Elucidation of the chemical nature of adsorbed species for Pt(111) in H2SO4 solutions by thermodynamic analysis.

The nature of the adsorbed species for Pt(111) in sulfuric acid solutions has been elucidated by a careful thermodynamic analysis of the effect of pH on charge density data. This analysis takes advantage of the fact that, for solutions of constant total sulfate + bisulfate concentration, an increase of pH would increase the sulfate concentration, at the expense of decreasing the bisulfate concentration. As a result, sulfate adsorption would be shifted toward lower potentials, while bisulfate adsorption would follow the opposite trend. In the present work, coulostatic data for Pt(111) in (0.2 - x) M Me(2)SO(4) + x M H(2)SO(4) (Me: Li, Na; x: 10(-4) - 0.2) and (0.1 - x) M KClO(4) + x M HClO(4) + 10(-3) M K(2)SO(4) (x: 10(-4) - 0.1) solutions are carefully analyzed. It is concluded that sulfate rather than bisulfate adsorption takes place at potentials higher than the potential of zero charge. This result agrees with the fact that similar FTIRRAS bands for adsorbed sulfate species are observed for pH 0.8-3.5 in (0.2 - x) M K(2)SO(4) + x M H(2)SO(4) solutions.