JPH09323039A - Method for catalytically reducing nitrogen oxide - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method for catalytically reducing the NOx in an exhaust gas stably and efficiently with hydrocarbons as a reducing agent even in coexistence of oxygen, SOx and moisture at a low temp. without using a large amt. of the reducing agent. SOLUTION: The NOx contained in an exhaust gas is catalytically reduced in the presence of a catalyst with hydrocarbons as a reducing agent. In this case, the exhaust gas is brought into contact with an NOx oxidizing catalyst to oxidize the nitrogen monoxide (NO) contained in the exhaust gas to nitrogen dioxide (NO2 ) in the first stage. Hydrocarbons are added to such an exgaust gas, and the exhaust gas is brought into contact with an NOx reducing catalyst selected from metallic rhodium and rhodium oxide to reduce the NOx to nitrogen in the second stage.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for catalytic reduction of nitrogen oxides using hydrocarbons as a reducing agent, and more specifically, it removes harmful nitrogen oxides contained in exhaust gas discharged from factories, automobiles and the like. The present invention relates to a method for catalytic reduction of nitrogen oxides, which enables stable reduction and removal at a high removal rate.

[0002]

2. Description of the Related Art Conventionally, nitrogen oxides contained in exhaust gas are obtained by oxidizing the nitrogen oxides and then absorbing them into an alkali, or by using a reducing agent such as ammonia, hydrogen, carbon monoxide, or a hydrocarbon. It has been removed by a method of converting to nitrogen.

However, according to the former method, it is necessary to treat the produced alkaline waste liquid to prevent pollution. On the other hand, according to the latter method, when ammonia is used as the reducing agent, it reacts with the sulfur oxide in the exhaust gas to form salts, and as a result, the reduction activity of the catalyst is reduced. Also, hydrogen, carbon monoxide,
Even when using hydrocarbons and the like as reducing agents, a large amount of reducing agent is required to reduce nitrogen oxides because they react with oxygen present in higher concentrations than nitrogen oxides present in low concentrations. There is a problem that

[0004] For this reason, recently, a method of directly decomposing nitrogen oxides with a catalyst in the absence of a reducing agent has been proposed. However, such a conventionally known catalyst has been proposed.
There is a problem that it is difficult to put into practical use due to low decomposition activity of nitrogen oxides.

Further, as a new catalyst for catalytic reduction of nitrogen oxides using a hydrocarbon or an oxygen-containing compound as a reducing agent, H
Zeolite and Cu ion exchange ZSM-5 have been proposed. Among them, H-type ZSM-5 (SiO ₂ / Al ₂
O ₃ molar ratio = 30 to 40) is considered to be optimal.
However, even with such H-type ZSM-5, it is still difficult to say that the H-type ZSM-5 has a sufficient reducing activity. In particular, when moisture is contained in the gas, the aluminum in the zeolite structure is dealuminated and the performance is reduced. There is a demand for a catalyst for catalytic reduction of nitrogen oxides which has a higher reduction activity because of a rapid decrease and has excellent durability even when the gas contains moisture.

[0006]

Accordingly, a catalyst comprising silver or silver oxide supported on an inorganic oxide has been proposed. However, such a catalyst has a high oxidation activity and a selective reaction to nitrogen oxide. The removal rate of nitrogen oxides is low because of low properties. Further, since the temperature range in which the catalyst has a decomposition activity of nitrogen oxides is high, it is necessary to preheat the exhaust gas in order to effectively decompose the nitrogen oxides in the exhaust gas, which poses a problem in practical use. is there. Further, the catalyst in which silver or silver oxide is supported on an inorganic oxide has a problem that the catalytic activity is significantly deteriorated in the presence of sulfur oxide (Japanese Patent Laid-Open No. 5-31).
7647 publication). In addition, conventional catalysts for catalytic reduction of nitrogen oxides generally do not have sufficient heat resistance, and there is a strong demand for even higher heat resistance depending on the application.

Therefore, the present inventors have already proposed a catalyst for catalytic reduction of nitrogen oxides, which comprises a solid acid carrier on which silver aluminate is supported (Japanese Patent Application No. 7-306070). Although the above problems are improved, the reduction rate of nitrogen oxides is still insufficient.

The present invention has been made in view of the above circumstances, and an object thereof is a catalytic reduction method of nitrogen oxides using a hydrocarbon as a reducing agent, such as oxygen and sulfur. Provided is a method for catalytic reduction of nitrogen oxides which can stably and efficiently catalytically reduce nitrogen oxides in exhaust gas at low temperature without using a large amount of reducing agent even in the presence of oxides and water. Especially.

[0009]

The present invention is a method for catalytically reducing nitrogen oxides contained in exhaust gas by using hydrocarbon as a reducing agent in the presence of a catalyst.
The exhaust gas is converted into a nitrogen oxide oxidation catalyst (hereinafter, the oxidation catalyst or the first
Sometimes called a catalyst. ) To oxidize nitric oxide (NO) contained in the exhaust gas into nitrogen dioxide (NO ₂ ), and then add hydrocarbons to such exhaust gas,
In a step, the exhaust gas is brought into contact with a nitrogen oxide reduction catalyst (hereinafter sometimes referred to as a reduction catalyst or a second catalyst) selected from rhodium metal and rhodium oxide to reduce the nitrogen oxide to nitrogen. Characterize.

[0010]

According to the method of the present invention, as a first step, the exhaust gas is brought into contact with a nitrogen oxide oxidation catalyst so that nitric oxide (NO) contained in the exhaust gas is converted into nitrogen dioxide (NO).
₂ ) to oxidize. Compared to nitric oxide, nitrogen dioxide
In the reduction reaction by the reduction catalyst described later, since it is superior due to the selective reactivity with the hydrocarbon that is the reducing agent, prior to the catalytic reduction of nitrogen oxides in the second step, in this way, in the first step, it is possible By oxidizing nitrogen oxide to nitrogen dioxide, the removal rate of nitrogen oxides contained in the exhaust gas can be increased.

As the above-mentioned oxidation catalyst, that is, the first catalyst,
Although not particularly limited, preferably platinum,
A catalyst composed of a metal selected from manganese, palladium, iridium, ruthenium and copper or an oxide thereof is used. These metals or oxides thereof are usually used by being supported on an oxide having a large specific surface area, for example, a solid acid carrier such as alumina, silica, silica-alumina, zirconia, titania or zeolite. In order to support the above metal or its oxide on such a carrier, an appropriate method such as a conventionally known ion exchange method or impregnation method may be used.

The amount of the above-mentioned metal or its oxide supported on the carrier, that is, the ratio of the above-mentioned metal or its oxide to the weight of the above-mentioned metal or its oxide and the carrier depends on the metal species to be used and the reaction conditions under which the catalyst is placed. Although it is not limited to this, it is usually 0.001 in terms of metal.
It is in the range of 10 to 10% by weight, and preferably in the range of 0.1 to 5% by weight. When the amount of the above metal or its oxide supported on the carrier is less than 0.001% by weight, the above-described ability to oxidize nitric oxide to nitrogen dioxide is insufficient. However, even if the supported amount exceeds 10% by weight, the ability to oxidize nitric oxide to nitrogen dioxide does not increase correspondingly, which is also disadvantageous from the economical point of view.

In the first stage, the space velocity at which the exhaust gas containing nitric oxide is brought into contact with such a first catalyst is
From the viewpoint of the oxidation rate, the lower the better, the better, but from the viewpoint of effectively oxidizing nitric oxide practically, it is usually in the range of 50,000 to 500,000 hr ⁻¹ .

According to the method of the present invention, the exhaust gas is brought into contact with the first catalyst in this manner to convert nitrogen monoxide contained in the exhaust gas into nitrogen dioxide, and then the hydrocarbon as a reducing agent is added to the exhaust gas. And by contacting it with the second catalyst, the nitrogen dioxide can be efficiently reduced to nitrogen, which in turn can increase the removal rate of nitrogen oxides in the exhaust gas.

Examples of the reducing agent composed of the above hydrocarbons include hydrocarbon gases such as methane, ethane, propane, propylene and butylene as gaseous substances, and pentane, hexane, octane and heptane as liquid substances. Single-component hydrocarbons such as benzene, toluene, xylene, and mineral oil hydrocarbons such as gasoline, kerosene, light oil, and heavy oil can be used. In particular, in the present invention,
Among the above, lower alkenes such as ethylene, propylene, isobutylene, 1-butene and 2-butene, lower alkanes such as propane and butane, and light oil are preferably used as the reducing agent. These hydrocarbons may be used alone or in combination of two or more as needed.

The hydrocarbon as the reducing agent varies depending on the specific hydrocarbon used, but is usually in a molar ratio with respect to nitrogen oxides in the exhaust gas (substantially consisting of nitric oxide and nitrogen dioxide). Therefore, it is used in the range of about 0.1 to 2. When the amount of the hydrocarbon used is less than 0.1 in terms of the molar ratio to the nitrogen oxides, sufficient reduction activity for the nitrogen oxides cannot be obtained. On the other hand, when the molar ratio exceeds 2, Since a large amount of unreacted hydrocarbons is emitted, a post-treatment for recovering the nitrogen oxides after the catalytic reduction treatment of the nitrogen oxides is required.

Incidentally, unburned substances or incompletely burned products such as fuel existing in the exhaust gas, that is, hydrocarbons and particulates are also effective as the reducing agent, and these are also included in the hydrocarbon in the present invention. Be done. From this point of view, it can be said that the method of the present invention is also useful as a method for reducing or removing hydrocarbons and particulates in exhaust gas.

The second catalyst is selected from rhodium metal and rhodium oxide. These are usually used by being supported on a solid acid carrier such as an oxide having a large specific surface area, for example, alumina, silica, silica-alumina, zirconia, titania, zeolite and the like. In order to support the rhodium metal or the rhodium oxide on such a carrier, an appropriate method such as an ion exchange method or an impregnation method known in the related art may be used as in the case of the first catalyst.

In the present invention, as the carrier, alumina among the above is particularly preferably used because it has excellent heat resistance and an excellent loading effect. Among alumina, as described in JP-A-7-171347, the content of alkali metal and alkaline earth metal is 0.5% by weight or less, and it is formed from pores having a diameter of 60 angstroms or less. that a pore volume of 0.06 cm ³ / g or more, a pore volume formed from the following pore diameters 80 Å are particularly preferably used alumina is 0.1 cm ³ / g or more. Porous alumina having such a pore volume promotes appropriate oxidation of the reducing agent, and cooperates with the rhodium metal or rhodium oxide supported on it to effectively catalytically reduce nitrogen oxides. can do.

As described above, the second catalyst made of rhodium metal or rhodium oxide can be formed into a honeycomb shape, a spherical shape or the like by itself or after being supported on a carrier by a conventionally known molding method. Can be molded into the shape of. At the time of this molding, a molding aid, a molded body reinforcement, an inorganic fiber, an organic binder, and the like may be appropriately compounded. The second catalyst can also be supported by coating on a preformed inert base material by a wash coat method or the like. As the substrate, for example, it can be supported on a honeycomb structure made of clay such as cordierite. Further, if necessary, any other known method for preparing a catalyst may be used.

The amount of the rhodium metal or rhodium oxide supported on the carrier is preferably 0.01 to 1% by weight. When the supported amount is less than 0.01% by weight, the reducing activity of nitrogen oxides is not sufficient, while when it is more than 1% by weight, the oxidizing activity is too high, resulting in poor selectivity. . According to the present invention, the amount of rhodium metal or rhodium oxide loaded on the carrier is particularly preferably in the range of 0.05 to 0.5% by weight.

In the second stage, the space velocity at which the exhaust gas containing hydrocarbons together with nitrogen oxides mainly consisting of nitrogen dioxide is brought into contact with such a second catalyst is usually 50.
The range is from 00 to 50,000 hr ^-1 . The catalyst in the second stage has a smaller oxidizing activity and a better selectivity with respect to nitrogen oxides than the catalyst in the first stage. Therefore, in order to obtain a high denitration rate, it is preferable that the space velocity is small. Usually, practically, the space velocity in the above range is adopted. According to the method of the present invention, the reaction temperature in the first step and the second step is in the range of 150 to 450 ° C. If necessary, the reaction temperature may be changed in the first stage and the second stage.

According to the present invention, as described above, in the first step, the exhaust gas is brought into contact with the oxidation catalyst to oxidize the nitric oxide contained in the exhaust gas into nitrogen dioxide, and then the exhaust gas is carbonized. Hydrogen is added, and as a second step, this exhaust gas is brought into contact with a nitrogen oxide reduction catalyst selected from rhodium metal and rhodium oxide to reduce nitrogen dioxide to nitrogen, so that nitrogen oxides are removed even in a low temperature range. Stable and efficient reductive decomposition can be performed.

[0024]

EXAMPLES The present invention will be described below with reference to examples along with examples of catalyst preparation for each step, but the present invention is not limited to these examples.

(1) Preparation of First Catalyst Preparation Example 1 5.31 g of chloroplatinic acid (H ₂ PtCl ₆ .6H ₂ O) was dissolved in 100 mL of deionized water. 100 mL of γ-alumina (KHA-24 manufactured by Sumitomo Chemical Co., Ltd.) having an average particle diameter of 3 mm, which had been dried in advance at 120 ° C. for 24 hours, was added to the above chloroplatinic acid aqueous solution and stirred for 30 minutes to form alumina fine particles. The pores were thoroughly impregnated with a chloroplatinic acid aqueous solution.

Next, γ-alumina was separated from the chloroplatinic acid aqueous solution, the excess aqueous solution adhering to the surface was removed, and then dried at 100 ° C. for 12 hours, and further, in air, 50
It was calcined at 0 ° C. to obtain a catalyst (A-1) in which platinum was supported on γ-alumina in a supported amount of 1% by weight.

Preparation Example 2 A catalyst prepared by supporting manganese dioxide on γ-alumina in an amount of 1% by weight was carried out in the same manner as in Preparation Example 1 except that 2.87 g of manganese nitrate was used instead of chloroplatinic acid. A-2) was obtained.

Preparation Example 3 Instead of chloroplatinic acid, palladium chloride (PdCl ₂ ) 3.
A catalyst (A-3) was obtained in which palladium was supported on γ-alumina in a supported amount of 1% by weight in the same manner as in Preparation Example 1 except that 3 g was used.

Preparation Example 4 Instead of chloroplatinic acid, iridium chloride (IrCl ₄ ) 1
A catalyst (A-4) was obtained in which iridium was supported on γ-alumina in a supported amount of 5% by weight in the same manner as in Preparation Example 1 except that 7.4 g was used.

Preparation Example 5 Instead of chloroplatinic acid, ruthenium chloride (RuCl ₃ ) 4.
A catalyst (A-5) was obtained in which ruthenium was loaded on γ-alumina in a loading amount of 1% by weight, in the same manner as in Preparation Example 1 except that 1 g was used.

[0031] Instead of Preparation 6 chloroplatinic acid, copper nitrate _{(Cu (NO 3) 2 ·} 3H
₂ O) 2.42 g was used, and in the same manner as in Preparation Example 1, a catalyst (A-6) was obtained in which cupric oxide was supported on γ-alumina in a supported amount of 1% by weight.

(2) Preparation of Second Catalyst Preparation Example 7 Rhodium Nitrate (Rh (NO ₃ ) ₂ .2H ₂ O) 0.64 g
Was dissolved in 100 mL of ion-exchanged water. 120 ° C in advance
100 mL of γ-alumina (KHA-24 manufactured by Sumitomo Chemical Co., Ltd.) having an average particle size of 3 mm dried for 24 hours was charged into the above rhodium nitrate aqueous solution, stirred for 30 minutes, and the rhodium nitrate in the pores of the alumina The aqueous solution was thoroughly impregnated.

Then, γ-alumina was separated from the rhodium nitrate aqueous solution, and after removing the excess aqueous solution adhering to the surface, it was dried at 100 ° C. for 12 hours, and further, in air, 50
0.1% by weight of rhodium in γ-alumina by firing at 0 ° C
A catalyst (B-1) supported in the supported amount was obtained.

Preparation Example 8 Rhodium was converted to γ-alumina in the same manner as in Preparation Example 8 except that 0.06 g of rhodium nitrate was used in Preparation Example 7.
A catalyst (B-2) supported in an amount of 0.01% by weight was obtained.

Preparation Example 9 Rhodium was converted into γ-alumina in the same manner as in Preparation Example 8 except that 3.20 g of rhodium nitrate was used in Preparation Example 7.
A catalyst (B-3) supported in an amount of 0.5% by weight was obtained.

Preparation Example 10 Rhodium was converted into γ-alumina in the same manner as in Preparation Example 8 except that 6.40 g of rhodium nitrate was used in Preparation Example 7.
A catalyst (B-4) supported in an amount of 1.0% by weight was obtained.

Examples 1 to 11 (Evaluation test) A gas having the following composition was used in the first stage in the first catalyst (A-
1 to 6), then a reducing agent (hydrocarbon) is added to this gas, and the gas is treated with the second catalyst (B-1 to 4) in the second stage to obtain nitrogen gas containing nitrogen oxides. Oxide catalytic reduction was performed, and in the first step, the conversion rate of nitric oxide to nitrogen dioxide and the removal rate of nitrogen oxides after the second step were determined by the chemiluminescence method. The results are shown in Tables 1 and 2.

(Test conditions) (1) Gas composition (first step) NO 500ppm O ₂ 10 volume% water 6 volume% nitrogen balance (2) Gas composition (second step) Gas treated in the first step and reducing agent (Hydrocarbon) 500
ppm was added. When light oil was used as the reducing agent, the light oil was C12 in terms of C. ) (3) Space velocity 1st stage 50000, 100000 or 20000
0 (hr ^-1 ) 2nd stage 20000 or 50000 (hr ^-1 ) (4) Reaction temperature 250 ° C, 300 ° C, 350 ° C,
400 ° C or 450 ° C

Comparative Examples 1 to 4 (Evaluation Test) The nitrogen oxide-containing gas was treated in the first stage (after adding the reducing agent to the nitrogen oxide-containing gas, then treated only with the oxidation catalyst) or in the second stage ( After adding the reducing agent to the nitrogen oxide-containing gas,
Treat with reducing catalyst only. ), Except that only one of the above) was treated, catalytic reduction of nitrogen oxide-containing gas was carried out in the same manner as in Example, and when the nitrogen oxide-containing gas was treated only in the first stage, The conversion of nitrogen dioxide to nitric oxide and the nitrogen oxide containing gas
When the treatment was performed only in stages, the removal rate of nitrogen oxides was obtained by the chemiluminescence method. The results are shown in Tables 1 and 2.

[0040]

[Table 1]

[0041]

[Table 2]

[0042]

As is clear from the results shown in Tables 1 and 2, according to the method of the present invention, even in the presence of oxygen, sulfur oxides and water, a large amount of reducing agent is not used.
In a low temperature range, nitrogen oxides in exhaust gas can be stably and efficiently catalytically reduced.

─────────────────────────────────────────────────── ─── Continuation of front page (51) Int.Cl. ⁶ Identification number Office reference number FI technical display location B01J 23/46 301 B01J 23/46 311A 311 23/72 A 23/72 B01D 53/36 102B 102G ( 72) Inventor Keiichi Tabata 5-1, Ebishima-cho, Sakai City, Osaka Prefecture Central Research Laboratory, Sakai Chemical Industry Co., Ltd.

Claims (4)

[Claims] 1. A method for catalytically reducing nitrogen oxides contained in exhaust gas by using hydrocarbon as a reducing agent in the presence of a catalyst, wherein the exhaust gas is brought into contact with a nitrogen oxide oxidation catalyst as a first step. Is oxidized to nitrogen dioxide (NO ₂ ), then hydrocarbon is added to such exhaust gas, and as a second step, this exhaust gas is treated with nitrogen selected from rhodium metal and rhodium oxide. A catalytic reduction method for contacting nitrogen oxides, which comprises contacting an oxide reduction catalyst to reduce nitrogen oxides to nitrogen. 2. The method according to claim 1, wherein the nitrogen oxide oxidation catalyst is a catalyst composed of a metal selected from platinum, manganese, palladium, iridium, ruthenium and copper or an oxide thereof. 3. The method according to claim 1, wherein the hydrocarbon is light oil. 4. The method according to claim 1, wherein in the first and second stages,
The method according to claim 1 or 2, wherein the exhaust gas is brought into contact with the catalyst at a temperature in the range of 450 ° C.