JPH1176824A - NOx purification catalyst material - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide high NOx-removing performance in both oxidative and reductive atmosphere in a prescribed temperature range by using a composition having a lamellate perovskite structure with a specified formula or a composition with another specified formula. SOLUTION: This NOx-removing catalyst material has a formula I; A3 Cu2 Oy (in the formula, A stands for at least one element selected from rare earth elements, barium, calcium, and yttrium; and y satisfies 5.5<=y<=6.5) and has a lamellate perovskite structure. In this case A is selected from rare earth elements, barium, calcium, yttrium, and their combinations and combinations of rare earth elements with barium or calcium are preferable. In this case, the NOx-removing catalytic material is expressed by a formula II; Ln2x Bax CaCu2 Oy (in the formula, Ln stands for a rare earth element; x satisfies 0.5<=x<=1.0; and y satisfies 5.5<=y<=6.5).

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a catalyst material capable of absorbing and purifying nitrogen oxides (hereinafter referred to as "NOx") in both an oxidizing atmosphere and a reducing atmosphere. It has a complex oxide structure, and can be used as a catalyst for purifying exhaust gas emitted from internal combustion engines, an exhaust gas purifying catalyst in the combustion of natural gas, etc. And a NOx purification catalyst material that can also be used in a denitration process.

[0002]

2. Description of the Related Art Conventionally, as a catalyst for purifying exhaust gas of an internal combustion engine, for example, 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. In addition, a three-way catalyst for purifying exhaust gas carrying a noble metal such as Pt, Pd, and Rh is typical.

[0003] In recent years, with increasing awareness of the environment on a global scale, the quality and quantity requirements for improving the combustion efficiency and fuel efficiency of internal combustion engines, purifying exhaust gas, and the like have been increasing. Under such circumstances, studies have been particularly made on improving the combustion of the internal combustion engine. At present, operation in a lean combustion (lean) region in which combustion is performed with an oxygen-excess mixture is actively performed. Therefore, a catalyst that can sufficiently purify NOx is desired.

[0004] Even in such a lean region, NOx
(1) A method using a zeolite catalyst for purifying NOx using hydrocarbon (HC) in a gaseous phase under a lean atmosphere (Machida, Murak)
ami, Kijima; Mater. Chem. ,
4 (1994) 1621) and (2) barium oxide,
A method in which lanthanum oxide and platinum are combined, NOx is absorbed under a lean atmosphere, and NOx is purified by a three-way catalyst in a three-way region (Japanese Patent Laid-Open Nos. 5-511556 and 5-261).
287) has been proposed.

Machida et al. (1)
In the document of La., La _2-x Ba _x SrCu ₂ O ₆ is simultaneously absorbed with NO at a high temperature of 600 ° C. or higher.
Has been reported to be released after being decomposed into oxygen and nitrogen.
Also, JP-A-5-511556 and JP-A-5-26
No. 1287 discloses a NOx purifying performance under an oxygen-excess atmosphere by using a NOx absorbent composed of an alkali metal, an alkaline earth metal, and a rare earth element in combination with a noble metal catalyst for purifying exhaust gas under an oxygen-excess atmosphere. Is disclosed.

[0006]

However, in such a conventional exhaust gas purifying catalyst, in a temperature range of 400 ° C. or more, the release of NOx starts simultaneously with the adsorption of NOx, and the NOx absorption amount decreases. In addition, NOx purification in a low temperature range of 300 ° C. or less is insufficient, and therefore, an exhaust gas purification catalyst exhibiting NOx purification performance in a wide temperature range and atmosphere in a use environment has been desired. .

The present invention has been made in view of the above-mentioned problems of the prior art, and an object of the present invention is to provide a high NOx purifying ability both in a temperature range of 350 ° C. or lower and in an oxidizing / reducing atmosphere. It is an object of the present invention to provide a NOx purification catalyst material exhibiting the above.

[0008]

Means for Solving the Problems The present inventors have conducted intensive studies to solve the above problems, and as a result, it has been found that a specific copper-based layered perovskite composite oxide absorbs nitrogen oxides in both an oxidizing atmosphere and a reducing atmosphere. In addition, they have found that they have a characteristic that they can effectively purify NOx even in a temperature range of 350 ° C. or lower, and have completed the present invention.

That is, the NOx purification catalyst material of the present invention has the following general formula (1) A ₃ Cu ₂ O _y (1) (where A is a rare earth element, barium, calcium and yttrium) At least one element selected from the group, y is 5.5 ≦ y ≦ 6.5), and has a layered perovskite structure represented by the following formula:

The preferred form of the NOx purification catalyst material of the present invention is represented by the following formula (2): Ln _2-x Ba _x CaCu ₂ O _y (2) (where Ln represents a rare earth element, x is 0.5 ≦ x
≦ 1.0, y is 5.5 ≦ y ≦ 6.5. ).

[0011]

BEST MODE FOR CARRYING OUT THE INVENTION Hereinafter, the present invention will be described in detail. As described above, the NOx purification catalyst material of the present invention has the following general formula (1): A ₃ Cu ₂ O _y (1) (where A is a group consisting of a rare earth element, barium, calcium and yttrium. Represents at least one element selected from the group consisting of: y is 5.5 ≦ y ≦ 6.5), and has a layered perovskite structure, but is an ordinary copper-based layered perovskite oxide NOx absorption catalyst Compared to materials,
The optimum temperature for NOx absorption is reduced to 350 ° C. or less.

As shown in the formula (1), A is selected from rare earth elements, barium, calcium or yttrium and any combination thereof.
A combination of barium and calcium is preferable. In this case, the NOx purification catalyst material of the present invention uses the following formula (2): Ln _2-x Ba _x CaCu ₂ O _y (2) , Represents a rare earth element, and x is 0.5 ≦ x
≦ 1.0, y is 5.5 ≦ y ≦ 6.5. ).

In the equations (1) and (2), y is 5.5
When the value of y is out of this range, it is difficult to form a layered perovskite structure, which is not preferable. Further, in the equation (2), x takes a value of 0.5 to 1.0. If x is out of this range, sufficient NOx purification performance is not exhibited, which is not preferable. As the rare earth element, La, Nd, Pr, and Gd can be preferably selected.

The NOx purifying catalyst material of the present invention as described above can be used in the form of powder as it is, but it can be used after being formed into various shapes such as granules and pellets. It is also possible to use it by supporting it on a conventionally known carrier substrate. Further, it is also possible to coat and use a monolithic carrier or a metal carrier made of a refractory material, and in particular, when purifying NOx in automobile exhaust gas, coating a honeycomb-like carrier with catalyst and exhaust gas. This is extremely effective because the contact area can be increased and the pressure loss can be suppressed.

As this honeycomb-shaped carrier, cordierite-based ones such as ceramics are generally used in many cases, but it is also possible to use a honeycomb-shaped carrier made of a metal material such as ferritic stainless steel. The material powder itself may be formed in a honeycomb shape.

As described above, the NOx purifying catalyst material of the present invention can absorb and purify NOx in both an oxidizing atmosphere and a reducing atmosphere, but is used in combination with a conventionally known noble metal component such as Pt, Pd and Rh. It is also possible,
For example, by combining with at least one noble metal of Pt, Pd and Rh, NOx in an oxidizing atmosphere
In addition to the absorption and purification characteristics, the NOx decomposition and purification performance under a reducing atmosphere can be improved.

[0017]

EXAMPLES Hereinafter, the present invention will be described in more detail with reference to Examples, Comparative Examples and Test Examples, but the present invention is not limited to these Examples.

(Example 1) Starting materials were carbonates or hydroxides of lanthanum, barium, strontium and copper,
These have a composition ratio of La: Ba: Ca: Cu = 1: 1: 1.
1: 2, pulverized and mixed by a ball mill, and reacted with citric acid in the same manner as described in JP-A-2-74505 to produce a composite citrate powder. By firing at 10 ° C for 10 hours,
LaBaCaCu (the _{δ 0.5~-0.5) 2 O 6-} δ obtain a perovskite powder represented by (NOx purifying catalytic material).

Example 2 Starting materials were carbonates or hydroxides of lanthanum, barium, strontium and copper.
The composition ratio is La: Ba: Ca: Cu = 1.5: 0.5:
The same operation as in the example was repeated except that the ratio was set to be 1: 2, and La _1.5 Ba _0.5 CaCu ₂ O ₆ -δ (δ was
(0.5 to -0.5).

Example 3 Neodymium, barium, strontium and copper carbonates or hydroxides as starting materials
The composition ratio is Nd: Ba: Sr: Cu = 1.5: 0.5:
1: except that added to a 2, the same procedure as in Example _{1, Nd 1.5 Ba 0.5 CaCu 2} O 6- δ (δ
Obtained a perovskite powder represented by 0.5 to -0.5).

(Comparative Example 1) A carbonate or hydroxide of lanthanum, barium, and manganese was used as a starting material, and the composition ratio was changed to La: Ba: Mn = 5: 5: 10. in except firing at 10 hours, the same procedure as in example _{1, La 0.5 Ba 0.5 MnO 3-} δ (δ
Obtained a perovskite powder represented by 0 to 0.3).

(Comparative Example 2) A carbonate or hydroxide of lanthanum, barium, and cobalt was used as a starting material, and added so that the composition ratio was La: Ba: Co: Mn = 5: 5: 5: 5. The same operation as in Example 1 was repeated except that calcination was performed at 850 ° C. for 10 hours in the air, and La _0.5 Ba _0.5 Co
A perovskite powder represented by _0.5 Mn _0.5 O _3- δ (δ is 0 to 0.3) was obtained.

[0023] (Comparative Example 3) the same procedure as in Example 1 by adjusting the composition ratio of each component, La _1.5 Ba _{0. 5} SrC
A perovskite powder represented by u ₂ O _6- δ (δ is 0 to 0.5) was obtained.

(Test Examples) Examples 1 to 3 and Comparative Examples 1 to 3
The NOx absorption characteristics of the perovskite-based catalyst materials were evaluated by placing the calcined perovskite oxide powder obtained in each example in a thermal analyzer and measuring the amount of NO absorbed and purified by thermogravimetric analysis. The evaluation conditions at this time will be described in detail below.

(Method of evaluating NOx absorption characteristics) The amount of NOx absorbed on the surface of each perovskite oxide was evaluated by calorimetric analysis and determining the amount of NOx absorption based on the change in weight due to NOx absorption due to a catalytic reaction.

(1) Thermal analysis reaction conditions 1) Reaction conditions (I) A composition gas of NO: N ₂ = 0.5: 99.5 was supplied at a flow rate of 100
NO was absorbed by flowing through the apparatus at cc / min. 2) Reaction conditions (II) N ₂ gas was passed through the apparatus at a flow rate of 100 cc / min to absorb NO. 3) Based on the data obtained from each of the above measurements, the amount of oxygen released from the oxide was estimated under the reaction condition (II), and the measurement result under the reaction condition (I) was regarded as the net NO absorption amount. The maximum absorption temperature was determined. (2) Measurement temperature Measurement was performed at a heating rate of 10 ° C./min from room temperature to 800 ° C.

Some of the results of thermogravimetric analysis (Examples 1 and 2) obtained as described above are shown in FIGS. 1 and 2. For reference, FIG. 1 additionally shows data of La ₂ CaCu ₂ Ox. Figure 1 shows the results of the results in NO, 2 minus the data in N ₂ from the data in NO. Table 1 summarizes the temperatures at which the amount of absorbed NO is maximized in Examples and Comparative Examples.

[0028]

[Table 1]

As is clear from these results, the NOx purifying catalyst material of the present invention is a copper-based layered perovskite composite oxide, which has a general formula Ln ₂₋ containing rare earth elements, barium, calcium and copper. _x Ba _x SrCu
By using a novel copper-based layered perovskite composite oxide represented by ₂ O _6- δ (Ln: rare earth element), nitrogen oxides can be absorbed even in an oxidizing atmosphere and effectively used even in a temperature range of 350 ° C. or lower. It can be seen that NOx purification can be realized. However, it goes without saying that the effect of the present invention can be similarly obtained when NO ₂ gas is used as the reaction gas.

[0030]

As described above, according to the present invention, since a specific copper-based layered perovskite composite oxide is used, a high NOx purifying ability can be obtained both in a temperature range of 350 ° C. or lower and in an oxidizing / reducing atmosphere. An effective NOx purification catalyst material can be provided. Moreover, a preferred form of the NOx purifying catalyst material of the present invention have the general formula La _2X Ba _X SrCu ₂ O containing rare earth elements, barium, each element of calcium and copper
A novel copper-based layered perovskite composite oxide represented by _6- δ (Ln: rare earth element), which promotes NOx absorption in a temperature range of 350 ° C or lower as compared with a normal NOx absorption catalyst, and effectively promotes NOx absorption. NOx can be captured.

[Brief description of the drawings]

FIG. 1 is a diagram showing a thermogravimetric analysis result of a NOx purification catalyst material in an NO—N ₂ atmosphere.

FIG. 2 is a graph showing a net change in weight obtained by subtracting data in N ₂ from data in a NO—N ₂ atmosphere of a NOx purification catalyst material.

Claims (2)

[Claims] 1. The following general formula (1): A ₃ Cu ₂ O _y (1) wherein A is at least one kind selected from the group consisting of rare earth elements, barium, calcium and yttrium. An element, wherein y is 5.5 ≦ y ≦ 6.5.) A NOx purification catalyst material having a layered perovskite structure represented by the following formula: 2. The following formula (2): Ln _2-x Ba _x CaCu ₂ O _y (2) (wherein Ln represents a rare earth element, and x is 0.5 ≦ x)
≦ 1.0, y is 5.5 ≦ y ≦ 6.5. The NOx purification catalyst material according to claim 1, wherein