铝酸盐尖晶石选择催化还原去除NOx的机理研究

In view of the traditional three-way catalysts with low efficiency in removing the NOx under the condition of oxygen-rich and low temperature, looking for catalysts with higher catalytic efficiency has become a hot research topic. It has been reported that the aluminate spinels have high catalytic activity in the selective catalytic reduction of NOx with hydrocarbon as reducing agent (HC-SCR) under the above condition, but its mechanism in removing the NOx is unclear and need more further research. The applicants have discovered in the theoretical study that ZnGaAlO4 has high catalytic activity, and O2 and other components, such as NO2, NO, CH3, have a synergistic effect when they adsorbed in the surface of ZnGaAlO4. This topic is proposed to establish the adsorption model of aluminate spinel with different metal elements which have 3d electrons by using the DMOL3 and DISOVER module based on the density functional theory (DFT) to obtain the most stable structure and the active atom, and study the adsorption, co-adsorption and desorption mechanism of small molecules, such as CO2, NO, H2O, CxHx, and their multicomponent synergies, and reveal the thermodynamic mechanism of the reaction between oxygen, hydrocarbon and NOx. Furthemore, dynamic model is built to explore whether the molecular oxygen and hydrocarbons promote the reduction of NOx and whether there exists competition or synergies between hydrocarbons and NOx. This has important theoretical significance and practical value for the development of efficient and affordable HC-SCR catalyst used in low temperature.

鉴于传统三元催化剂在富氧、低温下去除NOx效率低下，寻找催化效率更高的催化剂已成为当前研究的热点。在富氧、低温并以HC-SCR还原NOx反应中，铝酸盐尖晶石具有较高催化活性，但其去除NOx的催化还原机理尚未明确，需进一步研究。申请者在前期理论研究中发现ZnGaAlO4具有较高催化活性，O2和NO2、NO、CH3等组分在ZnGaAlO4表面的吸附具有一定的协同效应。本课题拟采用基于密度泛函理论的DMOL3、DISOVER模块，建立含不同3d电子金属元素的铝酸盐尖晶石吸附模型，获得最稳定的吸附结构和催化反应的活性位置，研究小分子在其表面的各种吸附、脱附机理及多组分协同性，揭示氧、碳氢化合物与NOx反应的热力学机理；建立动力学模型，探究不同温度下分子氧和碳氢化合物是否促进NOx的还原及碳氢化合物与NOx之间是否存在竞争、协同效应。这对开发低温下高效廉价HC-SCR催化剂具有重要的理论指导意义。

随着经济的发展，汽车尾气、工厂废气等排放造成环境污染日益加剧，其中NOx是造成酸雨、光化学烟雾、雾霾的主要因素。选择性催化还原（SCR）技术是一种将NOx 固定污染源脱除的技术，以烃为还原剂的HC-SCR，有在富氧环境下催化反应温度低，催化活性高的特点。针对，尖晶石型催化剂具有宽范围的催化活化温度，良好的分散性，热稳定性和水热稳定性的性质和研究热点。.本项研究基于密度泛函理论的第一性原理计算了ZnAl2O4低指数表面的结构，建立表面结构与H2O、NO和 NO2分子在ZnAl2O4低指数表面的吸附行为的关系。发现ZnAl2O4 (100)、(110）和（111）表面的截止原子表面为Zn-O-Al时，表面能最低。ZnA12O4 (100)和（111)表面存在凸出截止原子层且配位数未满的Zn和 Al位点，可作为气体分子的吸附位点。ZnAl2O4 (111)表面Al位点在费米面存在的能级，该能级与H2O、NO、NO2的分子轨道发生杂化叠加后，增加气体分子的吸附稳定性。.过渡族掺杂金属均在一定程度上提升了其分子吸附能力。掺杂提高了NO和NO2在 ZnAl2O4低指数表面的吸附性能，Co、Fe掺杂更为显著。NO和NO2分子在ZnAl2O4低指数表面成单键或双键吸附，双键吸附更为稳定。掺杂金属的3d与Zn 3d 电子态存在协同作用并与NO、NO2 分子轨道杂化叠加提供NO、NO2 吸附的驱动力，且从实验中初步得以验证。.H2O和O2对NOx吸附催化具有协同性。H2O分子在ZnAl2O4低指数表面的吸附是结构-敏感过程，易吸附在凸出截止表面的Zn 原子上，H2O的存在一定程度上可以提高NOx的吸附效果。O2 分子更易在表面与过渡金属形成双配位结构，O2的存在会降低NOx的吸附性能，但会氧化NO，使NOx获得更多电子处于活化状态。ZnGaAlO4(100)面的计算发现增大O2的覆盖率使得C2H4还原NO的反应焓更低，对反应有利。掺杂Fe发现，作为还原剂CH4比C2H4还原NO的反应焓更低，稳定的CH4分子更能催化NO。计算过渡态发现，催化剂表面原子态的O可以大大降低CH4的分解势垒并提高其吸附能力。.因此通过研究，我们证明了以烃为还原剂的HC-SCR，有在富氧环境下Co、Fe掺杂的ZnAl2O4催化反应温度低，催化活性更高的特点，为实验制备铝酸盐尖晶石催化剂提供理论支撑。