Impact of Sulfur on NOx Trap Catalyst Activity - Study of the Regeneration Conditions

Laboratory and engine tests were carried out to describe the sulphur effect on the NOx adsorbers catalysts efficiency for gasoline lean burn engines.Two main aspects were studied. The first one deals with the NOx storage efficiency of the adsorber under laboratory conditions, especially regarding the SO₂ gas phase concentration. The rate of sulfur storing is greatly affected by the SO₂ gas concentration. While 6.5 hours are required to get from 70 % NOx reduction to only 35 % when the gas mixture contains 10 ppm SO₂, it takes 20 hours with 5 ppm SO₂ and more than 60 hours with the 2 ppm SO₂ condition. The relationship between the loss in NOx trap performance and SO₂ concentration appears to have an exponential shape. The same amount of sulphur (0.8 % mass) is deposited onto the catalyst within 10 hours with the feed gas containing 10 ppm of SO₂ and within 50 hours with 2 ppm SO₂. Nevertheless, It was shown that the loss in NOx-Tiap efficiency is not the same in these two cases. The efficiency decreased from 70 % to 25 % in the first case (with 10 ppm SO₂), and from 70 % to only 38 % in the second case (with 2 ppm SO₂). The second aspect describes a parametric study on engine bench concerning the sulphur effect on NOx trap efficiency and concerning the required conditions (temperature, air/fuel ratio) to obtain different rates of desulphation. For instance, after 70 hours, NOx efficiency decreased from 90 % to 25 % with a sulphur content in gasoline of 110 ppm. Complete regeneration requires various duration of desulphation depending on air/fuel ratio (λ=1 to 0.95) and temperature conditions (650 to 750°C). For example, complete regeneration occurs after several minutes at λ=1 and several sets of ten seconds at λ=0.95 at 650°C. Results show that sulfur content close to EURO III gasoline standards is the main obstacle for the introduction of NOx adsorber catalyst in Europe. Reduction of the fuel sulphur level under 50 ppm could be an interesting way to make the NOx adsorber practical.