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

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)(3)H2O (extra-framework aluminum) species, whereas for Cu-CHA, formation of framework bound Cu-Al species is thermodynamically preferred over Al(OH)(3)H2O, which results in the increased stability of Cu-CHA. The formation of mobile Al(OH)(3)H2O in Cu-CHA is, moreover, associated with a high energy barrier. The phase diagrams show the formation of Al(OH)(3)H2O 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)(3)H2O + CuO. The microkinetic model shows that the formation of Al(OH)(3)H2O 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)(3)H2O 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.