THE ELECTROREDUCTION OF CARBON-DIOXIDE BY MACROCYCLIC COBALT COMPLEXES CHEMICALLY-MODIFIED ON A GLASSY-CARBON ELECTRODE

Various macrocyclic cobalt complexes were chemically bonded to a glass y carbon (GC) electrode, on which CO, electroreduction was carried out to give CO in aqueous phosphate buffer solution of pH 6.3. The macroc yclic cobalt complexes were naphthalocyanato cobalt(II), phthalocyanat o cobalt(II), yimino-4,10-dipropyl-5,9-diazatrideca-4,9-dien-one oxima to) cobalt(III) (denoted as CoDO), two kinds of hydrophobic vitamin B( 12)s (heptamethyl cobyrinate perchlorate and heptapropyl cobyrinate pe rchlorate), 5,10,15,20-tetraphenylporphyrinato cobalt(II), and 5,10,15 ,20-tetrakis (4-methoxyphenyl)porphyrinato cobalt(II). Their redox pot entials of Co(I)/Co(II) are in the above order from positive to negati ve potentials in a 0.05 M TBAP DMSO solution, being between -0.23 and -1.0 V (SCE). These complexes were chemically bonded to GC through -CO NH-pyridine which locates perpendicularly to a planar or semi-planar c omplex structure, where the N of the pyridine forms a coordinate bond with the Co atom of the above complexes as a fifth ligand. The catalyt ic activity for hydrogen evolution in aqueous solution was observed to be high on Co naphthalocyanine and Co phthalocyanine modified GC elec trodes; the latter gave H-2 evolution at the most positive potentials among the Co complexes employed. When the lower potential limit in cyc lic voltammetry became less than the hydrogen evolution potential, in the reverse positive-going sweep, an anodic hump current was observed at -0.35 similar to -0.78 V, which is assigned to a cobalt hydride oxi dation process; the hydride is suggested to form when hydrogen evoluti on takes place, and the hump disappeared after the introduction of CO2 into the solution. It was observed that the Co complex chemically bon ded on GC can give CO from CO2 only at relatively low overvoltages, ex cept for CoDO which was not able to reduce CO2; phthalocyanato cobalt( II) gave CO at E = -1.0 V (0.26 V as overvoltage) at 20% current effic iency. The highest CO current efficiency was observed in the case of t etraphenyl-porphyrinato cobalt(II) chemically modified GC. (C) 1997 El sevier Science S.A.