JP3503766B2 - High heat resistant exhaust gas purification catalyst - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [0001] BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an exhaust gas purifying catalyst.
In particular, it has excellent heat resistance and can maintain high heat resistance for a long period of time.
And a catalyst for purifying exhaust gas. [0002] 2. Description of the Related Art Conventionally, catalysts for purifying exhaust gas of automobiles
Simultaneously, oxidation of CO and HC and reduction of NOx
A three-way catalyst for purifying exhaust gas is used.
Such catalysts include, for example, cordierite and the like.
From porous carrier consisting of γ-alumina to heat-resistant carrier substrate
A carrier layer is formed, and Pt, Pd, Rh, etc.
Those supporting a noble metal catalyst are widely known. [0003] By the way, such an exhaust gas purifying catalyst is used.
The purification performance of the engine depends on the air-fuel ratio (A / F) of the engine.
Different. That is, the air-fuel ratio is large,
On the lean side, the amount of oxygen in the exhaust gas is large.
And the oxidation reaction for purifying CO and HC is active, but N
The reduction reaction for purifying Ox becomes inactive. Conversely, the air-fuel ratio
On the small side, that is, on the rich side where the fuel concentration is rich,
Oxygen reaction becomes inactive while the amount of oxygen in
The reaction becomes active. [0004] On the other hand, in the driving of automobiles, driving in urban areas
In this case, acceleration and deceleration are frequently performed, and the air-fuel ratio
G (stoichiometric air-fuel ratio) to a rich state.
It changes frequently. Demand for low fuel consumption in such driving
In order to respond to the
It is necessary to drive on the road side. So on the lean side
Development of a catalyst that can sufficiently purify NOx
I have. Accordingly, the applicant of the present application has proposed an apparatus represented by Ba.
Contact with alkaline earth metal and Pt supported on activated alumina carrier
A medium (Japanese Patent Application No. 4-130904) has been proposed. This catalyst
NOx will be alkaline earth metal during lean operation
Is stored in a NOx storage material such as stoichiometric
Contained in the exhaust gas when operating on the switch side
Because it is purified by reacting with reducing gases such as C and CO,
Also on the lean side, it has excellent NOx purification performance. [0006] SUMMARY OF THE INVENTION However, alkaline earths
In a catalyst in which metal and Pt are supported on an activated alumina carrier, 8
Crystal composed of alkaline earth metal and alumina at around 00 ℃
Aluminate is generated and NOx storage amount decreases
And the heat resistance was insufficient. Also Stoy
In the transition region near the key, when oxygen decreases, Pt increases.
NO oxidation reaction does not take place and NOx storage amount is small
Conversely, when the amount of oxygen increases, the released NOx is released.
In any case, NOx purification performance is reduced due to reduced emissions
There is a problem of doing. The present invention has been made in view of such circumstances.
Crystalline aluminate and crystalline cell at high temperatures.
To prevent the formation of lithium oxide and satisfy heat resistance
High NOx purification even in the transient region near stoichiometry
The purpose is to have performance. [0008] [MEANS FOR SOLVING THE PROBLEMS]
Ming's highly heat-resistant exhaust gas purification catalyst is represented by the following composition formula.
Amorphous aluminate carrier and the carrier
Catalyst noble metal and body4.8 to 28.8 MO
Amount%, CeO _Two From 17.0 to 27.2% by weight of Al _Two O
_Three From 51.0 to 72.9% by weightIt is characterized by the following. Composition formula aMO.bCeO_Two・ CAl_TwoO_Three (Where MO is an alkali metal, alkaline earth metal and rare metal
Oxides of elements selected from earth elements, molar ratio a: c
1: 6 to 1: 3, molar ratio b: c is 2:11 to 3:11) [0009] The exhaust gas purifying catalyst of the present invention has a lean atmosphere.
NO in exhaust gas is oxidized on catalytic noble metal
NO_TwoAnd the NOx storage element absorbs as nitrate.
Is stored. And when it becomes rich atmosphere, NOx storage source
NOx stored in the element is released,
N reduced by HC and CO_TwoBecomes This
Higher NOx purification performance can be obtained. On the other hand, in the transition region near the stoichiometry, oxygen
When the amount fluctuates, cerium oxygen storage / release action
As a result, fluctuations in the amount of oxygen are reduced,
Since it is actively performed, it exhibits high NOx purification performance. And
In the exhaust gas purifying catalyst of the present invention, the composition formula aMO · bC
eO_Two・ CAl _TwoO_ThreeIn the amorphous aluminate,
The molar ratio of each oxide, a: c is 1: 6 to 1: 3, b: c
Is from 800 ° C. to 1 by setting 2:11 to 3:11.
Amorphous with each element homogeneously dispersed at high temperature of 000 ℃
Can be maintained. Therefore, high specific surface
And maintain high catalytic activity. And a high temperature of 800 ° C. to 1000 ° C.
Crystallization of NOx storage element and cerium
Therefore, the NOx storage element and cerium remain in a highly dispersed state.
Of the NOx storage function and the oxygen storage / release function
There is no drop. As a result, there is no decrease in NOx purification performance
Has high heat resistance. A is greater than 1: 6 in the molar ratio a: c.
When it becomes smaller, that is, NOx storage element for alumina
Is less than the predetermined amount, the NOx storage capacity decreases,
When a is greater than 1: 3, that is, for alumina
When the NOx storage element exceeds a predetermined amount, crystalline Al
Heat resistance is reduced due to the formation of minate. B is smaller than 2:11 in the molar ratio b: c.
Is reached, that is, a certain amount of cerium is
When the amount is too low, crystalline cerium oxide is likely to be formed.
Is larger than 3:11, that is, alumina
On the other hand, even if the amount of cerium exceeds
Heat resistance is reduced due to the formation of lithium oxide. [0013] 【Example】 (Specific Examples of the Invention) Platinum (Pt),
At least one of palladium (Pd) and rhodium (Rh)
Seeds are used. The supported amount of platinum or palladium depends on the carrier
A range of 0.1 to 20.0 g per liter is desirable.
And the range of 0.3 to 10.0 g is particularly preferable. Carry amount
Is less than 0.1 g / L, NOx purification after initial and endurance
Performance is reduced, and it is effective even if it exceeds 20.0 g / L.
Fruits are saturated and the excess catalyst noble metal is effectively used.
I can't. The amount of rhodium supported is per liter of carrier.
Is preferably in the range of 0.001 to 1.0 g, and 0.05
A range of .about.0.5 g is particularly preferred. 0.001 supported
g / L, the NOx purification performance at the initial stage and after the endurance is
If it decreases and exceeds 1.0 g / L, platinum or palladium
The effect of the system is reduced. Rhodium loading is platinum
Or palladium loading should be determined
1/1 of the total amount of platinum or palladium carried
It is preferably 3 or less, more preferably 1/5 or less. As the alkali metal, lithium and sodium
, Potassium, rubidium, cesium, and francium
No. Alkaline earth metals are defined as group 2A in the periodic table.
Element means barium, beryllium, magnesium, mosquito
Lucium and strontium. Also rare earth element
The raw materials are scandium, yttrium, lanthanum,
Cerium, praseodymium, neodymium and the like are exemplified. An amorphous aluminate carrier of the composition formula
To form, the alkoxide, acetate or
Is a sol-gel method, a coprecipitation method or
Use a method of homogeneous synthesis from the liquid phase using the citric acid method, etc.
It is desirable to use Example 1 In a flask with a reflux device for sol-gel synthesis
1 liter of 2-propanol into the flask and keep at 80 ° C
You. And while stirring aluminum isopropoxide
Add 480 g to dissolve and stir at 80 ° C for 2 hours
You. Next, while stirring the solution at 80 ° C.,
50 g of umdiisopropoxide and 41 g of cerium nitrate
Add dropwise and continue stirring at 80 ° C for another 2 hours.
You. Thereafter, while stirring the solution at 80 ° C.,
g and 0.45 liter of 2-propanol in drops
Down. The dripping speed was 20 cc / min, and 8
Stir at 0 ° C. for 12 hours for aging. In addition, water is
It is three times the molar amount of lucoxide. Thereafter, the mixture is heated in a vacuum to obtain propanol.
Collect. Nylon coat with iron core
Pulverize with a ball to powder, calcine at 450 ° C,
It was baked at 800 ° C. for 5 hours. If the firing temperature is 900 ° C or
Also made each baked at 1000 ° C for 5 hours
did. The three types of powders obtained were analyzed by X-ray diffraction analysis.
It was confirmed that each was amorphous. Table 1 shows the composition and specific surface area of the obtained carrier powder.
Shown in Next, a predetermined amount of dinitrodian is added to the carrier powder.
Impregnated with an aqueous solution of minplatinum, dried at 110 ° C, and dried at 250 ° C
For 1 hour. The amount of supported Pt is
1.5 g of Pt for 100 g of alumina. Further
Is impregnated with a predetermined amount of rhodium nitrate aqueous solution
After drying at 110 ° C., it was baked at 250 ° C. for 1 hour. Carrying
The amount of Rh thus determined is based on 100 g of alumina in the carrier powder.
And 0.4 g of Rh. The obtained powder is formed into a pellet and
The exhaust gas purifying catalyst of Example 1 was used. (Other Examples) Kinds and ratios of metal alkoxides as starting materials
The ratio was variously selected so as to have the composition shown in Table 1, and the same as in Example 1.
In the same manner as in Example 1, an amorphous carrier powder was prepared.
To carry the same amount of Pt and Rh to purify the exhaust gas of each embodiment.
A catalyst for chemical conversion was prepared. (Comparative Example 1) In the same manner as in Example 1 so as to have the composition shown in Table 1,
Thus, a carrier powder was prepared. And as a result of X-ray diffraction, 90
Cerium oxide crystallized on carrier powder fired at 0 ° C or higher
It was confirmed that it was included in the state. This carrier powder
In the same manner as in Example 1, the same amount of Pt and Rh
An exhaust gas purifying catalyst of Comparative Example 1 was prepared. (Comparative Example 2) In the same manner as in Example 1 so as to have the composition shown in Table 1,
Thus, a carrier powder was prepared. And as a result of X-ray diffraction, 80
The carrier powder calcined at 0 ° C or higher containsConclusionCrystalline barium oxide
Luminate and crystalline cerium oxide
And was confirmed. This carrier powder was prepared in the same manner as in Example 1.
To carry the same amount of Pt and Rh, and purify the exhaust gas of Comparative Example 1.
A catalyst for use was prepared. (Comparative Examples 3 and 4) Contains one of cerium and barium
In the same manner as in Example 1 except that there was no
A body powder was prepared, and Pt and Rh were the same as in Example 1.
To prepare the exhaust gas purifying catalysts of Comparative Examples 3 and 4.
did. [0021] [Table 1](Evaluation) Model gas flow evaluation device shown in FIG.
Model of the composition shown in Table 2 for each of the above catalysts
The gas was passed at 800 ° C. or 900 ° C. for 100 hours.
Lean and stoichiometric gas for 2 minutes
It was distributed at a distance. [0023] [Table 2] Thereafter, the specific surface area of each catalyst was measured. Also on
N when the model gas is circulated under the same conditions as
Measures Ox purification rate and HC 50% purification temperature (T50)
The results are shown in Table 3. [0024] [Table 3]As shown in Table 1, the catalysts of Comparative Examples 1 and 2
Is a crystalline cerium oxide and a crystalline
Luminates are formed and the specific surface area is reduced. this
Is that the molar ratio of a, b, and c in the above composition formula is within the range of the present invention.
It is clear that the
You. From Table 3, the exhaust gas purifying catalyst of each embodiment is
The specific surface area is large, and even in the transient region near stoichiometry, N
It is clear that Ox purification performance and HC purification performance are excellent.
Call However, Comparative Examples 1-4 were inferior in NOx purification rate.
Comparative Example 3 does not contain cerium, so that it does not purify HC.
NO in Comparative Example 4 because barium was not contained.
It can be seen that the x purification rate is particularly low. [0027] That is, the exhaust gas purifying catalyst of the present invention can be used.
According to this, high NOx purification is achieved even in the transient region near stoichiometric.
And high temperature range of 800-1000 ° C
Can maintain an amorphous state with a high specific surface area,
And maintain high dispersion carrying state of NOx storage element and cerium
Because it can be made, it is extremely excellent in heat resistance.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a block diagram illustrating a configuration of a model gas flow evaluation device used in an embodiment of the present invention.

Continuation of the front page (58) Field surveyed (Int. Cl. ⁷ , DB name) B01J 21/00-38/74 B01D 53/94

Claims (1)

(57) and carrier of the Claims 1 Amorphous aluminate represented by the following composition formula, a catalytic precious metal supported on the carrier, Tona is, the MO
4.8 to 28.8% by weight, CeO ₂ is 17.0 to 27.
2 wt%, high heat resistance exhaust gas purifying catalyst according to claim <br/> comprise Al ₂ O ₃ from 51.0 to 72.9 wt%. Formula _{aMO · bCeO 2 · cAl 2 O} 3 ( where MO is an alkali metal, oxide of an element selected from alkaline earth metals and rare earth elements, the molar ratio of a: c is 1: 6 to 1: 3 molar ratio b: c is 2:11 to 3:11)