Copper Nanoparticles Impregnated on Halloysite Support: A Highly Active, Flexible and Reusable Heterogeneous Nanocatalyst in the Reduction of 4-Nitrophenol Under Mild Conditions

Abstract

Nitroaromatic compounds originating from industrial wastes pose serious ecological threats to water resources due to their high toxicity and environmental persistence. Effective removal of these pollutants is critical for sustainable water treatment. Therefore, the development of low-cost, environmentally friendly, and highly efficient catalysts for selective reduction reactions is an inevitable necessity. In this study, the synthesis and characterization of halloysite (Hallo) supported copper nanoparticles (Cu NPs) as a highly active, flexible, and reusable heterogeneous nanocatalyst for the reduction of 4-nitrophenol (4-Np) to 4-aminophenol (4-Ap) under mild conditions are presented. The catalytic performance, reusability, and kinetic properties of the Cu/Hallo catalyst were systematically evaluated in the presence of sodium borohydride (NaBH4) as a reducing agent. Advanced characterization techniques such as ICP-OES, UV-vis, XRD, XPS, SEM, SEM-EDX, SEM-elemental mapping, TEM, and BET-N2 adsorption-desorption analyses confirmed the homogeneous distribution of Cu NPs on the halloysite surface (the average size of the NPs was determined as 2.10 +/- 0.85 nm) and the predominant presence of metallic Cu (Cu0) and copper(I) oxide (Cu2O). The catalyst exhibited remarkable efficiency by achieving complete reduction of 4-Np to 4-Ap in only 4.75 min under the optimized conditions (2 mM (10 mL) 4Np; 1 mmol NaBH4; 5 mg catalyst; 25 degrees C) and showed a turnover frequency (TOF) of 265.2 h-1. Kinetic studies revealed a pseudo-first-order reaction mechanism with an activation energy (Ea) of 15.10 kJ/mol. The Cu/Hallo catalyst also showed remarkable reusability by retaining 85 % of its initial activity after five consecutive cycles of use. These findings highlight the potential of Cu/Hallo as an economical, environmentally friendly, and highperformance nanocatalyst for the removal of nitroaromatic pollutants in industrial wastewater.