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 CO 2 to CO under photoirradiation in the presence of H 2 or CH 4 as a reductant, although MgO is an insulating material. The present study clarified the mechanism of the CO 2 photocatalytic reduction in the presence of H 2 or CH 4 over MgO. The electron paramagnetic resonance (EPR) spectra show that a CO 2 molecule adsorbed on MgO was activated to a CO 2 - 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 CO 2 -adsorbed MgO. The CO 2 - radical was reduced to a surface bidentate formate or a surface bidentate acetate by H 2 or CH 4 in the dark, respectively. The surface bidentate formate anchors on MgO as a photoactive species and reduces CO 2 in the gas phase to CO since the CO 2 photocatalytic reduction proceeded over MgO absorbing HCHO or CH 3 CHO and only 12 CO was formed in the presence of 12 CO 2 over MgO modified by a 13 C-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 CO 2 photocatalytic reduction.