Dynamic Behavior of the Oxygen–Peroxide Couple on Platinum

Abstract

The near-reversible nature of the oxygen–peroxide couple on platinum is demonstrated by substantial exchange currents. The transition between current by reduction of oxygen to peroxide and the reduction of peroxide to hydroxide ion is shown. Diagnostic criteria for determining the mechanism have been measured. The cathodic behavior of oxygen–peroxide systems in 1M potassium hydroxide is determined by the reduction of oxygen to peroxide when the concentration of peroxide is relatively low, and by the reduction of peroxide to water when its concentration is relatively large. At low peroxide concentrations Tafel slopes of approximately −2.3(2RT / 3F) are observed. With increasing oxygen activity these Tafel curves are translated towards increased currents in direct proportion to the pressure. In contrast, peroxide concentration changes do not influence the Tafel slopes in low concentration ranges. At high concentrations of peroxide, peroxide reduction becomes the predominant source of the observed current. The diffusion-limited currents are translated toward higher currents in direct proportion to the concentration of peroxide. A mechanistic interpretation of the reduction of oxygen to peroxide indicates that a fast one-electron step in quasiequilibrium precedes a slow one-electron rate-determining step. This latter may be a slow proton transfer to a superoxide species or a slow desorption of peroxide. The computed and observed diagnostic derivatives are in good agreement. The specific rate constant based on the proposed mechanisms show good constancy with change over two orders of magnitude in oxygen and peroxide concentration.