Irreversibility of catalytic reduction of dioxygen by dissolved hemin

•Hemin catalyzes reduction of O2 at glassy carbon electrodes.•The catalytic current belongs to a surface process rather than volume reactions.•The current is not under the steady state.•It varies with concentrations of O2 in a very narrow domain.

Hemin in aerated dimethylsulfoxide catalyzes the reduction of dioxygen at glassy carbon electrodes with a gain voltage for the catalysis, 0.5 V. The catalytic rate, observed as the reduction current, increases with an increase in concentrations of dioxygen, but reaches a maximum at 6% of the saturated concentration. The voltammetric peak current has a linear relation with the scan rate. These variations are different from the ordinary catalytic mechanism, in which hemin oxidized by dioxygen might be reused for the electrochemical reduction. The voltammetric peak current in deaerated hemin solution is diffusion controlled, whereas that in aerated solution is represented as a sum of the diffusion current and a surface wave. The catalytic current is caused by hemin incorporated with dioxygen, of which adsorption density is close to an amount of a monolayer. Therefore hemin films are not suitable for continuous reduction of dioxygen. Once the adsorbed layer is electrochemically oxidized to remove the adsorption film, the catalytic reduction wave is retrieved.

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