Transition metal oxides dispersed on Ti-MCM-41 hybrid core-shell catalysts for the photocatalytic degradation of Congo red colorant

Hybrid core-shell catalysts were synthesized by transition metal (Fe, V and Cu) oxides dispersed on Ti doped mesoporous Ti-MCM-41 (noted as TiM) and their crystalline structures, morphologies, metal oxidation states and optical properties were studied with a variety of characterization techniques. All the catalysts showed a core-shell morphological feature. Fe₂O₃, CuO and V₂O₅ enriched the catalyst surface forming an oxide shell on the Ti-MCM-41 core. Predominant Fe³⁺, Cu²⁺ and V⁵⁺ ions coexisted with a small part of Fe²⁺, Cu⁺ and V⁴⁺ ions generating Fe³⁺/Fe²⁺, Cu²⁺/Cu⁺ and V⁵⁺/V⁴⁺ couples in the corresponding catalyst. The UV–vis spectroscopic analysis showed a shift of the prohibited band energy towards the visible region after the metal oxides loading. In the photocatalytic degradation reaction of Congo red (CR) in an aqueous solution, the catalysts exhibited an activity order as: Cu/TiM > Fe/TiM > V/TiM > TiM. The photocatalytic activity resulted from the affinity of the anionic colorant with the positively charged surface of the catalysts and correlated well with the PZC values and variations of the textural properties. Under ultraviolet (UV) irradiation, the C–N, C–S, and N[dbnd]N bonds linking with different functional groups in CR structure can be attacked by surface active species and interacted with the Fe³⁺/Fe²⁺, Cu²⁺/Cu⁺ and V⁵⁺/V⁴⁺ couples, leading to breaking of the CR molecular structure into various intermediate species. The photo-enhanced chemical reaction cycles were discussed and the CR photodegradation mechanisms was presented.