In situ growth synthesis of heterostructured LnPO4–SiO2 (Ln = La, Ce, and Eu) mesoporous materials as supports for small gold particles used in catalytic CO oxidation

Abstract

A general in situ growth method was successfully employed to prepare lanthanide phosphate–SiO₂ mesostructured cellular foams (MCFs) (LnPO₄–MCFs; Ln = La, Ce, and Eu; MCFs = SiO₂). These heterostructured MCFs (LnPO₄–MCFs) feature binary interpenetrating LnPO₄ and silica frameworks, large surface areas, and uniform mesopore diameters. They were characterized by small-angle X-ray scattering, X-ray diffraction, nitrogen sorption, and transmission electron microscopy. The essence of this in situ growth synthesis lies in the controlled heterogeneous reaction of highly dispersed lanthanide oxides embedded in MCFs with phosphate ions in solution, leading to the formation of highly dispersed crystalline phosphate nanorods (nanocrystalline LnPO₄) on the walls of MCFs. The resultant heterostructured LnPO₄–MCFs were used as a novel support system for gold catalysts in CO oxidation at low temperatures. Gold precursor species can be readily introduced on LnPO₄ nanophases of LnPO₄–MCFs via a simple deposition–precipitation method. The resulting Au–LnPO₄–MCF (2 wt% Au) catalysts exhibited high catalytic activities even below room temperatures. Because of the alteration of surface properties engineered by the in situ growth methodology and the strong interaction of metallic gold species with LnPO₄, these catalysts are highly sinter-resistant. Although some cationic Au species are also present on the LnPO₄–MCF surfaces, the metallic gold species are shown to be the key catalytic active sites for CO oxidation via in situ infrared spectroscopy.