During the last decades photocatalytic CO₂ reduction has attracted lots of attention as a potential approach in the investigation of alternative renewable energy sources and solutions for the increasing CO₂ emissions.

One major issue in fundamental studies on one of the most famous reactions - the gas-phase photocatalytic CO₂ reduction over TiO₂ catalysts utilizing H₂O splitting as hydrogen source - is that often no oxygen can be found among the products in the gas phase or it is not included in the analysis, although the reduction of carbon dioxide with water to methan presupposes oxygen liberation.

To clear up the absence of gaseous O₂, Simon Ristig (Research group leader in Robert Schlögl's department) and Jennifer Strunk (Leibniz Institute for Catalysis) and their team modified P25-TiO₂ with IrOx, a well-known water oxidation catalyst, which allowed the group to observe gaseous O₂ and H₂ in almost stoichiometric amounts from photocatalytic water splitting. The study revealed that the conversion of CO₂ to CH₄ was completely inhibited as long as the water oxidation reaction proceeded, likely due to competition for charge carriers by H₂ formation and the backward reaction of CO₂ reduction. Furthermore, the team found that the O₂ evolution started with a significant delay in comparison to H₂, indicating that a certain amount of O₂ was consumed under the applied reaction conditions. A quantification showed it to be in the same order of magnitude as the oxygen which is missing as a byproduct in photocatalytic CO₂ conversion on bare P25-TiO₂.

A detailed interpretation of the results in the context of the general understanding of the photocatalytic CO₂ reduction with H₂O on TiO₂ allows the hypothesis that H₂O oxidation might actually not be the hydrogen source for C-H bond formation in CO₂ reduction and that the photocatalyst itself could be responsible for the absence of oxygen, only enabling the formation of CH₄ as long as it can consume oxygen and thus preventing an inhibiting effect of O₂ and O-derived species. This would imply that the photocatalytic gas-phase CO₂ reduction with H₂O over P25-TiO₂ is not based on a true catalytic cycle.

The results of this study were recently published in the journal PCCP and also featured as a back cover.

Publication: Martin Dilla, Alina Jakubowski, Simon Ristig, Jennifer Strunk, Robert Schlögl (2019). The fate of O₂ in photocatalytic CO₂ reduction on TiO₂ under conditions of highest purity
Phys.Chem.Chem.Phys.,2019,21, 15949-1595, DOI: 10.1039/C8CP07765G