JP2005000829A - Catalyst for purifying exhaust gas and production method therefor - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To control the particle growth of a noble metal and to efficiently use most of the noble metal for developing an oxygen occlusion release capacity. <P>SOLUTION: A catalyst comprises a carrier containing a composite particle comprising an oxygen absorption release material particle and an alumina layer covering at least a part of the surface of the particle and the noble metal supported on at least the alumina layer of the carrier. The noble metal supported on the alumina layer is resistant to gain growth. Oxygen easily passes through the alumina layer. Since the noble metal is an entrance for oxygen, OSC (oxygen absorption and release capacity) is high. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to an exhaust gas purifying catalyst that is disposed in an exhaust system of an automobile and purifies exhaust gas, and more particularly to an exhaust gas purifying catalyst having a stable purifying activity regardless of fluctuations in oxygen concentration in the exhaust gas and a method for producing the same.
[0002]
[Prior art]
The exhaust gas purifying catalyst (three-way catalyst) includes, for example, a carrier substrate made of heat-resistant ceramics such as cordierite, a catalyst carrier layer made of activated alumina or the like formed on the carrier substrate, and a catalyst carrier layer And a noble metal such as supported Pt. This three-way catalyst oxidizes and purifies hydrocarbons (HC) and carbon monoxide (CO) in the exhaust gas of the internal combustion engine, and reduces and purifies nitrogen oxides (NO _x ).
[0003]
However, since the oxygen concentration in the exhaust gas varies greatly depending on operating conditions and the like, the oxidation and reduction purification activity may become unstable in the three-way catalyst. Therefore, ceria is added to the catalyst support layer. Ceria has an oxygen absorption / release capacity (hereinafter referred to as OSC) that stores oxygen in an oxidizing atmosphere and releases oxygen in a reducing atmosphere, thereby obtaining a stable purification activity even when the oxygen concentration in the exhaust gas fluctuates. It is done.
[0004]
Further, when a three-way catalyst containing ceria is used at a high temperature of 800 ° C. or higher, the OSC tends to be lowered due to the crystal growth of ceria and the accompanying noble metal grain growth. For this reason, a solid solution of ceria and zirconia is also used to suppress ceria crystal growth and maintain a high OSC.
[0005]
However, the exhaust gas temperature has risen remarkably with the recent improvement in engine performance and the increase in high-speed driving. Therefore, the temperature of the exhaust gas purifying catalyst at the time of use is considerably increased as compared with the prior art, and it is difficult to suppress the noble metal grain growth even if a solid solution of ceria and zirconia is used.
[0006]
Japanese Patent Application Laid-Open No. 08-131830 proposes an exhaust gas purification catalyst in which Pt is supported on alumina particles and the surface thereof is covered with a ceria layer. According to this exhaust gas purifying catalyst, alumina is excellent in heat resistance, so that the grain growth of Pt supported on alumina is suppressed. The ceria layer suppresses Pt from becoming platinum oxide even in a lean atmosphere, and the ceria layer suppresses the vapor phase movement of platinum oxide, thereby further suppressing the Pt grain growth.
[0007]
However, even with this catalyst, it is difficult to suppress the growth of ceria grains, so it is difficult to suppress the decrease in OSC after durability.
[0008]
Japanese Patent Application Laid-Open No. 10-202102 proposes an exhaust gas purifying catalyst in which a noble metal is supported on an Al—Ce—Zr composite oxide carrier prepared from a metal alkoxide. By using the Al—Ce—Zr composite oxide as a carrier, the growth of ceria grains can be suppressed, and the decrease in OSC after durability can be suppressed.
[0009]
However, in this catalyst, grain growth occurs in the noble metal, and it has been difficult to further suppress the decrease in purification activity after durability. It has also been revealed that noble metals are not effectively used, and there are considerable amounts of noble metals that do not contribute to the expression of OSC.
[0010]
[Patent Document 1] JP-A-08-131830 [Patent Document 2] JP-A-10-202102
[Problems to be solved by the invention]
The present invention has been made in view of such circumstances, and an object thereof is to further suppress the grain growth of the supported noble metal and to efficiently use the noble metal for the expression of OSC.
[0012]
[Means for Solving the Problems]
The exhaust gas purifying catalyst of the present invention that solves the above problems is characterized in that a carrier comprising composite particles comprising oxygen storage / release material particles having OSC and an alumina layer covering at least a part of the surface of the oxygen storage / release material particles; And at least a noble metal supported on an alumina layer of the support.
[0013]
The oxygen storage / release material particles preferably contain at least ceria, and more preferably contain zirconia.
[0014]
The exhaust gas purifying catalyst of the present invention is particularly effective when the noble metal is Pt.
[0015]
The alumina layer preferably has a thickness of 1 to 50 nm, and preferably covers 50% or more of the surface of the oxygen storage / release material particles.
[0016]
The feature of the production method of the present invention capable of producing the exhaust gas purifying catalyst is that at least a part of the surface of the oxygen storage / release material particles is obtained by attaching an aluminum-based coupling agent to the oxygen storage / release material particles having OSC and firing. A carrier made of composite particles on which an alumina layer is formed is prepared, and a noble metal is carried on the carrier.
[0017]
Also in the production method of the present invention, the oxygen storage / release material particles preferably contain at least ceria, and more preferably contain zirconia. The alumina layer preferably has a thickness of 1 to 50 nm, and preferably covers 50% or more of the surface of the oxygen storage / release material particles.
[0018]
DETAILED DESCRIPTION OF THE INVENTION
In order to know the ease of Pt grain growth in alumina and ceria-zirconia solid solution, the inventors of the present application are concerned with a catalyst supporting Pt on Al—Ce—Zr composite oxide particles produced by a coprecipitation method. A rich / lean repeated durability test at 800 ° C. was performed, and the subsequent Pt particle size was observed with STEM.
[0019]
First, when the carrier distribution was examined using STEM EDX mapping or the like, the Al—Ce—Zr composite oxide particles had a region where Al ₂ O ₃ and CeO ₂ —ZrO ₂ solid solution coexist, and only Al ₂ O _3. It was found to have a region where
[0020]
Next, when the particle size of Pt was observed, Pt particles on the CeO ₂ —ZrO ₂ solid solution were about 15 nm at the maximum, and no fine Pt particles of 2 nm or less were observed at all (FIG. 1), whereas Al ₂ Many fine Pt particles of 2 nm or less were observed on O ₃ (FIG. 2). From this, it is judged that Pt is less likely to grow when it is supported on Al ₂ O ₃ than on the CeO ₂ —ZrO ₂ solid solution.
[0021]
On the other hand, it is known that OSC is expressed at a site where Pt and CeO ₂ are close to each other. That is, Pt supported on Al ₂ O ₃ that does not coexist with CeO ₂ is one of the most important characteristics of the three-way catalyst, despite the fact that grain growth is suppressed and deterioration is prevented. Does not contribute. Therefore, in the conventional catalyst in which Pt is supported on the conventional Al—Ce—Zr composite oxide particles, Pt on the Al ₂ O ₃ dissociated from CeO _{2 is} part of the OSC while having Pt that is difficult to grow grains. There was a defect that it was not possible to contribute to the expression of.
[0022]
Therefore, in the exhaust gas purifying catalyst of the present invention, a noble metal is applied to at least the alumina layer of the carrier including the composite particles composed of the oxygen storage / release material particles having OSC and the alumina layer covering at least a part of the surface of the oxygen storage / release material particles. Carrying.
[0023]
Since the noble metal supported on the alumina layer is suppressed in grain growth as described above, it remains as fine particles after endurance and has many active points on the surface. Accordingly, a decrease in purification activity after durability is suppressed. In addition, since the alumina layer is amorphous and porous, oxygen adsorbed on the noble metal supported on the alumina layer easily passes through the pores of the alumina layer and is occluded by the internal oxygen storage / release material particles. The oxygen released from the oxygen storage / release material particles easily passes through the pores of the alumina layer and is released through the noble metal. Thereby, the noble metal contributes sufficiently to the expression of OSC. Accordingly, since many fine noble metals can contribute to the expression of OSC after the endurance, a high purification activity is exhibited even after the endurance.
[0024]
As the oxygen storage / release material particles having OSC, rare earth metal oxides such as CeO ₂ and PrO ₄ , transition metal oxides such as NiO, Fe ₂ O ₃ , CuO, and Mn ₂ O ₅ can be used. Among them, CeO ₂ is desirable, and it is particularly preferable to use a CeO ₂ —ZrO ₂ solid solution stabilized with ZrO ₂ .
[0025]
The particle size of the oxygen storage / release material particles is not particularly limited, but is preferably about 3 × 10 ^{− to} 3 μm, which is the particle size of a general oxide powder.
[0026]
The thinner the alumina layer on the surface of the oxygen storage / release material particles, the better, but the average thickness should be 1 to 50 nm. If the average thickness is less than 1 nm, the precious metal tends to grow due to the influence of the internal oxygen storage / release material, and if the average thickness exceeds 50 nm, it is difficult for oxygen to pass through the alumina layer and the OSC may decrease. is there.
[0027]
The alumina layer only needs to cover at least part of the surface of the oxygen storage / release material particles, but preferably covers 50% or more of the surface area of the oxygen storage / release material particles. If the area covered by the alumina layer is less than 50%, the amount of noble metal supported on the alumina layer is small, so that the amount of fine noble metal is reduced even after endurance, and the decrease in activity after endurance is increased.
[0028]
Note carrier may be included composite particle alumina layer is formed of formed oxygen-absorbing material particles may be composed of only the composite _{particles, Al 2 O 3, TiO 2} , ZrO 2, SiO 2 , etc. Other porous oxide powders may be mixed. However, the mixing amount of other porous oxide powders should be refrained so as to contain 50% by volume or more of composite particles.
[0029]
As the noble metal to be supported, one kind or plural kinds of Pt, Rh, Pd, Ir and the like can be used. However, the present invention is effective particularly when Pt which easily grows grains. The amount of noble metal supported is suitably in the range of 0.05 to 10 g per liter of catalyst. If it is less than this, sufficient activity cannot be obtained, and even if it is supported more than this, the effect is saturated and there is a problem in terms of cost.
[0030]
In order to produce the exhaust gas purifying catalyst of the present invention, an aluminum coupling agent is adhered to the oxygen storage / release material particles having OSC. As the aluminum coupling agent, for example, acetoalkoxyaluminum diisopropylate can be used. The adhesion method can be performed using a dry method, a wet method, or an integral blend method, and the adhesion area and thickness of the alumina layer formed can be adjusted by adjusting the adhesion amount.
[0031]
The aluminum-based coupling agent is chemically bonded to the surface of the oxygen absorbing / releasing material particles, which are inorganic substances, to form an organic film containing Al. Therefore, by firing this, the organic matter is burned out, Al is oxidized, and an alumina layer is formed. The baking conditions should just be the conditions which an organic substance burns away.
[0032]
Next, a noble metal is supported on a carrier including composite particles made of oxygen storage / release material particles on which an alumina layer is formed. This supporting method may be the same as the conventional method for producing a catalyst, and an adsorption supporting method or a water absorbing supporting method can be used. Use of the adsorption support method is particularly preferable because the noble metal is preferentially supported on the alumina layer.
[0033]
The exhaust gas purifying catalyst of the present invention may be formed as a pellet catalyst by forming a pellet from a catalyst powder in which a noble metal is supported on a carrier containing composite particles, or a coating layer is formed from a catalyst powder on a honeycomb substrate or a foam substrate. You can also When a honeycomb catalyst or a foam catalyst is used, a coat layer may be formed from a carrier containing composite particles, and then a noble metal may be supported.
[0034]
【Example】
Hereinafter, the present invention will be specifically described with reference to Examples and Comparative Examples.
[0035]
(Example 1)
[0036]
[Chemical 1]
[0037]
12 g of acetoalkoxyaluminum diisopropylate represented by the above formula was dissolved in 30 ml of n-propanol, and 240 g of CeO ₂ —ZrO ₂ solid solution powder prepared by a coprecipitation method was mixed and stirred. The composition of the CeO ₂ —ZrO ₂ solid solution is CeO ₂ : ZrO ₂ = 1: 1 in molar ratio.
[0038]
The well-stirred solution was transferred to a milling vessel, and an appropriate number of ceramic balls were placed and milled for 3 hours. After milling, the solvent was removed to some extent by decantation, washed with ethanol, and then centrifuged to remove the solvent. This washing was repeated three times, then dried and baked at 500 ° C. for 2 hours.
[0039]
As a result of observation with an electron microscope, the obtained carrier powder has an average thickness of ₂ to 5 nm of Al ₂ on the entire surface of CeO ₂ —ZrO ₂ solid solution particles (CZ particles) having an average particle diameter of about 100 nm as shown in FIG. It was composed of composite particles coated with an O ₃ layer. The Al ₂ O ₃ layer was amorphous and porous.
[0040]
This carrier powder was impregnated with a predetermined amount of a dinitrodiammine platinum solution having a predetermined concentration, evaporated to dryness, and calcined at 500 ° C. for 2 hours to obtain a catalyst powder carrying Pt. The amount of Pt supported is 2% by weight. And it shape | molded to the pellet shape by the usual method, and prepared the pellet catalyst of a present Example.
[0041]
(Comparative Example 1)
A CeO ₂ —ZrO ₂ solid solution powder similar to that in Example 1 was impregnated with a predetermined amount of a dinitrodiammine platinum solution having a predetermined concentration, evaporated to dryness, and calcined at 500 ° C. for 2 hours to obtain a catalyst powder. The amount of Pt supported is 2% by weight. And it shape | molded to the pellet shape by the usual method, and prepared the pellet catalyst of this comparative example.
[0042]
(Comparative Example 2)
Precipitation was produced by a coprecipitation method from a mixed aqueous solution in which cerium nitrate, zirconium oxynitrate, and aluminum nitrate were mixed at a predetermined ratio, and dried and fired to prepare an Al—Ce—Zr composite oxide powder. The composition of the obtained Al—Ce—Zr composite oxide powder is Al ₂ O ₃ : CeO ₂ : ZrO ₂ = 1: 1: 1 in molar ratio.
[0043]
Using this Al—Ce—Zr composite oxide powder, Pt was supported in the same manner as in Example 1. The amount of Pt supported is the same amount as in Example 1 with respect to CeO ₂ —ZrO ₂ contained. And it shape | molded to the pellet shape by the usual method, and prepared the pellet catalyst of this comparative example.
[0044]
<Test and evaluation>
Each evaluation catalyst is filled with 5 g of each pellet catalyst, and a lean gas of λ = 0.21 and a rich gas of λ = 3.98 are alternately flowed under the conditions of rich / lean = 1 min / 1 min and a flow rate of 5 L / min. An endurance test was performed in which treatment was performed at 800 ° C. for 5 hours.
[0045]
Each pellet catalyst after the durability test evaluation device and 3g respectively filled, flow rate 20L / min and _{N 2} gas containing _{O 2} 1%, while switching the _{N 2} gas containing CO 2% alternately at 1 minute intervals, The oxygen storage amount was measured by flowing at 400 ° C. and measuring the change in weight. This is repeated 5 times, and the average value of the second to fourth oxygen storage amounts is shown in FIG.
[0046]
FIG. 4 shows that the catalyst of Example 1 shows a higher OSC than the catalysts of the respective comparative examples, and also shows a high OSC after the durability test. This is because the use of a support in which an Al ₂ O ₃ layer is formed on the surface of CeO ₂ —ZrO ₂ solid solution particles suppresses the Pt grain growth during the durability test, and the fine Pt becomes CeO ₂ —ZrO ₂ solid solution. This is thought to be because it contributed to the expression of OSC.
[0047]
【The invention's effect】
That is, according to the exhaust gas purifying catalyst of the present invention, high OSC is expressed even after durability, and high purifying activity can be maintained. And according to the manufacturing method of this invention, such a catalyst for exhaust gas purification can be manufactured easily and stably.
[Brief description of the drawings]
FIG. 1 is a TEM photograph showing a particle structure after a durability test of a CeO ₂ —ZrO ₂ solid solution supporting Pt.
FIG. 2 is a TEM photograph showing a particle structure after durability test of Al ₂ O ₃ supporting Pt.
FIG. 3 is a cross-sectional view schematically showing the structure of composite particles prepared in one example of the present invention.
FIG. 4 is a graph showing OSC after endurance test of exhaust gas purifying catalysts of Examples and Comparative Examples.

Claims (7)

A carrier comprising composite particles comprising oxygen storage / release material particles having oxygen absorption / release capacity and an alumina layer covering at least a part of the surface of the oxygen storage / release material particles; and a noble metal supported on at least the alumina layer of the support And an exhaust gas purifying catalyst characterized by comprising: The exhaust gas-purifying catalyst according to claim 1, wherein the oxygen storage / release material particles include at least ceria. The exhaust gas purifying catalyst according to claim 2, wherein the oxygen storage / release material particles further contain zirconia. The exhaust gas purifying catalyst according to any one of claims 1 to 3, wherein the noble metal is platinum. The exhaust gas purifying catalyst according to any one of claims 1 to 4, wherein the alumina layer has a thickness of 1 to 50 nm. The exhaust gas purifying catalyst according to any one of claims 1 to 5, wherein the alumina layer covers 50% or more of the surface of the oxygen storage / release material particles. A carrier comprising composite particles in which an alumina layer covering at least a part of the surface of the oxygen absorbing / releasing material particles is prepared by attaching an aluminum coupling agent to the oxygen absorbing / releasing material particles having oxygen absorbing / releasing capability and firing it. And a method for producing an exhaust gas-purifying catalyst, wherein a noble metal is supported on the carrier.