(572f) Mechanism of CO2 Reduction to Hydocarbons Over Ti-SBA-15

Different gas mixtures containing CO, CO₂, H₂O or H₂ were exposed to illumination in the presence of to this date the most effective catalyst for CO₂ reduction, Ti-SBA-15, in order to clarify the route to hydrocarbons in photocatalytic CO₂ reduction over this photocatalyst. A mixture of CO and H₂O led to the highest quantities of CH₄, C₂H₄, and C₂H₆ after 7 hours of reaction, whereas a mixture of CO₂ and H₂ lead to the lowest production rate of these products. H₂O is thus 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 and CO₂ activation. Finally, the backward reaction, hydrocarbon oxidation into CO or CO₂, was found significant over this catalyst system. Based on i) results indicated above, and ii) available literature, a mechanism for photocatalytic CO₂ reduction is proposed involving 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). Implications of this study to further improve on the activity of this class of photocatalysts are discussed.