Process and catalyst for reducing nitrogen oxides

A process for reducing nitrogen oxides present in a lean exhaust gas from an internal combustion engine by selective catalytic reduction on a reduction catalyst using ammonia, comprising oxidizing in the absence of an electrical gas discharge some of the nitrogen monoxide present in the exhaust gas to nitrogen dioxide so that the exhaust gas contains 30 to 70 vol. % of nitrogen dioxide before contact with the reduction catalyst wherein oxidation of the nitrogen monoxide present in the exhaust gas takes place in the presence of an oxidation catalyst, passing the exhaust gas, together with ammonia, over said reduction catalyst wherein the reduction catalyst comprises a zeolite exchanged with a transition metal.