EP2155388A2

EP2155388A2 - Textured particle filter for catalytic use

Info

Publication number: EP2155388A2
Application number: EP08805801A
Authority: EP
Inventors: Patricia Andy; Caroline Tardivat; Ahmed Marouf; Damien Mey; Catherine Jacquiod; Valérie GOLETTO; Alexandra Dekoninck
Original assignee: Saint Gobain Centre de Recherche et dEtudes Europeen SAS
Current assignee: Saint Gobain Centre de Recherche et dEtudes Europeen SAS
Priority date: 2007-05-23
Filing date: 2008-05-19
Publication date: 2010-02-24
Also published as: US20100158774A1; WO2008142353A2; FR2916366A1; WO2008142353A3; JP2010529343A; KR20100017157A; FR2916366B1

Abstract

The invention relates to a catalytic filter comprising a porous matrix made from an inorganic material in the form of grains which are interconnected such as to form cavities therebetween, with an open porosity of between 30 and 60 % and a median pore diameter of between 5 and 40 μm. The filter is characterised in that at least part of the surface of the grains, and optionally the grain boundaries, of the inorganic material is covered with a texturing material, said texturisation consisting of surface irregularities having dimensions of between 10 nm and 5 micrometres, and in that the catalytic coating at least partially covers the texturing material and, optionally, the grains of the inorganic material.

Description

PARTICLE FILTER TEXTURE FOR CATALYTIC APPLICATIONS

The present invention relates to the field of porous filter materials. More particularly, the invention relates to typically honeycomb structures that can be used for the filtration of solid particles contained in the exhaust gases of a diesel engine or gasoline and additionally incorporating a catalytic component that makes it possible, for example, jointly elimination of NO _x type pollutants, carbon monoxide CO or unburned HC hydrocarbons.

The filters according to the invention have a matrix of an inorganic material, preferably ceramic, chosen for its ability to form a structure with porous walls and for acceptable thermomechanical resistance for application as a particulate filter in an automobile exhaust system. . Such a material is typically based on silicon carbide, in particular recrystallized silicon carbide. Other oxide, carbide or nitride materials, such as cordierite-based matrices, for example, are also included within the scope of the present invention, even if the SiC-based materials are preferred, because of their high refractoriness and their strong chemical inertness.

The increase in porosity and in particular the average pore size is generally sought for catalytic filtration gas treatment applications. Such an increase makes it possible to limit the pressure drop caused by the positioning of a particulate filter as previously described in an automobile exhaust line. Pressure loss means the difference in gas pressure existing between the inlet and the filter output. However, this increase in porosity finds its limits with the associated decrease in thermomechanical strength properties of the filter, especially when it is subjected to successive phases of accumulation of soot particles and regeneration, that is to say say soot removal by their combustion within the filter. During these regeneration phases, the filter can be raised to average inlet temperatures of the order of 600 to 700 ^° C., while local temperatures of more than 1000 ^° C. can be reached. These hot spots are all defects that are likely over the life of the filter to alter its performance, or even disable it. At very high porosity levels, for example greater than 60%, it has been found in particular on silicon carbide filters a sharp decrease in thermomechanical resistance properties.

This antagonism between the pressure drop caused by a filter and its thermomechanical resistance becomes all the more noticeable if it is sought to associate with the filtration function of the particles an additional component for eliminating or treating the polluting gas phases contained in the gases. Exhaust, type NOx, CO or HC. While effective catalysts for the treatment of these pollutants are now very well known, their integration into particle filters clearly raises the problem of their efficiency when they are present in the porosity of the inorganic matrix constituting the filter. and on the other hand their additional contribution to the pressure drop associated with the filter integrated in an exhaust line. In order to improve the efficiency of the catalytic treatment of gaseous pollutants, the solution currently the most studied is the increase in the amount of catalytic solution deposited per volume of filter typically by impregnation. To then maintain the pressure drop at acceptable values for an application in an automobile exhaust line, an evolution of these structures to the strongest porosity is then necessary. As explained above, such an evolution quickly finds its limits because it inevitably leads to an excessive drop in the thermomechanical properties of the filter for such an application.

In addition, other problems arise because of this increase in the catalyst load. The greater thickness of the catalyst layer substantially increases the local problems of hot spots already mentioned, especially during the regeneration phases because of the low ability of current catalyst compositions to transfer the heat of combustion of soot to the inorganic matrix.

Finally, the greater thickness of the catalyst deposit can lead to a lower catalytic efficiency as mentioned in US2007 / 0049492, paragraph [005], which can result from a bad distribution of the active sites, that is to say - say sites sites of the catalyzed reaction, making them less accessible to the gases to be treated. This has a significant impact on the initiation temperature of the catalytic reaction and consequently on the activation time of the catalyzed filter, that is to say on the time required for the cold filter to reach a temperature permitting effective treatment of pollutants.

In addition, this tendency towards greater loading of the catalyst filters leads to increasingly concentrated deposition suspensions, which poses problems of productivity, the deposition then being carried out in several impregnation cycles. Problems of feasibility also arise, because of the high viscosity of these suspensions. Indeed, beyond a certain viscosity depending on the chemical nature of the catalyst solution used for the impregnation, it becomes no longer possible, with the usual means of production, to effectively impregnate the porous substrate.

In addition to the difficulties mentioned above, related in particular to the increase of the pressure drop, the integration of a catalytic component in a particulate filter also poses the following problems:

- The adhesion of the impregnating solution on the porous substrate must be as uniform and homogeneous as possible but also allow to fix a large amount of catalyst solution. This problem is even more critical on matrices in the form of grains bonded to each other and whose surface is relatively smooth and / or convex, including SiC-based matrices.

to overcome the aging problem of the catalyst, especially in the sense described in application EP 1 669 580 A1, the catalytic coating deposited in the porosity of the walls of the filter must be sufficiently stable over time, that is to say that the catalytic activity must remain acceptable throughout the life of the filter, within the meaning of current and future anti-pollution standards.

At the present time, to guarantee acceptable catalytic performances during the entire duration of the filter, the solution adopted is to impregnate a larger quantity of catalytic solution and therefore of noble metals, in order to compensate for the loss of catalytic activity in the filter. time as described in JP 2006/341201. This solution leads not only to increase the pressure drop, as mentioned above, but also the cost of the process, due to the necessarily greater use of noble metals. The problem therefore still remains to limit the aging of the catalyst to ensure the stability of its performance. The object of the present invention is to provide an improved solution to all of the previously discussed problems. More particularly, one of the objects of the present invention is to provide a porous filter suitable for application as a particulate filter in an automobile exhaust line, which is subjected to successive stages of accumulation and combustion of soot, and having a catalytic component whose effectiveness is enhanced. More particularly, with equal porosity, the catalytic filters according to the invention can have a catalytic charge substantially greater than current filters. According to another possible mode, the catalytic filters according to the invention may have a better homogeneity, that is to say a more uniform distribution of the catalytic charge in the porous matrix.

Such an increase and / or the better homogeneity of the catalytic charge notably makes it possible to appreciably improve the treatment efficiency of the polluting gases without a joint increase in the pressure drop generated by the filter. The invention thus makes it possible in particular to obtain porous structures having thermomechanical properties that are acceptable for the application and a catalytic efficiency that is substantially increased throughout the lifetime of the filter.

Another object of the present invention is to obtain catalyzed filters having a better resistance to aging, as previously described.

More specifically, the invention relates to a catalytic filter for the treatment of solid particles and gaseous pollutants resulting from the combustion gases of an internal combustion engine, comprising a porous matrix consisting of an inorganic material, in the form of grains connected to each other so as to form between them cavities such that the open porosity is between 30 and 60% and the median pore diameter of between 5 and 40 μm, said filter being characterized in that: the grains and possibly the grain boundaries of the inorganic material are covered on at least a part of their surface of a texturizing material, said texturing consisting of irregularities whose dimensions are between 10 nm and 5 microns, a catalytic coating at least partially covers the texturizing material and optionally, at least partially, the grains of the material inorganic.

For example, said irregularities being for example in the form of beads, crystallites, polycrystalline clusters, or rods or acicular structures, cavities or craters, said irregularities having an average diameter of between about 10 nm and about 5 microns and a mean height h or a mean depth p of between about 10 nm and about 5 microns.

By mean diameter d, it is understood in the sense of the present description the mean diameter of the irregularities, these being individually defined from the plane tangent to the surface of the grain or grain joint on which they are located.

By average height h, it is understood in the sense of the present description the average distance between the top of the relief formed by the texturing and the plane mentioned above.

By average depth p, it is understood in the sense of the present description the average distance between on the one hand the deepest point formed by the impression, for example the trough or the crater of the texturing and on the other hand the plane cited previously.

In one possible embodiment, the average diameter of the irregularities is between 100 nm and 2.5 microns. For example, the height h or the average depth p of the irregularities is between 100 nm and 2.5 microns.

According to a preferred embodiment, the texturizing material covers at least 10% of the total surface of the grains and possibly grain boundaries of the inorganic material constituting the porous matrix. Preferably, the texturizing material covers at least 15% of the total surface of the grains and possibly grain boundaries of the inorganic material constituting the porous matrix.

Typically, the average equivalent diameter d and / or the height h or the average depth p of the irregularities are smaller than the average grain size of the inorganic material constituting the matrix by a factor of between 1/2 and 1/1000.

For example, the average equivalent diameter d and / or the height h or the average depth p of the irregularities are smaller than the average grain size of the inorganic material constituting the matrix by a factor of between 1/5 and 1/100.

In one possible mode, the texturizing material is of the same nature as the inorganic material constituting the matrix.

According to a first embodiment, the irregularities are constituted by crystallites or by a mass of crystallites of a material cooked or sintered on the surface of the grains of the porous matrix.

In another embodiment, the irregularities consist essentially of alumina or silica beads.

Alternatively, the irregularities may also be in the form of craters dug in a material such as silica or alumina, said material being baked or sintered on the surface of the grains of the porous matrix. According to a preferred embodiment, the material constituting the matrix consists of or comprises silicon carbide.

The invention also relates to the intermediate structure for obtaining a catalytic filter for the treatment of solid particles and gaseous pollutants according to one of the preceding claims and comprising a porous matrix consisting of an inorganic material, under the form of grains connected to each other so as to form between them cavities such that the open porosity is between 30 and 60% and the median pore diameter of between 5 and 40 microns, said grains of the inorganic material being covered on at least part of their surface of a texturizing material according to one of the preceding claims.

The invention further relates to a method for obtaining a filter as previously described and comprising the following steps: shaping and baking a honeycomb structure consisting of a porous matrix of an inorganic material in the form of grains connected to each other so as to form between them cavities such that the open porosity is between 30 and 60% and the median pore diameter is between 5 and 40 microns,

- Deposition on the surface of at least a portion of the grains of the honeycomb structure of a texturizing material, for example in the form of beads, crystallites, polycrystalline clusters, hollows or craters ,

impregnation of the textured honeycomb structure with a solution comprising a catalyst or a precursor of a catalyst.

According to the method, the deposition of the texturizing material can be obtained by applying a slip of said covering material to the surface of the grains, followed by a heat treatment of baking or sintering, by the application of a sol-gel solution comprising a filler in the form of beads or inorganic particles, followed by a heat treatment for firing or sintering or by the application of a sol-gel solution comprising a filler in the form of beads or organic particles, followed by a heat treatment for baking or sintering.

The preceding sol-gel solution is, for example, a silica sol.

More specifically, the texturing method according to the invention is obtained either:

1 °) by depositing a suspension, such as for example a slip consisting of a powder and a mixture of powders, preferably in a liquid such as water, or a sol-gel loaded with mineral particles, or an organic or organo-mineral sol-gel, which, after a heat treatment, leads to a material of crystalline and / or glassy inorganic nature, preferably ceramic, and of a thermal stability at least equal to that of the alumina which is the main constituent washcoat. The deposition is followed by one or more heat treatment (s) of the substrate, preferably under air but possibly under a controlled atmosphere, for example under argon or under nitrogen, if this is necessary in particular to avoid deterioration or oxidation. substrate or deposit for example. It can also be envisaged to do this texturing on the raw or partially cooked substrate, provided that the mechanical strength and the integrity of the substrate are sufficient to perform the texturing operation and since the cooking conditions make it possible to obtain the texturing characteristics mentioned above. In the case of suspensions, in addition to the inorganic or preferably inorganic powders or their precursors, for example an organometallic compound (for example a silicon alkoxide such as TEOS and liquid), the formulation may contain additions from the following list: one or several dispersants (for example an acrylic resin or an amine derivative), a binder of organic nature (for example an acrylic resin or a cellulose derivative) or even of a mineral nature (clay), a wetting or film-forming agent (for example a polyvinyl alcohol PVA), one or more porogens (for example polymers, latex, polymethylmethacrylate). Some of these components can combine many of these functions. As the shape and size of the powders or precursors and the nature of the suspension liquid, the nature and quantity of these additions will impact the size of micro-texturations and their location on the substrate. The preferred texturing should be performed on the surface of the grains but also partly on the grain boundaries.

2 °) from a powder or a mixture of powders via a carrier gas. A direct deposit from liquid or gaseous species, for example by PVD ("physical vapor deposition" according to the English term) or CVD ("chemical vapor deposition" according to the English term) is also possible.

Other texturing methods may also be employed according to the invention such as heat treatment under gas (for example O ₂ , N ₂ in the case of an SiC-based substrate). Plasma etching processes or chemically can also provide, depending on the conditions of implementation and depending on the nature of the substrate, texturations according to the invention.

For the purposes of the present invention, a catalytic coating is defined as a coating comprising or consisting of a material known to catalyze the reaction of the transformation of gaseous pollutants, that is to say mainly carbon monoxide (CO) and unburnt hydrocarbons and oxides of nitrogen (NO _x ) to less harmful gases such as nitrogen gas (N ₂ ) or carbon dioxide carbon (CO2) and / or to facilitate the combustion of soot stored on the filter.

This coating, in a well-known manner, most often comprises an inorganic support material of high specific surface area (typically of the order of 10 to 100 m ² / g) ensuring the dispersion and the stabilization of an active phase, such as metals, generally noble, acting as a center of catalysis proper oxidation or reduction reactions. The support material is typically based on oxides, more particularly on alumina or silica, or other oxides, for example based on ceria, zirconia or titanium oxide, or even mixed mixtures of these different oxides. The size of the support material particles constituting the catalytic coating on which the catalytic metal particles are arranged is of the order of a few nanometers to a few tens or exceptionally a few hundred nanometers.

The catalytic coating is typically obtained by impregnating a solution comprising the catalyst, in the form of the support material or its precursors and an active phase or a precursor of the active phase. In general, the precursors used are in the form of salts or organic or inorganic compounds, dissolved or suspended in an aqueous or organic solution. The impregnation is followed by a heat treatment aimed at obtaining the final deposition of a solid and catalytically active phase in the porosity of the filter.

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 catalysts deposited, which most often contain as active phase platinum group precious metals (Pt, Pd, Rh) on an oxide support, represents a significant part of the overall cost of the impregnation process. For the sake of economy, it is therefore important that the catalyst is deposited in the most uniform manner possible, so as to be easily accessible by the gaseous reactants.

A filter according to the invention and as previously described can typically be used in an exhaust line of a diesel engine or gasoline.

The invention and its advantages will be better understood on reading the following non-limiting examples of embodiment of the present invention and provided exclusively for illustrative purposes.

Example 1 (comparative)

In this example, an SiC-based catalytic filter is typically synthesized.

Specifically mixed initially 70 wt% of an SiC powder whose grains have a median diameter d ₅ o of 10 microns, with a second SiC powder whose grains have a median diameter d ₅ o 0, 5 micron, in a first mode comparable to the powder mixture described in EP 1 142 619. For the purposes of the present description, the median pore diameter d ₅ o denotes the diameter of the particles such that respectively 50% of the total population of grain is smaller than this diameter. To this mixture is added a porogen of the polyethylene type in a proportion equal to 5% by weight of the total weight of the SiC grains and a methylcellulose type shaping additive in a proportion equal to 10% by weight of the total weight of the SiC grains. , as reported in Table 2.

The quantity of water required is then added and kneaded to obtain a homogeneous paste whose plasticity allows the extrusion through a die of a honeycomb structure so as to produce monolithic blocks characterized by a wave arrangement of the internal channels such as those described in relation with FIG. 3 of the application WO 05/016491 are obtained. According to a cross section, the undulation of the walls is characterized by an asymmetry rate, as defined in WO 05/016491, equal to 7%.

The dimensional characteristics of the structure after extrusion are given in Table 1:

Table 1

The green microwave monoliths are then dried for a time sufficient to bring the water content not chemically bound to less than 1% by weight.

The channels of each face of the monolith are alternately plugged according to well-known techniques, for example described in application WO2004 / 065088.

The monolith is then baked under Argon according to a rise in temperature of 20 ° C / hour until a maximum temperature of 2200 ⁰ C is reached which is maintained for 6 hours.

A crude SiC filtering structure is thus obtained. FIG. 1 shows a SEM (scanning electron microscope) photograph of the filtering walls of the filter thus obtained, constituted by a matrix of SiC grains of smooth surface and interconnected by grain boundaries, the porosity of the material being provided by the cavities formed between the grains.

Example 2 (according to the invention):

In this example, the raw structure obtained according to Example 1 was then subjected to a first texturing treatment, the material used for texturing being introduced into the porosity of the filter in the form of a slip. More specifically, an SiC-based slurry was used in the form of a slurry.

The suspension comprises, as a weight percentage, 96% of water, 0.1% of nonionic type dispersant, 1.0% of a PVA type binder (polyvinyl alcohol) and 2.8% of an SiC powder. with a median diameter of 0.5 μm and a purity greater than 98% by weight.

The slurry or suspension is prepared according to the following steps:

The PVA, used as a binder, is firstly dissolved in water heated to 80 ^° C. In a stirred tank containing the PVA dissolved in water, the dispersant and then the SiC powder are introduced. to obtain a homogeneous suspension.

The slip is deposited in the filter by simple immersion, the excess of the suspension being removed by suction under vacuum, under a residual pressure of 10 mbar.

The filter thus obtained is subjected to a drying step in

120 ° C for 16 hours and then a sintering heat treatment at 1700 ^° C. under argon for 3 hours.

FIG. 2 shows a SEM photograph of the filtering walls of the texture filter thus obtained, showing the irregularities at the surface of the SiC grains constituting the porous matrix, which, according to this example, are in the form of crystallites and SiC crystallite clusters. According to this embodiment, the parameter d measured corresponds to the mean diameter, in the sense previously described, of the crystallites present on the surface of the SiC grains. The parameter h corresponds to the average height h of said crystallites.

Example 3 (according to the invention):

In this example, the raw structure obtained according to Example 1 was subjected to another texturizing treatment, the material used for the texturing being introduced into the porosity of the filter in the form of a silica sol comprising an inorganic filler.

Specifically, a silica sol loaded with alumina particles was used.

The soil comprises, as a percentage by weight, 45.6% of water, 34.7% of an aqueous solution containing 10.5% by weight of alumina particles marketed by Nissan under the reference Chemical Aluminasol 200®, 1 , 7% of TEOS

(tetraethoxysilane), 17.0% 2-propanol and 1.0% of a 37% hydrochloric acid solution.

The soil loaded with inorganic particles is prepared according to the following steps:

In a first step TEOS is hydrolyzed in propanol-2 in the presence of the hydrochloric acid solution to form the sol. In a second step, the filler is added through the aqueous solution containing the alumina particles, the third step consisting of dilution in water. The charged sol-gel is then allowed to stand for 18 hours before the next step. After maturation, the solution is then deposited in the monolith by simple immersion, the excess being removed by suction under vacuum, under a residual pressure of 10 mbar. The monolith thus obtained is then dried at 150 ^° C. for 1 h and then subjected to a heat treatment of 250 ^° C. under air for one hour.

The texture monolith thus obtained shows irregularities on the surface of the SiC grains constituting the porous matrix, which, according to this example, are in the form of rods fixed on the surface of the SiC grains and / or at the grain boundaries. In the sense previously described, the irregularities have, on the surface of the grains, an average height h = 2 μm and a mean diameter d = 1 μm.

Example 4 (according to the invention):

In this example, the raw structure obtained according to Example 1 was subjected to another texturizing treatment, the material used for the texturing being introduced into the porosity of the filter in the form of a silica sol comprising an inorganic filler according to the same principles as those described in Example 2. Unlike Example 3, this time we used a silica sol loaded with silica microspheres.

The sol comprises, as a weight percentage, 45% of a colloidal aqueous solution of silica beads with a diameter of between 300 and 400 nm, in the form, the mass concentration of beads being approximately 40%, sold under the reference MP4540. Nyacol ®, 3.3% TEOS (tetraethoxysilane),

32.4% propanol-2 used for the preparation of the soil, 17.3% propanol-2 as a diluent and 2.0% of a 37% hydrochloric acid solution.

In a first step the TEOS is hydrolyzed in propanol-

2 in the presence of the hydrochloric acid solution to form the soil. In a second step, the filler is added via the aqueous colloidal solution containing the silica beads, the third step consisting of a dilution in propanol-2. The charged sol-gel is then allowed to stand for 18 hours before the next step. After maturation, the solution is then deposited in the monolith by simple immersion, the excess being removed by suction under vacuum, under a residual pressure of 10 mbar. The monolith thus obtained is then dried at 150 ^° C. for 1 h and then subjected to a heat treatment of 250 ^° C. under air for one hour.

FIG. 3 shows a SEM photograph of the filtering walls of the texture monolith thus obtained, showing the irregularities at the surface of the SiC grains constituting the porous matrix, which, according to this example, are in the form of silica beads encapsulated in an envelope obtained by the sintering the silica sol and establishing the junction and bond with the grains of SiC constituting the matrix.

The texturing according to this embodiment is formed of spherical balls contiguous or isolated, characterized by their mean diameter which corresponds, in the sense of the preceding definitions, to the values h and d according to the invention.

Example 5 (according to the invention):

In this example, the crude structure obtained according to Example 1 was subjected to another texturing treatment, the material used for the texturing being introduced into the porosity of the monolith in the form of a silica sol comprising an organic filler.

The soil comprises, as a weight percentage, 4% of polymethyl methacrylate beads with a diameter of approximately 2 μm, sold by the company SEPPIC under the reference Micropearl M-201®, 16.3% of TEOS (tetraethoxysilane), 72.3. % of ethanol and 7.4% of an aqueous solution containing 4.4% by weight of HCl. The soil loaded with inorganic particles is prepared according to the following steps:

The organic filler consisting of polymethylmethacrylate beads is first mixed with ethanol. The TEOS is then added gradually, with stirring. The aqueous solution containing the HCl is then gradually added and with vigorous stirring, in order to allow gradual and homogeneous hydrolysis of the TEOS and the obtaining of the gel.

The sol-gel is then deposited in the monolith by simple immersion, the excess being removed by suction under vacuum, under a residual pressure of 10 mbar.

The monolith thus obtained is then dried at 110 ^° C. for 16 hours and then subjected to a heat treatment of 550 ^° C. under air for five hours.

FIG. 4 shows a SEM photograph of the filtering walls of the texture monolith thus obtained, showing the irregularities at the surface of the SiC grains constituting the porous matrix. As can be seen in FIG. 4, the irregularities are, according to this example, this time in the form of cavities or craters present in the texturizing material consisting of SiO 2 silica, obtained by sintering the silica sol, after heat treatment and organic removal.

According to this embodiment, the parameter d measured corresponds to the average diameter, in the sense previously described, of the craters dug by the elimination of the organic spheres within the SiO 2 texturing layer on the surface of the SiC grains. The average depth p of said craters is equal to 2 μm.

Example 6 (according to the invention):

In this example, the crude structure obtained according to Example 1 was subjected to another texturizing treatment, the material used for texturing being introduced into the porosity of the monolith in the form of a silica sol. comprising an organic charge different from that of Example 5.

The sol comprises, in weight percentage, 2% of 120 nm diameter Latex beads, 16.3% TEOS (tetraethoxysilane) and 81.7% of an aqueous solution containing 0.38% by weight of HCl.

The soil loaded with inorganic particles is prepared by first mixing the latex beads with the aqueous HCl solution and then gradually adding the

TEOS with vigorous stirring to obtain a homogeneous hydrolysis of the silicate and to obtain a gel.

The monolith thus obtained is then dried at 110 ^° C. for

16h and then subjected to a heat treatment of 550 ⁰ C under air for five hours.

FIG. 5 shows a SEM photograph of the filtering walls of the texture monolith thus obtained, showing the irregularities covering the surface of the SiC grains constituting the porous matrix. As can be seen in FIG. 5, the irregularities are, according to this example, this time in the form of cavities or craters present within the texturizing material constituted by a SiO 2 silica coating, obtained by sintering the silica sol. , after the heat treatment and the elimination of the organic ones.

According to this embodiment, the parameter d measured corresponds to the average diameter, in the sense previously described, of the craters dug by the elimination of the organic spheres within the SiO 2 texturing layer on the surface of the SiC grains. The parameter p corresponds to the average depth p of said craters.

The properties of these microtextured monoliths according to Examples 2 to 6 according to the invention were measured and compared with those of the non-textured reference monolith of Example 1. Since the drying and the various heat treatments implemented during the texturing process have no effect on the structure of the reference monoliths, it is possible to directly compare the results of the measurements made on the monoliths according to the invention and those of the reference monolith. These properties were measured according to the following experimental protocols:

A- Mass capture during texturizing deposition after heat treatment.

The weight gain due to the deposition of the texturizing material was measured on each monolith after heat treatment and relative to the weight of the reference monolith.

B- Measurement of the porosity of the material constituting the matrix. The open porosity of the material constituting the walls of the monoliths according to Examples 1 to 6 was determined according to conventional high-pressure mercury porosimetry techniques, with a 9500 micromeritics porosimeter.

C- Measurement of the geometrical characteristics of the irregularities of the texturing coating:

The parameters d, h or p as previously defined, characterizing the irregularities present on the surface of grains of SiC, were measured on a series of observations under a scanning electron microscope, on a series of images representative of the deposit produced and at different points of the monolith.

These images, from which FIGS. 1 to 5 are extracted, correspond to characteristic views of the internal structure, in particular open porosity, transversely fractured channel walls, within the monolith. Other SEM observations, made on a series of photographs at different points of the monolith, also make it possible to measure the surface covered by the texturizing material, relative to the total surface area of the grains and grain boundaries of the inorganic material constituting the porous matrix.

D- Measurement of the quantity of catalytic coating (washcoat) after impregnation:

The monoliths according to the invention (Examples 2 to 6) and the reference monolith (Example 1) were subjected to an impregnation treatment of a catalytic solution representative of the solutions currently used, according to the following experimental protocol:

The monolith is immersed in a bath of an aqueous solution containing the appropriate proportions of a platinum precursor in the form H ₂ PtCl 6, and a precursor of cerium oxide CeO ₂ (in the form of cerium nitrate) and a precursor of zirconium oxide ZrO ₂ (in the form of zirconyl nitrate) according to the principles described in the publication EP 1 338 322 A1. The monolith is impregnated with the solution according to an embodiment similar to that described in US Pat. No. 5,866,210. The monolith is then dried at about 150 ⁰ C and then heated to a temperature of about 500 ⁰ C.

E- Measurement of the pressure loss:

The pressure drop of the monoliths obtained after the catalytic impregnation previously described (see point D above), was measured according to the techniques of the art, for an air flow rate of 30 m ³ / h in a stream of air ambient. By pressure loss is meant within the meaning of the present invention the differential pressure existing between the upstream and downstream of the monolith.

F- Test of catalytic efficiency called "light off" This test aims to measure the catalyst initiation temperature, often referred to in the art as the "light off" temperature of the catalyst. This temperature is defined under constant pressure and gas flow conditions, such as the temperature for which a catalyst converts 50% by volume of the polluting gases. The CO and HC conversion temperature has here been determined according to an experimental protocol identical to that described in application EP 1759763, in particular in its paragraphs 33 and 34. According to the measurement, the lower the conversion temperature, the more the catalytic system is powerful.

The test was carried out on samples of about 25 cm ³ cut into a monolith.

G- Catalytic efficiency test called "light off", after aging

A non-microtextured fired monolith and a textured monolith according to each example of the invention are previously impregnated with catalyst as described in paragraph D and then placed in an oven at 800 ^° C. under a humid air atmosphere for a period of 5 hours such that the molar concentration of water is kept constant at 3%.

On each sample of monolith thus aged, the conversion rate of CO at 420 ^° C. and the light-off temperature of the HCs are measured according to the same experimental protocol as that described in the previous section F. The HC light off temperature increase is calculated as the difference between the light off temperature of the HC on the aged sample and that measured on the unaged sample. According to these tests, the lower the temperature of light off on aged sample or the increase in light-off temperature due to aging, the higher the aging resistance of the catalytic system. The higher the conversion rate after aging, the better the catalytic system. The main results obtained for the various measures A to F which preceded were grouped in table 2:

Table 2

The monoliths of Examples 23 and 5 show a catalytic coating load level substantially greater than that of the reference (example 1), for equivalent porosity characteristics. It is noted that the loss of charge caused by the monoliths according to the invention is also very little affected by the significant increase in the catalytic charge present in the textured filters according to the invention. The measured pressure drop values thus remain quite acceptable for the filtering application. All the monoliths of the invention show a catalytic activity more efficient than the reference.

Those of Examples 4 and 6 show a much higher catalytic efficiency despite a load significantly lower than the reference (Example 1), which could be interpreted as the result of a better distribution of the catalyst or easier access to sites assets for the gases to be purified.

The monolith of Example 2 shows a high load in wash coat and a high catalytic efficiency despite a low percentage of microtextured surface, which shows a very significant effect of microtexturation, even if it is present only on a minimal part of the grain surface.

All the products of the invention show, after aging, a higher catalytic performance than the reference. In particular, Examples 4 and 6 show the best resistance to aging despite the lowest wash-coat loads. Example 2 shows a lower light off temperature increase of the HCs.

In addition, the products according to the invention retain all their mechanical strength properties, while maintaining their filtration efficiency, unlike the solutions known to date for increasing the catalyst load present in the porosity of the filtering structures, in particular by by increasing the porosity quantities (open porosity, pore diameter).

Claims

Catalytic filter for the treatment of solid particles and gaseous pollutants from the combustion gases of an internal combustion engine, comprising a porous matrix consisting of an inorganic material, in the form of grains connected to each other so as to spare between them cavities such that the open porosity is between 30 and 60% and the median pore diameter between 5 and 40 microns, said filter being characterized in that:

the grains and optionally the grain boundaries of the inorganic material are covered on at least a part of their surface with a texturizing material, said texturing consisting of irregularities whose dimensions are between 10 nm and 5 microns, a catalytic coating; at least partially covers the texturizing material and optionally, at least partially, the grains of the inorganic material.

A filter according to claim 1 wherein said texturing consisting of irregularities, for example in the form of beads, crystallites, polycrystalline clusters, even rods or acicular structures, depressions or craters, said irregularities having a diameter mean equivalent d ranging from about 10 nm to about 5 microns and average height or depth p from about 10 nm to about 5 microns.

3. The filter of claim 1 or 2, wherein the average diameter of irregularities is between 100 nm and 2.5 microns.

4. Filter according to one of the preceding claims, wherein the height h or the average depth p of the irregularities is between 100 nm and 2.5 microns.

5. Filter according to one of the preceding claims, wherein the texturizing material covers at least 10%, and preferably at least 15%, of the total surface of the grains and possibly grain boundaries of the inorganic material constituting the porous matrix. .

6. Filter according to one of the preceding claims, wherein the average equivalent diameter d and / or the height h or the average depth p irregularities are less than the average grain size of the inorganic material constituting the matrix of a factor included between 1/2 and 1/1000.

7. Filter according to one of the preceding claims, wherein the average equivalent diameter d and / or the height h or the average depth p of the irregularities are less than the average grain size of the inorganic material constituting the matrix of a factor included between 1/5 and 1/100.

8. Filter according to one of the preceding claims, wherein the texturing material is of the same nature as the inorganic material constituting the matrix.

9. Filter according to one of the preceding claims, wherein the irregularities are constituted by crystallites or by a cluster of crystallites of a material cooked or sintered on the surface of the grains of the porous matrix.

10. Filter according to one of the preceding claims, wherein the irregularities consist essentially of alumina or silica beads.

11. Filter according to one of the preceding claims, wherein the irregularities are in the form of craters dug in a material such as silica or alumina, said material being baked or sintered on the surface of the grains of the porous matrix. .

12. Filter according to one of the preceding claims, wherein the material constituting the matrix is constituted by or comprises silicon carbide.

13. Intermediate structure for obtaining a catalytic filter for the treatment of solid particles and gaseous pollutants according to one of the preceding claims, comprising a porous matrix consisting of an inorganic material, in the form of grains connected together. to the others so as to provide between them cavities such that the open porosity is between 30 and 60% and the median pore diameter of between 5 and 40 microns, said grains of the inorganic material being covered on at least a portion of their surface of a texturizing material according to one of the preceding claims.

14. Process for obtaining a filter according to one of claims 1 to 12 comprising the following steps: shaping and baking a honeycomb structure constituted by a porous matrix of an inorganic material, in the form of grains connected to each other so as to form between them cavities such that the open porosity is between 30 and 60% and the median pore diameter is between 5 and 40 μm,

The method of claim 14, wherein depositing the texturizing material is obtained by applying a slip of said coating material to the surface of the grains, followed by a baking or sintering heat treatment.

16. The method of claim 14, wherein the deposition of the texturizing material is obtained by the application of a sol-gel solution comprising a filler in the form of beads or inorganic particles, followed by a baking heat treatment. or sintering.

17. The method of claim 14, wherein the deposition of the texturizing material is obtained by the application of a sol-gel solution comprising a filler in the form of beads or organic particles, followed by a baking heat treatment. or sintering.

18. The method of claim 16 or 17, wherein the sol-gel solution is a silica sol.

19. Use of a filter according to one of the preceding claims in an exhaust line of a diesel engine or gasoline.