JP2017185459A - Exhaust gas purification catalyst, method for producing the same and exhaust gas purification method using the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an exhaust gas purification catalyst excellent in catalytic performance such as NOx purification performance in a low temperature range and the like.SOLUTION: The exhaust gas purification catalyst comprises a base material and a catalyst-coated layer containing catalyst particles formed on the surface of the base material. Fibrous pores having an average value of an equivalent circle diameter of 8 μm or more with regard to pores in a cross-sectional image of a catalyst-coated layer cross-section perpendicular to an exhaust gas flow direction of the base material and having a pore length of 15 μm or more with regard to a length parallel to an exhaust gas flow direction of the base material are formed in the catalyst-coated layer. The fibrous pores are flat fibrous pores which have an average value of minor axis of pore in the cross-sectional image within the range of 8 to 15 μm, an average value of a prolate axial ratio (major axis/minor axis) of pore in the cross-sectional image of 2 to 5 and has an average value of an aspect ratio (length/minor axis) based on the minor axis of pore in the cross-sectional image within the range of 2 to 10. The proportion of the flat fibrous pores in the apparent volume of the catalyst-coated layer is 0.6 to 6 vol.%.SELECTED DRAWING: Figure 16

Description

  The present invention relates to an exhaust gas purification catalyst, a method for producing the same, and an exhaust gas purification method using the same.

  Conventionally, as a catalyst for exhaust gas purification mounted on automobiles, etc., harmful components such as harmful gases (hydrocarbon (HC), carbon monoxide (CO), nitrogen oxide (NOx)) contained in exhaust gas are purified. For this purpose, a three-way catalyst, an oxidation catalyst, a NOx occlusion reduction type catalyst, and the like have been developed. Due to the recent increase in environmental awareness, regulations on exhaust gases emitted from automobiles and the like have been further strengthened, and improvements in these catalysts have been promoted accordingly.

  As such an exhaust gas purification catalyst, Japanese Patent Application Laid-Open No. 2012-240027 (Patent Document 1) is an exhaust gas purification catalyst provided with a catalyst layer, and a plurality of voids exist in the catalyst layer, The mode of frequency distribution regarding the aspect ratio (D / L) of the cross section of the void is 2 or more, the void ratio in the catalyst layer is 15 vol% or more and 30 vol% or less, and the thickness in the thickest portion of the catalyst layer An exhaust gas purification catalyst having a length of 150 μm or less is disclosed, and as a method for obtaining the exhaust gas purification catalyst, a carbon compound material having a mode of frequency distribution relating to the aspect ratio (D / L) of the cross section of 2 or more is used. The method used is disclosed. However, the exhaust gas purifying catalyst disclosed in Patent Document 1 is not necessarily sufficient with respect to catalyst performance such as NOx purification performance in a low temperature region.

  In recent years, required characteristics for exhaust gas purification catalysts have been increasing, and an exhaust gas purification catalyst having excellent catalyst performance such as NOx purification performance in a low temperature region has been demanded.

JP 2012-240027 A

  The present invention has been made in view of the above-described problems of the prior art, and has an exhaust gas purification catalyst excellent in catalytic performance such as NOx purification performance in a low temperature range, a method for producing the same, and an exhaust gas purification method using the same. The purpose is to provide.

  As a result of intensive research to achieve the above object, the present inventors have mixed a metal oxide particle, a noble metal raw material, and a fibrous organic substance having a flat cross section having a specific dimension to obtain a catalyst slurry. Fabricate and apply this to the surface of the substrate to form a catalyst slurry layer, and then calcinate to remove at least a portion of the flat fibrous organic matter, so that the cross section having a specific dimension is a flat fibrous shape An exhaust gas purifying catalyst having pores formed in the catalyst coat layer can be obtained. In such an exhaust gas purifying catalyst, NOx can be efficiently occluded and reduced even in a low temperature range. It has been found that the catalyst performance such as NOx purification performance in the region is improved, and the present invention has been completed.

That is, the exhaust gas-purifying catalyst of the present invention includes a base material and a catalyst coat layer containing catalyst particles formed on the surface of the base material.
The average value of the equivalent circle diameters of the pores 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 is 8 μm or more and the length of the pores parallel to the exhaust gas flow direction of the base material is Fibrous pores having a size of 15 μm or more are formed in the catalyst coat layer,
The fibrous pores have an average value of the minor axis of the pores in the cross-sectional image in the range of 8 to 15 μm, and the average value of the aspect ratio (major axis / minor axis) of the pores in the sectional image is 2. Are flat fibrous pores having an average aspect ratio (length / minor axis) in the range of 2 to 10 based on the minor axis of the pores in the cross-sectional image.
It is characterized by this.

  In the exhaust gas purifying catalyst of the present invention, the porosity measured by X-ray CT of the catalyst coat layer is in the range of 15 to 45% by volume, and / or the flatness relative to the apparent volume of the catalyst coat layer. The ratio of the fibrous pores is preferably in the range of 0.1 to 6% by volume.

  In the exhaust gas purifying catalyst of the present invention, at least one NOx occlusion component selected from the group consisting of Li, Na, K, Rb, Cs, Be, Mg, Ca, Sr and Ba is further added to the catalyst particles. It is preferably supported.

The method for producing the exhaust gas purifying catalyst of the present invention comprises:
The average value of the equivalent circle diameter of the cross section is 8 μm or more, the average value of the short axis of the cross section is in the range of 8 to 15 μm, and the cross sectional length (long diameter / short) The average value of the diameter) is 2 to 5, and a flat fibrous organic material having an average aspect ratio (length / minor axis) in the range of 10 to 100 based on the minor axis of the cross section is mixed. And obtaining a catalyst slurry,
Applying the catalyst slurry to a surface of a substrate to form a catalyst slurry layer;
A firing step of obtaining an exhaust gas purifying catalyst by removing at least a part of the flat fibrous organic matter in the catalyst slurry layer;
It is the method characterized by including.

  In the method for producing an exhaust gas purifying catalyst of the present invention, the amount of the flat fibrous organic matter in the catalyst slurry is preferably in the range of 0.1 to 9% by mass in terms of solid content.

  In the method for producing an exhaust gas purifying catalyst of the present invention, the raw material of at least one NOx storage component selected from the group consisting of Li, Na, K, Rb, Cs, Be, Mg, Ca, Sr and Ba is provided. It is preferable that the catalyst slurry is further added.

  The exhaust gas purifying method of the present invention is an exhaust gas purifying method characterized by purifying exhaust gas by bringing the exhaust gas discharged from the internal combustion engine into contact with the exhaust gas purifying catalyst of the present invention.

  The reason why the above object is achieved by the exhaust gas purifying catalyst of the present invention, the production method thereof, and the exhaust gas purification method using the same is not necessarily clear, but the present inventors speculate as follows.

  That is, in the present invention, the average value of the equivalent circle diameter of the cross section is 8 μm or more, the average value of the short diameter of the cross section is in the range of 8 to 15 μm, and the cross-sectional length (major axis / minor axis) is Use is made of a fibrous organic material having an average value of 2 to 5 and a flat cross-section in which the average value of the aspect ratio (length / minor axis) in the range of 10 to 100 based on the minor axis of the cross section is used. Accordingly, the average value of the minor axis of the cross section is within the range of 8 to 15 μm, and the average value of the oblateness ratio (major axis / minor axis) of the cross section is 2 to 5, derived from the flat fibrous organic matter. In addition, fibrous pores having a flat cross section with an average aspect ratio (length / minor axis) in the range of 2 to 10 based on the minor axis of the cross section are formed in the catalyst coat layer. become. Therefore, the formation of such flat fibrous pores facilitates the mass transfer of gas derived from fine pressure changes accompanying NOx storage and reduction. Even if it exists, the present inventors speculate that NOx is efficiently occluded and reduced, and that the catalyst performance such as NOx purification performance in a low temperature region is improved.

  ADVANTAGE OF THE INVENTION According to this invention, it becomes possible to provide providing the exhaust gas purification catalyst excellent in catalyst performances, such as NOx purification performance in a low temperature range, its manufacturing method, and the exhaust gas purification method using the same.

BRIEF DESCRIPTION OF THE DRAWINGS It is the schematic which shows an example of the preparation method of the test piece for X-ray CT measurement, (A) is the schematic which shows the state which cut out one cell from the honeycomb-shaped exhaust gas purification catalyst, (B) is a horizontal cell. Fig. 6 is a schematic diagram showing a position cut in the axial direction by a dotted line, and Fig. 8C is a schematic diagram showing a position where the member cut at the position of the dotted line shown in (B) is further cut in the axial direction. It is the schematic which shows the test piece for X-ray CT measurement obtained by the method shown in FIG. It is sectional drawing (sectional image) which shows one sheet among the radial direction continuous cross-section images obtained in Example 1. FIG. FIG. 4 is a cross-sectional view (cross-sectional image) showing a state in which pores having an equivalent circle diameter of 8 μm or more are extracted from the radial-direction continuous cross-sectional image shown in FIG. 3. (A) is a schematic diagram showing how to determine the major axis of the pores, and (B) is a schematic diagram showing how to determine the minor axis of the pores (Mitani Corporation, “two-dimensional image analysis software WinROOF”). Quoted from the manual). It is the schematic which shows how to obtain | require the Ferre angle of a pore (it quotes from the manual of Mitani Corporation, "two-dimensional image analysis software WinROOF"). It is the analysis result about one test piece obtained in Example 2, Comprising: It is a graph which made the distance from the continuous slice image of the 1st sheet the horizontal axis, and made the distance from the left end of each cross-sectional image the vertical axis. . It is the analysis result about one test piece obtained in Example 2, Comprising: It is the graph which made the distance from the continuous cross-section image of the 1st sheet the horizontal axis, and made the equivalent circle diameter of the pore the vertical axis. It is a scanning electron micrograph of the cross section of the organic fiber used as the raw material of the flat fibrous organic substance used in the Example. It is a scanning electron micrograph of the cross section of the flat fibrous organic substance used in the Example. It is a microscope picture of the side surface of the flat fibrous organic substance used in the Example. It is a graph which shows distribution of the major axis of the cross section of the flat fibrous organic substance used in the Example. It is a graph which shows distribution of the fiber length of the flat fibrous organic substance used in the Example. 2 is a graph showing Log differential pore volume distribution of a catalyst coat layer in the catalysts obtained in Examples 1 to 3 and Comparative Example 1. FIG. It is a graph which shows the relationship between the addition amount of the added flat fibrous organic substance (average value of equivalent circle diameter of a cross section: 12 μm (φ12)) and the NOx purification rate in the catalysts obtained in Examples 1 to 3 and Comparative Example 1. Yes, It is a graph which shows the relationship between the average value (cross-sectional area equivalent diameter) of the cross-sectional equivalent diameter of the cross-section of the flat fibrous organic substance added in the catalyst obtained by Example 1 and Comparative Examples 1-3, and a NOx purification rate.

  Hereinafter, the present invention will be described in detail with reference to preferred embodiments thereof.

[Exhaust gas purification catalyst]
The exhaust gas purifying catalyst of the present invention comprises a base material, and a catalyst coat layer containing catalyst particles formed on the surface of the base material,
The average value of the equivalent circle diameters of the pores 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 is 8 μm or more and the length of the pores parallel to the exhaust gas flow direction of the base material is Fibrous pores having a size of 15 μm or more are formed in the catalyst coat layer,
The fibrous pores have an average value of the minor axis of the pores in the cross-sectional image in the range of 8 to 15 μm, and the average value of the aspect ratio (major axis / minor axis) of the pores in the sectional image is 2. Are flat fibrous pores having an average aspect ratio (length / minor axis) in the range of 2 to 10 based on the minor axis of the pores in the cross-sectional image.
It is characterized by this. By using such an exhaust gas purification catalyst, NOx is efficiently occluded and reduced even in a low temperature range, and it is possible to improve catalyst performance such as NOx purification performance in a low temperature range.

(Base material)
The base material in the exhaust gas purification catalyst of the present invention is not particularly limited, and a known base material that can be used as the base material of the exhaust gas purification catalyst can be used, and a honeycomb-shaped base material is preferable. Such a honeycomb-shaped substrate is not particularly limited, and a known honeycomb-shaped substrate that can be used as a substrate for an exhaust gas purifying catalyst can be used. Specifically, a honeycomb-shaped monolith substrate can be used. A material (honeycomb filter, high-density honeycomb, etc.) is preferably employed. In addition, the material of such a base material is not particularly limited, and a base material made of ceramics such as cordierite, silicon carbide, silica, alumina, mullite, or a base material made of metal such as stainless steel including chromium and aluminum is preferable. Adopted. 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, contains catalyst particles, preferably consists of catalyst particles, and is made of porous catalyst particles. Are particularly preferred.

The catalyst particles of such a catalyst coat layer are not particularly limited as long as they have a purification performance against exhaust gas, and specifically, aluminum oxide (Al ₂ O ₃ , alumina), cerium oxide (CeO ₂ , ceria). ), Zirconium oxide (ZrO ₂ , zirconia), titanium oxide (TiO ₂ , titania), silicon oxide (SiO ₂ , silica), yttrium oxide (Y ₂ O ₃ , yttria), neodymium oxide (Nd ₂ O ₃ ), oxide A catalyst base particle (oxide particles (preferably porous oxide particles)) made of an oxide such as zinc (ZnO) or magnesium oxide (MgO) or a composite oxide thereof or the like is used. Can be used.

  The noble metal is not particularly limited, but is selected from the group consisting of platinum (Pt), palladium (Pd), rhodium (Rh), gold (Au), silver (Ag), iridium (Ir), and ruthenium (Ru). It is preferable to use at least one kind. Among these, from the viewpoint of catalyst performance, at least one selected from the group consisting of Pt, Rh, Pd, Ir and Ru is more preferable, and at least one selected from the group consisting of Pt, Rh and Pd is particularly preferable.

  The amount of the noble metal supported is not particularly limited, and it may be appropriately supported according to the intended design. The amount of such noble metal supported is preferably 0.01 to 10 parts by weight, and 0.01 to 5 parts by weight, based on 100 parts by weight of the catalyst base particles (oxide particles). More preferably, it is a part. If the amount of the noble metal supported is less than the lower limit, the catalytic activity tends to be insufficient. On the other hand, even if the noble metal is supported exceeding the upper limit, the catalytic activity is saturated and the cost tends to increase. .

  In the exhaust gas purifying catalyst of the present invention, at least one NOx occlusion component selected from the group consisting of Li, Na, K, Rb, Cs, Be, Mg, Ca, Sr and Ba is further added to the catalyst particles. It is preferably supported. Such alkali metal elements as lithium (Li), sodium (Na), potassium (K), rubidium (Rb), and cesium (Cs), beryllium (Be), magnesium (Mg), calcium (Ca), strontium ( Alkaline earth metal elements such as Sr) and barium (Ba) have NOx occlusion ability, and the former alkali metal tends to have higher NOx occlusion ability in a relatively high temperature range, while the latter alkaline earth metal element. The similar metals tend to have a high NOx storage capacity in a relatively low temperature range. Therefore, you may use both together.

  Such NOx storage component is preferably at least one selected from the group consisting of Ba, Sr, K, Na and Li from the viewpoint of thermal stability. Moreover, it is more preferable to use Ba and / or K from the viewpoint that the NOx occlusion amount is larger and the NOx purification performance tends to be exhibited.

  The amount of the NOx occlusion component supported is not particularly limited, and a necessary amount may be appropriately supported according to the target design or the like. The supported amount of such NOx occlusion component is preferably 0.01 to 0.5 mol / L per 1 liter (unit volume) of the base material in terms of metal, 0.03 to 0 More preferably, it is 3 mol / L. If the amount of the NOx occlusion component is less than the lower limit, sufficient NOx occlusion performance tends to be difficult to obtain. On the other hand, if the upper limit is exceeded, the NOx occlusion component causes pore clogging or noble metal coating. Therefore, the activity of the precious metal and the NOx purification performance tend to decrease.

  In the catalyst coat layer in the exhaust gas purifying catalyst of the present invention, “consisting of catalyst particles” means that the catalyst coat layer is composed only of catalyst particles or mainly composed of catalyst particles, and the effects of the present invention are achieved. It means that it is configured to include other components within a range that does not deteriorate. As other components, other metal oxides and additives used as a catalyst coat layer for this type of application can be used. Specific examples of such other components include one or more of rare earth elements such as lanthanum (La), yttrium (Y), and cerium (Ce), and transition metals such as iron (Fe).

  The catalyst coating layer in the exhaust gas purifying catalyst of the present invention preferably has a coating amount in the range of 50 to 300 g / L per liter (unit volume) of the base material. When the coating amount is less than the lower limit, the catalytic activity of the catalyst particles tends to be insufficiently obtained. On the other hand, when the upper limit is exceeded, pressure loss increases and fuel consumption tends to deteriorate. The coating amount of the catalyst coat layer is more preferably in the range of 50 to 250 g / L per 1 liter (unit volume) of the base material from the viewpoint of a balance between pressure loss, catalyst performance, and durability. A range of 50 to 200 g / L is particularly preferable.

  The catalyst coat layer in the exhaust gas purifying catalyst of the present invention preferably has an average thickness in the range of 25 to 160 μm. If the average thickness is less than the lower limit, sufficient catalyst performance tends to be difficult to obtain. On the other hand, if the average thickness exceeds the upper limit, pressure loss when exhaust gas or the like passes tends to increase. The average thickness of the catalyst coat layer is more preferably in the range of 30 to 96 μm, more preferably in the range of 32 to 92 μm, from the viewpoint of a balance between pressure loss, catalyst performance, and durability. Is particularly preferred. The “thickness” refers to a length (height) in a direction perpendicular to the center of the flat portion of the base material of the catalyst coat layer. Such a thickness is obtained by observing the catalyst coat layer by scanning electron micrograph (SEM) observation, optical microscope observation, etc., and measuring the thickness of any 10 or more portions, and the “average thickness” Can be measured by calculating the average thickness.

  Furthermore, the particle diameter of the catalyst particles in the catalyst coat layer in the exhaust gas purifying catalyst of the present invention is the cumulative value based on the cross-sectional area of the catalyst particles by scanning electron microscope (SEM) observation of the cross section of the catalyst coat layer. The cumulative 15% diameter value in the particle size distribution is preferably in the range of 3 to 10 μm. When the particle size of the catalyst particles (the value of the cumulative 15% diameter based on the cross-sectional area) is less than the lower limit, the porosity of the catalyst coat layer is reduced and the gas diffusibility is deteriorated, so that sufficient catalyst performance is difficult to obtain. On the other hand, if the upper limit is exceeded, the gas diffusion resistance inside the catalyst particles increases, so that sufficient catalyst performance tends to be difficult to obtain. The particle diameter of the catalyst particles of the catalyst coat layer is a cumulative 15% diameter value based on the cross-sectional area from the viewpoint of the balance of diffusion resistance in the catalyst coat layer and the catalyst particles and the coatability of the slurry. More preferably, it is in the range of 3-9 μm, particularly preferably in the range of 3-7 μm.

The particle size of such catalyst particles (the value of the cumulative 15% diameter based on the cross-sectional area) can be measured by observation with a scanning electron microscope (SEM). Specifically, for example, an exhaust gas purification catalyst is embedded with an epoxy resin or the like, and a cross section cut in the radial direction of a base material (honeycomb-shaped base material) is observed with a scanning microscope (SEM) (magnification: 700 to 1500). Double, pixel resolution: 0.2 mm / pixel (pixel) or more), and the value of the cumulative 15% diameter in the cumulative particle size distribution based on the cross-sectional area of the catalyst particles is calculated. The cumulative 15% diameter of the catalyst particles means that the sum of the cross-sectional areas of the catalyst particles is less than 0.3 μm ² when the cross-sectional area of the catalyst particles is counted from those having a large catalyst particle size (cross-sectional area). This means the particle size of the catalyst particles (hereinafter sometimes referred to as “D15”) when it corresponds to 15% of the entire cross-sectional area of the catalyst coat layer excluding the pores (the area standard integration frequency is 15%). Such observation can be obtained by obtaining a rectangular region of 200 μm or more in the horizontal direction with respect to the substrate flat portion of the catalyst coat layer and 25 μm or more in the direction perpendicular to the substrate flat portion. The particle diameter means the diameter of the minimum circumscribed circle when the cross section is not circular.

  The porosity of the catalyst coat layer in the exhaust gas purifying catalyst of the present invention is preferably in the range of 15 to 45% by volume as measured by X-ray CT. If the porosity is less than the lower limit, gas diffusibility is deteriorated, so that sufficient catalyst performance tends to be difficult to obtain.On the other hand, if the upper limit is exceeded, the diffusivity is too large, and the catalyst active point The ratio of the gas that passes through the coating layer without contact increases, and it tends to be difficult to obtain sufficient catalyst performance. The porosity of the catalyst coat layer is preferably in the range of 15 to 40% by volume, preferably in the range of 20 to 40% by volume, from the viewpoint of the balance between gas diffusibility and catalyst performance. Is particularly preferred.

  Further, as the catalyst coat layer in the exhaust gas purifying catalyst of the present invention, there are at least two peaks in the pore size distribution measured by the mercury intrusion method, and the mode value of these peaks The mode pore diameter of the peak having the largest pore diameter (first peak) is preferably in the range of 0.4 to 10 μm. If the pore diameter of the mode value of the first peak is less than the lower limit, gas diffusibility is insufficient, so that sufficient catalytic performance tends to be difficult to be obtained. The ratio of the gas that passes through the coat layer without coming into contact with the point increases, and it tends to be difficult to obtain sufficient catalyst performance.

  In the catalyst coat layer in the exhaust gas purifying catalyst of the present invention, the average value of the equivalent circle diameters of the pores in the cross-sectional image of the cross section of the catalyst coat layer perpendicular to the flow direction of the exhaust gas of the substrate is 8 μm or more. It is necessary to form fibrous pores having a length of pores of 15 μm or more parallel to the flow direction of the exhaust gas. In the exhaust gas purifying catalyst of the present invention, the fibrous pores need to be fibrous pores having a flat cross section.

  Thus, in the exhaust gas purifying catalyst of the present invention, first, fibrous pores having an average value of the equivalent circle diameter of 8 μm or more and the length of 15 μm or more are formed in the catalyst coat layer. There is a need. If such a relatively large fibrous pore is not formed in the catalyst coat layer, the mass transfer of the gas resulting from the fine pressure change accompanying NOx occlusion and reduction will not proceed at a sufficient rate, Improvement in catalyst performance such as NOx purification performance in can not be obtained.

Furthermore, in the exhaust gas purifying catalyst of the present invention, the fibrous pores formed in the catalyst coat layer have the following conditions (i) to (iii):
(I) Regarding the short diameter of the pores in the cross-sectional image, the average value of the short diameter of the fibrous pores is in the range of 8 to 15 μm.
(Ii) Regarding the aspect ratio (long diameter / short diameter) of the pores in the cross-sectional image, the average value of the aspect ratio of the fibrous pores is 2 to 5,
(Iii) Regarding the aspect ratio (length / minor axis) based on the minor axis of the pores in the cross-sectional image, the average aspect ratio of the fibrous pores is in the range of 2 to 10,
It is necessary that the cross section satisfying all of the above is a flat pore (substantially oval) fibrous pore. If the fibrous pores formed in the catalyst coat layer are not such flat fibrous pores, the mass transfer of gas derived from fine pressure changes accompanying NOx occlusion and reduction is sufficiently accelerated. Thus, the catalyst performance such as NOx purification performance in the low temperature region cannot be sufficiently improved.

  With respect to the condition (i), when the average value of the minor diameter of the flat fibrous pores according to the present invention is less than 8 μm, mass transfer of gas resulting from fine pressure change accompanying NOx occlusion and reduction is inhibited, No improvement in catalyst performance such as NOx purification performance in a low temperature range can be obtained. On the other hand, if it exceeds 15 μm, the ratio of gas passing through the coating layer without contacting the catalyst active point increases, and sufficient catalyst performance is obtained. Absent. Moreover, from the same viewpoint regarding the condition (i), the average value of the minor axis of the flat fibrous pores according to the present invention is particularly preferably in the range of 10 to 15 μm.

  In addition, regarding the condition (ii), when the average value of the flattened pores of the flat fibrous pores according to the present invention is less than 2, the mass transfer of gas resulting from fine pressure change accompanying NOx occlusion and reduction is reduced. If not sufficiently promoted, catalyst performance such as NOx purification performance in a low temperature region cannot be sufficiently improved. On the other hand, if it exceeds 5, the proportion of gas passing through the coating layer without contacting the catalyst active point increases. Therefore, sufficient catalyst performance cannot be obtained. Moreover, from the same viewpoint regarding the condition (ii), the average value of the oblateness ratio of the flat fibrous pores according to the present invention is particularly preferably in the range of 2.1 to 4.5.

  Further, regarding the condition (iii), if the average value of the aspect ratio of the flat fibrous pores according to the present invention is less than 2, the mass transfer of gas derived from the fine pressure change accompanying NOx occlusion and reduction is sufficient. The catalyst performance such as NOx purification performance in the low temperature region is not sufficiently improved, and on the other hand, when it exceeds 10, the ratio of the gas passing through the coat layer without contacting the catalyst active point increases, Sufficient catalyst performance cannot be obtained. Moreover, from the same viewpoint regarding the condition (iii), the average value of the aspect ratio of the flat fibrous pores according to the present invention is particularly preferably in the range of 2-8.

  In the exhaust gas purifying catalyst of the present invention, the ratio of the flat fibrous pores to the apparent volume of the catalyst coat layer is preferably in the range of 0.1 to 6% by volume, preferably 0.1 to 4% by volume. % Is more preferable. When the proportion of the flat fibrous pores is less than the lower limit, the mass transfer of gas derived from fine pressure changes accompanying NOx storage and reduction is not sufficiently promoted, and catalyst performance such as NOx purification performance in a low temperature range. On the other hand, if the upper limit is exceeded, the proportion of gas that passes through the coat layer without contacting the catalytic active point increases, and sufficient catalytic performance tends to be difficult to obtain. It is in.

  In the exhaust gas purifying catalyst of the present invention, the ratio of the flat fibrous pores to the entire voids of the catalyst coat layer is preferably in the range of 2 to 8% by volume, and 2 to 7% by volume. It is more preferable to be within the range. When the proportion of the flat fibrous pores is less than the lower limit, the mass transfer of gas derived from fine pressure changes accompanying NOx storage and reduction is not sufficiently promoted, and catalyst performance such as NOx purification performance in a low temperature range. On the other hand, if the upper limit is exceeded, the proportion of gas that passes through the coat layer without contacting the catalytic active point increases, and sufficient catalytic performance tends to be difficult to obtain. It is in.

Furthermore, in the exhaust gas purifying catalyst of the present invention, the pore density of the fibrous pores in which the average value of the equivalent circle diameter in the catalyst coat layer is 8 μm or more and the length is 15 μm or more is 0.1-6. preferably in the range of [× ^{10 -3} cells / 100 [mu] m ^3], and more preferably in the range of 0.4 to 5 [× ^{10 -3} cells / 100 [mu] m ^3]. When the pore density of the fibrous pores is less than the lower limit, mass transfer of gas derived from fine pressure changes accompanying NOx storage and reduction is not sufficiently promoted, and catalyst performance such as NOx purification performance in a low temperature range. On the other hand, if the upper limit is exceeded, the proportion of gas that passes through the coat layer without contacting the catalytic active point increases, and sufficient catalytic performance tends to be difficult to obtain. It is in.

  In the exhaust gas purifying catalyst of the present invention, the flat fibrous pores formed in the catalyst coat layer are pores in a cross-sectional image of a cross section of the catalyst coat layer perpendicular to the exhaust gas flow direction of the substrate. It is preferable that the average value of the angle (Ferret angle) formed by the longitudinal vector and the horizontal vector parallel to the surface of the flat portion of the substrate is oriented within a range of 0 to 40 degrees. By doing in this way, the mass transfer of the gas resulting from the fine pressure change accompanying NOx occlusion and reduction is further promoted, and the catalyst performance such as NOx purification performance in a low temperature region tends to be further improved. In addition, when the average value of the ferret angle exceeds the upper limit, the mass transfer of gas in the horizontal direction of the catalyst coat layer is insufficient, and it is difficult to sufficiently improve the catalyst performance. The average value of the ferret angle is more preferably in the range of 10 to 30 degrees from the viewpoint of catalyst performance.

  Furthermore, in the exhaust gas purifying catalyst of the present invention, the flat fibrous pores formed in the catalyst coat layer have an angle (conical) formed by a length direction vector thereof and an exhaust gas flow direction vector of the base material. The angle is preferably in the range of 0 to 40 degrees with the value of the accumulated 80% angle in the angle-based cumulative angle 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 tends to be improved. In addition, when the value of the cumulative 80% angle of the cone angle exceeds the upper limit, the axial component of the gas diffusibility becomes insufficient, and the utilization efficiency of the active point tends to decrease. The cumulative 80% cone angle value is more preferably in the range of 0 to 30 degrees from the viewpoint of catalyst performance.

(Analysis method of flat fibrous pores in the catalyst coating layer)
“Circular equivalent diameter of fibrous pores”, “Length of fibrous pores”, “Short diameter of flat fibrous pores”, “Vertical ratio of flat fibrous pores” in such a catalyst coating layer , “Aspect ratio of flat fibrous pores”, “porosity of flat fibrous pores”, “Ferret angle of flat fibrous pores” and “cumulative 80% angle of cone angle” are X-ray CT, FIB -From the two-dimensional information and three-dimensional information of the pores of the catalyst coat layer obtained by SEM or the like, the catalyst perpendicular to the exhaust gas flow direction of the substrate (the axial direction of the honeycomb substrate) is as follows. It can be measured by analyzing a cross-sectional image (radial cross-sectional image) of the coat layer cross-section.

  Specifically, for example, when measuring an exhaust gas purification catalyst using a honeycomb substrate by X-ray CT analysis, first, one cell is cut out from the honeycomb catalyst as shown in FIG. Next, for example, the resin is filled with an epoxy resin, and then horizontally cut at the position of the dotted line shown in FIG. 1B to form the member shown in FIG. 1C. Next, FIG. A test piece having the shape shown in FIG. 2 is obtained by longitudinally cutting in the axial direction at the position of the dotted line shown in FIG. The size of the test piece is such that the width (width) of the cross section in the honeycomb radial direction of the catalyst coat layer to be measured is 100 μm or more, the length (height) is 30 μm or more, and the length (depth) in the honeycomb axial direction is 400 μm or more. The number of test pieces is 5 or more.

  The obtained test piece was measured using an X-ray CT apparatus (RIKEN, SPring-8 (Super Photoring-8), beam line: Toyota BL, voxel resolution: 0.325 μm / vox). From the obtained X-ray CT image, a continuous cross-sectional image (radial continuous cross-sectional image) parallel to the Hamicam radial direction is obtained as shown by the dotted line in FIG. The slice pitch is set to 0.325 μm, which is equivalent to the voxel resolution.

  Next, from the radial direction cross-sectional image obtained by the X-ray CT analysis, first, brightness is obtained using the “Kringing function” of commercially available image analysis software (“analysis software EXFact Analysis Particle / Pore” manufactured by Nippon Visual Science Co., Ltd.). Perform binarization as a reference. Since the pores are shown in black in the continuous cross-sectional image obtained from the X-ray CT image, the pores can be easily extracted by performing binarization based on the luminance.

  Next, based on the radial continuous cross-sectional image after the binarization process described above, image analysis is performed using a commercially available image analysis software (Math to Market, “analysis software GeoDict”), and the tertiary of the pores. An original image is constructed, and a radial continuous cross-sectional image is reconstructed from the obtained three-dimensional image of the pores (slice pitch: 0.325 μm). FIG. 3 shows an example of the radial continuous cross-section image reconstructed in this way (one of the radial continuous cross-sectional images obtained in Example 1 described later). In FIG. 3, white portions are pores.

  Next, based on the radial continuous cross-section image reconstructed in this way, pore structure analysis in each cross-section image using commercially available image analysis software (“Two-dimensional image analysis software WinROOF” manufactured by Mitani Corporation) I do. That is, first, after performing the opening process (repetition number 5) and the closing process (repetition number 5) attached to the analysis software, the area within the contour of each pore is obtained, and the particle size of the same area as each pore is obtained. The equivalent circle equivalent diameter is obtained, and pores having an equivalent circle diameter of 8 μm or more are extracted and set as analysis targets. As an example, FIG. 4 shows a state where pores having an equivalent circle diameter of 8 μm or more are extracted from the radial continuous cross-sectional image shown in FIG. In FIG. 4, the portion (arrow) shown in gray is a pore with an equivalent circle diameter of 8 μm or more extracted.

  Subsequently, for each of the pores with an equivalent circle diameter of 8 μm or more extracted as described above, “major axis”, “minor axis”, “flatness ratio”, and “Ferret angle” are obtained as follows. That is, regarding the “major axis”, the minimum circumcircle of the pore is assumed, and the absolute maximum length shown in FIG. 5A corresponding to the diameter of the minimum circumcircle is obtained as the major axis (a). As for the “minor axis”, two tangents of pores parallel to the vector of the major axis (a) shown in FIG. 5A are assumed, and the width corresponding to the major axis shown in FIG. Obtained as b). Then, from the major axis (a) and minor axis (b) obtained in this way, the “oblongation ratio” of the pore is obtained as the ratio of major axis (a) / minor axis (b). Further, regarding the “Ferret angle”, as shown in FIG. 6, the angle formed by the major axis (a) vector shown in FIG. 5A and the horizontal (H) vector parallel to the surface of the flat portion of the substrate. Is determined as the ferret angle (θ).

  By performing the pore structure analysis in such cross-sectional images for all radial continuous cross-sectional images, pores having a circle-equivalent diameter of 8 μm or more are extracted in all cross-sectional images, and all the extracted pores The major axis, minor axis, aspect ratio, and ferret angle are required.

  Subsequently, a three-dimensional structural analysis of the pores is performed based on all the radial continuous cross-sectional images analyzed as described above. That is, since the pores have a three-dimensional structure, cross-sectional images derived from the same pore are continuously observed in a plurality of adjacent cross-sectional images. As an example, as a result of analysis for one test piece obtained in Example 2 to be described later, the distance from the first continuous cross-sectional image is the horizontal axis, and the distance from the left end of each cross-sectional image is the vertical axis. A graph is shown in FIG. In addition, FIG. 8 shows a graph in which the horizontal axis represents the distance from the first continuous cross-sectional image and the vertical axis represents the equivalent circle diameter of the pores as the analysis result for the same test piece. As shown in FIGS. 7 and 8, by analyzing all the radial continuous cross-sectional images in the analysis region (test piece), the average value of the equivalent circle diameter in the cross-sectional image is 8 μm or more and the axial direction of the substrate ( Fibrous pores (16 fibrous pores indicated by 1 to 16 in FIG. 7) having a length of the exhaust gas flow direction of the substrate of 15 μm or more are extracted.

  In addition, in such an analysis, fibrous pores may be observed intersecting, but in that case, the place where the pores are recognized to intersect is excluded from the fibrous pores, The fibrous pores are extracted as one independent pore. The fibrous pores 6 to 9 shown in FIG. 7 are the four fibrous pores extracted as described above.

  Then, the circle equivalent diameter, major axis, minor axis, and aspect ratio of the pores in each sectional image obtained in advance for each of the fibrous pores extracted by the analysis of all the radial continuous sectional images in this way And the ferret angle and the length of each fibrous pore, the short diameter (average value) of each fibrous pore, the oblateness ratio (average value) of each fibrous pore, and the short diameter of each fibrous pore A reference aspect ratio (average value), a ferret angle (average value) of each fibrous pore, and a void (volume) of each fibrous pore are obtained, and for all the fibrous pores in the test piece. The average value of the minor axis, the average value of the aspect ratio, the average value of the aspect ratio based on the minor axis, and the average value of the ferret angle are obtained. Further, the ratio of the fibrous pores to the apparent volume of the catalyst coat layer (the porosity of the fibrous pores) based on the apparent volume of the catalyst coat layer and the voids (volume) of the fibrous pores obtained in advance. ), And the ratio of the fibrous pores to the entire voids of the catalyst coat layer is obtained from the porosity of the catalyst coat layer and the voids (volume) of the fibrous pores obtained in advance. It is done.

  Further, the orientation angle (conical angle) of the fibrous pores extracted by analysis of all radial continuous cross-sectional images can be obtained as follows. That is, an angle (cone angle) formed by the length direction vector of the fibrous pores and the exhaust gas flow direction (honeycomb axial direction) vector of the substrate is obtained, and the obtained cone angle cumulative reference is obtained. A cumulative 80% angle value in the angular distribution is determined. In addition, the orientation angle of the fibrous pores (cumulative 80% angle value) is calculated by randomly extracting 20 fibrous pores and accumulating them in the cumulative angular distribution based on the angle of the cone angle of the fibrous pores. It is obtained by averaging 80% angle values.

(Exhaust gas purification catalyst)
The exhaust gas purifying catalyst of the present invention is an exhaust gas purifying catalyst comprising the base material and the catalyst coat layer containing catalyst particles formed on the surface of the base material. The exhaust gas purifying catalyst of the present invention may be used 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]
Next, a method for producing the exhaust gas purifying catalyst of the present invention will be described. The method for producing the exhaust gas purifying catalyst of the present invention comprises:
The average value of the equivalent circle diameter of the cross section is 8 μm or more, the average value of the short axis of the cross section is in the range of 8 to 15 μm, and the cross sectional length (long diameter / short) The average value of the diameter) is 2 to 5, and a flat fibrous organic material having an average aspect ratio (length / minor axis) in the range of 10 to 100 based on the minor axis of the cross section is mixed. To obtain a catalyst slurry (catalyst slurry preparation step),
Applying the catalyst slurry to the surface of the substrate to form a catalyst slurry layer (catalyst slurry layer forming step);
A firing step (firing step) for obtaining an exhaust gas purifying catalyst by removing at least a part of the flat fibrous organic matter in the catalyst slurry layer;
It is the method characterized by including.

(Preparation of oxide particles)
In the method for producing an exhaust gas purifying catalyst of the present invention, first, metal oxide particles having a cumulative 50% diameter value in a volume-based cumulative particle size distribution measured by a laser diffraction method within a range of 3 to 10 μm are prepared. It is preferable (oxide particle preparation step).

  The metal oxide particles prepared in the oxide particle preparation step are the same as the catalyst base particles (oxide particles) in the catalyst particles described in the catalyst coat layer in the exhaust gas purifying catalyst of the present invention. Use. In addition, it does not restrict | limit especially as a preparation method of the metal oxide particle prepared in such an oxide particle preparation process, A well-known method is employable suitably. As such metal oxide particles, commercially available particles may be used. As the form of the metal oxide particles prepared in the oxide particle preparation step of the present invention, metal oxide particles (including composite oxide particles) prepared by a known method, commercially available metal oxide particles (composite oxide particles) Or a mixture thereof, or a dispersion obtained by dispersing these in a solvent such as ion-exchanged water.

  The particle size of the metal oxide particles used in the oxide particle preparation step according to the production method of the present invention is 3 to 3 in terms of a cumulative 50% diameter value in a volume-based cumulative particle size distribution measured by a laser diffraction method. It is preferable that it exists in the range of 10 micrometers. When the particle diameter of the metal oxide particles (volume-based cumulative 50% diameter value) is less than the lower limit, the particle diameter of the catalyst particles of the catalyst coat layer in the obtained exhaust gas purification catalyst (cumulative 15% diameter based on the cross-sectional area) Of the catalyst coating layer becomes too small, and the gas diffusibility becomes poor, so that sufficient catalytic performance tends to be difficult to obtain. On the other hand, if the upper limit is exceeded, the exhaust gas obtained Since the particle size of the catalyst particles in the catalyst coat layer in the purification catalyst (the value of the cumulative 15% diameter on the basis of the cross-sectional area) becomes too large and the gas diffusion resistance inside the catalyst particles increases, it is difficult to obtain sufficient catalyst performance. Tend to be. In addition, the particle diameter of such metal oxide particles is within a range of 3 to 9 μm in terms of a volume-based cumulative 50% diameter value from the viewpoint of the balance between coatability, diffusion resistance in the catalyst particles, and catalyst performance. More preferably, it is particularly preferably in the range of 3 to 7 μm.

  The particle diameter of such metal oxide particles (volume-based cumulative 50% diameter value) can be measured by a laser diffraction method. Specifically, for example, a randomly extracted (arbitrary) 1000 or more metal oxide 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 metal oxidation is performed. The cumulative 50% diameter value in the volume-based cumulative particle size distribution of the product particles is calculated. The cumulative 50% diameter of the metal oxide particles based on the volume is the total number of metal oxide particles when the number of metal oxide particles is counted from the one having a small metal oxide particle size (area). Is equivalent to 50% (volume-based cumulative frequency is 50%). The particle diameter means the diameter of the minimum circumscribed circle when the cross section is not circular.

  The method for preparing the metal oxide particles having such a particle size (volume-based cumulative 50% diameter value) is not particularly limited. For example, first, a raw material for metal oxide particles such as metal oxide particle powder is used. Next, after mixing the metal oxide particle powder and the like with a solvent such as ion-exchanged water, the resulting solution is subjected to metal oxidation using a media mill such as a bead mill or other stirring type pulverizer. A method of adjusting the metal oxide particles to a predetermined particle size by stirring and dispersing physical particle powder or the like in a solvent such as water may be mentioned. In addition, it does not restrict | limit especially as stirring conditions at the time of using media mills, such as a bead mill, As a bead diameter, it exists in the range of 100-5000 micrometers, As a processing time, it exists in the range of 3 minutes-1 hour, As stirring speed It is preferable to be within the range of 50 to 500 rpm.

(Catalyst slurry preparation process)
Next, in the method for producing an exhaust gas purifying catalyst of the present invention, the average value of the circle equivalent diameter of the cross section is 8 μm or more and the average value of the short diameter of the cross section is 8 μm or more. The average value of the aspect ratio (length / minor axis) based on the minor axis of the cross section is within a range of ˜15 μm, the average value of the cross-sectional oblateness ratio (major axis / minor axis) is 2-5. A catalyst slurry is obtained by mixing with a flat fibrous organic material having a value in the range of 10 to 100 (catalyst slurry preparation step).

  The noble metal raw material used in the catalyst slurry preparation step according to the production method of the present invention is not particularly limited. For example, a salt of a noble metal (for example, Pt, Rh, Pd, Ru, etc., or a compound thereof) (for example, Acetate, carbonate, nitrate, ammonium salt, citrate, dinitrodiammine salt, etc.) or a complex thereof (for example, tetraammine complex) dissolved in a solvent such as water or alcohol. Further, the amount of the noble metal is not particularly limited, and may be supported in a necessary amount as appropriate according to the intended design, etc., so that the amount of the noble metal supported in the obtained catalyst coat layer is within the above range. preferable. When platinum is used as the noble metal, the platinum salt is not particularly limited. For example, platinum (Pt) acetate, carbonate, nitrate, ammonium salt, citrate, dinitrodiammine salt, or a complex thereof may be used. Among them, dinitrodiammine salts are preferable from the viewpoint of easy loading and high dispersibility. In addition, when palladium is used as the noble metal, the palladium salt is not particularly limited. For example, palladium (Pd) acetate, carbonate, nitrate, ammonium salt, citrate, dinitrodiammine salt, or a complex thereof. Among them, nitrates and dinitrodiammine salts are preferable from the viewpoint of easy loading and high dispersibility. Furthermore, the solvent is not particularly limited, and examples thereof include a solvent that can be dissolved in an ionic form such as water (preferably pure water such as ion-exchanged water and distilled water).

  In addition, the fibrous organic substance used in such a catalyst slurry preparation step is not particularly limited as long as it is a substance that can be removed by a firing step described later. For example, polyethylene terephthalate (PET) fiber, acrylic fiber, nylon fiber, rayon A fiber and a cellulose fiber are 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 the subsequent process, it is possible to form voids having the same shape as the fibrous organic matter in the catalyst coat layer. It becomes possible.

  As such a fibrous organic material, the average value of the equivalent circle diameter of the cross section is 8 μm or more, the average value of the short axis of the cross section is in the range of 8 to 15 μm, and the cross-sectional length ratio (long diameter / short diameter) ) Having an average value of 2 to 5 and an average aspect ratio (length / minor axis) in the range of 10 to 100 with respect to the minor axis of the cross section being flat. It is necessary to use a (substantially elliptical) fibrous organic material. When the average value of the equivalent circle diameter of the cross section of the fibrous organic material used is less than 8 μm, fibrous pores having the average equivalent circle diameter of the present invention of 8 μm or more are not sufficiently formed in the obtained catalyst coat layer. For this reason, mass transfer of gas derived from minute pressure changes accompanying NOx storage and reduction is hindered, and improvement in catalyst performance such as NOx purification performance in a low temperature region cannot be obtained.

  In addition, a flat fibrous organic material that satisfies the condition that the average value of the minor axis of the cross section is in the range of 8 to 15 μm and the average value of the cross-sectional length (major axis / minor axis) is 2 to 5 is used. In addition, since the above-described flat fibrous pores according to the present invention are not sufficiently formed in the obtained catalyst coat layer, the mass transfer of gas derived from a fine pressure change accompanying NOx occlusion and reduction is not sufficiently promoted, Improvement in catalyst performance such as NOx purification performance in a low temperature region cannot be obtained sufficiently.

  Furthermore, if the average value of the aspect ratio (length / minor axis) based on the minor axis of the cross-section of the flat fibrous organic material to be used is less than 10, the pore connectivity in the resulting catalyst coat layer is insufficient, On the other hand, when the average value of the aspect ratio (length / minor axis) exceeds 100, the diffusibility in the resulting catalyst coat layer is too large without contacting the catalyst active point. The ratio of gas passing through the coating layer increases, and sufficient catalyst performance cannot be obtained. In addition, it is especially preferable that the average value of the aspect ratio (length / minor axis) of the flat fibrous organic material is in the range of 10 to 50 from the viewpoint of balance between gas diffusibility and catalyst performance.

  It should be noted that such flat fibrous organic matter has an “equivalent circular diameter of cross section”, “short diameter of cross section”, “longitudinal ratio of cross section (long diameter / short diameter)”, and “aspect ratio based on short diameter of cross section” (Length / minor axis) ”is the“ equivalent circular diameter of the cross section ”,“ minor axis of the cross section ”,“ longitudinal ratio of the cross section (major axis / minor axis) ”,“ cross section ”of the above-mentioned flat fibrous pores, respectively. Is defined in the same manner as the aspect ratio (length / minor axis) with reference to the minor axis of and can be obtained from an SEM image or a micrograph of the flat fibrous organic material to be used. In addition, “average value of the equivalent circle diameter of the cross section”, “average value of the short axis of the cross section”, “average value of the length of the cross section (major axis / minor axis)”, “cross section of such flat fibrous organic matter” The average value of the aspect ratio (length / minor axis) based on the minor axis of each "can be obtained by averaging measured values obtained by randomly extracting 50 or more fibrous organic substances. it can.

  Furthermore, the amount of the flat fibrous organic material used in such a catalyst slurry preparation step is preferably in the range of 0.1 to 9% by mass in terms of solid content in the catalyst slurry. If the blended amount of the flat fibrous organic material is less than the lower limit, sufficient pore connectivity in the resulting catalyst coat layer cannot be obtained, so that the catalyst performance tends to be insufficient, and on the other hand, when the upper limit is exceeded. The pressure loss tends to increase as the thickness of the resulting catalyst coat layer increases. In addition, it is especially preferable that the compounding quantity of a flat fibrous organic substance exists in the range of 0.5-5 mass% in conversion of solid content in the said catalyst slurry from a viewpoint of the balance of catalyst performance and pressure loss.

  Further, the amount of the flat fibrous organic material used in the catalyst slurry preparation step is preferably 80% by mass or more, and 90% by mass or more in the total solid organic material added to the catalyst slurry. It is more preferable that all the solid organic substances added to the catalyst slurry are the flat fibrous organic substances (100% by mass). When the blending ratio of the flat fibrous organic material in the total solid organic material is less than the lower limit, the flat fibrous pores according to the present invention are not sufficiently formed in the obtained catalyst coat layer, which accompanies occlusion and reduction of NOx. There is a tendency that mass transfer of gas derived from a minute pressure change is not sufficiently promoted and it is difficult to sufficiently improve catalyst performance such as NOx purification performance in a low temperature range.

  Furthermore, in the method for producing an exhaust gas purifying catalyst of the present invention, the raw material of at least one NOx storage component selected from the group consisting of Li, Na, K, Rb, Cs, Be, Mg, Ca, Sr and Ba is provided. It is preferable that the catalyst slurry is further added. Examples of the raw material for the NOx storage component include salts of the NOx storage component element (for example, acetate, carbonate, nitrate, ammonium salt, citrate, dinitrodiammine, etc.) or their complexes (for example, tetraammine). Complex). Further, the amount of the NOx storage component is not particularly limited, and it may be appropriately supported according to the intended design and the like, so that the amount of the NOx storage component in the obtained catalyst coat layer is within the above range. It is preferable to make it.

  Further, the method for preparing the catalyst slurry in such a catalyst slurry preparation step is not particularly limited, and the metal oxide particles, the noble metal raw material, the flat fibrous organic material, and, if necessary, the NOx storage component raw material, And a known method can be appropriately employed. The mixing conditions are not particularly limited. For example, the stirring speed is preferably in the range of 100 to 400 rpm, and the treatment time is preferably 30 minutes or more. It is only necessary that the organic substance can be uniformly dispersed and mixed in the catalyst slurry. The order of mixing is not particularly limited, and the method of mixing the fibrous organic matter after mixing the noble metal raw material with the dispersion containing the metal oxide particles and supporting the noble metal, the dispersion containing the metal oxide particles A method of mixing the noble metal raw material after mixing the fibrous organic material, a method of simultaneously mixing the noble metal raw material and the fibrous organic material into the dispersion containing the metal oxide particles, and the metal oxide particles and the fibrous organic material in the solution containing the noble metal raw material. Any of the method of mixing etc. may be sufficient. The treatment conditions are not particularly limited, and are appropriately selected according to the design of the target exhaust gas purification catalyst.

(Catalyst slurry layer forming step)
Next, in the method for producing an exhaust gas purifying catalyst of the present invention, the catalyst slurry is applied to the surface of a substrate to form a catalyst slurry layer (catalyst slurry layer forming step).

  The average thickness of the catalyst coat layer in such a catalyst slurry layer forming step is preferably in the range of 25 to 160 μm, and in the range of 30 to 96 μm, as the average thickness of the catalyst coat layer after firing. It is more preferable that the thickness is in the range of 32 to 92 μm.

  Further, the coating amount of the catalyst coat layer in such a catalyst slurry layer forming step is in the range of 50 to 300 g / L per 1 liter (unit volume) of the base material as the coating amount of the catalyst coat layer after firing. It is preferable that it is in the range of 50 to 250 g / L, more preferably in the range of 50 to 200 g / L.

  The substrate used in such a catalyst slurry layer forming step is not particularly limited, and the same substrates as those described as the substrate in the exhaust gas purifying catalyst of the present invention can be used.

  Moreover, it does not restrict | limit especially as a method of apply | coating the said catalyst slurry to the surface of a base material, A well-known method is employable suitably. Specifically, a method of immersing the base material in the catalyst slurry (immersion method), a wash coating method, a method of press-fitting the catalyst slurry by press-fitting means, and the like can be mentioned.

(Baking process)
Next, in the method for producing an exhaust gas purifying catalyst of the present invention, at least a part of the flat fibrous organic matter in the catalyst slurry layer is removed to obtain the exhaust gas purifying catalyst of the present invention (firing step).

In the firing step according to the method for producing the exhaust gas purifying catalyst of the present invention, the substrate on which the catalyst slurry layer is formed (catalyst slurry layer supporting substrate) is fired at a temperature in the range of 300 to 800 ° C. It is preferable to bake, and it is more preferable to bake at a temperature within the range of 400 to 700 ° C. If the firing temperature is less than the lower limit, the fibrous organic matter tends to remain, whereas if it exceeds the upper limit, the noble metal particles tend to be sintered. Further, the firing (heating) time varies depending on the firing temperature, and thus cannot be generally described, but is preferably 20 minutes or more, and more preferably 30 minutes to 2 hours. Furthermore, the atmosphere in such a firing step is not particularly limited, but is preferably in the air or in an inert gas such as nitrogen (N ₂ ).

[Exhaust gas purification method]
Next, the exhaust gas purification method of the present invention will be described. The exhaust gas purification method of the present invention is a method characterized by purifying exhaust gas by bringing exhaust gas discharged from an internal combustion engine into contact with the exhaust gas purification catalyst of the present invention.

  In such an exhaust gas purification method of the present invention, the method of 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, gas discharged from an internal combustion engine By arranging the exhaust gas purifying catalyst of the present invention in the exhaust gas pipe through which the gas flows, a method of bringing the exhaust gas from the internal combustion engine into contact with the exhaust gas purifying catalyst may be adopted.

  EXAMPLES Hereinafter, although this invention is demonstrated more concretely based on an Example and a comparative example, this invention is not limited to a following example.

<Preparation and analysis of fibrous organic matter>
Organic fibers (PET fibers) were processed to obtain flat fibrous organic materials used in the examples. When the cross section of the organic fiber used as a raw material was observed by SEM, the cross section was circular as shown in FIG.

  On the other hand, when the cross section of the obtained flat fibrous organic substance was observed by SEM, all the cross sections were flat (substantially oval) as shown in FIG. In photographing the SEM image, a sample obtained by filling a fibrous organic substance with a resin (resin is an epoxy resin) and cutting a cross section by ion milling was used as a sample. From the cross-sectional SEM image of the flat fibrous organic matter, “average value of equivalent circle diameter of cross section”, “average value of short axis of cross section”, and “average value of cross sectional length (major axis / minor axis)” were obtained. The results obtained are shown in Table 1.

  Further, when the side surface of the obtained flat fibrous organic matter was observed with a microscope (trade name: VHX-5000, manufactured by Keyence Corporation), as shown in FIG. 11, the cross-sectional major axis (24 μm in FIG. 11) and the fiber length (FIG. 11 was 81 μm). The distribution of the major axis of the cross section thus obtained is shown in FIG. 12, and the distribution of the fiber length is shown in FIG. When photographing with a microscope, the fibrous organic matter is dispersed in water and observed by applying light from below to the sandwich between two sapphire glass plates having a thickness of 0.5 mm, and sandwiched between the glass plates. By observing from above, the major axis of the cross section of the flat fibrous organic matter could be selectively observed. The “average aspect ratio (length / minor axis) based on the minor axis of the cross section” was determined from the SEM image and micrograph of the flat fibrous organic matter, and the obtained results are shown in Table 1.

Example 1
First, 100 g of Al ₂ O ₃ powder (manufactured by Sasol: specific surface area 100 m ² / g, average particle diameter 30 μm) and 100 g of Al ₂ O ₃ —ZrO ₂ —TiO ₂ composite oxide powder are added to 500 g of ion-exchanged water. Then, a bead mill (manufactured by ASONE, trade name “alumina ball”, used beads: microbeads having a diameter of 5000 μm alumina) was used for the dispersion obtained by mixing, and the processing time was 25 minutes and the stirring speed was 400 rpm. And a dispersion liquid containing metal oxide particles made of a mixture of Al ₂ O ₃ —ZrO ₂ —TiO ₂ composite oxide powder and Al ₂ O ₃ powder (composite metal oxide: AZT) was prepared. .

  Next, in the obtained dispersion, 0.05 L of a dinitrodiammine platinum solution containing 4 g of platinum (Pt) as a noble metal raw material in terms of metal, 76.5 g of barium acetate as a NOx storage component raw material, and flat fiber as a fibrous organic substance Organic matter (PET fiber, average value of equivalent circle diameter of cross section: 12 μm, average value of short axis of cross section: 9 μm, average value of cross sectional length (major axis / minor axis): 2.7, short axis of cross section 1% by mass in terms of solid content in the catalyst slurry was added, 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 wash-coated (applied) to a hexagonal cell cordierite monolith honeycomb substrate (Denso, 600 cells, 3 mil, substrate volume: 0.9 liter) as a substrate, After drying for 0.5 hours at a temperature of 100 ° C., the coating amount of the catalyst slurry is 200 g (200 g / 200 liters) per liter (unit volume) of the base material. By repeating until L), a catalyst slurry layer was formed on the substrate.

  Thereafter, an exhaust gas purifying catalyst (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 after firing for 2 hours at 500 ° C. in the atmosphere. Got.

(Example 2)
Exhaust gas purification catalyst (catalyst sample) was obtained in the same manner as in Example 1 except that the blending amount of the flat fibrous organic matter was 3% by mass in terms of solid content in the catalyst slurry.

(Example 3)
Exhaust gas purification catalyst (catalyst sample) was obtained in the same manner as in Example 1 except that the blending amount of the flat fibrous organic matter was 5% by mass in terms of solid content in the catalyst slurry.

(Comparative Example 1)
Exhaust gas purification catalyst (comparative catalyst sample) was obtained in the same manner as in Example 1 except that the fibrous organic material was not blended.

(Comparative Example 2)
Flat fibrous organic matter as the fibrous organic matter (PET fiber, average value of equivalent circle diameter of cross section: 0.7 μm, average value of short axis of cross section: 0.7 μm, average of cross section length (major axis / minor axis) Exhaust gas purification catalyst (comparative catalyst sample) was obtained in the same manner as in Example 2 except that the value: 1, the average value of aspect ratio based on the minor axis of the cross section: 57) was used.

(Comparative Example 3)
As a fibrous organic material, a flat fibrous organic material (PET fiber, average value of equivalent circle diameter of cross section: 4 μm, average value of short axis of cross section: 4 μm, average value of cross sectional length (major axis / minor axis): 1, Exhaust gas purification catalyst (comparative catalyst sample) was obtained in the same manner as in Example 2 except that the average aspect ratio based on the minor axis of the cross section was 45).

<Measurement test of average thickness of catalyst coat layer>
The catalyst samples obtained in Examples 1 to 3 and the comparative catalyst samples obtained in Comparative Examples 1 to 3 were embedded with an epoxy resin, cut in the radial direction of the substrate (honeycomb-shaped substrate), Then, 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 Table 2.

<Measurement test of porosity of catalyst coat layer>
The porosity of the catalyst coating layer of the catalyst samples obtained in Examples 1 to 3 and the comparative catalyst samples obtained in Comparative Examples 1 to 3 was measured by X-ray CT. The obtained results are shown in Table 2.

<Analysis of pore structure in catalyst coat layer>
The catalyst sample layers obtained in Examples 1 to 3 and the catalyst coat layers of the comparative catalyst samples obtained in Comparative Examples 1 to 3 were measured according to the above-described “analysis method of fibrous pores in the catalyst coat layer”. Analyzing the structure, from the two-dimensional information and three-dimensional information of the pores of the obtained catalyst coat layer, "average value of equivalent circle diameter of fibrous pores", "average value of the length of fibrous pores" ”,“ Average value of the major axis of the cross section of the fibrous pore ”,“ average value of the minor axis of the cross section of the fibrous pore ”,“ average value of the aspect ratio of the cross section of the fibrous pore ”,“ fibrous The average aspect ratio based on the minor axis of the cross section of the pores, the porosity of the fibrous pores, the average value of the ferret angles of the fibrous pores, and the cumulative 80% angle of the cone angle were obtained. . The obtained results are shown in Table 2.

  From the results shown in Table 1 and Table 2, in the catalyst samples obtained in Examples 1 to 3, the flat fibrous pores having the same shape corresponding to the shape of the flat fibrous organic material used as the raw material were catalyst coating layers. In the present invention, by using a fibrous organic material having a flat cross section (substantially elliptical shape), the fibrous pores having a flat cross section (substantially elliptical shape) corresponding to the shape. It was confirmed that

In addition, in the catalyst coat layers of the comparative catalyst samples obtained in Comparative Examples 1 to 3, the fibrous fine particles having an average equivalent circle diameter of the cross section of 8 μm or more and a length in the substrate axial direction of 15 μm or more. Although the presence of pores was not confirmed, in the catalyst coat layer of the comparative catalyst sample obtained in Comparative Example 3, the following pores were confirmed as pores recognized as being attributable to the added fibrous organic matter. It was.
Comparative Example 3 Average pore diameter: about 3 μm, average aspect ratio: about 12.

<Measurement test of pore size distribution of catalyst coat layer>
Regarding the catalyst coating layers of the catalyst samples obtained in Examples 1 to 3 and the comparative catalyst sample obtained in Comparative Example 1, the Log differential pore volume distribution of the catalyst coating layer was measured with a mercury intrusion porosimeter. The obtained result is shown in FIG. In addition, in the obtained Log differential pore volume distribution, it was confirmed that at least two peaks existed, and the mode of the peak (first peak) having the largest pore diameter among these peaks (first peak) is confirmed. When the pore diameter of the value was read, the following results were obtained.
[Mode 1 pore size pore size]
Example 1: 0.7 μm
Example 2: 4.4 μm
Example 3: 2.9 μm
Comparative Example 1: 3.5 μm.

<NOx purification rate measurement test>
With respect to the catalyst samples obtained in Examples 1 to 3 and the comparative catalyst samples obtained in Comparative Examples 1 to 3, the NOx purification rate in the transient atmosphere at the time of transition 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 280 ° C. The obtained results (NOx purification rate) are shown in Table 2 and FIGS. 15 and 16. FIG. 15 is a graph showing the relationship between the amount of added flat fibrous organic matter (average value of equivalent circular diameter of cross section: 12 μm (φ12)) and the NOx purification rate, and FIG. 16 shows the added flat fibrous shape. It is a graph which shows the relationship between the average value (cross-sectional area equivalent diameter) of the equivalent circle diameter of a cross section of organic substance, and a NOx purification rate.

  As is apparent from the results shown in Table 2, FIG. 15 and FIG. 16, the exhaust gas purifying catalyst of the present invention in which the fibrous pores having a flat cross section having the specific dimensions described above are formed in the catalyst coat layer ( In Examples 1 to 3), it was confirmed that NOx was efficiently purified even in a low temperature range of 280 ° C.

  As described above, according to the present invention, it is possible to provide an exhaust gas purification catalyst excellent in catalytic performance such as NOx purification performance in a low temperature region, a method for producing the same, and an exhaust gas purification method using the same. Become.

  Therefore, the exhaust gas purifying catalyst, the manufacturing method thereof, and the exhaust gas purifying method using the same of the present invention purify harmful components such as NOx contained in exhaust gas discharged from an internal combustion engine such as an automobile engine. The present invention is particularly useful as an exhaust gas purification catalyst, a production method thereof, and an exhaust gas purification method using the same.

Claims (8)

A base material, and a catalyst coat layer containing catalyst particles formed on the surface of the base material,
The average value of the equivalent circle diameters of the pores 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 is 8 μm or more and the length of the pores parallel to the exhaust gas flow direction of the base material is Fibrous pores having a size of 15 μm or more are formed in the catalyst coat layer,
The fibrous pores have an average value of the minor axis of the pores in the cross-sectional image in the range of 8 to 15 μm, and the average value of the aspect ratio (major axis / minor axis) of the pores in the sectional image is 2. Are flat fibrous pores having an average aspect ratio (length / minor axis) in the range of 2 to 10 based on the minor axis of the pores in the cross-sectional image.
An exhaust gas purifying catalyst characterized by that.   The exhaust gas purifying catalyst according to claim 1, wherein a ratio of the flat fibrous pores to an apparent volume of the catalyst coat layer is in a range of 0.1 to 6% by volume.   The exhaust gas-purifying catalyst according to claim 1 or 2, wherein the porosity measured by X-ray CT of the catalyst coat layer is in the range of 15 to 45 vol%.   The at least one NOx storage component selected from the group consisting of Li, Na, K, Rb, Cs, Be, Mg, Ca, Sr, and Ba is further supported on the catalyst particles. The exhaust gas purifying catalyst according to any one of? The average value of the equivalent circle diameter of the cross section is 8 μm or more, the average value of the short axis of the cross section is in the range of 8 to 15 μm, and the cross sectional length (long diameter / short) The average value of the diameter) is 2 to 5, and a flat fibrous organic material having an average aspect ratio (length / minor axis) in the range of 10 to 100 based on the minor axis of the cross section is mixed. And obtaining a catalyst slurry,
Applying the catalyst slurry to a surface of a substrate to form a catalyst slurry layer;
A firing step of obtaining an exhaust gas purifying catalyst by removing at least a part of the flat fibrous organic matter in the catalyst slurry layer;
A method for producing an exhaust gas purifying catalyst, comprising:   The method for producing an exhaust gas purifying catalyst according to claim 5, wherein a blending amount of the flat fibrous organic matter in the catalyst slurry is within a range of 0.1 to 9% by mass in terms of solid content.   The raw material of at least one NOx storage component selected from the group consisting of Li, Na, K, Rb, Cs, Be, Mg, Ca, Sr and Ba is further added to the catalyst slurry. Item 7. A method for producing an exhaust gas purifying catalyst according to Item 5 or 6.   An exhaust gas purification method comprising purifying the exhaust gas by bringing the exhaust gas discharged from the internal combustion engine into contact with the exhaust gas purification catalyst according to any one of claims 1 to 4.