JPH0671175A - Catalyst and method for removing nitrogen oxide - Google Patents

Abstract

PURPOSE:To provide a catalyst capable of removing nitrogen oxide effectively from the combustion exhaust gas containing nitrogen oxide and an excessive amount of oxygen. CONSTITUTION:The nitrogen oxide removing catalyst comprises a porous inorganic oxide and silver or silver oxide supported thereon in an amount of 2-15 pts.wt. per 100 pts.wt. of the inorganic oxide based on the weight of silver element converted therefrom and the catalizing method thereof comprises reducing the nitrogen oxide in the exhaust gas at a temperature of 250-600 deg.C by using as a reducing agent the hydrocarbon added to the exhaust gas from outside and/or the residual hydrocarbon therein.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a catalyst capable of effectively removing nitrogen oxides from combustion exhaust gas containing nitrogen oxides and excess oxygen, and a method for removing nitrogen oxides 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, although these methods can remove nitrogen oxides with high efficiency from a simulated exhaust gas containing no water, the actual exhaust gas contains about 10% of water, so It was found that the oxide removal rate was significantly reduced. Also, with these methods,
There is also the disadvantage that the optimum temperature for the reduction reaction of nitrogen oxides is as high as 400 to 600 ° C.

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,
It is an object of the present invention to provide a nitrogen oxide removal catalyst and a removal method capable of efficiently removing nitrogen oxides.

[0010]

As a result of earnest research in view of the above problems, the present inventor has used a catalyst in which a specific amount of a silver component is supported on a porous inorganic oxide, and the catalyst has a specific temperature. When the exhaust gas is brought into contact with the exhaust gas for a contact time, even if the exhaust gas contains about 10% of water, the amount of hydrocarbons added to the exhaust gas and //
Alternatively, the inventors have found that nitrogen oxides can be effectively removed by residual hydrocarbons in exhaust gas, and completed the present invention.

That is, the catalyst of the present invention for removing nitrogen oxides from a combustion exhaust gas containing nitrogen oxides and oxygen in an amount larger than the theoretical reaction amount for coexisting unburned components is 100 parts by weight of a porous inorganic oxide. 2 to 15 parts by weight of silver or silver oxide converted into elemental silver is supported, and the hydrocarbon added from the outside into the exhaust gas and / or the residual hydrocarbon in the exhaust gas is used as a reducing agent, It is characterized in that nitrogen oxides in the exhaust gas are reduced at 600 ° C.

Further, the method of the present invention for removing nitrogen oxides from a combustion exhaust gas containing nitrogen oxides and oxygen in an amount larger than the theoretical reaction amount with respect to the coexisting unburned components is adjusted to 100 parts by weight of a porous inorganic oxide. A catalyst formed by supporting 2 to 15 parts by weight of silver or silver oxide converted to silver element is installed in the middle of the exhaust gas conduit, and the exhaust gas is brought into contact with the catalyst at 250 to 600 ° C. The time in which 1 ml (value converted to the standard state) is in contact with 1 g of the catalyst is 0.3 seconds or less, and the residual hydrocarbons in the exhaust gas are reacted with the nitrogen oxides to remove the nitrogen oxides. Is characterized by.

The present invention will be described in detail below. In the present invention, the catalyst shown below is used, and by contacting the exhaust gas with this catalyst, the hydrocarbon in the exhaust gas and / or the hydrocarbon added to the exhaust gas on the upstream side of the site where the catalyst is installed are used as a reducing agent in the exhaust gas. The nitrogen oxides of are reduced and removed.

The catalyst of the present invention comprises a porous inorganic oxide carrying a silver component. As the porous inorganic oxide, porous alumina, titania, zirconia, and their composite oxides can be used, but preferably γ-
Alumina or an 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 inorganic oxide such as γ-alumina can be used in the form of pellets, powder, honeycomb, foam, plate or the like.

On the surface of a honeycomb-shaped or foam-shaped molded body (carrier) made of cordierite, mullite, or another ceramic material or metal having excellent heat resistance,
-A layer made of an inorganic oxide such as alumina may be formed by a known method (for example, a wash coat method) and used as a silver component carrying layer.

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 dispersion of the silver component (silver active species) in the inorganic oxide becomes poor, and the nitrogen oxide cannot be removed well.

The amount of the silver component supported on the above-described inorganic oxide such as γ-alumina is 2 to 15 parts by weight (silver element conversion value) with 100 parts by weight of the inorganic oxide. When the silver component is less than 2 parts by weight, the removal rate of nitrogen oxides on the low temperature side decreases. In particular, this tendency becomes remarkable when the contact time of the exhaust gas becomes short. On the other hand, if silver is loaded in an amount of more than 15 parts by weight, the hydrocarbon itself is easily burned, and the nitrogen oxide removal rate is rather lowered. Preferably, the amount of silver supported is more than 5 parts by weight and 15 parts by weight or less with respect to 100 parts by weight of the inorganic oxide. 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 of supporting the silver component on the inorganic oxide such as γ-alumina, a known dipping method can be used. At that time, the porous inorganic oxide is 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 00 to 600 ° C. and perform the firing. The firing is preferably performed in air or under air circulation, or under hydrogen gas circulation, or while evacuating.
The catalyst calcined under the flow of hydrogen gas is then calcined again under the flow of nitrogen gas containing oxygen, preferably at 500 to 600 ° C, more preferably at 550 to 600 ° C.

In the present invention, an alkali metal element or a platinum group element can be used in combination with silver. The supported amount of alkali metal element or platinum group element is 1 of inorganic oxide.
It should be less than or equal to weight%.

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

The exhaust gas contains acetylene, methane, ethane, propylene and the like as residual hydrocarbons, but when the residual hydrocarbons are not contained in an amount sufficient to reduce NOx in the exhaust gas, Hydrocarbons are introduced into the exhaust gas from the outside. The introduction position of the hydrocarbon is upstream of the position where the catalyst is installed.

As the hydrocarbon introduced from the outside, in addition to a gaseous hydrocarbon in a standard state such as propylene, acetylene, propane, a hydrocarbon in a liquid state in a standard state can be used. As a liquid hydrocarbon in the standard state,
Specific examples include light oil, cetane, heptane, and kerosene. These liquid hydrocarbons can be introduced into the exhaust gas by a method such as spraying.

The amount of hydrocarbons introduced from the outside is preferably 400% by volume or less of NOx in the exhaust gas when the added hydrocarbons are gaseous. More preferably, it is 300% by volume or less. When liquid hydrocarbons are used, it is preferable that the ratio (weight of liquid hydrocarbons added / weight of NOx in exhaust gas) be 0.2 to 5.

In the present invention, the time during which the exhaust gas containing hydrocarbon contacts the above-mentioned catalyst is adjusted so that the reaction between hydrocarbon and nitrogen oxides proceeds efficiently. From a practical standpoint, the contact time between exhaust gas containing hydrocarbons and the catalyst is 0.3.
Seconds ・ g / ml or less Here, the contact time represents the time (second) 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 catalyst.
For example, if the contact time is 0.1 sec · g / ml, 1
With g catalyst, 1 ml of exhaust gas is contacted with this catalyst for 0.1 seconds. If the flow rate of the exhaust gas is too high, the purification efficiency will be too low. Therefore, it is preferable to set the lower limit of the contact time to 0.01 sec · g / ml.

Further, in the present invention, the temperature of the exhaust gas at the catalyst installation site, which is the site where hydrocarbons and nitrogen oxides react, is maintained at 250 to 600 ° C. Exhaust gas temperature is 250
If the temperature is lower than ° C, the reaction between hydrocarbons and nitrogen oxides does not proceed, and it is impossible to remove nitrogen oxides satisfactorily. 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.

[0027]

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,
10 g of a length of about 6 mm and a specific surface area of 200 m ² / g) was immersed in an aqueous solution of silver nitrate and 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
It was calcined at 00 ° C. for 2 hours, and 6% by weight (elemental conversion value) of silver was supported on the pelletized γ-alumina.

About 1.5 g of the obtained catalyst was placed in a reaction tube, and a gas having the composition shown in Table 1 (nitrogen monoxide, carbon dioxide,
A total of 1 dry ingredients consisting of oxygen, propylene, and nitrogen
(00% by volume added with 10% by volume of water)
At a flow rate of 1.7 liters per minute (converted to standard conditions) (contact time 0.05 sec.g / ml) to keep the exhaust gas temperature in the reaction tube in the range of 300 to 600 ° C and oxidize propylene and nitrogen. Reacted with things.

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. The removal rate here indicates how much of the nitric oxide in the gas in Table 1 was removed, as a percentage, and the number of moles of nitric oxide in the gas in Table 1 is A And the number of moles of nitrogen oxides (total amount of nitric oxide and nitrogen dioxide) in the gas after passing through the reaction tube is B
Is expressed as [(A−B) / A] × 100 (%).

Table 1 Component Concentration Nitric oxide 800 ppm Carbon dioxide 10% by volume Oxygen 10% by volume Propylene 1714 ppm Nitrogen balance Moisture 10% by volume based on the amount of gas composed of the above components

Example 2 In the same manner as in Example 1, a catalyst in which 8% by weight of silver was supported on γ-alumina pellets was prepared. Using this catalyst and the gases shown in Table 1, a nitrogen oxide removal test was conducted in the same manner as in Example 1. The test results are shown in FIG.

Example 3 Powdery γ-alumina (average particle size 40 μm, specific surface area 200
m ² / g) 10 g was immersed in an aqueous solution of silver nitrate, dried at 70 ° C., and then 150 ° C. under a nitrogen stream containing 5% by volume of hydrogen.
200 ° C, 300 ° C, 400 ° C, 500 ° C, and 600
It was fired at each temperature of ° C for 2 hours, and 5% by weight of silver was supported on the powdery γ-alumina. About 1 powder obtained
g is a commercially available cordierite honeycomb molded body (diameter 3
The surface of 0 mm, length 12.5 mm, cell number 400 / square inch) is coated by the wash coat method and oxygen is applied to 10
%, A purifying material having a nitrogen oxide removing catalyst was obtained by firing at 600 ° C. for 2 hours in a nitrogen stream.

Using this purifying material, the flow rate of gas is 4.4 per minute.
A nitrogen oxide removal test was conducted in the same manner as in Example 1 except that the liter (space velocity: 30000 h ^-1 ) was used. The test results are shown in FIG.

Comparative Example 1 A nitrogen oxide removal test was conducted in the same manner as in Example 1 except that the same γ-alumina pellets used in Example 1 (without supporting silver) were used as a catalyst. The results are shown in Fig. 1.

As can be seen from the above, in each example, good removal of nitrogen oxides was observed at the exhaust gas temperature of 350 to 600 ° C. On the other hand, when a catalyst that does not support silver or silver oxide is used, good removal of nitrogen oxide is not observed at an exhaust gas temperature of 500 ° C. or lower.

[0036]

As described in detail above, according to the method of the present invention, nitrogen oxides in exhaust gas containing excess oxygen can be efficiently removed. Further, according to the method of the present invention, nitrogen oxides can be efficiently removed even when the exhaust gas contains about 10% of water.

The nitrogen oxide removing method of the present invention can be widely used for removing nitrogen oxides contained in exhaust gas from 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 to 3 and Comparative Example 1.

Claims (4)

[Claims] 1. A catalyst 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 100 parts by weight of a porous inorganic oxide is silver. Alternatively, the silver oxide can be converted into elemental silver to be 2 to
15 parts by weight are carried, and the nitrogen oxides in the exhaust gas are reduced at 250 to 600 ° C. by using the hydrocarbon added to the exhaust gas from the outside and / or the residual hydrocarbon in the exhaust gas as a reducing agent. A nitrogen oxide removal catalyst characterized by the following. 2. A method for removing nitrogen oxides from a combustion exhaust gas containing nitrogen oxides and oxygen in an amount larger than the theoretical reaction amount for coexisting unburned components, wherein silver or silver is added to 100 parts by weight of a porous inorganic oxide. 2 to convert oxides to elemental silver
A catalyst supporting 15 parts by weight was installed in the middle of an exhaust gas conduit, and the exhaust gas was brought into contact with the catalyst at 250 to 600 ° C., at that time, 1 ml of the exhaust gas (value converted to a standard state).
For contacting 1 g of the catalyst for 0.3 seconds or less, and removing the nitrogen oxides by reacting the residual hydrocarbons in the exhaust gas with the nitrogen oxides. . 3. The method according to claim 2, wherein a hydrocarbon is added upstream of the site where the catalyst is installed, and the nitrogen oxide is reduced and removed by the residual hydrocarbon and the added hydrocarbon in the exhaust gas. A method characterized by: 4. The method according to claim 2 or 3, wherein
The method, wherein the porous inorganic oxide is alumina or an alumina-based composite oxide.