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Photocatalytic conversion of CO 2  into mainly methane using Pd/TiO 2  photocatalyst proceeded faster at 0.80 MPa using water rather than hydrogen as a reductant. The former reaction (CO 2  + water) consists of two steps: first, water photosplitting and second, the latter reaction (CO 2  + 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 CO 2  photoconversion using H 2  or moisture and the exchange reaction of  13 CO 2  at Pd/TiO 2  surface was also monitored. As a result, the coordination number,  N (Ti–O) and  N [Ti(–O–)Ti] values, decreased from original values for TiO 2  crystalline (6 and 12) to 4.9–5.7 and 9.7–10.6 under CO 2  and moisture, respectively, in contrast to significantly smaller decreases under CO 2  and H 2  and under Ar. The exchange of gas-phase  13 CO 2  with preadsorbed  12 CO 2  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 H 2  could be explained owing to the oxygen vacancy (O  v  ) sites as confirmed by XAFS. The reaction of TiO 2  surface with water formed O  v   sites responsible for water oxidation, specially separated from Pd nanoparticle sites for CO 2  reduction. In contrast, Pd nanoparticle sites were competed by CO 2  and H species, and the photoconversion of CO 2  was suppressed at the elevated pressure of CO 2  + H 2 .

frontiers in chemistry

Why Is Water More Reactive Than Hydrogen in Photocatalytic CO2 Conversion at Higher Pressures? Elucidation by Means of X-Ray Absorption Fine Structure and Gas Chromatography–Mass Spectrometry