Photocatalytic CO₂ Reduction Under Continuous Flow High-Purity Conditions: Quantitative Evaluation of CH₄ Formation in the Steady-State

In this study, the photocatalytic CO₂ reduction on TiO₂ P25 was investigated for the first time under high-purity continuous flow conditions with gas chromatographic (GC) online detection of CH₄ as the main product. The thorough photocatalytic cleaning procedure in humid helium prior to all measurements was conducted under continuous flow too and we were able to monitor the decrease of carbonaceous contaminant concentration. On addition of CO₂ to the feed under illumination, an increase in CH₄ concentration was observed, which clearly follows the increase in CO₂ concentration in the reactor. It was also demonstrated that CH₄ formation ceases as soon as the lamp is switched off, providing clear evidence that the formation of CH₄ from CO₂ is a photoinduced process. It was shown that higher CO₂ concentration accelerated CH₄ formation under steady-state conditions up to a certain optimum. Higher CO₂ concentrations lead to a decrease in CH₄ formation. 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. 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. This study represents a first step towards performing true steady-state kinetic studies of photocatalytic CO₂ reduction.