JPH11319572A - Gas purifying catalyst and gas purifying process - Google Patents

Abstract

PROBLEM TO BE SOLVED: To remove efficiently Nox by using a porous carrier containing precious metals such as Pt and Pd, and an adsorbent containing P is zeolite of MFI structure having SiO2 /Al2 O3 molar ratio in the specified range in a catalyst for removing Nox in exhaust gas. SOLUTION: A gas purifying catalyst is prepared by using a porous carrier containing at least one kind of Pt, Pd, Rh and Ir and an adsorbent containing P in zeolite and having the MFI structure with SiO2 /Al2 O3 of 20-200 molar ratio. In this case, the particle diameter of zeolite to be used is 0.1-10 μm, and the absorbent is heat treated preferably at 800-1000 deg.C. As the porous carrier, it is not particularly limited so long as the catalyst contains Pt, Pd, Rh and Ir, however alumina and zeolite having a SiO2 /Al2 O3 molar ratio of at least 100 is preferred from the viewpoint of heat resistance, and mordenite having a SiO2 /Al2 O3 molar ratio of at least 100 is more preferred.

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 from exhaust gas discharged from an internal combustion engine of an automobile or the like, and to a method for removing nitrogen oxides from an exhaust gas. .

[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, while the problems of resources and environmental problems of global warming have been highlighted, the spread of lean-burn or direct-injection gasoline engines and diesel engines has been increasing in order to reduce carbon dioxide emissions. Have been. Since these engine exhaust gases contain excess oxygen, it is difficult to remove nitrogen oxides using a conventional three-way catalyst.

[0004] As a method for removing nitrogen oxides from an exhaust gas having an excess of oxygen, V ₂ O ₅ / Ti using ammonia as a reducing agent is used.
A selective reduction method on O ₂ and an absorption method in an alkaline solution are known. However, ammonia is a poison specified by law and has a high danger, and it is difficult to apply it to a mobile object such as an automobile. On the other hand, it is difficult to apply the absorption method to mobile objects such as general automobiles in terms of operability such as replenishment and treatment of an alkaline solution.

[0005] Hitherto, a purification method using a catalyst has been proposed for purifying exhaust gas under an excess of oxygen, and various catalysts have been disclosed. For example, as a reducing agent and methanol or ethanol in JP-1-135541 discloses, H type zeolite, Al ₂ O _3, a method of using one or more catalysts selected from the fourth period transition metal supported Al ₂ O ₃ Proposed.

[0006] Also, Japanese Patent Application Laid-Open No. 1-135541,
In Japanese Unexamined Patent Publication No. Hei 3-232533, a zeolite containing a noble metal is disclosed.
Olite catalysts are disclosed in JP-A-6-315635.
Ib, IIa, IIb, IIIa, IIIb, IV
a, IVb, Va, VIIa, at least of group VIII
A catalyst in which one type is supported on H-type zeolite or the like is disposed, and the inner layer
Catalyst in which a catalyst component composed of a noble metal is disposed
In Japanese Patent No. 327980, the outer layer is made of H-type zeolite or the like.
A medium is arranged, and a catalyst component composed of a noble metal is arranged in the inner layer.
Catalysts, disclosed in JP-A-6-327978, are treated with HCl.
The modified zeolite and the group VIIa, iron group, platinum group
A catalyst comprising a supported ceramic carrier is disclosed
I have. Further, the present inventors have previously described Pt, Pd, Ir and R
h. containing at least one active metal selected from the group consisting of
Porous carrier and SiO_Two/ Al_TwoO _ThreeMolar ratio of 20 or more 2
Proposing a catalyst comprising less than 00 MFI type zeolite
(Japanese Patent Application No. 9-230730).

[0007] However, the catalysts disclosed so far have insufficient durability, and once the catalyst is exposed to a high temperature, the catalytic activity is greatly reduced. For this reason, it has not yet been put to practical use.

[0008]

SUMMARY OF THE INVENTION An object of the present invention is to solve the problems of the prior art described above, and it is possible to remove nitrogen oxides from gas even after the catalyst is exposed to high temperatures. It is an object of the present invention to provide a gas purifying catalyst and a gas purifying method for removing gas efficiently.

[0009]

The inventor of the present invention has made intensive studies on the problems to be solved and found that Pt,
From a porous carrier containing at least one noble metal selected from Pd, Rh, and Ir, and an adsorbent containing P in a zeolite having an MFI structure having a SiO ₂ / Al ₂ O ₃ molar ratio of 20 to less than 200 It has been found that such a catalyst has a high activity of removing nitrogen oxides, has a small decrease in activity after being exposed to a high temperature, and has excellent durability, and has completed the present invention.

That is, the present invention provides a porous support containing at least one selected from the group consisting of Pt, Pd, Rh, and Ir, and a zeolite having an MFI structure having a SiO ₂ / Al ₂ O ₃ molar ratio of 20 to less than 200. And a gas purifying catalyst comprising a P-containing adsorbent. Further, the present invention is a gas purification method, which comprises bringing the above-mentioned gas purification catalyst into contact with a gas. Hereinafter, the present invention will be described in detail.

The porous carrier constituting the gas purification catalyst of the present invention will be described. The porous carrier is composed of the above Pt, Pd,
There is no particular limitation as long as it can contain Rh and Ir, but oxides such as alumina, silica, titania and zirconia, and composite oxides such as silica alumina, silica zirconia, silica titania and titania zirconia and mordenite, ferrierite, Zeolites such as beta zeolite, MFI zeolite and Y zeolite can be used. Among them, alumina and SiO ₂ /
Al ₂ O ₃ molar ratio is preferably used in more than 100 zeolite, SiO ₂ / Al ₂ O ₃ molar ratio is more preferably 100 or more mordenite.

It is essential that the porous carrier contains at least one noble metal selected from Pt, Pd, Rh and Ir. When the noble metal is contained in the porous carrier, the stability is improved. The raw material used to contain the noble metal is not particularly limited, but ammine complex salts, chlorides, nitrates and the like can be suitably used. The method for containing the noble metal in the porous carrier is not particularly limited, and a known method such as an impregnation-supporting method, an evaporation to dryness method, a physical mixing method, or an ion exchange method can be employed. The amount of the noble metal contained in the porous carrier is not particularly limited, but is preferably 0.1% by weight or more based on the porous carrier in order to further enhance the activity of removing nitrogen oxides. Further, it is preferable that the porous carrier containing the noble metal is subjected to a heat treatment in order to stabilize the metal. The conditions for the heat treatment are not particularly limited.
It can be processed at a temperature of 0 to 1000C. 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. These gases may contain water, hydrocarbons, hydrogen, nitrogen oxides and sulfur oxides.

The adsorbent according to the present invention will be described. First, it is essential that the adsorbent according to the present invention is composed of zeolite having an MFI structure. And generally of MFI _{zeolite, 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 a number of 20 or more, and z is a number of 0 or more). Regarding its structure, for example, US Pat.
No. 702886 and the like, and are defined as a structure having an X-ray diffraction pattern as shown in Table 1.

[0014]

[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 used for the adsorbent according to the present invention 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 the catalyst activity will be reduced when exposed to high temperatures. SiO ₂ / Al ₂ O ₃ molar ratio of 200
Above, the details are unknown, but the catalytic performance is not sufficiently exhibited. In order to further enhance the activity of removing nitrogen oxides and its durability, the SiO ₂ / Al ₂ O ₃ molar ratio should be 20 to 50.
It is preferably less than.

The particle size of the zeolite used in the adsorbent according to the present invention is not particularly limited, but is preferably 0.1%.
A zeolite having a size of at least 10 μm and more preferably at least 1 μm and at most 5 μm can be used. In general, the particle size of zeolite can be measured using a Coulter counter, microtrack measurement, and electron microscope observation (SE
(M, TEM). In the present invention, S
The average primary particle diameter is evaluated by EM observation, and the primary particle diameter means the minimum unit crystal diameter of the zeolite crystal.

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

It is essential that the adsorbent according to the present invention contains P. Raw materials used for containing P are not particularly limited, and are phosphoric acid compounds such as orthophosphoric acid, metaphosphoric acid, pyrophosphoric acid, diammonium hydrogenphosphate, and ammonium ammonium phosphate, and phosphite compounds such as trimethyl phosphite. 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 a phosphorus compound. Although the content of P is not particularly limited, P may be in the range of 0.05 to 30% by weight with respect to the zeolite in order to sufficiently enhance the activity of removing nitrogen oxides and the durability.
More preferably, it is in the range of 0.1 to 15% by weight.

The zeolite containing P by the above method may be used after heat treatment such as calcination. The heat treatment conditions are not particularly limited. Usually, the heat treatment can be performed at a temperature in the range of 400 to 1200 ° C. for a time in the range of 0.5 to 20 hours. Preferably, the temperature is 800-1000 ° C. and the time is 0.5-10 hours. The atmosphere for the heat treatment is not particularly limited, and the treatment can be performed in an atmosphere of an inert gas such as air, nitrogen, helium, or a mixture of these gases. Further, water, hydrocarbons, nitrogen oxides, and sulfur oxides may be contained in the atmosphere gas. Preferably, the treatment is performed in humidified air containing water.

The gas purification catalyst of the present invention comprises the above-described porous carrier and adsorbent. The combination thereof is not particularly limited as long as the porous carrier and the adsorbent are in contact with each other. Or a method of mixing and using in a molded state, or wash-coating a molded body or a honeycomb-shaped substrate, and contacting a gas with a catalyst to flow a gas from a catalyst to a porous carrier and an adsorbent from an upstream side to a downstream side. And a method in which a porous carrier and an adsorbent are laminated and used. Among them, a method of mixing both with powder is preferably used.

The porous carrier and the adsorbent constituting the gas purification catalyst of the present invention can be used alone or in combination and then mixed with a binder such as silica, alumina, clay mineral or the like and molded. Examples of the clay mineral include clay minerals such as kaolin, attapulgite, montmorillonite, bentonite, allophane, and sepiolite. Further, the gas purification catalyst of the present invention may be wash-coated on a cordierite or metal honeycomb substrate and used. The prepared gas purification catalyst may be subjected to a heat treatment before use. The conditions at that time are not particularly limited, but the treatment can be performed at a temperature of 400 to 1000 ° C. The atmosphere for the heat treatment is not particularly limited, and the treatment can be performed with air, nitrogen, hydrogen, or a gas containing hydrogen.

The removal of nitrogen oxides from the gas can be carried out by bringing the gas containing nitrogen oxides into contact with the above-mentioned gas purification catalyst, and is particularly effective for exhaust gas containing excess oxygen. Oxygen-excessive exhaust gas refers to exhaust gas containing an excess amount of oxygen that is necessary to completely oxidize carbon monoxide, hydrogen, and hydrocarbons contained in the exhaust gas. Specific examples of the 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. Further, the gas may contain hydrocarbon, carbon monoxide, carbon dioxide, hydrogen, nitrogen, sulfur compounds, and water.

The kind of hydrocarbon contained in the gas to be treated in the present invention is not particularly limited. As paraffin and olefin, hydrocarbons having 1 to 20 carbon atoms can be used, and benzene, naphthalene and derivatives thereof can be used. Further, two or more kinds of hydrocarbons selected from the above paraffins, olefins, and aromatic compounds can be used as a mixture, and gas oil, kerosene, gasoline, and the like can also be used. The concentration of each component in the gas is not particularly limited. For example, in an exhaust gas, for example, usually 50 to 2000 ppm of nitrogen oxide, 10 to 10000 ppm of hydrocarbon (in terms of methane), and 0.1% of oxygen.
1 to 20%. In order to further enhance the activity of removing nitrogen oxides, the above-mentioned appropriate hydrocarbon may be added to the exhaust gas.

The space velocity and temperature of the exhaust gas to be treated are not particularly limited, but are preferably space velocity (based on volume):
500 to 500,000 hr ^-1 , temperature: 100 to 800
° C, more preferably space velocity: 2000-300000
hr ⁻¹ , temperature 100 to 600 ° C.

[0025]

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 Preparation of Pt-Containing Porous Carrier 1 Mordenite having a SiO ₂ / Al ₂ O ₃ molar ratio of 224: 1
After adding 00 g to a 1000 mL aqueous solution in which 2.9 g of tetraamminedichloroplatinum was dissolved, the pH was adjusted to 7 with an aqueous ammonia solution, followed by stirring at 30 ° C. for 2 hours. After that, solid-liquid separation, washing with a sufficient amount of pure water,
Dry at 110 ° C. for 20 hours, and further 500
Calcination was carried out at 1 ° C. for 1 hour to obtain a Pt-containing porous carrier 1. When this Pt-containing porous carrier 1 was analyzed by ICP emission spectrometry, the Pt content was 1.7% by weight.

Preparation of Adsorbent 1 40 g of an MFI-structured zeolite having a SiO ₂ / Al ₂ O ₃ molar ratio of 40 and a particle diameter of 4 μm was admixed with NH ₄ Cl: 1
The solution was added to a 400 mL aqueous solution in which 8 g was dissolved, and an ion exchange operation was performed at 60 ° C. for 20 hours. After repeating this ion exchange operation twice, solid-liquid separation is performed, washed with pure water until Cl ions cannot be detected, dried at 110 ° C. for 20 hours, and ammonium-type MFI (NH ₄ -MFI-1)
I got

NH ₄ -MFI-1: 20 g (in terms of anhydrous)
Was dissolved in 10 g of diammonium hydrogen phosphate: 1 g.
0 mL aqueous solution was added and evaporated to dryness under reduced pressure at 60 ° C.
Then, it is dried at 110 ° C. for 20 hours, and dried in dry air for 5 hours.
It was baked at 00 ° C. for 1 hour to obtain an adsorbent 1. Adsorber 1 is IC
Analysis from P emission spectroscopy, had a composition of _{0.48P 2 O 5 · Al 2 O} 3 · 40SiO 2 in oxide-based anhydrous. 1: 2 g of the Pt-containing porous carrier obtained by the above operation and 1: 2 g of the adsorbent were mixed in a mortar, pressed and molded, crushed and sized to 12 to 20 mesh to obtain Catalyst 1.

Example 2 Preparation of Catalyst 2 Preparation of Pt-Containing Porous Carrier 2 Alumina (ACP-1) manufactured by Catalyst Chemicals: 100
g was added to a 1000 mL aqueous solution in which 3.1 g of tetraamminedichloroplatinum was dissolved and supported by impregnation.
Dry at 0 ° C. for 20 hours. Then, under air circulation, 80
Calcination was carried out at 0 ° C. for 1 hour to obtain a Pt-containing porous carrier 2. Further, the Pt content was analyzed by ICP emission analysis, and was 1.7% by weight. Pt-containing porous carrier 2: 2
g and adsorbent 1: 2 g were mixed in a mortar and pressed and molded.
The powder was pulverized and sized to 12 to 20 mesh to obtain Catalyst 2.

Example 3 Preparation of Catalyst 3 The adsorbent 1 obtained in Example 1 was subjected to a heat treatment at 850 ° C. for 5 hours in humidified air containing 10% by volume of water.
And Pt-containing porous carrier 1: 2 g and adsorbent 2: 2 g
Are mixed in a mortar, pressed and molded, and then crushed to 12 to 20.
The granules were sized to obtain Catalyst 3.

<Example 4> Preparation of catalyst 4 Preparation of adsorbent 3 The procedure of Example 1 was repeated except that 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 was used. Adsorbent 3 was obtained by performing the same operation as for adsorbent 1. When the adsorbent 3 was analyzed from ICP emission spectrometry, in the oxide-based anhydrous had a composition of _{0.75P 2 O 5 · Al 2 O} 3 · 70SiO 2. 1: 2 g of the Pt-containing porous carrier and 3: 2 g of the adsorbent were mixed in a mortar, pressed and molded, and then crushed and sized to 12 to 20 mesh to obtain a catalyst 4.

Example 5 The same operation as that of the adsorbent 1 of Example 1 was carried out except that a zeolite having an MFI structure having a SiO ₂ / Al ₂ O ₃ molar ratio of 24 and a particle diameter of 0.2 μm was used. Was carried out to obtain an adsorbent 4. The adsorbent 4 was analyzed from ICP emission spectrometry, it had a composition of _{0.30P 2 O 5 · Al 2 O} 3 · 24SiO 2 in oxide-based anhydrous. 1: 2 g of the Pt-containing porous carrier and 4: 2 g of the adsorbent were mixed in a mortar, pressed and molded, and then crushed and sized to 12 to 20 mesh to obtain a catalyst 5.

Comparative Example 1 Preparation of Comparative Catalyst 1 The adsorbent 1 of Example 1 was used except that a zeolite having an MOR structure having a SiO ₂ / Al ₂ O ₃ molar ratio of 26 and a particle diameter of 1 μm was used. By performing the same operation, the adsorbent 5 was obtained. When the adsorbent 5 was analyzed from the ICP emission _{spectrometry, 0.32P 2 O 5 · Al 2} O 3 · 26SiO 2 in oxide-based anhydrous
Having the following composition: 1: 2 g of the Pt-containing porous carrier and 5: 2 g of the adsorbent were mixed in a mortar, pressed and molded, and then crushed and sized to 12 to 20 mesh to obtain Comparative Catalyst 1.

Comparative Example 2 Preparation of Comparative Catalyst 2 The SiO ₂ / Al ₂ O ₃ molar ratio was 27 and the particle size was 0.2 μm.
The adsorbent 6 was obtained by performing the same operation as the adsorbent 1 of Example 1 except that the zeolite having a BEA structure of m was used. The adsorbent 6 was analyzed from ICP emission spectrometry, it had a composition of _{0.33P 2 O 5 · Al 2 O} 3 · 27SiO 2 in oxide-based anhydrous. 1: 2 g of the Pt-containing porous carrier and 6: 2 g of the adsorbent 6 were mixed in a mortar, pressed and molded, crushed and sized to 12 to 20 mesh to obtain Comparative Catalyst 2.

Comparative Example 3 Preparation of Comparative Catalyst 3 The NH ₄ -MFI-1 obtained in Example 1 was calcined in dry air at 500 ° C. for 1 hour to obtain an adsorbent 7. Pt-containing porous carrier 1: 2 g and adsorbent 7: 2 g were mixed in a mortar, pressed and molded, crushed and sized to 12 to 20 mesh,
Comparative catalyst 3 was obtained.

Comparative Example 4 Preparation of Comparative Catalyst 4 The SiO ₂ / Al ₂ O ₃ molar ratio was 2100 and the particle size was 1 μm.
The adsorbent 8 was obtained by performing the same operation as the adsorbent 1 of Example 1 except that the zeolite having the MFI structure of m was used. The adsorbent 8 was analyzed from ICP emission spectrometry, it had a composition of _{24.1P 2 O 5 · Al 2 O} 3 · 2100SiO 2 oxide-based anhydrous. 1: 2 g of the Pt-containing porous carrier and 8: 2 g of the adsorbent 8 were mixed in a mortar, pressed and molded, crushed and sized to 12 to 20 mesh to obtain Comparative Catalyst 4.

<Comparative Example 5> Preparation of Comparative Catalyst 5 The Pt-containing porous carrier 1 obtained in Example 1 was calcined for 1 hour at 500 ° C. in an air flow, pressed and then pulverized. The mixture was sized to 20 mesh to obtain Comparative Catalyst 5.

<Catalyst activity test> With respect to catalysts 1 to 5 and comparative catalysts 1 to 5, 1 g of each catalyst was filled in a normal-pressure fixed-bed flow-type reaction tube and subjected to a reaction. Table 2 shows the composition of the reaction gas. The reaction gas is 4000 mL / mi as a pre-reaction treatment.
n and raise the temperature of the catalyst layer to 550 ° C.
Hold for 30 minutes. Thereafter, the temperature was lowered to 100 ° C. and maintained for 1 hour. Then, while the temperature of the catalyst was raised at a rate of 10 ° C./min, the exhaust gas composition at the catalyst inlet and outlet was continuously analyzed to examine the nitrogen oxide removing activity. . Table 3 shows 150-3
The NOx removal rate in the range of 00 ° C. is shown. The space velocity (volume basis) at this time was 120,000 hr ^-1 . still,
The NOx removal rate is expressed by the following equation.

NOx removal rate = ｛([NOx] _in − [NO
x] _out ) / [NOx] _in ｝ × 100 [NOx] _in : NO flowing into the catalyst at 150 to 300 ° C.
x amount [NOx] _out : NOx amount flowing out of the catalyst at 150 to 300 ° C

[0040]

[Table 2]

[0041]

[Table 3]

<Catalyst Durability Test> With respect to Catalysts 1 to 5 and Comparative Catalysts 1 to 5, 1 g of each catalyst was filled in a normal-pressure fixed-bed flow type reaction tube and subjected to a catalyst durability test. The catalyst durability test was carried out by flowing a mixed gas containing 10% and 25 ppm by volume of H ₂ O and SO ₂ in Air gas at a flow rate of 200 mL / min.
After a durability treatment at 00 ° C. for 100 hours, NOx removal activity was examined under the same reaction pretreatment and activity evaluation conditions as in <Catalytic Activity Test>. Table 4 shows the NOx removal rate after the durability test.

[0043]

[Table 4]

As is clear from Tables 3 and 4, the exhaust gas purifying catalyst of the present invention has a higher activity of removing nitrogen oxides than the comparative catalyst, and furthermore has a lower activity of removing nitrogen oxides in a durability test. It turns out that it is small and has excellent durability.

[0045]

By using the exhaust gas purifying catalyst of the present invention, nitrogen oxides can be efficiently removed from exhaust gas containing nitrogen oxides even after the catalyst is exposed to a high temperature.

Claims (5)

[Claims] 1. A porous carrier containing at least one selected from the group consisting of Pt, Pd, Rh, and Ir, and SiO ₂ / A
A gas purification catalyst comprising a zeolite having an MFI structure having a l ₂ O ₃ molar ratio of 20 or more and less than 200 and comprising a P-containing adsorbent. 2. The gas purification catalyst according to claim 1, wherein the zeolite having an MFI structure constituting the adsorbent has a molar ratio of SiO ₂ / Al ₂ O _{3 of} 20 to less than 50. 3. The gas purification catalyst according to claim 1, wherein the zeolite having an MFI structure constituting the adsorbent has a particle diameter of 0.1 μm or more and 10 μm or less. 4. The gas purification catalyst according to claim 1, wherein the adsorbent is heat-treated at 800 to 1000 ° C. 5. A gas purification method comprising bringing the gas purification catalyst according to claim 1 into contact with a gas.