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Voltammograms of stepped and kinked stepped surfaces of palladium: Pd(S)-[n(111)×(100)] and Pd(S)-[n(100)×(110)]
Journal of Electroanalytical Chemistry 521 (2002) 155– 160 www.elsevier.com/locate/jelechem
Voltammograms of stepped and kinked stepped surfaces of palladium: Pd(S)-[n(111)×(100)] and Pd(S)-[n(100) ×(110)] Nagahiro Hoshi *, Makiko Kuroda, Yoshio Hori Department of Applied Chemistry, Faculty of Engineering, Chiba Uni6ersity, 1 -33, Yayoi-cho, Inage-ku, Chiba 263 -8522, Japan Received 21 November 2001; accepted 6 January 2002
Abstract Voltammograms of stepped and kinked stepped surfaces of Pd (Pd(S)-[n(111)× (100)] and Pd(S)-[n(100) ×(110)]) were measured in 0.5 M H2SO4. Both series gave redox peaks around 0.25 V (RHE) for which the intensity decreases with the decrease of the terrace width in 0.5 M H2SO4. No peak was observed around 0.25 V in 0.1 M HClO4. In the oxide film formation region, Pd(S)-[n(111) × (100)] and Pd(S)-[n(100)× (110)] electrodes gave single anodic peaks at 1.1 and 0.9 V (RHE), respectively. The peak intensity diminished with the decrease of the terrace width. Both series presented no peak for which the intensity was enhanced with the increase of step atom density. Charges of the anodic peaks equaled those calculated on the assumption that a monolayer of the oxide film (Pd–OH or Pd2O) covers the entire terrace. © 2002 Elsevier Science B.V. All rights reserved. Keywords: Pd(S)-[n(111) × (100)]; Pd(S)-[n(100)× (110)]; Voltammetry; Sulfuric acid anion; Adsorption; Nano-structure
1. Introduction Electrochemistry using well-defined single crystal electrodes has the potentiality for revealing the nanostructures that enhance the activity and selectivity of catalytic reactions. Hydrogen adsorption and oxide film formation are fundamental reactions in electrochemistry, and voltammetry is one of the convenient methods for studying the reactions. After Clavilier reported a simple method for preparing Pt single crystals, many papers demonstrated that Pt single crystal electrodes give voltammograms characteristic of their orientations [1 – 11], and voltammetric peaks were discussed in relation to hydrogen adsorption sites and anion adsorption. A series of voltammograms was also studied on Ir single crystals [12 –14]. Pt and Ir single crystal electrodes provide ‘finger print’ voltammograms of the surface structures, especially in H2SO4 solutions. Although the catalytic activity of Pd is as high as that of Pt, voltammetric studies on Pd single crystal electrodes are few and limited to low index planes, since preparation of a Pd single crystal is difficult using * Corresponding author. Tel.: + 81-43-290-3384; fax: +81-43-2903401. E-mail address: [email protected] (N. Hoshi).
Clavilier’s method. Solomun first reported voltammograms of a Pd single crystal electrode: the Pd(100) electrode gives a sharp anodic peak at 0.9 V (RHE) in H2SO4 solution, and the adsorbates were measured using X-ray photoelectron spectroscopy (XPS) and low energy electron diffraction (LEED) [15 –17]. Sashikata and co-workers [18,19] measured voltammograms of Pd low index planes in H2SO4. They found that the peak potentials of the oxide film formation depend on crystal orientations, and studied the anodic dissolution of I/Pd electrodes using in-situ scanning tunneling microscopy (STM) [18]. STM measurement also reveals that adsorbed sulfuric acid anion forms a 3 × 7 superstructure on a Pd(111) electrode between 0.5 and 0.9 V (RHE) [19]. Some papers also reported the adsorbates and electrochemical reactions on Pd low index planes. Soriaga and coworkers studied surface structures of bare and iodine modified Pd low index planes using LEED [20 – 23]. Adsorbed CO on Pd single crystal electrodes was examined using infrared reflection absorption spectroscopy (IRAS) [24 –26]. Electrochemical reduction of CO2 was studied on Pd low index planes: the reduction rates remarkably depend on the crystal orientation [27]. Ordered structures of Pd were studied on Pd deposited Au(111) and Au(100) electrodes [28 –30]. No paper,
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however, has reported the electrochemical reaction on high index planes of Pd. Our previous paper reported voltammograms of high index planes of Pd, Pd(S)-[n(111) ×(111)] and Pd(S)[n(100)×(111)], in 0.5H2SO4 [31]. The terrace and step structures affect the voltammograms in the ‘adsorbed hydrogen region’ and oxide film formation region significantly. In this paper, we extend the study to stepped Pd(S)-[n(111)× (100)] and kinked-stepped Pd(S)[n(100)×(110)] surfaces.
2. Experimental A spherical Pd single crystal (3 mm in diameter) was prepared from Pd wire (1 mm in diameter, 99.99% purity) according to Clavilier et al. [1]. The procedure is described in detail in our previous paper [31]. The crystal was oriented using the reflection beams of a He –Ne laser from the (111) and (100) facets [32], and then mechanically polished with diamond slurries down to 0.125 mm. The single crystal electrode was annealed in a H2 +O2 flame at about 1300 °C and cooled in an
Fig. 1. Voltammograms of Pd(S)-[n(111)× (100)] electrodes in (a) 0.5 M H2SO4 and (b) 0.1 M HClO4 solutions saturated with Ar. Scanning rate is 0.020 V s − 1.
Fig. 2. Charge of the cathodic peak (Q) at 0.25 V plotted against the density of atoms at the first layer (dF) on Pd(S)-[n(111)×(100)]. The results of Pd(S)-[n(111) × (111)] electrodes are reproduced for comparison [31]. The inset shows the atoms at the first layer (gray circles).
Ar atmosphere (99.9999%) before each voltammetric measurement. The surfaces examined were the following: Pd(S)-[n(111)×(100)]: Pd(111), Pd(544) (n = 9), Pd(322) (n = 5), Pd(211) (n= 3), Pd(311) (n =2) Pd(S)-[n(100)× (110)]: Pd(100), Pd(910) (n= 9), Pd(510) (n= 5), Pd(310) (n = 3), Pd(210) (n=2). The electrolytic solutions were prepared with suprapur grade chemicals (Merck) and ultrapure water treated with Milli Q low TOC (Millipore). All the potentials were referred to RHE. Electrochemical measurements were conducted at room temperature.
3. Results and discussion
3.1. Pd(S) -[n(111)×(100)] series 3.1.1. Potentials between 0.15 and 0.55 V Fig. 1(a) shows voltammograms of Pd(S)-[n(111)× (100)] electrodes in 0.5 M H2SO4. Surfaces with wide terraces give sharp redox peaks around 0.25 V, as is shown by the Pd(S)-[n(111)× (111)] series [31]. The peak intensity diminishes with the decrease of the terrace width. Voltammograms in 0.1 M HClO4, in which the anion will not be adsorbed in this potential range, do not present redox peaks around 0.25 V (Fig. 1(b)). These results strongly suggest that the redox peaks around 0.25 V are closely related to the desorption and 2− the adsorption of sulfuric acid anion (HSO− 4 or SO4 ). Fig. 2 presents the cathodic charge of the redox peak (Q) plotted against the density of atoms at the first layer (dF). We define circles edged in gray in the inset of Fig. 2 as atoms at the first layer, since sulfuric acid anion will not be adsorbed at the bottom of the step because of the steric hindrance. The value of Q is calculated from the gray area in Fig. 1(a) on the assumption that the background current of the cathodic peak changes linearly with the potential. Voltammograms in HClO4 solution cannot be used as the background in H2SO4 solution, since adsorbed sulfuric acid anion may affect the background current [31]. The plot in Fig. 2 gives a linear line, supporting the fact that the cathodic peak at 0.25 V is strongly associated with the sulfuric acid anion desorbed from the atoms at the first layer. The slope of the line of Pd(S)-[n(111)× (100)] is lower than that of Pd(S)-[n(111)× (111)]. Assuming that all the charge of the cathodic peak around 0.25 V originates from desorption of sulfuric acid anion, we estimate the coverage of the anion giving the peak at 0.25 V (q) from the following equation: q=(Q/zqe)/dF
(1)
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Fig. 3. Estimated coverage of adsorbed sulfuric acid anion (q) plotted against the density of atoms at the first layer (dF).
Fig. 4. Voltammograms of Pd(S)-[n(111)× (100)] electrodes in 0.5 M H2SO4 saturated with Ar. Scanning rate is 0.020 V s − 1.
where z is the number of transferred electrons per 2− sulfuric acid anion (1 for HSO− 4 , 2 for SO4 ), and qe is the elementary electron charge. In the case of Pd(111), Q= 1.2× 102 mC cm − 2. According to Eq. (1), adsorbed HSO− gives q =0.50, whereas adsorbed SO24 − gives 4 q = 0.25. STM measurement reveals the superstructure and coverage of the species that are stably adsorbed on surfaces. Sulfuric acid anion adsorbed stably on Pd(111) forms the 3 × 7 superstructure according to in-situ STM [19], giving a coverage of 0.22, which is close to q calculated from SO24 − adsorption. On the other hand, it is not plausible that all the adsorbed species are HSO− 4 . From the analogy of Pt(111) [33– 38], Pd(111) will adsorb sulfuric acid anion with 3-fold geometry which occupies three Pd atoms per anion, leading to a full coverage of 0.33. The q calculated from
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HSO− 4 adsorption (0.50) exceeds this value. Thus, we presume that SO24 − adsorbed stably on Pd(S)-[n(111)× (100)] gives peaks at 0.25 V. Fig. 3 shows q plotted against dF. The values of q on the Pd(S)-[n(111)× (100)] series are higher than those on the Pd(S)-[n(111)× (111)] series. This result can be explained using the following model. Pd(311) (= 2(111)−(100)) gives a small peak at 0.25 V in Fig. 1, whereas Pt(110) (=2(111)−(111)) produces no peak according to our previous report [31]. These facts demonstrate that sulfuric acid anion adsorbed on the (100) step of Pd gives a small peak, but the (111) step does not. The Pd(S)-[n(111)×(100)] series may adsorb sulfuric acid anion stably at the step as well as the terrace, thus the values of q on Pd(S)-[n(111)×(100)] are higher than those of Pd(S)-[n(111)× (111)]. Although Pd(110) gives no peak at 0.25 V, Pd(110) probably adsorbs sulfuric acid anion; IRAS studies show that Pt(110) [39], Rh(110) [40] and Au(110) [41] adsorb sulfuric acid anion. Desorption and adsorption of sulfuric acid anion may produce no voltammetric peak on Pd(110), as is the case of I− adsorption in the double layer region of Au(111) [42–44]. IRAS measurement and determination of the coverage of sulfuric acid anion are necessary to reveal the origin of the redox peaks at 0.25 V.
3.1.2. Potentials between 0.5 and 1.4 V Fig. 4 shows voltammograms of Pd(S)-[n(111)× (100)] electrodes in the oxide film formation region. A single anodic peak appears at 1.1 V on the surfaces with wide terraces. The peak intensity diminishes with the decrease of the terrace atomic rows, and finally disappears on Pd(311) (n=2) for which the first layer is composed of only step atoms. These facts suggest that the anodic peak originates from oxide film (Pd–OH or Pd2O) formation on the terrace. Pd(S)-[n(111)×(111)] electrodes gave an additional anodic peak at 0.9 V for which the intensity is enhanced with the increase of the step atom density, whereas the Pd(S)-[n(111)× (100)] series produces no peak from the oxide film formation on the step. The charge of the anodic peak (QOX) is calculated on the assumption that the background current changes linearly as shown in Fig. 4. QOX is plotted against the terrace atom density in Fig. 5. The plots give a straight line. Fig. 5 also depicts QOX calculated on the assumption that a monolayer of Pd–OH or Pd2O occupies the entire terrace site of Pd(S)-[n(111)× (100)] surfaces. Both plots provide nearly identical lines, supporting the fact that a monolayer oxide film covers the terrace of Pd(S)-[n(111)× (100)] surfaces at 1.1 V. The other anodic part of the voltammograms above 0.8 V may be due to the oxidation of Pd at the step and the second layer [18], or further oxidation of Pd (PdO, PdSO4, PdO(HSO4)) [17]. The absence of the anodic peak due to the step will be discussed in Section 3.3.
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Adsorbed sulfuric acid anion might be desorbed from the surface in the oxide film formation region, reducing the apparent anodic charge of the oxide film formation due to the cathodic current from anion desorption. However, a study of XPS shows that sulfuric acid anion is still adsorbed on the surface in the oxide film formation region [15]. Our experimental QOX almost equals the calculated QOX (Fig. 5). This result supports the fact that sulfuric acid anion is adsorbed with the oxide film of Pd(S)-[n(111) ×(100)] series.
Fig. 8. Estimated coverage of adsorbed sulfuric acid anion (q) plotted against the density of atoms at the first layer (dF) on Pd(S)-[n(100) × (110)] and Pd(S)-[n(100) ×(111)] electrodes.
3.2. Pd(S) -[n(100)×(110)] series
Fig. 5. Charge of the anodic peak at 1.1 V (QOX) plotted against terrace atom density on Pd(S)-[n(111)×(100)] electrodes. Also shown are the values of QOX calculated on the assumption that a monolayer of oxide film occupies the entire terrace. The inset shows the terrace (gray circles) and step atoms (black circles).
Fig. 6. Voltammograms of Pd(S)-[n(100)× (110)] electrodes in (a) 0.5 M H2SO4 and (b) 0.1 M HClO4 solutions saturated with Ar. Scanning rate is 0.020 V s − 1.
Fig. 7. Charge of the cathodic peak (Q) at 0.25 V plotted against the density of atoms at the first layer (dF) on kinked-step Pd(S)-[n(100) × (110)] electrodes. The results of stepped Pd(S)-[n(100)× (111)] electrodes are reproduced for comparison [31]. The inset shows the atoms at the first layer (gray circles).
3.2.1. Potentials between 0.15 and 0.55 V Pd(S)-[n(100)×(110)] electrodes give cathodic and anodic peaks at 0.25 and 0.30 V in 0.5 M H2SO4, respectively (Fig. 6). The peak intensity diminishes with the decrease of the terrace atomic rows. The voltammograms show no peak in 0.1 M HClO4. These results are identical with those on the Pd(S)-[n(100)×(111)] series reported previously [31]. The charge of the cathodic peak (Q) is plotted against the density of atoms at the first layer (dF) in Fig. 7, giving almost a straight line. The plots of Pd(S)-[n(100)× (110)] electrodes provide almost the same line as those of Pd(S)-[n(100)× (111)], except Pd(210) (= 2(100)− (110)) for which dF is the lowest. This difference arises from the fact that Pd(311) (= 2(100)− (111)) gives a small peak at 0.25 V, but Pd(210) (=2(100)−(110)) gives no peak. The (100) terrace of Pd(311) has enough space to adsorb sulfuric acid anion stably. In the case of Pd(210), however, kinked atoms protrude into the (100) terrace, possibly diminishing the stability of adsorbed sulfuric acid anion. The estimated coverage of sulfuric acid anion (q) is derived from Eq. (1) assuming SO24 − adsorption. The values of q are plotted against dF in Fig. 8. The plots of the Pd(S)-[n(100)×(110)] and Pd(S)-[n(100)× (111)] series are fitted well with the same straight lines. This result suggests that both Pd(S)-[n(100)× (110)] and Pd(S)-[n(100)× (111)] series adsorb sulfuric acid anion on a (100) terrace with the same geometry. 3.2.2. Potentials between 0.5 and 1.4 V Pd(S)-[n(100)× (110)] electrodes give a single anodic peak at 0.9 V in the oxide film formation region (Fig. 9). The peak intensity diminishes with the decrease of the terrace atomic rows. No peak due to oxide film formation on the step is observed, as is the case of the Pd(S)-[n(100)×(111)] series [31]. The charge of the anodic peak (QOX) is plotted against the terrace atom density in Fig. 10. The plots of Pd(S)-[n(100)×(110)] and Pd(S)-[n(100)×(111)] electrodes provide almost
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monolayer of Pd–OH or Pd2O occupies the entire terrace of the Pd(S)-[n(100)× (110)] and Pd(S)[n(100)× (111)] surfaces.
3.3. Structural effect on the oxide film formation
Fig. 9. Voltammograms of Pd(S)-[n(100)×(110)] electrodes in the oxide film formation region in 0.5 M H2SO4.
Fig. 10. Anodic charge at 0.9 V (QOX) plotted against terrace atom density on Pd(S)-[n(100)× (110)] electrodes. Results of Pd(S)[n(100) ×(111)] electrodes are also shown for comparison. The inset shows the terrace (gray circles) and kink atoms (black circles) on the Pd(S)-[n(100) ×(110)] surface.
the same linear lines. The values of QOX are almost identical with those calculated on the assumption that a
Our previous paper reported that Pd(S)-[n(111)× (111)] electrodes give two anodic peaks in the voltammograms in the oxide film formation region: the peak potential from steps (0.93 V) is lower than that from terraces (1.1 V) [31]. This fact shows that (111) steps of Pd(S)-[n(111)×(111)] surfaces have a higher activity than (111) terraces for the oxide film formation reaction (Pd +H2O Pd –OH +H+ + e, or 2Pd+H2O Pd2O+ 2H+ + 2e). The other series (Pd(S)-[n(100)× (111)], Pd(S)-[n(111)×(100)], Pd(S)-[n(100)×(110)]), however, give no anodic peak of oxide film formation at steps. These facts suggest that the step sites of the other series produce oxide film at a very much lower rate. Pd(S)-[n(100)× (111)] and Pd(S)-[n(100)×(110)] have the same step notation as Pd(S)-[n(111)×(111)] (= Pd(S)-[(n − 1)(111)×(110)]), but the activity for the oxide film formation is very low. Hard sphere models illustrate the difference of the step structures of these surfaces in Fig. 11. Pd(S)-[n(111)×(111)] surfaces have a ‘stepped’ (110) structure on which step atoms align linearly along the step line. Pd(S)-[n(100)× (111)] surfaces have a (111) step, but do not contain the (110) structure. Pd(S)-[n(100)×(110)] surfaces comprise (110) structure, but its ‘kinked-step’ atoms are protruded, so that they do not form a linear step line. These models suggest that the stepped (110) structure is the most favorable site for oxide film formation of Pd.
4. Conclusion 1. Both Pd(S)-[n(111)× (100)] and Pd(S)-[n(100)× (110)] series give redox peaks around 0.25 V in the voltammograms in 0.5 M H2SO4, in which the sulfuric 2− acid anion (HSO− 4 or SO4 ) is strongly adsorbed on the surfaces. The charges of the peaks decrease with the
Fig. 11. Hard sphere models of Pd(S)-[n(100)× (110)], Pd(S)-[n(111) ×(111)] and Pd(S)-[n(100) × (111)] surfaces.
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decrease of the terrace width. In 0.1 M HClO4 in which no anion is adsorbed on the electrodes, the voltammograms do not give such redox peaks. 2. Voltammograms of Pd(S)-[n(111) × (100)] and Pd(S)-[n(100)× (110)] electrodes provide single anodic peaks at 1.1 and 0.9 V, respectively. The charges of the anodic peaks are nearly identical to those calculated on the assumption that a monolayer of the oxide film (Pd –OH or Pd2O) occupies the entire terrace. Both series give no peak due to oxide film formation at the step.
[15] [16] [17] [18] [19] [20] [21] [22] [23] [24]
Acknowledgements This study was supported by grants-in-aid for Scientific Research of the Ministry of Education, No. 11118214 and No. 12650805.
References
[25] [26] [27] [28] [29] [30] [31]
[1] J. Clavilier, R. Faure, G. Guinet, R. Durand, J. Electroanal. Chem. 107 (1980) 205. [2] S. Motoo, N. Furuya, Ber. Bunsenges Phys. Chem. 91 (1987) 457. [3] N.M. Markovic´ , N.S. Marinkovic´ , R.R. Adzˇ ic´ , J. Electroanal. Chem. 241 (1988) 309. [4] N. Furuya, S. Koide, Surf. Sci. 220 (1989) 18. [5] J. Clavilier, K. El Achi, A. Rodes, J. Electroanal. Chem. 272 (1989) 253. [6] J. Clavilier, K. El Achi, A. Rodes, Chem. Phys. 141 (1990) 1. [7] A. Rodes, K. El Achi, M.A. Zamakhchari, J. Clavilier, J. Electroanal. Chem. 284 (1990) 245. [8] N.M. Markovic´ , N.S. Marinkovic´ , R.R. Adzˇ ic´ , J. Electroanal. Chem. 314 (1991) 289. [9] P.N. Ross, J. Chim. Phys. 88 (1991) 1353. [10] J. Clavilier, A. Rodes, K. El Achi, M.A. Zamakhchari, J. Chim. Phys. 88 (1991) 1291. [11] J.M. Feliu, A. Rodes, J.M. Orts, J. Clavilier, Polish J. Phys. Chem. 68 (1994) 1575. [12] S. Motoo, N. Furuya, J. Electroanal. Chem. 167 (1984) 309. [13] S. Motoo, N. Furuya, J. Electroanal. Chem. 181 (1984) 301. [14] N. Furuya, S. Koide, Surf. Sci. 226 (1990) 221.
[32] [33] [34] [35] [36] [37] [38] [39] [40] [41] [42] [43] [44]
T. Solomun, J. Electroanal. Chem. 217 (1987) 435. T. Solomun, J. Electroanal. Chem. 255 (1988) 163. T. Solomun, J. Electroanal. Chem. 302 (1991) 31. K. Sashikata, Y. Matsui, K. Itaya, M.P. Soriaga, J. Phys. Chem. 100 (1996) 20027. L.-J. Wan, T. Suzuki, K. Sashikata, J. Okada, J. Inukai, K. Itaya, J. Electroanal. Chem. 484 (2000) 189. G.J. Cali, G.M. Berry, M.E. Bothwell, M.P. Soriaga, J. Electroanal. Chem. 297 (1991) 523. M.E. Bothwell, G.J. Cali, G.M. Berry, M.P. Soriaga, Surf. Sci. 249 (1991) L322. G.M. Berry, J.R. McBride, J.A. Schimpf, M.P. Soriaga, J. Electroanal. Chem. 353 (1993) 281. J.A. Schimpf, J.B. Abreu, M.P. Soriaga, J. Electroanal. Chem. 364 (1994) 247. K. Yoshioka, F. Kitamura, M. Takeda, M. Takahashi, M. Ito, Surf. Sci. 227 (1990) 90. S. Zou, R. Go´ mez, M.J. Weaver, Surf. Sci. 399 (1998) 270. S. Zou, R. Go´ mez, M.J. Weaver, Langmuir 15 (1999) 2931. N. Hoshi, M. Noma, T. Suzuki, Y. Hori, J. Electroanal. Chem. 421 (1997) 15. M. Baldauf, D.M. Kolb, Electrochim. Acta 38 (1993) 2145. H. Naohara, S. Ye, K. Uosaki, J. Phys. Chem. B 102 (1998) 4366. H. Naohara, S. Ye, K. Uosaki, J. Electroanal. Chem. 473 (1999) 2. N. Hoshi, K. Kagaya, Y. Hori, J. Electroanal. Chem. 485 (2000) 55. S. Motoo, N. Furuya, J. Electroanal. Chem. 172 (1984) 339. P.W. Faguy, N.M. Markovic´ , R.R. Adzˇ ic´ , C.A. Fierro, E.B. Yeager, J. Electroanal. Chem. 289 (1990) 245. R.J. Nichols, in: J.L. Lipkowski, P.N. Ross (Eds.), Adsorption of Molecules at Metal Electrodes, VCH, New York, 1991 Ch. 7. Y. Sawatari, J. Inukai, M. Ito, J. Electron Spectrosc. 64/65 (1993) 515. F.C. Nart, T. Iwasita, M. Weber, Electrochim. Acta 39 (1994) 961. Y. Shingaya, M. Ito, Chem. Phys. Lett. 256 (1996) 438. P.W. Faguy, N.S. Marinkovic´ , R.R. Adzˇ ic´ , J. Electroanal. Chem. 407 (1996) 209. T. Iwasita, F.C. Nart, A. Rodes, E. Pastor, M. Weber, Electrochim. Acta 40 (1995) 53. I.R. Moraes, F.C. Nart, J. Braz. Chem. Soc. 12 (2001) 138. I.R. Moraes, F.C. Nart, J. Electroanal. Chem. 461 (1999) 110. B.M. Ocko, G.M. Watson, J. Wang, J. Phys. Chem. 98 (1994) 897. T. Yamada, N. Batina, K. Itaya, Surf. Sci. 335 (1995) 204. H.-W. Lei, H. Uchida, M. Watanabe, J. Electroanal. Chem. 413 (1996) 131.
Voltammograms of stepped and kinked stepped surfaces of palladium: Pd(S)-[n(111)×(100)] and Pd(S)-[n(100) ×(110)] Nagahiro Hoshi *, Makiko Kuroda, Yoshio Hori Department of Applied Chemistry, Faculty of Engineering, Chiba Uni6ersity, 1 -33, Yayoi-cho, Inage-ku, Chiba 263 -8522, Japan Received 21 November 2001; accepted 6 January 2002
Abstract Voltammograms of stepped and kinked stepped surfaces of Pd (Pd(S)-[n(111)× (100)] and Pd(S)-[n(100) ×(110)]) were measured in 0.5 M H2SO4. Both series gave redox peaks around 0.25 V (RHE) for which the intensity decreases with the decrease of the terrace width in 0.5 M H2SO4. No peak was observed around 0.25 V in 0.1 M HClO4. In the oxide film formation region, Pd(S)-[n(111) × (100)] and Pd(S)-[n(100)× (110)] electrodes gave single anodic peaks at 1.1 and 0.9 V (RHE), respectively. The peak intensity diminished with the decrease of the terrace width. Both series presented no peak for which the intensity was enhanced with the increase of step atom density. Charges of the anodic peaks equaled those calculated on the assumption that a monolayer of the oxide film (Pd–OH or Pd2O) covers the entire terrace. © 2002 Elsevier Science B.V. All rights reserved. Keywords: Pd(S)-[n(111) × (100)]; Pd(S)-[n(100)× (110)]; Voltammetry; Sulfuric acid anion; Adsorption; Nano-structure
1. Introduction Electrochemistry using well-defined single crystal electrodes has the potentiality for revealing the nanostructures that enhance the activity and selectivity of catalytic reactions. Hydrogen adsorption and oxide film formation are fundamental reactions in electrochemistry, and voltammetry is one of the convenient methods for studying the reactions. After Clavilier reported a simple method for preparing Pt single crystals, many papers demonstrated that Pt single crystal electrodes give voltammograms characteristic of their orientations [1 – 11], and voltammetric peaks were discussed in relation to hydrogen adsorption sites and anion adsorption. A series of voltammograms was also studied on Ir single crystals [12 –14]. Pt and Ir single crystal electrodes provide ‘finger print’ voltammograms of the surface structures, especially in H2SO4 solutions. Although the catalytic activity of Pd is as high as that of Pt, voltammetric studies on Pd single crystal electrodes are few and limited to low index planes, since preparation of a Pd single crystal is difficult using * Corresponding author. Tel.: + 81-43-290-3384; fax: +81-43-2903401. E-mail address: [email protected] (N. Hoshi).
Clavilier’s method. Solomun first reported voltammograms of a Pd single crystal electrode: the Pd(100) electrode gives a sharp anodic peak at 0.9 V (RHE) in H2SO4 solution, and the adsorbates were measured using X-ray photoelectron spectroscopy (XPS) and low energy electron diffraction (LEED) [15 –17]. Sashikata and co-workers [18,19] measured voltammograms of Pd low index planes in H2SO4. They found that the peak potentials of the oxide film formation depend on crystal orientations, and studied the anodic dissolution of I/Pd electrodes using in-situ scanning tunneling microscopy (STM) [18]. STM measurement also reveals that adsorbed sulfuric acid anion forms a 3 × 7 superstructure on a Pd(111) electrode between 0.5 and 0.9 V (RHE) [19]. Some papers also reported the adsorbates and electrochemical reactions on Pd low index planes. Soriaga and coworkers studied surface structures of bare and iodine modified Pd low index planes using LEED [20 – 23]. Adsorbed CO on Pd single crystal electrodes was examined using infrared reflection absorption spectroscopy (IRAS) [24 –26]. Electrochemical reduction of CO2 was studied on Pd low index planes: the reduction rates remarkably depend on the crystal orientation [27]. Ordered structures of Pd were studied on Pd deposited Au(111) and Au(100) electrodes [28 –30]. No paper,
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however, has reported the electrochemical reaction on high index planes of Pd. Our previous paper reported voltammograms of high index planes of Pd, Pd(S)-[n(111) ×(111)] and Pd(S)[n(100)×(111)], in 0.5H2SO4 [31]. The terrace and step structures affect the voltammograms in the ‘adsorbed hydrogen region’ and oxide film formation region significantly. In this paper, we extend the study to stepped Pd(S)-[n(111)× (100)] and kinked-stepped Pd(S)[n(100)×(110)] surfaces.
2. Experimental A spherical Pd single crystal (3 mm in diameter) was prepared from Pd wire (1 mm in diameter, 99.99% purity) according to Clavilier et al. [1]. The procedure is described in detail in our previous paper [31]. The crystal was oriented using the reflection beams of a He –Ne laser from the (111) and (100) facets [32], and then mechanically polished with diamond slurries down to 0.125 mm. The single crystal electrode was annealed in a H2 +O2 flame at about 1300 °C and cooled in an
Fig. 1. Voltammograms of Pd(S)-[n(111)× (100)] electrodes in (a) 0.5 M H2SO4 and (b) 0.1 M HClO4 solutions saturated with Ar. Scanning rate is 0.020 V s − 1.
Fig. 2. Charge of the cathodic peak (Q) at 0.25 V plotted against the density of atoms at the first layer (dF) on Pd(S)-[n(111)×(100)]. The results of Pd(S)-[n(111) × (111)] electrodes are reproduced for comparison [31]. The inset shows the atoms at the first layer (gray circles).
Ar atmosphere (99.9999%) before each voltammetric measurement. The surfaces examined were the following: Pd(S)-[n(111)×(100)]: Pd(111), Pd(544) (n = 9), Pd(322) (n = 5), Pd(211) (n= 3), Pd(311) (n =2) Pd(S)-[n(100)× (110)]: Pd(100), Pd(910) (n= 9), Pd(510) (n= 5), Pd(310) (n = 3), Pd(210) (n=2). The electrolytic solutions were prepared with suprapur grade chemicals (Merck) and ultrapure water treated with Milli Q low TOC (Millipore). All the potentials were referred to RHE. Electrochemical measurements were conducted at room temperature.
3. Results and discussion
3.1. Pd(S) -[n(111)×(100)] series 3.1.1. Potentials between 0.15 and 0.55 V Fig. 1(a) shows voltammograms of Pd(S)-[n(111)× (100)] electrodes in 0.5 M H2SO4. Surfaces with wide terraces give sharp redox peaks around 0.25 V, as is shown by the Pd(S)-[n(111)× (111)] series [31]. The peak intensity diminishes with the decrease of the terrace width. Voltammograms in 0.1 M HClO4, in which the anion will not be adsorbed in this potential range, do not present redox peaks around 0.25 V (Fig. 1(b)). These results strongly suggest that the redox peaks around 0.25 V are closely related to the desorption and 2− the adsorption of sulfuric acid anion (HSO− 4 or SO4 ). Fig. 2 presents the cathodic charge of the redox peak (Q) plotted against the density of atoms at the first layer (dF). We define circles edged in gray in the inset of Fig. 2 as atoms at the first layer, since sulfuric acid anion will not be adsorbed at the bottom of the step because of the steric hindrance. The value of Q is calculated from the gray area in Fig. 1(a) on the assumption that the background current of the cathodic peak changes linearly with the potential. Voltammograms in HClO4 solution cannot be used as the background in H2SO4 solution, since adsorbed sulfuric acid anion may affect the background current [31]. The plot in Fig. 2 gives a linear line, supporting the fact that the cathodic peak at 0.25 V is strongly associated with the sulfuric acid anion desorbed from the atoms at the first layer. The slope of the line of Pd(S)-[n(111)× (100)] is lower than that of Pd(S)-[n(111)× (111)]. Assuming that all the charge of the cathodic peak around 0.25 V originates from desorption of sulfuric acid anion, we estimate the coverage of the anion giving the peak at 0.25 V (q) from the following equation: q=(Q/zqe)/dF
(1)
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Fig. 3. Estimated coverage of adsorbed sulfuric acid anion (q) plotted against the density of atoms at the first layer (dF).
Fig. 4. Voltammograms of Pd(S)-[n(111)× (100)] electrodes in 0.5 M H2SO4 saturated with Ar. Scanning rate is 0.020 V s − 1.
where z is the number of transferred electrons per 2− sulfuric acid anion (1 for HSO− 4 , 2 for SO4 ), and qe is the elementary electron charge. In the case of Pd(111), Q= 1.2× 102 mC cm − 2. According to Eq. (1), adsorbed HSO− gives q =0.50, whereas adsorbed SO24 − gives 4 q = 0.25. STM measurement reveals the superstructure and coverage of the species that are stably adsorbed on surfaces. Sulfuric acid anion adsorbed stably on Pd(111) forms the 3 × 7 superstructure according to in-situ STM [19], giving a coverage of 0.22, which is close to q calculated from SO24 − adsorption. On the other hand, it is not plausible that all the adsorbed species are HSO− 4 . From the analogy of Pt(111) [33– 38], Pd(111) will adsorb sulfuric acid anion with 3-fold geometry which occupies three Pd atoms per anion, leading to a full coverage of 0.33. The q calculated from
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HSO− 4 adsorption (0.50) exceeds this value. Thus, we presume that SO24 − adsorbed stably on Pd(S)-[n(111)× (100)] gives peaks at 0.25 V. Fig. 3 shows q plotted against dF. The values of q on the Pd(S)-[n(111)× (100)] series are higher than those on the Pd(S)-[n(111)× (111)] series. This result can be explained using the following model. Pd(311) (= 2(111)−(100)) gives a small peak at 0.25 V in Fig. 1, whereas Pt(110) (=2(111)−(111)) produces no peak according to our previous report [31]. These facts demonstrate that sulfuric acid anion adsorbed on the (100) step of Pd gives a small peak, but the (111) step does not. The Pd(S)-[n(111)×(100)] series may adsorb sulfuric acid anion stably at the step as well as the terrace, thus the values of q on Pd(S)-[n(111)×(100)] are higher than those of Pd(S)-[n(111)× (111)]. Although Pd(110) gives no peak at 0.25 V, Pd(110) probably adsorbs sulfuric acid anion; IRAS studies show that Pt(110) [39], Rh(110) [40] and Au(110) [41] adsorb sulfuric acid anion. Desorption and adsorption of sulfuric acid anion may produce no voltammetric peak on Pd(110), as is the case of I− adsorption in the double layer region of Au(111) [42–44]. IRAS measurement and determination of the coverage of sulfuric acid anion are necessary to reveal the origin of the redox peaks at 0.25 V.
3.1.2. Potentials between 0.5 and 1.4 V Fig. 4 shows voltammograms of Pd(S)-[n(111)× (100)] electrodes in the oxide film formation region. A single anodic peak appears at 1.1 V on the surfaces with wide terraces. The peak intensity diminishes with the decrease of the terrace atomic rows, and finally disappears on Pd(311) (n=2) for which the first layer is composed of only step atoms. These facts suggest that the anodic peak originates from oxide film (Pd–OH or Pd2O) formation on the terrace. Pd(S)-[n(111)×(111)] electrodes gave an additional anodic peak at 0.9 V for which the intensity is enhanced with the increase of the step atom density, whereas the Pd(S)-[n(111)× (100)] series produces no peak from the oxide film formation on the step. The charge of the anodic peak (QOX) is calculated on the assumption that the background current changes linearly as shown in Fig. 4. QOX is plotted against the terrace atom density in Fig. 5. The plots give a straight line. Fig. 5 also depicts QOX calculated on the assumption that a monolayer of Pd–OH or Pd2O occupies the entire terrace site of Pd(S)-[n(111)× (100)] surfaces. Both plots provide nearly identical lines, supporting the fact that a monolayer oxide film covers the terrace of Pd(S)-[n(111)× (100)] surfaces at 1.1 V. The other anodic part of the voltammograms above 0.8 V may be due to the oxidation of Pd at the step and the second layer [18], or further oxidation of Pd (PdO, PdSO4, PdO(HSO4)) [17]. The absence of the anodic peak due to the step will be discussed in Section 3.3.
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Adsorbed sulfuric acid anion might be desorbed from the surface in the oxide film formation region, reducing the apparent anodic charge of the oxide film formation due to the cathodic current from anion desorption. However, a study of XPS shows that sulfuric acid anion is still adsorbed on the surface in the oxide film formation region [15]. Our experimental QOX almost equals the calculated QOX (Fig. 5). This result supports the fact that sulfuric acid anion is adsorbed with the oxide film of Pd(S)-[n(111) ×(100)] series.
Fig. 8. Estimated coverage of adsorbed sulfuric acid anion (q) plotted against the density of atoms at the first layer (dF) on Pd(S)-[n(100) × (110)] and Pd(S)-[n(100) ×(111)] electrodes.
3.2. Pd(S) -[n(100)×(110)] series
Fig. 5. Charge of the anodic peak at 1.1 V (QOX) plotted against terrace atom density on Pd(S)-[n(111)×(100)] electrodes. Also shown are the values of QOX calculated on the assumption that a monolayer of oxide film occupies the entire terrace. The inset shows the terrace (gray circles) and step atoms (black circles).
Fig. 6. Voltammograms of Pd(S)-[n(100)× (110)] electrodes in (a) 0.5 M H2SO4 and (b) 0.1 M HClO4 solutions saturated with Ar. Scanning rate is 0.020 V s − 1.
Fig. 7. Charge of the cathodic peak (Q) at 0.25 V plotted against the density of atoms at the first layer (dF) on kinked-step Pd(S)-[n(100) × (110)] electrodes. The results of stepped Pd(S)-[n(100)× (111)] electrodes are reproduced for comparison [31]. The inset shows the atoms at the first layer (gray circles).
3.2.1. Potentials between 0.15 and 0.55 V Pd(S)-[n(100)×(110)] electrodes give cathodic and anodic peaks at 0.25 and 0.30 V in 0.5 M H2SO4, respectively (Fig. 6). The peak intensity diminishes with the decrease of the terrace atomic rows. The voltammograms show no peak in 0.1 M HClO4. These results are identical with those on the Pd(S)-[n(100)×(111)] series reported previously [31]. The charge of the cathodic peak (Q) is plotted against the density of atoms at the first layer (dF) in Fig. 7, giving almost a straight line. The plots of Pd(S)-[n(100)× (110)] electrodes provide almost the same line as those of Pd(S)-[n(100)× (111)], except Pd(210) (= 2(100)− (110)) for which dF is the lowest. This difference arises from the fact that Pd(311) (= 2(100)− (111)) gives a small peak at 0.25 V, but Pd(210) (=2(100)−(110)) gives no peak. The (100) terrace of Pd(311) has enough space to adsorb sulfuric acid anion stably. In the case of Pd(210), however, kinked atoms protrude into the (100) terrace, possibly diminishing the stability of adsorbed sulfuric acid anion. The estimated coverage of sulfuric acid anion (q) is derived from Eq. (1) assuming SO24 − adsorption. The values of q are plotted against dF in Fig. 8. The plots of the Pd(S)-[n(100)×(110)] and Pd(S)-[n(100)× (111)] series are fitted well with the same straight lines. This result suggests that both Pd(S)-[n(100)× (110)] and Pd(S)-[n(100)× (111)] series adsorb sulfuric acid anion on a (100) terrace with the same geometry. 3.2.2. Potentials between 0.5 and 1.4 V Pd(S)-[n(100)× (110)] electrodes give a single anodic peak at 0.9 V in the oxide film formation region (Fig. 9). The peak intensity diminishes with the decrease of the terrace atomic rows. No peak due to oxide film formation on the step is observed, as is the case of the Pd(S)-[n(100)×(111)] series [31]. The charge of the anodic peak (QOX) is plotted against the terrace atom density in Fig. 10. The plots of Pd(S)-[n(100)×(110)] and Pd(S)-[n(100)×(111)] electrodes provide almost
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monolayer of Pd–OH or Pd2O occupies the entire terrace of the Pd(S)-[n(100)× (110)] and Pd(S)[n(100)× (111)] surfaces.
3.3. Structural effect on the oxide film formation
Fig. 9. Voltammograms of Pd(S)-[n(100)×(110)] electrodes in the oxide film formation region in 0.5 M H2SO4.
Fig. 10. Anodic charge at 0.9 V (QOX) plotted against terrace atom density on Pd(S)-[n(100)× (110)] electrodes. Results of Pd(S)[n(100) ×(111)] electrodes are also shown for comparison. The inset shows the terrace (gray circles) and kink atoms (black circles) on the Pd(S)-[n(100) ×(110)] surface.
the same linear lines. The values of QOX are almost identical with those calculated on the assumption that a
Our previous paper reported that Pd(S)-[n(111)× (111)] electrodes give two anodic peaks in the voltammograms in the oxide film formation region: the peak potential from steps (0.93 V) is lower than that from terraces (1.1 V) [31]. This fact shows that (111) steps of Pd(S)-[n(111)×(111)] surfaces have a higher activity than (111) terraces for the oxide film formation reaction (Pd +H2O Pd –OH +H+ + e, or 2Pd+H2O Pd2O+ 2H+ + 2e). The other series (Pd(S)-[n(100)× (111)], Pd(S)-[n(111)×(100)], Pd(S)-[n(100)×(110)]), however, give no anodic peak of oxide film formation at steps. These facts suggest that the step sites of the other series produce oxide film at a very much lower rate. Pd(S)-[n(100)× (111)] and Pd(S)-[n(100)×(110)] have the same step notation as Pd(S)-[n(111)×(111)] (= Pd(S)-[(n − 1)(111)×(110)]), but the activity for the oxide film formation is very low. Hard sphere models illustrate the difference of the step structures of these surfaces in Fig. 11. Pd(S)-[n(111)×(111)] surfaces have a ‘stepped’ (110) structure on which step atoms align linearly along the step line. Pd(S)-[n(100)× (111)] surfaces have a (111) step, but do not contain the (110) structure. Pd(S)-[n(100)×(110)] surfaces comprise (110) structure, but its ‘kinked-step’ atoms are protruded, so that they do not form a linear step line. These models suggest that the stepped (110) structure is the most favorable site for oxide film formation of Pd.
4. Conclusion 1. Both Pd(S)-[n(111)× (100)] and Pd(S)-[n(100)× (110)] series give redox peaks around 0.25 V in the voltammograms in 0.5 M H2SO4, in which the sulfuric 2− acid anion (HSO− 4 or SO4 ) is strongly adsorbed on the surfaces. The charges of the peaks decrease with the
Fig. 11. Hard sphere models of Pd(S)-[n(100)× (110)], Pd(S)-[n(111) ×(111)] and Pd(S)-[n(100) × (111)] surfaces.
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decrease of the terrace width. In 0.1 M HClO4 in which no anion is adsorbed on the electrodes, the voltammograms do not give such redox peaks. 2. Voltammograms of Pd(S)-[n(111) × (100)] and Pd(S)-[n(100)× (110)] electrodes provide single anodic peaks at 1.1 and 0.9 V, respectively. The charges of the anodic peaks are nearly identical to those calculated on the assumption that a monolayer of the oxide film (Pd –OH or Pd2O) occupies the entire terrace. Both series give no peak due to oxide film formation at the step.
[15] [16] [17] [18] [19] [20] [21] [22] [23] [24]
Acknowledgements This study was supported by grants-in-aid for Scientific Research of the Ministry of Education, No. 11118214 and No. 12650805.
References
[25] [26] [27] [28] [29] [30] [31]
[1] J. Clavilier, R. Faure, G. Guinet, R. Durand, J. Electroanal. Chem. 107 (1980) 205. [2] S. Motoo, N. Furuya, Ber. Bunsenges Phys. Chem. 91 (1987) 457. [3] N.M. Markovic´ , N.S. Marinkovic´ , R.R. Adzˇ ic´ , J. Electroanal. Chem. 241 (1988) 309. [4] N. Furuya, S. Koide, Surf. Sci. 220 (1989) 18. [5] J. Clavilier, K. El Achi, A. Rodes, J. Electroanal. Chem. 272 (1989) 253. [6] J. Clavilier, K. El Achi, A. Rodes, Chem. Phys. 141 (1990) 1. [7] A. Rodes, K. El Achi, M.A. Zamakhchari, J. Clavilier, J. Electroanal. Chem. 284 (1990) 245. [8] N.M. Markovic´ , N.S. Marinkovic´ , R.R. Adzˇ ic´ , J. Electroanal. Chem. 314 (1991) 289. [9] P.N. Ross, J. Chim. Phys. 88 (1991) 1353. [10] J. Clavilier, A. Rodes, K. El Achi, M.A. Zamakhchari, J. Chim. Phys. 88 (1991) 1291. [11] J.M. Feliu, A. Rodes, J.M. Orts, J. Clavilier, Polish J. Phys. Chem. 68 (1994) 1575. [12] S. Motoo, N. Furuya, J. Electroanal. Chem. 167 (1984) 309. [13] S. Motoo, N. Furuya, J. Electroanal. Chem. 181 (1984) 301. [14] N. Furuya, S. Koide, Surf. Sci. 226 (1990) 221.
[32] [33] [34] [35] [36] [37] [38] [39] [40] [41] [42] [43] [44]
T. Solomun, J. Electroanal. Chem. 217 (1987) 435. T. Solomun, J. Electroanal. Chem. 255 (1988) 163. T. Solomun, J. Electroanal. Chem. 302 (1991) 31. K. Sashikata, Y. Matsui, K. Itaya, M.P. Soriaga, J. Phys. Chem. 100 (1996) 20027. L.-J. Wan, T. Suzuki, K. Sashikata, J. Okada, J. Inukai, K. Itaya, J. Electroanal. Chem. 484 (2000) 189. G.J. Cali, G.M. Berry, M.E. Bothwell, M.P. Soriaga, J. Electroanal. Chem. 297 (1991) 523. M.E. Bothwell, G.J. Cali, G.M. Berry, M.P. Soriaga, Surf. Sci. 249 (1991) L322. G.M. Berry, J.R. McBride, J.A. Schimpf, M.P. Soriaga, J. Electroanal. Chem. 353 (1993) 281. J.A. Schimpf, J.B. Abreu, M.P. Soriaga, J. Electroanal. Chem. 364 (1994) 247. K. Yoshioka, F. Kitamura, M. Takeda, M. Takahashi, M. Ito, Surf. Sci. 227 (1990) 90. S. Zou, R. Go´ mez, M.J. Weaver, Surf. Sci. 399 (1998) 270. S. Zou, R. Go´ mez, M.J. Weaver, Langmuir 15 (1999) 2931. N. Hoshi, M. Noma, T. Suzuki, Y. Hori, J. Electroanal. Chem. 421 (1997) 15. M. Baldauf, D.M. Kolb, Electrochim. Acta 38 (1993) 2145. H. Naohara, S. Ye, K. Uosaki, J. Phys. Chem. B 102 (1998) 4366. H. Naohara, S. Ye, K. Uosaki, J. Electroanal. Chem. 473 (1999) 2. N. Hoshi, K. Kagaya, Y. Hori, J. Electroanal. Chem. 485 (2000) 55. S. Motoo, N. Furuya, J. Electroanal. Chem. 172 (1984) 339. P.W. Faguy, N.M. Markovic´ , R.R. Adzˇ ic´ , C.A. Fierro, E.B. Yeager, J. Electroanal. Chem. 289 (1990) 245. R.J. Nichols, in: J.L. Lipkowski, P.N. Ross (Eds.), Adsorption of Molecules at Metal Electrodes, VCH, New York, 1991 Ch. 7. Y. Sawatari, J. Inukai, M. Ito, J. Electron Spectrosc. 64/65 (1993) 515. F.C. Nart, T. Iwasita, M. Weber, Electrochim. Acta 39 (1994) 961. Y. Shingaya, M. Ito, Chem. Phys. Lett. 256 (1996) 438. P.W. Faguy, N.S. Marinkovic´ , R.R. Adzˇ ic´ , J. Electroanal. Chem. 407 (1996) 209. T. Iwasita, F.C. Nart, A. Rodes, E. Pastor, M. Weber, Electrochim. Acta 40 (1995) 53. I.R. Moraes, F.C. Nart, J. Braz. Chem. Soc. 12 (2001) 138. I.R. Moraes, F.C. Nart, J. Electroanal. Chem. 461 (1999) 110. B.M. Ocko, G.M. Watson, J. Wang, J. Phys. Chem. 98 (1994) 897. T. Yamada, N. Batina, K. Itaya, Surf. Sci. 335 (1995) 204. H.-W. Lei, H. Uchida, M. Watanabe, J. Electroanal. Chem. 413 (1996) 131.