Preparation And Hydrogenation-Catalytic Performances Of Confined Metal Catalysts

Interaction of active sites and supports can directly adjust activation energy and modify reaction ways. Researchers, in recent years, has found confinement in microenvironment strongly impacts the structure of active sites and properties of electron structure, and studied "interface confinement" of metal catalysts, "pore/cave confinement" of pore materials, "lattice confinement" of crystal and so forth. Thus, the synthesis of novel confined catalysts and formation mechanism for structure of active sites are becoming a crucial scientific challenge.In this thesis, by controlling reduction in lattice confinement, we prepared double-confined Ni nanoparticles catalyst, Ni/Ni(Al)Ox/AlOx, derived from NiAl-LDH precursor. That Ni Metal nanoparticles are confined inside Ni(Al)Ox shell and AlOx shell. In addition, we prepared TiO2-supported Pd metal catalysts with flat-shaped Pd nanoparticles confined on TiO2 surface. Characterizing physical and chemical properties including structure, morphology, valence and reduction degree of these two catalysts is to explore the impact of confinement microenvironment for metal active sites. Our work focused on reduction process of NiAl-LDH and supposed the formation of double-confined Ni nanoparticles. Double-confined Ni nanoparticles and surface-confined Pd/TiO2 catalysts were exerted to catalytic evaluation inDMT and phenylacetylene hydrogenation. The research contents are as follows:(1) Double-confined Ni nanoparticles catalyst, Ni/Ni(Al)Ox/AlOx, was fabricated by NiAl-LDH precursor reduced at 600 ? in H2. Results from structure characterization exhibit each single particle containing Ni metal nucleus, internal Ni(Al)Ox shell and external amorphous AlOx shell. This catalyst possesses smaller Ni metal particles and high reduction degree. Ni/Ni(Al)Ox/AlOx exhibits best catalytic performances, with 99.9% DMT conversion and 93.3% DMCD selectivity, large amounts of H desorption and higher desorption temperature in Ni/Ni(Al)Ox/AlOx sample suggest H spillover from Ni metal to supports. So the high DMT conversion can be obtained by Ni/Ni(Al)Ox/AlOx sample.(2) According to characterization of samples derived from as-synthesized NiAl-LDH reduced at different temperature in hydrogen atmosphere, the direct reduction of NiAl-LDH could be considered as four steps:1. Layer-collapse of LDH; 2. Formation of Ni metal nuclei; 3. Growth of Ni particles; 4. Coalescence and calcination of Ni particles. Based on the reduction mechanism, modifying the confinement structure of catalysts and adjusting their catalytic performances are possible.(3) Interface confined Pd nanoparticles catalyst, Pd/TiO2-PD, was prepared by photodeposition method. The Pd metal particles are confined on TiO2, surface with small particles, flat-shaped morphology and low coordination sites. Contrast to deposition-precipitation prepared Pd particles (Pd/TiO2-DP) catalyst, higher selectivity to styrene in same phenylacetylene conversion.(4) The behaviors of decomposition of ?-PdH reveal the Pd/TiO2-DP catalyst, during hydrogenation, would generate more ?-PdH which result total hydrogenation to alkane. The Pd/TiO2-PD catalyst's unique structure due to TiO2 surface confinement can inhibit formation of ?-PdH, so higher styrene selectivity will be obtained.