JPH05138035A - Catalyst for purifying exhaust gas - Google Patents

Abstract

(57) [Abstract] [Purpose] To improve the heat resistance of silicate-based exhaust gas purification catalysts containing metals such as zeolite. When a catalyst material obtained by supporting a transition metal on a metal-containing silicate by ion exchange is mixed with a binder and wash-coated on a carrier to obtain an exhaust gas purifying catalyst, the amount of the binder is set to 2 of the metal-containing silicate.
By setting the content to 10% by weight, the reaction between the transition metal and the binder at a high temperature is suppressed to a minimum and the deactivation of the catalyst is prevented.

Description

Detailed Description of the Invention

[0001]

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

[0002]

2. Description of the Related Art C is used as a catalyst for purifying exhaust gas of automobiles.
O (carbon monoxide) and HC (hydrocarbon) oxidation, NO
A three-way catalyst that simultaneously reduces x (nitrogen oxide) is generally known. This three-way catalyst is formed by supporting Pt (platinum) and Rh (rhodium) on γ-alumina, for example, and has an engine air-fuel ratio (A / F) of 1 theoretical air-fuel ratio.
A high purification efficiency can be obtained when controlling to around 4.7.

On the other hand, in the field of automobiles, there is a demand for increasing the air-fuel ratio to reduce the fuel consumption of the engine. In this case, since the exhaust gas has a so-called lean atmosphere with excess oxygen, the three-way catalyst cannot oxidize and purify CO and HC, but cannot reduce and purify NOx.

Therefore, in recent years, studies have been conducted on zeolite catalysts in which a transition metal is supported by ion exchange.
In the case of this zeolite catalyst, even in a lean atmosphere,
A reducing agent that directly or coexists with NOx (for example, C
O, HC, etc.) can be catalytically decomposed into N ₂ and O ₂ .

An example of the above zeolite catalyst is disclosed in JP-A-1-
No. 130735. That is, it obtains a catalyst material by ion-exchange of cations such as Na ⁺ and K ^{+ of} zeolite with Cu and Co as transition metals, and the catalyst material and binder (alumina sol +
(Silica sol) is mixed at a ratio of 60:40 to form a slurry, which is wash-coated on a cordierite carrier having a honeycomb structure.

[0006]

However, the above zeolite catalyst has a problem of low heat resistance. That is, for example, the exhaust gas temperature of the automobile is 500 ° C.,
The temperature may be as high as 600 ° C. or higher, and in that case, the zeolite catalyst is deactivated due to heat, and there is a fear that purification of the exhaust gas thereafter becomes insufficient.

That is, an object of the present invention is to improve the heat resistance of a metal-containing silicate-based exhaust gas purifying catalyst which is a crystalline porous material having microscopic pores such as the above zeolite.

[0008]

Means for Solving the Problems and Their Actions The inventors of the present invention, as a result of earnest research on such problems, have found that the heat resistance of the catalyst is affected by the binder. Therefore, the heat resistance of the exhaust gas purifying catalyst could be improved by adjusting the ratio between the binder amount and the catalyst material.

That is, as the binder, for example, hydrated alumina is generally adopted, but in the case of this hydrated alumina, when the temperature becomes high, Cu and C as catalyst components are used.
Reacts with transition metals such as o. Therefore, the catalyst component does not exhibit the function as an active species, and as a result, the activity decreases when the catalyst is continuously used at high temperature.

However, the means for solving the above-mentioned problems is an exhaust gas purifying catalyst which is obtained by mixing a catalyst material in which a transition metal is supported on a metal-containing silicate by ion exchange with a binder and wash-coating the carrier with the mixture. And the amount of the binder is 2 to 10% by weight of the metal-containing silicate.
It is characterized by being set to.

As the metal-containing silicate body, an aluminosilicate (zeolite) in which Al is used as a metal forming a skeleton of a crystal is suitable, and in addition to Al or together with Al, other materials such as Ce, Mn, and Tb are used. A metal-containing silicate using a metal as a skeleton-forming material can also be preferably adopted. As zeolite, A
Mold, X-type, Y-type, mordenite, ZSM-5 and the like are suitable.

As the transition metal, Cu is preferable, and in addition, Co, Cr, Ni, Fe, Mn and the like can be preferably used. Cordierite is suitable as the carrier, but other inorganic porous materials can also be used.

The binder amount is set to 2 to 10% by weight, which is less than the amount conventionally used. That is, the conventional binder amount is usually determined in consideration of only the washcoat property of the catalyst material on the carrier, and therefore, as described in the above-mentioned prior art publication, the binder amount is As in the case of the original catalyst, a relatively large amount of binder is used.

On the other hand, in the case of the present invention, the catalyst material is not a three-way catalyst but a metal-containing silicate ion-exchanged with a transition metal, that is, the metal-containing silicate body is supported by ion exchange. Considering that the transition metal that has been allowed to react with the binder is likely to be deactivated,
The amount of the binder is reduced to 2 to 10% by weight. As a result, the reaction between the binder and the transition metal, which is the catalyst component, at a high temperature is suppressed, and its deactivation is prevented.

In this case, if the amount of the binder exceeds 10% by weight, the effect of preventing deactivation of the transition metal cannot be sufficiently obtained. In that sense, the amount of the binder is more preferably 5% by weight or less. On the other hand, when the amount of the binder is less than 2% by weight, it becomes difficult to wash coat the catalyst material on the carrier, and the catalyst material is easily separated from the carrier.

However, when the amount of the binder is reduced, the deactivation of the catalyst component can be suppressed to the minimum as described above, but it becomes difficult to wash coat the catalyst material on the carrier and the amount supported on the carrier decreases. Under high SV (space velocity), sufficient activity may not be obtained. On the other hand, if the carrier is porous with an average pore size of 1 μm or more, the amount of the catalyst material supported on the carrier and the strength of the carrier can be increased.

That is, the metal-containing silicate catalyst material is mixed with a binder, water is added to form a slurry, and the carrier is wash-coated. The size of the secondary particles of the catalyst material in the slurry depends on the amount of the binder or the amount of the binder. Although it depends on the pH of the slurry, it is about 0.5 to 7 μ, for example.
On the other hand, in the case of the conventional porous carrier, the average pore diameter is usually set to about 0.5 μ, but in the present invention (the invention of claim 2), it is further increased. Is. As a result, the secondary particles are adhered and supported also on the inner surfaces of the pores of the porous carrier, and the amount of the secondary particles is increased. In addition, the catalyst material that has entered the pores in this way exhibits the anchor effect of the catalyst material that is adhered and supported on the surface of the porous carrier, thereby increasing the supporting strength of the catalyst material.

In this case, the average pore size of the porous carrier is
It is desirable to set the upper limit to about 40 μ. That is, the increase in the average pore diameter leads to a decrease in the strength of the carrier when the thickness of the wall of the carrier is constant, and when the average pore size is set to about 40 μ, the required loading amount for the catalyst material can be obtained. This is because the NOx purification rate is hardly improved even if the closed equilibrium right tilt is further increased to increase the catalyst carrying amount. Of course, in the case of a honeycomb carrier, if the wall of the carrier is made thicker in accordance with the increase in the average pore diameter, the above-mentioned problem of strength will be eliminated.

[0019]

Therefore, according to the present invention, in the exhaust gas purifying catalyst, a catalyst material comprising a metal-containing silicate carrying a transition metal by ion exchange is mixed with a binder and wash-coated on the carrier. Since the amount of the binder is set to 2 to 10% by weight of the metal-containing silicate, it is possible to suppress the reaction of the binder and the transition metal at a high temperature to improve the heat resistance of the catalyst, and to suppress the reduction rate of NOx. be able to.

Further, in the case where the carrier is a porous body having an average pore diameter of 1 μm or more, the carrying amount of the catalyst material is increased while the carrying strength of the catalyst material on the carrier is increased, and the purification rate of NOx is increased. You can

[0021]

EXAMPLES Examples of the present invention will be described below. <Example 1> Cu was bonded to synthetic zeolite ZSM-5 by a wet ion exchange method to obtain an ion exchange rate of 120-
140% Cu ion-exchanged zeolite catalyst material was obtained.
By mixing this catalyst material with a binder at various ratios (binder amount / catalyst material amount) and adding water to each of the cordierite honeycomb carriers and washcoating, various exhaust gas purifying catalysts having different binder ratios can be obtained. Obtained. Hydrated alumina was used as the binder.

With respect to the above various catalysts, the initial activity (maximum purification rate of NOx) under the following conditions and the activity after aging at 650 ° C. for 6 hours were examined.

-Gas composition-NO; 2100 ppm, HC; 6000 ppm, O ₂ ;
8%, CO ₂ ; 10%, CO; 0.2%, H ₂ ; 650
ppm-Space Velocity-SV = 25000 hr- ^{1 The} results are shown in FIG. From the figure, it can be seen that the NOx purification rate after aging is less likely to decrease in the binder ratio range of 2 to 10% by weight. This result is considered to be because the decrease in the activity due to the binding of Cu with the binder was suppressed because the amount of the binder was small.

<Example 2> In this example, the porosity of the carrier was examined.

That is, two types of cordierite porous honeycomb carriers A and B having different average pore diameters were prepared.
Then, the catalyst material of Example 1 was mixed with a binder at a binder ratio of 5% by weight, water was added thereto, and each of the carriers A and B was wash-coated to obtain an exhaust gas purifying catalyst. Table 1 shows the specific gravity, average pore size and washcoat amount of the carrier.

[0026]

[Table 1]

Then, the initial activity and the activity after aging (650 ° C. × 6 hours) of the catalysts of the above A and B carriers were examined. The results are shown in FIG. 2 (initial activity) and FIG. 3 (activity after aging).

That is, the porous carrier B having an average pore diameter of 22μ
In the case of, the active region is expanded to the low temperature side and the deterioration of the catalytic activity due to aging is less than that in the case of the porous carrier A having an average pore diameter of 0.5 μ. . This result is partly because the amount of washcoat in the case of B is more than twice as large as that of A, and in addition, in the case of B, the pore size is large so that the catalyst during washcoating is large. It is considered that the macropores were generated in the and the diffusion or permeation effect of the exhaust gas was improved.

Therefore, the relationship between the average pore size of the porous carrier and the amount of washcoat was further investigated. That is, various cordierite porous honeycomb carriers having different average pore diameters (the number of cells and the wall thickness are the same) were prepared, and the same catalyst slurry (a mixture of a catalyst material and a binder to which water was added) was added to each. When washcoating was performed (coating was performed once), the results shown in FIG. 4 were obtained. From this, it is understood that an increase in the average pore size of the porous carrier leads to an increase in the amount of washcoat.

Next, using a cordierite porous honeycomb carrier having a wall thickness of 6 mil and 400 cells, the relationship between the washcoat amount of the catalyst slurry and the NOx purification rate was examined, and the results shown in FIG. 5 were obtained. .. According to the figure, the NOx purification rate tends to be saturated when the washcoat amount is around 40% by weight. In this case, according to FIG. 4 above, the washcoat amount can be set to about 40% by weight if the average pore size is set to about 40 μ, so that when the saturation of the NOx purification rate is taken into consideration, the average pore size is set to about 40 μ. It is understood that the upper limit should be set.

The influence of the average pore diameter and wall thickness of the porous honeycomb carrier on the strength of the carrier was investigated. That is, when the isostatic strength of the carrier was examined by changing the average pore diameter and the wall thickness to various values, the isointensity curve shown in FIG. 6 was obtained. From this, it is understood that when the average pore diameter is increased, it is desirable to increase the wall thickness of the honeycomb carrier correspondingly.

[Brief description of drawings]

FIG. 1 Binder ratio and NO before and after aging
x Characteristic diagram showing the relationship with purification rate

FIG. 2 is a characteristic diagram showing the effect of the average pore size of the porous honeycomb carrier on the initial activity of the catalyst.

FIG. 3 is a characteristic diagram showing the effect of the average pore diameter of a porous honeycomb carrier on the activity of the catalyst after aging.

FIG. 4 is a characteristic diagram showing the relationship between the average pore size of the porous honeycomb carrier and the washcoat amount of the catalyst slurry.

FIG. 5 is a characteristic diagram showing the relationship between the washcoat amount of the catalyst slurry and the NOx purification rate.

FIG. 6 is a characteristic diagram showing iso-intensity curves of average pore diameter and wall thickness of a porous honeycomb carrier.

[Explanation of symbols]

 None

Claims (2)

[Claims] 1. An exhaust gas purifying catalyst comprising a catalyst containing a metal-containing silicate carrying a transition metal by ion exchange mixed with a binder and wash-coating the carrier, wherein the amount of the binder is the metal-containing silicate. An exhaust gas purifying catalyst, which is set to 2 to 10% by weight. 2. The carrier is porous and has an average pore size of 1
The exhaust gas-purifying catalyst according to claim 1, which has a μ or more.