Effect of TiO2 crystallinity on the photocatalytic reduction of nitrogen and carbon dioxide

Industrial TiO2 suspension from sulphate technology was used for samples preparation. One sample was directly calcined in an argon stream at 800 °C. The second one, before the calcination in the same conditions, was modified with APTES in such a way that the final product contained about 2 wt% of silicon. Photocatalysts were tested in the CO2 reduction reaction containing the addition of gaseous nitrogen and water vapour. No activity was observed in the CO2 reduction reaction for a directly calcined sample that mainly consists of a rutile phase. It was found that the addition of silicon prevents the anatase-to-rutile phase transition by forming a Si–O–Ti chain containing the anatase phase and leads to gaseous products: hydrogen, carbon monoxide and methane, and ammonia. The presence of silicon generates paramagnetic recentres with g= 2.0028, which are associated with oxygen vacancies and delocalized electrons. This electronic structure is critical to the photocatalytic reduction of CO2 because the pure rutile phase (without silicon modification) does not reduce CO2, but only the presence of ammonia was found in the products.