Catalytic oxidation of thiol compounds by novel fuel cell-inspired co-porphyrin and co-imidazole catalysts

The catalytic performance of pyrolyzed carbon-supported cobalt-nitrogen donor (CoN₄) catalysts for the oxidation of thiol compounds by dioxygen in aqueous solution was studied. This paper continues our previous line of research, which was inspired by the electrocatalytic reduction of oxygen on pyrolyzed carbon-supported cobalt-porphyrins and related tetra-coordinated nitrogen donor-transition metal complexes (MeN₄, where Me stands for a transition metal atom). Both pyrolyzed carbon-supported Co-imidazole and Co-porphyrin exhibited fast catalytic oxidation of the different thiols. The rate of oxidation of different thiols on the pyrolyzed CoN₄ catalysts was compared to the homogeneous rate of oxidation using 5,10,15,20-tetrakis(4- sulfonatophenyl)porphyrin Co(II) tetrasodium salt as catalyst. Based on the cobalt content, the heterogeneous catalysts always outperformed the homogeneous one, and at times even exhibited 4,100-fold better catalysis. The dependence of the catalytic rate of oxidation on the preparation temperature was investigated, showing an optimal catalysis at ∼650 °C for the cobalt-imidazole catalyst. The decrease in catalytic performance after heat treatment at elevated temperature was attributed to the formation of cobalt metal acting as a generator of carbon nanotubes.