Facile synthesis of ultrafine Cu, Ag, and Ni nanoparticles supported on 3D hydrogel beads for the rapid reduction of 4-nitrophenol

Abstract The development of a catalyst with high catalytic activity and selectivity remains a crucial challenge in the field of heterogeneous catalysis. In this work, we demonstrated a versatile procedure for the immobilization of ultrafine and monodispersed metal nanoparticles within 5 nm onto glutaraldehyde-crosslinked polyethyleneimine-modified calcium alginate (GLA-PEI/Ca-SA) beads. The resulting hydrogel beads underwent comprehensive characterization through scanning electron microscopy (SEM), transmission electron microscopy (TEM), energy-dispersive X-ray spectroscopy (EDS), and X-ray photoelectron spectroscopy (XPS). The results revealed that well-dispersed Ag, Cu, and Ni nanoparticles, each with a mean size of 3.5, 2.6, and 4.1 nm, respectively, were successfully loaded onto the surface of GLA-PEI/Ca-SA beads with a 3D interconnected honeycomb-like porous structure. Notably, the Ag/GLA-PEI/Ca-SA exhibited outstanding catalytic activity and stability, achieving over 95% conversion within 7 successive cycles towards the reduction of 4-nitrophenol (4-NP) to 4-aminophenol (4-AP) by NaBH₄ in aqueous solution. Particularly, the Cu/GLA-PEI/Ca-SA beads displayed the highest catalytic activity for the reduction of 4-NP with a kinetic reaction rate constant of 1.56×10⁻ ² s⁻ ¹. We expect that this work provides a novel and effective strategy to design highly efficient catalysts across various catalytic reactions.