RESUMO
Catalytic oxidation is a promising purification technique for ammonia (NH3) emission. However, high ignition temperatures and NOx peroxide generation limit its effectiveness due to a lack of active sites. Herein, the effects of Si/Al ratio (SAR) modulation on the speciation of copper active sites and the reaction mechanism at different acidic sites were investigated by loading CuO-CeO2 onto SSZ-13 with different SARs (Cu-Ce/SAR15, 20, and 30). Among them, Cu-Ce/SAR20 exhibits the lowest induction temperature (T20 = 180 °C) and the highest nitrogen selectivity (above 95%), attributing to a higher number of Cu2+ exchange sites. In situ IR spectroscopy and isotopic (18O2) transient response experiments indicate that more active Cu2+ in Cu-Ce/SAR20 provides sufficient Lewis acidic sites for NH3 adsorption and favors the stability of Si-OH-Al structures (Brønsted acid sites). NH3 adsorbed at Lewis acidic sites tends to form peroxide byproducts (NOx), while the NH4+ adsorbed at Brønsted acidic sites generates the key intermediate NH4NO2, which decomposes to N2 at high temperatures, thus enhancing nitrogen selectivity. The whole process mainly follows the Mars-van Krevelen (M-K) mechanism, with the Langmuir-Hinshelwood (L-H) mechanism playing a supporting role. Z2Cu2+ coordinates with adjacent Al atoms within the six-membered ring (6MR) and undergoes a slight deformation at high temperatures, facilitating the migration of the lattice oxygen. SAR plays a crucial role in local environmental speciation of reactive Cu2+, where the sufficient isolated Al provided in SAR20 pulls Cu2+ into the eight-membered ring (8MR), allowing it to come into contact with NH3 more readily.
RESUMO
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 °C) and release the stored NOx at higher temperatures (normally 250-450 °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.
