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An in Situ Infrared Study on the Effect of pH on Anion Adsorption at Pt(111) Electrodes from Acid Sulfate Solutions Peter W. Faguy* and Nebojsˇa S. Marinkovic´† Department of Chemistry, University of Louisville, Louisville, Kentucky 40292
Radoslav R. Adzˇic´ Department of Applied Science, Molecular Sciences Division, Brookhaven National Laboratory, Upton, New York 11973 Received February 14, 1995. In Final Form: October 31, 1995X From the in situ FTIR spectroscopy of the electrode/electrolyte solution interface at pH values of 1.2, 2.0, and 3.4, it can be confirmed that the adsorbate associated with the anomalous peaks in the cyclic voltammetry of Pt(111) in sulfate- and bisulfate-containing solutions is not the sulfate anion. The structure of the bisulfate-like adsorbate is tentatively postulated to be a sulfate ion/hydronium ion ion pair: SO42-‚H3O+. Over the potentials in question, and only in solutions with appreciable HSO4- concentration, can IR bands be found that are associated with the adsorbed species.
Introduction An important tool for the in situ study of molecular adsorbates at electrode surfaces is Fourier transform infrared (FTIR) spectroscopy. This is evident in the body of work dealing with anion adsorption on Pt single crystal electrodes.1-11 A well-studied problem has been the elucidation of the molecular structure of the adsorbate associated with the anomalous region of the cyclic voltammetry of Pt(111) in sulfate- and bisulfate-containing solutions. While there has been agreement in experimental results,1-5 there has been some disagreement in the proposed structure of the adsorbate as deduced from infrared measurements. The purpose of this paper is to support the conclusion that the adsorbate associated with the anomalous peaks is not the sulfate anion, and to propose an adsorbate structure which is consistent with the experimental evidence. The experimental basis found in this paper is a comparison of in situ FTIR data taken over a range of pH values for equivalent electrochemical conditions. The anomalous peaks in the cyclic voltammetry of Pt(111) and the lack of a full hydrogen coverage in the normal adsorption region, for bisulfate-containing electrolyte solutions, have been the source of much discussion and * To whom correspondence should be addressed. † Present address: Department of Chemistry, University of California, Davis. X Abstract published in Advance ACS Abstracts, December 1, 1995. (1) Faguy, P. W.; Markovic, N. M.; Adzic, R. R.; Fierro, C. A.; Yeager, E. J. Electroanal. Chem. 1990, 289, 245. (2) Nichols, R. J. In Adsorption of Molecules at Metal Electrodes; Lipowski, J. L., Ross, P. N., Eds.; VCH: New York, 1991; Chapter 7. (3) Sawatari, Y.; Inukai, J.; Ito, M. J. Electron Spectrosc. 1993, 64/ 65, 515. (4) Ogasawara, H.; Sawatari, Y.; Inukai, J.; Ito, M. J. Electroanal. Chem. 1993, 358, 337. (5) Nart, F. C.; Iwasita, T.; Weber, M. Electrochim. Acta. 1994, 39, 961. (6) Faguy, P. W.; Markovic, N. M.; Ross, P. N. J. Electrochem. Soc. 1993, 140, 1638. (7) Nart, F. C.; Iwasita, T.; Weber, M. Electrochim. Acta 1994, 39, 2093. (8) Ogasawara, H.; Inukai, J.; Ito, M. Surf. Sci. 1994, 331, L665. (9) Kim, C. S.; Korzeniewski, C. J. Phys. Chem. 1993, 97, 9784. (10) Stuhlmann, C.; Villegas, I.; Weaver, M. J. J. Chem. Phys. Lett. 1994, 219, 319. (11) Ye, S.; Kita, H.; Aramata, A. J. Electroanal. Chem. 1992, 333, 299.
0743-7463/96/2412-0243$12.00/0
analysis in the recent literature.1-5,12-23 For sulfuric acid solutions, radiotracer results have confirmed that sulfurcontaining species adsorb on the electrode surface over the potential range encompassing the anomalous peaks.18,19 Monovalent anion adsorption has been indicated both in electrochemical studies using competing adsorbate probe molecules22 and in a recent chronocoulometric coverage study.23 While these techniques provide detailed information regarding coverage, the need for molecular structural information has been addressed almost exclusively by infrared spectroscopy. Several groups have performed IR measurements of anion adsorption on Pt(111) electrodes from sulfuric acid solution.1-5 The major feature reported has been a ∼1250 cm-1 band which has a large Stark tuning rate, >100 cm-1/V; i.e., the peak position is strongly potential dependent. The groups of Yeager,1 Bewick,2 and Ito3,4 all propose that this mode is due to bisulfate adsorption, as do researchers at IBM who see the same mode appearing in polycrystalline studies.24 Nart et al.,5 on the other hand, have interpreted essentially the same Pt(111) data as due to sulfate adsorption. They argue that the lack of a feature at 950 cm-1, the presence of a shoulder at ∼1200 cm-1, and the consideration of solution pH effects lead to sulfate adsorption as the more consistent hypothesis. In all of (12) Al Jaaf-Golze, K.; Kolb, D. M.; Scherson, D. J. Electroanal. Chem. 1986, 200, 353. (13) Motoo, S.; Furuya, N. Ber. Bunsenges. Phys. Chem. 1987, 91, 457. (14) Markovic, N. M.; Marinkovic, N. S.; Adzic, R. R. J. Electroanal. Chem. 1988, 241, 309. (15) Clavilier, J. In Electrochemical Surface Science; Soriaga, M. P., Ed.; ACS Symposium Series 378; American Chemical Society: Washington, DC, 1988; Chapter 14. (16) Ross, P. N. In Electrochemical Surface Science; Soriaga, M. P., Ed.; ACS Symposium Series 378; American Chemical Society: Washington, DC, 1988; Chapter 3. (17) Ross, P. N. J. Chim. Phys. 1991, 88, 1353. (18) Wieckowski, A.; Zelenay, P.; Varga, K. J. Chim. Phys. 1991, 88, 1247. (19) Gamboa-Aldeco, M. E.; Herrero, E.; Zelenay, P. S.; Wieckowski, A. J. Electroanal. Chem. 1993, 348, 451. (20) Molina, F. V.; Parsons, R. J. Chim. Phys. 1991, 88, 1339. (21) Marinkovic, N. S.; Markovic, N. M.; Adzic, R. R. J. Electroanal. Chem. 1992, 330, 433. (22) Adzic, R. R.; Feddrix, F.; Nikolic, B. Z.; Yeager, E. J. Electroanal. Chem. 1992, 341, 287. (23) Savich, W.; Sun, S.-G.; Lipkowski, J.; Wieckowski, A. J. Electroanal. Chem. 1995, 388, 233. (24) Samant, M. G.; Kunimatsu, K.; Seki, H.; Philipott, M. R. J. Electroanal. Chem. 1990, 280, 391.
© 1996 American Chemical Society
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Table 1. Solution Composition and Adsorbate-Associated Band Changes for PDIR Spectra of Pt(111) Electrodes in Sulfate/Bisulfate Solutions pH
[HSO4-] (mM)
Ebgnd (V)
Esample (V)
0.23b 1.2 2.0 2.8b 3.4
24 41 22 2.7 0.6
-0.24 -0.25 -0.30 -0.24 -0.40
+0.13 to +0.73 +0.10 to +0.40 +0.00 to +0.40 +0.23 to 0.73 +0.00 to +0.40
intensity changesa ν1(ads) ν1(sol) ν2(ads) + + + + 0
+/0 +
c + + c 0
a Direction of intensity changes with increasingly positive sample potentials. b These data are taken from ref 5. c 945 cm-1 band cannot be distinguished from the background.
the in situ IR work there has been some attention given to adsorbate orientation at the well-defined electrode surface as a function of electrode potential. Further efforts in this area are left for a more exhaustive treatment,25 this paper deals only with the effect of pH on the in situ spectra. Experimental Section The cell design and the optical layout are described in detail in ref 26. The salient features are that a ZnSe hemisphere window was used with an incident angle of 36° and that the single beam spectra result from 4096 (128 × 32 steps) scans co-added into each emissivity spectrum at 16 cm-1 resolution. With this optical configuration a dramatic improvement in sensitivity is obtained and reproducibility is enhanced.26 All potential difference infrared (PDIR) spectra27 have been calculated from sample and background spectra as indicated in Table 1. For the spectra presented, and at each pH used, the background potential lies in the hydrogen adsorption region, unless otherwise noted. The PDIR spectra are presented as -∆R/R spectra, where features pointing up indicate positive changes in coverage or concentration at the sample potential relative to the background potential. Due to throughput limitations, the spectra presented here all have a low frequency cutoff of 900 cm-1. All potentials are given against a Ag/AgCl reference electrode. The Pt(111) single crystal electrode was polished, cleaned, and flame-annealed following standard procedures.14-16 Cyclic voltammetry was used to confirm electrode surface integrity prior and subsequent to the FTIR spectroscopic data collection. Solutions were made of concentrated sulfuric acid (Fisher, Optima grade) and KOH (Fisher) and Barnstead NANOpure 18 MΩ water. The electrolyte solutions were made from mixtures of stock KOH and H2SO4 solutions so as to achieve the stated pH, with the resulting combined sulfate and bisulfate concentrations ranging from 0.017 to 0.05 M.
Results and Discussion The infrared spectra of dilute sulfate- and bisulfatecontaining aqueous solutions28,29 in the region between 1400 and 800 cm-1 consist of five major peaks due to various S-O stretching modes for the tetrahedral sulfate anion and the C3v-symmetric bisulfate anion. For sulfate there are two possible normal modes associated with S-O stretching: the totally symmetric SO stretch (980 cm-1), labeled ν1′ in this study, and the triply degenerate SO stretch, labeled ν3′ (1100 cm-1). For bisulfate there are three normal modes associated with S-O stretching: the totally symmetric SO3 stretch labeled ν1 (1050 cm-1), the totally symmetric S-(OH) stretch labeled ν2 (885 cm-1), and the doubly degenerate SO3 stretch labeled ν4 (1200 cm-1). More detailed analysis of the symmetry consid(25) Faguy, P. W.; Marinkovic, N. S.; Adzic, R. R. J. Electroanal. Chem., in press. (26) Faguy, P. W.; Marinkovic, N. S. Anal. Chem. 1995, 67, 2791. (27) Corrigan, D. S.; Weaver, M. J. J. Electroanal. Chem. 1988, 239, 55. (28) Giguere, P. A.; Savoie, R. Can. J. Chem. 1960, 38, 2467. (29) Gillespie, R. J.; Robinson, E. A. Can. J. Chem. 1962, 40, 644.
Figure 1. PDIR spectra, 1350-900 cm-1, for different bulk solution pH values: (a) pH 1.2, (b) 2.0, and (c) 3.4. Ranges of sample potentials are noted on the figure: (a) spectra shown every 76 mV; (b and c) spectra shown every 100 mV. Background potentials are listed in Table 1. Dotted reference lines indicate, from left to right, ν1(ads), ν1(sol), and ν2(ads) bands. Spectra are offset for clarity.
erations can be found in the literature on Raman spectroscopy of anionic equilibria30,31 and in the IR spectroelectrochemical literature.1-8,24,25,32 Each of the stack plots in Figure 1 contains five spectra taken at different sample potentials over a range which encompasses the anomalous CV region (see Figure 2). The sample potential ranges and background potentials are listed in Table 1, as are the pH values and calculated bulk bisulfate concentrations. For all of the spectra shown, the background potential was selected to be just positive of hydrogen evolution. Therefore, it is expected that the spectra will possess features arising from adsorption/ desorption at the Pt(111) surface, changes in the anion concentration distribution, and changes in the acid-base equilibrium in the thin layer cell as sampled by the spectroelectrochemical experiment.32,33 The largest feature in the PDIR spectra shown in Figure 1a (pH ) 1.2) is a band which changes position from 1227 cm-1 at +0.25 V to 1246 cm-1 at 0.4 V. Figure 1b (pH ) 2.0) contains similar features: a shoulder at 1227 cm-1 (+0.2 V) which shifts to 1250 cm-1 (+0.4 V). However, now the band is superimposed on large solution-phase features. The ∼1250 cm-1 mode is associated with the adsorbate responsible for the anomalous peaks in the CV (30) Chen, H.; Irish, D. E. J. Phys. Chem. 1971, 75, 2672. (31) Dawson, B. S. W.; Irish, D. E.; Toogood, G. E. J. Phys. Chem. 1986, 90, 334. (32) Nart, F. C.; Iwasita, T. J. Electroanal. Chem. 1991, 308, 277. (33) Bae, I.; Xing, X.; Yeager, E.; Scherson, D. Anal. Chem. 1989, 61, 1164.
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Figure 2. Comparison of positive linear sweep voltammetry for a Pt(111) electrode at different bulk solution pH values: (a) pH 1.2, (b) 2.0, and (c) 3.4.
and will be denoted here as ν1(ads). In the same stack plot, at less positive sample potentials, there are features in the difference spectra attributable to changes in the anion concentration profile. An excess of both HSO4- and SO42- relative to their concentrations in the bulk electrolyte solution will begin to appear in the diffuse layer as the electrode is made more positive. The difference in concentration profiles between the sample potential and the background potential results in bands in the PDIR spectra due to solution-phase anions.27,32-35 The bottom two spectra of Figure 1a are due to the superposition of ν1′, ν3′, ν1, and ν4 solution-phase bands. No intensity for the ν1(ads) band is found. This is expected as these PDIR spectra were obtained at sample potentials before the anomalous peak region. From the IR data presented here and in other studies,1,25 the onset of anion specific adsorption occurs approximately 90 mV more positive than its onset as determined from cyclic voltammetry. This is probably due to the differences in sensitivity between the spectroscopic and electrochemical techniques. Further analysis of in situ data obtained at pH values of 1.2 and 2.0 leads to the observation of two other potential-dependent spectral features which cannot be ascribed to pH or migration changes. Correlated with the ν1(ads) band, both a negative-going band at 1050 cm-1 and a positive-going band at 945 cm-1 appear. The former can be assigned to the ν1 band of HSO4- in solution and will hereafter be referred to as ν1(sol). Its appearance has been previously reported4 and is discernible, in retrospect, in earlier published spectra.1,2 The lower frequency mode, called ν2(ads) here, is not identifiable with any solution-phase species. In frequency, it lies between the ν1′ mode (SO42-) at 980 cm-1 and the (34) Ashley, K.; Samant, M. G.; Seki, H.; Philpott, M. R. J. Electroanal. Chem. 1989, 270, 349. (35) Samant, M. G.; Kunimatsu, K.; Seki, H. Anal. Chem. 1994, 66, 1781.
Figure 3. PDIR spectra, 1065-900 cm-1, for different bulk solution pH values: pH (a) 1.2, (b) 2.0, and (c) 3.4. Same spectra as shown in Figure 1 but with enlarged intensity scales and smaller offsets. Dotted reference lines indicate ν1(sol) and ν2(ads) bands.
ν2 mode (HSO4-) at 885 cm-1. A peak at 948 cm-1 in the Raman spectra of SO42-/HSO4--containing solutions has been ascribed to the H3O+‚SO42- ion pair.30 Considering the conceptualized adsorbate structure: Pt‚‚‚O3SO2-‚H3O+, the ν2(ads) mode seen in this study could be assigned to the S-O stretch of such an adsorbed ion-pair.25 The presence of this adsorbate mode is extremely important to the argument against sulfate adsorption. The absence of a ∼940 cm-1 has been used to rationalize sulfate adsorption.5 However, its appearance has been noted in a Pt(111) study,2 but without assignment. It cannot be discerned in other Pt(111) published spectra,1,5 probably due to decreased sensitivity. It is evident from Figure 1 that the ν1(ads) band can be found in all of the PDIR spectra taken at sample potentials above +0.22 V and for solution pH values below or close to the second equilibrium constant of sulfuric acid, pKa ) 1.89. In Figure 1b the adsorbate mode is superimposed on the shoulder of the growing ν4 band, while in the lowest pH solution, Figure 1a, the change in bisulfate concentration is much less dramatic. It has been pointed out before12,15,16 that the anomalous peak behavior is pH independent, as is the potential dependence of the ν1(ads) band. This infrared absorption cannot be found in any of the PDIR spectra shown in Figure 1c, spectra taken under conditions of nearly complete dissociation, R > 0.96, where HSO4- anions account for less than 3% of the anionic charge present in the bulk solution. Although the change in intensity in the difference spectra for the ν1(sol) band is evident in the spectra shown in Figure 3a, it is not apparent in Figure 3b, even though it is expected due to the presence of the ν1(ads) band at
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a solution pH of 2. For replicate experiments to Figure 3b, the band may always be positive or it may change sign. The 1050 cm-1 band behaves like this because the dominant difference in infrared absorptions between the two potentials at pH ) 2, which generates the PDIR spectra, is the conversion of SO42- to HSO4-. Two counteracting processes are taking place: the loss of solution bisulfate to adsorption and the conversion of sulfate to bisulfate. Upon careful inspection of Figure 3b it is evident that the rate of ν1(sol) intensity increase drops off as the anomalous CV region is entered. When equivalent experiments are performed with a background potential that does not inject protons into the thin-layer cell, the 1050 cm-1 band does indeed change sign at sample potentials commensurate with the onset of the features in the CV. Also evident in parts a and b of Figure 3 is the ν2(ads) band at 945 cm-1. The intensity of this mode correlates well with the ν1(ads) mode25 and, perhaps most compelling to the case presented here, it is not evident in Figure 3c. The PDIR spectra taken at a pH of 3.4 consist of features due solely to changes in the thin-layer bulk solution composition. In Figure 3c, the only features visible are the ν1 mode which increases in intensity over the whole Esample range, and the ν3′ mode which decreases in intensity. This simply reflects the fact that enough proton concentration change occurs to effect the conversion of sulfate to bisulfate. Spectra taken with IR radiation polarized perpendicularly with respect to the plane of incidence are often used as a tool to distinguish “surface” species from “bulk” species in IR spectroelectrochemistry. Features found in the s-polarized data can come from two sources, either they result from leakage of p-polarized light due to intrinsic limitations in the wire grid polarizer36 or they arise from differences in solution composition occurring some distance from the electrode surface, that is, in the bulk electrolyte solution.32-35 For the 945 cm-1 band the expected extinction ratio36 is high enough and, in the experiments presented here, the noise is large enough that if the band is due to an adsorbate it will not be seen in the s-polarized spectra. This is confirmed in the PDIR spectra shown in Figure 4a. The only feature seen in this overlay plot is the loss of the ν1(sol) mode in the bulk electrolyte due to conversion of bisulfate to sulfate. In contrast, when p-polarized light is selected under identical conditions, the ν2(ads) mode is easily seen (Figure 4b) and its potential dependency tracks the cyclic voltammetry. Notice that the 1050 cm-1 band changes direction as the sample potential is made more positive, as has been pointed out earlier. When the background potential is set positive of the hydrogen desorption region and the equivalent PDIR spectra obtained (Figure 4c), the ν2(ads) mode remains the same but the changes in the 1050 cm-1 band now reflect the difference in proton concentration change due to the different background potentials, Figure 4b vs Figure 4c. These data again indicate that the 1050 cm-1 band is due to a solution phase species and the 945 cm-1 band is due to an adsorbate and that both bands correlate to the ν1(ads) mode and to the cyclic voltammetry. As these experiments were performed in a thin-layer cell there is some concern that the solution pH was not recovering upon sequential cycling, thus the true pH of the thin-layer cell could be higher. By alternately poising the electrode at the background potential and then at the sample potential, with a total time at any one potential of ∼16 s, it was hoped that pH changes would be minimized. Certainly they are better maintained than if (36) Faguy, P. W.; Marinkovic, N. S. Surf. Sci. 1995, 339, 329.
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Figure 4. PDIR spectra, 1065-900 cm-1, at pH ) 1.2: (a) Ebgnd ) -0.25 V and s-polarized light selected, (b) Ebgnd ) -0.25 V and p-polarized light selected, and (c) Ebgnd ) +0.06 V and p-polarized light selected. Esample ranges from +0.10 to +0.36 V in 37 mV increments. Arrows denote direction of intensity change as Esample is made more positive. Numbered spectra in (b) show direction of change for ν1(sol) band. Spectra are offset with a pseudoisosbestic point at 975 cm-1.
the total 4096 scans were collected consecutively at the sample potential. Nevertheless, the pH could be higher in the experiment and the actual bisulfate concentration in the thin-layer cell at a bulk solution pH ) 3.4 could be even lower than that listed in Table 1. When these data are compared to similar data obtained by Nart et al.,5 the same trends are seen, but with a few minor differences: their experiments were performed in an electrolyte solution containing a large relative excess of fluoride ion, and the poor S/N ratio below 1000 cm-1 renders the ν2(ads) mode undetectable. For the two solution pH values studied, they report the ν1(ads) mode and, under high proton concentrations, the ν2(ads) is evident (Figure 4 in ref 5). This is all consistent with a bisulfate-like adsorbate. Signs of specific adsorption are found in their study at a pH of 2.8, while in our work experiments with pH ) 3.4 showed only solution-phase spectral features. This difference can be understood when the relative concentrations of bisulfate are considered. As is listed in Table 1, there is at least 4.5 times more bisulfate used in the pH ) 2.8 study found in ref 5 compared to our pH ) 3.4 study. Again the evidence points to the adsorption of a bisulfate-like, not sulfate, species. Figure 2 shows the CV profiles obtained in dilute sulfuric acid solutions adjusted to different pH values. As pointed out earlier12,15,16 the anomalous potential region is independent of solution pH; however, the sharp spike at +210 mV becomes much less prominent as the solution pH is raised. The actual origin of the spike is unclear, perhaps
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it is due to a structural rearrangement occurring at a critical adsorbate coverage. Regardless of its cause, the spike intensity correlates to the availability of bisulfate ions, not sulfate ions. In conclusion, there are three IR spectroscopic features related to the anomalous peak region of the cyclic voltammetry of Pt(111) electrodes in bisulfate-containing electrolyte solutionsstwo due to the adsorbate species itself and one due to the loss of the adsorbing anion in the thin-layer cell. The behaviors of these bands, in solutions of different pH, are summarized in Table 1. These feature are all evident at pH values where the predominant solution-phase anion is bisulfate. Other spectral features found in the PDIR spectra are consistent with pH changes and differences in the composition of the thin layer due to the applied electrode potential. From these observations and the facts that the anomalous peaks in the CV are associated with sulfur-containing species18,19 and that they are most likely monoanionic,22,23 it seems clear that the voltammetric features arising at +0.22 V vs Ag/AgCl in the CV of Pt(111) in sulfuric acid solutions are not due to the adsorption of sulfate. Although the evidence suggests that the adsorbate is the bisulfate anion, the actual structure may be better
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described as Pt‚‚‚O3SO2-‚H3O+ rather than Pt‚‚‚O3SOH-. Such a ternary adsorbate structure is consistent with the experimental data and with the expected changes to bisulfate acidity upon adsorption. The adsorbate may possess a unique structure somewhere between sulfate and bisulfate where the hydronium ion is necessary to stabilize the adsorbed complex. This is why no IR evidence of adsorption is seen unless both H3O+ and SO42- are in large enough concentrations. However, this hypothesis is tentative and further concentration, pH, and isotopic exchange experiments will be necessary to confirm the adsorbate configuration. Future in situ spectroscopic studies will include investigation into cation effects and measurements made at frequencies down to ∼850 cm-1. Acknowledgment. Financial support was provided by the National Science Foundation and the Commonwealth of Kentucky through NSF-Kentucky EPSCoR Advanced Development Program (Grant EHR-9108764), and by the Department of Energy, Division of Chemical Sciences, through the office of Basic Energy Sciences (Contract DE-AC02-76CH00016). LA950115U
