ENHANCEMENT OF THE CATALYTIC ACTIVITY OF A MACROCYCLIC COBALT(II) COMPLEX FOR THE ELECTROREDUCTION OF O-2 BY ADSORPTION ON GRAPHITE

In solution, the [(tim)Co](2+) complex (tim = 1,4,8,11-tetraazacyclote tradeca-1,3,8,10-tetraene) reacts only slowly with O-2, but upon adsor ption on graphite electrodes, it becomes an active catalyst for the re duction of O-2 to H2O2. The electroreduction of O-2 proceeds in a sing le voltammetric step at close to the diffusion-controlled rate at a re latively positive potential (0.25 V vs SCE). The remarkable enhancemen t in catalytic activity is attributed to a higher affinity for O-2 of the adsorbed complex as a result of its interactions with functional g roups on the surface of roughened or oxidized graphite. A possible mec hanism for the catalytic reduction of O-2 is proposed. It differs from the one employed by the analogous [(hmc)Co](2+) complex (hmc = 4,14-h examethyl-1,4,8,11-tetraazacyclotetradecane) which operates at less po sitive potentials and exhibits two separated voltammetric steps in the reduction of O-2, via [(hmc)CoOOH](2+), to H2O2.