JPH06246158A - Defective perovskite type nitrogen oxide decomposition catalyst and method for producing the same - Google Patents

Abstract

PURPOSE:To attain high NOx decomposition activity even in a lean burning region in which oxygen exists in excess in exhaust gas by using an NOx decomposition catalyst made of a defective perovskite type compd. having disordered oxygen defects represented by BaLa2O4. CONSTITUTION:An inorg. salt of Ba is mixed with an inorg. salt, oxide or alkoxide of La and this mixture is fired at 800-1,400 deg.C under a flow of inert gas to produce the objective NOx decomposition catalyst made of a defective perovskite type compd. having disordered oxygen defects represented by BaLa2 O4. This catalyst exhibits high NOx decomposition activity even in a lean burning region in which oxygen exists in excess in exhaust gas.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a nitrogen oxide having a high proportion of oxygen in combustion exhaust gas, that is, a so-called lean burn region and a high nitrogen oxide decomposing property even in a relatively low temperature region around 200 ° C. The present invention relates to a decomposition catalyst and a method for producing the same.

[0002]

2. Description of the Related Art A nitrogen oxide decomposing catalyst composed of a perovskite type compound (A site is a divalent atom and B site is a tetravalent atom) conventionally represented by ABO ₃ is one in which oxygen of this compound is dissociated. It is said that an oxygen deficiency is caused by this, the oxygen of the nitrogen oxide is bonded to the oxygen position of the oxygen deficiency, and then the reaction mechanism is decomposed into nitrogen and oxygen.
However, it is difficult for such a perovskite catalyst to obtain high activity at a temperature of 600 ° C. or lower because the temperature at which oxygen of this compound dissociates is high. Further, since such dissociation of oxygen is accompanied, in an environment where oxygen coexists with nitrogen oxides, dissociation of oxygen from the perovskite crystal is suppressed, so that the nitrogen oxide decomposing activity is significantly reduced. In addition to having drawbacks, in many cases, an atom whose valence changes at the B site is used, and the reductive decomposition reaction of nitrogen oxides is also caused by utilizing this valence change at the B site. There is also a drawback that the B site atom itself is oxidized by the oxygen which is generated, and as a result, the nitrogen oxide decomposing activity is reduced. Therefore, A ₂ B ₂ O ₅ (A site is a divalent atom, B
A brown mirror light type compound represented by the following formula is proposed. Since this compound occupies the B site with trivalent atoms, it has a large amount of oxygen vacancies in order to neutralize the entire electric charge, and the oxygen vacancies undergo an order-disorder transition at a specific temperature. By this, oxygen defects that had an ordered arrangement at low temperature become disordered at this transition point, and oxygen can be just dissociated from the ABO ₃ compound, and since the valence of the B site ion does not change, It is characterized in that the B-site metal is less oxidized by coexisting oxygen and the nitrogen oxide decomposing ability is less deteriorated by coexisting oxygen. But,
The brown mirror light type compound represented by A ₂ B ₂ O ₅ also has an order-disorder transition point of generally 700 ° C. or higher,
It has a drawback that the nitrogen oxide decomposing ability is extremely low at a low temperature of 600 ° C. or lower. (Shin Shigemitsu et al., Institute of Chemical Technology, Vol. 75, No. 3, (1980) 83-8
5)

[0003]

DISCLOSURE OF THE INVENTION The present invention utilizes the order-disorder transition when the B site of an AB ₂ 0 ₄ type defective perovskite compound is La, which does not change the valence even in a reducing atmosphere, By decomposing the nitrogen oxides, the activity of decomposing the nitrogen oxides in the presence of oxygen is less decreased, and the oxygen defects of BaLa ₂ O ₄ undergo an order-disorder transition at a low temperature of around 250 ° C. to cause a low temperature of around 250 ° C. The present invention provides a nitrogen oxide decomposing catalyst and a method for producing the same, wherein the nitrogen decomposing activity is increased.

[0004]

The present invention provides a nitrogen oxide decomposing catalyst comprising a defective perovskite compound represented by BaLa ₂ O ₄ having disordered oxygen defects and a method for producing the same. It is a thing.

Next, the present invention will be described in detail.

The composition of the nitrogen oxide decomposition catalyst of the present invention is B
It must be represented by aLa ₂ O ₄ , and compositions other than this composition are not preferable because the order-disorder transition of oxygen defects does not appear. This order-disorder transition is 250 ° C
BaLa ₂ O ₄ is present in the vicinity and undergoes a phase transition from the orthorhombic phase to the tetragonal or cubic phase at the boundary of this transition point, and at the same time, oxygen defects have a disordered arrangement. This phase transition causes nitrogen oxides to bond with disordered oxygen defects, and then decompose into nitrogen and oxygen.

This AB ₂ 0 ₄ type compound has many strains in its crystal structure as compared with the A ₂ B ₂ O ₅ type compound conventionally used as a nitrogen oxide catalyst. Since it is easier to disorder oxygen vacancies as compared with the generally-known A ₂ B ₂ O ₅ type compound, it is considered that an order-chaotic transition of oxygen vacancies appears at an extremely low temperature of around 250 ° C.

Further, the A ₂ B ₂ O ₅ type compound is Ba--
Since it is extremely unstable in the La-O system, it is not preferable as a practical catalyst.

Further, even if it mainly consists of the BaLa ₂ O ₄ phase, when unreacted substances remain, the nitrogen oxide decomposing activity decreases as the proportion of the remaining unreacted substances increases. It is preferable that the crystal phase of BaLa ₂ O ₄ is contained in an amount of 80 vol% or more by an X-ray diffraction test. Further preferably, it is suitable to consist of only a single phase of BaLa ₂ O ₄ phase in order to obtain high nitrogen oxide decomposition activity. X
The quantification of the BaLa ₂ O ₄ phase by the line diffraction test is obtained by the following formula.

Ratio of BaLa ₂ O ₄ (vol%) = (Ba
Sum of four peak areas of (040), (320), (121) and (140) of La ₂ O ₄ ) / {(BaLa ₂
O ₄ (040), (320), (121) and (14
0) sum of four peak areas + area of main peak of Ba compound + area of main peak of Y compound)} where (040), (320), (1 of BaLa ₂ O ₄
The four peaks of 21) and (140) are peaks appearing in the range of 2θ (tube is Cu) between 28 ° and 32 °.

Next, the method for producing the nitrogen oxide decomposition catalyst of the present invention will be described.

In the manufacturing method of the present invention, a Ba source and L
The sources a are mixed so as to satisfy the above compound formula. That is, when converted to the atomic ratio of Ba atoms and La atoms, B
Provided is a method for producing a nitrogen oxide decomposition catalyst, which comprises mixing raw materials so that the a / La atomic ratio is 1/2 and firing the mixture at a temperature of 800 ° C. to 1400 ° C. under an inert gas flow. It is a thing. Here, the inorganic salt of Ba and La used may be selected from inorganic salts such as carbonate, nitrate and oxalate, and the alkoxide may be selected from methoxide, ethoxide, propoxide and butoxide. As the raw material oxide, Ba oxide is unstable and easily sublimes, which is not preferable as a raw material. However, La oxide is stable, so La ₂ O ₃ or the like may be used. Further, there is no particular limitation on the mixing method of the inorganic salt of Ba and the oxide or inorganic salt of La, and various mixing pulverizers such as a ball mill and a vibration mill or a neutralization coprecipitation method is used to obtain a uniform mixture. Methods can also be used. Furthermore, when an alkoxide is used as a raw material, hydrolysis can be carried out under heating under reflux to obtain a uniform raw material mixture. Also, the firing atmosphere of the raw material mixture must be a reducing gas atmosphere. BaLa ₂ O ₄ having a large amount of oxygen defects is obtained by firing in air without firing in a reducing gas atmosphere.
No phase is obtained, unreacted Ba absorbs carbon dioxide gas existing in the air to form barium carbonate, and unreacted La is in the form of lanthanum oxide, which is not preferable. The firing temperature must be 800 ° C to 1400 ° C. Below this temperature range, the proportion of the BaLa ₂ O ₄ phase will not increase and nitrogen oxide decomposition activity will not develop, which is not preferable. Above this temperature, the specific surface area of BaLa ₂ O ₄ is 0.01 m
^It is less than ² / g, and the nitrogen oxide decomposition activity is not sufficiently high, which is not preferable. The larger the specific surface area, the more easily the activity of the nitrogen oxide decomposition catalyst increases. This is because the catalytic reaction mainly occurs on the catalyst surface, and the catalytic surface where the catalytic reaction occurs becomes larger.
That is, as the specific surface area increases, the number of places where the catalytic reaction occurs increases, and the activity of the entire catalyst improves. Practically, the specific surface area of the catalyst is preferably 0.01 m ² / g or more and 50 m ² / g or less, and if it is less than this range, the activity of the whole catalyst becomes low, and as a result, the decomposition activity of nitrogen oxides becomes extremely small. Not preferable. Further, although it may be acceptable to exceed the above range, 50 m ² / g or less is sufficient because a certain effect cannot be expected. The firing time is not particularly limited, but is preferably 1 hour or more and 20 hours or less. If the firing time is less than 1 hour, the firing temperature is 1
When the temperature is lower than 000 ° C., the production rate of the crystal phase of BaLa ₂ O ₄ may not be sufficiently increased, which is not preferable.
Even if the firing time is 20 hours or more, a sufficient effect cannot be obtained, so about 20 hours is sufficient.

[0013]

The mechanism by which the effects of the present invention are exerted has not been fully clarified, but it is an AB ₂ O ₄ type compound Ba.
Oxygen defects in La ₂ O ₄ are disordered at the order-disorder transition point. This disordering phenomenon is also in a lower temperature region than the A ₂ B ₂ O ₅ type compound. Since many disordered oxygen vacancies appear on the catalyst surface, oxygen in the nitrogen oxide is bonded to the position of the oxygen vacancies, and electrons involved in the bond between La and the nitrogen oxide are slightly on the nitrogen oxide side. As a result, the nitrogen oxides are reduced, decomposed into nitrogen and oxygen, and desorbed from the surface of the catalyst, and La returns to its original state after this decomposition reaction, thereby exhibiting the function as a nitrogen oxide decomposition catalyst. It is considered to be a thing. It is considered that by using this function, the effect of oxidation due to coexisting oxygen was extremely reduced.

[0014]

As described above, the present invention provides a catalyst exhibiting a high nitrogen oxide decomposing activity even in a lean burn region where excess oxygen exists in exhaust gas, and a method for producing the same. .

[0015]

【Example】

Example 1 18.2 g of barium carbonate (manufactured by Kishida Chemical Co., Ltd.) and lanthanum oxide (manufactured by Kishida Chemical Co., Ltd.) 3 so as to have a composition of BaLa ₂ O _4.
After mixing and drying 0.0 g in ethanol using a ball mill, a single-phase powder of BaLa ₂ O ₄ was obtained by firing under a nitrogen gas flow at a temperature of 1300 ° C. for 2 hours. The BET specific surface area value of the obtained powder was 0.2 m ² / g. Using this BaLa ₂ O ₄ ,
NO = 1400ppm by atmospheric pressure fixed bed flow reactor
Nitrogen oxide decomposition activity was measured in the temperature range of 50 ° C to 600 ° C at SV = 2500 / h using the reaction gas of. When oxygen coexists, 4% of the total flow rate of oxygen was mixed, and the catalyst activity was measured under the same conditions as above. The measurement results are shown below.

[0016]

[0017]

[Table 1]

Example 2 18.2 g of barium carbonate (manufactured by Kishida Chemical Co., Ltd.) and lanthanum oxide (manufactured by Kishida Chemical Co., Ltd.) 3 so as to have a composition of BaLa ₂ O _4.
After mixing and drying 0.0 g in ethanol using a ball mill, the mixture was baked at a temperature of 1000 ° C. for 6 hours under a nitrogen gas flow to obtain a powder containing 92 vol% of BaLa ₂ O ₄ . The remaining materials were barium carbonate and yttrium oxide. The BET specific surface area value of the obtained powder was 3.2 m ² / g. Using this BaLa ₂ O ₄ , a reaction gas of NO = 1400 ppm was used in an atmospheric pressure fixed bed flow reactor, and SV =
Nitrogen oxide decomposition activity at 2500 / h is 50 ° C to 6
It measured in the temperature range of 00 degreeC. When oxygen coexists, 4% of the total flow rate of oxygen was mixed, and the catalyst activity was measured under the same conditions as above. The measurement results are shown below.

[0019]

[0020]

[Table 2]

Comparative Example 1 36.4 g of barium carbonate (manufactured by Kishida Chemical Co., Ltd.) and lanthanum oxide (manufactured by Kishida Chemical Co., Ltd.) so that the composition was Ba ₂ La ₂ O _5.
After 30.0 g was mixed and dried in ethanol using a ball mill, it was baked at a temperature of 1300 ° C. for 2 hours under a nitrogen gas flow. The obtained powder is Ba ₂
Although it was a single phase of La ₂ O _5, it was completely decomposed into barium carbonate and lanthanum oxide within a few hours after firing, and these barium carbonate and lanthanum oxide produced by the decomposition reaction had extremely high water absorption. In addition, it was not a catalyst evaluation sample.

Comparative Example 2 48.5 g of barium carbonate (manufactured by Kishida Chemical Co., Ltd.) and lanthanum oxide (manufactured by Kishida Chemical Co., Ltd.) so that the composition was Ba ₈ La ₆ O _17.
After 30.0 g was mixed and dried in ethanol using a ball mill, it was baked at a temperature of 1300 ° C. for 2 hours under a nitrogen gas flow. The obtained powder is Ba ₈
Although it was a La ₆ O ₁₇ single phase, it was completely decomposed into barium peroxide and lanthanum oxide within a few hours after firing, and these barium peroxide and lanthanum oxide produced by the decomposition reaction were extremely water-absorbing. Due to its high price, it did not become a catalyst evaluation sample.

Comparative Example 3 18.2 g of barium carbonate (manufactured by Kishida Chemical Co., Ltd.) and lanthanum oxide (manufactured by Kishida Chemical Co., Ltd.) 3 so as to have a composition of BaLa ₂ O _4.
After mixing and drying 0.0 g of it in ethanol using a ball mill, it was calcined at a temperature of 600 ° C. for 4 hours under a nitrogen gas flow. 61% by volume of BaLa ₂ O ₄
A remaining mixed powder of barium carbonate and lanthanum oxide was obtained. The BET specific surface area value of the obtained powder is 46.1 m ^2.
/ G. Using this mixed powder mainly consisting of BaLa ₂ O ₄ , NO =
SV = 2500 / using reaction gas of 1400 ppm
When the nitrogen oxide decomposing activity was measured in the temperature range of 50 ° C. to 600 ° C. for h, no significant improvement in the nitrogen oxide decomposing activity due to the order-disorder transition was observed.
The decomposition activity value was also extremely low as a whole. When oxygen coexists, the total flow rate of the flow gas is 4
% Oxygen was mixed and the catalyst activity was measured under the same conditions as above. The measurement results are shown below.

[0024]

[0025]

[Table 3]

Comparative Example 4 27.3 g of barium carbonate (manufactured by Kishida Chemical Co., Ltd.) and lanthanum oxide (manufactured by Kishida Chemical Co., Ltd.) so as to have a composition of Ba ₃ La ₄ O _9.
After 30.0 g was mixed and dried in ethanol using a ball mill, it was baked at a temperature of 1300 ° C. for 2 hours under a nitrogen gas flow. The powder obtained is Ba ₃
The La ₄ O ₉ phase was 10 vol%, and the remaining crystal phase was a mixed powder of barium carbonate and lanthanum oxide. The BET specific surface area value of the obtained powder was 0.3 m ² / g. Based on this sample, NO
= 1400 ppm of reaction gas, SV = 2500
/ H, the nitrogen oxide decomposing activity was measured in the temperature range of 50 ° C. to 600 ° C., and the nitrogen oxide decomposing activity was not found within the measuring range.

Claims (2)

[Claims] 1. BaLa having disordered oxygen defects.
A nitrogen oxide decomposition catalyst comprising a defective perovskite compound represented by ₂ O ₄ . 2. A disordered oxygen defect characterized in that an inorganic salt of barium and an inorganic salt, oxide or alkoxide of lanthanum are mixed and fired at a temperature of 800 ° C. to 1400 ° C. under a flow of an inert gas. With, BaL
A method for producing a nitrogen oxide decomposition catalyst comprising a defective perovskite compound represented by a ₂ O ₄ .