Patent ID: 7047726

Claim:
A method of purifying exhaust gas of an internal combustion engine comprising steps of: disposing a NOx occluding and reducing catalyst in an exhaust gas passage of the internal combustion engine to selectively occlude and hold NOx in the exhaust gas by adsorption, by absorption or by both of them when the air-fuel ratio of the exhaust gas flowing in is lean, and to reduce the occluded NOx with reducing components in the exhaust gas when the air-fuel ratio of the exhaust gas flowing in becomes the stoichiometric air-fuel ratio or a rich air-fuel ratio; and causing said catalyst to occlude the NOx in the exhaust gas from the engine when the engine is operated at a lean air-fuel ratio, and purifying the NOx occluded by said catalyst by reduction with reducing components in the exhaust gas from said engine when the engine is operated at the stoichiometric air-fuel ratio or at the rich air-fuel ratio; wherein a sulfur-solidifying agent that forms a solid sulfate upon the reaction with SOx at the time of combustion is supplied to the engine to solidify the SOx in the exhaust gas thereby to prevent the SOx in the exhaust gas from being occluded by the NOx occluding and reducing catalyst, and the amount of supplying said sulfur-solidifying agent to the engine is controlled depending upon the condition of the atmosphere at said catalyst as detected by a sensor disposed in the exhaust gas passage downstream from the NOx occluding and reducing catalyst.