JP7754881B2 - Oxygen storage material and method for producing the same - Google Patents

Description

The present invention relates to an oxygen storage material containing a pyrochlore-type ceria-zirconia composite oxide and a method for producing the same.

So-called three-way catalysts are known as exhaust gas purification catalysts that can oxidize carbon monoxide (CO) and hydrocarbons (HC) in exhaust gases emitted from internal combustion engines such as automobile engines, while simultaneously reducing nitrogen oxides (NOx).

When purifying exhaust gas using an exhaust gas purification catalyst, it is known to use materials with oxygen storage capacity (Oxygen Storage Capacity (OSC)), which can store oxygen when the oxygen concentration in the exhaust gas is high and release oxygen when the oxygen concentration in the exhaust gas is low, as supports or co-catalysts for the exhaust gas purification catalyst in order to absorb fluctuations in the oxygen concentration in the exhaust gas and increase the exhaust gas purification capacity.

Ceria has traditionally been used as a suitable oxygen storage material with such an OSC, and in recent years, various types of composite oxides containing ceria have been researched, with various ceria-zirconia composite oxides being developed using methods such as coprecipitation, reverse coprecipitation, hydrothermal synthesis, melting, and solid-phase methods.

For example, Japanese Patent Application Laid-Open No. 2015-182931 (Patent Document 1) discloses a method for producing a ceria-zirconia composite oxide containing cerium, zirconium, and other transition metal elements such as iron, manganese, cobalt, nickel, and copper, and having a pyrochlore phase crystal structure, using a so-called melting method.

Furthermore, Japanese Patent Laid-Open No. 2022-59284 (Patent Document 2) describes an oxygen storage material containing a pyrochlore-type ceria-zirconia composite oxide and iron added to the pyrochlore-type ceria-zirconia composite oxide, in which the content ratio of Fe to the total amount of Ce and Zr (Fe/(Ce+Zr)×100) is 0.5 to 9 at %, the molar fraction of Zr to the total number of moles of Ce and Zr (X=Zr/(Ce+Zr)×100) is X=40 to 50%, and the lattice constant calculated from the X-ray diffraction pattern obtained by X-ray diffraction measurement using CuKα before and after heating at 1100° C. in air for 5 hours satisfies the formula: lattice constant≦−7.00× ¹⁰ X+10.874, and the intensity ratio [I(14/29) value] between the diffraction line near 2θ=14.5° and the diffraction line near 2θ=29°, as determined from X-ray diffraction patterns obtained by X-ray diffraction measurement using CuKα before and after heating at 1100°C in air for 5 hours, satisfies the condition expressed by the formula: I(14/29) value≦2.36×10 ⁻³ X−0.072.

However, in recent years, the required characteristics of exhaust gas purification catalysts have been increasing, and there is a need for oxygen storage materials that can exhibit excellent oxygen storage capacity (OSC) at low temperatures of 250°C, not only at the initial stage of use but also after prolonged exposure to high-temperature exhaust gases of around 1100°C. Conventional oxygen storage materials such as those described in Patent Documents 1 and 2 have not necessarily been sufficient.

JP 2015-182931 A JP 2022-59284 A

The present invention was made in consideration of the problems inherent in the prior art described above, and aims to provide an oxygen storage material that can exhibit excellent oxygen storage capacity (OSC) at a low temperature of 250°C, not only in the initial stage of use but also after prolonged exposure to high-temperature exhaust gases at approximately 1100°C, as well as a method for producing the same.

As a result of extensive research conducted by the inventors to achieve the above-mentioned objective, it was discovered that a scandium-containing ceria-zirconia solid solution powder was produced by using scandium in combination with praseodymium or lanthanum as an element added to a pyrochlore-type ceria-zirconia composite oxide, and adjusting the cerium content, praseodymium or lanthanum content, scandium content, and atomic ratio of cerium to zirconium within a predetermined range. The powder was then pressed under a predetermined pressure, reduced under predetermined high-temperature conditions, and then oxidized. This resulted in a high-temperature powder of approximately 1100°C. The researchers discovered that a composite oxide in which scandium and praseodymium or lanthanum are sufficiently dissolved in a pyrochlore-type ceria-zirconia composite oxide can be obtained both before and after exposure to exhaust gases for an extended period of time, and that this pyrochlore-type ceria-zirconia composite oxide containing scandium and praseodymium or lanthanum exhibits excellent oxygen storage capacity (OSC) at a low temperature of 250°C, not only in the initial stage of use but also after long-term exposure to exhaust gases at high temperatures of around 1100°C, leading to the completion of the present invention.

That is, the present invention provides the following aspects:

[1] A pyrochlore-type ceria (CeO ₂ )-zirconia (ZrO ₂ ) composite oxide containing scandium (Sc) and praseodymium (Pr) or lanthanum (La),
With respect to all cations in the pyrochlore-type ceria-zirconia-based composite oxide,
The Ce content is 43 to 49 at%;
The content of Pr or La is 0.5 to 4.9 at%;
The Sc content is 0.5 to 4.9 at%;
An oxygen storage material having an atomic ratio of Ce to Zr (Ce/Zr) of 45/55 to 51/49.

[2] The oxygen storage material described in [1], in which at least a portion of Sc and Pr or La is solid-solved in the pyrochlore-type ceria-zirconia composite oxide.

[3] The oxygen storage material described in [1] or [2], wherein at least a portion of Sc substitutes for Zr sites in the pyrochlore-type ceria-zirconia composite oxide, and at least a portion of Pr or La substitutes for Ce sites in the pyrochlore-type ceria-zirconia composite oxide.

[4] The zero-mean normalized correlation coefficient (ZNCC) values [Sc-Zr], [Sc-Ce], [M PL-Ce], and [M _PL -Zr] calculated based on the Sc distribution map, Pr or La distribution map, and Ce distribution map in an element mapping image obtained by energy dispersive _X -ray analysis are expressed by the following formulas (1) and (2):
[Sc-Zr]-[Sc-Ce]>0.000 (1)
[M _PL -Ce] - [M _PL -Zr]>0.028 (2)
(In the above formula, [Sc-Zr] represents the ZNCC coefficient value between the Sc distribution map and the Zr distribution map, [Sc-Ce] represents the ZNCC coefficient value between the Sc distribution map and the Ce distribution map, [M _PL -Ce] represents the ZNCC coefficient value between the Pr or La distribution map and the Ce distribution map, and [M _PL -Zr] represents the ZNCC coefficient value between the Pr or La distribution map and the Zr distribution map.)
The oxygen storage material according to any one of [1] to [3], which satisfies the condition represented by the following formula:

[5] A method for producing an oxygen storage material containing a pyrochlore-type ceria (CeO ₂ )-zirconia (ZrO ₂ ) composite oxide containing scandium (Sc) and praseodymium (Pr) or lanthanum (La), comprising:
a step of preparing a ceria-zirconia-based solid solution powder containing scandium and praseodymium or lanthanum, wherein the Ce content is 43 to 49 at%, the Pr or La content is 0.5 to 4.9 at%, the Sc content is 0.5 to 4.9 at%, and the atomic ratio of Ce to Zr (Ce/Zr) is 45/55 to 51/49, relative to all cations;
a step of compacting the ceria-zirconia solid solution powder containing scandium and praseodymium or lanthanum at a pressure of 30 to 350 MPa, reducing the powder at a temperature of 1400 to 2000°C, and then oxidizing the powder to obtain an oxygen storage material containing the pyrochlore-type ceria-zirconia composite oxide containing scandium and praseodymium or lanthanum according to claim 1;
A method for producing an oxygen storage material, comprising:

While the reason why the oxygen storage material of the present invention can exhibit an excellent OSC at a low temperature of 250°C, not only at the initial stage of use but also after long-term exposure to high-temperature exhaust gases at approximately 1100°C, is not entirely clear, the inventors speculate as follows _: The _CeO2 - _ZrO2 superlattice structure in the pyrochlore-type ceria-zirconia composite oxide containing scandium and praseodymium or lanthanum that constitutes the oxygen storage material of the present invention undergoes a phase change between the pyrochlore phase ( _Ce2Zr2O7 ) and the κ phase ( _{Ce2Zr2O8 )} depending on the oxygen partial pressure in _the gas phase, thereby exhibiting OSC. Because the ionic radius of Sc3 ⁺ is close to that _of Zr4 ⁺ , it is believed that Sc ions selectively substitute for Zr sites in the pyrochlore-type ceria-zirconia composite oxide containing scandium and praseodymium or lanthanum according to the present invention. When Zr sites are substituted with Sc ions, it is presumed that the superlattice structure containing oxygen vacancies generated by charge compensation is more stabilized while maintaining heat resistance. Furthermore, since the ionic radius of Pr ³⁺ or La ³⁺ is close to that of Ce ³⁺ , it is believed that Pr ions or La ions selectively substitute for Ce sites in the pyrochlore-type ceria-zirconia composite oxide containing scandium and praseodymium or lanthanum according to the present invention. It is presumed that when Ce sites are substituted with Pr ions or La ions, degradation of the pyrochlore structure is suppressed even after prolonged exposure to high-temperature exhaust gases of approximately 1100°C. For these reasons, it is presumed that the oxygen storage material of the present invention can exhibit an excellent OSC at a low temperature of 250°C, even after prolonged exposure to high-temperature exhaust gases of approximately 1100°C.

The present invention makes it possible to obtain an oxygen storage material that exhibits excellent oxygen storage capacity (OSC) at temperatures as low as 250°C, not only in the initial stage of use but also after prolonged exposure to high-temperature exhaust gases at approximately 1100°C.

FIG. 1 is a distribution diagram of the elements Ce, Zr, Pr, and Sc in the composite oxide powder obtained in Example 5 before the heat resistance test. FIG. 1 is a distribution diagram of the elements Ce, Zr, Pr, and Sc in the composite oxide powder obtained in Comparative Example 3 before the heat resistance test. 1 is a graph showing the results of plotting the difference ([Sc−Zr]−[Sc−Ce]) in ZNCC coefficient values of the composite oxide powders obtained in Examples 1 to 7 and Comparative Examples 1 to 4 and 7 to 8 before heat resistance testing against the Sc content. 1 is a graph showing the results of plotting the difference ([M _PL −Ce] − [M _PL −Zr]) in ZNCC coefficient values of the composite oxide powders obtained in Examples 1 to 7 and Comparative Examples 1 to 4 and 6 before heat resistance testing against the Pr or La content. 1 is a graph showing the results of plotting the oxygen storage capacity (OSC) of catalyst powders in which platinum is supported on composite oxide powders after heat resistance tests obtained in Examples 1 to 7 and Comparative Examples 1 to 8 against the Sc content. 1 is a graph showing the results of plotting the oxygen storage capacity (OSC) of catalyst powders in which platinum is supported on composite oxide powders after heat resistance tests obtained in Examples 1 to 7 and Comparative Examples 1 to 8 against the Pr or La content.

The present invention will be described in detail below based on its preferred embodiments.

First, the oxygen storage material of the present invention will be described. The oxygen storage material of the present invention contains a pyrochlore-type ceria (CeO ₂ )-zirconia (ZrO ₂ ) composite oxide containing scandium (Sc) and praseodymium (Pr) or lanthanum (La),
With respect to all cations in the pyrochlore-type ceria-zirconia-based composite oxide,
The Ce content is 43 to 49 at%;
The content of Pr or La is 0.5 to 4.9 at%;
The Sc content is 0.5 to 4.9 at%;
The atomic ratio of Ce to Zr (Ce/Zr) is 45/55 to 51/49.

The oxygen storage material of the present invention contains a ceria-zirconia composite oxide (hereinafter referred to as a "pyrochlore-type ceria-zirconia composite oxide") having a superlattice structure in which Ce and Zr are regularly arranged. Oxygen storage materials containing such pyrochlore-type ceria-zirconia composite oxides have a higher oxygen diffusion rate within the bulk than ceria-zirconia composite oxides with a fluorite structure, and therefore have excellent oxygen storage capacity (OSC). Furthermore, by recognizing the presence of peaks (peaks appearing at 2θ = 14.0° to 16.0°) attributable to the superlattice structure in X-ray diffraction measurements using CuKα radiation, it can be confirmed that the ceria-zirconia composite oxide is a pyrochlore-type having a superlattice structure.

In the oxygen storage material of the present invention, the Ce content relative to all cations in the pyrochlore-type ceria-zirconia composite oxide must be 43 to 49 at%, preferably 44 to 49 at%, and more preferably 46 to 49 at%. If the Ce content is below the lower limit, the OSC capacity that can be achieved decreases. On the other hand, if the Ce content exceeds the upper limit, _CeO2 undergoes phase separation when exposed to high temperatures, causing the superlattice structure to collapse and reducing the OSC.

In addition, in the oxygen storage material of the present invention, the Pr or La content relative to all cations in the pyrochlore-type ceria-zirconia composite oxide must be 0.5 to 4.9 at% and preferably 1.0 to 2.5 at%. If the Pr or La content is below the lower limit, heat resistance decreases, while if the Pr or La content exceeds the upper limit, the OSC at low temperatures decreases.

Furthermore, in the oxygen storage material of the present invention, the Sc content relative to all cations in the pyrochlore-type ceria-zirconia composite oxide must be 0.5 to 4.9 at% and preferably 1.0 to 2.5 at%. If the Sc content is below the lower limit, the effect of improving the OSC at low temperatures decreases, while if the Sc content exceeds the upper limit, heat resistance decreases.

Furthermore, in the oxygen storage material of the present invention, the atomic ratio of Ce to Zr (Ce/Zr) must be 45/55 to 51/49, and preferably 46/54 to 50/50. If the Ce/Zr ratio is below the lower limit, the OSC capacity that can be achieved decreases, while if the Ce/Zr ratio exceeds the upper limit, _CeO2 undergoes phase separation when exposed to high temperatures, causing the superlattice structure to collapse and reducing the OSC.

Furthermore, in the oxygen storage material of the present invention, it is preferable that at least a portion of Sc and Pr or La is solid-solved in the pyrochlore-type ceria-zirconia composite oxide, and it is more preferable that at least a portion of Sc is substituted at the Zr site in the pyrochlore-type ceria-zirconia composite oxide, and at least a portion of Pr or La is substituted at the Ce site in the pyrochlore-type ceria-zirconia composite oxide. This improves heat resistance and low-temperature OSC. On the other hand, if Sc and Pr or La are not solid-solved in the pyrochlore-type ceria-zirconia composite oxide, if Sc is not substituted at the Zr site in the pyrochlore-type ceria-zirconia composite oxide, or if Pr or La is not substituted at the Ce site in the pyrochlore-type ceria-zirconia composite oxide, heat resistance and low-temperature OSC are not sufficiently improved.

It should be noted that even if Sc and Pr or La are added to a pyrochlore-type ceria-zirconia composite oxide, it is difficult to form a sufficient solid solution of Sc and Pr or La in the pyrochlore-type ceria-zirconia composite oxide by methods such as so-called solid-phase synthesis and impregnation , and therefore the addition of Sc and Pr or La does not contribute to improving the heat resistance and OSC at low temperatures. In contrast, in the present invention, by the production method of the present invention described below, it is possible to obtain a pyrochlore-type ceria-zirconia composite oxide in which Sc and Pr or La are sufficiently solid-solved, which was not possible to obtain in the past, and it becomes possible to achieve improvements in the heat resistance and the OSC at low temperatures.

In the oxygen storage material of the present invention, the coefficient values [Sc-Zr], [Sc-Ce], [M PL-Ce], and [M PL-Zr] of zero-means normalized cross-correlation (ZNCC) calculated based on an Sc distribution map, a Pr or La distribution map, a Ce distribution map, and an Sc distribution map in an element mapping image (EDX element mapping image) obtained by energy dispersive _X -ray analysis ( _EDX ) are expressed by the following formulas (1) and (2):
[Sc-Zr]-[Sc-Ce]>0.000 (1)
[M _PL -Ce] - [M _PL -Zr]>0.028 (2)
(In the above formula, [Sc-Zr] represents the ZNCC coefficient value between the Sc distribution map and the Zr distribution map, [Sc-Ce] represents the ZNCC coefficient value between the Sc distribution map and the Ce distribution map, [M _PL -Ce] represents the ZNCC coefficient value between the Pr or La distribution map and the Ce distribution map, and [M _PL -Zr] represents the ZNCC coefficient value between the Pr or La distribution map and the Zr distribution map.)
An oxygen storage material in which the differences in ZNCC coefficient values, [Sc-Zr]-[Sc-Ce] and [M _PL -Ce]-[M _PL -Zr], satisfy the conditions expressed by the formulas (1) and (2) has Sc sufficiently substituted at the Zr site and Pr or La sufficiently substituted at the Ce site, and therefore has excellent heat resistance and low-temperature OSC.

The EDX elemental mapping image can be obtained by, for example, performing EDX analysis using a scanning transmission electron microscope equipped with an energy dispersive X-ray analyzer under the condition of a field magnification of 4.8×10 ⁷ times.

The ZNCC coefficient values [Sc-Zr], [Sc-Ce], [M _PL -Ce], and [M _PL -Zr] are calculated based on the comparison image (the Sc, Pr, or La distribution map) and the reference image (the Ce or Zr distribution map) in the EDX element mapping image, using the following formula (3):

(In the above formula, ZNCC represents the coefficient value of zero-mean normalized cross-correlation (ZNCC), I(i, j) represents the luminance value of the ith pixel in the horizontal direction and the jth pixel in the vertical direction in the comparison image (the Sc, Pr, or La distribution map), I _ave represents the average luminance value in the comparison image, T(i, j) represents the luminance value of the ith pixel in the horizontal direction and the jth pixel in the vertical direction in the reference image (the Ce or Zr distribution map), and T _ave represents the average luminance value in the reference image.)
Specifically, the ZNCC coefficient values of the comparison image (the Sc, Pr, or La distribution map) and the reference image (the Ce or Zr distribution map) can be calculated using image processing software ImageJ or Matlab.

The ZNCC coefficient value calculated in this way is ZNCC = 1.0 when the comparison image (the Sc, Pr, or La distribution map) and the reference image (the Ce or Zr distribution map) are completely identical, and ZNCC = -1.0 when they are completely contradictory. For example, when comparing the Sc distribution map with the Zr distribution map, if Sc is present in the Zr site (i.e., if the Zr site is substituted with Sc), the ZNCC coefficient value [Sc - Zr] will be positive and less than 1.0.

In the oxygen storage material of the present invention, the ceria-zirconia composite oxide is a pyrochlore type, and the cation sites in the crystal lattice are divided into Zr sites and Ce sites. When Sc is dissolved in such a pyrochlore type ceria-zirconia composite oxide and Sc is substituted at the Zr site, the ZNCC coefficient value [Sc - Zr] becomes positive. When Sc is not present at the Ce site, the ZNCC coefficient value [Sc - Ce] becomes negative. Therefore, the difference in the ZNCC coefficient values ([Sc - Zr] - [Sc - Ce]) can be used as an indicator of whether Sc is selectively substituted at the Zr site. The larger the [Sc - Zr] - [Sc - Ce], the more selectively Sc is substituted at the Zr site. Similarly, when Pr or La is dissolved in a pyrochlore-type ceria-zirconia composite oxide and Pr or La is substituted at the Ce site, the ZNCC coefficient value [M _PL - Ce] becomes positive. Furthermore, when Pr or La is not present at the Zr site, the ZNCC coefficient value [M _PL - Zr] becomes negative. Therefore, the difference in the ZNCC coefficient values ([M _PL - Ce] - [M _PL - Zr]) can be used as an index showing whether Pr or La is selectively substituted at the Ce site, and the larger the value of [M _PL - Ce] - [M _PL - Zr], the more selectively Pr or La is substituted at the Ce site.

Therefore, in the oxygen storage material of the present invention, Sc is further selectively substituted at the Zr site, and Pr or La is further selectively substituted at the Ce site, and from the viewpoint of further improving the heat resistance and the OSC at low temperatures, the ZNCC coefficient values [Sc-Zr], [Sc-Ce], [M _PL -Ce], and [M _PL -Zr] are expressed by the following formulas (1A) and (2A):
[Sc-Zr]-[Sc-Ce]>0.010 (1A)
[M _PL -Ce]-[M _PL -Zr]>0.100 (2A)
It is preferable that the conditions represented by the following formulas (1B) and (2B):
[Sc-Zr]-[Sc-Ce]>0.020 (1B)
[M _PL -Ce]-[M _PL -Zr]>0.120 (2B)
It is more preferable that the condition expressed by the following expression is satisfied.

The average crystallite size of the oxygen storage material of the present invention is preferably 0.1 to 10 μm, and more preferably 0.2 to 5 μm. If the average crystallite size of the oxygen storage material is below the lower limit, the superlattice structure tends to be difficult to maintain after prolonged exposure to high-temperature exhaust gases at approximately 1100°C. On the other hand, if the average crystallite size exceeds the upper limit, the OSC improvement effect tends to be difficult to achieve. Note that this average crystallite size can be calculated based on the Scherrer equation using commercially available analysis software (e.g., "JADE" manufactured by Materials Data) and the half-width of the diffraction line near 2θ = 29° obtained from the X-ray diffraction pattern obtained by X-ray diffraction measurement using CuKα.

Furthermore, the specific surface area of the oxygen storage material of the present invention is not particularly limited, but is preferably 0.01 to 20 m ² /g, more preferably 0.05 to 10 m ² /g, and even more preferably 0.1 to 5 m ² /g. If the specific surface area of the oxygen storage material is below the lower limit, the oxygen storage capacity tends to decrease, while if it exceeds the upper limit, the number of particles with small particle diameters increases, and heat resistance tends to decrease. Note that this specific surface area can be calculated as the BET specific surface area from the adsorption isotherm using the BET isothermal adsorption equation.

Furthermore, the oxygen storage material of the present invention may further contain at least one element selected from the group consisting of rare earth elements other than cerium, scandium, praseodymium, and lanthanum, and alkaline earth elements. By incorporating such an element, the oxygen storage material of the present invention tends to exhibit higher exhaust gas purification capabilities when used as a support for an exhaust gas purification catalyst. Examples of such rare earth elements other than cerium, scandium, praseodymium, and lanthanum include yttrium (Y), neodymium (Nd), samarium (Sm), gadolinium (Gd), terbium (Tb), dysprosium (Dy), ytterbium (Yb), and lutetium (Lu). Among these, Nd and Y are preferred because they tend to stabilize the superlattice structure. Furthermore, examples of alkaline earth metal elements include magnesium (Mg), calcium (Ca), strontium (Sr), barium (Ba), and radium (Ra), with Mg, Ca, and Ba being preferred from the viewpoint of their tendency to stabilize the superlattice structure.

When at least one element selected from the group consisting of rare earth elements other than cerium, scandium, praseodymium, and lanthanum and alkaline earth elements is further contained, the content of the element is preferably 0.5 to 20 at% and more preferably 1 to 10 at% relative to all cations in the pyrochlore-type ceria-zirconia composite oxide. If the content of such an element is below the lower limit, the effect of stabilizing the superlattice structure tends to decrease, while if the content exceeds the upper limit, the oxygen storage capacity tends to decrease.

The oxygen storage material of the present invention contains the pyrochlore-type ceria-zirconia composite oxide and scandium and praseodymium or lanthanum added to the pyrochlore-type ceria-zirconia composite oxide. It exhibits excellent OSC at temperatures as low as 250°C, not only at the initial stage of use but also after prolonged exposure to high-temperature exhaust gases at approximately 1100°C. Therefore, the oxygen storage material of the present invention is suitable for use as a support or co-catalyst for exhaust gas purification catalysts. A suitable example of the use of the oxygen storage material of the present invention is an exhaust gas purification catalyst comprising a support made of the oxygen storage material of the present invention and a precious metal supported on the support. Examples of such precious metals include platinum, rhodium, palladium, osmium, iridium, gold, and silver. Another example is an exhaust gas purification catalyst in which the oxygen storage material of the present invention is disposed around a precious metal supported on other catalyst support particles.

Next, a method for producing an oxygen storage material of the present invention will be described. The method for producing an oxygen storage material of the present invention is a method for producing an oxygen storage material containing a pyrochlore-type ceria (CeO ₂ )-zirconia (ZrO ₂ ) composite oxide containing scandium (Sc) and praseodymium (Pr) or lanthanum (La), comprising the steps of:
A step (first step) of preparing a ceria-zirconia-based solid solution powder containing scandium and praseodymium or lanthanum, wherein the Ce content is 43 to 49 at%, the Pr or La content is 0.5 to 4.9 at%, the Sc content is 0.5 to 4.9 at%, and the atomic ratio of Ce to Zr (Ce/Zr) is 45/55 to 51/49, relative to all cations;
a step (second step) of compacting the ceria-zirconia solid solution powder containing scandium and praseodymium or lanthanum at a pressure of 30 to 350 MPa, reducing the powder at a temperature of 1400 to 2000°C, and then oxidizing the powder to obtain an oxygen storage material containing the pyrochlore-type ceria-zirconia composite oxide containing scandium and praseodymium or lanthanum according to claim 1;
The method includes:

First, in the first step, a ceria-zirconia solid solution powder is prepared, containing scandium (Sc) and praseodymium (Pr) or lanthanum (La), with a Ce content of 43 to 49 at% (preferably 44 to 49 at%, more preferably 46 to 49 at%) relative to all cations, a Pr or La content of 0.5 to 4.9 at% (preferably 1.0 to 2.5 at%), a Sc content of 0.5 to 4.9 at% (preferably 1.0 to 2.5 at%), and an atomic ratio of Ce to Zr (Ce/Zr) of 45/55 to 51/49 (preferably 46/54 to 50/50).

In the ceria-zirconia solid solution powder containing scandium and praseodymium or lanthanum, if the Ce content is less than the lower limit, the OSC capacity that can be developed decreases, while if the Ce content exceeds the upper limit, _CeO2 undergoes phase separation when exposed to high temperatures, causing the superlattice structure to collapse and the OSC to decrease. Furthermore, in the ceria-zirconia solid solution powder containing scandium and praseodymium or lanthanum, if the Pr or La content is less than the lower limit, the heat resistance decreases, while if the Pr or La content exceeds the upper limit, the OSC at low temperatures decreases. Furthermore, in the ceria-zirconia solid solution powder containing scandium and praseodymium or lanthanum, if the Sc content is less than the lower limit, the effect of improving the OSC at low temperatures decreases, while if the Sc content exceeds the upper limit, the heat resistance decreases. Furthermore, if the Ce/Zr ratio is less than the lower limit, the OSC capacity that can be expressed decreases, while if the Ce/Zr ratio exceeds the upper limit, when exposed to high temperatures, _CeO2 undergoes phase separation, the superlattice structure collapses, and the OSC decreases.

Furthermore, in the ceria-zirconia solid solution powder containing scandium and praseodymium or lanthanum, it is preferable to use a solid solution in which ceria and zirconia are mixed at the atomic level, from the viewpoint of more fully forming a superlattice structure. Furthermore, such a ceria-zirconia solid solution powder containing scandium and praseodymium or lanthanum preferably has an average primary particle size of about 2 to 100 nm, more preferably about 5 to 70 nm, and further preferably has a specific surface area of 1.0 to 100 m ² /g, more preferably 10 to 80 m ² /g, and even more preferably 30 to 80 m ² /g.

There are no particular limitations on the method for preparing (preparing) such a ceria-zirconia solid solution powder containing scandium and praseodymium or lanthanum, and examples include a method employing a so-called coprecipitation method to produce the solid solution powder so that the content ratios of cerium, zirconium, scandium, and praseodymium or lanthanum are within the above-mentioned ranges. Examples of such coprecipitation methods include using an aqueous solution containing a cerium salt (e.g., nitrate), a zirconium salt (e.g., nitrate), a scandium salt (e.g., nitrate), a praseodymium salt (e.g., nitrate), or a lanthanum salt (e.g., nitrate) to form a coprecipitate in the presence of ammonia, centrifuging the resulting coprecipitate, washing it, drying it, calcining it, and then pulverizing it using a mill such as a ball mill to obtain the ceria-zirconia solid solution powder containing scandium and praseodymium or lanthanum. The aqueous solution containing the cerium salt, the zirconium salt, the scandium salt, and the praseodymium salt or the lanthanum salt is prepared so that the content ratios of cerium, zirconium, scandium, and praseodymium or lanthanum in the resulting solid solution powder are within a specified range.

Furthermore, the aqueous solution containing the cerium salt, the zirconium salt, the scandium salt, and the praseodymium salt or the lanthanum salt may optionally contain a salt of at least one element selected from the group consisting of cerium, scandium, and rare earth elements other than praseodymium or lanthanum, and alkaline earth elements, or a surfactant (e.g., a nonionic surfactant). By adding at least one element selected from the group consisting of cerium, scandium, and rare earth elements other than praseodymium or lanthanum, and alkaline earth elements, to a ceria-zirconia solid solution powder containing scandium and praseodymium or lanthanum, the oxygen storage material of the present invention tends to exhibit higher exhaust gas purification ability when used as a co-catalyst for an exhaust gas purification catalyst. Examples of rare earth elements other than cerium, scandium, and praseodymium or lanthanum include yttrium (Y), neodymium (Nd), samarium (Sm), gadolinium (Gd), terbium (Tb), dysprosium (Dy), ytterbium (Yb), and lutetium (Lu). Of these, Nd and Y are preferred because they tend to stabilize the superlattice structure. Examples of alkaline earth metal elements include magnesium (Mg), calcium (Ca), strontium (Sr), barium (Ba), and radium (Ra). Of these, Mg, Ca, and Ba are preferred because they tend to stabilize the superlattice structure.

When at least one element selected from the group consisting of cerium, scandium, and rare earth elements other than praseodymium or lanthanum, and alkaline earth elements, is further contained, the content of the element is preferably 0.5 to 20 at% and more preferably 1 to 10 at% relative to all cations in the ceria-zirconia solid solution powder containing scandium and praseodymium or lanthanum. If the content of such an element is below the lower limit, it tends to be difficult to stabilize the superlattice structure, while if it exceeds the upper limit, the oxygen storage capacity tends to decrease.

Next, in the second step, the ceria-zirconia solid solution powder containing scandium and praseodymium or lanthanum is pressure-molded at a pressure of 30 to 350 MPa (preferably, a pressure of 40 to 300 MPa). If the pressure is below the lower limit, the contact between the secondary particles of the powder is not improved, and crystal growth during the reduction treatment is not sufficiently promoted, resulting in a decrease in the stability of the superlattice structure when exposed to high-temperature exhaust gases at approximately 1100°C for a long period of time. On the other hand, if the pressure exceeds the upper limit, crystal growth during the reduction treatment tends to proceed too quickly, resulting in a decrease in OSC at low temperatures such as 250°C. There are no particular limitations on the pressure-molding method, and known pressure-molding methods such as isostatic pressing can be used as appropriate.

Next, in the second step, the pressure-molded solid solution powder compact is subjected to a reduction treatment in which it is heated under reducing conditions at a temperature of 1400 to 2000°C (preferably 1600 to 1900°C) for 0.5 to 24 hours (preferably 1 to 10 hours), and then subjected to an oxidation treatment to obtain the oxygen storage material powder of the present invention. If the reduction temperature is below the lower limit, crystal growth does not proceed sufficiently, resulting in a decrease in the stability of the superlattice structure. On the other hand, if the reduction temperature exceeds the upper limit, the balance between the energy (e.g., power) required for the reduction treatment and the improvement in performance becomes poor. Furthermore, if the heating time during the reduction treatment is below the lower limit, the superlattice structure tends to be difficult to form sufficiently, while if the heating time exceeds the upper limit, the balance between the energy (e.g., power) required for the reduction treatment and the improvement in performance becomes poor.

The reduction method is not particularly limited as long as it is capable of heating the solid solution powder in a reducing atmosphere at a predetermined temperature. Examples include: (i) a method in which the solid solution powder is placed in a vacuum heating furnace, evacuated, and then reduced by flowing a reducing gas into the furnace to create a reducing atmosphere and heating at a predetermined temperature; (ii) a method in which the solid solution powder is placed in a graphite furnace, evacuated, and then heated at a predetermined temperature to create a reducing atmosphere using reducing gases such as CO and HC generated from the furnace body and heating fuel; and (iii) a method in which the solid solution powder is placed in a crucible filled with activated carbon, heated at a predetermined temperature, and then reduced by flowing a reducing gas such as CO and HC generated from the activated carbon to create a reducing atmosphere.

The reducing gas used to achieve such a reducing atmosphere is not particularly limited, and reducing gases such as CO, HC, _H2 , and other hydrocarbon gases can be used as appropriate. Among such reducing gases, it is more preferable to use one that does not contain carbon (C) from the viewpoint of preventing the generation of by-products such as zirconium carbide (ZrC) when reducing treatment is performed at higher temperatures. When such a reducing gas that does not contain carbon (C) is used, reduction treatment can be performed at higher temperatures close to the melting point of zirconium, etc., and the structural stability of the crystalline phase can be sufficiently improved.

Furthermore, in the second step, an oxidation treatment is further carried out after the reduction treatment. By carrying out such an oxidation treatment, the oxygen lost during reduction is replenished in the resulting ceria-zirconia composite oxide containing scandium and praseodymium or lanthanum, improving the stability of the oxide. The method of such an oxidation treatment is not particularly limited, and for example, a method in which the ceria-zirconia composite oxide containing scandium and praseodymium or lanthanum is heat-treated in an oxidizing atmosphere (e.g., in air) can be suitably employed. The heating temperature conditions for such an oxidation treatment are not particularly limited, but are preferably approximately 300 to 800°C. Furthermore, the heating time for the oxidation treatment is also not particularly limited, but is preferably approximately 0.5 to 5 hours.

Furthermore, in the second step, after the reduction treatment and/or the oxidation treatment, it is preferable to further subject the ceria-zirconia composite oxide containing scandium and praseodymium or lanthanum to a pulverization treatment. The method for such pulverization is not particularly limited, and suitable methods include, for example, wet pulverization, dry pulverization, and freeze pulverization.

The present invention will be explained in more detail below based on examples and comparative examples, but the present invention is not limited to the following examples.

Example 1
First, 218.77 g of an aqueous cerium nitrate solution having a concentration of 28% by mass, calculated as _CeO2 , 281.54 g of an aqueous zirconium nitrate solution having a concentration of 18% by mass, calculated as _ZrO2 , an aqueous solution in which 6.1 g of scandium nitrate tetrahydrate was dissolved in 100 ml of pure water, and an aqueous solution in which 8.7 g of praseodymium nitrate hexahydrate was dissolved in 100 ml of pure water were mixed together. The resulting mixed solution was added to a solution in which 162.2 g of 25% aqueous ammonia was diluted with 450 ml of pure water, and the mixture was stirred at 1,000 rpm for 10 minutes using a homogenizer (manufactured by AS ONE Corporation) to produce a coprecipitate. The resulting coprecipitate was dried in a degreasing furnace in air at 150°C for 7 hours and then calcined in air at 400°C for 5 hours to obtain a ceria-zirconia solid solution containing scandium and praseodymium (Sc—Pr-containing ceria-zirconia solid solution). The solid solution was then pulverized using a pulverizer ("Wonder Blender" manufactured by AS ONE Corporation) to a particle size of 75 μm or less by sieving, yielding a Sc—Pr-containing ceria-zirconia solid solution powder having an atomic ratio ([Ce]:[Zr]:[Pr]:[Sc]) of 43.5:51.5:2.5:2.5. In this Sc—Pr-containing ceria-zirconia solid solution powder, the atomic ratio of Ce to Zr (Ce/Zr) was 45.8/54.2.

Next, 20 g of this Sc—Pr-containing ceria-zirconia solid solution powder was packed into a polyethylene bag (volume 0.05 L), the inside was degassed, and the opening of the bag was heated and sealed. Subsequently, using an isostatic press (CK4-22-60 manufactured by Nikkiso Co., Ltd.), the bag was subjected to cold isostatic pressing (CIP) at a pressure (molding pressure) of 3000 kgf/cm ² (294 MPa) for 1 minute to obtain a molded body of the Sc—Pr-containing ceria-zirconia solid solution powder. The molded body had a length of 20 mm, a width of 20 mm, and an average thickness of 3 mm.

The resulting molded body was then placed in a small vacuum pressure sintering furnace (FVPS-R-150, manufactured by Fuji Radio Industrial Co., Ltd.) and the atmosphere was replaced with argon. After heating to 1000°C over a 1-hour period, it was then heated to 1500°C (reduction treatment temperature) over a 4-hour period and held at that temperature for 5 hours. It was then cooled to 1000°C over a 4-hour period and then allowed to cool naturally to room temperature, yielding a reduced product.

The resulting reduction-treated product was heated in air at 500°C for 5 hours to obtain a ceria-zirconia composite oxide containing scandium and praseodymium (Sc-Pr-containing ceria-zirconia composite oxide). This Sc-Pr-containing ceria-zirconia composite oxide was then pulverized using the pulverizer to a particle size of 75 μm or less, yielding a Sc-Pr-containing ceria-zirconia composite oxide powder with an atomic ratio ([Ce]:[Zr]:[Pr]:[Sc]) of cerium, zirconium, praseodymium, and scandium of 43.5:51.5:2.5:2.5. In this Sc-Pr-containing ceria-zirconia composite oxide powder, the atomic ratio of Ce to Zr (Ce/Zr) was 45.8/54.2.

Example 2
An Sc—Pr-containing ceria-zirconia composite oxide powder having an atomic ratio ([Ce]:[Zr]:[Pr]:[Sc]) of cerium to zirconium to praseodymium to scandium of 44.5:51.5:1.5:2.5 was obtained in the same manner as in Example 1, except that the amount of the cerium nitrate aqueous solution was changed to 218.69 g and the amount of the praseodymium nitrate hexahydrate was changed to 5.2 g. In this Sc—Pr-containing ceria-zirconia composite oxide powder, the atomic ratio of Ce to Zr (Ce/Zr) was 46.4/53.6.

Example 3
An Sc—Pr-containing ceria-zirconia composite oxide powder having an atomic ratio ([Ce]:[Zr]:[Pr]:[Sc]) of cerium to zirconium to praseodymium to scandium of 48.5:47.5:1.5:2.5 was obtained in the same manner as in Example 1, except that the amount of the cerium nitrate aqueous solution was changed to 238.35 g, the amount of the zirconium nitrate aqueous solution was changed to 259.67 g, and the amount of the praseodymium nitrate hexahydrate was changed to 5.22 g. In this Sc—Pr-containing ceria-zirconia composite oxide powder, the atomic ratio of Ce to Zr (Ce/Zr) was 50.5/49.5.

Example 4
An Sc—Pr-containing ceria-zirconia composite oxide powder having an atomic ratio ([Ce]:[Zr]:[Pr]:[Sc]) of cerium, zirconium, praseodymium, and scandium was obtained in the same manner as in Example 1, except that the amount of the zirconium nitrate aqueous solution was changed to 289.74 g, the amount of the scandium nitrate tetrahydrate was changed to 2.4 g, and the reduction treatment temperature was changed to 1700° C. In this Sc—Pr-containing ceria-zirconia composite oxide powder, the atomic ratio of Ce to Zr (Ce/Zr) was 45.1/54.9.

Example 5
An Sc—Pr-containing ceria-zirconia composite oxide powder having an atomic ratio ([Ce]:[Zr]:[Pr]:[Sc]) of cerium to zirconium to praseodymium to scandium of 44.5:45.0:1.5:1.0 was obtained in the same manner as in Example 1, except that the amount of the cerium nitrate aqueous solution was changed to 218.69 g, the amount of the zirconium nitrate aqueous solution was changed to 289.47 g, the amount of the scandium nitrate tetrahydrate was changed to 2.4 g, the amount of the praseodymium nitrate hexahydrate was changed to 5.2 g, and the reduction treatment temperature was changed to 1700° C. In this Sc—Pr-containing ceria-zirconia composite oxide powder, the atomic ratio of Ce to Zr (Ce/Zr) was 45.6/54.4.

Example 6
A ceria-zirconia composite oxide powder containing scandium and lanthanum (Sc—La-containing ceria-zirconia composite oxide powder) was obtained in the same manner as in Example 1, except that 8.7 g of lanthanum nitrate hexahydrate was used instead of the praseodymium nitrate hexahydrate and the heating temperature of the reduction-treated product was changed to 900° C. The atomic ratio of cerium to zirconium to lanthanum to scandium ([Ce]:[Zr]:[La]:[Sc]) was 43.5:51.5:2.5:2.5. In this Sc—La-containing ceria-zirconia composite oxide powder, the atomic ratio of Ce to Zr (Ce/Zr) was 45.8/54.2.

Example 7
A ceria-zirconia composite oxide powder containing scandium and lanthanum (Sc—La-containing ceria-zirconia composite oxide powder) having an atomic ratio ([Ce]:[Zr]:[La]:[Sc]) of 47.5:47.5:2.5:2.5 was obtained in the same manner as in Example 1, except that 8.7 g of lanthanum nitrate hexahydrate was used instead of the praseodymium nitrate hexahydrate, the amount of the cerium nitrate aqueous solution was changed to 233.43 g, and the amount of the zirconium nitrate aqueous solution was changed to 259.67 g. In this Sc—La-containing ceria-zirconia composite oxide powder, the atomic ratio of Ce to Zr (Ce/Zr) was 50.0/50.0.

(Comparative Example 1)
An Sc—Pr-containing ceria-zirconia composite oxide powder having an atomic ratio ([Ce]:[Zr]:[Pr]:[Sc]) of cerium to zirconium to praseodymium to scandium of 41.0:49.0:5.0:5.0 was obtained in the same manner as in Example 1, except that the amount of the cerium nitrate aqueous solution was changed to 201.49 g, the amount of the zirconium nitrate aqueous solution was changed to 267.87 g, the amount of the scandium nitrate tetrahydrate was changed to 12.1 g, the amount of the praseodymium nitrate hexahydrate was changed to 17.4 g, and the reduction treatment temperature was changed to 1700° C. In this Sc—Pr-containing ceria-zirconia composite oxide powder, the atomic ratio of Ce to Zr (Ce/Zr) was 45.6/54.4.

(Comparative Example 2)
An Sc—Pr-containing ceria-zirconia composite oxide powder having an atomic ratio ([Ce]:[Zr]:[Pr]:[Sc]) of cerium to zirconium to praseodymium to scandium of 36.0:44.0:10.0:10.0 was obtained in the same manner as in Example 1, except that the amount of the cerium nitrate aqueous solution was changed to 176.92 g, the amount of the zirconium nitrate aqueous solution was changed to 240.54 g, the amount of the scandium nitrate tetrahydrate was changed to 24.3 g, the amount of the praseodymium nitrate hexahydrate was changed to 34.8 g, and the reduction treatment temperature was changed to 1700° C. In this Sc—Pr-containing ceria-zirconia composite oxide powder, the atomic ratio of Ce to Zr (Ce/Zr) was 45.0/55.0.

(Comparative Example 3)
First, a ceria-zirconia composite oxide powder having a cerium to zirconium content ratio (atomic ratio [Ce]:[Zr]) of 46.0:54.0 was obtained in the same manner as in Example 1, except that the amount of the cerium nitrate aqueous solution was changed to 226.06 g, the amount of the zirconium nitrate aqueous solution was changed to 295.20 g, the scandium nitrate tetrahydrate and the praseodymium nitrate hexahydrate were not used, and the reduction treatment temperature was changed to 1700°C.

Next, this ceria-zirconia composite oxide powder was immersed in an aqueous solution containing scandium nitrate and praseodymium nitrate in a 1:1 mass ratio to impregnate the ceria-zirconia composite oxide powder with scandium nitrate and praseodymium nitrate so that the scandium content was 5.0 at% and the praseodymium content was 5.0 at%. The solid components were then evaporated to dryness, and the resulting powder was heated in air at 900°C for 5 hours to obtain a Sc-Pr-containing ceria-zirconia composite oxide powder having an atomic ratio ([Ce]:[Zr]:[Pr]:[Sc]) of cerium, zirconium, praseodymium, and scandium of 41.4:48.6:5.0:5.0. In this Sc-Pr-containing ceria-zirconia composite oxide powder, the atomic ratio of Ce to Zr (Ce/Zr) was 46.0/54.0.

(Comparative Example 4)
An Sc—Pr-containing ceria-zirconia composite oxide powder having an atomic ratio ([Ce]:[Zr]:[Pr]:[Sc]) of cerium to zirconium to praseodymium to scandium of 36.8:43.2:10.0:10.0 was obtained in the same manner as in Comparative Example 3, except that the ceria-zirconia composite oxide powder was impregnated with scandium nitrate and praseodymium nitrate so that the scandium content was 10.0 at% and the praseodymium content was 10.0 at%. In this Sc—Pr-containing ceria-zirconia composite oxide powder, the atomic ratio of Ce to Zr (Ce/Zr) was 46.0/54.0.

(Comparative Example 5)
A ceria-zirconia composite oxide powder having a cerium to zirconium content ratio of 50.0:50.0 in atomic ratio ([Ce]:[Zr]) was obtained in the same manner as in Example 1, except that the amount of the cerium nitrate aqueous solution was changed to 245.72 g, the amount of the zirconium nitrate aqueous solution was changed to 273.34 g, the scandium nitrate tetrahydrate and the praseodymium nitrate hexahydrate were not used, and the reduction treatment temperature was changed to 1700°C.

(Comparative Example 6)
A praseodymium-containing ceria-zirconia composite oxide powder (Pr-containing ceria-zirconia composite oxide powder) having an atomic ratio ([Ce]:[Zr]:[Pr]) of cerium to zirconium to praseodymium of 43.5:54.0:2.5 was obtained in the same manner as in Example 1, except that the amount of the zirconium nitrate aqueous solution was changed to 295.20 g and scandium nitrate tetrahydrate was not used. In this Pr-containing ceria-zirconia composite oxide powder, the atomic ratio of Ce to Zr (Ce/Zr) was 44.6/55.4.

(Comparative Example 7)
A scandium-containing ceria-zirconia composite oxide powder (Sc-containing ceria-zirconia composite oxide powder) having a cerium:zirconium:sc content ratio ([Ce]:[Zr]:[Sc]) of 46.0:51.5:2.5 was obtained in the same manner as in Example 1, except that the amount of the cerium nitrate aqueous solution was changed to 226.06 g, the amount of the zirconium nitrate aqueous solution was changed to 281.54 g, and the praseodymium nitrate hexahydrate was not used. In this Sc-containing ceria-zirconia composite oxide powder, the atomic ratio of Ce to Zr (Ce/Zr) was 47.2/52.8.

(Comparative Example 8)
A Sc-containing ceria-zirconia composite oxide powder having an atomic ratio ([Ce]:[Zr]:[Sc]) of cerium to zirconium to scandium of 46.0:49.0:5.0 was obtained in the same manner as in Example 1, except that the amount of the cerium nitrate aqueous solution was changed to 226.06 g, the amount of the zirconium nitrate aqueous solution was changed to 267.87 g, the amount of the scandium nitrate tetrahydrate was changed to 12.1 g, the praseodymium nitrate hexahydrate was not used, and the reduction treatment temperature was changed to 1700° C. In this Sc-containing ceria-zirconia composite oxide powder, the atomic ratio (Ce:[Zr]:[Sc]) of Ce to Zr was 48.4:51.6.

<Heat resistance test 1>
The composite oxide powders obtained in the examples and comparative examples were heated in air at 1100° C. for 5 hours.

<Heat resistance test 2>
1.5 g of the composite oxide powders obtained in the examples and comparative examples were heated at 1050°C for 5 hours while passing a rich gas (H ₂ (2%) + CO ₂ (10%) + H ₂ O (3%) + N 2 (balance)) and a lean gas (O ₂ (2%) + CO ₂ (10%) + H ₂ O (3%) + N 2 (balance)) alternately every 5 minutes at a gas flow rate of 500 ml/min.

<Energy dispersive X-ray analysis>
The composite oxide powders obtained in the examples and comparative examples before the heat resistance test were thinned to about 500 nm by FIB (Focused Ion Beam) processing to prepare observation samples. For this observation sample, a spherical aberration corrected scanning transmission electron microscope equipped with an energy dispersive X-ray analyzer ("JEM-ARM200F NEOARM" manufactured by JEOL Ltd.) was used, and energy dispersive X-ray (EDX) analysis was performed under the conditions of an acceleration voltage of 200 kV and a field of view magnification of 4.8 × 10 ⁷ times. An EDX element mapping image of 512 vertical pixels × 512 horizontal pixels was obtained, and a distribution map of the elements Ce, Zr, Pr or La, Sc was obtained. Figures 1 and 2 show the distribution maps of the elements Ce, Zr, Pr, and Sc of the composite oxide powders obtained in Example 5 and Comparative Example 3 before the heat resistance test, respectively.

Zero-Mean Normalized Correlation (ZNCC) Coefficient
Based on the distribution map of each element of Ce, Zr, Pr or La, Sc obtained by the EDX analysis, the following formula (3):

(In the above formula, ZNCC represents the coefficient value of zero-mean normalized cross-correlation (ZNCC), I(i, j) represents the luminance value of the ith pixel in the horizontal direction and the jth pixel in the vertical direction in the comparison image (the Sc, Pr, or La distribution map (M=512, N=512)), I _ave represents the average luminance value in the comparison image, T(i, j) represents the luminance value of the ith pixel in the horizontal direction and the jth pixel in the vertical direction in the reference image (the Ce or Zr distribution map (M=512, N=512)), and T _ave represents the average luminance value in the reference image.)
The ZNCC coefficient value [Sc-Ce] between the Sc distribution map and the Ce distribution map, the ZNCC coefficient value [Sc-Zr] between the Sc distribution map and the Zr distribution map, the ZNCC coefficient value [ _MPL -Ce] between the Pr or La distribution map and the Ce distribution map, and the ZNCC coefficient value [ _MPL -Zr] between the Pr or La distribution map and the Zr distribution map were calculated using the image processing software ImageJ. The results are shown in Table 1.

Furthermore, based on the results shown in Table 1, the difference between the ZNCC coefficient value [Sc-Zr] and the ZNCC coefficient value [Sc-Ce] ([Sc-Zr]-[Sc-Ce]) and the difference between the ZNCC coefficient value [M _PL -Ce] and the ZNCC coefficient value [M _PL -Zr] ([M _PL -Ce]-[M _PL -Zr]) were calculated. The results are shown in Table 2.

<X-ray diffraction (XRD) measurement>
The X-ray diffraction patterns of each composite oxide powder obtained in the Examples and Comparative Examples before the heat resistance test and after heat resistance test 1 were measured using an X-ray diffractometer (RINT-Ultima manufactured by Rigaku Corporation) under conditions of a tube voltage of 40 kV, a tube current of 40 mA, and a scanning speed of 2θ = 10°/min, using CuKα rays as an X-ray source.

In the obtained X-ray diffraction pattern, the ratio of the peak intensity I(14) of the diffraction line near 2θ=14.5° to the peak intensity I(29) of the diffraction line near 2θ=29° [I(14/29)=I(14)/I(29)] was calculated.
Structural retention rate = I (14/29) after heat resistance test 1 / I (14/29) before heat resistance test × 100
The structural maintenance rate was calculated by the following method. The results are shown in Table 2.

<Catalyst preparation>
Each of the composite oxide powders obtained in the Examples and Comparative Examples before the heat resistance test and after heat resistance test 1 (Comparative Examples 1, 2, and 4) or after heat resistance test 2 (Examples 1 to 7 and Comparative Examples 3, 5 to 8) was impregnated with a dinitrodiammineplatinic (II) acid solution, evaporated to dryness, and then calcined at 300°C for 3 hours to prepare a catalyst powder in which platinum (Pt) was supported on the composite oxide powder (Pt supported amount: 1% by mass).

<Oxygen release measurement>
15 mg of each catalyst powder prepared as described above was placed in a thermogravimetric analyzer ("TGA-50" manufactured by Shimadzu Corporation), and a reducing gas (H ₂ (5%) + N ₂ (balance)) and an oxidizing gas (O ₂ (5%) + N ₂ (balance)) were passed through the catalyst powder alternately every 5 minutes at a gas flow rate of 100 ml/min at a temperature of 250°C, and the increase or decrease in mass of the catalyst powder during this period was measured. The average mass loss of the catalyst powder during the second and third passes of the reducing gas was calculated and used as the oxygen storage capacity (OSC). The results are shown in Table 2.

The I(14/29) values shown in Table 2 confirm that the composite oxide powders obtained in the examples and comparative examples are pyrochlore-type ceria-zirconia composite oxide powders.

Based on the results shown in Table 2, the difference in ZNCC coefficient values ([Sc-Zr]-[Sc-Ce]) is plotted against the Sc content in FIG. 3 , and the difference in ZNCC coefficient values ([M _PL -Ce]-[M _PL -Zr]) is plotted against the Pr or La content in FIG. 4 . The straight line in FIG. 3 is based on the ZNCC coefficient value of the Sc-Pr-containing ceria-zirconia composite oxide powder (Comparative Example 3) prepared under conditions in which Sc is hardly dissolved in the crystal lattice, and indicates the boundary between whether Sc is substituted at the Zr site or not. The straight line in FIG. 4 is based on the ZNCC coefficient value of the Sc-Pr-containing ceria-zirconia composite oxide powder prepared under conditions in which Pr is hardly dissolved in the crystal lattice in Comparative Example 3, and indicates the boundary between whether Pr or La is substituted at the Ce site or not.

Furthermore, based on the results shown in Table 2, the oxygen storage capacity (OSC) of catalyst powders in which platinum (Pt) was supported on composite oxide powders after heat resistance testing was plotted against the Sc content and Pr or La content, and the results are shown in Figures 5 and 6.

3 and 4, the composite oxide powders obtained in Examples 1 to 7 and Comparative Examples 1, 2, and 4 have ZNCC coefficient value differences satisfying [Sc-Zr]-[Sc-Ce]>0.000 and [M _PL -Ce]-[M _PL -Zr]>0.028, which confirms that Sc is solid-dissolved in the ceria-zirconia crystal lattice and substituted at the Zr site, and Pr or La is solid-dissolved in the ceria-zirconia crystal lattice and substituted at the Ce site. Furthermore, the composite oxide powder obtained in Comparative Example 6 has ZNCC coefficient value differences satisfying [M _PL -Ce]-[M _PL -Zr]>0.028, which confirms that Pr is solid-dissolved in the ceria-zirconia crystal lattice and substituted at the Ce site. Furthermore, the composite oxide powders obtained in Comparative Examples 7 and 8 had a difference in ZNCC coefficient value that satisfied [Sc-Zr]-[Sc-Ce]>0.000, confirming that Sc was dissolved in the ceria-zirconia crystal lattice and substituted for the Zr site.

5 and 6 , it was found that the composite oxide powders obtained in Examples 1 to 7 had superior OSC compared to the composite oxide powders obtained in Comparative Examples 1 to 8. This is thought to be because, in the composite oxides obtained in Examples 1 to 6, Sc ³⁺ was replaced with Zr ⁴⁺ , and oxygen vacancies generated by charge compensation improved the mobility of the oxygen lattice, and further, Pr ³⁺ (1.16 Å) or La ³⁺ (1.13 Å), which has an ionic radius larger than that of Ce ⁴⁺ (0.97 Å), was replaced with Ce ⁴⁺ , which not only introduced oxygen vacancies generated by the charge compensation described above, but also partially formed Pr ₂ Zr ₂ O ₇ or La ₂ Zr ₂ O ₇ , which had high thermal stability, improving the heat resistance of the composite oxide powder and the low-temperature OSC after the heat resistance test.

On the other hand, in the composite oxide powders obtained in Comparative Examples 1 and 2, Sc is substituted for the Zr site and Pr or La is substituted for the Ce site, as in the composite oxides obtained in Examples 1 to 7. However, since the Sc content and Pr or La content were 5 at % or more, the structure became unstable and lattice changes during the oxidation-reduction of Ce were suppressed, which is thought to have resulted in a lower OSC compared to the composite oxides obtained in Examples 1 to 7. The reason for this is not necessarily clear, but the inventors speculate as follows: In other words, in the ceria-zirconia crystal lattice, the Zr site (Zr ⁴⁺ ) does not change in valence during the oxidation-reduction reaction and contributes greatly to the structural stability of the composite oxide powder during the oxidation-reduction reaction; however, in composite oxide powders with a Sc content and a Pr or La content of 5 at % or more, Zr ⁴⁺ is substituted with excess Sc ³⁺ , which generates excess oxygen vacancies near the Zr site due to charge compensation, resulting in an unstable structure. Furthermore, the substitution of Ce ⁴⁺ with excess Pr ³⁺ or La ³⁺ generates excess Pr ₂ Zr ₂ O ₇ or La ₂ Zr ₂ O ₇ as a structural anchor, suppressing lattice changes during the oxidation-reduction of Ce. These reasons are presumed to be the reason for the decrease in low-temperature OSC after the heat resistance test.

Furthermore, in the composite oxide powder obtained in Comparative Example 3, Sc and Pr were hardly dissolved in the ceria-zirconia crystal lattice, and therefore the substitution effect of Sc and Pr was not fully achieved, which is thought to have resulted in a lower OSC compared to the composite oxides obtained in Examples 1 to 7.

Furthermore, in the composite oxide powder obtained in Comparative Example 4, as in the composite oxides obtained in Examples 1 to 7, Sc is substituted for the Zr site, and Pr or La is substituted for the Ce site. However, the content of Pr or La, which has a larger ionic radius than Ce ⁴⁺ , is high at 10 at%, and inhibits the reduction of Ce ⁴⁺ to Ce ³⁺ , which is accompanied by an increase in the ionic radius. This is thought to have resulted in a decrease in OSC compared to the composite oxides obtained in Examples 1 to 7.

Furthermore, the composite oxide powders obtained in Comparative Examples 5 to 8 did not contain Sc and at least one of Pr and La, and therefore the substitution effects of Sc and Pr or La could not be obtained simultaneously. This is thought to have resulted in a lower OSC compared to the composite oxides obtained in Examples 1 to 7.

As explained above, the present invention makes it possible to obtain an oxygen storage material that exhibits excellent oxygen storage capacity (OSC) at low temperatures of approximately 250°C, not only at the initial stage of use but also after prolonged exposure to high-temperature exhaust gases of approximately 1100°C. Therefore, the oxygen storage material of the present invention possesses both excellent oxygen storage capacity (OSC) at low temperatures and high-temperature durability, making it useful as a support or co-catalyst for exhaust gas purification catalysts, a catalyst atmosphere adjuster, etc.

Claims (5)

The composite oxide contains a pyrochlore-type ceria (CeO ₂ )-zirconia (ZrO ₂ )-based composite oxide containing scandium (Sc) and praseodymium (Pr) or lanthanum (La),
With respect to all cations in the pyrochlore-type ceria-zirconia-based composite oxide,
The Ce content is 43 to 49 at%;
The content of Pr or La is 0.5 to 4.9 at%;
The Sc content is 0.5 to 4.9 at%;
the atomic ratio of Ce to Zr (Ce/Zr) is 45/55 to 51/49;
An oxygen storage material characterized by: The oxygen storage material described in claim 1, characterized in that at least a portion of Sc and Pr or La is solid-dissolved in the pyrochlore-type ceria-zirconia composite oxide. The oxygen storage material described in claim 1, characterized in that at least a portion of Sc substitutes for Zr sites in the pyrochlore-type ceria-zirconia composite oxide, and at least a portion of Pr or La substitutes for Ce sites in the pyrochlore-type ceria-zirconia composite oxide. The zero-mean normalized correlation coefficients (ZNCC) [Sc-Zr], [Sc-Ce], [M PL-Ce], and [M _{PL -} Zr] calculated based on the Sc distribution map, Pr or La distribution map, and Ce distribution map in the element mapping image obtained by energy dispersive X-ray analysis are expressed by the following formulas (1) and (2):
[Sc-Zr]-[Sc-Ce]>0.000 (1)
[M _PL -Ce] - [M _PL -Zr]>0.028 (2)
(In the above formula, [Sc-Zr] represents the ZNCC coefficient value between the Sc distribution map and the Zr distribution map, [Sc-Ce] represents the ZNCC coefficient value between the Sc distribution map and the Ce distribution map, [M _PL -Ce] represents the ZNCC coefficient value between the Pr or La distribution map and the Ce distribution map, and [M _PL -Zr] represents the ZNCC coefficient value between the Pr or La distribution map and the Zr distribution map.)
2. The oxygen storage material according to claim 1, wherein the oxygen storage material satisfies the condition expressed by the following formula: A method for producing an oxygen storage material containing a pyrochlore-type ceria (CeO ₂ )-zirconia (ZrO ₂ ) composite oxide containing scandium (Sc) and praseodymium (Pr) or lanthanum (La), comprising:
a step of preparing a ceria-zirconia-based solid solution powder containing scandium and praseodymium or lanthanum, wherein the Ce content is 43 to 49 at%, the Pr or La content is 0.5 to 4.9 at%, the Sc content is 0.5 to 4.9 at%, and the atomic ratio of Ce to Zr (Ce/Zr) is 45/55 to 51/49, relative to all cations;
a step of compacting the ceria-zirconia solid solution powder containing scandium and praseodymium or lanthanum at a pressure of 30 to 350 MPa, reducing the powder at a temperature of 1400 to 2000°C, and then oxidizing the powder to obtain an oxygen storage material containing the pyrochlore-type ceria-zirconia composite oxide containing scandium and praseodymium or lanthanum according to claim 1;
A method for producing an oxygen storage material, comprising: