JP2003063827A - Mixed conductive oxide and method for manufacturing the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain a mixed conductive oxide having high oxygen permeability in case of industrial application and to provide a method for manufacturing the same. SOLUTION: A raw material containing component metals with the ratio represented by the formula (1); La:Sr:Co:Fe=x:1-x:1-y:y (0<x<0.15, 0<y<0.15); is heated at the temperature lower than 1,100 deg.C to obtain a mixed oxide, and then fired at 1,100 deg.C or higher to obtain a dense mixed conductive oxide with 0.7% or less of the open porosity containing lanthanum, strontium, cobalt, and iron as main components having the molecular ratio represented by the formula (1).

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention relates to a mixed conductive oxide preferably used for selectively transporting oxygen in air or an oxygen-containing mixed gas, and a method for producing the mixed conductive oxide.

[0002]

2. Description of the Related Art Mixed conductive oxides are widely known as substances that selectively transport only oxygen components from an oxygen-containing mixed gas such as air. Here, the mixed conductive oxide is an oxide that conducts electrons or holes and simultaneously conducts oxygen ions (also referred to as oxide ions).

In order to industrially carry out selective oxygen transport utilizing such a mixed conductive oxide, the oxygen ion conductivity under the operating conditions needs to be about 0.1 S / cm or more, preferably. , 0.5 S / cm or more, and most preferably 1 S / cm or more. Further, it is necessary that the electronic conductivity has a value equal to or higher than those. That is, the mixed conductive oxide targeted by the present invention refers to an oxide whose electronic conductivity and oxygen ion conductivity are both about 0.1 S / cm or more under operating conditions.

As a mixed conductive oxide satisfying this definition
In the old days, JP-A 56-92103 and JP-A 6-92103
As disclosed in JP 1-21717 A, La_x
Sr _1-xCoO_3-aAnd La_1-xSr_xCo_1-yFe_yO_3-zWhen
Oxides having such a perovskite type crystal structure are known.
ing.

More recently, US Pat. No. 5,356,728.
And No. 5580497,
A typical composition is Sr ₄ Fe _6-x Co _x O _z (13−δ ≦ z ≦ 1
3 + δ), and one of the main crystalline phases is a non-perovskite type (Sr ₄ Fe ₆ in the above publication) at least near room temperature.
The same layered perovskite type as the O ₁₃ compound), and has a crystal structure of 10 to 30 S / cm in the atmosphere.
An oxide having an electronic conductivity of about (800 to 950 ° C.) has been invented.

Further, an academic paper by Teraoka et al. (Journal of the Chemical Society of Japan, vol.1988, No.7, pp1084-1089, 1988, "L"
a _1-x Sr _x Co _1-y Fe _y O ₃ oxygen permeation capacity of perovskite type oxide ”), as a result of changing x and y from 0 to 1.0, except for SrCoO _2.5 , x is A composition having a large y and a small y, for example, SrCo _0.8 Fe _0.2 O ₃
In that case, it is described that a good oxygen permeability can be obtained.

[0007] Teraoka et al. Further published the proceedings of the 68th conference of the Institute of Electrochemistry (1J26, pp175, April 1-3, 2001, "Sr,
“Synthesis and oxygen permeability of La—Sr—Co—Fe-based perovskite in the Co-rich composition region”), La at the A site.
Since the oxygen permeability at low temperature is improved by containing
La-Sr-Co-Fe consisting of four components, and Sr
And Co having the highest possible composition, for example La _0.1
It is described that good oxygen permeability can be obtained with a composition of Sr _0.9 Co _0.9 Fe _0.1 O ₃ . In addition, in this composition, SrCo _0.8
Although it is slightly improved in oxygen permeability at 900 ° C. as compared with Fe _0.2 O ₃ , it is reported that it is effective in improving oxygen permeability in a lower temperature region.

[0008]

However, although the composition of the mixed conductive oxide is examined in the above-mentioned prior art documents, the other conditions such as the state of the sintered body and the manufacturing method are industrially applied. Although it is an indispensable technical element when considering the above, it is scarcely described, and the present situation is that it cannot be said that the examination considering the industrial application is sufficient.

The present invention has been made in view of the above circumstances, and an object of the present invention is to provide a mixed conductive oxide having a high oxygen permeability when applied industrially and a method for producing the same.

[0010]

The present inventors have found that La-Sr-Co-Fe-based oxides having good oxygen permeability are high in industrial applications such as oxygen separation and recovery applications and diaphragm reactor applications. As a result of repeated studies to obtain a mixed conductive oxide having an oxygen permeability, it was found that not only the composition of the mixed conductive oxide but also the sintered body needs to be dense above a certain level. Then, it was found that in order to obtain such a dense sintered body, it is important to control the firing temperature and the temperature during the heat treatment performed prior to firing within a specific range.

The present invention has been made based on such findings, and provides the following (1) to (6).

(1) Lanthanum, strontium, cobalt and iron are the main components, the molar ratio of these component elements is represented by the following formula (1), and the open porosity is 0.7% or less. Characterized mixed conductive oxide. La: Sr: Co: Fe = x: 1-x: 1-y: y (1) (where 0 <x <0.15, 0 <y <0.15)

(2) The raw material compound containing the constituent metals in the composition ratio of the mixed conductive oxide shown in the above (1) is added at 1100 ° C.
A method for producing a mixed conductive oxide, comprising heat-treating at a temperature of less than 1 to obtain a mixed oxide, and then firing at 1100 ° C. or higher.

(3) A method for producing a mixed conductive oxide as described in (2) above, wherein the raw material compound is pulverized and used to have a median diameter of 15 μm or less.

(4) In the above (2) or (3), the heat treatment for converting the raw material compound into a mixed oxide is performed by holding at 800 to 1000 ° C. for 0.1 to 30 hours. And a method for producing a mixed conductive oxide.

(5) In any one of the above (2) to (4), the mixed oxide after the heat treatment has a median diameter of 10
A method for producing a mixed conductive oxide, which is used by pulverizing to a particle size of μm or less.

(6) In the above (2) to (5), the step of firing the mixed oxide is 1100 to 1300 ° C.
The method for producing a mixed conductive oxide is characterized in that the mixed conductive oxide is held for 0.1 to 20 hours.

[0018]

BEST MODE FOR CARRYING OUT THE INVENTION The present invention will be described in more detail below. The mixed conductive oxide according to the present invention contains lanthanum, strontium, cobalt and iron as main components, and the molar ratio of these component elements is represented by the following formula (1),
It is a dense material having an open porosity of 0.7% or less. La: Sr: Co: Fe = x: 1-x: 1-y: y (1) (where 0 <x <0.15, 0 <y <0.15)

In the mixed conductive oxide of the present invention, the cation is L
Chemical formula ABO _x composed of a-Sr-Co-Fe components
It is a mixed oxide that can be expressed as. La and Sr are located at the site represented by A in this chemical formula, while Co and Fe are located at the site represented by B.

When the substitution ratio of Sr with La in the A site is represented by x, 0 <x <0.15. If La is not present, the oxygen transmission rate in the low temperature region of 500 to 900 ° C. is low, but the overall oxygen transmission rate decreases as x increases, so that the above range provides a high oxygen transmission rate. Will be needed.

When the substitution rate of Co in the B site by Fe is represented by y, 0 <y <0.15. If Fe is not present, the symmetry of the crystal structure is lowered, and the presence of Fe is essential because the migration of oxide ions or oxygen vacancies is less likely to occur, but if y is increased to a certain extent or more, the overall oxygen permeability is lowered. Therefore, the above range is required to obtain high oxygen permeability.

The mixed conductive oxide of the present invention is required to have a high oxygen transmission rate (selective oxygen transport rate) when it is applied to oxygen separation and recovery applications and diaphragm reactor applications. It is extremely important not only to have a composition of the mixed conductive oxide, but also to increase the oxygen permeability as a dense microstructure. That is, in the present invention, the La-Sr-Co-Fe mixed conductive oxide having the above composition has a dense structure with an open porosity of 0.7% or less. When the mixed conductive oxide of the present invention is used for separation and recovery of oxygen or a membrane reactor, if there is a physical void in it, atmospheric gas passing through the void is mixed as an impurity and the oxygen permeability is lowered. I will let you. In order to sufficiently suppress the mixing of impurities, it is necessary to set the open porosity to 0.7% or less.

By defining the composition and open porosity as described above, a mixed conductive oxide having a high oxygen permeability can be obtained in industrial applications such as oxygen separation and recovery applications and diaphragm reactor applications. .

Next, a method for producing such a mixed conductive oxide will be described. First, a raw material compound corresponding to the composition of the mixed conductive oxide is prepared. Since the target mixed conductive compound is required to be a single phase, it is advantageous to synthesize it by using an extremely fine solid phase raw material, or a vapor phase or liquid phase compound as a raw material. For example, as shown in the above-mentioned Teraoka et al. Document, nitrates (Fe) and acetates (La, Sr, Co), etc. are used as starting materials for the constituents of the mixed conductive oxide, and the mixture of these is used. It is conceivable to calcine a fine powder obtained from an aqueous solution.

However, in the case of mass production, it is desirable that the raw material itself is inexpensive and the cost in the manufacturing process is also low. With these raw materials, not only the raw material itself is expensive, but also in the manufacturing process. It is also difficult to keep costs low. Specifically, extremely fine solid phase powder not only has a large bulk volume but also easily floats, which requires a large working space and equipment that considers the effects on human health and the surroundings. There is.

In addition, when the liquid phase or gas phase compound is used as the raw material, the above problems are not only the same, but in the case of the liquid phase, equipment and heat supply for removing the solvent are required, If it is a phase, it has a problem that a closed space is required, so that it is difficult to reduce the manufacturing cost.
Further, since the powder is fine, there is a problem that the composition is likely to shift during the calcination and firing steps.

Therefore, in consideration of mass production, it is preferable to obtain the target compound by using a general ceramic raw material. Typical examples of such a raw material include oxides, but carbonates, hydroxides, nitrates, oxalates, sulfates, chlorides and the like may be used in combination.

However, an oxide of an alkaline earth element such as Sr is unstable in air, and by forming a carbonate or a hydroxide during storage, the content of the metal element in the charged weight is an object. It may deviate from the value. In order to avoid this, it is necessary to carry out a sealing process or a facility that can block carbon dioxide and moisture, but even if these measures are taken, it is necessary to pay attention to artificial negligence.

Since the particle diameter of the above-mentioned raw material powder is usually about 3 to 30 μm in median diameter, if these are mixed as they are, sufficient component diffusion does not proceed in the calcination treatment,
In addition to the non-uniform composition, formation of coarse higher-order particles by necking may cause problems such as suppressing the density of the compact and not obtaining a dense sintered body. Regarding the non-uniformity of the composition among these, it is possible to eliminate it by raising the calcination temperature or extending the calcination time, but this reduces the density of the sintered body due to the coarsening of the powder, and the production. There is a problem that causes deterioration of sex. For this reason, in order to obtain a powder that is easy to pulverize so that a dense fired body can be obtained while sufficiently diffusing the constituent elements of the target compound, calcination is performed at the lowest temperature and the shortest time possible. In order not to cause inconvenience, it is necessary to make the powder used fine and uniform.

For this purpose, various media mills such as ball mills, attrition mills, and vibration mills can be used without particular limitation as they are usually used. As the mixing and pulverizing method at this time, there are a dry method using only powder and a wet method using water, alcohol, etc., but any method may be used. The particle size of the raw material powder mixture used for calcination is preferably 15 μm or less in median diameter, and more preferably 0.5 to 3 μm.

Heat treatment, that is, calcination is performed prior to the sintering in order to obtain a powder that is easily crushed so that a dense fired body can be obtained while sufficiently diffusing the constituent elements of the target compound. In order to obtain such powder in this calcination, the calcination temperature is important, and for that purpose, the temperature is set to 1
It should be below 100 ° C. Calcination temperature is 1100 ° C
In the above case, the diffusion of the constituents proceeds and the target phase single phase is formed depending on the conditions. In addition to that, the progress of necking formation between the powders makes it possible to perform a dense firing even if it is directly subjected to the steps after molding. In addition to the case where a lump cannot be obtained, the coarse particles that have grown to a diameter of 10 mm or more sometimes have to be crushed prior to the re-grinding, which complicates the process, and the accompanying production. There are problems that lead to an increase in cost and an increase in the amount of impurities mixed.

More preferable calcining conditions are 800 to 100.
Hold at 0 ° C for 0.1 to 30 hours. Calcination condition is 800
If the temperature is less than ℃ and / or less than 0.1 hours, not only the target phase is less likely to be generated in the powder after calcination, but if carbonate or hydroxide is used as the raw material compound of the alkaline earth element In some cases, decomposition may not be completed, and the effect of calcination may not be sufficiently obtained. On the other hand, if the calcination condition exceeds 1000 ° C. and / or exceeds 30 hours, necking formation between the powders slightly progresses, and the compacts of the sintered body may be slightly inferior when directly subjected to the steps after molding.

Even if the powder is sufficiently mixed / pulverized to diffuse the constituents in the calcination step, and even if the powder is calcinated under appropriate conditions, it is partially due to necking between the powders. Coarse particles of 10 μm are often mixed. It is possible to obtain a dense sintered body by subjecting it to the subsequent steps as it is, but by firing at the lowest possible temperature, and
In order to obtain a dense sintered body with good reproducibility, it is preferable to remove the coarse particles.

It is possible to use a fine filter for removing coarse particles, but in that case, clogging or non-uniformity of components may occur, which is not desirable. In order to avoid such inconvenience, it is preferable to carry out re-grinding in the same manner as mixing and grinding of starting materials. This has the effect of not only avoiding separate equipment investment but also reducing the possibility of impurities being mixed by making production equipment common to other processes. In other words, all processes do not operate every day at the production site in the same way, and especially the operation time of the equipment for the mixing and crushing process is limited. In such a case, another raw material may be processed in a non-operating mixing / grinding facility, and if the subsequent cleaning is incomplete, or if the raw material enters an unexpected location, the cause of contamination of impurities However, by using the equipment for mixing and pulverizing raw materials for mixing and pulverizing after calcination in this way, it is possible to use specific equipment for multiple processes that handle the same raw material, and to prevent the possibility of contamination with impurities. It can be further reduced.

The powder after calcination is molded into a desired shape such as a thin plate or a cylinder by a commonly used molding means such as a doctor blade method, an extrusion molding method, a powder molding method such as uniaxial pressing or isostatic pressing. However, if the powder is too fine, it becomes bulky and inconvenient to handle.Because a large amount of binder component is required, it takes a long time to remove the binder and a dense sintered body can be obtained. There is a problem that cannot be controlled. On the other hand, if the powder to be fired is coarse, it will not be densified, or even if it is densified, it will require a high temperature. Since the mixed conductive oxide according to the present invention contains a transition metal, there is a problem that such a component is diffused into a firing jig at a high temperature, or an alkaline earth metal is volatilized to shift the composition. Considering the above points, the median particle diameter of the powder after calcination is preferably 10 μm or less,
m is more preferred.

The calcined powder that has been re-ground is molded and then calcined to obtain a calcined body. In this case, in order to obtain the dense sintered body of the present invention, the firing temperature needs to be 1100 ° C. or higher. If the upper firing temperature is lower than 1100 ° C., it is substantially difficult to obtain a dense sintered body in a realistic processing time.

More desirable firing conditions are 1100 to 130.
It is to hold at 0 ° C. for 0.1 to 20 hours. 1300
If the temperature exceeds ℃, a dense sintered body can be obtained, but diffusion of transition metals, volatilization of alkaline earth metals and the like are likely to occur, and further, fusion to a firing jig is also unfavorable.

By adopting the above manufacturing method, it is possible to obtain a dense mixed conductive oxide having the above composition and having an open porosity of 0.7% or less.

[0039]

EXAMPLES Examples of the present invention will be described below together with comparative examples.

(Example 1) As shown in Table 1, as starting materials, La ₂ O ₃ (Kishida chemistry, purity 99.99%), Sr were used.
CO ₃ (Kanto Kagaku, 99.9%), Co ₂ O ₃ (Kanto Kagaku, purity 99.95%), and Fe ₂ O ₃ (Wako Pure Chemicals, 9
9.9%) to La: Sr: Co: Fe = 1: 9: 9: 1.
Were weighed so that the cation ratio was. Using a wet ball mill, mix these raw materials to the median diameter shown in Table 1.
Crushed. The median diameter is a value measured by using a laser diffraction / scattering particle size distribution measuring device with water as a dispersion medium and sodium hexametaphosphate as a dispersant.

The calcination was performed at 950 ° C. for 10 hours in an alumina bowl to obtain a perovskite single phase. This powder was finely pulverized again by a wet ball mill to a median diameter shown in the table. The median diameter at this time was also measured in the same manner as the raw material powder.

The finely pulverized powder was dried and then uniaxially pressed and isotropically hydrostatically formed into a disk having a diameter of 15 mm and a thickness of 2 mm. This is baked at 1300 ° C. for 5 hours,
The obtained sintered body was used as an evaluation pellet. The open porosity was measured by the underwater Archimedes method in order to investigate the denseness of the above pellets as a sintered body.

The above pellets have a diameter of about 11 mm and a thickness of about 1
After processing to mm, the ring was fused and fixed to the upper part of the mullite tube by heating to a silver melting point in a tubular furnace using a pure silver ring. Measurement temperature (900 ℃) in this state
To a pellet surface from the upper side of the mullite tube as a raw material gas pure air (mixed gas, O ₂ : 21.05%, N ₂ :
78.95%) was supplied at 20 ml / min. Also,
2 He was used as a purge gas on the surface of the permeated oxygen outlet pellet.
It was supplied at 0 ml / min. Oxygen separated from the raw material air and permeated through the pellets passed through the mullite tube together with the purge gas, and was then subjected to gas chromatography (GC-
TCD), and oxygen permeability was calculated from the detected oxygen. Furthermore, by detecting the N ₂ concentration, the presence or absence of air leakage due to cracking of pellets, defective fusion, etc. was confirmed. At the time of measurement, the measurement temperature was maintained for about 30 minutes, and then the measurement was performed four times at intervals of about 10 minutes. As shown in Table 1, the oxygen transmission rate obtained, about 2.2cm ³ (STP) · cm ^- was high and ² · min ^-1.

Example 2 Example 2 except that the calcination conditions were 950 ° C. for 1 hour and the firing conditions were 1200 ° C. for 5 hours.
Pellets for evaluation were prepared by the same procedure as in 1, and the oxygen permeability was calculated in the same manner. The results are also shown in Table 1. Also in this case, the oxygen transmission rate obtained is about 2.2 cm ³ (ST
P) · cm ⁻² · min ^−1, which was a high value.

(Example 3) Pellets for evaluation were prepared in the same procedure as in Example 1 except that the calcination conditions were 850 ° C for 10 hours and the firing conditions were 1300 ° C for 5 hours. The oxygen permeability was calculated. The results are also shown in Table 1.
Also in this case, the oxygen transmission rate obtained is about 2.1 cm ³ (S
TP) · cm ⁻² · min ^−1, which was a high value.

(Comparative Example 1) Acetic acid salts of La, Sr and Co and nitrates of Fe were used as starting materials, and these were mixed at the same cation ratio as in the example to evaporate and dry to powder. As a result, calcination is performed at 850 ° C. for 10 hours, and molding is performed in the same manner as in the example to obtain 1100 ° C.-
Firing was performed under the condition of 5 hours to obtain pellets for evaluation. Example
Pellets for evaluation were prepared by the same procedure as in 1, and the oxygen permeability was calculated in the same manner. The results are also shown in Table 1. In Comparative Example 1, the open porosity was higher than that of the pellets of the Examples, and the oxygen permeability was accordingly low.

When the fired pellets were left to stand, many of them cracked. Since no particular defects were found on the crack surface, it was speculated that the cause of this was that the strain remaining in the compact due to the use of the fine powder had an effect.

(Comparative Example 2) L which is a constituent of A site
Pellets for evaluation were prepared in the same procedure as in Example 1 except that the a: Sr ratio was changed as shown in the table. As a result, the open porosity was smaller than that of the pellets of Examples 1 to 3. This is because the median diameter of the powder used was made finer than that of the example. However, the oxygen permeability was about 20% lower than that of Examples 1-3. As described above, it was confirmed that even in the case of a dense sintered body, the oxygen permeability is significantly reduced by slightly changing the constituent cation ratio.

(Comparative Example 3) C which is a component of B site
Pellets for evaluation were prepared in the same procedure as in Example 1 except that the o: Fe ratio was changed as shown in the table. As a result, the open porosity was smaller than that of the pellets shown in the examples. This is because the median diameter of the powder used was made finer than that of the example. However, the oxygen permeability was about 20% lower than that of Examples 1-3. As described above, it was confirmed that even with a denser sintered body, the oxygen permeability was significantly reduced by slightly changing the constituent cation ratio.

Comparative Examples 4 to 6 In Comparative Example 4, evaluation pellets were prepared in the same procedure as in Example 1 except that the mixture of starting materials was subjected to the steps after calcination without crushing. No. 5 produced pellets for evaluation by the same procedure as in Example 1 except that the powder after calcination was subjected to the steps after firing without crushing, and in Comparative Example 6, the calcination conditions were 1150 ° C. for 10 hours. Otherwise, the evaluation pellets were produced in the same procedure as in Example 1. As a result, in Comparative Examples 4 to 6, the open porosity in the fired body was large and therefore the oxygen permeability was low, and in Comparative Examples 4 and 6, it was less than half of Examples 1 to 3.
Further, in Comparative Example 5, it was impossible to distinguish between oxygen that passed through the voids of the sintered body and oxygen that diffused in the solid in the amount of oxygen transmitted, and the oxygen permeability could not be measured.

[0051]

[Table 1]

[0052]

As described above, according to the present invention,
In industrial applications such as selectively transporting oxygen in air or oxygen-containing mixed gas, it is possible to provide a mixed conductive oxide having a high oxygen permeability and a method for producing the same. INDUSTRIAL APPLICABILITY The mixed conductive oxide of the present invention is suitably applied to the purpose of selectively transporting oxygen in oxygen gas or oxygen-containing mixed gas, and is industrially used for separation and recovery of oxygen in air, and light carbonization. It is suitable for so-called diaphragm reactor applications such as the oxidation and partial oxidation of hydrogen (methane, natural gas, ethane, etc.).

Continued front page    F-term (reference) 4D006 GA41 JA02C KE16Q KE28Q                       MA03 MB04 MB06 MC03 MC03X                       NA39 NA65 PA01 PA02 PB17                       PB62 PC71                 4G002 AA08 AA09 AA10 AB01 AD04                       AE05                 4G030 AA09 AA13 AA27 AA28 BA03                       GA08 GA11 GA27                 4G048 AA05 AB05 AC08 AD04 AE05

Claims (6)

[Claims] 1. A lanthanum, strontium, cobalt and iron as a main component, the molar ratio of these component elements is represented by the following formula (1), and the open porosity is 0.7% or less and is a dense substance. And mixed conductive oxides. La: Sr: Co: Fe = x: 1-x: 1-y: y (1) (where 0 <x <0.15, 0 <y <0.15) 2. A raw material compound containing a constituent metal in a composition ratio of the mixed conductive oxide according to claim 1 is heat-treated at a temperature of less than 1100 ° C. to form a mixed oxide, and then firing is performed at a temperature of 1100 ° C. or more. A method for producing a mixed conductive oxide, which is characterized. 3. The method for producing a mixed conductive oxide according to claim 2, wherein the raw material compound is pulverized to have a median diameter of 15 μm or less and used. 4. The heat treatment for converting the raw material compound to a mixed oxide is performed by holding the material compound at 800 to 1000 ° C. for 0.1 to 30 hours. Of the mixed conductive oxide of. 5. The method for producing a mixed conductive oxide according to claim 2, wherein the mixed oxide after the heat treatment is pulverized to have a median diameter of 10 μm or less and used. . 6. The step of firing the mixed oxide comprises 11
The method for producing a mixed conductive oxide according to any one of claims 2 to 5, wherein the method is performed by holding the material at 00 to 1300 ° C for 0.1 to 20 hours.