JPH11156196A - Oxide catalyst material for nitrogen oxide decomposition and nitrogen oxide decomposition removal method - Google Patents

Abstract

(57) [Summary] [PROBLEMS] To remove NO contained in various exhaust gas discharged from a fixed source and exhaust gas of a mobile source, particularly, a lean burn engine (lean burn engine) at a high temperature exceeding 600 ° C. Provided is a useful catalyst material which can be directly decomposed and removed into N ₂ and O ₂ with high catalytic activity without using a reducing agent even under exhaust gas conditions, and a method for decomposing and removing NO using the same. SOLUTION: Magnesium (Mg) and gallium (Ga)
Is an oxide catalyst material in which PdO is supported at 0.5 to 20.0% by weight on a spinel-type crystalline composite oxide containing as a main metal element, the exhaust gas containing the oxide catalyst material and NO and no reducing gas is present. By contacting the gas, 6
At 00 ° C. or higher, NO contained in exhaust gas is directly decomposed and removed efficiently into N ₂ and O ₂ .

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to nitrogen oxides (N) contained in various exhaust gases from internal combustion engines for various industrial furnaces and power plants in stationary factories and internal combustion engines for mobile automobiles and the like.
O) is an oxide catalyst material capable of directly decomposing O) into nitrogen (N ₂ ) and oxygen (O ₂ ), and is used to directly reduce NO in various exhaust gases to N ₂ without using a reducing agent. The present invention relates to a method for decomposing and removing O ₂ , particularly an oxide catalyst material for decomposing nitrogen oxides and an oxide catalyst suitable for purifying automobile exhaust gas such as lean burn engines having a low hydrocarbon concentration in exhaust gas. The present invention relates to a method for directly decomposing NO in exhaust gas into N ₂ and O ₂ by using a material and removing it.

[0002]

2. Description of the Related Art Conventionally, NO contained in exhaust gas discharged from various stationary industrial furnaces and internal combustion engines, and internal combustion engines typified by mobile automobiles, has only a bad effect on the human body. In addition, since it is a causative substance of acid rain and photochemical smog, its release into the atmosphere is a major environmental problem.

Therefore, the NO contained in the exhaust gas
As a method for removing NO, a catalytic reduction method has been mainly used for a long time. For example, since a fixed NO emission source such as the factory and the power plant contains a large amount of oxygen in the exhaust gas, a reducing agent is used. Using ammonia as vanadia (V ₂ O
₅ ) / NO is reduced and removed by a titania (TiO ₂ ) catalyst.

[0004] On the other hand, undesired carbon monoxide (CO) and hydrocarbons (C _x H _y ) remaining in the exhaust gas are removed from the mobile sources such as automobiles because the amount of oxygen in the exhaust gas is small. It is used as a reducing agent, and NO is reduced and removed by a three-way catalyst.

As a three-way catalyst used in such a NO reduction reduction method, for example, a noble metal such as palladium (Pd), platinum (Pt), rhodium (Rh) is converted to γ-alumina (Al ₂ O ₃ ). Those supported on a coated refractory carrier such as cordierite have been used.

[0006] However, the catalytic reduction method using ammonia, which is used as a method for removing NO from the fixed generation source, involves the problem that ammonia is expensive and dangerous, and requires sufficient care in its handling. It could not be used as a source.

[0007] On the other hand, in a mobile source such as an automobile,
At present, in lean-burn engines (lean-burn engines), which are attracting attention for energy saving, the amounts of unburned carbon monoxide and hydrocarbons in exhaust gas are extremely small.
There remains a problem of not exhibiting the reducing action of NO.

In order to solve this problem, it is easiest and ideal to remove NO without using a reducing agent. Therefore, NO is reduced to N ₂ and O ₂ without using a reducing agent. in which the study of directly decomposing catalyst is variously made, until the NO in the exhaust gas using a copper ion-exchanged zeolite catalyst may be directly decomposed and removed into N ₂ and O ₂ have been proposed to (Hei 4-349938, JP-A-1-
130735).

[0009]

However, in the case of using the copper ion exchanged zeolite catalyst, 400-
Although it is said that NO can be directly decomposed into N ₂ and O ₂ in the temperature range of 500 ° C., NO decomposition activity is rapidly deteriorated at temperatures exceeding 600 ° C., so high-temperature exhaust gas conditions exceeding 600 ° C. Under the above, there is a problem that the above-mentioned NO decomposition / removal ability is not provided, and it is not practical.

[0010]

SUMMARY OF THE INVENTION The present invention has been made in consideration of the above problems, and has as its object to provide various exhaust gases emitted from fixed sources such as factories and power plants, and mobile sources represented by automobiles, especially NO contained in the exhaust gas of a lean burn engine (lean burn engine) is directly decomposed and removed into N ₂ and O ₂ with high catalytic activity without using a reducing agent even under high-temperature exhaust gas conditions exceeding 600 ° C. It is intended to provide a useful catalyst material which can be used as well as a method for decomposing and removing NO using the same.

[0011]

Means for Solving the Problems The present inventors have made intensive studies in view of the above problems, and as a result, by carrying PdO on a spinel-type crystalline composite oxide, NO decomposition of a catalyst at a high temperature of 600 ° C. or more was carried out. It has been found that the removal ability can be increased, and the present invention has been completed.

That is, the oxide catalyst material for decomposing nitrogen oxides of the present invention comprises a spinel-type crystalline composite oxide containing Mg and Ga as main metal elements, and a PdO-containing compound for the spinel-type crystalline composite oxide. 0.5 to 20.0% by weight.

Further, the method for decomposing and removing nitrogen oxides according to the present invention is characterized in that the spinel-type crystalline composite oxide containing Mg and Ga as main metal elements and PdO is added to the spinel-type crystalline composite oxide in an amount of 0.1 to 0.1%. Using an oxide catalyst material for nitrogen oxide decomposition supported at a rate of 5 to 20.0% by weight, the catalyst material is brought into contact with exhaust gas containing NO, and NO is reduced to N ₂ and O ₂ without using a reducing agent. And directly decomposed and removed.

[0014]

According to the oxide catalyst material for decomposing nitrogen oxides and the method for decomposing and removing nitrogen oxides of the present invention, the catalyst material of the present invention is a spinel-type crystalline composite oxide containing Mg and Ga as main metal elements. And the amount of PdO supported is 0.5 to 2 with respect to the spinel-type crystalline composite oxide.
Since the content is 0.0% by weight, PdO and Mg-G
Due to the interaction of the a-based oxide catalyst, NO decomposition activity capable of directly decomposing NO in exhaust gas into N ₂ and O ₂ up to a high temperature exceeding 600 ° C. is developed. Since the amount of adsorbed NO increases, the catalytic activity is further improved.

[0015]

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

In the oxide catalyst material for decomposing nitrogen oxides according to the present invention, the spinel-type crystalline composite oxide contains Mg and Ga as main metal elements, and has an atomic ratio of Ga to Mg of n (Ga / Mg). Is a composite oxide having a ratio of 2.5 to 3.3, and MgGanO _{4 + Z} (n = 2.5 to 3.
It is represented by the general formula of 3).

In the above formula, (O _{4 + Z} ) is the amount of oxygen necessary for stable existence 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. Is what you do.

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

Next, when the supported amount of PdO of the present invention is less than 0.5% by weight based on the spinel-type crystalline composite oxide, even if a small amount of PdO is supported, catalytic activity is recognized, The decomposition rate is extremely low and not practical,
When the content exceeds 0.0% by weight, the effect of improving the catalytic activity is not recognized, and the supported amount is specified to be 0.5 to 20% by weight based on the spinel-type crystalline composite oxide.

In particular, from the viewpoint of catalytic activity, 1.0 to 10.
0% by weight is more preferable, and 3.0 to 8.0% by weight shows the most desirable tendency.

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

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 and Mg (Ga / M
g) is within the range of 2.5 to 3.3, and after sufficiently stirring and mixing, is mixed in an oxidizing atmosphere at 500 to 160.
By performing a heat treatment at a temperature of 0 ° C. for 5 to 30 hours, a composite oxide powder having a main crystal phase of a spinel crystal containing Mg and Ga as metal elements can be produced.

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.

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.

Further, in any of the above-mentioned manufacturing methods, in any of the heat treatments, if the heat treatment temperature is lower than 500 ° C., the crystallization becomes insufficient, and if the heat treatment temperature exceeds 1600 ° C., on the other hand, densification occurs. , In an oxidizing atmosphere,
The heat treatment is carried out for 5 to 30 hours. Heat treatment at a particularly 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 40 m ² / g or more.

Next, a predetermined amount of PdO is supported on the obtained composite oxide. The method for supporting the PdO is not particularly limited, and a known method such as an evaporation to dryness method or an impregnation method is used. For example, the composite oxide is immersed in a Pd solution prepared by dissolving a Pd salt in a solvent, and the solvent is evaporated off and dried.
Heat treatment is performed at a temperature of ° C.

Here, the salt of Pd is not particularly limited, but is usually a nitrate, a sulfate,
Inorganic salts such as carbonates, hydrochloric acid, and bromates (halides); organic salts such as acetate; hydroxides, oxides, and the like. Water or an organic substance such as an alcohol or a carbonyl compound.

The heat treatment is performed at a temperature of 600 ° C.
If it is lower than PdO, no PdO is generated, and PdO remains as a Pd salt. On the other hand, if the temperature exceeds 800 ° C., the generated PdO is reduced to Pd, and the catalytic activity decreases.

Particularly, the heat treatment temperature is most preferably 650 ° C. to 750 ° C. from the viewpoint of heat resistance of the spinel type crystalline composite oxide, catalytic activity and crystallization of PdO.

The time of the heat treatment in the presence of the oxidizing gas is not particularly limited, but usually 2 to 10 hours is sufficient, and the temperature may be changed stepwise during the heat treatment.

The catalyst material thus obtained can directly decompose NO into N ₂ and O ₂ without using a reducing agent by contacting the catalyst material with an exhaust gas containing NO.

[0032]

Next, the present invention was evaluated as described in detail below.

First, Mg (NO ₃ ) _{2.
6H 2 O, Ga (NO 3} ) with reagents ₂ · 9H ₂ O,
The reagents are weighed so that the metal ratio of Mg to Ga becomes 1: 3, and these reagents are separately dissolved in distilled water. The pH of Mg is adjusted to 11.0 and the pH of Ga is adjusted to 8.5 with aqueous ammonia with stirring. The resulting precipitate was mixed, filtered, washed, and lyophilized.

The dry powder thus obtained was placed in air at 700
Heat treatment at a temperature of 30 ° C. for 30 hours to have a specific surface area of 40 to 50.
A spinel-type crystalline composite oxide powder of m ² / g was obtained.

Thereafter, a Pd salt was added to the spinel-type crystalline composite oxide powder in the ratio shown in Table 1 and then heat-treated in air for 10 hours to carry out PdO-supported MgGa _3. O _{4 + Z} was produced.

Next, the MgGa ₃ supporting PdO was used.
The O _{4 + Z} powder was molded by a mold press, and the molded product was crushed and sieved, and sized to exceed 500 μm and 700 μm or less to prepare an evaluation sample.

The catalytic activities of the spinel-type crystalline composite oxide alone not supporting PdO and the conventional copper ion-exchanged zeolite were used as comparative examples.

Next, NO as a simulated exhaust gas was 300
A space gas (SV) of 9000 / hr. Was used as a condition that a reaction gas consisting of 0 ppm, 2% of O ₂ and the balance of He was in contact with the catalyst material. Then, the mixture was passed through the catalyst layer filled with the catalyst material for evaluation, and N ₂ gas generated by passing through the catalyst layer at a temperature in the range of 600 to 800 ° 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 decomposition rate (%) was calculated by dividing the value twice the concentration (ppm) by the NO concentration (ppm) on the catalyst layer inlet side, and calculating the NO decomposition rate at each temperature.

From the results, those having a minimum value of NO decomposition activity of less than 10% at 600 ° C. or higher were defective, and the lowest value was 10%.
If the maximum value is 18% or less, the maximum value is 19% or more and less than 20%.
Those with 0% or more were evaluated as excellent.

The crystal phase of the catalyst material for evaluation according to the present invention was identified by X-ray diffraction measurement (XRD), and it was confirmed that each of the catalyst materials had a crystal phase of PdO.

[0042]

[Table 1]

As is clear from the results of the evaluation of the catalyst performance in the table, Sample Nos. 1 and 16, which are Comparative Examples, all have a low NO decomposition activity in a predetermined temperature range, and are outside the scope of the present invention. Sample Nos. 2 and 11 also showed that the NO decomposition activity in the predetermined temperature range was low overall and impractical.

On the other hand, in the present invention, in each case, 600
It can be seen that a sufficient NO decomposition activity is exhibited in a high temperature range of not less than ° C.

[0045]

As described above, according to the oxide catalyst material for decomposing nitrogen oxides and the method for decomposing and removing nitrogen oxides of the present invention, magnesium (Mg) and gallium (Ga) are contained as main metal elements. An oxide catalyst material in which PdO is supported in an amount of 0.5 to 20.0% by weight on a spinel-type crystalline composite oxide, and the exhaust gas containing NO and no reducing gas is brought into contact with the oxide catalyst material. , 60
Since NO is directly and efficiently decomposed into N ₂ and O ₂ in a high temperature range of 0 ° C. or higher, NO contained in exhaust gas can be effectively decomposed and removed.

As a result, it is extremely useful for purifying various harmful substances containing NO, including exhaust gas from 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.

Claims (2)

[Claims] 1. Magnesium (Mg) and gallium (Ga)
Is characterized in that palladium oxide (PdO) is supported on a spinel-type crystalline composite oxide containing 0.5 to 20.0% by weight of the spinel-type crystalline composite oxide containing as a main metal element. Oxide catalyst material for decomposition of nitrogen oxides. 2. Magnesium (Mg) and gallium (Ga)
Palladium oxide (PdO) is supported on a spinel-type crystalline composite oxide containing 0.5 to 20.0% by weight of the spinel-type crystalline composite oxide containing 0.5% to 20.0% by weight as a main metal element. Oxide decomposition characterized in that the catalyst material is brought into contact with exhaust gas containing nitrogen oxides (NO) and directly decomposed into nitrogen (N ₂ ) and oxygen (O ₂ ) without using a reducing agent. Method.