JPH08173761A - Method for removing nitrogen oxide - Google Patents

Abstract

PURPOSE: To provide a method for removing more efficiently nitrogen oxides from an exhaust gas with excess oxygen exhausted from an internal combustion engine by using a catalyst with high performance in the wide temp. range, especially at a low temp. CONSTITUTION: In a method for removing nitrogen oxides in an exhaust gas contg. nitrogen oxides and hydrocarbons with excess oxygen, a catalyst wherein at least one of transition metals such as Cu, Co, Pt, Rh and In is incorporated in zeolite with a particle diameter of smaller than 0.1μm is brought into contact with the above described exhaust gas.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an exhaust gas purifying catalyst for removing nitrogen oxides, carbon monoxide and hydrocarbons in exhaust gas discharged from internal combustion engines such as automobiles, and more particularly to exhaust gas under excess oxygen. To a method for reducing nitrogen oxides.

[0002]

2. Description of the Related Art At present, nitrogen oxides and carbon monoxide which are harmful to the human body in exhaust gas emitted from gasoline engines are removed by a three-way catalyst in which platinum, rhodium and palladium are supported on a carrier. Has been done. However,
Regarding the exhaust gas from a Zel engine, the oxygen concentration in the exhaust gas is higher than that from a gasoline engine, so reduction and denitration by a three-way catalyst is difficult.

Further, in recent years, a lean-burn gasoline engine has been developed in order to reduce fuel consumption due to reduction of carbon dioxide emission. However, since the exhaust gas of this lean burn gasoline engine is in an oxygen excess atmosphere, it is difficult to denitrate with the conventional three-way catalyst as described above, and there is a demand for technological development related to nitrogen oxide removal.

As a method for removing nitrogen oxides from an oxygen-excessive exhaust gas, a selective catalytic reduction method on V ₂ O ₅ / TiO ₂ using ammonia as a reducing agent and an absorption method to an alkaline solution have been used so far. Although known, in any case, the range of use is limited, and it is difficult to apply it to mobile sources such as automobiles.

In recent years, it has been reported that a zeolite catalyst supporting a transition metal can reduce and remove nitrogen oxides in exhaust gas in the presence of excess oxygen without adding a selective reducing agent such as ammonia. JP-A-63-283727
No. JP-A-1-130735, etc.).

Among transition metal-containing zeolite catalysts, ZSM-5, mordenite, ferrierite, USY, and zeolite have been used as zeolites, which have been components of the catalyst so far.
Using zeolite β etc., Cu, Cu,
A nitrogen oxide removal catalyst supporting a transition metal such as Co, Fe, or Ni has been proposed. Among these zeolites, the nitrogen oxide removal catalyst using ZSM-5 has a specific high catalytic activity, and has attracted attention as the most promising denitration catalyst.

However, the above-mentioned transition metal-containing zeolite catalyst has problems such as low catalyst activity at low temperature in addition to catalyst deterioration due to high temperature as pointed out in JP-A-5-168863. ing. For this reason,
In order to apply to the exhaust gas purification of a diesel engine whose exhaust gas temperature is lower than that of a gasoline engine, it is necessary to further improve the catalyst performance in a low temperature range.

On the other hand, among the transition metals, the zeolite catalyst containing a noble metal such as platinum, palladium, or rhodium has the characteristics of high activity at low temperature and high heat resistance as compared with the above-mentioned transition metal-containing zeolite catalyst. There is. However, its activity is not sufficient, and performance improvement such as activity improvement is desired.

To date, many improved catalysts have been proposed for improving the catalyst performance in the low temperature range. For example, JP-A-5-168942 proposes a transition metal-containing zeolite catalyst to which Au is added, and JP-A-5-317720 proposes a Pt-supported zeolite catalyst in which Mg coexists.

[0010]

However, the effect is not sufficiently satisfied and the low temperature activity is insufficient, so that it has not been put to practical use. The purpose of the present invention is to
It is another object of the present invention to provide a method for more efficiently removing nitrogen oxides from an oxygen-excess exhaust gas discharged from an internal combustion engine by using a catalyst having a high performance in a wider temperature range, particularly at a low temperature.

[0011]

In view of these circumstances, the inventors of the present invention have made earnest studies on the ability of a zeolite catalyst to remove nitrogen oxides in exhaust gas with an excess of oxygen, and as a result, have a particle diameter of 0.1.
The present invention has found that a catalyst containing one or more kinds of transition metals in a submicron zeolite has an excellent nitrogen oxide removing ability in a wide temperature range as compared with the zeolite catalysts proposed so far. Has been completed.

That is, the present invention comprises contacting a zeolite having a particle size of less than 0.1 micron with a catalyst containing one or more kinds of transition metals in an oxygen-excess exhaust gas containing nitrogen oxides and hydrocarbons. The present invention provides a characteristic method for removing nitrogen oxides.

The present invention will be described in detail below.

It is important that the zeolite constituting the catalyst according to the present invention has a particle size of less than 0.1 micron. When the particle size of zeolite is 0.1 micron or more, a sufficient performance improving effect is not exhibited. Preferably 0.
It is 08 microns or less, more preferably 0.05 microns or less. Here, the particle size means primary particles of zeolite, and can be usually confirmed by an electron microscope. The structure of zeolite is not particularly limited,
For example, ZSM-5, -11, -12, -20, mordenite, ferrierite, Y type, zeolite β and the like can be mentioned. ZSM-5 and zeolite β are preferred. The method for producing zeolite is not particularly limited, and it can be produced by the known methods reported so far. Further, the method for controlling the particle size of zeolite is not particularly limited. If the particle size of the produced zeolite is 0.1 micron or more, it is possible to subject it to a pulverization process, but the pulverization may lower the crystallinity and may cause a decrease in performance. At 0.1
It is desirable to use those of less than micron.

The composition is not particularly limited, but in order to obtain high heat resistance and high durability, Si is used.
It is desirable to use an O ₂ / Al ₂ O ₃ molar ratio of about 10 to 1000. Further, this zeolite can be used as the catalyst of the present invention as it is or after being treated with an ammonium salt, a mineral acid or the like and subjected to ion exchange into NH ₄ type or H type.

The catalyst according to the present invention comprises the above zeolite
It is prepared by containing at least one kind of transition metal.

The transition metal contained in the zeolite is not particularly limited, but titanium, vanadium, chromium, manganese, iron, cobalt, nickel, copper, zinc, zirconium, molybdenum, platinum, ruthenium, palladium, rhodium, indium, gold. Etc. can be used. Copper, iron, cobalt, nickel, platinum, palladium, rhodium and indium are preferable, and copper, cobalt, platinum, rhodium and indium are more preferable.
These transition metals can be generally contained by an ion exchange method, an impregnation-supporting method, an evaporation-drying method, a physical mixing method, etc., but the ion exchange method is preferable.

As the ion exchange method, a generally used method, for example, a method of performing ion exchange using an aqueous solution containing a predetermined metal can be adopted. These metals can be used in the form of soluble salts, and as the soluble salts, nitrates, acetates, oxalates, chlorides and the like or complex salt compounds such as ammine complexes can be used.

The amount of metal added during ion exchange is preferably 0.1 to 10 in atomic ratio with respect to aluminum in the zeolite. If the amount of the metal added is less than 0.1, the metal content of the catalyst of the present invention may be low, and sufficient catalyst performance may not be obtained. May not be obtained. Regarding the processing conditions, the room temperature is usually 1 to 1.
The temperature may be 00 ° C. and the time may be several hours to several tens of hours. Further, the ion exchange operation can be repeated if necessary. Further, at the time of ion exchange, an ammonium aqueous solution may be added to the exchange mother liquor for ion exchange.
When two or more transition metals are contained, these metals may be contained either sequentially or simultaneously.

The zeolite containing these metals may further contain alkali metals and alkaline earth metals. The method for containing the alkali metal and the alkaline earth metal is not particularly limited, but an ion exchange method is preferable, and when the alkali metal and the alkaline earth metal are contained, the alkali metal and the alkaline earth metal and the above metal may be sequentially contained. Preferably, the alkali metal and the alkaline earth metal are first added, and then the above metal is added.

The metal content of the catalyst prepared by the above method is not particularly limited, but is preferably 0.1 to 3 times the atomic ratio of aluminum in the zeolite. If the contained metal has an atomic ratio of less than 0.1 with respect to aluminum in the zeolite, sufficient catalytic performance may not be obtained. Moreover, even if it exceeds 3, there is a possibility that the effect corresponding to it may not be obtained.

The exhaust gas purifying catalyst according to the present invention can also be used by mixing with a binder such as clay mineral and molding. It is also possible to form zeolite in advance and to add a transition metal, an alkali metal or an alkaline earth metal to the formed body. The binder used when molding the zeolite is not particularly limited, but clay minerals such as kaolin, attapulgite, montmorillonite, bentonite, allophane and sepiolite, silica, alumina and the like can be used. Alternatively, it may be a binderless zeolite molded product obtained by directly synthesizing a molded product without using a binder. Further, zeolite can be used as a washcoat on a honeycomb-shaped substrate made of cordierite or metal.

The removal of nitrogen oxides from the oxygen-excess exhaust gas can be carried out by bringing the exhaust gas purification catalyst described above into contact with the exhaust gas. The exhaust gas contains nitrogen oxides and hydrocarbons, and the oxygen-excessive exhaust gas targeted by the present invention means oxygen in excess of the amount of oxygen necessary to completely oxidize the hydrocarbons contained in the exhaust gas. Examples of such exhaust gas include exhaust gas discharged from an internal combustion engine such as a diesel engine, particularly exhaust gas burned in a state where the air-fuel ratio is large. .

The kind of hydrocarbon contained in the exhaust gas treated in the present invention is not particularly limited, and paraffin, olefin, aromatic compound and a mixture thereof can be used.
Specifically, hydrocarbons having 1 to 20 carbon atoms can be used as paraffins and olefins, and benzene, naphthalene, anthracene and derivatives thereof can be used as aromatic compounds. Also, as a mixture, light oil, kerosene,
Gasoline etc. can be used. In the present invention, it is considered that the catalytic reaction occurs between the zeolite particles, so that the effect is further enhanced when the hydrocarbon species that are difficult to enter the pores coexist. The hydrocarbon species that are difficult to enter into the zeolite pores have a carbon number of 10 or more. Is a mixture of. Furthermore, it is preferable to use light oil as the hydrocarbon.

The concentration of each component gas in the exhaust gas is not particularly limited, but normally, the nitrogen oxide content is 50 to 2000 pp.
m, hydrocarbons 10 to 10000 ppm, oxygen 0.
1 to 20%.

The space velocity and temperature of the exhaust gas to be treated are not particularly limited, but preferably space velocity (volume basis).
500 to 500,000 hr ^-1 , temperature 100 to 800
° C, more preferably space velocity 2000 to 20000
The temperature is ⁰ hr ⁻¹ and the temperature is 100 to 600 ° C.

[0027]

EXAMPLES The present invention will be further described below with reference to examples, but the present invention is not limited to these examples.

Example 1 <Preparation of catalyst 1> ZSM-5 type zeolite was prepared according to Example 15 of JP-A-53-147699. It had the following composition, expressed as the molar ratio of oxides in the anhydrous base.

1.38 (TPA) ₂ O · 0.73Na ₂ O
・ Al ₂ O ₃ .45SiO ₂ (TPA: tetrapropylammonium) Further, this ZSM-5 type zeolite was analyzed by an electron microscope (SE
According to M), it had a particle size of 0.04 to 0.045 micron.

This zeolite was 550 ° C. under air flow.
Tetrapropylammonium was removed by baking for 4 hours.

30 g of the calcined zeolite was added to 500 cc of an aqueous solution containing 11.5 g of ammonium acetate, and the mixture was stirred at 60 ° C. for 20 hours, washed, and then subjected to ammonium ion exchange. After repeating this operation twice, it was dried at 110 ° C. for 20 hours to obtain an ammonium-type zeolite.

This ammonium type zeolite: 10 g
Was added to an aqueous solution of copper acetate having a concentration of 0.1 mol / L, which was precisely weighed so that the atomic ratio was twice that of aluminum contained therein, and 2.5% -ammonia water was immediately added. In addition, the pH of the slurry was set to 10.5 and the slurry was kept at room temperature for 20
Stirred for hours. After solid-liquid separation, thoroughly wash and 2 at 110 ℃
After drying for 0 hours, catalyst 1 was obtained. As a result of chemical analysis, catalyst 1
Had the following composition, expressed as the molar ratio of oxides on an anhydrous basis.

1.86 CuO.Al ₂ O ₃ .45SiO ₂ Example 2 <Preparation of catalyst 2> Ammonium-type zeolite prepared in Example 1; 10 g
Was accurately weighed so that the atomic ratio was 0.5 times that of aluminum contained therein, and the concentration was 0.1 mol /
L tetraammine platinum nitrate aqueous solution, and immediately add 2.5% -ammonia water to adjust the pH of the slurry to 1.
It was set to 0.5 and stirred at room temperature for 20 hours. After solid-liquid separation, it was thoroughly washed and dried at 110 ° C. for 20 hours to obtain catalyst 2.
As a result of chemical analysis, catalyst 2 had the following composition expressed as the molar ratio of oxide on an anhydrous basis.

0.43PtO.Al ₂ O ₃ .45SiO ₂ Example 3 <Preparation of catalyst 3> Sodium aluminate; 1.88 g and sodium hydroxide; 8.08 g of tetraethylammonium hydroxide aqueous solution (20% aqueous solution); It was added to 442 g and completely dissolved. Nip seal (Nip made by Nippon Silica Industry Co., Ltd.)
VN-3); 82 g was gradually added, and the mixture was stirred for 30 minutes. The composition of the reaction mixture was as shown below in terms of molar ratio.

SiO ₂ / Al ₂ O ₃ = 100 (TEA) ₂ O / SiO ₂ = 0.25 (TEA; tetraethylammonium) Na ₂ O / SiO ₂ = 0.09 H ₂ O / SiO ₂ = 16.9 The mixture was charged into an autoclave and crystallized by standing at 150 ° C. for 96 hours. The product was solid-liquid separated, washed with water and dried. When this product was analyzed by X-ray diffraction,
It was pure zeolite β. As a result of chemical analysis, the composition had the following composition expressed by the molar ratio of the oxide in the anhydrous base.

2.38 (TEA) ₂ O ・ 0.51Na ₂ O
· Al ₂ O ₃ · 36SiO ₂ also was observed with zeolite β with electron microscope (SEM), and a particle size of 0.2 to 1 micron.

This zeolite was 550 ° C. under air flow.
Tetraethylammonium was removed by baking for 4 hours. After calcination of the zeolite after ball milling for 3 hours, it was observed with an electron microscope that it was 0.0
It had a particle size of 45 to 0.05 microns.

30 g of this zeolite was added to 500 cc of an aqueous solution containing 11.5 g of ammonium acetate;
Ammonium exchange was carried out by stirring at 20 ° C. for 20 hours. After repeating this operation twice, it was dried at 110 ° C. for 20 hours to obtain an ammonium-type zeolite.

This ammonium type zeolite: 10 g
Was added to an aqueous solution of copper acetate having a concentration of 0.1 mol / L, which was precisely weighed so that the atomic ratio was twice that of aluminum contained therein, and 2.5% -ammonia water was immediately added. In addition, the pH of the slurry was set to 10.5 and the slurry was kept at room temperature for 20
Stirred for hours. After solid-liquid separation, thoroughly wash and 2 at 110 ℃
After drying for 0 hour, catalyst 3 was obtained. As a result of chemical analysis, the catalyst 3 had the following composition represented by the molar ratio of oxides in the anhydrous base.

1.91 CuO.Al ₂ O ₃ .36SiO ₂ Example 4 <Catalyst activity test> Catalysts 1 to 3 were each press-molded and then pulverized to be sized to 12 to 20 mesh. Each sized catalyst (2 cc) was filled in a normal pressure fixed bed flow reaction tube and subjected to reaction. As a reaction pretreatment, the reaction gas was flowed at 1000 mL / min, the temperature was raised to 550 ° C., and the temperature was maintained for 30 minutes. Then 150
Steady-state catalyst activity was investigated using the model gases of Table 1 at any temperature between ~ 550 ° C. The space velocity (volume basis) at this time was 30,000 hr ⁻¹ . Table 2 shows the NOx removal rate at each temperature. The NOx removal rate is expressed by the following equation.

[0041] NOx removal rate _{= {([NOx] in -} [NO
x] _out ) / [NOx] _in } × 100 [NOx] _in : NOx concentration of inlet gas [NOx] _out : NOx concentration of outlet gas Comparative Example 1 <Preparation of Comparative Catalyst 1> 2 L of actual volume in a stirring state Sodium silicate solution (SiO ₂ : 29.6%, Na ₂
O: 9.35%, Al ₂ O ₃ : 0.01%, H ₂ O: 6
1.04%) and an aluminum sulfate aqueous solution (Al ₂ O ₃ ;
8.8 g / L, H ₂ SO ₄ ; 370 g / L) were continuously fed at a rate of 3 L / hr and 1 L / hr, respectively. The reaction temperature was 30 to 32 ° C., and the pH of the discharged slurry was 6.7 to 7.0.

After solid-liquid separation of the discharged slurry and sufficient washing, Na ₂ O; 0.75 wt%, Al ₂ O ₃ ; 0.77 w
t%, SiO ₂ ; 36.1 wt%, H ₂ O; 62.5 wt
% Of granular amorphous aluminosilicate homogeneous compound was obtained. The homogeneous compound; 2,860 g and 3.2 wt% NaOH aqueous solution; 6,150 g were charged into an autoclave, and 160
Crystallized at 72 ° C under stirring for 72 hours. Solid-liquid separation of products,
It was washed with water and dried to obtain ZSM-5 type zeolite. As a result of chemical analysis, the composition had the following composition represented by the molar ratio of oxide on an anhydrous basis.

[0043] _{1.03Na 2 O · Al 2 O 3} · 41SiO 2 The electron microscope the ZSM-5 type zeolite (SE
According to M), it had a particle diameter of 1 to 3 microns.

200 g of the above sodium type ZSM-5
Was added to an aqueous solution of copper acetate having a concentration of 0.1 mol / L, which was precisely weighed so that the atomic ratio was twice that of aluminum contained therein, and 2.5% aqueous ammonia was immediately added. The slurry was adjusted to pH 10.5 and stirred at room temperature for 20 hours. After solid-liquid separation, wash thoroughly and leave at 110 ° C for 20
Comparative catalyst 1 was obtained after drying for an hour. As a result of investigating the copper ion content of this comparative catalyst 1 by chemical analysis, it had the following composition expressed in terms of oxide molar ratio on an anhydrous basis.

1.74 CuO.Al ₂ O ₃ .41SiO ₂ Comparative Example 2 <Preparation of Comparative Catalyst 2> Sodium-type zeolite prepared in Comparative Example 1; 10 g, atomic ratio relative to aluminum contained therein Was added to an aqueous solution of tetraammine platinum nitrate having a concentration of 0.1 mol / L, which was precisely weighed to 0.5 times, and 2.5% -ammonia water was immediately added to adjust the pH of the slurry to 10.5. Stir at room temperature for 20 hours. After solid-liquid separation, it was thoroughly washed and dried at 110 ° C. for 20 hours to obtain comparative catalyst 2. As a result of chemical analysis, Comparative Catalyst 2 had the following composition expressed as the molar ratio of oxide on an anhydrous basis.

0.45PtO.0.51Na ₂ O.Al ₂
O ₃ .41SiO ₂ Comparative Example 3 <Preparation of Comparative Catalyst 3> An ammonium type zeolite β was prepared in the same manner as in Example 3 except that the ball mill pulverization was not performed.

This ammonium type zeolite β: 10 g
Was added to an aqueous solution of copper acetate having a concentration of 0.1 mol / L, which was precisely weighed so that the atomic ratio was twice that of aluminum contained therein, and 2.5% -ammonia water was immediately added. In addition, the pH of the slurry was set to 10.5 and the slurry was kept at room temperature for 20
Stirred for hours. After solid-liquid separation, thoroughly wash and 2 at 110 ℃
After drying for 0 hour, comparative catalyst 3 was obtained. As a result of chemical analysis,
Comparative catalyst 3 had the following composition, expressed as the molar ratio of oxides in the anhydrous base.

1.95 CuO.Al ₂ O ₃ .36SiO ₂ Comparative Example 4 <Catalyst Activity Test> The NOx conversion activity of the catalysts 1 to 3 and the comparative catalysts 1 to 3 was measured under the same conditions as in Example 4. Table 2 shows the NOx removal rate at each temperature.

[0049]

[Table 1]

[0050]

[Table 2]

[0051]

EFFECTS OF THE INVENTION By using the catalyst of the present invention, nitrogen oxides can be more efficiently removed from an exhaust gas containing excess oxygen containing nitrogen oxides and hydrocarbons in a wider temperature range.

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁶ Identification code Internal reference number FI Technical display location B01J 29/44 ZAB A 29/46 A 29/48 A B01D 53/36 102 H 102 A

Claims (1)

[Claims] 1. A method for removing nitrogen oxides in an oxygen-excess exhaust gas containing nitrogen oxides and hydrocarbons, wherein zeolite having a particle size of less than 0.1 micron contains one or more transition metals. A method for removing nitrogen oxides, which comprises contacting the catalyst with an oxygen-excess exhaust gas containing nitrogen oxides and hydrocarbons.