Kinetic modeling of sulfur poisoning and regeneration of lean NOx traps

Abstract

Sulfur poisoning and regeneration of lean NOx traps were investigated using experiments and kinetic modeling. A commercial Pt, Rh and barium containing NOx storage catalyst was used. The catalyst also contained oxygen storage material. First the oxygen storage capacity (OSC) was investigated using steps with oxygen and hydrogen. The OSC was substantial with a total use of all hydrogen (1%) for about 20s. The results were similar at the three investigated temperatures (300, 400 and 500°C), indicating that it is a low activation barrier connected with the process. Further, no effect was observed when adding 15ppm SO2 to the feed. Since no SO2 was observed in the outlet it is possible that SO2 is adsorbed during the lean period and then reduced to form H2S in the rich period (not measured). Further, the NOx storage was found to decrease during SO2 exposure, and the decrease was linear and dose dependent. In addition, we investigated different regeneration strategies. When using 500ppm H2 for 60min at 700°C the regeneration was poor. However, when adding 5% CO2 to the 500ppm H2 the regeneration was increased drastically. Further, the regeneration was decreased when decreasing the temperature to 600°C, and further decreased when using 500°C. In addition, it was beneficial with increasing the hydrogen concentration. The kinetic model contains three sub-models: (i) NOx storage and regeneration, (ii) oxygen storage and reduction and (iii) sulfur poisoning and sulfur regeneration. It was crucial to add NOx storage on two sites; barium and alumina. The NOx adsorbed on alumina is more loosely bound. Further, in the model formation of sulfates were added on both components. This was important in order to describe the rate of the sulfur deactivation. If sulfur was adsorbed only on barium the deactivation would have been too rapid. The model could describe the experimental features well.