Exhaust gas purification apparatus for an internal combustion engine and a catalyst for purifying exhaust gas

1. An exhaust gas purification apparatus comprising an exhaust gas purification catalyst so disposed in an exhaust gas flow passage of an internal combustion engine that catalytically active components containing at least one member selected from the group consisting of alkali metals and alkaline earth metals is supported directly or by a porous carrier on the surface of a substrate containing Cr, wherein a Cr trapping agent containing at least one member selected from the group consisting of alkali metals and alkaline earth metals, the member having a higher reactivity with chromium than elements selected from the group consisting of the alkali metals and alkaline earth metals contained in the catalytically active components have with chromium contained in the substrate, is disposed in contact with components constituting the substrate.

2. The exhaust gas purification apparatus according to claim 1 , wherein the Cr trapping agent is formed to cover the whole or part of the surface of the substrate.

3. The exhaust gas purification apparatus according to claim wherein the Cr trapping agent is formed in a layer on the surface of the substrate.

4. The exhaust gas purification apparatus according to claim 1 , wherein the Cr trapping agent is present inside the substrate.

5. The exhaust gas purification apparatus according to claim 1 , wherein the Cr trapping agent is present inside the porous carrier.

6. The exhaust gas purification apparatus according to claim 1 , wherein the Cr trapping agent is constituted by an element, which is stronger in basicity strength than the alkali metals and the alkaline earth metals contained as the catalytic components in the catalyst.

7. The exhaust gas purification apparatus according to claim 1 , wherein the Cr trapping agent is contained in the substrate in a molar amount two times or more of the amount of Cr contained in the substrate.

8. The exhaust gas purification apparatus according to claim 1 , wherein the catalyst further contains noble metals as the catalytic active component.

9. The exhaust gas purification apparatus according to claim 1 , wherein the catalyst contains at least one member selected from the group consisting of alkali metals and alkaline earth metals, at least one member selected from the group consisting of Rh, Pt and Pd, and Mn.

10. The exhaust gas purification apparatus according to claim 1 , wherein the substrate is made of metal containing iron.

11. The exhaust gas purification apparatus according to claim 1 , wherein the engine is a lean burn engine, and wherein when an air fuel ratio of the exhaust gas is lean, all or part of NOx contained in the exhaust gas is trapped by the alkali metal and the alkaline earth metals in the catalytic active components and when the air fuel ratio of the exhaust gas is rich or stoichiometric, the NOx trapped by the alkali metals and the alkaline earth metals is reduced with a reducing agent in the exhaust gas.

12. The exhaust gas purification apparatus according to claim 1 , wherein the substrate is a metal honeycomb substrate.

13. An exhaust gas purification apparatus comprising an exhaust gas purification catalyst so disposed in an exhaust gas flow passage of an internal combustion engine that catalytically active components containing at least one member selected from the group consisting of alkali metals and alkaline earth metals is supported directly or by a porous carrier on the surface of a substrate that contains Cr, wherein a Cr trapping agent containing Ag is disposed in contact with components constituting the substrate.

14. A catalyst for purifying an exhaust gas from an internal combustion engine, which catalyst comprises catalytic active components selected from the group consisting of alkali metals and alkaline earth metals supported directly or by means of a porous carrier on a surface of a substrate containing Cr, wherein a Cr trapping agent containing at least one member selected from the group consisting of alkali metals and alkaline earth metals, the member having a higher reactivity with chromium contained in the substrate than that of the alkali metals and alkaline earth metals contained in the catalytically active components with the chromium is disposed in contact with constituting components of the substrate.

15. The catalyst according to claim 14 , wherein all or part of the surface of the substrate is covered with the Cr trapping agent.

16. The catalyst according to claim 14 , wherein the Cr trapping agent is formed in a layer on the surface of the substrate.

17. The catalyst according to claim 14 , wherein the Cr trapping agent is present in the substrate.

18. The catalyst according to claim 14 , wherein the Cr trapping agent is present in the porous carrier.

19. The catalyst according to claim 14 , wherein the Cr trapping agent is constituted by elements stronger in basicity strength than that of the alkali metals and the alkaline earth metals contained in the catalytic active components.

20. The catalyst according to claim 14 , wherein the substrate is a metal honeycomb substrate.

21. An exhaust gas purification apparatus comprising an exhaust gas flow passage of an internal combustion engine and an exhaust gas purification catalyst containing catalytically active components and at least one selected from the group consisting of alkali metals and alkaline earth metals, which is disposed in the passage, wherein the catalytically active components are directly supported or by means of a porous carrier on a substrate containing Cr, and silver as a Cr trapping agent is in contact with the substrate.