JP2007167707A - Exhaust gas purification catalyst and production method thereof - Google Patents

Abstract

A reduction in purification activity is suppressed by further suppressing grain growth or alloying of catalytic metal.
SOLUTION: A first catalyst powder 1 carrying a first catalyst metal 12 between layers of a first oxide 10 having a multilayer structure with a gap 11 between layers, and a second catalyst metal on a porous second oxide 20. And a second catalyst powder 2 supporting 21 with a particle size larger than the average interlayer distance of the first oxide 10.
Since it becomes difficult for the second catalyst metal 21 to enter the gap 11 between the layers of the first oxide 10, it is highly possible that the first catalyst metal 12 and the second catalyst metal 21 grow or alloy. Can be suppressed.
[Selection] Figure 1

Description

  The present invention relates to an exhaust gas purifying catalyst for purifying exhaust gas discharged from an automobile engine and the like, and a method for manufacturing the same.

Conventionally, a three-way catalyst that purifies exhaust gas by simultaneously performing oxidation of CO and HC and reduction of NO _x has been used as an exhaust gas purification catalyst for automobiles. As such a three-way catalyst, a porous carrier layer made of γ-alumina is formed on a heat-resistant substrate made of cordierite or the like, and platinum (Pt), rhodium (Rh) or the like is formed on the porous carrier layer. Those carrying precious metals are widely known.

  By the way, in recent years, the exhaust gas purification catalyst tends to be located directly under the manifold close to the engine, and the exhaust gas temperature becomes higher during high-speed driving, so the exhaust gas purification catalyst is often exposed to high temperatures. . However, the conventional exhaust gas purifying catalyst has a problem that the catalytic performance is deteriorated because the sintering of γ-alumina proceeds with high-temperature exhaust gas, and the catalytic active point decreases due to the accompanying noble metal grain growth.

In recent years, lean burn engines that supply an oxygen-rich mixture have become the mainstream in order to suppress carbon dioxide emissions. However, noble metal grain growth is particularly noticeable when high temperatures of 800 ° C or higher are applied in a lean atmosphere with excess oxygen. For example, Pt supported on an alumina surface becomes PtO _{2 in} an atmosphere where oxygen coexists at a high temperature, and diffusion and aggregation are promoted by gas phase movement. Therefore, in an oxygen-excess lean atmosphere or stoichiometric atmosphere, when exposed to a high temperature, grain growth occurs in Pt, and the catalytic performance is greatly reduced due to a reduction in surface area.

  Therefore, the applicant of the present application has proposed in JP-A-2004-141864 a catalyst for purifying exhaust gas in which a noble metal is supported between layers of an oxide having a multilayer structure formed by firing hydroxide and having voids between the layers. . According to the exhaust gas-purifying catalyst, the noble metal supported between the layers hardly moves, so that the grain growth is suppressed, and thus the heat resistance and durability are excellent.

In addition the three-way catalyst, and oxidation activity of HC and CO is high Pt, reduction activity of the NO _x is being performed to be used in combination with high Rh, a. However, for example, in a catalyst in which Pt and Rh are supported on alumina, there is a problem that Pt and Rh are alloyed in a high temperature atmosphere, and their respective activities are reduced. Therefore, it is conceivable to apply Pt and Rh in the gaps between the layers by applying the technique described in JP-A-2004-141864. However, even though it is difficult to move, there is a high probability that Pt and Rh are in contact with each other in the gap, and it is difficult to highly suppress alloying.

  Furthermore, Rh supported on alumina has a problem that the activity is low although grain growth is suppressed. Therefore, there is a proposal of mixing a catalyst powder having Pt supported on alumina and a catalyst powder having Rh supported on zirconia. There is also a proposal for a catalyst layer having a two-layer structure in which Pt and Rh are supported on each layer. Thus, Pt and Rh can be separated and supported, thereby preventing mutual alloying.

  For example, Japanese Patent Laid-Open No. 2005-246216 discloses a first catalyst powder in which Pt is supported on a first porous oxide, a second catalyst powder in which Rh is supported on a second porous oxide, and a particle size smaller than these. An exhaust gas-purifying catalyst obtained by mixing three porous oxide powders has been proposed. According to this catalyst, since the third porous oxide particles are likely to be interposed between the first catalyst particles and the second catalyst particles, and the third porous oxide particles serve as a barrier, the first catalyst particles and the second catalyst particles It is difficult to contact with catalyst particles. Therefore, the proximity of Pt and Rh is suppressed, and the separation support effect is maximized, so that alloying can be greatly suppressed.

However, even with the catalyst described in JP-A-2005-246216, some degree of alloying is unavoidable between Pt and Rh, and therefore, further reduction in purification activity is required.
JP2004-141864 JP2005-246216

  This invention is made | formed in view of the said situation, and makes it the problem which should be solved to further suppress the grain growth and alloying of a catalyst metal.

A feature of the exhaust gas purifying catalyst of the present invention that solves the above problem is that a first catalyst powder supporting a first catalyst metal between layers of a first oxide having a multilayer structure with a gap between layers,
And a second catalyst powder in which the second catalyst metal is supported on the porous second oxide with a particle size larger than the average interlayer distance of the first oxide.

  Preferably, the first catalyst metal is Pt and the second catalyst metal is Rh.

  The exhaust gas purifying catalyst production method of the present invention that can produce the exhaust gas purifying catalyst is characterized in that the second catalyst metal is grown by heating the second catalyst powder, and then the second catalyst powder and the first catalyst are grown. It is to mix the powder.

  In the exhaust gas purifying catalyst of the present invention, the first catalyst powder and the second catalyst powder are mixed. However, in the second catalyst powder, since the particle size of the supported second catalyst metal is larger than the average interlayer distance of the first oxide, the second catalyst metal enters the voids between the layers of the first oxide. Will be difficult. Therefore, when the first catalyst metal and the second catalyst metal are of the same type, the grain growth can be suppressed. Further, when the first catalyst metal and the second catalyst metal are different, the alloying can be suppressed, and the decrease in the activity of each of the first catalyst metal and the second catalyst metal can be highly suppressed. it can.

  And according to the manufacturing method of the exhaust gas purifying catalyst of the present invention, the exhaust gas purifying catalyst of the present invention can be manufactured easily and stably.

  The exhaust gas purifying catalyst of the present invention is a mixture of a first catalyst powder and a second catalyst powder.

  The first catalyst powder is one in which a first catalyst metal is supported between layers of a first oxide having a multilayer structure having voids between layers. The first oxide constituting the carrier of the first catalyst powder is an oxide having a multilayer structure formed by firing a hydroxide and having voids between layers, and zirconia, titania, ceria, etc. can also be used. However, α-alumina is a particularly preferred material. That is, α-alumina is thermally very stable, stable even at a high temperature of 1200 ° C., and does not sinter.

  The α-alumina having a multilayer structure with voids between layers can be produced, for example, by drying aluminum hydroxide crystal particles and then firing at a high temperature of about 1200 ° C. At the time of firing, it is considered that the aluminum hydroxide adhered in a layered manner in the crystal shrinks, thereby generating voids between the layers. As aluminum hydroxide, those commercially available from Sumitomo Chemical Co., Ltd. or the like can be used. It has become clear that it is difficult to produce α-alumina having a multilayer structure at a firing temperature of 1000 ° C. or lower.

  The average interlayer distance in the first oxide is preferably 5 nm to 10 nm. If the average interlayer distance is larger than this, the first catalyst metal is likely to move and there is a risk of grain growth in a high temperature atmosphere. When the average interlayer distance is smaller than this, it is difficult to support the first catalyst metal between the layers. The interlayer distance can be adjusted by adjusting the firing temperature of aluminum hydroxide.

  As the first catalyst metal supported between the layers of the first oxide, those used in conventional exhaust gas purification catalysts such as Pt, Rh, Pd, Ir, and Ru can be used. This is particularly effective in the case of Pt which has high catalytic activity but easily grows. The amount of the first catalyst metal supported is 0.1% by weight or more, preferably 0.5 to 20% by weight, based on the first oxide. If the loading amount is less than this range, the activity as an exhaust gas purifying catalyst is too low to be practical, and if the loading amount exceeds this range, the activity is saturated and the cost increases.

  In order to support the first catalyst metal between the layers of the first oxide, the metal catalyst can be supported by impregnating the void using a metal compound chemical solution and then evaporating to dryness. In this case, it is desirable to use a metal compound chemical that is difficult to adsorb to the first oxide. When a chemical solution that is easily adsorbed is used, there is a problem in that the amount of the first catalyst metal supported on the layers other than the interlayer increases and the grains grow at high temperatures.

  When evaporating to dryness, it is desirable to apply external stress such as stirring until the solvent is completely evaporated. In particular, it is desirable to continue to apply external shear stress such as stirring until the solvent has completely evaporated. If stirring or the like is stopped in a state where the solvent remains, the metal compound chemical solution and the first oxide are separated, and it becomes difficult to sufficiently support the first catalyst metal between the layers. However, if external shear stress such as stirring is continuously applied until the solvent is completely evaporated, separation of the metal compound chemical and the first oxide can be avoided, and the first catalyst metal can be uniformly and sufficiently supported between the layers. .

  The second catalyst powder is formed by supporting the second catalyst metal on the porous second oxide with a particle size larger than the interlayer distance of the first oxide. As the second oxide, alumina, zirconia, titania, ceria, or a single product or a mixture such as a composite oxide composed of a plurality of types selected from these can be used. Although different from the first oxide is desirable, it may be the same type as the first oxide.

  The second catalyst metal may be the same as the first catalyst metal, or may be different from the first catalyst metal, and selected from Pt, Rh, Pd, Ir, Ru, etc. Can do. Since Pt tends to grow in an oxidizing atmosphere, it is preferable to select Pt as the first catalyst metal supported between the layers of the first oxide. Rh is an essential catalyst metal for the three-way catalyst, but is easily alloyed with Pt. Therefore, when Pt is adopted as the first catalyst metal, it is desirable to adopt Rh as the second catalyst metal.

  The amount of the second catalytic metal supported is 0.1% by weight or more with respect to the second oxide, preferably 0.5 to 20% by weight. If the loading amount is less than this range, the activity as an exhaust gas purifying catalyst is too low to be practical, and if the loading amount exceeds this range, the activity is saturated and the cost increases.

  The second catalytic metal is supported with a particle size larger than the average interlayer distance of the first oxide. This makes it difficult for the second catalyst metal to enter the gap between the layers of the first oxide, and it is possible to reliably suppress grain growth or alloying with the first catalyst metal supported between the layers. In order to support the second catalyst metal with a particle size larger than the average interlayer distance of the first oxide, the second catalyst metal is supported on the second oxide in the same manner as in the past, such as an adsorption support method or a water absorption support method. There is a method in which the second catalyst metal is grain-grown by heating to a high temperature after that, as in the production method of.

  In addition, when supported using a metal colloid chemical solution of the second catalytic metal, it is supported as relatively large metal particles in which 10 to several thousand atoms are collected, so that the average interlayer distance of the first oxide can be obtained without the need for grain growth. The second catalytic metal can be supported with a large particle size. The metal colloid chemical solution can be prepared by mixing a water-soluble metal salt and an alcohol in an aqueous solution of a water-soluble polymer such as polyvinylpyrrolidone or polyvinyl alcohol and heating to form a polymer protective metal colloid. This method is called the polymer protection method. Then, the second oxide metal can be supported on the second oxide by dispersing the second oxide powder in the aqueous solution of the polymer protective metal colloid and drying and firing it. Moreover, you may carry | support using the method using an electrostatic effect, the method using adsorption | suction to the support | carrier of a polymer chain, etc.

  For example, when the average interlayer distance of the first oxide is 10 nm, the particle diameter of the second catalyst metal may be set to a size exceeding 10 nm. As a result, most of the second catalyst metal has a particle size larger than the interlayer distance of the first oxide and can be prevented from entering the gap between the layers. Therefore, grain growth or alloy of the first catalyst metal and the second catalyst metal Can be suppressed. It is particularly desirable that the particle size of the second catalytic metal is larger than the maximum value of the interlayer distance of the first oxide.

  If the particle diameter of the second catalytic metal is too large, the activity corresponding to the amount supported will not be expressed. Therefore, in order to exhibit sufficient activity as an exhaust gas purifying catalyst, it is desirable that the particle size of the second catalytic metal is 10 nm or less.

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

(Example)
FIG. 1 schematically shows the three-way catalyst of this example. This three-way catalyst has a catalyst layer made of a mixture of a first catalyst powder 1, a second catalyst powder 2, and a CeO ₂ —ZrO ₂ composite oxide powder 3 on the surface of a cordierite honeycomb substrate. is doing. The first catalyst powder 1 is composed of α-alumina particles 10 having a multilayer structure having voids 11 between layers, and Pt12 supported between the layers. The second catalyst powder 2 is composed of zirconia particles 20 and Rh21 supported on the surface thereof, and Rh21 is supported with a particle size larger than the interlayer distance of the α-alumina particles 10. Hereinafter, a method for preparing the three-way catalyst will be described, and the detailed description of the configuration will be substituted.

<Preparation of first catalyst powder>
Commercially available aluminum hydroxide crystal particles (manufactured by Sumitomo Chemical Co., Ltd.) are dehydrated by drying at 120 ° C. for 2 hours, and then fired in the atmosphere at 1200 ° C. for 5 hours to have a multilayer structure with voids between the layers. α-alumina was prepared. The α-alumina particles had an average primary particle diameter of 3 μm, a specific surface area of several m ² / g, and an average interlayer distance measured by TEM observation of 7 nm.

  This α-alumina powder is impregnated with a predetermined amount of a dinitrodiammine platinum aqueous solution having a predetermined concentration, evaporated to dryness with stirring until the solvent is completely evaporated, dried in the atmosphere at 120 ° C. for 2 hours, and then at 400 ° C. Firing for 2 hours supported Pt. The amount of Pt supported is 1% by weight. When the obtained catalyst particles were embedded in a resin and the section sliced thinly with FIB was observed with a transmission electron microscope, it was found that Pt was supported between layers of a multilayer structure α-alumina.

<Preparation of second catalyst powder>
A commercially available zirconia powder was put into water, and a predetermined amount of an aqueous rhodium nitrate solution was added with stirring. This was evaporated to dryness and calcined at 400 ° C. for 2 hours to prepare Rh / ZrO ₂ powder having Rh supported on zirconia. The amount of Rh supported is 1% by weight.

This Rh / ZrO ₂ powder was fired in the atmosphere at 1000 ° C. for 5 hours to grow Rh grains. As a result of observation by an electron microscope, the particle size of Rh is 7 nm or more, which is larger than the average interlayer distance of α-alumina having a multilayer structure having voids between layers.

<Preparation of three-way catalyst>
50 parts by weight of the first catalyst powder, 50 parts by weight of the second catalyst powder, 130 parts by weight of the CeO ₂ —ZrO ₂ composite oxide powder, and an appropriate amount of alumina binder are mixed in water and milled in a ball mill. A slurry was prepared.

  On the other hand, cordierite honeycomb substrate (square cell, 600cpsi, 0.9L) was prepared, washed with the above slurry, dried at 120 ° C for 1 hour, fired at 500 ° C for 2 hours, and cell walls A catalyst layer was formed on the surface. The amount of catalyst layer formed per liter of honeycomb substrate is 250 g, Pt is supported at 1.5 g / L, and Rh is supported at 0.4 g / L.

(Comparative example)
A three-way catalyst was prepared in the same manner as in Example except that Rh / ZrO ₂ powder (used in Example 1) not calcined at 1000 ° C. was used instead of the second catalyst powder. The particle size of the supported Rh is 10 nm or less.

(Test / Evaluation)
The three-way catalysts of Examples and Comparative Examples were converted into converters and attached to both banks of the V-type 8-cylinder engine. Then, an endurance test was performed for 50 hours under the condition of a catalyst bed temperature of 950 ° C. while repeating fuel cut for 3 seconds for 10 seconds. The bank was changed in 25 hours.

Each three-way catalyst after the endurance test was installed in the exhaust system of a 2.4-liter engine, and the NO _x purification rate at an inlet gas temperature of 400 ° C. and A / F = 14.3 was measured. The results are shown in FIG.

  In addition, each three-way catalyst after the endurance test was decomposed, and the catalyst layer in the exhaust gas upstream half was scraped to perform XRD analysis. And the alloying rate of Pt and Rh was measured from the XRD diffraction chart, and the result is shown in FIG.

From FIG. 2, the three-way catalyst of the example shows a higher NO _x purification rate even after the durability test than the comparative example. According to FIG. 3, the three-way catalyst of the example has a lower alloying rate than the comparative example. Therefore, the three-way catalyst of the example shows a high NO _x purification rate because of the alloy of Pt and Rh. This is thought to be due to the suppression of conversion. That is, according to the three-way catalyst of the example, since the particle size of Rh21 supported on zirconia 20 is large, it is prevented from entering the space 11 between the layers of α-alumina 10 and alloying with Pt12, and the alloy It is thought that the conversion was suppressed. Therefore, NO _x reduction performance, which is a characteristic of Rh, is sufficiently expressed even after the durability test.

In the above embodiment, alloying of Pt and Rh was suppressed, but not only Pt and Rh but also a combination of metal species that can be alloyed produces the same effect. Moreover, if the first catalyst metal and the second catalyst metal are the same type, grain growth can be suppressed. Therefore, the present invention can be used not only for a three-way catalyst but also for various exhaust gas purification catalysts such as an oxidation catalyst and a NO _x storage reduction catalyst.

It is explanatory drawing which shows typically the three way catalyst which concerns on one Example of this invention. Is a graph showing _the NO _x purification ratio after the durability test of the three-way catalyst according to examples and comparative examples. It is a graph which shows the alloying rate of Pt and Rh after the endurance test of the three way catalyst which concerns on an Example and a comparative example.

Explanation of symbols

1: First catalyst powder 2: Second catalyst powder 3: CeO ₂ —ZrO ₂ composite oxide powder
10: α-alumina particles 11: Void 12: Pt
20: Zirconia particles 21: Rh

Claims (3)

A first catalyst powder supporting a first catalyst metal between the layers of the first oxide having a multilayer structure having voids between the layers;
A catalyst for exhaust gas purification, comprising: a second catalyst powder in which a second catalyst metal is supported on a porous second oxide with a particle size larger than the average interlayer distance of the first oxide.   The exhaust gas-purifying catalyst according to claim 1, wherein the first catalyst metal is Pt and the second catalyst metal is Rh. A method for producing an exhaust gas purifying catalyst according to claim 1 or 2,
A method for producing an exhaust gas purifying catalyst, wherein the second catalyst metal is grain-grown by heating the second catalyst powder, and then the second catalyst powder and the first catalyst powder are mixed.

Images