JP4123185B2 - Method for producing metal oxide particles - Google Patents

Description

  The present invention relates to a method for producing metal oxide particles, and more particularly to a method for producing metal oxide particles that are preferable for supporting a noble metal and using it as an exhaust gas purification catalyst.

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 is stored when the oxygen concentration in the exhaust gas is high, and oxygen is stored when the oxygen concentration in the exhaust gas is low. A material having an oxygen storage capacity (OSC capacity) to be released is used as a carrier for an exhaust gas purification catalyst. A typical material having the 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 in the exhaust gas and maintain the oxygen concentration in the vicinity of the stoichiometric air-fuel ratio with the material having the ability 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 a metal oxide support composed of a plurality of types of materials is used and a combination of these properties is used, the particles of the plurality of types of metal oxides can be mixed. However, in this case, the size of the combined metal oxide particles may be large, and the properties of the 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.

  Therefore, this invention provides the manufacturing method of the metal oxide particle which has multiple types of metal oxide, and can exhibit the property of each metal oxide satisfactorily.

  The method of the present invention for producing metal oxide particles having different compositions in the central portion and the outer skin portion provides a sol containing at least first colloid particles and second colloid particles each having different isoelectric points. The pH of the sol is made closer to the isoelectric point of the first colloidal particle than the isoelectric point of the second colloidal particle, in particular the isoelectric point of the first colloidal particle ± 1.0, more particularly equal The first colloidal particles are agglomerated by bringing them closer to an electric point within a range of ± 0.5; In the second colloidal particle, in particular, within the isoelectric point ± 1.0, more particularly within the range of the isoelectric point ± 0.5, Agglomerating the colloidal particles; and drying and calcining the resulting agglomerates Including.

  According to the method of the present invention, a metal oxide particle having a central portion containing a relatively large amount of components derived from the first colloid particles and an outer skin portion containing a relatively large amount of components derived from the second colloid particles is obtained. Obtainable.

  Further, in the method of the present invention, metal oxide particles having a structure composed of a central portion and a skin portion can be obtained with an arbitrary particle size. For example, when the colloidal particles actually used as the raw material have an average particle diameter of about 5 nm, the average particle diameter of the metal oxide particles synthesized by the method of the present invention can be 50 nm or less. Thus, the metal oxide particles can have an average particle size of, for example, less than 10 μm, less than 5 μm, less than 1 μm, less than 500 nm, less than 200 nm, less than 100 nm, or less than 50 nm.

  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 “a central part containing a relatively large amount of components derived from the first colloidal particles”, the molar fraction of the metal constituting this component in the central part is higher than the molar fraction of this metal in the outer skin part. Means.

  The term “colloidal particle” is a particle having a metal oxide or oxygen-bound metal dispersed in a liquid, particularly water, and the metal oxide is generated by removing the dispersion medium and firing. Means things. 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.

  Furthermore, the term “sol” means a dispersion system in which colloidal particles are dispersed in a dispersion medium that is a liquid, and is sometimes 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.

  In another embodiment of the method of the present invention, the first colloidal particles are aggregated by changing the pH of the sol so as to pass through the isoelectric point of the first colloidal particles.

  According to this, when the pH of the sol passes through the isoelectric point of the first colloidal particle, the zeta potential of the first colloidal particle becomes zero, and thus the first colloidal particle is more reliably aggregated. be able to.

  In another embodiment of the method of the present invention, the second colloidal particles are aggregated by changing the pH of the sol so as to pass through the isoelectric point of the second colloidal particles.

  According to this, when the pH of the sol passes through the isoelectric point of the second colloidal particle, the zeta potential of the second colloidal particle becomes zero, so that the second colloidal particle is more reliably aggregated. be able to.

  In another embodiment of the method of the present invention, the first and second colloidal particles are each independently selected from the group consisting of colloidal particles of alumina, ceria, zirconia and titania.

  In another embodiment of the method of the present invention, the first colloidal particles are zirconia, alumina or titania, particularly zirconia, and the second colloidal particles are ceria.

  According to this, metal oxide particles having a central portion containing a relatively large amount of zirconia, alumina or titania and a skin portion containing a relatively large amount of ceria can be obtained.

  When platinum is supported on such metal oxide particles, good heat resistance by zirconia, alumina or titania, particularly zirconia in the center, and platinum sintering prevention effect by ceria can be achieved.

  Also here, based on the total number of moles of metal in the metal oxide particles, the total mole fraction of cerium and zirconium, aluminum or titanium is at least 85 mol%, in particular at least 90 mol%, more particularly at least 95 mol%. It's okay.

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

  As shown in FIG. 1, according to the method of the present invention, metal oxide particles having different compositions in the central portion 1 and the outer skin portion 2 can be produced. That is, in the method of the present invention, the first colloid particles are aggregated in a sol containing at least two types of colloid particles having different isoelectric points, and then the second colloid particles are aggregated around the first colloid particles. A metal having a central part 1 mainly composed of components derived from the first colloidal particles and an outer skin part 2 mainly composed of components derived from the second colloidal particles by aggregating the colloidal particles of Oxide particles are produced.

  Here, the central portion 1 and the outer skin portion 2 are each composed of a plurality of primary particles (1a, 2a) derived from the first colloid particles and the second colloid particles. However, the plurality of primary particles may or may not have a clear boundary between each primary particle. 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 central part 1 and the outer skin part 2 may be a mixture of a component derived from the first colloidal particle and a component derived from the second colloidal particle, particularly a solid solution. In FIG. 1, the outer skin portion 2 is shown to be discontinuous, but it may be continuous.

  Any metal oxide can be selected as the metal oxide constituting the metal oxide particles produced by the method of the present invention, and in particular, a metal oxide that is preferably retained at the center of the metal oxide particles. The metal oxide selected as the first metal oxide and preferably exposed to the outer skin of the metal oxide particles can be selected as the second metal oxide. 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 particles of the present invention contains zirconia or ceria, these central portions or outer skin portions are metals other than cerium (Ce) and zirconium (Zr), such as alkaline earth metals and A metal selected from the group consisting of rare earth elements, particularly yttrium (Y) may be included. These alkaline earth metals and rare earth elements, especially yttrium, tend to provide excellent heat resistance to zirconia and ceria.

  An exhaust gas purification catalyst can be produced by supporting a noble metal such as platinum, rhodium or palladium on the metal oxide particles obtained by the method of the present invention. In this case, since the metal oxide particles obtained by the method of the present invention can have a center portion and a skin portion, this exhaust gas purification catalyst can mainly carry a noble metal on the skin portion.

  The supporting of the noble metal on the metal oxide particles can be carried out using any known method. This can be performed, for example, by carrying a water-absorbing solution containing a noble metal salt and / or complex salt, drying and firing. 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.

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

  Below, each process of the method of this invention is demonstrated.

[Providing mixed sol]
In the method of the present invention, first, a sol containing at least first colloid particles and second colloid particles having different isoelectric points is provided.

  Specific examples of the sol provided here 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 is arbitrarily set by the surface modification of the colloidal particle, particularly the surface modification of the colloidal particle by an organic compound. It can be done. Accordingly, the colloidal particles of the first and second metal oxides used in the method of the present invention can be arbitrarily selected so as to have an appropriate pH for the present invention. For example, the first and second colloidal particles can be selected such that the pH of the isoelectric point of these colloidal particles is 3 or more, particularly 4 or more, more particularly 5 or more.

  In addition, although the isoelectric point of the colloid particle required regarding implementation of the method of this invention can be obtained by arbitrary methods, it can measure by the electrophoretic light scattering method, for example.

  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 this method of the present invention, the first colloidal particles are then agglomerated by bringing the pH of the sol closer to the isoelectric point of the first colloidal particles than the isoelectric point of the second colloidal particles.

  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 colloid particle and the second colloid particle, when the pH of the sol is changed to near the isoelectric point of the first colloid particle, the first colloid The zeta potential of the particles is reduced and the electrical repulsion between the particles is reduced, thereby promoting the aggregation of the first colloidal particles. Here, since the pH of the sol is relatively far from the isoelectric point of the second colloidal particle, the second colloidal particle has a relatively large zeta potential, and therefore the aggregation of the second colloidal particle is suppressed. Has been.

  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 pH of the sol 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 pH of the sol is then brought closer to the isoelectric point of the second colloidal particle than the isoelectric point of the first colloidal particle, around the aggregated first colloidal particle, Aggregate the second colloidal particles.

  As described above, when the pH of the sol containing the aggregated first colloidal particles is changed to the vicinity of the isoelectric point of the second colloidal particles, the zeta potential of the second colloidal particles becomes small and the inter-particles The electrical repulsion is reduced, thereby promoting the aggregation of the second colloidal particles. Here, since the pH of the sol is relatively far from the isoelectric point of the first colloid particles, the aggregation of the first colloid particles is suppressed, and the second colloid particles around the first colloid particles. Accumulates.

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

[Drying and firing of aggregates]
The agglomerate thus obtained is dried and fired, so that it is mainly composed of a central part mainly composed of components derived from the first colloidal particles and a component derived from the second colloidal particles. Metal oxide particles having a skin portion can be produced.

  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.

[Example 1]
In this embodiment, metal oxide particles having a relatively large amount of zirconia in the center and a relatively large amount of ceria in the outer portion are obtained from the alkali-stabilized zirconia aqueous sol and the acid-stabilized ceria aqueous sol.

An alkali-stabilized zirconia aqueous sol (isoelectric point pH 3.5) and an acid-stabilized ceria aqueous sol (isoelectric point pH 8.5) are mixed to obtain a molar ratio of zirconia (ZrO ₂ ) and ceria (CeO ₂ ). Was set to 1: 1. While stirring, a nitric acid (HNO ₃ ) aqueous solution was dropped into the mixed sol to adjust the pH to 3.0, and zirconia was aggregated. Thereafter, an ammonia (NH ₃ ) aqueous solution was dropped into the mixed sol while stirring to adjust the pH to 10 to aggregate ceria.

  The mixed sol was dried at 120 ° C. for 24 hours, and the obtained dried product was fired at 700 ° C. for 5 hours to obtain metal oxide particles.

[Example 2]
In this embodiment, metal oxide particles containing a relatively large amount of titania at the center and a relatively large amount of ceria at the outer skin are obtained from the aqueous solution of alkali-stabilized titania and acidic stabilized ceria.

Alkali-stabilized titania aqueous sol (isoelectric point pH 3.9) and acid-stabilized ceria aqueous sol (isoelectric point pH 8.5) are mixed to obtain a molar ratio of titania (TiO ₂ ) and ceria (CeO ₂ ). Was set to 1: 1. While stirring, an aqueous nitric acid solution was dropped into the mixed sol to adjust the pH to 3.0, thereby coagulating titania. Thereafter, with stirring, an aqueous ammonia solution was dropped into the mixed sol to adjust the pH to 10 to aggregate ceria. Thereafter, drying and firing were performed in the same manner as in Example 1 to obtain metal oxide particles.

Example 3
In this embodiment, metal oxide particles containing a relatively large amount of alumina in the center and a relatively large amount of ceria in the outer skin part are obtained from the alkali-stabilized alumina aqueous sol and the acid-stabilized ceria aqueous sol.

An alkali-stabilized alumina aqueous sol (isoelectric point pH 4.8) and an acid-stabilized ceria aqueous sol (isoelectric point pH 8.5) are mixed to obtain alumina (Al ₂ O ₃ ) and ceria (CeO ₂ ). The molar ratio was 1: 2. While stirring, an aqueous nitric acid solution was dropped into the mixed sol to adjust the pH to 3.0, thereby coagulating the alumina. Thereafter, with stirring, an aqueous ammonia solution was dropped into the mixed sol to adjust the pH to 10 to aggregate ceria. Thereafter, drying and firing were performed in the same manner as in Example 1 to obtain metal oxide particles.

Example 4
In this example, metal oxide particles containing a relatively large amount of zirconia in the central portion and a relatively large amount of ceria in the outer skin portion are obtained from the acid-stabilized zirconia aqueous sol and the alkali-stabilized ceria aqueous sol.

An acid-stabilized zirconia aqueous sol (isoelectric point pH 7.8) and an alkali-stabilized ceria aqueous sol (isoelectric point pH 4.0) are mixed to obtain a molar ratio of zirconia (ZrO ₂ ) and ceria (CeO ₂ ). Was set to 1: 1. While stirring, an aqueous ammonia solution was dropped into the mixed sol to adjust the pH to 10 to aggregate zirconia. Thereafter, while stirring, an aqueous nitric acid solution was dropped into the mixed sol to adjust the pH to 3.0, and ceria was aggregated. Thereafter, drying and firing were performed in the same manner as in Example 1 to obtain metal oxide particles.

Example 5
In this example, metal oxide particles containing a relatively large amount of titania at the center and a relatively large amount of ceria at the outer skin are obtained from the acid-stabilized titania aqueous sol and the alkali-stabilized ceria aqueous sol.

An acid-stabilized titania aqueous sol (isoelectric point pH 7.9) and an alkali-stabilized ceria aqueous sol (isoelectric point pH 4.0) are mixed to obtain a molar ratio of titania (TiO ₂ ) and ceria (CeO ₂ ). Was set to 1: 1. While stirring, an aqueous ammonia solution was dropped into the mixed sol to adjust the pH to 10 to aggregate titania. Thereafter, while stirring, an aqueous nitric acid solution was dropped into the mixed sol to adjust the pH to 3.0, and ceria was aggregated. Thereafter, drying and firing were performed in the same manner as in Example 1 to obtain metal oxide particles.

Example 6
In this example, metal oxide particles containing a relatively large amount of alumina in the center and a relatively large amount of ceria in the outer skin are obtained from the acid-stabilized alumina aqueous sol and the alkali-stabilized ceria aqueous sol.

An acid-stabilized alumina aqueous sol (isoelectric point pH 7.6) and an alkali-stabilized ceria aqueous sol (isoelectric point pH 4.0) are mixed to obtain alumina (Al ₂ O ₃ ) and ceria (CeO ₂ ). The molar ratio was 1: 2. While stirring, an aqueous ammonia solution was dropped into the mixed sol to adjust the pH to 10 to aggregate the alumina. Thereafter, while stirring, an aqueous nitric acid solution was dropped into the mixed sol to adjust the pH to 3.0, and ceria was aggregated. Thereafter, drying and firing were performed in the same manner as in Example 1 to obtain metal oxide particles.

[Comparative Example 1]
In this embodiment, zirconia powder and ceria powder are mixed.

Zirconia powder and ceria powder were mixed so that the molar ratio of zirconia (ZrO ₂ ) to ceria (CeO ₂ ) was 1: 1, and this was mixed in a ball mill for 100 hours.

[Comparative Example 2]
In this embodiment, titania powder and ceria powder are mixed.

Titania powder and ceria powder were mixed so that the molar ratio of titania (TiO ₂ ) to ceria (CeO ₂ ) was 1: 1, and this was mixed for 100 hours in a ball mill.

[Comparative Example 3]
In this embodiment, alumina powder and ceria powder are mixed.

Alumina powder and ceria powder were mixed so that the molar ratio of alumina (Al ₂ O ₃ ) to ceria (CeO ₂ ) was 1: 2, and this was mixed in a ball mill for 100 hours.

[Comparative Example 4]
In this comparative example, metal oxide particles having zirconia and ceria are obtained using a coprecipitation method.

  Ammonium cerium nitrate and zirconium oxynitrate dihydrate were added to distilled water so that the molar ratio of zirconium (Zr) to cerium (Ce) was 1: 1. An aqueous ammonia solution was added dropwise to the mixture to adjust the pH to 9, causing precipitation. Thereafter, drying and firing were performed in the same manner as in Example 1 to obtain metal oxide particles.

[Structural evaluation of metal oxide particles]
The surface CeO ₂ concentration of the metal oxide particles of Examples 1 to 6 and Comparative Examples 1 to 4 was determined using XPS (X-ray photoelectron spectroscopy) quantitative analysis. The results are shown in Table 1 below.

  As apparent from Table 1, compared with the coprecipitation methods of Comparative Examples 1 to 3 in which ceria powder and zirconia powder are mixed and Comparative Example 4 in which ceria-zirconia powder is obtained, metal oxidation by the method of the present invention is performed. In the case of physical particles, it is clear that a relatively large amount of ceria appears on the surface even though the molar ratio in the raw material used is the same.

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 (5)

Providing a sol containing at least first colloid particles and second colloid particles each having a different isoelectric point;
Bringing the pH of the sol closer to the isoelectric point of the first colloidal particle than the isoelectric point of the second colloidal particle to aggregate the first colloidal particle;
The second colloidal particles are disposed around the aggregated first colloidal particles by bringing the pH of the sol closer to the isoelectric point of the second colloidal particles than the isoelectric point of the first colloidal particles. And drying and calcining the resulting agglomerates,
The manufacturing method of the metal oxide particle which has a composition which is different in a center part and an outer_skin | epidermis part.   The method of claim 1, wherein the first colloidal particles are aggregated by varying the pH of the sol so as to pass through the isoelectric point of the first colloidal particles.   The method according to claim 1 or 2, wherein the second colloidal particles are aggregated by changing the pH of the sol so as to pass through the isoelectric point of the second colloidal particles.   The method according to any one of claims 1 to 3, wherein the first and second colloidal particles are independently selected from the group consisting of colloidal particles of alumina, ceria, zirconia and titania.   The method of claim 4, wherein the first colloidal particle is zirconia, alumina, or titania, and the second colloidal particle is ceria.

Images