JP2003275580A - Oxygen storage material - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an oxygen storage material which consists of a multi- component oxide of ceria and zirconia, develops high OSC from a low- temperature region near 200°C and can suppress the reaction with SOx. <P>SOLUTION: The oxygen storage material has a ratio of Ce and Zr ranging 0.7≤Zr/(Ce+Zr)≤0.85 and is subjected to a reduction treatment in a temperature range of from 1,000 to 1,300°C. The composition region of the boundary where a zirconia phase can assume both of a tetragonal phase and a monoclinic phase makes the structure unstable and exerts a microscopical stress on the ceria and the zirconia, thereby probably activating the diffusion, absorption and release of the oxygen. Since Ce is little, the approach of the SOx is prevented. <P>COPYRIGHT: (C)2003,JPO

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention relates to an oxygen storage material composed of a composite oxide of ceria and zirconia. Since the oxygen storage material of the present invention exhibits a large oxygen storage / release capacity (oxygen storage capacity, hereinafter referred to as OSC) even in a low temperature range near 200 ° C, it is a catalyst for exhaust gas purification, a diesel particulate oxidation catalyst, a solid catalyst. It can be used as an electrolyte, an electrode material, ceramics dispersion strengthening particles, an ultraviolet shielding material, and the like.

[0002]

2. Description of the Related Art Ceria, which has an OSC, is widely used as a co-catalyst for an exhaust gas purification catalyst that purifies exhaust gas from an internal combustion engine. In addition, since it is desirable to increase the specific surface area in order to increase the oxygen storage capacity,
Ceria is used as a powder.

However, when used as an exhaust gas purifying catalyst, it is necessary that the purifying activity at high temperatures is high. Therefore, it is required for ceria that even if it is used as a powder with a high specific surface area, the specific surface area does not decrease when it is used at high temperatures, that is, it has excellent heat resistance.

Therefore, it has been conventionally proposed to form a solid solution of zirconia or an oxide of a rare earth element in ceria.
For example, Japanese Patent Laid-Open No. 4-055315 discloses a method for producing fine cerium oxide powder by coprecipitating ceria and zirconia from a mixed aqueous solution of a water-soluble salt of cerium and a water-soluble salt of zirconium, and heat-treating it. There is. According to this manufacturing method, the coprecipitate is heat-treated to form an oxide solid solution in which ceria and zirconia are solid-solved with each other.

Further, Japanese Patent Application Laid-Open No. 4-284847 discloses that a powder in which zirconia or an oxide of a rare earth element and ceria are solid-dissolved is produced by an impregnation method or a coprecipitation method.

However, the conventional ceria-zirconia solid solution does not have sufficient OSC, and its improvement has been strongly desired. Therefore, JP-A-9-221304 proposes ceria-zirconia solid solution particles in which the solid solubility of zirconia is 50% or more and the average diameter of crystallites in the particles is 100 nm or less. According to this ceria-zirconia solid solution, the molar ratio of Zr / (Ce + Zr) is in the range of 0.25 to 0.75 and the amount of ceria is relatively large.
Excellent OSC of 250-800 μmol O ₂ / g or more.

Further, in JP-A-11-165067, a mixture or coprecipitate of a cerium (III) compound and a zirconium (IV) compound is heated in a non-oxidizing atmosphere at 500 to 1000 ° C.,
A ceria-zirconia composite oxide is disclosed, which is obtained by thermally decomposing a cerium (III) -zirconium (IV) composite oxide, and then heating this in an oxidizing atmosphere at 400 to 1000 ° C. The ceria-zirconia composite oxide thus obtained is a cubic crystal having a pyrochlore-like structure and exhibits a high OSC.

However, in these ceria-zirconia composite oxides, as is clear from FIG. 3 of JP-A No. 11-165067, a high OSC is shown at a temperature above 400 ° C., but a low temperature range below 300 ° C. Does not indicate OSC. Therefore, when used as an exhaust gas purifying catalyst for automobiles, it is difficult to improve the purifying activity in a low temperature range such as at the time of starting.

Further, Japanese Patent Application Laid-Open No. 8-103650 discloses a ceria-zirconia composite oxide containing hafnium oxide, which is obtained by subjecting it to a reduction treatment and then an oxidation treatment.
It is described that it exhibits a high OSC of molO ₂ / g or higher. It is described that OSC is slightly expressed even at about 300 ° C.

However, since the conventional ceria-zirconia composite oxide is used in a range in which the amount of ceria exhibiting a high OSC is relatively large, the sulfur oxide in the exhaust gas reacts with Ce in the low temperature range of 200 to 300 ° C. To produce cerium sulfate,
There is a problem that the ceria-zirconia solid solution decomposes. There is also a problem that it is not practical because the OSC in the low temperature range of about 200 ° C is too low.

[0011]

The present invention has been made in view of such circumstances, and in an oxygen storage material composed of a ceria-zirconia composite oxide, a high OSC is exhibited from a low temperature range around 200 ° C. Moreover, it is an object of the present invention to provide an oxygen storage material capable of suppressing the reaction with sulfur oxides.

[0012]

The oxygen storage material of the present invention for solving the above-mentioned problems is characterized by comprising a composite oxide of ceria and zirconia, wherein the ratio of cerium to zirconium is 0.7 ≦ Zr / (Ce + Zr in molar ratio). ) ≦ 0.85, 1000 ~
It consists of being reduced in the temperature range of 1300 ° C.

It is particularly desirable to carry out the reduction treatment in the temperature range of 1100-1250 ° C.

[0014]

BEST MODE FOR CARRYING OUT THE INVENTION The oxygen storage material of the present invention has a molar ratio of cerium and zirconium of 0.7 ≦ Zr / (Ce + Zr).
It is in the range of ≤0.85, and is reduced in the temperature range of 1000 to 1300 ° C, more preferably 1100 to 1250 ° C. As a result, high OSC is expressed in the low temperature range around 200 ° C. The reason is not clear, but since the zirconia phase is a boundary composition region that can take both a tetragonal phase and a monoclinic phase, the structure becomes unstable and microscopic stress is exerted on the coexisting ceria-zirconia phase. It is considered that oxygen diffusion and absorption / release are activated.

Since the reduction treatment is carried out in the temperature range of 1000 to 1300 ° C., Ce atoms and Zr in the ceria-zirconia phase are
It is thought that the atoms are regularly arranged, but no special crystalline phase has been observed in experiments to date.

The oxygen storage material of the present invention has a sufficient amount of Zr atoms with respect to Ce atoms. Therefore, the basicity due to the Ce atom is lowered, and the proximity of sulfur oxides is prevented. Therefore, when it is used as a catalyst for purifying exhaust gas of automobiles, generation of cerium sulfate in a low temperature range is prevented, and decomposition of a ceria-zirconia solid solution is suppressed.

The oxygen storage material of the present invention has a molar ratio of cerium and zirconium of 0.7≤Zr / (Ce + Zr) ≤.
It should be in the range of 0.85. Molar ratio Zr / (Ce +
When Zr) is less than 0.7, the temperature range in which OSC is expressed shifts to the high temperature side, and the basicity increases, so that the reaction with sulfur oxides causes decomposition of the ceria-zirconia solid solution. If the molar ratio Zr / (Ce + Zr) exceeds 0.85, OSC will drop over the entire temperature range.

The oxygen storage material of the present invention may contain other metal elements such as rare earth elements, alkaline earth metals, iron and titanium as long as the ratio of cerium to zirconium satisfies the above range. For example, the solid solution phase can be stabilized by including at least one of an oxide of an alkaline earth element and an oxide of a rare earth element other than cerium. However, the content of these other metal elements is preferably 10 mol% or less with respect to the total cations. If the content of these other metal elements is higher than this, the OSC will drop over the entire temperature range.

To produce the oxygen storage material of the present invention, first, a ceria-zirconia composite oxide is prepared. Although the alkoxide method can be used for this purpose, the coprecipitation method is preferably used. For example, it can be manufactured by adding a surfactant and an alkaline substance to an aqueous solution in which at least Ce element and Zr element are dissolved to form a precipitate, and baking the precipitate.

Although the action of the surfactant is not clear here, it is presumed as follows. That is, in the state just neutralized with the alkaline substance, at least the Ce element and the Zr element are precipitated in the state of a very fine hydroxide or oxide having a particle diameter of several nm or less. And by adding a surfactant,
Plural kinds of precipitated particles are uniformly incorporated into the micelle of the surfactant. Then, as the neutralization, aggregation, and aging proceed in the micelle, the production of solid solution particles proceeds in a small space where a plurality of components are uniformly contained and concentrated. Furthermore, the dispersive effect of the surfactant improves the dispersibility of the precipitated fine particles, reduces the segregation, and increases the degree of contact.

The surfactant may be added before the alkaline substance, at the same time as the alkaline substance, or after the alkaline substance. However, if the surfactant is added too late, segregation will occur, so it is desirable to add it at the same time as or before the addition of the alkaline substance.

Examples of the dissolved compounds of Ce element and Zr element include cerium (III) nitrate, cerium (IV) ammonium nitrate, cerium (III) chloride, and cerium sulfate (III).
), Cerium (IV) sulfate, zirconium oxynitrate,
Examples include zirconium oxychloride and the like. Further, as the alkaline substance, any substance that exhibits alkalinity as an aqueous solution can be used. Ammonia, which can be easily separated on heating, is particularly desirable. However, other alkaline substances such as alkali metal hydroxides can be used because they can be easily removed by washing with water.

As the surfactant, any of anionic, cationic and nonionic surfactants can be used.
Among them, a surfactant that can easily form a narrow space inside the formed micelle, for example, a spherical micelle is preferable. Further, it is desirable that the critical micelle concentration (cmc) is 0.1 mol / L or less. More preferably, the surfactant is 0.01 mol / L or less.

Examples of these surfactants include alkylbenzene sulfonic acid and its salt, α-olefin sulfonic acid and its salt, alkyl sulfate ester salt, alkyl ether sulfate ester salt, phenyl ether sulfate ester salt, and methyl taurate phosphate. , Sulfosuccinate, ether sulfate, alkyl sulfate, ether sulfonate,
Saturated fatty acids and salts thereof, unsaturated fatty acids such as oleic acid and salts thereof, other anionic surfactants such as carboxylic acid, sulfonic acid, sulfuric acid, phosphoric acid and phenol derivatives, polyoxyethylene polyprolene alkyl Ether, polyoxyethylene alkyl ether, polyoxyethylene alkylphenyl ether, polyoxyethylene polystyryl phenyl ether, polyoxyethylene polyoxypolypropylene alkyl ether, polyoxyethylene polyoxypropylene glycol, polyhydric alcohol; glycol; glycerin; sorbitol; Mannitol; pentaethritol; sucrose; etc. fatty acid partial ester of polyhydric alcohol, polyhydric alcohol; glycol; glycerin; sorbitol; mannitol; pentaethritol; sucrose Etc. Polyoxyethylene fatty acid partial ester of polyhydric alcohol, polyoxyethylene fatty acid ester, polyoxyethylenated castor oil, polyglycene fatty acid ester, fatty acid diethanolamide, polyoxyethylene alkylamine, triethanolamine fatty acid partial ester, trialkylamine Nonionic surfactants such as oxides, primary fatty amine salts, secondary fatty amine salts, tertiary fatty amine salts, tetraalkylammonium salts; trialkylbenzylammonium salts; alkylpyrrodinium salts; 2-alkyl-1 -Alkyl-1
-Hydroxyethyl imidazolinium salt; N, N-dialkylmorpholinium salt; Polyethylene polyamine fatty acid amide salt; quaternary ammonium salt such as; cationic surfactants such as; and zwitterionic surface active agents such as betaine compounds It is at least one selected from agents.

The critical micelle concentration (cmc) is the lowest concentration at which a surfactant forms micelles.

The amount of the surfactant added is preferably in the range of 1 to 50 parts by weight with respect to 100 parts by weight of the ceria-zirconia composite oxide. By adjusting the amount to be 1 part by weight or more, the solid solubility is further improved. If it exceeds 50 parts by weight, the surfactant may be difficult to effectively form micelles.

In the above manufacturing method, it is necessary to pay attention to the valence of cerium. In the case of tetravalent cerium, ceria relatively easily forms a solid solution with zirconia, and thus the ceria-zirconia composite oxide most suitable for the present invention can be manufactured by the above manufacturing method. However, in the case of trivalent cerium, ceria does not easily form a solid solution with zirconia, so it is desirable to adopt another means.

Therefore, in this case, it is preferable to add hydrogen peroxide. In this way, cerium (III
) Forms a complex with hydrogen peroxide and is oxidized to cerium (I
V), ceria can be easily dissolved in zirconia to form a solid solution.

The amount of hydrogen peroxide added is preferably 1/4 or more of cerium ions. If the amount of hydrogen peroxide added is less than ¼ of the cerium ion, the solid solution of ceria and zirconia will be insufficient. Excessive addition of hydrogen peroxide has no particular adverse effect, but it is economically disadvantageous and has no merit. It is more preferable that the amount of hydrogen peroxide be in the range of 1/2 to 2 times that of cerium ions.

The time of addition of hydrogen peroxide is not particularly limited, and it may be added before the addition of the alkaline substance and the surfactant, or at the same time as or after the addition thereof. Further, hydrogen peroxide is a particularly desirable oxidant because it does not require post-treatment, but in some cases, other oxidants such as oxygen gas, ozone, peroxides such as perchloric acid and permanganate can be used. .

Further, the aqueous solution should be 10 ³ sec ^-1 or more, preferably
It is also preferable to obtain a precipitate by adding an alkaline substance while high-speed stirring at a high shear rate of 10 ⁴ sec ^-1 or more. The components in the precipitated fine particles, which are neutralization products, are inevitably segregated to some extent. The degree of contact between the cerium source and the zirconium source is further improved by making this segregation uniform by vigorous stirring and improving the dispersibility. Also, when coprecipitating from an aqueous solution of cerium salt and zirconium salt, the precipitation pH of both is different, and thus precipitated particles of the same kind are likely to be aggregated. Then, high-speed stirring at a high shear rate destroys a group of precipitated fine particles of the same type, so that the precipitated particles are well mixed.

Therefore, according to this method, the solid solubility can be improved and the average grain size of the crystallite can be further reduced. When the shear rate is less than 10 ³ sec ^-1 , the solid solution promoting effect is not sufficient. The shear rate V is represented by V = v / D. Here, v is the speed difference (m / sec) between the rotor and the stator of the stirrer, and D is the gap (m) between the rotor and the stator. According to this method, solid solution is promoted even when trivalent cerium is used, so that a ceria-zirconia composite oxide having a high solid solubility can be produced without being oxidized to tetravalent with hydrogen peroxide. .

The obtained precipitate is filtered and washed and then calcined to obtain a ceria-zirconia composite oxide. This firing step can be performed by heating in air at 150 to 600 ° C.

The obtained ceria-zirconia composite oxide is subjected to reduction treatment in the temperature range of 1000 to 1300 ° C. By this reduction treatment, the cerium atom becomes trivalent in the composite oxide to generate oxygen defects, the phase diffusion of cerium and zirconium is facilitated, and the solid solution of ceria and zirconia is easily promoted. Furthermore, zirconium is regularly arranged, and the skeleton of zirconia can be surely formed in ceria. And the molar ratio is 0.7 ≦ Zr / (Ce + Zr) ≦
Since it is in the range of 0.85, the zirconia phase is a boundary composition region in which both the tetragonal phase and the monoclinic phase can be taken, and the structure becomes unstable and the microscopic stress is exerted on the coexisting ceria-zirconia solid solution phase, It is thought to activate oxygen diffusion and absorption / desorption.

When the heating temperature in the reduction treatment is lower than 1000 ° C., mutual diffusion becomes insufficient and the formation of the ceria-zirconia solid solution phase becomes insufficient, resulting in a decrease in OSC.
On the other hand, if the heating temperature exceeds 1300 ° C, the oxide may sinter and the particles may aggregate.

The atmosphere for the reduction treatment is hydrogen, C
There is no particular limitation as long as it is a reducing atmosphere containing O, hydrocarbons, ammonia, and other organic substances.

[0037]

EXAMPLES The present invention will be specifically described below with reference to Examples and Comparative Examples.

Example 1 Cerium (III) nitrate and zirconyl nitrate were mixed at a molar ratio of Ce / Zr = 25/75 (Zr / (Ce + Z
r) = 0.75) was mixed to prepare an aqueous solution, and ammonia water was added dropwise with stirring in a glass beaker to neutralize and form a precipitate. Then, a hydrogen peroxide solution containing hydrogen peroxide in a mole number of 1/2 of the cerium ion contained in this mixed aqueous solution and an aqueous solution containing 10% by weight of the obtained oxide of alkylbenzenesulfonic acid are added and mixed. It was stirred.

While keeping the obtained slurry in a glass beaker, it was heated to 400 ° C. at a heating rate of 50 ° C./hour, and 400 ° C.
After that, the coexisting ammonium nitrate was evaporated or decomposed to prepare a ceria-zirconia composite oxide powder.

Next, the obtained ceria-zirconia composite oxide powder was put into a crucible with a lid together with graphite powder, and the temperature was set to 1200 ° C. in a reducing atmosphere containing CO due to partial oxidation of graphite.
Was heat-treated for 5 hours to prepare the oxygen storage material of this example.
At this time, a double crucible was used so that the sample and the graphite powder did not mix, and the inside was used as the sample and the outside was used as the graphite powder to separate the two.

Example 2 A Ce / Zr molar ratio of 20/80 (Zr /
Ceria-zirconia composite oxide powder was prepared in the same manner as in Example 1 except that (Ce + Zr) = 0.80), and reduction treatment was performed in the same manner.

Example 3 A Ce / Zr molar ratio of 15/85 (Zr /
Ceria-zirconia composite oxide powder was prepared in the same manner as in Example 1 except that (Ce + Zr) = 0.85), and reduction treatment was performed in the same manner.

Example 4 A Ce / Zr molar ratio of 30/70 (Zr /
Ceria-zirconia composite oxide powder was prepared in the same manner as in Example 1 except that (Ce + Zr) = 0.70), and reduction treatment was performed in the same manner.

(Example 5) The Ce / Zr molar ratio was set to 27.5 / 72.5.
Example 1 except that (Zr / (Ce + Zr) = 0.725)
A ceria-zirconia composite oxide powder was prepared in the same manner as in, and the reduction treatment was performed in the same manner.

(Comparative Example 1) A Ce / Zr molar ratio of 10/90 (Zr /
Ceria-zirconia composite oxide powder was prepared in the same manner as in Example 1 except that (Ce + Zr) = 0.90), and reduction treatment was performed in the same manner.

(Comparative Example 2) The Ce / Zr molar ratio was 50/50 (Zr /
Ceria-zirconia composite oxide powder was prepared in the same manner as in Example 1 except that (Ce + Zr) = 0.50), and reduction treatment was performed in the same manner.

(Comparative Example 3) A Ce / Zr molar ratio of 60/40 (Zr /
Ceria-zirconia composite oxide powder was prepared in the same manner as in Example 1 except that (Ce + Zr) = 0.40), and reduction treatment was performed in the same manner.

<Testing / Evaluation> 1% by weight of platinum was supported on each of the oxygen storage materials of Examples and Comparative Examples by an impregnation method to prepare respective catalysts. OSC was measured for each of these catalysts. OSC is measured by TPR (Temper), which measures the weight change during reduction using a thermogravimetric analyzer.
(ature programmed reduction) method. First, each sample was oxidized in N ₂ containing 50% O ₂ at 500 ° C, cooled to room temperature, and then heated in N ₂ containing 20% H ₂ at 10 ° C / min to reduce the weight loss. It was measured. This weight loss corresponds to the amount of released O ₂ (OSC).

The results of the oxygen storage materials of Examples 1-5 and Comparative Examples 1-2 are shown in FIG. The vertical axis in Fig. 1 is 15 mg of sample
Weight loss per unit (mg), and the larger the absolute value of this value, the higher the OSC.

The weight loss at 500 ° C. and the weight loss at 200 ° C. of each sample are shown in Table 1 as OSC. The ratio (improvement rate) of the weight reduction amount at 200 ° C to the weight reduction amount at 500 ° C was calculated and is also shown in Table 1.

[0051]

[Table 1]

From FIG. 1 and Table 1, it is clear that the oxygen storage materials of the respective examples have a larger weight loss than the comparative example 1 and exhibit high OSC. However, since the oxygen storage material of Example 4 has a lower improvement rate than the other Examples, Zr /
It can be seen that the lower limit of the (Ce + Zr) molar ratio is 0.70. Further, in Comparative Example 1, the improvement rate is high, but the absolute value of the weight reduction amount is small. Therefore, it is understood that it is desirable to set the Zr / (Ce + Zr) molar ratio to 0.85 of Example 3 as the upper limit. Further, in Comparative Examples 2 and 3, the OSC is high because of the large amount of Ce, but the improvement rate is inferior to that of Examples 1 to 3, and when the amount of Ce is large as described above, the sulfur oxides in the exhaust gas are likely to come close to each other, and May become sulfate.

The oxygen storage materials of Comparative Examples 2 and 3 were in the composition range of the conventional ceria-zirconia solid solution, and although the OSCs at 500 ° C. were both high, the OSC at 200 ° C. increased sharply when the ceria content increased up to Comparative Example 3. Has fallen to. In other words, if the amount of ceria is large, the O
SC cannot be improved.

[0054]

That is, according to the oxygen storage material of the present invention,
Using a ceria-zirconia composite oxide that has a larger amount of zirconia than before, a high OSC is exhibited even at a low temperature range of about 200 ° C. Therefore, since the formation of sulfate is prevented, it is optimally used as a catalyst for exhaust gas containing sulfur oxides.

[Brief description of drawings]

FIG. 1 is a graph showing the relationship between temperature and weight loss in oxygen storage materials of Examples and Comparative Examples.

   ─────────────────────────────────────────────────── ─── Continued front page    (72) Inventor Yoshio Ukyo             Aichi Prefecture Nagachite Town Aichi District             Local 1 Toyota Central Research Institute Co., Ltd. (72) Inventor Tsuyoshi Sasaki             Aichi Prefecture Nagachite Town Aichi District             Local 1 Toyota Central Research Institute Co., Ltd. (72) Inventor Akira Morikawa             Aichi Prefecture Nagachite Town Aichi District             Local 1 Toyota Central Research Institute Co., Ltd. (72) Inventor Yoshie Yamamura             Aichi Prefecture Nagachite Town Aichi District             Local 1 Toyota Central Research Institute Co., Ltd. (72) Inventor Masaaki Sugiura             Aichi Prefecture Nagachite Town Aichi District             Local 1 Toyota Central Research Institute Co., Ltd. F-term (reference) 4D048 AA14 AB01 BA08X BA19X                       BA30X BA42X EA04                 4G048 AA03 AB01 AB03 AC08 AD03                       AE05                 4G066 AA12B AA23B AA43D BA36                       CA37 DA02 EA20 FA03 FA05                       FA21 FA34 FA37 GA01                 4G069 AA01 AA08 BA05A BA05B                       BB06A BB06B BC43A BC43B                       BC51A BC51B CA02 CA03                       CA07 CA18 FA01 FB44 FC07                       FC08

Claims (2)

[Claims] 1. A composite oxide of ceria and zirconia, wherein the molar ratio of cerium to zirconium is 0.7 ≦.
An oxygen storage material, which is in the range of Zr / (Ce + Zr) ≦ 0.85 and is reduced in the temperature range of 1000 to 1300 ° C. 2. The oxygen storage material according to claim 1, wherein the reduction treatment is performed in a temperature range of 1100-1250 ° C.