SCR CATALYST AND EXHAUST GAS CLEANING SYSTEM

Abstract

The present invention relates to a SCR catalyst comprising a carrier substrate of the length L, which is a flow-through substrate, and a coating A which comprises a small pore zeolite, copper and palladium.

Claims

1. SCR catalyst comprising a carrier substrate of the length L, which is a flow-through substrate, and a coating A which comprises a small pore zeolite, copper and palladium.

2. SCR catalyst according to claim 1, wherein the small pore zeolite belongs to a framework type having the framework type code AEI, AFX, CHA, ERI, KFI or LEV.

3. SCR catalyst according to claim 1, wherein coating A comprises copper in an amount of 1 to 20% by weight based on the weight of the small pore zeolite and calculated as CuO.

4. SCR catalyst according to claim 1, wherein coating A comprises palladium in an amount of 0.003 to 2% by weight based on the weight of the small pore zeolite and calculated as palladium metal.

5. SCR catalyst according to claim 1, wherein coating A comprises cerium.

6. SCR catalyst according to claim 1, wherein coating A is present in an amount of 50 to 300 g/L, based on the volume of the carrier substrate.

7. SCR catalyst according to claim 1, wherein the carrier substrate comprises catalytically active coating B which comprises a small pore zeolite and copper and is free of palladium.

8. System comprising a SCR catalyst comprising a carrier substrate of the length L, which is a flow-through substrate, and a coating A which comprises a small pore zeolite, copper and palladium and a dosing unit for reductant.

9. Exhaust gas cleaning system comprising in the following order a first dosing unit for reductant, a first SCR catalyst comprising, a carrier substrate of the length L, which is a flow-through substrate, and a coating A which comprises a small pore zeolite, copper and palladium an oxidation catalyst, a particulate filter, a second dosing unit for reductant and a second SCR catalyst.

10. Exhaust gas cleaning system according to claim 9, wherein the oxidation catalyst and the particulate filter are combined to form a catalyzed particulate filter (cDPF).

11. Exhaust gas cleaning system according to claim 9, wherein the particulate filter and the second SCR catalyst are combined to form a so-called SDPF.

12. Process for cleaning exhaust gas emitted from a lean burn engine and containing nitrogen oxides, which process comprises passing the exhaust gas stream through an exhaust gas cleaning system comprising in the following order a first dosing unit for reductant, a first SCR catalyst comprising, a carrier substrate of the length L, which is a flow-through substrate, and a coating A which comprises a small pore zeolite, copper and palladium an oxidation catalyst, a particulate filter, a second dosing unit for reductant and a second SCR catalyst; wherein the first dosing unit for reductant and the first SCR catalyst are arranged close coupled and wherein the oxidation catalyst, the particulate filter, the second dosing unit for reductant and the second SCR catalyst are arranged underfloor and wherein the exhaust gas enters the exhaust gas cleaning system before the first dosing unit for reductant and leaves it after the second SCR catalyst.

Description

EXAMPLE 1

[0088] a) A zeolite of the type SSZ-13 (framework type code CHA) containing copper in an amount of 3.85% by weight based on the weight of the zeolite and calculated as CuO was suspended in water.
b) Palladium in form of palladium nitrate was precipitated on a commercially available alumosilica carrier material to a weight of palladium of 1.4% by weight.
c) The zeolite containing slurry obtained in step a) above was mixed with demineralised water, then the palladium containing powder obtained in step b) above was added in an amount, that 30 ppm of palladium is achieved. Next, the slurry obtained is mixed with 12% by weight of a commercially available binder based on boehmite and milled in a ball mill. Subsequently, the washcoat obtained was coated on a commercially available flow through substrate of cordierite at a loading of 200 g/L.
d) The SCR catalyst obtained in step c) above (hereinafter called Catalyst E1) was combined with a commercially available dosing unit for dosing of aqueous solution of urea.

Comparison Example 1

[0089] Example 1 was repeated with the exception that step b) was omitted. The catalyst obtained is hereinafter called Catalyst CE1.

Experiments

[0090] Catalysts E1 and CE1 were characterized in a test procedure targeting their SCR as well as their oxidizing capability. The SCR capability was represented by the reaction of NO with NH.sub.3 in the presence of oxygen (so called standard SCR reaction) and the oxidation capability was represented by the oxidation of CO.

[0091] The test procedure was transient in terms of concentrations and temperatures. It comprised a preconditioning and a test cycle for different temperature steps. The gas mixtures applied are as follows:

TABLE-US-00001 Gas Mixture 1 2 3 N.sub.2 Balance Balance Balance O.sub.2 10 Vol.-% 10 Vol.-% 10 Vol.-% NOx 0 ppm 500 ppm 500 ppm NO.sub.2 0 ppm 0 ppm 0 ppm NH.sub.3 0 ppm 0 ppm 750 ppm CO 350 ppm 350 ppm 350 ppm C.sub.3H.sub.6 100 ppm 100 ppm 100 ppm H.sub.2O 5 Vol.-% 5 Vol.-% 5 Vol.-% GHSV/h1 60.000 60.000 60.000

[0092] Test procedure: [0093] 1. Preconditioning at 600 C. under N.sub.2 for 10 min, in parallel identify exact gas concentrations (gas mixture 3) via bypass line. [0094] 2. Test cycle, which is repeated for each target temperature (in this case 350, 250, 225 and 175 C.) [0095] a. Go to target temperature using gas mixture 1 [0096] b. Add NOx (gas mixture 2) [0097] c. Add NH.sub.3 (gas mixture 3), wait until a break-through of 20 ppm NH.sub.3 is reached [0098] d. Temperature programmed desorption until 500 C. is reached (gas mixture 3)

[0099] The test results are shown in FIG. 1 for the component NO, in FIG. 2 for the component NH3 and in FIG. 3 for the component CO.

[0100] As the emission data of NO and NH.sub.3 are nearly identical for E1 and CE1 (see FIG. 1 and FIG. 2), the SCR Reaction capability of catalysts E1 and CE1 are the same.

[0101] As the CO emissions of E1 are clearly reduced when compared to the CO emissions of CE1 (see FIG. 3), the addition of palladium in E1 results in an improvement of the oxidation capability. This can be used for the heat up function as well as for the catalyst's self desulfation capability.

EXAMPLE 2

[0102] The SCR catalyst obtained in Example 1d) above was integrated into an exhaust gas cleaning system by adding to the SCR catalyst at the opposite side of the dosing unit [0103] a commercially available wall flow filter of cordierite coated with 100 g/L based on the volume of the wall flow filter of an oxidation catalyst consisting of platinum supported on alumina, [0104] a second dosing unit and [0105] a SCR catalyst comprising a commercially available flow through substrate of cordierite coated with a washcoat comprising iron-exchanged -zeolite.