FR2886868A1 - Filter for gas containing carbon, nitrogen, sulfur and hydrocarbons has honeycomb structure of porous ducts bonded by cement and impregnated with catalyst solution - Google Patents

Abstract

The filter consists of a structure with a central section made up of a number of honeycomb filter elements joined together by a porous bonding cement and made in the form of ducts or channels with parallel axes. The ducts or channels are blocked by cement plugs at one or other end; they have porous walls for the filtered gas to pass through and outer surfaces coated with porous cement. The bonding, plugging and coating cement is preferably at least 5 - 15 per cent less porous than the walls of the ducts or channels, which are impregnated with a catalyst in the form of a water-based solution or suspension. All the components of the filter are based on the same ceramic material, preferably silicon carbide.

Description

FL9 2005028.1-EN

  The invention relates to the field of particulate filters used in an exhaust line of an engine for the elimination of gases and to a filter. soot typically produced by combustion of a diesel fuel in an internal combustion engine. More specifically, the invention relates to a filter structure and a particle filter, said filter further comprising a material conferring catalytic properties, and the method of preparation thereof.

  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 of axes parallel to each other separated by porous filtration walls, which ducts are closed at one or the other of their ends to delimit entrance chambers opening along the intake face and exit chambers opening along 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 pass through the sidewalls FL9 2005028.1-EN of the inlet channels to join the channels exit. 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 bonding by means of a cement of a ceramic nature, called in the following description seal cement or 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 FL9 2005028.1-FR coefficients of thermal expansion of the different parts of the structure (filtration elements, coating cement, joint cement) must be substantially of 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 on the basis 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 (NOX) or sulfur (SOX) and carbon monoxide (CO), or even hydrocarbons unburned) to less harmful gases (such as nitrogen gas (N2) or carbon dioxide (CO2)) 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 member, generally of open honeycomb structure, is suitable for the treatment of pollutants in the gas phase, while the particulate filter is adapted to remove the soot particles emitted by the engine. In addition to the complexity of the implementation of this solution and its cost, the succession of FL9 2005028.1FR 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 monolithic particle filter. 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 within at least a portion of the porous walls of the channels constituting the internal part of the structure and crossed by the exhaust gases.

  FL9 2005028.1-EN 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 5,866,210, 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 not only that the catalyst be uniformly deposited on the walls of the filter channels, but also that a minimum part of it is deposited on the parts of the honeycomb structure that do not enter. not in direct contact with the gases to be filtered or soot. Said parts are essentially the coating cement for a monolithic structure, added joint cement in the case of a filter block as previously described, that is to say associating several monolithic elements in honeycomb.

  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, the present invention relates in a first aspect to a usable structure, after deposition of a catalyst, for the filtration of a gas charged with soot particles and pollutants in the gas phase, said structure comprising - a central part comprising a honeycomb filter element or a plurality of honeycomb nesting filter elements connected together by a joint cement, the at least one element comprising a set of adjacent parallel axis conduits or channels; between them separated by porous walls, which conduits are closed by plugs at one or the other of their ends to define inlet chambers opening along a gas intake face and exit chambers opening on a gas evacuation face, such that the gas to be filtered passes through the porous walls and - a peripheral portion constituted by a protective coating cement the said element or elements, the said structure being characterized in that the porosity of the coating cement and / or the joint cement is less than the porosity of the material constituting the one or more elements.

  Preferably, said structure is constituted by a filter block associating several monolithic honeycomb filtering elements, said elements being bonded by a joint cement whose porosity is lower than the porosity of the material constituting said element or elements.

  Typically, said element or elements, the coating cement and possibly the joint cement are based on the same ceramic material, preferably based on silicon carbide SiC.

  In general, the porosity of the honeycomb filter element or elements is between 30 and 70%, preferably between 35 and 60%. By porosity is meant the open porosity measured by mercury porosimetry.

  Typically the porosity of the cement used for the coating and / or for the joint is between 20 and 60%, preferably between 25 and 45%.

  Advantageously, the porosity of the cement used for the coating and / or for the joint is less than 5%, preferably at least 10%, and preferably at least 15%. at least 20% of the porosity of the filter element or 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.

  According to a second aspect, the invention also relates to a process for obtaining a filter for the filtration of a gas charged with particles of soot and of pollutants in the gas phase such as carbon monoxide CO, oxides nitrogen NOx, SOx sulfur oxides, HC hydrocarbons, said process comprising the steps of: manufacturing a first structure as previously described, wherein the porosity of the cement is lower than the porosity of the material used for the synthesis or the filter elements and preferably less than 5%, preferably at least 10%, or even 15% or even 20% to the porosity of said material.

  impregnating said structure with a solution containing a precursor of the catalyst or the catalyst solubilized in a solvent such as water, a suspension of catalytic particles in a solvent such as water.

  According to the invention, said 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 the pressure of the 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 carried out 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 aforementioned patents or patent applications.

  The porosity of the cement used for the coating and / or for the joint can be adjusted according to several modes of preparation: In a first mode, during the step of preparing the cement and before its application on the filtering part, the composition of the cement and / or grain size distribution of the grains used in the cement composition and / or the amount of 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 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 pores of 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.

  FL9 2005028.1-EN 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 third aspect, the invention relates to the catalytic filter obtained by the manufacturing method as just described and which is characterized by a lower porosity of the coating and / or joint cement with respect to the porosity of the or filter 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 by a porosity of the coating cement and / or seal less than 5% or even 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.

  A cement for the joint and the coating is then prepared by mixing: 15% by weight of an SiC powder whose grain size is less than 10 m, - 20% by weight of a SiC powder whose grain size is less than 50 m, - 50% by weight of an SiC powder whose grain size is less than 200 m, - 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 deflocculant of the TPPNa type (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.

  Square channel geometry Channel density 180 cpsi (channels per square inch, 1 inch = 2.54 cm, or about 70 channels per cm2) Wall thickness 350 pm Number of assembled elements 16 Cylindrical structure length 6 "( 15.2 cm) Volume 2.48 liters Porosity of the filter elements 47% Porosity of the cement 43% Difference of relative porosity 8.5% filtering elements / cement% mass of the elements and o plugs 80 / o% by weight of the cement joint 13 % by weight of the cement of 7% coating Table 1: characteristics of the raw structure (before impregnation) This raw structure is then immersed in a bath of an aqueous solution containing the appropriate proportions of a platinum precursor in the form H2PtC16, and a precursor of cerium oxide CeO 2 (in the form of cerium nitrate) and of a precursor of zirconium oxide ZrO 2 (in the form of zirconyl nitrate) according to the principles described in publication EP 1 338 322 Al. The filter is impregnated with the solution in an embodiment similar to that described in US Pat. No. 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.

  Elements Localization of the catalyst inside the walls, filtering around the grains of SiC Formulation Pt-CeO2-ZrO2 BET surface area 5.1 m2 / g Porosity of the elements 43% Cement Presence of catalyst Yes on the cement joint Presence of catalyst Yes on the cement Coating presence Catalyst presence Yes in cement joint Catalyst presence Yes in coating cement Porosity 40% Table 2: Catalytic filter characteristics The chemical analysis shows a total Pt concentration of 52 g / ft3 (1 g / ft3 = 0.035 kg / m3), ie a concentration of 1.82 kg / m3 or 4.5 grams unevenly distributed over the various parts of the filter.

  More specifically, the analysis reveals the following distribution: 0.25% by weight of platinum in the honeycomb elements, ie 4.0 grams, 0.13% by weight of platinum in the coating cement, ie 0, 25 grams, over a thickness of a few tens of m from the outer surface of the cement, 0.08% by weight of platinum in the cement joint, or 0.25 grams, the platinum being distributed homogeneously throughout the thickness cement.

Example 2

  A catalytic filter was manufactured by repeating the same steps as those of Example 1, with the difference that the FL9 2005028.1-FR cement serving for the coating and the joint was prepared from the following proportions of the various constituents: % by weight of an SiC powder having a grain size of less than 10 m, - 9% by weight of an SiC powder having a grain size of less than 50 m, - 55% by weight of SiC powder whose grain size is less than 200 m, - 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 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 crude structure and of 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 following main differences: the porosity of the joint and coating 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%. Chemical analysis shows a total Pt concentration 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% by weight of platinum in honeycomb elements, ie 4.0 grams, 0.08% by weight of platinum in the coating cement is 0, 1 grams, 0.05% weight of platinum in the cement joint, ie 0.1 grams.

  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 and 2 shows that the application of the process according to the invention makes it possible to save a not insignificant quantity of catalyst and in particular of precious metal (0.3 gram per filter), thus generating a substantial savings in 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, such as the introduction of charges, must be considered as included in the context 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.

FL9 2005028.1-EN

Claims (12)

  1. A structure that can be used, after the deposition of a catalyst, for the filtration of a gas charged with particles of soot and pollutants in the gas phase and comprising: a central part comprising a honeycomb filtering element or 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 each other separated by porous walls, which ducts are closed off by plugs at their ends; one or the other of their ends to define inlet chambers opening along a gas intake face and outlet chambers opening along a gas evacuation face, so that the gas crosses the porous walls and - a peripheral portion constituted by a coating cement protecting said element or elements, said structure being characterized in that the porosity of the coating cement and / or the cement seal is less than the porosity of the material constituting the one or more elements.   2. Structure according to claim 1, consisting of a filter block associating several monolithic honeycomb filter elements, said elements being bonded by a joint cement whose porosity is less than the porosity of the material constituting the one or more elements.   3. Structure according to claim 1 or 2, wherein the one or more elements, the coating cement and the optional cement seal are based on the same ceramic material, preferably based on silicon carbide SiC.   4. Structure according to claim 1 to 3 wherein, the porosity of the honeycomb filter element or elements is between 30% and 70%, preferably between 35% and 60%.   5. Structure according to one of claims 1 to 4 wherein the porosity of the cement used for the coating and / or the seal is between 20% and 60%, preferably between 25% and 45%.   6. Structure according to one of claims 1 to 5 wherein the porosity of the cement used for the coating and optionally for the seal is less than 5%, preferably at least 10% and very preferably d at least 15% or even 20%, to the porosity of the filter element or elements.   7. Process for obtaining a filter for the filtration of a gas loaded 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: manufacturing a first structure according to one of claims 1 to 6, wherein the porosity of the cement is less than the porosity of the material constituting said walls and preferably less than less than 5%, preferably at least 10%, or even 15% or even 20% to the porosity of said material, impregnating said structure with a solution containing a precursor of the catalyst or the catalyst solubilized FL49 in a solvent such as water, a suspension in a solvent such as water of catalytic particles.   The process according to claim 7, wherein said impregnation can be carried out by pumping the solution through the structure, by applying a vacuum or a vacuum or under the pressure of the liquid comprising the impregnating solution. at least one end of the structure or a combination of these different techniques.   9. The method of claim 7 or 8, wherein the porosity of the coating cements and / or seal is adjusted during the manufacturing step of the first structure, depending on the composition of the cement and / or the granulometric distribution of the grains and / or the amount 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.   The method according to claim 7 or 8, wherein the porosity of the coating and / or seal cements is adjusted by introducing, during the step of fabricating the structure, a filler or filler, the size is adjusted to that of the pores of said cement.   Catalytic filter obtainable by the method according to one of claims 7 to 10 and characterized by a lower porosity of the coating cement and / or seal with respect to the porosity of the filter element or elements, as well as by the presence of a minimal amount of catalyst on said cement.   FL9 2005028.1-EN The catalytic filter of claim 11 wherein the porosity of the coating and / or joint cement is at least 5%, preferably at least 10%, and most preferably at least 15% less than the porosity of the filter element or elements.