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A laboratory study was performed to assess the effects of sulfur poisoning and desulfation temperature on the NO conversion of a LNT+(Cu/SCR) in-situ system. Four LNT+(Cu/SCR) systems were aged for 4.5 hours without sulfur at 600, 700, 750, and 800°C using A/F ratio modulations to represent 23K miles of desulfations at different temperatures. NO conversion tests were performed on the LNT alone and on the LNT+SCR system using a 60 s lean/5 s rich cycle. The catalysts were then sulfur-poisoned at 400°C and desulfated four times and re-evaluated on the 60/5 tests. This test sequence was repeated 3 more times to represent 100K miles of desulfations. After simulating 23K miles of desulfations, the Cu-based SCR catalysts improved the NO conversion of the LNT at low temperatures (e.g., 300°C), although the benefit decreased as the desulfation temperature increased from 600°C to 800°C.

This paper summarizes results from a large study on the release of NOx from a lean NOx trap during rich purges. Under certain purge conditions, some NOx trap formulations have the propensity to release some of the NOx stored during previous lean operation without reducing it. This purge NOx release was examined for different NOx trap formulations. The purge NOx release was evaluated for one of the formulations as a function of several variables, including the aging condition of the trap, the trap temperature, the trap volume, the purge A/F ratio, the purge flow rate, and the amount of NOx stored. The effect of hot lean pretreatments on the purge NOx release was studied. In addition, the effect of the rhodium level on the purge NOx release was examined. Mechanisms for the NOx release are proposed that are consistent with the observed data. The results indicate that the purge NOx release is very low for thermally aged traps and is primarily a concern for fresh or stabilized traps.