新型“似菱沸石催化体系”净化柴油机等稀燃NOx的研究

At present, selective catalytic reduction (SCR) of NO with NH3 is considered to be one of the most promising method for the lean-burn gasoline or diesel engine, however, some problems still remain in SCR-catalysts for their practical application such as low thermal stability, the narrow temperature window and the loss of activity at high temperature. Zeolite-based catalysts promoted by transition metals such as Fe and Cu are being considered for practical applications in diesel emission control due to broader temperature windows and better stability at high temperatures. On the other hand, during operation of diesel engines, the SCR-DeNOx over these zeolite-based catalysts are gradually deactivated by hydrothermal aging and catalyst poisoning by hydrocarbon coking and formation of N2O at high temperatures. More recently, it has been reported in our study that chabazite-like supported Cu and Fe mixed (Cu-Fe/CHA) catalysts are superior catalysts for NO reduction by NH3 for overcoming above drawbacks. However, an obstacle is their low activity at low-temperature, which maybe inhibit their practical application. Therefore, it is needed to develop superior SCR catalysts with high activities at low-temperature with broad temperature window. .To enhance the catalytic activity for low-temperature SCR or broad temperature window, preparation methods will be optimized. Furthermore, some other metals or small amount precious metals are also added to the Cu-Fe/CHA catalysts to enhance the catalytic activity, which modified their surface Lewis acidity, ability of adsorption NO and the redox properties. During operation, new highly active catalysts for NH3-SCR will be gained by the design of catalyst surface distribution, preparation methods and characterization of catalysts, catalytic tests and the research of their mechanism.

对于处理柴油机等稀燃NOx尾气较有前途的NH3-SCR技术来说，目前所用催化剂仍存在催化反应温度窗口窄、低温净化效率低及高温失活等问题。分子筛催化剂具有较宽的反应温度窗口和高的热稳定性，是目前研究的热点。但其存在水热稳定性差、高温N2选择性低及在水汽和碳氢等化合物存在下易失活等缺点。我们近来制备的似菱沸石分子筛催化剂（Cu-Fe/CHA）克服了分子筛以上的不足，是理想的催化材料，但其低温活性不高，限制了其更好地应用。基于此，本研究首先优化Cu-Fe/CHA的制备方法；随后为提高其低温催化活性，将金属氧化物或少量贵金属掺杂改性Cu-Fe/CHA，以调节其Lewis酸性、吸附NO性能及氧化还原性等，通过"协同效应"综合金属氧化物/贵金属和Cu-Fe/CHA的催化反应优点，获得高性能的催化体系。在操作过程中通过对催化剂表面分布的设计、制备和表征、反应活性评价和机理研究，优选出新型高效的催化剂。

机动车尾气排放NOx 净化已成为NOx 总量控制的一个重要领域。对于处理柴油机等稀燃NOx尾气较有前途的NH3-SCR技术来说，目前所用催化剂仍存在催化反应温度窗口窄、低温净化效率低及高温失活等问题。分子筛催化剂具有较宽的反应温度窗口和高的热稳定性，是目前研究的热点。但其存在水热稳定性差、高温N2选择性低及在水汽和碳氢等化合物存在下易失活等缺点。. 本研究制备的似菱沸石小孔分子筛（CHA，SAPO-34、SAPO-18 和 SSZ-13）负载型催化剂（M-Cu-Fe/CHA）克服了以上分子筛的不足，是理想的催化材料。首先通过优化制备方法、调节载体Si/Al比、铜铁负载含量、活性组分表面分布设计来控制催化剂表面的酸性（Lewis和Bronsted）、吸附NO\NH3\O2\H2O的能力及其氧化还原性等，进一步筛选出最优Cu-Fe/CHA催化剂。随后为进一步提高其催化活性，掺杂少量金属改性M-Cu-Fe/CHA，通过“协同效应”综合掺杂金属和Cu-Fe/CHA的催化反应优点，获得高性能的M-Cu-Fe/CHA催化体系。在进行实验的实际操作过程中，我们通过对催化剂表面分布的设计、制备方法改进和各种表征、反应活性评价、水热稳定性、抗积碳、抗硫性能的研究，进一步优选出高效催化消除NOx的优良催化剂。. 另外，我们通过对机理的研究，系统地探求了负载型CHA分子筛的物理化学性质和催化作用之间的关系，认识其中的“协同效应”和催化剂的双功能作用。研究结果表明，负载型CHA催化剂上的NH3-SCR低温反应机理与高温反应机理明显不同。同时，通过各种表征及原位技术系统地研究了水热老化温度、Cu/Al比和Si/Al比对CuSSZ-13催化剂的催化性能和水热稳定性的影响，结果表明，水热老化温度升高、铜/铝比和硅/铝比增加，使催化剂上孤立Cu2+（D6R和CHA）物种转化和数量降低，聚集CuOx物种迁移和CuSSZ-13催化剂结构产生变化，最终导致催化剂水热稳定性变差。.上述关于高效催化剂的制备和负载型CHA催化剂上NH3-SCR反应机理的研究等关键问题的探讨和研究，对进一步开发新型NH3-SCR高效催化净化NOx反应催化剂至关重要，在理论上也是一种有益的探讨，同时为今后催化剂分子设计，以及催化剂制备方法和处理条件的选择，提供有价值的参考数据。