J. Phys. Chem. B 2002, 106, 1985-1990
1985
Infrared Reflection Absorption Spectroscopy of Sulfuric Acid Anion Adsorbed on Stepped Surfaces of Platinum Single-Crystal Electrodes Nagahiro Hoshi,* Akihiko Sakurada, Sadatoshi Nakamura, Seiyu Teruya, Osamu Koga, and Yoshio Hori Department of Applied Chemistry, Faculty of Engineering, Chiba UniVersity, 1-33, Yayoi-cho, Inage-ku, Chiba 263-8522, Japan ReceiVed: June 27, 2001; In Final Form: October 23, 2001
Adsorption of sulfuric acid anion (HSO4- or SO42-) was studied using IRAS (infrared reflection absorption spectroscopy) on Pt(S)-[n(111) × (111)] and Pt(S)-[n(100) × (111)] electrodes in 0.05 M H2SO4. Pt(S)[n(111) × (111)] electrodes give two IRAS bands. One is from a sulfuric acid anion adsorbed on the terrace with 3-fold symmetry (∼1200 cm-1), and another is from that on the step with 2-fold symmetry (∼1200 cm-1 and ∼1100 cm-1). Relative band intensity from the 2-fold sulfuric acid anion (∼1100 cm-1) gets higher with the increase of step atom density. Pt(S)-[n(100) × (111)] electrodes also show two IRAS bands around 1200 and 1100 cm-1 which originate from the sulfuric acid anion adsorbed on the terrace and the step with 2-fold symmetry. The relative intensity of the lower frequency band increased with the increase of the step atom density.
Introduction Adsorbed anions on single-crystal electrodes have been studied in an attempt to reveal the structure of the electrodeelectrolyte interface. Sulfuric acid anion (HSO4- or SO42-) is strongly adsorbed on electrode surfaces and greatly changes the voltammograms of Pt single-crystal electrodes.1-4 Some papers reported that adsorbed sulfuric acid anion affects electrochemical reactions on Pt single-crystal electrodes, such as methanol oxidation,5,6 H2 oxidation,6 O2 reduction,7 and CO2 reduction.8 Therefore, studying the nature of adsorbed anions will give important information for revealing the origin of catalytic activity of electrodes. Infrared reflection absorption spectroscopy (IRAS) can observe sulfuric acid anion adsorbed on Pt electrodes sensitively, and many papers have been published. IRAS spectra of adsorbed sulfuric acid anion were first measured on a Pt poly-crystal electrode in 0.5 M H2SO4, giving two bands around 1200 and 1100 cm-1.9,10 The higher and lower frequency bands were initially assigned to S-O symmetric vibration of the adsorbed bisulfate (HSO4-) and the sulfate anion (SO42-), respectively.9,10 The study was extended to IRAS measurements in the solution containing the sulfuric acid anion at various pH.11-15 Later, some authors reported IRAS spectra of the adsorbed sulfuric acid anion on a well-defined single-crystal electrode and discussed the bands in more detail. On Pt(111), only one peak appears around 1200 cm-1 in the frequency between 1050 and 1400 cm-1.16-23 The peak is assigned to the S-O stretching vibration of sulfuric acid anion adsorbed with 3-fold geometry on Pt(111). IRAS spectra of the sulfuric acid anion were also reported using other low index planes of Pt (Pt(100) and Pt(110)).17,24,25 Nart and Iwasita measured IRAS spectra of the adsorbed sulfate anion on Pt(100) and Pt(110), and observed two bands around 1200 and 1100 cm-1 in 0.5 M KF + 0.69 M HF containing 10-2 M K2SO4, in which SO42- is the predomi* Author to whom correspondence should be addressed. Fax: +81-43290-3384. E-mail:
[email protected].
nant adsorptive anion in the bulk solution.24,25 They concluded that only the sulfate anion is adsorbed on Pt(100) and Pt(110) with 2-fold geometry. The peaks around 1100 and 1200 cm-1 were assigned to the stretching vibration of the S-O bond of SO42- coordinated to Pt atoms and that of the uncoordinated S-O bond, respectively. According to the study on coordination compounds, the coordinated S-O bond length is increased, while the uncoordinated S-O bond length is decreased.19 Theoretical calculation of the bond length of SO42- adsorbed on Ag(100) gives the same tendency.26 Increase of the bond length reduces the overlap of the orbital, resulting in a red shift of the IRAS band, whereas decrease of the bond length gives a blue shift of the band. Thus it is reasonable to assign the 1100 and 1200 cm-1 bands to the vibration of coordinated and uncoordinated S-O bonds, respectively. IRAS studies on Au and Rh electrodes suggest that the sulfuric acid anion is adsorbed with 1-fold geometry on (100) and (110) surfaces, since the IRAS spectra give a single peak around 1200 cm-1.27,28 It is not plausible, however, for Pt(100) and Pt(110) to adsorb the sulfuric acid anion with 1-fold geometry, because the IRAS spectra differ from those on Au and Rh significantly. Sulfuric acid is dissolved to form two adsorptive anions in aqueous solution, for example, the 0.05 M H2SO4 solution gives 0.038 M bisulfate (HSO4-) and 0.012 M sulfate (SO42-) anions according to the dissociation constants (Ka2 ) 1.02 × 10-2). IRAS measurement, however, has not clearly revealed which of the anions, SO42- or HSO4-, is adsorbed on Pt electrodes in H2SO4 solutions. Some papers reported a weak IRAS band around 950 cm-1.15,18,20,21 Some of them assigned it to S-OH stretching vibration, suggesting that the HSO4- anion is adsorbed on Pt(111).15,18,20 However, Faguy et al. assigned the band around 950 cm-1 to the stretching vibration of the ion pair of SO42- and H3O+.22,23 Nart and Iwasita et al. reported no peak around 950 cm-1 in the solution in which the HSO4- anion is the predominant species.19 Kolics and Wieckowski claimed that the adsorbate is a partially discharged SO42- anion by measuring the accurate coverage of the adsorbed anion using a radioactive
10.1021/jp012456o CCC: $22.00 © 2002 American Chemical Society Published on Web 02/02/2002
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Figure 1. Voltammograms of Pt(S)-[n(111) × (111)] electrodes in 0.05 M H2SO4 saturated with Ar. Scanning rate: 0.050 V s-1.
labeling method.29 No general consensus is reached on the adsorbed species on Pt electrodes in H2SO4, thus we will not discuss which anion is adsorbed on Pt electrodes in this paper. Voltammograms of Pt high index planes are known to provide redox peaks in H2SO4 solutions characteristic of their crystal orientation.3,4,30-37 However, no paper has reported IRAS spectra of sulfuric acid anion adsorbed on Pt high index planes. This paper presents IRAS spectra of sulfuric acid anions adsorbed on Pt(S)-[n(111) × (111)] and Pt(S)-[n(100) × (111)] electrodes in 0.05 M H2SO4 solution. We further attempt to describe the nature of adsorbed sulfuric acid anions on Pt high index planes. Experimental Section A single-crystal bead of Pt (about 3 mm in diameter, cross section 0.07 cm2) can be prepared easily from a Pt wire (1 mm in diameter, purity 99.99%) according to Clavilier et al.38 A larger single crystal electrode is necessary for obtaining IRAS spectra with a good signal-to-noise ratio. We melted the lower part of a bead and added Pt little by little, and finally obtained a Pt single crystal in teardrop shape with cross section between 0.30 and 0.35 cm2.39 The crystal was oriented using a reflection beam of a He/Ne laser from (111) and (100) facets,31,40 and polished mechanically using diamond slurry. The single-crystal electrode was annealed in H2/O2 flame at about 1300 °C, cooled in a stream of Ar/H2 (95/5),34 and transferred to an IRAS cell with a droplet of ultrapure water on the surface. We prepared Pt(S)-[n(111) × (111)] and Pt(S)-[n(100) × (111)] electrodes with terrace atomic rows (n) between 2 and 9. An electrolytic solution, 0.05 M H2SO4, was prepared from ultrapure water treated with Milli Q plus low TOC (Millipore) and suprapure grade chemicals (Merck). The purity of Ar and H2 is higher than 99.9999%. IRAS spectra were measured using a JIR-6000M spectrometer (JEOL) with p-polarized light with a resolution 4 cm-1. The window of the IRAS cell was a CaF2 plate with a thickness of 3 mm. Reflected IR light was detected using an MCT detector cooled by liquid nitrogen. The spectra were averaged over 1000 scans using SNIFTIRS (substantially normalized interfacial FTIR spectroscopy). The reference potential was 0.05 V (RHE),
whereas the sample potential was changed between 0.10 and 0.80 V (RHE). All the potentials are shown with respect to a reversible hydrogen electrode (RHE). Results and Discussion Pt(S)-[n(111) × (111)] Electrodes. Figure 1 shows voltammograms of Pt(S)-[n(111) × (111)] electrodes in 0.05 M H2SO4 saturated with Ar. Electric charges of sharp peaks at 0.12 V and the other broad peaks depend linearly on the step and terrace atom densities, respectively. These results agree with those reported previously;4,30-33,35 we judged that the surfaces are correctly oriented. Figure 2 shows the IRAS spectra of Pt(S)-[n(111) × (111)] electrodes in 0.05 M H2SO4. Pt(111), which is composed of a flat terrace, gives a single IRAS band around 1200 cm-1 above 0.2 V, as reported previously.16-23 This IRAS band is assigned to the S-O stretching vibration of the sulfuric acid anion adsorbed with 3-fold geometry on Pt(111). The band shifts to a higher wavenumber with the increase of the potential (1.3 × 102 cm-1 V-1), as reported previously.16-23 Pt(997) (n ) 9) gives an extra IRAS band around 1100 cm-1 in addition to the band around 1200 cm-1 at 0.15 V. The intensity of the lower frequency band increases up to 0.4 V gradually, lowering above 0.5 V, and finally disappears above 0.7 V. On the other hand, the intensity of the higher frequency band grows with the increase of the potential, as is the case of Pt(111). IRAS spectra of Pt(221) (n ) 4) and Pt(331) (n ) 3) also give two bands around 1200 and 1100 cm-1 above 0.15 V. The band intensities of higher and lower frequency bands show the potential dependence similar to those on Pt(997) (n ) 9). It is, however, not appropriate to discuss the absolute integrated intensities of IRAS bands of the individual electrodes, since the coverage of sulfuric acid anion is not determined on Pt stepped surfaces. We adopted the relative band intensity IL/ IH(0.8 V) as a measure, where IL is the integrated band intensity of the lower frequency band, and IH(0.8 V) is that of the higher frequency band at 0.8 V adapted as an internal reference of each electrode. Compared at the same potentials, I L/IH(0.8 V)
Sulfuric Acid Anion Adsorbed on Pt Electrodes
J. Phys. Chem. B, Vol. 106, No. 8, 2002 1987
Figure 2. IRAS spectra of sulfuric acid anion adsorbed on Pt(S)-[n(111) × (111)] in 0.05 M H2SO4. Er ) 0.05 V (RHE). Spectra were averaged over 1000 scans. Resolution: 4 cm-1.
increases with the increase of step atom density. These results suggest that species adsorbed near the step give rise to the band around 1100 cm-1. Pt(110) gives two IRAS bands around 1100 and 1200 cm-1, as reported previously.17,25 The IL/IH(0.8 V) of Pt(110) is the highest in Pt(S)-[n(111) × (111)] series. The surface structure of Pt(110) ((1 × 1) or (1 × 2)), however, depends sensitively on the annealing and cooling conditions;41,42 we would not discuss the IRAS intensity in connection with step atom density on Pt(110). Sulfuric acid anion adsorbed with 2-fold geometry (C2V symmetry) gives two IRAS bands around 1200 and 1100 cm-1, according to the study on Pt(100) and Pt(110).17,24,25 In this paper, we assume that the anion is adsorbed with its z-axis perpendicular to the surface from the following reason. The number of the IRAS bands observable on a surface depends on the surface selection rule, symmetry, and orientation of the adsorbate. The CaF2 window in 3 mm thickness can measure IRAS spectra above 1050 cm-1. A cobalt complex coordinated with sulfate with C2V symmetry gives two IR bands around 1100 and 950 cm-1 of which dipole moments are parallel to the z-axis.43 The former band is the symmetric stretching vibration of an uncoordinated SO bond (ν(SO,s)), and the latter is that of a coordinated SO bond (ν(SO*,s)). The adsorption on Pt electrodes may shift these bands to higher wavenumber (around 1200 and 1100 cm-1), as is the case of the IRAS on Pt(111).18,20 The cobalt complexes with C2V symmetry also provide IR bands around 1060 (asymmetric stretching vibration of coordinated SO, ν(SO*,as)) and 1150 cm-1 (asymmetric stretching vibration of uncoordinated SO, ν(SO,as)),43 which may be shifted to a higher wavenumber due to the adsorption. The dipole moments of the asymmetric vibrations are parallel to x-and y-axes of the anion;43 IRAS cannot detect these bands when the z-axis of the anion is perpendicular to the surface. If the adsorbed anion is tilted, the forbidden bands become observable and four IRAS bands (ν(SO*,s), ν(SO,s), ν(SO*,as), and ν(SO,as)) will appear above 1050 cm-1. Since IRAS spectra give only two bands in Figure 2, it is not plausible that the z-axis of the adsorbed sulfuric acid anion is tilted. Some papers report, however, that surface selection rule of the sulfuric acid anion appears to be broken on the electrode surfaces due to vibrational electronic coupling.16,44 In addition, we do not know exactly how much the bands will be shifted due to the adsorption. Taking these matters into account, we cannot discuss the orientation of the adsorbed sulfuric acid anion in detail. The values of IL/IH(0.8 V) on Pt(997) (n ) 9), Pt(221) (n ) 4), and Pt(331) (n ) 3) were lower than those of Pt(100) and
TABLE 1: Band Shift (dν/dE)of Bisulfate Ion on Pt(S)-[n(111) × (111)] Surfaces
Pt(111) Pt(997) (n ) 9) Pt(221) (n ) 4) Pt(331) (n ) 3) Pt(110)
dνhigher/dE (cm-1 V-1)
dνlower/dE (cm-1 V-1)
1.3 × 102 1.0 × 102 83 77 71
20 24 14 24
Pt(110) at 0.8 V. The following model may rationalize this fact: the step site of Pt(S)-[n(111) × (111)] electrodes adsorbs sulfuric acid anion with 2-fold geometry, whereas the terrace site adsorbs it with 3-fold geometry. The band from the 3-fold sulfuric acid anion overlaps with the band of the higher frequency band from the 2-fold one around 1200 cm-1. The larger terrace adsorbs more 3-fold sulfuric acid anions, thus IL/ IH(0.8 V) diminishes with the increase of (111) terrace width. Former papers report that IRAS band shift (dν/dE) of sulfuric acid anion depends on the adsorbed geometry significantly.19,24,25 A sulfuric acid anion adsorbed with 3-fold geometry shows a large band shift about 1 × 102 cm-1 V-1 on Pt(111).16-23 On the other hand, a 2-fold sulfuric acid anion gives smaller band shifts on Pt(100): 58 cm-1 V-1 for the uncoordinated S-O bond (∼1200 cm-1) and 13 cm-1 V-1 for the coordinated S-O bond (∼1100 cm-1).24,25 The large band shift of the 3-fold sulfuric acid anion is explained by back-donation of the electron from metal to LUMO of the sulfuric acid anion16 and distortion of the sulfuric acid anion adsorbed with 3-fold geometry.19 The rather small band shift of the coordinated S-O bond of the 2-fold sulfuric acid anion is rationalized by Coulomb interaction between polarized Sδ+-Oδ- bond of sulfuric acid anion and positively charged Pt surface.24 The difference of the coverage will not cause the substantial discrepancy of the band shift between the 3-fold and 2-fold sulfuric acid anion, since the estimated band shift of the singleton 3-fold sulfuric acid anion is still higher than that of the 2-fold one.19 Table 1 presents the IRAS band shift of Pt(S)-[n(111) × (111)] electrodes. The band shift around 1200 cm-1 diminishes with the increase of step atom density, approaching the value of Pt(100), whereas the band shift around 1100 cm-1 does not depend on the crystal orientation. This fact also supports the model that the band around 1200 cm-1 is composed of the S-O stretching vibration of the 3-fold sulfuric acid anion at the terrace and the uncoordinated S-O vibration of the 2-fold one at the step. The ratio of the 2-fold sulfuric acid anion, of which band shift is small, increases with the increase of the step atom density, lowering the band shift around 1200 cm-1. According
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Figure 5. IRAS of sulfuric acid anion adsorbed on Pt(S)-[n(100) × (111)] electrodes in 0.05 M H2SO4. Er ) 0.05 V (RHE). Spectra were averaged over 1000 scans. Resolution: 4 cm-1.
Figure 3. Model of the geometry of sulfuric acid anion adsorbed on Pt(S)-[n(111) × (111)] electrodes.
to this model, the band around 1200 cm-1 is possibly split into two bands at higher potentials. However, the broad bandwidth and low S/N ratio of the spectra in Figure 2 may obscure small band splitting. Integrated intensity of the IRAS band around 1100 cm-1 decreases above 0.7 V on Pt(997) (n ) 9), Pt(221) (n ) 4), and Pt(331) (n ) 3). This potential is close to the positive end of the redox peaks between 0.4 and 0.7 V in the voltammograms in Figure 1. The following mechanism may explain the change of the IRAS band intensity and the redox peaks between 0.4 and 0.7 V: (a) Adsorbed sulfuric acid anion with 2-fold geometry changes to 3-fold geometry above 0.7 V as shown in Figure 3. (b) Intensity transfer from lower to higher frequency bands occurs in the IRAS spectra of CO adsorbed on Pt(111).45,46 Superstructure of the adsorbed sulfuric acid anion is modified above 0.7 V, causing effective intensity transfer from the 1100 cm-1 band of 2-fold geometry to the 1200 cm-1 band of 3-fold geometry. Structural change of the adsorbate (a) or (b) may cause the redox peaks between 0.4 and 0.7 V. An STM image of the superstructure of adsorbed sulfuric acid anion is necessary for revealing the real mechanism. It is very difficult, however, to obtain an in-situ STM image of stepped surfaces in atomic resolution; no paper has reported an atomic image of the stepped surface using in-situ STM except Au stepped surfaces.47 Pt(S)-[n(100) × (111)] Electrodes. Figure 4 shows voltammograms of Pt(S)-[n(100) × (111)] electrodes in 0.05 M H2-
SO4. The voltammograms agree with those reported by Clavilier et al.,3,30,31,36 thus the surfaces are correctly oriented. Figure 5 depicts IRAS spectra of Pt(S)-[n(100) × (111)] electrodes in 0.05 M H2SO4. Two bands appear around 1100 and 1200 cm-1 above 0.2 V, which is the onset of the main peaks of the voltammograms (Figure 4). Intensities of the two bands increase monotonically with the increase of the applied potential, but showing no correlation with the other peaks of the voltammograms. The band position and shape resemble those of Pt(100) on which sulfuric acid anion adsorbed with 2-fold geometry.24 These results suggest that sulfuric acid anion is adsorbed with 2-fold geometry on Pt(S)-[n(100) × (111)] electrodes. IRAS spectra on Pt(S)-[n(100) × (111)] (Figure 5) depend on crystal orientation less significantly than those on Pt(S)[n(111) × (111)] (Figure 2). The relative band intensity, IL/ IH(0.8 V), however, increases slightly with the increase of the step atom density at 0.8 V. IL/IH(0.8 V) is plotted against the step atom density in Figure 6, giving a linear line. This result suggests that the step enhances the IR absorptivity of the coordinated S-O vibration around 1100 cm-1. Table 2 presents the IRAS band shift on Pt(S)-[n(100) × (111)] electrodes. The band shift of the higher frequency band (∼1200 cm-1) is less than that on Pt(S)-[n(111) × (111)] even on the surface with long terrace atomic rows of 9, whereas the band shift of the lower frequency band (∼1100 cm-1) nearly equals that on Pt(S)-[n(111) × (111)]. These features are identical with those of the 2-fold sulfuric acid anion adsorbed on Pt(100), also supporting that Pt(S)-[n(100) × (111)] electrodes adsorb sulfuric acid anion with 2-fold geometry. The band shift of the higher frequency band increases with the increase of step atom density gradually, jumping considerably on Pt(311)
Figure 4. Voltammograms of Pt(S)-[n(100) × (111)] electrodes in 0.05 M H2SO4.
Sulfuric Acid Anion Adsorbed on Pt Electrodes
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Figure 6. Relative integrated intensity of IRAS band (IL/IH(0.8 V)) at 0.80 V on Pt(S)-[n(100) × (111)] electrodes plotted against the step atom density.
TABLE 2: Band Shift (dν/dE) of Sulfuric Acid Anion on Pt(S)-[n(100) × (111)] Surfaces
Pt(100) Pt(17 1 1) (n ) 9) Pt(11 1 1) (n ) 6) Pt(311) (n ) 2)
dνhigher/dE (cm-1 V-1)
dνlower/dE (cm-1 V-1)
24-31 34-54 37 70-94
24 20 16 20
(n ) 2) of which step density is the highest. This fact suggests that uncoordinated S-O bond adsorbed at step gives larger band shift than that at terrace. There has been no report that discussed the effect of the step on the IRAS spectra of adsorbed sulfuric acid anion. In the case of the CO adsorbed on stepped surfaces of Pt in UHV, step is known to enhance the IRAS cross section and the band shift, and the mechanism is discussed using two models: (a) Local electric field at step is higher than that at terrace, enhancing the cross section for IRAS and band shift.48 (b) The electron density is smoothed on a stepped surface.49 On the terrace, carbon atoms of CO lie within smoothed electron density and are screened, whereas oxygen atoms are unscreened since they locate further from the surface. On the step, both carbon and oxygen atoms are unscreened since the electron density at the top of the step is low owing to the smoothed electron density. Therefore, CO molecules at the step have a higher cross section for IRAS and a larger band shift than those at the terrace.50 Model (b) can explain the high cross section and the band shift of adsorbed CO more quantitatively than model (a). In the case of the sulfuric acid anion, a higher local electric field at the step will enhance the uncoordinated SO bond as well as the coordinated one. Thus model (b) is more plausible for explaining the effect of the step on the IRAS spectra. We cannot discuss, however, the models quantitatively at present, since we know neither the absolute coverage nor the orientation of the anion adsorbed on Pt stepped surfaces. Measurement of the coverage and the orientation using other experimental methods is necessary for revealing the step effect on IRAS spectra of the sulfuric acid anion. Conclusion 1. Pt(S)-[n(111) × (111)] electrodes coadsorbs the 3-fold and 2-fold sulfuric acid anion on the terrace and step, respectively, giving two IRAS bands around 1200 and 1100 cm-1 in 0.05 M H2SO4 solution. The IRAS band around 1200 cm-1 is
composed of the S-O stretching vibration of the 3-fold anion and the uncoordinated S-O vibration of the 2-fold anion, whereas the lower frequency band around 1100 cm-1 is assigned to the coordinated S-O vibration of the 2-fold anion. 2. The relative band intensity of the lower frequency IRAS band diminishes above 0.7 V on Pt(S)-[n(111) × (111)] electrodes. This potential is close to the positive end of the redox peaks between 0.5 and 0.7 V in the voltammograms of Pt(S)[n(111) × (111)] electrodes. 3. The band shift (dν/dE) of the higher frequency band decreases with the increase of the the step atom density on Pt(S)[n(111) × (111)] electrodes. 4. Pt(S)-[n(100) × (111)] electrodes also give two IRAS bands around 1200 and 1100 cm-1. The lower and higher frequency bands on Pt(S)-[n(100) × (111)] electrodes are assigned to coordinated and uncoordinated S-O vibrations of the 2-fold sulfuric acid anion, respectively. Both the terrace and step adsorb the sulfuric acid anion with 2-fold geometry. The relative band intensity of the lower frequency band increases with the increase of the step atom density 5. The band shift (dν/dE) of the higher frequency band increases with the increase of the step atom density on Pt(S)[n(100) × (111)] electrodes. Acknowledgment. This study was supported by grant-inaid for scientific research of the Ministry of Education of Japan, No. 09450313, No. 11118214 and No. 12650805. References and Notes (1) Clavilier, J.; Armand, D.; Wu, B. L. J. Electroanal. Chem. 1982, 135, 159. (2) Ross, P. N. J. Chim. Phys. 1991, 88, 1353. (3) Markovic´, N. M.; Marinkovic´, N. S.; Azˇic´, R. R. J. Electroanal. Chem. 1988, 241, 309. (4) Markovic´, N. M.; Marinkovic´, N. S.; Azˇic´, R. R. J. Electroanal. Chem. 1991, 314, 289. (5) Markovic´, N. M.; Ross, P. N. J. Electroanal. Chem. 1992, 330, 499. (6) Kita, H.; Gao, Y.; Nakato, T.; Hattori, H. J. Electroanal. Chem. 1994, 373, 177. (7) Kita, H.; Gao, Y.; Ohnishi, K. Chem. Lett. 1994, 73. (8) Hoshi, N.; Suzuki, T.; Hori, Y. J. Electroanal. Chem. 1996, 416, 61. (9) Kunimatsu, K.; Samant, M. G.; Seki, H.; Philpott, M. R. J. Electroanal. Chem. 1988, 243, 203. (10) Kunimatsu, K.; Samant, M. G.; Seki, H. J. Electroanal. Chem. 1989, 258, 163. (11) Kunimatsu, K.; Samant, M. G.; Seki, H. J. Electroanal. Chem. 1989, 272, 185. (12) Samant, M. G.; Kunimatsu, K.; Seki, H.; Philpott, M. R. J. Electroanal. Chem. 1990, 280, 391. (13) Nart, F. C.; Iwasita, T. J. Electroanal. Chem. 1991, 308, 277. (14) Nart, F. C.; Iwasita, T. J. Electroanal. Chem. 1992, 322, 289. (15) Nart, F. C.; Iwasita, T. Electrochim. Acta 1992, 37, 2179. (16) Faguy, P. W.; Markovic´, N. M.; Azˇic´, R. R.; Fierro, C. A.; Yeager, E. B. J. Electroanal. Chem. 1990, 289, 245. (17) Nichols, R. J. In Adsorption of Molecules at Metal Electrodes; Lipkowski, J. L., Ross, P. N., Jr., Eds.; VCH: New York, 1991; Chapter 7. (18) Sawatari, Y.; Inukai, J.; Ito, M. J. Electron Spectrosc. 1993, 64/ 65, 515. (19) Nart, F. C.; Iwasita, T.; Weber, M. Electrochim. Acta, 1994, 39, 961. (20) Shingaya, Y.; Ito, M. Chem. Phys. Lett. 1996, 256, 438. (21) Thomas, S.; Sung, Y.-E.; Kim, H. S.; Wiekowski, A. J. Phys. Chem. 1996, 100, 11726. (22) Faguy, P. W.; Marinkovic´, N. S.; Azˇic´, R. R. Langmuir 1996, 12, 243. (23) Faguy, P. W.; Marinkovic´, N. S.; Azˇic´, R. R. J. Electroanal. Chem. 1996, 407, 209. (24) Nart, F. C.; Iwasita, T.; Weber, M. Electrochim. Acta 1994, 39, 2093. (25) Iwasita, T.; Nart, F. C.; Rodes, A.; Pastor, E.; Weber, M. Electrochim. Acta 1995, 40, 53. (26) Olivera, P. P.; Patrito, E. M.; Sellers, H. Surf. Sci. 1998, 418, 376.
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