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, and wherein coating A comprises palladium in an amount of 0.003 to 0.1% by weight based on the weight of the small pore zeolite and calculated as palladium metal, wherein coating A extends only to a part of the length L. and a coating B which comprises a small-pore zeolite and copper but is free from palladium, and wherein coating A extends starting from one end of the carrier substrate to 10 to 80% of the length L. and coating B extends starting from the other end of the carrier substrate to 20 to 90% of the length L, and wherein L is L.sub.A+L.sub.B wherein La is the length of coating A and L.sub.B is the length of coating B.

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 0.04 to 0.1% 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 or cerium/zirconium mixed oxide in an amount of 10 to 80 g/L, respectively, based on the volume of the carrier substrate.

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 coating A comprises palladium in an amount of 0.06 to 0.08 wt % by weight based on the weight of the small pore zeolite and calculated as palladium metal.

8. SCR catalyst according to claim 1, wherein the small pore zeolite is SSZ-13.

9. SCR catalyst according to claim 1, wherein coating A comprises copper in an amount of 2 to 6% by weight based on the weight of the small pore zeolite and calculated as CuO.

10. SCR catalyst according to claim 1, wherein coating A and B only differ in the absence of palladium in coating B.

11. SCR catalyst according to claim 4, wherein the small pore zeolite belongs to a framework type having the framework type code AEI, AFX, CHA, ERI, KFI or LEV and has a SAR value of 5 to 50.

12. 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 in an amount of 0.003 to 0.1% by weight based on the weight of the small pore zeolite and calculated as palladium metal; wherein coating A extends only to a part of the length L, and a coating B which comprises a small-pore zeolite and copper but is free from palladium, and wherein coating A extends starting from one end of the carrier substrate to 10 to 80% of the length L, and coating B extends starting from the other end of the carrier substrate to 20 to 90% of the length L, and wherein L is L.sub.A+L.sub.B wherein La is the length of coating A and L.sub.B is the length of coating B, and wherein coating A is arranged upstream and coating B is arranged downstream; and a dosing unit for reductant supply to the SCR catalyst.

13. 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 in an amount of 0.003 to 0.1% by weight based on the weight of the small pore zeolite and calculated as palladium metal, wherein coating A extends only to a part of the length L, and a coating B which comprises a small-pore zeolite and copper but is free from palladium, and wherein coating A extends starting from one end of the carrier substrate to 10 to 80% of the length L, and coating B extends starting from the other end of the carrier substrate to 20 to 90% of the length L. and wherein L is L.sub.A+L.sub.B wherein La is the length of coating A and L.sub.B is the length of coating B, and wherein coating A is arranged upstream and coating B is arranged downstream, a second dosing unit for reductant and a second SCR catalyst.

14. Exhaust gas cleaning system according to claim 13, further comprising a combined oxidation catalyst and particulate filter which combination forms a catalyzed particulate filter (cDPF) that is positioned between the first SCR catalyst and the second dosing unit.

15. Exhaust gas cleaning system according to claim 13, further comprising an oxidation catalyst upstream of the second dosing unit and wherein the second SCR catalyst is in the form of an SDPF.

16. 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, the SCR catalyst of claim 1 as a first SCR catalyst, 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.

17. Exhaust gas cleaning system comprising in the following order a first dosing unit for reductant, a first SCR catalyst according to claim 1 an oxidation catalyst, a particulate filter, a second dosing unit for reductant and a second SCR catalyst.

18. System according to claim 17, wherein coating A comprises copper in an amount of 2 to 6% by weight based on the weight of the small pore zeolite and calculated as CuO, and the palladium is in an amount of 0.04 to 0.1%.

19. 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 wherein coating A comprises palladium in an amount of 0.003 to 0.1% by weight based on the weight of the small pore zeolite and calculated as palladium metal, wherein the carrier substrate comprises catalytically active coating B which comprises a small pore zeolite and copper and is free of palladium, and SCR, wherein coating A and coating B are the same but for coating B being free of palladium.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 shows the test results for the component NO.

(2) FIG. 2 shows the test results fot the component NH3.

(3) FIG. 3 shows the test results for the component CO.

(4) Coating A comprising a small pore zeolite, copper and palladium usually extends to the total length L of the carrier substrate. However, in embodiments of the present invention coating A extends only to a part of the length L. In that case there can be one or more additional catalytically active coatings on the carrier. For example, there can be coating B which comprises a small pore zeolite and copper but is free of palladium. In particular, coating A and B only differ in the absence of palladium in coating B.

(5) Coatings A and B can be arranged on the carrier substrate in different manner. In one embodiment coating A extends starting from one end of the carrier substrate to 10 to 80% of the length L and coating B extends starting from the other end of the carrier substrate to 20 to 90% of the length L. Usually L=L.sub.A+L.sub.B applies wherein L.sub.A is the length of coating A and L.sub.B is the length of coating B.

(6) When in use, coating A is to be arranged upstream and coating B downstream.

(7) SCR catalysts according to the present invention can be manufactured by known methods, for example in accordance with the customary dip coating methods or pump and suck coating methods with subsequent thermal post-treatment (calcination and possibly reduction using forming gas or hydrogen). These methods are sufficiently known from the prior art. Accordingly, in a first step a washcoat is prepared which comprises the small pore zeolite, copper and palladium which is in a second step coated onto the carrier substrate.

(8) In embodiments of the present invention the SCR catalyst is connected with a dosing unit for reductant.

(9) Suitable dosing units can be found in literature (see for example T. Mayer, Feststoff-SCR-System auf Basis von Ammonium-carbamat, Dissertation, Technical University of Kaiserslautern, Germany, 2005) and the skilled person can select any of them. The ammonia can be dosed into the exhaust gas flow as such or in form of a precursor which forms ammonia at the ambient conditions of the exhaust gas flow. Suitable precursors are for example aqueous solutions of urea or ammonium format, as well as solid ammonium carbamate. The reductant and its precursor, respectively, is usually carried in a storage tank which is connected to the dosing unit.

(10) The present invention therefore also pertains to a 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.

(11) When in use the inventive system is arranged into the exhaust gas stream so that the dosing unit is upstream of the SCR catalyst.

(12) The SCR catalyst of the present invention has increased ability to oxidize hydrocarbons above 300 C. primarily for the purpose of generating sufficient internal exotherm to reach a temperature which allows its desulfation. Surprisingly, this has no detrimental effect on or even enhances low temperature SCR performance. With other words, the SCR catalyst provides sufficient NOx conversion early in the cycle, for example during cold start. In addition to the self desulfation, the heated exhaust, potentially with some or a significant portion of hydrocarbons still present, enables the downstream DOC and cDPF to undergo an active soot regeneration with or without additional fuel.

(13) It is in particular surprising that the ability of the inventive SCR catalyst to oxidize hydrocarbons, doesn't increase oxidation of ammonia, particularly below 300 C.

(14) The inventive SCR catalyst is therefore in particular suitable as a close coupled SCR catalyst. This means that when in use there is no catalyst upstream of the inventive SCR catalyst.

(15) The present invention therefore also pertains to 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.

(16) In an embodiment of the inventive exhaust gas cleaning system the oxidation catalyst and the particulate filter are combined to form a catalyzed particulate filter (cDPF).

(17) In another embodiment of the inventive exhaust gas cleaning system the particulate filter and the second SCR catalyst are combined to form a so-called SDPF. In this case the second dosing unit is arranged upstream of the SDPF.

(18) In a further embodiment of the inventive exhaust gas cleaning system the oxidation catalyst comprises a noble metal, like for example platinum, palladium or platinum and palladium, on a carrier material. In the latter case the weight ratio of platinum and palladium is for example 4:1 to 14:1. As carrier material all materials can be used which are known to the skilled person for that purpose. Usually, they have a BET surface of 30 to 250 m.sup.2/g, preferably of 100 to 200 m.sup.2/g (determined according to German standard DIN 66132) and are in particular alumina, silica, magnesia, titania, as well as mixtures or mixed oxides comprising at least two of these materials.

(19) Preferred are alumina, alumina/silica mixed oxides and magnesia/alumina mixed oxides. In case alumina is used, it is preferably stabilized, for example with 1 to 6 weight percent, in particular 4 weight percent, of lanthana.

(20) The oxidation catalyst is usually present in form of a coating on a carrier substrate, in particular a flow-through substrate made of cordierite or metal.

(21) In case the oxidation catalyst is combined with the particulate filter the oxidation catalyst is present in form of a coating on the particulate filter which is usually a wall flow filter substrate made of cordierite.

(22) The second SCR catalyst of the inventive exhaust gas cleaning system can principally be selected from all catalysts which are active in catalyzing the SCR reaction of nitrogen oxides with ammonia. This is in particular true because the first SCR catalyst, which when in use is arranged close coupled, has good performance at low operating temperatures and therefore relieves the second SCR catalyst of the need to have good low temperature performance. Accordingly, it is preferred to use a second SCR catalyst with good performance at moderate temperatures and available NO.sub.2 for the control of N.sub.2O, without compromising overall low temperature system performance. This enables the use of SCR catalysts of the mixed oxide type, which for example comprise vanadium, tungsten and titanium, as well as of catalysts on the basis of zeolites, in particular zeolites which are exchanged with transition metals, in particular with copper, iron or iron and copper.

(23) In embodiments of the present invention the second SCR catalyst comprises small pore zeolites with a maximum ring size of eight tetrahedral atoms and a transition metal, for example copper, iron or copper and iron. Such SCR catalysts are for example disclosed in WO2008/106519 A1, WO2008/118434 A1 and WO2008/132452 A2.

(24) In addition, large and medium pore sized zeolites which are exchanged with transition metals can be used as well. Of interest are in particular zeolites belonging to the structure code BEA.

(25) In particular preferred zeolites belong to the structure codes BEA, AEI, CHA, KFI, ERI, LEV, MER or DDR and are in particular ion-exchanged with copper, iron or copper and iron.

(26) The zeolites comprise transition metal in particular in an amount of 1 to 10 weight percent, preferred 2 to 5 weight percent, calculated as metal oxide, like for example Fe.sub.2O.sub.3 or CuO.

(27) In preferred embodiments of the present exhaust gas cleaning system the second SCR catalyst comprises zeolites or molecular sieves of the Beta-type (BEA), Chabazite-type (CHA) or Levyne-type (LEV). Such zeolites or molecular sieves are for example known as ZSM-5, Beta, SSZ-13, SSZ-62, Nu-3, ZK-20, LZ-132, SAPO-34, SAPO-35, AIPO-34 and AIPO-35, see for example U.S. Pat. Nos. 6,709,644 and 8,617,474.

(28) The inventive exhaust gas cleaning system optionally contains as an additional element a so-called ammonia slip catalyst (ASC). The purpose of an ammonia slip catalyst is to oxidize ammonia which breaks through an SCR catalyst and thus to avoid its release to atmosphere. Consequently, an ammonia slip catalyst is coated on a separate carrier substrate and located downstream of the second SCR catalyst or it is coated on a downstream part of the second SCR catalyst.

(29) In embodiments of the inventive exhaust gas cleaning system the ammonia slip catalyst comprises one or more platinum group metals, in particular platinum or platinum and palladium.

(30) The present invention in addition pertains to a 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.

(31) The inventive method can be fine-tuned depending on the temperature of certain components of the exhaust gas cleaning system in particular via dosing of reductant, for example as follows.

(32) In case the temperature of the second SCR catalyst (in underfloor position) is between about 200 and 225 C., the urea dosing at the first dosing unit (in close coupled position) is preferably done to optimize ammonia storage and NOx conversion taking into account of the need to limit ammonia slip oxidation over the downstream oxidation catalyst.

(33) In case the temperature of the first SCR catalyst (in close coupled position) is between about 225 and 275 C. where passive soot oxidation in the downstream cDPF may not be significant, the urea dosing at the first dosing unit (in close coupled position) is preferably controlled to maintain a certain minimum ammonia storage, maximize overall system performance while minimizing formation of N.sub.2O of the complete exhaust gas cleaning system. In case the temperature of the first SCR catalyst (in close coupled position) is above about 275 C., the urea dosing at the first dosing unit (in close coupled position) is preferably minimized or stopped completely to enable adequate passive soot oxidation while the second SCR catalyst (in underfloor position) is at full conversion potential.

(34) When the first SCR catalyst (in close coupled position) needs to be desulfated, engine measures are to be taken to raise the exhaust gas temperature to about 300 to 350 C., and preferably late in-cylinder post injection of fuel is applied so that the first SCR catalyst (in close coupled position) oxidizes some of this cracked, partially oxidized hydrocarbon to reach the internal desulfation temperature target of about 400 to 450 C. Hydrocarbon slip from the first SCR catalyst (in close coupled position) is oxidized in the DOC and/or cDPF, and can be supplemented with in-exhaust injected diesel fuel downstream of the first SCR catalyst (in close coupled position) to accomplish a cDPF regeneration at the same time. If sufficient NOx conversion is achieved with the first SCR catalyst (in close coupled position) while undergoing desulfation, the downstream cDPF can be aggressively regenerated without regard to the need to stay in the good SCR conversion temperature window below about 550 to 600 C. If insufficient NOx conversion is available from the first SCR catalyst (in close coupled position), the second SCR catalyst (in underfloor position) is preferably kept in a temperature range where better SCR conversion is available, below about 500 C.

(35) When a cDPF regeneration is needed but the first SCR catalyst (in close coupled position) doesn't need to be desulfated, NOx conversion can be provided by the first SCR catalyst (in close coupled position) only, and fuel injected primarily downstream of it will be used to accomplish the cDPF regeneration. However, fuel burning in the first SCR catalyst (in close coupled position) can still be desirable to raise the DOC or cDPF inlet temperature to enhance its fuel burning, and it can be advantageous to always combine the desulfation and DPF regeneration to manage the condition of both.

EXAMPLE 1

(36) 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.

(37) 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.

(38) 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

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

Experiments

(40) 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.

(41) 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:

(42) 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

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

(44) 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.

(45) 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.

(46) 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

(47) 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 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, a second dosing unit and a SCR catalyst comprising a commercially available flow through substrate of cordierite coated with a washcoat comprising iron-exchanged -zeolite.