Method of removing nitrogen oxides

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PURPOSE : To remove nitrogen oxide efficiently by reacting an exhaust gas containing nitrogen oxide in contact with a catalyst which decomposes nitrogen oxide at 350-600 deg.C and then with a catalyst which decomposes nitroge oxide at 150-400 deg.C. CONSTITUTION : An exhaust gas containing nitrogen oxide from an electronic- controlled engine 1 is introduced to a layer 3 comprising a catalyst A which can decompose nitrogen oxide at 350-600 deg.C by way of an exhaust manifold 2. The gas from the layer 3 is then led into a layer 4 comprising a catalyst B which can decompose nitrogen oxide at 150-400 deg.C. The gas, after removal of nitrogen oxide, is discharged outside through a muffler 5. A catalyst of an element selected from Cu, Co, Ni, Fe, and rare earth elements on a carrier such as zeolite is suitable for the catalyst A. For the catalyst B, a catalyst of elements comprising Pt and/or Pd and one or more elements of alkai metals, etc., on a carrier of fire resistant inorganic oxide such as active alumina is appropriate.