EXHAUST PURIFICATION DEVICE FOR AN INTERNAL COMBUSTION ENGINE

Inside of an engine exhaust passage, a hydrocarbon feed valve (15) and an exhaust purification catalyst (13) are arranged. At the time of engine operation, a first NO x purification method which maintains the air-fuel ratio of the exhaust gas flowing into the exhaust purification catalyst (13) lean while injecting hydrocarbons from the hydrocarbon feed valve (15) at predetermined feed Intervals to thereby remove the NO x which is contained in exhaust gas and a second NO x purification method which switches the air-fuel ratio of the exhaust gas flowing to the exhaust purification catalyst (13) from lean to rich by intervals longer than the above predetermined feed Intervals to thereby remove the NO x are selectively used in accordance with the sulfur poisoning of the exhaust purification catalyst (13).