| 题名 | Cu基小孔分子筛催化剂用于NH3选择性催化还原NOx的研究 |
| 作者 | 谢利娟 |
| 学位类别 | 博士 |
| 答辩日期 | 2014-05 |
| 授予单位 | 中国科学院研究生院 |
| 授予地点 | 北京 |
| 导师 | 贺泓 |
| 关键词 | 选择性催化还原 Cu-SSZ-13 一步合成法 NOx去除 柴油车尾气 selective catalytic reduction Cu-SSZ-13 one-pot synthesis method NOx removal diesel engine exhaust |
| 其他题名 | Selective Catalytic Reduction of NOx with NH3 over Cu/zeolites Catalysts with Small-pore Structure |
| 学位专业 | 环境科学 |
| 中文摘要 | NH3选择性催化还原 NOx（NH3-SCR）是目前最有效的柴油车尾气 NOx净化技术之一，NH3-SCR催化剂是该技术的核心组件。开发高效、稳定、环境友好的 NH3-SCR催化剂对提高柴油车尾气 NOx的净化效率具有非常重要的意义。由于 Chabazite (CHA)结构的铜基小孔分子筛催化剂 Cu-SSZ-13具有催化活性优、水热稳定性高、抗碳氢化合物(HCs)中毒能力好等优点，引起学者们的密切关注。但合成 CHA分子筛材料所用的有机模板剂价格昂贵，制约了该催化材料的推广应用。近期有研究者采用低廉的新型铜胺络合物为模板剂，成功通过一步合成法制备了 Cu-SSZ-13催化剂，值得深入研究。 本论文首先通过调整初始凝胶的配比，改良了原有一步合成法的配方，在降低模板剂用量的情况下制备高结晶度的纯相 Cu-SSZ-13初始样品。采用硝酸铵溶液和稀硝酸溶液离子交换法，进一步优化了初始样品，显著提高了原催化剂的高温活性并拓宽了温度窗口，优选的催化剂具备优异的抗高空速能力和良好的水热稳定性，对H2O、CO2和C3H6的共存也不敏感。通过 X射线衍射（XRD）、H2 程序升温还原（H2-TPR)和电子顺磁共振（EPR）等手段研究发现，具有强氧化还原能力和高稳定性的孤立 Cu2+是催化剂表现出优异 SCR活性的原因。 后处理方法在调整Cu负载量的同时也调整了催化剂中的Na+含量。研究Cu-SSZ-13催化剂中残存 Na+含量对其水热稳定性的影响规律，发现催化剂水热稳定性下降的重要原因是 Na+含量过高导致的催化剂中 Cu物种稳定性的下降。因此，采用一步合成法制备 Cu-SSZ-13催化剂时，应避免过多的 Na+残留于催化剂中。 不同于氧化物催化剂和其他分子筛催化剂，Cu-SSZ-13催化剂上不存在“快速 SCR”效应。NO2的存在对 NH3-SCR反应有较大的抑制作用，直接添加 NO2不是提高该催化剂催化活性的有效手段。原位漫反射傅立叶变换红外光谱（In situ DRIFTS）的研究表明，NO2的抑制效应是由 NH4NO3在催化剂表面沉积引起的。NOx在活性中心Cu位上的吸附是催化反应的关键步骤。由于 NO2的动力学直径较大，不能顺畅接触到活性位，部分未能及时反应的NO2可与吸附在Brønsted酸位上的NH4+结合生成NH4NO3。这是催化剂低温活性差的主要原因。该催化剂在 SCR反应中不依赖“快速 SCR”的反应路径。 一步合成法制备 Cu-SSZ-13催化剂的催化性能可与液相离子交换法制备催化剂相媲美。考虑到该催化剂的成本优势，一步合成制备 Cu-SSZ-13催化剂在柴油车尾气 NOx控制领域具有非常好的应用前景。 |
| 英文摘要 | The selective catalytic reduction of NOx with NH3 (NH3-SCR) is one of the most promising technologies for NOx emission control from diesel engine exhaust. The NH3-SCR catalyst is the key component of the system. Thus, the development of environmentally-benign NH3-SCR catalysts with excellent performance and stability is central to increasing the NOx removal efficiency from diesel exhaust. Cu-SSZ-13 catalysts with small-pore Chabazite zeolite structure have attracted much attention due to their high NH3-SCR activity, good hydrothermal stability and resistance to small HCs. The template used for the synthesis of CHA zeolite cost very high, which is the main impediment to the application of the material. The one-pot synthesis method, using Cu-amine complex as novel template, greatly decreases the cost of Cu-SSZ-13 catalysts, and is worthy of in-depth research. From the base of the original one-pot synthesis method, the procedure was further improved to decrease Cu loading in the initial Cu-SSZ-13 sample. A post-treatment procedure with NH4NO3 or HNO3 solution was carried out to optimize the Cu loading in the catalysts, which increased NH3-SCR activity at high temperatures greatly and widened the operation temperature window. The optimal catalyst obtained also showed excellent resistance to high gas hourly space velocity (GHSV) and good hydrothermal stability, and was also insensitive to H2O, CO2 and C3H6. The XRD, H2-TPR and EPR results showed that isolated Cu2+ with high redox ability and stability is very important for the high SCR activity of the catalyst. The post-treatment procedure adjusted the Cu content in the catalysts as well as the Na content. The effects of Na+ ions on the hydrothermal stability of the catalysts were investigated. High Na+ content in the catalyst decreased the stability of Cu species severely, which resulted in the poor hydrothermal stability of the catalyst. Thus, it is better to avoid excessive Na+ remaining in the catalyst when the one-pot synthesis method is used to prepare Cu-SSZ-13 catalyst. “Fast SCR” is a common phenomenon over oxide catalysts and other zeolite catalysts. However, it was not observed with the optimal one-pot-synthesized Cu-SSZ-13 catalyst. NO2 inhibited the SCR activity of the catalyst, and increasing the NO2 ratio directly in the feed gas is thus not an effective way to improve its NH3-SCR performance. The results of in situ DRIFTS indicated that NH4NO3 deposition was thereason for the inhibitory effect. The adsorption of NOx on Cu sites was the key step in the reaction. Due to the large kinetic diameter of NO2, NO2 could not establish good contact with active sites, and part of it combined with NH4+ on Brønsted acid sites to form NH4NO3. This is the main reason the catalyst showed poor activity at low temperatures. “Fast SCR” reaction path is not suitable for the optimal catalyst. The Cu-SSZ-13 catalyst prepared by the one-pot synthesis method showed equivalent NH3-SCR performance to that prepared by the liquid ion exchange method. Considering the cost, it is believed that the Cu-SSZ-13 catalyst prepared by the one-pot synthesis method is a very promising catalyst for the removal of NOx from diesel engine exhaust. |
| 公开日期 | 2015-06-12 |
| 内容类型 | 学位论文 |
| 源URL | [http://ir.rcees.ac.cn/handle/311016/13462]  |
| 专题 | 生态环境研究中心_大气污染控制中心 |
| 推荐引用方式 GB/T 7714 | 谢利娟. Cu基小孔分子筛催化剂用于NH3选择性催化还原NOx的研究[D]. 北京. 中国科学院研究生院. 2014. |
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