Thermally stable ultra-small Pd nanoparticles encapsulated by silica: elucidating the factors determining the inherent activity of noble metal catalysts

Abstract

With an improved one-step reverse micelle method, Pd@SiO₂-RM with thermally stable, 1.1 nm ultra-small Pd nanoparticles were prepared in one-pot. HRTEM results reveal that the ultra-small Pd nanoparticles are embedded in the bulk of the silica nanospheres around 30 nm to form a multi-core shell structure. Therefore, the migration and agglomeration of the ultra-small Pd nanoparticle cores can be impeded effectively at elevated temperatures. Compared with Pd/SiO₂-IMP prepared by impregnation, core–shell Pd@SiO₂-ST and Pd@SiO₂-ME catalysts prepared by Stöber and regular micro-emulsion processes, Pd@SiO₂-RM possesses a much higher metal surface area. As a consequence, this catalyst shows remarkable activity and superior thermal stability for CO oxidation. It is concluded that the Pd grain size and metal surface area are the determining factors for the activity, as evidenced by the strict linear relationship between the differential rates and the Pd sizes/metal surface areas.