JPH0824650A - Oxide catalyst material for removing nitrogen oxide and method for removing nitrogen oxide - Google Patents

Abstract

PURPOSE:To provide an oxide catalyst material having a NOx reduction- decomposition function even in an atmosphere of a high oxygen concentration and to purify exhaust gas from combustion engines such as diesel engines and lean-burn engines. CONSTITUTION:A material in which 5-75wt.% of CeO2 is incorporated into a spinel type crystalline complex oxide containing metal elements of Ni and Ga as an oxide catalyst material which reduces and decomposes nitrogen oxides such as NOx and in which a Ga/Ni atomic ratio (n) is 2.5 3.3 is used come into contact with exhaust gas containing nitrogen oxides in an oxidative atmosphere in which oxygen of a high concentration and carbon-containing gas which has a reducing function exist so as to reduce, to decompose, and to remove 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 oxide catalyst material.

[0002]

2. Description of the Related Art In recent years, environmental pollution has become a problem, and among them, there is an urgent need to develop a method for decomposing and removing harmful substances such as NOx and COx contained in exhaust gas of automobiles. Therefore, as a conventionally used method for purifying automobile exhaust gas, carbon monoxide (CO) and hydrocarbon (CxH
A three-way catalyst that simultaneously performs the oxidation of y) and the reduction of nitrogen oxides (NOx) is widely used. As such a three-way catalyst, one having a noble metal such as Pd, Pt, or Rh supported on a refractory carrier such as cordierite coated with γ-alumina is used.

[0003]

However, the above 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%. There is a problem that it cannot work effectively in the above high concentration atmosphere. Therefore, it is usually practiced to measure the oxygen concentration in the exhaust gas and control it so that the air-fuel ratio will always be the optimum value. is there.

In order to solve these problems, a catalyst capable of efficiently removing NOx even in the presence of high-concentration oxygen has been developed, and as a catalyst capable of removing NOx even in the presence of high-concentration oxygen, copper ion-exchanged zeolite is currently used. , Metal silicate and alumina catalysts have been proposed.

Further, the inventors of the present invention previously contacted the exhaust gas containing excess oxygen and nitrogen oxides with a nickel gallate (NiGa ₂ O ₄ ) spinel type composite oxide catalyst to obtain nitrogen in exhaust gas. A method of removing oxide was proposed.

However, the catalyst proposed above is NOx.
The removal activity temperature range is narrow, and for example, the temperature range in which the NOx removal rate of the nickel gallate spinel type composite oxide catalyst is 30% or more is the temperature range of 370 to 480 ° C, and NOx is removed only in the specified temperature range. There was a problem. Further, the characteristics of the above catalyst are remarkably deteriorated when water vapor is present in the reaction gas. For example, when 10% of water vapor coexists in the reaction gas, the NOx removal rate is reduced from 81% to 25%. It was difficult.

[0007]

[Means for Solving the Problems] The inventors of the present invention have conducted research on the above problems, and as a result, as a metal element, Ni
CeO ₂ is added to a spinel-type crystalline composite oxide containing Ga and Ni and having a Ga / Ni atomic ratio n of 2.5 to 3.3.
It was found that the catalyst material added by up to 75% by weight has a high NOx removal rate even in a high oxygen concentration atmosphere, has a catalytic action in a wide temperature range, and has little performance deterioration due to coexisting steam.

Further, it was found that NOx decomposition and removal can be efficiently performed by contacting the oxide catalyst material with the exhaust gas containing nitrogen oxide in an oxidizing atmosphere in which a carbon-containing gas having a reducing property with oxygen is present. It was.

The present invention will be described in detail below. The composite oxide used in the present invention contains nickel (Ni) and gallium (Ga) as metal elements and has a Ga / Ni atomic ratio n.
Is a spinel-type composite oxide having a ratio of 2.5 to 3.3. If this is expressed by a general formula, NiGanO ₄₊ δ
(N = 2.5 to 3.3). In addition, (O in the formula
₄₊ δ) is the amount of oxygen necessary for stable existence as a complex oxide, which changes as needed depending on the value of n, and changes in the range of 0 to 2.0. The complex oxide used in the present invention preferably has a Ga / Ni atomic ratio n of 2.8 to 3.1. Of these, n = 3.0 is particularly preferable.
The reason for limiting the n value to the above range is that if the n value deviates from this range, the activity as a catalyst decreases.

According to the present invention, CeO is added to this composite oxide.
_The ratio of ₂ to the total amount is 5 to 75% by weight, especially 20 to 4
It is added in an amount of 0% by weight. The amount of CeO ₂ added is 75
This is because if the amount exceeds 5% by weight, the catalytic activity decreases, and if it is less than 5% by weight, the temperature range having a high removal rate becomes narrow.

According to the present invention, the oxygen concentration in the exhaust gas is reduced to 1% by bringing the above-mentioned CeO ₂ -added composite oxide into contact with the exhaust gas containing nitrogen oxide (NOx).
As described above, it has excellent NOx reduction resolution over a wide temperature range even at a high concentration of 3% or more. Further, in the atmosphere, hydrocarbons such as C ₂ H ₄ , C ₃ H ₆ , C ₃ H ₈ and CH ₃ O
The NOx reducibility is further enhanced by incorporating an alcohol such as H, C ₂ H ₅ OH, etc., and a carbon-containing gas having a reducibility such as CO into contact with the catalyst material.

To produce the oxide material of the present invention, for example, oxides of Ni and Ga, and carbonates, nitrates, acetates and the like of these metals capable of forming oxides by heat treatment are used. These are weighed and mixed so that the Ga / Ni atomic ratio is 2.5 to 3.3, and then heat treated for 5 to 30 hours in an oxidizing atmosphere at 500 to 1600 ° C. to obtain N.
It is possible to obtain a complex oxide powder having a spinel type crystal of i and Ga as a main crystal phase.

As other methods, solid phase reaction synthesis with oxides and other metal salts, and sol-gel method synthesis using metal alkoxide can also be used for production. In each case, the heat treatment is performed in an oxidizing atmosphere at 500 to 1600 ° C.
It is effective to perform the heat treatment at a low temperature for about 30 hours to increase the specific surface area of the powder. However, if the heat treatment temperature is lower than 500 ° C, crystallization becomes insufficient and
If the temperature exceeds 600 ° C., the composition is densified, which is not preferable. A suitable heat treatment temperature is 650 to 750 ° C.

Further, as a method of adding CeO ₂ , C
There is a method of pulverizing and mixing the eO ₂ powder and the above complex oxide powder with a ball mill or in a mortar.
It is not limited to this. The specific surface area of the composite oxide powder at this time is 30 to 100 m ² / g, and especially 40
Preferably ~80m is ² / g, the specific surface area of CeO ₂ powder 10 to 100 m ² / g, especially 15 to 60 ²
/ G is preferable.

[0015]

According to the present invention, 5 to 75 weight% of CeO ₂ is added to a spinel type crystalline composite oxide containing Ni and Ga as metal elements and having a Ga / Ni atomic ratio n of 2.5 to 3.3. % Of NOx can be reduced and removed with high efficiency by contacting the exhaust gas containing nitrogen oxides with exhaust gas, and the performance deterioration due to coexisting steam is small.

This reductive decomposition is more reactive with NO ₂ than with NO, and NO of CeO ₂ and NiGa ₂ O ₄ alone is used.
x The material of the present invention is 300 to 55 than the reductive decomposition property.
Since high NOx reduction decomposition characteristics were obtained in a wide temperature range of 0 ° C, it is considered that the addition of CeO ₂ to the above composite oxide improved the ability to oxidize NO into NO ₂ and improved the catalyst characteristics. .

[0017]

EXAMPLES Ni (NO ₃₎ as the starting material ₂ · 6H
₂ O, Ga (NO ₃₎ using a _{2 · 9H 2 O, Ni:} Ga
Were weighed so that the metal ratio of was the ratio shown in Table 1. These reagents were dissolved in distilled water and neutralized with aqueous ammonia while stirring. The precipitate formed was filtered, washed and freeze-dried. The dried powder was heat-treated at 700 ° C. for 30 hours in the air to obtain a spinel type crystalline complex oxide powder having a specific surface area of 40 to 50 m ² / g.

Further, CeO ₂ powder having a specific surface area of 15 m ² / g in an amount shown in Table 1 was mixed with this composite oxide powder. Next, the mixed 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 powder of 25 mesh pass, 35 mesh on. Then, using this powder, NO as exhaust gas
= 1000 ppm, O ₂ = 5%, C ₂ H ₄ = 1000 p
pm, He = balance gas, SV (space velocity) = 4000
Under the condition of 0 / h, the NO reduction resolution and the ethylene resolution were measured in the range of 300 to 600 ° C. using a gas chromatograph. Table 1 shows the results of measuring the NO reduction resolution in the temperature range in which the maximum NO activity and the NO removal rate of 30% or more of each sample and the gas in which the exhaust gas contains 10% of water vapor.

[0019]

[Table 1]

As is apparent from Table 1, the addition of 5% by weight of CeO ₂ to the composite oxide having a Ga / Ni ratio of 3.0 widens the temperature range in which the NOx removal rate is 30% or more by 50 ° C. . When CeO ₂ was added in an amount of 35% by weight, the temperature was further increased by 40 ° C. However, when the amount added exceeds 90% by weight, the maximum activity value decreases and the value of n is 2.5 to
Highest activity value in the range of 3.3, NOx removal rate 30%
It was found that the temperature range showing the above is wide.

As is clear from Table 1, Ga / Ni
NOx in the coexistence of steam with a complex oxide having a ratio of 3.0 alone
The removal rate decreased from 81% to 25%. However, Ga
The NOx removal rate in the presence of water vapor was improved from 25% to 40% simply by adding 5% by weight of CeO ₂ to the composite oxide having a / Ni ratio of 3.0. Further, when CeO ₂ was added in an amount of 35% by weight, the content improved to 65%. However, it was found that the NOx removal rate decreases to 20% when added in excess of 90% by weight.

[0022]

As described above in detail, the catalyst material according to the present invention has high NOx in a wide temperature range even in a high oxygen concentration atmosphere.
It has a reducing decomposition action and is useful for purification of exhaust gas from combustion engines such as diesel engines and lean burn engines in the future.

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Claims (2)

[Claims] 1. A Ga / N containing Ni and Ga as metal elements.
An oxide catalyst material for removing nitrogen oxides, which is obtained by adding 5 to 75% by weight of CeO ₂ to a spinel type crystalline complex oxide having an i atomic ratio n of 2.5 to 3.3. 2. A spinel type which contains Ni and Ga as metal elements and has a Ga / Ni atomic ratio n of 2.5 to 3.3 in an oxidizing atmosphere in which a carbon-containing gas having a reducing property with oxygen is present. 5 to 75% by weight of CeO ₂ in the crystalline composite oxide
A method for removing nitrogen oxides, which comprises contacting the added oxide catalyst material for removing nitrogen oxides with an exhaust gas containing nitrogen oxides.