Anomalous behavior of visible light active TiO2 for the photocatalytic degradation of different Reactive dyes

Nanocrystalline undoped, N-doped, N and metal codoped titania with different particle size, surface area, anatase phase content, crystallinity, band gap and zeta potential were synthesized by the sol-gel method. The photocatalytic activities of the synthesized TiO 2 powders were compared by employing four different Reactive dyes. The order of photocatalytic activity observed for Reactive Red 198 dye (RR 198) was undoped = N,Cu codoped = N-doped > N,Fe codoped TiO 2 , Reactive Blue 4 dye (RB 4) was N,Cu codoped > N,Fe codoped > N-doped > undoped TiO 2 , Reactive Black 5 dye (RB 5) was N,Cu codoped* > undoped > N-doped > N,Fe codoped TiO 2 and negligible degradation was observed for Reactive Orange 16 dye (RO 16). In this paper, the anomalous trend of the photocatalytic activity of various photocatalysts for the degradation of a particular class of dyes has been observed and accounted for based upon three parameters: mechanism of degradation, physicochemical properties of the catalyst and adsorption behavior based on the zeta potential. It was concluded that apart from these parameters, the substrate-specificity of the catalyst is also of equal importance in developing new catalysts for the photodegradation of dyes present in textile effluents. This article illustrates important substrate-catalyst interaction studies using Reactive dyes and various modified TiO 2 powders under visible light irradiation and demonstrates the need for developing new catalysts for the degradation of dyes having different structural features present in textile effluents.