WO2006061330A2

WO2006061330A2 - Catalytic converter for oxidizing carbon-containing particles, and device for purifying gas mixtures, which comprises the same

Info

Publication number: WO2006061330A2
Application number: PCT/EP2005/056227
Authority: WO
Inventors: Jörg Jockel; Wilhelm Maier
Original assignee: Robert Bosch Gmbh
Priority date: 2004-12-07
Filing date: 2005-11-25
Publication date: 2006-06-15
Also published as: DE102004058780A1; WO2006061330A3

Abstract

Disclosed is a catalytic converter for oxidizing carbon-containing particles, especially soot particles. Said catalytic converter contains an alloy or mixed compound of cobalt and at least one other metal or semimetal, the cobalt concentration in the alloy or mixed compound being greater than 70 mole percent relative to the total metal and semimetal concentration in the alloy or mixed compound.

Description

Catalyst for the oxidation of carbonaceous particles and apparatus for the purification of

Gas mixtures containing this

The invention relates to a catalyst for the oxidation of carbonaceous particles, in particular of carbon black, and a device containing these and their use according to the preamble of the independent claims.

State of the art

The purification of exhaust gases containing carbonaceous particles is becoming increasingly important. For cleaning such gas mixtures are usually ceramic

Filter systems used. The challenge of optimizing such systems is primarily not in the filtration itself - many particulate filters allow a separation of more than 99 percent - but in the permanent and efficient use of the filter without clogging and without an associated excessive increase in flow resistance throughout the filter system.

Newer filter systems have instead of a porous ceramic body on a sintered metal-based filter element. These have the advantage that they show a much more homogeneous filtration behavior than conventional filter systems and can be used largely maintenance-free. Nevertheless, it can be used especially in long-term operation

Clogging of the pores come.

The carbon deposited as carbon black must therefore preferably be removed oxidatively at regular intervals. The direct oxidation of soot by oxygen takes place to a relevant extent only at temperatures above 600 ⁰ C. The temperature of exhaust gases of a Diesel engines, however, are usually only 150 to 350 ⁰ C. The exhaust gas temperature must therefore be increased by motor or other types of regeneration measures. This results in increased fuel consumption, particularly in the case of engine measures, and can impair the service life of the internal combustion engine. In addition, the corresponding filter systems are also damaged by the high temperatures. There is therefore the need to design filter systems so that the number of regeneration processes can be kept as low as possible.

From US 6,803,015 a ceramic filter assembly for purifying combustion exhaust gases is known in which the cleaning effect of conventional ceramic

Filter is improved in that in or on the surface of a catalyst is applied, which catalyzes the burning or the oxidation of the deposited soot. The catalyst described may contain a variety of metallic components. The achieved in this way lowering the burning temperature of the deposited soot is not sufficient.

Object of the present invention is to provide a catalyst for the oxidation of carbon black, which ensures the oxidation of carbonaceous particles at the lowest possible temperatures.

Advantages of the invention

The problem underlying the invention is achieved in an advantageous manner by the catalyst according to the invention with the characterizing features of claim 1.

This cobalt-containing catalyst lowers the temperature at which carbonaceous particles are oxidized to a significant extent or burn off so much that when using the same in an exhaust aftertreatment device, such as a soot filter, its regeneration in a gentle manner and avoiding the use of fuel additives can be done. For this purpose, for example, the exhaust aftertreatment device is at least partially covered with the erfϊndungsgemäßen catalyst on its side facing the exhaust gas to be treated. The catalyst is designed as an alloy or mixed compound of cobalt with at least one other metal or semimetal. The measures listed in the dependent claims advantageous developments of the catalyst specified in claim 1 are possible.

It is thus advantageous if the content of cobalt in the alloy or mixed compound, based on the total content of metals and semimetals in the alloy or mixed compound, is greater than 85 mol%. A high content of cobalt in the catalyst leads to a particularly pronounced reduction in the oxidation temperature of carbonaceous particles. In this case, an oxide, carbide or nitride of the cobalt and at least one further metal or semimetal is provided as mixed compound.

In a particularly advantageous embodiment, an alkali metal is used as a further metal in the alloy or mixed compound. In this way, particularly long-term stable catalysts are obtained. In this case, a content of the alkali metal in the alloy or mixed compound based on the total content of metals and semimetals in the

Alloy or mixed compound between 1 and 5 mol% particularly advantageous.

In a further particularly advantageous embodiment, manganese, titanium, molybdenum or vanadium is chosen as the further metal in the alloy or mixed compound. In this way, highly active catalysts are obtained. This is a content of manganese, titanium,

Molybdenum or vanadium in the alloy or mixed compound based on the total content of metals and semi-metals in the alloy or mixed compound between 2 and 9 mol% is particularly advantageous.

The catalytic effect can be further increased if the catalyst additionally contains platinum as another metal. In this case, a platinum content of 0.1 to 3% by volume, based on the total content of metals and semimetals in the alloy or mixed compound, is of particular advantage.

The catalyst can be advantageously integrated into a system for purifying gas mixtures, which in particular contain soot particles, for example in diesel particle filters. - A -

drawing

An embodiment of the device according to the invention is shown in the drawing and explained in more detail in the following description. FIG. 1 shows schematically a device in the form of a filter provided with a surface coating according to an exemplary embodiment of the present invention.

embodiment

The catalyst according to the invention is designed as an alloy or mixed compound of cobalt with at least one further metal or semimetal, wherein the catalyst has a relatively high content of cobalt. The content of cobalt in the alloy or mixed compound is more than 70 mol%, particularly more than 85 mol%, based on the total content of metals and semi-metals in the alloy or compound. Of the

Catalyst can be a binary, ternary, quaternary or higher cobalt-containing substance. If the catalyst is designed as a mixed compound, it may, for example, contain an oxide, carbide or nitride of cobalt and at least one further metal or semimetal.

Another metal which may be present in the catalyst is an alkali metal, in particular cesium, rubidium or potassium. Its content in the alloy or mixed compound, based on the total content of metals and semimetals in the alloy or mixed compound, is preferably between 1 and 5 mol%, in particular between 2.5 and 4.0 mol%.

Such a catalyst containing cobalt and an alkali metal may also be known as ternary

Compound and then preferably contains one of the platinum metals rhodium, iridium, palladium, platinum, silver or gold, wherein the use of platinum is preferred. In this case, a platinum content in the catalyst material of 0.1 to 3 mol%, in particular from 0.5 to 1.5 mol% based on the total content of metals and semi-metals in the alloy or mixed compound is preferred. This results in compounds of the type Pt ₍ o _{) 1-3)} M _{(1) 5-5)} CO _{(92-98) 4)} , in particular Pt ₍ o, _5-1 , ₅₎ M ₍₂ , ₅ - ₄₎ CO _{(94) 5-97)} , where M is an alkali metal and the anions which are optionally added in mixed compounds, such as oxide, nitride or carbide ions, are not taken into consideration. Alternatively, as another metal in the catalyst, one of manganese, molybdenum, titanium or vanadium may be provided instead of an alkali metal, with the use of manganese being preferred. Its content in the alloy or compound compound based on the total content of metals and semimetals in the alloy or mixed compound is preferably between 2 and 9 mol%, in particular 4 to 8 mol%. Such a,

Cobalt and one of the elements containing manganese, molybdenum, titanium or vanadium, however, is preferably carried out as a ternary compound and then additionally contains one of the platinum metals rhodium, iridium, palladium, platinum, silver or gold, wherein the use of platinum is preferred. In this case, a platinum content in the catalyst material of 0.1 to 3 mol%, in particular from 0.5 to 1.5 mol% based on the total content of metals and semi-metals in the alloy or mixed compound is preferred. This results in compounds of the type Pt ₍ o _{) 1-3)} M ² _(2-7) CO _{(90-97) 9)} , Pt ₍ o, _{5-1) 5)} M ² _(4-8) CO _{(91) 5-95) 5)} , wherein M ^{2 is} one of the elements manganese, molybdenum, titanium or vanadium and wherein the optionally added in mixed compounds anions, the oxide, nitride or carbide ions are still not considered.

A catalyst containing, in addition to cobalt, one of the elements manganese, molybdenum, titanium or vanadium and one of the Pt metals may also be in the form of a quaternary compound and contain as the fourth component one of the elements aluminum, silicon or germanium, the use of aluminum being preferred is. Here is a salary of the fourth

Component in the catalyst material of 0.1 to 3 mol%, in particular from 0.5 to 1.5 mol% based on the total content of metals and semi-metals in the alloy or mixed compound is preferred. Thereby resulting compounds of the type Pt _(o, _1-3) M ³ _{_(o) 1-3)} M ² ₍₂ - ₇₎ Cθ _(85- 9 _{5) 8),} especially Pt (o, _5-1, ₅₎ M ³ (o _{) 5-1)} 5) M ² ( _4-8 ) CO (9 ₁ ) 5-9 _{5) 5} ), wherein M ^{3 is} one of aluminum, germanium or silicon and wherein the compounds optionally added in mixed compounds Anions the oxide, nitride or carbide ions are also not taken into account.

The preparation of the catalyst is preferably carried out by means of a sol-gel process, since, via sol-gel processes, metals are homogeneously distributed, for example as complexes, in oxide

Materials can be introduced. Furthermore, doping of the material by further catalytically active component in almost any amount can take place and high-quality ceramic optionally metal-oxide coatings can be produced, with which, for example, ceramic fibers can be coated. For example, soluble organometallic compounds such as, for example, alkoxides or alcoholates, often propionates, which form a gel by means of a condensation step with dehydration are used.

If an alcoholic solution of hydrolyzable alcoholates of polyvalent metal ions (eg Ti,

Co, Mn, Mo ...) applied to a surface to be coated, so formed in Ggw. Of moisture already during the evaporation of the solvent at low temperatures, for example at room temperature, a metal hydroxide network. This contains numerous metal hydroxide groups and is therefore hydrophilic and antistatic. When the temperature is raised, the metal hydroxide groups react with elimination of water to give metal oxide

Groupings, whereby the surfaces become mechanically very stable.

Alternatively, by thermolytic decomposition of metal complexes or by treatment in oxygen plasma, fumed metal oxide or metal particles can be produced, which i.a. can be used as heterogeneous catalysts. They are characterized by very small, homogeneously distributed and non-agglomerated particles, narrow particle size distributions and a very variable degree of loading.

A further alternative representation can be carried out starting from metal salt solutions of nitrates, acetates, citrates or carbonates of the corresponding metals. The solutions are dried and then calcined at temperatures> 600 ⁰ C, the corresponding anions are thermally decomposed and formed the corresponding metal oxides. The metal oxides formed can then be applied by printing processes to surfaces to be coated as a constituent of corresponding printing pastes.

The described embodiments of the catalyst show a significant reduction in the burnup temperature of carbonaceous particles such as carbon black. In the following table the results of a differential thermal analysis (DTA) are listed, were mixed in the embodiments of the catalyst with carbon black and heated in synthetic air to 650 ⁰ C. In this case, that temperature was determined at which a loss of mass of

50% occurred. For comparison, this temperature of 50 percent mass loss is also listed for soot.

It can be seen that the listed embodiments of the catalyst lead to a significant reduction of the burnup temperature of soot. The combustion temperature is understood to be the temperature at which soot in air or oxygen-containing gas mixtures oxidatively converts to combustion products such as carbon monoxide or carbon dioxide to an appreciable extent.

The catalyst described is suitable for use in systems which are exposed to carbonaceous particles. These may be, for example, particle filters for cleaning combustion exhaust gases of motor vehicles, in particular with diesel engines, turbines or combustion processes in power plants.

The basic structure of an apparatus for purifying particle-containing combustion exhaust gases will be described below. The device for cleaning gas mixtures is preferably designed as a filter, as shown schematically in Figure 1. The filter is integrated into a system in which a gas mixture charged with preferably soot-containing particles is conducted. This can be, for example, the exhaust pipe of a diesel engine. Alternatively, it is possible to arrange the filter in a bypass of the exhaust gas system.

The filter 10 depicted in FIG. 1 is preferably designed as a stainless steel or sintered metal filter and has a first side 11 facing the gas mixture to be cleaned and a second side 12 facing the cleaned gas mixture. The laden with particles, in particular with soot gas mixture 13 is supplied to the filter 10 on the first side 11. The filter 10 comprises a housing 16, in which the actual filter structure is integrated. The filter structure comprises pockets 15, which are open at their end facing the first side 11 for access of the particle-laden gas mixture and are closed at their end facing the second side 12. The pockets 15 are on their long sides preferably limited by walls 18, which are made porous so that they allow the passage of the gas mixture while retaining the particles contained in the gas mixture.

The gas mixture penetrating the walls 18 passes into second pockets 20, which are closed at their end facing the first side 11 and open at their end facing the second side 12, so that the gas mixture freed from particles can escape. The housing 16 and the walls 18 are made of a metallic material such as a sintered metal or stainless steel. Furthermore, it is possible to make the housing 16 and the walls 18 of different materials.

To increase the surface active surface of the walls 18, these may be at least partially, preferably provided over the entire surface with a surface coating 22 of ceramic fibers. The ceramic fibers consist, for example, of an aluminum oxide or of an aluminum silicate, if appropriate with the addition of zirconium dioxide. The

Fibers have an average diameter of 3 to 10 .mu.m, in particular of 5 .mu.m, and an average length of 150 to 400 .mu.m, preferably 250 .mu.m. Such fibers are available, for example, from Saffil Ltd, Cheshire, WA8 ORY, UK.

The application of the fibers to the filter base material of the walls 18 to form the

Surface coating 22 takes place in such a way that the pore structure of the porous walls 18 is not bonded and the resulting fiber composite is distributed homogeneously on the walls 18. Furthermore, the individual fibers of the surface coating 22 are bonded to one another in such a way that no fibers can be released from the fiber composite even at high flow velocities of the gas mixture 13 to be cleaned. As sticky

Component are aluminum or aluminosilicates, which are initially present as liquid brine or colloidal solutions. These initially largely soluble or dispersed compounds form by a condensation step with elimination of water corresponding gels. An advantage of this sol-gel process is that ceramic coatings can be easily produced.

This effect can be enhanced if the surface coating 22 in addition to the ceramic fibers additionally contains spherical particles. These serve as spacers for the ceramic fibers and allow the targeted adjustment of the porosity or the permeability of the layer 22. At the same time, the addition of spherical particles is the mechanical stabilization of the surface coating 22. The spherical particles are preferably made of the same material as the ceramic fibers 26. Alternatively, it is possible to carry out the spherical particles of alumina, zirconia, titania or mixed oxides of transition metals. The spherical particles preferably have a diameter of 5 to 50 microns.

To improve the regeneration capacity of the filter 10, a catalyst of the type according to the invention is preferably applied to the spherical particles or the ceramic fibers or to the wall 18. The fibers and the spherical particles may contain the same or different catalytically active substances. The application of the catalytically active substances to the fibers or spherical particles is preferably carried out before they are introduced to produce the layer 22 in a suspension. This allows the application of different catalytically active materials to the fibers 26 and the spherical particles 28. The application can be carried out, for example, by impregnation.

Claims

claims

1. A catalyst for the oxidation of carbonaceous particles, in particular of carbon black, which contains an alloy or mixed compound of cobalt with at least one further metal or semimetal, characterized in that the content of cobalt in the alloy or

Mixed compound based on the total content of metals and semi-metals in the alloy or mixed compound is greater than 70 mol%.

2. Catalyst according to claim 1, characterized in that the content of cobalt in the alloy or mixed compound based on the total content of metals and semi-metals in the alloy or mixed compound is greater than 85 mol%.

3. Catalyst according to claim 1 or 2, characterized in that the mixed compound is an oxide, carbide or nitride of the cobalt and at least one further metal or semimetal.

4. Catalyst according to one of claims 1 to 3, characterized in that the further metal is an alkali metal.

5. Catalyst according to claim 4, characterized in that the content of the alkali metal in the alloy or mixed compound based on the total content of metals and semi-metals in the alloy or mixed compound is between 1 and 5 mol%.

6. Catalyst according to one of claims 1 to 3, characterized in that the further metal is manganese, titanium, molybdenum or vanadium.

7. Catalyst according to claim 6, characterized in that the content of the further metal in the alloy or mixed compound based on the total content of metals and semi-metals in the alloy or mixed compound is between 2 and 9 mol%.

8. Catalyst according to one of the preceding claims, characterized in that is contained as a further component of the alloy or mixed compound platinum.

9. A catalyst according to claim 8, characterized in that the content of platinum in the alloy or mixed compound based on the total content of metals and semi-metals in the alloy or mixed compound is between 0.1 and 3 mol%.

10. A device for cleaning gas mixtures containing particles, in particular of soot-containing exhaust gases of internal combustion engines, wherein the device is designed as a filter (10) having a the gas mixture to be cleaned exposed porous surface (18, 22) of a filter base material, characterized in that a catalyst according to any one of claims 1 to 9 is applied to the surface exposed to the gas mixture to be cleaned (18, 22).

11. The device according to claim 10, characterized in that the filter base material a

Sintered metal is.

12. Use of a device according to claim 10 or 11 as a diesel particulate filter.