Photocatalytic Reduction of Carbon Dioxide over Ag Cocatalyst-Loaded ALa₄Ti₄O₁₅ (A = Ca, Sr, and Ba) Using Water as a Reducing Reagent

Ag cocatalyst-loaded ALa₄Ti₄O₁₅ (A = Ca, Sr, and Ba) photocatalysts with 3.79–3.85 eV of band gaps and layered perovskite structures showed activities for CO₂ reduction to form CO and HCOOH by bubbling CO₂ gas into the aqueous suspension of the photocatalyst powder without any sacrificial reagents. Ag cocatalyst-loaded BaLa₄Ti₄O₁₅ was the most active photocatalyst. A liquid-phase chemical reduction method was better than impregnation and in situ photodeposition methods for the loading of the Ag cocatalyst. The Ag cocatalyst prepared by the liquid-phase chemical reduction method was loaded as fine particles with the size smaller than 10 nm on the edge of the BaLa₄Ti₄O₁₅ photocatalyst powder with a plate shape during the CO₂ reduction. CO was the main reduction product rather than H₂ even in an aqueous medium on the optimized Ag/BaLa₄Ti₄O₁₅ photocatalyst. Evolution of O₂ in a stoichiometric ratio (H₂+CO:O₂ = 2:1 in a molar ratio) indicated that water was consumed as a reducing reagent (an electron donor) for the CO₂ reduction. Thus, an uphill reaction of CO₂ reduction accompanied with water oxidation was achieved using the Ag/BaLa₄Ti₄O₁₅ photocatalyst.