Investigation of Ce/BEA as a passive NOx adsorber: 2. Hydrothermal aging deactivation mechanism

Ce/BEA has the potential to be applied as a novel passive NO_x absorber (PNA) in the after-treatment of vehicles due to its considerable NO_x storage capacity. However, as a vehicle exhaust after-treatment material, it must withstand the test of long-term hydrothermal aging. This work examined the deactivation mechanism of Ce/BEA during hydrothermal aging. 3.0 wt% Ce/BEA was prepared using the ion-exchange method, and then subjected to hydrothermal treatment at 650 °C with 10% H₂O for 1–12 h to obtain samples with different aging extent. For comparison, the H-BEA support was aged under the same conditions. Brunauer-Emmett-Teller (BET) method, X-ray diffraction (XRD), NH₃ temperature programmed reduction (NH₃-TPD), ²⁷Al MAS nuclear magnetic resonance (²⁷Al MAS NMR), H₂ temperature programmed reduction (H₂-TPR), and high resolution-transmission electron microscopy (HR-TEM) were performed to characterize the changes in PNA performance, structure, Ce species, and acidity. The HR-TEM and H₂-TPR results show that CeO_x particles appear after hydrothermal aging, which results from the detachment and aggregation of active Ce species. Based on the ²⁷Al MAS NMR results, we conclude that BEA zeolite dealumination leads to the loss of acidic sites and the transformation of active Ce species on the acidic sites into the less active CeO_x. This is the primary reason for the hydrothermal aging deactivation of Ce/BEA.