Synthesis Of Magnetic Yolk-shell/core-shell Nanocomposites And Their Catalytic Applications

The research of novel nanocomposites based on multi-functionalization and microstructure is a hotspot in the field of catalysis. The magnetic nanocomposite catalyst with hierarchical structure possesses both the high catalytic performance of nanocatalyst and the unique magnetic properties of the magnetic nanoparticles, the multi-functional integrated assembly and surface functionalization is the key to enhance the performance of magnetic nanocomposite catalysts. In this dissertation, magnetic yolk-shell/core-shell nanocomposite catalysts have been synthesized through loading noble metal nanocatalysts, coating TiO2 photocatalysts and metal-organic frameworks on the magnetic nanoparticles. Catalytic active components are introduced into the self-assembling yolk-shell/core-shell structure, realizing the controllable design and functional assembly of the novel hierarchical magnetic nanocomposite catalyst. The catalytic applications of the nanocomposite catalysts based on the catalytic properties of the active components were investigated in different reaction systems, such as reduction of nitro group, photocatalytic degradation, aerobic oxidation of alcohols and olefins.Research in this dissertation includes the design, preparation and catalytic applications of three kinds of magnetic yolk-shell/core-shell nanocomposite catalysts with different structure and compositions, the main conclusions are sumerized as follows:(1) Double-shelled sea urchin-like yolk-shell Fe3O4/TiO2 microspheres were successfully synthesized through loading Au nanoparticles on the support to obtain double-shelled sea urchin-like yolk-shell nanocomposite catalyst. The Au nanoparticles were loaded both on the TiO2 nanofibers and inside the cavities of sea urchin-like yolk-shell Fe3O4/TiO2 microspheres. The sea urchin-like structure composed of TiO2 nanofibers ensure the good distribution of the Au nanoparticles, while the novel double-shelled yolk-shell structure guarantees the high stability of the Au nanoparticles. The Fe3O4/TiO2/Au microspheres display excellent activities and recycling properties in the catalytic reduction of 4-nitrophenol, the reaction rate constant is 1.84 min-1 and the turnover frequency is 5457 h-1, the nanocomposite catalyst can realize the convenient recovery by applying an external magnetic field.(2) Fe3O4(PAA)-Au@TiO2 photocatalyst with yolk-shell structure were achieved through coating anatase TiO2 active shell with excellent photocatalytic activities on polyacrylic acid modified Fe3O4 functional core, and Au nanoparticles were deposited on the composite to enhance the photocatalytic activities. The TiO2 absorption spectrum was expanded to the visible light region by gold nanoparticles immobilization, and the unique yolk-shell structure can enhance the reflections of the light source in the inner space of the microspheres, improving the efficiency of light absorption. The nanocomposite photocatalyst displays good catalytic efficiency in the photocatalytic degradation of rhodamine B under visible light, the reaction rate constant is 0.0159 min-1. Furthermore, the magnetic nanocomposite catalysts can be conveniently recovered by applying an external magnetic field.(3) Core-shell structured Fe3O4/Cu3(btc)2 microspheres were successfully synthesized by coating the catalytic active Cu3(btc)2 metal-organic frameworks on poly(acrylic acid) (PAA) functionalized Fe3O4 microspheres. The nanocomposite catalyst exhibited excellent catalytic activity in the selective oxidation of benzyl alcohol to benzaldehyde, achieving 100% conversion in 6 h, and the catalyst exhibited excellent catalytic activity and selectivity in the aerobic oxidation of alcohols and the epoxidation of olefins under mild conditions. The abundant carboxylic acid groups of PAA can coordinate Cu2+ ions and favor the stable coating of the MOF layer onto the magnetite core, and the catalyst can be easily segregated from the catalytic system by using an external magnetic field due to its magnetic properties.