JP3532044B2 - 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. Oxide catalyst material for removing nitrogen oxides suitable for purifying automobile exhaust gas such as lean burn engines and diesel engines in which excess oxygen is present, and the use of the oxide catalyst material in exhaust gas in the presence of excess oxygen And a method for removing nitrogen oxides (NO _x ).

[0002]

2. Description of the Related Art In recent years, pollution of the air by various pollutants has become a major social problem. Among them, NO _x and C contained in exhaust gas of automobiles, which are sources of movement of air pollution.
Decomposing harmful substances O _x or the like, the development of a method for removing is a pressing need.

[0003] Conventionally, NO _{x in} exhaust gas of automobiles,
As a method for decomposing and removing harmful substances such as CO _x, a three-way catalyst has been widely used which simultaneously oxidizes unburned carbon monoxide and hydrocarbons remaining in exhaust gas and reduces nitrogen oxides. .

The three-way catalyst used in such a method includes cordierite and the like, in which a noble metal such as palladium (Pd), platinum (Pt), and rhodium (Rh) is coated with γ-alumina (Al ₂ O ₃ ). What was carried on the refractory carrier was used.

[0005] However, the three-way catalyst can efficiently purify the exhaust gas at a low oxygen concentration of about 0.5%, but the oxygen concentration in the exhaust gas exceeds 1%. There is a disadvantage that it does not work effectively in an extremely high concentration atmosphere.

On the other hand, in order to avoid the above-mentioned drawback, the oxygen concentration in the exhaust gas is measured, and CO, C _x H _y , NO
is also being done to control such that the air-fuel ratio close range to the theoretical equivalent value which may be treated with _x with a high reduction ratio, the generation mechanism of the CO and C _x H _y and NO _x are contradictory properties Therefore, combustion in a limited state must be maintained, and at present, exhaust gas purification in a high oxygen concentration as described above has hardly been performed.

Therefore, as a catalyst capable of efficiently removing nitrogen oxides even in the presence of such a high concentration of oxygen, a metal ion-supported copper ion-exchanged zeolite such as a hydrophobic zeolite, a metal silicate, an alumina catalyst, and the like have been proposed. (See JP-A-4-349938).

[0008]

However, the copper ion exchanged zeolites, metal silicates, alumina catalysts and the like proposed above are all inferior in heat resistance.
(Space velocity) property is poor, and the SV value is 200000 / hr.
Under the conditions of the actual engine exhaust gas exhibiting the above high speed, there is a problem that the reducing ability of nitrogen oxides is remarkably reduced, and there is a problem that it is difficult to effectively purify nitrogen oxides as it is.

[0009]

SUMMARY OF THE INVENTION The present invention relates to an exhaust gas having a high oxygen concentration containing water, such as a diesel engine and various automobile engines, having an SV value of 2000.
00 / hr. Provided is a useful catalyst material which exhibits a high nitrogen oxide reducing action even at the above high speed and can effectively reduce nitrogen oxides in exhaust gas, and a nitrogen oxide removing method using the same. Things.

[0010]

SUMMARY OF THE INVENTION The present invention has been made in view of the above problems, the spinel crystalline composite oxide containing Ni and Ga as the main metal element, Ni _1-X
N represented by Ga ₂ O ₄ (where 0.2 ≦ X <0.6)
It has been found that an i-deficient solid solution has high catalytic activity for a long period of time even in a high oxygen concentration atmosphere and can effectively reduce nitrogen oxides in exhaust gas.

That is, the oxide catalyst material for removing nitrogen oxides of the present invention mainly comprises nickel (Ni) and gallium (Ga).
Oxides with spinel structure as metal elements
_{In, Ni 1-X Ga 2 O} 4 ( where, 0.2 ≦ X <0.6) consists Ni deficient solid solution represented by the true density 5.5
54 to 5.933 g / cc
It is.

Furthermore, the nitrogen oxide removal method of the present invention are
Concentration of oxygen and reducing carbon-containing gas are present
Nickel (Ni) and gallium (Ga) in an oxidizing atmosphere
Of spinel-type structure containing iron as main metal element
The oxide is an Ni-deficient solid solution represented by Ni _1-X Ga ₂ O ₄ (where 0.2 ≦ X <0.6) and has a true density of 5.5.
Oxidation for removing nitrogen oxides of 54 to 5.933 g / cc
Catalyst material in contact with exhaust gas containing nitrogen oxides
It is characterized by the following.

[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 of the present invention is a spinel crystalline composite oxide containing Ni and Ga as metal elements. , Ni-deficient solid solution (Ni / Ga atomic ratio n is 0.20 <n ≦ 0.40 ),
The main crystal phase allows the deficiency of Ni from the A site of NiGa ₂ O ₄ having a stoichiometric spinel type (AB ₂ O ₄ ) structure.
It is considered that the nitrogen oxide acts to show high characteristics by the reducing action of the nitrogen oxide.

[0014]

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

The composite oxide of the present invention contains Ni and Ga as main metal elements, and has a Ni / Ga atomic ratio n of 0.1.
A Ni-deficient spinel-type composite oxide having a ratio of 20 <n ≦ 0.40 , and Ni _1-X Ga ₂ O ₄ (provided that
0.2 ≦ X <0.6).

The composite oxide used in the present invention is:
When the X value of the general formula is represented by the Ni / Ga atomic ratio n, the value of the Ni / Ga atomic ratio n is 0.20 <n ≦ 0.40.
If the value is outside the range, the catalytic activity is reduced. Therefore, the value is specified in the above range, and n = 0.25 to 0.40 is most preferable in consideration of the ability to reduce nitrogen oxides.

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.

In the composite oxide of the present invention, a raw material powder containing Ni and Ga as main metal elements is weighed so that the atomic ratio n of Ni / Ga satisfies 0.20 <n ≦ 0.40, and After stirring and mixing in an oxidizing atmosphere,
After heat-treating at a temperature of 200 ° C. for 5 to 30 hours, it is further held at a temperature of 600 to 650 ° C. for 10 hours in the process of lowering the temperature so that Ni and G
A composite oxide powder having a spinel-type structure containing a and having a Ni-deficient solid solution as a main crystal phase can be 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.

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 done.

In any of the above-mentioned manufacturing methods, the crystallization becomes insufficient when the heat treatment temperature is lower than 700.degree. C., whereas the densification occurs when the heat treatment temperature exceeds 1200.degree. At the temperature of 5
Although the heat treatment is performed for 30 hours, heat treatment at a particularly low temperature is effective in increasing the specific surface area of the powder, and practically, it is desirable to set the specific surface area to be 35 m ² / g or more.

In the method for producing a composite oxide having a Ni-deficient spinel type crystal as a main crystal phase, particularly, during the heat treatment, a temperature of 600 to 650.degree.
It is important to hold for about an hour.

Further, the spinel-type composite oxide of the present invention comprises:
By contact with the exhaust gas containing nitrogen oxides, even if the concentration of oxygen contained in the exhaust gas is as high as 3% or more, and even in an atmosphere where water vapor is present, it can be widely used. It has excellent nitrogen oxide reduction performance in the temperature range.

Furthermore, in the exhaust gas atmosphere, hydrocarbons such as C ₂ H ₄ , C ₃ H ₆ , C ₃ H ₈ , CH 3
_When an alcohol such as ₃ OH and C ₂ H ₅ OH and a carbon-containing gas having a reducing property such as CO are mixed and brought into contact with the catalyst material comprising the composite oxide, the reducing performance of the nitrogen oxide is further improved. Become.

[0025]

EXAMPLES Next, the oxide catalyst material for removing nitrogen oxides and the method for removing nitrogen oxides of the present invention were evaluated as described in detail below.

First, Ni (NO ₃ ) _2.
6H ₂ O, and Ga _{(NO 3)} with reagents ₂ · 9H 2 O, the formula _{Ni 1-X Ga 2 O 4} ( where, 0 <X <
0.6) is set as shown in Table 1, specifically, the molar ratio of Ni / Ga is variously set and weighed, and these reagents are dissolved in distilled water. The mixture was neutralized with aqueous ammonia with stirring, and the precipitate formed at this time was filtered, washed, and freeze-dried.

The dry powder thus obtained is placed in air at 70
After heat treatment at a temperature within the range of 0 to 1200 ° C. for 30 hours, the temperature is lowered at 650 ° C. or 600 ° C. for 10 hours.
After holding for a time, a Ni-deficient spinel-type crystalline composite oxide powder having a specific surface area of 40 to 50 m ² / g was obtained.

Next, the spinel-type composite oxide powder is molded by a mold press, and further compressed by a cold isostatic pressing method, and then the molded product is crushed and sieved, and exceeds 500 μm to 700 μm or less. To prepare an evaluation sample.

A composite oxide prepared without performing a holding treatment at a predetermined temperature and for a predetermined time in the temperature decreasing process was used as a comparative example.

Using the powder of the evaluation sample thus obtained, a crystal phase was identified by X-ray diffraction measurement (XRD), and it was confirmed that the crystal phase consisted of spinel crystals.

Further, the true density and the surface area of the evaluation sample were measured by a gas replacement method and a BET method, respectively, and the gas replacement method was performed by using He gas having a purity of 99.999%.
The measurement results were averaged, Al ₂ O ₃ was measured as a standard sample, and calibration was performed. In the BET method, the surface area was determined by measuring the amount of adsorption using liquid nitrogen as the adsorption gas.

On the other hand, NO was used as a nitrogen oxide in the simulated exhaust gas, the NO being 1000 ppm, the O ₂ being 10%,
A reaction gas consisting of 666 ppm of C ₃ H ₆ and the balance of He was used as a condition for contacting the reaction gas with the catalyst material, and a space velocity (SV) of 200,000 / hr. Was passed through the catalyst layer filled with the evaluation sample, and the N ₂ gas generated by passing through the catalyst layer in a temperature range of 300 to 500 ° C. was measured by gas chromatography.

The NO reduction ability of the catalyst is determined to be twice the value of the N ₂ concentration (ppm) at the catalyst layer outlet side, and to the NO at the catalyst layer inlet side.
The percentage divided by the concentration (ppm) was defined as the NO reduction rate (%), and the NO reduction rate at 450 ° C. was obtained and evaluated as the nitrogen oxide reduction activity.

[0034]

[Table 1]

FIG. 1 shows the relationship between the Ni / Ga atomic ratio and the true density of the evaluation sample according to the present invention based on the results shown in Table 1.

As can be seen from FIG. 1, a linear relationship is obtained between the atomic ratio of Ni / Ga and the true density. When the atomic ratio of Ni / Ga in the stoichiometric spinel composition is 0.50, the relationship is extended. Since the theoretical density on the line passes through a point of 6.175 g / cc, the evaluation sample according to the present invention has a spinel type (AB ₂
From the A site of NiGa ₂ O _{4 having} an O ₃ ) structure, it can be seen that the Ni solution is a Ni-deficient solid solution that allows Ni deficiency.

On the other hand, as is clear from the table, Sample Nos. 12 and 14, which are comparative examples, have remarkably low NO reduction rates, and Sample Nos. 1, 2, 3, and 4 which are outside the scope of the present invention .
Even in 9, 10, 11, and 13, it became clear that the NO reduction rate was low and not practical.

[0038] On the other hand, 0.2 ≦ X <0.6, true tight
The degree is 5.554-5.933 g / cc and the specific surface area is 45.
m ² / g is more than 49 at 450 ° C. Both the present invention
% Or more, which indicates a sufficient NO reduction rate.

The samples for evaluation of the present invention all have an SV value of 300,000 / hr. Until the oxygen concentration is 15
%, There is no significant decrease in the evaluation characteristics, while 35
No significant change was observed in the evaluation characteristics even after contacting with the simulated exhaust gas for a reaction at a temperature of 0 ° C. for 100 hours for 100 hours.

Further, the evaluation sample was attached to an exhaust pipe of a 4-cylinder diesel engine bench test apparatus, and a durability test was performed in which the diesel engine was operated at the maximum rotation speed and full load for 100 hours. The evaluation sample was evaluated for nitrogen oxide reduction activity in the same manner as described above.
It can be confirmed that the NO reduction rate has hardly decreased,
It was proved that both water resistance and heat resistance were excellent.

[0041]

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 nickel (Ni) and gallium (Ga). in spinel crystalline composite oxide containing as a main metal _{element, Ni 1-X Ga 2 O} 4 ( where 0.
From Ni-deficient solid solution represented by 2 ≦ X <0.6) formed
And the true density is 5.554 to 5.933 g / cc, so that the oxide catalyst material and the nitrogen oxide are mixed in an oxidizing atmosphere in which a high concentration of oxygen and a carbon-containing gas having a reducing property are present. Since the exhaust gas containing substances is brought into contact with the exhaust gas, not only in an atmosphere in which water vapor is present but also in a high oxygen concentration atmosphere in which the oxygen concentration in the exhaust gas is 3% or more, the gas flow rate is high. Even so, it has excellent nitrogen oxide reduction performance, and can effectively reduce nitrogen oxides contained in exhaust gas.

As a result, various types of harmful substances containing nitrogen oxides, including exhaust gas from various internal combustion engines such as diesel engines and lean burn engines, which will be developed with the aim of saving energy and resources and preventing global warming, are developed. It will be very useful for purification.

[Brief description of the drawings]

FIG. 1 shows N of an oxide catalyst material for removing nitrogen oxides according to the present invention.
FIG. 4 is a diagram illustrating a relationship between an i / Ga atomic ratio and a true density.

Continuation of the front page (56) References JP-A-7-265703 (JP, A) JP-A-8-24650 (JP, A) JP-A 8-150338 (JP, A) JP-A 8-10002 (JP) , A) JP-A-9-225267 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) B01J 21/00-38/74 B01D 53/86, 53/94

Claims (2)

(57) [Claims] 1. A composite oxide having a spinel structure containing nickel (Ni) and gallium (Ga) as main metal elements, wherein Ni _1-X Ga ₂ O ₄ (where 0.2 ≦ X <0.6) ) Ri consists Ni deficient solid solution represented by the true density 5.5
54 to 5.673 g / cc, specific surface area of 45 m ² / g or less
An oxide catalyst material for removing nitrogen oxides, which is characterized by the above . 2. A composite oxide having a spinel structure containing nickel (Ni) and gallium (Ga) as main metal elements in an oxidizing atmosphere in which a high concentration of oxygen and a carbon-containing gas having a reducing property are present. _{, Ni 1-X Ga 2 O} 4 ( where, 0.2 ≦ X <0.6) Ri consists Ni deficient solid solution represented by the true density 5.5
A method for removing nitrogen oxides, comprising contacting an oxide catalyst material for removing nitrogen oxides of 54 to 5.673 g / cc with an exhaust gas containing nitrogen oxides.