JPH1176822A - Oxide catalyst material for removing nitrogen oxides and method for removing nitrogen oxides - Google Patents

Abstract

(57) [Summary] [PROBLEMS] To reduce nitrogen oxides contained in exhaust gas discharged from a factory or a power plant and exhaust gas of a lean burn engine of an automobile even under a high-temperature oxidizing atmosphere condition of 800 ° C. Disclosed are a catalyst material which can be directly decomposed and removed into nitrogen and oxygen without using an agent, and a method for removing nitrogen oxides using the same. A spinel-type crystalline composite oxide containing Mg and Ga as main metal elements, Pd in an amount of 0.5 to 15.0% by weight based on the spinel-type crystalline composite oxide,
The atomic ratio of Pd to Al (Pd / Al) is 0.15 to 1.0
At the same time to form an oxide catalyst material for removing nitrogen oxides, bringing the oxide catalyst material for removing nitrogen oxides into contact with an exhaust gas containing nitrogen oxides, and removing nitrogen oxides contained in the exhaust gas. The substance is directly decomposed into nitrogen and oxygen without using a reducing agent and removed.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION The present invention relates to a method for directly converting nitrogen oxides contained in various exhaust gases from various industrial furnaces in stationary factories, internal combustion engines in power plants, and internal combustion engines for mobile vehicles. The present invention relates to an oxide catalyst material capable of being decomposed and removed into nitrogen and oxygen, and a method for directly decomposing and removing nitrogen oxides in exhaust gas into nitrogen and oxygen without using a reducing agent using the same. Oxide catalyst material for removing nitrogen oxides suitable for purifying automobile exhaust gas such as lean burn engines having a low hydrocarbon concentration in the gas and nitrogen oxides in the exhaust gas using the oxide catalyst material directly to form nitrogen gas And a method of decomposing and removing oxygen.

[0002]

2. Description of the Related Art Conventionally, nitrogen oxides contained in exhaust gas discharged from various stationary industrial furnaces, internal combustion engines, and internal combustion engines typified by mobile vehicles have only adverse effects on human bodies. Rather, it is a substance that causes acid rain and photochemical smog, and its release into the atmosphere is a major environmental problem.

Therefore, as a method of removing nitrogen oxides from the exhaust gas, a catalytic reduction method has been mainly used for a long time. For example, fixed sources of nitrogen oxides in the factories and power plants have been used. Since the exhaust gas contains a large amount of oxygen, ammonia is used as a reducing agent and vanadia (V ₂ O
₅ ) / Nitrogen oxides are reduced and removed by a titania (TiO ₂ ) catalyst.

[0004] On the other hand, in mobile sources such as automobiles, since the amount of oxygen in the exhaust gas is small, unburned carbon monoxide (CO) and hydrocarbons (CxHy) remaining in the exhaust gas are used as reducing agents. Nitrogen oxide is reduced and removed by a three-way catalyst. As a three-way catalyst used in such a method, a noble metal such as Pd, Pt, Rh or the like is used in γ-alumina (A
(1 ₂ O ₃ ), which is supported on a refractory carrier such as cordierite.

[0005] However, the catalytic reduction method using ammonia, which is used as a method for removing nitrogen oxides from the fixed source, is not suitable because ammonia is expensive and dangerous, and it requires careful handling. It could not be used as a source.

[0006] Further, in a mobile source such as an automobile,
At present, lean-burn engines (lean-burn engines), which have been attracting attention for energy saving, do not show a reduction effect of nitrogen oxides because the amounts of unburned carbon monoxide and hydrocarbons in exhaust gas are extremely small. The problem remains.

In order to solve the above problem, it is simplest and ideal to remove nitrogen oxides that do not require a reducing agent. Therefore, nitrogen oxides are directly converted into nitrogen and oxygen without using a reducing agent. Various studies have been conducted on decomposing catalysts. Until now, copper ion-exchanged zeolite catalysts have been used.
It has been reported that nitrogen oxides can be directly decomposed into nitrogen and oxygen in a temperature range of up to 500 ° C. (Japanese Patent Laid-Open No. 1-1).
No. 30735, JP-A-63-283727).

[0008]

However, in the case of using the copper ion-exchanged zeolite catalyst, 600 ° C.
When the temperature exceeds 600 ° C., the nitrogen oxide decomposition activity rapidly deteriorates.
The nitrogen oxide decomposing ability is low, and the coexistence of oxygen significantly impairs the nitrogen oxide decomposing activity.

[0009]

SUMMARY OF THE INVENTION Accordingly, the present invention relates to an exhaust gas emitted from a fixed source such as a factory or a power plant, and a mobile source such as an automobile, particularly an exhaust gas containing oxygen such as a lean burn engine (lean burn engine). A useful catalyst material capable of maintaining a catalytic ability to directly decompose and remove nitrogen oxides contained therein into nitrogen and oxygen without using a reducing agent even under high temperature conditions exceeding 600 ° C. and nitrogen using the same It is intended to provide an oxide removing method.

[0010]

SUMMARY OF THE INVENTION The present invention has been made in view of the above-mentioned problems, and is directed to a spinel-type crystalline composite oxide containing Mg and Ga as main metal elements.
At the same time shows a high activity in the direct decomposition reaction of nitrogen oxides even under high temperature conditions exceeding 600 ° C.,
In particular, even in the case of an oxygen-containing exhaust gas containing nitrogen oxides, in which almost no reducing gas is discharged from a lean-burn internal combustion engine, it is necessary to purify the exhaust gas by effectively decomposing and removing the nitrogen oxides in the exhaust gas. It has been confirmed that the present invention can be performed, and the present invention has been achieved.

That is, the oxide catalyst material for removing nitrogen oxides of the present invention is a spinel-type crystalline composite oxide containing Pd and Al at the same time and containing Mg and Ga as main metal elements. Is 0.5 to 15.0% by weight based on the spinel-type crystalline composite oxide, and Al is supported at a ratio of 0.15 to 1.0 in an atomic ratio of Pd to Al (Pd / Al). It is characterized by the following.

Further, the method for removing nitrogen oxides according to the present invention
a spinel-type crystalline composite oxide containing Mg and Ga as the main metal elements that simultaneously supports
Is 0.5 to the spinel-type crystalline composite oxide.
15.0% by weight, Al is an atomic ratio of Pd to Al (Pd
/ Al) at a ratio of 0.15 to 1.0 using an oxide catalyst material for removing nitrogen oxides, contacting the catalyst material with an exhaust gas containing nitrogen oxides, and using a nitrogen gas without a reducing agent. The oxide is directly decomposed into nitrogen and oxygen.

[0013]

According to the oxide catalyst material for removing nitrogen oxides and the method for removing nitrogen oxides of the present invention, the oxide catalyst material for removing nitrogen oxides of the present invention comprises a spinel containing Mg and Ga as main metal elements. Pd and Al
And the loading amount of Pd is 0.5 to 15.0% by weight based on the spinel-type crystalline composite oxide, and Al is contained in an atomic ratio of Pd to Al (Pd / Al) of 0.15 to 1.0%. As a result, the aluminum compound supported simultaneously changes the electronic state of Pd, thereby suppressing the oxidation of Pd, improving the affinity with nitrogen oxide, and increasing the nitrogen oxide decomposition activity. Will be expressed.

[0014]

BEST MODE FOR CARRYING OUT THE INVENTION The oxide catalyst material for removing nitrogen oxides and the method for removing nitrogen oxides according to the present invention will be described in detail below.

In the present invention, the amount of Pd supported on the spinel-type crystalline composite oxide as an oxide catalyst material for removing nitrogen oxides is less than 0.5% by weight based on the spinel-type crystalline composite oxide. In this case, the catalyst activity is hardly exhibited. On the contrary, if it exceeds 15.0% by weight, no improvement effect of the catalyst activity is recognized.
In particular, from the viewpoint of catalytic activity, the supported amount is 1.0
To 10.0% by weight, more preferably 3.0 to 7.0% by weight.
% By weight indicates the most desirable trend.

When the ratio of the atomic ratio of Pd to Al (Pd / Al) is less than 0.15, the effect of improving the catalytic activity of the aluminum compound added to the spinel-type crystalline composite oxide is reduced. On the contrary, if it exceeds 1.0, the oxidation resistance of Pd is hardly improved, so that Pd / Al is 0.15 to 1.0.
0, and especially from 0.2 to 0.5 in terms of catalytic activity.
Is more preferable, and the addition amount of 0.25 to 0.3 shows the most desirable tendency.

When the spinel-type crystalline composite oxide is brought into contact with a simulated exhaust gas containing no reducing gas containing nitrogen oxide, the nitrogen oxide in the simulated exhaust gas is directly decomposed into nitrogen and oxygen. It is recognized that it has excellent properties.

The composite oxide contains Mg and Ga as main metal elements, and has an atomic ratio n of Ga / Mg of 2.
A spinel crystalline composite oxide consisting of the ratio of _{5~3.3, MgGa n O 4 + z} ( where, n = 2.5 to 3.
Represented by the general formula 3), wherein (O)
_{4 + z} ) is the amount of oxygen necessary to exist stably as a composite oxide, and the amount of oxygen changes as needed in the range of 0.2 or less depending on the value of n.

The composite oxide used in the present invention is:
If the value of the Ga / Mg atomic ratio n deviates from the range of 2.5 to 3.3, the catalytic activity decreases.
Particularly, 2.8 to 3.0 is most desirable.

Next, an example of the method for producing the oxide catalyst material for removing nitrogen oxides of the present invention will be described in detail.

The composite oxide of the present invention is obtained by mixing a raw material powder containing Mg and Ga with an atomic ratio n of Ga / Mg of 2.5 to 2.5.
After being weighed so as to fall within the range of 3.3 and sufficiently stirred and mixed, the mixture is heated in a oxidizing atmosphere at a temperature of 500 to 1600 ° C. for 5 minutes.
Heat treatment for up to 30 hours, Mg as a metal element
And a composite oxide powder having a spinel type crystal containing Ga as a main crystal phase.

As the raw material powder, for example, Mg and Ga oxides, and their carbonates, nitrates, acetates, etc., which form oxides by heat treatment, can be used.

In addition, the composite oxide can be synthesized by a solid phase reaction method using an oxide or another metal salt, or a sol-gel method such as a metal alkoxide. It is not something to be done.

In any of the above-mentioned manufacturing methods, in any of the heat treatments, if the heat treatment temperature is lower than 500 ° C., crystallization becomes insufficient, and if the heat treatment temperature exceeds 1600 ° C., densification occurs. In an oxidizing atmosphere, 5
Heat treatment at a low temperature is effective for increasing the specific surface area of the powder, and practically, it is desirable to set the specific surface area to 35 m ² / g or more.

Next, Pd and Al are supported on the composite oxide thus obtained as a support. The method for supporting Pd and Al is not particularly limited, and known methods such as an evaporation to dryness method and an impregnation method are used. For example, a solution containing the predetermined amount of Pd and a solution containing Al at the predetermined atomic ratio are added to the carrier powder, and then evaporated to dryness, and helium (H
e) in an atmosphere of gas or the like at a temperature of 400 to 600 ° C.
By performing the heat treatment for 5 hours, the oxide catalyst material for removing nitrogen oxides comprising the Mg—Ga-based spinel-type crystalline composite oxide supporting Pd and Al according to the present invention is obtained.

The salt of Pd and Al used for preparing the solution of Pd and Al is not particularly limited.
Usually, various inorganic salts such as nitrates, sulfates, carbonates, hydrochloric acid, halides and the like, organic salts such as acetates, hydroxides and oxides and the like can be mentioned.

As the solvent used for preparing the Pd and Al salts into a solution, water or an organic substance such as an alcohol or a carbonyl compound is preferably used.

Next, the atmosphere of the heat treatment is not particularly limited, but an inert gas such as He, N ₂ , argon (Ar) or a diluting gas of hydrogen with an inert gas can be used.

Further, the treatment time at the above-mentioned heat treatment temperature is not particularly limited, and usually 3 to 5 hours is sufficient, and the temperature can be changed stepwise during the heat treatment.

By contacting the catalyst material thus obtained with an exhaust gas containing nitrogen oxides, the nitrogen oxides can be directly decomposed into nitrogen and oxygen without using a reducing agent.

[0031]

EXAMPLES In evaluating the present invention, Mg (NO ₃ ) ₂ .6H ₂ O and Ga (NO ₃ ) were used as starting materials.
Using reagents ₂ · 9H ₂ O, metal ratio of Mg and Ga were weighed so that a ratio of 1: 3, dissolved the reagents in distilled water, neutralized with aqueous ammonia while stirring, the The precipitate formed at times was filtered, washed and lyophilized.

The dry powder thus obtained is placed in air at 70
Heat treatment at 0 ° C. for 30 hours for specific surface area of 40-5
A spinel-type crystalline composite oxide powder of 0 m ² / g was produced.

Thereafter, Pd and Al were loaded on the spinel-type composite oxide powder so that the loading amounts of Pd and Al were as shown in Table 1.
An aqueous solution containing d and Al was added, evaporated to dryness, and then heat-treated to obtain a spinel-type crystalline composite oxide powder supporting Pd and Al.

Next, the spinel-type crystalline composite oxide powder supporting Pd and Al is molded by a mold press.
Further, after being compressed by a cold isostatic pressing method, the molded product was crushed and sieved, and sized to a size exceeding 500 μm and not more than 700 μm to prepare a catalyst sample for evaluation.

Incidentally, the catalytic activity of only the spinel-type crystalline composite oxide catalyst not supporting Pd and Al at all and the catalytic activity of copper ion-exchanged zeolite were used as comparative examples.

Next, as a simulated exhaust gas, a space gas (SV) of 9000 / hr was used as a condition for contacting the catalyst material with the reaction gas consisting of 3000 ppm of NO as nitrogen oxide and the balance of He. And flowed through the catalyst layer filled with the catalyst sample for evaluation.
N ₂ gas generated through the catalyst layer in a temperature range of ° C. was measured by gas chromatography.

The NO resolution of the catalyst is determined by the N _{2 at the} outlet of the catalyst layer.
The NO removal rate (%) is obtained by dividing the value twice the concentration (ppm) by the NO concentration (ppm) on the catalyst layer inlet side, and the NO removal rate at each temperature is obtained. For clarity, O _{2 at} a temperature of 800 ° C. is 2
The NO removal rate in the simulated exhaust gas containing% was also determined.

The results show that the maximum value of the NO removal rate at the measurement temperature exceeds 20% in a region where the measurement temperature in the absence of O ₂ exceeds 600 ° C., and
NO removal rate in the presence of O _{2 at} a temperature of 800 ° C. is 5
% Or more was evaluated as good.

[0039]

[Table 1]

As is clear from the table, in Sample Nos. 18 and 19, which are comparative examples, NO decomposition activity was extremely low in the high temperature range exceeding 600 ° C. regardless of the presence or absence of O _2. Sample Nos. 1 and 9 outside the scope of claims
Shows that the NO decomposition activity is low irrespective of the presence or absence of O ₂ , and it was also found that the NO decomposition activity in the presence of O ₂ was low for all samples Nos. 10, 16 and 17, which were not practical. .

On the other hand, in the present invention, 800
It can be seen that sufficient NO decomposition activity is exhibited even in an oxidizing atmosphere at a high temperature of ° C.

[0042]

As described above, according to the oxide catalyst material for removing nitrogen oxides and the method for removing nitrogen oxides of the present invention, the oxide catalyst material contains Mg and Ga as main metal elements. 0.5 to the spinel-type crystalline composite oxide
~ 15.0% by weight of Pd and the atomic ratio of Pd to Al (Pd
/ Al) at the same time carrying Al having a ratio of 0.15 to 1.0, and contacting the oxide catalyst material with an exhaust gas containing no nitrogen oxide-containing reducing gas. Nitrogen oxides are directly decomposed into nitrogen and oxygen even under a high temperature oxidizing atmosphere of
It has a sufficient activity to decompose nitrogen oxides up to a high temperature range, which is higher than the temperature range above, so that nitrogen oxides contained in exhaust gas from mobile sources such as automobiles as well as stationary sources such as factories and power plants The object can also be directly decomposed and removed effectively.

As a result, it is extremely useful for purifying various harmful substances containing nitrogen oxides, including exhaust gas of various internal combustion engines such as lean burn engines which will be developed with the aim of saving energy and resources and preventing global warming. It will be useful.

Claims (2)

[Claims] 1. Magnesium (Mg) and gallium (Ga)
(Pd) and aluminum (Al) in spinel-type crystalline composite oxides containing
Is 0.5 to 15.0% by weight of palladium (Pd) based on the spinel-type crystalline composite oxide and aluminum (A
1) An oxide catalyst material for removing nitrogen oxides, characterized in that l) is supported at an atomic ratio (Pd / Al) of palladium (Pd) and aluminum (Al) of 0.15 to 1.0. 2. Magnesium (Mg) and gallium (Ga)
(Pd) and aluminum (Al) in spinel-type crystalline composite oxides containing
Is 0.5 to 15.0% by weight of palladium (Pd) based on the spinel-type crystalline composite oxide and aluminum (A
l) is an oxide catalyst material for removing nitrogen oxide, which is supported at an atomic ratio (Pd / Al) of palladium (Pd) and aluminum (Al) of 0.15 to 1.0, A method for removing nitrogen oxides, which comprises contacting an exhaust gas containing the nitrogen oxides to directly decompose the nitrogen oxides into nitrogen and oxygen.