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 TiO₂ 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 TiO₂, Reactive Blue 4 dye (RB 4) was N,Cu codoped > N,Fe codoped > N-doped > undoped TiO₂, Reactive Black 5 dye (RB 5) was N,Cu codoped* > undoped > N-doped > N,Fe codoped TiO₂ 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.

In this paper, we report the detailed kinetic and mechanistic study using N, Fe codoped TiO₂ for the degradation of Reactive Red dye 198 (RR dye 198) under visible light. The N, Fe codoped TiO₂ was prepared by sol–gel method using urea and ferric nitrate as the dopant source and well characterized by XRD, FTIR, DRS, TEM, XPS, EDX, and BET techniques. The N, Fe codoped TiO₂ nanoparticles were found to be 95.55% anatase and 4.45% rutile phase, with a narrow particle size distribution of 4–7 nm, having band bap of 2.4 eV, surface area of 98.01 m² g⁻¹, and are cubic in shape. The process conditions for photocatalytic degradation of RR dye 198 using N, Fe codoped TiO₂ under visible light have been optimized by varying operational parameters such as catalyst loading, pH, initial concentration of the substrate, and the effect of electron acceptors have been examined. The addition of H₂O₂ as a co-oxidant showed an anomalous behavior in the degradation pattern. The degradation followed first-order kinetics with r₀ of 3.036 mg L⁻¹ min⁻¹ at natural pH and obeyed the Langmuir–Hinshelwood model. The mechanism of degradation has been investigated with the help of radical quenchers. The GC–MS analysis of the residue after degradation depicted the degradation pattern of the dye under optimized conditions.

Nanocrystalline undoped, N-doped and N,Fe codoped TiO₂ have been synthesized using a sol–gel method for the photocatalytic degradation of Reactive Blue 4 dye under visible light, with N,Fe codoped TiO₂ exhibiting the best activity.