The real condition of electrochemically oxidized platinum surfaces: Part I. Resolution of component processes

Abstract

Summary The problem that 3 peaks can be resolved in the initial stages of Pt surface oxidation below a monolayer of electrosorbed OH (1 e per Pt atom), even on single crystal faces, is examined. A high degree of solution purity is required for this resolution to be observed. Reversibility with respect to these stages in the formation and reduction of the surface oxide layer has been evaluated. The stages of surface oxidation below the OH monolayer limit are related to the geometries of various sub-lattices of OH on the Pt surface rather than to specific stoichiometries of platinum-oxygen surface compounds. It is shown that the cathodic reduction profile can also be resolved into reversible and irreversible components depending on end potential in the anodic direction of polarization and on temperature. Low temperature measurements enable complete resolution of a fast reversible component from an irreversible one, associated with the normal main cathodic peak in Pt surface oxide reduction. The irreversible component arises from a rearranged form of the surface oxide and is formed already to a significant extent well below a monolayer of “PtOH”. It is reduced at less positive potentials and gives rise to the characteristic hysteresis between oxide formation and reduction processes at Pt. The origin of the shape of the i-V profile at Pt and other noble metals is explained. Optical studies by means of ellipsometry show that in pure solutions and with sensitive instrumentation there is no distinguishability, with respect to potential dependence, between the change of optical properties and the change of the surface coverage determined by surface coulometry down to the lowest coverages that can be detected. The surface oxidation and reduction is shown, however, to be optically irreversible with respect to degree of surface oxidation so that a real change of condition of the surface arises between anodic and cathodic directions of polarization.