DOCs are used to oxidize NO to NO₂, CO, and HCs to CO₂ and H₂O. The ubiquitous presence of Sulfur in Diesel fuel leads to the formation of SO₂in the exhaust, which oxidizes to SO₃ on DOCs. SOx (SO₂+SO₃) species can have a detrimental effect on DOC performance. S degradation of the NO and HC oxidation activities and SO₂ oxidation to SO₃ can influence the functioning of downstream components like Diesel Particulate Filter (DPF), and Selective Catalytic Reduction Catalysts (SCR). Therefore, DOC SOx model is required to simulate the diesel after-treatment system model with S in the feed. The kinetic model that can capture SOx storage/release, SO₂ oxidation to SO₃ and the degradation in oxidation activity of NO and HCs is presented. Temperature-programmed sulfur species release (TPO) of a DOC saturated with sulfur showed sites corresponding to two distinct species at ~550 and ~800C corresponding to surface and bulk sulfates, respectively. A pseudo-micro-kinetic model that can capture SOx storage/release and SO₂ oxidation to SO₃was developed based on TPO data. Reaction sensitivity analysis predicted that desorption of surface sulfates is the slowest step below 350°C; therefore, it is the rate-determining step of low-temperature SO₂ oxidation to SO₃. We discussed the role of Pt: Pd and hydrothermal aging on the decomposition of surface sulfates, which impact SO₂ oxidation to SO₃. NO oxidation and HC oxidation activities on a sulfur-saturated DOC were predominantly recovered after a thermal regeneration of 550°C-2h. SOx model demonstrates that the recovery is owed to the desorption of surface sulfates that directly impact PGM activity. SOx chemistry model used in conjunction with a global kinetic model for oxidation reactions can predict the real-time evolution of sulfur species on the catalyst and how these sulfur species influence the oxidation activity of DOCs.

Rama Krishna Dadi

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