Investigation of the active sites and optimum Pd/Al of Pd/ZSM-5 passive NO adsorbers for the cold-start application: Evidence of isolated-Pd species obtained after a high-temperature thermal treatment

We investigated the chemisorptive NO adsorption ability at a low temperature (120 degrees C) of the Pd/ZSM-5 passive NOx adsorbers (PNA) to address the cold-start NOx emission. The Pd/ZSM-5 showed a much higher NO adsorption ability after the oxidative treatment at 750 degrees C compared with the 500 degrees C treatment; according to the combined EXAFS, XPS and XRD results, atomically dispersed Pd species were formed over the former, while small PdO agglomerates were observed over the latter. The Pd species on the Pd/ZSM-5 were further examined by applying a NH4NO3-titration method, where an ion exchange occurred with only the ionic Pd in the zeolite. When the Pd(2)/ZSM-5 was treated at 750 degrees C, it exchanged ions with an NH4NO3 solution whereby most of the Pd was ion-exchanged, and this resulted in a decreased NO adsorption capability. Alternatively, a much lesser amount of ion-exchanged Pd was found on the Pd/ZSM-5 that was treated at 500 degrees C. The combined results indicate that the ionic-Pd species on the ZSM-5, not the bulk PdO, are the active sites for the chemisorptive NO adsorption at the low temperature. The NO adsorption capability was also investigated as a function of the Pd loading and the Si-to-Al-2 molar ratio of the ZSM-5. The results suggest that an optimum Pd-to-Al molar ratio exists for the Pd/ZSM-5 with the maximum NO adsorption capability, which was found at around 0.25, thereby suggesting that the Al in the ZSM-5 framework led to the high dispersion of the ionic-Pd species up to the optimum Pd/Al ratio. Above the optimum Pd/Al ratio, however, the bulk-PdO phase formed on the ZSM-5 resulting in a lower NO adsorption capability; that is, the ZSM-5 with the higher Al content required a higher Pd amount for the attainment of the optimum NO adsorption capability. In summary, the chemisorptive NO adsorption at the low temperature is significantly influenced by both the Si-to-Al-2 molar ratio and the Pd-to-Al molar ratio of the Pd/ZSM-5 catalysts.