MECHANISTIC ASPECTS OF THE ELECTROREDUCTION OF DIOXYGEN AS CATALYZED BY COPPER-PHENANTHROLINE COMPLEXES ADSORBED ON GRAPHITE-ELECTRODES

The complexes formed by Cu(II) with 1,10-phenanthroline, 5-chloro-1,10-phenanthroline, and 2,9-dimethyl-1,10-phenanthroline were irreversibly adsorbed onto graphite electrodes and examined as electrocatalysts for the reduction of O-2. Upon reduction, all three complexes catalyze the four-electron reduction of O-2. The adsorbed complexes with only a single ligand coordinated to the Cu center were much more reactive than those with two coordinated ligands. Analysis of the rising portions as well as the plateaus of current-potential curves recorded at rotating disk electrodes indicated that the coordination of O-2 to the Cu(I) forms of the complexes is the current-limiting step at all potentials. Significant differences exist between the kinetic behavior of the complexes adsorbed on graphite and that of the same complexes dissolved in solution. The electrochemistries and coordination chemistries exhibited by Cu(II) phenanthrolines and Co(II) porphyrins during their electrocatalysis of the reduction of O-2 are compared and contrasted.