CATALYTIC PATHWAYS FOR THE ELECTROREDUCTION OF O-2 AT GRAPHITE-ELECTRODES ON WHICH A MACROCYCLIC COBALT COMPLEX IS ADSORBED

The electroreduction of O-2 at graphite electrodes on which the comple x of Co(III) with the macrocyclic ligand 14,14-hexamethyl-1,4,8,11-tet raazacyclotetradecane (hmc) is adsorbed proceeds in two steps that are well separated in potential. The first step consumes two electrons pe r molecule of adsorbed (hmcCo3+ to yield adsorbed (hmc)CoOOH2+. The se cond step involves the reduction of the adsorbed (hmc)CoOOH2+ to (hmcC o2+ + H2O2 followed by the reduction of O-2 to H2O2 catalyzed by the a dsorbed (hmc)Co-3+/2+ complexes. A mechanistic scheme that accommodate s the observed electrochemical responses is presented and tested by co mparison with responses calculated by digital simulation methods. Diff erences between the electrochemical responses obtained with dissolved or adsorbed (hmcCo3+ are exposed and discussed. In addition, the behav ior of the adsorbed (hmc)Co-3+/2+ system as it acts to electrocatalyze the reduction of O-2 is compared and contrasted with that exhibited b y analogous adsorbed cobalt porphyrin electrocatalysts.