Photocatalytic CO2 reduction using water as an electron donor over Ag-loaded metal oxide photocatalysts consisting of several polyhedra of Ti4+, Zr4+, and Ta5+

Abstract

LaTa7O19 (BG: 4.1eV) and CaTa4O11 (BG: 4.5eV) with laminated structures consisting of layers of TaO6 octahedra and TaO7 decahedra were active for CO2 reduction to form CO using water as an electron donor when an Ag cocatalyst of an efficient CO2 reduction site was loaded. In contrast, the activity for the CO2 reduction of CaZrTi2O7 (BG: 3.6eV) with an anion-defect-type fluorite structure consisting of TiO4 tetrahedra, TiO6 octahedra, and ZrO7 decahedra was negligible even when the Ag cocatalyst was introduced. Selectivity for the CO formation (CO/(H2+CO)) over the optimized Ag/LaTa7O19 photocatalyst reached around 95% in an aqueous NaHCO3 solution. The rate of CO formation gradually decreased with a reaction time accompanied by an increase in the rate of H2 evolution. The results of scanning electron microscopy (SEM), diffuse reflectance spectroscopy (DRS), and X-ray photoelectron spectroscopy (XPS) revealed that aggregation of the Ag cocatalyst during the photocatalytic CO2 reduction caused the decrease in the CO formation.