Electrochemical Oxidation of Pt(111) Vicinal Surfaces: Effects of Surface Structure and Specific Anion Adsorption

The initial oxidation of Pt surfaces is an important process that could determine the reactivity of catalysts in a wide range of reactions, from electrocatalytic oxidation of organics to oxygen reduction, but the understanding of electrochemical Pt oxidation has been hindered by a lack of surface-structural definition. We have investigated the process at surfaces vicinal to Pt(111) and show that these oxidize in successive stages depending on site geometry as well as the adsorption behavior of the electrolyte anion. Step sites of {100} orientation slowly oxidize at low potential (0.7 V vs RHE) in a region overlapping that of the "butterfly" peak seen at Pt(111) in the absence of specific electrolyte anion adsorption. Almost regardless of the latter, both {110} and {100} steps also oxidize between 0.9 and 1.2 V, causing voltammetric peaks with shapes that are characteristic of step orientation. The complex oxidation behavior of Pt(111) in perchloric acid, ranging from 0.6 V to the onset of O(2) evolution at 1.5 V, is mostly suppressed in sulfuric acid. However, if steps are introduced, then the oxidation at lower potential is again facilitated, probably by breaking the ordered protective sulfate adlayer along the steps. It was found that {100} steps oxidize to the extent of one electron per step atom, whereas {110} steps show two consecutive oxidations, amounting to a total of two electrons per step atom. At the onset of O(2) evolution, Pt(111) terraces are oxidized with two electrons per surface atom.