PNA耦合SCRF降低柴油机尾气NOx及PM的机理研究

SCRF (SCR coated on filter) provides the benefits of fast light-off, packaging flexibility and reduction of the aftertreatment system volume and cost. However, in order to meet even more stringent emissions and fuel economy regulations, it is necessary to further reduce the NOx emissions during cold start conditions. The passive NOx adsorber (PNA) can adsorb NOx at low temperatures. Therefore, PNA coupled with SCRF is used to reduce NOx emissions over wide conditions. Pd-Zeolites based PNA catalysts are prepared and characterized. Catalyst sample tests are conducted to study the storage capacity, storage efficiency and desorption efficiency of NOx. The mechanism of impacts of PNA on NOx reduction and PM oxidation in SCRF is further studied. Some crucial kinetic parameters of elementary reactions can be determined and the reaction kinetic model are developed to investigate the PNA and SCRF. Numerical simulation is carried out to study the PNA and SCRF system. The engine bench test is conducted to study the mechanism of NOx and PM reduction in PNA coupled with SCRF system, which is helpful to achieve ultra low emissions and provides the theoretical basis and technical support for full aftertreatment system optimization.

选择性催化还原过滤器(SCRF)具有起燃快、安装灵活性好、体积小及节约成本等优势。然而为满足更加严格的排放及燃油经济性法规，需要进一步降低低温冷启动工况NOx排放。被动式氮氧化物吸附催化剂（PNA）能够在低温阶段存储NOx，因此拟将PNA与SCRF耦合使用来降低更宽发动机工况范围内的NOx排放。制备并表征Pd分子筛PNA催化剂，开展催化剂小样试验，研究PNA催化剂的NOx存储能力、吸附效率及解离效率。进一步探究PNA耦合SCRF系统中PNA对SCRF内NOx还原及PM氧化影响的机理。确定关键反应及其动力学参数，建立反应动力学机理模型，对PNA催化剂和SCRF进行数值仿真研究。开展发动机台架试验，研究PNA耦合SCRF系统降低尾气中NOx及PM的机理，实现超低排放，为优化柴油机后处理系统提供理论支撑和技术支持。

长期以来，柴油机的NOx和PM排放问题是人们关注的重点。选择性催化还原过滤器(SCRF)将SCR催化剂涂覆在颗粒捕集器(DPF)载体上，能够同时去除NOx和PM，相比于传统的DPF+SCR系统，能够减少后处理系统的热损失，从而使SCR快速起燃。通过耦合被动氮氧化物吸附催化剂（PNA），能够降低冷启动阶段的NOx排放，以满足未来超低排放法规的技术要求。建立Cu分子筛催化剂三吸附位NH3吸附模型，定量预测了催化剂NH3-TPD过程，深入研究了NOx选择性催化还原机理。考虑到目前SCRF因采用主动再生而带来的相比于传统SCR更为严重的水热老化问题，本项目从催化剂老化过程活性位迁移的角度发展了反应动力学模型，阐述了水热老化导致催化剂性能劣化的深层原因，成功地预测了催化剂老化后的性能。通过开展SCR催化剂小样试验，系统地研究了SCR催化剂的性能，进一步验证了SCR反应机理模型的可靠性。进而基于台架试验初步验证了模型的准确性，对开展后续的台架试验提供指导。针对SCRF中的碳烟氧化机理，首先开展了DPF中碳烟氧化机理的研究，通过耦合碳烟氧化机理和NOx还原机理，建立了SCRF总包反应模型，研究了NO2/NOx比、灰分对SCRF中NOx转化率的影响。此外，前期建立的铂催化剂（Pt）表面CO-H2-NO详细反应机理已经考虑了N2O、NH3等重要中间产物的生成机理。本项目为了研究PNA中CO对NO吸附的影响机制，新引入了HNCO的生成机理。研究发现Pt表面CO与NO反应生成的NCO(s)在促进NO吸附的同时也提高了NO的脱附温度。最后，开展了PNA+SCRF耦合系统的研究，探讨了PNA在使用过程中HC、贵金属负载量和贵金属种类对SCRF入口NO2/NOx比的影响。本项目的研究加深了对PNA+SCRF系统的理解，并为该系统的应用奠定了理论基础。