First principles study of sulfuric acid anion adsorption on a Pt(111) electrode

A first principles theory combined with a continuum electrolyte theory is applied to adsorption of sulfuric acid anions on Pt(111) in 0.1 M H2SO4 solution. The theoretical free energy diagram indicates that sulfuric acid anions adsorb as bisulfate in the potential range of 0.41 < U <= 0.48 V (RHE) and as sulfate in 0.48 V (RHE) < U. This diagram also indicates that sulfate inhibits formations of surface oxide and hydroxide. Charge analysis shows that the total charge transferred for the formation of the full coverage sulfate adlayer is 90 mu C cm(-2), and that the electrosorption valency value is -0.45 to -0.95 in 0.41 < U <= 0.48 V (RHE) and -1.75 to -1.85 in U > 0.48 V (RHE) in good agreement with experiments reported in the literature. Vibration analysis indicates that the vibration frequencies observed experimentally at 1250 and 950 cm(-1) can be assigned, respectively, to the S-O (uncoordinated) and symmetric S-O stretching modes for sulfate, and that the higher frequency mode has a larger potential-dependence (58 cm(-1) V-1) than the lower one.