Mechanistic study of hydrocarbon formation in photocatalytic CO₂ reduction over Ti-SBA-15

Ti-SBA-15 was exposed to illumination in the presence of different gas mixtures containing CO or CO₂, and H₂O or H₂, in order to clarify the route to hydrocarbon formation in photocatalytic CO₂ reduction over this photocatalyst. A mixture of CO and H₂O led to the highest quantities of CH₄, C₂H₄, and C₂H₆ after 7h of reaction, whereas a mixture of CO₂ and H₂ lead to the lowest production rate of these products. H₂O has been identified as more efficient in activation of CO and CO₂ than H₂. CH₃OH was not detected as significant product, and when fed to the catalyst, did not yield extensive product formation. Formaldehyde was found very reactive over the catalytic system, yielding a product distribution (C₁–C₂) of similar nature as obtained by CO activation. Finally, backward reactions, i.e., oxidation of hydrocarbon products into CO or CO₂, were found significant. Based on the experimental activity profiles, results indicated above, and available literature, a mechanism for photocatalytic CO₂ reduction is proposed involving the formation of CO in the initial stages, followed by consecutive formation of formaldehyde, which converts to CH₄, C₂H₄, and C₂H₆, presumably by reaction with photo-activated H₂O (OH radicals).