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.
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.
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