Effects of alkali and alkaline earth metals on the activity and stability of Pd/SSZ-13 for passive NOx adsorption

Abstract

Pd/SSZ-13 has been proposed as a passive NOx adsorber (PNA) to effectively alleviate NOx emission during vehicle cold start. However, the chemical poisons which are derived from biodiesel and lubricant oil are challenging for the practical application of PNA technology. In this paper, Pd/SSZ-13 was poisoned by alkali/alkaline earth metals to investigate the effects of chemical poisons on the adsorption performance of Pd/SSZ-13. It was found that the alkali/alkaline earth metals led to the decrease of surface area and Brønsted acid sites of Pd/SSZ-13. Strikingly, the 27Al MAS NMR demonstrated that the alkali/alkaline earth metals could effectively prevent Pd/SSZ-13 from dealumination during hydrothermal aging treatment. The H2-TPR results showed that more than 70% of Pd2+ ions were lost in the Na- and K-impregnated Pd/SSZ-13, while almost complete loss of Pd2+ ions was observed in the Mg- and Ca-impregnated Pd/SSZ-13, which resulted in the deactivation of Pd/SSZ-13. Further, the DRIFTS results showed that Mg2+ and Ca2+ ions exhibited preferential substitution for Pd2+ ions, while Na+ and K+ ions preferentially replaced the H+ ions. This work provides insights into the effects of alkali/alkaline earth metals on Pd/SSZ-13, expecting to attract attention to design PNA materials with resistance to chemical poisons.