Catalytic Effect of Adsorbed Iodine Atoms on Hydrogen Peroxide

J. Phys. Chem. B 2001, 105, 5751-5756

5751

Catalytic Effect of Adsorbed Iodine Atoms on Hydrogen Peroxide Reduction at Single-Crystal Pt Electrodes, Causing Enhanced Current Oscillations Shuji Nakanishi, Yoshiharu Mukouyama, and Yoshihiro Nakato* Department of Chemistry, Graduate School of Engineering Science, and Research Center for Photoenergetic of Organic Materials, Osaka UniVersity, Toyonaka, Osaka 560-8531, Japan ReceiVed: February 7, 2001; In Final Form: April 6, 2001

A current oscillation, observed for H2O2 reduction on atomically flat Pt(111) electrodes in a potential region from about 0.22 to 0.05 V vs NHE, and previously called oscillation E, was enhanced much by adsorbed iodine atoms of a submonolayer amount, showing an extension of its potential region toward the positive. Besides, the iodine adsorption led to an appearance of oscillation E even for atomically flat Pt(100), for which no oscillation E appeared in the absence of adsorbed iodine. The results were explained on the basis of a previously proposed model for oscillation E, including autocatalytic effect of adsorbed OH on dissociative adsorption of H2O2 (the first step of H2O2 reduction). Mathematical simulation by addition of catalytic effect of adsorbed iodine atoms on the dissociative adsorption of H2O2 has reproduced the observed enhancement of oscillation E. It is shown that the concept of the catalytic effect of adsorbed iodine atoms has the same theoretical basis as the autocatalytic effect of adsorbed OH.

Introduction Chemical and electrochemical oscillations are quite attractive phenomena as typical examples of dynamic self-organization of molecular systems. Much work has recently been done on oscillations in electrochemical systems1-4 especially from points of views of exploration of new-type oscillations,5 observation of spatiotemporal patterns,6-9 and investigations of their mechanisms.8-10 Electrochemical oscillations are also interesting from a point of view of investigation of mechanisms of electrochemical reactions themselves, because their studies sometimes reveal new mechanisms that might otherwise not be discovered. We reported previously11-19 that hydrogen peroxide (H2O2) reduction on Pt electrodes showed five oscillations of different types, named oscillation A, B, C, D, and E, depending on the electrode potential, the electrode-surface structure, and the presence or absence of halide ions in electrolytes. Of these, oscillation E, which is observed only for atomically flat Pt(111) electrodes,19 and oscillation C13,14 are interesting, in that they show the presence of autocatalytic effect of adsorbed OH (Pt-OH) on dissociative adsorption of H2O2 (the first step of the H2O2 reduction). A similar autocatalytic effect of adsorbed OH was reported for Ag electrodes.20 In the present paper, we report that oscillation E is enhanced much by the presence of adsorbed iodine atoms. Detailed studies including mathematical simulation have revealed that adsorbed iodine atoms have catalytic effect on the dissociative adsorption of H2O2, which leads to the enhancement of oscillation E. Experimental Section Single-crystal Pt(111), (100), and (110) electrodes with atomically flat surfaces were prepared by the method of Clavilier et al.,21 using Pt(99.97% pure) wires heated in a hydrogen flame. * Corresponding author. Phone: +81-6-6850-6235. Fax: +81-6-68506236. E-mail: [email protected]..

The details of electrode preparation were described in a previous paper.19 Just before measurements, all the electrodes were annealed in a hydrogen flame for 60 s and immediately quenched in Ar-bubbled pure water in order to obtain atomically flat surfaces. Iodine-modified Pt electrodes were obtained by immersing the above Pt electrodes in aqueous 10 µM KI (M ) mol/dm3) for a certain period of time, the amount of adsorbed iodine being regulated by a change in the immersion time. The adsorption of iodine on Pt was confirmed by X-ray photoelectron spectroscopy (Shimadzu ESCA-1000). Once iodine atoms were adsorbed on Pt, it was not desorbed at all during electrochemical measurements. An electrochemical cell, having two arms for the reference and the counter electrodes, was used in the present work, in the same way as reported in a previous paper.19 A Pt plate was used as the counter electrode (CE), and a hydrogen electrode, composed of platinized Pt, 1.0 M H2SO4, and 1 atm of hydrogen gas, was used as the reference electrode (RE). The electrolyte was 0.3 M H2SO4 (pH 0.55) with or without H2O2. They were prepared using special grade chemicals and pure water, the latter of which was obtained by purification of deionized water with a Milli-Q water purification system. Current density (j) vs potential (U) curves, and j vs time (t) curves were measured with a potentio-galvanostat (NikkoKeisoku NPGS-301) and a potential programmer (NikkoKeisoku NPS-2). They were either recorded with an X-Y recorder (Riken-Denshi F-35C) or stored digitally at 1 or 10 kHz with a Mac ADIOS II/16 (GW Instruments). Ohmic drops in the solution are not corrected in the present work. Results Figure 1a,b show, for reference, reported j-U curves for atomically flat Pt(111) in relatively low and high H2O2 concentrations (0.2 M H2O2 and 1.0 M H2O2 + 0.3 M H2SO4) under potential-controlled conditions. In Figure 1a, a cathodic

10.1021/jp0104684 CCC: $20.00 © 2001 American Chemical Society Published on Web 05/24/2001

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Figure 1. Effect of iodine adsorption on j-U curves for Pt(111) under potential-controlled conditions. Electrode: (a) and (b) naked Pt(111), and (c) and (d) iodine-modified Pt(111). Electrolyte: (a) and (c) 0.2 M H2O2 + 0.3 M H2SO4, and (b) and (d) 1.0 M H2O2 + 0.3 M H2SO4. Scan rate: 10 mV/s. The shape of the j-U curve in (c) is somewhat scattered in a region from 0.1 to 0.3 V.

current in a potential region from about +0.50 to +0.02 V vs NHE is attributed to H2O2 reduction, which starts at about +0.80 V. A hydrogen evolution current flows in a potential region more negative than ca. +0.02 V. It is to be noted in Figure 1a that the H2O2 reduction current, which is nearly independent of the electrode potential in 0.50 to 0.30 V, shows two negative differential resistances (NDR’s) in a regions from +0.30 to +0.15 V and +0.10 to +0.02 V.19 The potential-independent H2O2 reduction current is explained by assuming that the H2O2 reduction is initiated by dissociative adsorption of H2O2

2Pt + H2O2 9 8 2Pt-OH k

(1)

1

followed by electrochemical reduction of the resultant Pt-OH

8 Pt + H2O Pt-OH + H+ + e- 9 k

(2)

2

and that reaction 1 is the rate-determining step.13,16,19 The NDR in +0.10 to +0.02 V is caused by suppression of the dissociative adsorption of H2O2 (reaction ) by formation of underpotential deposited hydrogen (upd-H) of a nearly full coverage.13,16,19 On the other hand, the NDR in +0.30 to +0.15 V is caused by a decrease in the coverage (θOH) of adsorbed OH (acting as an autocatalyst for the dissociative adsorption of H2O2) with a negative potential shift.19 In a higher H2O2 concentration (1.0 M H2O2 + 0.3 M H2SO4), two oscillations, called oscillation A and E, appear in the potential regions of the above-mentioned NDR’s, respectively, as shown in Figure 1b.19 Slight shifts of the potential regions of the NDR’s between Figure 1a,b are due

Figure 2. The same as Figure 1, except that the electrodes are changed to naked and iodine-modified Pt(100). The shape of the j-U curve in (c) is somewhat scattered in a region from 0.1 to 0.3 V.

to the ohmic drops in the solution between the working and reference electrodes. Figure 1c,d show j-U curves for an iodine-modified Pt(111) electrode prepared by a 30 s immersion in 10 µM KI, drawn in the same way as Figure 1a,b, respectively. The coverage (θI) of adsorbed iodine is about 0.4 as estimated from a decrease in the current peaks of the upd-H formation in 0.3 M H2SO4. The potential region of the NDR due to adsorbed OH in Figure 1c is extended to more positive potentials, up to about +0.5 V, than that in Figure 1a, and the magnitude (or the slope) of the NDR also becomes higher, though the exact shape of the j-U curve in this region is somewhat scattered from experiment to experiment. In accordance with the changes of the NDR, the potential region where oscillation E appears is also extended to the positive (Figure 1d). Oscillation A for the iodinemodified electrode is not observed in Figure 1d owing to its long oscillation period compared with the speed of the potential scan. A similar result was obtained for atomically flat Pt(100) electrodes, as shown in Figure 2. The NDR due to adsorbed OH is weak for Pt(100) in a low H2O2 concentration (Figure 2a), and no oscillation E, except oscillation A, appears in a high H2O2 concentration (Figure 2b). The result is explained by the weak autocatalytic effect of adsorbed OH on Pt(100).19 For iodine-modified Pt(100) prepared by 30-s immersion in 10 µM KI, however, the NDR due to adsorbed OH becomes higher in magnitude and is extended to the positive potentials (Figure 2c). In harmony with the NDR change, oscillation E appears for iodine-modified Pt(100) in a higher H2O2 concentration (Figure 2d). Figure 3 shows j-U curves for atomically flat Pt(110), drawn in the same way as Figures 1 and 2. Contrary to the cases of

Hydrogen Peroxide Reduction

J. Phys. Chem. B, Vol. 105, No. 24, 2001 5753 Mathematical Simulation. The results of Figure 4 show that adsorbed iodine on Pt has the catalytic effect on the dissociative adsorption of H2O2, in addition to the site-blocking effect. We reported previously19 that the appearance of oscillation E for naked Pt(111) was explained by taking account of the autocatalytic effect of adsorbed OH on the same reaction. The catalytic effect of adsorbed iodine for iodine-modified Pt(111) will thus cooperate with the autocatalytic effect of adsorbed OH on a naked-Pt(111) part. The enhancement of oscillation E can be reproduced by mathematical simulation on the basis of this model, as explained below. In addition to reactions 1 and 2, the following reactions on Pt sites were considered in the same way as in our previous papers.13,16,19 k-3

Pt + H+ + e- [\ ] Pt-H (upd-H) k

(3)

8 2Pt + O2 + 2H+ + 2e2Pt-OH 9 k

(4)

3

4

k-5

] Pt-H (on-top-H) Pt + H+ + e- [\ k

(5)

8 H2 Pt-H (on-top-H) + Pt-H (on-top-H) 9 k

(6)

5

6

Figure 3. The same as Figure 1, except that the electrodes are changed to naked and iodine-modified Pt(110).

Pt(111) and (100), no extension of the potential region of the NDR nor appearance of oscillation E was observed by iodine adsorption for Pt(110), independent of the immersion time in 10 µM KI. Figure 4 shows in detail the NDR’s changes by changes in the coverage (θI) of adsorbed iodine, for (a) Pt(111), (b) Pt(100), and (c) Pt(110), in a low H2O2 concentration (0.2 M H2O2 + 0.3 M H2SO4). Curves 1, 2, and 3 are for Pt electrodes which were beforehand immersed in 10 µM KI for 0, 30, and 300 s, respectively. The coverages θI for these curves are estimated to be 0, 0.4, and 1.0, respectively. For Pt(111) in Figure 4a, the H2O2-reduction current in curve 3 is nearly zero throughout the potential, clearly showing the site-blocking effect of adsorbed iodine. On the other hand, the current in curve 2 is low (in the absolute value) in potentials from 0.00 to 0.05 V, whereas it is high in positive potentials from +0.2 to +0.4 V, even higher than that for a naked (no-iodine adsorbed) electrode (curve 1). The latter result clearly indicates that the adsorbed iodine has catalytic effect on the dissociative adsorption of H2O2, in addition to the site-blocking effect, as discussed in more detail in the discussion section. A similar result is obtained for Pt(100) in Figure 4b. Namely, the H2O2-reduction current in curve 2 is low on the negative-potential side, whereas it is high on the positive-potential side. For Pt(110) in Figure 4c, however, the current is decreased by the iodine adsorption uniformly throughout the potentials between 0.0 and 0.5 V, suggesting that almost no catalytic effect of adsorbed iodine is present for this surface, only the site-blocking effect playing a role. Figure 5 shows time courses of oscillation E in 1.0 M H2O2 + 0.3 M H2SO4, (a) and (b) for naked Pt(111), and (c) to (e) for iodine modified Pt(111) with θI ≈ 0.4. No notable change is caused in the wave shape by iodine adsorption.

where Pt represents schematically surface Pt site(s). The coverage (θI) of adsorbed iodine was taken to be constant, independent of oscillating reactions. The differential equations for a current balance, mass balances, and rate equations, were also obtained in the same way as in our previous work,13,16,19

I ) jA ) (U - E)/RΩ ) IC + IF ) ACDL(dE/dt) + IF

(7)

IF ) AF{-k3CH+S(1 - θH - θOH - θI) + k-3θH k2CH+sθOH + k4θOH2 - k5CH+s(1 - ΘH) + k-5ΘH} (8) (δHO/2) dCHOs/dt ) (DHO/δHO)(CHOb - CHOs) k1CHOs(1 - θH - θOH - θI)2 (9) N0dθOH/dt ) k1CHOs(1 - θH - θOH - θI)2 k3CH+sθOH - k4θOH2 (10) N0dθH/dt ) k3CH+s(1 - θH - θOH - θI) - k-3θH (11) N0dΘH/dt ) k5CH+s(1 - ΘH - θI) - k-5ΘH - 2k6ΘH2 (12) where I is the total current, A the electrode area, j the current density, U the external (or applied) electrode potential, E the true electrode potential, (U-E) the ohmic drop between the electrode surface and the position of the reference electrode, RΩ the solution resistance in the same place as above, IC ) ACDL(dE/dt) the charging current, CDL the double-layer capacitance, and IF the Faradaic current. CHOs and CH+s are the concentrations of H2O2 and H+ ions at the electrode surface, respectively; CHOb and CH+b the concentrations of H2O2 and H+ in the solution bulk, respectively; and θOH, θH, and ΘH are the surface coverages of Pt-OH, upd-H, and on-top-H, respectively. DHO and DH+ are the diffusion coefficients for H2O2 and H+, respectively, δHO and δH+ the thickness of the diffusion layers for H2O2 and H+, respectively. N0 schematically represents the total amount of surface Pt sites per unit area. In the

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Figure 4. Dependence of j-U curves for (a) Pt(111), (b) Pt(100), and (c) Pt(110) on the coverage (θI) of adsorbed iodine. Curve 1 is for naked Pt (θI ) 0), curve 2 for iodine-modified Pt (θI ≈ 0.4), and curve 3 for iodine-modified Pt (θI ≈ 1). Electrolyte: 0.2 M H2O2 + 0.3 M H2SO4. Scan rate: 10 mV/s. The exact shapes of the j-U curves in curve 2 for Pt(111) and (100) are somewhat scattered, depending on experiments.

present work, CH+s ) CH+b was assumed owing to a high mobility (high diffusion rate) of H+ compared with H2O2. The rate constants, k2, k3, k-3, k4, k5, and k-5, were expressed by the Butler-Volmer equations,

ki ) ki0exp[-RiniF(E - Ei0)/RT]

for i ) 2, 3, and 5 (13)

ki ) ki0exp[(1 - Ri)niF(E - Ei0)/RT] for i ) -3, 4, and -5 (14) where ki0 is the rate constant at E ) Ei0, Ei0 the equilibrium redox potential for the ith reaction, Ri the transfer coefficient, and ni the number of transferred electrons. The site-blocking effect of adsorbed iodine, namely, that the dissociative adsorption of H2O2 could not occur on adsorbed iodine, was taken into account by introducing such terms as (1 - θH - θOH - θI). The autocatalytic effect of adsorbed OH (Pt-OH) and the catalytic effect of adsorbed iodine on the dissociative adsorption of H2O2 were taken into account by expressing k1 as follows:

k1 ) k10 + γ1θOH + γ2θI

(15)

where k10 is a normal rate constant for reaction 1, and γ1 and γ2 are proportional constants expressing the strength of the autocatalytic effect and the catalytic effect, respectively. Figures 6a and b show calculated j-U curves with γ1 ) 8 and θI ) 0 (no iodine adsorption) and with γ1 ) 8, γ2 ) 0.01, and θI ) 0.3, respectively. The result of Figure 6a is essentially the same as reported,19 reproducing the appearance of oscillation E. In Figure 6b, enhanced oscillation E is reproduced, in

Figure 5. Time courses of oscillation E, (a) and (b) for Pt(111), and (c-e) for iodine-modified Pt(111) with θI ≈ 0.4, at the electrode potential given in the figure (vs NHE). Electrolyte: 0.2 M H2O2 + 0.3 M H2SO4.

agreement with the experiments. Figure 6c shows a calculated time course (j-t curve) at U ) +0.30 V with the same γ1, γ2, and θI values as Figure 6b. The calculated wave shape and period are namely the same as the experimental result (Figure 5). In addition, the calculated amplitude of oscillation E showed a tendency to become larger from zero with negative potential shifts, similar to the experimental results (see Figures 1b and 1d), as the rate of the potential scan was made lower. Discussion The experiments and mathematical simulation have clearly shown that adsorbed iodine has the catalytic effect on the dissociative adsorption of H2O2, in addition to the site-blocking effect. Experimentally, the site-blocking effect is clearly observed in the j-U curve for Pt(110) in Figure 4c, in which the H2O2-reduction current is decreased by (in the absolute value) iodine adsorption uniformly throughout the potentials between 0.00 and 0.45 V. Similar decreases in the H2O2-reduction current by iodine adsorption are observed for Pt(111) and (100) throughout the potentials in curve 3 (θI ≈ 1) of Figure 4a,b and also in potentials of about 0.00 to 0.05 V in curve 2 (θI ≈ 0.4) of Figure 4a,b. On the other hand, in curve 2 of Figure 4a,b, the H2O2-reduction current in positive potentials from +0.2 to +0.4 V does not show such a decrease, remaining only slightly lower or even higher than that for naked Pt (curve 1) in the same potentials. This high H2O2-reduction current in the positive potentials can be attributed to the catalytic effect of adsorbed iodine. The result indicates that the catalytic effect is comparable to or even exceeds the site-blocking effect in a region of moderate θI. The reason the catalytic effect of adsorbed iodine appears prominently in the positive potentials from 0.2 to 0.4 V can be

Hydrogen Peroxide Reduction

Figure 6. (a) and (b): Calculated j-U curves with (a) γ1 ) 8 and θI ) 0, and (b) γ1 ) 8, γ2 ) 0.01, and θI ) 0.3. U was scanned at a rate of 0.01 V/s. (c) Calculated j-t curve at U ) +0.30 V with γ1 ) 8, γ2 ) 0.01, and θI ) 0.3. Other parameter values are as follows: CHOb ) 0.7 × 10-3 mol cm-3, δHO ) 0.01 cm, DHO ) 1.7 × 10-5 cm2 s-1, CH+b ) 0.3 × 10-3 mol cm-3, δH+ ) 0.004 cm, DH+ ) 9.3 × 10-5 cm2 s-1, A ) 0.01 cm2, CDL ) 2.0 × 10-5 F cm-2, N0 ) 2.2 × 10-9 mol cm-2, RΩ ) 60 Ω, T ) 300 K, Ri ) 0.25 (i ) 2) and 0.50 (i * 2), n ) 1, k10 ) 4 × 10-2 cm s-1, k20 ) 1 × 10-5 cm s-1, k30 ) 1 × 10-2 cm s-1, k-30 ) 1 × 10-5 mol cm-2 s-1, k40 ) 1 × 10-8 mol cm-2 s-1, k50 ) 5 × 10-3 cm s-1, k-50 ) 5 × 10-6 mol cm-2 s-1, k6 ) 5 × 10-6 mol cm-2 s-1, E20 ) 1.37 V vs NHE, E50 ) E-50 ) 0.08 V, E60 ) 0.67 V, and E70 ) E-70 ) -0.03 V.

explained by taking account of its cooperation with the autocatalytic effect of Pt-OH. In a previous paper,19 the autocatalytic effect of Pt-OH was explained as follows: Pt-OH is present at hollow sites of surface Pt lattice and H2O2 is adsorbed with negatively polarized oxygen atoms of H2O2 directed to surface Pt atoms, as shown in Figure 7A. Thus, the adsorption of H2O2 is accelerated on surface Pt sites in the neighborhood of adsorbed OH because they are more or less positively polarized by an electronegativity difference between Pt atom and OH-group (Figure 7A). The catalytic effect of adsorbed iodine can be understood in the same way, because iodine atoms have a fairly large electronegativity than Pt, similarly to OH group. Thus, the cooperation occurs such that the increased adsorbed OH, formed by the catalytic effect of adsorbed iodine, further increases the adsorbed OH by its autocatalytic effect. The coverage of adsorbed OH (θOH) can be significant only in the positive potentials from +0.2 to +0.4 V, as the rate of its electrochemical reduction (reaction ) becomes high in more negative potentials. The extension of the potential region where oscillation E appears toward the positive can be explained by the same reasoning. The catalytic effect of adsorbed iodine is more prominent in the order, Pt(111) >> Pt(100) . Pt(110). This can be explained by considering the differences in the values of γ1 and γ2 in eq 15. We reported previously19 that the γ1 value decreased in the order Pt(111) > Pt(100) > Pt(110), owing to the difference in the number of Pt pairs on which accelerated H2O2 adsorption occurred, as shown in Figure 7B. The same ordering is expected for the catalytic effect of adsorbed iodine (Figure 7B).

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Figure 7. Schematic illustration of the autocatalytic effect of adsorbed OH and the catalytic effect of adsorbed iodine. (A) The dissociative adsorption of H2O2 is accelerated on positively polarized surface Pt atoms in the neighborhood of adsorbed OH and iodine. (B) The difference in the number of Pt pairs on which accelerated adsorption of H2O2 can occur among Pt(111), (100), and (110).

In conclusion, the present work has revealed that adsorbed iodine has the catalytic effect on the dissociative adsorption of H2O2, causing the enhancement of oscillation E. The presence of the catalytic effect of adsorbed iodine, as well as the autocatalytic effect of Pt-OH, and their strong dependence on the atomic-level structure of the Pt surface, are of much interest, showing new mechanisms (or aspects) of electrochemical reactions. Acknowledgment. This work was partly supported by a Grant-in-Aid for Scientific Research on Priority Area of “Electrochemistry of Ordered Interfaces” (Grant 09237105) from the Ministry of Education, Culture, Sports, Science and Technology, Japan. References and Notes (1) Hudson, J. L.; Tsotsis, T. T. Chem. Eng. Sci. 1994, 49, 1493. (2) Fahiday, T. Z.; Gu, Z. H. Modern Aspects of Electrochemistry; White, R. E., Bockris, J. O’M., Conway, R. E., Eds.; Plenum: New York, 1995; Vol 37, p 383. (3) Koper, M. T. M. AdVances in Chemical Physics Prigogine, I., Rice, S. A. Eds.; John Wiley & Sons: New York, 1996; Vol. 92, p 161. (4) Krischer, K. Modern Aspects of Electrochemistry; Conway, B. E., Bockris, J, O′M., White, R. E., Eds.; Plenum: New York 1999; Vol. 32, p 1. (5) Erne´, B, H.; Ozanam, F.; Stchakovsky, M.; Vanmaekelbergh, D.; Chazalviel, J. N. J. Phys. Chem. B 2000, 104, 5961. (6) Fla¨tgen, G.; Krischer, K.; Pettinger, B.; Dobelhofer, K.; Junkes, H.; Ertl, G. Science 1995, 269, 668. (7) Kiss, I. Z.; Gaspar, V.; Hudson, J. L. J. Phys. Chem. B 2000, 104, 7554. (8) Christoph, J.; Otterstedt, R. D.; Eiswirth, M.; Jaeger, N. I.; Hudson, J. L. J. Chem. Phys. 1999, 110, 8614. (9) Strasser, P.; Charistoph, J.; Lin, W. F.; Eiswirth, M.; Hudson, J. L. J. Phys. Chem. A 2000, 104, 1854. (10) Mazouz, N.; Krischer, K.; Fla¨tgen, G.; Ertl, G. J. Phys. Chem. B 1997, 101, 2403. (11) Matsuda, T.; Hommura, H.; Mukouyama, Y.; Yae, S.; Nakato, Y. J. Electrochem. Soc. 1997, 144, 1988. (12) Mukouayama, Y.; Hommura, H.; Nakanishi, S.; Nishimura, T.; Konishi, H.; Nakato, Y. Bull. Chem. Soc. Jpn. 1999, 72, 1247. (13) Mukouyama, Y.; Nakanishi, S.; Konishi, H.; Nakato, Y. J. Electroanal. Chem. 1999, 473, 156.

5756 J. Phys. Chem. B, Vol. 105, No. 24, 2001 (14) Mukouyama, Y.; Nakanishi, S.; Chiba, T.; Murakoshi, K.; Nakato, Y. J. Phys. Chem. B 2001. Submitted for publication. (15) Mukouyama, Y.; Nakanishi, S.; Konishi, H.; Ikeshima, Y.; Nakato, Y. J. Phys. Chem. B 2001. Submitted for publication. (16) Mukouyama, Y.; Nakanishi, S.; Nakato, Y. Bull. Chem. Soc. Jpn. 1999, 72, 2573. (17) Nakanishi, S.; Hommura, H.; Mukouyama, Y.; Matsuda, T.; Nakato, Y. Chem. Lett. 1998, 977.

Nakanishi et al. (18) Mukouyama, Y.; Nishimura, T.; Nakanishi, S.; Nakato, Y. J. Phys. Chem. B 2000, 104, 11186. (19) Nakanishi, S.; Mukouayma, Y.; Karasumi, K.; Imanishi, A.; Furuya, N,; Nakato, Y. J. Phys. Chem. B 2000, 104, 4181. (20) Fla¨tgen, G.; Wasle, S.; Lu¨bke, M.; Eickes, C.; Radhakirishnan, G.; Doblhofer, K.; Ertl, G. Electrochim. Acta 1999, 44, 4499. (21) Clavilier, J.; Faure, R.; Guinet, G.; Durand, D. J. Electroanal. Chem. 1980, 107, 205.