Particulate filter for exhaust gases of gasoline engines

Abstract

The invention relates to a wall-flow filter, in particular for use in an exhaust system of a vehicle driven by a gasoline engine. The filter has a three-way activity and filters fine particles resulting from the combustion of gasoline from the exhaust gas stream. The invention also relates to a method for producing a corresponding filter and to preferred uses thereof.

Description

[0001] describe

[0002] This invention relates to a wall-flow filter, specifically for use in the exhaust system of a vehicle driven by a gasoline engine. The filter exhibits improved three-way catalytic converter activity and filters fine particles generated by gasoline combustion from the exhaust gas stream. The invention also relates to a method for manufacturing the corresponding filter and its preferred applications.

[0003] For example, the exhaust gas from internal combustion engines typically contains harmful gases such as carbon monoxide (CO), hydrocarbons (HC), and nitrogen oxides (NOx). x ) and possible sulfur oxides (SO x As well as particulate matter, which mainly includes soot residues and potentially attached organic agglomerates, these are referred to as major emissions. CO, HC, and particulate matter are products of incomplete combustion of fuel in the engine's combustion chamber. When the combustion temperature locally exceeds 1400°C, nitrogen and oxygen in the intake air form nitrogen oxides in the cylinder. Sulfur oxides are caused by the combustion of organic sulfur compounds, small amounts of which are always present in non-synthetic fuels. To remove these harmful emissions from motor vehicle exhaust, various catalytic technologies for exhaust gas purification have been developed. Their basic principle is generally based on guiding the exhaust gas to be purified through 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 chemical reactions of various exhaust gas components to form harmless products such as carbon dioxide and water, while simultaneously removing soot particles in the case of a wall-flow filter.

[0004] Particulate filters are highly effective at removing particles from exhaust gases. Wall-flow filters made of ceramic materials have proven particularly successful. These filters have two end faces and are constructed of multiple parallel channels of a certain length, formed by porous walls and extending from one end face to the other. The channels alternately close at one end of the filter, forming: a first channel that is open on a first side of the filter and closed on a second side; and a second channel that is closed on a first side and open on a second side. Depending on the arrangement of the filter in the exhaust gas flow, one of the end faces forms the exhaust gas inlet end face, and the second end face forms the exhaust gas outlet end face. The flow channel open on the inlet side forms the inlet channel, and the flow channel open on the outlet side forms the outlet channel. For example, exhaust gas flowing into the first channel can only leave the filter again through the second channel, and for this purpose must flow through the wall between the first and second channels. For this reason, the material constructing the wall-flow filter exhibits an open porosity. As the exhaust gas passes through the wall, the particles remain unchanged.

[0005] Wall-flow filters can be catalytically active. Catalytic activity is achieved by coating the filter with a suspension containing a catalytically active material. The contact between the catalytically active material and the wall-flow filter is referred to in the art as a "coating." The coating has actual catalytic function and typically contains a storage material and / or a catalytically active metal, which is in most cases deposited in a highly dispersed form on a temperature-stable metal compound (especially an oxide) with a large surface area. In most cases, the coating is achieved by applying an aqueous suspension (also known as a repair substrate coating) of the storage material and the catalytically active component onto or within the wall-flow filter. After applying the suspension, the substrate is typically dried and, where appropriate, calcined at an elevated temperature. The coating can consist of a single layer or multiple layers applied to the corresponding filter in a manner that is sequentially on top (multilayer) and / or sequentially offset relative to each other (partition). The catalytically active material can be applied to the porous walls between the channels (known as a wall coating). However, this coating can lead to a significant increase in the back pressure of the filter. Against this backdrop, for example, JPH01-151706 and WO2005016497A1 propose coating wall-flow filters with a catalyst, allowing the catalyst to permeate the porous wall (known as an internal wall coating). The coating area is understood to mean the presence of catalytically active material (coating) on ​​or within the filter wall over a length less than the entire length of the wall-flow filter. In this invention, when referring to the length of the wall-flow filter, it means the entire length, including the plugs forming the channel closures.

[0006] So-called three-way catalysts are used to reduce exhaust gases from stoichiometric combustion engines. Three-way catalysts (TWCs) have long been known to those skilled in the art and have been legally mandated since the 1980s. The actual catalyst mass here comprises a large area of ​​metal compounds, specifically the bulk of the oxide support material, on which the catalytically active component is deposited with minimal distribution. Platinum group metals (platinum, palladium, and / or rhodium) are particularly suitable as the catalytically active component for cleaning stoichiometric exhaust gases. For example, alumina, silica, titanium dioxide, zirconium oxide, cerium oxide, and their mixed oxides, as well as zeolites, are suitable as support materials. Preferably, a catalyst with a surface area exceeding 10 m² is used. 2 Materials with a specific surface area (BET surface area, measured according to DIN 66132, latest version at the time of submission) of / g are referred to as active alumina. Furthermore, three-way catalysts include oxygen storage components that enhance dynamic conversion. These include cerium / zirconium mixed oxides, optionally containing lanthanum oxide, praseodymium oxide, and / or yttrium oxide. Meanwhile, partitioned and multilayer systems with ternary activity are also known (US8557204; US8394348). If such a three-way catalyst is located on or within a particulate filter, this is referred to as cGPF (catalyst-driven gasoline particulate filter; e.g., EP2650042B1).

[0007] The quality of catalytically coated exhaust filters is measured according to standards of filtration efficiency, catalytic performance, and pressure loss. To meet these diverse requirements, filters with, for example, catalytically active regions are provided. As mentioned above, these regions can exist on the walls of the filter matrix units or within porous walls.

[0008] A set 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 of length L, and extends from the inlet end face to the outlet end face through multiple alternately closed flow channels. A coating suspension is applied by vertically oriented the flow channels of the wall-flow filter, such that one end face is at the bottom and a second end face is at the top. A pressure differential is applied to introduce the coating composition into the filter body through the flow channels of the wall-flow filter, which open to a desired height above the lower end face. Excess coating composition is removed downwards by applying a suction pulse. Specific modifications of the methods in WO 06042699A1 and WO 11098450A1 are based on the same coating principle. A coating apparatus using this method principle is proposed in WO 13070519A1. Here, the principle of excess coating suspension and pressure differential reversal is also used.

[0009] This coating principle is also applicable to the preparation of particulate filters including regions with catalytically active materials on the inlet and outlet sides. WO 09103699A1 describes a method for coating a filter with two different repair substrate coatings, the steps of which are: the filter substrate is vertically oriented; a first coating suspension is pumped from below (with the pressure differential having the highest pressure at the lower end); excess coating suspension is removed by suction (pressure differential reversal); and after rotating 180°, the filter body is refilled from below with a second repair substrate coating, and excess coating suspension is removed by suction. After the coating process, the filter is dried and calcined. The same coating principle is disclosed in US 7094728B2. Coated wall-flow filters produced in this way typically have a gradient in the coating, such as in… Figure 1 The diagram is shown in a schematic and enlarged form.

[0010] For example, the methods described in WO 06021339A1, WO 15145122A2, and WO 0110573A2 belong to the second category of coating methods, wherein the filter body is coated without excessive repair substrate paint and without pressure differential reversal. In this case, the vertically oriented filter carrier may be coated with repair substrate paint from either the lower or upper end face.

[0011] WO06021339A1 discloses a method for coating a wall-flow particulate filter with a coating composition, wherein the particulate filter is made of an open-pore material, has a cylindrical shape of length L, and has a plurality of flow channels from an inlet end face to an outlet end face, the channels being closed alternately. The method is characterized in that the flow channels of the wall-flow filter are vertically oriented such that one end face is at the bottom and a 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 amount of coating composition, applying a negative pressure to an opening in the outlet channel in the upper end face, and drawing the entire amount of coating composition into the inlet and outlet channels through an opening in the inlet channel in the lower end face. The amount of coating composition presented is selected according to the desired coating concentration and coating height. There is no pressure differential reversal after the pressure differential applied for coating is applied. The coating suspension is measured and not used in excess.

[0012] WO 0110573A2 also describes a method for coating a particulate filter, wherein a measured amount of repair substrate coating is applied from below to a filter carrier. A loaded amount of coating suspension is drawn into channels of the substrate by applying a pressure differential (vacuum at the upward-oriented end face). The substrate is then rotated, and the repair substrate coating is distributed in the channels by an action of injecting pressurized air onto the upper end of the substrate. In this method, the pressure differential does not reverse because the second pressure pulse is also in the same direction as the movement of the repair substrate coating relative to the first pressure pulse, thus there is no pressure differential reversal.

[0013] WO 15145122A2 is another example of this type of coating method. However, in contrast to the methods described above, a predetermined amount of coating suspension is applied to the upper end face of a vertically oriented filter, as measured thereon, and distributed into the channels of the particulate filter by applying a pressure differential (by applying a vacuum to the lower end face). No further pressure differential reversal occurs after this coating step.

[0014] The object of this invention is to provide a wall-flow filter that improves catalytic activity and is substantially non-inferior to prior art gasoline particulate filters (GPF, OPF) in terms of filtration efficiency and exhaust back pressure. The desired filter should have correspondingly high filtration efficiency, especially under conditions of high catalytic activity, provided that the exhaust back pressure is not excessively compromised.

[0015] These and other objectives, obvious from the prior art, are achieved by a wall-flow filter having the features of the present invention. The present invention relates to a method for manufacturing or using a wall-flow filter according to the present invention. The invention also relates to corresponding preferred embodiments.

[0016] The aforementioned objective is surprisingly achieved by providing a wall-flow filter with a length L for reducing particulate emissions in the exhaust gas of a gasoline engine. The wall-flow filter includes channels E and A, which extend parallel to each other between a first end and a second end of the filter and are separated by porous walls forming surfaces OE or OA, respectively. Channel E is closed at the second end, and channel A is closed at the first end. The wall-flow filter has two catalytically active coatings applied to surfaces OE and OA in separate coating steps. A first coating extends from the first end of the wall-flow filter beyond 55% to 96% of the length L, a second coating extends from the first end of the wall-flow filter beyond 10% to 40% of the length L, a third coating extends from the second end of the wall-flow filter beyond 55% to 96% of the length L, and a fourth coating extends from the second end of the wall-flow filter beyond 10% to 40% of the length L. This extended wall-flow filter (… Figures 2a-d It exhibits better catalytic activity than existing wall-flow filters with the same loading capacity. Figure 3 The latter is coated with an excess of coating suspension from both sides, and in each case the excess coating suspension is removed by pressure differential reversal. Figure 1 Surprisingly, the filter according to the invention produces only low exhaust back pressure. Figure 4 ), and in filtration efficiency ( Figure 5 In terms of technology, filters are by no means inferior to those of existing technologies. Figure 1 The first to fourth coatings will be numbered 1 to 4 below.

[0017] According to the invention, the respective regions of the catalytic coating of the wall-flow filter are positioned on the walls of the input surface OE and the output surface OA from the corresponding open end of the channel toward the other end in each case. The term "on the wall" means that one of the catalytically active coatings penetrates into the porous wall surface of the input or output channel to a small extent, not exceeding 33%, more preferably less than 15%, and most preferably less than 10%, based on the total amount of the catalytically active coating. The corresponding analysis can be performed by means of CT or SEM images of cross-sections of the filter wall and image analysis methods (Blazek et al., Chem. Eng. J. 409 (2021) 128057; Greiner et al., Chem. Eng. J. 378 (2019) 121919).

[0018] The formation of the various regions can be modified by those skilled in the art within the given limits. This will be guided by the quality standards of the wall-flow filter mentioned at the beginning. Importantly, the longer regions of the catalytic coating (the first and third coatings) overlap by at least a small portion of the length L, while the shorter regions (the second and fourth regions) do not overlap. In a preferred embodiment, the first coating thus extends from the first end of the wall-flow filter beyond 55% to 80% of the length L, the second coating extends from the first end of the wall-flow filter beyond 20% to 40% of the length L, the third coating extends from the second end of the wall-flow filter beyond 55% to 80% of the length L, and the fourth coating extends from the second end of the wall-flow filter beyond 20% to 40% of the length L. More preferably, the first coating extends from the first end of the wall-flow filter beyond 55% to 70% of the length L, the second coating extends from the first end of the wall-flow filter beyond 25% to 35% of the length L, the third coating extends from the second end of the wall-flow filter beyond 55% to 70% of the length L, and the fourth coating extends from the second end of the wall-flow filter beyond 25% to 35% of the length L. The filter mentioned in this document by way of example has a region that is 60% of the length L of the long region and 30% of the length L of the short region.

[0019] It has been shown that having corresponding catalytic activity is advantageous for both long and short regions. The catalytic activity of the coating is also determined by the amount of coating. The amount of coating on the filter, in g / l, can be varied by those skilled in the art. It has been shown that it is particularly advantageous if the ratio of the amount of catalytically active coating, in g / l, between the first and second coatings or between the third and fourth coatings is less than or equal to 1:1 and greater than or equal to 1:3. This means that the short region is preferably applied to the filter at a higher concentration than the long region. The coating concentration ratio is particularly preferably 1:2 to 1:3 (measured in g / l). However, this specification is particularly applicable to cases where the coatings on surfaces OE (input) and OA (output) correspond in layout and construction. In one embodiment of the invention, it is particularly preferred if the coatings on surfaces OE and OA are different in each case. In another embodiment of the invention, it is particularly preferred if the coatings on surfaces OE and OA are the same in each case. All four coatings according to the invention can also be the same. The same coating on surface OE can also be different from the same coating on surface OA.

[0020] "Identical" means that the chemical composition, quantity, and extent of the coatings are the same. However, as just described, it is also possible that the four coating zones discussed can be designed differently chemically, quantitatively, and / or in length, depending on the required profile. It should be mentioned that additional coatings may also be present in and on the filter according to the invention. For example, it is conceivable to additionally coat the inner walls of the channels with a corresponding catalytically active composition. This would help to further increase the catalytic activity of the wall-flow filter described herein. However, it is also conceivable to have a combination of optional non-catalytically active powder coatings in and / or on the inlet side of the wall-flow filter. This would help to further increase the filtration efficiency without significantly increasing the exhaust back pressure.

[0021] The order in which the coating zones are applied can also be configured differently. It has been found that, particularly when the same coating is applied on both the inlet and outlet sides of the filter, one embodiment of the invention is preferred, in which the short zone is applied last and the long zone is applied first. Therefore, it is particularly advantageous if the first and third coatings are applied to the wall-flow filter before the second and fourth coatings are applied (Figures 2c and 2d).

[0022] The coatings are ternary catalytically active, particularly at operating temperatures from 250°C to 1100°C. They typically contain one or more noble metals fixed to one or more support materials and one or more oxygen storage components. The coatings preferably contain different but preferably equal amounts of the same oxygen storage component and the same noble metal support material. The coatings also contain the same or different amounts of the same or different noble metals. Particularly preferred is that, in each case, coatings 1 and 2, and coatings 3 and 4, are chemically identical. It has been found advantageous for the production of wall-flow filters 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 may preferably lie in the type of noble metal used and / or the type of oxygen storage material used. For example, coatings 1 and 3 may preferably contain platinum, palladium, or mixtures thereof, and coatings 2 and 4 may preferably contain platinum, palladium, rhodium, or mixtures thereof. Furthermore, coatings 2 and 4 may have an oxygen storage material, while coatings 1 and 3 have two oxygen storage materials. This architecture has shown particular robustness in aging tests.

[0023] Platinum, palladium, and rhodium are particularly suitable as precious metals for the above-mentioned coatings, wherein palladium and rhodium or platinum, palladium, and rhodium are preferred, and palladium and rhodium are particularly preferred. Based on the particulate filter according to the invention, the proportion of rhodium in the total precious metal content is specifically greater than or equal to 5% by weight, but less than or equal to 50% by weight, preferably less than 30% by weight. The porous wall of the particulate filter according to the invention is preferably substantially free of precious metals. Based on the volume of the wall-flow filter, the precious metal is typically used in an amount of 0.15 g / L to 5 g / L, more preferably 0.3 g / L to 4 g / L.

[0024] As a carrier material for precious metals, all materials familiar to those skilled in the art can be considered. Such materials, particularly metal oxides, have a BET surface area of ​​30 m². 2 / g to 250m 2 / g, preferably 100m 2 / g to 200m 2 / g (determined according to DIN 66132). Particularly suitable carrier materials for precious metals are selected from the series consisting of: alumina, doped alumina, silicon oxide, titanium dioxide, and mixed oxides of one or more of these. Doped alumina is, for example, alumina doped with lanthanum oxide, silicon oxide, zirconium oxide, and / or titanium oxide. Lanthanum-stabilized alumina is advantageously used, and it is further advantageous that, in each case, lanthanum is preferably used in an amount of 1% to 10% by weight, preferably 3% to 6% by weight, calculated as La2O3 and based on the weight of stabilized alumina. Another suitable carrier material is lanthanum-stabilized alumina coated with lanthanum oxide, barium oxide, or strontium oxide. In a particularly preferred embodiment, at least one of the coatings 1 to 4 contains 20% to 70% by weight of stabilized alumina based on the total weight of the coating, 30% to 80% by weight of rhodium, palladium, or palladium and rhodium, and one or more oxygen-storing components based on the total weight of the coating. This is highly preferred for all coatings 1 to 4.

[0025] Cerium / zirconium / rare earth metal mixed oxides are particularly suitable as oxygen storage components. Within the meaning of this invention, the term "cerium-zirconium-rare earth metal mixed oxide" does not include physical mixtures of cerium oxide, zirconium oxide, and rare earth oxides. Instead, "cerium / zirconium / rare earth metal mixed oxide" is characterized by a substantially homogeneous three-dimensional crystal structure and ideally does not contain phases of pure cerium oxide, zirconium oxide, or rare earth oxides. However, depending on the manufacturing process, imperfectly homogeneous products may be produced, which are generally usable without defects. In all other respects, the terms "rare earth metal" or "rare earth metal oxide" within the meaning of this invention do not include cerium or cerium oxide.

[0026] Lanthanum oxide, yttrium oxide, praseodymium oxide, neodymium oxide, and / or samarium oxide can be considered, for example, as rare earth metal oxides in cerium-zirconium-rare earth metal mixed oxides. Lanthanum oxide, yttrium oxide, and / or praseodymium oxide are preferred. Lanthanum oxide and / or yttrium oxide are particularly preferred, and more particularly preferred are combinations of lanthanum oxide and yttrium oxide, yttrium oxide and praseodymium oxide, and lanthanum oxide and praseodymium oxide. In embodiments of the invention, the oxygen storage component is particularly preferably free of neodymium oxide.

[0027] In a preferred embodiment of the invention, one or all of the coatings contain alkaline earth compounds, such as strontium oxide, barium oxide, or barium sulfate. Specifically, the amount of barium sulfate in each coating is from 2 g / L to 20 g / L wall-flow filter volume, preferably from 3 g / L to 10 g / L wall-flow filter volume. Coatings 1 and / or 3 specifically comprise strontium oxide or barium oxide.

[0028] In another advantageous embodiment of the invention, one or all of the coating contains additives, such as rare earth compounds, such as lanthanum oxide, and / or binders, such as aluminum compounds. The amounts of these additives can vary within a wide range and can be determined by those skilled in the art in specific circumstances using simple methods. If desired, these contribute to improving the rheological properties of the coating.

[0029] In embodiments of the invention, at least one of coatings 1 to 4, preferably each, comprises lanthanum-stabilized alumina, palladium, rhodium, or palladium and rhodium, and an oxygen storage component comprising zirconium oxide, cerium oxide, lanthanum oxide, and yttrium oxide and lanthanum oxide. If present, the yttrium oxide content in the coating is specifically 2% to 15% by weight, preferably 3% to 10% by weight, based on the weight of the oxygen storage component. The weight ratio of lanthanum oxide to yttrium oxide is particularly 0.1 to 1, preferably 0.3 to 1. If present, the praseodymium oxide content is specifically 2% to 15% by weight, preferably 3% to 10% by weight, based on the weight of the oxygen storage component. The weight ratio of lanthanum oxide to praseodymium oxide is particularly 0.1 to 1, preferably 0.3 to 1. One of coatings 1 to 4 may specifically include an additional oxygen storage component comprising zirconium oxide, cerium oxide, lanthanum oxide, and yttrium oxide and / or praseodymium oxide. This is preferred for all coatings 1 to 4.

[0030] In each embodiment, based on the total weight of the respective coatings, at least one of coatings 1 to 4 preferably comprises 35% to 60% by weight, particularly preferably 40% to 60% by weight, of lanthanum-stabilized alumina, and 40% to 50% by weight, particularly preferably 45% to 50% by weight, of an oxygen storage component. In each embodiment, based on the total weight of the respective coatings, at least one of coatings 1 to 4 preferably comprises 30% to 50% by weight, particularly preferably 40% to 50% by weight, of lanthanum-stabilized alumina, and 50% to 80% by weight, particularly preferably 55% to 80% by weight, of an oxygen storage component. In an advantageous embodiment of the invention, in at least one of coatings 1 to 4, the weight ratio of alumina to the oxygen storage component is at least 0.7 to at most 1.5, preferably 0.8 to 1.2. In an advantageous embodiment of the invention, in at least one of coatings 1 to 4, the weight ratio of alumina to the oxygen-storing component is at least 0.3 to at most 0.8, preferably 0.4 to 0.6. The embodiments mentioned herein are preferably applicable to all coatings 1 to 4.

[0031] The present invention also provides a method for producing a wall-flow filter according to the invention, comprising the following steps:

[0032] i) Excess first coating suspension is introduced into the wall-flow filter via the first end by applying a pressure differential through the vertically locked wall-flow filter, and the excess first coating suspension is removed from the wall-flow filter by reversing the pressure differential;

[0033] ii) Excess third coating suspension is introduced into the wall-flow filter via the second end by applying a pressure differential through the vertically locked wall-flow filter, and the excess third coating suspension is removed from the wall-flow filter by reversing the pressure differential;

[0034] iii) By applying a pressure differential over the vertically locked wall-flow filter, a second coating suspension is introduced into the wall-flow filter via the first end;

[0035] iv) By applying a pressure differential over the vertically locked wall-flow filter, a fourth coating suspension is introduced into the wall-flow filter via the second end.

[0036] The order of steps i)-iv) is not decisive in the first approximation. It is important to note that in steps i) and ii), a pressure reversal occurs, where excess coating suspension is removed from the wall-flow filter. Between steps i) and ii) and between steps iii) and iv), the filter is rotated 180° in lock in each case. Rotation may preferably be omitted if the coating according to i) and iii) is applied first from the first side of the filter, and then the coating according to ii) and iv) is applied from the second side of the filter. It should be noted that applying steps iii) and / or iv) can also be performed in the case of pressure reversal and excess coating suspension, as in steps i) or ii). In another preferred embodiment, the coating suspension from steps iii) and iv) is introduced into a wall-flow filter without excess coating suspension. In this case, as described at the beginning, the coating suspension is first provided and then completely drawn into and / or forced into the filter.

[0037] Additional intermediate steps in the coating process can occur between steps i)-iv). For example, intermediate drying, calcination, or rotation of the substrate can be performed within the scope of the invention, provided that the success of the invention is not unduly compromised. It should also be mentioned that coating can be performed with the same or different catalytically active materials in each case, with or without intermediate drying. Thus, in a preferred embodiment of the invention, it is conceivable to introduce an excess of the first coating suspension into a vertically locked wall-flow filter via a first end face by applying a pressure differential over the wall-flow filter, and then remove the excess first coating suspension from the wall-flow filter by reversing the pressure differential (step i). The pressure differential reversal thereby removes the excess coating suspension from the channels of the wall-flow filter against the coating direction. Then, a second coating suspension without excess is introduced into the wall-flow filter via a second end face by applying a pressure differential over the wall-flow filter (step iii). Separate drying preferably occurs only after the introduction of the second coating suspension. However, it is also preferable to perform separate intermediate drying in a furnace before the introduction of the second coating suspension. The same operation is then performed on the third and fourth coatings. However, one embodiment is particularly preferred, wherein there is no separate drying in a heating furnace between coating steps. Specifically, the coatings under i) and iii) or ii) and iv) can occur as a “wet-on-wet” coating without separate drying (refer to US10183287BB; WO2019008078A1). Optionally—if the various coating suspensions are too liquid—warm, dry air (about 50°C-80°C; <20% humidity) can be passed through the filter for a short time (usually less than 10 seconds, preferably less than 5 seconds) after coating i) or iii). Subsequently, the coating from step ii) or iv) is immediately applied onto the coating from i) or iii). The method for producing filters according to the invention is very efficient by eliminating separate drying cycles. It is extremely preferred to produce filters in which the coatings from steps i) and iii) or ii) and iv) are compositionally identical without separate drying.

[0038] The wall-flow filter according to the invention is preferably used for filtering exhaust gases from internal combustion engines. This filter is particularly preferred for gasoline engines. These typically emit relatively small particles, necessitating good filtration efficiency. Exhaust back pressure should not increase excessively during the process. For catalytic activity, the wall-flow filter unit according to the invention can be combined with additional exhaust gas reduction devices, such as those selected from three-way catalysts, SCR catalysts, nitrogen oxide storage catalysts, hydrocarbon traps, and nitrogen oxide traps. The combination of the wall-flow filter according to the invention with one or two additional TWC catalysts on a flow-through substrate is particularly preferred. In this case, one or both of the TWC catalysts can be located upstream of the filter according to the invention at a known location close to the engine. The filter can also be positioned as a unit close to the engine. However, it is particularly preferred if the TWC catalyst is arranged close to the engine, followed by the filter according to the invention, and then the TWC catalyst is again arranged close to the engine.

[0039] This invention enables the use of particularly advantageous wall-flow filters in exhaust systems. However, the fact that these filters allow for such a good balance between catalytic activity, exhaust back pressure, and filtration efficiency is unknown from the background of the prior art. Attached Figure Description

[0040] Figure 1 A prior art particulate filter not according to the invention is shown, comprising a wall-flow filter (1) of length L, having channels E (2) and A (3) extending parallel between a first end (4) and a second end (5) of the wall-flow filter and separated by porous walls (6) forming surfaces OE (7) and OA (8), respectively, wherein channel E (2) closes at the second end (5) and channel A (3) closes at the first end (4). A first coating (9) is located in channel E (2) on surface OE (7), and a second coating (10) is located in channel A (3) on surface OA (8). This filter is an intermediate product after the first and third coatings have been applied.

[0041] Figures 2a-d A schematic architecture according to the present invention is shown. Figures 2a-d Consider Figure 1 The layout is as follows. The same reference numerals apply. The dimensions of the rectangles (I, II, III, IV) symbolize the corresponding coating amounts. Therefore, with corresponding coating ratios between I / II and III / IV, advantageous embodiments according to the invention become clear.

[0042] Figure 3 Showing according to Figure 1Compared with the reference, the improved catalytic activity of the embodiments according to the present invention is shown in Figures 2a=[1]; 2b=[2]; 2c=[3]; 2d=[4].

[0043] Figure 4 It shows the relationship with the data. Figure 1 Compared with the reference, the exhaust gas back pressure of the implementation scheme is shown in Figure 2a=[1]; 2b=[2]; 2c=[3]; 2d=[4].

[0044] Figure 5 It shows the relationship with the data. Figure 1 Compared with the reference, the filtration efficiency of Figures 2a=[1]; 2b=[2]; 2c=[3]; 2d=[4] according to the embodiment of the present invention.

[0045] Example :

[0046] refer to

[0047] Lanthanum oxide-stabilized alumina was suspended in water along with a first oxygen storage component and a second oxygen storage component. The first oxygen storage component comprised 40 wt% cerium oxide, zirconium oxide, lanthanum oxide, and praseodymium oxide, while the second oxygen storage component comprised 24 wt% cerium oxide, zirconium oxide, lanthanum oxide, and yttrium oxide. The two oxygen storage components were used in equal proportions. The weight ratio of alumina to the oxygen storage component was 30:70. The resulting suspension was then mixed with a palladium nitrate solution and a rhodium nitrate solution under constant stirring. The resulting coating suspension was directly applied to a commercially available wall-flow filter substrate. The coating suspension was first applied to the filter wall of the substrate in the inlet channel, reaching 60% of the filter length. The inlet channel loading reached 83.33 g / L; the noble metal loading reached 1.06 g / L, with a palladium to rhodium ratio of 5:1. The resulting coated filter was dried and then calcined. The same coating suspension was then used to coat the output channel of the filter to 60% of the filter length. The resulting coated filter was dried again and then calcined. Therefore, the total load of this filter reaches 100 g / L; the total precious metal load reaches 1.27 g / L, with a palladium to rhodium ratio of 5:1. This will be referred to as a reference below.

[0048] Example 1

[0049] Lanthanum oxide-stabilized alumina was suspended in water along with a first oxygen storage component and a second oxygen storage component. The first oxygen storage component comprised 40 wt% cerium oxide, zirconium oxide, lanthanum oxide, and praseodymium oxide, while the second oxygen storage component comprised 24 wt% cerium oxide, zirconium oxide, lanthanum oxide, and yttrium oxide. The two oxygen storage components were used in equal proportions. The weight ratio of alumina to the oxygen storage component was 30:70. The resulting suspension was then mixed with a palladium nitrate solution and a rhodium nitrate solution under constant stirring. The resulting coating suspension was directly applied to a commercially available wall-flow filter substrate. The coating suspension was first applied to the filter wall of the substrate in the inlet channel to 25% of the filter length (II). The loading in this area reached 100 g / L; the noble metal loading reached 1.27 g / L, with a palladium to rhodium ratio of 5:1. The resulting coated filter was dried and then calcined. The same coating suspension was then applied to the outlet channel of the filter to 25% of the filter length (IV). The resulting coated filter was dried again and then calcined. In the third step, a coating suspension is applied to the input channel to reach 60% of the filter length (I). The loading in this area reaches 41.67 g / L; the precious metal loading reaches 0.53 g / L, with a palladium to rhodium ratio of 5:1. The resulting coated filter is dried and then calcined. In the final step, the same coating suspension is used to coat the output channel of the filter to reach 60% of the filter length (III). The resulting coated filter is dried again and then calcined. Thus, the total loading of the filter reaches 100 g / L; the total precious metal loading reaches 1.27 g / L, with a palladium to rhodium ratio of 5:1. This will be referred to as step 1 below.

[0050] Example 2

[0051] Lanthanum oxide-stabilized alumina was suspended in water along with a first oxygen storage component and a second oxygen storage component. The first oxygen storage component comprised 40 wt% cerium oxide, zirconium oxide, lanthanum oxide, and praseodymium oxide, while the second oxygen storage component comprised 24 wt% cerium oxide, zirconium oxide, lanthanum oxide, and yttrium oxide. The two oxygen storage components were used in equal proportions. The weight ratio of alumina to the oxygen storage component was 30:70. The resulting suspension was then mixed with a palladium nitrate solution and a rhodium nitrate solution under constant stirring. The resulting coating suspension was directly applied to a commercially available wall-flow filter substrate. The coating suspension was first applied to the filter wall of the substrate in the inlet channel, reaching 25% of the filter length (II). The loading in this area reached 66.67 g / L; the noble metal loading reached 0.85 g / L, with a palladium to rhodium ratio of 5:1. The resulting coated filter was dried and then calcined. The same coating suspension was then used to coat the outlet channel of the filter to reach 25% of the filter length (IV). The resulting coated filter is dried again and then calcined. In the third step, the coating suspension is applied to the input channel to 60% of the filter length (I). The loading in this area reaches 55.56 g / L; the precious metal loading reaches 0.71 g / L, with a palladium to rhodium ratio of 5:1. The resulting coated filter is dried and then calcined. In the final step, the same coating suspension is applied to the output channel of the filter to 60% of the filter length (III). The resulting coated filter is dried again and then calcined. Thus, the total loading of the filter reaches 100 g / L; the total precious metal loading reaches 1.27 g / L, with a palladium to rhodium ratio of 5:1. This will be referred to as 2 below.

[0052] Example 3

[0053] Lanthanum oxide-stabilized alumina was suspended in water along with a first oxygen storage component and a second oxygen storage component. The first oxygen storage component comprised 40 wt% cerium oxide, zirconium oxide, lanthanum oxide, and praseodymium oxide, while the second oxygen storage component comprised 24 wt% cerium oxide, zirconium oxide, lanthanum oxide, and yttrium oxide. The two oxygen storage components were used in equal proportions. The weight ratio of alumina to the oxygen storage component was 30:70. The resulting suspension was then mixed with a palladium nitrate solution and a rhodium nitrate solution under constant stirring. The resulting coating suspension was directly applied to a commercially available wall-flow filter substrate. The coating suspension was first applied to the filter wall of the substrate in the inlet channel, reaching 60% of the filter length (I). The loading in this area reached 41.67 g / L; the noble metal loading reached 0.53 g / L, with a palladium to rhodium ratio of 5:1. The resulting coated filter was dried and then calcined. The same coating suspension was then used to coat the outlet channel of the filter to 60% of the filter length (III). The resulting coated filter is dried again and then calcined. In the third step, a coating suspension is applied to the input channel to reach 25% of the filter length (II). The loading in this area reaches 100 g / L; the precious metal loading reaches 1.27 g / L, with a palladium to rhodium ratio of 5:1. The resulting coated filter is dried and then calcined. In the final step, the same coating suspension is applied to the output channel of the filter to reach 25% of the filter length (IV). The resulting coated filter is dried again and then calcined. Thus, the total loading of the filter reaches 100 g / L; the total precious metal loading reaches 1.27 g / L, with a palladium to rhodium ratio of 5:1. This will be referred to as step 3 below.

[0054] Example 4

[0055] Lanthanum oxide-stabilized alumina was suspended in water along with a first oxygen storage component and a second oxygen storage component. The first oxygen storage component comprised 40 wt% cerium oxide, zirconium oxide, lanthanum oxide, and praseodymium oxide, while the second oxygen storage component comprised 24 wt% cerium oxide, zirconium oxide, lanthanum oxide, and yttrium oxide. The two oxygen storage components were used in equal proportions. The weight ratio of alumina to the oxygen storage component was 30:70. The resulting suspension was then mixed with a palladium nitrate solution and a rhodium nitrate solution under constant stirring. The resulting coating suspension was directly applied to a commercially available wall-flow filter substrate. The coating suspension was first applied to the filter wall of the substrate in the inlet channel, reaching 60% of the filter length (I). The loading in this area reached 55.56 g / L; the noble metal loading reached 0.71 g / L, with a palladium to rhodium ratio of 5:1. The resulting coated filter was dried and then calcined. The same coating suspension was then used to coat the outlet channel of the filter to 60% of the filter length (III). The resulting coated filter is dried again and then calcined. In the third step, the coating suspension reaches 25% of the filter length in the input channel (II). The loading in this area reaches 66.67 g / l; the precious metal loading reaches 0.85 g / l, with a palladium to rhodium ratio of 5:1. The resulting coated filter is dried and then calcined. In the final step, the same coating suspension is used to coat the filter's output channel to 25% of the filter length (IV). The resulting coated filter is dried again and then calcined. Thus, the total loading of the filter reaches 100 g / l; the total precious metal loading reaches 1.27 g / l, with a palladium to rhodium ratio of 5:1. This will be referred to as step 4 below.

[0056] Catalytic characterization

[0057] All five particulate filters were aged together on an engine test bench. This aging process included an over-limit cutoff aging process, with an exhaust gas temperature of 950°C (maximum bed temperature of 1030°C) before the catalyst inlet. The aging time was 38 hours (see Motortechnische Zeitschrift, 1994, 55, 214–218). The catalytically active particulate filters in the aged state were then tested on an engine test bench in a so-called “ignition test.” In the ignition test, ignition behavior was determined under stoichiometric exhaust gas composition with a constant mean air-to-fuel ratio of λ (λ = 0.999 and ±3.4% amplitude). The results were presented in… Figure 3 As shown in the figure. The particulate filters 1-4 according to the invention, and especially 1 and 3, show a significant improvement in ignition behavior compared to the reference.

[0058] Physical characteristics

[0059] The exhaust back pressure of all five particulate filters was compared on the engine test bench. The average exhaust back pressure in the WLTC drive cycle is shown below. Figure 4 As expected, the altered distribution of the coating suspension resulted in a slight increase in back pressure compared to the reference. Furthermore, the filtration efficiency of all five particulate filters was tested in a WLTC drive cycle on an engine test bench. Figure 5 The filtration efficiency in the first WLTC cycle is shown. Compared with the reference, the filtration efficiency of all particulate filters 1 to 4 according to the invention is improved, especially for 2 and 4.

Claims

1. A wall-flow filter for reducing particulate emissions in the exhaust gas of a gasoline engine having a length L, wherein the wall-flow filter includes channels E and A extending parallel to each other between a first end and a second end of the wall-flow filter and separated by porous walls forming surfaces OE and OA, respectively, wherein channel E is closed at the second end and channel A is closed at the first end. Its features are, The wall-flow filter has two catalytically active coatings applied to the surfaces OE and OA in separate coating steps, wherein a first coating extends from the first end of the wall-flow filter beyond 55% to 96% of the length L, a second coating extends from the first end of the wall-flow filter beyond 10% to 40% of the length L, a third coating extends from the second end of the wall-flow filter beyond 55% to 96% of the length L, and a fourth coating extends from the second end of the wall-flow filter beyond 10% to 40% of the length L.

2. The wall-flow filter according to claim 1, Its features are, The first coating extends from the first end of the wall-flow filter beyond 55% to 80% of the length L, and the second coating extends from the first end of the wall-flow filter beyond 20% to 40% of the length L, wherein the third coating extends from the second end of the wall-flow filter beyond 55% to 80% of the length L, and the fourth coating extends from the second end of the wall-flow filter beyond 20% to 40% of the length L.

3. The wall-flow filter according to claim 1 or 2, Its features are, The ratio of the amount of catalytically active coating, in g / L, between the first coating and the second coating or between the third coating and the fourth coating is less than or equal to 1:1 and greater than or equal to 1:

3.

4. The wall-flow filter according to claim 1 or 2, Its features are, The coatings on the OE and OA surfaces are different in each case.

5. The wall-flow filter according to claim 1 or 2, Its features are, First, the first coating and the third coating are applied to the wall-flow filter before the second coating and the fourth coating are applied.

6. The wall-flow filter according to claim 1 or 2, Its features are, At least one of the first, second, third, and fourth coatings contains 20% to 70% by weight of stable alumina based on the total weight of the coatings, 30% to 80% by weight of rhodium, palladium, or palladium and rhodium based on the total weight of the respective coatings, and one or more oxygen storage components.

7. A method for producing a wall-flow filter according to any one of claims 1 to 6, Its features are, It has the following steps: i) Excess first coating suspension is introduced into the wall-flow filter via the first end by applying a pressure differential through the vertically locked wall-flow filter, and the excess first coating suspension is removed from the wall-flow filter by reversing the pressure differential; ii) Excess third coating suspension is introduced into the wall-flow filter via the second end by applying a pressure differential through the vertically locked wall-flow filter, and the excess third coating suspension is removed from the wall-flow filter by reversing the pressure differential; iii) By applying a pressure differential over the vertically locked wall-flow filter, a second coating suspension is introduced into the wall-flow filter via the first end; iv) By applying a pressure differential over the vertically locked wall-flow filter, a fourth coating suspension is introduced into the wall-flow filter via the second end.

8. The method according to claim 7, Its features are, The coating suspension described in steps iii) and iv) is introduced into a wall-flow filter without excess coating suspension.

9. The method according to claim 7 or 8, Its features are, No separate drying of the wall-flow filter occurs between the coating steps.

10. Use of the wall-flow filter according to any one of claims 1 to 6 in an exhaust system for gasoline engine exhaust gases.

11. The use according to claim 10, Its features are, The exhaust gas reduction unit, selected from three-way catalysts, SCR catalysts, nitrogen oxide storage catalysts, hydrocarbon traps, and nitrogen oxide traps, exists as an additional exhaust gas reduction unit in the exhaust system.

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