JP4601462B2 - Perovskite complex oxide and method for producing the same - Google Patents

Description

  The present invention relates to a perovskite complex oxide and a method for producing the same.

A perovskite complex oxide represented by ABO ₃ (A represents a rare earth metal, and B represents an alkaline earth metal or a transition metal) is an inexpensive three-ply gas purification catalyst that purifies CO, HC, NO _{x and the} like. Practical use is expected as an original catalyst, a catalyst carrier, and the like. At present, in addition to the above characteristics, there is a demand for the characteristics of burning soot, fuel particulates and the like contained in exhaust gas from a diesel engine. In order to satisfy these required performances, the composite oxide having a large specific surface area, particularly one having a small decrease in specific surface area under high temperature conditions (ie, having a high specific surface area heat resistance) Development is underway.

  As an exhaust gas combustion catalyst using a perovskite complex oxide, for example, those shown in Patent Documents 1 to 4 below are known.

Patent Document 1 relates to an oxygen-deficient perovskite catalyst. The claims include a gas purification catalyst, the composition of which is A ₁ A ′ ₂ B ₃ O _7-α (A is one or more elements selected from Y and rare earth elements, A ′ Is one or more elements selected from Ba, Sr and Ca, B is one or more elements selected from Cu, Mn and iron group elements, and 0 ≦ α ≦ 1) An oxygen-deficient perovskite catalyst characterized by being a powder having a specific surface area of 1 to 10 m ² / g is disclosed.

Patent Document 2 relates to an oxidation catalyst. Claim 1 includes the general formula: La _X A _1-X B _Y B ′ _1-Y O ₃ (wherein A is Ba, Sr, Zn, Ag, Ce; B is Mn or Co; B ′ is Co, Fe, Ni, Cu, Ti, Zr or Cr; and a perovskite complex oxide having a specific surface area represented by 0 ≦ X ≦ 1, 0 ≦ Y ≦ 1) of 20 m ² / g or more is supported on a solid acid carrier An oxidation catalyst is disclosed which is characterized in that

Patent Document 3 relates to a method for producing a powder for an LaMn-based oxide catalyst. In Claim 1, in the manufacturing method of the powder for LaMn system oxide catalyst which makes La and Mn essential, and has at least 1 sort of metal elements of Pb, K, Ce, Co, Ni, and Mg as a main component, A method for producing a powder for a LaMn-based oxide catalyst, characterized in that a solution of a complex of a nitrate of a metal element constituting a powder and an amino acid is heated to a boiling point or higher of the solution is disclosed. In the examples, it is described that a perovskite complex oxide represented by La _0.8 Ce _0.2 MnO ₃ having a BET specific surface area of 38.1 was obtained.

Patent Document 4 relates to an oxygen-deficient perovskite catalyst. In claim 1, the following formula: A ₁ A ′ ₂ B ₃ O _7-α (A is at least one element selected from Y and rare earth elements, A ′ is at least selected from Ba, Sr and Ca) One element, B represents at least one element selected from Cu, Mn, and an iron group element, and 0 ≦ α ≦ 1, and a composite oxide represented by MgO, ZrO ₂ , ZnO, It is characterized by comprising a catalyst having a specific surface area of 10 to 35 m ² / g supported on at least one oxide carrier selected from SrTiO ₃ , CoAl ₂ O ₄ , ZnAl ₂ O ₄ and MgAl ₂ O _4. An oxygen-deficient perovskite catalyst for gas purification is described.

However, such a conventionally known perovskite type complex oxide has an insufficient specific surface area, or has a large reduction rate of the specific surface area under high temperature conditions, and is further improved in order to satisfy the above-mentioned required performance. There is room for.
Japanese Examined Patent Publication No. 5-86257 JP-A-5-49943 JP-A-8-229390 Japanese Patent Publication No. 6-16851

  The main object of the present invention is to provide a perovskite complex oxide in which the decrease in specific surface area under high temperature conditions is suppressed.

  As a result of intensive studies to achieve the above object, the present inventors have found that a perovskite complex oxide manufactured by a specific manufacturing method can achieve the above object, and have completed the present invention.

  That is, the present invention relates to the following perovskite complex oxide and a method for producing the same.

1. General composition formula: LaFe _x Ti _1-x O _α
Wherein, x is the numerical of 0.17 ≦ x ≦ 0.64. α represents a numerical value of 2.5 ≦ α ≦ 3. ]
A perovskite complex oxide represented by:
A composite oxide having a BET specific surface area of 20 m ² / g or more after firing at 1000 ° C. for 3 hours in an air atmosphere.

2. In the X-ray diffraction result (relation between diffraction angle and diffraction X-ray intensity) of the composite oxide, the maximum value of the diffraction X-ray intensity caused by the perovskite structure is Imax, and the diffraction X-ray intensity not caused by the perovskite structure Is the maximum value of I′max,
The following formula: Phase Purity (%) = Imax / (Imax + I′max)
Item 2. The composite oxide according to Item 1, wherein the phase purity calculated from the above is 95% or more.

3 . General composition formula: LaFe _x Ti _1-x O _α
Wherein, x is the numerical of 0.17 ≦ x ≦ 0.64. α represents a numerical value of 2.5 ≦ α ≦ 3. ]
A method for producing a perovskite complex oxide represented by:
A composite hydroxide is produced by adding a mixed solution in which 1) a metal salt containing La , 2) a metal salt containing Fe , and 3) a titanium salt is dissolved in an alkaline solution, and then the composite hydroxide. The manufacturing method characterized by baking.

4. The rate at which the mixed solution is added to the alkaline solution is
1) General composition formula generated in the solution after the addition: LaFe _x Ti _1-x (OH) _α
Wherein, x is the numerical of 0.17 ≦ x ≦ 0.64. α represents a numerical value of 2.5 ≦ α ≦ 3. ]
The amount of the composite hydroxide represented by P is (mol),
2) Let the total amount of the solution after addition be Q (L),
3) When the time from the start of addition to the end of addition is R (time),
4) 0.01 ≦ P / (Q · R) ≦ 0.5
Item 4. The production method according to Item 3 , wherein the production rate is satisfied.

  Hereinafter, the composite oxide of the present invention and the production method thereof will be described in detail.

Perovskite complex oxide The perovskite complex oxide of the present invention has a general composition formula: LaFe _x Ti _1-x O _α
Wherein, x is the numerical of 0.17 ≦ x ≦ 0.64. α represents a numerical value of 2.5 ≦ α ≦ 3. ]
The BET specific surface area after baking for 3 hours at 1000 degreeC in air | atmosphere is 20 m < ² > / g or more.

In the above composition formula , La is preferable from the viewpoint that the metal salt (raw material) serving as the supply source has few industrial impurities.

In the above composition formula , Fe is preferable from the viewpoint of imparting good combustion catalyst performance to the composite oxide.

In the above composition formula, x represents the number of Fe atoms. The proportion of Fe and the proportion of Ti are correlated, and Fe is included in the form of replacing a part of Ti. That is, x may be a numerical value satisfying 0.17 ≦ x ≦ 0.64 .

  In the above composition formula, α represents the number of O (oxygen) atoms. The perovskite complex oxide may cause oxygen deficiency depending on the type of constituent elements. Therefore, in the present specification, 2.5 ≦ α ≦ 3 is described.

In the perovskite complex oxide of the present invention, the decrease in specific surface area under high temperature conditions is suppressed, and the BET specific surface area after firing at 1000 ° C. for 3 hours in an air atmosphere is 20 m ² / g or more. The optimization of the embodiment the composition of the composite oxide, BET specific surface area after firing under the same conditions becomes 2 5 m 2 / ^g or more. Although the details of the reason why the reduction in specific surface area under high temperature conditions is suppressed are unclear, it is thought to be due to the structural stabilization of the entire composite oxide by containing Ti as a constituent element.

Next, as a specific example, characteristics of the composite oxide represented by LaFe _x Ti _1-x O _α (wherein the symbols are the same as those described above) will be described.

  FIG. 1 is a graph in which x (ratio of Fe: molar ratio) of this composite oxide is taken on the horizontal axis, and the BET specific surface area (Aged SA) after firing at 1000 ° C. for 3 hours in the atmosphere is taken on the vertical axis. It is shown.

From FIG. 1, it can be seen that x (the ratio of Fe) needs to be set in a range of 0.17 ≦ x ≦ 0.64 in order to have a predetermined specific surface area characteristic in the composite oxide. Also , 0 . In the case of setting 24 ≦ x ≦ 0.6, it can be seen that the specific surface area after firing is 25 m ² / g or more.

The perovskite complex oxide of the present invention preferably consists essentially of a perovskite crystal structure. Specifically, in the X-ray diffraction result of the composite oxide (relation between the diffraction angle and the diffraction X-ray intensity), the maximum value of the diffraction X-ray intensity attributed to the perovskite structure is Imax, and it does not result from the perovskite structure. Let the maximum value of the diffracted X-ray intensity be I′max,
The following formula: Phase Purity (%) = Imax / (Imax + I′max)
It is preferable that the phase purity calculated from is 95% or more. When the composite oxide is substantially composed only of a perovskite structure, I′max is substantially 0, so that the phase purity of the composite oxide is substantially 100%. Thus, when the crystal structure is substantially composed of a perovskite single phase, the constituent elements of the composite oxide are uniformly dispersed at the atomic level, and active sites having different catalytic actions (in the above example, La, Fe And Ti) are close to each other, so that a good catalytic activity effect is obtained.

The perovskite-type composite oxide of the present invention is not only useful as a three-way catalyst for exhaust gas purification that purifies CO, HC, NO _x, etc., because the reduction in specific surface area under high temperature conditions is suppressed. It is also useful as a combustion catalyst for decomposing soot or fuel particles contained in engine exhaust gas. It can also be used as a catalyst carrier. The perovskite complex oxide of the present invention is also applicable as a catalyst and a catalyst carrier other than the catalyst for automobile exhaust gas purification.

Method for Producing Perovskite Type Composite Oxide The method for producing the perovskite type composite oxide of the present invention is not limited. For example, 1) a metal salt containing La , 2) a metal salt containing Fe , and 3) a titanium salt A mixed solution in which the compound hydroxide is added to an alkaline solution to form a composite hydroxide, and then the composite hydroxide is fired (hereinafter referred to as “the manufacturing method of the present invention”). ) .

The gold Shokushio Both nitrates, sulfates, acetates, chlorides, alkoxides, bromides, and the like. Among these, nitrates, alkoxides, and the like are preferable, and alkoxides are more preferable, from the viewpoint of avoiding contamination with impurities in the subsequent steps.

Although it does not specifically limit as a solvent which melt | dissolves a metal salt, Usually, water can be used. Concentration of the mixed _{solution, LaFe x Ti 1-x O} α ( although the description of the symbols are as defined above) is preferably about 1 to 20% by weight in terms of concentration, more preferably about 5 to 15 wt%. Less than 1% by weight is not preferable from the viewpoint of production efficiency. If it exceeds 20% by weight, dissolution in an ionic state becomes difficult, which is not preferable from the viewpoint of the stability of the mixed solution.

  The alkaline solution is not particularly limited as long as a composite hydroxide is formed by mixing with the mixed solution. For example, what dissolved at least 1 sort (s), such as ammonia, caustic soda, sodium carbonate, in water, can be used. Note that ammonia is preferably used from the viewpoint of avoiding impurities from being mixed into the composite oxide. The concentration of the alkaline solution is preferably about 5 to 30% by weight. When set within such a range, the base diffuses in the alkaline solution, a uniform precipitate is easily obtained, and the precipitate formation rate is unlikely to be slow.

In producing the composite hydroxide, a mixed solution of metal salts is added to the alkaline solution. By this addition, a composite hydroxide represented by LaFe _x Ti _1-x (OH) _α (wherein the symbols are the same as those described above, except that α indicates the amount of OH, hereinafter the same) is generated. The addition mode is opposite to the generally employed mode of adding an alkaline solution to a mixed solution of metal salts, and is a so-called reverse neutralization mode. The reason for adopting such an embodiment is that a composite metal hydroxide having a uniform composition is easily obtained because the precipitation reaction is always performed in a strongly alkaline region, which is much higher than the isoelectric point. On the other hand, when metal ions having different isoelectric points are added to the acidic region to the weakly alkaline region to form precipitates (an embodiment that is not reverse neutralization), the composition of the composite hydroxide generated by the difference in isoelectric points Variation occurs and it is difficult to obtain a composite hydroxide having a uniform composition.

  In the addition mode employed in the production method of the present invention, the pH of the solution after the precipitation is preferably 9.5 or higher, more preferably about 9.5 to 10.5. When the pH is less than 9.5, not all metal ions may be used for hydroxide generation. A pH exceeding 10.5 is not preferable because it is not economical.

In the addition mode employed in the production method of the present invention, the rate at which the mixed solution of the metal salt is added to the alkaline solution is:
1) General composition formula generated in the solution after addition: LaFe _x Ti _1-x (OH) _α (however, the symbols are the same as described above)
The amount of the composite hydroxide represented by P is (mol),
2) Let the total amount of the solution after addition be Q (L),
3) When the time from the start of addition to the end of addition is R (time),
4) 0.01 ≦ P / (Q · R) ≦ 0.5, preferably 0.05 ≦ P / (Q · R) ≦ 0.3
It is preferable that the speed is satisfied. Therefore, it is preferable to set the concentrations and amounts of the metal salt mixed solution and the alkali solution in advance so as to satisfy the condition shown in 4) above.

  When the value of P / (Q · R) is less than 0.01, the formation efficiency of the precipitate is lowered. In addition, when it exceeds 0.5, the pH of the solution may be locally less than 9.5 to acidic, resulting in a difference in the rate of precipitation (hydroxide) formation of metals with different isoelectric points. There is a risk that it may be difficult to obtain a composite hydroxide having a uniform composition.

The composite hydroxide represented by the general composition formula obtained through the above process: LaFe _x Ti _1-x (OH) _α may be immediately subjected to calcination, but if necessary, the solution containing the precipitate is heated. You may use for aging. By aging by heating, the constituent metals ( La , Fe and Ti) in the composite hydroxide are repeatedly dispersed and re-agglomerated, so that the composition of the composite hydroxide can be made more uniform. This contributes to the improvement of the phase purity of the composite oxide.

  The heat aging conditions are not particularly limited as long as the heat treatment conditions allow the dispersion and reaggregation described above, and the temperature is preferably 50 ° C. or higher, more preferably 80 ° C. or higher, and most preferably 100 ° C. or higher. The heating time may be appropriately set according to the temperature condition, but is preferably about 3 to 7 hours. In such a condition setting, heat aging can be performed efficiently. The heating means is not limited, but autoclave or the like is preferable from the viewpoint of improving powder characteristics.

  The composite hydroxide may be recovered by a solid-liquid separation method. Examples of the solid-liquid separation method include filtration, centrifugation, and decantation. When solid-liquid separation is performed by combining cleaning with pure water or the like, it is preferable because impurities can be easily removed.

  The composite hydroxide cake obtained by solid-liquid separation is dried as necessary. The drying method is not particularly limited, and any of natural drying and heat drying may be used. In addition, vacuum drying is preferable from the viewpoint of preventing adsorption of carbon dioxide in the air.

The dried composite hydroxide may be appropriately pulverized and classified. Further, the composition of the composite hydroxide is further adjusted by impregnating the dry composite hydroxide with a solution containing at least one of 1) a metal salt containing La and 2) a metal salt containing Fe. You may go. In such a case, the concentration of the metal salt solution is preferably about 0.1 to 10% by weight.

  The composite hydroxide is then fired to form a perovskite composite oxide. The firing conditions are not particularly limited as long as crystallization into a perovskite crystal proceeds. Usually, the firing may be performed at about 500 to 1000 ° C. for about 1 to 10 hours, and the firing may be stopped at the stage of crystallization into a perovskite crystal. When the temperature is lower than 500 ° C., crystallization into a perovskite crystal may be insufficient. When it exceeds 1000 ° C., the specific surface area of the composite oxide itself may be lowered. Although the firing atmosphere is not limited, it may normally be an air atmosphere.

The perovskite-type composite oxide of the present invention is not only useful as a three-way catalyst for exhaust gas purification that purifies CO, HC, NO _x, etc., because the reduction in specific surface area under high temperature conditions is suppressed. It is also useful as a combustion catalyst for decomposing soot or fuel particles contained in engine exhaust gas. It can also be used as a catalyst carrier. The perovskite complex oxide of the present invention is also applicable as a catalyst and a catalyst carrier other than the catalyst for automobile exhaust gas purification.

  According to the production method of the present invention, it is possible to efficiently produce the perovskite type complex oxide of the present invention in which the decrease in specific surface area under high temperature conditions is suppressed.

Examples , reference examples and comparative examples are shown below to further clarify the features of the present invention. However, the present invention is not limited to the contents of the examples.

In Examples , Reference Examples and Comparative Examples , the BET specific surface area was measured using a specific surface area meter “Flowsorb-II” (manufactured by Micromeritics).

Example 1
<Production of _{LaFe x Ti 1-x O α} >
Lanthanum nitrate solution (20 wt% as _La 2 _{O 3} reduced concentration) 335 g, iron nitrate (III) 9 hydrate _{(Fe (NO 3) 3 ·} 9H 2 O, 99% or more) 30 g and titanium tetrachloride solution ( After mixing 175 g of TiO ₂ equivalent concentration (15 wt%), 460 g of pure water was added and stirred until a uniform solution was obtained.

Concentration of the mixed solution of the metal _salt, in terms of the concentration of _{LaFe x Ti 1-x O α} , was x = 0.2. That is, the molar ratio of La: Fe: Ti was 1: 0.2: 0.8.

  This mixed solution was added to 1.5 L of diluted aqueous ammonia obtained by diluting 500 g of 25% aqueous ammonia three times with water at a constant rate over about 1 hour to form a composite hydroxide.

The total amount of the solution after the addition, where P (mol) is the amount of the composite hydroxide represented by LaFe _0.2 Ti _0.8 (OH) _{2.5 ≦ α ≦ 3} generated in the solution after the addition Is Q (L), and the time from the start of addition to the end of addition is R (time), the value of P / (Q · R) was 0.16.

  The solution after precipitation was aged by heating at 55 ° C. for 3 hours. Next, the produced composite hydroxide was filtered and washed with pure water. Next, the composite hydroxide was dried and then baked at 600 ° C. for 10 hours in an air atmosphere using an electric furnace.

Through the above process, 100 g of a perovskite complex oxide represented by LaFe _0.2 Ti _0.8 O _{2.5 ≦ α ≦ 3} was obtained. The composite oxide had a BET specific surface area of 29.6 m ² / g.

The BET specific surface area after firing the composite oxide at 1000 ° C. for 3 hours in the air atmosphere was 21.7 m ² / g.

  The X-ray diffraction result of the composite oxide is shown in FIG. The phase purity of the composite oxide calculated from the results of FIG. 2 was 68%.

Examples 2-3, Reference Examples and Comparative Examples 1-3
A perovskite complex oxide was produced in the same manner as in Example 1 except that the molar ratio of La: Fe: Ti was changed as shown in Table 1.

  Table 1 shows the characteristics of the obtained composite oxide.

Comparative Example 4
A perovskite complex oxide was prepared in the same manner as in Example 1 except that diluted aqueous ammonia was added to the mixed solution of metal salts and the molar ratio of La: Fe: Ti was 1: 0.6: 0.4. Manufactured.

  Table 1 shows the characteristics of the obtained composite oxide.

* 1: Specific surface area after calcining the composite hydroxide in the dry state at 600 ° C. for 10 hours in the air atmosphere * 2: Specific surface area after calcining at 1000 ° C. for 3 hours in the air atmosphere From the results in Table 1, It can be seen that the perovskite complex oxides of Examples 1 to 3 are suppressed from decreasing in specific surface area under high temperature conditions.

  The perovskite type complex oxide of Comparative Example 4 (manufacturing method in which an alkali solution is added to a mixed solution of metal salts) has a specific surface area decrease under a high temperature condition, but the perovskite type complex oxide of Example 3 having the same molar ratio. The Phase Purity was very low.

It is a graph which shows the relationship between Fe / (Ti + Fe) (molar ratio) and the BET specific surface area (m < ² > / g) after baking at 1000 degreeC for 3 hours (in air | atmosphere atmosphere). It is a figure which shows the X-ray-diffraction result (relationship between a diffraction angle and diffraction X-ray intensity) of the perovskite type complex oxide obtained by the Example , the reference example, and the comparative example.

Claims (4)

General composition formula: LaFe _x Ti _1-x O _α
Wherein, x is the numerical of 0.17 ≦ x ≦ 0.64. α represents a numerical value of 2.5 ≦ α ≦ 3. ]
A perovskite complex oxide represented by:
A composite oxide having a BET specific surface area of 20 m ² / g or more after firing at 1000 ° C. for 3 hours in an air atmosphere. In the X-ray diffraction result (relation between diffraction angle and diffraction X-ray intensity) of the composite oxide, the maximum value of the diffraction X-ray intensity caused by the perovskite structure is Imax, and the diffraction X-ray intensity not caused by the perovskite structure Is the maximum value of I′max,
The following formula: Phase Purity (%) = Imax / (Imax + I′max)
2. The composite oxide according to claim 1, wherein the phase purity calculated from is 95% or more. General composition formula: LaFe _x Ti _1-x O _α
Wherein, x is the numerical of 0.17 ≦ x ≦ 0.64. α represents a numerical value of 2.5 ≦ α ≦ 3. ]
A method for producing a perovskite complex oxide represented by:
A composite hydroxide is produced by adding a mixed solution in which 1) a metal salt containing La , 2) a metal salt containing Fe , and 3) a titanium salt is dissolved in an alkaline solution, and then the composite hydroxide. The manufacturing method characterized by baking. The rate at which the mixed solution is added to the alkaline solution is
1) General composition formula generated in the solution after the addition: LaFe _x Ti _1-x (OH) _α
Wherein, x is the numerical of 0.17 ≦ x ≦ 0.64. α represents a numerical value of 2.5 ≦ α ≦ 3. ]
The amount of the composite hydroxide represented by P is (mol),
2) Let the total amount of the solution after addition be Q (L),
3) When the time from the start of addition to the end of addition is R (time),
4) 0.01 ≦ P / (Q · R) ≦ 0.5
The manufacturing method of Claim 3 which is the speed | rate which satisfy | fills.

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