Extended X-ray absorption fine structure (EXAFS) study of cobalt–porphyrin catalysts supported on active carbon

Abstract

Two samples of an active-carbon-supported cobalt–porphyrin catalyst have been studied by EXAFS. The first was the ‘as prepared’ catalyst, the second had been heated in nitrogen at 973 K. For the ‘as prepared’ catalyst the experimental result agreed well with that calculated with two coordination shells: (1) Co—N, R_j= 1.95 ± 0.03 Å, CN = 4 and (2) Co—C, R_j= 3.08 ± 0.03 Å, CN = 8. The structure is thus very similar to that in the unsupported porphyrin, indicating that little decomposition had occurred. The EXAFS result shows no evidence for a cobalt–support interatomic distance. For the treated sample the experimental spectrum agrees well with that calculated for only a single shell, Co—N, R_j= 1.95 ± 0.03 Å, CN = 4. It is suggested that the Co—C interatomic distances have been randomized by the heat treatment through elimination of the meso-carbon atoms of the porphyrins and subsequent reaction of the remaining fragments with the carbon surface. The CoN₄ group is retained in the heat-treated catalyst.