Photochemical generation of carbon monoxide and hydrogen by reduction of carbon dioxide and water under visible light irradiation

Abstract

Visible light irradiation of solutions of Ru(2,2'-bipyridine)(3) (2+), cobalt(II) chloride, and carbon dioxide in acetonitrile/water/triethylamine generates simultaneously carbon monoxide and hydrogen. The reaction involves photoinduced reduction of CO(2) and H(2)O, triethylamine serving as electron donor in the Ru(2,2'-bipyridine)(3) (2+)/Co(2+) system. The amount of gas (CO + H(2)) produced and the selectivity ratio CO/H(2) depend markedly on the composition of the system. Addition of free bipyridine strongly decreases CO generation but increases H(2) production. With different tertiary amines, NR(3), both the quantity (CO + H(2)) and the ratio CO/H(2) increase markedly along the sequence R = methyl, ethyl, propyl. Higher selectivity for CO(2) reduction to CO in preference to water reduction occurs when triethanolamine is used instead of triethylamine. CoCl(2) is the most efficient mediator for both CO and H(2) generation and specifically promotes CO formation, whereas salts of other cations studied only yield H(2). The mechanism of the reaction may involve intermediate formation of Co(I) species. These processes represent an abiotic photosynthetic system allowing simultaneous generation of CO and H(2) and regulation of the CO/H(2) ratio. Mechanistic studies and explorations of other components that may increase efficiency and product selectivity should be carried out. The results obtained are also of significance for solar energy conversion with consumption of a pollutant, CO(2).