Patent ID: 7040087

Claim:
A device for purifying exhaust gas for an engine having an exhaust passage, the engine being operated with a lean air-fuel ratio, the device comprising: a SO x storage arranged in the exhaust passage for temporarily storing SO x contained in an exhaust gas inflowing therein; an auxiliary catalyst arranged in the exhaust passage downstream of the SO x storage, the auxiliary catalyst having an oxidizing ability; SO x discharging means for discharging SO x stored in the SO x storage therefrom; and atmosphere control means for controlling an atmosphere of the auxiliary catalyst, wherein the auxiliary catalyst converts the SO x discharged from the SO x storage to sulfate and increases an amount of sulfate being discharged into the outside air when an atmosphere of the auxiliary catalyst is in a sulfate forming atmosphere in which an amount of a reducing agent contained in the exhaust gas flowing to the auxiliary catalyst is smaller than an allowable minimum amount and a temperature of the auxiliary catalyst is higher than an allowable maximum temperature, and when the atmosphere of the auxiliary catalyst is changed to an atmosphere other than the sulfate forming atmosphere, the non-sulfate forming atmosphere of the auxiliary catalyst reduces the amount of sulfate being discharged into the outside air.