METHOD FOR PRODUCEING AN EXHAUST PURIFICATION MATERIAL AND METHOD FOR PRODUCEING AN EXHAUST PURIFICATION DEVICE

Abstract

A method for producing an exhaust gas purification material, wherein the method comprises the steps in this order: (a) Impregnating a metal oxide support with a solution of a rhodium compound; (b) Drying the metal oxide support impregnated with a solution of a rhodium compound 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 at a temperature within a range of 700 °C to 900 °C under an inert atmosphere; and (d) Mixing the rhodium-containing catalyst with a material that has a higher basicity than the basicity of the metal oxide support.

Description

BACKGROUND Technical area

[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. State of the art

[0002] Exhaust gas emitted by an internal combustion engine used in a vehicle, such as a car, contains harmful components like carbon monoxide (CO), hydrocarbons (HC), and nitrogen oxides (NOx). Regulations regarding emission levels of these harmful components have been tightened year after year. To remove these harmful components, precious metals such as platinum (Pt), palladium (Pd), and rhodium (Rh) have been used as catalysts.

[0003] Meanwhile, with regard to resource risk and resource conservation, a reduction in precious metals was demanded. One method for reducing the use of precious metals in an exhaust gas purification device is a known process in which a precious metal is supported in the form of fine particles on a support. For example, JP 2016-147256A discloses a process for producing an exhaust gas purification material, comprising a step of supporting precious metal particles on an oxide support to produce a precious metal-supported catalyst, and a step of carrying out a heat process on the precious metal-supported catalyst under a reducing atmosphere to control the size of the precious metal particles within a predetermined range.

[0004] Furthermore, DE 602 16 573 T2 and DE 602 16 572 T2 disclose exhaust gas purification catalysts from the prior art. SUMMARY

[0005] Through intensive studies, the inventors have found that the use of an exhaust gas purification material obtained by the manufacturing process disclosed in JP 2016 - 147 256 A leads in some cases to a reduction in catalytic activity in a high-temperature environment.

[0006] The present invention provides a method for producing an exhaust gas purification material and a method for producing an exhaust gas purification device, which enable efficient removal of a harmful component, even after exposure to a high-temperature environment.

[0007] The present invention offers, for example, the following aspects. [1] A method for producing an exhaust gas purification material, the method comprising the steps in this order: (a) Impregnating a metal oxide support with a solution of a rhodium compound; (b) Drying the metal oxide support impregnated with a solution of a rhodium compound 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 at a temperature within a range of 700 °C to 900 °C under an inert atmosphere; and (d) Mixing the rhodium-containing catalyst with a material that has a higher basicity than the basicity of the metal oxide support. [2] The procedure according to aspect [1], wherein in the rhodium-containing catalyst after step (c) there is a mean value of a particle size distribution of the rhodium particles of 1.5 nm to 18 nm, and a standard deviation of the particle size distribution of the rhodium particles is less than 1.6 nm. [3] The procedure according to aspect [2], where in the rhodium-containing catalyst after step (c) the mean value of the particle size distribution of the rhodium particles is from 4 nm to 14 nm. [4] The procedure according to aspect [2], wherein in the rhodium-containing catalyst after step (c) the mean value of the particle size distribution of the rhodium particles is from 2 nm to 8 nm. [5] The procedure according to one of the aspects [1] to [4], wherein the rhodium-containing catalyst contains the rhodium particles in an amount of 0.01 wt.% to 2 wt.%, based on a total weight of the metal oxide support and the rhodium particles. [6] The procedure according to one of the aspects [1] to [5], wherein the metal oxide support is an oxide containing zirconium dioxide as a main component, a composite oxide containing zirconium dioxide and aluminum oxide as main components, or a composite oxide containing zirconium dioxide, aluminum oxide, and cerium oxide as main components. [7] The procedure according to one of the aspects [1] to [6], wherein the metal oxide support is a composite oxide containing zirconium dioxide, aluminum oxide, and cerium oxide as main components, and the material has a higher basicity than the basicity of the metal oxide support, is a composite oxide containing cerium oxide and zirconium dioxide as main components. [8] The procedure according to one of the aspects [1] to [7], where the inert atmosphere is a nitrogen atmosphere. [9] A method for manufacturing an exhaust gas purification device, the method comprising: Obtaining the exhaust gas purification material by the process according to one of the aspects [1] to [8]; and Arranging the exhaust gas purification material on a substrate.

[0008] The exhaust gas purification material and the exhaust gas purification device produced according to the methods of the present invention enable efficient removal of a harmful component, even after exposure to a high-temperature environment. DETAILED DESCRIPTION

[0009] In the following, embodiments of the present invention are described, optionally with reference to the drawing. The present invention is not limited to the embodiments described below. A numerical range, here expressed by the term "to," comprises the respective values ​​described before and after the term "to" as a lower limit and an upper limit, respectively. Upper and lower limits in the numerical ranges disclosed herein can be combined arbitrarily. (1) Exhaust gas purification material

[0010] First, an exhaust gas purification material produced by a process according to the embodiment is described. The exhaust gas purification material is a mixture of an rhizome-containing catalyst, which contains a metal oxide support and rhizome particles supported on the metal oxide support, and a material which has a higher basicity than that of the metal oxide support.

[0011] Examples of metal oxide supports include an oxide of at least one metal selected from the group consisting of the metals of Group 3, Group 4, and Group 13 in the periodic table of elements, and lanthanide-based metals. If 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] For example, the metal oxide support can be 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 aluminium (Al); an oxide of at least one metal selected from the group consisting of Y, La, Ce, Ti, Zr, and Al in some embodiments; and an oxide of at least one metal selected from the group consisting of Al, Ce, and Zr in some embodiments. The metal oxide support can be an oxide containing zirconium dioxide (ZrO2) as the main component, an Al-Zr-based composite oxide containing zirconium dioxide and aluminum oxide (Al2O3) as the main components, or an Al-Ce-Zr-based composite oxide containing zirconium dioxide, aluminum oxide, and cerium oxide (CeO2) as the main components.Zirconium dioxide can serve to maintain the catalytic activity of the Rh particles. Cerium oxide can act as an oxygen storage capacity (OSC) material, storing oxygen in an oxygen-rich atmosphere and releasing oxygen in an oxygen-deficient atmosphere. Aluminum oxide can be used to control the diffusion of the Rh particles. The metal oxide support can be particles of a composite oxide containing aluminum oxide, cerium oxide, and zirconium dioxide as the main components, and furthermore at least one of yttrium oxide (Y₂O₃), lanthanum oxide (La₂O₃), neodymium oxide (Nd₂O₃), or praseodymium oxide (PreO₃). 11) contains. Yttrium oxide, lanthanum oxide, neodymium oxide, and praseodymium oxide improve the heat resistance of the composite oxide. Note that the phrase "contains as the main component(s)" here means that the content of the named component is 50 wt.% or more, 70 wt.% or more, 80 wt.% or more, or 90 wt.% or more of the total weight. If multiple main components are present, the phrase means that the sum of the contents of the named components is 50 wt.% or more, 70 wt.% or more, 80 wt.% or more, or 90 wt.% or more.

[0013] The metal oxide support can be particulate and can have any particle size, depending on its purpose.

[0014] The rhrhodium particles, supported on a metal oxide substrate, function as a catalyst to remove harmful components contained in exhaust gas. The average particle size distribution of the rhrhodium particles can range from 1.5 nm to 18 nm. Generally, the smaller the rhrhodium particles, the larger their specific surface area, and therefore the higher their catalytic performance. However, excessively small rhrhodium particles tend to coarsen under high-temperature conditions due to Ostwald ripening and aggregation, leading to a deterioration in catalyst performance.If the mean particle size distribution of the Rh particles is 1.5 nm or greater, particle coarsening under high-temperature conditions is controlled, thus reducing or preventing catalyst performance degradation. Rh particles with a mean particle size distribution of 18 nm possess a sufficiently high specific surface area, enabling high catalyst performance. The mean particle size distribution of the Rh particles can be within the range of 3 nm to 17 nm or within the range of 4 nm to 14 nm. The mean particle size distribution of the Rh particles can be within the range of 2 nm to 8 nm.

[0015] Furthermore, the standard deviation of the Rh particle size distribution can be less than 1.6 nm. As shown in the reference examples described below, a standard deviation of less than 1.6 nm in the Rh particle size distribution enables efficient removal of harmful components, even after the exhaust aftertreatment material has been exposed to a high-temperature environment. The Rh particles with a standard deviation of less than 1.6 nm comprise a small number of coarse Rh particles and a small number of fine Rh particles, which are likely to coarsen under high-temperature conditions. Therefore, such Rh particles can possess a sufficiently high specific surface area, even after the exhaust aftertreatment material has been exposed to a high-temperature environment, and consequently, high catalyst performance can be achieved.The standard deviation of the particle size distribution of the Rh particles can be 1nm or less.

[0016] The particle size distribution of the Rh particles is here a particle size distribution based on a number (i.e. a number-weighted particle size distribution) which is determined by measuring a projected area with an equivalence circle diameter of 50 or more of the Rh particles using an image obtained with a transmission electron microscope (TEM).

[0017] The amount of rhrhodium particles carried, that is, the proportion of rhrhodium particles relative to the total weight of the metal oxide support and the rhrhodium particles, can range from 0.01 wt.% to 2 wt.%. A proportion of rhrhodium particles of 0.01 wt.% or more allows for satisfactory removal of harmful components from the exhaust gas due to the action of a sufficient quantity of rhrhodium particles. A proportion of rhrhodium particles of 2 wt.% or less allows for a reduction in the amount of rhrhodium used and also ensures sufficient resistance to high temperatures, as coarsening of the rhrhodium particles under high-temperature conditions is avoided or controlled, due to the sparse distribution of the rhrhodium particles on the metal oxide support. The proportion of rhrhodium particles relative to the total weight of the metal oxide support and the rhrhodium particles can range from 0.2 wt.% to 1.8 wt.%.

[0018] The material with a higher basicity than that of the metal oxide support (hereinafter referred to as the "high-basic material") can be particulate. The high-basic material can, for example, be a material that functions as an OSC material. For instance, cerium oxide and a composite oxide containing cerium oxide (for example, a composite oxide containing cerium oxide as the main component, a Ce-Zr-based composite oxide containing cerium oxide and zirconium dioxide as the main components, and an Al-Ce-Zr-based composite oxide containing aluminum oxide, cerium oxide, and zirconium dioxide as the main components) can function as the OSC material. In particular, the Ce-Zr-based composite oxide can be used in some embodiments because it has a high oxygen storage capacity and is relatively inexpensive.In addition to the main component(s), the composite oxide containing cerium oxide may also contain at least one additive, such as praseodymium oxide, lanthanum oxide, yttrium oxide, or neodymium oxide. These additives may combine with the main component(s) to form a composite oxide. If the exhaust gas purification material contains the material acting as the OSC material, it can exhibit satisfactory exhaust gas purification performance under both oxygen-excess and oxygen-deficient atmospheres.

[0019] The high basicity material can be particulate and can have any particle size, depending on its purpose.

[0020] “A material with a higher basicity than that of the metal oxide support” is defined here as a material that has a lower mean or average electronegativity than the mean electronegativity of the metal oxide support. The “mean electronegativity” is an average of the Pauling electronegativities (hereafter referred to simply as “electronegativity”) of the constituent elements, weighted according to the number of each element per unit weight. For example, the mean electronegativity of the ACZ particles, which are composite oxide particles containing Al₂O₃, CeO₂, ZrO₂, La₂O₃, Y₂O₃, and Nd₂O₃, is calculated according to the following weight fractions: Al₂O₃: 30 wt%, CeO₂: 20 wt%, ZrO₂: 44 wt%, La₂O₃: 2 wt%, Y₂O₃: 2 wt%, and Nd₂O₃: 2 wt%. Average electronegativity of the ACZ particles = Electronativity of Al × Weight fraction of Al₂O₃ / Formula weight of Al₂O₃ × 2 + Electronativity of Ce × Weight fraction of CeO₂ / Formula weight of CeO₂ + Electronativity of Zr × Weight fraction of ZrO₂ / Formula weight of ZrO₂ + Electronativity of La × Weight fraction of La₂O₃ / Formula weight of La₂O₃ × 2 + Electronativity of Y × Weight fraction of Y₂O₃ / Formula weight of Y₂O₃ × 2 + Electronativity of Nd × Weight fraction of Nd₂O₃ / Formula weight of Nd₂O₃ × 2 + Electronativity 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,

[0021] The average electronegativity of the CZ particles, which are composite oxide particles, containing CeO2, ZrO2, and Pr6O 11 contained, with the following weight proportions: CeO2: 51.4 wt.%, ZrO2: 45.6 wt.%, and Pr6O 11 : 3.0 wt.%, is calculated as follows. Average electronegativity of the CZ particles = Electronativity of Ce × Weight fraction of CeO2 / Formula weight of CeO2 + Electronativity of Zr × Weight fraction of ZrO2 / Formula weight of ZrO2 + Electronativity of Pr × Weight fraction of Pr6O11 / Formula weight of Pr6O11 × 6 + Electronativity of O × (Weight fraction of CeO2 / Formula weight of CeO2 × 2) + Weight fraction of ZrO2 / Formula weight of ZrO2 × 2 + Weight fraction of Pr6O11 / Formula weight of Pr6O11×11)=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

[0022] The calculation described above shows that the CZ particles with the aforementioned composition have a lower average electronegativity than the ACZ particles with the aforementioned composition, and therefore they have a higher basicity than the ACZ particles with the aforementioned composition. (2) Method for producing the exhaust gas purification material

[0023] The process for producing the exhaust gas purification material described above comprises: impregnating a metal oxide support with a solution of a rhodium compound (step S1); drying the metal oxide support impregnated with the solution of a rhodium compound to obtain an rhodium-containing catalyst, which contains the metal oxide support and rhodium particles supported on the metal oxide support (step S2); heating the rhodium-containing catalyst at a temperature within a range of 700 °C to 900 °C under an inert atmosphere (step S3); and mixing the rhodium-containing catalyst with a material of high basicity (step S4), in this order. The respective processes are described in this order.

[0024] First, the metal oxide support is impregnated with a solution of a rhodium compound (step S1). Examples of rhodium compound solutions include aqueous solutions of rhodium hydroxide and aqueous solutions of rhodium nitrate. The impregnation method is not particularly restricted. For example, adding the metal oxide support and the rhodium compound solution to distilled water while stirring the distilled water allows the metal oxide support to be impregnated with the rhodium compound solution.

[0025] Next, the metal oxide support impregnated with a solution of a rhodium compound is dried (step S2). This yields the rhodium-containing catalyst, which comprises the metal oxide support and the rhodium particles carried on the support. If necessary, firing or baking can be carried out after drying. The fraction of rhodium particles in the rhodium-containing catalyst, based on the total weight of the catalyst (i.e., the sum of the weights of the metal oxide support and the rhodium particles), can range from 0.01 wt% to 2 wt%, particularly from 0.2 wt% to 1.8 wt%.

[0026] The rhenium-containing catalyst is heated to a temperature within the range of 700 °C to 900 °C under an inert atmosphere (step S3). Examples of inert atmospheres include nitrogen and argon. The heating period can be a suitable duration and may, for example, range from one to eight hours.

[0027] Heating in an inert atmosphere allows for the appropriate control of the mean and standard deviation of the particle size distribution of the Rh particles in the Rh-containing catalyst. Specifically, the mean particle size distribution of the Rh particles can be within the range of 1.5 nm to 18 nm, within the range of 3 nm to 17 nm, within the range of 4 nm to 14 nm, or within the range of 2 nm to 8 nm, and the standard deviation of the particle size distribution of the Rh particles can be less than 1.6 nm or 1 nm or less.

[0028] Note that, as described in the examples below, it is difficult to obtain the particle size distribution described above by firing in a reducing atmosphere, such as a hydrogen atmosphere, because the Rh particles cannot be sufficiently enlarged in a reducing atmosphere. It should also be noted that heating in an oxidizing atmosphere, such as air, causes the Rh particles to separate into the metal oxide support to form a solid mixture, solid solution, or solid solution, thus reducing the number of Rh particles on the surface of the metal oxide support.

[0029] The rhenium-containing catalyst is then mixed with the high-basic material (step S4). Although the mixing process is not particularly restricted, the rhenium-containing catalyst and the high-basic material can, for example, be mixed during pulverization. This allows for the production of a powdered exhaust gas purification material. The resulting powdered exhaust gas purification material can be pressed or shaped into any form, such as pellets, by compression molding or similar methods.

[0030] In general, exposure of fine Rh particles with a particle size of several nm to a high-temperature environment (for example, 1000 °C or more) causes the formation of Rh particles with increased particle sizes through Ostwald ripening. According to the present inventors, Rh is more stable on the high-basic material in a trivalent state (i.e., an oxidized state) than in a state where the valence is zero (i.e., a metallic state). Rh in the oxide state is readily vaporized and removed. Therefore, the collision frequency of the Rh atoms on the high-basic material is high, making the formation of coarse Rh particles more likely on the high-basic material than on the metal oxide support.The use of the exhaust gas purification material, which contains the rhenium-containing catalyst and the high-basic material, causes the rhenium atoms in the fine rhenium particles on the metal oxide support to migrate in the high-temperature environment to form coarse rhenium particles on the high-basic material. Therefore, when the rhenium-containing catalyst is used together with the high-basic material, compared to when the rhenium-containing catalyst is used alone, the rhenium particles are more likely to coarsen, and the purification performance is reduced. As described above, in the manufacturing process according to the embodiment, the mean and standard deviation of the particle size distribution of the rhenium particles on the metal oxide support are controlled by heating in an inert atmosphere, thereby reducing the number of excessively small rhenium particles.This prevents or controls the movement of Rh atoms towards the material with high basicity 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 material produced by the manufacturing process according to the embodiment exhibits an exhaust gas purification performance that is less likely to decrease in a high-temperature environment.

[0031] Furthermore, in the manufacturing process according to the embodiment, the rhodium particles with suitably controlled particle sizes are formed by a simple impregnation process using a rhodium compound solution and heating treatment in an inert atmosphere. Therefore, the manufacturing process according to the embodiment is highly efficient in terms of production and suitable for mass production. (3) Method for manufacturing the exhaust gas purification device

[0032] The exhaust gas purification device can be produced by arranging the exhaust gas purification material described above on the substrate.

[0033] The exhaust gas purification material can be arranged on the substrate together with, for example, a binder and an additive.

[0034] Although the substrate is not particularly restricted, a monolithic substrate with a honeycomb structure, for example, can be used. The substrate can be made from a ceramic material with high heat resistance, such as cordierite (2MgO·2Al2O). 3· 5SiO2), aluminum oxide, zirconium dioxide, and silicon carbide, and a metallic material made from a metal foil, such as stainless steel foil. Regarding cost considerations, the substrate can be made from cordierite in some embodiments.

[0035] If the substrate is a porous body with multiple pores, the exhaust gas purification material can be arranged on an internal surface that defines the pores of the substrate. That is to say, "arranged on the substrate" here includes both arrangement on an external surface of the substrate and arrangement on the internal surface of the substrate.

[0036] The exhaust gas cleaning material can be arranged on the substrate by the following described method. First, a slurry containing the exhaust gas cleaning material is prepared. The slurry may also include a binder, an additive, and the like. Properties of the slurry, such as viscosity and the particle diameter of any solid components, can be adjusted as appropriate. The prepared slurry is applied over a predetermined area of ​​the substrate. For example, the predetermined area of ​​the substrate is immersed in the slurry, and after a predetermined time has elapsed, the substrate is removed from the slurry, allowing the slurry to be applied over the predetermined area of ​​the substrate. Alternatively, the slurry can be poured into the substrate and blown out with a fan to distribute and apply it over the substrate.Next, the sludge is dried and then baked or cured for a predetermined period. This is how the exhaust gas cleaning material is applied to the substrate.

[0037] The exhaust gas purification device according to the embodiment is applicable to various types of vehicles which include internal combustion engines. EXAMPLES

[0038] The following describes the present invention in particular with the examples, but the present invention is not limited to the examples. Examples 1 to 5(1) Sample preparation

[0039] Composite oxide particles, containing Al₂O₃, CeO₂, and ZrO₂ as the main components, and further containing La₂O₃, Y₂O₃, and Nd₂O₃, were prepared as the metal oxide support. These composite oxide particles, containing Al₂O₃, CeO₂, and ZrO₂ as the main components, and further containing La₂O₃, Y₂O₃, and Nd₂O₃, will be referred to below as "ACZ particles" where necessary. The weight fractions of the respective components in the ACZ particles were: Al₂O₃: 30 wt%, CeO₂: 20 wt%, ZrO₂: 44 wt%, La₂O₃: 2 wt%, Y₂O₃: 2 wt%, and Nd₂O₃: 2 wt%.

[0040] While stirring distilled water, 10 g of ACZ particles and 8.0 g of an aqueous solution of rhodium hydroxide (concentration: 0.5 wt%) were added in the order mentioned, and the mixture was stirred for 10 minutes. The resulting mixture was dried, and firing was carried out by heating it in an air atmosphere for two hours at 500 °C in an electric furnace. This yielded an rhodium-containing catalyst containing the ACZ particles and rhodium (Rh) particles supported on the ACZ particles. The rhodium-containing catalyst contained the rhodium particles in an amount of 0.34 wt%, based on the total weight of the ACZ particles and the rhodium particles.

[0041] The rhenium-containing catalysts were heated in a nitrogen atmosphere for five hours at the temperatures described in Table 1. After heating, the rhenium-containing catalyst was observed using a transmission electron microscope (TEM) to determine the particle size distribution of the rhenium particles supported on the ACZ particles (original rhenium particles). Table 1 shows the mean and standard deviation of the particle size distribution of the original rhenium particles.

[0042] After heating, 10 g of composite oxide particles, containing CeO2 and ZrO2 as the main components, and furthermore Pr6O, were added to the Rh-containing catalyst. 11 The components were added, pulverized, and mixed in a mortar. The composite oxide particles, which contain CeO2 and ZrO2 as the main components, and furthermore Pr6O 11The components contained herein are referred to as "CZ particles" where necessary. The weight percentages of the respective components in the CZ particles were: CeO2: 51.4 wt.%, ZrO2: 45.6 wt.%, and PreO 11 : 3.0 wt.%). 2 g of the obtained powder were weighed and pressed into a pellet. (2) Aging process and measurement of the mean particle size of the Rh particles after the aging process

[0043] During heating to 1100 °C, the pellet was alternately exposed to a stoichiometric air-fuel mixture (air-fuel mixture A / F = 14.6) and a lean air-fuel mixture containing excess oxygen (A / F > 14.6) at a 1:1 time ratio over a fixed five-hour period. Subsequently, the mean particle sizes of the Rh particles in the pellets of Example 2 and Example 4 were determined using the carbon monoxide pulse method. Table 1 shows the results. (3) Evaluation of exhaust gas purification performance

[0044] While the gas with the composition described in Table 2 was flowed to the pellet after the aging process at a flow rate of 15 L / minute, the pellet was heated to 600 °C and held at this temperature for five minutes, and then allowed to cool to 150 °C. Subsequently, while the gas flow continued, the pellet temperature was increased to 600 °C at a rate of 20 °C / minute, and the NOx-T50 temperature, which is the temperature of the pellet when 50% of the NOx in the gas has been removed, was measured. The results were as shown in Table 1. Comparative example 1

[0045] With the exception that the rh-containing catalyst was not heated under a nitrogen atmosphere, the pellet was produced similarly to Example 1. The mean and standard deviation of the particle size distribution of the original rh particles were as shown in Table 1. The aging process and evaluation of the pellet's flue gas purification performance were carried out similarly to Example 1. Table 1 shows the results. Comparative examples 2 and 3

[0046] With the exception that the heating temperature of the rhr-containing catalyst under a nitrogen atmosphere was as described in Table 1, the pellet was produced similarly to Example 1. The mean and standard deviation of the particle size distribution of the original rhr particles were as shown in Table 1. The aging process and the evaluation of the pellet's flue gas purification performance were carried out similarly to Example 1. Table 1 shows the results. Comparative example 4

[0047] With the exception that the rhenium-containing catalyst was heated under an air atmosphere instead of a nitrogen atmosphere, the pellet was produced similarly to Example 3. The rhenium-containing catalyst was observed after heating using TEM, but the rhenium particles carried on the ACZ particles could not be detected. It is assumed that rhenium dissolved in the ACZ particles to form a solid mixture due to heating under an air atmosphere. The aging process and evaluation of the pellet's flue gas cleaning performance were carried out similarly to Example 1. Table 1 shows the results. Comparative example 5

[0048] With the exception that the rhenium-containing catalyst was heated under a hydrogen atmosphere instead of a nitrogen atmosphere, the pellet was produced similarly to Example 3. The mean and standard deviation of the particle size distribution of the original rhenium particles were as shown in Table 1. The aging process and evaluation of the pellet's flue gas purification performance were carried out similarly to Example 1. Table 1 shows the results. Comparative example 6

[0049] With the exception that ACZ particles were used instead of CZ particles, the pellet was manufactured similarly to Example 4. The aging process and the evaluation of the pellet's flue gas cleaning performance were carried out similarly to Example 1. Table 1 shows the results. Comparative example 7

[0050] With the exception that the rhizome-containing catalyst was not heated under a nitrogen atmosphere, the pellet was produced similarly to that in comparative example 6. The aging process and the evaluation of the pellet's flue gas purification performance were carried out similarly to example 1. Table 1 shows the results.

[0051] By comparing the NOx-T50 of Examples 1 to 5 with that of the comparison examples 1 to 5, it was found that heating the rhizome-containing catalyst in a nitrogen atmosphere at a temperature within the range of 700 °C to 900 °C improved the NOx reduction performance. As shown in Table 1, in Examples 1 to 5, heating to a temperature within the range of 700 °C to 900 °C in a nitrogen atmosphere allowed the mean particle size distribution of the rhizome particles to be within the range of 1.5 nm to 18 nm and the standard deviation of the particle size distribution to be less than 1.6 nm. This would have prevented or controlled coarsening of the rhizome particles and a reduction in the specific surface area of ​​the rhizome particles during the aging process, and consequently, the high NOx reduction performance was maintained.In particular, the comparison between the NOx-T50 of Example 2 and Example 4 shows that heating at 850 °C in a nitrogen atmosphere yielded superior NOx reduction performance compared to heating at 750 °C in a nitrogen atmosphere. The higher NOx reduction performance in Example 4, where the heating temperature was 850 °C, is thought to be due to the smaller mean particle size of the Rh particles after the aging process compared to Example 2, and the larger specific surface area of ​​the Rh particles. Furthermore, the comparison between the NOx-T50 of Example 3 and the comparison examples 4 and 5 demonstrates that the Rh-containing catalyst must be heated to achieve adequate control of the Rh particle size distribution in the nitrogen atmosphere.

[0052] The NOx-T50 of comparison example 6 was higher than the NOx-T50 of comparison example 7. This suggests that heating in the nitrogen atmosphere in comparison example 6 did not improve the NOx reduction performance. It is assumed that in comparison examples 6 and 7, because the pellet did not contain any material with a higher basicity than that of the ACZ particles used as the catalyst support, no significant coarsening of the Rh particles occurred during the aging process, even without intentional control of the Rh particle size distribution. [Table 1] Atmosphere during heating Heating temperature [°C] Mean particle size distribution of the original Rh particles [nm] Standard deviation of the particle size distribution of the original Rh particles [nm] Average particle size of Rh particles after aging [nm] NOx-T50[°C] Comparative example 1 - - 0,70 0,23 345,1 Comparative example 2 Nitrogen 600 1,22 0,37 330,3 Example 1 Nitrogen 700 2,20 0,89 320,4 Example 2 Nitrogen 750 3,36 1,16 9,55 310,2 Example 3 Nitrogen 800 4,32 1,22 308,2 Example 4 Nitrogen 850 5,49 1,49 9,21 299,8 Example 5 Nitrogen 900 7,85 1,59 315,2 Comparative example 3 Nitrogen 1000 10,02 2,385 328,5 Comparative example 4 Air 800 - - 393,6 Comparative example 5 hydrogen 800 1,40 0,36 327,9 Comparative example 6 Nitrogen 850 290,9 Comparative example 7 - - 276,2 [Table 2] component Portion CO 0.52 vol. O2 0.50 vol. C3H6 3000 ppmC NO 0.32% by volume CO2 14% by volume H2 3% by volume N2 rest

[0053] The following reference examples show results of the experiment conducted to determine the particle size distribution of the original Rh particles, which are suitable for preventing or controlling the reduction in exhaust gas purification performance under a high-temperature environment. In the reference examples, the Rh particles are supported on the ACZ particles by a different method than the embodiments described above. However, it should be understood that the particle size distribution of the original Rh particles, which may be determined based on the reference examples, can prevent or control the reduction in exhaust gas purification performance under a high-temperature environment, even if the exhaust gas purification material is produced by the method according to the embodiments. Reference Example 1(1) Sample Preparation

[0054] 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 yielded a rhodium particle dispersion (Rh particle dispersion).

[0055] The Rh particle dispersion and the ACZ particles were added to distilled water, and the resulting mixture was dried by heating while stirring. The resulting particles were placed in a dryer at 120 °C for two hours to further remove the water content, and subsequently, the resulting particles were fired by heating in an electric furnace in an air atmosphere at 500 °C for two hours.

[0056] The particles were observed with a TEM after firing / baking, and it was confirmed that the Rh particles were carried on the ACZ particles. Furthermore, based on the TEM image, the particle size distribution of the Rh particles carried on the ACZ particles (the original Rh particles) was determined. Table 3 shows the mean and standard deviation of the particle size distribution of the original Rh particles. The weight fraction of the Rh particles (that is, the weight fraction of the Rh particles based on the total weight of the ACZ particles and the Rh particles) in the fired particles was as shown in Table 3.

[0057] Composite oxide particles of CeO2 and ZrO2, equal in weight to the burnt particles, were added, pulverized, and mixed in a mortar. Hereinafter, the composite oxide particles of CeO2 and ZrO2, in which the respective components were CeO2: 46 wt% and ZrO2: 54 wt%, may be referred to as "CZ-2 particles". Two grams of the resulting powder were weighed and formed or pressed into a pellet. (2) Measurement of the mean particle size of Rh particles after the aging process

[0058] Similar to Example 2, the mean particle size of the Rh particles was measured after the pellet aging process. Table 3 shows the result. (3) Evaluation of exhaust gas purification performance

[0059] Similar to Example 1, the exhaust gas cleaning performance of the pellet was measured after the aging process. The result was as shown in Table 3. Reference Example 2

[0060] Except that the aqueous solution of rhodium nitrate was used instead of the Rh particle dispersion, the pellet was prepared similarly to Reference Example 1. The mean and standard deviation of the particle size distribution of the original Rh particles and the weight fraction of Rh particles in the fired particles were as shown in Table 3.

[0061] Similar to reference example 1, the mean particle size of the Rh particles was measured after the aging process and the exhaust gas purification performance was evaluated. Table 3 shows the results. Reference Example 3

[0062] Except that a rhodium particle dispersion prepared as described below was used instead of the rhodium particle dispersion prepared in Reference Example 1, the pellet was prepared similarly to that in Reference Example 1. 0.2 g of rhodium nitrate(III) was dissolved in 50 mL of ion-exchanged water to prepare an aqueous solution of rhodium nitrate (pH 1.0). A 175 g / L aqueous solution of tetraethylammonium hydroxide (pH 14) was also prepared. Using a reactor (microreactor) with two flat plates as a spacer, the aqueous solution of rhodium nitrate and the aqueous solution of tetraethylammonium hydroxide were reacted together.Specifically, aqueous solutions of rhodium nitrate and tetraethylammonium hydroxide (with a molar ratio of tetraethylammonium hydroxide to rhodium nitrate of 18:1) were introduced into a reaction field at a distance of 10 µm to react with each other, producing the rhodium particle dispersion. The resulting rhodium particle dispersion had a pH of 14.

[0063] The mean and standard deviation of the particle size distribution of the original Rh particles and the weight fraction of Rh particles in the burned particles were as shown in Table 3.

[0064] Similar to reference example 1, the mean particle size of the Rh particles was measured after the aging process and the exhaust gas purification performance was evaluated. Table 3 shows the results. Reference example 4

[0065] Except for changing the amount of sodium hydroxide used to prepare the Rh particle dispersion, the pellet was prepared similarly to Reference Example 1. The mean and standard deviation of the particle size distribution of the original Rh particles and the weight fraction of Rh particles in the fired particles were as shown in Table 3.

[0066] Similar to reference example 1, the mean particle size of the Rh particles was measured after the aging process and the exhaust gas purification performance was evaluated. Table 3 shows the results. Reference examples 5 to 7

[0067] Except for changing the amount of sodium hydroxide used to prepare the Rh particle dispersion, the pellet was prepared similarly to Reference Example 1. The mean and standard deviation of the particle size distribution of the original Rh particles and the weight fraction of Rh particles in the fired particles were as shown in Table 3.

[0068] Similar to reference example 1, the flue gas cleaning performance of the pellet was evaluated after the aging process. Table 3 shows the result. Reference example 8

[0069] Similar to Reference Example 1, the mixture of distilled water, the Rh particle dispersion, and the ACZ particles was prepared, dried, and calcined. During heating to 900 °C, the resulting particles were alternately exposed to a stoichiometric air-fuel mixture (air-fuel mixture A / F = 14.6) and a lean air-fuel mixture containing excess oxygen (A / F > 14.6) at a time ratio of 1:1 over a fixed five-hour period.

[0070] Next, the particles exposed to the air-fuel mixture were observed using a TEM. Based on the TEM image, the particle size distribution of the Rh particles carried on the ACZ particles (the original Rh particles) was determined. The mean and standard deviation of the particle size distribution of the original Rh particles and the weight fraction of Rh particles in the burned particles were as shown in Table 3.

[0071] CZ-2 particles were added to the particles exposed to the air-fuel mixture at the same weight as the exposed particles, pulverized, and mixed in a mortar. Two grams of the resulting powder were weighed and formed or pressed into a pellet.

[0072] Similar to reference example 1, the flue gas cleaning performance of the pellet was evaluated after the aging process. Table 3 shows the result. Reference example 9

[0073] Except for changing the mixing ratio of the Rh particle dispersion and the ACZ particles, the pellet was produced similarly to Reference Example 1. The mean and standard deviation of the particle size distribution of the original Rh particles and the weight fraction of the Rh particles in the fired particles were as shown in Table 3.

[0074] Similar to reference example 1, the flue gas cleaning performance of the pellet was evaluated. Table 3 shows the result. Reference example 10

[0075] Except for changing the mixing ratio of the Rh particle dispersion and the ACZ particles, the pellet was produced similarly to Reference Example 3. The mean and standard deviation of the particle size distribution of the original Rh particles and the weight fraction of the Rh particles in the fired particles were as shown in Table 3.

[0076] Similar to reference example 1, the flue gas cleaning performance of the pellet was evaluated. Table 3 shows the result.

[0077] It was shown that the NOx-T50 in reference examples 1 and 4 to 6, in which the mean values ​​of the particle size distributions of the original Rh particles were within the range of 1.5 nm to 18 nm, were lower than the NOx-T50 in reference examples 2, 3, and 7, and thus the pellets of reference examples 1 and 4 to 6 showed the higher NOx reduction performance. From the measurement results of the mean particle sizes of the Rh particles after the aging process in reference examples 1 and 4 and reference examples 2 and 3, it was shown that coarsening of the Rh particles during the aging process was controlled more efficiently in reference examples 1 and 4, in which the mean values ​​of the particle size distributions of the original Rh particles were 1.5 nm or more, than in reference examples 2 and 3, in which the mean values ​​of the particle size distributions of the original Rh particles were less than 1.5 nm.Accordingly, it is assumed that in reference examples 1 and 4 to 6, where the mean particle size distributions of the original Rh particles were 1.5 nm or greater, the coarsening of the Rh particles was controlled, resulting in the control of the reduction in the specific surface area of ​​the Rh particles and thus obtaining the higher NOx reduction performance. Furthermore, it is assumed that in reference example 7, where the mean particle size distribution of the original Rh particles was greater than 18 nm, the NOx reduction performance was inferior, since the specific surface area of ​​the Rh particles was small even before the aging process.

[0078] In reference example 8, similar to reference examples 1 and 4 to 6, the mean particle size distribution of the original Rh particles was within the range of 1.5 nm to 18 nm, but the standard deviation of the particle size distribution of the original Rh particles was 1.6 nm or more, which was larger than that of reference examples 1 and 4 to 6. This indicates that the pellet of reference example 8 contained a greater number of fine Rh particles than the pellets of reference examples 1 and 4 to 6. It is assumed that in reference example 8, the fine Rh particles were coarsened during the aging process, resulting in a smaller specific surface area of ​​the Rh particles after aging compared to reference examples 1 and 4 to 6. Consequently, the NOx reduction performance of reference example 8 was lower than that of any of reference examples 1 and 4 to 6.

[0079] Similarly, the pellet of Reference Example 9, in which the mean particle size distribution of the original Rh particles was within the range of 1.5 nm to 18 nm, showed a lower NOx T50 (i.e., a higher NOx reduction performance) than that of the pellet of Reference Example 10, in which the mean particle size distribution of the original Rh particles was less than 1.5 nm. The difference in NOx reduction performance between Reference Example 9 and Reference Example 10 was smaller than the difference in NOx reduction performance between Reference Example 1 and Reference Example 3. This provides evidence for the following: that is, if the weight fraction of Rh particles in the burned particles is within the range of 0.01 wt% to 2 wt%, and in particular within the range of 0.2 wt% to 1.8 wt%, then the NOx reduction performance may be significantly lower.If the weight fraction of Rh particles in the burned particles is less than -%, the mean particle size distribution of the original Rh particles can be within the range of 1.5 nm or more to provide a sufficient improvement in NOx reduction performance. However, if the weight fraction of Rh particles in the burned particles is greater than this (for example, greater than 2 wt%), the mean particle size distribution of the original Rh particles can no longer be within the range of 1.5 nm or more to provide a sufficient improvement in NOx reduction performance. [Table 3] Percentage of Rh particles in burned particles [wt%] Mean particle size distribution of the original Rh particles [nm] Standard deviation of the particle size distribution of the original Rh particles [nm] Average particle size of Rh particles after aging [nm] NOx-T50[°C] Reference Example 1 0,2 6,17 0,89 9,01 295,7 Reference Example 2 0,2 0,70 0,41 12,41 345,1 Reference Example 3 0,2 1,42 0,48 10,82 329,1 Reference example 4 0,2 4,20 0,74 9,39 308,1 Reference example 5 0,2 13,16 1,31 302,7 Reference example 6 0,2 16,81 1,53 322,4 Reference example 7 0,2 19,10 1,55 331,3 Reference example 8 0,2 5,64 1,61 332,5 Reference example 9 1,8 6,31 0,94 235,6 Reference example 10 1,8 1,48 0,51 237,4

Claims

A process for producing an exhaust gas purification material, comprising the steps in this order: (a) impregnating a metal oxide support with a solution of a rhodium compound; (b) drying the metal oxide support impregnated with the solution of a rhodium compound 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 at a temperature within a range of 700 °C to 900 °C under an inert atmosphere; and (d) mixing the rhodium-containing catalyst with a material having a higher basicity than the basicity of the metal oxide support. The method of claim 1, wherein in the rhodium-containing catalyst after step (c) there is a mean value of a particle size distribution of the rhodium particles of 1.5 nm to 18 nm, and a standard deviation of the particle size distribution of the rhodium particles is less than 1.6 nm. The method according to claim 2, wherein in the rhodium-containing catalyst after step (c) the mean value of the particle size distribution of the rhodium particles is from 4 nm to 14 nm. The method according to claim 2, wherein in the rhodium-containing catalyst after step (c) the mean value of the particle size distribution of the rhodium particles is from 2 nm to 8 nm. Method according to any one of claims 1 to 4, wherein the rhodium-containing catalyst contains the rhodium particles in an amount of 0.01 wt.% to 2 wt.%, based on a total weight of the metal oxide support and the rhodium particles. Method according to any one of claims 1 to 5, wherein the metal oxide support is an oxide containing zirconium dioxide as a major component, a composite oxide containing zirconium dioxide and aluminum oxide as major components, or a composite oxide containing zirconium dioxide, aluminum oxide, and cerium oxide as major components. Method according to any one of claims 1 to 6, wherein the metal oxide support is a composite oxide containing zirconium dioxide, aluminum oxide, and cerium oxide as main components, and the material has a higher basicity than the basicity of the metal oxide support, is a composite oxide containing cerium oxide and zirconium dioxide as main components. Method according to any one of claims 1 to 7, wherein the inert atmosphere is a nitrogen atmosphere. Method for manufacturing an exhaust gas purification device, wherein the method comprises: obtaining the exhaust gas purification material by the method according to any one of claims 1 to 8; and arranging the exhaust gas purification material on a substrate.