铁铈和铁钨复合氧化物的制备及其氨选择催化还原NO_x研究

发布时间：2018-05-13 01:39

本文选题：氨选择性催化还原 + 铁铈复合氧化物　；参考：《大连理工大学》2017年博士论文

【摘要】：氮氧化物(NO_x)是主要的大气污染物之一,其会对生态环境和人体健康产生不利影响。氨选择性催化还原氮氧化物(NH3-SCR)技术是一种有效的NO_x去除技术,在固定源的烟气脱硝和移动源的机动车尾气NO_x去除领域均有良好的应用。针对商业V2O5-WO3/TiO2催化剂温度窗口窄、高温条件下N2选择性差、成本高以及对环境造成二次污染等问题,本论文设计开发了高效且价格低廉的环境友好型铁基复合氧化物催化剂。通过各种表征手段和理论计算分析了铁基复合氧化物的物理化学性质,揭示了催化剂结构与NH3-SCR活性之间的构效关系,并采用原位红外光谱技术及质谱技术探讨了催化剂上吸附物种的形成及其反应行为。取得的主要研究成果如下:(1)通过表面活性剂辅助法制备了一系列具有高比表面积的介孔铁铈复合氧化物(FexCe1-xOC2-δ)催化剂。XRD和Raman结果表明铁掺杂到氧化铈中可形成铁铈固溶体(Fe-O-Ce结构),导致了催化剂活性的提高及活化能的降低。其中,Fe0.4Ce0.6O2-δ催化剂的SCR反应活化能最低(26 kJ·mol-1)且催化性能最优(在250到350℃反应温度范围内NO_x转化率大于80%)。XPS和DFT计算结果表明,Fe-O-Ce结构的形成促进了 Fe3+与Ce4+之间的电子相互作用,有利于增加催化剂的Lewis酸性位和提高催化剂的氧化还原性。当铁添加量较少时(0x0.3),可通过空位补偿机制形成铁铈固溶体且氧空位显著增加,从而提高了催化剂的SCR反应活性。当铁添加量较高时(1x ≥ 0.3),可通过间隙位补偿机制形成铁铈固溶体同时出现间隙位Fe3+物种。间隙位Fe3+物种具有富电子性有利于NO氧化成NO2,改善铁铈催化剂的催化性能。(2)设计开发了在宽工作温度窗口内具有优异NH3-SCR性能的铁钨复合氧化物(Fe1-xWxOδ)催化剂。其中,Feo.75W0.25Oδ催化剂活性最佳,在225至450℃温度范围内NO_x转化率可维持在90%以上。同时,该铁钨催化剂还具有较高的热稳定性和优良的抗高空速能力。钨物种的引入有利于形成较小颗粒的α-Fe2O3和FeWO4物种。FeWO4具有八面体[FeO6]/[WO6]结构可提供丰富的Br(?)nsted酸性位,有利于NH3吸附活化形成NH4+物种。结合DFT计算与XPS和UV-vis结果发现,α-Fe2O3和FeWO4之间的界面电子相互作用促使电子从W6+位点转移到Fe3+位点,有利于NO2活性物种的生成。因此,铁钨催化剂的高活性归功于α-Fe2O3和FeWO4物种之间的协同作用。(3)采用原位红外光谱技术探究了铁钨(FeW)催化剂的NH3-SCR反应机理。结果表明,在低温区(≤250℃),NO2、配位NH3和NH4+为主要的吸附物种,它们之间反应形成的中间产物NO2(NH3)2和NO2(NH4+)2复合物可以与NO反应产生N2和H2O。因此,该FeW催化剂的低温NH3-SCR遵循"快速SCR"反应路径,其中NO氧化生成NO2是该反应的速控步骤。在高温区(≥250℃),配位NH3和NH4+为主要的吸附物种而NO主要以气态形成参与反应。配位NH3/NH4+物种和NO之间反应形成NH2NO中间物种,进一步反应以产生N2和H2O,遵循Eley-Rideal(E-R)反应机理。(4)通过红外光谱和程序升温等技术系统研究了硫化过程对FeW催化剂结构、反应性能以及机理的影响。在低温区间内(300℃),硫化作用抑制了 NH3-SCR活性,而在高温(≥300℃)下反应活性没有明显变化。经过硫化作用,FeW催化剂上形成了具有共价S=O键的硫酸盐物种。结合TPD与原位红外光谱测定的数据发现,S=O共价双键的强吸电子效应,增强了催化剂金属物种的Lewis和Br(?)nsted酸性,有利于配位NH3、NH4+吸附物种和硝酸盐的生成。在低温区,NO2的形成因硫化过程受阻导致"快速SCR"路径被切断,从而低温SCR活性有所损失;在高温区,硝酸盐吸附物种的形成,使SCR反应遵循 Langmuir-Hinshelwood 反应机理。
[Abstract]:Nitrogen oxide (NO_x) is one of the main atmospheric pollutants, which will have adverse effects on the ecological environment and human health. Ammonia selective catalytic reduction nitrogen oxide (NH3-SCR) technology is an effective NO_x removal technology. It has a good application in the field of fixed source flue gas denitrification and mobile vehicle exhaust NO_x removal in mobile vehicles. The V2O5-WO3/TiO2 catalyst has a narrow temperature window, low selectivity of N2 at high temperature, high cost and two pollution to the environment. This paper designed and developed a highly efficient and inexpensive environmentally friendly iron based composite oxide catalyst. The physicochemical properties of iron based composite oxides were analyzed by various characterization methods and theoretical calculations. The structure-activity relationship between the structure of the catalyst and the activity of NH3-SCR was revealed, and the formation and reaction of the adsorbed species on the catalyst were investigated by in situ infrared spectroscopy and mass spectrometry. The main results obtained were as follows: (1) a series of mesoporous iron with high specific surface area was prepared by the surfactant assisted method. The results of.XRD and Raman of the cerium complex oxide (FexCe1-xOC2- delta) catalyst show that iron doped to cerium oxide can form the iron cerium solid solution (Fe-O-Ce structure), which leads to the improvement of the activity of the catalyst and the reduction of the activation energy. Among them, the SCR reaction activation energy of the Fe0.4Ce0.6O2- delta catalyst is lowest (26 kJ. Mol-1) and the catalytic performance is optimal (at 250 to 350 degrees C). The conversion rate of NO_x within the temperature range is greater than 80%).XPS and DFT calculation results show that the formation of Fe-O-Ce structure promotes the electronic interaction between Fe3+ and Ce4+, is beneficial to increase the Lewis acidity of the catalyst and to increase the oxidation-reduction of the catalyst. When the amount of iron is less (0x0.3), the iron cerium solid solution can be formed by the vacancy compensation mechanism. As the oxygen vacancy increased significantly, the SCR reaction activity of the catalyst was increased. When the amount of iron was higher (1x > 0.3), the gap position compensation mechanism was used to form the iron cerium solid solution and the gap position Fe3+ species appeared at the same time. The gap position Fe3+ species had rich electron property, which was beneficial to NO oxidation to NO2 and improved the catalytic performance of the iron cerium catalyst. (2) design opening. The Fe / W composite oxide (Fe1-xWxO delta) catalyst with excellent NH3-SCR properties in a wide working temperature window has been developed. Among them, the Feo.75W0.25O delta catalyst has the best activity, and the NO_x conversion rate can be maintained over 90% at the temperature range of 225 to 450. The fe w catalyst has high thermal stability and excellent high air velocity resistance. The introduction of tungsten species is beneficial to the formation of smaller particles of alpha -Fe2O3 and FeWO4 species.FeWO4 with a eight body [FeO6]/[WO6] structure that provides a rich Br (?) nsted Acid position and is beneficial to NH3 adsorption and activation to form NH4+ species. The site transfer to the Fe3+ site is beneficial to the generation of NO2 active species. Therefore, the high activity of the iron tungsten catalyst is attributed to the synergism between the alpha -Fe2O3 and the FeWO4 species. (3) the mechanism of the NH3-SCR reaction of the iron tungsten (FeW) catalyst was investigated by in situ infrared spectroscopy. The results show that in the low temperature region (less than 250 degrees C), NO2, the coordination NH3 and NH4+ are the main factors. The adsorbed species, NO2 (NH3) 2 and NO2 (NH4+) 2 complex formed by the reaction between them, can react with NO to produce N2 and H2O., so the low temperature NH3-SCR of the FeW catalyst follows the "fast SCR" reaction path, in which NO oxidation produces NO2 is the speed control step. NO mainly takes part in the formation of the gaseous state. The reaction between the coordination NH3/NH4+ species and NO forms the intermediate species of NH2NO, and further reacts to produce N2 and H2O and follows the Eley-Rideal (E-R) reaction mechanism. (4) the effects of the vulcanization process on the structure, the reaction properties and the mechanism of the FeW catalyst are studied by the infrared spectrum and the temperature programmed system. In the low temperature range (300 degrees C), the vulcanization inhibits the NH3-SCR activity, but the reactive activity is not obviously changed at high temperature (> 300 degrees C). After the vulcanization, the sulfate species with covalent S=O bond is formed on the FeW catalyst. The strong electron absorption effect of the S=O covalent double bond is enhanced by the number of TPD and in situ infrared spectroscopy. The Lewis and Br (?) nsted acidity of the metal species of the catalyst is beneficial to the coordination of the ligand NH3, the NH4+ adsorbed species and the formation of nitrate. In the low temperature zone, the formation of the NO2 is blocked by the vulcanization process that causes the "fast SCR" path to be cut off, so that the low temperature SCR activity is lost; in the high temperature region, the nitrate adsorbed the species to follow Langmuir-Hinshelw. The reaction mechanism of ood.

【学位授予单位】：大连理工大学
【学位级别】：博士
【学位授予年份】：2017
【分类号】：X701

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