JP3346653B2 - 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, there is an urgent need to develop a method for decomposing and removing harmful substances such as NOx and COx contained in exhaust gas from automobiles. Therefore, conventional methods for purifying automobile exhaust gas include carbon monoxide (CO) and hydrocarbon (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]

However, the above three-way catalyst can efficiently purify 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 the above-mentioned high concentration atmosphere does not work effectively. Therefore, usually, the oxygen concentration in the exhaust gas is measured to control the air-fuel ratio to always be the optimum, and exhaust gas purification with a higher oxygen concentration is hardly performed at present. is there.

[0004] In order to solve these problems, a catalyst capable of efficiently removing NOx even in the presence of high-concentration oxygen has been developed. At present, a catalyst capable of removing NOx in the presence of high-concentration oxygen is copper ion-exchanged zeolite. , Metal silicate and alumina catalysts have been proposed.

Further, the present inventors first contacted an exhaust gas containing an excessive amount of oxygen and containing nitrogen oxides with a nickel gallate (NiGa ₂ O ₄ ) spinel-type composite oxide catalyst to form nitrogen in the exhaust gas. A method for removing oxides was proposed.

[0006] However, the catalyst proposed above is NOx
The temperature range in which the removal activity temperature range is narrow, 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 a temperature range of 370 to 480 ° C., and NOx is removed only in the specified temperature range. There was a problem. In addition, the characteristics of the above catalyst are significantly reduced when water vapor is present in the reaction gas. For example, when 10% of water vapor is present in the reaction gas, the NOx removal rate is reduced from 81% to 25%. Had made it difficult.

[0007]

[Means for Solving the Problems] The present inventors have conducted research on the above problems, and as a result, 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 has been found that the catalyst material to which about 75% by weight is added 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, they have found that NOx decomposition and removal can be performed efficiently by contacting the oxide catalyst material with an exhaust gas containing nitrogen oxides in an oxidizing atmosphere in which a carbon-containing gas having oxygen and reducing properties is present. It was.

Hereinafter, the present invention will be described in detail. 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. When this is expressed by a general formula, NiGan O ₄₊ δ
(N = 2.5 to 3.3). Note that (O)
_{4 +} δ) is the amount of oxygen necessary to exist as a composite oxide stably, which changes as needed depending on the value of n, and changes in the range of 0 to 2.0. The composite oxide used in the present invention preferably has a Ga / Ni atomic ratio n of 2.8 to 3.1. Of these, those with n = 3.0 are particularly preferred.
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 the composite oxide.
₂ is 5 to 75% by weight based on the total amount, especially 20 to 4% by weight.
0% by weight is added. The amount of CeO ₂ added is 75
If the amount exceeds 5% by weight, the catalytic activity decreases, and if the amount is less than 5% by weight, the temperature range having a high removal rate becomes narrow.

According to the present invention, the composite oxide containing CeO ₂ is brought into contact with the exhaust gas containing nitrogen oxides (NOx) to reduce the oxygen concentration in the exhaust gas to 1%.
As described above, in particular, even at a high concentration of 3% or more, it has excellent NOx reduction resolution over a wide temperature range. Further, a hydrocarbon such as C ₂ H ₄ , C ₃ H ₆ , C ₃ H ₈ , CH ₃ O
When an alcohol such as H or C ₂ H ₅ OH or a carbon-containing gas having a reducing property such as CO is contained and brought into contact with the catalyst material, the NOx reducing property is further enhanced.

In order to produce the oxide material of the present invention, for example, an oxide of Ni or Ga or a carbonate, nitrate, acetate or the like of these metals capable of forming an oxide by heat treatment is used. These are weighed and mixed so that the atomic ratio of Ga / Ni becomes 2.5 to 3.3, and then heat-treated in an oxidizing atmosphere at 500 to 1600 ° C. for 5 to 30 hours.
A composite oxide powder having i and Ga spinel crystals as the main crystal phase can be obtained.

In addition, it can also be produced by a solid-phase reaction synthesis using an oxide or another metal salt, or a sol-gel synthesis using a metal alkoxide. In any case, heat treatment is performed in an oxidizing atmosphere at 500 to 1600 ° C for 5 minutes.
Heat treatment at a low temperature is effective for increasing 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., it is not preferable because it is densified. A preferred heat treatment temperature is 650-750 ° C.

Further, as a method of adding CeO ₂ ,
There is a method of pulverizing and mixing eO ₂ powder and the composite oxide powder in a ball mill or in a mortar.
It is not limited to this. At this time, the specific surface area of the composite oxide powder is 30 to 100 m ² / g, particularly 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.

[0015]

According to the present invention, 5 to 75 weight CeO ₂ in the spinel crystalline composite oxide containing Ni and Ga as the metal element Ga / Ni atomic ratio n becomes the ratio of 2.5 to 3.3 The NOx can be reduced and removed with high efficiency and in a wide temperature range by contacting the catalyst material added with% with the exhaust gas containing nitrogen oxides, and the performance decrease due to coexisting steam is small.

In this reductive decomposition, NO ₂ is more reactive than NO, and the NO ₂ of CeO ₂ and NiGa ₂ O ₄ alone
x 300-55 for the material of the present invention rather than x-reductive decomposition properties
Since a high NOx reductive decomposition property is obtained in a wide temperature range of 0 ° C., it is considered that the performance of oxidizing NO to NO ₂ is improved by adding CeO ₂ to the composite oxide, and the catalytic property is improved. .

[0017]

EXAMPLE Ni (NO ₃ ) ₂ .6H as starting material
₂ O, Ga (NO ₃₎ using a _{2 · 9H 2 O, Ni:} Ga
Were weighed so that the metal ratio of Table 1 became the ratio shown in Table 1. These reagents were 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. for 30 hours in the air to obtain a spinel-type crystalline composite 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 the composite oxide powder. Next, the mixed powder was molded by a die press, and then further compression-molded by a cold isostatic pressing method, and the molded product was crushed and sized into a powder of 25 mesh pass and 35 mesh on. Next, using this powder, NO
= 1000 ppm, O ₂ = 5%, C ₂ H ₄ = 1000p
pm, He = remaining gas, SV (space velocity) = 4000
Under the condition of 0 / h, the resolution of NO reduction and the resolution of ethylene were measured in the range of 300 to 600 ° C. using a gas chromatograph. Table 1 shows the temperature range showing the highest NO activity and the NO removal rate of 30% or more of each sample, and the results of measuring the NO reduction ability of the above exhaust gas containing 10% water vapor.

[0019]

[Table 1]

As is clear from Table 1, the temperature range in which the NOx removal rate was 30% or more was widened by 50 ° C. only by adding 5% by weight of CeO ₂ to the composite oxide having a Ga / Ni ratio of 3.0. . Further addition of 35% by weight of CeO ₂ further increased the temperature by 40 ° C. However, if it is added in excess of 90% by weight, the maximum activity value decreases, or the value of n becomes 2.5 to
The highest activity value is high in the range of 3.3, and the NOx removal rate is 30%
It was also found that the above temperature range was wide.

As is clear from Table 1, Ga / Ni
NOx in the presence of water vapor of a single composite oxide with a ratio of 3.0
The removal rate dropped from 81% to 25%. However, Ga
/ Ni ratio NOx removal rate in the water vapor presence in only the addition of CeO ₂ 5 wt% to the composite oxide of 3.0 was improved to 25% or <br/> et 51%. Further, when CeO ₂ was added at 35% by weight, the content was improved to 65%. However, it was found that the NOx removal rate was reduced to 20% when added in excess of 90% by weight.

[0022]

As described in detail above, 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 reductive decomposition action and is useful for purifying exhaust gas and the like of combustion engines such as diesel engines and lean burn engines in the future.

Continued on the front page (58) Fields surveyed (Int. Cl. ⁷ , DB name) B01J 21/00-38/74 B01D 53/86 B01D 53/94 JICST file (JOIS) WPI (DIALOG)

Claims (2)

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