A Laboratory Study of NOX Reduction During the Rich Operating Period Over a NOX Storage Catalyst

The behaviour of a NO_x 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 NO_x 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% NO_x conversion as treatment time was increased. To obtain 90% NO_x conversion required longer times, and 23% of the CO did not participate in the reduction of NO_X. 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. Thus in an actual exhaust the NO_x 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 NO_x 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 NO_X adsorbed during lean phase operations).