Surfactant-free synthesis of Ag@TiO₂ and CuO@TiO₂ photocatalyst: a comparative study of their photocatalytic activity for reduction of nitroaromatics in water

Abstract The reduction of harmful nitroaromatics to useful amino-aromatics have significant opportunities in synthetic chemistry. Here a visible-light-driven eco-friendly method for the selective reduction of aromatic nitro compounds to their corresponding amines in aqueous solution by using Ag@TiO₂ and CuO@TiO₂ is described. It was observed that both Ag@TiO₂ and CuO@TiO₂ are photo-catalytically more efficient compared to bare TiO₂ and CuO@TiO₂ has higher activity over Ag@TiO₂ for the said conversion. The structural and morphological characterization of the as-synthesized catalysts has been done with SEM-EDX, TEM, powder XRD, ICP-AES, XPS, Photoluminescence, and UV-vis spectroscopic techniques. The nanocomposites Ag@TiO₂ and CuO@TiO₂ exhibit pure anatase phase with average crystallite size of 5.89 nm and 5.87 nm respectively as calculated from the Debye-Scherrer equation depending on the (101) plane. UV-visible results inferred enhanced optical properties of both the synthesized catalysts and revealed a reduced band gap (3.07 eV for Ag@ TiO₂ and 2.5 eV for CuO@ TiO₂) as compared to neat TiO₂ (3.36 eV). Various nitro compounds were tolerated under 150 W LED as a light source (13.9 lumens for an area of 0.2 ft²) in an aqueous medium at room temperature (30 °C) using NaBH₄ as a reducing agent to access corresponding amines in satisfying yields (78%–99%). The catalyst can be separated from the reaction mixture by simple centrifugal precipitation and reused for up to six consecutive cycles without apparent loss of its catalytic activity. The products were characterized by ¹H-NMR spectroscopic techniques and compared with authentic samples.