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Within the framework of the French research program PREDIT, a study was undertaken by ADEME, IFP, LGRE, PSA Peugeot Citroën and Umicore, whose main objective was a better understanding of the NOx storage and reduction phenomena on an aged, sulfated and desulfated NOx-trap. The target of this work was to use the information on catalyst working conditions to optimize catalyst management for a gasoline direct injection engine. The catalysts were characterized on both engine and synthetic gas benches. Aging and poisoning phenomena were studied and a variety of different chemical analytical tools were used. The behavior of two different thermally aged cores was investigated under rich conditions on a synthetic gas test bench. The dependence of the NOx regeneration efficiency of the traps is reported for several operating parameters, including reductant concentrations, durations of the rich pulse and trap loadings.

Exhaust stream from automotives and trucks will be subjected to new and more stringent emissions standards. In the case of Diesel engine the lowering of carbon particulates emission (soot) is based on a filtration system in which trapped soot is periodically removed by controlled oxidation with O2 at 550°C or above. It is known that in presence of NO2 the carbon-O2 reaction is significantly speeded-up. Since NO2 may be formed in the gas stream by catalytical oxidation of NO present in the exhaust stream, it is thought that small amounts of NO2 (500 ppm by vol) will promote the continuous oxidation of the deposited carbon particulates in the temperature range 200 - 500°C. The behaviour of the CRT™ system is based on this principle. Very little is known about the reactivity of soot with NO2. Moreover, the effect of other species present in the exhaust stream like O2 and H2O is also not well established.

The behaviour of a NOx storage catalyst in powdered form and containing a storage component based on alkaline metal was investigated under rich conditions. Experiments were conducted in a fixed-bed flow reactor with the space velocity set at 45,000 h-1. From these experiments it was possible to extract the fractional NOx reduction and the efficiency of use of the reductant. With 0.9% CO as a reductant at 350°C, complete utilisation of CO was achieved up to 70% NOx conversion as treatment time was increased. To obtain 90% NOx conversion required longer times, and 23% of the CO did not participate in the reduction of NOX. A reductant balance shows that about 40% of the CO added is used to reduce the catalyst surface when the flow is switched from lean to rich. The ranking of efficiencies of different reductant gases at 350°C gave the following sequence: 0.9% H2 ≈ 0.9% CO > 1285 ppm toluene > 3000 ppm propene ≈ 1125 ppm i-octane > 3000 ppm propane.

Particulates (soot) were sampled from the regenerative trap of an automotive diesel engine run under three speed/load conditions. The fuel was doped with a cerium-based catalyst to promote oxidation of the soot bed. The soots were subjected to combustion testing in a DTG under both temperature ramping and isothermal conditions, and under temperature ramping in a small fixed bed. The combustion gas was 10% oxygen in nitrogen, which was supplemented in the case of the fixed bed with other gases found in diesel exhausts. The temperatures of initiation of combustion Tin were measured in the DTG and in the fixed bed. The kinetic rates of oxidation were calculated from the DTG results taking into account the influence of oxygen transport. The effect of gas composition on ignition and burnout in the fixed bed was determined. Ignition did not occur in the DTG, but rather burning progressed steadily.