JP2006341152A - Catalyst carrier manufacturing method and manufacturing method of exhaust gas purifying catalyst - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a manufacturing method of a catalyst carrier which develops good catalytic capacity when rhodium is carried by the catalyst carrier, and an exhaust gas purifying catalyst using the catalyst carrier. <P>SOLUTION: The manufacturing method of the catalyst carrier 10 which is a mixture of primary particles 1 of zirconia and primary particles 2 of alumina with each other includes a process for providing a salt solution containing a zirconium salt, an aluminum salt and urea, a process for heating the salt solution to decompose urea to produce ammonia and making the salt solution alkaline to form a deposit containing zirconium hydroxide and aluminum hydroxide and a process for drying and baking the yielded flocs. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a catalyst carrier and a method for producing an exhaust gas purification catalyst.

The exhaust gas from an internal combustion engine such as an automobile engine includes nitrogen oxide (NO _x ), carbon monoxide (CO), hydrocarbon (HC), and the like. Therefore, in general, after purifying by an exhaust gas purifying catalyst that oxidizes CO and HC and reduces NO _x , these exhaust gases are released into the atmosphere. A typical example of this exhaust gas purification catalyst is a three-way catalyst in which a noble metal such as platinum (Pt), rhodium (Rh), palladium (Pd) is supported on a porous metal oxide carrier such as γ-alumina. It has been.

The metal oxide support can be made of various materials, but conventionally, alumina has generally been used to obtain a high surface area. In recent years, however, various other materials such as ceria (CeO ₂ ), zirconia (ZrO ₂ ), titanium (TiO ₂ ), etc., have been used with alumina in order to promote exhaust gas purification utilizing the chemical nature of the support. It has also been proposed to use it in combination or not.

For example, in order to absorb fluctuations in the oxygen concentration in the exhaust gas and enhance the exhaust gas purification capacity of the three-way catalyst, oxygen is stored when the oxygen concentration in the exhaust gas is high, and oxygen is stored when the oxygen concentration in the exhaust gas is low. A material having an oxygen storage capacity (OSC capacity) to be released is used for an exhaust gas purification catalyst. A typical material having OSC ability is ceria. In addition, according to recent research, ceria not only has OSC ability, but also has a strong affinity with noble metals supported thereon, especially platinum, and therefore, noble metal grain growth (sintering or sintering) has occurred. It has been found that it can be suppressed. When platinum during use of the exhaust gas purifying catalyst is sintering, the active sites of the catalyst is reduced, thereby lowering the oxidation reduction efficiency of NO _x. Therefore, it is very important to suppress platinum sintering.

  Thus, ceria is important for providing OSC capability to the catalyst support and preventing platinum sintering. However, since ceria is relatively easy to sinter, ceria itself sinters, which may reduce the platinum sintering prevention effect of ceria.

  In this regard, Patent Documents 1 and 2 disclose that an alkaline solution is added to a solution containing an aluminum salt, a cerium salt, and a zirconium salt to obtain a catalyst carrier in which alumina, ceria, and zirconia are dispersed with each other. ing.

  Patent Document 3 proposes manufacturing such a catalyst carrier in which alumina, ceria and zirconia are dispersed from each other from a metal alkoxide. According to Patent Document 3, alumina, ceria, and zirconia are particularly uniformly mixed in the catalyst carrier manufactured in this way, thereby having stable OSC ability even after endurance at high temperatures. .

  Further, Patent Document 4 discloses a catalyst carrier in which at least one of ceria and zirconia particles and alumina particles are dispersed highly uniformly and a method for producing the same. According to Patent Document 4, such a catalyst carrier is obtained by mixing a salt solution containing at least one salt of cerium and zirconium and an aluminum salt with an alkaline solution in a short time. In such a catalyst support, at least one particle of ceria and zirconia is highly uniformly dispersed with alumina particles, so that sintering of at least one particle of ceria and zirconia is caused by alumina particles therebetween. It is suppressed.

  In addition, the use of zirconia as a catalyst carrier has been conventionally known. Particularly, it is known that when rhodium is supported on zirconia, excellent catalytic performance is provided as compared with the case where rhodium is supported on alumina. .

JP-A-10-202101 JP-A-11-267501 Japanese Patent No. 3379369 Japanese Patent No. 3262444

  The present invention provides a novel method for producing a catalyst carrier containing zirconia and alumina, and a method for producing an exhaust gas purification catalyst using such a catalyst carrier.

  The method of the present invention for producing a catalyst support containing zirconia and alumina provides a salt solution containing zirconium salt, aluminum salt and urea; heating the salt solution to decompose urea and generate ammonia Forming a precipitate containing hydroxides of zirconium and aluminum by making the salt solution alkaline; and drying and calcining the agglomerates.

  According to the method of the present invention for producing a catalyst support, a salt solution containing urea can be heated to decompose urea to generate ammonia, whereby the salt solution can be uniformly and rapidly changed to basic. . Making the salt solution basic in this way uniformly and quickly makes it possible to obtain a precipitate in which zirconium and aluminum are uniformly dispersed. The use of urea is also preferable because it can be easily removed by heating or the like.

  In the catalyst carrier obtained by firing the agglomerates thus obtained, primary particles of zirconia and primary particles of alumina are mixed with each other. Although zirconia is generally relatively easy to sinter, zirconia and alumina do not substantially form a solid solution. Therefore, in the catalyst support obtained by the method of the present invention, zirconia and alumina are not allowed to react with each other even when used at high temperatures. And each sintering is suppressed.

In one embodiment of the process of the invention for producing a catalyst support, the mass ratio of ZrO ₂ : Al ₂ O ₃ in the catalyst support is 95: 5 to 40:60, in particular 90:10 to 40:60, even more particularly 80. : 20 to 60:40.

  According to the catalyst carrier produced by this aspect of the method of the present invention, when rhodium is supported and used as an exhaust gas purification catalyst, rhodium sintering is prevented by the newness of zirconia and rhodium, and preferable catalyst performance is obtained. It can be demonstrated.

In the method for producing an exhaust gas purification catalyst of the present invention, the catalyst support obtained by the method of the present invention carries rhodium and optionally a NO _x storage material selected from the group consisting of alkali metals, alkaline earth metals and rare earth elements. Including.

[Catalyst support]
The present invention will be described with reference to FIG. Here, FIG. 1 is a conceptual diagram of a catalyst carrier obtained by the method of the present invention.

  As shown in FIG. 1, the catalyst carrier 10 obtained by the method of the present invention comprises zirconia primary particles 1 and alumina primary particles 2 mixed with each other.

  Thus, in the catalyst carrier in which the primary particles of zirconia and the primary particles of alumina are mixed, since zirconia and alumina do not form a solid solution with each other, sintering is suppressed at the interface between these primary particles. That is, in such a catalyst carrier, sintering of the catalyst carrier as a whole is suppressed. This suppression effect of sintering is exhibited particularly well when primary particles that do not dissolve in each other are evenly mixed with each other.

[Method for producing catalyst carrier]
Providing a salt solution The method of the present invention for producing a catalyst support first provides a salt solution, particularly an aqueous salt solution, containing a zirconium salt, an aluminum salt and urea.

  As the zirconium or aluminum salt that can be used here, any salt that can be dissolved in a medium such as water to generate zirconium or aluminum ions can be used. Therefore, these salts may have the same group or different groups. For example, the salts which can be used here are chloride salts, nitrates, oxynitrates or acetates, in particular nitrates.

Formation of Precipitate In the process of the present invention for producing the catalyst support, the salt solution containing the zirconium salt, aluminum salt and urea is then heated to decompose urea and generate ammonia, thereby producing this salt solution. Is made alkaline to produce a precipitate containing zirconium and aluminum hydroxides.

Drying and calcination of the precipitate Finally, in the method of the present invention for producing the catalyst carrier, the obtained aggregate is dried and calcined to produce a catalyst carrier containing zirconia and alumina.

  Removal of the solvent component from the agglomerate and drying can be done in any manner and at any temperature, for example, the agglomerate can be accomplished in a 120 ° C. oven. Thus, the catalyst carrier can be obtained by calcining the raw material obtained by removing the solvent component from the aggregate and drying it. Firing can be performed at a temperature generally used in metal oxide synthesis, for example, 250 to 600 ° C.

  In addition, it is preferable to wash | clean before drying an aggregate, This washing | cleaning can be performed with water, a C1-C5 lower alcohol, etc., for example.

  In using the catalyst carrier of the present invention, it can also be used by mixing with other carrier materials such as titania, zirconia, alumina and silica.

[Method for producing exhaust gas purification catalyst]
Exhaust gas purifying catalyst obtained by the method of the present invention, the _the NO _x storage material such as barium on the rhodium and optionally a catalyst support obtainable by the process of the present invention can be achieved by carrying. Supporting these rhodium and NO _x storage materials can be achieved by any method. For example, rhodium can be impregnated with a rhodium nitrate aqueous solution on a catalyst carrier, dried and calcined, and 0.5 to 2% by weight of rhodium can be supported. In addition, the loading of the NO _x storage material such as barium can be achieved by impregnating a catalyst carrier with a solution containing these metal salts, for example, a barium acetate solution, and drying and calcining it.

  The present invention will be described below based on examples, but the present invention is not limited thereto.

[Reference example]
Zirconium oxynitrate (217.5 g) and urea (5000 g) were dissolved in ion-exchanged water (1000 cc) and stirred for 2 hours. By heating this salt solution, urea was decomposed and a composite hydroxide was precipitated.

  From the salt solution containing the composite hydroxide precipitate thus obtained, water was filtered, washed, dried at 120 ° C. for 2 hours, and calcined at 300 ° C. for 2 hours to obtain the catalyst of the reference example. A carrier powder (zirconia powder) was obtained.

  Thereafter, 100 g of the catalyst support powder of Reference Example was immersed in an aqueous solution in which an aqueous solution of rhodium nitrate corresponding to 1.0 g of rhodium was added to 500 cc of ion-exchanged water, and stirred for 2 hours. This aqueous solution was dried at 120 ° C. for 2 hours and calcined at 300 ° C. for 1 hour to obtain a noble metal-supported powder. Thereafter, the noble metal-supported powder is immersed in a solution of 38.3 g of barium acetate in 500 cc of ion-exchanged water, stirred, dried at 120 ° C. for 2 hours, and calcined at 480 ° C. for 5 hours. The catalyst of the reference example was obtained.

[Example 1]
Except that 195.6 g of zirconium oxynitrate, 73.5 g of aluminum nitrate nonahydrate and 5000 g of urea were dissolved in 1000 cc of ion-exchanged water and stirred for 2 hours to obtain a mixed salt solution. The catalyst of Example 1 was obtained. In the carrier powder of Example 1, the mass ratio of ZrO ₂ : Al ₂ O ₃ was 90:10.

[Examples 2 to 4]
By changing the amounts of zirconium oxynitrate and aluminum nitrate nonahydrate contained in the raw salt solution, the mass ratios of ZrO ₂ : Al ₂ O _{3 in} the carrier powder were 70:30, 40:60 and 20:80, respectively. Except for this, the catalysts of Examples 2 to 4 were obtained in the same manner as Example 1.

[Comparative Example 1]
A catalyst of Comparative Example 1 was obtained in the same manner as in the Reference Example except that 100 g of zirconia powder was used as the catalyst support powder.

[Comparative Example 2]
The catalyst of Comparative Example 2 was obtained in the same manner as in the Reference Example except that the same mixed powder of 90 g of zirconia powder and 10 g of high specific surface area alumina powder as in Comparative Example 1 was used as the catalyst carrier powder. Therefore, in the carrier powder of Comparative Example 2, the mass ratio of ZrO ₂ : Al ₂ O ₃ was 90:10.

[Comparative Example 3]
A catalyst of Comparative Example 3 was obtained in the same manner as in the Reference Example except that the same mixed powder of 70 g of zirconia powder and 30 g of high specific surface area alumina powder as in Comparative Example 1 was used as the catalyst carrier powder. Therefore, the carrier powder of Comparative Example 3 had a ZrO ₂ : Al ₂ O ₃ mass ratio of 70:30.

[Comparative Example 4]
A catalyst of Comparative Example 4 was obtained in the same manner as in the Reference Example except that the same mixed powder of 40 g of zirconia powder and 60 g of high specific surface area alumina powder as in Comparative Example 1 was used as the catalyst carrier powder. Therefore, in the carrier powder of Comparative Example 4, the mass ratio of ZrO ₂ : Al ₂ O ₃ was 40:60.

[Comparative Example 5]
217.5 g of zirconium oxynitrate was dissolved in 1000 cc of ion exchange water and stirred for 2 hours. Thereafter, this salt solution was dropped into an aqueous sodium hydroxide solution to precipitate a composite hydroxide while maintaining the pH at 9.

  In the same manner as in the reference example, a catalyst carrier powder (zirconia powder) was obtained from the composite hydroxide precipitate thus obtained, and rhodium and barium were supported thereon, whereby the catalyst of Comparative Example 5 was obtained.

[Comparative Example 6]
Zirconium oxynitrate (152.3 g) and aluminum nitrate nonahydrate (220.5 g) were dissolved in ion-exchanged water (1000 cc) and stirred for 2 hours. Thereafter, this salt solution was dropped into an aqueous sodium hydroxide solution to precipitate a composite hydroxide while maintaining the pH at 9.

In the same manner as in the reference example, a catalyst carrier powder was obtained from the composite hydroxide precipitate thus obtained, and rhodium and barium were supported thereon to obtain a catalyst of Comparative Example 6. In the carrier powder of Comparative Example 6, the mass ratio of ZrO ₂ : Al ₂ O ₃ was 70:30.

  In the evaluation of the catalysts of Reference Examples, Examples, and Comparative Examples, these catalysts were used after being formed into 1 mm square pellets.

[Evaluation]
(Durable)
In order to evaluate the catalyst performance after endurance of the catalysts of Reference Examples, Examples and Comparative Examples, these catalysts were calcined in air at 750 ° C. for 24 hours.

(Carrier specific surface area)
The specific surface area of the supported catalyst was measured. This evaluation was performed by adsorbing N ₂ , which is a gas molecule having a known adsorption occupation area, and measuring the amount of adsorption. The results are shown in FIG.

(Rhodium particle size)
The rhodium particle diameter of the endured catalyst was evaluated. In this evaluation, the rhodium particle size was determined based on the amount of CO adsorbed on the catalyst. Since CO is adsorbed uniformly only to rhodium, a large amount of CO adsorption means that the rhodium particle size is small. The results are shown in FIG.

(NO _x purification rate)
1 g of the durable catalyst pellets was charged into a fixed bed flow type reactor, and rich gas (gas flow rate: 6.6 L / min) having the composition shown in Table 1 below was allowed to flow at 600 ° C. for 10 minutes. A lean gas and a rich gas having a composition of 1 were alternately circulated for 2 minutes each. Because the inlet side concentration of NO _x during the lean / rich cycle (ppm) and the outlet concentration of NO _x (ppm), it was evaluated _the NO _x purification rate. The results are shown in FIG.

〔result〕
(Carrier specific surface area)
As shown in FIG. 2, the catalyst carriers of Examples 1 to 4 of the present invention maintain a relatively large specific surface area after endurance as compared with the catalyst carriers of Reference Examples and Comparative Examples. This is because in the catalyst carrier of the present invention, zirconia and alumina, which do not form a solid solution, are well mixed with each other, so that zirconia and alumina act as a reaction barrier with each other to suppress sintering. It is thought that.

The catalyst of Example 2 and the catalyst of Comparative Example 6 are both manufactured by the coprecipitation method with a mass ratio of ZrO ₂ : Al ₂ O ₃ of 70:30. However, the catalyst of Example 2 using urea as the alkali source for coprecipitation maintains a higher specific surface area than the catalyst of Comparative Example 6 using sodium hydroxide as the alkali source. This is because urea is used as an alkali source, ammonia is decomposed by heating to generate ammonia, and the salt solution is made alkaline so that coprecipitation is performed. Zirconia and alumina are mixed well with each other. This is considered to be preferable for obtaining a carrier.

(Rhodium particle size)
As shown in FIG. 3, the catalyst carriers of Examples 1 to 4 of the present invention, particularly Examples 1 to 3 of the present invention, have relatively small rhodium particles after endurance as compared with the catalyst carriers of Reference Example and Comparative Example The diameter is maintained. This is considered to be due to the relatively large support specific surface area being maintained in the catalyst support of the example.

  Moreover, among the catalysts of Examples 1 to 4, the catalysts of Examples 1 to 3 having a relatively large zirconia content have a particularly small rhodium particle diameter. This is considered to be because the sintering of rhodium particles supported on zirconia is well suppressed by the affinity between zirconia and rhodium.

(NO _x purification rate)
As shown in FIG. 4, the catalyst carriers of Examples 1 to 4 of the present invention, particularly of Examples 1 to 3, have superior NO _x purification performance as compared with the catalyst carriers of Reference Examples and Comparative Examples. Show. This is considered to be due to the fact that the catalyst carrier of the example exhibits good catalytic performance with rhodium supported on zirconia.

(Other)
In Comparative Examples 5 and 6, sodium hydroxide was used as the alkali source for coprecipitation, but similar experiments were also conducted using potassium hydroxide as the alkali source. According to this, when potassium hydroxide was used as the alkali source, the same results as Comparative Examples 5 and 6 using sodium hydroxide as the alkali source were obtained in any evaluation.

It is a conceptual diagram of the catalyst carrier of this invention. It is a figure which shows the specific surface area after durability of the catalyst support | carrier of a reference example, an Example, and a comparative example. It is a figure which shows the rhodium particle diameter after durability of the catalyst of a reference example, an Example, and a comparative example. Reference Example illustrates the _the NO _x purification rate of the catalysts of Examples and Comparative Examples.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Primary particle | grains of zirconia 2 Primary particle | grains of alumina 10 The catalyst support | carrier obtained by the method of this invention

Claims (3)

Providing a salt solution containing a zirconium salt, an aluminum salt and urea;
Heating the salt solution to decompose urea, generating ammonia to make the salt solution alkaline, thereby forming a precipitate containing zirconium and aluminum hydroxides; and Drying and firing,
A process for producing a catalyst carrier containing zirconia and alumina. The method according to claim 1, wherein a mass ratio of ZrO ₂ : Al ₂ O ₃ in the catalyst support is 95: 5 to 40:60.   A method for producing an exhaust gas purification catalyst, comprising supporting rhodium on a catalyst carrier obtained by the method according to claim 1.

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