NOX PURIFICATION METHOD OF AN EXHAUST-GAS PURIFYING SYSTEM FOR INTERNAL-COMBUSTION ENGINE

In an internal combustion engine, inside of an engine exhaust passage, a hydrocarbon feed valve (17), exhaust purification catalyst (13), and intermediate NO x adsorption catalyst (15) are arranged. The intermediate NO x adsorption catalyst (15) adsorbs nitrogen-containing intermediate and NO x which are exhausted from the exhaust purification catalyst (13). To make the nitrogen-containing intermediate or NO x which is adsorbed at the intermediate NO x adsorption catalyst (15) desorb from it, the concentration of hydrocarbons flowing into the exhaust purification catalyst (13) is made to vibrate within a predetermined range of amplitude of 200 ppm or more and within a predetermined range of period of 5 seconds or less, whereby NO x which is contained in exhaust gas is removed at the exhaust purification catalyst (13) while the heat of oxidation reaction of the hydrocarbons is used to make the intermediate NO x adsorption catalyst (15) rise in temperature.