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 ($ΔH^{\ddagger}$), Gibbs free energy of activation ($ΔG^{\ddagger}$), and entropy of activation ($ΔS^{\ddagger}$), 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.

💡 Research Summary

In this work, the authors explore high‑entropy oxides (HEOs) as inexpensive, robust heterogeneous catalysts for the hydrogenation of nitro‑aromatic compounds, using p‑nitrophenol (p‑NP) as a benchmark reaction. Two multicationic oxides were prepared by a solution‑combustion synthesis (SCS) route: a medium‑entropy oxide (HEO‑4) containing Ni, Cu, Co and Zn in equimolar ratios, and a high‑entropy oxide (HEO‑5) that adds Mg to form a five‑component system (Ni, Mg, Cu, Co, Zn). Glycine served both as a complexing agent and as a fuel, enabling a self‑sustaining combustion at 180 °C that yields a porous, ash‑like precursor, which is subsequently calcined at 100 °C to give single‑phase face‑centered cubic (fcc) materials. Rietveld refinement of powder X‑ray diffraction data confirmed the formation of a single‑phase NiO‑based fcc lattice (space group Fm‑3m) with lattice parameters a = 4.240 Å for HEO‑4 and a = 4.228 Å for HEO‑5. Williamson‑Hall analysis indicated average crystallite sizes of ~25 nm (HEO‑4) and ~20 nm (HEO‑5), the latter showing higher lattice strain due to the additional cationic species.

Electron microscopy (SEM, TEM, HRTEM) revealed uniformly distributed spherical nanoparticles (~19 nm) that assemble into a highly porous architecture. BET surface area measurements gave 2.25 m² g⁻¹ (HEO‑4) and 2.53 m² g⁻¹ (HEO‑5), with average pore diameters of 47.4 nm and 25.3 nm respectively, indicating that the higher configurational entropy leads to a finer pore network. X‑ray photoelectron spectroscopy confirmed the presence of Ni²⁺/Ni³⁺, Cu²⁺, Co²⁺/Co³⁺, Zn²⁺, Mg²⁺ and a significant concentration of surface oxygen vacancies, which are known to facilitate electron transfer and H₂ activation.

Catalytic performance was evaluated in aqueous solutions of p‑NP (10⁻⁴ M) with excess NaBH₄ (7 M) as the reducing agent. The reaction progress was monitored by UV‑Vis spectroscopy, and kinetic data fitted a pseudo‑first‑order model. HEO‑5 displayed a remarkable apparent rate constant k_app ≈ 0.5 min⁻¹ and a turnover frequency (TOF) of 2.1 × 10⁻³ mol g⁻¹ s⁻¹, whereas HEO‑4 gave k_app ≈ 0.02 min⁻¹ and TOF ≈ 7.2 × 10⁻⁴ mol g⁻¹ s⁻¹, demonstrating roughly a 25‑fold enhancement for the high‑entropy composition. Temperature‑dependent studies yielded activation energies of 28 kJ mol⁻¹ (HEO‑5) and 55 kJ mol⁻¹ (HEO‑4), confirming that the additional metal species lower the energetic barrier. Thermodynamic parameters derived from the Eyring equation showed a positive activation entropy (ΔS‡) for HEO‑5, indicating an entropy‑driven transition state that further accelerates the reaction.

The catalyst’s scope was extended to ortho‑, meta‑nitrophenol, 2,4‑dinitrophenol and p‑aminophenol, all of which were reduced rapidly under identical conditions, confirming the generality of the catalytic system. Reusability tests demonstrated that HEO‑5 retained >90 % of its initial activity after five consecutive cycles, underscoring its structural stability and resistance to leaching.

Key insights from the study include: (i) high configurational entropy promotes uniform distribution of multiple redox‑active cations and generates abundant surface oxygen vacancies, both of which synergistically enhance electron transfer and H₂ activation; (ii) the solution‑combustion route provides a scalable, low‑cost method to produce nanostructured HEOs with intrinsic porosity and fine crystallite size, attributes that are advantageous for catalytic applications; (iii) the observed reduction in activation energy and favorable entropy contribution illustrate an entropy‑driven catalytic mechanism unique to high‑entropy oxides.

Overall, the work establishes high‑entropy oxide nanostructures as a viable, sustainable alternative to noble‑metal catalysts for nitro‑group hydrogenation, offering a pathway toward large‑scale, cost‑effective amine production. The findings also broaden the functional landscape of HEOs, suggesting their potential utility across a range of redox‑driven transformations in energy and environmental technologies.