Photoeatalytic Conversion of CO2 in Water over Layered Double Hydroxides

摘要

Conversion of the greenhouse gas CO2 into carbon and useful carbon sources such as CO, HCOOH, HCHO, CH3OH, and CH4 (the corresponding reactions and reduction potentials are reported elsewhere) is a potential method of solving both energy and environmental problems. Photoeatalytic conversion of CO2 is attractive from the viewpoint of sustainable energy production and greenhouse gas reduction. A few decades ago, some groups attempted the photoeatalytic conversion of CO2 using heterogeneous and homogeneous photocatalysts. However, interest in the photoeatalytic conversion of CO2 by water over heterogeneous photocata-lysts decreased because of competition from overall water splitting (decomposition of water into H2 and O). Further, complexes that function as homogeneous photocatalysts in aqueous media have not yet been identified. Kudo and co-workersreported that CO2 underwent reduction into CO and HCOOH more rapidly in the presence of silver-cocata-lyst-loaded BaLa4Ti4O_(15) than did H2O (with the reduction of H~+ into H2), although in the absence of cocatalyst overall water splitting predominantly occurred over BaLa4Ti4O_(15). Their finding indicated that novel photocatalysts can be designed by creating active sites on typical photocatalyst surfaces. Our group found that CO is generated in the photoeatalytic conversion of CO2 over Zr02, MgO, ATaO3 (A = Li, Na, or K), and Ga2O3, which have the properties of solid-base catalysts, in the presence of H2 or CH4 (reductants). Acidic substances such as CO2 can be adsorbed on a solid base, indicating that solid bases determine, alter, and control the structure of CO2 on their surface. The nonlinear CO2 molecule generated on the surface of solid bases is more destabilized than the linear CO2 molecule, Distortion of CO2 upon adsorption onto a solid base and subsequent conversion into the active species show the high reactivity of the photocatalyst for CO2 conversion.