Abstract:

Ammonia (NH₃) selective catalytic reduction of nitrogen oxides (NH₃-SCR) is currently the most promising diesel exhaust purification technology. The core of this technology is the development of catalysts with excellent catalytic performance. In recent years, Cu modified SSZ-13 zeolites with chabazite (CHA) topology as support have attracted extensive attention due to their excellent catalytic performance and hydrothermal stability. Therefore, in this study, a series of Cu_x/SSZ-13 catalysts with different Cu/Al ratios were prepared. The location and state of Cu ions on SSZ-13 zeolites with high catalytic activity was determined by insitu diffuse reflection infrared spectrum (DRIFT) using CO as probe molecule and H₂-TPR. By means of CO adsorption experiments, it was found that the Cu_x/SSZ-13 catalysts prepared by the ion exchange method of Cu(NO₃)₂ aqueous solution had a variety of Cu⁺ active sites. For catalysts with low Cu ion exchange degree, Cu⁺ firstly located in the micro-channel of SSZ-13 zeolites. As Cu exchange degree increased, Cu⁺ would locate in the double-six-membered ring of SSZ-13 zeolites. H₂-TPR characterization results show that there are also a large number of unstable Cu²⁺ located in the eight membered ring on Cu_x/SSZ-13 catalyst, which can be easily reduced to Cu⁺. For Cu/SSZ-13 catalyst aged by steam at 800℃ for 16 h, most of framework aluminum removed and Cu species agglomerated, both of which resulted in zeolites framework collapse. While, to a certain extent, the addition of Ce can improve the hydrothermal stability of Cu/SSZ-13 and protect the zeolites framework from collapse. The structure-activity relationship of the catalyst shows that the catalyst with a certain amount of stable Cu⁺ and a large amount of unstable Cu²⁺ has catalytic performance for NH₃-SCR in a wide temperature range.

Key words: NH₃-SCR reaction, Cu/SSZ-13 catalysts, Cu ion, reactivity

摘要：

NH₃选择性催化还原NO_x（NH₃-SCR）是目前最有应用前景的柴油车尾气净化技术，该技术的核心是开发具备优异催化性能的催化剂。以具有菱沸石（chabazite，CHA）结构的小孔分子筛SSZ-13为载体制备的Cu/SSZ-13催化剂，因具有优异的催化性能和水热稳定性能而受到广泛关注。制备了系列Cu_x/SSZ-13催化剂，并通过CO原位漫反射红外光谱（DRIFT）和H₂-TPR等方法能够确定具有高催化活性的铜离子在SSZ-13分子筛上的落位和存在状态。CO红外吸附实验发现，采用Cu(NO₃)₂水溶液离子交换法制备的Cu/SSZ-13催化剂上存在多种落位的Cu⁺活性中心。在较低的Cu⁺交换度条件下，Cu⁺优先落位于SSZ-13分子筛的八元环位置，随着交换度的提高，Cu⁺开始落位于SSZ-13分子筛双六元环的位置。H₂-TPR结果表明Cu_x/SSZ-13催化剂上也存在大量落位在八元环位置不稳定的Cu²⁺，这些Cu²⁺很容易被还原为Cu⁺。Cu_x/SSZ-13催化剂经800℃水蒸气连续老化16 h，分子筛骨架崩塌程度随着Cu含量的增加而提高，骨架铝的脱除，导致Cu物种发生团聚，而第二金属Ce元素的引入能够在一定程度上提高Cu/SSZ-13的水热稳定性。催化剂构效关系研究表明，具有一定量稳定存在的Cu⁺，并且拥有大量不稳定存在Cu²⁺的催化剂具有较宽温度范围的脱硝性能。

关键词: NH₃-SCR 反应, Cu/SSZ-13催化剂, 铜离子, 活性