45 and 0.05 V. Presumably, overlap of this chloride-induced feature with the H UPD region leads to the generation of the peak at 0.0 V. It is interesting that similar behavior is observed for the more strongly interacting bromide anion although in this case the peak at the onset of the H UPD region is more intense and shifted to more negative potentials.33 In what follows, it is suggested that it is the stronger interaction of chloride with palladium relative to platinum which leads to analogous behavior in the alloys and palladium-modified surfaces; that is, the attenuation in the butterfly region and the growth in the electrosorption peak at approximately 0.0 V are linked to specific adsorption/ desorption of anions and the overlap of this process with H UPD desorption/adsorption. The sharp H UPD peaks of the palladium adlayer on Pt{111} in sulfuric acid are almost certainly a manifestation of increased anion interaction with palladium as compared to platinum.18,19 The greater the amount of palladium in the surface, the stronger the interaction of chloride with the surface and the greater the shift to more negative potentials of the H UPD peak and the increase in its intensity. Moreover, by a combination of the surface coverage data for the flameannealed surface obtained from the LEIS results and the charge density determined from the area of the peak situated between -0.15 and 0 V and the peak potential, an almost linear relationship is obtained (Figure 8). Hence, voltammetry is highly sensitive to palladium surface coverage. However, is it sensitive to the distribution of (33) Gasteiger, H. A.; Markovic, N. M.; Ross, P. N. Langmuir 1996, 12, 1414.
Figure 9. Voltammetry in 0.1 M HClO4 + 10-3 M Cl- + 10-3 M Cu2+ of (a) Pt{111}, (b) Pd-Pt{111}-6.25%, (c) Pd-Pt{111}25%, and (d) Pt{111}-1 ML Pd. 10 mV/s, 293 K. The vertical dotted lines highlight the potential shifts alluded to in the text.
palladium? From inspection of the palladium adlayer results, the electrochemistry would suggest that islands of palladium are not forming in the surface of the alloy electrodes since from Figures 6 and 7, palladium islands give rise to sharp, intense, and narrow peaks at relatively more negative potentials than the alloys. Although secondand third-layer palladium on Pt{111} is reported to give rise to slightly broader peaks at more positive potentials,11,14 they are still not as broad as those for the alloy. The presence of a modified butterfly peak in all alloys tested would also preclude the possibility of the formation of monolayers of palladium segregating as islands at the surface. Therefore, our model for the alloy surface, taking into account the bulk phase diagram of Pt-Pd and the complete miscibility of both alloy components, would be of a randomly substituted surface alloy enriched in palladium in the top surface layer but possibly depleted in palladium in the second. Increasing amounts of palladium in the alloy surface gradually transform “Ptlike” behavior (butterfly peak, relatively “flat” H UPD region) into “Pd-like” regions, these giving rise to a much reduced butterfly peak, reversible oxide adsorption at 0.75
Well-Defined Pt-Pd{111} Single-Crystal Surfaces
V, and an increase in both total charge and peak definition in the H UPD region. The final comment to make concerning Figures 6 and 7 would be the complete absence of hydrogen absorption into the bulk which would be observed for pure palladium under these conditions. Clearly, a palladium bulk concentration much greater than 25% is required to support such behavior. The idea of tuning the electronic properties of the alloy electrodes by changing the bulk composition (the major theme of the present study) is reflected in gradual shifts in the potentials of electrosorption peaks relative to those of pure Pt{111}. (d) Copper Underpotential Deposition. UPD whereby the deposition of one metal upon another is strongly influenced by both the structure and composition of the substrate34 is also expected to show systematic trends as the platinum-palladium alloy structure is varied. Figure 9 shows a comparison of copper UPD in the presence of chloride anions on going from pure Pt{111} via the alloys to the palladium monolayer adsorbed on Pt{111}. One of the key differences between Pt{111} and Pd-Pt{111} (and Pd{111} also35) so far as copper UPD is concerned is that in the former, the process of metal deposition takes place in two distinct stages whereas for the latter it takes place in a single step. This has been noted previously and has been rationalized in terms of the strength of interaction of anions, those anions tending to interact most strongly with the surface generally giving rise to a single UPD peak.36 This is clearly reflected in Figure 9 in that as the surface becomes more enriched in palladium, not only does the potential separation of the two UPD peaks narrow, but the initial copper deposition peak becomes much broader relative to that of Pt{111} although the total charge under both peaks changes very little (charge values for this peak: Pt(111), ca. 265 µC cm-2; PtPd(111)-6.25%, ca. 280 µC cm-2; Pt-Pd(111)-25%, ca. 270 (34) Wang, J. X.; Adzic, R. R.; Ocko, B. M. In Interfacial Electrochemistry. Theory, Fundamentals and Applications; Wieckowski, A., Ed.; Marcel Dekker: New York, 1999. (35) Rigano, P. M.; Mayer, C.; Chierchie, T. Electrochim. Acta 1990, 35, 1189. (36) Al-Akl, A.; Attard, G. A. J. Phys. Chem. B 1997, 101, 4597. (37) Attard, G. A.; Ahmadi, A. J. Electroanal. Chem. 1995, 389, 175.
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µC cm-2). For the two alloys and Pt{111}, the second UPD peak remains sharp and its potential almost constant (the single copper UPD peak for the palladium monolayer has been reported previously as has the apparent stripping of some of the palladium adlayer to reveal clean regions of Pt{111} (broad feature between 0.5 and 0.25 V)).36 Once again, the absence of characteristics attributable to either pure Pd{111} or a Pd monolayer on Pt{111} in the copper UPD exhibited by the alloy electrodes appears to indicate a lack of palladium island formation in the surface. Conclusions A combination of electrochemical and UHV methods has been used to establish the validity of the Clavilier method of crystal manufacture to prepare well-defined Pt-Pd alloy single-crystal surfaces. The top layer of surface atoms is perfectly ordered after flame-annealing and found to be enriched in palladium, although after mild ion etching of the selvedge, the nominal bulk concentration of the crystal is obtained. A smooth gradation in the electrochemical response of the {111} alloy surfaces relative to those of Pt{111} and a Pd-Pt{111} monolayer is found. The stronger interaction of anions with palladium in comparison to platinum is reflected in the quenching of butterfly peak intensity together with an enhancement in H UPD charge and the formation of sharper H UPD peaks. The intensity of the H UPD peak associated with palladium is strongly correlated with surface coverages of palladium determined using LEIS and Auger spectroscopy. The tuning of the electrocatalytic properties of platinum in, for example, methanol oxidation by alloying with palladium will be the subject of future work. Acknowledgment. This work was supported by the Assistant Secretary for Conservation and Renewable Energy, Office of Transportation Technologies, Electric and Hybrid Propulsion Division of the U.S. Department of Energy, under Contract No. DE-AC03-76SF00098. G.A.A. also thanks the EPSRC for financial support (Grant Number M65724) and a studentship to D.W. LA025521+
