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New Results on the Adsorption of Sulfate Species at Polycrystalline Gold Electrodes. An in Situ FTIR Study M. Weber† and F. C. Nart* Instituto de Quı´mica de Sa˜ o Carlos, Universidade de Sa˜ o Paulo, C.P. 780, 13360-970 Sa˜ o Carlos (SP), Brazil Received February 16, 1995. In Final Form: December 27, 1995X The adsorption of sulfate species at polycrystalline gold electrodes was studied with in situ FTIR spectroscopy in a HF/KF buffer solution of pH ) 2.8. This base electrolyte diminishes the effect of ion migration in the thin layer produced by the loss of solution ions upon adsorption. This allows a clear distinction between adsorbate and solution features. Adsorption of sulfate starts at 0.4 V vs Pd/H2, i.e. about 0.28 V positive to the reported potential of zero charge and reaches a maximum at 1.2 V. This result is in a good agreement with radiotracer data obtained in acid and neutral sulfate solutions. One band between 1165 and 1193 cm-1 is observed at all studied potentials and is assigned to adsorbed SO42-. Below 0.7 V, coadsorption of HSO4- produces a very weak feature at 1100 cm-1. Below 1.0 V the adsorbate behaves as a lattice gas adlayer, but the ions seem to become more immobilized at higher potentials. Lateral interactions increase with potential due to the increase in coverage.
Introduction The understanding of adsorption processes at the electrode/electrolyte interface is a subject of general interest in electrochemistry. Much work has been done at the mercury/electrolyte interface;1 gold electrodes have been studied as a model system for the interfacial properties of solid electrodes due to their large double layer region.2 The application of in situ spectroscopic methods, such as IR spectroscopy, to the study of the electrochemical interface has made it possible to investigate also the electrochemical interfacial structure of other noble metals, among which platinum electrodes have gained the most interest, so far. Vibrational reflectance spectroscopy is generally considered to be a powerful tool to investigate adsorption processes on electrodes at the molecular level.3-5 However, the use of a base electrolyte is necessary to minimize spectral features due to changes in the composition of the solution for the ions studied (for a detailed discussion see refs 6-9. On platinum electrodes, faradaic processes, as in the hydrogen adsorption-desorption reaction and the oxide formation, are the main causes of these changes in the experimental thin layer between working electrode and IR window. Even working within the double layer region of gold electrodes, where no faradaic processes occur, migration effects can influence the obtained spectral data, when no suitable base elec† Present address: Institut fu ¨ r Physikalische Chemie, Universita¨t Bonn, Wegelerstr., 53115 Bonn, Germany. * Author to whom correspondence should be adressed. X Abstract published in Advance ACS Abstracts, March 1, 1996.
(1) Gonzalez, E. R. J. Electroanal. Chem. 1989, 258, 391. (2) Hamellin, A. In Nato ASI Series C, July 10-July 20, 1993, Sophia Antipolis (France); Gewirth, A. A., Ed.; Kluwer Academic Publishers: Dordrecht, The Neetherlands, 1993. (3) Iwasita, T.; Nart, F. C. In Advances in Electrochemical Science and Engineering; Gerischer, C. W., Tobias, C. W., Eds.; VCH: Weinheim, Vol. 4, p 126. (4) Christensen, P. A.; Hamnett, A. In Comprehensive Chemical Kinetics; Compton, R. G., Hamnett, A., Eds.; Elsevier: Amsterdam, 1989; Vol. 29, Chapter 1, p 1. (5) Ashley, K.; Pons, S. Chem. Rev. 1988, 88, 673. (6) Bae, I. T.; Xiang, X.; Yeager, E. B.; Scherson, D. Anal. Chem. 1989, 61, 1164. (7) Bae, I. T.; Scherson, D.; Yeager, E. B. Anal. Chem. 1990, 62, 45. (8) Iwasita, T.; Nart, F. C. J. Electroanal. Chem. 1990, 295, 215. (9) Corrigan, D. S.; Weaver, M. J. J. Electroanal. Chem. 1988, 239, 55.
trolyte is used, as demonstrated by Corrigan and Weaver in the case of the adsorption of N3- on polycrystalline gold surfaces.9 Recently, the adsorption of sulfate and bisulfate anions on polycrystalline and single-crystal gold electrodes was investigated using in situ FTIR spectroscopy.10,11 In the case of gold (111) single-crystal electrodes, it has been shown that practically the same results were obtained in pure sulfuric acid solutions as in the case of a large excess of perchloric acid.10 One interesting point is that no solution loss features have been observed in either case, which was interpreted as a fast migration of sulfate ions from the solution outside the thin layer. In mildly acid solutions, however, the sulfate solution loss features at 1100 cm-1 were clearly observed. The reason for this interesting difference has not been discussed in detail.10 The relative amount of adsorbed ions was compared with radiotracer and capacitance results showing good agreement. The strong potential-dependent feature observed between 1156 and 1220 cm-1 has been assigned to the adsorbed SO42- ions.10 The adsorption of sulfate from sulfuric acid and neutral Na2SO4 solutions on polycrystalline gold surfaces, as studied by Parry et al.,11 shows a completely different behavior. Bipolar bands are observed in both solutions. It has been suggested that sulfate and bisulfate are coadsorbed and that a reversible conversion of sulfate into bisulfate occurs as the potential is changed. In neutral solutions, bisulfate is reported to adsorb predominantly at higher potentials and converted to sulfate at more cathodic potentials. The opposite effect was suggested in the case of acidic solutions.11 Surprisingly, the change in band intensity with applied potential, which is proportional to the adsorbate amount, did not agree with previous radiotracer data for the case of polycrystalline gold electrodes.12 According to these investigations, practically no adsorption is observed below 0 V vs Ag/AgCl ([Cl-] ) 1 M). However FTIR data have been interpreted12 in terms of adsorbed bisulfate (for acidic solutions) or adsorbed sulfate (for neutral solutions) below (10) Edens, G. J.; Gao, X.; Weaver, M. J. J. Electroanal. Chem. 1994, 375, 357. (11) Parry, D. B.; Samant, M. G.; Seki, H.; Philpott, M. R.; Ashley, K. Langmuir 1993, 9, 1878. (12) Zelenay, P.; Rice-Jackson, L. M.; Wieckowski, A. J. Electroanal. Chem. 1990, 283, 389.
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0.1 V vs Ag/AgCl ([Cl-] ) 3 M).12 Furthermore, practically no frequency shift with potential was observed for the spectral features attributed to adsorbed species. In the present publication we present new in situ FTIR data on the sulfate adsorption at polycrystalline gold. Following the experimental approach applied to the study of other systems,13,14 a HF/KF base electrolyte was used for the present investigation. It will be shown that with this electrolyte it is possible to clearly distinguish between solution and adsorbate spectral features. The nature of the adsorbed species will be discussed on the basis of spectral data obtained at different potentials, solution pHs, and concentrations. The results will be compared with the ones obtained by the radiotracer method. Experimental Section A BOMEM DA-8 spectrometer with a liquid nitrogen cooled MCT detector was used for the experiments. The electrochemical cell was made of PTFE with a CaF2 window placed at the bottom of the cell. The gold working electrode (diameter 7 mm) was mounted in a Teflon holder and polished successively with 1, 0.3, and 0.05 µM Al2O3 (Buehler). A gold ring was used as the counter electrode. A Pd/H2 electrode served as a reference. Solutions were prepared with Merck Suprapur chemicals and Millipore MilliQ water. The base electrolyte consisted of HF for the solution pH ) 0.23 (7.3 M HF) and of a mixture of HF and KF for the solution pH ) 2.8 (0.67 M HF, 0.5 M KF), respectively. The reference potential in all experiments was 0.03 V vs Pd/ H2. At this potential negligible adsorption of sulfate is expected, as indicated by radiotracer data.12 Spectra were taken at the reference (R0) and the sample potentials (R) with a resolution of 8 cm-1 by switching the potential several times each 1000 scans. The resulting spectra are presented as the reflectance ratio (R/ R0). Using this ratio, positive bands are assigned to the loss of solution species during the adsorption and negative bands are assigned to species formed at the studied potential.
Results and Discussion 1. Identification of the Adsorbate and Solution Features. In Figure 1, three spectra at E ) 1.2 V recorded with s- (a) and p-polarized light (b) at pH ) 2.8 and p-polarized light (c) at pH 3.5 are shown. The spectrum taken with s-polarized light shows only one positive going band at 1108 cm-1. The same band can be observed in the spectra taken with p-polarized light, indicating clearly that this feature is a solution band. The 1108 cm-1 band is assigned to the ν3 triply degenerated mode of the free sulfate ion.15 The presence of only the sulfate loss band from solution is consistent with the solution composition, due to the sulfate excess in solution (ratio: SO42-/HSO4- ) 6.67). The sulfate loss in the solution during the adsorption process can be clearly observed, which means that migration into and out of the thin layer due to sulfate consumption and double layer charging is largely compensated by the fluoride ions. The p-polarized light spectrum shows a strong negative going feature at 1190 cm-1 and very weak band at 958 cm-1, which are not present in the spectrum taken with s-polarized light. Considering the surface selection rule, s-polarized light does not interact with adsorbed species since it has a zero electric field at the metallic surface. Therefore, it can be concluded that the negative bands belong to an adsorbed species formed at the sample potential. (13) Nart, F. C.; Iwasita, T. J. Electroanal. Chem. 1992, 322, 289. (14) Nart, F. C.; Iwasita, T. Electrochimica Acta 1992, 37, 385. (15) Nakamoto, K. Infrared and Raman Spectra of Inorganic and Coordination Compounds; Wiley: New York, 1978.
Figure 1. In situ FTIR reflectance spectra electrode/electrolyte interface at E ) 1.2 V with (a) s-polarized light, (b) p-polarized light at pH 2.8 (solution composition: 0.69 M HF, 0.5 M KF, 1.5 × 10-2 M Na2SO4), and (c) p-pol light at pH 3.5 (solution composition: 0.18 M HF, 1.0 M KF, 1.5 × 10-2 M Na2SO4); E0 ) 0.03 V. (Positive going features are due to the loss of solution species during the adsorption, and negative going bands are due to the adsorbed species.)
An interesting result is that the strong adsorbate band is located at wavenumbers significantly higher (here at about 1190 cm-1) than those of the solution species (1108 cm-1). This contrasts with the observation of Parry et al. who reported spectral features at about 1110 cm-1. Strong frequency shifts upon adsorption of sulfate species have already been reported for polycrystalline16 and single-crystal platinum surfaces17,18 as well as in the case of the Au(111) surface.10 The second very weak band is located at the wavenumber region typical of the forbidden symmetric band of the free sulfate ion. 2. Amount of Adsorbed Sulfate Species as a Function of Potential. Since the IR cross section of vibrational modes for adsorbed species is strongly influenced by the electric field as well as by lateral interactions and interactions with the substrate, the use of adsorbate bands to evaluate the surface coverage is not appropriate. On the other hand, the loss of solution sulfate upon adsorption is a useful way to measure the relative amount of species adsorbed at the surface. It is more suitable to use spectra taken with s-polarized light which are not affected by the adsorbate features. The spectra for different potentials are shown in Figure 2, and the integrated band intensity is plotted as a function of the applied potential in Figure 3. The onset of sulfate adsorption starts at about 0.4 V, about 280 mV more positive than the potential of zero charge (pzc), which was reported to be 0.12 V vs RHE in 0.1 M Na2SO4 on polycrystalline gold in the pH range 2-8.19 A maximum coverage is observed at about 1.01.2 V. (16) Nart, F. C.; Iwasita, T. .J. Electroahal. Chem. 1991, 308, 277. (17) Nart, F. C.; Iwasita, T.; Weber, M. Electrochimica Acta 1994, 39, 2093. (18) Nart, F. C.; Iwasita, T.; Weber, M. Electrochimica Acta 1994, 39, 961. (19) Bode, D. D., Jr.; Andersen, T. N.; Eyring, H. J. Phys. Chem. 1967, 71, 792.
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Figure 2. In situ FTIR reflectance spectra for the electrode/ electrolyte interface with s-polarized light at different potentials. Solution composition: 0.69 M HF, 0.5 M KF, 1.5 × 10-2 M Na2SO4 (pH ) 2.8); E0 ) 0.03 V.
Figure 3. Integrated band intensity calculated from the spectra of Figure 2 (Is) for the sulfate loss during adsorption as a function of the applied potential.
These results are in a good agreement with the adsorption characteristic of sulfate species in neutral and acid solution, as studied with the radiotracer method.12 Similar results have been reported for sulfate adsorbed on polycrystalline platinum20,21 and gold (111) single crystals.10 This adsorption behavior contrasts distinctively with the spectra reported by Parry et al.11 for (20) Kazarinov, V. E.; Balashova, N. A. Dokl. Phys. Chem. 1964, 157, 795. (21) Bockris, J. O’M.; Gamboa-Aldeco, M.; Szklarczyk, M. J. Electroanal. Chem. 1992, 339, 355.
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polycrystalline gold electrodes, where strong vibrational features at potentials negative of the pzc are observed. These differences are probably due to the absence of a suitable base electrolyte.8 It should be emphasized that the fluoride buffer solution used as supporting electrolyte in in situ FTIR experiments yields the same change in the adsorbate amount with potential as in the case of the radiotracer method. This illustrates again the importance of the compensation of migration effects. 3. Nature of the Adsorbed Species on Polycrystalline Gold Electrodes. The spectra taken with p-polarized light for the adsorption of sulfate from a solution pH of 2.8 are plotted in Figure 4 as a function of the potential (see also Figure 6a), where a spectrum at 0.55 V is plotted in expanded scale. Besides the solution loss at 1108 cm-1, the spectra show at all potentials one adsorbate feature between 1175 and 1193 cm-1. Another very weak band can be seen at about 958 cm-1, but in several spectra it is difficult to detect since the signal/noise ratio increases in this wavenumber region due to the cutoff of the CaF2 window used in these experiments. Below 0.7 V an additional, very weak feature can be seen at 1090 cm-1. A noticeable dependence of the band center frequency on the potential is observed for the adsorbate feature located between 1175 and 1195 cm-1 (see Figure 5). The band center frequency shifts linearly with increasing potential with a slope of dν/dE ) 44 cm-1/V (for c ) 1.5 × 10-2 mol/L). The same adsorbate feature on a polycrystalline platinum surface shows a value of 76 cm-1 (for c ) 2.5 × 10-2 mol/L).11 As discussed previously,14 this band shift is caused by the increasing interfacial electrical field as well as the increasing lateral interactions, which is due to increasing surface coverage. The relatively small band shift in comparison to polycrystalline platinum or even to Au(111) single-crystal electrodes could be due to a smaller coverage degree in the case of polycrystalline gold. 3.1. Coadsorption of Sulfate and Bisulfate Species. Unless the acid/base equilibria change drastically at the surface, SO42- ions are expected to be the dominant adsorbed species. A change in the acid/base equilibrium at the polarized electrode surface should be expected. It is very likely that in the case of a positive polarization, the contact adsorbed HSO4- ions are converted to adsorbed sulfate ions. Furthermore, the bisulfate ion adsorbed on polycrystalline platinum electrodes exhibits one strong feature at about 1100 cm-1.13 This feature has been assigned to the symmetric stretching vibration of the SO3 group frequency of adsorbed bisulfate with a 3-fold coordination. The asymmetric mode of the bisulfate ion in solution located at ca. 1190 cm-1, is forbidden by the surface selection rule and cannot be observed unless the surface selection rule is broken. Finally, bisulfate ions in solution present also a feature at about 950 cm-1 due to the S-OH stretching vibration. It seems reasonable that the 1090 and the 958 cm-1 bands may be identified with the presence of a small amount of coadsorbed bisulfate ions. This hypothesis seems to agree with the fact that the 1090 cm-1 band disappears for potentials higher than 0.9 V in the spectra shown in Figure 4. To clarify this point we have obtained spectra at a very low pH, where bisulfate ions are the predominant species in solution. In Figure 6 the spectra taken at pH 2.8 and 0.23 are compared. At the lower pH the expected positive going bands caused by the solution loss of HSO4- ions at 1195 and 1050 cm-1 overlap with two adsorption bands
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Figure 4. In situ FTIR reflectance spectra of adsorbed sulfate at polycrystalline gold with p-polarized light at different potentials. Solution composition: 0.69 M HF, 0.5 M KF, 1.5 × 10-2 M Na2SO4 (pH ) 2.8); E0 ) 0.03 V.
Figure 5. Frequency shift of the adsorption band as a function of the applied potential. Solution composition: 0.69 M HF, 0.5 M KF, 1.5 × 10-2 M Na2SO4 (pH ) 2.8).
located at ca. 1150 and ca. 1090 cm-1. Comparing the relative intensities of the adsorbate bands at the different pH values it seems reasonable to assign the feature at ca. 1090 cm-1 to adsorbed bisulfate. The band at 1150 cm-1 belongs to the adsorbed sulfate as it is the dominating species at higher pH (i.e. pH ) 2.8) and increasing potential. The question whether the band at about 958 cm-1 belongs to both adsorbed sulfate and bisulfate cannot have a definite answer based on these spectroscopic results as sulfate is still coadsorbed with bisulfate at low solution pHs. On the other hand the weak band at 958 cm-1 is also observed at higher solution pHs and therefore seems to belong to adsorbed sulfate also. In view of these arguments, it can be assumed that the adsorbate in weak acid solution is predominately the
Figure 6. In situ FTIR reflectance spectra of adsorbed sulfate at polycrystalline gold with p-polarized light at 0.55 V. Solution composition: (a) pH ) 2.8: 0.69 M HF, 0.5 M KF, 1.5 × 10-2 M Na2SO4; (b) pH ) 0.23: 7.3 M HF, 1.5 × 10-2 M Na2SO4; E0 ) 0.03 V.
sulfate ion. Bisulfate ions seem to be present in small amounts at pH ) 2.8. For potentials above 0.9 V the adsorbed bisulfate ions practically disappear from the surface, as indicated by the absence of the 1090 cm-1 band. In the case of Au(111) this feature is also assigned to the adsorbed sulfate ions.10 3.2. Geometry of the Adsorbed SO42-. The presence of two adsorption bands for the adsorbed sulfate on gold suggests a break up of the Td symmetry of the free sulfate after adsorption. In this case both a C3v or a C2v coordination symmetry are possible. In both cases, it is expected that the totally symmetric mode is activated. Moreover, a lower symmetry would lead to a splitting of the 3-fold degenerated mode located at 1100 cm-1, producing two or three bands for the C3v or C2v symmetry,
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Figure 7. Surface selection rule allowed SO4 stretching vibration for adsorbed sulfate ions under C3v (a) and C2v (b) symmetry.
respectively. Nevertheless, in both cases only one mode has a dipole moment change perpendicular to the surface, according to the character table, and thus only one active mode will be seen. The allowed active stretching modes for the case of a C2v and C3v symmetry are shown in Figure 7. In both cases the active mode corresponds to a SO4 vibration with dipole moment perpendicular to the surface. It is very likely that due to the difference in the number of equivalent oxygen atoms (two equivalent SO2 groups in the case of the C2v symmetry and the SO3 and SO groups for a C3v symmetry) at least small difference in the wavenumber for each case may occur. Nevertheless, it is difficult to show to which of the splitted modes (C3v or C2v symmetry) the observed feature belongs. A detailed normal coordinate analysis will be required to distinguish between the relative position of these two modes, which lies beyond the scope of the present work. Moreover, the overlap of the positive band may affect the real band position, introducing more complications into the analysis. The goal of these measurements is to establish the approximate band position of the adsorbed sulfate and bisulfate on gold polycrystalline surfaces and to verify the possibility of species interconversion at the surface as a function of potential as reported recently by Parry et al.11 From our spectra, it is possible to conclude that in weak acid solutions only a small amount of adsorbed bisulfate is present at the surface at lower potentials. Interestingly, the adsorption of sulfate on Au(111) single crystals10 has revealed that only sulfate ions are adsorbed even at very acidic solutions. 4. Interactions within the Adsorbed Adlayer. The surface coverage which is proportional to the loss of sulfate from the solution can be evaluated through the integrated intensity of the solution loss band at 1108 cm-1 observed with s-polarized light. In the plots of Figure 8 the integrated band intensity is linearly proportional to the logarithm of the sulfate ion concentration in solution, which resembles a Temkin isotherm. The Temkin isotherm for a very low coverage degree is an unexpected result. For adsorbed sulfate ions on polycrystalline platinum14 the possibility of the Temkin isotherm was explained, assuming that the sulfate ions are adsorbed in islands, which could account for the strong lateral interaction observed in that case. The lateral interaction can be deduced from the plot of the band center wavenumber as a function of the sulfate
Figure 8. Integrated band intensity for spectra obtained with s-polarized light (Is) of the sulfate loss from solution as a function of the solution concentration at 0.6 V (b), 0.7 V (9), 0.8 V (2).
Figure 9. Dependence of the band center frequency of the adsorbed sulfate at different solution concentrations at E ) 1.2 V (b), 1.0 V (9), 0.8 V (2).
coverage at a given potential. In Figure 9 the band center wavenumber is plotted as a function of log c to evaluate the effect of lateral interactions. A constant value for the band center frequency is observed at 0.8 V, while a slight dependence on coverage is observed at higher potentials. This result shows that at low potentials, lateral interactions are very weak, thus the hypothesis of island formation does not apply to sulfate adsorbed on gold electrodes, and the Temkin isotherm seems not to be applicable in this case. Zelenay et al.12 have found through thermodynamic analysis a strong lateral interaction for adsorbed sulfate on polycrystalline gold in neutral and acid media. This result contrasts with our observation of a small interaction
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between the adsorbed sulfate ions adsorbed on gold in weak acid solution. Further investigations, especially those on well-defined single-crystal surfaces, will be necessary to clarify this question. Conclusions The adsorption of sulfate species on polycrystalline gold was studied in HF/KF containing electrolytes which allows a clear distinction between adsorbate and solution features. (i) The adsorption of sulfate at polycrystalline gold starts at about 0.4 V vs Pd/H2, about 0.28 V higher than the pzc reported for this system. The amount of adsorbed species on the electrode surface reaches a maximum at potentials at about 1.0-1.2 V. (ii) Mainly one adsorption feature at 1163-1195 cm-1 has been observed in weak acid solutions showing a potential dependent frequency shift of about 44 cm-1/V. It is assigned to adsorbed SO42-.
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(iii) For adsorption potentials lower than 0.7 V, an additional very weak negative going vibrational feature located at about 1090 cm-1 can be seen in solutions of pH ) 2.8 as well as in strongly acid solutions (pH ) 0.23), which is assigned to traces of bisulfate coadsorbed at these lower potentials. (iv) Lateral interactions practically do not exist at potentials below 1.0 V. Above 1.0 V lateral interactions increase and cause a change in the band center frequency with the coverage. Acknowledgment. The authors are grateful to T. Iwasita for critical reading of the manuscript and helpful comments. They also thank FAPESP (93/1411-9), FINEP, and CNPq from Brazil for financial support. M. Weber thanks the DAAD of Germany and the CAPES of Brazil for a fellowship. LA950119Z
