Catalyst Sulfur Poisoning and Recovery Behaviors: Key for Designing Advanced Emission Control Systems 2017-26-0133

Advanced emission control systems for diesel engines usually include a combination of Diesel Oxidation Catalyst (DOC), Diesel Particulate Filter (DPF), Selective Catalytic Reduction (SCR), and Ammonia Slip Catalyst (ASC). The performance of these catalysts individually, and of the aftertreatment system overall, is negatively affected by the presence of oxides of sulfur, originating from fuel and lubricant. In this paper, we illustrated some key aspects of sulfur interactions with the most commonly used types of catalysts in advanced aftertreatment systems. In particular, DOC can oxidize SO₂ to SO₃, collectively referred to as SOx, and store these sulfur containing species. The key functions of a DOC, such as the ability to oxidize NO and HC, are degraded upon SOx poisoning. The impact of sulfur poisoning on the catalytic functions of a DPF is qualitatively similar to DOC. Additionally, the amount and composition of SOx species has an impact on the total particulate matter amount at the tailpipe location of the system. The SO₃-related particulates can’t be directly controlled by a DPF.

For Cu- or Fe- containing zeolite-based SCR catalysts, sulfur poisoning decreases their ability to convert NOx and NH₃, especially at low temperatures. As shown in this paper, the deactivation depends on SOx concentration, SO₃/SOx ratio, and temperature. The impact of upstream DOC on SCR performance is multifaceted, as on the one hand it generates SO₃ which aggravates the SCR poisoning as compared to SO₂ alone; but on the other hand DOC can bring NO₂/NOx ratio in the feed gas to SCR closer to the optimum value, thus improving its efficiency even in a poisoned state. Finally, the impact of sulfur on the last catalyst element, ASC, represents a complex product of poisoning of its two major components - a Ptcontaining under-layer, and SCR-containing over-layer. In particular, oxidation of SO₂ to SO₃ on the Pt-layer degrades SCR-layer performance more severely, increasing selectivity towards undesired byproducts of NH₃ oxidation such as NOx and N₂O. Ultimately, all these individual features superimpose to produce the overall effect of sulfur species on an advanced SCR system.