Exhaust purification system of internal combustion engine

An exhaust purification system of an internal combustion engine, comprising:

• an exhaust purification catalyst provided in an exhaust passage of the internal combustion engine and carrying a catalyst precious metal;
• an NOX sensor provided in the exhaust purification catalyst or in the exhaust passage at a downstream side of the exhaust purification catalyst in the direction of flow of exhaust;
• an air-fuel ratio sensor provided in the exhaust passage at the downstream side of the exhaust purification catalyst in the direction of flow of exhaust;
• and a control device for controlling the target air-fuel ratio of exhaust gas flowing into the exhaust purification catalyst and estimating a state of the exhaust purification catalyst based on an output of the NOX sensor,
• wherein the control device is configured to alternately set the target air-fuel ratio to a rich air-fuel ratio richer than a stoichiometric air-fuel ratio and a lean air-fuel ratio leaner than the stoichiometric air-fuel ratio, and switch the target air-fuel ratio from a rich air-fuel ratio to a lean air-fuel ratio when the air-fuel ratio of exhaust gas detected by the air-fuel ratio sensor becomes equal to or less than a rich judged air-fuel ratio richer than the stoichiometric air-fuel ratio, and
• wherein the control device is configured to estimate a degree of irreversible deterioration of the exhaust purification catalyst accompanying sintering of the catalyst precious metal, not based on the output of the NOX sensor when the air-fuel ratio of exhaust gas flowing into the exhaust purification catalyst is a lean air-fuel ratio, but based on the output of the NOX sensor when the air-fuel ratio of exhaust gas flowing into the exhaust purification catalyst is a rich air-fuel ratio, in first deterioration degree estimation control.