281809 Understanding the Effects of Thermal Treatments On Structure and Catalytic Properties of Ruthenium Nanoparticles On Mesoporous Silica
Recent advances in colloidal chemistry offer promising opportunities for rational design of well-defined metal nanocatalysts, which is of great importance to fundamental studies of heterogeneous catalysis and the development of new catalysts1-4. However, the preparation of supported catalysts using this strategy usually involves the additional challenge of removing the capping stabilizer from the resulting nanocatalysts without changing the particle morphology or dispersion. The difficulty in removing the capping agent stems from the strong bonds formed between these organic molecules and particle surfaces at approximately monolayer coverage1,4,5. In this study, we report on the influence of thermal treatments on the structure and catalytic properties of ruthenium nanoparticles (Ru NPs) supported on mesoporous silica (MSU-F). The monodisperse Ru NPs were synthesized by a polyol reduction method6-8 using poly-N-vinyl-2-pyrrolidone (PVP) as the stabilizing agent, followed by deposition onto MSU-F via a sonication-assisted colloidal impregnation approach to form the supported Ru nanocatalysts9. Three different protocols were explored to remove the PVP capping agent: gentle thermal oxidation at 150°C, thermal reduction at 350°C under a hydrogen atmosphere, and argon-protected calcination at 650°C. Transmission electron microscopy (TEM) characterization revealed negligible changes in morphology of the supported Ru nanoparticles following these treatments. However, thermal reduction and argon-protected calcination appeared to have led to migration of the nanoparticles from the inner wall to the edge of the support. Various physical and chemical characterizations confirmed that, of the three methods explored, only argon-protected calcination completely removes the capping polymer, promoting high metal dispersion as a result. Higher treatment temperatures also led to increased annealing effects and thus crystallinity of the NPs. It was shown by hydrogen chemisorption that the catalysts that underwent the other two thermal treatments retained only a fraction of available surface catalytic sites, confirming insufficient removal of the capping polymer agent from the surface. The reactivities of the catalysts were evaluated by the aqueous phase hydrogenation of pyruvic acid to lactic acid, with commercial Ru on silica used as a control for comparison. The results showed that the Ru nanocatalysts prepared in this work are more active than conventional Ru catalysts. The supported Ru NPs calcined under inert gas exhibited the highest activity. The new protocol thus enables more efficient use of the precious Ru metal. References:
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