JP4165442B2 - Metal oxide particles, production method thereof, and exhaust gas purification catalyst - Google Patents

Description

  The present invention relates to metal oxide particles, a production method thereof, and an exhaust gas purification catalyst produced from the metal oxide particles.

The exhaust gas from an internal combustion engine such as an automobile engine contains nitrogen oxides (NO _x ), carbon monoxide (CO), hydrocarbons (HC), etc., and these substances oxidize CO and HC. At the same time, it can be removed by an exhaust gas purifying catalyst capable of reducing NO _x. Typical examples of exhaust gas purification catalysts include three-way catalysts in which a noble metal such as platinum (Pt), rhodium (Rh), palladium (Pd) is supported on a porous metal oxide carrier such as γ-alumina. It has been.

Although this metal oxide support can be made of various materials, conventionally, alumina (Al ₂ O ₃ ) has been generally used to obtain a high surface area. In recent years, however, various other materials such as ceria (CeO ₂ ), zirconia (ZrO ₂ ), titanium (TiO ₂ ), etc., have been combined with alumina in order to promote exhaust gas purification utilizing the chemical nature of the support. It has also been proposed to use in combination or not.

  For example, in order to absorb fluctuations in the oxygen concentration in the exhaust gas and enhance the exhaust gas purification capacity of the three-way catalyst, oxygen can be stored when the oxygen concentration in the exhaust gas is high, and oxygen can be stored when the oxygen concentration in the exhaust gas is low. A material having an oxygen storage ability (OSC ability) that can be released is used as a carrier for an exhaust gas purification catalyst. A typical material having OSC ability is ceria.

In order for the oxidation of CO and HC and the reduction of NO _x to proceed efficiently by the action of the three-way catalyst, the air-fuel ratio of the internal combustion engine needs to be the stoichiometric air-fuel ratio (stoichiometric). It is preferable to absorb the fluctuation of the oxygen concentration therein and maintain the oxygen concentration in the vicinity of the theoretical air-fuel ratio because the three-way catalyst exhibits the exhaust gas purification ability. Further, according to recent research, ceria not only has OSC ability, but also has a strong affinity with noble metals supported thereon, particularly platinum, and therefore can suppress grain growth (sintering) of the noble metals. Has been found.

  Thus, ceria has favorable properties for use in exhaust gas purification catalysts, but may not have the heat resistance required in this application. Therefore, a method for improving the heat resistance by solidifying ceria and zirconia has been developed (see, for example, Patent Documents 1 and 2).

Japanese Patent Laid-Open No. 10-194742 JP-A-6-279027

  As described above, when using a metal oxide support composed of a plurality of types of materials and using a combination of these properties, the particles of the plurality of types of metal oxides can be mixed. Since the size of each combined metal oxide particle is large, the properties of those metal oxides may not be combined well.

  Although substantially uniform metal oxide particles can be obtained from a sol in which a plurality of different colloidal particles are mixed, uniform mixing does not necessarily give the best results.

  For example, a composite metal oxide in which ceria and zirconia are uniformly mixed is known to have good OSC ability and heat resistance, but may not sufficiently prevent sintering of noble metals such as platinum by ceria. . That is, since both ceria and zirconia exist on the surface of the composite metal oxide, some noble metals are supported not on the ceria part but on the zirconia part, and thus sintering may not be prevented.

  The present invention relates to metal oxide particles having a plurality of types of metal oxides, which can be used as a catalyst support capable of exhibiting the properties of each metal oxide, a method for producing the same, and exhaust gas obtained from the metal oxide particles. A purification catalyst is provided.

The metal oxide particles of the present invention have a central portion that contains a relatively large amount of the first metal oxide and an outer skin portion that contains a relatively large amount of the second metal oxide. It consists of a plurality of primary particles and the primary particle size of the second metal oxide is smaller than the primary particle size of the first metal oxide, in particular 70% or less, more particularly 50% or less, and even more particularly 30%. It is as follows. In the metal oxide particles of the present invention, the first metal oxide is zirconia, the second metal oxide is ceria, and the primary particle diameter of zirconia constituting the center is 100 nm or less.

According to the metal oxide particles of the present invention, the properties of the respective metal oxides can be combined by changing the composition of the central portion and the outer skin portion. Further, the primary particle diameter of the second metal oxide mainly constituting the outer skin part is smaller than the primary particle diameter of the first metal oxide mainly constituting the center part. It is preferable to cover securely. Thereby, when this metal oxide particle is used as a catalyst carrier, a good interaction between the supported catalyst metal and the second metal oxide can be obtained. Further, according to the metal oxide particles of the present invention, heat resistance is provided by zirconia at the center, and when noble metals such as platinum are supported on the metal oxide particles by ceria at the outer skin, Sintering can be prevented. Furthermore, in the metal oxide particles of the present invention, the primary particle diameter of zirconia constituting the central part is preferably 100 nm or less, which is preferable with respect to the performance of the exhaust gas purification catalyst in which noble metal is supported on the metal oxide particles.

  The expression “relatively contained” used for the metal oxide contained in the central portion and the outer skin portion is used in terms of a mole fraction based on the total number of moles of metal in each of the central portion and the outer skin portion. Therefore, for example, in the “center portion containing a relatively large amount of the first metal oxide”, the mole fraction of the metal constituting the first metal oxide in the center portion is higher than the mole fraction of this metal in the outer skin portion. It means that.

  In these metal oxide particles, based on the total number of moles of metal in the metal oxide particles, the total mole fraction of cerium and zirconium is at least 85 mol%, particularly at least 90 mol%, more particularly at least 95 mol%. It may be.

  In one embodiment of the metal oxide particles of the present invention having ceria and zirconia, the metal oxide particles have a particle size of 2.3 μm to 8.1 μm.

  This is preferable with respect to the performance of an exhaust gas purification catalyst in which a noble metal is supported on the metal oxide particles.

  In the exhaust gas purifying catalyst of the present invention, the metal oxide particles of the present invention having ceria and zirconia carry a noble metal, particularly platinum.

  According to this exhaust gas purification catalyst, platinum sintering can be prevented by the affinity between ceria and noble metal, and good catalytic performance can be provided.

  The method for producing metal oxide particles of the present invention is a sol containing at least first metal oxide colloidal particles and second metal oxide colloidal particles each having a different isoelectric point. A sol in which the particle diameter of the metal oxide colloidal particles is smaller than the particle diameter of the first metal oxide colloidal particles, particularly 70% or less, more particularly 50% or less, and even more particularly 30% or less. The pH of the sol is made closer to the isoelectric point of the first metal oxide colloidal particle than the isoelectric point of the second metal oxide colloidal particle, in particular of the first metal oxide colloidal particle. The colloidal particles of the first metal oxide are agglomerated close to the isoelectric point ± 1.0, more particularly within the range of the isoelectric point ± 0.5; the pH of the sol is changed to the colloid of the first metal oxide. Colloidal particles of the second metal oxide rather than the isoelectric point of the particles Agglomerating the second metal oxide colloidal particles around the agglomerated first metal oxide colloidal particles close to their isoelectric point; and drying and calcining the resulting agglomerates .

  According to the method of the present invention, it has a central portion that contains a relatively large amount of the first metal oxide and an outer skin portion that contains a relatively large amount of the second metal oxide. It is possible to obtain metal oxide particles that are composed of primary particles of which the primary particle diameter of the second metal oxide is smaller than the primary particle diameter of the first metal oxide.

  Here, the term “colloidal particles” is a metal oxide dispersed in a liquid, particularly water, or a particle having a metal bonded to oxygen, and the metal oxide is produced by removing the dispersion medium and firing. Means what to do. This “colloidal particle” is generally understood as having a diameter of 1-1000 nm, in particular 1-500 nm, for example those having a diameter of less than 100 nm or less than 50 nm are available.

  Here, the term “sol” means a dispersion system in which colloidal particles are dispersed in a dispersion medium that is a liquid, and may be referred to as a colloidal solution. The dispersion medium contained in the sol is generally water, but an organic dispersion medium such as alcohol or acetylacetone may be included as necessary.

  The present invention will be described with reference to FIG. Here, FIG. 1 is a cross-sectional view of the metal oxide particles of the present invention.

  As shown in FIG. 1, the metal oxide particles of the present invention have a central portion 1 (portion surrounded by a dotted line) containing a relatively large amount of the first metal oxide such as zirconia, and ceria-like particles. And a skin portion 2 (a portion outside the dotted line) containing a relatively large amount of the second metal oxide. Further, the central portion and the outer skin portion are each composed of a plurality of primary particles (1a, 2a).

  The plurality of primary particles constituting the central portion and the outer skin portion correspond to the colloidal particles in the sol when the metal oxide particles of the present invention are formed from the sol, and are between the primary particles. May or may not have a clear boundary. Further, the boundary between the central portion 1 and the outer skin portion 2 is not necessarily clear, and may appear as a portion where the composition gradually changes. Furthermore, the boundary part between the center part 1 and the outer skin part 2 may be a mixture of the first metal oxide and the second metal oxide, particularly a solid solution. Although the outer skin portion 2 is shown as discontinuous in FIG. 1, this may be substantially continuous.

  As the metal oxide constituting the metal oxide particles of the present invention, any metal oxide can be selected, and the metal oxide that is preferably held in the center of the metal oxide particles is the first metal oxide. As a second metal oxide, it is possible to select a metal oxide that is preferably exposed to the outer skin portion of the metal oxide particles. For example, zirconia is preferable as the first metal oxide, and ceria is preferable as the second metal oxide. Here, zirconia has high heat resistance, and ceria can prevent platinum sintering when platinum is supported.

  When the outer skin portion or the central portion of the metal oxide particle of the present invention contains zirconia, ceria and / or ceria-zirconia solid solution, the central portion or outer skin portion is a metal other than cerium (Ce) and zirconium (Zr). For example, a metal selected from the group consisting of alkaline earth metals and rare earth elements, in particular yttrium (Y). These alkaline earth metals and rare earth elements, especially yttrium, tend to provide excellent heat resistance to zirconia, ceria and / or ceria-zirconia solid solutions.

  The exhaust gas purification catalyst of the present invention can be produced by supporting the noble metal such as platinum, rhodium and palladium, particularly platinum, on the metal oxide particles of the present invention. The noble metal can be supported on the metal oxide particles by using a known method. For example, a method of absorbing and supporting a solution containing a salt and / or a complex salt of a noble metal, drying and firing can be mentioned. The amount of the noble metal supported on the metal oxide particles may be 0.01 to 5% by mass, particularly 0.1 to 2% by mass with respect to the metal oxide particles.

  The exhaust gas purification catalyst of the present invention can be used not only by molding itself but also by coating a monolith support, for example, a ceramic honeycomb.

  The metal oxide particles of the present invention can be produced by any method, and in particular, can be produced by the method of the present invention.

  Hereinafter, each step of the method of the present invention will be described.

[Providing mixed sol]
In the method of the present invention, first, a sol containing at least a first metal oxide colloidal particle and a second metal oxide colloidal particle each having a different isoelectric point, the second metal A sol in which the particle size of the colloidal particles of oxide is smaller than the particle size of the colloidal particles of the first metal oxide is provided.

  Specific examples of the sol include substances obtained by hydrolysis and condensation of metal alkoxides, acetylacetonates, acetates, nitrates, and the like. Further, sols such as alumina sol, zirconia sol, titania sol and ceria sol are known materials, and commercially available ones can also be obtained.

  Commonly sold metal oxide sols have a pH remote from the isoelectric point of the contained colloidal particles, so that the contained colloidal particles repel each other electrostatically to prevent agglomeration. ing. That is, a sol containing colloidal particles having an isoelectric point on the alkali side is stabilized by making the sol acidic (acid-stabilized sol). In addition, a sol containing colloidal particles having an isoelectric point on the acidic side is stabilized by making the sol alkaline (alkali stabilization sol).

  Here, the isoelectric point of this colloidal particle is not limited by the material itself such as an oxide constituting the particle, but can be arbitrarily determined by surface modification of the colloidal particle, particularly by surface modification of the colloidal particle by an organic compound. It can be set. Accordingly, the first and second colloidal particles used in the method of the present invention can be arbitrarily selected so as to have an appropriate pH of the present invention. For example, the pH of the isoelectric point of these colloidal particles can be selected to be at least 3 or more, particularly 4 or more, more particularly 5 or more.

  The isoelectric point of the colloidal particles necessary for carrying out the method of the present invention can be obtained by any method, and can be measured, for example, by electrophoretic light scattering.

  The sol containing at least two types of colloidal particles that can be used in the method of the present invention can be obtained by any method, but can be obtained by mixing different sols. The mixing ratio of these colloidal particles can be arbitrarily determined depending on the desired properties of the metal oxide particles.

  In the method of the present invention, elements such as alkaline earth metals and rare earths preferably contained in the metal oxide particles can be contained in the sol not only as colloidal particles but also as metal salts such as nitrates. .

[Agglomeration of first colloidal particles]
In the method of the present invention, the pH of the sol is then brought closer to the isoelectric point of the first metal oxide colloidal particle than the isoelectric point of the second metal oxide colloidal particle, Aggregate colloidal particles of metal oxide.

  As mentioned above, commonly sold metal oxide sols have a pH remote from the isoelectric point of the contained colloidal particles so that the colloidal particles repel each other electrostatically to prevent agglomeration. Has been.

  Therefore, as in the present invention, in the sol containing the first metal oxide colloidal particles and the second metal oxide colloidal particles, the pH of the sol is changed to the isoelectricity of the first metal oxide colloidal particles. When the point is varied, for example, within a range of ± 1.0, in particular, within a range of ± 0.5, the zeta potential of the colloidal particles of the first metal oxide is reduced, and the electrical potential between the particles is reduced. Repulsion is reduced, which promotes aggregation of the first metal oxide colloidal particles. Here, since the pH of the sol is relatively far from the isoelectric point of the second metal oxide colloidal particles, the second metal oxide colloidal particles have a relatively large zeta potential, and thus this Aggregation of colloidal particles of the second metal oxide is suppressed.

  When the colloidal particles are aggregated, if the pH of the sol is changed so as to pass through the isoelectric point of the colloidal particles to be aggregated, the colloidal particles are used when the pH of the sol passes through the isoelectric point. Therefore, the agglomeration of the colloidal particles can be performed more reliably.

  The pH of the sol can be adjusted by adding any acid or alkali. For example, mineral acids such as nitric acid and hydrochloric acid can be used as the acid, and aqueous ammonia and sodium hydroxide can be used as the alkali. The adjustment of the sol pH can also be achieved by simply mixing a plurality of types of sols.

  The pH of the sol can be adjusted by adding an acid or alkali to the sol while measuring the pH of the sol with a pH meter. It also measures the amount of acid or alkali required for pH adjustment using a pre-sampled sol, determines the amount of acid or alkali required for the entire sol based on it, and adds it to the entire sol Can also be achieved.

[Agglomeration of second colloidal particles]
In this method of the present invention, the sol is then agglomerated by bringing the pH of the sol closer to the isoelectric point of the colloidal particles of the second metal oxide than the isoelectric point of the colloidal particles of the first metal oxide. The colloidal particles of the second metal oxide are aggregated around the colloidal particles of the first metal oxide.

  When the pH of the sol containing the aggregated first metal oxide colloidal particles is changed to near the isoelectric point of the second metal oxide colloidal particles, the zeta of the second metal oxide colloidal particles is obtained. The potential is reduced and the electrical repulsion between the particles is reduced, thereby promoting the aggregation of the second metal oxide colloidal particles. Here, since the pH of the sol is relatively far from the isoelectric point of the first metal oxide colloidal particles, aggregation of the first metal oxide colloidal particles is suppressed, and the first metal oxide is oxidized. Second metal oxide colloidal particles are deposited around the colloidal particles of the object.

  The adjustment of the sol pH is the same as in the case of the aggregation of the first metal oxide colloidal particles.

[Drying and firing of aggregates]
In this method of the present invention, finally, the aggregate obtained as described above is dried and fired to produce metal oxide particles.

  Here, the metal oxide particles have a central portion that contains a relatively large amount of the first metal oxide and an outer skin portion that contains a relatively large amount of the second metal oxide. The primary particle diameter of the second metal oxide consisting of a plurality of primary particles and constituting the outer skin part is smaller than the primary particle diameter of the first metal oxide constituting the central part.

  Removal of the dispersion medium from the sol and drying can be performed by any method and at any temperature. This can be achieved, for example, by placing the sol in a 120 ° C. oven. The raw material obtained by removing the dispersion medium from the sol and drying in this way can be fired to obtain metal oxide particles. This calcination can be performed at a temperature generally used in the synthesis of metal oxides, for example, at a temperature of 500 to 1100 ° C.

  EXAMPLES Hereinafter, although this invention is further demonstrated based on an Example, this invention is not limited to these.

  The sol pH in the following experiments was measured by using a pH meter and immersing the pH meter electrode directly in the sol. The particle diameter of the colloidal particles in the sol was measured by a dynamic light scattering method (photon correlation method) using N4 type manufactured by Beckman Coulter. Moreover, the particle diameter of the obtained metal oxide particles (secondary particles) was measured using a laser diffraction / scattering particle size distribution measuring apparatus manufactured by Horiba.

[Example 1]
In this example, metal oxide particles (ceria (CeO ₂ ): zirconia (ZrO ₂ ): barium oxide (BaO) = 58) having a central part containing a relatively large amount of zirconia and a skin part containing a relatively large amount of ceria. 38: 4 (weight ratio)), and platinum is supported on the metal oxide particles. Here, the primary particle diameter of ceria constituting the outer skin part is 29 nm, the primary particle diameter of zirconia constituting the central part is 47 nm, and the particle diameter of the obtained metal oxide particles is 5.8 μm.

116.0 g of acid-stabilized ceria sol (CeO ₂ content 15 wt%, Nidral manufactured by Taki Chemical, colloid particle diameter 29 nm, isoelectric point pH 8.5) and alkali-stabilized zirconia sol (ZrO ₂ content 10.2 wt) %, Manufactured by Taki Chemical Co., Ltd., colloidal particle diameter 47 nm, isoelectric point pH 3.5) 111.7 g and barium nitrate 1.9 g were mixed, whereby the mixed sol was acidified to aggregate zirconia.

Thereafter, an ammonia (NH ₃ ) aqueous solution was dropped into the mixed sol while stirring to adjust the pH to 10 to aggregate ceria. This solution was dried at 120 ° C. for 24 hours, and the obtained dried product was calcined at 700 ° C. for 5 hours. When XRD measurement was performed on the metal oxide particles, peaks of zirconia and ceria were obtained independently.

  15 g of the metal oxide particles thus obtained are dispersed in 150 g of water, 3.41 g of a dinitrodiammine Pt solution (platinum content 4.4 wt%) is added, and platinum is added to the metal oxide particles. 1.0% by weight, and this was stirred for 2 hours. Then, the water | moisture content was dried at 120 degreeC and baked over 2 hours at 500 degreeC. The obtained catalyst was molded into 1 mm square pellets and used for performance evaluation.

[Example 2]
In this example, metal oxide particles (ceria: zirconia: barium oxide = 58: 38: 4 (weight ratio)) having a central portion containing a relatively large amount of zirconia and a skin portion containing a relatively large amount of ceria were obtained. The metal oxide particles carry platinum. Here, the primary particle diameter of ceria constituting the outer skin part is 29 nm, the primary particle diameter of zirconia constituting the central part is 95 nm, and the particle diameter of the obtained metal oxide particles is 8.1 μm.

As raw materials, acid-stabilized ceria sol (CeO ₂ content 15% by weight, Nidral manufactured by Taki Chemical, colloidal particle diameter 29 nm) 116.0 g, alkali-stabilized zirconia sol (ZrO ₂ content 11.7% by weight, colloidal particles A catalyst was obtained in the same manner as in Example 1 except that 97.4 g (diameter 95 nm) and 1.9 g of barium nitrate were used.

Example 3
In this example, metal oxide particles (ceria: zirconia: barium oxide = 58: 38: 4 (weight ratio)) having a central portion containing a relatively large amount of zirconia and a skin portion containing a relatively large amount of ceria were obtained. The metal oxide particles carry platinum. Here, the primary particle diameter of ceria constituting the outer skin part is 5 nm, the primary particle diameter of zirconia constituting the central part is 47 nm, and the particle diameter of the obtained metal oxide particles is 5.4 μm.

As raw materials, acid-stabilized ceria sol (CeO ₂ content 15.2% by weight, manufactured by Nissei Sangyo, colloidal particle diameter 5 nm) 114.4 g, alkali-stabilized zirconia sol (ZrO ₂ content 10.2% by weight, many A catalyst was obtained in the same manner as in Example 1 except that 111.7 g (Ecolite manufactured by Ki Chemical Co., Ltd., colloidal particle diameter 47 nm) and 1.9 g of barium nitrate were used.

Example 4
In this example, metal oxide particles (ceria: zirconia: barium oxide = 58: 38: 4 (weight ratio)) having a central portion containing a relatively large amount of zirconia and a skin portion containing a relatively large amount of ceria were obtained. The metal oxide particles carry platinum. Here, the primary particle diameter of ceria constituting the outer skin part is 5 nm, the primary particle diameter of zirconia constituting the central part is 32 nm, and the particle diameter of the obtained metal oxide particles is 2.3 μm.

As raw materials, acid-stabilized ceria sol (CeO ₂ content 15.2% by weight, manufactured by Nissei Sangyo, colloid particle diameter 5 nm) 114.4 g, alkali-stabilized zirconia sol (ZrO ₂ content 10.7% by weight, colloid) A catalyst was obtained in the same manner as in Example 1 except that 106.5 g (particle diameter 32 nm) and 1.9 g of barium nitrate were used.

[ Reference example ]
In this reference example , metal oxide particles (ceria: zirconia: barium oxide = 58: 38: 4 (weight ratio)) having a central portion containing a relatively large amount of zirconia and a skin portion containing a relatively large amount of ceria were obtained. The metal oxide particles carry platinum. Here, the primary particle diameter of ceria constituting the outer skin part is 29 nm, the primary particle diameter of zirconia constituting the central part is 153 nm, and the particle diameter of the metal oxide particles is 9.5 μm.

As raw materials, acid-stabilized ceria sol (CeO ₂ content 15% by weight, Nidral manufactured by Taki Chemical, colloidal particle diameter 29nm) 116.0g, alkali-stabilized zirconia sol (ZrO ₂ content 10.8% by weight, Dowa Mining) Manufactured, colloidal particle diameter 153 nm) and a catalyst was obtained in the same manner as in Example 1, except that 105.6 g and barium nitrate 1.9 g were used.

Example 6
In this example, metal oxide particles (ceria: zirconia: barium oxide = 58: 38: 4 (weight ratio)) having a central portion containing a relatively large amount of zirconia and a skin portion containing a relatively large amount of ceria were obtained. The metal oxide particles carry platinum. Here, the primary particle diameter of ceria constituting the outer skin part is 5 nm, the primary particle diameter of zirconia constituting the central part is 24 nm, and the particle diameter of the metal oxide particles is 1.1 μm.

As raw materials, acid-stabilized ceria sol (CeO ₂ content 15.2% by weight, manufactured by Nissei Sangyo, colloid particle diameter 5 nm) 114.4 g, alkali-stabilized zirconia sol (ZrO ₂ content 6.1% by weight, colloid) A catalyst was obtained in the same manner as in Example 1 except that 186.9 g (particle diameter: 24 nm) and 1.9 g of barium nitrate were used.

[Comparative Example 1]
In this comparative example, ceria particles are used as the metal oxide particles, and platinum is supported on the ceria particles.

  80.0 g of ammonium cerium nitrate was added to 500 g of water, and an aqueous ammonia solution was added dropwise thereto to adjust the pH to 9 to cause precipitation. This solution was dried at 120 ° C. for 24 hours, and the obtained dried product was calcined at 700 ° C. for 5 hours. The metal oxide particles thus obtained were loaded with 1.0% by weight of platinum in the same manner as in Example 1, and the obtained catalyst was molded into 1 mm square pellets and used for performance evaluation. .

[Comparative Example 2]
In this comparative example, ceria-zirconia-barium oxide solid solution particles (ceria: zirconia: barium oxide = 58: 38: 4 (weight ratio)) were obtained as metal oxide particles, and platinum was supported on the solid solution particles. .

  To 500 g of water, 73.89 g of ammonium cerium nitrate, 32.96 g of zirconium oxynitrate dihydrate, and 2.53 g of barium nitrate were added and stirred to make uniform. An aqueous ammonia solution was added dropwise thereto to adjust the pH to 9 to cause precipitation. This solution was dried at 120 ° C. for 24 hours, and the obtained dried product was calcined at 700 ° C. for 5 hours. The metal oxide particles thus obtained were loaded with 1.0% by weight of platinum in the same manner as in Example 1, and the obtained catalyst was molded into 1 mm square pellets and used for performance evaluation. .

[Comparative Example 3]
In this comparative example, metal oxide particles (ceria: zirconia: barium oxide = 58: 38: 4 (weight ratio)) having a central portion containing a relatively large amount of zirconia and a skin portion containing a relatively large amount of ceria were obtained. The metal oxide particles carry platinum. Here, the primary particle diameter of ceria constituting the outer skin part is 87 nm, the primary particle diameter of zirconia constituting the central part is 47 nm, and the particle diameter of the metal oxide particles is 10.2 μm.

As raw materials, 303 ml of acid-stabilized ceria sol (colloid particle diameter 87 nm), 67.1 g of alkali-stabilized zirconia sol (ZrO ₂ content 10.2% by weight, Ecolite manufactured by Taki Chemical, colloid particle diameter 47 nm), nitric acid A catalyst was obtained in the same manner as in Example 1 except that 1.22 g of barium was used. However, the acid-stabilized ceria sol used here was obtained by dissolving 40 g of ammonium cerium nitrate in 365 ml of distilled water and aging at 120 ° C. for 24 hours in a pressure-resistant vessel.

[Catalyst performance evaluation]
For the pelletized catalysts of Examples 1 to 4 and 6, Reference Examples, and Comparative Examples 1 to 3, the durable rich gas and the lean gas in Table 1 were switched every minute, and the flow rate was 5 L / min at 1,000 ° C. And endured for 5 hours. The rich gas and the lean gas for test shown in Table 2 were circulated at a flow rate of 20 L / min (space velocity: about 200,000 h ⁻¹ ) while switching the test rich gas and lean gas in Table 2 to 3 g of the durable catalyst. These gases were heated at a rate of 25 ° C./min, and the temperature (HC-T50) at which the C ₃ H ₆ purification rate was 50% was examined. The specific surface area (SSA) was measured using the BET single point method. Furthermore, the platinum particle diameter of the catalyst was determined by a CO pulse adsorption method at −80 ° C. The results obtained are shown in Table 3.

As is apparent from Table 3, the carrier of Comparative Example 1 composed of ceria particles has lower heat resistance and therefore has a smaller specific surface area than the carriers of Examples 1 to 4 and 6 of the present invention.

  Further, the support of Comparative Example 2 composed of a ceria-zirconia-barium oxide solid solution has improved heat resistance due to the presence of zirconia compared to the support composed of ceria particles of Comparative Example 1. However, the particle diameter of the supported platinum is larger than that of the carrier of Example 4 having the same specific surface area, and therefore the ability to prevent platinum sintering by ceria is not sufficiently exhibited. This is considered to be due to the fact that both ceria and zirconia exist on the surface of the solid solution, and a part of platinum is supported not on the ceria part but on the zirconia part.

  Furthermore, the carrier of Comparative Example 3 having a shell-center structure but made of relatively large ceria colloidal particles and relatively small zirconia colloidal particles is similar to the carrier of Example 2 having a similar specific surface area. In comparison, the supported platinum particles have a large particle size, and thus the platinum sintering ability is not sufficiently exhibited. This is considered to be because the ceria does not sufficiently cover the particle surface due to the large primary particle diameter of ceria in the carrier particles of Comparative Example 3.

  Among Examples, Examples 1 to 4 in which the primary particle diameter of zirconia is 100 μm or less and the average secondary particle diameter is 2.3 μm to 8.1 μm are relatively excellent with respect to HC-T50. That is, it has high activity even at a relatively low temperature.

It is sectional drawing showing the metal oxide particle of this invention.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 ... Center part 1a ... Primary particle | grains 2 which comprise center part ... Outer skin part 2a ... Primary particle which comprises outer skin part

Claims (4)

A central portion containing a relatively large amount of the first metal oxide and a skin portion containing a relatively large amount of the second metal oxide, each of the central portion and the skin portion comprising a plurality of primary particles ;
The primary particle size before Symbol second metal oxide, rather smaller than the primary particle diameter of the first metal oxide,
The first metal oxide is zirconia, the second metal oxide is ceria, and
The primary particle diameter of zirconia constituting the center is 100 nm or less.
Metal oxide particles. The metal oxide particles according to claim 1 , wherein the metal oxide particles have a particle diameter of 2.3 μm to 8.1 μm. An exhaust gas purification catalyst, wherein the metal oxide particles according to claim 1 or 2 carry a noble metal. A sol containing at least a first metal oxide colloidal particle and a second metal oxide colloidal particle having different isoelectric points, wherein the second metal oxide colloidal particle has a particle size of Providing a sol smaller than the particle size of the colloidal particles of the first metal oxide,
The first metal oxide colloidal particles are brought closer to the isoelectric point of the first metal oxide colloidal particles than the isoelectric point of the second metal oxide colloidal particles. Agglomerate
The pH of the sol is made closer to the isoelectric point of the colloidal particles of the second metal oxide than the isoelectric point of the colloidal particles of the first metal oxide, and the aggregated first metal oxides Agglomerating the colloidal particles of the second metal oxide around the colloidal particles, and drying and calcining the obtained agglomerates;
including,
A method for producing metal oxide particles.

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