JP7652379B2 - Metal composite oxide and method for producing same - Google Patents

Description

The present invention relates to a metal composite oxide and a method for producing the same.

Many oxides having perovskite or perovskite-like crystal structures have unique electrical or magnetic properties and are used in a variety of applications, including electronic components. For example, the following documents disclose technology related to perovskite-type oxides and/or oxides having a perovskite-like Rdolphin-Popper oxide structure:

Patent Document 1 (JP 2009-263225 A) discloses a compound of the general formula: (M ¹ _x1 M ² _X2 ) _(m+1) (Ti _(1-y-z) M ³ _y M ⁴ _z ) _m O _3m+1-δ (M ¹ is an alkaline earth metal, M ² is a rare earth metal, M ³ is a specific transition metal, and M ⁴ is a specific transition metal, rare earth metal, or base metal), and describes that the compound is known as a Rdolphin-Popper structure derived from perovskite, and that the compound can be used in electrodes for electrochemical devices (claims 3, [0040] and [0048] of Patent Document 1).

Patent document 2 (JP Patent Publication 2015-198065A) discloses a metal-air secondary battery that includes a positive electrode having a Rudolfden-Popper type layered perovskite oxide and a solid electrolyte having a Rudolfden-Popper type layered perovskite oxide that has been subjected to a humidification reduction treatment, and describes that the reaction overvoltage at the positive electrode can be significantly reduced (claims 1 to 5 and [0012] of patent document 2).

Patent Document 3 (JP Patent Publication No. 2014-047307) discloses a photochromic material in which rare earth ions are contained in oxide crystals having a Rdolphin-Popper type perovskite structure, and states that oxide crystals having a Rdolphin-Popper type perovskite structure are suitable for exhibiting a photochromic phenomenon (claims 1 to 5 and [0014] of Patent Document 3). Furthermore, Patent Document 4 (JP Patent Publication No. 10-114570) discloses a method for producing a shape-anisotropic ceramic powder consisting of ceramic particles having a Rdolphin-Popper type layered perovskite structure, in which ceramic raw materials are heated in a melt or solution (claim 2 of Patent Document 4).

Thus, oxides having perovskite or a perovskite-like crystal structure are used in various applications. Among these, oxides containing strontium (Sr) and transition metals (Me) in particular often exhibit catalytic functions. Catalysts are the core materials in clean energy technology. For this reason, perovskite or oxides having a perovskite-like crystal structure containing strontium (Sr) and transition metals (Me) are expected to be used as catalytic materials that contribute to oxidation-reduction reactions (ORR) and oxygen evolution reactions (OER) in fields such as fuel cells, metal-air batteries, and water splitting.

The larger the surface area of a catalyst, the more active sites it has, and the better its catalytic performance. Therefore, it is desirable for the catalyst material to have a large specific surface area, i.e., to be fine. Also, in order to achieve excellent catalytic performance, it is desirable for the catalyst material to contain as few foreign phases as possible.

Complex polymerization is a method widely used to synthesize metal oxide catalysts with large specific surface areas. In this method, a metal salt is dissolved in a glycol solution containing an oxycarboxylic acid such as citric acid to form a metal oxycarboxylic acid complex. The solution is then heated to form a polymer gel through a dehydration ester reaction. Finally, this gel is calcined and pyrolyzed to form a powder material. With this method, uniformity of the metal ions is achieved in the starting solution, and this uniformity is maintained in the polymer gel. As a result, it is said that the calcination temperature (pyrolysis temperature) can be lowered, and a fine, uniform powder material can be obtained.

An example of a document disclosing such a technique is Patent Document 5 (JP Patent Publication 2009-214033 A). Patent Document 5 describes a photocatalyst having a structure in which a co-catalyst is supported on the surface of a catalyst substance made of a layered perovskite compound represented by the general formula: ALa ₄ Ti ₄ O ₁₅ , and describes that a catalyst substance made of a layered perovskite compound can be obtained by a complex polymerization method, that is, by mixing a raw material corresponding to the layered perovskite compound to be synthesized with a solvent such as ethanol, reacting them by heating, and calcining the obtained reaction product (catalyst precursor) (claims 1 to 5 and [0025] of Patent Document 5).

JP 2009-263225 A JP 2015-198065 A JP 2014-047307 A Japanese Patent Application Publication No. 10-114570 JP 2009-214033 A

Perovskite or oxides with a perovskite-like crystal structure containing strontium (Sr) and a transition metal (Me) are expected to be useful as catalytic materials. In addition, complex polymerization is often used to synthesize metal oxide catalysts. However, when the metal oxide contains a cation with a large ionic radius such as strontium (Sr), it is difficult to synthesize a compound with a uniform composition even with the complex polymerization method. This is because it is difficult to maintain a complex containing a cation with a large ionic radius stably for a long period of time. As a result, the cation precipitates during the drying of the solvent, compromising the uniformity of the composition.

In particular, when the goal is to synthesize a Rdolphin-Popper type compound, which has a crystal structure similar to that of perovskite, even a slight deviation in composition can result in the production of a compound with different crystal symmetry and crystal structure. For this reason, it is difficult to synthesize a single-phase Rdolphin-Popper type compound by pyrolyzing the gel while maintaining a homogeneous state at the atomic level. On the other hand, in order to achieve a homogenous composition, it is possible to pyrolyze the gel using a complex polymerization method to produce a compound, which is then repeatedly crushed and fired. However, when the complex polymerization method is used, a fine single-phase compound cannot be obtained even with repeated crushing and firing.

The present inventors have conducted extensive research into the synthesis of a metal composite oxide having a perovskite or perovskite-like crystal structure that contains strontium (Sr) as the A-site element and a metal element (M) as the B-site element, and as a result have discovered that a compound with fine particles and few heterophases can be synthesized by freezing and solidifying a raw material solution containing the components of the metal composite oxide and a solvent, sublimating the solvent while this solid is still frozen to form a metal salt mixture, and heating the resulting metal salt mixture, thereby completing the present invention.

The present invention therefore aims to provide a manufacturing method that can obtain a fine compound with few heterophases, even if it is a metal composite oxide having a perovskite or perovskite-like crystal structure that contains strontium (Sr) as the A-site element and a metal (M) as the B-site element.

The present invention encompasses the following aspects (1) to (11). In this specification, the expression "-" includes both ends of the expression, i.e., "X to Y" is synonymous with "X or more and Y or less."

(1) A method for producing a metal composite oxide having a perovskite or perovskite-like crystal structure, which contains strontium (Sr) as an A-site element and a metal element (M) as a B-site element, comprising the steps of:
preparing a raw material solution containing components of the metal composite oxide and a solvent;
a step of freezing and solidifying the raw material solution, and sublimating and removing the solvent from the resulting solidified product while it is in a frozen state to obtain a metal salt mixture; and a step of heating the metal salt mixture to obtain a metal composite oxide.
A method comprising:

(2) The method according to (1) above, wherein the metal composite oxide is a Rudolfden-Popper type compound represented by the general formula: A _n+1 B _n O _3n+1 (where 0<n≦4).

(3) The method of (1) or (2) above, wherein the metal element (M) is one or more selected from the group consisting of titanium (Ti), zirconium (Zr), iron (Fe), copper (Cu), manganese (Mn), cobalt (Co), nickel (Ni), niobium (Nb), tantalum (Ta), tin (Sn), ruthenium (Ru), iridium (Ir), rhodium (Rh), chromium (Cr), hafnium (Hf), aluminum (Al) and magnesium (Mg).

(4) The method of (1) above, wherein the metal composite oxide is a Rdolphin-Popper type compound containing strontium (Sr) and a rare earth metal (Re) as A-site elements and a metal element (M) as a B-site element.

(5) The method according to (4) above, wherein the metal composite oxide has a composition represented by the general formula Re _2-x Sr _x MO _4±δ (where 0.5≦x≦1.5, 0≦δ≦0.25).

(6) The method according to (4) or (5) above, wherein the rare earth metal (Re) is one or more selected from the group consisting of lanthanum (La), praseodymium (Pr), neodymium (Nd), samarium (Sm), europium (Eu), gadolinium (Gd), terbium (Tb), dysprosium (Dy), holmium (Ho), yttrium (Y) and erbium (Er).

(7) Any of the methods (4) to (6) above, wherein the metal element (M) is at least one selected from the group consisting of nickel (Ni), cobalt (Co), manganese (Mn), iron (Fe), titanium (Ti), zirconium (Zr), hafnium (Hf), niobium (Nb), tantalum (Ta), tin (Sn), ruthenium (Ru), iridium (Ir), rhodium (Rh), chromium (Cr) and copper (Cu).

(8) The method according to any one of (4) to (7) above, wherein the metal composite oxide is in the form of a powder having a BET specific surface area of 0.4 m ² /g or more.

(9) Any of the methods (1) to (8) above, wherein the raw material solution is an aqueous solution of one or more components selected from the group consisting of nitrates, acetates, chlorides, and sulfates.

(10) Any of the methods (1) to (9) above, in which the raw material solution does not contain organic matter.

(11) A metal composite oxide having a perovskite or perovskite-like crystal structure, which contains strontium (Sr) as an A-site element and a metal element (M) as a B-site element,
The metal composite oxide is a Rudolfden-Popper type crystal compound represented by the general formula: A _n+1 B _n O _3n+1 (where 0<n≦4),
The metal element (M) is at least one selected from the group consisting of titanium (Ti), zirconium (Zr), iron (Fe), copper (Cu), manganese (Mn), cobalt (Co), nickel (Ni), niobium (Nb), tantalum (Ta), tin (Sn), ruthenium (Ru), iridium (Ir), rhodium (Rh), chromium (Cr), hafnium (Hf), aluminum (Al), and magnesium (Mg);
The metal composite oxide is a powder having a BET specific surface area of 2.0 m ² /g or more.

The present invention provides a manufacturing method that can obtain a fine compound with few heterophases, even if it is a metal composite oxide having a perovskite or a perovskite-like crystal structure that contains strontium (Sr) as an A-site element and a metal element (M) as a B-site element.

1 shows an X-ray diffraction pattern of an example sample. 4 shows an SEM image and compositional surface analysis results of the example sample. 1 shows a linear sweep voltammogram of an example sample. 1 shows an X-ray diffraction pattern of a comparative sample. 4 shows an SEM image and compositional surface analysis results of a comparative sample.

Manufacturing method of metal composite oxide In the manufacturing method of a metal composite oxide of the present invention, the metal composite oxide contains strontium (Sr) as an A-site element and a metal element (M) as a B-site element. In addition, this metal composite oxide has a perovskite or a crystal structure similar to that of a perovskite. Here, perovskite is an oxide containing an A-site element and a B-site element and having a crystal structure represented by the general formula: _ABO3 . In addition, the crystal structure similar to that of a perovskite refers to a double-perovskite, Ruddlesden-Popper and/or Dion-Jacobson type structure.

The method for producing a metal composite oxide of the present invention includes the following steps: preparing a raw solution containing the components of the metal composite oxide and a solvent (raw solution preparation step), freezing and solidifying the raw solution, and sublimating and removing the solvent from the resulting solidified product while it is still frozen to obtain a metal salt mixture (freeze-drying step), and heating the metal salt mixture to obtain a metal composite oxide (calcination step). Each step will be described in detail below.

<Raw material solution adjustment process>
In the raw solution preparation step, a raw solution containing the constituent components (A-site elements and B-site elements) of the metal composite oxide is prepared. The raw solution is a solution in which the inorganic acid salts of the constituent components are dissolved in a solvent. In this respect, the raw solution is clearly distinguished from a slurry or suspension in which solid components are suspended. The inorganic acid salt is preferably one or more selected from the group consisting of nitrates, acetates, chlorides, and sulfates. Furthermore, the solvent may be selected from those having a melting point of around 0° C. and a high vapor pressure. Examples of such solvents include water, cyclohexane, butanol, and the like. However, water is preferred because it is easy to handle. Therefore, the raw solution is preferably an aqueous solution of one or more selected from the group consisting of nitrates, acetates, chlorides, and sulfates of the constituent components. Such a raw solution may be prepared by a known method. For example, the raw solution may be prepared by dissolving the inorganic acid salts of the constituent components, such as nitrates, in a solvent such as water. Alternatively, the raw solution may be prepared by dissolving the metal, oxide, or hydroxide of the constituent components in an inorganic acid such as nitric acid.

The raw material solution preferably does not contain organic matter. When synthesizing a compound by the complex polymerization method, an oxycarboxylic acid such as citric acid is added to an aqueous solution of a metal salt to prepare an oxycarboxylic acid complex, which is then heated and dried to form a complex gel. Alternatively, a glycol such as ethylene glycol is dissolved in a solution of a metal citrate complex to form a polymer gel. In either case, in the complex polymerization method, an organic matter (polymer) is contained in the gel, and this organic matter must be thermally decomposed. Therefore, if the constituent metals contain a metal that is prone to forming carbonates, undesirable carbonates are likely to be formed during thermal decomposition. In contrast, in the manufacturing method of the present invention, it is not necessary to add organic matter such as oxycarboxylic acid or glycol to the raw material solution. Therefore, carbonates are unlikely to be formed, and high-temperature heat treatment to thermally decompose them is not required.

<Freeze-drying process>
In the freeze-drying process, the raw material solution is frozen and solidified, and the solvent is sublimated and removed from the resulting solidified material while it is still frozen to obtain a metal salt mixture. By freezing and solidifying the raw material solution, even if a raw material solution containing strontium (Sr), which has a large ionic radius, is used, all metal ions are frozen and solidified while maintaining a uniform state in the form of metal salts in the solution. Therefore, it is possible to make the distribution of metal ions in the resulting solidified material uniform at the atomic level.

Freezing and solidifying the raw material solution can be performed by cooling the raw material solution to its freezing temperature. The cooling temperature can be determined according to the component composition of the raw material solution and is not limited to any particular temperature. For example, when the raw material solution is an aqueous solution, the raw material solution can be brought into contact with or poured into a refrigerant at -10°C or lower. In this case, it is preferable to perform freezing quickly, as this will result in a more uniform distribution of metal ions in the solidified product. Therefore, it is preferable to rapidly cool the raw material solution. As a rapid cooling method, for example, liquid nitrogen at -196°C can be used as the refrigerant, and the raw material solution can be brought into contact with or poured into this liquid nitrogen. It is also effective to turn the raw material solution into fine droplets and spray them into a refrigerant such as liquid nitrogen. By turning the raw material solution into fine droplets, the raw material solution is instantly frozen. The liquid nitrogen can be removed by evaporating it after freezing and solidifying.

Next, the solvent is removed by sublimation while the solidified material is still in a frozen state, to obtain a metal salt mixture. The form of the obtained metal salt mixture is not particularly limited, but may be, for example, a powder. Metal ions are uniformly distributed in the frozen solidified material. Therefore, by removing the solvent by sublimation while the material is still in this state, the metal ions maintain a uniform distribution in the metal salt mixture after the solvent is removed. In contrast, if the raw material solution is heated and dried to evaporate and remove the solvent, the metal components will segregate in the dried material due to differences in the solubility of the metal components, and a uniform metal salt mixture cannot be obtained.

Sublimation of the solvent is carried out under reduced pressure. The lower the atmospheric pressure, the higher the sublimation rate. The atmospheric pressure is preferably 1 kPa or less, more preferably 100 Pa or less, and even more preferably 50 Pa or less. The higher the atmospheric temperature, the higher the sublimation rate. Therefore, as long as the solidified material remains frozen, the higher the atmospheric temperature, the more preferable it is. However, if the solidified material melts, it becomes necessary to dry the solvent, which will impair the uniform distribution of the metal ions. Therefore, when the solvent is removed by sublimation, it is necessary to maintain the solidified material in a frozen state.

<Firing process>
In the calcination process, the metal salt mixture is heated to form a compound. Heating causes the decomposition of the metal salts and also causes reactions between the constituent components to form a compound. The metal ions contained in the metal salt mixture are uniformly distributed. Therefore, by using such a metal salt mixture, it is possible to synthesize a compound at a low temperature. As a result, grain growth can be suppressed, and a fine compound with few heterogeneous phases can be obtained. In contrast, in the metal salt mixture obtained by heating and drying the raw material solution, the metal components are segregated. Therefore, in order to obtain a single-phase compound, the synthesis temperature must be high, and a fine compound cannot be obtained.

The heating temperature in the calcination step may be determined according to the type of raw material and the target compound, and is not limited to any particular temperature. However, the heating temperature is preferably 500 to 1500°C, and more preferably 700 to 1300°C. The atmosphere during heating is not limited as long as the desired compound is obtained. However, an oxygen-containing atmosphere such as an oxygen atmosphere or an air atmosphere is preferable. By heating in an oxygen-containing atmosphere, it is possible to prevent the resulting compound (metal composite oxide) from being excessively reduced. In addition, heating in the calcination step may be performed multiple times. For example, the metal salt mixture may be pre-calcined to obtain a pre-calcined product, and the resulting pre-calcined product may be subjected to main calcination. In this case, the pre-calcined product may be crushed, and the crushed pre-calcined product may be subjected to main calcination. By performing pre-calcination and main calcination in this way, or by inserting a crushing step, the synthesis of the compound can be further promoted.

The manufacturing method of the present invention makes it possible to obtain a fine compound with few heterophases, even if it is a metal composite oxide having a perovskite or perovskite-like crystal structure that contains strontium (Sr) as the A-site element and a metal element (M) as the B-site element. This compound can be used for various purposes, including as a catalyst, and is particularly useful as a material for an oxygen reduction catalyst and/or an oxygen reaction catalyst.

Metal Composite Oxide The metal composite oxide of the present invention contains strontium (Sr) as an A-site element and a metal element (M) as a B-site element, and has a perovskite or a perovskite-like crystal structure. Here, perovskite is an oxide containing an A-site element and a B-site element and having a crystal structure represented by the general formula: _ABO3 . The perovskite-like crystal structure refers to a double-perovskite, Ruddlesden-Popper, and/or Dion- _Jacobson type structure. Here, the double perovskite is a crystal structure represented by the general formula: _A2BB'O6 , and the Ruddlesden-Popper is a crystal structure represented by the general formula: An _{+ 1BnO3n +1} . Moreover, Dion-Jacobson has a crystal structure represented by the general formula: A'A _n-1 B _n O _3n+1 . Compounds having these crystal structures have a crystal structure similar to that of perovskite, and exhibit properties similar to those of perovskite compounds.

The metal composite oxide is preferably a compound having a Rudolfden-Popper type crystal structure (Rudolfden-Popper type compound) represented by the general formula: A _n+1 B _n O _3n+1 (where 0<n≦4). The Rudolfden-Popper type crystal structure is a type of layered perovskite type structure, and has a structure in which [ABO ₃ ] layers and [AO] layers are alternately stacked. As can be seen from this, the Rudolfden-Popper type compound has a crystal structure similar to that of a perovskite compound (ABO ₃ ), and therefore has a variety of properties such as giant magnetoresistance, superconductivity, ferroelectricity and catalytic activity.

In the above general formula, as long as the desired crystal structure is obtained, the composition fluctuation is allowed. For example, in the general formula _ABO3 of perovskite, the ratio of A to B does not need to be strictly "1:1", and as long as the perovskite crystal structure is shown, the deviation of the ratio is allowed. In addition, depending on the valence of the ions in the crystal structure, lattice defects such as oxygen vacancies may exist, in which case the amount of oxygen (O) in the general formula may deviate.

The metal composite oxide of the present invention contains strontium (Sr) and a metal element (M) other than strontium (Sr) as a constituent. That is, strontium (Sr) occupies the A site of the crystal structure, and the metal element (M) occupies the B site. The metal element (M) may be a transition metal (Me). The metal element (M) is preferably one or more selected from the group consisting of titanium (Ti), zirconium (Zr), iron (Fe), copper (Cu), manganese (Mn), cobalt (Co), nickel (Ni), niobium (Nb), tantalum (Ta), tin (Sn), ruthenium (Ru), iridium (Ir), rhodium (Rh), chromium (Cr), hafnium (Hf), aluminum (Al), and magnesium (Mg).

As long as the perovskite or perovskite-like crystal structure is maintained, the A site may contain other components (ions) other than strontium (Sr). For example, a portion of strontium (Sr) may be substituted with other alkaline earth metal cations such as barium (Ba) or calcium (Ca) that are divalent. However, the proportion of strontium (Sr) occupying the A site is preferably 50 atomic % or more, more preferably 75 atomic % or more, and even more preferably 90 atomic % or more. The A site may be occupied only by strontium (Sr). In addition, a portion of the oxygen ions (O ^2- ) in the compound may be substituted with other anions (Cl ^- , F ^-, etc.) such as chlorine or fluorine.

The metal complex oxide may be a Rdolphin-Popper type compound containing strontium (Sr) and a rare earth metal (Re) as A-site elements and a metal element (M) as a B-site element. Here, rare earth metal (Re) is a general term for elements ranging from lanthanum (La) with atomic number 57 to lutetium (Lu) with atomic number 71 in the periodic table, plus scandium (Sc) with atomic number 21 and yttrium (Y) with atomic number 39. In this way, Rdolphin-Popper type compounds containing strontium (Sr), a rare earth metal (Re) and a metal element (M) exhibit various functions, including catalytic function.

This metal composite oxide may have a composition represented by the general formula Re2 _{- xSrxMO4 ±δ} (where 0.5≦x≦1.5, 0≦δ≦0.25), where the rare earth metal (Re) is preferably at least one selected from the group consisting of lanthanum (La), praseodymium (Pr), neodymium (Nd), samarium (Sm), europium (Eu), gadolinium (Gd), terbium (Tb), dysprosium (Dy), holmium (Ho), yttrium (Y) and erbium (Er). The metal element (M) is preferably at least one selected from the group consisting of nickel (Ni), cobalt (Co), manganese (Mn), iron (Fe), titanium (Ti), zirconium (Zr), hafnium (Hf), niobium (Nb), tantalum (Ta), tin (Sn), ruthenium (Ru), iridium (Ir), rhodium (Rh), chromium (Cr) and copper (Cu). The metal element (M) may be a transition metal (Me). Here, transition metal (Me) is a general term for elements present between group 3 elements and group 11 elements in the periodic table. In addition, in the above general formula, fluctuations in the composition are allowed as long as a desired crystal structure is obtained. For example, the ratio of the total molar amount of strontium and rare earth metals to the molar amount of the metal element (Sr+Re:M) does not need to be strictly "2:1", and deviations in the ratio are allowed as long as the Rdolphin-Popper type crystal structure is exhibited.

The metal composite oxide is preferably a powder having a BET specific surface area of 0.4 m ² /g or more, more preferably a powder having a BET specific surface area of 2.0 m ² /g or more. The upper limit of the BET specific surface area is not particularly limited. However, it is typically 10.0 m ² /g or less. After the synthesis of the metal composite oxide, it is also effective to perform ultrafine pulverization to increase the specific surface area without newly generating a different phase. For the ultrafine pulverization, a wet medium stirring type pulverizer such as a bead mill or an attritor, or a mechanical pulverization such as a planetary mill or a ball mill is effective, but is not limited to these. In addition, when stress distortion occurs in the oxide particles due to the ultrafine pulverization, the stress distortion can be removed by heat treatment at a temperature at which sintering between the particles does not occur. The heat treatment temperature at which sintering between the particles does not occur varies depending on the material, but is generally 400 to 800°C.

The heterogeneous phase content of the metal composite oxide is preferably 10% by mass or less, more preferably 5% by mass or less, and even more preferably 1% by mass or less. The heterogeneous phase content may be 0% by mass, i.e., a single phase. In this way, by making the metal composite oxide have a high specific surface area and a low heterogeneous phase content, various functions such as catalytic performance can be excellent.

An example of a metal composite oxide is an oxide that has a perovskite or perovskite-like crystal structure containing strontium (Sr) as an A-site element and a metal element (M) as a B-site element, is a Rudolfden-Popper type compound represented by the general formula: A _n+1 B _n O _3n+1 (where 0<n≦4), in which the metal element (M) is one or more selected from the group consisting of titanium (Ti), zirconium (Zr), iron (Fe), copper (Cu), manganese (Mn), cobalt (Co), nickel (Ni), niobium (Nb), tantalum (Ta), tin (Sn), ruthenium (Ru), iridium (Ir), rhodium (Rh), chromium (Cr), hafnium (Hf), aluminum (Al) and magnesium (Mg), and is a powder having a BET specific surface area of 2.0 m ² /g or more.

In this way, according to the present invention, even if a compound (metal composite oxide) having a perovskite or a perovskite-like crystal structure containing strontium (Sr) as the A-site element and a metal element (M) as the B-site element is obtained in a fine state with few heterogeneous phases. The present invention is particularly useful for synthesizing Rudersden-Popper type compounds and compounds containing metals that easily form carbonates. Such compounds can be used for various purposes including as catalysts, and are particularly useful as materials for oxygen reduction catalysts and/or oxygen reaction catalysts.

In contrast, according to conventional methods, it was difficult to produce a perovskite or a compound with a perovskite-like crystal structure containing strontium (Sr), which has a large ionic radius, in a fine state with few heterophases. In particular, when a Rudersden-Popper type compound is synthesized, compounds with different crystal symmetries and crystal structures are likely to be produced, and therefore it was difficult to produce a single-phase compound. In addition, when a compound contains a metal that easily forms carbonates, such as lanthanum (La), it was difficult to produce such a compound in a fine state with few heterophases. This was because carbonates were produced during synthesis, and high-temperature heat treatment was required to decompose the carbonates.

The present invention will be further illustrated by the following examples.

Example 1
(1) Preparation of Compounds <Raw Material Solution Preparation Step>
Lanthanum nitrate hexahydrate (La(NO ₃ ) _{3.6H 2} O, purity 99.9%, Kanto Chemical Co., Ltd.), strontium nitrate (Sr(NO ₃ ) ₂ , purity 98.0%, Kanto Chemical Co., Ltd.) and nickel nitrate hexahydrate (Ni(NO ₃ ) _{2.6H 2} O, purity 98%, Kanto Chemical Co., Ltd.) were mixed in a ratio that would result in the stoichiometric composition of LaSrNiO ₄ to prepare a raw material solution consisting of a nitrate aqueous solution (0.03 M). Note that LaSrNiO ₄ corresponds to the general formula: Re _2-x Sr _x MO _4±δ , where the rare earth metal (Re) is lanthanum (La), the metal element (M) is nickel (Ni), x = 1.0, and δ = 0.

<Freeze-drying process>
100 ml of the obtained raw material solution (aqueous nitrate solution) was placed in a Teflon (registered trademark) beaker. Next, liquid nitrogen was placed around the beaker to rapidly cool and freeze the raw material solution. The frozen solution was depressurized to 20 Pa and left for three days to dry without melting the solvent (water). As a result, a metal salt mixture consisting of a uniform dry powder was obtained.

<Firing process>
The obtained metal salt mixture (dry powder) was placed in an alumina crucible and calcined in air at 700-1000°C for 1 hour to obtain a mixed powder of oxides and carbonates. The obtained mixed powder (calcined powder) was pulverized in an agate mortar and placed in the alumina crucible again and calcined in air at 1200°C for 8 hours to obtain a product (compound). This product was a powder consisting of completely oxides.

(2) Evaluation of the Product The properties of the resulting product were evaluated as follows.

<X-ray diffraction>
The crystal phase of the product was identified by powder X-ray diffraction (XRD) under the following analytical conditions:

-X-ray diffraction equipment: X'Pert Pro MRD (Spectris Corporation)
- Radiation source: CuKα (Kβ filter: Ni film)
-Tube voltage: 45 kV
-Tube current: 40mA
- Scan speed: 5.6°/min - Scan range (2θ): 20 to 60°

<BET specific surface area>
The BET specific surface area of the product was measured. The measurement was performed as follows. Using a fully automatic gas adsorption measurement device AS1-MP (manufactured by Quantachrome), the measurement was performed by the BET method, which is a gas adsorption method using krypton gas. The sample was degassed in vacuum at 100°C for 5 hours, and then the specific surface area was measured at the temperature under liquid nitrogen.

<Structural Observation>
The product was subjected to secondary electron image (SEM) observation using a scanning electron microscope (ULTRA55 manufactured by Carl Zeiss) and composition (Ni, Sr and a) surface analysis using an energy dispersive X-ray analyzer (EDS) (Quan Tax manufactured by BRUCKER) to evaluate the uniformity of the structure and composition.

<Oxidation-reduction activity>
The oxygen reduction activity of the obtained product was evaluated by convection voltammetry using a rotating disk electrode (RDE). A disk electrode (surface area: 0.196 cm ² ) made of glassy carbon was used. The evaluation was performed as follows.

Using the obtained product (powder), a catalyst ink was prepared by the following procedure. First, ion-exchanged water and 2-propanol were mixed in a ratio of 4:1 to prepare a solvent for the ink, and 5 mg of the synthesized product (powder) was mixed with 5 mL of this solvent, 0.16 mL of a 10 wt% Nafion solution (527114 manufactured by SIGMA-ALDRICH) and 21.3 mg of acetylene black (VLCAN XC72 manufactured by Cabot Corporation). The resulting mixture was ultrasonically dispersed for 30 minutes using a homogenizer while cooling the surroundings with water to prepare a catalyst ink. Next, 8.3 μL of this catalyst ink was dropped onto a glassy carbon electrode using a microsyringe, and the mixture was dried using hot air to form a catalyst electrode film of 0.042 mg/cm ^2. The rotating disk electrode on which the catalyst electrode film was formed was attached to a potentiostat, and the electrode surface was faced downward and brought into contact with the water surface of a three-electrode cell filled with an alkaline aqueous solution. In this state, the electrochemical reaction was measured while the disk was rotated.

When a disk is rotated in water, the amount of aqueous solution flowing onto the electrode surface changes depending on the rotation speed. In general, three types of phenomena contribute to electrode reactions: (A) diffusion of reactants to the electrode surface, (B) charge transfer on the electrode surface, and (C) escape of products from the electrode surface. However, convective voltammetry makes it possible to evaluate the effect of (B), which is due to the nature of the material, by observing electrochemical reactions in which the convective speed is changed in the electrolyte.

The measurement was carried out as follows. An oxygen-saturated 1.0 M potassium hydroxide aqueous solution obtained by bubbling oxygen for 30 minutes was used as the electrolyte, and the catalytic electrode was placed in the solution. The measurement temperature was 25°C, the reference electrode was a reversible hydrogen electrode (RHE), and the counter electrode was a platinum wire. Using a potentiostat (HZ-5000 manufactured by Hokuto Denko Corporation), the potential scanning speed was set to 5 mVs ^-1 , and the current value was measured while rotating the electrode at a rotation speed of 1600 rpm.

(3) Results The X-ray diffraction pattern of the product (metal composite oxide) obtained in Example 1 is shown in Figure 1. In addition, Figure 1 also shows the diffraction lines _{of LaSrNiO4} (space group F4/mmm), _LaSrNiO4 (space group I4/mmm), Sr(OH) _2.H2O , _SrCO3 , and NiO recorded in the powder X _- ray diffraction intensity database (Powder Diffraction File; PDF).

As shown in Fig. 1, only the diffraction _lines of LaSrNiO4 (PDF#01-089-8315) of space group I4/mmm were observed in the X _- ray diffraction pattern, and the diffraction lines of _SrCO3 (PDF#00-005-0418), Sr(OH) _2.H2O (PDF#01-077-2336) and NiO (PDF#00-047-1049) were not observed. Therefore, it was found that in Example 1, a single phase of _LaSrNiO4 of space group I4/mmm could be synthesized. In addition, this product had a BET specific surface area of 0.43 ^m2 /g, which was a large specific surface area that could exert a catalytic function.

Figure 2 shows an SEM image of the product obtained in Example 1 and the results of surface analysis of the composition (Ni, Sr, La). It was confirmed that this product has a uniform structure and a uniform composition (Ni, Sr, La).

The evaluation results of the oxygen reduction activity of the product of Example 1 are shown in Figure 3. As shown in Figure 3, a linear sweep voltammogram (LSV) was obtained. This confirmed that the product exhibits oxygen reduction activity.

Comparative Example 1
(1) Preparation of Compounds <Raw Material Solution Preparation Step>
The synthesis of _LaSrNiO4 was attempted by the citrate complex method. First, lanthanum nitrate hexahydrate (La( _NO3 ) _3.6H2O , purity _99.9 %, Kanto Chemical Co., Ltd.), strontium nitrate (Sr( _NO3 ) ₂ , purity 98.0%, Kanto Chemical Co., Ltd.) and nickel nitrate hexahydrate (Ni( _NO3 ) _2.6H2O , purity 98% _, Kanto Chemical Co., Ltd.) were dissolved in ion-exchanged water in a ratio that would give the stoichiometric composition of _LaSrNiO4 to prepare a raw material solution consisting of a nitrate aqueous solution (0.02 M).

<Gelling step>
The obtained raw material solution (nitrate aqueous solution) was mixed with a citric acid aqueous solution (0.02M) in an equimolar ratio of citric acid to metal ions to form a citric acid complex of metal ions. Next, the obtained citric acid complex was heated on a hot plate heated to 80°C to evaporate and remove the water solvent to form a gel. It was found that a white precipitate was observed in the green gel, impairing uniformity. When a gel was prepared from lanthanum nitrate hexahydrate and nickel nitrate hexahydrate in the same manner without using strontium nitrate, a white precipitate was not formed and a green, uniform dried product was obtained. From this, it was found that in Comparative Example 1, strontium (Sr) ions did not form a citric acid complex, but formed a white precipitate consisting of a strontium compound.

<Firing process>
The resulting heterogeneous gel was transferred to an alumina crucible and calcined in air at 350°C for 2 hours in an electric furnace to obtain a calcined powder. The calcined powder was taken out into a mortar and crushed. The crushed calcined powder was pre-calcined in an electric furnace at 1000°C for 1 hour and then fired at 1200°C for 8 hours. The resulting product (calcined product) was composed of complete oxides.

(2) Evaluation of the Product For the product obtained in Comparative Example 1, the crystal phase was identified by powder X-ray diffraction (XRD) in the same manner as in Example 1. In addition, SEM images were observed using a scanning electron microscope, and composition (Ni, Sr, La) surface analysis was performed using EDS.

(3) Results The X-ray diffraction pattern of the product obtained in Comparative Example 1 is shown in Figure 4. As shown in Figure 4, the product did not contain by-products such as SrCO ₃ (PDF#00-005-0418), Sr(OH) _{2.H 2} O (PDF#01-077-2336), NiO (PDF#00-047-1049), etc. However, two types of LaSrNiO ₄ with different symmetries (space group I4/mmm (PDF#01-089-8315) and space group F4/mmm (PDF#01-076-9307)) were synthesized in a mixed form.

Figure 5 shows an SEM image of the product obtained in Comparative Example 1 and the results of surface analysis of the composition (Ni, Sr, La). This product had non-uniform particle size and shape, and the composition (Ni, Sr, La) was also non-uniform.

Examples 2 to 6
(1) Preparation of Compounds In the raw material solution preparation step, in the general formula: Re2 _{- xSrxMO4 ±δ} , x was changed to x = 0.4 (Example 2), x = 0.6 (Example 3), x = 0.8 (Example 4), x = 1.2 (Example 5), or x = 1.4 (Example 6), instead of x = 1.0, in the formula: Re2-xSrxMO4±δ, a product was prepared in the same manner as in Example 1.

(2) Evaluation and Results For the products obtained in Examples 2 to 6, the crystal phase was identified by powder X-ray diffraction (XRD) in the same manner as in Example 1. As a result, it was confirmed that a single-phase compound was obtained. In addition, when the oxygen reduction activity was evaluated by convection voltalmetry using a rotating disk electrode in the same manner as in Example 1, a linear sweep voltammogram could be drawn, and it was confirmed that the product exhibited oxygen reduction activity.

Example 7
(1) Preparation of Compound In the raw material solution preparation process, yttrium nitrate hexahydrate (Y(NO ₃ ) _{3.6H 2} O, purity 99.99%, Kanto Chemical Co., Ltd.) was used instead of lanthanum nitrate hexahydrate to synthesize the YSrNiO ₄ compound. A product was prepared in the same manner as in Example 1.

(2) Evaluation and Results For the product obtained in Example 7, the crystal phase was identified by powder X-ray diffraction (XRD) in the same manner as in Example 1. As a result, it was confirmed that a single-phase compound was obtained. In addition, when the oxygen reduction activity was evaluated by convection voltalmetry using a rotating disk electrode in the same manner as in Example 1, a linear sweep voltammogram could be drawn, and it was confirmed that the product exhibited oxygen reduction activity.

Example 8
(1) Preparation of Compound In the raw material solution preparation process, neodymium nitrate hexahydrate (Nd(NO ₃ ) _{3.6H 2} O, purity 99.95%, Kanto Chemical Co., Ltd.) was used instead of lanthanum nitrate hexahydrate to synthesize NdSrNiO ₄ compound. A product was prepared in the same manner as in Example 1.

(2) Evaluation and Results For the product obtained in Example 8, the crystal phase was identified by powder X-ray diffraction (XRD) in the same manner as in Example 1. As a result, it was confirmed that a single-phase compound was obtained. In addition, when the oxygen reduction activity was evaluated by convection voltalmetry using a rotating disk electrode in the same manner as in Example 1, a linear sweep voltammogram could be drawn, and it was confirmed that the product exhibited oxygen reduction activity.

Example 9
(1) Preparation of Compound In the raw material solution preparation process, gadolinium nitrate hexahydrate (Gd(NO ₃ ) _{3.6H 2} O, purity 99.95%, Kanto Chemical Co., Ltd.) was used instead of lanthanum nitrate hexahydrate to synthesize GdSrNiO ₄ compound. A product was prepared in the same manner as in Example 1.

(2) Evaluation and Results For the product obtained in Example 9, the crystal phase was identified by powder X-ray diffraction (XRD) in the same manner as in Example 1. As a result, it was confirmed that a single-phase compound was obtained. In addition, when the oxygen reduction activity was evaluated by convection voltalmetry using a rotating disk electrode in the same manner as in Example 1, a linear sweep voltammogram could be drawn, and it was confirmed that the product exhibited oxygen reduction activity.

Claims (1)

A method for producing a metal composite oxide, which is a material for an oxygen reduction catalyst and has a crystal structure similar to perovskite and contains strontium (Sr) as an A-site element and a metal element (M) as a B-site element, comprising the steps of:
preparing a raw material solution containing components of the metal composite oxide and a solvent;
a step of freezing and solidifying the raw material solution, and sublimating and removing the solvent from the resulting solidified product while it is in a frozen state to obtain a metal salt mixture; and a step of heating the metal salt mixture to obtain a metal composite oxide.
Including,
the metal composite oxide is a Rudolfden-Popper type compound containing strontium (Sr) and a rare earth metal (Re) as A-site elements and a metal element (M) as a B-site element;
The metal composite oxide has a composition represented by the general formula Re2 _{- xSrxMO4 ±δ} (where 0.5≦x≦1.5, 0≦δ≦0.25),
The rare earth metal (Re) is at least one selected from the group consisting of lanthanum (La), neodymium (Nd), gadolinium (Gd), and yttrium (Y);
The metal element (M) is nickel (Ni),
The metal composite oxide is a powder having a BET specific surface area of 0.4 m ² /g or more,
The raw material solution contains components of a nitrate of strontium (Sr), a nitrate of a rare earth metal (Re), a nitrate of a metal element (M), and a solvent ,
The method, wherein the raw material solution does not contain organic substances such as oxycarboxylic acids and glycols .

Images