JPH05208816A - Production of cerium hydroxide and cerium oxide - Google Patents

Abstract

PURPOSE:To provide a producing method of cerium hydroxide and cerium oxide small in crystal diameter of cerium oxide finally obtained and capable of giving a catalyst larger in oxygen occluding ability than a conventional catalyst and capable of carrying in good dispersibility on a support. CONSTITUTION:When an alkali is added to an aqueous solution of cerium (IV) sulfate and cerium oxide is precipitated by hydrolysis with the alkali, an aqueous solution of the alkali is dropwise added to an aqueous solution of cerium (IV) sulfate, but the addition is temporarily stopped until cerium hydroxide once produced is re-dissolved in the aqueous solution of cerium sulfate, and after cerium hydroxide is completely re-dissolved, addition of the aqueous solution of alkali is again carried out. The reaction for depositing cerium hydroxide is finished so that pH of the reaction liquid in a state where the produced cerium hydroxide is not dissolved may be <=2.0 and the deposited cerium hydroxide is washed, dried and burned to produce the objective cerium oxide.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to cerium hydroxide and cerium oxide used as a cocatalyst for purifying exhaust gas, particularly automobile exhaust gas.

[0002]

2. Description of the Related Art Catalytic metals such as Pt, Pd and Rd are used to purify three components of CO, HC and Nox in exhaust gas at the same time. Although Ni was conventionally added to promote the performance of these catalysts, it has recently been reported in "Catalyst" 1989, Vol. 31, pp. 566-571 that CeO ₂ has better performance than Ni. . CeO ₂ on the catalyst is said to have an oxygen storage effect, a water gas shift reaction promoting effect, a noble metal dispersibility improving effect, and a high temperature heat resistance improving effect of the carrier.

According to this report, the crystallization of CeO ₂ on the catalyst proceeds almost in proportion to the temperature rise, and the O ₂ adsorption amount drops sharply when the temperature exceeds 800 ° C. A high temperature heat resistant catalyst to which Ba and Zr are added in order to suppress the growth of the crystallite diameter of CeO ₂ is already on the market. The amount of O ₂ adsorption has a great relationship with the purification performance. The smaller the crystallite diameter of CeO ₂ , the larger the amount of O ₂ adsorbed. Therefore, if the crystallite size of CeO ₂ in alumina of the calcined support is small, the catalyst in which the platinum group metal is supported on the support has improved performance as a catalyst.

To support cerium oxide on a catalyst carrier, an aqueous solution of cerium salt or cerium hydroxide is adhered to a catalyst carrier such as alumina and dried or calcined, or an aqueous solution of cerium salt and a carrier are mixed, This is carried out by a method of adding an alkali to the mixture to precipitate cerium hydroxide on the carrier, drying and firing the cerium hydroxide, and a method of attaching a slurry of alumina powder and cerium oxide powder to the porous carrier. When an alkali is added to an aqueous solution of cerium salt to form cerium hydroxide, it is disclosed in JP-A-56-14.
As disclosed in Japanese Patent No. 7634, Japanese Patent Application Laid-Open No. 57-56041, and Japanese Patent Application Laid-Open No. 57-119835, a trivalent water-soluble cerium salt is generally used, and the pH is adjusted to 8 at an alkali.
The cerium hydroxide is made to be -11.

[0005]

DISCLOSURE OF THE INVENTION According to the present invention, the cerium oxide finally obtained for such use has a smaller crystallite size, a larger oxygen storage capacity than before, and a good dispersibility on a carrier. An object of the present invention is to provide a method for producing cerium hydroxide and cerium oxide that can obtain a catalyst that can be obtained.

[0006]

[Means for Solving the Problems] In order to solve the above problems, the present invention is directed to the addition of an alkaline aqueous solution to a ceric sulfate aqueous solution to precipitate cerium hydroxide by hydrolysis. , And once the cerium hydroxide that was once generated was redissolved in the cerium sulfate aqueous solution, the addition of the alkaline aqueous solution was temporarily stopped,
After completely re-dissolving, the operation of adding an alkaline aqueous solution again is performed, and the reaction for precipitating cerium hydroxide is completed when the pH of the reaction solution in which the produced cerium hydroxide is not re-dissolved is 2.0 or less. The purpose is to obtain cerium hydroxide and to wash, dry, and calcine it to obtain cerium oxide.

Ce (SO ₄ ) ₂ .4H ₂ O is generally used as the ceric sulfate used in the present invention. As the alkali, it is possible to use sodium hydroxide, potassium hydroxide, calcium hydroxide, magnesium hydroxide, or aqueous ammonia which is generally used as an alkali.

An aqueous solution of cerium salt is 10 to 500
A concentration of about mmol / l, generally about 50 to 200 mmol / l, is used. If the concentration is lower than this, the productivity deteriorates. If the concentration is higher than this, it becomes difficult to control the pH during the reaction. 10 ~ 2000mmo as alkaline aqueous solution
l / l, about 1 to 1000 times the cerium salt aqueous solution concentration,
It is usually about 1 to 10 times.

[0009]

In the production method of the present invention, an aqueous alkali solution is added dropwise to the prepared aqueous cerium salt solution to obtain a liquid temperature of 20-80.
The hydrolysis reaction is carried out at ℃, usually 20 to 40 ℃ while vigorously stirring the solution. At this time, when an alkaline aqueous solution is dropped into the cerium salt aqueous solution, fine cerium hydroxide crystals are formed in the reaction solution. To generate. If alkali is added continuously to the reaction solution in which crystals are generated, the generated cerium hydroxide will not be uniformly dispersed in the reaction solution, and the particle size of the generated cerium hydroxide will be uneven and the crystal particles of cerium hydroxide will increase. As a result, it becomes impossible to obtain an acidic cerium having a small crystallite size by calcining the obtained cerium hydroxide. In order to avoid this, the operation of adding the alkaline aqueous solution by temporarily dropping the alkaline aqueous solution until the cerium hydroxide once generated is redissolved in the aqueous cerium sulfate aqueous solution and completely re-dissolved It is a thing.

In this hydrolysis reaction, the pH of the reaction solution is adjusted to pH.
Perform while monitoring with a meter. For example, the pH of the cerium sulfate aqueous solution is 1.0 at the start of the reaction, and the pH increases with the addition of the alkaline aqueous solution, but the manner of the increase is not constant with respect to the amount of the added alkali. We are late,
Once the neutralization point where the cerium hydroxide crystals are stably present in the reaction solution is reached, the pH suddenly rises. For this reason, if the alkali is not added gradually, the pH of the reaction solution will rise sharply even if it is monitored with a pH meter. The addition rate of alkali is 0.1 mmol / min or less, preferably about 0.01 mmol / min with respect to 1 mmol of cerium salt in the cerium salt aqueous solution.

The pH at which cerium hydroxide aqueous solution is hydrolyzed and cerium hydroxide is stably present is 1.4. However, cerium hydroxide is allowed to stably exist at such a low pH as possible and the reaction is carried out there. By stopping the above, the crystallite diameter of cerium oxide obtained from this can be minimized. It is not clear why, unlike the conventional method using a trivalent cerium salt, cerium hydroxide can be produced in such a low pH range,
It is believed that this is due to the use of a tetravalent cerium salt as a starting material.

In the present invention, the maximum value of pH during hydrolysis is set to 2.0, that is, when the reaction is completed when the pH exceeds 2.0, the crystallite size of the cerium oxide finally obtained is This is because the oxygen storage capacity becomes large and the oxygen storage capacity decreases. The reason is,
It is considered that when the pH exceeds 2.0, the number of hydrated ions coordinated with the generated cerium hydroxide increases.

The obtained cerium hydroxide precipitate is separated from the reaction mother liquor by a method such as centrifugal filtration or suction filtration. This operation may be before or after cooling the reaction to ambient temperature of 10-25 ° C. The cerium hydroxide separated from the mother liquor is washed several times with distilled water or ion-exchanged water at 5 to 90 ° C to remove the acidic anions adhering to the cerium hydroxide, and the cerium hydroxide is removed from the atmosphere or at a temperature of 10 ^{-2 to} 100 mmHg. It is dried under reduced pressure, dried at a temperature of 90 to 200 ° C. for 10 to 48 hours, and further baked at 300 to 500 ° C. for 2 to 12 hours to obtain cerium oxide.

In the present invention, the crystallite diameter L of cerium oxide is obtained by the following calculation from the line width of the (331) plane in the X-ray diffraction diagram using Scherrer's equation. L = Kλ / β · cos θ where K = 0.9: Sherrer constant λ (Å): Wavelength of Kα1 line 1.54Å β (rad): Peak line width (half width) θ: Incident angle

Further, the evaluation of the oxygen storage capacity of cerium oxide was carried out according to the method described in Journal of Catalysis., Vol.86, p.254 (1984) with respect to 0.05 g of cerium oxide as follows. Went to. 1) 3 at 500 ° C in helium gas flow (100 ml / min)
Pre-process for 0 minutes. 2) Helium flow set to 400 ℃ and 500 ℃. 3) Next, a cycle of flowing a 1% O ₂ / He gas stream for 10 seconds (2.5 ml) and then flowing He for 4 minutes and 50 seconds is continued until the introduced O ₂ is no longer consumed (cerium oxide is no more than O Repeat until ₂ no longer absorbs).

4) An air flow of 1% CO / He was applied for 10 seconds (5.
0 ml) and then He is flown for the next 4 minutes and 50 seconds, and is stopped when CO is no longer consumed (cerium oxide no longer absorbs CO). 5) Next, air flow of 1% O ₂ / He for 10 seconds (2.5 ml)
Repeat the operation of flowing He and flowing He for the next 4 minutes and 50 seconds.
Stop when ₂ is no longer consumed, and divide the total O ₂ consumption (integrated value mol) at this time by the molar amount of cerium oxide to give the OS.
Calculate CC (Oxygen Storage Capacity Cumulative).

[0017]

【Example】

Example 1 1 l of an aqueous solution of Ce (SO ₄ ) _{2 having a} concentration of 100 mmol / l was kept at 25 ° C., and the solution was vigorously stirred, and a 1 mol / l aqueous solution of sodium hydroxide was dripped from a buret at a rate of 1.0 ml / min. It was dripped at. When about 40 ml of the aqueous sodium hydroxide solution was added dropwise, formation of white microcrystalline cerium hydroxide was observed. At this point, the dropping of the aqueous sodium hydroxide solution was temporarily stopped, and when stirring was continued, the crystals completely dissolved and disappeared. The dropping of the aqueous sodium hydroxide solution was started again, and the white microcrystals formed when 180 ml of the aqueous sodium hydroxide solution was dropped were not dissolved. The pH at this time was 1.4,
The reaction was terminated at the point of time when 200 ml of the sodium hydroxide aqueous solution was added by continuing the dropwise addition of the sodium hydroxide aqueous solution. The pH at this time was 1.5.

The yellow precipitate obtained was filtered off and washed with 8
Cerium oxide powder was dispersed in 100 ml of 0 ° C. ion-exchanged water, stirred and dispersed for 5 minutes and filtered three times, dried at 120 ° C. for 12 hours, and further calcined in air at 900 ° C. for 3 hours. Got The crystallite diameter of this cerium oxide powder is 400 calculated from the above X-ray diffraction pattern.
It was Å. As a result of a test conducted according to the above-mentioned evaluation method of oxygen storage capacity of cerium oxide, OSCC was 1.
_{3mmol-O 2 / mol-CeO} 2, 5.7 at 500 ° C.
It was _{mmol-O 2 / mol-CeO} 2.

Example 2 A cerium oxide powder was obtained in the same manner as in Example 1 except that the pH at the end of the reaction was 1.9. The crystallite size of this cerium oxide powder was calculated in the same manner as in Example 1 and the result was 410Å. In addition, as a result of performing a test according to the evaluation method of the oxygen storage capacity of cerium oxide, OSCC was 1.4 mmol-O ₂ / m at 400 ° C.
ol-CeO ₂ , 5.2 mmol-O ₂ / mo at 500 ° C.
It was 1-CeO ₂ .

Comparative Example 1 A cerium oxide powder was obtained in the same manner as in Example 1 except that the pH at the end of the reaction was 2.1. The crystallite size of this cerium oxide powder was calculated in the same manner as in Example 1 and found to be 470Å. In addition, as a result of performing a test according to the evaluation method of the oxygen storage capacity of cerium oxide, the OSCC was 1.1 mmol-O ₂ / m at 400 ° C.
ol-CeO ₂ , 3.9 mmol-O ₂ / mo at 500 ° C.
It was 1-CeO ₂ .

Comparative Example 2 In Example 1, as the cerium salt, Ce (SO ₄ ) ₂
Example 1 except that 4H ₂ O was used and the final pH was 8.1.
Cerium oxide powder was obtained in the same manner as in. The crystallite size of this cerium oxide powder was calculated in the same manner as in Example 1 and the result was 530Å. In addition, CO of cerium oxide described above
As a result of performing a test according to the evaluation method of oxygen storage capacity, OSC
C is 1.0 mmol-O ₂ / mol-Ce at 400 ° C.
O ₂ , 2.4 mmol-O ₂ / mol-CeO ₂ at 500 ° C.
Met.

Comparative Example 3 In Example 1, trivalent Ce ₂ (S) was used as the cerium salt.
O ₄₎ except for using ₃ · 8H ₂ O was obtained cerium oxide powder in the same manner as in Example 1. The crystallite size of this cerium oxide powder was calculated in the same manner as in Example 1 and was found to be 500Å. Further, as a result of the test according to the evaluation method of the oxygen storage capacity of the cerium oxide, OSCC is 2.7mmol-O ₂ / mol-CeO in _{1.0mmol-O 2 / mol-CeO} 2, 500 ℃ at 400 ° C. _{Was 2} .

Comparative Example 4 Cerium sulfate tetrahydrate was decomposed by heating at 300 ° C. and further calcined at 900 ° C. for 3 hours to obtain cerium oxide. The crystallite size of this cerium oxide powder was 700Å as calculated from the results of the above X-ray diffraction pattern. As a result of performing a test according to the above-mentioned evaluation method of oxygen storage capacity of cerium oxide,
CC is 0.4mmol-O ₂ / mol-CeO at 400 ℃
₂ was 1.6 mmol-O ₂ / mol-CeO ₂ at 500 ° C.

[0024]

INDUSTRIAL APPLICABILITY According to the present invention, hydroxylation can be obtained in which the finally obtained cerium oxide has a smaller crystallite size, a larger oxygen storage capacity than before, and can be supported on the carrier with good dispersibility. A method for producing cerium and cerium oxide can be provided.

Claims (2)

[Claims] 1. When cerium hydroxide is precipitated by hydrolysis by adding an alkali to an aqueous solution of cerium sulfate, the aqueous alkali solution is dropped into the aqueous solution of cerium sulfate, and the cerium hydroxide once generated becomes an aqueous solution of cerium sulfate. The operation of adding the alkaline aqueous solution again after suspending the dropwise addition of the alkaline aqueous solution until it is redissolved and completely re-dissolving it, the pH of the reaction solution in a state in which the generated cerium hydroxide is not redissolved is 2.0. A method for producing cerium hydroxide, which comprises terminating the reaction for precipitating cerium hydroxide below. 2. When precipitating cerium hydroxide by hydrolysis by adding an alkali to an aqueous solution of cerium sulfate, the aqueous alkali solution is dropped into the aqueous solution of cerium sulfate, and the cerium hydroxide once produced becomes an aqueous solution of cerium sulfate. The operation of adding the alkaline aqueous solution again after suspending the dropwise addition of the alkaline aqueous solution until it is redissolved and completely re-dissolving it, the pH of the reaction solution in a state in which the generated cerium hydroxide is not redissolved is 2.0. A method for producing cerium oxide, in which the reaction for precipitating cerium hydroxide is terminated below, and the precipitated cerium hydroxide is washed, dried and calcined.