CATALYTIC ACTIVE PARTICLE FILTER WITH HIGH FILTRATION EFFICIENCY

Description

[0001] The present invention relates to a method for manufacturing a wall flow filter for reducing harmful exhaust gases and fine particles from an internal combustion engine.

[0002] Diesel particulate filters and gasoline particulate filters, with and without additional catalytically active coatings, are suitable components for removing particulate emissions and reducing pollutants in exhaust gases. These are honeycomb-shaped structures, also known as catalyst carriers, substrates, or substrate monoliths. To meet legal standards, it is desirable for current and future applications in the exhaust aftertreatment of combustion engines to combine particulate filters with other catalytically active functionalities, both for cost reasons and due to space constraints. The catalytically active coating can be located on the surface or within the walls of the channels that form this surface. The catalytically active coating is often applied to the catalyst carrier in a coating process as a suspension.Many such legal proceedings have been published in the past by automotive catalytic converter manufacturers (EP1064094B1, EP2521618B1, WO10015573A2, EP1136462B1, US6478874B1, US4609563A, WO9947260A1, JP5378659B2, EP2415522A1, JP2014205108A2). The use of a particulate filter – whether catalytically coated or not – leads to a noticeable increase in exhaust backpressure compared to a flow-through filter of the same dimensions, and thus to a reduction in engine torque or potentially increased fuel consumption. In order to avoid further increasing the exhaust back pressure, the quantities of oxide support materials for the catalytically active precious metals of the catalyst or oxide catalyst materials are generally applied in smaller quantities to a filter than to a flow carrier.As a result, the catalytic effectiveness of a catalytically coated particle filter is often inferior to that of a flow monolith of the same size.

[0003] Several efforts have already been made to provide particulate filters that exhibit good catalytic activity through an active coating while still maintaining the lowest possible exhaust backpressure. With regard to low exhaust backpressure, it has proven advantageous if the catalytically active coating is not applied as a layer to the channel walls of a porous wall-flow filter, but rather if the filter channel walls are permeated with the catalytically active material (WO2005016497A1, JPH01-151706, EP1789190B1). For this purpose, the particle size of the catalytic coating is selected so that the particles can penetrate the pores of the wall-flow filter and be fixed there by calcination. A disadvantage of catalytically active filters with an in-wall coating is that the amount of catalytically active substance is limited by the absorption capacity of the porous wall.

[0004] It has been shown that applying catalytically active substances to the surfaces of the channel walls of a wall flux honeycomb structure can increase the conversion of pollutants in the exhaust gas. Combinations of surface and in-wall coatings with catalytically active material are also possible, which can further increase the catalytic performance without significantly increasing the back pressure (WO2018172299A1; WO2015082892A2).

[0005] Besides its catalytic effectiveness, another function of the filter that can be improved by a coating is its filtration efficiency, i.e., the filtering effect itself. Special wet-applied filtration coatings are mentioned, for example, in EP2832962A1.

[0006] WO2011151711A1 describes a method for applying a dry aerosol to an uncoated or catalytically coated filter that carries the catalytic active material in the channel walls (in-wall coating with washcoat). The aerosol is provided by distributing a powdered high-melting-point metal oxide and guided over the inlet side of a wall-flow filter by a gas stream. Here, the individual particles, with a particle size of 0.2 µm to 5 µm, agglomerate into a bridged network of particles and are deposited as a layer on the surface of the individual inlet channels through the wall-flow filter. The typical powder loading of a filter is between 5 g and 50 g per liter of filter volume. It is explicitly stated that it is not desirable to achieve a coating of the metal oxide within the pores of the wall-flow filter.

[0007] Another method for increasing the filtration efficiency of catalytically inactive filters is described in WO2012030534A1. In this method, a filtration layer ("discriminating layer") is created on the walls of the inlet-side flow channels by depositing ceramic particles via a particle aerosol. The layers consist of oxides of zirconium, aluminum, or silicon, preferably in fiber form from 1 nm to 5 µm, and have a layer thickness of more than 10 µm, typically 25 µm to 75 µm. After the coating process, the applied powder particles are calcined in a thermal process.

[0008] Another method for increasing the filtration efficiency of non-catalytically active wall-flow filters, in which a membrane ("trapping layer") is created on the surfaces of the filter inlet channels, is described in patent US8277880B2. The filtration membrane on the inlet channel surfaces is created by drawing a gas stream loaded with ceramic particles (e.g., silicon carbide, cordierite) through it. After the filter layer is applied, the honeycomb structure is fired at temperatures above 1000°C to increase the adhesion of the powder layer to the channel walls. Further coating methods using powder application are mentioned in EP2502661A2 and EP2502662B1.

[0009] A coating applied within the pores of a wall flow filter assembly by atomizing dry particles is described in US 8388721B2. Here, the powder is intended to penetrate deep into the pores. Between 20% and 60% of the wall surface should remain accessible to soot particles, and thus open. Depending on the flow velocity of the powder-gas mixture, a more or less pronounced powder gradient between the inlet and outlet sides can be established. The pores of the channel walls of the filter coated with powder according to US 8388721B2 can subsequently be coated with a catalytically active component. Again, the catalytically active material is located within the channel walls of the filter.

[0010] The introduction of the powder into the pores, e.g., using an aerosol generator, is also described in EP2727640A1. Here, a non-catalytically coated wall flow filter is coated with a gas stream containing, for example, aluminum oxide particles, such that the complete particles, which have a particle size of 0.1 µm to 5 µm, are deposited as a porous filling in the pores of the wall flow filter. The particles themselves can provide an additional functionality of the filter beyond its filtration effect. For example, these particles are deposited in the pores of the filter at a quantity of more than 80 g / l, based on the filter volume. They fill 10% to 50% of the volume of the filled pores in the channel walls. This filter exhibits improved filtration efficiency compared to the untreated filter, both when loaded with soot and when unloaded, while the soot-loaded filter has a lower exhaust backpressure.

[0011] In WO2018115900A1, wall flow filters are coated with a possibly dry synthetic ash in such a way that a continuous membrane layer is created on the walls of the possibly catalytically coated wall flow filter.

[0012] All patents listed in this prior art aim to increase the filtration efficiency of a filter by coating it with a powder. Filters optimized for filtration efficiency in this way can also have a catalytically active coating applied to the porous channel walls prior to powder coating. However, none of the examples show any evidence of simultaneously optimizing the catalytic activity of a filter and increasing its filtration efficiency. Therefore, there remains a need for particulate filters in which both catalytic activity and filtration efficiency are optimized with respect to exhaust backpressure.

[0013] The object of the present invention is to provide a corresponding particulate filter in which sufficient filtration efficiency is coupled with the lowest possible increase in exhaust back pressure and high catalytic activity.

[0014] These and other problems arising obviously from the prior art are fulfilled by the specification of a particle filter according to the claims.

[0015] The present invention relates to a method for manufacturing a wall flow filter for removing particles from the exhaust gas of internal combustion engines according to claim 1. By applying a dry powder gas aerosol to a wall flow filter coated in a conventional wet-technical zoned manner, in which the catalytically active coating is located on the surface of the channel wall and which is coated with a dry powder gas aerosol after drying and / or calcining, a wall flow filter with extremely good filtration efficiency and only slightly increased exhaust back pressure and at the same time excellent catalytic effectiveness is obtained.

[0016] The catalytically coated filters described here, which are subsequently dusted with powder, differ from those formed in a vehicle's exhaust system by ash deposition during operation. According to the invention, the catalytically active filters are selectively dusted with a specific dry powder. This allows the balance between filtration efficiency and exhaust backpressure to be precisely adjusted from the outset. Therefore, wall-flow filters, in which undefined ash deposits result from fuel combustion, e.g., in the cylinder during driving or by means of a burner, are not covered by the present invention.

[0017] All conventional ceramic materials can be used as wall flow monoliths or wall flow filters. Porous wall flow filter substrates made of cordierite, silicon carbide, or aluminum titanate are preferred. These wall flow filter substrates have inlet and outlet channels, with the outlet ends of the inlet channels and the outlet channels being offset from each other and sealed with gas-tight plugs. The exhaust gas to be cleaned, flowing through the filter substrate, is forced to pass through the porous wall between the inlet and outlet channels, resulting in a particle filtering effect. The filtration performance for particles can be tailored by adjusting the porosity, pore / radius distribution, and wall thickness. The porosity of uncoated wall flow filters is typically more than 40%, generally between 40% and 75%, and particularly between 50% and 70% [measured according to DIN 66133 - latest version on the filing date].The average pore size of the uncoated filters is at least 7 µm, e.g., from 7 µm to 34 µm, preferably more than 10 µm, particularly preferably from 10 µm to 25 µm or most preferably from 15 µm to 20 µm [measured according to DIN 66134 latest version on the filing date]. The finished filters (catalytically coated) with a pore size of typically 10 µm to 20 µm and a porosity of 50% to 65% are particularly preferred.

[0018] The catalytic coating Z of the wall flow filter to be coated with powder does not extend over the entire length of the wall flow filter. Starting from each end of the wall flow filter, the coating Z is located over a length of 10 to 90% of the length L. The minimum length of the coating Z is at least 1.25 cm, preferably at least 2.0 cm, and most preferably at least 2.5 cm, measured from each end of the filter. The coating Z is located on the surface OA. This so-called application coating extends from each end of the wall flow filter over 10 to 90% of the length L of the wall flow filter, preferably 30 to 80%, and particularly preferably 60 to 80%. Application coating means that this coating extends from the surface OA into the channel A of the wall flow filter, thus reducing the channel cross-section.Only a subordinate proportion of the surface pores OA are filled with the catalytically active material. More than 80%, preferably more than 90%, of the catalytically active material is not located in the porous wall of channel A.

[0019] In a further preferred embodiment, the coating Z has a thickness gradient over the length L such that the thinnest coating Z is found at the respective ends of the wall flow filter. The thickness thus increases along the length L of the wall flow filter (see Fig. 3This coating may preferably have a thickness at one end of the coating that is more than twice, and more preferably up to three times, greater than that at the other end. The thickness is the height by which the coating Z rises above the surface OA. The thickness gradient of the coating on the channel wall also ensures that the filtration efficiency is uniform along the entire length L of the filter. This results in more uniform soot separation across the entire filter wall, and thus improved exhaust backpressure and potentially better soot combustion.

[0020] The wall flow filter, which according to the invention is exposed to the powder gas aerosol, therefore already possesses catalytic activity (here denoted by Z). In this context, catalytic activity is understood to mean the ability to convert harmful components of the exhaust gas from combustion engines into less harmful ones. In particular, the exhaust gas components NOx, CO, and HC, or particulate combustion, are to be mentioned here. According to the skilled person, this catalytic activity is provided by coating the walls of the wall flow filter with a catalytically active material, either within its walls or on its surface. Accordingly, the term "coating" refers to the application of catalytically active materials to a wall flow filter. The coating performs the actual catalytic function.The coating is applied by incorporating a low-viscosity aqueous suspension of the catalytically active components – also known as a washcoat – into or onto the wall of the wall-flow filter, e.g., according to EP1789190B1. After application of the suspension, the wall-flow filter is dried and, if necessary, calcined at an elevated temperature. The catalytically coated filter has a loading of 20 g / l to 200 g / l, preferably 30 g / l to 150 g / l. The most suitable loading for a wall-coated filter depends on its cell density, wall thickness, and porosity.

[0021] In principle, all coatings known to those skilled in the art for automotive exhaust systems, as well as combinations thereof, are suitable for the present invention. The catalytic coating of the filter can preferably be selected from the group consisting of three-way catalysts, SCR catalysts, nitrogen oxide storage catalysts, oxidation catalysts, and soot combustion coatings. For details of the individual catalytic functions under consideration and their explanation, reference is made to WO2011151711A1.

[0022] Coatings possessing three-way functionality and being particularly active at operating temperatures of 250 to 1100 °C are especially preferred. They typically contain one or more precious metals fixed to one or more carrier materials, as well as one or more oxygen storage components. Platinum, palladium, and rhodium are particularly suitable as precious metals, with palladium, rhodium, or palladium and rhodium being preferred and palladium and rhodium being particularly preferred. With regard to the particle filter according to the invention, the proportion of rhodium in the total precious metal content is particularly greater than or equal to 10 wt.%. The precious metals are typically used in amounts of 0.15 to 5 g / l, based on the volume of the wall flow filter.

[0023] All materials known to those skilled in the art for this purpose are suitable as carrier materials for the precious metals. Such materials are, in particular, metal oxides with a BET surface area of ​​30 to 250 m² / g, preferably 100 to 200 m² / g (determined according to DIN 66132 - latest version on the filing date). Particularly suitable carrier materials for the precious metals are selected from the following: aluminum oxide, doped aluminum oxide, silicon dioxide, titanium dioxide, and mixed oxides of one or more of these. Doped aluminum oxides include, for example, lanthanum oxide-, zirconium oxide-, and / or titanium oxide-doped aluminum oxides. Lanthanum-stabilized aluminum oxide is advantageously used, wherein lanthanum is used in amounts of 1 to 10 wt.%, preferably 3 to 6 wt.%, in each case calculated as La₂O₃ and based on the weight of the stabilized aluminum oxide.Another suitable substrate material is lanthanum-stabilized aluminum oxide, the surface of which is coated with lanthanum oxide, barium oxide or strontium oxide.

[0024] Cerium / zirconium / rare earth metal mixed oxides are particularly suitable as oxygen storage components. The term "cerium / zirconium / rare earth metal mixed oxide" as used in the present invention excludes physical mixtures of cerium oxide, zirconium oxide, and rare earth oxide. Rather, "cerium / zirconium / rare earth metal mixed oxides" are characterized by a largely homogeneous, three-dimensional crystal structure, which is ideally free of phases of pure cerium oxide, zirconium oxide, or rare earth oxide (solid solution). Depending on the manufacturing process, however, products that are not completely homogeneous may also be formed, which can generally be used without disadvantage. Furthermore, the terms rare earth metal and rare earth metal oxide as used in the present invention do not include cerium or cerium oxide, respectively. Examples of rare earth metal oxides in the cerium / zirconium / rare earth metal mixed oxides include lanthanum oxide, yttrium oxide, praseodymium oxide, neodymium oxide, and / or samarium oxide.Preferred materials include lanthanum oxide, yttrium oxide, and / or praseodymium oxide. Lanthanum oxide and / or yttrium oxide are particularly preferred, and lanthanum oxide and yttrium oxide, yttrium oxide and praseodymium oxide, as well as lanthanum oxide and praseodymium oxide are especially preferred. In embodiments of the present invention, the oxygen storage components are free of neodymium oxide. According to the invention, the mass ratio of cerium oxide to zirconium oxide in the cerium / zirconium / rare earth metal mixed oxides can vary widely. It is, for example, 0.1 to 1.5, preferably 0.2 to 1 or 0.3 to 0.5.

[0025] If the cerium / zirconium / rare earth metal mixed oxides contain yttrium oxide as a rare earth metal, its proportion is, in particular, 5 to 15 wt.% based on the oxygen storage material. If the cerium / zirconium / rare earth metal mixed oxides contain praseodymium oxide as a rare earth metal, its proportion is, in particular, 2 to 10 wt.% based on the oxygen storage material. If the cerium / zirconium / rare earth metal mixed oxides contain lanthanum oxide and yttrium oxide as rare earth metals, their mass ratio is, in particular, 0.1 to 1. If the cerium / zirconium / rare earth metal mixed oxides contain lanthanum oxide and praseodymium oxide as rare earth metals, their mass ratio is, in particular, 0.1 to 1. The coatings Y, Y', and Z typically contain oxygen storage components in amounts of 15 to 120 g / l, based on the volume of the wall flow filter.The mass ratio of carrier materials and oxygen storage components in the coating Z is typically 0.3 to 1.5, for example 0.4 to 1.3.

[0026] In embodiments of the present invention, the coating Z comprises lanthanum-stabilized aluminum oxide, rhodium, palladium or palladium and rhodium, and an oxygen storage component comprising zirconium oxide, cerium oxide, yttrium oxide, and lanthanum oxide. In other embodiments of the present invention, the coating Z comprises lanthanum-stabilized aluminum oxide, rhodium, palladium or palladium and rhodium, and an oxygen storage component comprising zirconium oxide, cerium oxide, praseodymium oxide, and lanthanum oxide.

[0027] In embodiments of the present invention, the coating Z contains no zeolite and no molecular sieve.

[0028] The composition and application method of the respective coating (on the wall), as well as the amount of catalytic coating applied, can be varied and combined to achieve the desired catalytic activity. This variation of the catalytically active coating, as described above, creates different wall permeabilities within the filter. Additionally, due to the manufacturing process, the catalytic layers on the filter walls sometimes exhibit inhomogeneities, small cracks, varying layer thicknesses, and occasionally uncoated areas. While these exhibit high catalytic activity and acceptable back pressure, they have a relatively low initial filtration efficiency. Areas lacking a catalytic coating may also have lower filtration efficiency.All of this leads to the channel walls having varying degrees of gas permeability and potentially exhibiting areas with different gas permeability.

[0029] Applying the dry powder gas aerosol to the wall flow filter under consideration causes the powder particles to be deposited in the filter's pores, following the gas flow. The previously mentioned varying wall permeability of the filter (e.g., due to inhomogeneities in the filter wall itself or different coating zones) leads to selective powder deposition on the filter wall or in the wall pores where the flow is strongest. This effect also means that cracks or pores in the washcoat layer caused by coating defects are filled by the porous powder, resulting in increased retention of soot particles in the exhaust gas as it passes through the filter. Consequently, improved filtration efficiency is achieved.

[0030] According to the invention, the zoned, coated, dry exhaust gas filter is covered with a powder in the direction of exhaust gas flow such that the cell wall areas with the highest flow rate are covered with loose, porous powder accumulations in the pores and, if necessary, also on the wall, in order to achieve a desired increase in filtration efficiency. Surprisingly, the formation of these porous powder accumulations results in a relatively low increase in back pressure. In a preferred embodiment, the wall flow filter is exposed to a powder gas aerosol such that the powder is deposited on the channel walls of the inlet side during exposure, forming a continuous layer there.In a further highly preferred embodiment, the wall flow filter is supplied with a powder gas aerosol in such a way that the powder is deposited in the pores of the filter walls upon supply and fills them up to the inlet surface without forming a continuous layer on the walls of the filter.

[0031] The amount of powder used depends on the type of powder, the type of coating (see above), and, if applicable, the volume of available pores. Under the given boundary conditions (not too high exhaust backpressure), this amount can be determined by a person skilled in the art in preliminary tests. As a rule, the powder loading of the filter is no more than 50 g / l based on the filter volume. Preferably, the value is no more than 20 g / l, and most preferably no more than 10 g / l. Naturally, the desired increase in filtration efficiency sets a lower limit. The exact amount of powder required to increase filtration efficiency is determined depending on the length and type of catalytic coating. If only a powder coating in the surface pores of the filter is desired, an amount of powder of up to 10 g / l, preferably up to 5 g / l, is advantageously sufficient under the aforementioned conditions.

[0032] Powders preferably used in the present invention to generate the aerosol are well known to those skilled in the art. These are high-melting-point metal compounds commonly used as support materials for catalysts in automotive exhaust systems. Preferably, suitable metal oxide, metal sulfate, metal phosphate, metal carbonate, or metal hydroxide powders, or mixtures thereof, are used. The metals suitable for the metal compounds are, in particular, those selected from the group of alkali metals, alkaline earth metals, or transition metals. Preferably, metals selected from the group consisting of calcium, magnesium, strontium, barium, aluminum, silicon, titanium, zirconium, and cerium are used. As mentioned, these metals can preferably be used as oxides.The use of cerium oxide, titanium dioxide, zirconium dioxide, silicon dioxide, aluminum oxide, iron oxide, zinc oxide, or mixtures or mixed oxides thereof is particularly preferred. The use of an aerosol comprising a mixture of air and one of these metal oxide powders is particularly preferred. The term "mixed oxide" (solid solutions of one metal oxide in at least one other) is understood here to also include zeolites and zeotypes. Within the scope of the invention, zeolites and zeotypes are defined as in WO2015049110A1.

[0033] In addition to the commonly used chemically precipitated metal oxides, so-called pyrogenic metal oxide powders can also be used. Generally, pyrogenic metal oxide powders are understood to be those obtained from a metal oxide precursor by flame hydrolysis or flame oxidation in an oxyhydrogen flame. (https: / / de.wikipedia.org / w / index.php?title=Pyrogenes Siliciumdioxid&oldid=182147815;Pater Albers et al. Chemie in unserer Zeit, 2016, 50, 162–171; Hans Ferkel et al. MTZ – Motortechnische Zeitschrift, 2010, 71, 128–133). These exhibit properties similar to those described for flame-synthesized particulate products in the following publications: Gutsch A. et al. (2002) KONA (No. 20); Li S. et al. (2016) Progress in Energy and Combustion Science (55); Ulrich G. (1971) Combustion Science and Technology (Vol. 4). Pyrogenic metal oxides are generally characterized by a high specific surface area and a low bulk density. In general, high-surface-area oxides of various metals can be produced using this process. These oxides are particularly advantageous when produced from the group of metals consisting of silicon, aluminum, titanium, zirconium, cerium, or from mixtures of these metals.

[0034] To ensure that the powder in the powder gas aerosol can be sufficiently deposited in the pores of the catalytically coated wall flow filter or adhere to the filter wall, the particle diameter in the aerosol should be at least smaller than the pores of the wall flow filter. The powder has a ratio of mean particle diameter (Q3 distribution; measured according to the latest version of ISO 13320 on the filing date) d50 in the dry aerosol to the mean pore diameter of the wall flow filter after coating (measured according to DIN 66134 - latest version on the filing date) between 0.03 and 2, preferably between 0.05 and 1.43, and most preferably between 0.05 and 0.63. This ensures that the powder particles in the aerosol can be deposited in the pores of the wall flow filter walls, following the gas flow.

[0035] The powders should advantageously be fixed to the substrate without prior or subsequent treatment. For a suitable powder to produce the filters according to the invention, an optimization between the largest possible surface area of ​​the powders used, the crosslinking, and the adhesive strength is advantageous. During operation in the vehicle, small particles follow the streamlines almost without inertia due to their short particle relaxation time. This uniform, convection-driven movement is superimposed with a random "jittering" motion. According to this theory, for good filtration efficiency of a powder-coated filter, it follows that the powder should offer the largest possible surface area exposed to the flow. The powder should therefore have a high fines content, since, for the same total volume of oxide, small particles offer significantly larger surface areas. At the same time, however, the pressure drop must only increase negligibly.This requires a loose cross-linking of the powders. For a coating that increases filtration efficiency, powders with a tapped density between 50 g / l and 900 g / l are preferably used, more preferably between 200 g / l and 850 g / l, and most preferably between 400 g / l and 800 g / l.

[0036] According to the invention, the powders can be used as described above. However, the use of dry powder with catalytic activity for exhaust aftertreatment is also conceivable. Accordingly, the powder itself can also be catalytically active with regard to reducing pollutants in the exhaust gas of an internal combustion engine. All activities known to those skilled in the art are suitable for this purpose, such as TWC, DOC, SCR, LNT, or catalysts that accelerate soot combustion. Generally, the powder will exhibit the same catalytic activity as any catalytic coating applied to the filter. This further increases the overall catalytic activity of the filter compared to filters not coated with catalytically active powder. Therefore, for example, aluminum oxide impregnated with a precious metal could be used to produce the powder gas aerosol.It is also possible to apply two or more powders of different composition and functionality as a dry powder gas aerosol in two or more successive coating steps.

[0037] In this context, a three-way activity of the powder, applied as an aerosol, is preferred, with activation including palladium and rhodium as well as an oxygen storage material such as cerium zirconia. It is also conceivable to use catalytically active material for the SCR reaction. The powder could, for example, consist of zeolites or zeotypes exchanged with transition metal ions. The use of iron and / or copper-exchanged zeolites is particularly preferred; CuCHA (copper-exchanged chabazite) is most preferred as the material for producing the powder gas aerosol. http: / / europe.iza-structure.org / IZA-SC / framework.php?STC=CHA) or CuAEl ( http: / / europe.iza-structure.org / IZA-SC / framework.php?STC=AEI ) used.

[0038] A powder whose catalytic activity leads to improved soot combustion is also advantageous. A powder consisting of an aluminum oxide impregnated with one or more noble metals is preferred. Preferred noble metals in this case are platinum, palladium, rhodium, or mixtures thereof. An aluminum oxide impregnated with platinum and palladium is particularly preferred. Other materials catalyzing soot combustion are pure or doped cerium oxides and / or cerium-zirconium mixed oxides. Dopants known to those skilled in the art include those from the rare earth group, such as lanthanum, yttrium, neodymium, and praseodymium. Other known elements catalyzing soot combustion come from the alkali, alkaline earth, and transition metal groups, such as magnesium, calcium, iron, copper, and manganese. These can be added directly in powder form, e.g.,as sulfate, carbonate, oxide or analogous compounds, or as a compound in combination with aluminium oxide, cerium oxide and / or cerium / zirconium mixed oxide applied to the filter.

[0039] It is particularly noteworthy that the powder-coated filters described here differ from those formed in a vehicle's exhaust system by ash deposition during operation. According to the invention, the filters are specifically mixed with various dry powders. This allows the balance between filtration efficiency, catalytic activity, and exhaust backpressure to be precisely tailored to the intended application (diesel soot, gasoline engine soot) from the outset.

[0040] In principle, a person skilled in the art knows how to produce an aerosol from a powder and a gas in order to then pass it through the filter to which the powder is applied. According to the invention, to produce a wall-flow filter for reducing pollutants in the exhaust gas of an internal combustion engine, a dry filter with a catalytically active coating, which has areas of varying permeability, is supplied with powder gas aerosols at the inlet side. The filter is supplied with the powder gas aerosol by dispersing the powder in a gas, which is then fed into a gas stream and subsequently drawn through the filter at the inlet side.

[0041] The aerosol made from the gas and the powder can be produced according to the instructions of a person skilled in the art or as described further below. For this purpose, a powder is generally mixed with a gas ( http: / / www.tsi.com / Aerosol generators-and-dispersers / ; https: / / www.palas.de / de / product / aerosolqeneratorssolidparticlesThis mixture of gas and powder, thus prepared, is then advantageously fed into the inlet side of the wall flow filter via a gas stream. The inlet side is understood to be the part of the filter formed by the inlet / flow channels. The inlet surface is formed by the wall surfaces of the inlet / flow channels on the inlet side of the wall flow filter. The same applies to the outlet side.

[0042] Any gas suitable for the purpose, as perceived by those skilled in the art, can be used to produce the aerosol and introduce it into the filter. The use of air is particularly preferred. However, other reactant gases can also be used, which can exhibit either oxidizing (e.g., O₂, NO₂) or reducing (e.g., H₂) activity towards the powder used. The use of inert gases (e.g., N₂) or noble gases (e.g., He) can also prove advantageous with certain powders. Mixtures of the gases listed are also conceivable.

[0043] To ensure that the powder is deposited sufficiently deep into the inlet area E and adheres well, a certain suction capacity is necessary. Those skilled in the art can determine this through preliminary tests for the respective filter and powder. According to the invention, the aerosol (powder-gas mixture) is drawn through the filter at a speed of 5 m / s to 60 m / s, more preferably 10 m / s to 50 m / s, and most preferably 15 m / s to 40 m / s. This also achieves advantageous adhesion of the applied powder.

[0044] The dispersion of the powder in the gas to establish a powder gas aerosol is achieved in various ways. Preferably, the powder is dispersed by at least one or a combination of the following methods: compressed air, ultrasound, sieving, in-situ milling, blowing, gas expansion, or fluidized bed. Other dispersion methods not mentioned here may also be used by those skilled in the art. In principle, those skilled in the art are free to choose any method for producing the powder gas aerosol. As described above, the powder is first converted into a powder gas aerosol by dispersion and then introduced into a gas stream.

[0045] This mixture of gas and powder, thus prepared, is then introduced into an existing gas flow, which carries the finely dispersed powder into the inlet side E of the wall flow filter. This process is preferably supported by a suction device positioned in the pipeline downstream of the filter. This contrasts with the method described in the Fig. 3The device shown in US 8277880B, in which the powder gas aerosol is generated directly in the gas stream, allows for a much more uniform and thorough mixing of the gas stream with the powder gas aerosol. This ultimately ensures an advantageous distribution of the powder particles in the filter in both radial and axial directions, thus helping to standardize and control the deposition of the powder particles on the filter's inlet surface. The powder is dry when applied to the wall-flow filter according to the invention. Preferably, the powder is mixed with ambient air before being applied to the filter. By mixing the powder gas aerosol with particle-free gas, preferably dry ambient air, the particle concentration is reduced to such a level that no significant agglomeration occurs before deposition in the wall-flow filter. This preserves the particle size established during dispersion in the aerosol.

[0046] A preferred device for manufacturing such a wall flow filter is described in Fig. 12 schematically represented. Such a device is characterized by the fact that at least one unit for dispersing powder in a gas; a unit for mixing the dispersion with an existing gas stream; at least two units for holding a filter, designed such that the gas stream flows through the filter without any further gas supply; a unit for generating suction, which maintains the gas stream through the filter; optionally, a unit for generating vortices upstream of the filter, so that powder deposits on the filter's inlet plug are prevented as far as possible; and optionally, a unit through which at least a partial gas stream is extracted downstream of the suction device and added to the gas stream being drawn through the filter before the powder is added.

[0047] In this preferred embodiment of the method according to the invention - as shown in the sketch Fig. 12The diagram shows that at least a portion of the gas flow is extracted from the downstream side of the suction device and added back to the gas flow being drawn through the filter before the powder is added. This allows the powder to be metered into an already heated airflow. The suction fans, which provide the necessary pressure, generate an exhaust air temperature of approximately 70°C, as the installed suction capacity is preferably greater than 20 kW. To optimize energy efficiency, the waste heat from the suction fan is used to heat the supply air, thereby reducing its relative humidity. This, in turn, reduces particle adhesion to each other and to the inlet plugs. The powder deposition process can thus be better controlled.

[0048] In the present method for manufacturing a wall flow filter, a gas stream is charged with a powder gas aerosol and drawn into a filter. This ensures that the powder is sufficiently dispersed in the gas stream to penetrate the inlet channels of the wall flow filter. The homogeneous distribution of the powder in the gas / air requires intensive mixing. Diffusers, Venturi mixers, and static mixers are known to those skilled in the art for this purpose. Mixing devices that prevent powder deposits on the surfaces of the coating system are particularly suitable for the powder coating process. Therefore, diffusers and Venturi tubes are preferred for this process. Introducing the dispersed powder into a rapidly rotating flow with high turbulence has also proven effective.

[0049] To achieve an advantageously uniform distribution of the powder across the filter's cross-section, the gas transporting the powder should exhibit a piston flow (preferably with a uniform velocity across the cross-section) upon impact with the filter. This is preferably achieved by an accelerated flow upstream of the filter. As is known to those skilled in the art, a continuous reduction in the cross-section without abrupt changes produces such an accelerated flow, as described by the continuity equation. Furthermore, it is also known to those skilled in the art that this results in the flow profile more closely approximating a piston profile. To selectively modify the flow, internal components such as sieves, rings, discs, etc., can be installed below and / or above the filter.

[0050] In a further advantageous embodiment of the present method, the powder coating apparatus comprises one or more devices (turbulators, vortex generators) with which the gas stream carrying the powder gas aerosol can be swirled before it strikes the filter. Suitable sieves or grids, positioned at a sufficient distance upstream of the filter, can serve as examples. The distance should be neither too great nor too small, ensuring sufficient swirling of the gas stream directly in front of the filter. A person skilled in the art can determine the distance through simple experiments. The advantage of this measure is that no powder particles are deposited on the inlet plugs of the outlet channels, and all powder can penetrate the inlet channels.According to the invention, it is therefore preferred if the powder is swirled before flowing into the filter in such a way that powder deposits on the inlet plug of the wall flow filter are avoided as far as possible. In aerodynamics, a turbulator or turbulence or vortex generator refers to devices that cause an artificial disturbance of the flow. As is known to those skilled in the art, vortices (especially microvortices) form behind bars, grids, and other flow-disturbing components at appropriate Reynolds numbers. The Karman vortex street (H. Benard, CR Acad. Sci. Paris Ser. IV 147, 839 (1908); 147, 970 (1908); T. von Karman, Nachr. Ges. Wiss. Göttingen, Math. Phys. Kl. 509 (1911); 547 (1912)) and the wake vortex behind aircraft, which can cover roofs, are well known.In the case according to the invention, this effect can be particularly advantageously enhanced by vibrating self-cleaning screens (so-called ultrasonic screens) that are advantageously positioned within the flow. Another method is to disrupt the flow with sound fields, which, through pressure amplitudes, excite the flow to turbulence. These sound fields can even clean the surface of the filter without flow. The frequencies can range from ultrasound to infrasound. These latter methods are also used for pipe cleaning in large-scale industrial plants.

[0051] The wall-flow filter produced in this way can be used to reduce harmful exhaust gases from an internal combustion engine. In principle, all catalytic exhaust aftertreatment systems (see above) suitable for this purpose can be used with a filter, but especially those in which the filter is used in an exhaust system together with one or more catalytically active components selected from the group consisting of nitrogen oxide storage catalysts, SCR catalysts, three-way catalysts, and diesel oxidation catalysts. The filter is particularly advantageous when used in combination with a three-way catalyst, especially downstream of it. It is especially advantageous if the filter itself is a three-way catalytically active filter. The filters produced according to the inventive method, optionally coated with catalytically active powder, are suitable for all these applications.The filters are preferably used for treating the exhaust gases of a stoichiometrically operated internal combustion engine. The [filters] are suitable for [the specific application].

[0052] The requirements for gasoline particulate filters (GPFs) differ significantly from those for diesel particulate filters (DPFs). Diesel engines without DPFs can exhibit up to ten times higher particulate emissions, based on particle mass, than gasoline engines without GPFs (Maricq et al., SAE 1999-01-01530). Furthermore, gasoline engines produce significantly fewer primary particles, and the secondary particles (agglomerates) are considerably smaller than those from diesel engines. Emissions from gasoline engines range in particle sizes from less than 200 nm (Hall et al., SAE 1999-01-3530) to 400 nm (Mathis et al., Atmospheric Environment 38 4347), with the maximum size in the range of approximately 60 nm to 80 nm. Therefore, nanoparticle filtration in GPFs must primarily be achieved through diffusion separation. For particles smaller than 300 nm, deposition by diffusion (Brownian molecular motion) and electrostatic forces becomes increasingly important with decreasing size (Hinds, W.: Aerosol technology: Properties and behavior and measurement of airborne particles. Wiley, 2. Auflage 1999).

[0053] "Dry" in the context of the present invention accordingly means the exclusion of the use of a liquid, in particular water. Specifically, the preparation of a suspension of the powder in a liquid for atomization into a gas stream should be avoided. A certain level of moisture may be tolerable for both the filter and the powder, provided that the achievement of the objective—the finest possible dispersion of the powder in or on the inlet surface—is not negatively affected. The powder is generally free-flowing and dispersible by the input of energy. The moisture content of the powder or the filter at the time of contact with the powder should be less than 20%, preferably less than 10%, and most preferably less than 5% (measured at 20°C and standard pressure according to ISO 11465, latest version, on the filing date).

[0054] The catalytically coated wall flow filter produced according to the invention exhibits excellent filtration efficiency with only a moderate increase in exhaust back pressure compared to a fresh wall flow filter not coated with powder. Preferably, the wall flow filter shows an improvement in soot particle separation (filter efficiency) of at least 5%, preferably at least 10%, and most preferably at least 20%, with a relative increase in the exhaust back pressure of the fresh wall flow filter of at most 40%, preferably at most 20%, and most preferably at most 10%, compared to a fresh filter coated with catalytically active material and not treated with powder. As mentioned, the powder is deposited in the open pores of the filter and forms a porous matrix there.The low increase in back pressure is probably due to the fact that, according to the invention, the cross-section of the channels on the inlet side of the filter is not significantly reduced by applying a powder. It is assumed that the powder forms a porous structure, which presumably has a positive effect on the back pressure. Therefore, a corresponding filter should also exhibit better exhaust back pressure compared to prior art filters in which a powder is deposited on the walls of the inlet side of a filter or a conventional wet-coating is used.

[0055] By providing a wall flow filter, which contains a zoned catalytic coating Z on the surface OA of the channel wall, for the reduction of pollutants in the exhaust gas of an internal combustion engine, wherein the dry filter is selectively exposed on its inlet surface with a dry powder gas aerosol, which advantageously contains at least one high-melting compound, a highly successful solution to the stated problem is achieved. The invention is explained in more detail with reference to several examples and figures.

[0056] The Fig. 1 This image shows a light microscopy photograph of a filter coated with powder. The photograph shows a top view of several channel walls in an area of ​​the filter where there is no catalytically active on-wall layer. The powder selectively deposits in the pores of the wall and fills them.

[0057] The Figures 2 - 8show different coating arrangements in catalytically active particle filters. This is described in more detail below. (E) the inlet / flow channel of the filter (A) the outlet / flow channel of the filter (L) the length of the filter wall (X) catalytic in-wall coating (not according to the invention) (Y - not according to the invention) and (Z - according to the invention) catalytic on-wall coating (P) areas of the filter wall coated with powder

[0058] Fig. 2 Figure 1 schematically shows a filter not according to the invention, which has a zone of an on-wall layer on the inlet (Y) and outlet (Z) side, each with a zone length of approximately 60% of the total length, and is coated with powder (P) at the end of the uncoated part of the inlet channel (E).

[0059] Fig. 3 shows the same, non-inventive filter construction as Fig. 2with the difference that the two on-wall layers (X, Y) have a coating gradient from the end faces to the center of the substrate.

[0060] Fig. 4 shows a filter not according to the invention with a wall-mounted layer (X) in the inlet area E, which is charged with a powder (P) in the inlet channel (E).

[0061] Fig. 5 shows a filter not according to the invention with an on-wall layer (Z) in the outlet area A and a zone with an in-wall coating (X) in which the powder (P) is deposited in the area of ​​highest permeability.

[0062] Fig. 6 shows a filter not according to the invention, in which the walls of the channels are completely activated with an in-wall coating (X) and which has a zone with an on-wall layer (Y) on the inlet side (E) and has been treated with powder (P).

[0063] Fig. 7 shows the same filter as Fig. 5however without catalytically active in-wall coating (X).

[0064] Fig. 8 Figure 1 schematically shows a filter not according to the invention with a wall-mounted layer in the inlet (Y) and outlet (Z) channels, each with a length of approximately 30% of the total length L, and a powder-laden area (P) in the inlet channel (E).

[0065] Fig. 9 shows the results of the investigations into the filtration effect of all particle filters and state-of-the-art particle filters described here.

[0066] Fig. 10 shows a photograph of an open filter with a zone of an on-wall layer in the inlet channels (grey, left side) and the area exposed to powder (right side) - not according to the invention.

[0067] Fig. 11 shows magnifications of the filter from Fig. 10 with photos of the entrance (1), the middle (2) and the exit (3) of the filter

[0068] Fig. 12Figure 1 shows a schematic drawing of an advantageous device for applying a powder to the filters. The powder 420 or 421 is mixed with the pressurized gas 451 through the atomizing nozzle 440 in the mixing chamber with the gas stream 454 and then drawn or forced through the filter 430. The particles that pass through are filtered out in the exhaust gas filter 400. The blower 410 ensures the necessary volume flow. The exhaust gas is split into an exhaust gas 452 and a warm recirculated gas 453. The warm recirculated gas 453 is mixed with the fresh gas 450. Examples: Manufacturing and testing of particle filters

[0069] Conventional highly porous cordierite filters with a round cross-section were used to produce the catalytically active particle filters described in the examples and comparative examples. The wall-flow filter substrates had a cell density of 46.5 cells per square centimeter and a cell wall thickness of 0.22 mm. They exhibited a porosity of 65% and a mean pore size of 18 µm.

[0070] In comparative example 1, a coating suspension was applied to the outflow channels. In comparative example 2, coating suspensions were applied to the inflow and outflow channels in two steps. The filter described in comparative example 3 contains both a coating arranged in the wall and a coating suspension applied to the inflow channels. After the application of each coating suspension, the wall flow filters were dried and calcined at 500°C for 4 hours. The coating suspension was applied according to the instructions of a person skilled in the art (as described in DE102010007499A1).

[0071] In the case of the particle filters (examples 1 to 5), the filters from comparison examples 1 - 3 were additionally coated with a powder in the inlet channels.

[0072] The resulting catalytically active filters were tested for their fresh filtration efficiency on an engine test bench using the real exhaust gas of an engine that operated predominantly (>50% of operating time) and on average (mean lambda over the operating time) with a stoichiometric air / fuel mixture. A globally standardized test procedure for determining exhaust emissions, known as WLTP (Worldwide Harmonised Light Vehicles Test Procedure), was applied. The driving cycle used was WLTC Class 3. Each filter was installed close to the engine, directly downstream of a conventional three-way catalytic converter. This three-way catalytic converter was the same for all filters tested. Each filter was subjected to a WLTP test. To detect particulate emissions during the test, particle counters were installed upstream of the three-way catalytic converter and downstream of the particulate filter.

[0073] Some catalytically active particulate filters were also subjected to additional engine test bench aging. The aging process consisted of overrun fuel cut-off aging with an exhaust gas temperature of 950°C upstream of the catalyst inlet (maximum bed temperature 1030°C). The aging time was 38 hours. After aging, the filters were tested for their catalytic activity.

[0074] In the investigation of the catalytic activity, the start-up behavior (light-off) of the particulate filters at a constant mean air number λ and the dynamic conversion when λ is changed were tested on an engine test bench. Comparative example 1:

[0075] A precious metal-containing coating suspension, comprising a cerium / zirconium mixed oxide, lanthanum-doped aluminum oxide, and barium sulfate, was applied to 80% of the length of the filter outlet channel of a wall-flow filter with a diameter of 118 mm and a length of 152 mm. The suspension was then calcined at 500°C. The particle size of the oxides in the coating suspension was selected such that the suspension was predominantly applied to the filter wall (only a small fraction of the fine coating particles penetrated the pores of the wall; less than 10%). After calcination, the coating quantity of VGPF1 corresponded to 67 g / l, based on the substrate volume. Comparative example 2:

[0076] In the first step, a precious metal-containing coating suspension, comprising a cerium / zirconium mixed oxide, lanthanum-doped aluminum oxide, and barium sulfate, was applied to 60% of the length of the inlet channel of a wall-flow filter with a diameter of 118 mm and a length of 118 mm. This suspension was then calcined. In a second coating step, another precious metal-containing coating suspension was applied to 60% of the length of the outlet channel and subsequently calcined. This second coating suspension also contained a cerium / zirconium mixed oxide, lanthanum-doped aluminum oxide, and barium sulfate. The particle size of the oxides in the coating suspension was selected to ensure that the suspension was predominantly applied to the filter wall. The coating quantity of VGPF2 after calcination was 100 g / l, based on the substrate volume. Comparative example 3:

[0077] In the first step, a precious metal-containing coating suspension, comprising a cerium / zirconium mixed oxide, lanthanum-doped aluminum oxide, and barium sulfate, was applied to the entire length of a highly porous wall-flow filter with a diameter of 132 mm and a length of 102 mm. The suspension was then calcined. The particle size of the oxides in the coating suspension was selected such that the suspension was predominantly located within the filter wall (>90%). The coating quantity after calcination was 100 g / l, based on the substrate volume.

[0078] In a second coating step, another precious metal-containing coating suspension was applied to 60% of the inlet channel and subsequently calcined. This coating suspension also contained a cerium / zirconium mixed oxide, lanthanum-doped aluminum oxide, and barium sulfate. The particle size of the oxides in the coating suspension was selected to ensure that the suspension was predominantly applied to the filter wall. The coating quantity of the VGPF3 after calcination was 132 g / l, based on the substrate volume.

[0079] To increase the filtration efficiency of the catalytically coated filters described in comparative examples 1 to 3, their inflow channels were treated with different types and quantities of powder. The coating parameters were selected so that the powder was primarily deposited in the area of ​​the substrate without the coating (areas of highest permeability). The production of filters GPF1 to GPF5 is explained in the following descriptions.

[0080] The examples investigated the influence of the type and amount of powder used on the filtration efficiency and catalytic effectiveness of different coating variants of the catalytic materials. Example 1 (according to the invention):

[0081] To increase the filtration efficiency of the catalytically coated filter VGPF1 described in comparative example 1, the filter's inlet channels were treated with 4 g / l of a highly porous aluminum oxide. The relative increase in back pressure of the GPF1 compared to the VGPF1 was 8.6 mbar. The GPF1 filter described in example 1 is in Fig. 7 sketched. Example 2 (according to the invention):

[0082] To increase the filtration efficiency of the catalytically coated filter VGPF1 described in comparative example 1, the filter's inlet channels were treated with 0.2 g / l of a high-melting-point pyrogenic aluminum oxide. The relative increase in back pressure of GPF1 compared to VGPF1 was 5 mbar. The filter GPF2 described in example 2 is in Fig. 7 sketched. Example 3 (not according to the invention):

[0083] To increase the filtration efficiency of the catalytically coated filter VGPF2 described in comparative example 2, the filter's inflow channels were treated with 10 g / l of a highly porous aluminum oxide. The filter GPF3 described in example 3 is in Fig. 2 sketched. Example 4 (not according to the invention):

[0084] To increase the filtration efficiency of the catalytically coated filter VGPF3 described in comparative example 3, the filter's inflow channels were treated with 4 g / l of a highly porous aluminum oxide. The filter GPF4 described in example 4 is in Fig. 4 sketched. Example 5 (not according to the invention):

[0085] To increase the filtration efficiency of the catalytically coated filter VGPF3 described in comparative example 3, the filter's inflow channels were treated with 7 g / l of a highly porous aluminum oxide. The filter GPF5 described in example 5 is in Fig. 4 sketched. Discussion of the results from filtration efficiency measurements of the particle filters VGPF1 to VGPF3 and GPF1 to GPF5 described in comparison examples and examples.

[0086] As previously described, the catalytically active filters produced in the comparative examples and case studies were each subjected to a WLTP test on an engine test bench to examine their filtering effectiveness. The results of these tests were presented in the Fig. 9 depicted. Fig. 9 shows filtration values ​​that resulted from the raw particle emissions and particle emissions after the respective filter during a WLTP procedure.

[0087] The advantages of the GPF1 to GPF5 filters are clearly observable during filter efficiency measurements. Applying a powder to the filter leads to an increase in filtration efficiency of up to 20%. The desired filtration efficiency can be adjusted by varying the amount of powder used.

[0088] Replacing a highly porous aluminum oxide (GPF1) with a pyrogenic aluminum oxide (GPF2) reduces the amount of powder used from 4 g / l to 0.2 g / l. This results in a saving of 500% by weight of powder used, while maintaining the same filtration efficiency and reducing back pressure.

[0089] To test whether the powder-coated filters exhibit high catalytic activity, the non-inventive particle filters GPF4 and GPF5 were subjected to engine test bench aging and a subsequent measurement of the "light-off" start-up behavior.

[0090] The following table lists the temperatures T50 at which 50% of the component under consideration is converted. The start-up behavior was determined at a stoichiometric exhaust gas composition (λ = 0.999 with ±3.4% amplitude). The standard deviation for this test is ±2°C. Table 1 contains the "light-off" data for the aged filters VGPF3, GPF4, and GPF5. Table 1: T 50 HC stöch T 50 CO stoch T 50 NOx stöch VGPF3 381 398 395 GPF4 386 401 401 GPF5 383 401 402

[0091] As the results show, applying a powder to the catalytically active filters leads to a significant increase in filtration efficiency while maintaining high catalytic activity and a low increase in back pressure. Furthermore, the choice of powder can significantly reduce the amount of powder used.

[0092] It was successfully demonstrated that the catalytic effectiveness of the zoned coated wall flow filters, the exhaust gas back pressure, and the filtration efficiency can be specifically tailored to customer requirements. A wall flow filter manufactured in this way was previously unknown in the art.

Claims

1. Method for producing a catalytically active wall flow filter for removing particles from the exhaust gas of internal combustion engines, the wall flow filter comprising a length L, a catalytically active coating Z and the channels E and A, which extend in parallel between a first end and a second end of the wall flow filter and which are separated by porous walls forming surfaces OE and OA, and the channels E being closed at the second end and the channels A being closed at the first end, the coating Z being deposited with a loading of 20 g / l to 200 g / l in the channels A on the walls of the surfaces OA, and the coating extending over the walls of the surfaces OA, starting from the second end of the wall flow filter, over a length of 10% to 90% of the length L, characterized in that subsequently at least one dry powder gas aerosol comprising high-melting metal compounds has additionally been applied to the inlet region E of the catalytically coated filter, the powder gas aerosol being applied to the catalytically coated filter by dispersing the powder in a gas which is then fed to a gas flow and subsequently sucked through the filter on the inlet side, and the ratio of the average particle diameter d50 of the Q 3 distribution, measured according to the latest ISO13320 on the filing date, in the dry powder aerosol to the average pore diameter of the filter after coating with washcoat, measured according to the latest version of DIN66134 on the filing date, being between 0.03 and 2, and the aerosol being sucked through the filter at a speed of 5 m / s to 50 m / s, so that the cell wall regions which have the strongest through-flow are covered with loose, inherently porous powder accumulations in the pores.

2. Method according to claim 1, characterized in that the coating Z is at least 1.25 cm long.

3. Method according to any of the preceding claims, characterized in that the coating Z has a thickness gradient over the length L such that the least thickness of the coating Z prevails at the ends of the wall flow filter.

4. Method according to any of the preceding claims, characterized in that the coating Z contains one or more noble metals, which are fixed to one or more carrier materials, as well as one or more oxygen storage components.

5. Method according to any of the preceding claims, characterized in that the coating Z contains a cerium / zirconium / rare earth metal mixed oxide as the oxygen storage component.

6. Method according to any of the preceding claims, characterized in that the amount of powder in the particulate filter is less than 50 g / l.

7. Method according to any of the preceding claims, characterized in that the powder is selected from the group consisting of silicon dioxide, aluminum oxide, titanium dioxide, zirconium dioxide, cerium oxide, iron oxide, zinc oxide, zeolites, metal-substituted zeolites or mixtures thereof.

8. Method according to any of the preceding claims, characterized in that the powder settles in the pores of the filter walls and does not form a cohesive layer on the walls of the filter.

9. Method according to any of the preceding claims, characterized in that the tapped density of the powder is at most 900 g / l.

10. Method for producing a catalytically active wall flow filter according to claim 1, characterized in that the powder has a moisture content of less than 20% at the time of the application on the wall flow filter.