Exhaust gas purification apparatus for an internal combustion engine

An exhaust gas purification apparatus for an internal combustion engine which performs lean burn operation, the apparatus comprising:

• an NOx storage reduction catalyst that is arranged in an exhaust passage of the internal combustion engine;
• and
• a controller comprising at least one processor is configured to carry out rich spike to temporarily change an air fuel ratio of an exhaust gas flowing into the NOx storage reduction catalyst from a lean air fuel ratio higher than a stoichiometric air fuel ratio into a rich air fuel ratio lower than the stoichiometric air fuel ratio;
• wherein
• the controller calculates a storage amount of NOx which is an amount of NOx stored in the NOx storage reduction catalyst;
• the controller that calculates, based on a temperature of the NOx storage reduction catalyst, a storage amount of nitrates which is an amount of nitrates stored in the NOx storage reduction catalyst;
• and
• the controller calculates a nitrate ratio, which is a ratio of the storage amount of nitrates with respect to the storage amount of NOx, based on the storage amount of NOx and the storage amount of nitrates;
• wherein in cases where the storage amount of NOx is the same, the controller makes a period of execution of the rich spike longer, and/or makes a degree of richness of the rich air fuel ratio achieved by the execution of the current rich spike larger, in the case where the nitrate ratio is high than in the case where it is low.