JP2851769B2 - Oxide catalyst material for removing nitrogen oxides and method for removing nitrogen oxides - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a novel oxide catalyst material capable of reducing and removing nitrogen oxides and a method for removing nitrogen oxides in exhaust gas using the same.

[0002]

2. Description of the Related Art In recent years, environmental pollution has become a problem, and among them, development of a method for decomposing and removing harmful substances such as NOx and COx in automobile exhaust gas has been urgently required. Therefore,
Conventionally used automobile exhaust gas purifying catalysts include carbon monoxide (CO) and hydrocarbons (CxH).
Three-way catalysts that simultaneously oxidize y) and reduce nitrogen oxides (NOx) are widely used. As such a three-way catalyst, a catalyst in which a noble metal such as Pd, Pt, and Rh is supported on a refractory carrier such as cordierite coated with γ-alumina is used.

[0003] In addition to the above, there are known a selective catalytic reduction method using ammonia, and a method of removing NOx while contacting a hydrocarbon with a metal-supporting hydrophobic zeolite as a catalyst.

[0004]

However, the three-way catalyst can efficiently purify the exhaust gas only at a low oxygen concentration of about 0.5%, and the oxygen concentration of the exhaust gas is 1%. However, there is a problem that it does not work effectively in a high concentration region exceeding the above. Therefore, it is usual to measure the oxygen concentration in the exhaust gas and control it so that the optimum air-fuel efficiency is always obtained.At present, it is hardly possible to purify the exhaust gas with a higher oxygen concentration. is there.

[0005] Therefore, in the case of gasoline engines, research and development of a lean burn system has been carried out in order to achieve low fuel consumption. There is a drawback that exhaust gas cannot be purified. Also for diesel engines,
At present, exhaust gas is not purified at all in the current combustion method due to high oxygen concentration in the exhaust gas.

Further, the method using ammonia is used in a fixed combustion device such as a factory and can reduce NOx in exhaust gas having a high oxygen concentration. There is a problem in terms of safety when attached to the equipment, and furthermore, it is difficult to apply a system using a metal-supported hydrophobic zeolite as a catalyst to a diesel engine or the like, and each method has a limited use. There is a problem.

Therefore, the present invention provides a catalyst useful as a catalyst which can effectively purify exhaust gas in an exhaust gas having a high oxygen concentration such as a diesel engine and which does not require a toxic reducing agent such as ammonia. A material and a method for removing nitrogen oxides using the same are provided.

[0008]

[Means for Solving the Problems] As a result of research on the above problems, the present inventors have found that zinc (Zn) and gallium (Ga) are contained as metal elements, and the spinel type crystal structure is used. It has been found that such a composite oxide has a high catalytic action even at a high oxygen concentration, and the present invention has been achieved.

That is, the oxide catalyst material for removing nitrogen oxides of the present invention is characterized by comprising a composite oxide having a spinel structure containing at least Zn and Ga as main metal elements. Further, the method for removing nitrogen oxides of the present invention is a method for removing a nitrogen-containing oxide having a spinel structure containing at least Zn and Ga as main metal elements in an oxidizing atmosphere in which excess oxygen and a carbon-containing gas having a reducing property are present. It is characterized by being brought into contact with an exhaust gas containing an oxide.

Hereinafter, the present invention will be described in detail. The composite oxide used in the present invention is zinc (Zn) as a metal element.
And gallium (Ga), and has a spinel-type crystal structure. This spinel-type composite oxide is represented by the general formula ZnGa ₂ O ₄ , and FIG. 1 shows its X-ray diffraction chart. In the composite oxide used in the present invention, the ratio of Zn to Ga is basically constituted by an atomic ratio of 1: 2, but the ratio of Zn: Ga is 1: 1.
If the ratio is out of the range of 95 to 2.05, the generation of crystals and glass other than the spinel type crystal structure may increase, and the catalyst performance may deteriorate.

The above-mentioned composite oxide can reduce and remove nitrogen oxides (NOx) contained in the exhaust gas by bringing the exhaust gas into contact with the exhaust gas containing nitrogen. It has excellent NOx reduction performance even at such a high concentration. At this time, C ₂ H
₄ , C ₃ H ₆ , C ₃ H ₈ and other hydrocarbons, CH ₃ OH,
When an alcohol such as C ₂ H ₅ OH or a carbon gas having a reducing property such as CO is contained and brought into contact with the catalyst material, the NOx reducing property is increased.

Next, in order to produce the oxide catalyst material of the present invention, for example, as a starting material, an oxide of Zn or Ga, a carbonate or a nitrate capable of forming an oxide by heat treatment, and the like are used. Each was weighed and dissolved in water or the like so that the metal element ratio of Zn: Ga became 1: 2, and the mixture was sufficiently stirred, and then neutralized with an alkali to obtain a precipitate. 5 in an oxidizing atmosphere of
By performing heat treatment for 30 hours, a composite oxide powder of a spinel crystal of Zn and Ga can be obtained.

As a method for producing the above-mentioned composite oxide,
In addition to the above, the sample is synthesized by a solid phase reaction synthesis using an oxide or another metal salt, or a sol-gel synthesis of a metal alkoxide or the like, but is not limited to these manufacturing methods. In any case, the heat treatment is performed in an oxidizing atmosphere at 500 ° C. to 1600 ° C. for 5 to 30 hours. Heat treatment at a particularly low temperature is effective for increasing the specific surface area of the powder. If the heat treatment temperature is lower than 500 ° C., the crystallization is insufficient, and if it exceeds 1600 ° C., it becomes dense and unsuitable.

[0014]

According to the present invention, a spinel-type composite oxide essentially containing Zn and Ga as main constituent metal elements is NOx
NOx can be reduced and decomposed and removed by contact with an exhaust gas containing.

Although the mechanism of this reductive decomposition is not clear, the present invention is more effective than the NOx reductive decomposition action of ZnO alone.
Since high characteristics are obtained by the Ox reductive decomposition action,
It is considered that a slight difference in electronic state around the metal element due to the compounding of the metal oxide affects the NOx reductive decomposition characteristics.

[0016]

EXAMPLE As a starting material, Zn (NO ₃ ) ₂ .6H ₂
O, Ga (NO ₃₎ using the ₃ · 9H ₂ O, the metal element ratio of 1: were weighed respectively so as to 2. It was dissolved in distilled water and neutralized with aqueous ammonia with stirring. The resulting precipitate was filtered, washed and lyophilized. The dried powder was heat-treated at 700 ° C., and the crystal phase was confirmed by XRD.
The BET specific surface area of the obtained powder was 11.8 m ² /
g.

Next, the obtained powder was molded by a die press, and further compression molded by a cold isostatic pressing method, and the molded product was crushed and sized to a powder of 40 mesh pass and 80 mesh on. . Then, using this powder, NO = 1000 ppm, O ₂ = 2%, C ₂ H ₄ = 1 as exhaust gas
000 ppm, He = remaining gas, SV (space velocity) =
Under the condition of 10,000 / h, the NO reduction ability and the ethylene reducing ability were measured using a gas chromatograph in the range of 200 ° C. to 600 ° C., and the relationship between the NO conversion rate and the ethylene decomposition rate and the measurement temperature was shown in FIG. Indicated.

As is clear from FIG. 2, the NO conversion at 500 ° C. was 56.4%. The catalytic activity is 300
Observed from 600 ° C, even at 600 ° C
It was found that the temperature range of the catalyst characteristics was wide. Further, according to the relationship between the ethylene decomposition rate and the NO conversion rate, it was found that the conversion amount of NO with respect to the decomposition of ethylene was large, so that it was an effective catalyst material.

[0019]

As described in detail above, the oxide catalyst material of the present invention has a NOx reduction / decomposition effect even in a high oxygen concentration atmosphere, and the exhaust gas of future combustion engines such as diesel engines and lean burn engines. It is useful for purification.

[Brief description of the drawings]

FIG. 1 is an X-ray diffraction chart of a composite oxide according to the present invention.

FIG. 2 shows the temperature and NO during measurement of the catalyst material in the present invention.
It is the figure which showed the relationship between conversion rate and ethylene decomposition rate.

Claims (3)

(57) [Claims] 1. An oxide catalyst material for removing nitrogen oxides, comprising a composite oxide having a spinel structure containing at least Zn and Ga as main metal elements. 2. The oxide catalyst material for removing nitrogen oxides according to claim 1, wherein said composite oxide is represented by ZnGa ₂ O ₄ . 3. In an oxidizing atmosphere containing an excess of oxygen and a reducing carbon-containing gas, at least Zn and Ga
A nitrogen oxide removal method, which comprises contacting a spinel-type composite oxide containing nitrogen as a main metal element with an exhaust gas containing nitrogen oxide.