CN113226546A

CN113226546A - Catalytically active filter substrate, method for the production thereof and use thereof

Info

Publication number: CN113226546A
Application number: CN202080007833.4A
Authority: CN
Inventors: B·巴尔特; A·舒勒; M·弗尔斯特
Original assignee: Umicore Corp And Lianghe Co
Current assignee: Umicore Corp And Lianghe Co; Umicore AG and Co KG
Priority date: 2019-01-04
Filing date: 2020-01-02
Publication date: 2021-08-06
Also published as: WO2020141191A1; DE102019100107A1

Abstract

The present invention relates to a method for producing a particle filter. The invention relates to a method for producing a coated wall-flow filter, to a wall-flow filter produced using said method, and to the use of a wall-flow filter for reducing hazardous components in exhaust gases of a combustion process. Particulate filters are commonly used to filter exhaust gases from combustion processes. The invention likewise relates to novel filter substrates and to their specific use in exhaust gas aftertreatment.

Description

Catalytically active filter substrate, method for the production thereof and use thereof

Detailed Description

The present invention relates to a method for producing a particle filter. Particulate filters are commonly used to filter exhaust gases from combustion processes. The invention likewise relates to novel filter substrates produced accordingly according to the invention and to their specific use in exhaust gas aftertreatment.

Exhaust gas of an internal combustion engine, for example, usually contains harmful gases such as carbon monoxide (CO), Hydrocarbons (HC), and Nitrogen Oxides (NO)_x) And possibly Sulfur Oxides (SO)_x) And particulates consisting essentially of smoke residue and possibly attached organic agglomerates. These are referred to as primary emissions. The CO, HC, and particulates are products of incomplete combustion of the fuel inside the combustion chamber of the engine. When the combustion temperature locally exceeds 1400 ℃, nitrogen and oxygen in the intake air form nitrogen oxides in the cylinder. Sulfur oxides are the result of combustion of organic sulfur compounds, a small amount of which is always present in non-synthetic fuels. In order to remove these emissions, which are harmful to health and the environment, in the exhaust gases of motor vehicles, a multiplicity of catalytic technologies for purifying the exhaust gases have been developed, the basic principle of which is generally based on the guidance of the exhaust gases to be purified over a catalyst consisting of a flow-through or wall-flow honeycomb body (wall-flow filtration period) and a catalytically active coating applied thereto and/or therein. Such catalysts promote chemical reactions of different exhaust gas components while forming harmless products such as carbon dioxide and water.

The particles can be removed from the exhaust gas very efficiently by means of the particle filter. Wall-flow filters made of ceramic materials have proven particularly successful. These filters have 2 end faces and are constructed from a plurality of parallel channels formed by porous walls and extending from one end face to the other. The channels are alternately sealed in an air-tight manner at one of the two ends of the filter, so that a first channel is formed which is open at the first side of the filter and sealed at the second side of the filter, and a second channel is formed which is sealed at the first side of the filter and open at the second side of the filter. For example, exhaust gas flowing into the first channels may leave the filter again only through the second channels and for this purpose must flow through the porous walls between the first and second channels. The particles remain unchanged as the exhaust gas passes through the wall.

One known method for removing nitrogen oxides from exhaust gases in the presence of oxygen is the selective catalytic reduction method with the aid of ammonia over a suitable catalyst (SCR method). With this method, ammonia is used to convert the nitrogen oxides to be removed in the exhaust gas into nitrogen and water. Ammonia used as reducing agent can be made available by metering an ammonia precursor compound, such as urea, ammonium carbamate or ammonium formate, into the offgas tract and subsequent hydrolysis. Ammonia may also be generated in situ by over-reducing NOx over an upstream nitrogen oxide storage catalyst.

It is also known to coat wall-flow filters with SCR-active materials in order to simultaneously remove particulates and nitrogen oxides from exhaust gases. Such products are commonly referred to as SDPF,

Or SCRAF. As regards the application of the required amount of SCR active material to the porous walls between the channels (known as wall-coating), this may however lead to an unacceptable increase in the filter back pressure. Against this background, for example, JPH01-151706 and WO2005016497A1 propose coating wall-flow filters with SCR catalysts such that the SCR catalyst penetrates the porous walls (known as an in-wall coating). It has also been proposed (see US2011274601a1) to introduce a first SCR catalyst into the porous walls, i.e. to coat the inner surfaces of the pores, and to place a second SCR catalyst on the surface of the porous walls. In this case, the average particle size of the first SCR catalyst is smaller than the average particle size of the second SCR catalyst.

For lean-burn or stoichiometric-burn engines, the application of different catalytically active materials to wall-flow filters has been described. Nitrogen oxide storage catalysts (NSC, LNT, NSR) are used to remove nitrogen oxides contained in Lean-burn exhaust gases of so-called Lean-burn mixture engines (diesel engines, Lean-GDI). The purification effect is based on the following facts: during the lean-burn operating phase (storage phase, lean-burn operation) of the engine, nitrogen oxides are stored in the form of nitrates by the storage material of the storage catalyst. During the subsequent rich-burn operating phases (regeneration phase, rich-burn operation, DeNOx phase) of the engine, the previously formed nitrates are decomposed and during the rich-burn operation, the nitrogen oxides released again are converted at the storage catalyst into nitrogen, carbon dioxide and water with the reduction of the rich-burn components of the exhaust gas (SAE 950809). Hydrocarbons, carbon monoxide, ammonia and hydrogen, among others, are designated as rich fuel components of the exhaust gas. Such filters coated with a nitrogen oxide storage catalyst are known as NDPF (e.g. in EP 1608854B 1), among others.

So-called three-way catalysts are used for reducing the exhaust gases of stoichiometric combustion engines. Three-way catalysts (TWCs) have long been known to those skilled in the art and have been regulated by law since the eighties of the twentieth century. The actual catalyst mass here comprises the majority of the oxidic substrate material with a high surface area, on which the catalytically active components are deposited with a minimum distribution. The noble metals of the platinum group (platinum, palladium, rhodium, iridium, ruthenium and osmium) are particularly suitable as catalytically active components for cleaning exhaust gases of stoichiometric composition. For example, alumina, silica, titania, zirconia and mixed oxides thereof, and zeolites are suitable as the substrate material. Preferably, use is made of a material having a thickness of more than 10m²A specific surface area (BET surface, measured according to DIN 66132, from the filing date) of a material known as activated alumina. In addition, the three-way catalyst includes an oxygen storage component that enhances dynamic conversion. These include cerium oxide, praseodymium oxide and cerium/zirconium mixed oxides (see description above for NSC; EP1181970A 1). Also, partitioned and multi-layered systems with ternary activity are known (US 8557204; US 8394348). If such a three-way catalyst is located on or in a particulate filter, this is called cGPF (e.g. EP 2650042B 1).

Oxidation catalysts for removing the polluting gases carbon monoxide (CO) and Hydrocarbons (HC) from the exhaust gases of diesel engines and lean-burn operating internal combustion engines are likewise well known in the art and are based primarily on platinum and alumina. Examples of diesel oxidation catalysts can be found in patent applications DE 10308288 a1, DE 19614540a1, DE 19753738 a1, DE 3940758 a1, EP 0427970 a2 and DE 4435073a 1. They oxidize the pollutant gases with the large amounts of oxygen contained in the diesel exhaust to carbon dioxide (CO2) and water vapor. A cerium oxide containing catalyst for the oxidation of carbon monoxide and hydrocarbons is described in WO 2013/149881. These forms of catalyst are also present on wall flux filters and are known in the art as, for example, cdpfds (e.g. EP 1309775B 1).

Common to all particulate filters is that they have to be regenerated at specific time intervals. That is, the accumulated particulate matter must be burned off to maintain the exhaust back pressure within an acceptable range. Exhaust gas temperatures of about 600 c are required for filter regeneration and soot combustion initiation. During combustion, very high temperatures may occur, which may be >800 ℃.

Furthermore, as already reported above, it must be ensured that a wall-flow filter correspondingly coated with a catalytically active material exerts as neutral an influence on the exhaust gas back pressure as possible, since an increased exhaust gas back pressure has an adverse effect on fuel consumption and thus on CO₂Emissions have an adverse effect. As noted above, techniques for reducing exhaust backpressure from a catalytically coated wall-flow filter have been described in the prior art.

Specific loading of porous filters with coating suspensions is described in EP 2727640a 1. The pores on the inflow side of the filter have internal particles which completely fill the pores at least in their cross section substantially transverse to the flow direction, and by virtue of this porous filling the pore volume of the filled pores is 50% to 90% of the pore volume of the unfilled pores. The pore volume of the filling pores at the inflow side of the filter is smaller than the pore volume of the filling pores farther from the inflow side of the filter in the flow direction, and the pore volume of the filling pores in the flow direction further increases as the pores are farther from the inflow side of the filter.

EP1716903B1 proposes a method of coating a filter body in which the filter releases excess coating dispersion after coating by repeatedly applying pressure pulses to one end of the filter body so that excess coating suspension is forced out of the filter body until its optimum coating weight is reached. The object here also appears to be, in particular, a reduction in the exhaust back pressure of the filter.

It is an object of the present invention to provide a method for producing a catalytically coated ceramic wall flux filter substrate, which method in particular allows for the production of an improved wall flux filter substrate. Wall-flow filters produced in this way should be superior to substrates produced accordingly according to the prior art, in particular with regard to the requirement of a low exhaust gas back pressure as possible but sufficient catalytic activity. Another object of the invention is to specify filter substrates prepared by the above-described method and their use in exhaust gas aftertreatment.

These and other objects, which are apparent from the prior art, are achieved by a method having the features of the object claims 1, 5 and 7. The dependent claims dependent on these claims relate to preferred embodiments of the method according to the invention.

Since in the method for producing a catalytically coated ceramic wall flux filter with an optimized exhaust gas back pressure for treating the exhaust gases of a combustion process, wherein after the catalytically active material has been applied to the filter substrate the filter has a porosity of > 50% (DIN 66133-latest version according to the filing date) and a mean pore diameter (resulting from the port volume) of 5 μm-50 μm (DIN 66134-latest version according to the filing date), in the next step the pores determine a volume flow through the filter wall of ≧ 50% by increasing and/or decreasing the pressure in the form of one or more pressure pulses relative to the coating direction, are particularly depleted without reducing the amount of catalytically active material on the filter substrate by more than 20%, the solution to achieve said object is very surprisingly achieved, but less advantageous. The amount of catalytically active material removed refers to the amount removed from the wall-flow filter by one or more pressure pulses. This therefore means the difference between the load before and after the pressure pulse is completed.

Extensive studies have shown that the exhaust gas flows through the filter wallThe volume flow is mainly realized by a large hole. If a pressure differential is applied across the orifice, laminar gas flow passes through the orifice. Then the volume flow rate and d⁴And (4) in proportion. I.e. 256 times the gas flow through 4 x larger diameter. If the pores release the catalytically active material by at least one short pressure pulse, the volume flow can flow through the walls of the ceramic filter without a high exhaust gas back pressure. In the smaller pores that make up the majority of the total porosity of the filter material, the catalytically active material may survive without obstructing the passing exhaust gas. Substrates prepared in this manner exhibit sufficient catalytic activity at exhaust back pressure, which is reduced compared to prior art catalytically active filters.

The pressure pulses (overpressure and/or underpressure) emanating from one or both end faces of a freshly coated wall-flow filter are a measure for the amount of catalytically active substance applied to be sufficiently removed from larger channels or pores (e.g.. gtoreq.40 μm pore size) passing through the wall. Typically, as already indicated above, only the large holes or channels that are reached through the wall are "blown out" or "released". The catalytically active species remain in the smaller pores of the filter wall (e.g.. ltoreq.40 μm pore size), which, however, constitute the majority of the porosity of the filter material (e.g. > 50%, more preferably > 80%, and particularly preferably > 90%). In the usual manner, therefore, it is also possible to react heterogeneously catalytically with harmful exhaust gas constituents. If distributions of pore sizes or particle sizes are mentioned below, the Q3 distribution always means (https:// de. wikipedia. org/w/index. phitlite ═ Partikelgr%. C3%. B6%. C3% 9 Fenverteileung & oldid ═ 181716548).

The filter substrate used preferably has an average pore size (d50) of 10 μm to 40 μm. More preferably, the filter substrate used has an average pore size of from 13 μm to 30 μm. Furthermore, the filter substrates used have a porosity of > 50%, more preferably from 50% to 70%, and very particularly preferably from 59% to 65%. For this method for producing a catalytically active exhaust gas filter with as low an exhaust gas back pressure as possible, it is preferred to use a filter substrate with a bimodal pore size distribution. A bimodal pore size distribution means that the filter substrate has substantially 2 maxima in the Q3 distribution. In this case, the first maximum should be in the range from 5 μm to 30 μm, particularly preferably from 10 μm to 25 μm, and very particularly preferably from 12 μm to 20 μm.

In contrast, the second maximum is from 20 μm to 100 μm, preferably from 25 μm to 80 μm, and very particularly preferably from 30 μm to 60 μm. The pores which are preferably released and correspond to the larger maximum are responsible for the backpressure of the exhaust gas, while the smaller pores contain the catalytically active compound.

As already indicated, the volume flow is achieved via the porous walls of the filter substrate, mainly by the corresponding macropores. Typically, these volume flows exceed 50%, preferably 60%, and very particularly preferably 70% of the total volume flow through the filter. These large pores, which typically have a pore size ≧ the d80 value of the filter substrate, are much lower in number and volume than the smaller pores in the filter substrate (less than the d60 value of the filter substrate). Thus, the proportion of the latter of the total porosity of the filter is the same.

Generally, the intensity and/or duration of the pressure pulse is adapted to the substrate of the filter. In a first approximation, it may be assumed that the product of the pressure difference and the suction time to be applied depends on the shape of the orifice. The larger the length of the hole, the smaller the diameter of the hole and the higher the viscosity of the coating suspension, the larger the product of the pressure difference and the suction time must be chosen. The magnitude and/or duration of the pressure pulse is preferably adapted to the Q3 value of the pore size > d60 value of the filter substrate, more preferably > d80 value, and very particularly preferably > d90 value.

By a simplified approximation such as ignoring transient terms, the following applies:

flow rate in the hole:

time to empty the hole: t to L/v

Thus:

reformulation, the result is:

v: volume flow through one hole [ m ]³/s]

v: velocity in the hole, in [ m/s ]

t: until the holes are sucked or blown [ s ]

d: pore diameter [ mu m ]

L: length of the hole [ μm ]

Eta: dynamic viscosity [ Pas. s ]

Δ p: differential pressure [ mbar ]

Preferably, the thickness and/or duration of the pressure pulse is adapted to have a pore size of the pores of the filter substrate of ≧ 30 μm, more preferably ≧ 40 μm, and most preferably ≧ 45 μm.

As described above in the equation, the desired product of the duration of the suction pulse and the pressure differential also depends on the square of the orifice length. The cell length is primarily proportional to the cell wall thickness. Thus, for varying cell wall thicknesses, a varying quadratic product of the pressure difference and the duration of the suction pulse is required.

In the case of conventional substrates for ceramic wall flow filters and common viscosities (see below), the duration of the pressure pulse at a constant level is advantageously between 0.1 and less than 5. The duration of the pressure pulse is more preferably between 0.5 and 4 seconds, and most preferably between 1 and 3 seconds. As mentioned, the intensity of the pressure pulse should be sufficient to release the catalytically active material precisely located in the respective macropores. Typically, the intensity of the pressure pulse will be in the range of 5kPa-80kPa, more preferably in the range of 20kPa-50kPa, and most preferably in the range of 30kPa-45 kPa.

To further distinguish between large and small holes, it may preferably be advantageous that the pressure pulse reaches full development within a relatively short period of time, and then the suction pressure remains stable and reproducible at a defined level for a defined period of time (corresponding to a formula). The maximum pressure difference should be reached within 0.5s or less, more preferably 0.2s or less, and most preferably 0.1s or less. The duration of the pressure pulse should in any case be less than 5 minutes. A short strong pressure pulse opens the large pores almost completely, but results in only a small removal of the coating suspension. In this connection, the coating suspension on the filter is reduced by the treatment by less than 20%, preferably less than 15%, and very particularly preferably less than 10%.

The contact of the catalytically active material with the wall flux filter is known in the art as a coating. The term refers to the use of catalytically active materials and/or storage components on a mostly inert support body/substrate. The coating has a practical catalytic function and typically comprises a storage material and/or a catalytically active metal which is deposited in most cases in highly dispersed form on a temperature-stable metal oxide having a large surface area. In most cases, the coating is effected by applying an aqueous suspension of storage material and catalytically active component (also referred to as washcoat) onto or into the walls of the wall-flow filter. After application of the suspension, the substrate is typically dried and, where appropriate, calcined at elevated temperature. The coating may consist of one layer or of a plurality of layers which are applied in a sequentially on top (multilayer) and/or sequentially offset relative to one another (zoned) manner on the respective filter.

In this case, the suspension with catalytically active material advantageously has a high surface oxidation component with an average particle diameter (d50 of the Q3 distribution; DIN 66160-latest edition according to filing date) which is smaller than the average pore diameter of the filter substrate. For in-wall coating, the particle size d99 of the Q3 distribution in the suspension should preferably be <0.6:1, more preferably <0.5:1, and particularly preferably <0.4:1, relative to the average pore size of the pores in the wall of the filter (d50 of the Q3 distribution). Thus, the suspension may be introduced into the pores of the walls of the wall-flow filter in a large proportion of > 80%, more preferably > 90%, and very preferably > 95% or more. By appropriate selection of the average particle size, it is possible to control how much catalytically active material is located in or on the walls of the wall-flow filter. Advantageously, the smaller the particle size of the high temperature stable oxide components, the more these components can be located in the pores. The ratio of oxide components present on the walls to oxide components present in the walls naturally also has a significant effect on the resulting exhaust gas backpressure of the filter substrate. The coating suspension may have a bimodal particle size distribution. Thus, the filter wall can be coated in one step. The application of the coating is preferably effected by a catalytically active material comprising a high-surface metal compound, in particular an oxide of which the average particle size of the Q3 distribution (DIN 66160-according to the latest version of the application) d50 is preferably >1:6 and <1:1, and particularly preferably >1:3 and <1:2, relative to the average pore size of the filter d50 of the Q3 distribution (https:// de. wikipedia. org/wiki/Partikelgr% C3% B6% C3% 9 Fenverteilung). The upper limit generally forms a value which is reasonable to the person skilled in the art in respect of the respective applied coating. In the present invention, it is particularly preferable to establish an in-wall coating. The scanning electron microscope images were evaluated by means of statistical grey scale evaluation in order to determine the proportion of the washcoat in the walls of the wall-flow filter and the proportion of the washcoat on the walls of the wall-flow filter. In this case, the free pores/air in the catalytically coated filter appear black, while the heaviest elements appear white. By appropriate selection of measurement settings known to the person skilled in the art, the difference between the active substance and the filter substrate can be evaluated in this way on the basis of the separation of the grey levels.

The coating process can advantageously be carried out several times in succession with the catalytically active material being identical in each case or different in each case. It is important here that, in the still moist state, suitable pressure pulses according to the invention are set from time to time, which ensures that the macropores are blocked as little as possible by the washcoat components of the catalyst, as described above. It should be mentioned that the coating can be carried out in each case with the same or in each case with different catalytically active materials, with intermediate drying and without drying.

In another aspect, the invention also relates to a catalytically coated ceramic wall-flow filter for treating exhaust gases of combustion processes, wherein the filter has a porosity of > 50% and a mean pore size of 5 μm to 50 μm and is prepared by a method as described above. Of course, the preferred embodiments specified for the production process are also applied, mutatis mutandis, to wall-flow filters.

In this connection, it should be mentioned that suitable filters are advantageously specified in which the macropores having a pore diameter of at least ≥ 40 μm, preferably at least ≥ 45 μm and particularly preferably at least ≥ 50 μm are substantially not filled with catalytically active material. Here, the expression "substantially" denotes the fact that the pressure pulse removes the oxide material in the coating as completely as possible from the corresponding holes (at least >80 wt.%, preferably >90 wt.%) through which the gas flow is allowed. The amount of material actually present in the pores also depends on the strength, the duration of the pressure pulse and the geometry of the pores, and on the material used accordingly and its degree of dryness. Generally, no more than 20% by weight, preferably no more than 10% by weight and very particularly preferably no more than 5% by weight of the catalytically active material present in the wall-flow filter is then formed, located in the macropores described above. The percentages here are based on the weight of the solid material after the application of the respective pressure pulse.

In a final aspect, the invention also relates to the use of a filter according to the invention in a method for oxidizing hydrocarbons and/or carbon monoxide and/or in a method for reducing nitrogen oxides originating from a combustion process, preferably of an automotive engine. The filter according to the invention is particularly preferably used in the exhaust system of an internal combustion engine as SDPF (SCR catalyst coated on a wall-flow filter) cGPF (three-way catalyst coated on a wall-flow filter), NDPF (NOx storage catalyst coated on a wall-flow filter) or cDPF (diesel oxidation catalyst coated on a wall-flow filter).

A preferred application is the removal of nitrogen oxides from lean-burn exhaust gas mixtures by the SCR method. For this SCR treatment of preferably lean-burn exhaust gases, ammonia or ammonia precursor compounds are injected into the exhaust gas and both are guided over the SCR catalytically coated wall-flow filter according to the invention. The temperature above the SCR filter should be between 150 ℃ and 500 ℃, preferably between 200 ℃ and 400 ℃ or between 180 ℃ and 380 ℃ so that the reduction can take place as completely as possible. The temperature range of 225 ℃ to 350 ℃ for the reduction is particularly preferred. In addition, only whenWith a molar ratio of nitric oxide to nitrogen dioxide (NO/NO)₂1) or NO₂The optimum nitrogen oxide conversion is only achieved at a/NOx ratio of about 0.5 (G.Tuentry et al, development of Industrial and engineering chemical research (Ind. Eng. chem. Prod. Res. Dev.), 1986, 25 th, 633 th. minus 636, EP1147801B 1; DE2832002A 1; Kasaoka et al, Japan chemical society (Nippon Kagaku Kaishi), 1978 th, 6 th, 874 th. 881. pages, Avila et al, Atmospheric environment 443, 443 th, 27A, 447 th. pages 447). Optimum conversion starting with a conversion of 75% already at 250 ℃ and at the same time optimum selectivity to nitrogen according to the stoichiometry of the reaction equation,

2NH₃+NO+NO₂→2N₂+3H₂O

only about 0.5 of NO is realized₂the/NOx ratio. This applies not only to SCR catalysts based on metal-exchanged zeolites, but also to all common (i.e. commercially available) SCR catalysts (so-called fast SCR). Corresponding NO: NO₂The content may be achieved by an oxidation catalyst located upstream of the SCR catalyst.

The injection device used can be selected at the discretion of the person skilled in the art. Suitable systems can be found in the literature (t.mayer, Solid SCR system based on ammonium carbamate]A academic paper, University of Kaiserlaunt technology (Technical University of Kaiserslauter),2005 the year). The ammonia may be introduced into the exhaust gas stream via an injection device either as such or in the form of a compound that generates ammonia at ambient conditions. Examples of possible compounds include urea or aqueous solutions of ammonium formate, as well as solid ammonium carbamate and the like. These substances can be obtained from supply sources known per se to the person skilled in the art and can be added to the exhaust gas stream in a suitable manner. The person skilled in the art particularly preferably uses an injection nozzle (EP0311758a 1). By these, NH is adjusted₃Optimum ratio of NOx so that nitrogen oxides can be converted as completely as possible into N₂。

Wall-flow filters with SCR catalytic function are called SDPF. These catalysts are used in many casesThe nitrogen-containing ammonia gas has the function of storing ammonia and the function of forming harmless nitrogen by the reaction of nitrogen oxides and ammonia. The NH can be designed according to the type known to the person skilled in the art₃Storing the SCR catalyst. In this case, this is a wall-flow filter which is coated with a catalytically active material for the SCR reaction and in which the catalytically active material (often referred to as "washcoat") is present in the pores of the wall-flow filter. However, in addition to the "catalytically active" component in the correct sense of the term, the wall-flow filter may also comprise other materials, such as binders consisting of transition metal oxides and large-surface support oxides, such as titanium oxide, aluminum oxide (in particular γ -Al)₂O₃) Zirconium oxide or cerium oxide. Those catalysts consisting of one of the materials listed below are also suitable for use as SCR catalysts. However, it is also possible to use a zoned or multilayered arrangement, or even an arrangement consisting of a plurality of components (preferably two or three components) one after the other, of the same material as the SCR component or of a different material. Mixtures of different materials on the substrate are also contemplated.

The actual catalytically active material used in accordance with the invention in this connection is preferably selected from transition metal-exchanged zeolites or zeolite-like materials (zeotypes). Such compounds are well known to those skilled in the art. In this regard, materials selected from levyne, AEI, KFI, chabazite, SAPO-34, ALPO-34, zeolite beta, and ZSM-5 are preferred. Zeolites of the chabazite type or zeotype materials, in particular CHA or SAPO-34, and LEV or AEI are particularly preferred. In order to ensure sufficient activity, these materials preferably have a transition metal selected from iron, copper, manganese and silver. In this connection it should be mentioned that copper is particularly advantageous. The ratio of metal to framework aluminum, or for SAPO-34, the ratio of metal to framework silicon is generally between 0.3 and 0.6, preferably 0.4 to 0.5. It is known to the person skilled in the art how to provide zeolite or zeolite-like materials with transition metals (EP0324082a1, WO1309270711a1, WO2012175409a1 and the documents cited therein) in order to be able to provide good activity in connection with the reduction of nitrogen oxides with ammonia. Furthermore, vanadium compounds, cerium oxide, cerium/zirconium mixed oxides, titanium oxide and tungsten-containing compounds and mixtures thereof may also be used as catalytically active material.

Furthermore, it has proven advantageous to store NH₃The materials used are known to the person skilled in the art (US20060010857a1, WO2004076829a 1). In particular, microporous solid materials such as so-called molecular sieves are used as storage materials. Such compounds selected from the following may be used: zeolites such as Mordenite (MOR), Y-zeolite (FAU), ZSM-5(MFI), Ferrierite (FER), Chabazite (CHA); and other "small pore zeolites" such as LEV, AEI or KFI, and beta-zeolite (BEA); and zeolite-like materials such as aluminophosphates (alpos) and silicoaluminophosphates SAPOs or mixtures thereof (EP0324082a 1). Particular preference is given to using ZSM-5(MFI), Chabazite (CHA), Ferrierite (FER), ALPO-or SAPO-34, and also beta-zeolite (BEA). CHA, BEA and AlPO-34 or SAPO-34 are particularly preferably used. It is highly preferred to use an LEV or CHA type material and most preferred here is CHA or LEV or AEI. If the zeolite or zeotype compound mentioned immediately above is used as catalytically active material in an SCR catalyst, it may be advantageous to add further NH naturally₃Storing the material. In general, the storage capacity of the ammonia storage component used may exceed 0.9g NH in the fresh state at a measured temperature of 200 ℃₃Per liter of catalyst volume, preferably between 0.9g and 2.5g NH₃Between 1.2g and 2.0g NH/liter of catalyst volume₃Between the catalyst volumes per liter, and very particularly preferably between 1.5g and 1.8g NH₃Per liter of catalyst volume. The ammonia storage capacity may be determined using a syngas plant. For this purpose, the catalyst is first conditioned with NO-containing synthesis gas at 600 ℃ to completely remove the ammonia residues in the core. After cooling the gas to 200 ℃, the ammonia is then added for 30,000h, for example^-1Until the core is completely filled with ammonia storage, and the ammonia concentration measured downstream of the core corresponds to the starting concentration. The ammonia storage capacity results from the difference between the metered total ammonia amount and the catalyst volume-based ammonia amount measured on the downstream side. The synthesis gas here is generally composed of 450ppm NH₃5% oxygen, 5% water and nitrogen.

All filter bodies made of ceramic materials typical of the prior art can be used as particle filters. Porous wall-flow filter substrates made of cordierite, silicon carbide or aluminum titanate are preferably used. These wall-flow filter substrates have inflow channels and outflow channels, wherein the respective downstream ends of the inflow channels and the respective upstream ends of the outflow channels are alternately closed by gas-tight "plugs". The diameter of the inflow channel may be equal to or different from the diameter of the outflow channel. In this case, as already mentioned at the outset, the exhaust gas to be purified which flows cocurrently through the filter substrate is forced through the porous walls between the inflow channels and the outflow channels, which leads to an excellent particle filtration effect. The filtration properties of the particles can be designed by means of porosity, pore/diameter distribution and wall thickness. The catalyst material may be provided in and/or on the porous walls between the inlet and outlet channels in the form of a coating. It is also possible to use filters extruded directly or with a binder from the respective catalyst material, i.e. the porous walls are made directly of the catalyst material. Filter substrates which are preferably used are known from EP1309775a1, EP2042225a1 or EP1663458a 1.

With the present invention, it is possible in particular to prepare wall-flow SCR catalytically coated particulate filters with an in-wall coating which have excellent activity at a not significantly increased exhaust gas backpressure. This is beyond expectations in the context of the known prior art.

FIG. 1: pressure drop curves plotted over time. The underpressure was calibrated to 380mbar (maximum). Substrate: SiC-filter with zeolite coating, 300CPSI, 12 mil wall thickness. (according to an embodiment of the present invention.)

FIG. 2: the 3 parts coated according to the invention were loaded with a pressure loss at an initial underpressure of 380mbar (using the conditions of figure 1). Substrate: SiC-filter with zeolite coating, 300CPSI, 12 mil wall thickness. (according to an embodiment of the present invention.)

FIG. 3: pressure drop curves plotted over time. The underpressure was calibrated to 380mbar (maximum). Substrate: SiC-filter with zeolite coating, 300CPSI, 12 mil wall thickness. (not in accordance with embodiments of the invention.)

FIG. 4: the pressure loss to the load of 3 parts which were not coated according to the invention at an initial underpressure of 380mbar (using the conditions of fig. 3). Substrate: SiC-filter with zeolite coating, 300CPSI, 12 mil wall thickness. (not in accordance with embodiments of the invention.)

FIG. 5: in embodiments according to the invention and not according to the invention, the pressure loss of the coated filter is compared to the pressure loss of the uncoated substrate. Substrate: SiC-filter with zeolite coating, 300CPSI, 12 mil wall thickness.

FIG. 6: pressure drop curves of the automatic false air flap during pressure pulses plotted over time. Substrate: cordierite-a filter with an oxide coating containing precious metal, 300CPSI, 8 mil wall thickness. (according to an embodiment of the present invention.)

FIG. 7: pressure drop profile during pressure pulse without automatic false air flap. Substrate: cordierite-a filter with an oxide coating containing precious metal, 300CPSI, 8 mil wall thickness. (not in accordance with an embodiment of the invention.) while fig. 7 has a peak at 100%, the suction has a level effect of about 80% of the initial negative pressure, thus creating a worse pressure loss after coating than the condition in fig. 6.

Examples：

According to the method, the proportion of the pore volume of the coating as a function of the pore diameter after coating and suction is compared. The large pores, which determine the pressure loss to a large extent, are transparent to a large extent.

Cordierite 300/8

Coated with 75g/l

The table shows, by way of example, that the filter substrate in the macroporous region hardly loses any pore volume due to the coating according to the method described herein.

In the following experiments, the pumping time and the solids concentration were varied. All other processes and feed parameters were kept constant. The results show that the pressure loss at the filter can be significantly reduced with increasing suction time.

According to the formula

Larger holes may be released or blown out

Pumping time second	Solids concentration of the slurry	Dry absorption	BP increase
				2	35％	100％	16％
4	35％	99％	14％
				2	42％	160％	131％
4	42％	153％	61％

In the following experiments, the suction pressure difference and the solid concentration were varied. All other processes and feed parameters were kept constant. The results show that as the initial evacuation pressure increases, the pressure loss at the filter can decrease.

According to the formula

Larger holes may be released or blown out

Pumping time second	Solids concentration of the slurry	Dry absorption	BP increase
				410	35％	99％	14％
380	35％	99％	17％
				410	42％	157％	98％
380	42％	159％	109％

Claims

1. A method for producing a catalytically coated ceramic wall-flow filter having an optimized exhaust gas backpressure for treating the exhaust gas of a combustion process, the filter having a porosity of > 50% and a mean pore diameter of 5 μm to 50 μm,

it is characterized in that the preparation method is characterized in that,

after the application of the catalytically active material to the filter substrate, in a directly following step, the pores, which determine a volume flow through the filter wall of > 50%, in the form of one or more pressure pulses opposite to the coating direction, are emptied in a targeted manner without reducing the amount of catalytically active material on the filter substrate to more than 20%.

2. The method of claim 1, wherein the first and second light sources are selected from the group consisting of,

it is characterized in that the preparation method is characterized in that,

the wall-flow filter has a bimodal pore size distribution.

3. The method according to claim 1 and/or 2,

it is characterized in that the preparation method is characterized in that,

the thickness and/or duration of the pressure pulse is adapted such that the pore size of the pores is larger than the d60 value of the filter substrate.

4. The method of any one of claims 1 to 3,

it is characterized in that the preparation method is characterized in that,

the catalytically active material comprises a high surface metal compound, preferably an oxidic component, having an average particle size (D50) smaller than the average pore size of the filter substrate.

5. The method of any one of claims 1 to 4,

it is characterized in that the preparation method is characterized in that,

in each case, the process is carried out several times in succession with the same or different catalytically active materials.

6. A catalytically coated ceramic wall-flow filter for treating exhaust gases of a combustion process prepared according to any preceding claim, the filter having a porosity of > 50% and an average pore size of from 5 to 50 μ ι η.

7. The filter as set forth in claim 6, wherein,

it is characterized in that the preparation method is characterized in that,

the pores having a pore diameter of 40 μm or more are substantially not filled with the catalytically active material.

8. Use of a filter according to any one of claims 6 and/or 7 in a process for oxidizing hydrocarbons and/or carbon monoxide and/or in a process for reducing nitrogen oxides.

9. Use of the filter of claim 8 as an SDPF.