Abstract. The potential of a number of tetraaza-macrocyclic complexes of cobalt and nickel to mediate the photoreduction of carbon dioxide has been studied. Carbon monoxide and hydrogen are the main products, which result from illumination of an aqueous solution containing Ru(2,2'-bipyridine):+ as sensitizer, ascorbic acid as sacrificial eletron donor and a tetraaza-macrocyclic complex as relay. Their ratio strongly depends upon the relay compound used. The complexes can be used as electrocatalysts for CO, reduction at a mercury electrode since they lower the overpotential for CO, reduction showing turnover numbers/hour of approximately 3. A minimum potential is required to induce reaction of electrocatalysts and CO, (and H,O). Finally, the redox properties of the macrocyclic metal complexes at p-GaP and p-GaAs photocathodes in acetonitrile are compared with those observed at Pt.

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