Dcsign,Synthcsis,and Mechanistic Study Of Nanostructurcd Oxides For Heterogeneous Catalysis

Because of their high surface areas, special electronic properties, and quantum effects, nanostructured materials have found important applications in catalysis. Using different preparation methods, nanostructured catalysts can be obtained with different sizes, exposed crystal planes, and various composite structures. As a result, these catalysts have shown high activity and stability in a series of important catalytic reactions. For a realistic heterogeneous system, however, the physical and chemical processes are associated with great complexity. Since the key obstacle of the structural complexity and heterogeneity commonly exists for conventional catalysts, design and construction of model nanocatalysts with uniformed and well-defined structures are essential for the physical and chemical studies of the catalytic processes. It is, therefore, conceivable to correlate the catalytic activity with the unique characteristics of the nanocatalyst structure unambiguously. Investigation based on this concept would lead to insights into the structure-activity correlation and catalytic mechanism at nanoscale under real reaction conditions.This thesis mainly focuses on the design, synthesis, and mechanistic investigations of semiconducting metal oxides and acidic zeolites (aluminium silicate) nanocatalysts. Taking advantage of high surfaces, unique flower-like nanomicrostructure and well-defined pore size of these synthetical catalysts, we investigated the catalytic mechanisms of gas sensing process catalyzed by semiconducting metal oxides and the methanol-to-olefins process catalyzed by acidic zeolites, respectively. The main issues addressed in this thesis are described in the following.In chapter1, the research background is presented, including the application of typical nanocatalyst for heterogeneous catalysis, recent development of semiconducting metal oxides as gas sensors, and the mechanistic study of methanol-to-olefin process.In chapter2, design, synthesis, and application of nano/microstructure SnO2for the gas sensing are described. Hierarchical three-dimensional flower-like SnO2spheres were synthesized by simple hydrothermal method. Because of its high surface area and unique nano/microstructure, this material shows a good performance to sense carbon monoxide, methane, methanol, and ethanol. In a parallel solid-state NMR study, many important surface species, such as surface carbonate species, surface formate species, and surface acetate species, were observed in the SnO2samples after exposed to the various sensing gases at different temperatures. Formation of these species consumes surface adsorbed oxygen species, resulting in the electron exchanges between the surface adsorbed species and SnO2, and thus changes the resistance of the SnO2for sensing signal. This combined sensing and solid-state NMR study provides crucial information for the surface chemistry during the sensing process.In chapter3, the synthesis of zeolites with different framework structures and mechanistic study of the methanol-to-olefin (MTO) process have been described.Two one-dimensional zeolite catalysts with similar pore structures but with a0.3A difference in pore sizes were exploited to gauge the critical sizes of the key intermediate species formed during the methanol conversion to olefins. In the MTO process, the hydrocarbon-pool mechanism is dominant in the H-ZSM-12zeolite with a pore size of6.0A, while the catalytic cycle could not be completed in the H-ZSM-22zeolite with a smaller pore size of5.7A. For the H-ZSM-22zeolite no olefin was produced, while in the H-ZSM-12zeolite, a large amount of olefins, paraffins, and aromatics were formed. This finding strongly confirmed that the hydrocarbon-pool mechanism is a space-demanding process. Our results also suggested that the size of key reactive hydrocarbon-pool intermediates for the MTO process is between5.7and6.0A.In chapter4, the synthesis and application of superparamagnetic Fe3O4nanoparticles is presented.By thermal-decomposition method, we have successfully synthesized the monodispersed Fe3O4nanoparticles with the adjustable particle size with uniformed size distribution and controllable shape. SiO2-coated Fe3O4magnetic nanoparticles were also synthesized via the reverse microemulsion. Finally, we attempted to apply these superparamagnetic nanoparticles to a study of heterogeneous catalysis and Zn2+sensing.Finally, in chapter5, a brief summary and outlook have been addressed.