The redox potentials of iron phthalocyanine (FePc), cobalt phthalocyanine (CoPc) and cobalt tetraaxaammlene (CoTAA), irreversibly adsorbed on pyrolytic graphite, were investigated as a function of the pH by cyclic voltammetry. The reduction of oxygen was studied under the same conditions with the rotating disc electrode technique. At high pH, where the reduction to H,Or on the cobalt-containing chelates is more reversible, the value of the redox potential is less important. At low pH, where this reduction is irreversible, there is a clear correlation between the redox potential of the central metal ion and the Oa reduction behaviour. In acid media, a 470 mV more positive redox potential of CoTAA compared to that of CoPc, resulted in a half-wave potential, Elfl, for the Or reduction being 400 mV more positive than the El12 of CoPc.
A review is given of the pyrolysis behaviour of carbon-supported transition metal chelates and the developed theories explaining the increased activity and stability towards oxygen reduction. Additional measurements with a new carbon-modified rotating ring-disc electrode show that the central metal ion-N, unit remains the active site after pyrolysis. The selectivity is increased after pyrolysis since relatively less H,O, is detected at the ring. The number of available active sites is an important parameter for the resulting activity. During the heat treatment, different processes take place: migration of the chelates over the carbon support, increasing the number of available active sites; reaction of the outer fringes of the organic skeleton retaining the metal ion-N, unit; and degradation of the catalyst with loss of activity. The condition of the pyrolysed catalyst and its corresponding activity are determined by the relative rate of these processes at the given pyrolysis temperature.
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