High-Entropy Oxide Nanostructures for Rapid and Sustainable Nitrophenol Reduction

High-entropy materials have emerged as a promising class of catalysts, driven by their high configurational entropy originating from structural disorder in single-phase multicomponent systems. Despite their potential, the catalytic performance of high-entropy oxides (HEOs) remains relatively underexplored. 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-entropy (HEO-4) and high-entropy (HEO-5) regimes. Rietveld refinement of powder X-ray diffraction data confirmed single-phase formation with a face-centered cubic (fcc) crystal structure for both nanostructures. The morphology, particle size, and multicationic elemental distribution were investigated using scanning and transmission electron microscopy. The catalytic performance of the synthesized HEOs was evaluated in the hydrogenation of a series of nitrophenol derivatives. Notably, HEO-5 exhibited significantly enhanced catalytic activity ($k_{\mathrm{app}} \approx 0.5~\mathrm{min^{-1}}$, TOF $= 2.1 \times 10^{-3}~\mathrm{mol\,g^{-1}\,s^{-1}}$), achieving rapid conversion of \emph{p}-nitrophenol compared to the medium-entropy oxide nanostructures ($k_{\mathrm{app}} \approx 0.02~\mathrm{min^{-1}}$, TOF $= 7.2 \times 10^{-4}~\mathrm{mol\,g^{-1}\,s^{-1}}$). Furthermore, the kinetic and thermodynamic parameters of the reaction, including the activation energy ($E_a$), enthalpy of activation ($\Delta H^{\ddagger}$), Gibbs free energy of activation ($\Delta G^{\ddagger}$), and entropy of activation ($\Delta S^{\ddagger}$), were determined to gain mechanistic insight into the reduction process. This study opens new avenues for the rational design and facile synthesis of high-entropy oxide catalysts, highlighting their potential for efficient and sustainable large-scale amine production.