ON THE KINETICS AND MECHANISM OF OXYGEN REDUCTION AT OXIDE FILM-COVERED PT ELECTRODES IN ACID-SOLUTIONS .2. ELECTRON-TRANSFER THROUGH THE OXIDE FILM AND MECHANISM OF THE REDUCTION

The kinetics and mechanism of oxygen reduction at oxide film-covered Pt electrodes in acid solution is analyzed. Two linear E-log i regions are observed when it is ensured that the thickness of the oxide film for each E-log i point is the same. The kinetics in the high current density (cd) region is characterized by the Tafel slope of -120 mV. In the low cd region, the slope is -60 mV. The reaction order with respect to H3O+ is 1/2 and 1 in the high- and the low-cd region, respectively. Because of the observed fractional reaction order in the high-cd region, the kinetics in either cd region cannot be explained in terms of any classical, commonly used procedure in mechanistic analysis with a simple model of electrified interfaces. Currents at a constant potential in the cd region with the -120 mV slopes decrease exponentially with the thickness of the oxide film. In contrast, currents in the region with the -60 mV slope are invariant with thickness. Three models are used in mechanism analysis to account for the observed pH dependencies. The model of splitting the interfacial potential difference into two components, and the acid-base equilibria model can explain the observed pH dependencies. However, they do not account for the observed kinetic behavior with respect to the thickness of the oxide film. The model of the distribution of the unoccupied electron energy levels in solution and electron tunneling through the oxide film accounts both for the observed pH dependencies and for the dependence of the rates on oxide film thickness. With this model it is shown that the electron energy level in the ground state of the reacting complex increases 60 meV as pH increases one unit. This dependence of the electron energy levels on pH provides physical significance for the observed pH dependence and fractional reaction order with respect to H3O+.