【作者】 姜冬宇；

【导师】 张敏华；

【作者基本信息】 天津大学 ， 化学工艺， 2012， 博士

【摘要】 氮氧化物排放可对人体健康和环境安全造成极大危害。在我国,多数氮氧化物来自火电厂,其最有效的控制方法是以NH3作为还原剂的选择性催化还原法。最近新型低温选择性催化还原催化剂的开发引起许多学者的关注,其中锰铈复合氧化物（MnO_x-CeO₂）是性能较好的一种。研究者已经开发许多新的制备方法以提高其反应性能,如等离子体法和溶液燃烧法等。本文试图采用超临界抗溶剂法（Supercritical Antisolvent process, SAS）来制备MnO_x-CeO₂催化剂,同时考察其选择性催化还原和储放氧性能,且通过对催化剂表征揭示其构效关系。另外,本文还通过密度泛函理论计算研究催化剂选择性催化还原和储放氧性能的相关机理。研究表明,利用SAS法可以成功制备MnO_x-CeO₂催化剂,并且发现其颗粒具有中空纳米球结构。根据实验现象,本文探讨了中空纳米球结构形成机理,主要包括液滴缩小、球壁形成和中空出现三个阶段。对SAS操作参数的影响研究发现,在前驱体溶液浓度较低的条件下可得到具有中空纳米球结构的催化剂,而浓度较高则为实心结构;还可以通过调控温度和压力对粒径进行控制。通过对初始和最终的Ce/Mn摩尔比的关系进行研究,可以实现前驱体中Ce/Mn比例可控。利用该简捷的制备方法可以同时实现催化剂的形貌、粒径和比例可控。MnO_x-CeO₂中空纳米球的储放氧能力和实心球相当,但储氧速率较高,可归结于其晶格氧迁移速率大和其较高的比表面积。中空球的选择性催化还原性能优于实心球的催化剂可归结于中空球比表面积较大,表面的活性氧物种多和储氧速率较高。随着Mn含量增加,由于MnO_x-CeO₂中空球催化剂的晶格氧的迁移性能增强,使得其储放氧性能也增强。而催化剂选择性催化还原性能有先升高后降低的趋势,主要是比表面积的影响。随着焙烧温度增加,MnO_x-CeO₂中空球催化剂的选择性催化还原和储放氧性能均出现先增大后减小的趋势,可分别归结于比表面积、表面氧物种数和晶格氧的迁移性能的先增后减。对Mn掺杂增强CeO₂（111）面储放氧性能的机理的密度泛函计算发现Mn的加入使得体系的几何结构和电子结构都发生了变化,增强了表面氧的活泼性,使得空位形成能降低,从而改善了体系的OSC性能。对在Mn掺杂CeO₂（111）面上ER型选择性催化还原反应机理进行密度泛函计算发现,NH₃吸附在Ce上和修复空位的活化氧作用而活化,再和气相中的NO作用形成NH₂NO;吸附在Mn附近的NH₂NO通过一系列氢转移过程形成N2和H₂O。整个反应路径为热力学有利,NH3的活化能垒最高。本文的计算可以为设计新型低温脱硝催化剂提供参考。更多还原

【Abstract】 Nitrogen oxides （NO_x） exhaust can cause serious damage to both human health and environmental safety. The emission of NO_x from power plants takes majority in China. The most effective method for reducing NO_x from power plants is selective catalytic reduction （SCR） with NH3 as reducing agent. In recent years, many efforts have been made to develop new SCR catalysts with good catalytic activity at low temperature. Among them, the manganese and cerium composite oxide （MnO_x-CeO₂） catalysts showed excellent activities for the SCR at low temperature. The recent reports have highlighted some new routes for the preparation of the MnO_x-CeO₂ catalysts to enhance its performance, such as plasma method and solution combustion method etc. In this work, it is attempted to synthesize MnO_x-CeO₂ catalysts via a supercritical antisolvent （SAS） process. The SCR and oxygen storage capacity （OSC） performances of catalysts were concerned. And some physicochemical characterizations of the prepared catalysts were also carried out to reveal the inherent structure-activity relationship. Furthermore, density functional theory （DFT） calculations were performed to explore the mechanisms about OSC and SCR of the MnO_x-CeO₂ catalysts.Firstly, the MnO_x-CeO₂ hollow nanospheres were successfully obtained in the SAS preparation. A new possible formation mechanism of the nano-sized hollow spheres had been proposed based on the experimental results. It included three stages: the shrinkage of droplets, the formation of spherical wall and the appearance of the hollow structure. Investigations on the effects of the SAS parameters revealed that the hollow MnO_x-CeO₂ nanospheres could be synthesized at low precursor concentration, but not at high solution concentration, which resulted in solid nanospheres. The size control was also realized by adjusting the pressure and temperature of the SAS process. Based on the exploration of the relationship between the Ce/Mn ratio in the initial solution and that in final product, the control of the Ce/Mn ratio was also realized. Thus, the particle morphology, size and element ratio of MnO_x-CeO₂ catalysts could be simultaneously controlled with a facile approach.Secondly, it was found that the total OSC of the hollow and solid MnO_x-CeO₂ nanospheres were equal. But the dynamic oxygen storage rate of hollow sample was better because of its better lattice oxygen mobility and higher specific surface area. Compared to the solid one, the MnO_x-CeO₂ hollow nanospheres exhibited a better NO conversion, which was related to their higher surface area, richer surface active oxygen species and higher oxygen storage rate. With the increase of Mn content, the OSC performance of the hollow MnO_x-CeO₂ nanospheres was enhanced due to the improvement of lattice oxygen mobility, but the NO conversion increased firstly and then decreased, which was mainly caused by the specific surface area of the catalysts. And with the increase of calcination temperature, the OSC and SCR performances of the MnO_x-CeO₂ hollow nanospheres increased firstly and declined later, which was due to the initial increase and the later decline of their specific surface area, surface active oxygen species and lattice oxygen mobility, respectively.Finally, the origin of the enhanced OSC performance of the Mn-doped CeO₂（111） was studied based on DFT calculations. It was found the modifications of geometric and electronic structures were caused by the incorporation of Mn in CeO₂, resulting in activated oxygen species on the surface. The formation energy of oxygen vacancies was lowered by the Mn doping. These changes could be responsible for the OSC enhancement of the Mn-doepd CeO₂ catalyst. The SCR mechanism according to the Eley-Rideal （ER） type on the Mn-doped CeO₂（111） was also studied by DFT calculations. It was found that NH3 adsorbed on the Ce atom could be activated by the active oxygen species, which could heal the vacancy on surface. The generated NH₂ species could react with NO in gas phase to form the adsorbed NH₂NO species near the Mn atom, which would experience a series of hydrogen-transfer steps to form N2 and H₂O. The overall reaction path was thermodynamically favorable and NH3 activation exhibited a highest energy barrier. The results could provide instructions for designing new type catalysts for the low-temperature SCR.更多还原