Investigation of the Kinetics and Mechanism of Co-Porphyrin Catalyzed Oxygen Reduction by Hydrophobic Carbon-Ceramic Electrodes

The electrochemical mechanism of pyrolyzed carbon supported Co-tetramethoxymeso-porphyrin (Co-TMMP) catalysis of oxygen reduction is studied by a fuel-cell-type hydrophobic ceramic-carbon composite electrode (CCE), made of sol-gel-derived graphite-silica material. Only a thin layer at the outermost surface of the hydrophobic porous CCE is in contact with the electrolyte, thus minimizing effects of electrode structure on the current density. This property makes CCEs most useful for elucidation of kinetic mechanisms of gas electrodes. At low cathodic overpotentials, oxygen reduction in acidic solution on heat-treated Co-TMMP supported graphite powder exhibits first-order kinetics with respect to adsorbed di-oxygen, which obeys Langmuir adsorption isotherm. The activation energy of oxygen reduction equals 7.5 ± 0.4 kcal/mol. Half-order oxygen dependence is observed at high overpotentials, and the reaction is limited by the rate of hydrogen peroxide conversion.