JP2006326477A - Method for manufacturing catalyst for cleaning exhaust gas - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To deposit a NOx storage material on a carrier oxide in a minuter state and deposit a noble metal and the NOx storage material respectively in highly dispersed states. <P>SOLUTION: The noble metal and the NOx storage material are deposited on the carrier oxide by using a target containing a noble metal source, a NOx storage material source and a carrier oxide source and by a laser ablation method. When the target is irradiated with laser beams, the noble metal source and the NOx storage material source are mixed with each other on the atomic levels in ionized states or neutral particle states and the obtained mixture is then mixed with the carrier oxide source on the atomic levels. As a result, the noble metal and the NOx storage material can be deposited dispersedly on the carrier oxide by atomic units and respectively in highly dispersed states. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

The present invention relates to a method for producing a NO _x storage reduction type exhaust gas purification catalyst.

Normally occluding NO _x in _the NO _x storage material in an oxygen excess lean atmosphere, by the rich atmosphere of the pulse to the reducing agent excess, _the NO _x storage to reduce and purify the released NO _x from _the NO _x storage material Reduction-type exhaust gas purification catalysts are known. This exhaust gas-purifying catalyst is obtained by supporting a noble metal such as Pt and a NO _x storage material such as an alkali metal or alkaline earth metal on a carrier oxide such as alumina.

In exhaust gas in a lean atmosphere, HC and CO are oxidized and purified by noble metals, and NO _x that is oxidized by NO is stored in the NO _x storage material as nitrate. When the pulse to be a rich atmosphere, it is NO _x is released from _the NO _x storage material, HC present in excess in the atmosphere by the noble metal is reduced and purified by the reaction such as CO.

However, in the conventional NO _x storage-and-reduction type exhaust gas purifying catalyst, grain growth is the noble metal exposed to high temperatures occurs, there is a problem that activity is reduced. There is also a problem that the NO _x storage capacity is rapidly deteriorated by the sulfur component contained in the fuel.

From the reaction mechanism described above, it is desirable that the noble metal and the NO _x storage material be as close as possible to each other. That is, since NO in exhaust gas is oxidized to NO ₂ and reacts with the NO _x storage material to become nitrate, it is desirable that the noble metal that oxidizes NO and the NO _x storage material are close to each other. . In a rich atmosphere, NO _x released by the decomposition of nitrate is reduced by the noble metal. In this case, it is desirable that the noble metal and the NO _x storage material be close to each other.

Furthermore, the problem of sulfur poisoning, in which the NO _x storage capacity deteriorates rapidly due to the sulfur component, is caused by the fact that the NO _x storage material reacts with the sulfur component to become sulfate, which is more stable than nitrate. Yes. However, even a sulfate can be decomposed by the catalytic action of a noble metal. In this case as well, it is desirable that the noble metal and the NO _x storage material be close to each other.

Further, it is obvious that the precious metal and the NO _x storage material are more active as the area in contact with the exhaust gas is larger. Therefore, in order to support the NO _x storage material and the noble metal close to each other, it is desirable to support both close to each other in a highly dispersed state. For example, a noble metal can be supported in a fine state by impregnating a carrier oxide with an aqueous solution of a water-soluble noble metal compound and firing it.

However, noble metals such as Pt have a problem that grain growth occurs when used at a high temperature, thereby reducing the activity. Therefore, Japanese Patent Application Laid-Open No. 2003-080077 describes that nanometer-sized base particles are prepared from CeO ₂ or the like using a physical evaporation method and coated with a noble metal layer at an atomic level. Japanese Patent Application Laid-Open No. 2004-195339 describes a method in which a nanometer-sized carrier layer is formed using a laser ablation method, and a noble metal is supported while maintaining a dispersed state. According to these methods, the size of one catalyst particle as a whole can be reduced to 100 nm or less, and the specific surface area can be large and highly active.

However in the conventional method of manufacturing a NO _x storage-and-reduction type exhaust gas purifying catalyst, in carrying _the NO _x storage material is an aqueous solution, such as nitrates or acetates of the NO _x storage material impregnated with a carrier oxide, then calcined The method to do is common. Therefore, crystallization and sintering occur during firing, and the NO _x storage material is supported as large particles having a particle size of several tens of nanometers or more. Therefore, there is a limit to supporting the noble metal and the NO _x storage material in a highly dispersed state, and development of a method for supporting the NO _x storage material in a finer state is desired.
JP2003-080077A JP2004-195339

The present invention has been made in view of the above circumstances, carrying _the NO _x storage material further fine state, and object to be achieved by carrying the noble metal and _the NO _x storage material in a highly dispersed state with each other To do.

Features of the method of manufacturing an exhaust gas purifying catalyst of the present invention for solving the aforementioned problems is use of a target containing a noble metal source, and _the NO _x storage material source and a carrier oxide source, a noble metal on the carrier oxide by laser ablation It is to carry the _the NO _x storage material and.

The carrier oxide preferably contains Al ₂ O ₃ and CeO ₂ . In this case, the target includes a first target including at least a noble metal source and a CeO ₂ source, and a second target including an Al ₂ O ₃ source, and the first target and the second target can be laser ablated simultaneously. desirable.

According to the production method of the present invention, since a target including a noble metal source, a NO _x storage material source, and a carrier oxide source is used, the noble metal source and the NO _x storage material source are in an ionic state by laser irradiation. Alternatively, it is mixed at the atomic level in the neutral particle state and mixed with the carrier oxide source at the atomic level. As a result, the noble metal and the NO _x storage material are dispersed and supported in atomic units on the carrier oxide, and can be supported in a highly dispersed state.

Therefore, according to the NO _x storage-and-reduction type catalyst produced by the production method of the present invention is excellent in _the NO _x storage capacity and _the NO _x reduction ability, is superior in recovery ability from sulfur poisoning.

Further, if the carrier oxide contains Al ₂ O ₃ and CeO ₂ , the grain growth of the noble metal can be further suppressed by CeO ₂ . If the first target containing at least a noble metal source and a CeO ₂ source and the second target containing an Al ₂ O ₃ source are laser ablated at the same time, the effect of suppressing the grain growth of noble metal by CeO ₂ is maximized in the obtained catalyst. To express.

In the method for producing an exhaust gas purifying catalyst of the present invention, a target including a noble metal source, an NO _x storage material source, and a carrier oxide source is used, and the noble metal and the NO _x storage material are supported on the carrier oxide by a laser ablation method. is doing. Hereinafter, the noble metal source, the NO _x storage material source, and the carrier oxide source are referred to as catalyst raw materials. The target may be one target including all kinds of catalyst raw materials, or a plurality of targets including any one or plural kinds selected from catalyst raw materials, and each target may be irradiated with a laser simultaneously or with a slight time difference. Also good.

Examples of the noble metal include Pt, Rh, Pd, and Ir, and examples of the NO _x storage material include alkali metals, alkaline earth metals, and rare earth elements. It is desirable to use at least one of alkali metals and alkaline earth metals having a high NO _x storage capacity. Rare earth elements may also be used but that _the NO _x storage capacity low, Ce is not classified as _the NO _x storage material. Examples of the carrier oxide include Al ₂ O ₃ , TiO ₂ , ZrO ₂ , CeO ₂ , and SiO ₂ .

Therefore, the noble metal source included in the target may be generated by laser ablation, and various noble metal compounds can be used. As the NO _x storage material source, a NO _x storage material (oxide, carbonate, etc.) may be generated by laser ablation, and hydroxide, oxide, carbonate, nitrate, etc. can be used. Furthermore, as the carrier oxide source, a carrier oxide may be generated by laser ablation, and metals, oxides, various metal salts, hydroxides, and the like can be used.

  The ratio of each catalyst raw material in the target is not equal to the composition ratio of the target catalyst. Since the degree of ablation varies depending on the raw material species, it is necessary to adjust the ratio of each catalyst raw material so that the final composition ratio of the catalyst is obtained. In the case of distribution to a plurality of targets, the ratio of each catalyst raw material in the total of all targets may be as described above.

  When the target is irradiated with a laser, neutral particles such as ions, atoms, molecules, and clusters are desorbed from the target. Moreover, from the target containing a plurality of types of catalyst raw materials, a plurality of types of ions or neutral particles are desorbed, and a catalyst is obtained in which each exists in the vicinity of each other. Therefore, it is desirable that catalyst raw materials that are preferably present in the vicinity of each other are included in the same target.

For example, when Pt is used as the noble metal, it is desirable to use CeO ₂ having a grain growth suppressing effect of Pt as the support oxide, and therefore it is desirable that the Pt source and the CeO ₂ source are included in the same target. In addition, when Rh is used as the noble metal, ZrO ₂ is desirable for the carrier oxide, and therefore it is desirable that the Rh source and the ZrO ₂ source be included in the same target. Further, when Pt and Rh are contained, it is desirable to separate Pt and Rh as much as possible in order to maximize the mutual activity. That is, the Pt source and the Rh source are preferably included in different targets. Therefore, when producing a catalyst supporting Pt and Rh, it is particularly desirable to use two types of targets, a target including a Pt source and a CeO ₂ source, and a target including an Rh source and a ZrO ₂ source. In addition, since the NO _x storage material may reduce the activity of Rh, it is desirable to include it in a separate target from the NO _x storage material source and the Rh source.

  The conditions for laser ablation may be any conditions in which the respective catalyst raw materials are desorbed from the target almost simultaneously as ions or neutral particles and then mixed. Each catalyst raw material desorbed from the target is led out from the laser ablation device to obtain catalyst powder. If a substrate or the like is placed in the laser ablation apparatus, a catalyst is formed on the substrate in the form of a film.

  Hereinafter, the present invention will be specifically described with reference to Examples and Comparative Examples.

Example 1
FIG. 1 schematically shows the laser ablation apparatus used in this example. This apparatus includes an excimer in a vacuum chamber 1, a gas introduction system 2 for supplying argon gas to the vacuum chamber 1, two holders 3 disposed in the vacuum chamber 1, and targets A and B held in the holder 3. It comprises two laser optical systems 4 that irradiate laser beams almost simultaneously, and an exhaust system 5 that exhausts the inside of the vacuum chamber 1 and collects the generated catalyst powder.

5 parts by weight of platinum black, 20 parts by weight of CeO ₂ powder, and 20 parts by weight of barium carbonate powder are mixed, pelletized with a uniaxial molding machine, fired at 500 ° C. in the atmosphere, and target A Was prepared. On the other hand, Rh / Al ₂ O ₃ powder in which Rh is supported on the high specific surface area Al ₂ O ₃ powder by the water absorption method is 2 parts by weight as Rh, and high specific surface area Al ₂ O ₃ powder is 120 parts by weight. 20 parts by weight of a specific surface area ZrO ₂ powder was mixed, pelletized with a uniaxial molding machine, and then fired in air at 500 ° C. to prepare target B.

  In addition, in order to prepare a target, it is not restricted to the said method, The impregnation method (water absorption carrying | support method) etc. which are used for the manufacturing method of a general catalyst can also be used. However, in order to make the ablation rate uniform, it is desirable to bake at around 500 ° C.

The targets A and B were respectively held in the holder 3, the argon gas flow rate by the gas introduction system 2 was 30 L / min, and the exhaust speed by the exhaust system 6 was 30 L / min to stabilize supply and exhaust. Then, laser ablation was performed by irradiating the targets A and B with a pulsed laser at 0.5 Hz under the conditions of output: 400 mJ / 1 pulse, output density: 50 J / cm ² , and irradiation time: 50 nsec.

  The powder collected by the exhaust system 5 was molded by a uniaxial molding machine and lightly pulverized to prepare a pellet catalyst having an average particle diameter of 1 to 2 mm.

(Example 2)
5 parts by weight of platinum black and 20 parts by weight of barium carbonate powder were mixed, and a target A was prepared in the same manner as in Example 1. On the other hand, the same Rh / Al ₂ O ₃ powder as in Example 1 was used as Rh, 2 parts by weight, 120 parts by weight of high specific surface area Al ₂ O ₃ powder, 20 parts by weight of high specific surface area ZrO ₂ powder, and CeO. ₂ parts of the powder was mixed with 20 parts by weight, and a target B was prepared in the same manner as in Example 1. The targets A and B were respectively held in the holder 3 and laser ablation was performed in the same manner as in Example 1 to prepare a pellet catalyst in the same manner.

(Example 3)
Target A was prepared in the same manner as Example 1 using only barium carbonate. On the other hand, 5 parts by weight of platinum black, 2 parts by weight of Rh / Al ₂ O ₃ powder similar to Example 1 as Rh, 120 parts by weight of high specific surface area Al ₂ O ₃ powder, and high specific surface area ZrO ₂ 20 parts by weight of the powder and 20 parts by weight of the CeO ₂ powder were mixed, and the target B was prepared in the same manner as in Example 1. The targets A and B were respectively held in the holder 3 and laser ablation was performed in the same manner as in Example 1 to prepare a pellet catalyst in the same manner.

Example 4
5 parts by weight of platinum black, 2 parts by weight of Rh / Al ₂ O ₃ powder as in Example 1 as Rh, 120 parts by weight of high specific surface area Al ₂ O ₃ powder, and high specific surface area ZrO ₂ powder 20 parts by weight, 20 parts by weight of CeO ₂ powder, and 20 parts by weight of barium carbonate powder were mixed, and a target A was prepared in the same manner as in Example 1. After holding the target A in both holders 3, laser ablation was performed in the same manner as in Example 1 to prepare a pellet catalyst in the same manner.

(Comparative Example 1)
120 g of the high specific surface area Al ₂ O ₃ powder used in Example 1, 2.0 g of dinitrodiammine platinum nitrate aqueous solution in terms of Pt, 0.5 g of rhodium nitrate aqueous solution in terms of Rh, and 1000 cc of ion-exchanged water were mixed. After stirring for 2 hours, it was dried at 120 ° C. for 2 hours and calcined at 300 ° C. for 1 hour. To this, 20 g of high specific surface area ZrO ₂ powder and 20 g of CeO ₂ powder were mixed, and this mixed powder was put into an aqueous solution in which 51.1 g of barium acetate was dissolved in 500 cc of ion-exchanged water. After drying at 120 ° C. for 2 hours, it was calcined at 500 ° C. for 5 hours.

  Using the obtained catalyst powder, it was molded by a uniaxial molding machine and lightly pulverized to prepare a pellet catalyst having an average particle diameter of 1 to 2 mm.

(Comparative Example 2)
The catalyst powder prepared in Comparative Example 1 was further calcined at 700 ° C. in the atmosphere, then molded with a uniaxial molding machine, and lightly pulverized to prepare a pellet catalyst having an average particle diameter of 1 to 2 mm.

(Comparative Example 3)
120 g of the high specific surface area Al ₂ O ₃ powder used in Example 1, 2.0 g of dinitrodiammine platinum nitrate aqueous solution in terms of Pt, 0.5 g of rhodium nitrate aqueous solution in terms of Rh, and 1000 cc of ion-exchanged water were mixed. After stirring for 2 hours, it was dried at 120 ° C. for 2 hours and calcined at 300 ° C. for 1 hour. This was further fired at 900 ° C. in the atmosphere to grow noble metal grains.

Thereafter, in the same manner as in Example 1, 20 g of high specific surface area ZrO ₂ powder and 20 g of CeO ₂ powder were mixed, and this mixed powder was put into an aqueous solution in which 51.1 g of barium acetate was dissolved in 500 cc of ion-exchanged water, and stirred for 2 hours. Thereafter, the water was evaporated, dried at 120 ° C. for 2 hours, and calcined at 500 ° C. for 5 hours.

  Using the obtained catalyst powder, it was molded by a uniaxial molding machine and lightly pulverized to prepare a pellet catalyst having an average particle diameter of 1 to 2 mm.

<Test and evaluation>
Table 1 summarizes the composition of the target in each example.

  About the pellet catalyst of each Example and each comparative example, the particle size of Ba was measured by XRD. The results are shown in FIG.

Next, the same amount of the pellet catalyst of each Example and each Comparative Example was filled in the evaluation apparatus, and the durability test was performed using the model gas and N ₂ gas shown in Table 2 in the pattern shown in FIG. Each gas flow rate is 25 L / min.

  Each pellet catalyst after the endurance test was charged in 2 g in a fixed bed flow reactor, and first treated with rich gas at 600 ° C. for 10 minutes using the model gas shown in Table 3, and then lean gas / rich gas at 600 ° C. = Repeated distribution at 20 minutes / 10 seconds. Each gas flow rate is 6.6 L / min.

Then the concentration of NO output gas immediately after flowing lean 20 minutes were measured respectively, it was assessed _the NO _x storage amount. The results are shown in FIG. 3 in terms of the NO _x storage amount per liter of the catalyst. Further, each NO _x storage amount is arranged with respect to the particle size of Ba and is shown in FIG.

From FIG. 3 and FIG. 4, it is clear that the catalyst produced in each example shows a high NO _x storage capacity, which is caused by the small particle size of Ba of about 15 nm or less. That is, according to the production method of the present invention, it is apparent that a catalyst carrying the NO _x storage material in a fine state can be produced easily and stably.

In addition, when the Examples are compared, the catalyst produced in Example 1 shows a particularly high NO _x occlusion amount. By including Pt and Ce in the same target, the grain growth of Pt by CeO ₂ is achieved. It is thought that the inhibitory effect was expressed well.

It is typical explanatory drawing of the laser ablation apparatus used for one Example of this invention. It is a temperature-time chart which shows the test conditions of the catalyst manufactured in the Example of this invention. Is a bar graph showing _the NO _x storage amount. It is a graph showing the relationship between the Ba particle size and _the NO _x storage amount.

Explanation of symbols

1: Vacuum tank 2: Gas introduction system 2 3: Holder 4: Laser optical system 5: Exhaust system A, B: Target

Claims (3)

Production of a catalyst for exhaust gas purification using a target containing a noble metal source, a NO _x storage material source, and a carrier oxide source, and carrying the noble metal and NO _x storage material on the carrier oxide by a laser ablation method Method. The method for producing an exhaust gas purifying catalyst according to claim 1, wherein the carrier oxide contains Al ₂ O ₃ and CeO ₂ . The target comprises at least a first target including the noble metal source and a CeO ₂ source and a second target including an Al ₂ O ₃ source, and laser ablating the first target and the second target simultaneously. The manufacturing method of the catalyst for exhaust gas purification as described in any one of.

Images