Photocatalytic reduction of carbon dioxide over Ag cocatalyst-loaded ALa ₄Ti ₄O ₁₅ (A = Ca, Sr, and Ba) using water as a reducing reagent

Ag cocatalyst-loaded ALa ₄Ti ₄O ₁₅ (A = Ca, Sr, and Ba) photocatalysts with 3.79-3.85 eV of band gaps and layered perovskite structures showed activities for CO ₂ reduction to form CO and HCOOH by bubbling CO ₂ gas into the aqueous suspension of the photocatalyst powder without any sacrificial reagents. Ag cocatalyst-loaded BaLa ₄Ti ₄O ₁₅ 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 BaLa ₄Ti ₄O ₁₅ photocatalyst powder with a plate shape during the CO ₂ reduction. CO was the main reduction product rather than H ₂ even in an aqueous medium on the optimized Ag/BaLa ₄Ti ₄O ₁₅ photocatalyst. Evolution of O ₂ in a stoichiometric ratio (H ₂+CO:O ₂ = 2:1 in a molar ratio) indicated that water was consumed as a reducing reagent (an electron donor) for the CO ₂ reduction. Thus, an uphill reaction of CO ₂ reduction accompanied with water oxidation was achieved using the Ag/BaLa ₄Ti ₄O ₁₅ photocatalyst.