CATALYTIC ACTIVE PARTICLE FILTER WITH HIGH FILTRATION EFFICIENCY

Description

[0001] The present invention relates to a wall flow filter, a method for its manufacture and its use for reducing harmful exhaust gases 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 processes have been published in the past by automotive catalytic converter 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 filter of the same dimensions, and thus to a reduction in engine torque or potentially increased fuel consumption. To avoid further increasing the exhaust backpressure, the amount of oxide support material for the catalytically active precious metals of the catalyst, or the oxide catalyst materials themselves, is generally lower in a filter than in a flow-through filter. As a result, the catalytic effectiveness of a catalytically coated particulate filter is often inferior to that of a flow-through monolith of the same dimensions.

[0004] Several efforts have already been made to provide particulate filters that exhibit good catalytic activity through an active coating while still displaying 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 channel walls of the filter are permeated 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 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.

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

[0006] In addition to 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. 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 high-melting-point metal oxide and is guided over the inlet side of a wall-flow filter by means of 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 running 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 expressly pointed out that it is not desirable to create a coating of metal oxide in 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 with a length of 1 nm to 5 µm and a 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 catalytically inactive 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 or 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 within the pores of a wall flow filter substrate by atomizing dry particles is described in US 8388721B2. Here, however, the powder is intended to penetrate deep into the pores. 20% to 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 set. 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. Here, too, the catalytically active material is located in the channel walls of the filter. The introduction of powder into the pores, e.g., using an aerosol generator, is also described in EP 2727640A1. Here, a non-catalytically coated wall flow filter with, for example, a...The gas stream containing aluminum oxide particles is coated in such a way 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.

[0010] 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.

[0011] In DE102018127955A1, wall flow filters with multiple layers or zones in the inlet and outlet channels are disclosed, whereby a powder aerosol can also be introduced into the inlet channels.

[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 applied to the porous channel walls prior to powder coating. However, none of the examples provide evidence of simultaneously optimizing the catalytic effect of a filter and increasing its filtration efficiency.

[0013] There is therefore a continued 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.

[0014] These and other problems arising obviously from the prior art are solved by a particulate filter according to claims 1 to 13. Claim 14 is directed to the manufacture of a particulate filter according to the invention. Claim 15 aims at the use of the particulate filter for exhaust aftertreatment of internal combustion engines.

[0015] 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 coatings Z and F that are different from one another. wherein the wall flow filter substrate has channels E and A which extend 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, wherein the channels E at the second end and the channels A at the first end are closed, 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 particulate metal compound and not a precious metal, and the wall-flow filter substrate has a coating Y that is different from the coatings Z and F, comprising platinum, palladium or platinum and palladium, which does not contain rhodium and not a cerium / zirconium mixed oxide, and which is located in the porous walls, but not on the surfaces Oa.

[0016] When the wall flow filter according to the invention is used as intended for cleaning exhaust gases from combustion engines, the exhaust gas flows into the filter at one end and exits at the other end after passing through the porous walls. For example, if the exhaust gas enters the filter at the first end, the 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 the channels A designate the outlet channels or downstream channels.

[0017] All ceramic wall-flow filter substrates known from the prior art and commonly used in automotive exhaust gas catalysis can be employed as wall-flow substrates. Porous wall-flow filter substrates made of cordierite, silicon carbide, or aluminum titanate are preferred. These wall-flow filter substrates have channels E and channels 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 upflow ends of the outflow channels are offset from each other and are typically sealed with gas-tight plugs. This forces the exhaust gas to be cleaned, which flows through the filter substrate, to pass through the porous wall between the inflow and outflow channels, thus creating a particulate filter effect.The filtration properties for particles can be designed by the porosity, pore / radius distribution, and wall thickness. 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 filing date]. The mean 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 most preferably from 15 µm to 20 µm [measured according to DIN 66134 latest version on the filing date], 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 specified 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 %.

[0018] Those skilled in the art are aware that, due to the plugs that seal channels E and A in a staggered arrangement, the entire length L of the wall flow filter substrate may not be available for coating. For example, channels E at the far end of the wall flow filter substrate are sealed, so that the surface area OE available for coating may consequently be slightly smaller than the length L. This, of course, only applies if the coating covers 100% of the length L or slightly less. For the sake of simplicity, the following text will nevertheless refer to 100% of the length L in these cases.

[0019] 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.

[0020] The coating on the surfaces OA is a so-called application coating. This means that the coating extends from 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 located within the porous wall.

[0021] The coatings exhibit a certain elevation above the wall surface. The thickness of layers Z and Y is typically 5 to 250 µm, preferably 7.5 to 225 µm, and most preferably 10 to 200 µm, with the layer thickness preferably being determined in the center of a respective channel web and not in the corners. Commonly used analytical methods known to those skilled in the art, such as scanning electron microscopy, are suitable for determining the layer thickness.

[0022] 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.

[0023] 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 coated with the coating Z to a secondary extent.

[0024] 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 the second end of the wall flow filter substrate.

[0025] The coating Z can have a thickness gradient along its length L such that the thickness of the coating Z increases from the second end to the first end along the length L of the wall flow filter. The coating may preferably be more than twice, and more preferably up to three times, thicker at one end than at the other. The thickness is the height to which the coating Z rises above the surface OA. This thickness gradient of the coating on the channel walls also ensures that the filtration efficiency is uniform along the entire length L of the filter. The result is a more uniform separation of soot across the entire filter wall, and thus an improved exhaust backpressure rise and potentially better soot combustion.

[0026] The coating Z can also have a thickness gradient along its length L such that the thickness of the coating Z decreases from the second end to the first end along the length L of the wall flow filter. The coating may preferably be more than twice, and more preferably up to three times, thicker at one end than at the other. The thickness is the height to which the coating Z rises above the surface OA. This thickness gradient of the coating on the channel walls also ensures that the filtration efficiency is uniform along the entire length L of the filter. The result is a more uniform separation of soot across the entire filter wall, and thus an improved exhaust gas backpressure rise and potentially better soot combustion.

[0027] Coating Z is a catalytically active coating, particularly due to its palladium and / or rhodium content. Within the scope of the present invention, "catalytically active" refers to the ability to convert harmful components of the exhaust gas from internal combustion engines into less harmful ones. The exhaust gas components NOₓ, CO, and HC are particularly relevant in this context. Consequently, coating Z is especially preferably three-way catalytically active, particularly at operating temperatures of 250 to 1100 °C.

[0028] Coating Z contains the precious metals palladium and / or rhodium, with platinum being present only exceptionally as an additional precious metal. Coating Z preferably contains palladium and rhodium and no platinum.

[0029] In another embodiment, coating Z contains the precious metals platinum and / or rhodium, with palladium being present only exceptionally as a further precious metal.

[0030] In another 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.

[0031] 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 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 quantities of 0.10 to 5 g / l, based on the volume of the wall-flow filter substrate.

[0032] The precious metals are usually fixed to one or more carrier materials. Any material known to those skilled in the art for this purpose is suitable as a carrier material. 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 oxide, titanium dioxide, and mixed oxides of one or more of these. Doped aluminum oxides include, for example, lanthanum oxide, zirconium oxide, barium oxide, and / or titanium oxide-doped aluminum oxides. Aluminum oxide or lanthanum-stabilized aluminum oxide is advantageously used, with lanthanum in the latter case being present 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 aluminium oxide, is used.

[0033] In the case of aluminum oxide doped with barium oxide, the proportion of barium oxide is in particular 1 to 10 wt.%, preferably 3 to 6 wt.%, in each case calculated as BaO and based on the weight of the stabilized aluminum oxide.

[0034] Another suitable substrate material is lanthanum-stabilized aluminum oxide, the surface of which is coated with lanthanum oxide, barium oxide and / or strontium oxide.

[0035] Coating Z preferably comprises at least one aluminum oxide or doped aluminum oxide.

[0036] 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 can vary widely in these products. For example, it is 0.1 to 1.5, preferably 0.15 to 1 or 0.2 to 0.9.

[0037] 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" 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. The same applies to cerium / zirconium mixed oxides that do not contain a rare-earth metal oxide. 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.

[0038] Suitable rare earth metal oxides in the cerium / zirconium / rare earth metal mixed oxides include, for example, lanthanum oxide, yttrium oxide, praseodymium oxide, neodymium oxide, and / or samarium oxide. Lanthanum oxide, yttrium oxide, and / or praseodymium oxide are preferred. 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.

[0039] The proportion of rare earth metal oxide in the cerium / zirconium / rare earth metal mixed oxides is particularly between 3 and 20 wt.% based on the cerium / zirconium / rare earth metal mixed oxide.

[0040] 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, their mass ratio is particularly 0.1 to 1.25, preferably 0.1 to 1.

[0041] The coating Z typically contains oxygen storage components in amounts 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 the coating Z is typically 0.25 to 1.5, for example, 0.3 to 1.3.

[0042] For example, the weight ratio of the sum of the masses of all aluminium oxides (including doped aluminium oxides) to the sum of the masses of all cerium / zirconium mixed oxides in coating Z is 10:90 to 75:25.

[0043] In 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 yttrium 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 and a cerium / zirconium / rare earth metal mixed oxide containing praseodymium oxide and lanthanum oxide as rare earth metal oxides.

[0045] 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.

[0046] Coating Z preferably contains no zeolites and no molecular sieve.

[0047] If coating Z contains aluminium oxide or doped aluminium oxide, the weight ratio of the sum of the masses of all aluminium oxides or doped aluminium 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.

[0048] According to the invention, coating F comprises a particulate metal compound and not a precious metal. It is therefore not catalytically active within the meaning of the present invention; that is, it is not capable of oxidizing the exhaust gas components CO and HC and reducing NOₓ.

[0049] Preferably, coating F consists of one or more particulate metal compounds and therefore contains no other components.

[0050] Suitable metal compounds are well known to those skilled in the art, as they are generally high-melting-point metal compounds commonly used as support materials for catalysts in automotive exhaust systems. Metal oxides, metal sulfates, metal phosphates, metal carbonates, or metal hydroxides, or mixtures thereof, are preferred. Metal oxides are particularly preferred.

[0051] Suitable metal oxides include primary, binary or ternary metal oxides or mixtures thereof.

[0052] The metals under consideration are selected in particular from the group consisting of alkali metals, alkaline earth metals, other earth metals, and transition metals. Preferably, the metals are selected from the group consisting of calcium, potassium, magnesium, strontium, barium, iron, zinc, aluminum, silicon, titanium, zirconium, lanthanum, praseodymium, bismuth, cobalt, nickel, copper, and cerium.

[0053] Particularly preferred metal compounds are cerium oxide, titanium dioxide, zirconium dioxide, silicon dioxide, aluminum oxide or mixtures or mixed oxides thereof.

[0054] In addition, so-called pyrogenic metal oxides can also be used. Generally, pyrogenic metal oxides 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, this process can be used to produce oxides of various metals with high surface areas. These oxides are advantageously produced from the group of metals consisting of silicon, aluminum, titanium, zirconium, cerium or from mixtures of these metals.

[0055] 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. According to the invention, "increasing gradient" means 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.

[0056] In the case of intended use of the wall flow filter, in which the exhaust gas flows in at its first end and out at the 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.

[0057] Simulations of the gas flow in a wall-flow filter have shown that the last third of the substrate is primarily responsible (more than 50%) for the overall filter's filtration performance. Increasing the coating F in the last third of the filter significantly raises the stagnation pressure there, due to the lower permeability, and shifts the flow direction more towards the first two-thirds of the filter. Therefore, the filter should have a steeper gradient of the coating F from the first to the second end to enhance its filtration efficiency.

[0058] The same applies mutatis mutandis to setting an advantageous exhaust back pressure. In a further embodiment of the invention, the gradient of the concentration of coating F therefore increases less sharply, or rather, the gradient of the concentration of coating F from the first to the second end decreases.

[0059] The coating F is preferably located in the porous walls of the wall-flow filter substrate, which means that the particle size of the metal compound must be adapted to the pore size of the wall-flow filter substrate. The particles of the metal compound therefore exhibit, in particular, a defined particle size distribution.

[0060] Since wall flow filter substrates typically contain pores of varying sizes, ideally, larger particles are present in the larger pores and smaller particles in the smaller pores. This means that the metal compound preferably exhibits a multimodal or broad q3 particle size distribution.

[0061] For the definition of the particle size distribution of the metal compound, a distinction is made, depending on the method used to determine the quantity of particles, between number-related (q0) and volume-related (q3) particle size distributions (M. Stieß, Mechanical Process Engineering - Particle Technology 1, Springer, 3rd edition 2009, page 29).

[0062] The size of the coarse particles (defined by the d90 value of the q3 particle size distribution, measured with a Beckmann Tornado dry dispersion modulator according to the latest version of ISO 13320-1 on the filing date) of the metal compound 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 metal compound (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 particle size distribution of the metal compound, which describes the fines content, should be 20% to 60% of the mean q3 particle size (d50) of the metal compound, preferably 25% to 50% and particularly preferably 25% to 40%.The d10 value of the number-related q0 particle size distribution should generally be greater than 0.05 µm, preferably greater than 0.08 µm and particularly preferably greater than 0.1 µm.

[0063] The particles of the metal compound have, in particular, a total surface area greater than 5 m² / l, preferably greater than 10 m² / l and most preferably greater than 15 m² / l, based on the external filter volume in liters.

[0064] The total surface area of ​​the particles SV is calculated using 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

[0065] (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

[0066] A person skilled in the art can easily determine the particle size distribution and the total surface area of ​​the metal compound in a finished wall flow filter according to the invention by washing the metal compound 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 mentioned above.

[0067] Particularly due to the manufacturing process, a portion of the coating F may also be located on the surface OE. Specifically, 1 to 90% of the total mass of the coating F may be located on the surface OE, preferably 2 to 70% and particularly preferably 3 to 50%.

[0068] The coating F does not preferably form a continuous, contiguous layer on the surface OE, but selectively clogs the large pores of the wall flow substrate, resulting in an island-like deposition pattern.

[0069] The coating F can be present wholly or partially as a closed layer on the surfaces OE. In this case, the layer thickness of the coating F is typically 1 to 75 µm, but preferably 5 to 65 µm.

[0070] In an embodiment of the invention, in which the coating Z is located on the surfaces OA, the layer thickness of coating F is less than or equal to the layer thickness of coating Z. The ratio of the layer thickness of coating F to the layer thickness of coating Z is preferably 0.1 to 1, more preferably 0.15 to 0.95, and more preferably 0.2 to 0.9. Furthermore, the mean particle diameter d50 of the oxides of coating F is less than or equal to the mean particle diameter d50 of coating Z. Preferably, the ratio of the d50 of the particles of coating F to the d50 of the particles of coating Z is 0.01 to 1, more preferably 0.05 to 0.9, and more preferably 0.15 to 0.8.

[0071] In an embodiment of the invention, in which the coating Z is located in the pores of the filter wall, the layer thickness of coating F is greater than or equal to the layer thickness of coating Z. Furthermore, the mean particle diameter d 50 of the oxides of coating F is greater than or equal to the mean particle diameter d 50 of coating Z. Preferably, the ratio of d 50 of the particles of coating F to d 50 of the particles of coating Z is 1 to 7, more preferably 1.05 to 6, and particularly preferably 1.1 to 5.

[0072] In a non-inventive embodiment, where coating Y is located on the surfaces OE, the layer thickness of coating F is less than or equal to the layer thickness of coating Y. The ratio of the layer thickness of coating F to the layer thickness of coating Y is preferably 0.1 to 1, more preferably 0.15 to 0.95, and more preferably 0.2 to 0.9. Furthermore, the mean particle diameter d50 of the oxides of coating F is less than or equal to the mean particle diameter d50 of coating Y. Preferably, the ratio of the d50 of the particles of coating F to the d50 of the particles of coating Y is 0.01 to 1, more preferably 0.05 to 0.9, and more preferably 0.15 to 0.8.

[0073] In the 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 mean particle diameter d 50 of the oxides of coating F is greater than or equal to the mean particle diameter d 50 of the coating Y. Preferably, the ratio of the d 50 of the particles of coating F to the d 50 of the particles of coating Y is 1 to 7, more preferably 1.05 to 6, and particularly preferably 1.1 to 5.

[0074] Based on the volume of the wall flow filter substrate, coating F is present, for example, in amounts of less than 50 g / l, and in particular less than 40 g / l. Preferably, coating F is present in amounts of 2.5 to 40 g / l based on the volume of the wall flow filter substrate.

[0075] 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.

[0076] According to the invention, the wall-flow filter substrate has a coating Y that differs from coatings Z and F and comprises platinum, palladium, or platinum and palladium, which does not contain rhodium or a cerium / zirconium mixed oxide, and which is located in the porous walls but not on the surfaces OA. Preferably, coating Y contains platinum and palladium with a platinum-to-palladium mass ratio of 25:1 to 1:25, more preferably 15:1 to 1:2.

[0077] In coating Y, platinum, palladium or platinum and palladium are usually fixed to one or more substrate materials.

[0078] Suitable carrier materials include all materials known to those 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 filing date). Particularly suitable carrier materials 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, barium oxide, and / or titanium oxide-doped aluminum oxides. Aluminum oxide or lanthanum-stabilized aluminum oxide is advantageously used, with the latter containing lanthanum in amounts of 1 to 10 wt.%, preferably 3 to 6 wt.%, each calculated as La₂O₃ and based on the weight of the stabilized aluminum oxide.In the case of aluminum oxide doped with barium oxide, the proportion of barium oxide is in particular 1 to 10 wt.%, preferably 3 to 6 wt.%, in each case calculated as BaO and based on the weight of the stabilized aluminum oxide.

[0079] Another suitable substrate material is lanthanum-stabilized aluminum oxide, the surface of which is coated with lanthanum oxide, barium oxide and / or strontium oxide.

[0080] Coating Y preferably comprises at least one aluminum oxide or doped aluminum oxide.

[0081] In a non-inventive 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.

[0082] 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.

[0083] The mass ratio of coating Y to coating Z is preferably 0.05 to 8.5.

[0084] For example, the substrate material of coating Y has a larger pore volume than the substrate material of coating Z. The ratio of the specific surface areas of the substrate oxides of coating Y to coating Z is preferably 0.5 to 2, more particularly 0.7 to 1.5.

[0085] For example, the ratio of the pore volume of the particulate metal compound of coating F to the pore volume of the carrier material of coating Z is preferably from 0.01 to 3, particularly from 0.05 to 2.5. The ratio of the specific surface area of ​​the particulate metal compound of coating F to the specific surface area of ​​the carrier oxides of coating Z is preferably from 0.1 to 4, particularly from 0.25 to 3.

[0086] The coatings Z, F, and Y can be arranged on the wall flow filter substrate in various ways. Figures 1 to 10 They will illustrate this with an example, whereby the Figures 1 to 4 non-inventive wall flow filters that only include coatings Z and F, whereas the wall flow filters according to the Figures 5 to 10 additionally include coating Y.

[0087] Figure 1This relates to a non-inventive wall flow filter 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 to 50% of the length L. The coating F is located in the channels E and extends over the entire length L.

[0088] Also Figure 2 This relates to a non-inventive wall flow filter 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, it extends over 80% of the length L. The coating F is located in the channels E and extends over the entire length L.

[0089] Figure 3This relates to a non-inventive wall flow filter 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.

[0090] Figure 4 This relates to a non-inventive wall flow filter in which the coating Z is located in the porous walls and extends from the second end of the wall flow filter substrate to 50% of the length L. The coating F is located in the channels E and extends over the entire length L.

[0091] Figure 5 relates to a wall flow filter according to the invention, which differs from that of the Figure 4This is distinguished by the fact that coating Z is located on surfaces OA over 50% of the length L, and coating Y is additionally 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.

[0092] Figure 6 relates to a wall flow filter according to the invention, which differs from that of the Figure 4 The difference lies in the fact that coating Z extends over 50% of the length L on the surfaces OA, and coating Y additionally extends into the porous walls, starting from the first end of the wall flow filter substrate and covering 50% of the length L. Coating F is located in the channels E and extends over the entire length L.

[0093] Figure 7This relates to a non-inventive wall flow filter 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. 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.

[0094] Figure 8This relates to a non-inventive wall flow filter in which the coating Z is located in the porous walls and extends over the entire length L. Additionally, 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. 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.

[0095] Figure 9This relates to a non-inventive wall flow filter in which the coating Z is located in the porous walls and extends from the second end of the wall flow filter substrate to 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 to 50% of the length L. Coating F is located in the channels E and extends from the second end of the wall flow filter substrate to 50% of the length L.

[0096] Figure 10This 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 to 80% of the length L. Furthermore, coating Y is located in the porous walls and extends over the entire length L. Coating F is located in the channels E and extends over the entire length L.

[0097] The wall flow filter according to the invention can be produced by applying the coatings Z, F and Y to a wall flow filter substrate.

[0098] The catalytic activity is provided according to the specifications of the person skilled in the art by coating the wall flow filter substrate with coating Z and with coating Y.

[0099] The term "coating" 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 achieved by applying a sufficiently low-viscosity aqueous suspension of the catalytically active components—also called a washcoat—to 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 necessary, calcined at an elevated temperature. The catalytically coated filter preferably has a loading of 20 g / l to 200 g / l, more preferably 30 g / l to 150 g / l (coating Z or the sum of coatings Z and Y). The most suitable loading of a wall-coated filter depends on its cell density, wall thickness, and porosity.

[0100] The coating F is applied to the wall flow filter substrate, in particular by applying a dry powder-gas aerosol to the channels E of the dry wall flow filter substrate, which is already coated with coating Z and coating Y. The powder contains a particulate metal compound and, in particular, consists of a particulate metal compound. By applying a dry powder-gas aerosol to a wall flow filter substrate that has been conventionally coated with coating Z and Y using a wet-process method, dried, and optionally calcined, a wall flow filter according to the invention is obtained with extremely good filtration efficiency, only a slight increase in exhaust gas backpressure, and simultaneously excellent catalytic effectiveness.

[0101] The wall flow filters according to the invention, which are catalytically coated and subsequently dusted with powder, differ from those that form 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, wall flow filters in which undefined ash deposits result from the combustion of fuel, e.g., in the cylinder during driving or by means of a burner, are not covered by the present invention.

[0102] Applying the dry powder-gas aerosol to the wall-flow filter substrates under consideration causes the powder particles to be deposited in the pores of the filter substrate and, if applicable, on the surfaces (OE), following the gas flow. The varying wall permeability of the filter substrate (e.g., due to inhomogeneities in the filter wall itself or different coating zones) leads to selective deposition of the powder in the pores of the wall or on the surfaces (OE) where the flow is strongest. This effect also means that, for example, 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.

[0103] According to the invention, the dry wall-flow filter substrate coated with coatings Z and Y is coated with a powder from its first end towards its second end (i.e., in the direction of exhaust gas flow with respect to its intended use) in such a way that the areas of the cell walls most exposed to the flow are covered with loose, internally porous powder accumulations in the porous walls and / or on the surfaces OE to achieve the desired increased filtration efficiency. Surprisingly, the formation of these internally porous powder accumulations results in a relatively low increase in back pressure. In a preferred embodiment, the wall-flow filter substrate is exposed to a powder-gas aerosol such that the powder is deposited in the pores of the porous wall and on the surfaces OE, forming a continuous layer.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 surfaces OE without forming a continuous layer on the surfaces OE.

[0104] To ensure that the powder in the powder-gas aerosol can be sufficiently deposited in the pores of the wall-flow filter substrate coated with coatings Z and Y, or 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 ensuring that the ratio of the 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) is 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 walls of the wall-flow filter substrate, following the gas flow.

[0105] A suitable powder has, in particular, a specific surface area of ​​at least 100 m² / g and a total pore volume of at least 0.3 ml / g.

[0106] For a suitable powder used to manufacture the wall flow filters according to the invention, an optimization of the powder's surface area, crosslinking, and adhesion 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 a wall flow filter coated with powder to achieve good filtration efficiency, 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 the metal compound, smaller particles offer significantly larger surface areas. At the same time, however, the pressure drop must only increase minimally. This necessitates a loose crosslinking of the powder.For a coating that increases filtration efficiency, powders with a tapped 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.

[0107] 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 below. For this purpose, a powder is generally mixed with a gas. (http: / / www.tsi.com / Aerosolgeneratoren-und-dispergierer / ; https: / / www.palas.de / de / product / aerosolgeneratorssolidparticles). This mixture of gas and powder produced in this way is then advantageously guided into the channels E of the wall flow filter substrate via a gas stream.

[0108] Any gas suitable for this purpose, as perceived by those 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 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.

[0109] To ensure the powder is deposited deep enough into the channels E and adheres well, a certain suction power is necessary. Those skilled in the art can determine this through preliminary tests for the specific wall flow filter and powder. It has been found that the aerosol (powder-gas mixture) is preferably drawn through the wall flow filter at a velocity 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.

[0110] 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.

[0111] This mixture of gas and powder, thus prepared, is then introduced into an existing gas flow, 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 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. According to the invention, the powder is dry when applied to the wall-flow filter substrate. 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 substrate. This preserves the particle size established during dispersion in the aerosol.

[0112] A preferred device for manufacturing a wall flow filter according to the invention is in Figure 11 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 that 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 by 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; are available.

[0113] In this preferred embodiment of the method according to the invention - as shown in the sketch Figure 11The 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.

[0114] In the present method for producing a wall flow filter according to the invention, a gas stream is charged with a powder-gas aerosol and drawn into a wall flow filter substrate. This ensures that the powder is sufficiently distributed 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.

[0115] To achieve an advantageous, uniform 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 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 results in 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 approximate 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.

[0116] 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 wall-flow filter substrate, 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 wall-flow filter substrate. 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 plugs of channels A, and all powder can penetrate into channels E.According to the invention, it is therefore preferred if the powder is swirled before flowing into the filter in such a way as to avoid powder deposits on the plug of the wall-flow filter substrate 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.

[0117] The preferred embodiments for the wall flow filter also apply mutatis mutandis to the method. In this regard, explicit reference is made to what was said above concerning the wall flow filter.

[0118] "Dry" in the context of the present invention means the exclusion of the use of any liquid, particularly 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 it does not negatively affect the achievement of the objective—the finest possible dispersion of the powder in the porous walls and / or surfaces (OE) of the wall-flow filter substrate. The powder is generally free-flowing and dispersible by the application of energy. The moisture content of the powder or the wall-flow filter substrate at the time of impregnation 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).

[0119] 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 coated with powder. Preferably, the wall flow filter according to the invention shows an improvement in soot particle separation (filter efficiency) in the filter of at least 5%, preferably at least 10%, and most preferably at least 20%, with a relative increase in the exhaust gas backpressure 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. The low 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 coating of the filter with powder according to the invention.It is assumed that the powder forms a porous structure, which has a positive effect on the back pressure. Therefore, a wall flow filter according to the invention should also exhibit better exhaust gas 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-technical coating is used.

[0120] Coating Z gives the wall flow filter according to the invention excellent three-way activity, while the optional coating Y is able to lower the soot ignition temperature and thus facilitates soot combustion.

[0121] The present invention 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 for treating exhaust gases from a stoichiometrically operated internal combustion engine, and in particular a gasoline-powered internal combustion engine, is preferred.

[0122] The wall flow filter according to the invention is particularly advantageous when used in combination with at least one three-way catalyst. It is especially advantageous if a three-way catalyst is located in a position 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. Likewise, it is advantageous if a three-way catalyst is located both upstream and downstream of the wall flow filter.

[0123] The preferred embodiments described for the wall flow filter according to the invention also apply mutatis mutandis to the use mentioned here.

[0124] The present disclosure 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. Preferably, this is a three-way catalyst, an oxidation catalyst, or a NOₓ storage catalyst. In another embodiment of this system, at least one further catalyst is arranged downstream of the filter according to the invention. Preferably, this is a three-way catalyst, an SCR catalyst, a NOₓ 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 NOx storage catalyst, and the downstream catalyst is a three-way catalyst, an SCR catalyst, a NOx 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.

[0125] The filter according to the invention is typically used primarily in internal combustion engines, especially those with direct injection or port injection. Preferably, these are stoichiometrically operated gasoline or natural gas engines. Turbocharged engines are also preferred.

[0126] 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 as the size decreases. (Hinds, W.: Aerosol technology: Properties and behavior and measurement of airborne particles. Wiley, 2nd edition 1999).

[0127] The Figures 1 to 10 The different coating arrangements of wall flow filters according to the invention and those not according to the invention, which have already been described in more detail above, are shown. Therein, the following are designated: (E) the inlet channel / flow channel of the wall flow filter (A) the outlet channel / flow 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

[0128] Figure 11Figure 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.

[0129] Figure 12 This is a light micrograph of a wall flow filter coated with powder. The photograph shows a top view of several channel walls in an area of ​​the wall flow filter where there is no catalytically active on-wall layer. The powder selectively deposits in and fills the pores of the wall.

[0130] Figure 13shows a comparison of the back pressure of the catalysts GPF1 and VGPF1 after soot loading.

[0131] Figure 14 This relates to a soot burn-off test performed with the VGPF3, VGPF4 and VGPF5 catalysts and shows the time required for the back pressure of the soot-laden filter to drop to 25% of the original back pressure without soot (defined as p25).

[0132] The advantages of the invention are explained below by means of examples. Comparison example 1: Coating Z only:

[0133] 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 mixed with a palladium nitrate solution and a rhodium nitrate solution under continuous stirring. The resulting coating suspension was used directly to coat a commercially available wall-flow filter substrate, with the coating being applied to the surface OA over 80% of the substrate length. The total loading of this filter was 75 g / l, the total precious metal loading 1.27 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.

[0134] Non-inventive Example 1: Coating Z in combination with coating F: Aluminum oxide stabilized with lanthanum 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 suspension thus obtained was then mixed 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 the coating being applied to the surfaces OA over 80% of the substrate length.The total loading of this filter was 75 g / l, the total precious metal loading 1.27 g / l with a palladium to rhodium ratio of 5:1. The resulting coated filter was dried and then calcined. Subsequently, the filter was exposed to a dry powder-gas aerosol, introducing 7 g / l of a metal oxide into channels E. It is referred to below as GPF1.

[0135] The two filters obtained in this way, VGPF1 and GPF1, were measured on a cold blow test rig to determine the pressure drop across each filter. At room temperature and a volume flow rate of 900 m³ / h of air, the back pressure of VGPF1 was 111 mbar and that of GPF1 was 122 mbar. As already described, the filtration coating F leads only to a moderate increase in back pressure.

[0136] Furthermore, the back pressure of both filters was examined 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. Figure 13 shows that the GPF1 according to the invention, despite the higher back pressure in the clean state, has a lower back pressure after soot loading than the comparison filter VGPF1.

[0137] In parallel, fresh filters VGPF1 and GPF1 were tested in the vehicle with regard to their particle filtration efficiency. For this purpose, the filters were measured in an RTS-95 driving cycle, also known as RTC-aggressive, in a position close to the engine between two particle counters. In both cases, a three-way catalytic converter was located upstream in the exhaust system, via which the vehicle's lambda control was implemented. Here, the filter GPF1 according to the invention exhibits a filtration efficiency of 84%, calculated from the particle values ​​of the two particle counters, while the comparison filter VGPF1 achieves a filtration efficiency of only 55.5%. In summary, it is evident that the combination of the filtration coating F and the three-way coating Z is particularly advantageous with regard to back pressure after soot loading and filtration efficiency. Comparison example 2: Coating Z only

[0138] 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 mixed with a palladium nitrate solution and a rhodium nitrate solution under continuous stirring. The resulting coating suspension was used directly to coat a commercially available wall-flow filter substrate, with the coating 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 1.24 g / l with a palladium to rhodium ratio of 6:1. The resulting coated filter was dried and then calcined. It is referred to below as VGPF2.

[0139] Non-inventive example 2: Coating Z in combination with coating F: Aluminum oxide stabilized with lanthanum 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 suspension thus obtained was then mixed 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, the coating 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 1.24 g / l with a palladium to rhodium ratio of 6:1. The resulting coated filter was dried and then calcined. Subsequently, the filter was exposed to a dry powder-gas aerosol, introducing 10 g / l of a metal oxide into channels E. It is referred to below as GPF2.

[0140] The two filters, VGPF2 and GPF2, were tested in the vehicle with regard to their particle filtration efficiency. For this purpose, the filters were measured in a WLTP driving cycle in a position close to the engine, between two particle counters. In both cases, a three-way catalytic converter was located upstream in the exhaust system, via which the vehicle's lambda control was implemented. The filter GPF2 according to the invention exhibits a filtration efficiency of 85%, calculated from the particle values ​​of the two particle counters, while the comparison filter VGPF2 achieves a filtration efficiency of only 65%.

[0141] Furthermore, the VGPF2 and GPF2 filters were aged in an engine test bench. This 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 76 hours (see Motortechnische Zeitschrift, 1994, 55, 214-218). Subsequently, the catalytically active particulate filters in their aged state were tested on an engine test bench in the so-called "light-off test" and the "lambda sweep test" to compare their catalytic activity. In the light-off test, the start-up behavior is determined with a stoichiometric exhaust gas composition and a constant mean air-fuel ratio λ (λ = 0.999 with ±3.4% amplitude).

[0142] The following Table 1 contains the temperatures T 50 at which 50% of the components under consideration are converted. Table 1 T 50 HC stöch T 50 CO stoch T 50 NOx stöch VGPF2 366 373 372 GPF2 366 373 373

[0143] The dynamic conversion behavior of the particulate filters in the lambda sweep test was determined in a range of λ = 0.99 - 1.01 at a constant temperature of 510°C. The amplitude of λ was ±6.8%. Table 2 shows the conversion at the intersection of the CO and NOx conversion curves, as well as the corresponding HC conversion of the aged particulate filters. Table 2 CO / NOx turnover at the intersection HC turnover at the λ of the CO / NOx intersection point VGPF2 93 92 GPF2 92 93

[0144] These tests show that the filtration coating F is particularly suitable for significantly increasing the filtration efficiency of the GPF2 filter according to the invention, without affecting the catalytic activity of the filter. Comparison example 3: Coating Z only

[0145] 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 mixed with a palladium nitrate solution and a rhodium nitrate solution under continuous stirring. The resulting coating suspension was used directly to coat a commercially available wall-flow filter substrate, with the coating being applied to the surface OA over 60% of the substrate length. The total loading of this filter was 75 g / l, the total precious metal loading 0.88 g / l with a palladium to rhodium ratio of 4:1.The resulting coated filter was dried and then calcined. It is referred to below as VGPF3. Comparison example 4: Coating Z only

[0146] 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 mixed with a palladium nitrate solution, a platinum nitrate solution, and a rhodium nitrate solution under continuous stirring. The resulting coating suspension was used directly to coat a commercially available wall-flow filter substrate, with the coating being applied to the surfaces OA over 60% of the substrate length.The total plating load of this filter was 75 g / l, the total precious metal plating load 0.88 g / l with a platinum-to-palladium-to-rhodium ratio of 92:108:50. The resulting coated filter was dried and then calcined. It is referred to below as VGPF4. Comparison example 5: Coating Y in combination with coating Z:

[0147] Stabilized aluminum oxide was suspended in water. The resulting suspension was then mixed with a palladium nitrate solution and a platinum nitrate solution under continuous stirring. The resulting coating suspension was used directly to coat a commercially available wall-flow filter substrate, with the coating being incorporated into the porous filter wall over 100% of the substrate length. The total loading of this filter was 10 g / l, the total precious metal loading 0.35 g / l with a palladium to rhodium ratio of 12:1. The coated filter was dried and then calcined. Subsequently, 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 mixed 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 the coating being applied to the surface OA over 60% of the substrate length. The total loading of this filter was 85 g / l, the total precious metal loading 0.88 g / l with a platinum-to-palladium-to-rhodium ratio of 92:108:50. The coated filter was dried and then calcined. It is referred to below as VGPF5.

[0148] The filters VGPF3, VGPF4, and VGPF5, manufactured in this manner, were first loaded with a defined amount of soot and then examined for their soot oxidation properties in a soot burn-off test on an engine test bench with a lean exhaust gas composition and a constant temperature of 450°C upstream of the catalyst inlet. It was found that the combination of coating Y and coating Z is best suited for VGPF5 to completely regenerate the particulate filter. Considering the time required for the back pressure of the soot-loaded filter to drop to 25% of the original back pressure without soot (defined as p25), the following results are available. Figure 14 It is evident that the combination of coating Y and coating Z reaches the p25 value after approximately 1700 seconds for VGPF5, while VGPF3 and VGPF4 only reach the p25 value after a time of 4100 and 2250 seconds respectively. Comparison example 6: Coating Y in combination with coating Z:

[0149] Stabilized aluminum oxide was suspended in water. The resulting suspension was then mixed with a palladium nitrate solution and a platinum nitrate solution under continuous stirring. The resulting coating suspension was used directly to coat a commercially available wall-flow filter substrate, with the coating being incorporated into the porous filter wall over 100% of the substrate length. The total loading of this filter was 10 g / l, the total precious metal loading 0.28 g / l with a palladium to rhodium ratio of 12:1. The coated filter was dried and then calcined. Subsequently, lanthanum oxide-stabilized aluminum oxide was suspended in water together with an oxygen storage component comprising 24 wt% cerium oxide, zirconium oxide, lanthanum oxide, and yttrium oxide. The weight ratio of aluminum oxide to oxygen storage component was 55:45.The suspension thus obtained was then mixed 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 the coating being applied to the surfaces OA over 80% of the substrate length. After the second step, the total loading of this filter was 60 g / l, the total precious metal loading 0.48 g / l with a platinum-to-palladium-to-rhodium ratio of 738:262:359. The resulting coated filter was dried and then calcined. It is referred to below as VGPF6. Example 3 according to the invention: Coating Y in combination with coating Z and coating F:

[0150] Stabilized aluminum oxide was suspended in water. The resulting suspension was then mixed with a palladium nitrate solution and a platinum nitrate solution under continuous stirring. The resulting coating suspension was used directly to coat a commercially available wall-flow filter substrate, with the coating being incorporated into the porous filter wall over 100% of the substrate length. The total loading of this filter was 10 g / l, the total precious metal loading 0.28 g / l with a palladium to rhodium ratio of 12:1. The coated filter was dried and then calcined. Subsequently, lanthanum oxide-stabilized aluminum oxide was suspended in water together with an oxygen storage component comprising 24 wt% cerium oxide, zirconium oxide, lanthanum oxide, and yttrium oxide. The weight ratio of aluminum oxide to oxygen storage component was 55:45.The suspension thus obtained was then mixed with a palladium nitrate solution and a rhodium nitrate solution under continuous stirring. The resulting coating suspension was used directly to coat a commercially available wall-flow filter substrate, with the coating being applied to surfaces OA over 80% of the substrate length. After the second step, the total loading of this filter was 60 g / l, the total precious metal loading 0.48 g / l with a platinum-to-palladium-to-rhodium ratio of 738:262:359. The coated filter was dried and then calcined. Subsequently, the filter was treated with a dry powder-gas aerosol, introducing 4 g / l of a metal oxide into channels E. It is referred to below as GPF3.

[0151] The two filters, VGPF6 and GPF3, were tested in a vehicle with regard to their particle filtration efficiency. For this purpose, the filters were measured in a WLTP driving cycle in a position close to the engine, between two particle counters. In both cases, a three-way catalytic converter was located upstream in the exhaust system, via which the vehicle's lambda control was implemented. The GPF3 filter according to the invention exhibits a filtration efficiency of 96%, calculated from the particle values ​​of the two particle counters, while the comparison filter, VGPF6, achieves a filtration efficiency of only 71%.

[0152] Additionally, the two filters obtained in this way were measured in their fresh state on a cold blow test rig to determine the pressure drop across each filter. At room temperature and a volume flow rate of 300 m³ / h of air, the back pressure of the VGPF6 was 69 mbar and that of the GPF3 was 122 mbar. As already described, the filtration coating F leads only to a moderate increase in back pressure.

[0153] Furthermore, the two filters were examined with regard to back pressure after soot loading. For this purpose, both filters were sooted on an engine test bench with a direct-injection turbocharged engine. At a soot load of 3 g, the GPF3 according to the invention, despite the higher back pressure in the clean state, exhibits a lower back pressure than the comparison filter VGPF6 (Table 3). Table 3 Back pressure in a clean state Dynamic pressure with 3 g of soot VGPF6 69 244 GPF3 122 205

[0154] In summary, it is evident that the additional regeneration coating Y can be optionally combined with the filtration coating F and three-way coating Z, whereby the advantages in back pressure behavior after soot loading and in filtration efficiency compared to a filter without coating F remain.

Claims

1. Wall-flow filter for removing particles from the exhaust gas of combustion engines, comprising a wall-flow filter substrate of length L and coatings Z and F that differ from one another, 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 form surfaces OE and OA respectively, 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 particulate metal compound and no precious metal, and 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 does not contain rhodium or cerium / zirconium mixed oxide, and which is located in the porous walls, but not on the surfaces OA.

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.

5. Wall-flow filter according to one or more of claims 1 to 4, characterized in that coating Z does not contain platinum.

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

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

8. Wall-flow filter according to one or more of claims 1 to 7, 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.

9. Wall-flow filter according to one or more of claims 1 to 7, 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.

10. Wall-flow filter according to one or more of claims 1 to 9, characterized in that coating F consists of one or more particulate metal compounds.

11. Wall-flow filter according to claim 10, characterized in that the particulate metal compound of the coating F is cerium oxide, titanium dioxide, zirconium dioxide, silicon dioxide, aluminum oxide, or mixtures or mixed oxides thereof.

12. Wall-flow filter according to one or more of claims 1 to 11, 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.

13. Wall-flow filter according to claim 12, characterized in that coating Y extends over a length of 50 to 100% of the length L.

14. Method for producing a wall-flow filter according to one or more of claims 1 to 13, characterized in that the channels E of the dry wall-flow filter substrate already coated with coating Z and coating Y are impinged on by a dry powder / gas aerosol, wherein the powder contains a particulate metal compound.

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