Designing Nanostructured Oxides for Supported Metal Catalysts

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Supported metal catalysts find many important uses in areas including chemical production, petroleum refining and emission control. The catalytic behavior of a supported metal catalyst is influenced by size and type of reaction sites on metal nanoparticles. For many structure insensitive reactions catalyzed by the supported metal catalysts, smaller metal nanoparticles have more surface metal atoms per unit mass of metal, which are active sites for reactants, thus giving higher reactivity. Therefore it is critical to develop methods for controlling the metal particle size at synthesis stage. Meanwhile, preserving the metal particle size in the high temperature environment, as is typical of many industrial relevant reactions, remains another unsolved scientific challenge. Many studies before have proposed different synthesis strategies to control the size of the metal nanoparticles, though studying the size and the reactivity of the catalyst post synthesis are both important. To modify the support in this thesis, a support material (TiO2 and Al2O3) is first grafted with bulky organic templates then sol-gel coated with a nanometer thick oxide overcoat followed by subsequent treatment to remove the organic templates. After this, metals (Ag and Pt) are deposited onto the modified support materials via photodeposition, wetness impregnation or strong electrostatic adsorption to generate the supported metal catalysts. In the first study of photodepositing Ag nanoparticles onto SiO2 partially overcoated TiO2 support, the combined use of sol-gel overcoat and organic templates helped generate highly dispersed Ag nanoparticles (<5nm) on TiO2 supports and preserved the metal particle size up to 450°C for prolonged time. In the next study of synthesizing highly dispersed TiO2 supported Pt nanoparticles, Pt nanoparticles were deposited onto TiO2 supports with partially overcoated SiO2 through wetness impregnation. The as-synthesized Pt nanoparticles are 1-2 nm by electron microscope characterization and maintain dispersion (percent of surface atoms) >45% by CO chemisorption even after prolonged heating at 500 °C, whereas Pt nanoparticles on unmodified TiO2 are less dispersed (~33%) and their dispersion falls further upon prolonged heating. Ethylene hydrogenation reaction study demonstrated that the Pt nanoparticles on modified TiO2 preserve the catalytic activities of Pt on unmodified TiO2. In the following study, strong electrostatic adsorption is proved to be a self-limiting approach to prepare highly dispersed supported Pt catalysts. The structure of active sites not only affects the catalytic behavior of supported metal nanoparticle catalysts but also influences the catalytic behavior of supported oxide catalysts. Therefore controlling the structure of supported oxides is important when preparing supported oxide catalysts, and various physical and chemical tools have been used to characterize the materials post synthesis. Among all characterization tools, direct visualization of active sites on supported oxide catalysts using electron microscope remains challenging. Here, an approach that uses molecular precursors and 2D oxide supports to enable direct visualization of highly dispersed supported oxide catalysts has been developed to prove the existence of isolated Ta sites on the TiO2 support. In summary, novel approaches for synthesizing highly dispersed thermal stable supported metal catalysts have been developed in this thesis and can be applied to scale-up synthesis of many other supported metal catalysts

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  • 10/21/2018
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