JP2008094698A - Method for producing cerium-based oxide - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a cerium-based oxide production method capable of stably producing ceric oxide having a specific surface area and excellent heat resistance with an inexpensive facility.
[Solution]
(1) a step of adjusting the free acid concentration of the cerium salt-containing solution, (2) a step of adding / dissolving urea to the solution, (3) a step of generating cerium hydroxide by heating the solution, (4) In the method for producing a cerium-based oxide having at least the step of filtering cerium hydroxide and (5) the step of firing cerium hydroxide, the urea addition equivalent in the step (2) is X [X (equivalent ) = {(Amount of urea added) − (amount of urea necessary to neutralize free acid)} / (amount of urea necessary to produce cerium hydroxide)]] It is characterized by doing.
[Selection figure] None

Description

  The present invention relates to a method for producing a cerium-based oxide.

Cerium oxide has an oxygen storage capacity (Oxygen Storage Capacity: OSC) because the redox potential of Ce ⁴⁺ and Ce ³⁺ is as low as about 1.6 V, and the reaction of the following formula proceeds reversibly. When pure cerium oxide is used as a co-catalyst or catalyst support for a three-way catalyst for automobiles, its dispersibility is very good when platinum is supported, that is, the high temperature of platinum particles. Therefore, it is used as a co-catalyst or catalyst carrier for a three-way catalyst for automobiles.
CeO ₂ ⇔CeO _2-X + X / 2O ₂ (X = 0 to 0.5)
However, when cerium oxide is exposed to a temperature exceeding 900 ° C, which is high speed and high load, from the atmospheric temperature, sintering of cerium proceeds, and as a result, the specific surface area becomes small, and noble metal particles present on the cerium oxide. Growth is promoted, and the OSC of cerium oxide decreases and the activity of the catalyst decreases.
Therefore, recently, cerium oxide that can maintain a high specific surface area even in such a high-temperature atmosphere, that is, excellent in heat resistance of the specific surface area, has been demanded.

Patent Document 1 describes a product prepared by hydrolyzing an aqueous solution of a cerium (IV) salt in an acidic medium, filtering the resulting precipitate, washing, and optionally drying and then calcining. Furthermore, “cerium oxide characterized by having a specific surface area (measured by the BET method) of 100 to 130 m ² / g after calcination at a temperature between 400 to 450 ° C. for 6 hours” is described.

Further, Patent Document 2 states that “the pore volume is 0, which is produced by adding a base to destabilize the aqueous colloidal dispersion of the cerium IV compound, separating the resulting precipitate, and then heat-treating the precipitate. “Cerium oxide having a specific surface area of 85 m ² / g or more after firing at a temperature of 350 to 550 ° C., characterized in that the average pore diameter is greater than .15 cm ³ / g and the average pore diameter is larger than 50 angstroms” Has been.
However, the cerium oxide described in Patent Document 1 and Patent Document 2 has a problem that the specific surface area is reduced to about 10 m ² / g when calcined at 800 ° C. for 6 hours.

On the other hand, Patent Document 3 discloses that “a) cerium hydroxide is produced by reacting in the presence of an oxidizing agent if a cerium salt solution and a base are required, and the ratio of the base is determined according to the pH of the reaction medium. And then separating the resulting precipitate, washing it if necessary, b) suspending ceric hydroxide in water or an aqueous solution of a degradable base, c) Is heated in a closed vessel to a temperature and pressure lower than the critical temperature and pressure of the reaction medium, respectively, d) the reaction mixture is cooled and returned to atmospheric pressure, and e) so-treated ceric hydroxide. F) and then calcining it, characterized by exhibiting a specific surface area of at least 15 m ² / g measured after calcining for at least 2 hours at a temperature between 800 and 900 ° C. Second cerium oxide "I have been described.

Further, Patent Document 4 states that “in an aqueous solution of a decomposable base, the following general formula (I) Ce (M) _x (OH) _y (NO ₃ ) _z (I)” (where M is quaternary ammonium A second hydroxyl group corresponding to x, wherein 0.01 is from 0.2 to 0.2, y is such that y = 4-z + x, and z is from 0.4 to 0.77. The cerium was suspended and the suspension was heated in a closed vessel to a temperature and pressure lower than the critical temperature and critical pressure of the medium, respectively, the reaction medium was cooled and returned to atmospheric pressure, and thus treated. Producing a specific surface area of at least 190 m < ² > / g measured after calcining at a temperature of 350 [deg.] C. to 450 [deg.] C., produced by separating ceric hydroxide and then calcining it, and When baked at a temperature of 800 to 900 ° C. for at least 2 hours Cerium oxide characterized by maintaining a specific surface area of at least 15 m ² / g.
However, in the methods described in Patent Document 3 and Patent Document 4, it is necessary to use an expensive closed container (autoclave), and the heat resistance reproducibility of the specific surface area of the obtained cerium oxide is poor, that is, However, it is becoming clear that there is a problem that a product having heat resistance with a specific surface area cannot be stably produced.

  Japanese Patent No. 2655138   Japanese Patent Publication No. 6-92253   Japanese Patent Publication No. 8-18833   Japanese Patent Publication No. 8-32555

  The present invention has been made in view of the above-mentioned problems, and the object thereof is a cerium-based material that can stably produce ceric oxide having excellent specific surface area heat resistance with inexpensive equipment. The object is to provide a method for producing an oxide.

As a result of diligent research to achieve the above object, the present inventors have produced a cerium hydroxide from a cerium salt by a uniform precipitation method using urea, and calcining this to produce a cerium-based oxide. It has been surprisingly found that a cerium-based oxide having a specific surface area excellent in heat resistance can be stably produced by adding urea at a specific equivalent amount or more.
Based on this finding, the present invention
(1) (1) a step of adjusting the free acid concentration of the cerium salt-containing solution, (2) a step of adding and dissolving urea in the solution, and (3) heating the solution to produce cerium hydroxide. And (4) a process of filtering cerium hydroxide and (5) a process of calcining cerium hydroxide, wherein the urea addition equivalent in step (2) is X [ X (equivalent) = {(urea addition amount) − (amount of urea necessary to neutralize free acid)} / (amount of urea necessary to produce cerium hydroxide)]] A method for producing a cerium-based oxide, characterized by comprising 3 or more.
(2) The method for producing a cerium-based oxide as described in (1) above, wherein the urea-containing filtrate in the step (4) is repeated to the step (2).
(3) The method for producing a cerium-based oxide according to (1) or (2) above, wherein the specific surface area after firing the cerium-based oxide at 350 ° C. for 10 hours is 120 m ² / g or more. .
(4) The method for producing a cerium-based oxide according to any one of (1) to (3) above, wherein the specific surface area of the cerium-based oxide after baking at 800 ° C. for 4 hours is 40 m ² / g or more. .
(5) The method for producing a cerium-based oxide according to any one of (1) to (4) above, wherein the specific surface area of the cerium-based oxide after baking at 900 ° C. for 4 hours is 25 m ² / g or more. .
(6) The cerium-based oxide is 0.01, at least one metal oxide selected from Ti, Y, Sc, Zr, transition metals containing rare earth elements, alkali metals, alkaline earth metals, Al, Si. The method for producing a cerium-based oxide according to any one of (1) to (5) above, wherein the content is -30%.
Is to provide.

  ADVANTAGE OF THE INVENTION According to this invention, the simple manufacturing method which can manufacture stably the cerium-type oxide excellent in the heat resistance of a specific surface area can be provided, and the manufactured cerium-type oxide is three for motor vehicles. It can be suitably used in this field as a cocatalyst for the original catalyst or a catalyst carrier.

Below, the manufacturing method of the cerium-type oxide of this invention is demonstrated in detail.
In the present invention, “%” means “wt% = mass%” unless otherwise specified.

1. Method for producing cerium-based oxide (first step)
First, in the present invention, the free acid concentration of the cerium salt-containing solution is adjusted.
Examples of cerium salts include hydrochlorides, nitrates, sulfates, etc., but nitrates are preferred from the standpoint that impurities are less mixed into the product. The solvent may be appropriately selected according to the type of cerium salt used, but it is usually desirable to use water (preferably pure water or ion-exchanged water).
The concentration of the cerium salt solution is not particularly limited, as generally cerium oxide in the solvent 1000g (CeO ₂₎ 5~200g, it is particularly desirable that the 50 to 100 g.
Examples of the free acid include sulfuric acid, nitric acid, hydrochloric acid and the like, and are not particularly limited. However, nitric acid is preferable from the viewpoint that impurities are not mixed into the product.
The free acid concentration is 0.1N to 5N, preferably 1N to 3N, and particularly preferably 1.5 to 2.5N. If it is less than 0.1N or exceeds 5N, the sedimentation and filterability of the produced cerium hydroxide are poor, and the production efficiency is deteriorated, which is not preferable.
The cerium-based oxide produced according to the present invention is at least one metal selected from Ti, Y, Sc, Zr, transition metals containing rare earth elements, alkali metals, alkaline earth metals, Al, Si. The oxide can be contained in an amount of 0.01 to 30%, but in order to contain these, it is preferable to contain a predetermined amount of these water-soluble salts in this step.
By adding these metal oxides, the heat resistance of the specific surface area is further improved.

(Second step)
Next, urea is added and dissolved in the acidic cerium salt-containing solution.
In this step, the urea addition equivalent is X [X (equivalent) = {(urea addition amount) − (urea amount necessary for neutralizing free acid)} / (necessary for producing cerium hydroxide. (Urea amount)], X is 3 or more.
X is usually from 3 to 20, preferably from 5 to 15, and more preferably from 8 to 12.
If it is less than 3, the filterability of the produced cerium hydroxide is deteriorated, the residual amount of impurities is increased, and the thermal stability of the specific surface area is decreased, which is not preferable. In addition, since it is not economical when it exceeds 20, it is not preferable.
When a water-soluble salt of at least one metal selected from Ti, Y, Sc, Zr, transition metals containing rare earth elements, alkali metals, alkaline earth metals, Al, Si is added in the first step In addition, an amount of urea (at least 1 equivalent) necessary to make these salts into hydroxides is further added (3 to 20 equivalents may be added as required for cerium salts). Needless to say.
On the other hand, when the urea-containing filtrate in the fourth step is repeatedly used, it is a matter of course that a deficient amount of urea is newly added in consideration of the amount of urea contained in the filtrate. .
Thus, the reason why urea is added excessively will be described in detail in the next step.

(Third process)
And cerium hydroxide is produced | generated by heating the urea containing acidic cerium salt solution produced at the 2nd process.
The heating temperature is 70 ° C. or higher, preferably 95 ° C. or higher.
If it is less than 70, decomposition of urea is delayed, and production efficiency is lowered, which is not preferable.
Urea (CH ₄ N ₂ O) in the liquid is decomposed into ammonia gas and carbon dioxide gas at a temperature of 70 ° C. or higher, and the following reaction proceeds (the reaction formula is exemplified when cerium nitrate is used). ).
Ce (NO ₃ ) ₄ + 2CH ₄ N ₂ O + 6H ₂ O = Ce (OH) ₄ + 4NH ₄ NO ₃ + 2CO ₂
This method for producing hydroxide using the decomposition reaction of urea is a well-known method as a uniform precipitation method, but is hardly carried out industrially.
The good point of this method is that ammonia gas, that is, nascent ammonia gas reacts with cerium ions, the reaction rate is very fast, and as a result, cerium hydroxide of very fine and uniform size Can be generated.

And, as described above, the characteristic of the method of the present invention is that, in performing the decomposition reaction of urea, 3 to 20 equivalents of urea amount necessary for cerium ions to become cerium hydroxide is added, Of these, at least 3 equivalents are to be reacted.
That is, at least 2 equivalents or more of urea is excessively decomposed and this decomposition is performed in a short time, a large amount of ammonia gas is generated, and a large amount of cerium hydroxide is generated. Cerium hydroxide can be produced.
The detailed reason has not been clarified at the present time, but by producing cerium hydroxide having a very fine and uniform size by such an operation, cerium oxide excellent in heat resistance of specific surface area is produced. be able to.
That is, (1) the specific surface area after firing at 350 ° C. for 10 hours is 120 m ² / g or more, preferably 150 m ² / g or more, (2) the specific surface area after firing at 800 ° C. for 4 hours is 40 m ² / g Above, preferably 50 m ² / g or more and (3) a specific surface area after firing at 900 ° C. × 4 hours, having a specific surface area of 25 m ² / g or more, preferably 30 m ² / g or more, having heat resistance Can be produced stably.
In addition, although heating time changes with heating temperature, it is 0.5 to 5 hours normally, Preferably, it is 1-2 hours.

(Fourth process)
Subsequently, in the fourth step, cerium hydroxide is filtered.
In addition, after completion | finish of neutralization reaction, it is preferable from a viewpoint of making the obtained precipitation age | cure and filter-separating to hold | maintain a cerium hydroxide containing solution at 60-100 degrees C or less for 1 hour or more.
And the deposit which consists of produced | generated cerium hydroxide is collect | recovered by the solid-liquid separation method.
The solid-liquid separation method may be a known method such as filtration, centrifugation, decantation, or the like.
Since the filtrate still contains urea, it is preferable to repeat the second step depending on the concentration of impurities contained in the solution.
After the collection, it is preferable to wash the cerium hydroxide with water as necessary to remove adhering impurities.
In addition, you may dry the obtained cerium hydroxide further as needed. The drying method may follow a well-known method, for example, any of natural drying, heat drying, etc. may be sufficient. If necessary, a pulverization process, a classification process, or the like may be performed after the drying process.

(Fifth process)
Finally, cerium hydroxide is heat-treated to obtain a cerium oxide-based oxide.
Although heat processing temperature is not specifically limited, Usually, 1Hr-10Hr are performed at about 300-900 degreeC.
Although the heat treatment atmosphere is not particularly limited, it may be usually in the air or an oxidizing atmosphere.
The composite oxide obtained in this way can be pulverized as necessary. Although it does not specifically limit about grinding | pulverization, It can grind | pulverize with grinders, such as a planetary mill, a ball mill, or a jet mill.

  Examples are given below to further clarify the features of the present invention. In addition, this invention is not limited to the aspect of these Examples.

Each physical property in the examples was measured by the following methods.
(1) Specific surface area Using a specific surface area meter (“Flowsorb-II” manufactured by Micromeritics), the specific surface area was measured by the BET method.

[Example 1]
986 g of pure water, 30.3 g of 63% nitric acid, and 600 g of urea were added to 412.5 g of a cerium (IV) nitrate solution (containing 100 g of CeO ₂ ) (about the amount necessary to make cerium hydroxide). 8.5 times).
This was heated to 97 ° C. and maintained for 3 hours for neutralization to produce cerium hydroxide (urea was reacted about 4 times as much as that required for cerium hydroxide).
The slurry was filtered and then washed with water to obtain a hydroxide.
This hydroxide was calcined at 350 ° C. for 10 hours to obtain a cerium oxide powder. The specific surface area was 179 m ² / g, and the specific surface area when further heat-treated at 800 ° C. for 4 hours was 50 m ² / g, and the specific surface area when heat-treated at 900 ° C. for 4 hours was 33 m. ² / g.
The crystallite diameter of the cerium oxide powder obtained by baking at 800 ° C. for 4 hours was 170 mm.

[Example 2]
Instead of 986 g of pure water added at the time of preparation, the supernatant liquid after neutralization reaction (containing urea = 280 g) and 100 g of pure water were used, and the amount of urea added was changed to 320 g, as in [Example 1]. A hydroxide was obtained.
This hydroxide was calcined at 350 ° C. for 10 hours to obtain a cerium oxide powder. The specific surface area was 182 m ² / g, and the specific surface area when further heat-treated at 800 ° C. for 4 hours was 50 m ² / g, and the specific surface area when heat-treated at 900 ° C. for 4 hours was 30 m. ² / g.
The crystallite diameter of the cerium oxide powder obtained by firing at 800 ° C. for 4 hours was 172 mm.

[Comparative Example 1]
A hydroxide was obtained in the same manner as in Example 1 except that the amount of urea added was 149 g (about twice the amount necessary for cerium hydroxide) (the urea reacted in its entirety).
This hydroxide was calcined at 350 ° C. for 10 hours to obtain a cerium oxide powder. The specific surface area is 68 m ² / g, and the specific surface area is 2.1 m ² / g when heat-treated at 800 ° C. for 4 hours and the specific surface area when heat-treated at 900 ° C. for 4 hours. Was 0.7 m ² / g.
The crystallite diameter of the cerium oxide powder obtained by baking at 800 ° C. for 4 hours was 381 mm.

  As is clear from the results of Example 1 and Comparative Example 1, the amount of urea added is X [X (equivalent) = {(urea added amount) − (urea amount necessary for neutralizing free acid)} / (The amount of urea necessary to produce cerium hydroxide)], X is 3 or more, and by reacting 3 equivalents or more, very fine and uniform cerium hydroxide is obtained. It turns out that cerium-type oxide excellent in heat resistance of a specific surface area can be manufactured by a simple method with good reproducibility by using as cerium oxide.

Claims (6)

  (1) a step of adjusting the free acid concentration of the cerium salt-containing solution, (2) a step of adding / dissolving urea to the solution, (3) a step of generating cerium hydroxide by heating the solution, (4) In the method for producing a cerium-based oxide having at least the step of filtering cerium hydroxide and (5) the step of firing cerium hydroxide, the urea addition equivalent in the step (2) is X [X (equivalent ) = {(Amount of urea added) − (amount of urea necessary to neutralize free acid)} / (amount of urea necessary to produce cerium hydroxide)]] A method for producing a cerium-based oxide.   The method for producing a cerium-based oxide according to claim 1, wherein the urea-containing filtrate in the step (4) is repeated to the step (2). The method for producing a cerium-based oxide according to claim 1 or ² , wherein the specific surface area of the cerium-based oxide after baking at 350 ° C for 10 hours is 120 m ² / g or more. The method for producing a cerium-based oxide according to any one of claims 1 to 3, wherein the specific surface area of the cerium-based oxide after firing at 800 ° C for 4 hours is 40 m ² / g or more. 5. The method for producing a cerium-based oxide according to claim 1, wherein a specific surface area of the cerium-based oxide after firing at 900 ° C. for 4 hours is 25 m ² / g or more.   The cerium-based oxide contains 0.01 to 30% of at least one metal oxide selected from Ti, Y, Sc, Zr, transition metals containing rare earth elements, alkali metals, alkaline earth metals, Al and Si. It contains, The manufacturing method of the cerium type oxide of Claims 1-5 characterized by the above-mentioned.