DE102010016465A1 - Coating and process for catalytic emission control - Google Patents

Abstract

The present application relates to a method for coating a carrier body with a coating composition, a coating composition for producing a ceramic coating, a ceramic coating from a coating composition for cleaning gases, in particular exhaust gases, and the use of a ceramic coating for cleaning gases, in particular exhaust gases.

Description

The present application relates to a method for coating a carrier body with a coating composition, a coating composition for producing a ceramic coating, a ceramic coating of a coating composition for cleaning gases, in particular exhaust gases, and the use of a ceramic coating for cleaning gases, in particular exhaust gases.

In modern industrial plants, various exhaust gases are produced. These exhaust gases are often contaminated with volatile or gaseous hydrocarbons (VOCs, volatile organic compounds), aerosol mixtures or with soot. The substances present in the exhaust gases may include solvents, fat and / or wax droplets. However, current emission standards require compliance with certain limits for such substances. They must therefore be removed from the exhaust stream, or at least reduced in their amount.

The degradation of VOCs and soot can take place on catalysts that are integrated into the exhaust stream. Frequently, however, suitable catalysts are expensive because they are based on the use of precious metals such as platinum, palladium, rhodium or iridium, or by residues in the degradation, i. H. a closure of the pores, becoming increasingly ineffective and therefore need to be replaced.

Another disadvantage of conventional catalysts is their production. Usually, honeycomb bodies are used as carriers for the catalytically active materials. However, these honeycomb bodies are difficult to homogeneously coat internally with catalytically active materials, especially if they have a high number of honeycombs, d. h., When the individual channels of the honeycomb body are very small. Often, so-called washcoats, which usually contain ceramic coatings and are not involved in the reaction, d. H. serve as support materials, and provide only a large surface, equipped with a catalytically active component.

It is therefore an object of the present invention to provide a coating which can be easily and uniformly applied to a carrier body which can be produced inexpensively and which is suitable for the catalytic purification of exhaust gases, in particular exhaust gases containing VOCs. A further object of the present invention is to provide a coating which, even at relatively low temperatures, can catalytically degrade a high proportion of VOCs in exhaust gases and are not relatively ineffective due to laking processes. In addition, it is an object of the present invention to provide a system which is more robust than conventional catalyst poisons, particularly over a noble metal catalyst system. Finally, it is an object of the present invention to provide a simple and inexpensive coating which can be used for the catalytic purification of exhaust gases.

These and other objects are achieved by the inventive method for coating a carrier body, by a carrier body of a ceramic coating according to the invention, by the use of a carrier body with a ceramic coating for the purification of exhaust gases, in particular VOC-containing exhaust gases, and by a method for the purification of Exhaust gases, in particular exhaust gases containing VOCs, using a carrier body of a ceramic coating according to the invention.

• a) Bereitstellen einer Beschichtungszusammensetzung umfassend i) eine Pulverkomponente; ii) eine Binderkomponente; iii) einen Verflüssiger; iv) eine Säure; und v) wahlweise einen Alkohol;
• b) Aufbringen der Beschichtungszusammensetzung auf mindestens eine Oberfläche des Trägerkörpers;
• c) Trocknen der aufgebrachten Beschichtungszusammensetzung; und
• d) Verfestigen der getrockneten Beschichtungszusammensetzung, um eine Beschichtung auszubilden; wobei die Pulverkomponente eine Mischung ist von – zumindest einem ersten porösen Aluminiumoxidpulver mit einer mittleren Partikelgröße d₅₀ im Bereich von 5–80 μm; – zumindest einem zweiten Aluminiumoxidpulver mit einer mittleren Partikelgröße d₅₀ im Bereich von 0,3–2 μm; und – zumindest einem Pigment mit einer mittleren (Primär-)partikelgröße d₅₀ im Bereich von 80–200 nm; wobei die Pulverkomponente in einer Menge von zwischen 80 und 97 Gew.-%, bezogen auf den Feststoffanteil in der Beschichtungszusammensetzung, enthalten ist; und wobei die Binderkomponente oberflächenmodifizierte Nanopartikel, ausgewählt aus n-ZrO₂ und n-TiO₂, mit einer mittleren Partikelgröße d₅₀ von weniger als 150 nm enthält.

A first aspect of the present invention relates to a method for coating a carrier body, comprising:

• a) providing a coating composition comprising i) a powder component; ii) a binder component; iii) a liquefier; iv) an acid; and v) optionally an alcohol;
• b) applying the coating composition to at least one surface of the carrier body;
• c) drying the applied coating composition; and
• d) solidifying the dried coating composition to form a coating; wherein the powder component is a mixture of - at least a first porous alumina powder having an average particle size d ₅₀ in the range of 5-80 microns; At least one second alumina powder having an average particle size d ₅₀ in the range of 0.3-2 μm; and - at least one pigment having an average (primary) particle size d ₅₀ in the range of 80-200 nm; wherein the powder component is contained in an amount of between 80 and 97% by weight, based on the solids content in the coating composition; and wherein the binder component contains surface-modified nanoparticles selected from n-ZrO ₂ and n-TiO ₂ , with an average particle size d ₅₀ of less than 150 nm.

A second aspect of the present invention relates to a coating preparable by the method for coating a carrier body according to the first aspect, wherein the coating has a pore volume of 50-80% by volume.

A third aspect of the present invention relates to a carrier body with a coating according to the second aspect.

A fourth aspect of the present invention relates to the use of the coating according to the second aspect or the carrier body according to the third aspect for the catalytic purification of a gas, preferably a VOC-containing exhaust gas.

A fifth aspect of the present invention relates to a method for catalytically purifying a gas, preferably a VOC-containing exhaust gas, using the coating according to the second aspect or the carrier body according to the third aspect.

Finally, a sixth aspect of the present invention relates to a coating composition, in particular a storage-stable coating composition, as provided in the first step of the method according to the first aspect. In particular, all embodiments of the coating composition as set forth herein in the context of the coating process are also intended to apply to the coating composition itself.

In the following, the advantages and embodiments of the various aspects of the present invention will be explained in more detail. When referring to a specific aspect of the present invention, it is to be understood that the described advantages or embodiments also apply to the other aspects of the present invention. In addition, the individual embodiments or continuations without limitation are combined, unless expressly stated otherwise.

The present invention will also be explained with reference to the attached figures:

1 shows the conversion of ethanol to coatings according to the present invention as a function of temperature;

2 shows the conversion of ethanol on an uncoated carrier body as a function of the temperature;

3 shows the conversion of toluene to coatings according to the present invention as a function of temperature;

4 shows the conversion of toluene on an uncoated support body as a function of the temperature; and

5 shows the degradation rate of ethanol at different space velocities.

A first aspect of the present invention relates to a method for coating a carrier body. In a first step a) of the process for coating a carrier body, a coating composition is provided. To prepare the coating composition, different components are mixed to form a coating composition.

Advantageously, the individual components of the coating composition are mixed together immediately prior to use of the coating composition in the further steps of the process. This mixing can be carried out, for example, in a solvent with a mechanical stirrer.

The coating composition is composed of at least four components, namely a powder component, a binder component, a plasticizer and an acid. Optionally, an alcohol may be used as the fifth component.

To prepare the coating composition, advantageously, the different components are mixed together in a solvent to form a slurry. In the present application, the solvent is referred to as a solvent, although it additionally or alternatively also acts as a dispersant or dispersant. The solvent is advantageously selected from water, alcohol, in particular ethanol and propanol, and mixtures thereof, particular preference being given to using water as solvent. If one speaks of a solvent or solvent in the context of the present invention, it is obvious to a person skilled in the art that this refers to substances which can serve as solvents or dispersants. However, it should also be noted that some of the substances, such as alumina, are not dissolved, but only dispersed or slurried. Other substances, however, such as the acid, are dissolved in the solvent.

Before mixing the individual components, these are storable. The shelf life or storage stability of the substances can be, for example, half a year or more. However, after blending the components, the coating composition should be processed promptly. The coating composition is advantageously subjected to a ripening process over a period of from 3 to 48 hours, preferably from 6 to 36 hours, and more preferably from 12 to 24 hours. The provision of the coating composition therefore comprises blending the individual components of the coating composition and then ripening the coating composition, preferably over a period of time as indicated above. During the ripening, the solution may optionally be stirred. When you tire different balances set. For example, the surface modifier may adsorb or desorb onto the powder surfaces, or during stirring, mechanical forces may be released which may cause powder agglomerates to break.

The coating composition is composed of at least four components. The first component forms a powder component. This powder component in turn is composed of at least three different powders. The mixture of the different powders comprises at least a first porous alumina powder, at least a second alumina powder and at least one pigment.

The first porous alumina powder has an average particle size d ₅₀ in the range of 5-80 microns. The first alumina powder is usually the largest particle size powder. Since the particle size has a direct influence on the minimum layer thickness, the minimum layer thickness can be set via the mean particle size, if it represents the largest particle size in the coating composition. By reducing the particle size of the first alumina powder, therefore, a reduction of the minimum layer thickness can be achieved.

According to a preferred embodiment, the particles of the first porous alumina powder may have an average particle size d ₅₀ in the range of 30-80 .mu.m, preferably 40-80 .mu.m, more preferably 35-60 .mu.m, and particularly preferably 37-45 .mu.m. According to another preferred embodiment, the particles of the first porous alumina powder may have an average particle size d ₅₀ in the range of 5-60 μm, preferably 5-30 μm, more preferably 6-20 μm, and most preferably 6-15 μm.

The second alumina powder has an average particle size d ₅₀ in the range of 0.3-2 microns. According to a preferred embodiment, the particles of the second alumina powder may have an average particle size d ₅₀ in the range of 0.4-1.5 μm, preferably of 0.5-0.8 μm.

The alumina of the first and second alumina powders may be different alumina modifications. According to a preferred embodiment, the first porous alumina powder comprises an alumina of γ-alumina modification. Aluminum oxide of the γ-modification is characterized by a high internal surface area. According to a preferred development, the first porous alumina powder consists of γ-alumina.

The second alumina powder may, according to a preferred embodiment, comprise an alpha-modification alumina. According to a preferred development, the second alumina powder consists of α-alumina. Usually, an alumina of the α-modification has less to no porosity.

The first porous alumina powder according to a preferred embodiment has a BET specific surface area in the range 50-100 m ² / g, more preferably 60-85 m ² / g, particularly preferably 65-80 m ² / g. It may also be alumina powder of the γ-modification.

In a further preferred embodiment, the first porous alumina powder is a porous alumina powder having pores with a pore size in the range of less than 10 nm. The porosity of the first porous alumina powder is preferably an open porosity, i. H. the opening of the pores is large in relation to the pore size. This may mean that the openings of the pores are at least equal to or even larger than the pores themselves. Due to an open porosity, reactants can easily penetrate into the pores of the alumina powder and be converted there.

According to another preferred embodiment, the second alumina powder may have a BET specific surface area of 5-15 m ² / g, more preferably 6-10 m ² / g. This is particularly preferably an aluminum oxide of the α-modification.

According to a further preferred embodiment, the particles of the pigment can have a BET specific surface area in the range from 4 to 4. 50 m ² / g, preferably 20-50 m ² / g, or preferably from 4.7 to 5.7 m ² / g.

In the coating composition, the particles of the pigment may be in the form of agglomerated particles whose size and distribution may depend on the state of dispersion. In particular, a bimodal particle size distribution of the pigment particles may be present in the coating composition. Primary and secondary particles may then be present, the primary particles being the originally used, non-agglomerated particles, and the secondary particles being the agglomerated particles in the composition. According to a preferred embodiment, therefore, the particles of the pigment may have a mean (primary) particle size d ₅₀ in the range of 50-250 nm, preferably of 80-200 nm, more preferably of 90-150 nm. According to another preferred embodiment, the average (secondary) particle size d _{50 may be} in the range of 0.5-5 μm, preferably of 0.6-3 μm, more preferably of 0.8-1.2 μm.

In still another preferred embodiment, the shape of the particles of the first porous alumina powder, the second alumina powder and / or the pigment is approximately spherical. By using spherical particles for the first porous alumina powder, the second alumina powder and / or the pigment, a macroscopically smoother surface of the ceramic coating can be achieved. In addition, the ceramic coating has a higher strength when using Spährischer particles in contrast to, for example, platelet-shaped particles.

By using three different materials in the powder component, a porous ceramic coating having an open porosity can be obtained in the production of the coating. This porous ceramic coating preferably has a porosity of at least 30% by volume, more preferably of at least 50% by volume, more preferably of at least 60% by volume. The porosity may be below 90% by volume, preferably below 80% by volume.

In addition, by using three materials with different particle size in the ceramic coating, a trimodal pore size distribution can be achieved. The pores in the ceramic coating preferably have three different sizes. The smallest pores may have a pore size of, for example, 3 to 6 nm, the average pores a size in the range of 100 to 200 nm, preferably 130 to 150 nm, and the largest pores may have a pore size in the range of 5 to 30 microns, preferably from 10 to 20 .mu.m. Due to the different pore sizes different reactants can be catalytically converted according to their size in an ideal environment, since different reaction chambers can be made available for the different reactants. It is believed that the different pores are formed between the different particles. However, the pores within the porous first alumina powder are also available.

The at least three constituents of the powder component are preferably mixed together in certain ratios. Advantageously, a powder mixture of the at least three constituents can be prepared before this powder component is added as a mixture of the coating composition.

In this mixture of the powder component, the first porous alumina powder may preferably be in an amount of 60 to 90% by weight, more preferably in an amount of 65 to 80% by weight, further preferably in an amount of 70 to 75% by weight. , based on the total weight of the dry powder component.

The second alumina powder may preferably be contained in the powder component in an amount of 10 to 20% by weight, more preferably in an amount of 12 to 18% by weight, more preferably in an amount of 14 to 16% by weight, and most preferably in an amount ranging from 14.3 to 15.8% by weight, based on the total weight of the dry powder component.

Finally, the pigment in the powder component may preferably be present in an amount of from 8 to 15% by weight, more preferably in an amount of from 10 to 13% by weight, more preferably in an amount of from 10.5 to 12% by weight, each based on the total weight of the dry powder component.

In a further preferred embodiment, the pigment is selected from the group consisting of Fe, Mn, Cu spinel, (Mn, Cu) Fe ₂ O ₄ , Mn, Fe spinel, MnO ₂ , transition metal oxides and ferrites.

The powder component is mixed in the coating composition with a binder component. This binder component acts as a type of adhesive in the finished ceramic coating and combines the individual components of the powder component in the ceramic coating.

The binder component is surface-modified nanoparticles selected from n-ZrO ₂ and / or n-TiO ₂ . These are nano-ZrO ₂ (n-ZrO ₂ ) and nano-TiO ₂ (n-TiO ₂ ). The binder component preferably has an average particle size d ₅₀ of less than 150 nm, preferably less than 100 nm, more preferably less than 80 nm, and most preferably less than 60 nm. According to another preferred embodiment, the particles have the binder component an average particle size d ₅₀ of at least 10 nm, preferably of at least 15 nm. In the case of TiO ₂ , the average particle size may be at least 40 nm. This is preferably the mean particle size of the primary particles of the surface-modified nanoparticles, ie, before these primary particles come together in a possible further reaction step to form larger agglomerates.

According to the invention, the particles of the binder component are surface-modified. By this surface modification, the binder particles can be retained in finely divided form in the coating composition. A disadvantage of previous binder compositions is the agglomeration of the nanoscale binder particles into larger particle agglomerates. By using a surface-modified binder particle, this agglomeration can be reduced or avoided. Thus, the smaller size nanoscale binder particles are retained in the coating composition.

Preferably, the surface modifying agent is an oxaalkanoic acid, ie, an aliphatic carboxylic acid in which one or more carbon atoms have been replaced by oxygen atoms. Preferably, an oxaalkanoic acid is based on a straight-chain, aliphatic carboxylic acid, but it is also possible to use branched oxaalkanoic acids. More preferably, the oxaalkanoic acid is a C ₄ -C ₁₅ aliphatic carboxylic acid based compound in which one or more carbon atoms, ie methylene groups, have been replaced by oxygen atoms. More preferably, every third methylene group of an aliphatic carboxylic acid is replaced by an oxygen atom to obtain an oxacarboxylic acid.

In a particularly preferred embodiment of the present invention, the nanoparticles of the binder components, which are also referred to below as nanobinders, are reacted with a surface-modifying agent selected from the group consisting of 3,6,9-trioxadecanoic acid (2- [2- (2 -Methoxyethoxy) ethoxy] acetic acid) (TODS), 3,6-dioxaheptanoic acid (2- (2-methoxyethoxy) acetic acid), acetic acid, polyacrylic acid and its salts, and alkaline polycarbonates, surface-modified. Preferably, 3,6,9-trioxadecanoic acid is used as the surface-modifying agent. Polyacrylic acid (PAS) and its salts may, for example, have a molecular weight in the range from 500 to 8000 g / mol, or from 2000 to 6000 g / mol.

For surface modification of the nanoparticles of the binder component, the nanoparticles, such as ZrO ₂ or TiO ₂ , may be mixed with a surface modifying agent. Preferably, between 1 and 50 g of surface-modifying agent are used per 100 g of nanoparticles.

The surface modification of the binder particles can on the one hand reduce or prevent the agglomeration of the particles by sterically shielding the particles from one another. On the other hand, the surface-modifying agents contribute to an electrostatic stabilization of the particles. Ideally, the steric and electrostatic effects are balanced, which, on the one hand, can prevent the agglomeration of the particles but, on the other hand, does not disturb the processing of the coating composition. For example, if the electrostatic effect is too strong, the formation of homogeneous droplets may be disturbed when spraying the coating composition, and thus a homogeneous coating difficult.

Without wishing to be bound by theory, it is believed that the surface-modified particles may beneficially affect the viscosity characteristics of the coating composition.

Also important in the preparation of the coating composition according to the present invention are the proportions between the powder component and the binder component. The powder component is used in a preferred embodiment in an amount of 85-97% by weight, preferably 90-97% by weight, based on the solids content in the coating composition.

The binder component can be used in different amounts depending on the type of binder component. When ZrO _{2 is used} as the binder component, it may be present in an amount in the range of from 3 to 10% by weight, preferably in an amount of from 5 to 8% by weight, more preferably from 5.5 to 7.5% by weight. %, and most preferably from 5.8 to 7.1% by weight, based on the amount of the dry powder component.

When using surface-modified TiO ₂ as the binder component, the amount of binder component can be reduced relative to the use of ZrO ₂ . Thus, in a preferred embodiment of the present invention, when TiO _{2 is used} as the binder component, it may preferably be contained in an amount in the range of 2 to 7% by weight, more preferably 3 to 6.5% by weight, more preferably 3 , 2 to 6.0 wt%, and most preferably from 3.5 to 5.9 wt%, based on the amount of dry powder component.

According to a preferred embodiment, only one of the two abovementioned binders is used for the binder component, ie TiO ₂ and ZrO ₂ are not used side by side as binder component. According to another preferred embodiment, a mixture of the two binder components of TiO ₂ and ZrO _{2 is} used.

The particle sizes of the surface-modified nanoparticles can also differ depending on the type of binder used. According to a preferred embodiment, the binder component comprises surface-modified ZrO ₂ and the particles have an average particle size distribution (d ₅₀ ) of 10 to 30 nm, more preferably 15 to 25 nm, more preferably 18 to 22 nm.

In an alternative embodiment, the binder component comprises surface-modified TiO ₂ and the particles have an average particle size (d ₅₀ ) of 40 to 60 nm, preferably 45 to 55 nm, more preferably 48 to 52 nm.

According to another preferred embodiment, the surface-modified nanoparticles of the binder component can be prepared separately in a suspension before this suspension is added to the coating composition. This suspension of nanoparticles can be prepared in conventional solvents, as are also used to prepare the coating composition. Preferably, this suspension has an amount of nanoparticles in the range of 30 to 70 wt .-%, preferably from 35 to 60 wt .-%, more preferably from 40 to 55 wt .-% of nanoparticles, based on the total amount of the suspension of nanoparticles.

In a preferred embodiment, the solvent is initially introduced, then the condenser is added, followed by the powder components and finally the nanobinder.

In providing the coating composition, a plasticizer is preferably used in the present invention. This condenser is added to raise a stable and homogeneous suspension sufficiently quickly. In this case, the condenser can additionally act homogenizing, dispersing and / or stabilizing for the coating composition. In the present invention, 3,6,9-trioxadecanoic acid and / or 3,6-dioxaheptanoic acid is preferably used as the plasticizer.

The liquefier is preferably used in an amount of 0-1%, particularly preferably 0.3-0.7% by weight, based on the total amount of the liquid coating composition or coating composition suspension. An amount less than 0.3% by weight of liquefier can lead to agglomeration of the particles present in the coating composition, in particular to agglomeration or destabilization of the pigment particles, and thus to a destabilization of the coating composition.

If the coating composition is applied by spraying, spraying dust may form on spraying the coating composition. This spray dust is usually undesirable. In order to reduce the formation of spray dust during the spraying process, an alcohol may also be added to the coating composition in an amount of up to 1% by weight. Propanol and / or butyl glycol (2-butoxyethanol) are preferably added for this purpose. By this addition of an alcohol in an amount of less than 1 wt .-%, the formation of spray dust can be reduced or prevented. In addition, the addition of an alcohol can also positively influence the flow properties of the coating composition.

As the fourth component, an acid is added to the coating composition according to the invention. The acid improves the formation of the ceramic coating. As the acid, any inorganic acid can be used. Preferably, the acid is selected from the group consisting of hydrochloric acid, nitric acid and sulfuric acid, in which case especially the nitric acid is preferred. Preferably, the acid is used in a concentration in the range of 1-13 mol%. Preferably, the acid, preferably nitric acid, in an amount of 0.35 to 0.8 wt .-% based on the total amount of the coating composition.

The coating composition provided in step a) has a pseudoplastic rheological behavior. Depending on the shear rate, for example, dynamic viscosities between 300 mPas (at 20 l / s) and 50 mPas (at 1200 l / s) are achieved. To reduce In addition, one of the abovementioned solvents, which is already used to prepare the coating composition, may be added to the viscosity. Alternatively or additionally, the viscosity can be adjusted by adding at least one alcohol. Preference is given to using an aliphatic alcohol, more preferably selected from the group consisting of methanol, ethanol, n-propanol, isopropanol, n-butanol, isobutanol, tert-butanol and 2-butoxyethanol, and preferably ethanol and / or iso-propanol. It is also possible to use mixtures of different alcohols.

An aliphatic alcohol, or also referred to as alcohol for short, is to be understood in the present invention to mean a straight-chain or branched, saturated alcohol. An aliphatic alcohol is preferably a straight-chain or branched C ₁ -C ₁₀ -alcohol, more preferably a straight-chain or branched C ₁ -C ₆ -alcohol, more preferably selected from the group consisting of methanol, ethanol, n-propanol, isobutyl, Propanol, n-butanol, iso-butanol, and tert-butanol.

By adjusting the viscosity, different carrier materials can be uniformly coated. In particular, absorbent carrier bodies, such as cordierite, require adjustment of the viscosity in accordance with the pumping speed in order to be able to apply coatings of different thicknesses. In addition, the alcohol has the advantageous property that when drying the applied coating composition, it can dry evenly and form a green coating with or without reduced cracking. By using an alcohol in the coating composition, therefore, cracking during drying of the coating composition can be reduced or completely prevented.

In a preferred embodiment of the present invention, alcohol is used to adjust the viscosity of the coating composition. When using alcohol for adjusting the viscosity, it may be used in an amount of from 1 to 20% by weight, preferably in an amount of from 3 to 20% by weight, more preferably in an amount of from 3 to 15% by weight, more preferably in an amount of from 3 to 10% by weight, and most preferably in an amount of from 3 to 5% by weight, based on the total weight of the liquid coating composition.

After adjusting the viscosity of the coating composition, the final coating composition may have a viscosity in the range of 50-90 mPas at a shear rate of 1200 l / s.

After providing the coating composition, the coating composition is applied in step b) on at least one surface of a carrier body. The application of the coating composition to the carrier body can be carried out by conventional methods, for example by means of dip coating, flooding, spraying or spin coating. The coating composition is preferably applied in a thickness such that the final ceramic coating after firing of the coating composition has a thickness in the range of 1 to 100 μm, preferably in the range of 5 to 70 μm, more preferably in the range of 10 to 50 has μm.

In a preferred embodiment, the carrier body to which the coating composition is applied is selected from a honeycomb body, in particular a honeycomb body with rectangular honeycombs, ceramic bulk material, in particular spheres or rings, and / or a metal sheet. However, it is also possible to apply the coating composition to an enamel layer. Particularly advantageous here is the possibility of burning or solidifying both the enamel coating and the coating composition according to the invention in a single step. For this purpose, the enamel coating can first be applied, and after drying the enamel coating without prior firing of the enamel coating, the coating composition according to the invention can be applied directly. In a subsequent firing step, both the enamel coating and the coating composition according to the invention can be baked.

When applying the coating composition to a honeycomb body, the coating may be done by dip coating. The dip coating may be done depending on the number of honeycombs in the honeycomb body. For honeycomb bodies with a high number of honeycombs per area, d. H. a small diameter of the individual honeycombs, usually less viscous coating compositions are used. If a coating composition having too high a viscosity is used, it may not be possible to form a uniform coating in all regions of the honeycomb body.

In a preferred embodiment of the present invention, a honeycomb body has at least 20 cells per square inch, preferably at least 40 cells per square inch, more preferably at least 100 cells per square inch, more preferably at least 150 cells per square inch.

In another preferred embodiment of the present invention, the carrier material is cordierite ceramics or aluminum silicate ceramics, in particular a cordierite honeycomb body or aluminum silicate honeycomb body.

After applying the coating composition to the carrier body, the coating composition is dried in step c). This can be done in the air, but it is also possible to accelerate the drying. This can be done for example by reducing the ambient pressure or by increasing the temperature. In a preferred embodiment, the applied coating composition is dried by means of warm air which is flowed through the inner channels of the honeycomb body. In this case, the evaporated solvent can be easily removed. The drying of the carrier body can be carried out at a temperature in the range from 25 to 80 ° C, more preferably in the range from 30 to 40 ° C.

In a final process step according to the present invention, the dried coating composition as produced in step c) is solidified. By solidifying, which involves a firing process, a ceramic coating is formed from the dried coating composition. In a preferred embodiment of the present invention, the dried coating composition may be fired at a temperature in the range of 400 to 1500 ° C, preferably in the range of 500 to 1000 ° C, more preferably in the range of 600 to 850 ° C. The firing temperature can be maintained, for example, over a period of at least 30 minutes, preferably at least 1 hour, more preferably at least 2 hours.

When heating or cooling the coated carrier material, it must be taken into account, above all, that the carrier material is not damaged by excessive temperature fluctuations. It is therefore preferable to select the heating rate or the cooling rate such that the carrier material is present unchanged after the firing process. Exemplary heating rates include rates of not more than 200 ° C / hr, preferably not more than 100 ° C / hr, more preferably not more than 80 ° C / hr. The cooling can be done with just such rates.

Preferably, drying and firing are performed in separate process steps to ensure that drying is complete before starting firing. However, it is also conceivable that both steps directly adjoin one another and thus be combined into one step.

In the following, some terms as used in the present application will be explained in more detail.

"Nanoparticles" in the sense of the present invention are particles having an average particle diameter (also referred to as average particle size) of not more than 100 nm or else re-dispersible agglomerates of such particles. Unless stated otherwise, the average particle diameter is taken to mean the particle diameter based on the volume average (D ₅₀ value). The D ₅₀ value is determined by dynamic light scattering, for example with an UPA (Ultrafine Particle Analyzer) or a Malverngerät. The principle of dynamic light scattering is also known by the terms "photon correlation spectroscopy" (PCS) or "quasi-elastic light scattering" (QELS). For particularly small particles, quantitative electron microscopic methods (in particular TEM) can also be used. In addition, X-ray diffraction (XRD) can also be used to determine primary particle size.

The measurement of the viscosity of a coating measurement is a measurement of the dynamic viscosity. It can be done in a thermostatable rotation viscometer at 20 ° C. The specification of the corresponding shear rate must be observed.

The coated carrier bodies according to the present invention can be advantageously used in the catalytic purification of VOCs offset exhaust gases. The coated carrier bodies according to the present invention are particularly suitable for the oxidation-catalytic treatment of gaseous organic hydrocarbons (VOCs) or of aerosol mixtures which may contain, for example, organic solvents, fat droplets or wax droplets.

By applying a coating according to the invention, the surface of the carrier body can be significantly increased. Depending on the layer thickness of the ceramic coating, an increase of the reactive surface at a layer thickness of 50 μm may result from about 5,600, with a layer thickness of 400 μm about a factor of 45,000. Thus, according to a preferred embodiment, the ceramic coating provides an increase in surface area per 10 μm layer thickness by a factor of at least 200, preferably at least 500, more preferably at least 800, more preferably at least 1000, and most preferably at least 1200.

Due to the open-pored structure of the ceramic coating according to the present invention, VOCs can penetrate into the pores of the coating and be oxidized with oxygen stored there. Without wishing to be bound by theory, it is believed that there is sufficient oxygen in the open cell structure to convert the VOCs into pores.

VOCs according to the present invention are to be understood as meaning carbon-based organic compounds which have a boiling point of at least 50 ° C., or of at least 80 ° C., or of at least 100 ° C., at ambient pressure. The VOCs can boil below a boiling point of 400 ° C, or below 300 ° C, or below 250 ° C at ambient pressure.

The exhaust gases laden with VOCs can be passed in a customary manner over the surface of the carrier body in order to enable the oxidative degradation of the VOCs. Preference is given to a temperature of 150 to 500 ° C is used, more preferably from 200 to 450 ° C. However, temperatures of up to 800 ° C are possible.

The space velocities can be between 500 / h and 20,000 / h with concentrations of VOCs from 1 to 100 g / m ³ , preferably from 1 to 25 g / m ³ .

To aid in oxidative degradation on the ceramic coatings of the present invention, the coatings may additionally be provided with a noble metal catalyst. This catalyst may, for example, comprise one or more noble metals selected from platinum, palladium, rhodium and / or iridium. The noble metals may be conventionally applied as salt solutions of the metals and subsequently reduced in-situ to the precious metals.

The use of the ceramic coatings of the invention are compared to conventional noble metal catalysts significantly cheaper, easier to manufacture and more robust against catalyst poisons, such as sulfur compounds, halogens and heavy metals. Moreover, the ceramic coatings of the present invention also do not exhibit the higher temperature wear typically exhibited by noble metal catalysts. Noble metal catalysts can lose efficiency at elevated temperature because the noble metal particles grow together and thus lose surface. In contrast, the ceramic catalyst of the present invention is extremely temperature stable.

In the following, the present invention will be explained in more detail with reference to some examples, without being limited to these examples.

Examples

Example 1: Comparison of the decomposition rates of ethanol

Two cordierite carrier bodies with 40 cells / inch ² were coated with a coating according to the invention with ZrO ₂ as binder. As a comparison, a structurally identical cordierite carrier body without coating was used.

All three carrier bodies had a volume of 0.176 l and were charged with an air flow of 29 l / min. This corresponds to a space velocity of about 10,000 / h. The air stream was provided with 1 g or 5 g of ethanol per m ³ .

1 shows the rate of degradation or conversion in% of ethanol at 1 g (graph with squares) or 5 g (graph with diamonds) ethanol per m ³ as a function of the temperature (in ° C) for the provided with the coatings according to the invention carrier body. It can be seen that even at temperatures of about 275 ° C over 50% of the ethanol is degraded in the air stream. A decomposition rate of over 90% is already achieved at a temperature of 300 ° C.

On the contrary is off 2 showing the rate of degradation or conversion in% of 1 g / m ^{3 of} ethanol at a space velocity of 16 100 / h as a function of the temperature (in ° C) for the uncoated carrier, it can be seen that at 250 ° C no appreciable degradation of ethanol and even at a temperature of 500 ° C, only about 40% of the ethanol are degraded.

Example 2: Comparison of the decomposition rates of toluene

Two cordierite carrier bodies with 40 cells / inch ² were coated with a coating according to the invention with ZrO ₂ or TiO ₂ as binder. As a comparison, a structurally identical cordierite carrier body without coating was used.

All three support bodies had a volume of 0.176 l and were charged with an air flow of 47 l / min. This corresponds to a space velocity of about 16 100 / h. The air flow was provided with 1 g of toluene per m ³ .

3 shows the rate of reduction or% conversion of toluene as a function of the temperature (in ° C) for the carrier bodies provided with the coatings according to the invention (graph with circles for ZrO ₂ coating, with squares for TiO ₂ coating). It can be seen that even at temperatures of about 370 ° C over 50% of the toluene is degraded in the air stream, regardless of the type of binder. A decomposition rate of more than 90% is already achieved at a temperature of 450 ° C.

On the contrary is off 4 showing the rate of degradation or% conversion of toluene as a function of temperature (in ° C) for the uncoated support, it can be seen that at 370 ° C no appreciable degradation of toluene took place and even at a temperature of 500 ° C only about 10% of the toluene are degraded.

Example 3: Degradation of ethanol at different space velocities

With the aid of the ceramic coatings of the present invention, a VOC-laden exhaust gas stream can be stably cleaned. Using the example of an exhaust gas stream charged with 1 g / m ^{3 of} ethanol, it has been shown that an almost 100% degradation rate can be achieved almost independently of the space velocity (GHSV) even at temperatures below 350 ° C. on a carrier body coated according to the invention. Out 5 It can be seen that at space velocities of 5,000 / h to 20,000 / h at about 330 ° C well over 90% of the ethanol are degraded.

The coating suspension was prepared as previously described. For coating the carrier body they are immersed in a sufficiently filled vessel. After the air has escaped from the honeycomb body, after about 1-2 minutes, the coated support body is pulled out of the suspension, placed on a draining rack and blown off in the inner channels still protruding coating with compressed air so that the coating is not destroyed. As a result, clogging of the inner channels is avoided with supernatant suspension. Thereafter, the coated carrier body is dried on the drying rack by internal flow at 40 to 60 ° C and fired after complete drying in the oven for one hour at 830 ° C.

Claims (32)

A process for coating a carrier body comprising a) providing a coating composition comprising i) a powder component; ii) a binder component; iii) a liquefier; iv) an acid; and v) optionally an alcohol; b) applying the coating composition to at least one surface of the carrier body; c) drying the applied coating composition; and d) solidifying the dried coating composition to form a coating; wherein the powder component is a mixture of - at least a first porous alumina powder having an average particle size d ₅₀ in the range of 5-80 microns; At least one second alumina powder having an average particle size d ₅₀ in the range of 0.3-2 μm; and - at least one pigment having an average (primary) particle size d ₅₀ in the range of 80-200 nm; wherein the powder component is contained in an amount of between 80 and 97% by weight, based on the solids content in the coating composition; and wherein the binder component contains surface-modified nanoparticles selected from n-ZrO ₂ and n-TiO ₂ , with an average particle size d ₅₀ of less than 150 nm. The method of claim 1, wherein the carrier body is selected from a honeycomb body and ceramic bulk material, preferably balls and / or rings. The method of claim 1 or 2, wherein the particles of the first porous alumina powder has an average particle size d ₅₀ in the range of 30-80 .mu.m, preferably 40-80 .mu.m, more preferably 35-60 .mu.m, and particularly preferably 37-45 .mu.m , or have a particle size d ₅₀ in the range of 5-60 .mu.m, preferably 5-30 .mu.m, more preferably 6-20 .mu.m, and particularly preferably 6-15 .mu.m. Method according to one of the preceding claims, wherein the particles of the first porous alumina powder comprise γ-alumina, preferably with a BET specific surface area of 50-100 m ² / g, more preferably of 60-85 m ² / g, particularly preferably of 65 -80 m ² / g. Method according to one of the preceding claims, wherein the particles of the second alumina powder have an average particle size d ₅₀ in the range of 0.4-1.5 microns, preferably from 0.5 to 0.8 microns. Method according to one of the preceding claims, wherein the particles of the second alumina powder comprise α-alumina, preferably having a BET specific surface area of 5-15 m ² / g, more preferably 6-10 m ² / g. Method according to one of the preceding claims, wherein the particles of the pigment have a BET specific surface area in the range of 4-50 m ² / g, preferably 20-50 m ² / g, or preferably 4.7-5.7 m ² / own g. Method according to one of the preceding claims, wherein the particles of the pigment have an average (primary) particle size d ₅₀ in the range of 50-250 nm, preferably from 80-200 nm, more preferably from 90-150 nm, and / or an average particle size d ₅₀ in the range of 0.5-5 microns, preferably from 0.6 to 3 microns, more preferably from 0.8 to 1.2 microns own. A process according to any one of the preceding claims, wherein the first alumina powder in the powder component is present in an amount of 60-90% by weight, preferably 65-80% by weight, more preferably 70-75% by weight, based on the Total weight of the powder component is included. A method according to any one of the preceding claims wherein the second alumina powder is present in the powder component in an amount of 10-20% by weight, preferably 10-18% by weight, more preferably 10-16% by weight, and most preferably of 14-15% by weight, based on the total weight of the powder component. Method according to one of the preceding claims, wherein the pigment in the powder component in an amount of 8-18 wt .-%, preferably from 9-17 wt .-%, more preferably from 10-16 wt .-%, based on the total weight the powder component is contained. A method according to any one of the preceding claims, wherein the pigment in the powder component is selected from the group consisting of Fe, Mn, Cu spinel (Mn, Cu) Fe ₂ O ₄ , Mn, Fe spinel, MnO ₂ , transition metal oxides and ferrites , The method of any one of the preceding claims, wherein the surface modified nanoparticles are surface modified with a surface modifying agent selected from the group consisting of 3,6,9-trioxadecanoic acid, 3,6-dioxaheptanoic acid, acetic acid and alkaline polyacrylates. Method according to one of the preceding claims, wherein the binder component comprises surface-modified ZrO ₂ and in an amount in the range of 2-5 wt .-%, preferably from 2.5 to 5 wt .-%, more preferably from 2.5 to 4, 5 wt .-%, and most preferably from 3-4 wt .-%, based on the powder component is added. Method according to one of the preceding claims, wherein the binder component comprises surface-modified ZrO ₂ and the particles has an average particle size distribution d ₅₀ of 10 nm-30 nm, preferably 15 nm-25 nm, more preferably 18 nm-22 nm. Method according to one of the preceding claims, wherein the binder component comprises surface-modified TiO ₂ and in an amount in the range of 1.5-7 wt .-%, preferably 2-5 wt .-%, more preferably 2-4 wt. %, and most preferably from 2.4 to 3.6% by weight, based on the powder component. Method according to one of the preceding claims, wherein the binder component comprises surface-modified TiO ₂ and the particles has an average particle size distribution (d ₅₀ ) of 40 nm-60 nm, preferably from 45 nm-55 nm, more preferably 48 nm-52 nm. A process according to any one of the preceding claims wherein the liquefier is selected from the group consisting of 3,6,9-trioxadecanoic acid and 3,6-dioxaheptanoic acid. A process according to any one of the preceding claims wherein the plasticizer is added in an amount ranging from 0-1%, more preferably from 0.3-0.7% by weight, based on the total amount of the coating composition. A process according to any one of the preceding claims, wherein the acid is selected from the group consisting of hydrochloric acid, nitric acid and sulfuric acid, and is preferably nitric acid. Method according to one of the preceding claims, wherein the alcohol is an aliphatic alcohol, preferably selected from the group consisting of methanol, ethanol, n-propanol, iso-propanol, n-butanol, iso-butanol, tert-butanol, 2-butoxyethanol and mixtures thereof, and preferably ethanol and / or iso-propanol. Method according to one of the preceding claims, wherein the alcohol in an amount of 1-20 wt .-%, preferably 3-20 wt .-%, more preferably 3-15 wt .-%, more preferably 3-10 wt %, and most preferably from 3-5% by weight, based on the total weight of the coating composition. The method of any one of the preceding claims wherein providing the coating composition comprises mixing the individual components of the coating composition and thereafter ripening the coating composition over a period of from 3 to 48 hours, preferably from 6 to 36 hours, and more preferably from 12 to 24 hours , Method according to one of the preceding claims, wherein the coating composition in step a) has a viscosity in the range of 50- 90 mPas at a shear rate of 1200 l / s. Method according to one of the preceding claims, wherein the coating has a thickness in the range of 1-100 microns, preferably in the range of 5-70 microns, more preferably in the range of 10-50 microns. Method according to one of the preceding claims, wherein the drying in step c) is effected by means of warm air, preferably at a temperature in the range of 25-80 ° C, more preferably in the range of 30-60 ° C. A method according to any one of the preceding claims, wherein the solidifying in step d) comprises firing the dried coating composition at a temperature in the range of 400-1500 ° C, preferably in the range of 500-1000 ° C, more preferably in the range of 600-850 ° C takes place. Coating preparable by a method according to one of the preceding claims. Carrier body with a coating according to claim 28. Use of a coating according to claim 28 or a carrier body according to claim 29 for the catalytic purification of a gas, preferably a VOC-containing exhaust gas. A process for the catalytic purification of a gas, preferably a VOC-containing exhaust gas, using a coating according to claim 28 or a carrier body according to claim 29. A coating composition comprising i) a powder component; ii) a binder component; iii) a liquefier; iv) an acid; and v) optionally an alcohol; wherein the powder component is a mixture of - at least a first porous alumina powder having an average particle size d ₅₀ in the range of 5-80 microns; At least one second alumina powder having an average particle size d ₅₀ in the range of 0.3-2 μm; and - at least one pigment having an average (primary) particle size d ₅₀ in the range of 80-200 nm; wherein the powder component is contained in an amount of between 80 and 97% by weight, based on the solids content in the coating composition; and wherein the binder component contains surface-modified nanoparticles selected from n-ZrO ₂ and n-TiO ₂ , with an average particle size d ₅₀ of less than 150 nm.

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