COMPARISON OF THE BEHAVIOR OF SEVERAL COBALT PORPHYRINS AS ELECTROCATALYSTS FOR THE REDUCTION OF O-2 AT GRAPHITE-ELECTRODES

Four cobalt porphyrins were adsorbed on graphite electrodes and used t o catalyze the electroreduction of O-2. The two porphyrins without sub stituent groups in the meso positions of the porphyrin ring operated a t the most positive potentials and catalyzed the reduction of O-2 to b oth H2O2 and H2O, but the H2O did not result from significant reductio n of H2O2. The porphyrins containing meso substituents catalyzed only the reduction of O-2 to H2O2. The catalysts that accomplish the four-e lectron reduction of O-2 are argued to consist of dimeric (or higher o ligomeric) forms of the adsorbed porphyrins. The present results and t hose of two recent related studies(1,2) indicate that the presence of only hydrogen or small alkyl groups in the meso positions of porphyrin rings facilitates the spontaneous formation of van der Waals dimers w ith greater catalytic activity for the reduction of O-2 by four electr ons. Such cobalt porphyrins were also found to be unusually active cat alysts for the electro-oxidation of H2O2.