JP2018150207A - Core-shell type oxide material, exhaust gas purification catalyst using the same, and exhaust gas purification method - Google Patents

Abstract

[PROBLEMS] To have excellent oxygen storage / release capacity (oxygen storage capacity (OSC) and oxygen storage / release rate (OSC-r)) and excellent NOx purification performance even when exposed to high temperatures. Provided is an oxide material capable of obtaining a manifested exhaust gas purifying catalyst. A core comprising a ceria-zirconia-based solid solution powder having at least one of a pyrochlore phase and a kappa phase, and a shell comprising 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, and is measured by X-ray photoelectron spectroscopy. A core-shell oxide material, wherein the average concentration of the Al element in a region having a depth of 3 nm from the surface of the shell is 25 to 75 at%. [Selection diagram] None

Description

  The present invention relates to a core-shell type 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.

  Conventionally, composite oxides containing various metal oxides have been used as carriers for exhaust gas purification catalysts, promoters, and the like. As a metal oxide in such a complex oxide, ceria has been used favorably because it can absorb and release oxygen according to the oxygen partial pressure in the atmosphere (has oxygen storage and release ability). . In recent years, various types of complex oxides containing ceria have been studied, and various ceria-zirconia based complex oxides and methods for producing the same have been disclosed.

  For example, Japanese Unexamined Patent Application Publication No. 2007-144290 (Patent Document 1) discloses a core-shell structure including a core portion made of oxygen storage / release material particles such as ceria and a shell portion made of a carrier oxide such as zirconia or titania. An exhaust gas-purifying catalyst is disclosed in which noble metal particles containing at least rhodium particles are in contact with this carrier, and it is also described that the oxidation of rhodium is suppressed and the catalytic activity is improved by the oxygen storage / release ability.

Japanese Patent Laying-Open No. 2005-830 (Patent Document 2) discloses a composite comprising CeO ₂ —ZrO ₂ solid solution particles and an Al ₂ O ₃ layer covering at least 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, and it is described that grain growth of noble metal is suppressed and oxygen storage / release capability is improved. Yes.

  Furthermore, 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. An exhaust gas purifying catalyst containing co-catalyst particles such as the above is disclosed, and agglomeration of noble metal particles is suppressed by the anchor effect, so that high catalytic activity is maintained even under fluctuations in the air-fuel ratio, and the catalyst purifying performance is improved. It is also described that the decrease 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 compound oxide material in which at least a part of the crystals of the lantana-zirconia complex oxide is solid-dissolved on the crystal particle surface of the ceria-zirconia complex oxide is disclosed. It is described that the storage capacity is hardly lowered.

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 diffusibility in the shell is low and oxygen storage / release rate is slow. It was. Further, when this exhaust gas purification catalyst is exposed to a high temperature, the ceria in the core and the zirconia in the shell are mutually diffused to destroy the core-shell structure, so that there is a problem that the catalytic activity of rhodium is lowered.

Further, in the exhaust gas purifying catalysts described in Patent Documents 2 to 3, the oxygen utilization efficiency of ceria in CeO ₂ —ZrO ₂ solid solution particles and promoter particles is low, and a sufficiently high oxygen storage / release capability is not necessarily obtained. It was.

  Furthermore, the catalyst in which rhodium is supported on the composite oxide material described in Patent Document 4 has a problem in that although it exhibits an excellent oxygen storage / release capability when exposed to high temperatures, the NOx purification performance decreases. The present inventors have found that.

  The present invention has been made in view of the above-described problems of the prior art, and has excellent oxygen storage / release capacity (oxygen storage amount (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 oxide material, and an exhaust gas purification method Objective.

  As a result of intensive studies to achieve the above object, the present inventors have a ceria-zirconia solid solution powder having at least one ordered phase of a pyrochlore phase and a κ phase and having a predetermined particle size. By coating the surface of the core with an alumina-based oxide at a predetermined ratio, even when the catalyst in which the noble metal is in contact with the obtained core-shell type oxide material is exposed to high temperature, excellent oxygen It has been found that it has occlusion / release capacity (oxygen occlusion amount (OSC) and oxygen occlusion / release rate (OSC-r)) and exhibits excellent NOx purification performance, and has completed the present invention.

That is, the core-shell type oxide material of the present invention is disposed on a core made of ceria-zirconia solid solution powder having at least one ordered phase of a pyrochlore phase and a κ phase, and a part of the surface of the core. A shell made of alumina-based oxide,
The secondary particle diameter D50 at which the cumulative volume in the volume-based particle size distribution of the ceria-zirconia solid solution powder is 50% is 0.2 to 8.0 μm,
The average concentration of Al element in a region 3 nm deep from the surface of the shell, measured by X-ray photoelectron spectroscopy, is 25 to 75 at%.
It is characterized by this.

  In such a core-shell type 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. Moreover, it is preferable that the core further contains a rare earth element other than Ce. Furthermore, it is preferable that the average concentration of Al element in a region 3 nm deep from the surface of the shell, measured by X-ray photoelectron spectroscopy, is 30 to 75 at%.

  The exhaust gas purifying catalyst of the present invention is characterized by comprising the core-shell type oxide material of the present invention and a noble metal in contact with the core-shell type oxide material. Furthermore, the exhaust gas purification method of the present invention is characterized in that exhaust gas containing nitrogen oxides is brought into contact with the exhaust gas purification catalyst of the present invention.

The reason why the core-shell type oxide material of the present invention has an excellent oxygen storage / release capability even when exposed to high temperatures is not necessarily clear, but the present inventors speculate as follows. That is, the core in the core-shell type oxide material of the present invention is made of a ceria-zirconia 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, and oxygen Expresses the ability to occlude and release. The oxygen storage / release ability expressed by the 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 ability expressed in the fluorite phase, which is almost the theoretical limit. In order to reach the value, the ceria-zirconia solid solution powder having the ordered phase exhibits a very high oxygen storage capacity (OSC) and O ₂ bulk diffusion rate. For this reason, even if at least a part of the surface of the core made of the ceria-zirconia solid solution powder having the ordered phase is coated with the alumina-based oxide, the oxygen storage amount (OSC) and oxygen storage / release rate (OSC) by the coating It is presumed that the decrease in -r) is small and an excellent oxygen storage / release ability is exhibited. In addition, since the core-shell type oxide material of the present invention is reduced at a high temperature of 1500 ° C. or higher, the core-shell type oxide material has excellent high-temperature stability compared to a normal ceria-zirconia solid solution and is exposed to a high temperature. However, it is presumed that an excellent oxygen storage / release ability is exhibited.

  Further, the reason why the catalyst in which the noble metal is brought into contact with the core-shell type oxide material of the present invention exhibits excellent NOx purification performance even when exposed to a high temperature is not necessarily clear. We infer as follows. That is, in the catalyst in which the noble metal is brought into contact with the core-shell type oxide material of the present invention, the noble metal is in contact with the shell of the core-shell type oxide material, that is, the alumina-based oxide coating layer. ) And the NOx purification activity is estimated to be improved as compared with the case where a noble metal is brought into contact with the core made of the ceria-zirconia solid solution powder having the ordered phase. Furthermore, even when exposed to high temperatures, the growth of precious metal grains is suppressed, and the decrease in NOx purification activity is presumed to be one of the reasons for exhibiting excellent NOx purification performance. .

  According to the present invention, even when exposed to high temperatures, 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 purification catalyst that exhibits purification performance.

It is a graph which shows the X-ray-diffraction pattern of the core-shell type oxide material powder obtained in Examples 1-5 and Comparative Examples 1-2.

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

  First, the core-shell type oxide material of the present invention will be described. The core-shell type oxide material of the present invention includes a core made of ceria-zirconia-based solid solution powder having at least one ordered phase of a pyrochlore phase and a κ phase, and an alumina-based material 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 solid solution powder is 50% is 0.2 to 8.0 μm, The average concentration of Al element in the region of 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 type oxide material of the present invention has an excellent oxygen storage / release capacity (oxygen storage capacity (OSC) and oxygen storage / release speed (OSC-r)) even when exposed to high temperatures. Is.

  The core-shell type oxide material of the present invention includes a core made of 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 there. Since the core-shell type oxide material having a core made of ceria-zirconia solid solution powder having such an ordered phase has a larger oxygen diffusion rate in the bulk than the ceria-zirconia solid solution having a fluorite structure, oxygen storage / release Performance (oxygen storage amount (OSC) and oxygen storage / release rate (OSC-r)). 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 45:55 to 55:45. Is more preferable. When the molar ratio (Ce: Zr) deviates from the above range, the ordered phase changes to a fluorite structure due to rearrangement when exposed to a high temperature, and the oxygen storage / release ability tends to decrease.

  The core made of the ceria-zirconia solid solution powder having such an ordered phase may further contain a rare earth element other than Ce or an additive element such as Ti. When such an additive element is contained, a decrease in oxygen storage / release ability when exposed to high temperatures is suppressed. Examples of the additive element include Sc, Y, La, Pr, Nd, Sm, Gd, Tb, Dy, Yb, Lu, Ti, and the like. Among these, oxygen storage / release ability when exposed to high temperatures is mentioned. From the viewpoint that the decrease is further suppressed, Y, La, Pr, and Nd are preferable, and Pr is more preferable. In addition, these additional elements may be contained individually by 1 type, or may contain 2 or more types. Further, the additive element is usually contained in the core as an oxide, and further preferably present in the ceria-zirconia solid solution powder having the ordered phase in a solid solution, dispersed state, etc. In order to obtain the effect of the additive element with certainty, it is more preferable that the element is dissolved.

  In the core according to the present invention, the content of the additive element is preferably 20 mol% or less, preferably 10 mol% or less, and particularly preferably 5 mol% or less in terms of element. When the content of the additive element exceeds the upper limit, the heat resistance of the ordered phase decreases, and the oxygen storage / release ability tends to decrease when exposed to high temperatures. In addition, although there is no restriction | limiting in particular as a minimum of content of the said additional element, In order to acquire the effect by the said additional element reliably, 0.1 mol% or more is preferable.

In the ceria-zirconia solid solution powder having the regular phase forming the core according to the present invention, the secondary particle diameter D50 at which the cumulative volume in the volume-based particle size distribution is 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 κ phase structure are changed to a fluorite structure due to thermal deterioration, and the oxygen utilization efficiency of CeO ₂ is reduced, so that the oxygen storage / release ability is lowered. On the other hand, when the secondary particle diameter D50 exceeds the upper limit, the specific surface area is relatively decreased and the oxygen diffusion distance from the inside of the particle is increased, so that the oxygen storage / release capability, particularly the oxygen storage / release rate. (OSC-r) decreases. The secondary particle diameter D50 of the ceria-zirconia solid solution powder having the ordered phase is preferably 1.0 to 7.5 μm from the viewpoint that a higher oxygen storage / release capability is exhibited. ˜7.0 μm is more preferable. The secondary particle diameter D50 of the ceria-zirconia solid solution powder having the ordered phase is, for example, that of the ceria-zirconia solid solution powder having the ordered phase by a dynamic light scattering method using a particle size distribution analyzer. A volume-based particle size distribution curve can be obtained, and the particle size can be obtained as the particle size at which the cumulative volume in the particle size distribution curve is 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 ability tends to decrease. On the other hand, when the upper limit is exceeded, particles having a small particle diameter increase, and high-temperature durability tends to decrease. Such a specific surface area can be calculated as a BET specific surface area from an adsorption isotherm using a BET isotherm adsorption equation.

The core-shell type oxide material of the present invention includes a core made of ceria-zirconia solid solution powder having such an ordered phase, and a shell made of alumina-based oxide arranged at a predetermined ratio on the surface of the core. Is provided. In such a core-shell type oxide material, the average concentration of Al element in the region 3 nm deep from the surface of the shell (hereinafter referred to as “outermost surface layer”) is 25 to 75 at%. When the average concentration of 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 type oxide material, the noble metal (especially rhodium) is likely to come into contact with the ceria in the core, , Rhodium) and ceria make it difficult for the reduction of the noble metal to proceed, and the NOx purification performance decreases. 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 tends to aggregate, thereby inhibiting oxygen diffusion and lowering the oxygen storage / release capability. The average concentration of the Al element in the outermost surface layer is preferably 30 to 75 at%, more preferably 40 to 72 at%, from the viewpoint that the oxygen storage / release ability and the NOx purification performance are improved in a balanced manner. The average concentration of the Al element in the outermost surface layer is determined by using an X-ray photoelectron spectroscopy (XPS) spectrum of the core-shell type oxide material, for example, using an X-ray photoelectron spectrometer, and monochromatic AlKα (1486.6 eV). , X-ray source, photoelectron extraction angle: 45 °, analysis region: about 200 μmφ, charge-up correction: Zr3d 182.2 eV (ZrO ₂ ), and present in the outermost surface layer based on the obtained XPS spectrum The element to be quantified can be determined as a ratio of the amount of Al element to the amount of all metal elements (amount of Al element / total amount of metal elements × 100).

  In such a core-shell type 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 weight is particularly preferable. 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 oxide material, the noble metal (especially rhodium) easily comes into contact with the ceria in the core, and the noble metal (especially rhodium) and the ceria. The reduction of the noble metal is less likely to proceed due to the interaction with NOx, and the NOx purification performance tends to decrease.On the other hand, when the upper limit is exceeded, the alumina-based oxide tends to aggregate, thereby inhibiting oxygen diffusion, Oxygen storage / release ability 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 these, from the viewpoint that the high-temperature durability of the shell is further improved. Is preferred. In addition, these rare earth elements may be contained individually by 1 type, or may contain 2 or more types. 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, preferably 5 mol% or less, and 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 high temperature durability tends to decrease, such as a decrease in the specific surface area of the shell. In addition, although there is no restriction | limiting in particular as a minimum of content of the said rare earth element, In order to acquire the effect by the said rare earth element reliably, 0.1 mol% or more is preferable.

  The thickness of the shell is preferably 1 to 50 nm, and 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 type oxide 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 reduction of the precious metal is less likely to proceed due to the interaction of NOx, and the NOx purification performance tends to be reduced. 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 be reduced. .

  In the core-shell type oxide material of the present invention, 2θ = 14.5 ° diffraction 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. The intensity ratio [I (14/29) value] of the line and the diffraction line of 2θ = 29 ° is preferably 0.02 or more, and more preferably 0.030 or more. When the I (14/29) value is less than the lower limit, the maintenance rate of the ordered phase is low, and the oxygen storage / release ability tends to decrease when exposed to high temperatures. The upper limit of the I (14/29) value is not particularly limited, but as will be described later, the I (14/29) value of the pyrochlore phase calculated from the PDF card (01-075-2694) is the upper limit. In view of the above, 0.05 or less is preferable.

Here, the diffraction line of 2θ = 14.5 ° is a diffraction line belonging to the (111) plane of the regular phase (κ phase). The diffraction line at 2θ = 29 ° is a combination of the diffraction line belonging to the (222) plane of the regular phase and 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 of both diffraction lines, this can be defined as an index indicating the maintenance rate (presence rate) of the ordered phase. When obtaining the diffraction line intensity, the calculation is performed by subtracting the average diffraction line intensity of 2θ = 10 ° to 12 ° as the background value from the value of each diffraction line intensity. In addition, the perfectly 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 ) from which oxygen is completely removed. I (14/29) value of κ phase calculated from the card (PDF2: 01-070-4048 for κ phase, PDF2: 01-075-2694 for pyrochlore phase) is 0.04, I (14/29) for pyrochlore phase ) The value is 0.05. Furthermore, the ordered phase, that is, the crystal phase having an ordered arrangement structure formed of 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 a crystal arrangement structure (φ ′ phase (the same phase as κ phase) type ordered arrangement phase: occurring in a fluorite structure Superlattice structure). Here, the “peak” means that the height from the baseline to the peak top is 30 cps or more.

  In the core-shell type oxide material of the present invention, 2θ = 28.5 ° determined from an 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 of 2θ and the diffraction line of 2θ = 29 ° is preferably 0.08 or less, more preferably 0.06 or less, and 0.04 It is particularly preferred that When the I (28/29) value exceeds the upper limit, the oxygen storage / release ability tends to decrease when exposed to high temperatures. In addition, there is no restriction | limiting in particular as a minimum of said I (28/29) value, It is preferable that it becomes a smaller value.

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

  The X-ray diffraction measurement method (for example, Rigaku Corporation) can be used as a method of X-ray diffraction measurement when obtaining the diffraction line intensity ratio [I (14/29) value] and [I (28/29) value]. Using “RINT2100” manufactured by RINT 2100, it is possible to adopt a method in which CuKα rays are used as an X-ray source and measurement is performed under conditions of 40 KV, 30 mA, 2θ = 2 ° / min.

Moreover, there is no restriction | limiting in particular as a specific surface area of the core-shell type oxide material of this invention, 0.1-20 m < ² > / g is preferable and 0.5-10 m < ² > / g is more preferable. When the specific surface area is less than the lower limit, the oxygen storage / release ability tends to decrease. On the other hand, when the upper limit is exceeded, particles having a small particle diameter increase, and high-temperature durability tends to decrease. Such a specific surface area can be calculated as a BET specific surface area from an adsorption isotherm using a BET isotherm adsorption equation.

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

  The ceria-zirconia-based solid solution used when producing the core-shell type oxide material of the present invention has a molar ratio of Ce and Zr (Ce: Zr) of 35:65 to 65:35. Preferably, the ratio is 45:55 to 55:45. When a ceria-zirconia solid solution whose molar ratio (Ce: Zr) deviates from the above range is used, when the obtained core-shell type oxide material is exposed to a high temperature, the ordered phase changes to a fluorite structure by rearrangement, Oxygen storage / release ability tends to decrease.

  Such a ceria-zirconia solid solution may further contain a rare earth element other than Ce, or an additive element such as Ti. When such an additive element is contained, a decrease in oxygen storage / release capability when the obtained core-shell type oxide material is exposed to a high temperature is suppressed. Examples of such additive elements include the additive elements exemplified as those that may be contained in the core of the core-shell type oxide material. Among them, the obtained core-shell type oxide material was exposed to a high temperature. From the viewpoint of further suppressing the decrease in oxygen storage / release capacity in this case, Y, La, Pr, and Nd are preferable, and Pr is more preferable. In addition, these additional elements may be contained individually by 1 type, or may contain 2 or more types. The additive element is usually contained in the core as an oxide, and is preferably present in the ceria-zirconia solid solution in a solid solution, dispersed state, etc. In order to reliably obtain the above, it is more preferable that the solid solution.

  In the ceria-zirconia solid solution, the content of the additive element is preferably 20 mol% or less, more preferably 10 mol% or less, and particularly preferably 5 mol% or less in terms of element. When the content of the additive element exceeds the upper limit, the heat resistance of the ordered phase decreases, and the oxygen storage / release ability tends to decrease when exposed to high temperatures. In addition, although there is no restriction | limiting in particular as a minimum of content of the said additional element, In order to acquire the effect by the said additional element reliably, 0.1 mol% or more is preferable.

  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 (for example, nitrate) and a zirconium salt (for example, nitrate), and if necessary, a salt of the additive element (for example, nitrate) and a surfactant, in the presence of ammonia. A co-precipitate is produced by separating, recovering and washing the obtained co-precipitate, followed by drying treatment, firing treatment and pulverization treatment, whereby a powdered ceria-zirconia solid solution can be obtained. In addition, content of each raw material in the said aqueous solution is suitably adjusted so that content of each component in the obtained ceria-zirconia solid solution may become a predetermined amount.

When producing the core-shell type oxide material of the present invention, first, such a ceria-zirconia solid solution is pressure-molded. The pressure during the 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). When the molding pressure deviates from the above range, the oxygen storage / release ability tends to decrease when the obtained core-shell type oxide material is exposed to a high temperature. In addition, there is no restriction | limiting in particular as such a pressure forming method, Well-known pressure forming methods, such as a hydrostatic pressure press, can be employ | adopted suitably.

  Next, the obtained pressure-molded body 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 ordered phase of the pyrochlore phase and the κ phase according to the present invention is formed. The ceria-zirconia solid solution having such an ordered phase has excellent surface thermal stability, and has a dense structure in which a solid-phase reaction hardly proceeds. When the reduction treatment temperature is less than the lower limit, the stability of the ordered phase is low, and the oxygen storage / release ability is lowered when the obtained core-shell type oxide material is exposed to a high temperature. In addition, the reduction treatment temperature is 1600 ° C. or higher from the viewpoint that stability of the ordered phase is improved and a decrease in oxygen storage / release capability is reliably suppressed when the obtained core-shell type oxide material is exposed to a high temperature. Is preferred. The reduction treatment time is preferably 0.5 hours or longer, and more preferably 1 hour or longer. When the reduction treatment time is less than the lower limit, the stability of the ordered phase is low, and when the obtained core-shell type oxide material is exposed to a high temperature, the oxygen storage / release ability tends to decrease. In addition, although there is no restriction | limiting in particular as an upper limit of reduction process temperature and reduction process time, From a viewpoint of reduction of energy efficiency or a by-product, 2000 degrees C or less (more preferably 1900 degrees C or less) and 24 hours or less (more 10 hours or less) is preferable.

  The reduction treatment method is not particularly limited as long as it is a method capable of performing reduction treatment at a predetermined temperature on the pressure-molded body in a reducing atmosphere. For example, (i) the pressurization in a vacuum heating furnace (Ii) a graphite furnace, in which a molded body is placed and evacuated, and then a reducing gas is introduced into the furnace to make the atmosphere in the furnace a reducing atmosphere and heated at a predetermined temperature for reduction treatment; The pressure molded body is placed in the furnace using a vacuum and evacuated, and then heated at a predetermined temperature and the atmosphere in the furnace is reduced by reducing gas such as CO or HC generated from the furnace body or heated fuel. A method of performing a reduction treatment as a reducing atmosphere, or (iii) by placing the pressure-molded body in a crucible filled with activated carbon and heating it at a predetermined temperature to generate a reducing gas such as CO or HC from the activated carbon Method of reducing the atmosphere in the crucible as a reducing atmosphere And the like.

As such a reducing gas to be used to achieve the reducing atmosphere is not particularly _limited, CO, HC, H 2, other reducing gases such as hydrocarbon gas. Further, 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 reduction treatment is performed at a higher temperature. Those are preferred. When such a reducing gas not containing carbon (C) is used, 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 sufficiently improved. It becomes.

  In the method for producing a core-shell type oxide 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. Thereby, oxygen lost during the reduction treatment is compensated, and the stability as an oxide material tends to be improved. There is no restriction | limiting in particular as a method of such an oxidation process, For example, the method of heat-processing the ceria-zirconia-type solid solution which has the said ordered phase in oxidizing atmosphere (for example, in air | atmosphere) can be employ | adopted suitably. Moreover, there is no restriction | limiting in particular as a heating temperature in such an oxidation process, About 300-800 degreeC is preferable. Furthermore, the heating time during the oxidation treatment is not particularly limited, but is preferably about 0.5 to 5 hours.

  Next, in the ceria-zirconia solid solution having the ordered phase thus obtained, the secondary particle diameter D50 at which the cumulative volume in the volume-based particle size distribution is 50% is within a predetermined range. A crushing process is performed to obtain a ceria-zirconia solid solution powder having the ordered phase. The pulverization method 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 solid solution powder having the ordered phase thus obtained is brought into contact with an alumina precursor, and the alumina is added to the surface of the ceria-zirconia solid solution powder having the ordered phase at a predetermined ratio. A precursor is attached (attachment step). The alumina precursor used here is not particularly limited as long as it can form 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 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 (for example, nitrate) and the interface. Examples include a method in which the ceria-zirconia solid solution powder having the ordered phase is immersed in an aqueous solution containing an activator and the ceria-zirconia solid solution powder having the ordered phase is impregnated with the alumina precursor aqueous solution. In addition, content of each raw material in the said aqueous solution is suitably adjusted so that content of each component in the obtained core-shell type oxide material may become predetermined amount.

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

  As temperature of heat processing, 300-1100 degreeC is preferable and 500-900 degreeC is more preferable. When the heat treatment temperature is less than the lower limit, a stable shell tends to be hardly formed. On the other hand, when the upper limit is exceeded, the specific surface area of the obtained core-shell type 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 comprises the core-shell type oxide material of the present invention and a noble metal in contact with the core-shell type 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 speed (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, as the noble metal, Rh, Pd and Pt are preferable, Rh and Pd are more preferable, and Rh is particularly preferable from the viewpoint that excellent NOx purification performance is obtained. 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 oxide material, and the noble metal is directly supported on the surface of the core-shell type oxide material. However, from the viewpoint that the operation is simple, the core-shell type oxide material and another oxide material supporting a noble metal may be mixed and brought into contact with each other.

  The exhaust gas purification catalyst of the present invention may be used by filling a reaction tube or the like in the form of a pellet, but from the viewpoint of practicality, the exhaust gas purification catalyst of the present invention is formed on the inner wall of the pores of the honeycomb substrate. It is preferably used as a honeycomb catalyst in which a layer made of a catalyst and a catalyst layer containing alumina are formed. Further, among such honeycomb catalysts, from the viewpoint of having excellent oxygen storage / release ability even when exposed to high temperature and high flow rate gas, and exhibiting excellent NOx purification performance. It is preferable to include 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 made of the exhaust gas purifying catalyst of the present invention formed on the catalyst lower layer. However, it is more preferable to use a mixture of the core-shell type oxide material of the present invention and a zirconia supporting a noble metal.

  EXAMPLES Hereinafter, although this invention is demonstrated more concretely based on an Example and a comparative example, this invention is not limited to a following example. The ceria-zirconia-praseodymium composite oxide used in Examples and Comparative Examples was prepared by the following method.

(Preparation Example 1)
A ceria-zirconia composite oxide having a molar ratio of cerium, zirconium, and praseodymium ([cerium]: [zirconium]: [praseodymium]) of 45: 54: 1 was prepared as follows. That is, first, it contains 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. 100 g of an aqueous solution and 197 g of hydrogen peroxide containing hydrogen peroxide in a 1.1-fold molar amount of cerium contained are added to 1217 g of an aqueous solution containing 1.2 times equivalent of ammonia relative to the neutralization equivalent. Then, a coprecipitate was produced, 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) was obtained. Thereafter, the solid solution was pulverized by sieving using a pulverizer (“Wonder Blender” manufactured by AS ONE Co., Ltd.) so that the particle size was 75 μm or less, thereby obtaining 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 deaerated, and then the mouth of the bag was heated and sealed. Subsequently, using a hydrostatic pressure press device (“CK4-22-60” manufactured by Nikkiso Co., Ltd.), the hydrostatic pressure press is applied to the bag 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 long, 4 cm wide, an average thickness of 7 mm, and a mass of about 20 g.

  Next, the obtained molded body (2 sheets) was placed in a crucible (inner volume: diameter 8 cm, height 7 cm) filled with 70 g of activated carbon, covered, and then placed in a high-speed temperature rising electric furnace. 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 degreeC over 4 hours, it cooled to room temperature by natural cooling, and obtained the reduction | restoration baking products.

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

(Comparative Preparation Example 2)
A ceria-zirconia solid solution powder having a molar ratio of cerium and zirconium ([cerium]: [zirconium]) of 45.5: 54.5 was prepared as follows. That is, first, it contains 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 1.1 times the molar amount of hydrogen peroxide as the contained cerium. 197 g of hydrogen peroxide solution was added to 1217 g of an aqueous solution containing 1.2 times equivalent of ammonia relative to the neutralization equivalent to form a coprecipitate, and the resulting coprecipitate was subjected to centrifugation, Washed with exchange water. Next, the obtained coprecipitate was dried at 110 ° C. for 10 hours or more and then calcined in the atmosphere 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 by sieving using a pulverizer (“Wonder Blender” manufactured by AS ONE Co., Ltd.) so as to have a particle size of 75 μm or less to obtain the ceria-zirconia solid solution powder.

Example 1
The ceria-zirconia-praseodymium composite oxide obtained in Preparation Example 1 was pulverized (manufactured by ASONE Co., Ltd.) so that the secondary particle diameter D50 at which the cumulative volume in the volume-based particle size distribution was 50% was 4 μm. Using a “wonder blender”), ceria-zirconia-praseodymium composite oxide powder (hereinafter abbreviated as “CZP powder”) was obtained. The volume-based particle size distribution of the CZP powder was measured by a dynamic light scattering method using a particle size distribution measuring device (“Laser diffraction / scattering particle size distribution measuring device MT3300EX” manufactured by Nikkiso Co., Ltd.).

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

(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 carboxylic acid chelating agent was changed to 2.8 g, and the amount of quaternary amine was changed to 4.8 g. A core-shell type oxide material powder coated with an alumina layer (CZP powder D50: 4 μm, CZP amount: 100 parts by mass, alumina coating amount: 0.25 parts by mass) was obtained.

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

(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 was 6 μm. Instead of CZP powder having a D50 of 4 μm, this CZP powder was used, and the amount of aluminum nitrate was changed to 5.49 g, the amount of carboxylic acid chelating agent was changed to 8.4, and the amount of quaternary amine was changed to 14.4 g. The core-shell type oxide material powder (CZP powder D50: 6 μm, CZP amount: 100 parts by mass, alumina coating amount: 0.75) was the same as Example 1 except that the surface of the CZP powder was coated with an alumina layer. Part by mass) was obtained.

(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 carboxylic acid chelating agent was changed to 10.2 g, and the amount of quaternary amine was changed to 19.2 g. A core-shell type oxide material powder coated with an alumina layer (CZP powder D50: 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 size D50 was pulverized to 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. To this La-containing alumina precursor aqueous solution, 100 g of the CZP powder (D50: 10 μm) was added and stirred for 15 minutes. The obtained CZP powder-containing dispersion is heated at 200 ° C. with stirring to evaporate to dryness, and the obtained dried product is fired at 900 ° C. for 5 hours, and the surface of the CZP powder contains an lanthanum-containing alumina layer. The core-shell type oxide material powder coated with (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) was obtained.

(Comparative Example 2)
The ceria-zirconia-praseodymium composite oxide obtained in Preparation Example 1 was used in place of the ceria-zirconia solid solution powder obtained in Comparative Preparation Example 1, and pulverized so that the secondary particle diameter D50 was 10 μm. Was obtained in the same manner as in Example 1 to obtain a ceria-zirconia solid solution powder (hereinafter abbreviated as “CZ powder”).

  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. To this La-containing alumina precursor aqueous solution, 100 g of the CZ powder (D50: 10 μm) was added and stirred for 15 minutes. The obtained CZP powder-containing dispersion is heated at 200 ° C. with stirring to evaporate to dryness, and the resulting dried product is fired at 900 ° C. for 5 hours. The alumina layer in which the surface of the CZ powder contains lanthanum The core-shell type oxide material powder (C50 powder D50: 10 μm, CZ amount: 100 parts by mass, alumina coating amount: 0.5 parts by mass, lantana coating amount: 0.015 parts by mass) was obtained.

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

<X-ray diffraction (XRD) measurement>
Each core-shell type oxide material powder obtained in Examples and Comparative Examples was heated in air at 1100 ° C. for 5 hours. The X-ray diffraction pattern of the regular phase (core regular phase) of the core-shell type oxide material powder after heating was measured using an X-ray diffractometer (“RINT2100” manufactured by Rigaku Corporation) and CuKα as an X-ray source. Measured by 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 of 2θ = 14.5 ° and the diffraction line of 2θ = 29 ° was determined. The results are shown in Table 1.

<X-ray photoelectron spectroscopy>
The X-ray photoelectron spectroscopy (XPS) spectra of the core-shell type oxide material powders obtained in Examples and Comparative Examples were monochromatized using an X-ray photoelectron spectrometer (“Quantera SXM” manufactured by ULVAC-PHI). In addition, AlKα (1486.6 eV) was used as an X-ray source, and measurement was performed under the conditions of photoelectron extraction angle: 45 °, analysis region: about 200 μmφ, charge-up correction: Zr3d 182.2 eV (ZrO ₂ ). Based on the obtained XPS spectrum, the element present in the region (about 200 μmφ) at a depth of 3 nm from the surface of the core-shell type oxide material powder (shell surface) is quantified, and the average concentration of Al element [= Al amount / (Al amount + Ce amount + Zr amount + Pr amount) × 100]. The results are shown in Table 1.

<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) at a mass ratio of 1: 1 The resulting mixture is pressure-molded with a hydrostatic pressure of 1 t, and the resulting molded body is pulverized and classified so that the particle size is 0.5 to 1 mm to obtain a pellet catalyst. It was.

<High temperature durability test>
1.5 g of the obtained pellet catalyst was packed in a cylindrical reaction tube having a diameter of 10 mm, and the pellet catalyst was mixed with rich gas [H ₂ (2%) + CO _{2 at} a gas flow rate of 10 L / min under a temperature condition of 1100 ° C. (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 for 5 minutes each.

<Measurement of oxygen storage / release rate (OSC-r) and oxygen storage / release amount (OSC)>
A mixture of 0.25 g of the pellet catalyst after the high temperature durability test and 0.25 g of quartz sand was charged into the reaction tube. A rich gas [CO (2% by volume) + N ₂ (remainder)] was circulated through the catalyst at a gas temperature of 500 ° C. and a gas flow rate of 10 L / min for 3 minutes, and then the flow gas was lean gas [O ₂ (1 volume). %) + N 2 _(allowed to flow by switching to the remainder)] 3 minutes, again switching the flow gas to the rich gas. From the amount of CO ₂ in the catalyst outgas for 5 seconds and 3 minutes after the second flow gas switching, the oxygen storage / release rate (OSC-r, unit: μmol / (g · s)) and the oxygen storage / release amount (OSC) , Unit: μmol / g). The results are shown in Table 1.

<Measurement of 50% NOx purification temperature>
The reaction tube was filled with 0.5 g of the pellet catalyst after the high temperature durability test. To this catalyst, 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 ppmC) + H ₂ (0 .233 volume%) + H ₂ O (10 volume%) + N ₂ (remainder)] at a gas flow rate of 10 L / min while heating from 100 ° C. to 600 ° C. at a heating rate of 50 ° C./min. The NOx purification rate was calculated by measuring the concentration of NO in the catalyst-containing gas and the catalyst outlet gas at the catalyst-containing gas temperature, and the catalyst temperature (50% NOx purification temperature) when NOx was purified by 50% was determined. . The results are shown in Table 1.

<NOx transient purification rate measurement>
The reaction tube was filled with 0.5 g of the pellet catalyst after the high temperature durability test. To this catalyst, at a gas flow rate of 10 L / min, 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)], and the temperature of the gas containing the catalyst is raised to 600 ° C., and then the flow gas is rich gas [NO (1500 volume ppm) + CO ₂ (10 Volume%) + CO (0.65 volume%) + C ₃ H ₆ (3000 volume ppm C) + H ₂ O (5 volume%) + N ₂ (remainder)] and let it flow for 5 minutes. The pre-treatment was carried out after switching for 5 minutes. Thereafter, the temperature of the gas containing the catalyst was lowered to 500 ° C., and the rich gas and the lean gas were circulated alternately at intervals of 5 minutes at a gas flow rate of 10 L / min. The NOx transient purification rate was determined by measuring the average NO concentration in the catalyst-containing gas and the catalyst output gas during the rich gas flow (5 minutes) in the third cycle. The results are shown in Table 1.

  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 all have diffraction line intensities 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 to have at least one ordered phase of the pyrochlore phase and the κ phase. On the other hand, in the core-shell type oxide material powder obtained in Comparative Example 2, the diffraction line of 2θ = 14.5 ° was not observed in the X-ray diffraction pattern using CuKα as the X-ray source (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 type oxide material powder obtained in Examples 1 to 5 is more oxygen storage than the pellet catalyst using the core-shell type oxide material powder obtained in Comparative Examples 1 and 2. It was found that the release amount (OSC) and oxygen storage / release rate (OSC-r) were excellent, the 50% NOx purification temperature was low, and the NOx purification activity at low temperatures was excellent.

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

  As described above, by using the core-shell type oxide material of the present invention, even when exposed to high temperature, excellent oxygen storage / release capacity (oxygen storage capacity (OSC) and oxygen storage / release rate (OSC) -R)) and an exhaust gas purifying catalyst exhibiting excellent NOx purifying performance can be obtained.

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

Claims (6)

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 a shell made of an alumina-based oxide disposed on a part of the surface of the core,
The secondary particle diameter D50 at which the cumulative volume in the volume-based particle size distribution of the ceria-zirconia solid solution powder is 50% is 0.2 to 8.0 μm,
The average concentration of Al element in a region 3 nm deep from the surface of the shell, measured by X-ray photoelectron spectroscopy, is 25 to 75 at%.
A core-shell type oxide material characterized by the above.   The core-shell type 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.   The core-shell type oxide material according to claim 1 or 2, wherein the core further contains a rare earth element other than Ce.   4. The average concentration of Al element in a region 3 nm deep from the surface of the shell, measured by X-ray photoelectron spectroscopy, is 30 to 75 at%. 5. The core-shell type oxide material described in 1.   An exhaust gas purification catalyst comprising the core-shell type oxide material according to any one of claims 1 to 4 and a noble metal in contact with the core-shell type oxide material.   An exhaust gas purification method comprising contacting exhaust gas containing nitrogen oxides with the exhaust gas purification catalyst according to claim 5.