An overview on the photocatalytic degradation of azo dyes in the presence of TiO₂ doped with selective transition metals

This article presents an overview on the degradation of azo dyes using transition metal doped TiO₂ as photocatalysts in aqueous solutions. Such dopants reduce the recombination of e^-_cb and h⁺_vb and decrease the band gap or create intra-band gap states, which result in more light absorption. Moreover, the addition of these dopants can alter the surface properties of TiO₂ catalyst such as surface acidity and surface area. Therefore, the photocatalysis on modified TiO₂ can be promoted using visible light. An important role in this process is played by molecular oxygen and other active species such as O₂^-, HO₂, H₂O₂ and OH which are generated in a sequence of reactions. Besides this, the degradation of dyes depends on several other parameters such as pH, catalyst concentration, substrate concentration and the nature of the doping substances. Depending on the structure of the azo dye, the major identified intermediates are hydroxylated derivatives, aromatic amines, naphthoquinone, phenolic compounds and several organic acids. This review also presents the literature findings on the available pathways and mechanisms of degradation of some azo dyes in the presence of metal doped TiO₂ catalysts.