DE10114328A1 - Process for applying washcoat to a honeycomb body - Google Patents

Abstract

The invention relates to a method for applying a washcoat dispersion (3) to a honeycomb structure (1) that is provided with channels (2). The washcoat dispersion (3) is preferably a substrate material (13) evenly distributed in a substrate fluid and comprising a mixture of stabilized, high-surface aluminum oxide of the transition group elements and at least one promoter oxide. The inventive method is further characterized in that the honeycomb structure (1) is contacted with the washcoat dispersion (3), the washcoat dispersion (3) being induced to vibrate at least while it is applied, thereby preventing the washcoat dispersion from prematurely gelling and ensuring a uniform washcoat thickness in the channels.

Description

The invention relates to a method for applying a washcoat. Dispersion on a honeycomb body with channels. Coated like this Honeycomb bodies are used in particular as catalytic converters for exhaust gas purification an internal combustion engine used.

Known honeycomb bodies are of many essentially parallel channels crossed and consist for example of a metallic honeycomb body, that of a large number of partially structured and possibly wound Sheet metal foils is built up, or from a ceramic molded body. As ceramic material can e.g. B. cordierite, mollite or α-alumina be used. With a view to a high effectiveness of the catalytic Implementation of pollutants in the exhaust gas requires the largest possible to provide catalytically active contact surface of the honeycomb body. The For this purpose, honeycomb bodies have very high channel densities, for example metallic honeycomb bodies with a cell density of up to 1500 cpsi ("cells per square inch ").

This further enlarges the catalytically active surface achieved that the relatively smooth channel walls with a so-called washcoat coated with a very jagged surface. This rugged surface ensures a sufficiently large one Space for fixing a catalyst (e.g. platinum, rhodium etc.) and serves on the other hand to swirl the exhaust gas flowing through, a particularly intensive contact with the catalyst is effected.  

Applying the high-surface washcoat layer that promotes catalysis is known to take place in such a way that the honeycomb body in a liquid Washcoat dispersion is dipped or sprayed with it. Then will the excess washcoat dispersion is removed, the washcoat in the honeycomb body dried and finally calcined at temperatures mostly above 450 ° C. During the calcination, the volatile components of the washcoat Dispersion expelled, so that a temperature-resistant and catalyst-promoting Layer with a high specific surface is generated. If necessary this process is repeated several times to achieve a desired layer thickness to reach.

The washcoat usually consists of a mixture of an aluminum oxide Transitional series and at least one promoter oxide such as Rare earth oxides, zirconium oxide, nickel oxide, iron oxide, germanium oxide and Barium oxide. The washcoat dispersion has to be applied during application the honeycomb body has the best possible flow property in order to to achieve the desired, uniform layer thickness over the entire channel length. This is especially true with regard to recent developments decreasing channel cross sections of particular importance.

In order to achieve such a flow property, known washcoat Dispersions have a certain pH, with only a limited Solid content is allowed. However, tests have shown that a such washcoat dispersion has a time-dependent viscosity. this has As a result, the washcoat dispersion gels very quickly and the generation a uniform layer thickness prevented. This gelling is particularly in the case of ceramic honeycomb bodies reinforced by the fact that the ceramic Base material absorbs the liquid component of the washcoat dispersion, whereby the solids content of the washcoat dispersion is increased as a percentage.  

It is therefore the object of the present invention to apply a method a washcoat dispersion on a honeycomb body with channels to be specified, with premature gelation of the washcoat dispersion preventing and a washcoat layer with a uniform washcoat thickness on the Canal walls is ensured.

This object is achieved by a method according to the characteristics of the Claim 1. Further advantageous embodiments of the method are in the dependent claims described.

According to the invention for applying a washcoat dispersion to a Channel honeycomb body proposed the honeycomb body with the Bring the washcoat dispersion into contact, the washcoat dispersion is stimulated to vibrate at least during application. The washcoat Dispersion preferably comprises an even one in a carrier fluid distributed carrier material with a mixture of stabilized, high-surface aluminum oxide of the transition series and if necessary at least one promoter oxide.

The process has the advantage that the washcoat Dispersion is prevented during application. This essentially has its origin is that the washcoat dispersion has many components different specific density or mass. As a result, the individual components due to the vibrations different movements To run. This leads, for example, to relative movements of the Carrier material particles against the carrier fluid, where forces and Deformations occur that reduce the viscosity of the washcoat Prevent dispersion over a longer period of time (e.g. several minutes) or at least significantly reduce it.  

According to a further embodiment of the method, the washcoat Dispersion excited to vibrate even after application, especially until A substantially uniform length over a predefinable length of the channels Washcoat thickness is produced, which is preferably between 5 to 50 microns. The Vibration of the washcoat dispersion does not only guarantee one over one certain period of time constant viscosity of the washcoat dispersion, but at the same time supports the distribution of the washcoat dispersion in the channels of the honeycomb body. The washcoat thickness produced is particularly of Thickness of the channel wall or the free cross section of the channels depending on the washcoat thickness with decreasing channel walls and cross sections is reduced. A washcoat thickness of 10 to 20 µm is preferred, especially for duct walls with a thickness of less than 0.03 mm.

It is also proposed that the washcoat dispersion be sprayed or to apply dipping methods. In a spraying process, the washcoat Dispersion finely atomized and over at least one end face of the honeycomb body brought in. In the immersion process, the honeycomb body is at least partially in a basin with the washcoat dispersion immersed in this process preferably only from one end of the honeycomb body. At a one-sided immersion, the washcoat is evenly distributed Dispersion on the channel walls of the honeycomb body using the capillary effect of the channels.

According to a further advantageous embodiment of the method, the The washcoat dispersion is stimulated by vibrating the honeycomb body itself. The generation of a vibration of the honeycomb body is particularly under manufacturing and process engineering aspects easier and more practicable, especially with regard to the use of the spraying method for Apply the washcoat dispersion.  

With regard to the type and direction of those carried out by the honeycomb body Vibration must always take the washcoat dispersion used into account, since ultimately in it the relative movements of the carrier materials compared to the Carrier fluid to be generated. Studies have shown that a vibration of the honeycomb body approximately perpendicular to an axis to which the Channels are arranged substantially in parallel, maintaining one certain viscosity of the washcoat dispersion is possible for a particularly long time. However, for example, is the uniform distribution of the washcoat Dispersion over a predefinable length of the channels in the foreground, it is so advantageous to vibrate the honeycomb body approximately in the direction of an axis to which the channels are essentially parallel.

According to a further embodiment of the method, the honeycomb body guides one Torsional vibration, which preferably has an amplitude close to a circumference of the Honeycomb body of less than 5 mm, in particular less than 2 mm. That means, that the channels, which are arranged near the circumference of the honeycomb body, with a relatively small amplitude in an alternating direction of the circumference swing. This also has the consequence that in particular centrally arranged Channels can be excited to vibrate with lower intensity. This is For example, advantageous if the length and / or the cross section of the Channels across the cross section of the honeycomb body is not constant. Such Torsional vibration is therefore, for example, for conical honeycomb bodies particularly well suited, the channels in the outer edge areas with a longer channel length than in central areas. The stronger pronounced vibration in these edge areas of the honeycomb body supports the Flow property of the washcoat dispersion and thus provides an even Distribution of washcoat on the channel walls of the honeycomb body safely.

Since the flow behavior of the washcoat dispersion essentially depends on the Depending on the shape of the duct or the duct cross-section, it may also be be advantageous to combine the different types of vibration.  

According to yet another embodiment of the method, the excitation takes place the washcoat dispersion with a predeterminable frequency, which in particular in Depending on the consistency of the washcoat dispersion can be selected. The The consistency of the washcoat dispersion can be varied Parameters are described: z. B. the viscosity, the ratio of Carrier material to carrier fluid, type and / or size of the carrier material, density of the carrier fluid, temperature of the washcoat dispersion, pH of the dispersion, Surface structure of the carrier material etc.

If, for example, the washcoat dispersion is directly stimulated, then a frequency in the ultrasound range has proven to be particularly advantageous. The Excitation took place in a frequency range from 20 kHz to 10 MHz. Especially in the case of indirect excitation, for example due to a Vibration of the honeycomb body, there are frequencies in the audible range proven, in particular an excitation with a frequency between 20 Hz and 15 kHz a drop in viscosity over a very long period of time ensured.

The method is now explained in more detail with reference to the following drawings. It demonstrate:

Fig. 1 shows schematically and in perspective a metallic honeycomb body and

Fig. 2 schematically shows a detailed view of a channel coated with washcoat.

Fig. 1 shows schematically and in perspective a honeycomb body 1, wherein the honeycomb body shown is formed from metallic sheet-metal foils 1 8, the structured part, are spirally wound and disposed in a sheath tube 9. The partially structured sheet metal foils 8 form channels 2 which are arranged essentially parallel to an axis 6 of the honeycomb body 1 . When using such a honeycomb body 1, for example as a catalytic converter for exhaust gas cleaning of an internal combustion engine, the exhaust gas to be cleaned flows through the honeycomb body 1 , in which it enters the channels 3 from the end face 11 , is guided past the inner channel walls 14 and brought into contact and ultimately emerges on the opposite end.

Such a honeycomb body 1 is brought into contact with a washcoat dispersion according to the invention, the washcoat dispersion 3 being excited to vibrate at least during application. This takes place, for example, in the form that the illustrated honeycomb body 1 vibrates approximately in the direction of the axis 6 . In addition, this movement can be superimposed by a torsional vibration of the honeycomb body 1 , the torsional vibration preferably having an amplitude close to a circumference 7 of less than 5 mm. This ensures that the honeycomb body 1 has a substantially uniform washcoat thickness 5 (not shown) after the coating process up to a predeterminable length 4 of the channels 2 . The washcoat dispersion 3 is brought into contact with the honeycomb body 1 in such a way that the end face 11 of the honeycomb body 1 is immersed in a basin with the washcoat dispersion 3 . Due to the capillary action in the channels 2 , the washcoat dispersion 3 flows through the channels 2 .

Fig. 2 shows schematically in a detailed view a channel 2 , which is formed with smooth and corrugated metal foils 8 . Such a honeycomb structure can also be produced, for example, by stacking smooth and corrugated sheet metal foils 8 and then S-shaped or involute twisting of the sheet foils 8 . The sheet metal foils 8 preferably have a foil thickness 10 of less than 0.03 mm. The channels 2 have essentially rectangular cross-sectional areas when the honeycomb body 1 has been extruded from ceramic material.

The channel 2 shown has on its channel wall 14 a washcoat dispersion 3 with an essentially uniform washcoat thickness 5 of preferably 5 to 50 μm. The washcoat dispersion 3 shown comprises a temperature-resistant carrier material 13 and catalysts 12 . Oxidic minerals such as aluminum oxide, titanium oxide, silicon oxide, tin oxide, zirconium oxide, magnesium oxide, aluminum silicate, zeolites and / or alkaline earth metal titanate are preferably used as the carrier material. The carrier material 13 advantageously additionally comprises promoters such as rare earths, alkaline earth metals and / or compounds of the elements of III. to V. Main group of the Periodic Table of the Elements. Metals of the platinum group, such as platinum, palladium and / or rhodium, are usually used as catalyst 12 .

The method according to the invention prevents premature gelling of the Washcoat dispersion during and after application on a metallic or ceramic honeycomb body, with a uniform washcoat thickness in the Channels of the honeycomb body is ensured. This leads in particular with regard on the use of such a honeycomb body as a catalytic converter for cleaning exhaust gases from an internal combustion engine in automobile construction to a highly effective converter that also has high thermal and withstands dynamic loads in such an exhaust system.  

LIST OF REFERENCE NUMBERS

1

honeycombs

2

channel

3

Washcoat dispersion

4

length
S washcoat thickness

6

axis

7

scope

8th

metal sheets

9

casing pipe

10

film thickness

11

front

12

catalyst

13

support material

14

channel wall

Claims (10)

1. A method for applying a washcoat dispersion ( 3 ) to a honeycomb body ( 1 ) having channels ( 2 ), the washcoat dispersion ( 3 ) preferably being a carrier material ( 13 ) uniformly distributed in a carrier fluid with a mixture of stabilized, comprises high-surface aluminum oxide of the transition series and at least one promoter oxide, in which the honeycomb body ( 1 ) is brought into contact with the washcoat dispersion ( 3 ), the washcoat dispersion ( 3 ) being excited to vibrate at least during application. 2. The method as claimed in claim 1, in which the washcoat dispersion ( 3 ) is excited to vibrate even after application, in particular to produce a substantially uniform washcoat thickness ( 5 ) over a predefinable length ( 4 ) of the channels ( 2 ), which is preferably between 5 to 50 microns. 3. The method according to claim 1 or 2, wherein the washcoat dispersion ( 3 ) is applied by means of a spray or immersion process. 4. The method according to any one of claims 1 to 3, wherein the honeycomb body ( 1 ) vibrates. 5. The method according to claim 4, wherein the channels ( 2 ) extend substantially parallel to an axis ( 6 ) of the honeycomb body ( 1 ), in which the honeycomb body (I) vibrates approximately perpendicular to the axis ( 6 ). 6. The method according to claim 4 or 5, wherein the channels ( 2 ) extend substantially parallel to an axis ( 6 ) of the honeycomb body ( 1 ), in which the honeycomb body ( 1 ) vibrates approximately in the direction of the axis ( 6 ). 7. The method according to any one of claims 4 to 6, wherein the channels ( 2 ) extend substantially parallel to an axis ( 6 ) of the honeycomb body ( 1 ), in which the honeycomb body ( 1 ) executes a torsional vibration, preferably a torsional vibration with a Amplitude close to a circumference ( 7 ) less than 5 mm, in particular less than 2 mm. 8. The method according to any one of claims 1 to 6, in which the washcoat dispersion ( 3 ) is excited with a predetermined frequency, which can be selected in particular depending on a consistency of the washcoat dispersion ( 3 ). 9. The method according to claim 7, wherein the frequency in the ultrasonic range is preferably between 20 kHz and 10 MHz. 10. The method of claim 7, wherein the frequency is in the audible range, is preferably between 20 Hz and 15 kHz.