The fate of O2 in photocatalytic CO2 reduction on TiO2 under conditions of highest purity

Abstract

Although the photocatalytic reduction of CO₂ to CH₄ by using H₂O as the oxidant presupposes the formation of O₂, it is often not included in the product analysis of most of the studies dealing with photocatalytic CO₂ reduction or it is reported to be not formed at all. The present study aims to clarify the absence of O₂ in the photocatalytic gas phase CO₂ reduction on TiO₂. By modifying P25-TiO₂ with IrO_x co-catalysts it was possible to observe photocatalytic water splitting, i.e. the formation of gaseous O₂ and H₂ in almost stoichiometric amounts, without the use of sacrificial agents, while bare P25-TiO₂ showed no activity in H₂ and O₂ formation under similar reaction conditions. Investigating the effect of improved H₂O oxidation properties on the photocatalytic CO₂ reduction revealed that the CH₄ formation on P25 from CO₂ was completely inhibited as long as the H₂O splitting reaction proceeded. Furthermore, we found that a certain amount of O₂ is consumed under conditions of photocatalytic water oxidation. A quantification showed it to be in the same order of magnitude as the oxygen which is missing as a byproduct from photocatalytic CO₂ conversion. 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 P25-TiO₂ undergoes a stoichiometric reaction, meaning that the CH₄ formation is not based on a true catalytic cycle and runs only as long as TiO₂ can consume oxygen.