DE102009000208A1 - Structural element, preferably particle filter, particle sensor or waste gas catalyst for reducing penetration of ash melting during thermal loading of component, comprises coating contactable with burn exhaust gases - Google Patents

Abstract

Structural element comprises a coating contactable with burn exhaust gases, where the coating contains oxides. The oxides are selected from a group consisting of oxides of lanthanides, lanthanum, actinium, actinide, scandium, yttrium, alkaline earth metal oxides, preferably aluminum oxide, mixtures of alkaline earth metal oxide and a silicon oxide, mixtures of an alkaline earth metal oxide, aluminum oxide and silicon oxide or products of reaction of the given oxides with each other. Cordierite and catalytic washcoat are also included in the structural element. An independent claim is also included for a method for producing structural element, which involves applying suspension of oxides and then sintered at 500-2500[deg] C.

Description

The The present invention relates to a device which is in operation flows through or flows around combustion exhaust gases is, wherein the device further with the combustion exhaust gases contactable coating comprises. It continues to apply Process for the preparation of such a device and the use a coating comprising oxides to reduce penetration of ash melts during the thermal loading of a such device.

State of the art

to Separation of soot particles from the exhaust gas of diesel engines Particulate filters are used. Collect in the diesel particulate filters so-called ashes over their lifetime. These are substances which during the thermal regeneration of the filter will not be burned. Typical components of this Ashes are sulphates, phosphates and / or oxides of calcium, magnesium, Zinc, iron and other elements.

If during thermal stress during regeneration the filter temperatures occur above 1000 ° C the ash can cause damage to the filter material to lead. There are different possibilities from the literature known damage to the ashes. A path of damage is the direct reaction between the ash components and the Filter material. Furthermore, a melting of the filter material at temperatures below its actual melting point in Present melting point-lowering ash components done. After all can pass pores on the surface of the filter material the melted ashes are sealed.

EP 1 205 228 A1 discloses a ceramic filter for use at high temperatures. The filter contains a substance having a reactivity toward ash components in the materials preferred by comparison with the main constituents of the ceramic filter, which materials have been retained by the ceramic filter and which have not been removed from the ceramic filter by high-temperature treatment such as burning. The reaction of the said ash component which leads to melting of the filter, which is not removed by a high-temperature treatment such as burning off of the filter and therefore accumulates with the filter component, can be suppressed. This achieves a long life of the filter.

The Concept of preventing a melting point lowering effect of However, ash components on the filter materials may not all Cover damage mechanisms. In particular, treated not the concept, as a harmful effect already melted ash ingredients can be prevented.

Desirable would therefore remain ways to extend the life of diesel particulate filters or other exhaust gases flowed through or around Components subjected to thermal stress to increase.

Disclosure of the invention

Proposed according to the invention is a component which flows through during operation of combustion exhaust gases or is flowed around, wherein the device is still a comprising contactable with the combustion exhaust gases coating. The component is characterized in that the coating Includes oxides selected from the group comprising Oxides of lanthanides, with the exception of lanthanum, cerium and praseodymium oxides of actinium, actinides, scandium oxide, yttrium oxide, Mixtures of an alkaline earth metal oxide and alumina, mixtures from an alkaline earth metal oxide and silicon oxide, mixtures of a Alkaline earth metal oxide, alumina and silica and / or products the reaction of the aforementioned oxides with one another.

The Component in the context of the present invention is in the operation of Flue gases flows through or flows around. Accordingly, it is downstream of one Exhaust source arranged. The combustion gases can, for example come from internal combustion engines, turbines or combustion plants. in the The invention provides that the exhaust gases contain particles can, which are still combustible or which already built up of inorganic combustion residues are. In particular, the particles may be soot particles and ash particles. Preferably, the exhaust gases to exhaust gases from diesel engines.

If the exhaust gases flow through the component, they occur at least partially through the component. In this case, the device can porous or discontinuous at the microscopic level be executed. However, it is also envisaged that the Exhaust gases flow around the component, ie the coating contact the entire surface of the device, without to pass through the device. Entrained in the exhaust gases Particles can then be generated, for example, by electric fields be deposited on the coating As through-flow or umströmtes component, this example, honeycomb structure be, have channels for the combustion gases or be provided with porous layers containing the exhaust gases can happen.

The Material of the device itself is initially no further limited, as long as there are operating conditions and the conditions of thermal stress is used. there For example, thermal stress includes heating the device a temperature necessary for thermal regeneration of the device leads. The thermal regeneration takes place in diesel particulate filters instead of when the soot particles are burned. For example The material may be a metal or a ceramic.

That the coating is contactable, that means that through the Component flowing exhaust gases reach the coating can. In particular, it is provided that in the exhaust gases present particles can come into contact with the coating. It is possible that the inventive Coating the outermost layer within the the exhaust gas facing surface within the device represents. However, it can also be more layers on the exhaust side attached to the coating of the invention be, for example, as a mechanical protective layers or as additional functional layers. In this case, the Contact the coating of the invention with the exhaust gases take place when the exhaust gases through the other layers pass.

For the purposes of the present invention, the term "oxide" is not to be understood as a monooxide, but includes the possible stoichiometries for an oxide of the specified elements in different oxidation states. By way of example, it denotes, but is not limited to, scandium oxide Sc ₂ O ₃ , yttrium oxide Y ₂ O ₃ , aluminum oxide Al ₂ O ₃ and silicon oxide SiO ₂ . The lanthanides mentioned are specifically neodymium, promethium, samarium, europium, gadolinium, terbium, dysprosium, holmium, erbium, thulium, ytterbium and lutetium.

The Actinides mentioned are in particular thorium, protactinium, Uranium and the transuranium neptunium, plutonium, americium, curium, Berkelium, Californium, Einsteinium, Fermium, Mendelevium, Nobelium and Lawrencium.

In In another embodiment, the oxides the coating by supplying energy to each other to the reaction to be brought. It can, for example, in the Reaction of calcium oxide and silica or alumina calcium silicates or calcium aluminates.

The Advantages of the coated component according to the invention are explained below. If the component of soot particle deposits be freed, this can be by means of a thermal Achieve regeneration. By the occurring high temperatures It can cause a melt within the combustion residues the particles formed ash come. Then, however, can be melted Ashes in the pores of the material of the device, even in the pores below the surface, penetrate.

To the different phases, on the one hand the cooled ash melt and on the other hand, the material of the device, different thermal expansion coefficients. Thereby It can lead to voltages when component temperature changes come within the material, which can cause tears to destruction lead of the device. A resolution of the component material in the ash melt the damage can be further intensified.

Especially This also leads to the removal of material and increase the content of foreign phases within the material.

This hitherto poorly described damage mechanism, unlike the direct reaction between the material of the component and the ash, not only leads to damage directly on the ash-contacting surface of the component, but also in areas below the surface. Furthermore, it can already proceed at significantly lower temperatures than a melting of the component material in the presence of ash. After all, unlike sealing pores on the skin, it does Surface of the device by melting ash not only the differential pressure and the filtration effect of a filter component, but reduces the mechanical stability of a device.

The Ash melts, which are typically phosphate melts with a high levels of calcium, magnesium and zinc are reactive with the compounds of the coating according to the invention. This produces reaction products with a higher melting point, which are no longer liquid. Thus, no Ash melts penetrate more into the material of the device.

For example, yttria in the ash melt coating reacts to form a mixture of yttrium phosphate, calcium-rich phosphate, and oxides of magnesium and zinc. These reaction products show significantly higher melting points than the phosphate melt formed from the ash, which causes crystallization of the melt in the reaction with Y ₂ O ₃ within the protective layer. It was found that the coating comprising yttria oxide was effective protection against ashes forming strong phosphate melt at least up to a temperature of 1150 ° C.

mixtures from an alkaline earth metal oxide, alumina and / or silica react with the ash melt in a way that through the formation of alkaline earth phosphates the remaining constituents from the Ash melt are displaced. These can then with alumina and / or silica to high melting aluminates and silicates react. The alkaline earth phosphates show higher Melting points than the original composition of Ash melt.

In an embodiment of the invention is in the coating the molar ratio of alkaline earth metal oxide to silica to alumina ≥ 1.0 to ≤ 5.0 to ≥ 1.0 to ≤ 5.0 to ≥ 0.5 to ≤ 3.0. Preferably, the molar ratio of Alkaline earth metal oxide to silica to alumina ≥ 2.0 to ≤ 4.0 to ≥ 1.5 to ≤ 4.0 to ≥ 0.75 to ≤ 1.50. This is particularly preferred molar ratio of alkaline earth metal oxide to silica to alumina ≥ 2.99 to ≤ 3.01 to ≥ 1.99 to ≤ 2.01 to ≥ 0.99 to ≤ 1.01. this includes is to be understood that, for example, to 3 moles alkaline earth metal oxide 2 moles of silica and 1 mole of alumina come. With such a Stoichiometry can be particularly advantageous through The formation of alkaline earth phosphates from the melt displaced Compounds to higher melting aluminates and silicates implement.

In Another embodiment of the invention is the alkaline earth metal oxide in the coating calcium oxide (CaO). The calcium oxide reacts with the phosphate melt from the ashes and thereby forms calcium-rich Phosphate with a high melting point. Magnesium oxide and zinc oxide are displaced from the phosphate melt and silica and / or alumina can be displaced with these Oxides react to silicates or aluminates. This reaction facilitates the uptake of calcium oxide in the phosphate melt. The obtained Phases show significantly higher melting points than those the ash formed phosphate melt, causing it within the coating to a crystallization of the melt in the reaction with the Oxide mixture comes. For example, it was found that a Coating of calcium oxide + aluminum oxide + silicon oxide in molar Ratio 3: 1: 2 against a strong phosphate melt forming Ash at least up to a temperature of 1150 ° C effective protection.

In a further embodiment of the invention are in the coating, the oxides in the form of particles or particle aggregates before and the mean size of the particles or the Particle aggregates are in a range of ≥ 0.1 microns up to ≤ 100 μm. Particle aggregates denote this Formations of interconnected individual particles. Such aggregates have the advantage of a higher specific surface area. However, consideration should be given to their selection, that they are not during the coating of the component or during whose operation is decaying.

The Size of particles or particle aggregates is advantageously also as a function of the pore size of the component to be coated selected. Particles or Particle aggregates with a medium size, which below about 10% of the mean pore size can lie during the coating of the device penetrate into its pores, where they no longer have their protective effect can develop sufficiently. Furthermore could then with temperature changes, damage to the material of the component when the particles and the material itself have a different thermal expansion behavior. In order to prevent the penetration of the particles into pores, that should Ratio of average particle size to mean pore size beneficially in a range of ≥ 1:10 to ≤ 1: 2 are preferred from ≥1: 5 to ≤1: 2.5.

Larger particle sizes than 100 microns are unfavorable, since it is expected that the mechanical stability the coating, in particular their adhesion to the device is deteriorated. Furthermore, due to the smaller specific surface area, the reactivity of the coating to ash melts would be limited.

The mean size of the particles or particle aggregates can be determined for example by means of dynamic light scattering. Preferably, the average size of the particles or the particle aggregates in a range of ≥ 1 micron to ≤ 20 μm, more preferably ≥ 3 μm up to ≤ 10 μm. This preferred range is special well suited for diesel particulate filter.

In a further embodiment of the invention is the Thickness of the coating in a range of ≥ 1 μm up to ≤ 1000 μm. Preferably, the thickness is in a range of ≥ 5 μm to ≤ 500 μm, particularly preferably from ≥ 10 μm to ≤ 100 μm. The thickness of the coating depends on the expected Amount of ash during operation of the device. through cost-effective production process, for example dip coating, the specified thicknesses can be easily reached.

In a further embodiment of the invention, the component is a particle filter, particle sensor or an exhaust gas catalytic converter. In this case, the component is preferably a diesel particle filter. Particle sensors may in particular be resistive particle sensors. These components profit in particular from the coating according to the invention, since they provide thermal regeneration for the removal of soot particles or temperatures may occur at which an ash melt could form. In another embodiment of the invention, the material of the device comprises cordierite (Mg ₂ Al ₄ Si ₅ O ₁₈ ). For example, the component can be a cordierite-comprising particle filter. For example, cordierite has a lower mechanical strength and a lower coefficient of thermal expansion compared to silicon carbide, which would also be suitable as a filter material. Thus, the risk of damage by mechanical stresses between the material of the device and the foreign phases formed from the ash is higher. Therefore, the cordierite already used as particulate filter material to a particular extent profits from the coating according to the invention.

In Another embodiment of the invention comprises Component further a catalytic washcoat. Consequently, can the device be an exhaust gas catalyst. It is also possible, the washcoat catalyses the oxidation of soot particles. It can sort and amount of coating to the underlying Washcoat can be adjusted. For example, a washcoat may be on the base of alumina as part of an alumina Coating effect. A combination of the coating with standard used washcoats is advantageous because this process costs be saved.

One Another object of the invention is a process for the preparation a device according to the invention, wherein a Suspension of the oxides is applied and subsequently at a temperature of ≥ 500 ° C to ≤ 2500 ° C is sintered. Preferably, the oxides are suspended in water. In the suspension, other auxiliaries such as thickeners and / or pore formers to be uniform or to achieve porous coating.

The application of the suspension can be carried out by conventional methods, for example by means of dipping. Preferably, sintering is carried out at a temperature of ≥ 900 ° C to ≤ 2000 ° C. At the sintering temperatures mentioned, there is a slight solidification of the oxide particles among one another, which increases the mechanical strength of the coating. Care should be taken that this solidification does not take place too much, since otherwise the coating would become too compact and thus the throughflow of the coating decreases. During the sintering process, reactions between constituents of the powdery protective layer can also take place. For example, calcium silicates or calcium aluminates can form from CaO and SiO ₂ or Al ₂ O ₃ if these oxides are present together in a sintering powder mixture.

It It is conceivable that the elements which the inventive Make up coating, initially in a different form, for example as nitrates, carbonates or oxalates applied to the device are and then transferred during sintering in the oxides become.

The invention also relates to the use of a coating which comprises oxides selected from the group consisting of oxides of lanthanides, oxides of actinides, scandium oxide, yttrium oxide, mixtures of an alkaline earth metal oxide and aluminum oxide, mixtures of an alkaline earth metal oxide and silicon oxide, mixtures of an alkaline earth metal oxide, alumina and silica, boria, titania, zirconia, hafnia, chromia, molybdenum, manganese, tungsten, galli oxide, indium oxide and / or lead oxide for reducing the penetration of ash melts during the thermal loading of a component, which flows through the operation of combustible exhaust gas comprising exhaust gases or is flowed around. The advantages of such a coating have already been described above.

The The present invention will be further understood from the following examples explained.

specimen of porous cordierite with a honeycomb geometry analogous to Diesel particulate filters were used. The sample height was 3 channels each, corresponding to 6 mm. The sample width was also each 3 channels, corresponding to 6 mm. The Sample length was 55 mm. On their outsides were the specimens with the inventive Coatings provided. The coating was carried out by means of immersion in an aqueous suspension.

The following samples were obtained:
Sample group 1: Coating of Y ₂ O ₃ with an average particle size of 5 μm and a thickness of the coating of 30 μm.
Sample group 2: Coating of a mixture of CaO, SiO ₂ and Al ₂ O ₃ in a molar ratio of 3: 2: 1, an average particle size of 5 μm to 6 μm and a thickness of the coating of 100 μm.
Sample group 3: Coating of Al ₂ O ₃ with an average particle size of 6 μm and a thickness of the coating of 20 μm.

The the coatings-containing samples were on the outsides coated with an artificial ash. Likewise were uncoated specimen with the artificial Ash, whereby samples of Reference Group 1 were obtained.

For each of the three protective layers tested, an amount of about 0.04 millimoles of oxide or oxide mixture per cm ^{2 was} used. The stated layer thicknesses are calculated in each case from the molar masses and the densities of the oxides used assuming 50% porosity in the layers.

The ash used consisted mainly of calcium sulfate and phosphates of Ca, Mg and Zn. Their composition depicts a real ash from diesel particulate filters. Such an ash forms a melt at temperatures between 1000 ° C and 1100 ° C. The amount of ash applied to the samples was 17 mg / cm ² .

The Samples with the ash layers became a thermal cycling subjected to high temperature and fast loads Temperature changes simulate, as for example in diesel particulate filters may arise in unkon trolliertem Rußabbrand. It was ten times a temperature of 1100 ° C started and the samples each after 3 minutes hold at 1100 ° C cooled quickly. Here, the maximum cooling rate was about 300 ° C / min at the beginning of the cooling process.

Subsequently the breaking strength of the samples was determined. 24 or 25 each of the thermally treated samples with ash coating were Four point bend tested for breaking strength. additionally were 25 reference samples without protective layer and ashes that did not thermally treated, examined. These reference samples will be referred to as reference group 2.

A support with 40 mm span and 20 mm load width was used in accordance with the requirements of the standard DIN EN 843 , The breaking forces were measured and converted into breaking stresses taking into account the honeycomb geometry of the test specimens. From the breaking stresses of all samples of a batch were determined according to the standard DIN EN 843 calculated a characteristic strength and Weibull modulus according to the Weibull statistic for each sample lot.

The characteristic strengths and Weibull moduli given in the table below were determined. The values in parentheses indicate the lower and upper limits of the 90% confidence intervals of the calculated values. Characteristic strength σ ₀ [MPa] Weibull module m Reference Group 1 4.8 (4.6 / 5.1) 6.2 (4.7 / 8.2) Reference Group 2 17.5 (17.1 / 17.9) 15.3 (11,7 / 20,1) Sample group 1 14.3 (14.0 / 14.7) 13.7 (10.5 / 18.0) Sample Group 2 15.0 (14.6 / 15.5) 11.4 (8.7 / 15.0) Sample group 3 10.4 (10.0 / 10.9) 7.7 (5.9 / 10.1)

The data show that the strength of the filter material in the presence of the coatings of the invention in the Group 1 and Sample 2 samples is much less reduced upon ash attack than in the absence of coatings, as observed in Reference Group 1 samples. A Beschich device according to sample group 3, which consists only of Al ₂ O ₃ , has a lower strength compared to the sample groups 1 and 2.

Also for the Weibull module as a measure of the scattering of the measured values within a sample batch applies that this at ash attack in the presence of the invention Coatings is significantly less reduced than in their Absence.

higher Strengths show only the samples of Reference Group 2, which but were not exposed to thermal ash attack.

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

• EP 1205228 A1 [0004]

Cited non-patent literature

• - DIN EN 843 [0043]
• - DIN EN 843 [0043]

Claims (10)

Component which flows through or is bypassed during operation of combustion exhaust gases, wherein the component further comprises a contactable with the combustion exhaust coating, characterized in that the coating comprises oxides selected from the group comprising oxides of lanthanides, wherein lanthanum, cerium and praseodymium excluded are oxides of actinium, actinides, scandium oxide, yttrium oxide, mixtures of an alkaline earth metal oxide and aluminum oxide, mixtures of an alkaline earth metal oxide and silicon oxide, mixtures of an alkaline earth metal oxide, alumina and silica and / or products of the reaction of the aforementioned oxides with one another. Component according to claim 1, wherein in the coating, the molar ratio of alkaline earth metal oxide to silica to alumina ≥ 1.0 to ≤ 5.0 to ≥ 1.0 to ≤ 5.0 to ≥ 0.5 to ≤ 3.0 is. Component according to claim 1 or 2, wherein the alkaline earth metal oxide in the coating is calcium oxide. Component according to one of the claims 1 to 3, wherein in the coating, the oxides in the form of particles or particle aggregates and wherein the mean size the particle or particle aggregates in a range of ≥ 0.1 microns to ≤ 100 microns. Component according to one of the claims 1 to 4, wherein the thickness of the coating in a range of ≥ 1 micron to ≤ 1000 microns. Component according to one of the claims 1 to 5, wherein the device is a particulate filter, particle sensor or an exhaust gas catalyst. Component according to one of the claims 1 to 6, wherein the material of the device comprises cordierite. Component according to one of the claims 1 to 7, further comprising a catalytic washcoat. Method for producing a component according to a of claims 1 to 8, wherein a suspension of the oxides is applied and then at a temperature from ≥ 500 ° C to ≤ 2500 ° C is sintered. Use of a coating comprising oxides, which are selected from the group comprising oxides of Lanthanides, oxides of actinides, scandium oxide yttria, mixtures from an alkaline earth metal oxide and alumina, mixtures of an alkaline earth metal oxide and silica, mixtures of a Alkaline earth metal oxide, alumina and silica, boria, titania, Zirconium oxide, hafnium oxide, chromium oxide, molybdenum oxide, manganese oxide, Tungsten oxide, gallium oxide, indium oxide and / or lead oxide for reduction the penetration of ash melts during the thermal Loading of a component which is combustible during operation Particles comprehensive exhaust gases flows through or is flowed around