Reline-assisted synthesis of fcc-hcp Ni/Ni(OH)2 nanocatalyst for effective reductive hydrogenation of 4-nitrophenol

Abstract

This study presents a facile, eco-friendly, and controlled reline-assisted chemical reduction method to synthesize fcc-hcp Ni/Ni(OH)₂ nanocomposites. The conventional chemical reduction process was modified by replacing water with reline to produce the Ni/Ni(OH)₂ nanocomposite (Ni-reline). The incorporation of precursor salt in reline significantly influenced its physicochemical properties. Specifically, the optimized concentration of nickel salt in reline (0.4 mol dm⁻³) reduced its viscosity from 573 cP to 57 cP and increased its conductivity, which played a crucial role in the nucleation and growth of nanoparticles. As a consequence, coral reef-like nanostructures (∼39 nm) developed in reline, whereas in aqueous media, spherical Ni nanoparticles (Ni-Aq.) with a significantly larger particle size (∼92 nm) were formed. Elemental analysis further confirmed the presence of carbon in Ni-reline, derived from the DES matrix. Ni-Aq. and Ni-reline were evaluated as catalysts for the reductive hydrogenation of 4-nitrophenol to 4-aminophenol. Ni-reline exhibited superior catalytic activity, achieving 95% conversion in 20 minutes with a rate constant of 0.1336 min⁻¹, 11 times higher than Ni-Aq. This enhancement is attributed to the mixed-phase structure, increased surface area, and surface-bound carbon species promoting adsorption and electron transfer. Moreover, Ni-reline demonstrated excellent reusability over five consecutive cycles due to easy magnetic separation with minimal catalyst loss. This study highlights the potential of reline to modify nanostructure growth and phase behavior for efficient, environmentally friendly catalytic applications.