THE CATALYTIC ELECTROREDUCTION OF DIOXYGEN ON IRON PROTOPORPHYRIN-IX MODIFIED GLASSY-CARBON ELECTRODES

The catalytic electroreduction of dioxygen at iron(III) protoporphyrin IX (hemin) modified glassy carbon electrodes was investigated in an aqueous solution within the pH range 1-13. The reduction potential of dioxygen on a hemin chemically modified electrode was more anodic than that on a hemin adsorbed electrode by a few tens of mV. The results of rotating disk and ring-disk voltammetric measurements indicated that the oxygen molecule is reduced via a one-step reduction accompanying four-electron and four-proton transfer at pH<11, and via two successive reductions accompanying two-electron transfer and, subsequently, two-electron and four-proton transfer at pH>12. The quantity of peroxide formation at hemin-adsorbed electrodes decreased with increasing pH, up to 10, and then increased with increasing pH beyond 12. The possible overall mechanisms for the catalytic reduction of dioxygen are described.