An EXAFS study of pyrolyzed metal macrocyclic electrocatalysts

Pyrolysis of iron and cobalt N/sub 4/-chelates on carbon greatly improves their electrocatalytic activity and stability for oxygen reduction. The best results have been obtained by pyrolysis of absorbed iron and cobalt tetramethoxyphenyl porphyrins (TMPP) on carbon. A knowledge of the structure of these electrocatalysts could point the way toward the development of low cost substitutes of platinum. This paper reports an EXAFS study of pyrolyzed Fe-TMPP and Co-TMPP on Vulcan XC-72 carbon at several temperatures. Pyrolysis yields a mixture of metal, metal oxides and residual metal maorcycle. The presence of the latter is often masked by the metal and the oxide. Chemical leaching of the material to remove these simplifies spectroscopic studies on the residual macrocycle. In the case of Fe-TMPP EXAFS reveals a residual Fe-N/sub 4/ species even for pyrolysis temperatures as high as 1000/sup 0/C. Residual Co-N/sub 4/ species were found for Co-TMPP at temperatures below 800/sup 0/C. Above 900/sup 0/C the Co-TMPP is completely reduced to the metal. At intermediate pyrolysis temperatures (600 to 800/sup 0/C) the pyrolyzed Co-TMPP is hydrophobic in KOH.