Cold-Start NO<sub><i>x</i></sub> Mitigation by Passive Adsorption Using Pd-Exchanged Zeolites: From Material Design to Mechanism Understanding and System Integration
Ying Li, Dongdong Chen, Xin Xu, Xinyu Wang, Running Kang, Mingli Fu, Yanbing Guo, Peirong Chen, Yongdan Li, Daiqi Ye
Abstract
It remains a major challenge to abate efficiently the harmful nitrogen oxides (NO x ) in low-temperature diesel exhausts emitted during the cold-start period of engine operation. Passive NO x adsorbers (PNA), which could temporarily capture NO x at low temperatures (below 200 °C) and release the stored NO x at higher temperatures (normally 250–450 °C) to downstream selective catalytic reduction unit for complete abatement, hold promise to mitigate cold-start NO x 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 NO x 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 NO x mitigation.