The conversion of NO x to nitrogen in the exhaust of a lean burn engine using a hydrocarbon assisted-selective catalytic reduction system is benefited by a two-stage oxidation catalyst-reduction catalyst SCR reactor. The character and quantity of the oxidation catalyst is managed to just activate the hydrocarbon reductant at low temperatures (but not fully burn it at high temperatures) to increase its NO x reduction efficiency as the exhaust then contacts the larger reduction catalyst stage of the two-stage reactor.
1. A method of converting NO x content of the exhaust gas of a lean-bum engine to nitrogen where the exhaust comprises NO and NO 2 , the method comprising:
passing the exhaust through a catalytic reactor comprising an upstream stage of an oxidation catalyst for the hydrocarbon content of the exhaust and an adjacent, but separate, downstream stage of a reduction catalyst for NO x and the reaction product of the first reactor stage, the space velocity of the exhaust gas with respect to said upstream stage being sufficiently high so as to activate the hydrocarbon content of the exhaust without consuming it by combustion.
2. The method as recited in claim 1 comprising increasing the hydrocarbon content of the exhaust before the exhaust enters the catalytic reactor.
3. The method as recited in claim 2 comprising increasing the hydrocarbon content of the exhaust at catalytic reactor temperatures in excess of 425° C.
4. The method as recited in claim 1 in which the upstream stage catalyst comprises one or more platinum group elements selected from the group consisting of platinum, palladium and rhodium dispersed as particles on alumina.
5. The method as recited in claim 1 in which the downstream stage catalyst comprises silver dispersed as particles on alumina.
6. The method as recited in claim 1 in which the space velocity of the exhaust stream contacting the oxidation catalyst stage is at least ten times the space velocity of the stream contacting the reduction catalyst stage.
7. The method as recited in claim 1 in which the temperature of the exhaust gas entering the catalytic reactor is at least 200° C.
8. The method as recited in claim 2 in which the hydrocarbon content of the exhaust of a diesel engine is increased by adding diesel fuel to the exhaust.
9. The method as recited in claim 2 in which the hydrocarbon content of the exhaust of a gasoline-fuelled engine is increased by the addition gasoline hydrocarbons to the exhaust.
10. The method as recited in claim 2 in which the hydrocarbon content of the exhaust of a lean-burn engine is increased by the addition of ethanol to the exhaust.
11. The method as recited in claim 1 in which the downstream catalyst stage comprises a base metal substituted zeolite.
12. The method as recited in claim 2 comprising increasing the hydrocarbon content of the exhaust at exhaust gas temperatures in excess of 425° C.