Confinement of palladium nanoparticles in halloysite nanotubes for efficient and selective nitroaromatics reduction

Immobilizing ultrasmall metal nanoparticles without aggregation remains challenging. By exploiting the surface chemical asymmetry of halloysite nanotubes (HNTs), we developed an in-situ confinement strategy for the immobilization of palladium nanoparticles (Pd NPs) within the nanotube lumen. In this study, closo-[B₁₂H₁₂]^2- clusters were selectively anchored onto the inner surface of HNTs through a vacuum-assisted process, driven by electrostatic interactions and hydrogen bonding. These surface-anchored clusters subsequently orchestrate the in-situ reduction of Na₂PdCl₄, yielding uniformly dispersed Pd NPs confined within the HNTs cavity. This approach effectively suppresses nanoparticle aggregation during synthesis, markedly enhancing catalytic activity. The resulting catalyst with an ultralow Pd loading of only 0.02 wt% (Pd_0.02%@B₁₂H₁₂@HNT) accomplishes the complete reduction of 4-nitrophenol (4-NP) within 75 s, with a turnover frequency (TOF) as high as 59.60 min^-1. Furthermore, the catalyst demonstrates exceptional operational stability, sustaining its high catalytic performance over 15 consecutive recycling cycles. Notably, the closo-[B₁₂H₁₂]^2- clusters serve a dual function as both an in-situ reducing agent and a stabilizing/protective species, enabling the generation of Pd NPs while preventing their aggregation. This work establishes a versatile and promising paradigm for the rational design of supported metal nanocatalysts with ultralow noble metal loading and superior catalytic efficiency.