JP2010058006A - Catalyst for cleaning exhaust gas and method for manufacturing the same - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To secure high oxygen absorption/discharge function, using a ceria-based oxide as a carrier and inhibit the deterioration of rhodium (Rh) due to the dissolution of Rh into the carrier. <P>SOLUTION: A support part 2, made of a basic oxide with higher basicity than ceria, is formed on the surface of a ceria-based oxide particle 1, and Rh 3 is supported by at least the support part 2. As an electronically strong bond is formed between the basic oxide and Rh, dissolution of Rh into the interior of the ceria-based oxide even under the action of heat load is inhibited, and consequently, Rh in only confined to the surface of the particle, so that the deterioration of the activity does not occur. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to an exhaust gas purifying catalyst for purifying automobile exhaust gas such as a three-way catalyst, and a method for producing the same.

Three-way catalysts are widely used as exhaust gas purifying catalysts for purifying exhaust gas from automobiles. This three-way catalyst uses a porous oxide such as alumina as a carrier, and supports a catalytic noble metal such as Pt and Rh. Then, by use in combustion controlled in the exhaust gas so as to near stoichiometric, to reduce and purify NO _x as well as oxidizing purify HC and CO.

  However, depending on the driving conditions of the automobile, the exhaust gas atmosphere may deviate from the vicinity of the stoichiometric. For this reason, an oxide having an ability to absorb and release oxygen such as ceria is mixed with the carrier. An oxygen storage / release material such as ceria absorbs oxygen when leaning and releases oxygen when rich, so that fluctuations in the atmosphere of exhaust gas can be mitigated.

  Further, in recent years, ceria-zirconia composite oxide is often used instead of ceria. Ceria-zirconia composite oxide has high thermal stability and is superior in ability to absorb and release oxygen than ceria, and therefore has a high ability to alleviate atmospheric fluctuations.

The Pt has high oxidation activity of HC and CO, Rh is higher the reducing activity of NO _x, often both Pt and Rh are used. However, when carrying Rh on ceria, Rh is dissolved to a carrier at a high temperature durability, there is a problem that reducing activity of the NO _x is greatly reduced. If the ceria-zirconia composite oxide is used as a support, the problem of solid solution can be slightly improved.

  For example, in JP-A-2003-073123, a composite oxide of ceria, zirconia, and a metal oxide that does not react with ceria and zirconia, and having a pyrochlore phase in which Ce and Zr are regularly arranged, An exhaust gas purifying co-catalyst carrying a noble metal is disclosed. In this composite oxide, since the metal oxide that does not react with ceria and zirconia is interposed between the ceria-zirconia composite oxide, it becomes a barrier to each other, so that grain growth is suppressed and the surface area is high. Since this composite oxide has a pyrochlore phase in which Ce and Zr are regularly arranged, a high oxygen absorption / release capability is exhibited. Therefore, the co-catalyst for exhaust gas purification, in which a noble metal is supported on this composite oxide, has both a high specific surface area and a high oxygen absorption / release capability, and exhibits a high practical purification activity.

  However, the degree of suppression of solid solution of Rh depends on the amount of zirconia in the ceria-zirconia composite oxide. If the amount of zirconia is small, the problem of solid solution increases. On the other hand, when the amount of zirconia is large, the amount of ceria decreases, so that the ability to absorb and release oxygen becomes insufficient, and the atmosphere relaxation becomes difficult and the ternary activity is lowered.

Japanese Patent Application Laid-Open No. 2007-301526 describes an exhaust gas purifying catalyst capable of suppressing deterioration due to Rh solid solution or grain growth. However, there is no description about the relationship between the degree of solid solution of Rh in the carrier and the oxygen absorption / release capacity of the carrier.
Japanese Patent Laid-Open No. 2003-073123 JP 2007-301526 A

  The present invention has been made in view of the above circumstances, and it should be solved that a ceria-based oxide is used as a carrier to ensure high oxygen absorption / release capability and to greatly suppress deterioration due to solid solution of Rh into the carrier. Let it be an issue.

  The exhaust gas purifying catalyst of the present invention that solves the above problems is characterized in that particles comprising a ceria-based oxide containing at least ceria and a support formed on the surface of the particle comprising a basic oxide having a higher basicity than ceria. And rhodium supported on at least the supporting part.

  The supporting part preferably has a layer shape with a thickness of several nm to 1 μm and covers the surface of the particles, and the thickness of the supporting part is more preferably several nm to 100 nm.

  The ceria-based oxide is preferably a ceria-zirconia composite oxide containing 50% by mass or more of ceria, and the basic oxide is preferably at least one selected from alkaline earth metal oxides and spinel.

  In addition, a feature of the method for producing an exhaust gas purifying catalyst of the present invention is that a mixed aqueous solution of a water-soluble salt of an alkaline earth metal and a water-soluble binder is brought into contact with a powder comprising particles of a ceria-based oxide containing at least ceria. A step of drying to form a film on the particle surface, a step of baking the film to form a basic oxide layer on the particle surface, and a step of supporting rhodium on the basic oxide layer. is there.

  According to the exhaust gas purifying catalyst of the present invention, Rh is supported at least on the supporting portion made of a basic oxide. Since this basic oxide has a higher basicity than ceria, an electronically strong bond is formed between the basic oxide and Rh. For this reason, even if a heat load is applied, Rh is prevented from dissolving in the interior of the ceria-based oxide, and Rh remains on the particle surface, so that a decrease in activity can be suppressed.

  If the supporting part is a layer having a thickness of several nanometers to 1 μm, direct contact between ceria and Rh can be avoided even if the ceria concentration in the ceria-based oxide is high, so that the solid solution of Rh can be further suppressed. it can. Also, if the thickness of the supporting part is as thin as several nanometers to 100 nm, Rh is prevented from being buried in the supporting part and at least a part of the surface is exposed, so the contact probability between Rh and exhaust gas is increased and the purification performance is improved. improves.

  According to the method for producing an exhaust gas purifying catalyst of the present invention, the exhaust gas purifying catalyst of the present invention having a layered support can be stably and easily produced.

  The exhaust gas purifying catalyst of the present invention comprises particles comprising a ceria-based oxide containing at least ceria, a supporting part formed of a basic oxide having a higher basicity than ceria and formed on the surface of the particle, and at least a supporting part. And supported rhodium.

  The ceria-based oxide containing at least ceria may be composed only of ceria, but it is preferable to use a ceria-zirconia composite oxide. In this case, the constituent ratio of ceria and zirconia in the ceria-zirconia composite oxide is not particularly limited, but it is desirable to contain 50% by mass or more of ceria in order to ensure sufficient oxygen absorption / release capability. In addition, when using ceria-zirconia complex oxide, you may contain the other metal element which comprises complex oxide with Ce and Zr. Examples of other elements include La, Pr, Nd, Y, and Al.

As the basic oxide having a higher basicity than ceria, an alkaline earth metal oxide or spinel can be used. As the alkaline earth metal oxide, magnesium oxide and calcium oxide having high basicity are particularly preferable. As the spinel, MgAl ₂ O ₄ and CaAl ₂ O ₄ are particularly preferable.

  The support portion may be present in the form of particles on the surface of the ceria-based oxide particles, but in order to avoid direct contact between ceria and Rh, the particles are formed in layers of several nm to 1 μm in thickness. It is desirable to coat the surface. If this thickness exceeds 1 μm, Rh is buried in the carrier and the contact probability with the exhaust gas decreases, so that the purification activity decreases.

  The supporting part can be formed, for example, by mixing a ceria-based oxide powder in an aqueous solution in which a water-soluble salt of a metal constituting a basic oxide is dissolved, and baking after evaporating to dryness. In this case, the basic oxide becomes a particulate support, but a layered support can be formed by adjusting the concentration of the water-soluble salt of the metal constituting the basic oxide in the aqueous solution.

  In addition, in order to form the supporting portion by the production method of the present invention, a mixed aqueous solution of a water-soluble salt of an alkaline earth metal and a water-soluble binder is brought into contact with a powder composed of ceria-based particles containing at least ceria. Dry to form a film on the particle surface. This film uses a water-soluble binder as a matrix and holds alkaline earth metal ions in a highly dispersed state. By firing this film, the water-soluble binder is burned out, and a thin layered support portion made of an alkaline earth metal oxide is formed on the surface of the particles made of ceria-based oxide.

  As the water-soluble binder, water-soluble organic polymers such as polyvinyl alcohol, ammonium polyacrylate, starch, and casein can be used.

  The thickness of the supporting portion can be adjusted by a method of adjusting the alkaline earth metal concentration in the mixed aqueous solution or a method of adjusting the mixing ratio of the mixed aqueous solution and ceria-based oxide powder. Moreover, the process of baking a film | membrane can be performed at 500-800 degreeC in air | atmosphere. If the calcination temperature is less than 300 ° C, formation of the supporting part becomes difficult, and if it exceeds 1000 ° C, decomposition starts, which is not preferable.

  In order to carry Rh at least on the carrying part, the impregnation carrying method or the adsorption carrying method can be used as in the conventional case. If the supporting part is in the form of particles, Rh is also supported on the surface of the exposed ceria-based oxide particles, but even if a thermal load is applied, at least Rh supported on the supporting part is ceria-based oxidized. Solid solution in the product is suppressed. However, if the supporting part is layered, solid solution is suppressed in almost all Rh.

  The supported amount of Rh is preferably in the range of 0.01 to 1.0% by mass in the total mass of the particles made of ceria-based oxide, the supported part, and the supported Rh. If the loading amount of Rh is less than 0.01% by mass, the effect of loading Rh cannot be obtained. Even if the loading amount exceeds 1.0% by mass, the effect is saturated and grain growth occurs during heat load.

  The exhaust gas purifying catalyst of the present invention is a catalyst particle comprising particles made of a ceria-based oxide, a supporting part formed on the surface of the particle, and at least Rh supported on the supporting part. Therefore, in actual use, the catalyst particles are supplied in the form of countless aggregated powders, which are used by being coated on a honeycomb-shaped or foam-shaped substrate, or used after being pelletized.

For example, in order to use as a three-way catalyst, the catalyst powder according to the present invention is mixed with catalyst powder in which Pt is supported on ceria powder or alumina powder, and the mixed powder is washed on a honeycomb substrate. . Alternatively, a lower coat layer is formed on a honeycomb base material from a catalyst powder in which Pt is supported on ceria powder, alumina powder or the like, and the upper coat layer is formed on the surface of the lower coat layer from the catalyst powder according to the present invention. A three-way catalyst having a two-layer structure in which can be formed. Further, the catalyst is not limited to a three-way catalyst, but can be used as an oxidation catalyst, a lean NO _x catalyst, a NO _x storage reduction catalyst, or the like.

  Hereinafter, the present invention will be specifically described with reference to Examples and Comparative Examples.

Example 1
FIG. 1 shows an exhaust gas purifying catalyst according to this example. This catalyst is a powder which is an aggregate of catalyst particles composed of particles 1 made of ceria-zirconia composite oxide, a magnesia layer 2 formed in layers on the surface of the particles 1, and Rh3 supported on the magnesia layer 2. It is composed of The average particle diameter of the particles 1 measured by cross-sectional observation with an electron microscope is 10 μm, and the thickness of the magnesia layer 2 is 1 nm.

  Hereinafter, a method for producing the catalyst powder will be described, and a detailed description of the configuration will be given.

0.03 wt% magnesium chloride, polyvinyl alcohol - le is an aqueous solution 100 parts by weight was dissolved at a concentration of 0.03 wt%, ceria - zirconia composite oxide powder (average particle size 10 [mu] m, the weight ratio _{CeO 2 / ZrO 2 = 6/} 1 ) 100 parts by mass were mixed, stirred for 30 minutes and then evaporated to dryness at 120 ° C. As a result, a film having a thickness of about 10 nm was formed on the surface of the ceria-zirconia composite oxide particles.

  Next, the ceria-zirconia composite oxide powder on which the film was formed was fired at 800 ° C. for 2 hours in the air. Thereby, the magnesium chloride in the film was converted to magnesium oxide, and the magnesia layer 2 was formed on the surface of the ceria-zirconia composite oxide particles 1. The thickness of the magnesia layer 2 measured by electron microscope observation was 1 nm. Magnesium oxide is supported by 0.01 part by mass with respect to 100 parts by mass of the ceria-zirconia composite oxide.

  Subsequently, a powder composed of aggregates of ceria-zirconia composite oxide particles 1 having a magnesia layer 2 is impregnated with a predetermined amount of a rhodium nitrate aqueous solution having a predetermined concentration, evaporated to dryness, and then at 500 ° C. for 1 hour in the atmosphere. Heated to load Rh. In the obtained catalyst powder, 0.1% by mass of Rh is supported. This catalyst powder was formed into pellets by a conventional method to prepare a pellet catalyst.

(Example 2)
A magnesia layer 2 was formed in the same manner as in Example 1 except that an aqueous solution in which magnesium chloride was dissolved at a concentration of 0.3% by mass and polyvinyl alcohol at a concentration of 0.3% by mass was used. The thickness of the magnesia layer 2 measured in the same manner as in Example 1 was 10 nm. Magnesium oxide is supported by 0.1 part by mass with respect to 100 parts by mass of the ceria-zirconia composite oxide.

  Using this powder, Rh was supported in the same manner as in Example 1, and then a pellet catalyst was prepared.

(Example 3)
A magnesia layer 2 was formed in the same manner as in Example 1 except that an aqueous solution in which magnesium chloride was dissolved at a concentration of 3% by mass and polyvinyl alcohol at a concentration of 3% by mass was used. The thickness of the magnesia layer 2 measured in the same manner as in Example 1 was 100 nm. Further, 1 part by mass of magnesium oxide is supported with respect to 100 parts by mass of the ceria-zirconia composite oxide.

  Using this powder, after carrying Rh in the same manner as in Example 1, a pellet catalyst was prepared.

Example 4
A magnesia layer 2 was formed in the same manner as in Example 1 except that an aqueous solution in which magnesium chloride was dissolved at a concentration of 30% by mass and polyvinyl alcohol at a concentration of 10% by mass was used. The thickness of the magnesia layer 2 measured in the same manner as in Example 1 was 1000 nm. Further, 10 parts by mass of magnesium oxide is supported with respect to 100 parts by mass of the ceria-zirconia composite oxide.

  Using this powder, after carrying Rh in the same manner as in Example 1, a pellet catalyst was prepared.

(Example 5)
After mixing 100 parts by mass of the same ceria-zirconia composite oxide powder as in Example 1 with 100 parts by mass of an aqueous solution not containing polyvinyl alcohol and dissolving magnesium chloride at a concentration of 0.3% by mass, and stirring for 30 minutes And calcined at 800 ° C. for 2 hours in the air. Thereby, the magnesia layer 2 was formed on the surface of the ceria-zirconia composite oxide particles 1. The thickness of the magnesia layer 2 measured by electron microscope observation was 10 nm. Further, 1 part by mass of magnesium oxide is supported with respect to 100 parts by mass of the ceria-zirconia composite oxide.

  Using this powder, after carrying Rh in the same manner as in Example 1, a pellet catalyst was prepared.

(Comparative Example 1)
A ceria-zirconia composite oxide powder similar to that in Example 1 was impregnated with a predetermined amount of a rhodium nitrate aqueous solution having a predetermined concentration, evaporated to dryness, and heated in the atmosphere at 500 ° C. for 1 hour to carry Rh. In the obtained catalyst powder, 0.1% by mass of Rh is supported. This catalyst powder was formed into pellets having a diameter of 1 mm by a conventional method to prepare a pellet catalyst.

<Test and evaluation>
Each of the pellet catalysts of Examples and Comparative Example 1 was subjected to an endurance test that was held at 1100 ° C. for 5 hours in the air. Each pellet catalyst after the endurance test was loaded in 1.5 g in the evaluation device, and the temperature was raised from 100 ° C. to 500 ° C. over 20 minutes while flowing the model gas shown in Table 1 at a flow rate of 10 L / min. The purification rates of ₃ H ₆ , CO and NO _x were measured respectively. Each 50% purification temperature was calculated, and the results are shown in FIG.

  From FIG. 2, it can be seen that the catalyst of each example is superior in purification performance after endurance as compared with the catalyst of Comparative Example 1, and it is clear that this is the effect of forming the magnesia layer 2. That is, it is clear that the formation of the magnesia layer 2 suppressed the deterioration of Rh, which is considered to be because the dissolution of Rh in the ceria-zirconia composite oxide particles 1 was suppressed.

  Moreover, it is found from the comparison between the examples that the thickness of the magnesia layer 2 is preferably in the range of 100 nm or less. From the comparison between the examples 1 and 5, polyvinyl alcohol is used when forming the magnesia layer 2. It can also be seen that this is preferable.

  That is, as shown in FIG. 3, Rh3 is supported on the surface of the magnesia layer 2 in the initial state. After the endurance test, even if Rh3 is solid-dissolved in the magnesia layer 2, since magnesia has a higher basicity than ceria, Rh3 does not dissolve in the ceria-zirconia composite oxide particles 1. If the magnesia layer 2 is as thin as 100 nm or less, at least a part of the surface of Rh3 is exposed from the magnesia layer 2 and the contact probability with the exhaust gas is increased. Therefore, high purification performance is exhibited.

  The exhaust gas purifying catalyst of the present invention can be used for various exhaust gas purifying catalysts for automobiles.

It is typical explanatory drawing of the catalyst for exhaust gas purification which concerns on one Example of this invention. It is a graph which shows 50% purification temperature. In the catalyst of this invention, it is explanatory drawing which shows the structural change after an initial stage and an endurance test.

Explanation of symbols

1: Ceria-zirconia composite oxide particles 2: Magnesia layer (supporting part)
3: Rh

Claims (7)

  A particle composed of a ceria-based oxide containing at least ceria; a supporting portion formed on the surface of the particle composed of a basic oxide having a higher basicity than ceria; and at least rhodium supported on the supporting portion. An exhaust gas purifying catalyst comprising:   2. The exhaust gas purifying catalyst according to claim 1, wherein the supporting portion has a layer shape with a thickness of several nm to 1 μm and covers the surface of the particles.   The exhaust gas purifying catalyst according to claim 2, wherein the thickness of the supporting part is several nm to 100 nm.   The exhaust gas purifying catalyst according to any one of claims 1 to 3, wherein the ceria-based oxide is a ceria-zirconia composite oxide containing 50 mass% or more of ceria.   The exhaust gas-purifying catalyst according to any one of claims 1 to 4, wherein the basic oxide is at least one selected from alkaline earth metal oxides and spinel. Contacting a mixed aqueous solution of a water-soluble salt of an alkaline earth metal and a water-soluble binder with a powder composed of ceria-based oxide containing at least ceria, and drying to form a film on the particle surface;
Firing the coating to form a basic oxide layer on the particle surface;
A method for producing an exhaust gas purifying catalyst, comprising the step of supporting rhodium on the basic oxide layer.   The method for producing an exhaust gas purifying catalyst according to claim 6, wherein the ceria-based oxide is a ceria-zirconia composite oxide containing 50 mass% or more of ceria.

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