METHOD FOR PURIFYING LEAN EXHAUST GAS AND CATALYST SYSTEM FOR THE METHOD

PROBLEM TO BE SOLVED: To avoid consumption of a fuel caused by the concn. increasing period which periodically occurs, by successively introducing the exhaust gas of a lean engine into a permanent reduction catalyst and a nitrogen oxide storage catalyst to purify the gas. SOLUTION: The exhaust gas system 2 of a lean engine 1 is equipped with two converters 3, 4. The converter 3 in the first stage contains a permanent reduction catalyst, while the converter 4 in the second stage contains a nitrogen oxide storage catalyst. Both of the catalysts in the converters 3, 4 can be theoretically assembled in one converter casing, however, it is preferable to assemble these catalysts separately in two converter casings separated from each other so as to keep the exhaust gas temp. in the storage catalyst in the main duration time of operation to the optimum temp. range for the storage catalyst. As for the permanent reduction catalyst used, an iridium-contg. catalyst or copper- contg. zeolite catalyst can be used.