Abstract

We have investigated the electrocatalytic activity of several substituted and unsubstituted cobalt-phthalocyanines of substituted tetraphenyl porphyrins and of vitamin B 12, for the electro-oxidation of 2-mercaptoacetate, with the complexes pre-adsorbed on a pyrolytic graphite electrode. Several N4-macrocyclic were used to have a wide variety of Co(II)/(I) formal potentials. The electrocatalytic activity, measured as current at constant potential, increases with the Co(II)/(I) redox potential for porphyrins as Co-pentafluorotetraphenylporphyrin∈<∈Co- tetrasulfonatotetraphenylporphyrin∈<∈Co-2,2′,2″, 2‴tetra-aminotetraphenylporphyrin and decreases for cobalt phthalocyanines as Co-3,4-octaethylhexyloxyphthalocyanine∈>∈Co- octamethoxyphthalocyanine∈>∈Co-tetranitrophthalocyanine Co-tetraaminophthalocyanine∈>∈Co-unsubstituted phthalocyanine∈ >∈Co-tetrasulfonatophthalocyanine∈>∈Co-perfluorinated phthalocyanine. Vitamin B 12 exhibits the maximum activity. A correlation of log I (at constant potential) versus the Co(II)/(I) formal potential of the catalysts gives a volcano curve. This clearly shows that the search for better catalysts for this reaction point to those N 4-macrocyclic complexes with Co(II)/(I) formal potentials close to -0.84 V versus SCE, which correspond to an optimum situation for the interaction of the thiol with the active site. © 2007 Springer-Verlag.