Comprehensive studies of formation and reduction of surface oxides at rhodium electrodes at 298 K

Abstract

Surface oxides on Rh were formed by anodic polarization at various potentials, Ep, from 0.70 to 1.40 V, RHE, for polarization times, tp, up to 104, at 298 K; their thickness expressed as a charge density, qOX, was evaluated from cyclic voltammetry, CV, profiles. The oxide growth proceeds in two steps whereas its reduction occurs in one. Upon extension of Ep and (or) tp to higher values, qOX increases. Under the above experimental conditions, qOX varied from 110 ± 5 to 985 ± 10 μC cm−2; after conversion, these charge densities correspond to 0.5 monolayer, ML, of RhOH and 1.56 ML of Rh(OH)3, respectively. Augmentation of Ep and tp results in thermodynamically more stable oxides. Plots of qOX versus log tp are linear over the whole range of qOX whereas plots of 1/qOX versus log tp are linear only when qOX > 210 μC cm−2. Thus, the oxide growth is either logarithmic in time over the whole region of qOX or it is logarithmic when qOX < 210 μC cm−2 and inverse-logarithmic when qOX > 210 μC cm−2. It was observed that the oxide growth continues when the positive-going scan is stopped in the oxide-formation potential region. Similarly, the oxide reduction proceeds when the negative-going scan is stopped in the oxide-reduction potential range. Finally, the authors present the first-ever studies of the Rh oxide reduction. The results indicate that the reduction takes place at E ≤ 0.70 V, RHE. The oxide reduction kinetics are not understood but, in general, the lower the reduction potential and the longer the reduction time, the greater the amount of the reduced oxide. Keywords: rhodium oxides, oxide films, growth kinetics, reduction kinetics.