| SO2 is one of the most dangerous atmospheric pollutants since it contributes directly to acid rain formation. SO2 is mainly produced during the combustion of fuels from coal-fired power plant. It has been reported that flue gas cleaning techniques provided one of the most effective ways to remove SO2. Among them, the dry flue gas desulfurization technology in the use of metal oxide desulfurization is considered as the most promising green desulfurization technology, in line with sustainable development, thus it has attracted increasing attention.Based on the review of the current situation of the technology development, it was pointed out that the following shortcomings are the main obstacles:the proper material, the role of the active components and the relationship between the physical and chemical characteristics of the catalyst and SO2 adsorption performances. To address the above problems, in this dissertation, NiAl hydrotalcite-derived catalysts, as well as the Cu and Ce-doped ones, are investigated. The composition of catalysts and the calcination temperature are optimized during the preparation process. Moreover, the catalytic oxidation performances and reaction mechanisms for SO2 were investigated systematically. The detailed works are as follows:(1) NiAl mixed oxides (NiAlO) were prepared by urea hydrolysis method from hydrotalcites as precursors, and their catalytic oxidation performances for SO2 were investigated. They have large surface area in the rang 135-150 m2·g-1, high pore volume in the rang 0.33-0.40 cm3·g-1, and 80% of catalyst pores are mesopores. All the catalysts have a certain amount of the SO2 adsorption capacity. CO2-TPD and XPS results showed that there are a great quantity of surface oxygen, adsorbed oxygen and lattice oxygen on the surface of the catalyst.(2) Hydrotalcite-based NiAlO mixed oxides were synthesized by traditional constant-pH coprecipitation and urea hydrolysis methods, and their catalytic oxidation performances for SO2 were evaluated. The NiAlO catalyst prepared by urea method exhibited higher activity for SO2 removal reaction. It shows a flower-like hierarchical structure, whose size is in the range between 1 and 2μm. The structure is comprised of densely packed irregular nano flakes, with the thickness of ca.10-30 nm. It has greater surface area and pore volume, and more narrow pore structure. The NiAlO catalyst prepared by urea method has more surface active oxygen species (surface oxygen, adsorbed oxygen and lattice oxygen) and has more Lewis acidic and basic active sites, which are benefit for SO2 oxidative adsorption and storage. In situ FTIR experiments showed that the main products on the NiAlO catalyst surface are sulfite, bisulfite, bidentate binuclear sulfate.(3) Cu-doped NiAl mixed oxides (CuNiAlO) were prepared by urea hydrolysis method from hydrotalcites as precursors, and their catalytic oxidation performances for SO2 were investigated. Compared with NiAlO catalyst, the CuNiAlO catalysts have smaller crystalline size, larger surface area and pore volume, as well as more surface oxygen vacancies. Various characterization results showed that copper mainly exists in the form of Cu2+. The introduction of Cu can improve the structure and enhance the redox ability of the catalysts, which significantly improves the catalytic oxidation performance for SO2. The main products on the catalyst surface are sulfite, bisulfite, bidentate binuclear sulfate and bidentate mononuclear sulfate(4) Ce-doped NiAl mixed oxides (CeNiAlO) were prepared by urea hydrolysis method from hydrotalcites as precursors, and their catalytic oxidation performances for SO2 were investigated. Compared with NiAlO catalyst, the CeNiAlO catalysts have smaller crystalline size, larger surface area, higher pore volume, and more surface oxygen vacancies. The introduction of Ce can improve the lattice oxygen content and enhance the oxygen storage capacity and redox capabilities. Besides, Ce-doped catalysts can increase the strength of Lewis acidity and basicity, and form the Brφnsted acid centers, which significantly increases the catalytic oxidation performance for SO2. The deactivated catalysts were regenerated by different treatments and it was found that H2 reduction process could effectively recover the most catalyst activity.(5) Finally, density functional theory (DFT) based computations calculations have been applied to study the structure of adsorbed sulfate on the NiAlO catalyst and to predict the interactions of adsorbates with catalytic sites. These DFT calculation results are in agreement with prior in situ FTIR experimental studies, which proposed mainly a bidentate binuclear sulfate binding geometry formed on NiAlO surface after SO2 adsorption.The contribution by this dissertation has important significance for the development of independent intellectual property rights, and could promote the desulfurization technology with the concept of "green chemistry" and "cleaner production"... |
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