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

Abstract

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.

Judging the feasibility of TiO2 as photocatalyst for chemical energy conversion by quantitative reactivity determinants

“…As described in our previous study 20 the detection of CH 4 and CO 2 during the first three hours results from the cleaning procedure. The consistent observation in our previous studies 20,22 was that a residual CH 4 baseline could not be avoided, even after extended cleaning procedures. The only potential reason for this observation not previously excluded is a diffusion of carbonaceous impurities from the bulk of P25.…”

“…An additional shortcoming of P25 identified in our previous studies [20][21][22][23] is its inability to liberate gaseous O 2 as the stoichiometric byproduct. The often-represented reaction (1) supposes the formation of CH 4 and O 2 in a 1 : 2 ratio.…”

“…24 It should be also noted that no gas-phase O 2 was observed in any of the tested samples, inconsistent with the predicted stoichiometry of eqn (1) but consistent with our previous observations. 20,22 As in batch mode the reactants and the products of the CO 2 reduction remain in the reactor until the experiment ends, produced CH 4 molecules and other intermediate C-containing species could re-adsorb to the surface of the photocatalyst or react again in further cycles. The adsorbed CH 4 molecules could also facilitate the adsorption or reaction of more CO 2 molecules, thus improving the production of more Ccontaining molecules.…”

“…As main conclusions from our work we were able to prove true CH 4 formation from CO 2 as a reactant, although we were never able to detect gas-phase oxygen as a byproduct (eqn (1)) with pure P25-TiO 2 as the photocatalyst. [20][21][22][23] The variation of reactant concentration and light intensity revealed that the formation of CH 4 actually results from a photo-induced process on TiO 2 and that maximum CH 4 formation was achieved at low reactant concentrations. We have yet to demonstrate the possibility to obtain quantitative activity determinants under highpurity conditions, enabling a final conclusion on the potential of P25-TiO 2 in photocatalytic CO 2 reduction.…”

Photocatalytic CO2 Reduction on TiO2‐Based Materials under Controlled Reaction Conditions: Systematic Insights from a Literature Study

“…In reference , in which CO 2 :H 2 O ratio is 10, using P25 no H 2 could be identified. As is also stated in reference , large amounts of H 2 O have a negative impact on the photocatalytic CO 2 reduction. Additionally, an increase in initial CO 2 concentration could boost the products’ yields, but only up to a certain point (of about 7000 ppm).…”