Photocatalytic conversion of CO2 into mainly methane using Pd/TiO2 photocatalyst proceeded faster at 0.80 MPa using water rather than hydrogen as a reductant. The former reaction (CO2 + water) consists of two steps: first, water photosplitting and second, the latter reaction (CO2 + hydrogen). It was paradoxical that total steps proceeded faster than each step based on simple kinetics. To elucidate the reason, Pd and Ti K-edge X-ray absorption fine structure (XAFS) was monitored during CO2 photoconversion using H2 or moisture and the exchange reaction of 13CO2 at Pd/TiO2 surface was also monitored. As a result, the coordination number, N(Ti-O) and N[Ti(-O-)Ti] values, decreased from original values for TiO2 crystalline (6 and 12) to 4.9-5.7 and 9.7-10.6 under CO2 and moisture, respectively, in contrast to significantly smaller decreases under CO2 and H2 and under Ar. The exchange of gas-phase 13CO2 with preadsorbed 12CO2 reached the equilibrium in ~20 h with a rate constant of 0.20 h-1. The reason of the higher activity using water rather than H2 could be explained owing to the oxygen vacancy (O v ) sites as confirmed by XAFS. The reaction of TiO2 surface with water formed O v sites responsible for water oxidation, specially separated from Pd nanoparticle sites for CO2 reduction. In contrast, Pd nanoparticle sites were competed by CO2 and H species, and the photoconversion of CO2 was suppressed at the elevated pressure of CO2 + H2.
Keywords: 13CO2; CO2; X-ray absorption fine structure; gas chromatography–mass spectrometry; oxygen vacancy.