Effects of Treatment Conditions on Pd Speciation in CHA and Beta Zeolites for Passive NO _x Adsorption

The structure and evolution of Pd species in Pd-exchanged zeolite materials intended for use as passive NO _x adsorbers were examined under various pretreatment conditions. Using in situ CO-diffuse reflectance infrared spectroscopy, Pd structures were characterized after 500 °C pretreatments in inert (Ar), water (1-2% H₂O in Ar), oxidizing (air), and reducing (H₂, CO) atmospheres. Two zeolites of similar Si/Al ratios but different framework topologies (Beta, CHA) were found to show different distributions of Pd species, depending on the reducing agent used. Reduction in H₂ (500 °C; 10% H₂ in Ar) followed by re-oxidation (500 °C; air) led to higher amounts of single-site Pd ions on Pd-CHA than Pd-Beta, whereas high-temperature reduction in CO (500 °C; 1000 ppm CO in Ar) followed by re-oxidation (500 °C; air) led to significant loss of ionic Pd on both Pd-CHA and Pd-Beta, albeit H₂ temperature-programmed reduction and XPS experiments suggest that this phenomena may be limited to surface Pd. High-temperature treatments with water (500 °C; 1-2% H₂O in Ar) are shown to form either Pd metal or PdO particles, with Pd-Beta being more susceptible to these effects than Pd-CHA. This work suggests that the effects of CO are especially problematic with respect to the durability of these materials in passive NO _x adsorption applications, especially in the case of Beta zeolite.