JP2011230104A - Exhaust gas purifying catalyst and method for manufacturing the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an exhaust gas purifying catalyst which prevents the deterioration of the catalytic activity in a long-term use, and can retain a high gas purifying capability (the purification performance of gas including at least one of NO, HC, and CO) for a longer term than before.SOLUTION: A method for manufacturing the exhaust gas purifying catalyst includes: a first metal-support process for supporting a first catalytic metal on a surface of a rod-like or fibrous material removable by burning; a layer formation process for imparting a metal oxide precursor-containing composition to cover a metal-support part of the material where the first catalytic metal is supported in order to form a precursor composition layer on the material; an oxide carrier formation process for forming a metal oxide carrier having a hollow structure with the first catalytic metal supported on its inner wall by burning the material with the precursor composition layer formed thereon and removing the material; and a second metal-support process for supporting a second catalytic metal on an outer wall of the metal oxide carrier.

Description

The present invention relates to an exhaust gas purifying catalyst suitable for purifying NO _x , HC, and CO contained in exhaust gas discharged from an internal combustion engine, and a method for manufacturing the same.

  Conventionally, a three-way catalyst is generally put into practical use as a purification catalyst for purifying harmful substances in exhaust gas discharged in an automobile equipped with an internal combustion engine. As this three-way catalyst, metals such as platinum (Pt), rhodium (Rh), and palladium (Pd) are used, and those in which these metal components are present in a single layer are widely known. Yes.

As a structure of the purification catalyst, a purification catalyst having, for example, a two-layer structure in addition to a single-layer structure in which a metal component is present in a single layer has been proposed. In the case of a layer structure in which two or more kinds of catalyst metals such as Pt and Rh coexist in a single layer, it is easy to form a solid solution by interacting with the catalyst metals. Will decline. For example, Pt or Pd, Rh, the oxidation of NO _x, is excellent in reduction performance, the catalyst activity is lowered by the solid solution, the performance of the purification catalyst is remarkably lowered. Therefore, in consideration of the influence on the catalytic activity due to the solid solution of metals, for example, a catalyst provided with two upper and lower catalyst layers in which Pt and Rh are included in different layers has been proposed.

In recent years, as a method for efficiently purify NO _x, or a three-way catalyst 2 or more layers structure, it is a multi-layered structure, placing the Rh with high gas contacting outer layer (upper layer away from the substrate) Has been widely studied. Specifically, a catalyst using a complex oxide mainly composed of ZrO ₂ in the upper layer as a carrier for rhodium and a ceria zirconia complex oxide in the lower layer as a carrier is known.

  As a technology related to a catalyst for exhaust gas purification, a highly heat-resistant catalyst carrier made of an oxide having a multilayer structure formed by firing hydroxide and having a gap between layers, and exhaust gas purification with a noble metal supported in the gap A catalyst for use is disclosed (for example, see Patent Document 1).

  In addition, a catalyst carrier base material is disclosed in which a carbon nanotube is impregnated with a liquid containing a precursor of a composite oxide, the composite oxide is supported, and then the carbon nanotube is eliminated (for example, Patent Document 2). It is said that a high specific surface area can be obtained.

JP 2004-141864 A JP 2005-46669 A

  In a catalyst in which a noble metal is supported on a metal oxide, there is a problem that the catalytic activity deteriorates due to the movement of the noble metal during long-term use and grain growth. The movement of noble metal is a phenomenon that occurs freely because there is no substance that blocks movement in space.

Among the conventional techniques described above, in the exhaust gas purifying catalyst in which the noble metal is supported in the gap between the layers, the multilayer structure can only be made of Al ₂ O ₃ , and the carrier such as CeO ₂ , ZrO ₂ , CZ (CeO ₂ —ZrO ₂ ) There is a problem that there is no selectivity and a desired catalyst configuration cannot be taken. Although there is a it is preferable to carry an oxide particles such as CeO 2, _ZrO 2, _CZ between the layers of α-Al ₂ O ₃ having a multilayer structure, the supported amount as compared with the amount of the noble metal often makes it difficult to coat the inner wall of the interlayer in CeO ₂ or the like. Since it cannot be coated with CeO ₂ or the like, the noble metal has a region supported by Al ₂ O ₃ having a multilayer structure, and the O ₂ storage effect of CeO ₂ and the characteristics of ZrO ₂ when Rh is supported are fully exhibited. There is also a problem that it is not done.

The present invention has been made in view of the above, and prevents a decrease in catalyst activity during long-term use, and high gas purification with respect to exhaust gas discharged from an internal combustion engine or the like (also simply referred to as “exhaust gas” in this specification) Exhaust gas purification catalyst that can maintain the performance (purification performance of gas containing at least one of NO _x , HC, and CO; hereinafter the same) for a long period of time and gas purification capacity for a long period of time It aims at providing the manufacturing method of the exhaust gas purification catalyst which can produce the exhaust gas purification catalyst which can be hold | maintained easily and stably, and makes it a subject to achieve this objective.

  In the present invention, it is considered that the probability that the metal moves into the hollow inside of the carrier formed into a tube shape is low, and a noble metal that easily causes grain growth and alloying is maintained in that the desired catalytic activity is maintained for a long time. The structure in which the tube is selectively disposed inside the tube structure and the other metal is disposed outside the tube structure has been obtained based on the knowledge that an extremely high improvement effect can be expected.

In order to achieve the above object, a method for producing an exhaust gas purification catalyst according to the first invention comprises:
<1> Covering a first metal supporting step of supporting the first catalytic metal on the surface of the rod-like or fibrous material that can be baked and removed, and a metal supporting portion on which the first catalytic metal of the material is supported. Applying a metal oxide precursor-containing composition and forming a precursor composition layer on the material; firing the material on which the precursor composition layer is formed; and firing the material An oxide carrier forming step of forming a metal oxide carrier having a hollow structure in which the first catalytic metal is supported on the inner wall by removing, and a second catalytic metal on the outer wall of the metal oxide carrier. And a second metal supporting step for supporting.

In the method for producing an exhaust gas purifying catalyst according to the first aspect of the present invention, a desired catalytic metal (and other metal if necessary; preferably a noble metal that easily causes grain growth and alloying) is used as a material that can be fired and removed. ) Is applied in a layer form so as to cover the catalyst metal, and calcined, and a catalyst metal different from the desired catalyst metal (and other metal if necessary) is formed on the support formed by calcination. By supporting a metal (preferably a noble metal that is unlikely to cause grain growth or alloying), a desired type and amount of noble metal is supported inside and outside the metal oxide support formed in the hollow structure. be able to. Thereby, when exposed to a high temperature environment for a long period of time, it is possible to avoid grain growth and alloying of noble metals such as Pt and Pd that contribute to the purification of NO _x , HC, and CO. Accordingly, it is possible to provide an exhaust gas purifying catalyst that can be used for a longer period of time than in the past while maintaining a desired purifying ability for NO _x , HC, and CO while preventing a decrease in catalytic activity during long-term use. Can do.

  <2> In the method for producing an exhaust gas purification catalyst according to <1>, the first catalyst metal and the second catalyst metal are each selected from platinum (Pt), palladium (Pd), and rhodium (Rh). It is preferable to include different noble metals.

During exhaust gas purification, different noble metals are supported on the inside and outside of the hollow structure of the metal oxide support, so that noble metals that particularly contribute to the purification of HC, CO, and NO _x interact with each other to form alloys. It is possible to suppress a decrease in the catalytic activity of each noble metal that accompanies this.

  <3> In the method for producing an exhaust gas purification catalyst according to <1> or <2>, at least one of the first catalytic metals is at least one of Pt (platinum) and Pd (palladium), In a preferred embodiment, at least one of the catalytic metals is Rh (rhodium).

Since Pt and Pd are easy to grow grains and alloy with other metals such as rhodium (Rh), Pt and Pd are disposed as catalyst metals in the hollow structure (inner wall) of the metal oxide support. Thus, it is possible to prevent Pt and Pd from excessively growing and alloying. Therefore, when purifying exhaust gas, Pt and Pd, which are excellent in HC and CO oxidation activity, are stably retained.
In addition, since Rh is supported on the outside (outer wall) of the metal oxide support and separated from Pt and Pd, the movement of Rh to the Pt and Pd supporting region can be suppressed. Solid solution is suppressed. Therefore, the NO reduction activity possessed by Rh is maintained, and the loss of the catalytic activity of Pt and Pd can be suppressed.
Thereby, the fall from the expected performance of the purification performance at the time of long-term use can be suppressed effectively.

<4> In the method for producing an exhaust gas purifying catalyst according to any one of <1> to <3>, an aspect in which carbon fiber is used as a material that can be removed by firing is preferable. A carbon material having one or more hollow structures selected from nanohorns and carbon nanofibers is preferred.
When the carbon fiber is used as the material that can be removed by firing, the material can be stably fired with less impurities. Carbon fibers do not have to have a hollow structure, but when carbon fibers such as carbon nanotubes are used, they have a hollow structure, so heat transfer is good and they can be easily burned off at the firing stage. In addition, since there are few combustion products, rapid heat generation during combustion is suppressed, which is advantageous in that the risk of metal oxide damage is low.

  <5> In the method for producing an exhaust gas purifying catalyst according to any one of <1> to <4>, the formation of the precursor composition layer in the layer forming step is performed by a sol-gel method or a hydrothermal method. Is a preferred embodiment.

Next, the exhaust gas purification catalyst according to the second invention is:
<6> A metal oxide support having a hollow structure, a first catalyst metal disposed on the inner wall of the hollow structure of the metal oxide support, and a second catalyst metal disposed on the outer wall. It is.

The exhaust gas purifying catalyst according to the above <6> has, for example, NO when it is exposed to a high temperature environment over a long period of time because different catalytic metals are supported on the inner wall and the outer wall of the hollow metal oxide support. It is possible to avoid grain growth and alloying of catalytic metals such as noble metals (Pt, Pd, etc.) responsible for purification of _x , HC and CO. As a result, the catalyst activity is prevented from lowering over a long period of use, and can be used for a long period of time while maintaining the desired purification performance for NO _x , HC, and CO.

  <7> In the exhaust gas purification catalyst according to <6>, the first catalyst metal and the second catalyst metal are different noble metals selected from Pt (platinum), Pd (palladium), and Rh (rhodium). It is preferable to include. Among these, it is preferable that at least one of the first catalyst metals is at least one of platinum and palladium, and at least one of the second catalyst metals is rhodium.

In the exhaust gas purification catalyst according to <7>, as described above, the catalytic activities of Pt, Pd, and Rh that particularly contribute to the purification of NO _x , HC, and CO in the exhaust gas are impaired by grain growth and alloying. Can be held without being. In particular, grain growth of Pt and Pd and solid solution formation of Pt and Rh are avoided, and reduction in catalytic activity when exposed to a high temperature environment for a long period can be reduced.

According to the present invention, it is possible to prevent a decrease in catalyst activity in a long-term use, and to have a high gas purification ability (gas purification performance including at least one of NO _x , HC, and CO) with respect to exhaust gas discharged from an internal combustion engine or the like. It is possible to provide an exhaust gas purification catalyst that can be maintained for a longer period than before. Also,
ADVANTAGE OF THE INVENTION According to this invention, the manufacturing method of the exhaust gas purification catalyst which can produce the exhaust gas purification catalyst which can hold | maintain gas purification ability over a long period of time easily and stably can be provided.

It is a graph which shows the temperature required for the exhaust gas purification catalyst of the present invention to purify 50% of hydrocarbons (HC) in comparison with a comparative purification catalyst. It is a graph which shows the temperature required for the exhaust gas purification catalyst of the present invention to purify 50% of nitrogen oxide (NO _x ) in comparison with the comparative purification catalyst.

  Hereinafter, the manufacturing method of the exhaust gas purification catalyst of the present invention will be described in detail, and the exhaust gas purification catalyst of the present invention will be described in detail through the description.

  The method for producing an exhaust gas purifying catalyst of the present invention includes a first metal supporting step of supporting a first catalytic metal on the surface of a rod-like or fibrous sinterable material, and a first sinterable material first A layer forming step of applying a metal oxide precursor-containing composition over a metal supporting portion on which a catalytic metal is supported, and forming a precursor composition layer on the material capable of being removed by firing; and the precursor composition A metal oxide support having a hollow structure in which the first catalytic metal is supported on an inner wall by firing the fireable removable material on which a physical layer is formed and removing the fireable removable material. An oxide carrier forming step to be formed and a second metal supporting step for supporting the second catalytic metal on the outer wall of the metal oxide carrier are provided. The method for producing an exhaust gas purifying catalyst of the present invention may be configured by further providing other processes such as a drying process, if necessary.

-First metal loading process-
In the first metal supporting step of the present invention, the first catalytic metal is supported on the surface of a rod-like or fibrous material that can be removed by firing (hereinafter, also referred to as “fired removal material”). The calcination-removable material that can be removed by calcination functions as a mold that can be burned off during calcination and can form a carrier in the layer forming step described later, and does not carry a metal inside the baked carrier, but before firing. By making the first catalyst metal exist in advance, it is possible to obtain a structure in which the first catalyst metal is supported inside the support simultaneously with the formation of the support.

  In the present invention, the “material that can be removed by firing” refers to a material that can be removed by firing in a temperature range of 200 ° C. or higher (preferably 400 ° C. or higher).

  The fired removal material is not particularly limited as long as it is a material that can be burned off during the firing process described later and has a rod-like or fibrous structure, and any material may be selected. Examples of the material that can be removed by firing (fired removal material) include various resin materials such as polyester, organic materials such as rubber materials, and inorganic materials such as carbon materials. Of these, inorganic materials, particularly carbon materials are preferred in terms of purity, surface area, and external size.

As an example of the carbon material, carbon fiber is preferable, and among them, carbon nanotube (CNT), carbon nanohorn, carbon nanofiber (CNF), and the like can be preferably exemplified. Carbon nanotubes include single-walled single-walled nanotubes (SWNT) and multi-walled multi-walled nanotubes (MWNT) such as double-walled and triple-walled structures, and may be either straight tube or helical . From the viewpoint of atomization and thinning, it is desirable that the carbon nanotubes have a single-layer structure.
The carbon nanohorn is an aggregate of carbon nanotubes having a single-layer structure whose tip is closed in a conical shape.
Carbon materials may be used alone or in combination of two or more.

  The carbon fiber preferably has an outer diameter (average outer diameter) of 30 nm or less, more preferably 10 nm or less. When the outer diameter is in the above range, a tubular body having a small diameter is used as a carrier for supporting the catalyst metal, so that a large surface area capable of supporting the catalyst metal can be secured, and the catalyst metal can be dispersed in a highly dispersed state. It is possible to carry.

  Carbon nanotubes and the like are generally synthesized via a gas phase such as arc discharge, laser evaporation, and CVD, but can also be synthesized by a hydrothermal method.

The fire removal material has a rod-like or fibrous structure. A cylindrical metal oxide support having an opening through which exhaust gas (exhaust gas) discharged from the internal combustion engine can pass is obtained by making the structure of the fired removal material rod-shaped or fiber-shaped. The first noble metal is supported on the hollow inner wall surface of the metal oxide carrier, and NO _x , HC and CO in the flowing exhaust gas can be purified.

  The rod-like or fiber-like shape means a linear or curved shape having a length larger than the width (or thickness), and supports the second catalytic metal in the exhaust gas flow and the post-process as described above. The length (l) in the longitudinal direction of the fired removal material with respect to the maximum length (r) in a cross section perpendicular to the longitudinal direction is long (preferably from the viewpoint of preventing the second catalytic metal from entering the hollow interior during The shape having an aspect ratio in which the length 1 in the longitudinal direction is 3 times or more of the maximum cross-sectional length r, and further 5 times or more is preferable.

  The length l in the longitudinal direction of the fired removal material is longer than the maximum length r of the orthogonal cross section with the longitudinal direction, preferably the length l in the longitudinal direction is at least 3 times the maximum length r of the cross section, more preferably 5 times Due to the shape having the above aspect ratio, when the second catalytic metal is supported on the outer wall of the fired metal oxide support by, for example, the impregnation method or the adsorption method, the hollow structure of the metal oxide support. It is possible to prevent the second catalytic metal from entering the inside. Thereby, the metal seed | species distribute | arranged to the inner wall of a hollow structure can be comprised in the structure separated from the metal seed | species distribute | arranged to an outer wall more reliably.

The rod-like or fiber-like fired removal material has a linear or curved shape, and preferably has an outer diameter (average diameter) of 30 nm or less, and more preferably an inner diameter of 10 nm or less. When the outer diameter is in the above range, a large surface area capable of supporting the catalyst metal can be secured when the metal oxide support is molded, and when the catalyst metal is supported, it can be supported in a highly dispersed state. .
In this case, the length in the longitudinal direction of the rod-like or fibrous fired removal material is preferably 0.005 μm or more and 5 μm or less. Among these, carbon fibers (particularly carbon nanotubes, carbon nanohorns, carbon nanofibers) having a length in the longitudinal direction of 0.1 μm to 3 μm are preferable. When the length is in the above range, it is effective to effectively prevent the second catalytic metal from entering the hollow structure of the metal oxide support.

  It is preferable that the rod-like or fibrous calcination removing material is subjected to pretreatment for imparting hydrophilicity in advance before supporting the first catalyst metal. The pretreatment can be performed, for example, by refluxing with nitric acid or a mixed acid of sulfuric acid and nitric acid.

The first catalytic metal is platinum (Pt), palladium (Pd), rhodium (Rh), osmium (Os), iridium (Ir), gold, as a noble metal having HC, CO oxidation activity or NO _x reduction activity. (Au) etc. are mentioned, Among these, the noble metal chosen from Pt, Pd, and Rh is preferable. Since the first catalyst metal is a catalyst metal supported on the inner wall of the hollow structure of the metal oxide carrier as described later, it is a noble metal that easily grows in a high temperature environment or is alloyed with other metals. Is more preferable. From this viewpoint, Pt and / or Pd are more preferable as the first catalyst metal. Pt, Pd is excellent HC, the oxidation activity of CO, Rh has high reduction activity of NO _x, by the catalytic activity of these catalytic metal is maintained high, over the performance of the purification catalyst for long-term Can be held high.

In addition to supporting a noble metal, the surface of the fired removal material may further support a metal other than the noble metal. As metals other than noble metals, promoters such as Al and Fe, and NO _x storage materials such as alkali metals and alkaline earth metals can be supported. Details of these metals will be described in the section of metals that can be supported on the outer wall of the metal oxide support described later.

  The amount of Pt supported on the fired removal material (in other words, the amount supported on the metal oxide support described later) is preferably in the range of 0.05 to 10 g / L from the viewpoint of the oxidation efficiency of HC and CO. A range of 1 to 5 g / L is preferred.

  As a particle diameter of Pt and Pd, the range of 1-20 nm is preferable, and the range of 1-10 nm is more preferable.

  When supporting Pt and / or Pd, mix the fired removal material, which is the supported material, with dinitrodiammine platinum solution, platinum chloride solution, ammine platinum solution, etc. and / or palladium nitrate solution, palladium chloride solution, etc. Pt or the like can be supported on the surface of the fired removal material by reduction with ethanol or the like, and firing as necessary.

-Layer formation process-
The layer forming step in the present invention includes applying a metal oxide precursor-containing composition so as to cover the metal supporting part on which the first catalytic metal of the material that can be fired and removed (fired removing material) is supported. A precursor composition layer is formed thereon. This precursor composition layer forms a metal oxide carrier by firing in the oxide carrier forming step described later.

The metal oxide precursor-containing composition in the present invention contains at least a metal oxide precursor capable of generating a metal oxide by oxidative decomposition during firing. As the metal oxide precursor, a metal hydroxide formed from a metal alkoxide, nitrate, acetate or the like which forms a metal oxide is suitable.
The metal here is a metal that forms a carrier for supporting the first catalyst metal such as Pt, and a metal suitable for the noble metal to be supported is preferably used. Specifically, cerium, zirconium, silicon, aluminum, titanium and the like. As an example of the metal oxide precursor, a hydroxide made from a metal alkoxide, nitrate, acetate or the like, such as cerium hydroxide or zirconium hydroxide, is preferable.

  The precursor composition layer can be formed using a metal oxide precursor-containing composition using a sol-gel method, an impregnation method, a hydrothermal method, or the like. The sol-gel method is generally a method in which a metal alkoxide sol is made into a gel that loses fluidity by hydrolysis and polycondensation reaction, and the gel is heated to obtain an oxide.

-Oxide support forming step-
In the oxide carrier forming step in the present invention, the first catalyst is formed on the inner wall by firing the fireable and removable material (fired removal material) on which the precursor composition layer is formed, and removing the fired removal material. A metal oxide support having a hollow structure on which a metal is supported is formed. When the fired removal material is burned out in this step, a metal oxide support formed into a hollow structure according to the shape of the fired removal material is obtained.

  The firing can be arbitrarily selected without limitation as long as it can remove the firing removal material (material that can be removed by firing) used as a mold. Firing can be suitably performed, for example, at 200 to 400 ° C. or 200 to 300 ° C. for 0.5 to 10 hours.

The metal oxide support for supporting the catalyst metal is a support for catalyst support formed by oxidizing the metal oxide precursor contained in the metal oxide precursor-containing composition. For example, cerium oxide ( Oxidation of CeO ₂ ), zirconium dioxide (ZrO ₂ ), aluminum oxide (Al ₂ O ₃ ), titanium dioxide (TiO ₂ ), silica, silica-alumina, alumina-titania (Al ₂ O ₃ —TiO ₂ ), zeolite, etc. Product particles, and mixed particles thereof. Among these, a solid solution of cerium oxide and zirconium oxide is preferable because it is particularly suitable for supporting Pt. This solid solution adjusts the oxygen concentration of the flowing exhaust gas to promote catalytic activity, and interacts with Pt to suppress Pt grain growth.

  The metal oxide support formed in the oxide support forming step uses a metal oxide precursor-containing composition containing a metal oxide precursor containing cerium hydroxide and zirconium hydroxide, and a precursor on the fired removal material. It may be formed as a CZ carrier which is a solid solution of cerium oxide and zirconium oxide by forming and firing a composition layer.

  The wall thickness forming the hollow structure of the formed metal oxide support is preferably 1 to 100 nm, more preferably 3 to 30 nm.

  In the formed hollow structure of the metal oxide support, the maximum length of the cross section of the hollow structure perpendicular to the longitudinal direction is preferably 0.005 to 3 μm, and the more preferable maximum length is 0.15 to 2.0 μm. It is. When the maximum length of the cross section is 0.005 μm or more, gas flowability when used as a purification catalyst is good, and when it is 3 μm or less, pulverization is caused by stirring during the preparation of the washcoat slurry. It is easy to escape and is not shredded.

-Second metal loading process-
In the second metal supporting step in the present invention, the second catalytic metal is supported on the outer wall of the metal oxide support formed in the oxide support forming step. In this step, a noble metal different from the first catalyst metal supported on the inner wall is supported on the outer wall of the metal oxide support.

Examples of the second catalytic metal include platinum (Pt), palladium (Pd), rhodium (Rh), etc., as noble metals having HC, CO oxidation activity or NO _x reduction activity, among which Pt, Pd, and A noble metal selected from Rh is preferred. Since the second catalyst metal is a metal supported on the outer wall of the hollow structure of the metal oxide support, it is more preferable that the second catalyst metal is a noble metal that is difficult to grow grains or alloy with other metals in a high temperature environment. In this respect, Rh is more preferable as the second catalyst metal. Rh is excellent in reduction activity of grain growth was hardly NO _x, can be constituted by utilizing the properties possessed Rh originally.

  As a particle diameter of Rh, the range of 1-20 nm is preferable, and the range of 1-10 nm is more preferable. When the particle size is 1 nm or more, it is difficult to deactivate due to oxidation, and when it is 20 nm or less, most of the supported Rh can promote the reaction and the catalytic activity is improved.

In addition to supporting a noble metal, the outer wall of the metal oxide support may further support a metal other than the noble metal. As a metal other than the noble metal, an NO _x storage material such as Al, Fe or the like, an alkali metal, or an alkaline earth metal can be supported. Examples of the alkali metal include lithium, sodium, potassium, rubidium, cesium and the like. Examples of the alkaline earth metal include barium, magnesium, calcium, strontium and the like. Among these, potassium, rubidium, cesium, and barium are preferable, and barium is more preferable from the viewpoint of having high basicity and exhibiting high NO _x storage performance when an oxide is formed.

For example, Al ₂ O _3, which is an oxide of Al, has a large specific surface area and can enhance HC adsorption, and also suppresses HC adsorption poisoning of coexisting Rh. _the NO _x purification capability with can be effectively exhibited. Therefore, it is possible to enhance the NO _x purification ability under a rich steady atmosphere.

The amount of Rh supported on the metal oxide carrier is preferably in the range of 0.01 to 10 g / L, more preferably in the range of 0.03 to 5 g / L, from the viewpoint of NO _x reduction efficiency.

  The second catalyst metal can be supported by using an impregnation method, an adsorption method, or the like using a rhodium nitrate solution, a rhodium chloride solution, an ammine rhodium solution, or the like on the metal oxide support after firing.

In addition to supporting the second catalytic metal on the surface of the fired removal material, as a metal other than the noble metal, a promoter such as Fe, Li, K, Na, Mg, Ca, St, Ba, etc. An NO _x storage material such as an alkali metal or an alkaline earth metal may be supported.

  The exhaust gas purification catalyst of the present invention includes a metal oxide support having a hollow structure, a first catalyst metal disposed on the inner wall of the hollow structure of the metal oxide support, and a second catalyst disposed on the outer wall of the hollow structure. A catalyst metal is provided. The exhaust gas purifying catalyst of the present invention supports different catalytic metals on the inner wall and outer wall of the hollow metal oxide carrier, so that the catalyst metal can be used even when used in a high temperature environment for a long time. By avoiding grain growth and alloying, it is possible to suppress a decrease in catalyst activity after long-term use.

  The exhaust gas purifying catalyst of the present invention may be formed by any method as long as it is a method capable of supporting different catalytic metals on the inner wall and the outer wall of the metal oxide carrier, but preferably the first metal described above It is produced by a production method (a method for producing an exhaust gas purification catalyst of the present invention) having a supporting step, a layer forming step, an oxide carrier forming step, and a second metal supporting step.

  Details of the metal oxide support are as described above. The hollow structure of the metal oxide support has a linear or curved shape, and an inner diameter (average inner diameter) of 30 nm or less is preferable, and a more preferable inner diameter is 10 nm or less. When the inner diameter is in the above range, a wide surface area capable of supporting the catalyst metal can be secured, and the catalyst metal can be supported at a high density.

As the first and second catalyst metals supported on the metal oxide support, platinum (Pt), palladium (Pd), rhodium (Rh), osmium (Os), iridium (Ir), gold (Au) are used as noble metals. ) And the like. In addition, as a metal other than the noble metal, a promoter such as Al or Fe, or an NO _x storage material such as an alkali metal or alkaline earth metal can be supported. Especially, it is preferable that the first catalyst metal and the second catalyst metal are each configured to include different catalyst metals selected from platinum, palladium, and rhodium, and in particular, the first catalyst on the inner wall. A form containing Pt and / or Pd as the metal and Rh as the second catalytic metal of the outer wall is preferable.

  From the viewpoint of catalyst performance and resource saving (cost), the amount of the first catalyst metal supported on the inner wall of the metal oxide support is preferably in the range of 0.05 to 10 g / L. The amount of the second catalytic metal supported on the outer wall is preferably in the range of 0.01 to 10 g / L. Furthermore, the amount of the first catalyst metal supported on the inner wall is 0.1 to 3 g / L, and the amount of the second catalyst metal supported on the outer wall is 0.03 to 0.5 g / L. More preferred.

  EXAMPLES Hereinafter, the present invention will be described more specifically with reference to examples. However, the present invention is not limited to the following examples unless it exceeds the gist thereof. Unless otherwise specified, “part” is based on mass.

Example 1
Linear single-walled carbon nanotubes (mainly 1 μm in length, outer diameter φ1.5 nm; hereinafter referred to as “single-walled CNT”) are prepared, and this is refluxed with nitric acid at 140 ° C. for one day, thereby improving hydrophilicity. A pretreatment for giving was performed.

  50 mg of single-walled CNT that had been pretreated was impregnated in a dinitrodiamine platinum nitric acid solution (corresponding to Pt = 0.33 g), reduced with ethanol, and dried to obtain a Pt / single-walled CNT catalyst powder.

  Next, the obtained Pt / single-walled CNT catalyst powder was dispersed in a mixed solution of 4.4 g of cerium ethoxide and 3.7 g of zirconium ethoxide and stirred for about 20 minutes. Was filtered with suction. This sample was bubbled by supplying nitrogen gas into a water bath heated to about 60 ° C., heated at about 70 ° C. for about 30 minutes under a nitrogen gas flow, and then dried at 100 ° C. for 1 hour. Dried. This sample was again dispersed in a solution obtained by mixing 4.4 g of cerium ethoxide and 3.7 g of an ethanol solution of zirconium ethoxide, and further stirred, filtered and dried, and this repeated operation was performed four times.

The sample thus obtained was fired at 350 ° C. for 1 hour and then at 500 ° C. for 2 hours, and Pt / CeO ₂ —ZrO ₂ (= 1: 1) nanotubes (hereinafter referred to as “Pt / CeO ₂ —ZrO _2” ) supported on the inner wall. (Referred to as Pt / CZ nanotubes). Hereinafter, the CeO ₂ —ZrO ₂ nanotube is referred to as “CZ nanotube”. As a result of chemical analysis of the obtained Pt / CZ nanotube, it was confirmed that 2% by mass of Pt was supported on the Pt / CZ nanotube.
The Pt / CZ nanotube mainly had a shape with an inner diameter of about 2 nm and a length of 500 nm.
The Pt / CZ nanotube catalyst powder 5 g and a commercially available Al ₂ O ₃ carrier 5 g were mixed well with a mixer and then pressed to obtain a pellet catalyst of about several mm.

(Example 2)
A catalyst powder of Pt / CZ nanotubes carrying 2% by mass of Pt was prepared in the same manner as in Example 1, and 5 g of this Pt / CZ nanotube catalyst powder was added to a rhodium nitrate solution (corresponding to Rh = 0.015 g). And dried at 500 ° C. for 2 hours, and 0.3 g of Rh per CZ nanotube was supported on the outer wall of the Pt / CZ nanotube.
Thus, a Pt / CeO ₂ —ZrO ₂ / Rh nanotube (hereinafter referred to as a Pt / CZ / Rh nanotube) having Rh supported on the wall was obtained.
5 g of the obtained Pt / CZ / Rh nanotube catalyst powder and 5 g of a commercially available Al ₂ O ₃ carrier were mixed well with a mixer and then pressed to obtain a pellet catalyst of about several mm.

(Comparative Example 1)
5 g of commercially available CZ powder (CeO ₂ —ZrO ₂ powder) was mixed with a dinitrodiammine platinum solution (corresponding to Pt = 0.1 g), dried, and then calcined at 500 ° C. for 2 hours to obtain 2% by mass. A Pt / CZ catalyst powder carrying Pt was prepared. 5 g of this Pt / CZ catalyst powder and 5 g of a commercially available Al ₂ O ₃ carrier were mixed well with a mixer, and then pressed in the same manner as in Example 1 to obtain a pellet catalyst.

(Comparative Example 2)
After 5 g of commercially available CZ powder (CeO ₂ —ZrO ₂ powder) is mixed with a mixed solution of dinitrodiammine platinum solution (corresponding to Pt = 0.1 g) and rhodium nitrate solution (corresponding to Rh = 0.015 g) and dried. The Pt—Rh / CZ catalyst powder was produced by firing at 500 ° C. for 2 hours. The Pt / CZ catalyst powder supported 2% by mass of Pt and 0.3% of Rh.
5 g of the obtained Pt / CZ catalyst powder and 5 g of a commercially available Al ₂ O ₃ carrier were mixed well with a mixer and then pressed in the same manner as in Example 1 to obtain a pellet catalyst.

(Evaluation)
The pellet catalysts prepared in the above examples and comparative examples were sealed, and an endurance test was conducted in which a lean atmosphere gas and a rich atmosphere gas simulating exhaust gas were flown at a catalyst bed temperature of 1000 ° C. for 5 hours as shown in Table 1 below. . With respect to this durability test product, the catalyst performance was evaluated using a model gas having the composition shown in Table 2 below. The evaluation results show the HC-50% purification temperature of Example 1 and Comparative Example 1 in FIG. 1, and the NO _x -50% purification temperature of Example 2 and Comparative Example 2 in FIG.

As shown in FIG. 1, Example 1 showed superior HC purification performance compared to Comparative Example 1. This is presumably because the Pt grain growth was suppressed. In addition, as shown in FIG. 2, Example 2 showed superior NO _x purification performance compared to Comparative Example 2. This is presumably because the solid solution of Pt—Rh was suppressed.

  In the above embodiment, the case where Pt is used as the first catalyst metal in the present invention and Al and Rh are used as the second catalyst metal has been described as an example. Even when used, the same effects as in the above embodiment can be obtained.

Claims (9)

A first metal loading step of loading a first catalytic metal on the surface of a rod-like or fibrous fire-removable material;
A layer forming step of applying a metal oxide precursor-containing composition over the metal supporting portion on which the first catalyst metal of the material is supported, and forming a precursor composition layer on the material;
An oxide that forms a metal oxide support having a hollow structure in which the first catalytic metal is supported on an inner wall by firing the material on which the precursor composition layer is formed and removing the material A carrier forming step;
A second metal supporting step of supporting a second catalytic metal on the outer wall of the metal oxide support;
A method for producing an exhaust gas purifying catalyst.   2. The method for producing an exhaust gas purification catalyst according to claim 1, wherein the first catalyst metal and the second catalyst metal include different noble metals selected from platinum, palladium, and rhodium.   The method for producing an exhaust gas purification catalyst according to claim 1 or 2, wherein at least one of the first catalyst metals is at least one of platinum and palladium, and at least one of the second catalyst metals is rhodium.   The method for producing an exhaust gas purification catalyst according to any one of claims 1 to 3, wherein the material that can be removed by firing is carbon fiber.   The method for producing an exhaust gas purifying catalyst according to claim 4, wherein the carbon fiber is at least one selected from carbon nanotubes, carbon nanohorns, and carbon nanofibers.   The said layer formation process is a manufacturing method of the exhaust gas purification catalyst of any one of Claims 1-5 which forms a precursor composition layer by a sol-gel method or a hydrothermal method.   An exhaust gas purification catalyst comprising a metal oxide support having a hollow structure, a first catalyst metal disposed on an inner wall of the hollow structure of the metal oxide support, and a second catalyst metal disposed on an outer wall.   The exhaust gas purification catalyst according to claim 7, wherein the first catalyst metal and the second catalyst metal include different noble metals selected from platinum, palladium, and rhodium.   The exhaust gas purifying catalyst according to claim 7 or 8, wherein at least one of the first catalytic metals is at least one of platinum and palladium, and at least one of the second catalytic metals is rhodium.