An extended X-ray absorption fine structure study of heat-treated cobalt porphyrin catalysts supported on active carbon

Abstract

Transition-metal chelates are good catalysts for the electrochemical reduction of oxygen, and their activity can be further improved by heat treatment. The present EXAFS study of 5,10,15,20-tetra-(p-chlorophenyl)porphyrinatocobalt(III) supported on Norit BRX active carbon and heat-treated in dinitrogen at temperatures up to 850 °C shows (i) that upon adsorption the Co chelate remains intact, (ii) that after heating at 550 °C, when the oxygen-reduction activity is at its maximum, all Co is still present in its original (square-planar N₄) environment and (iii) that at higher temperatures the CoN₄ part decomposes to form, ultimately, metallic Co, with a concomitant decline in catalytic activity. Metallic Co dissolves in 4 mol dm^–3 H₂SO₄, but reacts in 4 mol dm^–3 NaOH to give an oxyhydroxide, which is thought to have a capacity for reducing oxygen electrochemically.