JP2005231951A - Multiple oxide and catalyst for purification of exhaust gas - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a multiple oxide which has a pyrochlore structure realizing both a high oxygen occlusion/release capacity and a high heat resistance, and to produce a catalyst for purification of exhaust gas, which uses the multiple oxide as a carrier. <P>SOLUTION: The multiple oxide has the pyrochlore structure containing Ce and Zr. The multiple oxide having the high oxygen occlusion/release capacity and the high heat resistance is obtained by replacing 40-90% of the Ce with rare-earth or alkaline-earth ions. The catalyst for purification of exhaust gas is obtained by using the multiple oxide as the carrier and depositing at least one noble metal such as Pt, Rh, Pd, Ir and Ru on the carrier. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to an oxygen storage material, in particular, a composite oxide having high oxygen storage / release capability (hereinafter referred to as OSC) and high heat resistance, and an exhaust gas purifying catalyst using the composite oxide as a carrier.

  Conventionally, a three-way catalyst has been used as a method for purifying carbon monoxide (CO), hydrocarbons (HC), and nitrogen oxides (NOx), which are harmful substances in automobile exhaust gas. The exhaust gas is purified by simultaneously performing oxidation of CO and HC and reduction of nitrogen oxide (NOx) with a three-way catalyst. As such a three-way catalyst, for example, a carrier layer made of γ-alumina is formed on a heat-resistant honeycomb substrate made of cordierite or the like, and platinum (Pt), rhodium (Rh), palladium (Pd) is formed on the carrier layer. And the like, on which a catalytic metal such as) is supported is widely known.

  By the way, the condition of the carrier used for the exhaust gas purifying catalyst is that the specific surface area is large and the heat resistance is high. Generally, alumina, silica, zirconia, titania and the like are often used.

The performance of the three-way catalyst is greatly influenced by the composition of the exhaust gas, particularly the oxygen concentration. Therefore, the air-fuel ratio is controlled while monitoring the oxygen concentration in the exhaust gas. Further, ceria (CeO ₂ ) with OSC is used as a promoter or support to mitigate fluctuations in the oxygen concentration of the exhaust gas. The conditions for this OSC material include a high specific surface area and high heat resistance as well as a high OSC.

However, when the conventional exhaust gas purifying catalyst is exposed to a high temperature exceeding 800 ° C., the specific surface area of the support is reduced due to sintering, and grain growth of the catalytic metal occurs. In particular, CeO ₂ has a low specific surface area at high temperature. Therefore, there is a problem that the OSC is lowered and the purification performance is remarkably lowered.

  In addition, due to the recent tightening of exhaust gas regulations, the necessity of purifying exhaust gas within a very short time after engine startup has become extremely high. When starting the engine, the exhaust gas temperature is low and the catalyst does not work sufficiently. Therefore, the exhaust gas temperature is raised by arranging the catalyst near the engine so that the catalyst works. On the other hand, when the catalyst is brought closer to the engine, the catalyst is exposed to exhaust gas reaching 800 ° C. and further 1000 ° C. during high-speed operation. For this reason, the request | requirement to the high heat resistance of a support | carrier is increasing further.

CeO ₂ as an OSC material was initially developed with BaO and La ₂ O ₃ added to CeO ₂ to improve heat resistance and OSC. Recently, a material in which CeO ₂ and ZrO _{2 at} an equimolar ratio are solid-dissolved has been developed, and a significant improvement in OSC and heat resistance has been achieved as compared with conventional CeO ₂ -based materials (for example, Non-Patent Document 1). reference). Furthermore, it has been shown that heat resistance is further improved by mixing a solid solution of CeO ₂ and ZrO ₂ with alumina (for example, see Non-Patent Document 1).

However, even in the solid solution of CeO ₂ and ZrO ₂ described above, the OSC still remains about half of the theoretical value, and further improvement of the OSC is required.

Therefore, a precipitate is produced from a solution containing a cerium (III) salt and a zirconium salt by a coprecipitation method, and the precipitate is heated and held at 800 to 1000 ° C. in an inert atmosphere or a non-oxidizing atmosphere, thereby belonging to the pyrochlore phase. It has been proposed to obtain a CeO ₂ —ZrO ₂ composite oxide having an X-ray diffraction peak that exhibits high OSC (see, for example, Patent Document 1).

However, when the CeO ₂ —ZrO ₂ composite oxide having a pyrochlore phase is exposed to an oxidizing atmosphere, particularly at a temperature of 1000 ° C., the ordered arrangement of Ce and Zr peculiar to the pyrochlore phase is broken, resulting in a problem that OSC is lowered.

Japanese Patent Laid-Open No. 11-165067

Material Integration Vol. 16, no. 4 (2003) pp. 3-14

According to the method described in Patent Document 1 (Japanese Patent Laid-Open No. 11-165067), a CeO ₂ —ZrO ₂ composite oxide having a high OSC can be obtained. However, when the composite oxide is exposed to an oxidizing atmosphere, particularly at a temperature of 1000 ° C., the ordered arrangement of Ce and Zr peculiar to the pyrochlore phase is broken, resulting in a disadvantage that OSC is lowered.

  The present invention has been made in view of such circumstances, and provides a composite oxide containing Ce and Zr having a pyrochlore structure having high OSC and high heat resistance, and is excellent in using the composite oxide as a carrier. An object of the present invention is to provide an exhaust gas purification catalyst.

The composite oxide of the present invention is a solution of the above-mentioned problems, and is a composite oxide containing Ce and Zr having a pyrochlore structure, wherein a part of Ce in the pyrochlore structure is replaced with a rare earth other than Ce or The composite oxide is characterized by being substituted with at least one of Ca, Sr, and Ba. More specifically, it is a composite oxide containing Ce and Zr, and having a pyrochlore structure represented by the following composition formula when the valence of Ce is trivalent: is there.
_{(Ce 1-x-y,} RE x, AE y) 2 (Zr 1-y, M y) 2 O 7
Here, RE is at least one kind of rare earth ions other than Ce, AE is at least one kind of Ca, Sr, and Ba, M is at least one kind of Nb and Ta, 0 ≦ x ≦ 0.9, 0 ≦ y ≦ 0 .9, 0.4 ≦ x + y ≦ 0.9.

The pyrochlore structure of the present invention is a structure represented by the chemical formula Ce ₂ Zr ₂ O ₇ when the constituent metal element is composed only of Ce and Zr, and Ce and Zr are alternately arranged regularly with oxygen interposed therebetween. In the reduced state, this composite oxide has a valence of Ce of 3+ and a pyrochlore phase. However, in the oxidized state, the valence of Ce becomes 4+ and has a κ structure represented by the chemical formula CeZrO ₄ . That is, in this composite oxide, in an oxidizing atmosphere, Ce ³⁺ having a pyrochlore structure is oxidized to Ce ⁴⁺ to change to a κ structure represented by the chemical formula CeZrO _4. Conversely, in a κ structure, Ce ⁴⁺ is converted to Ce ^{3+ in a} reducing atmosphere. To a pyrochlore structure represented by the chemical formula Ce ₂ Zr ₂ O ₇ . As described above, this composite oxide is a composite oxide of Ce and Zr that reversibly changes between the pyrochlore structure and the κ structure according to the redox state, and between this pyrochlore structure and the κ structure. Oxygen is occluded or released in accordance with the change in. In this composite oxide, almost 100% of Ce in Ce ₂ Zr ₂ O ₇ is used for oxygen storage and exhibits high OSC.

  On the other hand, with respect to thermal stability, there is a problem that the κ structure is broken by oxidation treatment near 1000 ° C., and OSC is lowered. As a cause of this, in the κ structure, the valence of Ce becomes 4+, which is the same as the valence of Zr, and its ionic radius is also close to the ionic radius of Zr. In the pyrochlore structure, Ce is 8-coordinate and Zr is 6-coordinate, but both are 8-coordinate in the κ structure. Thus, in the κ structure, the valence, ion radius, and coordination number of Ce and Zr are almost the same, and substitution of Ce and Zr occurs, the regularity is broken, and it is assumed that OSC is lowered.

  The present inventors have stated that the thermal stability of the pyrochlore structure under the oxidation conditions, that is, the thermal stability of the κ phase is such that a part of Ce in the pyrochlore structure is replaced with rare earth ions other than Ce or Ca, Sr, Ba. It has been found that it can be remarkably improved by substitution with at least one kind.

By substituting a part of Ce in the pyrochlore structure with a rare earth ion other than Ce having no change in valence, in the κ structure in which the valence of Ce becomes 4+, half of the number of Ce ions substituted in the above manner. As a result of the occurrence of a number of oxygen defects, the Zr site in the κ structure becomes 7-coordinate or 6-coordinated. As a result, Ce ⁴⁺ having a large ionic radius can easily select an 8-coordinate Ce site, and Zr ⁴⁺ having a small ionic radius can easily select a 7-coordinate or 6-coordinate Zr site. This makes it difficult for the substitution of Ce and Zr to occur, so that the ordered arrangement of Ce and Zr is not disturbed, that is, the thermal stability of the κ structure is considered to be improved.

  Although the substitution of Ce in the pyrochlore structure can be performed with at least one of Ca, Sr, and Ba having no change in valence, the same effect can be obtained. In this case, however, the charge as a whole is compensated. In addition, at least one of Nb and Ta having an ionic radius similar to that of Zr is substituted with the same amount of Zr as that of Ce.

  The feature of the composite oxide having a pyrochlore structure represented by the above composition formula of the present invention is that 40% to 90% of Ce constituting the pyrochlore structure is at least one kind of rare earth ions other than Ce, or Ca, Sr, Ba And at least one kind of alkaline earth metal ion. When Ce is replaced with an alkaline earth metal such as Ca, Sr, or Ba as described above, the same amount of Zr is replaced with Nb or Ta.

When the pyrochlore phase of the composite oxide of the present invention is converted to a κ phase by oxidation treatment, oxygen ion defects that are half the number of ions substituted with Ce occur, and the coordination number of Zr is mostly 7-coordinate or 6 coordination. The composition formula is as follows.
_{(Ce 1-x-y,} RE x, AE y) 2 (Zr 1-y, M y) 2 O 8- (x + y)
As described above, RE is at least one rare earth ion other than Ce, AE is at least one of Ca, Sr, and Ba, M is at least one of Nb and Ta, 0 ≦ x ≦ 0.9, 0 ≦ y ≦ 0.9, 0.4 ≦ x + y ≦ 0.9.

Ce sites are 8-coordinate, and most of the Zr sites are 7-coordinate or 6-coordinated. ^Therefore, Ce ^{4+ with} a large ionic radius is 8-coordinated, and Zr ^{4+ with} a small ionic radius is 7-coordinated. It is presumed that the position or hexacoordination is easy to take, the substitution of Ce and Zr hardly occurs, and the thermal stability under oxidation conditions is improved.

  The ions replacing the Ce site are at least one kind of rare earth ions other than Ce, or at least one kind of alkaline earth metal ions of Ca, Sr and Ba.

  Furthermore, among rare earth ions, La, Nd, Sm, Gd, and Y are preferable because they easily form a pyrochlore structure because of the relationship of the ionic radii. In view of cost, La and Y are particularly preferable. As alkaline earth ions, Ca and Sr are preferable because they can easily form a pyrochlore structure because of the ionic radius.

  The substitution amount x with rare earth ions and the substitution amount y with alkaline earth metal ions are 0 ≦ x ≦ 0.9, 0 ≦ y ≦ 0.9, and 0.4 ≦ x + y ≦ 0.9. As described above, since the effect of the composite oxide of the present invention can be obtained by substituting Ce with either RE or AE, x and y may each be 0 alone. However, the total substitution amount (x + y) is 0.4 or more. When (x + y) is smaller than 0.4, the amount of Ce substitution is small, and the thermal stability of the κ phase under oxidation conditions is greatly reduced, which is not preferable. Further, when (x + y) exceeds 0.9, the amount of Ce decreases, and the OSC amount decreases, which is not preferable.

  A more preferable range of (x + y) is 0.5 ≦ x + y ≦ 0.8. Within this range, the thermal stability of the κ phase under oxidizing conditions is high and the amount of OSC is also large, which is particularly preferable.

  Further, when Ce is substituted only by RE (at least one kind of rare earth ions other than Ce), that is, when y = 0, it is preferable that 0.4 ≦ x ≦ 0.9, More preferably, ≦ x ≦ 0.8. In consideration of the handling of raw materials and the ease of synthesis, the Ce substitution ion is preferably RE (at least one kind of rare earth ions other than Ce).

The OSC of the composite oxide of the present invention is determined by the amount of Ce contained. However, since almost the entire amount of Ce can participate in redox between Ce ³⁺ and Ce ^{4+ in} a gas atmosphere, high OSC is exhibited.

  The method for producing the composite oxide of the present invention is not particularly limited, such as a solid phase method, a liquid phase method, and an alkoxide method. An example of a manufacturing method is shown below. In the method for producing a composite oxide of the present invention, an aqueous solution of a cerium compound and a zirconium compound, an aqueous solution of an RE (rare earth ion other than Ce) compound, an aqueous solution of an AE (Ca, Sr, Ba) compound, and an M (Nb, Ta) compound An aqueous solution of the above and an aqueous solution of a precipitant are mixed to form a precipitate, and the generated precipitate is dried and then fired in a reducing atmosphere.

  As the cerium compound, for example, water-soluble compounds such as nitrates such as cerium nitrate and cerium chloride, sulfates, and chlorides can be used. As the zirconium compound, water-soluble compounds such as nitrates such as zirconium oxynitrate and zirconium oxychloride, sulfates and chlorides can be used. Moreover, as RE (rare earth ions other than Ce) compounds, water-soluble compounds such as nitrates, sulfates and chlorides can be used. As the AE (Ca, Sr, Ba) compound, water-soluble compounds such as nitrates, sulfates, carbonates, and chlorides can be used. As the M (Nb, Ta) compound, water-soluble compounds such as nitrates, sulfates, carbonates and chlorides can be used.

  As the precipitant, alkali metal hydroxides such as ammonia and sodium hydroxide, alkali metal carbonates such as ammonium carbonate and sodium carbonate, and oxalates such as urea, oxalic acid, and ammonium oxalate can be used.

  Examples of the method for producing a precipitate containing Ce and Zr that form the composite oxide of the present invention by firing include cerium compounds, zirconium compounds, RE (rare earth ions other than Ce) compounds, AE (Ca, Sr, Ba) compounds, It can be produced as a coprecipitate from a mixed aqueous solution in which M (Nb, Ta) compound coexists, by coprecipitation, Ce precipitation, Zr precipitation, RE (rare earth ions other than Ce) compound, AE (Ca , Sr, Ba) compound precipitates and M (Nb, Ta) compound precipitates, respectively, and these precipitates may be mixed.

  Further, a cation, anion, or nonionic surfactant may be added to the raw material for the purpose of reducing the dispersibility of the precipitate and agglomeration.

  The precipitation method includes a method of adding an aqueous solution containing Ce and Zr in a precipitant such as ammonia water, and conversely a method of instantly adding a precipitant such as ammonia water to an aqueous solution containing Ce and Zr. There is also a method of adjusting the pH at which the oxide precursor starts to precipitate by adding hydrogen peroxide or the like, and then depositing the precipitate with ammonia water or the like. Also, use urea as a precipitating agent, and add a uniform precipitation method that gradually neutralizes with ammonia generated by the decomposition of urea, a method that neutralizes by gradually changing the pH, or a buffer solution that maintains a specific pH. There are methods. In the method for producing a composite oxide of the present invention, any of the above methods may be used as a precipitation method.

  As a method for drying the precipitate containing Ce and Zr, a conventionally known method is used. In particular, azeotropic dehydration can be performed as a method for alleviating the aggregation of the dry powder.

  Next, a composite oxide having a pyrochlore structure of the present invention is obtained by a firing treatment. As firing conditions, it is performed by heating and holding at 800 ° C. to 1400 ° C. in a reducing atmosphere. For the purpose of obtaining a complex oxide having a high specific surface area, the firing temperature is more preferably 800 ° C. to 1100 ° C. When the heating and holding temperature is lower than 800 ° C., it is difficult to generate a pyrochlore phase and OSC is lowered.

Although the reducing atmosphere can be an inert gas atmosphere or a non-oxidizing atmosphere, it is desirable to use an atmosphere containing a reducing gas such as H ₂ or CO. The generation of Ce ³⁺ is important for the generation of the pyrochlore phase. If no reducing gas is contained, the reduction to Ce ³⁺ is insufficient, and as a result, the generation of the pyrochlore phase becomes insufficient and high OSC cannot be obtained. There is a case. The κ phase is a metastable phase that is generated only when the pyrochlore phase is oxidized, and cannot be directly generated by firing in an oxidizing atmosphere. Therefore, in order to obtain a high OSC, the reducing atmosphere is used as described above. It is necessary to produce a sufficient pyrochlore phase.

  The composite oxide having a pyrochlore structure obtained in the present invention is a composite oxide having a κ structure in which the valence of Ce becomes 4+ by being held at 500 ° C. to 1000 ° C. in an oxidizing atmosphere such as air or an oxygen atmosphere. It is also possible to make it.

  When the composite oxide of the present invention is used as a catalyst carrier, the composite oxide of the present invention can be used alone, or a ceria-zirconia composite oxide having no alumina, ceria, zirconia, or pyrochlore structure. It can also be used by mixing with things. In particular, a high specific surface area can be achieved by combining with alumina.

  Furthermore, the exhaust gas-purifying catalyst of the present invention comprises the composite oxide of the present invention as a support and supports a noble metal thereon. When the composite oxide of the present invention is used as a support, only the composite oxide of the present invention may be used as a support, or may be used by mixing with other oxides such as alumina. As the noble metal, one type or a plurality of types can be selected and used from Pt, Rh, Pd, Ir, Ru and the like, and the supported amount may be the same as that of the conventional exhaust gas purifying catalyst. As the loading method, a conventional loading method such as an adsorption loading method or a water absorption loading method can be used.

  In the exhaust gas purifying catalyst of the present invention, the composite oxide of the present invention, which is an oxygen storage material, has high heat resistance. Therefore, even when used at a high temperature of about 1000 ° C., high OSC is maintained, and as a result, it is sufficiently high. Exhaust gas purification characteristics can be obtained.

  According to the present invention, a composite oxide containing Ce and Zr having high OSC and high heat resistance that can be used at a high temperature of about 1000 ° C. can be provided. By using such a composite oxide as a carrier, it becomes possible to maintain a high OSC even when used at a high temperature of about 1000 ° C. As a result, an exhaust gas purification catalyst having remarkably improved exhaust gas purification characteristics can be obtained. Can be provided.

  EXAMPLES Hereinafter, although an Example demonstrates this invention concretely, this invention is not limited only to these Examples.

  Identification of the crystal structure and OSC measurement of the composite oxide powders obtained in Examples and Comparative Examples were performed as follows.

(1) Crystal structure The crystal structure of the powder was identified by a powder X-ray diffractometer (MPX3 manufactured by Mac Science). Cu-Kα rays were used as the X-ray source.

(2) OSC measurement The OSC measurement was performed using a self-made temperature-reduction apparatus (TPR). Gas chromatography with a thermal conductivity detector (TCD) for the amount of H ₂ that is heated from room temperature to 900 ° C. at a rate of 10 ° C./min in an H ₂ = 20% atmosphere and consumed in the range of room temperature to 900 ° C. Detected with. A curve in which temperature is plotted on the horizontal axis and H ₂ consumption is plotted on the vertical axis is called a TPR curve, and OSC was calculated from the amount of H ₂ consumed in the range of 200 to 700 ° C. The sample was previously subjected to an oxidation treatment in the atmosphere at 500 ° C. for 1 hour, and then the measurement was performed.

(Example 1)
A 0.5 mol / L (hereinafter referred to as L) cerium nitrate aqueous solution 0.5 L, a 0.5 mol / L zirconium oxynitrate aqueous solution 1 L, and a 0.5 mol / L lanthanum nitrate aqueous solution 0.5 L were mixed. The aqueous solution was mixed with 30 g of 31% hydrogen peroxide solution and stirred. To this mixed aqueous solution, 430 mL of 7 mol / L aqueous ammonia was added and stirred for 2 hours. The obtained precipitate was filtered and washed with water, and dried overnight in a dryer at 110 ° C. to obtain a dry powder. The obtained powder was subjected to reduction treatment at 1200 ° C. for 2 hours in an N ₂ stream containing 4% of H ₂ to obtain a composite oxide powder of the present invention. The X-ray diffraction data of the obtained composite oxide is shown in FIG. 1 (profile indicated by 1 in FIG. 1). The OSC values are shown in Table 1.

（実施例２）
０．５ｍｏｌ／Ｌの硝酸セリウム水溶液０．６Ｌと０．５ｍｏｌ／Ｌのオキシ硝酸ジルコニウム水溶液１Ｌと０．５ｍｏｌ／Ｌの硝酸ランタン水溶液０．４Ｌを混合し、この水溶液に３１％の過酸化水素水３６ｇを混合し攪拌した。この混合水溶液に７ｍｏｌ／Ｌのアンモニア水４４０ｍＬを添加し、２時間攪拌した。得られた沈殿をろ過・水洗し、１１０℃の乾燥器中で一晩乾燥し、乾燥粉末を得た。得られた粉末をＨ_２を４％含むＮ_２気流中にて１１００℃で２時間還元処理をして、本発明の複合酸化物粉末を得た。得られた複合酸化物のＸ線回折結果及びＯＳＣ値を表１に示す。

(Example 2)
A 0.5 mol / L cerium nitrate aqueous solution (0.6 L), a 0.5 mol / L zirconium oxynitrate aqueous solution (1 L), and a 0.5 mol / L lanthanum nitrate aqueous solution (0.4 L) were mixed. 36 g of water was mixed and stirred. To this mixed aqueous solution, 440 mL of 7 mol / L aqueous ammonia was added and stirred for 2 hours. The obtained precipitate was filtered and washed with water, and dried overnight in a dryer at 110 ° C. to obtain a dry powder. The obtained powder was subjected to reduction treatment at 1100 ° C. for 2 hours in an N ₂ stream containing 4% of H ₂ to obtain a composite oxide powder of the present invention. Table 1 shows the X-ray diffraction results and OSC values of the obtained composite oxide.

(Example 3)
A 0.5 mol / L cerium nitrate aqueous solution (0.1 L), a 0.5 mol / L zirconium oxynitrate aqueous solution (0.4 L), and a 0.5 mol / L lanthanum nitrate aqueous solution (0.3 L) were mixed. 1 L of oxalic acid aqueous solution of L was added and stirred for 1 hour. The obtained aqueous solution containing the precipitate was evaporated to dryness and dried overnight in a dryer at 110 ° C. to obtain a dry powder. The obtained powder was subjected to reduction treatment at 900 ° C. for 4 hours in an N ₂ stream containing 4% of H ₂ to obtain a composite oxide powder of the present invention. Table 1 shows the X-ray diffraction results and OSC values of the obtained composite oxide.

Example 4
Obtained in the same manner as in Example 1 using 0.5 L of 0.5 mol / L cerium nitrate aqueous solution, 1 L of 0.5 mol / L zirconium oxynitrate aqueous solution and 0.5 L of 0.5 mol / L neodymium nitrate solution. The obtained powder was subjected to reduction treatment at 1200 ° C. for 2 hours in an N ₂ stream containing 4% of H ₂ to obtain a composite oxide powder of the present invention. The X-ray diffraction data of the obtained complex oxide is shown in FIG. 1 (profile with 2 in FIG. 1). The OSC values are shown in Table 1.

(Example 5)
It was obtained in the same manner as Example 1 using 0.5 L of 0.5 mol / L cerium nitrate aqueous solution, 1 L of 0.5 mol / L zirconium oxynitrate aqueous solution and 0.5 L of 0.5 mol / L samarium nitrate aqueous solution. The obtained powder was subjected to reduction treatment at 1200 ° C. for 2 hours in an N ₂ stream containing 4% of H ₂ to obtain a composite oxide powder of the present invention. The X-ray diffraction data of the obtained composite oxide is shown in FIG. 1 (profile indicated by 3 in FIG. 1). The OSC values are shown in Table 1.

(Example 6)
Obtained in the same manner as in Example 1, using 0.5 L of 0.5 mol / L cerium nitrate aqueous solution, 1 L of 0.5 mol / L zirconium oxynitrate aqueous solution and 0.5 L of 0.5 mol / L gadolinium chloride aqueous solution. The obtained powder was subjected to reduction treatment at 1300 ° C. for 2 hours in an N ₂ stream containing 4% of H ₂ to obtain a composite oxide powder of the present invention. The X-ray diffraction data of the obtained complex oxide is shown in FIG. 1 (profile indicated by 4 in FIG. 1). The OSC values are shown in Table 1.

(Example 7)
0.5 mol / L cerium nitrate aqueous solution 0.4 L, 0.5 mol / L zirconium oxynitrate aqueous solution 1 L, 0.5 mol / L lanthanum nitrate aqueous solution 0.4 L and 0.5 mol / L yttrium nitrate aqueous solution 0.2 L The aqueous solution was mixed with 24 g of 31% hydrogen peroxide solution and stirred. To this mixed aqueous solution, 420 mL of 7 mol / L aqueous ammonia was added and stirred for 2 hours. The obtained precipitate was filtered and washed with water, and dried overnight in a dryer at 110 ° C. to obtain a dry powder. The obtained powder was subjected to reduction treatment at 1200 ° C. for 2 hours in an N ₂ stream containing 4% of H ₂ to obtain a composite oxide powder of the present invention. Table 1 shows the X-ray diffraction results and OSC values of the obtained composite oxide.

(Example 8)
0.5 mol / L cerium nitrate aqueous solution 0.4 L, 0.5 mol / L zirconium oxynitrate aqueous solution 1 L, 0.5 mol / L lanthanum nitrate aqueous solution 0.5 L and 0.5 mol / L calcium nitrate aqueous solution 0.1 L And 0.5 mol of a 0.1 mol / L niobium pentachloride aqueous solution, the powder obtained in the same manner as in Example 7 was reduced at 1200 ° C. for 2 hours in an N ₂ stream containing 4% of H _2. Thus, a composite oxide powder of the present invention was obtained. Table 1 shows the X-ray diffraction results and OSC values of the obtained composite oxide.

Example 9
0.5 mol / L cerium nitrate aqueous solution 0.5 L, 0.5 mol / L zirconium oxynitrate aqueous solution 1 L, 0.5 mol / L lanthanum nitrate aqueous solution 0.45 L and 0.5 mol / L strontium nitrate aqueous solution 0.05 L And a 0.1 mol / L niobium pentachloride aqueous solution (0.25 L), the powder obtained in the same manner as in Example 1 was reduced at 1300 ° C. for 2 hours in an N ₂ stream containing 4% of H _2. Thus, a composite oxide powder of the present invention was obtained. Table 1 shows the X-ray diffraction results and OSC values of the obtained composite oxide.

(Comparative Example 1)
1 L of 0.5 mol / L cerium nitrate aqueous solution and 1 L of 0.5 mol / L zirconium oxynitrate aqueous solution were mixed, and 60 g of 31% hydrogen peroxide solution was mixed and stirred in this aqueous solution. To this mixed aqueous solution, 470 mL of 7 mol / L aqueous ammonia was added and stirred for 2 hours. The obtained precipitate was filtered and washed with water, and dried overnight in a dryer at 110 ° C. to obtain a dry powder. The obtained powder was subjected to reduction treatment at 1200 ° C. for 2 hours in an N ₂ stream containing 4% of H ₂ to obtain a complex oxide powder of Ce and Zr. The X-ray diffraction data of the obtained composite oxide is shown in FIG. 2 (profile with 5 in FIG. 2). The OSC values are shown in Table 1.

(Comparative Example 2)
A 0.5 mol / L cerium nitrate aqueous solution (0.7 L), a 0.5 mol / L zirconium oxynitrate aqueous solution (1 L) and a 0.5 mol / L lanthanum nitrate aqueous solution (0.3 L) were mixed, and 31% hydrogen peroxide was added to the aqueous solution. 42 g of water was mixed and stirred. To this mixed aqueous solution, 450 mL of 7 mol / L aqueous ammonia was added and stirred for 2 hours. The obtained precipitate was filtered and washed with water, and dried overnight in a dryer at 110 ° C. to obtain a dry powder. The obtained powder was subjected to reduction treatment at 1200 ° C. for 2 hours in an N ₂ stream containing 4% of H ₂ to obtain a composite oxide powder of the present invention. The X-ray diffraction data of the obtained composite oxide is shown in FIG. 2 (profile indicated by 6 in FIG. 2). The OSC values are shown in Table 1.

(Comparative Example 3)
0.75 L of 0.5 mol / L cerium nitrate aqueous solution, 1 L of 0.5 mol / L zirconium oxynitrate aqueous solution and 0.25 L of 0.5 mol / L yttrium nitrate aqueous solution were mixed, and 31% hydrogen peroxide was added to this aqueous solution. 45 g of water was mixed and stirred. To this mixed aqueous solution, 450 mL of 7 mol / L aqueous ammonia was added and stirred for 2 hours. The obtained precipitate was filtered and washed with water, and dried overnight in a dryer at 110 ° C. to obtain a dry powder. The obtained powder was subjected to reduction treatment at 1200 ° C. for 2 hours in an N ₂ stream containing 4% of H ₂ to obtain a composite oxide powder of the present invention. The X-ray diffraction data of the obtained composite oxide is shown in FIG. 2 (profile with 7 in FIG. 2). The OSC values are shown in Table 1.

<Heat resistance test>
The sample powders of Examples 1 to 9 and Comparative Examples 1 to 3 were placed in an alumina crucible and held at 1000 ° C. for 5 hours in an electric furnace in an air atmosphere. The crystal structure measurement and OSC measurement were performed on the sample after the heat treatment. In the crystal structure measurement, the presence or absence of a peak on the (331) plane near 2θ = 37 ° due to the ordered arrangement of cations of pyrochlore structure and κ structure was observed. Moreover, OSC was evaluated by the following OSC maintenance rate from the OSC value before and after heat-resistant treatment.

OSC retention rate (%) = (OSC after heat resistance test) / (OSC before heat resistance test) × 100

The X-ray diffraction data of the sample powders of Example 1, Example 4, Example 5, and Example 6 after the heat resistance test are shown in FIG. 3 (Example 1: Profile marked with 8 in FIG. 3, Example 4) : Profile with 9 in FIG. 3, Example 5: Profile with 10 in FIG. 3, Example 6: Profile with 11 in FIG.

  The X-ray diffraction data of the sample powders of Comparative Example 1, Comparative Example 2, and Comparative Example 3 after the heat resistance test are shown in FIG. 4 (Comparative Example 1: Profile with 12 in FIG. 4, Comparative Example 2: in FIG. 4) Profile with 13 and Comparative Example 3: Profile with 13 in FIG.

  Table 2 shows the X-ray diffraction results and OSC measurement results after the heat resistance test.

＜評価結果の比較＞
図１、図２より実施例１から９、比較例１から３の試料は還元性雰囲気焼成によりすべてパイロクロア構造を有する複合酸化物が得られている。図１、図２の２θ＝３７°付近の（３３１）ピークがパイロクロア構造であることを示している。表１にＯＳＣ評価結果を示したが、実施例１から９、比較例１から３の試料はすべてＣｅ含有量のほぼ１００％近くがＯＳＣに関与する結果であった。

<Comparison of evaluation results>
1 and 2, all of the samples of Examples 1 to 9 and Comparative Examples 1 to 3 were obtained as complex oxides having a pyrochlore structure by firing in a reducing atmosphere. The (331) peak in the vicinity of 2θ = 37 ° in FIGS. 1 and 2 indicates a pyrochlore structure. Table 1 shows the results of the OSC evaluation. In all of the samples of Examples 1 to 9 and Comparative Examples 1 to 3, almost 100% of the Ce content was related to OSC.

  The results after the heat resistance test are shown in FIGS. From FIG. 3, the samples of Examples 1 to 9 maintain the κ structure in which the valence of Ce is oxidized to 4+ even after the heat-resistant treatment. Table 2 shows the OSC maintenance rate, and all of the samples of Examples 1 to 9 showed a high maintenance rate of 85% or more. On the other hand, it was confirmed from FIG. 4 that the samples of Comparative Examples 1 to 3 lost the (331) peak after the heat treatment and the κ structure was broken. Table 2 shows the OSC retention rate, but the samples of Comparative Examples 1 to 3 had a low retention rate of 65% or less.

  From the above results, the composite oxides of Examples 1 to 9 have a pyrochlore structure and are a composite oxide containing Ce and Zr having high OSC and high heat resistance.

It is a figure which shows the X-ray-diffraction pattern of the complex oxide obtained in Example 1, Example 4, Example 5, and Example 6 (1: Example 1, 2: Example 4, 3: Example 5). 4: Example 6). It is a figure which shows the X-ray-diffraction pattern of the complex oxide obtained by the comparative example 1, the comparative example 2, and the comparative example 3 (5: Comparative example 1, 6: Comparative example 2, 7: Comparative example 3). It is a figure which shows the X-ray-diffraction pattern after the heat resistance test of the complex oxide obtained in Example 1, Example 4, Example 5, and Example 6 (8: Example 1, 9: Example 4, 10: Example 5, 11: Example 6). It is a figure which shows the X-ray-diffraction pattern after the heat resistance test of the composite oxide obtained by the comparative example 1, the comparative example 2, and the comparative example 3 (12: Comparative example 1, 13: Comparative example 2, 14: Comparative example) 3).

Claims (4)

A composite oxide containing Ce and Zr, wherein the composite oxide has a pyrochlore structure represented by the following composition formula when the valence of Ce is trivalent.
_{(Ce 1-x-y,} RE x, AE y) 2 (Zr 1-y, M y) 2 O 7
RE is at least one kind of rare earth ions other than Ce, AE is at least one kind of Ca, Sr and Ba, M is at least one kind of Nb and Ta, 0 ≦ x ≦ 0.9, 0 ≦ y ≦ 0.9, 0.4 ≦ x + y ≦ 0.9. The composite oxide according to claim 1, wherein 0.4 ≦ x ≦ 0.9 and y = 0. The composite oxide according to claim 1 or 2, wherein RE comprises at least one of La, Nd, Sm, Gd, and Y. An exhaust gas purification catalyst comprising a noble metal supported on the composite oxide according to any one of claims 1 to 3.

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