Manufacturing method for exhaust gas purification material and manufacturing method for exhaust gas purification device

Abstract

To provide a method for producing an exhaust gas purifying material and an exhaust gas purification device capable of removing harmful components with high efficiency even after exposure to a high temperature environment.SOLUTION: There is provided a method for producing exhaust gas purifying material which comprises: (a) impregnating a metal oxide carrier with a rhodium compound solution, (b) drying the metal oxide carrier impregnated with the rhodium compound solution to obtain a rhodium-containing catalyst which contains the metal oxide carrier and rhodium particles supported on the metal oxide carrier, (c) heating the rhodium-containing catalyst to a temperature in the range of 700 to 900°C under an inert atmosphere and (d) mixing the rhodium-containing catalyst with a material having a higher basicity than te metal oxide support, in this order.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a method for producing an exhaust gas purification material and a method for producing an exhaust gas purification device. [Background technology]

[0002] Exhaust gases emitted from internal combustion engines used in automobiles and other vehicles contain harmful components such as carbon monoxide (CO), hydrocarbons (HC), and nitrogen oxides (NOx). Regulations on the emission of these harmful components are becoming stricter every year, and precious metals such as platinum (Pt), palladium (Pd), and rhodium (Rh) are used as catalysts to remove these harmful components.

[0003] On the other hand, from the viewpoint of resource risks, there is a demand for reducing the amount of precious metals used. One known method for reducing the amount of precious metals used in exhaust gas purification devices is to support the precious metals as fine particles on a support. For example, Patent Document 1 discloses a method for producing an exhaust gas purification material, which includes a step of supporting precious metal particles on an oxide support to form a precious metal-supported catalyst, and a step of heat-treating the precious metal-supported catalyst in a reducing atmosphere to control the particle size of the precious metal within a predetermined range. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Laid-Open No. 2016-147256 Summary of the Invention [Problem to be solved by the invention]

[0005] The inventors have conducted extensive research and found that the catalytic activity of the exhaust gas purification material obtained by the production method described in Patent Document 1 may decrease when used in a high-temperature environment.

[0006] Therefore, an object of the present invention is to provide an exhaust gas purification material and an exhaust gas purification device that can remove harmful components with high efficiency even after being exposed to a high-temperature environment. [Means for solving the problem]

[0007] The present invention can be embodied in the following manner, for example. [Section 1] A method for producing an exhaust gas purification material, (a) impregnating a metal oxide support with a rhodium compound solution; (b) drying the metal oxide support impregnated with the rhodium compound solution to obtain a rhodium-containing catalyst comprising the metal oxide support and rhodium particles supported on the metal oxide support; (c) heating the rhodium-containing catalyst to a temperature in the range of 700 to 900°C under an inert atmosphere; (d) mixing the rhodium-containing catalyst with a material having a higher basicity than the metal oxide support; A method comprising, in this order: [Section 2] Item 2. The method according to Item 1, wherein in the rhodium-containing catalyst after step (c), the rhodium particles have a mean particle size distribution of 1.5 to 18 nm and a standard deviation of the particle size distribution of less than 1.6 nm. [Section 3] Item 3. The method according to Item 2, wherein the rhodium particles in the rhodium-containing catalyst after step (c) have an average particle size distribution of 4 to 14 nm. [Section 4] Item 3. The method according to Item 2, wherein the rhodium particles in the rhodium-containing catalyst after step (c) have an average particle size distribution of 2 to 8 nm. [Section 5] Item 5. The method according to any one of Items 1 to 4, wherein the rhodium-containing catalyst contains 0.01 to 2 wt % of the rhodium particles based on the total weight of the metal oxide support and the rhodium particles. [Section 6] Item 6. The method according to any one of Items 1 to 5, wherein the metal oxide support is an oxide containing zirconia as a main component, a composite oxide containing zirconia and alumina as main components, or a composite oxide containing zirconia, alumina, and ceria as main components. [Section 7] Item 7. The method according to any one of Items 1 to 6, wherein the metal oxide support is a composite oxide containing zirconia, alumina, and ceria as main components, and the material having a higher basicity than the metal oxide support is a composite oxide containing ceria and zirconia as main components. [Section 8] 8. The method according to any one of items 1 to 7, wherein the inert atmosphere is a nitrogen atmosphere. [Section 9] Obtaining an exhaust gas purification material by the method according to any one of items 1 to 8; disposing the exhaust gas purification material on a substrate; A method for manufacturing an exhaust gas purification device, comprising: [Effects of the Invention]

[0008] The exhaust gas purification material and exhaust gas purification device produced by the method of the present invention can remove harmful components with high efficiency even after being exposed to a high-temperature environment. DETAILED DESCRIPTION OF THE INVENTION

[0009] Hereinafter, embodiments of the present disclosure will be described with reference to the accompanying drawings as appropriate. The present invention is not limited to the following embodiments, and various design modifications can be made without departing from the spirit of the present invention as set forth in the claims. In this application, numerical ranges expressed using the symbol "to" include the numerical values ​​before and after the symbol "to" as the lower and upper limits, respectively. The upper and lower limit values ​​of the numerical ranges described in this application can be combined in any way.

[0010] (1) Exhaust gas purification materials First, an exhaust gas purification material produced by the method according to the embodiment will be described. The exhaust gas purification material is a mixture of a metal oxide support, an Rh-containing catalyst including Rh particles supported on the metal oxide support, and a material having a higher basicity than the metal oxide support.

[0011] Examples of metal oxide supports include oxides of at least one metal selected from the group consisting of metals in Groups 3, 4, and 13 of the periodic table and lanthanoid metals. When the metal oxide support contains two or more metal elements, the metal oxide support may be a mixture of oxides of the two or more metal elements, a composite oxide containing the two or more metal elements, or a mixture of an oxide of at least one metal element and at least one composite oxide.

[0012] The metal oxide support may be, for example, an oxide of at least one metal selected from the group consisting of scandium (Sc), yttrium (Y), lanthanum (La), cerium (Ce), neodymium (Nd), samarium (Sm), europium (Eu), lutetium (Lu), titanium (Ti), zirconium (Zr), and aluminum (Al), preferably an oxide of at least one metal selected from the group consisting of Y, La, Ce, Ti, Zr, and Al, more preferably an oxide of at least one metal selected from the group consisting of Al, Ce, and Zr. The metal oxide support may be an oxide containing zirconia (ZrO) as a major component, a composite oxide containing zirconia and alumina (AlO) as major components (Al-Zr composite oxide), or a composite oxide containing zirconia, alumina, and ceria (CeO) as major components (Al-Ce-Zr composite oxide). Zirconia can have the function of maintaining the catalytic activity of Rh particles. Ceria can function as an OSC (Oxygen Storage Capacity) material that absorbs oxygen in an oxygen-rich atmosphere and releases oxygen in an oxygen-deficient atmosphere. Alumina can have the function of suppressing the diffusion of Rh particles. The metal oxide support contains alumina, ceria, and zirconia as main components, and may further contain yttria (YO), lanthana (LaO), neodymia (NdO), or praseodymia (PrO 11 ) may be particles of a composite oxide containing at least one of the following. Yttria, lanthana, neodymia, and praseodymia improve the heat resistance of the composite oxide. In this application, "containing as a main component" means that the content of the component is 50 wt % or more, 70 wt % or more, 80 wt % or more, or 90 wt % or more of the total weight, and when there are multiple main components, it means that the total content of those components is 50 wt % or more, 70 wt % or more, 80 wt % or more, or 90 wt % or more.

[0013] The metal oxide support may be in the form of particles and may have any particle size depending on the purpose.

[0014] Rh particles supported on a metal oxide support function as a catalyst for removing harmful components from exhaust gases. The mean particle size distribution of the Rh particles may be within the range of 1.5 to 18 nm. Generally, the smaller the particle size of Rh particles, the larger the specific surface area of ​​the Rh particles and the higher the catalytic performance. However, Rh particles with excessively small particle sizes tend to coarsen due to Ostwald ripening and aggregation in high-temperature environments, causing a deterioration in catalytic performance. When the mean particle size distribution of the Rh particles is 1.5 nm or more, coarsening of the Rh particles in high-temperature environments is suppressed, thereby suppressing a decrease in catalytic performance. Furthermore, when the mean particle size distribution of the Rh particles is 18 nm or less, the specific surface area of ​​the Rh particles is sufficiently large, allowing the Rh particles to exhibit high catalytic performance. The mean particle size distribution of the Rh particles may be within the range of 3 to 17 nm or within the range of 4 to 14 nm. The mean particle size distribution of the Rh particles may be within the range of 2 to 8 nm.

[0015] Furthermore, the standard deviation of the particle size distribution of the Rh particles may be less than 1.6 nm. As shown in the Reference Examples below, a standard deviation of the particle size distribution of the Rh particles less than 1.6 nm allows the exhaust gas purification material to remove harmful components with high efficiency even after exposure to a high-temperature environment. As a result, the number of coarse Rh particles and the number of fine Rh particles that tend to coarsen in a high-temperature environment are reduced. Therefore, the Rh particles can have a sufficiently large specific surface area even after the exhaust gas purification material is exposed to a high-temperature environment, thereby enabling the material to exhibit high catalytic performance. The standard deviation of the particle size distribution of the Rh particles may be 1 nm or less.

[0016] In the present application, the particle size distribution of Rh particles is a particle size distribution based on the number of particles, which is obtained by measuring the projected area equivalent circle diameter of 50 or more Rh particles based on an image obtained by a transmission electron microscope (TEM).

[0017] The amount of Rh particles supported, i.e., the proportion of Rh particles based on the total weight of the metal oxide support and Rh particles, may be within the range of 0.01 to 2 wt %. A proportion of Rh particles of 0.01 wt % or more ensures that a sufficient amount of Rh particles is present, enabling effective removal of harmful components in exhaust gas. A proportion of Rh particles of 2 wt % or less allows for a reduction in the amount of Rh used. Furthermore, because the Rh particles are sufficiently loosely supported on the metal oxide support, coarsening of the Rh particles in high-temperature environments is suppressed, enabling sufficient durability against high temperatures. The proportion of Rh particles based on the total weight of the metal oxide support and Rh particles may be within the range of 0.2 to 1.8 wt %.

[0018] A material having a higher basicity than the metal oxide support (hereinafter referred to as a "highly basic material") may be particulate. The highly basic material may be, for example, a material that functions as an OSC material. For example, ceria and composite oxides containing ceria (e.g., composite oxides containing ceria as the main component, composite oxides containing ceria and zirconia as the main components (Ce-Zr composite oxides), and composite oxides containing alumina, ceria, and zirconia as the main components (Al-Ce-Zr composite oxides)) can function as OSC materials. In particular, Ce-Zr composite oxides are preferred because they have high oxygen storage capacity and are relatively inexpensive. The ceria-containing composite oxide may contain at least one of praseodymia, lanthana, yttria, and neodymia as an additive in addition to the main component, and these additives may form a composite oxide together with the main component. When the exhaust gas purification material contains a material that functions as an OSC material, the exhaust gas purification material can exhibit good exhaust gas purification performance both in an oxygen-excess atmosphere and in an oxygen-deficient atmosphere.

[0019] The overbased material may be in particulate form and may have any particle size depending on the purpose.

[0020] In this application, a "material having a higher basicity than the metal oxide support" refers to a material having an average electronegativity lower than that of the metal oxide support. The "average electronegativity" is a value obtained by weighting the Pauling electronegativity (hereinafter simply referred to as "electronegativity") of the constituent elements according to the number of each element per unit weight. For example, the average electronegativity of composite oxide particles (ACZ particles) containing Al2O3, CeO2, ZrO2, La2O3, YO3, and Nd2O3 in the following weight fractions: Al2O3: 30 wt%, CeO2: 20 wt%, ZrO2: 44 wt%, La2O3: 2 wt%, YO3: 2 wt%, and Nd2O3: 2 wt% is calculated as follows:

[0021] Average electronegativity of ACZ particles = Electronegativity of Al × Weight fraction of Al2O3 / Formula weight of Al2O3 × 2 + Electronegativity of Ce × Weight fraction of CeO2 / Formula weight of CeO2 + Electronegativity of Zr × Weight fraction of ZrO2 / Formula weight of ZrO2 + Electronegativity of La × Weight fraction of La2O3 / Formula weight of La2O3 × 2 + Electronegativity of Y × Weight fraction of Y2O3 / Formula weight of Y2O3 × 2 + Electronegativity of Nd × Weight fraction of Nd2O3 / Formula weight of Nd2O3 × 2 + Electronegativity of O × (weight fraction of Al2O3 / formula weight of Al2O3 × 3 + weight fraction of CeO2 / formula weight of CeO2 × 2 + weight fraction of ZrO2 / formula weight of ZrO2 × 2 + weight fraction of La2O3 / formula weight of La2O3 × 3 + weight fraction of Y2O3 / formula weight of Y2O3 × 3 + weight fraction of Nd2O3 / formula weight of Nd2O3 × 3) =1.61×0.3 / 101.9×2 +1.12×0.2 / 172.1 +1.33×0.44 / 123.2 +1.10×0.02 / 325.8×2 +1.22×0.02 / 225.8×2 +1.14×0.02 / 336.4×2 +3.44×(0.3 / 101.9×3+0.2 / 172.1×2+0.44 / 123.2×2+0.02 / 325.8×3+0.02 / 225.8×3+0.02 / 336.4×3) =0.081

[0022] Also, CeO2, ZrO2, and Pr6O 11 The following weight fractions: CeO2: 51.4 wt%, ZrO2: 45.6 wt%, Pr6O 11 The average electronegativity of the composite oxide particles (CZ particles) contained at 3.0 wt % is calculated as follows.

[0023] Average electronegativity of CZ particles = Electronegativity of Ce × Weight fraction of CeO2 / Formula weight of CeO2 + Electronegativity of Zr × Weight fraction of ZrO2 / Formula weight of ZrO2 + Electronegativity of Pr × Pr6O 11 Weight fraction of PrO 11 Formula weight x 6 + Electronegativity of O × (weight fraction of CeO2 / formula weight of CeO2 × 2 + weight fraction of ZrO2 / formula weight of ZrO2 × 2 + PrO 11 Weight fraction of PrO 11 Formula weight x 11) =1.12×0.514 / 172.1 +1.33×0.456 / 123.2 +1.13×0.03 / 1021.4×6 +3.44×(0.514 / 172.1×2+0.456 / 123.2×2+0.03 / 1021.4×11) =0.056

[0024] From the above calculations, the CZ particles of the above composition have a smaller average electronegativity than the ACZ particles of the above composition, and therefore have a higher basicity than the ACZ particles of the above composition.

[0025] (2) Manufacturing method of exhaust gas purification material The method for producing the exhaust gas purification material includes the steps of impregnating a metal oxide support with a rhodium compound solution (Step S1), drying the metal oxide support impregnated with the rhodium compound solution to obtain an Rh-containing catalyst containing the metal oxide support and Rh particles supported on the metal oxide support (Step S2), heating the Rh-containing catalyst to a temperature in the range of 700 to 900°C in an inert atmosphere (Step S3), and mixing the Rh-containing catalyst with a highly basic material (Step S4), in this order. Each step will be described in turn.

[0026] First, a metal oxide support is impregnated with a rhodium compound solution (step S1). Examples of the rhodium compound solution include an aqueous rhodium hydroxide solution and an aqueous rhodium nitrate solution. The impregnation method is not particularly limited. For example, the metal oxide support and the rhodium compound solution can be added to distilled water while stirring, thereby impregnating the metal oxide support with the rhodium compound solution.

[0027] Next, the metal oxide support impregnated with the rhodium compound solution is dried (step S2). This results in an Rh-containing catalyst containing the metal oxide support and Rh particles supported on the metal oxide support. If necessary, calcination may be performed after drying. In the Rh-containing catalyst, the proportion of Rh particles based on the total weight of the Rh-containing catalyst (i.e., the sum of the weights of the metal oxide support and the Rh particles) may be in the range of 0.01 to 2 wt %, particularly 0.2 to 1.8 wt %.

[0028] The Rh-containing catalyst is heated to a temperature in the range of 700 to 900°C in an inert atmosphere (step S3). Examples of the inert atmosphere include a nitrogen atmosphere and an argon atmosphere. The heating time may be appropriately set, and may be, for example, 1 to 8 hours.

[0029] Heating in an inert atmosphere allows the average and standard deviation of the particle size distribution of the Rh particles in the Rh-containing catalyst to be appropriately controlled. Specifically, the average particle size distribution of the Rh particles can be set to a range of 1.5 to 18 nm, 3 to 17 nm, 4 to 14 nm, or 2 to 8 nm, and the standard deviation of the particle size distribution of the Rh particles can be set to less than 1.6 nm or 1 nm or less.

[0030] However, when heating in a reducing atmosphere such as a hydrogen atmosphere, it is difficult to obtain the above particle size distribution because the Rh particles cannot be made sufficiently large, as will be shown in the examples described later.When heating in an oxidizing atmosphere such as an air atmosphere, the Rh particles are dissolved in the metal oxide support, resulting in a decrease in the number of Rh particles on the surface of the metal oxide support.

[0031] Thereafter, the Rh-containing catalyst is mixed with the highly basic material (step S4). The mixing method is not particularly limited, but for example, the Rh-containing catalyst and the highly basic material may be mixed while being pulverized. This results in a powdered exhaust gas purification material. This may be molded into any shape, such as pellets, by press molding or the like.

[0032] Typically, when fine Rh particles with a particle size of several nanometers are exposed to a high-temperature environment (e.g., 1000°C or higher), Ostwald ripening causes the formation of Rh particles with increased particle size. According to the present inventors, Rh on a highly basic material is more stable in a trivalent (oxide) state than in a zero-valent (metal) state. Furthermore, Rh in the oxide state is more likely to evaporate and move. Therefore, Rh atoms collide more frequently on the highly basic material, making it more likely for coarse Rh particles to form than on a metal oxide support. When an exhaust gas purification material containing a Rh-containing catalyst and a highly basic material is used in a high-temperature environment, Rh atoms in the fine Rh particles on the metal oxide support migrate to the highly basic material, forming coarse Rh particles on the highly basic material. Therefore, when a Rh-containing catalyst is used in combination with a highly basic material, the Rh particles tend to coarsen, resulting in a decrease in purification performance, compared to when the Rh-containing catalyst is used alone. However, in the manufacturing method of the embodiment, as described above, the average and standard deviation of the particle size distribution of the Rh particles on the metal oxide support are controlled by heating in an inert atmosphere, thereby reducing the number of excessively small Rh particles. This prevents or reduces the migration of Rh atoms to the highly basic material and the formation of coarse Rh particles due to Ostwald ripening when the exhaust gas purification material is exposed to a high-temperature environment. Therefore, the exhaust gas purification performance of the exhaust gas purification material manufactured by the manufacturing method of the embodiment is less likely to deteriorate even in a high-temperature environment.

[0033] Furthermore, in the manufacturing method of the embodiment, Rh particles having an appropriately controlled particle size are formed by a simple process that uses an impregnation method using a rhodium compound solution and heating under an inert atmosphere, and therefore the manufacturing method of the embodiment has high production efficiency and is suitable for mass production.

[0034] (3) Manufacturing method of exhaust gas purification device An exhaust gas purification device can be manufactured by disposing the above exhaust gas purification material on a substrate.

[0035] The exhaust gas purification material may be disposed on a substrate together with a binder, additives, and the like.

[0036] The substrate is not particularly limited, but may be, for example, a monolith substrate having a honeycomb structure. The substrate may be formed from, for example, a ceramic material having high heat resistance, such as cordierite (2MgO·2Al2O3·5SiO2), alumina, zirconia, or silicon carbide, or a metal material made of metal foil, such as stainless steel. From the viewpoint of cost, the substrate is preferably made of cordierite.

[0037] When the substrate is a porous body having a plurality of pores, the exhaust gas purification material may be disposed on the inner surface of the substrate that defines the pores. In other words, in this application, "disposed on the substrate" includes both being disposed on the outer surface of the substrate and being disposed on the inner surface of the substrate.

[0038] The exhaust gas purification material can be disposed on a substrate, for example, as follows. First, a slurry containing the exhaust gas purification material is prepared. The slurry may further contain a binder, additives, etc. The properties of the slurry, such as viscosity and particle size of the solid components, may be adjusted as appropriate. The prepared slurry is applied to a predetermined region of the substrate. For example, the predetermined region of the substrate is immersed in the slurry, and after a predetermined time has passed, the substrate is pulled out of the slurry, thereby applying the slurry to the predetermined region of the substrate. Alternatively, the slurry may be applied to the substrate by pouring it onto the substrate and spreading it by blowing air with a blower. Next, the slurry is dried and fired at a predetermined temperature and time. This results in the exhaust gas purification material being disposed on the substrate.

[0039] The exhaust gas purification device according to the embodiment can be applied to various vehicles equipped with an internal combustion engine. [Example]

[0040] The present invention will be specifically explained below with reference to examples, but the present invention is not limited to these examples.

[0041] Examples 1-5 (1) Sample preparation As a metal oxide support, composite oxide particles containing Al2O3, CeO2, and ZrO2 as main components, and further containing La2O3, Y2O3, and Nd2O3 (hereinafter referred to as "ACZ particles" where appropriate. The weight fractions of each component in the ACZ particles were Al2O3: 30 wt%, CeO2: 20 wt%, ZrO2: 44 wt%, La2O3: 2 wt%, Y2O3: 2 wt%, and Nd2O3: 2 wt%) were prepared.

[0042] While stirring the distilled water, 10 g of ACZ particles and 8.0 g of an aqueous rhodium hydroxide solution (concentration: 0.5 wt%) were added in that order and stirred for 10 minutes. The resulting mixture was dried and calcined in an electric furnace under an air atmosphere at 500°C for 2 hours. This resulted in an Rh-containing catalyst containing ACZ particles and rhodium (Rh) particles supported on the ACZ particles. The Rh-containing catalyst contained 0.34 wt% Rh particles based on the total weight of the ACZ particles and Rh particles.

[0043] The Rh-containing catalyst was heated for 5 hours under a nitrogen atmosphere at the temperature shown in Table 1. After heating, the Rh-containing catalyst was observed under a transmission electron microscope (TEM) to determine the particle size distribution of the Rh particles (initial Rh particles) supported on the ACZ particles. The average and standard deviation of the particle size distribution of the initial Rh particles are shown in Table 1.

[0044] After heating, the Rh-containing catalyst contains CeO2 and ZrO2 as the main components, and also contains Pr6O 11 Composite oxide particles containing CeO2 (hereinafter referred to as "CZ particles" as appropriate). The weight fractions of the components in the CZ particles are CeO2: 51.4 wt%, ZrO2: 45.6 wt%, PrO 11 10 g of a soluble fiber (3.0 wt %) was added, and the mixture was crushed and mixed in a mortar. 2 g of the resulting powder was weighed out and molded into pellets.

[0045] (2) Aging treatment and measurement of the average particle size of Rh particles afterwards The pellets were heated to 1100°C and exposed to a stoichiometric (air-fuel ratio A / F = 14.6) mixture and an oxygen-rich (lean: A / F > 14.6) mixture alternately at a fixed time ratio of 1:1 for 5 hours. The average particle size of Rh particles in the pellets of Examples 2 and 4 was then determined by the carbon monoxide pulse method. The results are shown in Table 1.

[0046] (3) Exhaust gas purification performance evaluation After the aging treatment, a gas having the composition shown in Table 2 was passed through the pellet at a flow rate of 15 L / min. The pellet was heated to 600°C, maintained at this temperature for 5 minutes, and then cooled to 150°C. Then, while continuing to pass the gas, the pellet was heated to 600°C at a rate of 20°C / min, and the temperature of the pellet when 50% of the NOx in the gas was removed (hereinafter referred to as "NOx-T50") was measured. The results are shown in Table 1.

[0047] Comparative Example 1 Pellets were prepared in the same manner as in Example 1, except that the Rh-containing catalyst was not heated in a nitrogen atmosphere. The average and standard deviation of the particle size distribution of the initial Rh particles were as shown in Table 1. The pellets were subjected to an aging treatment and evaluated for exhaust gas purification performance in the same manner as in Example 1. The results are shown in Table 1.

[0048] Comparative Example 2-3 Pellets were prepared in the same manner as in Example 1, except that the heating temperature of the Rh-containing catalyst in a nitrogen atmosphere was set as shown in Table 1. The average and standard deviation of the particle size distribution of the initial Rh particles were as shown in Table 1. The pellets were subjected to an aging treatment and evaluated for exhaust gas purification performance in the same manner as in Example 1. The results are shown in Table 1.

[0049] Comparative Example 4 Pellets were prepared in the same manner as in Example 3, except that the Rh-containing catalyst was heated in an air atmosphere instead of a nitrogen atmosphere. When the heated Rh-containing catalyst was observed with a TEM, no Rh particles supported on the ACZ particles were confirmed. It is believed that heating in an air atmosphere caused Rh to form a solid solution in the ACZ particles. The pellets were subjected to an aging treatment and an evaluation of their exhaust gas purification performance was carried out in the same manner as in Example 1. The results are shown in Table 1.

[0050] Comparative Example 5 Pellets were prepared in the same manner as in Example 3, except that the Rh-containing catalyst was heated in a hydrogen atmosphere instead of a nitrogen atmosphere. The average and standard deviation of the particle size distribution of the initial Rh particles were as shown in Table 1. The pellets were subjected to an aging treatment and the exhaust gas purification performance of the pellets was evaluated in the same manner as in Example 1. The results are shown in Table 1.

[0051] Comparative Example 6 Pellets were produced in the same manner as in Example 4, except that ACZ particles were used instead of CZ particles. The pellets were subjected to an aging treatment and the exhaust gas purification performance of the pellets was evaluated in the same manner as in Example 1. The results are shown in Table 1.

[0052] Comparative Example 7 Pellets were prepared in the same manner as in Comparative Example 6, except that the Rh-containing catalyst was not heated under a nitrogen atmosphere. The pellets were subjected to an aging treatment and the exhaust gas purification performance was evaluated in the same manner as in Example 1. The results are shown in Table 1.

[0053] A comparison of the NOx-T50 catalysts of Examples 1-5 and Comparative Examples 1-5 reveals that heating the Rh-containing catalyst to a temperature in the range of 700 to 900°C under a nitrogen atmosphere improved the NOx reduction performance. As shown in Table 1, in Examples 1-5, heating at a temperature in the range of 700 to 900°C under a nitrogen atmosphere resulted in the average particle size distribution of the Rh particles falling within the range of 1.5 to 18 nm, with a standard deviation of less than 1.6 nm. This is thought to have suppressed the coarsening of the Rh particles during the aging treatment and the reduction in the specific surface area of ​​the Rh particles, resulting in high NOx reduction performance. In particular, a comparison of the NOx-T50 catalysts of Examples 2 and 4 reveals that heating at 850°C under a nitrogen atmosphere provided better NOx reduction performance than heating at 750°C under a nitrogen atmosphere. In Example 4, in which the heating temperature was 850°C, the average particle size of the Rh particles after the aging treatment was smaller and the Rh particles had a larger specific surface area than in Example 2, which is thought to be why higher NOx reduction performance was obtained. Furthermore, a comparison of NOx-T50 in Example 3 and Comparative Examples 4-5 shows that the Rh-containing catalyst needs to be heated in a nitrogen atmosphere in order to appropriately control the particle size distribution of the Rh particles.

[0054] The NOx-T50 of Comparative Example 6 was higher than the NOx-T50 of Comparative Example 7. This indicates that heating under a nitrogen atmosphere did not improve the NOx reduction performance in Comparative Example 6. In Comparative Examples 6-7, the pellets did not contain a material with a higher basicity than the ACZ particles that served as the catalyst support, and therefore it is thought that the Rh particles did not become significantly coarser during the aging treatment, even though the particle size distribution of the Rh particles was not controlled.

[0055] [Table 1]

[0056] [Table 2]

[0057] The following Reference Examples show the results of experiments conducted to determine the particle size distribution of initial Rh particles suitable for preventing or reducing the deterioration of exhaust gas purification performance in high-temperature environments. In the Reference Examples, Rh particles were supported on ACZ particles by a method different from that of the above-described embodiment, but it is understood that the suitable particle size distribution of initial Rh particles determined from the Reference Examples can similarly prevent or reduce the deterioration of exhaust gas purification performance in high-temperature environments in exhaust gas purification materials produced by the methods according to the embodiments.

[0058] Reference example 1 (1) Sample preparation Polyvinylpyrrolidone and rhodium chloride were dissolved in ethylene glycol. Sodium hydroxide was added to the resulting solution. The solution was heated to 200°C overnight. This resulted in a rhodium particle dispersion (Rh particle dispersion).

[0059] The Rh particle dispersion and ACZ particles were added to distilled water, and the resulting mixture was heated and dried while stirring. The resulting particles were placed in a dryer maintained at 120°C for 2 hours to further remove moisture, and then calcined in an electric furnace at 500°C for 2 hours in an air atmosphere.

[0060] The calcined particles were observed with a TEM, and it was confirmed that Rh particles were supported on the ACZ particles. Furthermore, the particle size distribution of the Rh particles supported on the ACZ particles (initial Rh particles) was determined based on the TEM images. The mean and standard deviation of the particle size distribution of the initial Rh particles are shown in Table 3. Furthermore, the weight ratio of the Rh particles in the calcined particles (i.e., the weight ratio of the Rh particles based on the total weight of the ACZ particles and Rh particles) was as shown in Table 3.

[0061] The calcined particles were mixed with the same weight of composite oxide particles of CeO2 and ZrO2 (hereinafter referred to as "CZ-2 particles" as appropriate. The weight fractions of each component in the CZ-2 particles were CeO2: 46 wt% and ZrO2: 54 wt%), crushed, and mixed in a mortar. 2 g of the resulting powder was weighed out and molded into pellets.

[0062] (2) Measurement of the average particle size of Rh particles after aging treatment The average particle size of the Rh particles after the aging treatment of the pellets was measured in the same manner as in Example 2. The results are shown in Table 3.

[0063] (3) Exhaust gas purification performance evaluation The exhaust gas purification performance of the pellets after the aging treatment was measured in the same manner as in Example 1. The results are shown in Table 3.

[0064] Reference example 2 Pellets were prepared in the same manner as in Reference Example 1, except that an aqueous rhodium nitrate solution was used instead of the Rh particle dispersion. The mean and standard deviation of the particle size distribution of the initial Rh particles and the weight ratio of the Rh particles in the fired particles were as shown in Table 3.

[0065] The average particle size of the Rh particles after the aging treatment was measured, and the exhaust gas purification performance was evaluated in the same manner as in Reference Example 1. The results are shown in Table 3.

[0066] Reference example 3 Pellets were prepared in the same manner as in Reference Example 1, except that a Rh particle dispersion prepared as follows was used instead of the Rh particle dispersion prepared in Reference Example 1. A rhodium nitrate aqueous solution (pH 1.0) was prepared by dissolving 0.2 g of rhodium (III) nitrate in 50 mL of ion-exchanged water. A 175 g / L tetraethylammonium hydroxide aqueous solution (pH 14) was also prepared. The rhodium nitrate aqueous solution and the tetraethylammonium hydroxide aqueous solution were reacted using a reactor (microreactor) equipped with two flat plates as clearance adjustment members. Specifically, the rhodium nitrate aqueous solution and the tetraethylammonium hydroxide aqueous solution were introduced into a reaction field with a clearance set to 10 μm at a molar ratio of tetraethylammonium hydroxide:rhodium nitrate = 18:1 and reacted to prepare the Rh particle dispersion. The pH of the resulting Rh particle dispersion was 14.

[0067] The mean and standard deviation of the particle size distribution of the initial Rh particles and the weight ratio of the Rh particles in the fired particles are shown in Table 3.

[0068] The average particle size of the Rh particles after the aging treatment was measured, and the exhaust gas purification performance was evaluated in the same manner as in Reference Example 1. The results are shown in Table 3.

[0069] Reference example 4 Except for changing the amount of sodium hydroxide used in preparing the Rh particle dispersion, pellets were produced in the same manner as in Reference Example 1. The mean and standard deviation of the particle size distribution of the initial Rh particles and the weight ratio of the Rh particles in the fired particles were as shown in Table 3.

[0070] The average particle size of the Rh particles after the aging treatment was measured, and the exhaust gas purification performance was evaluated in the same manner as in Reference Example 1. The results are shown in Table 3.

[0071] Reference example 5-7 Except for changing the amount of sodium hydroxide used in preparing the Rh particle dispersion, pellets were produced in the same manner as in Reference Example 1. The mean and standard deviation of the particle size distribution of the initial Rh particles and the weight ratio of the Rh particles in the fired particles were as shown in Table 3.

[0072] The exhaust gas purification performance of the pellets after the aging treatment was evaluated in the same manner as in Reference Example 1. The results are shown in Table 3.

[0073] Reference example 8 A mixture of distilled water, Rh particle dispersion, and ACZ particles was prepared, dried, and fired in the same manner as in Reference Example 1. The resulting particles were heated to 900°C and exposed to a stoichiometric (air-fuel ratio A / F = 14.6) air-fuel mixture and an oxygen-excess (lean: A / F > 14.6) air-fuel mixture alternately at a time ratio of 1:1 over a period of 5 hours.

[0074] The particles exposed to the gas mixture were then observed using a TEM. The particle size distribution of the Rh particles (initial Rh particles) supported on the ACZ particles was determined based on the TEM images. The mean and standard deviation of the particle size distribution of the initial Rh particles, as well as the weight ratio of the Rh particles in the calcined particles, are shown in Table 3.

[0075] The particles exposed to the gas mixture were mixed with the same weight of CZ-2 particles, crushed and mixed in a mortar. 2 g of the resulting powder was weighed out and molded into pellets.

[0076] The exhaust gas purification performance of the pellets after the aging treatment was evaluated in the same manner as in Reference Example 1. The results are shown in Table 3.

[0077] Reference example 9 Except for changing the mixing ratio of the Rh particle dispersion and the ACZ particles, pellets were prepared in the same manner as in Reference Example 1. The mean and standard deviation of the particle size distribution of the initial Rh particles and the weight ratio of the Rh particles in the fired particles were as shown in Table 3.

[0078] The exhaust gas purification performance of the pellets was evaluated in the same manner as in Reference Example 1. The results are shown in Table 3.

[0079] Reference example 10 Except for changing the mixing ratio of the Rh particle dispersion and the ACZ particles, pellets were prepared in the same manner as in Reference Example 3. The mean and standard deviation of the particle size distribution of the initial Rh particles and the weight ratio of the Rh particles in the fired particles were as shown in Table 3.

[0080] The exhaust gas purification performance of the pellets was evaluated in the same manner as in Reference Example 1. The results are shown in Table 3.

[0081] The NOx-T50 values ​​in Reference Examples 1 and 4-6, in which the average particle size distribution of the initial Rh particles was within the range of 1.5 to 18 nm, were lower than the NOx-T50 values ​​in Reference Examples 2, 3, and 7, indicating that the pellets of Reference Examples 1 and 4-6 had higher NOx reduction performance. Measurement results of the average particle size of the Rh particles after aging in Reference Examples 1 and 4 and Reference Examples 2 and 3 indicated that coarsening of the Rh particles due to aging was suppressed in Reference Examples 1 and 4, in which the average particle size distribution of the initial Rh particles was 1.5 nm or more, compared to Reference Examples 2 and 3, in which the average particle size distribution of the initial Rh particles was less than 1.5 nm. Therefore, it is thought that in Reference Examples 1 and 4-6, in which the average particle size distribution of the initial Rh particles was 1.5 nm or more, coarsening of the Rh particles was suppressed, thereby suppressing a decrease in the specific surface area of ​​the Rh particles, resulting in high NOx reduction performance. Furthermore, in Reference Example 7, in which the average particle size distribution of the initial Rh particles exceeded 18 nm, the specific surface area of ​​the Rh particles was small before the aging treatment, which is thought to have resulted in poor NOx reduction performance.

[0082] In Reference Example 8, the average particle size distribution of the initial Rh particles was within the range of 1.5 to 18 nm, as in Reference Examples 1 and 4-6, but the standard deviation of the particle size distribution of the initial Rh particles was 1.6 nm or more, which was larger than that of Reference Examples 1 and 4-6. This indicates that the pellets of Reference Example 8 contained more fine Rh particles than the pellets of Reference Examples 1 and 4-6. In Reference Example 8, the fine Rh particles coarsened due to the aging treatment, so the specific surface area of ​​the Rh particles after the aging treatment was smaller than that of Reference Examples 1 and 4-6, and as a result, it is thought that the NOx reduction performance was lower than that of Reference Examples 1 and 4-6.

[0083] Similarly, the pellets of Reference Example 9, in which the average particle size distribution of the initial Rh particles was within the range of 1.5 to 18 nm, exhibited a lower NOx-T50, i.e., higher NOx reduction performance, than the pellets of Reference Example 10, in which the average particle size distribution of the initial Rh particles was less than 1.5 nm. The difference in NOx reduction performance between Reference Examples 9 and 10 was smaller than the difference in NOx reduction performance between Reference Examples 1 and 3. This suggests the following: When the weight ratio of Rh particles in the calcined particles is within the range of 0.01 to 2 wt %, particularly 0.2 to 1.8 wt %, a sufficient improvement in NOx reduction performance can be achieved by setting the average particle size distribution of the initial Rh particles to 1.5 nm or more. However, when the weight ratio of Rh particles in the calcined particles is higher (for example, exceeding 2 wt %), a sufficient improvement in NOx reduction performance may not be achieved even if the average particle size distribution of the initial Rh particles is 1.5 nm or more.

[0084] [Table 3]

Claims

1. A method for producing an exhaust gas purification material, (a) impregnating a metal oxide support with a rhodium compound solution; (b) drying the metal oxide support impregnated with the rhodium compound solution to obtain a rhodium-containing catalyst comprising the metal oxide support and rhodium particles supported on the metal oxide support; (c) heating the rhodium-containing catalyst to a temperature in the range of 700 to 900°C under an inert atmosphere; (d) mixing the rhodium-containing catalyst with a material having a higher basicity than the metal oxide support; A method comprising, in this order:

2. 2. The method of claim 1, wherein in the rhodium-containing catalyst after step (c), the rhodium particles have a mean particle size distribution of 1.5 to 18 nm and a standard deviation of the particle size distribution of less than 1.6 nm.

3. 3. The method of claim 2, wherein the rhodium particles in the rhodium-containing catalyst after step (c) have a mean particle size distribution of 4 to 14 nm.

4. 3. The method of claim 2, wherein the rhodium particles in the rhodium-containing catalyst after step (c) have a mean particle size distribution of 2 to 8 nm.

5. 5. The method of claim 1, wherein the rhodium-containing catalyst comprises 0.01 to 2 wt. % of the rhodium particles, based on the total weight of the metal oxide support and the rhodium particles.

6. The method according to any one of claims 1 to 4, wherein the metal oxide support is an oxide containing zirconia as a main component, a composite oxide containing zirconia and alumina as main components, or a composite oxide containing zirconia, alumina, and ceria as main components.

7. The method according to any one of claims 1 to 4, wherein the metal oxide support is a composite oxide containing zirconia, alumina, and ceria as main components, and the material having a higher basicity than the metal oxide support is a composite oxide containing ceria and zirconia as main components.

8. The method according to any one of claims 1 to 4, wherein the inert atmosphere is a nitrogen atmosphere.

9. Obtaining an exhaust gas purification material by the method according to any one of claims 1 to 4; disposing the exhaust gas purification material on a substrate; A method for manufacturing an exhaust gas purification device, comprising:

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