The adsorption of bisulfate and sulfate anions over a Pt(1 1 1) electrode: A first principle study of adsorption configurations, vibrational frequencies and linear sweep voltammogram simulations

Specific anion adsorption of sulfate and bisulfate anions to the Pt(1 1 1) surface was studied using periodic density functional theory (DFT) methods. The origins of experimentally observed voltammetric features have been assigned based on comparison of calculated vibrational frequencies and simulated linear sweep voltammograms (LSVs) to experimental data available in the literature. The inclusion of solvation in the electrochemical model well-predicts the bisulfate and sulfate adsorption potentials. Combined with vibrational frequency analysis, sulfate is identified as the dominant adsorption species over the (bi)sulfate adsorption potential region. Rapid increase of coverage with electrode potential causes a sharp peak in the simulated voltammogram. We have assigned the experimental characteristic vibrational mode of adsorbed (bi)sulfate in the 1200-1300 cm^-1 range as an S-O _{uncoordinated} stretch. The Stark tuning slope of the S-O _{uncoordinated} frequency for sulfate containing (√3 × √7) superstructures was calculated for different co-adsorbed species to resolve possible molecular structures for this adlayer. The oxidation of co-adsorbed water to surface hydroxyl at (√3 × √7) adlayer causes a decrease of the Stark tuning slope and a high potential feature in the voltammogram. The adsorption of a series of sulfonic acids to Pt(1 1 1) is considered, and the calculated adsorption equilibrium potential is observed to correlate linearly with the sulfonate proton affinity.