Pourbaix Diagrams for H2O Oxidation to Adsorbed OH on Pt(111) and Why They Differ from Those for Bulk Solids

Density functional calculations show that Pourbaix diagrams for reactions involving surface species are not the same as for bulk solids. This finding is based on calculations for the onset potentials over the pH range 0–14 for 15 water oxidation reactions forming OH(ads) + H + (aq) + e – on Pt(111). The deviations are results of the dependencies of Gibbs energies of adsorbed reactants on the electrode potential, causing their activities to change with potential instead of being fixed at unity, which is the case for Pourbaix diagrams of solids. In a sense, the redox couple changes as the electrode potential changes because at each potential the adsorbed reactants and products have new structures.