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;
• a controller comprising at least one processor configured to carry out rich spike to temporarily change an air fuel ratio of 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, and a storage amount of nitrates which is an amount of nitrates stored in the NOx storage reduction catalyst, and further
• 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, and further
• the controller controls a timing at which the rich spike is carried out based on the nitrate ratio.