COATING PROCESS FOR A WALL-FLOW FILTER

Abstract

The present invention relates to a method for coating wall-flow filters. It also relates to correspondingly produced wall-flow filters and to their use in exhaust gas cleaning.

Claims

1. Method for coating a wall-flow filter with a coating suspension that gives the filter a catalytic activity during exhaust gas cleaning, wherein the wall-flow filter is brought into contact with a liquid comprising water in a first step and a coating suspension is applied to the wall-flow filter in a second step, characterized in that the coating suspension is one that meets the following criteria: Q3 values of the grain size distribution (d90?d10)/d50<5 of the particles present in the suspension; viscosity <2000 mPas at a shear of 20 1/s.

2. Method according to claim 1, characterized in that in the second step, the coating suspension is introduced in excess into the wall-flow filter from below by applying a pressure difference across the vertically locked wall-flow filter, and subsequently a pressure difference reversal removes an excess of the coating suspension from the wall-flow filter.

3. Method according to claim 1, characterized in that in the first step, the wall-flow filter is contacted with the liquid comprising water over less than the entire length of the filter.

4. Method according to claim 1, characterized in that the filter is already vertically locked in the first step and is rotated by 180? after the first step before the coating suspension is introduced into the filter in the second step.

5. Method according to claim 1, characterized in that the coating suspension is predominantly introduced into the wall of the filter.

6. Method according to claim 1, characterized in that the average particle size d50 of the Q3 distribution in the coating suspension in relation to the average pore diameter D50 of the Q3 distribution is >33%.

7. Method according to claim 1, characterized in that the coating suspension has a density between 1050 kg/m.sup.3 and 1700 kg/m.sup.3.

8. Method according to claim 1, characterized in that after the coating and subsequent drying of the coated filter, the gradient of the coating suspension in the longitudinal direction is less than 10%.

9. Wall-flow filter produced according to claim 1.

10. Wall-flow filter according to claim 9, characterized in that it is provided with an SCR-active coating suspension.

11. Use of the wall-flow filter according to claim 9 for reducing harmful exhaust gas components of internal combustion engines.

Description

[0029] In a further preferred embodiment of the present method, the applied pressure difference for filling the filter with washcoat is between 0.05 and 4 bar, preferably between 0.1 and 2 bar, and particularly preferably between 0.5 and 1.5 bar. For this purpose, the pressure difference used for filling is preferably between 0.05 bar and 2 bar, more preferably between 0.07 and 1 bar and particularly preferably between 0.09 and 0.7 bar. For the pressure difference reversal, the person skilled in the art will preferably refer to the method specified in DE102019100107A1.

[0030] A further subject matter of the present application is a wall-flow filter produced according to the invention. The embodiments mentioned as preferred for the method also apply mutatis mutandis to the wall-flow filter referred to herein. The wall-flow filter can be provided with various catalytically active coatings. In particular, these are coatings that have three-way activity and a diesel oxidation catalyst or are active in ammonia oxidation or nitrogen oxide reduction by means of ammonia. Most preferably, the filter is provided with an SCR-active coating suspension, preferably to the greatest extent possible in the wall.

[0031] The present invention likewise provides for the use of a filter according to the invention after drying and optionally calcination, preferably for reducing harmful exhaust gas components of internal combustion engines. In principle, all exhaust gas aftertreatments which are suitable to a person skilled in the art for this purpose can be used as such. Preferably, filters having the above-indicated catalytic properties, but in particular SCR catalysts, are used. The wall-flow filters produced using the method according to the invention are suitable for all these applications. The use of these filters for the treatment of exhaust gases of a lean burning car engine is preferred.

[0032] All ceramic materials customary in the prior art 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 preferably used. These wall-flow filter substrates have inflow and outflow channels, wherein the respective downstream ends of the inflow channels and the upstream ends of the outflow channels are alternately closed off with gas-tight plugs. In this case, the exhaust gas that is to be purified and that flows through the filter substrate is forced to pass through the porous wall between the inflow channel and outflow channel, which delivers an excellent particulate filtering effect. The filtration property for particulates can be designed by means of the porosity, pore/radii distribution, and thickness of the wall. The porosity of the uncoated wall-flow filters is typically more than 40%, generally from 40% to 75%, particularly from 50% to 70% [measured according to DIN 66133, latest version on the filing date]. The average pore size (diameter) of the uncoated filters is at least 7 ?m, for example from 7 ?m to 34 ?m, preferably more than 10 ?m, in particular more preferably from 10 ?m to 25 ?m or most preferably from 15 ?m to 20 ?m [measured according to DIN 66133, latest version on the date of application]. The completed filters with a pore size of typically 10 ?m to 20 ?m and a porosity of 50% to 65% are particularly preferred.

[0033] The use of the wall-flow filter as an SCR-active catalyst support (known as an SDPF) is preferred. For this SCR treatment of the preferably lean exhaust gas, ammonia or an ammonia precursor compound is injected into the exhaust gas and both are conducted over a SCR-catalytically coated wall-flow filter produced 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 reduction can take place as completely as possible. A temperature range of 225? C. to 350? C. for the reduction is particularly preferred. Furthermore, optimum nitrogen oxide conversions are only achieved when there is a molar ratio of nitrogen monoxide to nitrogen dioxide (NO/NO.sub.2=1) or the NO.sub.2/NOx ratio=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 conversions beginning with 75% conversion, already at 250? C. with simultaneously optimal selectivity to nitrogen, are only achieved, according to the stoichiometry of the reaction equation

##STR00001##

with an NO.sub.2/NOx ratio of around 0.5. This applies not only to SCR catalysts based on metal-exchanged zeolites but to all common, i.e., commercially available, SCR catalysts (so-called fast SCRs). A corresponding NO:NO.sub.2 content may be achieved with oxidation catalysts positioned upstream of the SCR catalyst.

[0034] Wall flow filters having an SCR-catalytic function are referred to as SDPF. These catalysts frequently possess a function for storing ammonia and a function whereby nitrogen oxides can react with ammonia to form harmless nitrogen. An NH.sub.3-storing SCR catalyst can be designed in accordance with types known to the person skilled in the art. In the present case, this is a wall-flow filter which is coated with a material catalytically active for the SCR reaction and in which the catalytically active material, commonly called the washcoat, is present in the pores of the wall-flow filter. However, along with thein the proper sense of the termcatalytically active component, this wall flow filter may also contain other materials, such as binders consisting of transition metal oxides, and large-surface carrier oxides, such as titanium oxide, aluminum oxide, in particular gamma-Al.sub.2O.sub.3, zirconium oxide, or cerium oxide. Also suitable as SCR catalysts are those that are made up of one of the materials listed below. However, it is also possible to use zoned or multilayer arrangements or even arrangements consisting of a plurality of components one behind the other (preferably two or three components) with the same materials as the SCR component or different materials. Mixtures of different materials on a substrate are also conceivable.

[0035] The actual catalytically active material used in this regard is preferably selected from the group of transition-metal-exchanged zeolites or zeolite-like materials (zeotypes). Such compounds are sufficiently familiar to the person skilled in the art. Preferred in this regard are materials from the group consisting of levynite, AEI, KFI, chabazite, SAPO-34, ALPO-34, zeolite ?, and ZSM-5. Zeolites or zeolite-like materials of the chabazite type, in particular CHA or SAPO-34, as well as LEV or AEI are particularly preferred. In order to ensure sufficient activity, these materials are preferably provided with transition metals from the group consisting of iron, copper, manganese, and silver. It should be mentioned in this respect that copper is especially advantageous. The ratio of metal to framework aluminum or, in the case of SAPO-34, the ratio of metal to framework silicon is normally between 0.3 and 0.6, preferably 0.4 to 0.5. The person skilled in the art knows how to equip the zeolites or the zeolite-like materials with the transition metals (EP0324082A1, WO1309270711A1, WO2012175409A1, and the literature cited therein) in order to be able to deliver good activity with respect to the reduction of nitrogen oxides with ammonia. Furthermore, vanadium compounds, cerium oxides, cerium/zirconium mixed oxides, titanium oxide, and tungsten-containing compounds, and mixtures thereof can also be used as catalytically active material.

[0036] Materials which in addition have proven themselves to be advantageous for the application of storing NH.sub.3 are known to the person skilled in the art (US20060010857A1, WO2004076829A1). In particular, microporous solid materials, such as so-called molecular sieves, are used as storage materials. Such compounds, selected from the group consisting of zeolites, such as mordenites (MOR), Y-zeolites (FAU), ZSM-5 (MFI), ferrierites (FER), chabazites (CHA), and other small pore zeolites, such as LEV, AEI, or KFI, and ?-zeolites (BEA), as well as zeolite-like materials, such as aluminum phosphate (AIPO) and silicon aluminum phosphate SAPO or mixtures thereof, can be used (EP0324082A1). Particularly preferably used are ZSM-5 (MFI), chabazites (CHA), ferrierites (FER), ALPO- or SAPO-34, and ?-zeolites (BEA). Especially preferably used are CHA, BEA, and AIPO-34 or SAPO-34. Extremely preferably used are materials of the LEV or CHA type, and here maximally preferably CHA or LEV or AEI. Insofar as a zeolite or a zeolite-like compound as just mentioned above is used as catalytically active material in the SCR catalyst, the addition of further NH.sub.3-storing materials can, advantageously, naturally be dispensed with. Overall, the storage capacity of the ammonia-storing components used can, in a fresh state at a measuring temperature of 200? C., be more than 0.9 g NH.sub.3 per liter of catalyst volume, preferably between 0.9 g and 2.5 g NH.sub.3 per liter of catalyst volume, and particularly preferably between 1.2 g and 2.0 g NH.sub.3/liter of catalyst volume, and very particularly preferably between 1.5 g and 1.8 g NH.sub.3/liter of catalyst volume. The ammonia-storing capacity can be determined using synthesis gas equipment. To this end, the catalyst is first conditioned at 600? C. with NO-containing synthesis gas to fully remove ammonia residues in the drilling core. After the gas has been cooled to 200? C., ammonia is then metered into the synthesis gas at a space velocity of, for example, 30,000 h.sup.?1 until the ammonia storage in the drilling core is completely filled, and the ammonia concentration measured downstream of the drilling core corresponds to the starting concentration. The ammonia-storing capacity results from the difference between the amount of ammonia metered overall and the amount of ammonia measured on the downstream side based on the catalyst volume. The synthesis gas is here typically composed of 450 ppm NH.sub.3, 5% oxygen, 5% water, and nitrogen.

[0037] So-called three-way catalysts are used for exhaust gas reduction for stoichiometrically burning engines. Three-way catalysts (TWCs) have long been known to those skilled in the art and have been legally prescribed since the eighties in the last century. The actual catalyst mass here consists for the most part of a high-surface, oxidic substrate material, on which the catalytically active components are deposited with the finest distribution. The precious metals of the platinum group, platinum, palladium and/or rhodium are particularly suitable as catalytically active components for cleaning stoichiometrically composed exhaust gases. For example, aluminum oxide, silicon dioxide, titanium oxide, zirconium oxide, cerium oxide and their mixed oxides, and zeolites are suitable as substrate materials. What are known as active aluminum oxides having a specific surface (BET surface, measured according to DIN 66132the latest version at the time of filing) of more than 10 m.sup.2/g are preferably used. Moreover, three-way catalysts include oxygen-storing components to improve the dynamic conversion. These include cerium/zirconium mixed oxides which are optionally provided with lanthanum oxide, praseodymium oxide and/or yttrium oxide. Meanwhile, zoned and multilayer systems having three-way activity have also become known (U.S. Pat. No. 8,557,204; 8,394,348). If such a three-way catalytic converter is located on or in a particle filter, this is referred to as a cGPF (catalyzed gasoline particle filter; for example EP 2650042B1).

[0038] In the context of the invention, in-wall coating means that generally more than 80% of the coating composition is present in the wall of the wall-flow filter. 80% of the coating composition is present in the longitudinal section of the wall of the wall-flow filter in a region below the surface of the wall. This can be determined by means of corresponding recordings and computer-assisted evaluation methods.

[0039] Surprisingly, the present invention makes it possible to achieve an improved coating of wall-flow filters when the coating is applied from below, for example by pumping the coating suspension (pressure difference across the wall-flow filter) and subsequent removal of excess coating suspension by a pressure difference reversal, preferably downwards. By applying a liquid zone, the capillary forces during coating with coating suspension can be minimized, because in particular the smaller pores are already filled with liquid when the coating suspension reaches the wetted position. This results in a lower concentration of the suspension and thus to a smaller increase in filter cake thickness in the channels. The gradient for the coating in the coating direction is reduced and the drop in permeability in the coating direction is reduced. Against the background of the known prior art, this was not to be expected.