In-Situ FT-IR Study on the Mechanism of CO2 Reduction with Water over Metal (Ag or Au) Loaded Ga2O3 Photocatalysts

Ag loaded Ga2O3 (Ag/Ga2O3) has exhibited photocatalytic activity for CO2 reduction with water to produce CO as well as for water splitting to H2 and O2. In-situ FT-IR measurements have shown CO3 stretching vibration bands assignable to carbonate and bicarbonate species when CO2 molecules chemisorbed on the catalyst surface. These species change to bidentate formate species under photoirradiation. It subsequently converts to CO by interacting with water molecules, not chemisorbed OH on the catalyst surface. This result suggests that formate species is an intermediate of the photocatalytic CO2 reduction. On the other hand, Au loaded Ga2O3 (Au/Ga2O3) has produced H2 predominantly with a very small amount of CO formation, since the less amount of CO2 molecules chemisorb on Au/Ga2O3. FT-IR measurements of the Ag/Ga2O3 with high Ag loading have shown carbonate species due to the adsorption of atmospheric CO2 on hydroxyl group. The carbonate species is too unstable under photoirradiation to convert into formate species, but is stabilized as CO32- species by the reaction with water. XANES analysis has revealed that atomically dispersed Ag metal species may be effective for the adsorption of CO2 and the subsequent conversion into formate species to promote the photocatalytic CO2 reduction. DOI: 10.1380/ejssnt.2014.299