Millan Reisel, Cnudde Pieter, van Speybroeck Veronique, Boronat Mercedes
Instituto de Tecnología Química, Universitat Politècnica de València-Consejo Superior de Investigaciones Científicas, Avenida de los Naranjos s/n, 46022 València, Spain.
Center for Molecular Modeling, Ghent University, Technologiepark 46, 9052 Zwijnaarde, Belgium.
JACS Au. 2021 Sep 17;1(10):1778-1787. doi: 10.1021/jacsau.1c00337. eCollection 2021 Oct 25.
The mobility of the copper cations acting as active sites for the selective catalytic reduction of nitrogen oxides with ammonia in Cu-CHA catalysts varies with temperature and feed composition. Herein, the migration of [Cu(NH)] complexes between two adjacent cavities of the chabazite structure, including other reactant molecules (NO, O, HO, and NH), in the initial and final cavities is investigated using ab initio molecular dynamics (AIMD) simulations combined with enhanced sampling techniques to describe hopping events from one cage to the other. We find that such diffusion is only significantly hindered by the presence of excess NH or NO in the initial cavity, since both reactants form with [Cu(NH)] stable intermediates which are too bulky to cross the 8-ring windows connecting the cavities. The presence of O modifies strongly the interaction of NO with Cu. At low temperatures, we observe NO detachment from Cu and increased mobility of the [Cu(NH)] complex, while at high temperatures, NO reacts spontaneously with O to form NO. The present simulations give evidence for recent experimental observations, namely, an NH inhibition effect on the SCR reaction at low temperatures, and transport limitations of NO and NH at high temperatures. Our first principle simulations mimicking operating conditions support the existence of two different reaction mechanisms operating at low and high temperatures, the former involving dimeric Cu(NH)-O-Cu(NH) species and the latter occurring by direct NO oxidation to NO in one single cavity.
在Cu-CHA催化剂中，作为氨选择性催化还原氮氧化物活性位点的铜阳离子的迁移率随温度和进料组成而变化。在此，利用从头算分子动力学（AIMD）模拟结合增强采样技术，研究了菱沸石结构两个相邻空穴之间[Cu(NH)]配合物的迁移，包括其他反应物分子（NO、O、HO和NH）在初始空穴和最终空穴中的迁移，以描述从一个笼到另一个笼的跳跃事件。我们发现，这种扩散仅在初始空穴中存在过量NH或NO时受到显著阻碍，因为这两种反应物都与[Cu(NH)]形成稳定的中间体，这些中间体体积太大，无法穿过连接空穴的8元环窗口。O的存在强烈改变了NO与Cu的相互作用。在低温下，我们观察到NO从Cu上脱离，[Cu(NH)]配合物的迁移率增加，而在高温下，NO与O自发反应形成NO。目前的模拟为最近的实验观察提供了证据，即低温下NH对SCR反应的抑制作用，以及高温下NO和NH的传输限制。我们模拟操作条件的第一性原理模拟支持了在低温和高温下存在两种不同的反应机制，前者涉及二聚体Cu(NH)-O-Cu(NH)物种，后者通过在单个空穴中将NO直接氧化为NO而发生。
