Pyrochlore oxide powder

Abstract

[Problem] To provide a bismuth-ruthenium-oxygen type pyrochlore oxide powder in which the particle size distribution is controlled so as to be advantageous for improving dispersibility in a liquid medium and the intermingling of impurity phases is remarkably reduced. [Solution] A pyrochlore oxide powder that is a powder that has a crystal structure of a bismuth-ruthenium-oxygen type pyrochlore oxide and is composed of particles containing bismuth and ruthenium or containing bismuth, ruthenium, and manganese as metal elements, wherein the cumulative 50% particle diameter D50 in a volume-based particle size distribution determined using a laser diffraction scattering method is 3.0 μm or smaller, and a bismuth-containing phase different from the crystal structure of the pyrochlore oxide is not detected in an X-ray diffraction pattern.

Description

Pyrochlore oxide powder

[0001] The present invention relates to a powder of a pyrochlore oxide (hereinafter, this type of composite oxide may be referred to as a "BRO oxide") that has a crystalline structure of a bismuth (Bi)-ruthenium (Ru)-oxygen (O) type pyrochlore oxide and contains bismuth and ruthenium, or bismuth, ruthenium, and manganese (Mn) as its constituent metal elements.

[0002] BRO oxide is a material that is expected to be used as a cathode catalyst in air secondary batteries and hydrogen production processes by water electrolysis.

[0003] Patent Document 1 describes a manganese-containing BRO oxide catalyst (claim 1). The inclusion of manganese is said to improve catalytic activity and other effects. In an experiment to evaluate the properties of the oxide catalyst, an NaOH aqueous solution was added to an aqueous solution containing metal salts of raw materials in a predetermined ratio, and the mixture was stirred at 75°C while oxygen was introduced for 24 hours to produce an oxidation reaction. The resulting product was dried, calcined at 600°C, suction filtered using distilled water, and then dried. (Paragraphs 0027 and 0041) According to the description in paragraph 0027 and Table 2, the Bi / (Ru+Mn) atomic ratio in the raw material mixture in the aqueous solution was calculated to be 0.82 (Example 1), 0.93 (Examples 2 and 3), and 1.00 (Examples 4 to 6).

[0004] Patent Documents 2 to 5 describe BRO oxide catalysts and air secondary batteries using the same. Patent Document 2 discloses that metal components (mainly bismuth) in by-products formed during the production of BRO oxide undergo dissolution-precipitation reactions during the charge-discharge cycle of the battery, resulting in dendrite growth on the electrode plates, which causes micro-short circuits and reduces battery performance and battery life (paragraph 0012). To remove these by-products (see paragraph 0030), Patent Document 2 discloses a manufacturing method in which a washed and dried fired BRO oxide product is subjected to an acid treatment by immersing it in an aqueous nitric acid solution (paragraphs 0051 to 0053). It also describes that in preparing the initial aqueous solution used in the reaction before firing, a bismuth-containing salt and a ruthenium-containing salt are added to achieve the same concentration (paragraphs 0051 and 0077).

[0005] In Patent Document 3, in order to further improve the energy efficiency and power output of air secondary batteries (see paragraph 0009), a method is adopted in which a secondary heat treatment is performed to form ruthenium oxide on the surface of the catalyst (paragraphs 0053 to 0056). In the process of obtaining a water-washed and dried BRO oxide product to be subjected to the secondary heat treatment, an acid treatment is performed by immersing the product in a nitric acid aqueous solution to remove by-products, as in Patent Document 2 (paragraph 0052). In Patent Document 3, too, in preparing the initial aqueous solution used in the reaction before calcination, the bismuth-containing salt and the ruthenium-containing salt are added so that they have the same concentration (paragraphs 0045 and 0082).

[0006] In the production process of a BRO oxide catalyst disclosed in Patent Document 4, similarly to Patent Document 2, an acid treatment is carried out by immersion in an aqueous nitric acid solution to remove by-products (paragraphs 0041 to 0043), and in preparing the initial aqueous solution used in the reaction before calcination, a bismuth-containing salt and a ruthenium-containing salt are added so as to have the same concentration (paragraphs 0040 and 0065).

[0007] In Patent Document 5, in order to further improve the energy efficiency and power output of air secondary batteries (see paragraph 0012), a method is adopted in which a powdered precursor is added with an aqueous sodium hydroxide solution, dried, and then calcined (paragraphs 0046 to 0047).In this manufacturing process, as in Patent Document 2, an acid treatment is performed by immersion in an aqueous nitric acid solution to remove by-products (paragraphs 0041 to 0043), and in preparing the initial aqueous solution used in the reaction before calcination, a bismuth-containing salt and a ruthenium-containing salt are added so that they have the same concentration (paragraphs 0046 and 0074).

[0008] International Publication No. 2020 / 153401 Japanese Patent Application Laid-Open No. 2019-179592 Japanese Patent Application Laid-Open No. 2020-80291 Japanese Patent Application Laid-Open No. 2020-126754 Japanese Patent Application Laid-Open No. 2021-99914

[0009] As described in Patent Documents 2 to 5, by-products containing raw material metals (such as bismuth) generated in the BRO oxide production process are factors that reduce the performance and lifespan of secondary batteries that use the BRO oxide as a catalyst. In order to prevent such by-products from remaining in the catalyst material, it has been necessary to subject the synthesized BRO oxide to an acid pickling treatment, such as immersing it in an aqueous nitric acid solution. The addition of an acid pickling treatment step increases the environmental load and the production costs of the BRO oxide catalyst.

[0010] To utilize BRO oxide as a catalyst for electrochemical reactions, a powder product of BRO oxide is often dispersed in a predetermined liquid medium to form a suspension or paste, which is then supported on a conductor. In this process, BRO oxide can exhibit catalytic activity upon contact with substances involved in electrochemical reactions (such as an electrolyte or oxygen gas). Therefore, a larger specific surface area of ​​BRO oxide is preferred, since a larger contact area with these substances is expected to result in higher catalytic activity. BRO oxide powder particles typically exist as secondary particles formed by agglomeration of many primary particles. Generally, reducing the particle size (secondary particle diameter) of the powder particles is an effective method for improving the specific surface area of ​​powder particles. Furthermore, reducing the secondary particle diameter is expected to improve the dispersion stability of BRO oxide in a liquid medium (smaller particles tend to maintain a dispersed state without settling when the liquid medium is left standing), which is advantageous in terms of production.

[0011] Conventionally, finishing BRO oxide into a powder product consisting of fine particles, for example, with an average particle size on the submicron order, by final dispersion treatment has not generally been carried out, as can be seen from the examples of Patent Documents 1 to 5. It is also unknown what residual behavior the above-mentioned by-products exhibit after the final dispersion treatment. An object of the present invention is to provide a BRO oxide powder that has a particle size distribution controlled to be advantageous for improving dispersibility in a liquid medium and in which the presence of impurity phases is significantly reduced.

[0012] According to the inventors' research, when BRO oxide synthesized by a conventional process in which a precursor formed by a wet oxidation reaction is calcined is subjected to a final dispersion treatment to reduce the powder particle size, the presence of bismuth-containing impurity phases (mainly basic bismuth carbonates) that were not present before the dispersion treatment becomes apparent. Since basic bismuth carbonates thermally decompose at temperatures lower than the calcination temperature, they are thought to be absent in the calcined BRO oxide powder. Therefore, the impurity phases that become apparent after the dispersion treatment are presumed to be the result of excess bismuth components (such as bismuth hydroxide and bismuth oxide) that were present between the primary particles being exposed to the surface during the dispersion treatment and reacting with carbon dioxide in the atmosphere to form carbonates. The presence of such excess bismuth components on the catalyst surface can impair the movement of substances involved in the electrochemical reaction, thereby impairing catalytic activity. Therefore, the inventors conducted detailed studies and found that by slightly reducing the atomic ratio of bismuth used as a raw material to the total number of ruthenium and manganese atoms to less than equal, it is possible to significantly prevent the appearance of impurity phases containing bismuth after dispersion treatment. In Patent Document 1, the atomic ratio of bismuth used as a raw material to the total number of ruthenium and manganese atoms is set to a range of 0.82 to 1.00 (as described above). However, a final dispersion treatment for refinement is not performed, and Patent Document 1 does not teach a method for achieving both refinement and prevention of impurity phase formation.

[0013] The above object can be achieved by the following inventions: [1] A powder composed of particles having a bismuth-ruthenium-oxygen pyrochlore oxide crystal structure and containing bismuth and ruthenium, or bismuth, ruthenium, and manganese as metal elements, wherein the cumulative 50% particle diameter D50 in a volume-based particle size distribution determined by a laser diffraction / scattering method is 3.0 μm or less, and no bismuth-containing phase different from the crystal structure of the pyrochlore oxide is detected in an X-ray diffraction pattern. [2] The pyrochlore oxide powder according to [1] above, wherein the Mn / (Ru+Mn) atomic ratio, which represents the atomic ratio of manganese to the total amount of ruthenium and manganese, is 0 or more and 0.35 or less.

[0014] The pyrochlore oxide powder according to the present invention has a controlled, fine particle size, which is advantageous for improving catalytic activity and dispersibility in liquid media. Furthermore, the presence of impurity phases is significantly reduced, which is advantageous for improving the performance of devices such as secondary batteries that use the powder as a catalyst. The method for producing pyrochlore oxide powder according to the present invention eliminates the need for an acid washing step to remove impurity phases, thereby minimizing environmental impact. Furthermore, it is possible to significantly increase the amount of metal salt raw material mixed per liter of initial aqueous solution used in the reaction (hereinafter sometimes referred to as the "feed concentration") compared to the feed concentration typically used in laboratory experiments. Therefore, the present invention contributes to the industrial spread of pyrochlore oxide powder.

[0015] 5 is a diagram illustrating the particle size distribution of the BRO oxide powder before and after dispersion treatment obtained in Comparative Example 1. FIG. 6 is a diagram illustrating X-ray diffraction patterns of the BRO oxide powder before and after dispersion treatment obtained in Comparative Example 1. FIG. 7 is a diagram enlarged in the peak height direction of the diffraction pattern of FIG. 2. FIG. 8 is a diagram illustrating the particle size distribution of the BRO oxide powder before and after dispersion treatment obtained in Example 1. FIG. 9 is a diagram illustrating X-ray diffraction patterns of the BRO oxide powder before and after dispersion treatment obtained in Example 1. FIG. 10 is a diagram enlarged in the peak height direction of the diffraction pattern of FIG. 5. FIG. 11 is a diagram illustrating the particle size distribution of the BRO oxide powder before and after dispersion treatment obtained in Example 2. FIG. 12 is a diagram illustrating X-ray diffraction patterns of the BRO oxide powder before and after dispersion treatment obtained in Example 2. FIG. 13 is a diagram enlarged in the peak height direction of the diffraction pattern of FIG. 8. FIG. 14 is a diagram illustrating the particle size distribution of the BRO oxide powder before and after wet dispersion obtained in Example 3. 14 is a diagram illustrating the X-ray diffraction patterns of the BRO oxide powder before and after wet dispersion obtained in Example 3. 15 is a diagram enlarged in the peak height direction of the diffraction pattern of FIG. 11. 16 is a diagram illustrating the particle size distribution of the BRO oxide powder before and after wet dispersion obtained in Example 4. 17 is a diagram illustrating the X-ray diffraction patterns of the BRO oxide powder before and after wet dispersion obtained in Example 4. 18 is a diagram enlarged in the peak height direction of the diffraction pattern of FIG. 14. 19 is a diagram illustrating the X-ray diffraction patterns of the BRO oxide powder (obtained in Comparative Example 2, Examples 5 and 6) before dispersion treatment, the BRO oxide powder having an Mn / (Ru+Mn) atomic ratio of 0.30. 19 is a diagram substituted for drawing, showing the appearance of the liquids at the start of standing, which were prepared by forming the BRO oxide powder before and after dispersion treatment produced in Example 2 into slurries. 19 is a diagram substituted for drawing, showing the appearance of the liquid of FIG. 17 after standing for 15 hours.

[0016] [Pyrochlore oxide] The typical composition formula of pyrochlore oxide is A ２ B ２ O xwhere x is 7 or a value close to it. "Bismuth-ruthenium-oxygen pyrochlore oxide" refers to a pyrochlore oxide in which the A site is bismuth (Bi) and the B site is ruthenium (Ru). The pyrochlore oxide (BRO oxide) targeted by the present invention has a crystal structure in which diffraction peaks from each crystal plane of a "bismuth-ruthenium-oxygen pyrochlore oxide" are observed in an X-ray diffraction pattern. When manganese is included as a metal element, it is generally believed that an atomic arrangement is exhibited in which some of the ruthenium atoms occupying the B site are replaced by manganese atoms. Furthermore, atomic arrangements in which atoms other than bismuth, ruthenium, and manganese replace some of the bismuth atoms occupying the A site or some of the ruthenium atoms occupying the B site, or occupy interstitial positions, are also permitted as long as they do not impair the effects of the present invention. For example, sodium (Na) atoms, which may be introduced into the liquid during the manufacturing process, are believed to be present in the crystal lattice by replacing some of the bismuth atoms occupying the A site or by occupying interstitial positions. When manganese or the above-mentioned sodium is present in the crystal lattice, the crystal plane spacing may vary slightly compared to the pure "bismuth-ruthenium-oxygen pyrochlore oxide."

[0017] As described above, manganese can be used as an element to substitute for a portion of ruthenium in BRO oxide, which is useful for saving expensive ruthenium. The Mn / (Ru+Mn) atomic ratio, which represents the atomic ratio of manganese to the total amount of ruthenium and manganese, can be set in the range of 0 to 0.35, and may be controlled to 0.3 or less. If manganese is not contained, the Mn / (Ru+Mn) atomic ratio will be 0. If manganese is contained, it is effective to set the Mn / (Ru+Mn) atomic ratio to 0.05 or more, and more effective to set it to 0.1 or more.

[0018] [Particle Size Distribution] The BRO oxide powder of the present invention has a particle size distribution controlled to a small average particle size. Specifically, the cumulative 50% particle size D50 in the volume-based particle size distribution measured by a laser diffraction / scattering method is 3.0 μm or less. BRO powder with such a small average particle size is advantageous for exhibiting good catalytic activity and dispersion retention properties in a liquid medium. The D50 is more preferably 2.0 μm or less, and even more preferably 1.0 μm or less. The BRO powder particles can be made finer by a dispersion treatment using, for example, a bead mill, as described below. Excessive particle size reduction can lead to a decrease in catalytic activity or productivity due to the breakdown of the crystal structure. Usually, the D50 can be adjusted to a range of 0.05 μm or more.

[0019] [Impurity Phase] If a bismuth-containing impurity phase is present on the particle surface of the BRO oxide powder, it may hinder the improvement of the performance of devices such as secondary batteries that use the BRO oxide as a catalyst. In the BRO oxide powder of the present invention, no bismuth-containing phase different from the crystal structure of "bismuth-ruthenium-oxygen pyrochlore oxide" is detected in the X-ray diffraction pattern. A representative example of a bismuth-containing phase different from the crystal structure of "bismuth-ruthenium-oxygen pyrochlore oxide" is basic bismuth carbonate (Bi ２ O ２ CO ３ ) are examples of such impurity phases containing bismuth. These impurity phases are likely to become apparent through the final micronization achieved by the dispersion process. Furthermore, there is a concern that the presence of such impurity phases on the surface of the "bismuth-ruthenium-oxygen pyrochlore oxide" may result in a decrease in catalytic activity. As will be described later, the formation of these impurity phases can be prevented by adjusting the blending ratio of the metal salt raw materials in the metal salt-containing liquid preparation step.

[0020] [Method for Producing BRO Oxide Powder] The BRO oxide powder of the present invention can be obtained by a production process including, in the above order, a metal salt-containing liquid preparation step, an oxidation step, a drying step, a calcination step, and a dispersion treatment step.

[0021] [Stock Solution Preparation Process] A bismuth-containing salt (e.g., bismuth(III) nitrate hydrate) and a ruthenium-containing salt (e.g., ruthenium(III) chloride hydrate) are prepared as stock materials. If manganese is to be added, a manganese-containing salt (e.g., manganese(II) nitrate hydrate) is also prepared. These are mixed in an aqueous solvent to produce a metal salt-containing solution. This initial metal salt-containing solution is referred to herein as the "stock solution." An aqueous solvent is a liquid medium whose main component is water (i.e., the mass ratio of water is 50% or more). Because bismuth-containing salts and ruthenium-containing salts have low solubility in water, when they are mixed with water, most of them remain undissolved as solids in the water, forming a slurry upon stirring. Therefore, in the present invention, it is preferable to use an aqueous solvent in which an organic substance acting as a dispersant is dissolved as a secondary component other than water. An example of a dispersant is tetra-n-propylammonium bromide.

[0022] Since the stoichiometric atomic ratio of the A site to the B site in BRO oxide is 1:1, assuming that bismuth occupies the entire A site and ruthenium and manganese occupy the entire B site, theoretically, when raw metal salts are blended so that the Bi / (Ru + Mn) atomic ratio is 1.000, the A site and B site can be occupied by the above elements in equal amounts. However, when a raw material solution is prepared with such a blending ratio, it was found that a bismuth-containing impurity phase is formed by the final dispersion treatment. Therefore, the inventors conducted detailed studies. As a result, the problem of impurity phase formation was resolved by setting the Bi / (Ru + Mn) atomic ratio to 0.990 or less when preparing the raw material solution. If the Bi / (Ru + Mn) atomic ratio is too low, the amount of excess ruthenium increases, which impairs economic efficiency. Typically, the Bi / (Ru + Mn) atomic ratio should be set in the range of 0.900 or more, and may be controlled to 0.940 or more.

[0023] When manganese is contained, the Mn / (Ru+Mn) atomic ratio can be set to a range of 0.35 or less, and may be controlled to 0.3 or less. In these cases, it is effective to set the Mn / (Ru+Mn) atomic ratio to 0.05 or more, and more effective to set it to 0.1 or more.

[0024] In preparing the raw material solution, the metal salt concentration (molar concentration of the metal salt contained per 1 L of the liquid containing the raw material and dispersant) is preferably set so that the bismuth content in terms of atomic number is 0.1 mol / L or more, and more effectively 0.3 mol / L or more, from the viewpoint of productivity. If the metal salt concentration is too high, the amount of alkali added in the oxidation step described below will increase, which will increase the cooling load of the equipment to suppress heat generation during this process and limit the rate of alkali addition. Typically, the metal salt concentration is preferably set so that the bismuth content in terms of atomic number is 0.5 mol / L or less. When a dispersant is used, for example, in the case of tetra-n-propylammonium bromide, the dispersant concentration in the raw material solution may be adjusted to a range of 0.05 to 0.50 mol / L.

[0025] [Oxidation Step] An alkali is added to the raw material solution, and an oxidation reaction is carried out while supplying oxygen to the solution, forming a solid reaction product. A higher solution temperature favors the progress of the dissolution-precipitation reaction. On the other hand, to ensure operational safety, it is desirable to keep the solution temperature below the boiling point. Therefore, the solution temperature is preferably in the range of 50 to 99°C. For example, sodium hydroxide can be used as the alkali. The amount of alkali added is at least the amount necessary to react with all of the anions provided by the metal-containing salts mixed in the solution (bismuth-containing salt and ruthenium-containing salt; however, if manganese-containing salt is also mixed, the amount is the bismuth-containing salt, ruthenium-containing salt, and manganese-containing salt). The alkali is added while stirring the solution, either all at once, intermittently, or continuously, depending on the cooling capacity of the reaction vessel, so that the solution temperature does not exceed the above-mentioned temperature range. Thereafter, the solution temperature is maintained within the above-mentioned temperature range, and stirring is continued while oxygen is supplied to the solution. Oxygen can be supplied by blowing oxygen gas or air into the solution (bubbling). During this process, a reaction product is slowly formed as the dissolution-precipitation reaction proceeds. The stirring time under oxygen supply is preferably 1 to 72 hours. The oxygen supply may be initiated before the alkali is added. The oxidation reaction proceeds in this manner, forming a solid reaction product containing bismuth and ruthenium, and possibly manganese as well. It is believed that this reaction product is primarily composed of oxyhydroxides of the above metals.

[0026] (Example of a method for calculating the amount of alkali required to react with all of the anions supplied from the bismuth-containing salt, the ruthenium-containing salt, and the manganese-containing salt) For example, when NaOH is used as the alkali, Bi(NO ３ ) ３ ・5H ２ O: 0.95 mol, RuCl ３ ・nH ２ O: 0.7 mol, Mn(NO ３ ) ２ ・6H ２ If the solution contains 0.3 moles of Na ions, NO ３Since the Cl ions and Cl ions are all monovalent, the amount of alkali required to react with all of the anions supplied from the bismuth-containing salt, ruthenium-containing salt, and manganese-containing salt is 3 × 0.95 + 3 × 0.7 + 2 × 0.3 = 5.55 moles.

[0027] [Drying-out Process] The reaction product produced by the oxidation reaction is recovered and then dried. One method for recovering the reaction product involves allowing a liquid containing the reaction product to stand for, for example, 24 hours or more to allow the solid reaction product to settle and produce a supernatant, which is then removed. If necessary, a sedimentation promotion procedure such as centrifugation may be performed. It is effective to remove an amount of supernatant that is, for example, 70% or more by volume relative to the liquid containing the reaction product. Drying-out can be performed by exposing the reaction product (solid content) together with the remaining liquid components to air at, for example, 80 to 140°C. In this way, a precursor substance is formed. The resulting product can then be crushed until it passes through a sieve with a mesh size of 100 μm, followed by a process such as drying at a temperature of 180 to 250°C.

[0028] [Firing step] The obtained precursor material is fired by heating in an oxidizing gas atmosphere to synthesize an oxide powder composed of particles having a bismuth-ruthenium-oxygen pyrochlore oxide crystal structure. The firing temperature is preferably set to 400 to 700°C, and the firing time is preferably set in the range of 0.1 to 12 hours. Air under atmospheric pressure can be used as the oxidizing gas atmosphere.

[0029] Conventionally, BRO oxide synthesized by calcination is generally subjected to washing and crushing as appropriate to remove adhering salts, and is then used in various applications as a final powder product. In the present invention, a dispersion treatment is then further carried out.

[0030] [Dispersion Treatment Step] The oxide powder synthesized in the firing step and, if necessary, washed with water or crushed to pass through a sieve with a mesh size of, for example, 100 μm, is subjected to a dispersion treatment using an aqueous solvent. A bead mill is preferably used for this purpose. The beads may be, for example, zirconia (ZrO ２It is preferable to use ceramic spheres such as bismuth hydroxide, bismuth oxide, etc. From the viewpoint of efficiently obtaining a pyrochlore oxide powder with a small D50, the average particle size of the beads is preferably 0.03 mm to 10 mm, more preferably 0.1 mm to 5 mm. This dispersion process breaks down coarse secondary particles to obtain a powder composed of fine secondary particles. In addition to adjusting the particle size, this dispersion process also has the effect of washing out impurity components remaining between the primary particles (e.g., metal components remaining in excess of the formation of BRO oxide crystals, and sodium salts (sodium chloride, sodium bromide, etc.) derived from the dispersant and raw materials). The presence of excess bismuth components (bismuth hydroxide, bismuth oxide, etc.) can generate impurity phases such as basic carbonates, which can form on the surface of the particles after dispersion. In the present invention, the atomic ratio of bismuth is slightly reduced to less than equal to the total number of ruthenium and manganese atoms in the raw material solution preparation process, thereby avoiding the problem of impurity phase formation. The fine particle size achieved by this dispersion treatment provides good catalytic activity and good dispersion retention properties in the liquid medium.

[0031] As described above, it is desirable that the final particle size distribution of the BRO oxide be such that the cumulative 50% particle diameter D50 in the volume-based particle size distribution as determined by the laser diffraction / scattering method is 3.0 μm or less. The D50 is more preferably 2.0 μm or less, and even more preferably 1.0 μm or less. Usually, the D50 can be adjusted to a range of 0.05 μm or more. BRO oxide powder having such a desired particle size distribution can be obtained directly by the dispersion treatment described above, but the particle size can also be adjusted by performing a classification operation after the dispersion treatment, if necessary.

[0032] Comparative Example 1 (Stock Solution Preparation Step) A 10 L separable beaker (manufactured by Asahi Seisakusho) was placed on a mantle heater (manufactured by Asahi Seisakusho), and 2,500 g of pure water was added and heated to 75°C while stirring. An anchor blade was used for stirring at a rotation speed of 200 rpm. A dispersant solution of 0.521 mol (141.5 g) of tetra-n-propylammonium bromide (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.) dissolved in 500 g of pure water was added with stirring. Furthermore, 2.083 mol (1,015 g) of bismuth(III) nitrate pentahydrate (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.) and 2.083 mol (466.3 g) of ruthenium(III) chloride n-hydrate (manufactured by Tanaka Kikinzoku Kogyo Co., Ltd.) were prepared as homogeneously turbid slurries in 2,000 g of pure water and added. The total amount of pure water used was 7,000 g. After adding each substance, the mixture was stirred for 1 hour at a liquid temperature of 75°C. Because bismuth(III) nitrate pentahydrate and ruthenium(III) chloride n-hydrate have low solubility in water, they could not be completely dissolved even at 75°C, resulting in a slurry-like raw solution. In this example, no manganese-containing salt was added, and the Bi / (Ru + Mn) atomic ratio in this raw solution = Bi / Ru atomic ratio was 1.000. The concentration of the raw metal salts was 0.2976 mol / L, in terms of the bismuth content in terms of atomic number. This concentration was calculated based on the liquid volume of 7 L, which is the total amount of pure water used.

[0033] (Oxidation Step) A sodium hydroxide solution with a concentration of 18 mol / L was prepared and added to the raw material solution. The amount of sodium hydroxide added was set to provide an amount of sodium that was 1.08 times the atomic ratio of the amount of sodium required to react with the total amount of anions provided by bismuth(III) nitrate pentahydrate and ruthenium(III) chloride n-hydrate. The entire amount of sodium hydroxide was added over 7.5 minutes to prevent excessive temperature rise. The solution temperature during the sodium hydroxide addition was maintained at approximately 75°C using a temperature controller.

[0034] Next, pure oxygen gas was blown into the liquid in the reaction vessel while maintaining the temperature at 75°C and stirring was continued for 18 hours to form a solid reaction product. The pure oxygen gas was supplied at a rate of approximately 100 mL / min per liter of liquid in the reaction vessel, calculated as a gas volume at room temperature and atmospheric pressure.

[0035] (Drying-out step) The liquid in the reaction tank after stirring (slurry in which the reaction product was suspended) obtained above was left to stand for 5 days, and then the supernatant was removed. Finally, by a method of centrifuging, 70% or more of the supernatant was removed relative to the volume of the "liquid in the reaction tank" to recover the reaction product. The recovered reaction product was dried and solidified by holding it together with the remaining liquid components in air at 120°C for 18 hours, thereby forming a precursor substance.

[0036] This precursor material was crushed into powder, sieved through a sieve with 100 μm openings, dried by keeping the material on the sieve in air at 200°C for 2 hours, and then crushed in a mortar to prepare a precursor material for firing.

[0037] (Firing Step) The precursor material was fired by holding it in air at 600° C. for 1 hour to synthesize BRO oxide.

[0038] The synthesized BRO oxide was stirred and washed with water to remove adhering salts, and then the solid content was recovered by suction filtration and dried in air at 120°C to obtain BRO oxide powder.

[0039] (Dispersion Treatment Step) 300 g of the obtained BRO oxide powder was subjected to dispersion treatment using a wet bead mill (1 / 2G-SG, manufactured by Imex Co., Ltd., vessel capacity 2 L). Water was used as the solvent for the dispersion treatment, and zirconia (ZrO ２) beads were used. The amount of solvent was approximately 700 g and the amount of zirconia beads was 1200 g relative to 300 g of BRO oxide powder to be treated, and dispersion treatment was carried out for 6 hours at a rotation speed of 1500 rpm, a rotation radius of 45 mm, and a tip peripheral speed of 7 m / s. During treatment, the solvent temperature was maintained at approximately 25°C by cooling the container with a chiller. The liquid after dispersion treatment was subjected to suction filtration to recover the solid content, which was then dried in air at 100°C, to obtain BRO oxide powder composed of particles refined by the dispersion treatment.

[0040] (Measurement of particle size distribution) The BRO oxide powder before and after dispersion treatment was added to an aqueous solution containing 0.2% by weight of sodium hexametaphosphate as a dispersant, and after 5 minutes of ultrasonic cleaning using an ultrasonic cleaner (AUC-06L, manufactured by AS ONE Corporation), the volume-based particle size distribution was measured using a wet laser diffraction particle size distribution analyzer (MT3300EX, manufactured by Microtrac-Bell Corporation) with water as the solvent and a powder refractive index of 1.9, by laser diffraction / scattering method. As a result, the cumulative 50% particle diameter D50 was 7.4 μm before dispersion treatment and 0.5 μm after dispersion treatment. The particle size distribution measurement values, including D10 and D90, are shown in Table 1 (the same applies to each of the following examples).

[0041] FIG. 1 illustrates the particle size distribution of the BRO oxide powder obtained in this example before and after the dispersion treatment.

[0042] (Measurement of X-ray diffraction pattern) The X-ray diffraction patterns of the BRO oxide powder before and after the dispersion treatment were measured using an X-ray diffractometer (Ultima IV, manufactured by Rigaku) ​​under the conditions of Cu-Kα radiation, tube voltage of 40 kV, tube current of 40 mA, measurement step of 0.02 degrees, and scan speed of 0.5 degrees / min.

[0043] Figure 2 shows the X-ray diffraction patterns of the BRO oxide powder obtained in this example before and after dispersion treatment. Before dispersion treatment, in addition to the diffraction peaks representing the crystal structure of the bismuth-ruthenium-oxygen pyrochlore oxide, no peaks of a bismuth-containing phase different from the crystal structure were observed. However, after dispersion treatment, Bi was observed at the locations indicated by the arrows in Figure 2. ２ O２ CO ３ Diffraction peaks due to Bi were observed. Figure 3 shows the diffraction pattern of Figure 2 enlarged in the peak height direction. After the dispersion treatment, a new diffraction peak (Bi) was observed at the diffraction angle 2θ position indicated by the dashed line. ２ O ２ CO ３ It can be seen that a diffraction peak due to the bismuth-containing phase appears. In this example, it was not possible to achieve a pyrochlore oxide powder in which the cumulative 50% particle diameter D50 was refined to 3.0 μm or less without a detectable bismuth-containing phase. These results are shown in Table 1 (the same applies to each of the following examples).

[0044] (Measurement of Amount of Adhered Salt) The amount of adhering salt was measured for the BRO oxide powder before and after dispersion treatment. Br derived from the dispersant tetra-n-propylammonium bromide and Cl derived from the raw material ruthenium (III) chloride n-hydrate were measured using an ion chromatograph (IC-2100, manufactured by Tosoh Corporation). Na derived from sodium hydroxide was measured using an atomic absorption spectrophotometer (ZA3300, manufactured by Hitachi High-Technologies). The measurement target was a liquid obtained by adding 50 mL of ultrapure water to 1 g of BRO oxide powder, extracting the adhering components into the liquid, and then filtering the liquid. The amounts of adhering salt before dispersion treatment were 640 ppm for Na, 87 ppm for Br, and 103 ppm for Cl, and after dispersion treatment were 160 ppm for Na, 14 ppm for Br, and 18 ppm for Cl.

[0045] Example 1 BRO oxide powder was produced under the same conditions as in Comparative Example 1, except that in the raw material solution preparation step, 0.521 mol of tetra-n-propylammonium bromide, 2.083 mol of bismuth (III) nitrate pentahydrate, and 2.126 mol of ruthenium (III) chloride n-hydrate were mixed to set the Bi / (Ru + Mn) atomic ratio = Bi / Ru atomic ratio in the raw material solution to 0.980. The materials used were the same as in Comparative Example 1 (the same applies to Comparative Example 2 below). The concentration of raw material metal salts in the prepared raw material solution before the addition of alkali was 0.2976 mol / L in terms of the bismuth content in terms of the number of atoms. Particle size distribution and X-ray diffraction pattern measurements were performed using the same methods as in Comparative Example 1.

[0046] FIG. 4 illustrates the particle size distribution of the BRO oxide powder obtained in this example before and after dispersion treatment. The cumulative 50% particle diameter D50 was 7.4 μm before dispersion treatment and 0.9 μm after dispersion treatment. FIG. 5 illustrates the X-ray diffraction patterns of the BRO oxide powder obtained in this example before and after dispersion treatment. FIG. 6 shows the diffraction pattern of FIG. 5 enlarged in the peak height direction. No peak of the bismuth-containing phase was observed even after dispersion treatment. In this example, a pyrochlore oxide powder was achieved in which the cumulative 50% particle diameter D50 was refined to 3.0 μm or less, and no bismuth-containing phase was detected.

[0047] Example 2 BRO oxide powder was produced under the same conditions as in Comparative Example 1, except that in the raw material solution preparation step, 0.521 mol of tetra-n-propylammonium bromide, 2.083 mol of bismuth (III) nitrate pentahydrate, and 2.192 mol of ruthenium (III) chloride n-hydrate were mixed to set the Bi / (Ru + Mn) atomic ratio = Bi / Ru atomic ratio in the raw material solution to 0.950. The concentration of the raw material metal salts in the prepared raw material solution before the addition of alkali was 0.2976 mol / L in terms of the bismuth content in terms of the number of atoms. Measurement of particle size distribution and X-ray diffraction pattern was performed using the same methods as in Comparative Example 1.

[0048] FIG. 7 illustrates the particle size distribution of the BRO oxide powder obtained in this example before and after dispersion treatment. The cumulative 50% particle diameter D50 was 7.4 μm before dispersion treatment and 0.9 μm after dispersion treatment. FIG. 8 illustrates the X-ray diffraction patterns of the BRO oxide powder obtained in this example before and after dispersion treatment. FIG. 9 shows the diffraction pattern of FIG. 8 enlarged in the peak height direction. No peak of the bismuth-containing phase was observed even after dispersion treatment. In this example, a pyrochlore oxide powder was achieved in which the cumulative 50% particle diameter D50 was refined to 3.0 μm or less, and no bismuth-containing phase was detected.

[0049] Example 3 BRO oxide powder was produced under the same conditions as in Comparative Example 1, except that in the raw material solution preparation step, 0.521 mol of tetra-n-propylammonium bromide, 2.083 mol of bismuth (III) nitrate pentahydrate, and 2.314 mol of ruthenium (III) chloride n-hydrate were mixed to set the Bi / (Ru + Mn) atomic ratio = Bi / Ru atomic ratio in the raw material solution to 0.900. The concentration of the raw material metal salts in the prepared raw material solution before the addition of alkali was 0.2976 mol / L in terms of the bismuth content in terms of the number of atoms. Measurement of particle size distribution and X-ray diffraction pattern was performed using the same methods as in Comparative Example 1.

[0050] FIG. 10 illustrates the particle size distribution of the BRO oxide powder obtained in this example before and after dispersion treatment. The cumulative 50% particle diameter D50 was 4.9 μm before dispersion treatment and 0.5 μm after dispersion treatment. FIG. 11 illustrates the X-ray diffraction patterns of the BRO oxide powder obtained in this example before and after dispersion treatment. FIG. 12 shows the diffraction pattern of FIG. 11 enlarged in the peak height direction. No peak of the bismuth-containing phase was observed even after dispersion treatment. In this example, a pyrochlore oxide powder was achieved in which the cumulative 50% particle diameter D50 was refined to 3.0 μm or less, and no bismuth-containing phase was detected.

[0051] Example 4 BRO oxide powder was produced under the same conditions as in Comparative Example 1, except that in the raw material solution preparation step, 0.521 mol of tetra-n-propylammonium bromide, 2.083 mol of bismuth (III) nitrate pentahydrate, and 2.604 mol of ruthenium (III) chloride n-hydrate were mixed to set the Bi / (Ru + Mn) atomic ratio = Bi / Ru atomic ratio in the raw material solution to 0.800. The concentration of the raw material metal salts in the prepared raw material solution before the addition of alkali was 0.2976 mol / L in terms of the bismuth content in terms of the number of atoms. Measurement of particle size distribution and X-ray diffraction pattern was performed using the same methods as in Comparative Example 1.

[0052] FIG. 13 illustrates the particle size distribution of the BRO oxide powder obtained in this example before and after dispersion treatment. The cumulative 50% particle diameter D50 was 4.9 μm before dispersion treatment and 0.5 μm after dispersion treatment. FIG. 14 illustrates the X-ray diffraction patterns of the BRO oxide powder obtained in this example before and after dispersion treatment. FIG. 15 shows an enlarged view of the diffraction pattern in FIG. 14 in the peak height direction. No peak of the bismuth-containing phase was observed even after dispersion treatment. In this example, a pyrochlore oxide powder was achieved in which the cumulative 50% particle diameter D50 was refined to 3.0 μm or less, and no bismuth-containing phase was detected.

[0053] Comparative Example 2 (Stock Solution Preparation Step) A 1 L separable beaker (manufactured by Asahi Seisakusho) was placed on a mantle heater (manufactured by Asahi Seisakusho), and 150 g of pure water was added and heated to 75°C while stirring. An anchor blade was used for stirring at a rotation speed of 300 rpm. A dispersant solution of 0.0521 mol (14.15 g) of tetra-n-propylammonium bromide (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.) dissolved in 50 g of pure water was added with stirring. Furthermore, 0.2083 mol (101.5 g) of bismuth(III) nitrate pentahydrate (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.) and 0.1458 mol (32.64 g) of ruthenium(III) chloride n-hydrate (manufactured by Tanaka Kikinzoku Kogyo Co., Ltd.) were each prepared as homogeneously turbid slurries in 200 g of pure water and added. Additionally, 0.0625 mol (17.94 g) of manganese(II) nitrate hexahydrate (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.) was dissolved in 100 g of purified water and added. The total amount of purified water used was 700 g. After adding each substance, the mixture was stirred for 1 hour at a liquid temperature of 75°C. Because bismuth(III) nitrate pentahydrate and ruthenium(III) chloride n-hydrate have low solubility in water, they could not be completely dissolved even at 75°C, resulting in a slurry-like raw material solution. In this example, a manganese-containing salt was added, and the Bi / (Ru + Mn) atomic ratio in this raw material solution was 1.000, and the Mn / (Ru + Mn) atomic ratio was 30. The feed concentration of the raw material metal salt was 0.2976 mol / L in terms of bismuth content in terms of number of atoms. This feed concentration was calculated assuming the liquid volume was 0.7 L, which is the total amount of purified water used.

[0054] (Oxidation Step) A sodium hydroxide solution with a concentration of 18 mol / L was prepared and added to the raw material solution. The amount of sodium hydroxide added was set to provide an amount of sodium that was 1.08 times the atomic ratio of the amount of sodium required to react with the total amount of anions provided by bismuth(III) nitrate pentahydrate, ruthenium(III) chloride n-hydrate, and manganese(II) nitrate hexahydrate. The entire amount of sodium hydroxide was added over 7.5 minutes to prevent excessive temperature rise. The solution temperature during the sodium hydroxide addition was maintained at approximately 75°C using a temperature controller.

[0055] Next, pure oxygen gas was blown into the liquid in the reaction vessel while maintaining the temperature at 75°C and stirring was continued for 18 hours to form a solid reaction product. The pure oxygen gas was supplied at a rate of approximately 100 mL / min per liter of liquid in the reaction vessel, calculated as a gas volume at room temperature and atmospheric pressure.

[0056] Using this reaction product, the steps from the "drying step" described in Comparative Example 1 onwards were carried out in the same manner as in Comparative Example 1 to produce BRO oxide powder. Measurement of particle size distribution and measurement of X-ray diffraction pattern were carried out in the same manner as in Comparative Example 1.

[0057] The cumulative 50% particle diameter D50 of the BRO oxide powder obtained in this example was 12.2 μm before dispersion treatment. Figure 16 shows X-ray diffraction patterns before dispersion treatment for a BRO oxide powder having an Mn / (Ru+Mn) atomic ratio of 0.30, when the Bi / (Ru+Mn) atomic ratio of the raw material solution was 1.000 (this example), 0.980 (Example 5 described later), and 0.950 (Example 6 described later). In the BRO oxide powder obtained in this example, Bi was present at the location indicated by the arrow in Figure 16 at the stage before dispersion treatment. ２ O ２ CO ３ Diffraction peaks due to

[0058] Example 5: BRO oxide powder was produced under the same conditions as in Comparative Example 2, except that in the raw material solution preparation step, 0.0521 mol of tetra-n-propylammonium bromide, 0.2083 mol of bismuth(III) nitrate pentahydrate, 0.1488 mol of ruthenium(III) chloride n-hydrate, and 0.0638 mol of manganese(II) nitrate hexahydrate were mixed to set the Bi / (Ru+Mn) atomic ratio in the raw material solution to 0.980. The materials used were the same products as in Comparative Example 2. The concentration of the raw material metal salts in the prepared raw material solution was 0.2976 mol / L in terms of the bismuth content in terms of atomic number. Particle size distribution and X-ray diffraction pattern measurements were performed using the same methods as in Comparative Example 1.

[0059] In the X-ray diffraction pattern of the BRO oxide powder obtained in this example after dispersion treatment, no peaks of the bismuth-containing phase were observed, as in the case before dispersion treatment (see Figure 16). In this example, a pyrochlore oxide powder was achieved in which the cumulative 50% particle diameter D50 was reduced to 3.0 µm or less, and no bismuth-containing phase was detected.

[0060] Example 6 BRO oxide powder was produced under the same conditions as in Comparative Example 2, except that in the raw material solution preparation step, 0.0521 mol of tetra-n-propylammonium bromide, 0.2083 mol of bismuth(III) nitrate pentahydrate, 0.1535 mol of ruthenium(III) chloride n-hydrate, and 0.0658 mol of manganese(II) nitrate hexahydrate were mixed to set the Bi / (Ru+Mn) atomic ratio in the raw material solution to 0.950. The concentration of the raw material metal salts in the prepared raw material solution was 0.2976 mol / L in terms of the bismuth content in terms of the number of atoms. Measurement of particle size distribution and X-ray diffraction pattern was performed in the same manner as in Comparative Example 1.

[0061] The cumulative 50% particle diameter D50 of the BRO oxide powder obtained in this example was 3.9 μm before dispersion treatment and 0.33 μm after dispersion treatment. In the X-ray diffraction pattern of the BRO oxide powder obtained in this example after dispersion treatment, no peak of the bismuth-containing phase was observed, as in the pattern before dispersion treatment (see FIG. 16 ). In this example, a pyrochlore oxide powder in which the cumulative 50% particle diameter D50 was refined to 3.0 μm or less and no bismuth-containing phase was detected was achieved.

[0062]

[0063] [Comparison of dispersibility of BRO oxide powder in liquid before and after dispersion treatment] Sodium hexametaphosphate was added as a dispersant to the BRO oxide powder before and after dispersion treatment produced in Example 2, and a slurry-like liquid was produced by ultrasonically cleaning the powder for 5 minutes using an ultrasonic cleaner (AUC-06L, manufactured by AS ONE Corporation), and the liquid was allowed to stand at room temperature. Fig. 17 shows an example of an external photograph of the liquid at the start of standing, and Fig. 18 shows an example of an external photograph of the liquid after standing for 15 hours. It can be seen that the dispersibility of the BRO oxide powder in liquid is significantly improved by micronizing the particles by dispersion treatment.

Claims

1. A powder composed of particles having a crystalline structure of a bismuth-ruthenium-oxygen pyrochlore oxide and containing bismuth and ruthenium, or bismuth, ruthenium, and manganese as metal elements, in which the cumulative 50% particle diameter D50 in a volume-based particle size distribution determined by a laser diffraction / scattering method is 3.0 μm or less, and in which no bismuth-containing phase different from the crystalline structure of the pyrochlore oxide is detected in the X-ray diffraction pattern.

2. The pyrochlore oxide powder according to claim 1, wherein the atomic ratio of Mn / (Ru+Mn), which represents the atomic ratio of manganese to the total amount of ruthenium and manganese, is 0 or more and 0.35 or less.

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