JP7237048B2 - Oxygen storage material and its manufacturing method - 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.

A so-called three-way catalyst is known as an exhaust gas purifying catalyst capable of oxidizing carbon monoxide (CO) and hydrocarbons (HC) and reducing nitrogen oxides (NOx) in exhaust gas discharged from internal combustion engines such as automobile engines. ing.

In purifying exhaust gas using an exhaust gas purification catalyst, in order to absorb fluctuations in the oxygen concentration in the exhaust gas and increase the exhaust gas purification ability, oxygen can be occluded when the oxygen concentration in the exhaust gas is high, and It is known to use a material having an oxygen storage capacity (OSC) capable of releasing oxygen when the oxygen concentration is low as a carrier or co-catalyst for an exhaust gas purifying catalyst.

As an oxygen storage material having such an OSC, ceria has conventionally been suitably used. , a hydrothermal synthesis method, a melting method, a solid-phase method, etc., have been developed.

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

Further, Japanese Patent Application Laid-Open No. 2015-080736 (Patent Document 2) describes an oxygen storage/release material obtained by adding iron to a ceria-zirconia composite oxide, wherein the ceria-zirconia composite oxide has a pyrochlore phase, κ An oxygen storage/release material having a phase or a combination thereof and containing no precious metals, wherein the iron is at least partially substituted for the cerium site and/or the zirconium site in the ceria-zirconia composite oxide. A method of production by a precipitation method and a high temperature treatment in a reducing atmosphere (coprecipitation method-high temperature reduction treatment) is disclosed.

Furthermore, Japanese Patent Application Laid-Open No. 2019-131455 (Patent Document 3) discloses an oxygen storage/release material comprising a ceria-zirconia-iron oxide-based composite oxide containing cerium, zirconium and iron, wherein the cerium and the zirconium At least part of the iron is solid-dissolved in the composite oxide of, and the ceria-zirconia-iron oxide-based composite oxide has a cationic ordered structure. disclosed.

However, in recent years, the required properties for exhaust gas purifying catalysts have been increasing more and more. Oxygen storage materials that can express storage capacity (OSC) and have high utilization efficiency are now required, and conventional oxygen storage materials such as those described in Patent Documents 1 to 3 are not necessarily sufficient. rice field.

JP 2015-182931 A JP 2015-080736 A JP 2019-131455 A

The present invention has been made in view of the above-mentioned problems of the prior art, and is excellent at a low temperature of about 300 ° C. not only at the beginning of use but also after long exposure to exhaust gas at a high temperature of about 1100 ° C. An object of the present invention is to provide an oxygen storage material capable of exhibiting oxygen storage capacity (OSC) and having high utilization efficiency, and a method for producing the same.

The inventors of the present invention have made intensive studies to achieve the above object. It is difficult to dissolve iron in the ceria-zirconia composite oxide by methods such as method, hydrothermal synthesis method, etc., and the oxygen storage material obtained by such a method has an oxygen storage capacity at a low temperature of about 300 ° C. (OSC ) was found to be insufficient. On the other hand, in the melting method including high-temperature treatment described in Patent Document 1 and the production method involving high-temperature treatment in a reducing atmosphere described in Patent Document 2, although iron dissolves in the ceria-zirconia composite oxide to some extent, it is not always possible. However, it has been found that the oxygen storage materials obtained by these methods are still unsatisfactory in terms of oxygen storage capacity (OSC) at low temperatures of about 300°C. In addition, in the solution combustion synthesis method described in Patent Document 3, an oxygen storage material in which at least part of iron is solid-dissolved in a ceria-zirconia composite oxide is obtained. It was found that the superlattice structure (cation ordered structure) disappeared when exposed to high temperature, and the high temperature durability was still insufficient.

As a result of further extensive research, the present inventors selected iron as an element to be added to the ceria-zirconia composite oxide, and adjusted the content ratio of cerium, zirconium, and iron within a predetermined range. The iron-containing ceria-zirconia solid solution powder is press-molded at a predetermined pressure, subjected to a reduction treatment under a predetermined high-temperature condition, and further subjected to an oxidation treatment before and for a long time to be exposed to exhaust gas at a high temperature of about 1100 ° C. An iron-containing ceria-zirconia-based composite oxide in which iron is sufficiently dissolved in the ceria-zirconia-based composite oxide is obtained in any of the after-treatments, and the iron-containing ceria-zirconia-based composite oxide is used at the initial stage of use. In addition, even after being exposed to exhaust gas at a high temperature of about 1100 ° C. for a long time, it can exhibit excellent oxygen storage capacity (OSC) at a low temperature of about 300 ° C., and the utilization efficiency as an oxygen storage material is high. We have found that it is high, and have completed the present invention.

That is, the oxygen storage material of the present invention is an oxygen storage material containing a pyrochlore-type ceria-zirconia-based composite oxide and iron added to the ceria-zirconia-based composite oxide,
The content ratio of iron to the total amount of cerium (Ce) and zirconium (Zr) (Fe/(Ce + Zr) × 100) is 0.5 to 9 at%,
The molar fraction of zirconium with respect to the total number of moles of cerium (Ce) and zirconium (Zr) (X = Zr / (Ce + Zr) × 100) is X = 40 to 50%,
The lattice constant obtained from the X-ray diffraction patterns obtained by X-ray diffraction measurement using CuKα before heating at 1100° C. in the atmosphere and after heating for 5 hours is given by the following formula (1):
Lattice constant≦−7.00×10 ⁻³ X+10.874 (1)
(In the above formula, X represents the mole fraction of the zirconium)
It satisfies the conditions represented by
Before heating at 1100 ° C. in the atmosphere and after heating for 5 hours, a diffraction line at 2θ = 14.5 ° and a diffraction line at 2θ = 29 ° obtained from X-ray diffraction patterns obtained by X-ray diffraction measurement using CuKα. The diffraction line and the intensity ratio [I (14/29) value] are represented by the following formula (2):
I (14/29) value ≤ 2.36 × 10 ^-3 X - 0.072 (2)
(In the above formula, X represents the mole fraction of the zirconium)
It is characterized in that it satisfies the conditions represented by

In the oxygen storage material of the present invention, the molar fraction X of zirconium is preferably X=45 to 50%.

Further, 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-zirconia-based composite oxide and iron added to the ceria-zirconia-based composite oxide,
The content ratio of iron to the total amount of cerium (Ce) and zirconium (Zr) (Fe/(Ce + Zr) × 100) is 0.5 to 9 at%, and the total mole of cerium (Ce) and zirconium (Zr) preparing an iron-containing ceria-zirconia solid solution powder having a zirconium mole fraction (Zr/(Ce+Zr)×100) of 40 to 50%;
The pyrochlore-type ceria according to claim 1 or 2, wherein the iron-containing ceria-zirconia solid solution powder is pressure-molded at a pressure of 30 to 350 MPa, then subjected to reduction treatment under temperature conditions of 1400 to 2000° C., and further to oxidation treatment. - obtaining an oxygen storage material containing a zirconia-based composite oxide and iron added to the ceria-zirconia-based composite oxide;
A method comprising:

In the method for producing an oxygen storage material of the present invention, it is preferable that the zirconium molar fraction (Zr/(Ce+Zr)×100) in the iron-containing ceria-zirconia solid solution powder is 45 to 50%.

In addition, in the present invention, the reason why it can be judged that the iron-containing pyrochlore-type ceria-zirconia-based composite oxide satisfying the condition represented by the above formula (1) is one in which iron is sufficiently dissolved is as follows. explain. That is, in iron-free pyrochlore-type ceria-zirconia composite oxides, the lattice constant shows a negative correlation with the mole fraction X of zirconium. This is because in the pyrochlore-type ceria-zirconia-based composite oxide, the molar fraction X of zirconium increases, that is, Ce ⁴⁺ with a large ionic radius (ionic radius: 0.97 Å (8 coordination)). It is considered that when the proportion of Zr ⁴⁺ (ionic radius: 0.84 Å (8-coordinated)) with small ions increases, the average ionic radius of all cations in the crystal lattice decreases, causing the lattice to contract.

Then, when Fe ³⁺ (ionic radius: 0.78 Å) having an ionic radius smaller than that of Zr ⁴⁺ is dissolved in a pyrochlore-type ceria-zirconia-based composite oxide containing no iron, all cations in the crystal lattice The average ionic radius of becomes smaller, and the lattice shrinks further compared to the case where iron is not contained even if the mole fraction X of zirconium is the same, so the lattice constant becomes smaller. Therefore, it can be determined that the iron-containing pyrochlore-type ceria-zirconia composite oxide that satisfies the condition represented by the above formula (1) contains a sufficient amount of iron as a solid solution. The lattice constant is obtained by fitting the X-ray diffraction pattern obtained by X-ray diffraction measurement by the least squares method using commercially available analysis software (eg, Rietveld analysis software "Jana2006"). It can be obtained by refining. In addition, as a method for the X-ray diffraction measurement, an X-ray diffractometer (for example, "RINT-Ultima" manufactured by Rigaku Co., Ltd.) is used, a CuKα ray is used as an X-ray source, a tube voltage is 40 KV, a tube current is 40 mA, and scanning is performed. A method of measuring under the condition of speed 2θ=20°/min is adopted.

In addition, in the present invention, the reason why it can be determined that the iron-containing pyrochlore-type ceria-zirconia-based composite oxide satisfying the condition represented by the above formula (2) is one in which iron is sufficiently dissolved is as follows. explain. That is, in iron-free pyrochlore-type ceria-zirconia composite oxides, the I(14/29) value shows a positive correlation with the mole fraction X of zirconium. This is because the I (14/29) value of the pyrochlore-type ceria-zirconia composite oxide is determined by the ion radius difference between the Ce site and the Zr site in the crystal lattice, and the smaller the difference, the more the I (14/29) value. becomes ^{smaller ,} the mole fraction X of ^zirconium decreases. This is probably because the ion radius difference with the Zr site becomes smaller.

When Fe ³⁺ is solid-dissolved in a pyrochlore-type ceria-zirconia-based composite oxide containing no iron, Fe ³⁺ is presumed to form a substitution solid solution at the Zr site from the relationship of ionic radii. When Fe ³⁺ forms a substitutional solid solution at the Zr site, since Fe ³⁺ has a smaller oxidation number than Zr ⁴⁺ , oxygen defects are generated in the crystal lattice due to the principle of charge compensation, and the structural packing degree near the Zr site is reduced. descend. As a result, it is thought that the ionic radius of the Zr site increases, the difference in ionic radius between the Ce site and the Zr site decreases, and the I(14/29) value decreases. Therefore, it can be determined that the iron-containing pyrochlore-type ceria-zirconia composite oxide that satisfies the condition represented by the above formula (2) contains a sufficient amount of iron as a solid solution. The I (14/29) value is the peak intensity I (14) of the diffraction line at 2θ = 14.5° and the peak of the diffraction line at 2θ = 29° in the X-ray diffraction pattern obtained by X-ray diffraction measurement. can be obtained from the intensity I (29). In addition, as a method for the X-ray diffraction measurement, an X-ray diffractometer (for example, "RINT-Ultima" manufactured by Rigaku Co., Ltd.) is used, a CuKα ray is used as an X-ray source, a tube voltage is 40 KV, a tube current is 40 mA, and scanning is performed. A method of measuring under the condition of speed 2θ=20°/min is adopted. When obtaining the intensity (peak intensity) of the diffraction line, the average diffraction line intensity at 2θ=10° to 12° is subtracted from the intensity of each diffraction line as a background value.

Furthermore, the oxygen storage material of the present invention exhibits an excellent oxygen storage capacity (OSC) at a low temperature of about 300°C not only at the initial stage of use but also after being exposed to exhaust gas at a high temperature of about 1100°C for a long time. Although the reason why it is possible to increase the utilization efficiency is not necessarily clear, the present inventors speculate as follows. That is, the superlattice structure of CeO ₂ —ZrO ₂ (Ce ₂ Zr ₂ O ₇ ) in the iron-containing ceria-zirconia-based composite oxide constituting the oxygen storage material of the present invention varies depending on the oxygen partial pressure in the gas phase. It undergoes a phase change with the κ phase and expresses oxygen storage capacity (OSC). Since the ionic radius of Fe ³⁺ is smaller than the ionic radius of Zr ⁴⁺ , it is believed that Fe ions selectively replace Zr sites in the iron-containing ceria-zirconia composite oxide according to the present invention. Such substitution with Fe ions causes oxygen defects in the iron-containing ceria-zirconia composite oxide, and these oxygen defects improve the mobility of oxygen in the iron-containing ceria-zirconia composite oxide. , the oxygen storage material of the present invention can exhibit excellent oxygen storage capacity (OSC) even at a low temperature of about 300° C., and is presumed to have high utilization efficiency. In addition, by replacing the Zr site with Fe ions, the ion radius difference with Ce ⁴⁺ is expanded, and the superlattice structure in the state containing oxygen defects generated by charge compensation is further stabilized. It is presumed that the oxygen storage material can exhibit excellent oxygen storage capacity (OSC) at a low temperature of about 300°C even after being exposed to exhaust gas at a high temperature of about 1100°C for a long time, resulting in high utilization efficiency. be.

According to the present invention, excellent oxygen storage capacity (OSC) can be exhibited at a low temperature of about 300 ° C. not only at the beginning of use, but also after long exposure to high temperature exhaust gas of about 1100 ° C., It becomes possible to obtain an oxygen storage material with high utilization efficiency.

4 is a graph showing the results of plotting the lattice constants of the composite oxide powders before and after the high-temperature endurance test against the mole fraction X of zirconium obtained in Examples 1 to 3 and Comparative Examples 1 to 3. FIG. A graph showing the results of plotting the I (14/29) values of the composite oxide powders before and after the high temperature durability test against the mole fraction X of zirconium obtained in Examples 1 to 3 and Comparative Examples 1 to 3. be. Graph showing the results of plotting the OSC material utilization rate of the catalyst powder against the zirconium mole fraction X for the composite oxide powders before the high temperature durability test, which were obtained in Examples 1 to 3 and Comparative Examples 1 to 3. is. Graph showing the results of plotting the OSC material utilization rate of the catalyst powder against the zirconium mole fraction X for the composite oxide powders after the high temperature endurance test obtained in Examples 1 to 3 and Comparative Examples 1 to 3. is.

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

First, the oxygen storage material of the present invention will be explained. The oxygen storage material of the present invention is an oxygen storage material containing a pyrochlore-type ceria-zirconia-based composite oxide and iron added to the ceria-zirconia-based composite oxide,
The content ratio of iron to the total amount of cerium (Ce) and zirconium (Zr) (Fe/(Ce + Zr) × 100) is 0.5 to 9 at%,
The molar fraction of zirconium with respect to the total number of moles of cerium (Ce) and zirconium (Zr) (X = Zr / (Ce + Zr) × 100) is X = 40 to 50%,
The lattice constant obtained from the X-ray diffraction patterns obtained by X-ray diffraction measurement using CuKα before heating at 1100° C. in the atmosphere and after heating for 5 hours is given by the following formula (1):
Lattice constant≦−7.00×10 ⁻³ X+10.874 (1)
(In the above formula, X represents the mole fraction of the zirconium)
It satisfies the conditions represented by
Before heating at 1100 ° C. in the atmosphere and after heating for 5 hours, a diffraction line at 2θ = 14.5 ° and a diffraction line at 2θ = 29 ° obtained from X-ray diffraction patterns obtained by X-ray diffraction measurement using CuKα. The diffraction line and the intensity ratio [I (14/29) value] are represented by the following formula (2):
I (14/29) value ≤ 2.36 × 10 ^-3 X - 0.072 (2)
(In the above formula, X represents the mole fraction of the zirconium)
It satisfies the conditions represented by

The oxygen storage material of the present invention comprises a ceria-zirconia composite oxide (hereinafter referred to as "pyrochlore-type ceria-zirconia composite oxide") having a superlattice structure in which Ce and Zr are regularly arranged. It is. An oxygen storage material comprising such a pyrochlore-type ceria-zirconia-based composite oxide has a higher oxygen diffusion rate in the bulk than a ceria-zirconia-based composite oxide having a fluorite structure. Are better. In addition, by recognizing the existence of a peak derived from the superlattice structure (peak appearing at 2θ = 14.0 ° to 16.0 °) in X-ray diffraction measurement using CuKα rays, ceria-zirconia-based composite oxide can be confirmed to be a pyrochlore type with a superlattice structure.

In addition, the oxygen storage material of the present invention further contains iron added to such a pyrochlore-type ceria-zirconia composite oxide. The content of such iron is required to be 0.5 to 9 at% as the content ratio of iron to the total amount of cerium (Ce) and zirconium (Zr) (Fe/(Ce+Zr)×100). and more preferably 1 to 5 at %. If the iron content is less than the lower limit, the effect of improving the oxygen storage capacity (OSC) at low temperature and the effect of improving the utilization efficiency as an oxygen storage material due to solid solution of iron cannot be sufficiently obtained. On the other hand, if the iron content exceeds the upper limit, the iron does not sufficiently dissolve, and the effect of improving the oxygen storage capacity (OSC) at low temperatures and the effect of improving the utilization efficiency as an oxygen storage material are sufficiently obtained. can't

Even if iron is added to the ceria-zirconia-based composite oxide, it is difficult to sufficiently dissolve iron in the ceria-zirconia-based composite oxide by a method such as a so-called solid-phase synthesis method or a hydrothermal synthesis method. Therefore, the addition of iron does not contribute to the improvement of the oxygen storage capacity (OSC) at low temperatures and the improvement of the utilization efficiency as an oxygen storage material, whereas in the present invention, the production method of the present invention described later can be used to improve the conventional It is possible to obtain an iron-containing ceria-zirconia composite oxide in which iron is sufficiently dissolved, which could not be obtained, improving the oxygen storage capacity (OSC) at low temperatures and improving the utilization efficiency as an oxygen storage material. can be achieved.

In the oxygen storage material of the present invention, the molar fraction of zirconium with respect to the total number of moles of cerium (Ce) and zirconium (Zr) (X=Zr/(Ce+Zr)×100) is X=40 to 50%. is required and preferably 45 to 50%. When the mole fraction of zirconium is less than the lower limit, it becomes difficult to obtain a sufficient oxygen storage capacity (OSC), while when it exceeds the upper limit, it becomes difficult to obtain a single phase.

In addition, the oxygen storage material of the present invention is obtained by X-ray diffraction measurement using CuKα before heating at 1100° C. in air (before high temperature durability test) and after heating for 5 hours (after high temperature durability test). The lattice constant obtained from the X-ray diffraction pattern is the following formula (1):
Lattice constant≦−7.00×10 ⁻³ X+10.874 (1)
(In the above formula, X represents the mole fraction of the zirconium)
satisfies the conditions represented by
A diffraction line at 2θ = 14.5° and a diffraction line at 2θ = 29° obtained from an X-ray diffraction pattern obtained by X-ray diffraction measurement using CuKα before and after the high temperature durability test and the intensity of the diffraction line The ratio [I (14/29) value] is the following formula (2):
I (14/29) value ≤ 2.36 × 10 ^-3 X - 0.072 (2)
(In the above formula, X represents the mole fraction of the zirconium)
It satisfies the conditions represented by The lattice constant before the high temperature durability test and after the high temperature durability test satisfies the condition represented by the above formula (1), and the I (14/29) value before the high temperature durability test and after the high temperature durability test is the above formula (2) ), iron is sufficiently dissolved in the pyrochlore-type ceria-zirconia-based composite oxide, and not only at the initial stage of use, but also in exhaust gas at a high temperature of about 1100 ° C. for a long time. Even after exposure, it can exhibit excellent oxygen storage capacity (OSC) at a low temperature of about 300°C, showing high utilization efficiency. On the other hand, if the lattice constant before and after the high temperature durability test does not satisfy the condition represented by the above formula (1), or if the I (14/29) value before and after the high temperature durability test does not satisfy the condition represented by the formula (2), iron is not sufficiently dissolved in the pyrochlore-type ceria-zirconia-based composite oxide. However, the oxygen storage capacity (OSC) at a low temperature of about 300° C. is not fully developed and the utilization efficiency as an oxygen storage material is also low after long-term exposure.

Here, the diffraction line at 2θ = 14.5° is a diffraction line attributed to the (111) plane of the ordered phase, and the diffraction line at 2θ = 29° is a diffraction line attributed to the (222) plane of the ordered phase and ceria -Since the diffraction lines belonging to the cubic phase (111) plane of the zirconia solid solution (CZ solid solution) overlap, the superlattice structure ( It is defined as an index showing the maintenance rate (presence rate) of regular phase). When calculating the diffraction line intensity, the average diffraction line intensity at 2θ=10° to 12° is subtracted as a background value from each diffraction line intensity.

Furthermore, the oxygen storage material of the present invention preferably has an average crystallite size of 0.1 to 10 μm, more preferably 0.2 to 5 μm. When such an average crystallite diameter is less than the lower limit, the I (14/29) value, which indicates the retention rate of the superlattice structure, tends to decrease after the high-temperature durability test. If it exceeds, it tends to be difficult to sufficiently obtain the effect of improving the oxygen storage capacity (OSC). In addition, such an average crystallite diameter is obtained from the X-ray diffraction pattern obtained by X-ray diffraction measurement using CuKα, using the half width of the diffraction line near 2θ = 29°, and using commercially available analysis software (for example, It can be calculated based on the Scherrer formula using XRD analysis software "JADE").

Although the specific surface area of the oxygen storage material of the present invention is not particularly limited, it is preferably 0.01 to 20 m ² /g, more preferably 0.05 to 10 m ² /g, and more preferably 0.1 Even more preferably ˜5 m ² /g. If the specific surface area is less than the lower limit, the oxygen storage capacity tends to be low. Such a specific surface area can be calculated as a BET specific surface area from the adsorption isotherm using the BET isotherm adsorption formula.

In addition, the oxygen storage material of the present invention may further contain at least one element other than cerium selected from the group consisting of rare earth elements and alkaline earth elements. By containing such an element, when the oxygen storage material of the present invention is used as a carrier of an exhaust gas purifying catalyst, a higher exhaust gas purifying ability tends to be exhibited. Rare earth elements other than cerium include scandium (Sc), yttrium (Y), lanthanum (La), praseodymium (Pr), neodymium (Nd), samarium (Sm), gadolinium (Gd), and terbium (Tb). , dysprosium (Dy), ytterbium (Yb), and lutetium (Lu). Y and Nd are more preferred. Alkaline earth metal elements include magnesium (Mg), calcium (Ca), strontium (Sr), barium (Ba), and radium (Ra), among which there is a tendency to stabilize the superlattice structure. From this point of view, Mg, Ca, and Ba are preferable.

Furthermore, when at least one element selected from the group consisting of rare earth elements and alkaline earth elements other than cerium is further contained, the content of said element is 1 to 20% by mass in said oxygen storage material. preferably 3 to 10% by mass. When the content of such elements is less than the lower limit, the effect of stabilizing the superlattice structure tends to decrease, while when 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-based composite oxide and iron added to the ceria-zirconia-based composite oxide. Even after being exposed to high temperature exhaust gas for a long time, it is possible to exhibit excellent oxygen storage capacity (OSC) at a low temperature of about 300°C. Therefore, the oxygen storage material of the present invention is suitably used as a carrier or co-catalyst for an exhaust gas purifying catalyst. A preferred example using the oxygen storage material of the present invention is an exhaust gas purifying catalyst comprising a carrier made of the oxygen storage material of the present invention and a noble metal supported on the carrier. Such noble metals include platinum, rhodium, palladium, osmium, iridium, gold, silver and the like. In another example, the oxygen storage material of the present invention is arranged around an exhaust gas purifying catalyst in which a noble metal is supported on other catalyst carrier fine particles.

Next, the method for producing the 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-zirconia-based composite oxide and iron added to the ceria-zirconia-based composite oxide,
The content ratio of iron to the total amount of cerium (Ce) and zirconium (Zr) (Fe/(Ce + Zr) × 100) is 0.5 to 9 at%, and the total mole of cerium (Ce) and zirconium (Zr) A step of preparing an iron-containing ceria-zirconia solid solution powder in which the molar fraction of zirconium to the number (Zr/(Ce+Zr)×100) is 40 to 50% (first step);
After pressure-molding the iron-containing ceria-zirconia solid solution powder at a pressure of 30 to 350 MPa, it is subjected to a reduction treatment at a temperature of 1400 to 2000° C., and further to an oxidation treatment to perform the pyrochlore type ceria-zirconia composite oxidation of the present invention. a step of obtaining an oxygen storage material containing iron added to the ceria-zirconia-based composite oxide (second step);
is a method that includes

First, in the first step, the content ratio of iron to the total amount of cerium (Ce) and zirconium (Zr) (Fe/(Ce + Zr) × 100) is 0.5 to 9 at% (preferably 1 to 5 at% ), and the mole fraction of zirconium with respect to the total number of moles of cerium (Ce) and zirconium (Zr) (Zr/(Ce + Zr) × 100) is 40 to 50% (preferably 45 to 50%) A containing ceria-zirconia solid solution powder is prepared.

In the iron-containing ceria-zirconia solid solution powder, when the iron content is less than the lower limit, in the obtained oxygen storage material, the effect of improving the oxygen storage capacity (OSC) at low temperature due to iron solid solution and oxygen storage On the other hand, when the above upper limit is exceeded, iron does not sufficiently dissolve in the resulting oxygen storage material, and the oxygen storage capacity (OSC) at low temperatures is reduced. It becomes difficult to sufficiently obtain the improvement effect and the effect of improving the utilization efficiency of the oxygen storage material. Further, when the zirconium mole fraction is less than the lower limit, it becomes difficult to obtain a sufficient oxygen storage capacity (OSC) in the obtained oxygen storage material. It becomes difficult to obtain as

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

The method of preparing (preparing) such an iron-containing ceria-zirconia solid solution powder is not particularly limited. Examples thereof include a method for producing the solid solution powder. As such a coprecipitation method, for example, an aqueous solution containing a cerium salt (e.g., nitrate), a zirconium salt (e.g., nitrate), and an iron salt (e.g., nitrate) is used and co-precipitated in the presence of ammonia. A method of obtaining the iron-containing ceria-zirconia solid solution powder by forming a precipitate, centrifuging the obtained co-precipitate, washing it, drying it, firing it, and pulverizing it with a pulverizer such as a ball mill. is mentioned. The aqueous solution containing the cerium salt, zirconium salt and iron salt is prepared so that the content ratio of cerium, zirconium and iron in the obtained solid solution powder is within a predetermined range. In such an aqueous solution, if necessary, a salt of at least one element selected from the group consisting of rare earth elements and alkaline earth elements, a surfactant (for example, a nonionic surfactant), etc. may be added.

In addition, such an iron-containing ceria-zirconia solid solution powder may further contain at least one element other than cerium selected from the group consisting of rare earth elements and alkaline earth elements. By containing such an element, when the oxygen storage material of the present invention is used as a co-catalyst of an exhaust gas purification catalyst, there is a tendency that higher exhaust gas purification performance is exhibited. Rare earth elements other than cerium include scandium (Sc), yttrium (Y), lanthanum (La), praseodymium (Pr), neodymium (Nd), samarium (Sm), gadolinium (Gd), and terbium (Tb). , dysprosium (Dy), ytterbium (Yb), and lutetium (Lu). Y and Nd are more preferred. Alkaline earth metal elements include magnesium (Mg), calcium (Ca), strontium (Sr), barium (Ba), and radium (Ra), among which there is a tendency to stabilize the superlattice structure. From this point of view, Mg, Ca, and Ba are preferable.

Furthermore, when it further contains at least one element selected from the group consisting of rare earth elements and alkaline earth elements other than cerium, the content of the element is 1 to 1 in the iron-containing ceria-zirconia solid solution powder. It is preferably 20% by mass, more preferably 3 to 10% by mass. When the content of such elements is less than the lower limit, it tends to be difficult to stabilize the superlattice structure, while when the content exceeds the upper limit, the oxygen storage capacity tends to decrease.

Next, in the second step, the iron-containing ceria-zirconia solid solution powder is pressure-molded at a pressure of 30 to 350 MPa (preferably a pressure of 40 to 300 MPa). If the compression molding pressure is less than the lower limit, the contact between the secondary particles of the powder is not improved, so the crystal growth during the reduction treatment is not sufficiently promoted, and the powder is exposed to exhaust gas at a high temperature of about 1100 ° C. for a long time. the stability of the superlattice structure when On the other hand, if the compression molding pressure exceeds the upper limit, the crystal growth during the reduction treatment proceeds too much, and the oxygen storage capacity (OSC) at a low temperature of about 300° C. tends to decrease. The method for such pressure molding is not particularly limited, and a known pressure molding method such as hydrostatic pressing can be employed as appropriate.

Next, in the second step, the pressure-molded solid-solution powder compact is heated at a temperature of 1400 to 2000° C. (preferably 1600 to 1900° C.) for 0.5 to 24 hours under reducing conditions. It is subjected to reduction treatment by heating (preferably for 1 to 10 hours) and further to oxidation treatment to obtain the oxygen storage material powder of the present invention. If the temperature of the reduction treatment is less than the lower limit, crystal growth does not proceed sufficiently, and the stability of the superlattice structure deteriorates. On the other hand, if the temperature of the reduction treatment exceeds the upper limit, the balance between the energy (for example, electric power) required for the reduction treatment and the improvement of performance becomes poor. In addition, if the heating time during the reduction treatment is less than the lower limit, it tends to be difficult to form a sufficient superlattice structure. becomes unbalanced with

The method of the reduction treatment is not particularly limited as long as it is a method capable of heat-treating the solid solution powder under a predetermined temperature condition in a reducing atmosphere. After placing the powder and evacuating, a reducing gas is flowed into the furnace to make the atmosphere in the furnace a reducing atmosphere and heating is performed under a predetermined temperature condition to perform a reduction treatment; The solid solution powder is placed in the furnace using a furnace, and after evacuating, the atmosphere in the furnace is changed by reducing gas such as CO and HC generated from the furnace body and heated fuel by heating under predetermined temperature conditions. and (iii) placing the solid solution powder in a crucible filled with activated carbon and heating it under predetermined temperature conditions to reduce it by reducing gases such as CO and HC generated from the activated carbon. A method of applying a reduction treatment by making the atmosphere in the crucible a reducing atmosphere can be mentioned.

The reducing gas used to achieve such a reducing atmosphere is not particularly limited, and reducing gases such as CO, HC, H ₂ and other hydrocarbon gases can be used as appropriate. In addition, among such reducing gases, from the viewpoint of preventing the production of duplicate products such as zirconium carbide (ZrC) when reducing treatment is performed at a higher temperature, carbon (C) is used. It is more preferable to use one without. When such a reducing gas that does not contain carbon (C) is used, reduction treatment can be performed at a higher temperature condition close to the melting point of zirconium or the like, so the structural stability of the crystal phase is sufficiently improved. It is possible to

Furthermore, in the second step, an oxidation treatment is further performed after the reduction treatment. By performing such an oxidation treatment, oxygen lost during reduction is compensated for in the obtained iron-containing ceria-zirconia composite oxide, and the stability as an oxide is improved. The method of such oxidation treatment is not particularly limited, and for example, a method of heat-treating the iron-containing ceria-zirconia-based composite oxide in an oxidizing atmosphere (for example, in the air) can be suitably employed. The heating temperature conditions for such oxidation treatment are not particularly limited, but are preferably about 300 to 800.degree. Furthermore, the heating time during the oxidation treatment is not particularly limited, but it is preferably about 0.5 to 5 hours.

Further, in the second step, it is preferable to further subject the iron-containing ceria-zirconia composite oxide to a pulverization treatment after the reduction treatment and/or the oxidation treatment. The method of such pulverization treatment is not particularly limited, and for example, a wet pulverization method, a dry pulverization method, a freeze pulverization method, or the like can be suitably employed.

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

(Example 1)
An iron-containing ceria-zirconia composite oxide powder containing cerium, zirconium and iron at an atomic ratio ([Ce]:[Zr]:[Fe]) of 50:49:1 was prepared as follows. That is, first, 245.7 g of an aqueous cerium nitrate solution with a concentration of 28% by mass in terms of CeO ₂ , 267.9 g of an aqueous zirconium nitrate solution with a concentration of 18% by mass in terms of ZrO ₂ , iron nitrate 9 water in 100 ml of pure water Hydrate (Fe(NO ₃ ) ₃ 9H ₂ O, manufactured by Wako Pure Chemical Industries, Ltd., purity 99.9%) was mixed with an aqueous solution in which 3.2 g was dissolved, and the resulting mixed solution was diluted with 25% ammonia. Add 163.2 g of water to a solution diluted with 900 ml of pure water and stir at 1100 rpm for 10 minutes using a propeller stirrer and homogenizer (manufactured by AS ONE Corporation) to form a coprecipitate. It was centrifuged and washed with deionized water. Next, the obtained coprecipitate was dried in air at 150° C. for 7 hours using a degreasing furnace, and then calcined in air at 400° C. for 5 hours to obtain an iron-containing ceria-zirconia solid solution. After that, the solid solution is pulverized using a pulverizer ("Wonder Blender" manufactured by AS ONE Corporation) so that the particle size becomes 75 μm or less by sieving, and the content ratio of cerium, zirconium and iron is the atomic ratio ([Ce ]:[Zr]:[Fe]) to obtain an iron-containing ceria-zirconia solid solution powder with a ratio of 50:49:1. In this iron-containing ceria-zirconia solid solution powder, the content ratio of iron to the total amount of cerium and zirconium (Fe/(Ce+Zr)×100) is 1.0 atomic %, and the amount of zirconium to the total number of moles of cerium and zirconium. The molar fraction (Zr/(Ce+Zr)×100) is 49.5%.

Next, 20 g of this iron-containing ceria-zirconia solid solution powder was packed in a polyethylene bag (capacity: 0.05 L), the inside of the bag was degassed, and the opening of the bag was heated and sealed. Subsequently, using a hydrostatic press (“CK4-22-60” manufactured by Nikkiso Co., Ltd.), the bag was subjected to cold hydrostatic pressure at a pressure (molding pressure) of 3000 kgf/cm ² (294 MPa) for 1 minute. Pressing (CIP) was performed to obtain a compact of the iron-containing ceria-zirconia solid solution powder. The size of the molded body was 20 mm long, 20 mm wide, 3 mm average thickness, and about 10 g in mass.

Next, the obtained molded body is put into a small vacuum pressure sintering furnace ("FVPS-R-150" manufactured by Fuji Dempa Kogyo Co., Ltd.), replaced with an argon atmosphere, and then heated to 1000 in 1 hour. After heating to ° C., heating to 1700 ° C. (reduction treatment temperature) for 4 hours, holding for 5 hours, cooling to 1000 ° C. for 4 hours, cooling to room temperature by natural cooling. Then, a reduced product was obtained.

The resulting reduction-treated product was heated in air at 500° C. for 5 hours to obtain an iron-containing ceria-zirconia composite oxide. This iron-containing ceria-zirconia composite oxide is pulverized using the pulverizer so that the particle size becomes 75 μm or less, and the content ratio of cerium, zirconium, and iron is the atomic ratio ([Ce]:[Zr]: An iron-containing ceria-zirconia composite oxide powder with a ratio of [Fe]) of 50:49:1 was obtained. In this iron-containing ceria-zirconia composite oxide powder, the content ratio of iron to the total amount of cerium and zirconium (Fe/(Ce + Zr) × 100) is 1.0 at%, and the total number of moles of cerium and zirconium The molar fraction of zirconium (X=Zr/(Ce+Zr)×100) is X=49.5%.

(Example 2)
In the same manner as in Example 1, except that the amount of the zirconium nitrate aqueous solution was changed to 259.7 g and the amount of the iron nitrate nonahydrate was changed to 8.1 g, the content ratio of cerium, zirconium and iron was changed to the atomic ratio ( An iron-containing ceria-zirconia composite oxide powder having a ratio of [Ce]:[Zr]:[Fe]) of 50:47.5:2.5 was obtained. In this iron-containing ceria-zirconia composite oxide powder, the content ratio of iron to the total amount of cerium and zirconium (Fe/(Ce + Zr) × 100) is 2.6 at%, and the total number of moles of cerium and zirconium The molar fraction of zirconium (X=Zr/(Ce+Zr)×100) is X=48.7%.

(Example 3)
In the same manner as in Example 1, except that the amount of the zirconium nitrate aqueous solution was changed to 246.0 g and the amount of the iron nitrate nonahydrate was changed to 16.2 g, the content ratio of cerium, zirconium, and iron was changed to the atomic ratio ( An iron-containing ceria-zirconia composite oxide powder having a ratio of [Ce]:[Zr]:[Fe]) of 50:45:5 was obtained. In this iron-containing ceria-zirconia composite oxide powder, the content ratio of iron to the total amount of cerium and zirconium (Fe/(Ce + Zr) × 100) is 5.3 at%, and the total number of moles of cerium and zirconium The molar fraction of zirconium (X=Zr/(Ce+Zr)×100) is X=47.4%.

(Comparative example 1)
In the same manner as in Example 1, except that the amount of the zirconium nitrate aqueous solution was changed to 273.4 g and the iron nitrate nonahydrate was not added, the content ratio of cerium, zirconium, and iron was changed to the atomic ratio ([Ce ]:[Zr]:[Fe]) to obtain a ceria-zirconia composite oxide powder of 50:50:0. In this ceria-zirconia composite oxide powder, the iron content ratio (Fe/(Ce+Zr)×100) with respect to the total amount of cerium and zirconium is 0 atomic %, and the mole fraction of zirconium with respect to the total mole number of cerium and zirconium. The ratio (X=Zr/(Ce+Zr)*100) is X=50%.

(Comparative example 2)
In the same manner as in Example 1, except that the amount of the zirconium nitrate aqueous solution was changed to 218.7 g and the amount of the iron nitrate nonahydrate was changed to 32.32 g, the content ratio of cerium, zirconium, and iron was changed to the atomic ratio ( An iron-containing ceria-zirconia composite oxide powder having a ratio of [Ce]:[Zr]:[Fe]) of 50:40:10 was obtained. In this iron-containing ceria-zirconia composite oxide powder, the content ratio of iron to the total amount of cerium and zirconium (Fe/(Ce + Zr) × 100) is 11.1 at%, and the total number of moles of cerium and zirconium The molar fraction of zirconium (X=Zr/(Ce+Zr)×100) is X=44.4%.

(Comparative Example 3)
The amount of the cerium nitrate aqueous solution was changed to 252.0 g, the amount of the zirconium nitrate aqueous solution was changed to 266.3 g, and the amount of the 25% aqueous ammonia was changed to 164.0 g, and the iron nitrate nonahydrate was not added. Ceria-zirconia composite oxidation was carried out in the same manner as in Example 1 except that the content ratio of cerium, zirconium and iron was 50:47.5:0 in atomic ratio ([Ce]:[Zr]:[Fe]). A powder was obtained. The content ratio of cerium, zirconium, and iron in the obtained iron-supporting ceria-zirconia composite oxide powder is 50:47 in atomic ratio ([Ce]:[Zr]:[Fe]). An aqueous solution of iron nitrate nonahydrate (Fe(NO ₃ ) _{3.9H 2} O, manufactured by Wako Pure Chemical Industries, Ltd., purity 99.9%) dissolved in pure water at a ratio of .5:2.5 to impregnate the ceria-zirconia composite oxide powder with iron nitrate, and then heated at 500° C. for 5 hours to obtain an iron-supported ceria-zirconia composite oxide. This iron-supporting ceria-zirconia composite oxide was pulverized in the same manner as in Example 1, and the atomic ratio ([Ce]:[Zr]:[Fe]) of cerium, zirconium and iron was 50:47. An iron-supporting ceria-zirconia composite oxide powder with a ratio of 5:2.5 was obtained. In this iron-supporting ceria-zirconia composite oxide powder, the content ratio of iron to the total amount of cerium and zirconium (Fe/(Ce + Zr) × 100) is 2.6 at%, and the total number of moles of cerium and zirconium The molar fraction of zirconium (X=Zr/(Ce+Zr)×100) is X=48.7%.

<High temperature endurance test>
The composite oxide powders obtained in Examples and Comparative Examples were heated in air at 1100° C. for 5 hours.

<X-ray diffraction (XRD) measurement>
The X-ray diffraction patterns of the composite oxide powders obtained in Examples and Comparative Examples before and after the high temperature durability test were analyzed using an X-ray diffractometer ("RINT-Ultima" manufactured by Rigaku Co., Ltd.) using CuKα. Using X-rays as an X-ray source, measurements were made under the conditions of a tube voltage of 40 KV, a tube current of 40 mA, and a scanning speed of 2θ=20°/min.

The obtained X-ray diffraction pattern was subjected to fitting by the method of least squares using Rietveld analysis software "Jana2006" to refine the lattice constant. Table 1 shows the obtained lattice constants.

In the obtained X-ray diffraction pattern, the ratio of the peak intensity I (14) of the diffraction line at 2θ = 14.5° to the peak intensity I (29) of the diffraction line at 2θ = 29° [I (14/ 29)=I(14)/I(29)] was obtained. Table 1 shows the results.

<Catalyst preparation>
The composite oxide powders obtained in Examples and Comparative Examples before and after the high-temperature durability test and Pd-supported alumina powder (Pd-supported amount: 0.25% by mass) were mixed at a mass ratio of 1:1. to prepare a catalyst powder.

<Oxygen Release Amount Measurement>
15 mg of each of the catalyst powders prepared as described above was charged into a thermogravimetric analyzer ("TGA-50" manufactured by Shimadzu Corporation), and the catalyst powder was exposed to a rich gas (H ₂ (H 2 ( 5% by volume) + N ₂ (remainder)) and lean gas (O ₂ (5 vol%) + N ₂ (remainder)) are alternately switched every 5 minutes, and the gas flow rate is 100 ml/min. The increase or decrease in powder mass was measured. The average value of the amount of decrease in mass of the catalyst powder during the second and third rich gas flows was obtained and used as the oxygen release amount (actual value). The ratio of the oxygen release amount (actual value) to the maximum oxygen release amount (theoretical value) based on the amount of cerium in the catalyst powder was obtained, and this ratio was taken as the oxygen storage material (OSC material) utilization rate. Table 1 shows the results.

From the I(14/29) values shown in Table 1, it was confirmed that the oxygen storage materials obtained in Examples 1 to 3 and Comparative Examples 1 to 3 contained a pyrochlore-type ceria-zirconia composite oxide. was done.

Also, based on the results shown in Table 1, the lattice constant and I(14/29) values were plotted against the mole fraction X of zirconium. These results are shown in Figures 1-2. The straight line in FIG. 1 is expressed by the following formula (1a):
Lattice constant (Y1)=−7.00×10 ⁻³ X+10.874 (1a)
and the molar fraction of zirconium with respect to the total number of moles of cerium and zirconium (X=Zr/(Ce+Zr)×100). be. Further, the straight line in FIG. 2 is expressed by the following formula (2a):
I (14/29) value (Y2)=2.36×10 ⁻³ X−0.072 (2a)
The relationship between the I (14/29) value (Y2) of the oxygen storage material powder and the molar fraction of zirconium with respect to the total number of moles of cerium and zirconium (X = Zr / (Ce + Zr) × 100), represented by is shown.

Furthermore, the OSC material utilization rate of the catalyst powder was plotted against the mole fraction X of zirconium for each composite oxide powder before and after the high temperature durability test. These results are shown in Figures 3A-3B.

As shown in FIGS. 1 and 2, iron-containing ceria-zirconia in which the content ratio of iron (Fe/(Ce+Zr)×100) and the molar fraction of zirconium (Zr/(Ce+Zr)×100) are within a predetermined range The iron-containing ceria-zirconia composite oxide powder (Examples 1 to 3) obtained by pressure-molding the solid solution powder, reducing the resulting molded body, and further oxidizing it was tested before and after the high-temperature durability test. After the high temperature durability test, the lattice constant satisfied the conditions represented by the above formula (1), and the I(14/29) value satisfied the conditions represented by the above formula (2). , it was found that the iron atoms are sufficiently dissolved in the lattice of the pyrochlore-type ceria-zirconia composite oxide.

On the other hand, an iron-containing ceria-zirconia solid solution powder having an iron content ratio (Fe/(Ce+Zr)×100) larger than a predetermined range is pressure-molded, and the resulting molded body is subjected to reduction treatment and further oxidation treatment. The iron-containing ceria-zirconia composite oxide powder obtained by (Comparative Example 2) has a lattice constant that satisfies the condition represented by the above formula (1) both before and after the high temperature durability test. However, since the I (14/29) value before the high temperature durability test did not satisfy the condition represented by the above formula (2), iron atoms were sufficiently present in the lattice of the pyrochlore-type ceria-zirconia composite oxide. It is thought that it is not dissolved in In addition, in the iron-containing ceria-zirconia composite oxide powder obtained in Comparative Example 2, the I (14/29) value after the high temperature durability test was equal to the I (14/29) value obtained by the above formula (2a). This is not because the iron atoms dissolved in the pyrochlore-type ceria-zirconia composite oxide during the high-temperature durability test, but because the iron atoms that did not dissolve in solid solution promoted the phase separation of ceria and the pyrochlore This is probably because the stability of the type ceria-zirconia composite oxide was lowered.

In addition, the iron-supported ceria-zirconia composite oxide powder (Comparative Example 3), in which iron was supported on the pyrochlore-type ceria-zirconia composite oxide by an impregnation method, showed I Although the (14/29) value satisfied the condition represented by the above formula (2), the lattice constant was the condition represented by the above formula (1) both before and after the high temperature durability test. , it is considered that the iron atoms are not sufficiently dissolved in the lattice of the pyrochlore-type ceria-zirconia composite oxide.

As shown in FIG. 3A, before the high-temperature durability test, the oxygen storage material powder (that is, the above-mentioned Catalyst powders (Examples 1 to 3) containing oxygen storage material powders satisfying both conditions represented by formulas (1) and (2) were iron-free pyrochlore-type ceria-zirconia composites Catalyst powder containing oxygen storage material powder made of oxide (Comparative Example 1), and oxygen made of composite oxide powder in which iron atoms are not sufficiently dissolved in the lattice of pyrochlore-type ceria-zirconia composite oxide Compared to catalyst powders (Comparative Examples 2 and 3) containing storage material powders (that is, oxygen storage material powders that do not satisfy at least one of the conditions represented by formulas (1) and (2) above), oxygen storage It turned out that the material (OSC material) utilization rate becomes high.

Further, as shown in FIG. 3B, even after the high-temperature durability test, the oxygen storage material powder ( That is, the catalyst powders (Examples 1 to 3) containing the oxygen storage material powder satisfying both the conditions represented by the above formulas (1) and (2) were iron-free pyrochlore-type ceria - A catalyst powder containing an oxygen storage material powder made of a zirconia composite oxide (Comparative Example 1), as described above, iron atoms are not sufficiently dissolved in the lattice of the pyrochlore-type ceria-zirconia composite oxide. and a catalyst powder containing an oxygen storage material powder composed of a composite oxide powder in which phase separation of ceria is promoted by iron atoms that are not solid-dissolved and the stability of the pyrochlore-type ceria-zirconia composite oxide is lowered (comparative Example 2), and an oxygen storage material powder composed of a composite oxide powder in which iron atoms are not sufficiently dissolved in the lattice of the pyrochlore-type ceria-zirconia composite oxide (that is, represented by the above formula (1) It was found that the utilization rate of the oxygen storage material (OSC material) is higher than that of the catalyst powder (Comparative Example 3) containing the oxygen storage material powder that does not satisfy the conditions.

As described above, not only at the beginning of use, but also after long-term exposure to high-temperature exhaust gas of about 1100 ° C., excellent oxygen storage capacity (OSC) can be exhibited at a low temperature of about 300 ° C. It becomes possible to obtain an oxygen storage material with high utilization efficiency. Therefore, the oxygen storage material of the present invention has both excellent oxygen storage capacity (OSC) at low temperatures and high temperature durability, and is therefore useful as a carrier or co-catalyst for an exhaust gas purification catalyst, a catalyst atmosphere control material, or the like.

Claims (4)

An oxygen storage material containing a pyrochlore-type ceria-zirconia composite oxide and iron added to the ceria-zirconia composite oxide,
The content ratio of iron to the total amount of cerium (Ce) and zirconium (Zr) (Fe/(Ce + Zr) × 100) is 0.5 to 9 at%,
The molar fraction of zirconium with respect to the total number of moles of cerium (Ce) and zirconium (Zr) (X = Zr / (Ce + Zr) × 100) is X = 40 to 50%,
The lattice constant obtained from the X-ray diffraction patterns obtained by X-ray diffraction measurement using CuKα before heating at 1100° C. in the atmosphere and after heating for 5 hours is given by the following formula (1):
Lattice constant≦−7.00×10 ⁻³ X+10.874 (1)
(In the above formula, X represents the mole fraction of the zirconium)
It satisfies the conditions represented by
Before heating at 1100 ° C. in the atmosphere and after heating for 5 hours, a diffraction line at 2θ = 14.5 ° and a diffraction line at 2θ = 29 ° obtained from X-ray diffraction patterns obtained by X-ray diffraction measurement using CuKα. The diffraction line and the intensity ratio [I (14/29) value] are represented by the following formula (2):
I (14/29) value ≤ 2.36 × 10 ^-3 X - 0.072 (2)
(In the above formula, X represents the mole fraction of the zirconium)
An oxygen storage material that satisfies the conditions represented by: 2. The oxygen storage material according to claim 1, wherein the zirconium mole fraction X is X=45-50%. A method for producing an oxygen storage material containing a pyrochlore-type ceria-zirconia composite oxide and iron added to the ceria-zirconia composite oxide, comprising:
The content ratio of iron to the total amount of cerium (Ce) and zirconium (Zr) (Fe/(Ce + Zr) × 100) is 0.5 to 9 at%, and the total mole of cerium (Ce) and zirconium (Zr) preparing an iron-containing ceria-zirconia solid solution powder having a zirconium mole fraction (Zr/(Ce+Zr)×100) of 40 to 50%;
The pyrochlore-type ceria according to claim 1 or 2, wherein the iron-containing ceria-zirconia solid solution powder is pressure-molded at a pressure of 30 to 350 MPa, then subjected to reduction treatment under temperature conditions of 1400 to 2000° C., and further to oxidation treatment. - obtaining an oxygen storage material containing a zirconia-based composite oxide and iron added to the ceria-zirconia-based composite oxide;
A method for producing an oxygen storage material, comprising: 4. The method for producing an oxygen storage material according to claim 3, wherein the zirconium molar fraction (Zr/(Ce+Zr)×100) in the iron-containing ceria-zirconia solid solution powder is 45 to 50%.

Images