COLUMN-SHAPED HONEYCOMB STRUCTURE

Abstract

Columnar honeycomb structure comprising an outer circumferential sidewall and several partitions arranged on an inner circumferential side of the outer circumferential sidewall, dividing several cells that form flow paths from a first end face to a second end face, wherein a mean pore diameter of the partitions, measured by a mercury intrusion method specified in JIS R1655: 2003, is in the range of 3 to 10 µm, and when a cross-section of the several partitions is observed with an X-ray microscope and a porosity (%) is measured in a thickness direction from one surface to the other surface of each partition, assuming that P AVE a medium porosity of each partition wall is, P 1MIN a smallest porosity from one surface up to a thickness of 5% of the partition wall and P 2MINThe smallest porosity from the other surface up to a thickness of 5% of the partition is 40% ≤ P AVE ≤ 70%, and {(P 1MIN + P 2MIN ) / 2} / P AVE ≤ 0.9 are fulfilled.

Description

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] The present invention claims priority from Japanese patent application No. 2022-060626, filed on March 31, 2022 with the Japanese Patent Office, the contents of which are incorporated herein in full by reference. AREA OF INVENTION

[0002] 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 OF THE INVENTION

[0003] Exhaust gases emitted by internal combustion engines, such as those in motor vehicles, contain pollutants such as soot, nitrogen oxides (NOx), soluble organic compounds (SOF), hydrocarbons (HC), and carbon monoxide (CO). For this reason, it is common practice to install a columnar honeycomb structure in the exhaust system of internal combustion engines. This structure carries a suitable catalyst (oxidation catalyst, reduction catalyst, three-way catalyst, etc.) depending on the pollutant, in order to clean the exhaust gas.

[0004] A columnar honeycomb structure comprises an outer circumferential sidewall and several porous partitions arranged on an inner circumferential side of the outer circumferential sidewall, dividing several cells that form flow paths from a first end face to a second end face. A catalyst layer containing one of the catalysts described above may be formed on the surfaces of the porous partitions.

[0005] In recent years, the development of a columnar honeycomb structure capable of quickly raising the temperature of the catalyst layer to its activation temperature after engine start has been advanced. To bring the catalyst layer to the activation temperature rapidly, it is necessary to reduce the weight of the columnar honeycomb structure. This means reducing the heat capacity of the partitions by making them thinner or increasing their porosity. In this way, the temperature of the partitions can be raised quickly after the exhaust gas flow begins, and the temperature of the catalyst layer formed on the surface of the partitions can be raised to the activation temperature in a short time. However, since there is a limit to how thin the partitions can be, increasing porosity is considered a method for weight reduction.In this case, a decrease in the strength of the columnar honeycomb structure can be considered a problem.

[0006] Against this background, the following invention is described in patent literature 1 (Japanese patent application JP 2016-204 208 A) to provide a honeycomb structure in which the temperature of the catalyst layer can be raised to an activation temperature in a short time and cracks are less likely to occur even after repeated heating and cooling cycles and exhaust gas purification performance is less likely to decrease.

[0007] A honeycomb structure comprising polygonal lattice-like cell walls, multiple cells surrounded by the cell walls, and a catalyst layer formed on a surface of the cell walls, wherein Several recesses are formed on the cell walls; and when a cross-section of the cell walls is considered, an opening ratio of the openings of the deep recesses, which are the recesses with a depth of 10 µm or more from the surface of the cell walls, is 10% or greater, a number of narrow recesses, which are the deep recesses, whose opening has a length of 8 µm or less, constitute 10% or more of the total number of deep recesses, and a number of wide recesses, which are the deep recesses, whose opening has a length of 20 µm or greater, constitute 10% or more of the total number of deep recesses.

[0008] Additionally, patent literature 2 (Japanese patent application no. 2016-190198) and patent literature 3 (Japanese patent application no. 2019-505365) can be cited as earlier literature that discloses the porosity of honeycomb structures.

[0009] The following invention is described in patent literature 2.

[0010] A honeycomb structure comprising polygonal grid-like partitions that divide multiple cells, extending from one end face to the other and forming flow paths for a fluid, wherein the partition walls are formed porous with an aggregate and a binding material different from the aggregate, a surface porosity of a surface area of ​​the partitions from a surface of the partitions to a depth of 15% of the thickness of the partitions and an internal porosity of an internal area from a depth of 15% of the thickness of the partitions from the surface of the partitions to a depth of 50% of the thickness of the partitions are different from each other, and a difference that results from subtracting the surface porosity from the internal porosity is greater than 1.5%.

[0011] The following invention is described in patent literature 3.

[0012] A particle filter with at least one porous ceramic wall, wherein the wall has a microstructure with: a mean volume porosity of more than 55%, measured with a mercury porosimeter; 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 of 10% or less of the volume porosity at a midpoint of the wall. STATE OF THE ART Patent literature [Patent Literature 1] Japanese patent application JP 2016-204 208 A [Patent Literature 2] Japanese patent application JP 2016-190 198 A [Patent Literature 3] Japanese patent application JP 2019-505 365 A SUMMARY OF THE INVENTION

[0013] The invention described in patent literature 1 focuses on several recesses formed on cell walls (synonymous with "partitions"), controls the depth and width of the recesses, and is intended to prevent the detachment of the catalyst layer from the cell walls. It is not a technique for controlling the porosity distribution of the partitions.

[0014] The invention described in patent literature 2 controls the ratio between the surface porosity of the surface region from the surface of the partitions to a depth of 15% of the partition thickness and the internal pores of the inner region from a depth of 15% of the partition thickness from the surface of the partitions to a depth of 50% of the partition thickness. Patent literature 2 further 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 that a wide range of porosities can be used. However, the invention described in patent literature 2 does not aim to reduce the weight of the columnar honeycomb structure, but rather to increase the heat capacity by keeping the average porosity low. It therefore does not specifically disclose a columnar honeycomb structure with a high overall porosity.

[0015] Since the invention described in patent literature 3 aims to reduce pressure loss, it requires an average volume porosity of more than 55% and a relatively large pore diameter with a d50 of more than 16 µm. The simultaneous presence of high porosity and a large pore diameter tends to have a negative impact on strength. Furthermore, if the porosity is increased to reduce weight, the catalyst can easily penetrate the interior of the partitions. If the catalyst penetrates the interior of the partitions, the frequency of contact between the exhaust gas flowing on the surface of the partitions and the catalyst is reduced, and the desired purification performance may not be achieved.

[0016] Thus, the inventions described in patent literature 1 to 3 still offer room for improvement with regard to the objective of achieving a good balance between the three functions of weight reduction of the columnar honeycomb structure, increased strength, and suppression of catalyst penetration into the interior of the partitions. The present invention was made with the above circumstances in mind, and in one embodiment, it is an object of the present invention to provide a columnar honeycomb structure with a partition wall structure suitable for balancing the three functions of weight reduction, high strength, and suppression of catalyst penetration into the partitions.

[0017] As a result of intensive investigations to solve the aforementioned problems, the inventors have found that, with regard to the partitions forming the columnar honeycomb structure, it is advantageous, while simultaneously increasing the average porosity to reduce weight, to locally reduce the porosity of the partition surface where stresses are likely to occur and furthermore to reduce the average pore diameter in order to ensure high strength and suppress the penetration of the catalyst into the interior of the partitions. The present invention, which has been completed based on these findings, is illustrated by the following examples.

[0018] [1] A columnar honeycomb structure comprising an outer circumferential side wall and several partitions arranged on an inner circumferential side of the outer circumferential side wall, dividing several cells that form flow paths from a first end face to a second end face, wherein a mean pore diameter of the partitions, measured by a mercury intrusion method specified in JIS R1655: 2003, is in the range of 3 to 10 µm, and when a cross-section of the several partitions is observed with an X-ray microscope and a porosity (%) is measured in a thickness direction from one surface to the other surface of each partition, assuming that P AVE a medium porosity of each partition wall is, P 1MIN a smallest porosity from one surface up to a thickness of 5% of the partition wall and P 2MINThe smallest porosity from the other surface up to a thickness of 5% of the partition is 40% ≤ P AVE ≤ 70%, and {( P 1MIN + P 2MIN ) / 2} / P AVE ≤ 0.9 are fulfilled.

[0019] [2] Columnar honeycomb structure according to [1], where 0.6 ≤ {( P 1MIN + P 2MIN ) / 2} / P AVE ≤ 0.9 is fulfilled.

[0020] [3] Columnar honeycomb structure according to [1] or [2], wherein the mean thickness of the multiple partitions is in the range of 50 to 150 µm.

[0021] [4] Columnar honeycomb structure according to one of [1] to [3], wherein a flexural strength measured according to JIS R1664: 2004 is 6.0 MPa or greater.

[0022] [5] Columnar honeycomb structure according to one of [1] to [4], where 50% ≤ P AVE ≤ 60% is fulfilled.

[0023] [6] Columnar honeycomb structure according to one of [1] to [5], wherein the bulk density is in the range of 0.15 g / cc to 0.25 g / cc.

[0024] [7] Columnar honeycomb structure according to one of [1] to [6], wherein the partitions are made of ceramic comprising 90% or more of cordierite by mass.

[0025] [8] Columnar honeycomb structure according to one of [1] to [7], wherein a catalyst layer is provided on the surfaces of the partitions.

[0026] According to one embodiment of the present invention, a columnar honeycomb structure is created which includes a partition wall structure suitable for balancing the three functions of weight reduction, high strength, and suppression of catalyst penetration into the partition walls. Accordingly, for example, by using the columnar honeycomb structure as a catalyst support, it is possible to achieve the function of raising the catalyst temperature to an activation temperature in a short time, while ensuring the desired strength. Additionally, since the catalyst supported on the partition walls does not readily penetrate into the interior of the partition walls, the catalyst can be utilized efficiently. BRIEF DESCRIPTION OF THE DRAWINGS Fig. Figure 1 is a perspective view that schematically shows a column-shaped honeycomb structure of the wall penetration type. Fig.Figure 2 is a schematic cross-sectional view of a columnar honeycomb body of the wall-perforation type, seen from a direction orthogonal to the direction in which the cells extend. Fig. Figure 3 is a perspective view that schematically shows a columnar honeycomb structure of the wall stream type. Fig. Figure 4 is a schematic cross-sectional view of a columnar honeycomb body of the wall stream type, seen from a direction orthogonal to the direction in which the cells extend. Fig. Figure 5 is a schematic, partially enlarged view of a columnar honeycomb structure in a cross-section orthogonal to the direction in which the cells extend. Fig. Figure 6 is an example of a porosity profile where the porosity (%) is measured along the thickness direction D of a partition from one surface to the other surface. Fig.Figure 7 is a conceptual diagram that illustrates a method for specifying the position of a surface of the partition in a cross-sectional image; DETAILED DESCRIPTION OF THE INVENTION

[0027] Embodiments of the present invention are now described in detail with reference to the drawings. It is understood that the present invention is not limited to the following embodiments, and any modification, improvement, or the like of the design may be added in a suitable manner based on the normal knowledge of those skilled in the art, without departing from the spirit of the present invention. <1. Columnar honeycomb structure>

[0028] In general, a columnar honeycomb structure has a columnar honeycomb section comprising an outer circumferential sidewall and several partitions arranged on an inner circumferential side of the outer circumferential sidewall, separating several cells that form flow paths from a first end face to a second end face.

[0029] Fig. 1 and Fig.Figure 2 shows a schematic perspective view and a cross-sectional view of a columnar honeycomb structure 100, which can be used as a wall-passage exhaust filter and / or catalyst carrier for motor vehicles. The columnar honeycomb structure 100 comprises a columnar honeycomb section that includes an outer circumferential side wall 102 and partitions 112 arranged on the inner circumferential side of the outer circumferential side wall 102, the partitions 112 dividing several cells 108 that form flow paths for a fluid from a first end face 104 to a second end face 106. In this columnar honeycomb structure 100, both ends of each cell 108 are open, and exhaust gas flowing from the first end face 104 into a cell 108 is cleaned as it passes through the cell and flows out at the second end face 106.

[0030] Fig. 3 and Fig.Figure 4 shows a schematic perspective view or a cross-sectional view of a columnar honeycomb structure 200, which can be used as a wall-flow type exhaust filter and / or catalyst carrier for motor vehicles. The columnar honeycomb structure 200 comprises a columnar honeycomb section that includes an outer circumferential side wall 202 and partitions 212 arranged on the inner circumferential side of the outer circumferential side wall 202, wherein the partitions 212 divide several cells 208a, 208b that form flow paths for a fluid from a first end face 204 to a second end face 206.

[0031] In the columnar honeycomb structure 200, the multiple cells 208a, 208b can be classified into multiple first cells 208a, which are arranged on the inner circumferential side of the outer circumferential side wall 202 and extend from the first end face 204 to the second end face 206, open at the first end face 204 and have sealing sections 209 at the second end face 206, and multiple second cells 208b, which are arranged on the inner circumferential side of the outer circumferential side wall 202, extend from the first end face 204 to the second end face 206, have sealing sections 209 at the first end face 204 and open at the second end face 206. Furthermore, in this columnar honeycomb structure 200, the first cells 208a and the second cells 208b are arranged alternately adjacent to each other, with the partition wall 212 lying between them.

[0032] When an exhaust gas containing particulate matter (PM), such as soot, is fed to the first end face 204 on the upstream side of the columnar honeycomb structure 200, the exhaust gas is introduced into the first cells 208a and moves downstream within them. Because the first cells 208a have sealing sections 209 on the second end face 206 on the downstream side, the exhaust gas penetrates the porous partitions 212 that separate the first cells 208a and the second cells 208b, and flows into the second cells 208b. Since particulate matter (PM) cannot pass through the partitions 212, it is trapped and deposited in the first cells 208a. After the fine dust (PM) has been removed, the purified exhaust gas that has flowed into the second cells 208b flows downstream in the second cells 208b and flows out at the second end face 206 on the downstream side.

[0033] The shape of the end faces of the columnar honeycomb structures 100, 200 is not restricted and can, for example, be round, such as circular, elliptical, racetrack-shaped, and elongated circular shapes; polygonal, such as triangular and quadrilateral shapes; and other irregular shapes. The illustrated columnar honeycomb structures 100, 200 have a circular end face and an overall cylindrical shape.

[0034] The height of the columnar honeycomb structure (the length from the first end face to the second end face) is not particularly restricted and can be appropriately determined depending on the application and desired performance. There is no particular restriction on the ratio between the height of the columnar honeycomb structure and the largest diameter of each end face (referring to the maximum length of the diameters passing through the centroid of each end face of the columnar honeycomb structure). Therefore, the height of the columnar honeycomb structure can be longer than the largest diameter of each end face, or the height of the columnar honeycomb structure can be shorter than the largest diameter of each end face.

[0035] Examples of materials used for the partitions and outer perimeter wall of the columnar honeycomb structure include, but are not limited to, ceramics. These ceramics may include cordierite, mullite, zirconium phosphate, aluminum titanate, silicon carbide (SiC), silicon-silicon carbide mixtures (e.g., Si-bonded SiC), cordierite-silicon carbide mixtures, zirconium dioxide, spinel, indialite, sapphire, corundum, titanium dioxide, silicon nitride, and the like. Furthermore, the ceramic may be of a single type, or it may be of two or more types.

[0036] In a preferred embodiment, the partitions are made of ceramic comprising 90% or more by mass of cordierite. This means that the total mass fraction of cordierite (2MgO · 2Al₂O₃ · 5SiO₂) in 100% by mass of the partition material is 90% or more by mass. The mass ratio of cordierite in 100% by mass of the partition material is further preferably 95% or more by mass, and even more preferably 99% or more by mass. It is also possible for the partition material to be 100% by mass of cordierite, apart from unavoidable impurities.

[0037] The average thickness of the partitions in the columnar honeycomb structure is preferably 50 µm or more, more preferably 60 µm or more, and even more preferably 70 µm or more, to ensure strength. Additionally, to suppress pressure loss, the average thickness of the partitions is preferably 150 µm or less, more preferably 130 µm or less, and even more preferably 100 µm or less. Fig.Figure 5 shows a schematic, partially enlarged view of the partitions 112 (212) of the columnar honeycomb structure 100 (200), viewed in a cross-section orthogonal to the direction in which the cells 108 (208a, 208b) extend. In this specification, the partition thickness refers to the intersection length of a line segment L that crosses the partition when the centroids C of adjacent cells are connected by this line segment in a cross-section orthogonal to the direction in which the cells extend (the vertical direction of the columnar honeycomb structure). The mean partition thickness refers to the mean thickness of all partitions.

[0038] In the columnar honeycomb structure, the partitions can be porous. The lower limit of the mean pore diameter of the partitions in the columnar honeycomb structure is preferably 3 µm or larger to support the catalyst. Additionally, the upper limit of the mean pore diameter of the partitions is preferably 10 µm or smaller, more preferably 8 µm or smaller, and even more preferably 6 µm or smaller to prevent the catalyst from penetrating the interior of the partitions. Therefore, the mean pore diameter of the partitions is, for example, preferably in the range of 3 to 10 µm, more preferably 3 to 8 µm, and even more preferably 3 to 6 µm.

[0039] As used in this specification, the mean pore diameter of the partitions means the median diameter (D50) of the pore diameters, measured according to the mercury intrusion method specified in JIS R1655:2003 using a mercury porosimeter. The mercury intrusion method is a procedure in which the sample is immersed in mercury under vacuum and a uniform pressure is applied, so that mercury is injected into the sample while the pressure is gradually increased, and the pore diameter distribution is calculated from the pressure and the volume of mercury entering the pores. As the pressure is gradually increased, the mercury enters in order from the pores of larger diameter, and the accumulated mercury volume increases. When finally all pores are filled with mercury, the accumulated volume reaches equilibrium.The volume accumulated at this point is the total pore volume (cm³). 3 / g), and the pore diameter (D50) at the time when mercury has penetrated 50% of the total pore volume is defined as the mean pore diameter.

[0040] To measure the mean pore diameter of the partitions, samples are taken from the partitions (cross-sectional size (length 10 mm × width 10 mm) × depth 10 mm), where the cross-section is exposed orthogonally to the cell orientation, near the central axis, near the midpoint in the radial direction (near the midpoint between the central axis and the outer circumferential wall), and near the outer circumferential sidewall (but excluding the outer circumferential sidewall itself). These samples are taken near the first end face, near the midpoint in the vertical direction, and near the second end face of the columnar honeycomb structure. The mean pore diameter of each sample is measured. The mean value of all nine samples is then taken as the "mean pore diameter of the partitions" of the columnar honeycomb structure being measured.

[0041] Furthermore, if a cross-section of several partition walls 112 (212) of the columnar honeycomb structure is viewed with an X-ray microscope and the porosity (%) in the thickness direction from one surface 112a (212a) to the other surface 112b (212b) of each partition wall 112 (212) is measured, assuming that P AVE the mean porosity of each partition 112 (212) is, P 1MIN the smallest porosity from one surface up to a thickness of 5% of the partition 112a (212a) is and P 2MIN the smallest porosity from the other surface up to a thickness of 5% of the partition 112b (212b) is preferable that {(P 1MIN + P 2MIN ) / 2} / P AVE ≤ 0.9 is satisfied. Although the lower bound of the value of {(P 1MIN + P 2MIN ) / 2} / P AVE Since it is not specifically defined, from the point of view of ease of manufacture it is common that 0.6 ≤ {(P 1MIN + P 2MIN ) / 2} / P AVE≤ 0.9 is satisfied, and typically 0.7 ≤ {(P 1MIN + P 2MIN ) / 2} / P AVE ≤ 0.9 is satisfied, and even more typically 0.8 ≤ {(P 1MIN + P 2MIN ) / 2} / P AVE ≤ 0.9 is fulfilled.

[0042] From the perspective of weight reduction of the columnar honeycomb structure, the lower limit of the mean porosity (P) AVE ) of each partition 112 (212) preferably 40% or greater, more preferably 45% or greater, and even more preferably 50% or greater. From the perspective of ensuring the strength of the columnar honeycomb structure, the upper limit of the mean porosity (P) AVE ) of each partition 112 (212) preferably 70% or less, further preferably 65% ​​or less, and even more preferably 60% or less. Accordingly, the average porosity (P AVE) each partition 112 (212) for example preferably in the range of 40 to 70%, more preferably 45 to 65% and even more preferably 50 to 60%.

[0043] The observation procedure of each partition wall with an X-ray microscope and the measurement of P AVE and {(P 1MIN + P 2MIN ) / 2} / P AVE Each partition wall will be constructed according to the following procedures.

[0044] First, samples of the partition walls (cross-sectional size (length 20 mm × width 2 mm) × depth 0.3 mm), in which the cross-section is exposed orthogonally to the direction in which the cells extend, are taken near the central axis, near the center in the radial direction (near the center between the central axis and the outer circumferential wall) and near the outer circumferential side wall (but without the outer circumferential side wall), specifically for the vicinity of the first end face, the vicinity of the center in the vertical direction and the vicinity of the second end face of the columnar honeycomb structure.Next, after examining the cross-section of each sample with an X-ray microscope and CT scanning, the resulting three-dimensional cross-sectional image is binarized based on brightness and divided into a large number of voxels consisting of spatial sections and base material sections (size of a voxel = a cube with a length in the wall surface direction of the partition (Y-direction): 0.8 µm, a length in the thickness direction of the partition (X-direction): 0.8 µm, and a length in the depth direction of the partition (Z-direction): 0.8 µm).

[0045] The measurement condition of the X-ray microscope is a 4x magnification.

[0046] The binarization process is performed using the Otsu binarization method.

[0047] Then, for a predetermined area (a length in the direction of the wall surface of the partition (Y-direction): 340 µm, a length in the thickness direction of the partition (X-direction): a length that includes the entire thickness and has a space of 50 µm or more on both sides of the partition, and a length in the depth direction of the partition (Z-direction): 300 µm) of any partition on the three-dimensional cross-sectional image based on the binarized voxel data, a porosity profile (%) is obtained every 0.8 µm along the thickness direction (X-direction) of the partition from one surface to the other (see Fig.6) For the region with a thickness of 0.8 µm (a length in the direction of the wall surface of the partition (Y-direction): 340 µm, a length in the direction of the thickness of the partition (X-direction): 0.8 µm, and a length in the direction of the depth of the partition (Z-direction): 300 µm), the porosity at a thickness of 0.8 µm is calculated according to the following formula: Porosity = (Number of voxels in the spatial section) / (Total number of voxels in the region) × 100 (%). By performing the calculation for the entire predetermined region every 0.8 µm from the left end of the screen, the porosity profile (%) is obtained at intervals of 0.8 µm along the thickness direction (X-direction) of the partition from one surface to the other.

[0048] At this point, as in Fig.Figure 7 shows that the position of a partition surface is defined as the position of a mode when measuring the distance M in the thickness direction D (X-direction) from a line segment parallel to the direction of the partition wall surface to a partition surface to be measured. The distance M is measured at intervals of 0.8 µm over a length of 340 µm in the direction of the wall surface (Y-direction perpendicular to the thickness direction (X-direction)) of the partition on the binarized image. The position of the other partition surface is specified in the same way.

[0049] In this way, the porosity profile of any partition wall is obtained from each sample, and P AVE and (P 1MIN + P 2MIN ) / 2 of the partition wall are obtained from the profile, and {(P 1MIN + P 2MIN ) / 2} / P AVE is calculated. Then the mean of P is determined. AVE of the total nine samples as P AVEthe columnar honeycomb structure to be measured. Furthermore, the mean value of {(P 1MIN + P 2MIN ) / 2} / P AVE of the nine samples as {(P 1MIN + P 2MIN ) / 2} / P AVE taken from the columnar honeycomb structure to be measured.

[0050] Furthermore, the porosity profile of any partition wall is obtained from each sample, and (P 1MIN + P 2MIN ) / 2 of the partition wall is obtained from the profile, and the mean of (P 1MIN + P 2MIN ) / 2 of the total nine samples will be considered (P 1MIN + P 2MIN ) / 2 of the columnar honeycomb structure to be measured. At this point, from the perspective of ensuring strength and suppressing the penetration of the catalyst into the interior of the partitions, the upper limit of (P 1MIN + P 2MIN ) / 2 preferably 70% or less, more preferably 60% or less. Although the lower bound of (P1MIN + P 2MIN ) / 2 unless specifically specified, from the point of view of ease of manufacture it is generally 20% or greater, typically 25% or greater.

[0051] For use as an exhaust filter and / or catalyst support for motor vehicles, the flexural strength of the columnar honeycomb structure is preferably 6.0 MPa or greater, more preferably 7.0 MPa or greater, and even more preferably 8.0 MPa or greater. Although the upper limit of the flexural strength is not specifically defined, it is usually 15.0 MPa or less, and typically 12.0 MPa or less. As used here, the flexural strength of the columnar honeycomb structure refers to the four-point flexural strength measured according to JIS R1664:2004. It should be noted that the specimen size is approximately 100 mm wide (w) × 20 mm thick (t), with an overall length (h) of about 100 mm. The distance between the inner supports (inner span) is 20 mm, and the distance between the outer supports (outer span) is 60 mm. Furthermore, the direction of the overall length of the specimen is the direction in which the cell extends.The sample is taken from a location near the central axis and near the center in the vertical direction of the columnar honeycomb structure.

[0052] The shape of the cell openings in cross-section, orthogonal to the direction in which the cells extend, is not restricted and is preferably square, hexagonal, octagonal, or a combination thereof. Squares and hexagons are preferred. By shaping the cell openings as described above, the pressure drop when exhaust gases flow through the honeycomb structure is reduced, and the cleaning performance when used as a filter is excellent. Furthermore, by shaping the cell openings as described above, the pressure drop when a fluid flows through the columnar honeycomb structure is reduced, and the cleaning performance of the catalyst is excellent.

[0053] The cell density (number of cells per unit cross-sectional area) of the columnar honeycomb structure is not particularly limited and can, for example, range from 6 to 2000 cells / square inch (0.9 to 311 cells / cm²). 2 ), preferably 50 to 1000 cells / square inch (7.8 to 155 cells / cm²). 2 ) and particularly preferably 100 to 600 cells / square inch (15.5 to 92.0 cells / cm²) 2 ). Here, the cell density is calculated by dividing the number of cells in the columnar honeycomb structure by the end face on one side of the columnar honeycomb structure, excluding the outer circumferential side wall.

[0054] The bulk density of the columnar honeycomb structure is preferably as low as possible to ensure the desired strength. The bulk density of the columnar honeycomb structure is preferably in the range of 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 according to the following formula: Bulk density (g / cc) = Mass (g) of the columnar honeycomb structure / Volume (cc) of the columnar honeycomb structure based on external dimensions.

[0055] When the columnar honeycomb structure is used as a catalyst support, the surface of the partitions can be coated with a catalyst, depending on the purpose. The type of catalyst is not limited; however, options include a DOC oxidation catalyst to increase exhaust gas temperature through the oxidative combustion of hydrocarbons (HC) and carbon monoxide (CO), a PM combustion catalyst to support 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 may contain, for example, precious metals (Pt, Pd, Rh and the like), alkali metals (Li, Na, K, Cs and the like), alkaline earth metals (Mg, Ca, Ba, Sr and the like), rare earths (Ce, Sm, Gd, Nd, Y, La, Pr and the like), transition metals (Mn, Fe, Co, Ni, Cu, Zn, Sc, Ti, Zr, V, Cr and the like) and the like. <2. Method for producing a columnar honeycomb structure>

[0056] The following describes a process for producing a columnar honeycomb structure as an example. First, a starting material composition containing a ceramic base material, a dispersion medium, a pore-forming agent, and a binder is kneaded to form a green body. Then, a columnar honeycomb structure can be produced by extruding and drying the green body. Additives such as a dispersing agent can be added to the starting material composition as needed. During extrusion, a die with a desired overall shape, cell shape, septum thickness, cell density, and the like can be used.

[0057] In the drying step, conventionally known drying methods such as hot gas drying, microwave drying, dielectric drying, reduced pressure drying, vacuum drying, and freeze-drying can be used. Of these, a drying method combining hot gas drying with microwave or dielectric drying is preferable because the entire molded body can be dried quickly and uniformly. The sealing sections can be formed by creating them at predetermined positions on both end faces of the dried honeycomb-shaped body and then drying them.

[0058] The ceramic starting material is a portion of a metal oxide, metal, or the like that remains after firing and forms the framework of the columnar honeycomb body (the columnar honeycomb structure) after firing as a ceramic. The ceramic starting material may be provided, for example, in powder form. Examples of ceramic starting materials include materials for obtaining a ceramic such as cordierite, mullite, zirconate, aluminum titanate, silicon carbide, silicon nitride, zirconium dioxide, spinel, indialite, sapphire, corundum, and titanium dioxide. In particular, examples include, but are not limited to, silicon dioxide, talc, aluminum oxide, kaolin, serpentine, pyroferrite, bluesite, boehmite, mullite, magnesite, and aluminum hydroxide. A single type of ceramic starting material may be used, or a combination of two or more types may be used.From the perspective of reducing the porosity of the partition surface while simultaneously decreasing the pore diameter, the median diameter (D50) of the silicon dioxide particles is preferably relatively large, for example in the range of 15 to 30 µm. On the other hand, with other ceramic starting materials, it is preferable to use fine particles with a median diameter (D50) close to the desired mean pore diameter of the partitions, for example in the range of 2 to 10 µm.

[0059] In filter applications such as DPFs and GPFs, cordierite can be preferably used as the ceramic material. In this case, a cordierite-forming feedstock can be used as the ceramic material. A cordierite-forming feedstock is a material that transforms into cordierite upon firing. It is desirable that the cordierite-forming feedstock has a chemical composition of aluminum oxide (Al₂O₃) (containing the amount of aluminum hydroxide that converts to aluminum oxide): 30 to 45% by mass, magnesium oxide (MgO): 11 to 17% by mass, and silicon dioxide (SiO₂): 42 to 57% by mass.

[0060] Examples of the dispersion medium include water or a mixed solvent of water and an organic solvent such as alcohol, with water being particularly preferred.

[0061] The pore-forming agent is not particularly restricted as long as it forms pores after firing, and examples include wheat flour, starch, foam resin, water-absorbing resin, silica gel, carbon (e.g., graphite, coke), ceramic balloon, polyethylene, polystyrene, polypropylene, nylon, polyester, acrylic, phenol, and the like. A single type of pore-forming agent may be used, or a combination of two or more types may be used. The content of the pore-forming agent, with a view to increasing the porosity of the honeycomb-shaped body, is preferably 0.5 parts by mass or more, more preferably 2 parts by mass or more, and still more preferably 3 parts by mass or more, based on 100 parts by mass of the ceramic starting material.To ensure the strength of the honeycomb-shaped body, the pore-forming agent content is preferably 10 parts by mass or less, more preferably 7 parts by mass or less, and even more preferably 4 parts by mass or less, based on 100 parts by mass of the ceramic starting material. To reduce the porosity of the surface of the partitions and simultaneously decrease the pore diameter, it is preferable to set the median diameter (D50) of the pore-forming agent at a relatively large value, for example, in the range of 20 to 30 µm.

[0062] Examples of binders include organic binders such as methylcellulose, hydroxypropylmethylcellulose, hydroxyethylcellulose, carboxymethylcellulose, polyvinyl alcohol, and the like. In particular, it is preferable to use methylcellulose and hydroxypropylmethylcellulose in combination. Furthermore, to increase the strength of the honeycomb-shaped body before firing, the binder content is preferably 4 parts by mass or more, more preferably 5 parts by mass or more, and even more preferably 6 parts by mass or more, based on 100 parts by mass of the ceramic starting material. To suppress the occurrence of cracks due to abnormal heat generation during the firing process, the binder content is preferably 9 parts by mass or less, more preferably 8 parts by mass or less, and even more preferably 7 parts by mass or less, based on 100 parts by mass of the ceramic starting material.A single type of binder can be used alone, or a combination of two or more types can be used.

[0063] Suitable dispersants include ethylene glycol, dextrin, fatty acid soap, polyether polyol, and the like. A single dispersant type can be used, or a combination of two or more types can be employed. The dispersant content is preferably in the range of 0 to 2 parts by mass, based on 100 parts by mass of the ceramic starting material.

[0064] The columnar honeycomb structure can be open at both ends of all cells, as in Fig. 1 and Fig. 2 shown. Furthermore, the columnar honeycomb body can have a cell structure in which one end of the cells is alternately sealed, as in Fig. 3 and Fig.4 shown. The method for sealing the end face of the columnar honeycomb body is not particularly restricted, and a known method can be used.

[0065] The material of the sealing sections is not particularly restricted; however, ceramic is preferred from the perspective of strength and heat resistance. The ceramic is preferably a ceramic comprising at least one selected from the group consisting of cordierite, mullite, zirconium, aluminum titanate, silicon carbide, silicon nitride, zirconium dioxide, spinel, indialite, sapphire, corundum, and titanium dioxide. The sealing sections are preferably formed from a material containing a total of 50% or more by mass of this ceramic, and more preferably from a material containing a total of 80% or more by mass of this ceramic. It is even more preferred that the sealing sections have the same material composition as the main body section of the honeycomb structure, as this ensures a uniform coefficient of thermal expansion during firing and improves durability.

[0066] A method for forming the sealing section is described as an example. A sealing slurry is held in a reservoir. Next, a mask with openings at positions corresponding to the cells on which the sealing sections are to be formed is attached to one of the end faces. The end face to which the mask is attached is immersed in the reservoir, and the openings are filled with a sealing slurry, thus forming the sealing sections. The sealing sections can be formed on the other end face in the same way.

[0067] A columnar honeycomb structure can be produced by degreasing and firing the dried columnar honeycomb body. Depending on the material composition of the honeycomb body, known conditions can be used for the degreasing and firing processes, and although no further explanation is necessary, specific examples of such conditions are given below.

[0068] The degreasing process is described. The combustion temperature of the binder is approximately 200 °C, and the combustion temperature of the pore-forming agent is in the range of 300 to 1000 °C. Therefore, the degreasing step can be carried out by heating the honeycomb structure in the range of approximately 200 to 1000 °C. The heating time is not particularly limited, but is typically in the range of approximately 10 to 100 hours. The honeycomb structure after the degreasing step is referred to as a calcined structure.

[0069] The firing process depends on the material composition of the honeycomb body, but can be carried out, for example, by heating the calcined body to 1350 to 1600 °C and holding the temperature for 3 to 10 hours. EXAMPLES (1. Creating the honeycomb structure)<Beispiele 1 und 2, Vergleichsbeispiele 1 und 2>

[0070] A cordierite-forming starting material, a pore-forming agent A, a pore-forming agent B, a binder, a dispersant, and a dispersion medium were added, mixed, and kneaded according to the formulation shown in Table 1 for each test number to prepare a green body. Talc, kaolin, aluminum oxide, aluminum hydroxide, silicon dioxide A, and silicon dioxide B were used as the cordierite-forming material. Silicon dioxide A and silicon dioxide B had different median diameters (D50). Pore-forming agent A and pore-forming agent B also had different median diameters (D50). Water was used as the dispersion medium, a polyacrylic acid-based polymer was used as the pore-forming agent, hydroxypropyl methylcellulose was used as the binder, and a fatty acid soap was used as the dispersant.The median diameter (D50) of each material shown in Table 1 is a volume-based value measured using a laser diffraction particle size distribution analyzer (model LA960, manufactured by HORIBA, Ltd.).

[0071] The green material was loaded into an extruder and extruded horizontally through a predetermined die to obtain a cylindrical honeycomb structure. After dielectric drying and hot gas drying of the resulting honeycomb structure, both end faces were trimmed to obtain a cylindrical honeycomb structure with predetermined dimensions.

[0072] The resulting cylindrical honeycomb body was degreased by heating it to 200 °C for 8 hours in an air atmosphere and then fired at 1430 °C for 4 hours in an air atmosphere to obtain a columnar honeycomb structure. The required number of columnar honeycomb structures for each test sample were produced for the following measurements. The specifications of the resulting columnar honeycomb structures are as follows.

[0073] Overall shape: cylindrical with a diameter of 118 mm and a height of 91 mm

[0074] Cell shape in cross-section perpendicular to the flow direction of the cells: square

[0075] Cell density (number of cells per unit cross-sectional area): 750 cells / square inch

[0076] Average partition thickness: 2.6 mil (66 µm) (nominal value based on mold specifications) (2. Measurement of the mean pore diameter of the partition walls)

[0077] For each columnar honeycomb structure obtained by the fabrication process described above, the mean pore diameter of the partitions was determined using Autopore 9505, manufactured by Micromeritics Instrument Corporation, in accordance with the procedure described above. The results are shown in Table 2. (3. Measurement of the porosity profile of partition walls)

[0078] For each columnar honeycomb structure obtained by the manufacturing process described above, an X-ray microscope (model Xradia 520 Versa, manufactured by Zeiss) was used to measure the porosity profile of the partitions according to the procedure described above. AVE , (P 1MIN + P 2MIN ) / 2, and {(P 1MIN + P 2MIN ) / 2} ÷ P AVE (P AVE : medium porosity in each partition wall, P 1MIN: smallest porosity from a surface up to a thickness of 5% of the partition, P 2MIN : smallest porosity from the other surface up to a thickness of 5% of the partition. The results are shown in Table 2. (4. Volume density)

[0079] The bulk density of each columnar honeycomb structure obtained by the manufacturing process described above was determined according to the procedure described above. The results are shown in Table 2. (5. Measurement of flexural strength)

[0080] The flexural strength of each columnar honeycomb structure produced by the above-described method was measured using an Instron 3366 Dual Column Desktop Type Tester, according to the above-described method. Table 2 shows the results. Table 2 Test No. Example 1 Example 2 Comparative example 1 Comparative example 2 Mean pore diameter of the partition (µm) 4,3 4,9 5,6 4,8 PAVE (%) 45,2 54,1 46,2 52,8 (P 1MIN + P 2MIN ) / 2 (%) 37,3 47,1 43,3 50,7 {(P 1MIN + P 2MIN ) / 2} / P AVE 0,83 0,87 0,94 0,96 Volume density (g / cc) 0,23 0,19 0,22 0,2 Flexural strength (MPa) 8,1 6,3 6,8 5,4 (6. Discussion)

[0081] When comparing Example 1 and Comparison Example 1, where the average porosity (P) AVE Example 1 showed that the individual partition walls are close together, whose {(P 1MIN + P 2MIN ) / 2} / P AVE A higher flexural strength was appropriate. Similarly, when comparing Example 2 and Comparison Example 2, where the average porosity (P) AVE Example 2 showed that the individual partition walls are close together, whose {( P 1MIN + P 2MIN ) / 2} / P AVE A higher flexural strength was appropriate. Description of the reference symbols 100 Columnar honeycomb structure 102 Outer perimeter side wall 104 First end face 106 Second front face 108 cells 112 Partition wall 112a A surface of the partition 112b The other surface of the partition 200 Columnar honeycomb structure 202 Outer perimeter side wall 204 First front face 206 Second front face 208a First cell 208b Second cell 209 Sealing section 212 Partition wall C Focus D Thickness direction L line segment M distance

Claims

[1] A columnar honeycomb structure comprising an outer circumferential side wall and several partitions arranged on an inner circumferential side of the outer circumferential side wall, dividing several cells that form flow paths from a first end face to a second end face, wherein a mean pore diameter of the partitions, measured by a mercury intrusion method specified in JIS R1655: 2003, is in the range of 3 to 10 µm, and when a cross-section of the several partitions is observed with an X-ray microscope and a porosity (%) is measured in a thickness direction from one surface to the other surface of each partition, assuming that P AVE a medium porosity of each partition wall is, P 1MIN a smallest porosity from one surface up to a thickness of 5% of the partition wall and P 2MINThe smallest porosity from the other surface up to a thickness of 5% of the partition is 40% ≤ P AVE ≤ 70%, and {(P 1MIN + P 2MIN ) / 2} / P AVE ≤ 0.9 are fulfilled. [2] Columnar honeycomb structure according to claim 1, wherein 0.6 ≤ {(P 1MIN + P 2MIN ) / 2} / P AVE ≤ 0.9 is fulfilled. [3] Columnar honeycomb structure according to claim 1 or 2, wherein the mean thickness of the multiple partitions is in the range of 50 to 150 µm. [4] Columnar honeycomb structure according to any one of claims 1 to 3, wherein a flexural strength measured according to JIS R1664: 2004 is 6.0 MPa or greater. [5] Columnar honeycomb structure according to any one of claims 1 to 4, wherein 50% ≤ P AVE ≤ 60% is fulfilled. [6] Columnar honeycomb structure according to any one of claims 1 to 5, wherein the bulk density is in the range of 0.15 g / cc to 0.25 g / cc. [7] Columnar honeycomb structure according to any one of claims 1 to 6, wherein the partitions are formed of ceramic comprising 90% or more by mass of cordierite. [8] Columnar honeycomb structure according to one of claims 1 to 7, wherein a catalyst layer is provided on the surfaces of the partitions.

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