Catalyst system for gasoline combustion engines, having three-way catalysts and SCR catalyst
The present invention is directed to a catalyst system for the reduction of the harmful exhaust-gas constituents hydrocarbons (THC), carbon monoxide (CO), nitrogen oxides (NO x ), and the environmentally detrimental secondary emissions ammonia (NH 3 ) and nitrous oxide (N 2 O) of combustion engines operated using gasoline and to a corresponding method for exhaust-gas purification. The system is characterized by a particular arrangement of catalysts and is used in the case of engines which are operated with predominantly, on average, stoichiometric air/fuel mixtures. Here, a three-way catalyst produced in accordance with the current prior art is preferably installed in a close-coupled position. In the underfloor position, there is situated an SCR catalyst produced in accordance with the current prior art, followed by a further three-way catalyst produced in accordance with the current prior art. The three-way catalyst in the underfloor region furthermore has a lower oxygen-storing capacity than the close-coupled three-way catalyst.
1. A catalyst system for reducing harmful exhaust gases from a vehicle having a gasoline combustion engine that is operated predominantly, on average, stoichiometrically, having, in the following sequence:
i) a three-way catalyst arranged close to the engine,
ii) an SCR catalyst arranged in the underfloor of the vehicle, the SCR catalyst having an NH 3 -storing capacity, and
iii) an additional three-way catalyst arranged downstream of the SCR catalyst,
wherein the additional three-way catalyst arranged downstream of the SCR catalyst has less oxygen-storing capacity in mg per liter of catalyst volume than an oxygen-storing capacity of the three-way catalyst close to the engine.
2. The catalyst system according to claim 1 , wherein
the three-way catalyst arranged downstream of the SCR catalyst comprises two different catalytic coatings lying one on top of the other.
3. The catalyst system according to claim 2 , wherein
a base catalytic coating of the three-way catalyst arranged downstream of the SCR catalyst has no oxygen-storing material.
4. The catalyst system according to claim 3 , wherein
the base catalytic coating has only palladium as a catalytically active metal.
5. The catalyst system according to claim 4 , wherein
a top catalytic coating of the three-way catalyst arranged downstream of the SCR catalyst has palladium and rhodium as the only catalytically active metals, and oxygen-storing material.
6. The catalyst system according to claim 1 , wherein
the SCR catalyst has no oxygen-storing material.
7. The catalyst system according to claim 1 , wherein
the SCR catalyst is positioned downstream of the three-way catalyst close to the engine by such a distance that, during a driving operation, a temperature of the SCR catalyst is 300° C. to 400° C.
8. A process for reducing harmful exhaust gases from a vehicle having a gasoline combustion engine that is operated predominantly, on average, stoichiometrically, comprising
passing the exhaust gas over a catalyst system according to claim 1 during a driving operation.
9. The catalyst system according to claim 1 , wherein
the oxygen-storing capacity of the three-way catalyst close to the engine makes up >50% of the oxygen-storing capacity of the catalyst system.
10. The catalyst system according to claim 1 , wherein
the oxygen-storing capacity of the three-way catalyst close to the engine makes up ≥60% of the oxygen-storing capacity of the catalyst system.
11. The catalyst system according to claim 1 , wherein
the oxygen-storing capacity of the three-way catalyst close to the engine makes up ≥70% of the oxygen-storing capacity of the catalyst system.
12. The catalyst system according to claim 1 , wherein
a ratio of the oxygen-storing capacity of the three-way catalyst arranged downstream of the SCR catalyst to the oxygen-storing capacity of the three-way catalyst arranged close to the engine is from 1:2 to 1:12.
13. The catalyst system according to claim 1 , wherein
a ratio of the oxygen-storing capacity of the three-way catalyst arranged downstream of the SCR catalyst to the oxygen-storing capacity of the three-way catalyst arranged close to the engine is from 1:2 to 1:7.
14. The catalyst system according to claim 1 , wherein
a ratio of the oxygen-storing capacity of the three-way catalyst arranged downstream of the SCR catalyst to the oxygen-storing capacity of the three-way catalyst arranged close to the engine is from 1:7 to 1:12.