Photocatalytic reduction of carbon dioxide by water on titania: Role of photophysical and structural properties

Photocatalytic reduction of carbon dioxide has been investigated on three different titania catalysts (two commercial titania, De Gussa P-25 and UV-100 Hombikat and another catalyst prepared by sol-gel method under reverse micelle environment) dispersed in aqueous alkaline medium under irradiation in visible region (300-700 nm) from Hg lamp. The catalysts have been characterized by XRD, diffuse reflectance and fluorescence spectroscopic techniques and textural analysis. Methane, methanol and ethanol are observed as the major products. Activity in terms of total hydrocarbons production and (methane + methanol) yields during six hours run follows the order: P-25>RM-TiO2>UV-100. Ethanol is formed in significant amounts in the case of UV-100 and RM-TiO2 but not with P-25. Probable reaction pathways that lead to the formation of these products have been proposed. While the catalytic activity is governed by the generation and life span of photo-induced charge carriers, it is envisaged that product selectivity is determined by the type of transformations that the transient surface species undergo on titania. Surface Ti ions with singly or doubly vacant coordination sites (four or five coordinate Ti ions) act as active centres for these transformations. The relative population of four/five coordinate Ti ions play a vital role in directing the transformations and hence selectivity. The coordination environment of surface Ti atoms evolves during catalyst preparation. Photophysical as well as structural properties of titania samples influence their overall catalyst performance.