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The present invention relates to a nitrogen oxide storage catalyst comprising: a substrate;a first washcoat layer disposed on the substrate, the first washcoat layer comprising metal oxide support particles and a nitrogen oxide storage material comprising at least one metal compound selected from th

The present invention relates to a nitrogen oxide storage catalyst comprising: a substrate;a first washcoat layer disposed on the substrate, the first washcoat layer comprising metal oxide support particles and a nitrogen oxide storage material comprising at least one metal compound selected from the group consisting of alkaline earth metal compounds, alkali metal compounds, rare earth metal compounds, and mixtures thereof, at least a portion of said at least one metal compound being supported on the metal oxide support particles; anda second washcoat layer disposed over the first washcoat layer, said second washcoat layer comprising Rh, wherein the first washcoat layer contains substantially no Rh, and wherein the second washcoat layer is disposed on 100-x % of the surface of the first washcoat layer, x ranging from 20 to 80.

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1. A nitrogen oxide storage catalyst comprising: a substrate;a first washcoat layer disposed on the substrate, the first washcoat layer comprising metal oxide support particles and a nitrogen oxide storage material comprising at least one metal compound selected from the group consisting of alkaline

1. A nitrogen oxide storage catalyst comprising: a substrate;a first washcoat layer disposed on the substrate, the first washcoat layer comprising metal oxide support particles and a nitrogen oxide storage material comprising at least one metal compound selected from the group consisting of alkaline earth metal compounds, alkali metal compounds, rare earth metal compounds, and mixtures thereof, at least a portion of said at least one metal compound being supported on the metal oxide support particles; anda second washcoat layer disposed over the first washcoat layer, said second washcoat layer comprising Rh,wherein the first washcoat layer contains substantially no Rh, andwherein the second washcoat layer is disposed on 100- x % of the surface of the first washcoat layer, x ranging from 20 to 80. 2. The nitrogen oxide storage catalyst according to claim 1, wherein the first and second washcoat layers are both disposed on the substrate starting from one end and/or side of the substrate body. 3. The nitrogen oxide storage catalyst according to claim 1, wherein the second washcoat layer further comprises metal oxide support particles on which Rh is at least partially supported. 4. The nitrogen oxide storage catalyst according to claim 1, wherein the metal oxide support particles comprise at least one metal oxide selected from the group consisting of alumina, zirconia, zirconia-alumina, baria-alumina, lanthana-alumina, lanthana-zirconia-alumina, and mixtures thereof. 5. The nitrogen oxide storage catalyst according to claim 1, wherein the nitrogen oxide storage material comprises at least one element selected from the group consisting of Li, Na, K, Rb, Cs, Mg, Ca, Sr, Ba, Ce, La, Pr, Nd, and mixtures thereof. 6. The nitrogen oxide storage catalyst according to claim 1, wherein the nitrogen oxide storage material comprises a cerium compound and a barium compound. 7. The nitrogen oxide storage catalyst according to claim 1, wherein the first washcoat layer further comprises at least one platinum group metal selected from the group consisting of platinum, palladium, iridium, and mixtures thereof. 8. The nitrogen oxide storage catalyst according to claim 7, wherein the first washcoat layer comprises Pd and/or Pt. 9. The nitrogen oxide storage catalyst according to claim 1, wherein the nitrogen oxide storage catalyst further comprises a third washcoat layer which comprises Pd and which is disposed onto at least a portion of the first washcoat layer, onto which the second washcoat layer is not disposed. 10. The nitrogen oxide storage catalyst according to claim 9, wherein Pd comprised in the third washcoat layer is at least partially supported on metal oxide support particles. 11. The nitrogen oxide storage catalyst according to claim 1, wherein the substrate comprises a honeycomb substrate comprising a plurality of longitudinally extending passages formed by longitudinally extending walls bounding and defining said passages. 12. The nitrogen oxide catalyst according to claim 11, wherein the first washcoat layer onto 100-x % of the surface of which the second washcoat layer is disposed is the first washcoat layer disposed on the walls of the inlet passages of the honeycomb substrate. 13. The nitrogen oxide catalyst according to claim 11, wherein the nitrogen oxide storage catalyst comprises a third washcoat layer which comprises Pd and which is disposed onto at least a portion of the first washcoat layer, onto which the second washcoat layer is not disposed, and wherein the first washcoat layer onto the surface of which the third washcoat layer is disposed is the first washcoat layer disposed on the outlet passages of the honeycomb substrate, said third washcoat layer being disposed on at least a portion of said first washcoat layer of the outlet passages of the honeycomb substrate. 14. A treatment system for an automobile exhaust gas stream, comprising: a combustion engine which operates periodically between lean and rich conditions;an exhaust gas conduit in communication with the engine; anda nitrogen oxide storage catalyst according to claim 1 provided within the exhaust gas conduit. 15. The treatment system according to claim 14, wherein the nitrogen storage catalyst comprises separate substrates with respect to the portions or sections of the catalyst which comprise the second washcoat layer, and those which do not comprise the second washcoat layer. 16. A method for the treatment of automobile engine exhaust gas comprising: (i) providing a nitrogen oxide storage catalyst according to claim 1, and(ii) conducting an automobile engine exhaust gas stream over and/or through the nitrogen oxide storage catalyst. 17. The method according to claim 16, wherein the automobile engine operates periodically between lean and rich conditions. 18. A method of producing a nitrogen oxide storage catalyst comprising the steps of: (i) providing a substrate, preferably a honeycomb substrate;(ii) providing a first washcoat layer on said substrate, said first washcoat layer comprising metal oxide support particles and at least one metal compound selected from the group consisting of alkaline earth metal compounds, alkali metal compounds, rare earth metal compounds, and mixtures thereof, at least a portion of said at least one metal compound being supported on the metal oxide support particles;(iii) drying and/or calcining the coated substrate;(iv) providing a second washcoat layer comprising Rh on 100-x % of the surface of the first washcoat layer, wherein x ranges from 20 to 80;(v) drying and/or calcining the coated substrate. 19. The method of claim 18, further comprising the steps of: (vi) providing a third washcoat layer onto at least a portion of the surface of the first washcoat layer, onto which the second washcoat layer has not been provided;(vii) drying and/or calcining the coated substrate; wherein alternatively, steps (vi) and (vii) are conducted after step (iii) and prior to step (iv). 20. The method according to claim 19, wherein step (ii) comprises the steps of: (a) mixing a solution of the at least one metal compound with particles of at least one metal compound;(b) drying and/or calcining the mixture to obtain a composite material;(c) impregnating metal oxide support particles with a solution comprising Pt;(d) impregnating metal oxide support particles with a solution comprising Pd;(e) providing and milling a slurry of the particles obtained in steps (c) and (d);(f) providing a slurry comprising the milled particles obtained in step (e) and the composite material obtained in step (b) and milling the resulting mixture;(g) coating the substrate with the slurry obtained in step (f). 21. The method according to claim 20, wherein step (iv) comprises the steps of: (aa) impregnating metal oxide support particles with a solution comprising Rh;(bb) providing and milling a slurry of the impregnated particles;(cc) coating the substrate with the slurry obtained in step (bb). 22. The method according to claim 21, wherein step (vi) comprises the steps of: (dd) impregnating metal oxide support particles with a solution comprising Pd;(ee) providing and milling a slurry of the impregnated particles;(ff) coating the substrate with the slurry obtained in step (ee). 23. The method according to claim 22, wherein in steps (e), (f), (bb) and/or (ee) the slurry is milled to a particle size (d90) ranging from 5 to 20 μm, preferably from 8 to 14 μm, more preferably from 9 to 13 μm, and even more preferably from 10 to 12 μm. 24. The method according to claim 22, wherein in steps (c), (d), (aa) and/or (dd) the metal oxide support particles are impregnated by an incipient wetness procedure. 25. The method according to claim 22, wherein in steps (g), (cc) and/or (ff) the coating is performed by dip coating. 26. The method according to claim 20, wherein the slurry in step (f) contains substantially no Rh. 27. The method according to claim 19, wherein the substrate is a honeycomb substrate, and wherein the method preferably comprises a further step of: (viii) alternatively closing the inlet our outlet ends of the honeycomb substrate to form inlet passages having an open inlet end and a closed outlet end, and outlet passages having a closed inlet end and an open outlet end. 28. A nitrogen oxide storage catalyst obtainable by a method according to claim 18.