Is water more reactive than H2 in photocatalytic CO2 conversion into fuels using semiconductor catalysts under high reaction pressures?

The reaction pressure in the photocatalytic conversion of CO 2 into fuels is optimized between 0 and 0.80 MPa under CO 2 and moisture. The higher reactivity of water than H 2 was observed at higher pressure and the reason was clarified using several ∼10-μm-thick semiconductor-based photocatalysts. The best Pd/TiO 2 photocatalyst produces methane with a reaction order of 0.39. The sum of independent total formation rates of C-containing compounds under UV and visible light does not account for that under UV–visible light, demonstrating synergetic reaction mechanism on Pd for CO 2 reduction by excited electrons via surface plasmon resonance and on TiO 2 for water oxidation. Active metallic Pd and O vacancy sites due to O 2 formation from H 2 O are confirmed by in situ monitoring of EXAFS [ N (Pd–Pd) = 5.9–6.2; N (Ti–O) = 5.2–3.5] and the decrease of the H-bound and bi/tri-coordinated OH peaks in FTIR. Effective redox-site separation explains the higher reactivity of water than H 2 .