JPH11319571A - Gas purification catalyst and gas purification method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a catalyst for purification of gas and a method for purification of gas wherein nitrogen oxides can be efficiently removed from a gas even after the catalyst is exposed to high temp. and in addition, they are converted to non-toxic nitrogen. SOLUTION: A catalyst for purification of gas obtd. by a method wherein after a zeolite with an MFI structure with a molar ratio of SiO2 /Al2 O3 of 20-200 is incorportaed with P, it is heat-treated at a temp. of 800-1,000 deg.C and then, at least one selected from Pt, Pd, Rh and Ir, is brought into contact with the gas.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a catalyst for removing nitrogen oxides from a gas, particularly exhaust gas discharged from an internal combustion engine of an automobile or the like, and a method for removing the same. The present invention relates to an exhaust gas purifying catalyst capable of efficiently removing nitrogen oxides from gas even afterwards, and a method for purifying the same.

[0002]

2. Description of the Related Art Among exhaust gas discharged from a gasoline engine, nitrogen oxides, carbon monoxide and hydrocarbons which are harmful to the human body are mainly composed of Pt, Pd and Rh supported on a carrier. Removed by the original catalyst.

[0003] In recent years, as environmental problems such as global warming have been highlighted, gasoline engines and diesel engines of the lean burn system or the direct injection burn system have been spread to reduce carbon dioxide emissions. . Since these engine exhaust gases contain excess oxygen, it has been difficult to remove nitrogen oxides using a conventional three-way catalyst.

[0004] In order to solve this problem, there has been proposed a catalyst for reducing and removing nitrogen oxides from exhaust gas containing excess oxygen. For example, Japanese Patent Application Laid-Open No. 1-135541 discloses Pt, Pd, R
An exhaust gas purifying catalyst comprising a zeolite ion-exchanged with one or more metals selected from h, Ir, and Ru. In JP-A-3-232533, Pt, Pd and Rh are supported on a zeolite. exhaust gas purification catalyst, exhaust gas purification catalyst made of SiO ₂ / Al ₂ O ₃ molar ratio in JP-6-198190 discloses that was contained by ion exchange of Zn and Pt is at least 15 or more ZSM-5 zeolite, JP No. 6,198,192 discloses F
SiO ₂ / A containing e and Pt by ion exchange
Exhaust gas purifying catalysts comprising ZSM-5 zeolite having a l ₂ O ₃ molar ratio of at least 15 have been proposed. JP-A-6-238131 discloses, in a method for removing nitrogen oxides, a catalyst in which zeolite having a SiO ₂ / Al ₂ O ₃ molar ratio of at least 15 or more contains phosphorus and one or more active metals. I have. However, since the nitrogen oxide removal catalyst disclosed above has insufficient durability, once exposed to a high temperature, the catalyst performance is greatly reduced. For this reason, it has not yet been put to practical use.

In consideration of both nitrogen oxide removal performance and durability, in the purification of exhaust gas of a lean burn engine, an alkaline earth metal and Pt as disclosed in Japanese Patent Application Laid-Open No. 5-317652 are used. Nitrogen oxides are adsorbed on alkaline earth metals in an oxygen-excess atmosphere using a catalyst supported on a porous carrier such as alumina, and a stoichiometric (stoichiometric air-fuel ratio) or rich (reducing atmosphere) atmosphere is obtained by periodic engine control. A method of reducing and removing nitrogen oxides has been adopted.

[0006]

However, in a diesel engine, the oxygen concentration in the exhaust gas is high, and it is difficult to operate in a stoichiometric or rich exhaust gas atmosphere by engine control such as that performed in the lean burn system. That is, Japanese Patent Application Laid-Open No.
When a catalyst such as that proposed in Japanese Patent No. 17256 is used, purification can be performed by adsorption of nitrogen oxides in an initial state, but nitrogen oxides cannot be removed from the time when adsorption reaches saturation. . Further, in the removal of nitrogen oxides in an oxygen-excess atmosphere, the catalyst containing Pt proposed above is not desirable because it involves generation of a large amount of nitrous oxide.

SUMMARY OF THE INVENTION An object of the present invention is to solve the above-mentioned problems of the prior art, and to efficiently remove nitrogen oxides from a gas even after the catalyst is exposed to a high temperature. Another object of the present invention is to provide a gas purifying catalyst and a gas purifying method for converting the gas into harmless nitrogen.

[0008]

Means for Solving the Problems As a result of intensive studies on the above-mentioned problems to be solved, the present inventor has found that in gas purification for removing nitrogen oxides from a gas containing both nitrogen oxides and sulfur oxides, Zeolite having an MFI structure having a SiO ₂ / Al ₂ O ₃ molar ratio of 20 or more and less than 200 has P
By using a catalyst containing at least one noble metal selected from t, Pd, Rh and Ir and P, the activity of reducing and removing nitrogen oxides is high even after the catalyst is exposed to high temperature, and Discovered that the production of nitric oxide was suppressed,
The present invention has been completed.

That is, according to the present invention, zeolite having an MFI structure having a SiO ₂ / Al ₂ O ₃ molar ratio of 20 or more and less than 200 contains at least one or more selected from Pt, Pd, Rh and Ir and P. It is a gas purification catalyst characterized by having been performed. Further, the present invention is a gas purification method, which comprises contacting a gas with the above-mentioned gas purification catalyst. Hereinafter, the present invention will be described in detail.

It is essential that the exhaust gas purifying catalyst of the present invention is composed of an MFI-structured zeolite. Generally MF
The zeolite structure _{I, xM n / 2 O · Al} 2 O 3 · ySiO 2 · zH 2 O ( where, n is the valence of the cation M, x is a number in the range of 0 to 1.5, y is (The number is 20 or more, and z is 0 or more). Regarding its structure, for example, US Pat.
No. 02886 and the like, and are defined as a structure having an X-ray diffraction pattern as shown in Table 1.

[0011]

[Table 1]

The zeolite having the MFI structure can be produced by a known method. Specifically, US Pat.
No. 86, JP-B-56-49851 and the like. The SiO ₂ / Al ₂ O ₃ molar ratio of the zeolite is 20 or more and less than 200. If the SiO ₂ / Al ₂ O ₃ molar ratio is less than 20, the heat resistance of the zeolite itself will be low, and if the catalyst is exposed to a high temperature, the nitrogen oxide removal activity will be reduced. Although the details are unknown when the SiO ₂ / Al ₂ O ₃ molar ratio is 200 or more, the performance of the catalyst of the present invention is not sufficiently exhibited.

Although the crystal size of the zeolite used in the present invention is not particularly limited, it is preferably 0.1 μm or more and 10 μm or more.
μm or less, and more preferably 1 μm or more and 5 μm or less. In general, the crystal size of zeolite can be measured using a Coulter counter, microtrack measurement, and electron microscopic observation (S
(EM, TEM) or the like. The particle size evaluated in the present invention is an average primary particle size observed by SEM, and the primary particle size means a minimum unit crystal size of zeolite crystals.

The zeolite used in the present invention may contain an alkali metal, an alkaline earth metal, a rare earth metal and the like. It is also possible to use an NH ₄ type treated with an ammonium salt or the like, or an H type obtained by an acid treatment with a mineral acid or the like or an NH ₄ type baking treatment. Preferably NH ₄
It is desirable to use a mold and an H-shape.

[0015] The catalyst according to the present invention comprises the above zeolite with P
It is manufactured by containing at least one or more selected from t, Pd, Rh, and Ir and P.

There are no particular restrictions on the raw materials used for the inclusion of the noble metal selected from Pt, Pd, Rh, and Ir. Can be. As a method for incorporating a noble metal into the above-mentioned zeolite, generally known ion exchange methods, impregnation-supporting methods, evaporation to dryness methods, physical mixing methods and the like can be adopted, and preferably, ion-exchange methods and impregnation-supporting methods are desirable.

There are no particular restrictions on the raw materials used to contain P. Phosphate compounds such as orthophosphoric acid, metaphosphoric acid, pyrophosphoric acid, diammonium hydrogen phosphate, and ammonium ammonium phosphate, and phosphorus phosphites such as trimethyl phosphite. Acid compounds may be used, and these phosphorus compounds may be used in combination. The method for containing P is not particularly limited.
For example, P can be contained on zeolite by an impregnation-supporting method or an evaporation-drying method using a solution in which a phosphorus compound is dissolved in a suitable solvent such as water. Further, P can be contained by a gas phase treatment method in which a gas containing a phosphorus compound is brought into contact with zeolite, or by physical mixing of the phosphorus compound.

The order in which the noble metal and P are contained is not limited. The noble metal and P may be contained at the same time, or one of them may be contained and then the other. In order to further enhance the activity of removing nitrogen oxides, it is desirable to add a noble metal after adding P.

The contents of the noble metal and P contained in the zeolite are not particularly limited. However, in order to sufficiently enhance the performance of removing nitrogen oxides and the durability and the effect of suppressing the generation of nitrous oxide, the content of zeolite is not limited. And noble metal is 0.1 ~
10 wt% and P may be in the range of 0.05 to 30 wt%.
More preferably, the noble metal is in the range of 0.1 to 5% by weight,
Is in the range of 0.1 to 15% by weight.

The zeolite containing the noble metal and P by the above method may be used after heat treatment such as firing. The heat treatment conditions are not particularly limited. Usually 400 to 1200
The heat treatment can be performed at a temperature in the range of ° C. for a time in the range of 0.5 to 20 hours. Preferably the temperature is 800-100
0 ° C. and time is 0.5 to 10 hours. The atmosphere for the heat treatment is not particularly limited, and the treatment can be performed in an atmosphere in which an inert gas such as air, nitrogen, or helium or a mixture of these gases is used. Further, water, hydrocarbons, nitrogen oxides, and sulfur oxides may be contained in the atmosphere gas.

The timing of the heat treatment step is not particularly limited.
A heat treatment may be performed after containing P and a noble metal,
The heat treatment may be performed after containing either one of the elements. More preferably, it is desirable to perform a heat treatment after containing P and then to contain a noble metal. This is because, if the catalyst of the present invention is produced in such an order, a catalyst having improved hydrocarbon adsorption performance can be obtained.

As described above, the gas purification catalyst of the present invention can be manufactured. The catalyst of the present invention can be mixed with a binder such as silica, alumina, and clay minerals, and then molded and used. Examples of the clay mineral include kaolin, attapulgite, montmorillonite, bentonite, allophane, sepiolite and the like. Further, it can be used by wash-coating a cordierite or metal honeycomb substrate. In the case of wash coating, either a method of coating a honeycomb substrate with a zeolite and then containing a noble metal and P, or a method of previously containing the above elements in a zeolite and then coating a honeycomb substrate may be employed. .

By bringing the gas into contact with the gas purification catalyst of the present invention, nitrogen oxides in the gas can be removed, and furthermore, it can be converted into harmless nitrogen. The gas purification catalyst of the present invention is particularly effective for exhaust gas containing excess oxygen. Exhaust gas with an excess of oxygen refers to an exhaust gas containing an excess amount of oxygen that is necessary to completely oxidize hydrocarbons contained in the exhaust gas. Examples of such an exhaust gas include a diesel engine and the like. Exhaust gas discharged from the internal combustion engine, particularly exhaust gas burned in a state where the air-fuel ratio is large, is specifically exemplified. Further, the gas may contain hydrocarbon, carbon monoxide, carbon dioxide, hydrogen, nitrogen, sulfur compounds, and water.

The type of hydrocarbon contained in the gas to be treated in the present invention is not particularly limited. Examples of the paraffin and olefin include hydrocarbons having 1 to 20 carbon atoms, and benzene, naphthalene and Derivatives can be exemplified. In addition, the above paraffin, olefin,
A mixture of two or more hydrocarbons selected from aromatic compounds can be used, and light oil, kerosene, gasoline and the like can also be used.

The concentration of each component in the gas is not particularly limited.
00 ppm, sulfur oxides are 0.1 to 1000 ppm, hydrocarbons are 10 to 10000 ppmC (in terms of methane), and oxygen is 0.1 to 20%. In order to further enhance the activity of removing nitrogen oxides and the activity of converting nitrogen, the above-mentioned appropriate hydrocarbons may be added to the gas.

The space velocity and temperature of the gas to be treated are not particularly limited, but are preferably space velocity (volume basis): 5
00 to 500,000 hr ⁻¹ , temperature: 100 to 800 ° C.,
More preferably, space velocity: 2000 to 300,000 hr
^-1 and a temperature of 100 to 600 ° C.

[0027]

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

Example 1 Preparation of Catalyst 1 100 g of an MFI-structured zeolite having a molar ratio of SiO ₂ / Al ₂ O ₃ of 40 and a particle diameter of 4 μm was prepared using NH ₄ Cl:
45 g was added to an aqueous solution of ammonium chloride dissolved in 1000 g of pure water, and an ion exchange operation was performed at 60 ° C. for 20 hours. After repeating this ion exchange operation twice, the solid-liquid separation was performed, and the solid was washed with pure water until Cl ions could no longer be detected, and dried at 110 ° C. for 20 hours.
I (NH ₄ -MFI-1) was obtained.

NH ₄ -MFI-1: 20 g (in terms of anhydrous)
Was added to an aqueous solution in which 1 g of diammonium hydrogen phosphate was dissolved in 100 g of pure water, and evaporated under reduced pressure at 60 ° C. to dryness. The P-containing MFI was dried at 100 ° C. for 20 hours and calcined in dry air at 500 ° C. for 1 hour. Subsequently, 10 g of the calcined P-containing MFI was added to Pt (N
H ₃ ) ₄ Cl ₂ .H ₂ O: 0.31 g was added to an aqueous solution of tetraammineplatinum dichloride dissolved in 100 g of pure water, and an ion exchange operation was performed at 30 ° C. for 2 hours. Thereafter, the mixture was separated into solid and liquid, washed with pure water until Cl ions could not be detected, and dried at 110 ° C. for 20 hours. The MFI containing P and Pt was calcined in dry air at 500 ° C. for 1 hour to obtain Catalyst 1. When the catalyst 1 was analyzed by ICP emission _{spectrometry, 0.21PtO · 0.35P 2 O 5 ·} Al 2 O 3 · 40Si oxide-based anhydrous
It had a composition of O ₂ .

Example 2 Preparation of Catalyst 2 Instead of the MFI-structured zeolite having a SiO ₂ / Al ₂ O ₃ molar ratio of 40 and a particle size of 4 μm, SiO ₂ / Al ₂ O was used.
Catalyst 2 was obtained in the same manner as in Example 1, except that zeolite having an MFI structure with a ₃ molar ratio of 24 and a particle diameter of 0.2 μm was used. When the catalyst 2 was analyzed by ICP emission _{spectrometry, 0.16PtO · 0.29P 2 O 5 ·} Al 2 O 3 · 24Si oxide-based anhydrous
It had a composition of O ₂ .

Example 3 Preparation of Catalyst 3 The same operation as in Example 1 was carried out by using a zeolite having an MFI structure having a SiO ₂ / Al ₂ O ₃ molar ratio of 70 and a particle diameter of 1 μm. 3 was obtained. When the catalyst 3 was analyzed by ICP emission _{spectrometry, 0.38PtO · 0.62P 2 O 5 ·} Al 2 O 3 · 70Si oxide-based anhydrous
It had a composition of O ₂ .

Example 4 Preparation of Catalyst 4 A mixture prepared by dissolving 10 g of NH ₄ -MFI-1 prepared in Example 1, 1 g of diammonium hydrogen phosphate and 0.31 g of tetraammineplatinum dichloride in 100 g of pure water. It was added to an aqueous solution and evaporated to dryness under reduced pressure at 60 ° C. Thereafter, the resultant was dried at 110 ° C. for 20 hours, and calcined in dry air at 500 ° C. for 1 hour to obtain Catalyst 4. When the catalyst 4 was analyzed by ICP emission _{spectrometry, 0.21PtO · 0.48P 2 O 5 ·} Al 2 O 3 · 40Si oxide-based anhydrous
It had a composition of O ₂ .

<Example 5> Preparation of catalyst 5 The calcination after adding P to the zeolite having the MFI structure was performed as follows.
85 in humidified air containing 10% of water by volume
Catalyst 5 was obtained by performing the same operation as in Example 1 except that the heat treatment was changed to 0 ° C. for 5 hours. When the catalyst 5 was analyzed by ICP emission _{spectrometry, 0.21PtO · 0.35P 2 O 5 ·} Al 2 O 3 · 40Si oxide-based anhydrous
It had a composition of O ₂ .

Example 6 Preparation of Catalyst 6 A catalyst 6 was obtained in the same manner as in Example 1, except that 0.115 g of tetraamminepalladium dichloride was used instead of tetraammineplatinum dichloride. When the catalyst 6 was analyzed by ICP emission _{spectrometry, 0.11PdO · 0.37P 2 O 5 ·} Al 2 O 3 · 40Si oxide-based anhydrous
It had a composition of O ₂ .

Comparative Example 1 Preparation of Comparative Catalyst 1 The same operation as in Example 1 was carried out using a zeolite having a MOR structure having a SiO ₂ / Al ₂ O ₃ molar ratio of 26 and a particle diameter of 1 μm. Comparative catalyst 1 was obtained. Comparative catalyst 1 with ICP
It was analyzed by emission _{analysis, 0.14PtO · 0.26P 2 O 5 ·} Al 2 O 3 · 26Si oxide-based anhydrous
It had a composition of O ₂ .

Comparative Example 2 Preparation of Comparative Catalyst 2 The SiO ₂ / Al ₂ O ₃ molar ratio was 27 and the particle diameter was 0.2 μm.
Comparative Catalyst 2 was obtained by performing the same operation as in Example 1 using the zeolite having the BEA structure as follows. Comparative catalyst 2
It was analyzed by CP emission _{spectrometry, 0.15PtO · 0.30P 2 O 5 ·} Al 2 O 3 · 27Si oxide-based anhydrous
It had a composition of O ₂ .

Comparative Example 3 Preparation of Comparative Catalyst 3 10 g of NH ₄ -MFI-1 obtained in Example 1 was added to P
t (NH ₃ ) ₄ Cl ₂ .H ₂ O: 0.31 g to pure water 100 g
Was added to an aqueous solution of tetraammineplatinum dichloride dissolved in water, and an ion exchange operation was performed at 30 ° C. for 2 hours.
Thereafter, the mixture was separated into solid and liquid, washed with pure water until Cl ions could not be detected, and dried at 110 ° C. for 20 hours. The dried Pt-containing MFI was calcined in dry air at 500 ° C. for 1 hour to obtain Comparative Catalyst 3. The comparative catalyst 3 was analyzed by ICP emission spectrometry, it had the oxide-based composition of _{0.21PtO · Al 2 O 3 · 40SiO} 2 in anhydrous.

<Comparative Example 4> Preparation of Comparative Catalyst 4 NH ₄ -MFI-1 obtained in Example 1 was used in an amount of 10 g.
2 wt% diammonium hydrogen phosphate aqueous solution: 70 cc
And then dried at 110 ° C. for 20 hours. Then
Calcination was performed at 550 ° C. for 5 hours in dry air to obtain a P-containing MFI.

This P-containing MFI was added to a 0.1 mol / L aqueous copper acetate solution (41 cc), adjusted to pH = 10.5 with aqueous ammonia, and stirred at room temperature for 20 hours.
After washing, a Cu ion exchange operation was performed. After repeating this operation twice, it was dried and calcined in dry air at 500 ° C. for 1 hour to obtain Comparative Catalyst 4. When the comparative catalyst 4 was analyzed by ICP emission _{spectrometry, 1.05CuO · 0.41P 2 O 5 ·} Al 2 O 3 · 40Si oxide-based anhydrous
It had a composition of O ₂ .

<Comparative catalyst 5> Preparation of comparative catalyst 5 Comparative catalyst 5 was obtained in the same manner as in Comparative Example 3, except that 0.115 g of tetraamminepalladium dichloride was used instead of tetraammineplatinum dichloride. . Comparative Catalyst 5 was analyzed by ICP emission spectrometry, had the oxide-based composition of _{0.11PdO · Al 2 O 3 · 40SiO} 2 in anhydrous.

<Catalyst Activity Test I> The catalysts 1 to 6 and the comparative catalysts 1 to 5 were each press-molded, pulverized and sized to 12 to 20 mesh. 1 g of each of the sized catalysts was filled in a normal-pressure fixed-bed flow-type reaction tube, and supplied to the reaction. Table 2 shows the composition of the reaction gas.
Shown in As a reaction pretreatment, the reaction gas was 4000 mL /
The temperature was raised to 550 ° C. while flowing the mixture for min and maintained for 30 minutes. Thereafter, at an arbitrary temperature of 150 to 550 ° C., the steady-state activity of the catalyst was examined. Table 3 shows the NOx removal rate and N ₂ O
Shows the generation rate. The space velocity (volume basis) at this time is 1
It was 20,000 hr ^-1 . The NOx removal rate and N ₂
The O generation rate is represented by the following equation.

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 N ₂ O generation rate = ｛([N ₂ O] _out × 2) / [NO
x] _in ｝ × 100 [N ₂ O] _out : N ₂ O concentration of outlet gas

[0043]

[Table 2]

[0044]

[Table 3]

<Catalyst Durability Test> After each of the catalysts 1 to 6 and the comparative catalysts 1 to 5 were molded under pressure, they were pulverized and sized to 12 to 20 mesh. 1 g of each of the sized catalysts was filled into a normal-pressure fixed-bed flow-type reaction tube and subjected to a catalyst durability test. For the catalyst durability test, a mixture gas containing Air gas containing H ₂ O and SO ₂ at a volume conversion of 10% and 25 ppm, respectively, was passed through the catalyst at a flow rate of 200 mL / min.
After endurance treatment at 600 ° C. for 50 hours, the steady state activity of the catalyst was examined under the same pre-reaction treatment and activity evaluation conditions as in <Catalytic Activity Test I>. Table 4 shows the NOx removal rate and N after the durability test.
_{The 2} O generation rate is shown.

[0046]

[Table 4]

<Catalyst Activity Test II> In the catalyst durability test, the catalyst was treated under the same conditions as in the <catalyst durability test> except that no sulfur oxide was added to the processing gas. The steady-state activity was evaluated under the same conditions as in <Catalytic activity test I> using a reaction gas from which sulfur oxides were excluded from the exhaust gas. Table 5 shows the NOx removal rate and N ₂ O generation rate after the endurance of the catalyst 1.

[0048]

[Table 5]

As can be seen from Tables 3 and 4, the catalyst according to the present invention is different from the catalysts which have been proposed so far.
Small decrease in activity after running. Furthermore, from Tables 4 and 5, even when the catalyst of the present invention is used for exhaust gas in which nitrogen oxides and sulfur oxides coexist, the amount of N ₂ O generated is small. It can be seen that is higher removed conversion activity to harmless N ₂ of NOx.

[0050]

By using the catalyst according to the present invention,
Nitrogen oxides in the gas can be efficiently removed, and in particular, more nitrogen oxides can be converted to harmless N ₂ . This effect is sufficiently exerted even after the catalyst is exposed to high temperatures, and even when sulfur oxides coexist in the processing gas. This is because Pt, Pd, R
This is considered to be the effect of using at least one or more selected from h and Ir and P in combination. Further, according to the present invention, nitrogen oxides can be efficiently removed from exhaust gas which is always in an oxygen-excess atmosphere such as a diesel engine. In addition, in the lean-burn exhaust gas purification as described above, the nitrogen oxides in the exhaust gas can be removed without adjusting the exhaust gas atmosphere (stoichiometric and rich atmosphere) by engine control. The system is simplified.

Claims (3)

[Claims] (1) The molar ratio of SiO ₂ / Al ₂ O ₃ is 20 or more and 20 or more.
Zeolite having an MFI structure that is less than 0 includes Pt,
A gas purification catalyst characterized by containing at least one selected from Pd, Rh, and Ir and P. 2. After the P is contained in the zeolite, 800-
Heat treatment at a temperature of 1000 ° C., and then Pt, Pd,
The gas purification catalyst according to claim 1, wherein the catalyst is produced by containing at least one selected from Rh and Ir. 3. A gas purification method comprising bringing a gas into contact with the gas purification catalyst according to claim 1 or 2.