Oxygen absorbing material and method for manufacturing the same

Abstract

An object of the present invention is to provide an oxygen absorbing and releasing material and a manufacturing method thereof which improve oxygen absorbing and releasing capacity at low temperatures and ensure heat resistance. The oxygen absorbing and releasing material of the present invention contains ceria-zirconia-based composite oxide containing ceria (CeO2) and zirconia (ZrO2), the ceria-zirconia-based composite oxide containing praseodymium (Pr) or neodymium (Nd) as an additive element, at least a part of the ceria-zirconia-based composite oxide having at least one ordered phase of a kappa phase and a pyrochlore phase, 40% to 100% of primary particles having a particle diameter of 0.4 μm to 1.5 μm existing on a basis of the number of particles, an X-ray diffraction pattern being obtained by X-ray diffraction using CuKα after heating at a temperature condition of 1100°C for 5 hours in the atmosphere, an I(14 / 29) value calculated from the X-ray diffraction pattern being 0.015 or more, and an I(28 / 29) value being 0.08 or less.

Description

TECHNICAL FIELD

[0001] The present application relates to an oxygen storage material and a method for producing the same, and particularly to an oxygen storage material for exhaust gas purification catalysts and a method for producing the same. BACKGROUND

[0002] Exhaust gas emitted from an internal combustion engine of an automobile or the like contains harmful gases such as carbon monoxide (CO), nitrogen oxides (NOx), and unburned hydrocarbons (HC). An exhaust gas purification catalyst (so-called three-way catalyst) that decomposes such harmful gases contains, in addition to a noble metal that purifies exhaust gas, an oxygen storage material (also referred to as an OSC material) having an oxygen storage and release ability (also referred to as an Oxygen Storage Capacity (OSC)). The oxygen storage material adjusts the air-fuel ratio (A / F) by storing and releasing oxygen, whereby a decrease in the purification rate accompanying a change in the composition of exhaust gas can be suppressed.

[0003] As the oxygen storage material, a ceria-zirconia-based composite oxide, particularly a ceria-zirconia-based composite oxide containing a pyrochlore phase or a Kappa phase (κ phase) as an ordered phase, is widely used because of a large amount of oxygen storage and release.

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

[0005] In addition, Patent Literature 2 discloses a ceria-zirconia-based composite oxide having a secondary particle size (D50) of 3 to 7 μm, and discloses that by controlling the secondary particle size, the oxygen storage and release speed can be increased.

[0006] In addition, Patent Literature 3 discloses a ceria-zirconia-based composite oxide that, in addition to ceria and zirconia, contains at least one additive element selected from praseodymium, lanthanum, and yttrium at a predetermined proportion, and discloses that by containing the additive element, reduction of ceria is promoted, and the oxygen storage and release function is promoted.

[0007] In recent years, in the course of improvement in fuel efficiency and electrification of vehicles in response to the increase in CO2 restrictions, the temperature of exhaust gas is reduced, and oxygen absorbing and releasing materials that exhibit oxygen absorbing and releasing functions at low temperatures are required. The oxygen absorbing and releasing materials having pyrochlore phases or kappa phases disclosed in Patent Documents 1 to 3 have a large amount of oxygen absorbing and releasing, but it is difficult for them to exhibit oxygen absorbing and releasing functions at low temperatures. In general, it is known that when the particle diameter of ceria-zirconia-based composite oxides is reduced and the specific surface area is increased, the oxygen absorbing and releasing capacity at low temperatures is improved, but when the particle diameter is reduced, the heat resistance deteriorates.

[0008] Prior Art Documents

[0009] Patent Document 1: Japanese Patent Application Publication No. 2015-34113

[0010] Patent Document 2: Japanese Patent Application Publication No. 2018-38999

[0011] Patent Document 3: Japanese Patent Application Publication No. 2015-818 SUMMARY

[0012] As described above, in the conventional oxygen absorbing and releasing materials, if the particle diameter of ceria-zirconia-based composite oxides is reduced to improve the oxygen absorbing and releasing capacity, the heat resistance deteriorates, and it is difficult to improve the oxygen absorbing and releasing capacity at low temperatures and ensure the heat resistance. Therefore, an object of the present application is to provide an oxygen absorbing and releasing material that improves the oxygen absorbing and releasing capacity at low temperatures and ensures the heat resistance.

[0013] The present inventors have conducted various studies on means for solving the aforementioned problems, and as a result, it has been found that by reducing the particle diameter to improve the oxygen absorbing and releasing capacity at low temperatures, and further by stabilizing the crystal structure by improving the uniformity of elements to ensure the heat resistance, it is possible to improve the oxygen absorbing and releasing capacity at low temperatures and ensure the heat resistance in the oxygen absorbing and releasing material, and thus the present application has been completed.

[0014] That is, the gist of the present application is as follows.

[0015] (1) An oxygen absorbing and releasing material comprising a ceria-zirconia-based composite oxide containing ceria (CeO2) and zirconia (ZrO2),

[0016] the ceria-zirconia-based composite oxide contains praseodymium (Pr) or neodymium (Nd) as an added element,

[0017] at least a part of the ceria-zirconia-based composite oxide has at least one ordered phase of a kappa phase and a pyrochlore phase,

[0018] 40% to 100% of primary particles having a particle diameter of 0.4 μm to 1.5 μm are present on a particle number basis with respect to all of the primary particles of the ceria-zirconia-based composite oxide,

[0019] The cerium dioxide-zirconia composite oxide was heated in the atmosphere at 1100°C for 5 hours, and the X-ray diffraction pattern was obtained by X-ray diffraction using CuKα. The intensity ratio {I(14 / 29)} of the diffraction line with 2θ = 14.5° and the diffraction line with 2θ = 29° was greater than or equal to 0.015, and the intensity ratio {I(28 / 29)} of the diffraction line with 2θ = 28.5° and the diffraction line with 2θ = 29° was less than or equal to 0.08.

[0020] (2) The oxygen absorption and desorption material described in (1) above is obtained using a fluorite-type cerium dioxide-zirconium dioxide solid solution powder containing praseodymium (Pr) or neodymium (Nd) as additive elements. The solid solution powder is heated in atmosphere at 1200°C for 5 hours, and Ce... 0.5 Zr 0.5 The proportion of O2 crystals relative to the total crystal volume, y (%), and the amount of praseodymium oxide (Pr6O) in the solid solution powder. 11 The content x (wt%) of neodymium oxide (Nd2O3) or neodymium oxide satisfies the following formula (1):

[0021] y≥-0.7079x 2 +5.3331x+9.4883 (1)

[0022] (where x>0).

[0023] (3) According to the oxygen absorption and release material described in (1) or (2) above, the added element is Pr.

[0024] (4) A method for manufacturing an oxygen absorption / desorption material, comprising the method for manufacturing the oxygen absorption / desorption material described in (1) above, including:

[0025] The process of preparing solid solution powder, wherein the solid solution powder is a fluorite-type cerium dioxide-zirconium dioxide solid solution powder containing praseodymium (Pr) or neodymium (Nd) as additive elements, is carried out by heating it in the atmosphere at 1200°C for 5 hours. 0.5 Zr 0.5 The proportion of O2 crystals relative to the total crystal volume, y (%), and the amount of praseodymium oxide (Pr6O) in the solid solution powder. 11 The content x (wt%) of neodymium oxide (Nd2O3) or neodymium oxide satisfies the following formula (1):

[0026] y≥-0.7079x 2 +5.3331x+9.4883 (1)

[0027] (where x>0);

[0028] a process of performing press molding on the ceria-zirconia-based solid solution powder; and

[0029] a process of performing a reduction treatment by heat-treating the ceria-zirconia-based solid solution powder molded body after the press molding at 1400°C or higher and less than 1700°C.

[0030] (5) The oxygen absorbing and releasing material manufacturing method according to the above (4), wherein the additive element is Pr.

[0031] According to the present application, an oxygen absorbing and releasing material that improves oxygen absorbing and releasing capacity at low temperatures and ensures heat resistance can be provided. BRIEF DESCRIPTION OF DRAWINGS

[0032] Figure 1 is a graph showing the relationship between the proportion of primary particles having a particle size of 0.4 μm to 1.5 μm, the element dispersion degree (Ce 0.5 Zr 0.5 O2 crystal, and the I (14 / 29) value.

[0033] Figure 2 is a graph showing the relationship between the proportion of primary particles having a particle size of 0.4 μm to 1.5 μm, the element dispersion degree (Ce 0.5 Zr 0.5 O2 crystal, and the oxygen absorbing and releasing amount (400°C).

[0034] Figure 3 is a graph showing the relationship between the content of Pr6O 11 ) in the solid solution powder in the examples and the element dispersion degree (Ce 0.5 Zr 0.5 O2 crystal after heat treatment. DETAILED DESCRIPTION

[0035] Hereinafter, a preferred embodiment of the present application will be described in detail.

[0036] The present application relates to an oxygen absorbing and releasing material composed of a ceria-zirconia-based composite oxide, that is, the ceria-zirconia-based composite oxide of the present application is used as an oxygen absorbing and releasing material. The ceria-zirconia-based composite oxide of the present application contains ceria (CeO2) and zirconia (ZrO2), and contains praseodymium (Pr) or neodymium (Nd) as an additive element. Therefore, the ceria-zirconia-based composite oxide of the present application is a ceria, zirconia, and praseodymium oxide (Pr6O 11Pr or Nd is considered to contribute to the crystal structure stabilization of the composite oxide after durability, and by adding Pr or Nd, a high oxygen storage capacity and a high heat resistance can be obtained in the composite oxide. In the ceria-zirconia-based composite oxide of the present application, the additive element is preferably Pr.

[0037] In the ceria-zirconia-based composite oxide of the present application, the content ratio of (cerium + praseodymium or neodymium) to zirconium is preferably in the range of 1 : 1 to 1 : 1.4 in terms of molar ratio ([Ce + Pr or Nd]: [Zr]), more preferably in the range of 1 : 1.1 to 1 : 1.4, or also in the range of 1 : 1 to 1 : 1.2. If the content ratio is in this range, the oxygen storage capacity after durability can be sufficiently improved.

[0038] In the ceria-zirconia-based composite oxide of the present application, the content of praseodymium (Pr) or neodymium (Nd) as an additive element is preferably in the range of 0.5 mol% to 5 mol%, more preferably in the range of 0.5 mol% to 2.5 mol% with respect to the total amount of cations from the viewpoint of balancing the heat resistance and the OSC performance.

[0039] In the ceria-zirconia-based composite oxide of the present application, the content of ceria is preferably in the range of 35% by weight to 60% by weight, more preferably in the range of 47% by weight to 53% by weight on a weight basis.

[0040] In the ceria-zirconia-based composite oxide of the present application, the content of zirconia is preferably in the range of 40% by weight to 50% by weight, more preferably in the range of 41% by weight to 47% by weight on a weight basis.

[0041] In the ceria-zirconia-based composite oxide of the present application, the content of praseodymium oxide is preferably in the range of 0.1% by weight to 6% by weight, more preferably in the range of 0.5% by weight to 3% by weight on a weight basis. In the ceria-zirconia-based composite oxide of the present application, the content of neodymium oxide is preferably in the range of 0.1% by weight to 6% by weight, more preferably in the range of 0.5% by weight to 3% by weight on a weight basis.

[0042] The ceria-zirconia-based composite oxide of the present application can further contain at least one additional element selected from rare earth elements other than cerium (Ce), praseodymium (Pr), and neodymium (Nd), and alkaline earth elements. By containing such an additional element, in the case where the ceria-zirconia-based composite oxide of the present application is used as an oxygen storage and release material for an exhaust gas purification catalyst, a higher exhaust gas purification capability can be obtained. As such rare earth elements, scandium (Sc), yttrium (Y), lanthanum (La), samarium (Sm), gadolinium (Gd), terbium (Tb), dysprosium (Dy), ytterbium (Yb), lutetium (Lu), and the like can be given, of which Sc, Y, La are preferred from the viewpoint that the interaction with a noble metal becomes stronger and the affinity becomes greater when the noble metal is supported, and Y, La are more preferred. In addition, as alkaline earth metal elements, magnesium (Mg), calcium (Ca), strontium (Sr), barium (Ba), and radium (Ra) can be given, of which Mg, Ca, and Ba are preferred from the viewpoint that the interaction with a noble metal becomes stronger and the affinity becomes greater when the noble metal is supported. Such rare earth elements and alkaline earth metal elements having a low electronegativity have a strong interaction with a noble metal, and therefore, in an oxidizing atmosphere, the noble metal is combined with oxygen, evaporation and sintering of the noble metal are suppressed, and the degradation of the noble metal as an active site at the time of exhaust gas purification can be sufficiently suppressed.

[0043] In the case where the ceria-zirconia-based composite oxide contains an additional element, the content of the additional element is preferably 1 to 20% by weight, and more preferably 3 to 7% by weight, with respect to the ceria-zirconia-based composite oxide. If the content of the additional element is within this range, the interaction with a noble metal can be sufficiently improved when the noble metal is supported on the resulting composite oxide, and the oxygen storage and release capability is sufficient.

[0044] The ceria-zirconia-based composite oxide of the present application has at least one ordered phase of a kappa phase and a pyrochlore phase in at least a part thereof. The kappa phase and the pyrochlore phase are crystal phases having an ordered arrangement structure formed of cerium ions and zirconium ions. A part of the cerium ions and the zirconium ions can also be substituted with ions of an additional element such as praseodymium or neodymium. In one embodiment, a part of the cerium ions is substituted with praseodymium ions or neodymium ions. In the ceria-zirconia-based composite oxide, by forming such an ordered phase, the heat resistance at high temperatures is improved, and a sufficiently high oxygen storage and release capability can be exerted even after exposure to high temperatures.

[0045] The pyrochlore phase (Ce2Zr2O7) has oxygen defect sites, and if an oxygen atom enters the site, the pyrochlore phase changes to the kappa phase (Ce2Zr2O8). On the other hand, the kappa phase can change to the pyrochlore phase by emitting an oxygen atom. The oxygen absorption and emission capacity of the ceria-zirconia-based composite oxide is due to absorption and emission of oxygen by changing between the pyrochlore phase and the kappa phase. In the ceria-zirconia-based composite oxide of the present application, the ratio of the ordered phase to the total crystal phase, which is calculated from the peak intensity ratio of the X-ray diffraction pattern described below, is preferably 50 to 100%, and more preferably 80 to 100%. If the ratio of the ordered phase is within this range, the composite oxide has a high oxygen absorption and emission capacity and high heat resistance.

[0046] The ceria-zirconia-based composite oxide of the present application, after being heated in the atmosphere at a temperature of 1100°C for 5 hours, gives an X-ray diffraction pattern by X-ray diffraction measurement using CuKa, and the intensity ratio of the diffraction line at 2Θ = 14.5° to the diffraction line at 2Θ = 29° {I(14 / 29) value} 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 X-ray diffraction measurement can be performed, for example, using a commercially available apparatus "RINT 2100" manufactured by Rigaku Denki K.K. as the measurement apparatus, using CuKa line, under the conditions of 40 KV, 30 mA, and 2Θ = 2° / minute.

[0047] Here, the diffraction line at 2θ = 14.5° is a diffraction line of the (111) plane of the ordered phase (κ phase), and the diffraction line at 2θ = 29° is a diffraction line of the (222) plane of the ordered phase and a diffraction line of the (111) plane of the cubic crystal phase of the ceria-zirconia-based solid solution, and thus the I(14 / 29) value, which is the intensity ratio of the two diffraction lines, is defined as an index indicating the maintenance rate (existence rate) of the ordered phase. Further, in measuring the intensity of the diffraction line, the average intensity of the diffraction line at 2θ = 10° to 12° as a background value is subtracted from the value of each intensity of the diffraction line to calculate. In addition, the perfect ordered phase has a κ phase (Ce2Zr2O8) in which oxygen is completely filled and a pyrochlore phase (Ce2Zr2O7) in which oxygen is completely removed, and the I(14 / 29) value of the κ phase calculated from each PDF card (PDF2: 01-070-4048 for the κ phase, and PDF2: 01-075-2694 for the pyrochlore phase) is 0.04, and the I(14 / 29) value of the pyrochlore phase is 0.05. In addition, the ordered phase can be determined by the presence of peaks at positions of 2θ angles of 14.5°, 28°, 37°, 44.5°, and 51° in the X-ray diffraction pattern obtained by the above-described X-ray diffraction measurement using CuKα. Further, the "peak" referred to herein means a peak having a height of 30 cps or more from the baseline to the peak top.

[0048] In the ceria-zirconia-based composite oxide of the present application, the I(14 / 29) value is 0.015 or more, more preferably 0.017 or more, and particularly preferably 0.020 or more. If the I(14 / 29) value is 0.015 or more, the composite oxide has high heat resistance.

[0049] In addition, the diffraction line at 2θ = 28.5° is a diffraction line of the (111) plane of CeO2monomer, and the intensity ratio I(28 / 29) value of the diffraction line at 2θ = 28.5° and the diffraction line at 2θ = 29° is defined as an index indicating the degree of phase separation of CeO2from the composite oxide.

[0050] In the ceria-zirconia-based composite oxide of the present application, the I(28 / 29) value is 0.08 or less, more preferably 0.05 or less, and particularly preferably 0.01 or less. If the I(28 / 29) value is 0.08 or less, the phase separation of ceria is sufficiently suppressed, and the oxygen absorption and desorption capacity after high-temperature durability is sufficiently high. The lower limit of the I(28 / 29) value is not particularly limited, and a smaller value is preferable.

[0051] In the ceria-zirconia-based composite oxide of the present application, the primary particles having a particle size of 0.4 μm to 1.5 μm are present at 40% to 100% by number, preferably at 40% to 99%, and more preferably at 40% to 97.5% with respect to the total primary particles of the ceria-zirconia-based composite oxide. In addition, the lower limit of the presence ratio of the 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. If the presence ratio of the primary particles having a particle size of 0.4 μm to 1.5 μm is 40% or more, the oxygen absorption and desorption capacity at a low temperature (for example, 300°C to 500°C) can be improved. The conventional ceria-zirconia-based composite oxide has a problem that the heat resistance is reduced when the particle size is reduced in order to improve the oxygen absorption and desorption capacity at a low temperature, whereas the ceria-zirconia-based composite oxide of the present application can ensure the heat resistance by stabilizing the crystal structure through the improvement of the uniformity of elements, and thus can improve the oxygen absorption and desorption capacity at a low temperature and ensure the heat resistance. Further, the particle size distribution of the primary particles of the ceria-zirconia-based composite oxide is obtained by observing the cross section of the composite oxide particles at two or more arbitrary observation points by a scanning electron microscope (SEM), and the average value of the distribution is obtained by quantitatively measuring the crystal grain size in each observation field. Further, the crystal grain size as referred to herein means the diameter of the smallest circumscribed circle in the case where the cross section is not circular.

[0052] The ceria-zirconia-based composite oxide of the present application has a sufficiently narrow particle size distribution, and the composite oxide has a high oxygen absorption and desorption capacity and a high heat resistance. In the ceria-zirconia-based composite oxide of the present application, the presence ratio of the primary particles having a particle size of less than 0.4 μm with respect to the total primary particles is preferably 3% or less by number, and more preferably none. In addition, the presence ratio of the primary particles having a particle size of more than 1.5 μm with respect to the total primary particles is preferably 10% or less by number, and more preferably none.

[0053] The manufacturing method of the ceria-zirconia-based composite oxide of the present application is described as follows. The ceria-zirconia-based composite oxide of the present application is preferably obtained using a fluorite-type ceria-zirconia-based solid solution powder having a specific element dispersion degree. The ceria-zirconia-based solid solution powder contains Pr or Nd as an additive element, i.e., a ceria-zirconia-praseodymium oxide (Pr6O 11 ) or ceria-zirconia-neodymium oxide (Nd2O3) solid solution powder.

[0054] In a preferred embodiment, 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 application, after being heated in the atmosphere at a temperature of 1200°C for 5 hours, has a Ce 0.5 Zr 0.5 The proportion y (%) of the O2 crystal phase with respect to the entire crystal, and the content x (wt%) of praseodymium oxide (Pr6O 11 ) or neodymium oxide (Nd2O3) in the solid solution powder satisfies the following formula (1):

[0055] y > -0.7079x 2 + 5.3331x + 9.4883 (1)

[0056] (in the formula, x > 0).

[0057] As the fluorite-type ceria-zirconia solid solution powder used to obtain the ceria-zirconia composite oxide of the present application, the following method can be used for the production method of the present application.

[0058] The present application also includes a production method of the aforementioned oxygen absorbing and releasing material (ceria-zirconia composite oxide). The production method of the ceria-zirconia composite oxide of the present application includes a step of preparing a fluorite-type ceria-zirconia solid solution powder containing praseodymium (Pr) or neodymium (Nd) as an additive element (step 1), a step of pressure molding the ceria-zirconia solid solution powder (step 2), and a step of performing a reduction treatment by heat treating the pressure-molded ceria-zirconia solid solution powder compact at 1400°C or higher and less than 1700°C (step 3).

[0059] In step 1, a fluorite-type ceria-zirconia solid solution powder containing Pr or Nd as an additive element is prepared. In the present application, by using a solid solution powder having high uniformity of elements, a ceria-zirconia composite oxide having high heat resistance due to uniform arrangement of elements and stabilization of crystal structure can be obtained. Furthermore, the additive element of the ceria-zirconia composite oxide of the present application is the same as that of the fluorite-type ceria-zirconia solid solution powder, and in the case where 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 in the case where 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.

[0060] In the present application, Ce 0.5 Zr 0.5The proportion of the O2 crystal phase to the entire crystal is defined as "elemental dispersity", and this is used as an index of element uniformity. Here, the cerium dioxide-zirconium dioxide-based solid solution powder of the present application contains praseodymium (Pr) or neodymium (Nd) as an additive element, and since praseodymium ions or neodymium ions replace part of the cerium ions in the crystal, the so-called "Ce 0.5 Zr 0.5 O2 crystal" means, more precisely, a (Ce + Pr or Nd) to Zr ratio of 1 : 1 (Ce 0.5 Zr 0.5 O2 crystal.

[0061] If the fluorite-type cerium dioxide-zirconium dioxide-based solid solution powder is subjected to heat treatment at a high temperature (for example, 1100°C or higher), the crystal structure of Ce 0.5 Zr 0.5 O2 is phase-separated into crystal structures of Ce-rich or Zr-rich composition. In the cerium dioxide-zirconium dioxide-based solid solution powder, it is expected that if there are unstable portions in which the additive element praseodymium or neodymium is segregated in the crystal, the arrangement of cerium and zirconium is not uniform, and the like, the proportion of phase separation increases. Therefore, it is considered that the Ce 0.5 Zr 0.5 O2 crystal that remains without phase separation after the fluorite-type cerium dioxide-zirconium dioxide-based solid solution powder is heated at a temperature of 1200°C in the atmosphere for 5 hours is high. When the proportion of the O2 crystal phase to the entire crystal is high, the elements are arranged more uniformly, the resulting cerium dioxide-zirconium dioxide-based composite oxide is less likely to deteriorate into a fluorite-type crystal structure, and deterioration can be suppressed.

[0062] The elemental dispersity after heat treatment at 1200°C is determined by measuring the X-ray diffraction pattern obtained by subjecting the fluorite-type cerium dioxide-zirconium dioxide-based solid solution powder to heat treatment under the above conditions using CuKa, calculating the areas of the peaks detected at 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 O2 crystal) in the X-ray diffraction pattern, and calculating the proportion of the peak area at 29.3° among all the peak areas.

[0063] The fluorite-type cerium dioxide-zirconium dioxide-based solid solution powder (containing Pr or Nd as an additive element) used in the production of the cerium dioxide-zirconium dioxide-based composite oxide of the present application, when heated at a temperature of 1200°C in the atmosphere for 5 hours, has a Ce 0.5 Zr 0.5the proportion y (%) of the O2 crystal phase with respect to the entire crystal, and the content x (wt%) of praseodymium oxide (Pr6O 11 ) or neodymium oxide (Nd2O3) in the solid solution powder satisfies the following formula (1):

[0064] y > -0.7079x 2 + 5.3331x + 9.4883 (1)

[0065] (in the formula, x > 0).

[0066] The ceria-zirconia-based 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 more uniformly arranged, and therefore, when the solid solution powder is used, the crystal structure is stabilized, and a ceria-zirconia-based composite oxide having high heat resistance can be obtained.

[0067] In the above formula (1), x > 0, and from the viewpoint of obtaining a high oxygen absorption and release capacity, x is preferably 0.6 or greater. From the viewpoint of balancing heat resistance and OSC performance, x is preferably 6 or less, and more preferably 3 or less. In a preferred embodiment, 0 < x < 6, and preferably 0.6 < x < 3.

[0068] The degree of element dispersion after heat treatment (y in formula (1)) is preferably 19% or greater, more preferably 22% or greater, and particularly preferably 25% or greater. If the degree of element dispersion after heat treatment is 19% or greater, the ceria-zirconia-based composite oxide obtained has high heat resistance.

[0069] In one embodiment, when the added element is Pr, the degree of element dispersion after heat treatment is preferably 19% or greater, more preferably 22% or greater, and particularly preferably 25% or greater. In one embodiment, when the content x (wt%) of praseodymium oxide (Pr6O 11 ) is in the range of 0 < x < 1, the degree of element dispersion after heat treatment is preferably 14% or greater, and more preferably 15% or greater. In another embodiment, when the content x (wt%) of praseodymium oxide (Pr6O 11 ) is in the range of 1 < x < 2, the degree of element dispersion after heat treatment is preferably 17% or greater, and more preferably 18% or greater. In another embodiment, when the content x (wt%) of praseodymium oxide (Pr6O 11 ) is in the range of 2 < x < 3, the degree of element dispersion after heat treatment is preferably 19% or greater, and more preferably 20% or greater. Furthermore, in these embodiments, x and y satisfy formula (1).

[0070] In one embodiment, when the additive element is Nd, the element dispersion degree after the heat treatment is preferably 21% or greater, more preferably 24% or greater, and particularly preferably 27% or greater. In one embodiment, when the content x (wt%) of neodymium oxide (Nd2O3) is in the range of 0 < x ≤ 1, the element dispersion degree after the heat treatment is preferably 16% or greater, and more preferably 17% or greater. In another embodiment, when the content x (wt%) of neodymium oxide (Nd2O3) is in the range of 1 ≤ x ≤ 2, the element dispersion degree after the heat treatment is preferably 19% or greater, and more preferably 20% or greater. In another embodiment, when the content x (wt%) of neodymium oxide (Nd2O3) is in the range of 2 ≤ x ≤ 3, the element dispersion degree after the heat treatment is preferably 21% or greater, and more preferably 22% or greater. Furthermore, in these embodiments, x and y satisfy formula (1).

[0071] The ceria-zirconia-based solid solution powder can be produced, for example, by a so-called coprecipitation method. In the coprecipitation method, for example, a mixed solution of raw compound is obtained using 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), the mixed solution of raw compound is mixed with an aqueous solution of an alkaline compound and stirred, thereby generating a coprecipitate. Further, for example, the obtained coprecipitate is filtered, washed, and dried, and is further calcined, and, if necessary, is pulverized using a pulverizer such as a ball mill, thereby obtaining a ceria-zirconia-based solid solution powder. In the mixed solution of raw compound, a salt of at least one additional element selected from rare earth elements and alkaline earth elements, a surfactant (e.g., a nonionic surfactant), or the like can be added as necessary.

[0072] The production method of the present application can make the element dispersion degree fall within the target range by controlling the stirring conditions when the mixed solution of the mixed solution of raw compound and the aqueous solution of alkaline compound (hereinafter also referred to as reaction liquid) is stirred and adjusting the amount of alkaline compound used in the coprecipitation method.

[0073] The alkaline compound is not particularly limited, and for example, ammonia water, ammonium carbonate, sodium hydroxide, potassium hydroxide, sodium carbonate, and the like can be given, of which, from the viewpoint of removal by volatilization when the precipitate is calcined and the composite oxide is calcined, ammonia water and ammonium carbonate are preferred, and ammonia water is more preferred.

[0074] The amount of alkaline compound used in the coprecipitation method is generally more than 1 equivalent, and is preferably 1.5 equivalents or greater, relative to the total amount (moles) of cerium salt, zirconium salt, praseodymium salt, or neodymium salt. By using such an excess amount of ammonia, the dispersibility of elements becomes higher, and a solid solution powder having a high element dispersion degree can be obtained.

[0075] In the coprecipitation method, the mixing of the mixed solution of the raw material compounds and the aqueous solution of the basic compound can be performed, for example, by adding or dropping the mixed solution of the raw material compounds to the aqueous solution of the basic compound, or by adding or dropping the aqueous solution of the basic compound to the mixed solution of the raw material compounds.

[0076] The stirring of the mixed solution of the mixed solution of the raw material compounds and the aqueous solution of the basic compound (reaction liquid) can be performed in a manner such that a cerium-zirconium-based solid solution powder having an element dispersity satisfying the above formula (1) is obtained after the heat treatment. Such a stirring condition can be determined by the stirring Reynolds number Re, which is preferably 5000 or more, and more preferably 10000 or more. Further, if the stirring Reynolds number Re is 25000 or more, a solid solution powder having a high element dispersity with a more uniform element arrangement can be obtained. Furthermore, the upper limit of the stirring Reynolds number Re varies depending on the type, scale, and the like of the stirring device, and is, for example, about 25000 in the case of a general laboratory scale, but can be more than 25000 as long as the effects of the present application are not hindered.

[0077] The stirring Reynolds number Re is calculated by the following formula.

[0078] Re = (p x n x d 2 ) / μ

[0079] (In the formula, p represents the density of the reaction liquid at 25°C (kg / m 3 ), n represents the stirring speed (rps), d represents the stirring blade span (wing diameter) (m), and μ represents the viscosity of the reaction liquid at 25°C (Pa-s))

[0080] The stirring of the reaction liquid can be performed by a publicly known or commercially available stirring device. For example, a kneader, a mixer, a rotating vessel stirrer, a stirred reaction tank, a V-type stirrer, a double-cone type stirrer, a screw stirrer, a sigma mixer, a rapid mixer, an air flow stirrer, a ball mill, a roll edge machine, a forced thin film type microreactor, or the like can be used, and from the viewpoint of the stirring force, a forced thin film type microreactor is preferably used. In addition, the stirring of the reaction liquid can be performed in a micro reaction field.

[0081] The shape of the stirring blade is not particularly limited, and, for example, a propeller shape, a turbine shape, a fan turbine shape, a paddle shape, an inclined paddle shape, a gate shape can be used. In addition, the stirring blades of these shapes can be combined in multiple stages.

[0082] The stirring speed is preferably stirred to a degree that the stirring blade does not protrude due to the vortex generated by the stirring. In addition, in order to suppress the vortex generated by the stirring, a cylindrical tank, a square tank, or a tank provided with a baffle, or the like can be preferably used.

[0083] In the production method of the present application, for example, the stirring Reynolds number Re can be adjusted by setting the optimum stirring rotation speed and stirring blade span in accordance with the relationship with the physical properties of the reaction liquid. 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, the density of the reaction liquid is in the range of 1 kg / m 3 2 kg / m 3 , and the viscosity of the reaction liquid is in the range of 0.001 Pa-s to 0.01 Pa-s, and can be appropriately set from the above ranges in accordance with the kind of the device used, etc.

[0084] The drying of the co-precipitate can be performed under conditions capable of sufficiently removing the solvent, and can be generally performed under conditions of 100°C to 120°C and 5 hours to 24 hours. In addition, the firing of the co-precipitate can be generally performed under conditions of 400°C to 600°C and 2 hours to 10 hours.

[0085] In Step 2, the ceria-zirconia-based solid solution powder prepared in Step 1 is pressure-molded to obtain a solid solution powder molded body. The pressure-molding is generally performed under a pressure of 1500 kgf / cm 2 4000 kgf / cm 2 , and preferably under a pressure of 2500 kgf / cm 2 3500 kgf / cm 2 . If the pressure at the time of pressure-molding is in this range, a high oxygen absorption and desorption capacity and a high heat resistance can be obtained in the ceria-zirconia-based composite oxide obtained. The method of pressure-molding is not particularly limited, and a known pressure-molding method such as hydrostatic pressing can be appropriately used.

[0086] In Step 3, the ceria-zirconia-based solid solution powder molded body after pressure-molding obtained in Step 2 is subjected to heat treatment at a temperature of 1400°C or higher and less than 1700°C to perform reduction treatment. By this reduction treatment, the fluorite-type ceria-zirconia solid solution is converted into a ceria-zirconia-based composite oxide having an ordered phase κ phase and / or a pyrochlore phase. The heat treatment temperature is preferably 1400°C or higher and 1650°C or lower, and more preferably 1500°C or higher and 1600°C or lower. If the heat treatment temperature is in this range, an ordered phase can be sufficiently generated, a high heat resistance can be obtained, and in addition, the proportion of primary particles having a particle size of 0.4 μm to 1.5 μm can be controlled within a target range. The heat treatment temperature is selected so that the proportion of primary particles having a particle size of 0.4 μm to 1.5 μm, the I(14 / 29) value, and the I(28 / 29) value in the ceria-zirconia-based composite oxide obtained fall within the predetermined ranges of the present application.

[0087] The heat treatment is usually performed for 0.5 hours to 24 hours, preferably for 1 hour to 10 hours. If the heat treatment is performed for a time within this range, the phase separation into a disordered phase can be controlled and the ordered phase can be sufficiently produced.

[0088] The reduction treatment can be performed by a method capable of performing a heat treatment of the solid solution powder compact in a reducing atmosphere at a predetermined temperature condition, and is not particularly limited. For example, the reduction treatment can be performed by (i) a method in which the solid solution powder compact is placed in a vacuum heating furnace, a reducing gas is introduced into the furnace after the vacuum is applied, the atmosphere in the furnace is made a reducing atmosphere, and the heat treatment is performed at a predetermined temperature condition; (ii) a method in which a graphite furnace is used, the solid solution powder compact is placed in the furnace, the heat treatment is performed at a predetermined temperature condition after the vacuum is applied, and the atmosphere in the furnace is made a reducing atmosphere by using a reducing gas such as CO or HC generated from the furnace body or a heating fuel; or (iii) a method in which the solid solution powder compact is placed in a crucible filled with activated carbon, the heat treatment is performed at a predetermined temperature condition, and the atmosphere in the crucible is made a reducing atmosphere by using a reducing gas such as CO or HC generated from the activated carbon.

[0089] The production method of the present application can further include a step of performing an oxidation treatment on the ceria-zirconia-based composite oxide after the step 3. By performing the oxidation treatment, oxygen lost during the reduction can be supplemented in the resulting ceria-zirconia-based composite oxide, and the stability as an oxide powder can be improved.

[0090] The oxidation treatment is not particularly limited, and can be performed by, for example, performing a heat treatment of the ceria-zirconia-based composite oxide in an oxidizing atmosphere (e.g., air). In addition, the heating temperature during the oxidation treatment is not particularly limited, and is preferably around 300°C to 800°C. Further, the heating time during the oxidation treatment is not particularly limited, and is preferably around 0.5 hours to 10 hours.

[0091] The production method of the present application can further include a step of performing a pulverization treatment on the ceria-zirconia-based composite oxide after the reduction treatment step or the oxidation treatment step of the step 3. By performing the pulverization treatment, the handling when the resulting ceria-zirconia-based composite oxide is used for an exhaust gas purification catalyst and coated on a substrate such as a cordierite honeycomb becomes easy. The pulverization treatment is not particularly limited, and can be performed by, for example, a wet pulverization method, a dry pulverization method, a freeze pulverization method, or the like.

[0092] The ceria-zirconia-based composite oxide of the present application has a high oxygen absorption and desorption capacity at a low temperature (e.g., 300°C to 500°C) and a high heat resistance, and thus can be preferably used as an oxygen absorption and desorption material for an exhaust gas purification catalyst.

[0093] [Examples]

[0094] The present application will be described more specifically using examples below. However, the technical scope of the present application is not limited to these examples.

[0095] [Preparation of oxygen absorbing and releasing material]

[0096] A cerium oxide-zirconium oxide-praseodymium oxide composite oxide of 51.3 wt% -CeO2, 45.6 wt% -ZrO2, 2.9 wt% -Pr6O 11 was prepared as follows.

[0097] Example 1

[0098] An aqueous solution of diammonium cerous nitrate, an aqueous solution of praseodymium nitrate, and an aqueous solution of zirconyl nitrate were mixed, and the mixed solution was added dropwise to an excess (1.5 equivalents) of aqueous ammonia, and high-speed stirring was performed (stirring was performed at 10,000 rpm using a stirring reaction tank). Thus, a coprecipitate having a uniform element distribution was produced. The resulting coprecipitate was filtered and washed (with ion exchange water). Next, the resulting coprecipitate was dried at 110°C for 10 hours or more, and then calcined at 400°C in the atmosphere for 5 hours. Thus, a solid solution of cerium, zirconium, and praseodymium (fluorite type) (a cerium oxide-zirconium oxide-praseodymium oxide (CeO2-ZrO2-Pr6O 11 ) solid solution, hereinafter also referred to as a CZP solid solution) powder was obtained.

[0099] Next, the resulting cerium oxide-zirconium oxide-praseodymium oxide solid solution powder was charged into a polyethylene bag, and the inside was degassed, and then the mouth of the bag was heat-sealed. Next, using a hydrostatic pressure molding device, the bag was subjected to hydrostatic pressure molding (CIP) at a pressure (molding pressure) of 3,000 kgf / cm 2 Next, the resulting molded body was disposed in a crucible filled with activated carbon, and after the lid was put on, it was placed in a high-speed heating electric furnace, heated to 1,000°C at a temperature increase time of 1 hour, heated to 1,600°C (reduction treatment temperature) at a temperature increase time of 4 hours, maintained for 5 hours, and then cooled to 1,000°C at a cooling time of 4 hours, and then cooled to room temperature by natural cooling. Thus, a reduction treatment product was obtained. Next, the resulting reduction treatment product was oxidized by heating at a temperature condition of 500°C in the atmosphere for 5 hours. Thus, a cerium oxide-zirconium oxide-praseodymium oxide composite oxide (hereinafter also referred to as a CZP composite oxide) was obtained.

[0100] Example 2

[0101] A cerium oxide-zirconium oxide-praseodymium oxide composite oxide of Example 2 was prepared in the same manner as in Example 1, except that the calcination temperature in the high-speed heating electric furnace was set to 1,550°C.

[0102] Example 3

[0103] Example 3 was produced in the same manner as Example 1 except that the firing temperature in the high-speed temperature raising electric furnace was set to 1500°C.

[0104] Example 4

[0105] An aqueous cerium diammonium nitrate solution, an aqueous praseodymium nitrate solution, and an aqueous zirconyl nitrate solution were mixed, and the mixed solution was reacted with an excess (1.5 equivalents) of aqueous ammonia in a micro reaction field while being stirred at high speed (using a forced thin film type micro reactor manufactured by M Technique Co.) to produce a co-precipitate having a uniform element distribution. The resulting co-precipitate was filtered and washed (with ion exchange water). Next, the resulting co-precipitate was dried at 110°C for 10 hours or more, and then fired at 400°C in the air for 5 hours to produce a solid solution (fluorite type) of cerium, zirconium, and praseodymium (CeO2-ZrO2-Pr6O 11 solid solution) powder.

[0106] Next, the resulting cerium dioxide-zirconium dioxide-praseodymium oxide solid solution powder was packed in a polyethylene bag, degassed inside the bag, and then the mouth of the bag was heat-sealed. Next, the bag was molded by hydrostatic pressure compression (CIP) using a hydrostatic pressure compression device at a pressure (molding pressure) of 3000 kgf / cm 2 Next, the resulting molded body was disposed in a crucible filled with activated carbon, and after the crucible was covered with a lid, the resulting molded body was heated in a high-speed temperature raising electric furnace at a temperature raising time of 1 hour to 1000°C, at a temperature raising time of 4 hours to a reduction treatment temperature of 1600°C, and maintained for 5 hours, and then cooled to 1000°C at a cooling time of 4 hours, and then cooled to room temperature by natural cooling to obtain a reduction treatment product. Next, the resulting reduction treatment product was oxidized by heating at a temperature of 500°C in the air for 5 hours to obtain a cerium dioxide-zirconium dioxide-praseodymium oxide composite oxide.

[0107] Example 5

[0108] Example 5 was produced in the same manner as Example 4 except that the firing temperature in the high-speed temperature raising electric furnace was set to 1500°C.

[0109] Example 6

[0110] The cerium diammonium nitrate aqueous solution, the praseodymium nitrate aqueous solution, and the zirconyl nitrate aqueous solution were mixed, and the mixed solution was added dropwise to excess (1.5 equivalents) of ammonia water while stirring at high speed (using a stirring reaction tank, stirring at 5,000 rpm). Thus, a coprecipitate having a uniform element distribution was produced. The resulting coprecipitate was filtered and washed (with ion exchange water). Next, the resulting coprecipitate was dried at 110°C for 10 hours or more, and then calcined at 400°C in air for 5 hours. Thus, a solid solution (fluorite type) of cerium, zirconium, and praseodymium (CeO2-ZrO2-Pr6O 11 solid solution) powder was obtained.

[0111] Next, the resulting ceria-zirconia-praseodymia solid solution powder was charged into a polyethylene bag, degassed inside the bag, and then the mouth of the bag was heat-sealed. Next, the bag was molded by static pressure pressing (CIP) using a static pressure pressing device at a pressure (molding pressure) of 3,000 kgf / cm 2 Next, the resulting molded body was placed in a crucible filled with activated carbon, covered with a lid, and then placed in a high-temperature electric furnace. The temperature was raised to 1,000°C over a period of 1 hour, and then raised to a reduction treatment temperature of 1,600°C over a period of 4 hours. The temperature was maintained at 1,600°C for 5 hours, and then the temperature was lowered to 1,000°C over a period of 4 hours. The temperature was then lowered to room temperature by natural cooling. Thus, a reduction treatment product was obtained. Next, the resulting reduction treatment product was oxidized by heating at a temperature of 500°C in air for 5 hours. Thus, a ceria-zirconia-praseodymia composite oxide was obtained.

[0112] Example 7

[0113] A ceria-zirconia-praseodymia composite oxide of Example 7 was produced in the same manner as in Example 6, except that the calcination temperature in the high-temperature electric furnace was set to 1,550°C.

[0114] Comparative Example 1

[0115] The cerium diammonium nitrate aqueous solution, the praseodymium nitrate aqueous solution, and the zirconyl nitrate aqueous solution were mixed, and the mixed solution was added dropwise to 1 equivalent of ammonia water while stirring (using a stirrer, stirring at 500 rpm). Thus, a coprecipitate was produced. The resulting coprecipitate was filtered and washed (with ion exchange water). Next, the resulting coprecipitate was dried at 110°C for 10 hours or more, and then calcined at 400°C in air for 5 hours. Thus, a solid solution (fluorite type) of cerium, zirconium, and praseodymium (CeO2-ZrO2-Pr6O 11 solid solution) powder was obtained.

[0116] Next, the obtained ceria-zirconia-praseodymium solid solution powder was packed in a polyethylene bag, degassed inside the bag, and the mouth of the bag was heat-sealed. Next, the bag was molded by hydrostatic pressure compression (CIP) at a pressure (molding pressure) of 3000 kgf / cm 2 for 2 minutes using a hydrostatic pressure compression device to obtain a molded body of ceria-zirconia-praseodymium solid solution powder. Next, the obtained molded body was placed in a crucible filled with activated carbon, covered with a lid, and placed in a high-speed heating furnace. The temperature was raised to 1000°C over 1 hour, then to 1700°C (reduction treatment temperature) over 4 hours, and held for 5 hours. Then, the temperature was lowered to 1000°C over 4 hours, and the sample was allowed to cool to room temperature by natural cooling to obtain a reduction treatment product. Next, the obtained reduction treatment product was oxidized by heating at 500°C for 5 hours in the atmosphere to obtain a ceria-zirconia-praseodymium composite oxide.

[0117] Comparative Example 2

[0118] A ceria-zirconia-praseodymium composite oxide of Comparative Example 2 was produced in the same manner as in Comparative Example 1, except that the firing temperature in the high-speed heating furnace was set to 1600°C.

[0119] Comparative Example 3

[0120] A ceria-zirconia-praseodymium composite oxide of Comparative Example 3 was produced in the same manner as in Comparative Example 1, except that the firing temperature in the high-speed heating furnace was set to 1500°C.

[0121] Comparative Example 4

[0122] A ceria-zirconia-praseodymium composite oxide of Comparative Example 4 was produced in the same manner as in Example 1, except that the firing temperature in the high-speed heating furnace was set to 1700°C.

[0123] Comparative Example 5

[0124] An aqueous solution of diammonium cerous nitrate, an aqueous solution of praseodymium nitrate, and an aqueous solution of zirconyl nitrate were mixed, and the mixed solution was added dropwise to 1 equivalent of aqueous ammonia while stirring at high speed (stirring was performed at 5000 rpm using a SA reactor) to produce a coprecipitate. The obtained coprecipitate was filtered and washed (with ion exchange water). Next, the obtained coprecipitate was dried at 110°C for 10 hours or more, and then fired at 400°C in the atmosphere for 5 hours to obtain a cerium-zirconium-praseodymium solid solution (fluorite type) (CeO2-ZrO2-Pr6O 11 solid solution) powder.

[0125] Next, the obtained ceria-zirconia-praseodymium oxide solid solution powder was charged into a polyethylene bag, degassed inside, and the mouth of the bag was heat-sealed. Next, the bag was subjected to hydrostatic pressing (CIP) at a pressure (molding pressure) of 3000 kgf / cm 2 for 2 minutes using a hydrostatic pressing device to form a molded body of the ceria-zirconia-praseodymium oxide solid solution powder. Next, the obtained molded body was placed in a crucible filled with activated carbon, covered with a lid, and placed in a high-speed heating electric furnace. The temperature was raised to 1000°C over 1 hour, then to 1600°C (reduction treatment temperature) over 4 hours, and maintained for 5 hours. Then, the temperature was lowered to 1000°C over 4 hours, and the sample was allowed to cool to room temperature by natural cooling to obtain a reduction treatment product. Next, the obtained reduction treatment product was oxidized by heating at 500°C for 5 hours in the atmosphere to obtain a ceria-zirconia-praseodymium oxide composite oxide.

[0126] Comparative Example 6

[0127] A ceria-zirconia-praseodymium oxide composite oxide of Comparative Example 6 was produced in the same manner as in Comparative Example 5, except that the firing temperature in the high-speed heating electric furnace was set to 1500°C.

[0128] 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 the oxygen absorption and release capacity of the catalyst samples was evaluated after durability testing.

[0129] <Preparation of Catalyst Samples>

[0130] The CZP composite oxide and a carrier (Al2O3) supporting a noble metal (Rh) were mixed with a blender, and the mixed powder was pressurized at a pressure of 1 t to consolidate the powder to obtain a press-molded body. The press-molded body was pulverized with a mortar to obtain a granular catalyst having a size of about 1 to 2 mm square.

[0131] <Durability Test>

[0132] The durability test was performed under the following conditions.

[0133] Temperature: 1100°C x 5 hours

[0134] Gas atmosphere: (Alternated every 5 minutes), H2O 10%, N2 balance, 10 L / min

[0135] <oxygen absorption and release capacity>

[0136] The oxygen absorption and release capacity was evaluated under the following conditions.

[0137] Temperature: 400°C

[0138] After the switching, the oxygen absorption and desorption amount (relative to 1 g of the particulate catalyst) was calculated from the amount of CO2 generated for 15 seconds.

[0139] The following evaluations were performed on the CZP composite oxides of Examples 1 to 7 and Comparative Examples 1 to 6.

[0140] <Crystal structure evaluation by X-ray diffraction (XRD)>

[0141] The CZP composite oxides were heat-treated at 1100°C for 5 hours in the atmosphere, and the crystal phase of the treated CZP composite oxides was measured by X-ray diffraction. As the X-ray diffraction apparatus, a product of Rigaku Corporation, trade name "RINT2100" was used, and the X-ray diffraction pattern was measured at 40 kV, 30 mA, and 2θ = 2° / minute using CuKα, and the intensity ratio of the diffraction line at 2θ = 14.5° to the diffraction line at 2θ = 29° {I(14 / 29) value} and the intensity ratio of the diffraction line at 2θ = 28.5° to the diffraction line at 2θ = 29° {I(28 / 29) value} were calculated.

[0142] <Particle size distribution measurement>

[0143] The particle size distribution of the primary particles of the CZP composite oxide was measured as follows. Specifically, a product of JEOL Ltd., trade name "JSM-7000F" was used as a scanning electron microscope, and the cross section of the CZP composite oxide was observed at two or more arbitrary points, and the average value of the crystal grain diameter in each observation field obtained (magnification 500 times, area of 240 μm x 240 μm) was calculated on a numerical basis, and the distribution was evaluated. Further, the crystal grain diameter referred to herein means the diameter of the smallest circumscribed circle in the case where the cross section is not circular.

[0144] The element 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.

[0145] <Element dispersity (Ce 0.5 Zr 0.5 O2 crystal ratio) evaluation>

[0146] The fluorite-type CZP solid solution powder was heat-treated at 1200°C for 5 hours in the atmosphere, and the crystal phase after the treatment was measured by X-ray diffraction. As the X-ray diffraction apparatus, a product of Rigaku Corporation, trade name "Smart Lab" was used, and the X-ray diffraction pattern was measured at 45 kV, 200 mA, and 2θ = 1° / minute using CuKα, and the intensity ratio of the diffraction line at 2θ = 29° (Ce 0.6Zr 0.4 O2 crystal), 29.3° (Ce 0.5 Zr 0.5 O2 crystal), and 29.8° (Ce 0.2 Zr 0.8 O2 crystal) were measured, and the proportion of the peak area at 29.3° among the total peak areas was calculated according to the following formula (element dispersibility). Element dispersibility = peak area at 29.3° / (peak area at 29° + peak area at 29.3° + peak area at 29.8°)

[0147] The evaluation results are shown in Table 1. Also, Figure 1 In the present embodiment, for the ceria-zirconia-praseodymium oxide composite oxides of Examples 1 to 7 and Comparative Examples 1 to 6, the proportion of primary particles having a particle size of 0.4 to 1.5 μm, the element dispersibility (Ce 0.5 Zr 0.5 O2 crystal) after heat treatment, and the I (14 / 29) value were shown in relation to each other. Figure 2 In the present embodiment, for the ceria-zirconia-praseodymium oxide composite oxides of Examples 1 to 7 and Comparative Examples 1 to 6, the proportion of primary particles having a particle size of 0.4 to 1.5 μm, the element dispersibility (Ce 0.5 Zr 0.5 O2 crystal) after heat treatment, and the oxygen absorption and desorption amount (400°C) were shown in relation to each other. Also, in the present embodiment, Figure 1 and Figure 2 In the present embodiment, Examples 1 to 7 mean Examples 1 to 7, respectively, and Comparative Examples 1 to 6 mean Comparative Examples 1 to 6, respectively.

[0148] Table 1

[0149]

[0150] As shown in Table 1 and Figure 1 , in the CZP composite oxides of Examples 1 to 7, primary particles having a particle size of 0.4 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, and high heat resistance was exhibited. As shown in Table 1 and Figure 1 , the I (14 / 29) value after the durability test and the element dispersibility (Ce 0.5 Zr 0.5In the case where the proportion of the primary particles having a particle size of 0.4 μm to 1.5 μm is equivalent, there is a tendency that the value of I(14 / 29) becomes higher and the heat resistance becomes higher as the value of the element dispersibility after the heat treatment becomes higher. In addition, it was confirmed that, in the CZP composite oxide of the composition, in order to obtain the primary particles having a particle size of 0.4 μm to 1.5 μm, the CZP composite oxide in which the proportion of the primary particles is 40% or more and the value of I(14 / 29) after the durability test is 0.015 or more is obtained, it is preferable to use the fluorite-type CZP solid solution powder in which the element dispersibility after the heat treatment is 19% or more.

[0151] In addition, as shown in Table 1 and Figure 2 , it was confirmed that the CZP composite oxides of Examples 1 to 7 in which the proportion of the primary particles having a particle size of 0.4 μm to 1.5 μm is 40% or more and the value of I(14 / 29) after the durability test is 0.015 or more have significantly high oxygen absorption and release capacity at a low temperature (400°C).

[0152] <Study of the amount of Pr and element dispersibility>

[0153] The cerium dioxide-zirconium dioxide-praseodymium (CZP) composite oxides of Examples 8 to 10 and the cerium dioxide-zirconium dioxide (CZ) composite oxide of Comparative Example 7 were produced in the same manner as in Example 7 except that the amount of the raw material compound was changed so that the content of praseodymium oxide (Pr6O 11 ) shown in Table 2.

[0154] The element dispersibility was evaluated for the fluorite-type CZP solid solution powder before the reduction treatment in Examples 8 to 10 and the fluorite-type CZ solid solution powder before the reduction treatment in Comparative Example 7 as described above. In addition, the XRD measurement and the particle size distribution measurement were performed for the CZP composite oxides of Examples 8 to 10 and the CZ composite oxide of Comparative Example 7 as described above. The evaluation results of Examples 8 to 10 and Comparative Example 7 are shown together with the evaluation results of Example 7 in Table 2. In addition, Figure 3 indicates the relationship between the content of praseodymium oxide (Pr6O 11 ) in the solid solution powder and the element dispersibility (Ce 0.5 Zr 0.5 O2crystal) after the heat treatment.

[0155] Table 2

[0156]

[0157] As shown in Table 2, the CZP composite oxides of Examples 7-10 all contain more than 40% primary particles with a particle size of 0.4 μm to 1.5 μm, exhibiting high oxygen uptake and desorption capacity at low temperatures (400°C) and an I(14 / 29) value of 0.015 or higher after durability, demonstrating high heat resistance. On the other hand, the CZ composite oxide of Comparative Example 7 without added Pr, while also possessing the characteristics of the proportion of primary particles and the I(14 / 29) value, has a significantly lower oxygen uptake and desorption capacity compared to the CZP composite oxide.

[0158] Since the CZP composite oxides of Examples 7-10 and the CZ composite oxide of Comparative Example 7 were prepared in the same order, it is expected that the elemental uniformity in the solid solution powder obtained during the manufacturing process will be of the same degree. However, as Figure 3 As shown, if the amount of Pr added is different, the Pr6O in the solid solution powder will vary. 11 Different content results in varying elemental dispersions (Ce) even when prepared in the same order and heated to 1200℃. 0.5 Zr 0.5 The proportion of O2 crystals also differs, as does the proportion of Pr6O. 11 When the content decreases, Ce is present. 0.5 Zr 0.5 The proportion of O2 crystals tends to decrease. Furthermore, regarding Pr6O... 11 Ce content of 2.9% by weight after heat treatment 0.5 Zr 0.5 The CZP solid solution powders of Comparative Examples 1-3, containing 15% O2 crystals, also exhibited the same tendency. It is believed that Pr has an effect on suppressing phase separation in fluorite-type CZP solid solutions. Due to the reduction or absence of Pr, even if the elemental dispersion in the solid solution powder before heat treatment is the same, the amount of phase separation increases upon heating at 1200°C. It is speculated that Ce... 0.5 Zr 0.5 The proportion of O2 crystals has decreased.

[0159] Regarding Examples 1-7 and Comparative Examples 1-6, as described above, Pr6O was confirmed. 11 To obtain a CZP composite oxide with a content of 2.9% by weight and a primary particle size of 0.4 μm to 1.5 μm of 40% or more, and an I(14 / 29) value of 0.015 or more after durability testing, fluorite-type CZP solid solution powder with an elemental dispersion of 19% or more after heat treatment is preferably used, even when prepared in the same order and with the same expected elemental uniformity. Furthermore, as mentioned above, even when prepared in the same order and with the same expected elemental uniformity, if the Pr6O in the solid solution powder... 11A decrease in content tends to lead to a decrease in elemental dispersion after heat treatment. Therefore, the preferred fluorite-type CZP solid solution powder used in the manufacture of the CZP composite oxide of the present invention has an elemental dispersion after heat treatment similar to that of Pr6O. 11 Content-related, such as Figure 3 As shown, Ce 0.5 Zr 0.5 The proportion of O2 crystals relative to the overall crystal structure, y (%), and Pr6O 11 The content x (weight%) satisfies the following formula (1):

[0160] y≥-0.7079x 2 +5.3331x+9.4883 (1).

[0161] Here, as shown in Table 2, the oxygen uptake and desorption of the CZ composite oxide without Pr in Comparative Example 7 are significantly lower than those of the CZP composite oxides in Examples 7-10. Therefore, it is shown that in the present invention, it is preferable to use Pr6O that satisfies the above formula (1). 11 Fluorite-type CZP solid solution powder with a content x>0 (i.e., containing Pr). By using such fluorite-type CZP solid solution powder, CZP composite oxides with high elemental uniformity and high heat resistance can be obtained.

[0162] Furthermore, the effect expected to be achieved in CZP composite oxides by adding Pr is due to the presence of tetravalent Ce ions. Crystal structure collapse caused by diffusion within the crystal is due to the presence of trivalent Pr ions with ionic radii larger than Ce within the crystal. This suppression allows the crystal structure to be maintained even after durability. It is hoped that trivalent Nd ions with ionic radii similar to Pr will be developed. The same effect can be obtained in the same way. Therefore, in the cerium dioxide-zirconia composite oxide of the present invention, even when the added element is changed to Nd, the same effect as when the added element is Pr can be expected.

Claims

1. An oxygen absorbing and releasing material comprising a ceria-zirconia-based composite oxide containing ceria (CeO2) and zirconia (ZrO2), the ceria-zirconia-based composite oxide containing praseodymium (Pr) or neodymium (Nd) as an additive element, at least a part of the ceria-zirconia-based composite oxide having at least one ordered phase of a kappa phase and a pyrochlore phase, 40 to 100% of primary particles having a particle diameter of 0.4 to 1.5 μm being present on a particle number basis with respect to all of the primary particles of the ceria-zirconia-based composite oxide, an X-ray diffraction pattern being obtained by X-ray diffraction using CuKa after the ceria-zirconia-based composite oxide is heated in the atmosphere at a temperature of 1100°C for 5 hours, an intensity ratio of a diffraction line at 2θ = 14.5° to a diffraction line at 2θ = 29°, i.e., an I(14 / 29) value, being 0.015 or more and an intensity ratio of a diffraction line at 2θ = 28.5° to a diffraction line at 2θ = 29°, i.e., an I(28 / 29) value, being 0.08 or less according to the X-ray diffraction pattern, and the ceria-zirconia-based composite oxide containing ceria (CeO2) and zirconia (ZrO2) in a ratio y of 50 to 99% and praseodymium (Pr) or neodymium (Nd) in an amount x of 1 to 50% (mass %).

2. The oxygen absorbing and releasing material according to claim 1, wherein the additive element is praseodymium (Pr).

3. A method for producing an oxygen absorbing and releasing material, which is a method for producing the oxygen absorbing and releasing material according to claim 1, comprising: the ceria-zirconia-based composite oxide containing ceria (CeO2) and zirconia (ZrO2) in a ratio y of 50 to 99% and praseodymium (Pr) or neodymium (Nd) in an amount x of 1 to 50% (mass %); a step of performing pressure molding on the ceria-zirconia-based solid solution powder; and a step of performing a reduction treatment by heat treating the ceria-zirconia-based solid solution powder molded body after the pressure molding at 1400°C or higher and less than 1700°C.

4. The method for producing an oxygen absorbing and releasing material according to claim 3, wherein the additive element is praseodymium (Pr). ​ The oxygen absorbing and releasing material is obtained using a fluorite-type ceria-zirconia-based solid solution powder containing praseodymium (Pr) or neodymium (Nd) as an additive element, which, after being heated in the atmosphere at a temperature condition of 1200°C for 5 hours, contains Ce 0.5 Zr 0.5 The proportion y of the O2 crystal phase with respect to the entire crystal and the content x of praseodymium oxide represented by Pr6O 11 or neodymium oxide represented by Nd2O3 in the solid solution powder satisfy the following formula (1): y > -0.7079x 2 + 5.3331x + 9.4883 (1) ​ ​ ​ a process for preparing a solid solution powder of a fluorite-type ceria-zirconia system containing praseodymium (Pr) or neodymium (Nd) as an additive element, which is heated in the atmosphere at a temperature of 1200°C for 5 hours, Ce 0.5 Zr 0.5 the proportion y of the O2 crystal phase with respect to the entire crystal, and the content x of praseodymium oxide represented by Pr6O 11 x of neodymium oxide represented by Nd2O3 in the solid solution powder satisfy the following formula (1): y > -0.7079x 2 + 5.3331x + 9.4883 (1) ​ ​ ​ ​

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