DE102006028636A1 - Filter for purifying a particle-containing gas stream and process for its preparation - Google Patents

Abstract

The invention relates to a filter for cleaning a particle-containing gas stream, in particular a soot-containing exhaust gas stream of an internal combustion engine, in which a filter device (14) with a filter element (18) of a filter substrate made of ceramic in a housing (16) is added. All free surfaces of the grains (40) of the filter substrate are provided with a coating (42). Furthermore, the invention relates to a method for producing the filter.

Description

State of the art

The The invention relates to a filter for cleaning a particle-containing Gas stream, in particular a soot-containing exhaust stream of a Internal combustion engine. The invention further relates to a Process for producing such a filter.

filter elements for particle filters of diesel internal combustion engines and the support structures of catalysts of internal combustion engines become common made of magnesium-aluminum-silicates, preferably cordierite, manufactured. Pure magnesium aluminum silicate, preferably cordierite, has a very small thermal expansion coefficient and thus has a good resistance against sudden temperature changes (Thermal shocks) on.

at the continuous or cyclic regeneration of the filter elements can Temperatures of over 1000 ° C occur, since the regeneration of soot is exothermic is. Because the soot distribution is not homogeneous within the filter element and also the options the heat dissipation are locally different, arise in the regeneration local Temperature differences in the filter element.

So far The life of particulate filters is made of magnesium-aluminum silicates limited, inter alia, that the magnesium-aluminum silicate with the alkalis contained in the exhaust gas, in particular sodium and Potassium, or alkaline earths, reacts. In this reaction arise additional Phases, e.g. Nepheline, which has a larger thermal expansion coefficient as magnesium aluminum silicate. This results in succession the local temperature differences in the filter element thermal stresses, which can lead to cracks in the filter element and thus to its destruction.

Disclosure of the invention

Advantages of the invention

at a filter designed according to the invention for cleaning a particle-containing gas stream, in particular a sooty Exhaust stream of an internal combustion engine, is a filter substrate made of ceramic in a housing added. All are according to the invention free surfaces of the Filter substrate provided with a coating. As a result of that all free surfaces the filter substrate are provided with the coating is avoided that foreign ions, for example, alkali or alkaline earth ions in the Ceramics can penetrate. As a result, the mechanical and thermochemical strength of the Increased filter material. By the coating of all free surfaces, i. also the surfaces in the Inside the pores of the filter substrate becomes the chemical resistance the ceramic opposite the processes in which only the outer surfaces are coated on, continue elevated. Because all surfaces the filter substrate are coated, it is still sufficient that the coating takes place only in very thin layers. hereby There are no or only minor influences on the internal pressure behavior of the filter substrate.

The Filter substrate is preferably a magnesium-aluminum-silicate, especially cordierite.

The Coating material is preferably selected from a metal oxide or a mixed metal oxide of an element of the 4th, 5th, 6th, 7th or 8th subgroup, an aluminosilicate, a borosilicate, a magnesium silicate, a Lithium aluminosilicate and an oxide of lanthanum or a lanthanum.

Furthermore, it is preferred that the coating material is a perovskite of the structure ABO ₃ , A _1-x A ' _x BO ₃ , AB _1-y B' _y O ₃ or A _1-x A ' _x B _1-y B' _y O ₃ is. The value for x and y is between 0 and 1, and any value in this range can be assumed.

A is selected from Sr ²⁺ , Ba ²⁺ or Ca ²⁺ or an ion of a rare earth metal, in particular La ³⁺ , Ce ³⁺ , Pr ³⁺ , Nd ³⁺ , Sm ³⁺ , Eu ³⁺ , Gd ³⁺ , Y ³⁺ , Ga ³⁺ or Nb ³⁺ .

B is selected from Ti ²⁺ , Ti ⁴⁺ , Cr ⁴⁺ , Hf ⁴⁺ , Sn ⁴⁺ , Ce ³⁺ , Ce ⁴⁺ , Mn ³⁺ , Mn ⁴⁺ , Fe ³⁺ , Co ³⁺ and Al ^{3 +} .

A 'is selected from Ca ²⁺ , Sr ²⁺ , Ba ²⁺ , La ²⁺ , Ce ³⁺ , Ce ⁴⁺ , Pr ³⁺ and Nd ³⁺ , in particular Sr ²⁺ , Ba ²⁺ and Ca ²⁺ .

Finally, B 'is selected from Zr ⁴⁺ , Al ³⁺ , Sc ³⁺ , V ⁴⁺ , Cr ³⁺ , Mn ³⁺ , Mn ⁴⁺ , Fe ³⁺ , Co ²⁺ , Co ³⁺ , Gd ³⁺ , Hf ⁴⁺ , Al ³⁺ or Ni ²⁺ .

Furthermore, it is preferred that the coating material is a spinel of structure AB ₂ O ₄ , wherein
A is selected from Mg ²⁺ , Cr ²⁺ , Mn ²⁺ , Fe ²⁺ , Co ²⁺ , Ni ²⁺ , Cu ²⁺ , Zn ²⁺ , Sn ²⁺ , Sn ⁴⁺ or V ⁴⁺ and
B is selected from Al ³⁺ , In ³⁺ , Ti ³⁺ , Mn ³⁺ , Fe ²⁺ , Fe ³⁺ , Co ²⁺ , Co ³⁺ , Mg ²⁺ or Zn ⁴⁺ .

advantage the oxides of lanthanides, perovskites and spinels that these serve as oxygen storage for catalytically assisted reactions can.

Furthermore, it is preferred that the Be coating material is a phosphate of lanthanum and / or one or more lanthanides an alkali zirconium phosphate, a Erdalkalizirkonphosphat or a mixture thereof. Preferred phosphates of lanthanum and / or a lanthanoid are lanthanum phosphate, cerium phosphate, neodymium phosphate, samarium phosphate and gadolinium phosphate. Particularly preferred are alkali zirconium phosphates and Erdalkalizirkonphosphate and occurring in nature as monazite cerium phosphates. These have a similar low thermal expansion coefficient as the ceramic of the filter substrate. In addition, the Alkalizirkonphosphate and Erdalkalizirkonphosphate are not further attacked by these already contained in alkali or alkaline earth metal ions of these.

In order to improve the chemical properties of the coating materials, in particular the aluminosilicate, the borosilicate or silicate of magnesium, these can with one or more oxides of an element of the 4th to 6th sub-group, preferably ZrO _2, be doped. In this case, the proportion of the oxide with which the aluminosilicate, borosilicate or magnesium silicate is doped is at most 10% by weight. By doping with ZrO ₂ , for example, the mechanical strength values can be increased.

In order to by coating all surfaces of the filter substrate the Counterpressure behavior of the filter is not or only slightly influenced is, is the layer thickness is preferably at most 1 .mu.m, particularly preferably at maximum 0.5 μm.

In order to all surfaces the filter substrate are coated, the coating takes place e.g. by wet-chemical processes, formation of aerosols and by impregnation with water and / or alcohol-soluble Metal salts. This forms a homogeneous coating from the filter substrate.

Prefers the coating material is in the form of a soluble precursor in one solvent solved, subsequently the filter base material is impregnated with the solution, if necessary with compressed air blown out and soaked Finally, filter base material a heat treatment or an energy input by generating a microwave-based plasma in which the metal salt is converted to a metal oxide becomes. By loosening the Metal salt in the solvent the metal salt is in ionic form and not in the form of particles, so that the metal ions also penetrate into the pores of the filter substrate can.

The Heat treatment by which the metal salt is converted into a metal oxide is e.g. Calcining or sintering. The calcining may e.g. over one or multiple temperature steps, with one or more for the Coating adapted calcination temperatures are selected. The maximum possible Temperature can reach up to 1,460 ° C be. This temperature is below the melting point of the Cordierite filter material. While of calcining it is possible gases supply. With regard to the chosen Coating may be an inert gas supply, e.g. pure nitrogen, but also air, dried air or pure Oxygen act.

Also it is preferable to the heat treatment in the presence of a gas stream having an oxygen content in the range from 19 to 100% by volume. Preferably, the gas stream is air enriched with additional oxygen can be.

The soluble Metal salts in the solvent solved are preferably nitrate, halide, carbonate, phosphate, Acetate or carboxylic acid derivative in front. The solvent with the ones dissolved in it Metal salts can be of any pH. That's the way it is possible, that the solution sour, neutral or basic. Also, it is possible that the pH during dipping the filter substrate in the solution, e.g. from sour to basic changed will, causing a precipitation the corresponding hydroxides by addition of bases or the carbonates can be carried out by adding carbonate reagents.

The Thickness of the coating applied to the filter substrate, can e.g. by the concentration of the solution and / or by repeated Soak of the filter can be adjusted with subsequent heat treatment.

Brief description of the drawings

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

It demonstrate

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

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

3 an enlarged view of grains of the filter substrate with a coating.

Embodiments of the invention

1 shows a schematic representation of an internal combustion engine with an exhaust gas aftertreatment device according to the invention tung. 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 over an 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 in the present embodiment, a rotationally symmetrical, also a total cylindrical filter element 18 is arranged.

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

The filter element 18 is, for example, as extruded molded body made of a ceramic material, for example magnesium-aluminum silicate, preferably cordierite produced. The filter element 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 element 18 and leaves this via an exit surface 24 ,

Parallel to a longitudinal axis 26 of the filter element 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 the entrance channels 28 to the exit surface 24 Rejuvenate until the wall 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.

3 shows an enlarged view of ceramic grains, which are provided with a coating.

The filter walls 38 are each made of a filter substrate made of ceramic. The ceramic consists of individual grains 40 according to the invention with a coating 42 are provided. As 3 can be seen encloses the coating 42 each the entire grain 40 , As a result, the entire free surface of the filter substrate with the coating 42 Mistake. Between the individual grains 40 are pores 44 formed, which are flowed through during operation of the exhaust gas to be cleaned. In general, by coating the grains 40 the pores 44 between the grains 40 reduced. This leads to the fact that the free flow cross-section in the filter substrate also decreases. The reduced free flow cross-section has the consequence that the pressure loss in the filter increases. It must be applied a higher pressure, so that the exhaust gas flows through the filter. Due to the coating of all free surfaces of the filter substrate, it is possible, however, only a very small layer thickness on the grains 40 Apply so that the pore volume through the coating 42 is reduced only to a small extent, so that the pressure loss in the filter substrate through the coating 42 hardly changes.

For the production of the inventively designed filter element 18 In a first step, the base materials of the magnesium-aluminum silicate, preferably of the cordierite, in particular Al ₂ O ₃ , SiO ₂ and MgO, ground in powder form and mixed in the desired composition.

Out The powder obtained in the first step is in a second step a body the desired Form, for example by extruding or giving another shape Process produced. If necessary, this body can be pre-dried and subsequently be sintered in a third step.

Around all free surfaces of the body thus produced from filter substrate with a metal oxide, a perovskite or to coat a spinel or a mixture thereof, the body after that in a solution immersed, in which the coating material in the form of one or more soluble Metal precursors in a solvent is solved. By dipping the filter substrate is soaked in the solution. There the solution contains no particles, this penetrates into narrow pores of the filter substrate.

preferred solvent to release the metal salts are water, alcohol, e.g. Isopropanol, or mixtures from water and alcohol.

In dependence from the porosity of the filter material, the filter substrate is up to a few seconds soaked for a maximum of 10 minutes, preferably 30 seconds to 2 minutes. The Temperature of the impregnation solution can hereby 5 ° C up to 80 ° C, preferably 15 ° C up to 35 ° C be.

Finally, it will in a fifth Step the soaked Filter base material of a heat treatment subjected to the metal salt, which is on the filter substrate has been converted into a metal oxide. The heat treatment can be carried out either at a constant temperature or there are several stages in which the impregnated filter substrate each to a higher or lower temperature and then a predetermined time is kept at this temperature.

During the heat treatment Is it possible, to supply any gases. The heat treatment is preferred in the presence of air whose oxygen content is enriched can, performed. The oxygen content is preferably in the range between 20 and 100% by volume.

often the calcination step may be preceded by a drying step. This is soaked Filter element at a temperature in the range between 60 ° C and 150 ° C dried. Dependent on of the coating material both the duration of drying and the duration of calcination ranging from a few minutes to several hours.

Depending on the desired metal oxide with which the grains 40 are to be coated, the soluble metal salt is preferably a nitrate, halide, carbonate, phosphate, acetate or carboxylic acid derivative of one or more metals of the 4th, 5th, 6th, 7th or 8th subgroup, the lanthanum or a lanthanum. Preferred metal salts are the nitrates of, for example, Ce ³⁺ , La ³⁺ , Pr ³⁺ , Al ³⁺ , Ti ⁴⁺ or the oxynitrates of Zr ⁴⁺ .

The Coating may e.g. by a precipitation of the corresponding hydroxides, which arise when the pH of the solution of the metal salt through Addition of bases is changed from acid to basic, or by precipitation of carbonates, with the addition of the corresponding carbonate reagents or by precipitation of phosphates, with the addition of appropriate phosphate reagents, improved become.

If The filter substrate is coated with a silicate, in In general, two different coating methods are performed. These are on the one hand the wet impregnation and on the other hand the application of solid particles.

at wet impregnation become water-soluble Silicates, e.g. Lithium silicate with the corresponding aluminum, Boron and / or magnesium salt in the corresponding stoichiometric Released ratio. Furthermore is also the use of water and / or alcohol soluble silicones (organic Silicon compounds) as a silicon source possible. When coating By solid particles, these are applied in a suspension. The particle size of the silicates is then between 10 and 500 nm, preferably 20 to 200 nm.

A comparable coating by application of solid particles occurs e.g. also with the Lanthanoidphosphaten, alkali and Erdalkalizirkonphosphaten. A subsequent one heat treatment at a temperature of up to 1,460 ° C, preferably between 350 ° C and 1,300 ° C causes sintering of the coating material.

to Coating the filter element with perovskites or spinels this will be in a solution dipped the desired ones stoichiometric Amounts of dissolved Contains metal salts. A subsequent heat treatment converts the mixed metal salts in the corresponding oxides, then in the perovskite or Sinter spinel structure.

To adjust the thickness of the coating 42 on the grain 40 it is possible to immerse the filter substrate several times in the solution of the corresponding soluble metal salt and to supply the subsequent heat treatment.

Instead of it is the dipping of the filter substrate into the solution of the soluble metal salt also possible, to flow through the filter substrate with the appropriate solution.

Claims (12)

Filter for cleaning a particle-containing gas stream, in particular a soot-containing exhaust gas stream of an internal combustion engine, in which a filter device ( 14 ) with a filter element ( 18 ) from a filter substrate made of ceramic in a housing ( 16 ), characterized in that all the free surfaces of the grains ( 40 ) of the filter substrate with a coating ( 42 ) are provided. Filter according to claim 1, characterized in that the filter substrate is a magnesium-aluminum-silicate, preferably cordierite, is. Filter according to claim 1, characterized in that the material for the coating ( 42 ) is selected from a metal oxide or a mixed metal oxide of one or more elements of the 4th, 5th, 6th, 7th or 8th subgroup, an aluminosilicate, a borosilicate, a magnesium silicate, a lithium aluminosilicate or an oxide of lanthanum or a lanthanide. Filter according to claim 1 or 2, characterized in that the material for the coating ( 42 ) is a perovskite of the structure ABO ₃ , A _1-x A ' _x BO ₃ , AB _1-y B' _y O ₃ or A _1-x A ' _x B _1-y B' _y O ₃ , where x and y have any value between 0 and 1, A is selected from an ion of a rare earth metal, Sr ²⁺ , Ba ²⁺ or Ca ²⁺ , in particular La ³⁺ , Ce ³⁺ , Pr ³⁺ , Nd ³⁺ , Sm ³⁺ , Eu ³⁺ , Gd ³⁺ , Y ³⁺ , Ga ³⁺ or Nb ³⁺ , B is selected from Ti ²⁺ , Ti ⁴⁺ , Zr ⁴⁺ , Hf ⁴⁺ , Sn ⁴⁺ , Ce ³⁺ , Ce ⁴⁺ , Mn ³⁺ , Mn ⁴⁺ , Fe ³⁺ , Co ³⁺ or Al ³⁺ , A 'is selected from Ca ²⁺ , Sr ²⁺ , Ba ²⁺ , La ²⁺ , Ce ³⁺ , Ce ^{4 +} , Pr ³⁺ or Nd ³⁺ , in particular Sr ²⁺ , Ba ²⁺ or Ca ²⁺ , and B 'is selected from Zr ⁴⁺ , Al ³⁺ , Sc ³⁺ , V ⁴⁺ , Cr ³⁺ , Mn ³⁺ , Mn ⁴⁺ , Fe ³⁺ , Co ²⁺ , Co ³⁺ , Gd ³⁺ , Hf ⁴⁺ , Al ³⁺ or Ni ²⁺ . Filter according to claim 1 or 2, characterized in that the material for the coating ( 42 ) is a spinel of the structure AB ₂ O ₄ , wherein A is selected from Mg ²⁺ , Cr ²⁺ , Mn ²⁺ , Fe ²⁺ , Co ²⁺ , Ni ²⁺ , Cu ²⁺ , Zn ²⁺ , Sn ^{2 +} , Sn ⁴⁺ or V ⁴⁺ and B is selected from Al ³⁺ , In ³⁺ , Ti ³⁺ , Mn ³⁺ , Fe ²⁺ , Fe ³⁺ , Co ²⁺ , Co ³⁺ , Mg ²⁺ or Zn ⁴⁺ . Filter according to claim 1 or 2, characterized in that the material for the coating ( 42 ) is a phosphate of the lanthanum and / or one or more lanthanides, preferably lanthanum phosphate, cerium phosphate, neodymium phosphate, samarium phosphate, gadolinium phosphate, an alkali zirconium phosphate, an alkaline earth zirconium phosphate or a mixture thereof. Filter according to claim 3, characterized in that the aluminosilicate, borosilicate or magnesium silicate with one or more oxides of an element of the 4th to 6th subgroup, preferably ZrO ₂ , doped, wherein the proportion of the oxide with which the aluminosilicate, borosilicate or Magnesium silicate is doped, a maximum of 10 wt .-% is. Filter according to claim 1 or 5, characterized in that the thickness of the coating ( 42 ) is smaller than 1 μm, preferably smaller than 0.5 μm. Method for producing a filter according to one of Claims 1 to 8, characterized in that the material for the coating ( 42 ) is dissolved in a solvent in the form of a soluble metal salt, the filter base material is impregnated with the solution and the impregnated filter base material is finally subjected to a heat treatment or an energy input by producing a microwave-assisted plasma in which the metal salt is converted into a metal oxide. Method according to claim 9, characterized in that that the heat treatment Calcining or sintering is. Method according to claim 9 or 10, characterized that the heat treatment in the presence of a gas stream having an oxygen content in the range from 19 to 100% by volume. Method according to one of claims 9 to 11, characterized in that the soluble salts of the material for the coating ( 42 ) are present as nitrate, halide, carbonate, phosphate, acetate or carboxylic acid derivative.