Characteristics of Pd-based Passive NOx adsorbers (PNA) for cold start application
Do Heui Kim, Seoul National University
As the exhaust regulation on NOx emission is getting more stringent, the removal of NOx during cold start, in other words, at low temperature becomes more crucial. It is known that Pd based catalysts can adsorb NOx at low temperature. Hence, those materials have potential to be applied as a cold start catalyst which has the function of removing NOx during start-up by adsorbing NOx at low temperature and releasing them at higher temperature, where SCR catalyst operates. In this work, we aimed at investigating several Pd supported catalysts with respect to the NOx adsorption/desorption behaviors and the role of Pd and supports by using the combination of NOx adsorption/desorption measurements and DRIFT experiments, which are of importance to develop novel cold start catalyst. Especially, two types of supports such as CeO2 and zeolite were compared in terms of intrinsic NO adsorption/desorption behavior and chemistry, and the resistance to SO2 poisoning.
In addition, Pd/SSZ-13 catalysts after hydrothermal aging (HTA) treatment at 750 °C for 25 hr showed enhanced passive NO adsorption performance at temperature below 120 °C, although those calcined at 500 °C did not possess good NO adsorption capacity. Combined H2-TPR and Pd XAFS results clearly demonstrate that PdO mainly existed over the fresh samples, whereas HTA treated samples contained ionic Pd2+ species, indicating the redistribution of PdO into highly dispersed Pd2+ species within the SSZ-13 structure arising from HTA treatment. Such phenomenon was visually confirmed by comparative STEM-EDS analysis of fresh and HTA Pd/SSZ-13 samples. DRIFT results display the formation of two nitrosyl complexes adsorbed on Pd2+ ions, which are directly related to two desorption peaks of NOx at 250 and 400 °C. All combined results provide the unambiguous evidence about the generation of Pd ions in SSZ-13 zeolite induced by HTA treatment, which play as the active sites for NO adsorption at low temperature.