LAUSR

Abstract A straightforward and environmentally-friendly approach for the fabrication of TiO2 supported nano-Au and Ag catalysts with enhanced activity in nitroarenes reduction is presented. The immobilization of the metal nanoparticles (NPs) onto TiO2 was performed through a first step of metal precursors adsorption (tetrachloroauric(III) acid trihydrate or silver(I) nitrate) onto TiO2, followed by a step of in situ metal cations reduction using three types of reducing agents: citric acid, NaBH4 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%. NaBH4 was the best reducing agent for the Au NPs immobilization onto TiO2, leading to the highest Au loading (19.8 wt%) and lowest Au+/Au0 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 TiO2, leading to ultrasmall grafted NPs, 10.1 wt% Ag loading and the highest amount of metallic silver (Ag+/Ag0 ratio = 0.03). The catalytic performance of the Au/Ag-based TiO2 nanocomposites was evaluated in the reduction of 4-nitrophenol (4-NP) and 4-nitroaniline (4-NA) in aqueous medium, at room temperature, using NaBH4 as reducing agent (substrate:reducing agent:catalyst weight ratio = 0.007:1.89:1). The Au- and Ag-based nanocomposites (prepared with NaBH4 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.