Sulfur Poisoning and Desulfation of the Lean NOx Trap 2001-01-2503
Lean NOx traps (LNT) are currently the technology of choice for treating the NOx emissions from lean burn gasoline engines. With the help of the newly established low sulfur fuel to be introduced in the European and the U.S. markets, the widespread application of LNT technology becomes more promising. However, we found that the sulfur tolerance of aged LNTs is much lower than that of the fresh LNTs. Moreover, the aged LNTs are more difficult to be desulfated relative to the fresh ones. The robustness of the LNT still needs to be improved.
This work is focused on understanding the fundamental processes of sulfur poisoning and desulfation as the trap ages. Three LNT samples were selected from different suppliers. These samples were subject to different aging conditions. Flow reactor and pulse flame combustor tests were conducted on the samples to evaluate their performances with regard to sulfur poisoning and desulfation. The samples were further characterized using different techniques including TPR, H2/CO2 chemisorption, FTIR, XRD, etc. Interactions of sulfur with active components of the trap such as precious metal, barium, and cerium are correlated with the performance of the traps.
Significant loss of surface barium sites occurs upon aging. This is extremely detrimental since sulfur preferentially attaches to these limited remaining sites, resulting in accelerated sulfur poisoning of the trap. Because of the loss of surface area upon aging, bulk sulfate forms not only with barium, but also with cerium. The formation of cerium sulfate significantly inhibits the water gas shift reaction in generating H2, which makes the desulfation of the aged samples with bulk barium sulfates much more difficult.