Advances in metal nanoparticle-based catalysts for nitroarene reduction: from mechanistic understanding to sustainable design.

The catalytic reduction of nitroarenes (NAr) into their corresponding aminoarenes (AAr) represents a pivotal transformation in both environmental remediation and fine-chemical synthesis. In recent years, both noble and non-noble metal nanoparticle (MNP) systems have emerged as highly efficient and recyclable catalysts for these reduction reactions under mild conditions with reductants such as sodium borohydride (NaBH4). This critical review comprehensively analyzes the synthesis, kinetics, capping agents, and catalytic mechanisms of MNP-based systems, which highlight the role of particle size, morphology, metal composition, and capping agent interactions in modulating reduction kinetics and selectivity. Hybrid systems, both synthesized and biological, such as polymer-stabilized, microgel-encapsulated, carbon-supported, and mono-, bi-, and tri-metallic composites, are discussed in detail. They reveal that the physicochemical environment governs the diffusion of reactants and the electron-transfer dynamics, which are essential for the Langmuir-Hinshelwood (L-H) and Eley-Rideal (E-R) mechanisms. Comparative evaluation of recent literature demonstrates that nanoparticle dispersion, surface charge, and metal-support synergy significantly enhance apparent rate constants while maintaining excellent recyclability over multiple cycles. This review further dissects thermodynamic and kinetic factors, such as activation energy and enthalpic contributions, that dictate catalytic efficiency. Several critical gaps are identified regarding active-site characterization, in situ mechanistic elucidation, and scalability toward industrial applications. This review provides a unified perspective on the structure-activity relationships guiding the rational design of next-generation metal nanoparticle catalysts for sustainable nitroarene reduction.