JPH04334526A - Method and catalyst for removing nitrogen oxide in exhaust gas - Google Patents

Abstract

PURPOSE:To provide a catalyst having capacity removing nitrogen oxide in exhaust gas especially at low temp. without undergoing the poisoning due to SOx using hydrocarbon as a reducing agent and easy to prepared. CONSTITUTION:Exhaust gas containing nitrogen oxide is brought into contact with a catalyst containing platinum in the presence of hydrocarbon within an oxidizing atmosphere. This catalyst is supported on a carrier and the reaction temp. thereof is held to 300 deg.C or lower, pref., 280 deg.C or lower.

Description

Detailed Description of the Invention [0001] [Industrial Application Field] The present invention is applicable to boilers, internal combustion engines,
The present invention relates to a method and catalyst for efficiently removing nitrogen oxides contained in exhaust gas discharged from a nitric acid manufacturing factory. [0002] As environmental conservation becomes a global issue, the problem of removing NOx (nitrogen oxides) and SOx (sulfur oxides), which are said to be the causes of acid rain, has attracted much attention. There is. The main sources of NOx are exhaust gas from boilers, automobile internal combustion engines, and nitric acid manufacturing plants. Conventionally, for example, in gasoline-powered automobiles, a three-way catalyst containing platinum, rhodium, and cesium has been used. In a gasoline engine, fuel is burned under conditions of approximately the stoichiometric air-fuel ratio (A/F = 14.6), so the oxygen concentration in the exhaust gas is low, and CO and NO in the exhaust gas are low.
and hydrocarbons can be removed simultaneously with the three-way catalyst. In this catalyst, platinum plays the role of oxidizing CO and hydrocarbons, rhodium plays the role of reducing NOx, and cesium plays the role of releasing or storing oxygen depending on the composition of the exhaust gas. [0004] However, when the air-fuel ratio is high and oxygen-rich, or when the exhaust gas contains excess oxygen as in diesel engines and boilers, even rhodium, which has high denitrification activity, loses most of its activity. , the above three-way catalyst will no longer function and cannot be used. Therefore, conventional methods for removing nitrogen oxides in such cases include, for example, V2 O5 -Ti
A method has been adopted in which nitrogen monoxide is selectively reduced with an ammonia reducing agent using an O2 catalyst. however,
Since this method uses ammonia, which is dangerous and difficult to handle, it is desirable to have a catalyst that can use hydrocarbons as a reducing agent instead.
Catalyst obtained by ion-exchanging H+ or Na+ of zeolite with copper ions (Japanese Patent Application Laid-open No. 63-100
Several types of catalysts have been proposed, such as No. 919). Problems to be Solved by the Invention By the way, the above-mentioned copper-
Zeolite catalysts are not only expensive, but also
There is a drawback that it takes time and effort to exchange copper ions, and the ability to remove nitrogen oxides is not always sufficient. Furthermore, copper catalysts have the problem of being easily poisoned by SOx, and their activity is low at low temperatures, making them difficult to say are sufficient for practical use. The present invention has been made in view of the above-mentioned problems of the conventional technology, and its purpose is to use hydrocarbons as reducing agents to reduce nitrogen oxidation without being poisoned by SOx. It is an object of the present invention to provide a method and a catalyst for removing nitrogen oxides from exhaust gas, which have the ability to remove nitrogen oxides particularly at low temperatures and are easy to prepare. [0007]In order to achieve the above objects, the method according to the invention comprises contacting exhaust gas containing nitrogen oxides with a platinum-containing catalyst in the presence of hydrocarbons in an oxidizing atmosphere. This is how it was done. The platinum-containing catalyst according to the invention is preferably a composite of one or more of alumina, silica, titania, zirconia, aluminum phosphate, aluminum borate, boria, silica alumina, zeolite and activated carbon. It is supported by a carrier obtained by. [0009] The reaction temperature is 300°C or less, preferably 2
The temperature is kept below 80℃. Furthermore, according to the method of the present invention, S
NOx is removed from exhaust gas containing nitrogen oxides that coexist with Ox.
is now removed. [Operation] The present inventors have discovered that nitrogen oxides in exhaust gas can be efficiently removed by contacting exhaust gas containing nitrogen oxides with a catalyst containing platinum in the presence of hydrocarbons in an oxidizing atmosphere. I found out. Furthermore, the activity of this catalyst can be increased by supporting it on a carrier to maintain a large surface area. As shown in Comparative Example 1, silica is
By themselves, they have no ability to remove nitrogen oxides, and alumina and aluminum phosphate exhibit only slight activity, as shown in Comparative Examples 2 and 3. However, when platinum is supported on such a compound, Examples 1 to 3
As shown in Figure 2, high nitrogen oxide removal activity can be obtained. The method for preparing the catalyst is not particularly limited as long as platinum can be dispersed on the carrier. [0011] The hydrocarbon may be anything as long as it can reduce nitrogen oxides, and it may be present in the exhaust gas, or if it is insufficient, it may be introduced from the outside. Examples of the hydrocarbons introduced include paraffinic hydrocarbons such as propane, butane, and cyclohexane, olefinic hydrocarbons such as ethylene and propylene, and aromatic hydrocarbons such as toluene and xylene. The amount of N2 introduced is stoichiometrically reacted with nitrogen oxides in the exhaust gas, together with the existing hydrocarbons in the exhaust gas.
, H2 O and CO2, or more. The platinum-containing catalyst also causes hydrocarbons to react preferentially with nitrogen oxides over oxygen in an oxidizing atmosphere. In this case, the oxidizing atmosphere refers to carbon monoxide in the exhaust gas,
A condition in which more oxygen is present than is required to convert all hydrogen and hydrocarbons to carbon dioxide and water. Furthermore, the present inventors set the reaction temperature to 300°C.
It has been found that the expected catalytic activity can be obtained by maintaining the following. It is well known that SOx is reduced to S or H2S in a reducing atmosphere, and platinum catalysts are poisoned by the generated S and H2S, reducing their performance. If the temperature is preferably maintained below 280° C., the above-mentioned poisoning mechanism will not work in an oxidizing atmosphere, and as shown in Example 7, there will be little poisoning by SOx. Moreover, especially at temperatures below 300°C, NOx
Demonstrates removal ability. [Example] 1 g of active experimental catalyst was filled in a stainless steel reaction tube with an inner diameter of 10 mm, and a reaction gas (gas composition NO: 1000 ppm
, C3 H6: 1000 ppm, O2: 5% by volume,
He (residual amount) was passed at a flow rate of 30 ml/min through a reaction tube whose catalyst layer temperatures were maintained at 150°C, 200°C, 250°C, and 300°C, respectively. NO and N at the outlet of each reaction tube
The O2 concentration was measured using a chemiluminescent NOx meter. As a criterion for evaluating the performance of the catalyst, the NO conversion rate expressed by {(NOx concentration at the inlet of the reaction tube−NOx concentration at the outlet)/(NOx concentration at the inlet of the reaction tube)}×100% was used. Note that the by-product N2O was detected using a gas chromatograph, and the conversion rate of NO was corrected. Example 1 0.4 g of chloroplatinic acid was dissolved in 200 ml of water, and 15 g of it was dissolved in 200 ml of water.
Silica carrier (CARIACT-10 manufactured by Fuji Davison,
By adding this to BET surface area: 300 m / g), evaporating to dryness, and baking in air at 500 ° C for 3 hours,
I made a catalyst. The results of the activity experiments using this are shown in Table 1. Comparative Example 1 An activity experiment was conducted on the same silica carrier as in Example 1 without supporting platinum. The results are shown in Table 1. Example 2 A solution of 0.4 g of chloroplatinic acid dissolved in 200 ml of water was mixed with 23 g of Al(OH)3 powder (Aluminum hydroxide Grade G, manufactured by Nippon Ketsuchen, BET surface area 34).
0 m2/g) and evaporated to dryness while stirring. This was dried at 110° C. for 24 hours and then calcined in air at 500° C. for 3 hours to produce a 1 wt% pt/Al2 O3 catalyst. It was reduced in 20% hydrogen at 400° C. for 2 hours before use. The results of the activity experiments using this are shown in Table 1. Comparative Example 2 An activity experiment was conducted using the same Al(OH)3 powder as in Example 2, which was fired in air at 500° C. for 3 hours without supporting platinum. The results are shown in Table 1. Example 3 75 g of Al(OH3)3. 9H2O and 23g
of H3PO4 was dissolved in 1 liter of water, aqueous ammonia was added dropwise at a rate of 1 ml/min until the final pH reached 7-8, stirred for about 2 hours, left for 24 hours, and then filtered. After washing with pure water, it was dried at 110°C for 24 hours, and then calcined at 500°C in air for 3 hours to obtain aluminum phosphate. A solution containing 0.4 g of chloroplatinic acid was added to 15 g of ammonium phosphate obtained in this way, and the mixture was evaporated to dryness with stirring, and heated to 20 °C at 110 °C.
After drying for an hour, a catalyst was prepared by calcining at 500° C. in air for 3 hours. 2 hours at 400℃ before use
Reduced in 0% hydrogen. The results of the activity experiments using this are shown in Table 1. Comparative Example 3 An activity experiment was conducted using the aluminum phosphate obtained in Example 3. The results are shown in Table 1. Example 4 A solution of 24.7 g of boric acid dissolved in 200 ml of hot water (80°C) was added to 32 g of the Al(OH)3 powder used in Example 2, stirred for 2 hours, and heated to 110°C. After evaporating to dryness, it was calcined in air at 500°C for 3 hours to obtain aluminum borate (AlBx Oy). A solution of 2 g of chloroplatinic acid was added to 15 g of the aluminum borate thus obtained, evaporated to dryness with stirring, dried at 110°C for 20 hours, and then calcined at 500°C in air for 3 hours. I made a catalyst. 20% at 400℃ for 2 hours before use
Reduced in hydrogen. The results of the activity experiments using this are shown in Table 1. Comparative Example 4 An activity experiment was conducted using the aluminum borate obtained in Example 4. The results are shown in Table 1. Example 5 In Example 3, propylene was used as the hydrocarbon for reducing nitrogen oxides, but in this example propane was used instead. pt/Al2 in this case
The reaction activity of O3 is shown in Table 1. Example 6 Using the same catalyst as used in Examples 1 and 3, activity experiments were conducted by varying the composition of the reaction gas. The changes in activity at that time are shown in Table 2. Example 7 Activity experiments were conducted using a catalyst similar to that used in Examples 1 and 3 in the presence of 100 ppm SO2. Table 3 shows the denitrification activity in that case. [0022] Table 2
NO conversion rate (%) ────────
──────────────────────
O2 C3 H6
Pt/SiO2 Pt/AlPO4
(%) (ppm)
(1wt) (5wt%)
0 10
00 1
6.5
5 1000 57
．． 1 80.5
5 400
21.9 47.
6 10
1000 48.6
66.6
10 400
17.9 62.8
──────────────
────────────────
NO: 1000ppm Reaction temperature: 200°C Table 3
NO conversion rate (%) ──────────
──────────────────────
SO2 Pt/SiO2
Pt/AlPO4 Pt/AlPO4
(ppm) (1wt)
(1wt%) (5wt%)
0 57.
1 84.1 90.5
100
50.0 83.4
84.5 ---
──────────────────────────
──── NO:10
00ppm Reaction temperature: 200℃
C3 H6: 1000ppm
O2: 5% by volume
[0025] As described above, according to the present invention, nitrogen oxides contained in exhaust gas emitted from boilers, internal combustion engines, nitric acid manufacturing plants, etc. can be efficiently removed, and SOx
It is possible to provide a method and a catalyst which exhibit high activity in an oxygen atmosphere without being poisoned by.

Claims (5)

[Claims] 1. A method for removing nitrogen oxides from exhaust gas by contacting the exhaust gas containing nitrogen oxides with a platinum-containing catalyst in the presence of hydrocarbons in an oxidizing atmosphere. 2. The method according to claim 1, wherein the catalytic reaction is carried out at a temperature of 300° C. or lower. 3. The method according to claim 1, wherein nitrogen oxides are removed from exhaust gas containing nitrogen oxides in which sulfur oxides coexist. 4. A catalyst containing platinum which is brought into contact with exhaust gas containing nitrogen oxides in the presence of hydrocarbons in an oxidizing atmosphere to remove the nitrogen oxides from the exhaust gas. Claim 5: Supported by a carrier obtained by combining one or more of alumina, silica, titania, zirconia, aluminum phosphate, aluminum borate, boria, silica alumina, zeolite, and activated carbon. The catalyst according to item 4.