Method for use in connection with an exhaust gas after-treatment system

Usage method of conjunction involves operating operated internal combustion engine with excess air, reducing nitrogen oxides, forming reducing agent in the exhaust gas upstream of catalyst, loading catalyst material to the selective catalystic reduction of nitrogen oxides, providing ammonia to nitrogen oxidesby changing nitrogen oxide raw material from internal combustion engine and oscillating feed ratio of alpha -phases to the predetermined value. An independent claim is included for device.