Pt(1Â 1Â 1) surface disorder kinetics in perchloric acid solutions and the influence of specific anion adsorption

A significant number of electro-catalytic reactions take place in a potential region in which the surface of platinum is partly covered by oxygenated species. In this respect, the initial oxidation of Pt surfaces is an important process that could determine the reactivity of this catalyst. The understanding of electrochemical Pt oxidation has been hindered by a lack of surface structural definition. In this work, the electro-oxidation of Pt(1 1 1) electrode in the absence and presence of weak and moderately strong specific anion adsorption, and the subsequent surface modification induced by oxygen adsorption are studied. Two different potential dependences are found for the surface reordering kinetics in perchloric acid solutions, at higher and lower potentials, whereas only one appears on sulphate containing solutions. Additionally, a dual role of sulphate anion is observed: at high sulphate concentrations the protective character of the ordered sulphate adlayer delays surface disordering while small concentrations of sulphate anions increase the rate of surface reordering. Water dissociation is at the origin of the double behaviour in HClO4 and also explains the dual role of sulphate anions. It is concluded that platinum oxidation is a complex process that involves several adsorbed species that appear at increasing potentials. All of these process are influenced by anion adsorption and coexist during the initial stages of Pt(1 1 1) oxidation.