Catalysis of the electroreduction of O2 by cobalt 5,10,15,20-tetraphenylporphyrin dissolved in thin layers of benzonitrile on graphite electrodes

The electroreduction of O2 was examined at graphite electrodes that were covered with thin layers (∼30 μm) of benzonitrile (BN) and equilibrated with aqueous solutions of HClO4 saturated with air. The presence of the BN layers enhanced the rate of the electroreduction and affected its stoichiometry compared with results obtained at uncoated electrodes. Dissolving cobalt 5,10,15,20-tetraphenylporphyrin (CoTPP) in the thin layers of BN produced further increases in reaction rates and greater fractions of the O2 were reduced to H2O. The primary factors affecting the rate of the catalyzed reduction were the quantities of acid that partitioned into the BN layers and the concentration of CoTTP in the layers. The catalytic efficiency of CoTTP is greater when it is dissolved in thin layers of acidified BN than when it is adsorbed on the surface of the graphite electrodes. A possible catalytic mechanism is offered that is compatible with the experimental observations.