JPH0952047A - Catalyst for contact reducing nitrogen oxide - Google Patents

Abstract

PROBLEM TO BE SOLVED: To efficiently reduce a nitrogen oxide in exhaust gas without using a large amount of reducing agent by carrying a silver chloride or a solid acid carrier in a nitrogen oxide contact reducing catalyst using hydrocarbon as a reducing agent. SOLUTION: A nitrogen oxide contact reducing catalyst suitable for reducing and removing a harmful nitrogen oxide contained in exhaust gas exhausted out of a factory, an automobile and the like is composed of a solid acid carrier carrying a silver chloride, and the carrying amount of the silver chloride is set as 0.01-10wt.%. As the solid acid carrier, a zeolite solid acid carrier or an oxide solid acid carrier is used, and as the zeolite solid acid carrier, Na - mordenite, Na - ZSM - 5, metallosilicate formed by substituting a part of the whole of aluminum - zeolite for some other metallic element or the like is used. As the oxide solid acid carrier Al2 O3 , ZrO2 , SiO2 /Al2 O3 or the like is used.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a catalyst for catalytic reduction of nitrogen oxides using a hydrocarbon as a reducing agent, and more particularly to harmful nitrogen oxides contained in exhaust gas discharged from factories, automobiles and the like. The present invention relates to a catalyst for catalytic reduction of nitrogen oxides, which is suitable for reducing and removing nitrogen oxides.

[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 take measures for treating the generated alkaline waste liquid to prevent the occurrence of 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, there is a problem that the reduction activity of the catalyst is reduced. In addition, even when hydrogen, carbon monoxide, hydrocarbons, and the like are used as a reducing agent, since they react with oxygen present at a higher concentration than nitrogen oxide present at a lower concentration, it is necessary to reduce nitrogen oxides. Has the problem that a large amount of reducing agent is required.

[0003] For this reason, recently, a method of directly decomposing nitrogen oxides with a catalyst in the absence of a reducing agent has been proposed.
There is a problem that it is difficult to be put to practical use due to low nitrogen oxide decomposition activity. In addition, as a new catalyst for catalytic reduction of nitrogen oxides using hydrocarbons and oxygenated compounds as reducing agents,
H-type zeolite and Cu ion exchange ZSM-5 have been proposed. In particular, H-type ZSM-5 (SiO ₂ / Al ₂ O
₃ mol ratio = 30 to 40) is considered to be optimal. However, it is difficult to say that even such H-type ZSM-5 has sufficient reducing activity, and in particular, when water is contained in the gas, aluminum in the zeolite structure is dealuminated, resulting in poor performance. Because it drops sharply
There is a demand for a catalyst for catalytic reduction of nitrogen oxides, which has higher reduction activity and further has excellent durability even when the gas contains water.

[0004]

SUMMARY OF THE INVENTION The present invention has been made in view of the above-mentioned circumstances, and its object is to use a hydrocarbon as a reducing agent in the presence of oxygen. Also, and particularly, in the presence of oxygen, sulfur oxides and water, nitrogen oxides selectively react with hydrocarbons, so that nitrogen oxides in exhaust gas can be efficiently treated without using a large amount of reducing agent. Another object of the present invention is to provide a catalyst for catalytic reduction of nitrogen oxides, which can be reduced well and has excellent durability even in the presence of water.

[0005]

A catalyst for catalytic reduction of nitrogen oxides using a hydrocarbon as a reducing agent according to the present invention is characterized by comprising silver chloride supported on a solid acid carrier.

The solid acid carrier in the present invention means a carrier which exhibits solid acidity in the temperature range where the catalyst is used.
Solid acidity can be confirmed by the temperature programmed desorption method using ammonia,
In situ FTIR with ammonia or pyridine
(Fourier transform infrared absorption spectrum) method. Examples of such solid acid carriers that can be preferably used in the present invention include the following zeolite-based solid acid carriers and oxide-based solid acid carriers.

Zeolite-based solid acid carriers include Na-mordenite, Na-ZSM-5, Na-USY (USY: ultrastable or ultra-stable Y-zeolite), and a part or all of the aluminum in the zeolite to other metal elements. ,
In particular, metallosilicates substituted with iron, gallium, zinc, lanthanum, copper, molybdenum, chromium, germanium, titanium, boron and the like, zeolite having excellent heat resistance are treated with an aqueous solution of ammonium salt such as ammonium sulfate or an acid such as sulfuric acid. After treatment, a part or all of the alkali metal in the zeolite can be obtained by ion exchange with ammonium ions or hydrogen ions. In the case of the method of performing ion exchange with ammonium ions, a calcination treatment is finally required.

As an example of the zeolite-based solid acid carrier, for example, the following formula

[0009]

Embedded image

An acid-type mordenite obtained by acid-treating a mordenite-type zeolite represented by
_{A 2} / Al ₂ O ₃ molar ratio of 13 to 20, and Si
An acid type mordenite having an O ₂ / H ₂ O molar ratio of 25 to 200 can be mentioned. However, in the above formula, M represents an alkali metal ion, and r is a value that varies depending on the synthesis conditions of zeolite.

Another example of the zeolite-based solid acid carrier is, for example, the following formula

[0012]

Embedded image

Of the ions M'in the zeolite represented by
Part or all of the lanthanum ion (La ³⁺), Gallium y
On (Ga³⁺), Cerium ion (Ce⁴⁺), Titanium
ON (Ti⁴⁺), Zirconium ion (Zr⁴⁺), Tin
Ion (Sn⁴⁺) Etc. to obtain the zeolite
Can be mentioned. However, in the above formula, M'is alkali gold
Group ions, alkaline earth metal ions or hydrogen ions
, NA = p (n is the valence of the ion M), q / p.
≧ 5.

As the oxide type solid acid carrier, Al is used.
₂ O ₃ , TiO ₂ , TiO ₂ / SO ₄ ^2- , ZrO ₂ , Z
Single metal oxides such as rO ₂ / SO ₄ ^2- , SiO ₂ / A
l ₂ O ₃ , TiO ₂ / Al ₂ O ₃ , TiO ₂ / ZrO ₂
And other complex oxides. Among these, from the viewpoint of heat resistance, Al ₂ O ₃ , ZrO ₂ , SiO
₂ / Al ₂ O ₃ is preferred.

As another example of the solid acid carrier, a kind of crystalline aluminum phosphate (ALPO) having a zeolite-like porous structure or a layered structure, and its related substance, crystalline aluminum silicate phosphate (SAPO). ), ALPO
The crystalline aluminum metal phosphate (MAPO) in which a part of phosphorus or phosphorus-aluminum of (1) is substituted with a metal such as titanium, iron, magnesium, zinc, manganese, or cobalt.

The ALPO type phosphate is a so-called template such as amine or quaternary ammonium in a desired combination selected from the above-mentioned phosphoric acid source and metal source and silica, silica sol, sodium silicate and the like. It can be prepared by a hydrothermal synthesis method from a mixed raw material under conditions similar to those for synthesizing zeolite. The main difference from the case of synthesizing zeolite is that it is generally synthesized in an acidic region at a higher temperature (approximately 150 ° C. or higher).

The composition of the ALPO type phosphate is generally Al ₂ O _3. (0.8 to 1.2) .P ₂ O ₅ .nH ₂ O
Is represented by Further, in the case of SAPO or MAPO, the maximum amount of silica and metal to be substituted is about 1/10 of the total amount of aluminum and phosphorus, but in the present invention, it does not necessarily fall within this composition range. That is, a material containing an amorphous material may be used.
When the ALPO-type phosphate obtained by the hydrothermal synthesis method is used as a carrier, generally, the one obtained by washing with water, drying, and then calcination in air to remove the remaining template by incineration is used.

In the present invention, among the above-mentioned various solid acid carriers, alumina is particularly preferably used because the catalyst obtained has high durability even in the presence of water and is excellent in the effect of supporting silver chloride. . In particular, among alumina, as described in JP-A-7-171347, the content of alkali metals and alkaline earth metals is 0.5 wt% or less, and the pores having a diameter of 60 angstroms or less are formed. pore volume formed is 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 in cooperation with silver chloride supported on the alumina, can effectively catalytically reduce nitrogen oxides. .

The catalyst according to the present invention can be prepared, for example, according to the following method (1) or (2). (1) A water-soluble silver salt such as silver nitrate is charged into a chiller in which a solid acid is dispersed, and the pH of the slurry is maintained at around 8.0 where no silver hydroxide is generated, so that the ion exchange site of the solid acid is Fix silver ions. Here, when alumina is used as the solid acid, the solid acid having silver ions fixed in this manner is added to an aqueous solution containing sufficient chlorine ions to fix the silver ions, for example, an aqueous hydrochloric acid solution. A solid acid catalyst supporting silver chloride can be obtained by generating silver chloride by immersion and then removing excess chlorine ions by washing with water or the like.

(2) For example, a water-soluble salt which is a precursor of a solid acid such as aluminum nitrate, and a water-soluble silver salt such as silver nitrate are uniformly mixed to prepare an aqueous solution. A precipitate is formed by a method such as neutralization in the presence, and then the precipitate is repeatedly filtered, washed with water, and repulped, dried, and calcined to produce a solid acid, and at the same time, chlorided. Silver is supported on the solid acid.

In the catalyst according to the present invention, the amount of silver chloride supported is 0.01 in terms of the total weight of the solid acid carrier and silver chloride.
It is preferably in the range of 10% by weight. When the supported amount of silver chloride exceeds 10% by weight, the oxidizing power of the obtained catalyst is too high and the selectivity is poor, and when the supported amount is less than 0.01% by weight, the catalytic activity is insufficient. Particularly, in the present invention, the supported amount of silver chloride is preferably in the range of 0.1 to 5% by weight. When the supported amount is within this range, it is possible to obtain the excellent property that the SV dependence of the catalytic reduction reaction of nitrogen oxide is extremely small.

According to the present invention, a catalyst in which silver chloride is supported in the above-mentioned supported amount has an appropriate oxidizing power as compared with a solid acid catalyst in which silver oxide or silver is supported, and chlorine is contained in the catalyst. Since it promotes partial oxidation or cracking of hydrocarbons, it is considered to have high activity and selectivity in the catalytic reduction reaction of nitrogen oxides using hydrocarbons as reducing agents.

The catalyst according to the present invention can be molded into various shapes such as a honeycomb shape and a spherical shape by itself by a conventionally known molding method. 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. Further, the catalyst according to the present invention can be coated and supported by a washcoat method or the like on a preformed inert substrate. As the base material, for example, a honeycomb structure made of clay such as cordierite can be supported. Further, if necessary, any other known method for preparing a catalyst may be used.

In the catalytic reduction of nitrogen oxides using the catalyst according to the present invention, examples of the reducing agent composed of hydrocarbon include gaseous compounds such as methane, ethane, propane, propylene, butylene, etc., and liquids. As the material, a single-component hydrocarbon such as pentane, hexane, octane, heptane, benzene, toluene, xylene, and a mineral oil hydrocarbon such as gasoline, kerosene, light oil, and heavy oil can be used. In particular, according to 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 to be used, but is usually used in a molar ratio of about 0.1 to 2 with respect to nitrogen oxides. 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.

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

The temperature at which the reducing agent shows a selective reduction reaction with respect to nitrogen oxides increases in the order of alkyne <alkene <aromatic hydrocarbon <alkane. Further, in the same type of hydrocarbon, the temperature becomes lower as the carbon number becomes larger. The optimum temperature at which the catalyst according to the present invention exhibits a reducing activity on nitrogen oxides varies depending on the reducing agent and the type of catalyst used, but is usually from 100 to 800 ° C. In this temperature range, space velocity (SV) 500 to 1000
It is preferable that the exhaust gas is circulated at about 00. In the present invention, a particularly suitable temperature range is 200 to 500 ° C.

[0028]

The present invention will be described below with reference to examples.
The present invention is not limited to these examples.

(1) Preparation of catalyst Example 1 0.95 g of silver nitrate (AgNO ₃ ) was added to 100 m of ion-exchanged water.
l. Γ-alumina (KC-50 manufactured by Sumitomo Chemical Co., Ltd.) previously dried at 120 ° C. for 24 hours
1) Charge 60 g of powder, and set the pH to 8 under stirring.
While adjusting the pH with an H controller, 1/10 normal ammonia water was added dropwise. After completion of dropping, the mixture was aged for 1 hour, and silver ions were supported on the γ-alumina by ion exchange.

The slurry thus obtained was filtered to collect γ-alumina powder carrying silver ions, which was thoroughly washed with ion-exchanged water, and then hydrochloric acid aqueous solution 1
Γ-alumina powder carrying 100 ml and stirring for 10 minutes, filtering the slurry, thoroughly washing with ion-exchanged water, and supporting silver chloride at a supported amount of 1% by weight in terms of silver weight. Got This catalyst is called A-1.

Example 2 In the same manner as in Example 1 except that 2.85 g of silver nitrate was used, γ-supported with silver chloride at a supported amount of 3% by weight in terms of silver weight. Alumina powder was obtained. This catalyst is referred to as A-2.

Example 3 In the same manner as in Example 1 except that 4.75 g of silver nitrate was used, .gamma.-supported with silver chloride at a supported amount of 5% by weight in terms of silver weight. Alumina powder was obtained. This catalyst is referred to as A-3.

Example 4 Example 3 was repeated except that 0.095 g of silver nitrate was used.
In the same manner as in Example 1, γ-alumina powder supporting silver chloride in a supported amount of 0.1% by weight in terms of silver weight was obtained. This catalyst is referred to as A-4.

Example 5 1 kg of the same γ-alumina as in Example 1, 79.2 g of silver nitrate, polyethylene oxide (PEO-10 manufactured by Sumitomo Seika Chemicals Ltd.)
After sufficiently kneading 1 kg and an appropriate amount of water, a honeycomb molded product having 200 cells was extruded by an auger screw type extruder. This honeycomb molded product was air-dried at room temperature, then heated and dried at 100 ° C overnight, and further dried at 500 ° C for 3 hours.
After firing for a period of time, a honeycomb formed article made of alumina supporting silver (and / or silver oxide) was obtained. Next, this honeycomb formed article made of alumina supporting silver (and / or silver oxide) was put into a hydrochloric acid aqueous solution to chlorinate silver (and / or silver oxide), and the supported amount was 5 in terms of silver weight. weight%
A honeycomb catalyst made of γ-alumina supporting silver chloride was obtained. This catalyst is designated as A-5.

Comparative Example 1 γ-supported with silver ions in an amount of 5% by weight in the same manner as in Example 1 except that 4.75 g of silver nitrate was used.
Alumina powder was obtained. This catalyst is called B-1. Comparative Example 2 A honeycomb catalyst made of γ-alumina having a silver supported amount of 5% by weight prepared in Example 5 is designated as B-2.

(2) Evaluation test Using the catalysts (A-1 to 5) according to the present invention and the catalysts (B-1 to 2) of the comparative examples, the nitrogen oxide-containing gas was tested under the following test conditions. Nitrogen oxide catalytic reduction was performed, and the nitrogen oxide removal rate was determined by the chemical luminescence method.

(Test conditions) (However, when light oil was used as the reducing agent, light oil was C12 in terms of C.) (2) Space velocity 25000 (Hr ^-1 ) (3) Reaction temperature 250 ° C, 300 ° C, 350 ° C,
The results are shown in Table 1 below.

[0038]

[Table 1]

As is clear from the results shown in Table 1, all the catalysts according to the present invention have a high removal rate of nitrogen in the nitrogen oxides, whereas the catalysts according to the comparative examples generally have a low removal rate.

[0040]

As described above, the catalyst for catalytic reduction of nitrogen oxides according to the present invention uses hydrocarbons as a reducing agent to efficiently contact nitrogen oxides in exhaust gas even in the presence of oxygen and moisture. It can be reduced and has excellent durability.

 ──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Keiichi Tabata 5-1-1 Ebisushimacho, Sakai City, Osaka Sakai Chemical Industry Co., Ltd.

Claims (3)

[Claims] 1. A catalyst for catalytic reduction of nitrogen oxides using a hydrocarbon as a reducing agent, which is obtained by supporting silver chloride on a solid acid carrier. 2. The catalyst for catalytic reduction of nitrogen oxides according to claim 1, wherein the supported amount of silver chloride is in the range of 0.01 to 10% by weight. 3. The catalyst for catalytic reduction of nitrogen oxides according to claim 1, wherein the solid acid carrier is alumina.