EP2059660A1

EP2059660A1 - Filter for removing particles from a gas stream and method for producing said filter

Info

Publication number: EP2059660A1
Application number: EP07787330A
Authority: EP
Inventors: Bernd Reinsch; Christoph Osemann
Original assignee: Robert Bosch GmbH
Current assignee: Robert Bosch GmbH
Priority date: 2006-08-31
Filing date: 2007-07-10
Publication date: 2009-05-20
Also published as: US20110091361A1; KR20090056999A; WO2008025603A1; DE102006040739A1

Abstract

The invention relates to a filter for removing particles from a gas stream, in particular soot particles from an exhaust gas stream of an internal combustion engine. Said filter comprises a filter body consisting of a ceramic filter substrate, the latter being coated with a porous protective layer of (46, 48) coating material. The invention is characterised in that the coating material contains an admixture of between 1 and 20 % by weight of at least one compound of an element of the 2nd main group of the periodic table, preferably an oxide of an element of the 2nd main group, and that cracks (44) in the ceramic filter substrate are partially filled with the coating material. The invention also relates to a method for producing a filter, according to which the coating material for the porous protective layer (46, 48) is applied to the sintered ceramic filter substrate, at least one compound of an element of the 2nd main group is added, other substances contained in the protective layer (46, 48) are optionally added and the porous protective layer (46, 48) is fixed.

Description

description

title

Filter for removing particles from a gas stream and method for his

manufacturing

State of the art

The invention relates to a filter for removing particles from a gas stream according to the preamble of claim 1 and to a method for producing the filter.

Such filters are used, for example, in the exhaust aftertreatment of self-igniting internal combustion engines, in particular in diesel-powered motor vehicles. Usually, such filters for the removal of particles, so-called particulate filter made of a ceramic material, for example silicon carbide, aluminum titanate or cordierite. The particle filters are generally in the form of a honeycomb ceramic with mutually closed channels. Such particle filters have a filtration efficiency of more than 80% to regularly greater than 90%. However, the difficulty is not only in the filtration of soot particles but also in the regeneration of the filter. For this purpose, fuel or its decomposition products are catalytically oxidized in an exhaust aftertreatment arrangement, which comprises the particle filter, in order to generate the temperatures necessary for the ignition of the soot. During the hotter regeneration phases, the highest demands are placed on the thermal stability of the filter. Such a filter is known for example from US-B 6,898,930. In order to react noxious gases contained in the exhaust gas, the diesel particulate filter described in US Pat. No. 6,898,930 can be provided with a coating which contains a catalytically active material, for example a noble metal.

Ceramic filter materials generally have microcracks that contribute to the thermal stability of the filter. These are simply called "expansion joints" because they close due to thermal expansion of the material and thus reduce the thermally induced stresses of the filter component As the number of microcracks increases, the thermal expansion coefficient and thermal conductivity of the ceramic filter decreases one, the stabilizing effect can As a result, such ceramic filters, especially after high thermal stress, especially if they consist of the filter materials cordierite or aluminum titanate, have a higher probability of failure.

From US 4,532,228 a catalyst is known from a ceramic material in which microcracks in the ceramic material are filled prior to the application of a coating with an organic substance, so that the coating can not penetrate into the microcracks. After application of the coating, the organic substance is removed by burning out. It is also known from US Pat. No. 4,451,517 to close microcracks in the ceramic with an organic substance in the case of a honeycomb catalyst before applying a coating of aluminum oxide. In this way, both US Pat. No. 4,532,228 and US Pat. No. 4,451,517 are intended to prevent coating material from penetrating into the microcracks.

Disclosure of the invention

Advantages of the invention

An inventively designed filter for removing particles from a gas stream, in particular soot particles from an exhaust gas stream of an internal combustion engine, comprising a filter body of a ceramic filter substrate, wherein the filter substrate is coated with a porous protective layer of a coating material. In the present invention, the coating material contains an admixture of 1 to 20% by weight of at least one member of the second main group, preferably an oxide of an element of the second main group, and cracks contained in the ceramic filter substrate are partially filled by the coating material.

By partially filling the cracks in the ceramic filter substrate, it is avoided that particles can penetrate into these cracks. This increases the stability of the filter.

Besides partially filling the cracks in the ceramic filter substrate, it is possible for the protective layer to be in the form of a thin film or a thin layer on the filter substrate.

The addition of the at least one compound of an element of the second main group increases the thermal resistance of the protective layer. The coating material for the porous protective layer is preferably selected from the group consisting of aluminum oxide, aluminum hydroxide, titanium dioxide, silicon dioxide, zirconium dioxide, cerium oxide, aluminum silicates, magnesium aluminum silicates, cordierite, multilite, silicon carbide, aluminum titanate, zeolites, quartz, glasses and Mixtures thereof. The coating material for the porous protective layer is preferably selected from the group consisting of aluminum oxide, aluminum hydroxide, titanium dioxide, silicon dioxide, zirconium dioxide, cerium oxide, mixtures and mixed oxides thereof. Thus, for example, mixed oxides of aluminum oxide and silicon oxide with a mass fraction of up to 15% by weight of silicon dioxide, based on aluminum oxide, are suitable for use with zeolites rich in silicon or mixed oxides of cerium oxide and zirconium oxide.

In a preferred embodiment, the coating material for the porous protective layer further contains at least one alkali metal compound, preferably an alkali metal oxide for adjusting the morphology of the protective layer. The proportion of the at least one alkali metal compound based on the coating material is preferably up to 0.5% by weight.

The coating material for the porous protective layer may further contain at least one compound of a third to fifth subgroup element or lanthanides including lanthanum, preferably an oxide of a third to fifth subgroup element or lanthanides including lanthanum. The proportion of the compound of the element of the third to fifth subgroup or the lanthanides including the lanthanum, preferably in the range up to 5 wt .-%. By adding the compound of an element of the third to fifth subgroups or the lanthanides including the lanthanum, the thermal stability of the support material is further increased.

Mixtures of the substances for the porous protective layer are possible in any ratio. Thus, for example, mixtures of aluminum oxide with up to 18% by weight of BaO, 0.03% by weight of K ₂ O, 6% by weight of CeO ₂ and 8% by weight of ZrO _{2 are} preferred.

In a further embodiment, at least one further protective layer is applied to the porous protective layer. The porous protective layers can consist of the same or of different materials. The individual porous protective layers can fulfill different functions. Furthermore, it is also possible that, for example, two or more different porous protective layers are applied alternately one above the other. In one embodiment, at least one of the porous protective layers contains a catalytically active component. As the catalytically active component are preferably one or more metals from the group of platinum metals, preferably platinum, rhodium and / or palladium. The catalytically active component can be contained in the porous protective layer, with which the ceramic filter substrate is coated or in one of the protective layers applied thereto. It is also possible, if only a porous protective layer is applied to the ceramic filter substrate, that this protective layer contains the catalytically active substance.

With the help of the catalytically active substance noxious gases, such as unburned fuel, its decomposition products, for example carbon monoxide, as well as nitrogen oxides, sulfur oxides and soot can be stored or converted catalytically. The catalytic function is suitable to withstand the thermochemical attack of exhaust gas components.

The at least one catalytically active component is treated in a manner customary for the preparation of catalysts. In this case, mixtures of a plurality of catalytically active substances in a porous protective layer or even a plurality of different catalytically active substances can be used on different porous protective layers. The catalytically active substances, preferably noble metals, may also be present as alloys or mixtures.

In one embodiment, the porous protective layer is applied in the downstream and / or central region of the filter. Furthermore, it is also possible for individual regions of the filter substrate to be coated with different layers, amounts or layer sequences.

In a further embodiment, the porous protective layer is applied in the inflow-side region of the filter. In addition, special applications allow or require application to radial edge areas of the filter.

In the finished filter, the coating materials are preferably in the form of their oxides. However, it is also possible that the coating materials are in the form of their nitrates, hydroxides, acetates, oxalates, carbonates or similar compounds. In general, however, these compounds decompose at least temporarily to oxides under the operating conditions of the filter. Furthermore, it is also possible that these compounds are temporarily formed from the oxides under the operating conditions of the filter. The process according to the invention for producing a filter for removing particles from a gas stream comprises the following steps:

(a) addition of the at least one compound of a second main group element in the form of a solid, as a gel, as a salt solution or as a hydrosol or a mixture thereof,

(b) applying the coating material for the porous protective layer to the sintered ceramic filter substrate,

(c) optionally adding further substances contained in the coating,

(d) fixing the porous protective layer.

The coating material for forming the porous protective layer preferably has a BET surface area of more than 10 m ² / g and a pore volume in the range of 0.1 to 1.5 ml / g in powder form. The average particle size (D50) of the coating materials suitable for forming the protective layer varies widely. Particularly suitable are particles in the size of 2 nm to 20 microns. Especially suitable are particles with a size of more than 1 micron. The particles suitable for coating can be obtained, for example, by precipitation processes or by pyrolytic processes. For adjusting the particle size and the particle size distribution, milling processes or precipitation processes are suitable, for example. Any other processes known to those skilled in the art for adjusting particle size and particle size distribution are also possible. To adjust the particle size and the particle size distribution by precipitation processes, for example, inorganic salt solutions and organometallic solutions can be used as precursors.

Suitable coatings are formed, for example, by combination of differently sized particles, sometimes with bimodal or polymodal particle size distribution.

The coating material for the porous protective layer is preferably applied in a sol-gel process, as a preformed sol or gel or as a suspension of solid particles. The rheological properties of this coating composition and the particle size distribution are adjusted so that the coating composition is suitable for partially infilling the cracks in the ceramic filter substrate. The application of the coating material for the porous protective layer can be carried out by all coating processes known to the person skilled in the art. Suitable coating processes are, for example, spraying, dipping, impregnating or the like. Furthermore, coating processes based on vacuum or pressure are also suitable.

For application of the coating material for the porous protective layer, in addition to aqueous solutions, hydrosols, hydrogels or aqueous suspensions, it is also possible to use organosols, organogels or organic solutions or dispersions which have a lower surface tension than their aqueous homologs. In particular, aqueous media whose surface tension has been reduced by inorganic or organic additives are also suitable. Suitable additives are, for example, long-chain alcohols and surfactants.

The at least one compound of an element of the second main group or also optionally further substances contained in the protective layer, for example, the at least one alkali metal compound, the compound of an element of the third to fifth subgroup or the lanthanides including the lanthanum or the catalytically active component, for example in the form of a solid, for example as an oxide,

Mixed oxide, hydroxide or salt, preferably carbonate, nitrate or acetate and / or as a gel, for example as a hydroxide, as a salt solution, preferably as a carbonate, nitrate or acetate, or added as a hydrosol. The at least one compound of an element of the second main group and optionally further contained in the coating

Substances may be added to the raw material of the coating material, the coating composition or the finished coating. The addition may be effected in any manner known to the person skilled in the art.

The fixing of the porous protective layer in step (d) is carried out by conventional methods known to the person skilled in the art. Suitable methods are, for example, drying, calcination and sintering.

The amount of coating materials to be applied to form the porous protective layer can be varied within wide limits. The loading of the filter with the coating material is based on the filter volume and is 1 g / l to 200 g / l, preferably 10 to 150 g / l, based on the total filter volume.

Brief description of the drawings Embodiments of the invention are illustrated in the drawings and explained in more detail in the following description.

Show it

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

FIG. 2 shows a longitudinal section of a filter element according to the invention,

FIG. 3 shows a schematic representation of the coated filter substrate with a layer,

4 shows an example of a grain of the filter substrate with a coating in several layers.

Embodiments of the invention

FIG. 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 here a filter in which soot particles are removed from the exhaust gas flow.

An internal combustion engine 10 is connected via an exhaust pipe 12, in which a filter device 14 is arranged. Optionally, the arrangement contains one or more catalysts 19, which are arranged in front of the filter device 14. With the filter device 14 soot particles are filtered out of the exhaust gas flowing in the exhaust pipe 12. This is especially necessary for diesel engines to comply with legal requirements.

The filter device 14 comprises a cylindrical housing 16, in which a rotationally symmetrical in the present embodiment, a total of cylindrical filter element 18 is arranged.

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

The filter element 18 is produced, for example, as an extruded shaped body made of a ceramic material, for example magnesium aluminum silicate, preferably cordierite. The filter element 18 is traversed by exhaust gas in the direction of the arrows 20. The exhaust gas occurs via an entrance surface 22 in the filter element 18 and leaves this via an exit surface 24th

Parallel to a longitudinal axis 26 of the filter element 18 extend a plurality of inlet channels 28 in alternation with outlet channels 30. The inlet channels 28 are closed at the outlet surface 24. In the embodiment shown here, sealing plugs 36 are provided for this purpose.

Accordingly, the outlet channels 30 are open at the outlet surface 24 and closed in the region of the inlet surface 22.

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 into one of the outlet channels 30. This is illustrated by the arrows 32 by way of example.

FIG. 3 shows a schematic representation of the coated filter substrate with a layer. A filter wall 38 is made of a ceramic filter substrate. The ceramic filter substrate consists of individual grains 40, which are generally interconnected by sintering. The ceramic filter substrate is preferably silicon carbide, aluminum titanate, mullite or cordierite. Also, mixtures of these materials are possible. Between the individual grains 40 of the ceramic filter substrate are pores 42, which are flowed through by the gas stream to be cleaned. Particles contained in the gas stream are retained by the ceramic filter substrate of the filter wall 38. The particles removed from the gas stream also settle in the pores 42. This reduces the free cross section of the filter wall 38 and the pressure loss across the filter wall 38 increases. 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 from the particle filter in gaseous form.

Microcracks 44 are generally included in the individual grains. The microcracks 44 contribute to the thermal stability of the filter. With a thermal expansion of material, the microcracks 44 close and thus reduce the thermally induced stresses in the filter wall 38. As the number of microcracks 44 increases, the thermal expansion coefficient and the thermal conductivity of the ceramic filter 14 decrease. However, if particles penetrate the microcracks 44 in the grains 40, the stabilizing effect may be limited since the microcracks 44 will not expand upon thermal expansion of the material can close more. This increases the thermally induced stresses in the grains. The occurring stresses can lead to rupture of the filter wall 38.

In order to avoid the penetration of particles into the microcracks 44, these are partially filled with a porous protective layer 46. The material for the porous protective layer is as already described above preferably selected from the group consisting of aluminum oxides, aluminum hydroxide, titanium dioxide, silicon dioxide, zirconium dioxide, cerium oxide, aluminum silicates, magnesium aluminum silicates, cordierite, mullite, silicon carbide, aluminum titanate, zeolites, Quartz, glasses, mixtures thereof and mixed oxides thereof. The coating material is 1 to 20 wt .-% of at least one compound of an element of the second main group, preferably an oxide of an element of the second main group, admixed. Further, the porous protective layer may contain alkali metal compounds or compounds of an element of the third to fifth subgroups or the lanthanides including the lanthanum.

In addition to the at least partially filled microcracks 44, the coating material may also form a thin layer or film on the grains 40 of the filter substrate. In particular, if the coating material also forms a thin film or a thin layer on the grains 40 of the filter substrate, the ceramic coating may further contain at least one catalytically active material. Particularly suitable catalytically active material are noble metals from the group of platinum metals, for example platinum, rhodium or palladium. The catalytically active material contained in the coating also stores noxious gases and soot particles and converts them thermally catalytically. The reaction of noxious gases is generally exothermic, releasing heat of reaction. This heat of reaction contributes to the achievement of the exhaust gas temperature necessary for the regeneration of the filter.

The rheological properties and optionally the particle sizes of the particles contained in the coating composition are preferably adjusted so that portions of the coating material can also penetrate into the microcracks 44. The coating material is preferably applied in a sol-gel process, as a performed sol or gel or as a suspension of solid particles.

FIG. 4 shows a grain 40 of the filter substrate with a coating of two layers. In the grain 40, a micro-crack 44 is formed. The grain 40 shown here comprises a first protective layer 48 consisting of a coating material as described above. The coating material 48 also partially contains the microcrack 44 contained in the grain 40. Both the microcrack 44 and a branch 50 of the microcrack 44 include an unfilled region 52 that was not filled by the coating material.

In the embodiment illustrated here, the grain 40 is completely covered by the first layer 48. The first layer 48, which has also partially filled the microcrack 44, increases the thermal and hydrothermal stability of the grain 40. In the embodiment shown here, a second layer 54 is applied to the first layer 48. The second layer 54 also consists essentially of a ceramic or mineral oxide, as described above. Furthermore, it is possible that either the first layer 48 or the second layer 54 contain a catalytically active material. The first layer 48 and the second layer 54 may also both contain the catalytically active material. Furthermore, it is possible for the first layer 48 and the second layer 54 to consist of the same coating material. However, also the first layer 48 and the second layer 54 may be made of different coating materials.

In addition to the embodiment illustrated in FIG. 4, it is furthermore possible for at least one further porous protective layer to be applied to the second layer 54. It is possible that protective layers of two different coating materials are each applied alternately to the grain 40. Furthermore, it is also possible that each protective layer has a different composition or consists of a different coating material. Furthermore, it is also possible that the protective layers are all made of the same material or have the same composition. The number of layers applied to the grain 40 is arbitrary and is limited only by the desired pore size remaining after coating.

Claims

claims

1. A filter for removing particles from a gas stream, in particular of soot particles from an exhaust gas stream of an internal combustion engine, with a filter body of a ceramic filter substrate, wherein the filter substrate with a porous

Protective layer (46, 48) is coated from a coating material, characterized in that the coating material contains an admixture of 1 to 20 wt .-% of at least one compound of an element of the 2nd main group, preferably an oxide of an element of the 2nd main group, and in the ceramic filter substrate contained cracks (44) are partially filled with the coating material.

2. Filter according to claim 1, characterized in that the coating material for the porous protective layer (46, 48) is selected from the group consisting of aluminum numumina, aluminum hydroxide, titanium dioxide, silicon dioxide, zirconium dioxide, cerium oxide, aluminum silicates, magnesium aluminum silicates , Cordierite, mullite, silicon carbide,

Aluminum titanate, zeolites, quartz, glasses, mixtures and mixed oxides thereof.

3. Filter according to claim 2, characterized in that the coating material for the porous protective layer (46, 48) further contains at least one alkali metal compound, preferably an alkali metal oxide.

4. Filter according to one of claims 1 to 3, characterized in that the coating material for the porous protective layer (46, 48) further comprises at least one compound of an element of the 3rd to 5th subgroup or the lanthanides including the lanthanum, preferably an oxide of a Element of the 3rd to 5th subgroup or the lanthanides including the lanthanum contains.

5. Filter according to one of claims 1 to 4, characterized in that on the porous protective layer (48) at least one further porous protective layer (54) is applied.

6. Filter according to claim 5, characterized in that the superimposed porous protective layers (48, 54) perform different functions.

7. Filter according to one of claims 1 to 6, characterized in that at least one of the porous protective layers (46; 48, 54) contains a catalytically active component, preferably one or more metals from the group of platinum metals, preferably platinum and / or Palladium.

8. Filter according to one of claims 1 to 7, characterized in that the at least one porous protective layer (46; 48, 54) is applied in the outflow-side and / or central region of the filter.

9. Filter according to one of claims 1 to 7, characterized in that the at least one porous protective layer (46; 48, 54) is applied in the upstream and / or radially outer region of the filter.

10. Filter according to one of claims 1 to 9, characterized in that individual areas of the filter substrate with different layers (46; 48, 54), quantities or

Layer sequences are coated.

11. A process for producing a filter for removing particles from a gas stream according to one of claims 1 to 9, comprising the following steps:

(a) addition of the at least one compound of a second main group element in the form of a solid and / or as a gel, as a salt solution or as a hydrosol,

(b) applying the coating material for the porous protective layer (46, 48) to the sintered ceramic filter substrate,

(c) optionally adding further substances contained in the protective layer (46, 48),

(d) fixing the porous protective layer (46, 48).

12. The method according to claim 10, characterized in that the coating material in powder form has a BET surface of more than 10 m ² / g and a pore volume in the range of 0.1 to 1.5 ml / g.

13. The method according to claim 10 or 11, characterized in that the coating material has a particle size in the range of 2 nm to 20 microns, preferably greater than 1 micron.

14. The method according to any one of claims 10 to 12, characterized in that the porous protective layer (46, 48) is applied in a sol-gel process, as preformed sol or gel or as a suspension of solid particles.

15. The method according to any one of claims 10 to 13, characterized in that the at least one compound of an element of the 2nd main group and the optionally further substances contained in the coating are added to the raw material of the coating material, the coating composition or the finished coating.

16. The method according to any one of claims 10 to 14, characterized in that on the first porous protective layer (48) at least one further porous protective layer (54) is applied, wherein either each porous protective layer (48, 54) is first fixed before the Coating material of the subsequent layer (54) is applied or it is first applied the coating material for at least two layers (48, 54) and then fixed together.