185 nm Photoreduction of CO₂ to Methane by Water. Influence of the Presence of a Basic Catalyst

Photolysis of CO(2) at 185 nm using a Hg lamp in the presence of H(2) or H(2)O leads to the formation of CH(4) (after an induction period) and a lesser amount of CO. Using H(2) gas as reducing agent, up to 40% conversion was achieved with almost complete selectivity to methane and energy consumption of 1.55 Gcal/mol. When CO(2) irradiation is carried out in the gas phase in the presence of H(2)O, both reagents can undergo photolysis, and H(2) generation from water is a competing process accompanying the formation of CO and CH(4) (CO(2) conversion 0.67% at 14 h). For the irradiation of CO(2) using H(2)O as reducing agent, basic solids (K(+)-exchanged Y zeolite or hydrotalcites) can increase CO(2) conversion up to 2.21% with total selectivity toward CH(4) and an energy consumption of 4.5 Gcal/mol. This result opens a door in the quest for efficient photocatalysts that could further increase the apparent quantum yield of the process, particularly at longer wavelengths.