**5. Application of TiO2 in catalyst**

TiO2-based composite materials have been widely used as catalysts [105–107]. TiO2 is used as support in commercial V2O5-WO3/TiO2 catalysts for the selective catalytic reduction (SCR) of NOx. In SCR technology, highly undesirable NOx acid gas emissions from various industrial sources are reduced to harmless N2 and H2O. The V2O5-WO3/TiO2 catalysts are widely used in commercial applications because of their excellent thermal stability and lower oxidation activity for the conversion of SO2 to SO3 [108, 109]. The V2O5-WO3/TiO2 catalysts have become the most widely used industrial catalysts for these SCR applications since the introduction of this technology in the early of 1970s [110].

TiO2 has the potential to induce the reductive chemical transformation. The reductive photocatalysis of ethyne and ethene have been reported [111–113]. TiO2 have been used as a useful catalyst for the reduction of carbonyl compounds such as aldehydes or ketones, nitro compounds, imines and for the some of the chemical transformations involving redox processes. Photocatalysis on TiO2 is a light-driven redox reaction. Redox reactions can be induced by electrons (e) generated in conduction band (CB) and holes (h<sup>+</sup> ) simultaneously generated in valance band (VB) under the irradiation of light. The electrons in the conduction band are readily available for transferring while the holes in the valence band are open for donations [114]. The photocatalytic reduction of an electron acceptor can be carried out in the presence of a large excess amount of electron donors such as alcohols or amines, which are used to scavenge (h<sup>+</sup> ). Oxygen (O2) is a competitive electron acceptor, and can influence the reduction reaction. Therefore, the reductive chemical transformation should be generally performed in an O2 free environment. Under these conditions, a photocatalytic reduction proceeds through transferring electrons (e) in CB or trapped at surface defects of TiO2 into the organic molecules adsorbed on TiO2 surface. The photocatalytic reduction of aldehydes, nitro compounds, and imines have been reported. Aromatic aldehydes and ketones were reduced to the corresponding alcohols using TiO2 as a photocatalyst [115, 116]. Aromatic and aliphatic nitro compounds were reduced to corresponding amines using TiO2 as catalyst [117]. The direct reduction of imines to corresponding secondary amines was studied [118].
