As an exhaust gas purification apparatus of an internal combustion engine such as a diesel engine or the like, there has been known a storage reduction type NOx catalyst provided in an exhaust system and carrying out an exhaust gas purification of nitrogen oxides (NOx).
In the exhaust gas purification apparatus mentioned above, a NOx storage capacity of the NOx catalyst is lowered by storage of sulfur oxides (SOx) or the like in the NOx catalyst. Accordingly, in many of this kind of exhaust gas purification apparatuses, a catalyst recovery control for recovering the NOx storage capacity of the NOx catalyst which has been lowered by the storage of the SOx, at a predetermined point in time. For example, in Patent Document 1, a point in time for executing the catalyst recovery control is defined by considering temporary deterioration caused by the storage of SOx in the NOx catalyst, and a permanent deterioration such as thermal deterioration in the NOx catalyst or the like. The greater the permanent deterioration of the NOx catalyst becomes, the shorter interval the catalyst recovery control is executed at.
Further, in the catalyst recovery control mentioned above, the temperature of the catalyst is raised, for example, to about 600° C. to 700° C. through supply of unburned fuel component to the NOx catalyst of the exhaust system, and the air-fuel ratio of exhaust gas is made rich (the air-fuel ratio of the exhaust gas is made small) under such a high temperature. Accordingly, release and reduction of SOx from the NOx catalyst are promoted, and the recovery of the NOx storage capacity of the catalyst is achieved.
In this case, in order to make the air-fuel ratio of the exhaust gas rich in a state of a raised temperature of the NOx catalyst, for example, unburned fuel component may be added to a section upstream of the NOx catalyst through a concentrated intermittent fuel addition, thereby setting an air-fuel ratio detected by an air-fuel ratio sensor provided in the exhaust system to a target air-fuel ratio which is richer than a stoichiometric air-fuel ratio.
If the state in which the air-fuel ratio of the exhaust gas is made rich through the concentrated intermittent fuel addition lasts long, the heat generation caused by the oxidation of the unburned fuel component in the exhaust gas or on the catalyst becomes large, and the temperature of the NOx catalyst may be excessively raised. Accordingly, in the catalyst recovery control, the adding period, in which the concentrated intermittent fuel addition mentioned above is carried out, and a suspend period, in which the fuel addition is stopped, are repeated, thereby intermittently making the air-fuel ratio of the exhaust gas rich and suppressing an excessive temperature rising of the NOx catalyst.
If the thermal deterioration occurs in the NOx catalyst or an abnormality occurs in the air-fuel ratio sensor during the adding period of the catalyst recovery control, the unburned fuel component is excessively supplied to the NOx catalyst, and that white smoke can be generated in the exhaust gas of the internal combustion engine.
In other words, in the case that the thermal deterioration occurs in the NOx catalyst, the processing capacity, that is, the purifying capacity of unburned fuel component of the NOx catalyst is lowered. Under such a condition, if the concentrated intermittent fuel addition is executed during the adding period of the catalyst recovery control, the unburned fuel component cannot be sufficiently processed by the NOx catalyst. In other words, unburned fuel component is excessively supplied to the NOx catalyst. As a result, white smoke is generated in the exhaust gas of the internal combustion engine.
Further, in the case where an abnormality occurs in the air-fuel ratio sensor, the air-fuel ratio detected by the sensor can become leaner than the actual air-fuel ratio. Under such a condition, unburned fuel component the amount of which cannot be processed by the NOx catalyst is supplied to the NOx catalyst during the adding period of the catalyst recovery control, in order to set the air-fuel ratio detected by the air-fuel ratio sensor to the target air-fuel ratio. As a result, white smoke is generated in the exhaust gas of the internal combustion engine.    Patent Document 1: Japanese Laid-Open Patent Publication No. 2002-256951