JP2022178440A - Exhaust gas purification device - Google Patents

Abstract

To provide an exhaust gas purification device exhibiting high HC purification performance and Nox purification performance under rich conditions.SOLUTION: An exhaust gas purification device 1 includes a base material 10 and a catalyst layer 30 arranged on the base material 10. The catalyst layer 30 includes first catalyst metal 33 and second catalyst metal 35. The first catalyst metal 33 is Rh, and the second catalyst metal 35 is at least one of Pd or Pt. The catalyst layer 30 has an outer surface 36, and an inner surface 37 defining a macropore 50. Concentration of the first catalyst metal 33 in the vicinity of the outer surface 36 and the inner surface 37 is equal to or less than averaged concentration of the first catalyst metal 33 in the whole catalyst layer 30. Concentration of the second catalyst metal 35 in the vicinity of the outer surface 36 and the inner surface 37 is higher than averaged concentration of the second catalyst metal 35 in the whole catalyst layer 30.SELECTED DRAWING: Figure 1

Description

TECHNICAL FIELD The present invention relates to an exhaust gas purifying device for purifying exhaust gas.

Exhaust gases emitted from internal combustion engines such as automobiles contain harmful components such as carbon monoxide (CO), hydrocarbons (HC) and nitrogen oxides (NOx). Exhaust gas purifiers are used to remove these harmful components. The exhaust gas purifier is provided downstream of the internal combustion engine, oxidizes CO and HC in the exhaust gas to convert them to water and carbon dioxide, and reduces NOx to nitrogen.

Patent document 1 describes an exhaust gas purifying catalyst provided with a catalyst layer having pores. Patent Document 1 describes that the catalyst layer contains catalyst powder containing a noble metal, and that palladium (Pd) and rhodium (Rh) are exemplified as the noble metal.

Japanese Patent Application Laid-Open No. 2008-279428

The air-fuel ratio (A/F) of the air-fuel mixture supplied to the internal combustion engine fluctuates around the stoichiometric air-fuel ratio (stoichiometric: A/F=14.7) depending on the running conditions of the automobile. An exhaust gas purifier is required to efficiently purify HC and NOx contained in exhaust gas from an internal combustion engine supplied with a fuel-rich (rich: A/F<14.7) mixture.

Therefore, it is an object of the present invention to provide an exhaust gas purifier that exhibits high HC purification performance and NOx purification performance under rich conditions.

According to one aspect of the invention,
a substrate;
a catalyst layer disposed on the substrate;
with
the catalyst layer comprises a first catalyst metal and a second catalyst metal;
the first catalytic metal is rhodium;
the second catalytic metal is at least one of palladium and platinum;
the catalyst layer has an outer surface and an inner surface defining macropores;
The concentration of the first catalytic metal in the vicinity of the outer surface and the concentration of the first catalytic metal in the vicinity of the inner surface are equal to or less than the concentration of the first catalytic metal averaged over the entire catalyst layer. is less than
the concentration of the second catalytic metal in the vicinity of the outer surface and the concentration of the second catalytic metal in the vicinity of the inner surface are higher than the concentration of the second catalytic metal averaged over the entire catalyst layer; An exhaust gas purification device is provided.

The exhaust gas purifier of the present invention exhibits high HC purifying performance and NOx purifying performance under rich conditions.

FIG. 1 is a schematic enlarged cross-sectional view of an exhaust gas purifier according to an embodiment. 2 is a cross-sectional SEM image of the specimen of Example 1. FIG. 3 is a Pd mapping image of the specimen of Example 1. FIG. 4 is an Rh mapping image of the specimen of Example 1. FIG. 5 is a cross-sectional SEM image of the specimen of Comparative Example 1. FIG. 6 is a Pd mapping image of the specimen of Comparative Example 1. FIG. 7 is an Rh mapping image of the specimen of Comparative Example 1. FIG. 8 is a cross-sectional SEM image of the specimen of Comparative Example 2. FIG. 9 is a Pd mapping image of the specimen of Comparative Example 2. FIG. 10 is an Rh mapping image of the specimen of Comparative Example 2. FIG. 11 is a cross-sectional SEM image of the specimen of Comparative Example 3. FIG. 12 is a Pd mapping image of the specimen of Comparative Example 3. FIG. 13 is an Rh mapping image of the specimen of Comparative Example 3. FIG. FIG. 14 is a graph showing the HC emissions of the test specimens of Example 1 and Comparative Examples 1-3. FIG. 15 is a graph showing the NOx emissions of the test bodies of Example 1 and Comparative Examples 1-3. FIG. 16 is a graph showing the relationship between the ratio of the amount of Pd arranged near the outer surface to the total amount of Pd in the catalyst layer and the amount of HC discharged. FIG. 17 is a graph showing the relationship between the ratio of the amount of Pd arranged near the outer surface to the total amount of Pd in the catalyst layer and the amount of NOx emissions.

Hereinafter, embodiments will be described with reference to the drawings as appropriate. The present invention is not limited to the following embodiments, and various design changes can be made without departing from the spirit of the invention described in the claims. For convenience of explanation, the dimensional ratio and shape of each part in the drawings are exaggerated, and may differ from the actual dimensional ratio and shape. In addition, in the present application, a numerical range represented using the symbol "~" includes the numerical values described before and after the symbol "~" as lower and upper limits, respectively.

I Exhaust Gas Purifying Apparatus An exhaust gas purifying apparatus 1 according to an embodiment will be described with reference to FIG. The exhaust gas purification device 1 includes a substrate 10 and a catalyst layer 30 arranged on the substrate 10 .

The base material 10 is not particularly limited, and any base material that can be used as a base material for an exhaust gas purifier can be used. As the substrate 10, a substrate having a honeycomb shape, for example, a honeycomb-shaped monolith substrate (honeycomb filter, high-density honeycomb, etc.) can be used. Examples of materials for the base material 10 include ceramics such as cordierite, silicon carbide, silica, alumina and mullite, and metals such as stainless steel containing chromium and aluminum. These materials allow the exhaust gas purifying device 1 to exhibit high exhaust gas purifying performance even under high temperature conditions. From the viewpoint of cost reduction, the substrate 10 may be made of cordierite.

Catalyst layer 30 includes a first catalyst metal 33 and a second catalyst metal 35 . The first catalytic metal 33 is rhodium and the second catalytic metal 35 is at least one of palladium and platinum. The catalyst layer 30 may contain other precious metals to the extent that the performance of the first catalyst metal 33 and the second catalyst metal 35 is not impaired.

Catalyst layer 30 may further include carrier 31 . As the carrier 31, any carrier that can be used in a normal exhaust gas purifier can be used without particular limitation. For example, the carrier 31 may be aluminum oxide (Al ₂ O ₃ , alumina), cerium oxide (CeO ₂ , ceria), zirconium oxide (ZrO ₂ , zirconia), silicon oxide (SiO ₂ , silica), yttrium oxide (Y ₂ O ₃ , yttria) and neodymium oxide (Nd ₂ O ₃ ), or a composite oxide thereof. At least one of the first catalytic metal 33 and the second catalytic metal 35 may be supported on the carrier 31 .

Catalyst layer 30 has an outer surface 36 and an inner surface 37 that defines macropores 50 .

Macropores 50 may have an average pore size of 1-20 μm. When the average pore diameter is 1 μm or more, the exhaust gas can sufficiently diffuse through the entire catalyst layer 30 through the macropores 50, and the exhaust gas can be purified efficiently. When the average pore size is 20 μm or less, the catalyst layer 30 can have sufficient strength, and it is possible to avoid an increase in pressure loss due to an excessively large volume of the catalyst layer 30 . In order to ensure sufficient diffusion of the exhaust gas over the entire catalyst layer 30, the macropores may have an average pore diameter of 2 to 10 μm, or may be 3 to 10 μm.

The average pore diameter of the macropores 50 can be obtained as follows. Using a scanning electron microscope (SEM), backscattered electron images of a plurality of arbitrary 50 μm square regions on the surface or cross section of the catalyst layer 30 are obtained. From the obtained backscattered electron image, the diameters of arbitrary 20 or more macropores 50 are obtained, and the average value is calculated. Here, the diameter of the macrohole 50 means the diameter in the direction perpendicular to the direction (longitudinal direction of the macrohole 50) in which the length of the macrohole 50 in the backscattered electron image is maximized when measured in all directions. , refers to the maximum length of the macropores 50 . When the macropores 50 are formed using a fibrous pore-forming material as described later, the macropores 50 observed in the backscattered electron image usually have an elongated shape. , the maximum length of the macropores in the direction perpendicular to the longitudinal direction of the macropores 50 can be assumed to be the diameter of the macropores, and the average pore diameter can be obtained.

The macropores 50 may have an average aspect ratio within the range of 9-40, preferably 9-30, more preferably 9-28. When the average aspect ratio of the macropores 50 is within the above range, the exhaust gas can diffuse in the catalyst layer 30 at an appropriate speed, so that the exhaust gas can be purified efficiently.

The average aspect ratio of macropores 50 can be obtained as follows. Backscattered electron images of a plurality of arbitrary 50 μm square regions on the surface or cross section of the catalyst layer 30 are obtained using SEM. From the obtained backscattered electron image, the aspect ratios of arbitrary 20 or more macropores 50 are obtained, and the average value is calculated. Here, the aspect ratio of the macropores 50 refers to the value of (longitudinal length of the macropores 50)/(maximum length of the macropores 50 in the direction perpendicular to the longitudinal direction of the macropores 50). .

The catalyst layer 30 may have a porosity of 2-30 vol %, preferably 5-20 vol %. When the porosity is within the above range, the exhaust gas purifying device 1 can have good exhaust gas purifying performance. The porosity of the catalyst layer 30 can be measured by a mercury intrusion method or a gas adsorption measurement method, or can be calculated by three-dimensional analysis such as FIB-SEM (Focused Ion Beam-Scanning Electron Microscope) or X-ray CT.

The second catalytic metal 35 is concentrated near the outer surface 36 of the catalyst layer 30 and the inner surface 37 of the catalyst layer 30 defining the macropores 50 . Specifically, both the concentration of the second catalytic metal 35 in the vicinity of the outer surface 36 and the concentration of the second catalytic metal 35 in the vicinity of the inner surface 37 are averaged over the entire catalyst layer 30 35 concentration higher. Substantially all of the second catalytic metal 35 included in the catalytic layer 30 may be located near the outer surface 36 and the inner surface 37 . Additionally, second catalytic metal 35 may be disposed near outer surface 36 in an amount of 30-70% of the total weight of second catalytic metal 35 contained in catalytic layer 30 . The second catalytic metal 35 may be disposed near the inner surface 37 in an amount of 30-70% of the total weight of the second catalytic metal 35 contained in the catalyst layer 30 . Also, the ratio of the weight of the second catalytic metal 35 arranged near the outer surface 36 to the weight of the second catalytic metal 35 arranged near the inner surface 37 is in the range of 3:7 to 7:3. can be As a result, the exhaust gas purifying device 1 exhibits particularly high HC purification performance and NOx emission performance, as will be shown in Examples described later.

The concentration of first catalytic metal 33 in the vicinity of outer surface 36 and inner surface 37 is equal to or less than the concentration of first catalytic metal 33 averaged over catalyst layer 30 . In particular, the first catalytic metal 33 may be disposed in a substantially uniform concentration throughout the catalytic layer 30, where the concentration of the first catalytic metal 33 near the outer surface 36 and the inner surface 37 is approximately equal to the catalytic It is equivalent to the concentration of first catalytic metal 33 averaged over layer 30 .

In the present application, the vicinity of the outer surface 36 refers to a region within 4 μm from the outer surface 36 toward the inside of the catalyst layer 30 . Similarly, the vicinity of the inner surface 37 refers to a region within 4 μm from the inner surface 37 toward the inside of the catalyst layer 30 . The concentration of the first catalytic metal 33 and the concentration of the second catalytic metal 35 at each location in the catalyst layer 30 are measured using an electron probe microanalyzer (EPMA). Further, in the present application, the concentration of the first catalyst metal 33 averaged over the entire catalyst layer 30 means the ratio of the total weight of the first catalyst metal 33 contained in the catalyst layer 30 to the total weight of the catalyst layer 30. means. The concentration of the second catalytic metal 35 averaged over the entire catalytic layer 30 means the ratio of the total weight of the second catalytic metal 35 contained in the catalytic layer 30 to the total weight of the catalytic layer 30 .

By arranging the first catalytic metal 33 and the second catalytic metal 35 in the catalyst layer 30 as described above, the exhaust gas purification device 1 exhibits high HC purification performance and NOx purification performance under rich conditions. , which the present inventors have discovered through intensive studies. The inventors consider the reason as follows.

The first catalytic metal 33 exhibits excellent catalytic action in the NOx purification reaction, and the second catalytic metal 35 exhibits excellent catalytic performance in the HC purification reaction. Part of the exhaust gas supplied to the exhaust gas purifier 1 according to the embodiment passes through the exhaust gas purifier 1 while contacting the outer surface 36 of the catalyst layer 30 . At this time, HC in the exhaust gas comes into contact with the second catalytic metal 35 with high probability. Thereby, HC in the exhaust gas is oxidized and converted into water and carbon dioxide. As a result, the HC content in the exhaust gas is reduced.

Part of the exhaust gas supplied to the exhaust gas purifier 1 enters the catalyst layer 30 through the outer surface 36 of the catalyst layer 30 and diffuses within the catalyst layer 30 . Also when the exhaust gas passes through the outer surface 36, the HC in the exhaust gas contacts the second catalytic metal 35 with high efficiency and is oxidized, thereby reducing the HC content in the exhaust gas. Therefore, the contact probability between the first catalytic metal 33 arranged in the region other than the vicinity of the outer surface 36 in the catalytic layer 30 and HC is low. Usually, HC coats the first catalytic metal 33 and deactivates the first catalytic metal 33, but in the exhaust gas purification device 1 of the embodiment, the contact probability between the first catalytic metal 33 and HC is is low, deactivation of the first catalytic metal 33 by HC is suppressed. As a result, NOx in the exhaust gas comes into contact with the active first catalytic metal 33 with high probability and is reduced and removed. Therefore, high NOx purification performance can be obtained.

Furthermore, at least part of the exhaust gas diffusing in the catalyst layer 30 passes near the inner surface 37 of the catalyst layer 30 or moves between the macropores 50 and the catalyst layer 30 through the inner surface 37 . At this time, HC contained in the exhaust gas contacts the second catalytic metal 35 with high probability and is oxidized. This further reduces the HC content in the exhaust gas.

II Method for Manufacturing Exhaust Gas Purifying Device An example of a method for manufacturing an exhaust gas purifying device according to the embodiment will be described. A method for manufacturing an exhaust gas purifier includes preparing a pore-forming material supporting a second catalyst metal (hereinafter also referred to as a "second catalyst metal-supporting pore-forming material"); preparing a slurry containing one catalyst metal, forming a slurry layer on a substrate, drying and calcining the slurry layer to form a catalyst layer, and applying a second catalyst metal on the outer surface of the catalyst layer and placing the second catalytic metal proximate the outer surface by imbibing a solution containing Each step will be described in order below.

(1) Preparation of Second Catalyst Metal-Supporting Pore-Forming Material First, a pore-forming material is prepared. A fibrous pore former may be used as the pore former. Examples of fibrous pore-forming materials include polyethylene terephthalate (PET) fibers, acrylic fibers, nylon fibers, rayon fibers, and cellulose fibers. The pore-forming material may be at least one selected from the group consisting of PET fibers and nylon fibers from the viewpoint of the balance between workability and firing temperature.

The fibrous pore formers may have an average diameter (mean fiber diameter) of 1-20 μm, preferably 2-10 μm, more preferably 3-10 μm. When the average diameter is within the above range, it is possible to form macropores having a size suitable for gas diffusion. The average diameter of the fibrous pore-forming material is calculated by randomly taking fifty or more fibrous pore-forming materials, measuring the fiber diameters, and calculating the average.

The fibrous pore former may have an average aspect ratio within the range of 9-40, preferably 9-30, more preferably 9-28. When the average aspect ratio is within the above range, macropores having an appropriate size are formed to allow the exhaust gas to diffuse at an appropriate speed, so that an exhaust gas purifying device capable of efficiently purifying the exhaust gas can be manufactured. Here, the average aspect ratio of fibrous pore formers is defined as (average fiber length)/(average diameter (average fiber diameter)). The fiber length means the linear distance between both ends of the fiber, and is calculated by taking out fifty or more fibrous pore-forming materials at random, measuring the fiber length, and calculating the average value.

The prepared pore-forming material is added to the solution of the compound containing the second catalytic metal, and the solvent is evaporated while stirring the solution. As a result, the second catalyst metal is supported on the pore-forming material, and a second catalyst metal-supporting pore-forming material is obtained.

Compounds containing the second catalyst metal include salts of the second catalyst metal (e.g., acetates, carbonates, nitrates, ammonium salts, citrates, dinitrodiammine salts), complexes of the second catalyst metal (e.g., , tetraammine complex) and the like can be used. The solution of the compound containing the second catalytic metal may contain water, alcohol, etc. as a solvent. The amount and concentration of this solution may be appropriately set according to the amount of the second catalytic metal to be placed near the inner surface of the catalyst layer of the target exhaust gas purifier.

A noble metal other than the second catalyst metal (excluding rhodium) may be further supported on the pore-forming material.

(2) Preparation of Slurry A slurry containing the pore-forming material supporting the second catalyst metal and the first catalyst metal is prepared. Specifically, the pore-forming material supporting the second catalytic metal, the compound containing the first catalytic metal, the carrier particles, and the dispersion medium (solvent) are mixed. Alternatively, the carrier particles preloaded with the first catalytic metal, the second catalytic metal-carrying pore-forming material, and the dispersion medium are mixed.

Compounds containing the first catalyst metal include salts of the first catalyst metal (e.g., acetates, carbonates, nitrates, ammonium salts, citrates, dinitrodiammine salts), complexes of the first catalyst metal ( For example, a tetraammine complex) and the like can be used.

As carrier particles, metal oxide particles such as aluminum oxide (Al ₂ O ₃ , alumina), cerium oxide (CeO ₂ , ceria), zirconium oxide (ZrO ₂ , zirconia), silicon oxide (SiO ₂ , silica), Particles of oxides such as yttrium oxide (Y ₂ O ₃ , yttria) and neodymium oxide (Nd ₂ O ₃ ) or composite oxides thereof can be used.

As the dispersion medium, any solvent capable of preparing a slurry may be appropriately used. For example, water, preferably pure water such as ion-exchanged water and distilled water may be used.

The slurry may further contain other ingredients as needed. Other components include binders, thickeners, compounds containing metals other than the first catalyst metal (e.g., salts and complexes), carrier particles preloaded with metals other than the first catalyst metal, and the like.

(3) Formation of catalyst layer A slurry layer is formed by coating the surface of the substrate with slurry. For example, the slurry layer can be formed by immersing the base material in the slurry, pressing the slurry into the base material using a press-fitting means, or using a wash coating method.

The slurry layer is then heated to dry and bake the slurry layer. That is, by heating, the solvent in the slurry layer is evaporated and the pore-forming material is removed. Thereby, a catalyst layer containing the first catalyst metal and the second catalyst metal is formed on the substrate.

From the viewpoint of preventing the pore-forming material from remaining and grain growth of the first catalytic metal and the second catalytic metal, the heating is carried out in an atmosphere at a temperature of 300 to 800° C., preferably 400 to 700° C. You can go with Heating may be performed for 20 minutes or more, and may be performed for 30 minutes to 2 hours. Moreover, heating may be performed in the atmosphere or in an inert gas such as nitrogen.

When the pore-forming material is burned off by heating, macropores having a shape corresponding to the shape of the pore-forming material are formed in the portions where the pore-forming material was present. The second catalytic metal supported on the pore-forming material remains near the inner surface surrounding the macropores. Thus, the catalytic layer has an inner surface defining macropores, with the second catalytic metal concentrated near the inner surface.

(4) Placement of Second Catalyst Metal Near Outer Surface Further, a solution of a compound containing the second catalyst metal is absorbed from the outer surface of the catalyst layer. Thereby, the second catalytic metal is concentrated in the vicinity of the outer surface. The compounds described above can be used as the compound containing the second catalytic metal. The amount and concentration of the solution to be used may be appropriately set according to the amount of the second catalytic metal to be arranged in the vicinity of the outer surface of the catalyst layer of the target exhaust gas purifier.

As described above, the exhaust gas purifier according to the embodiment is manufactured.

EXAMPLES The present invention will be specifically described below with reference to Examples and Comparative Examples, but the present invention is not limited to these Examples.

(1) Preparation of test body Example 1
A polyethylene terephthalate fiber was used as the fibrous pore former. The fibrous pore former was added to the aqueous palladium nitrate solution and the solution was heated with stirring to dryness. As a result, a Pd-supporting pore-forming material in which Pd was supported on the fibrous pore-forming material was obtained.

Alumina-ceria-zirconia composite oxide, alumina-zirconia composite oxide, alumina, Pd-supporting pore former, rhodium nitrate, water, alumina binder, and thickener were mixed to prepare slurry. The concentrations of alumina-ceria-zirconia composite oxide, alumina-zirconia composite oxide and alumina in the slurry were 99 g/L, 41 g/L and 45 g/L, respectively. In the slurry, the Pd content was 0.075 wt% and the Rh content was 0.25 wt% based on the total solid weight.

The slurry was coated over the entire surface of a honeycomb-shaped monolith substrate having a volume of 0.875 L. Coating was performed by a suction method. The slurry on the substrate was then dried and fired. Thereby, a catalyst layer was formed on the substrate. The outer surface of the catalyst layer was imbibed with an aqueous palladium nitrate solution containing Pd in an amount of 0.075 wt% based on the total solid weight in the slurry. Thus, a specimen of Example 1 was obtained.

In addition, the concentration of Pd averaged over the entire catalyst layer of the prepared test specimen is the content of Pd in the slurry based on the total solid weight in the slurry and the aqueous solution of palladium nitrate absorbed on the outer surface of the catalyst layer. is substantially equal to the sum of the Pd contents in (i.e., 0.15 wt%).

Example 2
By adjusting the concentration of the palladium nitrate aqueous solution mixed with the fibrous pore-forming material, the Pd content based on the total solid weight in the slurry was set to 0.03 wt%, and the Pd was absorbed on the outer surface of the catalyst layer. A test specimen was prepared in the same manner as in Example 1, except that the amount of Pd contained in the palladium nitrate aqueous solution was set to 0.12 wt % based on the total solid weight in the slurry.

Example 3
By adjusting the concentration of the palladium nitrate aqueous solution mixed with the fibrous pore-forming material, the Pd content based on the total solid weight in the slurry was set to 0.06 wt%, and the Pd content was absorbed on the outer surface of the catalyst layer. A test specimen was prepared in the same manner as in Example 1, except that the amount of Pd contained in the palladium nitrate aqueous solution was set to 0.09 wt% based on the total solid weight in the slurry.

Example 4
By adjusting the concentration of the palladium nitrate aqueous solution mixed with the fibrous pore-forming material, the Pd content based on the total solid weight in the slurry was set to 0.09 wt%, and the Pd was absorbed on the outer surface of the catalyst layer. A test specimen was prepared in the same manner as in Example 1, except that the amount of Pd contained in the palladium nitrate aqueous solution was set to 0.06 wt% based on the total solid weight in the slurry.

Example 5
By adjusting the concentration of the palladium nitrate aqueous solution mixed with the fibrous pore-forming material, the Pd content based on the total solid weight in the slurry was set to 0.12 wt%, and the Pd content was absorbed on the outer surface of the catalyst layer. A test specimen was prepared in the same manner as in Example 1, except that the amount of Pd contained in the palladium nitrate aqueous solution was set to 0.03 wt% based on the total solid weight in the slurry.

Comparative example 1
Alumina-ceria-zirconia composite oxide, alumina-zirconia composite oxide, alumina, fibrous pore former, palladium nitrate, rhodium nitrate, water, alumina binder, and thickener were mixed to prepare slurry. The concentrations of alumina-ceria-zirconia composite oxide, alumina-zirconia composite oxide and alumina in the slurry were 99 g/L, 41 g/L and 45 g/L, respectively. In the slurry, the Pd content was 0.15 wt% and the Rh content was 0.25 wt% based on the total solid weight.

A catalyst layer was formed on a substrate in the same manner as in Example 1 using the obtained slurry. Thus, a specimen of Comparative Example 1 was obtained.

Comparative example 2
Alumina-ceria-zirconia composite oxide, alumina-zirconia composite oxide, alumina, fibrous pore former, rhodium nitrate, water, alumina binder, and thickener were mixed to prepare slurry. The concentrations of alumina-ceria-zirconia composite oxide, alumina-zirconia composite oxide and alumina in the slurry were 99 g/L, 41 g/L and 45 g/L, respectively. In the slurry, the content of Rh based on total solid weight was 0.25 wt%.

A catalyst layer was formed on a substrate in the same manner as in Example 1 using the obtained slurry. The outer surface of the catalyst layer was imbibed with an aqueous palladium nitrate solution containing Pd in an amount of 0.15 wt% based on the total solid weight in the slurry. Thus, a specimen of Comparative Example 2 was obtained.

Comparative example 3
A catalyst layer was formed on a substrate in the same manner as in Example 1, except that the Pd content based on the total solid weight in the slurry was 0.15 wt%. Thus, a specimen of Comparative Example 3 was obtained.

(2) Analysis by Electron Probe Microanalyzer (EPMA) Cross-sectional SEM observation and elemental analysis of the catalyst layer of each test piece of Example 1 and Comparative Examples 1 to 3 were performed using EPMA. A cross-sectional SEM image of Example 1 is shown in FIG. 2, a mapping image (Pd mapping image) showing the concentration distribution of Pd is shown in FIG. 3, and a mapping image (Rh mapping image) showing the concentration distribution of Rh is shown in FIG. A cross-sectional SEM image of Comparative Example 1 is shown in FIG. 5, a Pd mapping image is shown in FIG. 6, and an Rh mapping image is shown in FIG. A cross-sectional SEM image of Comparative Example 2 is shown in FIG. 8, a Pd mapping image is shown in FIG. 9, and an Rh mapping image is shown in FIG. A cross-sectional SEM image of Comparative Example 3 is shown in FIG. 11, a Pd mapping image is shown in FIG. 12, and an Rh mapping image is shown in FIG.

From the SEM images of FIGS. 2, 5, 8, and 11, the catalyst layer has an inner surface defining macropores in all test specimens of Example 1 and Comparative Examples 1 to 3. was confirmed.

From the Pd mapping image in FIG. 3, in the test piece of Example 1, Pd is unevenly distributed near the outer surface of the catalyst layer and the inner surface of the catalyst layer that defines the macropores (that is, near the outer surface and the inner surface was higher than the Pd concentration averaged over the entire catalyst layer). From the Pd mapping image in FIG. 6, in the specimen of Comparative Example 1, Pd is distributed in a substantially uniform concentration throughout the catalyst layer (that is, the concentration of Pd near the inner surface is the average Pd concentration in the entire catalyst layer). is equivalent to the concentration of ). From the Pd mapping image in FIG. 9, in the specimen of Comparative Example 2, Pd is unevenly distributed near the outer surface of the catalyst layer (that is, the Pd concentration near the outer surface is the average Pd concentration of the entire catalyst layer higher) and confirmed that the concentration of Pd in the vicinity of the inner surface was equal to or less than the concentration of Pd averaged over the entire catalyst layer. From the Pd mapping image in FIG. 12, in the test piece of Comparative Example 3, Pd is unevenly distributed near the inner surface of the catalyst layer (that is, the Pd concentration near the inner surface is the average Pd concentration of the entire catalyst layer higher) and confirmed that the concentration of Pd in the vicinity of the outer surface was equal to or less than the concentration of Pd averaged over the entire catalyst layer.

From the Rh mapping images in FIGS. 4, 7, 10, and 13, Rh is distributed at a substantially uniform concentration throughout the catalyst layer in both the test specimens of Example 1 and Comparative Examples 1 to 3. (That is, it was confirmed that the concentration of Rh in the vicinity of the inner surface and the outer surface of the catalyst layer is equivalent to the concentration of Rh averaged over the entire catalyst layer).

(3) Performance Evaluation The HC purifying performance and NOx purifying performance of each test body of Examples 1 to 5 and Comparative Examples 1 to 3 were evaluated as follows.

Each test body was placed directly under the engine. The operating conditions of the engine were set so that the temperature of the specimen was 950°C. Air-fuel ratio feedback control and fuel cut were alternately repeated to age the specimen for 50 hours.

The operating conditions of the engine were set so that the temperature of the gas flowing into the test body was 550°C. The air-fuel ratio (A/F) of the mixed gas supplied to the engine was set to 14.4, and the exhaust gas from the engine was allowed to flow into the test specimen. After the temperature of the gas flowing into the test body stabilized at 550°C, the air-fuel ratio (A/F) of the mixed gas supplied to the engine was changed to 14.1 (rich) and 15.1 (lean (excess oxygen) at intervals of 3 minutes. )) alternately. Three minutes after the A/F was switched from 15.1 (lean) to 14.1 (rich) for the third time, HC emissions and NOx emissions were measured.

14 and 15 show the HC emissions and NOx emissions of the test specimens of Example 1 and Comparative Examples 1 to 3, respectively. The test specimen of Example 1 exhibited lower HC emissions and NOx emissions than any of the specimens of Comparative Examples 1-3.

16 and 17 show the ratio of the amount of Pd disposed near the outer surface of the catalyst layer to the total amount of Pd in the catalyst layer, based on the evaluation results of the specimens of Examples 1 to 5 and Comparative Examples 2 and 3. 1 is a graph plotting HC and NOx emissions versus percentage; FIG. 16 shows that the HC emission amount when 30% or more and less than 100% of Pd is arranged near the outer surface is equal to or less than the HC emission amount when all Pd is arranged near the outer surface. ing. FIG. 17 shows that the HC emissions when more than 0% and 70% or less of Pd were placed near the outer surface were less than or equal to the HC emissions when all the Pd was placed near the inner surface. ing. Therefore, it was shown that particularly low HC and NOx emissions are achieved when Pd is placed near the outer surface of the catalyst layer in an amount of 30-70% of the total Pd amount.

1: Exhaust gas purification device, 10: Base material, 30: Catalyst layer, 31: Carrier, 33: First catalyst metal, 35: Second catalyst metal, 36: Outer surface, 37: Inner surface, 50: Macropores

Claims (1)

a substrate;
a catalyst layer disposed on the substrate;
with
the catalyst layer comprises a first catalyst metal and a second catalyst metal;
the first catalytic metal is rhodium;
the second catalytic metal is at least one of palladium and platinum;
the catalyst layer has an outer surface and an inner surface defining macropores;
The concentration of the first catalytic metal in the vicinity of the outer surface and the concentration of the first catalytic metal in the vicinity of the inner surface are equal to or less than the concentration of the first catalytic metal averaged over the entire catalyst layer. is less than
the concentration of the second catalytic metal in the vicinity of the outer surface and the concentration of the second catalytic metal in the vicinity of the inner surface are higher than the concentration of the second catalytic metal averaged over the entire catalyst layer; Exhaust gas purifier.

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