Electron transfer coupling of diffusional pathways. Homogeneous redox catalysis of dioxygen reduction by the methylviologen cation radical in acidic dimethylsulfoxide

It is shown that reduction of dioxygen in acidic DMSO can be catalyzed by the MV2+/MV·+ couple. This homogeneous redox catalytic process is under the kinetic control of the MV·+ + O2 → MV2+ + O·−2 electron transfer. Dioxygen has a diffusion coefficient which is much larger (6 times) than that of MV2+. This difference must be taken into account in the treatment of the kinetic data, especially for large values of the catalytic efficiency. The corresponding theory is derived in the context of cyclic voltammetry and used to analyze the variation of the catalytic efficiency with the sweep rate and the concentrations of catalyst and substrate. The rate constant of the MV·+ + O2 → MV2+ + O·−2 reaction is found to be 2.3 x 105M−1 s−1 showing that the reverse reaction is under diffusion control. This is compatible with the standard activation Gibbs energy which can be derived from the kinetics of the electrochemical reduction of dioxygen according to the Hush-Marcus theory, provided than the attraction work term for MV2+ and O·− is taken into account.