Exhaust gas purification system for an internal combustion engine

A system for purifying exhaust gas from an internal combustion engine capable of executing fuel combustion at lean air-fuel ratios comprising:

• a nitrogen oxide reducing catalyst installed in an exhaust conduit of the engine and constructed of zeolite carrying at least one kind of metal selected from transition metals and noble metals, wherein the catalyst produces active species through partial oxidation of the hydrocarbons in an oxidizing gas condition and causes the active species to react with nitrogen oxides included in the exhaust gas to reduce the nitrogen oxides, wherein said catalyst reduces nitrogen oxides more strongly in a catalyst temperature rising condition than in other catalyst temperature conditions;
• and
• means for systematically producing a catalyst temperature rising condition while the engine is running by repeatedly executing a cycle of cooling the catalyst and then stopping the cooling to allow the catalyst to rise in temperature.