The superior hydrothermal stability of Pd/SSZ-39 in low temperature passive NOx adsorption (PNA) and methane combustion

Uniform SSZ-39 (Si/Al ratio similar to 12) crystals with an average size of about one micron were synthesized and used to support Pd (0.7-3 wt%) for PNA. The as-synthesized materials were characterized by FTIR, XRD, Helium Ion Microscopy, HAADF-STEM imaging, Al-27, Si-29, and H-1-Si-29 CP solid state NMR spectroscopic techniques. FTIR studies with CO and NO probe molecules reveal that the majority of Pd is dispersed as isolated Pd(II) and Pd (II) - OH centers and as such is suitable as a low-temperature passive NOx adsorber. Pd(II)-NO, Pd(II)(OH)(NO), and Pd(II)(CO)(NO) complexes form in this material during PNA. Comparison to Pd/SSZ-13 (Si/Al similar to 12) shows the superior hydrothermal stability of this new material, surviving hydrothermal aging up to 815 degrees C in 10 % H2O/Air for 16 h without a significant loss of activity. The SSZ-39 crystal structure remains intact during hydrothermal aging up to 1000 degrees C confirmed by XRD and HAADF-STEM imaging/EDS mapping. However, changes to the framework, as evidenced by high-field 27 Al NMR, during such severe hydrothermal treatment significantly alter the NOx release profiles. Besides PNA, this hydrothermally stable material (3 wt% Pd on SSZ-39; Si/Al similar to 12) can be used as a robust methane combustion catalyst under practically relevant conditions (i.e., GHSV similar to 300 L/g*hr). This catalyst shows minimal deactivation after both hydrothermal aging at 750 and 800 degrees C and prolonged time on stream (105 h) at 425 degrees C. In contrast, both 3 wt% Pd/alumina and 3 wt% Pd/SSZ-13 lose a significant portion of their activity under such conditions, marking an improvement over current technology.