| abstract |
In an engine exhaust passage, in order from an exhaust upstream side, an NO x storage and reduction catalyst, NO oxidation catalyst, NO 2 reduction catalyst, and selective reduction catalyst may be arranged. An air-fuel ratio of an exhaust gas which flows into the NO x storage and reduction catalyst may be temporarily switched to a rich air-fuel ratio which may be adapted for generating ammonia from NO x which may be stored in the NO x storage and reduction catalyst. The ammonia, which may be generated by the NO x storage and reduction catalyst, may be held at the selective reduction catalyst. The NO x which flows into the selective reduction catalyst may be reduced by the ammonia. The NO oxidation catalyst and NO 2 reduction catalyst may be used to make an NO ratio of the exhaust gas which flows into the selective reduction catalyst approach an optimum ratio of the selective reduction catalyst. |