Hydroxy-substituted Cobalt Tetraphenylporphyrins as Electrocatalysts for the Reduction of O 2

The possible effects of the electron-donating properties of p-hydroxyphenyl groups on the catalytic activity of cobalt porphyrins toward the electro-reduction of O 2 were explored with three cobalt porphyrins having p-hydroxyphenyl substituents in the meso position of the porphyrin ring. The porphyrins were examined as electrocatalysts adsorbed on the surfaces of pyrolytic graphite electrodes in aqueous solution and as homogeneous catalysts dissolved in methanolic solutions. The adsorbed molecules exhibited a higher catalytic activity than those in solution and accomplished the reduction of a portion of the O 2 molecules reaching the electrode to H 2 O . However, the four-electron reduction was indirect, with H 2 O 2 being formed as an intermediate that was subsequently reduced. Although the introduction of the electron-donating p-hydroxyphenyl substituents on the porphyrin ring altered the catalytic activity of the cobalt porphyrins, the effect was not as great as can be obtained by coordinating ruthenium (II) complexes to π-accepting ligands in the meso position of the porphyrin ring.