A Laboratory Study of NOxReduction During the Rich Operating Period Over a NOxStorage Catalyst

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/octane > 3000 ppm propane. Thus in an actual exhaust the NOx reduction is due mainly to H₂ and CO. A study of the effect of temperature showed that maximum efficiency is obtained in the range 350 to 400°C. A study of the effect of concentration showed that for the injection of a given mass of reductant (CO), it is better to use a shorter burst at higher concentration. The competition for reductant between the NOx coming from nitrate decomposition and that needed for catalyst reduction requires a 3:1 excess (i.e. an approximate ratio of two between the quantity of reductant injected and the amount of NOx adsorbed during lean phase operations).