Adsorption of Phosphate Species on Pt(111) and Pt(100) As Studied

Dec 19, 1996 - With increasing potentials these two original bands are replaced by a new band located between 1150 and 1180 cm-1, which is ..... Geris...
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J. Phys. Chem. 1996, 100, 19933-19938

19933

Adsorption of Phosphate Species on Pt(111) and Pt(100) As Studied by in Situ FTIR Spectroscopy M. Weber,† F. C. Nart,* and I. R. de Moraes Instituto de Quı´mica de Sa˜ o Carlos, UniVersidade de Sa˜ o Paulo, C.P. 780, 13360-970 Sa˜ o Carlos (SP), Brazil

T. Iwasita Institut fu¨ r Physik, UniVersita¨ t der Bundeswehr Mu¨ nchen, Werner-Heisenberg Weg 39, 85521 Munchen, Germany ReceiVed: March 29, 1996; In Final Form: August 22, 1996X

In situ FTIR results on the adsorption of phosphate species on Pt(111) and Pt(100) single crystal surfaces are presented and discussed. In weak acid solutions (fluoride electrolyte pH ) 2.8), the adsorption of phosphate species on Pt(111) starts at about 0.4 V vs Pd/H2 and the maximum adsorption occurs at about 0.6 V. At low potentials, two spectral features due to adsorbed H2PO4- are observed at 1110 and 1000 cm-1. With increasing potentials these two original bands are replaced by a new band located between 1150 and 1180 cm-1, which is assigned to adsorbed HPO42- species. In strongly acidic solution (pH ) 0.23), undissociated H3PO4 molecules are adsorbed at low potentials. This species is characterized by a band between 1035 and 1050 cm-1. With increasing adsorption potential adsorbed H3PO4 dissociates, generating H2PO4-. For both solution pHs the deprotonation of adsorbed species is observed after the maximum of the anomalous voltammetric wave of the Pt(111) electrode. On Pt(100) and Pt(111) the adsorption of phosphate species shows almost identical behavior in mildly acidic solutions. Transformation from H2PO4- to HPO42- occurs on both single crystal surfaces at the same potentials.

Introduction It has been emphasized that vibrational data on adsorbed anions do not only contribute to the description of the electrochemical double layer from a molecular point of view but also help to characterize the physicochemical properties of the electrified interface.1 Phosphoric acid and related anions constitute one of the most attractive systems for this kind of study and has been the subject of several investigations using radiotracer2 and infrared methods.3-7 In a previous study on the adsorption of phosphate species at polycrystalline platinum electrodes in a HF/KF supporting electrolyte, Nart et al. found that the nature and/or the geometry of the adsorbed phosphate species depends on the applied potential.5 It was shown that H2PO4- changes the symmetry from C2V to Cs at the onset of surface oxide formation, and H3PO4 dissociates on the surface with increasing potential.5 The latter result indicates that the positively charged Pt surface has a promoting effect on acid-base dissociation. Adsorption of phosphate species on single crystal platinum was studied with cyclic voltammetry and in situ FTIR spectroscopy in the pH range between pH ) 1.03 and 12.7.6 From the spectra taken in neutral phosphate buffer and alkaline NaOH/ phosphate buffer solutions, it was concluded that the adsorbed phosphate species become protonated with increasing potential on polycrystalline platinum as well as on Pt(111).6 Obviously, there are contradictory results in the literature (see above) with respect to the effect of potential on the acid-base strength of the adsorbed anions. Because of the polyprotic nature of phosphoric acid, several anionic species may be involved in the adsorption process, thus making the investigation of phosphate adsorption a challenge † On leave from the Institut fu ¨ r Physikalische Chemic, Universita¨t Bonn, Wegelerstrasse 12, 53115 Bonn, Germany. X Abstract published in AdVance ACS Abstracts, November 15, 1996.

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from the point of view of spectroscopy. Interpreting the results of phosphate adsorption on single crystal platinum electrodes has required an amount of experience sampled by observing its behavior on other, more accessible (just from the spectroscopic point of view) systems like Au(111). While adsorption at Pt electrodes in alkaline solutions is limited by the early formation of Pt oxide, at gold electrodes adsorption can be studied on a wide range of pH values.7 The latter investigation gave thus the key to the understanding of the spectroscopic features at single crystal platinum electrodes. The in situ FTIR investigation of phosphate adsorption on Au(111)7 has emphasized the necessity of using appropriate base electrolytes to compensate migration of ions in the thin layer, which causes IR features from the bulk of the solution.8 As an additional effect at platinum, faradaic processes like for example H desorption complicate the acid-base equilibrium situation in the thin layer.8 We stress therefore once again the necessity of controlling these factors by means of using appropriate electrolyte solutions. In the present work we show the results of phosphate adsorption on Pt(111) and Pt(100) in acid media. Mixtures of HF and KF are used as base electrolytes to minimize bulk solution changes in the thin layer between electrode and IR window. The observed spectral results are discussed, taking into account previous results on polycrystalline platinum5 and gold single crystal surfaces.7 Experimental Section The in situ FTIR experiment were carried out using a Digilab FTS-40 infrared spectrometer, equipped with a nitrogen cooled MCT detector. The electrochemical cell used for both cyclic voltammetry and in situ IR spectroscopy was made of PTFE and equipped with a CaF2 window. Single crystal electrodes, Pt(111) (diameter ) 0.9 cm) and Pt(100) (diameter ) 0.79 cm), were prepared at the Forschung© 1996 American Chemical Society

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Figure 1. Cyclic voltammogram of Pt(111) (a) in 0.67 HF, 0.5 KF and (b) in 0.67 HF, 0.5 KF, 1.5 × 10-2 M NaH2PO4; pH ) 2.8; sweep rate ) 50 mV/s.

szentrum Ju¨lich. Electrodes were pretreated by flame annealing as recommended in the literature9 and protected by a drop of deaerated Millipore MilliQ water while transferring to the spectroelectrochemical cell. A platinum flat ring was used as the counter electrode, and a hydrogen charged Pd net in the test electrolyte served as the reference electrode (Pd/H2). All potentials are referred to this electrode if not otherwise indicated. Solutions were prepared from Merck Suprapur chemicals and Millipore MilliQ water. A mixture of 0.69 M HF and 0.5 M KF was used as a base electrolyte of pH 2.8. A solution of 7.3 M HF served as a base electrolyte of pH 0.23. The in situ FTIR spectra were obtained by alternating the potential each 1000 scans between reference and sample potential. Typically 20 000 interferograms were collected at each potential. The reference potential was set at 0.03 V vs Pd/H2, where a negligible adsorption of phosphate species is expected. Spectra were coadded and computed as the reflectance ratio R/Ro (sample/reference). This procedure results in positive-going bands due to the loss of solution species and negative-going bands due to the formation of adsorbed species. Results and Discussion Adsorption of Phosphate on Pt(111) at pH ) 2.8. The cyclic voltammograms obtained in HF/KF mixtures with a solution pH of 2.8 are shown in Figure 1 for the Pt(111) single crystal surface. In the absence of phosphate species the cyclic voltammogram of the Pt(111) surface in the HF/KF-mixture is typical for solutions containing nonspecifically adsorbed ions. The so-called anomalous state, at potentials above 0.4 V, presents a well-defined spike at about 0.78 V vs Pd/H2. The addition of phosphate anions to the fluoride electrolyte shifts the anomalous wave toward lower potentials (Figure 1b) and cancels the spike. A similar voltammetric response was reported by Ye et al. for perchloric acid solutions containing phosphate species.6 Potential Dependence of the Amount of Adsorption. As we have shown in previous papers,10,11 it is possible to follow the potential dependence of anion adsorption by using spolarized light and observing the depletion in the thin layer of

Figure 2. In situ FTIR spectra taken with s-polarized light of Pt(111) in 0.67 HF, 0.5 KF, 1.5 × 10-2 M NaH2PO4 (pH ) 2.8); reference potential 0.03 V vs Pd/H2, sample potentials as indicated.

Figure 3. Potential dependence of the integrated band intensity for the 1077 cm-1 feature of the s-polarized spectra in Figure 2.

the ions undergoing adsorption. In Figure 2 spectra taken with s-polarized light in a phosphate-containing solution of pH 2.8 are shown. As expected, only loss (positive) IR bands are observed over the whole potential range. The bands observed at 1077 and 1157 cm-1 are respectively assigned to the asymmetric and symmetric stretching of H2PO4-,12 which is the main species in solution at pH 2.8. In Figure 3 the integrated band intensity of the solution feature at 1077 cm-1 is plotted as a function of the applied potential. The bands can be clearly observed at 0.45 V; thus, the onset of adsorption may be extrapolated to ca. 0.40 V vs Pd/H2, i.e., to about the onset potential of the voltammetric anomalous wave in phosphate-containing solution. It can be reasonably inferred that this wave is related to the processes occurring during phosphate adsorption. However, the current in the voltammo-

Adsorption of Phosphate Species on Pt(111) and Pt(100)

Figure 4. In situ FTIR spectra taken with p-polarized light for adsorbed phosphate species on Pt(111). Solution composition: 0.67 HF, 0.5 KF, 1.5 × 10-2 M NaH2PO4 (pH ) 2.8); reference potential 0.03 V vs Pd/H2, sample potentials as indicated.

gram goes through a well-defined maximum at 0.46 V (Figure 1a) while phosphate adsorption reaches a maximum at about 0.55 V and remains relatively high up to the highest potential studied, i.e., 0.9 V. Nature of the Adsorbed Phosphate Species at pH ) 2.8. In Figure 4 spectra taken at different potentials with p-polarized light in phosphate-containing solution (pH ) 2.8) are presented. A strong negative-going band due to the adsorbed species is observed in the 1110-1175 cm-1 range. This feature is superimposed by the solution loss band at 1157 cm-1, which is therefore not apparent. The only visible solution band is the symmetric PO2 mode at 1077 cm-1 reported above. To obtain the negative-going band at 1157 cm-1 free from the interference of the positively-going band from solution species at 1157 cm-1, a difference between s- and p-polarized spectra can be used. Since the intensities are not the same for both light polarization, the spectra obtained with s-polarized light must be scaled by a factor which makes both spectra with the same intensity, since the bands due to solutions species must have the same intensity. Differences in band intensity for solution species in the thin layer of solution are due to the differences in the magnitude of mean electric field vector for both polarization in the thin solution layer.13 The results of the difference between the s- and p-polarized light spectra are shown in Figure 5, where now the 1110-1175 cm-1 band is not more disturbed by solution bands. In addition to the 1110 cm-1 band, at potentials below 0.55 V a weak adsorbate band is observed near 1000 cm-1. Both adsorbate bands have also been observed at about the same frequencies on polycrystalline platinum5 and on the Au(111).7 In terms of group frequencies they have been assigned to the symmetric PO2 and P-(OH)2 stretching vibrations of H2PO4ions12 adsorbed through the two oxygen atoms. The C2V symmetry of the ions in solution is thus retained. There are two important characteristics in the spectra of Figures 4 and 5, which must be emphasized: (1) the feature at 1000 cm-1 disappears as the potential increases beyond 0.55 V, and (2) in the 0.45-0.55 V range the mode at higher frequency presents an extremely high band center shift (dν/dE ) 300 cm-1/V), which changes abruptly to 80 cm-1/V at ca. 0.60, as can be seen in Figure 6.

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Figure 5. Ratio between scaled s-polarized light spectra from Figure 2 and p-polarized light spectra from Figure 4 for adsorbed phosphate species on Pt(111) electrode. Solution composition: 0.67 HF, 0.5 KF, 1.5 × 10-2 M NaH2PO4 (pH ) 2.8); reference potential 0.03 V vs Pd/H2, sample potentials as indicated

Figure 6. Band center frequency of the high-frequency mode of the adsorbed specie in Figure 5 as a function of the applied potential.

A band center shift of 300 cm-1/V is too high to be interpreted in terms of lateral interactions or Stark effect of the adsorbed species. The most plausible explanation of these results is that a change in the nature and/or adsorption geometry of the surface species occurs in the 0.45-0.55 V potential range. It has been observed that weak acids become dissociated at platinum electrodes in the adsorbed state,8,14 dissociation being favored at high positive potentials.5,15 Partial charge transfer from the adsorbed ion to the Pt surface, as reported for sulfate, oxalate, acetate, and other systems by Orts et al.,16 can favor proton dissociation. Electron donation to the metal must attenuate the basicity of the adsorbed anion, thus causing proton dissociation. On these grounds it is very likely that dissociation of the originally adsorbed H2PO4- ions occurs, and the resulting HPO42- species cause the unique band observed in the spectra from 0.60 V onward, at about 1160 cm-1. The interpretation given above is supported by results of phosphate adsorption on Au(111) in weak alkaline medium,

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Figure 8. Comparison of the band center shift of the adsorbate bands in Figure 4 with the cyclic voltamogram of Pt(111) in 7.3 M HF, 1.5 × 10-2 M NaH2PO4; pH 0.23, sweep rate 50 mV/s.

Figure 7. In situ FTIR spectra taken with p-polarized light for adsorbed phosphate species on Pt(111) at pH 0.23. Solution composition: 7.3 M HF, 1.5 × 10-2 M NaH2PO4; reference potential 0.03 V vs Pd/H2, sample potentials as indicated.

where the main solution species is HPO42-.7 As demonstrated there, adsorbed HPO42- ions present only one band near 1120 cm-1, which has been interpreted as the A1 stretching of the ion adsorbed through three oxygen atoms.7 Such assignment requires the symmetric PO3 group frequency to become blueshifted (by about 130 cm-1) in the adsorbed state as compared to the ion in solution.12 Similar results were also reported for the adsorption of phosphate species on colloidal goethite with IR spectroscopy.17 This band shift, which is caused by the metal-ion interaction, indicates a high sensitivity of the PO3 mode to the environment, a fact that can explain a larger shift at Pt(111) than at Au(111) as a result of stronger metal-ion interactions on the former surface. Comparing the amount of adsorption in Figure 3 with the result for the band shift (Figure 6), we observe that at 0.6 V the amount of adsorption has practically reached its maximum value. Consequently, from this potential onward no frequency shift due to lateral interactions is expected, and the 80 cm-1/V shift can be regarded as the Stark tuning rate of adsorbed HPO42-. Adsorption of Phosphate Species at pH ) 0.23. In solutions of pH 0.23 totally associated H3PO4 are the dominating species. At this pH significant differences in the spectral features are observed (Figure 7) as compared with those at pH ) 2.8 (Figures 4 and 5). At the onset of adsorption, 0.4 V vs Pd/H2, only one absorption band at about 1035 cm-1 is observed. As the potential increases, the original band is shifted to higher wavenumbers and a second band at about 1110 cm-1 appears (first as a shoulder at about 1100 cm-1 and then as a welldefined band). The latter feature shifts with increasing potential up to frequencies of about 1140 cm-1 and becomes the dominating band at potentials above 0.6 V. From the two positive-going solution bands expected for H3PO4 in solution, only the degenerated P-(OH)3 at 1005 cm-1 can be observed in the spectra of Figure 7 at potentials above

0.5 V. The other one, the symmetric PdO stretching vibration located at 1165 cm-1,18 is superimposed on the negative feature observed in the range 1110-1145 cm-1. Unfortunately, spolarized light spectra from this solution are practically impossible due to the large changes in refraction index of the very concentrated HF solution used to achieved the desired solution pH. The drastic change in the refraction index of the solution causes probably total reflection at the window/solution interface, and practically no light is reflected at the electrode for the s-polarization. The changes in the spectra, from one adsorbate band at 1035 cm-1 to one at 1110 cm-1, suggest a change in the nature of the adsorbed species with increasing potentials as in the case of pH 2.8 analyzed above. Also in this case a deprotonation of the species originally adsorbed seems to occur as the metal surface becomes more positive. A similar behavior was proposed for adsorbed phosphoric acid on polycrystalline platinum electrodes.5 The 1035 cm-1 mode was observed in strongly acidic media on polycrystalline platinum5 and gold19 as well as on Au(111)7 and is assigned to adsorbed undissociated phosphoric acid, probably bonded to the surface through the oxygen atom (C3V symmetry). Increasing the potential of the Pt(111) electrode leads to the formation of adsorbed H2PO4-. The observed band center for this species is in the same frequency region as for adsorbed H2PO4- in the weakly acidic solution (see above). The other frequency mode for H2PO4-, which is observed near 1010 cm-1 in the solution of pH 2.8 (see Figures 4 and 5), is probably overlapped by the positive-going solution band of phosphoric acid in the solution of pH 0.23. In Figure 8 the band centers are plotted as a function of potential. For comparison, the cyclic voltammogram obtained in the same solution is also reproduced in the figure. The onset of adsorption can be approximately extrapolated to the beginning of the “anomalous” wave in the voltammogram. The frequency shifts are 119 cm-1/V for the 1035 cm-1 mode and 87 cm-1/V for the 1110 cm-1 mode. These values are surprisingly high, compared with those observed for the same solution composition on polycrystalline platinum (dν/dE ) 25 cm-1 V-1 and 12 cm-1 5) and polycrystalline gold electrodes (dν/dE ) practically constant19). In Figure 9 the band center frequencies for all observed modes are plotted against the potential in the normal hydrogen scale (NHE). We have thus a common reference potential for systems having different pH. Extrapolating the data at pH 0.23 to lower

Adsorption of Phosphate Species on Pt(111) and Pt(100)

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Figure 9. Comparison of the band center frequencies obtained for adsorbed phosphate species on Pt(111) at pH 2.8 (circles) and pH 0.23 (squares) as a function of the potential referred to NHE.

Figure 11. In situ FTIR spectra from Pt(100), taken with p-polarized light in 0.67 HF, 0.5 KF, 1.5 × 10-2 M NaH2PO4 (pH ) 2.8); reference potential 0.03 V vs Pd/H2, sample potentials as indicated.

Figure 10. Cyclic voltammogram of Pt(100) in 0.67 HF, 0.5 KF, 1.5 × 10-2 M NaH2PO4 (pH ) 2.8); pH ) 2.8, sweep rate 50 mV/s.

potentials indicates that the frequency for H2PO4- at 0.40 V in the solution of pH 2.8 is probably strongly affected by the coadsorption of HPO42-, or in other words, a partial deprotonation of H2PO4- seems to take place already at the lowest adsorption potential measured in the solution of pH 2.8 and the observed band center is the result of the superimposition of the corresponding features. Summarizing, the present FTIR results on the phosphate adsorption at Pt(111) allow a clear distinction between adsorbed H3PO4, which presents one band in the range 1035-1055 cm-1, H2PO4-, characterized by two absorption bands at about 1120 and 1000 cm-1, and probably HPO42-, exhibiting one band at 1150-1180 cm-1. Adsorption of Phosphate on Pt(100) at pH ) 2.8. The characteristic voltammetric behavior of the Pt(100) single crystal in the presence of phosphate species is observed in Figure 10. This voltammogram is characterized by a sharp peak at 0.34 V vs Pd/H, corresponding to H adsorption on (100) bidimensional sites and a peak at 0.24 V due to H adsorption at (100) step sites. Similar voltammograms have been reported in ref 20 for Pt(100) in 0.1 M H3PO4. In Figure 11 the in situ FTIR spectra taken at different potentials with p-polarized light at Pt(100) in a phosphate-

Figure 12. Ratio between scaled s-polarized light spectra and p-polarized light spectra from Figure 11 for adsorbed phosphate species on Pt(100) electrode. Solution composition: 0.67 HF, 0.5 KF, 1.5 × 10-2 M NaH2PO4 (pH ) 2.8); reference potential 0.03 V vs Pd/H2, sample potentials as indicated.

containing solution are shown. The same spectra, but rationed with respect to the corresponding spectra obtained with spolarized light (the same procedure as above), are shown in Figure 12. In the studied frequency range (above 900 cm-1), at low potentials two adsorbate bands are present, while with increasing potential only one adsorbate band is observed. Surprisingly, this result is very similar to that shown in Figures 4 and 5 for the Pt(111) surface at pH 2.8, except for the low potential range, where for the Pt(100) the presence of two bands are more pronounced.

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Weber et al. 1. Adsorbed phosphate species at Pt(111) and Pt(100) tend to dissociate on the surface as the electrode potential is made more positive. Adsorbed H2PO4- is transformed to adsorbed HPO42- with increasing potential even in mildly acidic solutions at Pt(111)/Pt(100) electrodes. This behavior marks the strongest difference to the polycrystalline platinum and gold surfaces. 2. The adsorption of phosphate species on platinum single crystals is almost insensitive to the surface structure. Thus, in mildly acidic solutions (pH ) 2.8) not only are the same modes observed for both Pt(100) and the Pt(111) surfaces but also the same ν-E behavior, except for the ca. 100 mV difference in onset of adsorption. Acknowledgment. The authors are indebted to the Brazilian FAPESP, CNPq, CAPES, and FINEP for financial support and to the Forschungszentrum Ju¨lich for the preparation of the single crystal surfaces used in this work. M.W. gratefully acknowledges the DAAD of Germany for a fellowship during his stay at S. Carlos. References and Notes

Figure 13. Potential dependence of the band center frequency for adsorbed phosphate species on Pt(111) (circles) and Pt(100) (squares) at pH 2.8. Solution: 0.67 HF, 0.5 KF, 1.5 × 10-2 M NaH2PO4.

A comparison of the adsorbate band center frequencies on both single crystals indicates also a similar behavior for the adsorbate on both surfaces (see Figure 13). There is only a difference in the potential for the onset of adsorption (on Pt(100) adsorption starts already at 0.30 V, i.e., ca. 100 mV lower than on Pt(111)). The results of Figures 5, 11, and 13 indicate that the adsorption of phosphate species is rather insensitive to the surface geometry, a behavior which clearly differs from that of sulfate.21,22 On the basis of the present results alone, this behavior is difficult to understand. Tentatively, we propose that this effect could be addressed to the ability of phosphate species of forming strong hydrogen bridging with water molecules. A relatively stable adlayer formed by phosphate species coadsorbed with water, which may include protons and other solution components, e.g. Na+ ions, could be the dominating factor determining the adsorption behavior of phosphate at the interface. Coadsorption of anions and cations has been proposed for adsorbed sulfate on Au(111) by Edens et al., on the basis of in situ FTIR spectra and STM data.23 STM data on Pt(111) and Pt(100) could be of help to give a closer insight into the organization of the adsorbate layer. It is difficult at present to explain the difference between Pt(100) and Pt(111) with respect to the potential for the onset of adsorption, Ead. Generally, anion adsorption is expected to begin around the potential of zero charge, and if this is proportional to the work function of the respective surface, the trend Ead(100) < Ead(111) as observed here would be correct. It is noteworthy that the onset potential of adsorption for other anions like sulfate21,22 and acetate14 also follows this trend. However, recent data on the potential of zero charge of Pt single crystals in HClO4 solutions indicate that Epzc(100) ) 0.37 V > Epzc(111) ) 0.34 V.26 Concluding Remarks Two main remarks characterizing the behavior of phosphate on single crystal platinum electrodes should be done.

(1) Iwasita, T.; Nart, F. C.; Rodes, A.; Pastor, E.; Weber, M. Electrochim. Acta 1995, 40, 53-59. (2) Horanyi, G.; Rizmayer, M.; Inzelt, G. J. Electroanal. Chem. 1978, 93, 183-194. (3) Habib, M. A.; Bockris, J. O’M. J. Electrochem. Soc. 1985, 132, 108-114. (4) Paulissen,V. B.; Korzeniewski, C. J. Electroanal. Chem. 1990, 290, 181-189. (5) Nart, F. C.; Iwasita, T. Electrochim. Acta 1992, 37, 385-391. (6) Ye, S.; Kita, H.; Aramata, A. J. Electroanal. Chem. 1992, 333, 299-312. (7) Weber, M.; Nart, F. C. Electrochim. Acta, in press. (8) Iwasita, T.; Nart, F. C. In situ Infrared Fourier Transform Spectroscopy. A Tool to Characterize the Electrode-Metal Interface at a Molecular Level. In AdVances in Electrochemical Science and Engineering; Gerischer, C. W., Tobias, C. W., Eds.; VCH: New York, 1994; Vol. 4, pp 126-215. (9) Clavilier, J.; Faure, R.; Guinet, G.; Durand, R. J. Electroanal. Chem. 1980, 107, 205-209. (10) Nart, F. C.; Iwasita, T. Electrochim. Acta 1996, 41, 653-659. (11) Weber, M.; Nart, F. C. Langmuir 1996, 12, 1895-1900. (12) Steger, E.; Herzog, K. Z. Anorg. Allg. Chem. 1964, 331, 169182. (13) Sa´, E. L.; Pinheiro, A. L. N.; Nart, F. C. Manuscript in preparation. (14) Rodes, A.; Pastor, E.; Iwasita, T. J. Electroanal. Chem. 1994, 376, 109-118. (15) Nart, F. C.; Iwasita, T. J. Electroanal. Chem. 1992, 322, 289300. (16) Orts, J. M.; Go´mez, R.; Feliu, J. M.; Aldaz, A.; Clavilier, J. Electrochim. Acta 1994, 39, 1519-1524. (17) Tejedor-Tejedor, M. I.; Anderson, M. A. Langmuir 1990, 6, 602611. (18) Chapman, A. C.; Thirlwell, E. Spectrochim. Acta 1964, 20, 937947. (19) Weber, M. Doctoral Thesis, University of Bonn, 1996. (20) Herrero, E.; Franaszczuk, K.; Wieckowski, A. J. Phys. Chem. 1994, 98, 5074-5083. (21) Nart, F. C.; Iwasita, T.; Weber, M. Electrochim. Acta 1994, 39, 961-968. (22) Nart, F. C.; Iwasita, T.; Weber, M. Electrochim. Acta 1994, 39, 2093-2096. (23) Edens, G. J.; Gao, X.; Weaver, M. J. J. Electroanal. Chem. 1994, 375, 357-366. (24) Attard, G. A.; Ahmadi, A. J. Electroanal. Chem. 1995, 389, 175190. (25) Iwasita, T.; Xia, X. J. Electroanal. Chem., in press. (26) The latter value was recently confirmed on the basis of IR spectroscopy.24

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