Oxygen storage material and method for manufacturing the same

Abstract

To provide an oxygen storage material with a high utilization efficiency capable of developing excellent oxygen storage capacity (OSC) at a low temperature such as about 250°C, not only in a use initial period but also after exposed to a high temperature exhaust gas of such as about 1100°C for a long time.SOLUTION: An oxygen storage material containing a pyrochlore-type ceria-zirconia composite oxide to which scandium (Sc) is added, a molar fraction of Zr with respect to a total number of moles of Ce and Zr (X=Zr / (Ce+Zr)×100) is X=51.7-53.8%, in the atmosphere, before heating at 1100°C and after heating for 5 hours, a lattice constant obtained from an X-ray diffraction pattern obtained by X-ray diffraction measurement using CuKα satisfies the condition expressed by the following equation (1): the lattice constant≤-4.15×10-3X+10.733 (1), and an intensity ratio [I(14 / 29) value] of a diffraction line near 2θ=14.5° and the diffraction line near 2θ=29°[I(14 / 29) value] satisfies the following equation (2): I(14 / 29) value≤-1.00×10-3X+0.096 (2).SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to an oxygen storage material containing a pyrochlore-type ceria-zirconia composite oxide and a method for producing the same. [Background technology]

[0002] A so-called three-way catalytic converter is known as an exhaust gas purification catalyst that can oxidize carbon monoxide (CO) and hydrocarbons (HC) in exhaust gas emitted from internal combustion engines such as automobile engines, while simultaneously reducing nitrogen oxides (NOx).

[0003] Furthermore, when purifying exhaust gas using an exhaust gas purification catalyst, it is known that materials having oxygen storage capacity (OSC) that can absorb oxygen when the oxygen concentration in the exhaust gas is high and release oxygen when the oxygen concentration in the exhaust gas is low are used as carriers or co-catalysts for the exhaust gas purification catalyst in order to absorb fluctuations in the oxygen concentration in the exhaust gas and enhance the exhaust gas purification capacity.

[0004] Ceria has traditionally been preferred as an oxygen storage material having such OSCs, and in recent years, various types of ceria-containing composite oxides have been studied, and various ceria-zirconia composite oxides have been developed by methods such as coprecipitation, inverse coprecipitation, hydrothermal synthesis, melting, and solid-phase methods.

[0005] For example, Japanese Patent Publication No. 2015-182931 (Patent Document 1) discloses a method for producing a ceria-zirconia composite oxide containing cerium, zirconium, and other transition metal elements such as iron, manganese, cobalt, nickel, and copper, and having a pyrochlore phase in its crystalline structure, by a so-called melting method.

[0006] Furthermore, Japanese Patent Publication No. 2022-59284 (Patent Document 2) describes an oxygen storage material containing a pyrochlore-type ceria-zirconia composite oxide and iron added to the pyrochlore-type ceria-zirconia composite oxide, wherein the Fe content ratio to the total amount of Ce and Zr (Fe / (Ce+Zr)×100) is 0.5~9at%, the mole fraction of Zr to the total number of moles of Ce and Zr (X=Zr / (Ce+Zr)×100) is X=40~50%, and the lattice constant obtained from the X-ray diffraction pattern obtained by X-ray diffraction measurement using CuKα in air, before heating at 1100°C and after heating for 5 hours is given by the formula: Lattice constant ≤ -7.00×10 -3 The condition expressed as X + 10.874 is satisfied, and the intensity ratio [I(14 / 29) value] of the diffraction line around 2θ = 14.5° and the diffraction line around 2θ = 29°, obtained from the X-ray diffraction pattern obtained by X-ray diffraction measurement using CuKα in air, before heating at 1100°C and after heating for 5 hours, is given by the formula: I(14 / 29) value ≤ 2.36 × 10 -3 An oxygen storage material that satisfies the conditions represented by X-0.072 is disclosed.

[0007] However, in recent years, the required characteristics for exhaust gas purification catalysts have been increasing. There is a growing demand for oxygen storage materials that can exhibit excellent oxygen storage capacity (OSC) at a low temperature of 250°C, not only during the initial period of use but also after prolonged exposure to high-temperature exhaust gas of around 1100°C, and that have high utilization efficiency. Conventional oxygen storage materials, such as those described in Patent Documents 1 and 2, have not always been sufficient. [Prior art documents] [Patent Documents]

[0008] [Patent Document 1] Japanese Patent Publication No. 2015-182931 [Patent Document 2] Japanese Patent Publication No. 2022-59284 [Overview of the project] [Problems that the invention aims to solve]

[0009] This invention has been made in view of the problems of the above-mentioned prior art, and aims to provide an oxygen storage material that exhibits excellent oxygen storage capacity (OSC) at a low temperature of 250°C, not only at the beginning of use but also after being exposed to high-temperature exhaust gas of about 1100°C for a long period of time, and has high utilization efficiency, as well as a method for manufacturing the same. [Means for solving the problem]

[0010] The inventors, through diligent research to achieve the above objective, selected scandium as the element to be added to the pyrochlore-type ceria-zirconia composite oxide, and adjusted the mole fraction of zirconium relative to the total number of moles of cerium and zirconium to be within a predetermined range. The scandium-containing ceria-zirconia solid solution powder was then pressure-molded at a predetermined pressure, reduced under predetermined high-temperature conditions, and further oxidized. This process ensures that the product exhibits properties both before and after prolonged exposure to exhaust gas at a high temperature of approximately 1100°C. We obtained a scandium-containing pyrochlore-type ceria-zirconia composite oxide in which scandium is sufficiently dissolved in a pyrochlore-type ceria-zirconia composite oxide. We discovered that this scandium-containing pyrochlore-type ceria-zirconia composite oxide exhibits excellent oxygen storage capacity (OSC) not only in the initial stages of use, but also at a low temperature of 250°C even after prolonged exposure to high-temperature exhaust gas at approximately 1100°C, demonstrating high utilization efficiency as an oxygen storage material. This led to the completion of the present invention.

[0011] In other words, the present invention provides the following embodiments.

[0012] [1] An oxygen storage material containing a pyrochlore-type ceria-zirconia composite oxide with scandium (Sc) added, The mole fraction of zirconium (X = Zr / (Ce + Zr) × 100) relative to the total number of moles of cerium (Ce) and zirconium (Zr) is X = 51.7 to 53.8%. The lattice constants determined from the X-ray diffraction patterns obtained by X-ray diffraction measurement using CuKα before heating at 1100°C and after heating for 5 hours in the atmosphere satisfy the following formula (1): Lattice constant ≤ -4.15×10 -3 X + 10.733 (1) (In the above formula, X represents the molar fraction of zirconium) and satisfy the conditions represented by: The intensity ratio [I(14 / 29) value] between the diffraction line near 2θ = 14.5° and the diffraction line near 2θ = 29° determined from the X-ray diffraction patterns obtained by X-ray diffraction measurement using CuKα before heating at 1100°C and after heating for 5 hours in the atmosphere satisfies the following formula (2): I(14 / 29) value ≤ -1.00×10 -3 X + 0.096 (2) (In the above formula, X represents the molar fraction of zirconium) An oxygen storage material that satisfies the conditions represented by:

[0013] [2] The oxygen storage material according to [1], wherein the content ratio of scandium (Sc) to the total amount of cerium (Ce) and zirconium (Zr) (Sc / (Ce + Zr) × 100) is 0.8 to 5 at%.

[0014] [3] The oxygen storage material according to [1] or [2], wherein the molar fraction X of zirconium is X = 52.6 to 53.8%.

[0015] [4] A method for producing an oxygen storage material containing a pyrochlore-type ceria-zirconia-based composite oxide to which scandium (Sc) is added, a step of preparing a ceria-zirconia-based solid solution powder containing scandium in which the molar fraction of zirconium (Zr / (Ce + Zr) × 100) to the total number of moles of cerium (Ce) and zirconium (Zr) is 51.7 to 53.8%, The process involves pressurizing the ceria-zirconia solid solution powder containing scandium at a pressure of 30 to 350 MPa, then reducing it at a temperature of 1400 to 2000°C, and further oxidizing it to obtain an oxygen storage material containing a pyrochlore-type ceria-zirconia composite oxide with scandium added as described in any one of [1] to [3], A method for producing oxygen storage materials, including

[0016] [5] The method for producing an oxygen storage material according to [4], wherein the ratio of scandium (Sc) to the total amount of cerium (Ce) and zirconium (Zr) in the ceria-zirconia solid solution powder containing scandium (Sc / (Ce+Zr)×100) is 0.8 to 5 at%.

[0017] In this invention, the reason why a scandium-containing pyrochlore-type ceria-zirconia composite oxide that satisfies the conditions represented by formula (1) can be determined to have sufficient scandium in solid solution is explained below. That is, in a pyrochlore-type ceria-zirconia composite oxide that does not contain scandium, the lattice constant shows a negative correlation with the mole fraction X of zirconium. This is because, in a pyrochlore-type ceria-zirconia composite oxide, the mole fraction X of zirconium increases, that is, Ce has a large ionic radius. 4+ Zr has a smaller ionic radius compared to (ionic radius: 0.97 Å (8-coordinate)). 4+ This is thought to be because an increase in the proportion of cations with an ionic radius of 0.84 Å (8-coordinate) reduces the average ionic radius of all cations in the crystal lattice, causing the lattice to contract.

[0018] Then, to the scandium-free pyrochlore-type ceria-zirconia composite oxide, Zr 4+ When metal ions with a larger ionic radius than Zr are dissolved in a solid solution, the average ionic radius of all cations in the crystal lattice increases, causing the lattice to expand. 4+ When metal ions with a lower ionic charge than Zr are dissolved in solid solution, oxygen vacancies are generated due to charge compensation, causing the lattice to contract. Therefore, Zr4+ Sc, which has a larger ionic radius and a smaller ionic valence than 3+ (ionic radius 0.87 Å), when solid-soluted, the lattice expansion effect due to the increase in ionic radius and the lattice contraction effect due to the generation of oxygen defects compete with each other. Since the ionic radius of oxide ions (1.38 Å) is sufficiently larger than the ionic radius of the Zr site, the influence on the lattice is that the generation of oxygen defects is larger than the increase in ionic radius caused by the solid-soluted Sc 3+ , and as the solid solution of Sc 3+ progresses, the lattice constant tends to decrease.

[0019] Therefore, in the present invention, when the following formula (1a): Lattice constant (Y1) = -4.15×10 -3 X + 10.733 (1a) is considered to represent the boundary between the state where scandium is sufficiently solid-soluted and the state where scandium is not sufficiently solid-soluted in the pyrochlore-type ceria-zirconia-based composite oxide. When the measured lattice constant is less than or equal to the lattice constant (Y1), that is, when the condition represented by the above formula (1) is satisfied, it is a state where scandium is sufficiently solid-soluted. When it exceeds the lattice constant (Y1), that is, when the condition represented by the above formula (1) is not satisfied, it can be determined that it is a state where scandium is not sufficiently solid-soluted. The lattice constant can be obtained by performing fitting by the least squares method using free analysis software (for example, the Rietveld analysis software "Jana2006") on the X-ray diffraction pattern obtained by X-ray diffraction measurement to refine the lattice constant. Also, as the method of the above X-ray diffraction measurement, a method of measuring using an X-ray diffractometer (for example, "RINT-Ultima" manufactured by Rigaku Corporation) with CuKα rays as the X-ray source under the conditions of a tube voltage of 40 KV, a tube current of 40 mA, and a scanning speed of 2θ = 10° / min is adopted.

[0020] Furthermore, in the present invention, the reason why a scandium-containing pyrochlore-type ceria-zirconia composite oxide that satisfies the conditions represented by formula (2) above can be determined to have sufficient scandium in solid solution is explained below. That is, in a pyrochlore-type ceria-zirconia composite oxide that does not contain scandium, the I(14 / 29) value is determined by the difference in ionic radii between the Ce site and the Zr site in the crystal lattice, and the smaller this difference, the smaller the I(14 / 29) value. And, in a pyrochlore-type ceria-zirconia composite oxide that does not contain scandium, Zr 4+ Sc has a larger ionic radius than 3+ When solid solution is formed, the difference in ionic radius between the Ce site and the Zr site decreases, resulting in a smaller I(14 / 29) value.

[0021] Therefore, in the present invention, the following formula (2a): I(14 / 29) value (Y2) = -1.00 × 10 -3 X+0.096 (2a) If we consider the straight line represented by the equation to represent the boundary between a state in which scandium is sufficiently dissolved in a pyrochlore-type ceria-zirconia composite oxide and a state in which scandium is not sufficiently dissolved, then if the measured I(14 / 29) value is less than or equal to the I(14 / 29) value (Y2), that is, if the condition represented by equation (2) is satisfied, then it can be determined that scandium is sufficiently dissolved. If it exceeds the lattice constant (Y2), that is, if the condition represented by equation (2) is not satisfied, then it can be determined that scandium is not sufficiently dissolved. The I(14 / 29) value can be determined from the peak intensity I(14) of the diffraction line at 2θ=14.5° and the peak intensity I(29) of the diffraction line at 2θ=29° in the X-ray diffraction pattern obtained by X-ray diffraction measurement. Furthermore, the X-ray diffraction measurement method employs an X-ray diffractometer (for example, RIGAK Corporation's "RINT-Ultima"), using CuKα rays as the X-ray source, under the conditions of a tube voltage of 40KV, a tube current of 40mA, and a scanning speed of 2θ = 10° / min. The intensity (peak intensity) of the diffraction lines is calculated after removing Kα2 and background using commercially available analysis software (for example, Materials Data's "JADE").

[0022] Furthermore, the oxygen storage material of the present invention exhibits excellent oxygen storage capacity (OSC) not only at the beginning of use, but also at a low temperature of 250°C even after being exposed to high-temperature exhaust gas of approximately 1100°C for a long period of time. Although the reason for this high utilization efficiency is not entirely clear, the inventors speculate as follows: In other words, the superlattice structure of CeO2-ZrO2 in the scandium-containing pyrochlore-type ceria-zirconia composite oxide constituting the oxygen storage material of the present invention undergoes a phase change between the pyrochlore phase (Ce2Zr2O7) and the κ phase (Ce2Zr2O8) in response to the partial pressure of oxygen in the gas phase, thereby exhibiting oxygen storage capacity (OSC). 3+ The ionic radius of is Zr 4+Because of its ionic radius, it is believed that in the scandium-containing pyrochlore-type ceria-zirconia composite oxide according to the present invention, Sc ions selectively substitute for Zr sites. When Zr sites are substituted with Sc ions, the superlattice structure containing oxygen vacancies generated by charge compensation becomes more stable. Therefore, it is presumed that the oxygen storage material of the present invention can exhibit excellent oxygen storage capacity (OSC) at a low temperature of 250°C even after being exposed to high-temperature exhaust gas at around 1100°C for a long period of time, resulting in high utilization efficiency. Furthermore, in the pyrochlore-type ceria-zirconia composite oxide, by setting the mole fraction of zirconium (X = Zr / (Ce + Zr) × 100) relative to the total number of moles of cerium and zirconium within a predetermined range, it is presumed that phase separation of CeO2 due to prolonged exposure to high-temperature exhaust gas at around 1100°C is suppressed, and excellent oxygen storage capacity (OSC) is maintained. [Effects of the Invention]

[0023] According to the present invention, it is possible to obtain an oxygen storage material with high utilization efficiency that can exhibit excellent oxygen storage capacity (OSC) at a low temperature of 250°C, not only during the initial period of use but also after being exposed to high-temperature exhaust gas of approximately 1100°C for a long period of time. [Brief explanation of the drawing]

[0024] [Figure 1] This graph shows the lattice constants of the composite oxide powders obtained in Examples 1-2 and Comparative Examples 1-4, plotted against the molar fraction X of zirconium, before and after the high-temperature durability test. [Figure 2] This graph shows the I(14 / 29) values ​​of the composite oxide powders obtained in Examples 1-2 and Comparative Examples 1-4, plotted against the molar fraction X of zirconium. [Figure 3] This graph shows the results of plotting the OSC material utilization rate of the catalyst powder against the molar fraction X of zirconium for the composite oxide powders obtained in Examples 1-2 and Comparative Examples 1-4 after high-temperature durability testing. [Modes for carrying out the invention]

[0025] The present invention will be described in detail below with reference to its preferred embodiments.

[0026] First, the oxygen storage material of the present invention will be described. The oxygen storage material of the present invention is an oxygen storage material containing a pyrochlore-type ceria-zirconia composite oxide to which scandium (Sc) is added, The mole fraction of zirconium (X = Zr / (Ce + Zr) × 100) relative to the total number of moles of cerium (Ce) and zirconium (Zr) is X = 51.7 to 53.8%. The lattice constant, obtained from the X-ray diffraction patterns using CuKα before heating at 1100°C in air and after heating for 5 hours, is given by the following equation (1): Lattice constant ≤ -4.15 × 10⁻⁶ -3 X + 10.733 (1) (In the above formula, X represents the mole fraction of zirconium.) It satisfies the conditions expressed as follows: The intensity ratio [I(14 / 29) value] of the diffraction lines around 2θ=14.5° and 2θ=29°, obtained from the X-ray diffraction patterns obtained by X-ray diffraction measurements using CuKα in air before heating at 1100°C and after heating for 5 hours, is given by the following formula (2): I(14 / 29) value ≤ -1.00 × 10 -3 X+0.096 (2) (In the above formula, X represents the mole fraction of zirconium.) It satisfies the conditions expressed by [the formula shown].

[0027] The oxygen storage material of the present invention contains a ceria-zirconia composite oxide having a superlattice structure in which Ce and Zr are regularly arranged (hereinafter referred to as "pyrochlore-type ceria-zirconia composite oxide"). Oxygen storage materials containing such pyrochlore-type ceria-zirconia composite oxide have a higher oxygen diffusion rate in the bulk than ceria-zirconia composite oxides having a fluorite structure, and therefore exhibit superior oxygen storage capacity (OSC). Furthermore, by recognizing the presence of a peak originating from the superlattice structure (a peak appearing at 2θ = 14.0° to 16.0°) in X-ray diffraction measurements using CuKα rays, it can be confirmed that the ceria-zirconia composite oxide is of the pyrochlore type having a superlattice structure.

[0028] In the oxygen storage material of the present invention, the mole fraction of zirconium (X = Zr / (Ce + Zr) × 100) relative to the total number of moles of cerium (Ce) and zirconium (Zr) must be X = 51.7 to 53.8%, and preferably 52.6 to 53.8%. If the mole fraction of zirconium falls below the lower limit, it becomes difficult to obtain sufficient oxygen storage capacity (OSC), while if it exceeds the upper limit, it becomes difficult to obtain it as a single phase.

[0029] Furthermore, the oxygen storage material of the present invention further contains scandium (Sc) added to such pyrochlore-type ceria-zirconia composite oxide. By solid-solving scandium, heat resistance is ensured while oxygen vacancies are created, improving the low-temperature operability of oxygen. As a result, the oxygen storage capacity (OSC) at low temperatures and the utilization efficiency as an oxygen storage material are improved.

[0030] The scandium content is preferably 0.8 to 5 at%, more preferably 0.9 to 4 at%, even more preferably 0.9 to 3.5 at%, and particularly preferably 0.9 to 2.7 at%, as the ratio of scandium (Sc) to the total amount of cerium (Ce) and zirconium (Zr) (Sc / (Ce+Zr)×100). If the scandium content falls below the lower limit, the effect of improving the oxygen storage capacity (OSC) at low temperatures and the improvement of the utilization efficiency as an oxygen storage material due to the solid solution of scandium tends not to be sufficiently obtained. On the other hand, if the scandium content exceeds the upper limit, the scandium does not solidly dissolve sufficiently, and the effect of improving the oxygen storage capacity (OSC) at low temperatures and the improvement of the utilization efficiency as an oxygen storage material tend not to be sufficiently obtained.

[0031] Furthermore, even when scandium is added to pyrochlore-type ceria-zirconia composite oxides, it is difficult to sufficiently dissolve the scandium in the pyrochlore-type ceria-zirconia composite oxide using methods such as so-called solid-phase synthesis or hydrothermal synthesis. Therefore, the addition of scandium does not contribute to improving the oxygen storage capacity (OSC) at low temperatures or improving the utilization efficiency as an oxygen storage material. However, in the present invention, by the manufacturing method of the present invention described later, it is possible to obtain a scandium-containing pyrochlore-type ceria-zirconia composite oxide in which scandium is sufficiently dissolved, which was not possible with conventional methods. This makes it possible to achieve improved oxygen storage capacity (OSC) at low temperatures and improved utilization efficiency as an oxygen storage material.

[0032] The lattice constant of the oxygen storage material of the present invention, as determined from the X-ray diffraction patterns obtained by X-ray diffraction measurements using CuKα in air before heating at 1100°C (before the high-temperature endurance test) and after heating for 5 hours (after the high-temperature endurance test), is given by the following formula (1): Lattice constant ≤ -4.15 × 10⁻⁶ -3 X + 10.733 (1) (In the above formula, X represents the mole fraction of zirconium.) The conditions expressed by and The diffraction lines around 2θ=14.5° and around 2θ=29°, and their intensity ratio [I(14 / 29) value] obtained from the X-ray diffraction patterns obtained by X-ray diffraction measurements using CuKα before and after the high-temperature endurance test, are given by the following formula (2): I(14 / 29) value ≤ -1.00 × 10 -3 X+0.096 (2) (In the above formula, X represents the mole fraction of zirconium.) The oxygen storage material satisfies the conditions expressed by the formula (1) above, where the lattice constant before and after the high-temperature endurance test satisfies the conditions expressed by the formula (1), and the I(14 / 29) value before and after the high-temperature endurance test satisfies the conditions expressed by the formula (2) above. This means that scandium is sufficiently dissolved in the pyrochlore-type ceria-zirconia composite oxide, and it can exhibit excellent oxygen storage capacity (OSC) at a low temperature of 250°C, not only in the initial stages of use but also after being exposed to high-temperature exhaust gas at around 1100°C for a long period of time, demonstrating high utilization efficiency. On the other hand, if the lattice constant before and after the high-temperature endurance test does not satisfy the conditions expressed by formula (1), or if the I(14 / 29) value before and after the high-temperature endurance test does not satisfy the conditions expressed by formula (2), then scandium is not sufficiently dissolved in the pyrochlore-type ceria-zirconia composite oxide. As a result, the oxygen storage capacity (OSC) at a low temperature of 250°C will not be sufficiently expressed, both in the initial stages of use and after prolonged exposure to exhaust gas at a high temperature of around 1100°C, and the utilization efficiency as an oxygen storage material will also be low.

[0033] Here, the diffraction lines around 2θ = 14.5° are attributed to the (111) plane of the ordered phase, and the diffraction lines around 2θ = 29° overlap with the diffraction lines attributed to the (222) plane of the ordered phase and the diffraction lines attributed to the (111) plane of the cubic phase of the ceria-zirconia solid solution (CZ solid solution). Therefore, the I(14 / 29) value, which is the intensity ratio of the two diffraction lines, is defined as an index indicating the maintenance rate (abundance) of the superlattice structure (ordered phase).

[0034] Furthermore, the average crystallite size of the oxygen storage material of the present invention is preferably 0.1 to 10 μm, and more preferably 0.2 to 5 μm. If the average crystallite size falls below the lower limit, the I(14 / 29) value, which indicates the maintenance rate of the superlattice structure, tends to decrease after the high-temperature durability test. On the other hand, if it exceeds the upper limit, it tends to become difficult to obtain a sufficient improvement in oxygen storage capacity (OSC). The average crystallite size can be calculated using commercially available analysis software (for example, "JADE" from Materials Data Inc.) based on the Scherrer formula, using the full width at half maximum of the diffraction lines around 2θ=29° obtained from the X-ray diffraction pattern obtained by X-ray diffraction measurement using CuKα.

[0035] Furthermore, the specific surface area of ​​the oxygen storage material of the present invention is not particularly limited, but is between 0.01 and 20 m². 2 It is preferable that the value be / g, and 0.05 to 10m 2 It is more preferable that the value be / g, and 0.1 to 5m 2 It is even more preferable that the specific surface area is / g. When such a specific surface area falls below the lower limit, the oxygen storage capacity tends to decrease, while when it exceeds the upper limit, the number of particles with small particle sizes increases, and the heat resistance tends to decrease. Such a specific surface area can be calculated as the BET specific surface area using the BET isothermal adsorption formula from the adsorption isotherm.

[0036] Furthermore, the oxygen storage material of the present invention may further contain at least one element selected from the group consisting of rare earth elements other than cerium and scandium, and alkaline earth elements. By including such elements, when the oxygen storage material of the present invention is used as a support for an exhaust gas purification catalyst, a higher exhaust gas purification capacity tends to be exhibited. Examples of such rare earth elements other than cerium and scandium include yttrium (Y), lanthanum (La), praseodymium (Pr), neodymium (Nd), samarium (Sm), gadolinium (Gd), terbium (Tb), dysprosium (Dy), ytterbium (Yb), lutetium (Lu), etc. Among these, La, Nd, Pr, and Y are preferred, and La, Y, and Nd are more preferred, from the viewpoint of tending to stabilize the superlattice structure. Furthermore, examples of alkaline earth metal elements include magnesium (Mg), calcium (Ca), strontium (Sr), barium (Ba), and radium (Ra). Among these, Mg, Ca, and Ba are preferred from the viewpoint of their tendency to stabilize the superlattice structure.

[0037] When the mixture further contains at least one element selected from the group consisting of rare earth elements other than cerium and scandium, and alkaline earth elements, the content of the element is preferably 0.5 to 20 at%, and more preferably 1 to 10 at%, relative to the total amount of cerium (Ce) and zirconium (Zr). If the content of such an element falls below the lower limit, the effect of stabilizing the superlattice structure tends to decrease, while if it exceeds the upper limit, the oxygen storage capacity tends to decrease.

[0038] The oxygen storage material of the present invention contains the pyrochlore-type ceria-zirconia composite oxide and scandium added to this pyrochlore-type ceria-zirconia composite oxide. It is capable of exhibiting excellent oxygen storage capacity (OSC) at a low temperature of 250°C, not only during initial use but also after prolonged exposure to exhaust gas at high temperatures of approximately 1100°C. Therefore, the oxygen storage material of the present invention is suitably used as a carrier or co-catalyst for exhaust gas purification catalysts. A suitable example using such an oxygen storage material of the present invention is an exhaust gas purification catalyst consisting of a carrier made of the oxygen storage material of the present invention and a noble metal supported on the carrier. Examples of such noble metals include platinum, rhodium, palladium, osmium, iridium, gold, and silver. Another example is one in which the oxygen storage material of the present invention is arranged around an exhaust gas purification catalyst in which a noble metal is supported on other catalyst carrier fine particles.

[0039] Next, the method for producing the oxygen storage material of the present invention will be described. The method for producing the oxygen storage material of the present invention is a method for producing an oxygen storage material containing a pyrochlore-type ceria-zirconia composite oxide to which scandium (Sc) has been added, The process involves preparing a ceria-zirconia solid solution powder containing scandium, wherein the molar fraction of zirconium (Zr / (Ce+Zr)×100) relative to the total number of moles of cerium (Ce) and zirconium (Zr) is 51.7-53.8% (the first step), The process involves pressurizing the ceria-zirconia solid solution powder containing scandium at a pressure of 30 to 350 MPa, then reducing it at a temperature of 1400 to 2000°C, and further oxidizing it to obtain an oxygen storage material containing a pyrochlore-type ceria-zirconia composite oxide to which scandium has been added (second step), This method includes [something].

[0040] First, in the first step, a scandium-containing ceria-zirconia solid solution powder is prepared, which contains scandium (Sc) and has a molar fraction of zirconium (Zr / (Ce+Zr)×100) of 51.7-53.8% (preferably 52.6-53.8%) relative to the total number of moles of cerium (Ce) and zirconium (Zr). If the molar fraction of zirconium in the scandium-containing ceria-zirconia solid solution powder falls below the lower limit, it becomes difficult to obtain sufficient oxygen storage capacity (OSC) in the resulting oxygen storage material. On the other hand, if it exceeds the upper limit, it becomes difficult to obtain the oxygen storage material as a single phase.

[0041] Furthermore, in the scandium-containing ceria-zirconia solid solution powder, the ratio of scandium (Sc) to the total amount of cerium (Ce) and zirconium (Zr) (Sc / (Ce+Zr)×100) is preferably 0.8 to 5 at%, more preferably 0.9 to 4 at%, even more preferably 0.9 to 3.5 at%, and particularly preferably 0.9 to 2.7 at%. If the scandium content is below the lower limit, it tends to be difficult to obtain sufficient improvements in low-temperature oxygen storage capacity (OSC) and utilization efficiency as an oxygen storage material due to the solid solution of scandium in the resulting oxygen storage material. On the other hand, if it exceeds the upper limit, scandium does not sufficiently solid dissolve in the resulting oxygen storage material, and it tends to be difficult to obtain sufficient improvements in low-temperature oxygen storage capacity (OSC) and utilization efficiency as an oxygen storage material.

[0042] Furthermore, in the scandium-containing ceria-zirconia solid solution powder, it is preferable to use a solid solution in which ceria and zirconia are mixed at the atomic level, from the viewpoint of more sufficiently forming a superlattice structure. In addition, such a scandium-containing ceria-zirconia solid solution powder preferably has an average primary particle diameter of about 2 to 100 nm, more preferably about 5 to 70 nm, and furthermore, a specific surface area of ​​1.0 to 100 m². 2 It is preferable that the amount is / g, and 10 to 80m 2It is more preferable that the amount be / g, and 30-80m 2 It is even more preferable that it be / g.

[0043] The method for preparing such scandium-containing ceria-zirconia solid solution powder is not particularly limited. For example, a method can be used to produce the solid solution powder by employing a so-called coprecipitation method such that the content ratios of cerium, zirconium, and scandium are within the above range. As an example of such a coprecipitation method, an aqueous solution containing a cerium salt (e.g., nitrate), a zirconium salt (e.g., nitrate), and a scandium salt (e.g., nitrate) is used to generate a coprecipitate in the presence of ammonia. The obtained coprecipitate is then centrifuged, washed, dried, and further calcined, and then pulverized using a pulverizer such as a ball mill to obtain the scandium-containing ceria-zirconia solid solution powder. The aqueous solution containing the cerium salt, zirconium salt, and scandium salt is prepared such that the content ratios of cerium, zirconium, and scandium in the obtained solid solution powder are within a predetermined range.

[0044] Furthermore, the aqueous solution containing the cerium salt, zirconium salt, and scandium salt may optionally contain a salt of at least one element selected from the group consisting of rare earth elements other than cerium and scandium, and alkaline earth elements, or a surfactant (e.g., a nonionic surfactant). By incorporating at least one element selected from the group consisting of rare earth elements other than cerium and scandium, and alkaline earth elements into the scandium-containing ceria-zirconia solid solution powder, a higher exhaust gas purification capacity tends to be exhibited when the oxygen storage material of the present invention is used as a co-catalyst for exhaust gas purification catalysts. Other rare earth elements besides cerium and scandium include yttrium (Y), lanthanum (La), praseodymium (Pr), neodymium (Nd), samarium (Sm), gadolinium (Gd), terbium (Tb), dysprosium (Dy), ytterbium (Yb), and lutetium (Lu). Among these, La, Nd, Pr, and Y are preferred, with La, Y, and Nd being more preferred, from the viewpoint of their tendency to stabilize the superlattice structure. Alkaline earth metal elements include magnesium (Mg), calcium (Ca), strontium (Sr), barium (Ba), and radium (Ra). Among these, Mg, Ca, and Ba are preferred, from the viewpoint of their tendency to stabilize the superlattice structure.

[0045] When the mixture further contains at least one element selected from the group consisting of rare earth elements other than cerium and scandium, and alkaline earth elements, the content of the element is preferably 0.5 to 20 at%, and more preferably 1 to 10 at%, relative to the total amount of cerium (Ce) and zirconium (Zr) in the scandium-containing ceria-zirconia solid solution powder. If the content of such elements falls below the lower limit, it tends to become difficult to stabilize the superlattice structure, while if it exceeds the upper limit, the oxygen storage capacity tends to decrease.

[0046] Next, in the second step, the scandium-containing ceria-zirconia solid solution powder is pressure-molded at a pressure of 30 to 350 MPa (preferably 40 to 300 MPa). If the pressure molding pressure falls below the lower limit, the contact between the secondary particles of the powder does not improve, so crystal growth during the reduction treatment is not sufficiently promoted, and the stability of the superlattice structure when exposed to high-temperature exhaust gas of about 1100°C for a long time decreases. On the other hand, if the pressure molding pressure exceeds the upper limit, crystal growth during the reduction treatment progresses too much, and the oxygen storage capacity (OSC) at a low temperature of 250°C tends to decrease. There are no particular limitations on the method of such pressure molding, and known pressure molding methods such as hydrostatic presses can be appropriately adopted.

[0047] Next, in the second step, the pressure-molded solid solution powder molded body is subjected to a reduction treatment by heating it under reducing conditions at a temperature of 1400 to 2000°C (preferably 1600 to 1900°C) for 0.5 to 24 hours (preferably 1 to 10 hours), and then subjected to an oxidation treatment to obtain the oxygen storage material powder of the present invention. If the temperature of the reduction treatment falls below the lower limit, crystal growth does not proceed sufficiently, and the stability of the superlattice structure decreases. On the other hand, if the temperature of the reduction treatment exceeds the upper limit, the balance between the energy required for the reduction treatment (e.g., electricity) and the improvement in performance becomes poor. Also, if the heating time during the reduction treatment falls below the lower limit, the superlattice structure tends not to form sufficiently, and on the other hand, if it exceeds the upper limit, the balance between the energy required for the reduction treatment (e.g., electricity) and the improvement in performance becomes poor.

[0048] The method for the reduction treatment is not particularly limited as long as it is a method that can heat-treat the solid solution powder under predetermined temperature conditions in a reducing atmosphere. For example, (i) a method in which the solid solution powder is placed in a vacuum heating furnace, vacuumed, and then a reducing gas is introduced into the furnace to create a reducing atmosphere, and the furnace is heated under predetermined temperature conditions to perform the reduction treatment; (ii) a method in which the solid solution powder is placed in a graphite furnace, vacuumed, and then heated under predetermined temperature conditions to create a reducing atmosphere in the furnace using reducing gases such as CO and HC generated from the furnace body and heating fuel, and the reduction treatment is performed; and (iii) a method in which the solid solution powder is placed in a crucible filled with activated carbon, heated under predetermined temperature conditions to create a reducing atmosphere in the crucible using reducing gases such as CO and HC generated from the activated carbon, and the reduction treatment is performed.

[0049] The reducing gas used to achieve such a reducing atmosphere is not particularly limited, and any reducing gas such as CO, HC, H2, or other hydrocarbon gases can be used as appropriate. Furthermore, among such reducing gases, it is more preferable to use one that does not contain carbon (C) in order to prevent the formation of by-products such as zirconium carbide (ZrC) when reducing treatment is performed at higher temperatures. When such a carbon (C)-free reducing gas is used, reduction treatment can be performed at higher temperatures close to the melting point of zirconium, etc., thereby significantly improving the structural stability of the crystalline phase.

[0050] Furthermore, in the second step, an oxidation treatment is further applied after the reduction treatment. By applying such an oxidation treatment, the oxygen lost during reduction is replenished in the resulting scandium-containing pyrochlore-type ceria-zirconia composite oxide, thereby improving its stability as an oxide. The method of such oxidation treatment is not particularly limited, and for example, a method of heat-treating the scandium-containing pyrochlore-type ceria-zirconia composite oxide in an oxidizing atmosphere (e.g., air) can be suitably employed. Furthermore, the heating temperature during such oxidation treatment is not particularly limited, but it is preferably around 300 to 800°C. Furthermore, the heating time during the oxidation treatment is not particularly limited, but it is preferably around 0.5 to 5 hours.

[0051] Furthermore, in the second step, it is preferable to further pulverize the scandium-containing pyrochlore-type ceria-zirconia composite oxide after the reduction treatment and / or the oxidation treatment. The method of such pulverization is not particularly limited, and for example, wet pulverization, dry pulverization, freeze pulverization, etc., can be suitably employed. [Examples]

[0052] The present invention will be described more specifically below based on examples and comparative examples, but the present invention is not limited to the following examples.

[0053] (Example 1) First, 226.1 g of an aqueous solution of cerium nitrate with a concentration equivalent to 28% by mass in terms of CeO2, 289.7 g of an aqueous solution of zirconium nitrate with a concentration equivalent to 18% by mass in terms of ZrO2, and an aqueous solution prepared by dissolving 2.4 g of scandium nitrate tetrahydrate in 100 ml of pure water were mixed. The resulting mixed solution was added to a solution prepared by diluting 161.2 g of 25% aqueous ammonia with 450 ml of pure water, and the mixture was stirred at 1000 rpm for 10 minutes using a homogenizer (manufactured by AS ONE Corporation) to produce a coprecipitation. The obtained coprecipitation was dried in air at 150°C for 7 hours using a degreasing furnace, and then calcined in air at 400°C for 5 hours to obtain a scandium-containing ceria-zirconia solid solution. Subsequently, the solid solution was pulverized using a pulverizer ("Wonder Blender" manufactured by AS ONE Corporation) to a particle size of 75 μm or less by sieving, thereby obtaining a scandium-containing ceria-zirconia solid solution powder in which the content ratio of cerium, zirconium, and scandium was 46:53:1 in atomic ratio ([Ce]:[Zr]:[Sc]). In this scandium-containing ceria-zirconia solid solution powder, the mole fraction of zirconium relative to the total number of moles of cerium and zirconium (Zr / (Ce+Zr)×100) was 53.5%, and the content ratio of scandium relative to the total amount of cerium and zirconium (Sc / (Ce+Zr)×100) was 1.0 at%.

[0054] Next, 20 g of this scandium-containing ceria-zirconia solid solution powder was packed into a polyethylene bag (capacity 0.05 L), the inside was degassed, and the opening of the bag was heated and sealed. Subsequently, a hydrostatic press (Nikkiso Co., Ltd. "CK4-22-60") was used to apply 3000 kgf / cm² to the bag. 2 A molded body of scandium-containing ceria-zirconia solid solution powder was obtained by cold isostatic pressing (CIP) for 1 minute at a pressure (molding pressure) of 294 MPa. The dimensions of the molded body were 20 mm in length, 20 mm in width, and an average thickness of 3 mm.

[0055] Next, the obtained molded body was placed in a small vacuum pressure sintering furnace (FVPS-R-150 manufactured by Fuji Denpa Kogyo Co., Ltd.), the atmosphere was replaced with an argon atmosphere, and the body was heated to 1000°C in 1 hour. Then, it was heated to 1700°C (reduction treatment temperature) in 4 hours and held there for 5 hours. After that, it was cooled to 1000°C in 4 hours, and then allowed to cool naturally to room temperature to obtain the reduced product.

[0056] The resulting reduced product was heated in air at 500°C for 5 hours to obtain a scandium-containing ceria-zirconia composite oxide. This scandium-containing ceria-zirconia composite oxide was pulverized using the aforementioned pulverizer to obtain a scandium-containing ceria-zirconia composite oxide powder in which the content ratio of cerium, zirconium, and scandium was 46:53:1 in atomic ratio ([Ce]:[Zr]:[Sc]). In this scandium-containing ceria-zirconia composite oxide powder, the mole fraction of zirconium relative to the total number of moles of cerium and zirconium (X=Zr / (Ce+Zr)×100) was X=53.5%, and the content ratio of scandium relative to the total amount of cerium and zirconium (Sc / (Ce+Zr)×100) was 1.0at%.

[0057] (Example 2) Except for changing the amount of the zirconium nitrate aqueous solution to 281.5 g and the amount of the scandium nitrate tetrahydrate to 6.1 g, the same procedure as in Example 1 was followed to obtain a scandium-containing ceria-zirconia composite oxide powder in which the content ratio of cerium, zirconium, and scandium was 46:51.5:2.5 in atomic ratio ([Ce]:[Zr]:[Sc]). In this scandium-containing ceria-zirconia composite oxide powder, the mole fraction of zirconium relative to the total number of moles of cerium and zirconium (X=Zr / (Ce+Zr)×100) is X=52.8%, and the content ratio of scandium relative to the total amount of cerium and zirconium (Sc / (Ce+Zr)×100) is 2.6at%.

[0058] (Comparative Example 1) Except for changing the amount of the cerium nitrate aqueous solution to 245.7 g and the amount of the zirconium nitrate aqueous solution to 273.3 g, and not adding the scandium nitrate tetrahydrate, the same procedure as in Example 1 was followed to obtain a ceria-zirconia composite oxide powder in which the content ratio of cerium, zirconium, and scandium was 50:50:0 in atomic ratio ([Ce]:[Zr]:[Sc]). In this ceria-zirconia composite oxide powder, the mole fraction of zirconium relative to the total number of moles of cerium and zirconium (X=Zr / (Ce+Zr)×100) is X=50%.

[0059] (Comparative Example 2) Except for changing the amount of the above-mentioned zirconium nitrate aqueous solution to 295.2 g and not adding the above-mentioned scandium nitrate tetrahydrate, the same procedure as in Example 1 was followed to obtain a ceria-zirconia composite oxide powder having an atomic ratio ([Ce]:[Zr]:[Sc]) of 46:54:0 for cerium, zirconium, and scandium. In this ceria-zirconia composite oxide powder, the mole fraction of zirconium relative to the total number of moles of cerium and zirconium (X=Zr / (Ce+Zr)×100) is X=54.0%.

[0060] (Comparative Example 3) Except for changing the amount of the zirconium nitrate aqueous solution to 267.9 g and the amount of the scandium nitrate tetrahydrate to 12.1 g, the same procedure as in Example 1 was followed to obtain a scandium-containing ceria-zirconia composite oxide powder having an atomic ratio ([Ce]:[Zr]:[Sc]) of 46:49:5 for cerium, zirconium, and scandium. In this scandium-containing ceria-zirconia composite oxide powder, the mole fraction of zirconium relative to the total number of moles of cerium and zirconium (X=Zr / (Ce+Zr)×100) is X=51.6%, and the content ratio of scandium relative to the total amount of cerium and zirconium (Sc / (Ce+Zr)×100) is 5.3 at%.

[0061] (Comparative Example 4) First, a ceria-zirconia composite oxide powder was obtained in the same manner as in Comparative Example 1, except that the amount of the cerium nitrate aqueous solution was changed to 233.4 g and the amount of the zirconium nitrate aqueous solution was changed to 287.0 g. The cerium to zirconium content ratio was 47.5:52.5 in atomic ratio ([Ce]:[Zr]).

[0062] Next, the ceria-zirconia composite oxide powder was immersed in an aqueous scandium nitrate solution to impregnate it with scandium nitrate so that the scandium content ratio (Sc / (Ce+Zr)×100) to the total amount of cerium and zirconium was 2.5 at%. After impregnation, the powder was heated in air at 900°C for 5 hours to obtain a scandium-containing ceria-zirconia composite oxide powder in which the content ratio of cerium, zirconium, and scandium was 46.3:51.2:2.4 in atomic ratio ([Ce]:[Zr]:[Sc]). In this scandium-containing ceria-zirconia composite oxide powder, the mole fraction of zirconium relative to the total number of moles of cerium and zirconium (X = Zr / (Ce + Zr) × 100) is X = 52.5%, and the ratio of scandium content relative to the total amount of cerium and zirconium (Sc / (Ce + Zr) × 100) is 2.5 at%.

[0063] (Comparative Example 5) Except for changing the amount of the above-mentioned zirconium nitrate aqueous solution to 218.7 g and using 15.0 g of titanium oxynitrate (TiO(NO3)2) aqueous solution (Ti content: 0.08 mol) instead of the scandium nitrate tetrahydrate, a titanium-containing ceria-zirconia composite oxide powder was obtained in the same manner as in Example 1, in which the content ratio of cerium, zirconium, and titanium was 50:40:10 in atomic ratio ([Ce]:[Zr]:[Ti]). In this titanium-containing ceria-zirconia composite oxide powder, the mole fraction of zirconium relative to the total number of moles of cerium and zirconium (X=Zr / (Ce+Zr)×100) is X=44.4%, and the content ratio of titanium relative to the total amount of cerium and zirconium (Ti / (Ce+Zr)×100) is 11.1 at%.

[0064] <High Temperature Durability Test> The composite oxide powders obtained in the examples and comparative examples were heated in air at 1100 °C for 5 hours.

[0065] <X-ray diffraction (XRD) measurement> The X-ray diffraction patterns of each composite oxide powder before and after the high-temperature durability test obtained in the examples and comparative examples were measured using an X-ray diffractometer ("RINT-Ultima" manufactured by Rigaku Corporation), with CuKα radiation as the X-ray source, under the conditions of a tube voltage of 40 kV, a tube current of 40 mA, and a scanning speed of 2θ = 10° / min.

[0066] For the obtained X-ray diffraction patterns, fitting by the least squares method was performed using the Rietveld analysis software "Jana2006" to refine the lattice constants. The obtained lattice constants are shown in Table 1.

[0067] Also, in the obtained X-ray diffraction patterns, the ratio [I(14 / 29) = I(14) / I(29)] of the peak intensity I(14) of the diffraction line near 2θ = 14.5° to the peak intensity I(29) of the diffraction line near 2θ = 29° was determined. The results are shown in Table 1.

[0068] <Catalyst preparation> After impregnating each composite oxide powder obtained in the examples and comparative examples after the high-temperature durability test with a solution of dinitrodiammineplatinum(II) acid, it was evaporated to dryness, and further, the obtained dry product was calcined at 300 °C for 3 hours to prepare a catalyst powder (Pt loading: 1 mass%) in which platinum (Pt) was supported on the composite oxide powder.

[0069] <Oxygen release amount measurement> 15 mg of each catalyst powder prepared as described above was placed in a thermogravimetric analyzer (Shimadzu Corporation, "TGA-50"). At a temperature of 250°C, reducing gas (H2 (5 vol%) + N2 (remainder)) and oxidizing gas (O2 (5 vol%) + N2 (remainder)) were alternately switched every 5 minutes at a gas flow rate of 100 ml / min, and the increase or decrease in the mass of the catalyst powder during this period was measured. The average value of the mass decrease of the catalyst powder during the second and third reduction gas flow was calculated and defined as the oxygen release amount (measured value). The ratio of the oxygen release amount (measured value) to the maximum oxygen release amount (theoretical value) based on the amount of cerium in the catalyst powder was calculated and defined as the oxygen storage material (OSC material) utilization rate. The results are shown in Table 1.

[0070] [Table 1]

[0071] From the I(14 / 29) values ​​shown in Table 1, it was confirmed that the oxygen storage materials obtained in Examples 1-2 and Comparative Examples 1-5 contain pyrochlore-type ceria-zirconia composite oxides.

[0072] Furthermore, based on the results shown in Table 1, the lattice constant and I(14 / 29) value were plotted against the molar fraction X of zirconium. These results are shown in Figures 1 and 2. The straight line in Figure 1 is represented by the following equation (1a): Lattice constant (Y1) = -4.15 × 10⁻⁶ -3 X + 10.733 (1a) This figure shows the relationship between the lattice constant (Y1) of the oxygen storage material powder and the mole fraction of zirconium (X = Zr / (Ce + Zr) × 100) relative to the total number of moles of cerium and zirconium. Furthermore, the straight line in Figure 2 represents the following equation (2a): I(14 / 29) value (Y2) = -1.00 × 10 -3 X+0.096 (2a) This shows the relationship between the I(14 / 29) value (Y2) of the oxygen storage material powder, represented by [formula], and the mole fraction of zirconium (X=Zr / (Ce+Zr)×100) relative to the total number of moles of cerium and zirconium.

[0073] Furthermore, for each composite oxide powder after high-temperature durability testing, the utilization rate of the catalyst powder as an OSC material was plotted against the molar fraction X of zirconium. These results are shown in Figure 3.

[0074] As shown in Figures 1 and 2, the scandium-containing ceria-zirconia solid solution powder obtained by coprecipitation was pressure-molded, the resulting molded body was subjected to reduction treatment, and then oxidation treatment. The resulting scandium-containing pyrochlore-type ceria-zirconia composite oxide powder (Examples 1 and 2 and Comparative Example 3) satisfied the conditions expressed by formula (1) for the lattice constant and the conditions expressed by formula (2) for the I(14 / 29) value, both before and after the high-temperature durability test. This indicates that the scandium atoms were sufficiently solid-dissolved in the lattice of the pyrochlore-type ceria-zirconia composite oxide.

[0075] On the other hand, as shown in Figures 1-2, the scandium-containing pyrochlore-type ceria-zirconia composite oxide powder (Comparative Example 4), obtained by impregnating pyrochlore-type ceria-zirconia composite oxide powder with scandium nitrate and then oxidizing it, satisfied the condition represented by formula (2) for the I(14 / 29) value both before and after the high-temperature durability test. However, the lattice constant did not satisfy the condition represented by formula (1) both before and after the high-temperature durability test. Therefore, it is considered that scandium is not sufficiently solid-dissolved in the lattice of the pyrochlore-type ceria-zirconia composite oxide.

[0076] Furthermore, as shown in Table 1, the titanium-containing ceria-zirconia composite oxide powder (Comparative Example 5), obtained by pressure molding titanium-containing ceria-zirconia solid solution powder, reducing the resulting molded body, and then oxidizing it, satisfied the condition expressed by formula (1) for the lattice constant both before and after the high-temperature durability test. However, it was found that the I(14 / 29) value before the high-temperature durability test did not satisfy the condition expressed by formula (2). This is because Ti 4+The ionic radius of Zr 4+ The ionic radius is very small compared to that of the Ce site, which is thought to be due to a large difference in ionic radii between the Ce site and the Zr site. In addition, in the titanium-containing ceria-zirconia composite oxide powder obtained in Comparative Example 5, the I(14 / 29) value after the high-temperature endurance test was smaller than the I(14 / 29) value obtained by the above formula (2a). This is not because titanium atoms dissolved in the pyrochlore-type ceria-zirconia composite oxide during the high-temperature endurance test, but rather because the phase separation of ceria was promoted by the titanium atoms that did not dissolve, and the stability of the pyrochlore-type ceria-zirconia composite oxide was reduced.

[0077] As shown in Table 1 and Figure 3, after the high-temperature endurance test, the molar fraction of zirconium (Zr / (Ce+Zr)×100) is within a predetermined range, and scandium is sufficiently solid-dissolved in the pyrochlore-type ceria-zirconia composite oxide lattice (i.e., both conditions represented by formulas (1) and (2) above are satisfied). The catalyst powder (Examples 1-2) contains an oxygen storage material powder made of a composite oxide powder that does not contain scandium, while the catalyst powder (Comparative Examples 1-2) contains an oxygen storage material powder made of a pyrochlore-type ceria-zirconia composite oxide that does not contain scandium. - A catalyst powder containing an oxygen storage material powder made of a composite oxide powder in which zirconium is sufficiently solid-dissolved in the lattice of the zirconia composite oxide (i.e., both conditions represented by formulas (1) and (2) above are satisfied), but the molar fraction of zirconium (Zr / (Ce+Zr)×100) is smaller than a predetermined range (Comparative Example 3), a catalyst powder containing an oxygen storage material powder made of a composite oxide powder in which scandium is not sufficiently solid-dissolved in the lattice of the pyrochlore-type ceria-zirconia composite oxide (i.e., the conditions represented by formula (1) above are not satisfied after the high-temperature endurance test) (Comparative Example 4), and Zr 4+It was found that compared to a catalyst powder (Comparative Example 5) containing an oxygen storage material powder made of a composite oxide powder in which titanium atoms with a much smaller ionic radius than those of other materials are solid-dissolved in the lattice of the pyrochlore-type ceria-zirconia composite oxide, and the phase separation of ceria is promoted by the titanium atoms that are not solid-dissolved, resulting in a reduced stability of the pyrochlore-type ceria-zirconia composite oxide, the utilization rate of the oxygen storage material (OSC material) was higher. [Industrial applicability]

[0078] As described above, according to the present invention, excellent oxygen storage capacity (OSC) can be exhibited not only during the initial period of use, but also after prolonged exposure to exhaust gas at high temperatures of approximately 1100°C, and even at low temperatures of approximately 250°C, making it possible to obtain an oxygen storage material with high utilization efficiency. Therefore, because the oxygen storage material of the present invention possesses both excellent oxygen storage capacity (OSC) at low temperatures and high-temperature durability, it is useful as a carrier, co-catalyst, catalyst atmosphere adjuster, etc., for exhaust gas purification catalysts.

Claims

1. An oxygen storage material containing a pyrochlore-type ceria-zirconia composite oxide with added scandium (Sc), The mole fraction of zirconium (X = Zr / (Ce + Zr) × 100) relative to the total number of moles of cerium (Ce) and zirconium (Zr) is X = 51.7 to 53.8%. The lattice constant, obtained from the X-ray diffraction patterns using CuKα before heating at 1100°C in air and after heating for 5 hours, is given by the following equation (1): Lattice constant ≤ -4.15 × 10⁻⁶ －３ X+10.733 (1) (In the above formula, X represents the mole fraction of zirconium.) It satisfies the conditions expressed as follows: The intensity ratio [I(14 / 29) value] of the diffraction line around 2θ = 14.5° and the diffraction line around 2θ = 29°, obtained from the X-ray diffraction pattern obtained by X-ray diffraction measurements using CuKα in air before heating at 1100°C and after heating for 5 hours, is given by the following formula (2): I(14 / 29) value ≤ -1.00 × 10 －３ X+0.096 (2) (In the above formula, X represents the mole fraction of zirconium.) An oxygen storage material characterized by satisfying the conditions represented by the following:

2. The oxygen storage material according to claim 1, characterized in that the content ratio of scandium (Sc) to the total amount of cerium (Ce) and zirconium (Zr) (Sc / (Ce+Zr)×100) is 0.8 to 5 at%.

3. The oxygen storage material according to claim 1, characterized in that the mole fraction X of the zirconium is X = 52.6 to 53.8%.

4. A method for producing an oxygen storage material containing a pyrochlore-type ceria-zirconia composite oxide to which scandium (Sc) has been added, A step to prepare a ceria-zirconia solid solution powder containing scandium, wherein the mole fraction of zirconium (Zr / (Ce+Zr)×100) relative to the total number of moles of cerium (Ce) and zirconium (Zr) is 51.7-53.8%. The process involves pressurizing the ceria-zirconia solid solution powder containing scandium at a pressure of 30 to 350 MPa, then reducing it at a temperature of 1400 to 2000°C, and further oxidizing it to obtain an oxygen storage material containing the scandium-added pyrochlore-type ceria-zirconia composite oxide described in claim 1. A method for producing an oxygen storage material, characterized by containing [the specified ingredient].

5. The method for producing an oxygen storage material according to claim 4, characterized in that the ratio of scandium (Sc) to the total amount of cerium (Ce) and zirconium (Zr) in the ceria-zirconia solid solution powder containing scandium (Sc / (Ce+Zr)×100) is 0.8 to 5 at%.

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