Electrochemical catalytic reduction of molecular oxygen by iron porphyrin ion-complex modified electrode

The iron porphyrin ion-complex, which is electrostatic aggregate of iron meso-tetrakis- (N-methyl-4-pyridyl)porphyrin (FeTMPyP, cationic) and iron meso-tetrakis(p-sulfophenyl)porphyrin (FeTPPS, anionic), has a catalytic activity for the reduction of molecular oxygen in aqueous solutions. Rotating ring-disk voltammetric measurements show the two different reaction pathways for the oxygen reduction; one is in a lower and other in a higher overpotential region. The main product in lower overpotentials is hydrogen peroxide but 12% of molecular oxygen is reduced to water directly via four-electron process. In higher overpotentials, the iron porphyrin ion-complex also acts as a catalyst for the reduction of hydrogen peroxide. The four-electron process and the two step pathway through intermediate hydrogen peroxide occur. From these results, it can be presumed that some of iron porphyrin aggregates form a "face-to-face"-like structure by the electrostatic interaction and provide the effective sites for the reduction of molecular oxygen.