PROCESS FOR PRODUCING A CATALYST AND CATALYST ARTICLE

Abstract

A process for producing a ceramic catalyst involves the steps of: a) providing functional particles having a catalytically inactive pore former as a support surrounded by a layer of a catalytically active material, b) processing the functional particles with inorganic particles to form a catalytic composition, c) treating the catalytic composition thermally to form a ceramic catalyst, wherein the ceramic catalyst comprises at least porous catalytically inactive cells which are formed by the pore formers in the functional particles, which are embedded in a matrix comprising the inorganic particles, which form a porous structure and which are at least partly surrounded by an active interface layer comprising the catalytically active material of the layer of the functional particles.

An SCR catalyst produced in by this method has an improved NO.sub.x conversion rate compared to a conventionally produced SCR catalyst.

Claims

1. A process for producing a ceramic catalyst, the process comprising: a) providing functional particles having a catalytically inactive pore former as a support surrounded by a layer of a catalytically active material, b) processing the functional particles with inorganic particles to form a catalytic composition, c) treating the catalytic composition thermally to form a ceramic catalyst, wherein the ceramic catalyst comprises at least porous catalytically inactive cells which are formed by the pore formers in the functional particles, which are embedded in a matrix comprising the inorganic particles, which form a porous structure and which are at least partly surrounded by an active interface layer comprising the catalytically active material of the layer of the functional particles.

2. The process according to claim 1, wherein the pore formers consist of an organic material comprising thermoplastics or elastomers, and which escapes in the course of the thermal treatment of the catalytic composition, such that the cells are formed as pores.

3. The process according to claim 1, wherein the pore formers consist of an inorganic porous material having at least mesoporosity.

4. The process according to claim 3, wherein the inorganic material is selected from a clay mineral, diatomaceous earth, silica gel and porous glass.

5. The process according to claim 1, wherein powder particles of the catalytically active material have been applied to the pore former.

6. (canceled)

7. The process according to claim 1, in which the layer is formed by a conversion of an outer layer of the catalytically inactive pore formers to the catalytically active layer.

8. The process according to claim 1 wherein the pore formers have a particle size in the range from 10 μm to 200 μm.

9. The process according to claim 1, wherein the inorganic particles comprise a binder component wherein, after the treatment in step c), the binder component forms a ceramic matrix.

10. The process according to claim 1, further comprising forming a shaped body from the catalytic composition by extrusion and then thermally treating the shaped body.

11. (canceled)

12. The process according to claim 1, wherein the catalytic composition is applied as a washcoat to a monolithic support and the coated support is then treated thermally.

13. The process according to claim 1, wherein the application of the layer to the pore former for formation of the functional particles and the processing of the catalytic composition are conducted in a single process step.

14. The process according to claim 1, wherein the pore formers account for a proportion in the range from 40% by volume to 80% by volume, based on the dry catalytic composition.

15. The process according to claim 1, wherein catalytically active materials used are any of or a combination of: crystalline molecular sieves, catalytic systems based on base metals, especially titanium-based systems tungsten/cerium-based catalytic systems metal oxides or mixed oxides catalytic systems based on base metals PGM-based systems.

16. The process according to claim 15, wherein the crystalline molecular sieves are present as individual crystallites.

17. The process according to claim 15, wherein the crystalline molecular sieves have at least mesoporosity as defined by the IUPAC definition in addition to their inherent microporosity.

18. The process according to claim 1, wherein further particles introduced into the catalytic composition in step b) are a catalytically active material.

19. The process according to claim 1, wherein pore formers of differing particle size distribution are used.

20. The process according to claim 1, in which the catalytic composition comprises two or more different functional particles which differ in terms of the catalytically active material used for the layer, wherein: a mixture of two or more different catalytically active materials is used for the layer, especially a mixture of different powder particles; and/or different catalytically active material is applied to each of the individual pore formers, especially by applying first powder particles to first pore formers and second powder particles to second pore formers, so as to obtain first and second functional particles which are mixed to form the catalytic composition.

21. A catalyst article comprising a catalyst having a composite structure having a porous structure embedded into an inorganic matrix, wherein the porous structure is formed from catalytically inactive, at least porous cells at least partly surrounded by an active interface layer comprising a catalytically active material, wherein said catalyst article is any of: an extruded catalyst article consisting of the catalyst, a honeycomb consisting of the catalyst, a plate catalytic converter consisting of the catalyst, pellets consisting of the catalyst, a monolithic support body coated with a washcoat consisting of the catalyst, a wall-flow filter comprising a honeycomb, wherein the honeycomb consists of the catalyst and/or the honeycomb has been coated with a washcoat composed of the catalyst.

22-25. (canceled)

26. The catalyst article according to claim 19, wherein the ceramic matrix is at least substantially free of catalytically active material.

27. (canceled)

28. The catalyst article according to claim 19, having two or more different catalytically active materials, wherein the catalytically active material in an active interface layer that bounds a first cell is different from the catalytically active material in an active interface layer that bounds a second cell and/or an active interface layer that bounds a particular cell contains two or more different catalytically active materials.

29-34. (canceled)

35. The catalyst article according to claim 19, in which the catalytically active material is selected from crystalline molecular sieves, catalytic systems based on base metals tungsten/cerium-based catalytic systems metal oxides or mixed oxides catalytic systems based on base metals PGM-based systems.

36-38. (canceled)

39. Functional particles which comprise a pore former which takes the form of a support surrounded by a layer of a catalytically active material.

Description

[0103] A working example of the invention is explained in detail hereinafter with reference to the figures. The figures show:

[0104] FIG. 1 a schematic diagram of the production process of a catalyst,

[0105] FIG. 2 a simplified schematic diagram for illustration of the use of different functional particles,

[0106] FIG. 3 an SEM image of the functional particles,

[0107] FIG. 4 a detail from an image of an extruded honeycomb catalyst,

[0108] FIGS. 5 and 6 the honeycomb catalyst shown in FIG. 4 at different enlargements.

[0109] With reference to FIG. 1, the production of a catalyst 2 and of a catalyst article 17 formed by the catalyst 2 is elucidated hereinafter. The catalyst article 17 is especially an extruded SCR honeycomb catalyst. However, the use of this process is not restricted to an SCR catalyst article 17 of this kind, nor to an extruded honeycomb catalyst. The principles of the process can generally be applied to a wide variety of different catalysts which differ both in terms of their structure and in terms of the catalytically active materials used therein.

[0110] In the embodiment shown in FIG. 1, in a first step, as pore formers, organic particles 4 and powder particles 6 of a catalytically active material are provided. This catalytically active material is especially a zeolite, preferably an ion-exchanged zeolite, more particularly a copper ion-exchanged zeolite. The individual powder particles typically have a particle size in the range of 1-10 μm. The organic particles 4 are polymer particles, especially spherical particles or the like, consisting, for example, of polyethylene or other thermoplastics or elastomers.

[0111] In the next step, functional particles 8 are produced from these two starting materials. For this purpose, the organic particles 4 and the powder particles 6 are mixed with one another and preferably heated to above the glass point of the organic particles 4, such that they soften. As a result, the powder particles 6 stick to the surface of the organic particles 4 after cooling and form an at least substantially continuous powder layer on the surface of the particles 4, which forms a layer 5 around the particles 4.

[0112] As an alternative to the functional particles 8 described here, having the organic particles 4 as pore formers, it is also possible to use inorganic, porous particles as pore formers. In the case of these too, in a first variant, the layer 5 is formed by applied powder particles 6. Alternatively, the layer 5 of catalytically active material can also be formed by a catalytic activation of the surface of the particles 4.

[0113] In all cases, the layer 5 is in at least substantially closed form and has a coverage level of the particle 4 preferably in the range from 60% to 95%.

[0114] In the next step, the functional particles 8 are mixed with a further component, especially a binder component 10, for formation of a catalytic composition 12. This is effected, for example, by a kneading operation or another mixing operation, especially with thermal action. Binder components used here are inorganic binders as generally employed for the formation of extrudable catalytic compositions 12. Suitable binder components 10 are especially clays, for example bentonite, refractory clay, kaolin, or else metal oxides such as aluminium oxide, cerium oxide, or else sols based on aluminium oxide, silicon oxide or titanium oxide. These binder components 10 may be supplemented, if required, with further organic sintering aids or else support elements, for example support fibres. The proportion of these additional binder components 10 is, for example, within the range from 25 to 35% by volume. The remaining 65-75% by volume of the catalytic composition 12 (based on the dry state, without addition of liquids) are therefore formed by the functional particles 8. In addition to these constituents, organic extruding aids may also be added.

[0115] The organic particles 4 have a particle size typically in the range from about 10 to a maximum of 200 μm and preferably from 10 to a maximum of 100 μm.

[0116] The catalytic composition 12 produced in this way is subsequently extruded to form a shaped body 14. In the course of extrusion, a pressure is applied to the catalytic composition 12. This presses the individual functional particles 8 against one another, such that they are in mutual contact with their powder particles 6. After the extrusion, as usual, the shaped body 14 is dried, calcined and subsequently sintered, so as to obtain the final catalyst 2 and hence also the catalyst article 17 in the form of the honeycomb catalyst.

[0117] This is shown in FIG. 1, merely in the form of a schematized structure. As can be inferred from this, the catalyst 2 has an open-pore pore structure 18 consisting of several cells joined to one another, which are formed when organic pore formers are used as pores 20. The individual pores 20 are bounded in each case by an active interface layer 22 which forms the cell wall. This active interface layer 22 is formed by the powder particles 6, which are now sintered to one another, i.e. the catalytically active material. These active interface layers 22 are therefore at least substantially free of the binder component 10. The side of the active interface layer 22 facing away from the pores 20 adjoins the binder material, which thus forms a binder matrix 24 as support structure and matrix. The pore structure 18 has therefore been formed within the binder matrix 24, with each of the pore walls formed by the active interface layers 22. The connection of the individual pores 20 to one another is formed in the extrusion and the sintering operation and is favoured by the close proximity of the individual functional particles 8 in the catalytic composition 12. It is particularly advantageous in this context that the catalytic composition 12 is produced by the kneading or subsequent extrusion, in which high compressive forces are exerted. Overall, an open porous cellular wall structure has therefore been formed.

[0118] The process has been elucidated in the context of FIG. 1 with reference to the use of a zeolite as catalytically active material. The process concept described here, however, is not restricted to the use of a zeolite. The basic process steps and stated parameters can also be applied to other catalytically active materials. The functional particles 8 produced at the intermediate stage can additionally also be employed for other production processes, for example for formation of a suspension which is then applied to a support body which has been extruded, for example, for formation of a washcoat.

[0119] Through the provision of the functional particles 8, various adjustments in the properties of the catalyst 2 are possible in a simple manner. One way of doing this is illustrated in detail by FIG. 2. This shows, in schematic form, three kinds of functional particles 8 which differ with regard to the coating thereof with the powder particles 6. Thus, one kind of functional particles 8 has been configured with a first kind of powder particles 6a, a second kind with a second kind of powder particles 6b, and a third kind of functional particles 8 with a mixture of these two powder particles 6a, 6b. In the final catalyst 2, the effect of this is that the pores 20 are bounded by different active interface layers 22a, 22b, 22c. In one case, the pores 20 are thus bounded by the first catalytic composition of the powder particles 6a/the second catalytic composition as per the powder particles 6b or else by a catalytic composition formed from the mixture of these two powder particles 6a, 6b. In this way, different catalytic sites are provided in the immediate surroundings.

[0120] A catalyst 2 produced by the process described using PVC granulates as organic particles 4 exhibits a distinct improvement in catalytic efficiency compared to conventionally produced catalysts. In the case of the SCR honeycomb catalyst based on a zeolite as catalytically active material, which has been described here. In the Table below, the nitrogen oxide conversion rate is given against temperature, once for an inventive catalyst 2 and once for a comparative catalyst: the comparative catalyst employed was an extruded honeycomb catalyst having a proportion of 67% by volume of a copper ion-exchanged zeolite having an aluminosilicate CHA framework type structure. The remaining fractions consist of a catalytically inactive binder component 10. The catalysts 2 are extruded honeycomb catalysts having a cell density of 200 cpsi (cells per square inch). The extrudate was freeze dried using the method disclosed in WO 2009/080155 and calcined at 600° C. for 2 hours.

[0121] The inventive catalyst 2 used for comparison is composed of the same constituents, but with different proportions. Thus, the zeolite content has been reduced from 67% by volume to 35% by volume. At the same time, the proportion of the binder components has been increased from 10% by volume to 65% by volume.

[0122] To check the catalytic activity, the honeycomb catalyst was contacted with an exhaust gas to be treated at a space velocity SV of 120 000 per hour. The exhaust gas here contained a proportion of 100 ppm of NO.sub.x (with zero NO.sub.2) with addition of 100 ppm of NH.sub.3. At the catalyst outlet, the residual proportion of nitrogen oxides NO.sub.x was then measured and expressed as a ratio to the proportion of nitrogen oxides in the exhaust gas on the inlet side to determine the NO.sub.x conversion rate. The NO.sub.x conversion rate was determined at a temperature range from about 180 to 500 degrees. The results are set out in the Table below.

TABLE-US-00001 TABLE 1 NO.sub.x conversion and derived catalyst activity (defined as NOx conversion normalized by catalyst mass) for different temperatures (conditions: NO.sub.x = NH.sub.3 = 100 ppm (NO.sub.2 = 0 ppm), H.sub.2O = 7%; O.sub.2 = 9.3%; SV = 120000 h.sup.−1). Catalyst Activity* NO.sub.x Conversion [%] [%/g.sub.catalyst] Temperature [° C.] 180 250 400 500 180 250 400 500 Catalyst A 36.3 64.8 74.5 69.7 2.1 3.7 4.3 4.0 Catalyst B 32.5 73.6 80.5 75.1 3.2 7.2 7.9 7.4 *NO.sub.x conversion normalized by catalyst mass

[0123] As is immediately apparent, the inventive catalyst 2, in spite of the lower proportion of catalytically active composition by nearly a factor of 2 therein, in terms of the conversion rate, is well above the conversion rate of the comparative catalyst in virtually all temperature ranges. A further advantage of the inventive catalyst 2 is its comparatively lower weight. The catalyst 2 has a significantly better conversion by mass, as also indicated in the Table.

[0124] The SEM image in FIG. 3 once again shows the actual features of the functional particles 8. Clearly apparent here are the individual powder particles 6 which have been applied to the surface of the organic particles 4 and form a more or less continuous layer. Uncovered surface regions are fundamentally desirable for achieving a certain porosity of the active interface layer 22 ultimately formed. In addition, this promotes the formation of the open-pore cellular structure. The individual powder particles 6 are deposited on the surface of the organic particles 4 preferably in one layer or in a few layers. There is isolated formation of agglomerates.

[0125] FIG. 4 shows an image of a detail of an extruded honeycomb catalyst 2, here with rectangular flow channels through which the exhaust gas flows in operation. The catalyst 2 thus takes the form of an all-active extrudate in which the individual walls that bound the individual flow channels are formed from the catalytic composition 12.

[0126] In FIGS. 5 and 6, the (coarse) pore structure that forms is clearly apparent. In addition, it is also possible to see, in the images shown here, how the individual pores 20 are at least partly joined directly to one another, such that an open pore structure is formed overall. Also clearly apparent are the porous active interface layers 22, which are formed by the individual powder particles 6, now ceramically joined to one another.

LIST OF REFERENCE NUMERALS

[0127] 2 catalyst [0128] 4 pore former [0129] 5 layer [0130] 6 powder particles [0131] 8 functional particles [0132] 10 binder component [0133] 12 catalytic composition [0134] 14 shaped body [0135] 17 catalyst article [0136] 18 pore structure [0137] 20 cells [0138] 22 active interface layer [0139] 24 matrix