Ultra-stable Pd ions at Al T1/T2 sites on a dealuminated Pd/beta passive NOx adsorber

Abstract

Pd ions as NO adsorbing sites on zeolites for PNA materials showed evident deactivation when exposed to reducing agents or treated under hydrothermal conditions. In this study, stable Pd²⁺ species are mainly obtained by loading palladium on a dealuminated beta zeolite, and can survive during cyclic evaluations with reducing agents and hydrothermal treatment at 850 °C. On the dealuminated beta zeolite, a large amount of framework Al T1/T2 is reserved by design. CO-DRIFTS and H₂-TPR results reveal that [Pd–OH] and isolated Pd(iso)_b are mainly present on the dealuminated beta zeolite and are quite stable because of their higher energy. These [Pd–OH]/Pd(iso)_b on the dealuminated beta zeolite contribute stable NO_x storage performance. ²⁷Al MAS NMR suggests that the Al T1/T2 sites of PDB-F are more stable and the [Pd–OH]/Pd(iso)_b simultaneously protects the occupied Al T1/T2 sites from being destroyed. NH₃-TPD shows that [Pd–OH]/Pd(iso)_b on the dealuminated beta zeolite contributes a more stable acid concentration after cycle evaluation or hydrothermal aging. Thus, the dealuminated Pd/beta materials, with ultra-stable Pd²⁺ species, can meet real exhaust conditions.