OXYGEN EVOLUTION REDOX CATALYSIS USING RU-ADAMS

The results of a kinetic study of the oxidation of water to oxygen by Ce(IV) ions mediated by a newly identified, highly active redox catalyst, Ru-Adams, in different acid media are reported. In a medium which is largely HClO4 the rate of reduction of Ce(IV) is diffusion-controlled, depends directly upon the catalyst concentration and has an activation energy of 11 +/- 2 kJ mol-1. Increasing the concentration of sulphate decreases the rate of catalysis and the kinetics appear to be neither simple first-order or second-order with respect to [Ce(IV)], and this also appears to be the case in a kinetic study carried out in 0. 5 mol dm-3 H2SO4. The kinetics of O2 catalysis in all the different acid media used are readily interpreted using an electrochemical model in which the catalyst particles are considered as acting as microelectrodes which mediate electron transfer between a Nernstian reduction reaction (Ce(IV) --> Ce(III)) and an irreversible oxidation reaction (H2O --> 2H+ + 1/2 O2). Using this electrochemical model to interpret the observed complex kinetics in 0.5 mol dm-3 H2SO4 it is possible to extract mechanistic information concerning the nature of the rate-determining step in the oxidation of water on Ru-Adams and, using data arising from a temperature variation study, to estimate a value of 49 +/- 1 kJ mol-1 for the activation energy for water oxidation with the microelectrodes of Ru-Adams poised at a calculated mixture potential of 1.40 V vs. nhe.