WO2024042935A1 - Exhaust gas purifying catalyst, and catalyst body using same - Google Patents

Abstract

The present invention provides an exhaust gas purifying catalyst which has high purification activity of Rh even when exposed to high temperatures for a long period of time. The exhaust gas purifying catalyst disclosed herein comprises a base material and a catalytic noble metal supported on the base material. The base material comprises La-containing alumina particles and a zirconia coating arranged on the surface of the alumina particles. The catalytic noble metal contains at least Rh. The mass ratio of zirconia in the base material is at least 5 mass%. The average particle diameter of the zirconia coating, as determined by a focused ion beam scanning electron microscope, is less than 100 nm.

Description

Exhaust gas purification catalyst and catalyst body using the same

The present invention relates to an exhaust gas purifying catalyst. The present invention also relates to a catalyst body for exhaust gas purification using the same. This application claims priority based on Japanese Patent Application No. 2022-134003 filed on August 25, 2022, and the entire content of that application is incorporated herein by reference. There is.

Exhaust gas emitted from internal combustion engines such as automobile engines contains harmful components such as hydrocarbons (HC), carbon monoxide (CO), and nitrogen oxides (NOx). In order to efficiently react and remove these harmful components from exhaust gas, exhaust gas purifying catalysts in which noble catalytic metals are supported on a base material such as alumina have been used. Rh, which has high NOx reduction performance, is mainly used as a catalytic noble metal for NOx purification.

A technique is known that uses alumina particles coated with zirconia as a base material that supports Rh (see, for example, Patent Documents 1 and 2). Patent Document 1 discloses that when a zirconia coating is provided on the alumina particles of the base material, the zirconia coating can suppress Rh from forming a solid solution in the alumina and improve the durability of the exhaust gas purification catalyst. is listed. In Patent Document 2, alumina particles coated with zirconia used as a base material are produced by a coprecipitation method.

Japanese Patent Application Publication No. 2005-103410 Japanese Patent Application Publication No. 2007-301530

As a result of intensive studies by the present inventors on exhaust gas purification catalysts in which Rh is supported on alumina particles coated with zirconia, it was found that in the above conventional technology, the Rh purification activity decreases when exposed to high temperatures for a long time. I found out that there is a problem.

The present invention has been made in view of the above circumstances, and its purpose is to provide an exhaust gas purifying catalyst that has high Rh purifying activity even when exposed to high temperatures for a long time.

As a result of intensive studies, the present inventors succeeded in making the zirconia coating finer by manufacturing alumina, which is a base material, with a zirconia coating using a spray drying method. They have also found that when Rh is supported on an alumina base material coated with finely divided zirconia, the purification activity of Rh becomes sufficiently high even when exposed to high temperatures for a long time.

That is, the exhaust gas purifying catalyst [1] disclosed herein includes a base material and a catalytic noble metal supported by the base material. The base material includes La-containing alumina particles and a zirconia coating disposed on the surface of the alumina particles. The catalytic noble metal contains at least Rh. The mass percentage of zirconia in the base material is 5% by mass or more. The average grain size of the zirconia coating, as determined by focused ion beam scanning electron microscopy, is less than 100 nm.

According to the findings of the present inventors, in the conventional exhaust gas purification catalyst described above, the average particle size of the zirconia coating of the base material is several hundred nm or more, and therefore, when exposed to high temperatures for a long period of time, The solid solution of Rh into alumina is not sufficiently suppressed. Therefore, when exposed to high temperatures for a long time, the Rh purification activity decreases. However, the exhaust gas purifying catalyst [1] disclosed herein contains a predetermined amount of zirconia, and the average particle size of the zirconia coating is small. Therefore, in the exhaust gas purifying catalyst [1] disclosed herein, the zirconia coating is highly dispersed on the surface of the alumina particles. Therefore, the proportion of Rh existing on or near the zirconia coating increases, and the zirconia coating prevents Rh from forming a solid solution in the alumina of the base material when exposed to high temperatures for a long time. Can be suppressed. Therefore, the exhaust gas purification catalyst [1] disclosed herein has high Rh purification activity even when exposed to high temperatures for a long time.

The exhaust gas purification catalyst [2] disclosed herein is the same as the exhaust gas purification catalyst [1], in which a quantitative line analysis of Zr is performed from the surface of the alumina particles toward the center using a field emission type electron probe analyzer. When the depth from the surface to the center is expressed with the surface as 0% and the center as 100%, the peak top of Zr intensity exists within the range of 0% to 20%.

According to such a configuration, the proportion of Rh existing on or near the zirconia coating becomes higher, and solid solution of Rh into the alumina of the base material is further suppressed when exposed to high temperatures for a long period of time. can do. Therefore, even when exposed to high temperatures for a long time, the Rh purification activity becomes higher.

In the exhaust gas purification catalyst [3] disclosed herein, in the exhaust gas purification catalyst [1] or [2], the mass proportion of zirconia in the base material is 9 mass% or more and 30 mass% or less. At this time, the Rh purification activity becomes particularly high when exposed to high temperatures for a long time.

In the exhaust gas purification catalyst [4] disclosed herein, in any of the above exhaust gas purification catalysts [1] to [3], the average particle size of the zirconia coating is 5 nm or more and 80 nm or less. At this time, the Rh purification activity becomes particularly high when exposed to high temperatures for a long time.

The exhaust gas purifying catalyst body [5] disclosed herein includes a base material and a catalyst layer provided on the base material. The catalyst layer contains any one of the exhaust gas purifying catalysts [1] to [4].

According to such a configuration, it is possible to provide an exhaust gas purifying catalyst body that has high Rh purifying activity even when exposed to high temperatures for a long time.

The catalyst body for exhaust gas purification [6] disclosed herein is the catalyst body for exhaust gas purification [5], in which the catalyst layer includes a first partial catalyst layer located on the base material side, and a surface layer of the catalyst layer. a second partial catalyst layer located on the side. The first partial catalyst layer contains an exhaust gas purifying catalyst in which the catalytic noble metal is at least one of Pt and Pd. The second partial catalyst layer contains any one of the exhaust gas purifying catalysts [1] to [4].

According to such a configuration, it is possible to provide a catalyst body for exhaust gas purification that not only has particularly high purification performance for NOx contained in exhaust gas but also has particularly high purification performance for HC and CO.

FIG. 1 is a cross-sectional view schematically showing an exhaust gas purifying catalyst according to the present embodiment. FIG. 1 is a schematic diagram showing an exhaust gas purification system according to the present embodiment. 3 is a perspective view schematically showing the exhaust gas purifying catalyst body of FIG. 2. FIG. FIG. 4 is a partial cross-sectional view of the exhaust gas purifying catalyst body of FIG. 3 cut in the cylinder axis direction. FIG. 5 is a partial cross-sectional view showing the configuration of a modification of the exhaust gas purifying catalyst body of FIG. 4. FIG. This is an FIB-SEM image of the exhaust gas purifying catalyst of Example 4 observed at a measurement magnification of 300,000 times. This is an FIB-SEM image of the exhaust gas purification catalyst of Comparative Example 4 observed at a measurement magnification of 3,000 times.

Hereinafter, preferred embodiments of the present invention will be described with reference to the drawings. Note that matters other than those specifically mentioned in this specification that are necessary for implementing the present invention can be understood as matters designed by those skilled in the art based on the prior art in the relevant field. The present invention can be implemented based on the content disclosed in this specification and the common general knowledge in the field. Further, in the following drawings, members and parts that have the same function are given the same reference numerals, and overlapping explanations may be omitted or simplified. The dimensional relationships (length, width, thickness, etc.) in each figure do not necessarily reflect the actual dimensional relationships. In addition, in this specification, the notation "A to B" (A and B are arbitrary numerical values) indicating a range means not less than A but not more than B, as well as "preferably larger than A" and "preferably smaller than B." includes the meaning of

<<Catalyst for exhaust gas purification>>
FIG. 1 is a schematic cross-sectional view of an exhaust gas purifying catalyst 100, which is an example of the exhaust gas purifying catalyst disclosed herein. The exhaust gas purifying catalyst 100 includes a base material 30 and a catalytic noble metal 40, as shown in FIG.

The catalytic noble metal 40 contains at least Rh. Since the catalyst noble metal 40 contains Rh, NOx in the exhaust gas can be efficiently purified. The catalyst noble metal 40 contains a noble metal other than Rh (i.e., gold (Au), silver (Ag), platinum (Pt), palladium (Pd), iridium (Ir), ruthenium (Ru), osmium (Os)). You can leave it there. The amount of Rh in the catalytic noble metal 40 is preferably 80% by mass, more preferably 90% by mass, even more preferably 95% by mass, and most preferably 100% by mass (that is, the catalytic noble metal 40 is Rh only).

The catalytic noble metal 40 is typically in the form of particles. The average particle size of the catalyst noble metal 40 is not particularly limited. From the viewpoint of increasing the contact area with exhaust gas, the catalyst noble metal 40 is preferably fine particles with a sufficiently small particle size. The average particle size of the catalyst noble metal 40 is, for example, 15 nm or less, preferably 10 nm or less, and more preferably 5 nm or less. The lower limit of the average particle size of the catalyst noble metal 40 is not particularly limited, and the average particle size of the catalyst noble metal 40 may be, for example, 1 nm or more. Note that the average particle diameter of the catalyst noble metal 40 can be determined as the average value of the particle diameters of 50 or more catalyst noble metals determined by transmission electron microscopy (TEM) observation.

The supported amount of the catalyst noble metal 40 is not particularly limited, and can be appropriately determined depending on the design of the exhaust gas purification catalyst body using the exhaust gas purification catalyst 100. The amount of catalyst noble metal 40 supported is, for example, 0.01% by mass or more, preferably 0.05% by mass or more, and more preferably 0.1% by mass or more, based on the mass of base material 30. Further, the amount of the catalyst noble metal 40 supported is, for example, 10% by mass or less, preferably 5% by mass or less, and more preferably 3% by mass or less, based on the mass of the base material 30.

As illustrated, the base material 30 includes La-containing alumina particles 32 (also simply referred to as "alumina particles 32") and a zirconia coating 34 disposed on the surface of the alumina particles 32. In other words, the base material 30 of the exhaust gas purifying catalyst 100 is La-containing alumina particles 32 having a coating 34 of zirconia.

The average particle size of the base material 30 is not particularly limited. The average particle size of the base material 30 is, for example, 0.5 μm or more, preferably 1 μm or more, and more preferably 3 μm or more. The average particle size of the base material 30 is, for example, 200 μm or less, preferably 100 μm or less, and more preferably 60 μm or less. Note that the average particle size of the base material 30 can be determined as a median diameter (D50: volume basis) by measurement based on a laser diffraction/scattering method.

The alumina particles 32 contain La. Therefore, the alumina particles 32 are particles of lanthanum-alumina composite oxide. When the alumina particles 32 contain La, the heat resistance of the exhaust gas purification catalyst 100 is improved. Further, the fact that the alumina particles 32 contain La also contributes to suppressing solid solution of Rh into alumina when exposed to high temperatures for a long time. The configuration of the La-containing alumina particles 32 may be the same as or similar to known La-containing alumina particles used as a support for a three-way catalyst. The content of La in the alumina particles 32 is not particularly limited, and is, for example, 0.1% by mass or more, preferably 0.5% by mass or more, more preferably 1% by mass or more in terms of La ₂ O ₃ It is. Further, the content of La in the alumina particles 32 is, for example, 30% by mass or less, preferably 20% by mass or less, and more preferably 10% by mass or less, in terms of La ₂ O ₃ . The content of La in the alumina particles 32 can be determined by measurement using an X-ray fluorescence analyzer (XRF).

The alumina particles 32 contain components other than Al ₂ O ₃ and La ₂ O ₃ (for example, Pr ₂ O ₃ , Nd ₂ O 3 ) within a range that does not significantly inhibit the effects of the present invention (for example, 10% by mass or _less) . , Y ₂ O ₃ or other rare earth element oxides other than La, preferably Y ₂ O ₃ ).

The shape of the alumina particles 32 is not particularly limited, and may be approximately spherical, approximately ellipsoidal, irregular, etc. The alumina particles 32 are usually secondary particles that are aggregates of primary particles. When the alumina particles 32 are secondary particles, a large specific surface area can be ensured. However, the alumina particles 32 may be primary particles or a mixture of primary particles and secondary particles.

The coating 34 of zirconia (ZrO ₂ ) is in the form of fine particles and is scattered on the surface of the alumina particles 32 . Note that the shape and number of the zirconia coating 34 are not limited to those shown in the drawings.

Regarding the size of the coating 34, the average particle size determined by a focused ion beam scanning electron microscope (FIB-SEM) is less than 100 nm. This average particle diameter can be determined by measuring the equivalent circle diameters (in other words, Heywood diameters) of six or more coatings arbitrarily selected from the SEM image, and determining the average value thereof. Note that, in order to determine the average particle diameter, it is preferable to perform SEM observation at a measurement magnification of at least 300,000 times. Image analysis software (eg, ImageJ, etc.) may be used to calculate the equivalent circle diameter and the average value.

Further, in the exhaust gas purifying catalyst 100, the mass ratio of zirconia (ZrO ₂ ) in the base material 30 (that is, relative to the mass of the base material 30 (total mass of the alumina particles 32 and the coating 34)) is 5% by mass or more. Since the zirconia coating 34 has an average particle size of less than 100 nm and zirconia is present in the base material 30 in a mass proportion of 5% by mass or more, the zirconia coating 34 is highly concentrated on the surface of the alumina particles 32. This allows more Rh to be present on or near the zirconia coating 34. As a result, when the exhaust gas purifying catalyst 100 is exposed to high temperatures for a long time, the zirconia coating 34 can highly suppress Rh from dissolving into alumina. As a result, Rh purification activity can be maintained at a high level even when exposed to high temperatures for a long time.

Since the degree of dispersion of the zirconia coating 34 becomes higher and the above-mentioned Rh purification activity becomes higher, the average particle size of the zirconia coating 34 is preferably 80 nm or less, more preferably 70 nm or less, More preferably, it is 60 nm or less, particularly preferably 50 nm or less. The lower limit of the average particle size of the zirconia coating 34 is determined by technological limitations and may be, for example, 5 nm, 10 nm, 15 nm, or 20 nm.

Further, since the degree of dispersion of the zirconia coating 34 becomes higher and the above-mentioned Rh purification activity becomes higher, in the exhaust gas purification catalyst 100, the mass proportion of zirconia (ZrO ₂ ) in the base material 30 is preferably set to The content is 9% by mass or more, more preferably 14% by mass or more, even more preferably 18% by mass or more, and most preferably 20% by mass or more. On the other hand, the mass proportion of zirconia in the base material 30 is preferably 30% by mass or less, more preferably 28% by mass or less, still more preferably 25% by mass or less, and most preferably 22% by mass or less. . Note that the mass proportion of zirconia in the base material 30 can be determined by measurement using an X-ray fluorescence spectrometer (XRF).

The zirconia coating 34 may contain components other than ZrO ₂ within a range (for example, 10% by mass or less) that does not significantly inhibit the effects of the present invention.

In the exhaust gas purifying catalyst 100, zirconia typically exists as a coating 34 on the surface of the alumina particles 32. However, zirconia may exist inside the La-containing alumina particles 32 within a range that does not impede the effects of the present invention. For example, when manufacturing the exhaust gas purifying catalyst 100 by the method described below, if the alumina particles 32 are in the form of secondary particles, there are gaps between the primary particles, so the zirconia precursor described below may enter through the gaps. , zirconia may be generated inside the alumina particles 32.

In the exhaust gas purifying catalyst 100, it is preferable that zirconia is distributed only on the surface of the alumina particles 32, or in a large amount on the surface and the vicinity thereof, from the viewpoint of suppressing the solid solution of Rh into alumina to a higher degree. Therefore, quantitative line analysis of Zr was performed from the surface of the alumina particle 32 toward the center using a field emission electron probe analyzer (FE-EPMA), and from the surface to the center, the surface was set as 0% and the center as 100%. It is preferable that the peak top of the Zr intensity exists within the range of 0% to 20% (preferably 0% to 15%, more preferably 0% to 12%) when expressed as the depth of .

There is no particular restriction on the method of manufacturing the exhaust gas purifying catalyst 100. The exhaust gas purifying catalyst 100 can be manufactured, for example, as follows.

As a raw material for the zirconia coating 34, a zirconia precursor that is converted into zirconia by firing and is soluble in a solvent is used. Since water can be used as a solvent, zirconium oxynitrate is preferred as the zirconia precursor.

A step of mixing La-containing alumina particles 32, a zirconia precursor, and a solvent to prepare a suspension in which the La-containing alumina particles 32 are dispersed and the zirconia precursor is dissolved, and spray-drying this suspension. and a step of spray drying using a device. Spray drying conditions may be determined with reference to known conditions employed to coat particles with solids of a solution by spray drying. Thereby, the zirconia precursor can be adhered to the surface of the alumina particles 32 in a highly dispersed state. Further drying may be performed to sufficiently remove the solvent. Note that the mass proportion of zirconia in the base material 30 can be adjusted by changing the concentrations of the La-containing alumina particles 32 and the zirconia precursor in the suspension. Although spray drying allows the particle size of the zirconia coating to be less than 100 nm, the particle size of the zirconia coating can be further adjusted by changing the spray drying conditions.

Next, a step of firing the obtained powder is performed. This firing converts the zirconia precursor into a coating 34 of zirconia. The firing conditions may be determined as appropriate depending on the type of zirconia precursor. As a result, La-containing alumina particles 32 (ie, base material 30) having a zirconia coating 34 can be obtained.

Next, a step of mixing a catalytic precious metal source containing Rh (for example, a solution containing Rh as ions) and the base material 30 in a dispersion medium and drying the obtained mixed solution, and drying the obtained dried product. A step of firing is carried out. Thereby, the catalytic noble metal 40 can be supported on the base material 30. That is, the exhaust gas purifying catalyst 100 can be obtained.

The exhaust gas purification catalyst 100 has excellent Rh purification activity even when exposed to high temperatures for a long time. Therefore, the exhaust gas purification catalyst 100 has excellent NOx purification performance even when exposed to high temperatures for a long time. Therefore, an example of an exhaust gas purifying catalyst body using the exhaust gas purifying catalyst 100 and an example of an exhaust gas purifying system including the same will be described below.

≪Exhaust gas purification system≫
FIG. 2 is a schematic diagram of the exhaust gas purification system 1. The exhaust gas purification system 1 includes an internal combustion engine 2, an exhaust gas purification device 3, and an engine control unit (ECU) 7. The exhaust gas purification system 1 is configured so that the exhaust gas purification device 3 purifies harmful components such as HC, CO, NOx, etc. contained in the exhaust gas discharged from the internal combustion engine 2. Note that the arrows in FIG. 2 indicate the flow direction of exhaust gas. In the following description, the side closer to the internal combustion engine 2 along the flow of exhaust gas will be referred to as the upstream side, and the side farther from the internal combustion engine 2 will be referred to as the downstream side.

The internal combustion engine 2 is mainly configured with a gasoline engine of a gasoline vehicle. However, the internal combustion engine 2 may be an engine other than gasoline, such as a diesel engine or an engine installed in a hybrid vehicle. Internal combustion engine 2 includes a combustion chamber (not shown). The combustion chamber is connected to a fuel tank (not shown). Gasoline is stored in the fuel tank here. However, the fuel stored in the fuel tank may be diesel fuel (light oil) or the like. In the combustion chamber, fuel supplied from the fuel tank is mixed with oxygen and combusted. This converts combustion energy into mechanical energy. The combustion chamber communicates with the exhaust port 2a. The exhaust port 2a communicates with the exhaust gas purification device 3. The combusted fuel gas becomes exhaust gas and is discharged to the exhaust gas purification device 3.

The exhaust gas purification device 3 includes an exhaust path 4 communicating with the internal combustion engine 2, a pressure sensor 8, a first catalyst body 9, and a second catalyst body 10. The exhaust path 4 is an exhaust gas flow path through which exhaust gas flows. The exhaust path 4 here includes an exhaust manifold 5 and an exhaust pipe 6. An upstream end of the exhaust manifold 5 is connected to an exhaust port 2a of the internal combustion engine 2. A downstream end of the exhaust manifold 5 is connected to an exhaust pipe 6. In the middle of the exhaust pipe 6, a first catalyst body 9 and a second catalyst body 10 are arranged in order from the upstream side. However, the arrangement of the first catalyst body 9 and the second catalyst body 10 may be arbitrarily changed. Further, the number of first catalyst bodies 9 and second catalyst bodies 10 is not particularly limited, and a plurality of each may be provided. Further, a third catalyst body may be further disposed downstream of the second catalyst body 10.

The first catalyst body 9 may be the same as the conventional one and is not particularly limited. The first catalyst body 9 is, for example, a diesel particulate filter (DPF) that removes PM contained in exhaust gas; a diesel oxidation catalyst (DOC) that purifies HC and CO contained in exhaust gas; A three-way catalyst that simultaneously purifies HC, CO, and NOx contained in exhaust gas; a reducing agent that stores NOx during normal operation (under lean conditions) and reduces HC and CO when a large amount of fuel is injected (under rich conditions). It may also be a NOx storage-reduction (NSR) catalyst that purifies NOx. The first catalyst body 9 may have a function of increasing the temperature of the exhaust gas flowing into the second catalyst body 10, for example. Note that the first catalyst body 9 is not an essential component and may be omitted in other embodiments.

The second catalyst body 10 is an example of an exhaust gas purification catalyst body using the above-described exhaust gas purification catalyst 100. Therefore, the second catalyst body 10 includes the above-mentioned exhaust gas purifying catalyst 100, and has a function of purifying harmful components (particularly NOx) in the exhaust gas. Note that, hereinafter, the second catalyst body 10 may be referred to as an "exhaust gas purification catalyst body." The configuration of the second catalyst body (exhaust gas purification catalyst body) 10 will be described in detail later.

The ECU 7 controls the internal combustion engine 2 and the exhaust gas purification device 3. The ECU 7 is electrically connected to the internal combustion engine 2 and sensors installed at various parts of the exhaust gas purification device 3 (for example, a pressure sensor 8, a temperature sensor, an oxygen sensor, etc.). Note that the configuration of the ECU 7 may be the same as the conventional one and is not particularly limited. The ECU 7 is, for example, a processor or an integrated circuit. The ECU 7 includes an input port (not shown) and an output port (not shown). The ECU 7 receives information such as the operating state of the vehicle, the amount of exhaust gas discharged from the internal combustion engine 2, the temperature, and the pressure. The ECU 7 receives information detected by the sensor (for example, the pressure measured by the pressure sensor 8) via an input port. The ECU 7 transmits a control signal via the output port, for example, based on the received information. The ECU 7 controls operations such as fuel injection control, ignition control, and intake air amount adjustment control of the internal combustion engine 2, for example. The ECU 7 controls driving and stopping of the exhaust gas purification device 3 based on, for example, the operating state of the internal combustion engine 2 and the amount of exhaust gas discharged from the internal combustion engine 2.

≪Catalyst body for exhaust gas purification≫
FIG. 3 is a perspective view schematically showing the exhaust gas purifying catalyst body 10. As shown in FIG. The exhaust gas purification catalyst body 10 is a second catalyst body 10 that includes the above-described exhaust gas purification catalyst 100. Note that the arrows in FIG. 3 indicate the flow of exhaust gas. In FIG. 3, the upstream side of the exhaust path 4 that is relatively close to the internal combustion engine 2 is shown on the left, and the downstream side of the exhaust path that is relatively far from the internal combustion engine 2 is shown on the right. Moreover, in FIG. 3, the symbol X represents the cylinder axis direction of the exhaust gas purifying catalyst body 10. The exhaust gas purifying catalyst body 10 is installed in the exhaust path 4 so that the cylinder axis direction X is along the flow direction of exhaust gas. The cylinder axis direction X is the flow direction of exhaust gas. Hereinafter, among the cylinder axis directions X, one direction X1 will be referred to as the upstream side (also referred to as the exhaust gas inflow side, front side), and the other direction X2 will be referred to as the downstream side (also referred to as the exhaust gas outflow side, rear side). Sometimes. However, this is only a direction for convenience of explanation, and does not limit the installation form of the exhaust gas purifying catalyst body 10 in any way.

The exhaust gas purifying catalyst body 10 includes a base material 11 with a straight flow structure and a catalyst layer 20 (see FIG. 4). An end of the exhaust gas purifying catalyst body 10 in one direction X1 is an exhaust gas inlet 10a, and an end in the other direction X2 is an exhaust gas outlet 10b. The outer shape of the exhaust gas purifying catalyst body 10 is cylindrical here. However, the external shape of the exhaust gas purifying catalyst body 10 is not particularly limited, and may be, for example, an elliptical cylinder shape, a polygonal cylinder shape, a pipe shape, a foam shape, a pellet shape, a fibrous shape, or the like.

The exhaust gas purifying catalyst body 10 may include members other than the base material 11 and the catalyst layer 20. For example, the exhaust gas purifying catalyst body 10 may further include a layer other than the catalyst layer 20.

The base material 11 constitutes the framework of the exhaust gas purifying catalyst body 10. The base material 11 is not particularly limited, and various materials and shapes conventionally used for this type of use can be used. The base material 11 may be a ceramic carrier made of ceramics such as cordierite, aluminum titanate, silicon carbide, etc., or may be a ceramic carrier made of ceramics such as cordierite, aluminum titanate, silicon carbide, stainless steel (SUS), Fe-Cr-Al alloy, Ni-Cr- A metal carrier made of an Al-based alloy or the like may also be used. As shown in FIG. 2, the base material 11 has a honeycomb structure here. The base material 11 includes a plurality of cells (cavities) 12 regularly arranged in the cylinder axis direction X, and partition walls (ribs) 14 that partition the plurality of cells 12. Although not particularly limited, the volume of the base material 11 (apparent volume including the volume of the cells 12) may be approximately 0.1 to 10L, for example, 0.5 to 5L. Further, the average length (total length) L of the base material 11 along the cylinder axis direction X may be approximately 10 to 500 mm, for example, 50 to 300 mm.

The cell 12 becomes a flow path for exhaust gas. The cell 12 extends in the cylinder axis direction X. The cell 12 is a through hole that penetrates the base material 11 in the cylinder axis direction X. The shape, size, number, etc. of the cells 12 may be designed in consideration of, for example, the flow rate and components of the exhaust gas flowing through the exhaust gas purifying catalyst body 10. The shape of the cross section of the cell 12 perpendicular to the cylinder axis direction X is not particularly limited. The cross-sectional shape of the cell 12 may be various geometric shapes, such as squares, parallelograms, rectangles, trapezoids, other polygons (triangles, hexagons, octagons, etc.), wavy shapes, circles, etc. It's fine. The partition wall 14 faces the cells 12 and separates adjacent cells 12 from each other. Although not particularly limited, the average thickness (dimension in the direction perpendicular to the surface; the same shall apply hereinafter) of the partition wall 14 is approximately 0.1 to 0.1 from the viewpoint of improving mechanical strength and reducing pressure loss. It may be 10 mil (1 mil is about 25.4 μm), for example 0.2 to 5 mil. The partition wall 14 may be porous so that exhaust gas can pass therethrough.

The catalyst layer 20 is a reaction field that purifies harmful components in exhaust gas. The catalyst layer 20 is a porous body having many pores (voids). The exhaust gas that has flowed into the exhaust gas purification catalyst body 10 comes into contact with the catalyst layer 20 while flowing in the flow path (cell 12) of the exhaust gas purification catalyst body 10. This purifies harmful components in the exhaust gas.

FIG. 4 is a partial sectional view schematically showing a part of the cross section of the exhaust gas purifying catalyst body 10 taken along the cylinder axis direction X. The catalyst layer 20 is provided here on the base material 11, specifically on the surface of the partition wall 14. However, part or all of the catalyst layer 20 may penetrate into the partition wall 14 .

The catalyst layer 20 contains the above-mentioned exhaust gas purifying catalyst 100. Since the exhaust gas purification catalyst 100 contains Rh as the noble metal catalyst 40, the exhaust gas purification catalyst body 10 can reduce NOx contained in the exhaust gas and convert it into nitrogen, thereby purifying the exhaust gas. be able to. In particular, the exhaust gas purification catalyst body 10 has high NOx purification performance even when exposed to high temperatures for a long time.

The catalyst layer 20 may contain components other than the exhaust gas purifying catalyst 100 (hereinafter also referred to as "optional components"). The optional components will be explained below.

As an optional component, the catalyst layer 20 may contain a material having an oxygen storage capacity (OSC material) such as a composite oxide containing ceria. When the catalyst layer 20 contains the OSC material, the exhaust gas purifying catalyst body 10 can stably exhibit excellent purification performance even when the air-fuel ratio of the exhaust gas fluctuates depending on the driving conditions of the vehicle.

Regarding the OSC material, examples of the composite oxide containing ceria include a composite oxide containing ceria and zirconia (ceria-zirconia composite oxide (so-called CZ composite oxide or ZC composite oxide)). When the OSC material contains zirconium oxide, thermal deterioration of cerium oxide can be suppressed, so ceria-zirconia composite oxide is preferable as the OSC material.

When the OSC material is a composite oxide containing cerium oxide, the content of cerium oxide is preferably 15% by mass or more, more preferably 20% by mass or more, from the viewpoint of fully demonstrating its oxygen storage capacity. be. On the other hand, if the content of cerium oxide is too high, the basicity of the OSC material may become too high. Therefore, the content of cerium oxide is preferably 40% by mass or less, more preferably 30% by mass or less.

The OSC material may contain an oxide of a rare earth element for the purpose of improving properties (especially heat resistance, oxygen absorption/release properties, etc.). Examples of rare earth elements include Sc, Y, La, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, and the like. Preferred rare earth element oxides are Pr ₂ O ₃ , Nd ₂ O ₃ , La ₂ O ₃ , and Y ₂ O ₃ .

The amount of OSC material per 1 L volume of the base material 11 is not particularly limited, but may be, for example, 10 g/L or more, 25 g/L or more, or 50 g/L or more, and, for example, 150 g/L or less, It may be 100 g/L or less, or 80 g/L or less.

In this specification, "per 1 L volume of the base material" refers to per 1 L bulk volume of the entire base material including the volume of the cell passageway in the pure volume of the base material. In the following description, (g/L) indicates the amount contained in 1 L of volume of the base material.

As an optional component, the catalyst layer 20 may contain an oxygen storage material such as aluminum oxide (Al ₂ O ₃ , alumina), titanium oxide (TiO ₂ , titania), zirconium oxide (ZrO ₂ , zirconia), silicon oxide (SiO ₂ , silica), etc. It may also contain a material (non-OSC material) that does not have this ability.

Oxides used as non-OSC materials contain small amounts of oxides of rare earth elements such as Pr ₂ O ₃ , Nd ₂ O ₃ , La ₂ O ₃ , and Y ₂ O ₃ in order to improve heat resistance. For example, 1% by mass or more and 10% by mass or less) may be added. Since it has particularly excellent heat resistance and durability, the non-OSC material is preferably Al ₂ O ₃ , and Al ₂ O ₃ composited with La ₂ O ₃ (La ₂ O ₃ -Al ₂ O ₃ composite oxide; LA composite oxide) is more preferable.

The amount of non-OSC material per 1 L volume of the base material 11 is not particularly limited, but may be, for example, 10 g/L or more, 25 g/L or more, or 50 g/L or more, and, for example, 150 g/L or less. , 100 g/L or less, or 80 g/L or less.

As an optional component, the catalyst layer 20 may include an exhaust gas purification catalyst (hereinafter also referred to as "any exhaust gas purification catalyst") containing a catalytic noble metal other than Rh. Therefore, in the catalyst layer 20, the above-described exhaust gas purification catalyst 100 and an exhaust gas purification catalyst containing a catalytic noble metal other than Rh may be used together. Particularly, when the above-mentioned exhaust gas purification catalyst 100 and an exhaust gas purification catalyst containing at least one of Pt and Pd are used together in the catalyst layer 20, the exhaust gas purification catalyst body 10 is contained in the exhaust gas. It has excellent not only NOx purification performance but also HC and CO purification performance.

In any exhaust gas purifying catalyst, catalytic noble metals other than Rh are typically supported on a carrier. Examples of carriers include the above non-OSC materials and OSC materials. The carrier may be the base material 30 used in the above-mentioned exhaust gas purifying catalyst 100.

In any exhaust gas purification catalyst, the catalytic noble metal other than Rh is preferably used in the form of fine particles with a sufficiently small particle size from the viewpoint of increasing the contact area with the exhaust gas. The average particle size of the catalytic noble metal (specifically, the average value of the particle sizes of 50 or more catalytic noble metals determined by transmission electron microscopy (TEM) observation) is approximately 1 to 15 nm, for example, 10 nm or less, and even 5 nm. It is good if it is below.

The amount of catalytic noble metal in the exhaust gas purifying catalyst body 10 (total amount if catalytic noble metal other than Rh is included) is not particularly limited, and can be determined as appropriate depending on the type of catalytic noble metal. From the viewpoint of particularly high exhaust gas purification performance, the amount of catalytic precious metal per 1 L of volume of the base material 11 is, for example, 0.01 g/L or more, 0.03 g/L or more, 0.05 g/L or more, 0.08 g. /L or more, or 0.10g/L or more. From the viewpoint of balance between exhaust gas purification performance and cost, for example, 15.00 g/L or less, 10.00 g/L or less, 5.00 g/L or less, 3.00 g/L or less, 1.50 g/L or less, It may be 1.00 g/L or less, 0.80 g/L or less, or 0.50 g/L or less.

As an optional component, the catalyst layer 20 may contain an alkaline earth element such as calcium (Ca) or barium (Ba). Poisoning of noble metal catalysts (particularly oxidation catalysts) can be suppressed by alkaline earth elements. Moreover, the alkaline earth element improves the dispersibility of the catalyst noble metal, and can suppress sintering accompanying grain growth of the catalyst noble metal. Furthermore, when the catalyst layer 20 contains an alkaline earth element together with the OSC material, it is possible to further improve the amount of oxygen absorbed into the OSC material in a lean atmosphere (oxygen-excess atmosphere) where the fuel is thinner than the stoichiometric air-fuel ratio. can. Alkaline earth elements may be contained in the form of oxides, hydroxides, carbonates, nitrates, sulfates, phosphates, acetates, formates, oxalates, halides, and the like.

As optional components, the catalyst layer 20 may contain a NOx adsorbent having NOx storage capacity, a stabilizer, and the like. Examples of the stabilizer include rare earth elements such as yttrium (Y), lanthanum (La), and neodymium (Nd). Note that the rare earth element may exist in the catalyst layer 20 in the form of an oxide.

As optional components, the catalyst layer 20 may contain binders such as alumina sol and silica sol, various additives, and the like.

Although not particularly limited, the coating amount (forming amount) of the catalyst layer 20 is approximately 30 g/L or more, typically 50 g/L per 1 L of the volume of the catalyst body 10 for exhaust gas purification (volume of the base material 11). /L or more, preferably 70g/L or more, for example 100g/L or more, and may be approximately 500g/L or less, typically 400g/L or less, for example 300g/L or less. By satisfying the above range, it is possible to improve purification performance and reduce pressure loss at a high level. Note that in this specification, the "coating amount" refers to the mass of solid content contained per unit volume of the exhaust gas purifying catalyst body 10.

The length and thickness of the catalyst layer 20 can be appropriately designed in consideration of, for example, the size of the cells 12 of the base material 11 and the flow rate of exhaust gas flowing through the exhaust gas purifying catalyst body 10. The catalyst layer 20 may be continuously provided on the partition wall 14 of the base material 11, or may be provided intermittently. For example, the catalyst layer 20 may be provided along the cylinder axis direction X from the exhaust gas inlet 10a, or may be provided along the cylinder axis direction X from the exhaust gas outlet 10b.

Although not particularly limited, the overall coating width (average length) of the catalyst layer 20 in the cylinder axis direction may be 80% or more, for example 90% or more, and may be the same length as the total length L of the base material 11. Although not particularly limited, the coating thickness (average thickness) of the catalyst layer 20 is approximately 1 to 300 μm, typically 5 to 200 μm, for example 10 to 100 μm. Thereby, it is possible to achieve a high level of both improvement in purification performance and reduction in pressure loss.

The exhaust gas purifying catalyst 100 may be contained only in a part of the catalyst layer 20. For example, the catalyst layer 20 has an upstream X1 portion (front portion) and a downstream X2 portion (rear portion) in the cylinder axis direction X, and the upstream X1 portion (front portion) and the downstream X2 portion (rear portion). ) contains the exhaust gas purification catalyst 100, and the other contains any of the above exhaust gas purification catalysts (particularly an exhaust gas purification catalyst in which the catalytic noble metal is at least one of Pt and Pd). You can leave it there.

Alternatively, the catalyst layer 20 may be configured as a laminated structure including two or more layers, one layer containing the exhaust gas purification catalyst 100, and the other layer containing any of the above exhaust gas purification catalysts (in particular, catalyst noble metals). The exhaust gas purifying catalyst may contain at least one of Pt and Pd (exhaust gas purification catalyst). Hereinafter, as a modification, a preferred example in which the catalyst layer 20 has a multilayer structure will be described.

<<Modified example of exhaust gas purification catalyst body 10>>
FIG. 5 is a partial sectional view schematically showing a part of a cross section of an exhaust gas purifying catalyst body 10', which is a modified example of the exhaust gas purifying catalyst body 10, taken along the cylinder axis direction X. The exhaust gas purifying catalyst body 10' includes a base material 11 and a multilayer catalyst layer 20' provided on the base material 11. Since the catalyst layer 20' has a multilayer structure, the exhaust gas purification performance can be further improved.

The base material 11 is the same as above. The catalyst layer 20' has a multilayer structure, unlike the example shown in FIG. Specifically, the catalyst layer 20' has a laminated structure in which a first partial catalyst layer (lower layer) 21 and a second partial catalyst layer (upper layer) 22 are laminated in the thickness direction. Therefore, the lower layer 21 is provided so as to be in contact with the surface of the base material 11, and the upper layer 22 is provided so as to be in contact with the upper surface of the lower layer 21. In the illustrated example, the catalyst layer 20' has a two-layer structure, but the catalyst layer 20' may have a laminated structure of three or more layers. For example, the catalyst layer 20' may have an intermediate layer between the lower layer 21 and the upper layer 22, or the catalyst layer 20' may further have another layer on the upper layer 22. .

The lower layer 21 contains an exhaust gas purifying catalyst in which the catalytic precious metal is at least one of Pt and Pd. The upper layer 22 contains an exhaust gas purifying catalyst 100 in which the noble metal catalyst contains Rh. In this case, the exhaust gas purifying catalyst body 10' has particularly excellent exhaust gas purifying performance. The amount of Pt and Pd (total amount when both are included) in the catalyst noble metal of the exhaust gas purification catalyst in the lower layer 21 is preferably 80% by mass, more preferably 90% by mass, and even more preferably 95% by mass. %, most preferably 100% by mass.

The lower layer 21 and the upper layer 22 may contain the same optional components as the catalyst layer 20 described above, and preferably, the lower layer 21 and the upper layer 22 each contain an OSC material.

The exhaust gas purifying catalyst body 10' configured in this manner not only has particularly high purification performance for NOx contained in exhaust gas, but also has particularly high purification performance for HC and CO.

<<Method for manufacturing exhaust gas purification catalyst body 10>>
The exhaust gas purifying catalyst body 10 can be manufactured, for example, by the following method. First, the base material 11 and a catalyst layer forming slurry for forming the catalyst layer 20 are prepared. The slurry for forming the catalyst layer can be prepared by mixing the exhaust gas purification catalyst 100 and other optional components (for example, non-OSC materials, OSC materials, binders, various additives, etc.) in a dispersion medium. I can do it. As the dispersion medium, for example, water, a mixture of water and a water-soluble organic solvent, etc. can be used. The properties of the slurry (for example, viscosity, solid content, etc.) can be appropriately determined depending on the size of the base material 11 used, the form of the cells 12 (partition walls 14), the characteristics required for the catalyst layer 20, etc.

Next, the catalyst layer 20 is formed on the base material 11 using the catalyst layer forming slurry. The catalyst layer 20 can be formed by a conventionally known method (for example, an impregnation method, a wash coating method, etc.). Specifically, for example, the prepared slurry for forming a catalyst layer is made to flow into the cell 12 from the end of the base material 11, and is supplied to a predetermined length along the cylinder axis direction X. The slurry may be made to flow in from either the inlet 10a or the outlet 10b. At this time, excess slurry may be sucked from the opposite end. Further, excess slurry may be discharged from the cell 12 by blowing air from the opposite end. Next, the base material 11 to which the slurry has been supplied is dried at a predetermined temperature and time. As a result, a catalyst layer 20 including the exhaust gas purifying catalyst 100 is formed. In the manner described above, the exhaust gas purifying catalyst body 10 can be obtained.

Alternatively, a catalytic noble metal source containing Rh (e.g., a solution containing Rh as ions), La-containing alumina particles 32 (i.e., base material 30) having a coating 34 of zirconia, and other optional components ( For example, a slurry for forming the second catalyst layer is prepared by mixing non-OSC materials, OSC materials, binders, various additives, etc.) in a dispersion medium.

Next, the catalyst layer 20 is formed on the base material 11 using the second catalyst layer forming slurry. Specifically, the slurry for forming the second catalyst layer is supplied to the base material 11 in the same manner as described above. Next, the base material 11 to which the slurry has been supplied is fired at a predetermined temperature and time. The firing method may be the same as conventional methods. Further, the dispersion medium may be removed by drying before firing. As a result, the exhaust gas purification catalyst 100 is generated, and a porous catalyst layer 20 containing the exhaust gas purification catalyst 100 is formed on the base material. In the manner described above, the exhaust gas purifying catalyst body 10 can be obtained.

≪Applications of catalyst body 10 for exhaust gas purification≫
The exhaust gas purification catalyst 10 is used in vehicles such as cars and trucks, motorcycles and motorized bicycles, marine products such as ships, tankers, watercraft, personal watercraft, outboard motors, lawn mowers, and chainsaws. It can be suitably used for purifying exhaust gas emitted from gardening products such as trimmers, leisure products such as golf carts and four-wheeled buggies, power generation equipment such as cogeneration systems, and internal combustion engines such as garbage incinerators. Among these, it can be suitably used for vehicles such as automobiles, and in particular, it can be suitably used for vehicles equipped with a gasoline engine.

Hereinafter, test examples related to the present invention will be described, but the present invention is not intended to be limited to those shown in the test examples below.

[Examples 1 to 4]
Zirconium oxynitrate and La-containing alumina powder were placed in a container and stirred while adding pure water to prepare a suspension in which zirconium oxynitrate was dissolved and La-containing alumina powder was dispersed. In Examples 1 and 2, a powder with a particle size (D50) of 10 to 20 μm and a pore volume of 0.59 to 0.86 mL/g was used as the La-containing alumina powder, and in Examples 3 and 4, , a powder with a particle size (D50) in the range of 27 to 43 μm and a pore volume of 0.3 to 0.55 mL/g was used. Further, in Examples 2 and 4, the mixing ratio of zirconium oxynitrate and La-containing alumina powder was changed from Examples 1 and 3, and the zirconia content was lower than in Examples 1 and 3.

This suspension was spray-dried using a nozzle-type spray drying device (ODT-8 manufactured by Okawara Kakoki Co., Ltd.) to adhere zirconium oxynitrate to the surface of the La-containing alumina powder. The spray drying conditions were an inlet gas temperature of 200°C, an outlet gas temperature of 110°C, and a pump flow rate of 20 ccm.

The obtained powder was dried in an electric furnace at 120°C for 8 hours, and then calcined at 500°C for 2 hours. As a result, a base material having a zirconia coating on the surface of the La-containing alumina powder was obtained.

This base material was impregnated with a predetermined amount of Rh nitric acid aqueous solution and evaporated to dryness. The obtained powder was fired at 500° C. for 2 hours to support Rh on the base material. Note that the amount of Rh supported was 0.5% by mass. In this way, exhaust gas purifying catalysts of Examples 1 to 4 were obtained.

[Comparative example 1]
La-containing alumina powder with a particle size (D50) range of 27 to 43 μm and a pore volume of 0.3 to 0.55 mL/g was impregnated with a predetermined amount of Rh nitric acid aqueous solution and evaporated to dryness. The obtained powder was fired at 500° C. for 2 hours to make the La-containing alumina powder support Rh. Note that the amount of Rh supported was 0.5% by mass. In this way, an exhaust gas purifying catalyst of Comparative Example 1 was obtained. Therefore, in the exhaust gas purifying catalyst of Comparative Example 1, the base material does not have a zirconia coating.

[Comparative Examples 2 to 4]
Zirconium oxynitrate and La-containing alumina powder were placed in a container and stirred while adding pure water to prepare a suspension in which zirconium oxynitrate was dissolved and La-containing alumina powder was dispersed. In Comparative Examples 2 to 4, the mixing ratio of zirconium oxynitrate and La-containing alumina powder was changed, and the zirconia content was changed.

Co-precipitation was performed by adding ammonia water to the prepared suspension so that the suspension became alkaline. The precipitate was collected and dried in an electric furnace at 200°C for 2 hours. The obtained powder was calcined at 500°C for 2 hours. As a result, a base material having a zirconia coating on the surface of the La-containing alumina powder was obtained.

This base material was impregnated with a predetermined amount of Rh nitric acid aqueous solution and evaporated to dryness. The obtained powder was fired at 500° C. for 2 hours to support Rh on the base material. Note that the amount of Rh supported was 0.5% by mass. In this way, exhaust gas purifying catalysts of Comparative Examples 2 to 4 were obtained.

[Comparative Examples 5 and 6]
In Comparative Example 5, zirconium oxynitrate and La-containing alumina powder with a particle size (D50) range of 10 to 20 μm and a pore volume of 0.59 to 0.86 mL/g were placed in a container and stirred while adding pure water. A suspension in which zirconium oxynitrate was dissolved and La-containing alumina powder was dispersed was prepared. In Comparative Example 6, zirconia sol and La-containing alumina powder with a particle size (D50) range of 27 to 43 μm and a pore volume of 0.3 to 0.55 mL/g were placed in a container and stirred while adding pure water. A suspension in which zirconia sol and La-containing alumina powder were dispersed was prepared.

The prepared suspension was evaporated to dryness in an electric furnace at 120°C for 8 hours. The obtained powder was calcined at 500°C for 2 hours. As a result, a base material having a zirconia coating on the surface of the La-containing alumina powder was obtained.

This base material was impregnated with a predetermined amount of Rh nitric acid aqueous solution and evaporated to dryness. The obtained powder was fired at 500° C. for 2 hours to support Rh on the base material. Note that the amount of Rh supported was 0.5% by mass. In this way, exhaust gas purifying catalysts of Comparative Examples 5 and 6 were obtained.

[Elemental analysis by XRF]
ZrO ₂ , Al ₂ O ₃ and The abundance ratio of La ₂ O ₃ in the oxide was determined in mass %. Note that the measurement conditions were X-ray output: 2.4 kW, acceleration voltage: 30 kV, and current value: 80 mA, and the abundance ratio was determined by the FP method. The results are shown in Table 1.

[FIB-SEM observation]
The exhaust gas purifying catalysts of Examples 1 to 4 and Comparative Examples 2 to 6 prepared above were observed using a focused ion beam scanning electron microscope (FIB-SEM: JXA-8530F manufactured by JEOL). The measurement conditions were acceleration voltage: 7 kV or 3 kV, current value: 0.4 nA, and WD: 4 mm. In Examples 1 to 4, when observed at a measurement magnification of 300,000 times, it was confirmed that a fine zirconia coating was formed on the surface of the particles. For reference, a SEM image of the exhaust gas purifying catalyst of Example 4 observed at a measurement magnification of 300,000 times is shown in FIG. In the SEM image at a measurement magnification of 300,000 times, six or more zirconia coatings were arbitrarily selected, their equivalent circle diameters (ie, Heywood diameters) were determined, and the average value was calculated. The values (ie, the values of the average particle size of the zirconia coating) are shown in Table 1.

In Comparative Examples 2 to 5, it was confirmed that a coarse zirconia coating was formed on the surface of the particles at a measurement magnification of 3,000 times. On the other hand, even when the measurement magnification was increased to 300,000 times, no fine zirconia coating was observed. For reference, a SEM image of the exhaust gas purifying catalyst of Comparative Example 4 observed at a measurement magnification of 3,000 times is shown in FIG. In the SEM image with a measurement magnification of 3,000 times, six or more zirconia coatings were arbitrarily selected, their equivalent circular diameters were determined, and the average value was calculated. The values (ie, the values of the average particle size of the zirconia coating) are shown in Table 1.

[Line analysis by FE-EPMA]
Quantitative line analysis of Zr was performed on the exhaust gas purification catalysts of Examples 1 to 4 and Comparative Examples 2 to 6 prepared above using a field emission electron probe analyzer (FE-EPMA: Helios G4UX manufactured by FEI).

Specifically, first, a measurement sample was prepared in which the cross section of the particles of the exhaust gas purification catalyst was exposed. On the other hand, using FE-EPMA, the upper limit of the intensity of Al Level was set to 3000, and an Al mapping image was obtained. This is an area where

Next, the upper limit of the intensity of Zr Level was set to 300, and the detection range was set to the intensity of Zr of 30 or more. Quantitative line analysis of Zr was performed on the region where particles of the exhaust gas purifying catalyst were present from the surface position to the center position, and the Zr intensity spectrum was measured with the surface as 0% and the center as 100%. Then, the position of the peak top of Zr intensity was determined. The results are shown in Table 1.

[Durability treatment]
The exhaust gas purifying catalysts of each Example and each Comparative Example were pelletized according to a conventional method. The pellets were placed in a chamber-type electric furnace and heat-treated at 1000° C. for 10 hours in an air atmosphere.

[Model gas evaluation]
The pellets subjected to the above durability treatment were set in a catalyst evaluation device manufactured by Horiba. As a pretreatment, a reduction treatment was performed by flowing hydrogen gas at 500° C. for 5 minutes. In this test, the temperature was raised to 80° C. while flowing a mixed gas containing CO ₂ , O ₂ , NO, CO, and C ₃ H ₆ into the device. H ₂ O was added to the mixed gas, and the gas was allowed to flow at 80° C. for 3 minutes and 30 seconds. Thereafter, the temperature was raised to 600°C at a rate of 20°C/min, and held at 600°C for 30 seconds. At this time, the temperature at which the NO purification rate reached 50% (NOx 50% purification temperature: T50-NOx) was determined. The evaluation results are shown in Table 1.

From the results in Table 1, it can be seen that because the average particle diameter of the Zr coating is small to the order of two digits, NOx can be removed at a lower temperature after high-temperature durability. Therefore, it can be seen that the exhaust gas purifying catalyst disclosed herein has a high Rh purifying activity even when exposed to high temperatures for a long time.

[Confirmation experiment based on vehicle evaluation]
As a base material, a cordierite honeycomb base material (volume: 1.075 L, total length of base material: 100 mm, number of cells: 600 cells, cell shape: square, thickness of partition wall: 2 mm) was prepared.

A slurry for forming a lower layer was prepared by mixing an aqueous Pd nitrate solution, Al ₂ O ₃ , CeO ₂ -ZrO ₂ -based composite oxide, barium sulfate, Al ₂ O ₃ -based binder, and pure water. This slurry for forming a lower layer was applied onto the surface of the base material by a wash coating method. Then, it was fired at 500° C. for 2 hours in an electric furnace. In this way, a lower layer containing a Pd catalyst was formed on the base material.

Next, the Rh nitric acid aqueous solution, the base material used in Example 1, Example 3, or Comparative Example 1, the CeO ₂ -ZrO _2- based composite oxide, the Al ₂ O ₃ -based binder, and pure water were mixed to form the upper layer. A forming slurry was prepared. This slurry for forming an upper layer was applied onto the lower layer formed on the surface of the base material by a wash coating method. Then, it was fired at 500° C. for 2 hours in an electric furnace. This calcination produced an exhaust gas purifying catalyst in which Rh was supported on the base material. In this way, an upper layer containing an exhaust gas purification catalyst on which Rh was supported was formed on the lower layer, and exhaust gas purification catalyst bodies of Example 1, Example 3, and Comparative Example 1 were obtained.

Using these catalyst bodies for purifying exhaust gas, an on-drive test was conducted to evaluate the amount of NOx emissions. The driving mode was WLTP (Worldwide harmonized Light duty driving Test Procedure) Phase 1 to enable evaluation in low temperature ranges. The NOx emission amount was less than 0.004 ppm in Example 1 and less than 0.0045 ppm in Example 3, whereas it was more than 0.0065 ppm in Comparative Example 1. From this, it was confirmed that NOx can be significantly reduced when the exhaust gas purifying catalyst is used to form an exhaust gas purifying catalyst body and mounted on a vehicle.

Several embodiments of the present invention have been described above, but the above embodiments are merely examples. The present invention can be implemented in various other forms. The present invention can be implemented based on the content disclosed in this specification and the common general knowledge in the field. The technology described in the claims includes various modifications and changes to the embodiments exemplified above.

Claims (6)

1. base material and
   a catalytic noble metal supported by the base material;
   An exhaust gas purification catalyst comprising:
   The base material includes La-containing alumina particles and a zirconia coating disposed on the surface of the alumina particles,
   The catalytic noble metal contains at least Rh,
   The mass proportion of zirconia in the base material is 5% by mass or more,
   The average grain size of the zirconia coating determined by a focused ion beam scanning electron microscope is less than 100 nm.
   Catalyst for exhaust gas purification.
2. When quantitative line analysis of Zr is performed from the surface of the alumina particle toward the center using a field emission electron probe analyzer, and the depth from the surface to the center is expressed with the surface as 0% and the center as 100%. 2. The exhaust gas purifying catalyst according to claim 1, wherein the peak top of Zr intensity exists within a range of 0% to 20%.
3. The exhaust gas purifying catalyst according to claim 1, wherein the mass proportion of zirconia in the base material is 9% by mass or more and 30% by mass or less.
4. The exhaust gas purifying catalyst according to claim 1, wherein the average particle diameter of the zirconia coating is 5 nm or more and 80 nm or less.
5. comprising a base material and a catalyst layer provided on the base material,
   The catalyst layer contains the exhaust gas purifying catalyst according to claim 1.
   Catalyst for exhaust gas purification.
6. The catalyst layer includes a first partial catalyst layer located on the base material side and a second partial catalyst layer located on the surface layer side of the catalyst layer,
   The first partial catalyst layer contains an exhaust gas purifying catalyst in which the catalytic noble metal is at least one of Pt and Pd,
   The exhaust gas purifying catalyst body according to claim 5, wherein the second partial catalyst layer contains the exhaust gas purifying catalyst according to claim 1.

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