JPH06114266A - Exhaust gas purifying material and exhaust gas purifying method - Google Patents

Abstract

PURPOSE:To effectively remove nitrogen oxide in an exhaust gas containing excess oxygen at a wide temp. range by mixing a 1st and a 2nd purifying material composed of a 1st and 1 2nd catalyst, respectively carrying a specific material in each of the 1st catalyst and the 2nd catalyst and specifying the quantity of active seed of the 1st and the 2nd catalyst. CONSTITUTION:The exhaust gas purifying material is made by mixing the 1st and the 2nd purifying material composed of the 1st and the 2nd catalyst, carrying silver or silver oxide of the active seed on a porous inorganic oxide to make the 1st catalyst and carrying at least one kind of alkali metal element, at least one kind of element selected from Cu, Co, Mn and V and at least one kind of rare earth metal, which are the active seed, on the porous inorganic oxide to make the 2nd catalyst. And the quantity of the 1st catalyst active seed is 0.1-15wt.% per 100wt.% porous inorganic oxide and the quantity of the 2nd catalyst active seed is 0.5-20wt.% per 100wt.% porous inorganic oxide.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an exhaust gas purifying material capable of effectively removing nitrogen oxides from a combustion exhaust gas containing nitrogen oxides and excess oxygen, and an exhaust gas purifying method using the same.

[0002]

2. Description of the Related Art Excessive amounts of combustion exhaust gas discharged from internal combustion engines such as automobile engines, combustion equipment installed in factories, household fan heaters, etc. Nitrogen oxides such as nitric oxide and nitrogen dioxide are contained together with oxygen. Here, "containing excess oxygen" means containing more oxygen than the theoretical oxygen amount necessary to burn unburned components such as carbon monoxide, hydrogen, and hydrocarbons contained in the exhaust gas. To do. Moreover, the nitrogen oxide in the following refers to nitric oxide and / or nitrogen dioxide.

This nitrogen oxide is considered to be one of the causes of acid rain and is a serious environmental problem. Therefore, various methods for removing nitrogen oxides in exhaust gas discharged from various combustion devices have been studied.

As a method for removing nitrogen oxides from combustion exhaust gas containing excess oxygen, a selective catalytic reduction method using ammonia is used, particularly for large-scale fixed combustion devices (large combustors such as factories). It has been put to practical use.

However, in this method, ammonia used as a reducing agent for nitrogen oxides is expensive, and ammonia is toxic, so that unreacted ammonia is discharged so that nitrogen oxides in exhaust gas are not discharged. There are problems that the amount of ammonia injection must be controlled while measuring the concentration and that the apparatus is generally large.

[0006] As another method, there is a non-selective catalytic reduction method for reducing nitrogen oxides by using a gas such as hydrogen, carbon monoxide or hydrocarbon as a reducing agent, but this method is effective. In order to reduce and remove nitrogen oxides, it is necessary to add a reducing agent in an amount equal to or larger than a theoretical reaction amount with oxygen in exhaust gas, and there is a drawback that a large amount of reducing agent is consumed. Therefore, the non-selective catalytic reduction method is practically effective only for the exhaust gas having a low residual oxygen concentration that is burned in the vicinity of the theoretical air-fuel ratio, and is not versatile and impractical.

Therefore, there has been proposed a method for removing nitrogen oxides by adding a reducing agent in an amount equal to or less than a theoretical reaction amount with oxygen in exhaust gas by using zeolite or a catalyst supporting a transition metal thereon (for example, a special method). Kaisho 63-100919, 63-28372
No. 7, JP-A-1-130735, and 59th Annual Meeting of the Chemical Society of Japan
(1990) 2A526, 60th Autumn Meeting (1990) 3L420, 3L42
2, 3L423, "Catalyst" vol.33 No.2, page 59, 1991 etc.).

However, in the above-mentioned catalyst and the purification method using the catalyst, when the purification temperature range of nitrogen oxides is narrow and the contact time between the exhaust gas and the catalyst is 0.15 g · sec / ml or less, nitrogen oxidation is performed. It was found that the removal rate of the substances was significantly reduced.

Therefore, an object of the present invention is to remove nitrogen oxides, carbon monoxide, hydrogen, hydrocarbons, etc., such as combustion exhaust gas from a fixed combustion device and a gasoline engine, a diesel engine, etc. that burn under an excess oxygen condition. From the combustion exhaust gas that contains more than the theoretical reaction amount of oxygen for unburned components,
An exhaust gas purifying material and an exhaust gas purifying method capable of efficiently removing nitrogen oxides.

[0010]

As a result of earnest research in view of the above problems, the present inventor has found that a first purifying material comprising a porous inorganic oxide carrying a specific amount of a silver component, Cu, Co Using an exhaust gas purifying material that is mixed with a second purifying material that carries components such as, if hydrocarbon is added to the exhaust gas and the exhaust gas is brought into contact with the above catalyst at a specific temperature, a wide range is obtained. The present invention has been completed by discovering that nitrogen oxides can be effectively removed in the temperature range.

That is, the exhaust gas purifying material for removing nitrogen oxides from the combustion exhaust gas containing nitrogen oxides and oxygen in a larger amount than the theoretical reaction amount for coexisting unburned components is the first purification material composed of the first catalyst. And a second purification material composed of a second catalyst, wherein the first catalyst comprises a porous inorganic oxide carrying silver or silver oxide as an active species, The second catalyst is an active species for the porous inorganic oxide (a) one or more alkali metal elements, and (b) C
One or more elements selected from the group consisting of u, Co, Mn and V and (c) one or more elements of a rare earth element are supported, and the first catalytic activity The amount of the seed is 0.1 to 1 with respect to 100% by weight of the porous inorganic oxide.
5% by weight (elemental conversion value), and the amount of the second catalytically active species is 0.5 to 20% by weight (elemental conversion value) relative to 100% by weight of the porous inorganic oxide. Characterize.

Further, in an exhaust gas purification method for removing nitrogen oxides from a combustion exhaust gas containing nitrogen oxides and oxygen in a larger amount than the theoretical reaction amount for coexisting unburned components, the exhaust gas purification material is installed in the middle of the exhaust gas conduit. And adding hydrocarbons to the exhaust gas on the upstream side of the purification material,
In step 1, the exhaust gas to which the hydrocarbon is added is brought into contact with the purification material, and thereby the hydrocarbon in the exhaust gas and the nitrogen oxide are reacted to remove the nitrogen oxide.

The present invention will be described in detail below. In the present invention, an exhaust gas purifying material obtained by mixing the following two purifying materials is installed in an exhaust gas conduit, and hydrocarbon is added to the exhaust gas upstream from the installation position of the purifying material to convert the exhaust gas to the purifying material. To reduce nitrogen oxides in the exhaust gas by using hydrocarbon as a reducing agent.

First, the purifying material of the present invention will be described.
The purifying material of the present invention is a mixture of the following two purifying materials (1) and (2). (1) First purification material

The first purifying material comprises a first catalyst in which a silver component is supported on a porous inorganic oxide. As the porous inorganic oxide, porous alumina, titania, zirconia,
And their complex oxides can be used,
Preferably, γ-alumina or alumina-based composite oxide is used. By using γ-alumina or an alumina-based composite oxide, the reaction between the added hydrocarbon and / or the residual hydrocarbon in the exhaust gas and the nitrogen oxide in the exhaust gas occurs efficiently.

The specific surface area of the porous inorganic oxide is 30 m ²
/ G or more is preferable. Specific surface area of 30m ² / g
If it is less than the above range, the contact area between the exhaust gas and the inorganic oxide (and the silver component carried on the exhaust gas) becomes small, and the nitrogen oxide cannot be removed well.

The amount of the silver component supported as an active species on the above-mentioned inorganic oxide such as γ-alumina is 10
0.1 to 15% by weight (elemental conversion value) relative to 0% by weight. If it is less than 0.1% by weight, the removal rate of nitrogen oxides decreases. On the other hand, when silver is loaded in an amount of more than 15% by weight, the hydrocarbon itself is easily burned, and the nitrogen oxide removal rate is rather lowered. The preferred amount of silver component supported is 0.5 to 1.
It is 0% by weight. The silver component is in the state of metal or oxide in the temperature range of exhaust gas and can be easily converted into each other.

As a method for supporting the silver component on the inorganic oxide such as γ-alumina, a known dipping method or the like can be used. At that time, the porous inorganic oxide was dipped in a solution having a silver component such as an aqueous solution of silver nitrate and dried at about 70 ° C.
It is preferable to raise the temperature stepwise at 0 to 600 ° C. and to perform the firing. In addition, it is preferable to finally perform an oxidation treatment at 500 ° C.

The first purifying material is SO ₂ before use.
It is preferable to perform a treatment. Specifically, it is preferable to contact 0.02 to 1 mmol of SO ₂ with respect to 1 g of the first purification material. By performing such SO ₂ treatment, the purification characteristics of nitrogen oxides are further improved especially on the low temperature side (about 250 to 400 ° C.).

The first purifying material can be used in the form of pellets, powder, honeycomb, foam or the like.

(2) Second Purifying Material The second purifying material is composed of a second catalyst having a porous inorganic oxide carrying active species. As the porous inorganic oxide, γ-
Alumina and its oxides (γ-alumina-titania, γ
-Alumina-silica, γ-alumina-zirconia, etc.),
Examples of the heat-resistant ceramics are porous and have a large surface area, such as zirconia and titania-zirconia. Preferably, γ-alumina or alumina-based composite oxide is used.

The above-mentioned second catalytically active species is (a) an alkali metal (Li, Na, K, Cs, etc.) and (b) one element selected from Cu, Co, Mn and V, or Use is made of two or more kinds and (c) rare earth elements (Ce, La, Nd, Sm, etc.).
As the component (b), at least one of the five transition elements Cu, Co, Mn, Mo and V is used. In particular, the Cu element is an essential component in the component (b), and Co, At least one of Mn, Mo and V is added.

Although the supported components (a) to (c) are shown as metallic elements, the supported components (a) to (c) exist in the state of oxides under the normal use temperature condition of the purification material. .

The amounts of the active species components (a), (b) and (c) supported on the porous inorganic oxide by the above-mentioned second catalyst are such that (a) is converted to the respective metal components. 20% by weight or less, (b)
Is preferably 60 to 100% by weight, and (c) is preferably 20% by weight or less. (a), (c) exceeds 20% by weight, (b) is 6
When it is less than 0% by weight, the nitrogen oxide removing property on the low temperature side is deteriorated. More preferably, (a) is 15% by weight or less,
(b) is 70% by weight or more, (c) is 15% by weight or less,
Particularly, (a) is 0.1 to 15% by weight, (b) is 70 to 99% by weight, and (c) is 0.1 to 15% by weight.

The amount of the active species supported on the inorganic oxide by the second catalyst (the total amount of (a) + (b) + (c)) is based on the above-mentioned porous inorganic oxide (100% by weight). ) Is 0.5 to 20% by weight, preferably 0.5 to 15% by weight. If the amount of the catalytically active species is less than 0.5% by weight based on the substrate, the effect of supporting the catalyst is not remarkable, and the NOx reduction property is deteriorated. On the other hand, if the amount of catalyst supported exceeds 10% by weight, only the oxidative combustion of hydrocarbons will proceed, and the nitrogen oxide reduction characteristics will deteriorate.

The second purifying material can be used in the form of pellets, powder, honeycomb, foam or the like.
As a method for supporting the metal component on the inorganic oxide such as γ-alumina with the second catalyst, a known impregnation method, precipitation method, sol-gel method or the like can be used.

In the present invention, the above-mentioned (1) first purification material and (2) second purification material are mixed and used. Among them, the mixing ratio of the first catalyst and the second catalyst is preferably 5: 1 to 1: 5 by weight. When both catalyst components are mixed and used in such a blending ratio, it is 250 to 600.
It is possible to effectively remove nitrogen oxides over a wide range of ° C. When the ratio is less than 1: 5 (the amount of the first catalyst is small), the purification rate of nitrogen oxides is generally lowered in a wide temperature range of 250 to 600 ° C. On the other hand, when the ratio exceeds 5: 1 (the amount of the first catalyst is large), the purifying ability of nitrogen oxides at 400 ° C or lower does not increase. That is, the reaction between hydrocarbons in exhaust gas and nitrogen oxides at a relatively low temperature does not proceed sufficiently.

In order to improve the heat resistance, especially the heat shock resistance, the heat resistant porous molded article can be coated with the above-mentioned porous inorganic oxide carrying a catalytically active species.
Further, the above-mentioned catalytically active species may be supported on a porous inorganic oxide coated on the heat resistant porous molded body for use.

When the heat resistant porous molded body is used, examples of the material for forming the molded body include a ceramic or metal honeycomb type or foam type such as cordierite or mullite. When a molded body is used, the coating amount of the porous inorganic oxide can be 10 to 70% by weight, and particularly preferably 20 to 70% by weight based on the molded body.

If the purifying material having the above-mentioned structure is used, 250
It is possible to perform good nitrogen oxide removal in a wide temperature range of up to 600 ° C., but it is believed that this is due to the following reasons.

First, the first purification material of (1) satisfactorily reduces nitrogen oxides by using hydrocarbon as a reducing agent at a temperature higher than 400 ° C., but at 400 ° C. or lower, this reduction reaction does not proceed sufficiently ( Cannot remove enough nitrogen oxides).
However, if an oxygen-containing organic compound is present, nitrogen oxides can be reduced using this oxygen-containing organic compound as a reducing agent even in a relatively low temperature range of 400 ° C. or lower.

On the other hand, the above (2) second purification material is 40
Even at 0 ° C or lower, hydrocarbons are used as a reducing agent to reduce nitrogen oxides satisfactorily.

Therefore, if (2) the second purification material is used together with (1) the first purification material, the nitrogen oxides in the exhaust gas can be satisfactorily reduced by adding hydrocarbon even at 400 ° C. or lower. Will be able to.

Next, the method of the present invention will be described. First, the exhaust gas purification material is installed in the middle of the exhaust gas conduit.

Although the exhaust gas contains ethylene, propylene and the like as residual hydrocarbons to some extent, it is generally not sufficient to reduce NOx in the exhaust gas, so hydrocarbons are introduced into the exhaust gas from the outside. . The introduction position of the hydrocarbon is upstream of the position where the purification material is installed.

As the hydrocarbon introduced from the outside, an alkane, an alkene and / or an alkyne, which are in a gaseous state or in a liquid state in a standard state, can be used. Specifically, propylene, ethylene, propane, butane and the like are listed as the gaseous hydrocarbons in the standard state. Particularly, alkane or alkene having 3 or more carbon atoms is preferable. Specific examples of liquid hydrocarbons in the standard state include light oil, cetane, heptane, and kerosene. Considering practicality, it is preferable to use light oil, kerosene, or the like. The hydrocarbon can be introduced into the exhaust gas by a known method such as spraying. Further, oxygen-containing organic compounds such as alcohols can also be used.

The amount of hydrocarbons introduced from the outside is preferably such that the weight ratio (weight of hydrocarbons added / weight of NOx in exhaust gas) is 0.2 to 5. If this weight ratio is less than 0.2, the nitrogen oxide removal rate does not increase. On the other hand, when it exceeds 5, fuel consumption is deteriorated.

In the present invention, it is preferable to adjust the time during which the exhaust gas containing hydrocarbon contacts the above-mentioned purifying material so that the reaction between hydrocarbon and nitrogen oxides proceeds efficiently. The contact time between the exhaust gas containing hydrocarbons and the purification material is 0.006 seconds.
It is preferably g / ml or more. Here, the contact time is
It represents the time (seconds) in which 1 ml of the exhaust gas containing hydrocarbon (however, the volume converted to the standard state) is in contact with 1 g of the purification material. For example, when the contact time is 0.1 sec · g / ml, it means that 1 g of the purification material is used and 1 ml of exhaust gas is brought into contact with this purification material for 0.1 seconds. Contact time is 0.
If it is less than 006 seconds · g / ml (because the exhaust gas flow rate is too high), the purification efficiency will be low.

Further, in the present invention, the temperature of the exhaust gas at the purification material installation site, which is the site where hydrocarbons and nitrogen oxides react, is maintained at 250 to 600 ° C. Exhaust gas temperature is 25
If the temperature is lower than 0 ° C., the reaction between hydrocarbon and nitrogen oxide does not proceed, and the nitrogen oxide cannot be removed well.
On the other hand, if the temperature exceeds 600 ° C., the combustion of the hydrocarbon itself starts, and the nitrogen oxide cannot be reduced and removed by the hydrocarbon. A preferable exhaust gas temperature is 300 to 600 ° C.

[0040]

The present invention will be described in more detail by the following specific examples. Example 1 Commercially available pelletized porous γ-alumina (diameter 1.5 mm
, Length 6 mm, specific surface area 200 m ² / g) is immersed in an aqueous solution of silver nitrate, dried at 70 ° C., then under a nitrogen stream containing 5% by volume of hydrogen, 150 ° C., 200 ° C., 300 ° C., 400
2 hours each at ℃, 500 ℃, and 600 ℃, then, under a nitrogen stream containing 10% oxygen, 5
The mixture was calcined at 00 ° C. for 2 hours, and 2% by weight (element conversion value) of silver was supported on the pelletized γ-alumina. This was used as the first purification material.

Next, pelletized porous γ-alumina (each having a diameter of 1.5 mm, a length of about 6 mm and a specific surface area of 200 m ² /
g) is dipped in an aqueous solution of Cu (NO ₃ ) ₂ , La (NO ₃ ) ₃ and CsNO ₃ and converted into simple metal elements of Cu, La and Cs, which are 10% by weight and 0.4% by weight, respectively. Cu (NO ₃ ) ₂ , La (NO ₃ ) ₃ and CsNO ₃ were impregnated in a proportion of 0.4% by weight, dried and then fired at 700 ° C. to obtain a pellet-like second purification material. In this purification material, Cu, La and Cs exist as oxides.

The first purification material and the second purification material were mixed at a weight ratio of 1: 1 to obtain a purification material, and 1.34 g was set in the reaction tube.

Next, a gas having the composition shown in Table 1 (nitrogen monoxide, carbon dioxide, oxygen, propylene, and nitrogen) was flowed at a flow rate of 1.75 liters per minute (standard state) (space velocity of 30,000 h). ^-1 , contact time 0.05 sec.g / ml),
The temperature of the exhaust gas in the reaction tube was kept in the range of 200 to 550 ° C, and propylene and nitrogen oxide were reacted.

The concentration of nitrogen oxides (total amount of nitric oxide and nitrogen dioxide) of the gas after passing through the reaction tube was measured by a chemiluminescence type nitrogen oxide analyzer to determine the removal rate of nitrogen oxides. The results are shown in Fig. 1.

[0045]

Comparative Example 1 Only the first purification material used in Example 1 (a porous γ-alumina carrying a silver component) was used, and this was used in Example 1.
It was put into a reaction tube in the same manner as in, and the removal rate of nitrogen oxide was determined in the same manner as in Example 1. The test results are shown in FIG.

Example 2 Using the purification material prepared in Example 1, the gases having the compositions shown in Table 2 were used to obtain the nitrogen oxide removal rate in the same manner as in Example 1. The test results are shown in FIG.

[0048]

Comparative Example 2 Using the purifying material prepared in Comparative Example 1 and using the gases having the compositions shown in Table 2, the removal rate of nitrogen oxides was determined in the same manner as in Example 1. The test results are shown in FIG.

As can be seen from the above, in Examples 1 and 2, good removal of nitrogen oxides was observed at the exhaust gas temperature of 250 to 550 ° C. On the other hand, in Comparative Examples 1 and 2, the removal rate of nitrogen oxides is significantly reduced at the exhaust gas temperature of 400 ° C. or lower.

[0051]

As described above in detail, by using the exhaust gas purifying material of the present invention, nitrogen oxides in exhaust gas containing excess oxygen can be efficiently removed in a wide temperature range. INDUSTRIAL APPLICABILITY The exhaust gas purifying material and the purifying method of the present invention can be widely used for removing nitrogen oxides contained in exhaust gas of various combustors, automobiles and the like.

[Brief description of drawings]

FIG. 1 is a graph showing the relationship between exhaust gas temperature and nitrogen oxide removal rate in Example 1 and Comparative Example 1.

FIG. 2 is a graph showing the relationship between exhaust gas temperature and nitrogen oxide removal rate in Example 2 and Comparative Example 2.

Claims (3)

[Claims] 1. An exhaust gas purification material for removing nitrogen oxides from a combustion exhaust gas containing nitrogen oxides and oxygen in a larger amount than the theoretical reaction amount for coexisting unburned components, the first catalyst comprising a first catalyst. Purification material, a second purification material consisting of a second catalyst is mixed, the first catalyst is a porous inorganic oxide carrying silver or silver oxide as an active species,
The second catalyst is an active species for the porous inorganic oxide.
(a) one or more alkali metal elements, and (b) Cu,
One or two elements selected from the group consisting of Co, Mn and V
At least one species and (c) one or more rare earth elements are supported, and the amount of the first catalytically active species is 0.1% with respect to 100% by weight of the porous inorganic oxide. ˜15% by weight (elemental conversion value), and the amount of the second catalytically active species is 0.5 with respect to 100% by weight of the porous inorganic oxide.
20% by weight (elemental conversion value), an exhaust gas purifying material. 2. The exhaust gas purifying material according to claim 1, wherein the porous inorganic oxide of the first purifying material is alumina or an alumina-based composite oxide. 3. An exhaust gas purification method for removing nitrogen oxides from combustion exhaust gas containing nitrogen oxides and oxygen in a larger amount than the theoretical reaction amount for coexisting unburned components, using the exhaust gas purification material according to claim 1 or 2. In the method, the exhaust gas purification material is installed in the middle of an exhaust gas conduit, hydrocarbons are added to the exhaust gas on the upstream side of the purification material, and the exhaust gas to which the hydrocarbon is added is contacted with the purification material at 250 to 600 ° C. Then, the hydrocarbon in the exhaust gas is caused to react with the nitrogen oxide, thereby removing the nitrogen oxide.