Effects of Treatment Conditions on Pd Speciation in CHA and Beta Zeolites for Passive NO_xAdsorption

The structure and evolution of Pd species in Pd-exchanged zeolite materials intended for use as passive NOx 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 NOx adsorption applications, especially in the case of Beta zeolite.