EP1871525A2

EP1871525A2 - Catalytic filter for filtering a gas comprising a coating and/or a joint with controlled porosity

Info

Publication number: EP1871525A2
Application number: EP06726320A
Authority: EP
Inventors: Patricia Chez Mme Biancotti ANDY; Sébastian BARDON; Anthony Briot
Original assignee: Saint Gobain Centre de Recherche et dEtudes Europeen SAS
Current assignee: Saint Gobain Centre de Recherche et dEtudes Europeen SAS
Priority date: 2005-04-08
Filing date: 2006-04-07
Publication date: 2008-01-02
Also published as: US20080170973A1; WO2006106275A3; JP2008537510A; WO2006106275A2

Abstract

The invention concerns a catalytic filter for depolluting a gas loaded with soot particles and gaseous pollutants, obtained by impregnating in a catalytic solution a filtering structure formed by an assembly of honeycomb-type monolithic elements, wherein the porosity of the cement used as coating and/or as joint and/or as plug is less than the porosity of the material constituting said element(s).

Description

CATALYTIC FILTER FOR FILTERING A GAS COMPRISING A CONTROLLED POROSITY COATING AND / OR SEAL

The invention relates to the field of particulate filters used in an exhaust line of an engine for the removal of soot typically produced by combustion of a diesel fuel in an internal combustion engine. More specifically, the invention relates to a particulate filter, said filter further comprising a material conferring catalytic properties, and a method of preparing it. Filtration structures for soot contained in the exhaust gas of an internal combustion engine are well known in the prior art. These structures most often have a honeycomb structure, one of the faces of the structure for the admission of the exhaust gases to be filtered and the other side the exhaust of the filtered exhaust gases. The structure comprises, between the intake and discharge faces, a set of adjacent ducts or channels of axes parallel to each other separated by porous filtration walls, which ducts are closed to one or the other of their ends for delimiting input chambers s 'opening according to the inlet face and outlet chambers s' opening according to the discharge face. For a good seal, the peripheral part of the structure is surrounded by a cement, called coating cement in the following description. The channels are alternately closed in an order such that the exhaust gases, during the crossing of the honeycomb body, are forced to cross the walls side of the input channels to join the output channels. In this way, the particles or soot are deposited and accumulate on the porous walls of the filter body. Advantageously, the filter bodies are porous ceramic material, for example cordierite or silicon carbide.

In known manner, during its implementation, the particulate filter is subjected to a succession of filtration phases (accumulation of soot) and regeneration (removal of soot). During the filtration phases, the soot particles emitted by the engine are retained and are deposited inside the filter. During the regeneration phases, the soot particles are burned inside the filter, in order to restore its filtration properties. The porous structure is then subjected to intense thermal and mechanical stresses, which can cause micro-cracking likely over time to cause a severe loss of filtration capacity of the unit, or even its complete deactivation. This phenomenon is particularly observed on monolithic filters of large diameter.

To solve these problems and increase the life of the filters, it has been proposed more recently more complex filtration structures, combining in a filter block several monolithic elements in honeycomb. The elements are most often assembled together by gluing by means of a cement of ceramic nature, called in the following description cement joint. Examples of such filter structures are for example described in patent applications EP 816 065, EP 1 142 619, EP 1 455 923 or WO 2004/090294. It is known that in this type of structure, in order to ensure a better relaxation of the stresses, the coefficients of thermal expansion of the different parts of the structure (filtration elements, coating cement, joint cement) must be of substantially the same order . As a result, said parts are synthesized on the basis of the same material, most often silicon carbide SiC or cordierite. This choice also makes it possible to homogenize the distribution of heat during the regeneration of the filter. By the expression "at the base of the same material" is meant in the sense of the present description that the material consists of at least 25% by weight, preferably at least 45% by weight and very preferably at least 70% by weight of said material. The filters or porous soot filtration structures as previously described are mainly used on a large scale in the exhaust gas pollution control devices of a diesel engine. In addition to the soot treatment problem, the transformation of polluting emissions into the gas phase (ie mainly nitrogen oxides (NO _x ) or sulfur (SO _x ) and carbon monoxide (CO), or even unburned hydrocarbons) to less harmful gases (such as nitrogen gas (N ₂ ) or carbon dioxide (CO ₂ )) requires additional catalytic treatment.

According to a first technology, to eliminate all pollutants, the exhaust line of the internal combustion engine comprises, in series, a catalytic purification member and a particulate filter.

The catalytic purification unit, generally of open honeycomb structure, is adapted for treatment of pollutants in the gas phase, while the particulate filter is adapted to remove soot particles emitted by the engine. In addition to the complexity of the implementation of this solution and its cost, the succession of filter elements on the exhaust line is however responsible for a significant loss of load on said line, which may affect the engine performance .

To solve these problems, it was sought to transfer the catalytic function on a particle filter of the monolithic type. According to the conventionally used methods, the honeycomb structure is impregnated with a solution comprising the catalyst or a precursor of the catalyst.

Such methods generally include an immersion step in either a solution containing a catalyst precursor or a catalyst solubilized in water (or other solvent), or a suspension in water of catalytic particles. An example of such a process is described in US Pat. No. 5,866,210. According to this method, the application at the other end of the filter of a vacuum allows in a second time the rise of the solution in the structure and consequently the coating of the internal walls of the honeycomb structure.

According to other embodiments of the process for impregnating the honeycomb filters, said impregnations can be obtained by pumping, by applying a vacuum or under the pressure of the liquid comprising the impregnation solution, on at least one end of the monolith. Most often the processes described are characterized by a combination of these different techniques, in successive steps, the final step allowing the removal of the excess solution in the filter by introduction of air under pressure or by suction. One of the essential goals sought by the implementation of these methods is to obtain a uniform coating of the catalyst on or even inside at least a portion of the porous walls of the channels constituting the internal part of the structure and traversed by the gases exhaust.

Such methods, as well as the devices for their implementation, are for example described in the patent applications or patents US 2003/044520, WO 2004/091786, US 6,149,973, US 6,627,257, US 6,478,874, US 4,609,563, US 4,550,034, US 6,599,570, US 4,208,454 or US 5,422,138. Whatever the method used, the cost of the deposited catalysts, which most often contain precious metals of the Platinum group (Pt, Pd, Rh) on an oxide support represents a significant part of the overall cost of the impregnation process. It is therefore important according to the invention not only that the catalyst is deposited uniformly on the walls of the filter channels, but also that a minimum portion thereof is deposited on the parts of the honeycomb structure which do not come into direct contact with the gases to be filtered or the soot. Said parts are essentially the coating cement for a monolithic structure, added with the joint cement in the case of a filter block as previously described, that is to say combining several monolithic elements in honeycomb and in to a lesser extent the cement used for plugs alternately obstructing the channels.

The object of the present invention is thus to limit the amount of catalyst present on the parts of the structure which do not come into direct contact with the gas to be filtered or soot, during an impregnation process of a particle filter having a honeycomb structure. More specifically, according to a first aspect, the present invention relates to a process for obtaining a filter for the filtration of a gas loaded with soot particles and gas phase pollutants such as carbon monoxide CO, oxides nitrogen NOx, SOx sulfur oxides, HC hydrocarbons, said process comprising the steps of: a) manufacturing a first structure comprising:

a central part comprising a plurality of honeycomb filter elements interconnected by a joint cement, the one or more elements comprising a set of adjacent ducts or channels with axes parallel to one another separated by porous walls, which conduits being closed by plugs at one or the other of their ends to define inlet chambers opening on a gas inlet face and outlet chambers opening on a discharge face of gas, such that the gas passes through the porous walls and a peripheral portion constituted by a coating cement protecting the said element or elements, the porosity of the joint cement and / or the coating cement and / or the cement constituting the plugs being less than the porosity of the material constituting said walls, b) impregnating said structure with a solution containing a precursor of the catalyst or the solubilized catalyst in a solvent such as water, a suspension in a solvent such as water of catalytic particles.

Preferably, said elements, the coating cement, the joint cement and the plugs are based on the same ceramic material, preferably based on silicon carbide SiC. Advantageously, the porosity of the cement used for the coating and / or for the seal and / or for the plugs is less than 5%, preferably at least 10%, and very preferably at least 10%. 15% or even 20%, to the porosity of the filter elements. For the purposes of the present description, this difference of 5, 10, 15 or 20% is calculated on the basis of the percentage difference in porosity between the filter element (s) and the coating or joint cement, based on the percentage of porosity of the filter element (s) in question.

In general, the porosity of the honeycomb filter elements is between 30% and 70%, preferably between 35% and 60%. Typically, the porosity of the cement used for the coating and / or for the seal and / or for the plugs is between 20% and 60%, preferably between 25% and 45%.

According to the invention, the impregnation of the structure can be carried out by any method known in the art and in particular by pumping the solution through the structure, by applying a vacuum or a vacuum or under pressure liquid comprising the impregnating solution on at least one end of the structure. Better impregnation is generally achieved by a combination of these different techniques, in successive steps, usually a final step allowing the removal of the excess solution in the filter by suction or by introduction of pressurized air .

According to the invention, the impregnation step may be implemented according to the methods and / or devices known from the prior art and in particular according to one of the methods or devices described in the patents or patent applications mentioned above. The porosity of the cement used for the coating and / or for the seal and / or for the plugs can be adjusted according to several methods of preparation:

According to a first mode, during the step of preparing the cement and before its application on the filtering part, the cement composition and / or the particle size distribution of the grains entering into the cement composition and / or the quantity of cement are adjusted. water mixed with grains as well as that of the other liquid ingredients, to obtain the desired porosity. For example, it is possible according to the invention to use dynamic compaction models to obtain maximum compactness of the cement by playing in particular on the relative proportions of the particle size classes of the various powders used.

According to another possible mode, the porosity of the coating cements and / or seal and / or plugs is adjusted by the introduction, during the preparation step, of a charge or filler according to the English term, whose size is adjusted to that of the pores of the cement. The filler, by occupying at least part of the pores of the cement, thus prevents catalyst precursors or the catalyst from infiltrating the porosity.

Typically, the charges are, for example, organic or inorganic molecules, the size of which is substantially identical to or smaller than that of said precursors or said catalysts.

To further improve the efficiency of the process, it is of course possible, without departing from the scope of the invention, to use a combination of the two modes that preceded.

According to a second aspect, the invention relates to a catalytic filter that can be obtained by the process θ

manufacturing method as just described and characterized by a lower porosity of the cement constituting the coating and / or the joints and / or plugs relative to the porosity of the filter element or elements, as well as by the presence of a minimal amount of catalyst on said cement. For the purposes of this description, a minor amount is understood to mean a smaller amount of catalyst relative to the theoretical amount of the catalyst contained in a cement of the same porosity as that of the filtering elements. Preferably, the catalytic filter is characterized in that the porosity of the cement constituting the coating and / or the joints and / or plugs is less than 5%, preferably at least 10%, and very preferably at least 15% to the porosity of the filter element or elements.

The invention and its advantages will be better understood on reading the examples which follow. It is understood that these examples should not be considered, in any of the aspects described, as limiting the present invention.

Example 1

A filtering structure comprising an assembly of silicon carbide filter elements bonded with a joint cement was synthesized according to the techniques described in patent EP 1 142 619.

Sixteen monolithic filter elements of square section are first extruded, dried and then cooked according to well-known techniques, for example described in EP 1 142 619. In the model described in application EP 1 142 619, the channels of the elements are alternately blocked. by plugs made with the same paste as that used for the extrusion step of the monolithic elements. A cement for the joint and the coating is then prepared by mixing:

15% by weight of a SiC powder whose grain size is less than 10 μm, - 20% by weight of an SiC powder whose grain size is less than 50 μm,

50% by weight of a SiC powder whose grain size is less than 200 μm,

4% by weight of a calcined alumina powder marketed by Almatis,

10% by weight of a reactive alumina powder marketed by Almatis, 0.9% by weight of a temporary binder and plasticizer of the Cellulose type, 0.1% by weight of a TPPNa type deflocculant

(Sodium tripolyphosphate).

A quantity of water corresponding to 10% of the weight of this mixture is added to obtain a cement of adequate viscosity. After assembly of the monoliths by means of said cement and machining of the outer surface of the structure thus obtained, it is then covered with the same cement for its coating. The assembly is annealed at a temperature sufficient to ensure satisfactory cohesion of the filter and its elements.

The characteristics of the crude filtration structure thus synthesized are reported in Table 1.

Table 1: characteristics of the gross structure

(before impregnation)

This crude structure is then immersed in a bath of an aqueous solution containing the appropriate proportions of a platinum precursor in the form of H ₂ PtClO, and a precursor of cerium oxide CeO ₂ (in the nitrate form of cerium) and a precursor of zirconium oxide ZrO ₂ (in the form of zirconyl nitrate) according to the principles described in publication EP 1 338 322 A1. The filter is impregnated with the solution according to an implementation method. similar to that described in US Patent 5,866,210. The filter is then dried at about 150 ^° C. and then heated to a temperature of about 600 ^° C. The main characteristics of the catalytic filter thus obtained are summarized in Table 2.

Table 2: characteristics of the catalytic filter

The chemical analysis shows a total Pt concentration of 52 g / ft ³ (1 g / ft ³ = 0.035 kg / m ³ ), that is to say a concentration of 1.82 kg / m ³ , ie 4.5 grams distributed in a non-uniform manner. homogeneous on the different parts of the filter.

More specifically, the analysis reveals the following distribution:

0.25% weight of platinum in the monolithic elements, ie 4.0 grams, of which 3.8 grams in the porosity of the walls of the honeycomb elements and 0.2 grams in the plugs. 0.13% weight of platinum in the coating cement, ie 0.25 gram, over a thickness of a few tens of μm from the external surface of the cement, 0.08% weight of platinum in the cement joint, ie 0.25 gram, the platinum being homogeneously distributed throughout the thickness of the cement.

Example 2

A catalytic filter was manufactured by repeating the same steps as those of Example 1, except that the cement used for the coating and the seal was prepared from the following proportions of the various constituents:

21% by weight of a SiC powder whose grain size is less than 10 μm,

9% by weight of a SiC powder whose grain size is less than 50 μm, - 55% by weight of an SiC powder whose grain size is less than 200 μm,

4% by weight of a calcined alumina powder marketed by Almatis, 10% by weight of a reactive alumina powder marketed by Almatis,

0.9% by weight of a temporary binder and plasticizer of the Cellulose type,

0.1% weight of a deflocculant of the TPPNa type. A quantity of water corresponding to 10% of the weight of this mixture is added to obtain a cement of adequate viscosity.

In this example, a different grain size distribution of the SiC grains is used to reduce the porosity of the cement. The characteristics of the raw structure and the filter obtained after impregnation according to this example are substantially the same as those obtained for example 1 and listed in Tables 1 and 2 with the main differences: the porosity of the joint and coating cements in the raw structure is 36.5%, which corresponds to a difference in porosity between the porosity of the filter elements and the porosity of the cement of 22.3%. the porosity of the seal and coating cements in the filter (after impregnation) is this time 35%, which corresponds to a difference in porosity between the porosity of the filter elements and the porosity of the cement of 18.6%.

The chemical analysis shows a total concentration of Pt of 48 g / ft ³ (1.68 kg / m ³ ), ie 4.2 grams distributed over the different parts of the filter.

More specifically, the analysis reveals the following distribution:

0.25% weight of platinum in the honeycomb elements, ie 4.0 grams, of which 3.8 grams in the porosity of the walls of the honeycomb elements and 0.2 grams in the plugs.

0.08% by weight of platinum in the coating cement, ie 0.1 gram, 0.05% by weight of platinum in the attached cement, ie 0.1 gram.

It is thus shown that by selecting the respective porosities of the various constituents of the filter, it is possible to achieve a substantial saving of Pt. The comparison of the results obtained according to Examples 1 and 2 shows that the application of the method according to the invention makes it possible to save a significant amount of catalyst and especially precious metal (0.3 gram by filter), thus generating a substantial saving of the overall cost of the catalyst deposition process on the structure.

Example 2b:

A catalytic filter was manufactured by repeating the same steps as those of Example 2, with the difference that the material used to make the caps is not identical to the extrusion paste used for the synthesis of the elements but is a specific mixture leading after cooking to a lower porosity. The proportions of the various constituents of the mixture used are as follows:

21% by weight of a SiC powder whose grain size is less than 10 μm,

19% by weight of a SiC powder whose grain size is less than 50 μm,

59% by weight of a SiC powder whose grain size is less than 200 μm, - 0.9% by weight of a temporary binder and plasticizer of the Cellulose type,

0.1% by weight of a deflocculant of the Na silicate type. An amount of water corresponding to 15% of the weight of this mixture is added to obtain a mixture which is deposited at the end of the channels of the honeycomb elements, as previously described.

The characteristics of the raw structure and the filter obtained after impregnation according to this example are substantially the same as those obtained for Example 1 and listed in Tables 1 and 2. The chemical analysis shows a total Pt concentration of 47 g / ft ³ , ie 4.1 grams, distributed over the different parts of the filter.

More specifically, the analysis reveals the following distribution:

0.24% by weight of platinum in the honeycomb elements, ie 3.9 grams, of which 3.8 grams in the wall porosity of the honeycomb elements and 0.1 gram in the plugs. 0.08% by weight of platinum in the coating cement, ie 0.1 gram,

0.05% weight of platinum in the cement joint, ie 0.1 gram.

It is thus shown that by selecting the respective porosities of the different constituents of the filter, it is possible to achieve a substantial saving of Pt.

The comparison of the results obtained according to Examples 1, 2 and 2a shows that the present process makes it possible to save an even larger quantity of catalyst and in particular of precious metal (0.4 gram per filter), thus generating a substantial saving. the overall cost of the catalyst deposition process on the structure.

It is understood that the present invention is not limited to this simple embodiment and that any known means of acting on the porosities of the cement and / or filter elements and / or their plugs, such as the introduction of fillers or sintering additives, should be considered as part of the present invention. Without departing from the scope thereof, it is also possible to use a combination of such means to exert even better control of said porosity.

Claims

A process for obtaining a filter for the filtration of a gas charged with soot particles and gas phase pollutants such as CO carbon monoxide, NOx nitrogen oxides, SOx sulfur oxides, HC hydrocarbons, said process comprising the steps of: a) manufacturing a first structure comprising:

- A central portion comprising a plurality of honeycomb filter elements interconnected by a joint cement, the one or more elements comprising a set of adjacent ducts or channels of axes parallel to each other separated by porous walls, which conduits being closed by plugs at one or the other of their ends to define inlet chambers opening on a gas inlet face and outlet chambers opening on a discharge face of gas, so that the gas passes through the porous walls and

- a peripheral portion constituted by a coating cement protecting the said element or elements, the porosity of the joint cement and / or the coating cement and / or the cement constituting the plugs being less than the porosity of the material constituting said walls, b) impregnating said structure with a solution containing a precursor of the catalyst or the catalyst solubilized in a solvent such as water, or a suspension in a solvent such as water of catalytic particles.

2. Method according to claim 1, wherein said elements, the coating cement, the joint cement and the plugs are based on the same ceramic material, preferably based on silicon carbide SiC.

3. Method according to one of claims 1 or 2 wherein the porosity of the cement used for the coating and / or for the seal and / or plugs is less than 5%, preferably at least 10%. and most preferably at least 15% or even 20%, to the porosity of the filter elements.

4. Method according to one of the preceding claims wherein the porosity of the honeycomb filter elements is between 30% and 70%, preferably between 35% and 60%.

5. Method according to one of the preceding claims wherein the porosity of the cement used for the coating and / or the seal and / or plugs is between 20% and 60%, preferably between 25% and 45%.

6. Method according to one of the preceding claims, wherein said impregnation is performed by pumping the solution through the structure, by applying a vacuum or a vacuum or under the pressure of the liquid comprising the solution of impregnation on at least one end of the structure or a combination of these different techniques.

7. Method according to one of the preceding claims, wherein the porosity of the coating cements and / or seal and / or plugs is adjusted, during the step of manufacture of the structure, according to the composition of the cement and / or the particle size distribution of the grains and / or the quantity of water mixed with grains as well as that of the other liquid ingredients used in the composition of said cement, for example using dynamic compaction models.

8. Method according to one of the preceding claims, wherein the porosity of the coating cements and / or seal and / or plugs is adjusted by the introduction, during the step of manufacturing the structure, a load or filler, whose size is adjusted to that of the pores of said cement.

9. Catalytic filter obtainable by the process according to one of the preceding claims, characterized by a lower porosity of the cement constituting the coating and / or the seal and / or plugs with respect to the porosity of the element or elements. filters, as well as by the presence of a minimal amount of catalyst on said cement.

Catalytic filter according to claim 9, in which the porosity of the cement constituting the coating and / or the joints and / or the plugs is at least 5%, preferably at least 10%, and very preferably less than 10%. at least 15% to the porosity of the filter element or elements.