A PROCESS AND CATALYST FOR REDUCING NITROGEN OXIDES

摘要

The invention provides a process for reducing the nitrogen oxides present in a lean exhaust gas from an internal combustion engine by selective catalytic reduction on a reduction catalyst using ammonia, wherein a fraction of the nitrogen monoxid e present in the exhaust gas is oxidised to nitrogen dioxide before the exhaust gas, together with ammonia, is passed over the reduction catalyst. The process is characterised in that the reduction catalyst contains a zeolite exchanged with transition metals and oxidation of the nitrogen monoxide is performed in such a way that the exhaust gas contai ns 30 to 70 vol.% of nitrogen dioxide before contact with the reduction catalyst.