O2 reduction kinetics on a graphite electrode modified with adsorbed vitamin B12

Vitamin B12 (acquo-cobalamine) adsorbs irreversibly on ordinary pyrolytic graphite (OPG) electrodes from aqueous solutions. The adsorbed vitamin exhibits cyclic voltammetric reversible waves corresponding to Co(II)/Co(I) and Co(III)/(II) couples and its redox chemistry is similar to that found in solution. The OPG/vitamin B12(ads) electrodes show substantial activity for the reduction of oxygen. In alkaline pH, a Tafel slope of −60 mV/decade and a chemical order in protons of −1 suggest a redox catalysis type of kinetics, governed by the Co(III)/Co(II) couple in the cobalamine. AtpH