Photocatalytic Coversion of CO2 By H2o As an Electron Donor Using Various Solid-State Photocatalysts

The use of heterogeneous photocatalysts to reduce CO2 into storable feedstock is of particular interest because of its simplicity and low cost of operation. We have reported that CO as a main product is obtained for the photocatalytic conversion of CO2 in the presence of H2 over ZrO2, MgO, ATaO3 (A = Li, Na, and K), and Ga2O3, of which Ga2O3 exhibited the highest conversion of CO2: 7.8% over 48 h of photoirradiation. Unfortunately, these bare metal oxides function as photocatalysts in the presence of H2O not for reduction of CO2 but for overall water splitting. As of late, we found that Zn-doped, Ag-modified Ga2O3 exhibits higher formation rate of CO and selectivity toward CO evolution than bare, Ag-modified Ga2O3 for the photocatalytic conversion of CO2 by H2O as an electron donor.  An isotope experiment using 13CO2 and mass spectrometry clarified that the carbon source of the evolved CO is not the residual carbon species on the photocatalyst surface, but the CO2 introduced in the gas phase. In addition, stoichiometric amounts of evolved O2 were generated together with CO. Only in recent years, XAFS measurement identified that ZnGa2O4 is generated on the surface of Ga2O3; therefore, it suppresses the evolution of H2, which results in the higher selectivity toward CO evolution.