JPH1057811A - Nox gas oxidizing agent and production thereof - Google Patents

Abstract

PROBLEM TO BE SOLVED: To efficiently oxidize and decompose nitrogen oxides even in low concentrations by producing a NOx gas oxidizing agent from a substance containing activated manganese dioxide having BET specific surface area within a specified range and bearing copper as an effective component. SOLUTION: A NOx gas oxidizing agent to efficiently remove NOx in low concentration in an exhaust gas emitted out of an internal combustion engine of an automobile, etc., by oxidation and decomposition is produced using activated manganese dioxide having 150-300m<2> /g BET specific surface area and bearing copper as an effective component. Such a NOx gas oxidizing agent is produced by following processes; a first process to precipitate a mixed salt crystal of manganese and copper by converting an aqueous solution of a mixed salt of manganese and copper to be mineral acid-acidic, a second process to produce manganese dioxide by adding potassium permanganate to the resultant strongly acidic slurry obtained through the first process and carrying out oxidation treatment, a third process to separate a precipitated substance from a mother solution, and a fourth process to neutralize the obtained precipitated substance with an alkali agent.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a NOx gas oxidizing agent capable of effectively oxidatively decomposing and removing low-concentration NOx in exhaust gas and a method for producing the same. More specifically, the present invention relates to a NOx gas oxidizing agent containing copper-supported active manganese dioxide as an active ingredient and a method for producing the same.

[0002]

2. Description of the Related Art Recently, since harmful nitrogen oxide components are contained in exhaust gas discharged from internal combustion engines such as automobiles, boilers and industrial plants, it is necessary to remove nitrogen oxides from these exhaust gas. It is being studied from various aspects.
Conventionally, nitrogen oxides contained in exhaust gas include NH ₃ , H ₂ ,
It has been carried out by a method of reducing to N ₂ using a reducing agent such as CO, or a method of oxidizing the nitrogen compound and then absorbing the compound into an alkali. However, the above-mentioned method using a reducing agent has a disadvantage that it reacts with SOx in exhaust gas to generate salts, and as a result, reduction activity is reduced or reductive decomposition concentration is increased.

[0003] As a method of using a reducing catalyst, for example, copper, nickel, cobalt, silica-magnesia composite oxide having Mg / Si (atomic ratio) of 2/1 to 1/4 is used.
A method using a catalyst supporting manganese or iron (Japanese Unexamined Patent Publication No.
JP-A-161846), a method using a catalyst in which titanium oxide, vanadium oxide and oxides such as manganese and cobalt are blended in a specific ratio (Japanese Patent Laid-Open No. 57-156040) are known. Conventionally, this kind of known reduction catalyst has a drawback that it is not practical because of its low activity of decomposing nitrogen oxides.

On the other hand, a method for oxidizing nitrogen oxides is as follows.
When the concentration of nitric oxide to be oxidized is low, the oxidation reaction is remarkably slowed down, so that there is a disadvantage that the volume of the reactor must be increased and the number of times of repeating the oxidation step must be increased. As a method using an oxidation catalyst,
A method using a catalyst composed of manganese dioxide and iron oxide (JP-A-49-45894) and a method using a catalyst composed of manganese dioxide and lead oxide (JP-A-50-62)
No. 859) is known. However, there are drawbacks that the ability as a catalyst is low and that the reaction must be performed at a high temperature of 100 ° C. or higher. As described above, although there have been many proposals for decomposition of NOx, none of them has practicality, and it is very difficult to treat NOx at normal temperature and normal pressure.

[0005]

SUMMARY OF THE INVENTION In view of the above problems, the present invention has made extensive studies on NOx gas oxidizing agents. The inventors have found that NOx can be efficiently oxidatively decomposed in a relatively low temperature range, and have completed the present invention. That is, an object of the present invention is to provide a NOx gas oxidizing agent which can efficiently oxidize and decompose even low-concentration nitrogen oxides and a method for producing the same.

[0006]

NOx gas oxidant SUMMARY OF to the present invention is to provide, 150m ² / g~300m ² / g
It has a BET specific surface area, and comprises active manganese dioxide carrying copper as an active ingredient.
Further, the production method includes a first step in which a mixed aqueous solution of a manganese salt and a copper salt is acidified with a mineral acid to precipitate crystals of a mixed salt of manganese and copper, and a permanganate is added to the strongly acidic slurry system obtained from the first step. A second step of adding potassium acid and oxidizing to form manganese dioxide, a third step of separating the precipitate of the second step from the mother liquor, and a fourth step of neutralizing the obtained precipitate with an alkali agent Is a structural feature.

[0007]

BEST MODE FOR CARRYING OUT THE INVENTION Hereinafter, the present invention will be described in detail.
The NOx gas oxidizer according to the present invention has the above-mentioned features. Particularly, active manganese dioxide as an active ingredient is BET
It has a specific surface area of usually 150 to 300 m ² / g, preferably 170 to 280 m ² / g, and is substantially amorphous by analysis based on X-ray diffraction. The reason for limiting the specific surface area is that if it becomes smaller than 150 m ² / g, N
This is because the contact area with Ox tends to be small and effective oxidative decomposition cannot be performed. On the other hand, if it is larger than 300 m ² / g, industrial production becomes difficult.

The amount of copper supported on the activated manganese dioxide is in the range of 0.9 to 9.0, preferably 1.5 to 7.5, in terms of Mn / Cu molar ratio. The reason for this is that the molar ratio is
If the ratio is less than 9, oxidation of nitrogen oxides cannot be performed effectively. On the other hand, if it is larger than 9.0, it is not preferable because of problems such as molding. As described above, the active manganese dioxide according to the present invention has another feature in that copper is supported.

The method for producing a NOx gas oxidizing agent according to the present invention is, for example, a method of mixing manganese dioxide and copper oxide and then calcining the mixture, neutralizing a metal nitrate mixed solution containing at least manganese and copper, A method in which hydroxide is coprecipitated and then oxidized, for example, may be mentioned. In the present invention, it is most preferable to carry out the production method described below, which can industrially obtain a product having more stable quality. preferable. That is, a first step in which a mixed salt aqueous solution of manganese and copper is acidified with a mineral acid to precipitate mixed salt crystals of manganese and copper. Potassium permanganate is added to the strongly oxidized slurry system obtained from the first step to oxidize. It comprises a second step of forming manganese dioxide by treatment, a third step of separating the precipitate of the second step from the mother liquor, and a fourth step of neutralizing the obtained precipitate with an alkaline agent.

The first step is, as described above, a step of depositing a mixed salt crystal of manganese and copper. Manganese and copper salts, which are starting materials, may be any soluble salts, but typically include sulfates and nitrates, and these salts may be either anhydrous or hydrated. Generally, nitrates are preferred for sulfates because the remaining sulfur content tends to be a catalyst poison. Sulfuric acid and nitric acid are typically used as the mineral acid as another raw material. Therefore,
In this step, it is particularly desirable to carry out the process using a sulfate-sulfuric acid or nitrate-nitric acid system in consideration of the circulating use of the mother liquor as described later. Depending on the purpose of use or in order to further improve the oxidizing ability, nickel, cobalt, zinc or silver may be supported in addition to copper, if necessary. In this case, these soluble salts can be supported by appropriately using them together with copper salts.

The aqueous solution of a mixed salt of manganese and copper reacts with a mineral acid to make it acidic with a mineral acid, whereby the solubility of the salt of manganese or copper decreases according to the acid concentration, and the mixed salt thereof is precipitated. In the case where the acid is a mineral acid, a mineral acid may be added to the aqueous metal mixed salt solution or vice versa.
As described above, in this step, it is important that the mixed salt is crystallized from the solution with the mineral acid, and it is preferable to precipitate a substantial amount of the metal salt in the raw material aqueous solution, but it is not necessary to precipitate the entire amount. There is no.

Here, the term “mixed salt to be crystallized” is not necessarily defined as being strictly chemically, but is widely defined as a mixture of manganese and copper salts having a solubility of manganese due to the action of mineral acid. It is defined as a crystal precipitated by the reduction, and it does not matter whether the crystal is a mixed crystal as a double salt or solid solution of manganese and copper. Therefore, the mixed salt crystals in this step are probably presumed to be at least partially a double salt dissolved in each other, but such crystals are fine, and in this crystalline state, potassium permanganate is used in the next step. To produce manganese dioxide. That is, although the details of the reaction mechanism are unknown, the oxidation resolution can be improved by allowing potassium permanganate to act on the mixed crystal particles of manganese and copper.

The amount of copper in this starting material is Mn / C
The molar ratio of u is from 1.4 to 14.0, preferably from 2 to 10. As described above, in the first step, a small amount of nickel, cobalt, zinc or silver salt can also be mixed as required. Next, the second step to be performed is a step of adding potassium permanganate to the strongly acidic slurry system obtained in the first step to cause an oxidation-reduction reaction to generate manganese dioxide.

That is, in the first step, a strongly acidic slurry in which mixed crystals of manganese and copper are finely dispersed in a strongly acidic solution is obtained. With the reaction, the manganese salt is converted to black-brown manganese dioxide to form a precipitate. The oxidation reaction in this step may be performed at room temperature or at elevated temperature, but it is desirable to carry out the reaction at a temperature of 50 to 60 ° C. in consideration of the reaction rate. After the addition of potassium permanganate, the mixture is aged while continuing to stir, so that the oxidation-reduction reaction can be sufficiently performed.

Although the details of the reaction are unknown, it is conceivable that copper partially forms a solid solution in the produced manganese dioxide, and other copper salts are adsorbed on the active manganese dioxide in a crystalline state of almost fine copper salts. It seems to co-precipitate. The amount of potassium permanganate added is around the theoretical amount at which the manganese salt reacts with potassium permanganate to produce manganese dioxide,
Preferably, a slight excess is preferable, and when added, either a crystal or an aqueous solution may be used.

Next, the third step is a step of separating the manganese dioxide-containing precipitate produced in the previous step from the mother liquor. The reason for this is to reduce the solubility of the copper salt to avoid the incorporation of copper in the mother liquor as much as possible, to increase the copper yield, and to recycle the recovered mother liquor as a mineral acid source to the first step as necessary. is there. The mother liquor thus separated is mainly a mineral acid, but is also a strongly acidic solution composed of a potassium salt by-produced in the previous step and a copper salt for the solubility. Therefore, this mother liquor is industrially advantageous to be circulated and used as a mineral acid source for dissolution or precipitation of a metal salt. At this time, the temperature at the time of separation and the mother liquor are concentrated or concentrated as necessary. By controlling the specific gravity or the like by dilution, the composition can be controlled to a substantially constant composition and recycled. In addition, when the salt concentration is increased by circulating the mother liquor, it is needless to say that the potassium salt is crystallized and used as necessary.

On the other hand, the precipitate is mainly composed of active manganese dioxide, to which a copper salt is adsorbed, and the mother liquor is also considerably adsorbed, and has a high water content. The separation operation may be performed by a conventional method, for example, a desired method such as centrifugation, filtration or decantation.

Next, the fourth step is a step of neutralizing the separated and recovered precipitate with an alkaline agent. That is,
The precipitate is dispersed in water and neutralized at room temperature or with stirring in a caustic or aqueous ammonia solution. This treatment, of course, neutralizes the acidic components of the adhesion, but converts the copper salt to a hydrated oxide and ensures that copper is supported on active manganese dioxide. P
It is preferable to carry out until H is in the range of 9 to 10. After the neutralization treatment, separation and washing are performed by a conventional method to obtain copper-supported manganese dioxide.

The activated manganese dioxide carrying copper thus produced can be used for a desired use as a NOx gas oxidizing agent by drying, if necessary, kneading with a desired binder, molding and drying. . Examples of the binder include clay, cement, silica sol, alumina sol, sodium silicate, and a water-soluble polymer. Of course, active manganese dioxide can be supported on a desired carrier and used. Examples of such a carrier include alumina, silica, calcia, magnesia, iron oxide, zirconia, titanium dioxide, metal oxides such as spinel-based composite oxides, metal silicates, aluminosilicates such as zeolite, and activated carbon.

NOx to which the present invention is applied is discharged from a nitric acid production plant, a metal surface treatment plant, a plant with a metal melting process, etc., a road tunnel, a road with a shelter, a deep underground space, a road intersection, and the like. It is a low-grade nitrogen oxide such as NO emitted from ventilation gas, air or combustion equipment used in the home. The concentration of NOx in the exhaust gas that can be used with the NOx gas oxidizing agent of the present invention varies depending on the source of NOx, but in many cases is 5 to 10,000.
0 ppm, preferably about 10 to 500 ppm.

[0021] The reaction temperature of the NOx gas oxidizing agent according to the present invention is usually 30 to 200 ° C, preferably 40 to 150 ° C. The contact time varies depending on the composition of the catalyst used, but is usually 0.036 to 3.6 seconds, preferably 0.072 to 3.6 seconds.
A very short time of 0.72 seconds is sufficient. The nitrogen oxide oxidatively decomposed under the above conditions may be adsorbed on an adsorbent such as activated carbon or zeolite, or may be dissolved in an aqueous alkaline solution. In the present invention, it is desirable that the NOx gas be adsorbed and removed by the activated carbon supporting the alkali metal and / or the alkaline earth metal. Alkali metal or /
The loading amount of the alkaline earth metal is usually 0.1 to 30% by weight, preferably 3 to 25% by weight as the metal.

[0022]

The present invention will be described below in detail with reference to examples, but the present invention is not limited to these examples. Example 1 315 g of manganese sulfate (MnSO ₄ .H ₂ O) and 286 g of copper sulfate (CuSO ₄ .5H ₂ O) were added to 50% sulfuric acid 193.
0 g to prepare a mixed metal salt aqueous solution. Then
208 ml of concentrated sulfuric acid (98% H ₂ SO ₄ ) is added to this aqueous solution, and stirring is continued at a temperature of 50 to 60 ° C. for 1 hour to precipitate fine mixed sulfate crystals of manganese and copper. Next, 225 g of potassium permanganate powder is gradually added to the stirred slurry, and the temperature is kept at 50 to 60 ° C. for about 2 hours to finish the oxidation reaction. Next, a small amount of water is added to the oxidized slurry to facilitate the separation operation, followed by filtration to separate the mother liquor from the precipitate. After re-dispersing the precipitate in water, a sodium hydroxide solution (48% NaOH)
Then, the mixture was neutralized to pH 9 to 10 with stirring, and separated by a conventional method, washed with water and dried to obtain copper-supported manganese dioxide.

Example 2 Crystals of 315 g of manganese sulfate (MnSO ₄ .H ₂ O) and 286 g of copper sulfate (CuSO ₄ .5H ₂ O) were used in Example 1.
Mother liquor (H ₂ SO ₄ : 50%, Mn: 0.1)
7%, Cu: 0.22%, K: 0.66%, specific gravity: 1.3
97) was dissolved in 1387 mL, and concentrated sulfuric acid (H ₂ SO ₄ :
(98%) 208 mL is added and stirring is continued for about 1 hour to crystallize fine mixed crystals of manganese sulfate and copper sulfate. Then, 225 g of potassium permanganate crystals are gradually added to the sulfuric acid slurry at a temperature of 50 to 60 ° C. to cause an oxidation reaction, and stirring is continued for about 2 hours. Next, 358 mL of water was added, and solid-liquid separation was performed at room temperature to collect a precipitate and a mother liquor, respectively. Next, the obtained precipitate was redispersed in water, and the pH was adjusted to 9.3 with 28% aqueous ammonia.
, And then separated, washed with water and dried by a conventional method to obtain copper-supported manganese dioxide.

Example 3 Manganese sulfate (MnSO ₄ .H ₂ O) 11 in 120 mL of water
9 g and 108 g of copper sulfate (CuSO ₄ .H ₂ O) were added to the mixture, and the mixed aqueous solution of metal salts was heated to 45 ° C. The mother liquor recovered and prepared in Example 2 (H ₂ SO ₄ : 55%, M
n: 0.16%, Cu: 0.20%, K: 0.65%, specific gravity: 1.455), 1340 mL, and 90 mL of concentrated sulfuric acid (98% H ₂ SO ₄ ) are added to about 1%. The mixture is stirred for a time to precipitate a mixed crystal of the metal salt. Next, 85 g of potassium permanganate crystal powder is gradually added to the obtained slurry at a temperature of 55 ° C., and after the addition, an oxidation reaction is caused under stirring for about 2 hours to ripen. After aging, the precipitate and the filtrate are filtered and separated. Thereafter, the same operation as in Example 1 was performed to obtain copper-supported manganese dioxide.

Example 4 The mother liquor recovered in Example 3 (H ₂ SO ₄ : 55%, Mn:
0.16%, Cu: 0.2%, K: 0.65%, specific gravity: 1.
455) 315 g of manganese sulfate (1.
86 mol), to prepare a metal salt mixture is the temperature at 40 to 50 ° C. by adding copper sulfate 214 g (0.86 mol) and nickel sulfate _{(NiSO 4 · 6H 2 O)} 250g (0.95 mol). 229m of concentrated sulfuric acid (98% H ₂ SO ₄ )
L is added, and the mixture is aged at a temperature of 50 to 60 ° C. for about 1 hour to precipitate a mixed crystal of a metal salt. Next, 225 g of potassium permanganate crystal powder is gradually added to the obtained strongly acidic slurry, and then the stirring is continued at 50 to 60 ° C. for about 2 hours to complete the oxidation reaction. Then, 35 parts of water were added to the slurry.
After adding 8 mL, the mixture is filtered to separate the precipitate from the filtrate.
Thereafter, copper-supported manganese dioxide was obtained in the same manner as in Example 1.

Comparative Example 1 315 g of manganese sulfate (MnSO ₄ .H ₂ O) and 286 g of copper sulfate (CuSO ₄ .5H ₂ O) were added to 50% sulfuric acid 193
0 g to prepare a mixed metal salt aqueous solution. Then
208 mL of concentrated sulfuric acid (98% H ₂ SO ₄ ) is added to this aqueous solution, and stirring is continued at a temperature of 50 to 60 ° C. for 1 hour to precipitate mixed sulfate crystals of manganese and copper. Next, electrolysis is performed for 2 hours using carbon as an anode, and manganese dioxide and copper oxide are deposited on the anode. Thereafter, the mother liquor and the precipitate were separated, further redispersed in water, and a sodium hydroxide solution (48% N
aOH) and neutralized to pH 9-10 with stirring.
Separation, washing and drying were carried out by conventional methods to obtain copper-supported manganese dioxide.

Comparative Example 2 A metal salt aqueous solution was prepared by adding 315 g of manganese sulfate (MnSO ₄ .H ₂ O) to 1430 g of 50% sulfuric acid. Next, 208 mL of concentrated sulfuric acid (98% H ₂ SO ₄ ) was added to this aqueous solution.
And stirring is continued at a temperature of 50 to 60 ° C. for 1 hour to precipitate manganese sulfate crystals. Next, 178 g of potassium permanganate powder was gradually added to the stirred slurry, and the mixture was maintained for about 2 hours to complete the oxidation reaction. Next, a small amount of water was added to the oxidized slurry to facilitate the separation operation, and then the mother liquor and the precipitate were separated.
Further, the mixture was redispersed in water, and a sodium hydroxide solution (48% NaO
H), the mixture was neutralized to pH 9 to 10 with stirring, and then separated, washed and dried by a conventional method to obtain manganese dioxide. The NOx gas oxidizing agents obtained in Examples 1 to 4 and Comparative Examples 1 and 2 had the physical properties shown in Table 1.

[0028]

[Table 1]

After kneading 0.5 g of bentonite as a binder with an appropriate amount of water, 10 g of each of the samples obtained in Examples 1 to 4 and Comparative Examples 1 and 2 (on a dry basis at 120 ° C. for 4 hours) was molded with a pelletizer. Then, what was dried at about 200 ° C. was used as a sample (diameter 3 mm × length 4 to 8 mm).

<NO Gas Oxidation Test> Each sample was packed 126 mm into a column having an inner diameter of 25 mm and a length of 300 mm around which a ribbon heater was wound. Next, the high-concentration standard NO gas was diluted with air at normal temperature to prepare 30 ppm NO gas, which was then introduced into the column. After that, the inflow continued for 60 minutes, and N
O and NO ₂ concentrations were measured. Further, the set temperature of the ribbon heater was set at 40 ° C. for 60 minutes and at 50 ° C. for 60 minutes, and the NOx concentration at the outlet was measured in the same manner. Table 2 summarizes the measurement conditions. The measurement results are shown in FIGS.

[0031]

[Table 2] Measurement conditions Space velocity; 15,000 / hour Entrance NO concentration: 30 ppm Humidity: 50% constant Ribbon heater set temperature: Normal temperature 60 minutes → 40 ° C / 60 minutes → 50 ° C / 60 minutes Flow rate: 15 L / min Measurement Method: Gas detection tube method

[0032]

According to the NOx gas oxidizing agent of the present invention, 5%
Since NOx, which causes photochemical smog, can be efficiently oxidatively decomposed in a relatively low temperature range of about 0 ° C., it is extremely useful as a NOx gas oxidizing agent.

[Brief description of the drawings]

FIG. 1 is a NO oxidation test result of a NOx gas oxidizing agent obtained in Example 1.

FIG. 2 shows the results of a NO oxidation test of a NOx gas oxidizing agent obtained in Example 2.

FIG. 3 shows the results of a NO oxidation test of a NOx gas oxidizing agent obtained in Example 3.

FIG. 4 shows the results of a NO oxidation test of a NOx gas oxidizing agent obtained in Example 4.

FIG. 5 is a NO oxidation test result of a NOx gas oxidizing agent obtained in Comparative Example 1.

FIG. 6 shows the results of a NO oxidation test of a NOx gas oxidizing agent obtained in Comparative Example 2.

Claims (6)

[Claims] BET method according to claim 1] 150m ² / g~300m ² / g
A NOx gas oxidizing agent having a specific surface area and comprising active manganese dioxide carrying copper as an active ingredient. 2. The NOx gas oxidizing agent according to claim 1, wherein the activated manganese dioxide supporting copper has a molar ratio of manganese to copper (Mn / Cu) of 0.9 to 9.0. 3. A first step in which a mixed aqueous solution of a manganese salt and a copper salt is acidified with a mineral acid to precipitate mixed salt crystals of manganese and copper, and potassium permanganate is added to the strongly acidic slurry obtained in the first step. And a oxidizing treatment to produce manganese dioxide, a third step of separating the precipitate of the second step from the mother liquor, and a fourth step of neutralizing the obtained precipitate with an alkali agent. A method for producing a NOx gas oxidizing agent. 4. The method according to claim 3, wherein the molar ratio of Mn / Cu in the mixed aqueous solution of the manganese salt and the copper salt is in the range of 1.4 to 14.0. 5. The method according to claim 4, wherein the strongly acidic slurry is a metal sulfate-sulfuric acid slurry. 6. The process for producing a NOx gas oxidizing agent according to claim 3, wherein the mother liquor from which the precipitate has been separated is recycled as a mineral acid source for dissolving or depositing manganese salts and copper salts.