Photochemical reduction of carbon dioxide to carbon monoxide in water using a nickel(II) tetra-azamacrocycle complex as catalyst

Abstract

Visible-light illumination of a CO₂-saturated aqueous solution containing tris(2,2′-bipyridyl)ruthenium(II) chloride as photosensitiser, ascorbic acid as electron donor, and 1,4,8,11-tetra-azacyclotetradecanenickel(II) chloride as catalyst results in the production of both carbon monoxide and hydrogen. The yields of both CO and H₂ are pH dependent. The production of CO is proposed to proceed via CO₂ insertion into the Ni–H bond that is formed upon reaction of the reduced nickel species with a proton. This is followed by dissociation to form CO and H₂O. The production of hydrogen is also proposed to proceed via the reaction of H⁺ with the same Ni–H species. Some H₂ is also produced in the absence of catalyst, presumably by a photoreaction of the sensitiser. By a mechanism that is not yet understood, both CO₂ and CO enhance the photoproduction of H₂.