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

Abstract In this study, the photocatalytic CO 2 reduction on TiO 2 P25 was investigated for the first time under high‐purity continuous flow conditions with gas chromatographic (GC) online detection of CH 4 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 CO 2 to the feed under illumination, an increase in CH 4 concentration was observed, which clearly follows the increase in CO 2 concentration in the reactor. It was also demonstrated that CH 4 formation ceases as soon as the lamp is switched off, providing clear evidence that the formation of CH 4 from CO 2 is a photoinduced process. It was shown that higher CO 2 concentration accelerated CH 4 formation under steady‐state conditions up to a certain optimum. Higher CO 2 concentrations lead to a decrease in CH 4 formation. This observation is tentatively assigned to a limited availability of photogenerated charge carriers at the TiO 2 surface, or a lack of suitable catalytically active sites. Traces of O 2 in the reactor completely hinder CH 4 formation, implying that the lack of concomitant oxygen evolution observed in previous measurements on TiO 2 is likely beneficial for the overall process. This study represents a first step towards performing true steady‐state kinetic studies of photocatalytic CO 2 reduction.