AUTHOR=Zhang Hongwei , Izumi Yasuo TITLE=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 JOURNAL=Frontiers in Chemistry VOLUME=6 YEAR=2018 URL=https://www.frontiersin.org/journals/chemistry/articles/10.3389/fchem.2018.00408 DOI=10.3389/fchem.2018.00408 ISSN=2296-2646 ABSTRACT=

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.