Spreading and Phase Transformations in Highly Dispersed CeO2/SiO2and Pd/CeO2/SiO2Systems

Abstract

The effect of Pd on spreading and reaction of CeO2nanocrystallites with silica support on high temperature reduction in hydrogen was studied by X-ray diffraction (XRD), high resolution transmission electron microscopy (HRTEM), and selected area electron diffraction (SAED). Without Pd, CeO2nanocrystallites (size ca. 5 nm) began to spread onto the silica at 870 K, producing at 970 K uniform nearly amorphous overlayer. With Pd, the spreading occurred at 100 K lower temperature and moreover at 970 K the raft-like Ce4.67(SiO4)3O silicate crystals were formed in contact with the Pd particles. Chlorine from the PdCl2precursor reacted with ceria at 770 and 870 K to form the ribbon-like CeOCl crystals, which at 970 K became unstable, probably undergoing transformation into Ce silicate. Formation of Ce silicate and CeOCl during the high temperature reduction is a possible mechanism of the deactivation of the ceria-promoted Pd/SiO2catalysts (irreversible Ce4+to Ce3+reduction).