Methyl viologen mediated oxygen reduction in ethanol solvent: the electrocatalytic reactivity of the radical cation

The study of methyl viologen (MV²⁺) mediated oxygen reduction in electrolytic ethanol media possesses potential application in the electrochemical synthesis of hydrogen peroxide mainly due to the advantages of the much increased solubility of molecular oxygen (O₂) and high degree of reversibility of MV²⁺/•+ redox couple. The diffusion coefficients of both MV²⁺ and O₂ were investigated via electrochemical techniques. For the first time, MV²⁺ mediated O2 reduction in electrolytic ethanol solution has been proved to be feasible on both boron-doped diamond and micro-carbon disc electrodes. The electrocatalytic response is demonstrated to be due to the radical cation, MV•+. The homogeneous electron transfer step is suggested to be the rate determining step with a rate constant of (1 ± 0.1) × 10⁵ M⁻¹s⁻¹. With the aid of a simulation program describing the EC’ mechanism, by increasing the concentration ratio of MV²⁺ to O₂ electrochemical catalysis can be switched from a partial to a ‘total catalysis’ regime.