Effect of various activation conditions on the low temperature NO adsorption performance of Pd/SSZ-13 passive NOx adsorber

Various activation conditions such as temperature, time and the concentration of H2O during hydrothermal aging (HTA) treatment were extensively investigated to identify the proper ion-exchange conditions of Pd/SSZ-13 catalyst for low temperature NO adsorption performance. In addition, we examined how the cation in SSZ-13 (i.e. NH4-, Na- or K-SSZ-13) affects the low temperature NO adsorption performance of Pd/SSZ-13 after HTA treatment. It is found that NO adsorption ability begins to increase substantially when Pd/SSZ-13 is hydrothermally aged above 650 °C. Pd/SSZ-13 demonstrated the maximum NO adsorption performance after hydrothermal aging treatment in the range of 700–750 °C, followed by the severe decline above 850 °C. Such deactivation is attributed to the agglomeration of Pd species due to the dealumination of SSZ-13, as indicated by XRD and NMR analysis. At least 15 h of HTA treatment at 750 °C is required to provide the optimum activation of Pd/SSZ-13. Water vapor is essential component to activate Pd/SSZ-13 during the hydrothermal aging treatment. Unlike NH4-SSZ-13, Na- or K-SSZ-13 cannot be activated even if the optimum hydrothermal aging condition (750 °C for 25 h with 10% H2O) is applied. EXAFS and XRD analysis provides the evidence about the sintering of Pd species of the Pd/Na- or Pd/K-SSZ-13 catalysts rather than Pd ion exchange, indicating that the pre-existing Na or K ion inhibits the ion exchange during hydrothermal aging treatment. It can be concluded that the selection of proper hydrothermal aging conditions and zeolite form is essentially required to obtain the excellent low temperature NO adsorption performance of Pd/SSZ-13 as a passive NOx adsorber.