DE3337903A1 - Catalyst arrangement - Google Patents

Abstract

A catalyst arrangement for reducing and oxidising pollutants in hot oxidising exhaust gases has, with respect to the direction of flow of the exhaust gas, firstly an oxidising region (10) and subsequently a reducing region (11). The region favouring oxidation is preferably formed as a catalyst filter (10). As a result it is possible to filter out, for example, soot from an exhaust gas and to burn it to carbon dioxide. As a result of this consumption of oxygen in the oxidising exhaust gas, only a more weakly oxidising exhaust gas passes into the reducing catalyst region (11), which can operate particularly effectively as a result. The catalyst arrangement is particularly suitable for converting pollutants in the exhaust gases of diesel engines. <IMAGE>

Description

DESCRIPTION

The invention relates to a catalyst arrangement according to the preamble of the main claim.

Such an arrangement is known from DE-AS 26 49 825.

An oxidizing exhaust gas from an internal combustion engine becomes the assembly fed, which is divided into two parts, the first area reductions and the second Promotes oxidations. Nitrogen oxides (NO) are reduced and there is oxidation, for example Carbon monoxide CO. The first, reducing area is with rhodium and iridium coated as a catalyst material. These precious metals are capable of the reduction of nitrogen oxides to bring about NOx even in an oxidizing atmosphere. The second area is coated with platinum and / or palladium as a catalyst material. These elements favor the oxidation of carbon monoxide CO and hydrocarbons HC.

The known catalyst arrangement is very expensive because it is exclusively Precious metals used as catalyst materials. It is desirable cheaper To use catalyst materials, but which are often somewhat opposed to their activity lag behind the precious metals.

The invention is based on the object of a catalytic converter arrangement of the type mentioned at the beginning, without impairing their degree of strength also gets by with less active and cheaper catalyst materials, or has a higher efficiency when using the same materials.

The solution according to the invention is characterized in the main claim. Beneficial Refinements and developments are the subject of subclaims.

The catalyst arrangement according to the invention is characterized in that that the reducing area in the flow direction of the exhaust gas is no longer in front, but is arranged behind the oxidizing area. Knowledge lies in this arrangement based on the fact that in arrangements according to the prior art, as they are except from the mentioned DE-AS 26 49 825 also in a similar manner from DE-OS 21 58 877 and of DE-AS 21 59 774 are known, the reducing area supplied more oxygen gets as the oxidizing area.

But then particularly good catalysts have to be used nevertheless in an oxidizing atmosphere the reduction and in its oxidizing power to be able to promote oxidation in a weakened atmosphere. When the invention Structure, on the other hand, represents the maximum amount of oxygen from the exhaust gas for oxidation available, while in the reducing range the oxygen content through the previously oxidation of certain pollutants in the hot exhaust gas has already been reduced.

The arrangement according to the invention works particularly effectively in FIG Exhaust gases that contain soot components when the soot component is in the oxidizing state Area is filtered out and burned there. One such catalyst filter is known from DE-OS 29 51 316. A metal is used as the catalyst material or a metal oxide applied to a metallic screen mesh.

However, it has been found to be even more advantageous to use the catalyst material not on a metal screen, but on a non-woven fabric, or the catalyst to be introduced into the fiber fleece in powder form, either during manufacture the same or by subsequent immersion in a dispersion of the catalyst powder.

The burning temperature of filtered out soot becomes very special then lowered when used as a catalyst material to promote burning off Iron oxide, fine Fe2 04 is used.

For exhaust gases from gas generators in different load ranges emitting oxidizing and reducing exhaust gases on the other hand, this is an advantage to connect a catalyst arrangement according to the invention in parallel with a known one, in which a reducing catalyst area precedes an oxidizing area. Switching between the two parallel sub-assemblies is carried out by a lambda probe is made in the exhaust gas flow, which determines whether the exhaust gas is oxidizing or is reducing.

Exhaust gas generators, which mainly deliver oxidizing exhaust gases, are for Take diesel engines as an example. Therefore, the use of a catalyst arrangement according to the invention particularly advantageous in these engines, especially if the oxidizing area is designed as a catalyst filter.

The invention is explained in more detail below with reference to figures. 1 shows a flow diagram of the flow of exhaust gas through various catalyst regions; Figure 2 is a flow diagram of the flow of exhaust gas through various catalyst areas two switchable Kata.lysator part arrangements; and FIG. 5 is a schematic Cross section through a catalyst arrangement arranged in a housing According to 1, an oxidizing exhaust gas, for example from a diesel engine, flows into one Catalyst filter 10. The exhaust gas contains, for example, nitrogen oxides, NOx, hydrocarbons HC, carbon monoxide CO and soot C. The catalyst filter 10 has an oxidizing effect that is, the oxidizing area of a catalytic converter arrangement. Accordingly, by the catalyst hydrocarbons by reacting with oxygen in water and carbon dioxide and carbon monoxide converted into carbon dioxide. Soot C sticks to the filter and becomes also burned in carbon monoxide.

The exhaust gas leaving the catalyst filter 10 does not contain any oxidizable gases Parts, especially no more soot.

By converting the oxidizable gas components and by the When the soot is burned, the exhaust gas leaving the catalyst filter 10 has a weaker effect oxidizing than the exhaust gas that has been introduced into it. This weaker oxidizing Exhaust gas is fed to a reducing catalyst area 11, in which, in the still oxidizing atmosphere, nitrogen oxide NOx in nitrogen and oxygen be split up. Catalyst materials that allow such a reduction in oxidizing Rhodium or iridium or Raney cobalt, for example, make an atmosphere possible.

Through the effect of the catalyst arrangement with the oxidizing effect Catalyst filter 10 and the reducing catalyst area 11 are the finally listed pollutants in water, carbon dioxide, nitrogen and oxygen converted.

It is also possible to use the arrangement according to FIG. 1 as a partial arrangement to be used in conjunction with a conventional arrangement as shown in FIG is. This is recommended when the exhaust gas depends on different Combustion conditions can have an oxidizing and a reducing effect once, as is the case with diesel engines in particular and occurs in house combustion exhaust gases. For this purpose is in the exhaust pipe 12 from A lambda probe 13 is connected to the exhaust gas generator. Determine such lambda sensors whether gas atmospheres of exhaust gases have an excess of oxygen in relation to oxidizable Contain or not contain exhaust gas components, d. H.

whether the exhaust gas is oxidizing or reducing. Becomes an oxidizing Exhaust gas detected, a switch to an upper subassembly takes place just like the arrangement in Fig. 1 is constructed and a first catalyst filter 10.1 and a first reducing catalyst area 11.1 a.

If, on the other hand, the lambda probe detects a reducing exhaust gas, so it switches over to the second sub-arrangement shown at the bottom in FIG. 2, which is constructed in a similar way to conventional catalytic converter arrangements. However, it is Please note that for the oxidizing catalyst range in the lower Subassembly uses a second catalyst filter 10.2, what is different from conventional ones Arrangements are not known where non-filtering oxidizing catalyst areas be used.

The lower sub-assembly according to FIG. 2 has before the already mentioned second catalyst filter 10.2 a second reducing catalyst area 11.2 on. After switching through the lambda probe, this receives the reducing exhaust gases supplied from the exhaust pipe 12. A conversion of nitrogen oxides then takes place NOx in nitrogen and oxygen.

The exhaust gas from the reducing catalyst area 11.2 is mixed with oxygen, whereby an oxidizing exhaust gas is formed, which is still Hydrocarbons, carbon monoxide and soot C. The supplied oxygen stirs from the supplied combustion air, which is also supplied by the lambda probe 15 is controlled.

In the second catalyst filter 10.2, the soot is filtered out and at low temperature burned to carbon dioxide C02.

In this oxidizing catalyst area, the Hydrocarbons and the carbon monoxide are converted into water and carbon dioxide.

Switching through the lambda probe 15 enables pollutants in oxidizing exhaust gases always with optimal cm efficiency in water, carbon dioxide, To convert nitrogen and oxygen.

In the flow charts according to FIGS. 1 and 2, the oxidizing effects Catalytic converter areas designed as a catalytic converter filter 10. These filters find how Already mentioned above, it is used when the exhaust gas contains soot, for example from a diesel engine or burnout.

In this case, the registration-compliant arrangement has a particularly strong effect, because then there is a particularly strong reduction in the oxidizing fraction in the exhaust gas, so that the downstream reducing catalyst area work particularly effectively can. For exhaust gases that do not contain soot, such as B. the exhaust gases from gasoline engines, but it is also possible to use the oxidizing catalyst area in a conventional manner and to train in a manner that is not filtering.

The cross section according to FIG. 5 shows one arranged in a housing 14 Catalyst arrangement 15. This has an inner sheet metal screen cylinder 16.1 and a concentrically with this arranged central sheet metal screen cylinder 16.2.

These are designed in the same way as conventional sheet metal screen cylinders of air filters.

The two sheet metal screen cylinders 16.1 and 16.2 serve as a support for a Nonwoven fabric 17 made of inorganic oxide fibers. In the oxide fibers is used as a catalyst material Raney powder embedded. This material is particularly effective for reduction of nitrogen oxides NOx even in an oxidizing atmosphere.

To the outside, the middle sheet metal screen cylinder 16.2 is closed Star filter 18 coated with an oxidizing catalyst material is. The star filter 18 acts as a catalyst filter 10 according to FIGS. 1 and 2. There is the star filter 18 made of a metal mesh, so it is self-supporting and does not require any further hold.

If, on the other hand, the star filter is made of a fiber fleece, then an outer sheet metal screen cylinder 16.3, which is shown in dashed lines in FIG. 3, is arranged around it is shown.

Oxidizing exhaust gas, the nitrogen oxides NOx, hydrocarbons HC, carbon monoxide Contains CO and carbon black C, is through a supply port 19 to the interior of the housing 14 and thus supplied to the catalytic converter arrangement 15. It flows through the outer sheet metal cylinder 16.) and the star filter 18, from which, however, the soot particles contained in the exhaust gas 20 be withheld. The soot particles 20 are shown in FIG. 3 as individual grains shown. In fact, however, they form agglomerates of different sizes. This soot particles 20 or a soot layer formed from them is at relatively low Exhaust gas temperatures of about 300 ° C burned off at the star filter 18. This is oxygen consumed, so that only less oxidizing exhaust gas through the middle sheet metal screen cylinder 16.2 flows to the reducing fiber fleece 17. There, nitrogen oxides become NOx converted to nitrogen. Flows into the interior of the inner sheet metal screen cylinder 16.1 then exhaust gas, which is not a pollutant, but only the conversion products Contains water, carbon dioxide, nitrogen and oxygen. This exhaust gas is through an opening, not shown, led away.

The structure according to FIG. 5 can undergo numerous modifications. In particular it should be noted that the feed opening 19 in the housing 14 is shown rather schematically is. Such a feed is selected that results in optimal flow conditions. This is particularly important when the catalytic converter arrangement 15 is used in an internal combustion engine is used. Depending on the exhaust gas generator whose exhaust gases have to be converted, there will be more reducing or more oxidizing pollutants. Depending on it are the structure, especially the dimensions in the direction of flow of the oxidizing or the reducing catalyst range to be selected.

The star filter 18 may, for example, be made of metals or metal oxides metallic meshes coated as catalyst materials exist, like it in DE-OS 29 51 316 is described. A greater depth of the star filter 18 in The direction of flow of the exhaust gases arises when instead of a metal screen a nonwoven fabric is used, in the oxidizing catalytically active components are embedded. When using such a fiber fleece, the mesh size is larger than the mean diameter of soot particles, but still due to of the many consecutive fibers are retained. The soot particles show before their agglomeration a diameter of about 25-250 nm.

As the fiber fleece, for example, such as it was called "Cerablanket" or "Cerachrome" by the company. Ma.nville, France is available.

The catalytically active material can be incorporated into such a fiber fleece either from the manufacture of the fleece or by subsequent immersion in a dispersion of the catalyst material should be introduced. Such procedures and other usable fiber materials are given in DE-OS 29 36 927.

The burn-off temperature of soot is usually around.

800 C. If the soot is filtered out with a fiber fleece, the temperature drops Burning temperature to about 560 - 580 OC because such fiber webs, especially if they contain Al205, are already catalytically active. The burning temperature can be further increased if the catalytically active material is iron oxide, Fe2IIFeIIIO4, is used.

This is generally true regardless of whether this material is composed is used with a reducing area or just by itself. With iron oxide as a catalyst, either on a metal screen or in a non-woven fabric, can reduce the burning temperature to about 300 ° C.

For the fiber fleece 17, which is the catalyst material with a reducing effect contains, the same applies to what has just been said for a fiber fleece for absorption acting catalyst materials was specified. As nitrogen oxide reducing catalyst material Powdered Raney cobalt is particularly suitable. But this can take place in a Fiber fleece can also be embedded on a metallic carrier. This must then be arranged in such a way that there is a particularly large effective surface for flow through Exhaust gas results.

The registration according to the arrangement works particularly effectively because the Oxidation of pollutants is made when the exhaust gas is still particularly has a lot of oxidizing agent and the reduction is only carried out if a part of the oxidizing agent has already been used up in the area with an oxidizing effect. Will If the same good catalysts are used as before, this results in an improvement the effect. If cheap, not so effective catalysts are used, so results the same efficiency as before. The increased effectiveness of the arrangement it also enables the Build arrangement more compact, so in particular a catalyst filter and the summarize reducing area in a catalytic converter component.

Claims (10)

Catalyst arrangement CLAIM 1. Catalyst arrangement (15) with a Catalytic converter area (11) favoring reductions and an oxidizing area Catalyst area (10) for converting pollutants in hot oxidizing exhaust gases in non-harmful substances, it is not indicated that the reducing area (11) in the flow direction of the exhaust gas behind the oxidizing Area (10) is arranged. 2. Arrangement according to claim 1, d a d u r c h g e k e n nz e i c h n e t that the oxidizing area is a catalyst filter (10) for filtering out and Separating carbon black (20) is. 3. Arrangement according to claim 2, d a d u r c h g e -k e n n z e i c h n e t that the catalyst filter (10) and the reducing catalyst area (11) are combined in a catalyst component. 4. Arrangement according to claim 2 or), d a d u r c h g e -k e n n z e i c h n e t that the catalyst filter (10) is on a metallic screen mesh Has applied catalyst material. 5. Arrangement according to claim 2 or 3, d a d u r c h g e -k e n n z e i c h n e t that the catalyst filter (10) is arranged in a nonwoven fabric Has catalyst material. 6. Arrangement according to one of claims 2 to 5, d a d u r c h g e k It is noted that the catalyst material of the catalyst filter (10) Is iron oxide. 7. Arrangement according to one of claims 1 to 6, d a -d u r c h g e It is clear that the arrangement according to claim 1 is a first sub-arrangement represents, to which a second sub-assembly is connected in parallel in terms of exhaust gas, in which a second reducing catalyst area (11.2) before a second oxidizing one Catalyst area (10.2) is (Fig. 2). 8. The arrangement according to claim 7, d a d u r c h g e -k e n n z e i c h n e t that the second oxidizing area is also designed as a catalyst filter (10.2) is. 9. Arrangement according to claim 7 or 8, d a d u r c h g e k e n n z e i c h n e t that a lambda probe in the exhaust gas flow in front of the two sub-assemblies (13) indicated is that arranges the exhaust gas in the first subassembly conducts when it is oxidizing and conducts into the second subassembly when it is has a reducing effect. 10. Arrangement according to one of claims 1 to 9, g e -k e n n z e i c h n e t d u r c h the use in the exhaust gas flow of a diesel engine.