The Effects of Sulfur Poisoning and Desulfation Temperature on the NOx Conversion of LNT+SCR Systems for Diesel Applications

摘要

A laboratory study was performed to assess the effects of sulfur poisoning and desulfation temperature on the NO_x 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_x 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_x 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. During the poisonings and desulfations, the SCR catalyst was effective at converting H_2S and COS produced by the LNT back into SO_2. After simulating 47K miles of desulfations, the benefit of the SCR catalyst for NO_x conversion on the 60/5 test was minimal, regardless of the aging temperature. However, the rich purge time was increased to 6 seconds after simulating 73K and 100K miles, and the benefit of the SCR catalyst for low temperature NO_x conversion was observed again for all four desulfation temperatures. These results suggested that, as the LNT+SCR system ages on a vehicle, the rich purge time may need to be increased to maintain the benefit of the SCR catalyst for low temperature NO_x conversion. This can be attributed to a decrease in the purge kinetics of the LNT with thermal aging. The optimal desulfation temperature for this LNT+SCR system was 750°C, as lower temperatures resulted in incomplete desulfations and consequently lower NO_x conversions at high temperatures, while 800°C desulfations resulted in thermal degradation.