JP7307710B2 - Core-shell type oxygen storage/release material, method for producing same, catalyst for purifying exhaust gas using the same, and method for purifying exhaust gas - Google Patents

Description

TECHNICAL FIELD The present invention relates to a core-shell type oxygen absorbing/releasing material containing a ceria-zirconia composite oxide whose surface is coated with an alumina-based oxide, a method for producing the same, an exhaust gas purifying catalyst using the same, and an exhaust gas purifying method.

BACKGROUND ART Conventionally, composite oxides containing various metal oxides have been used as carriers, co-catalysts, etc. for exhaust gas purification catalysts. As the metal oxide in such a composite oxide, ceria has been suitably used because it is capable of absorbing and desorbing oxygen depending on the oxygen partial pressure in the atmosphere (has oxygen absorption and desorption capacity). . In recent years, various types of composite oxides containing ceria have been studied, and various ceria-zirconia composite oxides and production methods thereof have been disclosed.

In addition, since the ceria-zirconia-based composite oxide has a problem that it is easily deteriorated by heat at high temperatures (for example, 1000 ° C. or higher), the ceria-zirconia-based composite oxide Core-shell type composite oxides, which consist of a core made of an oxide and a shell covering at least part of the surface thereof, have also been studied.

For example, Japanese Patent Application Laid-Open No. 2017-186225 (Patent Document 1) describes a core made of a ceria-zirconia solid solution powder having at least one ordered phase of a pyrochlore phase and a κ phase, and at least a part of the core. A core-shell oxide material is disclosed comprising a shell of an alumina-based oxide disposed on the surface. In addition, Japanese Patent Application Laid-Open No. 2018-150207 (Patent Document 2) describes a core made of a ceria-zirconia solid solution powder having at least one ordered phase of a pyrochlore phase and a κ phase, and at least a part of the core and a shell made of an alumina-based oxide disposed on the surface thereof, the ceria-zirconia-based solid solution powder having a secondary particle diameter D50 at which the cumulative volume is 50% in the volume-based particle size distribution of the powder is 0.2 to 8.0. It discloses a core-shell type oxide material having a thickness of 0 μm and an average Al element concentration of 25 to 75 at % in a region of 3 nm depth from the surface of the shell measured by X-ray photoelectron spectroscopy. Furthermore, Japanese Patent Application Laid-Open No. 2019-217464 (Patent Document 3) describes a core made of a ceria-zirconia solid solution powder having at least one ordered phase of a pyrochlore phase and a κ phase, and at least a part of the core A shell made of an alumina-based oxide containing α-alumina is arranged on the surface, and after heating at 1100 ° C. for 5 hours in the air, X obtained by X-ray diffraction measurement using CuKα The intensity ratio [I (14/29) value] between the diffraction line at 2θ = 14.5° and the diffraction line at 2θ = 29° obtained from the line diffraction pattern is 2% or more, and the 2θ = 35.2° A core-shell oxygen storage/release material is disclosed in which the intensity ratio [I(35/29) value] of the diffraction line and the diffraction line at 2θ=29° is 0.03 to 1.2%. Patent Documents 1 to 3 disclose that such core-shell type oxide materials and core-shell type oxygen storage/release materials have excellent oxygen storage/release capacity even when exposed to high temperatures, and It is also described that an exhaust gas purifying catalyst exhibiting excellent NOx purification performance can be obtained.

JP 2017-186225 A JP 2018-150207 A JP 2019-217464 A

However, in the exhaust gas purifying catalysts in which the noble metal is brought into contact with the core-shell type oxide material or the core-shell type oxygen storage/release material described in Patent Documents 1 to 3, when the exhaust gas flowing into the catalyst is switched from lean gas to rich gas, The present inventors have found that there is a problem that the NOx purification performance is lowered.

The present invention has been made in view of the above-mentioned problems of the prior art, and it is possible to obtain an exhaust gas purification catalyst that exhibits excellent NOx purification performance even when the exhaust gas flowing into the catalyst is switched from lean gas to rich gas. It is an object of the present invention to provide a core-shell type oxygen storage/release material, a manufacturing method thereof, an exhaust gas purifying catalyst using the same, and an exhaust gas purifying method.

As a result of intensive research to achieve the above object, the present inventors have found that the surface of a core made of a ceria-zirconia solid solution powder having at least one of ordered phases of a pyrochlore phase and a κ phase is treated by a rotary CVD method. By applying and coating with an alumina-based oxide, the catalyst in which the noble metal is in contact with the obtained core-shell type oxygen storage material is excellent even when the exhaust gas flowing into it is switched from lean gas to rich gas. The inventors have found that NOx purifying performance is exhibited, and have completed the present invention.

That is, the core-shell type oxygen storage/release material of the present invention comprises a core made of a ceria-zirconia solid solution powder having at least one of ordered phases of a pyrochlore phase and a κ phase, and an alumina-based oxide layer disposed on the surface of the core. a shell consisting of an object,
The formula below:
Alumina-based oxide coverage = C _Al / (C _Al + C _Ce + C _Zr + C _M ) x 100
(Wherein, C _Al , C _Ce , C _Zr and C _M represent the concentrations of metal elements determined by X-ray photoelectron spectroscopy of the particle surface, C _Al is the Al concentration, C _Ce is the Ce concentration, C _Zr is the Zr concentration, _CM represents the concentration of the additive metal element M other than Al, Ce and Zr)
The coverage of the alumina-based oxide obtained by is 80% or more,
The shell has an average thickness of 2 to 75 nm,
The intensity ratio [I (14/29) value] of the diffraction line at 2θ = 14.5° and the diffraction line at 2θ = 29° obtained from the X-ray diffraction pattern obtained by X-ray diffraction measurement using CuKα is 2. % or more,
The intensity ratio [I (35/29) value] of the diffraction line at 2θ = 35.2° and the diffraction line at 2θ = 29° obtained from the X-ray diffraction pattern obtained by X-ray diffraction measurement using CuKα is 0. .2% or less,
It is characterized by

In such a core-shell type oxygen storage/release material of the present invention, the specific surface area of the ceria-zirconia solid solution powder is preferably 1.0 m ² /g or less. Moreover, it is preferable that the alumina-based oxide contains θ-alumina.

In the method for producing a core-shell type oxygen storage/release material of the present invention, a molded body obtained by pressure-molding a ceria-zirconia solid solution is subjected to a reduction treatment at a temperature of 1500° C. or higher, and then subjected to a pulverization treatment to obtain pyrochlore. a step of obtaining a ceria-zirconia solid solution powder having at least one ordered phase of a phase and a κ phase;
The ceria-zirconia solid solution powder having the ordered phase is stirred by rotating the reactor into which the ceria-zirconia solid solution powder having the ordered phase is charged with the horizontal direction as the rotation axis. a step of contacting a ceria-zirconia solid solution powder having an ordered phase with a vaporized alumina precursor in an oxidizing atmosphere to adhere an alumina-based oxide to the surface of the ceria-zirconia solid solution powder having an ordered phase;
The ceria-zirconia solid solution powder having the alumina-based oxide attached to the surface is fired to form a core made of the ceria-zirconia solid solution powder having the ordered phase and an alumina-based oxide disposed on the surface of the core. a step of obtaining the core-shell type oxygen-absorbing material of the present invention , comprising a shell made of
A method comprising:

Further, the exhaust gas purifying catalyst of the present invention is characterized by comprising the core-shell type oxygen-absorbing material of the present invention and a noble metal in contact with the core-shell-type oxygen absorbing/releasing material. be. Further, the exhaust gas purifying method of the present invention is a method characterized by bringing the exhaust gas containing nitrogen oxides into contact with the exhaust gas purifying catalyst of the present invention.

Here, the intensity ratios [I(14/29) value] and [I(35/29) value] of the diffraction beams in the present invention respectively refer to the core-shell type oxygen storage/release material to be measured using CuKα. The ratio of the peak intensity I (14) of the diffraction line at 2θ = 14.5° to the peak intensity I (29) of the diffraction line at 2θ = 29° obtained from the X-ray diffraction pattern obtained by X-ray diffraction measurement [I (14/29) value: I (14) / I (29) × 100 [%]], and the peak intensity I (29) of the diffraction line at 2θ = 29 ° of the diffraction line at 2θ = 35.2 ° It is the ratio of peak intensity I(35) [I(35/29) value: I(35)/I(29)×100[%]]. As a method for the X-ray diffraction measurement, an X-ray diffractometer (for example, "Ultima IV" manufactured by Rigaku Co., Ltd.) is used, CuKα rays are used as an X-ray source, tube voltage: 40 V, tube current: 30 A, scanning speed: A method of measuring at 10°/min is adopted. When obtaining the intensity (peak intensity) of the diffraction line, the average diffraction line intensity at 2θ=10° to 12° is subtracted from the intensity of each diffraction line as a background value.

In addition, the diffraction line at 2θ = 14.5° is a diffraction line attributed to the (111) plane of the ordered phase (κ phase), and the diffraction line at 2θ = 29° is a diffraction line attributed to the (222) plane of the ordered phase. line and the diffraction line attributed to the cubic phase (111) plane of the ceria-zirconia solid solution (CZ solid solution). Therefore, the I(14/29) value calculated as the intensity ratio of the two diffraction lines is defined as an index indicating the maintenance rate (existence rate) of the ordered phase. In addition, the complete ordered phase includes a κ phase (Ce ₂ Zr ₂ O ₈ ) completely filled with oxygen and a pyrochlore phase (Ce ₂ Zr ₂ O ₇ ) completely depleted of oxygen. I (14/29) value of κ phase calculated from card (PDF2:01-070-4048 for κ phase, PDF2:01-075-2694 for pyrochlore phase) is 4%, I (14/29) for pyrochlore phase The value is 5%.

Furthermore, the ordered phase, that is, the crystal phase having an ordered structure formed by cerium ions and zirconium ions, has an X-ray diffraction pattern obtained by the above X-ray diffraction measurement using CuKα with a 2θ angle of 14.5°. , 28°, 37°, 44.5° and 51°, respectively. superlattice structure). The term "peak" used herein refers to a height of 30 cps or more from the baseline to the peak top.

Moreover, the diffraction line at 2θ=35.2° is a diffraction line attributed to the (104) plane of the α-alumina (Al ₂ O ₃ ) phase (α-phase). Therefore, the I(35/29) value calculated as the intensity ratio to the diffraction line at 2θ = 29° is an index indicating the retention rate (abundance rate) of the α phase in the core-shell oxygen storage/release material to be measured. defined as Furthermore, the α phase, that is, the crystal phase of α-alumina, is located at positions where the 2θ angles of the X-ray diffraction pattern obtained by the X-ray diffraction measurement using CuKα are 35.2°, 43.3° and 25.6°. It is an arrangement structure of crystals having peaks at . The term "peak" used herein refers to a height of 30 cps or more from the baseline to the peak top.

It should be noted that the reason why the exhaust gas purifying catalyst in which the core-shell type oxygen storage/release material of the present invention is brought into contact with a noble metal exhibits excellent NOx purification performance even when the exhaust gas flowing into the catalyst is switched from lean gas to rich gas is not necessarily clear. However, the present inventors conjecture as follows. That is, in a conventional exhaust gas purifying catalyst in which a noble metal is brought into contact with an oxygen storage/release material made of a ceria-zirconia solid solution powder, the precious metal is in contact with the ceria-zirconia solid solution powder, and the interaction of these The NOx purification performance (rich NOx transient purification performance) is lowered when the exhaust gas flowing into the catalyst is switched from lean gas to rich gas.

On the other hand, in the core-shell type oxide materials and core-shell type oxygen storage/release materials described in Patent Documents 1 to 3, a shell made of an alumina-based oxide is formed on at least a part of the surface of a core made of a ceria-zirconia solid solution powder. Therefore, in an exhaust gas purifying catalyst in which a noble metal is brought into contact with such a core-shell type oxide material or a core-shell type oxygen storage/release material, since the noble metal is in contact with the shell made of an alumina-based oxide, the precious metal (especially rhodium) and the ceria-zirconia solid solution powder (especially ceria) are inhibited from interacting with each other due to their contact, and the deterioration of rich NOx transient purification performance is suppressed. However, the core-shell type oxide materials and the core-shell type oxygen storage/release materials described in Patent Documents 1 to 3 form a shell made of an alumina-based oxide by an evaporation to dryness method. It is difficult to completely cover the surface of the core made of ceria-zirconia solid solution powder with a thin film shell made of alumina-based oxide, and the shell made of alumina-based oxide was not necessarily high enough. For this reason, contact between the noble metal (especially rhodium) and the ceria-zirconia solid solution powder (especially ceria) cannot be sufficiently prevented, and the noble metal (especially rhodium) and the ceria-zirconia solid solution powder (especially ceria) ) is in contact with the rich NOx transient purification performance.

On the other hand, in the core-shell type oxygen storage/release material of the present invention, the shell made of alumina-based oxide is formed by the rotary CVD method. It can be coated with a thin shell with high coverage. Therefore, in the exhaust gas purifying catalyst in which the core-shell type oxygen storage and release material of the present invention is brought into contact with a noble metal, contact between the noble metal (especially rhodium) and the ceria-zirconia solid solution powder (especially ceria) is sufficiently prevented. It is presumed that interaction due to contact between the noble metal (especially rhodium) and the ceria-zirconia solid solution powder (especially ceria) is sufficiently suppressed, and excellent rich NOx transient purification performance is exhibited.

According to the present invention, it is possible to obtain an exhaust gas purification catalyst that exhibits excellent NOx purification performance even when the exhaust gas flowing into the catalyst is switched from lean gas to rich gas.

1 is a schematic diagram showing a rotary CVD apparatus; FIG. 4 is a graph showing alumina coverage of the core-shell type oxygen storage/release material powders obtained in Examples 1 and 2 and Comparative Examples 1 and 5. FIG. 4 is a graph showing average thicknesses of alumina thin films of core-shell type oxygen storage/release material powders obtained in Examples 1 and 2 and Comparative Examples 1 and 5. FIG. 1 is a transmission electron micrograph showing the core-shell type oxygen storage/release material powder obtained in Example 1. FIG. 4 is a transmission electron micrograph showing the core-shell oxygen-absorbing material powder obtained in Example 2. FIG. 1 is a diagram showing an electron beam diffraction pattern of the core-shell oxygen storage/release material powder obtained in Example 1. FIG. 2 is a diagram showing an electron beam diffraction pattern of the core-shell type oxygen storage/release material powder obtained in Example 2. FIG. 2 is a diagram showing an electron beam diffraction pattern of the core-shell type oxygen storage/release material powder obtained in Comparative Example 1. FIG. 4 is a graph showing the rich NOx transient removal rate of the catalysts obtained in Examples 1-2 and Comparative Examples 1-5.

BEST MODE FOR CARRYING OUT THE INVENTION The present invention will now be described in detail with reference to its preferred embodiments.

First, the core-shell type oxygen-absorbing material of the present invention will be described. The core-shell type oxygen storage/release material of the present invention comprises a core made of a ceria-zirconia solid solution powder having at least one ordered phase of a pyrochlore phase and a κ phase, and an alumina-based oxide disposed on the surface of the core. with a shell that
The formula below:
Alumina-based oxide coverage = C _Al / (C _Al + C _Ce + C _Zr + C _M ) x 100
(Wherein, C _Al , C _Ce , C _Zr and C _M represent the concentrations of metal elements determined by X-ray photoelectron spectroscopy of the particle surface, C _Al is the Al concentration, C _Ce is the Ce concentration, C _Zr is the Zr concentration, _CM represents the concentration of the additive metal element M other than Al, Ce and Zr)
The coverage of the alumina-based oxide obtained by is 80% or more,
The shell has an average thickness of 2 to 75 nm,
The intensity ratio [I (14/29) value] of the diffraction line at 2θ = 14.5° and the diffraction line at 2θ = 29° obtained from the X-ray diffraction pattern obtained by X-ray diffraction measurement using CuKα is 2. % or more,
The intensity ratio [I (35/29) value] of the diffraction line at 2θ = 35.2° and the diffraction line at 2θ = 29° obtained from the X-ray diffraction pattern obtained by X-ray diffraction measurement using CuKα is 0. .2% or less.

The core-shell type oxygen storage/release material of the present invention comprises a core made of a ceria-zirconia solid solution powder having at least one of a pyrochlore phase and a κ phase in which Ce and Zr are regularly arranged. is. A core-shell type oxygen storage/release material having a core made of a ceria-zirconia solid solution powder having such an ordered phase has a higher oxygen diffusion rate in the bulk than a ceria-zirconia solid solution having a fluorite structure. Excellent release capacity (oxygen absorption/release rate (OSC-r)). The reason why the ceria-zirconia solid solution according to the present invention has at least one of the ordered phases of the pyrochlore phase and the κ phase is that the peak specific to the ordered phase is obtained by the X-ray diffraction measurement using the above CuKα. It can be confirmed by acknowledgment.

In the core-shell type oxygen storage/release material of the present invention, the retention rate (abundance rate) of the ordered phase, that is, the aforementioned I(14/29) value must be 2% or more, and 3% or more. 3.3% or more is more preferable. When the I(14/29) value is less than the above lower limit, the retention rate of the ordered phase is low, and thus the oxygen storage/release capacity is lowered when exposed to high temperatures. The upper limit of the I (14/29) value is not particularly limited, but from the viewpoint that the I (14/29) value of the pyrochlore phase calculated from the PDF card (01-075-2694) is the upper limit, 5% The following are preferable.

In addition, in the core-shell type oxygen storage/release material of the present invention, 2θ=28.5 obtained from the X-ray diffraction pattern obtained by X-ray diffraction measurement using CuKα after heating at 1100° C. for 5 hours in the air. The intensity ratio [I(28/29) value] between the diffraction line at 20° and the diffraction line at 2θ=29° is preferably 8% or less, more preferably 6% or less, and 4% or less. It is even more preferable to have When the I(28/29) value exceeds the above upper limit, the oxygen absorption/desorption capacity tends to decrease when exposed to high temperatures. The lower limit of the I(28/29) value is not particularly limited, and a smaller value is preferable.

Here, the diffraction line at 2θ = 28.5° is the diffraction line attributed to the (111) plane of single _CeO2 , and the peak intensity (I29) of the diffraction line at 2θ = 29° By calculating the ratio of the peak intensity (I28) of the diffraction line [I (28/29) value: I28/I29 × 100 [%]], the degree of phase separation of CeO ₂ from the composite oxide can be determined. It can be specified as an index to show. The method of X-ray diffraction measurement is as described above.

In addition, the content ratio of Ce and Zr in the ceria-zirconia solid solution powder having such an ordered phase is preferably 35:65 to 65:35 in terms of molar ratio (Ce:Zr), and 45: A ratio of 55 to 55:45 is more preferred. When the molar ratio (Ce:Zr) deviates from the above range, the ordered phase tends to rearrange to change to a fluorite structure when exposed to high temperature, and the oxygen storage capacity tends to decrease.

The core composed of the ceria-zirconia solid solution powder having such an ordered phase may further contain additional elements such as rare earth elements other than Ce and Ti. When such an additive element is contained, there is a tendency to further suppress the decrease in oxygen absorption/desorption capacity when exposed to high temperatures. Examples of the additive elements include Sc, Y, La, Pr, Nd, Sm, Gd, Tb, Dy, Yb, Lu, and Ti. Y, La, Pr, and Nd are preferable, and Pr is more preferable, from the viewpoint that the is further suppressed. These additive elements may be contained singly or in combination of two or more. In addition, when the additive element is contained, the element is usually contained in the core as an oxide, and is in a state of solid solution, dispersion, etc. in the ceria-zirconia solid solution powder having the ordered phase. In order to obtain the effect of the additive element with certainty, it is more preferable to be in solid solution.

In the core according to the present invention, when the additive element is contained, the content thereof is 20 mol % or less in terms of the ratio (element conversion) of the additive element to the total of the ceria element, the zirconia element and the additive element. preferably 10 mol % or less, and even more preferably 5 mol % or less. If the content of the additive element exceeds the above upper limit, the heat resistance of the ordered phase tends to decrease, and the oxygen absorption/desorption capacity tends to decrease when exposed to high temperatures. Although the lower limit of the content of the additive element is not particularly limited, it is preferably 0.1 mol % or more in order to reliably obtain the effect of the additive element.

In the ceria-zirconia solid solution powder having the ordered phase that forms the core according to the present invention, the secondary particle diameter D50 at which the cumulative volume is 50% in the volume-based particle size distribution is 0.1 to 30 μm. preferable. When the secondary particle diameter D50 is less than the lower limit, the pyrochlore phase structure and the κ phase structure change to a fluorite structure due to thermal deterioration, and the oxygen utilization efficiency of CeO ₂ decreases, so the oxygen storage capacity tends to decrease. It is in. On the other hand, when the secondary particle diameter D50 exceeds the upper limit, the specific surface area is relatively decreased and the diffusion distance of oxygen from the inside of the particle is increased, so the oxygen storage capacity, particularly the oxygen storage rate is reduced. (OSC-r) tends to decrease. The secondary particle diameter D50 of the ceria-zirconia solid solution powder having such an ordered phase is more preferably 1 to 15 μm, more preferably 3 to 10 μm, from the viewpoint of expressing higher oxygen absorption and release capacity. is more preferable. The secondary particle diameter D50 of the ceria-zirconia solid solution powder having the ordered phase is determined, for example, by a dynamic light scattering method using a particle size distribution measuring device. A volume-based particle size distribution curve is obtained, and the particle diameter at which the cumulative volume in this particle size distribution curve is 50% can be obtained.

The specific surface area of the ceria-zirconia solid solution powder having an ordered phase is not particularly limited, but is preferably 1.0 m ² /g or less, and is 0.1 to 0.9 m ² /g. is more preferable, and 0.3 to 0.8 m ² /g is even more preferable. When the specific surface area is less than the lower limit, the oxygen storage capacity tends to decrease. Such a specific surface area can be calculated as a BET specific surface area from the adsorption isotherm using the BET isotherm adsorption equation.

The core-shell type oxygen storage/release material of the present invention comprises a core made of the ceria-zirconia solid solution powder having such an ordered phase, and a shell made of an alumina-based oxide disposed on the surface of the core. is.

In the core-shell type oxygen storage/release material of the present invention, the retention rate (abundance rate) of the α phase, that is, the aforementioned I(35/29) value must be 0.2% or less. It is more preferably 0.1% or less, still more preferably 0.05% or less, particularly preferably 0.03% or less, and most preferably 0.02% or less. When the I(35/29) value exceeds the above upper limit, the oxygen absorption/desorption capacity decreases due to α-alumina. The lower limit of the I(35/29) value is not particularly limited, preferably a smaller value, most preferably 0%.

Further, in the core-shell type oxygen storage/release material of the present invention, the following formula:
Alumina-based oxide coverage = C _Al / (C _Al + C _Ce + C _Zr + C _M ) x 100
(Wherein, C _Al , C _Ce , C _Zr and C _M represent the concentrations of metal elements determined by X-ray photoelectron spectroscopy of the particle surface, C _Al is the Al concentration, C _Ce is the Ce concentration, C _Zr is the Zr concentration, _CM represents the concentration of the additive metal element M other than Al, Ce and Zr)
The coverage of the alumina-based oxide obtained by is required to be 80% or more, preferably 82% or more, more preferably 85% or more, and even more preferably 88% or more. . When the coverage of the alumina-based oxide is less than the lower limit, in the catalyst in which the precious metal is in contact with the core-shell type oxygen storage/release material, the precious metal (especially rhodium) easily contacts with ceria in the core, and the precious metal (especially , rhodium) and ceria reduce the NOx purification performance when the exhaust gas flowing into the catalyst is switched from lean gas to rich gas (hereinafter referred to as "rich NOx transitional purification performance"). Note that the upper limit of the coverage of the alumina-based oxide is preferably 100% or less. In the present invention, the Al concentration C _Al , the Ce concentration C _Ce , the Zr concentration C _Zr , and the concentration CM of the additive metal element _M other than Al, Ce, and Zr are determined by the X-ray concentration of the core-shell oxygen storage/release material. Photoelectron spectroscopy (XPS) spectrum, for example, using an X-ray photoelectron spectrometer, using monochromatic AlKα (1486.6 eV) as an X-ray source, photoelectron extraction angle: 45 °, analysis area: 200 μmφ, charge-up correction :Zr3d Exists in a region (approximately 200 μmφ) at a depth of 3 nm from the surface of the core-shell type oxygen storage/release material (shell surface) based on the XPS spectrum obtained under the conditions of 182.2 eV (ZrO ₂ ). It can be determined by quantifying the elements.

Furthermore, in the core-shell type oxygen storage/release material of the present invention, the average thickness of the shell is required to be 2 to 75 nm, preferably 2 to 50 nm, more preferably 3 to 30 nm, More preferably, it is 3 to 20 nm. When the average thickness of the shell is less than the lower limit, in the catalyst in which the noble metal is in contact with the core-shell type oxygen storage/release material, the noble metal (especially rhodium) is likely to come into contact with the ceria in the core, and the noble metal (especially rhodium) interaction with ceria degrades rich NOx transient cleanup performance. On the other hand, when the above upper limit is exceeded, the diffusion of oxygen is inhibited by the shell, and the oxygen absorbing/releasing capacity tends to decrease. In addition, in the present invention, the average thickness of the shell can be obtained by the following method. That is, first, a sample is prepared by embedding a core-shell type oxygen storage/release material in a cured epoxy resin using a room-temperature-curable epoxy resin, and this sample is thinned by focused ion beam (FIB) processing or microtome processing. . A cross section of the obtained flake is observed with a transmission electron microscope (TEM) or a scanning transmission electron microscope (STEM) to obtain a TEM image or an STEM image, and element distribution analysis is performed. Next, based on the obtained elemental distribution analysis results, the region where the alumina-based oxide exists is specified in the TEM image or the STEM image, and this region is defined as the shell. A total of 10 to 20 points for measuring the thickness of the shell are randomly selected from a plurality of TEM images or STEM images, the thickness of the shell at each measurement point is measured, and the average value is obtained. , which is the average thickness of the shell.

In the core-shell type oxygen storage/release material of the present invention, the content of the shell made of the alumina-based oxide (preferably the content of alumina) is 0.1 to 20 parts by mass with respect to 100 parts by mass of the core. is preferred, and 0.5 to 15 parts by mass is more preferred. When the content of the shell is less than the lower limit, in the catalyst in which the noble metal is in contact with the core-shell type oxygen storage/release material, the noble metal (especially rhodium) easily comes into contact with the ceria in the core, and the noble metal (especially rhodium) and ceria, the rich NOx transient purification performance tends to decrease. On the other hand, if the above upper limit is exceeded, the alumina-based oxide tends to aggregate, which hinders the diffusion of oxygen and causes oxygen absorption and release. performance tends to decline.

θ-alumina is preferable as the alumina-based oxide according to the present invention. By containing θ-alumina, the oxygen storage capacity is improved. The inclusion of θ-alumina can be confirmed by observing an electron beam diffraction pattern derived from θ-alumina in electron beam diffraction measurement using a transmission electron microscope.

Moreover, the shell made of such an alumina-based oxide may further contain a rare earth element (preferably a rare earth element other than Ce). When such a rare earth element is contained in the shell, the high temperature durability of the shell tends to be further improved. Examples of the rare earth elements include Sc, Y, La, Pr, Nd, Sm, Gd, Tb, Dy, Yb, and Lu. La is preferred. These rare earth elements may be contained singly or in combination of two or more. Also, when the rare earth element is contained, the element is usually contained in the shell as an oxide.

In the shell according to the present invention, when the rare earth element is contained, the content thereof is preferably 10 mol % or less in terms of the ratio of the rare earth element to the total of the Al element and the rare earth element (in terms of element). , is more preferably 5 mol % or less, and even more preferably 2 mol % or less. When the content of the rare earth element exceeds the upper limit, an aluminate phase is formed, which tends to lower the specific surface area of the shell and lower the high-temperature durability. Although the lower limit of the content of the rare earth element is not particularly limited, it is preferably 0.1 mol % or more in order to reliably obtain the effect of the rare earth element.

The specific surface area of the core-shell type oxygen storage/release material of the present invention is not particularly limited, but is preferably 1.0 m ² /g or less, more preferably 0.1 to 0.9 m ² /g. It is preferably 0.3 to 0.8 m ² /g, and more preferably 0.3 to 0.8 m 2 /g. When the specific surface area is less than the lower limit, the oxygen storage capacity tends to decrease. Such a specific surface area can be calculated as a BET specific surface area from the adsorption isotherm using the BET isotherm adsorption equation.

Next, a method for producing the core-shell type oxygen-absorbing material of the present invention will be described. The core-shell type oxygen-absorbing material of the present invention can be produced, for example, by the following method for producing the core-shell-type oxygen-absorbing material of the present invention, that is,
A molded body obtained by pressure molding a ceria-zirconia solid solution is subjected to a reduction treatment at a temperature of 1500° C. or higher and then to a pulverization treatment to have at least one ordered phase of a pyrochlore phase and a κ phase. A step of obtaining a ceria-zirconia solid solution powder (reduction treatment step);
The ceria-zirconia solid solution powder having the ordered phase is stirred by rotating the reactor into which the ceria-zirconia solid solution powder having the ordered phase is charged with the horizontal direction as the rotation axis. A step of contacting a ceria-zirconia solid solution powder having an ordered phase with a vaporized alumina precursor in an oxidizing atmosphere to attach an alumina oxide to the surface of the ceria-zirconia solid solution powder having an ordered phase (rotating CVD process) and
The ceria-zirconia solid solution powder having the alumina-based oxide attached to the surface is fired to obtain a core made of the ceria-zirconia solid solution powder having the ordered phase and an alumina-based oxide disposed on the surface of the core. a step of obtaining a core-shell oxygen-absorbing material having a shell made of a substance (firing step);
It can be manufactured by a manufacturing method including

The ceria-zirconia solid solution used in the production of the core-shell type oxygen storage/release material of the present invention has a content ratio of Ce and Zr in a molar ratio (Ce:Zr) of 35:65 to 65:35. is preferred, and a ratio of 45:55 to 55:45 is more preferred. When a ceria-zirconia-based solid solution having a molar ratio (Ce:Zr) outside the above range is used, when the obtained core-shell type oxygen storage/release material is exposed to high temperature, the ordered phase changes to a fluorite structure due to rearrangement. , the oxygen storage capacity tends to decrease.

Such a ceria-zirconia solid solution may further contain additional elements such as rare earth elements other than Ce and Ti. When such an additive element is contained, there is a tendency that the decrease in oxygen storage capacity when the obtained core-shell type oxygen storage material is exposed to high temperature is further suppressed. Examples of such additive elements include the additive elements exemplified as those that may be contained in the core of the core-shell oxygen-absorbing material. Y, La, Pr, and Nd are preferable, and Pr is more preferable, from the viewpoint of further suppressing the decrease in the oxygen storage capacity when it is used. These additive elements may be contained singly or in combination of two or more. In addition, when the additive element is contained, the element is usually contained in the core as an oxide, and is present in the ceria-zirconia solid solution in a solid solution, dispersed state, or the like. is preferred, and in order to obtain the effect of the additive element with certainty, it is more preferred that the additive element is in solid solution.

Such a ceria-zirconia solid solution may further contain additional elements such as rare earth elements other than Ce and Ti. When such an additive element is contained, there is a tendency that the decrease in oxygen storage capacity when the obtained core-shell type oxygen storage material is exposed to high temperature is further suppressed. Examples of such additive elements include the additive elements exemplified as those that may be contained in the core of the core-shell oxygen-absorbing material. Y, La, Pr, and Nd are preferable, and Pr is more preferable, from the viewpoint of further suppressing the decrease in the oxygen storage capacity when it is used. These additive elements may be contained singly or in combination of two or more. In addition, when the additive element is contained, the element is usually contained in the core as an oxide, and is present in the ceria-zirconia solid solution in a solid solution, dispersed state, or the like. is preferred, and in order to obtain the effect of the additive element with certainty, it is more preferred that the additive element is in solid solution.

In the ceria-zirconia solid solution, when the additive element is contained, the content thereof is 20 mol % or less in terms of the ratio (element conversion) of the additive element to the total of the ceria element, the zirconia element and the additive element. Preferably, 10 mol % or less is more preferable, and 5 mol % or less is even more preferable. If the content of the additive element exceeds the above upper limit, the heat resistance of the ordered phase tends to decrease, and the oxygen absorption/desorption capacity tends to decrease when exposed to high temperatures. Although the lower limit of the content of the additive element is not particularly limited, it is preferably 0.1 mol % or more in order to reliably obtain the effect of the additive element.

Such a ceria-zirconia solid solution can be produced, for example, by the following coprecipitation method. That is, using an aqueous solution containing a cerium salt (e.g., nitrate) and a zirconium salt (e.g., nitrate), and optionally a salt of the additive element (e.g., nitrate), a surfactant, etc., in the presence of ammonia A powdery ceria-zirconia-based solid solution can be obtained by forming a co-precipitate, separating, collecting and washing the obtained co-precipitate, followed by drying, firing, and pulverization. The content of each raw material in the aqueous solution is appropriately adjusted so that the content of each component in the resulting ceria-zirconia solid solution is a predetermined amount.

When producing the core-shell type oxygen storage/release material of the present invention, first, such a ceria-zirconia solid solution is pressure-molded. The pressure during pressure molding is preferably 400 to 3500 kgf/cm ² (39 to 343 MPa), more preferably 500 to 3000 kgf/cm ² (49 to 294 MPa). If the molding pressure deviates from the above range, the resulting core-shell type oxygen-absorbing material tends to lower its oxygen-absorbing capacity when exposed to high temperatures. There is no particular limitation on the method of such pressure molding, and a known pressure molding method such as hydrostatic pressing can be employed as appropriate.

Next, the pressure-molded body thus obtained is subjected to reduction treatment at a temperature of 1500° C. or higher (reduction treatment step). As a result, a ceria-zirconia solid solution having at least one of ordered phases of the pyrochlore phase and the κ phase according to the present invention is formed. A ceria-zirconia solid solution having such an ordered phase has excellent thermal stability on the surface, and has a dense structure in which a solid-phase reaction hardly progresses. If the reduction treatment temperature is below the above lower limit, the stability of the ordered phase is low, and the oxygen storage capacity of the resulting core-shell type oxygen storage/release material is lowered when exposed to high temperatures. In addition, the reduction treatment temperature is 1600° C. from the viewpoint that the stability of the ordered phase is improved and the reduction in oxygen storage capacity when the obtained core-shell type oxygen storage material is exposed to high temperature is suppressed. The above is preferable. The reduction treatment time is preferably 0.5 hours or longer, more preferably 1 hour or longer. If the reduction treatment time is less than the above lower limit, the stability of the ordered phase is low, and the oxygen storage capacity tends to decrease when the obtained core-shell type oxygen storage/release material is exposed to high temperatures. The upper limits of the reduction treatment temperature and the reduction treatment time are not particularly limited. preferably 10 hours or less).

The method of reduction treatment is not particularly limited as long as it is a method capable of reducing the pressure-molded body at a predetermined temperature in a reducing atmosphere. (ii) a method of performing a reduction treatment by heating at a predetermined temperature by introducing a reducing gas into the furnace to make the atmosphere in the furnace a reducing atmosphere after installing the molded body and evacuating the furnace; After the pressure-molded body is placed in the furnace and evacuated using (iii) placing the press-molded body in a crucible filled with activated carbon, heating it at a predetermined temperature, and reducing it with a reducing gas such as CO or HC generated from the activated carbon or the like; A method of applying a reduction treatment by setting the atmosphere in the crucible to a reducing atmosphere, or the like can be mentioned.

The reducing gas used to achieve such a reducing atmosphere is not particularly limited, and examples thereof include reducing gases such as CO, HC, H ₂ and other hydrocarbon gases. In addition, among such reducing gases, carbon (C) is not included from the viewpoint of preventing the generation of duplicate products such as zirconium carbide (ZrC) when the reduction treatment is performed at a higher temperature. things are preferred. The use of such a reducing gas that does not contain carbon (C) enables reduction treatment at a higher temperature close to the melting point of zirconium or the like, so that the stability of the ordered phase can be sufficiently improved. becomes.

In the method for producing a core-shell type oxygen storage/release material of the present invention, it is preferable that the ceria-zirconia solid solution having the ordered phase is further subjected to an oxidation treatment after the reduction treatment. As a result, the oxygen lost during the reduction treatment is compensated for, and the stability as an oxide material tends to be improved. The method of such oxidation treatment is not particularly limited, and for example, a method of heat-treating the ceria-zirconia solid solution having the ordered phase in an oxidizing atmosphere (for example, in the air) can be suitably employed. The heating temperature for such oxidation treatment is not particularly limited, but is preferably about 300 to 800.degree. Furthermore, the heating time during the oxidation treatment is not particularly limited, but is preferably about 0.5 to 5 hours.

Next, the ceria-zirconia solid solution having the ordered phase thus obtained is pulverized to obtain the ceria-zirconia solid solution powder having the ordered phase. The pulverization method is not particularly limited, and examples thereof include wet pulverization, dry pulverization, freeze pulverization, and the like. As the condition of the pulverization, it is preferable to pulverize so that the secondary particle diameter D50 at which the cumulative volume becomes 50% in the volume-based particle size distribution falls within a predetermined range.

Next, by a rotary CVD method, the ceria-zirconia solid solution powder having an ordered phase obtained as described above and an alumina precursor are brought into contact in an oxidizing atmosphere to obtain a ceria-zirconia system having an ordered phase. An alumina-based oxide is attached to the surface of the solid solution powder (rotary CVD process).

FIG. 1 shows a schematic diagram of a rotary CVD apparatus 1 used in the rotary CVD process. The rotary CVD apparatus 1 is installed in a rotary film formation chamber 11 rotatable around a horizontal direction (a direction perpendicular to the direction of gravity) as a rotation axis, a precursor vaporization section 12 , and the precursor vaporization section 12 . The precursor holder 13 and the heating furnace 14 are provided. A precursor supply pipe 15 and an oxygen-containing gas supply pipe 16 are connected to the rotary film forming chamber 11 . A blade 17 is installed inside the rotary film-forming chamber 11 , and the powder in the rotary film-forming chamber 11 is agitated by the rotation of the rotary film-forming chamber 11 .

Specifically, first, the ceria-zirconia solid solution powder having the ordered phase is introduced into the rotary film forming chamber 11 of the rotary CVD apparatus 1 . Also, the alumina precursor A is put into the precursor holder 13 . The alumina precursor used here is not particularly limited as long as it can be used to form an alumina-based oxide thin film by CVD (chemical vapor deposition). dipivaloylmethanato)aluminum (Al(DPM) ₃ ), tris(2,4-pentanedionato)aluminum (aluminum acetylacetonato, Al(acac) ₃ ), trimethylaluminum (TMA), etc.). These alumina precursors may be used singly or in combination of two or more. Among these alumina precursors, Al(DPM) ₃ and Al(acac) ₃ are preferable from the viewpoints of easy availability, high stability in the atmosphere, easy handling, and low vaporization decomposition temperature. (DPM) ₃ is more preferred.

The amount of the alumina precursor added is preferably 0.1 to 20 parts by mass, more preferably 0.5 to 15 parts by mass, with respect to 100 parts by mass of the ceria-zirconia solid solution powder having an ordered phase. more preferred. If the input amount of the alumina precursor is less than the above lower limit, the content of the shell in the core-shell type oxygen storage/release material becomes insufficient, and in the catalyst in which the noble metal is in contact with the core-shell type oxygen storage/release material, the noble metal (especially rhodium) is likely to come into contact with ceria in the core, and the interaction between noble metals (especially rhodium) and ceria tends to reduce the rich NOx transient purification performance. When the content of the shell becomes excessive, the alumina-based oxide tends to agglomerate, which hinders the diffusion of oxygen and tends to reduce the oxygen absorption and release capacity.

Next, the alumina precursor in the precursor holder 13 is heated and vaporized while supplying a carrier gas to the precursor vaporizer 12 . The heating temperature of the alumina precursor is preferably 160 to 220°C, more preferably 170 to 200°C, even more preferably 180 to 195°C. If the heating temperature of the alumina precursor is less than the lower limit, the evaporation rate of the alumina precursor tends to be slow. There is a tendency. Moreover, as said carrier gas, inert gas, such as argon gas and nitrogen gas, is mentioned. The flow rate of the carrier gas is preferably 5-100 sccm, more preferably 5-60 sccm, and even more preferably 6-50 sccm.

Next, the alumina precursor vaporized in this way is supplied together with the carrier gas through the precursor supply pipe 15 to the rotary film forming chamber 11 into which the ceria-zirconia solid solution powder having the ordered phase is charged. , Also, an oxygen-containing gas (oxygen gas or a mixed gas of oxygen and an inert gas) is supplied through the oxygen-containing gas supply pipe 16 independently of the vaporized alumina precursor to the ceria-zirconia solid solution having the ordered phase. The powder is supplied to the rotary film forming chamber 11 into which the powder is charged. At this time, while rotating the rotary film-forming chamber 11 about the horizontal direction as a rotation axis, the vaporized alumina precursor and the oxygen-containing gas are supplied to the rotary film-forming chamber 11, whereby the rotary film-forming chamber 11 is rotated. While stirring the ceria-zirconia solid solution powder having an ordered phase in the film forming chamber 11, the ceria-zirconia solid solution powder having an ordered phase and the vaporized alumina precursor are brought into contact with each other in an oxidizing atmosphere. can be done. As a result, the alumina precursor can be converted into an alumina-based oxide while adhering the alumina precursor to the surface of the ceria-zirconia solid solution powder having the ordered phase, and the ceria-zirconia having the ordered phase The alumina-based oxide can be adhered to the surface of the solid-solution powder. The alumina precursor may be converted to the alumina-based oxide just before it adheres to the surface of the ceria-zirconia solid solution powder having the ordered phase, may be converted at the same time as the adhesion, or may may be converted after (preferably immediately after).

The oxygen concentration in the oxygen-containing gas may be 100% or less than 100%. ) is preferably 1/10 to 5/1, more preferably 1/5 to 5/1, still more preferably 1/2 to 5/1, particularly preferably 1/1 to 5/1. It is set in consideration of the flow rates of the carrier gas and the oxygen-containing gas, which will be described later. When the volume ratio of oxygen and inert gas in the rotary film forming chamber 11 is less than the lower limit, the oxidation reaction of the alumina precursor is inhibited, and a shell made of a sufficient alumina-based oxide tends to be difficult to form. On the other hand, when the upper limit is exceeded, the alumina precursor is oxidized before the alumina precursor reaches the surface of the ceria-zirconia solid solution powder having the ordered phase, and a uniform shell made of alumina-based oxide is formed. tend to be difficult to form. Moreover, argon gas, nitrogen gas, etc. are mentioned as an inert gas.

In addition, the flow rate of the oxygen-containing gas is adjusted so that the volume ratio of oxygen and inert gas in the rotary film forming chamber 11 is within the above range. Although it is set in consideration of the concentration, it is preferably 1 to 300 sccm, more preferably 20 to 250 sccm, even more preferably 24 to 200 sccm. If the flow rate of the oxygen-containing gas is less than the lower limit, the oxygen concentration suitable for film formation cannot be achieved in the rotary film forming chamber 11, and a sufficient shell made of alumina-based oxide tends to be difficult to form. On the other hand, when the upper limit is exceeded, the alumina precursor is oxidized before it reaches the surface of the ceria-zirconia solid solution powder having the ordered phase, and a uniform shell made of alumina-based oxide is formed. tends to be difficult.

The rotational speed of the rotary film forming chamber is preferably 10 to 60 rpm, more preferably 20 to 45 rpm, even more preferably 25 to 30 rpm. When the rotation speed of the film forming chamber is less than the lower limit, the floating time of the ceria-zirconia solid solution powder having the ordered phase becomes short, and the thickness of the shell made of the alumina-based oxide tends to become thin. If the above upper limit is exceeded, the ceria-zirconia solid solution powder having the ordered phase tends to aggregate, making it difficult to form a uniform shell made of an alumina oxide.

In this way, by heating the ceria-zirconia solid solution powder having the ordered phase with the alumina precursor attached to the surface in an oxidizing atmosphere, the alumina precursor is converted to an alumina oxide, and the surface A ceria-zirconia-based solid solution powder having the ordered phase in which the alumina-based oxide adheres to is obtained.

The heating temperature (film formation temperature) of the ceria-zirconia solid solution powder having an ordered phase is preferably 400 to 1100°C, more preferably 600 to 1000°C, and even more preferably 800 to 1000°C. If the film formation temperature is less than the lower limit, it may not be possible to stably form a shell made of an alumina-based oxide. .

The pressure in the rotary film forming chamber (total pressure during film formation) is preferably 50 to 2000 Pa, more preferably 100 to 1000 Pa, and even more preferably 500 to 1000 Pa. When the total pressure during the film formation is less than the lower limit, the concentrations of oxygen and alumina precursor supplied to the surface of the ceria-zirconia solid solution powder having the ordered phase decrease, and the shell made of the alumina-based oxide is formed. The formation rate tends to be slow, and on the other hand, if the upper limit is exceeded, the alumina precursor tends to clog the piping.

The film formation time is preferably 30 to 240 minutes, more preferably 60 to 180 minutes, even more preferably 60 to 120 minutes. If the film formation time is less than the lower limit, the thickness of the shell made of the alumina-based oxide tends to be too thin, while if it exceeds the upper limit, the alumina-based oxide constituting the shell tends to aggregate easily. It is in.

Next, the ceria-zirconia solid solution powder having the ordered phase with the alumina-based oxide adhering to the surface thus obtained is fired (firing step). As a result, the core-shell type oxygen storage/release material of the present invention comprising a core made of the ceria-zirconia solid solution powder having the ordered phase and a shell made of an alumina oxide disposed on the surface of the core is produced. can get.

The firing temperature is preferably 300 to 1100°C, more preferably 500 to 900°C. When the firing temperature is less than the lower limit, it tends to be difficult to form a stable shell. Although the firing time is not particularly limited, it is preferably 2 to 10 hours.

Next, the exhaust gas purifying catalyst of the present invention will be described. The exhaust gas purifying catalyst of the present invention comprises the core-shell type oxygen storage/release material of the present invention and a noble metal in contact with the core-shell type oxygen storage/release material. Such an exhaust gas purifying catalyst of the present invention exhibits excellent NOx purification performance (rich NOx transient purification performance) even when the exhaust gas flowing into the catalyst is switched from lean gas to rich gas.

In the exhaust gas purification catalyst of the present invention, the noble metal is preferably Rh, Pd, or Pt, more preferably Rh or Pd, and particularly preferably Rh, from the viewpoint of obtaining excellent NOx purification performance. In the exhaust gas purifying catalyst of the present invention, the form of such noble metal is not particularly limited as long as it is in contact with the core-shell type oxygen-absorbing material. They may be carried and brought into contact, but from the viewpoint of simple operation, the core-shell type oxygen storage/release material and other oxide material carrying a noble metal may be mixed and brought into contact.

The exhaust gas purifying catalyst of the present invention may be used in the form of pellets filled in a reaction tube or the like. It is preferably used as a honeycomb catalyst in which a catalyst layer and an alumina-containing catalyst layer are formed. In addition, among such honeycomb catalysts, from the viewpoint of exhibiting excellent oxygen storage and release performance even when exposed to high temperature and high flow rate gas and exhibiting excellent NOx purification performance, honeycomb catalysts are preferred. It is preferable to have a catalyst lower layer containing noble metal and alumina formed on the inner walls of the pores of the material, and a catalyst upper layer composed of the exhaust gas purifying catalyst of the present invention formed on the catalyst lower layer, and the catalyst upper layer is more preferably a mixture of the core-shell type oxygen storage/release material of the present invention and zirconia supporting a noble metal.

Next, the exhaust gas purifying method of the present invention will be described. The exhaust gas purifying method of the present invention comprises contacting exhaust gas containing nitrogen oxides with the exhaust gas purifying catalyst of the present invention. The method for bringing the exhaust gas into contact with the exhaust gas purifying catalyst of the present invention is not particularly limited, and a known method can be appropriately adopted. By arranging the exhaust gas purifying catalyst of the invention, a method of contacting the exhaust gas purifying catalyst with the exhaust gas from the internal combustion engine may be employed.

Since the exhaust gas purification catalyst of the present invention used in the exhaust gas purification method of the present invention has excellent rich NOx transient purification performance, even when the exhaust gas flowing into the catalyst is switched from lean gas to rich gas, excellent It is possible to exhibit NOx purification performance, and for example, when purifying exhaust gas from an internal combustion engine of an automobile or the like using the exhaust gas purification catalyst of the present invention, even if the exhaust gas is switched from lean gas to rich gas, It is possible to sufficiently purify harmful gases such as NOx contained in the exhaust gas.

EXAMPLES The present invention will be described in more detail below based on examples and comparative examples, but the present invention is not limited to the following examples. The ceria-zirconia-praseodymium composite oxide powder used in Examples and Comparative Examples was prepared by the following method.

(Preparation Example 1)
A ceria-zirconia solid solution powder containing cerium, zirconium and praseodymium in a molar ratio ([cerium]:[zirconium]:[praseodymium]) of 45:54:1 was prepared as follows. That is, first, it contains 442 g of a 28% by mass cerium nitrate aqueous solution in terms of CeO2, 590 _g of an 18% by mass zirconium oxynitrate aqueous solution in terms of _ZrO2 , and 1.2 g of _praseodymium nitrate in terms of _Pr6O11 . 100 g of the aqueous solution and 197 g of hydrogen peroxide solution containing 1.1 molar amount of hydrogen peroxide as much as the contained cerium were added to 1217 g of the aqueous solution containing 1.2 molar equivalents of ammonia with respect to the neutralization equivalent. The resulting coprecipitate was centrifuged and washed with deionized water. Next, the obtained coprecipitate was dried at 110° C. for 10 hours or longer and then calcined in the atmosphere at 400° C. for 5 hours to form a solid solution of cerium, zirconium and praseodymium (CeO ₂ —ZrO ₂ —Pr ₆ O ₁₁ solid solution) was obtained. Thereafter, the solid solution was pulverized using a pulverizer ("Wonder Blender" manufactured by AS ONE Co., Ltd.) so that the particle size became 75 μm or less by sieving to obtain the ceria-zirconia-praseodymium solid solution powder.

Next, 20 g of this ceria-zirconia-praseodymium solid solution powder was packed in a polyethylene bag (capacity: 0.05 L), the inside was degassed, and the opening of the bag was heated and sealed. Subsequently, using a hydrostatic press (“CK4-22-60” manufactured by Nikkiso Co., Ltd.), the bag was subjected to a hydrostatic press (molding pressure) of 2000 kgf/cm ² (196 MPa) for 1 minute. CIP) molding was performed to obtain a molding of the ceria-zirconia-praseodymium solid solution powder. The size of the molded body was 4 cm long, 4 cm wide, 7 mm average thickness, and about 20 g in mass.

Next, the obtained moldings (2 sheets) were placed in a crucible (inner volume: 8 cm in diameter, 7 cm in height) filled with 70 g of activated carbon, covered with a lid, and then placed in a high-speed heating electric furnace and heated to 1000 ° C. The temperature was raised to 1700° C. over 1 hour, further raised to 1700° C. over 4 hours, and then heated at 1700° C. (reduction treatment temperature) for 5 hours. Then, after cooling to 1000° C. over 4 hours, it was naturally cooled to room temperature to obtain a reduction fired product.

Next, this reduced calcined product was heated in the atmosphere at a temperature of 500° C. for 5 hours to oxidize it, and the content ratio of cerium, zirconium, and praseodymium in the composite oxide was changed to a molar ratio ([cerium]:[zirconium]: [praseodymium]) to obtain a ceria-zirconia-praseodymium composite oxide of 45:54:1. This ceria-zirconia-praseodymium composite oxide is pulverized using a pulverizer ("Wonder Blender" manufactured by AS ONE Co., Ltd.) so that the secondary particle diameter D50 at which the cumulative volume is 50% in the volume-based particle size distribution is 10 μm or less. to obtain a ceria-zirconia-praseodymium composite oxide powder (hereinafter abbreviated as "CZP powder"). The specific surface area of this CZP powder was 0.6 m ² /g. The volume-based particle size distribution was measured by a dynamic light scattering method using a particle size distribution analyzer ("Laser diffraction/scattering particle size distribution analyzer MT3300EX" manufactured by Nikkiso Co., Ltd.). In addition, the specific surface area of the CZP powder is obtained by measuring the nitrogen adsorption and desorption isotherm at a pretreatment temperature of 250 ° C. using a fully automatic specific surface area measuring device (“MicroSorp 4232 system 3 SSA analyzer” manufactured by Microdata). It was obtained by the BET method based on the nitrogen adsorption/desorption isotherm.

(Comparative Preparation Example 1)
A ceria-zirconia solid solution powder containing cerium and zirconium in a molar ratio ([cerium]:[zirconium]) of 45.5:54.5 was prepared as follows. That is, first, 442 g of a 28% by mass cerium nitrate aqueous solution in terms of CeO ₂ , 590 g of an 18% by mass zirconium oxynitrate aqueous solution in terms of ZrO ₂ , and hydrogen peroxide in an amount 1.1 times the molar amount of cerium contained. 197 g of hydrogen peroxide water is added to 1217 g of an aqueous solution containing 1.2 equivalents of ammonia with respect to the neutralization equivalent to form a coprecipitate, and the obtained coprecipitate is centrifuged to remove ions. Washed with exchange water. Next, the obtained coprecipitate was dried at 110° C. for 10 hours or more, and then calcined in air at 400° C. for 5 hours to obtain a solid solution of cerium and zirconium (CeO ₂ —ZrO ₂ solid solution). After that, the solid solution is pulverized using a pulverizer ("Wonder Blender" manufactured by AS ONE Corporation) so that the particle size becomes 75 μm or less by sieving, and the cumulative volume in the volume-based particle size distribution is 50%. Ceria-zirconia composite oxide powder (hereinafter abbreviated as “CZ powder”) was obtained by pulverizing with the aforementioned pulverizer so that the secondary particle diameter D50 was 10 μm. The volume-based particle size distribution was measured by a dynamic light scattering method using a particle size distribution analyzer ("Laser diffraction/scattering particle size distribution analyzer MT3300EX" manufactured by Nikkiso Co., Ltd.).

(Example 1)
Alumina was deposited on the surface of the CZP powder obtained in Preparation Example 1 using the rotary CVD apparatus (manufactured by Toei Kagaku Sangyo Co., Ltd.) shown in FIG. Specifically, first, 5.00 g of the CZP powder was put into the rotary film forming chamber 11 of the rotary CVD apparatus 1 . Further, 5.20 g of tris(dipivaloylmethanato)aluminum (Al(DPM) ₃ , manufactured by Toshima Seisakusho Co., Ltd.) as alumina precursor A was placed in the precursor holder 13 of the precursor vaporizing unit 12 of the rotary CVD apparatus 1 . was put in. Al(DPM) ₃ in the precursor holder 13 is heated to 195° C. while supplying argon gas as a carrier gas to the precursor vaporization unit 12 at a flow rate of 6 sccm to vaporize the Al(DPM) ₃ , and the vaporized Al (DPM) ₃ is passed through the precursor supply pipe 15 together with the argon gas, the inside is heated to 700° C., the pressure is controlled to 500 Pa, and the rotation speed is 25 rpm with the rotation axis in the horizontal direction. It was supplied to the normal film-forming chamber 11 . In addition, oxygen gas (oxygen concentration 100%) is also supplied to the rotary film forming chamber 11 at the same time so that the volume ratio (oxygen/argon) between oxygen and argon in the film forming chamber is 4/1. It was supplied through supply tube 16 at a flow rate of 24 sccm. As a result, the CZP powder and vaporized Al(DPM) ₃ came into contact with each other in an oxidizing atmosphere, and the Al(DPM) ₃ adhered to the surface of the CZP powder and was converted to alumina. This operation was continued for 120 minutes to obtain a CZP powder with an alumina thin film attached to the surface. After that, the CZP powder with the alumina thin film adhered to the surface is fired in the atmosphere at 500° C. for 5 hours to obtain a core-shell oxygen storage/release material powder in which a shell made of an alumina thin film is formed on the surface of the core made of the CZP powder. got The amount of alumina supported in this core-shell type oxygen storage/release material powder was 13 parts by mass with respect to 100 parts by mass of the CZP powder.

(Example 2)
The amount of Al(DPM) ₃ is set to 0.60 g, the flow rate of oxygen gas is set to 200 sccm, the flow rate of argon gas is set to 50 sccm, the heating temperature of Al(DPM) ₃ in the precursor holder 13 is set to 200° C., and the above Core-shell type oxygen absorption/desorption in which a shell made of an alumina thin film was formed on the surface of the core made of the CZP powder in the same manner as in Example 1, except that the temperature in the rotary film forming chamber 11 was changed to 880 ° C. A material powder was prepared. The amount of alumina supported in this core-shell type oxygen storage/release material powder was 0.5 parts by mass with respect to 100 parts by mass of the CZP powder.

(Comparative example 1)
An aqueous alumina precursor solution was prepared by adding 150 ml of an aqueous solution containing 9.6 g of a quaternary amine to 50 ml of an aqueous solution containing 3.66 g of aluminum nitrate and 3.76 g of a carboxylic acid chelating agent. 100 g of the CZP powder obtained in Preparation Example 1 was added to this alumina precursor aqueous solution, and the mixture was heated with stirring to evaporate to dryness. The resulting dried material was calcined in the atmosphere at 500° C. for 5 hours to obtain a core-shell type oxygen storage/release material powder in which a shell made of an alumina thin film was formed on the surface of a core made of the CZP powder. The amount of alumina supported in this core-shell type oxygen storage/release material powder was 0.5 parts by mass with respect to 100 parts by mass of the CZP powder.

(Comparative example 2)
From the CZP powder in the same manner as in Comparative Example 1, except that the amount of aluminum nitrate was changed to 7.32 g, the amount of carboxylic acid chelating agent was changed to 7.52 g, and the amount of quaternary amine was changed to 19.2 g. A core-shell type oxygen absorbing/releasing material powder was obtained in which a shell consisting of an alumina thin film was formed on the surface of a core composed of The amount of alumina supported in this core-shell type oxygen storage/release material powder was 1.0 parts by mass with respect to 100 parts by mass of the CZP powder.

(Comparative Example 3)
29.4 g of aluminum nitrate nonahydrate was dissolved in 100 g of deionized water, and 20 g of 25% aqueous ammonia was added to prepare an aluminum hydroxide gel. After subjecting this aluminum hydroxide gel to centrifugation and washing with ion-exchanged water twice, 50 g of ion-exchanged water and 14.8 g of an 88 mass% concentration methanoic acid aqueous solution were added and stirred to obtain aluminum hydroxide methanoate. An aqueous solution of the complex (Al(HCOO) ₂ (OH)) was obtained. 100 g of the CZP powder obtained in Preparation Example 1 was added to this aqueous solution of aluminum hydroxide methanoate complex, and the mixture was heated with stirring to evaporate to dryness. The resulting dried material was calcined in the atmosphere at 500° C. for 5 hours to obtain a core-shell type oxygen storage/release material powder in which a shell made of an alumina thin film was formed on the surface of a core made of the CZP powder. The amount of alumina supported in this core-shell type oxygen storage/release material powder was 8 parts by mass with respect to 100 parts by mass of the CZP powder.

(Comparative Example 4)
CZP A core-shell type oxygen storage/release material powder was obtained in which a shell made of an alumina thin film was formed on the surface of a core made of powder. The amount of alumina supported in this core-shell type oxygen storage/release material powder was 16 parts by mass with respect to 100 parts by mass of the CZP powder.

(Comparative Example 5)
9.5 mmol of aluminum nitrate and 0.096 mmol of lanthanum nitrate were dissolved in 200 ml of ion-exchanged water to prepare an aqueous La-containing alumina precursor solution. To this La-containing alumina precursor aqueous solution, 100 g of the CZ powder obtained in Comparative Preparation Example 1 was added, stirred for 15 minutes, and heated while stirring to evaporate to dryness. The obtained dried product was calcined at 900° C. for 5 hours. The obtained powder was added again to the La-containing alumina precursor aqueous solution, stirred for 15 minutes, and heated while stirring to evaporate to dryness. The resulting dried material was calcined at 900° C. for 5 hours to obtain a core-shell type oxygen storage/release material powder in which a shell made of an alumina thin film was formed on the surface of a core made of the CZ powder. The amount of alumina supported in this core-shell type oxygen storage/release material powder was 1.0 parts by mass with respect to 100 parts by mass of the CZ powder.

<X-ray photoelectron spectroscopy (XPS) analysis>
The X-ray photoelectron spectroscopy (XPS) spectrum of each core-shell type oxygen storage/release material powder obtained in Examples and Comparative Examples was monochromatized using an X-ray photoelectron spectroscopy analyzer ("Quantera SXM" manufactured by ULVAC-PHI). AlKα (1486.6 eV) was used as an X-ray source, and the measurement was performed under the conditions of photoelectron extraction angle: 45°, analysis area: about 200 μmφ, charge-up correction: Zr3d 182.2 eV (ZrO ₂ ). Based on the obtained XPS spectrum, the elements present in a region (approximately 200 μmφ) at a depth of 3 nm from the surface of the core-shell type oxygen storage/release material powder (surface of the shell) were quantified using the following formula:
Alumina-based oxide coverage = C _Al / (C _Al + C _Ce + C _Zr + C _M ) x 100
(In the formula, C _Al represents the Al concentration, C _Ce the Ce concentration, C _Zr the Zr concentration, and _CM the concentration of the additive metal element M other than Al, Ce and Zr (Pr in the examples and comparative examples)).
The coverage of the alumina-based oxide was determined by the method. The results are shown in Table 1 and FIG.

<Measurement of average thickness of alumina-based oxide thin film (shell)>
First, using a room temperature curable epoxy resin, a sample was prepared by embedding each core-shell type oxygen storage/release material powder obtained in Examples and Comparative Examples in an epoxy resin cured product, and this sample was subjected to a focused ion beam ( FIB) processing or thinning by microtoming. A cross section of the obtained flake was observed with a transmission electron microscope (TEM) or a scanning transmission electron microscope (STEM) to obtain a TEM image or an STEM image, and an elemental distribution analysis was performed. Next, based on the obtained elemental distribution analysis results, the region where the alumina-based oxide was present was specified in the TEM image or the STEM image, and this region was used as the alumina-based oxide thin film. A total of 10 to 20 measurement points for the thickness of the alumina-based oxide thin film are randomly extracted from a plurality of TEM images or STEM images, and the thickness of the alumina-based oxide thin film at each measurement point is measured. Then, the average value of them was obtained, and this was taken as the average thickness of the alumina-based oxide thin film (shell). The results are shown in Table 1 and FIG.

<X-ray diffraction (XRD) measurement>
The X-ray diffraction patterns of the ordered phase (ordered core phase) and the α phase (α phase of the shell) of each core-shell type oxygen storage/release material powder obtained in Examples and Comparative Examples were analyzed using an X-ray diffractometer (Rigaku Corporation). "Ultima IV" (manufactured by Fujifilm Co., Ltd.), CuKα as an X-ray source, 2θ = 10 to 60°, scanning speed: 10°/min, tube voltage: 40V, tube current: 30A. In the obtained X-ray diffraction pattern, the peak intensity I (14) of the diffraction line at 2θ = 14.5°, the peak intensity I (29) of the diffraction line at 2θ = 29°, and the diffraction at 2θ = 35.2° Determine the peak intensity I(35) of the line and use the following formula:
I(14/29)=I(14)/I(29)×100
I(35/29)=I(35)/I(29)×100
Thus, the intensity ratio [I (14/29) value] of the diffraction line at 2θ = 14.5° and the diffraction line at 2θ = 29° and the diffraction line at 2θ = 35.2° and the diffraction line at 2θ = 29° and the intensity ratio [I (35/29) value] was determined. Table 1 shows the results.

<Transmission electron microscope observation>
Each core-shell type oxygen storage/release material powder obtained in Examples and Comparative Examples was observed under a transmission electron microscope ("HF-2100 Cold-FE-TEM" manufactured by Hitachi High-Technologies Corporation or "JEM-ARM200F" manufactured by JEOL Ltd. ), TEM observation and electron beam diffraction analysis were performed under the condition of an acceleration voltage of 200 kV. 4 and 5 show transmission electron micrographs of the core-shell type oxygen storage/release material powders obtained in Examples 1 and 2. FIG. 6 to 8 show electron beam diffraction patterns of the core-shell type oxygen storage/release material powders obtained in Examples 1 and 2 and Comparative Example 1. FIG.

<Catalyst preparation>
Each core-shell type oxygen storage/release material powder obtained in Examples and Comparative Examples and Rh-supported Al ₂ O ₃ —ZrO ₂ —La ₂ O ₃ —Nd ₂ O ₃ composite oxide powder (Rh supported amount: 0.2 mass %, Al ₂ O ₃ : ZrO ₂ : La ₂ O ₃ : Nd ₂ O ₃ = 30% by mass: 64% by mass: 4% by mass: 2% by mass, average particle diameter: 20 μm) at a mass ratio of 1:1 The mixture is mixed using a mortar, and the resulting mixture is pressure-molded under a hydrostatic pressure of 1 t. Obtained.

<High temperature endurance test>
A cylindrical reaction tube with a diameter of 10 mm was filled with 1.5 g of the obtained pellet catalyst, and the pellet catalyst was charged with a rich gas [H ₂ (2%) + CO ₂ (10%)+N ₂ (88%)] and lean gas [O ₂ (1%)+CO ₂ (10%)+N ₂ (89%)] were alternately switched for 5 minutes each for 25 hours.

<Rich NOx transition purification rate measurement>
After the high-temperature durability test, 0.5 g of the pelletized catalyst was packed in a reaction tube with a diameter of 10 mm, and attached to a fixed-bed flow-type catalytic activity evaluation device ("CATA-5000-7SP" manufactured by Best Sokki Co., Ltd.). To this catalyst, stoichiometric model gas [NO (1500 volume ppm) + CO ₂ (10 volume%) + O ₂ (0.7 volume%) + CO (0.65 volume%) + C ₃ H ₆ (3000 volume ppmC) + H ₂ O (5 vol%) + N ₂ (remainder)] was circulated at a gas flow rate of 10 L / min and a catalyst-containing gas temperature of 600 ° C. After pretreatment, lean gas [NO (1500 vol ppm) + CO ₂ (10 vol%) + O ₂ (0.8 vol%) + CO (0.65 vol%) + C ₃ H ₆ (3000 vol ppmC) + H ₂ O (5 vol%) + N ₂ (remainder)] gas flow rate 10 L / min, catalyst-containing gas After flowing for 240 seconds at a temperature of 500 ° C., the flowing gas was changed to a rich gas [NO (1500 volume ppm) + CO ₂ (10 volume%) + CO (0.65 volume%) + C ₃ H ₆ (3000 volume ppmC) + H ₂ O ( 5% by volume)+N ₂ (remainder)] and flowed for 240 seconds. The average NO concentrations in the catalyst-containing gas and the catalyst-exhausting gas were measured during the flow of the rich gas, and the rich NOx transient purification rate was obtained. The results are shown in Table 1 and FIG.

As shown in FIGS. 4 and 5, it was confirmed that the surfaces of the core-shell type oxygen storage/release material powders obtained in Examples 1 and 2 were coated with an alumina thin film. Further, as shown in FIGS. 6 and 7, it was confirmed that the alumina thin films of the core-shell type oxygen storage/release material powders obtained in Examples 1 and 2 contained θ-alumina. On the other hand, as shown in FIG. 8, it was found that the alumina thin film of the core-shell type oxygen storage/release material powder obtained in Comparative Example 1 consisted of amorphous-alumina.

As shown in Table 1, FIGS. 2 to 3, and 9, the core-shell type oxygen storage/release material powders (Examples 1 and 2) in which an alumina thin film was formed on the surface of the CZP powder by the rotary CVD method were coated with alumina. ratio, average thickness of the alumina thin film, I(14/29) value and I(35/29) value were within the predetermined ranges, and it was confirmed that the rich NOx transient purification performance was excellent.

On the other hand, core-shell type oxygen storage material powders (Comparative Examples 1 and 2) were prepared by forming an alumina thin film on the surface of the CZP powder by evaporation to dryness using an alumina precursor aqueous solution containing aluminum nitrate and a carboxylic acid chelating agent. ) has a small alumina coverage and is inferior in rich NOx transient purification performance.

In addition, the core-shell type oxygen storage material powder (Comparative Examples 3 and 4) in which an alumina thin film was formed on the surface of the CZP powder by an evaporation drying method using an aqueous solution of an aluminum hydroxide methanoate complex, the average thickness of the alumina thin film was It was found to be thick, the I(35/29) value was larger than the predetermined range (the alumina thin film contained α-alumina), and the rich NOx transient purification performance was poor.

Further, a core-shell type oxygen storage material powder (Comparative Example 5) in which an alumina thin film was formed on the surface of the CZ powder by an evaporation to dryness method using an La-containing alumina precursor aqueous solution containing aluminum nitrate and lanthanum nitrate , the alumina coverage is small, the I(14/29) value is smaller than the predetermined range (the ordered phase is insufficient), and the rich NOx transient purification performance is poor.

As described above, by using the core-shell type oxygen storage/release material of the present invention, it is possible to obtain an exhaust gas purification catalyst that exhibits excellent NOx purification performance even when the exhaust gas flowing into the catalyst is switched from lean gas to rich gas. becomes possible.

Therefore, the core-shell type oxygen-absorbing material of the present invention is useful as a catalyst carrier or co-catalyst for purifying exhaust gas containing nitrogen oxides discharged from internal combustion engines such as automobiles.

Reference Signs List 1: rotary CVD apparatus 11: rotary film formation chamber 12: precursor vaporizer 13: precursor holder 14: heating furnace 15: precursor supply pipe 16: oxygen-containing gas supply pipe 17: blade

Claims (6)

A core made of a ceria-zirconia solid solution powder having an ordered phase of at least one of a pyrochlore phase and a κ phase, and a shell made of an alumina-based oxide disposed on the surface of the core,
The formula below:
Alumina-based oxide coverage = C _Al / (C _Al + C _Ce + C _Zr + C _M ) x 100
(Wherein, C _Al , C _Ce , C _Zr and C _M represent the concentrations of metal elements determined by X-ray photoelectron spectroscopy of the particle surface, C _Al is the Al concentration, C _Ce is the Ce concentration, C _Zr is the Zr concentration, _CM represents the concentration of the additive metal element M other than Al, Ce and Zr)
The coverage of the alumina-based oxide obtained by is 80% or more,
The shell has an average thickness of 2 to 75 nm,
The intensity ratio [I (14/29) value] of the diffraction line at 2θ = 14.5° and the diffraction line at 2θ = 29° obtained from the X-ray diffraction pattern obtained by X-ray diffraction measurement using CuKα is 2. % or more,
The intensity ratio [I (35/29) value] of the diffraction line at 2θ = 35.2° and the diffraction line at 2θ = 29° obtained from the X-ray diffraction pattern obtained by X-ray diffraction measurement using CuKα is 0. .2% or less,
A core-shell type oxygen absorbing/releasing material characterized by: 2. The core-shell oxygen storage/release material according to claim 1, wherein the ceria-zirconia solid solution powder has a specific surface area of 1.0 m ² /g or less. 3. The core-shell oxygen absorbing/releasing material according to claim 1, wherein the alumina-based oxide contains θ-alumina. A molded body obtained by pressure molding a ceria-zirconia solid solution is subjected to a reduction treatment at a temperature of 1500° C. or higher and then to a pulverization treatment to have at least one ordered phase of a pyrochlore phase and a κ phase. obtaining a ceria-zirconia solid solution powder;
The ceria-zirconia solid solution powder having the ordered phase is stirred by rotating the reactor into which the ceria-zirconia solid solution powder having the ordered phase is charged with the horizontal direction as the rotation axis. a step of contacting a ceria-zirconia solid solution powder having an ordered phase with a vaporized alumina precursor in an oxidizing atmosphere to adhere an alumina-based oxide to the surface of the ceria-zirconia solid solution powder having an ordered phase;
The ceria-zirconia solid solution powder having the alumina-based oxide attached to the surface is fired to form a core made of the ceria-zirconia solid solution powder having the ordered phase and an alumina-based oxide disposed on the surface of the core. obtaining the core-shell type oxygen-absorbing material according to claim 1, comprising a shell made of
A method for producing a core-shell type oxygen-absorbing material, comprising: An exhaust gas purifying catalyst comprising: the core-shell type oxygen storage/release material according to any one of claims 1 to 3; and a noble metal in contact with the core-shell type oxygen storage/release material. . 6. A method for purifying an exhaust gas, which comprises contacting an exhaust gas containing nitrogen oxides with the exhaust gas purifying catalyst according to claim 5.

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