JP4779461B2 - Catalyst carrier, method for producing the same, and exhaust gas purification catalyst - Google Patents

Description

  The present invention relates to a catalyst carrier, a method for producing the same, and an exhaust gas purification catalyst having the catalyst carrier. In particular, the present invention has a good poison resistance against acidic substances such as sulfur compounds, and a catalyst carrier that is relatively easy to regenerate even when poisoned, a method for producing the same, and a catalyst carrier comprising the catalyst carrier. The present invention relates to an exhaust gas purifying 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. Thus in general, to oxidize CO and HC, also after purification by exhaust gas purifying catalyst for reducing NO _x, which emits these exhaust gases to 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 (TiO ₂ ), titanium (TiO ₂ ), etc., have been used with alumina 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 order for CO and HC oxidation and NO _x reduction to proceed efficiently by the action of the three-way catalyst, the air-fuel ratio of the internal combustion engine needs to be a stoichiometric air-fuel ratio (stoichiometric). Therefore, it is preferable to absorb the fluctuation of the oxygen concentration in the exhaust gas and maintain the oxygen concentration in the vicinity of the stoichiometric air-fuel ratio with the material having the OSC ability because the three-way catalyst exhibits the exhaust gas purification ability.

  Various studies have been made on the use of ceria for the purpose of obtaining such OSC ability. For example, in Patent Document 1, a base material is coated with a mixture of alumina particles and ceria particles, and a catalyst component such as a noble metal is provided here. Is disclosed. Further, Patent Document 1 also discloses that a ceria-supported catalyst carrier is obtained by impregnating a porous carrier with a solution of a cerium salt such as cerium nitrate, and drying and calcining it (paragraph 0017). Etc.).

In addition, as shown in Patent Document 2, for example, Ceria's OSC ability has been studied for use in a NO _x storage reduction catalyst that supports a noble metal such as platinum and a NO _x storage material such as barium. This NO _x storage reduction catalyst is a catalyst for purifying exhaust gas from a lean burn engine intended to improve fuel consumption.

As shown in Patent Documents 1 and 2, in the prior art, various studies have been conducted with the intention of using the OSC ability of 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 preventing platinum sintering. However, since ceria is relatively easy to sinter, ceria itself sinters, which may reduce the platinum sintering prevention effect of ceria.

  Regarding prevention of ceria sintering, Patent Documents 3 and 4 add an alkaline solution 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. It is disclosed.

  Patent Document 5 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 5, 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 6 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 6, 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 these Patent Documents 3 to 6, ceria and / or zirconia and alumina are mixed with each other, so that sintering of ceria and / or zirconia is suppressed by alumina between them.

In the NO _{x storage-and-reduction} catalysts, in particular using _the NO _x storage material, which may poisoning of the catalyst by acidic substances such as sulfur compounds in the exhaust gas becomes a problem. In order to solve this problem, as shown in Patent Document 7, an acidic metal oxide such as titania or silica is used as a carrier.

JP 2001-9280 A JP 2001-276622 A JP-A-10-202101 JP-A-11-267501 Japanese Patent No. 3379369 Japanese Patent No. 3262444 JP-A-8-57314

  In the catalyst supports shown in Patent Documents 3 to 6, the ceria and / or zirconia and alumina are uniformly mixed with each other, thereby at least partially solving the problem of ceria / or zirconia sintering. . Patent Document 7 uses an acidic metal oxide such as titania as a carrier to at least partially solve the problem of poisoning caused by sulfur compounds.

  Therefore, in the present invention, a catalyst carrier that achieves good sintering of ceria and / or zirconia, has high poisoning resistance to acidic substances such as sulfur compounds, and can be easily regenerated even when poisoned, and its production Provide a method. The present invention also provides an exhaust gas purification catalyst using such a catalyst carrier.

  The catalyst carrier of the present invention is a catalyst carrier in which primary particles of ceria and primary particles of alumina are mixed with each other, and further contains an acidic metal oxide.

  Since ceria and alumina do not substantially form a solid solution, according to the catalyst carrier of the present invention, even when used at high temperatures, ceria and alumina serve as a reaction barrier to suppress their sintering. In addition, heat resistance and OSC ability can be improved by dissolving zirconia in the primary particles of ceria.

Moreover, the catalyst carrier of the present invention has a relatively acidic property by containing an acidic metal oxide. The acid resistance is improved poisoning resistance against acidic substances such as sulfur compounds, to allow for relatively good reproduction even when poisoned. In the description of the present specification and claims, the “acidic metal oxide” refers to alumina that is contained in a catalyst carrier in which primary particles of ceria and primary particles of alumina are mixed with each other. Means a metal oxide having a large standard electrode potential. Therefore, examples of the acidic metal oxide include titanium, silica, zinc oxide, manganese oxide and the like.

  The acidic metal oxide may be dissolved in any of these ceria and alumina, or may form primary particles alone and be mixed with the primary particles of ceria and the primary particles of alumina. In this way, the acidic metal oxide is uniformly mixed with ceria and alumina at the primary particle level and is present in the entire catalyst carrier, so that the effect of ceria, such as OSC ability, platinum sintering, etc. is maintained, while oxidizing the acidic metal oxide. The effect of the product, that is, the effect of improving the poisoning resistance to acidic substances can be provided to the entire carrier.

  Examples of acidic metal oxides include titania and silica. Here, silica dissolves with alumina to provide an acidic property, and titania alone forms primary particles to provide an acidic property.

In the catalyst support of the present invention, preferably, the acidic metal oxide is titania, and the catalyst support has a mass ratio of Al ₂ O ₃ : CeO ₂ of 95: 5 to 75:25, (Al ₂ O ₃ + CeO ₂ ). weight ratio of TiO ₂ is 95: 5 to 50: 50, in particular 80: 20 to 60: 40.

Exhaust gas purifying catalyst of the present invention, the catalyst carrier of the present invention, platinum, and a noble metal selected from the group consisting of rhodium and palladium, an alkali metal, NO _x storage material that is selected from the group consisting of alkaline earth metals and rare earth And is carried.

  The method of the present invention for producing a catalyst carrier containing ceria, alumina, and acidic metal oxide comprises a raw material solution containing cerium, aluminum, and a metal salt or hydrolyzable compound constituting the acidic metal oxide. Providing, generating a precipitate containing cerium, aluminum and a hydroxide of the above metal from the raw material solution, and drying and calcining the resulting agglomerates.

  In one embodiment of the method of the present invention, the raw material solution contains cerium, aluminum and the above metal salt, and further urea, and the raw material solution is heated to decompose urea to generate ammonia, and the raw material solution Is made alkaline to produce a precipitate.

  According to this aspect, the raw material solution can be uniformly and rapidly changed to basic by heating the raw material solution containing urea to decompose urea and generate ammonia. Thus, making the raw material solution basic 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.

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

  The catalyst carrier 10 of the present invention is composed of fine primary particles containing ceria, alumina, and acidic metal oxide. For example, as shown in FIG. 1, primary particles 1 of acidic metal oxide-alumina solid solution. And the primary particles 2 of ceria are mixed with each other. In the catalyst carrier of the present invention, primary particles of the acidic metal oxide-ceria solid solution and primary particles of alumina may be mixed with each other. Thus, the acidic metal oxide can be dissolved in either ceria or alumina depending on its properties.

  Furthermore, in the catalyst carrier of the present invention, ceria, alumina, and acidic metal oxide constitute primary particles independently, and the primary particles of ceria, primary particles of alumina, and primary particles of acidic metal oxide are mixed with each other. May be.

  Therefore, for example, when the acidic metal oxide is silica, silica is solid-dissolved in alumina. Therefore, the catalyst carrier of the present invention is composed of primary particles of silica-alumina solid solution and primary particles of ceria mixed with each other. In addition, when the acidic metal oxide is titania, titania does not form a solid solution with either alumina or ceria. Therefore, the catalyst carrier of the present invention includes primary alumina particles, primary ceria particles, and primary titania particles mixed together. Become.

  The catalyst carrier of the present invention can be produced by any method, and in particular, can be produced by the method of the present invention.

[Method for producing catalyst carrier]
(Provision of raw material solution)
In the method of the present invention for producing a catalyst support, first, a raw material solution containing cerium, aluminum, and a metal salt or alkoxide constituting an acidic metal oxide is provided.

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

  Examples of hydrolyzable compounds that can be used here include alkoxides and β-keto acid salts.

(Precipitation formation)
In the method of the present invention for producing a catalyst carrier, a precipitate containing cerium, aluminum and a metal hydroxide constituting an acidic metal oxide is then generated from the raw material solution.

  When the raw material solution contains cerium, aluminum, and a metal salt constituting an acidic metal oxide, a precipitate is generated by making the raw material solution alkaline. Here, in order to make the raw material solution alkaline, a basic compound such as sodium hydroxide or ammonia can be added to the raw material solution.

  Alternatively, the raw material solution may further contain urea, and this solution is heated to decompose urea, generate ammonia, and make the solution alkaline, thereby generating a precipitate.

  When the raw material solution contains cerium, aluminum, and a metal hydrolyzable compound constituting an acidic metal oxide, a precipitate is generated by adding water to the raw material solution and hydrolyzing it.

(Drying and firing the precipitate)
Finally, in the method of the present invention for producing a catalyst carrier, the obtained aggregate is dried and calcined to produce a catalyst carrier containing ceria, alumina and acidic metal oxide.

  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.

[Manufacture of exhaust gas purification catalyst]
The exhaust gas purifying catalyst of the present invention can be achieved by supporting a noble metal such as platinum and a NO _x storage material such as barium on the catalyst carrier of the present invention. The loading of the noble metal can be achieved by any method. For example, in the case of loading platinum, the catalyst carrier is impregnated with a dinitrodiammine platinum nitrate aqueous solution, dried and calcined, and 0.5 to 2% by weight of platinum is loaded. The exhaust gas purification catalyst of the present invention can be manufactured. 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]
16.3 g of cerium nitrate hexahydrate and 234.5 g of aluminum nitrate nonahydrate were dissolved in 1000 cc of ion-exchanged water and stirred for 1 hour to obtain a raw material solution. This raw material solution was added dropwise to the aqueous ammonia to obtain a composite hydroxide precipitate while maintaining the pH at 9.

Moisture was filtered from the solution containing the composite hydroxide precipitate obtained in this way, washed, dried at 120 ° C. for 2 hours, and calcined at 300 ° C. for 2 hours. A powder was obtained. In the carrier powder of this reference example, the mass ratio of Al ₂ O ₃ : CeO ₂ was 84:16.

  Thereafter, 150 g of the carrier powder of the reference example was immersed in an aqueous solution in which a dinitrodiammine platinum nitric acid aqueous solution for 2 g of platinum and a rhodium nitrate aqueous solution for 0.5 g of rhodium were put in 500 cc of ion-exchanged water, and stirred for 2 hours. Further, 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]
16.3 g of cerium nitrate hexahydrate, 234.5 g of aluminum nitrate nonahydrate, and 7.6 g of titanium (IV) chloride were dissolved in 1000 cc of ion-exchanged water and stirred for 1 hour to obtain a raw material solution. Except for this, the catalyst of Example 1 was obtained in the same manner as in the Reference Example. In the carrier powder of Example 1, the mass ratio of Al ₂ O ₃ : CeO ₂ was 84:16, and the mass ratio of (Al ₂ O ₃ + CeO ₂ ): TiO ₂ was 95: 5.

[Examples 2 to 4]
The amount of titanium (IV) chloride contained in the raw material solution was changed so that the (Al ₂ O ₃ + CeO ₂ ): TiO ₂ ratio of the carrier powder was 70:30, 50:50 and 30:70, respectively. Except this, the catalyst of Examples 2 to 4 was obtained in the same manner as Example 1. In these carrier powders of Examples 2 to 4, the mass ratio of Al ₂ O ₃ : CeO ₂ was 84:16.

[Comparative Example 1]
A catalyst of Comparative Example 1 was obtained in the same manner as in the Reference Example except that 124.5 g of high specific surface area alumina powder and 25.5 g of ceria powder were mixed to obtain a carrier powder. In the carrier powder of Comparative Example 1, the mass ratio of Al ₂ O ₃ : CeO ₂ was 83:17.

[Comparative Example 2]
A catalyst of Comparative Example 2 was obtained in the same manner as in the Reference Example except that 124.5 g of high specific surface area alumina powder, 25.5 g of ceria powder and 64.3 g of titania powder were mixed to obtain a carrier powder. In the carrier powder of Comparative Example 2, the mass ratio of Al ₂ O ₃ : CeO ₂ was 83:17, and the (Al ₂ O ₃ + CeO ₂ ): TiO ₂ ratio was 70:30. That is, the carrier powder of Comparative Example 2 had almost the same composition as the carrier powder of Example 2.

[Comparative Example 3]
In the same manner as in the reference example, a carrier powder in which primary particles of ceria and primary particles of alumina were mixed with each other was obtained. The carrier powder had an Al ₂ O ₃ : CeO ₂ mass ratio of 84:16.

  70 g of the carrier powder thus obtained and 7.6 g of titania (IV) chloride were sequentially added to 1000 cc of ion-exchanged water being stirred and mixed. After stirring for 1 hour, it was dried overnight at 120 ° C. and calcined at 300 ° C. for 2 hours to obtain a carrier powder of Comparative Example 3. The catalyst carrier of Comparative Example 3 thus obtained was loaded with platinum, rhodium and barium in the same manner as in the Reference Example to obtain the catalyst of Comparative Example 3.

In the carrier powder of Comparative Example 3, the mass ratio of Al ₂ O ₃ : CeO ₂ was 84:16, and the mass ratio of (Al ₂ O ₃ + CeO ₂ ): TiO ₂ was 70:30. That is, the carrier powder of Comparative Example 3 (the primary particles of ceria and alumina mixed with each other were further supported with titania by impregnation) was the carrier powder of Example 2 (primary particles of ceria, alumina, and titania). Have the same composition, although the composition is different.

  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]
(Acid amount)
The acid amounts of the carrier powders of Examples and Comparative Examples are shown in FIG. Here, the acid amount means an NH ₃ desorption amount measured by a temperature-programmed desorption method (TPD) in which NH ₃ adsorbed on the catalyst is desorbed by a temperature rise.

(Durable)
Reference Example, in order to evaluate the _the NO _x purification performance of the catalyst after the durability test of the examples and comparative examples, these catalysts were calcined for 5 hours at 800 ° C. in air over the following 4 hours, shown in Tables 1 Durability was performed by alternately flowing lean gas and rich gas (gas flow rate: 5.0 L / min) at 400 ° C. for 4 minutes and 1 minute, respectively. The sulfur poisoning amount by this durability is shown in FIG. A carbon / sulfur analyzer (C / S analyzer, manufactured by HORIBA) was used for evaluating the sulfur poisoning amount.

(Sulfur desorption evaluation)
1 g of the durable catalyst pellets was charged into a fixed bed flow reactor, and a rich gas (gas flow rate: 6.6 L / min) having the composition shown in Table 2 below was passed at 600 ° C. for 10 minutes for regeneration treatment, Thereafter, the amount of sulfur poisoning was evaluated. The results are shown in FIGS. Also, from the sulfur poisoning amount after endurance (before regeneration treatment) and the sulfur poisoning amount after regeneration treatment with rich gas, the ratio of desorbed sulfur (= the amount of sulfur desorbed by regeneration treatment (g) / before regeneration treatment) Of sulfur poisoning (g) × 100). The results are shown in FIG.

(NO _x purification rate evaluation)
1 g of the durable catalyst pellets was packed into a fixed bed flow reactor, and a rich gas (gas flow rate: 6.6 L / min) having the composition shown in Table 3 below was passed at 600 ° C. for 10 minutes for regeneration treatment. After that, lean gas and rich gas 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), was evaluated _the NO _x purification rate. The results are shown in FIG.

〔result〕
(Acid amount)
As shown in FIG. 2, in the catalyst supports of the examples, reference examples, and comparative examples of the present invention, the acid amount increases as the titania content increases.

(Sulfur desorption)
As shown in FIG. 3, in the catalyst carrier of the example of the present invention, the degree of poisoning by sulfur decreases as the acid amount increases due to the increase in titania content. Here, the catalysts of Example 2 and Comparative Examples 2 and 3 in which both titania contents are 30 wt% are poisoned by sulfur to substantially the same extent. However, as understood from FIGS. 4 and 5, compared with the catalysts of Comparative Examples 2 and 3, the catalyst of Example 2 of the present invention was regenerated from sulfur poisoning relatively well by the regeneration treatment with rich gas. Yes. This is because, in the catalyst of Example 2 of the present invention, fine alumina, ceria and titania particles are mixed with each other in the carrier, so that the regeneration effect from sulfur poisoning by titania, which is an acidic metal oxide, is achieved. This is considered to be due to the fact that the carrier works well.

(NO _x purification rate)
As shown in Figure 6, by adjusting the acid quantity of the catalyst support, it is possible to change the _the NO _x purification performance. This is considered to be due to the fact that the acidity of the carrier affects the regeneration from sulfur poisoning. In particular, excellent NO _x purification is provided when the carrier powder has a mass ratio of (Al ₂ O ₃ + CeO ₂ ): TiO ₂ of 95: 5 to 50:50.

Further, as can be understood from FIG. 6 in particular, the catalyst of Example 2 (using a carrier powder in which primary particles of ceria, alumina and titania are alternately mixed) and the catalyst of Comparative Example 3 (ceria and primary alumina) are used. particles are mixed alternately, by the use) with a carrier powder titania is carried by impregnation here, significant differences occurs in _the NO _x purification rate. This is because, in the carrier powder of Example 2, since titania is mixed with ceria and alumina at the primary particle level, both the effect of OSC ability by ceria and the effect of sulfur poisoning resistance by titania are good. It is thought to be due to the provision.

  The difference between the reference example and the comparative example 1 is considered to be due to the relatively large surface area obtained with the support of the reference example.

It is a conceptual diagram of the catalyst carrier of this invention. It is a figure which shows the acidity of a catalyst support | carrier. It is a figure which shows the grade of the sulfur poisoning after durability of a catalyst. It is a figure which shows the grade of the sulfur poisoning of the catalyst after a regeneration process. It is a figure which shows the ratio of the sulfur removed from a catalyst by a regeneration process. Example illustrates the _the NO _x purification performance of the catalysts of Reference Examples and Comparative Examples.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Primary particle of acidic metal oxide-ceria solid solution (or ceria) 2 Primary particle of alumina (acidic metal oxide-alumina solid solution) 10 Catalyst carrier of the present invention

Claims (6)

A catalyst carrier in which primary particles of ceria, primary particles of alumina , and primary particles of titania are mixed with each other. The catalyst carrier according to claim 1 , wherein zirconia is dissolved in primary particles of the ceria. In the catalyst _support, Al ₂ O 3: mass ratio of CeO ₂ is 95: 5 to 75: A 25, and _{(Al 2 O 3 + CeO 2} ): the weight ratio of TiO ₂ is 95: 5 to 50: 50 The catalyst carrier according to claim 1 or 2 , wherein The catalyst carrier according to any one of claims 1 to 3, platinum, and a noble metal selected from the group consisting of rhodium and palladium, an alkali metal, NO _x storage material that is selected from the group consisting of alkaline earth metals and rare earth And an exhaust gas purification catalyst. A method for producing a catalyst carrier containing ceria, alumina and titania ,
Providing a raw material solution containing salts or hydrolyzable compounds of cerium, aluminum and titanium ;
Generating a precipitate containing hydroxides of cerium, aluminum and titanium from the raw material solution, and drying and calcining the obtained aggregate.
A process for producing a catalyst support containing ceria, alumina and titania . The raw material solution contains a salt of cerium, aluminum, and titanium , and further urea, the raw material solution is heated to decompose urea, generate ammonia, and make the raw material solution alkaline, thereby causing the precipitation. The method of claim 5 , wherein the product is produced.

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