Wall flow filters with high filtration efficiency

Abstract

Method for producing a wall flow filter for purifying gases of small particulate solids, wherein the inlet area of ​​the dry filter is supplied with a dry powder-gas aerosol, characterized in that the powder comprises a pyrogenic high-melting-point metal compound and the amount of pyrogenic high-melting-point compound in the filter is less than 5 g / l, wherein the pyrogenic high-melting-point compound is subjected to a shear force before the filter is supplied.

Description

[0001] The present invention relates to a method for manufacturing a wall flow filter for separating fine particulate solids from gases and its use for cleaning exhaust gases from an internal combustion engine. The invention also relates to a correspondingly manufactured exhaust gas filter with high filtration efficiency.

[0002] The exhaust gases from combustion engines in motor vehicles typically contain the harmful gases carbon monoxide (CO) and hydrocarbons (HC), nitrogen oxides (NOx). x ) and, if applicable, sulfur oxides (SO₄) x), as well as particles consisting largely of solid carbonaceous particles and, where applicable, adhering organic agglomerates. These are referred to as primary emissions. CO, HC, and particles are products of the incomplete combustion of fuel in the engine's combustion chamber. Nitrogen oxides are formed in the cylinder from nitrogen and oxygen in the intake air when combustion temperatures exceed 1200°C. Sulfur oxides result from the combustion of organic sulfur compounds, which are always present in small quantities in non-synthetic fuels. Compliance with future legal emission limits for motor vehicles in Europe, China, North America, and India requires the extensive removal of these pollutants from the exhaust gas.To remove these emissions, which are harmful to the environment and health, from the exhaust gases of motor vehicles, a variety of catalytic exhaust gas purification technologies have been developed. Their basic principle is usually based on passing the exhaust gas to be cleaned over a flow-through or wall-flow honeycomb structure with a catalytically active coating applied to it. The catalyst promotes the chemical reaction of various exhaust gas components, forming harmless products such as carbon dioxide, water, and nitrogen.

[0003] The flow-through or wall-flow honeycomb structures described above are also referred to as catalyst supports, carriers, or substrate monoliths, as they carry the catalytically active coating on their surface or within the walls that form this surface. The catalytically active coating is often applied to the catalyst support in a process called coating, usually as a suspension.

[0004] Many such processes have been published in the past by automotive catalytic converter manufacturers (EP 1 064 094 B1, EP 2 521 618 B1, WO 2010 / 015 573 A2, EP 1 136 462 B1, US 6 478 874 B1, US 4 609 563 A, WO 99 / 47 260 A1, JP S53- 78 659 B2, EP 2 415 522 A1, JP 2014- 205 108 A)

[0005] The operating mode of the combustion engine is crucial for the possible methods of pollutant conversion in the catalytic converter. Diesel engines are usually operated with excess air, while most gasoline engines use a stoichiometric mixture of intake air and fuel. Stoichiometric means that, on average, exactly as much air is available for the combustion of the fuel present in the cylinder as is required for complete combustion. The air-fuel ratio λ (air / fuel ratio) defines the actual mass of air m available for combustion. L,tats relative to the stoichiometric air mass m L,st : λ=mL.tatsmL,st If λ < 1 (e.g., 0.9), this means "lack of air," and the exhaust mixture is described as rich. If λ > 1 (e.g., 1.1), this means "excess air," and the exhaust mixture is described as lean. The statement λ = 1.1 means that there is 10% more air present than would be necessary for a stoichiometric reaction.

[0006] When this text refers to lean-burning motor vehicle engines, it primarily refers to diesel engines and, on average, lean-burning gasoline engines. The latter are predominantly gasoline engines that operate with a lean air-fuel ratio (A / F ratio). In contrast, most gasoline engines operate with a stoichiometric combustion mixture on average. The term "on average" takes into account the fact that modern gasoline engines do not operate statically at a fixed air-fuel ratio (A / F ratio; λ-value). Rather, the engine control unit specifies a mixture with a discontinuous air-fuel ratio (λ) around λ = 1.0, resulting in a periodic alternation of oxidizing and reducing exhaust gas conditions. This alternation of the air-fuel ratio (λ) is essential for the exhaust gas purification result. For this purpose, the λ-value of the exhaust gas is measured with very short cycle times (approx.The exhaust gas frequency (0.5 to 5 Hertz) and an amplitude Δλ of 0.005 ≤ Δλ ≤ 0.07 are regulated around the value λ = 1.0. On average, the exhaust gas in such operating conditions can therefore be described as stoichiometric. To prevent these deviations from adversely affecting the exhaust gas purification result when the exhaust gas passes through the three-way catalytic converter, the oxygen storage materials contained in the three-way catalytic converter compensate for these deviations by absorbing or releasing oxygen from the exhaust gas as needed (R. Heck et al., Catalytic Air Pollution Control - Commercial Technology, Wiley, 2nd edition 2002, page 87). However, due to the dynamic operation of the engine in the vehicle, further deviations from this state occur temporarily. For example, during strong acceleration or deceleration, engine operating conditions, and thus exhaust gas conditions, can be found that are, on average, above- or below-stoichiometric.Lean-burning gasoline engines, on the other hand, have an exhaust gas which predominantly, i.e., for the majority of the combustion operation, burns an average lean air / fuel ratio.

[0007] The harmful gases carbon monoxide and hydrocarbons can be rendered harmless from a lean exhaust gas by oxidation on a suitable oxidation catalyst. In a stoichiometrically operated combustion engine, all three harmful gases (HC, CO, and NOx) can be eliminated via a three-way catalytic converter.

[0008] Reducing nitrogen oxides to nitrogen (“deniosis” of the exhaust gas) is more difficult due to the high oxygen content of a lean-burn engine. A well-known method is selective catalytic reduction (SCR) of nitrogen oxides using a suitable catalyst. This method is currently considered the preferred approach for denitrification of lean-burn engine exhaust gases. In the SCR process, the reduction of nitrogen oxides in the exhaust gas is achieved with the aid of a reducing agent injected into the exhaust stream from an external source. Ammonia is used as the reducing agent, which converts the nitrogen oxides present in the exhaust gas to nitrogen and water at the SCR catalyst.The ammonia used as a reducing agent can be made available by dosing an ammonia precursor compound, such as urea, ammonium carbamate or ammonium formate, into the exhaust stream and subsequent hydrolysis.

[0009] Diesel particulate filters (DPF) and gasoline particulate filters (GPF), with and without additional catalytically active coatings, are suitable devices for removing particulate emissions. To meet legal standards, it is desirable for current and future applications in the exhaust aftertreatment of combustion engines to combine particulate filters—especially wall-flow type filters—with other catalytically active functionalities, both for cost reasons and due to space constraints. The use of a particulate filter—whether catalytically coated or not—leads to a noticeable increase in exhaust backpressure compared to a flow-through filter of the same dimensions, and thus to a reduction in engine torque or potentially increased fuel consumption. To prevent a further increase in exhaust backpressure, the quantities of oxide support materials for the catalytically active precious metals of the catalyst are reduced.Oxide catalyst materials are typically applied in smaller quantities to a filter than to a flow-through filter. As a result, the catalytic effectiveness of a catalytically coated particle filter is often inferior to that of a similarly sized flow-through monolith.

[0010] Several efforts have already been made to provide particulate filters that exhibit good catalytic activity through an active coating while still maintaining the lowest possible exhaust backpressure. One advantage has been found to be that the catalytically active coating is not applied as a layer to the wall of a porous wall-flow filter, but rather that the filter wall is penetrated by the catalytically active material (WO 2005 / 016 497 A1, JP H01-151 706 A, EP 1 789 190 B1). For this purpose, the particle size of the catalytic coating is selected so that the particles can penetrate the pores of the wall-flow filter and be fixed there by calcination.

[0011] Another filter function that can be improved by a coating is its filtration efficiency, i.e., the filtering effect itself. Increasing the filtration efficiency of non-catalytically active filters is described in WO2012030534A1. Here, a filtration layer ("discriminating layer") is created on the walls of the inlet-side flow channels by depositing ceramic particles via a particle aerosol. The layers consist of oxides of zirconium, aluminum, or silicon, preferably in fiber form from 1 nm to 5 µm, and have 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] A coating within the pores of a wall flow filter unit by atomizing dry particles is described in US 8,388,721 B2. Here, however, the powder is intended to penetrate deep into the pores. Between 20% and 60% of the wall surface is to remain accessible to soot particles, and thus open. Depending on the flow velocity of the powder-gas mixture, a more or less pronounced powder gradient between the inlet and outlet sides can be set.

[0013] The introduction of the powder into the pores, e.g., using an aerosol generator, is also described in EP 2 727 640 A1. Here, a non-catalytically coated wall flow filter is coated with a gas stream containing, for example, aluminum oxide particles, such that the complete particles, which have a particle size of 0.1 µm to 5 µm, are deposited as a porous filling in the pores of the wall flow filter. The particles themselves can provide an additional functionality of the filter beyond its filtration effect. For example, these particles are deposited in the pores of the filter at a quantity of more than 80 g / l, based on the filter volume. They fill 10% to 50% of the volume of the filled pores in the channel walls. This filter exhibits improved filtration efficiency compared to the untreated filter, both when loaded with soot and when unloaded, while the soot-loaded filter has a lower exhaust backpressure.

[0014] Further methods for coating filters by powder application are mentioned in EP 2 502 661 A2 and EP 2 502 662 B1. These documents also describe corresponding apparatus for applying a powder-gas aerosol to the filter, in which the powder applicator and the wall-flow filter are separated, allowing air to be drawn in through this space during the coating process. Another method, in which a membrane ("trapping layer") is created on the surfaces of the inlet channels of catalytically inactive wall-flow filters to increase their filtration efficiency, is described in patent US 8277880B2. The filtration membrane on the inlet channel surfaces is created by drawing through a gas stream loaded with ceramic particles (e.g., silicon carbide, cordierite).After the filter layer is applied, the honeycomb structure is fired at temperatures above 1000°C to increase the adhesion of the powder layer to the channel walls.

[0015] WO 2011 / 151 711 A1 describes a method for loading an uncoated or catalytically coated filter with a dry aerosol. The aerosol is provided by distributing a powdered high-melting-point metal oxide with a mean particle size of 0.2 µm to 5 µm and is guided by a gas stream over the inlet side of a wall-flow filter. Here, the individual particles agglomerate into a bridged network and are deposited as a layer on the surface of the individual inlet channels traversing the wall-flow filter. The typical powder loading of a filter is between 5 g and 50 g per liter of filter volume. It is explicitly stated that it is not desirable to achieve a coating of the metal oxide within the pores of the wall-flow filter.

[0016] EP 1 576 998 A2 describes the fabrication of a thin membrane, <5 µm, on the outlet side of the porous cell wall. The porous membrane is made from nanoparticles with diameters between 20 and 200 nm. A final calcination is performed to fix the membrane to the outlet side of the wall-flow filter. WO 2017 / 075 328 A1 also discloses filter articles comprising a porous ceramic structure with a plurality of channels separated by a plurality of porous inner walls, and a nanomembrane arranged on at least a portion of the surface of the porous ceramic structure. The nanomembrane comprises nanoparticles of at least one inorganic oxide, and the nanoparticles are present in a concentration ranging from approximately 0.001 g / l to approximately 1 g / l, based on the total volume of the porous ceramic structure.

[0017] US 9,745,227 B2 describes the production of a coating layer with porous particle agglomerates with a diameter between 10 and 200 µm. These agglomerates are, in turn, produced in a preceding process from particles with dimensions between 0.01 and 5 µm. The applied layer must then be calcined.

[0018] WO 2018 / 115 900 A1 refers to the oxide powders as synthetic ash with a d90 < 1 µm. The filters are coated with this material in such a way that a packed bed of synthetic ash is formed on the filter walls.

[0019] However, there are further requirements for particulate filters, for which solutions are still being sought. These include, for example, improved soot combustion and the introduction of additional reactive zones to control the catalytic reactions.

[0020] The object of the present invention was therefore to provide further and improved particulate filters, also called wall flow filters, which are optimized with regard to their filtration efficiency and the resulting exhaust backpressure. Furthermore, the filters should be easy to manufacture using a robust and flexible process and be cost-effective.

[0021] These and other problems, which are obvious to a person skilled in the art from the prior art, are solved by a method according to independent claim 1. Preferred embodiments of the method are set forth in the dependent claims of claim 1. Claim 10 relates to a wall flow filter manufactured accordingly. Claim 11 includes a preferred use.

[0022] By supplying the inlet area of ​​a dry filter with a dry powder-gas aerosol in a process for manufacturing a wall-flow filter for cleaning gases of small particulate solids, wherein the powder comprises a pyrogenic high-melting-point metal compound and the amount of pyrogenic high-melting-point compound in the filter is less than 5 g / l, and wherein the pyrogenic high-melting-point compound is subjected to a shear force before being supplied to the filter, the stated problem is solved very advantageously. The wall-flow filters manufactured according to the invention are preferably used in the cleaning of exhaust gases from internal combustion engines. They filter the soot formed during combustion from the exhaust gas and result in only a slight increase in the exhaust back pressure. This result can be achieved with a relatively small amount of pyrogenic material.This was not to be expected given the known state of the art.

[0023] Applying the dry, possibly catalytically coated filter with the dry powder-gas aerosol causes the powdered, pyrogenic metal compounds to be deposited on the surface of the filter's inlet side and, if necessary, within the filter's pores, following the gas flow. In principle, a specialist knows how to produce an aerosol from a pyrogenic powder and a gas in order to then pass it through the filter. For the production of a wall-flow filter, it is advantageous to apply the dry, pyrogenic powder-gas aerosol to the inlet surface of a dry filter by dispersing the powder in a gas, then introducing it into a gas stream, and finally drawing or pushing it into the filter's inlet side without any further gas supply. For occupational safety reasons, drawing is preferable to pushing.

[0024] The aerosol consisting of the gas and the pyrogenic metal compound can be produced according to the instructions of a person skilled in the art. For this purpose, a pyrogenically produced metal powder is generally mixed with a gas (http: / / www.tsi.com / Aerosolgeneratoren-und-dispergierer / ; https: / / www.palas.de / de / product / aerosolgeneratorssolidparticles). This mixture of gas and powder is then advantageously fed into the inlet side of the wall flow filter via a gas stream. The inlet side is defined as the part of the filter formed by the inlet / flow channels. The inlet surface is formed by the wall surfaces of the inlet / flow channels on the inlet side of the wall flow filter. The same applies to the outlet side.

[0025] Any gas suitable for the purpose, as perceived by those skilled in the art, can be used to produce the aerosol and introduce it into the filter. The use of air is particularly preferred. However, other reactant gases can also be used, which can exhibit either oxidizing (e.g., O₂, NO₂) or reducing (e.g., H₂) activity towards the powder used. The use of inert gases (e.g., N₂) or noble gases (e.g., He) can also prove advantageous with certain powders. Mixtures of the gases mentioned are also conceivable.

[0026] To ensure sufficient powder deposition on and / or within the filter wall surface on the inlet side of the filter, a certain suction or pressure output is required. Those skilled in the art can determine this through preliminary tests for the specific filter and powder. It has been found that the aerosol (powder-gas mixture) is preferably drawn through the filter at a velocity of 5 m / s to 60 m / s, more preferably 10 m / s to 50 m / s, and most preferably 15 m / s to 40 m / s, as this corresponds to the subsequent exhaust gas velocities. This also achieves advantageous adhesion of the applied powder. Gas velocities corresponding to those of the exhaust gas to be cleaned are most preferably used.By using specific gas velocities upstream of the filter during coating, corresponding to the subsequent exhaust gas velocities upstream of the filter, a powder distribution is achieved, and thus a distribution of the filtering surface within the filter, that is optimally suited to the particle flow in the exhaust gas. The velocities are adapted to the requirements of the specific filter application within the scope of the invention. For filters with a uniform permeability distribution across the filter, this means that, as a rule, more powder is deposited in the last third of the filter in the direction of flow. With an uneven permeability distribution within the filter, e.g., due to zone coatings, this generally means that the powder is preferentially deposited in the high-permeability areas, since the volume flow rate during coating, and later during operation as an exhaust gas filter, is also highest in the high-permeability area.

[0027] The amount of powder in the filter depends on the type of powder and the dimensions of the filter and can be determined by a person skilled in the art in preliminary tests under the given boundary conditions (not too high exhaust back pressure). As a rule, and according to the invention, the loading of the filter with the pyrogenic powder is less than 5 g / l based on the filter volume. Preferably, the value is no more than 3 g / l, and most preferably no more than 2 g / l. Naturally, the desired increase in filtration efficiency sets a lower limit. In this context, it is particularly preferred if the amount of powder remaining in the filter is below 2 g / l.

[0028] By applying the special powder-gas aerosol to the filter, the filtration efficiency and / or the exhaust backpressure of the filter, with optional additional catalytic functions, can be precisely adjusted to the specific conditions in the exhaust system, for example, of a car. Developing designs tailored to these diverse requirements is cost-effective and flexible. The manufacturing process is also cost-efficient, as the same system can be used, for example, with two or more applicators, to apply several different powder types, each with potentially different functions, in rapid succession, separated by mere fractions of a second.

[0029] The filters described here, which may be pre-coated with a catalytic material and subsequently dusted with pyrogenic powders, differ from those formed in a vehicle's exhaust system by ash deposition during operation. According to the invention, the optionally catalytically active filters are selectively dusted with a specific, dry powder in the form of a pyrogenically produced material, e.g., a metal oxide. This allows the balance between filtration efficiency and exhaust backpressure to be precisely adjusted from the outset. Therefore, wall-flow filters, in which more or less undefined ash deposits result from fuel combustion, e.g., in the cylinder during operation or by means of a burner, are not covered by the present invention.

[0030] All conventional ceramic materials can be used as wall flow monoliths or wall flow filters. Porous wall flow filter substrates made of cordierite, silicon carbide, or aluminum titanate are preferred. These wall flow filter substrates have inlet and outlet channels, with the outlet ends of the inlet channels and the outlet channels being offset from each other and sealed with gas-tight plugs. The exhaust gas to be cleaned, flowing through the filter substrate, is forced to pass through the porous wall between the inlet and outlet channels, resulting in excellent particle filtration. The filtration performance for particles can be tailored to the porosity, pore / radius distribution, and wall thickness.The porosity of the uncoated wall flow filters is generally more than 40%, typically from 40% to 75%, and particularly from 50% to 70% [measured according to DIN 66133 - latest version on the filing date]. The average pore size (mean pore diameter; d50) of the uncoated filters is at least 7 µm, e.g., from 7 µm to 34 µm, preferably more than 10 µm, and especially more preferably from 10 µm to 25 µm or most preferably from 15 µm to 20 µm [measured according to DIN 66134 latest version on the filing date]. Finished filters with a pore size (d50) of typically 10 µm to 20 µm and a porosity of 50% to 65% are particularly preferred.

[0031] In general and according to the invention, pyrogenically produced metal powders are understood to be those obtained from a metal precursor by flame hydrolysis or flame oxidation in a flame and exhibiting properties as described for flame-synthesized particulate products in the following literature references: Gutsch A. et al. (2002) KONA (No. 20); Li S. et al. (2016) Progress in Energy and Combustion Science (55); Ulrich G. (1971) Combustion Science and Technology (Vol. 4). Such processes have been used on an industrial scale since 1944, e.g., at Degussa AG. The first patents for this date from the years 1931 to 1954 (US 1,967,235 A, US 2,488,440 A, DE 948,415 B, DE 952,891 B). Pyrogenic silica (https: / / de.wikipedia.org / w / index.php?title=Pyrogenes Siliciumdioxid&oldid=182147815) is produced, for example, by the company Evonik under the name Aerosil. ® or pyrogenic aluminum oxide under the name Aerooxides ®(https: / / www.aerosil.com / product / aerosil / downloads / ti-1331-aerosil-and-aeroxide-for-glossy-photo-inkjet-media-en.pdf). In general, this process can be used to create high-surface-area compounds, especially oxides of various metals with very low tapped densities (<100 kg / m³). 3 , preferably <80 kg / m² 3 and preferably <60 kg / m 3 The materials are produced (measured according to standard DIN EN ISO 787-11 - latest version on the filing date) and preferably used according to the invention. The porosity of these pyrogenic materials, e.g., the metal oxides, is >90%, preferably >93%, and most preferably >95%. This is determined from the ratio of the tapped density to the primary particle density or the true non-porous density of the respective oxides. The formula for this is: Porosity = 1 − Bulk density / true density

[0032] Using aluminum oxide as an example, the true density (https: / / de.wikipedia.org / w / index.php?title=Reindichte&oldid=164022376) is 3200 to 3600 kg / m³. 3 and the tapped density of the pyrogenic oxides is only about 50 kg / m³. 3 . Thus, it follows that 1 m 3 The powder contains only about 1.5% aluminum oxide. Advantageous examples are those selected from the group consisting of silicon dioxide, aluminum oxide, titanium dioxide, zirconium dioxide, cerium oxide, iron oxide, zinc oxide, or their mixed oxides or mixtures thereof. However, other pyrogenic oxides, mixed oxides (so-called solid solutions), or doped mixed oxides can also be produced.

[0033] Flame pyrolysis initially produces nearly spherical primary particles in the nanometer range (d50: 5–50 nm), which sinter together during the subsequent reaction to form highly porous, chain-like aggregates. These aggregates typically have a mean particle size (d50) of <0.5 µm and can subsequently clump together to form agglomerates with a mean particle size of 10–100 µm ( Fig.1, (https: / / de.wikipedia.org / wiki / Pyrogenes Siliciumdioxid). In contrast to agglomerates, which can generally be separated relatively easily into aggregates by the input of energy, aggregates are only further broken down into primary particles by intensive energy input (Manuel Gensch, Dissertation, Mechanical Stability of Nanoparticle Agglomerates under Mechanical Loads, ISBN: 978-3-8440-6110-9, Shaker Verlag). Here, the particle sizes are determined both offline using laser diffraction according to the common standard ISO 13320 (latest version on the filing date). Whenever a d50 value for particles is mentioned in this text, it refers to the d50 value of a Q3 distribution.

[0034] According to the invention, a method is provided in which the pyrogenic high-melting-point compound is subjected to a fluid dynamic stress (shear force) before being applied to the filter. Depending on the intensity of the shear force, this modification can result in the powder being deagglomerated and / or deaggregated. The powder can thus be deposited either only on the wall of the wall-flow filter, on and within its wall, or exclusively within the wall. This allows the wall-flow filter to be well adapted to the underlying cleaning problem (e.g., filtering small-particulate or large-particulate soot from the exhaust gas of an internal combustion engine). The shear force can be determined from preliminary tests. The lower limit of the shear force will be found at the point where the agglomerates of the pyrogenic compound can be broken down into smaller compartments.In this respect, the upper limit will be the division of the aggregates into smaller units down to the primary particles.

[0035] The shear force exerted on the powder can be generated using methods known to those skilled in the art. To increase the powder's penetration into the filter pores, for example, at least some of the powder's particle diameters should be smaller than the pore diameters of the wall-flow filter. The particle diameters of the agglomerates can be reduced by grinding steps known to those skilled in the art for oxide powders. For the pyrogenic materials used in this invention, methods involving shear and / or impact are preferred for introducing the shear force. Atomizing nozzles, for example, are methods involving strong shear. Air classifiers, counterjet mills, and impaction onto impact plates, on the other hand, are methods that primarily break up the agglomerates of pyrogenic oxides through impact.Preferably, the shear force is therefore generated by one or more devices selected from the group consisting of a wear-free atomizing nozzle, air classifier, mill and impact plate.

[0036] By using, as is highly preferred according to the invention, shear-resistant and wear-protected atomizing nozzles in combination, for example, with an impact plate, the required particle sizes can be produced during the coating process (Break-Up and bounce of TiO2 agglomerates by impaction, Ihalainen, M.; Lind, T.; Arffman, A.; Torvela, T.; Jokiniemi, J. In: Aerosol Science and Technology, Vol. 48, No. 1, 2014, p. 31-41; Interparticle forces in silica nanoparticle agglomerates (English) Seipenbusch, M. Rothenbacher, S. Kirchhoff, M. Schmid, H.-J. Kasper, G. Weber, AP in Journal of Nanoparticle Research; 12, 6; 2037-2044; Manuel Gensch, Dissertation, Mechanical stability of nanoparticle agglomerates under mechanical loads, ISBN: 978-3-8440-6110-9, Shaker Verlag; Dry deagglomeration of nanoparticle agglomerates in a classifier mill, Sascha Füchsel*, Klaus Husemann and Urs Peuker, Chemie Ingenieur Technik 2011, 83, No. 8, 1262-1275).

[0037] The particle agglomerates are broken down to a d50 of approximately 2 to 8 µm fractions of a second before being applied to the filter, for example, by the wear-resistant atomizing nozzle. This, along with dilution of the aerosol with an additional transport gas, prevents re-agglomeration. For even finer particles, additional impact plate dispersion is advantageously employed. Here, too, dispersion is carried out fractions of a second before coating, thus preventing re-agglomeration of the particles. The particle sizes can be continuously adjusted, if necessary, by controlling the mass flow and the impact velocity on the impact plate. These two dispersion methods, in conjunction with the use of pyrogenic metal compounds, enable a rapidly adaptable, flexible, and cost-effective process that can be tailored to different filter qualities and requirements.

[0038] The techniques described here allow for very precise control of the particle sizes suitable for the intended purpose. Preferably, the particle sizes (d50) of the pyrogenic metal compounds, particularly the metal oxides, when applied to the pores of the wall-flow filter, are in the range of the primary particle size of 1–100 nm, more preferably 10–80 nm, and most preferably 20–50 nm. This ensures that these particles can be deposited in sufficient quantities in the surface pores of the wall-flow filter. If, as mentioned, powder deposition in the pores is not desired, the d50 value is more preferably 0.5–100 µm, more preferably 1–50 µm, and most preferably 5–20 µm. Mixtures of both size ranges during application, possibly with a time delay, are also conceivable.Typically, the particle size distribution of the powder before application will preferably be in a range of 0.1 - 50 µm (d50).

[0039] The surface area of ​​the particles in the powder is very high. Advantageously, the pyrogenic high-melting compound has a BET surface area of ​​> 50 m² / g, more preferably > 70 m² / g, and most preferably > 900 m² / g. The BET surface area is determined according to DIN ISO 9277:2003-05 (Determination of the specific surface area of ​​solids by gas adsorption using the BET method). The virtually pore-free primary particles with a particle diameter of < 100 nm even generate a high specific surface area of ​​up to over 200 m². 2 / g, which is available for the deposition of the nanoparticles.

[0040] Table 1 shows the dependence of the external surface area on the particle size. The true non-porous density of aluminum oxide was used for the calculation as 3600 kg / m³. 3 and the particle shape is assumed to be a sphere. Particle diameter (d50) [µm] 8 4 2 1 0,5 0,1 0,03 0,01 outer surface m 2 / g 0,2 0,4 0,8 1,7 3,3 16,7 55,6 166,7 Grey: Region of the pyrogenic metal compound: Primary particle diameter < 100nm

[0041] The large external surface area provides an excellent separation area for particles, especially soot particles in the nanometer range. The low tapped density of the pyrogenic metal compound and the virtually pore-free primary particles result in extremely high porosity available for flow. Consequently, the pressure rise caused by the pyrogenic powder on the filter is very low.

[0042] Table 2 shows some exemplary data for pyrogenic oxides compared to common highly porous aluminum oxides such as Puralox. ® represented by the company Sasol. Table 2: Key data compiled as an example for a pyrogenic aluminum oxide produced by Evonik. The pyrogenic aluminum oxide C has a primary particle size of 13 nm. The density of the primary particles is approximately 3200 g / l. Puralox from Sasol was used for comparison. porosity BET surface Stamped density medium primary grain size average agglomerate size depending on shear energy m 2 / g g / liter nm µm Pyrogenic aluminum oxide C 98,5% approximately 100 approximately 50 13 1 to 10 commercially available highly porous aluminum oxides in catalysis approximately 80% up to over 200 approximately 600 - 1200 >1000 unground 20 to 50

[0043] In a specific embodiment of the invention, the optionally catalytically activated pyrogenic high-melting-point compounds in the powder-gas aerosol can be mixed with other non-pyrogenic high-melting-point compounds. Advantageously, the pyrogenic metal compounds are also mixed with other non-pyrogenic materials, e.g., oxides selected from the group consisting of silicon dioxide, aluminum oxide, titanium dioxide, zirconium dioxide, cerium oxide, iron oxide, zinc oxide, or their mixed oxides or mixtures thereof, optionally coated with precious metal oxides or optionally ion-exchanged zeolites, and much more, before being introduced into the wall-flow filter. Mixed oxides with other metals, such as iron, based on the aforementioned group can exhibit catalytically interesting properties for specific reaction processes.This largely preserves the good filtration properties and low pressure drop resulting from the pyrogenic materials, while the other non-pyrogenic materials can provide additional catalytic properties. Consequently, it is also preferable to mix the pyrogenically produced metal oxide powders with the other metal oxide powders and then subject them to the wall flow filter. The individual powders, either separately or together, can exhibit catalytic activity, for example, due to the addition of precious metals. This enables a cost-effective standard process for a wide range of product properties. The strong cross-linking of the pyrogenic compounds also means that mixtures of pyrogenic metal compounds with common non-pyrogenic materials, e.g.,Metal oxides with a low external surface area offer the same advantages in terms of filtration efficiency and pressure drop, but the amount of powder can be significantly reduced.

[0044] In a preferred embodiment, the filter can be catalytically coated before being exposed to the powder-gas aerosol. In this context, catalytic coating refers to the ability to convert harmful components of the exhaust gas from combustion engines into less harmful ones. In particular, the exhaust gas components NOx, CO, and HC, as well as particulate matter, are relevant here. This catalytic activity is provided, according to the skilled person, by coating the wall-flow filter with a catalytically active material. Accordingly, the term "coating" refers to the application of catalytically active materials to the wall-flow filter. The coating performs the actual catalytic function. In this case, the coating is achieved by applying a correspondingly low-viscosity aqueous suspension—also called a washcoat—or solution of the catalytically active components to the wall-flow filter, see, for example, [reference to relevant example].According to EP 1 789 190 B1. After application of the suspension / solution, the wall-flow filter is dried and, if necessary, calcined at elevated temperature. The catalytically coated filter preferably has a loading of 20 g / l to 200 g / l, more preferably 30 g / l to 150 g / l. The most suitable loading of a wall-coated filter depends on its cell density, wall thickness, and porosity. For common medium-porous filters (<60% porosity) with, for example, a cell density of 200 cpsi and a wall thickness of 8 mil, the preferred loading is 20 g / l to 50 g / l (based on the external volume of the filter substrate). Highly porous filters (>60% porosity) with, for example, 300 cpsi and 8 mil have a preferred loading of 25 g / l to 150 g / l, particularly preferably 50 g / l to 100 g / l.

[0045] In principle, all coatings known to those skilled in the art for automotive exhaust systems are suitable for the present invention. Preferably, the catalytic coating of the filter can be selected from the group consisting of three-way catalysts, SCR catalysts, nitrogen oxide storage catalysts, oxidation catalysts, and soot ignition coatings. For details regarding the individual catalytic activities under consideration and their explanation, reference is made to WO 2011 / 151 711 A1. It is particularly advantageous for the filter to have a catalytically active coating comprising at least one metal ion-exchanged zeolite, cerium / zirconium mixed oxide, aluminum oxide, and palladium, rhodium, or platinum, or combinations of these precious metals.

[0046] The powders used here can be employed as described above according to the invention. However, the use of dry, pyrogenic metal compounds, particularly oxide powders, and / or non-pyrogenic metal compounds, particularly oxides, which exhibit catalytic activity with regard to exhaust aftertreatment, is also conceivable. Consequently, the powder itself can also be catalytically active with respect to reducing pollutants in the exhaust gas of an internal combustion engine. All activities known to those skilled in the art are suitable for this purpose, such as TWC, DOC, SCR, LNT, or soot combustion accelerating catalysts. Generally, the powder will exhibit the same catalytic activity as any catalytic coating applied to the filter. This further increases the overall catalytic activity of the filter compared to filters not coated with catalytically active powder.Therefore, it is possible, for example, for pyrogenic aluminum oxide impregnated with a noble metal to be used for the production of the powder-gas aerosol. A three-way activity with a coating comprising palladium and rhodium, as well as an oxygen storage material such as cerium zirconia, is preferred in this context. It is also conceivable that a catalytically active material is used for the SCR reaction. The powder can, for example, consist of zeolites or zeotypes exchanged with transition metal ions. The use of iron- and / or copper-exchanged zeolites is particularly preferred in this context. CuCHA (copper-exchanged chabazite; http: / / europe.iza-structure.org / IZA-SC / framework.php?STC=CHA) or CuAEI (http: / / europe.iza-structure.org / IZA-SC / framework.php?STC=AEI) is most preferably used as the material for the production of the powder-gas aerosol.Furthermore, the powder's activity can be advantageous in improving soot combustion.

[0047] In a further embodiment, the present invention relates to a wall flow filter as described above. The product-characteristic advantages described above in the context of the method according to the invention also apply mutatis mutandis to the wall flow filter according to the invention.

[0048] The wall flow filters according to the invention can be used in any application where fine particulate solids need to be separated from gases. This process is particularly advantageous in the field of automotive exhaust gases, which are laden with finely dispersed soot particles from incomplete combustion. The illustrated wall flow filters can be used in an automotive exhaust system with other catalysts selected from the group consisting of TWC, SCR catalyst, nitrogen oxide storage catalyst, diesel oxidation catalyst, and others.

[0049] The wall flow filter produced according to the invention exhibits excellent filtration efficiency with only a moderate increase in exhaust gas backpressure compared to a wall flow filter in its new state that has not been coated with pyrogenic powder. The lower the filter's filtration efficiency, the greater the increase in filtration efficiency due to the coating with pyrogenic metal compounds. This is particularly noticeable in filters that have been previously coated with a washcoat. Preferably, the wall flow filter according to the invention, previously coated with a washcoat, shows a relative increase in filtration efficiency of at least 5%, preferably at least 10%, and most preferably at least 20%, with a relative increase in exhaust gas backpressure of the new wall flow filter of at most 40%, preferably at most 20%, and most preferably at most 10%, compared to a new filter that has not been treated with powder.A particularly advantageous improvement in filtration efficiency of at least 20% is achieved with a maximum increase in back pressure of no more than 10%.

[0050] The wall-flow filters presented here are characterized by high filtration efficiency coupled with a very low increase in exhaust backpressure. It is assumed that the pyrogenically produced metal compounds, after comminution, only re-crosslink into larger agglomerates during the separation process within the filter. They then form a loose network on the wall and / or in the pores. Primarily, the particle size of the agglomerates after the described comminution, the amount of powder, the pore diameters, and the open porosity of the filter determine whether the pores, the cell surface, or both are coated. The extremely loose structure allows the gases to flow without significant pressure loss. The large external surface area of ​​the pyrogenic metal compounds, in turn, results in very good filtration efficiency for soot particles in the nanometer range. Very small quantities of pyrogenic oxides are sufficient to generate large external surface areas.The oxides commonly used in catalysis have large internal surfaces due to high porosity in the nanometer range, but these are not accessible for the filtration of nanoparticles.

[0051] "Dry" in the context of the present invention accordingly means the exclusion of the use of a liquid, in particular water. Specifically, the preparation of a suspension of the powder in a liquid for atomization into a gas stream should be avoided. A certain level of moisture may be tolerable for both the filter and the powder, provided that the achievement of the objective—the finest possible dispersion of the powder in or on the inlet surface—is not negatively affected. The powder is generally free-flowing and dispersible by the input of energy. The moisture content of the powder or the filter at the time of contact with the powder should be less than 20%, preferably less than 10%, and most preferably less than 5% (measured at 20°C and standard pressure according to ISO 11465, latest version, on the filing date).

[0052] In Fig.Figure 1 illustrates the flame pyrolysis process for the production of fumed silicon dioxide. The initially formed particles subsequently aggregate and eventually assemble into agglomerates (larger chains of primary particles). File:Fumed silica process.svg. (2016, November 28). Wikimedia Commons, the free media repository. Retrieved 08:28, November 7, 2018 from https: / / commons.wikimedia.org / w / index.php?title=File:Fumed silica process.svg&oldid=222460038.

[0053] In the Fig.Section 2 explains the inventive method and the comminution and mixing chamber in more detail: The pyrogenic oxide 400 is forced through the wear-free atomizing nozzle 700 under high shear with a gas 300 under pressure. The velocity can reach almost the speed of sound. After exiting the atomizing nozzle, the gas-powder mixture 500 strikes the baffle plate 600, which is located in the comminution and mixing chamber 100. The resulting gas-powder mixture with the comminuted powder agglomerates is then mixed with the gas stream 200 and proceeds as a diluted gas-powder mixture 800 to coat the filter. The dilution gas 200 is required so that the flow velocity of the filter during coating can be varied independently of the amount of atomizing gas. Reference symbol list: 100 grinding and mixing chamber 200 dilution gas stream 300 gas flow 400 pyrogenic oxide 500 Gas powder mixture 600 Baffle plate 700: wear-free atomizing nozzle 800 diluted gas-powder mixture Example 1:

[0054] Coating of a raw washcoat-free filter with dimensions 4.66"x6.00" 300 / 8 with powder.

[0055] The pyrogenically produced powder was dispersed using an atomizing nozzle at 2 bar and sucked into the filter at a speed of 20m / s. relative* increase in filtration efficiency relative* pressure increase 0.6g pyrogenic Al2O3 / liter filter volume 5,5% 2% 1.2g pyrogenic Al2O3 / liter filter volume 9% 3% 0.3g pyrogenic Al2O3 + 1.2g Al2O3 with a d50 of 3µm / liter filter volume 6% 1% *relative to an uncoated raw filter substrate without additional powder coating Example 2:

[0056] In a first step, the filter was coated with 50 g / l washcoat in the porous filter wall, dried, and calcined. It was then coated with 2 g / l pyrogenic powder. The powder was dispersed at 2 bar using a wear-free atomizing nozzle and drawn into the filter at a speed of 20 m / s. The increase in filtration efficiency and the rise in pressure drop were measured at 600 m. 3 / h relative to the powder-free filter. relative* increase in filtration efficiency relative* pressure increase 2g pyrogenic Al2O3 / liter filter volume 47% 10% *relative to the substrate coated only with washcoat

Claims

[1] Method for producing a wall-flow filter for cleaning gases from small particulate solids, wherein the inlet area of ​​the dry filter is exposed to a dry powder-gas aerosol, characterized by that the powder contains a pyrogenic refractory metal compound and the amount of pyrogenic refractory compound in the filter is below 5 g / l, wherein the pyrogenic refractory compound is subjected to a shear force before being applied to the filter. [2] Method according to claim 1, characterized by that the pyrogenic high-melting compound is selected from the group consisting of silicon dioxide, aluminum oxide, titanium dioxide, zirconium dioxide, cerium oxide, iron oxide, zinc oxide, mixed oxides of the aforementioned oxides or mixtures thereof. [3] Method according to claim 2, characterized bythat the shear force is caused by one or more devices selected from the group consisting of wear-free atomizer nozzle, air classifier, mill and impact plate. [4] Method according to one of the preceding claims, characterized by that the mean particle size (d50) of the pyrogenic refractory compound is between 0.1 µm and 50 µm. [5] Method according to one of the preceding claims, characterized by that the pyrogenic high-melting compound has a BET surface area of ​​> 50 m 2 / g. [6] Method according to one of the preceding claims, characterized by that the pyrogenic refractory compounds in the powder-gas aerosol are mixed with other non-pyrogenic refractory compounds. [7] Method according to one of the preceding claims, characterized by that the wall-flow filter has been catalytically coated before being exposed to the pyrogenic high-melting compound. [8] Method according to one of the preceding claims, characterized by that the pyrogenic compound itself is catalytically active. [9] Method according to claim 6, characterized by that the non-pyrogenic compounds themselves are catalytically active [10] Wall flow filter obtainable by a method according to any one of the preceding claims. [11] Use of a wall flow filter according to claim 10 for the purification of car exhaust gases.

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