JP2000024504A - Production of catalyst for cleaning exhaust gas - Google Patents

Abstract

PROBLEM TO BE SOLVED: To suppress the cleaning capacity of an exhaust gas cleaning catalyst using a carrier containing at least zeolite at a time of high temp. use. SOLUTION: A noble metal is supported on zeolite and the supported carrier is brought into contact with an alkaline soln. with a pH of 8-11 to perform alkali treatment. Since the amorphous part of zeolite is removed by alkali treatment, the noble metal is applied to the amorphous part at a time of high temp. use to suppress the lowering of activity.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a method for producing an exhaust gas purifying catalyst in which a noble metal is supported on a porous carrier containing zeolite, and more particularly to a method for producing an exhaust gas purifying catalyst having further improved durability.

[0002]

2. Description of the Related Art Zeolites, which are also called molecular sieves, have pores equivalent to the size of molecules and are used as adsorbents and as catalysts in many reactions. It also contains cations to neutralize the negative charge of the main component, aluminum oxide, and these cations are easily exchanged for other cations in aqueous solution, so they are also used as cation exchangers. .

[0003] Utilizing such properties of zeolite,
In recent years, its use in automobile exhaust gas purification catalysts has been studied.
For example, Japanese Unexamined Patent Publication No. 3-232533 discloses an exhaust gas purifying catalyst in which a noble metal such as platinum or palladium is supported on zeolite. Zeolites are chemically tectoaluminosilicates, and zeolites having various Si / Al ratios are known. It has been found that the catalyst characteristics of the zeolite greatly change depending on the value of the Si / Al ratio.

[0004] Zeolites having a small Si / Al ratio have many acid sites and exhibit high cracking ability and high HC (hydrocarbon) adsorption ability. However, zeolite having many acid sites has a problem that HC adsorbed in the pores is carbonized and easily coked, and the pores are closed, resulting in a decrease in the ability to adsorb HC over time. In addition, zeolite having many acid sites has a disadvantage that when subjected to hydrothermal durability, acid sites are easily eliminated due to Al removal (tetracoordinate in the zeolite structure becomes hexacoordinate), and cracking ability is reduced. Further, in such a catalyst in which a catalyst metal is supported on a zeolite, there is a problem that an Al removal reaction occurs during a hydrothermal durability test, and the activity of the catalyst metal decreases due to grain growth accompanying the reaction.

[0005] On the other hand, zeolites having a large Si / Al ratio have low acid sites and thus have low cracking ability. However, since coking does not occur, there is no decrease in HC adsorption capacity over time, and
Since the grain growth of the catalyst metal by Al is relatively suppressed, there is an advantage that the durability is excellent.

[0006]

However, in an exhaust gas purification catalyst in which a noble metal such as platinum or palladium is supported on a zeolite carrier, even if a zeolite having a large Si / Al ratio is used, a certain amount of There has been a problem that grain growth occurs, the purification performance decreases, and the amount of harmful components in the exhaust gas discharged increases.

Further, a metal oxide having an oxygen storage / release capability such as a ceria-zirconia solid solution is supported, and the purification performance is improved by adjusting the oxygen atmosphere in the exhaust gas by the metal oxide. However, in the case of a catalyst in which a metal oxide having an oxygen storage / release capability is supported on a zeolite carrier, there is a problem that the oxygen storage / release capability decreases when a high-temperature durability test is performed, and thus the purification performance decreases.

[0008] The present invention has been made in view of such circumstances, and an object of the present invention is to suppress a decrease in purification performance of a catalyst for purifying exhaust gas using a carrier containing at least zeolite at a high temperature. .

[0009]

According to a first aspect of the present invention, there is provided a method for producing an exhaust gas purifying catalyst, comprising the steps of: loading a noble metal on at least zeolite; and then contacting the alkaline solution with a pH of 8 to 11. To perform an alkali treatment. The feature of the production method according to claim 2 is that in the method for producing an exhaust gas purifying catalyst according to claim 1,
The alkaline solution is ammonia water.

The method for producing a catalyst for purifying exhaust gas according to claim 3 is characterized in that at least a noble metal is supported on a zeolite, and then an alkali treatment is carried out by bringing the zeolite into contact with an alkaline solution having a pH of 9 to 11. Is to carry a metal oxide having the formula: The method for producing a catalyst for purifying exhaust gas according to claim 4 is characterized in that at least a zeolite supports a noble metal and a metal oxide having an oxygen storage / release capability, and then contacts an alkaline solution having a pH of 9 to 11. Is to do.

[0011]

DESCRIPTION OF THE PREFERRED EMBODIMENTS In the manufacturing method according to the first aspect,
After the noble metal is supported on the zeolite, alkali treatment is performed by bringing the zeolite into contact with an alkaline solution having a pH of 8 to 11. This suppresses a reduction in the catalytic activity of the noble metal even after the high-temperature durability test. Therefore, it is possible to reliably manufacture an exhaust gas purifying catalyst having improved durability.

The reason for this is not completely clear, but is presumed as follows. Zeolites have a crystalline part and an amorphous part. Since the amorphous portion covers the noble metal carried near the amorphous portion, it has been clarified that the characteristics of the noble metal are not exhibited and the purification performance is reduced. This amorphous portion contains more silanol groups (-SiOH) than the crystalline portion.

Therefore, in the present invention, an alkali treatment in which a noble metal is supported and then brought into contact with an alkaline solution is performed. Thereby, the silanol group is preferentially eluted into the alkaline solution, and the amorphous portion is preferentially removed.
For this reason, even when the high-temperature durability test is performed, the amorphous portion does not cover the noble metal, so that a reduction in purification performance is suppressed.

When the noble metal is supported on the zeolite, the pH of the alkaline solution is adjusted to 8 to 11. pH 8
If it is less than 10, it becomes difficult to remove the amorphous portion, and it becomes difficult to suppress a decrease in purification performance after the high-temperature durability test. By the way, zeolite has several nanometers to several nanometers due to structural defects, etc.
It has 10 nm mesopores. As a result of recent research, it has been confirmed that the mesopores have a large amount of hydroxyl groups (OH) due to silanol groups, and that the amount of OH groups decreases after ion-exchange loading of the noble metal. Therefore, from these facts, it is considered that the mesopores have many amorphous parts due to many amorphous parts, and the noble metal is preferentially ion-exchange-carried on the OH groups. It was also found that the supported noble metal was physically inhibited from growing by the mesopores.

It has been found that, when treated with a strong alkaline solution having a pH of more than 11, the resulting catalyst has reduced purification performance after a high-temperature durability test. According to the above theory, it is considered that the amorphous portion of the mesopores is removed by the strong alkali treatment to increase the diameter of the mesopores, so that the supported noble metal has grown. Further, the alkali treatment referred to in the present invention is performed after the noble metal is supported, and when the alkali treatment is performed before the noble metal is supported, it becomes difficult to support the noble metal. This is because the silanol groups in the mesopores are removed by the alkali treatment, so that it becomes difficult to carry the ion exchange of the noble metal. Silanol groups in which the noble metal has been ion-exchanged are less likely to be eluted in an alkali solution than pure silanol groups. Therefore, if an alkali treatment is performed after the noble metal is supported, the amorphous portion can be removed while maintaining the amount of the noble metal carried. .

The removal of the amorphous portion by the alkali treatment is extremely effective even when a metal oxide having an oxygen storage / release capability is supported. That is, it has been clarified that the cause of the decrease in the oxygen storage / release capacity after the high-temperature durability test is that the oxygen storage / release ability is covered by the amorphous portion as in the case of the above-mentioned noble metal. Therefore, in the invention according to claim 3, first, a noble metal is supported on zeolite,
Thereafter, alkali treatment is performed by contacting with an alkaline solution having a pH of 9 to 11, and then a metal oxide having oxygen storage / release capability is supported. In other words, since the zeolite from which the amorphous portion has been removed carries a metal oxide having an oxygen storage / release capability, problems such as being covered by the amorphous portion are avoided, and a high oxygen storage / release capability is maintained even after a high-temperature durability test. Secured. Therefore, high purification performance is maintained even after the high-temperature durability test, in combination with the above-mentioned action of the noble metal.

Further, in the production method according to the fourth aspect, after the noble metal and the metal oxide having the ability to occlude and release oxygen are supported on the zeolite, an alkali treatment is performed by bringing the zeolite into contact with an alkaline solution having a pH of 9 to 11. As a result, the amorphous portion is removed, so that the same operation and effect as those of the third aspect can be obtained. According to the third and fourth aspects of the present invention, an alkaline solution having a pH of 9 to 11 is used. When the pH of the alkaline solution is less than 9, it is difficult to suppress a decrease in the oxygen storage / release capability, which is considered to be due to a large interaction between the metal oxide having the oxygen storage / release capability and the amorphous portion. In addition, even if the pH of the alkaline solution exceeds 11, the oxygen storage / release capacity does not decrease. However, the problem that the mesopores are widened and the noble metal grows in grains occurs, so the upper limit of the pH was set to 11.

Although the alkali treatment time varies depending on the pH of the alkaline solution, it is desirable to determine the alkali treatment time by trial and error so as to be the longest as long as the performance does not deteriorate. Generally, a treatment for 30 to 120 minutes is sufficient. If the processing time is too short, it is difficult to suppress performance degradation after the high-temperature durability test,
If the length is too long, the size of the mesopores becomes large and the noble metal grains grow during the high-temperature durability test, so that the purification performance decreases.

Examples of the alkaline substance supplied as the alkaline solution include ammonia, amine and the like.
Alkali metal hydroxides and the like can also be used,
When some alkali metal or the like remains in the catalyst, the performance of the catalyst may be adversely affected. Therefore, it is preferable to use ammonia, amine, or the like, which is burned or volatilized by heating.

As the zeolite, mordenite, ZS
Natural or synthetic zeolites such as M-5, Y-type zeolite, ferrierite and zeolite β can be used. A single type selected from these may be used, or a plurality of types may be mixed and used. The zeolite used is desirably a high silica zeolite having a Si / Al ratio of 800 or more in molar ratio. By using the high silica zeolite, the grain growth of the noble metal due to the Al removal reaction is suppressed.

The present invention may be applied to a method for producing a catalyst using only zeolite as a carrier, or to a method for producing a catalyst using a porous oxide containing at least zeolite as a carrier. Examples of the porous oxide other than zeolite include alumina, silica, zirconia, and titania. As the noble metal, platinum (Pt), rhodium (Rh), palladium (Pd), iridium (Ir), or the like can be used. In some cases, base metals such as iron, cobalt and nickel may be used in combination.

In order to support the above-mentioned noble metal on a carrier containing at least zeolite, a noble metal compound such as tetraammineplatinum hydroxide, hexaammineplatinum hydroxide, tetraamminepalladium hydroxide or hexaamminerhodium hydroxide is used. The precious metal can be supported by bringing these into contact with a carrier as an aqueous solution or an alcohol solution and drying and firing. The obtained catalyst is then subjected to an alkali treatment.

The loading amount of the noble metal is preferably in the range of 0.01 to 30 g per liter of the catalyst. Can hardly purify NO _x is less than 0.01 g, since activity was more than 30g bearing is saturated, further bearing is only results in an increase in cost. Examples of metal oxides having oxygen storage / release capability include rare earth metal oxides such as CeO ₂ and PrO ₄ , NiO, Fe ₂ O
₃ , transition metal oxides such as CuO, Mn ₂ O ₅ or CeO ₂
—ZrO ₂ composite oxide or the like can be used. Claim 3
And the production method according to claim 4 is particularly effective when a CeO ₂ -ZrO ₂ composite oxide is supported. The presence of the amorphous portion greatly affects the CeO ₂ -ZrO ₂ composite oxide, and the CeO _2-
This is because it is considered that the oxygen storage / release capability is reduced due to a decrease in the solid solubility of the ZrO ₂ composite oxide.

It is preferable that the metal oxide having oxygen storage / release capability is supported at a ratio of 10 to 400 parts by weight per 100 parts by weight of the carrier. When the ratio of metal oxide having oxygen storage capacity is less than 10 parts by weight, it is not expected improvement of the NO _x purifying ability in low temperature range becomes difficult activation of HC.
If the amount is more than 400 parts by weight, the catalytic activity decreases.

[0025]

The present invention will be described below in detail with reference to examples. (Example 1) 200 ml of an aqueous tetraammine platinum hydroxide solution containing 0.16 g of platinum and ZSM-5 (molar ratio Si / A
l = 2000) Mix with 10 g of powder, stir for 2 hours, and filter
It was dried and calcined at 500 ° C. for 2 hours to prepare a Pt / ZSM-5 catalyst powder.

Next, the whole amount of the Pt / ZSM-5 catalyst powder was added to 400 ml of an aqueous ammonia solution adjusted to pH 10, stirred for 2 hours, filtered, dried, and calcined at 500 ° C. for 1 hour. One catalyst was prepared. Pt / ZSM before and after this alkali treatment
When the weight of the -5 catalyst powder was measured, the amount of the catalyst powder eluted into the aqueous ammonia solution was about 0.2% by weight.

The resulting catalyst powder is prepared by a conventional method in a range of 0.5 to 1.4.
It was formed into a pellet having a diameter of mm to obtain a pellet catalyst. (Example 2) A pellet catalyst of Example 2 was prepared in the same manner as in Example 1 except that an aqueous ammonia solution adjusted to pH 8 was used. In addition, Pt / Z before and after alkali treatment
When the weight of the SM-5 catalyst powder was measured, the elution amount from the catalyst powder into the aqueous ammonia solution was about 0.03% by weight.

Example 3 The procedure of Example 1 was repeated except that an aqueous ammonia solution adjusted to pH 9 was used.
A pellet catalyst of Example 3 was prepared. When the weight of the Pt / ZSM-5 catalyst powder before and after the alkali treatment was measured, the elution amount from the catalyst powder in the aqueous ammonia solution was about 0.1% by weight.

Example 4 The procedure of Example 1 was repeated except that an aqueous ammonia solution adjusted to pH 11 was used.
A pellet catalyst of Example 4 was prepared. When the weight of the Pt / ZSM-5 catalyst powder before and after the alkali treatment was measured, the elution amount from the catalyst powder in the aqueous ammonia solution was about 0.4% by weight.

Comparative Example 1 A pellet catalyst of Comparative Example 1 was prepared in the same manner as in Example 1 except that the alkali treatment was not performed. Comparative Example 2 A pellet catalyst of Comparative Example 2 was prepared in the same manner as in Example 1 except that an aqueous ammonia solution whose pH was adjusted to 12 was used. In addition, Pt / Z before and after alkali treatment
When the weight of the SM-5 catalyst powder was measured, the elution amount from the catalyst powder into the aqueous ammonia solution was about 0.8% by weight.

Example 5 1.4 g of the alkali-treated catalyst powder obtained in Example 1 and CeO ₂ -ZrO ₂ composite oxide powder
0.9 g was mixed to prepare a catalyst powder, and a pellet catalyst of Example 5 was prepared in the same manner as in Example 1 using this catalyst powder. (Example 6) A catalyst powder was prepared by mixing 1.4 g of the alkali-treated catalyst powder obtained in Example 3 and 0.9 g of CeO ₂ -ZrO ₂ composite oxide powder. In the same manner as in Example 1, a pellet catalyst of Example 6 was prepared.

(Example 7) 1.4 g of the alkali-treated catalyst powder obtained in Example 4 and CeO ₂ -ZrO ₂ composite oxide powder
0.9 g was mixed to prepare a catalyst powder, and a pellet catalyst of Example 7 was prepared using this catalyst powder in the same manner as in Example 1. (Comparative Example 3) 1.4 g of the catalyst powder obtained in Comparative Example 1 and Ce
0.9 g of O ₂ —ZrO ₂ composite oxide powder was mixed to prepare a catalyst powder, and a pellet catalyst of Comparative Example 3 was prepared in the same manner as in Example 1 using this catalyst powder.

Comparative Example 4 1.4 g of the alkali-treated catalyst powder obtained in Example 2 and CeO ₂ -ZrO ₂ composite oxide powder
0.9 g was mixed to prepare a catalyst powder, and a pellet catalyst of Comparative Example 4 was prepared in the same manner as in Example 1 using this catalyst powder. (Comparative Example 5) A catalyst powder was prepared by mixing 1.4 g of the alkali-treated catalyst powder obtained in Comparative Example 2 and 0.9 g of the CeO ₂ -ZrO ₂ composite oxide powder. In the same manner as in Example 1, a pellet catalyst of Comparative Example 5 was prepared.

(Test / Evaluation) For each of the pellet catalysts of the above Examples and Comparative Examples, the model exhaust gas on the lean side and the model exhaust gas on the rich side shown in Table 1 were alternately repeated every 2 minutes at a flow rate of 10 L / min. The flowing endurance treatment was performed at an incoming gas temperature of 1000 ° C. for 10 hours. Example 1 after endurance treatment
And the pellet catalysts of Comparative Examples 1 and 2 were placed in the model gas flow passage, and the stoichiometric gas shown in Table 2 was used as the pellet catalyst.
At a flow rate of 10 liters / minute with respect to 1.4 g, the purification rate of HC at each temperature was measured. A 50% purification temperature of HC was calculated from the obtained data, and the result is shown in FIG.

[0035]

[Table 1]

[0036]

[Table 2] The catalyst pellets of Examples 5-7 and Comparative Examples 3-5 after the durability treatment, then placed in a model gas flow path, respectively, O ₂
The oxygen storage / release amount was measured based on the amount of CO ₂ generated at the time of switching (1%)-CO (2%) to determine the oxygen storage / release capacity. Each result is shown in FIG.

As shown in FIG. 1, the catalysts of the respective examples show higher purification performance than the catalyst of Comparative Example 1 which is not subjected to the alkali treatment, and the alkali treatment is extremely effective in suppressing a decrease in the purification performance after the durability test. It is clear that. It can also be seen that the catalyst of Example 1 has the best purification performance, and the pH of the aqueous ammonia in the alkaline treatment is optimally around 10.

However, as in Comparative Example 2, p
When H reaches 12, the purification performance is lower than that of the catalyst of Comparative Example 1 in which the alkali treatment is not performed. This is considered to be because the mesopores were expanded by the strong alkali treatment, and the Pt grain growth was promoted. As shown in FIG. 2, the catalysts of Examples 5 to 7 and Comparative Example 5 are superior to Comparative Examples 3 and 4 in oxygen absorbing and releasing ability, and a decrease in oxygen absorbing and releasing ability during the durability treatment is suppressed. From this, it can be seen that if the pH of the aqueous ammonia is 9 or more, the decrease in the oxygen absorbing and releasing ability is suppressed. However, in the catalyst of Comparative Example 5, the degree of reduction in purification performance was larger than that of the catalysts of Examples 5 to 7 due to the growth of Pt particles during the durability treatment, and was substantially the same as that of the catalyst of Comparative Example 3.

It is to be noted that 1.4 g of the Pt / ZSM-5 catalyst powder prepared in Example 1 and 0.9 g of the CeO ₂ -ZrO ₂ composite oxide powder were mixed, and the mixture was used in the same manner as in Examples 1-4. For each of the catalysts obtained by the treatment, the purification performance after the durability treatment was measured in the same manner as in Example 1. As a result, the same purification performance as in Examples 1 to 4 was confirmed, and CeO ₂ -Zr
No difference due to the loading time of the O ₂ composite oxide powder was observed.

[0040]

According to the catalyst produced by the method for producing an exhaust gas purifying catalyst according to claims 1 and 2,
While suppressing the growth of noble metal grains at high temperatures,
The coating of the noble metal with the amorphous portion of the zeolite is suppressed, and a reduction in purification performance after the high-temperature durability test can be suppressed.

Further, according to the catalyst produced by the method for producing an exhaust gas purifying catalyst according to claims 3 and 4, the above-mentioned effect is exhibited, and the metal oxide having the oxygen storage / release capability by the amorphous portion of zeolite. Is suppressed, so that a high oxygen storage / release capability is ensured even after a high-temperature durability test, and a decrease in purification performance can be further suppressed.

[Brief description of the drawings]

FIG. 1 is a bar graph showing the HC 50% purification temperatures of catalysts obtained in Examples and Comparative Examples after a durability test.

FIG. 2 is a bar graph showing the oxygen absorbing and releasing ability of the catalysts obtained in Examples and Comparative Examples after a durability test.

 ──────────────────────────────────────────────────続 き Continuing on the front page F-term (reference) 4D048 AA06 AA13 AA18 AB05 BA08X BA11X BA19X BA30X BA42X BB01 CA01 4G069 AA03 AA08 BA07A BA07B BB01C BB04A BB06B BC32A BC33A BC43B BC51B BC69A BC01B06 CB01B06 FB65 FC04 FC09 ZA11B ZC04 ZD01

Claims (4)

[Claims] 1. A method for producing an exhaust gas purifying catalyst, comprising carrying out an alkali treatment in which at least a noble metal is supported on a zeolite and then brought into contact with an alkaline solution having a pH of 8 to 11. 2. The method for producing an exhaust gas purifying catalyst according to claim 1, wherein the alkaline solution is aqueous ammonia. 3. An exhaust gas purification method comprising: supporting at least a noble metal on a zeolite, performing an alkali treatment by bringing the zeolite into contact with an alkaline solution having a pH of 9 to 11, and then supporting a metal oxide having an oxygen storage / release capability. Production method of catalyst for use. 4. An exhaust gas purifying catalyst characterized in that at least a zeolite carries a noble metal and a metal oxide capable of occluding and releasing oxygen, and then carries out an alkali treatment in which the catalyst is brought into contact with an alkaline solution having a pH of 9 to 11. Production method.