Au/Ag nanoparticles-decorated TiO2 with enhanced catalytic activity for nitroarenes reduction

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A straightforward and environmentally-friendly approach for the fabrication of TiO 2 supported nano-Au and Ag catalysts with enhanced activity in nitroarenes reduction is presented. The immobilization of the metal nanoparticles (NPs) onto TiO 2 was performed through a first step of metal precursors adsorption (tetrachloroauric(III) acid trihydrate or silver(I) nitrate) onto TiO 2 , followed by a step of in situ metal cations reduction using three types of reducing agents: citric acid, NaBH 4 and simulated sunlight irradiation. The type of reducing agent influenced the size, the oxidation state and loading of the grafted metal NPs. The Au-based nanocomposites contained Au NPs with sizes ranging between 19.3 and 28.2 nm and Au loadings in the range of 7.2–19.8 wt%. NaBH 4 was the best reducing agent for the Au NPs immobilization onto TiO 2 , leading to the highest Au loading (19.8 wt%) and lowest Au + /Au 0 ratio (0.07). In the case of the Ag-based materials, citric acid revealed to be the best reducing agent for the Ag NPs immobilization onto TiO 2 , leading to ultrasmall grafted NPs, 10.1 wt% Ag loading and the highest amount of metallic silver (Ag + /Ag 0 ratio = 0.03). The catalytic performance of the Au/Ag-based TiO 2 nanocomposites was evaluated in the reduction of 4-nitrophenol (4-NP) and 4-nitroaniline (4-NA) in aqueous medium, at room temperature, using NaBH 4 as reducing agent (substrate:reducing agent:catalyst weight ratio = 0.007:1.89:1). The Au- and Ag-based nanocomposites (prepared with NaBH 4 and citric acid, respectively) led to 100% conversion of 4-NP within 260 and 40 s, respectively, with rate constants ( k ) of 19.1 × 10 −3 and 94.2 × 10 −3 s −1 (pseudo-first-order kinetics), respectively, demonstrating the higher performance of the Ag-based catalyst. In contrast, in the case of the 4-NA reduction, the highest catalytic performance was achieved for the Au-based material, promoting the total substrate conversion within 90 s ( k = 29.4 ×10 −3 s −1 ), while for the Ag-based catalyst the total 4-NA conversion was achieved within 120 s ( k = 16.6 ×10 −3 and 27.5 ×10 −3 s −1 ). Both catalysts were reused in nine consecutive cycles in both 4-NP and 4-NA reduction, with the Au-based material presenting higher stability, especially for 4-NA reduction, with only a small reaction time increment. Successful immobilization of Au and Ag nanoparticles onto TiO 2 support through a straightforward and environmentally-friendly route using three types of reducing agents, producing efficient and recyclable catalysts for the room temperature reduction of nitroarenes (4-nitrophenol and 4-nitroaniline) in water in the presence of NaBH 4 . • E co-friendly in situ immobilization of Au and Ag nanoparticles onto TiO 2 support. • Metal nanoparticles formation promoted by citric acid, NaBH 4 and Xe lamp agents. • Highly active catalysts in 4-nitrophenol (4-NP) and 4-nitroaniline (4-NA) reduction. • Higher efficiency of Au catalyst for 4-NA reduction vs Ag catalyst for 4-NP reduction. • Au-based catalyst with better performance in both reactions upon 9 reuse cycles.