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 to catalyze the electroreduction of O_2. The two porphyrins without substituent groups in the meso positions of the porphyrin ring operated at the most positive potentials and catalyzed the reduction of O_2 to both H_2O_2 and H_2O, but the H_2O did not result from significant reduction of H_2O_2. The porphyrins containing meso substituents catalyzed only the reduction of O_2 to H_2O_2. The catalysts that accomplish the four-electron reduction of O_2 are argued to consist of dimeric (or higher oligomeric) forms of the adsorbed porphyrins. The present results and those of two recent related studies 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 with greater catalytic activity for the reduction of O_2 by four electrons. Such cobalt porphyrins were also found to be unusually active catalysts for the electro-oxidation of H_2O_2