JP2013180235A - Catalyst for cleaning exhaust gas and method for producing the catalyst - Google Patents

Abstract

PROBLEM TO BE SOLVED: To improve diffusibility of the exhaust gas inside a catalyst particle constituting a catalyst layer.SOLUTION: A catalyst (1) for cleaning exhaust gas: includes a plurality of catalyst units (8) each including a noble metal particle (3) and a noble metal particle-supporting anchor particle (4); and the catalyst particles (2) containing a clathrate material (6) including the plurality of the catalyst units (8) and parting the catalyst units from one another. A plurality of pores (7) are present inside the secondary particle of the catalyst particle (2). Further, the mode value of a frequency distribution of ratios of major axes (D) to minor axes (L) (D/L) in the cross sections of pores (7) is ≥2.0. The total value of the pore volume of pores having 0.1-10 μm pore size in the plurality of catalyst particles (2) is ≥0.5 cc/g.

Description

  The present invention relates to an exhaust gas purification catalyst for purifying exhaust gas discharged from an internal combustion engine and a method for manufacturing the same. Specifically, the present invention relates to an exhaust gas purifying catalyst capable of improving the diffusibility of exhaust gas in secondary particles of catalyst particles and a method for producing the same.

  As an exhaust gas purification catalyst mounted on automobiles, etc., a three-way catalyst that oxidizes or reduces harmful gases (hydrocarbon (HC), carbon monoxide (CO), nitrogen oxide (NOx)) contained in exhaust gas is known. It has been. Due to the recent increase in environmental awareness, regulations on exhaust gas emitted from automobiles and the like have been further strengthened, and the three-way catalyst has been improved accordingly.

  In such an exhaust gas purification catalyst, a catalyst layer for purifying exhaust gas is formed on the inner surface of a refractory inorganic carrier such as a honeycomb carrier. And in the conventional exhaust gas purification catalyst, in order to improve the diffusibility of the exhaust gas in the catalyst layer, pores are formed inside the catalyst layer (see, for example, Patent Document 1). In other words, Patent Document 1 improves the gas diffusibility in the catalyst layer by forming pores between the catalyst powders, and improves the purification performance by increasing the contact rate between the noble metal and the exhaust gas.

JP 2008-279428 A

  However, in order to further improve the gas diffusibility in the catalyst layer and increase the contact rate between the noble metal and the exhaust gas, it is necessary to further improve the porosity in the catalyst layer. Therefore, it has become necessary to form pores not only in the catalyst layer but also in the catalyst particles constituting the catalyst layer. In particular, in the exhaust gas purification catalyst of Patent Document 1, since noble metals are also present inside the catalyst particles, it is necessary to improve the porosity inside the catalyst particles.

  The size of the pores inside the catalyst particles is on the order of nanometers to submicrons. The pore volume in this region largely depends on the physical properties of the material used, and is often determined by the firing temperature at the time of production of the catalyst substrate, the synthesis method, and the like. For this reason, when the kind of material of the catalyst base is fixed, it is very difficult to control the pore volume greatly.

  The present invention has been made in view of such problems of the prior art. And the objective of this invention is providing the exhaust gas purification catalyst which can improve the diffusibility of the exhaust gas inside the catalyst particle | grains which comprise a catalyst layer, and its manufacturing method.

  The exhaust gas purifying catalyst according to the first aspect of the present invention includes a plurality of catalyst units including noble metal particles and anchor particles supporting the noble metal particles, a package containing the plurality of catalyst units, and separating the catalyst units from each other. Catalyst particles containing the contact material are provided. A plurality of pores exist inside the secondary particles of the catalyst particles, and the mode of the frequency distribution regarding the ratio (D / L) of the major axis (D) to the minor axis (L) in the cross section of the pores. Is 2.0 or more. Furthermore, in the plurality of catalyst particles, the total value of the pore volumes related to the pores having a pore diameter in the range of 0.1 to 10 μm is 0.5 cc / g or more.

  In the method for producing an exhaust gas purifying catalyst according to the second aspect of the present invention, the catalyst unit includes a precursor of an inclusion material, and a ratio of a major axis (D) to a minor axis (L) in a cross section (D / L). Is mixed with a slurry containing an extinguisher that is 2.0 or more, and is dried and fired.

  The exhaust gas purifying catalyst of the present invention has a plurality of elongated pores. Therefore, the pores are connected to each other, and the gas diffusion path in the secondary particles of the catalyst particles is increased, so that the diffusibility of the exhaust gas can be improved. Furthermore, a high exhaust gas purification performance is obtained by the synergistic effect of the elongated pores and the increased pore volume.

FIG. 1 is a schematic view showing an exhaust gas purification catalyst according to an embodiment of the present invention. FIG. 2 is a schematic view showing catalyst particles contained in the exhaust gas purification catalyst. FIG. 3 is a photomicrograph for explaining a method of measuring the ratio (D / L) of the major axis (D) to the minor axis (L) in the cross section of the pores in the exhaust gas purification catalyst. FIG. 3A is an enlarged photograph of secondary particles of catalyst particles in the exhaust gas purification catalyst. FIG. 3B is a diagram obtained by binarizing the micrograph of FIG. 4A to 4C are schematic diagrams for explaining the major axis and minor axis of the pores in the exhaust gas purification catalyst. FIG. 5 shows the ratio Da / Db between the average particle diameter Da of the composite particles before the exhaust durability test and the average pore diameter Db of the clathrate, and the CeO ₂ crystal growth ratio and the surface area of Pt after the exhaust durability test. It is a graph which shows these relationships by making ordinate the vertical axis. FIG. 6 is a graph showing the relationship between the particle diameter and surface area of the noble metal. FIG. 7 is a graph showing the relationship between the particle diameter, the number of atoms, and the surface area of the noble metal. Fig.8 (a) is a perspective view which shows the exhaust gas purification catalyst structure which concerns on embodiment of this invention. FIG. 8B is an enlarged schematic diagram of a portion indicated by reference sign B in FIG. FIG. 9A is a schematic view showing the catalyst precursor before the lost material is burned out. FIG. 9B is a schematic view showing the exhaust gas purification catalyst after the lost material is burned out. FIG. 10 is a graph showing the frequency distribution of pores in the catalysts of Example 1 and Comparative Example 1. FIG. 11A is an SEM photograph of the catalyst of Example 1. FIG. FIG. 11B is an SEM photograph of the catalyst of Comparative Example 1. FIG. 12 is a graph showing the relationship between the pore volume and the HC conversion rate in Examples 1 and 2 and Comparative Examples 1 to 3.

  In a vehicle exhaust gas purification catalyst, the improvement in gas diffusibility in the catalyst layer leads to an increase in the contact rate between noble metal particles, which are active sites, and exhaust gas under high space velocity conditions such as during acceleration. Therefore, improvement of gas diffusibility in the catalyst layer is an essential condition for improvement of catalyst performance.

  And the method of improving the gas diffusibility in an exhaust gas purification catalyst is mainly divided into two. The first is expansion of the pore volume in the catalyst layer. For example, the pore volume in the catalyst layer can increase the pore volume by indirectly expanding the interparticle voids by increasing the size of the catalyst particles constituting the catalyst layer. Moreover, the pore volume can be increased by generating voids directly in the catalyst layer using a pore-forming material such as a polymer material that disappears at the time of the final firing in the production process of the catalyst layer. The pore size of the catalyst layer thus formed is on the order of several microns, and the pore diameter and pore volume can be measured mainly with a mercury porosimeter or the like.

  And the 2nd method of improving gas diffusivity is expansion of the volume of the pore which exists in the inside of each catalyst particle | grain which is the main point of this invention. Hereinafter, an embodiment of an exhaust gas purification catalyst of the present invention to which such a technique is applied will be described in detail with reference to the drawings. In addition, the dimension ratio of drawing is exaggerated on account of description, and may differ from an actual ratio.

[Exhaust gas purification catalyst]
As shown in FIG. 1, an exhaust gas purification catalyst 1 (hereinafter also referred to as a catalyst) according to this embodiment includes a plurality of catalyst particles 2 shown in FIG. That is, the exhaust gas purification catalyst 1 is composed of secondary particles (aggregates) of the catalyst particles 2. And the catalyst particle 2 has the inclusion structure shown in FIG.1 and FIG.2. Specifically, the catalyst particles 2 contain noble metal particles 3 and anchor particles 4. The anchor particle 4 carries the noble metal particle 3 on the surface as an anchor material for the noble metal particle 3. Further, the catalyst particles 2 contain an inclusion material 6 that encloses the composite particles 5 of the noble metal particles 3 and the anchor particles 4 and separates the adjacent composite particles 5 from each other.

  In the catalyst particles 2, the noble metal particles 3 and the anchor particles 4 are brought into contact with each other, whereby the anchor particles 4 act as an anchor material for chemical bonding and suppress the movement of the noble metal particles 3. Further, by covering the anchor particles 4 carrying the noble metal particles 3 with the inclusion material 6 and enclosing them, the physical movement of the noble metal particles 3 beyond the section separated by the inclusion material 6 can be confirmed. To suppress. Further, by including the anchor particles 4 in the section separated by the enclosure material 6, the anchor particles 4 are prevented from contacting and aggregating beyond the section separated by the enclosure material 6. This not only prevents the anchor particles 4 from aggregating, but also prevents the noble metal particles 3 supported on the anchor particles 4 from aggregating. As a result, the catalyst particles 2 can suppress a decrease in catalyst activity due to the aggregation of the noble metal particles 3 without increasing the manufacturing cost and the environmental load. Moreover, the activity improvement effect of the noble metal particle 3 by the anchor particle 4 can be maintained.

  Thus, it becomes possible to suppress aggregation of the noble metal particles 3 by enclosing both the noble metal particles 3 and the anchor particles 4 with the inclusion material 6. However, since the surroundings of the noble metal particles 3 and the anchor particles 4 are covered with the inclusion material 6, the exhaust gas may not easily reach the noble metal particles 3 that are active points.

  On the other hand, in the exhaust gas purification catalyst 1 of the present embodiment, elongated pores 7 are formed inside the secondary particles of the catalyst particles 2. As a result, the exhaust gas easily reaches around the composite particles 5. Further, as will be described later, since the inclusion material 6 has a plurality of fine holes 6a, the exhaust gas easily comes into contact with the noble metal particles 3 which are active points, and the purification performance can be improved. .

  More specifically, in the exhaust gas purifying catalyst of the present embodiment, the pores 7 formed in the secondary particles of the catalyst particles 2 have a shape that extends with respect to a sphere, such as a cylindrical shape, a rectangular parallelepiped, or a disc shape. ing. For this reason, when the diameter, weight, and pore volume of the secondary particles are constant, the contacts between the pores relatively increase in the case of an elongated shape as compared with the spherical shape. That is, the ratio (D / L) compared to the state where the pores having a ratio (D / L) of the major axis (D) to the minor axis (L) in the cross section of 1 are evenly distributed in the secondary particles. In a state where non-symmetry pores exceeding 1 are evenly distributed in the secondary particles, the contacts between the pores increase. The number of contacts increases exponentially with respect to the ratio (D / L) of the major axis (D) to the minor axis (L) in the cross-sectional shape of the pores. As a result, since the connection between the pores increases, it becomes easier for the exhaust gas to reach the noble metal particles 3 of the catalyst particles 2 existing inside the secondary particles. Therefore, the contact rate between the active sites in the catalyst particles 2 and the exhaust gas is increased, and the catalyst performance is improved.

  In this embodiment, the mode of the frequency distribution regarding the ratio (D / L) of the major axis (D) to the minor axis (L) in the cross section of the pores 7 in the secondary particles of the catalyst particle 2 is 2. It is preferably 0 or more. That is, with respect to all the pores 7 present in the secondary particles of the catalyst particle 2, those having a pore length (major axis) of more than twice the pore diameter (minor axis) are most often present. preferable. Thereby, since the contact between pores increases more, the diffusibility of the exhaust gas in a secondary particle improves. Moreover, since the gas diffusibility in the secondary particles is improved, even if the particle diameter of the secondary particles is increased, the contact ratio between the noble metal particles inside the secondary particles and the exhaust gas can be improved.

  In addition, a plurality of long pores having a ratio (D / L) of the major axis (D) to the minor axis (L) in the cross section of 2.0 or more exist in the secondary particles of the catalyst particle 2, It is preferable that the long pores communicate with each other. That is, the long pores are preferably in contact with each other. Thereby, the diffusibility of the exhaust gas in the secondary particles is greatly improved, and the noble metal particles inside the secondary particles and the exhaust gas can efficiently contact each other.

  The mode value of the frequency distribution of the pores 7 is preferably 2 or more, but the mode value is more preferably 2 or more and 10 or less, and particularly preferably 2 or more and 5 or less. The effect of the present invention can be obtained even when the mode value of the pores exceeds 10. However, when the mode value exceeds 10, the pores in the secondary particles are based on the pores in the secondary particles. There is a possibility that destruction occurs and the particle diameter of the secondary particles is made smaller than necessary. In addition, when the mode value of the pores 7 is 10 or less, particularly 5 or less, it is possible to further suppress the occurrence of destruction of secondary particles.

Here, the frequency distribution of the ratio (D / L) of the major axis (D) to the minor axis (L) in the cross section of the pores 7 formed inside the secondary particles of the catalyst particle 2 is obtained as follows. be able to.
(1) First, an exhaust gas purification catalyst structure, which will be described later, is cut and embedded with resin, and secondary particles are imaged with a scanning electron microscope (SEM) as shown in FIG. At this time, it is desirable to take a plurality of images so that the measurement points are not biased toward the upper end portion, the central portion, the lower end portion, and the like of the catalyst layer in the structure. It is also desirable to photograph a plurality of cells in each structure. In addition, in order to observe the pores inside the catalyst particles, it is possible to obtain a clearer cross-sectional image by performing microtome processing and creating an ultrathin film section. Enlarge the next particle. That is, the magnification of the SEM image is increased so that the major and minor diameters of the pores in the secondary particles can be measured.
(3) Further, the pore portion in the enlarged SEM image is binarized and extracted. In a normal SEM image, a portion without catalyst particles, that is, a portion filled with a resin during sample pretreatment does not reflect an electron beam and appears black. Therefore, as shown in FIG. 3B, when the black and white of this image is inverted and further binarized, the pore portion is extracted white.
(4) Thereafter, the contour of the portion extracted in white is extracted, and the ratio (D / L) is calculated from the major axis (D) and minor axis (L) of each pore.
(5) For the obtained data, when the ratio (D / L) is plotted on the horizontal axis and the frequency is plotted on the vertical axis, the frequency distribution of the ratio (D / L) related to the pores in the secondary particles is obtained. Then, the mode value in this graph is read.

  As shown in FIG. 4, the major axis 7a is the maximum distance between two points on the outer periphery of the contour shape of the pore 7, and the minor axis 7b is a line passing through the midpoint of the major axis 7a and perpendicular to the direction of the major axis 7a. Refers to the length across the contour of the pore 7.

  Here, the exhaust gas purifying catalyst according to the present embodiment has the above frequency distribution, and the pores related to the pores in the plurality of catalyst particles 2 having a pore diameter in the range of 0.1 to 10 μm. The total volume is 0.5 cc / g or more. By setting the pore volume of the catalyst powder made of the catalyst particles 2 to 0.5 cc / g or more, the absolute amount of the pore volume inside the catalyst particles increases. Therefore, the exhaust gas is easily diffused inside the catalyst particles, and the catalytic reaction can be improved.

  In the present embodiment, the total value of the pore volumes in the plurality of catalyst particles 2 is preferably 0.5 cc / g or more, but the total value of the pore volumes is 0.5 cc / g or more and 0.7 cc / g. It is particularly preferred that Even when the total pore volume exceeds 0.7 cc / g, the effect of the present invention can be obtained. However, when the total pore volume exceeds 0.7 cc / g, the secondary particles are destroyed, and the secondary particles There is a risk that the particle size of the particles will be smaller than necessary. The pore volume of pores having a pore diameter in the range of 0.1 to 10 μm is preferably measured by a nitrogen gas adsorption method, but can also be measured by a mercury intrusion method.

  The secondary particles of the catalyst particles 2 preferably have an average particle diameter (D50) of 6 μm or less. Even when the secondary particles of the catalyst particles 2 exceed 6 μm, the effect of the present invention can be obtained. However, when it exceeds 6 μm, the catalyst layer is likely to be peeled off or biased when coated on the honeycomb carrier. The average particle diameter of the secondary particles of the catalyst particles 2 is more preferably in the range of 3 μm to 6 μm that can suppress peeling. In addition, the average particle diameter of the secondary particle | grains of the catalyst particle 2 can be calculated | required by applying the slurry containing these particles to a laser diffraction type particle size distribution measuring apparatus. Moreover, the average particle diameter in this case means a median diameter (D50).

  Here, the catalyst particles 2 according to the exhaust gas purification catalyst of the present embodiment will be described in more detail. In the catalyst particle 2 shown in FIG. 2, a catalyst unit 8 containing noble metal particles 3 and secondary particles in which primary particles of the anchor particles 4 are aggregated is included in a region separated by the inclusion material 6. ing. However, the anchor particles 4 may exist as primary particles in a region separated by the inclusion material 6. That is, the catalyst unit 8 may contain the noble metal particles 3 and the primary particles of the anchor particles 4.

  The noble metal particle 3 is at least one selected from the group consisting of platinum (Pt), palladium (Pd), rhodium (Rh), gold (Au), silver (Ag), iridium (Ir) and ruthenium (Ru). One can be used. Among these noble metals, platinum (Pt), palladium (Pd) and rhodium (Rh) are the most cost-effective and are suitable as noble metals for use in exhaust gas purification catalysts.

  Moreover, it is preferable that the anchor particle 4 contains a transition metal. In particular, the anchor particle 4 preferably contains at least one of cerium (Ce) and zirconium (Zr). When the anchor particles contain at least one element selected from Ce and Zr, the anchor function for the noble metal particles can be improved, and aggregation of the noble metal particles after durability can be suppressed.

The anchor particles 4 include aluminum oxide (Al ₂ O ₃ ), cerium oxide (CeO ₂ ), zirconium oxide (ZrO ₂ ), yttrium oxide (Y ₂ O ₃ ), and neodymium oxide (Nd ₂ O ₃ ). At least one selected from can be the main component. Among these, CeO ₂ and ZrO ₂ are excellent in high-temperature heat resistance and can maintain a high specific surface area. Therefore, it is preferable that the anchor particles 4 have CeO ₂ or ZrO ₂ as a main component. In the present specification, the main component means a component having a content of 50 mole percent or more in the particles.

The inclusion material 6 preferably contains at least one of aluminum (Al) and silicon (Si). The inclusion material 6 is preferably a material that can include anchor particles and ensure gas permeability. From such a viewpoint, a compound containing at least one of Al and Si, such as alumina (Al ₂ O ₃ ) and silica (SiO ₂ ), has a large pore volume and can ensure high gas diffusibility. Moreover, these compounds have an inclusion function for suppressing aggregation of anchor particles. Therefore, the enclosure material 6 is preferably composed mainly of _Al 2 _{O 3} and SiO _2. The inclusion material may be a composite compound of Al and Si.

  Here, it is preferable that the weight ratio of the anchor particles 4 in the catalyst particles 2 is 30 to 70 wt%, and the weight ratio of the inclusion material 6 is 70 to 30 wt%. When the weight ratio of the anchor particles is 30 wt% or more, the density of the noble metal supported on the anchor particles is reduced. That is, since the amount of the noble metal supported on one anchor particle is reduced, even if the noble metal particles aggregate on the anchor particle after durability, the noble metal particles do not greatly enlarge. As a result, a decrease in the surface area of the noble metal particles can be suppressed. Moreover, it can suppress that the amount of enclosure materials falls because the weight ratio of an anchor particle is 70 wt% or less. Therefore, even after durability, aggregation of anchor particles and accompanying aggregation of noble metal particles can be suppressed.

  Here, the enclosure material 6 used in the catalyst particles 2 does not completely surround the periphery of the catalyst unit 8. That is, the enclosure material 6 has micropores that allow the exhaust gas and active oxygen to pass through while covering the catalyst unit 8 so as to suppress physical movement. Specifically, as shown in FIG. 2, the enclosure material 6 appropriately encloses the catalyst unit 8 and suppresses aggregation of the units. Furthermore, since the enclosure material 6 has a plurality of fine holes 6a, exhaust gas and active oxygen can pass therethrough. The fine pore 6a has a pore diameter of preferably 30 nm or less, more preferably 10 nm to 30 nm. The pore diameter can be determined by a nitrogen gas adsorption method.

As described above, alumina (Al ₂ O ₃ ) or silica (SiO ₂ ) can be used as such an inclusion material 6. When the clathrate is made of alumina, it is preferable to use boehmite (AlOOH) as a precursor. That is, the anchor particles 4 carrying the noble metal particles 3 are put into a slurry in which boehmite is dispersed in a solvent such as water and stirred. Thereby, boehmite adheres around the anchor particles 4. Then, by drying and firing the mixed slurry, boehmite is dehydrated and condensed around the anchor particles 4, and an inclusion material made of boehmite-derived γ-alumina is formed. Such a clathrate made of boehmite-derived alumina has many fine pores of 30 nm or less while covering the anchor particles 4, and thus has excellent gas permeability.

  Similarly, when the clathrate is made of silica, it is preferable to use silica sol and zeolite as precursors. That is, the anchor particles 4 carrying the noble metal particles 3 are put into a slurry in which silica sol and zeolite are dispersed in a solvent, stirred, dried and fired, whereby a clathrate made of silica is formed. Such an inclusion material composed of silica sol and zeolite-derived silica also has excellent gas permeability because it has many fine pores of 30 nm or less while covering the anchor particles 4.

  In addition, it is preferable that the average particle diameter of the catalyst unit 8 included in the compartments separated by the inclusion material 6 is 300 nm or less. Therefore, it is preferable that the average secondary particle diameter of the anchor particles 4 included in the catalyst unit 8 is also 300 nm or less. In this case, the noble metal can be maintained in a fine particle state. The average particle diameter of the catalyst unit 8 and the average secondary particle diameter of the anchor particles are more preferably 200 nm or less. As a result, the amount of noble metal supported on the secondary particles of the anchor particles is further reduced, so that aggregation of noble metals can be suppressed. In addition, although the minimum of the average particle diameter of the catalyst unit 8 and the average secondary particle diameter of the anchor particle 4 is not specifically limited, For example, it can be 5 nm. However, as will be described later, the average particle diameter of the catalyst unit 8 is preferably larger than the average pore diameter of the micropores 6 a formed in the enclosure material 6. Therefore, it is more preferable that the average particle size of the catalyst unit 8 and the average secondary particle size of the anchor particle 4 exceed 30 nm.

  The average secondary particle diameter of the anchor particles can be determined by applying a slurry containing catalyst particles to a laser diffraction particle size distribution measuring apparatus. In addition, the average secondary particle diameter in this case means a median diameter (D50). The average secondary particle diameter of anchor particles and the particle diameter of noble metal particles described later can also be measured from a transmission electron microscope (TEM) photograph of the obtained catalyst powder. Furthermore, the average particle diameter of the catalyst unit 8 can also be measured from a TEM photograph.

  The average particle diameter of the noble metal particles 3 is preferably in the range of 2 nm to 10 nm. When the average particle diameter of the noble metal particles 3 is 2 nm or more, sintering due to the movement of the noble metal particles 3 themselves can be reduced. Moreover, when the average particle diameter of the noble metal particles 3 is 10 nm or less, a decrease in reactivity with the exhaust gas can be suppressed.

  Here, regarding the catalyst unit 8 containing the noble metal particles 3 and the anchor particles 4, the average particle diameter Da of the catalyst unit 8 and the average of the fine holes 6 a formed in the enclosure material 6 containing the catalyst unit 8 are included. The pore diameter Db preferably satisfies the relationship Db <Da. That is, as shown in FIG. 2, Db <Da means that the average particle diameter Da of the catalyst unit 8 is larger than the average pore diameter Db of the fine holes 6 a of the enclosure material 6. When Db <Da, the movement of the composite particles 5 of the noble metal particles 3 and the anchor particles 4 through the fine holes 6a formed in the inclusion material 6 is suppressed. Therefore, aggregation with the composite particles 5 included in other compartments can be reduced.

The effect of the inequality Db <Da has been confirmed by the experiments of the present inventors. FIG. 5 shows ceria crystals as anchor particles 4 after the exhaust durability test with the horizontal axis representing the ratio Da / Db between the average particle diameter Da of the composite particles 5 before the exhaust durability test and the average pore diameter Db of the inclusion material. It is a graph which shows these relationship, making the growth ratio and the surface area of platinum as the noble metal particle 3 a vertical axis. From FIG. 5, it can be seen that when Da / Db exceeds 1, the crystal growth ratio of CeO ₂ is remarkably lowered and the sintering of CeO ₂ is small. In addition, it can be seen that even after the durability test, the surface area of Pt is maintained in a high state, and aggregation of Pt is suppressed.

Further, it is desirable that 80% or more of the noble metal particles 3 are in contact with the anchor particles 4.
Even if the ratio of the noble metal particles 3 in contact with the anchor particles 4 is less than 80%, the effect of the present invention can be obtained. However, if the ratio is less than 80%, the noble metal particles 3 that do not exist on the anchor particles 4 increase, and thus sintering may be caused by the movement of the noble metal particles 3.

  The anchor particles 4 are preferably an oxide further containing at least one selected from iron (Fe), manganese (Mn), cobalt (Co) and nickel (Ni). That is, as described above, the anchor particles 4 are mainly composed of ceria and zirconia. And it is desirable to contain the transition metal as an additive in the anchor particles. By containing at least one of these transition metals, the catalytic performance, particularly the CO and NOx purification rate, can be improved by the active oxygen contained in the transition metal.

  The anchor particles 4 preferably further include at least one NOx adsorbent selected from barium (Ba), magnesium (Mg), calcium (Ca), strontium (Sr), and sodium (Na). Any compound containing these elements acts as a NOx adsorbent. Therefore, NOx purification performance can be improved by including the NOx adsorbent in the anchor particles. This is because the NOx adsorption reaction is very sensitive to gas contact. A catalyst containing these NOx adsorbents is suitable as a catalyst for a lean burn engine in which a large amount of NOx is generated than an engine that burns near the stoichiometric air-fuel ratio. Note that this is not the case when used in a gasoline engine mainly for combustion in a stoichiometric atmosphere.

  Further, the clathrate 6 is composed of at least one of cerium (Ce), zirconium (Zr), lanthanum (La), barium (Ba), magnesium (Mg), calcium (Ca), strontium (Sr), and sodium (Na). An oxide containing any one of these is preferable. By containing cerium, the oxygen storage and release ability can be imparted to the inclusion material, and the exhaust gas purification performance can be improved. Moreover, the heat resistance of an enclosure material can be improved by containing a zirconium and lanthanum. Furthermore, the NOx purification performance can be improved by containing a NOx adsorbent such as barium, magnesium, calcium, strontium and sodium in the clathrate. Note that this is not the case when used in a gasoline engine mainly for combustion in a stoichiometric atmosphere. Moreover, these elements can be contained by mixing a nitrate slurry or acetate of these elements into the precursor slurry of the clathrate.

Moreover, it is preferable to contain the noble metal particles 3 in a total amount of 8 × 10 ⁻²⁰ mol or less in the compartments separated by the inclusion material 6. That is, the number of moles of the noble metal particles 3 in one catalyst unit 8 is preferably 8 × 10 ⁻²⁰ moles or less. In a section separated by the inclusion material 6, a plurality of noble metal particles 3 may move and aggregate together in a high temperature state. In this case, the noble metal particles 3 do not move to the enclosure material 6 due to the effect of the anchor particles 4, but aggregate into one or a plurality of noble metal particles on the surface of the anchor particles 4.

  Here, when the noble metal particles 3 aggregate in one catalyst unit 8, if the particle diameter of the aggregated noble metal particles 3 is 10 nm or less, sufficient catalytic activity is exhibited and deterioration due to aggregation can be suppressed. . FIG. 6 is a graph showing the relationship between the particle diameter and the surface area for platinum and palladium as noble metals. In FIG. 6, since the same curve is shown in the case of platinum and palladium, it is shown as one curve. As is clear from FIG. 6, since the surface area is large if the particle diameter of the noble metal is 10 nm or less, it is possible to suppress the deterioration of the catalyst activity due to aggregation.

FIG. 7 is a graph showing the relationship between the particle diameter and the number of atoms for platinum and palladium as noble metals. In FIG. 7, since the same curve is shown in the case of platinum and palladium, it is shown as one curve. As is apparent from FIG. 7, when the particle diameter is 10 nm, the number of atoms of the noble metal is about 48000, and when this value is converted into the number of moles, it becomes about 8 × 10 ⁻²⁰ moles. From these viewpoints, by limiting the amount of noble metal in the catalyst unit 8 to 8 × 10 ⁻²⁰ mol or less, even if the noble metal aggregates in the catalyst unit 8, deterioration of catalyst activity is suppressed. can do. In addition, as a method of making the amount of noble metal contained in the catalyst unit 8 8 × 10 ⁻²⁰ mol or less, it is possible to reduce the particle diameter of the anchor particles 4 carrying the noble metal particles 3.

  Further, in the catalyst particle 2, when the adsorption stabilization energy of the noble metal particle 3 to the anchor particle 4 is Ea and the adsorption stabilization energy of the noble metal particle 3 to the enclosure 6 is Eb, Ea is more than Eb. A small value (Ea <Eb) is preferable. The fact that the adsorption stabilization energy Ea of the noble metal particles 3 on the anchor particles 4 is smaller than the adsorption stabilization energy Eb of the noble metal particles 3 on the enclosure material 6, so that the noble metal particles 3 move to the enclosure material 6. Can be suppressed. As a result, aggregation of the noble metal particles 3 can be further reduced.

  Further, the difference (Eb−Ea) between the adsorption stabilization energy Ea of the noble metal particle 3 on the anchor particle 4 and the adsorption stabilization energy Eb of the noble metal particle 3 on the enclosure 6 exceeds 10.0 cal / mol. It is more preferable. When the adsorption stabilization energy difference exceeds 10.0 cal / mol, it is possible to more reliably suppress the movement of the noble metal particles 3 to the inclusion material 6.

  The adsorption stabilization energy Ea of the noble metal particles 3 to the anchor particles 4 and the adsorption stabilization energy Eb of the noble metal particles 3 to the enclosure 6 can be calculated by simulation using a density functional method. it can. This density functional method is a method for predicting the electronic state of a crystal by introducing a Hamiltonian that incorporates a correlation effect between many electrons. The principle is based on the mathematical theorem that the total energy of the ground state of the system can be expressed by an electron density functional method. The density functional method is highly reliable as a method for calculating the electronic state of a crystal.

  Such a density functional method is suitable for predicting an electronic state at the interface between the anchor particle 4 or the inclusion material 6 and the noble metal particle 3. The catalyst of the present embodiment, which is designed based on a combination of precious metal particles, anchor particles and inclusion materials selected based on actual simulation values, is less prone to coarsening of precious metal particles and has high purification performance even after high temperature durability. Has been confirmed to maintain. Analysis software for simulation using such a density functional method is commercially available, and examples of calculation conditions of the analysis software include the following.

  Pre / post: Materials studio 3.2 (manufactured by Accelrys), solver: DMol3 (manufactured by Accelrys), temperature: absolute zero, approximation: GGA approximation

[Exhaust gas purification catalyst structure]
FIG. 8 shows an exhaust gas purification catalyst structure 20 according to the present embodiment. As shown in FIG. 8A, the exhaust gas purification catalyst structure 20 includes a honeycomb carrier (refractory inorganic carrier) 21 having a plurality of cells 21a. The exhaust gas flows through each cell 21 a along the arrow F and is purified by contacting with the catalyst layer 22 there.

  In the exhaust gas purification catalyst structure 20, a catalyst layer 22 is formed on the inner surface of the honeycomb carrier 21 as shown in FIG. The catalyst layer 22 is formed by the exhaust gas purification catalyst 1, that is, the secondary particles (aggregates) of the catalyst particles 2. When the exhaust gas purification catalyst structure 20 having such a configuration is installed in the exhaust gas passage of the internal combustion engine, the contact rate between the exhaust gas discharged from the internal combustion engine and the noble metal particles 3 in the catalyst layer 22 is increased. It becomes possible.

  As the honeycomb carrier 21, a cordierite honeycomb carrier can be used. Cordierite honeycomb carriers are excellent in heat resistance, impact resistance and production cost, and are generally used as carriers for exhaust gas purification catalysts for automobiles. The cross-sectional shape of the flow channel (cell 21a) includes a quadrangle and a hexagon, but any shape can be used in the present embodiment. As the refractory inorganic carrier 21, a stainless metal carrier can also be used. Since the metal carrier can be processed with a thin wall thickness, the metal carrier is mainly used for high-power vehicles that require a reduction in pressure loss. Since the metal carrier is processed so as to wind the corrugated stainless steel foil into a concentric shape, the channel shape is an irregular shape having corners mainly at three locations. In this embodiment, it can be used for a carrier having this shape.

  Furthermore, as shown in FIG. 8B, the catalyst layer 22 of the exhaust gas purification catalyst structure 20 in the present embodiment can have a structure in which a plurality of catalyst layers 22a and 22b are stacked. By providing a plurality of catalyst layers 22, the catalyst performance can be improved. For example, platinum (Pt), palladium (Pd), and rhodium (Rh) are easily alloyed with each other, and the aggregation at high temperature is facilitated by alloying. For this reason, it becomes possible to suppress alloying by dividing | segmenting the layer which carry | supports each noble metal.

  When a plurality of catalyst layers are provided in the exhaust gas purification catalyst structure, the pore volume of the catalyst particles contained in each catalyst layer may be different for each catalyst layer. Specifically, the pore volume of the catalyst particles in the surface catalyst layer may be increased from the pore volume of the catalyst particles in the inner catalyst layer adjacent to the honeycomb carrier. Thereby, the gas diffusibility from the surface catalyst layer to the inner catalyst layer can be further improved. Conversely, the pore volume of the catalyst particles in the inner catalyst layer may be increased from the pore volume of the catalyst particles in the surface catalyst layer.

  Here, an exhaust gas purification system for a vehicle equipped with a gasoline engine depends on the destination of the vehicle on which the gasoline engine is mounted and exhaust regulations, but often includes two catalysts, a manifold catalyst structure and an underfloor catalyst structure. The manifold catalyst structure is a catalyst that is directly connected to the exhaust manifold of the engine or disposed immediately below the exhaust manifold. The underfloor catalyst structure is a catalyst that is disposed at the lower part of the vehicle floor and that is disposed on the exhaust downstream side of the manifold catalyst structure.

  In general, the manifold catalyst structure purifies 95 to 97% of HC, CO and NOx discharged from the engine outlet, and plays a role of exhaust gas purification particularly at a low temperature immediately after starting the engine. In contrast, the underfloor catalyst structure is required to purify components that could not be purified by the manifold catalyst structure to a value that can finally satisfy the exhaust emission control value. On the other hand, the function with respect to the NOx purification rate tends to be more demanded.

  However, the exhaust gas treatment amount of the manifold catalyst structure and the underfloor catalyst structure is generally about 1/10 to 1/50 of the manifold catalyst structure, and the manifold catalyst structure is overwhelmingly large as the treatment amount. . In addition, from the viewpoint of consideration for the global environment and reduction of vehicle cost, reduction of the amount of noble metal used for the exhaust gas purification catalyst is required. However, since the manifold catalyst structure has a larger amount of exhaust gas treatment than the underfloor catalyst structure, it has been extremely difficult to achieve both reduction in the amount of noble metal in the manifold catalyst structure and exhaust gas purification performance. .

  On the other hand, in the exhaust gas purification catalyst structure in the present embodiment, a large number of elongated pores 7 are formed in the secondary particles of the catalyst particles 2 forming the catalyst layer. As a result, the exhaust gas diffusibility inside the secondary particles of the catalyst particles 2 is improved, so that the exhaust gas can easily come into contact with the noble metal particles 3 that are active points, and the exhaust gas can be efficiently purified. . And by using such an exhaust gas purification catalyst structure as a manifold catalyst structure, it becomes possible to improve exhaust gas purification performance while reducing the amount of noble metal.

[Method for producing exhaust gas purification catalyst]
Next, the manufacturing method of the exhaust gas purification catalyst according to the present embodiment will be described. The manufacturing method of the present embodiment includes a step of preparing a catalyst unit including noble metal particles and anchor particles. Further, the prepared catalyst unit is prepared by adding a precursor of the inclusion material and a disappearing material in which a ratio (D / L) of a major axis (D) to a minor axis (L) in a cross section is 2.0 or more. It has the process of mixing with the containing slurry, drying and baking.

  In the manufacturing method of the present embodiment, first, a catalyst unit including noble metal particles and anchor particles is prepared. Specifically, first, the noble metal particles 3 are supported on the anchor particles 4. At this time, the noble metal particles 3 can be supported by an impregnation method. Then, the anchor particles 4 carrying the noble metal particles 3 on the surface are pulverized using a bead mill or the like to obtain a desired particle diameter. As described above, the particle diameter of the anchor particle 4 can be set to, for example, 300 nm. Note that the crushing step can be omitted by using a fine raw material such as an oxide colloid as a raw material of the anchor particle 4.

  Next, a clathrate slurry containing a precursor of the clathrate 6 is prepared. As the precursor of the clathrate 6, as described above, when the clathrate is mainly composed of alumina, it is preferable to use boehmite (AlOOH). It is preferred to use zeolite. And the said clathrate slurry can be prepared by mixing the precursor of the clathrate 6 with solvents, such as water, and stirring.

  Next, the crushed anchor particles 4 carrying the noble metal particles 3 are put into the clathrate slurry and stirred. Thereby, the catalyst precursor slurry in which the precursor of the inclusion material 6 is attached around the catalyst unit 8 (composite particle 5) can be obtained.

  Thereafter, the disappearing material is mixed into the prepared catalyst precursor slurry. By mixing such a disappearing material into the catalyst precursor slurry and burning it off in the subsequent steps, it becomes possible to create pores having the same shape as the shape of the disappearing material in the secondary particles of the catalyst particles 2. The pores thus obtained serve as a diffusion flow path for the exhaust gas, and the catalyst performance can be improved particularly when the gas flow rate increases rapidly, such as during acceleration.

  However, in order to form a large number of elongated pores 7 in the secondary particles of the catalyst particles 2, it is necessary to mix an elongated shaped disappearing material. Specifically, the mode of the frequency distribution related to the ratio (D / L) of the major axis (D) to the minor axis (L) in the cross section of the lost material needs to be 2 or more. By using such a disappearing material, elongated pores 7 are formed, so that the gas diffusibility in the secondary particles of the catalyst particles 2 can be improved.

  Here, the mode value of the frequency distribution regarding the ratio (D / L) in the cross section of the lost material can be obtained in the same manner as the mode value of the pores 7. That is, the major axis and minor axis of the disappearing material are obtained using an electron microscope or the like, and the ratio (D / L) between the major axis (D) and the minor axis (L) is calculated. For the obtained data, the ratio (D / L) on the horizontal axis and the frequency on the vertical axis indicate the frequency of the ratio (D / L) of the major axis (D) to the minor axis (L) related to the lost material. A distribution is obtained and the mode value in this graph is read. In addition, the definition of the major axis and the minor axis of the disappearing material is the same as that of the pore 7. Specifically, the major axis of the vanishing material is the maximum distance between two points on the outer periphery of the contour shape of the vanishing material, and the minor axis of the vanishing material is a line perpendicular to the major axis direction through the midpoint of the major axis of the vanishing material. The length across the contour of the lost material.

  Then, the catalyst precursor 1a shown in FIG. 9A is prepared by drying the catalyst precursor slurry mixed with the disappearing material. And in this embodiment, at least one part of the loss | disappearance material 30 in the obtained catalyst precursor 1a is burned down. Specifically, as shown in FIG. 9 (b), the resulting catalyst precursor is calcined in air to eliminate the disappearing material and form pores 7 in the secondary particles of the catalyst particle 2. can do. The firing temperature at this time may be calcined at a temperature at which the disappearing material disappears. For example, by calcining at a temperature of 400 ° C. or higher for 1 hour or longer, the disappearing material in the catalyst precursor can be burned out. . In addition, by this baking process, when the clathrate precursor is boehmite, the boehmite is dehydrated and condensed around the anchor particles 4 to form a clathrate made of boehmite-derived alumina. When the clathrate precursor is silica sol and zeolite, a clathrate made of silica sol and zeolite-derived silica is formed.

  Here, in a state where the obtained exhaust gas purification catalyst is not used, it is preferable that the lost material in the secondary particles of the catalyst particles is completely burned out. However, in the unused state, the disappearing material in the secondary particles may not be completely burned off. That is, even if the vanishing material in the secondary particles remains in an unused state, the catalyst is installed and used in the exhaust gas flow path of the internal combustion engine, thereby increasing the temperature of the catalyst and It is also possible to burn off.

  In this way, after mixing the disappearing material, it is possible to complete the production without passing through the step of burning or burning only a part. However, depending on the conditions of use of the catalyst, it may not reach the temperature until the lost material is burned off during actual operation.

  As described above, as the disappearing material, the mode of the frequency distribution relating to the ratio (D / L) of the major axis (D) to the minor axis (L) in the cross section is 2 or more, and under an air stream It is necessary to use materials that can be burned out. As such a disappearing material, it is preferable to use at least one of carbon, a polymer compound, a polysaccharide and a carbohydrate. Further, it is desirable that the disappearing material to be mixed does not contain a component that lowers the catalyst performance, such as catalyst poisoning or coating of active sites. Specifically, a material that contains as little potassium (K), phosphorus (P), zinc (Zn), and chlorine (Cl) as possible is desirable.

  Specifically, carbon, cellulose, polycarbonate, glucose, polyacrylic acid and the like are preferable as the disappearing material. These are suitable materials because the catalyst performance is not easily lowered, and they are burned out at a relatively low temperature (200 to 400 ° C.), so that no precious metal is affected by heat such as sintering. Among these, cellulose is particularly preferable. Cellulose is easily burned off at low temperatures, and those having a ratio (D / L) of 2 or more are easily obtained.

  The cellulose used as the vanishing material preferably has a fiber length of about 1.0 to 50 μm and a fiber diameter of about 0.1 to 5 μm. Thus, elongated pores 7 having a major axis of 0.6 to 3 μm and a minor axis of 0.1 to 0.5 μm can be obtained after firing. In addition, the size of the cellulose described here refers to a value when charged into the catalyst precursor slurry.

  EXAMPLES Hereinafter, although an Example and a comparative example demonstrate this invention further in detail, this invention is not limited to these Examples.

[Example 1]
(Preparation of Rh catalyst powder)
First, a rhodium nitrate solution was supported on a base powder of active zirconium-cerium composite oxide (ZrO ₂ -CeO ₂ ) having a specific surface area of about 70 m ² / g so that the rhodium supporting concentration was 1.0 wt%. . This solution was dried at 150 ° C. for a whole day and night, and then calcined at 400 ° C. for 1 hour to obtain a zirconium-cerium composite oxide carrying 1.0 wt% rhodium. Next, the zirconium-cerium composite oxide carrying Rh was pulverized to make the average particle size (D50) 150 nm. For measurement of the average particle diameter, a laser diffraction / scattering particle size distribution measuring apparatus LA-920 manufactured by Horiba, Ltd. was used.

  Next, boehmite, nitric acid, and water were mixed and stirred for 1 hour. Then, the catalyst precursor slurry was prepared by slowly adding the pulverized Rh-supported zirconium-cerium composite oxide into this solution and further stirring for 2 hours using a high-speed stirrer.

  Then, the obtained catalyst precursor slurry was mixed with a separately pulverized disappearing material, and further stirred for 2 hours using a high-speed stirrer. Thereafter, the obtained slurry was rapidly dried and further dried at 150 ° C. for a whole day and night to remove moisture. And it baked in the air at 550 degreeC for 3 hours, and obtained the Rh catalyst powder of Example 1. In addition, the said loss | disappearance material used the cellulose of the elongate shape from which D / L mode value of the pore shown in Table 1 is obtained. Table 1 also shows the mixing amount of the disappearing material obtained from the following formula 1.

(Preparation of Pd catalyst powder)
First, a palladium nitrate solution was supported on a zirconia base powder (ZrO ₂ ) having a specific surface area of about 70 m ² / g so that the supporting concentration was 8.0 wt%. This solution was dried at 150 ° C. for a whole day and night, and then calcined at 400 ° C. for 1 hour to obtain ZrO ₂ powder supporting 8.0 wt% palladium. Next, the ZrO ₂ powder supporting Pd was pulverized, and the average particle diameter (D50) was set to 150 nm.

Next, boehmite, nitric acid, and water were mixed and stirred for 1 hour. Then, the pulverized Pd-supported ZrO ₂ powder was slowly put into this solution, and further stirred for 2 hours using a high-speed stirrer to prepare a catalyst precursor slurry.

  Then, the obtained catalyst precursor slurry was mixed with a separately pulverized disappearing material, and further stirred for 2 hours using a high-speed stirrer. Thereafter, the obtained slurry was rapidly dried and further dried at 150 ° C. for a whole day and night to remove moisture. And it baked in the air for 3 hours at 550 degreeC, and obtained Pd catalyst powder of Example 1. In addition, the kind and mixing amount of the used disappearance material are the same as in the case of the Rh catalyst powder.

(Preparation of catalyst layer)
200 g of the above Rh catalyst powder, 25 g of CeO ₂ —ZrO ₂ powder, 25 g of alumina sol, 230 g of water and 10 g of nitric acid were put into a magnetic ball mill, mixed and pulverized to prepare an Rh catalyst slurry.

Next, 175 g of the above Pd catalyst powder, 50 g of CeO ₂ -ZrO ₂ powder, 25 g of alumina sol, 230 g of water and 10 g of nitric acid were put in a magnetic ball mill, mixed and pulverized to prepare a Pd catalyst slurry.

  And the thing which grind | pulverized separately the commercially available cellulose powder whose average of ratio (D / L) of a major axis (D) and a minor axis (L) is 2.5 was mixed with the said Rh catalyst slurry. Thereafter, the Rh catalyst slurry containing cellulose was attached to a cordierite monolith support (0.12 L, 900 cells), and excess slurry in the cells was removed by an air flow. Furthermore, after drying the support | carrier with a slurry at 130 degreeC, it baked at 400 degreeC for 1 hour, and prepared the Rh catalyst layer.

  Furthermore, what grind | pulverized separately the commercially available cellulose powder whose average of ratio (D / L) of a major axis (D) and a minor axis (L) is 2.5 was mixed with the Pd catalyst slurry. Thereafter, the Pd catalyst slurry containing cellulose was adhered to the monolith support on which the Rh catalyst layer was supported, and excess slurry in the cell was removed by an air flow. Furthermore, after drying the support | carrier with a slurry at 130 degreeC, it baked at 400 degreeC for 1 hour, and prepared the Pd catalyst layer. In this way, the catalyst structure of Example 1 in which the Pd catalyst layer was provided as the surface layer and the Rh catalyst layer was provided as the inner layer was prepared.

[Examples 2 to 6]
Catalyst structures of Examples 2 to 6 were prepared in the same manner as in Example 1 except that the amount and type of noble metal species and disappearance material were changed as shown in Table 1. In Example 3, a palladium nitrate solution was used as a solution for supporting palladium on the active zirconium-cerium composite oxide base powder. Further, a rhodium nitrate solution was used as a solution for supporting rhodium on the zirconia base powder. In Example 6, a dinitrodiamine platinum solution was used as a solution for supporting platinum on the zirconia substrate powder.

[Example 7]
A catalyst structure of Example 7 was prepared in the same manner as in Example 1 except that the amount and type of noble metal species and disappearance material were changed as shown in Table 1. That is, platinum was used as a noble metal species for the surface layer, and the mixing amount of the disappearing material when preparing the Pt catalyst powder used for the surface layer and the Rh catalyst powder used for the inner layer was changed as shown in Table 1. In Example 7, a dinitrodiamine platinum solution was used as a solution for supporting platinum on the zirconia substrate powder.

[Comparative Examples 1-3]
Catalyst structures of Comparative Examples 1 and 2 were prepared in the same manner as in Example 1 except that the mixing amount and type of the disappearing material were changed as shown in Table 1. Moreover, about the comparative example 3, the catalyst structure of the comparative example 3 was prepared like Example 1 except not using a loss | disappearance material.

[Observation of pores in catalyst structure]
The catalyst structures of Examples 1 to 7 and Comparative Examples 1 to 3 were cut and the major axis (D) and minor axis (L) in the cross section of the pores formed inside the secondary particles of the catalyst particles by the above-described method. ) And the frequency distribution of the ratio (D / L).

  Furthermore, the catalyst layer of the surface layer and the catalyst layer of the inner layer in the catalyst structures of Examples 1 to 7 and Comparative Examples 1 to 3 were scraped off, and the powder of the catalyst layer was collected. Thereafter, the pore volume of pores having a pore diameter in the range of 0.1 to 10 μm in the catalyst layer powder was measured by a nitrogen gas adsorption method. Table 1 also shows the measurement results of the mode of the frequency distribution and the pore volume in each example and comparative example.

[Durability test of catalyst structure]
The catalyst structures of Examples 1 to 7 and Comparative Examples 1 to 3 were mounted on an exhaust system of a gasoline engine with a displacement of 3500 cc, and were operated for 50 hours at an inlet temperature of 920 ° C. . After that, each of the deteriorated catalyst structures is attached to the exhaust system of a 3500 cc gasoline engine, the inlet temperature is set to 480 ° C., and the hydrocarbon concentration at the inlet and the outlet of the catalyst structure The conversion rate (HC conversion rate) was measured.

Table 1 also shows the HC conversion ratios after endurance tests of Examples 1 to 7 and Comparative Examples 1 to 3. Furthermore, Table 1 also shows the noble metal species and noble metal-supporting substrate species of Examples 1 to 7 and Comparative Examples 1 to 3. Note that “ZrO ₂ —CeO ₂ / Al ₂ O ₃ ” in the noble metal-supporting base material in Table 1 represents that ZrO ₂ —CeO ₂ was used as the anchor particle and Al ₂ O ₃ was used as the inclusion material. Similarly, “ZrO ₂ / Al ₂ O ₃ ” represents that ZrO ₂ was used as the anchor particle and Al ₂ O ₃ was used as the inclusion material.

  FIG. 10 shows the frequency distribution of the ratio (D / L) of the major axis (D) and minor axis (L) of the pores in Example 1 and Comparative Example 1. FIG. 11A shows an SEM photograph of secondary particles of catalyst particles in Example 1, and FIG. 11B shows an SEM photograph of secondary particles of catalyst particles in Comparative Example 1. As shown in FIG. 10, in the catalyst of Example 1, the mode value of the frequency distribution is 2.5, and from FIG. 11 (a), it can be seen that a plurality of elongated pores 7 are formed. Further, it can be confirmed that the elongated pores are in contact with each other and communicate with each other. In contrast, in the catalyst of Comparative Example 1, the mode value of the frequency distribution is 1.4, and it can be seen from FIG. 11B that many spherical pores 7A are formed.

  FIG. 12 is a scatter diagram showing the relationship between the pore volume and the HC conversion rate in Examples 1 and 2 and Comparative Examples 1 to 3. From FIG. 12, Examples 1 and 2 in which the pore volume of pores having a pore diameter in the range of 0.1 to 10 μm is 0.5 cc / g or more have an HC conversion rate as compared with Comparative Examples 1 to 3. You can see that it is rising.

  Here, as shown in FIG. 12, when the regression line based on the plots of Comparative Examples 1 to 3 is extended, a difference occurs in the HC conversion rate by the amount corresponding to the reference line and the regression line based on the plots of Examples 1 and 2. I understand that. That is, even if the number of substantially spherical pores as in the comparative example is simply increased and the pore volume is increased, the pores are not sufficiently communicated with each other, so that the gas diffusibility inside the secondary particles is difficult to improve. On the other hand, when the elongated pores are increased as in the example, the pores communicate with each other, so that the gas diffusibility inside the secondary particles is improved, and the noble metal particles inside the secondary particles are improved. And the exhaust gas can contact efficiently. Therefore, it is estimated that the HC conversion rate has been greatly improved.

  Thus, when the mode of the frequency distribution regarding the D / L of the pores is 2.0 or more and the total value of the pore volumes is 0.5 cc / g or more, the elongated pores It can be seen that high exhaust gas purification performance can be obtained by the synergistic effect of the increased pore volume.

  Although the present invention has been described with reference to the examples and comparative examples, the present invention is not limited to these, and various modifications can be made within the scope of the gist of the present invention.

DESCRIPTION OF SYMBOLS 1 Exhaust gas purification catalyst 2 Catalyst particle 3 Noble metal particle 4 Anchor particle 6 Inclusion material 7 Pore 8 Catalyst unit 20 Exhaust gas purification catalyst structure 22 Catalyst layer

Claims (8)

A plurality of catalyst units including noble metal particles and anchor particles supporting noble metal particles as an anchor material of the noble metal particles;
An enclosure material containing the plurality of catalyst units and separating the catalyst units from each other;
Comprising a plurality of catalyst particles containing
A plurality of pores are present in the secondary particles of the catalyst particles, and the frequency distribution mode of the ratio of the major axis (D) to the minor axis (L) (D / L) in the cross section of the pores. Is 2.0 or more,
An exhaust gas purification catalyst, wherein a total value of pore volumes of the pores having a pore diameter in the range of 0.1 to 10 µm in the plurality of catalyst particles is 0.5 cc / g or more.   In the catalyst particles, there are a plurality of long pores having a ratio (D / L) of the major axis (D) to the minor axis (L) in the cross section of 2.0 or more. The exhaust gas purification catalyst according to claim 1, wherein   3. The exhaust gas purification catalyst according to claim 1, wherein a weight ratio of the anchor particles in the catalyst particles is 30 to 70 wt%, and a weight ratio of the enclosure material is 70 to 30 wt%.   The exhaust gas purification catalyst according to any one of claims 1 to 3, wherein the anchor particles contain at least one of cerium and zirconium.   The exhaust gas purification catalyst according to any one of claims 1 to 4, wherein the enclosure material contains at least one of aluminum and silicon.   The exhaust gas purification catalyst according to any one of claims 1 to 5, wherein the noble metal particles contain at least one selected from the group consisting of platinum, palladium, and rhodium. A plurality of catalyst layers containing the exhaust gas purifying catalyst according to any one of claims 1 to 6;
A refractory inorganic carrier carrying the catalyst layer;
An exhaust gas purification catalyst structure comprising: In the manufacturing method of the exhaust gas purification catalyst according to any one of claims 1 to 6,
Preparing a catalyst unit comprising the noble metal particles and anchor particles;
The prepared catalyst unit contained a precursor of the inclusion material and a disappearing material having a ratio (D / L) of a major axis (D) to a minor axis (L) in a cross section of 2.0 or more. Mixing with slurry, drying and firing;
A method for producing an exhaust gas purification catalyst, comprising: