JPH09299806A - Catalyst for nox-containing waste gas purification and its purification method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To remarkably improve the efficiency of NOx removal from a waste gas in a wide temperature range by using a NOx-containing waste gas purification catalyst produced by depositing Pd on a clinoptilolite carrier. SOLUTION: A catalyst for purification of NOx containing combustion gas is produced by depositing Pd on clinoptilolite and the quantity of Pd to be deposited on clinoptilolite is so controlled to be 0.1wt.% or more of Pd in the sum weight of the catalyst containing clinoptilolite. Synthesized clinoptilolite is used as the clinoptilolite carrier. Such a catalyst for purification of a NOx- containing waste gas is produced by depositing Pd on clinoptilolite by ion exchange using an aqueous solution of nitrate, chloride, acetate, or a complex of Pd.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a NOx-containing exhaust gas purifying catalyst and a purifying method thereof, and more specifically to Nx.
Ox-containing exhaust gas is added to clinoptilolite-type zeolite
NOx-containing exhaust gas purifying catalyst supporting d, method for producing the same, and NO containing oxygen and water vapor using this catalyst
The present invention relates to a method for purifying x-containing exhaust gas.

[0002]

Exhaust gas emitted from automobiles, aircraft, thermal power plants, various factories, etc. contains, in addition to carbon dioxide and water vapor produced by combustion of fuel, NOx, SOx, CO, odorous substances, dust, etc. Unburned hydrocarbons (H
C) is contained, and when incinerating industrial waste, municipal waste, etc., hydrogen chloride gas and the like are generated in addition to these substances, depending on the origin and composition of the waste.

Further, city gas and fuel gas containing methane, ethane, propane, etc. are used in gas engines, gas turbines, boilers, heating furnaces, etc.
In order to reduce the environmental load of the produced exhaust gas in them, or to increase combustion efficiency and thermal efficiency, the air ratio, that is, the ratio of air to fuel gas, is set to the fuel gas lean.
On the n) side, that is, a so-called lean combustion system in which the air amount is 1.0 to 5.0 times, in particular 1.0 to 3.0 times, the theoretical air amount required for complete combustion of the fuel gas with respect to the fuel gas is used. Has been applied.

The above point is that a so-called cogeneration system or GHP that produces electric power, mechanical energy and thermal energy from a single drive source (that is, a single energy source) and can use the energy with high efficiency. The same applies to a lean burn gas engine in a (Gas Heat Pump) system or the like. However, even when such a lean burn method is adopted, a large amount of oxygen, carbon dioxide and water vapor, as well as a small amount of unburned lower hydrocarbons (especially methane) and carbon monoxide in the exhaust gas,
A trace amount of nitrogen oxide (NOx) will be contained.

Regarding the treatment of NOx such as NO and NO ₂ among these components in various exhaust gases, so-called flue gas denitration technology, for example, non-catalytic reduction method, catalytic cracking method,
Various methods such as non-selective or selective catalytic reduction method, adsorption method, electron beam irradiation method, molten salt absorption method, reduction absorption method and the like are known. Of these, the catalytic reduction method of purifying using a catalyst for the treatment usually attracts particular attention because it finally changes NOx to N ₂ to make it harmless, but as the catalyst, Pt, Rh, Noble metals such as Pd, TiO ₂ , V ₂ O
There are various kinds of metal oxides such as ₅ , Cr ₂ O ₃ , Fe ₂ O _{3 and the} like, rare earth oxides, sulfides and the like, and a zeolite type is also known as one of them.

Zeolite has many kinds depending on the ratio of the alumina (Al ₂ O ₃ ) component and the silica (SiO ₂ ) component constituting the zeolite, its crystal structure, etc., and each has different characteristics. Some are used not only as catalysts themselves, but also as carriers for various catalysts. Among the various zeolites, the present inventors pay particular attention to mordenite, carry out research and development from various aspects to obtain a series of excellent results, and also develop a denitration method using ZSM-5. (JP-A-6-254352)
No., Japanese Patent Application No. 8-92029).

Of these, for example, in Japanese Patent Laid-Open No. 6-254352, ZSM-5 (= MFI type zeolite) is modified to H type, and a catalyst in which Pd is supported is used. Particularly, it is generated by a lean burn method. It is applied to the purification of combustion exhaust gas. Further, in Japanese Patent Application No. 8-92029, by using a catalyst in which Pd and Rh are supported on an H-type MFI-type zeolite, NOx in the NOx-containing exhaust gas is reduced to 3%.
It can be extremely effectively removed in a wide temperature range of 40 to 570 ° C., and since this catalyst has an excellent NOx removal effect even in the presence of oxygen and water, it is particularly effective for combustion exhaust gas from a lean burn system. It is valid.

Among the zeolites, clinoptilolite is a zeolite that is often found in natural sedimentary rocks, and is relatively abundantly produced in Japan. This naturally-occurring clinoptilolite is used for applications such as soil conditioners, adsorbents, and papermaking fillers, but almost no examples of using clinoptilolite as a catalyst or catalyst carrier are found. Further, it is considered very difficult to produce clinoptilolite by synthesis, and of course, a synthetic product is not commercially available. However, the present inventors have found that pure (K + Na) mixed ionic reaction mixture can be produced by stirring hydrothermal synthesis. It was previously found that clinoptilolite can be synthesized in the presence of seed crystals or without seed crystals. FIG.
Is a production example thereof, and the vertical axis in FIG. 1 is the X-ray diffraction peak (0
20) shows the relative intensity.

In FIG. 1, when seed crystals were added, the raw material composition was kept constant and the reaction temperatures were 180 ° C., 150 ° C. and 120 ° C.
The reaction temperature was changed to 150 ° C when the temperature was changed to 0 ° C, and when there was no seed crystal. As shown in FIG. 1, when hydrothermal synthesis was performed by adding 1 wt% of the synthetic solid clinoptilolite as a seed crystal to the reaction mixture as a seed crystal, a wide temperature range (120 to 180) was obtained as compared with the case where the seed crystal was not used. C) gives pure clinoptilolite. Even when the seed crystal is not used, the reaction time is longer than that in the case where the seed crystal is added, but similarly pure clinoptilolite is produced.
The thus-obtained synthetic clinoptilolite is particularly excellent in ion exchange property, and Pd can be uniformly supported by the ion exchange method as described later.

[0010]

Therefore, the present inventor focused on clinoptilolite as described above, and conducted various experiments and studies on the suitability of NOx-containing exhaust gas as a denitration catalyst. As a result, clinoptilolite was obtained. P for light type zeolite
By supporting d, from around 340 ° C to 500 ° C
It has been found that an excellent denitration rate can be obtained in a wide temperature range over the above range, and that this denitration effect is particularly remarkable when a synthetic clinoptilolite is used, and has arrived at the present invention. .

That is, according to the present invention, in the catalytic reduction denitration method for NOx-containing exhaust gas, by using a specific catalyst carrier called clinoptilolite among zeolites,
A NOx-containing exhaust gas purification catalyst, a method for producing the same, and a NOx-containing exhaust gas containing oxygen and water vapor that significantly improve the NOx removal rate in the exhaust gas over a wide temperature range from a temperature range of about 340 ° C to 500 ° C or higher. It is an object of the present invention to provide a method for purifying by using the catalyst, using unburned hydrocarbon (lower hydrocarbon, especially methane) in combustion exhaust gas.

[0012]

The present invention provides (1) a catalyst for purifying NOx-containing exhaust gas, which is characterized in that clinoptilolite is loaded with Pd. (2) Clinoptilolite is supported on Pd by ion-exchange with clinoptilolite by an aqueous solution of Pd in the form of nitrate, chloride, acetate or complex salt. A method for producing a catalyst for purifying NOx-containing exhaust gas, wherein Pd is supported on a light, and (3) NOx-containing exhaust gas containing oxygen and water vapor is contacted with a clinoptilolite catalyst comprising Pd. A method for purifying NOx-containing exhaust gas is provided.

[0013]

BEST MODE FOR CARRYING OUT THE INVENTION In the above catalyst for purifying NOx-containing exhaust gas, the amount of Pd supported on the clinoptilolite carrier is such that NOx in the exhaust gas exhibits an effective catalytic action. The catalyst is supported so that it is preferably 0.1 wt% or more, more preferably 0.5 wt% or more in the total amount of the catalyst containing lolite.

The method for producing the Pd-supported clinoptilolite catalyst according to the present invention is not particularly limited as long as it is a method capable of supporting Pd on clinoptilolite as uniformly as possible. It can be supported by an ion exchange method or an impregnation method, but an ion exchange method is more preferably applied. The mode will be described. First, Pd is used as an aqueous solution of its nitrate, chloride, acetate, complex salt (dichlorotetraamminepalladium, etc.) or other form (note that
In the case of a salt which is poorly soluble in water such as Pd acetate, for example, ammonia is added to be used as an aqueous solution), and clinoptilolite is added thereto for ion exchange or impregnation into the aqueous solution. Especially in the case of the ion exchange method, Pd can be adjusted by adding acetic acid and ammonia water to adjust the pH.
Can be carried more uniformly.

In the case of the ion exchange method, since Pd is carried in place of the cation component in the clinoptilolite, the clinoptilolite is obtained by the (K + Na) mixed ion reaction. For this purpose, it is desirable to apply the above-mentioned ion exchange method after previously exchanging K ions or Na ions in clinoptilolite with an aqueous solution of ammonium nitrate to form ammonia type or H type. After that, the Pd-supported clinoptilolite catalyst is obtained by drying and calcining according to a conventional method.

As the clinoptilolite, powders, granules, pellets and other shapes can be used as the raw materials as they are. However, in the case of powders or granules, the raw material stage or the manufacturing process, Alternatively, it is desirable to adjust the particle size and regulate the particle size in the final stage of the manufacturing process, whereby Pd can be carried uniformly and a homogeneous property as a catalyst can be secured.

The catalyst may be used in the form of powder, particles,
It can be used in an appropriate form such as a granular form (including: spherical form), a pellet form, or a honeycomb form (monolith body). However, because it is necessary to pass the exhaust gas to be processed through these,
When the catalyst obtained by supporting Pd is in the form of powder, it is granulated into granules or molded into pellets for use in order to prevent its escape. Regarding the honeycomb shape (monolith body), (1) the present catalyst is extruded together with a binder or the like as required to form a honeycomb shape (monolith body), (2) the powder, granular or other main catalyst is suspended. It is manufactured by a method such as supporting as a liquid on a ceramic honeycomb.

The molding into a granular form or another form can be carried out by an ordinary method. For example, in the case of the honeycomb form in the above-mentioned mode (2), one or two examples including the Pd loading time will be described. And (a) a powder catalyst in which Pd is supported on a clinoptilolite carrier is washed-coated as an aqueous suspension with a binder such as alumina sol to be supported on a cordierite substrate which is previously made into a honeycomb shape, and is supported. (B) Various methods such as washcoating powdered clinoptilolite with an binder such as alumina sol as an aqueous suspension and then carrying Pd on the clinoptilolite carrier by an ion exchange method or an impregnation method are adopted. be able to.

As a type of apparatus for carrying out the NOx-containing exhaust gas purification method of the present invention, regardless of vertical type or horizontal type, use is made of various types of reaction apparatus normally used for catalytic reaction of gas. However, FIG. 2 schematically shows an example of the horizontal device. In FIG. 2, A is a combustion device for city gas or the like, B is an exhaust gas conduit, and when the fuel is city gas, NOx, H ₂ O,
Such as _{CO 2, O 2, CH 4} , N 2, CO is contained.
C is a catalyst layer, D is an exhaust conduit, NOx in the exhaust gas is reduced to N ₂ by the action of the catalyst of the catalyst layer C, and C
H ₄ and CO are oxidized into CO ₂ and H ₂ O by the same action, and are discharged from the discharge conduit D as a gas containing H ₂ O, O ₂ , CO ₂ and N ₂ .

In the present invention, when the NOx-containing exhaust gas to be treated is a combustion exhaust gas from a city gas combustion device,
Since unburned lower hydrocarbons (especially methane) contained therein can be used as the reducing agent, the exhaust gas is passed through the layer of the Pd-supported clinoptilolite catalyst without the need for adding a separate reducing agent. NOx can be changed to N ₂ by itself. When a lower hydrocarbon (particularly methane) is not contained in the flue gas to be treated, or when it is contained but is less than the required effective amount, the amount necessary for the reaction is appropriately supplied and added. By these, it is possible to cope with the presence or absence of hydrocarbons in the exhaust gas to be treated, and regardless of the content thereof.

[0021]

EXAMPLES Hereinafter, the present invention will be described in more detail with reference to Examples, but it is needless to say that the present invention is not limited to these Examples. First, natural clinoptilolite (produced in Akita Prefecture) was prepared, clinoptilolite was synthesized, and Pd was ion-exchanged and supported on these to produce the test catalyst of the example. Further, Pd was loaded on the natural clinoptilolite (produced in Akita Prefecture) by impregnation.

<< Synthesis of clinoptilolite >> 27.4 g of sodium hydroxide, 42.4 g of potassium hydroxide and 39 g of aluminum hydroxide were mixed, and about 50 ml of water was added, and the temperature was adjusted to a transparent liquid. The mixture was heated and stirred at 90 ° C. On the other hand, 1500 ml of water is placed in another container, and finely powdered amorphous silica (Nipseal VN manufactured by Nippon Silica Industry Co., Ltd.)
(3, trade name) and stirred, and the transparent solution obtained above was added thereto, and then synthetic clinoptilolite as seed crystals 3.
3 g was added and stirred at room temperature for 10 minutes or longer to obtain an aqueous suspension.

Then, the above aqueous suspension was placed in an autoclave, hermetically sealed, and kept at 150 ° C. for 120 hours while stirring to be crystallized. After this crystallization reaction, it was allowed to cool, the resulting slurry was taken out, and after repeating filtration and washing with water,
It was dried at a temperature of 110 ° C. The Na / K type synthetic clinoptilolite thus obtained was subjected to ion exchange treatment in an ammonium nitrate aqueous solution at a temperature of 60 ° C. for 2 hours, and this ion exchange treatment was repeated twice and dried to obtain an ammonium type clinoptilolite. .

<< Support of Pd >> On the other hand, 30 ml of ammonia water was added to 100 ml of distilled water and palladium acetate was dissolved in an aqueous solution heated to 50.degree.
g / l, 3.1 g / l and 5.0 g / l aqueous palladium solutions were prepared. PH of each palladium aqueous solution is 7 with acetic acid
Then, 15 g of the ammonium type clinoptilolite obtained above was added and ion exchange treatment was carried out at a temperature of 50 ° C. for 3 hours. After ion exchange, filtration and washing with water were repeated 3 times, and subsequently the solid content was dried at a temperature of 110 ° C. overnight. Then, the dried sample is heated to a temperature of 5 in an air atmosphere in an electric furnace.
The powder was calcined at 00 ° C. for 3 hours to obtain a powdery clinoptilolite catalyst supporting Pd. After that, it is molded and crushed to have a particle size of 3
The particle size was adjusted to a range of 55 to 710 μm to obtain a test catalyst.

For natural clinoptilolite (produced in Akita Prefecture), the temperature is 60 ° C. in an aqueous ammonium nitrate solution.
Was subjected to ion exchange treatment for 2 hours, and this ion exchange treatment was repeated twice and dried to obtain ammonium type clinoptilolite. The same Pd concentration as above 3.1g / l
Was subjected to ion exchange with the palladium aqueous solution of to obtain a test catalyst in the same manner as above. However, since the amount of Pd supported was small due to ion exchange (the limit was 0.16 wt% of Pd supported under the above-mentioned ion exchange conditions), impregnation was carried out by a rotary evaporator at a temperature of 50 ° C. Baking for 3 hours, particle size 3
The sized test catalyst was in the range of 55 to 710 μm.

<Purification test> Next, a purification test of exhaust gas containing NOx was carried out using each of the test catalysts obtained above. A fixed bed flow reactor was used as the reactor, which consisted of a stainless steel reaction tube with an inner diameter of 1
0 mm. In this example, 4.5 ml of each of the test catalysts was filled in the tube. N as the exhaust gas to be treated
_{O = 91ppm, CH 4 = 1080ppm} , O 2 = 9.
1%, CO = 910 ppm, CO ₂ = 6.8%, H ₂ O
= 9.1%, the balance (Balance): using a gas containing N _2, changing the treatment temperature i.e. the reaction temperature over the range of three hundred twenty-five to five hundred ° C., also a space velocity (GHSV) 44,
It was carried out as 000 h ^-1 . Table 1 and FIGS. 3 to 4 show these results. In addition, in FIG.
The measured values in the range of 475 ° C. are plotted.

Here, the "NOx removal rate (%)" means the NOx concentration in the exhaust gas at the inlet of the catalyst layer at each measurement point is X, and the NOx concentration at the outlet of the catalyst layer is Y,
It is calculated by the following formula (1).

[Equation 1]

Table 1 shows the results at a reaction temperature of 425 ° C. depending on the type of carrier and Pd loading conditions. As is clear from Table 1, when natural clinoptilolite is used for removing NOx, it is of course effective, but when synthetic clinoptilolite is used as a carrier (in Table 1),
It can be seen that it is significantly superior to the case of using natural clinoptilolite. Further, in the case of natural clinoptilolite, Pd cannot be sufficiently supported by ion exchange (in Table 1), so the test catalyst supported by impregnation was also tested (in Table 1). It shows that the NOx removal rate does not depend on the amount.

[0029]

[Table 1]

FIG. 3 shows NO depending on the amount of Pd supported on the synthetic clinoptilolite carrier (Pd content: wt%).
It shows the change in x removal rate (temperature 425 ° C.).
As shown in Fig. 3, when the Pd content is 0.2 wt%, it is already about 33%.
The NOx removal rate is shown below, and thereafter the NOx removal rate increases with an increase in the Pd content rate, and is 65% at a Pd content rate of 0.8 wt%.
The NOx removal rate is as high as%, and the same high NOx removal rate is maintained even if Pd is further contained.

FIG. 4 is a plot of NOx removal characteristics depending on the loading amount of Pd on each of synthetic and natural clinoptilolite carriers, at each measurement temperature. As shown in FIG. 3, when the synthetic clinoptilolite is used as the carrier, the NOx removal characteristic is significantly higher than that of the natural clinoptilolite carrier. Amount of Pd supported by natural clinoptilolite carrier by ion exchange 0.16
If it is wt% (marked with Δ in FIG. 3), for example, the temperature 42
The NOx removal rate is slightly higher than 21% at 5 ° C, which is considerably effective, but even if Pd is loaded on the natural clinoptilolite carrier by impregnation, and 0.68 wt% is also supported. (∇ mark in FIG. 3) indicates that it does not contribute to the NOx removal characteristics.

[0032]

As described above, according to the present invention, the use of a catalyst in which clinoptilolite is loaded with Pd as the catalyst can reduce NOx in the NOx-containing exhaust gas to 34%.
It can be removed very effectively over a wide temperature range from 0 ° C to 500 ° C or higher. Further, since it is possible to obtain an excellent NOx removal effect even in the presence of oxygen and water, it is particularly effective for exhaust gas from the lean burn system.

In particular, since synthetic clinoptilolite is excellent in ion exchange property, the amount of Pd supported can be increased by the ion exchange method as compared with a natural product. Further, in the case of a Pd-supported clinoptilolite catalyst, its catalytic activity is characterized in that the catalytic activity on the high temperature side is higher than that of, for example, a Pd-supported mordenite catalyst. Further, since clinoptilolite has a small pore size, Various advantages such as suppression of sintering of Pd in the holes and improvement of durability can be obtained.

[Brief description of drawings]

FIG. 1 is a diagram showing a crystallization curve of clinoptilolite [X
According to the relative intensity of the line diffraction peak (020)].

FIG. 2 is a diagram schematically showing an example of an apparatus for carrying out the NOx-containing exhaust gas purification method of the present invention.

FIG. 3 is a graph showing the influence of Pd loading on a synthetic clinoptilolite carrier and the NOx removal rate (reaction temperature = 425).
° C).

FIG. 4 is a graph showing NOx removal rates of various Pd loading amounts on a clinoptilolite carrier (reaction temperature = 325 to 500).
° C).

[Explanation of symbols] A Combustion equipment for city gas, etc. B Exhaust gas conduit C Catalyst layer D Emission conduit

Claims (7)

[Claims] 1. A catalyst for purifying exhaust gas containing NOx, comprising:
A catalyst for purifying NOx-containing exhaust gas, comprising Pd supported on a clinoptilolite carrier. 2. In the catalyst for purifying NOx-containing exhaust gas, the amount of Pd supported on clinoptilolite is at least 0.1 wt% of the total amount of the catalyst containing clinoptilolite.
The NOx-containing exhaust gas purifying catalyst according to claim 1, which is as described above. 3. The catalyst for purifying NOx-containing exhaust gas according to claim 1, wherein the clinoptilolite carrier is a synthetic clinoptilolite. 4. The catalyst for purifying NOx-containing exhaust gas according to claim 1, 2 or 3, wherein the catalyst for purifying NOx-containing exhaust gas is granular, granular, pellet-shaped or honeycomb-shaped. 5. Purification of NOx-containing exhaust gas, characterized in that clinoptilolite is loaded with Pd by ion exchange with an aqueous solution of Pd in the form of a nitrate, chloride, acetate or complex salt. Method for producing catalyst. 6. A method for purifying a NOx-containing exhaust gas, which comprises contacting a NOx-containing exhaust gas containing oxygen and water vapor with a Pd-supported clinoptilolite catalyst. 7. The NOx-containing exhaust gas generated by the lean-burn combustion, wherein the NOx-containing exhaust gas is a NOx-containing exhaust gas.
A method for purifying NOx-containing exhaust gas as described.