Nitrogen oxide storage catalytic converter for use in a motor vehicle in a close-coupled position

1. A nitrogen oxide storage catalyst comprising at least two catalytically active coatings on a support body,

the lower coating applied directly to the support body comprising platinum as a catalytically active component applied to a homogeneous magnesium-aluminum mixed oxide with a magnesium oxide content between 5 and less than 28% by weight, based on the total weight of the magnesium-aluminum mixed oxide, in combination with a nitrogen oxide storage material

which comprises a nitrogen oxide storage component applied to a homogeneous magnesium-aluminum mixed oxide with a molar ratio of MgO to Al 2 O 3 of 1.1:1, wherein

the second layer applied thereto comprises palladium applied to aluminum oxide and barium oxide or strontium oxide, but not platinum.

2. The nitrogen oxide storage catalyst as claimed in claim 1 , wherein

the second layer does not comprise any oxygen-storing materials.

3. The nitrogen oxide storage catalyst as claimed in claim 1 , wherein

in the second layer barium oxide or strontium oxide and palladium have been deposited together on the aluminum oxide support material.

4. The nitrogen oxide storage catalyst as claimed in claim 1 , wherein

the nitrogen oxide storage components present are oxides, hydroxides, oxide hydroxides or carbonates of strontium or barium, or mixtures thereof.

5. The nitrogen oxide storage catalyst as claimed in claim 4 , wherein

the magnesium-aluminum mixed oxide which has a molar ratio of MgO to Al 2 O 3 of 1.1:1 and is used as support material for the nitrogen oxide storage component further comprises 0.5 to 20% by weight of cerium oxide or manganese oxide (MnO 2 ) or mixtures thereof, based on the total weight of the support material.

6. The nitrogen oxide storage catalyst as claimed in claim 4 , wherein

the magnesium-aluminum mixed oxide used as support material for platinum further comprises 0.5 to 20% by weight of cerium oxide, based on the total weight of the support material.

7. The nitrogen oxide storage catalyst as claimed in claim 6 , wherein

0.05 to 10% by weight of a further noble metal selected from gold, palladium and rhodium has additionally been applied to the magnesium-aluminum mixed oxide used as support material for platinum, based on the resulting total weight of noble metal and support material.

8. The nitrogen oxide storage catalyst as claimed in claim 1 , wherein

the lower coating applied directly to the support body additionally comprises a cerium/zirconium mixed oxide which has been coated with 0.05 to 5% by weight of rhodium or platinum and may comprise further rare earth metals selected from the group consisting of yttrium, lanthanum, praseodymium, neodymium, samarium and mixtures thereof, where the content stated is based on the total weight of noble metal and mixed oxide.

9. A process for cleaning the exhaust gases of gasoline engines with direct gasoline injection which are operated predominantly with a lean air/fuel mixture, by contacting the exhaust gases with a catalyst as claimed in claim 1 arranged in a close-coupled position.

10. The process as claimed in claim 9 , wherein an uncoated particulate filter or a particulate filter comprising a catalytically active coating is arranged downstream of a catalyst as claimed in claim 1 .

11. The process as claimed in claim 9 , wherein an SCR catalyst is arranged downstream of a catalyst as claimed in claim 1 .

12. The process as claimed in claim 9 , wherein a nitrogen oxide storage catalyst is arranged downstream of a catalyst as claimed in claim 1 .

13. The process as claimed in claim 11 wherein an apparatus for introducing a reducing agent into the exhaust gas line is positioned upstream of entry to the SCR catalyst and downstream of the catalyst as claimed in claim 1 .

14. The nitrogen oxide storage catalyst as claimed in claim 1 wherein the second layer does not comprise a cerium-zirconium mixed oxide.