JP2017100073A - Catalyst for exhaust purification - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a catalyst for exhaust purification that exhibits sufficient purification performance even in a condition with a high volume of intake air, for example, at the time of acceleration.SOLUTION: The present invention provides a catalyst for exhaust purification having two or more catalyst coat layers on base material, where a catalyst coat of the top layer has a structure with excellent gas diffusibility and a catalyst particle for use in the top layer has a structure with a noble metal particle carried on the surface of a secondary particle of carrier material.SELECTED DRAWING: Figure 21

Description

  The present invention relates to an exhaust gas purification catalyst. More specifically, the present invention relates to an exhaust gas purifying catalyst having two or more catalyst coat layers, which is characterized by the structure of the uppermost catalyst coat layer.

  Exhaust gas discharged from internal combustion engines such as automobiles contains harmful gases such as carbon monoxide (CO), nitrogen oxides (NOx), and unburned hydrocarbons (HC). Such a catalyst for purifying exhaust gas that decomposes harmful gases is also called a three-way catalyst. A honeycomb monolith substrate made of cordierite or the like, catalytic noble metal particles and oxygen storage capacity (OSC: Oxygen Storage Capacity). In general, a slurry containing a co-catalyst having a catalyst is washed and coated with a catalyst coating layer.

  Various attempts have been made to improve the purification efficiency of exhaust gas purification catalysts. For example, in order to improve the diffusibility of the exhaust gas in the catalyst coat layer, the particle size of the catalyst particles is increased, or a pore former that disappears when the catalyst is calcined at the final stage of production is used. A technique for forming voids inside the catalyst coat layer is known.

  However, if a void is provided inside the catalyst layer, the thickness of the catalyst layer increases accordingly, and the pressure loss of the catalyst increases, which may lead to a decrease in engine output and fuel consumption. In addition, when the voids are provided by the method as described above, there is a problem that the strength of the catalyst layer is reduced or the connection between the provided voids is poor and a sufficient effect cannot be obtained. In view of such a problem, for example, Patent Document 1 discloses a mode value of a frequency distribution related to the aspect ratio (D / L) of a cross section by mixing a carbon compound material having a predetermined shape and eliminating it during catalyst firing. Describes a method of providing a void in the catalyst layer with 2 or more.

  In addition, in order to efficiently exhibit the catalytic activity of the noble metal particles, attempts have been made to make more exhaust gas come into contact with the noble metal particles. For example, in Patent Document 2, a noble metal-supported powder particle and a binder are mixed in a liquid to form a slurry, and the slurry is vibrated to disperse the noble metal-supported powder particles, and then the slurry is spray-dried in that state. And a method for obtaining a noble metal-supported powder in which pores effective for gas diffusion are formed inside the particles is described.

JP 2012-240027 A JP 2009-131839 A

  For example, in a catalyst using two kinds of noble metals such as Pt and Rh, in view of the problem of reduction in catalytic activity due to solid solution of metals, a two-layer catalyst in which these two kinds of noble metals are contained in different layers is used. Techniques are known. In such a two-layer catalyst, the upper layer that is in direct contact with the exhaust gas is exposed to a high stress such as a high-temperature and high-concentration gas, and therefore a corresponding amount of catalyst coat is required to have sufficient heat resistance. However, when the coating amount is increased, there is a problem in that gas diffusibility in the catalyst is lowered, utilization efficiency of the catalyst active point is lowered, and purification performance is lowered. In particular, since the purification performance of the catalyst is gas diffusion-controlled under conditions with a large amount of intake air, such as during acceleration (high intake air amount or high Ga condition: synonymous with high space velocity or high SV condition), It becomes a problem. However, a method for forming a catalyst coat that achieves sufficient gas diffusivity even under high Ga conditions has not been found so far.

  In the noble metal-supported powder obtained by the method of Patent Document 2, the noble metal particles are supported on the surface of the primary particles of the carrier. In the secondary particles in which the primary particles are aggregated, noble metal particles present on the surface where the primary particles are in contact with each other cannot contact the exhaust gas, no matter how fine pores are formed inside the secondary particles. I can't demonstrate it.

  As a result of examining the problems as described above, the present inventors have used organic fibers having a predetermined shape as a pore-forming material, thereby having high aspect ratio pores excellent in communication and excellent gas diffusibility. It was found that a catalyst coat can be formed. The catalyst coat is used as the uppermost layer of the multilayer catalyst having two or more catalyst coats, and the catalyst particles having a structure in which noble metal particles are supported on the surface of the secondary particles of the support material are used as the uppermost layer. Thus, it was found that the purification performance under high Ga conditions can be remarkably enhanced while maintaining the heat resistance. The gist of the present invention is as follows.

(1) An exhaust gas purifying catalyst having two or more catalyst coat layers on a substrate,
Each catalyst coat layer contains catalyst particles having a composition different from that of the adjacent catalyst coat layer;
The catalyst particles contained in the uppermost catalyst coat have a structure in which noble metal particles are supported on the surfaces of secondary particles of a support material containing Al ₂ O ₃ .
In the uppermost catalyst coat,
The average thickness of the coat layer is in the range of 25-160 μm;
The porosity measured by the underwater gravimetric method is in the range of 50 to 80% by volume, and 0.5 to 50% by volume of the entire voids are composed of high aspect ratio pores having an aspect ratio of 5 or more,
The high aspect ratio pores have a circle equivalent diameter in the cross-sectional image of the cross section of the catalyst coat layer perpendicular to the flow direction of the exhaust gas in the range of 2 to 50 μm and the average aspect ratio in the range of 10 to 50. The exhaust gas-purifying catalyst.
(2) A method for producing an exhaust gas purifying catalyst having two or more catalyst coat layers on a substrate,
A step of preparing catalyst particles having a structure in which noble metal particles are supported on the surface of secondary particles of a support material containing Al ₂ O ₃ , and the catalyst particles and 100 parts by mass of metal oxide in the slurry Using a catalyst slurry containing 0.5 to 9.0 parts by mass of fibrous organic matter, and forming a top catalyst coat,
The said fibrous organic substance is the said method whose average fiber diameter is in the range of 1.7-8.0 micrometers, and whose average aspect-ratio is in the range of 9-40.

  The exhaust gas purifying catalyst of the present invention has high aspect ratio pores in which the uppermost catalyst coat satisfies a predetermined condition, so that the gas diffusibility in the uppermost catalyst coat is remarkably improved. It is possible to exhibit sufficient purification performance even under high Ga conditions, while having a catalyst coating amount sufficient to achieve the above. Furthermore, the catalyst particles contained in the uppermost catalyst coat have a structure in which noble metal particles are supported on the surface of secondary particles of the support material, so that the catalytic activity of the noble metal particles can be efficiently exhibited. it can. In the exhaust gas purification catalyst of the present invention, the top layer catalyst coat has a very excellent purification performance due to the synergistic effect of the coat structure with excellent gas diffusibility and the structure of catalyst particles that can efficiently exhibit the catalytic activity of noble metals. Have

BRIEF DESCRIPTION OF THE DRAWINGS It is the schematic which shows an example of the measurement method of FIB-SEM, (A) is schematic which shows a part of catalyst coat layer cross section perpendicular | vertical to the flow direction of the waste gas of the base material of the exhaust gas purification catalyst of this invention, (B ) Is a schematic view showing a test piece obtained by cutting the exhaust gas purifying catalyst in the axial direction at the position of the dotted line shown in FIG. (A), and (C) is a schematic view of an SEM image obtained by the FIB-SEM measurement method. 6 is a scanning electron micrograph (SEM photograph) of a cross section of the catalyst coat layer perpendicular to the flow direction of the exhaust gas of the base material of the exhaust gas purifying catalyst obtained in Example 5 of Test 1. FIG. It is the figure which binarized the SEM photograph of FIG. It is a two-dimensional projection figure which illustrates the three-dimensional information of the pore obtained by analyzing the continuous cross-sectional image of the catalyst coat layer cross section perpendicular | vertical to the flow direction of the exhaust gas of the base material of the exhaust gas purification catalyst. It is the schematic which shows the pore of the catalyst coat layer cross section in AE of FIG. FIG. 5 is a schematic diagram showing a cone angle of a high aspect ratio pore in the two-dimensional projection view of FIG. 4. It is a graph which shows the result of the catalyst performance evaluation test of the catalyst obtained by Examples 1-42 of Test 1, and Comparative Examples 1-133, and is a graph which shows the relationship between the coating amount of a catalyst coat layer, and a NOx purification rate. It is a graph which shows the result of the catalyst performance evaluation test of the catalyst obtained by Examples 1-42 of Test 1, and Comparative Examples 1-133, and is a graph which shows the relationship between the average thickness of a catalyst coat layer, and a NOx purification rate. It is a graph which shows the result of the catalyst performance evaluation test of the catalyst obtained by Examples 1-42 of Test 1, and Comparative Examples 1-133, and is a graph which shows the relationship between the porosity of a catalyst coat layer, and a NOx purification rate. It is a graph which shows the relationship between the aspect ratio and frequency of the high aspect ratio pore of the catalyst obtained by Example 5 of Test 1, and the aspect ratio and frequency of the pore of the catalyst obtained by Comparative Example 4. . The graph which shows the result of the catalyst performance evaluation test of the catalyst obtained by Examples 1-42 and Comparative Examples 1-133 of Test 1, and shows the relationship between the average aspect ratio of the high aspect ratio pores and the NOx purification rate It is. It is a graph which shows the result of the catalyst performance evaluation test of the catalyst obtained by Examples 1-42 and Comparative Examples 1-133 of Test 1, and shows the relationship between the ratio of the high aspect ratio pores to the entire voids and the NOx purification rate. It is a graph to show. It is a graph which shows the relationship between the cone angle of the high aspect ratio pore of the catalyst obtained by Example 16 of Test 1, and a cumulative ratio. It is a graph which shows the result of the catalyst performance evaluation test of the catalyst obtained by Examples 1-42 of Test 1, and Comparative Examples 1-133, and the relationship between the value of the cumulative 80% angle of high aspect ratio pores and the NOx purification rate It is a graph which shows. It is a graph which shows the measurement result of the OSC performance on the high Ga conditions of the catalyst (Comparative Examples 1-3 and Example 1) prepared by Test 2. FIG. It is a graph which shows the measurement result of the NOx purification rate under high Ga conditions of the catalyst (Comparative Examples 1-3 and Example 1) prepared by Test 2. FIG. It is a graph which shows the result of having measured the temperature (T50-NOx) in which the NOx purification rate after an endurance test will be 50% in the catalyst prepared in Test 2 (Comparative Examples 1-3 and Example 1). It is the figure which showed typically the loading state of Rh in each Rh carrying material prepared in Test 3. It is a HAADF-STEM image of the Rh / Al ₂ O ₃ material prepared by carrying Rh on the composite Al ₂ O ₃ material by sputtering as described in Example 1 of Test 3. It is a graph showing the result of the purification performance evaluation after the durability test of Example 1 of Test 3 and Comparative Examples 1 and 2 and under high temperature. It is a figure showing typically the coat structure of Example 1 of test 3, and the catalyst of comparative examples 3 and 4, and a graph which shows the result of purification performance evaluation after the endurance test of these catalysts, and under high temperature. It is a graph which shows the result of the purification performance evaluation after the durability test of the catalyst of Example 2 of Test 3 and Comparative Examples 5 to 9 and under high temperature. 4 is a graph showing the relationship between the secondary particle diameter of the carrier and the NOx purification performance after the durability test of the catalyst prepared in Test 3.

[Exhaust gas purification catalyst]
The exhaust gas purifying catalyst of the present invention has two or more catalyst coat layers on a substrate, and each catalyst coat layer contains catalyst particles having a composition different from that of the adjacent catalyst coat layer. The uppermost catalyst coat has an average thickness of the coat layer in the range of 25 to 160 μm, a porosity measured by the underwater weight method is in the range of 50 to 80% by volume, and 0% of the entire void. 5 to 50% by volume consist of high aspect ratio pores having an aspect ratio of 5 or more, and the high aspect ratio pores are pore circles in a cross-sectional image of the cross section of the catalyst coat layer perpendicular to the flow direction of the exhaust gas. The equivalent diameter is in the range of 2 to 50 μm and the average aspect ratio is in the range of 10 to 50, and the contained catalyst particles are on the surface of the secondary particles of the support material containing Al ₂ O _3. It has a structure in which noble metal particles are supported.

(Base material)
As the substrate in the exhaust gas purifying catalyst of the present invention, for example, a substrate having a known honeycomb shape can be used, and specifically, a honeycomb-shaped monolith substrate (honeycomb filter, high-density honeycomb, etc.), etc. Is preferably employed. In addition, the material of such a base material is not particularly limited, and there are base materials made of ceramics such as cordierite, silicon carbide, silica, alumina, mullite, and base materials made of metal such as stainless steel including chromium and aluminum. Preferably employed. Among these, cordierite is preferable from the viewpoint of cost.

(Catalyst coat layer)
The catalyst coat layer in the exhaust gas purifying catalyst of the present invention is formed on the surface of the substrate, and consists of two or more layers, that is, two layers, three layers, or four layers or more. Preferably, the catalyst coat layer in the exhaust gas purifying catalyst of the present invention has a two-layer structure. Each catalyst coat layer has a composition different from that of the adjacent catalyst coat layer. In addition, each catalyst coat layer does not necessarily have to be uniform over the entire base material of the exhaust gas purification catalyst, and for each zone, for example, upstream and downstream with respect to the exhaust gas flow direction. It may have a different composition. In addition, a different composition means that the component which comprises the catalyst particle mentioned later differs, for example. The catalyst coat layer composed of two or more layers can be classified into the uppermost catalyst coat and the lower layer catalyst coat existing in the lower layer, and the uppermost catalyst coat has many voids as described later. It has a structure including.

Each catalyst coat layer includes catalyst particles configured such that noble metal particles functioning as a main catalyst are supported on a carrier made of a metal oxide or the like. Support materials include metal oxides, specifically, aluminum oxide (Al ₂ O ₃ , alumina), cerium oxide (CeO ₂ , ceria), zirconium oxide (ZrO ₂ , zirconia), silicon oxide (SiO ₂ , silica). ), Yttrium oxide (Y ₂ O ₃ , yttria), neodymium oxide (Nd ₂ O ₃ ), and composite oxides thereof. Two or more metal oxides may be used in combination.

  Specific examples of the noble metal include platinum (Pt), palladium (Pd), rhodium (Rh), gold (Au), silver (Ag), iridium (Ir), and ruthenium (Ru). Among these, at least one selected from the group consisting of Pt, Rh, Pd, Ir and Ru is preferable from the viewpoint of catalyst performance, and at least one selected from the group consisting of Pt, Rh and Pd is particularly preferable. One kind of noble metal is preferably used per catalyst coat layer.

  The noble metal is preferably supported on the metal oxide as described above. The amount of the precious metal supported is not particularly limited, and a necessary amount may be supported appropriately according to the target design. The noble metal content is preferably 0.01 to 10 parts by mass and more preferably 0.01 to 5 parts by mass with respect to 100 parts by mass of the catalyst particles in terms of metal. If the amount of the precious metal supported is too small, the catalytic activity tends to be insufficient. On the other hand, if the amount is too large, the catalytic activity tends to be saturated and the cost tends to increase. Absent.

  In the two or more catalyst coat layers formed on the substrate, the uppermost catalyst coat layer has a high stress of high temperature and high concentration gas, and sulfur (S) or carbonization compared to the lower catalyst coat layer. Many are exposed to poisoning substances such as hydrogen (HC). Therefore, in order to obtain a catalyst having durability against them, it is preferable to use a noble metal whose catalytic activity is hardly impaired by stress or poisoning in the uppermost catalyst coat layer. An example of such a noble metal is Rh. Examples of noble metals that are relatively weak against stress and poisoning and are preferably used in a catalyst coat other than the uppermost layer include Pd and Pt.

  The coating amount per layer of the catalyst coat layer is preferably in the range of 50 to 300 g / L per unit volume of the substrate. If the coating amount is too small, the catalyst activity performance of the catalyst particles cannot be sufficiently obtained, so that sufficient catalyst performance such as NOx purification performance cannot be obtained. On the other hand, if too much, the pressure loss increases and the fuel consumption deteriorates. However, such a problem does not occur in the above range. In addition, from the viewpoint of a balance between pressure loss, catalyst performance, and durability, the coating amount per layer of the catalyst coat layer is in the range of 50 to 250 g / L, particularly 50 to 200 g / L, per unit volume of the base material. It is more preferable.

  Further, the thickness per layer of the catalyst coat layer is preferably within the range of 25 to 160 μm. If the catalyst coat layer is too thin, sufficient catalyst performance cannot be obtained.On the other hand, if it is too thick, pressure loss when exhaust gas or the like passes increases and sufficient catalyst performance such as NOx purification performance cannot be obtained. Such a problem does not occur in the above range. In addition, from the viewpoint of a balance between pressure loss, catalyst performance, and durability, it is more preferably in the range of 30 to 96 μm, particularly 32 to 92 μm. Here, the “thickness” of the catalyst coat layer is the length in the direction perpendicular to the center of the flat portion of the substrate of the catalyst coat layer, that is, the surface of the catalyst coat layer and the surface of the substrate (the lower layer catalyst coat is If present, it means the shortest distance between the lower catalyst coat). For example, the average thickness of the catalyst coat layer is measured by observing the catalyst coat layer using a scanning electron microscope (SEM) or an optical microscope, and measuring the thickness of any 10 or more portions. It is possible to calculate by calculating the average value.

  The catalyst coat layer is mainly composed of catalyst particles, but may further contain other components as long as the effects of the present invention are not impaired. Other components include other metal oxides and additives used in the catalyst coat layer for this type of application, specifically potassium (K), sodium (Na), lithium (Li), cesium (Cs). Alkaline metals such as barium (Ba), calcium (Ca), strontium (Sr), etc., rare earth elements such as lanthanum (La), yttrium (Y), cerium (Ce), iron (Fe), etc. 1 type or more of these transition metals etc. are mentioned.

(Top catalyst coat)
The uppermost catalyst coat has many voids, and the void ratio is preferably in the range of 50 to 80% by volume as measured by an underwater weight method. If the porosity of the uppermost catalyst coat is too low, the gas diffusibility will deteriorate and sufficient catalyst performance will not be obtained. Although the ratio of the gas passing through the layer increases and sufficient catalyst performance cannot be obtained, such a problem does not occur in the above range. The porosity of the uppermost catalyst coat is more preferably in the range of 50.9 to 78.8% by volume, particularly 54 to 78.0% by volume, from the viewpoint of the balance between gas diffusibility and catalyst performance.

  The “void” of the uppermost catalyst coat means a space that the catalyst coat layer has inside. The shape of the “void” is not particularly limited, and may be any shape such as a spherical shape, an elliptical shape, a cylindrical shape, a rectangular parallelepiped shape (rectangular column), a disk shape, a through-passage shape, and similar shapes. Such voids include micropores with a cross-sectional equivalent circle diameter of less than 2 μm, high-aspect ratio pores with a cross-sectional equivalent circle diameter of 2 μm or more and an aspect ratio of 5 or more, and a cross-sectional equivalent circle diameter of 2 μm. There are included pores such as pores having the aspect ratio of 5 or more. Such a porosity of the uppermost catalyst coat can be obtained, for example, by measuring an exhaust gas purification catalyst in which only the uppermost catalyst coat is formed by an underwater weight method. Specifically, the porosity can be measured by a method according to a method defined in JIS R 2205, for example.

  In the exhaust gas purifying catalyst of the present invention, among the voids of the uppermost catalyst coat, 0.5 to 50% by volume of the catalyst is composed of high aspect ratio pores having an aspect ratio of 5 or more. The high aspect ratio pores have an equivalent circle diameter of 2 to 50 μm in the cross-sectional image of the cross section of the catalyst coat layer perpendicular to the exhaust gas flow direction, and an average aspect ratio of 10 to 50. It is characterized by being. Accordingly, pores having an equivalent circle diameter of less than 2 μm are not regarded as high aspect ratio pores even if the aspect ratio is 5 or more.

  If the average aspect ratio of the high aspect ratio pores is too low, sufficient pore connectivity cannot be obtained, while if it is too high, the gas diffusivity is too high, so that the coating layer passes through the coating layer without contacting the catalytic active point. However, if the average aspect ratio is within the range of 10 to 50, such a problem does not occur. From the viewpoint of achieving both gas diffusibility and catalyst performance, the average aspect ratio of the high aspect ratio pores is more preferably in the range of 10 to 35, particularly 10 to 30.

  The average aspect ratio of the high aspect ratio pores in the uppermost catalyst coat is based on the three-dimensional information of the pores of the catalyst coat layer obtained by FIB-SEM (Focused Ion Beam-Scanning Electron Microscope) or X-ray CT. It can be measured by analyzing a cross-sectional image of the cross section of the catalyst coat layer perpendicular to the flow direction of the exhaust gas of the material (the axial direction of the honeycomb substrate).

  Specifically, for example, when performing FIB-SEM analysis, first, a continuous cross-sectional image (SEM image) of the cross section of the catalyst coat layer perpendicular to the exhaust gas flow direction of the substrate is obtained by FIB-SEM analysis. Next, the obtained continuous cross-sectional image is analyzed, and three-dimensional information of pores having a cross-sectional equivalent circle diameter of 2 μm or more is extracted. As an example of the analysis result of the three-dimensional information of the pores in FIG. 4, the tertiary of the pores obtained by analyzing the continuous cross-sectional image of the cross section of the catalyst coat layer perpendicular to the exhaust gas flow direction of the exhaust gas purification catalyst substrate. The example of the two-dimensional projection figure which illustrates the analysis result of original information is shown. As shown in the analysis result of the three-dimensional information of the pores illustrated in FIG. 4, the shape of the pores is indefinite, and the distance connecting the start point and the end point in the continuous cross-sectional image (SEM image) of the pores. Is defined as “major axis”. Note that the start point and the end point are the centers of gravity in the respective SEM images. Next, among the constrictions in the path connecting the start point and the end point of the pores in the continuous cross-sectional image (SEM image) with the shortest distance, the smallest equivalent circle diameter in the cross-sectional SEM image is 2 μm or more and is defined as “throat” The equivalent circle diameter in this cross-sectional SEM image is defined as “throat diameter”. (There may be multiple constrictions in the pores, but as the throat diameter for calculating the aspect ratio, select the minimum constriction in the path connecting the start point and the end point with the shortest distance, and this minimum constriction (throat) The equivalent circle diameter of the pores in the cross-sectional SEM image is defined as “throat diameter”.) Further, the aspect ratio of the pores is defined as “long diameter / throat diameter”.

  Next, (A) (the start point of the pore), (B) (the throat of the pore), (C) (the median point of the major axis of the pore), (D) (the maximum circle of the pores) in FIG. FIG. 5 shows examples of cross-sectional images (SEM images) of (maximum diameter portion which is an equivalent diameter) and (E) (end point of pore). FIG. 5 is a schematic view of a cross-sectional image (SEM image) showing pores in the cross section of the catalyst coat layer in FIGS. FIG. 5A is a schematic diagram of a cross-sectional image of a pore at the start point (one end where the equivalent circle diameter of the pore is 2 μm or more) of the two-dimensional projection diagram of the pore illustrated in FIG. G1 indicates the center of gravity of the pores in the cross-sectional image. (B) in FIG. 5 is the throat of the two-dimensional projection diagram of the pore illustrated in FIG. 4 (the equivalent circle diameter of the pore is 2 μm or more and the minimum in the path connecting the start point and the end point with the shortest distance). It is the schematic of the cross-sectional image of the pore in a constriction). FIG. 5C is a schematic diagram of a cross-sectional image of the pores at the midpoint of the path connecting the starting point and the ending point of the long diameter in the two-dimensional projection diagram of the pores illustrated in FIG. 4 with the shortest distance. FIG. 5D is a cross-sectional view of the pore in the portion where the equivalent circle diameter of the pore is the maximum in the path connecting the starting point and the ending point of the long diameter in the two-dimensional projection diagram of the pore illustrated in FIG. 4 with the shortest distance. It is an image. FIG. 5E is a schematic diagram of a cross-sectional image of a pore at the end point (the other end where the equivalent circle diameter of the pore is 2 μm or more) in the two-dimensional projection diagram of the pore illustrated in FIG. G2 indicates the center of gravity of the pores in the cross-sectional image. Here, in FIG. 5, the distance of the straight line connecting the start point of the pore (G1 shown in FIG. 5A) and the end point of the pore (G2 shown in FIG. 5E) is defined as “major axis”. . Also, among the constrictions in the path connecting the start point and the end point of the pores with the shortest distance, the smallest equivalent circle diameter in the cross-sectional SEM image is 2 μm or more is defined as “throat”. The diameter is defined as “throat diameter”. The aspect ratio of the pore is defined as “major diameter / throat diameter”. Furthermore, a range of 500 μm or more in the horizontal direction with respect to the substrate flat portion of the catalyst coat layer, 25 μm or more in the vertical direction with respect to the substrate flat portion, and 1000 μm or more in the axial direction, or a corresponding range is measured Then, by calculating the average aspect ratio of the high aspect ratio pores having an aspect ratio of 5 or more among the pores, the “average aspect ratio of the high aspect ratio pores in the catalyst coat layer” can be obtained.

  As described above, the ratio of the high aspect ratio pores in the uppermost catalyst coat to the entire voids is in the range of 0.5 to 50% by volume. If this ratio is too low, the pore connectivity is insufficient, while if it is too high, the gas diffusivity in the direction perpendicular to the flow direction of the exhaust gas is insufficient and sufficient catalytic performance cannot be obtained. Although peeling or the like due to a decrease in strength of the catalyst coat layer also occurs, such a problem does not occur within the above range. The ratio of the high aspect ratio pores to the entire voids is 0.6 to 40.9% by volume, particularly 1 to 31% by volume, from the viewpoint of the balance of gas diffusibility, catalyst performance, and catalyst coat layer strength. It is preferable to be within the range.

  The proportion of the high aspect ratio pores in the entire space in the uppermost catalyst coat is 500 μm or more in the horizontal direction with respect to the flat part of the substrate of the catalyst coat layer, and 25 μm or more in the direction perpendicular to the flat part of the substrate. The porosity of the high aspect ratio pores in the range of 1000 μm or more in the axial direction or a range corresponding thereto is divided by the porosity of the catalyst coat layer obtained by measurement by the underwater weight method.

  Further, in the uppermost catalyst coat, the high aspect ratio pores are accumulated based on the angle (cone angle) of the angle (conical angle) formed by the major axis direction vector of the high aspect ratio pores and the exhaust gas flow direction vector of the substrate. It is preferable to be oriented within a range of 0 to 45 degrees with a cumulative 80% angle value in the angular distribution. By doing so, the gas diffusibility in the flow direction of the exhaust gas (the axial direction of the honeycomb-shaped substrate) is particularly improved, and the utilization efficiency of the active points can be improved. If the cumulative 80% angle value is too large, the axial component of the gas diffusibility tends to be insufficient and the utilization efficiency of the active points tends to decrease, but such a problem does not occur in the above range. Note that. The cumulative 80% angle value is preferably in the range of 15 to 45 degrees, particularly 30 to 45 degrees, from the viewpoint of catalyst performance.

  The cone angle (orientation angle) of the high aspect ratio pores in the uppermost catalyst coat is determined from the three-dimensional information of the pores in the catalyst coat layer in the flow direction of the exhaust gas of the substrate (the axial direction of the honeycomb substrate). It can be measured by analyzing a cross-sectional image of a vertical cross section of the catalyst coat layer. Specifically, for example, when the analysis is performed by FIB-SEM analysis, from the angle formed by the major axis direction vector obtained by the “major axis” of the high aspect ratio pores obtained as described above and the exhaust gas flow direction vector of the substrate, “ Cone angle "can be determined. FIG. 6 is a schematic diagram showing the cone angle (orientation angle) of high aspect ratio pores, and shows an example of how to obtain the “cone angle”. FIG. 6 shows the major axis direction vector (Y) of the high aspect ratio pores and the exhaust gas flow direction vector (X) of the base material in the two-dimensional projection diagram of FIG. 4, and the major axis direction vector (Y). And the exhaust gas flow direction vector (X) of the base material is defined as a “conical angle”. By analyzing the three-dimensional information (three-dimensional image) of the pores, it is possible to calculate a cumulative 80% angle value in the cumulative angular distribution based on the cone angle. The cumulative 80% angle in the cumulative angular distribution based on the angle of the cone angle of the high aspect ratio pores means the number of high aspect ratio pores from the small cone angle (angle) of the high aspect ratio pores. When counted, it means the cone angle of the aspect ratio pore when the number of high aspect ratio pores corresponds to 80% of the total (cone angle standard integration frequency of cone angle is 80%). Note that the cumulative 80% angle value in the cumulative angular distribution of the cone angle of the high aspect ratio pores randomly extracted 20 or more high aspect ratio pores, and the cones of these high aspect ratio pores. It can be obtained by measuring and averaging the value of the cumulative 80% angle in the cumulative angular distribution of the angular reference.

(Catalyst particles contained in the uppermost catalyst coat)
In the catalyst of the present invention, the catalyst particles contained in the uppermost catalyst coat have a structure in which noble metal particles are supported on the surfaces of secondary particles of a support material containing Al ₂ O ₃ . In the present specification, “secondary particles” mean aggregates or aggregates formed by aggregating a plurality of particles (primary particles) that are judged to constitute one unit from the appearance. Therefore, “the noble metal particles are supported on the surface of the secondary particles” means that the noble metal particles are supported on the surface of the secondary particles as a unit, not the surface of the individual primary particles constituting the secondary particles. Means that From the viewpoint of reducing the amount of noble metal used, it is preferable that the catalyst particles have no noble metal particles inside the secondary particles, that is, other than the surface.

The support material has high heat resistance by containing Al ₂ O ₃ . Examples of the carrier material containing Al ₂ O ₃ include those consisting only of Al ₂ O ₃ , and metal oxides usually used as the carrier material as described above and containing Al ₂ O ₃ , such as alumina- A ceria-zirconia composite oxide may be mentioned. From the viewpoint of improving heat resistance, the support material preferably contains 40 at% or more of Al ₂ O ₃ in terms of aluminum element, based on the total metal elements contained.

  In the catalyst particles contained in the uppermost catalyst coat, as the secondary particles of the support material are smaller in size, the noble metal is supported at a lower density, that is, the distance between the particles is increased, which is advantageous. The heat resistance of itself will fall. From this viewpoint, the secondary particle diameter of the carrier material is preferably 6 μm or more in terms of a cumulative 50% diameter (D50) in a volume-based cumulative particle size distribution. This cumulative 50% diameter is preferably a cumulative 50% diameter in a volume-based cumulative particle size distribution measured by a laser diffraction method. On the other hand, if the particle size is too large, it becomes difficult to form a catalyst coat. Therefore, the secondary particle size (D50) of the carrier material is preferably less than 25 μm. In addition, it is preferable that the noble metal particle carry | supported by the support | carrier material has the average particle diameter measured by CO adsorption method in the range of 1-9 nm.

  As described above, the particle size (D50) of the secondary particles of the carrier material can be measured by a laser diffraction method. More specifically, for example, 1000 (randomly) randomly extracted particles are measured by a laser diffraction method using a laser diffraction device such as a laser diffraction particle size distribution measurement device, and the data is based on the data. Can be calculated. The volume-based cumulative 50% diameter means that the number of particles corresponds to 50% of the whole (volume-based cumulative frequency is 50%) when counting the number from the smallest particle size (area). It means the particle size of the particles. The particle diameter means the diameter of the minimum circumscribed circle when the cross section is not circular.

(Utilization of exhaust gas purification catalyst)
The exhaust gas purifying catalyst of the present invention may be used alone or in combination with other catalysts. Such other catalysts are not particularly limited, and are known catalysts (for example, in the case of automobile exhaust gas purification catalysts, oxidation catalysts, NOx reduction catalysts, NOx occlusion reduction catalysts (NSR catalysts), lean NOx traps). Catalyst (LNT catalyst), NOx selective reduction catalyst (SCR catalyst, etc.) may be used as appropriate.

[Method for producing exhaust gas-purifying catalyst]
The method for producing an exhaust gas purifying catalyst having two or more catalyst coat layers on a substrate according to the present invention has a structure in which noble metal particles are supported on the surface of secondary particles of a carrier material containing Al ₂ O _3. And a catalyst slurry containing 0.5 to 9.0 parts by mass of fibrous organic matter with respect to 100 parts by mass of the catalyst particles and the metal oxide material in the slurry. Forming a coat. The fibrous organic matter has the characteristics that the average fiber diameter is in the range of 1.7 to 8.0 μm and the average aspect ratio is in the range of 9 to 40. It is preferable that at least a part of the fibrous organic material is removed by applying the catalyst slurry to the substrate and then heated to form voids in the catalyst coat layer. Of the catalyst coat layers, the lower layer catalyst coat below the uppermost catalyst coat may be formed according to a conventionally known method such as using a catalyst slurry similar to the above except that it does not contain fibrous organic matter. it can.

Catalyst particles having a structure in which noble metal particles are supported on the surface of secondary particles of a carrier material containing Al ₂ O ₃ can be prepared, for example, by sputtering. As an example, a carrier material powder and a target material that is the source of noble metal particles are placed in a vacuum chamber, an inert gas such as argon gas is introduced, and then a high voltage is applied between the carrier material and the target material. Thus, there is a method in which ionized Ar is applied to the target material and the noble metal particles blown off from the target material are supported. However, the present invention is not limited to this, and it can be prepared by a known method such as spray support or reduction treatment. As the noble metal used for the uppermost catalyst coat, as described above, a catalyst whose catalytic activity is hardly damaged by stress or poisoning, particularly Rh is preferable.

(Preparation and application of catalyst slurry)
In the method for producing an exhaust gas purifying catalyst of the present invention, a catalyst slurry containing 0.5 to 9.0 parts by mass of fibrous organic matter is used with respect to the aforementioned catalyst particles and 100 parts by mass of metal oxide in the slurry. Although it will not restrict | limit especially if it is a substance which can be removed by the heating process mentioned later as a fibrous organic substance, For example, a polyethylene terephthalate (PET) fiber, an acrylic fiber, a nylon fiber, rayon fiber, a cellulose fiber is mentioned. Among these, it is preferable to use at least one selected from the group consisting of PET fibers and nylon fibers from the viewpoint of the balance between processability and firing temperature. By adding such fibrous organic matter to the catalyst slurry and removing at least a part of the fibrous organic matter in a subsequent step, a void having the same shape as the fibrous organic matter can be formed in the catalyst coat layer. It becomes possible. The voids thus prepared serve as a diffusion flow path for the exhaust gas, and can exhibit excellent catalytic performance even in a high load region with a high gas flow rate.

  The fibrous organic substance used in the method for producing a catalyst of the present invention has an average fiber diameter in the range of 1.7 to 8.0 μm. If the average fiber diameter is too small, effective high aspect ratio pores cannot be obtained, resulting in insufficient catalyst performance. On the other hand, if it is too large, the thickness of the catalyst coat layer increases, resulting in increased pressure loss and fuel consumption. Although it causes deterioration, such a problem does not occur in the above range. From the viewpoint of the balance between the catalyst performance and the coat thickness, the average fiber diameter of the fibrous organic material is preferably in the range of 2.0 to 6.0 μm, particularly 2.0 to 5.0 μm.

  Moreover, the fibrous organic substance used with the manufacturing method of the catalyst of this invention has the average aspect-ratio in the range of 9-40. If the average aspect ratio is too small, the gas diffusivity will be insufficient due to insufficient pore communication. On the other hand, if the average aspect ratio is too large, the diffusivity will be too large to pass through the coating layer without contacting the catalytic active point. However, such a problem does not occur in the above range, although the ratio of the gas to be increased increases and sufficient catalyst performance cannot be obtained. The average aspect ratio of the fibrous organic matter is preferably in the range of 9 to 30, particularly 9 to 28, from the viewpoint of the balance between gas diffusibility and catalyst performance. The average aspect ratio of the fibrous organic material is defined as “average fiber length / average fiber diameter”. Here, the fiber length is a linear distance connecting the start point and the end point of the fiber. The average fiber length can be obtained by randomly extracting 50 or more fibrous organic substances, and measuring and averaging the fiber lengths of these fibrous organic substances. Moreover, an average fiber diameter can be calculated | required by extracting 50 or more fibrous organic substances at random, and measuring and averaging the fiber diameter of these fibrous organic substances.

  In the method for producing a catalyst of the present invention, the fibrous organic matter is added in an amount in the range of 0.5 to 9.0 parts by mass with respect to 100 parts by mass of the metal oxide in the slurry, and the catalyst for forming the uppermost catalyst coat. Used for slurry. If the amount of the fibrous organic compound is too small, sufficient pore connectivity cannot be obtained, resulting in insufficient catalyst performance. On the other hand, if the amount is too large, the thickness of the catalyst coat layer increases and pressure loss increases. However, although this causes fuel consumption deterioration, such a problem does not occur in the above range. In addition, from the viewpoint of the balance between the catalyst performance and the pressure loss, the fibrous organic matter is 0.5 to 8.0 parts by mass, particularly 1.0 to 5.0 parts by mass with respect to 100 parts by mass of the metal oxide in the slurry. It is preferable to use the catalyst slurry in an amount within the above range. In addition, it is more preferable that the fibrous organic material has an average fiber diameter in the range of 2.0 to 6.0 μm and an average aspect ratio in the range of 9 to 30.

  The method for preparing the catalyst slurry is not particularly limited, and the catalyst particles and the fibrous organic matter may be mixed together with a known binder or other metal oxide material as necessary, and a known method is appropriately adopted. can do. The mixing conditions are not particularly limited. For example, the stirring speed is preferably in the range of 100 to 400 rpm, the processing time is preferably 30 minutes or more, and the fibrous organic matter is in the catalyst slurry. It only needs to be uniformly dispersed and mixed.

  The catalyst slurry containing the fibrous organic matter is coated on the surface of the substrate, more specifically on the lower layer catalyst coat on the substrate, and the coating amount and average thickness of the catalyst coat layer after firing are each unit volume of the substrate. It is preferable to form the catalyst slurry layer by coating so as to be within the range of 50 to 300 g / L and within the range of 25 to 160 μm. The application method is not particularly limited, and a known method can be appropriately employed. Specific examples include a method in which a honeycomb-shaped base material is applied by immersing it in a catalyst slurry (immersion method), a wash coating method, a method in which the catalyst slurry is press-fitted by press-fitting means, and the like. The coating conditions are as follows: the catalyst slurry is applied to the surface of a honeycomb-shaped substrate, and the coating amount of the catalyst coat layer after firing is in the range of 50 to 300 g / L per unit volume of the substrate. It is necessary to apply so that the average thickness of the material falls within the range of 25 to 160 μm.

In the method for producing a catalyst of the present invention, the catalyst slurry is applied to the substrate and then heated to evaporate the solvent and the like contained in the slurry and remove the fibrous organic matter. Heating is typically performed by firing a substrate coated with a catalyst slurry. Firing is preferably performed at a temperature in the range of 300 to 800 ° C, particularly 400 to 700 ° C. If the firing temperature is too low, fibrous organic matter tends to remain, whereas if it is too high, noble metal particles tend to sinter, but such a problem does not occur in the above range. Since the firing time varies depending on the firing temperature, it cannot be generally stated, but it is preferably 20 minutes or more, and more preferably 30 minutes to 2 hours. Furthermore, the atmosphere during firing is not particularly limited, but is preferably in the air or in an inert gas such as nitrogen (N ₂ ). An exhaust gas purifying catalyst having two or more catalyst coat layers is a catalyst in which the composition, that is, the amount and type of catalyst particles and the like differ on the catalyst coat formed on the substrate by applying and heating the catalyst slurry. It can be prepared by applying the slurry again and repeating heating.

  The exhaust gas purifying catalyst of the present invention is used in a method for purifying exhaust gas by contacting exhaust gas discharged from an internal combustion engine. The method for bringing the exhaust gas into contact with the exhaust gas purification catalyst is not particularly limited, and a known method can be appropriately employed. For example, the exhaust gas according to the present invention is disposed in an exhaust gas pipe through which the gas discharged from the internal combustion engine flows. A method of bringing the exhaust gas from the internal combustion engine into contact with the exhaust gas purifying catalyst may be adopted by arranging the purifying catalyst.

  Since the exhaust gas purifying catalyst of the present invention exhibits excellent catalytic performance even in a high load region with a high gas flow rate, the exhaust gas purifying catalyst of the present invention includes, for example, an internal combustion engine such as an automobile. By contacting the exhaust gas discharged from the exhaust gas, it becomes possible to purify the exhaust gas even in a high load region with a high gas flow rate. The exhaust gas purifying catalyst of the present invention is a harmful component such as harmful gas (hydrocarbon (HC), carbon monoxide (CO), nitrogen oxide (NOx)) in exhaust gas discharged from an internal combustion engine such as an automobile. Can be used to purify.

  EXAMPLES Hereinafter, although this invention is demonstrated in detail using an Example, this invention is not limited to these Examples.

[(Reference) Test 1: Preparation and Evaluation of Catalyst Having One Layer of Catalyst Coat Layer with Void]
1. Preparation of catalyst (1) Example 1
First, 500 g of ion-exchanged water, 150 g of Al ₂ O ₃ powder (manufactured by Sasol: specific surface area 100 m ² / g, average particle diameter 30 μm) and CeO ₂ —ZrO ₂ solid solution powder (manufactured by Daiichi Elemental Chemical Co., Ltd .: For a solution obtained by adding and mixing 300 g of CeO ₂ content 20 mass%, ZrO ₂ content 25 mass%, specific surface area 100 m ² / g, average particle diameter 10 nm), a bead mill (manufactured by AS ONE, product) Name “alumina ball”, used beads: microbeads with a diameter of 5000 μm), treatment time: 25 minutes, agitation treatment was performed at a stirring speed of 400 rpm, and the CeO ₂ —ZrO ₂ solid solution and Al ₂ O ₃ powder were A dispersion liquid containing metal oxide particles made of a mixture (complex metal oxide) was prepared. In addition, when the particle size of the metal oxide particles was measured by a laser diffraction method using a laser diffraction type particle size distribution measuring apparatus (trade name “LA-920” manufactured by Horiba, Ltd.), accumulation in an area-based cumulative particle size distribution was performed. The 50% diameter value was 3.2 μm.

  Next, in the obtained dispersion liquid, 0.05 L of a dinitrodiammine platinum solution containing 4 g of platinum (Pt) as a noble metal raw material in terms of metal and organic fibers (PET fibers, average diameter: 3 μm × length: 42 μm) as fibrous organic matter The average aspect ratio: 14) was added in an amount of 1.0 part by mass with respect to 100 parts by mass of the metal oxide particles, and the mixture was mixed for 30 minutes at a stirring speed of 400 rpm to prepare a catalyst slurry.

Next, the obtained catalyst slurry was a hexagonal cell cordierite monolith honeycomb substrate (trade name “D60H / 3-9R-08EK”, manufactured by Denso Corporation) as a substrate, diameter: 103 mm, length: 105 mm, volume: 875 ml. , Cell density: 600 cell / inch ² ), and after drying in the atmosphere at a temperature of 100 ° C. for 0.5 hours, the catalyst slurry is further washed, dried and calcined. The catalyst slurry layer was formed on the base material by repeatedly performing the coating amount on the base material until it reached 100 g (100 g / L) per liter of the base material.

  Thereafter, the catalyst for exhaust gas purification (catalyst sample) in which a catalyst coat layer made of catalyst particles is formed on the surface of a base material made of a honeycomb-shaped cordierite monolith base material by firing at 500 ° C. in the atmosphere for 2 hours. Got.

  Treatment time [minutes] of the stirring treatment in the oxide particle preparation step and the particle size of the metal oxide particles obtained (cumulative 50% diameter value) [μm], fibrous organic matter used in the catalyst slurry preparation step Table 1 shows the raw material species, the average fiber diameter [μm], the average aspect ratio, the mixing amount [parts by mass], and the coating amount [g / L] of the catalyst coat layer.

(2) Examples 2 to 42
The processing time by the bead mill is the time shown in Table 1 to Table 2, and the bead mill is such that the particle size of the metal oxide particles becomes the value of Table 1 with the value of the cumulative 50% diameter in the volume-based cumulative particle size distribution. The catalyst was treated in the same manner as in Example 1 except that the raw material species, the average fiber diameter, the average aspect ratio, and the mixed amount of the fibrous organic material shown in Tables 1 and 2 were used as the fibrous organic material. A slurry was obtained. Next, the obtained catalyst slurry was applied (coated) to a cordierite monolith honeycomb substrate in the same manner as in Example 1, and fired to obtain an exhaust gas purifying catalyst (catalyst sample).

In addition, the fibrous organic substance used in Examples 31-39 is titanium isopropoxide (Ti (OPri) ₄ ), polyethylene glycol (PEG), and polymethyl methacrylate resin (PMMA) particles (average diameter 3 μm) in isopropanol. Was used, and an organic fiber having a predetermined shape was prepared by radiating into distilled water.

  Treatment time [minutes] of the stirring treatment in the oxide particle preparation step and the particle size of the metal oxide particles obtained (cumulative 50% diameter value) [μm], fibrous organic matter used in the catalyst slurry preparation step Table 1 and Table 2 show the raw material species, average fiber diameter [μm], average aspect ratio, mixing amount [parts by mass], and coating amount [g / L] of the catalyst coat layer.

(3) Comparative Examples 1-7
The processing time by the bead mill is set to the time shown in Table 3, and the stirring process is performed in the bead mill so that the particle size of the metal oxide particles becomes the value of Table 3 with the cumulative 50% diameter value in the cumulative particle size distribution on the volume basis. A comparative catalyst slurry was obtained in the same manner as in Example 1 except that no organic material (fibrous organic material) was used. Next, the obtained comparative catalyst slurry was applied (coated) to a cordierite monolith honeycomb substrate in the same manner as in Example 1 and fired to obtain a comparative exhaust gas purification catalyst (comparative catalyst sample). It was.

  Treatment time [min] of the stirring treatment in the oxide particle preparation step, the particle size of the obtained metal oxide particles (volume-based cumulative 50% diameter value) [μm], the coating amount of the catalyst coat layer [g / L Is shown in Table 3.

(4) Comparative Examples 8 to 133
The processing time by the bead mill is the time shown in Tables 3 to 8, and the particle size of the metal oxide particles is set to the value of Table 3 to Table 8 with the value of the cumulative 50% diameter in the cumulative particle size distribution on the volume basis. Except for using the raw material species, average fiber diameter or average diameter, average aspect ratio, and mixing amount of fibrous organic matter or organic matter shown in Tables 3 to 8 as the fibrous organic matter or organic matter. In the same manner as in Example 1, a comparative catalyst slurry was obtained. Next, the obtained comparative catalyst slurry was applied (coated) to a cordierite monolith honeycomb substrate in the same manner as in Example 1 and fired to obtain a comparative exhaust gas purification catalyst (comparative catalyst sample). It was.

The organic substances (fibrous organic substances) used in Comparative Examples 127 to 131 were isopropanol, titanium isopropoxide (Ti (OPri) ₄ ), polyethylene glycol (PEG), and polymethyl methacrylate resin (PMMA) particles (average diameter). 3 μm) was added and the resulting mixture was radiated into distilled water to prepare an organic fiber having a predetermined shape.

  Treatment time [minutes] of the stirring treatment in the oxide particle preparation step and the particle size of the metal oxide particles obtained (cumulative 50% diameter value) [μm], fibrous organic matter used in the catalyst slurry preparation step Table 3 to Table 8 show organic material types, average fiber diameter or average diameter [μm], average aspect ratio and mixing amount [parts by mass], and catalyst coating layer coverage [g / L].

2. Evaluation Regarding the exhaust gas purification catalyst (catalyst sample) obtained in Examples 1-42 and the comparative exhaust gas purification catalyst (comparative catalyst sample) obtained in Comparative Examples 1-133, the average thickness of the catalyst coat layer [ μm], catalyst particle size (cumulative 15% diameter value based on cross-sectional area) [μm], catalyst coating layer porosity [volume%], average aspect ratio of high aspect ratio pores, high aspect ratio pores The percentage of the total voids [%] and the orientation angle [degree (°)] of high aspect ratio pores (cumulative 80% angle value) were measured.

(1) Measurement test of average thickness of catalyst coat layer Catalyst sample and comparative catalyst sample were embedded in epoxy resin, cut in the radial direction of the substrate (honeycomb-shaped substrate), and the cross section was polished The average thickness of the catalyst coat layer was measured by observation with a scanning electron microscope (SEM) (magnification: 700 times). In addition, average thickness was calculated | required by extracting ten catalyst coat layers at random, measuring the layer thickness of these catalyst coat layers, and averaging. The obtained results are shown in Tables 1 to 8.

(2) Measurement test of catalyst particle size The catalyst sample and the comparative catalyst sample were embedded in epoxy resin, cut in the radial direction of the substrate (honeycomb-shaped substrate), and the cross-section polished was measured. Then, observation with a scanning electron microscope (SEM) (magnification: 700 times) was performed, and the value of the cumulative 15% diameter in the cumulative particle size distribution based on the cross-sectional area of the catalyst particles was calculated. In addition, the value of the cumulative 15% diameter of the particle size of the catalyst particles is 200 μm or more in the horizontal direction with respect to the substrate flat portion of the catalyst coat layer and 25 μm in the vertical direction with respect to the substrate flat portion. When the catalyst particles in the rectangular area composed of the above are extracted and the cross-sectional area of the catalyst particles is counted from the catalyst particles having a large catalyst particle size (cross-sectional area), the sum of the cross-sectional areas of the catalyst particles is 0 It was determined by measuring the value of the particle size of the catalyst particles when corresponding to 15% of the entire cross-sectional area of the catalyst coat layer excluding pores less than 3 mm ² . The obtained results are shown in Tables 1 to 8.

(3) Measurement Test of Porosity of Catalyst Coat Layer The porosity of the catalyst sample was measured by the following formula by the underwater weight method according to JIS R 2205. The degassing was vacuum degassing.
Porosity (porosity) (volume%) = (W3−W1) / (W3−W2) × 100
W1: Dry mass (120 ° C x 60 minutes)
W2: Underwater mass W3: Saturated mass The obtained results are shown in Tables 1 to 8.

(4) Measurement test of pores in catalyst coat layer 1: equivalent circle diameter of pores FIB-SEM analysis was performed on the pores in the catalyst coat layer of the catalyst sample and the comparative catalyst sample.
First, the catalyst sample and the comparative catalyst sample were cut in the axial direction at the position of the dotted line shown in FIG. 1 (A) to obtain a test piece having the shape shown in FIG. 2 (B). Next, the range indicated by the dotted line in FIG. 1B is shown in FIG. 1C while cutting with a FIB (focused ion beam processing apparatus, product name “NB5000”, manufactured by Hitachi High-Technologies Corporation). Thus, SEM (scanning electron microscope, manufactured by Hitachi High-Technologies Corporation, trade name “NB5000”) images were taken at a depth of 0.28 μm. The conditions for the FIB-SEM analysis were SEM images of 25 μm or more in length, 500 μm or more in width, a measurement depth of 500 μm or more, a number of fields of view of 3 or more, and a magnification of 2000 times. FIG. 1 is a schematic view showing an example of a measurement method of FIB-SEM, and FIG. 1A is a schematic view showing a part of a cross section of a catalyst coat layer perpendicular to the exhaust gas flow direction of a substrate of an exhaust gas purification catalyst of the present invention. FIG. 4B is a schematic view showing a test piece obtained by cutting the exhaust gas purifying catalyst in the axial direction at the position of the dotted line shown in FIG. Indicates. As an example of the observation result by FIB-SEM analysis, one of the continuous cross-sectional SEM images measured for the catalyst sample of Example 5 is shown in FIG. The black portions in FIG. 2 are pores. FIG. 2 is a scanning electron micrograph (SEM photograph) of the cross section of the catalyst coat layer perpendicular to the exhaust gas flow direction of the exhaust gas purifying catalyst substrate obtained in Example 5. Note that a continuous image as shown in FIG. 1C can also be taken by X-ray CT or the like.

Next, a continuous cross-sectional image (SEM image) obtained by FIB-SEM analysis is obtained using commercially available image analysis software (Mitani Corporation, “Two-dimensional image analysis software WinROOF” using the difference in brightness between the pores and the catalyst. ]) Was used for image analysis, and binarization was performed to extract pores. As an example of the obtained results, FIG. 3 shows a binarized version of the SEM photograph of FIG. In FIG. 3, a black part shows a catalyst and a white part shows a pore, respectively. In the analysis of the pores, pores having an equivalent circle diameter of 2 μm or more in the cross-sectional image of the cross section of the catalyst coat layer perpendicular to the exhaust gas flow direction of the base material were analyzed. In addition, the function of extracting an object using the difference in luminance is not limited to WinROOF, but is standardly installed in general analysis software (for example, manufactured by Planetron Corporation) image-Pro Plus).
By such image analysis, the area within the outline of the pore was obtained, the equivalent circle diameter of the pore was calculated, and the equivalent circle diameter as the particle diameter of the pore was obtained.

(5) Measurement test 2 of pores in catalyst coat layer: average aspect ratio of high aspect ratio pores Next, the continuous cross-sectional image obtained by the above method was analyzed, and three-dimensional information of the pores was extracted. Here, the method for measuring the average aspect ratio of the high aspect ratio pores is the same as the method described with reference to FIGS. 4 and 5, and the average aspect ratio of the high aspect ratio pores is the same as that described above. 4 and FIG. 5, a two-dimensional projection diagram illustrating the three-dimensional information of the pores and a cross-sectional image of the pores are created. It was determined by analyzing a high aspect ratio pore (number of fields of view is 3 or more and the magnification of imaging was 2000 times). A two-dimensional projection diagram illustrating three-dimensional information of pores obtained by analyzing a continuous cross-sectional image of the cross section of the catalyst coat layer perpendicular to the exhaust gas flow direction of the exhaust gas purification catalyst substrate obtained in Example 5 These are the same as the two-dimensional projection view illustrating the three-dimensional information of the pores shown in FIG. As a result, the average aspect ratio of the high aspect ratio pores of Example 5 was 18.9. Moreover, the measurement results (average aspect ratio of high aspect ratio pores) of Examples and Comparative Examples other than Example 5 are shown in Tables 1 to 8, respectively.

(6) Measurement test of pores in catalyst coat layer 3: Ratio of high aspect ratio pores to the entire voids Next, the ratio of the high aspect ratio pores to the entire voids is determined by the voids of the high aspect ratio pores. The rate was determined by dividing the rate by the porosity of the catalyst coat layer.

  The porosity (volume%) of the high aspect ratio pores is as follows. First, SEM image: high aspect ratio pores in the range of 25 μm or more in length, 500 μm or more in width, and 500 μm or more in measurement depth (number of fields of view is 3 or more) The imaging magnification was 2000 times), and the respective volumes were calculated by the following method. That is, the volume of the high aspect ratio pores is calculated by multiplying the cross-sectional area of the high aspect ratio pores in the cross-sectional image obtained by FIB-SEM by the pitch of the continuous cross-sectional image 0.28 μm and integrating the values. did. Next, by dividing the obtained value of “volume of high aspect ratio pores” by the volume of the range in which the FIB-SEM was photographed (the range of the SEM image), the porosity of the high aspect ratio pores ( Volume%).

  Next, the porosity (volume%) of the obtained high aspect ratio pores is divided by the porosity (volume%) of the catalyst coat layer obtained in the above “measurement test of the porosity of the catalyst coat layer”. From the above, the ratio (volume%) of the high aspect ratio pores to the entire voids was determined (“the ratio of the high aspect ratio pores to the entire voids (%)” = “the porosity of the high aspect ratio pores (volume%). ) "/" Porosity of catalyst coat layer (volume%) "x 100).

  As a result, the ratio of the high aspect ratio pores of Example 5 to the entire voids was 11.1% by volume. Moreover, the measurement result (ratio which occupies for the whole space | gap of a high aspect ratio pore) of Examples other than Example 5 and a comparative example is shown in Table 1-Table 8, respectively.

(7) Measurement test of pores in catalyst coat layer 4: Orientation angle of high aspect ratio pores Next, as the orientation angle of the high aspect ratio pores, the major axis direction vector of the high aspect ratio pores and the base A cumulative 80% angle value in the cumulative angular distribution of the angle (conical angle) formed by the exhaust gas flow direction vector of the material was determined. Here, the method for measuring the orientation angle (cumulative 80% angle value) of the high aspect ratio pores is the same as the method described with reference to FIGS. Note that the two-dimensional projection obtained in Example 5 is the same as the two-dimensional projection shown in FIG. 4, and FIG. 6 shows the high aspect ratio pores in the two-dimensional projection obtained in Example 5. It is the same as the schematic diagram showing the cone angle. As shown in the schematic diagram of FIG. 6, the angle (cone angle) formed by the long diameter direction vector (Y) of the high aspect ratio pores and the exhaust gas flow direction (honeycomb axial direction) vector (X) of the substrate. The cumulative 80% angle value in the cumulative angular distribution based on the cone angle was calculated by image analysis of the three-dimensional image. The orientation angle (cumulative 80% angle value) of the high aspect ratio pores was randomly extracted from 20 high aspect ratio pores, and the cumulative angle based on the cone angle of these high aspect ratio pores. The cumulative 80% angle value in the distribution was measured and averaged. The obtained results (cumulative 80% angle values) are shown in Tables 1 to 8, respectively.

(8) Catalyst performance evaluation test About the catalyst sample obtained in Examples 1-42 and Comparative Examples 1-133, the NOx purification rate measurement test was each performed as follows, and the catalyst performance of each catalyst was evaluated.

(NOx purification rate measurement test)
With respect to the catalyst samples obtained in Examples 1-42 and Comparative Examples 1-133, the NOx purification rate in the transient atmosphere at the time of transient was measured as follows.
That is, first, using an in-line four-cylinder 2.4L engine, A / F feedback control was performed targeting 14.1 and 15.1, and the NOx purification rate was calculated from the average NOx emission at the time of A / F switching. . The engine operating conditions and piping setup were adjusted so that the intake air amount at that time was 40 (g / sec) and the inflow gas temperature to the catalyst was 750 ° C.

3. Results (1) Relationship between coverage of catalyst coat layer and catalyst performance As a graph showing the results of the catalyst performance evaluation tests of the catalysts obtained in Examples 1-42 and Comparative Examples 1-133, A graph showing the relationship with the NOx purification rate is shown in FIG. As is apparent from a comparison between the results of Examples 1 to 42 and the results of Comparative Examples 1 to 133 shown in FIG. 7 and Tables 1 to 8, the exhaust gas purifying catalysts of Examples 1 to 42 are catalyst coated layers. In the range of 50 to 300 g / L, it was confirmed that excellent catalytic performance was exhibited even in a high load region with a high gas flow rate.

(2) Relationship between average thickness of catalyst coat layer and catalyst performance As a graph showing the results of the catalyst performance evaluation tests of the catalysts obtained in Examples 1-42 and Comparative Examples 1-133, the average thickness of catalyst coat layer and NOx A graph showing the relationship with the purification rate is shown in FIG. As is apparent from a comparison between the results of Examples 1-42 and the results of Comparative Examples 1-133 shown in FIG. 8 and Tables 1 to 8, the exhaust gas-purifying catalysts of Examples 1-42 are catalyst coated layers. It was confirmed that the catalyst exhibited excellent catalyst performance even in a high load region with a high gas flow rate in a range of 25 to 160 μm in average thickness.

(3) Relationship between porosity of catalyst coat layer and catalyst performance As a graph showing the results of the catalyst performance evaluation tests of the catalysts obtained in Examples 1-42 and Comparative Examples 1-133, the porosity of catalyst coat layer (in water A graph showing the relationship between the porosity measured by the gravimetric method) and the NOx purification rate is shown in FIG. As is apparent from a comparison between the results of Examples 1 to 42 and the results of Comparative Examples 1 to 133 shown in FIG. 9 and Tables 1 to 8, the exhaust gas purifying catalysts of Examples 1 to 42 are catalyst coated layers. In the range of 50 to 80% by volume, it was confirmed that excellent catalytic performance was exhibited even in a high gas flow and high load region.

(4) Relationship between average aspect ratio of high aspect ratio pores and catalyst performance First, the aspect ratio of high aspect ratio pores of the catalyst obtained in Example 5 (catalyst coating perpendicular to the exhaust gas flow direction of the substrate) (Aspect ratio of high aspect ratio pores having an aspect ratio of 5 or more among the pores) and frequency obtained by analyzing pores having an equivalent circle diameter of 2 μm or more in the cross-sectional image of the layer cross section) %) Is shown in FIG. The relationship between the aspect ratio and the frequency (%) in the pores of the catalyst obtained in Comparative Example 4 is also shown in FIG. From the comparison between the results of Example 5 and Comparative Example 4 shown in FIG. 10, it was confirmed that the comparative exhaust gas purifying catalyst of Comparative Example 4 had very few high aspect ratio pores.

  Next, as a graph showing the results of the catalyst performance evaluation tests of the catalysts obtained in Examples 1-42 and Comparative Examples 1-133, the average aspect ratio of the high aspect ratio pores (in the exhaust gas flow direction of the substrate) The average aspect ratio of high aspect ratio pores having an aspect ratio of 5 or more among the pores obtained by analyzing pores having an equivalent circle diameter of 2 μm or more in a cross-sectional image of a vertical catalyst coat layer cross section FIG. 11 is a graph showing the relationship between the ratio) and the NOx purification rate. As is apparent from a comparison between the results of Examples 1 to 42 and the results of Comparative Examples 1 to 133 shown in FIG. 11 and Tables 1 to 8, the exhaust gas purifying catalysts of Examples 1 to 42 have a high aspect ratio. When the average aspect ratio of the pores is in the range of 10 to 50, it was confirmed that excellent catalytic performance was exhibited even in a high load region with a high gas flow rate.

(5) Relationship between the ratio of the high aspect ratio pores to the entire void and the catalyst performance As a graph showing the results of the catalyst performance evaluation tests of the catalysts obtained in Examples 1-42 and Comparative Examples 1-133, the high aspect ratio FIG. 12 shows a graph showing the relationship between the ratio of the pores to the entire voids (the ratio of the high aspect ratio pores) and the NOx purification rate. As is apparent from a comparison between the results of Examples 1 to 42 and the results of Comparative Examples 1 to 133 shown in FIG. 12 and Tables 1 to 8, the exhaust gas purifying catalysts of Examples 1 to 42 have a high aspect ratio. It was confirmed that excellent catalytic performance was exhibited even in a high load region with a high gas flow rate within a range of 0.5 to 50% of the total pore space.

(6) Relationship between Accumulated 80% Angle Value of High Aspect Ratio Pore and Catalyst Performance First, the cone angle (degree (°) of the high aspect ratio pore of the catalyst obtained in Example 16 was reduced. FIG. 13 is a graph showing the relationship between the cumulative ratio (%) of the major axis direction vector Y of the holes and the flow direction vector X of the exhaust gas of the base material. From FIG. 13, it was confirmed that the cone angle has a distribution.

  Next, as a graph showing the results of the catalyst performance evaluation test of the catalysts obtained in Examples 1 to 42 and Comparative Examples 1 to 133, the cumulative 80% angle value of high aspect ratio pores (high aspect ratio pores). A graph showing the relationship between the NOx purification rate and the cumulative 80% angle value in the angular reference cumulative angle distribution of the angle (cone angle) formed by the major axis direction vector Y and the exhaust gas flow direction vector X of the base material. 14 shows. As is apparent from a comparison between the results of Examples 1 to 42 and the results of Comparative Examples 1 to 133 shown in FIG. 14 and Tables 1 to 8, the exhaust gas purifying catalysts of Examples 1 to 42 have a high aspect ratio. It was confirmed that excellent catalyst performance was exhibited even in a high load region with a high gas flow rate when the orientation angle of the pores (accumulated 80% angle value based on the angle) was in the range of 0 to 45 degrees (°). .

[(Reference) Test 2: Preparation and evaluation of catalyst having two catalyst coat layers]
1. Catalyst Preparation (1) Comparative Example 1: Two-layer catalyst prepared without using a pore former (a) Lower Pd layer (Pd (1.0) / CZ (50) + Al ₂ O ₃ (75))
Using an aqueous palladium nitrate solution (Cataler Co., Ltd.) having a noble metal content of 8.8% by weight, Pd was converted to ceria-zirconia composite oxide material (30% by weight CeO ₂ , 60% by weight ZrO ₂ , 5% by weight). A Pd / CZ material supported on a composite oxide consisting of 5% Y ₂ O ₃ and 5% by weight La ₂ O ₃ (hereinafter referred to as CZ material) was prepared. Next, the Pd / CZ material, a composite Al ₂ O ₃ carrier containing 1% by weight of La ₂ O ₃ , and an Al ₂ O ₃ binder are added to distilled water with stirring and suspended. Was prepared. The 15% cumulative diameter in the cumulative particle size distribution on the basis of the cross-sectional area of the particles contained in the slurry was 3.3 μm.

Slurry 1 was poured into a honeycomb structure base material made of cordierite having a capacity of 875 cc (600H / 3-9R-08, manufactured by Denso Corporation), and then unnecessary portions were blown off with a blower to coat the base material wall surface. The coating contained 1.0 g / L Pd, 75 g / L composite Al ₂ O ₃ support, and 50 g / L Pd / CZ material relative to the substrate volume. After coating, moisture was removed for 2 hours with a 120 ° C. dryer, followed by firing for 2 hours in an electric furnace at 500 ° C. The thickness of the coating based on SEM observation was 35 μm, and the porosity of the coating based on the underwater weight method was 73%.

(B) Upper layer Rh layer (Rh (0.2) / CZ (60) + Al ₂ O ₃ (40))
An Rh / CZ material in which Rh was supported on a CZ material was prepared by an impregnation method using a rhodium hydrochloride aqueous solution (Cataler Co., Ltd.) having a noble metal content of 2.8 wt%. Next, the Rh / CZ material, a composite Al ₂ O ₃ carrier containing 1% by weight of La ₂ O ₃ , and an Al ₂ O ₃ binder are added to distilled water with stirring and suspended. Was prepared. The 15% cumulative diameter in the cumulative particle size distribution on the basis of the cross-sectional area of the particles contained in the slurry was 3.2 μm.

Slurry 2 was poured into the honeycomb structure base material coated with slurry 1, and then unnecessary portions were blown off with a blower to coat the base material wall surface. The coating included Rh 0.2 g / L, composite Al ₂ O ₃ support 40 g / L, and Rh / CZ material 60 g / L relative to the substrate volume. After coating, moisture was removed for 2 hours with a 120 ° C. dryer, followed by firing for 2 hours in an electric furnace at 500 ° C. The thickness of the coating based on SEM observation was 27 μm, and the porosity of the coating based on the underwater gravimetric method was 68%.

(2) Comparative Example 2: Two-layer catalyst prepared by using a pore-forming material only in the lower layer When a slurry 1 is prepared, PET fiber having a diameter (φ) of 2 μm and a length (L) of 80 μm is used as a metal oxide as a pore-forming material. A catalyst was prepared in the same manner as in Comparative Example 1 except that 3% by weight based on the weight of the particles was further added. The 15% cumulative diameter in the cumulative particle size distribution based on the cross-sectional area of the particles contained in the slurry 1 was 3.4 μm. Moreover, the lower layer coating formed using the slurry 1 to which the pore former was added had a coating thickness of 39 μm based on SEM observation, and the porosity of the coating based on the underwater weight method was 76%. The volume ratio of the high aspect ratio pores having an aspect ratio of 5 or more with respect to all voids in the coating was 9%, and the average aspect ratio of the high aspect ratio pores was 41 (both 3D measurement by FIB-SEM). based on).

(3) Comparative Example 3: Two-layer catalyst prepared using pore formers in the upper and lower layers In both preparations of slurry 1 and slurry 2, the diameter (φ) 2 μm, length (L) as the pore former A catalyst was prepared in the same manner as in Comparative Example 1 except that 80 μm of PET fiber was further added by 3 wt% with respect to the weight of the metal oxide particles. The 15% cumulative diameter in the cumulative particle size distribution based on the cross-sectional area of the particles contained in the slurry 2 was 3.0 μm. The upper layer coating formed using the slurry 2 to which the pore former was added had a coating thickness of 30 μm based on SEM observation, and the porosity of the coating based on the underwater weight method was 71%. The volume ratio of the high aspect ratio pores having an aspect ratio of 5 or more with respect to all the voids in the coating was 9%, and the average aspect ratio of the high aspect ratio pores was 40 (all in 3D measurement by FIB-SEM). based on). In addition, about each data of the lower layer coating formed using the slurry 1 which added the pore former, it was the same as that of said (2).

(4) Example 1: Two-layer catalyst prepared using a pore-forming material only in the upper layer When a slurry 2 was prepared, PET fiber having a diameter (φ) of 2 μm and a length (L) of 80 μm was used as a metal oxide as the pore-forming material. A catalyst was prepared in the same manner as in Comparative Example 1 except that 3% by weight based on the weight of the particles was further added. Each data of the upper coating formed using the slurry 2 to which the pore former was added was the same as the above (3). Each data of the lower layer coating formed using the slurry 1 to which no pore former was added was the same as (1) above.

2. Evaluation (1) Oxygen storage capacity (OSC) evaluation under high Ga conditions A catalyst is mounted on a 2AR-FE engine (manufactured by Toyota), and A / F is rich and lean under high intake air (high Ga) conditions. The amount of oxygen absorbed and released from the catalyst was calculated from the behavioral delay of the O ₂ sensor attached to the catalyst outlet at that time.

(2) NOx purification performance evaluation under high Ga conditions NOx purification performance under high intake air (high Ga) conditions by attaching a catalyst to 2AR-FE engine (manufactured by Toyota) and controlling A / F with stoichiometric feedback Evaluated. The intake air amount was set to 40 g / s or more.

(3) Durability test A catalyst was attached to a 1UR-FE engine (manufactured by Toyota), and a deterioration acceleration test was conducted at 1000 ° C (catalyst bed temperature) for 25 hours. Regarding the exhaust gas composition at that time, the throttle opening and the engine load were adjusted, and the deterioration was promoted by repeating the rich region to stoichiometric to lean region in a constant cycle. When exhaust gas in a stoichiometric atmosphere (air-fuel ratio A / F = 14.6) is supplied to the catalyst after the durability test and the temperature is raised to 500 ° C., the temperature at which the NOx purification rate becomes 50% (T50−NOx) Was measured.

3. Results FIG. 15 is a graph showing measurement results of OSC performance (g / cat) under high Ga conditions, and FIG. 16 is a graph showing measurement results of NOx purification rates under high Ga conditions. The catalyst of Example 1 prepared using the pore former only in the upper layer was superior to the catalysts of Comparative Examples 1 to 3 under high Ga conditions in both oxygen storage capacity and NOx purification performance. From this, the rate-limiting step in gas diffusion is the upper layer coating, and it is not possible to obtain a sufficient effect even if only the lower layer coating is improved in gas diffusion as in Comparative Example 2, and as in Comparative Example 3. When the gas diffusibility of both the lower layer coating is improved, the gas does not easily permeate the lower layer, but the upper layer is less susceptible to purification, and the upper layer is made into a highly active Rh layer in consideration of heat resistance. It was speculated that the purification performance could not be fully utilized.

  FIG. 17 is a graph showing the results of measuring the temperature at which the NOx purification rate becomes 50% (T50-NOx) in the catalyst after the durability test. The catalyst of Example 1 prepared using the pore former only in the upper layer was judged to have no significant difference in heat resistance even when compared with the catalyst of Comparative Example 1 prepared without using the pore former. .

[Test 3: Examination of influence of catalyst particle structure and coat structure in catalyst having two catalyst coat layers]
1. Catalyst Preparation (1) Comparative Example 1: Using Rh / Al ₂ O ₃ Material Prepared by Conventional Method Using Colloid Solution (a) Lower Pd Layer (Pd (1.0) / CZ (50) + Al ₂ O ₃ (75))
Using an aqueous solution of palladium nitrate by impregnation method, ceria Pd - zirconia composite oxide material (30 wt% of _CeO 2, 60 wt% of _ZrO 2, 5 wt% of _Y 2 _{O 3} and 5 wt% of _La 2 O A Pd / CZ material supported on a composite oxide composed of ₃ (hereinafter referred to as CZ material) was prepared. The average particle size (D50) of the used CZ material was 6 μm. Next, the Pd / CZ material, a composite Al ₂ O ₃ material containing 1% by weight of La ₂ O ₃ (D50 = 6 μm; hereinafter simply referred to as a composite Al ₂ O ₃ material), and Al ₂ A slurry 1 was prepared by adding and suspending an O ₃ binder in distilled water while stirring.

Slurry 1 was poured into a cordierite honeycomb structure base material (600 cell hexagonal base material, manufactured by Denso Corporation) having a capacity of 875 cc, and then unnecessary portions were blown away with a blower to coat the base material wall surface. The coating contained 1.0 g / L of Pd, 75 g / L of composite Al ₂ O ₃ support, and 50 g / L of CZ material with respect to the substrate capacity. After coating, moisture was removed for 2 hours with a 120 ° C. dryer, followed by firing for 2 hours in an electric furnace at 500 ° C.

(B) Upper layer Rh layer (Rh (0.2) / Al ₂ O ₃ (40) + ACZ (60): with pore former)
A Rh / Al ₂ O ₃ material in which Rh was supported on a composite Al ₂ O ₃ material (D50 = 6 μm) was prepared using a colloidal solution in which Rh having an average particle diameter of 2 nm was dispersed. Next, the Rh / Al ₂ O ₃ material and alumina-ceria-zirconia composite oxide material (30 wt% Al ₂ O ₃ , 20 wt% CeO ₂ , 46 wt% ZrO ₂ , 2 wt% A composite oxide composed of Nd ₂ O ₃ and 2% by weight of La ₂ O ₃ (hereinafter referred to as ACZ material) and an Al ₂ O ₃ binder are added to distilled water with stirring and suspended to prepare slurry 2 did. To the slurry 2, PET fiber having a diameter (φ) of 2 μm and a length (L) of 80 μm as a pore former was further added by 3% by weight based on the coating amount.

Slurry 2 was poured into the honeycomb structure base material coated with slurry 1, and then unnecessary portions were blown off with a blower to coat the base material wall surface. The coating included Rh of 0.2 g / L, composite Al ₂ O ₃ material of 40 g / L, and ACZ material of 60 g / L relative to the substrate capacity. After coating, moisture was removed for 2 hours with a 120 ° C. dryer, followed by firing for 2 hours in an electric furnace at 500 ° C.

(2) Example 1: Rh / Al ₂ O ₃ material prepared by sputtering is used Rh target, composite Al ₂ O ₃ material powder (D50 = 6 μm) and Ar gas are introduced into a vacuum chamber, and Rh target is used. Then, sputtering was performed by applying a high voltage between the powder and a Rh / Al ₂ O ₃ material in which Rh was supported on the composite Al ₂ O ₃ material. By adjusting the sputtering conditions, the average particle size of Rh (by the CO adsorption method) was set to 2 nm. A two-layer catalyst was prepared in the same manner as in Comparative Example 1 except that the Rh / Al ₂ O ₃ material prepared by sputtering was used when preparing the slurry 2.

(3) Comparative Example 2: Rh / Al ₂ O ₃ material prepared by a conventional impregnation method is used Rh / Al ₂ in which Rh is supported on a composite Al ₂ O ₃ material by an impregnation method using an aqueous rhodium nitrate solution. A two-layer catalyst was prepared in the same manner as in Comparative Example 1 except that O ₃ material was prepared and slurry 2 was prepared using it.

(4) Comparative Example 3: The coated catalyst was immersed in a rhodium nitrate solution. After coating the substrate wall surface with slurry 2, the catalyst was immersed in the rhodium nitrate solution and Rh was adsorbed and supported on the surface of the coating layer. Prepared a two-layer catalyst in the same manner as in Comparative Example 1.

(5) Comparative Example 4: No pore-forming material Except that no pore-forming material was added during the preparation of slurry 2, the same procedure as in Example 1 using the Rh / Al ₂ O ₃ material prepared by sputtering was performed. A bed catalyst was prepared.

(6) Example 2: Using Rh / ACZ material prepared by sputtering Sputtering was performed using ACZ material powder (D50 = 6 μm) instead of composite Al ₂ O ₃ material powder, and the average particle size (CO adsorption method) A two-layer catalyst was prepared in the same manner as in Example 1 except that an Rh / ACZ material carrying 2 nm of Rh was prepared, and slurry 2 was prepared using it.

(7) Comparative Example 5: Using Rh / ACZ material prepared by a conventional method using a colloid solution Rh is supported on an ACZ material (D50 = 6 μm) using a colloid solution in which Rh having an average particle diameter of 2 nm is dispersed. A two-layer catalyst was prepared in the same manner as in Comparative Example 1 except that the prepared Rh / ACZ material was prepared and the slurry 2 was prepared using it.

(8) Comparative Example 6: Using Rh / ACZ material prepared by conventional impregnation method Using a rhodium nitrate aqueous solution, an Rh / ACZ material in which Rh is supported on an ACZ material is prepared by using the impregnation method, and using it A two-layer catalyst was prepared in the same manner as Comparative Example 1 except that the slurry 2 was prepared.

(9) Comparative Example 7: Using Rh / CZ material prepared by sputtering Sputtering was performed using CZ material powder (D50 = 6 μm) not containing Al ₂ O ₃ instead of composite Al ₂ O ₃ material powder. A two-layer catalyst was prepared in the same manner as in Example 1 except that an Rh / CZ material supporting Rh having an average particle diameter (by CO adsorption method) of 2 nm was prepared and slurry 2 was prepared using the Rh / CZ material. .

(10) Comparative Example 8: Rh / CZ material prepared by a conventional method using a colloid solution is used. Rh is supported on a CZ material (D50 = 6 μm) using a colloid solution in which Rh having an average particle diameter of 2 nm is dispersed. A two-layer catalyst was prepared in the same manner as in Comparative Example 1 except that the prepared Rh / CZ material was prepared and slurry 2 was prepared using it.

(11) Comparative Example 9: Using Rh / CZ material prepared by conventional impregnation method Using a rhodium nitrate aqueous solution, an Rh / CZ material having Rh supported on CZ material was prepared by using the impregnation method, and using it A two-layer catalyst was prepared in the same manner as Comparative Example 1 except that the slurry 2 was prepared.

(12) Comparative Example 10: Use of Al ₂ O ₃ having a particle size of 1 μm for the preparation of Rh / Al ₂ O ₃ material Instead of the composite Al ₂ O ₃ material powder having an average particle size (D50) of 6 μm, average particles A two-layer catalyst was prepared in the same manner as in Example 1 except that sputtering was performed using a composite Al ₂ O ₃ material powder having a diameter (D50) of 1 μm.

(13) Comparative Example 11: Use of Al ₂ O ₃ having a particle size of 3 μm for the preparation of Rh / Al ₂ O ₃ material Instead of the composite Al ₂ O ₃ material powder having an average particle size (D50) of 6 μm, average particles A two-layer catalyst was prepared in the same manner as in Example 1 except that sputtering was performed using a composite Al ₂ O ₃ powder having a diameter (D50) of 3 μm.

(14) Example 3: Use of Al ₂ O ₃ having a particle size of 20 μm for the preparation of Rh / Al ₂ O ₃ material Instead of the composite Al ₂ O ₃ material powder having an average particle size (D50) of 6 μm, average particles A two-layer catalyst was prepared in the same manner as in Example 1 except that sputtering was performed using a composite Al ₂ O ₃ material powder having a diameter (D50) of 20 μm.

2. Evaluation (1) Noble metal particle size evaluation The Rh particle size of the material obtained by supporting Rh was calculated by the CO adsorption method.

(2) Durability test A catalyst was attached to a 1UR-FE engine (manufactured by Toyota), and a 50-hour degradation acceleration test was conducted at 950 ° C (catalyst bed temperature). Regarding the exhaust gas composition at that time, the throttle opening and the engine load were adjusted, and the deterioration was promoted by repeating the rich region to stoichiometric to lean region in a constant cycle. When the catalyst after the endurance test is mounted on a 2AR-FE engine (manufactured by Toyota), the A / F is stoichiometrically feedback controlled, and when the temperature is raised to 500 ° C. under a high intake air amount (high Ga) condition, For each of hydrocarbon (THC), nitrogen oxide (NOx), and carbon monoxide (CO), the temperature (T50) at which the purification rate was 50% was measured.

(3) Purification performance evaluation under high temperature A catalyst is mounted on a 2AR-FE engine (manufactured by Toyota), and A / F is stoichiometrically feedback controlled, and NOx at 450 ° C under high intake air (high Ga) conditions. The purification rate was evaluated as the purification performance in the gas diffusion controlled region.

3. Results For each Example and Comparative Example, the results of Rh particle size measurement are summarized in Table 9 together with the Rh loading state expected from the production method, the carrier material and the secondary particle size.

  FIGS. 18A to 18C are diagrams schematically showing the state of Rh supported. FIG. 18A shows a state in which Rh is supported on the surface of the support primary particles as a result of preparing the Rh support material using the colloidal solution in which Rh is dispersed as in Comparative Examples 1, 5 and 8. . FIG. 18B shows a result of preparing an Rh-supporting material by an impregnation method using an aqueous rhodium nitrate solution as in Comparative Examples 2, 6 and 9, and as a result, Rh is present in the interior or surface of the primary particles of the support. Represents the state. FIG. 18C shows a state in which Rh is present only on the surface of the secondary particles constituting the carrier as a result of preparing the Rh-supporting material by sputtering as in Examples 1 to 3.

FIG. 19 is a HAADF-STEM image of an Rh / Al ₂ O ₃ material prepared by supporting Rh on a composite Al ₂ O ₃ material by sputtering as described in Example 1. It was confirmed that countless fine Rh particles were supported on the surface of the composite Al ₂ O ₃ material secondary particles shown in gray.

  FIG. 20 is a graph showing the results of the purification performance evaluation after the durability test of the catalyst of Example 1 and Comparative Examples 1 and 2 that differ only in the loading state in the Rh-supporting material and under high temperature. In the Rh-supported material, the catalyst of Example 1 in which Rh is supported on the secondary particle surface of the support material was superior in durability compared to the catalysts of Comparative Examples 1 and 2. Further, the NOx purification rate at a high temperature at which the influence of gas diffusibility becomes strong was particularly excellent. This is considered to be due to the increased contact area with the gas due to the presence of Rh on the surface of the secondary particles.

  FIG. 21 is a diagram schematically showing the coat structure of the catalyst of Example 1 and Comparative Examples 3 and 4, and a graph showing the results of the purification performance evaluation after the durability test of these catalysts and under high temperature. The catalyst of Comparative Example 4 having a conventional coat structure without using a pore former was inferior to the catalyst of Example 1 in durability and NOx purification rate at high temperature. Further, in the catalyst of Comparative Example 3 in which Rh is supported on the surface of the coating layer for the purpose of improving the contact property between the gas and Rh, Rh is supported at a high density and sintering is promoted, so that the durability is particularly inferior. It was.

In FIG. 22, the graph which shows the result of the purification performance evaluation after the endurance test of the catalyst of Example 2 and Comparative Examples 5-9 under high temperature is shown. As in Example 2 and Comparative Examples 5 and 6 of the catalysts using ACZ material in place of the composite Al ₂ O ₃ material support material, as shown in FIG. 21, similar to the composite Al ₂ O ₃ material Results were obtained. On the other hand, in the catalysts of Comparative Examples 7 to 9 using the CZ material, even the catalyst of Comparative Example 7 prepared by sputtering the Rh-supporting material was inferior in purification performance after the durability test and under high temperature. From this, it was speculated that the carrier supporting Rh needs alumina (Al ₂ O ₃ ), which is a material having high heat resistance.

  FIG. 23 is a graph showing the relationship between the secondary particle diameter of the carrier and the NOx purification performance after the durability test. When the secondary particle size of the carrier is small, and thus the surface area is large, it can be said that it is advantageous in terms of carrying Rh at a low density. However, since the heat resistance tended to decrease on the other hand, it was considered that the secondary particle diameter of the carrier is preferably 6 μm or more. In the case where the carrier particle diameter D50 exceeds 25 μm, the catalyst coat could not be formed.

Claims (1)

An exhaust gas purifying catalyst having two or more catalyst coat layers on a substrate,
Each catalyst coat layer contains catalyst particles having a composition different from that of the adjacent catalyst coat layer;
The catalyst particles contained in the uppermost catalyst coat have a structure in which noble metal particles are supported on the surfaces of secondary particles of a support material containing Al ₂ O ₃ .
In the uppermost catalyst coat,
The average thickness of the coat layer is in the range of 25-160 μm;
The porosity measured by the underwater gravimetric method is in the range of 50 to 80% by volume, and 0.5 to 50% by volume of the entire voids are composed of high aspect ratio pores having an aspect ratio of 5 or more,
The high aspect ratio pores have a circle equivalent diameter in the cross-sectional image of the cross section of the catalyst coat layer perpendicular to the flow direction of the exhaust gas in the range of 2 to 50 μm and the average aspect ratio in the range of 10 to 50. The exhaust gas-purifying catalyst.

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