柴油机NOx存储耦合在线选择性催化还原的协同机理研究

The NOx storage coupled with in-situ selective catalytic reduction (NSR-SCR) is a combination system, which provides the benefits of simple structure, no external source of NH3, low precious metal consumption, and high NOx abatement efficiency. However, the mechanism of NOx reduction, especially for the synergetic mechanism of NH3 and HC over SCR catalyst, is not clear. On the basis of previous studies on NSR-only system within our group, Pt/Pd/Rh based storage and reduction catalyst and Cu-zeolite, Fe-zeolite based SCR catalyst are prepared and characterized for lean/rich transient and steady state experiments. Simulated diesel exhaust gases are employed both in transient and steady state experiments to characterize the processes of NOx reduction and the formation of the key intermediates. Some crucial kinetic parameters of elementary reactions are to be determined by density functional theory (DFT) and unity bond index-quadratic exponential potential (UBI-QEP) methods. Multi-dimensional modeling, from molecular level to multi-dimensional reactive flow scale, is carried out for thoroughly studying the NSR-SCR system. The model validation is performed by comparing the simulation results and the data achieved from diesel bench test. The purpose of this project is to explore possible solutions for minimizing the amount of precious metal and improving the low and high temperature activity of the catalyst. It is expected the project will offer some fundamental support for systematic design of NSR-SCR system.

NOx存储耦合在线选择性催化还原（NSR-SCR）技术具备结构简单、无需额外氨源、较低的贵金属使用量和较高的NOx去除效率等优点，然而该系统对NOx的协同去除机理，尤其是NH3/HC-双SCR反应机理并不明确。在前期研究NSR系统的基础上，制备并表征以Pt/Pd/Rh为主要活性成分的NSR催化剂及Cu分子筛、Fe分子筛SCR催化剂，设计模拟尾气成分稀、浓瞬变实验，研究关键组分和中间产物的生成规律及反应过程。综合运用微观反应动力学原理、密度泛函理论（DFT）、键指数归一-二次指数势（UBI-QEP）方法，确定关键反应步及动力学参数，建立表面催化详细化学反应动力学机理。对NSR-SCR系统进行从分子尺度到多维反应流动尺度的全过程多尺度仿真研究，开展发动机台架实验验证催化剂小样实验及模拟结果。探索降低贵金属使用量及提高催化剂低温、高温活性的方案，为优化NSR-SCR系统提供理论基础和技术支持。

NOx存储耦合在线选择性催化还原（NSR-SCR）技术能够实现更高的NOx转化率且降低了贵金属的使用量。然而对该系统的NOx协同去除机理以及非氨组分在NSR-SCR耦合系统中的作用并不明确。本项目在深入理解NSR-SCR耦合反应体系复杂性的基础上，对该体系进行了解耦研究以NH3的生成与转化作为实现NOx存储还原耦合SCR反应的关键。通过试验和数值仿真等方法对NSR催化剂进行了研究，探索了铂催化剂（Pt）表面H2、CO还原NO的详细反应机理，并阐明了NH3在NOx存储还原过程中的生成与转化规律。通过催化剂小样试验考察了Cu基SCR催化剂的性能，表明Cu分子筛催化剂具有较好的低温SCR活性且表面至少含有两种不同类型的吸附活性位；实现了Cu基催化剂反应动力学模型与三维流体动力学的耦合研究，综合考察了尿素水溶液的雾化质量和催化剂的NOx转化性能；开展模拟研究分析了Cu分子筛催化剂经水热老化后的SCR反应特性。对比分析了Fe分子筛催化剂和Cu分子筛催化剂的性能，发现Fe分子筛催化剂高温SCR活性更好；发展的Fe-Cu分区及分层组合SCR催化剂综合了两种催化剂的优势，拓宽了SCR反应的活性温窗且降低了N2O的生成。基于对Pt催化剂反应机理和分子筛催化剂NH3-SCR反应机理的理解，发展了Fe-Pt组合双功能催化剂模型，研究并阐明了分子筛催化剂选择性催化还原与Pt催化剂氧化反应的协同作用机理。本项目的研究结果为NSR-SCR系统在实际中的应用提供了理论支撑和指导，也为Fe-Cu组合催化剂在高效SCR系统中的应用奠定了很好的理论基础，同时也推动了Fe（Cu）-Pt组合双功能催化剂在实际中的使用。