Impact of Sulphur on the NOx Trap Catalyst Activity-Poisoning and Regeneration Behaviour 2000-01-1874

This presented paper deals with NOx trap sulphur poisoning and its regeneration. Sulphur poisoning has been studied with different SO₂ gas concentrations under laboratory and engine test bench conditions. The sulphur poisoning studies have shown that the different NOx-traps available in the market have different behaviours toward SO₂ poisoning and are all very sensitive to it. The results outline a non linear relationship of the NOx trap sulphur poisoning as a function of SO₂ concentration. For instance, engine bench tests show that with a 50 and a 110ppm sulphur containing gasoline, a decrease of 50% in the NOx-trap storage capacity is respectively observed after 20 and 15 hours. With a gasoline containing 20ppm of sulphur, the same deactivation level is observed after 90 hours. A simulation carried out from these experiments and a study dealing with the European customers driving profile suggests that with a gasoline containing very low amount of sulphur (for example: <10ppm), the NOx-trap regeneration could be executed naturally in most cases. These results have to be confirmed on long duration durability tests.

Concerning the sulphur regeneration, the results show that the NOx-trap initial activity recovery can take place without the complete desorption of SO₂ initially adsorbed by the NOx-trap.

In other respects, it is noticed that regeneration time does depend not only on conditions such as temperature, lambda or NOx-trap formulation but also on the NOx-trap ageing. For example, while between 2 and 5 min are required to purge the sulphur at 700°C with lambda=0.97 for a fresh catalyst, it takes 15 min for an aged one.

A parametric study concerning the regeneration has shown that a good strategy to decrease sulphur regeneration time could be to begin the regeneration at high temperature with low overrichness and continue it by lowering the temperature but increasing the richness.