Mechanism for Cu-enhanced hydrothermal stability of Cu–CHA for NH3-SCR†

Abstract

Exposure of acidic zeolite-based catalysts to water at high temperatures generally leads to deactivation due to dealumination. In Cu–CHA zeolite, which is a preferred catalyst for the selective catalytic reduction of NO by NH₃ (NH₃-SCR), the acidic protons in the zeolite are partially exchanged by Cu ions. The presence of Cu has been measured to reduce the rate of dealumination, thus stabilizing the catalyst. To understand the stabilizing effect of Cu, density functional theory calculations, ab initio thermodynamics and microkinetic modeling are used to compare the reaction mechanism for the dealumination of H–CHA to Cu–CHA. For H–CHA, we find that dealumination leads to the formation of mobile Al(OH)₃H₂O (extra-framework aluminum) species, whereas for Cu–CHA, formation of framework bound Cu–Al species is thermodynamically preferred over Al(OH)₃H₂O, which results in the increased stability of Cu–CHA. The formation of mobile Al(OH)₃H₂O in Cu–CHA is, moreover, associated with a high energy barrier. The phase diagrams show the formation of Al(OH)₃H₂O and Al₂O₃ from H–CHA and that high temperatures favor the formation of Al₂O₃. For Cu–CHA, high temperatures lead to the formation of CuO and Al₂O₃, which is favored over Al(OH)₃H₂O + CuO. The microkinetic model shows that the formation of Al(OH)₃H₂O in the presence of water starts at 380 K and 800 K in H–CHA and Cu–CHA, respectively. Additionally, the time evolution of the Al(OH)₃H₂O coverage at 923 K reveals that the process of dealumination is significantly faster for H–CHA as compared to Cu–CHA, which is in accordance with the measured increased stability.