Halloysite@Polydopamine Nanoplatform for Ultrasmall Pd and Cu Nanoparticles: Suitable Catalysts for Hydrogenation and Reduction Reactions

The design of sustainable nanomaterials for catalysis is a key challenge in green chemistry. Herein, we report the synthesis of halloysite nanotube (Hal)-based nanomaterials selectively functionalized with a bio-inspired polydopamine (PDA) coating, which enables the controlled anchoring of palladium and copper nanoparticles (PdNPs and CuNPs). This mild and ecofriendly strategy yields highly dispersed and ultrasmall (<5 nm) metal nanoparticles without the need for surfactants or harsh reagents. The resulting materials, Hal@PDA/PdNPs and Hal@PDA/CuNPs, were evaluated in two well-established model reactions commonly employed to probe catalytic performance: cinnamaldehyde hydrogenation and 4-nitrophenol reduction. Hal@PDA/PdNPs displayed complete conversion and >90% selectivity toward hydrocinnamaldehyde at low Pd loading (0.8 wt%) and maintained its efficiency over six catalytic cycles (TOF up to 0.1 s−1), while Hal@PDA/CuNPs retained high activity through eight consecutive runs in the reduction of 4-nitrophenol. Hal@PDA/CuNPs proved to be an excellent recyclable catalyst for the reduction of 4-nitrophenol, retaining high activity through eight consecutive runs. Overall, this study introduces a robust and modular approach to fabricating halloysite-based nanocatalysts, demonstrating their potential as green platforms for metal nanoparticle-mediated transformation.