ADSORPTION ON GRAPHITE AND CATALYTIC REDUCTION OF O-2 BY THE MACROCYCLIC COMPLEX OF COBALT(II) WITH -1,4,8,11-TETRAAZACYCLOTETRADECA-1,3,8,10-TETRAENE

The complex of Co(III) with the macrocyclic ligand ph-tim (ph-tim = 1, 4,8,11-tetraazacyclotetradeca-1,3,8,10-tetraene) adsorbs irreversibly on roughly polished pyrolitic graphite electrodes. The adsorbed comple x exhibits two, reversible, one-electron voltammetric waves correspond ing to Co(III)/Co(II) and Co(II)/Co(I) redox couples located at 0.32V and -0.07V, respectively. The adsorbed Co(II) complex generated at 0.3 2V catalyzes the electroreduction of O-2 to H2O2 in a separate voltamm etric step at more negative potentials which vary with pH. Catalytic p lateau currents for the reduction of O-2 at rotating pyrolytic graphit e disk electrodes coated with [(ph-tim)Co](ads)(3+) are limited by the rate of coordination of O-2 to the reduced catalyst, [(ph-tim)Co](ads )(2+). An upper limit of 150 M-1 was placed on the equilibrium constan t for the binding of O-2 to [(ph-tim)Co](ads)(2+). A catalytic mechani sm compatible with the results is proposed and discussed. (C) 1997 Els evier Science S.A.