Photoreduction of carbon dioxide catalysed by free and supported zinc and cadmium sulphide powders

Loading of ZnS onto large-surface-area SiO2 (340 m(2) g(-1)) affords photocatalysts for the reduction of carbon dioxide to formate using 2,5-dihydrofuran (2,5-DHF) as reducing agent. A 13% coverage gives the most active powders, producing 7 mmol of formate on irradiation with UV light, and a slight excess of zinc ions improves the yield to 10 mmol. This acceleration, together with the observation that no oxalate is formed, suggests that CO2 is reduced in a two-electron process. The activity decreases at higher and lower coverages, and photocorrosion to Zn(0) is observed when the latter is below 7%. When the SiO2 surface is modified by aminopropyl groups, increased activity is observed due to their reducing properties. Analogous experiments with CdS at 40% coverage on irradiation with visible light again affords higher yields of formate than unsupported CdS. However, when 2,5-DHF is replaced by sodium sulphite, the supported catalyst reaches only half of the activity exhibited by the unsupported sulphide. When CdS is platinized by 0.5 or 4.3 mol.% Pt, a mixture of CO2/KHCO3 affords, in addition to formate, formaldehyde and methanol in amounts of 0.1 and 0.51 mmol. (C) 1997 Elsevier Science S.A.