JP6630305B2 - Core-shell type oxide material, exhaust gas purification catalyst using the same, and exhaust gas purification method - Google Patents

Description

  The present invention relates to a core-shell oxide material containing a ceria-zirconia-based composite oxide whose surface is coated with an alumina-based oxide, 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 a carrier for a catalyst for purifying exhaust gas, a cocatalyst, and the like. As a metal oxide in such a composite oxide, ceria has been suitably used because it can absorb and release oxygen (has an oxygen storage and release capability) according to the oxygen partial pressure in the atmosphere. . In recent years, various kinds of composite oxides containing ceria have been studied, and various ceria-zirconia-based composite oxides and methods for producing the same have been disclosed.

  For example, Japanese Patent Application Laid-Open No. 2007-144290 (Patent Document 1) discloses a core-shell structure including a core made of oxygen storage / release material particles such as ceria and a shell made of a carrier oxide such as zirconia and titania. Discloses a catalyst for purifying exhaust gas in which noble metal particles containing at least rhodium particles are in contact with the support described above, and also describes that oxidation of rhodium is suppressed and catalytic activity is improved by oxygen storage / release capability.

Japanese Patent Application Laid-Open No. 2005-830 (Patent Document 2) discloses a composite including CeO ₂ —ZrO ₂ solid solution particles and an Al ₂ O ₃ layer covering at least a part of the surface of the CeO ₂ —ZrO ₂ solid solution particles. An exhaust gas purifying catalyst in which Pt and Pd are supported on at least an Al ₂ O ₃ layer of particles is disclosed, which describes that grain growth of a noble metal is suppressed and oxygen storage / release capability is improved. I have.

  Further, Japanese Patent Application Laid-Open No. 2007-69107 (Patent Document 3) discloses an alumina carrier, noble metal particles such as Pt, Pd, and Rh existing inside the alumina carrier, and ceria and zirconia in contact with the noble metal particles. A catalyst for purifying exhaust gas containing co-catalyst particles and the like is disclosed, and aggregation of noble metal particles is suppressed by an anchor effect, so that high catalyst activity is maintained even under fluctuations in air-fuel ratio, and purification performance of the catalyst is improved. It is also described that the reduction is prevented.

  Japanese Patent Application Laid-Open No. 2014-114180 (Patent Document 4) discloses a crystal particle having a pyrochlore structure of a ceria-zirconia composite oxide and a crystal having a pyrochlore structure of a lantana-zirconia composite oxide present on the particle surface. A composite oxide material in which the crystals of the lantana-zirconia composite oxide are at least partially dissolved as solid solutions on the crystal particle surfaces of the ceria-zirconia composite oxide. It is described that the storage capacity is hardly reduced.

JP 2007-144290 A JP-A-2005-830 JP 2007-69107 A JP 2014-114180 A

  However, since zirconia and titania are relatively dense oxides, the exhaust gas purifying catalyst described in Patent Document 1 has a problem that oxygen diffusivity in the shell is low and the oxygen storage / release speed is slow. Was. In addition, when the exhaust gas purifying catalyst is exposed to a high temperature, ceria in the core and zirconia in the shell interdiffuse to destroy the core-shell structure, so that there is a problem in that the catalytic activity of rhodium is reduced.

Further, in the exhaust gas purifying catalysts described in Patent Documents 2 and 3, ceria's oxygen utilization efficiency in CeO ₂ -ZrO ₂ solid solution particles and cocatalyst particles is low, and a sufficiently high oxygen storage / release capability is not necessarily obtained. Was.

  Further, in the catalyst described in Patent Document 4 in which rhodium is supported on a composite oxide material, when exposed to a high temperature, excellent oxygen storage / release capability is exhibited, but there is a problem that NOx purification performance is reduced. The present inventors have found that.

  The present invention has been made in view of the above-mentioned problems of the related art, and has excellent oxygen storage / release capacity (OSC) and oxygen storage / release rate (OSC) even when exposed to high temperatures. -R)) and an oxide material capable of obtaining an exhaust gas purification catalyst exhibiting excellent NOx purification performance, an exhaust gas purification catalyst using the same, and an exhaust gas purification method. Aim.

  The present inventors have conducted intensive studies to achieve the above object, and have at least one ordered phase of a pyrochlore phase and a κ phase, and are composed of a ceria-zirconia-based solid solution powder having a predetermined particle diameter. By coating the surface of the core with the alumina-based oxide at a predetermined ratio, the catalyst in which the noble metal is in contact with the obtained core-shell oxide material has excellent oxygen even when exposed to high temperatures. The inventors have found that they have occlusion and release capabilities (oxygen storage amount (OSC) and oxygen storage and release speed (OSC-r)) and exhibit excellent NOx purification performance, and have completed the present invention.

That is, the core-shell oxide material of the present invention is arranged on a core made of a ceria-zirconia-based solid solution powder having at least one ordered phase of a pyrochlore phase and a κ phase, and is disposed on a part of the surface of the core. A shell made of an alumina-based oxide,
A secondary particle diameter D50 at which the cumulative volume in the volume-based particle size distribution of the ceria-zirconia-based solid solution powder becomes 50% is 0.2 to 8.0 µm;
As measured by X-ray photoelectron spectroscopy, the average concentration of Al element in the region of the depth of 3nm from the surface of the shell is Ri 25～75At% der,
After heating at 1100 ° C. for 5 hours in the air, a 2θ = 14.5 ° diffraction line and a 2θ = 29 ° diffraction line obtained from an X-ray diffraction pattern obtained by X-ray diffraction measurement using CuKα were used. The intensity ratio [I (14/29) value] is 0.030 or more;
It is characterized by the following.

  In such a core-shell oxide material of the present invention, the content of the shell is preferably 0.05 to 2.0 parts by mass with respect to 100 parts by mass of the core. It is preferable that the core further contains a rare earth element other than Ce. Furthermore, it is preferable that the average concentration of the Al element in a region at a depth of 3 nm from the surface of the shell, measured by X-ray photoelectron spectroscopy, is 30 to 75 at%.

  Further, an exhaust gas purifying catalyst of the present invention includes the core-shell oxide material of the present invention and a noble metal in contact with the core-shell oxide material. Further, the exhaust gas purifying method of the present invention is characterized in that an exhaust gas containing nitrogen oxides is brought into contact with the exhaust gas purifying catalyst of the present invention.

The reason why the core-shell oxide material of the present invention has excellent oxygen storage / release properties even when exposed to a high temperature is not necessarily clear, but the present inventors presume as follows. That is, the core in the core-shell oxide material of the present invention is made of a ceria-zirconia-based solid solution powder having at least one ordered phase of a pyrochlore phase and a κ phase. The pyrochlore phase (Ce ₂ Zr ₂ O ₇ ) of such a ceria-zirconia solid solution undergoes a phase change with the κ phase (Ce ₂ Zr ₂ O ₈ ) according to the oxygen partial pressure in the gas phase. Develops occlusion and release ability. The oxygen storage / release capacity developed by such a phase change between the pyrochlore phase and the κ phase has an extremely high oxygen utilization efficiency of CeO ₂ compared to the oxygen storage / release capacity developed in the fluorite phase, and is almost the theoretical limit. In order to reach the value, the ceria-zirconia-based solid solution powder having the ordered phase exhibits a very high oxygen storage capacity (OSC) and a bulk diffusion rate of O ₂ . For this reason, even if at least a part of the surface of the core made of the ceria-zirconia-based solid solution powder having the ordered phase is coated with the alumina-based oxide, the oxygen storage amount (OSC) and the oxygen storage-release rate (OSC) due to the coating can be obtained. It is presumed that the decrease in -r) was small and excellent oxygen storage / release capability was exhibited. Further, since the core-shell oxide material of the present invention is reduced at a high temperature of 1500 ° C. or more, it has excellent high-temperature stability as compared with a normal ceria-zirconia solid solution, and is exposed to a high temperature. It is presumed that excellent oxygen storage / release capability is exhibited.

  Further, the reason why the catalyst in which the core-shell oxide material of the present invention is brought into contact with a noble metal exhibits excellent NOx purification performance even when exposed to high temperatures is not necessarily clear, but the present inventors have Infer as follows. That is, in the catalyst of the present invention in which the noble metal is brought into contact with the core-shell oxide material, the noble metal is in contact with the shell of the core-shell oxide material, that is, the alumina-based oxide coating layer. It is presumed that the NOx purification activity is improved as compared with the case where the noble metal is brought into contact with the core made of the ceria-zirconia-based solid solution powder having the ordered phase. Further, it is speculated that one of the reasons for exhibiting excellent NOx purification performance is that the grain growth of the noble metal is suppressed even when exposed to a high temperature, and the decrease in NOx purification activity is suppressed. .

  ADVANTAGE OF THE INVENTION According to this invention, even if it is exposed to high temperature, it has excellent oxygen storage / release capacity (oxygen storage capacity (OSC) and oxygen storage / release rate (OSC-r)) and excellent NOx. It is possible to obtain an exhaust gas purifying catalyst exhibiting purification performance.

3 is a graph showing an X-ray diffraction pattern of the core-shell oxide material powder obtained in Examples 1 to 5 and Comparative Examples 1 and 2.

  Hereinafter, the present invention will be described in detail with reference to preferred embodiments.

  First, the core-shell oxide material of the present invention will be described. The core-shell oxide material of the present invention includes a core comprising a ceria-zirconia-based solid solution powder having at least one of a regular phase of a pyrochlore phase and a kappa phase, and an alumina-based powder disposed on a part of the surface of the core. A secondary particle diameter D50 at which the cumulative volume in the volume-based particle size distribution of the ceria-zirconia-based solid solution powder becomes 50% is 0.2 to 8.0 μm; The average concentration of the Al element in a region at a depth of 3 nm from the surface of the shell, measured by linear photoelectron spectroscopy, is 25 to 75 at%. Such a core-shell oxide material of the present invention has excellent oxygen storage / release capacity (oxygen storage capacity (OSC) and oxygen storage / release rate (OSC-r)) even when exposed to high temperatures. Things.

  The core-shell oxide material of the present invention includes a core made of a ceria-zirconia-based solid solution powder having at least one ordered phase of a pyrochlore phase and a κ phase in which Ce and Zr are regularly arranged. is there. A core-shell oxide material having a core made of a ceria-zirconia-based solid solution powder having such an ordered phase has a higher oxygen diffusion rate in a bulk than a ceria-zirconia-based solid solution having a fluorite structure. Performance (oxygen storage amount (OSC) and oxygen storage release rate (OSC-r)). Further, the content ratio of Ce and Zr in the ceria-zirconia-based solid solution powder having such an ordered phase is preferably 35:65 to 65:35 in terms of molar ratio (Ce: Zr), and 45:55 to 55:45. Is more preferred. When the molar ratio (Ce: Zr) deviates from the above range, when exposed to a high temperature, the ordered phase changes to a fluorite structure due to rearrangement, and the oxygen storage / release capability tends to decrease.

  The core made of the ceria-zirconia-based solid solution powder having such an ordered phase may further contain a rare earth element other than Ce or an additional element such as Ti. When such an additional element is contained, a decrease in the oxygen storage / release capability when exposed to a high temperature is suppressed. Examples of the additional element include Sc, Y, La, Pr, Nd, Sm, Gd, Tb, Dy, Yb, Lu, and Ti. Among them, the oxygen storage / release capacity when exposed to high temperatures is particularly preferable. From the viewpoint that the decrease is further suppressed, Y, La, Pr, and Nd are preferable, and Pr is more preferable. These additional elements may be used alone or in combination of two or more. In addition, the additive element is usually contained in the core as an oxide, and further, is preferably present in a solid solution, a dispersed state or the like in the ceria-zirconia-based solid solution powder having the ordered phase, In order to surely obtain the effect of the additional element, it is more preferable that the solid solution is formed.

  In the core according to the present invention, the content of the additional element is preferably 20 mol% or less, more preferably 10 mol% or less, and particularly preferably 5 mol% or less in terms of element. If the content of the additional element exceeds the upper limit, the heat resistance of the ordered phase tends to decrease, and the oxygen storage / release capacity tends to decrease when exposed to high temperatures. The lower limit of the content of the additional element is not particularly limited, but is preferably 0.1 mol% or more in order to surely obtain the effect of the additional element.

In the ceria-zirconia-based 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 in the volume-based particle size distribution becomes 50% is 0.2 to 8.0 µm. . When the secondary particle diameter D50 is less than the lower limit, the pyrochlore phase structure and the kappa phase structure change to the fluorite structure due to thermal deterioration, and the oxygen use efficiency of CeO ₂ decreases, so that the oxygen storage / release capacity decreases. On the other hand, when the secondary particle diameter D50 exceeds the upper limit, the specific surface area is relatively reduced, and the diffusion distance of oxygen from the inside of the particles is long. Therefore, the oxygen storage / release capability, particularly the oxygen storage / release speed (OSC-r) decreases. The secondary particle diameter D50 of the ceria-zirconia-based solid solution powder having such an ordered phase is preferably from 1.0 to 7.5 μm, from the viewpoint of exhibiting higher oxygen storage / release capability, and is preferably 3.0. -7.0 µm is more preferable. In addition, the secondary particle diameter D50 of the ceria-zirconia-based solid solution powder having the ordered phase is, for example, a dynamic light scattering method using a particle size distribution analyzer, the ceria-zirconia-based solid solution powder having the ordered phase. A volume-based particle size distribution curve is obtained, and the particle size can be obtained as a particle size at which the cumulative volume in the particle size distribution curve becomes 50%.

Further, the ceria having a regular phase - is not particularly limited as specific surface area of the zirconia solid solution powder is preferably ^{0.1~20m 2 / g, 0.5~10m 2 /} g is more preferable. When the specific surface area is less than the lower limit, the oxygen storage / release capability tends to decrease, while when the specific surface area exceeds the upper limit, particles having a small particle diameter tend to increase, and high-temperature durability tends to decrease. In addition, such a specific surface area can be calculated as a BET specific surface area from an adsorption isotherm using a BET isothermal adsorption equation.

The core-shell type oxide material of the present invention includes a core made of a ceria-zirconia-based solid solution powder having such an ordered phase, and a shell made of an alumina-based oxide disposed at a predetermined ratio on the surface of the core. It is provided with. In such a core-shell oxide material, the average concentration of the Al element in a region having a depth of 3 nm from the surface of the shell (hereinafter, referred to as “outermost surface layer”) is 25 to 75 at%. When the average concentration of the Al element in the outermost surface layer is less than the lower limit, in the catalyst in which the noble metal is in contact with the core-shell oxide material, the noble metal (particularly, rhodium) easily contacts the ceria in the core, and the noble metal (particularly, , Rhodium) and ceria, the reduction of the noble metal becomes difficult to progress, and the NOx purification performance is reduced. On the other hand, when the average concentration of the Al element in the outermost surface layer exceeds the upper limit, the alumina-based oxide is likely to aggregate, thereby inhibiting the diffusion of oxygen and reducing the oxygen storage / release ability. The average concentration of the Al element in the outermost surface layer is preferably from 30 to 75 at%, and more preferably from 40 to 72 at%, from the viewpoint of improving the oxygen storage / release ability and the NOx purification performance in a well-balanced manner. Note that the average concentration of the Al element in the outermost surface layer can be determined by converting an X-ray photoelectron spectroscopy (XPS) spectrum of the core-shell oxide material into a monochromatic AlKα (1486.6 eV) using, for example, an X-ray photoelectron spectrometer. Is used as an X-ray source, photoelectron extraction angle: 45 °, analysis area: about 200 μmφ, charge-up correction: measured under conditions of Zr3d 182.2 eV (ZrO ₂ ), and present in the outermost surface layer based on the obtained XPS spectrum The amount of the element to be quantified can be determined and determined as the ratio of the amount of Al element to the total amount of metal element (Al element amount / total metal element amount × 100).

  In such a core-shell oxide material, the content of the shell made of an alumina-based oxide is preferably 0.05 to 2.0 parts by mass, and 0.1 to 1.5 parts by mass with respect to 100 parts by mass of the core. Part is more preferable, and 0.2 to 1.0 part by mass is particularly preferable. When the content of the shell is less than the lower limit, in a catalyst in which the noble metal is in contact with the core-shell oxide material, the noble metal (particularly, rhodium) easily contacts the ceria in the core, and the noble metal (particularly, rhodium) and the ceria The reduction of the noble metal is less likely to progress due to the interaction with NO, and the NOx purification performance tends to decrease.On the other hand, when the upper limit is exceeded, the alumina-based oxide is easily aggregated, thereby inhibiting the diffusion of oxygen, Oxygen storage / release capacity tends to decrease.

  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 is improved. Examples of the rare earth element include Sc, Y, La, Pr, Nd, Sm, Gd, Tb, Dy, Yb, and Lu. Among them, La is preferable from the viewpoint of further improving the high-temperature durability of the shell. Is preferred. These rare earth elements may be used alone or in combination of two or more. The rare earth element is usually contained in the shell as an oxide.

  In the shell according to the present invention, the content of the rare earth element is preferably 10 mol% or less, more preferably 5 mol% or less, particularly preferably 2 mol% or less in terms of element. When the content of the rare earth element exceeds the upper limit, an aluminate phase is formed, and the high-temperature durability such as a decrease in the specific surface area of the shell tends to decrease. The lower limit of the content of the rare earth element is not particularly limited, but is preferably 0.1 mol% or more in order to surely obtain the effect of the rare earth element.

  Further, the thickness of the shell is preferably 1 to 50 nm, more preferably 2 to 20 nm. When the 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 oxide material, the noble metal (particularly, rhodium) easily contacts the ceria in the core, and the noble metal (particularly, rhodium) and the ceria The reduction of the noble metal hardly progresses due to the interaction of NO, and the NOx purification performance tends to decrease. On the other hand, when the upper limit is exceeded, the diffusion of oxygen is inhibited by the shell, and the oxygen storage / release capability tends to decrease. .

  In the core-shell oxide material of the present invention, after heating at 1100 ° C. for 5 hours in the air, 2θ = 14.5 ° diffraction obtained from an X-ray diffraction pattern obtained by X-ray diffraction measurement using CuKα. The intensity ratio [I (14/29) value] between the line and the diffraction line at 2θ = 29 ° is preferably 0.02 or more, more preferably 0.030 or more. When the value of I (14/29) is less than the lower limit, the maintenance rate of the ordered phase is low, and the oxygen storage / release capacity tends to decrease when exposed to high temperatures. The upper limit of the I (14/29) value is not particularly limited, but as described later, the upper limit is the I (14/29) value of the pyrochlore phase calculated from the PDF card (01-075-2694). In light of this, 0.05 or less is preferable.

Here, the diffraction line at 2θ = 14.5 ° is a diffraction line belonging to the (111) plane of the ordered phase (κ phase). The diffraction line at 2θ = 29 ° is obtained by overlapping the diffraction line belonging to the (222) plane of the ordered phase with the diffraction line belonging to the cubic phase (111) plane of the ceria-zirconia solid solution (CZ solid solution). is there. Therefore, by calculating the I (14/29) value, which is the intensity ratio between the two diffraction lines, this can be defined as an index indicating the maintenance rate (existence rate) of the ordered phase. When calculating the diffraction line intensity, it is calculated by subtracting the average diffraction line intensity at 2θ = 10 ° to 12 ° as a background value from each diffraction line intensity value. The completely ordered phases include a kappa phase (Ce ₂ Zr ₂ O ₈ ) completely filled with oxygen and a pyrochlore phase (Ce ₂ Zr ₂ O ₇ ) completely removed from oxygen. The I (14/29) value of the kappa phase calculated from the card (PDF2: 01-070-4048 for the kappa phase, PDF2: 01-075-2694 for the pyrochlore phase) was 0.04, and I (14/29) for the pyrochlore phase. ) Value is 0.05. Furthermore, the ordered phase, that is, the crystalline phase having an ordered array structure formed by cerium ions and zirconium ions, has a 2θ angle of 14.5 ° in the X-ray diffraction pattern obtained by the X-ray diffraction measurement using CuKα. , 28 °, 37 °, 44.5 ° and 51 °, respectively, in the crystal array structure (regular array phase of φ ′ phase (same phase as κ phase): occurs in fluorite structure Superlattice structure). Here, the “peak” refers to a peak whose height from the baseline to the peak top is 30 cps or more.

  In the core-shell oxide material of the present invention, 2θ = 28.5 ° obtained from an X-ray diffraction pattern obtained by X-ray diffraction measurement using CuKα after heating in air at 1100 ° C. for 5 hours. Is preferably 0.08 or less, more preferably 0.06 or less, and 0.04 or less. It is particularly preferred that: If the I (28/29) value exceeds the upper limit, the oxygen storage / release capacity tends to decrease when exposed to high temperatures. The lower limit of the I (28/29) value is not particularly limited, and is preferably a smaller value.

Here, the diffraction line at 2θ = 28.5 ° is a diffraction line belonging to the (111) plane of CeO ₂ alone, and both the diffraction line at 2θ = 28.5 ° and the diffraction line at 2θ = 29 °. By calculating the I (28/29) value, which is the intensity ratio of the diffraction line, this can be defined as an index indicating the degree to which CeO ₂ is phase-separating from the composite oxide.

  In addition, as a method of X-ray diffraction measurement for obtaining the diffraction line intensity ratio [I (14/29) value] and [I (28/29) value], an X-ray diffractometer (for example, Rigaku Corporation) is used. Using "RINT2100" manufactured by Nissan Co., Ltd. and using a CuKα ray as an X-ray source under the conditions of 40 KV, 30 mA, 2θ = 2 ° / min.

No particular limitation is imposed on the specific surface area of the core-shell type oxide material of the present invention, preferably ^{0.1~20m 2 / g, 0.5~10m 2 /} g is more preferable. When the specific surface area is less than the lower limit, the oxygen storage / release capability tends to decrease, while when the specific surface area exceeds the upper limit, particles having a small particle diameter tend to increase, and high-temperature durability tends to decrease. In addition, such a specific surface area can be calculated as a BET specific surface area from an adsorption isotherm using a BET isothermal adsorption equation.

  Such a core-shell oxide material of the present invention can be produced, for example, by the following method. Specifically, a ceria-zirconia-based solid solution having a regular phase of at least one of a pyrochlore phase and a κ phase is subjected to a reduction treatment at a temperature of 1500 ° C. or more to a molded body obtained by pressure-forming a ceria-zirconia-based solid solution. A powder is prepared (reduction treatment step), and the ceria-zirconia-based solid solution powder having the ordered phase is brought into contact with an alumina precursor to form a predetermined ratio on the surface of the ceria-zirconia-based solid solution powder having the ordered phase. The core-shell oxide material of the present invention can be obtained by adhering the alumina precursor (adhering step) and heating the ceria-zirconia-based solid solution powder to which the alumina precursor is adhering (firing step).

  As the ceria-zirconia-based solid solution used when producing the core-shell oxide material of the present invention, those in which the content ratio of Ce and Zr is 35:65 to 65:35 in molar ratio (Ce: Zr) are used. Preferably, the ratio is 45:55 to 55:45. When a ceria-zirconia-based solid solution having a molar ratio (Ce: Zr) out of the above range is used, when the obtained core-shell oxide material is exposed to a high temperature, the ordered phase changes to a fluorite structure by rearrangement, Oxygen storage / release capacity tends to decrease.

  Such a ceria-zirconia solid solution may further contain a rare earth element other than Ce or an additional element such as Ti. When such an additive element is contained, a decrease in the oxygen storage / release capability when the obtained core-shell oxide material is exposed to a high temperature is suppressed. Examples of such additional elements include the above-described additional elements that may be contained in the core of the core-shell oxide material. Among them, the obtained core-shell oxide material was exposed to a high temperature. Y, La, Pr, and Nd are preferable, and Pr is more preferable, from the viewpoint that the decrease in the oxygen storage / release capacity in this case is further suppressed. These additional elements may be used alone or in combination of two or more. In addition, the additive element is usually contained in the core as an oxide, and is preferably present in the ceria-zirconia-based solid solution in a state of solid solution or dispersion. In order to surely obtain, it is more preferable that the solid solution is formed.

  In the ceria-zirconia solid solution, the content of the additional element is preferably 20 mol% or less, more preferably 10 mol% or less, particularly preferably 5 mol% or less in terms of element. If the content of the additional element exceeds the upper limit, the heat resistance of the ordered phase tends to decrease, and the oxygen storage / release capacity tends to decrease when exposed to high temperatures. The lower limit of the content of the additional element is not particularly limited, but is preferably 0.1 mol% or more in order to surely obtain the effect of the additional element.

  Such a ceria-zirconia solid solution can be produced, for example, by the following coprecipitation method. That is, an aqueous solution containing a salt of cerium (for example, nitrate) and a salt of zirconium (for example, nitrate) and, if necessary, a salt of the additional element (for example, nitrate) and a surfactant is used in the presence of ammonia. A co-precipitate is formed in the above, the obtained co-precipitate is separated and collected, washed, and then subjected to a drying treatment, a baking treatment, and a pulverization treatment, whereby a powdery ceria-zirconia-based solid solution can be obtained. The content of each raw material in the aqueous solution is appropriately adjusted so that the content of each component in the obtained ceria-zirconia solid solution becomes a predetermined amount.

When manufacturing the core-shell oxide material of the present invention, first, such a ceria-zirconia-based solid solution is pressure-formed. The pressure at the time of pressure molding is preferably 400 to 3500 kgf / cm ² (39 to 343 MPa), and more preferably 500 to 3000 kgf / cm ² (49 to 294 MPa). If the molding pressure deviates from the above range, the oxygen storage / release capacity tends to decrease when the obtained core-shell oxide material is exposed to high temperatures. Note that there is no particular limitation on such a pressure molding method, and a known pressure molding method such as a hydrostatic press can be appropriately employed.

  Next, a reduction treatment is performed on the obtained pressure-molded body at a temperature of 1500 ° C. or higher (reduction treatment step). Thereby, a ceria-zirconia-based solid solution having at least one of the pyrochlore phase and the kappa phase according to the present invention is formed. The ceria-zirconia-based solid solution having such an ordered phase has excellent thermal stability on the surface, and has a structure in which the solid phase reaction does not easily proceed. When the reduction treatment temperature is lower than the lower limit, the stability of the ordered phase is low, and the oxygen storage / release ability is reduced when the obtained core-shell oxide material is exposed to a high temperature. Further, from the viewpoint that the stability of the ordered phase is improved and the decrease in the oxygen storage / release capacity when the obtained core-shell oxide material is exposed to a high temperature is reliably suppressed, the reduction treatment temperature is 1600 ° C. or more. Is preferred. The reduction treatment time is preferably 0.5 hours or more, more preferably 1 hour or more. When the reduction treatment time is shorter than the lower limit, the stability of the ordered phase is low, and the oxygen storage / release capability tends to decrease when the obtained core-shell oxide material is exposed to a high temperature. The upper limit of the reduction treatment temperature and the reduction treatment time are not particularly limited, but from the viewpoint of energy efficiency and reduction of by-products, respectively, 2000 ° C or less (more preferably 1900 ° C or less) and 24 hours or less (more (Preferably 10 hours or less).

  The method of the reduction treatment is not particularly limited as long as the method can perform the reduction treatment at a predetermined temperature on the pressure-molded body under a reducing atmosphere. For example, (i) the pressure molding is performed in a vacuum heating furnace. A method in which a reducing gas is introduced into the furnace to reduce the atmosphere in the furnace after the molded body is placed and evacuated, and a reducing treatment is performed by heating the furnace at a predetermined temperature; (ii) a furnace made of graphite. After the above-mentioned press-molded body is set in the furnace using a vacuum and evacuated, the furnace is heated at a predetermined temperature and the atmosphere in the furnace is reduced by a reducing gas such as CO or HC generated from the furnace body or heated fuel. (Iii) placing the pressure-molded body in a crucible filled with activated carbon, heating at a predetermined temperature, and using a reducing gas such as CO or HC generated from the activated carbon. Method of performing reduction treatment using the atmosphere in the crucible as a reducing atmosphere And the like.

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 formation of double products such as zirconium carbide (ZrC) when the reduction treatment is performed at a higher temperature. Are preferred. If such a reducing gas containing no carbon (C) is used, a reduction treatment at a higher temperature close to the melting point of zirconium or the like becomes possible, so that the stability of the ordered phase can be more sufficiently improved. It becomes.

  In the method for producing a core-shell oxide material according to the present invention, it is preferable that the ceria-zirconia-based solid solution having the ordered phase is further subjected to an oxidation treatment after the reduction treatment. Thereby, oxygen lost during the reduction treatment is compensated for, and the stability as an oxide material tends to be improved. There is no particular limitation on the method of such oxidation treatment, and for example, a method of heat-treating a ceria-zirconia-based solid solution having the ordered phase in an oxidizing atmosphere (for example, in the air) can be suitably employed. The heating temperature in such an oxidation treatment is not particularly limited, but is preferably about 300 to 800 ° C. Further, the heating time in the oxidation treatment is not particularly limited, but is preferably about 0.5 to 5 hours.

  Next, in the ceria-zirconia-based solid solution having the ordered phase obtained in this manner, the secondary particle diameter D50 at which the cumulative volume in the volume-based particle size distribution becomes 50% is within a predetermined range. A pulverization treatment is performed to obtain a ceria-zirconia-based solid solution powder having the regular phase. The method of the pulverization treatment is not particularly limited, and examples thereof include a wet pulverization method, a dry pulverization method, and a freeze pulverization method.

  Next, the ceria-zirconia-based solid solution powder having the ordered phase thus obtained is brought into contact with an alumina precursor, and the surface of the ceria-zirconia-based solid solution powder having the ordered phase is coated with the alumina at a predetermined ratio. A precursor is attached (attachment step). The alumina precursor used here is not particularly limited as long as it forms an alumina-based oxide by heat treatment, and examples thereof include aluminum salts (for example, nitrates and acetates).

  The method for bringing the ceria-zirconia-based solid solution powder having the ordered phase into contact with the alumina precursor is not particularly limited. For example, the alumina precursor, if necessary, the rare earth element salt (eg, nitrate) and the interface The ceria-zirconia-based solid solution powder having the ordered phase is immersed in an aqueous solution containing an activator or the like, and the alumina precursor aqueous solution is impregnated in the ceria-zirconia-based solid solution powder having the ordered phase. The content of each raw material in the aqueous solution is appropriately adjusted so that the content of each component in the obtained core-shell oxide material becomes a predetermined amount.

  Next, after the ceria-zirconia solid solution powder impregnated with the alumina precursor aqueous solution is evaporated to dryness, a heat treatment is performed on the ceria-zirconia solid solution powder to which the alumina precursor is attached (firing treatment). As a result, a shell made of alumina-based oxide is formed at a predetermined ratio on the surface of the core made of ceria-zirconia-based solid solution powder having the ordered phase, and the core-shell oxide material of the present invention is obtained.

  The temperature of the heat treatment is preferably from 300 to 1100C, more preferably from 500 to 900C. When the heat treatment temperature is lower than the lower limit, a stable shell tends to be hardly formed, and when the heat treatment temperature exceeds the upper limit, the specific surface area of the obtained core-shell oxide material tends to be small. The heating time is not particularly limited, but 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 includes the core-shell oxide material of the present invention and a noble metal in contact with the core-shell oxide material. Such an exhaust gas purifying catalyst of the present invention has excellent oxygen storage / release capacity (oxygen storage capacity (OSC) and oxygen storage / release rate (OSC-r)) even when exposed to high temperatures. In addition, it exhibits excellent NOx purification performance.

  In the exhaust gas purifying catalyst of the present invention, Rh, Pd, and Pt are preferable, Rh and Pd are more preferable, and Rh is particularly preferable, from the viewpoint that excellent NOx purifying performance is obtained. In the exhaust gas purifying catalyst of the present invention, the form of such a noble metal is not particularly limited as long as it is in contact with the core-shell oxide material, and the noble metal is directly supported on the surface of the core-shell oxide material. However, from the viewpoint of easy operation, the core-shell oxide material and another oxide material supporting a noble metal may be mixed and contacted.

  The exhaust gas purifying catalyst of the present invention may be used by filling pellets into a reaction tube or the like.However, from the viewpoint of practicality, the exhaust gas purifying catalyst of the present invention It is preferable to use as a honeycomb catalyst having a catalyst layer and a catalyst layer containing alumina. Further, among such honeycomb catalysts, from the viewpoint that they have excellent oxygen storage and release performance even when exposed to high temperature or high flow rate gas, and exhibit excellent NOx purification performance, the honeycomb catalysts A catalyst lower layer containing a noble metal and alumina formed on the inner wall of the pores of the material, and a catalyst upper layer comprising the exhaust gas purifying catalyst of the present invention formed on the catalyst lower layer, preferably the catalyst upper layer Is more preferably composed of a mixture of the core-shell oxide material of the present invention and zirconia supporting a noble metal.

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

(Preparation Example 1)
A ceria-zirconia-based composite oxide having a content ratio of cerium, zirconium, and praseodymium of 45: 54: 1 in molar ratio ([cerium]: [zirconium]: [praseodymium]) was prepared as follows. That is, first, 442 g of a 28 mass% cerium nitrate aqueous solution in terms of CeO ₂ , 590 g of an 18 mass% zirconium oxynitrate aqueous solution in terms of ZrO ₂ , and praseodymium nitrate in an amount of 1.2 g in terms of Pr ₆ O ₁₁ are included. 100 g of the aqueous solution and 197 g of a hydrogen peroxide solution containing 1.1 times the molar amount of hydrogen peroxide of the contained cerium were added to 1217 g of an aqueous solution containing 1.2 times the equivalent of ammonia with respect to the neutralization equivalent. To form a coprecipitate, and the obtained coprecipitate was centrifuged and washed with ion-exchanged water. Then, after drying the obtained coprecipitate at 110 ° C. 10 hours or more in air, a solid solution of 5 hours firing to cerium and zirconium and praseodymium at _{400 ℃ (CeO 2 -ZrO 2 -Pr} 6 O 11 (Solid solution). Thereafter, the solid solution was pulverized using a pulverizer (“Wonder Blender” manufactured by As One Corp.) so as to have a particle size of 75 μm or less by sieving to obtain the ceria-zirconia-praseodymium solid solution powder.

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

  Next, the obtained molded bodies (two pieces) were placed in a crucible (internal volume: diameter 8 cm, height 7 cm) filled with 70 g of activated carbon, covered, and then placed in a high-speed electric heating furnace at 1000 ° C. The temperature was raised to 1700 ° C. over 4 hours, and then heated at 1700 ° C. (reduction temperature) for 5 hours. Then, after cooling to 1000 ° C. over 4 hours, the mixture was naturally cooled to room temperature to obtain a reduced fired product.

  Next, this reduced calcined product is oxidized by heating at 500 ° C. for 5 hours in the air, and the content ratio of cerium, zirconium, and praseodymium in the composite oxide is set to a molar ratio ([cerium]: [zirconium]: [Praseodymium]) to obtain a ceria-zirconia-praseodymium composite oxide of 45: 54: 1.

(Comparative Preparation Example 2)
A ceria-zirconia solid solution powder in which the content ratio of cerium and zirconium was 45.5: 54.5 in molar ratio ([cerium]: [zirconium]) 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 1.1 times the molar amount of cerium contained are included. 197 g of aqueous hydrogen peroxide was 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 was subjected to centrifugal separation. Washed with replacement water. Next, the obtained coprecipitate was dried at 110 ° C. for 10 hours or more, and calcined in the air at 400 ° C. for 5 hours to obtain a solid solution of cerium and zirconium (CeO ₂ -ZrO ₂ solid solution). Thereafter, the solid solution was pulverized using a pulverizer (“Wonder Blender” manufactured by As One Co., Ltd.) so as to have a particle size of 75 μm or less by sieving to obtain the ceria-zirconia solid solution powder.

(Example 1)
The ceria-zirconia-praseodymium composite oxide obtained in Preparation Example 1 was milled (manufactured by As One Corporation) so that the secondary particle diameter D50 at which the cumulative volume in the volume-based particle size distribution becomes 50% was 4 μm. This was pulverized using a “Wonder Blender”) to obtain a ceria-zirconia-praseodymium composite oxide powder (hereinafter abbreviated as “CZP powder”). The volume-based particle size distribution of the CZP powder was measured by a dynamic light scattering method using a particle size distribution analyzer (“Laser diffraction / scattering particle size analyzer MT3300EX” manufactured by Nikkiso Co., Ltd.).

  Next, an alumina precursor aqueous solution was prepared by adding 150 ml of an aqueous solution containing 1.92 g of a quaternary amine to 50 ml of an aqueous solution containing 0.73 g of aluminum nitrate and 1.12 g of a carboxylic acid chelating agent. 100 g of the CZP powder (D50: 4 μm) is put into the alumina precursor aqueous solution, stirred, heated by microwave to evaporate to dryness, and the obtained dried product is calcined at 500 ° C. in the air for 5 hours. Thus, a core-shell type oxide material powder (C50: 4 μm, CZP amount: 100 parts by mass, alumina coating amount: 0.1 parts by mass) in which the surface of the CZP powder was coated with an alumina layer was obtained.

(Example 2)
The surface of the CZP powder was changed in the same manner as in Example 1 except that the amount of aluminum nitrate was changed to 1.83 g, the amount of the carboxylic acid chelating agent was changed to 2.8 g, and the amount of the quaternary amine was changed to 4.8 g. A core-shell oxide material powder coated with an alumina layer (D50 of CZP powder: 4 μm, CZP amount: 100 parts by mass, alumina coating amount: 0.25 parts by mass) was obtained.

(Example 3)
A CZP powder was obtained in the same manner as in Example 1, except that the powder was pulverized so that the secondary particle diameter D50 became 5 μm. This CZP powder was used instead of the CZP powder having a D50 of 4 μm, and the amount of aluminum nitrate was changed to 3.66 g, the amount of the carboxylic acid chelating agent was changed to 5.6 g, and the amount of the quaternary amine was changed to 9.6 g. Except for the above, in the same manner as in Example 1, the core-shell type oxide material powder in which the surface of the CZP powder was coated with an alumina layer (D50 of CZP powder: 5 μm, CZP amount: 100 parts by mass, alumina coating amount: 0.5 Parts by mass).

(Example 4)
A CZP powder was obtained in the same manner as in Example 1, except that the powder was pulverized so that the secondary particle diameter D50 became 6 μm. This CZP powder was used instead of the CZP powder having a D50 of 4 μm, and the amount of aluminum nitrate was changed to 5.49 g, the amount of the carboxylic acid chelating agent was changed to 8.4, and the amount of the quaternary amine was changed to 14.4 g. Except for the above, in the same manner as in Example 1, a core-shell type oxide material powder in which the surface of the CZP powder was coated with an alumina layer (D50 of CZP powder: 6 μm, CZP amount: 100 parts by mass, alumina coating amount: 0.75 Parts by mass).

(Example 5)
The surface of the CZP powder was changed in the same manner as in Example 1 except that the amount of aluminum nitrate was changed to 7.32 g, the amount of the carboxylic acid chelating agent was changed to 10.2 g, and the amount of the quaternary amine was changed to 19.2 g. A core-shell oxide material powder coated with an alumina layer (D50 of CZP powder: 4 μm, CZP amount: 100 parts by mass, alumina coating amount: 1.0 parts by mass) was obtained.

(Comparative Example 1)
A CZP powder was obtained in the same manner as in Example 1 except that the secondary particle diameter D50 was pulverized so as to be 10 μm. Next, 9.5 mmol of aluminum nitrate and 0.096 mmol of lanthanum nitrate were dissolved in 200 ml of ion-exchanged water to prepare a La-containing alumina precursor aqueous solution. 100 g of the CZP powder (D50: 10 μm) was added to the La-containing alumina precursor aqueous solution, and the mixture was stirred for 15 minutes. The obtained CZP powder-containing dispersion is heated at 200 ° C. while stirring to evaporate to dryness, and the obtained dried material is calcined at 900 ° C. for 5 hours, so that the surface of the CZP powder has an alumina layer containing lanthanum. (D50 of CZP powder: 10 μm, CZP amount: 100 parts by mass, alumina coating amount: 0.5 parts by mass, lantana coating amount: 0.015 parts by mass).

(Comparative Example 2)
Instead of using the ceria-zirconia solid solution powder obtained in Comparative Preparation Example 1 in place of the ceria-zirconia-praseodymium composite oxide obtained in Preparation Example 1, pulverizing so that the secondary particle diameter D50 becomes 10 μm. In the same manner as in Example 1, ceria-zirconia solid solution powder (hereinafter abbreviated as “CZ powder”) was obtained.

  Next, 9.5 mmol of aluminum nitrate and 0.096 mmol of lanthanum nitrate were dissolved in 200 ml of ion-exchanged water to prepare a La-containing alumina precursor aqueous solution. 100 g of the CZ powder (D50: 10 μm) was added to the La-containing alumina precursor aqueous solution, and the mixture was stirred for 15 minutes. The obtained CZP powder-containing dispersion is heated at 200 ° C. while stirring to evaporate to dryness, and the obtained dried material is calcined at 900 ° C. for 5 hours, so that the surface of the CZ powder is an alumina layer containing lanthanum. (D50 of CZ powder: 10 μm, CZ amount: 100 parts by mass, alumina coating amount: 0.5 parts by mass, lantana coating amount: 0.015 parts by mass).

  Next, the surface of the CZ powder was coated with an alumina layer containing lanthanum in the same procedure as the above method except that 100 g of the core-shell type oxide material powder was used instead of the CZ powder (D50: 10 μm). A core-shell type oxide material powder (D50 of CZ powder: 10 μm, CZ amount: 100 parts by mass, alumina coating amount: 1.0 parts by mass, lantana coating amount: 0.030 parts by mass) was obtained.

<X-ray diffraction (XRD) measurement>
Each core-shell type oxide material powder obtained in the examples and comparative examples was heated at 1100 ° C. for 5 hours in the air. The X-ray diffraction pattern of the ordered phase (ordered phase of the core) of the core-shell oxide material powder after heating was measured using an X-ray diffractometer (“RINT2100” manufactured by Rigaku Corporation) using CuKα as an X-ray source. It was measured by a diffraction method. The result is shown in FIG. Further, in the obtained X-ray diffraction pattern, the intensity ratio [I (14/29) value] between the diffraction line at 2θ = 14.5 ° and the diffraction line at 2θ = 29 ° was determined. Table 1 shows the results.

<X-ray photoelectron spectroscopy analysis>
The X-ray photoelectron spectroscopy (XPS) spectrum of each core-shell type oxide material powder obtained in the examples and comparative examples was converted to a monochromatic color using an X-ray photoelectron spectrometer (ULQUA-PHI “Quantera SXM”). AlKα (1486.6 eV) was used as an X-ray source, and the measurement was carried out under the conditions of a photoelectron extraction angle: 45 °, an analysis region: about 200 μmφ, and a charge-up correction: Zr3d 182.2 eV (ZrO ₂ ). Based on the obtained XPS spectrum, the elements present in a region (about 200 μmφ) having a depth of 3 nm from the surface (shell surface) of the core-shell oxide material powder were quantified, and the average concentration of Al element [= Al amount / (Al amount + Ce amount + Zr amount + Pr amount) × 100] was obtained. Table 1 shows the results.

<Catalyst preparation>
Examples and core-shell type oxide material powder obtained in Comparative Example and Rh-supporting _{Al 2 O 3 -ZrO 2 -La 2} O 3 -Nd 2 O 3 composite oxide powder (Rh support amount: 0.2 wt% , 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 size: 20 μm) and a mortar at a mass ratio of 1: 1. , And the resulting mixture is press-molded under a hydrostatic pressure of 1 t, and the obtained molded body is pulverized and classified so that the particle size becomes 0.5 to 1 mm, thereby obtaining a pellet catalyst. Was.

<High temperature endurance test>
1.5 g of the obtained pellet catalyst was charged into a cylindrical reaction tube having a diameter of 10 mm, and the rich gas [H ₂ (2%) + CO _2] was added to the pellet catalyst at a temperature of 1100 ° C. and a gas flow rate of 10 L / min. (10%) + N ₂ (88%)] and lean gas [O ₂ (1%) + CO ₂ (10%) + N ₂ (89%)] were allowed to flow for 5 hours while alternately switching each for 5 minutes.

<Measurement of oxygen storage / release rate (OSC-r) and oxygen storage / release amount (OSC)>
A mixture of 0.25 g of the pellet catalyst and 0.25 g of quartz sand after the high temperature durability test was filled in the reaction tube. This catalyst entering the catalyst gas temperature 500 ° C., at a gas flow rate 10L / min, rich gas [CO (2 vol%) + _{N 2} (balance)] After circulating for three minutes, lean gas circulation gas _[O 2 (1 volume %) + N ₂ (remainder)] and circulated for 3 minutes, and again the flowing gas was switched to the rich gas. From the amount of CO ₂ in the catalyst outgas for 5 seconds and 3 minutes after the second circulation gas switching, the oxygen storage and release rate (OSC-r, unit: μmol / (g · s)) and the oxygen storage and release amount (OSC , Unit: μmol / g). Table 1 shows the results.

<50% NOx purification temperature measurement>
The reaction tube was filled with 0.5 g of the pellet catalyst after the high temperature durability test. A model gas [NO (1200 vol ppm) + CO ₂ (10 vol%) + O ₂ (0.646 vol%) + CO (0.7 vol%) + C ₃ H ₆ (1600 vol ppm C) + H ₂ (0 vol. .233% by volume) + H ₂ O (10% by volume) + N ₂ (remainder)] while heating at a heating rate of 50 ° C./min from 100 ° C. to 600 ° C., and flowing at a gas flow rate of 10 L / min. The NOx purification rate was calculated by measuring the NO concentration in the catalyst-containing gas and the catalyst exit gas at the catalyst-containing gas temperature, and the catalyst temperature at the time when 50% of NOx was purified (50% NOx purification temperature) was obtained. . Table 1 shows the results.

<Measurement of NOx transient purification rate>
The reaction tube was filled with 0.5 g of the pellet catalyst after the high temperature durability test. A 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% by volume) + N ₂ (remainder)], the temperature of the gas containing the catalyst was raised to 600 ° C., and the flowing gas was rich gas [NO (1500 ppm by volume) + CO ₂ (10 (Volume%) + CO (0.65 vol%) + C ₃ H ₆ (3000 vol ppm C) + H ₂ O (5 vol%) + N ₂ (remainder)], and circulate for 5 minutes. The mixture was switched and circulated for 5 minutes to perform pretreatment. Thereafter, the temperature of the catalyst-containing gas was lowered to 500 ° C., and the gas was flowed at a gas flow rate of 10 L / min while switching the rich gas and the lean gas alternately every 5 minutes as flowing gas. The average NO concentration in the gas entering the catalyst and the gas exiting the catalyst during the third cycle of the rich gas flow (5 minutes) were measured to determine the NOx transient purification rate. Table 1 shows the results.

  As is clear from the results shown in Table 1, the core-shell type oxide material powders obtained in Examples 1 to 5 and Comparative Example 1 each had a diffraction line intensity in an X-ray diffraction pattern using CuKα as an X-ray source. The ratio [I (14/29) value] was 0.02 or more, and it was confirmed that the composition had at least one of the pyrochlore phase and the kappa phase. On the other hand, in the core-shell oxide material powder obtained in Comparative Example 2, in the X-ray diffraction pattern using CuKα as the X-ray source, no diffraction line at 2θ = 14.5 ° was observed (I (14/29) = 0), it was found that neither the pyrochlore phase nor the kappa phase was formed.

  In addition, the pellet catalyst using the core-shell oxide material powder obtained in Examples 1 to 5 has a higher oxygen storage capacity than the pellet catalyst using the core-shell oxide material powder obtained in Comparative Examples 1 and 2. It was found to be excellent in release amount (OSC) and oxygen storage / release rate (OSC-r), and low in 50% NOx purification temperature and excellent in NOx purification activity at low temperature.

  Furthermore, the pellet catalyst using the core-shell type oxide material powder obtained in Examples 1 to 5 was compared with the pellet catalyst using the core-shell type oxide material powder obtained in Comparative Example 2 in terms of NOx transient purification. It was found that the ratio was high, and that it could respond quickly to changes in the composition of the flowing gas. Among them, the ceria-zirconia solid solution powder has a predetermined secondary particle diameter D50, and the pellet catalyst (Examples 2 to 5) using a core-shell type oxide material powder having an average concentration of Al element of 30 at% or more, It was found that the NOx transient purification rate was particularly excellent, and that it could respond more quickly to changes in the composition of the flowing gas.

  As described above, by using the core-shell oxide material of the present invention, even when exposed to high temperatures, excellent oxygen storage / release capacity (oxygen storage capacity (OSC) and oxygen storage / release rate (OSC) -R)), and it is possible to obtain an exhaust gas purifying catalyst which exhibits excellent NOx purifying performance.

  Therefore, the core-shell oxide material of the present invention is useful as a carrier or a promoter of an exhaust gas purifying catalyst for purifying exhaust gas containing nitrogen compounds, which is discharged from an internal combustion engine of an automobile or the like.

Claims (6)

A core comprising a ceria-zirconia-based solid solution powder having at least one regular phase of a pyrochlore phase and a κ phase, and a shell composed of an alumina-based oxide disposed on a partial surface of the core,
A secondary particle diameter D50 at which the cumulative volume in the volume-based particle size distribution of the ceria-zirconia-based solid solution powder becomes 50% is 0.2 to 8.0 µm;
As measured by X-ray photoelectron spectroscopy, the average concentration of Al element in the region of the depth of 3nm from the surface of the shell is Ri 25～75At% der,
After heating at 1100 ° C. for 5 hours in the air, a 2θ = 14.5 ° diffraction line and a 2θ = 29 ° diffraction line obtained from an X-ray diffraction pattern obtained by X-ray diffraction measurement using CuKα were used. The intensity ratio [I (14/29) value] is 0.030 or more;
A core-shell type oxide material characterized by the above-mentioned.   The core-shell oxide material according to claim 1, wherein the content of the shell is 0.05 to 2.0 parts by mass with respect to 100 parts by mass of the core.   3. The core-shell oxide material according to claim 1, wherein the core further contains a rare earth element other than Ce. 4.   The average concentration of the Al element in a region at a depth of 3 nm from the surface of the shell, measured by X-ray photoelectron spectroscopy, is 30 to 75 at%. The core-shell oxide material according to 1.   An exhaust gas purifying catalyst comprising: the core-shell oxide material according to claim 1; and a noble metal in contact with the core-shell oxide material.   An exhaust gas purification method comprising contacting the exhaust gas-purifying catalyst according to claim 5 with an exhaust gas containing nitrogen oxides.