CATALYST FOR DECREASING NITROGEN-CONTAINING POLLUTANT GASES IN THE EXHAUST GAS OF DIESEL ENGINES

摘要

The invention relates to exhaust purification units for reducing the nitr ogen oxides in lean-running exhaust gas from internal combustion engines by selective catalytic reduction using ammonia, wherein an excess administratio n of ammonia leads to undesired emission of unused ammonia. Said emissions c an be reduced with ammonia trap catalysts. In the ideal case, ammonia is oxi dised to nitrogen and water by said catalysts. This requires additional cons truction space in the exhaust system, which has to be taken from the volume provided for the SCR main catalyst where necessary. Furthermore, on applicat ion of such ammonia trap catalysts, over-oxidation of the ammonia to give ni trogen oxides can occur. According to the invention, the above problems can be overcome by means of a catalyst for removal of nitrogenous pollutants fro m diesel exhaust containing two superimposed layers. The lower layer has an oxidation catalyst and the upper layer can store at least 20 millilitres of ammonia per gramme of catalyst material. Said catalyst displays reduced ammo nia leakage with good SCR conversions in the low temperature range and can b e used as SCR catalyst with reduced ammonia leakage or as ammonia trap catal yst.