| 摘要 |
Excessive generation NO<SUB>2 </SUB>by oxidation catalyst arrangement upstream of a selective reduction catalyst is suppressed to prevent falling of NO<SUB>x </SUB>reduction rate. A selective reduction catalyst 4 capable of selectively reacting NO<SUB>x </SUB>with ammonia even in the presence of oxygen is incorporated in an exhaust pipe 3 from an engine 1 . A pair of oxidation catalysts 5 A and 5 B are arranged in parallel with each other and upstream of the selective reduction catalyst. In an operation condition with low exhaust temperature, amounts of the exhaust gas 2 distributed to the oxidation catalysts 5 A and 5 B are adjusted so as to make NO/NO<SUB>2 </SUB>ratio in the exhaust gas 2 to about 1-1.5. In an operation condition with higher exhaust temperature where the NO<SUB>x </SUB>reduction rate is less affected by the NO/NO<SUB>2 </SUB>ratio, as a countermeasure against increasing, leaking ammonia, passage changeover is conducted to pass the exhaust gas 2 in backflow through the selective reduction catalyst 4 and the oxidation catalysts 5 A and 5 B, so that ammonia passing through the selective reduction catalyst 4 without reacted is oxidized in the oxidation catalysts 5 A and 5 B.
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