Photocatalytic CO₂ Reduction under Continuous Flow High-Purity Conditions: Influence of Light Intensity and H₂O Concentration

The present study deals with fundamental investigations on the effect of light energy and intensity on the photocatalytic reduction of CO₂ on TiO₂ P25 under high purity continuous flow conditions. In accordance with previous works, gas chromatographic (GC) online detection identified CH₄ as the main product of photocatalytic CO₂ reduction. It was found that the product formation is dependent on the light intensity, which verifies that CH₄ is formed in a photon induced process on TiO₂. A variation of the light intensity revealed that charge carrier recombination is more strongly enhanced compared to the charge transfer reaction to adsorbed species. On these grounds, the rate of CH₄ formation increases only by the square root of the light intensity. Furthermore, product formation is predominantly a UV photon driven process. A further part of this study investigated the effect of H₂O on the CH₄ formation. The photocatalytic removal of carbon-containing species and the CO₂ reduction can already proceed with traces of adsorbed H₂O, whereas a continuous flow of gaseous H₂O results in an inhibition of product formation. Based on our study, we can identify highly promising routes for photocatalyst improvement.