Pd-promoted Beta Zeolite for Passive NOx Adsorber Applications

Yaying  Ji, University of Kentucky Center for Applied Energy Research

To abate NOx emissions during engine cold starts, one potential option is the use of a passive NOx adsorber (PNA) device in combination with a urea-SCR catalyst. In this system, the PNA adsorbs NOx emitted from the engine during cold starts, and then releases the NOx at higher temperatures, e.g., above 200 °C. At this point, the downstream SCR catalyst is sufficiently warm to convert NOx efficiently. Since Chen et al. reported the remarkable ability of Pd-promoted zeolites to store NOx below 200 °C [1], Pd-zeolite PNAs have attracted significant interest. However, fundamental understanding of the chemistry of NO/HC adsorption and reaction in Pd/zeolites is currently lacking. In order to probe the nature and function of the Pd species present, in this study a series of Pd-Beta samples with Pd loadings ranging from 0.1 to 1.0 wt% were prepared and investigated by means of microreactor and DRIFTS measurements. For comparison purposes, H-Beta and Pd-promoted Si-Beta were also included in the study. Samples were characterized using a combination of methods, including H2-TPR, NH4+ back exchange, 27Al MAS NMR, Pd K-edge XAS, XPS and TEM. NOx adsorption-desorption experiments were conducted in a microreactor with pre-treatment of samples in 10% O2 at 500 oC for 1 h. NOx adsorption was subsequently carried out at 50 oC in a gas mixture containing 1000 ppm NO, 10% O2, and He as balance (10 min), after which the sample was purged for 1 h in 10% O2/He to remove weakly physisorbed NOx, and finally NOx-TPD was performed in a flow of 10% O2/He with a ramp of 10 °C/min. Corresponding measurements were performed in an environmental DRIFTS cell in order to probe the nature of the adsorbates under these conditions and the effect of pre-treatment conditions on Pd speciation. In this presentation, we report the results of these studies, focusing on the relationship among sample pre-treatment, Pd speciation, and NOx uptake and release.