Study On Preparation Process Of Synth-maghemite Catalyst And Its Optimization Mechanism For NH₃-SCR Performance

Nitrogen oxide (NOx) emission from coal-fired power plants harms the environment by forming acid rain, haze and photo-chemical smog. The removal of nitrogen oxides has attracted world-wide attention due to the serious threat for human health and environment. In China, the standards of emission limitations and assessment for NOx emissions have been more rigorous in 2015. Among the various NOx emission control technologies, the selective catalytic reduction of NOx by NH3 (NH3-SCR) is the most efficient method that has been widely applied in coal-fired power plant. As the core of NH3-SCR technology, catalysts are the main factor affecting the NOx removal efficiency and economical efficiency of the whole SCR system. V2Os/TiO2 catalysts, as the most widely used catalysts, brings about some disadvantages in practical application, such as high manufacturing cost, heavy metal loss, and secondary pollution caused by volatility in high temperature.In autumn 2014, the Ministry of Environmental Protection stipulated that the vanadium based catalyst should be managed under tighter supervision as hazardous waste. Afterwards, a provincial standard issued by Shandong Province expressly identified that the V2O5 can not be detected in SCR catalysts. Stated thus, it should be urgent to develop the environment-friendly SCR catalysts which is vanadium-free. Iron based catalyst is a potential catalyst for NH3-SCR which possesses advantages such as plentiful source, low cost and eco-friendly components. The activity of iron based catalyst is strongly influenced by the crystalline phase of iron oxide. Research shows that synth-maghemite (γ-Fe2O3) which is widely used in high-tech field gains better NH3-SCR denitrification performance than α-Fe2O3. Meanwhile, synth-maghemite has positive effects on Hg0 oxidation and mercury removal. However, the systematic study of SCR dynamics characteristics, NH3-SCR optimization mechanism of denitrification performance, and reaction mechanism for SCR over synth-maghemite catalyst has not been reported. Moreover, simple preparation method is needed for the preparation and modification of synth-maghemite catalyst in order to reduce the manufacture cost. In this paper, a series of synth-maghemite catalysts with high SCR activity were obtained using simple preparation method. On this basis, the denitrification performance of synth-maghemite catalysts was further enhanced by optimizing the preparation conditions and doping agents. Also, necessary characterization measurements were employed to investigate the SCR reaction mechanism. The main research methods and conclusions are as follows:(1) The synth-maghemite catalyst was prepared using precipitation microwave pyrolysis method. Crystalline phase and microstructure of catalysts were characterized by x-ray diffraction (XRD), mercury intrusion porosimetry (MIP), N2 adsorption-desorption, and thermal gravimetric (TG) method. Denitrification efficiency, reaction rate, SO2 and H2O resistance, thermal stability over different catalysts were investigated. And the effect of operating parameters (O2 concentration, NH3/NO mole ratio, etc.) on NH3-SCR activity of synth-maghemite catalyst were also studied. The results indicated that synth-maghemite catalyst could be obtained via the precipitation microwave pyrolysis method, which showed higher NOx conversion (96%) than synth-hematite catalyst. Synth-maghemite catalyst possessed a broader pore size distribution with abundant macroporous in 100～50000 nm as well as appropriate ratios of slit-shaped mesoporous. Meanwhile, synth-maghemite catalyst exhibited high-purity γ-Fe2O3, sufficient surface lattice oxygens, excellent thermal stability and high resistance capacity to SO2 and H2O.(2) The effect of two adding methods for precipitators (FPM and RPM), two typical amino precipitators (NH4OH and (NH4)2CO3), two typical alkali metal precipitators (NaOH and NaCO3) and four calcination temperature conditions on NH3-SCR denitration performance over synth-maghemite catalyst were investigated. The crystal phase, pore structure, surface element distribution and microscopic morphology of synth-maghemite catalysts were characterized by XRD, N2 adsorption-desorption, energy dispersive spectrometer (EDS) and scanning electron microscope (SEM), and the influence of precipitation condition and calcination temperature on the NH3-SCR denitration activity over synth-maghemite catalyst was investigated. Optimal preparation parameters for the preparation of synth-maghemite catalyst with precipitation microwave pyrolysis method was observed. The results indicated that synth-maghemite catalyst prepared with NH4OH precipitator, using FPM, and calcined at 400℃ exhibited the highest SCR denitrification activity. High-purity γ-Fe2O3 crystal phase is the prerequisite for the excellent SCR denitration performance of synth-maghemite catalyst, and the formation of α-Fe2O3 phase was unfavorable to SCR reaction. Sufficient surface lattice oxygen, independent spherical particle morphology were the key factors of ensuring the high SCR denitrification activity over synth-maghemite catalyst. Suitable proportion of mesopores and macropores is also crucial for the improvement of SCR activity over synth-maghemite catalyst. Diffusion resistance of reaction gas decreased with the excessive growth of pore structures in range of 10～50 nm, and this would restrict the diffusion of reaction gas into small pore structures which suppressed the SCR activity of synth-maghemite catalyst.(3) A series of synth-maghemite catalysts doping with Cu, Ti, Sn, and Ce were prepared via the precipitation microwave pyrolysis method. The influence of Cu, Ti, Sn, and Ce on the NH3-SCR activity over synth-maghemite catalysts were investigated. The crystalline phase, pore structure, surface element and microscopic morphology of catalysts were characterized by XRD, N2 adsorption-desorption, EDS and SEM. Effect of promoters doping on crystalline phase, pore structure, surface elements and microstructure over synth-maghemite catalyst were systematically observed. The optimization mechanism of NH3-SCR denitrification activity over synth-maghemite catalyst was explored. The results indicated that synth-maghemite catalysts doped with Ti, Cu-Ti at a mole ratio of 0.05 obtained excellent NH3-SCR denitrification activity. Synth-maghemite catalyst doped with Ti exhibited the highest denitrification efficiency of 98.3% with a wide temperature windows for high SCR activity. The surface area and pore volume of synth-maghemite catalyst improved significantly when doping with Cu, Ti, Sn, and Ce. The surface area and pore volume of SM-Fe95Ti5 were 48.03 m2·g-1 and 0.209 cm3·g-1 respectively, which reached more than three times of the surface area and pore volume of SM-Fe. But Cu, Ti, Sn, and Ce showed different impacts on the crystalline phase over synth-maghemite catalyst. The addition of Cu led to the overgeneration of a-Fe2O3 in synth-maghemite catalyst, which was unfavorable to SCR reaction. However, the addition of Cu-Ti restained the generation of α-Fe2O3, and rationalized the distribution of pore structure over synth-maghemite catalyst at the pore diameter of 10～50 nm, which were beneficial to the SCR reaction.(4) Dynamic characteristics over SM-Fe95Ti5, SM-Fe95Cu2.5Ti2.5 and SM-Fe catalyst were studied using steady-state kinetics test method. The effect of reactants concentration and reaction temperature on the intrinsic SCR reaction rate over synth-maghemite catalyst was investigated. The intrinsic reaction rate, apparent rate constant, thereaetion order and activation energy were measured, and the reaction kinetic equation was founded. Besides, the chemical status of surface elemental, type and quantity of surface acid sites, redox properties over synth-maghemite catalyst were observed with methods of x-ray photoelectron spectroscopy (XPS), temperature programmed desorption of NH3 (NH3-TPD), H2-temperature programmed reduction (H2-TPR). Finally, the reaction mechanism of NH3-SCR over synth-maghemite catalyst was explored on the basis of the above research. The results indicated that Eley-Rideal mechanism and Langmuir-Hinshelwood mechanism were co-existed in-the SCR reaction over synth-maghemite catalyst. The presence of reaction path following the Langmuir-Hinshelwood mechanism reduced the activation energy of synth-maghemite catalyst significantly. The amount and intensity of Lewis acid sites and Bronsted acidic sites on the surface of Ti-doped synth-maghemite catalyst increased substantially, with the activation energy lowing to 21.836 kJ·mol-1, which was only half as much as the activation energy of commercial vanadium-based catalyst. Redox properties of synth-maghemite catalyst depends crucially on the reduction process for Fe3O4â†'FeO. Fe3+ and Ti4+ were the main existing and expressing form of iron and titanium over synth-maghemite catalyst. Simultaneously, NH3 adsorption in the process of SCR reaction also mainly occurred on the sites of surface Fe3+.-NH2, NH4+ and NH3 were the main form for the participation in SCR reaction of ammonia.The completion for research contents and research objectives of this dissertation is of great significance to the development of low-pollution and low-cost SCR denitration catalyst which is suitable for coal-fired power plants in China. Study, development and utilization of synth-maghemite catalyst with tremendous application potential will help to alleviate the critical current situation for pollutant emission in China.