Exhaust gas purification catalyst

Abstract

The objective is to provide an exhaust gas purification catalyst that can further improve purification performance. [Solution] The exhaust gas purification catalyst of the present invention comprises carrier particles, zirconium dioxide particles containing zirconium dioxide, wherein a first cation having a higher oxidation state than zirconium and a second cation having a larger ionic radius than zirconium are added together with the zirconium dioxide, and rhodium particles supported on the zirconium dioxide particles, characterized in that (i) the carrier particles are porous carrier particles and the zirconium dioxide particles and rhodium particles are supported in the pores of the porous carrier particles, or (ii) the carrier particles are composed of a plurality of primary particles and the zirconium dioxide particles and rhodium particles are dispersed inside the carrier particles.

Description

[Technical Field]

[0001] This invention relates to a catalyst for exhaust gas purification. [Background technology]

[0002] Exhaust gases emitted from internal combustion engines contain harmful components such as CO (carbon monoxide), HC (hydrocarbons), and NOx (nitrogen oxides). To remove these harmful components from exhaust gases, exhaust gas purification catalysts are used, which consist of noble metals such as Rh (rhodium) supported on a carrier such as Al2O3 (aluminum oxide).

[0003] Examples of such exhaust gas purification catalysts include catalysts containing noble metals, Al2O3 support particles, and ZrO2 (zirconium dioxide) semiconductor particles supported on the surface of the support particles (Patent Document 1). Another known catalyst is one in which a composite compound is formed in which a compound of at least one noble metal selected from the group such as Pt (platinum) and at least one metal element selected from the group such as Al (aluminum) is dispersed substantially uniformly in at least one oxide selected from the group such as Al2O3, and in which the noble metal is supported on the composite compound with a portion of its surface area covered by the composite compound (Patent Document 2). [Prior art documents] [Patent Documents]

[0004] [Patent Document 1] International Publication No. 2020 / 050464 [Patent Document 2] Japanese Patent Publication No. 2006-198594 [Overview of the project] [Problems that the invention aims to solve]

[0005] Regulations on the emission of harmful components in exhaust gases are becoming stricter year by year, and further improvements in the purification performance of exhaust gas purification catalysts are required.

[0006] This invention has been made in view of these points, and its purpose is to provide an exhaust gas purification catalyst that can further improve purification performance. [Means for solving the problem]

[0007] To solve the above problems, the exhaust gas purification catalyst of the present invention comprises carrier particles, zirconium dioxide particles containing zirconium dioxide, wherein a first cation having a higher oxidation state than zirconium and a second cation having a larger ionic radius than zirconium are added together with the zirconium dioxide, and rhodium particles supported on the zirconium dioxide particles, characterized in that (i) the carrier particles are porous carrier particles and the zirconium dioxide particles and rhodium particles are supported in the pores of the porous carrier particles, or (ii) the carrier particles are composed of a plurality of primary particles and the zirconium dioxide particles and rhodium particles are dispersed inside the carrier particles. [Effects of the Invention]

[0008] According to the present invention, purification performance can be further improved. [Brief explanation of the drawing]

[0009] [Figure 1] This is a schematic diagram of an exhaust gas purification catalyst according to the first embodiment. [Figure 2] This is a schematic diagram of an exhaust gas purification catalyst according to the second embodiment. [Figure 3] These are the XRD patterns of the samples after durability testing of Examples 4-6 and Comparative Examples 2 and 3, particularly in the 27°-32° range. [Figure 4] This graph shows the change in NOx 50% purification temperature [°C] with respect to the Nb2O5 content [mass%] in the first powder for the samples after durability test 1 of Comparative Examples 1-3, the samples after durability test 1 of Examples 1-3 (Y2O3 content in the first powder: 1 mass%), and the samples after durability test 1 of Examples 4-6 (Y2O3 content in the first powder: 4 mass%). [Figure 5] This graph shows the change in NOx 50% purification temperature with respect to the Nb2O5 content [mass%] in the first powder for the samples after durability test 2 of Comparative Examples 1-3, the samples after durability test 2 of Examples 1-3 (Y2O3 content in the first powder: 1 mass%), the samples after durability test 2 of Examples 4-6 (Y2O3 content in the first powder: 4 mass%), and the samples after durability test 2 of Examples 7-9 (Y2O3 content in the first powder: 8 mass%). [Modes for carrying out the invention]

[0010] The following describes embodiments of the exhaust gas purification catalyst of the present invention. It should be noted that the present invention is not limited to the following embodiments, but can be implemented with various modifications within the scope of the present invention.

[0011] 1. Catalyst for exhaust gas purification First, the exhaust gas purification catalyst according to the embodiment will be illustrated by first and second embodiments. Figure 1 is a schematic diagram of the exhaust gas purification catalyst according to the first embodiment, and Figure 2 is a schematic diagram of the exhaust gas purification catalyst according to the second embodiment.

[0012] As shown in Figure 1, the exhaust gas purification catalyst 1 according to the first embodiment comprises a powder of porous carrier particles 30, a powder of zirconium dioxide particles 20, and a powder of rhodium particles 10 supported on the zirconium dioxide particles 20. The zirconium dioxide particles 20 are particles containing ZrO2 (zirconium dioxide), in which a first cation having a higher oxidation state than Zr (zirconium) and a second cation having a larger ionic radius than Zr are added (doped) together with the ZrO2. The zirconium dioxide particles 20 and the rhodium particles 10 are supported in the pores of the porous carrier particles 30 and dispersed between the porous carrier particles 30.

[0013] As shown in FIG. 2, the exhaust gas purification catalyst 2 according to the second embodiment has powder of carrier particles 40, powder of zirconium dioxide particles 20, and powder of rhodium particles 10 supported on the zirconium dioxide particles 20. The carrier particles 40 are secondary particles in which a plurality of primary particles are aggregated. The zirconium dioxide particles 20 are particles containing ZrO2 (zirconium dioxide), and a first cation having an oxidation number higher than that of Zr (zirconium) and a second cation having an ionic radius larger than that of Zr are added (doped) to ZrO2 together. The zirconium dioxide particles 20 and the rhodium particles 10 are dispersed inside the carrier particles 40 and between the carrier particles 40.

[0014] Subsequently, the details of the exhaust gas purification catalyst according to the embodiment will be described. Generally, Rh (rhodium) as a catalyst metal exhibits particularly high catalytic activity with respect to NOx reduction compared to Pd (palladium) and Pt (platinum). By improving the catalytic activity of Rh, it is expected to reduce the amount of Rh used.

[0015] In this regard, the rate-determining step in the reduction of NOx in exhaust gas is the CO2 (carbon dioxide) production pathway. That is, the reaction between the O (oxygen) atoms adsorbed on the rhodium particles and CO (carbon monoxide) is the rate-determining step. Therefore, if the adsorption energy of the O atoms adsorbed on the rhodium particles can be reduced, the NOx reduction performance of the exhaust gas purification catalyst can be improved. In the exhaust gas purification catalyst, rhodium particles are supported on zirconium dioxide particles. The ZrO2 (zirconium dioxide) contained in the zirconium dioxide particles is electron-rich due to the addition of a primary cation (e.g., Nb) with a higher oxidation state than Zr (zirconium). These electrons flow into the rhodium particles, reducing the adsorption energy of the O atoms. Therefore, the NOx reduction performance of the exhaust gas purification catalyst can be improved. Thus, the purification performance of the exhaust gas purification catalyst can be improved. In addition, ZrO2 has a particularly high adsorption energy among metal oxides and readily adsorbs with Rh, for example, RhO2 (rhodium oxide). In particular, ZrO2 with a primary cation, especially Nb (niobium), added has a significantly higher adsorption energy compared to ZrO2 without the primary cation. This suppresses the evaporation of rhodium particles, resulting in high heat resistance.

[0016] On the one hand, in an exhaust gas purification catalyst, when a first cation (e.g., Nb) is added to ZrO2 of zirconium dioxide particles, due to reasons such as the crystal structure of the oxide of the first cation (e.g., Nb2O5) being stable in a monoclinic crystal, at high temperatures during use, the crystal structure of the zirconium dioxide particles becomes unstable and takes on a monoclinic crystal structure instead of a structure in which the tetragonal crystal is stabilized, and there is a risk of a decrease in purification performance. However, in an exhaust gas purification catalyst, a second cation (e.g., Y) with an ionic radius larger than Zr is added together with the first cation to ZrO2 of the zirconium dioxide particles. As a result, for example, based on the same principle as partially stabilized zirconia (e.g., yttrium-stabilized zirconia) described in papers such as Garvie, R.C., Hannink R.H. and Pascoe, R.T. (1975) Ceramic steel. Nature, 258(5537), 703-704. and Kobayashi, K., Kuwajima H. and Masaki, T., Solid State Ionics., Vol. 3 / 4, pp. 489-493 (1981). etc., even at high temperatures during use, the crystal structure of the zirconium dioxide particles can take on a structure in which the tetragonal crystal is stabilized. Therefore, it is possible to suppress a decrease in the purification performance of the exhaust gas purification catalyst due to the addition of the first cation. For this reason, the purification performance of the exhaust gas purification catalyst can be further improved.

[0017] In addition, in the exhaust gas purification catalyst according to the first embodiment, the zirconium dioxide particles are supported in the pores of the porous carrier particles or dispersed and supported between the porous carrier particles, so that aggregation of the zirconium dioxide particles is also suppressed, and thus the heat resistance is further improved. The rhodium particles are also supported on the zirconium dioxide particles and are supported in the pores of the porous carrier particles. Also, in the exhaust gas purification catalyst according to the second embodiment, the zirconium dioxide particles are dispersed and supported inside the carrier particles or between the carrier particles, so that aggregation of the zirconium dioxide particles is suppressed, and thus the heat resistance is further improved. The rhodium particles are also supported on the zirconium dioxide particles and are dispersed inside the carrier particles.

[0018] In other words, the exhaust gas purification catalyst achieves both high NOx reduction performance and high heat resistance. While the exhaust gas purification catalyst is not particularly limited, a three-way catalyst, for example, may be used.

[0019] The specific surface area of ​​the exhaust gas purification catalyst after durability testing was 90 m². 2 / g or less, 89.9m 2 / g or less, 89.8m 2 / g or less, or 89.7m 2 / g or less is acceptable, 88.0m 2 / g or more, 88.1m 2 / g or more, 88.2m 2 / g or more, or 88.3m 2 A value of 1 / g or higher is acceptable. The durability test is conducted, for example, by placing the exhaust gas purification catalyst in a flow-through durability furnace and repeatedly passing reducing gas (CO) and oxidizing gas (O2) alternately through it at 1000°C for 5 hours.

[0020] The NOx 50% purification temperature after the durability test of the exhaust gas purification catalyst may be, for example, 250°C or lower, 249°C or lower, 248°C or lower, 247°C or lower, or 246°C or lower, and may be 245°C or higher, 246°C or higher, 247°C or higher, or 248°C or higher. The NOx 50% purification temperature is determined as follows: First, the exhaust gas purification catalyst is placed in a flow-through reactor and heated to 500°C in an evaluation model gas at a heating rate of 50°C / min. After holding this temperature for 10 minutes, it is cooled to 100°C. Next, it is heated at a heating rate of 20°C / min, and the gas temperature at which the NOx purification rate reaches 50% is determined as the NOx 50% purification temperature. The composition of the evaluation model gas may be, for example, the composition used in the examples described later.

[0021] 1-1. Carrier particles The carrier particles in the exhaust gas purification catalyst may be porous carrier particles commonly used in exhaust gas purification catalysts, or secondary particles composed of multiple primary particles. The carrier particles may be, for example, metal oxides, specifically metal oxides containing Al (aluminum), more specifically Al2O3 (aluminum oxide), or composite oxides containing Al and Zr, such as Al2O3-ZrO2 composite oxides.

[0022] The average particle diameter of porous carrier particles and carrier particles composed of multiple primary particles is not particularly limited, but may be, for example, 1 μm or more, 10 μm or more, 50 μm or more, or 100 μm or more, and may be 1000 μm or less, 500 μm or less, 200 μm or less, or 100 μm or less. The average particle diameter is calculated by observing at least 200 particles using a scanning electron microscope (SEM), determining the equivalent diameter of a circle when a perfect circle equal to the area is defined as an equal-area circle, and then calculating the numerical average of these equivalent diameters.

[0023] The pore diameter of the pores in the porous carrier particles and the pore diameter of the pores inside the carrier particles are not particularly limited as long as they are large enough to support zirconium dioxide particles and rhodium particles within the pores. For example, they may be 10 nm or larger, 50 nm or larger, or 100 nm or larger, and may be 1000 nm or smaller, 500 nm or smaller, or 200 nm or smaller.

[0024] 1-2. Zirconium dioxide particles The zirconium dioxide particles in the exhaust gas purification catalyst are particles containing ZrO2 (zirconium dioxide), in which a first cation having a higher oxidation state than Zr (zirconium) and a second cation having a larger ionic radius than Zr are added (doped) to the ZrO2. The crystal structure of the zirconium dioxide particles of the exhaust gas purification catalyst after durability testing may be, for example, a tetragonal structure. The crystal structure of the zirconium dioxide particles is the crystal structure of the compound in which the first and second cations are added together to the ZrO2 contained in the zirconium dioxide particles.

[0025] The content of zirconium dioxide particles relative to the total of zirconium dioxide particles and carrier particles may be between 1.0% and 70.0% by mass. Specifically, this ratio may be 1.0% or more by mass, 5.0% or more by mass, 10.0% or more by mass, or 15.0% or more by mass, and 70.0% or less by mass, 50.0% or less by mass, 30.0% or less by mass, or 10.0% or less by mass. If this ratio is above these lower limits, the amount of zirconium dioxide particles that can act on the rhodium particles can be significantly increased. On the other hand, if this ratio is below these upper limits, the dispersibility of zirconium dioxide particles within the pores of the porous carrier particles and within the carrier particles is particularly good.

[0026] The first cation is not particularly limited, but examples include Nb (niobium). The content of the Nb (niobium) oxide portion in the zirconium dioxide particles may be 0.1% to 12.0% by mass. Specifically, this content may be 0.1% or more by mass, 0.5% or more by mass, 1.0% or more by mass, 1.5% or more by mass, 2.0% or more by mass, 5.0% or more by mass, or 11.0% or more by mass, and may be 12.0% or less by mass, 11.0% or less by mass, 10.0% or less by mass, 9.0% or less by mass, 8.0% or less by mass, 6.0% or less by mass, or 3.0% or less by mass. Note that the content of the Nb oxide portion in the zirconium dioxide particles is the ratio [mass%] of the mass of the Nb oxide portion to the total mass of the zirconium dioxide particles. The oxide portion of Nb in zirconium dioxide particles is specifically, for example, Nb2O5 (niobium pentoxide).

[0027] When the content is above these lower limits, the adsorption energy of zirconium dioxide particles becomes particularly large, making it easier to suppress the evaporation of rhodium particles. Furthermore, the electron-increasing effect due to the addition of Nb to ZrO2 is easily obtained. On the other hand, when the content is below these upper limits, electron transfer to rhodium particles is particularly good, and the NOx 50% purification temperature can be particularly reduced. Furthermore, at high temperatures during use, it is easier to suppress the instability of the zirconium dioxide particles, which would otherwise shift from a stabilized tetragonal crystal structure to a monoclinic crystal structure.

[0028] The second cation is not particularly limited, but examples include Y (yttrium). The content of the Y (yttrium) oxide portion in the zirconium dioxide particles may be 0.1% to 12.0% by mass. Specifically, this content may be 0.1% or more by mass, 0.5% or more by mass, 1.0% or more by mass, 1.5% or more by mass, 2.0% or more by mass, 5.0% or more by mass, or 11.0% or more by mass, and may be 12.0% or less by mass, 11.0% or less by mass, 10.0% or less by mass, 9.0% or less by mass, 8.0% or less by mass, 6.0% or less by mass, or 3.0% or less by mass. Note that the content of the Y oxide portion in the zirconium dioxide particles is the ratio of the mass of the Y oxide portion to the total mass of the zirconium dioxide particles [mass%]. Specifically, the Y oxide portion in the zirconium dioxide particles is, for example, Y2O3 (yttrium oxide).

[0029] If this content is above these lower limits, the effect of stabilizing the tetragonal crystal structure of zirconium dioxide particles at high temperatures during use, despite the addition of Nb to ZrO2, can be more effectively obtained. On the other hand, if this content is below these upper limits, the reduction in the electron increase effect due to the addition of Nb to ZrO2, which is caused by the addition of Y, can be more effectively suppressed.

[0030] The crystallite size of the zirconium dioxide particles may be 6.0 nm or larger, 6.2 nm or larger, 6.4 nm or larger, or 6.8 nm or larger, and may be 8.0 nm or smaller, 7.8 nm or smaller, 7.6 nm or smaller, or 7.4 nm or smaller. When the crystallite size is above these lower limits, the effect of suppressing sintering of catalyst metal particles is particularly good. On the other hand, when the crystallite size is below these upper limits, the heat resistance of the zirconium dioxide particles is particularly good, and aggregation of zirconium dioxide particles due to heating is particularly suppressed. The secondary particle size (D50) of the zirconium dioxide particles may be greater than 0 nm, 5 nm or larger, 10 nm or larger, or 15 nm or larger, and may be 40 nm or smaller, 35 nm or smaller, 30 nm or smaller, or 25 nm or smaller. When the secondary particle size (D50) is of such size, the dispersibility of the zirconium dioxide particles can be further improved. Furthermore, D50 (median diameter) can be measured as the particle diameter at 50% cumulative frequency by performing particle size distribution measurement using, for example, a laser diffraction particle size distribution analyzer (SALD-2300) manufactured by Shimadzu Corporation.

[0031] 1-3. Rhodium particles The rhodium particles in exhaust gas purification catalysts are particles containing Rh (rhodium) and supported on zirconium dioxide particles.

[0032] The size and shape of the rhodium particles may be any size and shape suitable for use as a catalytic metal in exhaust gas purification catalysts. Specifically, the particle diameter (D50) of the rhodium particles may be 0.1 nm or larger, 1.0 nm or larger, 2.0 nm or larger, or 2.5 nm or larger, and may be 10.0 nm or smaller, 5.0 nm or smaller, 3.0 nm or smaller, or 2.5 nm or smaller. The method for measuring the D50 of rhodium particles is the same as the method for measuring the D50 of zirconium dioxide particles described above.

[0033] The amount of rhodium particles supported on zirconium dioxide particles may be, for example, an amount such that the ratio of the mass of rhodium particles to the total mass of the exhaust gas purification catalyst is 0.1% by mass or more, 0.2% by mass or more, 0.3% by mass or more, or 0.4% by mass or more, and the ratio of the mass of rhodium particles to the total mass of the exhaust gas purification catalyst may be an amount such that it is 5.0% by mass or less, 2.5% by mass or less, 1.0% by mass or less, or 0.5% by mass or less. When the ratio of the mass of rhodium particles to the total mass of the exhaust gas purification catalyst is not less than these lower limits, NO X Since the number of rhodium particles contributing to purification increases, higher NO X purification performance can be achieved. On the other hand, when the ratio of the mass of rhodium particles to the total mass of the exhaust gas purification catalyst is not more than these upper limits, the amount of Rh, which is an expensive noble metal, can be reduced, and the cost performance is excellent.

[0034] 2. Method for manufacturing exhaust gas purification catalyst The method for manufacturing the exhaust gas purification catalyst is not particularly limited, and for example, the following first to third manufacturing methods and the like can be mentioned.

[0035] 2-1. First manufacturing method The first manufacturing method includes a step of obtaining a dispersion liquid by dispersing porous carrier particles, a zirconium dioxide source, a first cation source, and a second cation source in an acidic dispersion medium, a step of drying the dispersion liquid and then firing it to obtain porous carrier particles in which zirconium dioxide particles containing ZrO2 and in which the first and second cations are added (doped) together to ZrO2 are supported in the pores, and a step of supporting rhodium particles on the zirconium dioxide particles in the pores of the porous carrier particles, in this order. The rhodium particles are supported in the pores of the porous carrier particles and on the zirconium dioxide particles.

[0036] As an acidic dispersion medium, for example, citric acid can be used. As porous support particles, for example, Al2O3 can be used, specifically, for example, La2O3-compounded Al2O3 (La2O3(lanthanum(III) oxide): 1% by mass) can be used. As a zirconium dioxide source, for example, oxynitrate Zr can be used. As a first cation source, for example, oxalic acid Nb can be used. As a second cation source, for example, nitric acid Y can be used.

[0037] Drying of the dispersion can be performed by evaporating the dispersion to dryness to obtain a precipitate, and then drying it in a constant temperature furnace at 100°C to 200°C, preferably 100°C to 150°C, more preferably 100°C to 120°C for 1 to 36 hours, preferably 10 to 24 hours. Sintering can be performed, for example, in air at 600°C to 1000°C, preferably 700°C to 900°C, more preferably 700°C to 800°C for 1 to 10 hours, preferably 2 to 5 hours, more preferably 3 to 4 hours. In the process of drying the dispersion and sintering, a reduction treatment may be performed after sintering. The reduction treatment can be performed, for example, in a 3% hydrogen atmosphere at 800°C for 2 hours. The pH of the dispersion medium may be, for example, 1.0 or higher, 1.5 or higher, or 2.0 or higher, and may be 5.0 or lower, 4.5 or lower, or 4.0 or lower, but 2.5 to 3.5 is particularly preferred.

[0038] There are no particular limitations on the method for supporting rhodium particles on zirconium dioxide particles within the pores of porous carrier particles. One example is to disperse the porous carrier particles, on which zirconium dioxide particles are supported within the pores, in a dispersion medium (e.g., water such as distilled water), add a rhodium source (e.g., nitric acid Rh), stir, dry, and then calcine. In this method, drying and calcination can be carried out under the same conditions as drying and calcining of the dispersion in the process of obtaining porous carrier particles.

[0039] 2-2. Second manufacturing method The second manufacturing method comprises, in this order, the steps of: obtaining a first dispersion by dispersing porous carrier particles in an organic solvent; mixing a zirconium dioxide source, a first cation source, and a second cation source with the first dispersion; boiling and stirring the first dispersion; removing the organic solvent to obtain a powder; drying the powder and further calcining it to obtain porous carrier particles containing ZrO2, wherein zirconium dioxide particles in which the first and second cations are added together with ZrO2 are supported within the pores; and supporting rhodium particles on the zirconium dioxide particles within the pores of the porous carrier particles. The rhodium particles are supported within the pores of the porous carrier particles and also supported on the zirconium dioxide particles.

[0040] As an organic solvent, for example, hexane can be used. As porous support particles, for example, aluminum oxide (Al2O3) can be used, specifically, for example, La2O3-compounded Al2O3 (La2O3: 1% by mass) can be used. As a zirconium dioxide source, for example, zirconium butoxide can be used. As a first cation source, for example, niobium butoxide can be used. As a second cation source, for example, yttrium butoxide can be used.

[0041] The method for supporting rhodium particles on zirconium dioxide particles within the pores of porous carrier particles in the second manufacturing method, as well as drying and calcination, can be carried out in the same manner as in the first manufacturing method.

[0042] 2-3. Third manufacturing method The third manufacturing method comprises, in this order, the steps of: obtaining a first dispersion by dispersing a zirconium dioxide source (e.g., oxynitrate Zr), a first cation source (e.g., oxalic acid Nb), and a second cation source (e.g., nitrate Y) in a solvent (e.g., water); generating a precipitate by adjusting the pH of the first dispersion to a basic state using, for example, ammonia; adding raw materials for primary particles constituting carrier particles (e.g., aluminum nitrate and nitrate La) to the first dispersion and mixing; obtaining a first powder (a powder on which zirconium dioxide particles are supported on carrier particles) by removing the solvent from the first dispersion, for example, using a centrifuge, drying, and then calcining; and obtaining a second powder (a powder on which Rh is supported) by dispersing the first powder and a rhodium source (e.g., nitrate Rh) in a solvent (e.g., water), drying, and then calcining. The third manufacturing method may further include the steps of: dispersing the second powder and the raw materials for the primary particles constituting the carrier particles (for example, Al2O3-CeO2-ZrO2 composite oxide, CZ composite oxide, and an optional binder (for example, an Al2O3-based binder)) in a liquid (for example, water) to obtain a slurry in which these are suspended in the liquid; and pulverizing the obtained slurry after pressurizing it, for example by cold isostatic pressing (CIP).

[0043] 3. Exhaust gas purification method The exhaust gas purification method according to the embodiment includes contacting exhaust gas with an exhaust gas purification catalyst. The method of contacting the exhaust gas with the catalyst is not particularly limited. The exhaust gas may, for example, contain NOx (nitrogen oxides), CO (carbon monoxide), and HC (hydrocarbons). [Examples]

[0044] The exhaust gas purification catalyst according to the embodiment will be described in more detail below with reference to examples and comparative examples.

[0045] 1. Preparation of a sample of the exhaust gas purification catalyst. 1-1. Raw materials for exhaust gas purification catalyst samples • Material 1 (Al2O3 (aluminum oxide)): Powder containing particles of La2O3-compounded Al2O3 (La2O3 (lanthanum(III) oxide): 1% by mass) ·Material 2 (ACZ): Al2O3-CeO2-ZrO2 composite oxide (Al2O3: 30% by mass / CeO2 (ceria): 20% by mass / ZrO2 (zirconium dioxide): 44% by mass / Nd2O3 (neodymium(III) oxide): 2% by mass / La2O3: 2% by mass / Y2O3 (yttrium(III) oxide): 2% by mass) ·Material 3(CZ): Pyrochlore-type CZ composite oxide (CeO2: 51.5% by mass / ZrO2: 45.5% by mass / Pr6O 11 :mass%) • Material 4 (Aluminum Nitrate): Fujifilm Wako Pure Chemical Industries, commercially available reagent • Material 5 (Zr(zirconium) oxynitrate): Fujifilm Wako Pure Chemical Industries, commercially available reagent • Material 6 (Rh Nitrate (Rhium)): Nitric acid Rh aqueous solution (Rh concentration: 2wt%) • Material 7 (La Nitrate (Lanthane)): Fujifilm Wako Pure Chemical Industries, commercially available reagent • Material 8 (Nb oxalate (niobium)): Taniobis, commercially available reagents • Material 9 (Yttrium nitrate): Fujifilm Wako Pure Chemical Industries, commercially available reagent • Material 10 (Ammonia): Fujifilm Wako Pure Chemical Industries, commercially available reagent

[0046] 1-2. Preparation of a sample of the exhaust gas purification catalyst [Example 1] A test sample of the exhaust gas purification catalyst according to the second embodiment was prepared. First, while stirring, oxynitrate Zr (material 5), oxalic acid Nb (material 8), and nitric acid Y (material 9) were added to the solvent distilled water. Then, the pH was adjusted to a basic state using ammonia (material 10) to produce a precipitate. To this solution, a pre-mixed solution of nitrate Al (material 4) and nitrate La (material 7) was added and mixed. After removing the solvent using a centrifuge, the mixture was dried at 120°C overnight, and then calcined in air at 800°C for 3 hours to obtain the first powder (a powder in which zirconium dioxide particles are supported on carrier particles). The first powder is a powder having carrier particles composed of multiple primary particles and zirconium dioxide particles in which Nb and Y were added together with ZrO2 dispersed inside the carrier particles.

[0047] Next, while stirring, the first powder and nitrate Rh (material 6) were added to distilled water, then dried and further calcined to obtain the second powder (Rh-supported powder). In the second powder, rhodium particles are supported on zirconium dioxide particles and dispersed inside the carrier particles. The second powder corresponds to the exhaust gas purification catalyst according to the second embodiment.

[0048] Subsequently, while stirring, the second powder, ACZ (material 2), CZ (material 3), and an Al2O3-based binder were added to distilled water to obtain a suspended slurry.

[0049] Subsequently, the suspended slurry was dried, pressed and molded under a pressure of 1 ton using a cold isostatic press (CIP), and then sieved while being crushed to obtain a test sample of the exhaust gas purification catalyst of Example 1.

[0050] The content (mass ratio) of Nb2O5, Y2O3, ZrO2, and carrier particles (Al2O3) in the first powder was determined using XRF (X-ray fluorescence analysis). In this process, the content (mass ratio) of Nb, Y, Zr, and Al in the first powder was measured using XRF. Then, assuming that Nb, Y, Zr, and Al are present as constituent elements of Nb2O5, Y2O3, ZrO2, and carrier particles (Al2O3), respectively, the content of Nb2O5, Y2O3, ZrO2, and carrier particles (Al2O3) in the first powder was calculated from the content of Nb, Y, Zr, and Al. The content of Nb2O5, Y2O3, ZrO2, and carrier particles (Al2O3) in the first powder of Example 1 is shown in Table 1 below. Assuming that Nb2O5, Y2O3, and ZrO2 in the first powder are contained in the zirconium dioxide particles, the content (mass ratio) of Nb2O5 (oxide portion of niobium) and Y2O3 (oxide portion of yttrium) in the zirconium dioxide particles was calculated from the content of Nb2O5, Y2O3, and ZrO2 in the first powder. The calculation results are also shown in Table 1 below.

[0051] [Examples 2-9] Samples for the exhaust gas purification catalysts of Examples 2 to 9 were obtained in the same manner as in Example 1, except that the content of Nb2O5, Y2O3, and ZrO2 in the first powder, and the content of Nb2O5 and Y2O3 in the zirconium dioxide particles were adjusted by adjusting the amounts of oxynitrate Zr (material 5), oxalic acid Nb (material 8), and nitric acid Y (material 9) added to the solvent, as shown in Table 1 below.

[0052] [Comparative Example 1] A test sample for the exhaust gas purification catalyst of Comparative Example 1 was obtained in the same manner as in Example 1, except that the amount of oxynitrate Zr (material 5) and nitric acid Y (material 9) added to the solvent was adjusted so that the content of Nb2O5, Y2O3, and ZrO2 in the first powder, and the content of Nb2O5 and Y2O3 in the zirconium dioxide particles, were as shown in Table 1 below, by not adding oxalic acid Nb (material 8) to the solvent.

[0053] [Comparative Examples 2 and 3] Samples for the exhaust gas purification catalysts of Comparative Examples 2 and 3 were obtained in the same manner as in Example 1, except that nitric acid Y (material 9) was not added to the solvent, and the amounts of oxynitrate Zr (material 5) and oxalic acid Nb (material 8) added to the solvent were adjusted to determine the content of Nb2O5, Y2O3, and ZrO2 in the first powder, and the content of Nb2O5 and Y2O3 in the zirconium dioxide particles, as shown in Table 1 below.

[0054] 2. Exam 2-1. X-ray diffraction test For each of the samples in Examples 1-9 and Comparative Examples 1-3, the crystal structure of zirconium dioxide particles in the samples after the durability test was identified. Specifically, for each sample, a durability test was performed by firing at 1100°C for 5 hours under air (a test different from durability tests 1 and 2 in "2-2. Durability Test" described later) to obtain a sample after the durability test. Then, an X-ray diffraction test was performed on the sample after the durability test using an X-ray diffractometer, and the XRD pattern was measured. Based on the XRD pattern, the crystal structure of zirconium dioxide particles in the sample after the durability test for each example was identified. In the X-ray diffraction test, CuKα rays were excited, and the range from 0° to 90° was measured at a scan speed of 0.05° / min.

[0055] The results of identifying the crystal structure of zirconium dioxide particles in the samples after durability testing for each of Examples 1-9 and Comparative Examples 1-3 are shown in Table 1 below. Figure 3 shows the XRD patterns of the samples after durability testing for Examples 4-6 and Comparative Examples 2 and 3, particularly in the 27°-32° range.

[0056] 2-2. Durability Test (Durability Test 1) The samples from Examples 1-6 and Comparative Examples 1-3 were placed in a flow-through endurance furnace, and endurance test 1 was conducted by alternately flowing reducing gas (CO) and oxidizing gas (O2) at 1000°C for 5 hours. The flow rates and flow times of the reducing and oxidizing gases were set at 20 L / min and 10-minute intervals, respectively.

[0057] (Durability Test 2) The samples from each of Examples 1-9 and Comparative Examples 1-3 were placed in a flow-through endurance furnace, and endurance test 2 was conducted by alternately flowing reducing gas (CO) and oxidizing gas (O2) at 1050°C for 5 hours. The flow rates and flow times of the reducing and oxidizing gases were set at 20 L / min and 10-minute intervals, respectively.

[0058] 2-3. Exhaust Gas Purification Test (Exhaust gas purification test 1) First, 2 g of each sample from Examples 1-6 and Comparative Examples 1-3 after durability test 1 was placed in a flow-through reactor and heated to 500°C at a heating rate of 50°C / min in the evaluation model gas. This temperature was maintained for 10 minutes, and then the temperature was lowered to 100°C. Next, the gas was heated at a heating rate of 20°C / min, and the gas temperature at which the NOx purification rate reached 50% was determined as the NOx 50% purification temperature. The composition of the evaluation model gas was CO (0.65 vol%), CO2 (10.00 vol%), C3H6 (0.10 vol%), NO (0.15 vol%), O2 (0.70 vol%), H2 (3.00 vol%), and N2 (residue).

[0059] Table 1 below shows the NOx 50% purification temperature [°C] in exhaust gas purification test 1 for samples after durability test 1 for each of Examples 1-6 and Comparative Examples 1-3. Figure 4 is a graph showing the change in NOx 50% purification temperature [°C] with respect to the Nb2O5 content [mass%] in the first powder for samples after durability test 1 for Comparative Examples 1-3, samples after durability test 1 for Examples 1-3 (Y2O3 content in the first powder: 1 mass%), and samples after durability test 1 for Examples 4-6 (Y2O3 content in the first powder: 4 mass%).

[0060] (Exhaust gas purification test 2) First, for each of the samples from Examples 1-9 and Comparative Examples 1-3 after durability test 2, the NOx 50% purification temperature was determined using the same method as in exhaust gas purification test 1.

[0061] Table 1 below shows the NOx 50% purification temperature [°C] in exhaust gas purification test 2 for samples after durability test 2 for each of Examples 1-9 and Comparative Examples 1-3. Figure 5 is a graph showing the change in NOx 50% purification temperature with respect to the Nb2O5 content [mass%] in the first powder for samples after durability test 2 for Comparative Examples 1-3, samples after durability test 2 for Examples 1-3 (Y2O3 content in the first powder: 1 mass%), samples after durability test 2 for Examples 4-6 (Y2O3 content in the first powder: 4 mass%), and samples after durability test 2 for Examples 7-9 (Y2O3 content in the first powder: 8 mass%).

[0062] 3. Test results The manufacturing conditions for the samples in Examples 1-9 and Comparative Examples 1-3, as well as the results of the X-ray diffraction and exhaust gas purification tests, are summarized in Table 1 below.

[0063] [Table 1]

[0064] 4. Evaluation 4-1. X-ray diffraction test As shown in Table 1 and Figure 3 above, the samples from Examples 1-9 and Comparative Example 1 after the durability test are considered to have a tetragonal crystal structure based on the peaks in the XRD patterns. On the other hand, the sample from Comparative Example 2 after the durability test is considered to have a structure in which the zirconium dioxide particles are in the process of transitioning from a tetragonal to a monoclinic crystal, based on the peaks in the XRD patterns. Furthermore, the sample from Comparative Example 3 after the durability test is considered to have a monoclinic crystal structure based on the peaks in the XRD patterns.

[0065] These results show that, as with the samples in Comparative Examples 2 and 3, adding Nb to ZrO2 resulted in the zirconium dioxide particles adopting a monoclinic crystal structure instead of a tetragonal structure after the durability test. Since the crystal structure of Nb2O5 is monoclinic, it is thought that adding Nb to ZrO2 caused the zirconium dioxide particles to adopt a monoclinic crystal structure at high temperatures during the durability test. In contrast, as with the samples in Examples 1 to 9, adding Y along with Nb to ZrO2 resulted in the zirconium dioxide particles adopting a tetragonal crystal structure after the durability test. It is thought that further adding Y to ZrO2 caused the zirconium dioxide particles to adopt a tetragonal crystal structure at high temperatures during the durability test, similar to the principle for partially stabilized zirconia.

[0066] 4-2. Exhaust Gas Purification Test (Exhaust gas purification test 1) As shown in Table 1 and Figure 4 above, in the samples of Comparative Examples 1 to 3 after durability test 1, a tendency was observed for the NOx 50% purification temperature to rise as the Nb2O5 content increased. Although the addition of Nb to ZrO2 provides an electron-increasing effect, it is thought that as the amount of Nb added increases, the crystal structure of the zirconium dioxide particles becomes unstable, taking on a monoclinic structure instead of a stabilized tetragonal structure, thus reducing the purification performance of the catalyst. In contrast, in the samples of Examples 1 to 3 after durability test 1 (Y2O3 content in the first powder: 1 mass%), a tendency was observed for the NOx 50% purification temperature to rise as the Nb2O5 content increased, similar to Comparative Examples 1 to 3, but the NOx 50% purification temperature tended to be lower than that of the samples of Comparative Examples 1 to 3 after durability test 1. It is thought that by further adding Y to ZrO2, the crystal structure of the zirconium dioxide particles took on a stabilized tetragonal structure, resulting in the suppression of the decrease in the purification performance of the catalyst due to the increase in the amount of Nb added. Furthermore, in the samples from Examples 4-6 after durability test 1 (Y2O3 content in the first powder: 4% by mass), the NOx 50% purification temperature when the Nb2O5 content was 2% by mass was higher than that of Comparative Examples 1-3 and the samples from Examples 1-3 after durability test 1. This suggests that the electron-increasing effect due to Nb addition was reduced because the amount of Y added was excessive relative to the amount of Nb added. On the other hand, in this sample, the NOx 50% purification temperature when the Nb2O5 content was 8% by mass was much lower than that of Comparative Examples 1-3 and the samples from Examples 1-3 after durability test 1. This suggests that the effect of increasing the amount of Y added relative to the amount of Nb added was greater in stabilizing the tetragonal crystal structure of the zirconium dioxide particles.

[0067] (Exhaust gas purification test 2) As shown in Table 1 and Figure 5 above, in the samples of Comparative Examples 1-3 after Durability Test 2, there was a tendency for the NOx 50% purification temperature to rise as the Nb2O5 content increased. Although the addition of Nb to ZrO2 provides an electron-increasing effect, it is thought that as the amount of Nb added increases, the crystal structure of the zirconium dioxide particles becomes unstable, taking on a monoclinic structure instead of a stabilized tetragonal structure, thus reducing the purification performance of the catalyst. In contrast, in the samples of Examples 1-3 after Durability Test 2 (Y2O3 content in the first powder: 1 mass%), there was a tendency for the NOx 50% purification temperature to rise as the Nb2O5 content increased, similar to Comparative Examples 1-3, but the NOx 50% purification temperature tended to be lower than that of the samples of Comparative Examples 1-3 after Durability Test 2. It is thought that by further adding Y to ZrO2, the crystal structure of the zirconium dioxide particles took on a stabilized tetragonal structure, which suppressed the decrease in the purification performance of the catalyst due to the increase in the amount of Nb added. Furthermore, in the samples from Examples 4-6 after Durability Test 2 (Y2O3 content in the first powder: 4% by mass) and the samples from Examples 7-9 after Durability Test 2 (Y2O3 content in the first powder: 8% by mass), the NOx 50% purification temperature when the Nb2O5 content was 2% by mass was lower than that of the samples from Comparative Examples 1-3 after Durability Test 2, and higher than that of the samples from Examples 1-3 after Durability Test 2. Although the addition of Y stabilized the tetragonal crystal structure of the zirconium dioxide particles, it is thought that the electron-increasing effect due to the addition of Nb was reduced because the amount of Y added was excessive compared to the amount of Nb added. On the other hand, in this sample, the NOx 50% purification temperature when the Nb2O5 content was 8% by mass was much lower than that of Comparative Examples 1-3 and the samples from Examples 1-3 after Durability Test 2. It is thought that increasing the amount of Y added in relation to the amount of Nb added resulted in a greater effect in stabilizing the tetragonal crystal structure of the zirconium dioxide particles.

[0068] Although embodiments of the exhaust gas purification catalyst according to the present invention have been described in detail above, the present invention is not limited to the embodiments described above, and various design modifications can be made without departing from the spirit of the invention as described in the claims. [Explanation of Symbols]

[0069] 1: Exhaust gas purification catalyst, 2: Exhaust gas purification catalyst, 10: Rhodium particles, 20: Zirconium dioxide particles, 30: Porous carrier particles, 40: Carrier particles

Claims

1. Carrier particles and Zirconium dioxide particles containing zirconium dioxide, wherein a first cation having a higher oxidation state than zirconium and a second cation having a larger ionic radius than zirconium are added together with the zirconium dioxide, The zirconium dioxide particles are supported on rhodium particles, (i) The carrier particles are porous carrier particles, and the zirconium dioxide particles and the rhodium particles are supported within the pores of the porous carrier particles, or (ii) The carrier particles are composed of a plurality of primary particles, and the zirconium dioxide particles and the rhodium particles are dispersed inside the carrier particles.

2. The exhaust gas purification catalyst according to claim 1, characterized in that the content of the zirconium dioxide particles relative to the total of the zirconium dioxide particles and the carrier particles is 1.0% by mass to 70.0% by mass.

3. The exhaust gas purification catalyst according to claim 1 or 2, characterized in that the first cation is niobium.

4. The exhaust gas purification catalyst according to claim 3, characterized in that the content of the niobium oxide portion in the zirconium dioxide particles is 0.1% by mass to 12.0% by mass.

5. The exhaust gas purification catalyst according to claim 1 or 2, characterized in that the second cation is yttrium.

6. The exhaust gas purification catalyst according to claim 5, characterized in that the content of the yttrium oxide portion in the zirconium dioxide particles is 0.1% by mass to 12.0% by mass.

7. The exhaust gas purification catalyst according to claim 1 or 2, characterized in that the carrier particles are aluminum oxide.

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