Temperature Dependence of Growth of Surface Oxide Films on Rhodium Electrodes

Abstract

Surface oxides on Rh electrodes were formed by anodic polarization at potentials, E p , between 0.70 and 1.40 V, RHE, with an interval of 0.05 V for polarization times, t p , up to 10 000 s and at temperatures, T, between 278 and 348 K. This procedure results in thin films having their charge density, q ox , of less than 1260 μC cm -2 , thus their thickness, X, of up to 2 ML of Rh(OH) 3 . Cyclic-voltammetry, CV, reveals one states, OC1, in the oxide reduction profiles. Increase of Τ leads to augmentation of the oxide thickness but it does not influence its surface state; thermodynamics of their reduction are not affected by Τ variation. Plots of q ox versus log t p or 1 / q ox versus log t p for a wide range of Τ and E p allow one to discriminate between the logarithmic and the inverse-logarithmic oxide growth kinetics. Two kinetic regions are observed in the oxide formation plots, each one giving rise to a distinct growth mechanism. Oxides having X ≤ 1 ML of RhOH are formed according to the logarithmic kinetics and the process is limited by the rate of the place exchange between the Rh surface atoms and the electroadsorbed OH groups. Formation of oxides having X between 1 ML of RhOH and 2 ML of Rh(OH) 3 follows the inverse-logarithmic kinetics and the process is limited by the rate of escape of Rh 3+ from the metal into the oxide. Theoretical treatment of the data in the region corresponding to X > 1 ML of RhOH leads to determination of the potential drop across the film and the electric field within the oxide layer, the latter being of 10 9 V m -1 .Surface oxide films on various transition metals can be formed by application of electrochemical techniques or by exposure to an oxidizing atmosphere and the extent of surface oxidation, thus the oxide thickness, is affected by the oxidation conditions.