Linking hydrothermal deactivation of Cu-CHA catalysts for NH3-SCR to dealumination and reduced [Cu2(NH3)4O2]2+formation

Abstract

NH3-Selective Catalytic Reduction (NH3-SCR) is a critical technology for control of NOx emissions from diesel and H2 engines. Cu-exchanged chabazite (Cu-CHA) materials are preferred catalysts for the NH3-SCR reaction thanks to high activity, selectivity and stability during normal operating conditions. However, exposure of Cu-CHA to water at high temperatures (>650 ^○C) leads to dealumination, which deactivates the catalysts. Here, we have measured the activity and selectivity of hydrothermally aged Cu-CHA. The hydrothermal deactivation is linked to changes in the amount of Brønsted acid sites as measured by NH3-Temperature-Programmed Desorption (TPD) and the ability to form the key intermediate [Cu2(NH3)4O2]2+, as probed by NO-Temperature-Programmed Reaction (TPR). The activity decreases rapidly during the first 10 hours of aging, which correlates with a rapid loss of Brønsted acid sites. The activity after 10 hours is reduced at a slower rate, which coincides with a loss in ability to form [Cu2(NH3)4O2]2+. The measurements are corroborated by density functional theory based microkinetic modeling. The simulations show that the part of the reaction cycle that occur over Brønsted acid sites in fresh catalysts can occur at a slower rate over silanol nests in the aged catalyst. Roughly half of the activity loss after 100 hours of aging can be attributed to the decrease of Brønsted acid sites, whereas the remaining loss can be related to a reduced formation of the [Cu2(NH3)4O2]2+ complex.