Aminopyridyl cation radical method for bridging between metal complex and glassy carbon:: cobalt(II) tetraphenylporphyrin bonded on glassy carbon for enhancement of CO2 electroreduction

Aminopyridyl compound immobilization on glassy carbon (GC) was carried out by amine cation radical formation through electro-oxidation at GC in ethanol solution, and subsequently by chemical bonding of the radical to GC. The aminopyridyl group immobilized on GC takes part as a bridge between cobalt tetraphenylporphyrin (Co(II)TPP) and GC, where the nitrogen of the pyridyl group is coordinated as a fifth ligand of Co(II)TPP, being perpendicular to the porphyrin plane. The Co(II)TPP modified GC electrode was thus prepared by a bridging amine of 4-aminopyridine, 4-aminoethylpyridine, or imidazole in the electrochemical and chemical processes, which is denoted 'CoTPP bonded GC'. Whereas Co(II)TPP physically attached to GC, denoted CoTPP/GC, was not able to reduce CO2 at E = -1.1V(SCE) in pH 6.3 phosphate buffer solution, 'CoTPP bonded GC' gave CO from CO2 electroreduction at that potential with 60% current efficiency. By polarization of 'CoTPP bonded GC' at hydrogen evolution potentials, Co(II)TPP stacked on 'CoTPP bonded GC' is suggested to be converted to HCo(II)TPP, but Co(II)TPP on CoTPP/GC remains the same. Hydrogenated 'CoTPP bonded GC' is suggested to play an important role as one of the intermediates of CO2 electroreduction. (C) 1997 Elsevier Science S.A.