JPH1157475A - Exhaust gas cleaning catalyst material - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an exhaust gas cleaning catalyst material having a high NOx purification capacity under an oxidizing atmosphere at a temperature not lower than 300 deg.C and a thermal resistance. SOLUTION: This material comprises a complex oxide having a bedded perovskite structure represented by the formula: A3 B2 O7 (wherein A represents at least one kind selected from the group consisting of alkali elements, alkaline earth elements, rare earth elements and lead and B represents at least one kind selected from the group consisting of copper, cobalt and iron, and the complex oxide is represented by the formula: La2-x Bax SrCu2-y B'y O7 (wherein B' represents cobalt and/or iron, and 0<x<2, 0<y<0.5.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a catalyst material for purifying exhaust gas, and more particularly to an exhaust gas discharged from an internal combustion engine, an exhaust gas in the combustion of natural gas and the like, and a nitrogen oxide generated in a chemical process in a factory or the like. The present invention relates to a catalyst material for NOx purification that can be effectively used in a process of absorbing and purifying substances and a denitration process.

[0002]

2. Description of the Related Art Conventionally, as a catalyst for purifying exhaust gas of an internal combustion engine, a γ-alumina slurry is applied to a heat-resistant carrier such as cordierite and baked, as represented by an exhaust gas purifying catalyst for automobiles. There are three-way catalysts supporting noble metals such as Pt, Pd, and Rh.

[0003] In recent years, with increasing awareness of the environment on a global scale, demands for both quality and quantity have been increasing with respect to improvement of combustion efficiency and fuel efficiency of internal combustion engines, purification of exhaust gas, and the like. Under such circumstances, it has been studied to improve the fuel efficiency of the internal combustion engine, in particular, and operation is currently performed in a lean burn (lean) region in which combustion is performed with a mixture ratio of excess oxygen. Even in this lean region, NOx is sufficiently reduced. A catalyst that can be purified is desired.

[0004] Even in such a lean region, N
As a method for purifying Ox, (1) a method using a zeolite catalyst for purifying NOx using hydrocarbon (HC) in a gaseous phase under a lean atmosphere (Machida, Murakami, Kiji
ma; J. Mater. Chem., 4 (1994) 1621) and (2) a combination of barium oxide, lanthanum oxide and platinum, absorbing NOx under a lean atmosphere,
Methods for purifying Ox (JP-A-5-511556 and JP-A-5-261287) have been proposed.

[0005] As described in JP-A-5-511556 and JP-A-5-261287, an alkali element,
It is known that NOx purification performance can be obtained in an oxygen-excess atmosphere by using a combination of a NOx absorbent composed of an alkaline earth element or a rare earth element and a noble metal catalyst. However, such an exhaust gas purifying catalyst has a problem that, in a temperature range of 300 ° C. or more, the release of NOx starts simultaneously with the adsorption of NOx, which causes a reduction in the NOx absorption amount as a whole. .

[0006] In addition, Machida et al., The general formula La _2-x Ba
_The composite oxide represented by _x SrCu ₂ O ₆ simultaneously absorbs NO at a high temperature of 600 ° C. or higher at the same time as NO absorption.
O was decomposed into oxygen and nitrogen and released (Mch
ida, Murakami, Kijima; J. Mater. Chem., 4 (1994) 162
1). The composite oxide is used in combination with a noble metal catalyst.
Unlike an x-absorption catalyst, it is a catalyst material that can decompose and purify NOx with a composite oxide alone. But,
Since these copper oxides have a relatively low melting point, there is a problem that they are liable to be deteriorated when used in a high temperature region.

Therefore, there is a demand for an exhaust gas purifying catalyst having NOx purifying performance in a wide temperature range and an operating environment under an atmosphere.

[0008]

SUMMARY OF THE INVENTION Accordingly, an object of the present invention is to provide a catalyst material for purifying exhaust gas which has high NOx purifying ability even in a temperature range of 300 ° C. or higher and in an oxidizing atmosphere and has heat resistance. It is in.

[0009]

Means for Solving the Problems As a result of intensive studies to solve the above problems, the present inventors have found that a composite oxide having a certain structure absorbs and purifies nitrogen oxides even in an oxidizing atmosphere and a reducing atmosphere. However, the present inventors have found that the melting point of a conventional copper-based layered perovskite oxide is improved, and that it is excellent in heat resistance and durability.

That is, the exhaust gas purifying catalyst of the present invention has the following general formula: A ₃ B ₂ O ₇ (where A is selected from the group consisting of alkali elements, alkaline earth elements, rare earth elements and lead) At least one, B
Represents at least one selected from the group consisting of copper, cobalt and iron), and has a layered perovskite structure represented by the following general formula: La _2-x Ba _x SrCu _2-y B ' _y O ₇ (where B' is cobalt and / or iron, 0 <x <2,0
<Y <0.5).

[0011]

BEST MODE FOR CARRYING OUT THE INVENTION The composite oxide of the catalyst material for purifying exhaust gas of the present invention has the following general formula A ₃ B ₂ O ₇ (A in the formula)
Is at least one selected from the group consisting of alkali elements, alkaline earth elements, rare earth elements and lead (Pb);
Is a layered perovskite structure represented by at least one selected from the group consisting of copper (Cu), cobalt (Co) and iron (Fe). Such a composite oxide is
The melting point is higher than that of a normal catalyst for purifying exhaust gas. By increasing the melting point in this way, the heat resistance when NOx is absorbed and purified under an oxidizing atmosphere and a reducing atmosphere can be improved.

As the alkali element, Na, K, Rb,
Cs, as alkaline earth elements, Ca, Sr, Ba
And rare earth elements include Sc, Y, La, Ce,
Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, H
o, Er, Tm, Yb, and Lu can be used.

A particularly preferred complex oxide is represented by the following general formula L
a_2-xBa_xSrCu_1-yB '_yO ₇(Where B 'is
Baltic (Co) and / or iron (Fe), 0 <x <1,0 <
y <0.5) layered perovskite structure represented by
Body.

With such a composition, in particular, N
While maintaining Ox absorption performance, in addition to Cu, Co and / or
Alternatively, the addition of Fe makes it possible to improve the heat resistance more than that of plain Cu.

In the above formula, each of x and y is 0 <x <2,
0 <y <0.5. It is A ₃ B _{2 that} 0 <x < ₂
In order to maintain the O ₇ type crystal structure, in particular, 0.5 ≦ x
It is preferred that ≦ 1.0. The reason for satisfying 0 <y <0.5 is to enable element substitution for imparting heat resistance while maintaining the A ₃ B ₂ O ₇ type crystal structure.
Preferably, y <0.3.

Each of the constituent elements of the composite oxide exerts the above-mentioned effects to the maximum when all of these contained in the catalyst are complexed, but at least a part of the complex oxide forms a complex. Even if it is possible, the above effect can be sufficiently obtained. Each constituent element of the composite oxide can exist as a composite oxide without being separated as a separate oxide even after thermal endurance, and this can be confirmed by, for example, X-ray diffraction measurement.

The composite oxide used in the present invention is prepared by mixing nitrates, acetates, carbonates or the like of the respective constituent elements of the composite oxide in a desired composition ratio of the composite oxide, calcining the mixture, and pulverizing the mixture. After the solid-phase reaction to be subjected to heat treatment and firing, nitrate, acetate or carbonate, hydrochloride, citrate, etc. of each constituent element of the composite oxide are mixed in a desired composition ratio of the composite oxide, and then dissolved in water. If necessary, an alkaline solution such as NH ₄ OH or NH ₃ CO _{3 is} added dropwise to form a precipitate, which can be prepared by a known method such as coprecipitation in which the precipitate is filtered, dried and calcined. By such a method, at least a part of each component constituting the composite oxide can be composited.

Particularly preferably, a coprecipitate comprising a carbonate, a basic carbonate or a hydroxide of each constituent element of the composite oxide, or a mixture of a single compound is reacted with citric acid and dried.
After dehydration, the resulting composite citrate is prepared by a method of baking. The synthesis temperature of the composite citrate is 40-12
0 ° C., preferably 65 ± 5 ° C.
The dehydration temperature is 50 to 120 ° C, preferably 90 ± 5 ° C. The firing temperature may be 850 to 1100 ° C. for 4 hours or more.

[0019]

The present invention will be described below with reference to the following examples and comparative examples. Example 1 Starting from lanthanum, barium, strontium, copper and iron carbonates or hydroxides, the composition ratio was La: B.
a: Sr: Cu: Fe = 15: 5: 10: 16: 4 and pulverized and mixed in a ball mill. From among them, 100 g of the mixture, about 400 g of pure water and about 60 g of citric acid were added and reacted at 60 ± 5 ° C. After completion of the reaction, the obtained slurry was dehydrated at about 120 ° C. to obtain a composite citrate. The resulting citrate was calcined 10 hours at 900 ° C. to obtain a composite oxide having a perovskite powder represented by _{La 1.5 Ba 0.5 SrCu 1. 6 Fe} 0.4 O 7.

EXAMPLE 2 Starting materials were lanthanum, barium, strontium, copper and iron carbonates or hydroxides, and the composition ratio was La: B.
a: Sr: Cu: Fe = 15: 5: 10: 18: 2 and La _1.5 Ba _0.5 SrCu _1.8 Fe in the same manner as in Example 1 except that the mixture was pulverized and mixed by a ball mill.
A composite oxide of perovskite powder represented by _0.2 O ₇ was obtained.

Example 3 Starting from lanthanum, barium, strontium, copper and iron carbonates or hydroxides, the composition ratio is La: B.
a: Sr: Cu: Fe = 15: 5: 10: 19: 1 and so as added, except for ground and mixed in a ball mill, La _1.5 in the same manner as in Example 1 Ba _0.5 SrCu _1.9 Fe
A composite oxide of perovskite powder represented by _0.1 O ₇ was obtained.

Example 4 Starting materials were lanthanum, barium, strontium, copper and iron carbonates or hydroxides, and the composition ratio was La: B.
a: Sr: Cu: Fe = 15: 5: 10: 19.5:
0.5 and La _1.5 Ba _0.5 SrCu in the same manner as in Example 1 except that the mixture was pulverized and mixed with a ball mill.
A composite oxide of perovskite powder represented by _1.95 Fe _0.05 O ₇ was obtained.

Example 5 Starting from lanthanum, barium, strontium, copper and iron carbonates or hydroxides, the composition ratio is La: B.
a: Sr: Cu: Fe = 15: 5: 10: 19: 1 and so as added, except for ground and mixed in a ball mill, La _1.5 in the same manner as in Example 1 Ba _0.5 SrCu _1.9 Fe
A composite oxide of perovskite powder represented by _0.1 O ₇ was obtained.

Comparative Example 1 Starting from lanthanum, barium, strontium and copper carbonates or hydroxides, the composition ratio was La: Ba:
Sr: Cu = 15: 5: 10: 2 become as added, except for ground and mixed in a ball mill, a composite oxide of perovskite powder represented by _{La 1.5 Ba 0. 5 SrCu 2 O} 7 in the same manner as in Example 1 I got something.

Test Example The NOx absorption characteristics obtained in the above Examples 1 to 5 and Comparative Example 1 were judged by performing evaluation under the following conditions.

(Evaluation Method of Melting Point), (Evaluation Method of NO Absorbing Capacity) The melting point of the obtained composite oxide was measured using a commercially available thermal analyzer (TG-DTA2000, manufactured by Mac Science Co., Ltd.). Measured by thermal analysis. The NO absorption capacity was evaluated by thermogravimetric analysis using a thermobalance. (1) Measurement conditions for thermal analysis 1) Measurement conditions (I) (measurement of melting point) N ₂ gas was passed through the thermal analyzer at a flow rate of 100 cc / min, and differential thermal analysis was performed at a heating rate of 10 ° C./min. Was performed. 2) Measurement conditions (II) (measurement of NO absorption amount) A composition gas of NO: N ₂ = 0.5: 99.5 was supplied at a flow rate of 100
The sample was circulated through the thermal analyzer at cc / min, and the weight change was measured at a heating rate of 10 ° C./min.

Tables 1 and 2 show the results obtained as described above.
Shown in Table 1 shows the melting point of each oxide determined by differential thermal analysis.

[0028]

[Table 1]

Table 2 shows the NO absorption amount as the largest absorption amount in the NO absorption characteristic curve obtained by thermogravimetric analysis.
It shows the weight increase (%) at 0 ° C.

[0030]

[Table 2]

[0031]

The catalytic material for purifying exhaust gas of the present invention can improve the melting point of the conventional copper-based layered perovskite oxide, improve heat resistance and durability, and purify NOx in a temperature region of 300 ° C. or higher. Can be effectively implemented.

Further, the NOx purifying apparatus of the present invention comprises a composite oxide catalyst having a layered perovskite structure and at least one of Pt, Pd and Rh as a catalyst capable of absorbing and purifying nitrogen oxides in an oxidizing atmosphere and a reducing atmosphere. As a NOx purifying device combining a three-way purifying catalyst containing one kind of noble metal, in addition to the NOx absorption characteristics in a reducing atmosphere, the NOx analyzing and purifying performance can be improved.

Claims (1)

[Claims] 1. The following general formula: A ₃ B ₂ O _{7 wherein} A is at least one selected from the group consisting of alkali elements, alkaline earth elements, rare earth elements, and lead;
Represents at least one selected from the group consisting of copper, cobalt and iron), and has a layered perovskite structure represented by the following general formula: La _2-x Ba _x SrCu _2-y B ' _y O ₇ (where B' is cobalt and / or iron, 0 <x <2,0
<Indicating y <0.5), which is a catalyst material for purifying exhaust gas.