DE102007030283A1 - SCR catalyst assembly - Google Patents

Abstract

Catalyst arrangement (1) with at least one honeycomb body (3) with a ceramic carrier element (9) and a selectively catalytic coating (10) and a honeycomb body (3) enclosing the housing portion (2), wherein the housing portion (2) has a ferritic Includes steel material. In addition, a special housing material and a large amount of washcoat are proposed for an SCR application.

Description

The The present invention relates to a catalyst arrangement with at least a honeycomb body, for the selective catalytic reduction of nitrogen oxides (SCR, Selective catalytic reduction) from exhaust gases of mobile internal combustion engines is trained. Such a catalyst arrangement comes in particular in exhaust systems of mobile internal combustion engines, for example in automobiles, for use.

Statutory regulations have been made in many countries worldwide that set an upper limit for the content of certain undesirable substances in the exhaust gas of internal combustion engines, in particular diesel engines or lean-burn engines. These are mostly substances, such as nitrogen oxides (NO _x ), whose release into the environment is undesirable. To reduce the substances in the exhaust gas catalysts are used which are subject to special requirements. They are for example exposed to very large thermal cycling, with large temperature fluctuations from 0 ° -20 ° C ambient temperature at rest up to 800 ° C and more may occur regularly under full load. This is often accompanied by signs of aging and wear, so that it is desirable to improve the lifetime and efficiency of the catalysts.

Due to the general conditions, for example the design of internal combustion engines with regard to a favorable fuel consumption and the like, the internal engine avoidance of nitrogen oxide emissions to comply with the limits often insufficient, so that at least for the maintenance of relatively low limits exhaust after-treatment is required. It has been found that a selective catalytic reduction of the nitrogen oxides is advantageous. This SCR method requires a reducing agent that is nitrogen-containing. In particular, the use of ammonia (NH ₃ ) as a reducing agent has proven to be a possible alternative. Usually ammonia is not kept as pure ammonia, as this can lead to problems especially in motor vehicles or other mobile applications. Rather, instead of storing the reducing agent itself, reducing agent precursors are often stored and carried in mobile applications.

The Application of the reducing agent is then carried out in specially for the execution of SCR method trained catalysts specially designed for this purpose honeycombs exhibit. In the context of constantly increasing demands on such components, this is regularly the Problem with the devices in terms of their weight, their Size and her Efficiency and reliability to improve. In practice, this will be different priorities set. So it is required for some systems, this particular to build small, if, for example, little space on a planned Vehicle is present. In other cases, but also a special high reliability and Longevity in the foreground are, as for example in commercial vehicle the case is.

task It is thus a catalyst arrangement of the present invention indicate, at least with reference to one of the aforementioned problematic Aspects of relief the known prior art procures.

The The object is achieved with a catalyst arrangement according to the features of the patent claim 1 solved. Further advantageous embodiments of the invention are formulated in the dependent claims specified. It should be noted that in the dependent formulated claims individually listed Features in any, technologically meaningful way with each other can be combined and define further embodiments of the invention.

Furthermore are the in the claims specified features specified and explained in detail in the description, wherein further preferred embodiments of the invention are shown.

present At least some of the objects are achieved by a catalyst arrangement with at least one honeycomb body with a ceramic carrier element and a selective catalytic coating and a housing portion enclosing the honeycomb body solved, wherein the housing portion comprises a ferritic steel material. Under a ferritic Steel is understood to be a steel, for the most part made of ferrite. Such steels often have a carbon content of very low and especially less than 1.0 wt .-% is. Ferritic steels can open Iron base with components such as chromium, nickel, silicon, Aluminum, magnesium be formed. Ferrite is the structure of the Iron alloy, which in the case of iron, a cubic-body-centered Crystal form corresponds. It is a so-called Alpha-iron, which is also called alpha-ferrite.

The preferred use of a ferritic steel material as a housing or housing section makes the catalyst particularly resistant to external influences, such as occur during driving. The catalyst arrangement is within the scope of the present invention Invention including a honeycomb body and a ferritic housing. The honeycomb body carries the substances required for the intended catalytic reaction and is enclosed by a housing. The housing may represent once the housing enclosing the honeycomb body including the adjoining nozzle or pipe sections. The housing in turn can also be divided into a plurality of housing sections, which together form the housing. In particular, it is possible to divide the housing into an upper part and a lower part, which together completely enclose the honeycomb body.

in the Within the scope of the invention so that whole housing or also formed individual housing sections so that they comprise a ferritic steel material. The aforementioned influences can For example, thermal cycling or corrosion loads be. Particularly resistant will the housing, if the steel material used is a ferritic, non-rusting Steel is that has a low coefficient of expansion having. Advantageously, the thermal expansion coefficient of the used Steel material as possible close to the thermal expansion coefficient of the honeycomb body enclosed by it. As a result, a relative movement, which by heating or cooling down the catalyst arrangement during of driving occurs, between the individual components of the Arrangement noticeable reduced. As steel, in the context of the invention, in particular also Stainless steel can be used. Steel is then called stainless steel, if it is an alloyed or unalloyed steel with a particularly high Purity is.

In particular, the relative movements, for example between the honeycomb body and the housing section, can be reduced if the thermal expansion coefficient of the ferritic steel material lies in the range between 10 · 10 ^-6 K ^-1 and 12 · 10 ^-6 K ^-1 [1 / K]. Although it has been shown that a noticeable increase in the service life can be observed even when the coefficient of thermal expansion is in the range from 8 · 10 ^{-6 *} K ^-1 to 14 · 10 ^-6 K ^{-1, it} is particularly preferred the area above. Large differences in temperature usually lead to a rapid expansion behavior of the housing or the housing sections. As a result, break the relatively pressure-sensitive ceramic carrier of the honeycomb body and are considerably damaged. In addition, it may come to a shake loose the honeycomb body, which are held by means of mats within the housing. Loose honeycombs are subject to increased mechanical stress due to vibration and are lost over time. Particularly significant advantages of the invention therefore in connection with ceramic honeycomb bodies with very low thermal expansion coefficient.

All a steel material has proven particularly advantageous, the in addition to iron (FE) as further components a chromium content (Cr) from 18.5 wt% to 19.5 wt%, a molybdenum content (Md) of at most 0.1 wt .-% and a nickel content (Ni) of at most 1 wt .-%. Steel materials, that fulfill this have excellent resistance to corrosion and have z. B. also the above preferred thermal expansion coefficient. You point it over In addition, very good mechanical strength on. Especially preferred is a steel material with a chromium content of 19 wt .-% ± 0.1 wt .-%, a molybdenum content of not more than 0.03 Wt .-% and a nickel content of at most 0.05 wt .-%. Furthermore the ferrite steel material described here is at most nor impurities amounting to a maximum of 0.1 wt .-% in the sum. About that In addition, we also consider a ferritic steel material to be beneficial which is 19% by weight ± 0.1 Wt.% Chromium, at most 1.0 wt .-% nickel and at most 0.1 wt .-% molybdenum (remainder Impurities and iron). The compositions listed here for a Housing of a catalytic converters are an independent part of the invention with respect to any (e.g., metallic) honeycomb body, the optionally also independent can be followed up.

Further Also, a catalyst arrangement is proposed, wherein the selectively catalytic coating at least partially covers the carrier element and is applied in an amount that is a ratio of at least 200 g / l [grams per liter] of honeycomb volume equivalent. The listed here Coating quantity is an independent one Component of the invention with respect to any honeycomb body, the optionally also independent from the housing material can be followed up. In particular, the honeycomb body can be here instead of a ceramic material also of a metallic one Material (eg at least partially structured sheet metal layers) made be. In the following, reference is essentially made to the ceramic honeycomb body However, if necessary, this applies correspondingly to metallic honeycomb bodies.

The honeycomb body regularly has a plurality of channels through which the exhaust gas to be cleaned or treated can flow. The walls are formed by honeycomb walls enclosing channels, which may have, for example, rectangular, square, round or similar cross-sections. Become frequent Honeycomb body produced as a ceramic honeycomb body in an extrusion process, so that almost any cross-sectional shapes for the honeycomb channels to be produced are possible. On the honeycomb body walls, which are made of a ceramic material, a catalytically active coating is applied in addition, so that the honeycomb body consists of honeycomb walls, catalytic coating and the enclosed honeycomb channels.

The catalytic coating, otherwise called washcoat is, is on the relatively porous ceramic surfaces the honeycomb walls applied. The formed by all honeycomb walls Component is subsequently used as a carrier element referred to as the honeycomb body gives the necessary mechanical rigidity and serves as a carrier. The already porous surface or wall is additionally structured by the applied washcoat. The so rugged Guaranteed surface on the one hand a sufficiently large space for the Fixation of catalysts, eg. B. vanadium pentoxide, tungsten trioxide, Titanium dioxide or iron-zeolite coatings and mixtures thereof. On the other hand, the jagged surface also improve the turbulence of the flowing exhaust gas, being a particularly intensive contact with the Kata lysatorstoffen created in the catalytic coating. The washcoat can for example, from mixtures of an alumina of the transition series and at least one promoter oxide, such as at least a rare earth oxide, zirconium oxide, nickel oxide, iron oxide and / or Barium oxide exist. In particular, it is advantageous if the for the washcoat materials used have a coefficient of thermal expansion have, as possible is equal to the thermal expansion coefficient of for the carrier element used material. As particularly beneficial for the SCR process in automotive applications, it has been proven when the amount of Washcoats equivalent to at least 200 g / l of a honeycomb body volume. Under the honeycomb body volume is the entire volume of the honeycomb body enclosed space too understand. Thus, the honeycomb volume includes the volumes of the Support element, of the washcoat, as well as the channels enclosed by it.

Preferably is the minimum amount of washcoat is 220 or even 250 g / l and exceeds in particular not a value of 300 g / l. In this area have can be particularly efficient cleaning effects represent. In addition, a particularly large surface-specific surface at these amounts the heavily jagged Washcoat created which an excellent cleaning of the exhaust gas flowing through allows.

It is particularly advantageous to view the quantities as average quantities, which makes it possible, for example, for the same average values, to arrange larger washcoat amounts and thus larger amounts of catalytically active coating in some areas of the honeycomb body, whereas in other areas these quantities can be kept lower. Furthermore, the averaging can also take account of the heavily rugged surface of the washcoat coating by using an average layer height (H _Wm ) in the measurement of the layer height. This average layer height can serve as a reference for the amount of washcoat applied.

It has also proved to be particularly advantageous if the catalytic coating has a height of at least 10 μm [micrometers]. The height or layer height means the height of the coating measured in the normal direction to the coated surface. In particular, heights of 30 .mu.m to 80 .mu.m have been found to be particularly advantageous since they cause an additional enlargement of the surface. In addition to the use of a mean layer height H _Wm , further values for the layer height can also be recorded in order to describe them more precisely. These values are, on the one hand, the maximum layer height H _Wmax and the minimum layer height of the washcoat H _Wmin . From the maximum and minimum values, it is possible, for example, together with the mean layer height (H _Wm ), to make a statement about the strength of the rupture of the washcoat surface. Indirectly, it is also possible to make a statement about the additional surface created, since with increasing fracture, the additionally created effective surface of the washcoat is increased.

Since it may happen that a honeycomb body has different properties in different areas, it is advantageous in the context of the present invention if the catalytic coating has a plurality of zones of different washcoat amount and / or doping. Under dopings here are the distributions of catalyst materials to understand. Depending on the use of the honeycomb body, these zones can be arranged, for example, more intensively in the inlet and / or outlet area and / or also in the middle of the channels. For example, it is possible to arrange particularly high washcoat amounts in a particularly effective region, such as, for example, an axial subsegment of the honeycomb body. This is advantageous if the axia le subsegment has particularly good conditions for a catalytic reaction. Due to the high Washcoatmengen a particularly strong fracture is generated in the axial sub-segment, so that large differences between the maximum and minimum values of the washcoat height. At the same time a particularly high doping of catalyst materials can be provided at this point, which additionally increases the performance of the honeycomb body.

Around one possible compact honeycomb body with a particularly large one surface In the context of the invention, it is furthermore advantageous to obtain if this is formed with a channel density of at least 600 cpsi is. The unit cpsi stands for "cells per square inch", a value of 600 cpsi corresponds to approximately 93 cells per square centimeter. Of the Conversion factor from cpsi to cells per square centimeter is 6.4516. With a channel density of 600 cpsi or more, extra large surfaces can be used in the honeycombs created and thus the honeycomb body themselves are kept very compact overall.

To another advantageous developments of the invention is provided, that the honeycomb volume 0.2 to 1.2 times a combustion chamber volume of an internal combustion engine equivalent. The combustion chamber volume is for example the displacement a gasoline engine or a diesel engine. With accordingly high channel density, the honeycomb body according to the invention thus can be particularly compact be trained and at the same time an excellent exhaust treatment or reduction of exhaust gas components cause. So compactly built honeycombs is easy to integrate in a vehicle and points beyond a relatively small mass.

According to one Another particularly preferred embodiment of the invention provided that the honeycomb body at least one support element with porous ceramic walls having. The porous ones ceramic walls offer a very big one inner surface for the Washcoat. Next to the fracture of the washcoat, can through the porous ceramic material support element with an additional created enlarged surface become. As a ceramic material z. As silicon carbide (SiC), α-alumina, γ-alumina or similar be used. For this purpose, it is advantageously provided that the carrier element at least partially made of an organic oxide. in this connection in particular, the already mentioned alumina or else Silicon carbide into consideration. These materials can be used with relatively little effort to honeycomb bodies be processed and provide the necessary thermal and mechanical strength, as a carrier element to serve.

The The present invention may be particularly advantageous in motor vehicles with an exhaust treatment device, such as personal or Trucks are used. For this it is already sufficient if at least one catalyst arrangement or a honeycomb body, such as described herein, trained and in the exhaust treatment device is arranged.

The Invention and the technical environment are described below Figures closer explained. It should be noted that the figures were particularly preferred variants of the invention to which, however, it is not limited. In The drawing shows schematically:

1 : An oblique view of a catalyst arrangement according to the invention;

2 a sectional view of a flow channel of a honeycomb body in an enlarged view;

3 a greatly enlarged sectional view through a honeycomb wall with washcoat; and

4 FIG. 3: a sectional view of a ceramic carrier element with washcoat coating. FIG.

In 1 is a catalyst arrangement 1 shown in an oblique view. The catalyst arrangement 1 includes two housing sections 2 , a honeycomb body 3 and a mat 4 , At the front 18 of the honeycomb body 3 are a variety of channels 5 recognizable, through which the exhaust gas to be treated flows. To assemble the catalyst assembly 1 becomes the mat 4 around the honeycomb body 3 wrapped around and then the two housing sections 2 put over it. The mat 4 serves to protect the ceramic honeycomb body 3 so that this tight inside the housing sections 2 sitting. In particular, the mat serves 4 to prevent loose shaking, with particularly large thermal expansions. As the thermal expansion behavior of housing section 2 and honeycomb body 3 usually different. While the housing sections 2 expand more when heated than the honeycomb body 3 , takes care of the mat 4 , which is made of a compressible and high temperature resistant material, that the honeycomb body 3 Even at high temperatures still play within the housing section 2 sitting. Both housing sections 2 together form a housing 6 , The honeycomb body 3 is also in the direction of its axis 19 in a first zone 16 and a second zone 17 divided, which differ only in the coating.

In 2 is a channel 5 of the ceramic honeycomb body 3 shown in a greatly enlarged view. The honeycomb walls 7 together form a carrier element 8th , On the surface of the carrier element 8th is the catalytic coating 9 applied, which as a washcoat 10 is trained. The washcoat 10 includes the catalyst materials provisionally shown here 11 which cause the conversion or aftertreatment of the exhaust gas. The illustrated cross-sectional shape of the channel 5 is rectangular. Through the applied washcoat 10 this basic form is essentially retained.

In 3 is now part of a honeycomb wall 7 shown in a greatly enlarged sectional view. Well recognizable here is the ceramic material 12 from which the honeycomb wall 7 is made. The ceramic material 12 is porous and traversed by a variety of inclusions and channels. The inclusions and channels in the present case are washcoats 10 attached. Well recognizable, z. B. at a first location 13 and a second place 14 , that is already in the honeycomb wall 7 existing fissures in attached washcoat 10 with the given height 15 find again and be partially reinforced. In this way, the available surface of the honeycomb body 3 be significantly increased.

In 4 is finally a ceramic carrier element 8th shown in a greatly enlarged view on which a washcoat 10 is applied. This washcoat is again with catalyst materials 11 doped. In this schematic representation is particularly clearly seen that the washcoat 10 has a relatively strong rugged surface. That is, the height difference between the lowest point of the coating indicated by the value H _Wmin and the peak point of the coating indicated by the value H _Wmax is relatively large. However, this has the advantage that the effective surface of the honeycomb body is thereby greatly increased. In addition, this fissured outer surface also promotes turbulence in the exhaust gas, allowing more exhaust gas in contact with the catalytic coating 9 can occur. To assess the technical properties, it is also helpful if the characteristic size given is an average layer height of the washcoat. This value is in 4 as H _Wm and represents the average height of the washcoat. Together with the values H _Wmin and H _Wmax , these values can serve as clues as to how heavily rugged the washcoat surface is and thus how effective the surface is overall. Especially with very large average layer heights with correspondingly large differences between the coating _heights , and here in particular between the values H _Wmax and H _Wmin , the surface increases sharply, so that an improved catalytic _effect due to the increased surface area, in particular at layer heights of 10 .mu.m and 30 μm is observed. In addition, the effective surface can be significantly increased depending on the application but also in layer thicknesses of 50 microns to 80 microns in their effectiveness.

The The present invention is not limited to the illustrated embodiments limited. Rather, numerous variations of the invention are within the scope of the claims possible. So can for example, instead of the described metallic and ceramic Materials and numerous other likewise metallic or ceramic materials Find use. Also the combinations of different catalyst materials with different washcoat straps can within the scope of the invention in various Be varied, so as to the particular application in the best possible Way to be adapted.

1

A catalytic converter arrangement

2

housing section

3

honeycombs

4

mat

5

channels

6

casing

7

Honeycomb wall

8th

support element

9

coating

10

washcoat

11

catalyst material

12

Ceramic material

13

first Job

14

second Job

15

height

16

first Zone

17

second Zone

18

front

19

axis

Claims (10)

Catalyst arrangement ( 1 ) with at least one honeycomb body ( 3 ) with a ceramic carrier element ( 9 ) and a selective catalytic coating ( 10 ) and a honeycomb body ( 3 ), enclosing housing section ( 2 ), wherein the housing section ( 2 ) comprises a ferritic steel material. Catalyst arrangement ( 1 ) according to the preceding claim, wherein a thermal expansion coefficient of the ferritic steel material is in the range between 10 · 10 ^-6 K ^-1 and 12 · 10 ^-6 K ^-1 . Catalyst arrangement ( 1 ) according to one of the preceding claims, wherein the steel material has a chromium content of 18.5 wt .-% to 19.5 wt .-%, a molybdenum content of the highest 0.1 wt .-% and a nickel content of highest 1 wt .-% has. Catalyst arrangement ( 1 ) according to one of the preceding claims, with at least one carrier element ( 9 ) and a selective catalytic coating ( 10 ), this at least partially the carrier element ( 9 ) and applied in an amount corresponding to a ratio of at least 200 g / l of a honeycomb body volume. Catalyst arrangement ( 1 ) according to any one of the preceding claims, wherein the catalytic coating ( 1 ) a height ( 15 ) of at least 10 microns. Catalyst arrangement ( 1 ) according to any one of the preceding claims, wherein the catalytic coating ( 10 ) a plurality of zones ( 16 . 17 ) Has different Washcoatmenge or doping. Catalyst arrangement ( 1 ) according to one of the preceding claims, wherein a honeycomb structure of the honeycomb body ( 3 ) is formed with a channel density of at least 600 cpsi. Catalyst arrangement ( 1 ) according to one of the preceding claims, wherein the honeycomb body volume corresponds to 0.2 to 1.2 times a combustion chamber volume of an internal combustion engine. Catalyst arrangement ( 1 ) according to one of the preceding claims, wherein the honeycomb body ( 3 ) at least one carrier element ( 9 ) having porous ceramic walls. Motor vehicle with an exhaust gas treatment device, the at least one catalyst arrangement ( 1 ) according to one of the preceding claims.