Patent ID: 8051642

Claim:
An NOx purification system, comprising: an NOx occlusion reduction catalyst that occludes NOx, when an air-fuel ratio of an exhaust gas in an exhaust passage of an internal combustion engine is in a lean state, and releases and reduces the occluded NOx when the air-fuel ratio of the exhaust gas is in a rich state; and a control device including an NOx regeneration controller that includes a preset commencing condition for an NOx regeneration and a preset termination condition for the NOx regeneration and that, at the preset commencement condition, brings the air-fuel ratio of the exhaust gas to the rich state in order to recover an NOx occlusion ability of the NOx occlusion reduction catalyst, and a sulfur poisoning regeneration controller that brings the air-fuel ratio of the exhaust gas to the rich state in order to recover from catalyst deterioration due to sulfur poisoning of the NOx occlusion reduction catalyst, wherein the control device estimates a degree of the deterioration, immediately after terminating the rich air-fuel state of the NOx regeneration control brought on by the preset termination condition, based on a calculation of an integrated value of a difference between an oxygen concentration on an outlet side of the NOx occlusion reduction catalyst and an oxygen concentration on an inlet side of the NOx occlusion reduction catalyst, and wherein the calculation of the integrated value occurs when the oxygen concentration on the inlet side of the catalyst and the oxygen concentration on the outlet side of the catalyst become equal.