JPH08281071A - Waste gas purifying method and waste gas purifying catalyst - Google Patents

Abstract

PURPOSE: To maintain the performance of a ternary catalyst close to stoichiometry almost equal to that of the conventional one without using expensive Pt or Rh and to improve the NOx purifying performance in the lean region close to stoichiometry. CONSTITUTION: A waste gas is first brought into contact with a first catalyst 1 carrying Pd and Mo and then with a second catalyst 2 carrying Pd. NOx is efficiently reduced and purified by the first catalyst 1, and the remaining HC and CO are efficiently oxidized and purified by the second catalyst 2.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to nitrogen oxides (N) contained in exhaust gas discharged from internal combustion engines such as automobiles.
The present invention relates to an exhaust gas purification method and an exhaust gas purification catalyst for simultaneously purifying three components of O _x ), hydrocarbon (HC) and carbon monoxide (CO).

[0002]

During exhaust gas discharged from an internal combustion engine such as an automobile, NO _x, HC, contains harmful components CO. Therefore, it is necessary to reduce or oxidize these harmful components to change them into harmless components, and conventionally, an exhaust gas purifying catalyst has been used by being arranged in an exhaust gas passage.

An exhaust gas purifying catalyst for purifying NO _x , HC and CO at the same time is generally called a three-way catalyst. For example, a porous carrier layer made of γ-alumina is formed on a heat resistant monolith substrate made of cordierite or the like. It is widely known that the porous carrier layer carries a catalytic precious metal such as platinum (Pt) or rhodium (Rh). For example, JP-A-64-
Japanese Patent No. 58347 discloses an exhaust gas purifying catalyst containing at least one platinum group element as a catalytic metal component and activated alumina, cerium oxide, a barium compound and a zirconium compound. And as a platinum group element,
It is described that Pt and Rh are preferably contained for the purpose of simultaneously purifying NO _x , HC and CO.

However, Rh is an expensive metal, and Pt is the second most expensive element in the platinum group elements,
The exhaust gas-purifying catalyst containing Pt and Rh has a problem of being extremely expensive. Therefore, there has been a demand for the development of an exhaust gas purifying catalyst that does not contain Pt or Rh but has a purifying performance equivalent to that containing Pt or Rh. Then, JP-A-6
Japanese Patent Publication No. 99069-A discloses an exhaust gas purifying catalyst carrying palladium (Pd), activated alumina, cerium oxide and barium oxide. According to this exhaust gas purifying catalyst, activated alumina, cerium oxide, and barium oxide assist the catalytic action of Pd in an auxiliary manner, so that the exhaust gas purifying catalyst exhibits purifying performance equivalent to that of a three-way catalyst containing Pt and Rh. Moreover, since the cost of Pd is lower than that of Rh or Pt, the catalyst becomes an inexpensive exhaust gas purifying catalyst.

Also, Applied Catalysis B: Environmental,
The 2 (1993) 131-146, PdO- MoO 3 / γ-Al 2 O
There is a research report on the reduction and purification action of NO _x on the above ₃ .
This NO _x purification catalyst is constituted by impregnating an alumina carrier with Pd at 2 wt% and Mo at 20 wt%, can suppress the generation of ammonia in the rich region, and can also reduce NO _x in the lean region. It can be efficiently reduced and purified. Moreover, since Pt and Rh are not used, it is inexpensive.

[0006]

[Patent Document 1] Japanese Unexamined Patent Application Publication No.
In the exhaust gas-purifying catalyst disclosed in Japanese Patent Laid-Open No. -99069, Pd is oxidized in the lean region near stoichiometry so that the active sites on the Pd surface are almost occupied by oxygen, and oxygen is preferentially over NO _x. Since it is activated, there is a problem that the reducing and purifying action of NO _x in that region is low.

Also, Applied Catalysis B: Environmental,
2 (1993) 131-146, the catalyst is in the lean region.
NO_xAlthough it has excellent purification performance,
NO catalytic activity in the vicinity, NO_x, HC, CO
It is difficult to purify the components simultaneously and at a sufficiently high purification rate.
It was difficult. The present invention has been made in view of such circumstances.
Without using expensive Pt or Rh,
NO near one stoichiometry_x, HC, CO purification rate
While maintaining the same level as a conventional three-way catalyst,
NO in the lean area beside _xCan be purified with a high purification rate
The purpose is to do so.

[0008]

Means for Solving the Problems A first method for solving the above problems is described below.
The exhaust gas purification method of the invention is characterized in that the exhaust gas is first contacted with a first catalyst having palladium and molybdenum supported on a carrier and then contacted with a second catalyst having palladium supported on the carrier to be purified. The exhaust gas purifying catalyst of the second invention capable of carrying out this exhaust gas purifying method comprises a first catalyst in which palladium and molybdenum are supported on a carrier and a second catalyst in which palladium is supported on a carrier. It is characterized in that the first catalyst is arranged at the first side and the second catalyst is arranged at the downstream side.

The exhaust gas purifying catalyst of the third invention capable of carrying out the exhaust gas purifying method of the first invention is a second catalyst layer carrying palladium and a first catalyst carrying palladium and molybdenum formed on the surface of the second catalyst layer. And a catalyst layer.

[0010]

In the exhaust gas purification method of the first invention, the exhaust gas first comes into contact with the first catalyst carrying Pd and Mo. On the first catalyst, it is considered that the reactivity of HC and CO on the Pd surface with NO _x is improved by the electronic interaction between Mo and Pd, and as a result, NO _x is efficiently reduced and purified to nitrogen even in the lean region. can do.

At this time, a portion of HC and CO in the exhaust gas acts as a reducing agent for NO _x and is purified, and is also oxidized and purified by O _{2 in} the exhaust gas.
The exhaust gas from which NO _x has been reduced and purified is then contacted with the second catalyst carrying Pd. On the second catalyst, HC and CO remaining in the exhaust gas are efficiently oxidized and purified by Pd.

In the exhaust gas purifying catalyst of the second invention, since the first catalyst carrying Pd and Mo is arranged on the upstream side of the exhaust gas passage, the exhaust gas first comes into contact with the first catalyst. Therefore, it is considered that the reactivity of HC and CO on the Pd surface with NO _x is improved by the electronic interaction between Mo and Pd on the first catalyst, and as a result, NO _x is efficiently reduced to nitrogen even in the lean region. Can be purified.

At this time, a part of HC and CO in the exhaust gas is purified by acting as a reducing agent for NO _x , and is also oxidized and purified by O _{2 in} the exhaust gas.
Then, the exhaust gas, which has passed through the first catalyst and is reduced and purified of NO _x, is brought into contact with the second catalyst which is arranged on the downstream side and carries Pd. On the second catalyst, HC and CO remaining in the exhaust gas are efficiently oxidized and purified by Pd.

In the exhaust gas purifying catalyst of the third invention, the exhaust gas first comes into contact with the first catalyst layer existing on the outermost surface. Since Pd and Mo are supported on the first catalyst layer, Mo and Pd
HC and C on the Pd surface due to electronic interaction with
It is considered that the reactivity between O and NO _x is improved, and as a result, NO _x can be efficiently reduced and purified into nitrogen even in the lean region.

At this time, a part of HC and CO in the exhaust gas acts as a reducing agent for NO _x and is purified, and is also oxidized and purified by O _{2 in} the exhaust gas.
Then, the exhaust gas, which has passed through the first catalyst layer and is reduced and purified of NO _x, is brought into contact with the second catalyst layer located in the lower layer. Second
Since Pd is carried on the catalyst layer, HC and CO remaining in the exhaust gas are efficiently oxidized and purified by Pd.

Further, in the exhaust gas purifying catalyst of the third invention, it is not necessary to separately manufacture two kinds of catalysts and arrange them separately in the exhaust gas passage as in the second invention. Therefore, an exhaust gas purifying catalyst of the same size as the conventional one can be obtained, and the exhaust gas purifying system can be directly adapted to the conventional exhaust gas purifying system.

[0017]

【Example】

Specific Example of the Invention The first catalyst or the first catalyst layer may be configured such that Pd and Mo are supported on a porous carrier made of a material such as alumina, titania, silica, silica-alumina, zirconia, or zeolite. it can. Alumina is particularly preferable as the material of the carrier. The amount of Pd supported on the first catalyst or the first catalyst layer is preferably in the range of 0.1 to 10 g per 100 g of the carrier when the carrier is alumina.
If it is less than 0.1 g, the desired catalytic activity cannot be obtained, and if it exceeds 10 g, the effect is saturated and the material cost increases, which is not preferable.

Further, it is supported on the first catalyst or the first catalyst layer.
The supported amount of Mo is 1 to 20 times the molar ratio of Pd.
Is good. Below the same mole as Pd, in the lean region
NO _xPurification activity decreases, and the molar ratio exceeds 20 times
Then, ternary activity (HC, CO, NO in stoichiometry)_x
Purification activity of) decreases. About 10 times the molar amount supported
Is the best.

The second catalyst or the second catalyst layer may have a structure in which Pd is supported on a porous carrier made of a material such as alumina, titania, silica, silica-alumina, zirconia, or zeolite. Alumina is particularly preferable as the material of the carrier. Second catalyst or Pd supported on the second catalyst layer
When the carrier is alumina, the supported amount of is preferably 0.1 to 10 g per 100 g of the carrier. If it is less than 0.1 g, the desired catalytic activity cannot be obtained, and if it exceeds 10 g, the effect is saturated and the material cost increases, which is not preferable.

The shape of the first catalyst or the second catalyst is not limited, and a monolith type and a pellet type can be freely combined and used. For example, a monolith-type catalyst can be produced by forming a coat layer from a carrier such as alumina on a monolith substrate made of cordierite and impregnating and supporting Pd or Mo on the coat layer. As the base material, a metal carrier base material formed of a metal foil can also be used.

Further, the coating layer may contain ceria, lanthanum carbonate, barium acetate or the like. Since ceria has an oxygen storage / release function, when the atmosphere deviates from stoichiometric, oxygen is stored / released to maintain the atmosphere in stoichiometry. As the content of ceria, for example, 50 to 150 g is suitable for 100 g of the alumina carrier. In addition, barium and lanthanum have the effect of enhancing the catalytic activity of Pd through electronic interaction with Pd, and are used in an amount of 0.2 mol or 30 per 100 g of alumina carrier.
A content of about g is suitable.

To form the first catalyst layer and the second catalyst layer, for example, a cordierite monolith substrate is coated with a slurry of alumina or the like, and Pd is impregnated and supported to form the second catalyst layer. Then, the slurry is coated again, and Pd and Mo are impregnated and supported to form the first catalyst layer, which facilitates production. When a pellet made of alumina or the like is used as the base material, Pd is first impregnated and supported to form a second catalyst layer, and then a coat layer is formed and then Pd is formed.
It can be manufactured by supporting and Mo. Similarly to the above, it is also preferable that the first catalyst layer and the second catalyst layer contain ceria, lanthanum, barium and the like. [Examples] Hereinafter, specific examples will be described. (Example 1) (1) Preparation of first catalyst powder To 5.0 g of alumina carrier powder, 2.5 ml of an aqueous palladium nitrate solution having a concentration of 50 g / l was added, and distilled water was further added to about 50 parts.
After adding ml, the mixture was stirred at room temperature for about 5 hours. The obtained suspension was heated at 110 ° C. overnight to be dried, and was calcined in the air at 500 ° C. for 3 hours to prepare a Pd-supported alumina powder. The amount of Pd carried is 2.5 g of metal Pd per 100 g of alumina carrier.

Then 2.07 g of (NH ₄ ) ₆ Mo ₇ O ₂₄
4H ₂ O was dissolved in 50 ml of distilled water, 5.0 g of the above Pd-supported alumina powder was added, and the mixture was stirred at room temperature for about 5 hours. The obtained suspension was heated at 110 ° C. overnight to be dried, and then dried in a nitrogen gas containing 1% by volume of hydrogen gas at a temperature of 500.
A reduction treatment was performed by heating at 0 ° C. for 3 hours. The amount of Mo supported was 22.50 as metallic Mo per 100 g of the alumina carrier.
It is 5 g. (2) Preparation of second catalyst powder To 5.0 g of alumina carrier powder, 2.5 ml of a palladium nitrate aqueous solution having a concentration of 50 g / l was added, and distilled water was further added to about 50
After adding ml, the mixture was stirred at room temperature for about 5 hours. The obtained suspension was heated at 110 ° C. overnight to be dried, and calcined in the air at 500 ° C. for 3 hours. The amount of Pd carried is 2.5 g of metal Pd per 100 g of alumina carrier. (3) Test Example Using a normal atmospheric fixed bed flow reactor, the first catalyst powder 1 was placed on the upstream side of the gas flow path and the second catalyst powder 2 was placed on the downstream side as shown in FIG. I placed it. The amount of each of the first catalyst powder 1 and the second catalyst powder 2 used was 0.25 g, and the total amount was 0.5 g.

Then, using 23 kinds of exhaust model gases from the rich region to the lean region shown in Table 1, flowing at a flow rate of 3.3 liters / min at 400 ° C., HC, CO
And the purification rate of NO was measured. The results are shown in Figure 2. In Table 1 and FIG. 2, λ is λ = air-fuel ratio (A / F)
The value calculated by /14.6 is λ <1.0 is the rich region, λ = 1.0 is the stoichiometric region, and λ> 1.0 is the lean region.

[0025]

[Table 1] (Unit: vol%) (Comparative Example 1) The second catalyst powder of Example 1 was used as the catalyst of Comparative Example 1, and 0.25 g of each catalyst was placed on the upstream side and the downstream side to obtain HC, The purification rates of CO and NO were measured. The results are shown in Fig. 3. (Comparative Example 2) The first catalyst powder of Example 1 was used as the catalyst of Comparative Example 1, and 0.25 g of each was placed on the upstream side and the downstream side, and HC, CO and NO purification rates were the same as in Example 1. Was measured. FIG. 4 shows the results. In the case of this comparative example 2, the amount of Mo used is the total of the upstream side and the downstream side, which is twice as much as that of the example 1. (Evaluation) Comparing FIG. 2 and FIG. 3, it can be seen that in Comparative Example 1, the NO purification rate in the lean region is lower than that in Example 1 and Comparative Example 2. In other words, by supporting Mo as in Example 1 and Comparative Example 2, NO in the lean region
It is clear that the purification performance of is improved.

Comparing FIG. 2 with FIG. 4, there is no difference in the NO purification performance in the lean region, but in the rich region and stoichiometry, the first embodiment has HC, CO and NO.
It can be seen that all of the purification performance of (3 way purification performance) is excellent. From the above results, according to the purification method of Example 1, the purification performance of HC and CO is excellent in all regions from the rich region to the lean region, and the purification performance of NO in the stoichiometric region from the rich region is also excellent. Furthermore, it is clear that a high NO purification rate is exhibited even in the lean region near stoichiometry, and these excellent three-way purification characteristics show that the exhaust gas is first contacted with the first catalyst supporting Pd and Mo, and then Pd is supported. It is clear that this is the effect of contact with the second catalyst. (Aspects of Examples) In the above examples, tests were carried out using catalyst powders for convenience, but similar effects can be obtained with the following catalysts. (Aspect Example 1) A cordierite honeycomb-shaped monolithic carrier substrate is used, which is dipped in alumina slurry and pulled up to blow off excess slurry, and then dried and fired to form a coat layer.

A carrier having this coat layer is impregnated with a predetermined amount of an aqueous solution of palladium nitrate having a predetermined concentration, and then baked to carry Pd. Next, a predetermined concentration of (NH ₄ ) ₆ Mo ₇ O
_A predetermined amount of 24.4 H ₂ O aqueous solution is impregnated and baked to support Mo. As a result, the first catalyst supporting Pd and Mo is manufactured. Further, in the same manner as above, a monolith catalyst in which only Pd is supported is produced and is used as the second catalyst. Then, by arranging the first catalyst on the upstream side of the exhaust gas channel and the second catalyst on the downstream side, excellent purification performance can be obtained as in the first embodiment. The first catalyst and the second
The catalyst may be arranged in series as shown in FIG. 1 or may be arranged at a predetermined distance from each other. (Aspect Example 2) FIG. 5 shows an enlarged cross-sectional view of a main part of the exhaust gas purifying catalyst of this example. This exhaust gas-purifying catalyst comprises a cordierite monolith carrier substrate 3 having a honeycomb passage 30,
Second formed on the surface of the monolith carrier substrate 3 and carrying Pd
It is composed of a catalyst layer 4 and a first catalyst layer 5 formed on the surface of the second catalyst layer 4 and carrying Pd and Mo. This exhaust gas-purifying catalyst is manufactured as follows.

Using a honeycomb-shaped monolithic carrier substrate 3 made of cordierite, it is immersed in an alumina slurry and pulled up to blow off excess slurry, followed by drying and firing to form a coat layer. A carrier having this coat layer is impregnated with a predetermined amount of an aqueous solution of palladium nitrate and then fired to form a second catalyst layer 4 carrying Pd.

Next, a coat layer is formed from alumina slurry on the surface of the second catalyst layer 4 in the same manner as above. This is impregnated with a predetermined amount of a palladium nitrate aqueous solution, and then baked to carry Pd. Next, a predetermined concentration of (NH ₄ )
_{6 Mo 7 O 24 · 4H 2} O aqueous solution was a predetermined amount impregnated, supporting the Mo and fired. As a result, the first catalyst layer 5 supporting Pd and Mo is formed.

In the exhaust gas purifying catalyst of this example, the exhaust gas flowing through the honeycomb passage 30 is first contacted with the first catalyst layer 5 carrying Pd and Mo to be purified, and then with the second catalyst layer 4 carrying Pd. Since it is contacted and purified, Example 1
It also shows excellent three-way purification performance. (Aspect Example 3) FIG. 6 shows an exhaust gas purifying catalyst of this example. This exhaust gas-purifying catalyst is composed of a pellet carrier substrate 6 (second catalyst layer) carrying Pd, and a first catalyst layer 7 formed on the surface of the pellet carrier substrate 6 and carrying Pd and Mo. There is.

In order to produce this exhaust gas purifying catalyst, an alumina pellet carrier base material is prepared, impregnated with a predetermined amount of an aqueous palladium nitrate solution having a predetermined concentration, and then fired to obtain Pd.
Carry. Then, on the surface of the pellet carrier substrate 6 on which Pd is carried, a coat layer is formed from the alumina slurry in the same manner as described above, and Pd and Mo are similarly carried to form the first catalyst layer 7.

Also in the exhaust gas purifying catalyst of this example, the exhaust gas flowing on the catalyst surface is first contacted with the first catalyst layer 7 carrying Pd and Mo to be purified, and then contacted with the pellet carrier substrate 6 carrying Pd. As a result, the same three-way purification performance as in Example 1 is exhibited.

[0033]

According to the exhaust gas purifying method and the exhaust gas purifying catalyst of the present invention, the purifying performance superior to that of the conventional three-way catalyst can be obtained without using expensive Pt or Rh. , N in the lean region near stoichiometry
The O _x purification rate can be improved.

[Brief description of drawings]

FIG. 1 is an explanatory diagram showing a state of arrangement of a first catalyst and a second catalyst in a gas flow path in one embodiment of the present invention.

FIG. 2 is a graph showing the air-fuel ratio characteristic of the three-way catalyst performance in the example of the present invention.

FIG. 3 is a graph showing an air-fuel ratio characteristic of three-way catalyst performance in Comparative Example 1 of the present invention.

FIG. 4 is a graph showing an air-fuel ratio characteristic of three-way catalyst performance in Comparative Example 2 of the present invention.

FIG. 5 is a cross-sectional view of an essential part showing an embodiment of an exhaust gas purifying catalyst of the present invention.

FIG. 6 is a sectional view showing one embodiment of an exhaust gas purifying catalyst of the present invention.

[Explanation of symbols]

1: First catalyst powder 2: Second catalyst powder
3: Monolith carrier substrate 4: Second catalyst layer 5: First catalyst layer
6: Pellet carrier base material 7: First catalyst layer 30: Honeycomb passage

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁶ Identification code Internal reference number FI Technical display location F01N 3/28 301 B01D 53/36 102H 102A 102B 104A (72) Inventor Hirofumi Shinjo Nagakute, Aichi-gun, Aichi Prefecture 1 in 41 Chuo-dori, Chozai, Chodai, Toyota Central Research Institute Co., Ltd. (72) Inventor Hiroshi Hirayama 1 Toyota Town, Toyota City, Aichi Prefecture Toyota Automobile Co., Ltd.

Claims (3)

[Claims] 1. A method for purifying exhaust gas, which comprises first contacting an exhaust gas with a first catalyst having palladium and molybdenum supported on a carrier and then contacting a second catalyst having palladium supported on the carrier for purification. 2. A first catalyst comprising palladium and molybdenum supported on a carrier and a second catalyst comprising palladium supported on the carrier, wherein the first catalyst is disposed on the upstream side of the exhaust gas passage and the first catalyst is disposed on the downstream side. An exhaust gas-purifying catalyst characterized in that two catalysts are arranged. 3. An exhaust gas purifying catalyst, comprising: a second catalyst layer supporting palladium and a first catalyst layer formed on the surface of the second catalyst layer supporting palladium and molybdenum.