JP2005139029A - Oxide powder, its preparation process, and catalyst - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To enable the pore distribution of an oxide powder to be controlled finely; to make the pore diameter more smaller; and to further increase the pore volume. <P>SOLUTION: The oxide powder is prepared by a process comprising precipitating an oxide precursor from an aqueous solution at least containing a cerium compound dissolved therein, agitating a suspension containing the resultant precipitate and a surfactant, and baking the agitated suspension. In the agitation step, at least either of the following control operations is carried out: the suspension is prepared so that the weight of the oxide powder yielded is 1-30 wt.% of that of the solvent; and the agitation is conducted under such conditions as to control the shear rate to 10<SP>3</SP>-10<SP>4</SP>/sec. Because the shear efficiency is high, a pore distribution of which the central pore diameter is 20 nm or less and the volume of pores of 30 nm or less is 0.1 cc/g or larger can be formed. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to a ceria-based oxide powder useful as a catalyst carrier, a method for producing the same, and a catalyst using the oxide powder as a carrier.

As an exhaust gas purifying catalyst conventionally automobiles, three-way catalyst for purifying performing the reduction of the oxidized and NO _x CO and HC in the exhaust gas simultaneously is used. As such a three-way catalyst, for example, a carrier layer made of γ-Al ₂ O ₃ is formed on a heat-resistant honeycomb substrate made of cordierite, and platinum (Pt), rhodium (Rh), etc. are formed on the carrier layer. Those carrying a noble metal are widely known.

The carrier used for the exhaust gas purification catalyst has a large specific surface area and high heat resistance, and Al ₂ O ₃ , SiO ₂ , ZrO ₂ , TiO _{2 and the} like are often used in general. It is also known to add CeO ₂ with oxygen storage / release capacity (hereinafter referred to as OSC) and CeO ₂ -ZrO ₂ solid solution with improved heat resistance and CeO ₂ in order to mitigate atmospheric fluctuations in exhaust gas. It has been.

By the way, with the recent increase in exhaust gas temperature, improvement in heat resistance of a catalyst or catalyst support containing CeO ₂ is particularly required. Thus, for example, Japanese Patent Application Laid-Open No. 02-221119 discloses CeO ₂ having a specific surface area of at least 190 m ² / g measured after firing at a temperature of 350 to 450 ° C.

However, the conventional CeO ₂ —ZrO ₂ solid solution is excellent in heat resistance, but it is clear that OSC is insufficient and the solid solubility is low. Therefore, the applicant of the present application has proposed an exhaust gas purification catalyst using CeO ₂ —ZrO ₂ solid solution having high solid solubility and high OSC in Japanese Patent Application Laid-Open No. 09-211304. In addition, Japanese National Publication No. 10-512191 discloses a CeO ₂ —ZrO ₂ solid solution, which describes a high specific surface area even after firing at a high temperature of 1000 ° C., and is described as being optimal as a catalyst carrier. .

In CeO ₂ described in JP-A-02-221119, it is described that the pore volume measured after firing at 350 to 450 ° C. is 0.15 to 0.30 cc / g, and the average pore diameter is 2 to 10 nm. It is described that there is. In the publication, “average pore diameter is defined as a diameter in which all pores smaller than this diameter form 50% of the total pore volume of pores having a diameter smaller than 60 nm. Is described. Therefore, if the average pore diameter after firing at 350 to 450 ° C. is 2 to 10 nm, the pores with a very small diameter occupy most of the total pore volume, which causes a problem that the heat resistance is insufficient.

Further, in recent years, the temperature of automobile exhaust gas has been increasing, and the catalyst disclosed in Japanese Patent Application Laid-Open No. 09-211304 has a problem of insufficient heat resistance although the OSC is high. It has been found that this occurs because sintering occurs in the CeO ₂ —ZrO ₂ solid solution and the pore volume is reduced.

On the other hand, the CeO ₂ —ZrO ₂ solid solution disclosed in JP-T-10-512191 discloses that a specific surface area of 25 m ² / g or more is exhibited even when fired at 1000 ° C. However, when it is used as an exhaust gas purification catalyst for automobiles, it is sufficient to have heat resistance up to 900 ° C. In Japanese Patent Publication No. 10-512191, only a high specific surface area is described, and there is no description regarding the pore volume and its distribution necessary as a precious metal loading field or as a gas diffusion space.

Therefore, Japanese Patent Laid-Open No. 2002-220228 discloses that the pore volume with a pore diameter of 3.5 to 100 nm is 0.07 cc / g or more after firing at 600 ° C. for 5 hours, and the pore diameter after firing at 800 ° C. for 5 hours. Describes a ceria-based oxide powder having a characteristic that the pore volume of 3.5 to 100 nm is 0.04 cc / g or more, and a production method thereof. Therefore, this oxide powder has an optimum central pore diameter and pore volume for use as a catalyst in actual exhaust gas, and is excellent in heat resistance.
JP 02-221119 Special table 10-512191 JP 2002-220228

  However, in the production method described in Japanese Patent Application Laid-Open No. 2002-220228, the pore distribution of the obtained oxide powder is left to control rather than controlling.

  An object of the present invention is to make it possible to finely control the pore distribution and to further reduce the pore diameter and further increase the pore volume.

  A feature of the oxide powder of the present invention that solves the above problems is that the cerium oxide is a main component and the pore size distribution is such that the center pore diameter is 20 nm or less and the pore volume of 30 nm or less is 0.1 cc / g or more. is there.

In addition, the oxide powder production method of the present invention is characterized by the precipitation of the oxide precursor by adding at least an amount of a base equal to or more than an acid group contained in an aqueous solution in which a compound containing at least cerium is dissolved or a solution containing water. A method for producing an oxide powder, in which a precipitation step for precipitating a precipitate, a stirring step for stirring a suspension containing the precipitate and a surfactant, and a firing step for filtering and firing the resulting precipitate,
In the stirring step, control to prepare a suspension so that the weight of the obtained oxide powder is 1 to 30% with respect to the solvent, control to stir under conditions where the shear rate is 10 ³ to 10 ⁴ / sec, It is to perform control of at least one of the following.

  In the oxide powder of the present invention and the production method thereof, the oxide is preferably a ceria-zirconia composite oxide.

  In the method for producing oxide powder of the present invention, it is desirable to perform a washing step for washing the precipitate between the precipitation step and the stirring step. The washing step is preferably performed between the stirring step and the firing step. Moreover, it is preferable that 2-40% of surfactant is contained with respect to the weight of the oxide powder obtained in suspension.

  The catalyst of the present invention is characterized in that at least a noble metal is supported on a support containing the oxide powder of the present invention.

  According to the oxide powder of the present invention, the pore volume with a center pore diameter of 20 nm or less and a pore volume of 30 nm or less is 0.1 cc / g or more, so that the specific surface area is extremely large and useful as a catalyst support. is there. And according to the catalyst in which the noble metal is supported on the oxide powder, the noble metal is supported in a highly dispersed manner and the grain growth is suppressed even after the endurance, so there are many active sites and high purification activity, and heat resistance. Also excellent.

  And according to the manufacturing method of the oxide powder of this invention, the oxide powder of this invention can be manufactured stably and reliably, and it becomes possible to control fine pore distribution finely.

  A conventional catalyst using cerium oxide powder as a carrier has a problem that the pore volume decreases after a high temperature endurance as in actual exhaust gas, the specific surface area decreases, and the catalytic activity decreases.

  Therefore, the oxide powder of the present invention is an aggregate of secondary particles that are aggregates of primary particles mainly composed of cerium oxide, and has a pore diameter of 0.1 cc / min. It has the characteristic of having a pore distribution of g or more. That is, it has fine pores and a sufficient pore volume is secured.

  Therefore, in the catalyst of the present invention in which noble metal is supported on this oxide powder, there are sufficiently noble metal supporting sites or pores which are gas diffusion fields even after high temperature durability, and a sufficiently large specific surface area is ensured. Thus, a decrease in catalyst activity is suppressed. If the center pore diameter of the oxide powder exceeds 20 nm or the pore volume of 30 nm or less is less than 0.1 cc / g, the above-described characteristics are difficult to be expressed.

The oxide powder of the present invention is preferably a ceria-zirconia composite oxide. By using a composite oxide in which zirconia is dissolved in ceria, heat resistance is further improved and OSC is remarkably improved, so that it is particularly suitable as a catalyst carrier. In this case, the atomic ratio of Ce and Zr is preferably Ce: Zr = 80: 20 to 20:80, Ce: Zr = 70: 30 to 30:70, and further Ce: Zr = 60: 40 to 40 : 60 is more preferable. If the Ce content is less than this range, the OSC necessary for the catalyst cannot be obtained. If the Ce content is more than this range, the amount of ZrO ₂ solid solution is too small and the heat resistance decreases.

The oxide powder of the present invention containing CeO ₂ —ZrO ₂ solid solution as a main component preferably has a solid solubility of 50% or more. It is more preferably 70% or more, and particularly preferably 85% or more. If the solid solubility is less than 50%, the OSC is insufficient. The higher the solid solubility, the larger the OSC.

  Here, the solid solubility means a value defined by the following equation from the peak shift of X-ray diffraction.

Solid solubility (%) = 100 × (amount of ZrO ₂ dissolved in CeO ₂ ) / total amount of ZrO ₂ The solid solubility S (%) is calculated by the equation (1).

S = 100 × (x / C) × [(100−C) / (100−x)] (1)
Here, C is the content (%) of ZrO ₂ obtained from the blending ratio of Ce and Zr, and x is ZrO ₂ that is dissolved in CeO ₂ calculated by the formula (2) from the lattice constant obtained from X-ray diffraction. Concentration (%).

x = (5.423-a) /0.003 (2)
In the formula (2), a is a lattice constant (Å).

  In the production method of the present invention, a precipitation step is performed in which the precipitate of the oxide precursor is precipitated by adding an acid group equal to or more than an acid group contained in an aqueous solution in which a compound containing at least cerium is dissolved or a solution containing water. . Alternatively, the precipitate of the oxide precursor is deposited from an aqueous solution in which the cerium compound and the zirconium compound are dissolved or a solution containing water. As the cerium compound and zirconia compound, a salt is generally used, and as the salt, sulfate, nitrate, hydrochloride, acetate, and the like can be used. Moreover, water, alcohols, and mixtures thereof can be used as the solvent for uniformly dissolving the salt.

As the cerium compound, a tetravalent Ce salt is generally used, but there is a problem that the tetravalent Ce salt is expensive. Therefore, it is preferable to use an inexpensive trivalent Ce salt and make it tetravalent by oxidation during the reaction. For this purpose, for example, hydrogen peroxide (H ₂ O ₂ ) is preferably used as the oxidizing agent. The effect that the solid solution of CeO ₂ and ZrO ₂ is promoted by H ₂ O ₂ is also exhibited.

  And the precipitation of an oxide precursor precipitates by adding a base equal to or more than the acid group contained in this solution. By neutralizing with an equal amount or more of the base, the precipitation reaction of the oxide precursor is promoted. As the base, an aqueous solution or alcohol solution in which ammonia, ammonium carbonate, sodium hydroxide, potassium hydroxide, sodium carbonate or the like is dissolved can be used. Particularly preferred are ammonia and ammonium carbonate which volatilize during firing. The pH of the basic solution is more preferably 9 or higher.

  There are various adjustment methods for precipitation, such as adding ammonia water instantly and stirring vigorously, or adjusting the pH at which the oxide precursor starts to precipitate by adding hydrogen peroxide, etc. There is a method of depositing a precipitate with ammonia water or the like. Also, a buffer solution that sufficiently lengthens the time required for neutralization with aqueous ammonia, etc., and preferably neutralizes in 10 minutes or more, or neutralizes step by step while monitoring the pH, or maintains a predetermined pH. It is also preferable to add.

  It is also preferable to carry out an aging step in which the mixture is heated in a suspended state using water or a solution containing water as a dispersion medium or in a state where water is sufficiently present in the system after the precipitation has occurred. Thereby, although the mechanism is unknown, an oxide powder with controlled pores can be obtained.

  In order to mature the precipitate in a state where moisture is sufficiently present in the system, the solution containing the precipitate can be heated to evaporate the solvent, and then baked as it is. Alternatively, the precipitate separated by filtration may be calcined in the presence of water vapor. In this case, it is preferable to bake in a saturated steam atmosphere.

When the aging step is performed, dissolution / reprecipitation is promoted by the heat of heating and particle growth occurs. In the present invention, as described above, since neutralization is performed with an equal amount or more of a base, the ceria precursor and the zirconia precursor are aged more uniformly, effectively forming pores, and CeO ₂ —ZrO _2. In the case of an oxide powder made of a solid solution, solid solution during firing is further promoted.

  This aging step is desirably performed at room temperature or higher, preferably 100 to 200 ° C, more preferably 100 to 150 ° C. When heating at less than 100 ° C., the effect of promoting aging is small, and the time required for aging becomes long. Further, at a temperature higher than 200 ° C., a synthesis apparatus that can withstand 10 atm or more is required, and the equipment cost is high, so that it is not suitable for the catalyst carrier production method which is the main use of the present invention.

In the stirring step, which is a feature of the present invention, the suspension is controlled so that the weight of the obtained oxide powder is 1 to 30% with respect to the solvent, and the shear rate is 10 ³ to 10 ⁴ / sec. At least one control of stirring under conditions is performed. It is particularly preferable to perform both controls. If the concentration of the oxide precursor contained in the suspension exceeds 30% by weight as the oxide powder, the shear efficiency by stirring decreases, so the pore volume with a center pore diameter of 20 nm or less and 30 nm or less is 0.1 cc. It becomes difficult to form a pore distribution greater than / g. If it is less than 1% by weight, productivity is poor and efficiency is low. In the suspension, the weight of the obtained oxide powder is preferably 10% or less, and preferably 5% or less with respect to the solvent.

For example, in the case of an agitator having a rotor and a stator, the shear rate V is expressed by V = v / D. Here, v is a speed difference (m / second) between the rotor and the stator, and D is a gap (m) between the rotor and the stator. When the shear rate is less than 10 ³ / sec, the shear efficiency by stirring becomes low, so it becomes difficult to form a pore distribution with a pore size of 20 cc or less and a pore volume of 30 cc or less and 0.1 cc / g or more. _The solid solubility of _2- ZrO ₂ solid solution is also lowered. On the other hand, when the shear rate exceeds 10 ⁴ / sec, the shear efficiency is saturated and the wear of the stirrer easily proceeds. The shear rate is desirably 5 × 10 ³ / second or more.

Optimum shearing is achieved by using a suspension containing the oxide powder in an amount of 1 to 30% with respect to the solvent or by stirring at a shear rate of 10 ³ to 10 ⁴ / sec. Efficiency is manifested, and a pore distribution with a pore size of 20 cc or less and a pore volume of 0.1 cc / g or more can be formed. In addition, according to this method, since solid solution is promoted even when trivalent Ce is used, CeO ₂ —ZrO ₂ composite oxide having high solid solubility without being oxidized to tetravalent with hydrogen peroxide. Can be manufactured.

  In the stirring step, a surfactant is contained in the suspension. The action of the surfactant is not clear, but is presumed as follows. That is, in the state just neutralized with the base, the metal element is precipitated in the form of a very fine hydroxide or oxide having a particle size of several nm or less. By adding the surfactant, the precipitated particles are uniformly taken into the micelles of the surfactant, and generation of oxide particles proceeds in the concentrated small space in the firing step. Furthermore, the dispersibility of the precipitated fine particles is improved by the dispersing effect of the surfactant, the segregation is reduced and the contact degree is increased. Therefore, it is possible to form a pore distribution in which the pore diameter is 20 cc or less and the pore volume is 30 cc or less and the pore volume is 0.1 cc / g or more.

  The addition amount of the surfactant is desirably 2 to 40% with respect to the weight of the obtained oxide powder in the suspension. If the amount is less than 2% by weight, the added effect is not expressed. If the amount exceeds 40% by weight, aggregation of the surfactants occurs, making it difficult to highly disperse the precursor. Moreover, the calorific value at the time of baking is large, and sintering of the oxide powder is promoted.

  As the surfactant, any of an anionic type, a cationic type, and a non-ionic type can be used. Among them, a shape in which the micelle to be formed can form a narrow space inside, for example, an interface that easily forms a spherical micelle. An activator is desirable. Moreover, the critical micelle concentration (cmc) is preferably 0.1 mol / L or less. More desirably, a surfactant of 0.01 mol / L or less is desirable.

  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, methyl taurate, sulfosuccinic acid Salts, ether sulfates, alkyl sulfates, ether sulfonates, saturated fatty acids and salts thereof, unsaturated fatty acids such as oleic acid and salts thereof, other carboxylic acids, sulfonic acids, sulfuric acids, phosphoric acids, phenol derivatives Anionic surfactants such as polyoxyethylene polyprolene alkyl ether, polyoxyethylene alkyl ether, polyoxyethylene alkyl phenyl ether, polyoxyethylene polystyryl phenyl ether, polyoxyethylene polyoxypolypropylene alkyl Polyether, polyoxyethylene polyoxypropylene glycol, polyhydric alcohol; glycol; glycerin; sorbitol; mannitol; pentaesitol; sucrose; Pentaerythritol; Sucrose; Polyoxyethylene fatty acid partial ester of polyhydric alcohol, polyoxyethylene fatty acid ester, polyoxyethylenated castor oil, polyglycene fatty acid ester, fatty acid diethanolamide, polyoxyethylene alkylamine, triethanol Nonionic surfactants such as amine fatty acid partial esters and trialkylamine oxides, primary fatty amine salts, secondary fatty amine salts, tertiary fatty amine salts, tetraalkylamines Trialkylbenzylammonium salt; alkylpyrodinium salt; 2-alkyl-1-alkyl-1-hydroxyethylimidazolinium salt; N, N-dialkylmorpholinium salt; polyethylene polyamine fatty acid amide salt; It is at least one selected from cationic surfactants such as quaternary ammonium salt and amphoteric surfactants such as betaine compounds.

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

  The suspension can be prepared by adding a surfactant to the solution containing the precursor precipitate after the precipitation step, but a washing step for washing the precursor precipitate is performed between the precipitation step and the stirring step. Thereafter, it is desirable to prepare a new suspension. This is because if the base used in the precipitation step is present, the action of the above-described surfactant may be inhibited, and therefore it is desirable to remove the base.

  A baking process is a process of filtering and baking the obtained precipitation. This firing step can be performed by heating at 150 to 700 ° C. in the air. In addition, in order to suppress the heat_generation | fever by above-mentioned surfactant, it is desirable to perform the washing | cleaning process which wash | cleans precipitation between a stirring process and a baking process.

The catalyst of the present invention comprises at least a noble metal supported on the above-described oxide powder of the present invention. Pt, Rh, Pd, Ir and the like can be used as the noble metal, but Pt is particularly preferable. The amount of the precious metal supported can be arbitrarily selected in the range of 0.05 to 20% by weight. In order to support the noble metal, it can be supported using a known method such as an adsorption support method or an impregnation support method using a noble metal chemical solution such as a dinitrodiammine platinum aqueous solution. In addition to the noble metal, a NO _x storage material such as an alkali metal or an alkaline earth metal may be supported. The shape of the catalyst is not particularly limited, such as a pellet shape or a honeycomb shape.

The catalyst of the present invention can be used as an exhaust gas purification catalyst such as an oxidation catalyst, a three-way catalyst, a NO _x storage reduction catalyst, or a hydrogen production catalyst.

  Hereinafter, the present invention will be described specifically by way of examples.

(Example 1)
<Precipitation process>
In a 3 liter beaker, 442.29 g of cerium nitrate aqueous solution with a concentration of 28% by weight as CeO ₂ and 601.3 g of zirconium oxynitrate aqueous solution with a concentration of 18 wt% as ZrO ₂ were mixed with 1200 g of ion-exchanged water, and the propeller was mixed. While stirring with a stirrer, 319.9 g of 25% aqueous ammonia was added to precipitate the oxide precursor.

<Washing process>
The precipitate was washed several times with ion exchange water using a centrifuge.

<Stirring step>
The total amount of the precipitate was transferred to a 3 liter beaker, 1800 g of ion exchange water was added, and the mixture was stirred for 5 minutes using a propeller stirrer and a homogenizer. The concentration of the precipitate converted to CeO ₂ —ZrO ₂ is 8.2% by weight, and the shear rate is 1000 / sec. To this was added 5 g of a cationic surfactant (“Armac” Lion) and stirred for 5 minutes. Further, 5 g of an anionic surfactant (“Armoflow” Lion) was added, and the mixture was further stirred for 5 minutes at a shear rate of 1000 / sec.

<Washing process>
Thereafter, the precipitate was washed in the same manner as described above.

<Baking process>
The obtained precipitate was calcined at 400 ° C. for 5 hours in the air using a degreasing furnace, and further calcined at 700 ° C. for 5 hours.

(Example 2)
<Precipitation process>
In a 3 liter beaker, 110.6 g of cerium nitrate aqueous solution with a concentration of 28% as CeO ₂ and 150.3 g of zirconium oxynitrate with a concentration of 18% by weight as ZrO ₂ were mixed with 1500 g of ion-exchanged water, While stirring with a stirrer, 80 g of 25% aqueous ammonia was added to precipitate the oxide precursor.

<Washing process>
The precipitate was washed several times with ion exchange water using a centrifuge.

<Stirring step>
The total amount of the precipitate was transferred to a 3 liter beaker, 1800 g of ion exchange water was added, and the mixture was stirred for 5 minutes using a propeller stirrer and a homogenizer. The concentration of the precipitate converted to CeO ₂ —ZrO ₂ is 2.8% by weight, and the shear rate is 1000 / sec. To this was added 5 g of a cationic surfactant (“Armac” Lion) and stirred for 5 minutes. Further, 5 g of an anionic surfactant (“Armoflow” Lion) was added, and the mixture was further stirred for 5 minutes at a shear rate of 1000 / sec.

<Washing process>
Thereafter, the precipitate was washed in the same manner as in Example 1.

<Baking process>
The obtained precipitate was calcined in the same manner as in Example 1.

(Example 3)
<Precipitation process>
In into 3 liters beaker, and an aqueous solution of cerium nitrate 442.29g of a concentration of 28 wt% CeO _2, a concentration of 18 wt% as ZrO ₂ and zirconium oxynitrate solution 601.3G, mixed with ion-exchanged water 1200 g, propeller While stirring with a stirrer, 319.9 g of 25% aqueous ammonia was added to precipitate the oxide precursor.

<Washing process>
The precipitate was washed several times with ion exchange water using a centrifuge.

<Stirring step>
The total amount of the precipitate was transferred to a 3 liter beaker, 1800 g of ion exchange water was added, and the mixture was stirred for 5 minutes using a propeller stirrer and a homogenizer. The concentration of the precipitate converted to CeO ₂ —ZrO ₂ is 8.2% by weight, and the shear rate is 5000 / sec. To this was added 5 g of a cationic surfactant (“Armac” Lion) and stirred for 5 minutes. Further, 5 g of an anionic surfactant (“Armoflow” Lion) was added, and the mixture was further stirred for 5 minutes at a shear rate of 5000 / sec.

<Washing process>
Thereafter, the precipitate was washed in the same manner as in Example 1.

<Baking process>
The obtained precipitate was calcined in the same manner as in Example 1.

(Examination / Evaluation)
The conditions of the stirring step in each example are summarized in Table 1.

  The oxide powder obtained in each example was measured for the pore distribution using a mercury porosimeter and the BET specific surface area. The results are shown in FIG.

  1 and Table 1, the oxide powders obtained in Examples 2 and 3 have a smaller central pore diameter than the central pore diameter of the oxide powder obtained in Example 1, and the pore volume of Examples. 1 is larger than the oxide powder obtained in 1 and has a large specific surface area. That is, it is clear that the pore distribution can be finely controlled by adjusting the precursor concentration or shear rate in the suspension in the stirring step.

  Next, the oxide powder obtained in each example was impregnated with a predetermined amount of an aqueous platinum nitrate solution having a predetermined concentration, dried, and then fired in the atmosphere at 300 ° C. for 3 hours, so that 1 wt. % Supported. The obtained catalyst powder was compacted and pulverized to prepare pellet catalysts.

An endurance test was performed in which 1.5 g of each pellet catalyst was placed in an endurance test apparatus, and the rich gas and the lean gas shown in Table 2 were held at 1000 ° C. for 5 hours while alternately flowing for 5 minutes. 1 g of each pellet catalyst after the durability test was pretreated at 500 ° C. for 10 minutes in the model gas shown in Table 3, and then the model gas shown in Table 3 was alternately circulated for 1 second at a flow rate of 3500 cc / min. The temperature was raised from room temperature to 400 ° C. at a rate of 12 ° C./min, and the purification rates of CO, C ₃ H ₆ and NO at each temperature were measured, and the 50% purification temperature was calculated. The results are shown in FIG.

  From FIG. 2, it can be seen that the catalysts of Examples 2 and 3 both show higher purification performance than the catalyst of Example 1, and this is apparently due to the difference in pore distribution.

It is a graph which shows the pore distribution of the catalyst of each Example. It is a graph which shows the 50% purification temperature of the catalyst of each Example.

Claims (8)

  An oxide powder comprising cerium oxide as a main component and having a pore distribution with a pore diameter of 20 cc or less and a pore volume of 30 cc or less and 0.1 cc / g or more.   The oxide powder according to claim 1, which is a ceria-zirconia composite oxide. A precipitation step of precipitating the precipitate of the oxide precursor by adding a base equal to or more than an acid group contained in an aqueous solution in which a compound containing at least cerium is dissolved or a solution containing water; and the precipitation and the surfactant A method for producing an oxide powder, in which an agitation step of agitating a suspension containing the mixture and a firing step of filtering and firing the obtained precipitate are sequentially performed,
In the stirring step, the suspension is prepared so that the weight of the obtained oxide powder is 1 to 30% with respect to the solvent, and the stirring is performed under the condition that the shear rate is 10 ³ to 10 ⁴ / sec. A method for producing an oxide powder, wherein at least one of the controls is performed.   The method for producing oxide powder according to claim 3, wherein the aqueous solution or the solution containing water contains cerium and zirconium.   The manufacturing method of the oxide powder of Claim 3 or Claim 4 which performs the washing | cleaning process which wash | cleans the said precipitation between the said precipitation process and the said stirring process.   The said surfactant is a manufacturing method of the oxide powder in any one of Claims 3-5 contained 2-40% with respect to the weight of the oxide powder obtained in the said suspension liquid.   The manufacturing method of the oxide powder in any one of Claims 3-6 which perform the washing | cleaning process which wash | cleans the said precipitation between the said stirring process and the said baking process.   A catalyst comprising at least a noble metal supported on a carrier comprising the oxide powder according to claim 1 or 2.

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