JP2798690B2 - Method for producing exhaust gas purifying catalyst - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a method for producing an exhaust gas purifying catalyst using high specific surface area alumina used for purifying exhaust gas, particularly exhaust gas discharged from an internal combustion engine of an automobile or the like. .

(Prior Art) Recently, researches for applying a combustion catalyst technology to a boiler, a gas turbine, and the like to make a low NOx power source have been actively conducted. These combustion catalysts are heated to a high temperature of 1000 ° C. or more by combustion of a fuel such as methane on the catalyst, but a catalyst that can be stably used in such a high temperature range has not been found so far. In order to develop such a high heat resistant catalyst, first, a heat resistant support that maintains a high specific surface area even in such a high temperature range is required. Conventionally, a heat-resistant oxide such as activated alumina has been used as a carrier for the combustion catalyst. However, this alumina is transformed into α-alumina having a small specific surface area at a high temperature of 1000 ° C. or higher, and a high-temperature catalyst carrier is used. Not available.

Also, catalysts for purifying automobile exhaust gas have been used as combustion catalyst technology for medium temperatures of around 800 ° C. However, in recent years, the rise in exhaust temperature and the reaction of the catalyst by installing the catalyst in the high temperature area of exhaust gas temperature near the engine have been used. Increase the activity, platinum in the catalyst,
Moves to reduce the use of precious metals such as rhodium,
For example, it is required to have a high temperature durability of about 1000 ° C. However, conventional exhaust gas purifying catalysts also use activated alumina or the like as a carrier and support platinum, rhodium, palladium or other precious metals as catalysts.
At a high temperature of about ° C., the activated alumina is transformed into α-alumina, the specific surface area of the carrier is reduced, and the supported noble metal is agglomerated, so that sufficient catalytic activity cannot be maintained.

For this reason, various attempts have been made to add another type of metal oxide to alumina to increase the heat resistance and suppress the decrease in the specific surface area, and lanthanum oxide (La ₂ O ₃ ) (JP-A-48-146000, JP 60-226414 JP), or barium oxide (B _a O) (JP 50-950590 and JP Chemistry Letters (Chemistry Letters) 151, pp to 154, 1985, a surface (1986 Vol.24, No. 11, p. 658) is effective.

(Problems to be Solved by the Invention) However, although the heat resistance of the carrier is improved in the above proposal, the catalyst supporting the noble metal as a catalytic active component causes aggregation of the noble metal fine particles,
Sufficient catalytic activity cannot be maintained, and the catalyst as a whole does not always have sufficient heat resistance.

In general, for practical combustion catalysts, a heat-resistant support (honeycomb structure such as mullite or cordierite) is coated with a noble metal / alumina catalyst, or a stepwise preparation in which a support is coated with a support and then noble metal is impregnated. An approach is taken. In this case, it has been pointed out that in the case of the alumina composite oxide to which barium oxide or lanthanum oxide is added, the additives are easily eluted during coating, and coating is difficult, and in addition, the catalyst performance is reduced by elution and coating operation. The disadvantage is pointed out.

Therefore, the present invention overcomes the problems of the prior art, even under high temperature, does not cause a decrease in the specific surface area of the carrier,
It is an object of the present invention to provide a method for producing an exhaust gas purifying catalyst using a high specific surface area alumina capable of suppressing aggregation of a noble metal supported as a catalytically active component and maintaining sufficient catalytic activity.

(Means for Solving the Problems) The present inventors have conducted intensive studies on the improvement of the specific surface area of alumina in order to achieve the above object, and as a result, have found that hexylene glycol (2-methyl-2,4-pentane When aluminum alkoxide is dissolved in diol), heated, and then hydrolyzed, hexylene glycol controls the alkoxide hydrolysis and dehydration-condensation polymerization reaction, and shows high uniformity and high specific surface area with uniform pore diameter. The inventors have found the fact that alumina can be obtained, and have achieved the present invention based on this finding.

That is, in the method of the present invention, an aluminum alkoxide is placed in hexylene glycol and reacted by heating at a temperature in the range of 101 ° C. to 200 ° C., then a gel is obtained from the sol by hydrolysis, and the formed gel is dried, At 750
Calcination at ℃ or more, to obtain alumina with a high specific surface area, impregnated with cerium in this alumina, cerium oxide and alumina sol were mixed and pulverized to obtain a slurry, and the slurry was impregnated with a carrier, dried and fired. A method for producing an exhaust gas purifying catalyst, comprising supporting at least one noble metal selected from the group consisting of platinum, rhodium and palladium on a composite oxide or mixed oxide film formed on a carrier. is there.

(Action) In general, aluminum ions can form tetrahedral tetracoordinate and octahedral hexacoordinate compounds. Alumina is formed by interposing oxygen and hydroxyl groups at the apexes of these two polyhedrons. Polyhedra are macromolecules formed by sharing vertices, edges, or faces. Accordingly, alumina having various structures and various molecular weights is produced depending on the type and structure of the aluminum hydroxide produced by the hydrolysis and the progress of the dehydration condensation reaction of the hydroxide. As a matter of course, the specific surface area of alumina changes depending on its structure and molecular weight. In general, when the molecular weight of alumina particles is small, that is, when the particle size of primary particles is small, the specific surface area increases, and structurally, the tetrahedral structure has a larger specific surface area per unit weight than the octahedral structure. . However, if the particle size is too small, the surface energy increases, and the number of uncrosslinked hydroxyl groups in the particles increases.At high temperatures, simple fusion and dehydration fusion between particles easily occur, resulting in a decrease in specific surface area. It will be easier. Also, structurally, if too much aluminum takes a tetrahedral structure in the particles, from the relationship of charge neutralization,
Non-crosslinked coordinating water and hydroxyl groups increase, and at high temperatures, the specific surface area decreases as before. Therefore, in order to maintain high specific surface area in alumina even at high temperature, it is necessary to adjust and control the hydrolysis reaction and dehydration polymerization reaction, and to adjust the primary particles, molecular weight and structure of alumina to an appropriate one. is there.

The present invention provides an alumina exhibiting a high specific surface area even at a high temperature by adjusting and controlling the hydrolysis and polycondensation reaction of aluminum alkoxide with hexylene glycol from the above viewpoint, and using the same, a catalyst for exhaust gas purification. Is to be manufactured. That is, since the hexylene glycol used in the present invention has two hydroxyl groups, the aluminum alkoxide is subjected to a complex formation reaction or a ligand exchange reaction to monodentate an aluminum ion with one hydroxyl group. And two hydroxyl groups for chelate coordination or cross-link coordination.
Therefore, hexylene glycol blocks or joins the vertices of the octahedron and tetrahedron of the aluminum ion, in other words, by hydrolysis or dehydration polymerization of aluminum alkoxide, and thus aluminum. It controls the alkoxide solification and gelation reactions. As a result, homogeneous alumina having a uniform particle size and a high specific surface area even at a high temperature can be obtained.

Regarding the aluminum alkoxide, any alkoxide can be used in the present invention irrespective of the type of the alkyl group, and those which easily become alkoxide in hexylene glycol, such as anhydrous aluminum halide, can be used. Is also good. However, for example, aluminum methoxide, ethoxide, iso-propoxide, n-propoxide, n-butoxide, sec-butoxide, tert-butoxide and the like are preferable from the viewpoint of cost and operability.

As described above, hexylene glycol is not a simple solvent but reacts with aluminum alkoxide. Therefore, the use amount of hexylene glycol is preferably 0.5 mol or more per 1 mol of aluminum alkoxide. Further, the heating temperature is preferably from 101 to 200 ° C. However, if the amount of hexylene glycol used is too large, too much organic matter remains after hydrolysis, or the particle size becomes too small, and it is easy to cause fusion at high temperatures, and more than necessary. Since it is not economical to use a large amount of the aluminum alkoxide, the amount should be 5 mol or less, more preferably 3 mol or less, per 1 mol of the aluminum alkoxide.
In general, if the amount of hexylene glycol used is smaller than the amount of aluminum alkoxide used, the aluminum alkoxide is less likely to be dissolved, and thus the reaction is slower, but in this case, a solvent can be used. As the solvent, any one that dissolves aluminum alkoxide and hexylene glycol and has a weaker coordination force to aluminum than hexylene glycol can be used.
Various things such as monohydric alcohols, ethers and hydrocarbons can be mentioned.

Regarding the heating temperature, if the temperature is too low, the complexation reaction and the ligand exchange reaction between the aluminum alkoxide and hexylene glycol become too slow, and the polymerization reaction of the aluminum alkoxide mediated by hexylene glycol hardly occurs. Should be heated at 100 ° C.

The amount of water used during the hydrolysis is closely related to the alumina particles formed, that is, the specific surface area of the alumina. If the amount of water used is too small, the alumina gel after drying will contain a large amount of organic substances, and the alumina will be easily fused at high temperature. Also, if the amount of water is too large,
The molecular weight of the alumina particles generated in the gelation step is increased, and as a result, only alumina having a small specific surface area is obtained. Therefore, the amount of water to be added is 0.5 to 20 mol, preferably 2 to 10 mol, per 1 mol of the aluminum alkoxide. Regarding the method of adding water, it can be added directly, diluted with an appropriate solvent, and added.
It is possible in any desired way.

In the present invention, the solution generally becomes a gel via the sol after the addition of water, but in the drying step, the obtained gel can be dried as it is under a normal method or under reduced pressure. As another method, a sol or undried gel may be impregnated or applied to another substance, for example, a honeycomb support, and then dried under a normal method or under reduced pressure. Therefore, the drying method is
Any method is possible.

Regarding firing, the present invention is characterized in that the treatment is carried out at 750 ° C. or more, but before the treatment at 750 ° C. or more, 750 ° C.
The pretreatment may be performed at a temperature of not more than ℃. The temperature and atmosphere in the pretreatment are not particularly limited, and are generally used in various gas atmospheres and various temperatures such as air, oxygen, nitrogen, hydrogen, argon, and helium. Processing can be performed. The firing at 750 ° C. or higher is not particularly limited, and is merely performed in a normal heat treatment in an air or oxygen atmosphere. The heat treatment can be performed for a desired time.

Further, the alumina obtained above was impregnated with cerium, cerium oxide and alumina sol were mixed and pulverized into a slurry by mixing and pulverizing to obtain a slurry. The slurry was impregnated with a carrier, dried, calcined, and formed on the carrier. At least one catalytic metal selected from the group consisting of platinum, rhodium and palladium is supported on the coating. At this time, in order to improve the dispersion of the noble metal and to form fine particles, the specific surface area of the carrier is preferably larger. Is preferred. The powder obtained by impregnating and firing cerium into alumina is preferably obtained by impregnating and burning cerium to 3% by weight using cerium nitrate in alumina. On the other hand, when the heat resistance of the carrier is poor, the aggregation proceeds easily and the specific surface area decreases, and the noble metal carried during the aggregation of the carrier is embedded in the carrier, or the distance between the dispersed noble metal fine particles decreases. Therefore, the aggregation of the noble metal proceeds quickly. For this reason, the specific surface area of the carrier is required to be substantially 100 m ² / g, and it is particularly necessary to maintain this value even after being exposed to a high temperature. For combustion catalysts such as automobile exhaust gas purification catalysts, especially 1000
100m even after heat treatment at 3 ℃ for 3 hours, for example.
It is necessary to maintain a large specific surface area of ² / g.

(Effect of the Invention) According to the method of the present invention as described above, alumina exhibiting a high specific surface area even at a high temperature can be obtained, and these have a higher specific surface area than those produced by various conventional methods. It has a uniform particle size, is highly homogeneous, and maintains a high specific surface area even when used at 1000 ° C. for a long time.

Also, conventionally, cerium oxide has been known to have a cocatalyst effect of accelerating the oxidation reaction of carbon monoxide and olefinic hydrocarbons on a platinum catalyst. Since the cerium oxide is contained in a highly dispersed state in the catalyst for use, it has an excellent co-catalyst effect, and its aggregation at high temperatures is also suppressed. Sustained stably.

(Examples) Hereinafter, the present invention will be described in more detail based on examples and comparative examples.

Example 1 599.8 g of aluminum isopropoxide (Al (OisoPr) ₃ ) was placed in a 2000 ml beaker, 540.9 g of hexylene glycol was added thereto, and the mixture was stirred for 4 hours while heating in a 120 ° C. oil bath. Next, after cooling the oil bath temperature to 100 ° C., 449.5 g of water was added to the aluminum alkoxide solution. After standing overnight at the same temperature, the obtained gel was transferred to an eggplant-shaped flask and dried at 120 to 170 ° C under reduced pressure to obtain 195 g of a dried gel. 450 ° C, 600 ° C for 1 hour at 300 ° C
The mixture was calcined at a temperature of 800C, 800C and 1000C for 3 hours to obtain 150 g of white alumina powder. This powder was impregnated and dried using cerium nitrate to a cerium content of 3% by weight.
C. for 2 hours in an air atmosphere at a temperature of .degree. C. to obtain cerium-supported alumina powder. The cerium-supported alumina powder was 109.
9 g, 217.9 g of alumina sol (a sol obtained by adding 10% by weight of HNO ₃ to a suspension of 10% by weight of boehmite alumina) and 72.2 g of cerium oxide powder were mixed in a ball mill, and pulverized at 80 rpm for 6 hours. The monolithic carrier base material (60
ml, 400 cells / in ² ), immersion, air blow, and drying were repeated three times to deposit an oxide coat layer.
The firing was performed in an air atmosphere at 600 ° C. for 2 hours. At this time, the adhesion amounts of alumina and cerium oxide were 9.5 g /
And 5.5 g / piece. Further, the carrier having alumina and cerium oxide attached thereto was immersed in a mixed aqueous solution of chloroplatinic acid and rhodium chloride, and the attached amounts of platinum and rhodium were each reduced to 0.
After loading at a rate of 067 g / piece and 0.007 g / piece, calcination was performed at 600 ° C. for 2 hours in an air atmosphere to obtain Catalyst 1.

Example 2 A catalyst was prepared in the same manner as in Example 1 except that the amount of hexylene glycol used was 274.7 g, and heating and stirring were performed at 120 ° C. for 3 hours.

Example 3 A catalyst was prepared in the same manner as in Example 1, except that the amount of hexylene glycol used was changed to 749.1 g, to obtain a catalyst 3.

Example 4 A catalyst was prepared in the same manner as in Example 1 except that the temperature of the hot water bath was set at 145 ° C. and the heating and stirring time before adding water was 3 hours, to obtain Catalyst 4.

Example 5 A catalyst was prepared in the same manner as in Example 1, except that the temperature of the hot water bath was set at 190 ° C. and the heating and stirring time before adding water was 4 hours, to obtain a catalyst 5.

Example 6 A catalyst was prepared in the same manner as in Example 1 except that the temperature of the hot water bath was set at 105 ° C., and the heating and stirring time before adding water was set to 3 hours.

Example 7 As an aluminum alkoxide, 725.2 g of aluminum sec-butoxide (Al (Osec-Bu) ₃ ) was used.
A catalyst was prepared in the same manner as in Example 1 except that the temperature of the hot water bath was set to 145 ° C., and a catalyst 7 was obtained.

Example 8 Instead of a mixed aqueous solution of chloroplatinic acid and rhodium chloride,
Using a mixed aqueous solution of chloroplatinic acid, palladium chloride and rhodium chloride, the adhesion amount of platinum, palladium and rhodium is 0.03
A catalyst was prepared in the same manner as in Example 1 except that the catalyst was supported at 4 g / unit, 0.034 g / unit, and 0.007 g / unit, and catalyst 8 was obtained.

Comparative Example 1 600 g of (Al (OisoPr) ₃ ) was placed in isopropanol, heated and stirred at 70 ° C. for 4 hours, and then 450 g of water was added.
The following operation was performed in the same manner as in Example 1 to prepare a catalyst, and a catalyst A was obtained.

Comparative Example 2 503 g of aluminum sulfate was dissolved in 625 ml of water, and a solution obtained by diluting 290 ml of ammonia water with 250 ml of water was added thereto.
Aluminum hydroxide was precipitated. After decanting several times, the mixture was filtered and dried. The calcination was performed in the same manner as in Example 1, and the above operation was performed in the same manner as in Example 1 to prepare a catalyst.

Comparative Example 3 An aqueous sodium aluminate solution was added to an aqueous solution containing 503 g of aluminum sulfate until the pH became 7, and the resulting precipitate was separated by filtration. The washing solution was free from white precipitation by the addition of an aqueous barium nitrate solution. Washed until. After drying the precipitate,
The calcination was performed in the same manner as in Example 1, and the catalyst was adjusted to obtain Catalyst C.

Comparative Example 4 600 g of Al (OisoPr) ₃ was placed in 500 g of hexylene glycol, and heated and stirred in an 80 ° C. oil bath for 6 hours.
0 g of water was added and left overnight. Subsequent operations are described in Example 1.
The catalyst was adjusted to obtain catalyst D.

Test Example The catalysts 1 to 8 obtained in Examples 1 to 8 and the catalysts A to D obtained in Comparative Examples 1 to 4 were subjected to engine durability under the following conditions, and the conversion of each component was measured on an engine bench. did. The results are shown in the table.

Engine endurance conditions Catalyst container 8 simultaneous endurance containers Catalyst outlet temperature Approx. 850 ° C Space velocity Approx. 130,000 Hr ^-1 Endurance time 100 hours Engine Displacement equivalent to 2200cc Evaluation condition Evaluation temperature range Room temperature to 500 ° C Heating speed 30 ° C / min Engine Displacement 2000cc catalyst container 8 simultaneous evaluation containers

Continuation of the front page (72) Inventor Junichi Mine 2 Takaracho, Kanagawa-ku, Yokohama-shi, Kanagawa Nissan Motor Co., Ltd. Examiner Yoshihisa Seki (56) References JP-A-61-61645 (JP, A) JP-A Sho 60-206447 (JP, A) (58) Fields investigated (Int. Cl. ⁶ , DB name) B01J 21/00-38/00 B01D 53/94 C01F 7/02

Claims (1)

(57) [Claims] 1. An aluminum alkoxide is placed in hexylene glycol (2-methyl-2,4-pentanediol) and heated and reacted at a temperature in the range of 101 ° C. to 200 ° C.
Next, a gel is obtained from the sol by hydrolysis, and the resulting gel is dried and finally calcined at a temperature of 750 ° C. or more to obtain alumina, and a powder obtained by impregnating and calcining this alumina with cerium is mixed with cerium oxide and alumina sol. Is mixed and pulverized into a slurry, and the slurry is impregnated with a carrier, dried and calcined, and a coating of a composite oxide or a mixed oxide formed on the carrier is coated with at least one selected from the group consisting of platinum, rhodium and palladium. A method for producing an exhaust gas purifying catalyst for removing carbon monoxide, hydrocarbons, and nitrogen oxides in exhaust gas, wherein the catalyst supports various kinds of catalytic metals.