SPECIFIC ADSORPTION OF A BISULFATE ANION ON A PT(111) ELECTRODE - ULTRAHIGH-VACUUM SPECTROSCOPIC AND CYCLIC VOLTAMMETRIC STUDY

We addressed in this study the process of specific adsorption of anion s at the metal/solution interface; We focused on the nature of the sur face chemical bond that accounted for the phenomenon of adsorption spe cificity in the context of bisulfate coverage and structural informati on. While we limited our investigations to bisulfate adsorption on the Pt(111) electrode in sulfuric and mixed sulfuric/perchloric acid medi a, our conclusions have general significance in explaining ionic adsor ption events in electrochemistry. We used core-level electron energy l oss spectroscopy, auger electron spectroscopy, low energy electron dif fraction, and cyclic voltammetry. Our findings show that in the studie d range of sulfuric acid concentration (10(-4)-10(-1) M) the maximum a nion coverage is 0.34 +/- 0.02 monolayer (ML) and that this coverage c orresponds to a highly ordered Pt(111)(root 3 x root 3)R30 degrees sur face structure. S2p core-level and LMM Auger electron spectra indicate that the chemical state of bisulfate sulfur is +6, as in the sulfate anion ina sulfate salt matrix. However, the electron density on the ad lattice sulfur is higher than in the salt, evidently due to back-donat ion 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 p otential 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 po tential range. When OH adsorption and platinum oxidation begin, a 2D c ompressive effect of the O-type adsorbates causes the bisulfate-platin um O-Pt bond to besequentially cleaved. The ''flow'' of metal electron s to the adsorbate is therefore reduced, and the S2p loss energy appro aches the level characteristic of sodium sulfate unperturbed by surfac e interactions. Loss spectra from Pt4f(7/2) confirm that the oxidized surface is emersed to vacuum but in the double layer potential range f ail to respond to either the electrode potential change or the bisulfa te adsorption.