Honeycomb filter

Abstract

To provide a honeycomb filter capable of collecting a particulate material in exhaust gas by a collection layer, and oxidizing and burning the collected particulate material at a lower temperature.SOLUTION: A honeycomb filter includes a honeycomb structure 4 having a porous partition wall 1, and a plugging part 5. The honeycomb structure 4 further has a collection layer 14 on the inner surface side of the partition wall 1 enclosing an inflow cell 2a, and the collection layer 14 includes a part constituted of a sintered body of CeOparticles at least on a surface layer of the collection layer 14, and an average particle diameter of the CeOparticles constituting the collection layer 14 is 1.1 μm or less.SELECTED DRAWING: Figure 4

Description

Technical Field

[0001] The present invention relates to a honeycomb filter. More specifically, the present invention relates to a honeycomb filter capable of collecting particulate matter in exhaust gas by a collection layer and oxidatively burning the collected particulate matter at a lower temperature.

Background Art

[0002] In recent years, regulations regarding the removal of particulate matter contained in exhaust gas discharged from gasoline engines have become stricter worldwide, and honeycomb filters having a honeycomb structure are used as filters for removing particulate matter. Hereinafter, particulate matter may be referred to as "PM". PM is an abbreviation for "Particulate Matter".

[0003]

[0004] Conventionally, as a technique for improving the collection performance of a honeycomb filter, techniques such as thickening the partition walls of the honeycomb structure and reducing the size of the pores formed in the partition walls have been proposed. However, when the collection performance is improved by the above-described techniques, there is a problem that PM (for example, soot) is likely to clog the pores formed in the partition walls, and the pressure loss of the honeycomb filter increases. That is, since the above-described techniques have an antinomic relationship between the effect of improving the collection performance and the effect of suppressing the increase in pressure loss, it has been difficult to say that they are effective solutions.

[0005] ​For this reason, honeycomb filters have been proposed in which a collection layer for collecting PM is disposed on the surface of the partition walls of the honeycomb structure (see, for example, Patent Documents 1 to 4). For example, the collection layer is composed of a porous membrane having an average pore diameter smaller than the average pore diameter of the partition walls. According to such a honeycomb filter, since PM can be deposited on the surface of the collection layer, it is possible to improve the collection efficiency when collecting PM while suppressing a sharp increase in pressure loss due to clogging of PM in the pores of the partition walls.

Prior Art Documents

Patent Documents

[0006]

Patent Document 1

Patent Document 2

Patent Document 3

Patent Document 4

Summary of the Invention

Problems to be Solved by the Invention

[0007] As described above, although the honeycomb filter having a collection layer on the surface of the partition wall can prevent PM from being easily clogged in the pores of the partition wall and suppress a sharp increase in pressure loss, there is a problem that another problem as described below occurs and thus countermeasures are required. That is, in the honeycomb filter provided with the collection layer, a large amount of soot as PM accumulates on the collection layer, and thus it is necessary to suppress an increase in pressure loss caused by such a soot accumulation layer. In particular, in a usage environment where soot is difficult to burn, such as in a cold region, it is necessary to frequently perform a regeneration operation of the filter by backwashing or forced heating of soot. Further, instead of frequently performing the regeneration operation, a technique of supporting an oxidation catalyst on the collection layer and burning and removing the soot accumulation layer deposited on the collection layer by a catalytic reaction has also been studied. Regarding the technique of burning and removing the soot accumulation layer deposited on the collection layer by a catalytic reaction, since a reduction in the oxidation combustion start temperature of soot greatly contributes to the effect of suppressing an increase in pressure loss, there is a strong demand for the development of a technique capable of oxidizing and burning soot at a lower temperature.

[0008] The present invention has been made in view of such problems of the prior art. According to the present invention, there is provided a honeycomb filter capable of collecting particulate matter in exhaust gas by a collection layer and oxidizing and burning the collected particulate matter at a lower temperature.

Means for Solving the Problems

[0009] According to the present invention, there is provided a honeycomb filter as described below.

[0010] [1] A honeycomb structure having a porous partition wall disposed so as to surround a plurality of cells serving as fluid flow paths extending from an inflow end face to an outflow end face, and a plugging portion disposed so as to seal one end of either the inflow end face side or the outflow end face side of the cell, wherein the cell having the plugging portion disposed at the end on the outflow end face side and the inflow end face side being open is defined as an inflow cell, and the cell having the plugging portion disposed at the end on the inflow end face side and the outflow end face side being open is defined as an outflow cell. The honeycomb structure further has a collection layer for collecting particulate matter in the exhaust gas on the inner surface side of the partition wall surrounding the inflow cell. The collection layer includes a portion formed of a sintered body of CeO2 particles at least on the surface layer of the collection layer. The average particle diameter of the CeO2 particles constituting the collection layer is 0.7~ 1.1 μ in m There is a honeycomb filter, The entire collection layer includes a sintered body of CeO2 particles, and the honeycomb filter in which the content of CeO2 contained in the collection layer exceeds 70% by mass.

[0011] [2] The honeycomb filter according to [1], wherein the average pore diameter of the collection layer is smaller than the average pore diameter of the partition wall.

[0012] [3] The honeycomb filter according to [1] or [2], wherein the partition wall is made of cordierite.

[0013] [4] The honeycomb filter according to any one of [1] to [3], wherein the average pore diameter of the partition wall is 6 to 24 μm.

[0014] [5] The honeycomb filter according to any one of [1] to [4], wherein the porosity of the partition wall is 45 to 66%.

[0015] [6] The honeycomb filter according to any one of [1] to [5], wherein the thickness of the partition wall is 0.10 to 0.35 mm.

[0016] [7] The honeycomb filter according to any one of [1] to [6], wherein the thickness of the collection layer is 20 to 50 μm.

Advantages of the Invention

[0017] The honeycomb filter of the present invention can collect particulate matter (PM) in exhaust gas by a collection layer and oxidatively combust the collected PM at a lower temperature. That is, it includes a site composed of a sintered body of CeO2 particles at least on the surface layer of the collection layer, and the average particle diameter of the CeO2 particles constituting this collection layer is 0.7~ 1.1 μ in m such that the CeO2 particles exhibit catalytic activity as an oxidation catalyst at a lower temperature. For this reason, the PM collected by the collection layer can be oxidatively combusted at a lower temperature. The honeycomb filter of the present invention does not need to frequently perform filter regeneration work even in a usage environment where PM (especially soot) is difficult to burn, such as in cold regions. Further, the honeycomb filter of the present invention can satisfactorily oxidatively combust the PM collected by the collection layer without further supporting an oxidation catalyst on the collection layer.

Brief Description of the Drawings

[0018]

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

Mode for Carrying Out the Invention

[0019] Hereinafter, embodiments of the present invention will be described, but the present invention is not limited to the following embodiments. Therefore, it should be understood that within the scope not departing from the gist of the present invention, those obtained by appropriately changing and improving the following embodiments based on the ordinary knowledge of those skilled in the art also fall within the scope of the present invention.

[0020] (1) Honeycomb filter: An embodiment of the honeycomb filter of the present invention is a honeycomb filter 100 as shown in FIGS. 1 to 4. Here, FIG. 1 is a perspective view schematically showing one embodiment of the honeycomb filter of the present invention. FIG. 2 is a plan view of the inflow end face side of the honeycomb filter shown in FIG. 1. FIG. 3 is a plan view of the outflow end face side of the honeycomb filter shown in FIG. 1. FIG. 4 is a cross-sectional view schematically showing the A-A' cross-section of FIG. 2.

[0021] As shown in FIGS. 1 to 4, the honeycomb filter 100 of the present embodiment includes a honeycomb structure 4 and an end sealing portion 5. The honeycomb structure 4 has a porous partition wall 1 disposed so as to surround a plurality of cells 2 that serve as fluid flow paths extending from the inflow end face 11 to the outflow end face 12. The honeycomb structure 4 shown in FIGS. 1 to 4 is configured in a cylindrical shape having the inflow end face 11 and the outflow end face 12 as both end faces, and further has an outer peripheral wall 3 on its outer peripheral side surface. That is, the outer peripheral wall 3 is disposed so as to surround the partition walls 1 arranged in a lattice pattern.

[0022] The end sealing portion 5 is arranged so as to seal one end of the cell 2 on either the inflow end face 11 side or the outflow end face 12 side. Hereinafter, among the plurality of cells 2, the cell 2 in which the end sealing portion 5 is disposed at the end on the outflow end face 12 side and the inflow end face 11 side is open is referred to as an "inflow cell 2a". Also, among the plurality of cells 2, the cell 2 in which the end sealing portion 5 is disposed at the end on the inflow end face 11 side and the outflow end face 12 side is open is referred to as an "outflow cell 2b". In the honeycomb filter 100 of the present embodiment, it is preferable that the inflow cells 2a and the outflow cells 2b are alternately arranged with the partition wall 1 interposed therebetween.

[0023] The honeycomb filter 100 is characterized in that the honeycomb structure 4 is configured as follows. That is, as shown in FIG. 5, the honeycomb structure 4 further has a collection layer 14 for collecting particulate matter (hereinafter also referred to as "PM") in the exhaust gas on the inner surface side of the partition wall 1 surrounding the inflow cell 2a. And the collection layer 14 includes a site composed of a sintered body of CeO2 particles at least on the surface layer of the collection layer 14. That is, the collection layer 14 is a porous membrane including a site composed of a sintered body of CeO2 particles. And the average particle diameter of the CeO2 particles constituting the collection layer 14 is 0.7~ 1.1 μ in m There is. Here, FIG. 5 is a cross-sectional view schematically showing a cross-section of the partition wall. In FIG. 5, reference numeral 7 indicates pores formed in the partition wall 1.

[0024] The surface layer of the collection layer 14 means a site including a range of 10 μm in the thickness direction from the surface side of the collection layer 14. Therefore, the honeycomb filter 100 of the present embodiment includes a site composed of a sintered body of CeO2 particles in at least a range of 10 μm in the thickness direction from the surface side of the collection layer 14. Note that the honeycomb filter 100 of the present embodiment is configured such that the entire collection layer 14 includes a sintered body of CeO2 particles to . And when the entire collection layer 14 includes a sintered body of CeO2 particles, the content of CeO2 contained in the collection layer 14 exceeds 70% by mass to. The collection layer 14 in which the entire collection layer 14 includes a sintered body of CeO2 particles and the content of CeO2 contained in the collection layer 14 is 70% by mass may be hereinafter referred to as "the collection layer 14 composed of CeO2 particles".

[0025] The average particle diameter of the CeO2 particles constituting the collection layer 14 is 0.7~ 1.1 μ in mDue to this, the CeO2 particles exhibit catalytic activity as an oxidation catalyst at a lower temperature. Therefore, the PM collected by the collection layer 14 can be oxidatively combusted at a lower temperature. The honeycomb filter 100 of the present embodiment does not need to frequently perform filter regeneration work even in a usage environment where PM (especially soot) is difficult to burn, such as in cold regions. Further, the honeycomb filter of the present embodiment can satisfactorily oxidatively combust the PM collected by the collection layer 14 without further supporting an oxidation catalyst on the collection layer 14. Note that when the average particle diameter of the CeO2 particles constituting the collection layer 14 exceeds 1.1 μm, the temperature at which the CeO2 particles exhibit catalytic activity becomes high, and it becomes difficult to oxidatively combust PM at a low temperature.

[0026] In the honeycomb filter 100 of the present embodiment, the average particle diameter of the CeO2 particles constituting the collection layer 14 can be measured as follows. First, a part of the partition wall 1 and the collection layer 14 is cut out as a test piece from the honeycomb structure 4 constituting the honeycomb filter 100, and the cut-out test piece is embedded in resin. Next, the resin-embedded test piece is cut in a direction orthogonal to the extending direction of the cell 2, and the cut surface is polished. Next, the polished cut surface is imaged using a scanning electron microscope (hereinafter also referred to as "SEM") to obtain an SEM image at a magnification of 200 times. "SEM" is an abbreviation of "Scanning Electron Microscope". The SEM image is an image where 1 pixel is 0.261 μm in the vertical direction × 0.261 μm in the horizontal direction. As the scanning electron microscope, for example, a scanning electron microscope "model number: S3400-N" manufactured by Hitachi High-Technologies Corporation can be used. In the measurement of the average particle diameter of the CeO2 particles, one test piece is prepared from a range of 20 mm × 20 mm × 20 mm including the central position in the extending direction of the cell 2 of the honeycomb filter 100 and the central part farthest from the outer peripheral wall 3 of the honeycomb filter 100. The size of the test piece to be prepared is 6 mm × 6 mm × 6 mm. An SEM image is obtained for the test piece prepared in this way.

[0027] Next, image processing is performed on the collection layer 14 in the obtained SEM image, and the particle size of the CeO2 particles constituting the collection layer 14 is measured. Specifically, first, from the SEM image of the collection layer 14, at an arbitrary position where the mass ratio of CeO2 is 70% by mass or more, a region of 1 μm in the thickness direction × 100 μm in the horizontal direction of the collection layer 14 is surrounded. At this time, in the SEM image, the collection layer 14 is positioned horizontally with respect to the partition wall 1, and the above region is specified so as to be parallel to the horizontal line. The region specified in the SEM image in this way is hereinafter referred to as the "specified region". The specified region in this SEM image is binarized using "Image-Pro 9.3.2 (product name)" of Nippon Loper Co., Ltd. By this binarization process, the collection layer 14 in the specified region in the SEM image is separated into the solid part as a sintered body of CeO2 particles and the voids between the CeO2 particles. The binarization process is performed with 1 pixel as the minimum unit as described above. Next, the particle size of all the CeO2 particles in the specified region is measured by dividing the area of each part recognized as the solid part as a sintered body of CeO2 particles by a width of 1 μm. The average value of the measured particle sizes of the CeO2 particles is calculated, and this is done in two regions, and the calculated average value is taken as the average particle size of the CeO2 particles constituting the collection layer 14.

[0028] Also, that the particles constituting the collection layer 14 are CeO2 particles can be confirmed by the following qualitative analysis. EDS measurement is also performed during SEM imaging, and the total mass ratio of Ce element and O element is measured. If the above mass ratio is 90% by mass or more, it is regarded as CeO2 particles. Note that EDS is an abbreviation for Energy Dispersive x-ray Spectroscopy.

[0029] The collection layer 14 is preferably disposed only on the inner surface of the partition wall 1 surrounding the inflow cell 2a. If the collection layer 14 is disposed not only on the inner surface of the partition wall 1 surrounding the inflow cell 2a, the pressure loss of the honeycomb filter 100 may increase.

[0030] It is preferable that the average pore diameter of the collection layer 14 is smaller than the average pore diameter of the partition wall 1. By configuring in this way, the PM contained in the exhaust gas can be satisfactorily collected by the collection layer 14 disposed on the inner surface side of the partition wall 1 surrounding the inflow cell 2a.

[0031] The average pore diameter of the collection layer 14 is preferably 0.5 to 15 μm, more preferably 0.5 to 8 μm, and particularly preferably 0.5 to 1 μm.

[0032] The porosity of the collection layer 14 is preferably 50 to 90%, more preferably 70 to 90%, and particularly preferably 80 to 90%. If the porosity of the collection layer 14 is less than 50%, the pressure loss may increase. On the other hand, if the porosity of the collection layer 14 exceeds 90%, the collection efficiency may deteriorate.

[0033] The porosity and the average pore diameter of the collection layer 14 can be measured by the following method. First, the cross-sectional portion of the collection layer 14 is observed with a scanning electron microscope to obtain its SEM image. The SEM image shall be observed at a magnification of 200 times. Next, by image-analyzing the obtained SEM image, the solid portion of the collection layer 14 and the void portion in the collection layer 14 are binarized. Then, the percentage of the ratio of the void portion in the collection layer 14 to the total area of the solid portion and the void portion of the collection layer 14 is calculated, and this value is taken as the porosity of the collection layer 14. Separately, the voids of each particle between children in the SEM image are binarized, and their sizes are directly measured with a scale, and the pore diameter in the collection layer 14 is calculated from the measured values. The average value of the calculated pore diameters is taken as the average pore diameter of the collection layer 14.

[0034] The thickness of the collection layer 14 is preferably 20 to 50 μm, more preferably 20 to 40 μm, and particularly preferably 20 to 30 μm. If the thickness of the collection layer 14 is less than 20 μm, it is not preferable in that the improvement in collection efficiency may be low. On the other hand, if the thickness of the collection layer 14 exceeds 50 μm, it is not preferable in that the improvement in collection efficiency may plateau and the pressure loss may increase.

[0035] The thickness of the collection layer 14 can be measured by the following method. First, six intersections as follows are determined from a cross-section parallel to the partition wall 1 passing through the central axis in the extending direction of the cell 2 of the honeycomb filter 100. The six intersections are the six intersections where three straight lines that divide the cross-section into four equal parts in the extending direction of the cell 2 and two straight lines that divide the cross-section into three equal parts in a direction perpendicular to the extending direction of the cell 2 intersect. Then, a test piece including a region of 20 mm (longitudinal) × 20 mm (lateral) parallel to the cross-section is cut out with each intersection as the center. The thickness of the test piece (i.e., the depth parallel to the cross-section) can be arbitrarily determined. One arbitrary set of adjacent inflow cell 2a and outflow cell 2b is selected from the test piece, and the average value of the surface height of each cell 2 (specifically, the surface height in the direction perpendicular to the partition wall 1 of each cell 2) is measured within a range of about 8 mm in the extending direction of the cell 2 by a 3D shape measuring machine. Subsequently, the difference in the surface height between the inflow cell 2a and the outflow cell 2b is calculated, and this is taken as the thickness of the collection layer 14.

[0036] Since the soot contained in the exhaust gas is collected on the surface of the collection layer 14, it is preferable that the collection layer 14 has an oxidation catalyst function at least on its outermost surface. Note that the collection layer 14 may be the collection layer 14 composed of CeO2 particles as described above.

[0037] The average pore diameter of the partition wall 1 is preferably 6 to 24 μm, more preferably 9 to 24 μm, and particularly preferably 16 to 24 μm. The average pore diameter of the partition wall 1 is a value measured by the mercury intrusion method. The average pore diameter of the partition wall 1 can be measured, for example, using Autopore 9500 (trade name) manufactured by Micromeritics. If the average pore diameter of the partition wall 1 is less than 6 μm, the permeation resistance of the partition wall 1 increases, and the pressure loss may increase, which is not preferable. If the average pore diameter of the partition wall 1 exceeds 24 μm, it is not preferable in terms of the formability during the film formation of the collection layer 14.

[0038] The porosity of the partition wall 1 of the honeycomb structure 4 is preferably 45 to 66%, more preferably 52 to 66%, and particularly preferably 60 to 66%. The porosity of the partition wall 1 is a value measured by the mercury intrusion method. The porosity of the partition wall 1 can be measured, for example, using Autopore 9500 (trade name) manufactured by Micromeritics. If the porosity of the partition wall 1 is less than 45%, the permeation resistance of the partition wall 1 increases, and the pressure loss increases, which is not preferable. If the porosity of the partition wall 1 exceeds 66%, the strength may be significantly reduced, which is not preferable.

[0039] For the honeycomb structure 4, the thickness of the partition wall 1 is preferably 0.10 to 0.35 mm, more preferably 0.10 to 0.24 mm, and particularly preferably 0.10 to 0.18 mm. The thickness of the partition wall 1 can be measured, for example, using a Profile Projector. If the thickness of the partition wall 1 is less than 0.10 mm, sufficient strength may not be obtained. On the other hand, if the thickness of the partition wall 1 exceeds 0.35 mm, the pressure loss may increase when the collection layer 14 is disposed on the surface of the partition wall 1.

[0040] There is no particular limitation on the shape of the cells 2 formed in the honeycomb structure 4. For example, in a cross-section orthogonal to the extending direction of the cells 2, examples of the shape of the cells 2 include polygons, circles, ellipses, etc. Examples of polygons include triangles, quadrilaterals, pentagons, hexagons, octagons, etc. Note that the shape of the cells 2 is preferably a triangle, quadrilateral, pentagon, hexagon, or octagon. Also, regarding the shape of the cells 2, the shapes of all the cells 2 may be the same or different. For example, although not shown, a mixture of quadrilateral cells and octagonal cells may be possible. Also, regarding the size of the cells 2, the sizes of all the cells 2 may be the same or different. For example, although not shown, among a plurality of cells, the size of some cells may be increased and the size of other cells may be relatively decreased. In the present invention, the cell 2 means a space surrounded by the partition wall 1.

[0041] The cell density of the cells 2 partitioned by the partition wall 1 is preferably 30 to 60 cells / cm 2 and more preferably 30 to 50 cells / cm 2 By configuring in this way, it can be suitably used as a filter for collecting PM in the exhaust gas discharged from an engine of an automobile or the like.

[0042] The outer peripheral wall 3 of the honeycomb structure 4 may be integrally formed with the partition wall 1, or may be an outer peripheral coat layer formed by coating an outer peripheral coating material so as to surround the partition wall 1. Although not shown, the outer peripheral coat layer can be provided on the outer peripheral side of the partition wall after removing the formed outer peripheral wall by a known method such as grinding after integrally forming the partition wall and the outer peripheral wall during manufacturing.

[0043] There is no particular limitation on the shape of the honeycomb structure 4. Examples of the shape of the honeycomb structure 4 include columnar shapes such as circular, elliptical, and polygonal shapes for the inflow end face 11 and the outflow end face 12.

[0044] The size of the honeycomb structure 4, for example, the length in the extending direction of the cells 2 of the honeycomb structure 4 (hereinafter also referred to as "total length"), and the size of the cross-section orthogonal to the extending direction of the cells 2 of the honeycomb structure 4 (hereinafter also referred to as "cross-sectional area") are not particularly limited. Each size may be appropriately selected so as to obtain optimal purification performance when the honeycomb filter 100 is used. The total length of the honeycomb structure 4 is preferably 90 to 160 mm, and more preferably 120 to 140 mm. Further, the cross-sectional area of the honeycomb structure 4 is preferably 8000 to 16000 mm 2 and more preferably 10000 to 14000 mm 2 .

[0045] It is preferable that the material of the partition wall 1 includes at least one selected from the group consisting of cordierite, silicon carbide, silicon-silicon carbide composite material, mullite, alumina, aluminum titanate, silicon nitride, and silicon carbide-cordierite composite material. The material constituting the partition wall 1 is preferably a material containing 30% by mass or more of the materials listed in the above group, more preferably a material containing 40% by mass or more, and particularly preferably a material containing 50% by mass or more. In the honeycomb filter 100 of the present embodiment, cordierite is particularly preferable as the material constituting the partition wall 1.

[0046] (2) Method for manufacturing a honeycomb filter: There is no particular limitation on the method for manufacturing the honeycomb filter of the present invention, and for example, the following methods can be mentioned.

[0047] First, prepare a plastic clay for manufacturing the partition walls of the honeycomb structure. The clay for manufacturing the partition walls of the honeycomb structure can be prepared by appropriately adding additives such as binders, pore formers, and water to the raw material powder for manufacturing the suitable materials for the aforementioned partition walls. As the raw material powder, for example, powders of alumina, talc, kaolin, and silica can be used. Examples of the binder include methylcellulose and hydroxypropyl methylcellulose. In addition, examples of the additive include surfactants.

[0048] Next, by extruding the clay thus obtained, produce a columnar honeycomb formed body having partition walls that partition and form a plurality of cells and an outer peripheral wall disposed so as to surround the partition walls. Next, dry the obtained honeycomb formed body, for example, with microwaves and hot air.

[0049] Next, form a plugging portion on the dried honeycomb formed body. The method for forming the plugging portion can be carried out in accordance with the conventionally known method for manufacturing a honeycomb filter. For example, first, apply a mask to the inflow end face of the honeycomb formed body so that the inflow cells are covered. Then, brush the plugging slurry onto the end portion of the honeycomb formed body with the mask applied, and fill the openings of the outflow cells not covered with the mask with the plugging slurry. Then, also for the outflow end face of the honeycomb formed body, fill the openings of the inflow cells with the plugging slurry in the same manner as above. Then, further dry the honeycomb formed body with the plugging portion formed in a hot air dryer.

[0050] Next, by firing the honeycomb formed body with the plugging portion formed, produce a honeycomb filter precursor before the collection layer is disposed. Note that the firing temperature and firing atmosphere when firing the honeycomb formed body vary depending on the raw materials for manufacturing the honeycomb formed body, and those skilled in the art can select the firing temperature and firing atmosphere optimal for the selected materials.

[0051] Next, CeO2 particles for forming the collection layer are prepared. As the CeO2 particles, for example, CeO2 particles with an average particle diameter of 0.2 to 1.1 μm can be preferably used. Then, the prepared CeO2 particles are made into a slurry by adding water, a dispersant, a pore-forming agent, a flocculant, and a viscosity modifier, and the size is adjusted so that the aggregated particles containing CeO2 do not enter the pores of the honeycomb filter precursor. Such a slurry is supplied from the lower side in the vertical direction of the honeycomb filter precursor set in the jig of the film-forming apparatus, and the permeate is allowed to flow from the upper side. After flowing a predetermined amount of the slurry, the honeycomb filter precursor is turned over together with the jig, and the jig is removed from the honeycomb filter precursor. Then, the honeycomb filter precursor in a state where the aggregated particles in the slurry are arranged on the surface of the partition wall is dried and fired.

[0052] In this way, a collection layer composed of CeO2 particles is formed on the inner surface side of the partition wall surrounding the inflow cells of the honeycomb filter precursor. As described above, the honeycomb filter of the present invention can be manufactured.

Example

[0053] Hereinafter, the present invention will be described more specifically by way of examples, but the present invention is not limited to these examples in any way.

[0054] (Example 1) First, alumina, talc, kaolin, and silica raw materials for forming the partition walls of the honeycomb structure were prepared. To the prepared alumina, talc, kaolin, and silica raw materials, 2 parts by mass of a dispersion medium and 7 parts by mass of an organic binder were added respectively, and they were mixed and kneaded to prepare a clay. Water was used as the dispersion medium. Methyl cellulose was used as the organic binder. A surfactant was used as the dispersant.

[0055] Next, the clay was extruded using a die for producing a honeycomb molded body to obtain a honeycomb molded body having a cylindrical overall shape. The shape of the cells of the honeycomb molded body was square.

[0056] Next, the honeycomb formed body was dried with a microwave dryer and further completely dried with a hot air dryer. After that, both end faces of the honeycomb formed body were cut and adjusted to a predetermined dimension.

[0057] Next, a sealing portion was formed on the dried honeycomb formed body. Specifically, first, a mask was applied to the inflow end face of the honeycomb formed body so that the inflow cells were covered. Then, a sealing slurry was brushed onto the end portion of the honeycomb formed body with the mask applied, and the opening of the outflow cell not covered with the mask was filled with the sealing slurry. Then, regarding the outflow end face of the honeycomb formed body as well, the opening of the inflow cell was filled with the sealing slurry in the same manner as above. Then, the honeycomb formed body with the sealing portion formed was further dried with a hot air dryer.

[0058] Next, the dried honeycomb formed body was degreased and fired to produce a honeycomb filter precursor before the collection layer was disposed.

[0059] Next, a collection layer was formed on the inner surface side of the partition wall surrounding the inflow cells of the honeycomb filter precursor by the following method. Specifically, first, CeO2 particles with an average particle diameter of 1.3 μm were prepared. The CeO2 particles used were cerium oxide powder manufactured by Tribach. Next, the prepared CeO2 particles were aggregated to prepare a slurry for forming the collection layer. Next, the slurry for forming the collection layer was supplied from the lower side in the vertical direction of the honeycomb filter precursor set in the jig of the film forming apparatus, and the permeate was flowed from the upper side. After flowing a predetermined amount of the slurry for forming the collection layer, the honeycomb filter precursor was turned over together with the jig, and the jig was removed from the honeycomb filter precursor. Then, the honeycomb filter precursor was dried at room temperature for 22 hours, then dried at 80 °C for 24 hours, and further heated to 1200 °C at a heating rate of 200 °C / h and fired at 1200 °C for 2 hours to form the collection layer.

[0060] The honeycomb filter of Example 1 had a cylindrical shape with circular inflow and outflow end faces. The length of the honeycomb filter in the direction in which the cells extend was 127.1 mm. The diameter of the end face of the honeycomb filter was 118.5 mm. The honeycomb structure constituting the honeycomb filter had a partition wall thickness of 0.158 mm and a cell density of 33.3 cells / cm 2 . The partition walls of the honeycomb structure had a porosity of 48.6%.

[0061] Also, the honeycomb filter of Example 1 had a collection layer composed of CeO2 particles on the inner surface side of the partition walls surrounding the inflow cells. The thickness of the collection layer was 26 μm. The total mass of the CeO2 particles constituting the collection layer was 33 g. The results are shown in the column of "Mass of CeO2 particles (g)" in Table 1. Also, the mass of the collection layer per unit area was 24 g. The results are shown in the column of "Mass of collection layer per unit area (g / m 2 2)". Note that the mass of the collection layer per unit area means the mass per 1 m 2 of the porous layer that is composed of CeO2 particles and is effective as the collection layer. The average particle diameter of the CeO2 particles constituting the collection layer, measured by the following measurement method, was 1.1 μm. The results are shown in Table 1.

[0062] [Measurement method for CeO2 particles constituting the collection layer] First, a test piece for measurement was cut out from the honeycomb structure that constitutes the honeycomb filter. The test piece was produced with a size of 6 mm × 6 mm × 6 mm from a range of 20 mm × 20 mm × 20 mm including the central position in the extending direction of the cells of the honeycomb filter and the central part farthest from the outer peripheral wall of the honeycomb filter. Next, the cut-out test piece was embedded in resin. Next, in the direction perpendicular to the extending direction of the cells, the resin-embedded test piece was cut, and the cut surface was polished. Next, the polished cut surface was imaged using a scanning electron microscope to obtain an SEM image with a magnification of 200 times. The scanning electron microscope used was the scanning electron microscope "Model No.: S3400-N" manufactured by Hitachi High-Technologies Corporation. Next, image processing was performed on the image of the collection layer in the obtained SEM image, and the particle diameter of the CeO2 particles constituting the collection layer was measured. Specifically, first, from the SEM image of the collection layer, at an arbitrary position, a region of 1 μm in the thickness direction and 100 μm in the horizontal direction of the collection layer was enclosed. At this time, in the SEM image, the collection layer was horizontal with respect to the partition walls of the honeycomb structure, and the above region was specified so as to be parallel to the horizontal line. This region was binarized using "Image-Pro 9.3.2 (trade name)" of Nippon Loper Co., Ltd. Next, the particle diameter of all the CeO2 particles in the specified region was measured by dividing the area of each part recognized as the solid part of the sintered body of the CeO2 particles by a width of 1 μm. The average value of the measured particle diameters of the CeO2 particles was calculated, and this was done in two regions, and the calculated average value was taken as the average particle diameter of the CeO2 particles constituting the collection layer.

[0063] [Table 1]

[0064] Regarding the honeycomb filter of Example 1, the "soot oxidation start temperature (°C)" was measured by the following method. The results are shown in Table 1.

[0065] [Soot oxidation start temperature (°C)] First, exhaust gas containing soot was passed through the honeycomb filters of each example, and the soot in the exhaust gas was collected by the collection layer of the honeycomb filter. The ventilation of the exhaust gas was carried out until the amount of soot collected by the collection layer reached 1 g per 1 L of the volume of the honeycomb filter. Next, the partition walls and the collection layer of the honeycomb filter were cut so that the length of one side was 0.5 to 1.5 cm to to prepare a sample piece for measuring the soot oxidation start temperature. Since soot was attached to the surface of the collection layer, the sample piece was prepared so that the soot did not peel off from the surface of the collection layer. Next, the prepared sample piece was subjected to temperature programmed desorption-mass spectrometry (TPD-MS). Specifically, first, the prepared sample piece was placed in a measurement cell for temperature programmed desorption analysis, and He / O2 (20%) gas adjusted to a flow rate of 50 mL / min was passed through the measurement cell. After the temperature in the measurement cell was raised to 300 °C, the temperature in the measurement cell was further raised to 700 °C at a heating rate of 20 °C / min. At this time, the CO2 concentration in the gas discharged from the measurement cell was measured. That is, by raising the temperature in the measurement cell, the soot collected by the collection layer burns to generate CO2. Based on the measurement results of the CO2 concentration, a graph was created with the temperature in the measurement cell on the horizontal axis (200 to 700 °C) and the CO2 concentration on the vertical axis (arbitrary unit (a.u.)), and the total area of the CO2 concentration peak was designated as A 100% and the temperature at which the area ratio of the CO2 concentration peak reached 20% of A 20% was determined. The "temperature at which it reached A 20% " was defined as the "soot oxidation start temperature (°C)".

[0066] (Example 2, Comparative Examples 1 to 3) A honeycomb filter was fabricated in the same manner as in Example 1, except that the average particle diameter of the CeO2 particles for forming the trapping layer was changed as shown in Table 1. When the average particle diameter of the CeO2 particles constituting the trapping layer of the honeycomb filters of Example 2 and Comparative Examples 1 to 3 was measured, it was 0.7 μm for Example 2, 2.1 μm for Comparative Example 1, 4.5 μm for Comparative Example 2, and 5.5 μm for Comparative Example 3. The configurations of the partition walls and the trapping layer in the honeycomb filters of Example 2 and Comparative Examples 1 to 3 are shown in Table 1.

[0067] For the honeycomb filters of Example 2 and Comparative Examples 1 to 3 as well, the "soot oxidation start temperature (°C)" was measured in the same manner as in Example 1. The results are shown in Table 1.

[0068] (Results) The soot oxidation start temperature of the honeycomb filter of Example 1 was 464 °C, and it was confirmed that it was lower than the soot oxidation start temperatures of the honeycomb filters of Comparative Examples 1 to 3. Also, the soot oxidation start temperature of the honeycomb filter of Example 2 was 455 °C, and furthermore, it was confirmed that the soot oxidation start temperature became lower. From the above results, by 0.7~ 1.1 μ m and making the average particle diameter of the CeO2 particles constituting the trapping layer, it was found that combustible PM such as soot trapped by the trapping layer can be oxidized and burned at a lower temperature.

Industrial Applicability

[0069] The honeycomb filter of the present invention can be used as a filter for collecting particulate matter in exhaust gas.

Explanation of Signs

[0070] 1: Partition wall, 2: Cell, 2a: Inflow cell, 2b: Outflow cell, 3: Outer peripheral wall, 4:: Honeycomb structure, 5: Plugging portion, 7: Pore, 11: Inflow end face, 12: Outflow end face, 14: Trapping layer, 100: Honeycomb filter.

Claims

1. A honeycomb structure having a porous partition wall disposed so as to surround a plurality of cells serving as a flow path for a fluid extending from an inflow end face to an outflow end face, the honeycomb structure being obtained by extrusion molding and firing a clay containing alumina, talc, kaolin, and silica as raw materials, and a plugging portion disposed so as to seal one end of either the inflow end face side or the outflow end face side of the cell, The cell having the plugging portion disposed at the end on the outflow end face side and the inflow end face side being open is defined as an inflow cell, The cell having the plugging portion disposed at the end on the inflow end face side and the outflow end face side being open is defined as an outflow cell, The honeycomb structure further has a collection layer for collecting particulate matter in exhaust gas on the inner surface side of the partition wall surrounding the inflow cell, The collection layer includes a portion formed of a sintered body of CeO₂ particles at least on the surface layer of the collection layer, The average particle diameter of the CeO₂ particles constituting the collection layer is 0.7 to 1.1 μm, The entire collection layer includes a sintered body of CeO₂ particles, and the content of CeO₂ contained in the collection layer exceeds 70% by mass, The average pore diameter of the collection layer is smaller than the average pore diameter of the partition wall, The honeycomb filter, wherein the porosity of the partition wall is 45 to 66%.

2. The honeycomb filter according to claim 1, wherein the partition wall is made of cordierite.

3. The honeycomb filter according to claim 1 or 2, wherein the average pore diameter of the partition wall is 6 to 24 μm.

4. The honeycomb filter according to any one of claims 1 to 3, wherein the thickness of the partition wall is 0.10 to 0.35 mm.

5. The honeycomb filter according to any one of claims 1 to 4, wherein the thickness of the collection layer is 20 to 50 μm.

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