Redox behaviour of the dicobalt cofacial dicobalt porphyrin Co2FTF4 and related complexes adsorbed on graphite electrodes

Abstract

Adsorbed on graphite electrodes, Co2FTF4 in a potent catalyst for O2 reduction by a four-electron mechanism. The two observable redox surface waves have been previously assigned to the two cobalt centres. Using differential pulse polarography (DPP), the behaviour of this dicobalt cofacial dimer was re-examined at different pH values in aqueous solutions and in the presence of potential axial ligands for cobalt. From these observations and from a comparison with other adsorbed porphyrins it can be concluded that (a) the porphyrins are probably adsorbed by strong interactions between graphite and the aromatic rings, and (b) the more negative surface wave is cobalt-based but the more positive one is instead a porphyrin ring oxidation. This implies that the catalyst is in the CoIICoIIIFTF4 state when catalytic oxygen reduction begins.