JP3309024B2 - 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]

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 that.

[0003] Conventionally, NO _x and C in exhaust gas of automobiles have been developed.
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.

Examples of the three-way catalyst used in such a method include cordierite in which a noble metal such as palladium (Pd), platinum (Pt), and rhodium (Rh) is coated with γ-alumina (Al ₂ O ₃ ). Supported on a refractory.

[0005] 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 drawback that it does not work effectively in a high concentration region exceeding the above range.

[0006] Therefore, usually the oxygen concentration in the exhaust gas is measured and controlled to a air-fuel ratio of CO and C _X H _X, close range to the theoretical equivalent value which may be treated with NO _X at a high purification rate 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.

[0007] Furthermore, since energy savings and resource savings have been called for in recent years, research and development of a lean burn system for gasoline engines has been carried out in order to achieve low fuel consumption. Has a disadvantage that the exhaust gas cannot be purified due to oxygen poisoning of the noble metal of the catalyst.

[0008] Also, in the present combustion system, exhaust gas is not purified at all because of the high oxygen concentration in the exhaust gas in the current combustion system.

On the other hand, it is generated at a high temperature of about 1000 ° C. or more,
As a method for purifying NO _X concentration thereof becomes higher as the combustion temperature becomes higher effectively, the there is a selective catalytic reduction method using ammonia in addition, the oxygen concentration in the stationary combustion apparatus such as factories although there are effective for purification of the NO _X in the high exhaust gas, applying this method to a mobile combustion device such as an automobile has a problem in terms of 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 I have.

[0011]

However, those using a metal-supported hydrophobic zeolite as a catalyst have poor water resistance, and when exhaust gas contains moisture as in a diesel engine, the presence of water vapor does not exist. Therefore, there is a problem that the purification rate of NO _X is greatly reduced with time, and the use thereof is limited. Therefore, a catalyst material excellent not only in heat resistance but also in water resistance has been desired.

[0012]

SUMMARY OF THE INVENTION The present invention does not require a highly toxic reducing agent such as ammonia or the like, and is effective in discharging exhaust gas having a high oxygen concentration containing water, such as a diesel engine, even if the flow rate of the exhaust gas is high. there is provided a nitrogen oxide removal method using a catalyst useful materials and it can be purified NO _X in the exhaust gas.

[0013]

SUMMARY OF THE INVENTION The present invention has been made in view of the above-mentioned problems, and it is an object of the present invention to provide a composite oxide containing Ni and Ga as main metal elements having a spinel-type crystal phase containing Al ₂ O ₃ . It has been found that the mixed oxide catalyst material has excellent catalytic activity over a long period even at a 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 having a crystal phase having a spinel structure, and an Al ₂ O ₃ powder. Is a catalyst material obtained by mixing

Further, the method for removing nitrogen oxides according to the present invention is characterized in that 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. In the composite oxide having the structure, 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 a main metal element, and has a crystal phase of a spinel structure represented by NiGa ₂ O ₄ as a general formula, and the correlation between Ni and Ga of the composite oxide is as follows:
When discussing 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 especially 1.5 or more is most desirable.

It is considered that Al ₂ O ₃ having a high specific surface area is suitable, and practically 60 m ²
/ G or more, and the change in the catalytic activity due to the mixed amount of Al ₂ O ₃ is small, but the mixed amount is 20-70.
% By weight is preferred, and most preferably 25 to 60% by weight.

Furthermore, when a catalyst material in which Al ₂ O ₃ is mixed with the composite oxide and an exhaust gas containing nitrogen oxides are brought into contact, C ₂ H ₄ ,
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 a carbon gas having a reducing property such as CO is contained and brought into contact with the catalyst material, NO
_X- reduction is further improved.

[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 comprises 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
O is oxidized to promote the formation of NO _2, the high Ni-Ga-based oxide catalyst towards reducing activity against NO ₂ from NO, exhibit high properties in reducing decomposition of the NO _X.

[0020]

EXAMPLES 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 with reference to 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 an Al ₂ O ₃ powder is added. Then, by performing a heat treatment at a temperature of 500 to 1600 ° C. for 5 to 30 hours in an oxidizing atmosphere, a composite oxide of a spinel crystal containing Ni and Ga as main metal elements and A
A mixed powder consisting of l ₂ O ₃ is obtained.

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

Further, the raw material powder, it is considered a reductive decomposition characteristic of the NO _X is improved by solid solution of Ga metal NiGa ₂ O ₄ showing the spinel structure of stoichiometric, of Ga to Ni metal It is desirable to mix such that the element ratio becomes 2.1 or more.

The composite oxide can be synthesized by a solid phase reaction with an oxide or another metal salt, a sol-gel method such as a metal alkoxide, or the like. Not something.

[0025] 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. If the heat treatment temperature exceeds 1600 ° C, the crystallization becomes insufficient. In the atmosphere, 5
Although the heat treatment is performed for 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 be 60 m ² / g or more.

Next, when evaluating the present invention, Ni (NO ₃ ) ₂ .6H ₂ O and Ga (NO ₃ ) _{3 were used} as starting materials.
Using a 9H ₂ O reagent, weigh so that the metal element ratio of Ni and Ga is 1: 3, dissolve these reagents in distilled water, and neutralize 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 slurry, and the mixture was sufficiently stirred and mixed with an ultrasonic emulsifying and dispersing machine, and then freeze-dried.

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

A sample without Al ₂ O ₃ was prepared in the same manner as described above and used as a comparative example.

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

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

[0031] Then, NO is 1000ppm, O ₂ 5
% Or 10%, H ₂ O is 10%, and C ₂ H _{4 is} used as a reducing agent.
The reaction gas containing 1000 ppm or 666 ppm of C ₃ H ₆ , and the balance consisting of He, was mixed with 0.03 g of W / F 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, measure N ₂ generated by reduction of NO in a temperature range of 300 to 600 ° C. by gas chromatography,
The NO conversion rate at 450 ° C., which shows the highest activity based on the production amount of ₂ , was evaluated as NO reduction activity.

[0032]

[Table 1]

As is apparent from Table 1, Sample Nos. 1, 5, 11 and 15 which are comparative examples in which Al ₂ O ₃ was not mixed were used.
Indicate that the NO reduction activity at 450 ° C. is 33% or less,
It can be seen that the reducing activity shows 35% or more.

Further, using the composite oxide for evaluation described above,
A 4-cylinder diesel engine bench test device was set in the exhaust pipe, and a durability test was performed in which the diesel engine was operated for 100 hours under the conditions of the maximum rotation speed and full load. When the NO reduction activity was evaluated, it was confirmed that the NO conversion rate was hardly reduced, and it was also proved that the NO conversion rate was excellent.

[0035]

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 Ni and Ga as main metal elements. Al _{2 is} added to the spinel-type composite oxide.
Since it is a mixture of O ₃ , even in a high oxygen concentration atmosphere where the oxygen concentration in the exhaust gas is 3% or more as well as in an atmosphere where water vapor is present, the gas flow rate is high. even superior because it has a NO _X reduction performance was, it is possible to effectively reduce and remove NO _X contained in the exhaust gas, energy saving, future diesel engines and lean to resource saving is developed as the target including exhaust gas for various internal combustion engine such as burn engine, it is extremely useful for the purification of various harmful substances containing NO _X.

Claims (2)

(57) [Claims] 1. A nitrogen oxide removal method comprising 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. For oxide catalyst materials. 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 alumina (Al ₂ O) as a composite oxide having a spinel structure containing gallium (Ga) as a main metal element
_{3. A} method for removing nitrogen oxides, comprising contacting an oxide catalyst material obtained by mixing ₃ ) with an exhaust gas containing nitrogen oxides.