| abstract |
A first selective reduction catalyst that is composed of a silver-based catalyst is provided in an exhaust pipe of an engine, and a second selective reduction catalyst that is composed of a copper-based catalyst or the like is provided in the exhaust pipe in the exhaust downstream of the first selective reduction catalyst. An ozone generator generates ozone using oxygen in the atmosphere, and a hydrocarbon liquid supply means supplies a hydrocarbon liquid. Some of NO in the exhaust gas flowing in the exhaust pipe is oxidized into NO 2 by the ozone generated by the ozone generator, and the NO 2 is supplied to the first selective reduction catalyst. Meanwhile, some of the hydrocarbon liquid supplied by the hydrocarbon liquid supply means is partially oxidized, by the ozone generated by the ozone generator, into an active reducing agent that is composed of an oxygen-containing hydrocarbon such as aldehyde, and the active reducing agent is supplied to the first selective reduction catalyst. |