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 NOx absorber (PNA) in the after-treatment of vehicles due to its considerable NOx 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 TEM and H2-TPR results show that CeOx 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 CeOx. This is the primary reason for the hydrothermal aging deactivation of Ce/BEA.