Elrcfrochimica Acta, Vol. 38, No. 17. p9.2493-2% 1993 0013-4686/93 S6.00 + 0.00 Printed in Great Britain. 0 1993. Pergamon Prw Ltd PYROLYSIS OF IRON AND COBALT PORPHYRINS SUBLIMATED ONTO THE SURFACE OF CARBON BLACK AS A METHOD TO PREPARE CATALYSTS FOR O2 REDUCTION ANDERS WIDEL~V Inorganic Chemistry 1, Chemical Centre, PO Box 124,S-22100 Lund, Sweden (Received 26 zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA April 1993; in reuisedjorm 14 .rune 1993) Abstract-Cobalt and iron tetraphenylporphyrins were adsorbed on carbon black by sublimation of porphyrin in the presence of carbon black. Thus the formation of crystallites of porphyrin was avoided. Heat treatment of these preparations in an inert atmosphere increased catalytic activity for reduction of oxygen. The loss of the phenyl groups in the prophyrin molecule, with the concomitant formation of porphyrin-carbon-support bonds, was put forward as a possible explanation for the increase in activity. Key words: cobalt tetraphenylporphyrin, iron tetraphenylporphyrin, sublimation, pyrolysis, ESCA. INTRODUCTION In a previous investigation we undertook a study of pyrolysed porphyrins that were prepared in various ways[l]. A variety of catalyst preparations were sub- jected to heat treatment in an inert atmosphere: crystalline porphyrins, porphyrins mixed with carbon black and porphyrins precipitated on carbon black, plus a number of other methods of prep- aration. The primary aim of that investigation was to study the effect of heat treatment on these various catalysts with ESCA and to relate this to their cata- lytic properties. We found that in many respects the different preparations responded differently to heat- treatment (although in other respects their behaviour was completely analogous) and that they also dif- fered in their catalytic properties. In the present investigation porphyrins adsorbed on carbon black are examined in more detail. In order to do this study we had to find a new catalyst preparation method to avoid ambiguity in the results. In our previous study the possibility to draw firm conclusions as to whether chelates adsorbed on carbon black in monolayer form get an enhanced activity after pyrolysis was clouded by the fact that before pyrolysis all our catalyst preparations con- tained crystalline porphyrin in one or another form. In our preparation method (iii) of the previous paper[l], where porphyrin (CoTPP) was precipi- tated on carbon black by slow addition of ethanol to a chloroform solution of the porphyrin, we found that porphyrin precipitate as crystallites on the surface of carbon black rather than adsorb entirely in monolayer form. This is a drawback in that it could then be argued that the increase in activity after heat treatment of porphyrins precipitated on carbon black is not due to any change in the porphyrin molecule itself, but to an increase in chelate dispersion caused by the sublimation of porphyrin from crystallites followed by readsorption of porphyrin on the surface of carbon during pyro- lysis. In our previous investigation, pyrolysis of CoTPP precipitated on carbon black resulted in a five-fold increase of surface concentration of chelates. Thus, to find out if porphyrins adsorbed on carbon in monolayer form, upon heat treatment, undergo changes that increase their catalytic activity for oxygen reduction, it is very important to eliminate any suspicion that crystallites of porphyrin are present during pyrolysis. A way to overcome the formation of crystallites would be to let porphyrins adsorb on the surface of carbon black by equilibrium adsorption, zyxwvutsrqponmlkji ie letting porphyrin adsorb on carbon black in a saturated solution of porphyrin in, eg chloroform, without addition of an additional solvent to force precipi- tation of porphyrin[2]. This we have tried, but it seems that this method leads to an incomplete adsorption of chelates, probably due to desorption during washing. A more thorough study, trying dif- ferent solvents, might give better results. We, however, found another method of preparing cata- lysts which overcomes all such problems. Since porphyrin molecules sublime during pyrolysis, and, apparently, re-adsorb on the surface of carbon black; it was supposed that by subjecting separate batches of carbon black and porphyrin to a mild (<4OOC) heat treatment in vacuum, sublimated porphyrin should adsorb on the surface of carbon black. It was found that it did. Porphyrin and carbon black were physically separated from each other and since they were held at the same temperature there can be no question but that the chelates were adsorbed singly (in mono- layer form) on the surface of carbon black. A sub- sequent heat-treatment of the catalysts prepared in this way in an inert atmosphere, at a temperature higher than the sublimation-adsorption temperature, would reveal if pyrolysis increases the activity of che- lates adsorbed on carbon black in monolayer form. In the following the word “chelate” will be used to denote any intact MeN,-unit although the outer 2493