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 CON, part decomposes to form, ultimately, metallic Co, with a concomitant decline in catalytic activity. Metallic Co dissolves in 4 mol dm-3 H,S04, but reacts in 4 mol dm-3 NaOH to give an oxyhydroxide, which is thought to have a capacity for reducing oxygen electrochemically.

From the extended X-ray absorption fine structure spectra of Mo/A1203 catalysts, prepared via equilibrium adsorption of ammonium heptamolybdate, fairly accurate figures for the fractions of fourand six-coordinated Mo in the adsorbed phase have been derived. The results show that not only basic hydroxyl groups, as is commonly thought, but also some other sites are involved in the adsorption reaction, here tentatively identified as coordinatively unsaturated Al3+ surface sites.

ChemInform Abstract The EXAFS results indicate that two different sites are involved in the adsorption process: basic hydroxyl groups leading to adsorption of Mo(IV) and coordinatively unsaturated Al3+ surface sites leading to adsorption of Mo(VI).

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