Abstract:
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, H2O2and•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.