Honeycomb structure in columnar form

Abstract

To provide a pillar-shaped honeycomb structure having a partition wall structure suitable for both light weight and high strength.SOLUTION: There is provided a pillar-shaped honeycomb structure including an outer peripheral side wall, and a plurality of partition walls disposed on an inner peripheral side of the outer peripheral side wall, the plurality of partition walls partitioning a plurality of cells forming flow paths from a first end surface to a second end surface, wherein an average pore diameter of the partition walls measured by a mercury porosimeter is 3 to 10 μm, and when a cross section of the plurality of partition walls is observed with an X-ray microscope and porosities (%) of each partition wall is measured in a thickness direction from one surface to the other surface of each partition wall, an average porosity of each partition wall is 40 to 70%, and a difference between a maximum value and a minimum value of the porosity of each partition wall is 11% or less.SELECTED DRAWING: Figure 6

Description

Technical Field

[0001] The present invention relates to a columnar honeycomb structure. In particular, the present invention relates to a columnar honeycomb structure for exhaust gas purification.

Background Art

[0002] Exhaust gas discharged from internal combustion engines typified by automobile engines contains contaminants such as soot, nitrogen oxides (NOx), soluble organic components (SOF), hydrocarbons (HC), and carbon monoxide (CO). For this reason, in the exhaust gas system of an internal combustion engine, it is generally practiced to install a columnar honeycomb structure carrying an appropriate catalyst (oxidation catalyst, reduction catalyst, three-way catalyst, etc.) according to the contaminants to purify the exhaust gas.

[0003] The columnar honeycomb structure includes an outer peripheral side wall and a plurality of partition walls disposed on the inner peripheral side of the outer peripheral side wall and partitioning a plurality of cells that form flow paths from a first bottom surface to a second bottom surface. A catalyst layer containing the above-described catalyst can be formed on the surface of the partition wall.

[0004] In recent years, after starting the engine, development of a columnar honeycomb structure capable of raising the temperature of the catalyst layer to the activation temperature in a short time has been underway. In order to be able to raise the temperature of the catalyst layer to the activation temperature in a short time, it is necessary to reduce the weight of the columnar honeycomb structure. That is, it is necessary to reduce the heat capacity of the partition wall by thinning the partition wall or increasing the porosity. By doing so, after the exhaust gas starts to flow, the partition wall can be heated up in a short time, and the catalyst layer formed on the surface of the partition wall can be heated up to the activation temperature in a short time. However, since there is a limit to thinning the partition wall, increasing the porosity is considered as a method of weight reduction. A concern in that case is a decrease in the strength of the columnar honeycomb structure.

[0005] Under such circumstances, Patent Document 1 (Japanese Patent Application Laid-Open No. 2016-204208) discloses the following invention aimed at providing a honeycomb structure that can raise the temperature of the catalyst layer to the activation temperature in a short time, is less likely to crack even when repeating thermal cycles, and is less likely to reduce the purification performance of exhaust gas. A honeycomb structure comprising a polygonal lattice-shaped cell wall, a plurality of cells surrounded by the cell wall, and a catalyst layer formed on the surface of the cell wall, wherein a plurality of recesses are formed in the cell wall, when observing the cross-section of the cell wall, the opening ratio of the openings of the deep recesses, which are recesses with a depth of 10 μm or more from the surface of the cell wall, is 10% or more, the ratio of the number of narrow recesses, which are deep recesses with an opening length of 8 μm or less, to the total number of deep recesses is 10% or more, and the ratio of the number of wide recesses, which are deep recesses with an opening length of 20 μm or more, to the total number of deep recesses is 10% or more.

[0006] Moreover, as prior documents disclosing the porosity of the honeycomb structure, Patent Document 2 (Japanese Patent Application Laid-Open No. 2016-190198) and Patent Document 3 (Japanese Patent Application Laid-Open No. 2019-505365) can be cited.

[0007] Patent Document 2 discloses the following invention. A honeycomb structure comprising a polygonal lattice-shaped partition wall that partitions and forms a plurality of cells extending from one end face to the other end face that form fluid flow paths, wherein the partition wall is formed in a porous manner using an aggregate and a binder made of a material different from the aggregate, the surface porosity of the surface region from the partition wall surface to a depth of 15% of the partition wall thickness and the internal porosity of the internal region from the partition wall surface to a depth of 15% to 50% of the partition wall thickness are different from each other, and the honeycomb structure shows a relationship in which the difference obtained by subtracting the surface porosity from the internal porosity is more than 1.5%.

[0008] Patent Document 3 discloses the following invention. In a particulate filter having at least one porous ceramic wall, the wall has an average bulk porosity of more than 55% as measured by mercury porosimetry, a d50 (pore diameter) of more than 16 μm, a d90 (pore diameter) of less than 37 μm, and a surface porosity measured by X-ray topography that is within 10% of the bulk porosity at the midpoint of the wall, and has a microstructure, a particulate filter.

Prior Art Documents

Patent Documents

[0009]

Patent Document 1

Patent Document 2

Patent Document 3

Summary of the Invention

Problems to be Solved by the Invention

[0010] In the invention described in Patent Document 1, attention is paid to a plurality of recesses formed in the cell wall (synonymous with "partition wall"), and the depth and width of the recesses are controlled. However, even if the depth and width of the recesses near the surface of the partition wall are controlled, if there is a large variation in porosity inside the cell wall, stress concentrates in the places with high porosity, and cracks are likely to occur starting from there. Also, Patent Document 1 does not have specific descriptions about porosity.

[0011] In the invention described in Patent Document 2, the surface porosity of the surface region from the partition wall surface to a depth of 15% of the partition wall thickness and the internal porosity of the internal region from the partition wall surface to a depth from 15% to 50% of the partition wall thickness Ratecontrols the relationship. In the invention described in Patent Document 2, in order to suppress an increase in pressure loss, it is recommended that the difference between the two is large, but this rather has an adverse effect on the strength of the columnar honeycomb structure. Further, if there is a place with a locally high porosity in each region, stress concentrates at that place, and cracks are likely to enter from there. Further, Patent Document 2 describes that the surface porosity is in the range of 10% to 50%, and the internal porosity is in the range of 20% to 75%, and it is described that a wide range of porosities can be adopted. However, the invention described in Patent Document 2 is not aimed at reducing the weight of the columnar honeycomb structure, but rather aims at increasing the heat capacity by keeping the average porosity low, so a columnar honeycomb structure having a high porosity as a whole is not specifically disclosed.

[0012] Although the invention described in Patent Document 3 describes that the porosity on the surface of the partition wall is within 10% of the porosity at the midpoint of the partition wall, it is insufficient to control only the relationship between the surface and the midpoint. If there are portions with a high porosity other than the surface and the midpoint inside the partition wall, cracks are likely to enter from there. Further, since the main focus is on reducing the pressure loss, it requires an average bulk porosity exceeding 55% and a relatively large pore diameter with d50 exceeding 16 μm. However, when a high porosity and a large pore diameter coexist, an adverse effect on the strength is likely to occur.

[0013] As described above, in the inventions described in Patent Documents 1 to 3, there is still room for improvement regarding the compatibility of weight reduction and high strength of the columnar honeycomb structure. The present invention has been made in view of the above circumstances, and in one embodiment, an object is to provide a columnar honeycomb structure having a partition wall structure suitable for achieving both weight reduction and high strength.

Means for Solving the Problems

[0014] As a result of intensive studies to solve the above problems, the present inventor has found that it is advantageous to control the maximum value and the minimum value of the porosity in the thickness direction from one surface to the other surface of each partition wall constituting the columnar honeycomb structure when measuring the porosity with an X-ray microscope. The present invention completed based on the finding is exemplified below.

[0015] [1] A columnar honeycomb structure comprising an outer peripheral side wall and a plurality of partition walls disposed on the inner peripheral side of the outer peripheral side wall and partitioning a plurality of cells that form a flow path from a first bottom surface to a second bottom surface, wherein the average pore diameter of the partition walls measured by a mercury porosimeter is 10 μm or less, when the cross sections of the plurality of partition walls are observed with an X-ray microscope and the porosity (%) in the thickness direction is measured from one surface to the other surface for each partition wall, the average porosity of each partition wall is 40 to 70%, and the difference between the maximum value and the minimum value of the porosity in each partition wall is 11% or less. A columnar honeycomb structure. [2] The columnar honeycomb structure according to [1], wherein the average pore diameter of the partition walls measured by a mercury porosimeter is 3 to 10 μm. [3] The columnar honeycomb structure according to [1] or [2], wherein the average thickness of the plurality of partition walls is 50 to 150 μm. [4] The columnar honeycomb structure according to any one of [1] to [3], wherein when the average porosity (%) of each partition wall is x and the difference (%) between the maximum value and the minimum value of the porosity in each partition wall is y, the following formula (A) holds.

Number

Advantages of the Invention

[0016] According to an embodiment of the present invention, a columnar honeycomb structure having a partition structure suitable for achieving both weight reduction and high strength is provided. Thereby, for example, by using the columnar honeycomb structure as a catalyst carrier, it becomes possible to exhibit a function of raising the catalyst temperature to the activation temperature in a short time while ensuring a desired strength.

Brief Description of the Drawings

[0017]

Figure 1

Figure 2

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Figure 7

Figure 8

Embodiments for Carrying Out the Invention

[0018] Next, embodiments of the present invention will be described in detail with reference to the drawings. It should be understood that the present invention is not limited to the following embodiments, and design changes, improvements, etc. can be appropriately made based on the ordinary knowledge of those skilled in the art without departing from the spirit of the present invention.

[0019] <1. Columnar honeycomb structure> Generally, a columnar honeycomb structure has a columnar honeycomb structure portion including an outer peripheral side wall and a partition wall disposed on the inner peripheral side of the outer peripheral side wall and partitioning a plurality of cells that form a flow path from a first bottom surface to a second bottom surface.

[0020] FIGS. 1 and 2 respectively illustrate a schematic perspective view and a cross-sectional view of a columnar honeycomb structure 100 applicable as a wall-through type automotive exhaust gas filter and / or catalyst carrier. This columnar honeycomb structure 100 has a columnar honeycomb structure portion including an outer peripheral side wall 102 and a partition wall 112 disposed on the inner peripheral side of the outer peripheral side wall 102 and partitioning a plurality of cells 108 that form a fluid flow path from a first bottom surface 104 to a second bottom surface 106. In this columnar honeycomb structure 100, both ends of each cell 108 are open, and the exhaust gas flowing into one cell 108 from the first bottom surface 104 is purified while passing through the cell and flows out from the second bottom surface 106.

[0021] FIGS. 3 and 4 respectively illustrate a schematic perspective view and a cross-sectional view of a columnar honeycomb structure 200 applicable as a wall-flow type automotive exhaust gas filter and / or catalyst carrier. This columnar honeycomb structure 200 has a columnar honeycomb structure portion including an outer peripheral side wall 202 and a partition wall 212 disposed on the inner peripheral side of the outer peripheral side wall 202 and partitioning and forming a plurality of cells 208a, 208b that form a fluid flow path from a first bottom surface 204 to a second bottom surface 206.

[0022] In the columnar honeycomb structure 200, a plurality of cells 208a and 208b are disposed on the inner peripheral side of the outer peripheral side wall 202, extend from the first bottom surface 204 to the second bottom surface 206, and have a plurality of first cells 208a with the first bottom surface 204 being open and having a blind seal portion 209 on the second bottom surface 206, and are disposed on the inner peripheral side of the outer peripheral side wall 202, extend from the first bottom surface 204 to the second bottom surface 206, have a blind seal portion 209 on the first bottom surface 204, and can be classified into a plurality of second cells 208b with the second bottom surface 206 being open. And in this columnar honeycomb structure 200, the first cells 208a and the second cells 208b are alternately and adjacently arranged with the partition wall 212 interposed therebetween.

[0023] When exhaust gas containing particulate matter (PM) such as soot is supplied to the first bottom surface 204 on the upstream side of the columnar honeycomb structure 200, the exhaust gas is introduced into the first cell 208a and proceeds downstream in the first cell 208a. Since the first cell 208a has a blind seal portion 209 on the second bottom surface 206 on the downstream side, the exhaust gas permeates through the partition wall 212 partitioning the first cell 208a and the second cell 208b and flows into the second cell 208b. Since the particulate matter (PM) cannot pass through the partition wall 212, it is collected and deposited in the first cell 208a. After the particulate matter (PM) is removed, the clean exhaust gas that has flowed into the second cell 208b proceeds downstream in the second cell 208b and flows out from the second bottom surface 206 on the downstream side.

[0024] There is no limitation on the bottom surface shape of the columnar honeycomb structures 100 and 200, and for example, it can be a round shape such as a circular shape, an elliptical shape, a racetrack shape, and an oval shape, a polygonal shape such as a triangular shape and a quadrangular shape, and other irregular shapes. The illustrated columnar honeycomb structures 100 and 200 have a circular bottom surface shape and are columnar as a whole.

[0025] The height of the columnar honeycomb structure (the length from the first bottom surface to the second bottom surface) is not particularly limited and may be appropriately set according to the use and required performance. There is also no particular limitation on the relationship between the height of the columnar honeycomb structure and the maximum diameter of each bottom surface (the diameter with the maximum length among the diameters passing through the center of gravity of each bottom surface of the columnar honeycomb structure). Therefore, the height of the columnar honeycomb structure may be longer than the maximum diameter of each bottom surface, or the height of the columnar honeycomb structure may be shorter than the maximum diameter of each bottom surface.

[0026] Examples of the material constituting the partition walls and the outer peripheral side walls of the columnar honeycomb structure include, but are not limited to, ceramics. Examples of ceramics include cordierite, mullite, zirconium phosphate, aluminum titanate, silicon carbide (SiC), silicon-silicon carbide composite (e.g., Si-bonded SiC), cordierite-silicon carbide composite, zirconia, spinel, indialite, sapphirine, corundum, titania, silicon nitride, and the like. These ceramics may contain one kind alone or may contain two or more kinds simultaneously.

[0027] In a preferred embodiment, the partition walls are formed of a ceramic containing 90 mass% or more of cordierite. This means that the mass ratio of cordierite (2MgO·2Al 2 O 3 ·5SiO 2 ) quality in the material constituting the partition walls is 90 mass% or more. The mass ratio of cordierite in the material constituting the partition walls is more preferably 95 mass% or more, and even more preferably 99 mass% or more. It is also possible to make the 100 mass% of the material constituting the partition walls cordierite, excluding inevitable impurities.

[0028] In the columnar honeycomb structure, the average thickness of the partition walls is preferably 50 μm or more, more preferably 60 μm or more, and even more preferably 70 μm or more from the viewpoint of ensuring strength. Also, the average thickness of the partition walls is preferably 150 μm or less, more preferably 130 μm or less, and even more preferably 100 μm or less from the viewpoint of suppressing pressure loss. Fig. 5 shows a schematic partial enlarged view when observing the partition walls 112 (212) of the columnar honeycomb structure 100 (200) in a cross section orthogonal to the extending direction of the cells 108 (208a, 208b). In this specification, the thickness of the partition wall refers to the length of the line segment connecting the centroids C of adjacent cells when the line segment crosses the partition wall in a cross section orthogonal to the extending direction of the cells (the height direction of the columnar honeycomb structure). The thickness direction D of the partition wall refers to the direction parallel to the line segment L. The average thickness of the partition walls refers to the average value of the thicknesses of all the partition walls.

[0029] In the columnar honeycomb structure, the partition walls can be porous. When observing the cross sections of the plurality of partition walls 112 (212) of the columnar honeycomb structure with an X-ray microscope and measuring the porosity (%) in the thickness direction D from one surface 112a (212a) to the other surface 112b (212b) for each partition wall 112 (212), the lower limit of the average porosity of each partition wall is preferably 40% or more, more preferably 45% or more, and even more preferably 50% or more from the viewpoint of reducing the weight of the columnar honeycomb structure. The upper limit of the average porosity of each partition wall is preferably 70% or less, more preferably 65% or less, and even more preferably 60% or less from the viewpoint of ensuring the strength of the columnar honeycomb structure. Therefore, the average porosity of each partition wall is preferably, for example, 40 to 70%, more preferably 45 to 65%, and even more preferably 50 to 60%.

[0030] Further, cross-sections of a plurality of partition walls 112(212) of the columnar honeycomb structure are observed with an X-ray microscope, and when the porosity (%) in the thickness direction D from one surface 112a(212a) to the other surface 112b(212b) is measured for each partition wall 112(212), the difference between the maximum value and the minimum value of the porosity in each partition wall is preferably 11% or less, more preferably 9% or less, still more preferably 7% or less, and most preferably 5% or less. There is no particular lower limit for the difference between the maximum value and the minimum value of the porosity in each partition wall, but usually it is 1% or more, typically 3% or more, for the reason that production is easy. Therefore, the difference between the maximum value and the minimum value of the porosity in each partition wall is preferably, for example, 1 to 11%, more preferably 1 to 9%, still more preferably 1 to 7%, and still more preferably 1 to 5%.

[0031] The method for observing each partition wall by an X-ray microscope, the method for measuring the average porosity of each partition wall, and the method for measuring the difference between the maximum value and the minimum value of the porosity in each partition wall are carried out according to the following procedure. First, for each of the vicinity of the first bottom surface, the vicinity of the center in the height direction, and the vicinity of the second bottom surface of the columnar honeycomb structure, samples of partition walls (cross-section size (vertical 20 mm × horizontal 2 mm) × depth 0.3 mm) whose cross-sections perpendicular to the extending direction of the cells are exposed are taken from near the central axis, near the center in the radial direction (near the center between the central axis and the outer peripheral side wall), and near the outer peripheral side wall (however, the outer peripheral side wall is not included). Next, after observing and CT-scanning the cross-section of each sample with an X-ray microscope, the obtained three-dimensional cross-sectional image is binarized based on luminance and divided into a large number of voxels (size of one voxel = length in the wall surface direction (Y direction) of the partition wall: 0.8 μm, length in the thickness direction (X direction) of the partition wall: 0.8 μm, length in the depth direction (Z direction) of the partition wall: 0.8 μm cube) of the space part and the base material part. The measurement conditions of the X-ray microscope are a magnification of 4 times. The binarization process is carried out by the Otsu's binarization method.

[0032] Next, for a predetermined region of any one partition wall on the three-dimensional cross-sectional image (length in the wall surface direction (Y direction) of the partition wall: 340 μm, length in the thickness direction (X direction) of the partition wall: length including the entire thickness and having a space portion of 50 μm or more on both sides of the partition wall, length in the depth direction (Z direction) of the partition wall: 300 μm), based on the binarized voxel data, a profile of the porosity (%) every 0.8 μm along the thickness direction (X direction) of the partition wall from one surface to the other surface is obtained (see Fig. 6). The porosity at a thickness of 0.8 μm is calculated according to the formula porosity = (number of voxels in the space portion) / (total number of voxels in the region) × 100 (%) for a thickness region of 0.8 μm (length in the wall surface direction (Y direction) of the partition wall: 340 μm, length in the thickness direction (X direction) of the partition wall: 0.8 μm, length in the depth direction (Z direction) of the partition wall: 300 μm). By performing the above calculation every 0.8 μm from the left end of the screen for the entire predetermined region, a profile of the porosity (%) every 0.8 μm along the thickness direction (X direction) of the partition wall from one surface to the other surface is obtained.

[0033] At this time, as shown in Fig. 7, the position of one surface of the partition wall is set as the position of the mode value when measuring the distance M in the thickness direction D (X direction) from a line segment parallel to the wall surface direction of the partition wall to one surface of the partition wall to be measured. The distance M is measured at intervals of 0.8 μm over a length of 340 μm in the wall surface direction (Y direction perpendicular to the thickness direction (X direction)) of the partition wall on the binarized image. The position of the other surface of the partition wall is specified in the same manner.

[0034] In this way, a porosity profile of any one partition wall is obtained from each sample, and the average porosity of the partition wall, and the difference between the maximum value and the minimum value of the porosity are obtained from the profile. Then, the average values of a total of 9 samples are used as the "average porosity of each partition wall" and the "difference between the maximum value and the minimum value of the porosity in each partition wall" in the columnar honeycomb structure to be measured.

[0035] One measure of the mechanical strength of a columnar honeycomb structure is the isostatic fracture strength. In measuring the isostatic fracture strength of a columnar honeycomb structure, a test is conducted in which the columnar honeycomb structure is submerged in water inside a pressure vessel and an isotropic pressure is applied to the columnar honeycomb structure by gradually increasing the water pressure. As the water pressure inside the pressure vessel gradually increases, finally, fractures occur in the partition walls and the outer peripheral side walls of the columnar honeycomb structure. The value of the pressure (fracture strength) at the time of fracture is the isostatic fracture strength. The isostatic fracture strength is measured based on the automotive standard (JASO M505 - 87) issued by the Japan Automobile Manufacturers Association, Inc.

[0036] In applying the columnar honeycomb structure as an automotive exhaust gas filter and / or a catalyst carrier, the lower limit of the isostatic fracture strength is preferably 0.5 MPa or more, more preferably 1.0 MPa or more, and even more preferably 1.5 MPa or more. Although the upper limit of the isostatic fracture strength is not particularly set, it is usually 3.0 MPa or less, and typically 2.5 MPa or less.

[0037] Assuming that the average porosity of each partition wall is constant, the smaller the difference between the maximum value and the minimum value of the porosity in each partition wall, the higher the isostatic fracture strength of the columnar honeycomb structure. For example, when the average porosity of each partition wall is 55%, the isostatic fracture strengths of columnar honeycomb structures with various changes in the difference between the maximum value and the minimum value of the porosity in each partition wall (hereinafter also referred to as "porosity variation") are measured. When the porosity variation (%) (Y) is plotted on the horizontal axis and the isostatic fracture strength (MPa) (S) is plotted on the vertical axis in a two - dimensional coordinate system, the approximate formula S = 5.2176e -0.283Y approximately holds. This approximate formula has particularly high accuracy when the partition walls are formed of ceramics containing 90 mass% or more of cordierite, the average thickness of the partition walls is 2.3 - 2.9 mil (58 - 74 μm), and the cell density is 730 - 770 cells per square inch (113 - 119 cells / cm 2 ).

[0038] When the influence of the average porosity of each partition wall is taken into account in the above approximate formula, empirically, the following formula generally holds among the average porosity (%) (x) of each partition wall, the difference between the maximum and minimum values of the porosity in each partition wall (porosity variation (%)) (y), and the isostatic fracture strength (MPa) (S).

Number

[0039] Therefore, it is preferable that formula (A) holds, more preferably that formula (B) holds, and even more preferably that formula (C) holds.

Number

[0040] The lower limit of the average pore diameter of the partition wall is preferably 3 μm or more from the viewpoint of catalyst loading. Also, the upper limit of the average pore diameter of the partition wall is preferably 10 μm or less, more preferably 8 μm or less, and even more preferably 6 μm or less from the viewpoint of preventing the catalyst from penetrating into the base material. Therefore, the average pore diameter of the partition wall is preferably, for example, 3 to 10 μm, more preferably 3 to 8 μm, and even more preferably 3 to 6 μm.

[0041] In this specification, the average pore diameter of the partition wall means the median diameter (D50) of the pore diameter measured by the mercury intrusion method defined in JIS R1655:2003 using a mercury porosimeter. The mercury intrusion method is a method in which a sample is immersed in mercury in a vacuum state and an equal pressure is applied, and mercury is intruded into the sample while gradually increasing the pressure, and the pore size distribution is calculated from the pressure and the volume of mercury intruded into the pores. When the pressure is gradually increased, mercury is intruded into the pores in order of the larger diameter pores, and the cumulative volume of mercury increases. Finally, when all the pores are filled with mercury, the cumulative volume reaches the saturation amount. At this time, the cumulative volume becomes the total pore volume (cm 3 / g), and the pore diameter (D50) at the time when 50% of the total pore volume of mercury is intruded is taken as the average pore diameter.

[0042] When determining the average pore diameter of the partition walls, for each of the vicinity of the first bottom surface, the vicinity of the center in the height direction, and the vicinity of the second bottom surface of the columnar honeycomb structure, samples of the partition walls (cross-sectional size (10 mm in length × 10 mm in width) × 10 mm in depth) where the cross-sections orthogonal to the extending direction of the cells are exposed are taken from near the central axis, near the center in the radial direction (near the center between the central axis and the outer peripheral side wall), and near the outer peripheral side wall (however, the outer peripheral side wall is not included), and the average pore diameter of each sample is measured. Then, the average value of all nine samples is defined as the "average pore diameter of the partition walls" of the columnar honeycomb structure to be measured.

[0043] There is no limitation on the opening shape of the cells in the cross-section orthogonal to the extending direction of the cells, but it is preferably a quadrilateral, hexagon, octagon, or a combination thereof. Among these, a square and a hexagon are preferred. By making the opening shape of the cells like this, the pressure loss when exhaust gas flows through the honeycomb structure becomes small, and the purification performance when used as a filter becomes excellent. By making the opening shape of the cells like this, the pressure loss when fluid flows through the columnar honeycomb structure becomes small, and the purification performance of the catalyst becomes excellent.

[0044] There is no particular limitation on the cell density (the number of cells per unit cross-sectional area) in the columnar honeycomb structure either. For example, it can be 6 to 2000 cells / square inch (0.9 to 311 cells / cm 2 ), more preferably 50 to 1000 cells / square inch (7.8 to 155 cells / cm 2 ), and particularly preferably 100 to 600 cells / square inch (15.5 to 92.0 cells / cm 2 ). Here, the cell density is calculated by dividing the number of cells the columnar honeycomb structure has by one bottom area excluding the outer peripheral side wall of the columnar honeycomb structure.

[0045] The bulk density of the columnar honeycomb structure is preferably as small as possible as long as the desired strength can be ensured. The bulk density of the columnar honeycomb structure is preferably 0.15 g / cc to 0.25 g / cc, more preferably 0.15 g / cc to 0.23 g / cc, and even more preferably 0.15 g / cc to 0.20 g / cc. In this specification, the bulk density of the columnar honeycomb structure is measured by the formula: bulk density (g / cc) = mass (g) of the columnar honeycomb structure ÷ volume (cc) based on the outer dimensions of the columnar honeycomb structure.

[0046] When the columnar honeycomb structure is used as a catalyst carrier, a catalyst layer according to the purpose can be formed on the surface of the partition wall. Examples of the catalyst include, but are not limited to, an oxidation catalyst (DOC) for oxidatively combusting hydrocarbons (HC) and carbon monoxide (CO) to increase the exhaust gas temperature, a PM combustion catalyst for assisting the combustion of PM such as soot, an SCR catalyst and an NSR catalyst for removing nitrogen oxides (NOx), and a three-way catalyst capable of simultaneously removing hydrocarbons (HC), carbon monoxide (CO), and nitrogen oxides (NOx). The catalyst can appropriately contain, for example, noble metals (Pt, Pd, Rh, etc.), alkali metals (Li, Na, K, Cs, etc.), alkaline earth metals (Mg, Ca, Ba, Sr, etc.), rare earths (Ce, Sm, Gd, Nd, Y, La, Pr, etc.), transition metals (Mn, Fe, Co, Ni, Cu, Zn, Sc, Ti, Zr, V, Cr, etc.).

[0047] <2. Manufacturing Method of Columnar Honeycomb Structure> The manufacturing method of the columnar honeycomb structure will be exemplarily described below. First, after kneading a raw material composition containing a ceramic raw material, a dispersion medium, a pore former, and a binder to prepare a clay, the clay can be extruded and dried to produce a columnar honeycomb formed body. Additives such as a dispersant can be blended in the raw material composition as needed. When extruding, a die having a desired overall shape, cell shape, partition wall thickness, cell density, etc. can be used.

[0048] In the drying process, for example, conventionally known drying methods such as hot air drying, microwave drying, dielectric drying, vacuum drying, freeze drying, etc. can be used. Among them, a drying method combining hot air drying with microwave drying or dielectric drying is preferable in that the entire molded body can be dried quickly and uniformly. The eye-sealing part can be formed by forming the eye-sealing part at predetermined positions on both bottom surfaces of the dried honeycomb molded body and then drying the eye-sealing part.

[0049] The ceramic raw material is a raw material of a part that remains after firing of a metal oxide, metal, etc. and constitutes the skeleton of the fired columnar honeycomb molded body (columnar honeycomb structure) as ceramics. The ceramic raw material can be provided, for example, in the form of powder. Examples of the ceramic raw material include raw materials for obtaining ceramics such as cordierite, mullite, zircon, aluminum titanate, silicon carbide, silicon nitride, zirconia, spinel, indialite, sapphirine, corundum, titania, etc. Specifically, but not limited to, silica, talc, alumina, kaolin, serpentine, pyrophyllite, brucite, boehmite, mullite, magnesite, aluminum hydroxide, etc. may be mentioned. The ceramic raw material may be used alone or in combination of two or more. From the viewpoint of reducing the variation in porosity while refining the pore diameter, it is preferable to use fine silica particles having a median diameter (D50) close to the average pore diameter of the target partition wall, for example, 2 to 7 μm. Similarly, it is preferable to use fine particles for other ceramic raw materials.

[0050] In the case of filter applications such as DPF and GPF, cordierite can be preferably used as the ceramic. In this case, a cordierite-forming raw material can be used as the ceramic raw material. The cordierite-forming raw material is a raw material that becomes cordierite by firing. The cordierite-forming raw material contains alumina (Al 2 O 3 )(including the part of aluminum hydroxide converted to alumina): 30 to 45% by mass, magnesia (MgO): 11 to 17% by mass, and silica (SiO2 ): It is preferably composed of a chemical composition of 42 to 57% by mass.

[0051] Examples of the dispersion medium include water, or a mixed solvent of water and an organic solvent such as alcohol. In particular, water can be preferably used.

[0052] The pore-forming agent is not particularly limited as long as it becomes pores after firing. For example, wheat flour, starch, foamed resin, water-absorbing resin, silica gel, carbon (e.g., graphite, coke), ceramic balloon, polyethylene, polystyrene, polypropylene, nylon, polyester, acrylic, phenol, etc. can be mentioned. The pore-forming agent may be used alone or in combination of two or more. From the viewpoint of increasing the porosity of the honeycomb structure, the content of the pore-forming agent is preferably 0.5 part by mass or more, more preferably 2 parts by mass or more, and still more preferably 3 parts by mass or more with respect to 100 parts by mass of the ceramic raw material. From the viewpoint of ensuring the strength of the honeycomb structure, the content of the pore-forming agent is preferably 10 parts by mass or less, more preferably 7 parts by mass or less, and still more preferably 4 parts by mass or less with respect to 100 parts by mass of the ceramic raw material. From the viewpoint of reducing the variation in porosity while refining the pore diameter, it is preferable to use a fine pore-forming agent having a median diameter (D50) close to the average pore diameter of the target partition wall, for example, 5 to 25 μm.

[0053] Examples of the binder include organic binders such as methyl cellulose, hydroxypropyl methyl cellulose, hydroxyethyl cellulose, carboxymethyl cellulose, and polyvinyl alcohol. In particular, it is preferable to use methyl cellulose and hydroxypropyl methyl cellulose in combination. Further, from the viewpoint of increasing the strength of the honeycomb formed body before firing, the content of the binder is preferably 4 parts by mass or more, more preferably 5 parts by mass or more, and still more preferably 6 parts by mass or more with respect to 100 parts by mass of the ceramic raw material. From the viewpoint of suppressing the occurrence of flash due to abnormal heat generation in the firing process, the content of the binder is preferably 9 parts by mass or less, more preferably 8 parts by mass or less, and still more preferably 7 parts by mass or less with respect to 100 parts by mass of the ceramic raw material. The binder may be used alone or in combination of two or more kinds.

[0054] As the dispersant, ethylene glycol, dextrin, fatty acid soap, polyether polyol, etc. can be used. The dispersant may be used alone or in combination of two or more kinds. The content of the dispersant is preferably 0 to 2 parts by mass with respect to 100 parts by mass of the ceramic raw material.

[0055] As shown in FIGS. 1 and 2, both ends of all the cells of the columnar honeycomb formed body may be opened. Further, as shown in FIGS. 3 and 4, the columnar honeycomb formed body may have a cell structure in which one end of the cell is alternately plugged. The method for plugging the bottom surface of the columnar honeycomb formed body is not particularly limited, and a known method can be adopted.

[0056] There are no particular restrictions on the material of the eye seal portion, but it is preferably a ceramic from the viewpoints of strength and heat resistance. The ceramic is preferably a ceramic containing at least one selected from the group consisting of cordierite, mullite, zircon, aluminum titanate, silicon carbide, silicon nitride, zirconia, spinel, indialite, sapphirine, corundum, and titania. The eye seal portion is preferably formed of a material containing 50% by mass or more of these ceramics in total, and more preferably formed of a material containing 80% by mass or more. In order to make the expansion rate during firing the same and lead to an improvement in durability, it is even more preferable that the eye seal portion has the same material composition as the main body portion of the honeycomb molded body.

[0057] The method for forming the eye seal portion will be exemplarily described. The eye seal slurry is stored in a storage container. Next, a mask having an opening at a location corresponding to the cell where the eye seal portion is to be formed is attached to one bottom surface. The bottom surface with the mask attached is immersed in the storage container to fill the opening with the eye seal slurry to form the eye seal portion. The eye seal portion can also be formed on the other bottom surface in the same manner.

[0058] A columnar honeycomb structure can be manufactured by performing a debinding process and a firing process on the dried columnar honeycomb molded body. The conditions of the debinding process and the firing process may be known conditions according to the material composition of the honeycomb molded body, and no particular explanation is required, but examples of specific conditions are given below.

[0059] The debinding process will be described. The combustion temperature of the binder is about 200°C, and the combustion temperature of the pore former is about 300 to 1000°C. Therefore, the debinding process may be carried out by heating the honeycomb molded body in the range of about 200 to 1000°C. The heating time is not particularly limited, but is usually about 10 to 100 hours. The honeycomb molded body after the debinding process is called a green body.

[0060] The firing process can be carried out, for example, by heating the green body to 1350 - 1600 °C in an air atmosphere and holding it for 3 - 10 hours, although it depends on the material composition of the honeycomb formed body.

Example

[0061] <Test numbers 1 - 8> (1. Manufacture of honeycomb structure) According to the test number, the cordierite-forming raw material, pore former A, pore former B, binder, dispersant, and dispersion medium were added in the formulations shown in Table 1, respectively, and mixed and kneaded to prepare the clay. As the cordierite-forming raw material, talc, kaolin, alumina, aluminum hydroxide, and silica A and silica B were used. Silica A and silica B have different median diameters (D50). Pore former A and pore former B also have different median diameters (D50). Water was used as the dispersion medium, a polyacrylic acid-based polymer was used as the pore former, hydroxypropyl methylcellulose was used as the binder, and fatty acid soap was used as the dispersant. Also, the median diameter (D50) of each material shown in Table 1 is a volume-based value measured by a laser diffraction particle size distribution analyzer (Model LA960 manufactured by HORIBA).

[0062]

Table 1

[0063] This clay was put into an extrusion molding machine and extruded horizontally through a predetermined die to obtain a columnar honeycomb formed body. After dielectric drying and hot air drying the obtained honeycomb formed body, both bottom surfaces were cut to have a predetermined dimension to obtain a columnar honeycomb formed body.

[0064] The obtained columnar honeycomb formed body was heated and degreased at 200 °C for 8 hours in an air atmosphere, and further fired at 1430 °C for 4 hours in an air atmosphere to obtain a columnar honeycomb structure. The columnar honeycomb structures according to each test number were manufactured in the amounts required for the following measurements, respectively. The specifications of the obtained columnar honeycomb structures are as follows. Overall shape: Cylindrical with a diameter of 118 mm and a height of 91 mm Cell shape in the cross-section perpendicular to the flow path direction of the cell: Square Cell density (number of cells per unit cross-sectional area): 750 cells / square inch Average thickness of the partition wall: 2.6 mil (66 μm) (nominal value based on the specifications of the base)

[0065] (2. Measurement of the porosity profile of the partition wall) For each columnar honeycomb structure obtained by the above manufacturing method, according to the method described above, using an X-ray microscope (model Xradia520Versa manufactured by Zeiss), the porosity profile of the partition wall was measured, and the average porosity of each partition wall, as well as the difference between the maximum and minimum values of the porosity in each partition wall (porosity variation) were determined. The results are shown in Table 2.

[0066] (3. Measurement of the average pore diameter of the partition wall) For each columnar honeycomb structure obtained by the above manufacturing method, according to the method described above, using a Micrometrics model Autopore9505, the average pore diameter of the partition wall was determined. The results are shown in Table 2.

[0067] (4. Bulk density) For each columnar honeycomb structure obtained by the above manufacturing method, according to the method described above, the bulk density was determined. The results are shown in Table 2.

[0068] (5. Measurement of the isostatic fracture strength) For each columnar honeycomb structure obtained by the above manufacturing method, the isostatic fracture strength was measured based on the automotive standard (JASO M505-87) issued by the Japan Automobile Manufacturers Association. The results are shown in Table 2.

[0069]

Table 2

[0070] (6. Discussion) Among each of the columnar honeycomb structures obtained by the above manufacturing method, for Test Nos. 1 to 6 where the "average porosity of each partition wall" was 55%, the "difference between the maximum and minimum values of the porosity in each partition wall" (porosity variation) was different. For Test Nos. 1 to 6, when the porosity variation (Y) was plotted on the horizontal axis and the isostatic fracture strength (S) was plotted on the vertical axis in a two-dimensional coordinate system, and an approximate curve was obtained by the method of exponential approximation, S = 5.2176e -0.283Y was obtained (Fig. 8). Also, when predicting the isostatic fracture strength (S) of the columnar honeycomb structures of Test No. 7 with an average porosity of 40% and Test No. 8 with an average porosity of 70% according to the following formula, they are 0.97 MPa and 0.47 MPa respectively. Therefore, it can be seen that Test No. 7 with an average porosity of 40% and Test No. 8 with an average porosity of 70% have a good approximation to the isostatic fracture strength (S) predicted by the following formula.

Equation

Explanation of Symbols

[0071] 100: Columnar honeycomb structure 102: Outer peripheral side wall 104: First bottom surface 106: Second bottom surface 108: Cell 112: Partition wall 112a: One surface of the partition wall 112b: The other surface of the partition wall 200: Columnar honeycomb structure 202: Outer peripheral side wall 204: First bottom surface 206: Second bottom surface 208a: First cell 208b: Second cell 209: Plugging portion 212: Partition wall

Claims

1. A columnar honeycomb structure comprising an outer peripheral side wall and a plurality of partition walls disposed on the inner peripheral side of the outer peripheral side wall and partitioning a plurality of cells that form a flow path from a first bottom surface to a second bottom surface, wherein the average pore diameter of the partition walls measured by a mercury porosimeter is 10 μm or less, the plurality of partition walls have an average thickness of 50 to 150 μm and are formed of a ceramic containing 90% by mass or more of cordierite, when the cross section of the plurality of partition walls is observed with an X-ray microscope and the porosity (%) in the thickness direction from one surface to the other surface is measured for each partition wall, the average porosity of each partition wall is 40 to 70%, and the difference between the maximum value and the minimum value of the porosity in each partition wall is 11% or less. A columnar honeycomb structure.

2. The columnar honeycomb structure according to claim 1, wherein the average pore diameter of the partition walls measured by a mercury porosimeter is 3 to 10 μm.

3. The columnar honeycomb structure according to claim 1 or 2, wherein when the average porosity (%) of each partition wall is x and the difference (%) between the maximum value and the minimum value of the porosity in each partition wall is y, the following formula (A) holds. 【Number 1】

4. The columnar honeycomb structure according to any one of claims 1 to 3, having an isostatic breaking strength of 0.5 MPa or more.

5. The columnar honeycomb structure according to any one of claims 1 to 4, having a bulk density of 0.15 g / cc to 0.25 g / cc.

6. The columnar honeycomb structure according to any one of claims 1 to 5, further comprising a catalyst layer on the surface of the partition walls.

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