Specific Adsorption of a Bisulfate Anion on a Pt(111) Electrode

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J. Phys. Chem. 1996, 100, 11726-11735

Specific Adsorption of a Bisulfate Anion on a Pt(111) Electrode. Ultrahigh Vacuum Spectroscopic and Cyclic Voltammetric Study S. Thomas, Y.-E. Sung, H. S. Kim, and A. Wieckowski* Department of Chemistry and Frederick Seitz Materials Research Laboratory, UniVersity of Illinois at Urbana-Champaign, Urbana, Illinois 61801 ReceiVed: February 29, 1996; In Final Form: May 1, 1996X

We addressed in this study the process of specific adsorption of anions at the metal/solution interface. We focused on the nature of the surface chemical bond that accounted for the phenomenon of adsorption specificity in the context of bisulfate coverage and structural information. While we limited our investigations to bisulfate adsorption on the Pt(111) electrode in sulfuric and mixed sulfuric/perchloric acid media, our conclusions have general significance in explaining ionic adsorption events in electrochemistry. We used core-level electron energy loss spectroscopy, auger electron spectroscopy, low energy electron diffraction, and cyclic voltammetry. Our findings show that in the studied range of sulfuric acid concentration (10-4-10-1 M) the maximum anion coverage is 0.34 ( 0.02 monolayer (ML) and that this coverage corresponds to a highly ordered Pt(111)(x3 × x3)R30° surface structure. S2p core-level and LMM Auger electron spectra indicate that the chemical state of bisulfate sulfur is +6, as in the sulfate anion in a sulfate salt matrix. However, the electron density on the adlattice sulfur is higher than in the salt, evidently due to back-donation of electrons from the substrate to the adsorbate. We conclude that backdonation plays a major role in binding the anions to the surface. Further, the plot of the back-donated electron density vs electrode potential assumes a distorted parabolic shape. The descending parabola branch covers the potential range where bisulfate adsorption increases with potential, and a flat minimum coincides with the double layer potential range. When OH adsorption and platinum oxidation begin, a 2D compressive effect of the O-type adsorbates causes the bisulfate-platinum O-Pt bond to be sequentially cleaved. The “flow” of metal electrons to the adsorbate is therefore reduced, and the S2p loss energy approaches the level characteristic of sodium sulfate unperturbed by surface interactions. Loss spectra from Pt4f7/2 confirm that the oxidized surface is emersed to vacuum but in the double layer potential range fail to respond to either the electrode potential change or the bisulfate adsorption.

I. Introduction Anions, together with cations and solvent molecules, are building blocks of the boundary layer that develops in the interface between a metal (electronic conductor) and an electrolytic solution (ionic conductor).1 The extent of interactions of these solution components with the surface is, however, different. On platinum, for instance, water is weakly bonded and exhibits a desorption temperature (to vacuum) as low as 160 K. Cations are tightly hydrated and saved from intimate interactions with the metal by the water hydration shell and, possibly, by the surface hydration layer. To the contrary, anions’ hydration shell is not as compact, and anions are frequently adsorbed “specifically”. This is to say that they selectively bind to the surface from an environment containing nonspecifically adsorbed anions that, as a part of supporting electrolyte, are present in solution in large excess with respect to the specifically adsorbed anions. A hypothesis is formulated here that specific forces appear when an anion is contactadsorbed and the molecular and surface electron density has a finite probability to overlap and create an anion-surface chemical bond. In reality, a surface complex stabilized by such a bond remarkably modifies the electrode surface structure2,3 and reactivity.4-7 Therefore, understanding the surface chemistry of anions is essential in developing realistic theories of the electrochemical interface, in assessing the performance of electroanalytical methods, and in the theory and practice of electrocatalysis. This, and relatively little experimental informaX

Abstract published in AdVance ACS Abstracts, June 15, 1996.

S0022-3654(96)00632-6 CCC: $12.00

tion concerning the nature of the anion-surface chemical bond, warrants further research on surface anions in electrochemistry. One of such specifically adsorbed anions, bisulfate, has been studied on electrodes made of a variety of metal substrates. However, due to the unique role platinum plays in electrocatalysis, the Pt electrode has been investigated quite frequently (for recent results, see refs 8-13), including more recent focus on platinum single-crystal surfaces.14-26 Despite this rich research activity it appears thatswith the exception of the earlier work by Kolb and Hansen27-29 and our recent spectroscopic studies2,30sthere is a genuine lack of characterization of the electronic structure and chemical states of electrochemical adsorbates, including adsorbed anions. Our present approach involves the application of core-level electron energy loss spectroscopy (CEELS) and, to a lesser extent, auger electron spectroscopy (AES) and focuses on (i) the identification of 2p core electronic levels in the bisulfate adlattice as referenced to the energy levels in sodium sulfate and (ii) loss spectra from 4f7/2 levels in platinum and the electrode potential effect in these loss spectra. As is well known,2,30-36 in the core-level electron energy loss spectroscopy a monochromatic incident electron interacts with an inner shell electron of the target atoms and lifts the core-level electron to a final (empty) state above the Fermi level. The loss energy (∆E) is given by ∆E ) Ep - Ek, where Ep is the incident electron energy and Ek is the kinetic energy of the outgoing electron. Due to the low energy of incident electrons in our studies, 500 eV, the cross-section for energy losses is high and CEELS is surface sensitive.2,30 Since one probes only the difference in the energy levels between the core level states and the empty electronic states in the excited © 1996 American Chemical Society

Adsorption of a Bisulfate Anion on a Pt(111) Electrode adsorbate, the results are not affected by charging effects and the work function changes. Therefore, the loss energy shifts are more discernible than those involved in XPS spectra and provide genuine information on surface/adsorbate chemical states and on effects of electrode potential on such states. This work derives from our previous UHV-spectroscopic studies of bisulfate adsorbed on Au(111)30 and Rh(111),2 as well as from our recent chronocoulometric and radiochemical study of the Pt(111)/bisulfate system in 0.1 M HClO4 solutions containing varying amounts of H2SO4.26 The key finding2,30 was that surface structure and coverage of the anionic adsorbate on gold and rhodium of the same (111) surface geometry were different. Namely, on Au(111), the coverage is 0.20 monolayer (ML) and the LEED pattern shows a weak (x3 × x3)R30° pattern. With Rh(111), the coverage is 0.33 and the “sharp” (x3 × x3)R30° LEED pattern gives evidence as to a highly ordered bisulfate surface structure. We also found that sulfate (and perchlorate2) were much more strongly adsorbed on rhodium than on gold. Investigating the Pt(111) electrode,26 we found that at low bulk concentration of bisulfate in solution the coulometric and radiochemical results showed the same trends, but at higher concentrations two plateau regions were found by chronocoulometry but only one by radioactive labeling. At the first plateau region, at around 620 mV vs RHE, the bisulfate/sulfate coverage is approximately 0.33 ML. In the second chronocoulometric plateau region, which begins at 900 mV vs RHE, the coverage is about 0.40 ML, much higher that that of radiochemical or current AES determination. These observations will be addressed in this paper. However, the main focus is on the spectroscopic characterization of the bisulfate adsorbate using CEELS. In the context of our present investigations it is important to highlight a recent theoretical study by Attard et al.37 showing that for oxy-anions such as ClO4-, HSO4-, and H2PO4- neither anion polarizability nor hydration is a prominent factor in determining the strength of specific adsorption. Instead, the Hartee-Fock, self-consistent field level data show that the HOMO-LUMO gap follows the adsorption strength of these anions: ClO4- < HSO4- < H2PO4-. The HOMO level in HSO4- is lower than in ClO4-, making it easier for HSO4-, rather than for ClO4-, to form a bond via σ donation to the vacant Pt5d orbitals. As the HOMO-LUMO separation is lower for HSO4- than for ClO4- (14.36 vs 25 eV), the backdonation (and back-bonding) from Pt5d to the LUMO bisulfate orbital is similarly privileged. While indirect experimental evidence in support of the σ bonding may be found in the recent paper by White et al. (dealing with sulfur dioxide adsorption on platinum),38 we will show below that there is an effective electron back-donation pathway that may assist the bisulfate surface chemical bond. This observation is one of the principal results of this study. II. Experimental Procedure The combined ultrahigh vacuum/electrochemistry instrument was previously reported.2,3,30 The Pt(111) single crystal (Aremco) was polished and oriented to within 1° using the Laue X-ray diffraction. Prior to the electrochemical experiments, the electrode was cleaned by 1 keV Ar+ ion sputtering and by annealing at around 1100 K. A clean and ordered Pt(111) sample was then transferred to the system antechamber for electrochemical measurements using a conventional threeelectrode circuitry and a EG&G PAR 362 potentiostat. A meniscus position of the working electrode in solution ensured that only the clean and ordered face of the single crystal was exposed to the electrolyte. The Pt(111) electrode covered with

J. Phys. Chem., Vol. 100, No. 28, 1996 11727 adsorbed bisulfate was emersed at various adsorption potentials and transferred, without exposure to air, to the system UHV chamber for AES, LEED, and CEELS characterization. Tilting the electrode directly after emersion forced the remnants of solution to collect at its lower end and facilitated its removal using a Teflon capillary attached to a syringe. After 5 min of cryo- and ion-pumping, the pressure in the chamber was in low 10-8 Torr, quite adequate for the electron spectroscopy use. The base pressure of the UHV system was in low 10-10/high 10-11 Torr range. We used a primary electron beam of 500 eV to measure the core-level electron energy loss spectra utilizing a Perkin-Elmer PHI-10-155 cylindrical mirror electron energy spectrometer. The measurements were carried out in a differentiated mode with 2 eV modulation amplitude at 3 keV (0.5 µA, 10 eV/s, τ ) 30 ms) and at 500 eV (0.5 µA, 1 eV/s, τ ) 1 s or τ 30 ms). (The reduction in the primary electron energy reduces electronstimulated desorption and electron beam damage of the bisulfate anions.) The loss energy, ∆E, was obtained by subtracting the measured energy from the primary electron energy determined from the 500 eV elastic peak. Below, we will present the electron energy loss spectra in the differentiated mode (as obtained), as well as the integrated spectra. The integrated spectra were used to verify the peak position assignment, especially when the change in peak position versus electrode potential was examined. The quantitative analysis of bisulfate overlayers was performed using a standardization technique developed in our laboratory.2,30 This procedure involves a comparison of the spectral data from bisulfate adlayers and from a thin sodium sulfate film by the electrode emersion to vacuum from a 0.3 M Na2SO4 solution. The p/p intensities of sulfur and oxygen at 132 and 516 eV relative to the Pt p/p (64 eV) intensity of a clean Pt sample were used for the comparison. Peak-to-peak heights of Auger electron transitions were extrapolated to zero time in order to obtain signal intensities unperturbed by beam damage. Working solutions were made of Millipore water (18 MΩcm) and ultrapure grade sulfuric acid and perchloric acid. Working solutions were deaerated and blanketed with nitrogen (Linde, oxygen free, 99.99%) prior to the electrochemical studies. All measurements were conducted at room temperature. Electrode potentials are given vs Ag/AgCl reference with [Cl-] ) 1 M (the actual concentration of Cl- was 10-5 M, and the potentials are recalculated to the 1 M Cl- concentration.) III. Results 1. Structure/Coverage Determination by AES, LEED, and Voltammetry. A typical Auger electron spectrum from the clean Pt(111) surface obtained after several cycles of Ar+ bombardment and annealing is shown in Figure 1A. All of the major Pt Auger electron transitions, at 64, 150, 161, 239, and 253 eV, are easily identified, whereas the residual carbon signal (270 eV) is within the noise level. The Pt(111) sample emersed from sulfuric acid solutions (or from perchloric acid-containing sulfuric acid solutions, see below) exhibits an already reported Auger electron spectrum associated with bisulfate adsorption2,30 composed of three peaks at 122.4, 134, and 152 eV (Figure 1B). The Auger spectral morphology from bisulfate adsorbate is in excellent agreement with that obtained from the thin film of Na2SO42,30 (Figure 1B) and with the literature data on solid sulfate obtained by other investigators.39-42 Figure 2A,B show LEED patterns obtained from the clean Pt(111) substrate and from the surface covered by bisulfate at a maximum coverage. A careful inspection of LEED patterns

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Thomas et al.

Figure 1. Auger electron spectra of bisulfate anion adsorbed on the Pt(111) electrode at several electrode potentials. Also shown are spectra for the clean Pt(111) electrode and the electrode covered by a thin film of a Na2SO4 deposit (see the Experimental Section). Primary electron beam energy was 3 keV: (A) a wide spectrum from 50 to 550 eV showing Pt(NOO), S(LMM), and O(KLL) regions; (B) spectral data for the S(LMM) region.

indicates that this issin terms of LEED spots symmetry, size, and brightnesssthe same (x3 × x3)R30° pattern as previously found with rhodium.2 Emersion of the electrode at potentials characteristic of lower than maximum bisulfate coverage yields a diffuse (x3 × x3)R30° pattern that ultimately disappears at the extreme, either negative or positive, potentials. The LEED pattern shown in Figure 2B (and the corresponding real space structure, Figure 2C) was found in a broad bulk concentration range of the acid, from 10-4 to 10-1 M, with the intensity vs background decreasing somewhat with the decrease in sulfuric acid concentration. Cyclic voltammograms (CV’s) of the UHV-prepared Pt(111) electrode taken in the mixed H2SO4/HClO4 and clean H2SO4

Figure 2. (A) LEED pattern from a clean and ordered Pt(111). (B) Pt(111)(x3 × x3)R30° LEED pattern of surface bisulfate formed at E ) 0.30 V in 0.10 M H2SO4. The electron beam energy was 49.5 eV. (C) Real space (x3 × x3)R30° structure of the bisulfate adsorbate.

solutions are shown in Figure 3A and B, respectively. The CV’s were usually taken in the potential range from -0.28 to 0.70 V. The voltammetric features in the clean sulfuric acid are very similar to those obtained in the mixed HClO4/H2SO4 media (Figure 3). This shows that bisulfate adsorption on Pt(111) is much stronger than perchlorate adsorption (see below). CV

Adsorption of a Bisulfate Anion on a Pt(111) Electrode

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Figure 4. Bisulfate coverage from AES measurements and coulometry.26 Bisulfate adlayers were obtained in 0.10 M HClO4 + x M H2SO4 solutions. The dotted, dashed, and solid lines represent chronocoulometric coverage in 0.1, 1, and 5 mM H2SO4 solutions, respectively. Data points are the corresponding AES results (and those for 0.5 mM). Inset: bisulfate AES coverage obtained in clean (free of perchloric acid) H2SO4 solutions. Concentrations “x” associated with AES data are given in the figure.

Figure 3. Cyclic voltammograms (CV’s) of the UHV-prepared Pt(111) electrode. (A) CVs in x M H2SO4 + 0.10 M HClO4 solutions at 50 mV/s. The concentrations “x” are given in the figure. (B) CV’s in H2SO4 solutions free of perchloric acid (both concentrations and scan rates are shown in the figure). Inset: the CV of the Pt(111) electrode in 0.10 M H2SO4 in a full voltammetric range highlighting platinum surface oxidation current.

features in Figure 3A confirm all of the tendencies that we have identified before26 but show better resolution. This comparison, the quality of our LEED pattern, and data from previous work by other investigators25,26,43-46 indicate that our Pt(111) surface is practically defect-free. As already agreed upon,46-48 the voltammetric activity in the potential ranges from -0.25 to 0.05 V and from 0.05 to 0.25 V corresponds to hydrogen and bisulfate adsorption, respectively. The hydrogen adsorption range either does not overlap the HSO4- adsorption range at all or, at the highest concentrations of sulfuric acid investigated, overlaps to a small degree.49 This is a clear contrast to our conclusions with Rh(111) where bisulfate and hydrogen adsorption overlaps in the hydrogen range of the electrode potentials.2 Consequently, the present AES data, as well as previous radiochemical data,18 indicate a complete desorption of (bi)sulfate in the hydrogen range. In Figure 4 bisulfate coverage data are given for the mixed sulfuric/perchloric acid media; those obtained in pure sulfuric

Figure 5. Plot of bisulfate coverage versus sulfuric acid concentration. Bisulfate adlayers were obtained either in 0.10 M HClO4 solution containing sulfuric acid (empty triangles) or in clean (free of perchloric acid) H2SO4 solutions (solid squares).

acid are shown in the Figure 4 inset. The data in this figure, and in Figure 5, show that ClO4- ions present in solution do not reduce the bisulfate coverage, neither have we found evidence for ClO4- adsorption from the mixed media (Figure 5). In both cases the maximum coverage of bisulfate is 0.33 ( 0.02 ML. The coverage decreases considerably when the Pt surface starts to oxidize. For instance, for 1 mM H2SO4 the surface concentration at E ) 0.90 V is already as low as 0.15 ML. Figure 4 also shows the comparison of bisulfate coverage between the present quantitative Auger and previous chronocoulometric26 measurements. While there is a general agreement between the two data sets, there are some differences. At high potentials the AES bisulfate coverage is independent of the concentration of sulfuric acid in solution, unlike that given by chronocoulometry. The difference may still be within the experimental scatter since we cannot claim a relative precision better than 5%, and chronocoulometric scatter was not previ-

11730 J. Phys. Chem., Vol. 100, No. 28, 1996 ously given. The AES data, and the chronocoulometry, show that between the potential of 0.05 to 0.60 V the bisulfate coverage increases. In contrast to the chronocoulometric data, the spectroscopic data indicate that at 0.60 V bisulfate coverage reaches the maximum value of 0.34 ML ((0.02) that no longer increases with E. Notably, while the 0.60 V charge taken from Figure 3A is 83 µC cm-2, the charge at the most positive double layer potential is as high as 112 µC cm-2. Formally, this charge yields the coverage of 0.45 ML.26 However, we were unable to confirm such a high coverage in this spectroscopic study. Using the charge as a coverage gauge, one could also assign the small surface redox couple (hump) in the cyclic voltammogram centered around 0.45 V (Figure 3b, solid line) to additional uptake of bisulfate. Our data again do not substantiate such a claim.26 Ross et al.20 have associated this hump to OH adsorption (as the preoxidation state on Au surfaces50). Nishihara et al.51 suggest that the reason for the small redox couple appearance is the change in the chemical state of the adsorbate. This again would not give an increase in bisulfate coverage. 2. Auger Electron and Core-Level Electron Energy Loss Spectra. Typical electron spectra at 500 eV taken from the clean platinum electrode, from the electrode emersed from solutions containing sulfuric acid, and from the thin film of Na2SO4 are shown in Figure 6A-C, respectively. In Figure 6B and C the electron energy loss spectra are presented in differentiated and integrated modes, solid vs dotted lines, respectively. Both the derivative and integrated spectra were used and gave identical peak position assignments (Figures 7 and 8). As in our previous studies, we identify three Auger electron transitions at 122.4, 134, and 153 eV peaks characteristic of bisulfate (or sulfate) sulfur (Figure 1B). There are also S2p and Pt4f7/2 loss peaks from the adlayer and platinum substrate (Figure 6). Figure 6B shows the S2p loss spectra from the platinum electrode emersed at 0.35 and 0.93 V from a 0.1 M sulfuric acid, and from the thin film of Na2SO4. The energy transitions are at 191, 182.6, and 174.7 eV. These loss peaks arise from transfer of the S2p electron to an empty state close to the vacuum level. Spectral morphology from the adsorbed bisulfate is the same as that from Na2SO4 (cf. data in Figure 6B). The principal loss peak at 174.7 eV can be compared to the S2p binding energy at 169.5 eV (with respect to the Fermi level) measured by XPS for a Na2SO4 sample.52 The S2p loss energy decreases with adsorption potential, giving rise to a flat minimum at 0.40 V (Figure 7). However, further increase in adsorption potential causes an increase in the energy loss that then asymptotically approaches the electron loss energy for Na2SO4 (176 eV). The complete energy-potential plot has the appearance of a distorted parabola with a broad minimum and asymmetric branches. We have also seen the loss feature from the S2s level (not shown) at 233 eV. This S2s loss energy correlates well with the 229 eV binding energy of the S2s level.52 Figure 6C shows the Pt4f7/2 loss spectrum from clean Pt and from platinum emersed from sulfuric acid media at various potentials. The loss peak at 73.2 eV is characteristic of both clean and bisulfate covered substrate and is independent of adsorption potential until E ) 0.80 V, where the oxidation of platinum begins (Figure 8). At electrode potentials positive of 0.80 V, that is, in the platinum oxidation range, the loss energy increases sharply (Figure 8). IV. Discussion 1. Bisulfate Surface Structure. Our LEED studies reveal a (x3 × x3)R30° adsorbate structure at emersion potentials

Thomas et al. between 0.10 and 0.50 V. Knowing that the adsorbate coverage is 1/3, and using previous synchrotron X-ray information on similar surface systems,53 a possible arrangement of bisulfate with respect to the hexagonal Pt(111) mesh is shown in Figure 2C. In this adsorption model, three oxygen atoms of adsorbed bisulfate are on top of platinum atoms, and a sulfur atom sits on the top of the equilateral triangle of the O atoms. Obviously, this model yields the maximum coverage of bisulfate as 0.33 ML. However, recent in situ STM studies of bisulfate adsorption on Pt(111) from sulfuric acid media by Stimming et al.25 have indicated a (x3 × x7) adsorbate structure. This structure is interpreted as a coadsorbate of anions and solvent species 2 1 which form together a primitive [-1 2] adlattice. The adlattice of anions and coadsorbed solvent species could be sulfate ions, SO42-, and H3O+ or bisulfate ions, HSO4-, and water molecules. Of the two sets of maxima seen in STM images, the larger one is attributed to the anion-derived species (SO42- or HSO4-), and the smaller maxima is attributed to the solvent-derived species (H3O+ or H2O). This yields 20% coverage for the bisulfate. On the basis of our present data, we conclude that the (x3 × x7) adsorbate structure is characteristic of an intermediate coverage, not accessible for the LEED studies. However, more work is needed to reconcile the difference.2 2. Auger Electron Spectra. Auger electron spectra (Figure 1B and Results) from bisulfate monolayers closely resemble those from the thin Na2SO4 film, including spacing between the energy transitions. Therefore, we conclude that the oxidation state of sulfur in the bisulfate adsorbate is +6 (as in Na2SO4). The quantitative treatment of the spectra shows that the sulfurto-oxygen ratio remains at 4 until the platinum surface starts to oxidize. Extending the adsorption/emersion potential to the platinum oxidation range causes an increase in the O/S ratio. This is due to incorporation of oxygen atomssnot associated with bisulfatesinto the Pt surface. For atomic sulfur, the observed Auger electron transitions correspond to LIIMIMII,III and LIIMII,IIIMII,III processes41 resulting from cross-Auger electron transitions.54 In bisulfate anion, the MI, MII,III are sp3 hybridized into Mn, Mσ, and MO orbitals. Here, “Mn” stands for nonbonding orbitals predominantly localized on the oxygen, “Mσ” are bonding orbitals between sulfur and oxygen, and “MO” stands for O2s molecular orbitals.41,55 Under the assumption that one of the two electron vacancies in the final state after Auger electron emission occurs in the same set of orbitals,41 three groups of Auger transitions should develop: LIIMσMn, LIIMσMσ, and LIIMσMO. On the basis of the work of Farrell et al.,41 the position of the nonbonding orbital, Mn, was assumed to be 4.9 eV below the vacuum level. The kinetic energy of Auger electrons emitted from bisulfate is 7 eV higher than that from the thin sulfate film. The shift toward higher kinetic energy is due to extramolecular relaxation energy available in the former system. This extramolecular relaxation energy is most likely due to effective screening of electronic holes formed on adsorbed HSO4- during Auger processes by conduction electrons of platinum.42 3. Core-Level Spectra. Core-level electron energy loss spectra from S and Pt have provided new information regarding electronic interactions between the anion and the surface. As the spectra are not affected by charging effects and work function changes,56-58 they reflect the change in the electronbinding energies of bisulfate anions associated with the change in electronic interactions between the substrate and the adsorbate. There is a great deal of data on electronic configurations of tetrahedral species of the AB4 type, like HSO4- or SO42-, where the central atom (A) is surrounded by four electronegative atoms

Adsorption of a Bisulfate Anion on a Pt(111) Electrode

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Figure 6. (A) Electron energy spectra showing AES transitions, Pt(NOO) and S(LMM), and loss peaks, S2p for bisulfate and Pt4f7/2 for the substrate. The data are for clean Pt(111), a monolayer of adsorbed bisulfate, and the thin film of Na2SO4. (B) S2p electron energy loss spectra of bisulfate obtained in 0.10 M H2SO4 at 0.35 and 0.93 V and the loss spectrum of the thin film of Na2SO4. (C) Pt4f7/2 electron energy loss spectrum of clean Pt(111) and those of surface bisulfate obtained in 0.10 M H2SO4 at 0.29, 0.60, and 1.05 V. Dotted line represents integrated spectra. Inset: a spectral second derivative showing 4f5/2 and 4f7/2 components of the Pt loss spectra. All data in A-C were obtained with primary electron beam of 500 eV.

(B), with 32 valence electrons.59-65 From these data we assume that the configuration for bisulfate is (4a1)2(3t2)6(5a1)2(4t2)6(1e)4(5t2)6(1t1)6. In principle, the orbitals of the highest energy for HSO4-, 1t1, 5t2, and 1e, are nonbonding, consisting predomi-

nantly of oxygen 2p orbitals.66 However, the 5t2 and 1e have significant S3d components that give these orbitals some intramolecular bonding character. The major bonding intramolecular interaction comes from the second group of orbitals, 4t2

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Figure 7. Bisulfate S2p loss energy as a function of the electrode potential as referenced to S2p energy of the thin Na2SO4 film. The bisulfate adsorbate was obtained in 1 mM H2SO4 and in 0.1 M H2SO4 solution (inset).

Figure 8. Pt4f7/2 loss energy as a function of the electrode potential. The Pt(111) electrode was emersed from 1 mM H2SO4 and from 0.10 M H2SO4 solution (inset).

and 5a1 of S3p and S3s, with oxygen 2p and 2s orbitals. The final group of orbitals, 3t2 and 4a1, consists mainly of the oxygen 2s atomic orbitals.66 Further, for HSO4-, the HOMO is the 1t1 level while the LUMO is the 6a1 level.61 Sekiyama et al. in the L2,3 absorption spectra studies of SO42- anions have demonstrated an existence of vacant levels 6a1, 6t2, 2e, 8t2, 7a1, and 3e. Notably, the separation between the 6a1, 6t2, and 2e levels are approximately 10 eV each. Our core loss spectra from S2p of bisulfate sulfur (Figure 6B) show three loss features, at 174.7, 182.6, and 191 eV, showing the same separation of almost 10 eV each. Therefore, the main core loss feature at 174.7 eV can be attributed to a S2p to 6a1 transition, while the other two loss features at 182.5 and 191 eV can be from S2p to 6t2 and S2p to 2e levels. Further, the principal energy loss from sulfur, 174.7 eV, can be compared to the 169.5 eV binding energy of S2p core level electron measured for Na2SO4.52 However, another plausible explanation is that the 182.6 and

Thomas et al. 191 eV loss features may derive from the second loss that the primary electron suffers due to the excitation of a 4t2 or 5a1 and 3t2 electron to the vacant 6a1 level. The loss energy differences are around 9 and 18 eV from the main loss peak at 174.7 eV, which correlates reasonably well with the predicted values of the energy separations between the above mentioned levels, 8-10 and 19.7 eV.41,55 There can also be some contribution from Rydberg transitions in our loss spectra since Hitchcock et al.67 and Touke et al.68 observed inner electron transitions to such states. At low coverage of bisulfate, the energy loss is significantly lower than the one taken from bisulfate anion in the salt matrix (Figure 7). This lowering of the S2p loss energy is due to higher electron density on the sulfate sulfur as referenced to that in the salt. We conclude that the decrease in the energy is mainly due to electron back-donation from the substrate atoms into the unfilled orbitals of the bisulfate adsorbate. The most probable arrangement is that the electron density from Pt 5d is donated to the empty orbitals, LUMO, which are mainly S3s and S3p in nature. Following the Pt5d-HSO4- LUMO orbital interaction, the increased electron density around the S atom adds to the intramolecular electronic repulsion in bisulfate, hence lowering the electron-binding energies. However, the σ donation from the HOMO of SO42- (1t1) to Pt5d also takes place.37 In infrared studies of bisulfate adsorption on Pt electrodes, a strong potential dependent shift (positive) in peaks associated with the stretching modes of bisulfate has been observed by several groups.17,69-71 Ab-initio molecular orbital calculations performed on bisulfate anion by Ito et al.71 have shown that the HOMO is mainly localized on oxygen lone pairs. These lone pairs exhibit antibonding character to the O-S bond. Hence, these are favored to form the bond with the metal atoms and in doing so strengthen the O-S bond. The interpretation presented above, in terms of back-donation effects, is supported by earlier experimental and theoretical studies. For instance, backbonding has been observed for NO, SO2, and CO on Pt38,72-77 and on Rh(111).2 Goodman et al., using XPS,78,79 demonstrated that adsorption of CO on bimetallic surfaces induced a large positive shift in the core level of the metal overlayer. Holloway and Norskov discussed the electrode potential induced red shift of adsorbed CO, emphasizing the backbonding mechanism. Mehandru and Anderson80 were focusing on CO and potassium coadsorption on Pt(111) and, in particular, on the shift in CO adsorption from an on-top to a 2-fold (bridge-bonded) site. Apparently, the rearrangement of CO to higher coordination sites occurs due to the chargetransfer-induced shift in the Pt valence band that becomes closer in energy to the empty 2π* orbital of CO. This leads to an increase in the π orbital mixing with Pt5d orbitals and in a reduced donation from the CO 5σ orbital, which lies beneath the Pt valence band. This situation is similar for NO, where the 2π* orbital is occupied by a single electron. The electron transfer to the antibonding 2π* orbital weakens the N-O bond and may even break it.81,82 More recently, White et al.38 studied adsorption of SO2 on Pt(111) and found that, in addition to the σ bonding, there was a significant charge transfer from the Pt5d band to the lowest unoccupied molecular orbital of SO2. The decrease in the binding energy for the O1s, as well as the S2p levels, could be as high as 2.2 eV. The large shift in binding energy is attributed not only to charge transfer but also to finalstate screening effects. UPS data38 show a spectral feature at 2.7 eV due to a new surface orbital derived from the interaction between the LUMO of SO2 (3b1) and a Pt d-band. This is similar to what observed by Outka and Madix in SO2 adsorbed on Ag(110), where the equivalent spectral feature was at 1.6

Adsorption of a Bisulfate Anion on a Pt(111) Electrode eV.83 Calculations indicate that the LUMO of an isolated SO2 is predominantly centered on the S atom (60% S3px; 8% S3dx,y).84 Therefore, most of the charge transfer is localized on the S atom, with only a small percentage (15%) on each of the O atom. Since the LUMO is below the Pt Fermi level, upon adsorption, a charge transfer is expected from the metal d-band into the LUMO, as experimentally observed.38 The S2p core loss energy is strongly influenced by the electrode potential; see the distorted parabolic energy-electrode potential curve in Figure 7. First, in the range of 0.00-0.40 V, a downshift in loss energy is observed. This becomes understood if the increase in bisulfate coverage with E (Figure 4), tantamount with the increase in surface concentration of surface dipoles Pt+-HSO4-, augments repulsive lateral interactions between the negative dipole terminals. A molecular distortion resulting from such interactions adds to the intramolecular electronic repulsion in the bisulfate and lowers the electron binding energy (see above). As the adsorption potential further increases, the S2p core loss energy stabilizes since the bisulfate coverage is constant (AES data in Figure 4). The lowest S2p core level energy here is 174.5 eV. When the electrode surface becomes oxidized (at ca. 0.6 V), the loss energy increases and reaches 175.5 eV at 1.00 V (Figure 7). To interpret this result, one recalls that the assumed registry of bisulfate on a clean Pt(111) surface is such that three oxygen atoms of bisulfate are in contact with platinum atoms and sulfur atom sits on the top of the equilateral triangle of the atoms.85 However, when bisulfate is adsorbed on the oxidized platinum surface, it adsorbs in an inverted mode in which oxygen atoms are facing away from the platinum surface with the OH group directed toward the oxidized surface.14 One needs also to remember that, prior to the PtO formation, the platinum surface is covered with OH groups.86-90 We propose that with the uptake of OH or surface oxygen surface bisulfate either progressively desorbs or the Pt-O anion bonds are sequentially broken and HSO4- anions adopt the inverted orientation (Figure 9), as concluded by Seki et al. in ref 14. Hence, with the decreases in the bond order, there is less flow of the electron density to the adsorbate, and the S2p loss energy increases. Since the binding of bisulfate to the metal is reduced, the adsorbate loss energy approaches that in Na2SO4 (Figure 7). Pt4f7/2 loss energy spectra (Figure 8) show three distinctively different trends: (a) no change in loss energy in the potential range from -0.20 to 0.40 V or to 0.60 V, depending on the concentration of sulfuric acid, (b) a gradual change where the Pt4f7/2 loss energy increases from 73.2 to 73.5 eV (from 0.40 to 0.80 V in dilute H2SO4), and (c) a sharp increase in the energy above 0.80 V, with the final value 74.3 eV. Since there is no spectral change in range (a)swhere bisulfate adsorption commences and developsswe conclude that bisulfate adsorption does not detectably change the 4f7/2 energy level. Between 0.40 and 0.80 V, the surface electronic environment around a platinum atom is progressively changing due to OH adsorption (see above and Figure 9). This is reflected by a clear change in platinum loss energy (Figure 8). The increase in the electrode potential above 0.8 V causes an advancement in the oxidation of platinum, with the surface being covered with oxygen atoms, and this is indicated by an even sharper increase in the Pt4f7/2 loss energy. We noticed that we did not see any changes in the Pt4f core loss energy during bisulfate adsorption. However, Pt4f levels are far below the Fermi level; hence, they are not directly involved in backbonding. In contrast, Pt5d has a binding energy of ∼0.8 eV,91 which is very close to the Fermi level and close in energy to the LUMO of bisulfate. Hence, electron density

J. Phys. Chem., Vol. 100, No. 28, 1996 11733

Figure 9. Proposed configuration of bisulfate that was adsorbed in the electrode potential regions of interest.

overlap is favored and some charge is transferred from the surface to the adsorbate (back-donation). When this happens our CEELS data show a decrease in S2p loss energy, primarily due to the added intramolecular electronic repulsion. Moreover, the gap in energy between the Pt5d and Pt4f is too large to expect a noticeable shift in the Pt4f loss energy. When Pt forms chemical bonding with oxygen, all the Pt energy levels undergo a considerably larger shift in energy. They are then easily seen by CEELS (Figure 8). V. Electrochemical and Molecular Model of Bisulfate Adsorption We reiterate that adsorption of bisulfate on platinum is specific since it competes efficiently for surface sites with perchlorate anion that is present in solution in the large excess (Figures 4 and 5). Since the charge on the two anions, both in solution and on the surface, is identical no simple coulombic argument can be used to account for the specificity. Likewise, since the hydration energy for perchlorate is actually lower than for bisulfate,92,93 the hydration process cannot energetically favor bisulfate adsorption against perchlorate adsorption. This is in agreement with Attard et al.37 Since the adsorbate acquires high coverage in a relatively narrow potentials range, the electron flow to the circuit from the interface (needed to compensate for the negative bisulfate adsorption) is high, and the charge corresponds to a 1/3 coverage of a monocharged anionic adsorbate. The adsorption process begins when the electrode potential is positive of the potential of zero charge (pzc),94 showing that surface orbital vacancies are needed for the adsorption process to begin. This favors bonding via electron donation to empty

11734 J. Phys. Chem., Vol. 100, No. 28, 1996 surface orbitals or, at least, allows one to conclude that the backbonding is not sufficient to counteract the repulsive interaction of the negatively charged adsorbate and the negatively charged surface below the pzc. However, it is unknown whether the donation mechanism alone would be sufficient to account for the adsorption strength and resulting adsorption specificity. UPS experiments are planned to shed more light on the binding event that plays a key role in the understanding of anion adsorption on metal electrodes and in the electrode reactivity. Finally, we have not referred in the paper to the possibility of a coadsorption of H3O+ cation with sulfate anions, as postulated by Faguy,95 or to a hypothetical recombination of bisulfate with hydronium cation to produce a neutral sulfuric acid molecule adsorbed on platinum, as considered by Ito.96 The planned UPS measurements will allow us to comment on these important issues which have surfaced just very recently. VI. Conclusions 1. Using CEELS, AES, and LEED we have found that bisulfate adsorption on Pt(111) takes place in the potentials range of 0.05-0.50 V with the maximum coverage of 0.34 ( 0.02 ML. The sulfur to oxygen ratio of the adsorbate is 4, and the oxidation state of sulfur in the HSO4- adsorbate is +6. LEED studies indicate that bisulfate forms a well-ordered (x3 × x3)R30° adlattice on the hexagonal mesh of platinum. 2. Auger electron spectra from adsorbed bisulfate consists of three-electron energy transitions with relative intensities being very similar to those found in the thin film Na2SO4 deposit. The observed upshift in Auger electron energies between bisulfate adlattice and the Na2SO4 salt occurs due to the extramolecular relaxation process in the adsorbate system. 3. CEELS was used to monitor electronic interactions between the HSO4- adsorbate and the Pt(111) substrate. The S2p loss energy decreases from 174.5 to 174 eV as the emersion potential is increased from 0.00 to 0.40 V. This is due to an increase in the electron density around S atom from the Pt5dHSO4- LUMO orbital interaction via an electron back-donation process. At higher potentials, the increase in the loss energy results from the change in surface coordination of the bisulfate adsorbate induced by surface oxidation. The same oxidation process also accounts for the increase in Pt4f7/2 loss energy in the potentials range from 0.40 to 0.80 V. 4. We present a bisulfate binding model with platinum that assumes both electron donation and back-donation but from which only the latter effect has been experimentally documented in this work. Other components of bonding that could account for adsorption specificity, like anions’ charge or hydration difference, are considered nonessential. UPS measurements are planned to provide a statement on the bonding/back-bonding balance, which is the key to the final understanding of the studied system. Acknowledgment. This work is supported by the National Science Foundation under Grant No. CHE 94-11184 and by the National Science Foundation under Grant No. DMR 8920538 administered by the Frederick Seitz Materials Research Laboratory at the University of Illinois. References and Notes (1) Blum, L. AdVances in Chemical Physics; Prigogine, I., Rice, S. A., Eds.; John Wiley & Sons: New York, 1990; Vol. LXXVIII. (2) Sung, Y.-E.; Thomas, S.; Wieckowski, A. J. Phys. Chem. 1994, 99, 13513. (3) Zhang, J.; Sung, Y.-E.; Rikvold, P. R.; Wieckowski, A. J. Chem. Phys., in press. (4) Mrozek, P.; Sung, Y.-E.; Wieckowski, A. Surf. Sci. 1995, 335, 44.

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