Influencing the NOx Stability by Metal Oxide Addition to Pd/BEA for Passive NOx Adsorbers

Passive NOx adsorption performance was investigated for Pd-based BEA zeolite as a reference sample along with ceria, zirconium, and lanthanum as different promoters. The La-promoted sample showed favorable NOx desorption behavior regarding greater quantities of NOx release at temperatures higher than 200 degrees C. The study was therefore mainly focused on the La-promoted sample and Pd/BEA, since the largest effect was seen using La as a promoter. The effect of water and hydrogen pretreatments for passive NOx adsorption (PNA) processes was therefore studied for La-Pd/BEA and compared with that for pure Pd/BEA. It was shown that the presence of water reduces the NOx desorption at lower temperature. Hydrogen pretreatment decreases the NOx uptake and shifts the adsorption and desorption temperatures to higher values. Interestingly, the La-Pd/BEA sample was less sensitive to H-2 pretreatment. Temperature-programmed oxidation (TPO) and X-ray photoelectron spectroscopy (XPS) results indicated the presence of more stable Pd oxide sites for La-Pd/BEA. Furthermore, Diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) analysis suggested that NO is adsorbed with stronger and more stable bonds in the form of nitrates compared with the Pd/BEA reference sample.