Photocatalytic CO₂Reduction Under Continuous Flow High-Purity Conditions: Quantitative Evaluation of CH₄Formation in the Steady-State

In this study photocatalytic CO₂ reduction on TiO₂ P25 is studied for the first time under high-purity continuous flow conditions with gas chromatographic online detection of the main product CH₄. The thorough photocatalytic cleaning procedure in humid helium prior to all measurements is also conducted under continuous flow, and the decrease of carbonaceous contaminant concentration can be clearly monitored. Upon addition of CO₂ to the feed under illumination, an increase in CH₄ concentration is observed which clearly follows the increase in CO₂ concentration in the reactor. It is also demonstrated that CH₄ formation ceases as soon as the lamp is switched off, providing clear evidence for the formation of CH₄ from CO₂ in a photoinduced process. A variation of CO₂ concentration under steady-state conditions shows that higher CO₂ concentration accelerated CH₄ formation, but only up to a certain optimum. For higher CO₂ concentrations, CH₄ formation does not increase further or even decreases. This observation is tentatively assigned to a limited availability of photogenerated charge carriers at the TiO₂ surface, or a lack of suitable catalytically active sites. Even traces of O₂ in the reactor completely hinder CH₄ formation, implying that the lack of concomitant oxygen evolution observed in previous measurements on TiO2 is likely beneficial for the overall process. The studies are a first step towards performing true steady-state kinetic studies of photocatalytic CO₂ reduction.