The effect of heat treatment on group VIIIB porphyrins as electrocatalysts in the cathodic reduction of oxygen

Abstract

The electrocatalytic activities of heat-treated chloro(tetraphenylporphyrinato)iron(III)(ClTPPFe^III) for oxygen reduction have been studied using the rotating ring–disc electrode technique and compared with those of heat-treated TPPCo^II and TPPNi^II. ClTPPFe^III gives an unusual result, not observed for TPPCo^II or TPPNi^II, that the capability of reducing oxygen to water is increased on raising the heat-treatment temperature to 750 °C. This capability has been closely correlated with the anomalously high magnetic moment of the Fe site in the heat-treated porphyrin. It is further suggested that the increased catalytic activity of heat-treated ClTPPFe^III for the so-called four-electron reduction of oxygen is associated with increased activity in the electrochemical reduction of hydrogen peroxide, but not in its chemical decomposition. Measurements of the X-ray diffraction profile, the e.s.r. spectrum and the magnetic moment for powders of the heat-treated porphyrin have been carried out to interpret the variations in catalytic activities with heat-treatment temperature.