Cold-Start NOx Mitigation by Passive Adsorption Using Pd- Exchanged Zeolites: From Material Design to Mechanism Understanding and System Integration

It remains a major challenge to abate efficiently the harmful nitrogen oxides (NOx) in low-temperature diesel exhausts emitted during the cold-start period of engine operation. Passive NOx adsorbers (PNA), which could temporarily capture NOx at low temperatures (below 200 degrees C) and release the stored NOx at higher temperatures (normally 250-450 degrees C) to downstream selective catalytic reduction unit for complete abatement, hold promise to mitigate cold-start NOx emissions. In this review, recent advances in material design, mechanism understanding, and system integration are summarized for PNA based on palladium-exchanged zeolites. First, we discuss the choices of parent zeolite, Pd precursor, and synthetic method for the synthesis of Pd-zeolites with atomic Pd dispersions, and review the effect of hydrothermal aging on the properties and PNA performance of Pd-zeolites. Then, we show how different experimental and theoretical methodologies can be integrated to gain mechanistic insights into the nature of Pd active sites, the NOx storage/release chemistry, as well as the interactions between Pd and typical components/poisons in engine exhausts. This review also gathers several novel designs of PNA integration into modern exhaust after-treatment systems for practical application. At the end, we discuss the major challenges, as well as important implications, for the further development and real application of Pd-zeolite-based PNA in cold-start NOx mitigation.