JPH07155546A - Method for purifying exhaust gas - Google Patents

Abstract

PURPOSE:To provide a method for purifying exhaust gas at a higher rate of purification when exhaust gas excess in oxygen contg. NOx, CO and hydrocarbon is purified. CONSTITUTION:Zeolite contg. cobalt and magnesium sulfate or cobalt, palladium and magnesium sulfate is used as a catalyst and NOx is removed from exhaust gas excess in oxygen contg. NOx and hydrocarbon.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a catalyst and method for purifying exhaust gas discharged from an internal combustion engine, and more particularly to a method for removing nitrogen oxides in exhaust gas in excess of 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 also found in large quantities from various mobile sources such as internal combustion engines such as automobile gasoline engines, and fixed sources such as internal combustion engines such as boilers in plant plants, gas engines in cogeneration systems, and gas turbines. It is discharged to the United States and its purification is an urgent and serious social issue.

Currently, a three-way catalyst in which Pt, Rh, Pd, etc. are carried 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.

However, JP-A-63-283727
The conventional zeolite-based catalysts proposed in JP-A No. 1-130735 and JP-A No. 1-130735 have not reached the practical range.

Furthermore, in the case of an internal combustion engine using a gas fuel such as a gas engine or 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, Japanese Patent Laid-Open No. 4-244218,
JP-A-4-363144 and JP-A-5-208
No. 138, a catalyst for efficiently removing nitrogen oxides, carbon monoxide, and hydrocarbons from oxygen-rich exhaust gas containing nitrogen oxides, carbon monoxide, and hydrocarbons even if the main component of the hydrocarbons is methane. As such, a cobalt-containing zeolite or a cobalt- and palladium-containing zeolite has been proposed.

As an exhaust gas purifying catalyst using zeolite containing cobalt and magnesium, Japanese Patent Laid-Open No. 3-196842 proposes an exhaust gas purifying method for automobile exhaust gas.

[0011]

[Patent Document 1] Japanese Unexamined Patent Publication No. 4-24421
The cobalt-containing zeolites and the cobalt- and palladium-containing zeolite catalysts disclosed in JP-A-8, JP-A-4-363144 and JP-A-5-208138 certainly efficiently remove nitrogen oxides from exhaust gas in excess of oxygen. However, a higher nitrogen oxide purification capacity is required as an exhaust gas purification catalyst.

The object of the present invention is to solve the problems described in JP-A-4-244218.
Japanese Patent Application Laid-Open No. 4-363144 and Japanese Patent Application Laid-Open No.
-208138, in a method for removing nitrogen oxides, carbon monoxide, and hydrocarbons from oxygen-rich exhaust gas containing nitrogen oxides, carbon monoxide, and hydrocarbons, the exhaust gas has a higher exhaust gas purification rate. It provides a purification method.

[0013]

Means for Solving the Problems As a result of intensive studies to solve the above problems, the present inventors have found that cobalt and magnesium sulfate or a zeolite containing cobalt, palladium and magnesium sulfate is used as a catalyst to obtain nitrogen. The inventors have found that nitrogen oxides can be efficiently removed from oxygen-excess exhaust gas containing oxides and hydrocarbons, and have completed the present invention.

That is, in the present invention, when removing nitrogen oxides from exhaust gas in excess of oxygen containing nitrogen oxides and hydrocarbons, cobalt and magnesium or cobalt,
An exhaust gas purification method characterized by using zeolite containing palladium and magnesium as a catalyst.

The present invention will be described in more detail below.

The zeolite used in the present invention, _{generally, xM 2 / n O · Al} 2 O 3 · ySiO 2 · zH 2 O ( where, n is the valence of the cation M, x is 0.8 to 1.2
, Y is a number of 2 or more, and z is a number of 0 or more), and is a crystalline aluminosilicate, and many types are known as natural products and synthetic products. The type of zeolite used in the present invention is not particularly limited, but it is desirable that the SiO ₂ / Al ₂ O ₃ molar ratio is 10 or more. Typically, ferrierite, Y, mordenite, ZSM-5, ZSM-11 and the like can be mentioned. Of these, ZSM-5 is most preferred. Also,
These zeolites may be used as they are, but they are ion-exchanged with NH ₄ Cl, NH ₄ NO ₃ , (NH ₄ ) ₂ SO ₄ or the like in the NH ₄ type or they are calcined or ion-exchanged with protons by mineral acid or the like. However, there is no problem even if it is used as an 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 cobalt and magnesium sulfate or cobalt, palladium and magnesium sulfate.

The method of incorporating cobalt 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 carrying out ion exchange of cobalt, the zeolite may be added to a solution containing cobalt ions and stirred at 20 to 100 ° C. for several hours to several tens hours. Examples of the cobalt salt used include acetate, nitrate, oxalate, chloride and the like.

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 the palladium is ion-exchanged with the zeolite, the zeolite may be added to a solution containing palladium ions coordinated with ammonia and stirred at 20 to 100 ° C. for several hours to several tens of hours. Examples of the palladium salt used include acetate, nitrate, ammine complex salt, chloride and the like.

The method of incorporating magnesium sulfate into zeolite is not particularly limited, and it may be carried out by an impregnation supporting method, a physical mixing method, or the like. When impregnating and supporting magnesium sulfate, zeolite may be added to a solution containing magnesium sulfate and the solvent may be removed by heating.

The order of incorporating cobalt, palladium and magnesium sulfate in the zeolite is not particularly limited,
Finally, the zeolite may contain cobalt and magnesium sulfate or cobalt, palladium and magnesium sulfate.

The content of cobalt, palladium and magnesium sulfate is not particularly limited, but the content of cobalt is preferably 0.2 to 2.5, expressed as CoO / Al ₂ O ₃ molar ratio, and 0.8 to 2. 0 is more preferable. The content of palladium is 0.01 in terms of PdO / Al ₂ O ₃ molar ratio.
Is preferably 1.0 to 1.0, more preferably 0.05 to 0.4. The content of magnesium sulfate is preferably 0.15 to 5.0, expressed as a MgO / Al ₂ O ₃ molar ratio, and 0.2 to 4.
0 is more preferable. When the CoO / Al ₂ O ₃ molar ratio is lower than 0.2, sufficient activity cannot be obtained. In addition, CoO
Even if the / Al ₂ O ₃ molar ratio is higher than 2.5, it is difficult to obtain the effect of increasing cobalt. When the PdO / Al ₂ O ₃ molar ratio is lower than 0.01, sufficient activity cannot be obtained. Further, even if the PdO / Al ₂ O ₃ molar ratio is higher than 1.0, the effect of increasing the amount of palladium cannot be obtained. MgO / Al ₂ O ₃
When the molar ratio is lower than 0.15, sufficient activity cannot be obtained. Further, even if the MgO / Al ₂ O ₃ molar ratio is higher than 5.0, the effect of increasing magnesium sulfate cannot be obtained.

Zeolite containing cobalt and magnesium sulfate, or cobalt, palladium and magnesium sulfate, when used as a catalyst,
You may use it after performing pretreatment, such as drying and baking.

The zeolite catalyst containing cobalt and magnesium sulfate or cobalt, palladium and magnesium sulfate according to the present invention may have any shape, structure, etc. such as powder, pellets and honeycombs. Further, the introduction of the metal element can be performed after the molding.

The exhaust gas purifying catalyst of the present invention is formed 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, and alumina, magnesia, cordierite or the like in the shape of a honeycomb or the like is formed. It may be used after being coated on the refractory base material.

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 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. Normal,
The reactivity of hydrocarbons 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.

When the catalyst of the present invention is used, hydrocarbon may be added to the above exhaust gas. The hydrocarbon to be added is not particularly limited, but the catalyst of the present invention can efficiently purify the exhaust gas even if the added hydrocarbon is methane or a mixed gas of hydrocarbons containing methane as a main component. it can. 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 hydrocarbon added is
There is no particular limitation, and it may be about 50 ppm to 1%.
Although the addition amount may be increased, it is not preferable because it causes a decrease in economic efficiency and a reduction rate of hydrocarbons. 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 nitrogen oxides are not particularly limited, but the space velocity is 100 to 50,000.
It is preferable that the temperature is 0 hr ⁻¹ and the temperature is 200 to 800 ° C.

[0029]

The present invention will be described in more detail below with reference to examples, but the present invention is not limited to these examples.

Example 1 <Preparation of catalyst 1> 200 g of NH ₄ -ZSM-5 zeolite having a SiO ₂ / Al ₂ O ₃ molar ratio of 40 was mixed with 0.25 M of Co (CH ₃ COO) _2.
・ Pour into 1800 ml of 4H ₂ O aqueous solution and perform 20 at 80 ℃.
Ion exchange was performed with stirring for a time. After the solid-liquid separation of the slurry, the zeolite cake was again 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 20 liter of pure water,
After drying for an hour, Co-ZSM-5 was obtained. As a result of elemental analysis, the CoO / Al ₂ O ₃ molar ratio was 1.39. 10 of the Co-ZSM-5 zeolites thus obtained
1.0 g of MgSO _{4 with} respect to the number of moles of alumina
・ Put in 100 ml of an aqueous solution containing 7H ₂ O,
After drying under reduced pressure at ℃ and supporting MgSO ₄ , it was dried at 110 ℃ for 20 hours to obtain catalyst 1. As a result of elemental analysis, Co
The O / Al ₂ O ₃ molar ratio was 1.39 and the MgO / Al ₂ O ₃ molar ratio was 1.0.

Example 2 <Preparation of Catalyst 2> 10 g of Co-ZSM-5 obtained in Example 1 was added to MgSO ₄ .7H ₂ O in an amount of 1.0 times the molar number of alumina and 0.1. Aqueous solution 100 containing twice as much Pd (NO ₃ ) ₂
It is put in ml and dried under reduced pressure at 80 ° C to obtain Pd and MgS.
After supporting O ₄ , it was dried at 110 ° C. for 20 hours to obtain catalyst 2.
Got As a result of elemental analysis, CoO / Al ₂ O ₃ molar ratio was 1.39, MgO / Al ₂ O ₃ molar ratio was 1.0, and PdO /
The Al ₂ O ₃ molar ratio was 0.1.

Example 3 <Preparation of catalyst 3> MgSO ₄ .7H ₂ O was added in an amount of 3.0 with respect to the number of moles of alumina.
A catalyst 3 was obtained in the same manner as in Example 2 except that the amount was doubled.
As a result of elemental analysis, the CoO / Al ₂ O ₃ molar ratio was 1.39,
The MgO / Al ₂ O ₃ molar ratio was 3.0 and the PdO / Al ₂ O ₃ molar ratio was 0.1.

Comparative Example 1 <Preparation of Comparative Catalyst 1> The Co-ZSM-5 obtained in Example 1 was used as Comparative Catalyst 1.

Comparative Example 2 <Preparation of Comparative Catalyst 2> Comparative catalyst 2 was obtained in the same manner as in Example 1 except that Mg (NO ₃ ) ₂ .6H ₂ O was used instead of MgSO ₄ .7H ₂ O. As a result of elemental analysis, the molar ratio of CoO / Al ₂ O ₃ is 1.39, Mg
The O / Al ₂ O ₃ molar ratio was 1.0.

Comparative Example 3 <Preparation of Comparative Catalyst 3> 10 g of Co-ZSM-5 obtained in Example 1 was dissolved in Pd (NO ₃ ) ₂ in an amount 0.1 times the number of moles of alumina. It was put into 100 ml of an aqueous solution, dried under reduced pressure at 80 ° C. to support Pd, and then dried at 110 ° C. for 20 hours to obtain a comparative catalyst 3. As a result of elemental analysis, the CoO / Al ₂ O ₃ molar ratio was 1.39 and the PdO / Al ₂ O ₃ molar ratio was 0.1.

Comparative Example 4 <Preparation of Comparative Catalyst 4> MgSO ₄ .7H ₂ O was added to 0.1 mol of alumina.
Comparative catalyst 4 was obtained in the same manner as in Example 2 except that the amount was doubled. As a result of elemental analysis, the CoO / Al ₂ O ₃ molar ratio was 1.3.
9, MgO / Al ₂ O ₃ molar ratio is 0.1, PdO / Al ₂
The O ₃ molar ratio was 0.1.

[0037] except that Comparative Example 5 MgSO ₄ · 7H ₂ O <Preparation of Comparative Catalyst 5> was _{Mg (NO 3) 2 · 6H} 2 O is performed in the same manner as in Example 2 to obtain a comparative catalyst 5. As a result of elemental analysis, CoO / Al ₂ O ₃ molar ratio was 1.39, M
The molar ratio of gO / Al ₂ O ₃ was 1.0, and the molar ratio of PdO / Al ₂ O ₃ was 0.1.

Example 4 <Catalyst Evaluation 1> Catalyst 1 and Comparative Catalysts 1 and 2 were crushed after tableting, respectively.
The particles were sized to 12 to 20 mesh, and 1.2 g of the sized particles was packed in a fixed-pressure atmospheric reactor. After pretreatment at 500 ° C. for 1 hour under air flow, 500 ml of gas having the composition shown in Table 1 is used.
The catalyst activity was measured at 350 ° C, 400 ° C and 500 ° C. Table 2 shows the NOx purification rate when the temperature reached a steady state. The CO purification rate was almost 100% for all the catalysts. The NOx purification rate is a value calculated from the following equation.

NOx purification rate (%) = (NOxin−NOxout) / NOxin × 100 NOxin: Reaction tube inlet NOx concentration NOxout: Reaction tube outlet NOx concentration

[0040]

[Table 1]

[0041]

[Table 2]

Example 5 <Catalyst evaluation 2> Catalysts 2 to 3 and comparative catalysts 3 to 5 were crushed after tableting, respectively, and then crushed to 12 to 20 mesh, 1.2 g of which was fixed under atmospheric pressure. Filled. 1 at 500 ° C under air flow
After the pretreatment for an hour, the gas having the composition shown in Table 1 was added to 500
The catalyst activity was measured at 400 ° C. and 500 ° C. at a flow rate of ml / min. NOx when it reaches a steady state at each temperature
The purification rate is shown in Table 3. The CO purification rate was almost 100% for all the catalysts. The NOx purification rate is a value obtained in the same manner as in Example 4.

[0043]

[Table 3]

Example 6 <Catalyst Evaluation 3> Catalyst 1 and Comparative Catalysts 1 and 2 were crushed after tableting, respectively.
The particle size was adjusted to 12 to 20 mesh, and 5.0 g of the sized powder was charged into a fixed-pressure atmospheric reactor. After pretreatment at 500 ° C. for 1 hour under air flow, 500 ml of gas having the composition shown in Table 4 is used.
The catalyst activity was measured at 400 ° C and 500 ° C. Table 5 shows the purification rates of NOx when the temperature reaches a steady state. The CO purification rate was almost 100% for all the catalysts. The NOx purification rate is a value obtained in the same manner as in Example 4.

[0045]

[Table 4]

[0046]

[Table 5]

Example 7 <Catalyst Evaluation 4> Catalysts 2 to 3 and comparative catalysts 3 to 5 were crushed after tableting, respectively, and then crushed to 12 to 20 mesh, 5.0 g of which were fixed under atmospheric pressure in a fixed bed reactor. Filled. 1 at 500 ° C under air flow
After pretreatment for an hour, the composition of the gas shown in Table 4 was changed to 500
The catalyst activity was measured at 400 ° C. and 500 ° C. at a flow rate of ml / min. NOx when it reaches a steady state at each temperature
The purification rate is shown in Table 6. The CO purification rate was almost 100% for all the catalysts. The NOx purification rate is a value obtained in the same manner as in Example 4.

[0048]

[Table 6]

[0049]

From the results of Table 2, Table 3, Table 5 and Table 6, the use of the cobalt and magnesium sulfate of the present invention or the zeolite containing cobalt, palladium and magnesium sulfate as a catalyst results in nitrogen oxide and carbonization. It is clear that nitrogen oxides can be efficiently removed from oxygen-rich exhaust gas containing hydrogen. Further, according to the method of the present invention, nitrogen oxides can be removed at a higher purification rate than that of the comparative catalyst even if the exhaust gas is methane as a main component of hydrocarbons in the exhaust gas. Therefore, the present invention is extremely significant in terms of environmental protection.

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Claims (2)

[Claims] 1. When removing nitrogen oxides from an oxygen-excess exhaust gas containing nitrogen oxides and hydrocarbons, cobalt and magnesium sulfate or a zeolite containing cobalt, palladium and magnesium sulfate is used as a catalyst. Exhaust gas purification method. 2. The exhaust gas purification method according to claim 1, wherein the main component of the hydrocarbon contained in the exhaust gas is methane.