Photocatalytic Conversion of CO2 in H2o over Solid Based Materials

Conversion of the greenhouse gas CO₂ into carbon and useful carbon sources such as CO, HCOOH, HCHO, CH₃OH, and CH₄ is a potential method of solving energy and environmental problems. Photocatalytic conversion of manmade CO₂ is attractive from the viewpoint of sustainable energy production and greenhouse gas reduction. Layered double hydroxides (LDH) are synthetic or natural crystalline clays that consist of brucite (Mg(OH)₂)-like positively charged two-dimensional sheets in which some divalent cations such as Mg²⁺ are substituted by trivalent cations; further, water and exchangeable charge-compensating anions are present in the interlayer. In this study, we carried out the photocatalytic conversion of CO₂ in H₂O using various LDHs. Considerable amount of CO was obtained over various LDHs in the photocatalytic conversion of CO₂ and photocatalytic H₂ evolution from H₂O competitively took place in all cases. Ni-Al LDH exhibited high selectivity to CO because the amount of H₂ evolved was relatively low. Ni-Al LDH (Ni/Al = 4) which was aged for 20 h under hydrothermal condition at 383 K after coprecipitation showed the highest activity for the evolution of CO. GC/MS analysis for the photocatalytic conversion of ¹³C-labeled CO₂ clarified that ¹³CO (m/z = 29) was evolved in priority to ¹²CO (m/z = 28). Accordingly, CO₂ introduced in the gas phase was adsorbed on the surface of LDHs, and then reduced into CO in the photocatalytic conversion of CO₂ in water.