Kinetics of corrosion of ruthenium dioxide hydrate by CeIV ions

Abstract

The kinetics of oxidative dissolution of RuO₂·xH₂O to RuO₄ by Ce^IV ions are studied. Under conditions of a low [Ce^IV] : [RuO₂·xH₂O] ratio (e.g. 0.35 : 1) and a high background concentration of Ce^III ions (which impede dissolution) the initial reduction of Ce^IV ions is due to charging of the RuO₂·xH₂O microelectrode particles. The initial rate of charging depends directly upon [RuO₂·xH₂O] and has an activation energy of 25 ± 5 kJ mol^–1. Under conditions of a high [Ce^IV] : [RuO₂·xH₂O](e.g. 9 : 1) and a low background [Ce^III] the reduction of Ce^IV ions is almost totally associated with the dissolution of RuO₂·xH₂O to RuO₄, i.e. not charging. The kinetics of dissolution obey an electrochemical model in which the reduction of Ce^IV ions and the oxidation of RuO₂·xH₂O to RuO₄ are assumed to be highly reversible and irreversible processes, respectively, mediated by dissolving the microelectrode particles of RuO₂·xH₂O. Assuming this electrochemical model, from an analysis of the kinetics of dissolution the activation energy for this process was estimated to be 39 ± 5 kJ mol^–1 and the Tafel slope for RuO₂·xH₂O corrosion was calculated to be 15 mV per decade. The mechanistic implications of these results are discussed.