Optimizing Study On Selective Catalytic Reduction Of NO_x With NH₃over Iron-based Catalysts

Nitric oxides(NOx) are a major pollution which is severe harmful to environment. In our country, the total amount of NOx increases year by year with the development of the national economy, and the pollution caused by NOx emission became severe gradually. In recent ten years, the type of acid rain changed from the sulfuric acid type to the composite of sulfuric acid and nitric acid by degrees in some regions of China. As a major source of NOx emission, it is urgent to reduce NOx in exhaust flue gas from coal-fired power plant. Due to being a relatively mature with high NOx removal efficiency, selective catalytic reduction (SCR) is a well-known mainstream technology for NO*abatement emitted from coal-fired power plant. For the moment, V2O5/TiO2catalyst promoted by MoO3or WO3is the most effectively and widely commercial SCR catalyst used to reduce the nitric oxides emitted from the station source as the coal-fired power plant owing to the high activity and the durability to sulfur compounds. However, V2O5-WO3(MoO3)/TiO2catalyst would bring about some disadvantages in practical use, such as high denitrification temperature, high cost, and toxicity of vanadium pent-oxide to environment and human health. Therefore, it is in dire need of developing a novel SCR technology with high activity, low cost, and absence of toxicity which is suitable for the national conditions of China. Compared with the traditional V2O5-WO3(MoO3)/TiO2catalyst, iron-based catalyst has some advantages with low cost and absence of toxicity, and is a potential catalyst for SCR of NOx Nevertheless, iron-based catalyst shows lower activity at relatively low-temperature, which restricts its large-scare industrial application. On account of higher reaction temperature for iron-based catalyst, the purpose of this paper was to optimize its low-temperature SCR activity through doping other metal oxide and improving the preparation method of catalyst, and to make its SCR reaction temperature window shifted to the low-temperature. Finally, the promotional mechanisms for doping other metal oxide and improving the preparation method of catalyst have also been studied and given in paper. In this paper, we firstly obtained iron-based catalyst prepared through co-precipitation method showing high SCR activity. The effects of doping the additives as cerium oxide and the hydrothermal treatment of catalyst precursor by using microwave radiation on SCR activity over iron-based catalysts were investigated. The denitrification characteritics of iron-based catalyst were also under systematic investigation, and its denitration reaction model was founded. Finally, we gave the promotion mechanisms for doping the additives as cerium oxide and the hydrothermal treatment of catalyst precursor by using microwave radiation on SCR activity over iron-based catalysts.(1) The types of precipitants showed an important effect on SCR activity over iron-based catalysts prepared through co-precipitation method. Compared with the alkali metal precipitants, the catalysts prepared by using amino precipitants showed high SCR activity. From the IR and XRD results for iron-cerium mixed oxides catalysts and their precursors prepared with different precipitants, it can be seen that: the alkali metal precipitants such as NaOH and Na2CO3could promot the formation of α-FeOOH and α-Fe2O3in the precursors of iron-cerium mixed oxides catalysts. However, amino precipitants would depress the formation of α-FeOOH and α-Fe2O3in the precursors of iron-cerium mixed oxides catalysts, and made iron oxide and cerium oxide to form the solid solution in iron-cerium mixed oxides catalysts. The results of N2adsorption for different catalysts also indicated that the iron-cerium mixed oxides catalysts prepared with amino precipitants had large BET surface areas and pore volumes compared to the catalysts prepared with alkali metal precipitants. The larger BET surface areas and pore volumes could provide huge amount of reactive sites for SCR reaction, and thereby promoted the catalysts showed high medium-low temperature SCR activity.(2) The doping of cerium oxide could enhance the medium-low temperature SCR activity of iron oxide catalyst prepared through co-precipitation with NH4OH as the precipitant. The cerium doping amount showed an effect on SCR activity over iron-cerium mixed oxide catalysts through influencing the interaction of cerium oxide and iron oxide in catalysts. The results of research indicated that when themolar ratio of Ce/(Ce+Fe) was increased from0.025to0.1, the low-temperature SCR activity over iron-cerium mixed oxide catalysts increased gradually, meanwhile, its high-temprature SCR activity firstly increased and then decreased. The iron-cerium mixed oxide catalysts at the molar ratio of Ce/(Ce+Fe) being0.05showed the most wide temperature window for SCR reaction.(3) By means of the catalyst characterizations such as XRD, N2adsorption, TPD and so on, the promotion mechanism by doping cerium oixde on SCR activity over iron oxide catalyst was revealed. The results indicated that the doping cerium oxide could interact well with iron oxide in catalysts prepared though co-precipitation method, and led to the formation of the solid solution between them. The interaction between iron oxide and cerium oxide promoted a large mount of adsorbed oxygen and lattice oxygen with high activity in catalysts, and improved the low-temperature activity of oxidizing NO to NO2over iron oxide catalyst. Meanwhile, when the molar ratio of NO2/NOx was increased to0.3in the simulated flue gas, the conversions of NOx over Fe2O3and Fe0.95Ce0.05O2catalysts at150℃were enhanced by55.9%and19.9%, respectively. The doping of cerium oxide could optimize the microscopic pore structure of iron oxide, made its pore size become smaller, and enhanced the BET surface area and pore volume of iron oxide. The BET surface area and the pove volume of Fe2O3were enlarged from44.53m2/g and0.205cm3/g to107.25m2/g and0.285cm3/g after doping cerium oxide with the molar ratio of Ce/(Ce+Fe) being0.05, and were enhanced1.4and1.44times, respectively. At the same time, The doping of cerium oxide could promote the formation of Lewis acid over iron oxide, which improved the low-temperature adsorption of NH3and NO over iron oxide catalyst, thereby enhanced its medium-low temperature SCR activity.(4) The effects of operation parameters such as calcination temperature, Gas hourly space velocity(GHSV), O2concentration and NH3concentration were examined detailly in the intergral experiment system. And the research results demonstrated that the suitable calcination temperature was vital to the SCR activity over iron-cerium mixed oxide catalyst. When the calcination temperature is higher than the complete decompotition temperature for the precursors of iron-cerium mixed oxide catalyst, higher temperature would lead α-Fe2O3separated out from iron-cerium mixed oxide catalyst, and made the BET surface area and the pore volume of catalyst decrease, thereby depressed the promotion of doping cerium oxide on SCR activity over iron oxide. The suitable calcinations temperature was400℃. Oxygen played an important role on SCR activity over iron-cerium mixed oxide catalysts. The doping of cerium oxide could promote the formation of adsorbed oxygen and lattice oxygen with high activity, which made the iron-cerium mixed oxide catalyst show high SCR activity in the absence of gaseous O2within the simulated gas. Meanwhile, gaseous O2could oxidize the surface of iron-cerium mixed oxide catalyst and promoted the formation of absorbed and lattice oxygen on it, thereby enchanced the SCR activity over iron-cerium mixed oxide catalyst. When the concentration of gaseous O2in the simulated gas was lower than1%, the improvement of gaseous O2could lead the SCR activity of iron-cerium mixed oxide catalyst increase sharply. However, the concentration of gaseous O2showed no influence almost on SCR activity of catalyst when the concentration of gaseous O2was higher than3.0%. The SCR activity of catalyst firstly increased sharply and then slowly with the molar ratio of NH3/NO in the simulated gas increasing from0to1, and the SCR activity of catalyst could be not enhanced again by further improving the molar ratio of NH3/NO.(5) The kinetics of SCR over Fe0.95Ce0.05O2catalyst had been studied in a differential system, and its catalytic kinetics model for SCR reaction was founded. Under the reaction conditions in this paper, the rate of NO conversion on Fe0.95Ce0.05O2is first-order with respect to NO, zero-order with respect to NH3and nearly0.5-order with respect to O2. The apparent activation energy of the NH3-SCR reaction was42.6kJ/mol at175～275℃.(6) By doping other transition metal oxide into iron-cerium mixed oxide catalyst, and we investigated the effect of doping another transition metal oxide on SCR activity over iron-cerium mixed oxide catalyst. The research results indicated that, compared with W、Mo and Zr, the doping of Ti could obviously enhance the low-temperature SCR activity over iron-cerium mixed oxide catalyst, and made its reactive temperature window of SCR shift to the low-temperature region. Thereinto, the suitable molar ratio of Ti/(Ti+Fe+Ce) is0.15.(7) There existed better interaction between the doping titanium with other elements as iron and cerium, which could refine the pore diameter of iron-cerium mixed oxide catalyst, and enlarged the BET surface area and pore volume of catalyst. The doping of titanium led the concentration of iron and cerium elements on the surface of catalyst decrease, and enhanced the dispersion of iron and cerium groups on the surface of catalyst. The doping of titanium might change the surface structure of iron-cerium mixed oxide, enhanced the weak Lewis acid, and improved the low-temperature adsorption of NH3over catalyst. At the same time, the adsorted oxygen concentration over iron-cerium mixed oxide catalyst could also be improved after doping titanium which enhanced the low-temperature catalytic oxidation of NO to NO2over catalyst, and thereby promoted the low-temperature SCR activity over iron-cerium mixed oxide catalyst.(8) The SCR activity of iron-cerium-titanium mixed oxide catalyst could be enhanced further by introducing microwave hydrothermal treatment into the preparation of catalyst which could make the reactive SCR temperature window shift to the low-temperature region. Meanwhile, the promotional effect of microwave hydrothermal treatment on low-temperature SCR activity is closely related to the molar ratio of Fe and Ti in the mixed oxide catalyst. When the molar ratio of Fe and Ti is smaller, the promotion of microwave hydrothermal treatment is higher. The alternate microwave heating method and the microwave iddiation time played an important role on the promotion of low-temperature SCR activity over iron-cerium-titanium mixed oxide catalyst by using microwave to treat the precursors of catalyst. Under the same microwave iddiation time, when the alternate microwave heating method changed from P30to P80, the promotional effect of microwave hydrothermal treatment on low-temperature SCR activity over iron-cerium-titanium mixed oxide catalyst decreased gruadually. Under the condition of P30, the promotion of microwave hydrothermal treatment was the highest when the microwave iddiation time is15min.(9) Microwave hydrothermal treatment could accelerate the crystallite rate of the precursors for iron-cerium-titanium mixed oxide catalysts, adjusted the microscopic pore structure of catalyst, and enlarged the pore diameter and the pore volume of iron-cerium-titanium mixed oxide catalyst. Microwave hydrothermal treatment could strengthen the interaction between iron oxide and titanium oxide in mixed oxide, and promoted the formation of iron-titantium composite oxide. At the same time, the molar ratio of Ce3+/Ce4+on the surface of mixed oxide catalyst could be enhanced after microwave hydrothermal treatment, and improved the relative concentration of Ce3+on the surface of catalyst which enhanced the reduction and oxidation conversion between Ce3+and Ce+, and improved the defect value of lattice oxygen on the surface of iron-cerium-titanium mixed oxide catalyst. Therefore, the concentration of lattice oxygen over catalyst could be improved after microwave hydrothermal treatment, and enhanced the oxidation and reduction properties of catalyst. At the same time, microwave hydrothermal treatment enhanced the Lewis acid of the mixed oxide catalyst, and improved the low-temperature adsorption of NH3over catalyst, thereby enhanced the low-temperature SCR activity over iron-based catalyst.