JPH0938493A - Catalyst for purification of exhaust gas - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain a catalyst excellent in durability and preventing the deterioration of NOx removing performance by carrying a catalytic noble metal and an NOx storage material on a carrier made of Co and/or La oxide combined with alumina. SOLUTION: At least one of Co and La is added to alumina and fired to form Co and/or La oxide combined with alumina. A catalytic noble metal such as Pt and an NOx storage material such as an alkali metal or an alkaline earth metal are carried on a carrier made of the combined oxide. Since the stability of the alumina is increased and the reaction of the alumina with the NOx storage material is prevented even at a high temp., the deterioration of the function of the NOx storage material is prevented. The specific surface area of the carrier is hardly reduced and the sintering of the catalytic noble metal is also 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 for purifying exhaust gas discharged from an internal combustion engine, and more specifically, N having an action of absorbing and releasing NO _x in exhaust gas.
The present invention relates to an exhaust gas purifying catalyst carrying an O _x storage material.

[0002]

2. Description of the Related Art Conventionally, CO has been used as an exhaust gas purifying catalyst.
In addition, a three-way catalyst that purifies exhaust gas by oxidizing HC and reducing NO _x is used. As such a three-way catalyst, for example, a porous carrier layer made of γ-alumina is formed on a heat-resistant substrate made of cordierite or the like, and platinum (Pt), rhodium (Rh), or the like is formed on the porous carrier layer. What carried the catalyst noble metal is widely known.

On the other hand, in recent years, from the viewpoint of protecting the global environment, carbon dioxide (CO ₂ ) in exhaust gas discharged from internal combustion engines such as automobiles has become a problem, and as a solution to this problem, so-called lean combustion in which lean combustion is performed in an oxygen excess atmosphere is performed. Burn is promising. In this lean burn, the fuel consumption is reduced, so that the fuel efficiency is improved and the generation of CO ₂ which is the combustion exhaust gas can be suppressed.

On the other hand, the conventional three-way catalyst purifies by simultaneously oxidizing and reducing CO, HC and NO _x in the exhaust gas burned by the air-fuel mixture controlled to the stoichiometric air-fuel ratio (stoichiometric ratio). Therefore, it does not show sufficient purification performance for reducing and removing NO _x in the oxygen rich atmosphere of exhaust gas during lean burn. Therefore, it is desired to develop an exhaust gas purifying catalyst and an exhaust gas purifying system that can efficiently purify NO _x even in an oxygen excess atmosphere.

Therefore, the applicant of the present invention first found that barium (NO
_A catalyst for exhaust gas purification in which _x storage material) and Pt are supported on alumina (for example, JP-A-6-142458) is proposed. According to this exhaust gas purifying catalyst, NO on the lean side
_{Since x} is absorbed by the NO _x storage material, which is released on the stoichiometric or rich side and reacts with reducing components such as HC and CO, good purification performance of NO _x can be obtained on the lean side as well.

[0006]

However, the conventional NO
_{x In} the exhaust gas purifying catalyst supporting the occlusion material, 80
At a temperature of around 0 ° C., a reaction occurs between the NO _x storage material and alumina, and a compound having a low melting point is produced. As a result, the specific surface area of the alumina carrier is reduced and the pores are clogged, and the sintering of the catalytic precious metal also occurs,
In some cases, the purification performance of NO _{x in} the exhaust gas may decrease.

The present invention has been made in view of such circumstances, and an object thereof is to provide an exhaust gas purifying catalyst capable of preventing deterioration of NO _x purification performance after endurance.

[0008]

The features of the exhaust gas purifying catalyst of the present invention for solving the above-mentioned problems are that a carrier comprising a composite oxide of at least one of cobalt (Co) and lanthanum (La) and alumina, and a carrier. The catalyst is composed of a noble metal catalyst and an NO _x storage material carried on the substrate.

[0009]

BEST MODE FOR CARRYING OUT THE INVENTION The carrier characterizing the present invention is Co
And a composite oxide of at least one of La and alumina. By using such a carrier,
Although the reason is unknown, it is considered that the stability of alumina is increased, and the reaction between the NO _x storage material and alumina is prevented even at high temperature, so that the function of the NO _x storage material is prevented from being impaired. Further, the specific surface area of the carrier is less reduced, and the sintering of the catalytic noble metal is prevented.

The content of at least one of Co and La in this carrier is 100% by weight of the carrier as the amount of one element contained when only one of them is contained, or as the total amount of both when both are contained. Over 3% by weight in 1
It is preferably in the range of less than 0% by weight. If it is 3% by weight or less, the effect of addition is small, the reaction between the NO _x storage material and alumina is likely to occur, and the improvement of thermal stability is small.
On the other hand, if it is 10% by weight or more, the specific surface area of the carrier is reduced, and the heat resistance is rather reduced as compared with the conventional exhaust gas purifying catalyst containing only alumina.

To make the carrier such a composite oxide,
A composite oxide with alumina can be formed by adding at least one element of Co and La to alumina and firing at 800 ° C. or higher. If the firing temperature is less than 800 ° C,
When the formed exhaust gas-purifying catalyst is exposed to a temperature higher than that, the specific surface area is reduced, and the reaction between alumina and the NO _x storage material is likely to occur. However, if the calcination temperature exceeds 1000 ° C., the specific surface area of the obtained exhaust gas-purifying catalyst itself is small, and the initial purification performance will be low. Therefore, it is desirable to perform firing at a temperature in the range of 800 to 1000 ° C.

As the catalyst noble metal, for example, one or more of Pt, Pd and Rh can be used in combination. The supported amount is the amount of the carrier material 10 as the total amount of the noble metal catalyst.
0.2-40 g is preferable with respect to 0 g, and 1-20 g is particularly preferable. If converted to a volume of 1 liter of the exhaust gas purifying catalyst, 0.24 to 48 g is preferable,
1.2 to 24 g is particularly preferable.

Even if the supported amount of the catalytic noble metal is further increased, the activity is not improved and the effective utilization cannot be achieved. On the other hand, if the supported amount of the catalytic noble metal is less than this, practically sufficient activity cannot be obtained. In order to support the catalytic noble metal on the carrier, it is possible to carry it in the same manner as in the conventional method by using a solution of its chloride, nitrate or the like and utilizing an impregnation method, a spraying method, a slurry mixing method or the like.

As the NO _x storage material, at least one selected from alkali metals, alkaline earth metals and rare earth elements can be used. Alkali metals include lithium, sodium, potassium, rubidium, cesium,
Francium is mentioned. Further, the alkaline earth metal refers to an element of Group 2A of the periodic table, and examples thereof include barium, beryllium, magnesium, calcium, and strontium. Examples of the rare earth element include scandium, yttrium, lanthanum, cerium, praseodymium, and neodymium.

In order to support the NO _x storage material on the carrier, an aqueous solution of a water-soluble salt such as acetate is used in the same manner as in the conventional method by using an impregnation method, a spraying method, a slurry mixing method or the like. be able to.

[0016]

EXAMPLES The present invention will be described in more detail below with reference to examples. (Embodiment 1) Hereinafter, a method for producing an exhaust gas purifying catalyst according to Embodiment 1 will be described to replace the detailed description of the structure.

Alumina powder was impregnated with a predetermined amount of a cobalt nitrate aqueous solution having a predetermined concentration, dried at 110 ° C. for 2 hours, and then dried.
The powder was calcined at 0 ° C. for 5 hours, and a carrier powder composed of a composite oxide of Co and alumina was prepared by a dry evaporation method. The content of Co in the carrier powder is 1% by weight. Next, the carrier powder is impregnated with a predetermined amount of a dinitrodiammine platinum aqueous solution of a predetermined concentration and dried at 110 ° C. for 2 hours, then impregnated with a predetermined amount of a rhodium nitrate aqueous solution of a predetermined concentration, dried at 110 ° C. for 2 hours, and then at 300 ° C. Baking for 1 hour, 1.5 wt% Pt and R
0.1% by weight of h was supported. Further, a predetermined amount of a barium acetate aqueous solution having a predetermined concentration was impregnated, dried at 110 ° C. for 2 hours, and then baked at 500 ° C. for 2 hours to carry 20 wt% of Ba.

The catalyst powder thus obtained is compacted to give 0.5-
A 1 mm pellet catalyst was prepared as the pellet catalyst of Example 1. (Example 2) Co was changed by changing the concentration of the cobalt nitrate aqueous solution.
A pellet catalyst of Example 2 was prepared in the same manner as in Example 1 except that the content of was 3% by weight. (Example 3) Changing the concentration of the cobalt nitrate aqueous solution to Co
Was set to 5% by weight, and the firing temperature after Co was set to 800 ° C.
The pellet catalyst of Example 3 was prepared. (Example 4) Changing the concentration of the cobalt nitrate aqueous solution to Co
A pellet catalyst of Example 4 was prepared in the same manner as in Example 1 except that the content of was 5% by weight. (Example 5) Co was prepared by changing the concentration of the cobalt nitrate aqueous solution.
A pellet catalyst of Example 5 was prepared in the same manner as in Example 1 except that the content of was 7% by weight. (Example 6) Co was prepared by changing the concentration of the cobalt nitrate aqueous solution.
A pellet catalyst of Example 6 was prepared in the same manner as in Example 1 except that the content of was 10% by weight. (Example 7) Co was prepared by changing the concentration of the cobalt nitrate aqueous solution.
The pellet catalyst of Example 7 was prepared in the same manner as in Example 1 except that the content of was set to 5% by weight and the calcination temperature after containing Co was set to 1000 ° C. (Comparative Example 1) A pellet catalyst of Comparative Example 1 was prepared in the same manner as in Example 1 except that Co was not used and alumina powder was used as the carrier powder and the carrier was not calcined. (Comparative Example 2) Co was prepared by changing the concentration of the cobalt nitrate aqueous solution.
Was set to 5% by weight and the firing temperature after Co was set to 700 ° C.
The pellet catalyst of Comparative Example 2 was prepared. (Test / Evaluation) After the initial NO _x purification rate of each of the above pellet catalysts was measured, and after the endurance treatment of heating at 900 ° C. for 5 hours in an oxygen excess atmosphere under lean conditions shown in Table 1 was performed. The NO _x purification rate was measured. The measurement of the NO _x purification rate was performed under transient conditions in which two kinds of lean (A / F = 18) and rich (A / F = 14) model gases shown in Table 1 were alternately flowed for 2 minutes each. Inlet gas temperature is 350
C., space velocity SV = 100,000 / h. Table 2 shows the results.

[0019]

[Table 1]

[0020]

[Table 2] Except for Example 6, the pellet catalyst of each Example had N after durability.
It can be seen that the O _x purification rate is higher than that of Comparative Example 1, and the NO _x purification performance is excellent. It is clear that this is the effect of using the Co-alumina carrier. In Example 6, the specific surface area of the carrier is reduced because Co is contained in a large amount of 10% by weight, and the NO _x purification rate after endurance is lower than that of Comparative Example 1 having no Co. However, even in the case of Example 6, by increasing the firing temperature a little more, N
The O _x purification rate may be improved.

Further comparing the examples, when the Co content is 3% by weight or less, the NO _x purification rate after endurance is less than 50%, and Co exceeds 3% by weight and 10% by weight.
It can be seen that the range below is particularly preferable. Further, comparing with Comparative Example 2 and Example 4, towards the fourth embodiment is superior to _the NO _x purification ratio after endurance. This is due to the difference in firing temperature. In Comparative Example 2, since firing was performed at a low temperature of 700 ° C., Co and alumina did not form a composite oxide, whereas in Example 4, firing was performed at 900 ° C. Therefore, it is due to the difference that Co and alumina are present in the carrier as a complex oxide.

The ratio of Example 3 to Example 4 and Example 7
By comparison, 900 ℃ is more durable after 800 ℃ than NO_xPurification
When the conversion rate becomes high and reaches 1000 ° C
NO _xIt can be seen that the purification rate decreases. Therefore optimal
Presumably the firing temperature is between 800 and 1000 ℃
Is done. (Example 8) 100 parts by weight of alumina powder and alumina
70 parts by weight of 10% by weight alumina sol and 40% concentration
15 parts by weight of an aluminum nitrate aqueous solution of 30% by weight and 30 parts of water
Parts by weight were mixed to form a slurry. And Kojirai
Immerse a monolithic honeycomb monolith carrier in this slurry
Then, pull up and blow off the excess slurry.
Dry for 2 hours at ℃ and calcine for 2 hours at 650 ℃ to obtain alumina
The coating layer was formed. The alumina coating layer is
120 g are formed per liter of Norris carrier.

Next, the monolith carrier having the above-mentioned alumina coating layer was dipped in a cobalt nitrate aqueous solution having a predetermined concentration, pulled up to blow off excess water drops, dried at 110 ° C. for 2 hours, and calcined at 950 ° C. for 5 hours. To form a coat layer made of Co-alumina composite oxide. The Co content in the coat layer is 5% by weight. Then, the monolith carrier having a coat layer is dipped in a dinitrodiammineplatinum aqueous solution having a predetermined concentration, pulled up and blown off excess water droplets, and then 1
It was dried at 10 ° C. for 2 hours. Further, after immersing in a rhodium nitrate aqueous solution having a predetermined concentration, pulling it up and blowing off excess water droplets, it was dried at 110 ° C. for 2 hours and calcined at 300 ° C. for 1 hour to obtain Pt of 1.5% by weight and Rh of 0. 1 wt% was supported. Further, after immersing in an aqueous solution of barium acetate having a predetermined concentration, pulling it up and blowing off excess water droplets, it was dried at 110 ° C. for 2 hours and baked at 500 ° C. for 2 hours to carry 20 wt% of Ba, and the monolith of the present example. A catalyst was prepared. (Comparative Example 3) A monolith catalyst of Comparative Example 3 was prepared in the same manner as in Example 8 except that Co was not used and the coating layer was formed only from alumina and the carrier was not baked. (Test / Evaluation) The monolith catalysts of Example 8 and Comparative Example 3 were arranged in the exhaust system of an actual vehicle (lean burn engine (1.6 liter) lean burn engine), and initial NO _x purification was carried out when running in 10/15 mode. The rate was measured. Also, they are arranged in the exhaust system of the same type of engine, respectively, and the incoming gas temperature is 850
The NO _x purification rate after a 20-hour durability test was measured in the same manner as the initial purification rate. The results are shown in Table 3.

[0024]

[Table 3] From Table 3, it is clear that the monolith catalyst of Example 8 has a higher NO _x purification rate after endurance as compared with Comparative Example 3, which is an effect of using the Co-alumina composite oxide carrier. (Example 9) Alumina powder was impregnated with a predetermined amount of a lanthanum nitrate aqueous solution having a predetermined concentration, and dried at 110 ° C for 2 hours, and then 90
The powder was calcined at 0 ° C. for 5 hours, and a carrier powder composed of a composite oxide of La and alumina was prepared by an evaporation-drying method. The content of La in the carrier powder is 1% by weight.

Next, the carrier powder was impregnated with a predetermined amount of a dinitrodiammineplatinum aqueous solution of a predetermined concentration and dried at 110 ° C. for 2 hours, and then impregnated with a predetermined amount of a rhodium nitrate aqueous solution of a predetermined concentration and dried at 110 ° C. for 2 hours. It was calcined at 300 ° C. for 1 hour to carry 1.5 wt% of Pt and 0.1 wt% of Rh. Further, a predetermined amount of a barium acetate aqueous solution having a predetermined concentration was impregnated, dried at 110 ° C. for 2 hours, and then baked at 500 ° C. for 2 hours to carry 20 wt% of Ba.

The obtained catalyst powder is compacted into a powder, and 0.5 to
A 1 mm pellet catalyst was prepared and used as the pellet catalyst of Example 9. (Example 10) L was changed by changing the concentration of the lanthanum nitrate aqueous solution.
A pellet catalyst of Example 10 was prepared in the same manner as in Example 9 except that the content of a was 3% by weight. (Example 11) L was changed by changing the concentration of the lanthanum nitrate aqueous solution.
A pellet catalyst of Example 11 was prepared in the same manner as in Example 9 except that the content of a was 5% by weight and the calcination temperature after containing La was 800 ° C. (Example 12) Changing the concentration of the lanthanum nitrate aqueous solution to L
A pellet catalyst of Example 12 was prepared in the same manner as in Example 9 except that the content of a was 5% by weight. (Example 13) L was changed by changing the concentration of the lanthanum nitrate aqueous solution.
A pellet catalyst of Example 13 was prepared in the same manner as in Example 9 except that the content of a was 7% by weight. Example 14 L was changed by changing the concentration of the lanthanum nitrate aqueous solution.
A pellet catalyst of Example 14 was prepared in the same manner as in Example 9 except that the content of a was 10% by weight. (Example 15) L was changed by changing the concentration of the lanthanum nitrate aqueous solution.
A pellet catalyst of Example 15 was prepared in the same manner as in Example 9 except that the content of a was 5% by weight, and the calcination temperature after containing La was 1000 ° C. (Comparative Example 4) A pellet catalyst of Comparative Example 4 was prepared in the same manner as in Example 9 except that La was not used and alumina powder was used as the carrier powder. The pellet catalyst of Comparative Example 4 is the same as that of Comparative Example 1. (Comparative Example 5) The concentration of the lanthanum nitrate aqueous solution was changed to La.
Was set to 5% by weight, and the firing temperature after adding La was set to 700 ° C. in the same manner as in Example 9,
The pellet catalyst of Comparative Example 5 was prepared. (Test / Evaluation) With respect to each of the above pellet catalysts, NO _x after initial stage and after durability test was performed in the same manner as in Examples 1 to 7.
The purification rate was measured. The results are shown in Table 4.

[0027]

[Table 4] It is understood that the pellet catalysts of the respective examples have a higher NO _x purification rate after endurance as compared with Comparative Example 4 except for Example 14, and are excellent in NO _x purification performance. It is clear that this is the effect of using the La-alumina carrier. Example 14
In Comparative Example 4, since a large amount of La was contained at 10% by weight, the specific surface area of the carrier was reduced, and the NO _x purification rate after endurance was lower than in Comparative Example 4 having no La. However, even in the case of Example 14, there is a possibility that the NO _x purification rate after endurance will be improved by making the firing temperature a little higher.

Further comparing the examples, the La
If the content is less than 3% by weight, NO after endurance_xPurification rate
Less than 50%, La exceeds 3% by weight and 10% by weight
It can be seen that the range below is particularly preferable. Also, a comparative example
Comparing 5 and Example 12, Example 12 is more durable.
Later NO _xExcellent purification rate. This is the difference in firing temperature
Due to the above, in Comparative Example 5, firing was performed at a low temperature of 700 ° C.
Therefore, La and alumina do not form a composite oxide.
On the other hand, in Example 12, since the baking was performed at 900 ° C.
La and alumina are present in the carrier as complex oxides.
It is due to the difference that there is.

The eleventh embodiment, the twelfth embodiment, and the first embodiment.
From the comparison of No. 5, N after the endurance at 900 ℃ than 800 ℃
It can be seen that the O _x purification rate becomes high, and the NO _x purification rate after endurance decreases rather at 1000 ° C. Therefore, it is assumed that the optimum firing temperature is between 800 and 1000 ° C. (Example 16) 100 parts by weight of alumina powder, 70 parts by weight of alumina sol having an alumina content of 10% by weight, and a concentration of 4
15 parts by weight of 0% by weight aluminum nitrate aqueous solution and 3 parts of water
0 parts by weight was mixed to form a slurry. Then, a cordierite honeycomb monolithic carrier is dipped in this slurry and pulled up to blow off excess slurry.
The alumina coating layer was formed by drying at 2 ° C. for 2 hours and baking at 650 ° C. for 2 hours. The alumina coating layer is formed in an amount of 120 g per liter of the monolith carrier.

Next, the monolith carrier having the above-mentioned alumina coating layer was immersed in an aqueous lanthanum nitrate solution having a predetermined concentration, pulled up to blow off excess water droplets, dried at 110 ° C. for 2 hours, and calcined at 950 ° C. for 5 hours. To form a coat layer made of La-alumina composite oxide. The content of La in the coat layer is 5% by weight. Then, the monolith carrier having a coat layer is dipped in a dinitrodiammineplatinum aqueous solution having a predetermined concentration, pulled up and blown off excess water droplets, and then 1
It was dried at 10 ° C. for 2 hours. Further, after immersing in a rhodium nitrate aqueous solution having a predetermined concentration, pulling it up and blowing off excess water droplets, it was dried at 110 ° C. for 2 hours and calcined at 300 ° C. for 1 hour to obtain Pt of 1.5% by weight and Rh of 0. 1 wt% was supported. Further, after immersing in a barium acetate aqueous solution having a predetermined concentration, pulling it up and blowing off excess water droplets, it is dried at 110 ° C. for 2 hours and baked at 500 ° C. for 2 hours to convert Ba into an oxide of 2
The monolith catalyst of this example was prepared by supporting 0% by weight. (Comparative Example 6) A monolith catalyst of Comparative Example 6 was prepared in the same manner as in Example 16 except that La was not used and the support was not baked by forming the coat layer from only alumina. The monolith catalyst of Comparative Example 6 is the same as that of Comparative Example 3. (Test / Evaluation) With respect to the monolith catalysts of Example 16 and Comparative Example 6, the NO _x purification rates at the initial stage and after the durability test were measured in the same manner as in Example 8 and Comparative Example 3, and the results are shown in Table 5.

[0031]

[Table 5] From Table 5, it is clear that the monolith catalyst of Example 16 has a higher NO _x purification rate after endurance than that of Comparative Example 6, and this is due to the effect of using the La-alumina composite oxide carrier.

[0032]

According to the exhaust gas purifying catalyst of the present invention, the initial NO _x purification performance is excellent, and the degree of decrease in the NO _x purification rate after endurance is small, and the durability is excellent.

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁶ Identification code Office reference number FI Technical display location B01J 23/63 B01D 53/36 ZABH // B01J 32/00 102B 102H B01J 23/56 301A

Claims (1)

[Claims] 1. A catalyst for purifying exhaust gas, comprising a carrier composed of a composite oxide of at least one of cobalt and lanthanum and alumina, and a catalytic noble metal and a NO _x storage material carried on the carrier.