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Abstract

An exhaust gas purification system for an internal combustion engine includes a catalyst installed in an exhaust conduit of an engine and constructed of zeolite carrying at least one kind of metal selected from transition metals and noble metals to reduce nitrogen oxides included in the exhaust gas from the engine under an oxidizing gas condition and in the presence of hydrocarbons. The system further includes means for repeating a cycle of cooling the catalyst and then stopping the cooling to allow the catalyst to rise in temperature so that the temperature rising courses of the catalyst are positively and repeatedly produced. Since the catalyst can show a higher nitrogen oxides reducing ability in the temperature rising course than in the temperature lowering course, the NOx purification rate is greatly improved due to the positive production of the temperature rising courses.

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First Claim