(304g) Sulfur Poisoning of Diesel Exhaust Aftertreatment Catalysts

Significant advances in recent years have led to highly efficient catalytic aftertreatment systems for diesel engines, and such systems typically consist of catalysts-in-series that target emissions reductions in NO_X, CO, hydrocarbons and particulate matter. Even with these systems being commercialized, performance degradation is ever-present for each component used. In this presentation, one of the degradation modes, sulfur poisoning, will be highlighted through a brief review of selective catalytic reduction (SCR) catalysis and lean-NO_X trap (LNT) literature, and results obtained from model and commercial catalysts presented. Although not necessarily straightforward, sulfur poisoning of the LNT systems is relatively well understood, with sulfur impacting both the precious metal functionality as well as the basic sites used in the adsorption of NO_X to form nitrites and nitrates through formation of relatively stable sulfate species. For SCR systems, there is still significant ambiguity as to the chemistry that occurs on the surface upon exposure to sulfur. However, the results clearly show that although significant improvements have been made with regards to reducing the impact of sulfur poisoning of these systems, it still occurs. The data suggest that there are multiple catalytic functions affected, for example NO_X adsorption and the formation of surface species that block active sites, with the significance of these dependent on the sulfur species present (i.e. SO₂ vs. SO₃ as an example). Overall, this presentation will highlight surface chemistry changes as a function of sulfur exposure for the two key NO_X reduction systems used in diesel aftertreatment systems, SCR catalysts and LNTs.