High fuel efficiency combustion modes in new generation internal combustion engines cause considerable decrease in exhaust temperatures. The lower exhaust temperature poses a great challenge for current lean NOx (NO+NO2) control technologies to meet the strict NOx emission standards that mandate a significant reduction in cold-start NOx emission. A promising approach to address cold-start NOx emission is to employ a passive NOx adsorber (PNA) material upstream of the SCR catalyst bed. Efficient PNA is designed to adsorb NOx, preferably NO during the cold start period. The stored NOx should then be readily released once the SCR system becomes operational. In contrast to the lean NOx trap (LNT) catalyst that only adsorbs NO2 and requires periodical chemical reduction via rich purging to release stored NOx, the PNA thermally releases NOx under continuous lean conditions at higher temperatures, e.g., 200-350 °C, where the SCR catalyst functions efficiently. The elimination of the need for the rich purging significantly enhances fuel economy and engine durability, and simplifies electronic controlling. Zeolite-supported Pd catalysts for low-temperature NO adsorption have recently attracted great interest, due to their superb low-temperature NO trapping efficiency and storage capacity, good resistance to sulfur and hydrocarbon (HC) poisoning. The complex nature of Pd supported on zeolites makes it difficult to unravel the structure-function relations for NO adsorption/desorption. The present study aims to gain insights into the low-temperature NO storage mechanism and active site requirement for Pd in/on different zeolites: Beta, ZSM-5 and SSZ-13. TEM, in-situ XPS and DRIFTS, as well as temperature-programed experiments (TPD-IR/TPR-IR) were used to interrogate the state of Pd on different zeolites before, during and after NO adsorption. Surface reaction and associated changes in the state of Pd upon NO adsorption were examined. Effects from zeolite framework structures on the temperature-dependent NO trapping efficiency and NO desorption temperature were also investigated. Finally, approaches towards rational design of PNA catalysts for real-world applications were discussed.

Low-Temperature Pd/zeolite passive NOx adsorbers: Structure, performance and adsorption chemistry

254th National Meeting (2017)

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