Preparation and characterization of novel Pd/SiO 2 and Ca–Pd/SiO 2 egg-shell catalysts with porous hollow silica

Abstract

Novel egg-shell structured monometallic Pd/SiO 2 and bimetallic Ca–Pd/SiO 2 catalysts were prepared by an impregnation method using porous hollow silica (PHS) as the support and PdCl 2 and Ca(NO 3) 2·4H 2O as the precursors. It was found from transmission electron microscope (TEM), scanning electron microscope (SEM), energy dispersive spectroscopy (EDS) and X-ray diffraction (XRD) that Pd was loaded on PHS with a particle size of 5–12 nm in Pd/SiO 2 samples and the Pd particle size in Ca–Pd/SiO 2 was smaller than that in Pd/SiO 2 since Ca could prevent Pd particles from aggregating. X-ray photoelectron spectroscopy (XPS) analyses exhibited that Pd 3d 5/2 binding energies of Pd/SiO 2 and Ca–Pd/SiO 2 were 0.2 and 0.9 eV lower than that of bulk Pd, respectively, as a result of the shift of the electron cloud from Pd to oxygen in Pd/SiO 2 and to both oxygen and Ca in Ca–Pd/SiO 2. The activity of Ca–Pd/SiO 2 egg-shell catalyst for CO hydrogenation and the selectivity to methanol, with a value of 36.50 mmol CO mol −1 Pd s −1 and 100%, respectively, were much higher than those of the catalysts prepared with traditional silica gel as the support, owing to the porous core-shell structure of the PHS support.