DE102007021471A1 - Method for producing filter for removing particles from gas flow, particularly soot particles from exhaust gas flow of internal combustion engine, involves wetting filter substrate, which is made of ceramic material with sol - Google Patents

Abstract

Filter production involves wetting a filter substrate (18), which is made of a ceramic material with a sol. The sols are converted into a gel through a hydrolysis reaction. The filter substrate is dried. A catalytically active surface coating is applied on the filter substrate, where the surface coating is applied before or after drying. An independent claim is also included for removing particles from a gas flow, particularly soot particles from an exhaust gas flow of an internal combustion engine, which has a filter substrate made of a ceramic material, where the ceramic material has microcracks, which contain a temperature-stable material.

Description

State of the art

The The invention relates to a method for producing a filter for Removal of particles from a gas stream, in particular soot particles from an exhaust gas stream of an internal combustion engine. The invention continues to assume a filter to remove particles a gas stream according to the preamble of the claim 9 off. Such filters are for example in the exhaust aftertreatment self-igniting internal combustion engines, in particular used in diesel-powered vehicles. Usually such particulate removal filters, so-called particulate filters from the ceramic materials silicon carbide, aluminum titanate and / or cordierite. The particulate filters are generally in the form of a honeycomb ceramic with mutually closed Channels formed. Have such particle filter a filtration efficiency of more than 80% to regularly> 90%. The difficulty however, is not alone in the filtration of the soot particles but also in the regeneration of the filter. This will be fuel or its decomposition products in an exhaust aftertreatment containing the Particles, catalytically oxidized to those for ignition of the soot necessary to produce temperatures. While the hotter regeneration phases become highest Requirements for the thermal stability of the filter posed.

Thermochemical Reactions of the filter material with exhaust gas components and during operation over the life of the motor vehicle collecting ashes from the filter, for example from oil, fuel, Fuel additives or engine abrasion, reduce the mechanical and thermochemical strength of ceramic filters. By thermochemical Reaction aged filter, especially if these are made of the materials Cordierite and aluminum titanate are made have a higher Failure probability on as non-aged filter. With high Thermal stress increases the probability of failure.

by virtue of In particular, cordierite and others are used in the manufacturing process ceramic materials a series of microcracks in the microstructure on. These microcracks are partially desirable and perform a significant contribution for example to the filter effect or Catalyst effect of the functional elements. The micro cracks lead although to a lower strength of the ceramic material, but at the same time also to a lower elastic modulus and a lower one Coefficients of thermal expansion and contribute to that thermal stresses in the ceramic are degraded and the thermal Resilience is increased. A low thermal expansion coefficient and ensure a low modulus of elasticity low induced stresses under thermal stress of the substrate. This is particularly due to the fact that the micro-cracks in the ceramic carrier material are after and after close at temperature increase and form such a buffer for thermal expansion.

On the ceramic support usually a catalytic coating is applied. This coating is often also referred to as a "washcoat." For this coating, ceramic materials, for example porous aluminum oxide (A ₂ O ₃ ), are generally ground to a desired particle size, and then a suspension with a certain particle size distribution is produced, and this suspension, the also called Slorry, applied to the ceramic carrier.

The Problem of this known method, however, is that the Suspensions also contain the smallest particles in the microcracks of the ceramic carrier can penetrate. This causes these microcracks when heated of the ceramic carrier not in the above-described Can close measurements. This increases the thermal expansion coefficient of the ceramic support and there are unwanted, increased thermal Tensions.

Next The particles of the washcoat can be removed during the Operation of the filter also the accumulating ashes in the microcracks penetrate, which further enhances the effect described above.

For example, in order to avoid that no particles can penetrate into the microcracks during the application of the catalytic coating, it is not US-A 2005/0037147 known to close the microcracks first by organic polymers. For this purpose, the filter substrate is first impregnated with a solution containing therein dispersed or dissolved organic polymers. The solution penetrates the microcracks, where organic polymers are deposited, closing the microcracks. Then the washcoat is applied. In a subsequent thermal treatment, the organic polymers evaporate completely and thus release the microcracks again.

disadvantage However, it is here that after the removal of the organic Polymers an ash entry or a subsequent partial Fill the microcracks with Washcoatpartikeln can be done.

Disclosure of the invention

Advantages of the invention

• (a) Benetzen eines Filtersubstrates aus einem keramischen Material mit einem Sol,
• (b) Umwandeln des Sols in ein Gel und
• (c) Trocknen und/oder Kalzinieren des Filtersubstrates.

The inventive method for producing a filter for removing particles from a gas stream, in particular soot particles from an exhaust stream of an internal combustion engine, comprises the following steps:

• (a) wetting a filter substrate made of a ceramic material with a sol,
• (b) converting the sol to a gel and
• (c) drying and / or calcining the filter substrate.

At the Wetting the filter substrate penetrates the sol into microcracks of the ceramic Materials. When converting to a gel, individual separated ones crosslink Units that are present in the sol, with each other. This forms a network contained in the microcracks. At the Drying the solvent of the sol is removed. It finds a shrinking process is taking place. That's why the microcracks are not completely filled. By filling the micro-cracks are prevented from being applied to a subsequently applied Coating coating material penetrate into the microcracks can. Because the microcracks are not completely filled, they can close when the temperature rises, whereby stresses acting on the filter are reduced. As a result, a sufficient temperature stability is achieved. By filling the microcracks with the networks, the caused by the gelation, it is further avoided that the Example ashes during operation in the microcracks can penetrate.

The Ceramic material for the filter substrate is preferably selected from cordierite, aluminum titanate and silicon carbide. Particularly preferred is the ceramic material cordierite, since this is the Cheapest is. This makes it possible produce cheap filters.

Of the Separation efficiency of the sol, with which the filter substrate is wetted, in the microcracks, is about drying parameters or influences the capillary forces. The adjustment of the capillary forces is done by the diameter of the pores and microcracks.

The Sol, with which the filter substrate is wetted, preferably contains Metal alcoholates, organometallic compounds or mixtures it. Alcoholates of aluminum, silicon or are preferred Zircon used. These are generally also part of one optionally later applied to the filter substrate Coating. Thus, no further substances are applied. Further suitable substances for the coating are, for example Siloxanes, boehmite or silica. Also can Mixtures of various metal alcoholates and / or organometallic Connections are used.

The Amount of organic radicals of the metal alcoholates or organometallic Compounds can be adapted to the structure of the microcracks. The cracks should during a coating process with be completely closed with another coating, so that nothing of the coating material penetrates into the microcracks can. Furthermore, however, it is necessary that the microcracks when applying a high temperature during operation of the filter yet can close. By the amount and the kind the organic radicals, the amount of inorganic material is adjusted, after a calcination process in which the organic Burn leftovers, remaining in the microcracks. hereby will ensure that a sufficient and not too large amount of inorganic components remains in the microcracks. The amount of inorganic material remaining in the microcracks should be chosen so that the microcracks in operation still close, but no foreign particles in the microcracks can penetrate.

usual organic radicals of the metal alcoholates or organometallic compounds For example, methyl, ethyl, butyl, phenyl, silicon trimethyl or aromatic systems, for example tri-phenyl.

In the sol, with which the filter substrate is wetted from the ceramic material, individually separated units are present. These are the above-described metal alcoholates or organometallic compounds. The conversion of the sol into a gel occurs, for example, by a hydrolysis reaction. This initiates networking. Thus, for example, with the elimination of water, metal-oxygen-metal bonds are formed between the individual separated units. The crosslinking can be controlled, for example by means of additives, for example Bronsted or Lewis acids or bases, for example H ₂ O, acetic acid, citric acid, HNO ₃ , NH ₃ or NaOH or by a modification of the alcoholates used, for example by introduction of proton donor or acceptor groups.

By following the transformation of the sol into a gel Drying step becomes the solvent of the sol, in general Water, away. This leads to a shrinking process and a network of inorganic material remains in the microcrack. When using alcoholates of aluminum, silicon or zirconium Forms a network with Al-O, Si-O or Zr-O bonds.

The Wetting the filter substrate of the ceramic material with the Sol is done, for example, by spraying, dipping, watering or similar coating processes. This is preferred Filter substrate of ceramic material immersed in the sol.

In a preferred embodiment is applied to the filter substrate in a further step after the formation of the gel and drying applied a surface coating. The surface coating is, for example, a catalytically active coating, as the Is known in the art.

When Coating material for surface coating For example, alumina, alumina hydrates, zirconia, Silica or titania. These substances are in general with an oxide of a metal of the third to fifth subgroups, at least one oxide of a lanthanum, including the lanthanum or a mixture of one or more of these oxides doped. Especially preferred is an oxide of the ces, praseodymium or neodymium.

The Surface coating is for example in the form of particles as a slip or as a sol by spraying, dipping, watering or similar coating processes on the ceramic Applied filter substrate. Furthermore, they are also based on vacuum Coating processes suitable.

suitable Cover layers are produced, for example, by combining nanoparticles, that is particles with a mean diameter <1 μm, and microparticles, that is, particles having a middle one Diameter> 1 μm, sometimes with bi or polymodal particle size distribution, achieved.

by virtue of the gel networks contained in the microcracks can during the coating process for producing the Surface coating no particles in the microcracks penetration.

In order to the gel is during the coating process with the Surface coating does not dissolve, it is required that it is not too hydrophilic. On the other hand it is but also necessary that the gel is not too hydrophobic, so the application of the surface coating is not difficult becomes. This is done, for example, by inserting primary, achieved secondary or tertiary siloxane.

to Fixation of the surface coating becomes the filter substrate after application of the particulate slip or sol preferably dried and / or calcined in a further step. The drying is preferably carried out at temperatures in the range from 60 to 150 ° C, calcining at temperatures in the range from 300 to 700 ° C.

When calcining, if this is done, or even at higher temperatures during operation of the filter, the organic radicals of the gel are generally burned. This will destroy the gel network. There remain agglomerates of the inorganic compounds of the gel. When using aluminum, silicon or zirconium alcoholates as starting reagents for producing the gel, the inorganic compounds are aluminum oxide (Al ₂ O ₃ ), silicon oxide (SiO ₂ ) or zirconium oxide (ZrO ₂ ). These agglomerates remain in the microcracks and thus ensure that no foreign substances are introduced into the microcracks during operation. These foreign substances are, for example, ash particles, such as may occur during operation of the internal combustion engine. The ratio of the inorganic constituents to the organic radicals which are burned at high temperatures is adjusted so that the cracks can still close sufficiently to be able to compensate for thermal stresses. A suitable ratio of organic to inorganic constituents is preferably in the range from 95 to 5 to 70 to 30, in particular in the range from 90 to 10 to 80 to 20.

Brief description of the drawings

embodiments The invention is illustrated in the drawings and in the following Description explained in more detail.

It demonstrate

1 a schematic representation of an internal combustion engine with an exhaust gas aftertreatment device according to the invention,

2 a filter substrate according to the invention in longitudinal section,

3 a schematic representation of the coated filter substrate,

4 a procedure for the formation of the gel using the example of a micro-crack.

embodiments the invention

1 shows a schematic representation of an internal combustion engine with an exhaust gas aftertreatment device according to the invention. The exhaust aftertreatment device is a filter in which soot particles are removed from the exhaust gas flow.

An internal combustion engine 10 is over exhaust pipe 12 connected in which a filter device 14 is arranged. With the filter device 14 soot particles are out of the exhaust pipe 12 filtered exhaust gas filtered out. This is especially necessary for diesel engines to comply with legal requirements.

The filter device 14 includes a cylindrical housing 16 in which a rotationally symmetrical in the present embodiment, also a total cylindrical filter substrate 18 is arranged.

2 shows a filter substrate according to the invention in longitudinal section.

The filter substrate 18 For example, as an extruded molded body made of a ceramic material, for. For example, magnesium-aluminum silicate, preferably cordierite produced. The filter substrate 18 will be in the direction of the arrows 20 flows through exhaust gas. The exhaust gas passes over an entrance surface 22 in the filter substrate 18 and leaves this via an exit surface 24 ,

Parallel to a longitudinal axis 26 of the filter substrate 18 run several inlet channels 28 in alternation with outlet channels 30 , The entrance channels 28 are at the exit surface 24 locked. In the embodiment shown here are sealing plugs for this purpose 36 intended. Instead of the sealing plugs 36 However, it is also possible that entrance channels 28 to the exit surface 24 Rejuvenate until the walls of the entrance channel 28 touch and the entrance channel 28 so closed. In this case, the inlet channel 28 in the direction parallel to the longitudinal axis 26 a triangular cross section.

Accordingly, the outlet channels 30 at the exit surface 24 open and in the area of the entrance area 22 locked.

The flow path of the unpurified exhaust gas thus leads into one of the inlet channels 28 and from there through a filter wall 38 in one of the exit channels 30 , This is exemplified by the arrows 32 shown.

In 3 a schematic representation of the coated filter substrate is shown. A filter wall 38 is made of a ceramic filter substrate. The ceramic filter substrate consists of individual crystallites 40 which are generally interconnected by sintering. The ceramic filter substrate is preferably silicon carbide, aluminum titanate or cordierite. Also, mixtures of these materials are possible. Between the individual crystallites 40 of the ceramic filter substrate are pores 42 , which are flowed through by the gas stream to be purified. The particles contained in the gas stream become from the ceramic filter substrate of the filter wall 38 retained. The particles that are removed from the gas stream also settle in the pores 42 from. This reduces the free cross section in the filter wall 38 and the pressure loss across the filter wall 38 rises. For this reason, it is necessary to remove the particles from the pores at regular intervals. This is generally done by thermal regeneration by heating the filter to a temperature in excess of 600 ° C. At this temperature, the usually organic particles burn to carbon dioxide and water and are discharged in gaseous form out of the particle filter.

The crystallites 40 of the ceramic filter substrate are generally coated with a surface 44 coated. The surface coating is usually catalytically active to oxidize hydrocarbons contained in the exhaust gas, for example, contained CO to CO ₂ and in the exhaust gas to CO ₂ and H ₂ O as well as to reduce nitrogen oxides to nitrogen and oxygen. Usually, the surface coating contains 44 Particles containing a catalytically active material. The particles are usually sintered to a ceramic coating. The production of such surface coatings 44 is known in the art.

In the individual crystallites 40 are generally micro cracks 46 contain. At high temperatures, the material can expand by the microcracks 46 getting closed. As a result, tensions in the structure of the crystallites 40 reduced. Are the microcracks 46 due to components originating from the coating process or completely or partially blocked by ash, the filter substrate can not expand sufficiently. As a result, locally very high voltages can build up, which can lead to reversible cracking, especially at high temperatures under hydrothermal conditions. The filter substrate 18 tears open.

To close the microcracks 46 with particles of the coating material to prevent and to prevent the ingress of ash during ongoing operation of the internal combustion engine, the microcracks 46 filled with a gel.

In 4 a method for filling microcracks with a gel is exemplified for a microcrack.

In a first step, the filter substrate is wetted with a sol. Wetting is done, for example, by spraying, dipping, soaking or the like coating processes. By moistening sol penetrates 48 into the microcracks 46 that exist in the individual crystallites 40 of the filter substrate are included. The sol preferably contains organometallic compounds or alcoholates of aluminum, silicon or zirconium. Suitable constituents of the sol are, for example, aluminum alcoholates, siloxanes, boehmite or silicic acid. Mixtures of these substances can also be used. The alcoholates or organometallic compounds are present in the sol as individually separated units, which are not crosslinked with each other. Hydrolysis initiates crosslinking of the individual units separated from each other. This forms metal-oxygen-metal bonds with elimination of water. It creates a gel network 52 in the microcrack 46 , By adding suitable additives or by a modification of the alkoxides used, the crosslinking can be controlled.

In a subsequent drying step 54 the solvent is removed. These are generally water. Drying causes a shrinkage process. In the microcrack 46 remains a dry network 56 , This contains metal-oxygen bonds and organic components. The metal-oxygen bonds are dependent on the alkoxides or organometallic compounds used, generally aluminum-oxygen bonds, silicon-oxygen bonds or zirconium-oxygen bonds or combinations thereof.

The degree of separation of the sol 48 in the microcracks 46 as well as the formation of the gel network 52 in the microcracks 46 can be adjusted by the drying parameters or the capillary tension. So the microcracks 46 through the gel network 52 The drying is generally carried out in the range of 40 to 120 ° C over a period of 5 to 30 minutes.

The Capillary stresses can be, for example, by adding surface-active Adjust substances, for example surfactants. The more of the surfactant is added, the lower will be the surface tension. As surface-active Substances are both non-ionic, and anionic, cationic and amphoteric surfactants. When choosing the surfactants is However, make sure that these are not with the alcoholates used or organometallic compounds react. In particular as surfactants suitable are polyoxyethylene sorbitan monolaurate, polyoxyethylene sorbitan monopalmitate, Polyoxyethylene sorbitan monostearate, polyoxyethylene sorbitan monooleate, Polyoxyethylene sorbitan trioleate, polyoxyethylene (20) stearyl ether, Polyoxyethylene (10) lauryl ether, polyoxyethylene (4) lauryl ether, polyoxyethylene (2) cetyl ether, Polyoxyethylene (10) cetyl ether and polyoxyethylene (20) stearyl ether.

QUOTES INCLUDE IN THE DESCRIPTION

This list The documents listed by the applicant have been automated generated and is solely for better information recorded by the reader. The list is not part of the German Patent or utility model application. The DPMA takes over no liability for any errors or omissions.

Cited patent literature

• US 2005/0037147 A [0007]

Claims (11)

Method for producing a filter for removing particles from a gas stream, in particular soot particles from an exhaust gas stream of an internal combustion engine, comprising the following steps: (a) wetting a filter substrate ( 18 ) of a ceramic material with a sol ( 48 ), (b) converting the sol ( 48 ) into a gel ( 52 ) and (c) drying and / or calcining the filter substrate ( 18 ). Method according to claim 1, characterized in that the conversion of the sol ( 48 ) into a gel ( 52 ) by a hydrolysis reaction. Process according to claim 1 or 2, characterized in that the sol ( 48 ) Metal alkoxides, organometallic compounds or mixtures thereof. Method according to one of claims 1 to 3, characterized in that the sol ( 48 ) Aluminum alkoxides, silicon alkoxides, zirconium alcoholates, siloxanes, boehmite, silica or mixtures thereof. A method according to claim 4, characterized in that the alcoholates contain organic radicals selected from methyl, ethyl, butyl, phenyl, SiMe ₃ , Ph ₃ and aromatic systems. Method according to one of claims 1 to 5, characterized in that in a further step, a catalytically active surface coating on the filter substrate ( 18 ), wherein the surface coating may be applied before or after drying in step (c). A method according to claim 6, characterized in that the catalytically active surface coating in the form of particles as a slip or as a sol on the filter substrate ( 18 ) is applied. Method according to claim 6 or 7, characterized in that the filter substrate ( 18 ) is dried and / or calcined after application of the catalytically active surface coating. Filter for removing particles from a gas stream, in particular soot particles from an exhaust gas stream of an internal combustion engine, a filter substrate ( 18 ) comprising a ceramic material, the ceramic material having microcracks ( 46 ), characterized in that in the microcracks ( 46 ) Particles are contained in a temperature-stable material. Filter according to claim 9, characterized characterized in that the temperature-stable material is selected is of alumina, silica, zirconia and mixtures thereof. Filter according to claim 9 or 10, characterized in that the ceramic material of the filter substrate ( 18 ) Alumina, silicon carbide or cordierite.