Patent ID: 7506502

Claim:
An exhaust gas purifying system for an internal combustion engine, comprising: a first NOx storage reduction catalyst disposed on an exhaust passage; a second NOx storage reduction catalyst disposed on said exhaust passage at a location downstream of said first NOx storage reduction catalyst; and a controller that controls an air-fuel ratio changing device to control the air-fuel ratio of exhaust gas in such a manner that: when oxides occluded in said first NOx storage reduction catalyst or said second NOx storage reduction catalyst are reduced, the surrounding atmosphere of that one of said first and second NOx storage reduction catalysts, in which the oxides are reduced, is changed between an oxidative atmosphere and a reducing atmosphere by intermittently supplying a reducing agent to an exhaust gas at a location upstream of said first NOx storage reduction catalyst thereby to change the air-fuel ratio of the exhaust gas at an upstream side of said first NOx storage reduction catalyst; when the surrounding atmosphere of said second NOx storage reduction catalyst is changed into a reducing atmosphere, the air-fuel ratio of the exhaust gas at an upstream side of said first NOx storage reduction catalyst is made lower than when the surrounding atmosphere of said first NOx storage reduction catalyst is changed into a reducing atmosphere; and when the surrounding atmosphere of said second NOx storage reduction catalyst is changed into an oxidative atmosphere, the duration of the oxidative atmosphere is made longer than when the surrounding atmosphere of said first NOx storage reduction catalyst is changed into an oxidative atmosphere.