JP6015336B2 - Exhaust gas purification catalyst and method for producing the same - Google Patents

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. More specifically, the present invention relates to an exhaust gas purifying catalyst having high heat resistance and capable of purifying harmful substances in exhaust gas with high efficiency, and a method for producing the same.

  A three-way catalyst that oxidizes or reduces harmful gases (hydrocarbon (HC), carbon monoxide (CO), nitrogen oxide (NOx)) contained in exhaust gas as a catalyst for exhaust gas purification mounted on automobiles, etc. It has been known. 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.

  As a conventional three-way catalyst, the first compound that supports the noble metal particles and suppresses the movement and the first compound that supports the noble metal particles are included, and the first compound is brought into contact with the first compounds. A catalyst comprising a second compound that suppresses aggregation is disclosed. In this catalyst, since the periphery of the first compound carrying the noble metal particles is covered with the second compound, the noble metal particles are prevented from moving and aggregating from the first compound even after long-term use. Therefore, high purification performance can be maintained.

International Publication No. 2007/052627

  However, in recent years, exhaust regulations have been strengthened, so that the use of a three-way catalyst directly under the exhaust manifold is increasing for early activation. In this case, the use environment temperature is increased and may exceed 900 ° C. Further, the manifold catalyst has an exhaust gas processing amount of about 10 to 90 times that of the underfloor catalyst, and high exhaust gas processing performance is required. Therefore, it is necessary to increase the amount of noble metal supported. As a result, even if the exhaust gas purifying catalyst of Patent Document 1 is used, it may be difficult to suppress aggregation of noble metal particles. That is, since the noble metal particles supported on the first compound are aggregated after moving on the second compound covering the periphery, there is a problem that the catalyst performance is lowered.

  The present invention has been made in view of such problems of the prior art. An object of the present invention is to provide an exhaust gas purification catalyst capable of suppressing the aggregation of noble metal particles and maintaining the exhaust gas purification performance even when the use environment temperature is increased. .

The exhaust gas purifying catalyst of the present invention has a plurality of catalyst units including anchor particles supporting noble metal particles, and an enclosure material containing the plurality of catalyst units. Then, the comparative binding energy Ec1 between the anchor particle and the noble metal particle at 900 ° C. calculated by the density functional method and the comparative binding energy Ec2 between the inclusion material and the noble metal particle are in a relationship of Ec1 <Ec2. meets, noble metal particles is summarized in that carried on both the anchor particles and the enclosure material.

  The method for producing an exhaust gas purifying catalyst of the present invention comprises mixing an anchor particle precursor, an inclusion material precursor, a binder that forms an aggregate together with the anchor particle precursor, and a noble metal particle precursor in a solvent. The gist is to dry and bake.

  In the exhaust gas purifying catalyst of the present invention, at 900 ° C., the comparative binding energy Ec1 between the anchor particles and the noble metal particles is smaller than the comparative binding energy Ec2 between the inclusion material and the noble metal particles. Therefore, when a high temperature state of 900 ° C. or higher is reached, noble metal particles that are active sites move from the inclusion material onto the anchor particles and remain on the anchor material, thereby suppressing diffusion and aggregation of the noble metal. Can do.

FIG. 1 is a schematic view showing an exhaust gas purification catalyst and an exhaust gas purification catalyst structure according to an embodiment of the present invention. FIG. 1A is a perspective view showing an exhaust gas purifying catalyst structure. FIG.1 (b) is the schematic which expanded the part of the code | symbol B of Fig.1 (a). FIG.1 (c) is the schematic which expanded the part of the code | symbol C of FIG.1 (b). FIG. 2 is a schematic view showing the state during production and the state after heating to 900 ° C. in the exhaust gas purifying catalyst of the present embodiment. FIG. 3 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. 4 is a graph for explaining a method of manufacturing the exhaust gas purifying catalyst of the present embodiment. FIG. 5 is a schematic view showing an exhaust gas purification system according to an embodiment of the present invention. FIG. 6 is a TEM photograph at the time of manufacturing the exhaust gas purifying catalyst of Example 1. 7 is a TEM photograph after an endurance test on the exhaust gas purifying catalyst of Example 1. FIG.

  Hereinafter, embodiments of the present invention 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]
FIG. 1 shows an exhaust gas purification catalyst and an exhaust gas purification catalyst structure according to this embodiment. In this specification, the exhaust gas purifying catalyst may be simply referred to as “catalyst”, and the exhaust gas purifying catalyst structure may be simply referred to as “catalyst structure”.

  As shown in FIG. 1A, the catalyst structure 1 includes a honeycomb carrier (a refractory inorganic carrier) 2 having a plurality of cells 2a. The exhaust gas flows through each cell 2a along the exhaust gas flow direction F, and is purified by contacting with the catalyst layer there.

  In the catalyst structure 1, a catalyst layer is formed on the inner surface of the carrier 2. Specifically, a catalyst layer 3 is formed on the inner surface of the carrier 2 as shown in FIG. The catalyst layer 3 is formed by a plurality of catalyst particles (exhaust gas purifying catalyst) 5 as shown in FIG.

  The catalyst particles 5 constituting the catalyst layer 3 contain noble metal particles 6 and anchor particles 7. The anchor particle 7 carries the noble metal particle 6 on the surface as an anchor material for the noble metal particle 6. Further, the catalyst particles 5 contain an inclusion material 9 that encloses the composite particles 8 of the noble metal particles 6 and the anchor particles 7 and separates the adjacent composite particles 8 from each other.

  In the catalyst particles 5, the noble metal particles 6 are in contact with and supported on the anchor particles 7, so that the anchor particles 7 act as an anchor material due to chemical bonding and suppress the movement of the noble metal particles 6. Further, the anchor particles 7 carrying the noble metal particles 6 are covered with the inclusion material 9 so as to be encapsulated, so that the movement of the noble metal particles 6 beyond the section separated by the inclusion material 9 is physically performed. To suppress. Further, by including the anchor particles 7 in the section separated by the enclosure material 9, the anchor particles 7 are prevented from contacting and aggregating beyond the section separated by the enclosure material 9. This not only prevents the anchor particles 7 from aggregating, but also prevents the noble metal particles 6 supported on the anchor particles 7 from aggregating. As a result, the catalyst particles 5 can suppress a decrease in catalyst activity due to the aggregation of the noble metal particles 6 without increasing the manufacturing cost and the environmental load. Moreover, the activity improvement effect of the noble metal particle 6 by the anchor particle 7 can be maintained.

  Here, in the catalyst particles 5 shown in FIG. 1C, in the region separated by the inclusion material 9, the catalyst unit 10 containing the secondary particles in which the primary particles of the anchor particles 7 are aggregated and the noble metal particles 6. Is included. However, the anchor particles 7 may exist as primary particles in a region separated by the inclusion material 9. That is, the catalyst unit 10 may contain the primary particles of the anchor particles 7 and the noble metal particles 6.

  As described above, since both the noble metal particles 6 and the anchor particles 7 are encapsulated by the inclusion material 9, aggregation of the noble metal particles 6 can be suppressed. However, when the exhaust gas purifying catalyst according to the present embodiment is prepared by the manufacturing method described later, the noble metal particles are supported not only on the surface of the anchor particles 7 but also on the surface of the enclosure material 9. That is, in the exhaust gas purifying catalyst of this embodiment, the noble metal particles 6 are supported on the surface of the enclosure material 9 while the noble metal particles 6 are supported on the surface of the anchor particles 7.

  Here, when the noble metal particles 6A are present on the inclusion material 9, the noble metal particles 6A may come into contact with adjacent noble metal particles and aggregate. That is, the noble metal particles 6 supported on the anchor particles 7 are prevented from contacting and aggregating between the noble metal particles due to the chemical interaction with the anchor particles 7 and the inclusion structure of the inclusion material 9. However, the noble metal particles 6 </ b> A supported on the enclosure material 9 do not cause a chemical interaction with the anchor particles 7, and are not included in the enclosure material 9. Therefore, there is a possibility that the noble metal particles 6A on the enclosure material 9 aggregate with each other under a high temperature condition of 900 ° C. or higher.

  Therefore, in the exhaust gas purifying catalyst of the present embodiment, at 900 ° C., the comparative binding energy Ec1 between the anchor particle 7 and the noble metal particles 6 and 6A, and the inclusion material 9 and the noble metal particles 6 and 6A The comparative binding energy Ec2 needs to satisfy the relationship Ec1 <Ec2. The comparative binding energies Ec1 and Ec2 are calculated by simulation using the density functional method.

  More specifically, the 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, and its principle is as follows. When the electron density ρ of a ground state of a system is determined, the external potential having it in the ground state is determined in one way. Further, the number of electrons N of the system can also be obtained by integrating the electron density over the entire space. Solving the Hamiltonian H Schrödinger equation derived from the external potential and the number of electrons, the wave function Ψ of the electron system allowed under the external potential can be found, so that any physical quantity can be obtained from the wave function Ψ. . In other words, in principle, any physical quantity of the system can be calculated from the ground state electron density, including the quantity related to the excited state.

  The comparative binding energy calculated by the simulation using the density functional method is a numerical value indicating the stability between the noble metal particle, the anchor particle and the inclusion material. It shows that the nature is high. Therefore, at 900 ° C., the fact that the comparative binding energies Ec1 and Ec2 satisfy the relationship of Ec1 <Ec2 means that at this temperature, the anchor particles 7 and the noble metal particles are more stable than the stability between the inclusion material 9 and the noble metal particles 6A. It shows that the stability between 6 is higher. Therefore, when the exhaust gas purifying catalyst is 900 ° C. or higher, the noble metal particles 6A supported on the enclosure material 9 move onto the anchor particles 7 having higher stability. That is, as shown in FIG. 2, even when noble metal particles are supported on both the anchor particles 7 and the enclosure material 9 at the time of manufacture, the noble metal particles 6A are separated from the enclosure material 9 in a high temperature state. It moves onto the anchor particle 7 through the interface between the anchor particle 7 and the inclusion material 9. Therefore, in the high temperature region, the noble metal particles that are active sites remain on the anchor material, and as a result, aggregation of the noble metal on the enclosure material 9 can be suppressed. 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: 900 ° C, approximation: GGA approximation

  Moreover, the difference (Ec2-Ec1) between the comparative binding energy Ec1 between the anchor particle and the noble metal particle and the comparative binding energy Ec2 between the inclusion material and the noble metal particle is 10.0 cal / mol or more. Is preferred. When Ec2-Ec1 is 10.0 cal / mol or higher, the noble metal particles 6A supported on the inclusion material 9 easily move onto the anchor particles 7 when the temperature is 900 ° C. or higher. It becomes easier to suppress. In addition, it is more preferable that Ec2-Ec1 is 15.0 cal / mol or more.

  The noble metal particles 6 are at least one selected from the group consisting of platinum (Pt), palladium (Pd), rhodium (Rh), gold (Au), silver (Ag), iridium (Ir), and ruthenium (Ru). Can be contained. Of these noble metals, platinum, palladium and rhodium are the most cost-effective and are suitable as noble metals for use in exhaust gas purification catalysts.

  Here, as described above, when the exhaust gas purifying catalyst according to the present embodiment is used as a manifold catalyst, it is necessary to increase the amount of noble metal supported as compared with the underfloor catalyst. Therefore, it is preferable to use palladium, which is relatively inexpensive relative to platinum or rhodium. Accordingly, the noble metal particles 6 are preferably composed mainly of palladium, and specifically, the noble metal particles 6 preferably contain 50 mol% or more of palladium. As a result, the cost of the exhaust gas purification catalyst can be reduced.

In addition, as the anchor particles 7, it is preferable to use those that act as an anchor material by chemical bonding with respect to the noble metal particles 6 and further satisfy the relation of the comparative binding energy. Specifically, the anchor particle 7 preferably contains an inorganic oxide such as aluminum oxide (Al ₂ O ₃ ), zirconium oxide (ZrO ₂ ), cerium oxide (CeO ₂ ), and titanium oxide (TiO ₂ ). . By containing such an inorganic oxide having high heat resistance, it becomes possible to suppress aggregation of the anchor particles 7 under high temperature conditions while satisfying the relation of the comparative binding energy. From the viewpoint of improving heat resistance, the anchor particles 7 preferably contain at least one selected from the group consisting of aluminum oxide, zirconium oxide, cerium oxide, and titanium oxide as a main component. In the present specification, the main component means a component having a content of 50 mol% or more in the particles.

  Furthermore, in order to adjust the comparative binding energy Ec1 between the anchor particles 7 and the noble metal particles 6, lanthanum (La), iron (Fe), manganese (Mn), cobalt (Co), nickel (Ni), neodymium It may further contain at least one transition metal oxide selected from the group consisting of (Nd) and yttrium (Y). By containing at least one of these transition metal oxides, catalytic performance, in particular, CO and NOx purification performance can be improved by the active oxygen contained in the transition metal while controlling the comparative binding energy Ec1. In addition, in the anchor particle 7, it is preferable that content of the said transition metal oxide exists in the range of 1-10 mol%. When the content of the transition metal oxide is within this range, the crystal structure of the inorganic oxide can be maintained, so that anchor particles having excellent heat resistance can be obtained.

The anchor particles 7 are preferably composed mainly of aluminum oxide (Al ₂ O ₃ ). Further, the anchor particles 7 may be formed only from aluminum oxide, and further used are those obtained by adding zirconium oxide (ZrO ₂ ), cerium oxide (CeO ₂ ) and lanthanum oxide (La ₂ O ₃ ) to aluminum oxide. It is preferable to do. When the anchor particles 7 contain aluminum oxide as a main component, as will be described later, the comparative binding energies Ec1 and Ec2 can easily satisfy the relationship of Ec1 <Ec2, and the high-temperature purification performance can be improved.

The enclosure material 9 preferably contains at least one of zirconium oxide (ZrO ₂ ) and cerium oxide (CeO ₂ ). The inclusion material 9 is preferably a material that can include anchor particles and ensure gas permeability. Zirconium oxide and cerium oxide have a large pore volume, and can secure higher gas diffusibility. The inclusion material may be a complex oxide of zirconium oxide and cerium oxide. In addition, it is preferable that the inclusion material 9 is mainly composed of zirconium oxide. When the clathrate 9 is mainly composed of zirconium oxide, the comparative binding energies Ec1 and Ec2 can easily satisfy the relationship of Ec1 <Ec2.

  Here, it is preferable that the mass ratio of the anchor particles 7 in the catalyst particles 5 is 30 to 70 mass%, and the mass ratio of the inclusion material 9 is 70 to 30 mass%. When the mass ratio of the anchor particles 7 is 30% by mass or more, the density of the noble metal supported on the anchor particles is reduced. That is, since the amount of noble metal supported on one anchor particle is reduced, noble metal particles are not greatly enlarged even if the noble metal particles are aggregated on the anchor particles after many years of use. 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 mass ratio of an anchor particle is 70 mass% or less. Therefore, even after long-term use, aggregation of anchor particles and accompanying aggregation of noble metal particles can be suppressed.

  Here, the enclosure material 9 used in the catalyst particles 5 does not completely surround the periphery of the catalyst unit 10. In other words, the enclosure material 9 has pores that allow the exhaust gas and active oxygen to pass through while covering the catalyst unit 10 so as to suppress physical movement. Specifically, as shown in FIG. 1C, the enclosure material 9 appropriately encloses the catalyst unit 10 and suppresses aggregation of the catalyst units. Furthermore, since the enclosure material 9 has a plurality of pores 9a, exhaust gas and active oxygen can pass therethrough. The pore diameter of the pores 9a is preferably 30 nm or less, and more preferably 10 nm to 30 nm. This pore diameter can be determined by a gas adsorption method.

  In addition, it is preferable that the average particle diameter of the catalyst unit 10 contained in the division separated by the enclosure material 9 is 300 nm or less. Therefore, it is preferable that the average secondary particle diameter of the anchor particles 7 included in the catalyst unit 10 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 10 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 10 and the average secondary particle diameter of the anchor particle 7 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 10 is preferably larger than the average pore diameter of the pores 9 a formed in the enclosure material 9. Therefore, it is more preferable that the average particle diameter of the catalyst unit 10 and the average secondary particle diameter of the anchor particles 7 exceed 30 nm.

  The average secondary particle diameter of the enclosure material 9 is not particularly limited as long as the pores 9a can be formed while covering the periphery of the catalyst unit 10.

  The average secondary particle diameter of the anchor particles and the clathrate can be determined by applying a slurry containing these particles to a laser diffraction particle size distribution measuring device in the production process of the catalyst particles. In addition, the average secondary particle diameter in this case means a median diameter (D50). Moreover, the average secondary particle diameter and the particle diameter of noble metal particles described later can also be measured from a photograph of a transmission electron microscope (TEM) of the obtained catalyst powder. Furthermore, the average particle diameter of the catalyst unit 10 can also be measured from a TEM photograph.

  The average particle diameter of the noble metal particles 6 is desirably in the range of 2 nm to 10 nm. When the average particle diameter of the noble metal particles 6 is 2 nm or more, aggregation due to movement of the noble metal particles 6 themselves can be reduced. Moreover, when the average particle diameter of the noble metal particles 6 is 10 nm or less, a decrease in reactivity with the exhaust gas can be suppressed.

  Here, regarding the catalyst unit 10 containing the noble metal particles 6 and the anchor particles 7, the average particle diameter Da of the catalyst unit 10 and the average fineness of the pores 9 a formed in the enclosure material 9 that encloses the catalyst unit 10. It is preferable that the hole diameter Db satisfies the relationship Db <Da. That is, as shown in FIG. 1 (c), Db <Da means that the average particle diameter Da of the catalyst unit 10 is larger than the average diameter Db of the pores 9 a of the enclosure material 9. When Db <Da, the movement of the composite particles 8 of the noble metal particles 6 and the anchor particles 7 through the pores 9a formed in the inclusion material 9 is suppressed. Therefore, aggregation with the composite particles 8 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. 3 shows the ratio of the average particle diameter Da of the catalyst unit 10 before the exhaust durability test to the average pore diameter Db of the inclusion material (Da / Db) on the horizontal axis, and ceria as anchor particles 7 after the exhaust durability test. It is a graph which shows these relations, making the crystal growth ratio and the surface area of platinum as the noble metal particle 6 the vertical axis. FIG. 3 shows that when Da / Db exceeds 1, the ceria crystal growth ratio is remarkably lowered, and the ceria is less sintered. Further, it can be seen that even after the durability test, the platinum surface area is maintained in a high state, and aggregation of platinum is suppressed.

[Exhaust gas purification catalyst structure]
The exhaust gas purifying catalyst structure 1 of this embodiment includes a catalyst layer 3 containing an exhaust gas purifying catalyst 5 and a refractory inorganic carrier 2 that supports the catalyst layer 3. As described above, the exhaust gas purifying catalyst 5 of the present embodiment can maintain the fine state of the noble metal particles even at 900 ° C. or higher. And by applying such a catalyst 5 to the refractory inorganic carrier 2 to form the catalyst layer 3, the pressure loss of the catalyst layer 3 is reduced, and the thermal stability, thermal shock resistance and mechanical strength are increased. be able to.

  As the refractory inorganic carrier 2, a cordierite honeycomb carrier can be used. Cordierite honeycomb carriers are excellent in heat resistance, impact resistance and manufacturing cost, and are generally used as carriers for automobile exhaust gas purification catalysts. Although the cross-sectional shape of the flow path (cell 2a) includes a quadrangle and a hexagon, any shape can be used in this embodiment. As the refractory inorganic carrier 2, a stainless steel 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.

[Method for producing exhaust gas purifying catalyst]
Next, a method for manufacturing the exhaust gas purifying catalyst according to the present embodiment will be described.

  As shown in FIG. 4, in the manufacturing method of this embodiment, the anchor particle precursor is first pulverized to prepare an anchor particle precursor having a desired particle diameter. As described above, the particle diameter of the anchor particles can be set to 300 nm, for example. The crushing step can be omitted by using a fine raw material such as an oxide colloid as the raw material for the anchor particles.

  Next, the clathrate material is pulverized to prepare a clathrate precursor having a desired particle size. In addition, a crushing process can be skipped by using fine raw materials, such as an oxide colloid, as a raw material of an enclosure material.

  Then, the anchor particle precursor, the inclusion material precursor, the binder that forms an aggregate together with the anchor particle precursor, and the noble metal particle precursor are mixed in a solvent to prepare a mixture. As the binder that forms an aggregate with the anchor particle precursor, a compound that preferentially aggregates with the anchor particle precursor in a solvent can be used. For example, when the anchor particle precursor is mainly composed of alumina, boehmite can be used as the binder. In the case of platinum, examples of the noble metal particle precursor include chloroplatinic acid and an ammonia complex of platinum. In the case of palladium, examples include palladium chloride, palladium nitrate, palladium acetate, and an ammonia complex of palladium. In the case of rhodium, rhodium chloride, rhodium acetate, rhodium nitrate, and an ammonia complex of rhodium are exemplified. As the solvent, for example, ion exchange water can be used.

  Next, the mixture is further stirred and dispersed. At this time, the anchor particle precursor and the binder aggregate to form an aggregate, and the inclusion material precursor adheres around the aggregate. Then, the exhaust gas purifying catalyst according to the present embodiment can be obtained by drying and firing the mixture after dispersion. The drying is preferably performed by spray drying.

  In addition, the exhaust gas purifying catalyst of the present embodiment may have a small ratio of covering the periphery of the anchor particles with the inclusion material at the time of manufacture. However, as the temperature rises, the noble metal particles move onto the anchor particles having high stability, and the inclusion material is thermally aggregated to be present around the anchor particles. Therefore, finally, the inclusion structure shown in FIG. 2 is formed, and aggregation of the noble metal particles and the anchor material is suppressed by the inclusion material.

[Method of producing exhaust gas purification catalyst structure]
Next, the manufacturing method of the exhaust gas purifying catalyst structure of this embodiment will be described. In this manufacturing method, the exhaust gas purifying catalyst prepared as described above is first pulverized. This pulverization may be either wet or dry. Usually, the exhaust gas purifying catalyst is mixed with a solvent such as ion exchange water and stirred, and then pulverized using a ball mill or the like. Thereby, a catalyst slurry in which the exhaust gas purifying catalyst is dispersed in the solvent is obtained. At this time, a binder is added to the catalyst slurry as necessary. The average particle size (D50) of the exhaust gas purifying catalyst in the catalyst slurry can be 1 μm to 20 μm, but preferably 6 μm or less.

  Thereafter, the catalyst slurry is applied to the inner surface of a refractory inorganic carrier (honeycomb carrier), dried and fired, whereby an exhaust gas purification catalyst structure can be obtained.

[Exhaust gas purification system]
As shown in FIG. 5, the exhaust gas purification system 30 of the present embodiment can be configured such that exhaust gas purification catalyst structures 33 </ b> A and 33 </ b> B are disposed in the exhaust gas flow path 32 of the internal combustion engine 31. It is preferable to use a catalyst structure having an exhaust gas purification catalyst in at least one of the exhaust gas purification catalyst structures 33A and 33B.

  By configuring the exhaust gas purification system 30 of this embodiment in such a configuration, the exhaust gas purification catalyst structures 33A and 33B can be activated at an early stage, and the exhaust gas can be purified even in a low temperature range. In particular, the exhaust gas purifying catalyst structure of the present embodiment can suppress aggregation of noble metal particles even in an extremely high temperature state, and can therefore be disposed close to the exhaust manifold 34. Since the catalyst structure can be activated at an early stage by being provided directly under the exhaust manifold 34, the exhaust gas can be efficiently purified from a low temperature. In addition, the exhaust gas purification system of this embodiment is not restricted to the structure shown in FIG. For example, a three-way catalyst or a NOx adsorption catalyst may be further provided before and after the exhaust gas purification catalyst structures 33A and 33B. Moreover, the exhaust gas purification system 30 of this embodiment can use a gasoline engine, a lean burn engine, a direct injection engine, a diesel engine, etc. for various internal combustion engines.

  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]
First, a composite oxide of alumina (Al ₂ O ₃ ) and zirconia (ZrO ₂ ) is prepared as an anchor material precursor, and only the anchor material precursor is pulverized by a wet pulverizer, and the average particle diameter (D50) is 150 nm. I made it. Next, a composite oxide of zirconia and ceria was prepared as an inclusion material precursor, and only the inclusion material precursor was pulverized by a wet pulverizer, so that the average particle diameter (D50) was 700 nm. Note that the alumina and zirconia composite oxide has a molar ratio of alumina and zirconia of 97: 3, and the zirconia and ceria composite oxide has a molar ratio of zirconia and ceria of 80:20. For measurement of the average particle size, a laser diffraction / scattering particle size distribution measuring apparatus LA-920 manufactured by Horiba, Ltd. was used.

  Next, the pulverized anchor material precursor, the inclusion material precursor, and boehmite as a binder are mixed so as to have a mass ratio of 35:55:10, a palladium nitrate solution is further mixed, and the catalyst slurry is stirred. Was prepared. The amount of palladium nitrate solution added was adjusted so that the amount of palladium metal in the resulting exhaust gas purifying catalyst was 1.12% by mass.

  Further, the obtained catalyst slurry was dried and then calcined at 400 ° C. for 1 hour to prepare a Pd catalyst powder. In addition, drying used the spray drying apparatus and made inlet temperature 350 degreeC. Moreover, the muffle furnace was used for baking.

  Then, the Pd catalyst powder, alumina sol, water and nitric acid were put into a magnetic ball mill, mixed and pulverized to prepare a Pd catalyst slurry. Thereafter, the Pd catalyst slurry was adhered to a cordierite monolith support, and excess slurry in the cell was removed by an air flow. Furthermore, after drying the support | carrier with a slurry at 130 degreeC, the catalyst structure of Example 1 which provided the Pd catalyst layer was prepared by baking at 400 degreeC for 1 hour.

[Example 2]
A catalyst structure of Example 2 was prepared in the same manner as Example 1 except that a composite oxide of alumina and ceria (CeO ₂ ) having a molar ratio of 97: 3 was used as the anchor material precursor.

[Example 3]
A catalyst structure of Example 3 was prepared in the same manner as Example 1 except that a complex oxide of alumina and lanthanum oxide (La ₂ O ₃ ) having a molar ratio of 97: 3 was used as the anchor material precursor. .

[Example 4]
A catalyst structure of Example 4 was prepared in the same manner as Example 1 except that alumina was used as the anchor material precursor.

[Comparative Example 1]
First, a composite oxide of zirconia (ZrO ₂ ) and ceria (CeO ₂ ) having a molar ratio of 80:20 was prepared as an anchor material precursor. Next, after the composite oxide was dispersed in ion-exchanged water, a palladium nitrate solution was mixed and stirred to support palladium on the composite oxide . The amount of palladium nitrate solution added was adjusted so that the amount of palladium metal in the resulting exhaust gas purifying catalyst was 3.201% by mass. And after drying the slurry of complex oxide which carry | supported palladium, Pd carrying | support complex oxide was prepared by baking at 400 degreeC for 1 hour. In addition, the muffle furnace was used for baking.

  Next, the obtained Pd-supported composite oxide powder was pulverized by a wet pulverizer, and the average particle size (D50) was adjusted to 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.

Furthermore, a composite oxide of zirconia (ZrO ₂ ) and ceria (CeO ₂ ) having a molar ratio of 80:20 was separately prepared and pulverized with a wet pulverizer, so that the average particle size (D50) was 150 nm.

Boehmite (cladding material precursor), 10% nitric acid and water were mixed and stirred for 1 hour to prepare a boehmite aqueous solution. Then, the pulverized Pd-supported composite oxide and zirconia-ceria composite oxide were slowly added to the boehmite aqueous solution, and stirred for 2 hours using a high-speed stirrer. At this time, the total of Pd-supporting composite oxide and zirconia-ceria composite oxide and boehmite were mixed so that the mass ratio was 70:30 . The obtained slurry was rapidly dried and further dried at 150 ° C. for a whole day and night to remove moisture. Then, Pd catalyst powder was prepared by baking in the air at 550 degreeC for 3 hours. In addition, rapid drying used the spray-drying apparatus and made inlet temperature 350 degreeC. Moreover, the muffle furnace was used for baking.

  Then, the Pd catalyst powder, alumina sol, water and nitric acid were put into a magnetic ball mill, mixed and pulverized to prepare a Pd catalyst slurry. Thereafter, the Pd catalyst slurry was adhered to a cordierite monolith support, and excess slurry in the cell was removed by an air flow. Furthermore, after drying the support | carrier with a slurry at 130 degreeC, the catalyst structure of the comparative example 1 which provided the Pd catalyst layer was prepared by baking at 400 degreeC for 1 hour.

  Table 1 shows the precious metal species of Examples 1 to 4 and Comparative Example 1, the anchor material compound, the inclusion material compound, and the mixing ratio of the anchor material and the inclusion material. Table 1 also shows the comparative binding energy Ec1 between the anchor material and the noble metal, the comparative binding energy Ec2 between the inclusion material and the noble metal, and Ec2-Ec1.

[Durability test method]
The catalysts of Examples 1 to 4 and Comparative Example 1 were mounted on an exhaust system of a gasoline engine with a displacement of 3500 cc, and the exhaust gas temperature at the catalyst inlet was set to 900 ° C. for 30 hours to deteriorate each catalyst. After that, each deteriorated catalyst was attached to the exhaust system of a 3500 cc gasoline engine, and the HC residual ratio, CO residual ratio and NOx residual ratio at 400 ° C. were measured from the following equations.

HC remaining rate (%) = [HC emission amount at the catalyst outlet (g / test)] / [HC emission amount at the catalyst inlet (g / test)] × 100
CO residual rate (%) = [CO emission amount at the catalyst outlet (g / test)] / [CO emission amount at the catalyst inlet (g / test)] × 100
NOx remaining rate (%) = [NOx emission at catalyst outlet (g / test)] / [NOx emission at catalyst inlet (g / test)] × 100

  Table 2 shows the HC residual ratio, CO residual ratio, and NOx residual ratio of Examples 1 to 4 and Comparative Example 1 measured by the above measurement method.

  As shown in Tables 1 and 2, Examples 1 to 4 satisfying the relationship of Ec1 <Ec2 showed lower values of the HC residual rate, the CO residual rate, and the NOx residual rate than those of Comparative Example 1. In addition, as described above, the exhaust gas purifying catalysts of Examples 1 to 4 have a precious metal amount of 1.12% by mass, which is significantly smaller than the 3.201% by mass of Comparative Example 1, but the residual rate. Was low. This is presumably because, under high temperature conditions, the noble metal particles supported on the inclusion material moved onto anchor particles having higher stability, and aggregation was suppressed.

  FIG. 6 is a TEM photograph at the time of production of the exhaust gas purifying catalyst of Example 1, and FIG. 7 is a TEM photograph after the endurance test in the exhaust gas purifying catalyst of Example 1. In FIG. 6, since the particle diameter of palladium is several nm, it cannot be clearly discriminated, but in FIG. 7, an aggregate of palladium can be confirmed. Then, as shown in FIG. 7, since the periphery of palladium is covered with the clathrate, it is confirmed that the palladium has moved from the clathrate onto the anchor particles and remains on the anchor material during the durability test. it can.

  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.

5 Exhaust gas purification catalyst 6, 6A Precious metal particles 7 Anchor particles 9 Inclusion material 9a Pore 10 Catalyst unit

Claims (5)

A plurality of catalyst units comprising noble metal particles and anchor particles supporting noble metal particles as an anchor material for the noble metal particles;
An enclosure material containing the plurality of catalyst units and separating the catalyst units from each other;
Have
A comparative binding energy Ec1 between the anchor particle and the noble metal particle, and a comparative binding energy Ec2 between the inclusion material and the noble metal particle, which are calculated by a simulation at 900 ° C. using a density functional method. but, to satisfy the relationship of Ec1 <Ec2,
The exhaust gas purifying catalyst , wherein the noble metal particles are supported on both the anchor particles and the inclusion material .   The exhaust gas purifying catalyst according to claim 1, wherein a difference (Ec2-Ec1) between the comparative binding energy Ec1 and the comparative binding energy Ec2 is 10.0 cal / mol or more. When said exhaust gas purifying catalyst 900 ° C., the noble metal particles supported on the enclosure materials on the exhaust gas purification according to claim 1 or 2, characterized in that migrate to the surface of the anchor particles Catalyst. The noble metal particles containing palladium least 50 mol%, wherein the anchor member contains alumina least 50 mol%, wherein the packaging Sezzai is any one of claims 1 to 3, characterized by containing zirconia or 50 mol% The exhaust gas purifying catalyst according to Item. The precursor of the anchor particle, the precursor of the inclusion material, the binder that forms an aggregate together with the precursor of the anchor particle, and the precursor of the noble metal particle are mixed in a solvent to prepare a mixture. Process,
Drying and firing the mixture;
The method for producing an exhaust gas purifying catalyst according to any one of claims 1 to 4 , wherein: