Kinetic study of the oxidation of water by CeIV ions mediated by activated ruthenium dioxide hydrate

Abstract

The results of a kinetic study of the oxidation of water to oxygen by Ce^IVions, mediated by thermally activated ruthenium dioxide hydrate (RuO₂·yH₂O^*), are reported. At low [Ce⁴⁺], i.e. ca. 3.45 × 10^–5 mol dm^–3, the rate of reduction of Ce^IV ions (R) was diffusion controlled and depended directly upon [Ce⁴⁺]. and [redox catalyst], but was virtually independent of [Ce³⁺]. At higher [Ce⁴⁺], i.e. 3.45 × 10^–3 mol dm^–3, the kinetics were more complex, with R decreasing with increasing [Ce³⁺] and decreasing [Ce⁴⁺] and [redox catalyst]. The observed kinetics can be successfully described using an electrochemical model in which the catalyst particles are considered to act as microelectrodes which mediate the transfer of electrons between a Nernstian reaction (the reduction of Ce^IV ions) and an irreversible reaction (the oxidation of water). Using this model it proved possible to obtain from any [Ce⁴⁺]vs. time decay trace a Tafel plot of the variation of current as a function of potential for the oxidation of water on the surface of the particles of RuO₂·yH₂O^*. In addition to providing valuable mechanistic information concerning this reaction, it also allowed the original kinetic traces to be reconstructed and decay curves simulated for a wide range of conditions. From the variation in Tafel plot data as a function of temperature, an activation energy for water oxidation of 52 ± 8 kj mol^–1 was obtained.