Influence of underpotential deposition of copper with submonolayer coverage on hydrogen adsorption at the stepped surfaces Pt(955), Pt(322) and Pt(544) in sulfuric acid solution

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Journal of ElectroanalyticalChemistry396 (! 995) 139-142

Influence of underpotential deposition of copper with submonolayer coverage on hydrogen adsorption at the stepped surfaces Pt(955), Pt(322) and Pt(544) in sulfuric acid solution 1 C. Nishihara *, H. Nozoye National Institute of Materials and Chemical Research, Tsukuba, lbarakd305, Japan

Received2 February1995;in revisedform26 May 1995

Abstract The effects of coverage by an underpotentially deposited copper submonolayer on hydrogen adsorption were examined for three surfaces ((955), (322) and (544)) of platinum single crystals. A very small amount of copper preferentially blocks the hydrogen adsorption to the (100) step sites. This result supports our previous conclusion (J. Electroanal. Chem., 386 (1995) 75) that the most positive peak of copper underpotential deposition relates to the deposition of copper on the (100) step sites of these three surfaces. Keywords: Platinum;Underpotentialdeposition;Copper; Hydrogenadsorption;Steppedsurfaces

1. Introduction

platinum with a submonolayer coverage of copper, i.e. a platinum surface which is partially covered with upd copper, and showed that hydrogen adsorption to step sites is blocked by small amounts of copper on the surface.

The electrochemical behavior of stepped surfaces of single crystals has been attracting increasing attention recently [1]. We have studied the effect of electrode processes of hydrazine and copper at step sites of platinum single-crystal electrodes [2,3]. Underpotential deposition (upd) of metal is an old topic that is being actively investigated again in this decade. Copper on platinum is one of the most frequently studied examples [3,4]. The pioneering studies by Scortichini and Reilley [5,6] of copper upd on stepped surfaces Pt(331) and Pt(311) coincided with the development of a simple new technique for producing clean well-ordered surfaces of platinum single crystal electrodes [7,8]. Using this technique, Gomez et al. [9] recently investigated copper upd on Pt(311). In our previous paper [3] on copper upd at a series of stepped surfaces [ n ( l l l ) × (100)], we inferred that the most positive peak among the copper upd peaks observed in sulfuric acid solution corresponds to the deposition of copper on (100) step sites. In the work reported here we examined the hydrogen adsorption behavior at a series of stepped surfaces of

Three types of stepped surface were investigated: (955) = 3.5(111) × (100), 2 (322) = 5(111) × (100) and (544) = 9(111) × (100). The instruments and experimental procedures were essentially the same as described previously [2,3], but the electrolytic cell was modified so that the sample solution could be changed while keeping the electrode under argon atmosphere. All potentials are cited versus a reversible hydrogen electrode (RHE). The quantity of electricity was determined by integrating the voltammograms graphically; no correction for the doublelayer charging current was applied. Sulfuric acid solutions (0.5 M) were prepared from Merck Suprapur reagent. A stock solution of Cu 2÷ was prepared from CuSO 4 • 5H20 (99.999%, Aldrich). All the sample solutions were deoxygenated by passing argon through them.

Dedicated to ProfessorMatsuda, ProfessorTamamushiand Professor Honda on the occasionof their 70th birthdays. * Correspondingauthor.

2 The average step structureof the (955) surface is well describedby this nominal notation[3].

0022-0728/95/$09.50 © 1995ElsevierScienceS.A. All rights reserved SSDI 0022-0728(95)04181-8

2. Experimental

C. Nishihara, H. Nozoye/ Journal of Electroanalytical Chemistry 396 (1995) 139-/42

140

A platinum surface with a upd copper monolayer in copper-ion-free 0.5 M H2SO 4 was prepared as follows. The platinum surface was dipped in a deoxygenated solution containing 0.2 mM Cu 2÷ (CuSO 4 . 5H20) and 0.5 M H2SO 4 and its potential was scanned from 0.80 V to a potential slightly more positive than that where the bulk deposition began (scan rate, 1 mV s-l). This potential was 0.30 V for the (544) surface and 0.25 V for the (322) and (955) surfaces. The electrode was held at this potential for several minutes. The electrolytic solution was then replaced by deoxygenated 0.5 M H2SO 4, and the electrode was rinsed by dipping in the H2SO 4. During this operation the electrode was kept connected to the potentiostat (the deposited copper would have been lost if the electrode had been disconnected while it was touching the solution). The washing procedure was repeated twice.

3. Results and discussion Fig. 1 shows the current-potential profile of a Pt(322) surface with a upd copper monolayer in 0.5 M H2SO 4 (copper-ion-free) started from 0.06 V. Hydrogen desorption [10,11] was not observed between 0.06 and 0.30 V on

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E/Vvs. RHE Fig. 1. Cyclic voltammogram of the Pt(322) surface between 0.06 and 0.80 V, starting from 0.06 V, in 0.5 M H2SO4 solution at 20 mV s -~ . The surface was initially covered with one monolayer of copper. The inset shows the cyclic voltammogram of the clean surface in 0.5 M H2SO + at 20 mV s - I .

the first positive sweep. The anodic peaks in the potential range between 0.35 and 0.80 V are caused mostly by the dissolution of the upd copper [3], and the corresponding cathodic currents are caused by the partial redeposition in the vicinity of the electrode surface of the copper ions that had been produced in the preceding positive sweep. As the electrode surface was gradually stripped of the upd copper, the hydrogen adsorption-desorption peaks (0.06-0.35 V) [10,11] became larger. When the upd copper was entirely stripped off the voltammogram was identical with that of the clean surface (inset). As is evident from Fig. 1, the most positive anodic dissolution peak of copper at about 0.74 V began to decrease only in the final stage. In parallel with its disappearance, the pair of sharp peaks at 0.26 V, which are ascribed to hydrogen adsorption-desorption at the step sites [10,11], began to appear. This observation strongly suggests that the most positive anodic peak corresponds to the dissolution of the copper adsorbed at the step sites. Similar behavior was observed for the (955) and (544) surfaces. The effect of the upd copper on the adsorption-desorption of hydrogen can be examined in a more quantitative way by recording voltammograms at an electrode with controlled coverage of upd copper, which was prepared as follows. The upd copper monolayer on the single-crystal electrode with a specified surface, prepared as described in Section 2, was anodically stripped step by step in a copper-ion-free sulfuric acid by using a trapezoidal potential scanning pattern with successively increasing height or a stripping potential, E x (Fig. 2, inset). It was found that the upd copper was stripped quantitatively and a reproducible submonolayer state was attained if the electrode was held at each E x for 100 s or more. This holding time was found to be sufficient for the dissolved copper ions to diffuse away from the electrode surface so that they were not redeposited on the reverse scan. The voltammograms in Fig. 2 were recorded at the anodic branches of the potential scan. Hydrogen desorption becomes more pronounced with increasing stripping potential (curves (a)-(f)), i.e. as the upd copper is stripped. The voltammogram recorded after the stripping at 0.80 V where copper is completely stripped away (curve (f)) is the same as that of the clean (322) surface. The peak at 0.26 V is attributable to the oxidative desorption of hydrogen from the step sites and the broad peak at 0.06-0.35 V is due to the desorption of hydrogen from the terrace sites [11]. Observation of these voltammograms suggests that hydrogen desorption from the terrace sites recovered earlier than that from the step sites. This can be shown more clearly by plotting the quantities of electricity corresponding to each hydrogen desorption against the stripping potential E x. These quantities were determined from the areas under the voltammograms as indicated in Fig. 2, curve (e): the quantity of electricity QH-stcp for desorption from the step sites was determined from the double-hatched area, and the

C. Nishihara, H. Nozoye / Journal of Electroanalytical Chemistry 396 (1995) 139-142

quantity of electricity Qlt-terrace for desorption from the terrace sites was determined from the single-hatched area. In Figs. 3(a), 3(b) and 3(c), QH-,t~p(triangles), QH-tc~ce (circles) and their sum QH (filled squares) are plotted against E~ (for E x < 0.6 V only the sum is plotted because the two peaks cannot be discriminated). These figures also include the plots of Qc~ (open squares), the quantity of electricity corresponding to the areas under the voltammograms of copper UPD (insets) between 0.80 V and E x. This quantity can be regarded as a measure of the amount of UPD copper on the surface even though Qcu does not exactly correspond to the copper coverage [ 12]. Both QH-~t¢p and QH-ten'ace decrease with increasing Qc~; the former decreases faster than the latter. The ratio QH-~tep/QH-te~=e (crosses) indicates this trend more clearly. Plateaus in the ratio observed at E x > 0.7 V reflect the fact that in this potential range the copper coverage increases only slightly. The relation between the copper coverage and the amount of hydrogen adsorbed at step sites, terrace sites and both sites, represented by Qlt-step, QIt-terrace and Qrl, can be shown more clearly by plotting these quantities against Qcu. An example of the results is shown in Fig. 4

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Fig. 3. Plots of Qcu ( n ) , Qrl-slep (A), QH-terrace( Q ) and QH ( I ) against the electrode potential: (a) Pt(955); (b) Pt(322); (c) Pt(544). The ratio QIt-step//Qrl-terrace is shown by crosses. The insets show the voltammograms of copper UPD in 0.2 mM Cu 2+ and 0.5 M H~SO 4 at 0.2 mV s -I . Details are given in the text.

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for (322). The plots of QH versus Qcu are almost linear, as was reported by Adzic et al. [13] for Pt(111) in sulfuric acid, although there was some negative deviation from the straight line at small Qcu values. The degree of deviation depends on the step density. The arrow on the horizontal axis shows the value corresponding to the potential of the minimum current on the voltammogram of the copper upd as described below. With increasing Qcu the step sites are increasingly blocked for hydrogen adsorption, but terrace sites are not blocked so much. The extrapolation of the decreasing portion of the plots for the step sites intersects

C Nishihara, H. Nozoye/ Journal of Electroanalytical Chemistry 396 (1995) 139-142

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sulfate species on Pt(111). Shingaya and Ito [15] suggested the coadsorption of hydrogen sulfate species on the basis of the results of an in-situ IR reflection absorption study of copper upd at a Pt(111) electrode. If some terrace sites are blocked by the bulky hydrogen sulfate species coadsorbed with the copper at the step sites, these blocked terrace sites will not be available for hydrogen adsorption. The decrease of Ql-l-terrac¢ appears to be largest for the (955) surface (Fig. 2(a)) and smallest for the (544) surface (Fig. 2(c)), in accordance with the step density. This observation can be interpreted as an indication of the presence of an effect of coadsorbed species.

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the horizontal axis rather close to the arrow. Similar behavior was observed for the (955) and (544) surfaces. The voltammograms of copper upd consist of three pairs of peaks (see insets to Figs. 3(a), 3(b) and 3(c)), and the most positive pair A was assumed to correspond to the adsorption-desorption of copper at the step sites as discussed earlier [3]. If the assignment of peak A is correct, the terrace sites are almost free of the UPD copper when E x is more positive than the potential of the minimum current between peaks A and B, indicated by the arrows in these figures. Therefore the preferential blockage of hydrogen adsorption at step sites can be explained by the selective adsorption of copper at step sites for the most positive peak A. 3 Varga et al. [14] found, using the radiotracer technique, that upd copper enhanced the adsorption of hydrogen

3 Gomez et ai. [9] have recently measured the copper upd on a Pt(3 i 1) = 2(111) × (100) electrode in 0. I M H 2SO4. They concluded that the most positive peak of copper upd corresponds to reduction-oxidation at the (111) terrace sites and not at the (100) step sites. This conclusion appears to conflict with our observations. However, since Pt(31l) is a highly stepped surface, it may behave differently from our surfaces. For example, the (311) surface is not favorable for hydrogen sulfate species to be adsorbed, in contrast with (955), (322) and (544) surfaces, as found by Markovic et ai. [10] in copper-free sulfuric acid.

Acknowledgement The authors thank Professor Gen P. Sat& Sophia University, for discussions and for help in revising the manuscript.

References [1] R. Parsons and G. Ritzoulis, J. Electroanal. Chem., 318 (1991) i. [2] C. Nishihara, I.A. Raspini, H. Kondoh, H. Shindo, M. Kaise and H. Nozoye, J. Electmanal. Chem., 338 (1992) 299. [3] C. Nishihara and H. Nozoye, J. Electroanal. Chem., 386 (1995) 75. [4] N. Markovic and P.N. Ross, Langmuir, 9 (1993) 580, and references cited therein. [5] C.L. Scortichini and C.N. Reilley, J. Electmanal. Chem., 139 (1982) 247. [6] C.L. Scortichini and C.N. Reiiley, J. Electroanal. Chem., 152 (1983) 255. [7] J. Clavilier, R. Fanre, G. Guinet and R. Durand, J. Electroanai. Chem., 107 (1980) 205. [8] J. Clavilier, J. Electroanal. Chem., 107 (1980) 211. [9] R. Gomez, J.M. Feliu and H.D. Abruna, Langmuir, 10 (1994) 4315. [10] N.M. Markovic, N.S. Marinkovic and R.R. Adzic, J. Electroanal. Chem., 241 (1988) 309. [11] A. Redes, K. El Achi, M.A. Zamakhchari and J. Clavilier, J. Electroanai. Chem., 284 (1990) 245. [12] L.W.H. Leung, T.W. Gregg and D.W. Goodman, Chem. Phys. Lett., 188 (1992) 467. [13] R.R. Adzic, F. Feddrix, B.Z. Nikolic and E. Yeager, J. Electroanal. Chem., 341 (1992) 287. [14] K. Varga, P. Zelenay and A. Wieckowski, J. Electroanai. Chem., 330 (1992) 453. [15] Y. Shingaya and M. Ito, J. Electroanal. Chem., 372 (1994) 283.