JP3427419B2 - Method for producing exhaust gas purifying catalyst - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing an exhaust gas purifying catalyst.

[0002]

2. Description of the Related Art Various catalysts for zeolite are used as exhaust gas purifying catalysts suitable for decomposing and removing NOx (nitrogen oxide) contained in exhaust gas of automobile engines in an atmosphere in which oxygen exceeds the stoichiometric air-fuel ratio. Those carrying active species are generally known. For example, Japanese Patent Laid-Open No. 4-243545
Japanese Patent Laid-Open Publication No. 2003-242242 discloses that Pt and R as active species are used as a zeolite carrier.
It is described that h is supported by an ion exchange method or an impregnation method.

The above-mentioned ion exchange method is, for example, an ion exchange step of immersing zeolite in a solution of active species at room temperature to 80 ° C. for 24 to 48 hours, a drying step of 100 to 120 ° C. × 10 hours, and 400 to 700 ° C. The firing process for several hours is sequentially performed. In the impregnation method, zeolite is immersed in the active species solution for 1 to 2 hours and then dried in the atmosphere.

[0004]

However, in the above-mentioned prior art, when a plurality of active species is supported on zeolite,
From the viewpoint of improving the activity of the catalyst, no consideration has been made as to which active species is brought into contact with the zeolite first or which is brought later.

That is, an object of the present invention is to produce a catalyst by removing a liquid component from a slurry obtained by mixing a solution such as an active species solution with a carrier such as zeolite as in a dry evaporation method or a spray dry method. In the method, from the viewpoint of how to prepare a slurry when supporting a plurality of active species,
The purpose is to improve the activity of the catalyst.

[0006]

A method for producing an exhaust gas purifying catalyst according to the present invention comprises a solution of the metal-containing silicate and the Rh in supporting Pt, Ir and Rh as catalytically active species on the metal-containing silicate. And then mixed with the Pt solution, and then mixed with the iridium trichloride solution as the Ir solution to prepare a slurry. It is characterized by being removed.

Here, the metal-containing silicate means a crystalline porous body having micropores such as zeolite (aluminosilicate using Al as a metal forming a crystal skeleton). Of course, instead of Al or together with Al, a metal-containing silicate in which another metal such as Ga, Ce, Mn, or Tb is used as a skeleton-forming material can be used. As the zeolite, A type, X type, Y type
Molds, ZSM-5, etc. are preferred.

[0008] 2-valent platinum ammine crystal water solution as the Pt solution, but examples of the Rh solution is preferably an aqueous rhodium nitrate solution, other solutions may be used.

Further, the liquid component is removed from the slurry by evaporating to dryness by heating the slurry at a temperature below the boiling point to evaporate the liquid component, or by making the slurry into a droplet state and contacting it with hot air. A method of drying, for example, a spray drying method is suitable.

The particle size of the above iridium trichloride is preferably 3 μm or less, more preferably 1 μm or less. This is because the dispersibility of the particles in the Ir solution is improved, which results in less uneven distribution when loaded on the metal-containing silicate, which is advantageous in improving the catalytic activity.

[0011]

The above-mentioned Pt and Rh are water-soluble and can be present in the solution as ions in a highly dispersed state, so that they are easily supported in a highly dispersed state on the metal-containing silicate particles. On the other hand, three
Since iridium chloride is insoluble in water and alcohol and can only be dispersed as colloidal particles in a solvent, it is inherently difficult to support it on metal-containing silicate particles with high dispersion.

Therefore, first for metal-containing silicates
If the solution of iridium trichloride is brought into contact, Ir
Are not highly dispersed on the particles, and Pt and Rh subsequently placed on the particles are likely to be unevenly distributed under the influence of uneven distribution of Ir. For this reason, in the present invention, a solution of Pt or Rh is first contacted with the metal-containing silicate to achieve high dispersion thereof , and the trisalt is then added.
The iridium chloride solution is brought into contact with the metal-containing silicate.

The reason why the latter of the Pt solution and the Rh solution is brought into contact with the metal-containing silicate first is that Pt is supported on the metal-containing silicate in a large amount. That is, if the amount of Rh is small, the dispersibility on the metal-containing silicate particles is high, and since it is small, it does not significantly affect the dispersibility of Pt that comes into contact with the particles later. Is.

[0014]

According to the present invention, therefore, a metal-containing silicate and a solution of Rh are mixed, then a solution of Pt is mixed with the solution, and then the solution is mixed with iridium trichloride. Since the slurry is prepared by mixing the solution of <>, Pt, Rh, and Ir can be supported on the metal-containing silicate in the state of the highest dispersibility,
As a result, the activity of the catalyst can be improved.

[0015]

EXAMPLES Examples of the present invention will be described below in comparison with comparative examples.

(Example) In FIG. 1, 1 is an H-type Z as a metal-containing silicate.
SM-5 (SiO ₂ / Al ₂ O ₃ = 70) A slurry preparation tank containing particles 2 and a stirrer 3 attached. 4 is a container for the Rh solution, 5 is a container for the Pt solution,
6 is a container for Ir solution.

The Rh solution was prepared by dissolving rhodium nitrate in ion-exchanged water, the Pt solution was prepared by dissolving divalent platinum ammine crystals in ion-exchanged water, and the Ir solution was prepared by dissolving iridium trichloride in ethanol. It is a thing. Rhodium nitrate, divalent platinum ammine crystal and iridium trichloride were weighed so that Pt: Ir: Rh = 30: 10: 1. The particle size of the iridium trichloride is 1 μm or less.

In manufacturing the exhaust gas purifying catalyst,
First, the Rh solution was added to the slurry preparation tank 1 and stirred, and the Pt solution was added while continuing stirring, and then the Ir solution was added. Then, the mixed solution was stirred at room temperature for 2 hours, heated at 80 ° C. for 3 hours to evaporate the liquid content, and further dried at 150 to 200 ° C. for 6 hours, so that Pt, Ir and Rh contained ZSM. A catalyst powder carried on -5 particles was obtained.

Then, hydrated alumina was added as a binder to the catalyst powder, and an appropriate amount of water was added to make a slurry, which was wash-coated on a cordierite honeycomb carrier (400 cells / inch ² ).

(Comparative Examples 1 to 5) Catalyst powders were prepared by the same method as in the example except that the order of introducing the Rh solution, the Pt solution and the Ir solution into the slurry preparation tank 1 was different from that in the above example. Then, the same wash coat was performed. The order of charging in the comparative example is as shown in Table 1.

[0021]

[Table 1]

(Evaluation of catalytic activity) The following model gas (corresponding to exhaust gas of an automobile engine with A / F = 22) was passed through each of the honeycomb catalysts of Examples and Comparative Examples so as to have a space velocity of 55000 hr ^-1. , NO
The maximum value of x purification rate was obtained. The results are shown in Table 1 above.

NO; 2000 ppm HC; 5500 ppm O ₂ ; 8% H ₂ ; 650 ppm CO; 0.2% CO ₂ ; 10% N ₂ ; balance

According to the results shown in Table 1, the example has the highest NOx purification rate. This example and Comparative Example 1 are examples in which the Ir solution was charged last, but the NOx purification rate is higher than other comparative examples in which the Ir solution was charged first or second. This is because Pt and R have low dispersibility of Ir.
It was confirmed that this did not adversely affect the dispersion of h, and from this, it was found that the last addition of the Ir solution was good. In addition, Example 1 and Comparative Example 1 are different from Example in the order of introducing Pt and Rh, but from the comparison of both examples, even when the same ion-exchanged water is used as a solvent, the solution of Rh having a small loading amount is first. It turns out that it is better to throw in.

Comparative Examples 2 and 5 are examples in which the Ir solution was added first, but both have a low NOx purification rate. Among the two examples, it is considered that Comparative Example 2 shows a better result because of the order of feeding the Rh and Pt solutions.
Comparative Examples 3 and 4 are examples in which the Ir solution was placed in the second order, but it was recognized that Comparative Example 3 showed better results for the same reason as in Comparative Example 2 above. To be

[Brief description of drawings]

FIG. 1 is a schematic diagram showing the configuration of the present invention.

[Explanation of symbols]

1 Slurry preparation tank 2 Metal-containing silicate particles 3 stirrer Container for 4 Rh solution 5 Pt solution container 6 Ir Solution Container

─────────────────────────────────────────────────── ─── Continuation of the front page (56) References JP-A-4-371231 (JP, A) JP-A-4-74534 (JP, A) JP-A-63-119853 (JP, A) (58) Field (Int.Cl. ⁷ , DB name) B01J 21/00-38/74 B01D 53/94

Claims (2)

(57) [Claims] 1. In supporting Pt, Ir and Rh as a catalytically active species on a metal-containing silicate, the metal-containing silicate and the Rh solution are mixed, and then the Pt solution is mixed with the mixed solution. Then, a method for producing an exhaust gas purifying catalyst, characterized in that a slurry is prepared by subsequently mixing an iridium trichloride solution as the Ir solution into the mixed solution , and a liquid component is removed from the slurry. 2. The method for producing an exhaust gas purifying catalyst according to claim 1, wherein the Pt solution and the Rh solution are a divalent platinum ammine crystal aqueous solution and a rhodium nitrate aqueous solution, respectively.