Exhaust gas purification catalyst and its manufacturing method

Abstract

This exhaust gas cleaning catalyst comprises: a base material; and an adsorption part which is provided to the base material and which includes an adsorption material containing Si. The adsorption part has a plurality of openings. In a cross section orthogonal to an exhaust gas flow direction, the percentage of the total sum of the areas of openings that are included in the adsorption part and satisfy the following formula, with respect to the apparent area of the adsorption part present on the base material, is over 5% and no greater than 30%. Formula: L / \{2(πS)1 / 2\} ≤ 1.1 (where L is a circumferential length of an opening in the cross section; and S is an area of an opening in the cross section).

Description

[Technical Field]

[0001] The present invention relates to an exhaust gas purification catalyst and a method for producing the same. [Background technology]

[0002] Exhaust gas purification catalysts used in engines such as gasoline engines exhibit their exhaust gas purification performance by coming into contact with the high-temperature exhaust gas emitted from the engine and increasing in temperature. However, immediately after starting the engine, the exhaust gas purification catalyst may not be able to fully exhibit its exhaust gas purification performance due to insufficient temperature rise, resulting in the emission of hydrocarbons (HC) and other substances in the exhaust gas without purifying them. Therefore, the use of an HC trap material that adsorbs HC in the exhaust gas immediately after starting the engine and desorbs the HC after the temperature of the exhaust gas purification catalyst has increased due to the exhaust gas after some time has passed since engine start has started has been considered. Adsorbents containing Si (silicon), such as zeolite, have been known as HC trap materials. For example, Patent Documents 1 to 3 describe exhaust gas purification catalysts equipped with a catalyst layer containing zeolite.

[0003] Furthermore, in the field of exhaust gas purification catalysts, a technique is known in which a catalyst layer is provided with pores formed by a pore-forming agent in order to improve the contact efficiency with exhaust gas. For example, Patent Documents 4 and 5 describe exhaust gas purification catalysts with specific pores. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] EP3623048 A1 [Patent Document 2] US2019099749 A1 [Patent Document 3] WO2017178801 A1 [Patent Document 4] Patent No. 6751831 [Patent Document 5] WO2021 / 029098 Brochure Summary of the Invention

[0005] Si-containing adsorbents such as zeolite are typically formed as layered adsorption sections (adsorption layers) on a substrate. With such adsorption layers, exhaust gases pass near the surface of the adsorption layer at high speed during flow, resulting in poor contact efficiency with the zeolite in the adsorption layer, making it difficult to achieve HC adsorption performance. One possible solution to this problem is to provide voids in the adsorption layer to increase the contact efficiency between the exhaust gas and the zeolite. However, because Si-containing adsorbents have a high specific surface area, when they are formed on a substrate or other catalyst layer, they generally exhibit poor adhesion to the substrate or catalyst layer. Therefore, providing voids presents the problem of the adsorption layer easily peeling off from the substrate or catalyst layer. Patent Documents 1 to 3 do not consider the compatibility of HC adsorption performance and peeling resistance, and Patent Documents 4 and 5 consider the prevention of pressure loss and PM collection rate in particulate filters such as GPC, but do not consider the HC adsorption performance in an adsorption section having a Si-containing adsorbent.

[0006] The present invention aims to solve the problems of the conventional technology, and has an object to provide an exhaust gas purification catalyst that can achieve both improved HC adsorption performance and high exfoliation resistance in an adsorption part having a Si-containing adsorbent, which has been difficult to achieve in the past.

[0007] The present invention provides the following [1] to

[17] . [1] An exhaust gas purification catalyst comprising a substrate and an adsorption section provided on the substrate and containing an adsorbent containing Si, The adsorption portion has a plurality of voids, The present invention provides an exhaust gas purification catalyst in which, in a cross section perpendicular to the exhaust gas flow direction, the proportion of the total amount of voids in the adsorption portion that satisfies the following formula relative to the apparent area of ​​the adsorption portion present on the substrate is more than 5% and 30% or less. Formula: L / {2(πS) 1 / 2}≦1.1 (L is the perimeter of the gap in the cross section, and S is the area of ​​the gap in the cross section) [2] The exhaust gas purification catalyst according to [1], wherein the adsorbent containing Si is a zeolite. [3] The exhaust gas purification catalyst according to [2], wherein the zeolite has at least one pore structure selected from the group consisting of 10-membered rings and 12-membered rings. [4] The exhaust gas purification catalyst according to [2] or [3], wherein the zeolite is at least one type of zeolite selected from BEA type, MSE type, and MFI type. [5] The exhaust gas purifying catalyst according to any one of [2] to [4], wherein the zeolite content of the adsorption part is 10% by mass or more and 90% by mass or less. [6] The exhaust gas purification catalyst according to any one of [1] to [5], wherein the adsorption portion contains at least one kind of precious metal selected from Pt, Pd, and Rh. [7] The exhaust gas purifying catalyst according to any one of [1] to [6], wherein the value of the ratio to the thickness of the adsorption portion is 0.20% / μm or more and 1.00% / μm or less. [8] An exhaust gas purification catalyst according to any one of [1] to [7], wherein the average value of the equivalent circle diameter of voids present in the adsorption section and satisfying the formula is 1 μm or more and 60 μm or less. [9] The average number of voids present in the adsorption part and satisfying the formula is less than 1 mm2 of the cross-sectional area of ​​the adsorption part. 2 The exhaust gas purifying catalyst according to any one of [1] to [8], wherein the number of particles per particle is 200 or more and 800 or less.

[10] An exhaust gas purification catalyst according to any one of [1] to [9], wherein the standard deviation of the equivalent circular diameter of voids present in the adsorption section and satisfying the formula is less than 25% of the average equivalent circular diameter.

[11] The exhaust gas purifying catalyst according to any one of [1] to

[10] , wherein the substrate is a flow-through substrate.

[12] The exhaust gas purification catalyst according to any one of [1] to

[11] , which has an adsorption layer containing an adsorbent material containing Si as the adsorption portion, and further has a purification layer containing a catalytically active component.

[13] The exhaust gas purifying catalyst according to any one of [1] to

[12] , wherein the adsorption portion occupies 20 mass % or more and 50 mass % or less of the total coating amount.

[14] The exhaust gas purification catalyst according to any one of [1] to

[13] , which has an adsorption layer containing an adsorbent material containing Si as the adsorption portion, and the thickness of the adsorption layer accounts for 20% to 80% of the thickness of the entire coating layer on the substrate.

[15] An exhaust gas purification system having two or more exhaust gas purification catalysts in an exhaust gas flow path, the exhaust gas purification system having the exhaust gas purification catalyst according to any one of [1] to

[14] as the second or subsequent catalyst from the upstream in the exhaust gas flow direction.

[16] An adsorbent containing Si; L' / {2(πS') 1 / 2 a step of applying a slurry containing a pore-forming agent made of a crosslinked resin, the pore-forming agent containing 90% or more particles by number having a circularity of 1.05 or less, and water, to a substrate; and A step of firing the slurry after coating on the substrate to form an adsorption layer containing the adsorbent and voids formed by burning off the pore-forming agent. A method for producing an exhaust gas purification catalyst comprising: The method for producing an exhaust gas purification catalyst includes stirring the slurry at a rotation speed of 3000 rpm or more and 8000 rpm or less, and applying the slurry to a substrate in a state where the viscosity at 25°C is 2000 cP or more and 100,000 cP or less. (where L' is the perimeter of the image of the pore-forming agent observed with a scanning electron microscope, and S' is the area of ​​the image observed with a scanning electron microscope.)

[17] The method for producing an exhaust gas purification catalyst according to

[16] , wherein the amount of the pore-forming agent in the slurry is 10 parts by mass or more and 60 parts by mass or less per 100 parts by mass of the solid content excluding the pore-forming agent in the slurry. [Brief explanation of the drawings]

[0008] [Figure 1] FIG. 1 is a schematic perspective view of an exhaust gas purification catalyst according to one embodiment of the present invention. [Figure 2] FIG. 2 is an enlarged view of a part of a cross section along the axial direction of the substrate. [Figure 3] FIG. 3 is a cross-sectional view of the rectangular portion in FIG. 2 taken along the axial direction of the substrate. [Figure 4] FIG. 4 is a diagram showing an example of a sampling method for observing the cross section of an adsorption layer. [Figure 5] FIG. 5 is a diagram showing an example in which a provisional boundary line is drawn on a scanning electron microscope image of the adsorption layer 12 on the partition wall (substrate). [Figure 6] 6(a) and (b) are diagrams showing examples of observed images of the cross section of the adsorption layer. [Figure 7] FIG. 7 is a diagram showing an example of a scanning electron microscope image of the adsorption layer 12 on the partition wall (substrate) with dividing lines drawn. [Figure 8] 8(a) and (b) are schematic diagrams for explaining how to draw division lines to determine the void ratio of an approximately perfect circle. [Figure 9] 9(a) to 9(c) are schematic diagrams for explaining how to draw division lines to find the void ratio of an approximately perfect circle. [Figure 10] 10(a) and (b) are schematic diagrams for explaining how to draw division lines to find the void ratio of an approximately perfect circle. [Figure 11] 11(a) and (b) are schematic diagrams for explaining a method for measuring the porosity of an approximately perfect circle. DETAILED DESCRIPTION OF THE INVENTION

[0009] The present invention will be described below based on preferred embodiments thereof, but the present invention is not limited to the following embodiments. The exhaust gas purifying catalyst 10 of this embodiment will be described below. The exhaust gas purification catalyst 10 of this embodiment includes a substrate 11 and an adsorption section 12 (hereinafter also referred to as "adsorption layer 12") that contains an adsorbent containing Si and is provided on the substrate 11. The adsorbent containing Si and the adsorption section (adsorption layer) that contains it adsorb organic substances in exhaust gas, such as HC (hydrocarbons), aldehydes, alcohols, carboxylic acids, ketones, ethers, esters, etc.

[0010] The exhaust gas purification catalyst 10 has a substrate 11 as a support for forming the adsorption layer 12. Typical shapes of the substrate 11 include honeycomb, pellet, and foam shapes, with the honeycomb shape being preferred. Examples of honeycomb-shaped substrates include substrates having a large number of cells 15, which are long exhaust gas flow channels parallel to the axial direction, such as wall-flow and flow-through substrates. In the present invention, the use of a flow-through substrate is preferred from the viewpoint of forming a uniform adsorption layer 12. FIGS. 1 and 2 show an example in which the substrate 11 of the exhaust gas purification catalyst 10 is a flow-through honeycomb substrate. For example, a flow-through honeycomb substrate having approximately 200 to 900 cells per square inch, a volume of 0.01 to 2.0 L, an axial length of 15 to 200 mm, and partition walls 23 separating the cells with a thickness of 50 to 150 μm is preferred from the viewpoint of HC adsorption performance and conversion of adsorbed HC.

[0011] Examples of materials for the substrate 11 include ceramics such as alumina (Al2O3), mullite (3Al2O3-2SiO2), cordierite (2MgO-2Al2O3-5SiO2), aluminum titanate (Al2TiO5), and silicon carbide (SiC), as well as metal materials such as stainless steel. The substrate 11 typically has a cylindrical shape as shown in FIG. 1, and is disposed in the exhaust path of the internal combustion engine so that the axial direction of the columnar shape substantially coincides with the exhaust gas flow direction X. The overall outer shape of the substrate 11 may be an elliptical cylinder or a polygonal cylinder instead of a cylindrical shape. The substrate 11 is disposed so that its axial direction is parallel to the exhaust gas flow direction X.

[0012] Adsorption sections 12 containing a layer of Si-containing adsorbent are provided on the partition walls 23 of the substrate 11. The coating layer 16 provided on the substrate 11 in FIG. 2 is composed of layer of adsorption sections 12 containing Si-containing adsorbent, or has layer of adsorption sections 12 containing Si-containing adsorbent in a part of its thickness direction. Hereinafter, the layer of adsorption section 12 containing Si-containing adsorbent is also referred to as "adsorption layer 12." FIGS. 3(a) to 3(c) show examples of a configuration in which the adsorption layer 12 is provided on the partition walls 23 of the substrate 11. In the examples shown in FIGS. 3(a) and 3(b), the adsorption layer 12 is provided on the partition walls 23 so as to be in direct contact with the partition walls 23. In FIG. 3(b), another layer 13 is formed on the side of the adsorption layer 12 opposite the partition walls 23. Alternatively, the adsorption layer 12 may be provided on the partition walls 23 via another layer 13, as shown in FIG. 3(c). The adsorption layer 12 is formed on at least a part of the substrate 11 in the exhaust gas flow direction X. As described above, in this embodiment, as a specific example in which an adsorption section containing a Si-containing adsorbent is provided on the base material 11, the adsorption section containing a Si-containing adsorbent is provided on the partition wall 23 of the base material 11. Note that the "other layer 13" here refers to a layer that does not contain a Si-containing adsorbent. As shown in Figures 3(a) to 3(c), the adsorption layer 12 has a plurality of voids 12c.

[0013] In the present invention, it has been discovered that the HC adsorption performance of the adsorption layer 12 is excellent when the circular porosity in the cross section of the adsorption layer 12 perpendicular to the exhaust gas flow direction X is equal to or greater than a specific value. The greater the number of voids in the adsorption layer 12, the higher the probability of contact between the Si-containing adsorbent in the adsorption layer 12 and the HC in the exhaust gas, which is advantageous for the HC adsorption performance. On the other hand, the greater the number of voids in the adsorption layer 12, the more voids there are likely to be near the interface between the adsorption layer 12 and the other layer 13 or the partition wall 23. This is because the voids in the adsorption layer 12 are usually obtained by adding a pore-forming agent to the adsorption layer-forming slurry and then calcining the slurry to remove the pore-forming agent. However, due to differences in particle size and specific gravity, the pore-forming agent used to obtain the voids is likely to accumulate near the interface between the slurry-coated portion of the substrate 11 and the partition wall 23 or the other layer 13 during the coating and calcination processes of the slurry. Compared with voids of other shapes, circular voids in the cross section of catalyst 10 have fewer points of contact with the partition walls (substrate) and other layers. In addition, the spherical pore-forming agent that creates circular voids in the cross section of catalyst 10 has a smaller surface area per unit volume and is less likely to aggregate than needle-shaped or rectangular pore-forming agents, so the formation of large voids in adsorption layer 12 is suppressed. From the above viewpoints, the inventors further investigated the relationship between the HC adsorption performance and peeling performance and the area ratio of the circular voids in the adsorption layer, and found that a specific value of the circular void ratio makes it possible to obtain an adsorption layer 12 that has excellent HC adsorption performance while also having high peeling resistance.

[0014] Specifically, in the cross section of the exhaust gas purifying catalyst 10 perpendicular to the exhaust gas flow direction (hereinafter also referred to as the "X direction" or the "exhaust gas flow direction X"), the apparent area of ​​the adsorption layer 12 present on the substrate 11 is expressed by the formula: L / {2(πS) 1 / 2}≦1.1 (where L is the circumferential length of the void in the cross section, and S is the area of ​​the void in the cross section), is preferably more than 5% and not more than 30% (also referred to in this specification as "circular porosity").

[0015] The apparent area of ​​the adsorption layer 12 present on the substrate 11 in the cross section refers to the entire area including voids in the adsorption layer 12 present on the substrate 11 in the cross section. Formula: L / {2(πS) 1 / 2 The closer the value of} is to 1, the closer the shape is to a perfect circle, with the minimum value being 1. Therefore, void shapes whose values ​​derived by the above formula are 1.1 or less are close to a perfect circle. Hereinafter, voids that satisfy the above formula are referred to as approximately perfect circular voids. For the convenience of measuring the circular void ratio based on image processing, it is preferable that the approximately perfect circular void has an equivalent circle diameter of 1 μm or more. Therefore, the circular void ratio is preferably specifically the ratio of the total area of ​​voids that satisfy the above formula and have an equivalent circle diameter of 1 μm or more and 60 μm or less. Here, the equivalent circle diameter refers to the diameter when the void is considered to be a circle of the same area.

[0016] A circular porosity of more than 5% can improve the HC adsorption performance of the adsorption layer 12. Furthermore, a circular porosity of 30% or less makes the adsorption layer 12 less likely to peel off from the partition walls 23 or other layers 13. From these viewpoints, the circular porosity of the adsorption layer 12 on the partition walls 23 of the substrate 11 is preferably 5% or more and 30% or less, more preferably 10% or more and 25% or less, and even more preferably 15% or more and 25% or less.

[0017] The circular porosity is preferably measured based on scanning electron microscope observation. There is no limitation on the X-direction position of the cross section of the exhaust gas purification catalyst 10 to be observed with the scanning electron microscope, and the cross section can be observed at any position. The circular porosity can be measured, for example, as follows.

[0018] (1) Sampling In the exhaust gas purification catalyst 10, as shown in FIG. 4(a), a cylindrical sample Sp having a diameter of 25.4 mm and a central axis parallel to the exhaust gas flow direction is cut out. This cylindrical sample Sp has a length that spans the entire exhaust gas flow direction X of the exhaust gas purification catalyst 10. In a plane perpendicular to the exhaust gas flow direction (see FIG. 4(b)), there is no particular limitation on the position of the cylindrical sample Sp cut out in the exhaust gas purification catalyst 10. As shown in FIG. 4(b), it is preferable that the central axis of the cutout portion is located at a position 10% to 70% of the radial length from the center C of the substrate 11 toward the radially outward direction in the plane, since this allows multiple cylindrical samples Sp to be collected from one catalyst. Note that the center of the substrate 11 in the plane is defined as the point that bisects the largest line segment that intersects the outer shape of the substrate 11 in the plane. The radial length is defined as half of the largest line segment.

[0019] As mentioned above, the cross-sectional position of the exhaust gas purification catalyst 10 in the exhaust gas flow direction X is not limited. For example, if the position is 5 mm or more away from the upstream or downstream end of the cylindrical sample Sp in the X direction, it is easy to observe the cross section of the adsorption layer 12. For example, the central portion of the sample Sp in the X direction may be used as the observation surface. These observation surfaces are exposed by cutting the sample along a cross section perpendicular to the X direction. The observation surface is embedded with resin and polished. Note that the thickness (length in the X direction) of the observation sample is preferably 10 mm for ease of sample handling. Figure 4(c) shows an example of obtaining two samples (T and B in Figure 4(c)) for observing positions t and b, respectively, 10 mm from the upstream and downstream ends in the exhaust gas flow direction X, and a sample (M in Figure 4(c)) for observing the center position m in the exhaust gas flow direction X.

[0020] (2) Identifying the outer edge of the partition wall that comes into contact with the adsorption layer The observed surface is subjected to EPMA (Electron Probe Micro Analyzer) mapping to observe and determine the distribution of the adsorption layer 12 and the partition walls 23 or other layers 13 located above or below it, respectively, at a magnification of 40 to 1000 times (accelerating voltage 15 kV to 25 kV). The components used to determine the distribution of the adsorption layer 12, the partition walls 23, and the other layers 13 in the EPMA can be identified by subjecting each of these layers and the substrate 11 to X-ray fluorescence analysis or the like. For example, if the partition walls 23 (substrate 11) contain cordierite, the distribution of Mg is observed as a substrate component. Since the adsorption layer 12 contains an adsorbent containing Si, it can be identified by observing the distribution of Si. Alternatively, if the adsorption layer 12 contains a specific element other than Si, the adsorption layer 12 may be identified by observing the distribution of the specific element. As for the other layers 13, they may be any components not contained in the substrate 11 or the adsorption layer 12. Images of the same position on the same sample are taken with a scanning electron microscope (SEM) at the same magnification as in the EPMA mapping, and the images are compared with the previous EPMA mapping image to determine the distributions of the adsorption layer 12, the substrate 11, and (if necessary) the other layers 13. If the adsorption layer 12 is in direct contact with the substrate 11, the "tentative boundary" of the adsorption layer 12 on the substrate 11 is identified by the outer edge of the distribution range of the substrate 11 components. When the adsorption layer 12 is provided on the substrate 11 via another layer 13, the "tentative boundary line" of the adsorption layer 12 on the other layer 13 is identified by the outer edge of the distribution range of the other layer 13. The acceleration voltage for SEM observation is preferably 10 kV to 15 kV.

[0021] As described above, the cross section of the cells of a honeycomb substrate perpendicular to the X direction is usually polygonal. In SEM and EPMA observation images of the cells, images including the intersections Cn between sides, such as the corners of the polygon, are used (see FIG. 5). FIG. 5 shows an example of an SEM image of a cross section perpendicular to the X direction of an exhaust gas purification catalyst 10 in which an adsorption layer 12 is formed directly on the partition walls of a honeycomb substrate having cells with a square cross section. FIG. 5 shows an image of a corner of a cell with a square cross section in the exhaust gas purification catalyst 10. The intersections Cn between the sides of the substrate 11, such as the corners, have a large amount of adsorbent component attached per unit area on the substrate 11, which tends to reduce the contact efficiency with the exhaust gas. Therefore, it is believed that having a specific circular porosity is highly effective. Furthermore, due to the large amount of adsorbent component attached at the intersections Cn, many voids fit within the layer, making it easy to observe the voids. The corners may be rounded.

[0022] When using the observation image of the intersection Cn described above, it is preferable to use an observation image of a portion in which the longitudinal length L1 (see FIG. 5) of the observation image is 15% or more of the perimeter of the cell 15 in the cross section of the substrate 11 (the inner perimeter in the cross section perpendicular to the X direction of the cell 15 before slurry application). Furthermore, when the cell 15 has a polygonal shape in the cross section perpendicular to the X direction, the longitudinal direction L1 of the observation image is preferably 80% or less of the length of one side of the polygon. When the lengths of the sides of the polygon are not the same, the length of one side referred to here is the average length of the lengths of the sides. It is preferable that there be only one intersection Cn in one observation image. Specifically, "one intersection Cn in one observation image" means the following: As shown in the area surrounded by frame 38 in FIG. 1, one intersection (corner) in a cell partitioned by a partition wall is adjacent to another intersection (corner). Therefore, depending on the observation image, for example, multiple intersections Cn may be observed in one observation image. For example, four intersections Cn are observed in FIG. 6(a), and two intersections Cn are observed in FIG. 6(b). However, in this specification, it is preferable that there be only one intersection Cn in one observation image (e.g., FIG. 5). However, it is acceptable that adjacent intersections Cn may inevitably be included in the observation image. Furthermore, it is preferable that the observation image be an image in which an adsorption portion of a predetermined length or more exists within the range of the observation image. That is, it is preferable to adopt an observation image in which the adsorption layer 12 extending along one intersection Cn and the two sides E1 and E2 connected thereto is continuously present for 80% or more of the length in the longitudinal direction A of the observation image (see FIG. 5).It is also preferable to adopt an observation image in which the adsorption layer 12 extending along one intersection Cn and the two sides E1 and E2 connected thereto is continuously present for 80% or more of the length in the width direction B of the observation image (see FIG. 5).

[0023] (3) Identifying the width of the plot As described above, the outer edge of the adsorption layer 12 on the substrate 11 side is determined based on the SEM observation image and the EPMA mapping image by the outer edge of the distribution range of the substrate components, or, if another layer 13 is interposed, the components of the other layer 13. Furthermore, when another layer 13 is laminated on the side of the adsorption layer 12 opposite the substrate 11, the outer edge of the adsorption layer 12 on the side opposite the substrate 11 is defined by the outer edge of the distribution range of the adsorption layer 12. When no other layer 13 is laminated on the side of the adsorption layer 12 opposite the substrate 11, the outer edge is defined by the color difference between the adsorption layer 12 and the outside of the catalyst in the SEM image. The outer edges of these adsorption layers 12 are defined as "provisional boundaries" as shown in Figure 5. An example of color difference is shown in Figure 5, where the adsorption layer 12 is gray and the outside of the catalyst 10 is black. The "provisional boundaries" can be defined using image processing software for boundary line drawing, as described below, and the selection threshold can be set within the same range as described below. The "provisional boundaries" are intended to provisionally define approximately circular gaps in order to define the section width, as described below, and to define the substrate-side boundary line and outer boundary line, as described below, but are not used to measure the adsorption layer area.

[0024] In the adsorption layer on the substrate 11 or other layer 13 defined by the "provisional boundary line", S (void area) and L (void perimeter) of each void are measured, and the equivalent circle diameter is 1 μm or more and 60 μm or less and the above formula: L / {2(πS)} 1 / 2}≦1.1. Image processing to identify the shape of each void can be performed using image processing software for boundary line drawing. For example, Pictbear (provided by Fenrir Co., Ltd.) can be used as the image processing software for boundary line drawing. The selection threshold is preferably 20 or more and 40 or less, based on the color of the clear void region. The color of the clear void region is the color of the portion other than the components of the adsorption layer and the substrate 11, and is usually black, as shown in FIG. 5. The perimeter of each void is preferably drawn using 20 points or more. After boundary line drawing, the perimeter L and area S of the void can be calculated using image analysis software, specifically, ImageJ (public domain), Photoshop (provided by Adobe Systems Incorporated), or AreaQ (provided by Estec Co., Ltd.). The entire void is contained within the adsorption layer 12 on the partition wall 23 or other layer 13 defined by the above-mentioned "provisional boundary line," and L / {2(πS) 1 / 2 The area of ​​each void that satisfies the condition}≦1.1 and has an equivalent circular diameter of 1 μm to 60 μm is measured for 20 fields of view. The equivalent circular diameter of each void is measured. Next, as shown in Figure 7, for example, multiple parallel lines are drawn at equal intervals in the vertical and horizontal directions of the SEM image of the catalyst cross section, with a width equal to or less than the median diameter of the obtained equivalent circular diameters. Hereinafter, the width between the partition lines will also be referred to as the "partition width." Sufficient measurement accuracy can be ensured if the partition width is equal to or less than the median diameter of the equivalent circular diameter; for example, it is set to 75% of the median diameter of the equivalent circular diameter. In Figure 7, the longitudinal direction of the observation image is the horizontal direction A, and the width direction is the vertical direction B. If the observation image has a shape that does not have a longitudinal direction, such as a square, the terms "longitudinal direction" and "width direction" in the following explanation are replaced with "lateral direction" and "vertical direction."

[0025] The 20 fields of view are defined as the fields that are entirely included in the adsorption layer 12 on the partition wall defined by the "provisional boundary line", have a circular equivalent diameter of 1 μm or more and 60 μm or less, and are L / {2(πS) 1 / 2}≦1.1, and does not include fields of view that do not have such a gap.

[0026] (4) Identification of the apparent area of ​​the adsorption layer on the partition wall The SEM observation image is obtained by appropriately setting the measurement magnification and measuring in a field of view in which the number of partition lines in the longitudinal direction A of the observation image is 35 or more according to the above-mentioned partition width. The obtained SEM observation image is subjected to the following processing. As shown in FIG. 7 , a line connecting the intersections of the above-mentioned provisional boundary line and a partition line extending in the longitudinal direction A or the width direction B with a straight line is defined as the line defining the boundary of the adsorption layer 12 on the substrate side (hereinafter, also referred to as the "substrate-side boundary line" to distinguish it from the above-mentioned "provisional boundary line"). On the side opposite the substrate 11, a line connecting the intersections of the edge of the adsorption layer 12, which is the provisional boundary line, with a partition line extending in the longitudinal direction A or the width direction B with a straight line is defined as the line defining the boundary between the adsorption layer 12 and the outside (hereinafter, also referred to as the "outside boundary line" to distinguish it from the above-mentioned "provisional boundary line"). Hereinafter, the partition line extending in the longitudinal direction A will be referred to as partition line A, and the partition line extending in the width direction B will be referred to as partition line B. In the observation image, to determine whether the intersection of the provisional boundary line with demarcation line A or demarcation line B is to be connected, first, the intersection of the provisional boundary line with each demarcation line A is connected in the drawing direction (direction along demarcation line B) shown in Figure 8(a). In this case, as the observation image, an observation image is selected in which the lengths of the portion of the substrate 11 present are different at both end edges in the width direction B of the observation image (two end edges A1, A2 extending in the A direction), as in the example shown in Figure 8(a). Next, in the observation image, the intersection of the demarcation line A with the provisional boundary line is connected from the end edge A1 where the proportion of the portion of the substrate 11 present is small toward the other end side A2. When a provisional boundary line intersects with a demarcation line A at two or more points, the intersection point closest to the most recent intersection point is selected, except as described in Figure 10(b) below. In the example of Figure 8(a), after connecting intersection point P' along the drawing direction for the provisional boundary line, there is no intersection point with the next demarcation line A. In this case, when there is no intersection point between demarcation line A and the provisional boundary line in the drawing direction, the system returns to the most recent intersection point P' and connects point Q', the intersection point between demarcation line B and the provisional boundary line that is closest to P' and located forward of intersection point P' in the drawing direction (toward A2 in Figure 8(a)), with a straight line. Next, the system switches to connecting the intersection point between demarcation line B and the provisional boundary line toward the opposite side of the substrate 11 from line P'Q' in the A direction, and continues in this manner unless a similar event occurs. If there is no intersection between the demarcation line B and the provisional boundary line downstream of the intersection point P' in the drawing direction (Figure 8(b)), the intersection between the demarcation line B and the provisional boundary line that is closest to the intersection point P' on the opposite side of the substrate 11 from the straight line connecting the intersection point P' and the previous intersection point R' is searched for, and the intersection between the demarcation line B and the provisional boundary line is switched to connect the intersection between the demarcation line B and the provisional boundary line toward the opposite side of the substrate 11 from the line P'R' in the A direction, and the same process is carried out thereafter unless a similar event occurs. 8, 9, and 10, the adsorption layer 12 is omitted for the sake of convenience of explanation. As will be described later, as shown in Figure 7, the area in the observation image surrounded by the substrate side boundary line, the outer boundary line, the two outermost partition lines A' in the width direction B (two partition lines extending in the A direction shown by thick lines in Figure 7), and the two outermost partition lines B' in the longitudinal direction A (two partition lines extending in the B direction shown by thick lines in Figure 7) is determined to be the apparent area of ​​the adsorption layer 12.

[0027] In the following, exceptions to the case where the substrate-side boundary line is defined will be explained. As shown by symbols S1 and S2 in Fig. 7, for locations where the adsorbent components have permeated into the substrate 11, the substrate-side boundary line is set as follows: For S1, which is a permeation location other than a corner of a cell as shown in Fig. 7, when the intersections of the demarcation line B are connected along the line drawing direction as shown in Fig. 9(a), in the substrate-side boundary line connecting intersections P, R, and Q of the demarcation line B and the provisional boundary line, if the change in width direction B of intersection R adjacent to intersection P is equal to or greater than the demarcation width, intersection R is not used, and the straight line PQ, whose change in width direction B is less than the demarcation width, is set as the substrate-side boundary line.

[0028] Similarly, when connecting the intersections of the division line B and the provisional boundary line along the drawing direction of the A direction, if there are consecutive intersections where the change in width direction B is equal to or greater than the division width, starting from the intersection P (intersection R in Figure 9(b)), 1 and intersection point R 2 ), the intersection where the change in the B direction based on the intersection P is equal to or greater than the section width (intersection R 1 and intersection point R 2 ) is not used, and the straight line PQ where the change in width direction B is less than the division width is taken as the substrate-side boundary line. Figures 9(a) and (b) show the case where the intersection of division line B with the provisional boundary line is connected, but the same treatment is applied to the case where the intersection of division line A with the provisional boundary line is connected. In other words, as shown in Figure 9(c), when the intersection of division line A with the provisional boundary line is connected along the drawing line direction, even if there is an intersection where the change in length direction A is equal to or greater than the division width based on intersection point P (intersection point R in Figure 9(c)), R is not used, and the straight line PQ with the next intersection point Q where the change in length direction A is less than the division width is taken as the substrate-side boundary line.

[0029] However, for example, when drawing lines by connecting the intersections of the demarcation line B and the provisional boundary line as shown in Figure 10(a), if there are five or more consecutive intersections with a change in direction B that is greater than the demarcation width, do the following. As shown in Figure 10(a), intersection point P → intersection point R 2 →Intersection R 1 After drawing a line, five or more intersections after intersection Q are intersection R. 1 The starting point is the intersection point R, and the change in the direction B is greater than the width of the section. 1Intersection R, one point back from 2 Then, the intersection point R 2 Starting from point Q, five or more intersections after point Q are also intersections R. 2 If the change is greater than the section width, the intersection point R 2 The intersection point P, which is one step back from the intersection point R, is set as the starting point. The intersection point P does not change more than the division width in the direction B of the division line connecting the intersection point R to the intersection point Q. In this case, the intersection point P is set as the starting point, and the straight line PQ is set as the substrate side boundary line. For example, in Figure 10(a), the intersection point R 1 and intersection point R 2 The line PQ is used as the boundary line on the base material side. The same applies when drawing a line connecting the intersection of the division line A and the provisional boundary line.

[0030] Furthermore, in a location where the positional relationship between the adsorption layer 12 and the substrate 11 changes at a corner, such as S2 in Figure 7, the substrate-side boundary line is defined as follows: As shown in Figure 10(b), at intersection P' between demarcation line A and the provisional boundary line, demarcation line A including adjacent intersection R' has intersections with two or more provisional boundary lines on the opposite side of the substrate 11 in the direction A from the substrate-side boundary line formed by P'R'. In such a case, the intersection Q' between demarcation line B adjacent to P on the opposite side of the substrate in the direction A from P' is used. The straight line P'Q' is defined as the substrate-side boundary line, and hereafter, if the above situation does not apply, demarcation line B is similarly used to define the substrate-side boundary line.

[0031] (5) Identifying the area of ​​an approximately circular void Next, the adsorption layer region on the substrate 11 is redefined as a region surrounded by the substrate-side boundary line, the outer boundary line, the two outermost A-direction division lines A' in the width direction B in the observation image, and the two outermost B-direction division lines B' in the longitudinal direction A (hereinafter also referred to as the "apparent area" region) (FIG. 7). In this way, the adsorption layer region on the substrate 11 is redefined as a region included in the "apparent area" region, instead of the "provisional boundary line" defined in (2) above, where the circle equivalent diameter is 1 μm or more and 60 μm or less and the region satisfies the formula: L / {2(πS)} 1 / 2}≦1.1 and its area are re-specified. At this time, the following approximation process, which was not performed in the above-described boundary line specification process, is performed (see FIG. 11).

[0032] As shown in FIG. 11(a), since there are some protrusions or defects, depending on the actual perimeter and area, the 1 / 2 Approximately circular voids that do not fall within the range of}≦1.1 are treated as follows: If the ratio of the dotted line length connecting both ends of the protrusion or defect (the dotted line length in FIG. 11(a)) to the perimeter of the area other than the protrusion or defect (the solid line length in FIG. 11(a)) is 30% or less, the area enclosed by the dotted line and the solid line is considered to be a void. The area enclosed by the dotted line and the solid line and the perimeter, which is the sum of these lines, is calculated by the following formula: L / {2(πS) 1 / 2 If so, the area surrounded by the solid and dotted lines is considered to be the shape of the void, and is included in the calculation of the circular void ratio. Figure 11(b) shows a hypothetical void shape where α and β are considered as described above. On the other hand, if the ratio is more than 30%, the above process is not performed, and the area and perimeter are calculated as usual, and the above formula L / {2(πS) 1 / 2}≦1.1. For voids that are partially missing due to the substrate-side boundary line, the outer boundary line, or the two outermost demarcation lines B in the longitudinal direction A and the two outermost demarcation lines A in the width direction B, as shown by γ and δ in Figure 11(a), the same processing as for α and β is performed based on the perimeter (solid line length) of the void other than the boundary lines and the boundary portion (dotted line length) of the void. Also, for voids that are partially missing due to demarcation lines, the same processing as for α and β is performed. Furthermore, when an adsorbent component or the like is contained, as shown by ε in Figure 11(a), the void and its area that satisfy the above formula are identified, assuming that no component is contained.

[0033] Through the above process, the sum of the areas of approximately circular voids with an equivalent circular diameter of 1 μm or more and 60 μm or less that exist within the "apparent area" region of the adsorption layer 12 on the substrate 11 is calculated, and the ratio of this sum to the apparent area of ​​the adsorption layer 12 on the substrate 11 is calculated. This ratio is calculated for each field of view, and the circular porosity is calculated as the average value for the 20 fields of view. Note that if, as a result of the above-mentioned area redefinition, it turns out that there are no approximately circular voids with an equivalent circular diameter of 1 μm or more and 60 μm or less within the "apparent area" region in any of the fields of view, a dividing line is drawn in a new field of view using the previously determined dividing width, and the "apparent area" region is identified using the EPMA observation image using the procedure described above. The average value for the 20 fields of view in which there is at least one approximately circular void with an equivalent circular diameter of 1 μm or more and 60 μm or less within the "apparent area" region is then calculated.

[0034] In the present invention, it is sufficient that the circular porosity obtained by observing one or more observation surfaces of one sample Sp over a total of 20 fields of view falls within the above range. For example, when observing 20 fields of view of multiple observation surfaces at different positions in the exhaust gas flow direction X, if the circular porosity varies depending on the observation surface, the present invention is met if the circular porosity of any of the observation surfaces falls within the above range. The same applies to the circle equivalent diameter and the number of approximately perfect circular voids per mm, as well as the thickness of the adsorption layer 12, which will be described later.

[0035] In the adsorption layer 12 on the substrate 11, which is defined by the "substrate-side boundary line" and the "outer boundary line," the average equivalent circle diameter of the nearly circular voids is 1 μm or more and 60 μm or less. By setting this range, it is possible to measure the circular porosity, which contributes to peeling prevention and HC adsorption performance. From the viewpoint of further improving peeling prevention and HC adsorption performance, the average equivalent circle diameter is preferably 5 μm or more and 50 μm or less, and more preferably 10 μm or more and 30 μm or less. The average equivalent circle diameter is obtained by determining the average equivalent circle diameter of each of the nearly circular voids having a diameter of 1 μm or more and 60 μm or less for each visual field, and then averaging the average values ​​for each visual field for the 20 visual fields ultimately used to determine the circular porosity.

[0036] The standard deviation of the equivalent circle diameters of approximately perfect circles present in the adsorption layer 12 on the substrate 11 and having an equivalent circle diameter of 1 μm to 60 μm is preferably less than 25% of the average equivalent circle diameter. This indicates that the pore-forming agent used to form voids is present in the adsorption layer 12 in a highly dispersed state, which has the advantage of suppressing a decrease in the adhesion of the adsorption layer 12. The standard deviation of the equivalent circle diameters of approximately perfect circles is preferably 5% or more of the average equivalent circle diameter. This indicates that not all of the pore-forming agent is uniformly aligned on the same observation surface, but is present slightly toward the front or back, thereby avoiding excessive aggregation and maintaining high dispersion. High dispersion of the pore-forming agent is preferable because it has the advantage of suppressing a decrease in the adhesion of the adsorption layer 12. From these points of view, the standard deviation of the equivalent circle diameters is more preferably 10% to 23% of the average equivalent circle diameter, and particularly preferably 15% to 20%. The value of the standard deviation of the equivalent circle diameter is preferably, for example, from 0.1 μm to 15 μm, and more preferably from 1.0 μm to 5.0 μm.

[0037] The apparent area of ​​the adsorption layer 12 on the substrate 11 is 1 mm 2 From the viewpoint of HC adsorption performance, it is preferable that the average number of approximately perfect circular voids having a circle-equivalent diameter of 1 μm to 60 μm per 1 mm2 is 50 or more, and from the viewpoint of preventing peeling, it is preferable that the average number is 25,000 or less. 2 The average number of approximately circular voids having a circle-equivalent diameter of 1 μm to 60 μm per square millimeter is more preferably 100 to 20,000, and even more preferably 200 to 15,000. 2 The average number of approximately perfectly circular voids having a circle-equivalent diameter of 1 μm or more and 60 μm or less per field of view is the average value for the 20 fields of view finally used to determine the circular porosity.

[0038] The average thickness of the adsorption layer 12 on the substrate 11 is preferably 10 μm or more in terms of HC adsorption performance and the formation of nearly perfectly circular voids, and is preferably 100 μm or less in terms of maintaining practical pressure loss. From these points of view, the average thickness of the adsorption layer 12 on the substrate 11 (partition wall 23) is more preferably 15 μm or more and 60 μm or less, and even more preferably 20 μm or more and 50 μm or less. The average thickness of the adsorption layer 12 on the substrate 11 (partition wall 23) is calculated by dividing the apparent area of ​​the adsorption layer 12 on the substrate 11 in one observation image by the length of the substrate-side boundary line in the observation image. In this way, the thickness of the adsorption layer is calculated for each visual field. The average value of the thickness for each visual field over the 20 visual fields finally used to calculate the circular void ratio is calculated.

[0039] To obtain the above-mentioned circular void ratio, in a suitable method for producing an exhaust gas purification catalyst described below, a pore-forming agent having a specific shape, thermal decomposition onset temperature, particle size distribution, and swelling degree in a solvent is used, and the amount of the pore-forming agent, particle size of the pore-forming agent, particle size of the adsorbent and other metal oxide particles in the slurry, composition of the adsorbent and other metal oxides, and slurry viscosity described below are appropriately adjusted. The same applies to the number of approximately perfect circular voids, the equivalent circle diameter and its standard deviation, and the thickness of the adsorption layer 12.

[0040] In the present invention, it is particularly preferable that the circular porosity be 30% or less and that the value (% / μm) of the circular porosity (%) relative to the thickness (μm) of the adsorption layer 12 be 0.20 (% / μm) or more in terms of reducing the adhesion of the adsorption layer 12. The value of the circular porosity relative to the thickness of the adsorption layer 12 is a value that indicates the degree of aggregation of the pore-forming agent used when forming the adsorption layer 12. The more the aggregation of the pore-forming agent when forming the adsorption layer 12 increases, the smaller the circular porosity of the formed adsorption layer 12, and therefore the value of the circular porosity relative to the thickness of the adsorption layer 12 decreases. The applicant believes that by controlling the value of the circular porosity relative to the thickness of the adsorption layer 12 within the above range, circular pores are formed in the adsorption layer 12 in a well-dispersed state, effectively suppressing peeling. The upper limit of the value (% / μm) of the circular porosity (%) relative to the thickness (μm) of the adsorption layer 12 is preferably 1.00 (% / μm) or less, in order to prevent peeling of the adsorption layer 12 due to excessive voids. More preferably, the value of the circular porosity (%) relative to the thickness (μm) of the adsorption layer 12 is 0.30 (% / μm) or more and 0.80 (% / μm) or less. The value of the circular porosity (%) relative to the thickness (μm) of the adsorption layer 12 (% / μm) can be achieved by using a pore-forming agent having a specific shape, thermal decomposition onset temperature, particle size distribution, and degree of swelling in a solvent, adjusting the amount of pore-forming agent, particle size of the pore-forming agent, particle size of the adsorbent and other metal oxide particles in the slurry, and the composition of the adsorbent and other metal oxides, and then adjusting the slurry viscosity, as described below.

[0041] In order to achieve a balance between the HC adsorption performance and the prevention of peeling even more effectively, the volume of the pores having a diameter of 1 to 300 nm originating from the pores in the adsorption layer 12 is set to 0.05 cm 3 / g~0.3cm 3 / g. The pore volume is measured according to the BJH method (Barrett-Joyner-Halenda method) described in ISO 15901-2, and the pore volume is calculated from the measured nitrogen adsorption isotherm. The desorption curve is used for the calculation, and the adsorption cross section of the nitrogen molecule is 0.1620 nm 2 The measurement device used can be a BELSORP MAX II manufactured by Microtrac-Bell Corporation.

[0042] Examples of Si-containing adsorbents include zeolites, silica, and silica-alumina composite oxides such as silica alumina phosphate (SAPO),1 with the use of zeolites being preferred since they further enhance HC adsorption performance. Zeolites are crystalline substances in which TO4 units (T is the central atom) with a tetrahedral structure are connected three-dimensionally by sharing O atoms, forming open, regular micropores. Specific examples include silicates, germanium salts, arsenates, etc., as listed in the Data Collection of the Structure Committee of the International Zeolite Association (hereinafter sometimes referred to as "IZA").

[0043] Here, silicates include, for example, aluminosilicates, gallosilicates, ferrisilicates, titanosilicates, and borosilicates; germanium salts include, for example, aluminogermanium salts; and arsenates include, for example, aluminoarsenates. These include, for example, those in which Si or Al in the framework is substituted with divalent or trivalent cations such as Ti, Ga, Mg, Mn, Fe, Co, and Zn. In this embodiment, it is preferable to use a crystalline aluminosilicate as the zeolite. Zeolites have a variety of crystal structure shapes and are classified into types such as BEA type, MSE type, MFI type, YFI type, FER type, MOR type, and FAU type. In particular, in the present invention, it is preferable to employ a zeolite having at least one type of pore structure selected from 10-membered rings and 12-membered rings from the viewpoint of HC adsorption performance, and among these, it is particularly preferable to employ at least one type selected from the BEA type, MSE type, MFI type, and YFI type, since the pore size can be made to have excellent adsorption properties for toluene, propylene, pentane, and the like, which are contained in large amounts as HC molecular species in exhaust gases, and it is particularly more preferable to employ at least one type selected from the BEA type, MSE type, and YFI type, which have a 12-membered ring structure.

[0044] When using zeolite, the SiO2 / Al2O3 molar ratio is preferably 5 to 500, more preferably 5 to 250, and particularly preferably 10 to 40, in terms of excellent HC adsorption performance and excellent heat resistance under high-temperature exhaust gas from a gasoline engine or the like.

[0045] Zeolites can be loaded with various elements to improve their functionality. Examples of such elements include P, Zr, Ga, Sn, B, K, and Cs. In particular, the inclusion of P and / or Zr is preferred in terms of improving heat resistance, and a combination of both is most preferred. P and Zr can be loaded in zeolites by modifying the zeolite with, for example, phosphoric acid (H3PO4) or zirconium oxynitrate and then calcining the modified zeolite. When the zeolite contains P, the amount of P, calculated as phosphorus atoms, is preferably 0.5 to 10 mass%, more preferably 1 to 5 mass%, of the modified zeolite. When the zeolite contains zirconium, the amount of P, calculated as Zr atoms, is preferably 1 to 20 mass%, more preferably 5 to 10 mass%, of the modified zeolite. Here, the amount of modified zeolite refers to the amount including the amount of one or more elements or compounds modified by the zeolite. The amount of phosphorus and zirconium in the zeolite can be measured using an X-ray fluorescence spectrometer.

[0046] In this specification, the phrase "component A is supported" on particle B refers to a state in which component A is physically or chemically adsorbed or held on the outer surface or the inner surface of the pores of particle B. Specifically, the fact that particle B supports component A can be determined by, for example, confirming that component A and particle B are present in the same region in elemental mapping obtained by EDS analysis of a cross section of exhaust gas purification catalyst 10, and thus particle B "supports" component A. Here, the term "particle" also includes particles that have been bonded together by firing to form a sintered body.

[0047] The proportion of zeolite in the adsorption layer 12 is preferably, for example, 30% by mass or more and 95% by mass or less, more preferably 50% by mass or more and 90% by mass or less, and particularly preferably 57% by mass or more and 80% by mass or less, from the viewpoint of HC adsorption performance and adhesion.

[0048] Examples of components other than the Si adsorbent that can be contained in the adsorption layer 12 include alumina, titania, zirconia, ceria, ceria-zirconia, activated carbon, and metal-organic frameworks (MOFs). Alumina is particularly preferred because of its excellent heat resistance. The amount of metal oxides other than the Si adsorbent that can be contained in the adsorption layer 12 is, for example, more preferably 5% by mass to 50% by mass, and particularly preferably 10% by mass to 43% by mass, in the adsorption layer 12.

[0049] The adsorption layer 12 may contain a catalytically active component. Examples of the catalytically active component include platinum group metals, specifically, one or more of platinum (Pt), palladium (Pd), rhodium (Rh), ruthenium (Ru), iridium (Ir), and osmium (Os). From the viewpoint of purification performance of HC and the like, the catalytically active component contained in the adsorption layer 12 is preferably at least one selected from platinum (Pt), palladium (Pd), and rhodium (Rh). When the adsorption layer 12 contains a catalytically active component, the amount thereof is preferably 0.05% by mass or more and 5% by mass or less, and more preferably 0.5% by mass or more and 2.5% by mass or less.

[0050] The adsorption layer 12 may contain at least one transition metal selected from Cu, Ag, Ni, Fe, and Mn. When such a transition metal is contained in the adsorption layer containing the Si-containing adsorbent, the HC retention performance at high temperatures is improved, which is preferable.

[0051] The amount of zeolite, the amount of inorganic oxides such as alumina, and the amount of transition metals and catalytically active components in the adsorption layer 12 can be measured by measuring the amounts of various component elements in the solution obtained by completely dissolving the adsorption layer 12 using ICP-AES.

[0052] The mass of the adsorption layer 12, measured after drying, is preferably 30 g or more per 1 L of the volume of the substrate 11 in the portion on which the adsorption layer 12 is formed, in terms of improving HC adsorption performance, and more preferably 50 g or more. The mass of the adsorption layer 12, measured after drying, is preferably 200 g or less per 1 L of the volume of the substrate 11 in the portion on which the adsorption layer 12 is formed, in terms of improving peel resistance, and more preferably 150 g or less. The volume of the substrate 11 referred to here is the apparent volume including the partition walls 23 of the substrate 11, the adsorption layer 12, other layers 13 if present, the other layers 13, the pores in the partition walls 23, and the spaces within the cells 15. When the substrate 11 is cylindrical, the outer diameter of the substrate 11 is 2A and the length of the substrate 11 is B, and the apparent volume of the substrate 11 is then π×A. 2 × B. The volume of the base material 11 in the portion where the adsorption layer 12 is formed is the volume calculated by "apparent volume of base material 11 × length of adsorption layer 12 in the X direction / length of base material 11 in the X direction."

[0053] As shown in FIGS. 3(b) and 3(c), when the exhaust gas purification catalyst 10 includes another layer 13 as the coating layer 16 in addition to the adsorption layer 12, the other layer 13 preferably contains a catalytically active component. In this case, the catalytically active component may be the same as the catalytically active component that the adsorption layer 12 may contain. The other layer 13 preferably contains an inorganic oxide serving as an oxygen storage component or an inorganic oxide other than the oxygen storage component as a component supporting the catalytically active component. Examples of oxygen storage components include CeO2, CZ materials (ceria-zirconia composite oxides containing Ce and Zr, and solid solutions of CeO2 and ZrO2), iron oxide, and copper oxide. In addition to these, oxides of rare earth elements other than Ce may be used. Examples of inorganic oxides other than the oxygen storage component include alumina, titania, and zirconia.

[0054] When the exhaust gas purification catalyst 10 has an adsorption layer 12 and another layer 13 as coating layers 16 on a substrate 11, the proportion of the adsorption layer 12 to the entire coating layers 16 on the substrate 11 is preferably 20 mass% or more, and more preferably 20 mass% or more and 50 mass% or less.

[0055] When the exhaust gas purification catalyst 10 has the adsorption layer 12 and another layer 13 as the coating layer 16 on the substrate 11, it is more preferable that the ratio of the thickness of the adsorption layer 12 to the total thickness of the coating layer 16 present on the substrate 11 is 20% or more and 80% or less. The thickness of the adsorption layer 12 can be measured by the method described above. The thickness of the coating layer 16 can also be measured in accordance with the method for measuring the thickness of the adsorption layer 12.

[0056] The configuration of the exhaust gas purification catalyst 10 will be further explained. When adsorption layer 12 is provided so as to be in direct contact with partition wall 23, it is preferable that adsorption layer 12 be present mainly on the surface of partition wall 23 rather than inside partition wall 23, in order to further enhance HC adsorption performance and exhaust gas purification performance during high-speed operation. "Adsorption layer 12 being present mainly on the surface of partition wall 23" means that, in a cross section of substrate 11 on which adsorption layer 12 is provided, the mass of adsorption layer 12 present on the surface of partition wall 23 of substrate 11 is greater than the mass of adsorption layer 12 present inside partition wall 23. For example, the cross section of the partition wall on which adsorption layer 12 is provided can be observed with a scanning electron microscope ("JEM-ARM200F" manufactured by JEOL Ltd.) and analyzed with energy dispersive X-ray spectrometry (EDS) to confirm that adsorption layer 12 is present mainly on the surface by line analysis of the boundary between an element present in substrate 11 (e.g., Mg) and an element present in adsorption layer 12 (e.g., Si or a specific element present in the adsorption layer), or by analysis with an electron probe microanalyzer (EPMA), etc.

[0057] The length of the adsorption layer 12 in the X direction is preferably 50% to 100% of the length of the substrate 11 in the X direction, from the viewpoint of improving HC adsorption performance while reducing the suppression of peeling, and more preferably 70% to 100%. The length of the adsorption layer 12 can be measured by the following method. That is, it is preferable to visually observe the exhaust gas purification catalyst 10 and identify and measure the boundaries of the adsorption layer 12. In this case, it is preferable to measure the lengths of the adsorption layer 12 at, for example, 10 arbitrary locations on the exhaust gas purification catalyst 10 and determine the average value as the length of the adsorption layer 12. When the boundaries of the adsorption layer 12 in the exhaust gas flow direction cannot be visually determined, the composition of the exhaust gas purification catalyst can be analyzed at multiple locations (e.g., 8 to 16 locations) along the exhaust gas flow direction, and the length can be determined based on the concentration of catalytically active components at each location. The concentration of the catalytically active components at each location can be determined, for example, by X-ray fluorescence analysis (XRF) or inductively coupled plasma atomic emission spectroscopy (ICP-AES).

[0058] Next, a preferred method for producing the exhaust gas purifying catalyst of the present invention will be described below. The manufacturing method includes: L' / {2(πS') 1 / 2 a step of applying a slurry containing a pore-forming agent made of a crosslinked resin and containing particles having a circularity of 90% or more by number of particles having a circularity of 1.05 or less (hereinafter also simply referred to as "the pore-forming agent"); and A step of forming an adsorption layer on a substrate, the adsorbent layer containing the adsorbent and voids formed by burning off the pore-forming agent, by firing the slurry after coating on the substrate. A method for producing an exhaust gas purification catalyst comprising: The slurry is stirred at a rotation speed of 3000 rpm or more and 8000 rpm or less, and then coated onto a substrate in a state where the viscosity at 25° C. is adjusted to 2000 cP or more and 100,000 cP or less. (where L' is the perimeter of the image of the pore-forming agent observed with a scanning electron microscope (SEM), and S' is the area of ​​the image observed with the scanning electron microscope.)

[0059] The pore-forming agent is "L' / {2(πS')1 / 2 90% or more particles have a "circularity of 1.05 or less" on a number basis. This configuration makes it easy to obtain an exhaust gas purification catalyst 10 having the circular porosity. The circularity here is measured based on observation using an SEM. An observation sample is obtained by dropping a powder sample attached to a cotton swab from above onto an SEM sample stage with carbon tape attached, and then brushing off excess powder with an air gun. Conditions for spraying with the air gun include, but are not limited to, spraying air at 5 atmospheres (gauge pressure) for 1 second from a position 10 cm away. The SEM observation is preferably performed at an acceleration voltage of 5 kV to 15 kV and a magnification of 40 to 1000 times. SEM images of 50 randomly selected pore-forming agents are observed, and the outline of the SEM images is determined to be "L' / {2(πS') 1 / 2 The percentage of pieces having a circularity of 1.05 or less is calculated. Among the pore-forming agents, the "L' / {2(πS') 1 / 2 The proportion of the pore-forming agent having a circularity of 1.05 or less is more preferably 95% or more, and particularly preferably 98% or more.

[0060] Examples of the material for the pore-forming agent include polymers of monomers having ethylenically unsaturated bonds, including cross-linkable monomers. Cross-linkable acrylic resins and cross-linkable styrene resins can be used, and in particular, cross-linked polystyrene particles, cross-linked poly(meth)acrylate particles, etc. Examples of cross-linked poly(meth)acrylate particles include cross-linked polymethyl(meth)acrylate particles and cross-linked polybutyl(meth)acrylate particles. Using a cross-linked resin as the pore-forming agent makes it easier to leave highly round voids in the adsorption layer 12 without losing their shape during baking.

[0061] The pore-forming agent preferably has a thermal decomposition initiation temperature of 200°C or higher in the atmosphere. Porosity-forming agents with high thermal decomposition temperatures generally have a high degree of cross-linking of molecular chains. This allows highly circular voids to remain in the adsorption layer 12 without losing their shape during firing. The thermal decomposition initiation temperature of the pore-forming agent in the atmosphere is more preferably 230°C or higher, and particularly preferably 250°C or higher. The upper limit of the thermal decomposition initiation temperature is preferably 550°C or lower to ensure that the pore-forming agent disappears during firing, and more preferably 500°C or lower. The thermal decomposition initiation temperature is measured, for example, by increasing the temperature from room temperature to 500°C in an air atmosphere. The heating rate can be, for example, 5°C / min to 20°C / min. The thermal decomposition initiation temperature is defined as the intersection point between a line parallel to the horizontal axis passing through the mass before the start of test heating and a tangent drawn so that the gradient between the inflection points on the decomposition curve is maximized. In the following examples, the thermal decomposition initiation temperature in the atmosphere is simply referred to as the "thermal decomposition initiation temperature."

[0062] The pore-forming agent preferably has a value of (D90-D10) / D50 of 0.1 or more and 1.1 or less, where D10 is the particle diameter at 10% cumulative volume, D50 is the particle diameter at 50% cumulative volume, and D90 is the particle diameter at 90% cumulative volume, as measured by a laser diffraction particle size distribution method. The value of (D90-D10) / D50 (hereinafter also referred to as "monodispersity") is a measure of the particle size distribution of the pore-forming agent, and the smaller the monodispersity value, the sharper the particle size distribution. A pore-forming agent with a sharp particle size distribution has a high degree of crosslinking, which makes it even more excellent in that the pore-forming agent undergoes less deformation during firing due to thermal expansion, making it easier to achieve the circular porosity described above. From this perspective, the monodispersity of the pore-forming agent is more preferably 0.7 or less, and particularly preferably 0.3 or less. In order to obtain voids with the above-mentioned equivalent circle diameter, the pore-forming agent preferably has a D50 of 5 μm or more and 50 μm or less, and more preferably 10 μm or more and 30 μm or less. Furthermore, in terms of monodispersity, the pore-forming agent preferably has a D90 of 6 μm or more and 60 μm or less, and more preferably 10 μm or more and 30 μm or less. From the same perspective, the D10 is preferably 1 μm or more and 30 μm or less, and more preferably 2 μm or more and 20 μm or less. The monodispersity and particle size can be measured using a laser diffraction / scattering particle size / particle size distribution analyzer, such as the Microtrac HRA or Microtrac 3000 series manufactured by Microtrac-Bell. For example, they can be measured as follows. Specifically, using an automatic sample feeder for laser diffraction particle size distribution measurement equipment (Microtrac SDC, manufactured by Microtrac-Bell), the pore-forming agent was added to the aqueous dispersion medium, and after irradiating it with 40W ultrasound at a flow rate of 40% for 360 seconds, measurements were taken using a laser diffraction scattering particle size distribution analyzer (Microtrac MT3300EXII, manufactured by Microtrac-Bell). The measurement conditions were: particle refractive index 1.5, particle shape spherical, solvent refractive index 1.3, set zero 30 seconds, measurement time 30 seconds, and the average of two measurements was calculated. Pure water was used as the aqueous dispersion medium.

[0063] The pore-forming agent preferably has a low degree of swelling in solvents, since a pore-forming agent with a low degree of swelling has a high degree of cross-linking. For example, after immersion at 50°C for 48 hours, the solvent absorption (g / g) per gram of polymer is preferably 0.05 g / g or less for ethanol, 0.7 g / g or less for acetone, 0.15 g / g or less for 2-methoxyethanol, and 0.15 g / g or less for toluene.

[0064] In the present production method, the proportion of the pore-forming agent in the total pore-forming agent used to form the adsorption layer is preferably 80% by mass or more, more preferably 90% by mass or more, and particularly preferably 100% by mass.

[0065] The pore-forming agent-containing slurry further contains Si-containing adsorbent particles and preferably metal oxide particles. Examples of the Si-containing adsorbent particles and other metal oxide particles include the inorganic oxide particles described above as components of the adsorption layer 12. The particle size of the metal oxide particles, including the Si-containing adsorbent particles, is preferably D50 or more of 3 μm, and more preferably D90 or more of 10 μm, from the viewpoint of improving the dispersibility of the catalytically active component and enhancing the HC adsorption performance. The upper limit of the particle size of the metal oxide particles is preferably D50 or less of 15 μm, and more preferably D90 or less of 30 μm, from the viewpoint of improving the dispersibility of the catalytically active component. The D50 and D90 of the metal oxide particles can be measured in the same manner as the D50 and D90 of the pore-forming agent.

[0066] The particle size of the particles in the slurry containing the pore-forming agent is preferably D50 or more of 2 μm, D90 or more of 8 μm, more preferably D50 or more of 3 μm, and more preferably D90 or more of 9 μm.D50 is preferably 40 μm or less, more preferably D90 or less of 120 μm, more preferably D50 or less of 30 μm, and more preferably D90 or less of 70 μm.

[0067] The present inventors have found that when a slurry containing a Si-containing adsorbent and the pore-forming agent is directly applied to a substrate, an exhaust gas purification catalyst with sufficient peeling resistance cannot be obtained, and that it is difficult to obtain an exhaust gas purification catalyst having the above-mentioned circular porosity in the adsorption layer 12. They have also found that applying the slurry to a substrate after stirring at a predetermined rotation speed and achieving a predetermined viscosity can produce an exhaust gas purification catalyst with high peeling performance and excellent HC adsorption performance, and also facilitate the formation of an adsorption layer with the above-mentioned circular porosity. This is thought to be due to the following reasons: Si-containing adsorbents such as zeolite are hydrophobic and tend to aggregate in aqueous slurries. Furthermore, circular pore-forming agents made of crosslinked resins are similarly hydrophobic. Thus, when a Si-containing adsorbent is combined with the pore-forming agent, if the stirring is insufficient or the viscosity is below a predetermined value, the pore-forming agent will not disperse sufficiently in the slurry, forming large voids and reducing adhesion to the substrate 11 or other layer 13. That is, while a method of adding a pore-forming agent to a slurry for forming a purification layer on a substrate is conventionally known, the present inventors have discovered that adding a pore-forming agent to a slurry containing a Si-containing adsorbent using the conventional method can easily cause aggregation due to the interaction between the hydrophobic Si-containing adsorbent and the hydrophobic pore-forming agent in the slurry. In response to this, the present inventors have discovered that by maintaining a predetermined stirring state and controlling the viscosity within an appropriate range, the dispersibility of the Si-containing adsorbent and the pore-forming agent can be improved, resulting in the successful production of an exhaust gas purification catalyst having the above-mentioned circular porosity and excellent peeling resistance and HC adsorption performance. From this perspective, in the present production method, it is preferable that the coating be applied to the substrate at a viscosity of 2000 to 100,000 cP after stirring at a rotation speed of 3000 to 8000 rpm. By maintaining a rotation speed of 3000 rpm or higher and a viscosity of 2000 cP or higher, the above-mentioned aggregation suppression effect can be achieved. Furthermore, a rotation speed of 8000 rpm or less makes it easier to prevent physical damage to the pore-forming agent. A viscosity of 100,000 cP or less ensures the coatability of the slurry. From these points of view, it is more preferable that the slurry be coated on a substrate with a viscosity of 3,000 to 50,000 cP after the stirring, and it is particularly preferable that the slurry be coated on a substrate with a viscosity of 5,000 to 20,000 cP after the stirring.The stirring speed is more preferably 5000 to 8000 rpm, and the temperature of the slurry during viscosity measurement is, for example, 25°C. Furthermore, the viscosity after stirring is preferably measured within 30 minutes, and more preferably within 10 minutes, of stirring. A thickener may be used to adjust the viscosity. Examples of thickeners include xanthan gum, carboxymethyl cellulose, hydroxypropyl methyl cellulose, and hydroxyethyl cellulose.

[0068] For 2500 g of slurry, the stirring time is preferably 5 to 20 minutes, more preferably 10 to 15 minutes. When the amount of slurry is changed, the preferable stirring time can be increased or decreased in proportion to the increase or decrease in the amount of slurry.

[0069] Furthermore, it is preferable that the stirred slurry is applied to the substrate within 60 minutes after the viscosity measurement. In this case, applying to the substrate includes applying the slurry to the other layer 13 when another layer 13 has already been formed on the substrate.

[0070] In order to easily obtain the above viscosity, the amount of solids in the adsorption layer forming slurry excluding the pore-forming agent is preferably 20% by mass or more and 40% by mass or less, more preferably 25% by mass or more and 35% by mass or less.

[0071] Furthermore, the amount of the pore-forming agent relative to the solid content excluding the pore-forming agent of the adsorption layer forming slurry is, for example, preferably 10% by mass or more and less than 75% by mass, more preferably 20% by mass or more and 60% by mass or less, and particularly preferably 30% by mass or more and 50% by mass or less.

[0072] In the first step, a slurry containing the above-mentioned components is applied to the partition walls. Methods for applying the slurry to the partition walls include, but are not limited to, immersing one end of the substrate in the exhaust gas flow direction in the slurry and sucking the slurry from the opposite side. When the slurry is dried before firing, a drying temperature of 40 to 120°C is preferred. Firing is usually carried out in the air. A preferred firing temperature is, for example, 350 to 550°C.

[0073] The exhaust gas purification catalyst 10 manufactured in this manner can be used for various purposes as an exhaust gas purification catalyst for internal combustion engines powered by fossil fuels, such as gasoline engines, by taking advantage of its anti-peeling properties and HC adsorption properties. Furthermore, according to this embodiment, an exhaust gas purification method using such an exhaust gas purification catalyst 10 can also be provided. For example, by providing the exhaust gas purification catalyst 10 in the exhaust path of an internal combustion engine, such as a gasoline engine, and utilizing its HC adsorption properties, it becomes possible to effectively purify the exhaust gas from the gasoline engine. In particular, when the exhaust gas purification catalyst 10 is used in an exhaust gas purification device in which two or more exhaust gas purification catalysts are arranged along the exhaust gas flow direction, it is preferable to use it as the second or subsequent catalyst from the upstream side. This is because, due to the characteristics of organic trap catalysts, the amount of coating slurry is limited, so if it is used as the first catalyst, which has a high thermal load, it is prone to deterioration. However, by using it as the second or subsequent catalyst, high purification performance and HC adsorption performance can be maintained. [Example]

[0074] The present invention will be described in more detail below with reference to examples. However, the scope of the present invention is not limited to these examples. All drying and firing were carried out in the atmosphere. The thermal decomposition onset temperatures in the following examples were all measured in the atmosphere. The solid content referred to below is the amount excluding the pore-forming agent. The conditions for adding the thickener (type and amount) were the same in each example and comparative example. In each of the Examples and Comparative Examples, the stirring conditions for the slurry were as follows. (Mixing conditions) 2500 g of the slurry is placed in a cylindrical container having a diameter of 15 cm to a height of 15 cm, and stirred for 10 minutes using a mechanical stirrer equipped with a 4 cm long cross-shaped stirring blade. The viscosity of the slurry was measured at 25°C within 10 minutes after stirring using a Brookfield digital viscometer (model number: DV-E). The spindle used was LV-2 (No. 62) for Comparative Examples 4 and 5, LV-4 (No. 64) for Examples 6 and 8, and LV-3 (No. 63) for all other examples. The rotation speed was 12 rpm, and the viscosity was measured after 5 or more rotations from the start of rotation. In each example, the pore volume of pores with diameters of 1 to 300 nm originating from the pores in the adsorption layer was 0.05 cm 3 / g~0.3cm 3 / g. In each example, the particle size of the particles in the slurry was measured in the same manner as the particle size measurement method for the pore-forming agent described above, except that the particles in the slurry were not drained, and instead of a dispersion in which a pore-forming agent was dispersed in an aqueous dispersion, the slurry itself was supplied to an automatic sample feeder for a laser diffraction particle size distribution analyzer and subjected to ultrasonic treatment.

[0075] Example 1 [1. Preparation of slurry for forming adsorption layer] Zeolite powder (BEA type, SiO / AlO molar ratio = 40, phosphorus atom 2.3 mass%, zirconium atom 6.6 mass%) and alumina powder were prepared. The zeolite powder and alumina powder were mixed and immersed in pure water to a solid content of 30 mass%. Next, this suspension was mixed with a spherical pore-forming agent (crosslinked polymethyl(meth)acrylate particles, D50=20 μm, D90=22 μm, D10=18 μm, monodispersity=0.165, thermal decomposition onset temperature=250°C, circularity ratio: 95%, solvent absorption (g / g) per gram of polymer after immersion at 50°C for 48 hours was 0.02 g / g for ethanol, 0.24 g / g for acetone, 0.05 g / g for 2-methoxyethanol, and 0 g / g for toluene), boehmite powder, and alumina sol, and a thickener was added. Further, the mixture was stirred under the above stirring conditions at a rotation speed of 7000 rpm for 10 minutes to prepare a slurry for forming an adsorption layer. The viscosity of the slurry for forming an adsorption layer was 6800 cP. The circularity ratio was determined by the ratio of L / {2(πS) 1 / 2 The ratio of particles based on the number of particles satisfying the following relationship:}≦1.05. When the components other than the pore-forming agent in the solid content of the slurry were taken as 100 parts by mass, the zeolite powder was 60 parts by mass, the alumina powder was 30 parts by mass, the boehmite powder was 5 parts by mass, and the alumina sol was 5 parts by mass. The mass ratio of the pore-forming agent to the slurry solid content was 30%. The amount of thickener was set to achieve the specified viscosity. The D50 and D90 in the slurry were 9 μm and 18 μm, respectively. When measuring the roundness of the raw pore-forming agent, SEM observation was performed at an accelerating voltage of 15 kV and a magnification of 300x.

[0076] [2. Formation of adsorption portion precursor layer (adsorption portion before firing)] The substrate is made of 600 cells / inch in a plane perpendicular to the axial direction, with cells extending in the axial direction and separated by partition walls with a thickness of 80 to 100 μm. 2 A flow-through substrate having a volume of 0.174 L and a total length of 30 mm was used.

[0077] One end of the substrate was immersed in the adsorption layer forming slurry for 10 to 50% of the total length in the exhaust gas flow direction, and then the substrate was suctioned from the other end. The suctioned surface was similarly immersed in the adsorption layer forming slurry, and the suction was then suctioned from the other end. The substrate was then dried at 90°C for 10 minutes to form an adsorption portion precursor layer made of the solid content of the adsorption layer forming slurry.

[0078] [3. Firing] Thereafter, the substrate was calcined for 1 hour at 450° C. As a result, an exhaust gas purifying catalyst of Example 1 was obtained, in which the adsorbing portion was formed on the substrate.

[0079] In the exhaust gas purification catalyst of Example 1, the adsorbent portions were formed on the partition wall surfaces over the entire length L in the exhaust gas flow direction X. The mass of the adsorbent portions relative to the volume of the base material in the portion where the adsorbent portions were formed was 97.9 g / L after firing.

[0080] <Example 2> In Example 1, the mass ratio of the pore-forming agent to the solid content of the slurry was changed to 10%. Under the above stirring conditions, stirring was carried out for 10 minutes at a rotation speed of 7000 rpm to prepare a slurry for forming an adsorption layer. The viscosity of the slurry for forming an adsorption layer was 7500 cP. The D50 and D90 of the slurry were 8 μm and 17 μm, respectively. Except for this, an exhaust gas purification catalyst was obtained in the same manner as in Example 1.

[0081] Example 3 In Example 1, the mass ratio of the pore-forming agent to the solid content of the slurry was changed to 20%. Under the above stirring conditions, stirring was carried out for 10 minutes at a rotation speed of 7000 rpm to prepare a slurry for forming an adsorption layer. The viscosity of the slurry for forming an adsorption layer was 7300 cP. The D50 of the slurry was 9 μm and the D90 was 18 μm. Except for this, an exhaust gas purification catalyst was obtained in the same manner as in Example 1.

[0082] Example 4 In Example 1, the mass ratio of the pore-forming agent to the solid content of the slurry was changed to 40%. Under the above stirring conditions, stirring was carried out for 10 minutes at a rotation speed of 7500 rpm to prepare a slurry for forming an adsorption layer. The viscosity of the slurry for forming an adsorption layer was 6300 cP. The D50 of the slurry was 10 μm and the D90 was 20 μm. Except for this, an exhaust gas purification catalyst was obtained in the same manner as in Example 1.

[0083] <Example 5> In Example 1, the mass ratio of the pore-forming agent to the solid content of the slurry was changed to 50%. Under the above stirring conditions, stirring was carried out for 10 minutes at a rotation speed of 7500 rpm to prepare a slurry for forming an adsorption layer. The viscosity of the slurry for forming an adsorption layer was 5500 cP. The D50 of the slurry was 12 μm and the D90 was 23 μm. Except for this, an exhaust gas purification catalyst was obtained in the same manner as in Example 1.

[0084] Example 6 In Example 1, the pore-forming agent in the adsorption layer-forming slurry was changed to a spherical pore-forming agent (crosslinked polymethyl (meth)acrylate particles, D50 = 5 μm, D90 = 7 μm, D10 = 3 μm, monodispersity = 0.120, thermal decomposition onset temperature = 250 °C, circularity rate: 95%, solvent absorption (g / g) per gram of polymer after immersion at 50 °C for 48 hours was 0.03 g / g for ethanol, 0.43 g / g for acetone, 0.08 g / g for 2-methoxyethanol, and 0.02 g / g for toluene). The mass ratio of the pore-forming agent to the slurry solids was changed to 50%. Under the above stirring conditions, stirring was performed at a rotation speed of 7500 rpm for 10 minutes to prepare a slurry for forming an adsorption layer. The viscosity of the slurry for forming an adsorption layer was 19800 cP. The D50 and D90 of the slurry were 5 μm and 12 μm, respectively. Except for this point, an exhaust gas purifying catalyst was obtained in the same manner as in Example 1.

[0085] Example 7 In Example 1, the pore-forming agent in the adsorption layer-forming slurry was changed to a spherical pore-forming agent (crosslinked polymethyl (meth)acrylate particles, D50 = 50 μm, D90 = 80 μm, D10 = 28 μm, monodispersity = 1.039, thermal decomposition onset temperature = 250 °C, circularity rate: 90%, solvent absorption (g / g) per 1 g of polymer after immersion at 50 °C for 48 hours was 0.04 g / g for ethanol, 0.66 g / g for acetone, 0.12 g / g for 2-methoxyethanol, and 0.12 g / g for toluene). The mass ratio of the pore-forming agent to the slurry solids was changed to 20%. Under the above stirring conditions, stirring was performed at a rotation speed of 7000 rpm for 10 minutes to prepare a slurry for forming an adsorption layer. The viscosity of the adsorption layer-forming slurry was 4800 cP. The D50 and D90 of the slurry were 28 μm and 85 μm, respectively. Except for this point, an exhaust gas purifying catalyst was obtained in the same manner as in Example 1.

[0086] Example 8 In Example 1, the amounts of zeolite, alumina powder, alumina sol, and boehmite sol were increased in the same proportions to make the solid content of the slurry 32% by mass, and the slurry was stirred for 10 minutes at a rotation speed of 7000 rpm under the stirring conditions described above to prepare a slurry for forming an adsorption layer. The viscosity of the slurry for forming an adsorption layer was 11000 cP. The D50 and D90 of the slurry were 8 μm and 19 μm, respectively. The mass of the adsorption portion relative to the volume of the substrate at the portion where the adsorption portion was formed was 120.3 g / L after firing. Except for this, an exhaust gas purification catalyst was obtained in the same manner as in Example 1.

[0087] Example 9 In Example 1, the powdered zeolite in the adsorption layer forming slurry was changed to MSE-type zeolite (SiO2 / Al2O3 molar ratio = 200). Under the above stirring conditions, stirring was carried out for 10 minutes at a rotation speed of 7500 rpm to prepare an adsorption layer forming slurry. The viscosity of the adsorption layer forming slurry was 8800 cP. The D50 of the slurry was 9 μm and D90 was 19 μm. Except for this, an exhaust gas purification catalyst was obtained in the same manner as in Example 1.

[0088] Example 10 1. Preparation of upper layer forming slurry CeO2-ZrO2 solid solution powder (containing 40 mass% CeO2, 50 mass% ZrO2, and 10 mass% oxides of rare earth elements other than Ce) and alumina powder were prepared. The CeO2-ZrO2 solid solution powder and alumina powder were mixed and impregnated in an aqueous palladium nitrate solution. Next, this suspension was mixed with alumina sol, zirconia sol, and water as a liquid medium to prepare a slurry for the upper layer as an example of the "other layer." [2. Upper layer stacking] In Example 1, the substrate on which the adsorption layer precursor layer had been formed was immersed in the above-described upper layer forming slurry from one end face to 10% to 50% of the total length in the exhaust gas flow direction, and then aspirated from the other end face. Similarly, another layer forming slurry was immersed in the aspirated face, and aspirated from the other end face. The substrate was then dried at 90°C for 10 minutes to form an upper layer precursor layer composed of the solid content of the upper layer forming slurry. Firing was carried out in the same manner as in Example 1 to obtain an exhaust gas purification catalyst having an adsorption portion and an upper layer formed on the substrate. The mass of the adsorption portion relative to the volume of the substrate where the adsorption portion was formed was 200.8 g / L after firing. The thickness of the coating layer consisting of the adsorption layer and the upper layer was 48.22 μm.

[0089] <Comparative Example 1> In Example 1, no pore-forming agent was added to the adsorption layer forming slurry. Under the above stirring conditions, stirring was carried out for 10 minutes at a rotation speed of 7000 rpm to prepare the adsorption layer forming slurry. The adsorption layer forming slurry viscosity was 9800 cP. The slurry had a D50 of 7 μm and a D90 of 16 μm. Except for this, an exhaust gas purification catalyst was obtained in the same manner as in Example 1.

[0090] <Comparative Example 2> In Example 1, the mass ratio of the pore-forming agent to the solid content of the slurry was changed to 75%. Under the above stirring conditions, stirring was carried out for 10 minutes at a rotation speed of 7000 rpm to prepare a slurry for forming an adsorption layer. The viscosity of the slurry for forming an adsorption layer was 3800 cP. The D50 and D90 of the slurry were 15 μm and 25 μm, respectively. Except for this, an exhaust gas purification catalyst was obtained in the same manner as in Example 1.

[0091] <Comparative Example 3> In Example 1, acicular particles (material: cellulose) with an average diameter of 28 μm were used as the pore-forming agent in the adsorption layer-forming slurry at 30 mass % relative to the slurry solid content. Under the above stirring conditions, stirring was carried out for 10 minutes at a rotation speed of 7000 rpm to prepare the adsorption layer-forming slurry. The viscosity of the adsorption layer-forming slurry was 5800 cP. The D50 of the slurry was 9 μm and the D90 was 18 μm. Except for these points, an exhaust gas purification catalyst was obtained in the same manner as in Example 1.

[0092] <Comparative Example 4> In Example 1, under the above stirring conditions, stirring was carried out at a rotation speed of 1500 rpm for 10 minutes to prepare a slurry for forming an adsorption layer. The viscosity of the slurry for forming an adsorption layer was 1500 cP. The D50 of the slurry was 10 μm and the D90 was 20 μm. Except for these points, an exhaust gas purification catalyst was obtained in the same manner as in Example 1.

[0093] <Comparative Example 5> In Example 1, the solid content of the slurry was set to 25% by mass, and the slurry was stirred for 10 minutes at a rotation speed of 3000 rpm under the above stirring conditions to prepare a slurry for forming an adsorption layer. The viscosity of the slurry for forming an adsorption layer was 800 cP. The D50 of the slurry was 11 μm and the D90 was 19 μm. Except for these points, an exhaust gas purification catalyst was obtained in the same manner as in Example 1.

[0094] (Circular void ratio (%), apparent area 1mm 2(average number of approximately perfect circular voids with a circular equivalent diameter of 1 μm to 60 μm per particle, the circular equivalent diameter of the approximately perfect circular voids (μm), the standard deviation σ (μm) of the circular equivalent diameter, the ratio of the standard deviation to the circular equivalent diameter (%), the thickness of the adsorption layer (μm), etc.) For the exhaust gas purification catalysts obtained in Examples 1 to 10 and Comparative Examples 1 to 5, various parameters shown in Table 1, including the circular porosity, were measured using the above-mentioned method, in which the median diameter of the equivalent circle diameter of approximately circular voids in 20 fields of view was defined as the section width. The sampling method was the same as described above. The sample observation surface was set at any position 5 mm or more away from the upstream end and downstream end in the exhaust gas flow direction X, and a total of 20 fields of view were observed on the observation surface. As in Figure 4(c), the sample had a thickness of 10 mm in the X direction from each observation surface. The observation magnification for EPMA mapping was 300x, and the accelerating voltage was 15 kV (when measuring Al, Ba, Ce, La, Mg, Si, and Zr) and 25 kV (when measuring Pd, Rh, and Pt). In EPMA mapping, the substrate component element was Mg, and the adsorption portion component elements were Si and Al. The observation magnification for SEM when determining the section width was the same as for EPMA mapping, and the accelerating voltage was 15 kV. The width of the compartments used to measure various parameters was set to 0.75 times the average diameter of the provisionally calculated equivalent circle. The results are shown in Table 1.

[0095] (peeling rate) As in the case of determining the circular porosity, cylindrical samples were prepared by hollowing out the exhaust gas purification catalyst along the exhaust gas flow direction. The longitudinal direction (same as the exhaust gas flow direction X) of the obtained cylindrical samples was 30 mm. For the peeling test, air at 6 atmospheres (gauge pressure) was sprayed from an air gun 10 cm away from one end face of the cylindrical sample, which was placed so that the exhaust gas flow direction was horizontal, for 10 seconds. The other end face was also subjected to the same treatment. The weight loss rate of the sample after each treatment (= (weight before injection - weight after injection) / weight before injection × 100 (%)) was determined, and a weight loss of less than 3% was assigned an A rating, a weight loss of 3% to less than 5% was assigned a B rating, a weight loss of 5% to less than 10% was assigned a C rating, and a weight loss of 10% or more was assigned a D rating.

[0096] (Measurement of hydrocarbon adsorption / desorption amounts) The exhaust gas purification catalysts obtained in the examples and comparative examples were placed in a catalyst evaluation device (SIGU series, manufactured by Horiba, Ltd.). A test gas with the composition shown below was heated from 50°C to 500°C at 50°C / min using a temperature-programmed reaction method, and the amount of hydrocarbon desorption (=adsorption amount) was measured while flowing at flow rates of 25 mL / min and 62.5 mL / min. The amount of desorption was measured as the total amount of hydrocarbons (isooctane, propylene, and toluene) in the test gas. The amount of desorption was measured using an exhaust gas analyzer (MEXA-ONE, manufactured by Horiba, Ltd.). Note that the exhaust gas purification catalysts of Comparative Examples 2 to 5 had a high peeling rate, which could have resulted in the adsorption layer peeling off during measurement and contaminating the device, so the amount of hydrocarbon adsorption / desorption was not measured.

[0097] (Evaluation Gas Composition) The evaluation gas has the following composition: carbon dioxide: 14.00 vol%, oxygen: 0.49 vol%, isooctane: 24 ppmC, nitric oxide: 500 ppm (volume basis), carbon monoxide: 0.50 vol%, hydrogen: 0.17 vol%, propylene: 240 ppmC, toluene: 840 ppmC, water: 10.00 vol%

[0098] [Table 1]

[0099] As shown in Table 1, in each example where the circular porosity was more than 5% and 30% or less, the peeling rate of the adsorption layer was suppressed and the HC adsorption amount was high. Furthermore, in the HC adsorption test, the adsorption amount ratio between 62.5 ml / min and 25 ml / min was close to 2.5 times the flow rate ratio, indicating that sufficient adsorption performance was achieved even when exhaust gas flowed at high speeds. In contrast, Comparative Example 1, which uses no pore-forming agent and has a circular porosity of 0%, has a good peeling evaluation but poor HC adsorption performance. Comparative Example 2, which uses a circular porosity of over 30%, does not provide peeling resistance. Similarly, Comparative Example 3, which uses a needle-shaped pore-forming agent, has needle-shaped pores that are prone to peeling. Furthermore, when the slurry viscosity does not satisfy the specified range, as in Comparative Examples 4 and 5, the circular porosity is not included in the scope of the present invention, and the peeling performance is poor. [Industrial Applicability]

[0100] According to the present invention, it is possible to provide an exhaust gas purification catalyst that can achieve both improved HC adsorption performance and high exfoliation resistance in an adsorption section having a Si-containing adsorbent. Also, according to the present invention, it is possible to provide an exhaust gas purification catalyst method that can produce, by an industrially advantageous method, an exhaust gas purification catalyst that can achieve both improved HC adsorption performance and high exfoliation resistance in an adsorption section having a Si-containing adsorbent.

Claims

1. An exhaust gas purification catalyst comprising a substrate and an adsorption section provided on the substrate and containing zeolite, The adsorption portion has a plurality of voids, An exhaust gas purification catalyst in which, in a cross section perpendicular to the exhaust gas flow direction, the ratio of the total area of ​​voids in the adsorption portion that satisfies the following formula to the apparent area of ​​the adsorption portion present on the substrate is more than 5% and 30% or less. Formula: L / {2(πS)} １／２ }≦1.1 In the above formula, L is the perimeter of the gap in the cross section, and S is the area of ​​the gap in the cross section.

2. 2. The exhaust gas purification catalyst according to claim 1, wherein the zeolite has a pore structure of at least one of 10-membered rings and 12-membered rings.

3. 3. The exhaust gas purification catalyst according to claim 1, wherein the zeolite is at least one type of zeolite selected from the group consisting of BEA type, MSE type, MFI type and YFI type.

4. 3. The exhaust gas purification catalyst according to claim 1, wherein the content of zeolite in the adsorption portion is 10% by mass or more and 90% by mass or less.

5. 3. The exhaust gas purification catalyst according to claim 1, wherein the adsorption portion comprises at least one kind of precious metal selected from the group consisting of Pt, Pd, and Rh.

6. 3. The exhaust gas purifying catalyst according to claim 1, wherein the value of the ratio to the thickness of the adsorption portion is 0.20% / μm or more and 1.00% / μm or less.

7. 3. The exhaust gas purification catalyst according to claim 1, wherein the average value of the equivalent circle diameter of voids present in the adsorption portion and satisfying the formula is 1 μm or more and 60 μm or less.

8. The average number of voids that exist in the adsorption portion and that satisfy the above formula is equal to or greater than 1 mm of the cross-sectional area of ​​the adsorption portion. ２ The exhaust gas purifying catalyst according to claim 1 or 2, wherein the number of particles per one of the particles is 200 or more and 800 or less.

9. 3. The exhaust gas purification catalyst according to claim 1, wherein the standard deviation of the equivalent circle diameter of voids present in the adsorption portion and satisfying the formula is less than 25% of the average equivalent circle diameter.

10. 3. The exhaust gas purification catalyst according to claim 1, wherein the substrate is a flow-through substrate.

11. 3. The exhaust gas purification catalyst according to claim 1, which has an adsorption layer containing zeolite as the adsorption portion, and further has a purification layer containing a catalytically active component.

12. 3. The exhaust gas purifying catalyst according to claim 1, wherein the adsorption portion occupies 20% by mass or more and 50% by mass or less of the entire coating layer on the substrate.

13. 3. The exhaust gas purifying catalyst according to claim 1, wherein the adsorption portion comprises an adsorption layer containing zeolite, and the thickness of the adsorption layer accounts for 20% to 80% of the thickness of the entire coating layer on the substrate.

14. An exhaust gas purification system having two or more exhaust gas purification catalysts in an exhaust gas flow path, the exhaust gas purification system having the exhaust gas purification catalyst according to claim 1 or 2 as the second or subsequent catalyst from the upstream in the exhaust gas flow direction.

15. Zeolite; L' / {2(πS') １／２ a step of applying a slurry containing a pore-forming agent made of a crosslinked resin, the pore-forming agent containing 90% or more particles by number having a circularity of ≦1.05, and water, onto a substrate; and A step of forming an adsorption layer containing the zeolite and voids formed by burning off the pore-forming agent on the substrate by firing the slurry after coating on the substrate. A method for producing an exhaust gas purification catalyst comprising:

2. The method for producing an exhaust gas purifying catalyst according to claim 1, wherein the slurry is stirred at a rotational speed of 3,000 rpm or more and 8,000 rpm or less, and then coated on a substrate in a state where the viscosity at 25°C is 2,000 cP or more and 100,000 cP or less. In the above formula, L' is the perimeter of the pore-forming agent in an image obtained by observing the pore-forming agent with a scanning electron microscope, and S' is the area of ​​the pore-forming agent in an image obtained by observing the pore-forming agent with a scanning electron microscope.

16. The method for producing an exhaust gas purifying catalyst according to claim 15, wherein the amount of the pore-forming agent in the slurry is 10 parts by mass or more and 60 parts by mass or less per 100 parts by mass of the solid content excluding the pore-forming agent in the slurry.

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