Exhaust gas purifying catalyst for internal combustion engine

An exhaust gas purifying system for an internal combustion engine, has an exhaust gas purifying catalyst disposed in an exhaust gas passage of the engine to remove an exhaust gas component; a concentration sensor disposed in the exhaust gas passage downstream of the catalyst to detect a concentration of the component; and a control unit programmed to detect an activity transition time and to judge a catalyst deterioration at the transition time. An exhaust gas purifying system for an internal combustion engine, comprises an exhaust gas purifying catalyst disposed in an exhaust gas passage of the engine to remove an exhaust gas component; a concentration sensor disposed in the exhaust gas passage downstream of the exhaust gas purifying catalyst to detect a concentration of the exhaust gas component; and a control unit programmed to detect an activity transition time at which the exhaust gas purifying catalyst changes from an inactive state to an active state, in accordance with the concentration of the exhaust gas component detected by the concentration sensor, and programmed to judge a deterioration of the exhaust gas purifying catalyst at the activity transition time. An independent claim is also included for a method of purifying exhaust gas discharged from an internal combustion engine provided with an exhaust gas purifying catalyst disposed in an exhaust gas passage of the engine to remove an exhaust gas component, comprising detecting a concentration of the exhaust gas component in the exhaust gas passage downstream of the exhaust gas purifying catalyst; detecting an activity transition time at which the exhaust gas purifying catalyst changes from an inactive state to an active state, in accordance with the detected concentration of the exhaust gas component; and judging a deterioration of the exhaust gas purifying catalyst at the activity transition time.