The Desulfation Characteristics of Lean NOx Traps

This paper summarizes results from a large study on the desulfation characteristics of lean NO_x traps. Degreened and thermally aged NO_x trap formulations were poisoned with SO₂ multiple times in a flow reactor. The effect of desulfation time and temperature on the recovery of NO_x trapping efficiency was evaluated after the poisonings. The relative effectiveness of the reducing agents (CO and H₂) used during the desulfation was evaluated for one of the formulations.

The results indicate that the high temperatures required for desulfation (e.g., 700°C and above) result in thermal degradation of NO_x traps that had been degreened at a lower temperature (e.g., 600°C), and therefore desulfation studies should be performed with traps that have already been exposed to the desulfation temperatures or higher prior to the poisoning. NO_x trap formulations can differ in their desulfation characteristics, as two of the formulations studied could be desulfated effectively at 725°C, whereas the third formulation was more difficult to desulfate and required a desulfation temperature near 750°C. The results indicate that hydrogen is the more effective desulfation agent, as higher levels of H₂ in the feedgas or additional H₂ produced by the water-gas-shift reaction resulted in improved NO_x trapping efficiency following the desulfation.