EXHAUST-GAS PURIFICATION SYSTEM FOR THE SELECTIVE CATALYTIC REDUCTION OF NITROGEN OXIDES IN THE LEAN EXHAUST GAS OF INTERNAL COMBUSTION ENGINES AND METHOD OF EXHAUST-GAS PURIFICATION

摘要

The present invention relates to an exhaust-gas purification system for the selective catalytic reduction of nitrogen oxides. The system includes at least one catalyst having catalytically active components for the selective catalytic reduction (SCR components). An NOx storage catalyst (5) is arranged upstream of the SCR catalyst (3) in the exhaust-gas purification system. For performing the selective catalytic reduction, metering means (8) for supplying a compound decomposable into ammonia is provided between the NOx storage catalyst and the SCR catalyst (3). At low exhaust­-gas temperatures, the NOx storage catalyst (5) adsorbs the nitrogen oxides contained in the exhaust gas and desorbs them only at rising exhaust-gas temperatures, so that they can afterwards be converted by the SCR catalyst (3) which is active then. This results in an altogether improved conversion rate for the nitrogen oxides.