High-entropy oxide nanostructures for rapid and sustainable nitrophenol reduction

Abstract

High-entropy materials have emerged as a promising class of catalysts, driven by their high configurational entropy that originates from structural disorder in single phase multi-component systems. Despite their potential, the catalytic performance of high-entropy oxides (HEOs) is relatively less explored. In this study, we present a simple solution-based combustion route to synthesize two low-cost, transition metal-rich multicationic oxides positioned in the medium- (HEO-4) and high-entropy (HEO-5) regions. Rietveld refinement of the powder X-ray diffraction data confirmed single phase formation and an fcc crystal structure in both these nanostructures. Morphological, size and multicationic elemental analyses were performed using both scanning and transmission electron microscopy studies. The catalytic performance of these HEOs was evaluated in the hydrogenation of a series of nitrophenol derivatives. Notably, HEO-5 displayed significantly higher catalytic activity (k_app ≈ 0.5 min⁻¹, TOF = 2.1 × 10⁻³ mol g⁻¹ s⁻¹), achieving rapid conversion of p-nitrophenols compared to medium-entropy oxide nanostructures (k_app ≈ 0.02 min⁻¹, TOF = 7.2 × 10⁻⁴ mol g⁻¹ s⁻¹). Furthermore, the reaction kinetic and thermodynamic parameters (E_a, ΔH^‡, ΔG^‡ and ΔS^‡) were determined in order to gain mechanistic insight into the reduction process. This study opens avenues for developing rational design and facile synthetic strategies for HEOs as efficient catalysts toward large-scale sustainable amine production.