Oxygen absorbing / releasing material and its manufacturing method

Abstract

To provide an oxygen absorbing / releasing material capable of both improving oxygen absorbing / releasing ability at a low temperature and securing heat resistance.SOLUTION: The present invention relates to an oxygen absorbing / discharging material comprising a ceria-zirconia composite oxide containing ceria (CeO2) and zirconia (ZrO2). The ceria-zirconia composite oxide contains praseodymium (Pr) or neodymium (Nd) as an additive element, has at least one of an ordered phase of a κ phase and a pyrochlore phase in at least part thereof, has 40% to 100% primary particles having a particle diameter of 0.4 μm to 1.5 μm as a particle number standard, and has I(14 / 29) value of 0.015 or more and I(28 / 29) value of 0.08 or less as determined from an X-ray diffraction pattern using CuKα obtained by X-ray diffraction measurement after heating at a temperature of 1100°C in an atmosphere for 5 hours. The present invention also relates to a method for producing the same.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to an oxygen absorbing / releasing material and a method for producing the same, and more particularly to an oxygen absorbing / releasing material for an exhaust gas purification catalyst and a method for producing the same. [Background technology]

[0002] Exhaust gases emitted from internal combustion engines, such as automobiles, contain harmful gases such as carbon monoxide (CO), nitrogen oxides (NOx), and unburned hydrocarbons (HC). Exhaust gas purification catalysts (so-called three-way catalysts) that decompose such harmful gases contain, in addition to precious metals that purify exhaust gases, oxygen storage materials (also called oxygen storage materials or OSC materials) that have the ability to absorb and release oxygen. Oxygen storage materials absorb and release oxygen, adjusting the air-fuel ratio (A / F), thereby suppressing the decline in purification efficiency that accompanies fluctuations in exhaust gas composition.

[0003] Ceria-zirconia composite oxides are widely used as oxygen absorbing / releasing materials, and in particular, ceria-zirconia composite oxides containing an ordered pyrochlore phase or kappa phase (κ phase) are preferred because of their high oxygen absorbing / releasing capacity.

[0004] As a ceria-zirconia composite oxide containing such a pyrochlore phase or κ phase, Patent Document 1 discloses a ceria-zirconia composite oxide in which primary particles with a particle size of 1.5 to 4.5 μm account for 50% or more of all primary particles of the composite oxide on a particle number basis. In Patent Document 1, heat resistance is ensured by increasing the proportion of primary particles with a relatively large particle size and decreasing the proportion of fine particles with a particle size of less than 1.5 μm.

[0005] Furthermore, Patent Document 2 discloses a ceria-zirconia composite oxide having a secondary particle diameter (D50) of 3 to 7 μm, and discloses that the oxygen absorption / release rate can be improved by controlling the secondary particle diameter.

[0006] Furthermore, Patent Document 3 discloses a ceria-zirconia composite oxide containing, in addition to ceria and zirconia, at least one additive element selected from the group consisting of praseodymium, lanthanum, and yttrium in a predetermined ratio, and discloses that the inclusion of the additive element can promote the reduction of ceria and enhance the oxygen absorption / release function.

[0007] In recent years, in response to stricter CO2 regulations, fuel efficiency has improved, and vehicle electrification has progressed, leading to a decrease in exhaust gas temperature and a demand for oxygen-absorbing / releasing materials that exhibit oxygen-absorbing / releasing functions even at low temperatures. Oxygen-absorbing / releasing materials having a pyrochlore phase or a κ phase, as disclosed in Patent Documents 1 to 3, have a high oxygen absorption / releasing capacity but are less likely to exhibit oxygen-absorbing / releasing functions at low temperatures. It is generally known that reducing the particle size of ceria-zirconia composite oxides to increase their specific surface area improves their oxygen-absorbing / releasing capacity at low temperatures, but reducing the particle size also leads to a deterioration in heat resistance. [Prior art documents] [Patent documents]

[0008] [Patent Document 1] Japanese Patent Application Laid-Open No. 2015-34113 [Patent Document 2] Japanese Patent Application Publication No. 2018-38999 [Patent Document 3] Japanese Patent Application Laid-Open No. 2015-818 Summary of the Invention [Problem to be solved by the invention]

[0009] As described above, in conventional oxygen-absorbing / releasing materials, when an attempt is made to improve the oxygen-absorbing / releasing capacity by reducing the particle size of the ceria-zirconia composite oxide, the heat resistance deteriorates, making it difficult to simultaneously improve the oxygen-absorbing / releasing capacity at low temperatures and ensure heat resistance. Therefore, an object of the present invention is to provide an oxygen-absorbing / releasing material that simultaneously improves the oxygen-absorbing / releasing capacity at low temperatures and ensures heat resistance. [Means for solving the problem]

[0010] The inventors have investigated various means for solving the above problems and have found that it is possible to improve the oxygen absorption / release capacity at low temperatures by reducing the particle size, and further ensure heat resistance by increasing the uniformity of the elements and stabilizing the crystal structure, thereby making it possible to achieve both improved oxygen absorption / release capacity at low temperatures and ensure heat resistance in an oxygen absorbing / releasing material, and have completed the present invention.

[0011] That is, the gist of the present invention is as follows. (1) An oxygen absorbing / releasing material comprising a ceria-zirconia composite oxide containing ceria (CeO2) and zirconia (ZrO2), wherein the ceria-zirconia composite oxide is Contains praseodymium (Pr) or neodymium (Nd) as an additive element, At least one of an ordered phase of a κ phase and a pyrochlore phase is present in at least a portion of the alloy, primary particles having a particle diameter of 0.4 μm to 1.5 μm account for 40% to 100% by particle number of all primary particles of the ceria-zirconia-based composite oxide, The intensity ratio {I(14 / 29) value} of the diffraction line at 2θ=14.5° to the diffraction line at 2θ=29°, determined from the X-ray diffraction pattern using CuKα obtained by X-ray diffraction measurement after heating in air at a temperature of 1100°C for 5 hours, is 0.015 or more, and the intensity ratio {I(28 / 29) value} of the diffraction line at 2θ=28.5° to the diffraction line at 2θ=29°, determined from the X-ray diffraction pattern using CuKα, is 0.08 or less. Oxygen absorbing and releasing material. (2) The oxygen absorbing / releasing material according to (1) above, which is obtained by using a fluorite-type ceria-zirconia solid solution powder containing praseodymium (Pr) or neodymium (Nd) as an additive element, wherein the solid solution powder is heated in air at a temperature of 1200°C for 5 hours, and the Ce 0.5 Zr 0.5 The ratio y (%) of O2 crystals to the whole crystals and the praseodymium oxide (PrO 11 ) or the neodymium oxide (Nd2O3) content x (wt%) is expressed by the following formula (1): y≧-0.7079x 2+5.3331x+9.4883 (1) (wherein x>0) An oxygen absorbing and releasing material that satisfies the above requirements. (3) The oxygen-absorbing material according to (1) or (2), wherein the additional element is Pr. (4) A method for producing the oxygen absorbing / releasing material according to (1), Fluorite-type ceria-zirconia solid solution powder containing praseodymium (Pr) or neodymium (Nd) as an additive element, and after heating in air at a temperature of 1200°C for 5 hours, 0.5 Zr 0.5 The ratio y (%) of O2 crystals to the whole crystals and the praseodymium oxide (PrO 11 ) or the neodymium oxide (Nd2O3) content x (wt%) is expressed by the following formula (1): y≧-0.7079x 2 +5.3331x+9.4883 (1) (wherein x>0) preparing the solid solution powder that satisfies the above; a step of press-molding the ceria-zirconia solid solution powder; a step of subjecting the pressure-molded ceria-zirconia solid solution powder compact to a reduction treatment by heat treating the compact at 1400°C or higher but lower than 1700°C; A method for producing an oxygen-absorbing / releasing material, comprising: (5) The method for producing an oxygen absorbing / releasing material according to (4) above, wherein the additive element is Pr. [Effects of the Invention]

[0012] According to the present invention, it is possible to provide an oxygen absorbing / releasing material that achieves both improved oxygen absorbing / releasing capacity at low temperatures and ensures heat resistance. [Brief explanation of the drawings]

[0013] [Figure 1]1 is a graph showing the relationship between the proportion of primary particles with a particle size of 0.4 μm to 1.5 μm, the element dispersion degree after heat treatment (proportion of CeZrO crystals), and the I(14 / 29) value for the ceria-zirconia-praseodymium oxide composite oxides of Examples 1 to 7 and Comparative Examples 1 to 6. [Figure 2] 1 is a graph showing the relationship between the proportion of primary particles with a particle size of 0.4 μm to 1.5 μm, the element dispersion degree after heat treatment (proportion of CeZrO crystals), and the oxygen absorption / release amount (400°C) for the ceria-zirconia-praseodymium oxide composite oxides of Examples 1 to 7 and Comparative Examples 1 to 6. [Figure 3] 1 is a graph showing the relationship between the content of praseodymium oxide (Pr6O11) in a solid solution powder and the element dispersion degree (proportion of Ce0.5Zr0.5O2 crystals) after heat treatment in an example. DETAILED DESCRIPTION OF THE INVENTION

[0014] Preferred embodiments of the present invention will now be described in detail.

[0015] The present invention relates to an oxygen absorbing / releasing material comprising a ceria-zirconia composite oxide. That is, the ceria-zirconia composite oxide of the present invention is used as an oxygen absorbing / releasing material. The ceria-zirconia composite oxide of the present invention contains ceria (CeO2) and zirconia (ZrO2), and contains praseodymium (Pr) or neodymium (Nd) as an additive element. Therefore, the ceria-zirconia composite oxide of the present invention is a composite oxide of ceria, zirconia, and praseodymium oxide (PrO 11 The composite oxide is preferably a composite oxide of ceria, zirconia, and neodymium oxide (Nd2O3), or a composite oxide of ceria, zirconia, and neodymium oxide (Nd2O3). Pr or Nd contributes to stabilizing the crystal structure of the composite oxide after durability testing, and it is believed that adding Pr or Nd can provide the composite oxide with high oxygen absorption / release capacity and high heat resistance. In the ceria-zirconia composite oxide of the present invention, the additive element is preferably Pr.

[0016] In the ceria-zirconia composite oxide of the present invention, the molar ratio of (cerium + praseodymium or neodymium) to zirconium ([Ce + Pr or Nd]:[Zr]) is preferably in the range of 1:1 to 1:1.4, more preferably in the range of 1:1.1 to 1:1.4, or may be in the range of 1:1 to 1:1.2. When the molar ratio is within this range, the oxygen absorption / release capacity after endurance testing can be sufficiently high.

[0017] In the ceria-zirconia-based composite oxide of the present invention, the content of praseodymium (Pr) or neodymium (Nd) contained as an additive element is preferably 0.5 mol % to 5 mol %, and more preferably 0.5 mol % to 2.5 mol %, based on the total amount of all cations, from the viewpoint of achieving both heat resistance and OSC performance.

[0018] In the ceria-zirconia-based composite oxide of the present invention, the ceria content is preferably 35 to 60% by weight, and more preferably 47 to 53% by weight, on a weight basis.

[0019] In the ceria-zirconia-based composite oxide of the present invention, the zirconia content is preferably 40% by weight to 50% by weight, and more preferably 41% by weight to 47% by weight, on a weight basis.

[0020] In the ceria-zirconia composite oxide of the present invention, the praseodymium oxide content is preferably 0.1 to 6% by weight, more preferably 0.5 to 3% by weight.In the ceria-zirconia composite oxide of the present invention, the neodymium oxide content is preferably 0.1 to 6% by weight, more preferably 0.5 to 3% by weight.

[0021] The ceria-zirconia composite oxide of the present invention may contain at least one additional element selected from the group consisting of rare earth elements other than cerium (Ce), praseodymium (Pr), and neodymium (Nd) and alkaline earth elements. By incorporating such an additional element, the ceria-zirconia composite oxide of the present invention may be able to achieve higher exhaust gas purification performance when used as an oxygen absorbing / releasing material for an exhaust gas purification catalyst. Examples of such rare earth elements include scandium (Sc), yttrium (Y), lanthanum (La), samarium (Sm), gadolinium (Gd), terbium (Tb), dysprosium (Dy), ytterbium (Yb), and lutetium (Lu). Among these, Sc, Y, and La are preferred, with Y and La being more preferred, because they tend to have a stronger interaction with and affinity for the precious metal when the precious metal is supported on the oxide. Examples of alkaline earth metal elements include magnesium (Mg), calcium (Ca), strontium (Sr), barium (Ba), and radium (Ra), and among these, Mg, Ca, and Ba are preferred because they tend to have a stronger interaction with the precious metal and a greater affinity when the precious metal is supported. Such rare earth elements and alkaline earth metal elements with low electronegativity have a strong interaction with the precious metal, so they bond with the precious metal via oxygen in an oxidizing atmosphere, suppressing the evaporation and sintering of the precious metal and sufficiently suppressing the deterioration of the precious metal, which is the active site during exhaust gas purification.

[0022] When the ceria-zirconia composite oxide contains an additional element, the content of the additional element is preferably 1 to 20% by weight, more preferably 3 to 7% by weight, relative to the ceria-zirconia composite oxide. When the content of the additional element is within this range, when a precious metal is supported on the obtained composite oxide, the interaction with the precious metal can be sufficiently improved, and the composite oxide has sufficient oxygen absorption / release capacity.

[0023] The ceria-zirconia composite oxide of the present invention at least partially has an ordered phase of at least one of a κ phase and a pyrochlore phase. The κ phase and the pyrochlore phase are crystalline phases having an ordered structure formed by cerium ions and zirconium ions. The cerium ions and zirconium ions may be partially substituted with ions of an additional element such as praseodymium or neodymium. In one embodiment, the cerium ions are partially substituted with praseodymium ions or neodymium ions. The formation of such an ordered phase in the ceria-zirconia composite oxide improves heat resistance at high temperatures, and allows the oxide to exhibit sufficiently high oxygen absorption / release capacity even after exposure to high temperatures.

[0024] The pyrochlore phase (CeZrO) has oxygen vacancy sites, and when oxygen atoms enter these sites, the pyrochlore phase changes to the κ phase (CeZrO). On the other hand, the κ phase can change to the pyrochlore phase by releasing oxygen atoms. The oxygen absorption / release capacity of the ceria-zirconia composite oxide is due to the mutual phase change between the pyrochlore phase and the κ phase, which absorbs and releases oxygen. In the ceria-zirconia composite oxide of the present invention, the content ratio of the ordered phase to all crystalline phases, as determined by the peak intensity ratio of an X-ray diffraction pattern described below, is preferably 50% to 100%, more preferably 80% to 100%. When the content ratio of the ordered phase is within this range, the composite oxide exhibits high oxygen absorption / release capacity and high heat resistance.

[0025] The ceria-zirconia composite oxide of the present invention is heated in air at 1100°C for 5 hours, and the X-ray diffraction pattern obtained using CuKα has an intensity ratio (I(14 / 29) value) of 0.015 or more between the diffraction line at 2θ=14.5° and the diffraction line at 2θ=29° (I(28 / 29) value) of 0.08 or less between the diffraction line at 2θ=28.5° and the diffraction line at 2θ=29° (I(28 / 29) value) of 0.08 or less. The X-ray diffraction measurement can be performed, for example, using a measuring device manufactured by Rigaku Corporation (product name: RINT2100) with CuKα radiation under conditions of 40 kV, 30 mA, and 2θ=2° / min.

[0026] Here, the diffraction line at 2θ=14.5° is a diffraction line attributable to the (111) plane of the ordered phase (κ phase), and the diffraction line at 2θ=29° is a diffraction line attributable to the (222) plane of the ordered phase overlapping with a diffraction line attributable to the (111) plane of the cubic phase of the ceria-zirconia solid solution. Therefore, the intensity ratio of these two diffraction lines, I(14 / 29), is defined as an index of the maintenance rate (abundance rate) of the ordered phase. Note that when calculating the diffraction line intensity, the average diffraction line intensity from 2θ=10° to 12° is subtracted from each diffraction line intensity value as a background value. The fully ordered phase includes the κ phase (CeZrO), which is completely filled with oxygen, and the pyrochlore phase (CeZrO), which is completely deoxygenated. The I(14 / 29) value calculated from the respective PDF cards (PDF2:01-070-4048 for the κ phase and PDF2:01-075-2694 for the pyrochlore phase) is 0.04 and 0.05, respectively. The ordered phase can be identified by the presence of peaks at 2θ angles of 14.5°, 28°, 37°, 44.5°, and 51° in the X-ray diffraction pattern obtained using CuKα. Note that the term "peak" here refers to a peak with a height from the baseline to the peak top of 30 cps or more.

[0027] In the ceria-zirconia composite oxide of the present invention, the I(14 / 29) value is 0.015 or more, more preferably 0.017 or more, and particularly preferably 0.020 or more. When the I(14 / 29) value is 0.015 or more, the composite oxide has high heat resistance.

[0028] The diffraction line at 2θ=28.5° is a diffraction line attributable to the (111) plane of CeO2 alone, and the I(28 / 29) value, which is the intensity ratio of the diffraction line at 2θ=28.5° to the diffraction line at 2θ=29°, is defined as an index indicating the degree to which CeO2 is phase-separated from the composite oxide.

[0029] In the ceria-zirconia composite oxide of the present invention, the I(28 / 29) value is 0.08 or less, more preferably 0.05 or less, and particularly preferably 0.01 or less. When the I(28 / 29) value is 0.08 or less, phase separation of ceria is sufficiently suppressed, and the oxygen absorption / release capacity after high-temperature durability is sufficiently high. The lower limit of the I(28 / 29) value is not particularly limited, but a smaller value is preferable.

[0030] In the ceria-zirconia composite oxide of the present invention, primary particles having a particle size of 0.4 μm to 1.5 μm account for 40% to 100% of the total primary particles of the ceria-zirconia composite oxide, based on the number of particles, preferably 40% to 99%, and more preferably 40% to 97.5%. The lower limit of the proportion of primary particles having a particle size of 0.4 μm to 1.5 μm is preferably 45% or more, more preferably 60% or more, and particularly preferably 80% or more. When the proportion of primary particles having a particle size of 0.4 μm to 1.5 μm is 40% or more, the oxygen absorption / release capacity at low temperatures (e.g., 300°C to 500°C) can be improved. Conventional ceria-zirconia composite oxides have a problem in that reducing the particle size to improve their oxygen absorption / release capacity at low temperatures reduces their heat resistance. However, the ceria-zirconia composite oxide of the present invention ensures heat resistance by increasing the uniformity of elements and stabilizing the crystal structure, thereby achieving both improved oxygen absorption / release capacity at low temperatures and ensuring heat resistance. The particle size distribution of the primary particles of the ceria-zirconia composite oxide was determined by scanning electron microscope (SEM) observation. The cross section of the composite oxide particle was observed at two or more random locations, and the crystal grain size within each observation field was calculated numerically. The crystal grain size referred to here refers to the diameter of the smallest circumscribing circle if the cross section is not circular.

[0031] The ceria-zirconia composite oxide of the present invention has a sufficiently narrow particle size distribution, and the composite oxide has high oxygen absorption / release capacity and high heat resistance. In the ceria-zirconia composite oxide of the present invention, the proportion of primary particles with a particle diameter of less than 0.4 μm relative to all primary particles is preferably 3% or less, and more preferably none, based on the number of particles. Furthermore, the proportion of primary particles with a particle diameter of more than 1.5 μm relative to all primary particles is preferably 10% or less, and more preferably none, based on the number of particles.

[0032] As will be explained below regarding the method for producing the ceria-zirconia composite oxide of the present invention, the ceria-zirconia composite oxide of the present invention is preferably obtained by using a fluorite-type ceria-zirconia solid solution powder having a specific element dispersion. This ceria-zirconia solid solution powder contains Pr or Nd as an additive element, i.e., ceria-zirconia-praseodymium oxide (PrO 11 ) or ceria-zirconia-neodymium oxide (Nd2O3) solid solution powder.

[0033] In a preferred embodiment, the fluorite-type ceria-zirconia solid solution powder (containing Pr or Nd as an additive element) used for producing the ceria-zirconia composite oxide of the present invention is heated in air at a temperature of 1200° C. for 5 hours, and then the Ce 0.5 Zr 0.5 The ratio y (%) of O2 crystals to the whole crystals and the praseodymium oxide (PrO 11 ) or the neodymium oxide (Nd2O3) content x (wt%) is expressed by the following formula (1): y≧-0.7079x 2 +5.3331x+9.4883 (1) (wherein x>0) It satisfies the following.

[0034] As the fluorite-type ceria-zirconia solid solution powder used to obtain the ceria-zirconia composite oxide of the present invention, the one produced by the method of the present invention described below can be used.

[0035] The present invention also includes a method for producing the oxygen absorbing / releasing material (ceria-zirconia composite oxide). The method for producing the ceria-zirconia composite oxide of the present invention includes the steps of: preparing a fluorite-type ceria-zirconia solid solution powder containing praseodymium (Pr) or neodymium (Nd) as an additive element (step 1); pressure-molding the ceria-zirconia solid solution powder (step 2); and reducing the molded ceria-zirconia solid solution powder by heat-treating the molded compact at 1400°C or higher but lower than 1700°C (step 3).

[0036] In step 1, a fluorite-type ceria-zirconia solid solution powder containing Pr or Nd as an additive element is prepared. In the present invention, by using a solid solution powder with high elemental uniformity, the elements are uniformly arranged, the crystal structure is stabilized, and a ceria-zirconia composite oxide with high heat resistance can be obtained. Note that the additive element of the ceria-zirconia composite oxide of the present invention and the fluorite-type ceria-zirconia solid solution powder is the same. When the additive element of the ceria-zirconia composite oxide is Pr, the additive element of the fluorite-type ceria-zirconia solid solution powder is also Pr, and when the additive element of the ceria-zirconia composite oxide is Nd, the additive element of the fluorite-type ceria-zirconia solid solution powder is also Nd.

[0037] In the present invention, Ce and Zr are 1:1. 0.5 Zr 0.5 The ratio of O2 crystals to the entire crystal is defined as the "element dispersion degree," and this element dispersion degree is used as an index of element uniformity. Here, the ceria-zirconia solid solution powder of the present invention contains praseodymium (Pr) or neodymium (Nd) as an added element, and since praseodymium ions or neodymium ions replace some of the cerium ions in the crystal, "Ce where Ce and Zr are 1:1" is used. 0.5 Zr 0.5 O2 crystal" is, precisely, (Ce + Pr or Nd) and Zr 1:1, (Ce + Pr or Nd) 0.5 Zr 0.5 It means O2 crystals.

[0038] When fluorite-type ceria-zirconia solid solution powder is heat-treated at high temperatures (for example, 1100°C or higher), Ce and Zr are mixed in a 1:1 ratio. 0.5 Zr 0.5The O2 crystal structure undergoes phase separation into a crystalline structure with a Ce-rich or Zr-rich composition. In ceria-zirconia solid solution powder, if there are unstable areas where the elements are not uniformly arranged, such as when the added elements praseodymium or neodymium are segregated in the crystal or when there is unevenness in the arrangement of cerium and zirconium, it is expected that the rate of phase separation will increase. Therefore, the Ce that remains without phase separation after heating fluorite-type ceria-zirconia solid solution powder in air at a temperature of 1200°C for 5 hours is 0.5 Zr 0.5 When the ratio of O2 crystals to the entire crystal is high, the elements are arranged more uniformly, and the resulting ceria-zirconia composite oxide is less likely to change into a fluorite-type crystal structure, which is thought to suppress deterioration.

[0039] The element dispersion after heat treatment at 1200°C was measured by X-ray diffraction measurement using CuKα after heat treatment of the fluorite-type ceria-zirconia solid solution powder under the above conditions, and the element dispersion was measured by X-ray diffraction measurement using CuKα after heat treatment of the fluorite-type ceria-zirconia solid solution powder under the above conditions. 0.6 Zr 0.4 O2 crystal), 29.3°(Ce 0.5 Zr 0.5 O2 crystal) and 29.8° (Ce 0.2 Zr 0.8 The area of ​​the peak detected in the O2 crystal is calculated, and the ratio of the peak area at 29.3° to the total peak area is calculated.

[0040] The fluorite-type ceria-zirconia solid solution powder (containing Pr or Nd as an additive element) used in the production of the ceria-zirconia composite oxide of the present invention is heated in air at a temperature of 1200°C for 5 hours, and then the Ce 0.5 Zr 0.5 The ratio y (%) of O2 crystals to the whole crystals and the praseodymium oxide (PrO 11 ) or the neodymium oxide (Nd2O3) content x (wt%) is expressed by the following formula (1): y≧-0.7079x 2 +5.3331x+9.4883 (1) (wherein x>0) It satisfies the following.

[0041] The ceria-zirconia solid solution powder satisfying the above formula (1) has a high degree of element dispersion even after heat treatment at 1200 °C, and the elements are arranged more uniformly. Therefore, when this solid solution powder is used, a ceria-zirconia composite oxide with a stabilized crystal structure and high heat resistance can be obtained.

[0042] In the above formula (1), x > 0, and from the viewpoint of obtaining a high oxygen absorption and release ability, preferably x ≥ 0.6. From the viewpoint of achieving both heat resistance and OSC performance, preferably x ≤ 6, more preferably x ≤ 3. In a preferred embodiment, 0 < x ≤ 6, preferably 0.6 ≤ x ≤ 3.

[0043] The degree of element dispersion (y in formula (1)) after heat treatment is preferably 19% or more, more preferably 22% or more, and particularly preferably 25% or more. When the degree of element dispersion after heat treatment is 19% or more, the obtained ceria-zirconia composite oxide has high heat resistance.

[0044] In one embodiment, when the additive element is Pr, the degree of element dispersion after heat treatment is preferably 19% or more, more preferably 22% or more, and particularly preferably 25% or more. In one embodiment, in the range where the content x (wt%) of praseodymium oxide (Pr6O 11 ) is 0 < x ≤ 1, the degree of element dispersion after heat treatment is preferably 14% or more, more preferably 15% or more. In another embodiment, in the range where the content x (wt%) of praseodymium oxide (Pr6O 11 ) is 1 ≤ x ≤ 2, the degree of element dispersion after heat treatment is preferably 17% or more, more preferably 18% or more. In another embodiment, in the range where the content x (wt%) of praseodymium oxide (Pr6O 11 ) is 2 ≤ x ≤ 3, the degree of element dispersion after heat treatment is preferably 19% or more, more preferably 20% or more. In these embodiments, x and y satisfy formula (1).

[0045] In one embodiment, when the additive element is Nd, the element dispersion degree after heat treatment is preferably 21% or more, more preferably 24% or more, and particularly preferably 27% or more. In one embodiment, in the range where the content x (wt%) of neodymium oxide (Nd2O3) satisfies 0 < x ≦ 1, the element dispersion degree after heat treatment is preferably 16% or more, more preferably 17% or more. In another embodiment, in the range where the content x (wt%) of neodymium oxide (Nd2O3) satisfies 1 ≦ x ≦ 2, the element dispersion degree after heat treatment is preferably 19% or more, more preferably 20% or more. In another embodiment, in the range where the content x (wt%) of neodymium oxide (Nd2O3) satisfies 2 ≦ x ≦ 3, the element dispersion degree after heat treatment is preferably 21% or more, more preferably 22% or more. In these embodiments, x and y satisfy formula (1).

[0046] The ceria-zirconia-based solid solution powder can be prepared, for example, using a so-called coprecipitation method. In the coprecipitation method, for example, an aqueous solution containing a cerium salt (e.g., nitrate), a zirconium salt (e.g., nitrate), and a praseodymium salt (e.g., nitrate) or a neodymium salt (e.g., nitrate) is used to obtain a mixed solution of raw material compounds. This mixed solution of raw material compounds is mixed with an aqueous solution of an alkaline compound and stirred to form a coprecipitate. Further, for example, the obtained coprecipitate is filtered, washed, dried, and then fired, and in some cases, pulverized using a pulverizer such as a ball mill to obtain the ceria-zirconia-based solid solution powder. To the mixed solution of raw material compounds, salts of at least one additional element selected from the group consisting of rare earth elements and alkaline earth elements, a surfactant (e.g., nonionic surfactant), etc. may be added as necessary.

[0047] In the production method of the present invention, in the coprecipitation method, by controlling the stirring conditions when stirring the mixed solution of the mixed solution of raw material compounds and the aqueous solution of the alkaline compound (hereinafter also referred to as the reaction solution), and adjusting the amount of the alkaline compound used, the element dispersion degree can be made within the target range.

[0048] The alkaline compound is not particularly limited, and examples thereof include ammonia water, ammonium carbonate, sodium hydroxide, potassium hydroxide, and sodium carbonate. Among these, ammonia water and ammonium carbonate are preferred, and ammonia water is more preferred, from the viewpoint that they can be removed by volatilization when the precipitate is calcined or the composite oxide is calcined.

[0049] The amount of alkaline compound used in the coprecipitation method is usually more than 1 equivalent, preferably 1.5 equivalents or more, based on the total amount (mol) of cerium salt, zirconium salt, and praseodymium salt or neodymium salt. By using such an excess amount of ammonia, the dispersibility of elements is further improved, and a solid solution powder with high element dispersion can be obtained.

[0050] In the coprecipitation method, the mixed solution of the raw material compounds and the aqueous solution of the alkaline compound can be mixed, for example, by adding or dropping the mixed solution of the raw material compounds into the aqueous solution of the alkaline compound, or by adding or dropping the aqueous solution of the alkaline compound into the mixed solution of the raw material compounds.

[0051] The mixed solution (reaction solution) of the mixed solution of the raw material compounds and the aqueous solution of the alkaline compound may be stirred so as to obtain a ceria-zirconia solid solution powder having an element dispersion satisfying the above formula (1) after heat treatment. Such stirring conditions can be determined by the stirring Reynolds number Re, which is preferably 5,000 or more, and more preferably 10,000 or more. Furthermore, a stirring Reynolds number Re of 25,000 or more can obtain a solid solution powder with a high element dispersion in which the elements are more uniformly arranged. The upper limit of the stirring Reynolds number Re varies depending on, for example, the type and scale of the stirring apparatus. For example, on a typical laboratory scale, it is sufficient to set it to about 25,000, but it may exceed 25,000 as long as the effects of the present invention are not impaired.

[0052] The stirring Reynolds number Re is calculated using the following formula: Re=(ρ×n×d 2 ) / μ (where ρ is the density of the reaction solution at 25°C (kg / m 3 ), n is the stirring speed (rps), d is the stirring blade span (blade diameter) (m), and μ is the viscosity of the reaction solution at 25°C (Pa s).

[0053] The reaction solution can be stirred using a known or commercially available stirring device. For example, a kneader, a mixer, a rotary vessel stirrer, a stirred reaction tank, a V-shaped stirrer, a double cone stirrer, a screw mixer, a sigma mixer, a flash mixer, an airflow stirrer, a ball mill, an edge runner, a forced thin film microreactor, etc. can be used. From the viewpoint of stirring power, a forced thin film microreactor is preferably used. The reaction solution may also be stirred in a microreaction field.

[0054] The shape of the stirring blade is not particularly limited, and for example, a propeller type, a turbine type, a fan turbine type, a paddle type, an inclined paddle type, or a gate type can be used. Furthermore, stirring blades of these shapes can be combined in multiple stages.

[0055] The stirring speed is preferably set to such a level that the stirring blades are not exposed to vortexes generated by stirring. In order to suppress vortexes generated by stirring, a cylindrical tank, a square tank, or a tank equipped with baffles can be suitably used.

[0056] In the production method of the present invention, the stirring Reynolds number Re can be adjusted by, for example, setting the stirring rotation speed and stirring blade span optimally in relation to the physical properties of the reaction solution. For example, the stirring rotation speed is in the range of 80 rps to 900 rps, the stirring blade span is in the range of 0.01 m to 0.5 m, and the density of the reaction solution is 1 kg / m 3 ~2kg / m 3 The viscosity of the reaction solution can be appropriately set within the range of 0.001 Pa·s to 0.01 Pa·s depending on the type of device used, etc.

[0057] The coprecipitate can be dried under conditions that allow sufficient removal of the solvent, typically at 100°C to 120°C for 5 to 24 hours, and calcined at 400°C to 600°C for 2 to 10 hours.

[0058] In step 2, the ceria-zirconia solid solution powder prepared in step 1 is compacted under pressure to obtain a compacted solid solution powder. 2 ~4000kgf / cm 2 pressure, preferably 2500 kgf / cm 2 ~3500kgf / cm 2 When the pressure in the pressure molding is within this range, the resulting ceria-zirconia composite oxide can have high oxygen absorbing / releasing capacity and high heat resistance. The pressure molding method is not particularly limited, and known pressure molding methods such as isostatic pressing can be appropriately used.

[0059] In step 3, the pressure-molded ceria-zirconia solid solution powder compact obtained in step 2 is subjected to a reduction treatment by heat treatment at 1400°C or higher but lower than 1700°C. This reduction treatment converts the fluorite-type ceria-zirconia solid solution into a ceria-zirconia composite oxide having an ordered κ phase and / or pyrochlore phase. The heat treatment temperature is preferably 1400°C or higher but 1650°C or lower, more preferably 1500°C or higher but 1600°C or lower. A heat treatment temperature within this range ensures sufficient generation of an ordered phase, resulting in high heat resistance and controlling the proportion of primary particles with a particle size of 0.4 μm to 1.5 μm within the desired range. The heat treatment temperature is selected so that the proportion of primary particles with a particle size of 0.4 μm to 1.5 μm, the I(14 / 29) value, and the I(28 / 29) value in the resulting ceria-zirconia composite oxide fall within the specified ranges of the present invention.

[0060] The heat treatment time is usually 0.5 to 24 hours, and preferably 1 to 10 hours. When the heat treatment time is within this range, phase separation into a disordered phase can be controlled and an ordered phase can be sufficiently generated.

[0061] The reduction treatment may be carried out by any method that can heat-treat the solid solution powder molded body under predetermined temperature conditions in a reducing atmosphere, and is not particularly limited. For example, (i) a method in which the solid solution powder molded body is placed in a vacuum heating furnace, evacuated, and then a reducing gas is introduced into the furnace to create a reducing atmosphere and heated under predetermined temperature conditions to perform the reduction treatment; (ii) a method in which the solid solution powder molded body is placed in a graphite furnace, evacuated, 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; or (iii) a method in which the solid solution powder molded body 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, etc. to perform the reduction treatment.

[0062] The production method of the present invention may further include a step of subjecting the ceria-zirconia-based composite oxide to an oxidation treatment after step 3. By subjecting the ceria-zirconia-based composite oxide to an oxidation treatment, oxygen lost during reduction is replenished in the resulting ceria-zirconia-based composite oxide, thereby improving the stability of the oxide powder.

[0063] The oxidation treatment is not particularly limited, and can be carried out, for example, by heat-treating the ceria-zirconia composite oxide in an oxidizing atmosphere (e.g., air). The heating temperature conditions during such oxidation treatment are not particularly limited, but are preferably about 300°C to 800°C. The heating time during oxidation treatment is also not particularly limited, but is preferably about 0.5 hours to 10 hours.

[0064] The production method of the present invention may further include a step of subjecting the ceria-zirconia composite oxide to a pulverization treatment after the reduction treatment step or oxidation treatment step of step 3. By subjecting the ceria-zirconia composite oxide to a pulverization treatment, when the resulting ceria-zirconia composite oxide is used in an exhaust gas purification catalyst, it becomes easier to handle when coating a substrate such as a cordierite honeycomb. The pulverization treatment is not particularly limited, and can be carried out by, for example, a wet pulverization method, a dry pulverization method, a freeze pulverization method, or the like.

[0065] The ceria-zirconia composite oxide of the present invention has high oxygen absorption / release capacity at low temperatures (for example, 300° C. to 500° C.) and high heat resistance, and therefore can be suitably used as an oxygen absorption / release material for exhaust gas purification catalysts. [Example]

[0066] The present invention will be described in more detail below using examples, although the technical scope of the present invention is not limited to these examples.

[0067] <Preparation of oxygen absorbing / releasing material> 51.3 wt%-CeO2, 45.6 wt%-ZrO2, 2.9 wt%-Pr6O 11 The ceria-zirconia-praseodymium oxide composite oxide was prepared as follows.

[0068] Example 1 A diammonium cerium nitrate solution, a praseodymium nitrate solution, and a zirconyl oxynitrate solution were mixed, and the mixed solution was dropped into an excess amount (1.5 equivalents) of ammonia water. The mixture was stirred at high speed (10,000 rpm using a stirred reactor) to produce a coprecipitate with a uniform elemental distribution. The coprecipitate was then filtered and washed with ion-exchanged water. The coprecipitate was then dried at 110°C for 10 hours or more, and then calcined in air at 400°C for 5 hours to produce a solid solution (fluorite type) of cerium, zirconium, and praseodymium (ceria-zirconia-praseodymium oxide (CeO2-ZrO2-Pr6O 11 ) solid solution. Hereinafter, this will also be referred to as CZP solid solution. ) powder was obtained.

[0069] Next, the obtained ceria-zirconia-praseodymium oxide solid solution powder was packed into a polyethylene bag, and after the inside was degassed, the opening of the bag was heated and sealed. Subsequently, using an isostatic press, the bag was subjected to isostatic pressing (CIP) at 3000 kgf / cm. 2 The resulting compact was molded at a pressure (molding pressure) of 1000°C for 2 minutes to obtain a molded body of ceria-zirconia-praseodymium oxide solid solution powder. The resulting molded body was then placed in a crucible filled with activated carbon, covered, and placed in a rapid heating electric furnace. It was heated to 1000°C in 1 hour, then heated to 1600°C (reduction treatment temperature) in 4 hours, and held there for 5 hours. It was then cooled to 1000°C in 4 hours and then allowed to cool naturally to room temperature to obtain a reduced product. The reduced product was then heated in air at 500°C for 5 hours to oxidize it, obtaining a ceria-zirconia-praseodymium oxide composite oxide (hereinafter also referred to as CZP composite oxide).

[0070] Example 2 A ceria-zirconia-praseodymium oxide composite oxide of Example 2 was prepared in the same manner as in Example 1, except that the firing temperature in the rapid heating electric furnace was set to 1550°C.

[0071] Example 3 A ceria-zirconia-praseodymium oxide composite oxide of Example 3 was prepared in the same manner as in Example 1, except that the firing temperature in the rapid heating electric furnace was set to 1500°C.

[0072] Example 4 An aqueous solution of diammonium cerium nitrate, an aqueous solution of praseodymium nitrate, and an aqueous solution of zirconyl oxynitrate were mixed, and this mixture was reacted with an excess amount (1.5 equivalents) of ammonia water while stirring at high speed in a micro-reaction field (using a forced thin film microreactor manufactured by M Technique Co., Ltd.), producing a coprecipitate with a uniform elemental distribution. The resulting coprecipitate was then filtered and washed (with ion-exchanged water). Next, the resulting coprecipitate was dried at 110°C for more than 10 hours and then calcined in air at 400°C for 5 hours to produce a solid solution (fluorite type) of cerium, zirconium, and praseodymium (CeO2-ZrO2-Pr6O 11A solid solution powder was obtained.

[0073] Next, the obtained ceria-zirconia-praseodymium oxide solid solution powder was packed into a polyethylene bag, and after the inside was degassed, the opening of the bag was heated and sealed. Subsequently, using an isostatic press, the bag was subjected to isostatic pressing (CIP) at 3000 kgf / cm. 2 The compact was then pressed under a pressure (molding pressure) of 0.01 for 2 minutes to obtain a compacted ceria-zirconia-praseodymium oxide solid solution powder. The resulting compact was then placed in a crucible filled with activated carbon, covered, and placed in a rapid heating electric furnace. The compact was heated to 1000°C in 1 hour, then heated to 1600°C (reduction treatment temperature) in 4 hours, and held there for 5 hours. The resulting compact was then cooled to 1000°C in 4 hours and then allowed to cool naturally to room temperature to obtain a reduced product. The reduced product was then heated in air at 500°C for 5 hours to oxidize it, yielding a ceria-zirconia-praseodymium oxide composite oxide.

[0074] Example 5 A ceria-zirconia-praseodymium oxide composite oxide of Example 5 was prepared in the same manner as in Example 4, except that the firing temperature in the rapid heating electric furnace was set to 1500°C.

[0075] Example 6 A diammonium cerium nitrate solution, a praseodymium nitrate solution, and a zirconyl oxynitrate solution were mixed, and the mixed solution was dropped into an excess amount (1.5 equivalents) of ammonia water. The mixture was stirred at high speed (5000 rpm using a stirred reactor) to produce a coprecipitate with a uniform elemental distribution. The coprecipitate was then filtered and washed with ion-exchanged water. The coprecipitate was then dried at 110°C for 10 hours or more, and then calcined in air at 400°C for 5 hours to produce a solid solution (fluorite type) of cerium, zirconium, and praseodymium (CeO2-ZrO2-Pr6O 11 A solid solution powder was obtained.

[0076] Next, the obtained ceria-zirconia-praseodymium oxide solid solution powder was packed into a polyethylene bag, and after the inside was degassed, the opening of the bag was heated and sealed. Subsequently, using an isostatic press, the bag was subjected to isostatic pressing (CIP) at 3000 kgf / cm. 2 The compact was then pressed under a pressure (molding pressure) of 0.01 for 2 minutes to obtain a compacted ceria-zirconia-praseodymium oxide solid solution powder. The resulting compact was then placed in a crucible filled with activated carbon, covered, and placed in a rapid heating electric furnace. The compact was heated to 1000°C in 1 hour, then heated to 1600°C (reduction treatment temperature) in 4 hours, and held there for 5 hours. The resulting compact was then cooled to 1000°C in 4 hours and then allowed to cool naturally to room temperature to obtain a reduced product. The reduced product was then heated in air at 500°C for 5 hours to oxidize it, yielding a ceria-zirconia-praseodymium oxide composite oxide.

[0077] Example 7 A ceria-zirconia-praseodymium oxide composite oxide of Example 7 was prepared in the same manner as in Example 6, except that the firing temperature in the rapid heating electric furnace was set to 1550°C.

[0078] Comparative Example 1 An aqueous solution of diammonium cerium nitrate, an aqueous solution of praseodymium nitrate, and an aqueous solution of zirconyl oxynitrate were mixed, and the mixed solution was added dropwise to one equivalent of aqueous ammonia. The mixture was stirred (using a stirrer at 500 rpm) to produce a coprecipitate. The resulting coprecipitate was filtered and washed with ion-exchanged water. The resulting coprecipitate was then dried at 110°C for 10 hours or more and then calcined in air at 400°C for 5 hours to produce a solid solution (fluorite type) of cerium, zirconium, and praseodymium (CeO2-ZrO2-Pr6O 11 A solid solution powder was obtained.

[0079] Next, the obtained ceria-zirconia-praseodymium oxide solid solution powder was packed into a polyethylene bag, and after the inside was degassed, the opening of the bag was heated and sealed. Subsequently, using an isostatic press, the bag was subjected to isostatic pressing (CIP) at 3000 kgf / cm. 2The compact was then pressed under a pressure (molding pressure) of 0.01 for 2 minutes to obtain a compacted ceria-zirconia-praseodymium oxide solid solution powder. The resulting compact was then placed in a crucible filled with activated carbon, covered, and placed in a rapid heating electric furnace. The compact was heated to 1000°C in 1 hour, then heated to 1700°C (reduction treatment temperature) in 4 hours, and held there for 5 hours. The resulting compact was then cooled to 1000°C in 4 hours and then allowed to cool naturally to room temperature to obtain a reduced product. The reduced product was then heated in air at 500°C for 5 hours to oxidize it, yielding a ceria-zirconia-praseodymium oxide composite oxide.

[0080] Comparative Example 2 A ceria-zirconia-praseodymium oxide composite oxide of Comparative Example 2 was prepared in the same manner as in Comparative Example 1, except that the firing temperature in the rapid heating electric furnace was set to 1600°C.

[0081] Comparative Example 3 A ceria-zirconia-praseodymium oxide composite oxide of Comparative Example 3 was prepared in the same manner as in Comparative Example 1, except that the firing temperature in the rapid heating electric furnace was set to 1500°C.

[0082] Comparative Example 4 A ceria-zirconia-praseodymium oxide composite oxide of Comparative Example 4 was prepared in the same manner as in Example 1, except that the firing temperature in the rapid heating electric furnace was set to 1700°C.

[0083] Comparative Example 5 An aqueous solution of diammonium cerium nitrate, an aqueous solution of praseodymium nitrate, and an aqueous solution of zirconyl oxynitrate were mixed, and the mixed solution was added dropwise to one equivalent of aqueous ammonia. A coprecipitate was generated while stirring at high speed (5000 rpm using an SA reactor). The coprecipitate was filtered and washed with ion-exchanged water. The coprecipitate was then dried at 110°C for 10 hours or more and then calcined in air at 400°C for 5 hours to obtain a solid solution (fluorite type) of cerium, zirconium, and praseodymium (CeO2-ZrO2-Pr6O 11 A solid solution powder was obtained.

[0084] Next, the obtained ceria-zirconia-praseodymium oxide solid solution powder was packed into a polyethylene bag, and after the inside was degassed, the opening of the bag was heated and sealed. Subsequently, using an isostatic press, the bag was subjected to isostatic pressing (CIP) at 3000 kgf / cm. 2 The compact was then pressed under a pressure (molding pressure) of 0.01 for 2 minutes to obtain a compacted ceria-zirconia-praseodymium oxide solid solution powder. The resulting compact was then placed in a crucible filled with activated carbon, covered, and placed in a rapid heating electric furnace. The compact was heated to 1000°C in 1 hour, then heated to 1600°C (reduction treatment temperature) in 4 hours, and held there for 5 hours. The resulting compact was then cooled to 1000°C in 4 hours and then allowed to cool naturally to room temperature to obtain a reduced product. The reduced product was then heated in air at 500°C for 5 hours to oxidize it, yielding a ceria-zirconia-praseodymium oxide composite oxide.

[0085] Comparative Example 6 A ceria-zirconia-praseodymium oxide composite oxide of Comparative Example 6 was prepared in the same manner as in Comparative Example 5, except that the firing temperature in the rapid heating electric furnace was set to 1500°C.

[0086] Catalyst samples were prepared using the ceria-zirconia-praseodymium oxide composite oxides (CZP composite oxides) of Examples 1 to 7 and Comparative Examples 1 to 6, and after a durability test was carried out, the oxygen adsorption / release capacity of the catalyst samples was evaluated.

[0087] <Catalyst sample preparation> After mixing the CZP composite oxide and a support (Al2O3) carrying a precious metal (Rh) in a blender, the mixed powder was pressed under a pressure of 1 ton to solidify the powder and obtain a pressed compact. The pressed compact was crushed in a mortar to obtain catalyst pellets of approximately 1 to 2 mm square.

[0088] <Durability test> The durability test was carried out under the following conditions. Temperature: 1100℃ x 5 hours Gas atmosphere: CO 2% ⇔ O2 5% (switched every 5 minutes), H2O 10%, N2 balance, 10 L / min

[0089] <Oxygen absorption and release capacity> The oxygen absorption and release capacity was evaluated under the following conditions. Temperature: 400 °C After switching from O2(1%) to CO(2%), the oxygen absorption and release amount was calculated from the amount of CO2 generated in 15 seconds (oxygen absorption and release amount per 1 g of pellet catalyst).

[0090] The following evaluations were performed on the CZP composite oxides of Examples 1 to 7 and Comparative Examples 1 to 6.

[0091] <Crystal structure evaluation by X-ray diffraction (XRD) measurement> The CZP composite oxide was heat-treated at 1100 °C in air for 5 hours for a durability test, and the crystal phase of the CZP composite oxide after the treatment was measured by X-ray diffraction. As the X-ray diffractometer, the product name "RINT2100" manufactured by Rigaku Corporation was used. Using CuKα, the X-ray diffraction pattern was measured under the conditions of 40 kV, 30 mA, and 2θ = 2° / min, and the intensity ratio {I(14 / 29) value} between the diffraction line at 2θ = 14.5° and the diffraction line at 2θ = 29°, and the intensity ratio {I(28 / 29) value} between the diffraction line at 2θ = 28.5° and the diffraction line at 2θ = 29° were determined.

[0092] <Particle size distribution measurement> The particle size distribution of the primary particles of the CZP composite oxide was measured as follows. Specifically, using the product name "JSM-7000F" manufactured by JEOL Ltd. as a scanning electron microscope, the cross-section of the CZP composite oxide was arbitrarily observed at two or more locations, and the average value of the crystal particle sizes in each observed field of view (region of 500 times magnification, 240 μm × 240 μm) was determined based on several criteria, and the distribution was evaluated. Here, the crystal particle size refers to the diameter of the minimum circumscribed circle when the cross-section is not circular.

[0093] The elemental dispersity of the fluorite-type ceria-zirconia-praseodymium oxide solid solution (CZP solid solution) powder before reduction treatment in Examples 1 to 7 and Comparative Examples 1 to 6 was evaluated.

[0094] <Elemental dispersity (Ce 0.5 Zr 0.5O2 crystal ratio) evaluation> The fluorite-type CZP solid solution powder was heat-treated in air at 1200°C for 5 hours, and the crystalline phase after the treatment was measured by X-ray diffraction. The X-ray diffraction pattern was measured using a Rigaku Corporation "Smart Lab" X-ray diffractometer under the conditions of CuKα, 45 KV, 200 mA, and 2θ = 1° / min. The 2θ = 29° (Ce 0.6 Zr 0.4 O2 crystal), 29.3°(Ce 0.5 Zr 0.5 O2 crystal) and 29.8° (Ce 0.2 Zr 0.8 The area of ​​the peak detected in the O2 crystal was calculated, and the ratio of the peak area at 29.3° to the total peak area (element dispersion) was calculated using the following formula. Elemental dispersion = 29.3° peak area / (29° peak area + 29.3° peak area + 29.8° peak area)

[0095] The evaluation results are shown in Table 1. In addition, FIG. 1 shows the proportion of primary particles with particle diameters of 0.4 μm to 1.5 μm and the element dispersion degree (Ce 0.5 Zr 0.5 2 shows the relationship between the proportion of primary particles with particle diameters of 0.4 μm to 1.5 μm and the element dispersion degree (CeO2 crystal ratio) after heat treatment for the ceria-zirconia-praseodymium oxide composite oxides of Examples 1 to 7 and Comparative Examples 1 to 6. 0.5 Zr 0.5 1 and 2, Examples 1 to 7 represent Examples 1 to 7, respectively, and Comparative Examples 1 to 6 represent Comparative Examples 1 to 6, respectively.

[0096] [Table 1]

[0097] As shown in Table 1 and FIG. 1, the CZP composite oxides of Examples 1 to 7 had primary particles with a particle size of 0.4 μm to 1.5 μm present at 40% or more, and an I(14 / 29) value of 0.015 or more after the durability test, and had high heat resistance. As shown in Table 1 and FIG. 1, the I(14 / 29) value after the durability test was correlated with the elemental dispersion degree (the ratio of the Ce 0.5 Zr 0.5 O2 crystal) after heat treatment at 1200 °C for 5 hours in the atmosphere. When the ratio of primary particles with a particle size of 0.4 μm to 1.5 μm was about the same, the I(14 / 29) value tended to increase and the heat resistance tended to increase as the value of the elemental dispersion degree after heat treatment increased. Also, in the CZP composite oxide of this composition, in order to obtain a CZP composite oxide in which primary particles with a particle size of 0.4 μm to 1.5 μm were present at 40% or more and the I(14 / 29) value after the durability test was 0.015 or more, it was confirmed that it was preferable to use a fluorite-type CZP solid solution powder with an elemental dispersion degree of 19% or more after heat treatment.

[0098] Also, as shown in Table 1 and FIG. 2, it was confirmed that the CZP composite oxides of Examples 1 to 7 in which primary particles with a particle size of 0.4 μm to 1.5 μm were present at 40% or more and the I(14 / 29) value after the durability test was 0.015 or more had a significantly high oxygen absorption and release ability at low temperature (400 °C).

[0099] <Examination of Pr content and elemental dispersion degree> Cerium-zirconia-praseodymium oxide (CZP) composite oxides of Examples 8 to 10 and a Pr-free cerium-zirconia (CZ) composite oxide of Comparative Example 7 were prepared in the same manner as in Example 7, except that the amount of the raw material compounds was changed so that the content of praseodymium oxide (Pr6O 11 ) shown in Table 2 was obtained.

[0100] The element dispersion was evaluated as described above for the fluorite-type CZP solid solution powders before reduction treatment in Examples 8 to 10 and the fluorite-type CZ solid solution powder before reduction treatment in Comparative Example 7. XRD measurement and particle size distribution measurement were also performed as described above for the CZP composite oxides of Examples 8 to 10 and the CZ composite oxide of Comparative Example 7. The evaluation results for Examples 8 to 10 and Comparative Example 7 are shown in Table 2 together with the evaluation result for Example 7. FIG. 3 shows the distribution of praseodymium oxide (PrO 11 ) content and element dispersion after heat treatment (Ce 0.5 Zr 0.5 The relationship between the ratio of O2 crystals and the temperature is shown.

[0101] [Table 2]

[0102] As shown in Table 2, the CZP composite oxides of Examples 7 to 10 all had 40% or more primary particles with particle sizes of 0.4 μm to 1.5 μm, high oxygen absorption / release capacity at low temperatures (400°C), and an I(14 / 29) value of 0.015 or more after durability testing, indicating high heat resistance. On the other hand, the CZ composite oxide of Comparative Example 7 without added Pr also had these properties in both the proportion of primary particles and the I(14 / 29) value, but its oxygen absorption / release capacity was significantly lower than that of the CZP composite oxide.

[0103] The CZP composite oxides of Examples 7 to 10 and the CZ composite oxide of Comparative Example 7 were prepared using the same procedure, and therefore the uniformity of elements in the solid solution powders obtained during the manufacturing process is expected to be comparable. However, as shown in Figure 3, the amount of Pr added differed, and the PrO 11 If the content is different, the element dispersion (Ce) after heat treatment at 1200°C will be different even if they are prepared using the same procedure. 0.5 Zr 0.5 O2 crystal ratio) are different, and Pr6O 11 As the content decreases, Ce 0.5 Zr 0.5 The proportion of O2 crystals tended to be low. 11Ce content of 2.9 wt% after heat treatment 0.5 Zr 0.5 A similar tendency was confirmed for the CZP solid solution powders of Comparative Examples 1 to 3, in which the proportion of O2 crystals was 15%. Pr is thought to have the function of suppressing phase separation of the fluorite-type CZP solid solution. When the amount of Pr is reduced or absent, the amount of phase separation increases when heated at 1200°C, even if the element dispersion in the solid solution powder before heat treatment is the same, and Ce 0.5 Zr 0.5 It is presumed that the proportion of O2 crystals has decreased.

[0104] As described above, in Examples 1 to 7 and Comparative Examples 1 to 6, PrO 11 It was confirmed that, when the content is 2.9 wt%, in order to obtain a CZP composite oxide in which 40% or more of primary particles have a particle size of 0.4 μm to 1.5 μm and an I(14 / 29) value after durability testing is 0.015 or more, it is preferable to use a fluorite-type CZP solid solution powder with an elemental dispersion of 19% or more after heat treatment. Furthermore, as mentioned above, even when prepared by the same procedure and expected to have the same degree of elemental uniformity, the PrO 11 As the content decreases, the element dispersion after heat treatment tends to decrease. Therefore, the fluorite-type CZP solid solution powder preferably used for producing the CZP composite oxide of the present invention has an element dispersion after heat treatment of PrO 11 This correlates with the Ce content, as shown in Figure 3. 0.5 Zr 0.5 The ratio y (%) of O2 crystals to the whole crystal and Pr6O 11 The content x (wt%) of the compound is expressed by the following formula (1): y≧-0.7079x 2 +5.3331x+9.4883 (1) It was found that the above conditions were satisfied.

[0105] As shown in Table 2, the CZ composite oxide of Comparative Example 7 to which no Pr was added had a significantly lower oxygen absorption / release amount than the CZP composite oxides of Examples 7 to 10. Therefore, in the present invention, the CZP composite oxide satisfying the above formula (1) and 11It has been shown that it is preferable to use a fluorite-type CZP solid solution powder in which the content x of Pr is greater than 0 (i.e., Pr is included). By using such a fluorite-type CZP solid solution powder, it is possible to obtain a CZP composite oxide with high element uniformity and high heat resistance.

[0106] Furthermore, this effect obtained by adding Pr to the CZP composite oxide is expected to be due to the fact that the collapse of the crystal structure caused by the diffusion of tetravalent Ce ions (0.97 Å) within the crystal is suppressed by the presence of trivalent Pr ions (1.126 Å), which have an ionic radius larger than that of Ce, within the crystal, thereby maintaining the crystal structure even after durability testing. The same effect is expected to be obtained with trivalent Nd ions (1.109 Å), which have an ionic radius similar to that of Pr. Therefore, even if the additive element in the ceria-zirconia composite oxide of the present invention is changed to Nd, it is expected that the same effect as when the additive element is Pr will be obtained.

Claims

1. Ceria (CeO ２ ) and zirconia (ZrO ２ ) The oxygen absorbing / releasing material is made of a ceria-zirconia based composite oxide containing containing praseodymium (Pr) or neodymium (Nd) as an additive element, At least one of an ordered phase of a κ phase and a pyrochlore phase is present in at least a portion of the alloy, primary particles having a particle diameter of 0.4 μm to 1.5 μm account for 40% to 100% by particle number of all primary particles of the ceria-zirconia-based composite oxide, the intensity ratio of the diffraction line at 2θ=14.5° to the diffraction line at 2θ=29° {I(14 / 29) value} obtained from an X-ray diffraction pattern using CuKα obtained by X-ray diffraction measurement after heating in air at a temperature of 1100°C for 5 hours is 0.015 or more, and the intensity ratio of the diffraction line at 2θ=28.5° to the diffraction line at 2θ=29° {I(28 / 29) value} is 0.08 or less, The solid solution powder is obtained using a fluorite-type ceria-zirconia solid solution powder containing praseodymium (Pr) or neodymium (Nd) as an additive element, and the solid solution powder is heated in air at a temperature of 1200°C for 5 hours, and the ratio y (%) of Ce 0.5 Zr 0.5 O 2 crystals to the entire crystals and the content x (wt %) of praseodymium oxide (Pr 6 O 11 ) or neodymium oxide (Nd 2 O 3 ) in the solid solution powder satisfy the following formula (1): y≧-0.7079x 2 +5.3331x+9.4883 (1) (Wherein, x≧0.6) fulfill, Oxygen absorbing and releasing material.

2. 2. The oxygen absorbing / releasing material according to claim 1, wherein the additional element is Pr.

3. A method for producing the oxygen absorbing / releasing material according to claim 1, A fluorite-type ceria-zirconia solid solution powder containing praseodymium (Pr) or neodymium (Nd) as an additive element, wherein the Ce after heating in air at a temperature of 1200°C for 5 hours is ０．５ Zr ０．５ O ２ The ratio y (%) of the crystals to the whole crystals and the praseodymium oxide (Pr ６ O １１ ) or neodymium oxide (Nd ２ O ３ ) content x (wt%) is expressed by the following formula (1): y≧-0.7079x ２ +5.3331x+9.4883 (1) (Wherein, x≧0.6) preparing the solid solution powder that satisfies the above; a step of press-molding the ceria-zirconia solid solution powder; a step of subjecting the pressure-molded ceria-zirconia solid solution powder compact to a reduction treatment by heat treating the compact at 1400°C or higher but lower than 1700°C; A method for producing an oxygen-absorbing / releasing material, comprising:

4. The method for producing an oxygen absorbing / releasing material according to claim 3, wherein the additional element is Pr.

Images