Facile Synthesis of Natural Kaolin-Based CuO Catalyst:\nAn Efficient Heterogeneous Catalyst for the Catalytic Reduction of\n4‑Nitrophenol

Water contamination\nby nitro compounds from various industrial\nprocesses has significantly contributed to environmental pollution\nand severely threatened aquatic ecosystems. Inexpensive, efficient,\nand environmentally benign catalysts are required for the catalytic\nreduction of such nitro compounds. This study reports the fabrication\nof various nanocomposites (NCs) of copper oxide nanoparticles (CuO\nNPs) supported on a kaolin sheet using straightforward and simple\none-pot synthesis procedures that control the metal precursor to kaolin\nratios. The selected as-synthesized CuO/kaolin NC was characterized\nusing a range of advanced spectroscopic and microscopic methods, such\nas X-ray diffraction (XRD), Fourier transform infrared spectroscopy\n(FT-IR), ultraviolet–visible (UV–vis) spectroscopy,\nfield emission scanning electron microscopy (FE-SEM), transmission\nelectron microscopy (TEM), energy dispersive X-ray spectroscopy (EDX),\nhigh-angle annular dark-field scanning TEM (HAADF-STEM), and N<sub>2</sub> adsorption/desorption analysis. The characterization results\nconfirmed the successful incorporation of CuO NPs into the kaolin\nsheets, which had an average size of about 18.7 nm. The fabricated\nCuO/kaolin NC was used as a heterogeneous catalyst for the efficient\nreduction of 4-nitrophenol (4-NP) to 4-aminophenol (4-AP) in the presence\nof sodium borohydride (NaBH<sub>4</sub>) in an aqueous system at room\ntemperature. The catalyst demonstrated superior catalytic performance\nwith high 4-NP conversion into 4-AP (>99%) in the aqueous phase\n(50\nmL, 20 mg L<sup>–1</sup>) within 6 min. In addition, the reaction\nkinetics of 4-NP reduction was also investigated, and the reaction\nfollowed the pseudo-first-order kinetics equation with the apparent\nrate constant of 1.76 min<sup>–1</sup>. Furthermore, the Arrhenius\nand Eyring parameters for the catalytic hydrogenation reaction of\n4-NP were calculated in order to investigate the catalytic reaction\nprocess in more detail. Moreover, the catalyst exhibited excellent\nreusability and stability over seven repeated catalytic test cycles\nwithout any noticeable decline in catalytic activity. Therefore, this\npaper could provide a novel, efficient, and environmentally promising\nclay-based non-noble metal oxide nanocatalyst to reduce nitro compounds\nin the aqueous system.