Elucidating the key active sites and impact of silanol defects over Pd/SSZ-13 as PNA catalyst

Utilizing Pd/SSZ-13 as a passive NOx adsorber (PNA) offers a significant approach to mitigate cold start NOx emissions from diesel engines. However, despite extensive research, identifying active Pd adsorption sites and the mechanism of NO adsorption on Pd/SSZ-13 still need to be explored. Herein, our study reveals that as the two main active NO adsorption sites, a marked preference for NO molecules to adsorb on the [Pd(OH)](+)-Z sites over the Pd2+-Z(2) sites, with the former exhibiting triple the adsorption capacity of the latter. Further, NO adsorption on the Pd2+-Z(2) sites is a single-step process. In contrast, NO adsorption on the [Pd(OH)](+)-Z sites is a multi-step process critically influenced by their adjacent silanol defects. These defects are crucial for forming the vital intermediate species, i.e., HONO. Our findings facilitate the design of more effective PNA materials, vital for their real-world application in mitigating vehicle emissions.