Ceria-zirconia-based composite oxide and method for producing the same, and exhaust gas purification catalyst containing ceria-zirconia-based composite oxide

Abstract

Cerium oxide-zirconium oxide-based composite oxide, characterized in that the cerium oxide-zirconium oxide-based composite oxide comprises a composite oxide containing cerium oxide and zirconia, wherein Primary particles with a particle diameter of 1.5 to 4.5 µm in the cerium oxide-zirconium oxide-based composite oxide constitute at least 50% of all primary particles in the composite oxide on a particle number basis, and the cerium to zirconium content ratio ([Cer]:[Zirconium]) in the cerium oxide-zirconium oxide-based composite oxide is in the range of a molar ratio of 43:57 to 55:45; and The cerium oxide-zirconium oxide-based composite oxide, when heated in the atmosphere for 5 hours under a temperature condition of 1,100°C and subsequently examined by X-ray diffractometry using CuKα, exhibits an X-ray diffraction pattern in which an intensity ratio of the diffraction line at a 2θ of 14.5° to the diffraction line at a 2θ of 29° {I(14 / 29) value} and an intensity ratio of the diffraction line at a 2θ of 28.5° to the diffraction line at a 2θ of 29° {I(28 / 29) value} satisfy the requirement I(14 / 29) ≥ 0.015 and I(28 / 29) ≤ 0.08.

Description

BACKGROUND OF THE INVENTION 1. Field of the invention

[0001] The invention relates to a cerium oxide-zirconium oxide-based composite oxide, a method for producing the composite oxide and an exhaust gas purification catalyst comprising the cerium oxide-zirconium oxide-based composite oxide. 2. Description of the related prior art

[0002] Composite oxides containing various metal oxides are currently used as supports, promoters, and the like for exhaust gas purification catalysts. Since cerium oxide is capable of absorbing and releasing oxygen depending on the partial pressure of oxygen in the atmosphere (exhibiting oxygen storage capacity (OSC)), it is used as a metal oxide in such composite oxides. In recent years, various types of cerium oxide-containing composite oxides have been investigated, and a variety of cerium oxide-zirconium oxide-based composite oxides and processes for their preparation have been disclosed.

[0003] For example, Japanese patent application publication no. 2011-219329 (JP 2011-219329 A) discloses a cerium oxide-zirconium oxide-based composite oxide comprising a composite oxide of cerium oxide and zirconium oxide, and a method for producing the same, wherein the composite oxide is characterized in that the molar ratio of cerium to zirconium ([Cer]:[Zirconium]) is in the range of 43:57 to 48:52, and also in that the intensity ratio between the diffraction line 2θ = 14.5° and the diffraction line 2θ = 29° (I(14 / 29) value) and the intensity ratio between the diffraction line at 2θ = 28.5° and the diffraction line at 2θ = 29° (I(28 / 29) value), which are obtained from an X-ray diffraction pattern using the CuKα line, obtained by X-ray diffractometry (XRD) after 5 hours of heating the composite oxide in the atmosphere at 1,100°C, must be determined if the following conditions are met: I(14 / 29) ≥ 0.015 and I(28 / 29) ≤ 0.08.According to this patent publication, it is possible to provide a cerium oxide-zirconium oxide-based composite oxide that possesses high heat resistance and is capable of exhibiting excellent oxygen storage capacity even after long-term exposure to elevated temperatures. However, since increasingly higher-quality properties are now required for exhaust gas purification catalysts, there has recently been a demand for cerium oxide-zirconium oxide-based composite oxides that, by possessing both sufficiently improved oxygen storage capacity and sufficiently higher heat resistance, are able to exhibit sufficiently improved oxygen storage capacity even after long-term exposure to elevated temperatures.

[0004] Furthermore, International Patent Application No.2006 / 030763 (WO 2006 / 030763 A1) both a cerium oxide-zirconium oxide-based composite oxide, which is a cerium and zirconium-containing composite oxide and is characterized by having (1) an oxygen release initiation temperature of 380°C or less, (2) an oxygen release quantity of 485 µmol / g or more and (3) an oxygen release quantity at 400°C of 15 µmol / g or more; as well as a process for producing cerium oxide-zirconium oxide-based composite oxides, which includes: mixing a cerium source with a zirconium source in a predetermined proportion, melting the resulting starting material mixture at a temperature not below the melting point, then cooling the melt to form a block, subsequently grinding the block as desired to obtain a powder, then relieving internal stresses in the powder crystals by heating, and further grinding the powder.However, the cerium oxide-zirconium oxide-based composite oxide disclosed in WO 2006 / 030763 A1 does not have an adequate effect of inhibiting deterioration of the oxygen-storing material, does not always show sufficient oxygen absorption / release function after long-term exposure to elevated temperatures, and has insufficient stability.

[0005] Furthermore, the published patent application WO 2011 / 129460 A1 discloses a cerium oxide-zirconium oxide-based composite oxide from the prior art. SUMMARY OF THE INVENTION

[0006] This invention was made in view of the problems in the related prior art and provides: a cerium oxide-zirconium oxide-based composite oxide which is equipped with both a sufficiently good oxygen storage capacity and a sufficiently high heat resistance and is thus able to exhibit a sufficiently good oxygen storage capacity even after long-term exposure to elevated temperatures; a method for producing such a composite oxide; and an exhaust gas purification catalyst which includes such a cerium oxide-zirconium oxide-based composite oxide.

[0007] To solve the aforementioned problems, the inventors carried out extensive investigations, as a result of which they found that by using a cerium oxide-zirconium oxide-based composite oxide of a composite oxide of cerium oxide and zirconium oxide that meets the specific conditions of having a relative content of cerium and zirconium in a certain ratio and a high content of particles with a relatively large particle diameter as primary particles, the resulting cerium oxide-zirconium oxide-based composite oxide achieves both a sufficiently good oxygen storage capacity and a sufficiently good heat resistance at a high level and is able to exhibit a sufficiently good oxygen storage capacity even after long-term exposure to elevated temperatures.

[0008] The inventors also discovered that, surprisingly, by adjusting the cerium to zirconium ratio ([Cer]:[Zirconium]) in a cerium oxide-zirconium oxide-based mixed crystal powder in the range of a molar ratio of 43:57 to 55:45, compression molding this cerium oxide-zirconium oxide-based mixed crystal powder at a predetermined high pressure, and subsequently carrying out a two-stage reducing treatment under predetermined temperature conditions, the oxygen storage capacity and the heat resistance of the resulting cerium oxide-zirconium oxide-based composite oxide are both achieved at a high level, and the cerium oxide-zirconium oxide-based composite oxide is able to exhibit a sufficiently good oxygen storage capacity even after long-term exposure to elevated temperatures.

[0009] According to one aspect of the invention, a cerium oxide-zirconium oxide-based composite oxide comprises a composite oxide containing cerium oxide and zirconium oxide, wherein primary particles with a particle diameter of 1.5 to 4.5 µm constitute at least 50% of all primary particles in the cerium oxide-zirconium oxide-based composite oxide on a particle number basis, and the cerium to zirconium content ratio ([Cer]:[Zirconium]) in the cerium oxide-zirconium oxide-based composite oxide is in the range of a molar ratio of 43:57 to 55:45; and the cerium oxide-zirconium oxide-based composite oxide, when exposed to the atmosphere for 5 hours at a temperature of 1.When heated to 100°C and subsequently examined by X-ray diffractometry using CuKα, the X-ray diffraction pattern exhibits an intensity ratio of the diffraction line at a 2θ of 14.5° to the diffraction line at a 2θ of 29° {I(14 / 29) value} and an intensity ratio of the diffraction line at a 2θ of 28.5° to the diffraction line at a 2θ of 29° {I(28 / 29) value} that meets the requirements I(14 / 29) ≥ 0.015 and I(28 / 29) ≤ 0.08.

[0010] A method for producing the cerium oxide-zirconium oxide-based composite oxide, which includes a cerium oxide and zirconium oxide-containing composite oxide according to one aspect of the invention, comprises: providing a cerium oxide-zirconium oxide-based mixed crystal powder with a cerium to zirconium content ratio ([Cer]:[Zirconium]) in the range of a molar ratio of 43:57 to 55:45; and compression molding the cerium oxide-zirconium oxide-based mixed crystal powder at a pressure of 1,500 to 3,500 kgf / cm². 2; Subjecting a cerium oxide-zirconium oxide-based mixed crystal powder pellet to a first reducing treatment, which includes 0.5 to 24 hours of heat treatment under reducing conditions at a temperature between 1,400 and 1,550°C; and subjecting the first-reduced cerium oxide-zirconium oxide-based mixed crystal powder pellet to a second reducing treatment, which includes 0.5 to 5 hours of heat treatment under reducing conditions at a temperature between 1,600 and 2,000°C and at least 100°C higher than the temperature in the first reducing treatment, such that the cerium oxide-zirconium oxide-based composite oxide is obtained according to one aspect of the invention.

[0011] An exhaust gas purification catalyst may contain the aforementioned cerium oxide-zirconium oxide-based composite.

[0012] In the aforementioned cerium oxide-zirconium oxide-based composite oxide, it can be ensured that primary particles with a particle diameter of less than 1.5 µm do not constitute more than 40% of all primary particles in the composite oxide on a particle number basis.

[0013] In the aforementioned cerium oxide-zirconium oxide-based composite oxide, it can be ensured that primary particles with a particle diameter of less than 1.5 µm do not constitute more than 20% of all primary particles in the composite oxide on a particle number basis.

[0014] In the aforementioned cerium oxide-zirconium oxide-based composite oxide, it can be ensured that primary particles with a particle diameter of less than 1.5 µm do not constitute more than 10% of all primary particles in the composite oxide on a particle number basis.

[0015] In the aforementioned cerium oxide-zirconium oxide-based composite oxide, it can be ensured that primary particles with a particle diameter of 1.5 to 4.5 µm constitute at least 70% of all primary particles in the composite oxide on a particle number basis.

[0016] In the aforementioned cerium oxide-zirconium oxide-based composite oxide, it can be ensured that primary particles with a particle diameter of 1.5 to 4.5 µm constitute at least 80% of all primary particles in the composite oxide on a particle number basis.

[0017] The above method for producing the cerium oxide-zirconium oxide-based composite oxide may further include subjecting the cerium oxide-zirconium oxide-based composite oxide to an oxidizing treatment following the second reducing treatment.

[0018] The I(14 / 29) and I(28 / 29) values ​​are, in one aspect of the invention, the intensity ratio between the diffraction line at 2θ = 14.5° and the diffraction line at 2θ = 29° or the intensity ratio between the diffraction line at 2θ = 28.5° and the diffraction line at 2θ = 29°, which is determined from an X-ray diffraction pattern using the CuKα line obtained by X-ray diffractometry after heating the cerium oxide-zirconium oxide-based composite oxide in the atmosphere at 1,100°C for 5 hours. X-ray diffractometry can be performed by measurement with an X-ray diffractometer (RINT 2100, Rigaku Corporation) using the CuKα line under the following conditions: 40 kV, 30 mA, 2θ = 2° / min.

[0019] Here, the diffraction line at 2θ = 14.5° is a diffraction line that can be assigned to the (111) plane of the ordered phase (κ-phase), and the diffraction line at 2θ = 29° is the location of an intersection between the diffraction line that can be assigned to the (222) plane of the ordered phase and the diffraction line that can be assigned to the (111) plane of the cubic phase of a cerium oxide-zirconium oxide solid solution (CZ solid solution). Therefore, by calculating the intensity ratio between the two diffraction lines, here referred to as the I(14 / 29) value, an indicator of the percentage retention (frequency) of the ordered phase is obtained. When determining the intensities of the diffraction lines, the average diffraction line intensity for the 2θ range of 10° to 12° is subtracted from each diffraction line intensity as a background value.Furthermore, fully ordered phases include the κ-phase (Ce2Zr2O8), in which all oxygen sites are occupied, and the pyrochlore phase (Ce2Zr2O7), in which all oxygen sites are free; the I(14 / 29) value for the κ-phase and the I(14 / 29) value for the pyrochlore phase, calculated from the corresponding pair distribution function (PDF) cards (PDF-2: 01-070-4048 for the κ-phase and PDF-2: 01-075-2694 for the pyrochlore phase), is 0.04 and 0.05, respectively. The crystalline phase with an ordered phase, i.e., an ordered arrangement formed from cerium ions and zirconium ions, is an arrangement of crystals with peaks at 2θ angles of 14.5°, 28°, 37°, 44.5° and 51° (ϕ' phase (same phase as κ phase) of the ordered arrangement phase type: a superlattice structure occurring in a fluorite structure) in the X-ray diffraction pattern using the CuKα line obtained by X-ray diffractometry.Here, "peak" refers to a peak with a height of at least 30 cps from the baseline to the top.

[0020] The diffraction line at 2θ = 28.5° is a diffraction line that can be attributed solely to the (111) plane of CeO2. By calculating the I(28 / 29) value, which is the intensity ratio between the diffraction line at 2θ = 28.5° and the diffraction line at 2θ = 29°, an indicator of the degree to which CeO2 is present as a separate phase in the composite oxide is obtained.

[0021] Although it is not entirely clear why the aforementioned problems are solved with cerium oxide-zirconium oxide-based composite oxides obtained by the manufacturing process according to one aspect of the invention, the inventors believe the reasons are as follows. Firstly, in one aspect of the invention, by ensuring that primary particles with a particle diameter of 1.5 to 4.5 µm constitute at least 50% of all primary particles in the cerium oxide-zirconium oxide-based composite oxide on a particle number basis, the proportion of primary particles with a relatively large particle diameter can be increased, and a uniform particle size distribution can be obtained, while simultaneously preventing an increase in the surface area.For example, it is hypothesized that even upon exposure to an oxidizing high-temperature atmosphere, a phase transition from the pyrochlore phase (an ordered phase) to a cubic phase (fluorite structure) can be prevented, thus preventing a decrease in oxygen storage capacity (OSC). Furthermore, the inventors believe that because the cerium to zirconium ratio in the composite oxide ([Cer]:[Zirconium]) is in the range of a molar ratio of 43:57 to 55:45, the effect of increasing the stability of the composite oxide, achieved by preventing the phase separation of cerium oxide, outweighs the decrease in oxygen storage capacity due to the zirconium-rich composition described above. This results in the composite oxide possessing increased heat resistance and exhibiting sufficiently good oxygen storage capacity even after long-term exposure to elevated temperatures.

[0022] In one aspect of the invention, a cerium oxide-zirconium oxide mixed crystal powder, in which the ratio of cerium to zirconium ([Cer]:[Zirconium]) is in the range of a molar ratio of 43:57 to 55:45, is produced at a pressure of 1,500 to 3,500 kgf / cm². 2The material is compression-pressed and then subjected to a first reducing treatment, which includes 0.5 to 24 hours of heat treatment under reducing conditions at a temperature between 1,400 and 1,550°C, followed by a second reducing treatment, which includes 0.5 to 5 hours of heat treatment under reducing conditions at a temperature between 1,600 and 2,000°C, at least 100°C higher than in the first reducing treatment. By compression-pressing the unreduced composite oxide at a predetermined high pressure, grain boundaries within the powder are controlled and the contact between particles is increased. This facilitates crystal growth during a reducing treatment and simultaneously promotes ion rearrangement, thus facilitating the formation of an ordered phase.Moreover, because the particles are densely packed, the probability of contact between particles is relatively uniform, and the rate of grain growth is consistent, resulting in greater crystal stability. Consequently, the inventors believe that by subjecting a composite oxide pressed in this manner at a predetermined pressure to a reducing treatment under predetermined temperature and time conditions, excessive grain growth and the transition of particles to the crystalline phase in a subsequent high-temperature resistance test will be completely prevented. This will enhance the stability of the composite oxide by preventing the aforementioned phase separation of cerium oxide and, as a result, maintain a high oxygen storage capacity even during prolonged exposure to elevated temperatures.It also appears that by subjecting the compression-pressed cerium oxide-zirconium oxide-based mixed crystal powder pellet to a two-stage reducing treatment at higher treatment temperatures, the proportion of primary particles present in the cerium oxide-zirconium oxide-based composite oxide, which have a relatively large particle diameter, can be increased, resulting in a composite oxide with a uniform particle size distribution, while preventing an increase in the surface area. This, in turn, presumably allows a phase transformation from the pyrochlore phase (ordered phase) to a cubic phase (fluorite structure) to be prevented, even when the composite oxide has been exposed to an oxidizing high-temperature atmosphere, and enables a decrease in oxygen storage capacity to be prevented.Furthermore, the inventors hypothesize that carrying out a two-stage reducing treatment at specific temperatures and for specific periods has the following effects: First, the initial reducing treatment enables the effective formation of an ordered phase while preventing phase separation of cerium oxide; second, the intermediate reducing treatment promotes grain growth of the primary particles, thus providing the composite oxide with both a sufficiently good oxygen storage capacity (OSC) and a sufficiently high heat resistance, and enabling the composite oxide to maintain a sufficiently good oxygen storage capacity (OSC) even after long-term exposure to elevated temperatures.

[0023] The above aspects of the invention thus enable the provision of a cerium oxide-zirconium oxide-based composite oxide which is equipped with both a sufficiently good oxygen storage capacity (OSC) and a sufficiently high heat resistance and is able to exhibit a sufficiently good oxygen storage capacity (OSC) even after long-term exposure to elevated temperatures, a method for producing such a composite oxide and an exhaust gas purification catalyst which includes this cerium oxide-zirconium oxide-based composite oxide. BRIEF DESCRIPTION OF THE DRAWING

[0024] Features, advantages, and the technical and industrial significance of exemplary embodiments of the invention are described below with reference to the accompanying drawings, in which the same symbols denote the same elements. It shows: Fig. Figure 1 is a table showing the Ce / Zr ratios, the pressure applied in the compression molding step, reduction treatment temperatures and times, particle size distributions of cerium oxide-zirconium oxide-based composite oxides, I(14 / 29) values, I(28 / 29) values ​​and oxygen storage capacity for Example 1 to 8 and Comparative Example 1 to 6. DETAILED DESCRIPTION OF EXECUTION FORMS

[0025] The invention is described in more detail below with reference to preferred embodiments thereof.

[0026] First, the cerium oxide-zirconium oxide-based composite oxide according to an example of the invention is described. The cerium oxide-zirconium oxide-based composite oxide according to the example of the invention is a cerium oxide-zirconium oxide-based composite oxide comprising a composite oxide containing cerium oxide and zirconium oxide. Primary particles with a particle diameter of 1.5 to 4.5 µm constitute at least 50% of all primary particles in the cerium oxide-zirconium oxide-based composite oxide on a particle number basis, and the cerium to zirconium content ratio ([Cer]:[Zirconium]) in the cerium oxide-zirconium oxide-based composite oxide is in the range of a molar ratio of 43:57 to 55:45. When exposed to the atmosphere for 5 hours at a temperature of 1.When heated to 100°C and subsequently examined by X-ray diffractometry using CuKα, the cerium oxide-zirconium oxide-based composite oxide exhibits an X-ray diffraction pattern in which an intensity ratio of the diffraction line at a 2θ of 14.5° to the diffraction line at a 2θ of 29° {I(14 / 29) value} and an intensity ratio of the diffraction line at a 2θ of 28.5° to the diffraction line at a 2θ of 29° {I(28 / 29) value} satisfy the requirements I(14 / 29) ≥ 0.015 and I(28 / 29) ≤ 0.08.

[0027] In the cerium oxide-zirconium oxide-based composite oxide of the example of the invention, it is crucial that primary particles with a particle diameter of 1.5 to 4.5 µm constitute at least 50% of all primary particles in the composite oxide on a particle number basis. If the primary particle content is less than 50% in the aforementioned size range, it is not possible to have a high proportion of primary particles with a relatively large diameter. For example, if the composite oxide has been exposed to an oxidizing high-temperature atmosphere, it will not be sufficiently effective in preventing the phase transition from the pyrochlore phase (ordered phase) to the cubic phase (fluorite structure), and the effect of preventing a decrease in oxygen storage capacity will tend to be inadequate.The particle size distribution of primary particles of the cerium oxide-zirconium oxide-based composite oxide is an average value obtained by using a scanning electron microscope (SEM) to arbitrarily examine cross-sections of CZ composite oxide particles at two or more locations, then determining, on a numerical basis, crystal grain diameters within each obtained field of view and estimating their distribution. In cases where the cross-section is not circular, "crystal grain size" here refers to the diameter of the smallest circle circumscribed around the crystal grain.

[0028] The primary particles with a particle diameter of 1.5 to 4.5 µm in the aforementioned cerium oxide-zirconium oxide-based composite oxide preferably constitute at least 70% of all primary particles in the composite oxide on a particle number basis. This reduces the proportion of particle surfaces exposed to an oxidizing high-temperature atmosphere, thus tending to decrease the proportion of phase transformations of the ordered phase.

[0029] It is particularly preferred that the primary particles, with a particle diameter of 1.5 to 4.5 µm, constitute at least 80% of all primary particles in the aforementioned cerium oxide-zirconium oxide-based composite oxide on a particle number basis. This further reduces the exposed fraction of particle surfaces, thus tending to decrease the proportion of phase transitions.

[0030] In the cerium oxide-zirconium oxide-based composite oxide according to the example of the invention, primary particles with a particle diameter of less than 1.5 µm preferably constitute no more than 40%, more preferably no more than 20%, and most preferably no more than 10% of all primary particles in the composite oxide on a particle number basis. If the content of primary particles in this particle diameter range exceeds the aforementioned limit, the surface area of ​​the particles increases. Consequently, upon exposure to an oxidizing high-temperature atmosphere, for example, the inhibition of phase transformations from the pyrochlore phase (ordered phase) to the cubic phase (fluorite structure) is insufficient, and the inhibition of oxygen storage capacity (OSC) tends to be inadequate.

[0031] The cerium oxide-zirconium oxide-based composite oxide according to the example of the invention has a cerium to zirconium content ratio ([Cer]:[Zirconium]) that must be in the range of a molar ratio of 43:57 to 55:45, preferably in the range of 43:57 to 48:52, and most preferably in the range of 44:56 to 47:53. If the cerium content is below the aforementioned lower limit, the decrease in oxygen storage capacity due to the zirconium-rich composition exceeds the effect of increasing the stability of the composite oxide by preventing phase separation of cerium oxide, resulting in insufficient oxygen storage capacity after a high-temperature resistance test.In contrast, if the cerium content exceeds the aforementioned upper limit, the effect of increasing the stability of the composite oxide due to the inhibition of phase separation of cerium oxide is not achieved, and thus the oxygen storage capacity is inadequate after a high-temperature resistance test.

[0032] Furthermore, the cerium oxide-zirconium oxide-based composite oxide according to the example of the invention must have an I(14 / 29) value of at least 0.015, preferably at least 0.030, and most preferably at least 0.032. If this I(14 / 29) value is below the aforementioned lower limit, the percentage of ordered phase retention will be low, and the oxygen storage capacity after a high-temperature resistance test will be inadequate. Although the I(14 / 29) value does not have a specific upper limit, it is preferably not to exceed 0.05, given that the I(14 / 29) value of the pyrochlore phase, calculated from the PDF chart (01-075-2694), is the upper limit.

[0033] Furthermore, the cerium oxide-zirconium oxide-based composite oxide according to the example of the invention must have an I(28 / 29) value of 0.08 or less, preferably 0.06 or less, and even more preferably 0.04 or less. If this I(28 / 29) value exceeds the aforementioned upper limit, the phase separation of cerium oxide will not be sufficiently prevented, and the oxygen storage capacity after a high-temperature resistance test will be inadequate. Although the I(28 / 29) value does not have a specific lower limit, a smaller value is more preferred.

[0034] In the cerium oxide-zirconium oxide-based composite oxide according to the example of the invention, a crystalline phase (ordered phase, pyrochlore phase) with an ordered arrangement structure is formed from the cerium ions and zirconium ions in the composite oxide. Due to the formation of such an ordered phase, the heat resistance to elevated temperatures increases, and even after exposure to elevated temperatures, a sufficiently high oxygen storage capacity is maintained. Furthermore, in the cerium oxide-zirconium oxide-based composite oxide according to the example of the invention, the content of this ordered phase, based on the total crystalline phase, as determined from the peak intensity ratios in the X-ray diffraction pattern described above, is preferably 50 to 100% and more preferably 80 to 100%.If the ordered phase content is below the lower limit specified above, the effects of preventing deterioration of the oxygen-storing material and the heat resistance of the composite oxide tend to diminish.

[0035] The cerium oxide-zirconium oxide-based composite oxide according to the example of the invention can also include at least one element selected from the group consisting of rare earth elements other than cerium and alkaline earth elements. By including such an element, the inventive cerium oxide-zirconium oxide-based composite oxide tends to exhibit higher exhaust gas purification performance when used as a support for an exhaust gas purification catalyst. Examples of such rare earth elements other than cerium include scandium (Sc), yttrium (Y), lanthanum (La), praseodymium (Pr), neodymium (Nd), samarium (Sm), gadolinium (Gd), terbium (Tb), dysprosium (Dy), ytterbium (Yb), and lutetium (Lu).When a noble metal is supported on the inventive cerium oxide-zirconium oxide-based composite oxide, La, Nd, Pr, Y, and Sc are preferred, with La, Y, and Nd being more preferred, given the tendency for interactions with the noble metal to become stronger and the affinity to increase. Examples of alkaline earth metals include magnesium (Mg), calcium (Ca), strontium (Sr), barium (Ba), and radium (Ra). When a noble metal is supported on the inventive cerium oxide-zirconium oxide-based composite oxide, Mg, Ca, and Ba are preferred, given the tendency for interactions with the noble metal to become stronger and the affinity to increase when a noble metal is supported.Since rare earth elements other than cerium and alkaline earth metals, which all have low electronegativity, interact strongly with precious metals, these elements tend to combine with precious metals through oxygen in an oxidizing atmosphere, thereby preventing evaporation and sintering of the precious metal and making it possible to completely prevent degradation of the precious metal that forms the active sites during exhaust gas purification.

[0036] If at least one element selected from the group consisting of rare earth elements other than cerium and alkaline earth elements is included, the content of such elements is preferably 1 to 20 wt% and more preferably 3 to 7 wt% of the cerium oxide-zirconium oxide-based composite oxide. If the content of such elements is below this lower limit, it tends to be difficult to fully enhance interactions with a noble metal when a precious metal is supported on the resulting composite oxide. Conversely, if the content exceeds the upper limit, the oxygen storage capacity tends to decrease.

[0037] The cerium oxide-zirconium oxide-based composite oxide described above possesses a specific surface area which, although not subject to any particular restrictions, is preferably 0.01 to 20 m². 2 / g and preferably 0.05 to 10 m 2The specific surface area is defined as a certain value per gram. If the specific surface area is below this lower limit, interactions with the precious metal are minimal and the oxygen storage capacity tends to be low. Conversely, if the specific surface area exceeds this upper limit, there is a tendency for the number of particles with a small diameter to increase and the heat resistance to decrease. Such a specific surface area can be calculated from an adsorption isotherm using the BET isothermal adsorption equation as the specific BET surface area.

[0038] Next, an example of a process for producing the cerium oxide-zirconium oxide-based composite oxide according to the example of the invention will be described.

[0039] The method for producing the cerium oxide-zirconium oxide-based composite oxide according to the example of the invention is a method for producing a cerium oxide-zirconium oxide-based composite oxide comprising a composite oxide of cerium oxide and zirconium oxide, wherein this method comprises: providing a cerium oxide-zirconium oxide-based mixed crystal powder with a cerium to zirconium content ratio ([Cer]:[Zirconium]) in the range of a molar ratio of 43:57 to 55:45; and compression molding the cerium oxide-zirconium oxide-based mixed crystal powder at a pressure of 1,500 to 3,500 kgf / cm². 2; Subjecting a cerium oxide-zirconium oxide-based mixed crystal powder pellet to a first reducing treatment, which includes 0.5 to 24 hours of heat treatment under reducing conditions at a temperature between 1,400 and 1,550°C; and subjecting the first-reduced cerium oxide-zirconium oxide-based mixed crystal powder pellet to a second reducing treatment, which includes 0.5 to 5 hours of heat treatment under reducing conditions at a temperature between 1,600 and 2,000°C and at least 100°C higher than the temperature in the first reducing treatment.

[0040] The cerium oxide-zirconium oxide-based mixed crystal powder according to the invention (a mixed crystal powder containing cerium oxide and zirconium oxide) has a cerium to zirconium content ratio ([Cer]:[Zirconium]) that must be in the range of a molar ratio of 43:57 to 55:45, preferably in the range of 43:57 to 48:52, and most preferably in the range of 44:56 to 47:53. If the cerium content in the cerium oxide-zirconium oxide-based mixed crystal powder used is below the aforementioned lower limit, the reduction in oxygen storage capacity due to the zirconium-rich composition is greater than the effect of increasing the stability of the composite oxide due to the inhibition of phase separation of cerium oxide, which leads to insufficient oxygen storage capacity after a high-temperature resistance test.If, on the other hand, the cerium content exceeds the aforementioned upper limit, the effect of increasing the stability of the composite oxide by preventing the phase separation of cerium oxide will not be achieved, resulting in insufficient oxygen storage capacity after a high-temperature resistance test.

[0041] To enable sufficient formation of an ordered phase, it is preferable to use a mixed crystal such as this cerium oxide-zirconium oxide-based mixed crystal powder in which cerium oxide and zirconium oxide have been mixed at the atomic level. Such a cerium oxide-zirconium oxide-based mixed crystal powder has a mean primary particle diameter of preferably about 5 to about 50 nm. If the mean primary particle diameter of the cerium oxide-zirconium oxide-based mixed crystal powder is below the aforementioned lower limit, the formation of a mixed crystal of cerium oxide and zirconium oxide in the powder does not occur to its full extent, which tends to make obtaining an ordered phase difficult.If, on the other hand, the mean primary particle diameter of the cerium oxide-zirconium oxide-based mixed crystal powder exceeds the aforementioned limit, the contact condition between primary particles deteriorates during compression molding and grain growth during a reducing treatment tends to be inadequate.

[0042] The process for producing such a cerium oxide-zirconium oxide-based mixed crystal powder is not subject to any particular restrictions and can be illustrated, for example, by a process for producing the mixed crystal powder by using a co-precipitation process to adjust the cerium to zirconium ratio within the range specified above. The co-precipitation process is illustrated by a process in which an aqueous solution containing a cerium salt (e.g., a nitrate salt) and a zirconium salt (e.g., a nitrate salt) is used to form a co-precipitate in the presence of ammonia, the resulting co-precipitate is filtered and washed, then dried and calcined, and then milled using a milling machine, such as a ball mill, to yield the cerium oxide-zirconium oxide-based mixed crystal powder.The aqueous solution, containing at least one type of salt selected from the group consisting of cerium salts and zirconium salts, is prepared such that the cerium and zirconium content in the resulting mixed-crystal powder falls within predetermined ranges. Salts of at least one element selected from the group consisting of rare-earth elements and alkaline-earth elements, surfactants (e.g., nonionic surfactants), and other components may optionally be added to such an aqueous solution.

[0043] The individual steps will be described next. In this invention, the first step is a compression molding step in which the cerium oxide-zirconium oxide-based mixed crystal powder is molded at a pressure of 1,500 to 3,500 kgf / cm². 2 (and preferably a pressure of 1,750 to 3,000 kgf / cm²) 2The powder is subject to a minimum compression molding pressure. If the pressure during this molding step is below this lower limit, the powder's packing density does not increase adequately, and consequently, crystal growth during a reducing treatment does not proceed to its full extent. This results in insufficient oxygen storage capacity for the resulting cerium oxide-zirconium oxide-based composite oxide after a high-temperature resistance test. Conversely, if the pressure during the molding step exceeds the aforementioned limit, phase separation of the cerium oxide occurs, again resulting in insufficient oxygen storage capacity for the resulting cerium oxide-zirconium oxide-based composite oxide after a high-temperature resistance test. Such a molding process is not subject to any particular limitations; however, a conventional molding process, such as isostatic pressing, can be used appropriately.

[0044] In this invention, a first reducing treatment step is next, in which the cerium oxide-zirconium oxide-based mixed crystal powder pellet formed in the compression molding step is subjected to a first reducing treatment, which includes 0.5 to 24 hours (preferably 1 to 10 hours) of heat treatment under reducing conditions at a temperature of 1400 to 1550°C (preferably 1450 to 1550°C). If the temperature of this first reducing treatment is below the aforementioned lower limit, the stability of the ordered phase is low, resulting in inadequate oxygen storage capacity for the resulting cerium oxide-zirconium oxide-based composite oxide after high-temperature resistance tests.If, however, the temperature of the first reducing treatment exceeds the aforementioned upper limit, phase separation of the cerium oxide readily occurs, which tends to decrease the oxygen storage capacity. If the heating time during this first reducing treatment is below the lower limit, the formation of the ordered phase tends to be insufficient. If, on the other hand, it exceeds the aforementioned upper limit, phase separation of the cerium oxide readily occurs.

[0045] The first reduction treatment method is not subject to any particular restrictions, provided that the mixed-crystal powder can be heat-treated under a reducing atmosphere and under predetermined temperature conditions. Illustrative examples of suitable methods include: (i) the process of placing the mixed-crystal powder in a vacuum heating furnace, evacuating the furnace, then introducing a reducing gas into the furnace to change the atmosphere inside the furnace to a reducing atmosphere, and heating the powder under predetermined temperature conditions to effect a reducing treatment;(ii) the process of using a graphite furnace, placing the mixed crystal powder into the furnace, evacuating the furnace, then heating the furnace to predetermined temperature conditions to cause the furnace body, heated fuel, and the like to produce reducing gases, such as carbon monoxide (CO) and hydrocarbons (HC), thereby changing the atmosphere inside the furnace to a reducing atmosphere and effecting a reducing treatment; and (iii) the process of placing the mixed crystal powder into a crucible filled with activated carbon (C) and heating it under predetermined temperature conditions to cause the activated C and the like to produce reducing gases, such as CO and HC, thereby changing the atmosphere inside the crucible to a reducing atmosphere and effecting a reducing treatment.

[0046] The reducing gas used to create such a reducing atmosphere is not subject to any particular restrictions, although reducing gases such as CO, HC, H₂, and other hydrocarbon gases can be used appropriately. To prevent the formation of byproducts such as zirconium carbide (ZrC) during reducing treatment at significantly elevated temperatures, a reducing gas that does not contain carbon (C) is preferred. In cases where such a carbon-free reducing gas is used, the structural stability of the crystalline phase can be suitably increased, as reducing treatment at higher temperature conditions, close to the melting point of zirconium, is possible.

[0047] Next, the cerium oxide-zirconium oxide-based mixed crystal powder pellet, which has undergone a first reducing treatment, is subjected to a second reducing treatment step. This second treatment includes a heating treatment under reducing conditions at a temperature of 1,600 to 2,000°C, which is at least 100°C higher than the temperature in the first reducing treatment (preferably a temperature of 1,650 to 1,750°C, which is at least 150°C higher than the temperature in the first reducing treatment), for a period of 0.5 to 5 hours (preferably 1 to 4 hours), yielding the cerium oxide-zirconium oxide-based composite oxide according to the example of the invention. If the temperature in the second reducing treatment is below the aforementioned lower limit, grain growth tends to be inadequate.If, however, the temperature of the second reducing treatment exceeds the aforementioned upper limit, the balance between the energy required for the reducing treatment (e.g., electrical energy) and the increase in efficiency deteriorates. Furthermore, if the temperature difference between the second reducing treatment and the first reducing treatment is less than 100°C, it tends to be difficult to induce grain growth while adequately preventing phase separation of the cerium oxide. Moreover, grain growth tends to be insufficient if the heating time during the second reducing treatment is below the lower limit. Conversely, if the heating time exceeds the aforementioned upper limit, sufficient grain growth occurs, rendering further measures unnecessary, which tends to decrease economic efficiency.

[0048] The second reducing treatment process and the reducing gases used to achieve a reducing atmosphere therein may be the same reducing treatment process and reducing gas as those used in the first reducing treatment.

[0049] In carrying out this invention, it is preferred that, after the second reducing treatment step, an additional oxidation treatment is performed on the cerium oxide-zirconium oxide-based composite oxide (oxidation treatment step). By performing such an oxidation treatment, oxygen released during the reduction is compensated in the resulting cerium oxide-zirconium oxide-based composite oxide, and the stability as an oxide powder tends to increase.

[0050] The method used for such oxidation treatment is not subject to any particular restrictions, although the method of heat-treating the cerium oxide-zirconium oxide-based composite oxide in an oxidizing atmosphere (e.g., in the atmosphere) is preferred. The heating temperature conditions for such oxidation treatment are not subject to any particular restrictions, although a temperature of about 300 to about 800°C is preferred.

[0051] Furthermore, the heating time during such an oxidation treatment is not subject to any particular restrictions, although a heating time of about 0.5 to about 5 hours is preferred.

[0052] In carrying out this invention, it is preferable to additionally perform a grinding treatment on the cerium oxide-zirconium oxide-based composite oxide after the second reducing treatment step or the oxidizing treatment step (grinding step). Performing such a grinding treatment tends to make the handling of the resulting cerium oxide-zirconium oxide-based composite oxide easier, for example, when it is applied to a cordierite honeycomb structure.

[0053] The method used for such a grinding treatment is not subject to any particular restrictions, although, for example, wet grinding, dry grinding or freeze grinding may be preferred.

[0054] By carrying out the process for producing a cerium oxide-zirconium oxide-based composite oxide according to the example of the invention in this manner, it is possible to obtain a cerium oxide-zirconium oxide-based composite oxide according to the invention which simultaneously satisfies the following three conditions: (1) Primary particles with a particle diameter of 1.5 to 4.5 µm constitute at least 50% of all primary particles in the cerium oxide-zirconium oxide-based composite oxide on a particle number basis; (2) the I(28 / 29) value is ≥0.015; and (3) the I(28 / 29) value is ≤0.08.

[0055] To obtain a cerium oxide-zirconium oxide-based composite oxide according to the example of the invention, in which primary particles with a particle diameter of 1.5 to 4.5 µm constitute at least 70% of all primary particles in the composite oxide on a particle number basis, it is preferred that the pressure applied in the compression molding step be 1,750 to 3,000 kgf / cm². 2 The first reducing treatment is carried out at a temperature between 1450 and 1550°C for a period of 1 to 10 hours, the second reducing treatment is carried out at a temperature between 1650 and 1750°C for a period of 1 to 4 hours, and the difference between the temperature of the first reducing treatment and the temperature of the second reducing treatment is at least 150°C.

[0056] The cerium oxide-zirconium oxide-based composite oxide and the process for producing such a composite oxide according to the example of the invention have been described above. The inventive exhaust gas purification catalyst, which uses such a cerium oxide-zirconium oxide-based composite oxide, is described below analogously.

[0057] The exhaust gas purification catalyst according to the example of the invention comprises the cerium oxide-zirconium oxide-based composite oxide according to the example of the invention described above. This exhaust gas purification catalyst according to the example of the invention possesses both a sufficiently good oxygen storage capacity (OSC) and a sufficiently high heat resistance, and moreover exhibits a sufficiently good oxygen storage capacity even after long-term exposure to elevated temperatures.

[0058] Preferred examples of the exhaust gas purification catalyst according to the invention include exhaust gas purification catalysts comprising a support containing the cerium oxide-zirconium oxide-based composite oxide of the invention and a precious metal supported on the support. Illustrative examples of such precious metals include platinum, rhodium, palladium, osmium, iridium, gold, and silver. The method for supporting the precious metal on such a support is not subject to any particular limitations. A conventional method can be used in a suitable manner, such as one involving immersion of a powder of the cerium oxide-zirconium oxide-based composite oxide (support) in a solution consisting of a precious metal salt (e.g., nitrate, hydrochloride, acetate) or precious metal complex dissolved in a solvent such as water or alcohol, removal of the solvent, and subsequent firing.The quantity of precious metal carried on the support is not subject to any special restrictions, provided that a necessary and suitable quantity is carried for considerations such as the intended design of the catalyst, although setting the quantity to at least 0.01 wt% is preferred.

[0059] Another preferred example of an exhaust gas purification catalyst according to the invention is an exhaust gas purification catalyst formed from a first catalyst consisting of catalyst-bearing fine particles and a noble metal supported on the catalyst-bearing fine particles, wherein the cerium oxide-zirconium oxide-based composite oxide according to the example of the invention is arranged around the first catalyst. Such catalyst-bearing fine particles are not subject to any particular restrictions; a support composed of a metal oxide or metal oxide composite that can be used as the support for an exhaust gas purification catalyst (e.g., aluminum oxide particles, particles consisting of aluminum oxide / cerium oxide, and particles consisting of aluminum oxide / cerium oxide / zirconium oxide) can be used appropriately.The method described above can be used as a method for carrying a precious metal on such catalyst-bearing fine particles. The amount of precious metal carried on the catalyst-bearing fine particles is not subject to any particular limitations, provided that a necessary and suitable amount is carried for considerations such as the intended design of the catalyst, although adjusting the amount to at least 0.01 wt% is preferred. Likewise, the method of arranging the cerium oxide-zirconium oxide-based composite oxide around this first catalyst, according to the example of the invention, is not subject to any particular limitations; for example, a method can be used that includes mixing the first catalyst and the cerium oxide-zirconium oxide-based composite oxide according to the example of the invention.Furthermore, from the point of view of obtaining higher catalyst activity, it is preferable that the cerium oxide-zirconium oxide-based composite oxide according to the example of the invention is arranged in a highly dispersed state around the first catalyst.

[0060] The invention is described in more detail below with reference to working examples and comparative examples, although the invention is not to be interpreted as being limited by these examples. Example 1

[0061] A cerium oxide-zirconium oxide-based mixed crystal powder with a cerium to zirconium molar ratio ([Cer]:[Zirconium]) of 45:55 was prepared as described below. First, 442 g of a 28 wt% (CeO₂-based) aqueous cerium nitrate solution, 601 g of an 18 wt% (ZrO₂-based) aqueous zirconium oxynitrate solution, and 200 g of an aqueous solution containing 1.1 mol of hydrogen peroxide per mol of cerium were added to 1,220 g of an aqueous solution containing 1.2 ammonia equivalents per neutralization equivalent, forming a co-precipitate. The resulting co-precipitate was separated centrifugally and washed (ion-exchanged water). Next, the co-precipitate was dried in the atmosphere at 110°C for at least 10 hours and then fired in the atmosphere at 400°C for 5 hours, resulting in a mixed crystal of cerium and zirconium (CeO2-ZrO2 mixed crystal).The mixed crystal was then ground on a sieve using a grinding mill (available from AS ONE Corporation under the trade name “Wonder Blender”) to a particle diameter of no more than 75 µm, resulting in a cerium oxide-zirconium oxide-based mixed crystal powder.

[0062] Twenty grams of the resulting cerium oxide-zirconium oxide-based mixed crystal powder were then packaged in a polyethylene bag (0.05 l volume) and the contents degassed. The bag's mouthpiece was then heated and sealed. Next, the bag was subjected to a pressure (applied in one press cycle) of 1,500 kgf / cm² for one minute using an isostatic press (available under the trade name "CK4-22-60" from NIKKISO CO., LTD.). 2Cold isostatic pressing (CIP) was performed, yielding a cerium oxide-zirconium oxide-based mixed-crystal powder pellet. The pellet size was set to 4 cm in length, 4 cm in width, 7 mm in average thickness, and approximately 20 g in weight.

[0063] The resulting pellets (2 surface elements) were then placed in a crucible (internal volume: diameter 8 cm; height 7 cm) filled with 70 g of activated carbon, and a lid was placed on the crucible. The crucible was then placed in an electric high-speed heating oven and heated to 1000°C over a temperature rise time of 1 hour, then to 1400°C (temperature of the first reducing treatment) over a rise time of 2 hours and held at that temperature for 3 hours. Next, it was heated to 1600°C (temperature of the second reducing treatment) over a rise time of 2 hours and held at that temperature for 2 hours. Finally, the crucible was cooled to 1000°C over a cooling time of 4 hours and then gradually allowed to cool to room temperature, resulting in a reduction-treated product.

[0064] This reduction-treated product was then oxidized by heating at 500°C in the atmosphere for 5 hours, yielding a cerium oxide-zirconium oxide-based composite oxide with a cerium to zirconium molar ratio ([Cer]:[Zirconium]) of 45:55. This cerium oxide-zirconium oxide-based composite oxide was then sieved to a particle size of no more than 75 µm. Test for measuring particle size distribution

[0065] The particle size distribution of primary particles of the resulting cerium oxide-zirconium oxide-based composite oxide was measured as follows. This is an average value obtained by using a scanning electron microscope (available from JEOL Ltd. under the trade name “JSM-7000F”) to randomly examine cross-sections of cerium oxide-zirconium oxide-based composite oxide particles at two or more locations, then determining, on a numerical basis, crystal grain diameters within each obtained field of view (240 µm x 240 µm at a magnification of 500x) and estimating their distribution. In cases where the cross-section is not circular, “crystal grain size” refers to the diameter of the smallest circle circumscribed around the crystal grain. The results thus obtained are shown in Table 1 ( Fig. 1) shown. X-ray diffractometry (XRD) measurement

[0066] The cerium oxide-zirconium oxide-based composite oxide was heated in the atmosphere at 1100°C for 5 hours (high-temperature resistance test), and the crystalline phase of the cerium oxide-zirconium oxide-based composite oxide after treatment was measured by X-ray diffraction. The X-ray diffraction pattern was measured using an X-ray diffractometer (available from Rigaku Corporation under the trade name "RINT-2100"), and the I(14 / 29) and I(28 / 29) values ​​were measured. The results are shown in Table 1 ( Fig. 1) shown. Measurement of oxygen storage capacity: Assessment of oxygen storage capacity

[0067] One gram of cerium oxide-zirconium oxide-based composite oxide powder according to the above resistance test and 1 g of a Pd / Al2O3 catalyst containing supported Pd (0.25 wt%) were physically mixed in a mortar, whereupon the mixture was pressed into a pellet and then ground, yielding an exhaust gas purification catalyst in the form of pellets with a diameter of 0.5 mm to 1 mm.

[0068] Next, 0.5 g of the resulting catalyst was placed in a quartz reaction tube (internal volume: diameter 1.7 cm; length 9.5 cm), and a rich gas (CO (2 vol%) + N₂ (balance)) and a lean gas (O₂ (1 vol%) + N₂ (balance)) were alternately passed through a fixed-bed flow reactor, switching between them every 3 minutes. The oxygen storage capacity (OSC) was determined from the amount of CO₂ formed in the rich gas atmosphere. The gas flow rate was set to 10 L / min, and the measurement temperature was set to 600°C. The instrument used was the exhaust gas analyzer available under the trade name "Bex-5900Csp" from BEST INSTRUMENTS CO., Ltd. The results are shown in Table 1 ( Fig. 1) shown. Example 2

[0069] Apart from the fact that the pressure applied in the compression molding step is 2,000 kgf / cm² 2The cerium oxide-zirconium oxide-based mixed crystal powder obtained in Example 1 was used in the same manner as in Example 1 to obtain a cerium oxide-zirconium oxide-based composite oxide. Particle size distribution testing, X-ray diffractometry, and oxygen storage capacity measurements were performed on the resulting cerium oxide-zirconium oxide-based composite oxide in the same manner as in Example 1. The results are shown in Table 1 ( Fig. 1) shown. Example 3

[0070] Apart from the fact that the pressure applied in the compression molding step is 2,000 kgf / cm² 2After the reducing treatment conditions were changed, with the first reducing treatment set to a rise time of 3 hours, a reducing treatment temperature of 1500°C, and a holding time of 3 hours, and the second reducing treatment set to a rise time of 2 hours, a reducing treatment temperature of 1700°C, and a holding time of 2 hours, the cerium oxide-zirconium oxide-based mixed crystal powder obtained in Example 1 was used in the same manner as in Example 1 to obtain a cerium oxide-zirconium oxide-based composite oxide. Particle size distribution testing, X-ray diffractometry, and oxygen storage capacity measurements were performed on the resulting cerium oxide-zirconium oxide-based composite oxide in the same manner as in Example 1. The results are shown in Table 1 ( Fig. 1) shown. Example 4

[0071] Apart from adjusting the amount of 28 wt% (CeO2-based) aqueous cerium nitrate solution to 423 g and the amount of 18 wt% (ZrO2-based) aqueous zirconium oxynitrate solution to 623 g, a cerium oxide-zirconium oxide-based mixed crystal powder with a content ratio in the form of the molar ratio of cerium to zirconium ([Cer]:[Zirconium]) of 43:57 was obtained in the same way as in Example 1, whereupon a cerium oxide-zirconium oxide-based composite oxide with a content ratio in the form of the molar ratio of cerium to zirconium ([Cer]:[Zirconium]) of 43:57 was obtained in the same way as in Example 3. Tests to measure particle size distribution, X-ray diffractometry, and oxygen storage capacity were performed on the resulting cerium oxide-zirconium oxide-based composite oxides in the same manner as in Example 1. The results are shown in Table 1 ( Fig. 1) shown. Example 5

[0072] Apart from adjusting the amount of 28 wt% (CeO2-based) aqueous cerium nitrate solution to 491 g and the amount of 18 wt% (ZrO2-based) aqueous zirconium oxynitrate solution to 547 g, a cerium oxide-zirconium oxide-based mixed crystal powder with a content ratio in the form of the molar ratio of cerium to zirconium ([Cer]:[Zirconium]) of 50:50 was obtained in the same way as in Example 1, whereupon a cerium oxide-zirconium oxide-based composite oxide with a content ratio in the form of the molar ratio of cerium to zirconium ([Cer]:[Zirconium]) of 50:50 was obtained in the same way as in Example 3. Tests to measure particle size distribution, X-ray diffractometry, and oxygen storage capacity were performed on the resulting cerium oxide-zirconium oxide-based composite oxides in the same manner as in Example 1. The results are shown in Table 1 ( Fig. 1) shown. Example 6

[0073] Apart from adjusting the amount of 28 wt% (CeO2-based) aqueous cerium nitrate solution to 541 g and the amount of 18 wt% (ZrO2-based) aqueous zirconium oxynitrate solution to 492 g, a cerium oxide-zirconium oxide-based mixed crystal powder with a content ratio in the form of the molar ratio of cerium to zirconium ([Cer]:[Zirconium]) of 55:45 was obtained in the same way as in Example 1, whereupon a cerium oxide-zirconium oxide-based composite oxide with a content ratio in the form of the molar ratio of cerium to zirconium ([Cer]:[Zirconium]) of 55:45 was obtained in the same way as in Example 3. Tests to measure particle size distribution, X-ray diffractometry, and oxygen storage capacity were performed on the resulting cerium oxide-zirconium oxide-based composite oxides in the same manner as in Example 1. The results are shown in Table 1 ( Fig. 1) shown. Example 7

[0074] Apart from the fact that the pressure applied in the compression molding step is 3,000 kgf / cm² 2After the reducing treatment conditions were changed, with the first reducing treatment set to a rise time of 3 hours, a reducing treatment temperature of 1500°C, and a holding time of 3 hours, and the second reducing treatment set to a rise time of 2 hours, a reducing treatment temperature of 1700°C, and a holding time of 2 hours, the cerium oxide-zirconium oxide-based mixed crystal powder obtained in Example 1 was used in the same manner as in Example 1 to obtain a cerium oxide-zirconium oxide-based composite oxide. Particle size distribution testing, X-ray diffractometry, and oxygen storage capacity measurements were performed on the resulting cerium oxide-zirconium oxide-based composite oxide in the same manner as in Example 1. The results are shown in Table 1 ( Fig. 1) shown. Example 8

[0075] Apart from adjusting the amount of 28 wt% (CeO2-based) aqueous cerium nitrate solution to 472 g and the amount of 18 wt% (ZrO2-based) aqueous zirconium oxynitrate solution to 569 g, a cerium oxide-zirconium oxide-based mixed crystal powder with a content ratio in the form of the molar ratio of cerium to zirconium ([Cer]:[Zirconium]) of 48:52 was obtained in the same way as in Example 1, whereupon a cerium oxide-zirconium oxide-based composite oxide with a content ratio in the form of the molar ratio of cerium to zirconium ([Cer]:[Zirconium]) of 48:52 was obtained in the same way as in Example 3. Tests to measure particle size distribution, X-ray diffractometry, and oxygen storage capacity were performed on the resulting cerium oxide-zirconium oxide-based composite oxides in the same manner as in Example 1. The results are shown in Table 1 ( Fig. 1) shown. Comparative example 1

[0076] Apart from the fact that no compression molding was performed, and the reducing treatment conditions in the first reducing treatment were set to a rise time of 3 hours, a reducing temperature of 1,500°C, and a holding time of 3 hours, and the reducing treatment conditions in the second reducing treatment were set to a rise time of 2 hours, a reducing temperature of 1,700°C, and a holding time of 2 hours, the cerium oxide-zirconium oxide-based mixed crystal powder obtained in Example 1 was used in the same manner as in Example 1 to obtain a cerium oxide-zirconium oxide-based composite oxide. A particle size distribution test, X-ray diffractometry, and an oxygen storage capacity measurement were performed on the resulting cerium oxide-zirconium oxide-based composite oxide in the same manner as in Example 1.The results are shown in Table 1 ( . Fig. 1) shown. Comparative example 2

[0077] Apart from the fact that the pressure applied in the compression molding step is 500 kgf / cm² 2After modifying the conditions of the reducing treatment in the first reducing treatment to a rise time of 2 hours, a reducing treatment temperature of 1400°C, and a holding time of 5 hours, and omitting the second reducing treatment and subsequently cooling to 1000°C over a cooling period of 4 hours, the cerium oxide-zirconium oxide-based mixed crystal powder obtained in Example 1 was used in the same manner as in Example 1 to obtain a cerium oxide-zirconium oxide-based composite oxide. Particle size distribution testing, X-ray diffractometry, and oxygen storage capacity measurements were performed on the resulting cerium oxide-zirconium oxide-based composite oxide in the same manner as in Example 1. The results are shown in Table 1 ( Fig. 1) shown. Comparative example 3

[0078] Apart from the fact that the pressure applied in the compression molding step is 2,000 kgf / cm² 2 After modifying the conditions of the reducing treatment in the first reducing treatment to a rise time of 3 hours, a reducing treatment temperature of 1,700°C, and a holding time of 5 hours, and omitting the second reducing treatment and subsequently cooling to 1,000°C over a cooling time of 4 hours, the cerium oxide-zirconium oxide-based mixed crystal powder obtained in Example 1 was used in the same manner as in Example 1 to obtain a cerium oxide-zirconium oxide-based composite oxide. Particle size distribution testing, X-ray diffractometry, and oxygen storage capacity measurements were performed on the resulting cerium oxide-zirconium oxide-based composite oxide in the same manner as in Example 1. The results are shown in Table 1 ( Fig. 1) shown. Comparative example 4

[0079] Apart from the fact that the pressure applied in the compression molding step is 4,000 kgf / cm² 2After the conditions of the reducing treatment in the first reducing treatment were changed to a rise time of 3 hours, a reducing treatment temperature of 1500°C, and a holding time of 3 hours, and the conditions of the reducing treatment in the second reducing treatment were changed to a rise time of 2 hours, a reducing treatment temperature of 1700°C, and a holding time of 2 hours, the cerium oxide-zirconium oxide-based mixed crystal powder obtained in Example 1 was used in the same manner as in Example 1 to obtain a cerium oxide-zirconium oxide-based composite oxide. A particle size distribution test, X-ray diffractometry, and an oxygen storage capacity measurement were performed on the resulting cerium oxide-zirconium oxide-based composite oxide in the same manner as in Example 1. The results are shown in Table 1 ( Fig. 1) shown. Comparative example 5

[0080] Apart from adjusting the amount of 28 wt% (CeO2-based) aqueous cerium nitrate solution to 413 g and the amount of 18 wt% (ZrO2-based) aqueous zirconium oxynitrate solution to 634 g, a cerium oxide-zirconium oxide-based mixed crystal powder with a content ratio in the form of the molar ratio of cerium to zirconium ([Cer]:[Zirconium]) of 42:58 was obtained in the same way as in Example 1, whereupon a cerium oxide-zirconium oxide-based composite oxide with a content ratio in the form of the molar ratio of cerium to zirconium ([Cer]:[Zirconium]) of 42:58 was obtained in the same way as in Example 3. Tests to measure particle size distribution, X-ray diffractometry, and oxygen storage capacity were performed on the resulting cerium oxide-zirconium oxide-based composite oxides in the same manner as in Example 1. The results are shown in Table 1 ( Fig. 1) shown. Comparative example 6

[0081] Apart from adjusting the amount of 28 wt% (CeO2-based) aqueous cerium nitrate solution to 550 g and the amount of 18 wt% (ZrO2-based) aqueous zirconium oxynitrate solution to 481 g, a cerium oxide-zirconium oxide-based mixed crystal powder with a content ratio in the form of the molar ratio of cerium to zirconium ([Cer]:[Zirconium]) of 56:44 was obtained in the same way as in Example 1, whereupon a cerium oxide-zirconium oxide-based composite oxide with a content ratio in the form of the molar ratio of cerium to zirconium ([Cer]:[Zirconium]) of 56:44 was obtained in the same way as in Example 3. Tests to measure particle size distribution, X-ray diffractometry, and oxygen storage capacity were performed on the resulting cerium oxide-zirconium oxide-based composite oxides in the same manner as in Example 1. The results are shown in Table 1 ( Fig. 1) shown.

[0082] As can be seen from a comparison of the data in Table 1 ( Fig. 1) The results shown in Examples 1 to 8 compared with the results in Comparative Examples 1 to 6 were confirmed in cases where cerium oxide-zirconium oxide-based mixed crystal powders obtained by the manufacturing process according to the example of the invention with a cerium to zirconium content ratio ([Cer] : [Zirconium]) in the range of a molar ratio of 43:57 to 55:45 were used, and the pressure applied in the compression molding step was 1,500 to 3,500 kgf / cm². 2where the temperature in the first reducing treatment was in the range of 1,400 to 1,550°C and the temperature in the second reducing treatment was in the range of 1,600 to 2,000°C (Examples 1 to 8), cerium oxide-zirconium oxide-based composite oxides are obtained according to the example of the invention, which simultaneously satisfy the following three conditions: primary particles with a particle diameter of 1.5 to 4.5 µm constitute at least 50% of all primary particles of the composite oxide on a particle number basis in the cerium oxide-zirconium oxide-based composite oxide; the I(14 / 29) value ≥ 0.15; and the I(28 / 29) value ≤ 0.08. Furthermore, it was confirmed that the cerium oxide-zirconium oxide-based composite oxides of the invention (Examples 1 to 8) possess both a sufficiently good oxygen storage capacity and a sufficiently good heat resistance, and moreover, exhibit a sufficiently good oxygen storage capacity even after long-term exposure to elevated temperatures.

[0083] In contrast, for composite oxides made of cerium oxide and zirconium oxide, in which primary particles with a particle diameter of 1.5 to 4.5 µm constitute less than 50% of all primary particles in the composite oxide (comparative examples 1 and 2), it was confirmed that the pyrochlore phase had reduced heat resistance and poor oxygen storage capacity.

[0084] In the case where the Ce / Zr ratio in the composite oxide of cerium oxide and zirconium oxide and the pressure conditions in the compression molding step fell within the prescribed range, but no two-stage treatment was carried out (comparative example 4), it was confirmed that there was no good oxygen storage capacity.

[0085] In cases where primary particles with a particle diameter of 1.5 to 4.5 µm in the complex oxide of cerium oxide and zirconium oxide represent a percentage of all primary particles in the composite oxide that falls within the prescribed range, but the I(14 / 29) value or the I(28 / 29) value is outside the prescribed range (comparative examples 4 to 6), it was confirmed that there was no good oxygen storage capacity.

[0086] As explained above, this invention enables the provision of cerium oxide-zirconium oxide-based composite oxides that are equipped with both a sufficiently good oxygen storage capacity and a sufficiently high heat resistance, and which are also able to exhibit a sufficiently good oxygen storage capacity even after long-term exposure to elevated temperatures; a method for producing such cerium oxide-zirconium oxide-based composite oxides; and exhaust gas purification catalysts that use such cerium oxide-zirconium oxide-based composite oxides.

[0087] Since the cerium oxide-zirconium oxide-based composite oxides according to the examples of the invention are thus equipped with both a sufficiently good oxygen storage capacity and a sufficiently high heat resistance, they can be advantageously used, for example, as supports, promoters and catalyst atmosphere modifiers for exhaust gas purification catalysts.

Claims

[1] Cerium oxide-zirconium oxide-based composite oxide, characterized by , that the cerium oxide-zirconium oxide-based composite oxide comprises a composite oxide containing cerium oxide and zirconia, wherein Primary particles with a particle diameter of 1.5 to 4.5 µm in the cerium oxide-zirconium oxide-based composite oxide constitute at least 50% of all primary particles in the composite oxide on a particle number basis, and the cerium to zirconium content ratio ([Cer]:[Zirconium]) in the cerium oxide-zirconium oxide-based composite oxide is in the range of a molar ratio of 43:57 to 55:45; and The cerium oxide-zirconium oxide-based composite oxide, when heated in the atmosphere for 5 hours under a temperature condition of 1,100°C and subsequently examined by X-ray diffractometry using CuKα, exhibits an X-ray diffraction pattern in which an intensity ratio of the diffraction line at a 2θ of 14.5° to the diffraction line at a 2θ of 29° {I(14 / 29) value} and an intensity ratio of the diffraction line at a 2θ of 28.5° to the diffraction line at a 2θ of 29° {I(28 / 29) value} satisfy the requirement I(14 / 29) ≥ 0.015 and I(28 / 29) ≤ 0.

08. [2] Cerium oxide-zirconium oxide-based composite oxide according to claim 1, wherein primary particles with a particle diameter of less than 1.5 µm in the cerium oxide-zirconium oxide-based composite oxide do not constitute more than 40% of all primary particles in the composite oxide on a particle number basis. [3] Cerium oxide-zirconium oxide-based composite oxide according to claim 1, wherein primary particles with a particle diameter of less than 1.5 µm in the cerium oxide-zirconium oxide-based composite oxide do not constitute more than 20% of all primary particles in the composite oxide on a particle number basis. [4] Cerium oxide-zirconium oxide-based composite oxide according to claim 1, wherein primary particles with a particle diameter of less than 1.5 µm in the cerium oxide-zirconium oxide-based composite oxide do not constitute more than 10% of all primary particles in the composite oxide on a particle number basis. [5] Cerium oxide-zirconium oxide-based composite oxide according to any one of claims 1 to 4, wherein primary particles with a particle diameter of 1.5 to 4.5 µm constitute at least 70% of all primary particles in the composite oxide on a particle number basis in the cerium oxide-zirconium oxide-based composite oxide. [6] Cerium oxide-zirconium oxide-based composite oxide according to any one of claims 1 to 4, wherein primary particles with a particle diameter of 1.5 to 4.5 µm constitute at least 80% of all primary particles in the composite oxide on a particle number basis in the cerium oxide-zirconium oxide-based composite oxide. [7] Method for producing a cerium oxide-zirconium oxide-based composite oxide according to any one of claims 1 to 6, characterized by , that the procedure includes: Providing a cerium oxide-zirconium oxide-based mixed crystal powder with a cerium to zirconium content ratio ([Cer]:[Zirconium]) in a molar ratio range of 43:57 to 55:45; Compression molding of the cerium oxide-zirconium oxide-based mixed crystal powder at a pressure of 1,500 to 3,500 kgf / cm² 2 ; Subjecting a cerium oxide-zirconium oxide-based mixed crystal powder pellet to a first reducing treatment, which includes 0.5 to 24 hours of treatment under reducing conditions at a temperature between 1,400 and 1,550°C; and Subjecting the first-reduction-treated cerium oxide-zirconium oxide-based mixed crystal powder pellet to a second reducing treatment, which includes 0.5 to 5 hours of heat treatment under reducing conditions at a temperature between 1,600 and 2,000°C and at least 100°C higher than the temperature in the first reducing treatment, such that the cerium oxide-zirconium oxide-based composite oxide is obtained according to any one of claims 1 to 6. [8] Method for producing a cerium oxide-zirconium oxide-based composite oxide according to claim 7, further comprising, following the second reducing treatment, subjecting the cerium oxide-zirconium oxide-based composite oxide to an oxidation treatment. [9] Exhaust gas purification catalyst, characterized by , that the exhaust gas purification catalyst comprises the cerium oxide-zirconium oxide-based composite oxide according to one of claims 1 to 6.