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The kinetics of oxygen reduction have been studied on a ruthenium electrode in alkaline solutions using the rotating disc and rotating disc-ring methods. The reaction kinetics and mechanism were found to depend on the oxidation state of the ruthenium surface. For a surface having up to two monolayers of oxide a "parallel" mechanism of O₂ reduction has been found, which goes predominantly through a four-electron direct reduction to OH^-. All the rate constants of the O₂ and HO^-₂ reactions, including those of the adsorption equilibrium of HO^-₂ on the Ru surface, have been determined. No catalytic decomposition of HO^-₂ was found. Although the Tafel slope is higher than -120 mV/dec it appears that an initial electron exchange is the rate-determining step. The kinetics are slower on a Ru surface with a thicker oxide layer. The mechanism of the reaction is also different, showing a potential-dependent ratio of the rate constants for a direct and series reduction of O₂ and a catalytic decomposition of HO^-₂. © 1986.