Active SCR Catalyst

Abstract

The invention relates to a catalyst containing a BEA-type zeolite which contains iron as well as 0.05 to 1 percent by weight of sodium, the weight percentage being relative to the iron-exchanged zeolite and being calculated as metallic sodium.

Claims

1-15. (canceled)

16. A BEA-type zeolite, which comprises iron and sodium, wherein the amount of sodium is calculated as metallic sodium and is 0.05 to 1% by weight of the iron content of the BEA-type zeolite.

17. A catalyst comprising the BEA-type zeolite according to claim 16, wherein the iron content is calculated as Fe.sub.2O.sub.3 and ranges from 1 to 10% by weight of the BEA-type zeolite.

18. The catalyst according to claim 17, wherein the iron content is calculated as Fe.sub.2O.sub.3 and ranges from 3 to 6% by weight of the BEA-type zeolite.

19. The catalyst according to claim 17, wherein the BEA-type zeolite has a SAR (silica-to-alumina molar ratio) value of 1 to 50.

20. The catalyst according to claim 17, wherein the BEA-type zeolite has a SAR (silica-to-alumina molar ratio) value of 5 to 35.

21. The catalyst according to claim 17, wherein the catalyst comprises a sodium content of 0.1 to 1% by weight.

22. The catalyst according to claim 17, wherein the catalyst comprises a sodium content of 0.5 to 1% by weight.

23. The catalyst according to claim 17, wherein the catalyst is present in the form of a coating on a carrier substrate.

24. The catalyst according to claim 23, wherein the carrier substrate is a flow-through substrate or a wall-flow filter.

25. The catalyst according to claim 23, wherein the carrier substrate is inert and consists of silicon carbide, aluminum titanate, or cordierite.

26. The catalyst according to claim 23, wherein the catalyst forms a part of a carrier substrate.

27. A method for purifying exhaust gas of lean-operated combustion engines, which comprises passing an exhaust gas over a BEA-type zeolite according to claim 1 or a catalyst comprising the BEA-type zeolite.

28. A system for purifying exhaust gas from lean-operated combustion engines, which comprises a BEA-type zeolite according to claim 1 or a catalyst comprising the BEA-type zeolite downstream of an injector for an aqueous urea solution.

29. The system according to claim 28, and further comprising an oxidation catalyst.

30. The system according to claim 29, wherein the oxidation catalyst comprises platinum on a carrier material.

Description

[0039] The invention is explained in more detail in the following examples and figures.

[0040] FIG. 1 shows the SCR activity of K1 to K4, as well as VK1 and VK2 in hydrothermally aged condition (100 hours at 550° C.)

EXAMPLE 1

[0041] A commercially available BEA-type zeolite with a SAR of 10 is mixed in water with a quantity of Fe(NO.sub.3).sub.3 having an iron content of 4.5% by weight (relative to the iron-containing zeolite and calculated as Fe.sub.2O.sub.3) and stirred overnight. A quantity of sodium nitrate corresponding to 0.5% by weight (relative to the iron-containing zeolite and calculated as metallic Na) is then added and the mixture is stirred for 30 minutes. The suspension obtained in this way is used directly as a coating suspension (washcoat) for coating a commercially available flow-through substrate of cordierite. The catalyst obtained (hereinafter referred to as K1) is dried at 90° C., then calcined step-by-step in air at 350° C. and at 550° C.

EXAMPLE 2

[0042] Example 1 is repeated with the difference that the quantity of sodium nitrate is measured in such a way that a loading with sodium of 1% by weight results. The catalyst thus obtained is referred to below as K2.

EXAMPLE 3

[0043] Example 1 is repeated with the difference that the quantity of sodium nitrate is measured in such a way that a loading with sodium of 0.1% by weight results. The catalyst thus obtained is referred to below as K3.

EXAMPLE 4

[0044] Example 1 is repeated with the difference that the quantity of sodium nitrate is measured in such a way that a loading with sodium of 0.2% by weight results. The catalyst thus obtained is referred to below as K4.

COMPARATIVE EXAMPLE 1

[0045] Example 1 is repeated with the difference that the quantity of sodium nitrate is measured in such a way that a loading with sodium of 2% by weight results. The catalyst thus obtained is referred to below as VK1.

COMPARATIVE EXAMPLE 2

[0046] Example 1 is repeated with the difference that the addition of sodium nitrate is omitted. The loading with sodium is thus 0% by weight. The catalyst thus obtained is referred to below as VK2.

Comparative Experiments

[0047] a) The catalysts K1 to K4 as well as VK1 and VK2 were aged and then compared. The aging was carried out at 550° C. in 10% H.sub.2O and 10% O.sub.2 in N.sub.2 for 100 hours.

[0048] b) The SCR activity of the aged catalysts K1 to K4 as well as VK1 and VK2 was tested in a laboratory model gas system under the conditions given in the table below.

TABLE-US-00001 Gas/Parameter Concentration/Conditions NH.sub.3 1100 ppm NO 1000 ppm H.sub.2O  5% O.sub.2 10% N.sub.2 Remainder Temperature Cooling step-by-step 550 to 150° C. Space velocity 60.000 h.sup.−1

[0049] The results are shown in FIG. 1.

[0050] Accordingly, VK1 overall shows the poorest NOx conversion over the entire temperature range. Although VK2 is nearly equivalent to catalysts K1 to K4 at higher temperatures, it exhibits as poor an NOx conversion as VK1 at lower temperatures. In contrast, the catalysts K1 to K4 according to the invention have significantly better results, in particular at low temperatures. This applies in particular to K1 and K2 (sodium contents of 0.5 or 1% by weight).