JPH05168862A - Catalyst for purification of exhaust gas - Google Patents

Abstract

PURPOSE:To improve heat resistance of a catalyst for purifying exhaust gas by constituting the catalyst of a cordierite carrier and a catalyst layer comprising transition metal deposited on a metal-contg. silicate. CONSTITUTION:This catalyst has a base layer 3 comprising a metal-contg. silicate which does not carry a transition metal between the carrier 1 and the catalyst layer 2. Thereby, decrease in activity due to the reaction of the transition metal of the catalyst layer 2 with Al in the carrier 1 is prevented. The catalyst layer 2 consists of zeolite carrying the transition metal deposited by ion exchanging of Cu.

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, 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 . In addition, Japanese Patent Publication No. 2-500822 discloses a catalyst in which H-type zeolite is loaded with a transition metal salt such as Fe or Cu.

[0005]

The catalyst using zeolite as described above is often used by supporting it on a honeycomb carrier made of cordierite. Although such a catalyst has a relatively high purifying ability, it has a low heat resistance, that is, when it is used at a high temperature, there is a problem that the catalytic activity decreases. On the other hand, in an actual vehicle, the exhaust gas temperature sometimes becomes a high temperature close to 800 ° C. to 900 ° C. Therefore, it is an important issue to improve the heat resistance of the catalyst.

As the cause of the low heat resistance of the above-mentioned conventional catalysts, structural destruction due to heat of zeolite itself, thermal deterioration of active species (transition metal), etc. are considered. The purpose is to improve the heat resistance of the catalyst by taking measures against deterioration.

[0007]

Means for Solving the Problems and Actions Thereof The present inventor has made intensive studies on such problems, and as a result, the thermal degradation of the above-mentioned active species is carried out by the carrier as compared with the case of the catalyst material alone. It has been found that it is accelerated in the state of being supported on. The cause is that the active species and the carrier (Al of cordierite) react at a high temperature, and it has been found that blocking the reaction improves the heat resistance of the catalyst.

That is, the means for solving the above problems is an exhaust gas purifying catalyst in which a catalyst layer in which a transition metal is supported on a metal-containing silicate is provided on a cordierite carrier. An underlying layer made of a metal-containing silicate that does not carry a transition metal is provided between the carrier and the carrier.

In the above means, since the underlayer prevents direct contact between the catalyst layer and the carrier, the reaction between the transition metal of the catalyst layer and Al of the carrier is prevented. And, by this, the deterioration of the transition metal is prevented and the heat resistance of the catalyst is obtained. In this case, the underlayer is
Since it is composed of a porous metal-containing silicate that does not carry a transition metal, it works advantageously in increasing the diffusion of exhaust gas in the catalyst layer.

The catalyst layer will be described below.
The metal-containing silicate must be a crystalline porous body having micropores such as the above-mentioned zeolite (alumina silicate). And, as the metal-containing silicate, instead of the zeolite, for example, Al, Fe, Ce, and M are used as the metal forming the skeleton of the crystal.
It is also possible to employ a silicate formed by combining with another metal such as n, Tb, Cu or B, or a non-aluminosilicate containing no Al.

The crystal structure of the metal-containing silicate is as follows:
Pentasil type is preferable, and A type, X type, Y type,
Mordenite types may also be used. Further, the metal-containing silicate may be Na type or other alkali metal type or alkaline earth metal type, but H type is preferable from the viewpoint of heat resistance.

That is, the thermal deterioration of the above-mentioned active species (transition metal) is caused by the reaction with the above-mentioned carrier, and in the case of Na-type metal-containing silicate, the metal (for example, Al) constituting the skeleton thereof is heated by heat. It also occurs due to desorption and accompanying movement of Na ⁺ for charge compensation. That is, the Na ⁺ attacks the active site, and the transition metal of the active site causes migration aggregation, resulting in a decrease in the activity of the catalyst. From such a viewpoint, it is preferable that the metal-containing silicate use H ⁺ as a charge-compensating cation, instead of an alkali metal or an alkaline earth metal.

Although Cu is preferable as the transition metal to be supported on the metal-containing silicate, other transition metals such as Co, Cr, Ni, Fe, and Mn can be used. As a method for supporting the transition metal, a method by ion exchange is preferable, but a method in which a transition metal salt is supported by impregnation may be used. Also, two or more different transition metals can be used in combination.

In this case, the catalyst layer may be a single layer, but may be a multilayer. For example, the uppermost layer may be a layer having a high transition metal loading amount, and may have a multi-layer structure in which the transition metal loading amount is gradually reduced toward the lower layer. Of course, a single-layer structure in which the amount of the transition metal supported continuously decreases from the outermost surface to the lowermost surface of the catalyst layer may be adopted.

Further, it is preferable that a small amount of a noble metal be supported on the metal-containing silicate. That is, when the exhaust gas contains HC,
However, there is a problem that the transition metal is poisoned by the carbon deposited by the decomposition of the above, and the transition metal is poisoned by the carbon. On the other hand, the noble metal promotes the combustion of the deposited carbon and prevents the poisoning.

As for the metal-containing silicate constituting the underlayer, the same metal-containing silicate as the catalyst layer can be used. Also in this case, when an alkali metal or an alkaline earth metal is used as the charge-compensating cation, the movement of the alkali-metal causes deterioration of the active species of the catalyst layer, and therefore an H-type metal-containing silicate is preferably used.

[0017]

According to the present invention, therefore, an underlayer made of a metal-containing silicate not supporting a transition metal is provided between a catalyst layer in which a transition metal is supported on a metal-containing silicate and a cordierite carrier. Since it is provided, the reaction between the transition metal as an active species and the support can be prevented by the underlayer, and the heat resistance of the catalyst can be increased. Further, in the case of a catalyst in which the catalyst layer is composed of Cu ion-exchanged zeolite and the underlayer is composed of H-type zeolite, it becomes more advantageous in terms of heat resistance and high activity can be obtained.

[0018]

EXAMPLES Examples of the present invention will be described below based on a purification test. <Test 1> -Example 1-The catalyst structure of Example 1 is shown in FIG. In the figure, 1 is a carrier, 2 is a catalyst layer, and 3 is a base layer provided between the carrier 1 and the catalyst layer 2. The carrier 1 is made of cordierite and has a honeycomb structure (400 cells / inch ² ). The catalyst layer 2 is composed of zeolite in which Cu is supported by ion exchange. This zeolite is Na-type ZSM-5, and is SiO ₂ / Al
₂ O ₃ = 30, and the Cu ion exchange rate is 80 to
It is 120%. Underlayer 3 is SiO ₂ / Al ₂ O ₃ = 7
0 type H zeolite (ZSM-5).

In the present specification, the ion exchange rate is the exchange rate of the transition metal (Cu in this case) with respect to the ion exchange site of the metal-containing silicate (zeolite in this case), and the transition metal is divalent, It is calculated by assuming that the amount of supported transition metal which is 1/2 of the amount of metal (Al in this case) contained in the metal-containing silicate is 100% of the ion exchange rate.

(Preparation of catalyst) After forming 10% by weight of H-type zeolite on the carrier 1 by wash coating to form the base layer 3, 20% by weight of Cu ion-exchanged zeolite was supported by wash coating to form the catalyst layer 2. .. The wash coating was carried out by mixing each material with 10 wt% of hydrated alumina (binder) and adding water to obtain a slurry liquid.

Example 2 The same Cu ion-exchanged zeolite as that of Example 1 (however, SiO ₂ / Al ₂ O ₃ = 70) was pretreated (heat-treated) at 700 ° C. for 6 hours. A catalyst having a layer structure similar to that shown in FIG. 1 was obtained.

Comparative Example 1 A catalyst was obtained by wash-coating the same Cu ion-exchanged zeolite as in Example 1 on the same carrier without providing the underlayer as in each of the above Examples.

Comparative Example 2 The same Cu ion-exchanged zeolite as in Example 1 was used.
Using a catalyst that had been subjected to a pretreatment (heat treatment) at 00 ° C. for 6 hours, a catalyst similar to that of Comparative Example 1 having no underlayer was obtained.

-Purification test-Using the catalysts prepared as described above as test materials, the initial purification rate (NOx decomposition rate) and the post-heat treatment purification rate (NOx decomposition rate) were measured in the following test modes. The heat treatment in this case was 700 ° C. × 6 hours.

Simulated exhaust gas composition; NOx: 2000 pp
m, HC: 5500 ppm C, O ₂ : 8%, CO: 0.
15%, H ₂ : 650 ppm space velocity; 25000 h ^{−1 The} results are shown in Table 1.

[0026]

[Table 1] According to the test results, there is almost no difference in the initial purification rate between the example and the comparative example. In addition, Example 2 and Comparative Example 2 using the Cu ion-exchanged zeolite subjected to the pretreatment were compared with Example 1 and Comparative Example 1 not subjected to the pretreatment as a result of the thermal deterioration of the active species due to the pretreatment. Activity is low.

On the other hand, looking at the purification rate after heat treatment, in both Examples 1 and 2, the NOx purification rate was higher than that of the Comparative Example.
The reduction of the purification rate is small compared to the initial purification rate. This proves that the existence of the underlayer (H-type zeolite) contributes to the improvement of heat resistance, and the present invention is effective even when the Cu ion-exchange zeolite pretreatment (heat treatment) is performed. I understand.

<Test 2> -Example 3-FIG. 2 shows the catalyst structure of this example. In the case of the catalyst of this example, the carrier 1 is the same as that of Example 1, but the underlayer 5
Is a H-type zeolite (ZS / SiO ₂ / Al ₂ O ₃ = 30)
M-5). The catalyst layer has a three-layer structure including an upper layer 6, an intermediate layer 7 and a lower layer 8. In each of the upper, middle, and lower layers 6 to 8, each metal-containing silicate is H-type zeolite (ZSM-5), but the ion exchange rates of Cu as a transition metal are 120%, 80% in order from the top, It is 30%. Preparation of the catalyst of this example was carried out according to Example 1.

-Comparative Example 3-As Comparative Example 3, a catalyst having the same catalyst layer structure as that of Example 3 and having no underlayer was prepared.

-Purification test-Using the catalysts prepared as described above as test materials, the initial purification rate (NOx decomposition rate) and the post-heat treatment purification rate (NOx decomposition rate) were measured in the following test modes. The heat treatment in this case was 700 ° C. × 10 hours.

Simulated exhaust gas composition; NOx: 1000 pp
m, HC: 2500 ppm C, O ₂ : 8% space velocity; 25000 h ^{−1 The} results are shown in FIG.

Regarding the initial purification rate, no difference was found between Example 3 and Comparative Example 3. However, N after heat treatment
Regarding the Ox purification rate, the result of Example 3 was higher than that of Comparative Example 3, and particularly in the high temperature range of 450 ° C. or higher, the value of Example was more than doubled. From the above results, simply reducing the amount of the transition metal supported in the catalyst layer toward the lower layer does not improve the heat resistance, and the presence of the underlayer due to the metal-containing silicate not supporting the transition metal is heat resistant. It turns out that it is effective for improvement.

<Test 3> -Example 4-The catalyst structure of this example is shown in FIG. In the case of the catalyst of this example, the carrier 1 and the underlayer 3 are the same as those of Example 1, but the lower three layers 6, 7, 8 of the catalyst layer are the same as those of Example 3, and Na-type ZSM similar to that of Example 1
-5 having _{(SiO 2 / Al 2 O 3} = 30) top layer 9 which was allowed carrying with Cu ion exchange ratio of 140% to.
Then, as the test materials of Example 4, those in which the amount of supported catalyst was changed (the thickness of each of the catalyst layers was changed) were prepared.

-Comparative Example 4- As a test material of Comparative Example 4, a material having the same catalyst layer structure as that of the above-mentioned Example 4 and having no underlayer (catalyst loading amount is different) was prepared.

-Purification test-For each catalyst test material prepared as described above, the initial purification rate (NOx decomposition rate) and the post-heat treatment purification rate (NOx decomposition rate) were measured in the following test modes. The heat treatment in this case was 700 ° C. × 6 hours.

Simulated exhaust gas composition; NO: 2500 pp
m, HC: 2500 ppm C, O ₂ : 2%, CO: 0.
18%, H ₂ : 500 ppm space velocity; 55000 h ^{−1 The} results are shown in FIG.

Regarding the initial purification rate, no difference was found between Example 4 and Comparative Example 4. However, N after heat treatment
Regarding the Ox purification rate, the result of Example 4 was higher than that of Comparative Example 4, regardless of the amount of catalyst supported.

(Test 4 to Test 8) Next, Tests 4 to 8 will be described based on Table 2. First, the meanings of the symbols in each column of Table 2 are as follows.

Column of configuration of catalyst layer Each symbol described side by side through commas in this column represents a catalyst material, the one on the left side is located in the upper layer, and the one on the right side is in the lower layer. Means to be located. "Cu / Z" Na-type zeolite ZSM-5 loaded with Cu at an ion exchange rate of 100% "H-Cu / Z (70)" H-type zeolite ZSM-5 loaded with Cu at an ion exchange rate of 70% What has been "H-Cu / Z (120)" H-type zeolite ZSM-5
Cu loaded with 120% ion exchange rate "Same (80)" H-type zeolite ZSM-5 Cu loaded with 80% ion exchange rate "Same (30)" H-type zeolite ZSM-5 Cu loaded with an ion exchange rate of 30% Catalyst loading amount value (g); Catalyst amount per liter of carrier volume Underlayer column "H / Z (70)" C-ban ratio 70 H type zeolite " H / Z (120) "H-type zeolite having a Cavan ratio of 120 As described above.

[0040]

[Table 2] <Test 4> Purification tests were carried out for Examples 5 and 6 and Comparative Example 5 in which the catalyst layer had a two-layer structure. As a result, the NOx purification rate of Example 5 having the underlayer was twice or more higher than that of Comparative Example 5 having no underlayer. Further, although the heat treatment temperature was increased in Example 6, Comparative Example 5 was still used.
It showed a higher NOx purification rate. From this result, it is supported that the underlayer contributes to the improvement of heat resistance.

<Test 5> Purification tests were conducted on Examples 7 and 8 and Comparative Example 6 in which the catalyst layer had a three-layer structure.
When Example 6 and Example 7 are compared, the latter differs from the former only in that the catalyst layer has a three-layer structure (having the H-type zeolite with a Cu ion exchange rate of 30% as the lowermost layer). However, the result shows that the NOx purification rate is higher in the latter case, that is, in Example 7. This is Example 7
In this case, it is considered that the reaction between the upper layer having a large amount of Cu and the carrier was prevented by the lowermost layer having a small amount of Cu.

Although Example 8 and Comparative Example 6 are different in the presence or absence of the underlayer, this relationship is the same as the relationship between Example 5 and Comparative Example 5, and therefore, the NOx purification rate has the same difference. Has occurred.

<Test 6> With respect to Example 9 and Comparative Example 7 in which the catalyst layer had a three-layer structure, a purification test was conducted by changing the test gas composition from those in Example 8 and Comparative Example 6. As a result, the difference in NOx purification rate showed the same tendency as in Example 8 and Comparative Example 6. Therefore, it can be said that the base layer contributes to the heat resistance even if the composition of the exhaust gas changes.

<Test 7> A purification test was conducted on Example 10 in which the catalyst layer had a four-layer structure. This Example 10
Is different from Example 9 in the structure of the catalyst layer, but N
The Ox purification rate was higher than that in the case of Example 9. These results are also seen between Example 6 and Example 7, but from these results, the transition metal loading amount of the catalyst layer increases stepwise or continuously from the upper layer to the lower layer. It can be said that it is effective to improve the heat resistance.

<Test 8> Purification tests were carried out on Examples 11 to 4 and Comparative Example 8 in which the catalyst layer had a three-layer structure and the Cavan ratio and the catalyst loading amount of the underlayer were changed. in this case,
In Example 11 and Example 12, the catalyst layers have the same structure and the underlayer has a different Cavern ratio. However, Example 11 having a lower underlayer has a higher NOx purification rate. Therefore, it can be said that the Cayvan ratio of the underlayer is 70. Of course, the above-mentioned Example 11 and Example 12 are
The NOx purification rate is higher than that of Comparative Example 8 having no underlayer. Further, comparing Examples 11, 13, and 14,
Although these catalysts have different catalyst loadings, those having a low catalyst loading also have a low NOx purification rate. However, even in Example 14 in which the amount of supported catalyst is the smallest,
More NOx than Comparative Example 8 having a catalyst loading amount more than twice that
High purification rate. This is due to the existence of the underlayer.

<H in the metal-containing silicate of the catalyst layer
Consideration of type and Na type> Cu ion-exchanged zeolite (Na type) having a Na ⁺ amount of 0.30 wt% and Na ⁺ amount of 0.
008 wt% Cu ion-exchanged zeolite (H type) was prepared and subjected to a purification test. The Cu ion exchange rate is 120%.

In preparing the Na type, predetermined amounts of water glass, aluminum sulfate, sodium chloride and tetrapropylammonium bromide (TPAB) are dissolved in water and mixed, and the pH is adjusted to 10 to 1 with an aqueous NaOH solution.
After adjusting to 3, Na-type ZSM-5 powder was obtained by hydrothermal synthesis using an autoclave. Then, after drying and firing this powder (550 ° C. × 3 hours), the powder was subjected to Cu ion exchange treatment under the following conditions to obtain a Na type.

Aqueous copper nitrate solution 0.05 mol / l Powder treatment amount 10 g / l Aqueous solution temperature 50 ° C. Holding time 24 hours On the other hand, for the H type, the Na type ZSM-5 powder was added to 0.05 mol / l ammonium nitrate. By placing in an aqueous solution and stirring with heating (50 ° C. × 5 hours),
An NH ₄ type was prepared, which was dried (120 ° C. × 3 hours), and then subjected to the above Cu ion exchange treatment to obtain it. The concentration of the aqueous ammonium nitrate solution is preferably about 0.005 to 0.1 mol / l, and the remaining amount of Na ⁺ is 0.1.
It is preferable to set it to 01 wt% or less.

As the purification test, an initial activity test and
An activity test was performed after the heat treatment (650 ° C. × 6 hours). Each test condition is A / F = 22, SV = 250
It is 00h- ¹ . Results are shown in FIG.

In the Na type, the activity after the heat treatment is much lower than the initial activity (a decrease of 20 to 30% is seen), whereas in the H type, the activity of the catalyst is almost reduced even after the heat treatment. I can't. From this, it is understood that the H-type is better than the Na-type in terms of heat resistance as the metal-containing silicate of the catalyst layer.

<Countermeasures against carbon poisoning> H in exhaust gas
With respect to the problem that carbon is deposited on the catalyst due to the decomposition of C and the transition metal is poisoned by this, it is preferable that a small amount of noble metal is supported on the catalyst layer. That is,
A purification durability test was conducted using the following simulated exhaust gas for each test material in which a predetermined amount of Ru was carried by a Cu ion-exchanged zeolite (Na type ZSM-5) catalyst by the ion-exchange method.

Simulated exhaust gas composition; NO: 1000 pp
m, C ₃ H ₆ : 1000 ppm, O ₂ : 10% Table 3 shows the Ru loading amount (% by weight based on Cu) of each test material. Further, the test result (the result of the purification test after the exhaust gas has been purified for a predetermined time) is shown in FIG.

[0053]

[Table 3] According to FIG. 7, the test material having a Ru supported amount of 0.05 to 0.5 wt% exhibits high catalytic activity. This is because Ru promotes the combustion of carbon produced by the decomposition of HC and removes the carbon. On the other hand, the catalytic activity is low in both small and large amounts than the above range. It is considered that when the amount is small, the effect of removing carbon by Ru is not sufficiently obtained, and when the amount is large, the combustion of HC proceeds too much and the reducing property of NOx is impaired. Therefore, from the above results, a small amount of Ru
It can be seen that supporting C on the catalyst layer is effective in solving the problem of carbon poisoning. In addition, in supporting the noble metal such as Ru, a method of impregnating an aqueous solution of a noble metal salt may be adopted.

[Brief description of drawings]

FIG. 1 is a sectional view showing a catalyst structure of Example 1.

FIG. 2 is a sectional view showing a catalyst structure of Example 3.

FIG. 3 is a graph showing the results of test 2 (cleaning test).

FIG. 4 is a sectional view showing a catalyst structure of Example 4.

FIG. 5 is a graph showing the test 3 (purification test) K result.

FIG. 6 is a graph comparing the catalytic activities of Na-type zeolite and H-type zeolite.

FIG. 7 is a graph chart comparing the activities of catalysts having different loadings of noble metal.

[Explanation of symbols]

 1 carrier 2 catalyst layer 3,5 underlayer

Claims (2)

[Claims] 1. An exhaust gas purifying catalyst in which a catalyst layer comprising a metal-containing silicate carrying a transition metal is provided on a cordierite carrier, wherein the transition metal is present between the catalyst layer and the carrier. An exhaust gas purifying catalyst, characterized in that an underlayer of a metal-containing silicate that does not support is provided. 2. The exhaust gas purifying catalyst according to claim 1, wherein the catalyst layer is made of zeolite in which Cu is carried by ion exchange, and the underlayer is made of H-type zeolite.