The upgrading role of Al at T1/T2 sites in stabilizing Pd ions over Pd-beta passive NOx adsorbers under a reducing atmosphere

Abstract

Using Pd/beta as a passive NO_x adsorber can effectively decrease the cold-start NO_x emission from automotive diesel engines. However, the presence of a reducing agent leads to the degradation of NO_x storage during cyclic applications. To deeply understand how to prevent Pd ions from being reduced, a series of dealuminated Pd/beta samples were prepared. It was found that oxalic acid could effectively remove non-framework Al and T3–T9 framework Al, leading to samples with a high ratio of Al at T1–T2 sites. H₂-TPR results revealed that Pd ions anchored near Al at T1–T2 sites as γ and β sites were stable, and remained unchanged after tests. Meanwhile, Pd ions near T3–T9 Al as α sites could be easily reduced. The quantitative relationship further illustrated that the preserved Pd ions as γ and β sites corresponded to the stabilized NO_x storage capability and rendered a suitable desorption temperature. Particularly on the most dealuminated beta supported Pd sample, as 80% of Pd^II ions were located at γ and β sites, almost all Pd ions were preserved and the desorption temperature remained unchanged. This work provides a valuable method for the preparation of stable Pd/beta PNA catalysts.