DE102019100107A1 - Catalytically active filter substrate and process for its manufacture and use - Google Patents

Abstract

Particulate filter manufacturing process Particulate filters are commonly used to filter exhaust gases from a combustion process. New filter substrates and their specific use in exhaust gas aftertreatment are also specified.

Description

The present invention is directed to a method of making particle filters. Particulate filters are commonly used to filter exhaust gases from a combustion process. Also specified are new filter substrates produced according to the invention and their specific use in exhaust gas aftertreatment.

The exhaust gas from, for example, internal combustion engines in motor vehicles typically contains the harmful gases carbon monoxide (CO) and hydrocarbons (HC), nitrogen oxides (NO _x ) and possibly sulfur oxides (SO _x ), as well as particles which mainly consist of soot residues and possibly adhering organic agglomerates. These are referred to as primary emissions. CO, HC and particles are products of the incomplete combustion of the fuel in the combustion chamber of the engine. Nitrogen oxides are generated in the cylinder from nitrogen and oxygen in the intake air when the combustion temperatures locally exceed 1400 ° C. Sulfur oxides result from the combustion of organic sulfur compounds, which are always contained in small amounts in non-synthetic fuels. A large number of catalytic exhaust gas purification technologies have been developed to remove these emissions, which are harmful to the environment and health, from motor vehicles, the basic principle of which is usually based on the fact that the exhaust gas to be cleaned is passed over a catalytic converter which consists of a flow or wall flow honeycomb body (wall flow filter) and a catalytically active coating applied thereon and / or therein. The catalyst promotes the chemical reaction of various exhaust gas components with the formation of harmless products such as carbon dioxide and water.

Particles can be removed from the exhaust gas very effectively using particle filters. Wall-flow filters made from ceramic materials have proven particularly useful. These have two end faces and are made up of a large number of parallel channels, which are formed by porous walls and extend from one end face to the other. The channels are mutually sealed at one of the two ends of the filter so that first channels are formed which are open on the first side of the filter and closed on the second side of the filter, and second channels which are on the first side of the filter closed and open on the second side of the filter. The exhaust gas flowing into the first channels, for example, can only leave the filter again via the second channels, and for this purpose has to flow through the porous walls between the first and second channels. When the exhaust gas passes through the wall, the particles are retained.

A known method for removing nitrogen oxides from exhaust gases in the presence of oxygen is the selective catalytic reduction (SCR method) using ammonia on a suitable catalyst. In this process, the nitrogen oxides to be removed from the exhaust gas are converted with ammonia to nitrogen and water. The ammonia used as the reducing agent can be made available by metering an ammonia precursor compound, such as urea, ammonium carbamate or ammonium formate, into the exhaust line and subsequent hydrolysis. It is also possible to generate the ammonia in situ by over-reducing NOx via an upstream nitrogen oxide storage catalytic converter.

It is also already known to coat wall flow filters with SCR-active material and thus to remove particles and nitrogen oxides from the exhaust gas at the same time. Such products are commonly referred to as SDPF, SCRF ^® or SCRoF. If the required amount of SCR-active material is applied to the porous walls between the channels (so-called on-wall coating), this can lead to an unacceptable increase in the back pressure of the filter. Against this background, for example, JPH01-151706 and WO2005016497A1 propose to coat a wall flow filter with an SCR catalyst in such a way that the latter penetrates the porous walls (so-called in-wall coating). It has also been suggested - see US2011274601A1 - to introduce a first SCR catalyst into the porous wall, ie to coat the inner surfaces of the pores and to place a second SCR catalyst on the surface of the porous wall. The average particle size of the first SCR catalytic converter is smaller than that of the second SCR catalytic converter.

The allocation of wall flow filters with other catalytically active material has already been described for lean or stoichiometric combustion engines. Nitrogen oxide storage catalysts (NSCs; LNT, NSR) are used to remove the nitrogen oxides contained in the lean exhaust gas from so-called lean-burn engines (diesel, Lean-GDI). The cleaning effect is based on the fact that in a lean operating phase (storage phase, lean operation) of the engine, the nitrogen oxides are stored in the form of nitrates by the storage material of the storage catalytic converter. In a subsequent rich operating phase (regeneration phase, rich operation, DeNOx phase) of the engine, the previously formed Nitrates decompose and the nitrogen oxides released with the reducing, rich components of the exhaust gas are converted to nitrogen, carbon dioxide and water on the storage catalytic converter during rich operation (SAE 950809). The rich components of the exhaust gas include hydrocarbons, carbon monoxide, ammonia and hydrogen. Such filters coated with nitrogen oxide storage catalysts are referred to as NDPF (eg in the EP1608854B1 ).

So-called three-way catalytic converters are used to reduce emissions in stoichiometric combustion engines. Three-way catalysts (TWCs) are well known to those skilled in the art and have been required by law since the 1980s. The actual catalyst mass here mostly consists of a high-surface, oxidic support material on which the catalytically active components are deposited in the finest distribution. The noble metals of the platinum group, platinum, palladium, rhodium, iridium, ruthenium and osmium are particularly suitable as catalytically active components for the purification of stoichiometrically composed exhaust gases. Suitable carrier materials are, for example, aluminum oxide, silicon dioxide, titanium oxide, zirconium oxide and their mixed oxides and zeolites. So-called active aluminum oxides with a specific surface area (BET surface area measured according to DIN 66132 as of the filing date) of more than 10 m ² / g are preferably used. To improve the dynamic conversion, three-way catalysts also contain oxygen-storing components. These include cerium oxide, praseodymium oxide and cerium / zirconium mixed oxides (see above for NSC; EP1181970A1 ). Zoned and multilayer systems with three-way activity are now also known ( US8557204 ; US8394348 ). If such a three-way catalytic converter is located on or in a particle filter, one speaks of a cGPF (e.g. EP2650042B1 ).

Oxidation catalysts for removing the harmful gases carbon monoxide (CO) and hydrocarbons (HC) from the exhaust gas of diesel and lean-burn internal combustion engines are also well known from the prior art and are predominantly based on platinum and aluminum oxide. Examples of diesel oxidation catalysts can be found in the patent applications DE10308288 A1 , DE19614540 A1 , DE19753738 A1 , DE3940758 A1 , EP 0427970 A2 and DE4435073 A1 being found. They use the oxygen present in large quantities in the diesel exhaust gas in order to oxidize the aforementioned harmful gases to carbon dioxide (CO2) and water vapor. Cerium oxide-containing catalysts for the oxidation of carbon monoxide and hydrocarbons are in the WO2013 / 149881 described. These forms of catalysts are also found on wall-flow filters and are referred to in the technical term as cDPF (e.g. EP1309775B1 ).

It is common to all particle filters that they have to be regenerated at certain time intervals. This means that the accumulated soot particles have to be burned off in order to keep the exhaust gas back pressure in an acceptable range. Exhaust gas temperatures of approx. 600 ° C are required for filter regeneration and initiation of soot combustion. Very high temperatures can occur during the burn-up, which can be> 800 ° C.

Furthermore, as already reported above, care must be taken to ensure that wall flow filters coated with catalytically active material have a neutral influence on the exhaust gas back pressure, since increased exhaust gas back pressure has a negative effect on fuel consumption and thus on CO ₂ emissions. As described above, techniques have been described in the prior art on how the exhaust gas back pressure of catalytically coated wall flow filters can be reduced.

The targeted loading of a porous filter with a coating suspension is carried out in the EP2727640A1 described. The pores on the inflow side of the filter have particles in the interior which completely fill the pores at least across their cross-section, essentially transversely to the direction of flow, and due to this porous filling, the pore volume of the filled pores is 50 to 90% based on the pore volume of unfilled pores. The pore volume of the filled pores on the upstream side of the filter is less than the pore volume of the filled pores in the flow direction further away from the upstream side of the filter and the pore volume of the filled pores in the flow direction increases the further away the pores are from the upstream side of the filter .

The EP1716903B1 proposes a method for coating filter bodies, in which the filter, after coating, is freed from too much coating dispersion by repeatedly applying pressure pulses to one end of the filter body in such a way that excess coating suspension is forced out of the filter body until it reaches its optimum Coating weight has reached. The goal seems to be to reduce the exhaust gas back pressure of the filter.

The object of the present invention is to provide a manufacturing process for catalytically coated, ceramic wall-flow filter substrates, which in particular allows improved wall-flow filter substrates to be generated. The wall-flow filters produced in this way should be superior to the substrates produced according to the prior art, especially with regard to the requirement of the lowest possible exhaust gas back pressure with sufficient catalytic activity. It was also an object of the present invention to specify filter substrates produced by the above process and their use in exhaust gas aftertreatment.

These and other tasks that result from the prior art in an obvious manner are solved by a method with the features of the claims 1, 5 and 7 in question. The dependent claims dependent on these claims are directed to preferred embodiments of the method according to the invention.

The fact that in a process for the production of catalytically coated ceramic wall flow filters with optimized exhaust gas back pressure for the treatment of exhaust gases from a combustion process, the filter has a porosity of> 50% (DIN 66133 - latest version on the filing date) and an average pore diameter (which is from the pore volume) of 5 - 50 µm (DIN 66134 - latest version on the filing date), after the catalytically active material has been applied to the filter substrate, in a directly subsequent step by increasing and / or reducing the pressure in the form of one or more pressure pulses contrary to the coating direction, the pores that determine ≥ 50% of the volume flow through the filter wall are deliberately emptied, without the amount of catalytically active material on the filter substrate being more than 20%, which is extremely surprising, but no less advantageous in solving the problem. The amount of catalytically active material that is removed refers to that that is removed from the wall flow filter by the one or more pressure pulses. Accordingly, this means the difference in loading before and after the pressure pulse.

Intensive investigations have shown that the volume flow of the exhaust gas flowing over the wall of a filter is largely caused by large pores. If a pressure difference is applied to pores, the gas flows laminarly through the pores. The volume flow is then proportional to d ⁴ . Ie, through a 4 x larger diameter, 256 times the gas flows. If these pores are freed of catalytically active material by at least one short pressure pulse, the volume flow can flow through the wall of the ceramic filter without a higher exhaust gas back pressure. The catalytically active material can continue to be present in the smaller pores, which make up the majority of the total porosity of the filter material, without significantly obstructing the exhaust gas flowing through. The substrates produced in this way show sufficient catalytic activity with reduced exhaust gas back pressure compared to the catalytically active filters of the prior art.

The pressure impulse (overpressure and / or underpressure) emanating from one or both end faces of the wall flow filter just coated is a measure which is sufficient to remove the larger channels or pores (for example ≥40 µm pore diameter) through the wall from the applied ones to rid catalytically active substances as much as possible. As a rule, only the large pores or channels that reach through the wall are "blown free" or "sucked free" as already shown above. The catalytically active substance remains in the smaller pores (e.g. ≤40 µm pore diameter) of the filter walls, which however make up the majority (e.g.> 50%, more preferably> 80% and particularly preferably> 90%) of the filter material's porosity. In the usual way, it can therefore be brought to the heterogeneous catalytic reaction with the harmful exhaust gas components. If a distribution of pore diameters or particle sizes is mentioned below, a Q3 distribution is always meant (https://de.wikipedia.org/w/index.php?title=Partikelgr%C3%B6%C3%9Fen Distribution&oldid=181716548) .

The filter substrate used preferably has an average pore diameter (d50) of 10-40 μm. The average pore diameter of the filter substrate used is more preferably 13-30 μm. Furthermore, the filter substrate used has a porosity of ≥50%, more preferably 50-70% and very particularly preferably 59-65%. The use of filter substrates with a bimodal pore diameter distribution is preferred for this method for producing a catalytically active exhaust gas filter with the lowest possible exhaust gas back pressure. The bimodal pore diameter distribution means that the filter substrate has essentially 2 maxima in the Q3 distribution. The first maximum should be in a range of 5 to 30 µm, particularly preferably 10 to 25 µm and very preferably 12 to 20 µm.

The second maximum, on the other hand, is 20-100 µm, preferably 25-80 µm and very particularly preferably 30-60 µm. The pores corresponding to the larger maximum, which preferably sucked free are responsible for the exhaust gas back pressure, while the smaller pores contain the catalytically active mass.

As already indicated, the volume flow over the porous wall of the filter substrate is mainly accomplished through correspondingly large pores. As a rule, they make up more than 50%, preferably more than 60% and very particularly preferably more than 70% of the total volume flow through the filter. These large pores with a pore diameter of generally ≥ the d80 value of the filter substrate are far inferior in number and volume corresponding to the smaller pores (<the d60 value of the filter substrate) in the filter substrate. As a result, the proportion of the latter in the total porosity of the filter is so large.

As a rule, the strength and / or duration of the pressure pulse is adapted to the substrate of the filter. In a first approximation, it can be assumed that the product to be used, consisting of pressure difference and suction time, depends on the shape of the pores. The greater the length of the pores, the smaller the diameter of the pores and the higher the viscosity of the coating suspension, the greater the product to choose from the pressure difference and suction time. The strength and / or duration of the pressure pulse is preferably adapted to the Q3 value of the pore diameter> the d60 value, more preferably> the d80 value and very particularly preferably> the d90 value of the filter substrate.

• Strömungsgeschwindigkeit in der Pore: $$v\sim \frac{d^2}{32\ast \eta \ast L}\ast \mathrm{\Delta }p$$
  
• Zeit bis Pore entleert ist: $$t\sim \text{L}/\text{v}$$
  
• Damit gilt: $$L/t\sim \frac{d^2}{32\ast \eta \ast L}\ast \mathrm{\Delta }p$$
  
• Umgeformt ergibt sich: $$\mathrm{\Delta }p\ast t\sim 32\ast \eta \ast \frac{L^2}{d^2}$$
  
• V : Volumenstrom durch eine Pore [m³/s]
• v : Geschwindigkeit in der Pore in [m/s]
• t: Zeit bis die Pore freigesaugt oder freigeblasen ist [s]
• d : Porendurchmesser [µm]
• L : Länge der Pore [µm]
• η : dynamische Viskosität [Pas*s]
• Δp : Druckdifferenz [mbar]

In approximation with simplifications such as neglecting the transient terms:

• Flow velocity in the pore: $$v\sim \frac{d^{\mathrm{2nd}}}{32\ast \eta \ast L}\ast \mathrm{\Delta }p$$
  
• Time until pore is empty: $$t\sim \text{L}/\text{v}$$
  
• The following applies: $$L/t\sim \frac{d^{\mathrm{2nd}}}{32\ast \eta \ast L}\ast \mathrm{\Delta }p$$
  
• Formed results: $$\mathrm{\Delta }p\ast t\sim 32\ast \eta \ast \frac{L^{\mathrm{2nd}}}{d^{\mathrm{2nd}}}$$
  
• V: volume flow through a pore [m ³ / s]
• v: velocity in the pore in [m / s]
• t: time until the pore is sucked or blown free [s]
• d: pore diameter [µm]
• L: length of the pore [µm]
• η: dynamic viscosity [Pas * s]
• Δp: pressure difference [mbar]

The strength and / or duration of the pressure pulse for the pore diameter of the pores is preferably adjusted with ≥ 30 μm, more preferably 40 40 μm and very particularly preferably 45 45 μm of the filter substrate.

The product required from the duration and pressure difference of the suction pulse also depends on the square of the pore length, as described in the formula above. In a first approximation, the pore length is proportional to the cell wall thickness. A square product of pressure difference and duration of the suction pulse is therefore required for a changed cell wall thickness.

With the usual substrates for ceramic wall flow filters and common viscosities (see below), the duration of the pressure pulses at the constant level is advantageously between 0.1 and less than 5 seconds. A pressure pulse with a duration between 0.5-4 seconds is more preferred, and it is very particularly preferably between 1-3 seconds. As stated, the strength of the pressure pulse should be sufficient to free the correspondingly large pores from the catalytically active materials contained therein. As a rule, the strength of the pressure pulse will be in the range from 5 to 80 KPa, more preferably in the range from 20 to 50 KPa and very particularly preferably in the range from 30 to 45 kPa.

To further discriminate between large and smaller pores, it may be beneficial for the pressure pulse to develop fully in a relatively short period of time and then for the suction pressure to be stable and reproducible at the defined level over a defined period of time in accordance with the equation. The maximum pressure difference should be reached within ≤ 0.5 s, more preferably ≤ 0.2 s and very preferably ≤ 0.1 s. The duration of the pressure pulse should in any case be less than 5 seconds. The short, strong pressure pulses only open the large pores, but only result in a small amount of coating suspension being removed. In this respect, the coating suspension on the filter is reduced by this treatment by less than 20%, preferably by less than 15% and very particularly preferably by less than 10%.

Contacting the catalytically active materials with the wall flow filter is known in the art as coating. This term is understood to mean the application of catalytically active materials and / or storage components to a largely inert support body / substrate. The coating takes on the actual catalytic function and often contains storage materials and / or catalytically active metals, which are usually deposited in a highly dispersed form on temperature-stable, high-surface metal oxides. The coating is usually carried out by applying an aqueous suspension of the storage materials and catalytically active components - also called washcoat - to or in the wall of the wall flow filter. After the suspension has been applied, the carrier is generally dried and, if appropriate, calcined at elevated temperature. The coating can consist of one layer or be composed of several layers, which are applied one above the other (multilayer) and / or offset from one another (as zones) on a corresponding filter.

In the present case, the suspension with the catalytically active materials advantageously has high-surface area oxidic components whose average particle diameter (d50 of the Q3 distribution; DIN 66160 - Latest version on the filing date) is less than the average pore diameter of the filter substrate. For an in-wall coating, the particle size d99 of the Q3 distribution in the suspension in relation to the average pore diameter of the pores in the filter walls (d50 of the Q3 distribution) should be preferably <0.6: 1, more preferably <0.5: 1 and particularly preferably <0.4: 1. It is thus possible to get a large proportion of the suspension of> 80%, more preferably> 90% and very preferably> 95% and more into the pores of the wall of a wall flow filter. The correct choice of the average particle diameter can be used to control how much of the catalytically active materials should be positioned in the wall or on the wall of the wall flow filter. The smaller the particle diameter of the advantageously high temperature stable oxide components, the more of these components can be positioned in the small pores. The ratio of the oxidic components present on the wall to the wall naturally also has a significant influence on the exhaust gas back pressure of the filter substrate produced in this way. The coating suspension can have a bimodal particle diameter distribution. It can be coated on and into the filter wall in one step. Expensive coatings are preferably achieved in that the catalytically active material contains high-surface area metal compounds, in particular oxides, whose average particle size (DIN 66160 - latest version on the filing date) preferably d50 the Q3 distribution in relation to the average pore diameter of the filter d50 the Q3 distribution > 1: 6 and <1: 1 and particularly preferably at> 1: 3 and <1: 2 (https://de.wikipedia.org/wiki/Partikelgr%C3%B6%C3%9Fen Distribution). An upper limit is generally the value which the person skilled in the art can judge to be sensible in the case of appropriate cost coatings. In the present invention, it is particularly preferred if an in-wall coating is established. To determine the proportion of the washcoat in the wall of the wall flow filter and the proportion of the washcoat on the wall of the wall flow filter, scanning electron microscope images are evaluated with the aid of a statistical grayscale evaluation. Free pores / air appear black in a catalytically coated filter, while the heaviest elements appear white. Through a suitable choice of the measurement settings known to the person skilled in the art, the difference between active mass and filter substrate can be evaluated in this way due to the separation of the gray levels.

The coating process can advantageously be carried out several times in succession with the same or different catalytically active materials. It is important that one intermittently in the still moist state sets a corresponding pressure pulse according to the invention, which ensures that the large pores are clogged as little as possible by the washcoat components of the catalyst. It should be mentioned that the coating can be carried out with the same or different catalytically active materials, with and without intermediate drying.

In a further aspect, the present invention also relates to a catalytically coated ceramic wall-flow filter for the treatment of exhaust gases from a combustion process, the filter having a porosity of> 50% and an average pore diameter of 5-50 μm and being produced according to one of the above described method. It goes without saying that the preferred embodiments specified for the production process also apply mutatis mutandis to the wall flow filter.

In this context, it should be mentioned that a corresponding filter is advantageously specified in which large pores with a pore diameter of ≥ 40 μm, preferably 45 45 μm and particularly preferably 50 50 μm are essentially not filled with catalytically active material. The expression “essentially” here means the fact that the corresponding pores that let the gas flow through the pressure pulse are freed as completely as possible of the oxidic material from the coating (at least> 80% by weight, preferably> 90% by weight) . How much material is actually still present in the pores then also depends on the intensity, the duration of the pressure pulse and the geometry of the pores, as well as the corresponding material used and its degree of dryness. As a rule, no more than 20% by weight, preferably no more than 10% by weight and very particularly preferably no more than 5% by weight of the catalytically active material in the wall flow filter will then be present in the large pores specified above are located. The percentages here are based on the weight of the solid material after application of the corresponding pressure pulse.

In a last aspect, the present invention also relates to the use of the filter according to the invention in a process for the oxidation of hydrocarbons and / or carbon monoxide and / or in a process for nitrogen oxide reduction, which originate from a combustion process, preferably that of an automobile engine. The filter according to the invention is particularly preferred in exhaust systems of an internal combustion engine as SDPF (SCR catalyst coated on a wall flow filter), cGPF (3-way catalyst coated on a wall flow filter), NDPF (NOx storage catalyst coated on a wall flow filter) or cDPF ( Diesel oxidation catalyst coated on a wall flow filter) is used.

A preferred application is the removal of nitrogen oxides from lean exhaust gas mixtures using the SCR process. For this SCR treatment of the preferably lean exhaust gas, ammonia or an ammonia precursor compound is injected into this and both are passed through an SCR-catalytically coated wall flow filter according to the invention. The temperature above the SCR filter should be between 150 ° C and 500 ° C, preferably between 200 ° C and 400 ° C or between 180 ° C and 380 ° C, so that the reduction can take place as completely as possible. A temperature range from 225 ° C. to 350 ° C. is particularly preferred for the reduction. Furthermore, optimal nitrogen oxide conversions are only achieved if there is a molar ratio of nitrogen monoxide to nitrogen dioxide (NO / NO ₂ = 1) or the ratio NO ₂ / NOx = 0.5 ( G. Tuenter et al., Ind. Eng. Chem. Prod. Res. Dev. 1986, 25, 633-636 ; EP1147801B1 ; DE2832002A1 ; Kasaoka et al., Nippon Kagaku Kaishi (1978), 6, 874-881 ; Avila et al., Atmospheric Environment (1993), 27A, 443-447) . Optimal sales starting with 75% conversion at 250 ° C with optimal selectivity to nitrogen are according to the stoichiometry of the reaction equation 2 NH ₃ + NO + NO ₂ → 2 N ₂ + 3 H ₂ O only achieved with a NO ₂ / NOx ratio of around 0.5. This applies not only to SCR catalysts based on metal-exchanged zeolites, but also to all common, ie commercially available, SCR catalysts (so-called Fast SCR). A corresponding NO: NO ₂ content can be achieved by means of oxidation catalysts which are positioned upstream of the SCR catalyst.

The injection devices used can be chosen arbitrarily by a person skilled in the art. Suitable systems can be found in the literature (T. Mayer, solid SCR system based on ammonium carbamate, dissertation, TU Kaiserslautern, 2005). The ammonia can be introduced into the exhaust gas stream as such or in the form of a connection via the injection device, which ammonia is produced under the ambient conditions. As such, among other things, aqueous solutions of urea or ammonium formate come into question, as well as solid ammonium carbamate. These can be taken from a provided source known to the person skilled in the art and added to the exhaust gas stream in a suitable manner become. The person skilled in the art particularly preferably uses injection nozzles ( EP0311758A1 ) a. This is used to set the optimum ratio of NH ₃ / NOx so that the nitrogen oxides can be converted to N ₂ as completely as possible.

Wall flow filters with an SCR catalytic function are called SDPF. These catalysts often have a function for storing ammonia and a function in that nitrogen oxides can react together with ammonia to form harmless nitrogen. An NH ₃ -storing SCR catalytic converter can be designed according to the types known to those skilled in the art. In the present case, this is a wall flow filter coated with a material that is catalytically active for the SCR reaction, in which the catalytically active material — commonly called the “wash coat” - is present in the pores of the wall flow filter. However, in addition to the component which is catalytically active in the actual sense, it can also contain other materials such as binders made of transition metal oxides and high-surface carrier oxides such as titanium oxide, aluminum oxide, in particular gamma-Al ₂ O ₃ , zirconium or cerium oxide. Also suitable as SCR catalysts are those which are composed of one of the materials listed below. However, zoned or multi-layer arrangements or arrangements of several components in succession (preferably two or three components) with the same or different materials can also be used as the SCR component. Mixtures of different materials on one substrate are also conceivable.

The actual catalytically active material used according to the invention in this regard is preferably selected from the group of transition metal-exchanged zeolites or zeolite-like materials (zeotypes). Such compounds are well known to the person skilled in the art. In this regard, materials from the group consisting of Levynit, AEI, KFI, Chabazit, SAPO-34, ALPO-34, Zeolite β and ZSM-5 are preferred. Zeolites or zeolite-like materials of the chabazite type, in particular CHA or SAPO-34, and LEV or AEI are particularly preferably used. To ensure sufficient activity, these materials are preferably provided with transition metals from the group consisting of iron, copper, manganese and silver. Copper is particularly advantageous in this context. The metal to framework aluminum or in the SAPO-34 framework silicon ratio is usually between 0.3 and 0.6, preferably 0.4 to 0.5. The person skilled in the art knows how to provide the zeolites or the zeolite-like material with the transition metals ( EP0324082A1 , WO1309270711A1 , WO2012175409A1 as well as the literature cited there) in order to be able to provide good activity against the reduction of nitrogen oxides with ammonia. Vanadium compounds, cerium oxides, cerium / zirconium mixed oxides, titanium dioxide and tungsten-containing compounds and mixtures thereof can also be used as catalytically active material.

Materials which have also proven to be advantageous for the use for storing NH ₃ are known to the person skilled in the art ( US20060010857A1 , WO2004076829A1 ). Microporous solids, for example so-called molecular sieves, are used in particular as storage materials. Such compounds can be selected from the group consisting of zeolites, such as, for example, mordenite (MOR), Y zeolite (FAU), ZSM-5 (MFI), ferrierite (FER), chabazite (CHA) and other “small pore zeolites” such as LEV, AEI or KFI, and β-zeolites (BEA) as well as zeolite-like materials such as aluminum phosphates (AIPO) and silicon aluminum phosphate SAPO or mixtures thereof are used ( EP0324082A1 ). ZSM-5 (MFI), chabazite (CHA), ferrierite (FER), ALPO or SAPO-34 and β-zeolites (BEA) are particularly preferably used. CHA, BEA and AIPO-34 or SAPO-34 are very particularly preferably used. Materials of the LEV or CHA type are used with extreme preference and here CHA or LEV or AEI are most preferably used. If a zeolite or zeolite-like compound is used as the catalytically active material in the SCR catalyst, the addition of further NH ₃ -storing material can of course advantageously be omitted. Overall, the storage capacity of the ammonia storage components used in the fresh state at a measuring temperature of 200 ° C. should be more than 0.9 g of NH ₃ per liter of catalyst volume, preferably between 0.9 g and 2.5 g of NH ₃ per liter of catalyst volume and particularly preferably between 1.2 g and 2.0 g of NH ₃ / liter of catalyst volume, and most preferably between 1.5 g and 1.8 g of NH ₃ / liter of catalyst volume, respectively. The ammonia storage capacity can be determined using a synthesis gas system. For this purpose, the catalyst is first conditioned at 600 ° C with synthesis gas containing NO in order to completely remove ammonia residues in the drill core. After cooling the gas to 200 ° C is then, at a space velocity of z. B. 30,000 h ^-1 , as long as ammonia metered into the synthesis gas until the ammonia storage of the drill core is completely filled and the measured ammonia concentration after the drill core corresponds to the input concentration. The ammonia storage capacity results from the difference between the total metered amount and the amount of ammonia measured on the downstream side, based on the catalyst volume. The synthesis gas is typically composed of 450 ppm NH ₃ , 5% oxygen, 5% water and nitrogen.

All filter bodies made of ceramic materials which are customary in the prior art can be used as the particle filter. Porous wall flow filter substrates made of cordierite, silicon carbide or aluminum titanate are preferred. These wall-flow filter substrates have inflow and outflow channels, the outflow-side ends of the inflow channels and the upstream-side ends of the outflow channels being sealed off from one another with gas-tight “plugs”. The diameter of the inflow channels can be the same or different from the diameter of the outflow channels. As already mentioned at the beginning, the exhaust gas to be cleaned, which flows through the filter substrate, is forced to pass through the porous wall between the inflow and outflow channels, which requires an excellent particle filter effect. Due to the porosity, pore / diameter distribution, and thickness of the wall, the filtration property can be designed for particles. The catalyst material can be in the form of coatings in and / or on the porous walls between the inflow and outflow channels. Filters can also be used which have been extruded directly or with the aid of binders from the corresponding catalyst materials, that is to say that the porous walls consist directly of the catalyst material. Preferred filter substrates can be the EP1309775A1 , EP2042225A1 or EP1663458A1 be removed.

• 1 Druckabfallkurven aufgetragen über Zeit. Unterdruck ist normiert auf 380 mbar (Maximalwert). Substrat: SiC - Filter mit Zeolithbeschichtung, 300 CPSI, 12 mil Wandstärke. (Erfindungsgemäße Ausführung.)
• 2 Druckverlust vs Beladung für 3 Teile die erfindungsgemäß mit initialem Unterdruck von 380 mbar (mit den Bedingungen aus 1) beschichtet worden sind. Substrat: SiC - Filter mit Zeolithbeschichtung, 300 CPSI, 12 mil Wandstärke. (Erfindungsgemäße Ausführung.)
• 3 Druckabfallkurven aufgetragen über Zeit. Unterdruck ist normiert auf 380 mbar (Maximalwert). Substrat: SiC - Filter mit Zeolithbeschichtung, 300 CPSI, 12 mil Wandstärke. (Nicht erfindungsgemäße Ausführung.)
• 4 Druckverlust vs Beladung für 3 Teile die nicht erfindungsgemäß mit initialem Unterdruck von 380 mbar (mit den Bedingungen aus 3) beschichtet worden sind. Substrat: SiC - Filter mit Zeolithbeschichtung, 300 CPSI, 12 mil Wandstärke. (Nicht Erfindungsgemäße Ausführung.)
• 5 Vergleich der Druckverlusterhöhung der beschichteten Filter in Relation zum Druckverlust des unbeschichteten Substrates in erfindungsgemäßer und nicht erfindungsgemäßer Ausführung. Substrat: SiC - Filter mit Zeolithbeschichtung, 300 CPSI, 12 mil Wandstärke.
• 6 Druckabfallkurve während Druckimpuls aufgetragen über Zeit mit automatischer Falschluftklappe. Substrat: Cordierit - Filter mit Edelmetallhaltiger Oxidbeschichtung, 300 CPSI, 8 mil Wandstärke. (erfindungsgemäße Ausführung.)
• 7 Druckabfallkurve während Druckimpuls ohne automatische Falschluftklappe. Substrat: Cordierit - Filter mit Edelmetallhaltiger Oxidbeschichtung, 300 CPSI, 8 mil Wandstärke. (nicht erfindungsgemäße Ausführung.). Die 7 hat zwar einen Peak bei 100 %, aber die Absaugung hat die Wirkung des Niveaus von ca. 80 % des initialen Unterdrucks und erzeugt damit einen schlechteren Druckverlust nach Beschichtung als die Bedingungen in 6.

With the present invention, it is possible to produce, in particular, SCR-catalytically coated particle filters of the wall-flow type with an in-wall coating, which have an extremely good activity with a not significantly increased exhaust gas back pressure. Against the background of the known prior art, this was not to be expected.

• 1 Pressure drop curves plotted over time. Vacuum is standardized to 380 mbar (maximum value). Substrate: SiC filter with zeolite coating, 300 CPSI, 12 mil wall thickness. (Design according to the invention.)
• 2nd Pressure loss vs loading for 3 parts according to the invention with an initial negative pressure of 380 mbar (with the conditions out 1 ) have been coated. Substrate: SiC filter with zeolite coating, 300 CPSI, 12 mil wall thickness. (Design according to the invention.)
• 3rd Pressure drop curves plotted over time. Vacuum is standardized to 380 mbar (maximum value). Substrate: SiC filter with zeolite coating, 300 CPSI, 12 mil wall thickness. (Execution not according to the invention.)
• 4th Pressure loss vs loading for 3 parts which are not according to the invention with an initial negative pressure of 380 mbar (with the conditions out 3rd ) have been coated. Substrate: SiC filter with zeolite coating, 300 CPSI, 12 mil wall thickness. (Execution not according to the invention.)
• 5 Comparison of the pressure loss increase of the coated filter in relation to the pressure loss of the uncoated substrate in the inventive and not inventive embodiment. Substrate: SiC filter with zeolite coating, 300 CPSI, 12 mil wall thickness.
• 6 Pressure drop curve during pressure pulse plotted over time with automatic false air damper. Substrate: Cordierite filter with oxide coating containing precious metals, 300 CPSI, 8 mil wall thickness. (Execution according to the invention.)
• 7 Pressure drop curve during pressure pulse without automatic false air damper. Substrate: Cordierite filter with oxide coating containing precious metals, 300 CPSI, 8 mil wall thickness. (Execution not according to the invention.). The 7 has a peak at 100%, but the suction has the effect of the level of approx. 80% of the initial negative pressure and thus produces a worse pressure loss after coating than the conditions in 6 .

Examples:

Comparison of the proportion of coated pore volume depending on the pore diameter after a coating and suction according to the process. The large pores, which largely determine the pressure loss, are largely free. Cordierite 300/8 coated with 75g / l Pore diameter Pore volume in ml / g µm coated uncoated <20 0.25 0.30 > 20 0.35 0.55 > 35 0.10 0.09 > 50 0.07 0.07 > 100 0.04 0.04 > 150 0.03 0.03 > 200 0.02 0.02 Total: 0.61 0.86

The table shows an example that the filter substrates in the area of the large pores hardly lose any pore volume due to the coating according to the method described here.

In the following experiment, the suction time and the solids concentration were varied. All other process and feed parameters were kept constant. The results show that the pressure loss at the filter can be significantly reduced with increasing suction time.

konnten mehr große Poren freigesaugt bzw. freigeblasen werden suction time sec solid concentration of the slurry Dry uptake BP increase 2 35% 100% 16% 4 35% 99% 14% 2 42% 160% 131% 4 42% 153% 61% According to the formula $$\mathrm{\Delta }p\ast t\sim 32\ast \eta \ast \frac{L^{\mathrm{2nd}}}{d^{\mathrm{2nd}}}$$

more large pores could be sucked or blown free suction time sec solid concentration of the slurry Dry uptake BP increase 2nd 35% 100% 16% 4th 35% 99% 14% 2nd 42% 160% 131% 4th 42% 153% 61%

In the following experiment, the suction pressure difference and the solids concentration were varied. All other process and feed parameters were kept constant. The results show that the pressure loss at the filter can be reduced with increasing starting suction pressure.

konnten mehr große Poren freigesaugt bzw. freigeblasen werden suction pressure mbar solid concentration of the slurry Dry uptake BP increase 410 35% 99% 14% 380 35% 99% 17% 410 42% 157% 98% 380 42% 159% 109% According to the formula $$\mathrm{\Delta }p\ast t\sim 32\ast \eta \ast \frac{L^{\mathrm{2nd}}}{d^{\mathrm{2nd}}}$$

more large pores could be sucked or blown free suction pressure mbar solid concentration of the slurry Dry uptake BP increase 410 35% 99% 14% 380 35% 99% 17% 410 42% 157% 98% 380 42% 159% 109%

QUOTES INCLUDE IN THE DESCRIPTION

This list of documents listed by the applicant has been generated automatically and is only included for the better information of the reader. The list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions.

Patent literature cited

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Claims (9)

Process for the production of catalytically coated ceramic wall flow filters with optimized exhaust gas back pressure for the treatment of exhaust gases from a combustion process, the filter having a porosity of ≥50% and an average pore diameter of 5 - 50 µm, characterized in that after the catalytically active material applied to the filter substrate in a directly following step by increasing and / or reducing pressure in the form of one or more pressure pulses opposite to the coating direction, specifically emptying the volume flow through the filter wall to> 50% determining pores, without increasing the amount of catalytically active material the filter substrate is reduced to more than 20%. Procedure according to Claim 1 , characterized in that the wall flow filter has a bimodal pore size distribution. Procedure according to Claim 1 and / or 2, characterized in that one adjusts the strength and / or duration of the pressure pulse on the pore diameter of the pores greater than the d60 value of the filter substrate. Procedure according to one of the Claims 1 - 3rd , characterized in that the catalytically active material contains high-surface area metal compounds, preferably oxidic components, whose average particle diameter (D ₅₀ ) is less than the average pore diameter of the filter substrate. Procedure according to one of the Claims 1 - 4th , characterized in that the method is carried out several times in succession with the same or different catalytically active materials. Catalytically coated ceramic wall-flow filter for the treatment of exhaust gases from a combustion process, the filter having a porosity of> 50% and an average pore diameter of 5-50 µm, produced according to one of the preceding claims. Filter by Claim 6 , characterized in that pores with a pore diameter of ≥40 µm are essentially not filled with catalytically active material. Use the filter according to one of the Claims 6 and / or 7 in a process for the oxidation of hydrocarbons and / or carbon monoxide and / or in a process for nitrogen oxide reduction. Use of the filter after Claim 8 as SDPF.

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