SCR METHOD FOR REDUCING OXIDES OF NITROGEN AND METHOD FOR PRODUCING A CATALYST FOR SUCH METHOD

Abstract

A method of reducing nitrogen oxides in exhaust gas of an internal combustion engine by selective catalytic reduction (SCR) comprises contacting the exhaust gas also containing ammonia and oxygen with a catalytic converter comprising a catalyst (2) comprising at least one crystalline small-pore molecular sieve catalytically active component (Z.sub.M,I) having a maximum ring opening of eight tetrahedral basic building blocks, which crystalline small-pore molecular sieve catalytically active component (Z.sub.M,I) comprising mesopores.

Claims

1.-17. (canceled)

18. A catalyst composition comprising at least one small-pore zeolite having mesopores introduced by alkaline treatment.

19. The catalyst composition of claim 18, wherein the zeolite has at least one framework selected from CHA, AEI, ERI, and AFX.

20. The catalyst composition of claim 18, further comprising a metallic activator.

21. The catalyst composition of claim 18, further comprising a post-treatment exchanged metallic activator.

22. The catalyst composition of claim 21, wherein the metallic activator is at least one of copper and iron.

23. The catalyst composition of claim 18, having fraction of the small-pore, microporous catalytically active component is about 50 to 95 wt %.

24. The catalyst of claim 18, wherein the catalyst is in the form of an extruded catalyst, more particularly a honeycomb catalyst or a wall-flow filter.

25. A method for producing a catalyst comprising the step of using alkaline treatment to introduce mesopores in a small-pore zeolite.

26. The method of claim 25, further comprising, subsequent to the alkaline treatment, the step of metal ion exchange to form a metallically activated zeolite catalyst.

27. The method of claim 26, further comprising the step of extruding said catalyst into a shaped honeycomb body.

Description

[0060] Working examples of the invention are elucidated in more detail below using two figures, which in schematized form illustrate the method for producing the catalyst in two different variants.

[0061] In both variants, an extruded SCR honeycomb catalyst 2 is produced as a fully manufactured sintered body. In both cases, from different starting components, an extrudable catalyst material E is first of all provided, and is extruded into a honeycomb body 4 having flow channels 6. After drying, the honeycomb body is sintered to form the fully fabricated catalyst 2. In both method variants, the catalyst 2 consists of a small-pore zeolite Z.sub.M,I, catalytically active, ion-exchanged and provided with mesopores, and of a catalytically activated binder component B.sub.A, and also, as and when required, of a further solid component R.

[0062] The indices M and I here stand for a small-pore zeolite with incorporated mesopores (index M) and also for an ion-exchanged zeolite (index I), in which case, in particular, copper ions or else iron ions have been introduced into the microstructure. The index A for the binder component B indicates that the individual particles of the binder component B are catalytically activated.

[0063] The zeolite Z.sub.M,I preferably comprises a zeolite with the framework type CHA. Alternatively or in combination, as small-pore zeolites, zeolites of framework types AEI/ERI are used. Instead or additionally, zeolites of framework types AFX, AFR and/or AFS are used.

[0064] Employed preferably as binder component B.sub.A is a catalytically activated diatomaceous earth. The catalytic activation in this case is accomplished in particular by partial or complete conversion of the microstructure into a zeolite microstructure, preferably of the same type as that of the zeolite Z.sub.M,I used as active component.

[0065] The binder component B.sub.A need not necessarily be catalytically activated. Studies have shown that simply by the introduction of a porous binder component B, such as diatomaceous earth, in spite of an accompanying reduction in the amount of catalytically active material, the catalytic activity of the catalyst (given identical overall weight) is at least constant, since the meso-or macroporosity of the binder component B enables improved accessibility to the active centres within the catalyst material.

[0066] In the variant version according to FIG. 1, a small-pore zeolite Z, which has not been ion-exchanged or provided with mesopores, is employed initially as starting material. This zeolite is customarily in powder form. In a first treatment stage, mesopores are introduced in the manner described into this small-pore zeolite, producing a small-pore zeolite Z.sub.M provided with mesopores. Finally, in a way known per se, an ion exchange is performed, in which copper ions, in particular, are introduced into the framework structure, producing an ion-exchanged zeolite Z.sub.M,I, provided with mesopores, in powder form.

[0067] The binder component B is catalytically activated in a preparatory step, producing a catalytically activated binder component B.sub.A. This component, together with the ion-exchanged small-pore zeolite Z provided with mesopores, and optionally with admixture of a residual fraction R, comprising for example an inorganic porous filler or else fibre fraction, is combined to form the extrudable compound E. The only subsequent steps are the extrusion to form the honeycomb body 4, and finally the drying and sintering to form the catalyst 2.

[0068] In the variant version according to FIG. 2, the formation of the mesopores and the metal ion exchange take place only after extrusion, or, generally, after shaping of a catalyst body from a catalyst material. In the case of a washcoat, therefore, these steps would not take place until after the application of the catalyst material on the inert support.

[0069] Consequently, a small-pore zeolite Z, which has not been ion-exchanged and has not been provided with mesopores either, together with a binder component B, which in this working example has not been activated, and also, as and when necessary, with a fraction R, is combined to form the extrudable compound E, and is subsequently extruded to give the honeycomb body 4. In the subsequent method step, the honeycomb body 4 produced is subjected to an alkaline treatment, converting the zeolite Z into a zeolite Z.sub.M provided with mesopores. This is followed by metal ion exchange, producing the desired state of the ion-exchanged zeolite Z.sub.M, I provided with mesopores. After that, there is sintering to give the fully fabricated catalyst 2.

[0070] The particular advantage in this case is to be seen in the fact that the mesopores begin from the flow channels 6, and so have a defined preferential orientation. As a consequence, in subsequent deployment, more effective transport of exhaust gas into the volume of the catalyst material is made possible.

[0071] The invention can also be defined according to one or more of the following: [0072] 1. Catalyst (2), especially SCR catalyst, comprising at least one small-pore, microporous catalytically active component (Z.sub.M,I), this small-pore catalytically active component (Z.sub.M,I) comprising mesopores introduced by alkaline treatment. [0073] 2. Catalyst (2) according to 1, the small-pore, microporous catalytically active component being a molecular sieve, more particularly a zeolite (Z.sub.M,I). [0074] 3. Catalyst (2) according to 2, the molecular sieve comprising a metallic activator and being more particularly an ion-exchanged zeolite (Z.sub.M,I). [0075] 4. Catalyst (2) according to 2 or 3, a molecular sieve having the framework structure CHA, AEI, EM or AFX being used alternatively or in combination as small-pore catalytically active molecular sieve (Z.sub.M,I). [0076] 5. Catalyst (2) according to any of 1 to 4, wherein the fraction of the small-pore, microporous catalytically active component (Z.sub.M,I) being in the range from 50 to 95 wt %. [0077] 6. Catalyst (2) according to any of 1 to 5, comprising an inorganic binder component (B,B.sub.A). [0078] 7. Catalyst (2) according to 6, in which the inorganic binder component (B,B.sub.A) comprises porous particles. [0079] 8. Catalyst converter (2) according to 6 or 7, in which the inorganic binder component (B.sub.A) is catalytically activated. [0080] 9. Catalyst (2) according to 8, in which the inorganic binder component (B.sub.A) comprises particles coated with a catalytically active layer or converted at least partially into a zeolite framework structure with retention of their particle form. [0081] 10. Catalyst (2) according to any of 1 to 9, in the form of an extruded catalyst, more particularly a honeycomb catalyst or a wall-flow filter. [0082] 11. Method for producing a catalyst (2) more particularly according to any of 1 to 10, comprising a small-pore catalytically active component (Z.sub.M,I), mesopores being introduced into the small-pore component (Z.sub.M,I) by alkaline treatment. [0083] 12. Method according to 11, in which a molecular sieve, more particularly a zeolite (Z.sub.M,I), is used as small-pore active component. [0084] 13. Method according to 12, in which following the introduction of the mesopores by ion exchange, catalytically active metal ions are introduced into the small-pore component in order to form catalytically active cells. [0085] 14. Method according to 13, in which the molecular sieve following the introduction of the mesopores is alternatively directly metal ion-exchanged or is first converted into an intermediate form before the metal ion exchange takes place. [0086] 15. Method according to any of 11 to 14, in which a formable catalyst composition (E) is provided and is formed into a shaped body (4), in particular by extrusion, and the mesopores are introduced only after formation of the shaped body (4).

LIST OF REFERENCE SYMBOLS

[0087] catalyst [0088] honeycomb body [0089] 6 flow channels [0090] Z small-pore zeolite [0091] Z.sub.M small-pore zeolite provided with mesopores [0092] Z.sub.M,I small-pore zeolite provided with mesopores and ion-exchanged [0093] B binder component [0094] B.sub.A catalytically activated binder component [0095] R residual component