Photocatalytic Reduction of CO₂ to CO in the Presence of H₂ or CH₄ as a Reductant over MgO

Abstract

MgO exhibits activity for the reduction of CO2 to CO under photoirradiation in the presence of H2 or CH4 as a reductant, although MgO is an insulating material. The present study clarified the mechanism of the CO2 photocatalytic reduction in the presence of H2 or CH4 over MgO. The electron paramagnetic resonance (EPR) spectra show that a CO2 molecule adsorbed on MgO was activated to a CO2- radical under photoirradiation. In addition, it was confirmed by photoluminescence that new acceptor level built up between the valence band and the conduction band of MgO on CO2-adsorbed MgO. The CO2- radical was reduced to a surface bidentate formate or a surface bidentate acetate by H2 or CH4 in the dark, respectively. The surface bidentate formate anchors on MgO as a photoactive species and reduces CO2 in the gas phase to CO since the CO2 photocatalytic reduction proceeded over MgO absorbing HCHO or CH3CHO and only 12CO was formed in the presence of 12CO2 over MgO modified by a 13C-labeled formate under irradiation. The active species was generated from the side-on adsorption-type bidentate carbonate selectively, although the two types of bidentate carbonates were detected by Fourier transform infrared (FT-IR) spectroscopy. On the other hand, the role of the surface bidentate acetate is under discussion. It is the first report that the substrate-modified insulating material exhibits activity in the CO2 photocatalytic reduction.