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 O-2 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.