JP2006256911A - Preparation method of ceria-zirconia-based oxide - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a preparation method of a ceria-zirconia-based oxide suitable for a catalyst carrier for emission gas purification etc., whereby the specific surface area of ceria, zirconia or the ceria-zirconia oxide etc., can be increased and the large specific surface area can be retained, even after exposure to ≥800°C, especially 1,000°C. <P>SOLUTION: The ceria-zirconia-based oxide is prepared by adding an alkaline aqueous solution to an aqueous solution of at least one chosen from a cerium salt, a zirconium salt and a mixed salt thereof to obtain a precipitate or heating the aqueous solution to obtain a deposit through thermal hydrolysis, subsequently dispersing the obtained precipitate or deposit in an organic solvent, removing the solvent to obtain a metal salt-supported ceria-zirconia-based hydrous oxide and firing the obtained hydrous oxide e.g. at ≥200°C. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a method for producing ceria, zirconia, or ceria / zirconia oxide having a high specific surface area suitable for an exhaust gas purifying promoter support and the like.

  Conventionally, a three-way catalyst has been used as a method for purifying carbon monoxide (CO), hydrocarbons (HC), and nitrogen oxides (NOx), which are harmful substances in automobile exhaust gas. The exhaust gas is purified by simultaneously performing oxidation of CO and HC and reduction of nitrogen oxide (NOx) with a three-way catalyst. As such a three-way catalyst, for example, a carrier layer made of γ-alumina is formed on a heat-resistant honeycomb substrate made of cordierite or the like, and platinum (Pt), rhodium (Rh), palladium (Pd) is formed on the carrier layer. And the like, on which a catalytic metal such as) is supported is widely known.

  By the way, the conditions of the carrier used for the exhaust gas purifying catalyst include a large specific surface area and high heat resistance. Generally, alumina, silica, zirconia, titania and the like are often used.

Further, since the performance of the three-way catalyst is greatly influenced by the atmosphere of the exhaust gas, particularly the oxygen concentration, the air-fuel ratio is controlled while monitoring the oxygen concentration in the exhaust gas. Furthermore, the ceria (CeO ₂ ) having OSC (oxygen storage and release ability) is used as a promoter or support to mitigate fluctuations in the atmosphere of the exhaust gas. The conditions for this OSC material include a high specific surface area and high heat resistance as well as a high OSC.

However, when the conventional exhaust gas purifying catalyst is exposed to a high temperature exceeding 800 ° C., the specific surface area of the support is reduced due to sintering, and grain growth of the catalytic metal occurs. In particular, CeO ₂ has a low specific surface area at high temperature. Therefore, there is a problem that the OSC is lowered and the purification performance is remarkably lowered.

  In addition, due to the recent tightening of exhaust gas regulations, the necessity of purifying exhaust gas within a very short time after engine startup has become extremely high. When starting the engine, the exhaust gas temperature is low and the catalyst does not work sufficiently. Therefore, the exhaust gas temperature is raised by arranging the catalyst near the engine so that the catalyst works. On the other hand, when the catalyst is brought closer to the engine, the catalyst is exposed to exhaust gas reaching 800 ° C. and further 1000 ° C. during high-speed operation. For this reason, the request | requirement to the high heat resistance of a support | carrier is increasing further.

  Various methods have been proposed for increasing the specific surface area of ceria, zirconia, ceria-zirconia solid solution, and the like. For example, Non-Patent Document 1 describes a method of increasing the specific surface area by adding a surfactant to form a mesopore structure when ceria powder is synthesized by an ammonia neutralization method. Patent Document 1 discloses a method of washing a precipitate with an alcohol compound when synthesizing a ceria-zirconia mixed powder by a hydrolysis method or a neutralization method. Patent Document 2 describes a method for increasing the specific surface area by synthesizing a ceria-zirconia solid solution by a reverse micelle method. Patent Document 3 describes a method in which a surfactant micelle is used as a reaction field when a ceria-zirconia solid solution is synthesized by an ammonia neutralization method. Further, Non-Patent Document 2 describes a method of mixing ceria, zirconia, and ceria-zirconia solid solution with alumina.

JP 2001-504428 A JP 10-258231 A JP 09-221304 A Journal of Catalysis vol. 178 (1998), pages 299-308. Material Integration Vol. 16, no. 4 (2003), pp. 3-14

  As described above, methods for increasing the specific surface area of ceria, zirconia and ceria-zirconia oxides have been proposed, but these methods also provide a high ratio at a high temperature of 800 ° C. or higher, particularly 1000 ° C. Maintenance of the surface area was insufficient.

  The present invention provides a method for producing ceria, zirconia or ceria-zirconia oxide that can maintain a high specific surface area even at a high temperature of 800 ° C. or higher, particularly 1000 ° C.

The present invention has solved the above problems, the general _{formula: Ce 1-X Zr X O} 2 · nH 2 O (x = 0~1, n = 0~2) ceria-zirconia-based hydrous represented by After mixing an oxide and a metal (M) salt soluble in an organic solvent (M: at least one selected from the group consisting of alkaline earth metals and rare earths) in an organic solvent, the organic solvent is removed to remove the metal The present invention relates to a method for producing a ceria-zirconia hydrated oxide, characterized in that the salt is supported on a ceria-zirconia hydrated oxide and calcined, and the metal salt is efficiently applied to the surface of the ceria-zirconia hydrated oxide. It is possible to obtain a ceria / zirconia-based oxide having a large specific surface area by supporting and then firing.

  Hereinafter, the present invention will be described in detail.

The ceria / zirconia hydrated oxide of the present invention is represented by the general formula: Ce _1-X Zr _X O ₂ .nH ₂ O (x and n are the same as described above), and cerium, zirconium or cerium and zirconium These are compounds having oxygen and OH groups, and are hydroxides, oxides, or compounds having an intermediate composition.

  Among the ceria-zirconia-based hydrated oxides of the present invention, when Ce and Zr are included, the constituent ratio is 2/8 to 8 / in Ce / Zr atomic ratio in order to ensure sufficient OSC and specific surface area. 2 is preferable, and 3/7 to 7/3 is particularly preferable.

  The ceria / zirconia hydrated oxide includes rare earth ions such as Y, La, Pr, and Nd, and alkaline earth metals such as Mg, Ca, Sr, and Ba, in amounts that do not affect the present invention. It may contain ions, transition metal ions such as Cu and Fe, and ions such as Ti, Zn, Sn, Al, and Ga.

  The average diameter (primary particle diameter) of the ceria / zirconia hydrated oxide is preferably 1 to 20 nm. In this range, an oxide having a high specific surface area can be obtained. If it is smaller than 1 nm, the sintering of the particles proceeds during firing treatment, and the specific surface area of the obtained oxide may be reduced. If it exceeds 20 nm, the obtained oxide may not have a high specific surface area.

  The production method of the ceria / zirconia hydrated oxide of the present invention is not particularly limited, but a liquid phase synthesis method such as a coprecipitation method or a hydrothermal method is particularly preferable in order to increase the specific surface area.

  Specifically, for example, a ceria precipitate or a zirconia precipitate is formed from an aqueous solution of a cerium compound or a zirconium compound, and then mixed or coprecipitated from a mixed aqueous solution in which the cerium compound and the zirconium compound coexist. Good. At this time, a cation, anion, or nonionic surfactant may be added to the raw material for the purpose of reducing the dispersibility of the precipitate and the aggregation.

  Examples of the cerium compound that can be used in the present invention include water-soluble compounds such as nitrates such as cerium nitrate and cerium chloride, sulfates, and chlorides. As the zirconium compound, water-soluble compounds such as nitrates such as zirconium oxynitrate and zirconium oxychloride, sulfates and chlorides can be used.

  As the precipitating agent, alkali metal hydroxides such as ammonia and sodium hydroxide, alkali metal carbonates such as ammonium carbonate and sodium carbonate, and oxalates such as urea, oxalic acid, and ammonium oxalate can be used.

  There are various precipitation methods. For example, a method of adding an aqueous solution of ceria or zirconia compound to an alkaline aqueous solution such as ammonia, or conversely a method of instantly adding aqueous ammonia or the like to an aqueous solution of ceria or zirconia compound or adding hydrogen peroxide or the like to an oxide. Examples thereof include a method of adjusting the pH at which the precursor starts to precipitate and then depositing the precipitate with aqueous ammonia.

  Also, using urea as a precipitating agent, a uniform precipitation method that gradually neutralizes with ammonia generated by decomposition of urea, a method of adding a buffer solution that neutralizes by changing the pH stepwise or maintains a specific pH and so on.

  Further, there is a method in which a solution of a cerium compound and a zirconium compound is heated at, for example, 100 ° C. to 250 ° C. to obtain a precipitate by thermal hydrolysis. In the method for producing an oxide of the present invention, any of the precipitation / deposition methods described above may be used.

  The obtained precipitate is hydrothermally treated at 100 ° C. to 250 ° C. before carrying the metal salt to improve the crystallinity of the precipitate and adjust the particle size, or calcined to obtain a ratio of the precipitate. The surface area can also be adjusted.

  Next, the ceria / zirconia hydrated oxide and the metal salt are dispersed in an organic solvent, and the organic solvent is removed to support the metal salt on the surface of the hydrate. By carrying the supporting operation in an organic solvent, a ceria / zirconia oxide having a large specific surface area can be obtained. This is presumed to be because the use of an organic solvent alleviates the strong aggregation of particles when the solvent is removed, and the supported metal element is efficiently supported on the surface of the ceria / zirconia hydrated oxide. However, this assumption does not affect the present invention.

As metal ions of the metal salts used, alkaline earth metal ions such as Mg ²⁺ , Ca ²⁺ , Sr ²⁺ , Ba ²⁺ , La ³⁺ , Ce ³⁺ , Pr ³⁺ , Nd ³⁺ , Pm ³⁺ , Sm ³⁺ , Eu ³⁺ , It is at least one kind of rare earth ions such as Gd ³⁺ and Y ³⁺ . Since the specific surface area can be kept high, it is preferable to use rare earth ions.

  As the metal salt anion, inorganic acid ions such as nitrate ions, halogen ions such as chlorine ions, organic acid ions such as acetate ions, and the like that can be used in an organic solvent can be used. Among these, nitrate is preferable because impurities derived from the raw material hardly remain after firing.

  The organic solvent as used in this invention refers to the solvent which consists of organic substance. In the case of an organic solvent soluble in water, it may contain water. As the usable organic solvent, any metal salt supported on the ceria / zirconia hydrated oxide may be used, and examples thereof include alcohol and acetone. Of these, alcohol is preferred from the viewpoint of dispersibility of ceria / zirconia hydrated oxide and solubility of metal salts.

  Examples of the alcohol include linear or branched alcohols having 1 to 8 carbon atoms such as methanol, ethanol, propanol, iso-propanol, butanol, sec-butanol, tert-butanol, pentanol, hexanol, heptanol and octanol. In particular, methanol, ethanol, propanol, and iso-propanol are preferable.

  To highly disperse ceria / zirconia hydrated oxide in an organic solvent, after synthesizing ceria / zirconia hydrated oxide, disperse it in the organic solvent to be used without drying, and perform solvent replacement operation. It is the most efficient to do.

  The metal salt is preferably added in a molar ratio of added metal / ceria / zirconia hydrated oxide of 0.05 to 0.15. However, when the hydrated oxide contains Ce and Zr, the total molar amount of the molar amount of Ce and the molar amount of Zr is used as the denominator.

  If the content of the additive metal is less than 0.05, the surface coating is insufficient, the sintering of the particles proceeds during the firing treatment, and the maintenance ratio of the specific surface area of the resulting oxide may be reduced. If it exceeds 1, the surface concentration increases, and the OSC characteristics of the ceria / zirconia oxide may be deteriorated. More preferably, it is 0.05 to 0.1. Within this range, the specific surface area and the OSC characteristics are balanced, so that the characteristics are the best as a catalyst carrier.

  Subsequently, the dispersion of the metal salt and the ceria / zirconia hydrated oxide is sufficiently stirred in the range of room temperature to the boiling point of the solvent. The stirring time is not particularly limited, but 24 hours or less is sufficient.

  After stirring, the solvent is removed by filtration or distillation, and dried to obtain ceria / zirconia hydrated oxide particles carrying a metal salt. When almost all of the added metal salt is supported, only the organic solvent is evaporated and dried.

  Next, the ceria / zirconia hydrated oxide particles supported by the obtained metal salt are heated and held at 200 ° C. to 1200 ° C. to obtain a ceria / zirconia oxide having a large specific surface area. Can do. More preferably, baking is performed at 300 ° C. to 1000 ° C. If the heating and holding temperature is lower than 200 ° C., impurities may remain, and if it is higher than 1200 ° C., the specific surface area may be significantly reduced.

  The atmosphere during firing may be any of oxidizing, reducing and inert atmospheres. In order to improve the OSC characteristics, firing is performed in a reducing and inert atmosphere.

  The ceria / zirconia oxide of the present invention is an oxide containing a metal element of a metal salt on the surface of any one or a mixture of ceria, zirconia or ceria-zirconia solid solution.

  In the present invention, the composition ratio of cerium and zirconium in the ceria-zirconia solid solution is preferably such that the Ce / Zr atomic ratio is 2/8 to 8/2 in order to ensure sufficient OSC and specific surface area. 3/7 to 7/3 is particularly preferable.

  The ceria / zirconia oxide of the present invention includes rare earth ions such as Y, La, Pr, and Nd, and alkaline earth metals such as Mg, Ca, Sr, and Ba in amounts that do not affect the present invention. Metal ions, transition metal ions such as Cu and Fe, and ions such as Ti, Zn, Sn, Al, and Ga may be included.

The ceria / zirconia oxide of the present invention has a specific surface area of 20 m ² / g or more even after being kept at 1000 ° C. for 1 hour. In particular, in the case of ceria-zirconia oxide, a specific surface area of 30 m ² / g or more can be easily achieved even after holding at 1000 ° C. for 1 hour.

A ceria / zirconia oxide containing ceria and zirconia has an oxygen storage capacity of 800 μmol-O ₂ / g-CeO ₂ or more.

  Thus, the ceria / zirconia oxide of the present invention has a high specific surface area and can maintain a high specific surface area even at a high temperature of 800 ° C. or higher. It is useful as a carrier for exhaust gas purification catalysts. When the oxide of the present invention is used as a catalyst support for exhaust gas purification, only the oxide of the present invention may be used as a support, or may be used by mixing with other oxides such as alumina. May be used as a catalyst.

  As the noble metal, for example, one type or a plurality of types can be selected and used from Pt, Rh, Pd, Ir, Ru and the like, and the supported amount may be the same as that of the conventional exhaust gas purification catalyst. As the loading method, a conventional loading method such as an adsorption loading method or a water absorption loading method can be used.

  The carrier referred to in the present invention includes those having a function of simply supporting a catalyst component and those having a function of a promoter for a catalytic reaction.

  The exhaust gas purification catalyst of the present invention has sufficiently high exhaust gas purification characteristics because the specific surface area of the carrier is large even after being exposed to a high temperature of 800 ° C. or higher, particularly 1000 ° C.

  According to the present invention, a ceria, zirconia or ceria-zirconia oxide capable of maintaining a large specific surface area even after being exposed to a high temperature of 800 ° C. or higher, particularly 1000 ° C. can be provided. Further, by using the oxide as a catalyst carrier, it is possible to provide an exhaust gas purification catalyst having a large specific surface area and high exhaust gas purification characteristics even under a high temperature condition of 800 ° C. or higher.

EXAMPLES Hereinafter, although an Example demonstrates this invention concretely, this invention is not limited only to these Examples. In addition, the measurement of the various physical-property values of the powder obtained by the Example and the comparative example was performed as follows.
(1) Composition analysis The composition of the powder was measured by ICP emission spectrometry.
(2) Crystal Structure The crystal structure of the powder was identified by a powder X-ray diffractometer (manufactured by Mac Science, trade name “MPX3”). Cu-Kα rays were used as the X-ray source.
(3) Specific surface area measurement The specific surface area of the powder was measured with a one-point specific surface area measurement device (manufactured by Yuasa Ionics, trade name “MONOSORB”).
(4) OSC measurement The OSC measurement was performed using a self-made temperature-reduction apparatus (TPR). The temperature was raised at a H ₂ = 20% atmosphere to 900 ° C. from room temperature 10 ° C. / min, and _{H 2} amount consumed in the range of 200 to 700 ° C. TCD-GC - detected by, OSC from _{H 2} amount Was calculated. The sample was previously subjected to an oxidation treatment in the atmosphere at 500 ° C. for 1 hour, and then the measurement was performed.
Example 1
An H ₂ O ₂ liquid (H ₂ O ₂ ) was added to a mixed liquid (Ce / Zr molar ratio = 1/1) of a Ce (NO ₃ ) ₃ solution (0.15 M) and a ZrO (NO ₃ ) ₂ solution (0.15 M). / Ce molar ratio = 1/1) was added, and ammonia water was poured until pH = 9 while stirring to obtain a precipitate. The precipitate was filtered and washed with water, then dispersed in ethanol, and the solution was stirred.

Subsequently, La (NO ₃ ) ₃ · 6H ₂ O / (Ce + Zr) molar ratio = 0.1 La (NO ₃ ) ₃ · 6H ₂ O was dissolved in ethanol, and then the precipitate was dispersed in the ethanol solution. Added and stirred for 1 hour. Next, this solution was dried using an evaporator to obtain La-supported ceria / zirconia hydrated oxide.

The La-supported ceria / zirconia hydrated oxide thus obtained was calcined at 800 ° C. or 1000 ° C. for 1 hour in the air to obtain the ceria / zirconia oxide of the present invention.
(Example 2)
As the metal salt, _{Y (NO 3)} the _{3 · 6H 2 O, Y (} NO 3) 3 · 6H 2 O / (Ce + Zr) except for using a molar ratio = 0.05 by following the procedure of Example 1 A Y-supported ceria / zirconia hydrated oxide of the present invention was obtained. Moreover, it carried out similarly to Example 1, and baked at 800 degreeC or 1000 degreeC in air | atmosphere for 1 hour, respectively, and obtained the ceria zirconia type oxide of this invention.
(Example 3)
As the metal salt, La (NO ₃ ) ₃ · 6H ₂ O is used, La (NO ₃ ) ₃ · 6H ₂ O / (Ce + Zr) molar ratio = 0.12, Ca (NO ₃ ) ₂ · 4H ₂ O is used as Ca The same operation as in Example 1 was carried out except that the (NO ₃ ) ₂ .4H ₂ O / (Ce + Zr) molar ratio = 0.03 was used to obtain La and Ca-supported ceria / zirconia hydrated oxide of the present invention. It was.

The thus obtained La and Ca-supported ceria / zirconia hydrated oxide is calcined at 800 ° C. or 1000 ° C. for 1 hour in a nitrogen gas containing 4% of H ₂ for ceria / zirconia oxidation according to the present invention. A product powder was obtained.
Example 4
While adding H ₂ O ₂ liquid (H ₂ O ₂ / Ce molar ratio = 1/1) to Ce (NO ₃ ) ₃ solution (0.3 M) and stirring, aqueous ammonia was poured until pH = 9, After obtaining the precipitate, the same operation as in Example 1 was performed to obtain the La-supported ceria hydrated oxide of the present invention. Further, in the same manner as in Example 1, the ceria / zirconia oxide of the present invention was obtained by firing at 800 ° C. or 1000 ° C. for 1 hour in the air.
(Example 5)
A ZrOCl ₂ solution (0.3 M) was heated at 100 ° C. for 5 days to obtain a white turbid liquid in which a hydrolyzed precipitate was formed, and ammonia water was added thereto until pH = 9 to cause aggregation, followed by filtration and separation. . The precipitate was filtered and washed with water, then dispersed in ethanol, and the solution was stirred. Nd (NO ₃ ) ₃ .6H ₂ O / Zr molar ratio = 0.1 Nd (NO ₃ ) ₃ · 6H ₂ O was dissolved in ethanol and then added and stirred for 1 hour.

  Next, this solution was dried using an evaporator to obtain an Nd-supported zirconia hydrated oxide.

Further, in the same manner as in Example 1, the ceria / zirconia oxide of the present invention was obtained by firing at 800 ° C. or 1000 ° C. for 1 hour in the air.
(Comparative Example 1)
A sample was obtained in the same manner as in Example 1 except that distilled water was used as a solvent when La (NO ₃ ) ₃ · 6H ₂ O was supported. The sample thus obtained was fired at 800 ° C. or 1000 ° C. for 1 hour in the air to obtain an oxide.
(Comparative Example 2)
Mixed solution (Ce / Zr / La molar ratio = 1) of Ce (NO) ₃ solution (0.15M), ZrO (NO ₃ ) ₂ solution (0.15M) and La (NO ₃ ) ₃ solution (0.15M) H ₂ O ₂ liquid (H ₂ O ₂ / Ce molar ratio = 1/1) was added to /1/0.2), and ammonia water was poured to pH = 9 while stirring to obtain a precipitate. The obtained precipitate was filtered, washed with water, solvent-substituted with ethanol, and dried to obtain a sample. The sample thus obtained was fired at 800 ° C. or 1000 ° C. for 1 hour in the air to obtain an oxide.
(Comparative Example 3)
Except for the use of distilled water as a solvent at the time of loading of _{La (NO 3) 3 · 6H} 2 O was obtained a sample in the same manner as in Example 4. The sample thus obtained was fired at 800 ° C. or 1000 ° C. for 1 hour in the air to obtain an oxide.

  About the ceria zirconia-type oxide obtained in Examples 1-5 and the oxide obtained in Comparative Examples 1-3, those specific surface areas (calculated at 800 ° C. or 1000 ° C. for 1 hour each) were measured by the BET method. did. Moreover, the crystal phase of these oxides was identified from the XRD measurement. The results are shown in Table 1.

  It can be seen that the ceria and zirconia oxides obtained in Examples 1 to 5 have a larger specific surface area after firing at 800 ° C. and 1000 ° C. than the oxides obtained in Comparative Examples 1 to 3.

実施例１〜４で得られたセリア・ジルコニア系酸化物（８００℃又は１０００℃で各１時間焼成）について、２００〜７００℃における酸素貯蔵能を試料から放出される酸素によって消費されるＨ_２ガス量を測定するＴＰＲ曲線から求めた。結果を表２に示す。

For ceria-zirconia-based oxide obtained in Examples 1 to 4 (each 1 hour calcination at 800 ° C. or 1000 ℃), _{H 2} consumed by oxygen released oxygen storage capacity at 200 to 700 ° C. from the sample It calculated | required from the TPR curve which measures gas amount. The results are shown in Table 2.

Claims (3)

Formula _{Ce 1-X Zr X O 2} · nH 2 O (x = 0~1, n = 0~2) and ceria-zirconia hydrous oxide represented by the soluble metal organic solvent (M ) After mixing the salt (M: at least one selected from the group consisting of alkaline earth metals and rare earths) in an organic solvent, the organic solvent is removed and the metal salt is supported on the ceria / zirconia hydrated oxide And firing the ceria-zirconia-based oxide. The production method according to claim 1, wherein nitrate is used as the metal salt. The production method according to claim 1, wherein the supported amount of metal (M) is 0.05 to 0.15 in terms of a molar ratio of metal (M) / ceria / zirconia hydrated oxide.