Laboratory Study of Lean NOx Trap Desulfation Strategies

Desulfation characteristics of several model and fully-formulated monolithic lean NO_x trap materials were studied in a laboratory flow reactor employing a chemical ionization mass spectrometer. For all samples, desulfation at elevated temperatures under reducing conditions resulted in appearance of sulfur dioxide (SO₂) followed by carbonyl sulfide (COS) and hydrogen sulfide (H₂S). The data appear consistent with a desulfation mechanism involving elimination of SO₂ from stored sulfates under reducing conditions, followed by reaction of the SO₂ with CO and H₂ to produce COS and H₂S, respectively. Based on these observations, several cyclic and multistage desulfation strategies were devised which greatly decreased H₂S emissions while achieving relatively rapid and complete sulfur removal.