Method for producing exhaust gas purification material

Abstract

To provide a method for producing an exhaust gas purification material which can remove a harmful component with high efficiency even after exposed to a high temperature environment.SOLUTION: There is provided a method for producing an exhaust gas purification 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-supported metal oxide containing the metal oxide carrier and rhodium particles supported on the metal oxide carrier; and (c) heating the rhodium-supported metal oxide to a temperature in the range of 850 to 1000°C in an atmosphere containing carbon monoxide at a concentration of 0.01 to 5 vol.% and the remainder of inert gas to obtain the exhaust gas purification material.SELECTED DRAWING: None

Description

[Technical field]

[0001] The present disclosure relates to a method for producing an exhaust gas purification material. [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 risk, it is required to reduce the amount of precious metals used. In an exhaust gas purification device, one method for reducing the amount of precious metals used is known 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 catalyst, which includes a step of heat-treating a Rh-supported Zr-based composite oxide at 550°C or more and 800°C or less in a reducing atmosphere containing CO, and a step of heat-treating a Rh-supported CeZr-based composite oxide at 500°C or more and 800°C or less in a reducing atmosphere containing CO. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] JP 2014-168751 A Summary of the Invention [Problem to be solved by the invention]

[0005] It is desirable for an exhaust gas purification material to be able to remove harmful components with high efficiency even after being exposed to a high-temperature environment. Therefore, an object of the present disclosure is to provide a method for producing an exhaust gas purification material that can remove harmful components with high efficiency even after being exposed to a high-temperature environment. [Means for solving the problem]

[0006] Aspects of the present disclosure include the following. [Aspect 1] A method for producing an exhaust gas purification material, comprising the steps of: (a) impregnating a metal oxide support with a solution of a rhodium compound; (b) drying the metal oxide support impregnated with the rhodium compound solution to obtain a rhodium-supported metal oxide comprising the metal oxide support and rhodium particles supported on the metal oxide support; (c) heating the rhodium-supported metal oxide in an atmosphere containing carbon monoxide at a concentration of 0.01 to 5 volume % and the remainder being an inert gas to a temperature in the range of 850 to 1000°C to obtain the exhaust gas purification material; A method comprising: [Aspect 2] 2. The method of claim 1, wherein after step (c), the percentage of rhodium dissolved in the metal oxide support is less than 17 wt % based on the total weight of rhodium contained in the rhodium-supported metal oxide. [Aspect 3] 2. The method of claim 1, wherein after step (c), the percentage of rhodium dissolved in the metal oxide support is 3 wt % or less, based on the total weight of rhodium contained in the rhodium-supported metal oxide. [Aspect 4] The method of any one of aspects 1 to 3, wherein after step (c), the rhodium particles have a mean of particle size distribution of 2.2 to 18 nm. [Aspect 5] A method according to any one of aspects 1 to 4, wherein after step (c), the rhodium particles have a coefficient of variation (σ / μ) obtained by dividing the standard deviation (σ) by the mean (μ) of the particle size distribution of the rhodium particles is 1 or less. Effect of the Invention

[0007] The exhaust gas purification material produced by the method of the present disclosure can remove harmful components with high efficiency even after being exposed to a high-temperature environment. [Brief description of the drawings]

[0008] [Figure 1] Figure 1 is a graph showing the relationship between the coefficient of variation σ / μ of the particle size distribution of the initial Rh particles in Examples 2 to 7 and NOx-T50. [Diagram 2] Figure 2 is a graph showing the relationship between the solid solution Rh ratio in Reference Examples 1 to 5 and NOx-T50.

Mode for Carrying Out the Invention

[0009] In the present application, a numerical range represented by using the symbol "~" includes each of the numerical values described before and after the symbol "~" as the lower limit value and the upper limit value. The upper limit value and the lower limit value of the numerical range described in the present application can be used alone or arbitrarily combined.

[0010] (1) Exhaust gas purification material First, the exhaust gas purification material produced by the method according to the embodiment will be described. The exhaust gas purification material includes a metal oxide carrier and Rh particles supported on the metal oxide carrier.

[0011] Examples of the metal oxide carrier include oxides of at least one metal selected from the group consisting of metals of Groups 3, 4, and 13 of the periodic table of elements and lanthanoid series metals. When the metal oxide carrier contains two or more metal elements, the metal oxide carrier 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 zirconia (ZrO 2 ) as a main component, and may be an oxide containing zirconia and alumina (Al 2 O 3 ) as a main component (Al-Zr-based composite oxide), or zirconia, alumina, and ceria (CeO 2 The metal oxide support may be a composite oxide (Al-Ce-Zr composite oxide) containing, as a main component, yttria (Y 2 O 3 ), Lantana (La 2 O 3 ), neodymia (Nd 2 O 3 ), or praseodymia (Pr 6 O 11) may be composite oxide particles containing at least one of the above. 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% by weight or more of the total weight. When there are multiple main components, it means that the total content of these components is 50% by weight or more. The content of the component described as the main component may be 70% by weight or more, 80% by weight or more, or 90% by weight or more of the total weight.

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

[0014] The Rh particles supported on the metal oxide carrier function as a catalyst for removing harmful components contained in exhaust gas. The average particle size distribution of the Rh particles may be within a range of 2.2 to 18 nm. In general, the smaller the particle size of the Rh particles, the larger the specific surface area of ​​the Rh particles, and therefore the higher the catalytic performance. However, Rh particles with an excessively small particle size tend to become coarse due to Ostwald ripening and aggregation in a high-temperature environment, causing deterioration of the catalytic performance. When the average particle size distribution of the Rh particles is 2.2 nm or more, the coarsening of the Rh particles in a high-temperature environment is suppressed, and the deterioration of the catalytic performance is suppressed. In addition, when the average particle size distribution of the Rh particles is 18 nm or less, the specific surface area of ​​the Rh particles is sufficiently large, so that the Rh particles can exhibit high catalytic performance. The average particle size distribution of the Rh particles may be within a range of 2.8 to 6 nm.

[0015] Further, the standard deviation of the particle size distribution of the Rh particles may be less than 5.5 nm. When the standard deviation of the particle size distribution of the Rh particles is less than 5.5 nm, the exhaust gas purification material can efficiently remove harmful components even after being exposed to a high-temperature environment. Since the number of large Rh particles and the number of small Rh particles that are likely to coarsen in a high-temperature environment decrease due to the standard deviation of the particle size distribution of the Rh particles being less than 5.5 nm, 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, and as a result, high catalytic performance can be exhibited. The standard deviation of the particle size distribution of the Rh particles may be 5.36 nm or less.

[0016] The coefficient of variation of the particle size distribution of the Rh particles (i.e., the value obtained by dividing the standard deviation by the average) may be 1 or less. When the coefficient of variation of the particle size distribution of the Rh particles is 1 or less, the exhaust gas purification material can efficiently remove harmful components even after being exposed to a high-temperature environment. Since the number of large Rh particles and the number of small Rh particles that are likely to coarsen in a high-temperature environment decrease due to the coefficient of variation of the particle size distribution of the Rh particles being 1 or less, 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, and as a result, high catalytic performance can be exhibited. The coefficient of variation of the particle size distribution of the Rh particles may be 0.54 or less. In this case, since the number of large Rh particles and the number of small Rh particles that are likely to coarsen in a high-temperature environment are particularly small, the Rh particles can have a particularly large specific surface area even after the exhaust gas purification material is exposed to a high-temperature environment, and as a result, particularly high catalytic performance can be exhibited.

[0017] In the present application, the particle size distribution of the Rh particles is a number-based particle size distribution obtained by measuring the equivalent circle diameters of the projected areas of 50 or more Rh particles based on an image obtained by a transmission electron microscope (TEM).

[0018] The amount of Rh particles supported, i.e., the ratio of Rh particles based on the total weight of the metal oxide carrier and the Rh particles, may be within the range of 0.01 to 2% by weight. When the ratio of Rh particles is 0.01% by weight or more, a sufficient amount of Rh particles is present, and harmful components in exhaust gas can be effectively removed. When the ratio of Rh particles is 2% by weight or less, the amount of Rh used can be reduced. In addition, since the Rh particles are sufficiently sparsely supported on the metal oxide carrier, coarsening of the Rh particles in a high-temperature environment is suppressed, and sufficient durability against high temperatures can be exhibited. The ratio of Rh particles based on the total weight of the metal oxide carrier and the Rh particles may be within the range of 0.2 to 1.8% by weight.

[0019] Based on the total weight of Rh supported on the metal oxide support, the proportion of Rh dissolved in the metal oxide support may be less than 17% by weight. As a result, as shown in the Reference Examples described later, the exhaust gas purification material can remove harmful components with high efficiency even after the exhaust gas purification material is exposed to a high-temperature environment. Based on the total weight of Rh supported on the metal oxide support, the proportion of Rh dissolved in the metal oxide support may be 3% by weight or less. As a result, as shown in the Reference Examples described later, the exhaust gas purification material can remove harmful components with particularly high efficiency even after the exhaust gas purification material is exposed to a high-temperature environment.

[0020] The proportion of Rh dissolved in the metal oxide support can be determined as follows: The X-ray absorption spectrum Sm(x) of a standard sample of pure Rh metal and Rh 2 O 3 The X-ray absorption spectrum Sox(x) of the standard sample is measured using an X-ray absorption fine structure (XAFS) measurement device. The X-ray absorption spectrum S(x) of the exhaust gas purification material at the Rh K absorption edge is measured using an XAFS measurement device in a hydrogen atmosphere at 400°C. Using the obtained X-ray absorption spectra Sm(x), Sox(x), and S(x), the following formula: S(x) = a Sm(x) + b Sox(x) The values ​​of a and b are obtained by least squares fitting with a and b as parameters. In a hydrogen atmosphere at 400°C, Rh present on the surface of the metal oxide carrier (i.e., Rh not dissolved in the metal oxide carrier) is reduced and exists in a metal (0 valence) state, while Rh dissolved in the metal oxide carrier is not reduced and exists in an oxide (3 valence) state combined with oxygen in the metal oxide carrier. Therefore, the ratio of a to b obtained by least squares fitting corresponds to the ratio of the amount of Rh present on the surface of the metal oxide carrier in the exhaust gas purification material to the amount of Rh dissolved in the metal oxide carrier. Therefore, the ratio of Rh dissolved in the metal oxide carrier based on the total weight of Rh supported on the metal oxide carrier can be obtained by calculating b / (a+b).

[0021] (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 a Rh-supported metal oxide containing the metal oxide support and Rh particles supported on the metal oxide support (step S2), and heating the Rh-supported metal oxide to a temperature in the range of 850 to 1000°C in an atmosphere containing carbon monoxide (CO) at a concentration of 0.01 to 5% by volume with the remainder being an inert gas to obtain the exhaust gas purification material (step S3). Each step will be described in order.

[0022] First, the metal oxide support is impregnated with a rhodium compound solution (step S1). Examples of the rhodium compound solution include an aqueous solution of rhodium hydroxide and an aqueous solution of rhodium nitrate. The impregnation method is not particularly limited. For example, the metal oxide support and the rhodium compound solution are added to distilled water while stirring, so that the metal oxide support can be impregnated with the rhodium compound solution.

[0023] Next, the metal oxide support impregnated with the rhodium compound solution is dried (step S2). As a result, a Rh-supported metal oxide containing the metal oxide support and Rh particles supported on the metal oxide support is obtained. If necessary, calcination may be performed after drying. In the Rh-supported metal oxide, the proportion of Rh particles based on the total weight of the Rh-supported metal oxide (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%.

[0024] The Rh-supported metal oxide is heated to a temperature in the range of 850 to 1000°C in an atmosphere containing CO at a concentration of 0.01 to 5% by volume with the remainder being an inert gas (step S3). Examples of the inert gas include nitrogen and argon. The heating temperature may be 850°C or higher and lower than 950°C, or 850°C or higher and 900°C or lower. The heating time may be appropriately set and may be, for example, 1 to 30 hours.

[0025] By heating to a temperature in the range of 850 to 1000°C in an atmosphere containing CO and an inert gas, it is possible to appropriately control the particle size distribution of the Rh particles in the Rh-supported metal oxide while preventing or reducing the formation of a solid solution of Rh in the metal oxide support. Specifically, it is possible to control the average particle size distribution of the Rh particles to within the range of 2.2 to 18 nm, the standard deviation of the particle size distribution of the Rh particles to less than 5.5 nm, and the coefficient of variation of the particle size distribution of the Rh particles to 1 or less.

[0026] Heating in an inert atmosphere such as a nitrogen atmosphere or an oxidizing atmosphere such as an air atmosphere induces the dissolution of Rh into the metal oxide support, resulting in a decrease in the number of Rh particles on the surface of the metal oxide support, making it difficult to achieve high catalytic performance.

[0027] In this manner, an exhaust gas purification material is obtained. The obtained exhaust gas purification material may be mixed with another metal oxide, a binder, an additive, and the like before use. The exhaust gas purification material may be molded into any shape, such as a pellet shape, by press molding or the like before use. Alternatively, a slurry containing the exhaust gas purification material may be prepared, applied to a substrate, such as a monolith substrate having a honeycomb structure, and then dried and fired to manufacture an exhaust gas purification device. The exhaust gas purification material can be used to purify exhaust gas from various vehicles equipped with internal combustion engines.

[0028] In the manufacturing method of the embodiment, Rh particles having an appropriately controlled particle size distribution are formed while preventing or reducing the formation of a solid solution of Rh in the metal oxide support by a simple process using an impregnation method using a rhodium compound solution and heating in an atmosphere containing CO. Therefore, the manufacturing method of the embodiment has high production efficiency and is suitable for mass production.

[0029] Although the embodiments of the present disclosure have been described in detail above, the present disclosure is not limited to the above-described embodiments, and various design modifications can be made without departing from the technical scope described in the claims. EXAMPLES

[0030] The present disclosure will be specifically described below using examples, but the present disclosure is not limited to these examples.

[0031] Example 1 (1) Preparation of exhaust gas purification materials As a metal oxide support, Al 2 O 3 and ZrO 2 Contains as the main component, and also contains La 2 O 3 , Y 2 O 3 , and Nd 2 O 3 Composite oxide particles containing Al 2 O 3 :30wt%, ZrO 2 :60% by weight, La 2O 3 :4% by weight, Y 2 O 3 :4% by weight, Nd 2 O 3 :2% by weight) was prepared.

[0032] 10 g of AZ particles and 4.00 g of rhodium hydroxide aqueous solution (Rh concentration 1.00 wt%) were added in order to the stirred distilled water and stirred for 10 minutes. The resulting mixture was dried and fired in an electric furnace in an air atmosphere at 500°C for 2 hours. As a result, Rh-supported metal oxide including AZ particles and rhodium (Rh) particles supported on the AZ particles was obtained.

[0033] The Rh-loaded metal oxide was mixed with CO at a concentration of 1% by volume and the balance was N. 2 The mixture was heated at 850°C for 5 hours in an atmosphere of 0.1% by weight, thereby obtaining an exhaust gas purification material.

[0034] (2) Analysis of exhaust gas purification materials The exhaust gas purification material was observed with a transmission electron microscope (TEM) to determine the particle size distribution of the Rh particles (initial Rh particles) supported on the AZ particles. The average, standard deviation, and coefficient of variation of the particle size distribution of the initial Rh particles are shown in Table 1.

[0035] The ratio of Rh dissolved in the AZ particles based on the total weight of Rh contained in the exhaust gas purification material was calculated as follows. The X-ray absorption spectrum S(x) at the K-absorption edge of Rh was measured for the exhaust gas purification material in a hydrogen atmosphere at 400°C using an X-ray absorption fine structure (XAFS) measurement device. In addition, the X-ray absorption spectrum Sm(x) of a standard sample of pure Rh metal and Rh 2 O 3 The X-ray absorption spectrum Sox(x) of the standard sample was also measured using an XAFS measurement device. Using the obtained X-ray absorption spectrum, the following formula: S(x) = a Sm(x) + b Sox(x) The values ​​of a and b were obtained by least squares fitting with a and b as parameters. b / (a+b) was calculated to obtain the proportion of Rh dissolved in the AZ particles based on the total weight of Rh contained in the exhaust gas purification material. The results are shown in Table 1.

[0036] (3a) Preparation of pellets for evaluating exhaust gas purification performance (Pellet preparation method A) Add 10g of exhaust gas purification material and CeO 2 and ZrO 2 Contains as the main component, and Pr 6 O 11 Composite oxide particles containing CeO 2 :51.4% by weight, ZrO 2 :45.6% by weight, Pr 6 O 11 The dispersion was heated in an electric furnace in an air atmosphere at 500° C. for 2 hours for firing. The fired body was crushed to a predetermined size to obtain pellets.

[0037] (4) Aging treatment of pellets The pellets were heated to 1000°C and exposed to alternating fuel-rich (rich, A / F<14.6) and oxygen-rich (lean, A / F>14.6) mixtures every 5 minutes for 5 hours.

[0038] (5) Measurement of the average particle size of Rh particles after aging The average particle size of the Rh particles in the pellets after the aging treatment was measured by the carbon monoxide pulse method, and the results are shown in Table 1.

[0039] (6) Evaluation of exhaust gas purification performance after aging treatment The pellets after the aging treatment were heated to 600°C while passing a gas having the composition shown in Table 2 through them at a flow rate of 15 L / min, and were then cooled to 150°C after being maintained at that temperature for 5 minutes. Thereafter, while continuing to pass the gas through the pellets, the pellets were heated to 600°C at a rate of 20°C / min, and the temperature of the pellets when 50% of the NOx in the gas was removed (hereinafter referred to as "NOx-T50") was measured. The results were as shown in Table 1.

[0040] Comparative Example 1 Except for not carrying out heat treatment on the Rh-supported metal oxide, an exhaust gas purifying material was produced in the same manner as in Example 1. Analysis of the obtained exhaust gas purifying material, preparation of pellets and aging treatment thereof, measurement of the average particle size of Rh particles after aging treatment, and evaluation of exhaust gas purifying performance were carried out in the same manner as in Example 1. The results are shown in Table 1.

[0041] Comparative Example 2 Heat treatment of Rh-loaded metal oxide 2 Except for the fact that the experiment was carried out in an atmosphere, an exhaust gas purifying material was produced in the same manner as in Example 1. Measurement of the dissolved Rh ratio in the obtained exhaust gas purifying material, preparation of pellets and aging treatment thereof, measurement of the average particle size of Rh particles after aging treatment, and evaluation of exhaust gas purifying performance were carried out in the same manner as in Example 1. The results are shown in Table 1.

[0042] Examples 2 to 5 Except for using 6 g of AZ particles and carrying out the heat treatment of the Rh-supported metal oxide at the temperature and time shown in Table 1, an exhaust gas purifying material was prepared in the same manner as in Example 1, and the obtained exhaust gas purifying material was analyzed. The results are shown in Table 1.

[0043] (3b) Preparation of pellets for evaluating exhaust gas purification performance (Pellet preparation method B) Add 6g of exhaust gas purification material and CeO 2 and ZrO 2 Contains as the main component, and Pr 6 O 11 Composite oxide particles containing CeO2 :51.4% by weight, ZrO 2 :45.6% by weight, Pr 6 O 11 3.0% by weight) 4 g and Al 2 O 3 , CEO 2 , and ZrO 2 Contains as the main component, and also contains La 2 O 3 , Y 2 O 3 , and Nd 2 O 3 Composite oxide particles containing Al (hereinafter referred to as "ACZ particles" as appropriate). The weight fraction of each component in the ACZ particles is 2 O 3 :30wt%, CeO 2 :20wt%, ZrO 2 :44% by weight, La 2 O 3 :2% by weight, Y 2 O 3 :2% by weight, Nd 2 O 3 10 g of cellulose acetate (2% by weight) was added. Hydroxyethyl cellulose (HEC) and citric acid were added to adjust the viscosity, and the mixture was stirred for 6 hours to obtain a slurry. The slurry was fired in an electric furnace in an air atmosphere at 500°C for 2 hours. The fired body was crushed to a specified size to obtain pellets.

[0044] The pellets were subjected to an aging treatment, and 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 Example 1. The results are shown in Table 1.

[0045] Comparative Example 3 Except for carrying out the heat treatment of the Rh-supported metal oxide at 800° C. for 5 hours, exhaust gas purification materials were produced in the same manner as in Examples 2 to 5. Analysis of the obtained exhaust gas purification materials, preparation of pellets and aging treatment thereof, measurement of the average particle size of Rh particles after aging treatment, and evaluation of exhaust gas purification performance were carried out in the same manner as in Examples 2 to 5. The results are shown in Table 1.

[0046] Comparative Example 4 Except for not carrying out heat treatment on the Rh-supported metal oxide, exhaust gas purification materials were produced in the same manner as in Examples 2 to 5. Analysis of the obtained exhaust gas purification materials, preparation of pellets and aging treatment thereof, measurement of the average particle size of Rh particles after aging treatment, and evaluation of exhaust gas purification performance were carried out in the same manner as in Examples 2 to 5. The results are shown in Table 1.

[0047] Examples 6 and 7 Except for using 1.45 g of an aqueous rhodium nitrate solution (Rh concentration: 2.75 wt%) instead of 4.00 g of an aqueous rhodium hydroxide solution (Rh concentration: 1.00 wt%), and for carrying out the heat treatment of the Rh-supported metal oxide at the temperature and for the time shown in Table 1, exhaust gas purification materials were produced in the same manner as in Examples 2 to 5. Analysis of the obtained exhaust gas purification materials, preparation of pellets and aging treatment thereof, measurement of the average particle size of Rh particles after aging treatment, and evaluation of exhaust gas purification performance were carried out in the same manner as in Examples 2 to 5. The results are shown in Table 1.

[0048] Comparative Example 5 Except for not carrying out heat treatment on the Rh-supported metal oxide, exhaust gas purification materials were produced in the same manner as in Examples 6 and 7. Analysis of the obtained exhaust gas purification materials, preparation of pellets and aging treatment thereof, measurement of the average particle size of Rh particles after aging treatment, and evaluation of exhaust gas purification performance were carried out in the same manner as in Examples 2 to 5. The results are shown in Table 1.

[0049] [Table 1]

[0050] [Table 2]

[0051] Comparing NOx-T50 of Example 1 and Comparative Examples 1 and 2, it can be seen that when the Rh-supported metal oxide was heat-treated in a CO-containing atmosphere, the NOx reduction performance was improved compared to when the heat treatment was not performed or when the heat treatment was performed in nitrogen. In Example 1, where the heat treatment was performed in a CO-containing atmosphere, the average particle size distribution of the initial Rh particles was relatively large at 3.03 nm, so that the coarsening of the Rh particles during the aging treatment was suppressed and the decrease in the specific surface area of ​​the Rh particles was suppressed, and as a result, high NOx reduction performance was obtained. In Comparative Example 1, where the heat treatment was not performed, the average particle size distribution of the Rh particles was small at 1.4 nm, so that the Rh particles coarsened during the aging treatment and the specific surface area of ​​the Rh particles decreased, and as a result, the NOx reduction performance was reduced. 2 In Comparative Example 2, in which heat treatment was performed in an atmosphere, the average particle size of the Rh particles after the aging treatment was smaller than that of Example 1, which is considered to have suppressed the coarsening of the Rh particles during the aging treatment, but the NOx-T50 was higher than that of Example 1. This is considered to be due to the fact that 17% by weight of the Rh contained in the exhaust gas purification material of Comparative Example 2 was dissolved in the AZ particles.

[0052] Comparison of NOx-T50 between Examples 2 to 5 and Comparative Examples 3 and 4, and comparison of NOx-T-50 between Examples 6, 7 and Comparative Example 5 also showed that the NOx reduction performance was improved by controlling the particle size distribution of the initial Rh particles by heat treatment of the Rh-supported metal oxide in a CO-containing atmosphere. Comparative Example 3, in which the Rh-supported metal oxide was heat-treated at 800°C, had lower NOx reduction performance than Examples 2 to 5, in which the heat treatment was performed at a temperature of 850°C or higher. In Comparative Example 3, the average particle size distribution of the Rh particles (i.e., the initial Rh particles) after heat treatment was smaller than in Examples 2 to 5, so that the coarsening of the Rh particles during the subsequent aging treatment was not suppressed as much as in Examples 2 to 5, decreasing the specific surface area of ​​the Rh particles, and as a result, the NOx reduction performance was lowered. In addition, since the average particle size distribution of the initial Rh particles in Examples 2 to 4 was the same value, it was shown that the effect of the heat treatment time on the average particle size distribution of the initial Rh particles was small. Since the average particle size distribution of the initial Rh particles was equivalent in Examples 5 and 7, which used different materials as the Rh raw materials, it is considered that the effect of the difference in the Rh raw material used on the average particle size distribution of the initial Rh particles is also small. From Examples 2 to 7, it is considered that the average particle size distribution of the initial Rh particles can be controlled by the heat treatment temperature.

[0053] The relationship between the coefficient of variation σ / μ of the particle size distribution of the initial Rh particles in Examples 2 to 7 and NOx-T50 is shown in Figure 1. When the coefficient of variation σ / μ was 1 or less, the NOx-T50 was lower (i.e., the NOx reduction performance was higher). When the coefficient of variation σ / μ was 0.54 or less, the NOx-T50 was particularly low (i.e., the NOx reduction performance was particularly high).

[0054] Reference examples 1~5 In Reference Examples 1 to 5, experiments were carried out to examine the effect of the proportion of Rh dissolved in the AZ particles on the NOx reduction performance.

[0055] Exhaust gas purifying material a was prepared in the same manner as in Example 2. Exhaust gas purifying material b was prepared in the same manner as in Examples 6 and 7, except that the heat treatment of the Rh-supported metal oxide was performed in an air atmosphere at 1000°C for 5 hours. Exhaust gas purifying material a and exhaust gas purifying material b in the amounts shown in Table 3 were mixed to obtain an exhaust gas purifying material. The proportion of Rh dissolved in the AZ particles based on the total weight of Rh contained in the exhaust gas purifying material was measured in the same manner as in Example 1. The results are shown in Table 3.

[0056] Using the prepared exhaust gas purification material, pellets were produced by the same pellet production method (pellet production method B) as in Examples 2 to 5. The aging treatment of the pellets and the evaluation of the exhaust gas purification performance were carried out in the same manner as in Example 1. The results are shown in Table 3.

[0057] [Table 3]

[0058] The relationship between the dissolved Rh proportion and NOx-T50 is shown in FIG. 2. In FIG. 2, the plot of the dissolved Rh proportion of 0% by weight corresponds to Example 2. FIG. 2 shows that the smaller the dissolved Rh proportion, the lower the NOx-T50 (i.e., the higher the NOx reduction performance). When the dissolved Rh proportion was less than 17% by weight, a higher NOx reduction performance was achieved than those of Comparative Examples 3 to 5. When the dissolved Rh proportion was 3% by weight or less, a particularly high NOx reduction performance was achieved.

Claims

1. A method for producing an exhaust gas purification material, comprising the steps of: (a) impregnating a metal oxide support with a solution of a rhodium compound; (b) drying the metal oxide support impregnated with the rhodium compound solution to obtain a rhodium-supported metal oxide comprising the metal oxide support and rhodium particles supported on the metal oxide support; and (c) heating the rhodium-supported metal oxide in an atmosphere containing carbon monoxide at a concentration of 0.01 to 5 volume % and the remainder being an inert gas to a temperature in the range of 850 to 1000°C to obtain the exhaust gas purification material; A method comprising:

2. 2. The method of claim 1, wherein after step (c), the percentage of rhodium dissolved in the metal oxide support is less than 17% by weight, based on the total weight of rhodium contained in the rhodium-supported metal oxide.

3. 2. The method according to claim 1, wherein after step (c), the proportion of rhodium dissolved in said metal oxide support is 3% by weight or less, based on the total weight of rhodium contained in said rhodium-supported metal oxide.

4. 2. The method of claim 1, wherein after step (c), the rhodium particles have a mean particle size distribution of 2.2 to 18 nm.

5. 5. The method according to claim 1, wherein after step (c), the coefficient of variation (σ / μ), calculated as the standard deviation (σ) divided by the mean (μ), of the particle size distribution of the rhodium particles is 1 or less.

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