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

Surface oxides on Ph were formed by anodic polarization at various potentials, E-p, from 0.70 to 1.40 V, RHE, for polarization times, t(p), up to 10(4) at 298 K; their thickness expressed as a charge density, q(ox), was evaluated from cyclic voltammetry, CV, profiles. The oxide growth proceeds in two steps whereas its reduction occurs in one. Upon extension of E-p and (or) t(p) to higher values, q(ox) increases. Under the above experimental conditions, q(ox), varied from 110 +/- 5 to 985 +/- 10 mu C 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 E-p and t(p) results in thermodynamically more stable oxides. Plots of q(ox) versus log t(p) are linear over the whole range of q(ox) whereas plots of 1/q(ox) versus log t(p) are linear only when q(ox) >210 mu C cm(-2). Thus, the oxide growth is either logarithmic in time over the whole region of q(ox) or it is logarithmic when q(ox) <210 mu C cm(-2) and inverse-logarithmic when q(ox) >210 mu C 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 less than or equal to 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.