Which is an Intermediate Species for Photocatalytic Conversion of CO2 by H2O as the Electron Donor: CO2 Molecule, Carbonic Acid, Bicarbonate, or Carbonate Ions?

In the photocatalytic conversion of CO2 using H2O as the electron donor, there are four candidates of the intermediate species in the solution: hydrated CO2 molecule (CO2(aq)), carbonic acid (H2CO3), bicarbonate (HCO3–), and carbonate (CO32–) ions. The concentrations of all four species in the system at equilibrium can be controlled by controlling the temperature, pH value, and concentration of the countercation. According to our experiments conducted under different conditions, we narrowed the possible intermediates down to two: CO2(aq) and HCO3–. The isotopic experiment using 13CO2 and in situ infrared spectroscopy revealed that CO2(aq) reacts with the hydroxyl group anchored on the Ga atom on the surface of ZnGa2O4/Ga2O3 to form bidentate HCO3–Ga, followed by the formation of bidentate HCOO–Ga via monodentate HCO3–Ga. We conclude that bidentate HCOO–Ga is the real intermediate species for the photocatalytic conversion of CO2 by H2O as the electron donor over Ag/ZnGa2O4/Ga2O3 catalyst, and it decomposes...