Ultrathin Films of Ruthenium on Low Index Platinum Single Crystal

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Langmuir 1997, 13, 5974-5978

Ultrathin Films of Ruthenium on Low Index Platinum Single Crystal Surfaces: An Electrochemical Study W. Chrzanowski and A. Wieckowski* Department of Chemistry, University of Illinois at Urbana-Champaign, Urbana, Illinois 61801 Received February 21, 1997. In Final Form: September 4, 1997X We report on electrochemical properties of ultrathin films of ruthenium on platinum single crystal surfaces, Pt(100), Pt(111) and Pt(110), and demonstrate that such films can be obtained by spontaneous deposition. We also show that the spontaneously deposited ruthenium coverage is surface structure dependent. Using the spontaneous deposition process and a constant potential electrolysis, a variety of Pt/Ru surfaces up to ca. 0.4 monolayer of ruthenium were prepared. All such Ru films are stable in the electrode potential range that precedes platinum oxidation. A strong surface structure effect in the electrochemical properties of these thin films was found. On Pt(100)/Ru at a fixed Ru coverage, there is a transition from a reversible to irreversible surface redox behavior that is not observed on other platinum single crystal faces. In contrast to Pt(100)/Ru and Pt(110)/Ru, the individual voltammetric phases of the Pt(111)/Ru electrode are not resolved, and ruthenium surface oxides appear to be the most stable on the Pt(111) electrode.

Introduction Electrochemical deposition of submonolayer-to-monolayer metal amounts on foreign metal substrates usually belongs to the category of underpotential deposition processes (UPD), namely, to the class of deposition processes occurring at electrode potentials more positive with respect to the potential for bulk deposition.1-4 A typical UPD adlayer is formed when a metal possessing a work-function φ is deposited on a metal substrate of higher work function, frequently a noble metal.5 The formation of such UPD films is usually reversible or quasireversible, and the processes can be modeled on the interfacial thermodynamics basis using, e.g., Monte Carlo simulations.3,4 The films tend to create an array of twodimensional (2D) surface structures that depend on the type of cationic precursor, electrode material, surface crystallography, electrode potential, and anion coadsorption. The UPD films usually dissolve as metal cations without passing through a metal oxide phase (and without causing surface oxidation) after a sufficiently positive electrode potential has been reached. Electrodeposition of submonolayer-to-monolayer amounts of noble metals occurs along different lines. First, the relationship between the work function of the substrate and the deposit may be the inverse of what is needed for the underpotential deposition process to occur.5 In such a casesunless some unique catalytic or additional chemical processes are involvedsthe deposition is limited to bulk deposition. Second, the electrode potential6 for the submonolayer-to-monolayer deposition process may be high enough to cause electrooxidation or electrodissolution of the substrate. Third, the electrooxidation of noble metal films usually does not lead to dissolution; instead, the * Author to whom correspondence should be sent. X Abstract published in Advance ACS Abstracts, October 15, 1997. (1) Kolb, D. M. In Advances in Electrochemistry and Electrochemical Engineering; Gerischer, H., Tobias, C. W., Eds.; Wiley-Interscience: New York, 1978; Vol. 11. (2) Adzic, R. R. In Advances in Electrochemistry and Electrochemical Engineering; Gerischer, H., Ed.; Wiley-Interscience: New York, 1984; Vol. 13. (3) Blum, L.; Huckaby, D. A. J. Chem. Phys. 1991, 94, 6887-6894. Huckaby, D. A.; Blum, L. J. Electroanal. Chem. 1991, 315, 255-261. Langmuir 1995, 11, 4583-4587. (4) Zhang, J.; Sung, Y.-E.; Rikvold, P. A.; Wieckowski, A. J. Chem. Phys. 1996, 104, 5699-5712. (5) Kolb, D. M.; Przasnycki, M.; Gerischer, H. J. Electroanal. Chem. 1974, 54, 25-38. (6) Llopis, J. F.; Tordesillas, I. M. In Encyclopedia of Electrochemistry of Elements; Bard, A. J., Ed.; Marcel Dekker: New York, 1976; Vol. VI, Chapter 8, pp 227-298.

S0743-7463(97)00193-5 CCC: $14.00

films remain on the surface as stable oxides. One of the consequences in this case is that the Me/Men+ system is significantly shifted away from the reversibility that makes the application of interfacial thermodynamics3,4 difficult, if not impossible. Fourth (and finally), when noble metal single crystal surfaces are used as substrates for the noble metal deposits, the long range surface structure characteristic of the UPD films is absent, and three-dimensional islands may form even in the submonolayer deposition range.7-9 The absence of the long range order indicates that an appropriate balance of forces between adsorbate-adsorbate and adsorbate-surfaces needed for the adlattice formationsis not realized.10 Below, to discriminate between the noble metal deposition and underpotential deposition, Goodman’s terminology: “ultrathin metallic films”,11 will be used for the noble metal deposits. The scope of such films extends from a submonolayer to a few monolayers andsbecause of the surface forces involvedsthe films may have different physicochemical properties vs those of the corresponding thick film deposits. The focus of this paper is on ruthenium deposition on platinum. Ruthenium is known to be important catalytic material in hydrogenolysis reactions and in ammonium synthesis12 and is a common additive to the main metal catalysts to affect efficiency and selectivity of a variety of surface reactions.13 Therefore, ruthenium surface chemistry has already been investigated,13,14 although not as intensely as that of platinum or rhodium. Used as a platinum supplement, ruthenium enhances catalytic electrooxidation of methanol, the process of key interest (7) Friedrich, K. A.; Geyzers, K.-P.; Henglein, F.; Marmann; A.; Stimming, U. In Electrode Processes VI; Wieckowski, A., Itaya, K., Eds.; The ECS Proceedings; Electrochemical Society: Pennington, NJ, 1996; Vol. 96-98, pp 119-135. (8) Cappadonia, M.; Schmidberger, J.; Schwegle, W.; Stimming, U. In Electrode Processes VI; Wieckowski, A., Itaya, K., Eds.; The ECS Proceedings; Electrochemical Society: Pennington, NJ, 1996; Vol. 9698, pp 269-275. (9) Aberdam, D.; Razafimaharo, F.; Faure, R.; Kabbai, A.; Durand, R. In Electrode Processes VI; Wieckowski, A., Itaya, K., Eds.; The ECS Proceedings; Electrochemical Society: Pennington, NJ, 1996; Vol 9698, pp 280-290. (10) Sung, Y.-E.; Chrzanowski, W.; Zolfaghari, A.; Jerkiewicz, G.; Wieckowski, A. J. Am. Chem. Soc. 1997, 119, 194-200. (11) Berlowitz, P. J.; Goodman, D. W. Langmuir 1988, 4, 10911095. Leung, L. H.; Gregg, T. W.; Goodman, D. W. Chem. Phys. Lett. 1992, 188, 467-470. (12) Seddon, E. A.; Seddon, K. R. The Chemistry of Ruthenium; Elsevier: Amsterdam, 1984. Seddon, K. R. Platinum Met. Rev. 1996, 40, 128-134.

© 1997 American Chemical Society

Ruthenium Films on Platinum

in fuel cell science and technology.15 Previously, we pointed out that ruthenium submonolayer-to-monolayer coverages on platinum single crystal faces are ideal templates for getting an insight into surface structure effects in Pt/Ru heterogeneous electrocatalysis.16 In the same paper, we investigated methanol electrooxidation processes and found, for the first time, a distinctively different oxidation behavior between the Pt(111)/Ru and Pt(110)/Ru catalysts. Since then, several important studies have been conducted on electrochemistry of ruthenium on platinum single crystal7-9,17 and polycrystalline substrates.18,19 The data obtained by some of the quoted groups will be used in the Results and Discussion section of this paper. Below, we will report on surface structure effects in the electrochemical behavior of submonolayer amounts of ruthenium on platinum in perchloric acid medium. Ruthenium was added to platinum low index surfaces by electrolysis9,16,20 and also by spontaneous deposition. The distinctive surface structure effects in electrochemistry of ruthenium on platinum may have, we believe, major consequences in Pt/Ru electrocatalysis since the surface crystallographic order of platinum particles, as templates for ruthenium deposition, may be adjusted through the particle size and thermal/electrochemical treatment.21,22 Experimental Section Three platinum single crystal faces of ca. 0.2 cm in diameter were used as working electrodes: Pt(111), Pt(110), and Pt(100).16,23,24 The crystals were flame annealed and cooled in nitrogen/hydrogen atmosphere, as previously reported.24,25 The meniscus configuration ensured that other than needed crystallographic faces were not exposed to the electrolytic solutions.24,25 Before ruthenium deposition, platinum surfaces were activated by several cyclic voltammograms (CV) at 50 mV s-1 and stable CV profiles were taken to confirm the cleanliness and order of the surfaces. Ruthenium deposition was next carried out at 300 mV vs a reversible hydrogen electrode, RHE (to which all potentials quoted in this paper are referred) in solutions of commercial RuCl3 in 0.1 M HClO4. The solutions were aged for 2-3 weeks to obtain a stable yellow/orange solution color and reproducible electrolysis data. Using UV-vis spectrophotometry, we found that the stable solution contained aqua complexes of ruthenium, [RuO(H2O)4]2+, in which ruthenium(IV) is present12,26 (13) Sinfelt, J. H. Bimetallic Catalysts; Wiley: New York, 1983. Peden, C. H. F.; Goodman, D. W. J. Catal. 1985, 95, 321-324; 1987, 104, 347358. Rodriguez, J. A.; Goodman, D. W. Surf. Sci. Rep. 1991, 14, 1-107. (14) Yates, J. T., Jr.; Peden, C. H. F.; Houston, J. E.; Goodman, D. W. Surf. Sci. 1985, 160, 37-45. Egawa, C.; Iwasawa, Y. Surf. Sci. 1988, 198, L329. Ko¨tz, R. Appl. Surf. Sci. 1991, 47, 109-114. Kuhn, M.; Rodriguez, J. A. J. Vac. Sci. Technol., A 1995, 13, 1569-1573. Goodman, D. W. J. Phys. Chem. 1986, 100, 13090-13102. (15) Hogarth, M. P.; Christensen, P. A.; Hamnett, A. In New Materials for Fuel Cells Systems 1; Savadogo, O., Ed.; Editions de L’Ecole Polytechnique de Montreal, 1995; pp 310-325 and references therein. (16) Herrero, E.; Franaszczuk, K.; Wieckowski, A. J. Electroanal. Chem. 1993, 361, 269-273. (17) Friedrich, K. A.; Geyzers, K.-P.; Linke, U.; Stimming, U.; Stumper, J. J. Electroanal. Chem. 1996, 402, 123-128. (18) Colom, F.; Gonza´lez-Tejera, M. J. J. Appl. Electrochem. 1994, 24, 426-433. (19) Frelink, T.; Visscher, W.; van Veen, J. A. R. Langmuir 1996, 12, 3702-3708. (20) Watanabe, M.; Motoo, S. J. Electroanal. Chem. 1975, 60, 267273. (21) Lee, W. H.; Vanloon, K. R.; Petrova, V.; Woodhause, J. B.; Lexton, C. M.; Masel, R. I. J. Catal. 1990, 126, 658-670. (22) Palaikis, L.; Wieckowski, A. Catal. Lett. 1989, 3, 143-148 and references therein. (23) The crystals were built in the laboratory of Professor J. Feliu, University of Alicante, Spain. (24) Franaszczuk, K.; Herrero, E.; Zelenay, P.; Wieckowski, A.; Wang J.; Masel, R. I. J. Phys. Chem. 1992, 96, 8508-8516. Herrero, E.; Franaszczuk, K.; Wieckowski, A. J. Phys. Chem. 1994, 98, 5074-5083. Herrero, E.; Chrzanowski, W.; Wieckowski, A. J. Phys. Chem. 1995, 99, 10423-10424. (25) Clavilier, J. J. Electroanal. Chem. 1980, 107, 211-216. (26) Gortsema, P. F.; Cobble, J. W. J. Am. Chem. Soc. 1961, 83, 43174321.

Langmuir, Vol. 13, No. 22, 1997 5975 (this complex compound is further abbreviated as RuO2+). The electrolysis current was integrated to produce ruthenium deposition charges from which ruthenium coverage data were obtained (Results and Discussion). When the experiment with a given Pt/Ru surface ended, ruthenium was removed by several negativeand positive-going scans at 50 mV s-1 in the range terminated by onset potentials of hydrogen and oxygen evolution. For each data point or CV presented in this paper (except for Figure 1 that deals with platinum only24,25) the following experimental cycle was executed: voltammetric ruthenium removal and platinum surface cleaning, platinum flame annealing (ordering), voltammetric characterization of the Pt clean electrode, ruthenium deposition, and voltammetric characterization of the Ru-modified Pt electrode using two subsequent voltammetric cycles in the electrode potential range from 0 to 0.8 V (vs RHE). Reproducibility of cyclic voltammetric curves involving the Ru-covered surfaces (Figures 2 and 4) was better than 5%. Integrated ruthenium deposition charges for a given electrolysis time, used for the preparation of Figure 3, displayed the relative error of 2%. Ruthenium coverage data obtained from the spontaneous deposition (see the Results and Discussion section and Table 1) represent the average of three points. The regression analysis (Figure 3) was carried out at a 95% confidence level. Voltammetric measurements with clean platinum and ruthenium-modified platinum surfaces were conducted in 0.1 M HClO4. Real surface area of the clean Pt electrodes was determined using hydrogen adsorption-desorption charges.24 Two electrochemical cells equipped with Luggin capillaries were used in parallel, and all solutions were protected against air oxygen by ultrahigh purity argon (99.999%). Chemicals were Millipore water (18 MΩ cm) and perchloric acid double distilled from Vycor (GFS). All experiments were carried out at room temperature of 25 ( 1 °C. The electrode potential was controlled via a PAR 273 potentiostat interfaced to the IBM PC/AT computer. Data were collected by the use of HEADSTART and M-270 programs (PAR-EG&G) and were processed using Sigma Plot (Jandel Scientific Software) on a Gateway 2000, 486PC.

Results and Discussion Typical cyclic voltammetric (CV) profiles of the clean, well-ordered platinum single crystal electrodes24,25 in 0.1 M HClO4 are presented in Figure 1. After the voltammetric characterization, such electrodes were ready to be covered by ruthenium using RuO2+ solutions and a constant potential electrolysis, as described in the Experimental Section. However, surprisingly, after a simple immersion of platinum into the RuO2+ solution, we found that a substantial amount of ruthenium was deposited spontaneously. Since the coverage of the spontaneously deposited ruthenium on Pt(100) was the highest (Table 1), we have chosen the Pt(100) electrode to demonstrate the spontaneous deposition process. The details of the experiment were the following. The electrode was first immersed in the RuO2+ solution at open circuit. We found that after a short drift, the open circuit potential (OCP) stabilized at 830 mV. After 60 s of immersion, the electrode was removed from the solution, placed into 0.1 M HClO4 electrolyte, and characterized by voltammetry. As shown in Figure 2, the CV profile is now very much different from that of Figure 1A, including a noticeable suppression of the hydrogen deposition charge. From the hydrogen suppression, and from the observation that the new CV profile is similar to that of the Pt(100)/Ru electrode obtained by electrolysis (see below), we conclude that the voltammetric change is due to the presence of a rutheniumcontaining species spontaneously deposited on the surface. We have made yet another observation from the data in Figure 2. Namely, the first CV cycle (solid line) taken from the OCP deviates toward more negative current from the subsequently stable CV curve (dashed line). This indicates that the spontaneously deposited ruthenium oxide is, at least in part, susceptible to reduction. We propose that RuO2 is spontaneously formed on platinum according to reaction 1

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RuO[(H2O)4]2+ + A- + e f RuOads + Aads + 4H2O

(3)

which would explain the high open circuit potential after the deposition (830 mV, see above). Therefore, after the first potential cycle, the main and apparently stable surface species is Ru(II), abbreviated here as RuOads, coadsorbed with discharged anions, Aads. These data and interpretation give support to the recent quartz microbalance data showing that a Ru(II) oxide/hydroxide, strongly interacting with solution anions, resides on platinum in the double layer range.19 More molecular-level work is however needed to verify this surface oxide assignment that, at this point, is considered tentative. The data obtained with Pt(111) and Pt(110) were qualitatively similar to those obtained with Pt(100) and yielded stable open circuit potentials of 880 mV for Pt(111) and 790 mV for Pt(110). We notice that our observation of the spontaneous formation of noble metal deposits on a noble metal substrate is not unprecedented since palladium has previously been observed to be spontaneously deposited on platinum.27-29 Additional amounts of ruthenium were added to the surfaces by electrolysis (Experimental Section). The procedure to evaluate the ruthenium coverage, θRu, from electrolysis, and from spontaneous deposition, θ0, was as follows. First, the “apparent” ruthenium coverage from the electrolysis, θ(q)′, not containing the spontaneously deposited contribution, was obtained using the formula

θ(q)′ ) Figure 1. Cyclic voltammograms of clean Pt single crystal surfaces at 50 mV s-1 in 0.1 M HClO4 solution: A, Pt(100); B, Pt(110); C, Pt(111).

qNA AnFNPt

where A is electrode surface area, NA is Avogadro’s number, F is the Faraday constant, NPt is the known number of Pt sites per cm2, and q is the electrolysis charge involved in reaction

RuO2+ + 2H+ + 4e f Pt-Ru0 + H2O

Figure 2. Cyclic voltammogram of a Pt(100) electrode in 0.1 M HClO4 after spontaneous deposition of ruthenium in 5 × 10-5 M RuCl3 + 0.1 M HClO4 (RuO2+) solution during 60 s. First scan from an open circuit potential is shown by the solid line (see START for the negative-going scan beginning). Stable CV profile is depicted by the dashed line. Sweep rate: 50 mV s-1.

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

(1)

and that RuO2ads is involved in the reduction process leading to RuOads formation

RuO2ads + 2H+ + 2e f RuOads + H2O

(2)

However, the Ru(II) oxide may also be formed directly upon ruthenium deposition

(4)

(5)

The number of electrons in reaction 5, n ) 4, was chosen because (i) RuO2+ is present in electrolytic bath containing 0.1 M perchloric acid (see above) and (ii) the evidence is already available from a quartz microbalance study that at 300 mV vs RHE, that is, under the reported conditions for electrolysis, metallic ruthenium is present on platinum.19 The value of q was measured in several experiments using different electrolysis times. Second, cyclic voltammetric curves of the three Pt(hkl) surfaces electrolytically covered by ruthenium were analyzed to identify a voltammetric feature, defined as ∆, that was clearly responding to ruthenium addition. For instance, with Pt(100), ∆ is the difference between the height of hydrogen stripping peak at 220 mV for clean Pt(100) and the height at the same potential for the Ru-treated (100) surface (µA). Third, ∆ was plotted as a function of the apparent θ(q)′ (Figure 3). As shown in Figure 3A for 5 × 10-5 M RuO2+ solution (as well as for an auxiliary 5 × 10-6 M of RuO2+ solution in Figure 3B, see below) a straight line of ∆ vs θ(q)′ was found. This line does not pass through the origin and yields θ(t)′ at ∆ ) 0 defined as θ0′ (a negative value). θ0, equal to -θ0′, is the coverage of the spontane(27) Attard, G. A.; Bannister, A. J. Electroanal. Chem. 1991, 300, 467-485. Attard, G. A.; Price, R. In Surface Science Electrochemistry; Trasatti, S., Wandelt, K., Eds.; Elsevier: Amsterdam, 1995; pp 63-74. (28) Clavilier, J.; Llorca, M. J.; Feliu, J. M.; Aldaz, A. J. Electroanal. Chem. 1991, 310, 429-435. Llorca, M. J.; Feliu, J. M.; Aldaz, A.; Clavilier, J. J. Electroanal. Chem. 1993, 351, 299-319. Go´mez, R.; Rodes, A.; Pe´rez, J. M.; Feliu, J. M.; Aldaz, A. Surf. Sci. 1995, 327, 202-215; 1995, 344, 85-97. (29) Llorca, M. J.; Feliu, J. M.; Aldaz, A.; Clavier, J. J. Electroanal. Chem. 1994, 376, 151-160.

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Figure 3. (A) Plot of ∆ (µA) versus apparent ruthenium coverage, θ(q)′, obtained from eq 2 for the Pt(100) electrode in 5 × 10-5 M RuO2+. (B) Plot of ∆ (µA) versus apparent ruthenium coverage, θ(q)′, for Pt(100) in 5 × 10-6 M RuO2+. ∆ is the difference in the voltammetric peak currents at 220 mV for hydrogen oxidation measured with the clean and Ru-covered electrode, respectively (see text). Added are ∆ values found for the spontaneously deposited ruthenium in two solutions: the filled large square for 5 × 10-5 M RuO2+ and the hollow large square for 5 × 10-6 M RuO2+. (C) Plot of ∆ (µA) versus total ruthenium coverage, θRu (including the coverage of spontaneous Ru deposit, see text). Data are presented at 95% confidence level. Table 1. Coverage of Spontaneously Deposited Ruthenium, θ0, on Three Platinum Single Crystal Surfaces Immersed in 5 × 10-5 M RuCl3 + 0.1 M HClO4 (RuO2+ Solution) for 60 sa face of Pt(hkl) electrodes

coverage, θ0 (Ru atom/Pt site)

Pt(100) Pt(110) Pt(111)

0.24 ( 0.04 0.05 ( 0.03 0.10 ( 0.02

a

Data are presented at 95% confidence level.

ously deposited ruthenium, and the total coverage is θRu ) θ0 + θ(q). The final plot, ∆ vs θRu, is presented in Figure 3C. (Notice that the data corresponding to the spontaneous deposition are indicated by large squares.) The data in Table 1 show that the coverage of the spontaneously deposited ruthenium (θ0, as defined above) strongly depends on platinum geometry and is very high for Pt(100). Of course, θRu and θ0 are related to the number of sites taken by ruthenium oxide and the anions coadsorbed with the Ru species (reaction 3). The components of this complicated adlayer are now being investigated by Auger electron spectroscopy and related techniques.30 The quantitative procedure described above is correct provided that the Ru layer obtained by spontaneous deposition and electrolysis is sufficiently homogeneous (30) Thomas, S.; Sung, Y.-E.; Kim, S. H.; Wieckowski, A. J. Phys. Chem. 1996, 100, 11726-11735.

under all measuring conditions of ∆. Although the previous scanning tunneling microscopy and synchrotron X-ray surface diffraction data in the submonolayer range, θRu < 0.6, testify to such a homogeneity (ruthenium is deposited as monoatomic islands7-9), the detailed conditions between the experiments from several laboratories may not be identical. However, a strong argument in favor of our procedure is that ∆ for the spontaneous deposition is equal to the intercept of the straight line at θ(q)′ ) 0, and ∆ - θ(t)′ pairs (large squares) are excluded from regression. This is unlikely if the spontaneously obtained deposit was different than that obtained from the electrolysis (the properties of the Ru deposits obtained in the two processes, spontaneous deposition and electrolysis, are brought close to each other by the short CV treatment, see Experimental Section). Also, the data presented in Figure 3B for 5 × 10-6 M of RuO2 indicate that ∆ for the spontaneous deposit is consistently lower than in 5 × 10-5 M of RuO2 (compare the data depicted by large squares, parts A and B of Figure 3). Finally, in this experiment (Figure 3B), when the electrolysis charge is referred to the spontaneously deposited ruthenium reference, the data fit to the straight line obtained from the experiment with the more concentrated RuO2+ solution (Figure 3C). As verified by numerous voltammetric tests, the ruthenium films are stable in the electrode potential range investigated (Figures 2 and 4). The dissolution of ruthenium adlayers was observed when the positive potential bias was shifted to more positive values, especially to the beginning of oxygen evolution, which was accompanied by the well-known disorder of platinum single crystal surfaces.31 Unless used for the voltammetric removal of ruthenium, as explained in the Experimental Section, the potentials higher than ca. 0.9 V were avoided. Having been able to prepare the Ru-covered Pt(hkl) surfaces in a broad coverage range, we began the voltammetric characterization of such surfaces, and the data are shown in Figure 4. Inspection of these data shows major differences in the CV current-potential profiles at all potentials. Focusing on the electrode potential range of interest to this studyswhere ruthenium oxidation/ reduction takes place (roughly from 300 to 800 mV)swe may notice the following: 1. On Pt(100)/Ru,9 near the ruthenium oxidation threshold, but at potentials lower that 0.6 V, a reversible oxidation/reduction behavior was found (shaded areas in Figure 4A). The reversible peak is followed by two other voltammetric features that have a common, more cathodic reductive component, at 500 mV, that does not increase with the increase in ruthenium coverage. This shows that the two CV peaks correspond to irreversible ruthenium oxidation reactions,32 or the Ru oxide(s) once formed at E > 0.6 V cannot be reduced under the voltammetric conditions of this work. Along with the increase in ruthenium coverage, the charge under the first CV peak is reduced and that under the consecutive more anodic peaks increases (Figure 4D). Further, when ruthenium coverage increases, the reversible CV peak loses resolution and asymptotically merges into the current-potential curve that is dominated by the irreversible oxidation processes. (The last CV peak in Figure 4A is a remnant of the clean Pt(100) behavior; cf. the data at Figure 1A at 700 mV.) 2. On Pt(110)/Ru,16 two broad oxidation/reduction redox systems are observed, both symmetrically arranged (31) Wieckowski, A.; Rosasco, S. D.; Schardt, B. C.; Stickney, J. L.; Hubbard, A. T. Inorg. Chem. 1984, 23, 565-569. Wieckowski, A.; Schardt, B. C.; Rosasco, S. D.; Stickney, J. L.; Hubbard, A. T. Surf. Sci. 1984, 146, 115-125. (32) Conway, B. E. Prog. Surf. Sci. 1995, 49, 331-452.

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in the double layer range of platinum, ruthenium exists in the form of a Ru(II) oxide/hydroxide (that strongly interacts with solution anions) instead of the previously postulated Ru(OH)20 or RuO2 species.33,34 However, since the disagreement between the published ruthenium oxidation state data is significant, an independent confirmation of the most recent (quartz microbalance) results may be needed. Only when the nature of ruthenium oxides on platinum is known can a more quantitative treatment of data such as those presented in this paper be unambiguously afforded, and the fundamental understanding of ruthenium adlayers on the Pt(hkl) substrates may be obtained. Conclusions

Figure 4. Cyclic voltammograms of the Ru-covered Pt single crystal electrodes at ca. 0.3 monolayer of Ru in 0.1 M HClO4 solution: A, Pt(100)/Ru; B, Pt(110)/Ru; C, Pt(111)/Ru; D, the voltammetric morphology of Pt(100) with increasing Ru coverage; solid line, 0.24 monolayer from spontaneous deposition; dashed line, 0.27; dotted line, 0.32 monolayer (all from electrolysis). Shaded area in A highlights the surface redox reversibility for Pt(100)/Ru below 0.6 V. Sweep rate was 50 mV s-1.

around the E-axis, showing a substantial reversibility in the full potential range examined. The distribution of the film redox states (the CV morphology) does not depend on ruthenium coverage and provides evidence that the structure obtained at low coverage replicates itself at high coverage. 3. On the Pt(111)/Ru electrode, as in the case of Pt(110)/ Ru, the CV morphology does not depend on ruthenium coverage (Figure 4C). However, in contrast to the voltammetry with two other surfaces, the Pt(111)/Ru CV profile is genuinely devoid of any resolution of the film redox processes. Since the current is much higher than the one measured in the absence of ruthenium (cf. Figure 1C), the electrode is electroactive, that is, there is a pseudocapacitive, not only capacitive charging. Notably, we have found no evidence that the film redox reactions ended before the hydrogen adsorption began (Figure 4C). Such a tenacious ruthenium oxide/hydroxide is unique to the Pt(111)/Ru system. (The oxidation feature between 580 and 800 mV (Figure 4C) coincides with the “butterfly”25 position of clean Pt(111) and may be considered as a remnant of the clean surface behavior.) Qualitatively, the data discussed above indicate that the Ru films have different redox properties on these Pt single crystal electrodes. However, and overall, the results open a list of issues that need to be addressed in further research by molecule sensitive techniques. It is not yet known, for instance, why there is such a dependence of the voltammetric behavior of the Ru films on the surface structure as presented above. Since film redox reactions including oxygen transfer are involved, there may be a connection between the ruthenium oxidation propensity and ruthenium surface crystallography through the binding sites available for water adsorption, that is, the source of oxygen needed for the oxidation. Nor it is clear why the coverage of the spontaneously deposited ruthenium depends on platinum surface geometry. On the basis of quartz microbalance data19 it is presently believed that

This paper belongs to a recently published series of articles7-9,16,17,29,35,36 that may indicate a change in emphasis from the underpotential deposition research to studies of thin films of noble metals on noble metal single crystal substrates. Such noble metal films are very reactive to interfacial organic molecules and may be used as versatile models of real-world heterogeneous catalysts. The new focus requires a clear identification of the electrochemical component of the overall physical events as they relate to the substrate surface structure. In this study, we have provided the description of such events on several Pt/Ru single crystal electrodes. Interestingly, to our knowledge, the results on ruthenium added to single crystal platinum substrates by vacuum deposition have not been reported. Electrochemistry has a clear head start16 to exploit properties of such systems in both fundamentally and applied oriented electrochemical surface science. Specifically in this report we have presented the first observation of the spontaneously deposited ruthenium and its surface structure specificity. We have also shown that the oxidation process involving submonolayer ruthenium deposits is strongly surface structure sensitive. The surface structure effects are both qualitative (e.g., different distribution of voltammetric current-potential peaks from one surface to another) and quantitative (e.g., different uptake of the spontaneously adsorbed ruthenium). Apparently, the surface crystallographic effects are as important in ruthenium deposition on platinum as in the investigations of the UPD films. Some other observations, e.g., the unique development of the voltammetric current with Pt(111)/Ru at the negative end of the ruthenium oxidation range have also been highlighted thatswe believeswas not given appropriate attention before. Since ruthenium oxide (hydroxide) is instrumental in accelerated oxidation of organic molecules such as, for instance, carbon monoxide chemisorbed on platinum,24 the Pt(111)/ Ru catalyst may be capable of an enhanced CO removal at the low potentials as compared to other Pt/Ru surfaces. Among several others, this aspect of the Pt/Ru/solution interfaces is being investigated by this group. Acknowledgment. This work is supported by the National Science Foundation, under Grant No. CHE 9411184. Assistance by Hongsun Kim in the manuscript preparation is appreciated. LA970193C (33) Goodenough, J. B.; Hamnett, A.; Kennedy, B. J.; Manoharan, R.; Weeks, S. A. J. Electroanal. Chem. 1988, 240, 133-145. Hamnett, A.; Kennedy, B. J.; Wagner, F. E. J. Catal. 1990, 124, 30-40. (34) Hable, C. T.; Wrighton, M. S. Langmuir 1993, 9, 3284-3290. (35) Han, M.; Mrozek, P.; Wieckowski, A. Phys. Rev. B 1993, 48, 8329-8335. (36) Baldauf, M.; Kolb, D. J. Phys. Chem. 1996, 100, 11375-11381.