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

Abstract

Uniform SSZ-39 (Si/Al ratio ∼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, 27Al, 29Si, and 1H-29Si 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 ∼12) shows the superior hydrothermal stability of this new material, surviving hydrothermal aging up to 815 °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 °C confirmed by XRD and HAADF-STEM imaging/EDS mapping. However, changes to the framework, as evidenced by high-field 27Al 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 ∼12) can be used as a robust methane combustion catalyst under practically relevant conditions (i.e., GHSV∼300 L/g*hr). This catalyst shows minimal deactivation after both hydrothermal aging at 750 and 800 °C and prolonged time on stream (105 h) at 425°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.