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.

Using a high-purity gas phase photoreactor and highly sensitive trace gas analysis, new insights into the mechanism of photocatalytic CO2 reduction on TiO2 P25 have been obtained. The reactor design and sample pretreatment excludes product formation from intermediates. Apart from CO2, the only other reactant offered to the catalyst is water. The main products found on this prominent photocatalyst are methane and carbon monoxide. To distinguish between the three possible mechanisms reported in previous studies, likely intermediates of the reaction were added to the TiO2 photocatalyst and their reactivity was followed by gas chromatographic analysis. Based on the results, we can clearly rule out CO as intermediate of any photocatalytic reaction pathway on TiO2, because CO was not converted at all within a course of six hours. An improvement of carbonate formation on TiO2 brought about by surface-doping with sodium decreased product yields, so carbonates are unlikely intermediates as well. Methanol, formaldehyde and formic acid were exclusively oxidized back to CO2. We thus support a mechanism running over C2-intermediates, and we tested our hypothesis by reacting glyoxal, glyoxylic acid, acetic acid and acetaldehyde on TiO2. The reactions of acetaldehyde and acetic acid led to product distributions very similar to those obtained from CO2 under the standard reaction conditions, strongly supporting the C2 mechanism. This mechanism can also explain the small amounts of ethane usually found in the product mixture.

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.

Au/TiO catalysts in different geometrical arrangements were designed to explore the role of morphology and structural properties for the photocatalytic reduction of CO with H O in the gas-phase. The most active sample was a Au@TiO core-shell catalyst with additional Au nanoparticles (NPs) deposited on the outer surface of the TiO shell. CH and CO are the primary carbon-containing products. Large amounts of H are additionally formed by photocatalytic H O splitting. Shell thickness plays a critical role. The highest yields were observed with the thickest layer of TiO , stressing the importance of the semiconductor for the reaction. Commercial TiO with and without Au NPs was less active in the production of CH and CO. The enhanced activation of CO on the core-shell system is concluded to result from electronic interaction between the gold core, the titania shell, and the Au NPs on the outer surface. The improved exposure of Au-TiO interface contributes to the beneficial effect.

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