JPH06319998A - Material and method for decontaminating exhaust gas - Google Patents

Abstract

PURPOSE:To eliminate efficiently nitrogen oxide from an exhaust combustion gas contg. at last the theoretical amt. of reaction of oxygen to unburnt part by a method wherein a specified decontaminating material for exhaust gas is used and a hydrocarbon is incorporated as a reducing agent and nitrogen oxide in the exhaust as is reduced at a specified temp. CONSTITUTION:In a decontaminating material for exhaust gas wherein the first catalyst is set on an exhaust gas inlet side and the second catalyst is set on the exhaust gas outlet side, the first catalyst carried 0.2-15wt.% (in terms of the element) silver or silver oxide as an active species on a porous inorg. oxide and the second catalyst carries more than 2wt.% and less than 5wt.% at least one of elements selected from the group of Pt, Pd, Ru, Rh and Ir as an active species on a porous inorg. oxide. In addition, silver or silver oxide has a mean diameter of 10-500nm and a hydrocarbon is incorporated as a reducing agent in the exhaust gas from the outside to reduce nitrogen oxide in the exhaust gas at 200-650 deg.C.

Description

Detailed Description of the Invention

[0001]

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

[0002]

2. Description of the Related Art Excessive amounts of combustion exhaust gas emitted 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, it has been found that these methods have a narrow temperature range for removing nitrogen oxides, and that the exhaust gas containing water and having a large temperature change depending on operating conditions has a significantly low nitrogen oxide removal rate. .

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 inventors have found that the first catalyst comprising a specific amount of a silver component of a specific shape supported on a porous inorganic oxide, Pt, etc. The second catalyst supporting the above components is prepared separately, and the exhaust gas purifying material formed by combining them is used to add hydrocarbons to the exhaust gas and contact the exhaust gas with the above catalyst at a specific temperature. By doing so, it was discovered that nitrogen oxides can be effectively removed in a wide temperature range even with exhaust gas containing 10% of water, and the present invention has been completed.

That is, the first exhaust gas purifying material of the present invention has the first catalyst on the exhaust gas inflow side and the second catalyst on the exhaust gas outflow side, and the first catalyst is porous. 0.2 to 15% by weight of silver or silver oxide which is an active species in the inorganic oxide
(Element conversion value), wherein the second catalyst is at least one element 2 selected from the group consisting of Pt, Pd, Ru, Rh and Ir, which are active species in the porous inorganic oxide. More than 5% by weight and 5% by weight or less are supported, the silver or silver oxide has an average diameter of 10 to 500 nm, and hydrocarbon is added to the exhaust gas as a reducing agent from the outside to obtain 200 to 65%.
It is characterized in that nitrogen oxides in the exhaust gas are reduced at 0 ° C.

In the second exhaust gas purifying material of the present invention, the first catalyst and the second catalyst are mixed and used, and the first catalyst is a porous inorganic oxide containing silver as an active species or Pt containing 0.2 to 15% by weight of silver oxide (elemental conversion value), and the second catalyst is a porous inorganic oxide which is an active species.
At least one element selected from the group consisting of Pd, Ru, Rh, and Ir is contained in an amount of more than 2% by weight and 5% by weight or less, and the silver or silver oxide has an average diameter of 10 to 500 nm. A hydrocarbon is externally added to the exhaust gas as a reducing agent to reduce nitrogen oxides in the exhaust gas at 200 to 650 ° C.

Furthermore, 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. The exhaust gas added with hydrocarbons on the upstream side of the purification material is heated to 200 to 600 ° C.
In step 2, the nitrogen oxides are removed by contact with the purifying material and thereby reacting with the hydrocarbon in the exhaust gas.

The present invention will be described in detail below. In the purifying material of the first aspect of the present invention, on the exhaust gas inflow side, 0.2 to 15% by weight (element conversion value) of silver or silver oxide, which is an active species, is supported by a porous inorganic oxide. Pt, Pd, Ru, Rh, which form a catalyst and are active species in the porous inorganic oxide on the outflow side,
An exhaust gas purifying material is formed in the exhaust gas conduit by installing an exhaust gas purifying material in which a second catalyst is formed in the amount of more than 2% by weight of at least one element selected from the group consisting of Ir and 5% by weight or less. Hydrocarbons are added to the exhaust gas upstream of the installation position of the material to bring the exhaust gas into contact with this purification material, and the nitrogen oxides in the exhaust gas are reduced and removed using the hydrocarbon as a reducing agent.

In the second purifying material of the present invention, 0.2 to 15% by weight of silver or silver oxide, which is an active species, is added to the porous inorganic oxide.
The first catalyst supporting (element conversion value), and 2 weight of at least one element selected from the group consisting of Pt, Pd, Ru, Rh, and Ir which are active species in the porous inorganic oxide. %, And an exhaust gas purifying material mixed with a second catalyst supporting 5% by weight or less is installed in the exhaust gas conduit, and hydrocarbons are contained in the exhaust gas upstream from the installation position of the purifying material. The exhaust gas is added to bring the exhaust gas into contact with the purification material, and the hydrocarbon is used as a reducing agent to reduce and remove nitrogen oxides in the exhaust gas.

Both of the above exhaust gas purifying materials can take the following two forms. A first preferred form is a purification material obtained by coating a purification material substrate with a catalyst in which a powdery porous inorganic oxide carries a catalytically active species. As the ceramic material forming the substrate of the purification material, γ-alumina and its oxides (γ-alumina-titania, γ-alumina-silica, γ-alumina-zirconia, etc.), zirconia, titania-zirconia, and other porous materials are used. A heat-resistant material having a large surface area can be used. When high heat resistance is required, it is preferable to use cordierite, mullite, alumina and their composites. Also, a known metal material can be used for the substrate of the exhaust gas purifying material.

The shape and size of the substrate of the exhaust gas purifying material are
Various changes can be made according to the purpose. Practically, it is preferable to have two or more parts including an inlet part and an outlet part. Examples of the structure include a honeycomb structure type, a foam type, a plate type, a three-dimensional network structure type such as a fibrous refractory, a granular type, a pellet type and the like.

A second preferred form is a purifying material obtained by filling a pellet-shaped, granular-shaped, or powdered porous inorganic oxide with a catalyst having a catalytically active species described below.

The following two catalysts are formed in the purification material of the present invention. (1) First catalyst The first catalyst comprises a porous inorganic oxide carrying a silver component, and is formed on the exhaust gas inflow side. As the porous inorganic oxide, porous alumina, silica, titania, zirconia, and their composite oxides can be used, but γ-alumina or alumina-based composite oxide is preferably 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 10 m ²
/ G or more is preferable. Specific surface area of 10 m 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 silver component supported as an active species on the above-mentioned inorganic oxide such as γ-alumina is 10
0.2 to 15% by weight (elemental conversion value) relative to 0% by weight. If it is less than 0.2% by weight, the removal rate of nitrogen oxides is lowered. 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.

The silver or silver oxide supported on the porous inorganic oxide is granular, but in the purifying material of the present invention, the average diameter of the silver or silver oxide particles is 10 to 500 nm. In general, the smaller the particle size of the silver component, the higher the reaction characteristics. However, when the average particle size is less than 10 nm, only the oxidation reaction of the hydrocarbon that is the reducing agent proceeds, and the removal rate of nitrogen oxides increases. descend. On the other hand, when the average particle size exceeds 500 nm, the reaction characteristics of the silver component are reduced and the nitrogen oxide removal rate is reduced. A preferable average particle size is 10 to 200 nm.
In addition, the average here means an arithmetic mean.
Further, the particulate silver component exists in the form of metallic silver or silver oxide, but exists in the form of silver oxide on the surface of the particles.

As a method for supporting the silver component on the inorganic oxide such as γ-alumina, a known impregnation method, a kneading method or the like can be used. Adjustment of the purifying material after loading is 50 to 15
After drying at about 0 ° C., it is preferable to raise the temperature stepwise at 100 to 600 ° C. and bake. The firing is preferably performed in air or under nitrogen flow, or under hydrogen gas flow, or while evacuating. By calcining at 600 ° C. for one hour or more in an oxidizing atmosphere, the particle size of the supported silver component can be set to a desired size. Baking temperature is 600 ℃
In the above case, it is preferable to shorten the firing time. For example, when firing at 700 ° C., the firing time is within 3 hours. At this time, if nitrogen oxide is present, the firing can be performed more effectively. In addition, it is preferable that the purification material fired under the flow of nitrogen gas or hydrogen gas is finally oxidized at 700 ° C. or lower.

In the first preferred embodiment of the purification material, the thickness of the first catalyst provided on the purification material substrate is generally limited due to the difference in thermal expansion characteristics between the substrate material and this catalyst. In many cases. The thickness of the catalyst provided on the purification material substrate is preferably 200 μm or less. With such a thickness, it is possible to prevent the purification material from being damaged by thermal shock during use. The method of forming the catalyst on the surface of the purifying material substrate is a known washcoat method, sol-gel method, powder method or the like.

The amount of the first catalyst provided on the surface of the purification material substrate is preferably 5 to 70% by weight of the purification material substrate. If the amount of the catalyst is less than 5% by weight, good NOx cannot be removed. On the other hand, when the amount of the catalyst exceeds 70% by weight, the removal characteristics do not improve so much and the pressure loss increases. More preferably, the first catalyst provided on the surface of the purification material substrate is 10 to 70% by weight of the purification material substrate.

(2) Second catalyst The second catalyst comprises a porous inorganic oxide carrying a catalytically active species and is formed on the exhaust gas outflow side. As the porous inorganic oxide, γ-alumina and its oxide (γ-alumina-titania, γ-alumina-silica, γ-alumina-
Examples thereof include heat-resistant ceramics having a large surface area such as zirconia), zirconia, and titania-zirconia. Preferably, γ-alumina, titania, zirconia and a composite oxide containing them are used. Like the first catalyst, the porous inorganic oxide has a specific surface area of 10 m ² /
It is preferably at least g.

As the active species of the above-mentioned second catalyst, Pt, P
At least one element selected from the group consisting of d, Ru, Rh, and Ir is used. In particular, Pt and Rh; Pd and Rh; P
The combination of t, Pd and Rh is effective. The total amount of active species supported on the inorganic oxide by the second catalyst is more than 2% by weight and 5% by weight or less based on the above-mentioned porous inorganic oxide (100% by weight). When the amount of the catalytically active species exceeds 5% by weight with respect to the substrate, only the oxidative combustion of hydrocarbons proceeds, and the nitrogen oxide reduction property deteriorates. Further, if it is less than 2% by weight, the effect is not sufficiently exhibited.

Further, it is preferable that 10% by weight or less of at least one element selected from rare earth elements such as La and Ce is further supported as the active species of the second catalyst.
By supporting the rare earth element, the heat resistance of the platinum-based catalyst can be improved.

For supporting the active species on the second catalyst, a known impregnation method, precipitation method, sol-gel method or the like can be used. When using the impregnation method, the porous inorganic oxide is immersed in an aqueous solution of a carbonate, a hydrochloride, a nitrate, an acetate, or a hydroxide of a catalytically active species element, dried at 70 ° C., and then 100 to 700 ° C.
It is carried out by gradually heating and firing at. Although the supported component is shown as a metal element, the supported component exists in the state of a metal and an oxide under normal use temperature conditions of the purification material.

In the first preferred embodiment of the purification material, the thickness of the second catalyst provided on the purification material substrate is 100 μm.
It is preferably m or less. Further, the amount of the second catalyst provided on the surface of the purification material substrate is preferably 5 to 70% by weight of the purification material substrate. The method of coating the purifying material substrate with the second catalyst is the same as the method of the first catalyst described above.

In the present invention, the weight ratio of the first catalyst to the second catalyst is preferably 5: 1 to 1: 5. The ratio is less than 1: 5 (less first catalyst)
Then, 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 the reducing agent and the nitrogen oxide does not proceed sufficiently at a relatively low temperature.
More preferable weight ratio of the first catalyst to the second catalyst is from 4: 1 to
It is 1: 4.

If the purifying material having the above-mentioned constitution is used,
In a wide temperature range of up to 650 ° C., good nitrogen oxides can be removed even with exhaust gas containing about 10% of water.

Next, the method of the present invention will be described. In the first exhaust gas purifying material of the present invention, the first catalyst faces the inlet of the exhaust gas, and the second catalyst faces the outlet of the exhaust gas. In the second exhaust gas purification material of the present invention, the 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 a reducing agent composed of a hydrocarbon is externally supplied to the exhaust gas. Introduce inside. The introduction position of the reducing agent is upstream of the position where the purification material is installed.

As the hydrocarbon introduced from the outside, gaseous or liquid alkane, alkene and / or alkyne in a standard state can be used. Particularly in the case of alkane or alkene, carbon number of 2 or more is preferable. Specific examples of the liquid hydrocarbon in the standard state include hydrocarbons such as acetylene, light oil, cetane, heptane, and kerosene.

The amount of hydrocarbons introduced from the outside is preferably such that the weight ratio (weight of reducing agent to be added / weight of nitrogen oxide in exhaust gas) is 0.1 to 5. If this weight ratio is less than 0.1, the nitrogen oxide removal rate does not increase. On the other hand, when it exceeds 5, fuel consumption is deteriorated.

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 200 to 650 ° C. Exhaust gas temperature is 20
If the temperature is lower than 0 ° C, the reaction between the reducing agent and the nitrogen oxide does not proceed, and the nitrogen oxide cannot be removed satisfactorily. On the other hand, if the temperature exceeds 650 ° C., the combustion of hydrocarbon itself starts, and the nitrogen oxide cannot be reduced and removed. A preferable exhaust gas temperature is 300 to 600 ° C.

[0038]

The present invention will be described in more detail by the following specific examples. Example 1 5 g of commercially available pelletized γ-alumina (diameter: 1.5 mm, length: about 6 mm, specific surface area: 200 m ² / g) was loaded with 3% by weight of silver using an aqueous solution of silver nitrate, and dried up to 600 ° C. The catalyst 1 was prepared by calcining stepwise. The average diameter of the silver or silver oxide particles was 45 nm. Also, 5 g of the same pellet-like alumina was loaded with 3% by weight of Pt using an aqueous solution of chloroplatinic acid, dried and then calcined to 700 ° C.
Was prepared.

A purification material in which 3.7 g of the silver-based catalyst and 1.8 g of the Pt-based catalyst were mixed was set in the reaction tube. Next, a gas having a composition shown in Table 1 (nitrogen monoxide, carbon monoxide, oxygen, propylene, and nitrogen) was flowed at a flow rate of 4.4 liters per minute (standard state) (total apparent space velocity of about 20). 1,000 h
^-1 , the contact time between silver-based catalyst and Pt-based catalyst is about 0.0
5, 0.025 sec · g / ml), the temperature of the exhaust gas in the reaction tube was kept in the range of 150 to 650 ° C, and propylene was reacted with nitrogen oxides.

The concentration of nitrogen oxides in 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.

Table 1 Component Concentration Nitric oxide 800 ppm Carbon monoxide 100 ppm Oxygen 10% by volume Moisture 10% by volume Propylene 1714 ppm Nitrogen balance

Example 2 Powdery γ-alumina (specific surface area 2
A catalyst having 5% by weight of silver supported on 00 m ² / g) was prepared, and about 1.0 g of this catalyst was commercially available and made of a cordierite honeycomb-shaped compact (diameter 30 mm, length 12.6 mm, 400 cells / square inch). ) Was coated, dried and then fired stepwise to 600 ° C. to prepare a purification material. The average diameter of the silver or silver oxide particles was 50 nm. In addition, the same honeycomb shaped body (length 6 mm) was coated with 0.4 g of a catalyst in which 4 wt% of Pt was supported on powdery γ-alumina using a chloroplatinic acid solution, and the temperature was 700 ° C. after drying. Was fired to prepare a purification material. A silver-based purification material on the inflow side of the exhaust gas,
It was set in the reaction tube so as to be a platinum-based purification material on the outflow side. Example 1 except that, in the composition shown in Table 1, instead of propylene, light oil having a mass three times that of nitric oxide was used.
Was evaluated in the same manner as (total apparent space velocity 20,000
h ^-1 ). The experimental results are shown in FIG.

Comparative Example 1 In the same manner as in Example 1, 10 g of γ-alumina pellets
A purification material was prepared by supporting 5% by weight of silver. The average diameter of the silver particles on the purification material was 8 nm. This purification material 5.4
g was set in a reaction tube, and the gas having the composition shown in Table 1 was evaluated. The experimental results are shown in FIG.

As can be seen from the above, in Examples 1 and 2, good removal of nitrogen oxides was observed over a wide range of exhaust gas temperatures. On the other hand, in Comparative Example 1, the temperature range for removing nitrogen oxides was narrow.

[0045]

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 purifying 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 the removal rate of nitrogen oxides in exhaust gas in Examples 1 and 2 and Comparative Example 1.

Claims (6)

[Claims] 1. An exhaust gas purification material having a first catalyst on the exhaust gas inflow side and a second catalyst on the exhaust gas outflow side,
The first catalyst is a porous inorganic oxide carrying 0.2 to 15% by weight (element conversion value) of silver or silver oxide as an active species, and the second catalyst is a porous inorganic oxide. The oxide contains at least one element selected from the group consisting of Pt, Pd, Ru, Rh, and Ir which is an active species in an amount of more than 2% by weight and 5% by weight or less. The average diameter is 10-5
An exhaust gas purifying material having a thickness of 00 nm and externally adding hydrocarbon as a reducing agent to the exhaust gas to reduce nitrogen oxides in the exhaust gas at 200 to 650 ° C. 2. An exhaust gas purifying material which is used as a mixture of a first catalyst and a second catalyst, wherein the first catalyst is a porous inorganic oxide containing silver or silver oxide 0.2 to 0.2 as an active species. Pt, Pd, Ru, Rh and Ir, which are 15% by weight (elemental conversion value) supported and in which the second catalyst is an active species in the porous inorganic oxide.
2% by weight of at least one element selected from the group consisting of
5% by weight or less is supported, the silver or silver oxide has an average diameter of 10 to 500 nm, and a hydrocarbon is added to the exhaust gas from the outside as a reducing agent.
An exhaust gas purification material, which reduces nitrogen oxides in the exhaust gas at 650 ° C. 3. The exhaust gas purifying material according to claim 1 or 2, wherein the purifying material is formed by coating the surface of a ceramic or metal three-dimensional structure with the first and second catalysts. Characteristic exhaust gas purification material. 4. The exhaust gas purifying material according to claim 1 or 2, wherein the porous inorganic oxides of the first and second catalysts are in the form of pellets or granules, respectively. 5. The exhaust gas purifying material according to claim 1, wherein the porous inorganic oxide is alumina or an alumina-based composite oxide in the first catalyst and alumina in the second catalyst. An exhaust gas purification material, which is a composite oxide composed of either titania or zirconia or two or more thereof. 6. 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.
~ 5, using the exhaust gas purification material, the exhaust gas purification material is installed in the middle of the exhaust gas conduit, the exhaust gas to which hydrocarbon is added upstream of the purification material, 200 ~ 650 ℃
The method for purifying exhaust gas, wherein the nitrogen oxide is removed by contacting with the purifying material and reacting with the hydrocarbon in the exhaust gas.