CATALYTIC ACTIVE PARTICLE FILTER WITH HIGH FILTRATION EFFICIENCY

Description

[0001] The present invention is directed to a wall-flow filter, a method for its production and its use for reducing harmful exhaust gases from an internal combustion engine.

[0002] Diesel particulate filters and gasoline particulate filters, with or without an additional catalytically active coating, are suitable units for removing particulate emissions and reducing pollutants in exhaust gases. These are wall-flow honeycomb structures referred to as catalyst supports, carriers, or substrate monoliths. To meet legal standards, it is desirable for current and future exhaust aftertreatment applications in combustion engines to combine particulate filters with other catalytically active functionalities, both for cost reasons and for space reasons. The catalytically active coating can be located on the surface of the channels or in the walls forming this surface. The catalytically active coating is often applied to the catalyst support in the form of a suspension in a so-called coating process.Many such processes have been published in the past by automotive catalyst manufacturers, see for example EP1064094B1, EP2521618B1, WO10015573A2, EP1136462B1, US6478874B1, US4609563A, WO9947260A1, JP5378659B2, EP2415522A1 and JP2014205108A2.

[0003] The use of a particulate filter—whether catalytically coated or not—leads to a noticeable increase in exhaust backpressure compared to a flow-through monolith of the same dimensions, thus reducing engine torque or potentially increasing fuel consumption. To prevent further increasing exhaust backpressure, 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-through monolith. As a result, the catalytic effectiveness of a catalytically coated particulate filter is often inferior to a flow-through monolith of the same size.

[0004] There have already been some efforts to provide particulate filters that exhibit good catalytic activity through an active coating while still exhibiting the lowest possible exhaust backpressure. With regard to low exhaust backpressure, it has proven advantageous if the catalytically active coating is not located as a layer on the channel walls of a porous wall-flow filter, but rather the channel walls of the filter are interspersed with the catalytically active material, see, for example, WO2005016497A1, JPH01-151706 and EP1789190B1. For this purpose, the particle size of the catalytic coating is selected such that the particles penetrate the pores of the wall-flow filter and can 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.

[0005] It has been shown that applying catalytically active substances to the surface of the channel walls of a wall-flow honeycomb structure can increase the conversion of pollutants in the exhaust gas. Combinations of on-wall and in-wall coating with catalytically active material are also possible, allowing catalytic performance to be further enhanced without significantly increasing back pressure.

[0006] In addition to catalytic effectiveness, another functionality of the filter that can be improved by a coating is its filtration efficiency, i.e. the filter effect itself. WO2011151711A1 describes a method by which an uncoated or catalytically coated filter, which carries the catalytically active material in the channel walls (in-wall coating with washcoat), is exposed to a dry aerosol. The aerosol is provided by distributing a powdered mineral material and is guided over the inlet side of a wall-flow filter by means of a gas stream. The individual particles with a particle size of 0.2 µm to 5 µm agglomerate to form a bridged network of particles and are deposited as a layer on the surface of the individual inlet channels running through the wall-flow filter. The typical loading of a filter with the powder is between 5 g and 50 g per liter of filter volume.It is expressly pointed out that it is not desirable to achieve a coating in the pores of the wall flow filter with the metal oxide.

[0007] Another method for increasing the filtration efficiency of catalytically inactive filters is described in WO2012030534A1. A filtration layer ("discriminating layer") is created on the walls of the flow channels on the inlet side by depositing ceramic particles via a particle aerosol. The layers consist of oxides of zirconium, aluminum, or silicon, preferably in fiber form with a length of 1 nm to 5 µm and 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 heat process.

[0008] Another method for increasing the filtration efficiency of catalytically non-active wall-flow filters by creating a membrane ("trapping layer") on the surfaces of the filter inlet channels is described in patent US8277880B2. The filtration membrane on the surfaces of the inlet channels is created by sucking through a gas stream loaded with ceramic particles (e.g., silicon carbide or cordierite). After the filter layer has been applied, the honeycomb body is fired at temperatures greater than 1000°C to increase the adhesion of the powder layer to the channel walls. EP2502661A2 and EP2502662B1 mention further coatings applied by powder application.

[0009] A coating within the pores of a wall-flow filter substrate by atomizing dry particles is described in US8388721 B2. However, here the powder is intended to penetrate deeply into the pores. 20% to 60% of the wall surface is intended to be accessible to soot particles, thus remaining open. Depending on the flow velocity of the powder-gas mixture, a more or less pronounced powder gradient can be set between the inlet and outlet sides. The pores of the channel walls of the filter coated with powder in the pores according to US8388721 B2 can be subsequently coated with a catalytically active component. Here, too, the catalytically active material is located in 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 in such a way that the entire 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 in addition to the filtering effect. For example, these particles are deposited in the pores of the filter in an amount 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, both loaded with soot and without soot, exhibits improved filtration efficiency compared to the untreated filter, while the soot-loaded filter exhibits 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 formed on the walls of the possibly catalytically coated wall-flow filter.

[0012] All of the prior art patents listed above aim to increase the filtration efficiency of a filter by coating it with a powder. Filters optimized in this way can also have a catalytically active coating in the porous channel walls prior to powder coating. However, none of the examples provide any indication of simultaneously optimizing the catalytic effect of a filter and increasing filtration efficiency.

[0013] An attempt to simultaneously optimize catalytic effect and filtration efficiency is disclosed in DE102018127955A1.

[0014] However, there is still a need for particulate filters in which both catalytic activity and filtration efficiency are optimized with regard to exhaust backpressure. 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 backpressure and high catalytic activity.

[0015] These and other objects arising in a clear manner from the prior art are achieved by a particulate filter according to claims 1 to 11. Claim 12 is directed to the production of a particulate filter according to the invention. Claim 13 is directed to the use of the particulate filter for exhaust gas aftertreatment of internal combustion engines.

[0016] The present invention relates to a wall-flow filter for removing particles from the exhaust gas of internal combustion engines, comprising a wall-flow filter substrate of length L and different coatings Z and F, wherein the wall-flow filter substrate has channels E and A which extend in parallel between a first and a second end of the wall-flow filter substrate, are separated by porous walls and have surfaces OE and A, respectively.OA and wherein the channels E are closed at the second end and the channels A are closed at the first end and wherein the coating Z is located in the porous walls and / or on the surfaces OA , but not on the surfaces OE , and comprises palladium and / or rhodium and a cerium / zirconium mixed oxide, characterized in that the coating F is located in the porous walls and / or on the surfaces OE , but not on the surfaces OA and comprises a mineral material and not a noble metal, wherein the mineral material is a phyllosilicate such as kaolin, sepliolite, montmorillonite or bentonite, muscovite, vermiculite.

[0017] When the wall-flow filter according to the invention is used as intended for cleaning exhaust gas from internal combustion engines, the exhaust gas flows into the filter at one end and exits again at the other end after passing through the porous walls. For example, if the exhaust gas enters the filter at the first end, channels E designate the inlet channels or upstream channels. After passing through the porous walls, it then exits the filter at the second end, so that channels A designate the outlet channels or downstream channels. All ceramic wall-flow filter substrates known from the prior art and commonly used in the field of automotive exhaust gas catalysis can be used as the wall-flow substrate. Porous wall-flow filter substrates made of cordierite, silicon carbide, or aluminum titanate are preferred.These wall-flow filter substrates have channels E and A, which, as described above, function as inlet channels (also called inflow channels) and outlet channels (also called outflow channels). The outflow ends of the inflow channels and the upstream ends of the outflow channels are offset from each other and sealed with generally gas-tight "plugs." The exhaust gas to be cleaned, which flows through the filter substrate, is forced to pass through the porous wall between the inflow and outflow channels, resulting in a particle filtering effect. The porosity, pore / radius distribution, and wall thickness can be used to tailor the filtration properties for particles. According to the invention, the porosity of the uncoated wall-flow filter substrates is generally more than 40%, for example, from 40% to 75%, particularly from 50% to 70% [measured according to DIN 66133 - latest version on the date of filing].The average pore size d 50 of the uncoated wall flow filter substrates is at least 7 µm, for example from 7 µm to 34 µm, preferably more than 10 µm, in particular more preferably from 10 µm to 25 µm or very preferably from 15 µm to 20 µm [measured according to DIN 66134 latest version on the date of filing], wherein the d 50 value of the pore size distribution of the wall flow filter substrate is understood to mean that 50% of the total pore volume determinable by mercury porosimetry is formed by pores whose diameter is less than or equal to the value stated as d 50. In the case of the wall flow filters according to the invention, the wall flow filter substrates provided with the coatings Z and F and optionally coating Y (see below) particularly preferably have a pore size d 50 of 10 µm to 20 µm and a porosity of 50% to 65%.Those skilled in the art are aware that, due to the plugs that close off channels E and A in a mutually offset manner, the entire length L of the wall-flow filter substrate may not be available for coating. For example, channels E are closed at the second end of the wall-flow filter substrate, so that the surface Oε available for coating may be slightly smaller than the length L. This, of course, only applies if a coating is present over 100% of the length L or slightly less. For the sake of simplicity, however, in these cases, 100% of the length L will be referred to below.

[0018] If the coating Z is located on the surfaces OA of the wall-flow filter substrate, it preferably extends from the second end of the wall-flow filter substrate to 50 to 90% of the length L.

[0019] The coating on the surfaces OA is a so-called overlay coating. This means that the coating extends above the surfaces OA into the channels A of the wall-flow filter substrate, thus reducing the channel cross-section. In this embodiment, the pores of the porous wall adjacent to the surfaces OA are only partially filled with the coating Z. More than 80%, preferably more than 90%, of the coating Z is not present in the porous wall.

[0020] Added coatings exhibit a certain elevation above the wall surface. However, the thickness of the layers is generally between 5 and 250 µm, preferably between 7.5 and 225 µm, and most preferably between 10 and 200 µm. The thickness of the layer is preferably determined in the center of each channel web rather than in the corners. Common analytical methods known to those skilled in the art, such as scanning electron microscopy, are suitable for determining the layer thickness.

[0021] If the coating Z is located in the porous walls of the wall-flow filter substrate, it preferably extends from the first end of the wall-flow filter substrate to 50 to 100% of the length L.

[0022] The coating in the porous walls is a so-called in-wall coating. In this embodiment, the surfaces OA adjacent to the porous walls are only partially coated with the coating Z.

[0023] 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, calculated from the second end of the wall flow filter substrate.

[0024] The coating Z can have a thickness gradient over the length L such that the thickness of the coating Z increases along the length L of the wall-flow filter from the second end to the first end. In this case, the coating can preferably have more than 2 times, more preferably up to more than 3 times the thickness at one coating end than at the other coating end. The thickness is the height to which the coating Z rises above the surface OA. The thickness gradient of the coating on the channel walls also ensures that the filtration efficiency is uniform over the entire length L of the filter. The result is a more uniform deposition of the soot over the entire filter wall and thus an improved increase in exhaust gas backpressure and possibly better burn-off of the soot.

[0025] However, the coating Z can also have a thickness gradient over the length L such that the thickness of the coating Z decreases along the length L of the wall-flow filter from the second end to the first end. In this case, the coating can preferably have more than 2 times, more preferably up to more than 3 times the thickness at one coating end than at the other coating end. The thickness is the height to which the coating Z rises above the surface OA. The thickness gradient of the coating on the channel walls also ensures that the filtration efficiency is uniform over the entire length L of the filter. The result is a more uniform deposition of the soot over the entire filter wall and thus an improved increase in exhaust gas backpressure and possibly better burn-off of the soot.

[0026] Coating Z is a catalytically active coating, particularly due to its palladium and / or rhodium components. For the purposes of the present invention, "catalytically active" refers to the ability to convert harmful components of the exhaust gases of internal combustion engines into less harmful ones. In particular, the exhaust gas components NOx, CO, and HC are mentioned here. Consequently, coating Z is particularly preferably three-way catalytically active, especially at operating temperatures of 250 to 1100 °C.

[0027] Coating Z contains the precious metals palladium and / or rhodium, with platinum being present only in exceptional cases. Coating Z preferably contains palladium and rhodium, and no platinum.

[0028] In a further embodiment, coating Z contains the precious metals platinum and / or rhodium, with palladium only being present as an additional precious metal in exceptional cases.

[0029] In a further embodiment, coating Z contains the precious metals platinum and palladium, and optionally rhodium. In this embodiment, it is advantageous if the mass ratio of platinum to palladium is 15:1 to 1:15, in particular 10:1 to 1:10.

[0030] With respect to the particulate filter according to the invention, the proportion of rhodium in the total precious metal content is in particular greater than or equal to 5 wt.%, preferably greater than or equal to 10 wt.%. For example, the proportion of rhodium in the total precious metal content is 5 to 20 wt.% or 5 to 15 wt.%. The precious metals are typically used in amounts of 0.10 to 5 g / l, based on the volume of the wall-flow filter substrate.

[0031] The precious metals are usually fixed to one or more support materials. All materials familiar to the person skilled in the art for this purpose can be used as support materials. 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 date of filing). Particularly suitable support materials for the precious metals are selected from the series consisting of aluminum oxide, doped aluminum oxide, silicon oxide, titanium dioxide and mixed oxides of one or more thereof. Doped aluminum oxides are, for example, aluminum oxides doped with lanthanum oxide, zirconium oxide, barium oxide and / or titanium oxide. Advantageously, aluminum oxide or lanthanum-stabilized aluminum oxide is used, with lanthanum being used in amounts of in particular 1 to 10 wt. %, preferably 3 to 6 wt.-%, each calculated as La 2 O 3 and based on the weight of the stabilized aluminum oxide.

[0032] Also in the case of alumina doped with barium oxide, the proportion of barium oxide is in particular 1 to 10 wt.%, preferably 3 to 6 wt.%, each calculated as BaO and based on the weight of the stabilized alumina.

[0033] Another suitable support material is lanthanum-stabilized aluminum oxide, the surface of which is coated with lanthanum oxide, barium oxide and / or strontium oxide.

[0034] Coating Z preferably comprises at least one aluminum oxide or doped aluminum oxide.

[0035] Coating Z contains at least one cerium / zirconium mixed oxide, which acts as an oxygen storage component. The mass ratio of cerium oxide to zirconium oxide in these products can vary widely. It is, for example, 0.1 to 1.5, preferably 0.15 to 1, or 0.2 to 0.9.

[0036] Preferred cerium / zirconium mixed oxides comprise one or more rare earth metal oxides and can thus be referred to as cerium / zirconium / rare earth metal mixed oxides. The term "cerium / zirconium / rare earth metal mixed oxide" within the meaning of 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, ideally free of phases of pure cerium oxide, zirconium oxide, or rare earth oxide (solid solution). Depending on the production process, however, not completely homogeneous products may also be formed, which can generally be used without disadvantage. The same applies to cerium / zirconium mixed oxides that do not contain a rare earth metal oxide. Furthermore, the term rare earth metal or rare earth metal oxide within the meaning of the present invention does not include cerium or cerium oxide.

[0037] Examples of rare earth metal oxides considered in the cerium / zirconium / rare earth metal mixed oxides are lanthanum oxide, yttrium oxide, praseodymium oxide, neodymium oxide, and / or samarium oxide. Lanthanum oxide, yttrium oxide, and / or praseodymium oxide are preferred. Particular preference is given to lanthanum oxide and / or yttrium oxide, and very particular preference is given to lanthanum oxide and yttrium oxide, yttrium oxide and praseodymium oxide, and lanthanum oxide and praseodymium oxide. In embodiments of the present invention, the oxygen storage components are free of neodymium oxide.

[0038] The proportion of rare earth metal oxide in the cerium / zirconium / rare earth metal mixed oxides is in particular 3 to 20 wt.% based on the cerium / zirconium / rare earth metal mixed oxide.

[0039] If the cerium / zirconium / rare earth metal mixed oxides contain yttrium oxide as a rare earth metal, its proportion is preferably 4 to 15 wt.%, based on the cerium / zirconium / rare earth metal mixed oxide. If the cerium / zirconium / rare earth metal mixed oxides contain praseodymium oxide as a rare earth metal, its proportion is preferably 2 to 10 wt.%, based on the cerium / zirconium / rare earth metal mixed oxide. If the cerium / zirconium / rare earth metal mixed oxides contain lanthanum oxide and another rare earth oxide, such as yttrium oxide or praseodymium oxide, as a rare earth metal, their mass ratio is in particular 0.1 to 1.25, preferably 0.1 to 1.

[0040] Typically, coating Z contains oxygen storage components in quantities of 15 to 120 g / l, based on the volume of the wall-flow filter substrate. The mass ratio of carrier materials to oxygen storage components in coating Z is typically 0.25 to 1.5, for example, 0.3 to 1.3.

[0041] In one embodiment, the weight ratio of the sum of the masses of all aluminum oxides (including doped aluminum oxides) to the sum of the masses of all cerium / zirconium mixed oxides in coating Z is 10:90 to 75:25.

[0042] For example, coating Z comprises lanthanum-stabilized aluminum oxide, rhodium, palladium or palladium and rhodium and a cerium / zirconium / rare earth metal mixed oxide containing yttrium oxide and lanthanum oxide as rare earth metal oxides.

[0043] In other embodiments of the present invention, the coating Z comprises lanthanum-stabilized aluminum oxide, rhodium, palladium or palladium and rhodium and a cerium / zirconium / rare earth metal mixed oxide containing praseodymium oxide and lanthanum oxide as rare earth metal oxides.

[0044] In other embodiments of the present invention, the coating Z comprises lanthanum-stabilized aluminum oxide, rhodium, palladium or palladium and rhodium, a cerium / zirconium / rare earth metal mixed oxide containing praseodymium oxide and lanthanum oxide as rare earth metal oxides, and a second cerium / zirconium / rare earth metal mixed oxide containing yttrium oxide and lanthanum oxide as rare earth metal oxides.

[0045] The coating Z preferably contains no zeolite and no molecular sieve.

[0046] If coating Z contains aluminum oxide or doped aluminum oxide, the weight ratio of the sum of the masses of all aluminum oxides or doped aluminum oxides to the sum of the masses of all cerium / zirconium mixed oxides or cerium / zirconium / rare earth metal mixed oxides is in particular 10:90 to 75:25.

[0047] According to the invention, coating F comprises a mineral material and not a precious metal. It is therefore not catalytically active within the meaning of the present invention, meaning it is not capable of oxidizing the exhaust gas components CO and HC and reducing NOx.

[0048] Coating F preferably consists of one or more mineral materials and therefore does not contain any other components.

[0049] Suitable mineral materials contain in particular one or more elements selected from the group consisting of silicon, aluminum, titanium, zirconium, cerium, iron, zinc, magnesium, calcium, potassium and sodium.

[0050] The mineral material is a phyllosilicate such as kaolin, sepiolite, montmorillonite or bentonite, muscovite, vermiculite.

[0051] The coating F advantageously has a specific surface area of ​​at least 50 m 2 / g, preferably from 100 to 350 m 2 / g.

[0052] It is furthermore advantageous if the coating F has a bulk density of 300 to 3000 g / l, preferably 500 to 2500 g / l.

[0053] The wall-flow filter according to the invention can have an increasing concentration gradient of the coating F in the longitudinal direction of the filter from its first to its second end. "Increasing gradient" is understood according to the invention to mean the fact that the gradient of the concentration of the coating F in the filter increases in the axial direction from one end to the other, possibly from negative values ​​to more positive values.

[0054] In the case of a proper use of the wall-flow filter, in which the exhaust gas flows in at its first end and out at its second end, a larger amount of coating F is preferably located near the second end of the wall-flow filter substrate and a significantly smaller amount of coating F is located near the first end of the wall-flow filter substrate.

[0055] Simulations of gas flow in a wall-flow filter have shown that the last third of the substrate is primarily responsible (more than 50%) for the filtration properties of the overall filter. Increased application of coating F in the last third of the filter increases the back pressure there, which is due to the lower permeability, and the flow shifts more to the first two-thirds of the filter. Therefore, the filter should have a steeper gradient of coating F from the inlet to the outlet to increase its filtration efficiency. This applies mutatis mutandis to setting a favorable exhaust backpressure. Accordingly, a less steeply increasing gradient of the concentration of coating F should be selected here if necessary.

[0056] The coating F is preferably located in the porous walls of the wall-flow filter substrate, which means that the particle size of the mineral material must be adapted to the pore size of the wall-flow filter substrate. The particles of the mineral material therefore have a defined particle size distribution. Since wall-flow filter substrates usually contain pores of different sizes, ideally there is a proportion of larger particles in the large pores and a proportion of smaller particles in the smaller pores. This means that the mineral material preferably has a multimodal or broad q3 particle size distribution. For the definition of the particle size or grain size distribution of the mineral material, a distinction is made, depending on the method used to determine the quantity of particles, between number-related (q0) and volume-related (q3) grain size distributions (M.Stieß, Mechanical Process Engineering - Particle Technology 1, Springer, 3rd edition 2009, page 29).

[0057] The size of the coarse particles (defined by the d90 value of the q3 particle size distribution, measured using a Beckmann Tornado dry dispersion module according to the latest ISO 13320-1 on the filing date) of the mineral material should be less than or equal to 60% of the mean volume-related q3 pore size (d50) of the filter used (measured according to DIN 66134 - latest version on the filing date), preferably less than 50%. The mean q3 particle size of the mineral material (d50) should correspond to 5% to 30% of the mean q3 pore size (d50) of the filter used, preferably 7% to 25%, and very preferably 10% to 25%. The d10 value of the q3 grain size distribution of the mineral material, which describes the fine fraction, should be 20% to 60% of the mean q3 grain size (d50) of the mineral material, preferably 25% to 50% and particularly preferably 25% to 40%.The d10 value of the number-related q0 grain size distribution should generally be greater than 0.05 µm, preferably greater than 0.08 µm and particularly preferably greater than 0.1 µm.

[0058] The particles of the mineral material have in particular a total surface area of ​​greater than 5 m 2 < / l, preferably greater than 10 m 2 < / l and most preferably greater than 15 m 2 < / l, based on the external filter volume in liters.

[0059] The total surface area of ​​the particles SV is determined with the particle size x according to: S V m − 1 = 6 ⋅ ∫ x _ min x _ max x i − 1 ⋅ q 0 x i ⋅ dx = 6 ⋅ ∑ min max Δ Q 3 x i x i (M. Stieß, Mechanical Process Engineering - Particle Technology 1, Springer, 3rd edition 2009, page 35), and with the density of the particles ρ, the mass-related surface area is obtained (M. Stieß, Mechanical Process Engineering - Particle Technology 1, Springer, 3rd edition 2009, page 16): S m m 2 kg = S V ρ Partikel ä u ß e r e O b e r f l ä c h e d e s P u l v e r s S ä u ß e r e m 2 = S m ⋅ m P u l v e r

[0060] A person skilled in the art can easily determine the particle size distribution and the total surface area of ​​the mineral material of a finished wall-flow filter according to the invention by washing the mineral material out of the wall-flow filter substrate with water. They simply need to collect the washed-out material, dry it, and then determine the desired parameters using methods known to them or those mentioned above.

[0061] Particularly for manufacturing reasons, a portion of the coating F may also be located on the surface OE. In particular, 1 to 90% of the total mass of the coating F may be located on the surface OE, but preferably 2 to 70% and particularly preferably 3 to 50%.

[0062] The coating F preferentially does not form a coherent, continuous layer on the surface Oε, but selectively clogs the large pores of the wall flow substrate, resulting in an island-like deposition pattern.

[0063] The coating F can be present entirely or partially as a continuous layer on the surfaces Oε. In this case, the layer thickness of the coating F is generally 1 to 75 µm, but preferably 5 to 65 µm.

[0064] In an embodiment according to the invention in which the coating Z is located on the surfaces OA, the layer thickness of the coating F is less than or equal to the layer thickness of the coating Z. The ratio of the layer thickness of the coating F to the layer thickness of the coating Z is preferably 0.1 to 1, particularly preferably 0.15 to 0.95 and particularly preferably 0.2 to 0.9. Furthermore, the average particle diameter d 50 of the mineral material of coating F is less than or equal to the average particle diameter d 50 of the coating Z. The ratio of the d 50 of the particles of coating F to the d 50 of the particles of coating Z is preferably 0.01 to 1, preferably 0.05 to 0.9 and particularly preferably 0.15 to 0.8.

[0065] In an embodiment according to the invention in which the coating Z is located in the pores of the filter wall, the layer thickness of the coating F is greater than or equal to the layer thickness of the coating Z. Furthermore, the average particle diameter d 50 of the mineral material of coating F is greater than or equal to the average particle diameter d 50 of the coating Z. The ratio of the d 50 of the particles of coating F to the d 50 of the particles of coating Z is preferably 1 to 7, preferably 1.05 to 6 and particularly preferably 1.1 to 5.

[0066] In an embodiment according to the invention in which the coating Y is located on the surfaces OE, the layer thickness of the coating F is less than or equal to the layer thickness of the coating Y. The ratio of the layer thickness of the coating F to the layer thickness of the coating Y is preferably 0.1 to 1, particularly preferably 0.15 to 0.95 and particularly preferably 0.2 to 0.9. Furthermore, the average particle diameter d 50 of the mineral material of coating F is less than or equal to the average particle diameter d 50 of the coating Y. The ratio of the d 50 of the mineral material of coating F to the d 50 of the particles of coating Y is preferably 0.01 to 1, preferably 0.05 to 0.9 and particularly preferably 0.15 to 0.8.

[0067] In an embodiment according to the invention in which the coating Y is located in the pores of the filter wall, the layer thickness of the coating F is greater than or equal to the layer thickness of the coating Y. Furthermore, the average particle diameter d 50 of the mineral material of coating F is greater than or equal to the average particle diameter d 50 of the coating Y. Preferably, the ratio of the d 50 of the mineral material of coating F to the d 50 of the particles of coating Y is 1 to 7, preferably 1.05 to 6 and particularly preferably 1.1 to 5.

[0068] Based on the volume of the wall-flow filter substrate, coating F is present, for example, in quantities of less than 50 g / l, in particular less than 40 g / l. Preferably, coating F is present in quantities of 2.5 to 40 g / l, based on the volume of the wall-flow filter substrate.

[0069] The coating F can extend over the entire length L of the wall-flow filter substrate or only over a portion of it. For example, coating F extends over 10 to 100, 25 to 80, or 40 to 60% of the length L.

[0070] In one embodiment of the wall-flow filter according to the invention, the wall-flow filter substrate has a coating Y that is different from coatings Z and F, which comprises platinum, palladium, or platinum and palladium, which contains no rhodium and no cerium / zirconium mixed oxide, and which is located in the porous walls and / or on the surfaces Oε, but not on the surfaces OA. Coating Y preferably contains platinum and palladium with a platinum to palladium mass ratio of 25:1 to 1:25, particularly preferably 15:1 to 1:2.

[0071] In coating Y, platinum, palladium or platinum and palladium are usually fixed on one or more carrier materials.

[0072] Suitable support materials include all materials familiar to the person skilled in the art for this purpose. 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 date of filing). Particularly suitable support materials are selected from the series consisting of aluminum oxide, doped aluminum oxide, silicon oxide, titanium dioxide, and mixed oxides of one or more thereof. Doped aluminum oxides are, for example, lanthanum oxide-, zirconium oxide-, barium oxide-, and / or titanium oxide-doped aluminum oxides. Advantageously, aluminum oxide or lanthanum-stabilized aluminum oxide is used, with lanthanum being 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.

[0073] Also in the case of alumina doped with barium oxide, the proportion of barium oxide is in particular 1 to 10 wt.%, preferably 3 to 6 wt.%, each calculated as BaO and based on the weight of the stabilized alumina.

[0074] Another suitable support material is lanthanum-stabilized aluminum oxide, the surface of which is coated with lanthanum oxide, barium oxide and / or strontium oxide.

[0075] Coating Y preferably comprises at least one aluminum oxide or doped aluminum oxide.

[0076] In one embodiment, the coating Y is located exclusively on the surfaces OE of the wall-flow filter substrate and extends from its first end over a length of 50 to 90% of the length L.

[0077] In another embodiment, the coating Y is located in the porous walls of the wall-flow filter substrate and extends from its first end preferably over a length of 50 to 100% of the length L.

[0078] If coating Y is present, the mass ratio of coating Y to coating Z is preferably 0.05 to 8.5.

[0079] For example, the carrier material of coating Y has a larger pore volume than the carrier material of coating Z. The ratio of the specific surface areas of the carrier oxides of coating Y to coating Z is preferably 0.5 to 2, in particular 0.7 to 1.5.

[0080] For example, the ratio of the pore volume of the mineral material of coating F to the pore volume of the carrier material of coating Z is preferably 0.01 to 3, in particular 0.05 to 2.5. The ratio of the specific surface areas of the mineral material of coating F to the specific surface areas of the carrier oxides of coating Z is preferably 0.1 to 4, in particular 0.25 to 3.

[0081] The coatings Z, F and, if present, Y can be arranged on the wall-flow filter substrate in various ways. Figures 1 to 10 explain this by way of example, whereby the Figures 1 to 4 inventive wall flow filters which comprise only the coatings Z and F, while the inventive wall flow filters according to the Figures 5 to 10 additionally include the coating Y.

[0082] Figure 1relates to a wall-flow filter according to the invention, in which the coating Z is located in the channels A on the surfaces OA and extends from the second end of the wall-flow filter substrate over 50% of the length L. The coating F is located in the channels E and extends over the entire length L.

[0083] Also Figure 2 relates to a wall-flow filter according to the invention, in which the coating Z is located in the channels A on the surfaces OA. Starting from the second end of the wall-flow filter substrate, however, it extends over 80% of the length L.

[0084] The coating F is located in the channels E and extends over the entire length L.

[0085] Figure 3relates to a wall-flow filter according to the invention, in which the coating Z is located in the porous walls and extends over the entire length L. The coating F is located in the channels E and also extends over the entire length L.

[0086] Figure 4 relates to a wall-flow filter according to the invention, in which the coating Z is located in the porous walls and extends from the second end of the wall-flow filter substrate over 50% of the length L. The coating F is located in the channels E and extends over the entire length L.

[0087] Figure 5 relates to a wall flow filter according to the invention, which differs from that of Figure 4 only differs in that additional coating Y is located in the porous walls over the entire length L. Coating F is located in the channels E and extends over the entire length L.

[0088] Figure 6relates to a wall flow filter according to the invention, which differs from that of Figure 4 differs only in that additional coating Y extends in the porous walls, starting from the first end of the wall-flow filter substrate, over 50% of the length L. Coating F is located in the channels E and extends over the entire length L.

[0089] Figure 7 relates to a wall-flow filter according to the invention, in which the coating Z is located in the channels A on the surfaces OA and extends over 50% of the length L. Furthermore, coating Y is located in the channels E on the surfaces OE and extends from the first end of the wall-flow filter substrate over 50% of the length L. The coating F is located in the channels E and extends from the second end of the wall-flow filter substrate over 50% of the length L.

[0090] Figure 8relates to a wall-flow filter according to the invention, in which the coating Z is located in the porous walls and extends over the entire length L. In addition, coating Y is located in the channels E on the surfaces OE and extends from the first end of the wall-flow filter substrate over 50% of the length L. The coating F is located in the channels E and extends from the second end of the wall-flow filter substrate over 50% of the length L.

[0091] Figure 9relates to a wall-flow filter according to the invention, in which the coating Z is located in the porous walls and extends from the second end of the wall-flow filter substrate over 50% of the length L. Furthermore, coating Y is located in the channels E on the surfaces OE and extends from the first end of the wall-flow filter substrate over 50% of the length L. The coating F is located in the channels E and extends from the second end of the wall-flow filter substrate over 50% of the length L.

[0092] Figure 10 relates to a wall-flow filter according to the invention, in which the coating Z is located in the channels A on the surfaces OA and extends from the second end of the wall-flow filter substrate over 80% of the length L. Furthermore, coating Y is located in the porous walls and extends over the entire length L. The coating F is located in the channels E and extends over the entire length L.

[0093] The wall flow filter according to the invention can be produced by applying the coatings Z, F and, if present, Y to a wall flow filter substrate.

[0094] The catalytic activity is provided according to the expert's specifications by coating the wall-flow filter substrate with coating Z and, if present, with coating Y.

[0095] The term "coating" therefore refers to the application of catalytically active materials to a wall-flow filter substrate. The coating performs the actual catalytic function. In this case, the coating is applied by applying a correspondingly low-viscosity aqueous suspension of the catalytically active components—also called a washcoat—in or onto the wall of the wall-flow filter substrate, for example, according to EP1789190B1. After application of the suspension, the wall-flow filter substrate is dried and, if appropriate, calcined at elevated temperature. The catalytically coated filter preferably has a loading of 20 g / l to 200 g / l, preferably 30 g / l to 150 g / l (coating Z or the sum of coatings Z and Y). The most suitable loading amount for a wall-coated filter depends on its cell density, wall thickness, and porosity.

[0096] The coating F is applied to the wall-flow filter substrate in particular by exposing the channels E of the dry wall-flow filter substrate already coated with coating Z and optionally coating Y to a dry powder-gas aerosol, wherein the powder contains a mineral material and in particular consists of a mineral material.

[0097] By applying a dry powder-gas aerosol to a wall-flow filter substrate that has been conventionally wet-coated with coating Z and optionally Y, dried and optionally calcined, a wall-flow filter according to the invention is obtained with extremely good filtration efficiency and only slightly increased exhaust gas back pressure and, at the same time, excellent catalytic effectiveness.

[0098] The catalytically coated and subsequently powder-coated wall-flow filters according to the invention differ from those that develop in the exhaust system of a vehicle due to ash deposition during operation. According to the invention, the catalytically active wall-flow filter substrates 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, the present invention does not encompass wall-flow filters in which undefined ash deposits have occurred from the combustion of fuel, for example, in the cylinder during driving or by means of a burner.

[0099] Exposure to the wall-flow filter substrates considered here with the dry powder-gas aerosol results in the powder particles following the gas flow and depositing in the pores of the wall-flow filter substrate and, if applicable, on the surfaces Oε. The varying wall permeability of the wall-flow filter substrate (e.g., due to inhomogeneities in the filter wall itself or different coating zones) leads to a selective deposition of the powder in the pores of the wall or on the surfaces OE where the flow is greatest. This effect also leads to, for example, cracks or pores in the washcoat layer caused by coating defects being filled by the porous powder in such a way that the soot particles in the exhaust gas are subsequently retained to a greater extent as the exhaust gas passes through the filter. This therefore results in improved filtration efficiency.

[0100] According to the invention, the dry wall-flow filter substrate coated with coating Z and optionally coating Y is coated with a powder starting from its first end and towards its second end (i.e. with reference to the intended use in the direction of exhaust gas flow) in such a way that the cell wall regions through which the flow is greatest are coated with loose, inherently porous powder accumulations in the porous walls and / or on the surfaces OE in order to achieve a desired increase in filtration efficiency. The formation of the inherently porous powder accumulations surprisingly leads to a relatively low increase in back pressure. In a preferred embodiment, the wall-flow filter substrate is exposed to a powder-gas aerosol in such a way that the powder is deposited in the pores of the porous wall and on the surfaces OE upon exposure, building up a coherent layer there.In a further very preferred embodiment, the wall-flow filter is subjected to a powder-gas aerosol in such a way that the powder precipitates in the pores of the filter walls upon exposure and fills them up to the surfaces OE and does not form a coherent layer on surfaces OE.

[0101] In order for the powder of the powder-gas aerosol to deposit sufficiently well in the pores of the wall-flow filter substrate coated with coating Z and optionally coating Y, or to adhere to the surfaces OE, the particle diameter in the aerosol should be at least smaller than the pores of the wall-flow filter substrate. This can be expressed by the ratio of the mean particle diameter (Q 3 distribution; measured according to the latest ISO 13320 on the filing date) dso in the dry aerosol and the mean pore diameter of the wall-flow filter after coating (measured according to DIN 66134 - latest version on the filing date) being between 0.03 and 2, preferably between 0.05 and 1.43, and very particularly preferably between 0.05 and 0.63. This ensures that the powder particles in the aerosol can precipitate into the pores of the walls of the wall-flow filter substrate, following the gas flow.

[0102] In particular, a suitable powder has a specific surface area of ​​at least 100 m 2 / g and a total pore volume of at least 0.3 ml / g.

[0103] For a suitable powder for producing the wall-flow filters according to the invention, an optimization between the largest possible surface area of ​​the powder used, cross-linking, and adhesive strength is advantageous. During operation in the vehicle, small particles follow the streamlines with almost no inertia due to their short particle relaxation time. This uniform, convection-driven movement is superimposed on a random "dithering" movement. Following this theory, for a good filtration effect of a wall-flow filter loaded with powder, it should offer the largest possible surface area. The powder should therefore have a high fines content, since small particles offer significantly larger surface areas for the same total volume of mineral material. At the same time, however, the pressure drop must only increase insignificantly. This requires loose cross-linking of the powder.For a filtration efficiency-enhancing coating, powders with a tamped density between 50 g / l and 900 g / l can preferably be used, preferably between 200 g / l and 850 g / l and most preferably between 400 g / l and 800 g / l.

[0104] The aerosol from the gas and powder can be prepared according to the instructions of the person skilled in the art or as described below. For this purpose, a powder is generally mixed with a gas. (http: / / www.tsi.com / Aerosolgeneratoren-und-dispersier / ; https: / / www.palas.de / de / product / aerosolgeneratorssolidparticles ). This mixture of gas and powder thus produced is then advantageously fed into the channels E of the wall-flow filter substrate via a gas stream.

[0105] Any gases considered suitable for the present purpose by a person skilled in the art can be used to produce the aerosol and introduce it into the wall-flow filter substrate. The use of air is particularly preferred. However, other reaction gases that can develop either an oxidizing (e.g., O 2 , NO 2 ) or a reducing (e.g., H 2 ) activity toward the powder used can also be used. Likewise, for certain powders, the use of inert gases (e.g., N 2 ) or noble gases (e.g., He) can prove advantageous. Mixtures of the listed gases are also conceivable.

[0106] In order to deposit the powder sufficiently deep into the channels E and ensure good adhesion, a certain suction power is required. Experts can gain an idea of ​​this by conducting preliminary tests for the respective wall-flow filter and the respective powder. It has been found that the aerosol (powder-gas mixture) is preferably drawn through the wall-flow 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.

[0107] 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 measures: compressed air, ultrasound, sieving, in-situ grinding, blower, gas expansion, fluidized bed. Other dispersion methods not mentioned here may also be used by the person skilled in the art. In principle, the person skilled in the art is free to choose a method for producing the powder-gas aerosol. As just described, the powder is first converted into a powder-gas aerosol by dispersion and then introduced into a gas stream.

[0108] The resulting mixture of gas and powder is then introduced into an existing gas stream, which carries the finely dispersed powder into the channels E of the wall-flow filter substrate. This process is preferably supported by a suction device positioned in the pipe downstream of the filter. This is in contrast to the Fig. 3the device shown in US8277880B, in which the powder-gas aerosol is generated directly in the gas stream. The method according to the invention allows for a much more uniform and thorough mixing of the gas stream with the powder-gas aerosol, which ultimately ensures an advantageous distribution of the powder particles in the filter in both the radial and axial directions and thus helps to standardize and control the deposition of the powder particles on the filter. The powder is dry when applied to the wall-flow filter substrate within the meaning of the invention. Preferably, the powder is mixed with ambient air and applied to the filter. By mixing the powder-gas aerosol with particle-free gas, preferably dry ambient air, the concentration of the particles is reduced to such a level that no significant agglomeration occurs until deposition in the wall-flow filter substrate. This preserves the particle size in the aerosol established during dispersion.

[0109] A preferred device for producing a wall flow filter according to the invention is described in Figure 11 shown schematically. 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 receiving a filter, which are designed such that the gas stream flows through the filter without any further supply of gas; a unit for generating suction, which maintains the gas flow through the filter; optionally a unit for generating vortices in front of the filter, so that deposition of powder on the inlet plug of the filter is avoided as far as possible; and optionally a unit through which at least a partial gas stream is taken downstream of the suction device and added to the gas stream which is sucked through the filter before the powder is added; are present.

[0110] In this preferred embodiment of the method according to the invention - as shown in the sketch Figure 11- At least a partial gas stream is taken from the downstream side of the suction device and added back to the gas stream being drawn through the filter before the powder is added. This way, the powder is metered into an already heated air stream. The suction fans for the required pressures generate an exhaust air temperature of approximately 70°C, since the installed suction power is preferably >20 kW. For energy optimization, the waste heat from the suction fan is used to heat the supply air to reduce the relative humidity of the supply air. This, in turn, reduces the adhesion of the particles to each other and to the inlet plugs. The powder deposition process can thus be better controlled.

[0111] In the present method for producing a wall-flow filter according to the invention, a gas stream is subjected to a powder-gas aerosol, which is sucked into a wall-flow filter substrate. This ensures that the powder can be distributed sufficiently well in the gas stream to penetrate the channels E. 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. Diffusers and venturi tubes are therefore preferred for this process. Introducing the dispersed powder into a rapidly rotating flow with high turbulence has also proven effective.

[0112] To achieve an advantageous, even distribution of the powder across the cross-section of the wall-flow filter substrate, the gas transporting the powder should exhibit a piston flow (preferably 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 steady reduction in the cross-section without abrupt changes causes such an accelerated flow, described by the continuity equation. Furthermore, it is also known to those skilled in the art that this causes the flow profile to more closely resemble a piston profile. To specifically modify the flow, internals such as sieves, rings, disks, etc. can be used below and / or above the filter.

[0113] In a further advantageous embodiment of the present method, the powder coating apparatus has one or more devices (turbulators, vortex generators) with which the gas stream carrying the powder-gas aerosol can be swirled before it hits the filter. Examples of this could be appropriate sieves or grids placed at a sufficient distance upstream of the wall-flow filter substrate. The distance should not be too large or too small so that sufficient swirling of the gas stream is achieved directly in front of the wall-flow filter substrate. A person skilled in the art can determine the distance in simple experiments. The advantage of this measure is that no powder components are deposited on the plugs of channels A, and all powder can penetrate into channels E.Accordingly, it is preferred according to the invention if the powder is swirled before flowing into the filter in such a way that deposits of powder on the plugs of the wall-flow filter substrate 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 (particularly microvortices) form behind rods, grids, and other flow-disturbing fittings at certain Re 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 of the invention, this effect can be particularly advantageously enhanced by vibrating self-cleaning screens (so-called ultrasonic screens), which advantageously move within the flow. Another method is to disrupt the flow with sound fields, which stimulate turbulence through the pressure amplitudes. These sound fields can even clean the surface of the filter without any flow. The frequencies can range from ultrasonic to infrasound. The latter measures are also used for pipe cleaning in large-scale plants.

[0114] The preferred embodiments for the wall-flow filter also apply mutatis mutandis to the method. In this regard, explicit reference is made to the above statements regarding the wall-flow filter.

[0115] Dry in the context of the present invention means the exclusion of the use of a liquid, especially water. In particular, the preparation of a suspension of the powder in a liquid for atomization into a gas stream should be avoided. A certain degree of humidity may be tolerable for both the filter and the powder, provided that the goal of achieving the most finely distributed deposition of the powder in the porous walls and / or the surfaces Oε of the wall-flow filter substrate is not negatively affected. The powder is generally free-flowing and dispersible by energy input. The moisture content of the powder or the wall-flow filter substrate at the time of exposure to the powder should be less than 20%, preferably less than 10%, and most preferably less than 5% (measured at 20°C and atmospheric pressure according to ISO 11465, latest version on the filing date).

[0116] The wall-flow filter according to the invention exhibits excellent filtration efficiency with only a moderate increase in exhaust gas backpressure compared to a fresh wall-flow filter not treated with powder. Preferably, the wall-flow filter according to the invention exhibits an improvement in soot particle deposition (filtering effect) in the filter of at least 5%, preferably at least 10%, and most preferably at least 20%, with a relative increase in exhaust gas backpressure of the fresh wall-flow filter of at most 40%, preferably of at most 20%, and most preferably of at most 10%, compared to a fresh, non-powder-treated filter coated with catalytically active material. The slight increase in backpressure is probably due to the fact that the cross-section of the channels on the inlet side is not significantly reduced by the inventive application of powder to the filter.It is assumed that the powder forms a porous structure within itself, which has a positive effect on back pressure. Therefore, a wall-flow filter according to the invention should also exhibit improved exhaust back pressure compared to prior art, in which a powder is deposited on the walls of the inlet side of a filter or a conventional wet coating is used.

[0117] Coating Z provides the inventive wall-flow filter with excellent three-way activity, while the optional coating Y is able to reduce the soot ignition temperature and thus facilitates soot burnoff.

[0118] The present invention thus also relates to the use of a wall-flow filter according to the invention for reducing harmful exhaust gases from an internal combustion engine. The use of the wall-flow filter according to the invention is preferred for treating exhaust gases from a stoichiometrically operated internal combustion engine, i.e., in particular, a gasoline-powered internal combustion engine.

[0119] The wall-flow filter according to the invention is very advantageously used in combination with at least one three-way catalyst. It is particularly advantageous if a three-way catalyst is located close to the engine, upstream of the wall-flow filter according to the invention. It is also advantageous if a three-way catalyst is located downstream of the wall-flow filter according to the invention. It is also advantageous if a three-way catalyst is located upstream and downstream of the wall-flow filter.

[0120] The preferred embodiments described for the wall flow filter according to the invention also apply mutatis mutandis to the use mentioned here.

[0121] The preferred embodiments described for the wall flow filter according to the invention also apply mutatis mutandis to the use mentioned here.

[0122] The present invention further relates to an exhaust gas purification system comprising a filter according to the invention and at least one further catalyst. In one embodiment of this system, at least one further catalyst is arranged upstream of the filter according to the invention. This is preferably a three-way catalyst or an oxidation catalyst or a NO x storage catalyst. In a further embodiment of this system, at least one further catalyst is arranged downstream of the filter according to the invention. This is preferably a three-way catalyst or an SCR catalyst or a NO x storage catalyst or an ammonia slip catalyst. In a further embodiment of this system, at least one further catalyst is arranged upstream of the filter according to the invention and at least one further catalyst is arranged downstream of the filter according to the invention.Preferably, the upstream catalyst is a three-way catalyst, an oxidation catalyst, or a NO x storage catalyst, and the downstream catalyst is a three-way catalyst, an SCR catalyst, a NO x storage catalyst, or an ammonia slip catalyst. The preferred embodiments described for the wall-flow filter according to the invention also apply, mutatis mutandis, to the exhaust gas purification system mentioned here.

[0123] The filter according to the invention is typically used primarily in internal combustion engines, particularly in internal combustion engines with direct fuel injection or intake manifold injection. These are preferably stoichiometrically operated gasoline or natural gas engines. These are preferably turbocharged engines.

[0124] The requirements for gasoline particulate filters (GPFs) differ significantly from those for diesel particulate filters (DPFs). Diesel engines without DPFs can have particle emissions up to ten times higher, 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 significantly smaller than those from diesel engines. Emissions from gasoline engines range from particle sizes smaller than 200 nm (Hall et al., SAE 1999-01-3530) to 400 nm (Mathis et al., Atmospheric Environment 38 4347), with the maximum in the range of approximately 60 nm to 80 nm. Therefore, the filtration of nanoparticles in GPFs must primarily be achieved by diffusion deposition. For particles smaller than 300 nm, deposition by diffusion (Brownian motion) and electrostatic forces becomes increasingly important with decreasing size (Hinds, W.: Aerosol technology: Properties and behavior and measurement of airborne particles. Wiley, 2nd edition 1999).

[0125] The Figures 1 to 10 show the different coating arrangements of the wall-flow filters according to the invention, which have already been described in more detail above. (E) the inlet channel / inflow channel of the wall-flow filter (A) the outlet channel / outflow channel of the wall-flow filter (OE ) the surfaces formed by the inlet channels (E) (OA ) the surfaces formed by the outlet channels (A) (L) the length of the filter wall (Z) the coating Z (Y) the coating Y (F) the coating F

[0126] Figure 11shows 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 atomizer nozzle 440 in the mixing chamber with the gas stream 454 and then sucked or forced through the filter 430. The particles that pass through are filtered out in the exhaust filter 400. The blower 410 ensures the necessary volume flow. The exhaust gas is divided into an exhaust gas 452 and a warm recycle gas 453. The warm recycle gas 453 is mixed with the fresh gas 450.

[0127] The advantages of the invention are illustrated below by reference examples. Comparative Example 1: Coating Z only: Lanthanum oxide-stabilized aluminum oxide was suspended in water together with a first oxygen storage component comprising 40 wt.% cerium oxide, zirconium oxide, lanthanum oxide, and praseodymium oxide, and a second oxygen storage component comprising 24 wt.% cerium oxide, zirconium oxide, lanthanum oxide, and yttrium oxide. Both oxygen storage components were used in equal parts. The weight ratio of aluminum oxide to oxygen storage component was 30:70. The resulting suspension was then admixed with a palladium nitrate solution and a rhodium nitrate solution while stirring continuously. The resulting coating suspension was used directly to coat a commercially available wall-flow filter substrate, with coating Z being incorporated into the porous filter wall over 100% of the substrate length.The total loading of this filter was 75 g / l, the total precious metal loading was 2.12 g / l, with a palladium to rhodium ratio of 5:1. The resulting coated filter was dried and then calcined. It is referred to below as VGPF1.

[0128] Reference Example 1: Coating Z in combination with coating F: Lanthanum oxide stabilized alumina was suspended in water together with a first oxygen storage component comprising 40 wt% cerium oxide, zirconium oxide, lanthanum oxide and praseodymium oxide and a second oxygen storage component comprising 24 wt% cerium oxide, zirconium oxide, lanthanum oxide and yttrium oxide.

[0129] Both oxygen storage components were used in equal parts. The weight ratio of aluminum oxide to oxygen storage component was 30:70. A palladium nitrate solution and a rhodium nitrate solution were then added to the resulting suspension under continuous stirring. The resulting coating suspension was used directly to coat a commercially available wall-flow filter substrate, with coating Z being introduced into the porous filter wall over 100% of the substrate length. The total loading of this filter was 75 g / l, the total precious metal loading was 2.12 g / l, with a palladium to rhodium ratio of 5:1. The coated filter thus obtained was dried and then calcined. The filter was then exposed to a dry powder-gas aerosol, with 4 g / l of a mineral material being introduced into the channels E. It is referred to below as GPF1.

[0130] The two resulting filters were then measured on a cold blow test bench to determine the pressure drop across each filter. At room temperature and a volume flow of 600 m³ / h of air, the back pressure for the VGPF1 was 29 mbar and for the GPF1 39 mbar. As already described, the filtration coating F only leads to a moderate increase in back pressure.

[0131] Furthermore, the two filters were tested for backpressure after soot loading. For this purpose, both filters were sooted on an engine test bench with a direct-injection turbocharged engine. The final soot loading was approximately 3 g. Under these conditions, the soot backpressure for the VGPF1 was 73 mbar and for the GPF1 only 44 mbar.

[0132] In parallel, fresh filters VGPF1 and GPF1 were tested in the vehicle for their particulate filtration efficiency. For this purpose, the filters were measured in an RTS-95, also known as RTC-aggressive, driving cycle in a close-coupled position between two particulate counters. In both cases, a three-way catalyst was located upstream in the exhaust tract, via which the vehicle's lambda control was carried out. The inventive filter GPF1 exhibits a filtration efficiency of 96%, calculated from the particulate values ​​of the two particulate counters, while the comparison filter VGPF1 achieves only a filtration efficiency of 42%. Overall, it is evident that the combination of filtration coating F and three-way coating Z is particularly advantageous with regard to backpressure after soot loading and filtration efficiency.

Claims

1. Wall-flow filter for removing particles from the exhaust gas of combustion engines, the filter comprising a wall-flow filter substrate of length L and coatings Z and F that differ from one another, the wall-flow filter substrate having channels E and A, which extend in parallel between a first and a second end of the wall-flow filter substrate, being separated by porous walls and forming surfaces OE and OA, respectively, and the channels E being closed at the second end and the channels A being closed at the first end, and the coating Z being located in the porous walls and / or on the surfaces OA, but not on the surfaces OE, and comprising palladium and / or rhodium and a cerium / zirconium mixed oxide, characterized in that the coating F is located in the porous walls and / or on the surfaces Oε, but not on the surfaces OA, and comprises a mineral material and no precious metal, the mineral material being a phyllosilicate such as kaolin, sepiolite, montmorillonite or bentonite, muscovite, vermiculite.

2. Wall-flow filter according to claim 1, characterized in that coating Z is located on the surfaces OA of the wall-flow filter substrate and extends from the second end of the wall-flow filter substrate to 50 to 90% of the length L.

3. Wall-flow filter according to claim 1, characterized in that coating Z is located in the porous walls of the wall-flow filter substrate and extends from the first end of the wall-flow filter substrate to 50 to 100% of the length L.

4. Wall-flow filter according to one or more of claims 1 to 3, characterized in that coating Z contains palladium and rhodium and no platinum.

5. Wall-flow filter according to one or more of claims 1 to 4, characterized in that the cerium / zirconium mixed oxide of coating Z contains one or more rare earth metal oxides.

6. Wall-flow filter according to claim 5, characterized in that the rare earth metal oxide is lanthanum oxide, yttrium oxide, praseodymium oxide, neodymium oxide and / or samarium oxide.

7. Wall-flow filter according to one or more of claims 1 to 6, characterized in that coating Z comprises lanthanum-stabilized aluminum oxide, rhodium, palladium or palladium and rhodium, and a cerium / zirconium / rare earth metal mixed oxide containing yttrium oxide and lanthanum oxide as rare earth metal oxides.

8. Wall-flow filter according to one or more of claims 1 to 6, characterized in that coating Z comprises lanthanum-stabilized aluminum oxide, rhodium, palladium or palladium and rhodium, and a cerium / zirconium / rare earth metal mixed oxide containing praseodymium oxide and lanthanum oxide as rare earth metal oxides.

9. Wall-flow filter according to one or more of claims 1 to 8, characterized in that it has an increasing concentration gradient of the coating F in the longitudinal direction of the filter from the first to the second end thereof.

10. Wall-flow filter according to one or more of claims 1 to 9, characterized in that the wall-flow filter substrate has a coating Y which is different from the coatings Z and F, which comprises platinum, palladium or platinum and palladium, which contains no rhodium and no cerium / zirconium mixed oxide and which is located in the porous walls and / or on the surfaces Oε, but not on the surfaces OA.

11. Wall-flow filter according to claim 10, characterized in that coating Y extends over a length of 50 to 100% of the length L.

12. Method for producing a wall-flow filter according to one or more of claims 1 to 11, characterized in that the channels E of the dry wall-flow filter substrate already coated with coating Z and optionally coating Y are impinged on by a dry powder / gas aerosol, the powder containing a mineral material.

13. Use of a wall-flow filter according to one or more of claims 1 to 11 for reducing harmful exhaust gases of a combustion engine.