JPH08309150A - Method for removing nitrogen oxide - Google Patents

Abstract

PURPOSE: To provide a method for removing nitrogen oxides with the use of a catalyst having a higher nitrogen oxide purification capacity in the removal of nitrogen oxides in oxygen-excessive exhaust gas containing nitrogen oxides and hydrocarbons. CONSTITUTION: In the removal of nitrogen oxides from oxygen-excessive exhaust gas containing nitrogen oxides and hydrocarbons, zeolite containing palladium and zinc, or palladium, zinc, and at least one kind of the first series transition element (Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu) is used as a catalyst.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for purifying exhaust gas discharged from combustion equipment, and more particularly to a method for removing nitrogen oxides in exhaust gas with excess oxygen.

[0002]

2. Description of the Related Art At present, purification of nitrogen oxides, carbon monoxide, hydrocarbons and the like is regarded as important because of serious environmental problems. Nitrogen oxides are used for combustion of various mobile sources such as internal combustion engines such as automobile gasoline engines, and fixed sources such as boilers of factory plants, gas engines of cogeneration systems, and internal combustion engines such as gas turbines. A large amount of exhaust gas is emitted from equipment, and its purification is an urgent and serious social issue.

Currently, a three-way catalyst having Pt, Rh, Pd, etc. supported on a carrier is used as a purification catalyst for exhaust gas discharged from an internal combustion engine. The three-way catalyst is nitrogen in exhaust gas with excess oxygen. Since it cannot purify oxides, it is used together with a system for controlling the ratio of air to fuel (so-called air-fuel ratio).

On the other hand, a lean burn system has been developed for the purpose of reducing fuel consumption and reducing carbon dioxide emissions. However, since lean burn exhaust gas contains excess oxygen, the three-way catalyst removes nitrogen oxides. Can not do it.

As a method for removing nitrogen oxides from an oxygen-excess exhaust gas, reductive denitration by adding ammonia has been carried out, but its range of use is limited due to the large size of the apparatus and the danger of ammonia.

Recently, a zeolite-based catalyst has been proposed which can purify nitrogen oxides in exhaust gas with excess oxygen without adding a special reducing agent such as ammonia. For example, JP-A-63-283727 and JP-A-1-13073.
Japanese Patent Laid-Open No. 5 proposes that a zeolite catalyst ion-exchanged with a transition metal can purify nitrogen oxides by using unburned hydrocarbons contained in a small amount in oxygen-rich exhaust gas as a reducing agent.

Further, Japanese Patent Laid-Open No. 4-256444 discloses a catalyst containing zinc in zeolite as a catalyst for removing nitrogen oxides from combustion exhaust gas containing nitrogen oxides. Furthermore, JP-A-4-2191
In Japanese Patent Publication No. 45, a method for removing nitrogen oxides using a catalyst containing cobalt and alkaline earth metals and nickel and / or zinc in zeolite is disclosed.

However, the conventional zeolite-based catalyst as described above is required to have further improved catalytic ability.

Further, in the case of combustion equipment using a gaseous fuel such as a gas engine and a gas turbine, the trace hydrocarbons contained in the exhaust gas are methane having a carbon number of 1 as a main component,
The previously proposed zeolite-based catalysts have particularly low nitrogen oxide purification performance.

Therefore, for example, Japanese Patent Laid-Open No. 4-363144
In Japanese Patent Laid-Open No. 5-208138 and Japanese Patent Laid-Open No. 5-208138,
Cobalt-containing zeolite as a catalyst for efficiently removing nitrogen oxides, carbon monoxide, and hydrocarbons from exhaust gas with excess oxygen containing nitrogen oxides, carbon monoxide, and hydrocarbons, even if the main component of hydrocarbons is methane Alternatively, cobalt- and palladium-containing zeolites have been proposed.

However, the cobalt- and palladium-containing zeolite catalysts disclosed in the above publication can certainly remove nitrogen oxides from oxygen-excess exhaust gas, but have a higher nitrogen oxide-purifying ability as an exhaust gas purification catalyst. It is the current situation that is required.

On the other hand, as a catalyst for removing nitrogen oxides from oxygen-rich exhaust gas containing nitrogen oxides and methane,
For example, in Japanese Unexamined Patent Publication No. 6-254352, a catalyst in which Pd is supported on H-type ZSM-5, and US Pat.
No. 64606, a ZSM-5 catalyst impregnated with zinc is disclosed. However, the evaluation gas shown in the examples is a mixed gas that does not contain water, and the nitrogen oxide removal performance for the actual exhaust gas containing water was unclear.

[0013]

DISCLOSURE OF THE INVENTION An object of the present invention is to remove nitrogen oxides in exhaust gas in excess of oxygen containing nitrogen oxides and hydrocarbons, the hydrocarbons containing methane as a main component. Even if the exhaust gas contains water, a method for removing nitrogen oxides using a catalyst having a higher nitrogen oxide purification capacity is provided.

[0014]

Means for Solving the Problems As a result of intensive studies for solving the above problems, the present inventors have found that palladium and zinc, or a zeolite containing palladium, zinc and at least one first transition series element is used as a catalyst. As a result, they have found that nitrogen oxides can be efficiently removed from the oxygen-excess exhaust gas containing nitrogen oxides and hydrocarbons, and have completed the present invention.

That is, according to the present invention, when removing nitrogen oxides from oxygen-rich exhaust gas containing nitrogen oxides and hydrocarbons, palladium and zinc, or palladium, zinc and at least one first transition series element are used. The present invention provides a method for removing nitrogen oxides, which comprises using the contained zeolite as a catalyst.

The present invention will be described in detail below.

The exhaust gas purifying catalyst used in the present invention is
It comprises palladium and zinc, or a zeolite zeolite containing palladium, zinc and at least one first transition series element.

The zeolite is generally _{M 2 / n O · Al 2} O 3 · ySiO 2 · zH 2 O ( where, n is the valence of the cation M, y is a number of 2 or more, z
Is a crystalline aluminosilicate having a composition of 0 or more), and many types are known as natural products and synthetic products. The kind of the zeolite used in the present invention is not particularly limited, but in order to obtain high durability, S
It is desirable that the iO ₂ / Al ₂ O ₃ molar ratio be 10 or more. Typically, ferrierite, Y, mordenite,
Examples thereof include ZSM-5 and ZSM-11, and among them, ZSM-5 is preferable.

Although these zeolites may be used as they are, they are used as NH ₄ Cl, NH ₄ NO ₃ , (NH ₄ )
There is no problem even if the NH ₄ type ion-exchanged with ₂ SO ₄ or the like, or the ion-exchanged NH ₄ type with calcination or mineral acid etc. is used as the H-type. Further, it does not matter even if it contains cations such as alkali metals and alkaline earth metals.

The exhaust gas purifying catalyst of the present invention is characterized by containing (1) palladium and zinc, or (2) palladium, zinc and at least one first transition series element. The first transition series element is, for example, F.I. A.
Cotton, G.I. As described in Wilkinson, Translated by Katsunori Nakahara, Inorganic Chemistry (Baifukan), pages 617-618, nine elements of scandium, titanium, vanadium, chromium, manganese, iron, cobalt, nickel, and copper. Is. In the method of the present invention, at least one element of the nine kinds of first transition series elements can be used, and among them, cobalt is preferable.

The method of incorporating palladium into zeolite is not particularly limited, and it may be carried out by an ion exchange method, an impregnation supporting method or the like. When ion-exchanging palladium with zeolite, the zeolite may be added to a solution containing ammonia and palladium ions and stirred at 20 to 100 ° C. for several hours to several tens hours. Examples of usable palladium compounds include acetates, nitrates, ammine complex salts and chlorides.

The method of incorporating zinc into zeolite is not particularly limited, and it may be carried out by an ion exchange method, an impregnation supporting method or the like. When ion-exchanging zinc, zeolite may be added to a solution containing zinc ions and stirred at 20 to 100 ° C. for several hours to several tens of hours. Examples of zinc compounds that can be used include acetate, nitrate, oxalate, and chloride.

The method of incorporating the first transition series element into zeolite is not particularly limited, and it may be carried out by an ion exchange method, an impregnation supporting method or the like. When the first transition series element is ion-exchanged, zeolite may be added to the solution containing the first transition series element ion and stirred at 20 to 100 ° C. for several hours to several tens hours. Examples of usable first transition series element compounds include acetates, nitrates, oxalates and chlorides.

The content of each of palladium, zinc and the first transition series element is not particularly limited, but in order to obtain high catalytic performance, the content of palladium is 0.01 expressed as a PdO / Al ₂ O ₃ molar ratio. ~ 1.0 is preferable, and 0.03 ~
0.8 is more preferable. The content of zinc is ZnO / Al ₂
0.05 to 2.0 are preferred represented by O ₃ molar ratio, 0.
1 to 1.5 is more preferable. When the content of the first transition series element is M and the cation thereof is divalent, the content of the first transition series element is 0.2 to 0.2 in terms of MO / Al ₂ O ₃ molar ratio.
2.5 is preferable, and 0.25 to 2.0 is more preferable.

The order in which the zeolite contains palladium, zinc and at least one first transition series element is not particularly limited, and finally palladium and zinc, or palladium, zinc and at least one first transition series element are It may be contained in the zeolite.

Zeolite containing palladium and zinc or at least one element of palladium, zinc and the first transition series element may be used after being subjected to pretreatment such as drying and calcination when used as a catalyst. Good.

The shape of the zeolite catalyst used in the present invention,
The structure and the like are not particularly limited, and examples thereof include powder, pellets, and honeycombs. further,
The above-mentioned introduction of the metal element can also be performed after the catalyst molding.

The exhaust gas purifying catalyst used in the present invention is molded into a predetermined shape by adding a binder such as alumina sol, silica sol or clay, or is made into a slurry by adding water.
It may be used after being applied on a refractory base material such as honeycomb, alumina, magnesia, cordierite or the like.

The exhaust gas targeted by the catalyst of the present invention is an oxygen-excess exhaust gas containing nitrogen oxides. Exhaust gas with excess oxygen means exhaust gas containing oxygen in excess of the amount of oxygen required to completely oxidize reducing components such as carbon monoxide and hydrocarbons contained in the exhaust gas. Further, the hydrocarbon contained in the exhaust gas is not particularly limited, but the catalyst of the present invention can efficiently purify the exhaust gas even for the exhaust gas whose main component of the hydrocarbon is methane having 1 carbon atom. You can

Generally, most of the hydrocarbons contained in the exhaust gas discharged from an engine using a liquid fuel such as an automobile are hydrocarbons having 2 or more carbon atoms. On the other hand, the main component of the hydrocarbon contained in the exhaust gas discharged from an engine using a gaseous fuel such as a gas engine is methane. Usually, the reactivity of hydrocarbon tends to increase as the number of carbon atoms increases, and the reactivity is particularly low in the case of methane having 1 carbon atom. Here, the exhaust gas containing methane as a main component of hydrocarbon means an exhaust gas in which 80% or more of the hydrocarbon contained in the exhaust gas is methane. Examples of such exhaust gas include exhaust gas discharged from a lean-burn gas engine using city gas as fuel.

Further, in the method of the present invention, hydrocarbon may be further added to the exhaust gas from the engine. The hydrocarbon to be added is not particularly limited, but the catalyst used in the present invention can efficiently purify the exhaust gas even if the hydrocarbon is methane or a mixed gas of hydrocarbons containing methane as a main component. . The mixed gas of hydrocarbons containing methane as a main component means a mixed gas in which 80% or more of the hydrocarbons in the mixed gas are methane.
The concentration of the added hydrocarbon is not particularly limited and is 50 pp
If it is about m to 1%, the economical efficiency and the hydrocarbon purification rate are not caused, which is preferable. Further, if the hydrocarbon concentration in the exhaust gas is sufficiently high, the hydrocarbon need not be added.

The space velocity, temperature, etc. for removing the nitrogen oxides are not particularly limited, but the space velocity is 100 to 50000.
It is preferable that the temperature is 0 hr ⁻¹ and the temperature is 200 to 800 ° C. Hereinafter, the present invention will be described in more detail with reference to Examples, but the present invention is not limited to these Examples.

[0033]

【Example】

Example 1 NH ₄ -ZSM-5,20 of <Preparation of Catalyst 1> silica / alumina ratio of 40
0 g was added to 1800 ml of a 0.25 M zinc acetate aqueous solution, and ion exchange was performed by stirring at 80 ° C. for 20 hours.
After solid-liquid separation of the slurry, wash with 20 l of pure water,
It dried at 10 degreeC for 10 hours, and obtained zinc containing ZSM-5.

As a result of elemental analysis, zinc is 1.2% of that of alumina.
It was 5 times.

The zinc-containing ZSM-5, 1 thus obtained
0 g was added to 90 ml of pure water. After that, 7% ammonia water was added to adjust the pH to 10.0, and the [Pd (N
_{H 3) 4] Cl 2 ·} H 2 O was added and stirred for 2 hours at 30 ° C., was palladium ion exchange. After the solid-liquid separation of the slurry, it was washed with 1 liter of pure water and dried at 110 ° C. for 20 hours to obtain ZSM-5 containing zinc and palladium, and catalyst 1
And

As a result of elemental analysis, zinc is 1.2% of that of alumina.
It was 0 times and palladium was 0.1 times.

Example 2 <Preparation of catalyst 2> 10 g of ZSM-5 containing zinc obtained in Example 1
It was air-baked at 00 ° C. for 1 hour. Then Co of 0.25M
(CH ₃ COO) was added to ₂ · 4H ₂ O aqueous solution 90 ml,
Ion exchange was performed by stirring at 80 ° C. for 20 hours. After solid-liquid separation of the slurry, the zeolite cake was put into an aqueous solution having the same composition as above, and the ion exchange operation was performed again. After solid-liquid separation, it was washed with 1 liter of pure water and dried at 110 ° C. for 10 hours to obtain zinc- and cobalt-containing ZSM-5.

As a result of elemental analysis, cobalt was 1.29 times that of alumina and zinc was 0.38 times.

10 g of the ZSM-5 containing zinc and cobalt thus obtained was added to 90 ml of pure water. Then, 7% ammonia water was added to adjust the pH to 10.0, and the amount was adjusted to 0.1 with respect to the number of moles of alumina in the zeolite.
Double [Pd (NH ₃ ) ₄ ] Cl ₂ · H ₂ O was added, and the temperature was 30 ° C.
The mixture was stirred for 2 hours, and palladium ion exchange was performed. After the solid-liquid separation of the slurry, the slurry was washed with 1 l of pure water and dried at 110 ° C. for 20 hours to obtain ZSM-5 containing zinc, cobalt and palladium, which was designated as catalyst 2.

As a result of elemental analysis, cobalt was 1.29 times that of alumina, zinc was 0.38 times, and palladium was 0.1 times.

Example 3 <Preparation of catalyst 3> NH ₄ -ZSM-5, 20 having a silica / alumina ratio of 40
0 g was added to 1800 ml of 0.025 M zinc nitrate aqueous solution, and ion exchange was performed by stirring at 80 ° C. for 20 hours. After the solid-liquid separation of the slurry, wash with 20 l of pure water,
It was dried at 10 ° C. for 10 hours to obtain zinc-containing ZSM-5.

As a result of elemental analysis, zinc was 0.4% of alumina.
It was double.

The zinc-containing ZSM-5, 1 thus obtained
0 g was air-baked at 500 ° C. for 1 hour. Then 0.2
5M of _{Co (CH 3 COO) 2 ·} 4H 2 O aqueous solution 90ml
Then, the mixture was stirred at 80 ° C. for 20 hours for ion exchange. After solid-liquid separation of the slurry, the zeolite cake was put into an aqueous solution having the same composition as above, and the ion exchange operation was performed again. After solid-liquid separation, wash with 1 liter of pure water and
After drying for 0 hour, ZSM-5 containing zinc and cobalt was obtained.

As a result of elemental analysis, cobalt was 1.36 times that of alumina and zinc was 0.1 times. 10 g of ZSM-5 containing zinc and cobalt thus obtained was added to 90 ml of pure water. Then, 7% ammonia water was added to adjust the pH to 10.0, and [Pd (NH ₃ ) ₄ ] C was added 0.1 times the number of moles of alumina in the zeolite.
l ₂ · H ₂ O was added, and the mixture was stirred at 30 ° C. for 2 hours to carry out palladium ion exchange. After solid-liquid separation of the slurry, 1 liter
Was washed with pure water and dried at 110 ° C. for 20 hours to obtain ZSM-5 containing zinc, cobalt and palladium, which was designated as catalyst 3.

As a result of elemental analysis, cobalt was 1.36 times that of alumina, zinc was 0.1 times, and palladium was 0.1 times.

Example 4 <Preparation of catalyst 4> NH ₄ -ZSM-5, 20 having a silica / alumina ratio of 40.
0 g of 0.25M Co (CH ₃ COO) ₂ .4H ₂ O
The solution was added to 1800 ml of an aqueous solution and stirred at 80 ° C. for 20 hours for ion exchange. After solid-liquid separation of the slurry, the zeolite cake was put into an aqueous solution having the same composition as above, and the ion exchange operation was performed again. After solid-liquid separation, it is washed with 20 l of pure water and dried at 110 ° C for 10 hours to obtain cobalt-containing ZS.
M-5 was obtained.

As a result of elemental analysis, cobalt was 1.39 times that of alumina.

Co-containing ZSM-5, 1 thus obtained
0 g was added to 90 ml of a 0.025 M zinc acetate aqueous solution, and ion exchange was performed by stirring at 80 ° C. for 20 hours. After solid-liquid separation of the slurry, the zeolite cake was put into an aqueous solution having the same composition as above, and the ion exchange operation was performed again.
After solid-liquid separation, it was washed with 2 l of pure water and dried at 110 ° C for 10 hours to obtain cobalt- and zinc-containing ZSM-5.

As a result of elemental analysis, cobalt was 1.15 times that of alumina and zinc was 0.25 times.

Co and zinc containing ZS thus obtained
M-5, 10 g was added to 90 ml of pure water. afterwards,
The pH was adjusted to 10.0 by adding 7% aqueous ammonia, and 0.1 times [Pd (NH ₃ ) ₄ ] Cl ₂ .H ₂ O was added to the number of moles of alumina in the zeolite. The mixture was stirred at 30 ° C. for 2 hours to carry out palladium ion exchange. After the solid-liquid separation of the slurry, it was washed with 1 liter of pure water, and then at 110 ° C for 2
After drying for 0 hour, ZSM-5 containing cobalt, zinc, and palladium was obtained and designated as catalyst 4.

As a result of elemental analysis, cobalt was 1.15 times that of alumina, zinc was 0.20 times, and palladium was 0.1 times.

Example 5 <Preparation of catalyst 5> Catalyst 5 was prepared in the same manner as in Example 4 except that the concentration of the zinc acetate aqueous solution was 0.05M.

As a result of elemental analysis, cobalt was 1.15 times that of alumina, zinc was 0.40 times, and palladium was 0.1 times.

Example 6 <Preparation of catalyst 6> Example 4 except that the zinc acetate aqueous solution was changed to the zinc nitrate aqueous solution.
In the same manner as above, a catalyst 6 was obtained.

As a result of elemental analysis, cobalt was 1.15 times that of alumina, zinc was 0.22 times, and palladium was 0.1 times.

Example 7 <Preparation of catalyst 7> Example 5 except that the zinc acetate aqueous solution was changed to the zinc nitrate aqueous solution.
In the same manner as above, a catalyst 7 was obtained.

As a result of elemental analysis, cobalt was 1.13 times that of alumina, zinc was 0.29 times, and palladium was 0.1 times.

Comparative Example 1 <Preparation of Comparative Catalyst 1> Comparative catalyst 1 was prepared in the same manner as in Example 1 except that palladium ion exchange was not carried out.

As a result of elemental analysis, zinc was 1.2% of that of alumina.
It was 5 times.

Comparative Example 2 <Preparation of Comparative Catalyst 2> NH ₄ -ZSM-5, 10 having a silica / alumina ratio of 40.
g was added to 90 ml of pure water. Then, 7% ammonia water was added to adjust the pH to 10.0, and [Pd (NH
₃ ) ₄ ] Cl ₂ .H ₂ O was added, and the mixture was stirred at 30 ° C. for 2 hours to perform palladium ion exchange. After the solid-liquid separation of the slurry, the slurry was washed with 1 liter of pure water and dried at 110 ° C. for 20 hours to obtain palladium-containing ZSM-5, which was used as Comparative Catalyst 2.

As a result of elemental analysis, palladium was 0.1 times that of alumina.

Comparative Example 3 <Preparation of Comparative Catalyst 3> Comparative catalyst 3 was prepared in the same manner as in Example 4 except that zinc ion exchange was not carried out.

As a result of elemental analysis, cobalt was 1.39 times that of alumina, and palladium was 0.1 times that of alumina.

Comparative Example 4 <Preparation of Comparative Catalyst 4> Comparative catalyst 4 was prepared in the same manner as in Example 4 except that palladium ion exchange was not carried out.

As a result of elemental analysis, cobalt was 1.15 times that of alumina and zinc was 0.25 times.

Example 8 <Performance Evaluation 1> Catalysts 1 to 7 and comparative catalysts 1 to 4 were crushed after tableting,
The particle size was adjusted to 12 to 20 mesh, and 2 cc of the sized powder was charged into a fixed pressure fixed bed reactor. After pretreatment at 500 ° C. for 1 hour under air flow, the gas having the composition shown in Table 1 was passed at 400 ° C. at 500 ml / min to measure the catalytic activity. 40
Table 2 shows the NOx purification rate when it reached a steady state at 0 ° C.
The NOx purification rate is a value calculated from the following equation.

[0067] NOx purification rate _{(%) = {(NOx in} -NOx out) / NOx in} × 100 NOx in: the reaction tube inlet NOx concentration NOx _out: reaction tube exit NOx concentration

[0068]

[Table 1]

[0069]

[Table 2]

Example 9 <Performance evaluation 2> Catalysts 1 to 7 and comparative catalysts 1 to 4 were crushed after tableting,
The particle size was adjusted to 12 to 20 mesh, and 5.0 g of the sized powder was charged into a fixed-pressure fixed-bed reactor. 1 at 500 ° C under air flow
After pretreatment for an hour, a gas having a composition shown in Table 3 was passed at 500 ml / min at 400 ° C. to measure the catalytic activity. Four
Table 4 shows the NOx purification rate when the steady state was reached at 00 ° C. The NOx purification rate is a value obtained in the same manner as in Example 8.

[0071]

[Table 3]

[0072]

[Table 4]

[0073]

From the results shown in Tables 2 and 4, the use of palladium and zinc of the present invention, or the zeolite containing palladium, zinc and at least one first transition series element as a catalyst resulted in a nitrogen oxide and It is clear that nitrogen oxides can be efficiently removed from exhaust gas containing excess oxygen containing hydrocarbons, even if the hydrocarbons contain methane as the main component and the exhaust gas contains water. Is. Therefore, the present invention is extremely significant in terms of environmental protection.

[0074]

Claims (3)

[Claims] 1. A zeolite containing palladium and zinc, or palladium, zinc and at least one first transition series element in removing nitrogen oxides from oxygen-excess exhaust gas containing nitrogen oxides and hydrocarbons. A method for removing nitrogen oxides, which comprises using as a catalyst. 2. The method for removing nitrogen oxides according to claim 1, wherein the first transition series element is cobalt. 3. The method for removing nitrogen oxides according to claim 1, wherein the main component of the hydrocarbon contained in the exhaust gas is methane.