JPH09253490A - Catalyst for clarification of exhaust gas and its preparation - Google Patents

Abstract

PROBLEM TO BE SOLVED: To prevent Pt particles from growing and to suppress reaction of Rh with a carrier. SOLUTION: A metal cluster consisting of an alloy wherein at least parts of Pt and Rh are under solid soln. condition each other by reducing a soln. contg. Pt and Rh in a liq. phase and and the metal cluster is carried on a porous carrier. Rh with higher affinity with oxygen is deposited partially and the metal cluster particles are prevented thereby from growing and decrease in active points of the catalyst is suppressed. In addition, the reaction of Rh with alumina is also suppressed by preparing the metal cluster.

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention relates to an exhaust gas purifying catalyst and a method for producing the same. According to the exhaust gas-purifying catalyst of the present invention, it is possible to suppress grain growth of the catalytic noble metal and the reaction with the carrier, and the durability of the exhaust gas-purifying catalyst is improved.

[0002]

2. Description of the Related Art Conventionally, a three-way catalyst for purifying exhaust gas by oxidizing CO and HC and reducing NO _x has been used as an exhaust gas purifying catalyst for automobiles. As such a three-way catalyst, for example, a porous carrier layer made of γ-alumina is formed on a heat resistant base material made of cordierite, and platinum (Pt) and rhodium (R) are formed on the porous carrier layer.
A catalyst carrying a precious metal such as h) is widely known.

It is known that in this three-way catalyst, particularly high activity is obtained when Pt and Rh coexist. Therefore, for example, in JP-A-57-207544, P is used as a carrier by using a platinum compound solution in an alkaline state.
There is disclosed a method of supporting t and supporting Rh on a carrier by using a rhodium compound solution in an acidic state. According to the exhaust gas-purifying catalyst obtained by this production method, the layer thickness of the Pt-supported portion and the Rh-supported portion in the carrier are approximately equal, so that Pt and Rh easily coexist and high ternary activity is obtained.

Further, in JP-A-62-282641, a coat layer is formed on a carrier from a slurry containing zirconium oxide containing Rh in advance and activated alumina containing cerium in advance, and then platinum is formed on the coat layer. A method of impregnating a compound solution to support Pt is disclosed.
According to this method, even if the supported amounts of Pt and Rh are reduced, the purification performance does not deteriorate, and an exhaust gas purification catalyst with improved heat resistance can be manufactured.

[0005]

By the way, the exhaust gas temperature of automobiles is increasing due to the recent tightening of exhaust gas regulations and improvement of engine performance. Therefore, in the conventional exhaust gas purifying catalyst, when exposed to an oxidizing atmosphere at a high temperature, particle growth occurs in the fine particles of the catalytic noble metal supported on the carrier, and as a result, the catalytic activity decreases due to a decrease in active points. Was occurring. There is also a problem that Rh reacts with alumina as a carrier to lower the activity.

The present invention has been made in view of such circumstances, and an object of the present invention is to provide an exhaust gas-purifying catalyst capable of preventing Pt grain growth and suppressing the reaction between Rh and a carrier.

[0007]

The features of the exhaust gas purifying catalyst of the present invention for solving the above-mentioned problems are characterized by a porous carrier and an alloy in which at least a part of Pt and Rh are supported on the porous carrier as a solid solution. To have a metal cluster. Further, the feature of the method for producing an exhaust gas purifying catalyst of the present invention, which is most suitable for producing this exhaust gas purifying catalyst, is that a solution containing Pt and Rh is reduced in a liquid phase so that at least a part of Pt and Rh are mutually solid. It is to include a first step of preparing a metal cluster composed of a molten alloy and a second step of supporting the metal cluster on a porous carrier.

[0008]

BEST MODE FOR CARRYING OUT THE INVENTION The metal cluster referred to in the present invention refers to a cluster of a plurality of atoms at the atomic level to a cluster of metal fine particles having an average particle size of about 100 nm. In the exhaust gas purifying catalyst of the present invention, Pt and Rh are supported as metal clusters made of an alloy in which at least some of them are solid-solved with each other. Therefore, even when the catalyst is exposed to a high temperature oxidizing atmosphere, a decrease in catalytic activity is suppressed.

The reason why the catalytic activity is suppressed is that Rh, which has a high affinity for oxygen, is segregated on the surface of the metal clusters, which suppresses the particle growth of Pt and thus suppresses the particle growth of the metal clusters. It is considered that this is because the decrease in active sites is suppressed. Further, by using a metal cluster composed of an alloy of Pt and Rh, the reaction between Rh and the porous carrier is also suppressed.

Examples of the porous carrier used in the exhaust gas purifying catalyst of the present invention include γ-alumina, silica, titania, zirconia, silica-titania, zeolite and the like. The amount of Pt supported as a metal cluster is 0.1 to 100 g per 100 g of the porous carrier.
About 20 g is preferable, and about 1 to 10 g is particularly preferable. The amount of Pt supported is 0.1 g per 100 g of the porous carrier.
If the amount is less, the catalytic activity will be insufficient and the purification performance will be insufficient. Even if more than 20 g is carried, the catalytic activity will be saturated and the cost will increase.

The amount of Rh forming a metal cluster made of an alloy solid-soluted with Pt is preferably about 1/100 to 1/2 by weight ratio with respect to Pt, and 1/20 to 1/1.
About 3 is particularly preferable. If the supported amount of Rh is less than 1/100 by weight ratio with respect to Pt, the function of suppressing the particle growth of Pt becomes low and the purification performance by the particle growth of metal clusters deteriorates. On the other hand, when it is more than 1/2, the effect is saturated and the cost is increased.

The Pt and Rh alloyed in the metal cluster may be at least a part of the supported Pt and Rh, but 50% by weight or more of the whole is preferably alloyed. More preferably, it is desirable that substantially all Pt and Rh are alloyed in the metal cluster. The larger the proportion of alloying, the more effectively grain growth can be prevented, and the further reduction in catalyst activity can be prevented.

The exhaust gas-purifying catalyst of the present invention may contain cerium oxide (ceria) or lanthanum carbonate. Since ceria has an oxygen storage / release function, when the exhaust gas atmosphere deviates from the stoichiometry, the atmosphere can be kept stoichiometric by storing and releasing oxygen, further improving the purification performance. The content of this ceria is preferably 50 to 100 g per 100 g of the porous carrier.

Further, lanthanum carbonate has a function of enhancing the catalytic activity of Pt by an electronic interaction with Pt, and the purification performance is further improved by further containing lanthanum carbonate in the exhaust gas purifying catalyst of the present invention. A suitable content of lanthanum carbonate is about 30 g per 100 g of the porous carrier. In the method for producing an exhaust gas purifying catalyst of the present invention, which is optimal for producing the exhaust gas purifying catalyst, the solution containing Pt and Rh is reduced in the liquid phase in the first step. That is, the salt of Pt and the salt of Rh are simultaneously reduced in a solution state. As a result, a metal cluster made of an alloy in which at least part of Pt and Rh is solid-solved can be easily obtained, and a metal cluster having a uniform composition in which Pt and Rh are sufficiently solid-solved can be easily prepared by adjusting reducing conditions. be able to.

The solution used in the first step is not particularly limited as long as it is a solution of Pt salt and Rh salt dissolved at the same time, and various salts and solvents can be used. The reducing agent used to reduce this solution is preferably an organic reducing agent that disappears by combustion and does not leave impurities, and lower alcohols, formaldehyde, acetaldehyde, formic acid, citric acid, and oxalic acid which are also used as solvents. Examples thereof include acids. Further, the reduction may be performed by introducing gaseous hydrogen into the reaction solution.

The first step can be carried out by adding a reducing agent to a solution of Pt salt and Rh salt and reacting by heating. It is desirable that the reduction reaction of Pt salt and Rh salt proceed simultaneously. In addition, it is also preferable to add a high molecular compound such as polyvinylpyrrolidone or polyvinyl alcohol, a low molecular compound such as citric acid having a coordination action to a metal ion, triphenylphosphine, or phenanthroline to the solution during the reduction. By doing so, the stability of the produced metal clusters in the solution can be improved, and the composition of the metal clusters in the obtained exhaust gas-purifying catalyst can be made more uniform.

In the second step, the metal cluster formed in the first step is supported on the porous carrier. As the supporting method, an adsorption supporting method in which a porous carrier is immersed and supported in a solution containing a metal cluster, or a porous carrier powder is impregnated with a solution containing a metal cluster in a predetermined amount and the solvent is evaporated to carry the carrier. An impregnation supporting method or the like can be used. The porous carrier powder on which the metal clusters are supported may be granulated as it is to be used as a pellet catalyst, or may be coated on a honeycomb carrier base made of cordierite or metal to be a straight flow type catalyst. .

Therefore, in the exhaust gas-purifying catalyst produced by the production method of the present invention, Pt-Rh is contained in the liquid phase.
Since the metal clusters made of the alloy are obtained, the composition ratio of Pt and Rh in the supported metal clusters is uniform in each metal cluster particle. That is, since there is almost no metal cluster having a small amount of solid solution of Rh or Rh which is not solid-dissolved with Pt, grain growth of the metal cluster at high temperature due to Pt is further suppressed, and Rh and the porous carrier are separated. Is further suppressed.

[0019]

EXAMPLES The present invention will be specifically described below with reference to Examples and Comparative Examples. (Example 1) <First Step> 400 ml of water and 400 ml of ethanol were placed in a 1-liter three-necked flask, and the concentration was 0.0178 mol.
/ L RhCl ₃ aqueous solution 28.8 ml, concentration 0.02
11.3 ml of a 15 mol / L H ₂ PtCl ₆ aqueous solution was added and dissolved. 2.419 g of polyvinylpyrrolidone was further added to this solution, and the mixture was heated with an oil bath while stirring with a mechanical stirrer and reacted under reflux for 2 hours.

Thereafter, the mixture was allowed to cool, 30 g of an ion exchange resin washed with distilled water in advance was added, and the mixture was stirred for 10 minutes. Thereafter, the ion exchange resin was separated from the solution by filtration, whereby unreacted metal ions and chlorine ions existing in the solution were removed from the solution to obtain a colloidal solution containing a metal cluster made of a Pt-Rh alloy. . <Second Step> Next, the colloidal solution 500 obtained above.
ml and 5 g of γ-alumina powder were mixed and stirred at room temperature for 8 hours. Then, the mixture was heated to 100 ° C. with stirring, and the solvent was evaporated to dryness to support the Pt—Rh metal clusters on the alumina powder. The loading amount of Pt—Rh metal clusters in the catalyst powder was Pt based on 100 g of alumina.
Is 1.0 g and Rh is 0.22 g.

Then, the obtained powder is dried in a drier 11
It was dried overnight at 0 ° C., and further calcined at 350 ° C. for 3 hours under air flow to obtain a catalyst powder of Example 1. (Comparative Example 1) A predetermined amount of a nitric acid solution of dinitrodiammineplatinum adjusted to a predetermined concentration was impregnated with a predetermined amount of γ-alumina powder, and water was evaporated to dryness to support Pt. Next, this powder is impregnated with a predetermined amount of a nitric acid aqueous solution of rhodium nitrate adjusted to a predetermined concentration, and the water is evaporated to dryness to form R
h was supported. As a result, 1 g of Pt was loaded on 100 g of alumina and 0.22 g of Rh was loaded.

Then, the obtained powder is dried in a dryer for 11
The catalyst powder of Comparative Example 1 was obtained by drying overnight at 0 ° C. and further calcining at 350 ° C. for 3 hours under air circulation. (Activity Evaluation) Each of the above-mentioned catalyst powders was compacted into a pellet catalyst having a particle size of 0.3 to 0.7 mm. Each pellet catalyst was filled in an endurance test device, and a stoichiometric endurance test was performed in which the oxidizing gas A and the reducing gas shown in Table 1 were alternately flowed at a cycle of 10 minutes and each was heated at 800 ° C. for 5 hours. In addition, a lean endurance test was conducted in which the oxidizing gas A (O ₂ 5%) and the oxidizing gas B (O ₂ 20%) shown in Table 1 were respectively circulated while heating at 800 ° C. for 5 hours.

[0023]

[Table 1] With respect to each pellet catalyst after each endurance test, the noble metal particle diameter was measured from the diffraction line width of Pt metal by the powder X-ray diffraction method, and the results are shown in Table 2. For each pellet catalyst in the stoichiometric durability test and lean durability test (O ₂ 20%), the evaluation gas shown in Table 1 was used, and the catalyst amount was 0.5.
g, a gas flow rate of 3.3 L / min, the gas temperature is increased from 100 ° C. to 500 ° C., and HC, CO and NO
The purification rate of _x was measured, and the 50% purification temperature of each was determined. Table 3 shows the results.

[0024]

[Table 2]

[0025]

[Table 3] From Table 3, it can be seen that the catalyst of Example 1 has higher HC and C than Comparative Example 1.
It can be seen that the purification performance of O and NO _x is improved.
The improvement of the purifying ability in the examples is largely seen after the lean endurance test. From Table 2, it is considered that this difference is due to the fact that the catalyst of Example 1 suppressed the grain growth of the noble metal even after the durability test.

That is, the Pt salt and the Rh salt are previously reduced in solution to form a solid solution and alloyed to support an alloyed metal cluster on an alumina carrier, whereby grain growth of the noble metal after the durability test is suppressed and durability is improved. It is clear that a good catalyst is obtained.

[0027]

That is, the exhaust gas purifying catalyst of the present invention is extremely excellent in durability because the growth of precious metal particles is prevented even after the high temperature durability test. Further, according to the method for producing an exhaust gas purifying catalyst of the present invention, an alloy cluster in which Pt and Rh form a solid solution with each other can be prepared at an arbitrary composition ratio, and the exhaust gas purifying catalyst carrying an alloy cluster having a uniform composition is supported. Can be manufactured easily and reliably.

Claims (2)

[Claims] 1. An exhaust gas purifying catalyst comprising a porous carrier and a metal cluster made of an alloy which is supported on the porous carrier and in which at least a part of platinum and rhodium are solid-solved with each other. 2. A first step of preparing a metal cluster composed of an alloy in which at least a part of platinum and rhodium are solid-solved by reducing a solution containing platinum and rhodium in a liquid phase, and the metal cluster is a porous carrier. A second step of supporting
A method for producing an exhaust gas-purifying catalyst, comprising: