Catalytically active particulate filter with high filtration efficiency and oxidation function

Abstract

The invention relates to a wall-flow filter having a catalytically active coating in the channel as a particulate matter filter, which is hermetically closed at opposite closed ends. First, a coating F that increases filtration, a dry powder-gas aerosol having a compound such as AI2O3, SiO2, FeO2, TiO2, ZnO2, ZrO2, zeolite, cordierite or a different silicate, is applied to the filter. A catalytically active coating Z having Pt and / or Pd is then applied over the sub-region or over the entire length (L) of the filter.

Description

[0001] The invention relates to a wall flow filter and a corresponding system comprising a wall flow filter according to the invention. A method for reducing exhaust gas is also a subject of the invention.

[0002] The exhaust gas from internal combustion engines in motor vehicles usually contains harmful gases such as carbon monoxide (CO) and hydrocarbons (HC), nitrogen oxides (NO x ) and possibly sulfur oxides (SO x ) and particles consisting largely of solid carbonaceous particles and possibly adhering organic agglomerates. They are referred to as primary emissions. CO, HC and particles are products of incomplete combustion of the fuel in the engine combustion chamber. When the combustion temperature exceeds 1200°C, nitrogen oxides are formed from nitrogen and oxygen in the cylinder from the intake air. Sulfur oxides are produced by the combustion of organic sulfur compounds, which are always contained in small quantities in non-synthetic fuels. In the future, compliance with the regulatory emission limits applicable to motor vehicles in Europe, China, North America and India will require the removal of the mentioned pollutants from the exhaust gas in large quantities. In order to remove these emissions that are harmful to the environment and health from the exhaust gas of motor vehicles, various catalytic exhaust cleaning technologies have been developed, the basic principle of which is generally based on: the exhaust gas to be cleaned is guided through a flow-through or wall-flow honeycomb body having a catalytically active coating applied thereto. The catalyst promotes the chemical reaction of various exhaust gas components to form harmless products, such as carbon dioxide, water and nitrogen. For removing particle emissions, diesel particulate filters (DPF) or gasoline particulate filters (GPF) / Otto particulate filters (OPF) with and without an additional catalytically active coating are suitable complete sets of equipment.

[0003] The flow-through or wall-flow honeycomb bodies described above are also referred to as catalyst supports, supports, substrates or substrate monoliths, because they carry a catalytically active coating on their surface or in the walls forming the surface. The catalytically active coating is usually applied to the catalyst support in the form of a suspension in a so-called coating process. Many such methods have been published in the past by manufacturers of automotive exhaust gas catalysts (EP1064094B1, EP2521618B1, WO10015573A2, EP1136462B1, US6478874B1, US4609563A, WO9947260A1, JP5378659B2, EP2415522A1, JP2014205108A2).

[0004] Exhaust gases from internal combustion engines (i.e., gasoline engines, for example, running on gasoline or natural gas) which are operated with a predominantly stoichiometric air / fuel mixture (>50% of the operating time) are cleaned in conventional methods using three-way catalysts (TWCs). These catalysts are able to simultaneously convert the three main gaseous pollutants of the engine (i.e., hydrocarbons, carbon monoxide and nitrogen oxides) into harmless components. Stoichiometry means that, on average, the amount of air available for combustion of the fuel present in the cylinder is exactly the amount of air required for complete combustion. The combustion air ratio λ (A / F ratio; air / fuel ratio) sets the air mass m actually available for combustion. Ltats With stoichiometric air mass m Lst The relationship between:

[0005]

[0006] If λ<1 (e.g. 0.9), this means "not enough air" and is called a rich exhaust gas mixture; λ>1 (e.g. 1.1) means "excess air" and the exhaust gas mixture is called a lean exhaust gas mixture. Stating λ=1.1 means that there is 10% more air than required for the stoichiometric reaction. The same applies to the exhaust gas from an internal combustion engine.

[0007] In the known three-way catalysts, platinum group metals are usually used as catalytically active materials, in particular platinum, palladium and rhodium, which are present, for example, on gamma alumina as a support material. In addition, the three-way catalyst contains an oxygen storage material, for example a cerium / zirconium mixed oxide. In the latter, cerium oxide (a rare earth metal oxide) is the basic component for oxygen storage. In addition to zirconium oxide and cerium oxide, these materials may also contain further constituents, such as other rare earth metal oxides or alkaline earth metal oxides. The oxygen storage material is activated by applying a catalytically active material such as a platinum group metal and therefore also serves as a support material for the platinum group metal.

[0008] In order to comply with regulatory standards, for cost reasons and for installation space reasons, it may be desirable to combine a particulate filter with a catalytically active function for current and future internal combustion engine exhaust aftertreatment applications. The use of a particulate filter (whether or not it is catalytically coated) results in a significant increase in exhaust back pressure compared to a flow-through substrate of the same size, and therefore results in a reduction in engine torque or a possible increase in fuel consumption. In order not to further increase the exhaust back pressure, the amount of oxidizing support material or oxidizing catalyst material for the catalytically active element of the catalyst is usually applied to the filter in a smaller amount than the flow-through substrate. Some efforts have been made to provide a particulate filter that has good catalytic activity due to the active coating while still exhibiting the lowest possible exhaust back pressure. With regard to low exhaust back pressure, it has been proven to be advantageous that the catalytically active coating is not located as a layer on the channel wall of a porous wall flow filter, but that the channel wall of the filter is interspersed with catalytically active material, see, for example, WO2005016497A1, JPH01-151706 and EP1789190B1. For this purpose, the particle size of the catalytic coating is selected so that the particles can penetrate into the pores of the wall flow filter and can be fixed there by calcination. A disadvantage of catalytically active filters with inner wall coating is that the amount of catalytically active substance is limited by the retention capacity of the porous wall.

[0009] It has been shown that applying catalytically active substances to the surface of the channel walls of wall-flow honeycombs can achieve an increase in the conversion of pollutants in the exhaust gas. Combinations of wall and wall coatings with catalytically active materials are also possible, which can further increase the catalytic performance without significantly increasing the dynamic pressure, EP 3501648 A1.

[0010] In addition to the catalytic effectiveness, another function of the filter that can be improved by the coating is its filtration efficiency, i.e. the filtering effect itself. WO2011151711A1 describes a method by means of which a dry aerosol is applied to an uncoated or catalytically coated filter carrying a catalytically active material in the channel walls (inner wall coating with a patching substrate coating). The aerosol is provided by distributing a powdered mineral material and is passed through the inlet side of the wall flow filter by means of an air flow. In the process, individual particles with a particle size of 0.2 μm to 5 μm agglomerate to form a bridging network of particles and are deposited as a layer on the surface of each inlet channel extending through the wall flow filter. The typical powder loading of the filter is between 5 g and 50 g per liter of filter volume. It is explicitly pointed out that it is not desirable to obtain a coating in the pores of the wall flow filter with metal oxides.

[0011] Another method for improving the filtration efficiency of catalytically inactive filters is described in WO2012030534A1. In this case, a filter layer ("discrimination layer") is generated on the wall of the flow channel on the inlet side by depositing ceramic particles via a particle aerosol. This layer consists of zirconium, aluminum or silicon oxides, preferably in the form of fibers with a length of 1 nm to 5 μm, and has a layer thickness of more than 10 μm, typically 25 μm to 75 μm. After the coating process, the applied powder particles are calcined in a thermal process.

[0012] Another method is described in patent specification US8277880B2, in which a film ("capture layer") is generated on the surface of the inlet channel of the filter to improve the filtration efficiency of a catalytically inactive wall-flow filter. The filter film on the surface of the inlet channel is realized by sucking a gas stream loaded with ceramic particles (for example, silicon carbide or cordierite) through the inlet channel. After applying the filter layer, the honeycomb is burned at a temperature above 1000° C. to increase the bonding strength of the powder layer on the channel wall. EP2502661A2 and EP2502662B1 mention additional wall coatings applied by powder.

[0013] US8388721B2 describes the coating within the pores of a wall-flow filter substrate by means of atomization of dry particles. In this case, however, the powder should penetrate deeply into the pores. 20% to 60% of the surface of the wall should be accessible to the soot particles and thus remain open. Depending on the flow rate of the powder-gas mixture, a stronger or weaker powder gradient can be adjusted between the inlet side and the outlet side. The pores of the channel walls of the filter coated with powder in the pores according to US8388721B2 can subsequently be coated with catalytically active components. Here, the catalytically active material is also located in the channel walls of the filter.

[0014] EP2727640A1 describes the introduction of powders into the pores, for example by means of an aerosol generator. Here, a wall-flow filter that is not catalytically coated is coated with a gas stream containing, for example, aluminum oxide particles in a manner such that intact particles with a particle size of 0.1 μm to 5 μm are deposited as porous fillers in the pores of the wall-flow filter. In addition to the filtering effect, the particles themselves can also achieve other functions of the filter. For example, these particles are deposited in the pores of the filter in an amount greater than 80 g / L relative to the filter volume. They fill 10% to 50% of the volume of the filled pores in the channel wall. When loaded with soot and without soot, the filter has an improved filtration efficiency at a lower soot-loaded filter exhaust back pressure compared to an untreated filter.

[0015] In WO2018115900A1, a wall-flow filter is coated with a possibly dried synthetic ash such that a continuous film layer is formed on the walls of the possibly catalytically coated wall-flow filter.

[0016] WO 2020 / 047708 and WO 2020 / 047503 also describe wall flow filters in which inorganic secondary particles composed of aluminum oxide and a silicon-containing binder are deposited on the filter with the aid of a gas flow, which leads to an increased durability of the filter layer.

[0017] All prior art patents listed above aim to increase the filtration efficiency of filters by covering the filters with powders or membranes. In this way, wall-flow filters can be produced that are able to filter incoming soot particles almost completely. In this case, deposition rates and filtration efficiencies of more than 99% can be achieved in common test cycles. On the other hand, the catalytic coating of these filters with common washcoats consisting of oxide particles and precious metal components leads to a reduction in the original filtration efficiency of the starting filter, since the introduction of the coating affects the gas flow through the filter and the pore structure of the filter. In this case, it is generally irrelevant whether the catalytic coating is located in the pores of the porous filter wall or on the channel surface of the outlet channel.

[0018] With the extension of the particle number measurement range from the previous 23 nm to 10 nm in the Euro 7 regulation, high filter efficiencies will be indispensable in the future (PN10 measurement). In addition, the new particle number measurement method requires that even very small particles consisting of volatile organic components can be detected, which in turn manifests itself in higher measured particle numbers at the end of the exhaust line. This, in combination with the further statutory reduction of the particle number limit values ​​for internal combustion engines, leads to a future need for wall-flow filters that have very high filter efficiencies and at the same time are able to oxidize the volatile organic particles to form CO2 so that they are no longer detected as particles within the framework of the future required PN10 measurement.

[0019] The object of the present invention is therefore to provide a correspondingly coated particle filter which, on the one hand, retains without reducing the excellent filtration efficiency of a wall flow filter which has been provided with a layer which increases the filtration efficiency and, on the other hand, has applied a precious metal component which catalyzes the oxidation of volatile organic particles. Preferably, the application of the precious metal component changes neither the filtration performance nor the back pressure of the filter.

[0020] These and other objects which are obvious from the prior art are achieved by a particle filter according to claims 1 to 14. Claims 15 and 16 relate to an exhaust system. Claims 17 and 18 relate to a method for exhaust gas cleaning by exhaust gas aftertreatment of an internal combustion engine, in particular an internal combustion engine which is mainly operated in a stoichiometric manner, using a particle filter according to the invention.

[0021] The invention relates to a wall-flow filter for removing particles from the exhaust gas of an internal combustion engine, in particular an internal combustion engine operating predominantly in a stoichiometric manner, comprising a wall-flow filter substrate having a length L and different coatings Z and F, wherein the wall-flow filter substrate comprises channels E and A extending in parallel between a first end and a second end of the wall-flow filter substrate, separated by porous walls and forming a surface O in each case. E and O A , and wherein the channel E is closed at the second end, and the channel A is closed at the first end, and wherein the coating F is located in the porous wall and / or surface O E On and / or on the surface O A and includes a layer that increases filtration, the coating Z being located in the porous wall and / or on the surface O E and / or A On and / or on the end faces of channels E and / or A and / or on components of the coating F and contains platinum and / or palladium and is characterized in that the coating Z extends over a sub-region of at least 3 mm to 100% of the length L of the filter.

[0022] In the intended use of the wall-flow filter according to the invention for cleaning exhaust gases from an internal combustion engine, the exhaust gases flow into the filter at one end and leave the filter again at the other end after passing through the porous wall. For example, if the exhaust gases enter the filter at a first end, channel E represents the input channel or the inflow side channel. After passing through the porous wall, the exhaust gases then leave the filter at a second end, so that channel A represents the output channel or the outflow side channel. L represents the average coatable length of the filter. Due to the plugs at the ends of the channels, it is not necessary to coat the entire length of the filter. L represents the sum of the lengths actually available for coating in the coating channels, averaged over the number of coating channels.

[0023] All ceramic wall-flow filter substrates known from the prior art and commonly used in the field of automotive exhaust catalysis can be used as wall-flow substrates. Preferably, porous wall-flow filter substrates made of cordierite, silicon carbide or aluminum titanate are used. These wall-flow filter substrates have channels E and channels A, which, as described above, act as input channels (also referred to as inflow side channels) and as output channels (also referred to as outflow side channels). The outflow side end of the inflow side channel and the inflow side end of the outflow side channel are closed with a generally airtight "plug" in an offset manner to each other. In this case, the exhaust gas to be cleaned flowing through the filter substrate is forced to pass through the porous wall between the inflow side and outflow side channels, which will result in a particle filtering effect. The porosity, pore / radius distribution and thickness of the wall allow the filtration performance to be designed for particles. According to the present invention, the porosity of the uncoated wall-flow filter substrate is generally greater than 40%, for example 40% to 75%, in particular 50% to 70% [measured according to the latest version of DIN 66133 on the date of submission]. The average pore size d of the uncoated wall-flow filter substrate 50 is at least 4 μm, for example 4 to 34 μm, preferably greater than 6 μm, particularly preferably 6 to 25 μm, or most preferably 7 to 17 μm [measured according to the latest version of DIN 66134 at the date of submission], wherein d 50 The value is understood to mean the value for which 50% of the total pore volume, which can be determined by mercury porosimetry, consists of pores with a diameter less than or equal to that designated as d 50 In the case of the wall flow filter according to the invention, the wall flow filter substrate provided with coating Z preferably has no change in pore size distribution compared to the pore size distribution of the original filter provided only with coating F.

[0024] Coating F:

[0025] According to the invention, the term "increased filtration" is understood to mean that by means of the coating F, the wall-flow filter acquires a better ability to retain particles from the exhaust gas flow than the original filter, without the exhaust gas back pressure increasing excessively compared to the starting substrate. Preferably, this means that the filtration efficiency of the filter increases by more than 2 percentage points, more preferably by more than 5 percentage points, most particularly preferably by more than 10 percentage points compared to the starting substrate, while the exhaust gas back pressure increases by no more than 60%, preferably by less than 50%, most preferably by less than 40% compared to the starting substrate.

[0026] The increase in filtration efficiency of the filter with coating F compared to the original filter is calculated according to the following formula:

[0027] FE increase by coating F> / =(100%-FE F之前 )x increase factor

[0028] The increase factor in the case according to the invention is in the range of 0.2-0.99, preferably in the range of 0.35-0.95, particularly preferably in the range of 0.5-0.85. This is particularly advantageous for filter substrates whose starting substrate already has high filter properties.

[0029] The increase in exhaust back pressure can be calculated according to the following formula:

[0030] Increase in back pressure through coating F = ((back pressure with F / back pressure without F) – 1) x 100

[0031] The coating F advantageously does not contain any noble metals. It is therefore preferably not catalytically active in the sense of the present invention. In particular, it is substantially incapable of oxidizing carbonaceous particles such as soot or volatile hydrocarbon particles, nor of oxidizing the exhaust gas components CO and HC or of reducing NO. x Therefore, the coating F preferably consists of a ceramic or oxide composition and does not contain any other catalytic components.

[0032] Particularly suitable components of the coating F contain aluminum oxide, zirconium dioxide, cerium oxide, yttrium oxide, mullite, tin oxide, silicon nitride, zeolite, titanium dioxide, silicon dioxide, aluminum titanate, silicon carbide, cordierite, island silicates, phyllosilicates, industrial silicates, inosilicates, group silicates or mixtures thereof.

[0033] Wall F:

[0034] The membrane layer preferably has a continuous structure on the surface of the filter wall. E It may happen that a small part of the components of the membrane layer also penetrates into the pores of the filter. However, in another preferred embodiment, the filtration-enhancing coating F is essentially located on the surface O E This means that the components of the coating F are present in the pores of the filter wall in an amount less than 20%, preferably less than 10%, and most preferably less than 5% of the total mass of the coating. The amount of the coating component in the wall can be determined as described in WO2019197177A1.

[0035] The coating F is preferably on the surface O E In particular, the membrane layer is defined as a continuous porous layer, the porosity of which is in the range of 30-90%, preferably 40-80% (measured according to the latest version of DIN 66133 at the date of submission). The average pore size d of the membrane layer F is 50 is at least 50 nm, for example, 50 nm to 5 μm, preferably greater than 100 nm to 4 μm, and more preferably 200 nm to 2.5 μm, wherein the pore size distribution d50 The value is understood to mean the value for which 50% of the total pore volume, which can be determined by mercury porosimetry, consists of pores with a diameter less than or equal to that designated as d 50 The value of pore formation.

[0036] Therefore, the coating F is preferably applied on the wall surface, preferably O E The coating F preferably has a layer thickness of 1 to 150 μm, preferably 2 to 50 μm. The wall flow filter is characterized in that the ratio of the wall thickness of the wall flow filter substrate to the thickness of the film layer of the coating F is preferably 0.8 to 400, preferably 1 to 300, particularly preferably 1.25 to 250. In addition, when coating in the axial direction, accumulation of the coating F on the plug area at the channel end may occur. In a preferred embodiment, the thickness of the coating F here is preferably 0.1 to 10 mm, in particular 0.25 to 5 mm.

[0037] In the embodiments discussed herein, the coating F is therefore preferably located on the porous walls of the wall-flow filter substrate. The particle size of the ceramic membrane is adapted to the pore size of the wall-flow filter substrate. Therefore, in particular, the particles of the coating F have a defined particle size distribution. The coating F preferably has a unimodal or multimodal or broad-peaked q3 particle size distribution. For the definition of the particle size distribution or grain size distribution of the coating F, a distinction is made according to the method by which the amount of particles is determined, in particular between number-related (q0) and volume-related (q3) particle size distributions (M. Stieβ, Mechanische Verfahrenstechnik-Partikeltechnologie 1 [Mechanical Process Technology-Particle Technology 1], Springer, 3rd edition, 2009, page 29).

[0038] If the coating F is located on the porous wall of the wall-flow filter substrate, the particle size distribution of the membrane particles is 50 The value (q3; latest version of DIN 66143 as of the date of submission) is advantageously greater than or equal to d 2 of the pore size distribution of the wall flow filter substrate. 50 Value (measured according to the latest version of DIN 66133 on the date of submission). In addition, the d90 value of the particle size distribution of the coating F is preferably greater than or equal to the d95 value of the pore size distribution of the wall-flow substrate. Alternatively, the d90 value of the particle size distribution of the coating F is preferably less than the d95 value of the pore size distribution of the wall-flow substrate.

[0039] If the coating F penetrates into the porous filter wall, the penetration depth is limited. In particular, the penetration depth of the membrane coating F into the filter wall is a maximum of 50% of the wall thickness, preferably a maximum of 40%, most particularly preferably a maximum of 25%. If the coating F is partially located in the porous wall of the wall flow filter substrate, 1% to 50% of the total mass of the coating F is preferably located in the porous wall of the wall flow filter substrate, preferably 1.5% to 40%, most particularly preferably 2% to 25%. This evaluation is carried out analogously to WO2019197177A1. For example, the coating F is a ceramic membrane, preferably a silicon carbide membrane.

[0040] Intramural F:

[0041] In an alternative embodiment, the coating F is preferably not present on the surface of the filter, preferably on the surface O. E A continuous coating (without membrane) is formed on the surface of the wall-flow substrate. In this case, the filtration-enhancing coating is particularly defined as a discontinuous layer selectively formed over the pores of the wall-flow substrate. In these regions, the porosity of the coating F is in the range of 30-90%, preferably 40-80% (determined similarly to WO2019197177A1). The average pore size d of the coating F is 50 is at least 50 nm, for example, 50 nm to 5 μm, preferably greater than 100 nm to 4 μm, and more preferably 200 nm to 2.5 μm, wherein the pore size distribution d 50 The value is understood to mean the value for which 50% of the total pore volume, which can be determined by mercury porosimetry, consists of pores with a diameter less than or equal to that designated as d 50 These values ​​are determined as described above.

[0042] In the embodiments discussed herein, the coating F is located entirely or partially in the porous walls of the wall flow filter substrate. Preferably, the coating F is present in the porous walls of the wall flow filter substrate in a proportion of 50-100%, preferably 60-90%, most particularly preferably 70-80% of the total mass of the coating F. The evaluation is performed analogously to WO2019197177A1.

[0043] The particle size of the coating F is adapted to the pore size of the wall-flow filter substrate. Thus, in particular, the particles of the coating F have a defined particle size distribution. The coating F preferably has a unimodal or multimodal or broad-peaked q3 particle size distribution. If the coating F is located entirely or partly in the porous walls of the wall-flow filter substrate, the particle size distribution of the oxide of the membrane particles is d 50 The value (q3; latest version of DIN 66143 as of the date of submission) is preferably smaller than d 2 of the pore size distribution of the wall flow filter substrate. 50Value (measured in accordance with the latest version of DIN 66133 as of the filing date). It is further preferred that the d90 value of the particle size distribution of the oxide is in particular smaller than the d95 value of the pore size distribution of the wall flow substrate. In this case, the filtration-increasing coating F preferably consists predominantly of aluminum oxide.

[0044] The coating F, whether continuous or not, can be produced according to methods known to those skilled in the art. The production of the coating F is described in detail in, for example, EP2776144A1, WO2020 / 047506A1, WO2020 / 047708A1. The coating F can also alternatively be applied to the surface O by a wet chemical coating step. E For example, a film coating F is first applied to the surface O E and then apply a coating Z after calcination (EP1789190B1 or US11161098BB).

[0045] In another embodiment, the wall-flow filter is characterized in that the coating F has an increasing concentration gradient (first derivative of the concentration curve) from its first end to its second end in the longitudinal direction of the filter. According to the invention, an "increasing gradient" is understood to mean that the gradient of the powder concentration in the filter increases in the axial direction from the inlet side to the outlet side, possibly from negative values ​​to more positive values. In a preferred embodiment, more powder is present near the output plug of the inlet channel, while the powder at the inlet of the filter is significantly less. In order to describe the gradient, the filter can be divided into three continuous areas of equal length along its longitudinal axis. In a preferred form, the filter is covered with powder in the area near the inlet side and in the central area of ​​the filter on less than 40% of the input channel wall surface in each case, and in the area near the outlet side on more than 40% of the input channel wall surface, wherein in a particularly preferred form, between 5% and 35% of the input channel wall surface in the area near the inlet side, between 8% and 38% of the input channel wall surface in the central area of ​​the filter, and between 40% and 60% of the input channel wall surface in the area near the outlet side are covered with powder, and in a most particularly preferred form, between 5% and 25% of the input channel wall surface in the area near the inlet side, between 8% and 30% of the input channel wall surface in the central area of ​​the filter, and between 45% and 60% of the input channel wall surface in the area near the outlet side are covered with powder. The coverage of the wall surface is determined from the optical microscopy image by means of image analysis. Corresponding images of the inlet channel and the outlet channel are created. In this type of analysis, the average color of the outlet channel wall surface not covered with powder is determined as a reference. This reference was subtracted from the corresponding image of the area covered with powder in the entrance channel, where the color difference was determined according to CIE76 of the International Commission on Illumination, with a minimum distinguishable color difference of 2.33 (https: / / en.wikipedia.org / wiki / Color_difference#CIE76).

[0046] The gradient produced by the powder coating helps to further improve the filtration efficiency. In one embodiment, the concentration gradient is designed, for example, by changing the powder deposition rate, so that more powder is deposited on the inlet side of the filter than in the center of the filter and on the outlet side. In a more preferred embodiment, the concentration gradient can be designed so that more powder is deposited on the inlet side of the filter than in the center of the filter, and more powder is deposited on the outlet side (at the other end of the filter) than on the inlet side. In this regard, the simulation results provide the following images (Table 1).

[0047] Table 1: Powder distribution on the filter at different gas space velocities:

[0048]

[0049] Simulations of the air flow in a wall-flow filter have shown that the last third of the substrate is primarily (more than 50%) responsible for the filtration performance of the total filter. By increasing the application of coating F in the last third of the filter, the dynamic pressure there increases more due to the lower permeability and the flow is diverted more into the first two thirds of the filter. Therefore, the filter should have a stronger increasing gradient of coating F from the first end to the second end in order to increase its filtration effect. This applies mutatis mutandis to the adjustment of a favorable exhaust gas back pressure. Therefore, a less strong increasing gradient of the concentration of coating F should be chosen here, if possible.

[0050] Preferably, the wall-flow filter comprises a coating F having a mass of 1 to 50 g / L relative to the volume of the wall-flow filter substrate. Further preferably, the mass of the coating F is 5 to 40 g / L, most preferably 10 to 30 g / L relative to the volume of the wall-flow filter substrate.

[0051] Preferably, the components of the coating F are fixed to the ceramic wall-flow substrate. This can preferably be achieved by using a suitable binder. Such binders are well known to those skilled in the art and are disclosed, for example, in WO2020047506A1. Thermal fixation of the coating F is also preferred. Those skilled in the art can also learn known methods in this regard, for example, from WO2012030533A1 and EP21164198.0.

[0052] As an alternative to the powder process just discussed, the coating F can also be applied by wet techniques, just like a normal washcoat. In this case, it can extend over the entire length L of the wall-flow filter substrate or only over a portion thereof. For example, the coating F extends over 0 to 100%, 25% to 80% or 40% to 60% of the length L of the filter substrate. An extension over 80%–100%, in particular over 90%–100% of the length L is preferred. In one embodiment of the wall-flow filter according to the invention, the coating F extends over the entire length L of the channel E and adjoins the plug at the end of the channel.

[0053] Coating Z:

[0054] The coating Z is an oxidation-catalytically active coating. With the aid of this coating, HC and CO as well as soot particles and volatile hydrocarbon particles in the exhaust line are oxidized. The person skilled in the art is familiar with the appearance of such coatings (e.g., US11161098BB). In the present case, the coating Z is characterized in that it has almost no ceramic or oxide parts. Therefore, the coating preferably has a precious metal mass in the coating Z of 0.1-20 g / L filter volume. In this context, 1-15 g / L filter volume is preferred, 2-10 g / L filter volume is particularly preferred. If the coating Z extends only over a sub-region of the filter having a length L, a higher mass of 5-30 g / L, preferably 5-25 g / L, may occur in this region.

[0055] In order to achieve better dispersion and better distribution, the coating Z may also contain a small amount of oxide carrier material in addition. Advantageously, there may be 5-40 g / L of ceramic or oxide carrier material in the coating Z, preferably between 7-25 g / L, and particularly preferably between 10-15 g / L of the filter substrate volume. In this case, common carrier materials are aluminum oxide, boehmite, aluminum sol or other materials known to those skilled in the art and disclosed in EP3505246A1. If present, the ceramic or oxide component is preferably selected from aluminum oxide or boehmite. The coating Z preferably does not contain zeolite and does not contain molecular sieves. If the coating Z extends only over a sub-region of a filter having a length L, a higher mass of 10-85 g / L, preferably 20-60 g / L, may occur in this region.

[0056] In a preferred embodiment, the coating Z extends over at least an axial sub-area of ​​the length L of the filter. In a preferred embodiment, the minimum length of the coating Z is at least 3 mm, preferably at least 5 mm, most preferably at least 10 mm from the end of the filter. In another preferred embodiment, the maximum length of the coating Z is 5-100%, preferably 10-100%, most preferably 20-100% relative to the length L of the filter. In another preferred embodiment, the coating Z extends over the entire length L of the filter.

[0057] Platinum and / or palladium are preferably considered as precious metals in the coating Z. The mass ratio of platinum to palladium is preferably 0.05-20, more preferably 0.1-10, most preferably 1.5-5. Platinum and / or rhodium are also preferably considered as precious metals in the coating Z. The mass ratio of platinum to rhodium is preferably 0.05-20, more preferably 0.1-10, most preferably 1.5-5. Platinum and / or rhodium and / or palladium are also preferably considered as precious metals in the coating Z. The mass ratio of platinum to palladium to rhodium is preferably 1–10:1–10:1–2, more preferably 1–10:1–7:1–1.5. The coating Z particularly preferably contains the precious metals platinum and / or palladium, only as an exception, wherein rhodium is also advantageously present as a further precious metal. Particularly preferably, the coating Z contains platinum and palladium without rhodium. In another embodiment, the coating Z contains the precious metals platinum and / or rhodium, only as an exception, wherein palladium is also preferably present as a further precious metal. Particularly preferably, the coating Z contains platinum and rhodium without palladium. In another embodiment, the coating Z contains the noble metals platinum, palladium and / or rhodium. It is also particularly advantageous in this embodiment if the mass ratio of platinum to palladium is 0.05-20, preferably 0.07-15. The noble metals of the coating Z are preferably not fixed on a support material (ceramic or oxide part of the coating Z). However, they can be deposited on oxides and constituents of the coating F during the coating process.

[0058] The generation of the coating medium of coating Z is known to those skilled in the art (e.g., US11161098BB). Coating Z can be applied to a wall-flow filter according to methods known to those skilled in the art (e.g., US11161098BB). In the simplest case, one or more precious metals are dissolved in water and applied to a wall-flow filter in this form. Since there is almost no oxide or ceramic component in coating Z, the coating has a very low viscosity. The viscosity of the coating medium is preferably less than 0.1Pas*s, more preferably less than 0.05Pas*s, and most preferably less than 30Pas*s, in each case measured at a shear rate (revolutions per second) of 100 1 / s (similar to WO2020109778A1 measurement). Viscosity can be adjusted with common thickeners such as methylcellulose.

[0059] In one embodiment according to the invention, the coating Z is located in the porous wall and / or on the surface O E and / or A On the end face of channel E and / or A and / or on a component of coating F. Preferably, channel O E and / or AThe end faces of the filter substrate are also provided with a coating Z. In this embodiment, the ends E and / or A of the filter are coated with a coating Z on the outside of the filter at the plugs at the respective ends. It is also particularly preferred that the coating Z enters the pores of the porous filter wall. In another advantageous embodiment, the coating Z is completely located in the pores of the filter substrate. The coating Z can also be deposited additionally on particles of the coating F. In addition, the coating Z can also be located on the surface O A superior.

[0060] The coating Z is a catalytically active coating, in particular due to the constituents platinum and / or palladium and / or rhodium. In the context of the present invention, the term "catalytically active" is understood to mean the oxidation of carbonaceous particles such as soot or volatile hydrocarbon particles or the oxidation of exhaust gas components CO and HC or the reduction of NO x The coating Z is catalytically active in particular at operating temperatures of 200 to 1100° C. In particular, the term “catalytically active” is understood to mean that at an exhaust gas temperature of 500° C., a lambda value of 1.05 and an exhaust gas mass flow of 70 kg / h, a filter comprising coatings F and Z can oxidize 10% more soot and / or hydrocarbons, preferably 15% more, most particularly preferably 20% more, within the same time unit than a filter comprising only coating F.

[0061] Preferably, the coating Z has no appreciable layer thickness and does not extend beyond the surface O. A and / or E The layer thickness of the coating Z is preferably less than 5 μm, particularly preferably less than 3 μm, most particularly preferably less than 1 μm.

[0062] The wall flow filter according to the invention can be produced by applying coatings Z and F to a wall flow filter substrate. In a preferred embodiment of the wall flow filter according to the invention, the filter substrate is firstly applied with coating F and then with coating Z. This has the advantage that coating Z is not only deposited on the surface O E , O A on and in the porous filter walls, but can also additionally be deposited on the particles of coating F.

[0063] The application of the coating Z in the case according to the invention is accompanied by only a very small increase in the exhaust gas back pressure. Compared to a filter comprising only the coating F, the back pressure increases by only 15%, preferably only 10%, particularly preferably only 5%, by the application of the coating Z, wherein the increase in the exhaust gas back pressure can be calculated according to the following formula:

[0064] Backpressure increase through Z = ((backpressure with F and Z / backpressure with F but no Z) – 1) x 100

[0065] The application of the coating Z in the case according to the invention is accompanied by only a very small change in the filter efficiency of the filter. Compared to a filter comprising only the coating F, the filter efficiency changes by applying the coating Z by only 3 percentage points, preferably by only 2 percentage points, particularly preferably by only 1 percentage point. In the sense of the present invention, the term "change" is understood to mean both an increase and a decrease within the specified range.

[0066] The increase in filtration efficiency is calculated according to the following formula.

[0067] Increased filtration efficiency through Z = ((filtration efficiency with F and Z / filtration efficiency with F but no Z) – 1) x 100

[0068] If the filtration efficiency is slightly reduced due to the introduction of Z, the reduction is calculated according to the following formula.

[0069] Filtration efficiency reduction through Z = (1-(filtration efficiency with F and Z / filtration efficiency with F but no Z)) x 100

[0070] The subject of the present invention is also a system for cleaning exhaust gases from an internal combustion engine, in particular from an internal combustion engine that is primarily operated in a stoichiometric manner, the system comprising a wall-flow filter according to the invention. Further preferred is a system for cleaning exhaust gases, characterized in that, in the direction of the exhaust gases, the wall-flow filter according to the invention is arranged as a separate unit in the exhaust gas line after a three-way catalyst close to the engine. In particular, it will be advantageous if the three-way catalyst is located directly on the inflow side of the wall-flow filter according to the invention in a position close to the engine. It will also be advantageous if the three-way catalyst is located on the outflow side of the wall-flow filter according to the invention. It will also be advantageous if the three-way catalyst is located on the inflow side and on the outflow side of the wall-flow filter according to the invention. In particular, a system for cleaning exhaust gases is also preferred, characterized in that, in the direction of the exhaust gases, after the wall-flow filter according to the invention, there is at least one additional catalyst, the at least one additional catalyst being selected from the group consisting of a three-way catalyst, an oxidation catalyst, a NO x Storage catalyst, hydrocarbon trap, SCR catalyst, ammonia slip catalyst. The preferred embodiments described for the wall-flow filter according to the invention also apply mutatis mutandis to the system mentioned here.

[0071] Alternatively, the present invention also relates to an exhaust gas cleaning system comprising a filter according to the present invention and at least one further catalyst. In one embodiment of this system, the at least one further catalyst is arranged upstream of the filter according to the present invention. Preferably, the catalyst is a three-way catalyst or an oxidation catalyst or a NO xIn another embodiment of the system, at least one additional catalyst is arranged downstream of the filter according to the invention. Preferably, the catalyst is a three-way catalyst or an SCR catalyst or a NO x In another embodiment of the system, at least one additional catalyst is arranged upstream of the filter according to the invention and at least one additional catalyst is arranged downstream of the filter according to the invention. Preferably, the upstream catalyst is a three-way catalyst or an oxidation catalyst or a NO x Storage catalyst, downstream catalyst is three-way catalyst or SCR catalyst or NO x Storage catalyst or ammonia slip catalyst.The preferred embodiments described for the wall flow filter according to the invention also apply mutatis mutandis to the exhaust gas cleaning system mentioned here.

[0072] The subject of the invention is also a method for cleaning the exhaust gas of an internal combustion engine, wherein the exhaust gas of the engine is conducted through a wall flow filter according to the invention or a system comprising the wall flow filter. Preferably, the internal combustion engine is an internal combustion engine which is mainly operated in a stoichiometric manner. Preferred and alternative embodiments of the wall flow filter and the system according to the invention also apply mutatis mutandis here.

[0073] In the sense of the present invention, the term "close to the engine" refers to a region in the exhaust line which is located close to the engine, ie approximately 10-80 cm, preferably 20-60 cm from the engine outlet.

[0074] The term "coating" is therefore to be understood as meaning the application of catalytically active material to a wall-flow filter substrate. The coating Z assumes the actual catalytic function. In the present case, the coating is carried out by applying an aqueous suspension of a corresponding, preferably catalytically active component to the wall of the wall-flow filter substrate, for example according to EP 1 789 190 B1 or US 1 1 161 098 BB. After application of the suspension, the wall-flow filter substrate is in each case dried and, where appropriate, calcined at elevated temperature. The most suitable loading of the filter coating in the wall depends on its cell density, wall thickness and porosity.

[0075] The catalytically coated wall-flow filters according to the invention are different from those filters which are formed in the exhaust gas line of a vehicle during operation by ash deposition. According to the invention, the catalytically active wall-flow filter substrate is provided specifically with a coating or, where appropriate, with a membrane coating F. This results in the balance between filter efficiency and exhaust gas back pressure being able to be adjusted specifically from the outset. The invention therefore does not include wall-flow filters in which undefined ash deposition has occurred from the combustion of the fuel (for example in the cylinder, during driving operation or by means of a burner).

[0076] Compared to a wall-flow filter comprising only coating F but not coating Z, the wall-flow filter according to the invention shows an excellent filtration efficiency and no increase or only a very small increase in the exhaust gas back pressure. Preferably, the wall-flow filter according to the invention shows an unchanged soot particle deposition (filtering effect) compared to the starting filter comprising only coating F. The low total back pressure may be attributed to the fact that the presence of coating F does not significantly reduce the cross-section of the channel on the input side. It is presumed that coating F forms a porous structure, which has a positive effect on the dynamic pressure. Through coating F, the filter according to the invention also has a lower dynamic pressure after soot loading than a similar filter without coating F, because the coating largely prevents soot from penetrating into the porous filter wall. By being able to lower the ignition point of soot particles or hydrocarbon particles and thus promote their oxidation, coating Z gives the wall-flow filter according to the invention an excellent catalytic ability to oxidize soot particles or hydrocarbon particles.

[0077] The filter according to the present invention is usually mainly used in internal combustion engines, in particular direct injection or intake manifold injection internal combustion engines. Preferably, these engines are gasoline or natural gas engines that mainly operate in a stoichiometric manner. Preferably, these engines are turbocharged engines. The requirements of gasoline particulate filters (GPF) are significantly different from those of diesel particulate filters (DPF). In terms of particle mass, a diesel engine without a DPF can have up to ten times higher particle emissions than a gasoline engine without a GPF (Maricq et al., SAE 1999-01-01530). In addition, there are significantly fewer primary particles in a gasoline engine, and the secondary particles (agglomerates) are significantly smaller than those in a diesel engine. The emissions of gasoline engines are in the particle size range of less than 200nm (Hall et al., SAE 1999-01-3530) to 400nm (Mathis et al., Atmospheric Environment 38, 4347), with a maximum value in the range of about 60nm to 80nm. Therefore, the filtration of nanoparticles in GPF must be mainly carried out via diffusion separation. For particles less than 300 nm, as the size decreases, separation by diffusion (Brownian molecular motion) and electrostatic forces becomes increasingly important (Hinds, W.: Aerosol technology: Properties and behavior and measurement of airborne particles. Wiley, 2nd edition, 1999). However, the wall-flow filter according to the present invention can also be used in the diesel field.

[0078] The coatings Z, F can be arranged in various ways on the wall-flow filter substrate. Figures 1 to 10 This is explained by way of example, where Figures 2 to 10 It is a wall-flow filter according to the invention which comprises coatings Z and F only.

[0079] Figure 1 Involving surface O E and O A Wall-flow filter substrate.

[0080] Figure 2 Involved on the surface O E A wall-flow filter including coating F is provided.

[0081] Figure 3 A wall-flow filter according to the present invention comprises a E Coating F on the filter and a short zone of coating Z in the input area of ​​the filter.

[0082] Figure 4 A wall-flow filter according to the present invention comprises a E The coating F on the stopper and a short zone of coating Z in the inlet area of ​​the filter, wherein the coating F is also located on the end face of the stopper.

[0083] Figure 5 A wall-flow filter according to the present invention comprises a E The coating F on the filter and the longer area of ​​the coating Z in the input area of ​​the filter.

[0084] Figure 6 A wall-flow filter according to the present invention comprises a E A short zone of coating F on the filter and coating Z in the output area of ​​the filter.

[0085] Figure 7 A wall-flow filter according to the present invention comprises a E The coating F on the filter and a longer area of ​​coating Z in the output area of ​​the filter.

[0086] Figure 8 A wall-flow filter according to the present invention comprises a E A coating F and a coating Z on the filter, wherein the coating Z extends over the entire filter.

[0087] Fig. 9 It relates to a wall-flow filter according to the present invention and schematically shows that the coating Z can be located on the surface O of the filter substrate. A on and / or in the pores and / or on the particles of coating F.

[0088] Fig.10It relates to a wall-flow filter according to the invention and schematically shows an image on which it can be seen that the coating Z extends over approximately 50% of the length of the filter.

[0089] Fig.11 A comparison is shown between a filter without coating Z (black) and a filter according to the invention with coating Z (grey) in a particle oxidation test.

[0090] Figures 2 to 10 Different coating arrangements described in more detail above are shown for a wall flow filter according to the invention. The reference numerals used therein are:

[0091] (E) Input channel / inflow side channel of wall flow filter

[0092] (A) Outlet channel / outflow side channel of wall flow filter

[0093] (O E )The surface formed by the input channel (E)

[0094] (O A ) The surface formed by the output channel (A)

[0095] (L) Length of filter wall

[0096] (Z) Coating Z

[0097] (F) Coating F

[0098] The present invention is explained below with reference to Examples.

[0099] According to embodiment 1 of the present invention:

[0100] The ceramic filter substrate is provided with a filter coating F consisting of a mineral phyllosilicate of approximately 14 g / L by means of the aerosol method (EP 3774036 A1), which is then thermally fixed at 900° C. The filter is then coated with a noble metal solution over 50% of the total filter length, then dried and calcined. The noble metal concentration calculated based on the total filter volume is 3.355 g / L. The noble metal concentration in the region of the coating Z is correspondingly 6.708 g / L.

[0101] The back pressure and particle oxidation capacity of the filters obtained in this way were then investigated on an engine test bench and compared with a control filter without coating Z.

[0102] First, on a cold gas test bench at 600m 3 The pressure loss of the filter was measured at an air flow of 1 / h before and after coating with Z. Before coating with Z, the back pressure of the filter was 68.5 mbar, while after coating with Z, the back pressure was 68.6 mbar.

[0103] Both filters were then initially loaded with 7 g of soot on a direct injection gasoline engine and subsequently weighed. Thereafter, the filter was again installed in the exhaust line of the engine and brought to a temperature of 500° C. with rich exhaust gas with exclusion of oxygen. Subsequently, the exhaust gas composition was changed from rich exhaust gas to lean exhaust gas (λ1.1), and the reduction in back pressure over time was then recorded. Fig.11 As can be seen in the graph, the back pressure of the soot-laden filter according to the invention (grey / Example 1) decreases significantly faster than the back pressure of the comparative filter without coating Z. This is particularly surprising since, contrary to the applicable technical literature, no support materials with a high specific surface area such as aluminum oxide, cerium oxide, cerium / zirconium mixed oxides or other oxides known to the person skilled in the art are required to catalyze the oxidation of the particles. Fig.11 It can also be seen that within the first 500 seconds of the test, the filter according to the invention comprising coating Z has the same back pressure as an equivalent filter without coating F. The filtration efficiency of the filter according to the invention in the WLTP cycle is 84.7%, which also remains unchanged compared to the filtration efficiency of the reference filter.

[0104] Therefore, the combination of coating F and coating Z provides an elegant way to catalyze the oxidation of soot particles, especially in the range of 10-23 nm, without degrading the original physical properties of the filter, such as pressure loss.

Claims

1. A wall-flow filter for removing particles from the exhaust gas of an internal combustion engine, in particular an internal combustion engine operating predominantly in a stoichiometric manner, the wall-flow filter comprising a wall-flow filter substrate having a length L and different coatings Z and F, wherein the wall-flow filter substrate comprises channels E and A extending in parallel between a first end and a second end of the wall-flow filter substrate, separated by porous walls and forming a surface O respectively. E and O A , and wherein the channel E is closed at the second end, and the channel A is closed at the first end, and wherein the coating F is located in the porous wall and / or the surface O E On and / or on the surface O A and comprises a layer that increases filtration, and the coating Z is located in the porous wall and / or the surface O E and / or A on the surface and / or on the end surface of the channel E and / or A and / or on a component of the coating F and contains platinum and / or palladium, characterized in that The coating Z extends over a sub-region of at least 3 mm to 100% of the length L of the filter.

2. The wall flow filter according to claim 1, Features The coating F is selected from alumina, zirconium dioxide, cerium oxide, yttrium oxide, mullite, tin oxide, silicon nitride, zeolite, titanium dioxide, silicon dioxide, aluminum titanate, silicon carbide, cordierite, island silicate, phyllosilicate, industrial silicate, inosilicate, group silicate or a mixture thereof.

3. The wall flow filter according to claim 1 and / or 2, Features The filtration-enhancing coating F is substantially located on the surface O E superior.

4. The wall-flow filter according to claim 3, Features The coating F has a layer thickness of 1 to 150 μm.

5. A wall flow filter according to any one of claims 3 or 4, Features The ratio of the wall thickness of the wall flow filter substrate to the thickness of the coating F is 0.8 to 400.

6. A wall flow filter according to claim 3, 4 or 5, Features The coating F is located in the porous wall of the wall flow filter substrate at 1% to 50% of the total mass of the coating F.

7. The wall flow filter according to any one of claims 1 or 2, Features The coating F that increases filtration is not on the surface O E A continuous coating is formed on the surface.

8. A wall-flow filter according to any one of the preceding claims, Features The coating F has an increasing concentration gradient in the longitudinal direction of the filter from its first end to its second end.

9. A wall-flow filter according to any one of the preceding claims, Features Coating F advantageously has a mass of 1 to 50 g / L relative to the volume of the wall-flow filter substrate.

10. A wall flow filter according to any one of the preceding claims, Features The mass of the noble metal of the coating Z is 0.1-20 g / L filter volume.

11. A wall-flow filter according to any one of the preceding claims, Features The coating Z also contains a small amount of ceramic or oxide components in the range of 5-40 g / l filter substrate volume.

12. A wall flow filter according to any one of the preceding claims, Features The ceramic or oxide component of the coating Z is alumina or boehmite.

13. A wall-flow filter according to any one of the preceding claims, Features The mass ratio of platinum to palladium is 0.05-20.

14. A wall flow filter according to any one of the preceding claims, Features To the filter substrate, coating F is first applied and then coating Z is applied.

15. A system for cleaning exhaust gas from an internal combustion engine, in particular an internal combustion engine operating predominantly in a stoichiometric manner, the system comprising a wall-flow filter similar to that described in claims 1-14, the wall-flow filter being arranged downstream of a three-way catalyst close to the engine.

16. The system for cleaning exhaust gas according to claim 15, characterized in that Downstream of the wall flow filter, there is at least one additional catalyst selected from a three-way catalyst, an oxidation catalyst, a NO x Storage catalysts, hydrocarbon traps, SCR catalysts, ammonia slip catalysts.

17. A method for reducing the exhaust gas of an internal combustion engine, in particular an internal combustion engine operating predominantly in a stoichiometric manner, wherein the exhaust gas of the engine is conducted through a wall-flow filter according to any one of claims 1 to 14.

18. A method for reducing exhaust gas from an internal combustion engine operating primarily in a stoichiometric manner, wherein the exhaust gas from the engine is directed through A system according to claim 15 or 16.

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