Cu/CHA催化剂酸性调控及中温段NH3-SCR反应特性研究

Cu/CHA catalysts, Cu/SSZ-13 and Cu/SAPO-34, have become the first choice in SCR research due to their superior SCR activity, high hydrothermal durability and good regeneration ability under sulfur poisoning. However, the declined SCR activity in the middle temperature region restricts its future application and the cause needs to be further studied. According to our previous study, we found ammonia would migrate from Brønsted to Lewis acid sites to participate in NH3-SCR reaction in the middle temperature region and therefore, speculate the ammonia migration can directly influence NH3-SCR mechanism in the middle temperature region. To justify the above suggestion, one of the focuses of this work is to understand the structural-activity relationship between acidity properties and the NH3-SCR performance by regulating the acidity properties over CHA. Moreover, using both in-situ technology and TPSR method, the key active intermediates and the elementary reaction during SCR reaction can be clearly revealed. Naturally, the ammonia migration effect on elementary reaction of NH3-SCR processes can be also obtained. The last but not the least, using first principle studies, the microcosmic cause of NH3-SCR mechanism in the middle temperature region can be clearly obtained by establishment the NH3-SCR model. The project will expand our horizons of SCR mechanism over Cu/CHA catalysts as well as will provide a new idea for illumination the NH3-SCR mechanism in the middle temperature region.

Cu/CHA催化剂已成为新一代柴油车用尾气脱硝催化剂首选。然而在中温段Cu/CHA催化剂却表现出先降后升NH3-SCR反应特性，该现象严重影响催化剂脱硝效率，抑制Cu/CHA催化剂高效利用。质子酸在反应中起到的动态微观催化作用对SCR反应有重要影响。前期研究发现质子酸在中温段会引发氨迁移现象，由此推测氨迁移现象是导致中温段NH3-SCR反应特性的重要原因。为证实以上假说，本项目采用可控制备技术调控Cu/CHA催化剂酸性，首先明确酸中心性质与中温段反应特性间构效关系；进而利用原位表征技术结合程序升温手段，通过建立氨迁移及反应定量描述方法，明确氨迁移对中温段NH3-SCR基元反应步的影响规律，获得中温段反应决速步；最后，结合实验表征，采用量子计算手段，建立中温段NH3-SCR反应机理模型，从而在电子水平上还原NH3-SCR真实反应机理。本项目为阐明中温段NH3-SCR反应特性提供新思路。

本项目主要考察了Cu/CHA催化剂在NH3-SCR活性评价过程中温段不寻常反应的形成机理，在动力学测试过程中，TOF曲线在225-300℃之间出现一个平台，且不受Cu含量，反应物NO、NH3和O2浓度的影响。NH3滴定结果证明NH3能从Brønsted酸性位迁移至Lewis酸性位上参与反应，且通过对In-situ DRIFTS实验结果的半定量计算发现，在225-275℃范围内NH3迁移速率与NH3-SCR反应速率几乎相等，说明在该温度段NH3-SCR反应的速控步骤是NH3迁移。同时，NH3迁移速率同时受NH3覆盖度CNH3和迁移常数k两个因素影响，而NH3覆盖度随温度的升高而升高，迁移常数随温度的升高而降低，故NH3迁移速率在中温段为常数，不受温度影响，形成活性评价中的特殊反应特性。.此外，样品酸性对NH3-SCR反应在不同温度段存在不同影响：在低温段，Brønsted及Lewis酸性位上吸附足够多NH3，该温度段速控步骤为表面反应，且在一系列不同K含量的Cu/K-CHA样品中TOF值不受Brønsted酸性位含量影响，这是由于Lewis酸性位上吸附足够多NH3，Brønsted酸性位未参与到反应中。在中温段，能有效吸附NH3的Lewis酸性位减少，NH3开始从Brønsted酸性位迁移至Lewis酸性位上参与反应，同时NH3迁移速率比表面反应速率慢，NH3迁移为速控步骤，且NH3迁移速率在该温度段为常数，与温度无关，形成TOF曲线中的平台。在高温段，TOF值随温度的升高而升高，且随着CNH3的减小而不断减小，可以预测该温度段的速控步骤仍是NH3迁移，且NH3迁移速率随温度的升高而升高，这是由于在高温段迁移常数k迅速增加。此外，在一系列不同K含量的Cu/K-CHA样品中TOF值随Brønsted酸性位含量的减少而减小，说明Lewis酸性位上吸附NH3极少，Brønsted酸性位在高温段的反应中起到重要作用。在此基础上，利用第一性原理计算，初步对最稳定Cu位点、单分子吸附行为模拟及Cu氧化还原过程进行模拟计算，从而得到了SCR反应机理图。