Mechanistic aspects of formation of sintering-resistant palladium nanoparticles over SiO2 prepared using Pd(acac)2 as precursor

Abstract

SiO2-supported Pd(acac)2 was prepared by impregnation of the support material in Pd(acac)2/acetylacetone solution. Interaction between Pd(acac)2 and SiO2 surface and thermal decomposition of the Pd(acac)2/SiO2 sample in air were characterized in detail by a variety of techniques including thermogravimetric-mass spectroscopy analyses, in situ infrared spectroscopy, X-ray photoelectron spectroscopy, in situ X-ray powder diffraction, and transmission electron microscope. The results indicate that the surface silanol groups on SiO2 acted as the centers of interaction with Pd(acac)2 molecules when SiO2 was impregnated in the Pd(acac)2 solution, leading to the formation of hydrogen bonded Pd(acac)2 on SiO2 surface. Concentration of silanol groups on SiO2 surface has a direct impact on the dispersion of Pd(acac)2 on SiO2 surface. In the Pd(acac)2/SiO2 sample prepared with a fully hydroxylated silica, no multiple layer Pd(acac)2 species was detected on SiO2 surface when Pd loading was below 8wt%. When a silica supported Pd(acac)2 sample was heated to ∼200°C in air, the single layer of Pd(acac)2 species on SiO2 surface decomposed into the metallic Pd nanoparticles with an average size of about 2.8nm. The resulting Pd nanoparticles demonstrated superior stability against sintering at high temperature. No significant aggregation of the Pd nanoparticles was observed when the sample was heated to 800°C in air or to 600°C in a mixture of H2/Ar=5/95 (volume ratio). In the Pd(acac)2/SiO2 samples with Pd loadings higher than 9wt% or the samples prepared with a silica of low concentration of surface hydroxyl groups, however, multiple layers of Pd(acac)2 could also form on the SiO2 surface, because there were no enough surface OH sites to allow all Pd(acac)2 molecules to “fit” into the single layer. Decomposition of multiple layer Pd(acac)2 species in air at elevated temperature resulted in the formation of large Pd particles on SiO2.