Silver-loaded sodium titanate photocatalysts for selective reduction of carbon dioxide to carbon monoxide with water

To obtain more efficient photocatalyst for photocatalytic reduction of CO₂ with H₂O and figure out the reason for nonstoichiometric O₂ evolution, silver-loaded sodium titanate photocatalysts were further studied in the improved reaction conditions. After preliminary tests for two kinds of sodium titanate samples with different ratio of sodium to titanium (Na₂Ti₆O₁₃ and Na₂Ti₃O₇), several sodium hexatitanate (Na₂Ti₆O₁₃) photocatalysts were further prepared in the flux method by changing the various parameters such as the flux, the loading amount of the Ag cocatalyst, and the loading method of the Ag cocatalyst. As a result, a Ag/Na₂Ti₆O₁₃ sample prepared in a sodium chloride flux, with 1.0 wt% of Ag cocatalyst loaded by a photodeposition method, exhibited the highest production rate (4.6 μmol h⁻¹) and the highest selectivity (74%) to carbon monoxide among the examined samples, which are more than 29 times higher production rate and 2.7 times higher selectivity to carbon monoxide than those in our previous report. Furthermore, although required oxygen production rate equivalent to the formation rates of the reduced products was not observed in our previous study, it was found that the developed Ag/Na₂Ti₆O₁₃(NaCl) photocatalyst produced enough amount of oxygen after a long induction period of 50 h in the present condition. The reasons for the insufficient oxygen formation in the initial period were also investigated and clarified, i.e., the chloride residues and the photoadsorption of O₂ on the surface are responsible for the insufficient O₂ evolution at the initial period.