JPH06198195A - Purifying material for exhaust gas and purifying method thereof - Google Patents

Abstract

PURPOSE:To provide a purifying material and purifying method for exhaust gas by which nitrogen oxides can be efficiently removed from combustion exhaust gas containing nitrogen oxides and oxygen in a larger amt. than the stoichiometric amt. for unburnt component such as nitrogen oxide, carbon monoxide, hydrogen, and hydrocarbon. CONSTITUTION:This purifying material for exhaust gas contains a first catalyst in the entrance side for exhaust gas and a second catalyst in the exit side for exhaust gas. The first catalyst consists of a porous inorg. oxide carrying silver or silver oxide by 0.2-15wt.% (calculated as element) as the active component. The second catalyst consists of a porous inorg. oxide carrying (a) 0.2-15wt.% (calculated as element) silver or silver oxide and (b) <=2wt.% copper or copper oxide (calculated as element) as the active component.

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 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, it has been found that these methods have a narrow temperature range for removing nitrogen oxides and that the exhaust gas containing water 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 porous inorganic oxide carrying a specific amount of a silver component and the silver and copper components. Using an exhaust gas purifying material formed by separating the second catalyst supporting the catalyst, the hydrocarbon or oxygen-containing organic compound is added to the exhaust gas, and the exhaust gas is contacted with the catalyst at a specific temperature. For example, they have found that nitrogen oxides can be effectively removed in a wide temperature range even with exhaust gas containing 10% of water, and completed the present invention.

That is, the exhaust gas purifying material of the present invention 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 purification material has a first catalyst on the exhaust gas inflow side and a second catalyst on the exhaust gas outflow side, and the first catalyst is silver or silver oxide that is an active species of a porous inorganic oxide. Things 0.2-1
5% by weight (elemental conversion value) is supported, and the second catalyst is an active species in the porous inorganic oxide (a) silver or silver oxide 0.2 to 15% by weight (elemental conversion value) ) And (b) 2% by weight or less of copper or copper oxide (elemental conversion value).

Further, the first method of the present invention for removing nitrogen oxides from a combustion exhaust gas containing nitrogen oxides and oxygen in a larger amount than the theoretical reaction amount with respect to the coexisting unburned components is the exhaust gas purifying material described above. The exhaust gas, which is installed in the middle of the conduit and is added with a hydrocarbon or an oxygen-containing organic compound on the upstream side of the purification material, is brought into contact with the purification material at 200 to 600 ° C., and thus the hydrocarbon or the oxygen-containing organic material in the exhaust gas is provided. It is characterized in that the nitrogen oxide is removed by a reaction with a compound.

Furthermore, the second method of the present invention 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 is the above exhaust gas purification material. It is installed in the middle of the conduit, and the exhaust gas is brought into contact with the purification material at 200 to 600 ° C.
Therefore, the nitrogen oxides are removed by the reaction with the residual hydrocarbons in the exhaust gas.

The present invention will be described in detail below. In the present invention, on the exhaust gas inflow side, a first catalyst formed by supporting silver or silver oxide which is an active species on a porous inorganic oxide is formed,
Exhaust gas purification material formed on the outflow side by forming a second catalyst comprising (a) silver or silver oxide, which is an active species in a porous inorganic oxide, and (b) copper or copper oxide. Is installed in the exhaust gas conduit, and the residual hydrocarbons in the exhaust gas and / or the hydrocarbons or oxygen-containing organic compounds added to the exhaust gas upstream of the installation position of the purification material are used as reducing agents, and nitrogen oxides in the exhaust gas are used. Is reduced and removed.

A first preferred form of the exhaust gas purifying material of the present invention is a purifying material in which a purifying material base is coated with a catalyst comprising a powdery porous inorganic oxide carrying 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 can be variously changed according to the purpose. Practically, it is preferably composed of a combination of two or more molded bodies having an inlet portion and an outlet portion. Examples of the structure include a honeycomb structure type, a foam type, and a three-dimensional network structure type such as a fibrous refractory.

A second preferred form of the exhaust gas purifying material of the present invention is a purifying material obtained by filling a pellet-like or granular porous inorganic oxide with a catalyst comprising a catalytically active species.

The following two catalysts are formed on the purification material of the present invention. (1) First catalyst The first catalyst is formed by supporting silver or silver oxide on a porous inorganic oxide and is formed on the exhaust gas inflow side of the purification material. As the porous inorganic oxide, porous alumina, silica,
Although titania, zirconia, and their composite oxides can be used, 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, the oxygen-containing organic compound 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 20 m ²
/ G or more is preferable. Specific surface area of 20 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. Particularly preferred specific surface area is 30 m ²
/ G or more.

The amount of silver or silver oxide supported as an active species on the above-mentioned inorganic oxide such as γ-alumina is 0.2 to 15% by weight (element conversion value) based on 100% by weight of the inorganic oxide. And If it is less than 0.2% by weight, the removal rate of nitrogen oxides is lowered. On the other hand, if it exceeds 15% by weight, combustion of the hydrocarbon itself is likely to occur, and the removal rate of nitrogen oxides is rather lowered. A preferable total supported amount is 0.5 to 12% by weight (elemental conversion value). It is considered that silver exists in the form of metal or oxide in the temperature range of exhaust gas.

As a method for supporting the silver component on the inorganic oxide such as alumina, a known impregnation method, precipitation method, sol-gel method or the like can be used. When the impregnation method is used, it is preferable to immerse the porous inorganic oxide in an aqueous solution of silver nitrate or the like, dry at about 70 ° C., and then gradually raise the temperature at 100 to 600 ° C. and bake. This firing can be performed in air, under an oxygen atmosphere, under a nitrogen atmosphere, or under a hydrogen gas flow. When firing in a nitrogen atmosphere or under a flow of hydrogen gas, it is preferable to perform the oxidation treatment last.

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 catalyst is formed on the surface of the purification material substrate by a known washcoat method, sol-gel method, or the like.

Further, 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, the nitrogen oxide cannot be removed well. 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 of the purifying material. As the porous inorganic oxide, similar to the case of the first catalyst, γ-alumina, zirconia, titania, and composite oxides thereof and the like can be mentioned, but preferably γ-alumina, titania, zirconia and them are included. A complex oxide is used.
As with the first catalyst, the specific surface area of the porous inorganic oxide is preferably 20 m ² / g or more, and particularly 30 m ²
/ G or more is preferable.

As the active species of the second catalyst, (a) silver or silver oxide and (b) copper or copper oxide are mixed and used. The supported amount of silver is 0.2 to 15% by weight (metal element conversion value) with 100% by weight of the porous inorganic oxide.
If the supported amount is less than 0.2% by weight or exceeds 15% by weight,
The removal rate of nitrogen oxides decreases. The amount of silver supported is preferably 0.5 to 12% by weight (metal element conversion value).

When the porous inorganic oxide is 100% by weight,
The amount of copper or copper oxide supported is 2% by weight or less (metal element conversion value). When the supported amount exceeds 2% by weight, the nitrogen oxide removal rate decreases. The preferable loading amount of copper or copper oxide is 0.1 to 1.5% by weight (metal element conversion value).

The total amount of active species supported on the inorganic oxide by the second catalyst ((a) + (b)) is 0.2 to 17 based on the above-mentioned porous inorganic oxide (100% by weight). The weight% (metal element conversion value), preferably 0.5 to 14 weight% (metal element conversion value). If the amount of catalytically active species exceeds 17% by weight, the amount of supported catalyst will increase only the oxidative combustion of hydrocarbons,
The reduction characteristics of nitrogen oxides will deteriorate. Further, if the supported amount is less than 0.2% by weight, the nitrogen oxide removing performance in the low temperature region is deteriorated.

The second catalyst can further support an alkali metal and a rare earth element to improve heat resistance. As the alkali metal element, it is particularly preferable to use cesium, sodium and potassium. As the rare earth element, lanthanum, cerium or neodymium is preferably used, but a misch metal which is a mixture of rare earth elements can also be used. When an alkali metal element and a rare earth element are used together in the second catalyst, the total supported amount of the alkali metal element and the rare earth element is 0.1% by weight or less (element conversion value).
And

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 the impregnation method is used, the porous inorganic oxide is immersed in an aqueous solution of a carbonate, a hydrochloride, a nitrate, an acetate, a hydroxide or the like of a catalytically active species element, dried at 70 ° C, and then staged at 100 to 600 ° C. The temperature is increased and the firing is performed. Although the supported component is shown as a metal element, it is considered that the supported component exists in the state of a metal and an oxide under the normal use temperature condition 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 preferably 200 μm 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.

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 removal 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 nitrogen oxide removing ability 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 and 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 600 ° 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. First, the exhaust gas purifying material is installed in the middle of the exhaust gas conduit so that the first catalyst faces the exhaust gas inlet and the second catalyst faces the exhaust gas outlet.

The exhaust gas contains ethylene, propylene, etc. to some extent as residual hydrocarbons. NO in exhaust gas
If the amount is not sufficient to reduce x, a reducing agent composed of a hydrocarbon or an oxygen-containing organic compound is externally introduced into the exhaust gas. 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, the number of carbon atoms is preferably 3 or more. Specific examples of the liquid hydrocarbon in the standard state include hydrocarbons such as acetylene, light oil, cetane, heptane, and kerosene. As the oxygen-containing organic compound, alcohols such as ethanol can be used.

The amount of the hydrocarbon or the oxygen-containing organic compound introduced from the outside should be such that the weight ratio (weight of reducing agent added / weight of nitrogen oxide in exhaust gas) is 0.1 to 5. preferable. 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.

In the present invention, the contact time between the exhaust gas and the catalyst is adjusted in order to adjust the time during which the exhaust gas containing the hydrocarbon or the oxygen-containing organic compound is in contact with the above-mentioned catalyst, and to promote the reduction reaction of nitrogen oxides efficiently. Is preferably 0.006 seconds · g / ml or more. If the contact time is less than 0.006 seconds · g / ml, the removal efficiency will be low. The preferred contact time is 0.
007 seconds / g / ml or more.

Further, in the present invention, the temperature of the exhaust gas is maintained at 200 to 600 ° C. at the purification material installation site where the hydrocarbon or oxygen-containing organic compound reacts with the nitrogen oxide. If the temperature of the exhaust gas is less than 200 ° 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, when the temperature exceeds 600 ° C, the combustion of the hydrocarbon or oxygen-containing organic compound itself starts,
Nitrogen oxide cannot be reduced and removed. A preferable exhaust gas temperature is 300 to 600 ° C.

[0039]

The present invention will be described in more detail by the following specific examples. Example 1 Using an aqueous solution of silver nitrate, 2% by weight of silver was supported on commercially available pelletized γ-alumina (specific surface area 200 m ² / g),
After drying, it was calcined stepwise in air to 600 ° C. to prepare a silver-based catalyst. Similarly, using copper nitrate and silver nitrate aqueous solution, 2% by weight of silver and 0.07% by weight of copper were supported on pelletized γ-alumina, and after drying, stepwise in air 6
It was baked to 00 ° C. to prepare a silver-copper catalyst.

A purifying material was combined in the reaction tube so that the inflow side of the exhaust gas was 3.75 g and the outflow side was 3.75 g of silver and copper catalyst. Next, gases having the composition shown in Table 1 (nitric oxide, oxygen, propylene, and nitrogen)
At a flow rate of 4.4 liters per minute (standard state) (total apparent space velocity of about 15,000 h ⁻¹ , contact time between silver-based catalyst and copper-based catalyst is both 0.05 sec · g / ml) The exhaust gas temperature in the reaction tube was kept in the range of 200 to 600 ° C., and propylene and nitrogen oxide were reacted.

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 nitrogen oxide removal rate. The results are shown in Fig. 1.

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

Example 2 In the same manner as in Example 1, powdery γ-alumina (average particle size 40 μm, specific surface area 200) was prepared using an aqueous solution of silver nitrate.
m ² / g) and 1.4 g of a catalyst in which 3% by weight of silver is supported on a commercially available cordierite honeycomb molded body (diameter 30 mm,
A length of 12.5 mm) was coated by a wash coating method, dried, and baked to 600 ° C. to prepare a silver-based purification material. Similarly, using a copper nitrate and silver nitrate aqueous solution, 1.4 g of a catalyst in which 3% by weight of silver and 0.09% by weight of copper were supported on powdery γ-alumina was coated on the same honeycomb formed body, and then dried. A silver / copper purification material was prepared by firing to 600 ° C.

A purifying material was set in the reaction tube so that a silver-based purifying material was placed on the inflow side of the exhaust gas and silver and a copper-based purifying material were placed on the outflow side. Evaluation was carried out under the same reaction conditions as in Example 1 (overall apparent space velocity: about 15,000 h ⁻¹ ) using a gas having the composition shown in Table 1. The results are shown in Fig. 1.

Comparative Example 1 In the same manner as in Example 1, a purification material was prepared by loading 2% by weight of silver on γ-alumina pellets. The purification material 7.50
g was set in a reaction tube and evaluated using the gas having the composition shown in Table 1 under the same conditions as in Example 1. The experimental results are shown in FIG.

As can be seen from the above, in Examples 1 and 2 in which the silver-based catalyst and the silver-copper-based catalyst were combined, good removal of nitrogen oxides was observed over a wide range of exhaust gas temperatures. On the other hand, in Comparative Example 1 using only the silver-based catalyst, the temperature range for removing nitrogen oxides is narrow.

[0047]

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 nitrogen oxide removal rate in Examples 1 and 2 and Comparative Example 1.

Claims (6)

[Claims] 1. An exhaust gas purification material for removing nitrogen oxides from a combustion exhaust gas containing nitrogen oxides and oxygen in an amount larger than a theoretical reaction amount for coexisting unburned components, wherein a first catalyst is provided on the exhaust gas inflow side of the purification material. And has a second catalyst on the exhaust gas outflow side, and the first catalyst has 0.2 to 15% by weight of silver or silver oxide which is an active species in a porous inorganic oxide.
(Element-converted value), and the second catalyst is a porous inorganic oxide which is an active species (a) silver or silver oxide.
An exhaust gas purifying material, which carries 2 to 15% by weight (elemental conversion value) and (b) 2% by weight or less of copper or copper oxide (elemental conversion value). 2. The exhaust gas purifying material according to claim 1, wherein the purifying material is obtained by coating the surface of a ceramic or metal base with the first and second catalysts. Material. 3. The exhaust gas purifying material according to claim 1, wherein the porous inorganic oxides of the first and second catalysts are in the form of pellets or granules, respectively. 4. The exhaust gas purifying material according to claim 1, wherein the porous inorganic oxide is alumina.
An exhaust gas purification material, which is a composite oxide composed of one or more of titania and zirconia. 5. 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 purifying material according to any one of to 4 is installed in the middle of an exhaust gas conduit, and the exhaust gas to which a hydrocarbon or an oxygen-containing organic compound is added on the upstream side of the purifying material is used as the purifying material at 200 to 600 ° C. A method for purifying an exhaust gas, which comprises contacting and then removing the nitrogen oxide by reacting with a hydrocarbon or an oxygen-containing organic compound in the exhaust gas. 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.
The exhaust gas purifying material according to any one of 4 to 4 is installed in the middle of an exhaust gas conduit, and the exhaust gas is brought into contact with the purifying material at 200 to 600 ° C., whereby the nitrogen is produced by a reaction with residual hydrocarbons in the exhaust gas. An exhaust gas purification method characterized by removing oxides.