PARTICLE FILTER FOR PETROL ENGINE EXHAUST

Description

[0001] The present invention is directed to a wall-flow filter, which is used particularly in exhaust systems of gasoline-powered vehicles. The filter has improved three-way activity and filters out the fine particles produced during gasoline combustion from the exhaust stream. A method for producing a corresponding filter and its preferred use are also claimed.

[0002] The exhaust gases from, for example, internal combustion engines in motor vehicles typically contain the harmful gases carbon monoxide (CO) and hydrocarbons (HC), nitrogen oxides (NO x ) and possibly sulfur oxides (SO x ), as well as particulates consisting predominantly of soot residues and any adhering organic agglomerates. These are referred to as primary emissions. CO, HC and particulates are products of the incomplete combustion of the fuel in the engine's combustion chamber. Nitrogen oxides are formed in the cylinder from nitrogen and oxygen in the intake air when combustion temperatures locally exceed 1400°C. Sulfur oxides result from the combustion of organic sulfur compounds, which are always present in small quantities in non-synthetic fuels.To remove these environmentally and health-damaging emissions from motor vehicle exhaust gases, a variety of catalytic exhaust gas purification technologies have been developed. Their basic principle is typically based on passing the exhaust gas to be purified over a catalyst consisting of a flow-through or wall-flow honeycomb structure (wall-flow filter) and a catalytically active coating applied to and / or within it. The catalyst promotes the chemical conversion of various exhaust gas components to form harmless products such as carbon dioxide and water. In the case of a wall-flow filter, it simultaneously removes the soot particles.

[0003] Particles can be removed from exhaust gases very effectively using particulate filters. Wall-flow filters made of ceramic materials have proven particularly effective. These have two end faces and are constructed from a multitude of parallel channels of a specific length, formed by porous walls that extend from one end face to the other. The channels are alternately closed at one of the two ends of the filter, forming first channels that are open on the first side of the filter and closed on the second side, and second channels that are closed on the first side of the filter and open on the second side. Depending on the arrangement of the filter in the exhaust gas flow, one of the end faces forms the inlet face and the second face the outlet face for the exhaust gas.The flow channels open on the inlet side form the inlet channels, and the flow channels open on the outlet side form the outlet channels. For example, exhaust gas flowing into the first channels can only leave the filter via the second channels and must therefore flow through the walls between the first and second channels. For this reason, the material from which the wall-flow filters are constructed has an open-pore porosity. As the exhaust gas passes through the wall, the particles are trapped.

[0004] Wall-flow filters can be catalytically active. The catalytic activity is achieved by coating the filter with a coating suspension containing the catalytically active material. The contact of the catalytically active materials with the wall-flow filter is referred to in the technical world as "coating." The coating performs the actual catalytic function and often contains storage materials and / or catalytically active metals, which are usually deposited in highly dispersed form on temperature-stable, high-surface-area metal compounds, especially oxides. The coating is usually achieved by applying an aqueous suspension of the storage materials and catalytically active components—also called a washcoat—on or into the wall of the wall-flow filter. After applying the suspension, the support is usually dried and, if necessary, calcined at elevated temperature.The coating can consist of a single layer or be constructed from multiple layers, which are applied one above the other (multilayered) and / or offset from one another (as zones) to a corresponding filter. The catalytically active material can be applied to the porous walls between the channels (so-called overlay coating). However, this coating can lead to a significant increase in the filter's backpressure. Against this background, JPH01-151706 and WO2005016497A1, for example, propose coating a wall-flow filter with a catalyst in such a way that the catalyst penetrates the porous walls (so-called in-wall coating). A coating zone is understood to mean the presence of a catalytically active material (coating) on ​​or in the wall of the filter over less than the entire length of the wall-flow filter.When we talk about the length of the wall flow filter, we mean the entire length, including the plugs that close off the channels.

[0005] Three-way catalysts are used to reduce emissions in stoichiometrically combustion engines. Three-way catalysts (TWCs) are well known to those skilled in the art and have been legally required since the 1980s. The actual catalyst mass usually consists of high-surface-area metal compounds, particularly oxide support materials, on which the catalytically active components are deposited in extremely fine distribution. The precious metals of the platinum group, platinum, palladium, and / or rhodium, are particularly suitable as catalytically active components for purifying stoichiometrically composed exhaust gases. Suitable support materials include aluminum oxide, silicon dioxide, titanium oxide, zirconium oxide, cerium oxide, and their mixed oxides and zeolites.So-called active aluminum oxides with a specific surface area (BET surface area measured according to DIN 66132 - latest version on the date of application) of more than 10 m² / g are preferred. To improve dynamic conversion, three-way catalysts also contain oxygen-storing components. These include cerium / zirconium mixed oxides, which may be provided with lanthanum oxide, praseodymium oxide, and / or yttrium oxide. Zoned and multilayer systems with three-way activity are also known (US8557204; US8394348). If such a three-way catalyst is located on or in a particulate filter, it is referred to as a cGPF (catalyzed gasoline particle filter; e.g., E P2650042B1).

[0006] The quality of a catalytically coated exhaust filter is measured by the criteria of filtration efficiency, catalytic performance, and pressure drop. To meet these diverse requirements, filters are equipped with, among other things, catalytically active zones. These zones can be located on the cell walls or in the porous wall of the filter matrix.

[0007] A group of coating techniques for wall-flow filters is described in WO06021338A1. Here, the wall-flow filter is made of an open-pore material, has a cylindrical shape with a length L, and is traversed from an inlet face to an outlet face by a plurality of alternately closed flow channels. The coating suspension can be applied by vertically aligning the flow channels of the wall-flow filter such that one face is at the bottom and the second face is at the top, introducing the coating composition into the filter body through the flow channels of the wall-flow filter, which are open in the lower face, up to a desired height above the lower face by applying a pressure difference, and removing excess coating composition downwards by applying a suction pulse.The specific modifications of the process in WO06042699A1 and WO11098450A1 are based on the same coating principle. A coating apparatus using this process principle is described in WO13070519A1. Here, too, an excess of coating suspension is used and the principle of pressure difference reversal is applied.

[0008] This coating principle is also suitable for the production of particulate filters that have zones with catalytically active material on the inlet and outlet sides. For example, WO09103699A1 describes a method for coating filters with two different washcoats. The method steps consist of orienting the filter substrate vertically, pumping in a first coating suspension from below (pressure difference with the highest pressure at the lower end), removing the excess coating suspension by suction (pressure difference reversal), and rotating the filter body 180°, refilling it from below with the second washcoat, and removing the excess by suction. The filter is dried and calcined after the coating process. The same coating principle is disclosed in US7094728B2.Coated wall-flow filters manufactured in this way often have a gradient in the coating, as shown schematically and exaggeratedly in . Fig. 1 is shown.

[0009] The second group of coating processes, in which the filter bodies are coated without excess washcoat and without pressure difference reversal, includes, for example, the processes described in WO06021339A1, WO15145122A2, and WO0110573A2. The vertically oriented filter carrier can be coated with the washcoat from the lower or upper end face.

[0010] WO06021339A1 discloses a method for coating a wall-flow particle filter with a coating composition, wherein the particle filter is made of an open-pore material, has a cylindrical shape with the length L and has a plurality of flow channels from an inlet end face to an outlet end face, which are mutually closed.The process is characterized in that the flow channels of the wall-flow filter are aligned vertically so that one end face is at the bottom and the second end face is at the top. The filter is filled by immersing the lower end face of the wall-flow filter in a defined, pre-filled amount of the coating composition, applying negative pressure to the openings of the outlet channels in the upper end face, and sucking the entire amount of coating composition into the inlet and outlet channels through the openings of the inlet channels in the lower end face. The pre-filled amount of coating composition is selected according to the desired coating concentration and coating height. There is no reversal of the pressure difference after the pressure difference for coating has been applied. The coating suspension is measured and not used in excess.

[0011] WO0110573A2 also describes a method for coating particle filters, in which a measured amount of washcoat is applied to the filter carrier from below. By applying a pressure difference (vacuum on the upwardly facing end face), the applied amount of coating suspension is sucked into the channels of the substrate. The substrate is then rotated, and the washcoat is distributed into the channels by the action of a jet of pressurized air on the upper end of the substrate. In this method, there is no reversal of the pressure difference, since the second pressure pulse also points in the same direction as the first with regard to the movement of the washcoat, and thus no reversal of the pressure difference occurs.

[0012] WO15145122A2 is another example of this group of coating processes. In contrast to the processes described above, however, here a predefined amount of coating suspension is applied to the upper face of the vertically aligned filter in a measured amount and distributed into the channels of the particulate filter by applying a pressure difference (suction by applying a vacuum to the lower face). After this coating step, no further pressure difference reversal occurs.

[0013] The documents US2020182115A1 and DE102015212788A1 disclose particulate filters for gasoline engines that have two superimposed catalytic coatings on the upstream side and another catalytic coating on the downstream side.

[0014] The object of the present invention is to provide a wall-flow filter that is improved in terms of its catalytic activity and not significantly inferior to prior art gasoline particulate filters (GPF, OPF) in terms of filtration efficiency and exhaust backpressure. The envisioned filter should, particularly with high catalytic activity, exhibit a correspondingly high filtration efficiency without excessively deteriorating exhaust backpressure.

[0015] These and other objects arising from the prior art are achieved by a wall-flow filter having the features of claim 1. Claims 6 and 9 are directed to a method for producing and a use of the wall-flow filter according to the invention, respectively. The claims dependent on these claims are directed to preferred embodiments of the respective independent claims.

[0016] By specifying a wall-flow filter for reducing particulate emissions in the exhaust gas of gasoline engines having a length L, the wall-flow filter comprising channels E and A which extend in parallel between a first and a second end of the wall-flow filter and which are separated by porous walls having 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 this has two catalytically active coatings applied in separate coating steps on both the surfaces OE and OA, wherein a first coating extends from the first end of the wall flow filter over 55 to 96% of the length L and a second coating extends from the first end of the wall flow filter over 10 to 40% of the length L and wherein a third coating extends from the second end of the wall flow filter over 55 to 96% of the length L and a fourth coating extends from the second end of the wall flow filter over 10 to 40% of the length L, the stated problem is surprisingly solved. The wall flow filters (. Fig. 2a-d ) show better catalytic activity ( Fig. 3 ), the latter being coated with an excess of coating suspension from both sides and the excess of coating suspension being removed by a pressure difference reversal ( Fig. 1 ). It is also surprising that the filter according to the invention produces only a slightly increased exhaust back pressure ( Fig. 4 ) and regarding filtration efficiency, the state-of-the-art filters ( Fig. 1 ) is in no way inferior ( Fig. 5 ). The first to fourth coatings are addressed below with the numbers 1 - 4.

[0017] According to the invention, the individual zones of the catalytic coating of the wall-flow filter are positioned on the wall of the inlet surface OE and the outlet surface OA, from the corresponding open ends of the channels to the other end. The term "on the wall" implies that one of the catalytically active coatings penetrates into the porous wall surface of the inlet or outlet channels only to a small extent, namely a maximum of 33%, more preferably less than 15%, and very preferably less than 10%, based on the total amount of this catalytically active coating. Corresponding analyses can be determined using CT images or SEM images of sections through the filter wall and image evaluation methods (Blazek et al., Chem. Eng. J. 409 (2021) 128057; Greiner et al., Chem. Eng. J. 378 (2019) 121919).

[0018] The design of the individual zones can be varied by a person skilled in the art within the limits specified above. In doing so, they will be guided by the criteria for the quality of a wall-flow filter mentioned at the outset. It is important that the longer zones (1st and 3rd coating) of the catalytic coating overlap at least over a small proportion of the length L, while the short zones (2nd and 4th zones) do not. In a preferred embodiment, therefore, a first coating extends from the first end of the wall-flow filter over 55 to 80% of the length L and a second coating extends from the first end of the wall-flow filter over 20 to 40% of the length L, with a third coating extending from the second end of the wall-flow filter over 55 to 80% of the length L and a fourth coating extending from the second end of the wall-flow filter over 20 to 40% of the length L.More preferably, a first coating extends from the first end of the wall-flow filter over 55 to 70% of the length L, and a second coating extends from the first end of the wall-flow filter over 25 to 35% of the length L, and a third coating extends from the second end of the wall-flow filter over 55 to 70% of the length L, and a fourth coating extends from the second end of the wall-flow filter over 25 to 35% of the length L. The filters exemplified in this document had zones of 60% of the length L for the long zones and 30% of the length L for the short zones.

[0019] It has proven advantageous if both the long and the short zones have a corresponding catalytic activity. The catalytic activity of a coating is also determined by the amount of coating. The amount of coating in g / L on the filter can be varied by the person skilled in the art. It has proven particularly advantageous if the ratio of the amount of catalytically active coating in g / L between the first coating and the second coating or between the third and the fourth coating is equal to or less than 1:1 and equal to or more than 1:3. This means that the short zones are preferably applied to the filter in a higher concentration than the long zones. The ratio of the coating concentrations is very particularly preferably 1:2 to 1:3 (measured in g / L).However, this statement applies in particular if the coatings on surfaces OE (input) and OA (output) correspond in layout and structure. In one embodiment of the invention, it is very particularly preferred if the coatings on surfaces OE and OA are each different. In another embodiment of the invention, it is very particularly preferred if the coatings on surfaces OE and OA are each identical. All four coatings according to the invention can also be identical. It is also possible for the same coatings on surface OE to be different from the same coatings on surface OA.

[0020] By "equal" we mean that the chemical composition as well as the quantity and extent of the coatings are the same. However, as just described, it is also possible for the four coating zones discussed to be designed differently in terms of chemical composition, quantity and / or length depending on the requirements profile. It should be mentioned that further coatings can also be present in and on the filter according to the invention. One possibility, for example, is the additional coating of the inner wall of the channels with a corresponding catalytically active mass. This would help to further increase the catalytic activity of the wall-flow filter described here. However, combinations with a possibly non-catalytically active powder coating in and / or on the inlet side of the wall-flow filter are also conceivable. This would help to further increase the filtration efficiency without the exhaust backpressure increasing immensely.

[0021] The order of application of the individual coating zones can also be varied. It has been found that—particularly when the same coatings are applied on the inlet and outlet sides of the filter—an embodiment of the present invention is preferred in which the short zones are applied last and the long zones first. Accordingly, it is particularly advantageous if the first coating and the third coating are applied to the wall-flow filter first, before the second and fourth coatings are applied ( Fig. 2c und 2d ).

[0022] The coatings are three-way catalytically active, particularly at operating temperatures of 250 to 1100 °C. They typically contain one or more precious metals fixed on one or more support materials, as well as one or more oxygen storage components. The coatings preferably comprise identical oxygen storage components and identical support materials for precious metals in different, but preferably equal, amounts. The coatings also contain identical or different precious metals in equal or different amounts. Particularly preferably, coatings one and two, as well as coatings three and four, are chemically identical. For the production of wall-flow filters, it has proven advantageous if at least coatings 1 and 3 are chemically identical and coatings 2 and 4 are chemically identical but different from coatings 1 and 3.The difference can preferably lie in the type of precious metals used and / or in the type of oxygen storage materials used. For example, coatings 1 and 3 can preferably comprise platinum, palladium, or mixtures thereof, and coatings 2 and 4 can preferably comprise platinum, palladium, rhodium, or mixtures thereof. Furthermore, coatings 2 and 4 can comprise one oxygen storage material, while coatings 1 and 3 can comprise two oxygen storage materials. Such architectures have proven particularly robust in aging tests.

[0023] Suitable precious metals for the above coatings are, in particular, platinum, palladium, and rhodium, with palladium, rhodium, or platinum, palladium, and rhodium being preferred, and of these, palladium and rhodium being particularly preferred. Based on the particle filter according to the invention, the proportion of rhodium in the total precious metal content is, in particular, greater than or equal to 5% by weight but equal to or less than 50% by weight, preferably less than 30% by weight. The porous walls of the particle filter according to the invention are preferably substantially free of precious metals. The precious metals are typically used in amounts of 0.15 to 5 g / L, more preferably 0.3 to 4 g / L, based on the volume of the wall-flow filter.

[0024] Suitable support materials for the precious metals 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). Particularly suitable support materials for the precious metals are selected from the group 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, silicon oxide, zirconium oxide, and / or titanium oxide. Advantageously, aluminum oxide stabilized with lanthanum is used, with lanthanum being used advantageously 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.Another suitable support material is lanthanum-stabilized aluminum oxide, the surface of which is coated with lanthanum oxide, barium oxide, or strontium oxide. In a particularly preferred embodiment, at least one of the coatings 1-4 comprises stabilized aluminum oxide in an amount of 20 to 70 wt.%, based on the total weight of the coating, rhodium, palladium, or palladium and rhodium, and one or more oxygen storage components in an amount of 30 to 80 wt.%, based on the total weight of the coating. This most preferably applies to all coatings 1-4.

[0025] Cerium / zirconium / rare earth metal mixed oxides are particularly suitable as oxygen storage components. 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. Depending on the manufacturing process, however, not completely homogeneous products may also be produced, which can generally be used without disadvantage. 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.

[0026] 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 combinations of 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 particularly preferably free of neodymium oxide.

[0027] In preferred embodiments of the present invention, one or all coatings contain an alkaline earth compound such as strontium oxide, barium oxide, or barium sulfate. The amount of barium sulfate per coating is in particular 2 to 20 g / L, preferably 3-10 g / L of the wall-flow filter volume. In particular, coating 1 and / or 3 contains strontium oxide or barium oxide.

[0028] In further advantageous embodiments of the present invention, one or all of the coatings contain additives such as rare earth compounds, such as lanthanum oxide, and / or binders, such as aluminum compounds. These additives are used in amounts that can vary within wide limits and can be determined by a person skilled in the art using simple means. These additives may help improve the rheology of the coating.

[0029] In embodiments of the present invention, at least one, preferably each of the coatings 1-4 comprises lanthanum-stabilized aluminum oxide, palladium, rhodium, or palladium and rhodium, and an oxygen storage component comprising zirconium oxide, cerium oxide, lanthanum oxide, and yttrium oxide and / or praseodymium oxide. If present in the coating, the yttrium oxide content is in particular 2 to 15 wt. %, preferably 3 to 10 wt. %, based on the weight of the oxygen storage component. The weight ratio of lanthanum oxide to yttrium oxide is in particular 0.1 to 1, preferably 0.3 to 1. If present, the praseodymium oxide content is in particular 2 to 15 wt. %, preferably 3 to 10 wt. %, based on the weight of the oxygen storage component. The weight ratio of lanthanum oxide to praseodymium oxide is in particular 0.1 to 1, preferably 0.3 to 1.In particular, one of the coatings 1-4 may comprise an additional oxygen storage component containing zirconium oxide, cerium oxide, lanthanum oxide, as well as yttrium oxide and / or praseodymium oxide. This preferably applies to all coatings 1-4.

[0030] At least one of the coatings 1-4 preferably comprises, in embodiments, lanthanum-stabilized aluminum oxide in amounts of 35 to 60 wt. %, particularly preferably 40 to 60 wt. %, and an oxygen storage component in amounts of 40 to 50 wt. %, particularly preferably 45 to 50 wt. %, in each case based on the total weight of the respective coating. At least one of the coatings 1-4 preferably comprises, in embodiments, lanthanum-stabilized aluminum oxide in amounts of 30 to 50 wt. %, particularly preferably 40 to 50 wt. %, and an oxygen storage component in amounts of 50 to 80 wt. %, particularly preferably 55 to 80 wt. %, in each case based on the total weight of the respective coating. In advantageous embodiments of the present invention, in at least one of the coatings 1-4, the weight ratio of aluminum oxide to the oxygen storage component is at least 0.7 to a maximum of 1.5, preferably 0.8 to 1.2.In advantageous embodiments of the present invention, the weight ratio of aluminum oxide to the oxygen storage component in at least one of the coatings 1-4 is at least 0.3 to a maximum of 0.8, preferably 0.4-0.6. The embodiments mentioned here preferably apply to all coatings 1-4.

[0031] The present invention also relates to a method for producing a wall flow filter according to the invention, which comprises the following steps: i) a first coating suspension is introduced in excess via the first end by applying a pressure difference across the vertically locked wall-flow filter, and an excess of the first coating suspension is removed from the wall-flow filter by reversing the pressure difference; ii) a third coating suspension is introduced in excess via the second end by applying a pressure difference across the vertically locked wall-flow filter, and an excess of the third coating suspension is removed from the wall-flow filter by reversing the pressure difference; iii) a second coating suspension is introduced via the first end by applying a pressure difference across the vertically locked wall-flow filter; iv) a fourth coating suspension is introduced via the second end by applying a pressure difference across the vertically locked wall-flow filter.

[0032] The sequence of steps i) - iv) is not crucial to a first approximation. What is important is that a pressure difference reversal takes place in steps i) and ii), during which excess coating suspension is removed from the wall-flow filter. Between steps i) and ii) as well as between steps iii) and iv), the filter is rotated by 180° in its locking device. It is preferable to omit rotation if the coatings according to i) and iii) are first applied from the first side of the filter and then the coatings according to ii) and iv) are applied from the second side. It should be noted that coating steps iii) and / or iv) can also be carried out like steps i) or ii) with a pressure difference reversal and an excess of coating suspension.In a further preferred embodiment, however, the coating suspensions in steps iii) and iv) are introduced into the wall-flow filter without an excess of coating suspension. As mentioned above, the coating suspension is initially introduced and then completely sucked and / or forced into the filter.

[0033] Further intermediate steps of the coating process can take place between steps i) - iv). For example, intermediate drying, calcination, or rotation of the substrate can be carried out within the scope of the invention, provided that the inventive success is not unduly impaired thereby. It should also be mentioned that the coating can be carried out with identical or different catalytically active materials, with or without intermediate drying. In a preferred embodiment of the present invention, it is therefore conceivable that a first coating suspension is introduced in excess into a vertically locked wall-flow filter via the first end face by applying a pressure difference across the wall-flow filter, and that an excess of the first coating suspension is then removed from the wall-flow filter by reversing the pressure difference (step i)).The reversal of the pressure difference removes the said excess coating suspension from the channels of the wall-flow filter in the opposite direction to the coating direction. A second coating suspension without excess is then introduced into the wall-flow filter via the first end face by applying a pressure difference across the filter (step iii)). Separate drying preferably only takes place after the second coating suspension has been introduced. However, it is preferably also possible for a separate intermediate drying in an oven to take place before the second coating suspension is introduced. The same procedure is then followed for the third and fourth coatings. However, an embodiment in which no separate drying in an oven takes place between the coating steps is particularly preferred. In particular, the coatings under i) and iii) orii) and iv) can be carried out without separate drying as a "wet-on-wet" coating (Lit. US10183287BB; WO2019008078A1). If necessary, if the individual coating suspensions are too liquid, warm, dry air (approx. 50°C - 80°C; <20% humidity) can be passed through the filter for a short time (generally less than 10, preferably less than 5 seconds) after coating i) or iii). Following this, the coating in step ii) or iv) is immediately applied to the coating i) or iii). The elimination of separate drying cycles makes the process for producing the filter according to the invention extremely efficient. Filters in which the coatings in steps i) and iii) or ii) and iv) have the same composition can be produced with extreme preference without separate drying.

[0034] The wall-flow filter according to the invention is preferably used for filtering exhaust gases from internal combustion engines. The filter is preferably used for exhaust gases from gasoline engines. These generally emit relatively small particles, so good filtration efficiency is essential. The exhaust backpressure should not be excessively high. For catalytic activity, the wall-flow filter assemblies according to the invention can be combined with other exhaust gas reduction devices, e.g., those selected from the group consisting of three-way catalysts, SCR catalysts, nitrogen oxide storage catalysts, hydrocarbon traps, and nitrogen oxide traps. Very particular preference is given to combining the wall-flow filter according to the invention with one or two further TWC catalysts on flow-through substrates. Either one or both TWC catalysts can be located upstream of the filter according to the invention in a position known to be close to the engine.Positioning the filter close to the engine is also possible. However, it is particularly preferred if a TWC catalyst is arranged close to the engine, followed by the filter according to the invention, and then another TWC catalyst.

[0035] The present invention allows the use of particularly advantageous wall-flow filters in exhaust systems. However, the fact that these filters allow such a good balance between catalytic activity, exhaust backpressure, and filtration efficiency was unknown in the prior art. Figures:

[0036] Fig. 1 shows a prior art particulate filter not according to the invention, which comprises a wall-flow filter of length L (1) with channels E (2) and channels A (3) which extend parallel between a first end (4) and a second end (5) of the wall-flow filter and which are separated by porous walls (6) which form surfaces OE (7) and OA (8), respectively, and wherein the channels E (2) are closed at the second end (5) and the channels A (3) are closed at the first end (4). A first coating (9) is located in the channels E (2) on the surfaces OE (7) and a second coating (10) is located in the channels A (3) on the surfaces OA (8). This filter is an intermediate product after application of the first and third coatings. Fig. 2a - d shows schematic architectures according to the invention. The layout of the Fig. 1 is in accordance with the Fig. 2a - d The same reference numerals apply. The size of the rectangles (I, II, III, IV) symbolizes the respective amount of coating. This is intended to clearly illustrate the advantageous embodiments according to the invention with a corresponding coating ratio between I / II and III / IV. Fig. 3 shows the improved catalytic activity of the inventive embodiments of the Fig. 2a = [1]; 2b = [2]; 2c = [3]; 2d = [4] compared to the reference according to Fig. 1 . Fig. 4 shows the exhaust back pressure of the inventive embodiments of the Fig. 2a = [1]; 2b = [2]; 2c = [3]; 2d = [4] compared to the reference according to Fig. 1 . Fig. 5 shows the filtration efficiency of the inventive embodiments of the Fig. 2a = [1]; 2b = [2]; 2c = [3]; 2d = [4] compared to the reference according to Fig. 1 . Examples: reference

[0037] 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 continuously stirring. The resulting coating suspension was used directly to coat a commercially available wall-flow filter substrate. The coating suspension was coated onto the filter walls of the substrate, initially in the inlet channels over a length of 60% of the filter length.The loading of the inlet channel was 83.33 g / l, the precious metal loading was 1.06 g / l, and the palladium to rhodium ratio was 5:1. The coated filter thus obtained was dried and then calcined. Subsequently, the outlet channels of the filter were coated with the same coating suspension over a length of 60% of the filter length. The coated filter thus obtained was dried again and then calcined. The total loading of this filter was thus 100 g / l, the total precious metal loading was 1.27 g / l, and the palladium to rhodium ratio was 5:1. It is referred to below as the reference. Example 1

[0038] 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 continuously stirring. The resulting coating suspension was used directly to coat a commercially available wall-flow filter substrate. The coating suspension was coated onto the filter walls of the substrate, initially in the inlet channels over a length of 25% of the filter length (II).The loading of this zone was 100 g / l, the precious metal loading 1.27 g / l with a palladium to rhodium ratio of 5:1. The coated filter thus obtained was dried and then calcined. The filter outlet channels were then coated with the same coating suspension over a length of 25% of the filter length (IV). The coated filter thus obtained was dried again and then calcined. In the third step, the coating suspension was coated in the inlet channels over a length of 60% of the filter length (I). The loading of this zone was 41.67 g / l, the precious metal loading 0.53 g / l with a palladium to rhodium ratio of 5:1. The coated filter thus obtained was dried and then calcined. In the final step, the filter outlet channels were coated with the same coating suspension over a length of 60% of the filter length (III).The resulting coated filter was dried again and then calcined. The total loading of this filter was thus 100 g / l, the total precious metal loading 1.27 g / l, with a palladium to rhodium ratio of 5:1. This will be referred to as 1 below. Example 2

[0039] 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 continuously stirring. The resulting coating suspension was used directly to coat a commercially available wall-flow filter substrate. The coating suspension was coated onto the filter walls of the substrate, initially in the inlet channels over a length of 25% of the filter length (II).The loading of this zone was 66.67 g / l, the precious metal loading was 0.85 g / l, and the palladium to rhodium ratio was 5:1. The coated filter thus obtained was dried and then calcined. Subsequently, the filter outlet channels were coated with the same coating suspension over a length of 25% of the filter length (IV). The coated filter thus obtained was dried again and then calcined. In the third step, the coating suspension was applied to the inlet channels over a length of 60% of the filter length (I). The loading of this zone was 55.56 g / l, the precious metal loading was 0.71 g / l, and the palladium to rhodium ratio was 5:1. The coated filter thus obtained was dried and then calcined. In the final step, the filter outlet channels were coated with the same coating suspension over a length of 60% of the filter length (III).The resulting coated filter was dried again and then calcined. The total loading of this filter was thus 100 g / l, the total precious metal loading 1.27 g / l, with a palladium to rhodium ratio of 5:1. It is referred to below as 2. Example 3

[0040] 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 continuously stirring. The resulting coating suspension was used directly to coat a commercially available wall-flow filter substrate. The coating suspension was coated onto the filter walls of the substrate, initially in the inlet channels over a length of 60% of the filter length (I).The loading of this zone was 41.67 g / l, the precious metal loading was 0.53 g / l, and the palladium to rhodium ratio was 5:1. The coated filter thus obtained was dried and then calcined. Subsequently, the filter outlet channels were coated with the same coating suspension over a length of 60% of the filter length (III). The coated filter thus obtained was dried again and then calcined. In the third step, the coating suspension was applied to the inlet channels over a length of 25% of the filter length (II). The loading of this zone was 100 g / l, the precious metal loading was 1.27 g / l, and the palladium to rhodium ratio was 5:1. The coated filter thus obtained was dried and then calcined. In the final step, the filter outlet channels were coated with the same coating suspension over a length of 25% of the filter length (IV).The resulting coated filter was dried again and then calcined. The total loading of this filter was thus 100 g / l, the total precious metal loading 1.27 g / l, with a palladium to rhodium ratio of 5:1. It is referred to below as 3. Example 4

[0041] 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 continuously stirring. The resulting coating suspension was used directly to coat a commercially available wall-flow filter substrate. The coating suspension was coated onto the filter walls of the substrate, initially in the inlet channels over a length of 60% of the filter length (I).The loading of this zone was 55.56 g / l, the precious metal loading 0.71 g / l with a palladium to rhodium ratio of 5:1. The coated filter thus obtained was dried and then calcined. The filter outlet channels were then coated with the same coating suspension over a length of 60% of the filter length (III). The coated filter thus obtained was dried again and then calcined. In the third step, the coating suspension in the inlet channels was coated over a length of 25% of the filter length (II). The loading of this zone was 66.67 g / l, the precious metal loading 0.85 g / l with a palladium to rhodium ratio of 5:1. The coated filter thus obtained was dried and then calcined. In the final step, the filter outlet channels were coated with the same coating suspension over a length of 25% of the filter length (IV).The resulting coated filter was dried again and then calcined. The total loading of this filter was thus 100 g / l, the total precious metal loading 1.27 g / l, with a palladium to rhodium ratio of 5:1. It is referred to below as 4. Catalytic characterization

[0042] All five particulate filters were aged together in an engine test bench aging test. This consists of an overrun cutoff aging test with an exhaust gas temperature of 950°C upstream of the catalyst inlet (maximum bed temperature 1030°C). The aging time was 38 hours (see Motortechnische Zeitschrift, 1994, 55, 214-218). Subsequently, the catalytically active particulate filters were tested in their aged state on an engine test bench in the so-called "light-off test." In the light-off test, the light-off behavior is determined at a stoichiometric exhaust gas composition with a constant average air / fuel ratio λ (λ = 0.999 with ±3.4% amplitude). The results are shown in Fig. 3 The particulate filters 1-4 according to the invention, and especially 1 and 3, show a significant improvement in light-off behavior compared to the reference. Physical characterization

[0043] All five particulate filters were compared on the engine test bench with regard to exhaust back pressure. The average exhaust back pressure in a WLTC driving cycle is Fig. 4 As expected, the modified distribution of the coating suspension leads to a slight increase in back pressure compared to the reference. In addition, all five particulate filters were tested for filtration efficiency on the engine test bench in WLTC driving cycles. Fig. 5 shows the filtration efficiency in the first WLTC cycle. Filtration efficiency was improved for all particulate filters 1-4 according to the invention compared to the reference, particularly significantly for 2 and 4.

Claims

1. Wall flow filter for reducing particulate emissions in the exhaust gas of gasoline engines, the wall flow filter having a length L, wherein the wall flow filter comprises channels E and A which extend in parallel between a first end and a second end of the wall flow filter and which are separated by porous walls forming surfaces OE and OA, and wherein the channels E are closed at the second end and the channels A are closed at the first end, characterized in that this filter has two catalytically active coatings applied in separate coating steps on both the surfaces OE and OA, wherein a first coating extends from the first end of the wall flow filter over 55 to 96% of the length L and a second coating extends from the first end of the wall flow filter over 10 to 40% of the length L and wherein a third coating extends from the second end of the wall flow filter over 55 to 96% of the length L and a fourth coating extends from the second end of the wall flow filter over 10 to 40% of the length L.

2. Wall flow filter according to claim 1, characterized in that the first coating extends from the first end of the wall flow filter over 55 to 80% of the length L and the second coating extends from the first end of the wall flow filter over 20 to 40% of the length L and wherein the third coating extends from the second end of the wall flow filter over 55 to 80% of the length L and the fourth coating extends from the second end of the wall flow filter over 20 to 40% of the length L.

3. Wall flow filter according to claim 1 or 2, characterized in that the ratio of the amount of catalytically active coating in g / L between the first coating and the second coating or between the third and the fourth coating is equal to or less than 1:1 and equal to or more than 1:3.

4. Wall flow filter according to any one of the preceding claims, characterized in that the coatings on the surface OE and OA are unequal.

5. Wall flow filter according to any one of the preceding claims, characterized in that at least one of the coatings 1 - 4 comprises stabilized aluminum oxide in an amount of from 20 to 70 wt.%, based on the total weight of the coating, rhodium, palladium or palladium and rhodium and one or more oxygen storage components in an amount of from 30 to 80 wt.%, based on the total weight of the particular coating.

6. Method for producing a wall flow filter according to any one of claims 1 to 5, characterized in that it comprises the following steps: i) a first coating suspension is introduced into the vertically locked wall flow filter in excess via the first end by applying a pressure difference across the wall flow filter, and an excess of the first coating suspension is removed from the wall flow filter by means of a pressure difference reversal; ii) a third coating suspension is introduced into the vertically locked wall flow filter in excess via the second end by applying a pressure difference across the wall flow filter, and an excess of the third coating suspension is removed from the wall flow filter by means of a pressure difference reversal; iii) a second coating suspension is introduced into the vertically locked wall flow filter via the first end by applying a pressure difference across the wall flow filter; iv) a fourth coating suspension is introduced into the vertically locked wall flow filter via the second end by applying a pressure difference across the wall flow filter.

7. Method according to claim 6, characterized in that the coating suspensions in steps iii) and iv) are introduced into the wall flow filter without an excess of coating suspension.

8. Method according to claim 6 or 7, characterized in that the wall flow filter is not separately dried between the coating steps.

9. Use of a wall flow filter according to any of claims 1 to 5 in an exhaust gas system for gasoline engine exhaust gases.

10. Use according to claim 9, characterized in that further exhaust gas reduction units selected from the group consisting of three-way catalytic converters, SCR catalytic converters, nitrogen oxide storage catalytic converters, hydrocarbon traps, nitrogen oxide traps are present in the exhaust gas system.