EP3496853A1

EP3496853A1 - Particle filter having scr-active coating

Info

Publication number: EP3496853A1
Application number: EP17748810.3A
Authority: EP
Inventors: Anke Schuler; Katja Adelmann; Franz Dornhaus; Michael Schiffer; Stephan Eckhoff
Original assignee: Umicore AG and Co KG
Current assignee: Umicore AG and Co KG
Priority date: 2016-08-11
Filing date: 2017-08-11
Publication date: 2019-06-19
Also published as: US10989089B2; CN109641195A; KR20190040242A; KR102513661B1; WO2018029330A1; EP3281699A1; US20190203625A1; CN109641195B

Abstract

The invention relates to a particle filter, which comprises a wall flow filter and SCR-active material, wherein the wall flow filter comprises ducts which extend in parallel between the first and the second end of the wall flow filter and which are alternately closed in a gas-tight manner either at the first or the second end and which are separated by porous walls, the pores of which have inner surfaces, and the SCR-active material is located in the form of a coating on the inner surfaces of the pores of the porous walls, characterized in that the coating has a gradient, such that the side of the coating facing the exhaust gas has a higher selectivity in the SCR reaction than the side of the coating that faces the inner surfaces of the pores. The SCR-active material is preferably a small-pored zeolite, which has a maximum ring size of eight tetrahedral atoms and is exchanged with copper and/or iron.

Description

Particulate filter with SCR active coating

The present invention relates to a particulate filter with SCR active

Coating for the simultaneous reduction of particles and nitrogen oxides in the exhaust gas of internal combustion engines.

Exhaust gases from motor vehicles with a predominantly lean-burn internal combustion engine contain, in addition to particulate emissions, in particular the primary emissions carbon monoxide CO, hydrocarbons HC and

Nitrogen oxides NOx. Due to the relatively high oxygen content of up to 15 vol.%, Carbon monoxide and hydrocarbons can be made relatively harmless by oxidation, but the reduction of nitrogen oxides to nitrogen is much more difficult. One known method for removing nitrogen oxides from exhaust gases in the presence of oxygen is the Selective Catalytic Reduction (SCR) process using ammonia on a suitable catalyst, the SCR catalyst. In this method, the nitrogen oxides to be removed from the exhaust gas are reacted with ammonia to nitrogen and water.

The ammonia used as the reducing agent can be made available by metering in an ammonia-decomposable compound such as urea, ammonium carbamate or ammonium formate into the exhaust gas line and subsequent hydrolysis.

Particles can be removed very effectively with the help of particle filters from the exhaust gas. Wall flow filters made of ceramic materials have proven particularly useful. These are composed of a plurality of parallel channels formed by porous walls. The channels are mutually sealed gas-tight at one of the two ends of the filter to form channels A which are open on the first side of the filter and closed on the second side of the filter, and channels B located on the first side of the filter closed and on the second side of the filter are open. The exhaust gas flowing, for example, into the channels A can leave the filter only via the channels B, and must flow through the porous walls between the channels A and B for this purpose. As the exhaust passes through the wall, the particles are retained.

It is also already known to coat wall-flow filters with SCR-active material and at the same time to remove particles and nitrogen oxides from the exhaust gas.

However, if the required amount of SCR active material is applied to the porous walls between the channels, this can result in an unacceptable increase in the back pressure of the filter.

Against this background, for example, JPH01-151706 and WO2005 / 016497 suggest coating a wall-flow filter with an SCR catalyst in such a way that the latter penetrates the porous walls. The porous walls of the wall flow filter are intended for this purpose

Have a porosity of at least 50% and an average pore size of at least 5 microns.

It has also been proposed, see US 2011/274601, to introduce a first SCR catalyst into the porous wall, i. coat the inner surfaces of the pores and place a second SCR catalyst on the surface of the porous wall. The average particle size of the first SCR catalyst is smaller than that of the second SCR catalyst.

From US 2012/186229 a wall flow filter is known, which is coated with SCR catalyst. Preferably, the SCR catalyst is located on the surfaces of the pores in the filter wall.

JP 2013-139035 A discloses a wall-flow filter coated with two washcoats, each containing iron-exchanged β-zeolites. US 2010/077737 describes a system for the reduction of nitrogen oxides which comprises two different zeolite-based catalysts which differ in their reaction properties of nitrogen oxides. In one embodiment, a copper-exchanged zeolite is direct coated on the carrier substrate. This layer serves as a support for a second layer comprising an iron-exchanged zeolite. The carrier substrate may be a diesel particulate filter. The known SCR catalysts coated wall-flow filters have the disadvantage that their selectivity in the SCR reaction against the undesired oxidation of ammonia, especially at high

Temperatures is too low. The object of the present invention is thus to provide wall-flow filters coated with SCR-active material which have an improved selectivity in the SCR reaction, in particular in the case of

Temperatures of over 600 ° C, have. The present invention relates to a particulate filter comprising a wall-flow filter and SCR-active material, wherein

the wall flow filter comprises channels extending in parallel between a first and a second end of the wall flow filter which are alternately gas-tightly sealed at either the first or second end and which are separated by porous walls whose pores have internal surfaces,

and the SCR-active material is in the form of a coating on the inner surfaces of the pores of the porous walls,

characterized in that the coating has a gradient, so that the side of the coating facing the exhaust gas has a higher selectivity in the SCR reaction than the side of the coating facing the inner surfaces of the pores.

Wall-flow filters which can be used in accordance with the present invention are known and available on the market. They consist for example of silicon carbide, aluminum titanate or cordierite. They have uncoated state, for example, porosities of 30 to 80, especially 50 to 75%. Their average pore size when uncoated, for example, 5 to 30 microns.

As a rule, the pores of the wall-flow filter are so-called open pores, that is to say they have a connection to the channels. Furthermore, the pores are usually interconnected. This allows, on the one hand, the slight coating of the inner pore surfaces and, on the other hand, an easy passage of the exhaust gas through the porous walls of the wall-flow filter.

Within the scope of the present invention, SCR-active material is understood as meaning a material which effects the SCR reaction, ie the conversion of

Nitrogen oxides can catalyze with ammonia to nitrogen and water in the exhaust lean-burned combustion engines. A suitable SCR active material thus has to implement nitrogen oxides effectively under the conditions prevailing in the exhaust gas, including, for example, temperatures of 200 to 750 °. In principle, all known SCR catalytically active materials can be used according to the invention as the SCR-active material.

Examples are, for example, vanadium-containing or vanadium-free mixed oxides, as described, for example, in WO2008 / 049491 A1, WO2011 / 116907 A2 and US Pat

WO2011 / 131324 AI known, and mixtures of mixed oxides with SCR catalysts based on zeolite, as for example

WO2009 / 124643 AI, EP 2 335 810 AI and WO2012 / 168277 AI are known.

In particular embodiments of the present invention, the SCR active material comprises a small pore zeolite exchanged with copper and / or iron.

Small-pore zeolites have a maximum ring size of eight tetrahedral atoms. Well-known zeolites of this type can be used. These include naturally occurring, but preferably synthetically produced small-pore zeolites. Examples of synthetically produced small-pore zeolites include the structure types ABW, ACO, AEI, AEN, AFN, AFT, AFX, ANA, APC, APD, ATN, ATT, ATV, AWO, AWW, BIK, BRE, CAS, CDO CHA, DDR, DFT, EAB, EDI, EPI, ERI, ESV, GIS, GOO, IHW, ITE, ITW, JBW, KFI, LEV, LTA, LTJ, MER, MON, MTF, NSI, OWE, PAU, PHI, RHO, RTE, RTH, SAS, SAT, SAV, SIV, THO, TSC, UEI, UFI, VNI, YUG and ZON.

Preferred small-pore zeolites are those which belong to the structure types AEI, CHA (chabazite), ERI (erionite), LEV (Levyne), AFX, DDR and KFI.

Particularly preferred are the structure types CHA, AEI, ERI and LEV, most preferably CHA and LEV.

In the context of the present invention, the term zeolite is intended to include not only aluminosilicates, but also silicoaluminophosphates and aluminophosphates, sometimes referred to as zeolite-like compounds.

In embodiments of the present invention, the small pore aluminosilicate-type zeolites have a SAR of from 5 to 50, preferably from 14 to 40, more preferably between 20 and 35.

Suitable silicoaluminophosphates or aluminophosphates in particular also belong to the structure types AEI, CHA (chabazite), ERI

(Erionite), LEV (Levyne), AFX, DDR and KFI. Such materials can be found under the related three-letter code of the Structure Database of the International Zeolite Association under Related Materials (http://www.iza-structure.org/databases/).

Examples are SAPO-17, SAPO-18, SAPO-34, SAPO-35, SAPO-39, SAPO-43, SAPO-47 and SAPO-56 and AIPO-17, AIPO-18, AIPO-34 and AIPO-35 , For these materials, the above-mentioned preferred SAR values of aluminosilicates are not applicable.

The zeolites mentioned are exchanged with iron and / or copper, in particular with copper. The amounts of iron or copper are in particular 0.2 to 6 wt .-%, calculated as Fe ₂ 03 or CuO and based on the total weight of the exchanged zeolite.

According to the invention, the coating of the inner surfaces of the pores of the porous walls with SCR-active material has a gradient. This can be continuous, i. the selectivity of the SCR active material continuously decreases from the exhaust side of the coating to the side of the coating facing the interior surfaces of the pores.

In embodiments of the present invention, however, the gradient is discontinuous. The coating consists, for example, of two or more layers which differ with regard to the selectivity of the SCR-active material. In this case, the outermost layer, that is to say the layer facing the exhaust gas, has the highest selectivity and the innermost layer has the lowest selectivity.

In a particular embodiment of the present invention, the coating consists of two layers, wherein the layer facing the exhaust gas has a higher selectivity in the SCR reaction than the layer facing the inner surface of the pores.

In the context of the present patent application, the term selectivity is defined as the quotient of the conversion of NO _x and the conversion of

S = X (NOx) / X (NH3)

where S is the selectivity, X (NOX) the conversion of NOx in% and X (NH3) the conversion of N H3 in%.

The larger the quotient, the more selective the SCR-active material or SCR-active coating is with respect to the desired selective catalytic reaction of NOx and N H3 with nitrogen and water over the undesirable oxidation of N H3 with oxygen

Nitrogen or even NOx. For the Determination of X (NOX) and X (NH3) A wall-flow filter is coated with the SCR-active material of each coating, then hydrothermally aged for 16 hours at 800 ° C and then the conversion of NOx or N H3 at 500 ° C in a Test gas of the following composition determines:

Subsequently, the quotient X (NOX) / X (NH3) is calculated. The coating with the highest quotient is inventively arranged so that it faces the exhaust gas, while the coating with the

lowest quotient is arranged so that it faces the wall flow filter.

It is also important to ensure that, in addition to the above test conditions, all other test conditions are the same. In particular, identical wall flow filters and identical coating methods should be used.

A concrete example for the determination of the quotient S = X (NOX) / X (NH3) is described in Example 2. In a further embodiment of the present invention, the amount of coating with SCR catalytically active material is 70 to 150, in particular 90 to 130 g / L, based on the volume of the wall-flow filter. If the coating with SCR catalytically active material consists of two layers, the amount of one layer is, for example, 40 to 100 g / L and that of the other 30 to 50 g / L.

In one embodiment of the present invention, the

Coating with SCR-active material of two layers, each containing a copper-exchanged small-pore zeolite.

In this case, the small-pore zeolite of the layer facing the exhaust gas contains less than 3, preferably 0.3-2.5, particularly preferably 0.5-1.5 wt .-% Cu, calculated as CuO and based on the exchanged zeolite of this layer, while the small pore zeolite of the inner pore surface facing layer Ibis 6, preferably 2-5, more preferably 3-4.5 wt .-% Cu, calculated as CuO and based on the exchanged zeolite of this layer contains. In this case, the amounts of copper in both layers are to be coordinated so that the coating facing the exhaust gas has the higher selectivity (determined as described above).

In a particular embodiment of the invention, the layer facing the exhaust gas may also contain a small-pore zeolite which has not been exchanged with copper but is in the form of an H or NhU.

In a preferred embodiment of the invention, the layer facing the exhaust gas contains less copper calculated as CuO per unit weight washcoat than the layer facing the inner pore surface. It should be noted that the preferred amount of copper in the

Relationship to the zeolite depends on the SiO ₂ / Al ₂ O 3 ratio of the

Zeolites. In general, as the Si0 ₂ / Al ₂ O ₃ ratio of the zeolite increases, the amount of exchangeable copper decreases. The preferred atomic ratio of zeolite exchanged copper to framework aluminum in the zeolite, hereinafter Cu / Al ratio

denotes according to the invention, in particular at 0.1 to 0.6.

This corresponds to a theoretical degree of exchange of the copper with the zeolite of 20% to 120%, starting from a complete one

Charge compensation in the zeolite by divalent Cu ions at a degree of exchange of 100%.

In a preferred embodiment of the invention, the ratio of exchanged copper to aluminum (Cu / Al) in the zeolite, the

in the exhaust gas facing layer, at values of 0.1 to 0.3, preferably 0.15 to 0.25, and the Cu / Al ratio of the zeolite contained in the layer, that of the inner pore surface is at values of 0.3 to 0.6, preferably at values of 0.35 to 0.5.

In another embodiment of the invention, the layer facing the exhaust gas contains a small pore zeolite having a lower SiO 2 / Al 2 O 3 ratio (SAR) than the small pore zeolite contained in the layer facing the inner pore surface.

In a further preferred embodiment of the invention, the amount of coating of the layer facing the exhaust gas at 20-70g / L, preferably at 30-60g / L and the amount of coating of the inner pore surface facing layer at 50-120g / L, is preferred 70-100g / L. Preferably, the mass of the coating of the layer facing the exhaust gas is less than the mass of the inner pore surface

facing layer.

In one embodiment of the present invention, the

Coating with two-layer SCR-active material, each containing a copper-exchanged CHA-type zeolite. The chabazite of the layer facing the exhaust gas contains 1% by weight of Cu, calculated as CuO and based on the exchanged chabazite this layer, while the chabazite of the inner pore surface facing layer 3 wt .-% Cu, calculated as CuO and based on the exchanged chabazite of this layer contains. Both

Coatings are preferably present in amounts of 50% of the total coating amount.

In another embodiment of the present invention, the coating of SCR active material consists of two layers, each containing a copper-exchanged zeolite of the LEV type. The levyne of the layer facing the exhaust gas contains 1% by weight of Cu, calculated as CuO and based on the exchanged Levyne of this layer, while the Levyne of the layer facing the inner pore surface contains 3% by weight of Cu, calculated as CuO and based on the

Levyne exchanged this layer containing. Both coatings are preferably present in amounts of 50% of the total coating amount.

It is particularly surprising for a person skilled in the art that the particle filter according to the invention achieves the stated object. In fact, the contact times of the nitrogen oxides and ammonia to be reacted with the catalytically active material are particularly low in the particle filter according to the invention, at least lower than conventionally coated flow and wall flow filter substrates.

Throughflow of laminar flow through the exhaust gas, wherein the molecules to be reacted over the entire length of the substrate in contact with the located on the substrate walls catalytically active

Have coatings. For reaction, the reactants diffuse into the catalytically active layer and the resulting reaction products diffuse from the catalytically active layer back into the exhaust gas stream. This mechanism can take place over the entire length of the substrate.

In Wandflussfiltersubstraten, in which the catalytically active

Coatings are on the filter walls, the flow is turbulent, but the mechanism described above can also for Wear come. In addition, the reactants flow through the

catalytically active layer and the flow through the filter wall is a particularly intense contact. See, for example, EP 1 300 193 AI.

In the particulate filter according to the invention, in which the catalytically active coating is located on the pore surfaces in the filter wall, the exhaust gas does not flow through the catalytically active coating, nor is contact possible over the entire filter length. Rather, there is only a short contact time during the passage of the exhaust gas through the filter wall, during which the reactants and the catalytically active layer can touch. It was therefore not foreseeable that the

According to the invention provided gradient of the coating comes into play at all and the particulate filter according to the invention thus solves the problem.

The preparation of the particulate filter according to the invention can be carried out by methods familiar to the person skilled in the art, for example by the customary dip coating method or pump and suction coating method with subsequent thermal aftertreatment (calcination and optionally reduction with forming gas or hydrogen).

It is known to the person skilled in the art that the average pore size of the wall-flow filter and the average particle size of the SCR-catalytically active materials must be coordinated with one another such that a

Coating of the inner pore surfaces takes place. In particular, the average particle size of the SCR catalytically active materials must be small enough to penetrate the pores of the wallflow filter.

The particulate filter according to the invention can be used with advantage for purifying exhaust gas from lean-burn internal combustion engines, in particular diesel engines. It removes particles from the exhaust gas and converts nitrogen oxides contained in the exhaust gas into the innocuous compounds nitrogen and water. The present invention accordingly also relates to a method for

Purification of exhaust gas from lean-burn internal combustion engines, which is characterized in that the exhaust gas is passed through a particulate filter according to the invention.

As a rule, this transition takes place in the presence of a reducing agent. The reducing agent used in the process according to the invention is preferably ammonia. The required ammonia can

For example, be formed in the exhaust system upstream of the particle filter according to the invention, for example by means of an upstream nitrogen oxide storage catalytic converter (Jean NOx trap - LNT), especially during operation under rich (rieh) exhaust conditions. This method is known as "passive SCR".

However, ammonia can also be carried in the form of aqueous urea solution in the "active SCR process", which is metered in as required via an injector upstream of the particle filter according to the invention.

The present invention thus also relates to an apparatus for purifying exhaust gas of lean-burn internal combustion engines, which is characterized in that it comprises a particulate filter according to the invention, and a means for providing a reducing agent.

As a rule, ammonia is used as the reducing agent. In one embodiment of the device according to the invention, the means for providing a reducing agent is thus an injector for aqueous urea solution. The injector is usually fed with aqueous urea solution, which comes from an entrained reservoir, so for example a tank container. In another embodiment, the means for providing a

Reducing agent, a nitrogen oxide storage catalyst, which is able to form ammonia from nitrogen oxide. Such nitrogen oxide storage catalysts are known to the person skilled in the art and comprehensively described in the literature. For example, from SAE-2001-01-3625 it is known that the SCR reaction with ammonia is faster if the nitrogen oxides are present in a 1: 1 mixture of nitrogen monoxide and nitrogen dioxide or at least approximate this ratio. Because the exhaust gas is operated by lean

Internal combustion engines usually has an excess of nitrogen monoxide over nitrogen dioxide, the document proposes to increase the proportion of nitrogen dioxide with the aid of an oxidation catalyst.

In one embodiment, the device according to the invention thus also comprises an oxidation catalyst. In embodiments of the present invention, platinum on a support material is used as the oxidation catalyst.

Suitable carrier material for the platinum are all those skilled in the art for this purpose materials into consideration. They have a BET surface area of from 30 to 250 m ² / g, preferably from 100 to 200 m ² / g (determined to ISO 9277) and are in particular aluminum oxide, silicon oxide,

Magnesium oxide, titanium oxide, zirconium oxide, cerium oxide and mixtures or mixed oxides of at least two of these oxides.

Preference is given to aluminum oxide and aluminum / silicon mixed oxides. If alumina is used, it is particularly preferably stabilized, for example with lanthanum oxide.

The device according to the invention is, for example, constructed so that in the flow direction of the exhaust gas first the oxidation catalyst, then the injector for aqueous urea solution and then the particle filter according to the invention are arranged.

Alternatively, in the flow direction of the exhaust gas, first a nitrogen oxide storage catalyst and then the particle filter according to the invention are arranged. In the regeneration of the nitrogen oxide storage catalyst, ammonia can be formed under reductive exhaust gas conditions.

Oxidation catalyst and injector for aqueous urea solution are dispensable in this case. The invention is explained in more detail in the following examples and figures. shows a cross section through the porous wall of a particulate filter according to the invention, comprising two layers with SCR catalytically active material, wherein

(1) the pores of the porous wall

(2) the porous wall

(3) the layer of SCR catalytically active material which has the lower selectivity and

(4) the layer of SCR catalytically active material which has the higher selectivity and

The arrows show the direction of the exhaust gas

shows the NOx conversion of the catalysts Kl, VK1 and

VK2 (Example 1 and Comparative Examples 1 and 2) shows the NOx conversion of K2 and VK3 (Example 2 and

Comparative Example 3), and the catalysts VKB and VKC from Example 2. Example 1

a) A commercial wall-flow filter made of silicon carbide with a

Porosity of 65% and an average pore size of 23 pm was coated by a conventional dipping method with 60 g / L of a washcoat containing a copper-exchanged chabazite, with a

SiO ₂ / Al ₂ O 3 ratio (SAR) of 30, with a copper content of 3% by weight (calculated as CuO and based on the exchanged chabazite). The mean particle size of the copper-exchanged chabazite was 1.16 pm. Subsequently, the coated wall-flow filter was dried and calcined. b) In a second step, the according to a) was coated

Wall flow filter provided with a second coating. This was done by means of a conventional dipping process with 60 g / L of a washcoat coated containing a copper-exchanged chabazite (SAR = 30) with a copper amount of 1 wt.% (calculated as CuO and based on the exchanged chabazite). The average particle size of the copper exchanged chabazite was 1.05 pm. Subsequently, the coated wall-flow filter was dried and calcined.

Calculated over the entire wall flow filter, the amount of copper was 2 wt.% (Calculated as CuO and based on the exchanged

Chabaziten).

The catalyst is referred to below as Kl.

Comparative Example 1

A commercially available silicon carbide wall flow filter having a porosity of 65% and an average pore size of 23 μm was coated by a conventional dipping method with 120 g / L of a washcoat containing a copper-exchanged chabazite (SAR = 30) with a

Copper amount of 2 wt.% (Calculated as CuO and based on the exchanged chabazite) contained. The mean particle size of the copper-exchanged chabazite was 1.06 pm. Subsequently, the coated wall-flow filter was dried and calcined.

Chabaziten)

The catalyst is hereinafter called VK1. Comparative Example 2

a) A commercial wall-flow filter made of silicon carbide with a

Porosity of 65% and an average pore size of 23 pm was coated on the inlet side with 60 g / L of a washcoat containing a copper-exchanged chabazite (SAR = 30) with a copper amount of 1% by weight by a conventional dipping process to 50% of its length. (calculated as CuO and based on the exchanged chabazite). The mean particle size of the copper-exchanged chabazite was 1.05 μη.ΐ. Subsequently, the coated wall-flow filter was dried and calcined, b) in a second step, that coated according to a)

Wandflussfilter also provided on the uncoated part of its length (50%) on the outlet side with a second coating. For this purpose, 60 g / L of a washcoat was coated by means of a conventional dipping process, which contained a copper-exchanged chabazite (SAR = 30) with a copper amount of 3 wt.% (Calculated as CuO and based on the exchanged chabazite). The mean particle size of the copper-exchanged chabazite was 1.16 pm. Subsequently, the coated

Wall flow filter dried and calcined.

Chabaziten)

The catalyst is hereinafter called VK2.

Determination of NOx conversion of Kl, VK1 and VK2

a) First, Kl, VK1 and VK2 were aged for 16 h at 800 ° C in a hydrothermal atmosphere (10% water, 10% oxygen, balance nitrogen). b) The NOx conversion of the particle filter Kl according to the invention and the comparison particle filter VK1 and VK1 as a function of the temperature upstream of the catalyst was determined in a model gas reactor in the so-called NOx conversion test.

The NOx turnover test consists of a test procedure, which is a

Pretreatment and a test cycle covers that for different

Go through target temperatures. The applied gas mixtures are noted in Table 1.

Test procedure:

1. Preconditioning at 600 ° C under nitrogen for 10 min 2. Test cycle repeated for the target temperatures

a. Approaching the target temperature under gas mixture 1

b. Switching on NO _x (gas mixture 2)

c. Switch on N hb (gas mixture 3), wait until N hb breakthrough> 20ppm, or a maximum of 30min d. Temperature programmed desorption up to 500 ° C

(Gas mixture 3)

For each temperature point, the maximum rate for the test procedure area 2c is determined. The application of the maximum NOx rate for the various temperature points results in a job as shown in Figure 2.

As can be seen from FIG. 2, Kl shows a significantly better NOx conversion compared to VK1 and VK2. Example 2

I) Determination of the selectivity in the SCR reaction

a) A commercial wall-flow filter made of silicon carbide with a

Porosity of 63% and an average pore size of 20 microns was coated by a conventional dipping method with 80 g / L of a washcoat containing a copper-exchanged chabazite (SAR = 30) with a copper content of 4.5 wt.% (Calculated as CuO and related to the exchanged chabazite). The mean particle size of the copper-exchanged chabazite was 1.43 pm. Subsequently, the coated wall flow filter was dried, calcined at 350 ° C and annealed at 550 ° C.

The catalyst is hereinafter called VKB. b) A commercial wall-flow filter made of silicon carbide with a

Porosity of 63% and an average pore size of 20 microns was coated by a conventional immersion method with 30 g / L of a washcoat containing a copper-exchanged chabazite (SAR = 30) with a copper amount of 2 wt.% (Calculated as CuO and on the exchanged Chabazites). The mean particle size of the copper-exchanged chabazite was 1.93 pm. Subsequently, the coated wall flow filter was dried, calcined at 350 ° C and annealed at 550 ° C.

The catalyst is hereinafter called VKC. c) The catalysts VKB and VKC were at 800 ° C for 16 hours

hydrothermal and then their NO x and N H 3 conversions are determined under the experimental conditions specified in the table below.

Temperature 500 ° C

N ₂ balance

O2 10 vol.%

Hereinafter, the quotient of the conversion of NOx and the conversion of NH3 was determined for VKB and VKC. The following results were obtained:

VKB VKC

X (NOx) /% 66.20 24.74

X (NH ₃ ) /% 96.18 30.06

X (NOx) / x (NH3) 0.69 0.82 It was found that VKC has a higher ratio of NOx and NH3 conversions than VKB. Thus, VKC is to be coated on the side facing the gas and VKB on the side facing the wall flow filter. II) According to the results according to paragraph Ic) above, a particle filter according to the invention was obtained as follows:

A commercially available silicon carbide wall-flow filter having a porosity of 63% and an average pore size of 20 microns was coated by conventional immersion with 80 g / L of a washcoat containing a copper-exchanged chabazite (SAR = 30) with a copper amount of 4 , 5% by weight (calculated as CuO and based on the exchanged Chabaziten) contained. The mean particle size of the copper-exchanged chabazite was 1.43 pm. Subsequently, the coated

Dried wall flow filter and calcined at 350 ° C. In a second step, the wall flow filter coated according to a) was provided with a second coating. For this purpose, it was coated by means of a customary dipping process with 30 g / L of a washcoat containing a copper-exchanged chabazite (SAR = 30) with a copper amount of 2% by weight (calculated as CuO and based on the exchanged chabazite). The mean particle size of the copper-exchanged chabazite was 1.93 pm. Subsequently, the coated wall flow filter was dried, calcined at 350 ° C and annealed at 550 ° C.

Calculated over the entire wall flow filter, the amount of copper was 3.8 wt.% (Calculated as CuO and based on the exchanged

Chabaziten).

The catalyst is referred to below as K2. Comparative Example 3

A commercially available silicon carbide wall-flow filter having a porosity of 63% and an average pore size of 20 microns was coated by a conventional dipping method with 110 g / L of a washcoat containing a copper-exchanged chabazite (SAR = 30) with a

Copper amount of 3.8 wt.% (Calculated as CuO and based on the exchanged chabazite) contained. The mean particle size of the copper-exchanged chabazite was 1.61pm.) Subsequently, the coated wall-flow filter was dried and calcined.

The catalyst is hereinafter called VK3. The determination of the NOx conversion of K2 and VK3 (and of VKB and VKC) was carried out as described in Example 1. The result is shown in FIG. 3.

Claims

claims

A particulate filter comprising a wall-flow filter and SCR-active material, wherein

the wall-flow filter comprises channels extending in parallel between first and second ends of the wall-flow filter, which are alternately gas-tightly sealed at either the first or second end and which are separated by porous walls whose pores have internal surfaces, and

the SCR-active material is in the form of a coating on the inner surfaces of the pores of the porous walls,

characterized in that the coating has a gradient, so that the side of the coating facing the exhaust gas has a higher selectivity in the SCR reaction than the side of the coating facing the inner surface of the pores.

2. Particle filter according to claim 1, characterized in that the wall-flow filter consists of silicon carbide, aluminum titanate or cordierite.

3. Particle filter according to claim 1 and / or 2, characterized in that the wall flow filter in the uncoated state has a porosity of 30 to 80%.

4. Particle filter according to one or more of claims 1 to 3, characterized in that the wall-flow filter has an average pore size in the uncoated state of 5 to 30 microns.

5. Particle filter according to one or more of claims 1 to 4, characterized in that the SCR-active material comprises a small-pore zeolite, which is exchanged with copper and / or iron.

6. Particle filter according to claim 5, characterized in that the small-pore zeolite of the structure type AEI, CHA (chabazite), ERI (erionite), LEV (Levyne), AFX, DDR or KFI belongs.

7. Particle filter according to claim 5 and / or 6, characterized in that the small-pore zeolite is an aluminosilicate, Silicoaluminophosphate or aluminophosphate.

8. Particle filter according to one or more of claims 5 to 7, characterized in that the iron or copper amounts 0.2 to 6 wt .-%, calculated as Fe ₂ 03 or CuO and based on the total weight of the exchanged small-pore zeolite , amount.

Particulate filter according to one or more of claims 1 to 8, characterized in that the coating consists of two or more layers differing in the selectivity of the SCR active material, the outermost layer facing the exhaust gas having the highest selectivity and the innermost layer has the lowest selectivity.

10. Particle filter according to one or more of claims 1 to 9, characterized in that the coating consists of two layers, wherein the exhaust gas facing layer has a higher selectivity in the SCR reaction than the inner surface of the pores facing layer.

11. Particle filter according to one or more of claims 1 to 10, characterized in that term selectivity defined as the quotient of the conversion of NO _x and the conversion of Nhb, ie

S = X (NOx) / X (NH3)

12. A particulate filter according to claim 11, characterized in that for the determination of X (NOX) and X (NH3) coated with the SCR-active material of each coating, a wall flow filter, then hydrothermally aged for 16 hours at 800 ° C, then the conversion of NO _x or Nhb at 500 ° C in a test gas of the following composition

and then the quotient X (NOX) / X (NH3) is calculated.

13. Particle filter according to one or more of claims 1 to 12, characterized in that the amount of coating with SCR catalytically active material is 70 to 150 g / L, based on the volume of the wall flow filter.

Particulate filter according to one or more of claims 1 to 13, characterized in that the coating with SCR-active material consists of two layers, each containing a copper-exchanged small-pore zeolites, wherein the small-pore zeolite of the exhaust-facing layer 0, 3 to 3 wt .-% Cu, calculated as CuO and Based on the exchanged zeolite of this layer and the small pore zeolite of the inner pore surface facing layer 0.5 to 5 wt .-% Cu, calculated as CuO and based on the

exchanged zeolites of this layer containing.

Particulate filter according to one or more of claims 1 to 14, characterized in that the coating with SCR active material consists of two layers, each containing a copper-exchanged zeolite, wherein the zeolite contained in the layer, the to the exhaust gas, less Cu, calculated as CuO and based on the exchanged zeolite, contains this layer as the zeolite contained in the layer facing the inner pore surface.

16. Particle filter according to one or more of claims 1 to 15, characterized in that the coating with SCR-active material consists of two layers, each containing a copper-exchanged zeolites of the structure type chabazite (CHA), wherein the zeolite of the exhaust gas facing layer less than 3 wt .-% Cu, calculated as CuO and based on the exchanged zeolite of this layer and the zeolite of the inner pore surface facing layer more than 3 wt .-% Cu, calculated as CuO and based on the exchanged zeolites of these Layer containing.

17. Particle filter according to one or more of claims 1 to 15, characterized in that the coating with SCR-active material consists of two layers, each containing a copper-exchanged zeolites of the structure type Levyne (LEV), wherein the zeolite of the exhaust gas facing layer less than 3 wt .-% Cu, calculated as CuO and based on the exchanged zeolite of this layer and the zeolite of the inner pore surface facing layer more than 3 wt .-% Cu, calculated as CuO and based on the exchanged zeolites of these Layer containing.

18. A process for purifying exhaust gas from lean operated

Internal combustion engines, characterized in that the exhaust gas is passed through a particulate filter according to one or more of claims 1 to 17.

19. Apparatus for purifying exhaust gas from lean operated

Internal combustion engines, characterized in that they include

Particulate filter according to one or more of claims 1 to 17, as well as a means for providing a reducing agent.

20. The device according to claim 19, characterized in that the means for providing a reducing agent is an injector for aqueous urea solution.

21. Device according to one of claim 19 and / or 20, characterized in that it comprises an oxidation catalyst.

22. The device according to claim 19, characterized in that the means for providing a reducing agent is a nitrogen oxide storage catalyst.