Impact of sulfur on NO_x trap catalyst activity - study of the regeneration conditions

摘要

Laboratory and engine tests were carried out to describe the sulphur effect on the NO_x adsorbers catalysts efficiency for gasoline lean-burn engines. Two main aspects were studied. The first one deals with the NO_x storage efficiency of the adsorber under laboratory conditions, especially regarding the SO_2 gas phase concentration. The rate of sulphur storing is greatly affected by the SO_2 gas concentration. While 6.5 hours are required to get from 70% NO_x reduction to only 35% when the gas mixture contains 10 ppm SO_2, it takes 20 hours with 5 ppm SO_2 and more than 60 hours with the 2 ppm SO_2 condition. The relationship between the loss in NO_x trap performance and SO_2 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 SO_2 and within 50 hours with 2 ppm SO_2. Nevertheless, it was shown that the loss in NO_x-trap efficiency is not the same in these two cases. The efficiency decreased from 70% to 25% in the first case (with 10 ppm SO_2), and from 70% to only 38% in the second case (with 2 ppm SO_2).The second aspect describes a parametric study on engine bench concerning the sulphur effect on NO_x trap efficiency and concerning the required conditions (temperature, air/fuel ratio) to obtain different rates of desulphation. For instance, after 70 hours, NO_x 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 II gasoline standards is the main obstacle for the introduction of NO_x adsorber catalyst in Europe. Reduction of the fuel sulphur level under 50 ppm could be an interesting way to make the NO_x adsorber practical.