Palladium nanoparticles encapsulated in porous silica shells: an efficient and highly stable catalyst for CO oxidation

Abstract

Porous silica-supported metallic Pd catalysts were prepared in three steps: (1) synthesis of oleylamine-capped Pd nanoparticles, (2) silica polymerization around the oleylamine-capped Pd in a water-in-oil microemulsion system and (3) removal of the capping agent through calcination. With the characterization of transmission electron microscopy, X-ray photoelectron spectroscopy, X-ray diffraction and N₂ physisorption, the resultant 3 nm Pd nanoparticles were identified to be homogenously encapsulated within 10 nm-thick porous silica shells. During the oxidation of CO at 443 K, the as-prepared Pd@SiO₂-673 delivered a turnover frequency up to 33 times greater than that associated with the reference Pd/SiO₂-673 catalyst prepared by a conventional immobilization method. Meanwhile, the as-prepared Pd@SiO₂-673 also exhibited significantly improved stability for a continuous reaction of 576 h than the control catalyst. The remarkable enhancement of the catalytic performance was found to depend on the core-shell configuration. The Pd particle cores have a small size (3 nm), contributing to weakening the strength of CO adsorption and thus enhancing catalytic activity. On the other hand, the porous silica shells allow the reactants to penetrate into the core-shell structured Pd@SiO₂ composite and thus increase the accessibility of the Pd cores. Furthermore, they are separate Pd nanoparticles and therefore improve the thermal stability.