JPH08150337A - Oxide catalyst material for removing nitrogen oxide and method for removing nitrogen oxide - Google Patents

Abstract

(57) [Summary] [Purpose] Not only in the presence of water vapor, but also in a high oxygen concentration atmosphere, W
Even if the / F value is high, it has a high NO _X reductive decomposition action and is useful for purification of various harmful substances containing NO _X , including exhaust gas of various internal combustion engines under development for energy saving and resource saving. . [Structure] Ni as an oxide catalyst material for removing nitrogen oxides
In an oxidizing atmosphere in which the main crystal phase containing Al and Ga as the main metal element is a mixture of a complex oxide having a spinel structure and alumina, in which a high concentration of oxygen exists and a reducing gas such as hydrocarbon exists. Then, the exhaust gas containing nitrogen oxides is brought into contact with the exhaust gas to reduce and decompose the nitrogen oxides.

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]

BACKGROUND ART In recent years, contamination of the atmosphere by various pollutants is a major social problem, NO _X in the exhaust gas of an automobile that is the moving source of air pollution among them, decompose harmful substances such as CO _X, removed There is an urgent need to develop ways to do it.

Conventionally, NO _x , C in the exhaust gas of automobiles
Decomposing harmful substances O _X such as a method of removing, using an oxidation of carbon monoxide (CO) and hydrocarbons (C _X H _X), nitrogen oxide three-way catalyst to perform reduction at the same time the (NO _X) The method has been adopted.

As a three-way catalyst used in such a method, cordierite obtained by coating a noble metal such as palladium (Pd), platinum (Pt) or rhodium (Rh) with γ-alumina (Al ₂ O ₃ ) is used. What was carried on the refractory of was used.

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%.
It has a drawback that it does not work effectively in a high concentration range exceeding the range.

Therefore, normally, the oxygen concentration in the exhaust gas is measured, and the air-fuel ratio is controlled so that CO, C _X H _X , and NO _X can be processed at a high purification rate, and the air-fuel ratio is in a range close to the theoretical equivalent value. However, the CO and C _X H _X , and N
Since O _X generation mechanisms have contradictory properties, it is necessary to support combustion in a state where limited exhaust gas purification at a high oxygen concentration in than it is at present, not hardly.

Further, since energy saving and resource saving have recently been demanded, in a gasoline engine, research and development of a lean combustion system have been carried out in order to achieve low fuel consumption. Has a disadvantage that the noble metal of the catalyst cannot be purified by exhaust gas due to oxygen poisoning.

Also in the diesel engine, in the present combustion system, the exhaust gas is not purified at all because the oxygen concentration in the exhaust gas is high.

On the other hand, it occurs at a high temperature of about 1000 ° C. or higher,
As a method of effectively purifying NO _X , the concentration of which increases as the combustion temperature increases, there is a selective catalytic reduction method using ammonia in addition to the above method. Although effective for purifying NO _{x in} high exhaust gas, applying this method to a mobile combustion device such as an automobile has a problem in safety.

[0010] Therefore, as to solve the above problems, a method of the hydrophobic zeolite carrying the metal removing NO _X while in contact with the hydrocarbon as a catalyst, is proposed in JP-A 4-349938 Patent Publication There is.

[0011]

However, the one using the above-mentioned metal-supporting hydrophobic zeolite as a catalyst has poor water resistance, and in the case where the exhaust gas contains water as in a diesel engine, the presence of water vapor is present. Therefore, there is a problem that the purification rate of NO _X is greatly decreased with time and its use is limited, and a catalyst material excellent in not only heat resistance but also water resistance has been desired.

[0012]

OBJECT OF THE INVENTION The present invention does not require a highly toxic reducing agent such as ammonia, and is effective for exhaust gas with high oxygen concentration containing moisture such as diesel engine even if the flow speed of the exhaust gas is high. The present invention also provides a useful catalyst material that can purify NO _x in exhaust gas and a method for removing nitrogen oxides using the catalyst material.

[0013]

The present invention has been made in view of the above problems, and Al ₂ O ₃ is added to a complex oxide having a spinel structure as a crystal phase containing Ni and Ga as main metal elements. It has been found that the mixed oxide catalyst material has excellent catalytic activity for a long period of time even at high oxygen concentration in the presence of water vapor.

That is, the oxide catalyst material for removing nitrogen oxides of the present invention contains a composite oxide powder containing Ni and Ga as main metal elements, and the crystal phase of which has a spinel type structure, and Al ₂ O ₃ powder. Is a catalyst material formed by mixing

Further, according to the method for removing nitrogen oxides of the present invention, the crystal phase containing Ni and Ga as main metal elements is spinel type in an oxidizing atmosphere in which a high concentration of oxygen and a carbon-containing gas having a reducing property are present. The structure of the complex oxide, A
It is characterized in that a catalyst material mixed with l ₂ O ₃ is brought into contact with an exhaust gas containing nitrogen oxides.

In the present invention, the composite oxide is Ni
And Ga as main metal elements and has a crystal phase of spinel structure represented by NiGa ₂ O ₄ as a general formula, and the correlation between Ni and Ga of the composite oxide is
In terms of the count ratio of Ga to Ni by fluorescent X-ray analysis, 1.4 or more is more desirable from the viewpoint of catalytic activity, and most preferably 1.5 or more.

Further, it is considered that the Al ₂ O ₃ having a high specific surface area is suitable, and 60 m ² is practically used.
/ G or more is desirable, and although the change in catalyst activity due to the Al ₂ O ₃ mixed amount is small, the mixed amount is 20 to 70.
Weight percent is preferred and 25-60 weight percent is most desirable.

Further, when the catalyst material obtained by mixing the composite oxide with Al ₂ O ₃ and the exhaust gas containing nitrogen oxide are brought into contact with each other, C ₂ H ₄ as a reducing agent in the exhaust gas atmosphere,
Hydrocarbons such as C ₃ H ₆ and C ₃ H ₈ , CH ₃ OH, C ₂ H
_When an alcohol such as ₅ OH or the like, a carbon gas having a reducing property such as CO or the like is contained and brought into contact with the catalyst material, NO
_X- reducing property becomes higher.

[0019]

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 is N
i and Ga contained as the main metal element, a complex oxide crystal phase having a spinel structure, since it is a mixture of _{Al 2 O 3, Al 2 O} 3 in the catalyst material is N
Oxidation of O promotes the generation of NO ₂ , and the Ni—Ga-based oxide catalyst, which has a higher reduction activity for NO ₂ than NO, exhibits high characteristics due to the reductive decomposition action of NO _X.

[0020]

EXAMPLES Hereinafter, the oxide catalyst material for removing nitrogen oxides and the method for removing nitrogen oxides of the present invention will be described in detail based on examples.

In the method for producing an oxide catalyst material for removing nitrogen oxides according to the present invention, first, a predetermined amount of a raw material powder containing Ni and Ga is weighed, sufficiently stirred and mixed, and then Al ₂ O ₃ powder is added. Then, by heat-treating at a temperature of 500 to 1600 ° C. for 5 to 30 hours in an oxidizing atmosphere, a composite oxide of spinel type crystals containing Ni and Ga as main metal elements and A
A mixed powder of 1 ₂ O ₃ is obtained.

As the raw material powder, for example, oxides of Ni and Ga, and their carbonates, nitrates, acetates and the like which produce oxides by heat treatment can be used.

Further, since it is considered that the raw material powder improves the reductive decomposition property of NO _x by solid-dissolving Ga metal in NiGa ₂ O ₄ having a stoichiometric spinel structure, the metal of Ga with respect to Ni is It is desirable to mix them so that the element ratio is 2.1 or more.

In addition to the above, the complex oxide can be synthesized by a solid-state reaction with an oxide or another metal salt, a sol-gel method using a metal alkoxide or the like, and is not limited to any production method thereof. Not something.

In any of the above-mentioned manufacturing methods, if the heat treatment temperature is lower than 500 ° C., the crystallization becomes insufficient, and if the heat treatment temperature exceeds 1600 ° C., densification occurs. Therefore, oxidation is performed at a temperature of 500 to 1600 ° C. 5 in the atmosphere
It is carried out for 30 hours, but heat treatment at a particularly low temperature is effective for increasing the specific surface area of the powder, and in practice, it is desirable to set the specific surface area to be 60 m ² / g or more.

Next, in evaluating the present invention, Ni (NO ₃ ) ₂ .6H ₂ O and Ga (NO ₃ ) _{3 were used} as starting materials.
-Using a reagent of 9H ₂ O, weighed so that the metal element ratio of Ni and Ga was 1: 3, dissolved these reagents in distilled water, and neutralized with ammonia water while stirring.
25 to 75 parts by weight of Al ₂ O ₃ powder was added to 100 parts by weight of the formed precipitate slurry, and the mixture was sufficiently stirred and mixed by an ultrasonic emulsification disperser, and then freeze-dried.

Next, the dry powder is heat-treated in the air at a temperature of 700 ° C. for 30 hours to prepare an oxide catalyst powder, which is molded by a die press and then CIP molded to obtain the molded body. The sample was crushed, sieved, and sized to exceed 500 μm and 700 μm or less to prepare a sample for evaluation.

Incidentally, a material not mixed with Al ₂ O ₃ was prepared in the same manner as the above and used as a comparative example.

The count ratio of Ga to Ni determined by fluorescent X-ray analysis using the powder of the evaluation material thus obtained was 1.93.

The crystal phase was identified by X-ray diffraction measurement (XRD), and it was confirmed that the crystal phase consisted of a spinel crystal and a γ-Al ₂ O ₃ phase.

Next, NO 1000 ppm and O ₂ 5
% Or 10%, H ₂ O 10%, C ₂ H ₄ as reducing agent
Is 1000 ppm or C ₃ H ₆ is 666 ppm, and the balance is a reaction gas containing water vapor consisting of He, and W / F is 0.03 g as a condition for contacting the reaction gas with the catalyst material.
Set to sec / cc, flow through the catalyst layer filled with the evaluation material, and measure N ₂ produced by NO reduction in the temperature range of 300 to 600 ° C. by gas chromatograph.
The NO conversion rate at 450 ° C., which shows the highest activity, was evaluated as the NO reduction activity from the production amount of ₂ .

[0032]

[Table 1]

As is clear from Table 1, sample numbers 1, 5, 11 and 15 which are comparative examples in which Al ₂ O ₃ is not mixed.
Has a NO reduction activity at 450 ° C. of 33% or less, whereas in the present invention, NO at 450 ° C.
It can be seen that the reducing activity shows 35% or more.

Further, using the above-mentioned composite oxide for evaluation,
A four-cylinder diesel engine bench test device was set on the exhaust pipe, and a durability test was conducted in which the diesel engine was operated for 100 hours at maximum rotation speed and full load. The NO reduction activity was evaluated, but it was confirmed that the NO conversion rate was hardly reduced, and it was also proved that the water resistance was excellent.

[0035]

As described above in detail, 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 Ni and Ga as main metal elements. Al _{2 is} added to the complex oxide of spinel structure.
Since it is a mixture of O ₃ , the flow velocity of the gas is high even when the oxygen concentration in the exhaust gas is 3% or more in the high oxygen concentration atmosphere as well as in the atmosphere in which water vapor exists. However, since it has excellent NO _X reduction performance, it can effectively reduce and remove NO _X contained in exhaust gas, and future diesel engines and lean engines developed with the goal of energy saving and resource saving. It is extremely useful for purifying various harmful substances containing NO _x , including exhaust gas from various internal combustion engines such as a burn engine.

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁶ Identification code Office reference number FI technical display location B01D 53/36 102 C 102 H

Claims (2)

[Claims] 1. Nitrogen oxide removal characterized by mixing alumina (Al ₂ O ₃ ) with a composite oxide having a spinel structure containing at least nickel (Ni) and gallium (Ga) as main metal elements. Oxide catalyst material. 2. In an oxidizing atmosphere in which a high concentration of oxygen and a carbon-containing gas having a reducing property are present, at least nickel (N
i) and gallium (Ga) as the main metal elements, a spinel-type composite oxide containing alumina (Al ₂ O
_A method for removing nitrogen oxides, which comprises contacting an exhaust gas containing nitrogen oxides with an oxide catalyst material formed by mixing ₃ ).