Oxygen reduction on pyrolysed carbon-supported transition metal chelates

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 which a new carbon-modified rotating ring-disc electrode show that the central metal ion---N4 unit remains the active site after pyrolysis. The selectivity is increased after pyrolysis since relatively less H2O2 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 availableactive sites; reaction of the outer fringes of the organic skeleton retaining the metal ion---N4 unit; and degration 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.