JPH06210137A - Denitration method - Google Patents

Abstract

PURPOSE:To provide a denitration method applicable to a reduction reaction of NOx in an atmosphere contg. excess oxygen over a wide temp. region. CONSTITUTION:Exhaust gas is first brought into contact with a catalyst obtd. by carrying Pt and at least one among Fe, Co, Cu and Mn on a carrier having a large specific surface area by 0.01-5wt.% and 0.01-10wt.% of the amt. of the carrier, respectively, and then the exhaust gas is brought into contact with a catalyst obtd. by carrying Pt on a carrier having a large specific surface area by 0.01-5wt.% of the amt. of the carrier. Since this denitration method enables a reduction reaction of NOx over a wide temp. range, it is fit to remove NOx in exhaust gas from an automobile, etc.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for purifying nitrogen oxides in exhaust gas, especially exhaust gas from automobiles and the like.

[0002]

2. Description of the Related Art Nitrogen oxides generally mean nitrogen oxides existing in the atmosphere, and include dinitrogen monoxide (N ₂ O), nitric oxide (NO), and dinitrogen trioxide (N ₂ O ₃ ). , Nitrogen dioxide (NO ₂ ), dinitrogen tetraoxide (N ₂ O ₄ ), dinitrogen pentoxide (N ₂ O ₅ ), and the like. Of these, NO and NO ₂ are referred to as Knox (NOx). This NOx is contained in various exhaust gases such as exhaust gases from internal combustion engines such as boilers and automobiles, exhaust gases emitted from nitric acid manufacturing plants, etc., and affects not only the human body but also increases respiratory disease morbidity. It is also one of the causes of acid rain, which is regarded as a problem from the viewpoint of global environmental protection. Therefore, it is desired to develop a denitration technique for efficiently removing nitrogen oxides from these various exhaust gases.

The ideal removal method of NO in NOx is the direct decomposition of NO as shown in the equation (1). Equation (1) is overwhelmingly advantageous to the right-hand side generator in terms of equilibrium. 2NO → N ₂ + O ₂ (1)

Japanese Patent Application Laid-Open No. Sho 60 (1993) discloses that it is based on this reaction.
There is a denitration technology described in Japanese Patent No. 125250. The denitration technology uses a catalyst in which Cu is supported on zeolite by an ion exchange method, and the catalyst promotes a direct decomposition reaction of NO. However, in the denitration technique, oxygen generated by the reaction of the formula (1) is preferentially adsorbed on the catalytic active site, and as a result, the removal efficiency gradually decreases. Further, there is a drawback that the reaction of the formula (1) is completely hindered under the condition that excess oxygen exists in the reaction system (oxygen excess atmosphere).

In addition, when it is tried to remove NOx,
A number of methods using a catalyst in the presence of a reducing agent such as carbon monoxide, hydrogen, hydrocarbons, ammonia, and hydrazine have been proposed, and the denitration catalyst used in these methods is also a catalyst in which a platinum group element is supported on a carrier. In addition, various other catalysts have been proposed, and denitration methods using these catalysts have been proposed.

However, these denitration methods are not suitable for use in an atmosphere containing less oxygen than the theoretical amount of oxygen required to completely oxidize reducing agents such as carbon monoxide, hydrocarbons and hydrogen, that is, in a reducing atmosphere. Although it exhibits good denitration efficiency, it has a drawback that the denitration efficiency sharply decreases in an oxygen-excess atmosphere with a large amount of oxygen. That is, in the catalytic reduction of NOx in an oxygen-excess atmosphere, the combustion reaction of coexisting reducing agents such as carbon monoxide, hydrogen and hydrocarbons is prioritized and becomes the main reaction, the selectivity of the reduction reaction is significantly reduced, and the side reaction It is because

In order to solve this drawback, the present applicants have disclosed a denitration method using a hydrocarbon as a reducing agent in an oxygen excess atmosphere and a catalyst supporting Pt as an active metal as a denitration catalyst, as disclosed in JP-A-04-334562. It is disclosed in the official gazette. However, the denitration method is affected by temperature, and although NOx is reduced to N ₂ at a high rate in the temperature range of 100 to 250 ° C., when it exceeds 250 ° C., the reduction rate to N ₂ is rapidly reduced. . Therefore, the method cannot be applied to an actual exhaust gas purification system for automobile engines. Because the exhaust gas temperature is 50 depending on the position of the catalytic converter.
It becomes about 0 ℃, a heat exchanger to lower this temperature,
This is because it is not realistic to provide the automobile with a drain treatment facility or the like that is generated as the temperature drops. Taking this point into consideration, it must be said that at present, a denitration method that can be effectively applied in an oxygen excess atmosphere has not been completed.

[0008]

DISCLOSURE OF THE INVENTION The present invention has been made in view of the above circumstances, and it is a denitrification that can be applied in a wide temperature range to a NOx reduction reaction under an excess oxygen atmosphere using a hydrocarbon as a reducing agent. The purpose is to provide a method.

[0009]

As a result of various investigations by the present inventors to solve the above problems, the Pt catalyst promotes the oxidation of hydrocarbons in the high temperature range of 300 ° C. or higher, and NOx
Found that they did not contribute to the reduction of
It was found that NOx reduction proceeds even at a higher temperature by using a catalyst which carries, as an active metal, at least one selected from the group consisting of Fe, Co, Cu and Mn. That is, the method of the present invention is a denitrification method of removing the nitrogen oxides from the exhaust gas by contacting the exhaust gas containing the nitrogen oxides in the presence of hydrocarbons in an oxygen excess atmosphere, and the exhaust gas is first heated at a high temperature. The catalyst A that exerts a denitration function, and then the catalyst B that exerts a denitration function at a low temperature are brought into contact with each other.
A catalyst in which 5 wt% of platinum and 0.01 to 10 wt% of at least one selected from the group consisting of iron, cobalt, copper and manganese with respect to the amount of the carrier is supported on a carrier having a high specific surface area is used, and a low temperature is used. As the catalyst B exhibiting the denitration function, a catalyst in which 0.01 to 5 wt% of platinum is supported on the carrier having a high specific surface area is used.

[0010]

The materials that can be used as the carrier material of the catalyst used in the method of the present invention are silica, alumina, titania, zirconia, silica-alumina, silica-titania and the like. It is immaterial whether these are crystalline or amorphous. It is preferable to use silica, alumina, or silica-alumina from the viewpoint of easily obtaining a carrier having a high specific surface area. The shape of the carrier obtained by using these materials is not particularly limited, and may be spherical, cylindrical, honeycomb, rod-shaped, helical, granular or the like.

The platinum raw material used in the present invention is platinum chloride (I
V) acid hexahydrate, potassium platinum (II) chloride, platinum chloride (I
V) Platinic acid salts such as potassium, platinum (II) chloride hexahydrate, ammonium chloride (IV) chloride, or platinum complex salts such as platinum tetraammine complex dichloride and platinum tetraammine complex nitrate can be used.

The platinum salt aqueous solution is prepared by dissolving the above-mentioned platinum acid, platinum salt or platinum complex salt in distilled water or ion exchange water. The concentration of platinum in the solution may be adjusted so that the total amount of platinum in all the solutions used is within a range sufficient to support a catalytically effective amount of platinum with respect to the finished catalyst, and can be appropriately adjusted as necessary. .

In the present invention, the amount of platinum supported can be variously changed, but 0.01 to 5% by weight, preferably 0.1 to 1% by weight, relative to the carrier, is suitable. Considering that platinum is expensive because the catalyst activity is not sufficiently exerted when the amount of platinum supported is less than the above range, and there is no improvement in the catalytic activity particularly with an increase in the amount supported even when the amount is more than the above range, The above range is suitable.

The iron sources used in the catalyst A used in the present invention in the high temperature range include ferrous chloride (II) hydrate and ferric chloride.
(III), ferrous nitrate (II), water-soluble iron salts such as ferric nitrate (III) can be used, cuprous chloride (I) hydrate as a copper source,
Cuprous sulfate (I), cupric sulfate (II), cuprous nitrate (I), water-soluble salts such as cupric nitrate (II) can be used, and the cobalt source is cobalt (II) carbonate. , Water-soluble salts such as cobalt (II) chloride hydrate, cobalt (II) sulfate, cobalt (II) acetate, and cobalt (II) nitrate can be used, and manganese (II) chloride and manganese acetate (II) can be used as manganese sources. ) Hydrate, manganese oxalate (I
I) hydrate, manganese (II) carbonate hydrate, manganese (II) nitrate
Water-soluble salts such as hydrates and manganese (II) sulfate hydrates can be used.

The impregnating solution containing these active metals is prepared by dissolving these water-soluble salts in distilled water or ion-exchanged water. The metal concentration in the impregnating solution may be such that the amount of active metal in the total solution used is the amount to be supported on the carrier,
Adjust accordingly. There is no problem as long as the amount of the active metal salt aqueous solution used in the ion exchange method or the dipping method is such that the carrier can be sufficiently impregnated with the active metal salt. It is usually about 2 to 20 times that of the carrier. The amount of the active metal salt aqueous solution used in the incipient wetness method is the amount of water that the carrier can occlude in its pores, and is determined by measurement prior to the impregnation treatment.

In the present invention, the supported amounts of iron, copper, cobalt and manganese can be variously changed, but it is necessary to set it to 0.01 to 10% by weight, preferably 0.1 to 5% by weight based on the weight of the carrier. is there. Iron, copper, cobalt, and manganese are all NO alone under oxygen excess atmosphere.
The reducing activity of x is extremely small. Nevertheless, it is speculated that all of these metals can be used as a denitration catalyst in the present invention because they are cocatalysts for platinum. Therefore, it is preferable to determine the supported amount of iron, copper, cobalt, and manganese according to the supported amount of platinum.
It is preferably set to 3.

According to the above method, a predetermined amount of iron, copper, cobalt,
A carrier containing at least one of manganese and platinum, or a carrier containing platinum is dried and calcined according to a conventional method to obtain the denitration catalysts A and B of the present invention.
It is preferable to carry out the calcination in the air at 400 to 600 ° C. economically, from the viewpoint of catalytic activity and durability of the catalyst.

In the method of the present invention, the catalyst A is first contacted with the exhaust gas, and then the catalyst B is contacted therewith, but the ratio of the respective catalyst amounts to be used can be arbitrarily selected.
For example, if the amount of catalyst A is increased and the amount of catalyst B is decreased, denitration is performed on the high temperature side, and conversely, denitration is performed on the low temperature side. Even if the catalyst A and the catalyst B used in the method of the present invention are mixed and exhaust gas is brought into contact with the mixture, a sufficient effect cannot be obtained.

The denitration method of the present invention can be applied to denitration of various exhaust gases such as exhaust gases from internal combustion engines such as boilers and automobiles, exhaust gases discharged from nitric acid manufacturing plants, etc., and particularly NOx reduction in an oxygen excess atmosphere. This is an effective method for the reaction.

[0020]

The present invention will be described in more detail with reference to the following examples, but it goes without saying that the present invention is not limited to these examples.

Example 1 Alumina hydrate having a pseudo-boehmite structure manufactured by Nippon Ketjen KK (trade name G
200g of base) is calcined at 500 ° C for 3 hours to obtain alumina powder
170 g were produced. According to the nitrogen adsorption method, the specific surface area of this alumina powder was 295 m ² / g, and according to the mercury intrusion method, the pore volume was 2.8 ml / g.

Next, 100 ml of a chloroplatinic acid solution containing 1 g of Pt was diluted to 280 ml with ion-exchanged water, and this was gradually added to 100 g of the above-mentioned alumina powder with stirring to impregnate the alumina powder with platinum. Let Then the impregnated material
And dried at 500 ° C. for 3 hours and sized to 250 to 500 μm to obtain catalyst B.

90 ml of platinum chloride solution containing 1 g of Pt
And 50 ml of an aqueous ferric chloride solution containing 1 g of Fe were mixed, and ion-exchanged water was added to the mixture to make 280 ml. This solution was gradually added to 100 g of the alumina powder while stirring, and platinum was added. Alumina powder was impregnated with iron. After that, it was dried at 120 ° C., then calcined at 500 ° C. for 3 hours, and sized to 250 to 500 μm to obtain catalyst A. According to the ICP emission analysis method, the platinum loading amount of the catalyst A was 0.85 wt% and the iron loading amount was 0.48 wt%.

A stainless steel reaction tube having an inner diameter of 10 mm was charged with 0.5 g of catalyst A in the front stage and 0.5 g of catalyst B in the rear stage, and a reaction gas (gas composition NO: 1,000 ppm,
_{C 3 H 6: 1,000ppm, O} 2: 5vol%, He: remaining amount)
Was passed at a flow rate of 30 ml / min (SV = 1,000 / h) so that the reaction gas would flow from the front stage to the rear stage of the reaction tube, and a denitration test was conducted. The reaction tube outlet gas composition was measured for NO and NO ₂ concentrations using a chemiluminescence type NOx analyzer, and for N ₂ O concentration, a gas chromatograph thermal conductivity detector equipped with a porous polymer packed column. Was measured using. The temperature of the catalyst layer was set to a predetermined temperature within the range of 100 to 550 ° C., and the value at the time when the composition of the gas at the outlet of the reaction tube became stable was determined for each predetermined temperature and adopted as the measured value. The results obtained at each temperature are shown in Table 1.

When the reaction gas passes through the catalyst, NO in the reaction gas is converted into NO ₂ , N ₂ O, and N _{2. The} NO conversion rate, N ₂ O production rate, and N ₂ production rate are as follows. Defined as In the present invention, the NOx removal rate of NO is evaluated by the above N ₂ production rate.

[0026]

[0027]

N ₂ production rate (%) = NO conversion rate−N ₂ O production rate

Example 2 A catalyst A was prepared in the same manner as in Example 1 using an aqueous solution of cobalt chloride instead of the aqueous solution of ferric chloride, and a denitration test was conducted in the same manner as in Example 1 and the results were shown. The results are shown in Table 1. The amount of cobalt carried is 0.52.
It was wt%.

(Example 3) A catalyst A was prepared in the same manner as in Example 1 except that an aqueous solution of cupric chloride was used instead of the aqueous solution of ferric chloride.
Was prepared, a denitration test was conducted in the same manner as in Example 1, and the results are shown in Table 1. The amount of copper supported was 0.49 wt%.

Example 4 A catalyst A was prepared in the same manner as in Example 1 by using an aqueous solution of manganese chloride instead of the aqueous solution of ferric chloride, and a denitration test was conducted in the same manner as in Example 1, and the results were shown. The results are shown in Table 1. The supported amount of manganese is 0.58.
It was wt%.

Comparative Example 1 A denitration test was conducted in the same manner as in Example 1 using only the catalyst B of Example 1. The obtained results are shown in Table 1.

Comparative Example 2 A denitration test was conducted in the same manner as in Example 1 using only the catalyst A of Example 1. The obtained results are shown in Table 1.

(Comparative Example 3) Catalyst A and catalyst B of Example 1 were mixed and filled, and a denitration test was conducted in the same manner as in Example 1. The obtained results are shown in Table 1.

(Comparative Example 4) The catalyst A of Example 1 was filled in the latter stage, and the catalyst B was filled in the former stage, and a denitration test was conducted in the same manner as in Example 1. The obtained results are shown in Table 1. From Table 1, in the range of 100 to 300 ° C, the catalyst B packed in the latter stage of the reaction tube
It is understood that NOx is reduced by, and NOx is reduced by the catalyst A filled in the preceding stage of the reaction tube in the range of 300 to 550 ° C.

[0036]

[0037]

INDUSTRIAL APPLICABILITY The denitration method of the present invention has a wide range of N
It is suitable for a method for denitrifying NOx in exhaust gas of automobiles or the like because it enables an Ox reduction reaction.

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁵ Identification code Office reference number FI technical display location B01J 23/89 ZAB A 8017-4G F01N 3/08 ZAB B

Claims (2)

[Claims] 1. A denitrification method for removing nitrogen oxides from exhaust gas by bringing the exhaust gas into contact with exhaust gas containing nitrogen oxides in the presence of hydrocarbons in an oxygen-excess atmosphere, wherein the exhaust gas is first subjected to a denitration function at high temperature. A denitration method comprising contacting with a catalyst A that exerts, and then contacting with a catalyst B that exerts a denitration function at a low temperature. 2. The catalyst A as 0.01 to the amount of carrier.
Using a catalyst in which 5 wt% of platinum and 0.01 to 10 wt% of at least one selected from the group consisting of iron, cobalt, copper and manganese with respect to the amount of the carrier are supported on a carrier having a high specific surface area, a catalyst B is used. The denitration method according to claim 1, wherein a catalyst in which 0.01 to 5 wt% of platinum is supported on a carrier having a high specific surface area is used as the carrier.