JPH04341324A - Method for decomposing and removing nitrous oxide - Google Patents

Abstract

PURPOSE:To effectively decompose and remove nitrous oxide at relatively low temp. while reducing the effect of moisture or sulfur oxide. CONSTITUTION:Nitrous oxide-containing gas is brought into contact with a catalyst containing rhodium sesquioxide, cobalt sesquioxide or a mixture of them as an active component at 100-600 deg.C to decompose nitrous oxide. Nitrous oxide in exhaust gas can be efficiently decomposed into oxygen and nitrogen without requiring a reducing agent such as ammonia or hydrogen. This catalyst has high activity at relatively low temp. and is extremely reduced in the lowering of activity due to the poisoning caused by moisture or sulfur oxide and can keep a stable high denitration rate over a long time.

Description

[Detailed description of the invention]

0001

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for decomposing and removing nitrous oxide contained in various industrial exhaust gases.

0002

BACKGROUND OF THE INVENTION Various regulatory measures have been taken with respect to the release of various industrial exhaust gases into the atmosphere, such as combustion exhaust gases and chemical factory exhaust gases, from the viewpoint of pollution prevention and environmental conservation. In particular, nitrogen oxides are strictly regulated from being released into the atmosphere as they are the cause of photochemical smog, acid rain, and the like. Conventionally, the nitrogen oxides that have been subject to emission regulations are nitric oxide (NO) and nitrogen dioxide (NO2), and denitrification technology has been researched for these substances, using reducing substances such as ammonia. Catalytic reduction methods and methods for decomposing nitrogen into nitrogen and oxygen using catalysts such as metal catalysts have been developed. However, in recent years, nitrous oxide, which is contained in trace amounts in these exhaust gases but is said to be stable and harmless compared to other nitrogen oxides, has been decomposed in the stratosphere to produce nitrogen monoxide. It has also become a problem as it has a strong greenhouse effect and has a long half-life of approximately 150 years, suggesting that it may have an impact on global warming. However, in the denitrification method described above, nitrous oxide cannot be decomposed or removed at all, and furthermore, in the denitrification method using ammonia as a reducing agent, nitrogen monoxide, nitrogen dioxide, and ammonia may be produced depending on the operating conditions of the denitrification equipment. It has even become clear that nitrous oxide can be produced in relatively high concentrations through this reaction.

[0003] Therefore, various methods for decomposing and removing nitrous oxide contained in various exhaust gases have been studied and proposed. The main method that has been proposed to decompose nitrous oxide in exhaust gas is the catalytic decomposition method (Japanese Unexamined Patent Application Publication No. 63-7-7), which decomposes nitrous oxide by contacting it with a metal catalyst at high temperature.
826, etc.), catalytic reduction method in which reductive decomposition is carried out by contacting with a catalyst together with a reducing gas such as ammonia or hydrogen (Japanese Patent Publication No. 55-47933, JP-A No. 2-68120, etc.)
Alternatively, a method of decomposition using light or radiation (Japanese Unexamined Patent Publication No. 6
3-111927, JP-A-63-111929, etc.), but in these methods, the treatment temperature is high and the catalyst is exposed to oxygen, sulfur oxides, moisture, etc. present in the exhaust gas. The reality is that it has not been put into practical use because of many problems such as poisoning, reduced decomposition activity, and the need for special equipment.

0004

Among the above-mentioned conventional techniques, the catalytic cracking method is considered to be the simplest and most practical, but this method generally requires treatment at high temperatures. The method described in JP-A No. 63-7826 is a method in which a gas containing nitrous oxide is mixed with a metal of Group Ib or Group VIII of the Periodic Table of the Elements, or an oxide or composite oxide of the metal. Although this is a method of contacting with a catalyst contained, in order to obtain a denitrification rate of 50% or more in view of the examples, a high temperature of 350° C. or higher is required except for the noble metal catalyst. Additionally, according to experiments conducted by the present inventors, the catalysts described here such as NiO, Fe2O3, CoO, and CuO can be used in a short period of time if the exhaust gas being treated contains moisture or sulfur oxides. It was found that there was a problem in that the deactivation of nitrous oxide resulted in a decrease in the decomposition activity of nitrous oxide. The purpose of the present invention is to
A method for decomposing and removing nitrous oxide that solves the problems with conventional catalytic cracking methods, allows processing at relatively low temperatures, and can treat exhaust gas containing nitrous oxide that coexists with water and sulfur oxides. Our goal is to provide the following.

[0005]

[Means for Solving the Problems] As a result of searching for a catalyst for decomposing nitrous oxide, the present inventors found that a metal oxide catalyst containing Rh2 O3 and Co2 O3 as active components can effectively decompose moisture and sulfur oxides in exhaust gas. The present invention was completed based on the discovery that the decrease in activity due to poisoning of nitrous oxide is extremely small and that nitrous oxide can be decomposed into oxygen and nitrogen with high activity even at relatively low temperatures. That is, the present invention converts nitrous oxide-containing gas into rhodium sesquioxide (Rh2O3), cobalt sesquioxide (Co
A method for decomposing and removing nitrous oxide, and a method for decomposing and removing water and/or nitrous oxide, which is characterized in that nitrous oxide is decomposed by contacting with a catalyst containing 2O3) or a mixture thereof as an active ingredient at a temperature of 100 to 600°C. Nitrous oxide-containing gas in which sulfur oxides coexist is converted into rhodium sesquioxide (Rh2O3
), cobalt sesquioxide (Co2O3), or a mixture thereof as an active component;
This is a method for decomposing and removing nitrous oxide, which is characterized by contacting at a temperature of 0° C. to decompose nitrous oxide.

The catalyst used in the process of the invention is R
It contains either h2O3, Co2O3 or a mixture thereof as an active ingredient. These catalysts can be used by granulating each active component with a binder component such as colloidal silica mainly composed of SiO2, or by supporting it on a carrier such as titania, alumina, silica/alumina, or magnesia. is convenient. In addition, the shape and size of the catalyst may be appropriately selected depending on the purpose of use, usage conditions, etc., and shapes such as granules, bales, spheres, rings, cylinders, plates, and honeycombs can be used. , honeycomb shape, plate shape, etc. are particularly preferable from the viewpoint of contact efficiency with gas and pressure loss.

[0007] The method for producing the catalyst is not particularly limited, but powdered Rh2O3 or Co2O3 or a mixed powder of these in any proportion is used as a raw material, kneaded with water together with a binder component, and mixed with water as required. A method in which a carrier component is added, mixed, molded into an appropriate size, dried, and then pulverized to adjust the particle size; a method in which the binder component and the carrier component are kneaded with water, molded into an arbitrary shape, and then dried; Any method can be used, such as mixing the binder component with water to form a slurry, adhering it to a carrier of any shape, and drying it. The content ratio of the active components in the catalyst varies depending on the processing, from supported catalysts in which each active component or a mixture thereof is supported on various carriers, to a catalyst with a content of 98% by weight or more in which the active components are molded together with a small amount of binder component. It can be arbitrarily set within a wide range depending on processing conditions such as gas properties, processing equipment, processing temperature, and required decomposition rate of nitrous oxide. In addition, when supporting on a carrier, the supported amount is 0 as the metal oxide.
．． Preferably, the content is in the range of 1 to 30% by weight. If it is less than 0.1% by weight, the catalyst activity will be low, and if it exceeds 30% by weight, the reinforcing effect of the carrier will be reduced.

[0008] Nitrous oxide can be decomposed into oxygen and nitrogen by filling a reaction tank with the catalyst thus prepared and causing a reaction by passing a nitrous oxide-containing gas through the catalyst. The reaction temperature and gas space velocity (SV) vary depending on the nitrous oxide concentration in the gas, the form and amount of catalyst used, the shape of the reactor, etc., but the reaction temperature is in the range of 100 to 600°C,
In particular, the range of 150 to 600°C is preferable, and the space velocity is
The range of 3000 to 20000 (hr-1) is preferable. If the temperature is less than 100°C, the decomposition of nitrous oxide will be difficult to proceed, and if it exceeds 600°C, the deterioration of the catalyst will become severe, which is not preferable. If the space velocity is less than 3,000 (hr-1), there is no change in the decomposition rate of nitrous oxide, but the gas processing capacity becomes small, making it impractical;
(hr-1) is not preferable because the decomposition rate of nitrous oxide decreases.

According to the method of the present invention, nitrous oxide in exhaust gas can be decomposed into oxygen and nitrogen without requiring a reducing agent such as ammonia or hydrogen. In addition, the catalyst used in the present invention has high activity even at relatively low temperatures, has very little reduction in activity due to poisoning by water or sulfur oxides, and can maintain a stable high denitrification rate over a long period of time. In other words, conventional CoO and RhO catalysts are greatly affected by moisture and sulfur oxides, and no nitrous oxide decomposition activity is observed at temperatures below 250°C.
O2 O3, Rh2 O3 based catalysts, especially mixed catalysts of both, can achieve a decomposition rate of 10% or more at a temperature around 150°C even under conditions where less than 20% by volume of moisture or less than 10% by volume of sulfur oxides coexist. be able to. A catalyst that exhibits nitrous oxide decomposition activity in the presence of moisture or sulfur oxides at such low temperatures has not been previously known.

0010

EXAMPLES The method of the present invention will be explained in more detail with reference to Examples below. (Preparation of catalyst) Commercially available Co2 O3 (purity 99.5%
), Rh2O3 (purity 99.0%), CoO (purity 99.5%) and RhO (purity 99.0%),
A catalyst was prepared according to the following procedure. (1) Single catalyst of Co2 O3 or Rh2 O3 and mixed catalyst of Co2 O3 and Rh2 O3 100 parts by weight of Co2 O3 or Rh2 O3 or 98 parts of Co2 O3 and Rh2 O3 respectively
3 parts by weight of colloidal silica as a binder (forming aid) as SiO2 was added to 100 parts by weight of the mixture mixed in a ratio of 2 or 50/50 and kneaded with water. This kneaded product was molded into a sphere with a diameter of about 30 mm, dried in an air atmosphere at 120°C for 24 hours, and crushed.
A granular catalyst of mm was obtained. Each of these catalysts (Co
2 O3 ), (Rh2 O3 ), (Co2 O3
-Rh2O3, 98/2) and (Co2O3-
Rh2 O3, 50/50). (2) Alumina-supported catalyst using Co2 O3 and Rh2 O3 Co2 O3 or Rh2 O3 was added to 100 parts by weight of γ-alumina heat-treated at 600°C for 5 hours in a nitrogen atmosphere.
20 parts by weight of colloidal silica and SiO2
A mixture of 3 parts by weight was added and kneaded with water. This kneaded material was formed into a sphere with a diameter of about 30 mm, dried in an air atmosphere at 120°C for 24 hours, and crushed.
A 3 mm granular catalyst was obtained. Each of these catalysts (C
o2 O3 /Al2 O3 ) and (Rh2 O3
/Al2O3).

(3) Co2 O3 and Rh2 O3
3 parts by weight of colloidal silica as SiO2 was added to a mixture of 50 parts by weight of Co2 O3 and 50 parts by weight of Rh2 O3, and 100 parts by weight of titania (anatase type) was added and kneaded with water. This kneaded material is approximately 30mm in diameter.
The catalyst was molded into a spherical shape, dried at 120° C. for 24 hours in an air atmosphere, and crushed to obtain a granular catalyst of 1 to 3 mm. This catalyst (Co2 O3 -Rh2 O3 /TiO
2) is displayed. (4) Alumina-supported catalyst using CoO and RhO 100 parts by weight of γ-alumina heat-treated at 600°C for 5 hours in a nitrogen atmosphere was immersed in an aqueous solution of cobalt nitrate or rhodium nitrate, and 20 parts by weight of each of CoO or RhO was added. A catalyst was prepared by adsorbing a portion of the catalyst, drying it in an air atmosphere at 110°C for 24 hours, and then calcining it in a nitrogen atmosphere at 600°C for 5 hours. Each of these catalysts (CoO/A
12 O3 ) and (RhO/Al2 O3 ). (Nitrous oxide decomposition and removal test) 25 ml of each of the catalysts prepared as described above was filled into a test device consisting of a quartz tube with an inner diameter of 20 mm, and a gas adjusted to a prescribed composition was passed through the reaction tube under prescribed temperature conditions. The concentration of nitrous oxide in the gas at the inlet and outlet was measured. From this value, the decomposition rate of nitrous oxide was calculated and the activity of the catalyst was compared.

(Example 1) The temperature of the catalyst layer was set as shown in Table 1, and air containing 60 ppm of N2O was
It was passed through at a space velocity of 6000 hr-1, and the decomposition rate of N2O was measured. The results are shown in Table 1, and the catalyst used in the present invention exhibits higher nitrous oxide decomposition activity than conventional CoO or RhO-based catalysts, and in particular
A remarkable synergistic effect was observed in the catalyst containing 2 O3 and Rh2 O3, and even at a low temperature of 150°C, approximately 2
It can be seen that a decomposition rate of 0% was obtained. In addition, N2
When a similar test was conducted by changing the concentration of O, N2
Almost the same decomposition rate was obtained up to an O concentration of about 1000 ppm.

[0013]

[Table 1]

(Example 2) The temperature of the catalyst layer was set as shown in Table 2, and air containing 150 ppm N2O and 14% moisture was passed through the catalyst layer at a space velocity of 5000 hr-1. The decomposition rate of N2O was measured after 100 hours. The results are shown in Table 2, and the conventional C
oO or RhO type catalysts are severely deactivated and a decomposition rate of only about 20% can be obtained even at 400°C, whereas when the catalyst of the present invention is used, there is less deactivation and even a single catalyst can be decomposed at 300°C. Approximately 40-60%, approximately 75-80 at 400℃
% decomposition rate, and with a catalyst using a combination of Co2 O3 and Rh2 O3, it is about 10% at 150°C, about 40% at 200°C, about 80% at 250°C, and about 90% at 300°C.
, it can be seen that a high decomposition rate of 99% or more is maintained at 400°C.

[0015]

[Table 2]

(Example 3) The temperature of the catalyst layer was set as shown in Table 3, and air containing 150 ppm N2 O and 50 ppm SO2 was passed through it at a space velocity of 5000 hr-1. The rate of N2O decomposition after hours was measured. The results are shown in Table 3,
Conventional CoO or RhO catalysts are severely deactivated at 400℃
However, when the catalyst of the present invention is used, there is little deactivation, and even with a single Co2 O3 catalyst, the decomposition rate is only about 20-40% at 300°C.
%, shows a decomposition rate of about 90% at 400°C, and shows a decomposition rate of about 90% at 400°C.
3 and Rh2 O3 in combination, the temperature at 150°C
approx. 20% at 200℃, approx. 50% at 200℃, approx. 90% at 250℃
%, it can be seen that a high decomposition rate of 99% or more is maintained at 300 to 400°C.

[0017]

[Table 3]

[0018]

[Effects of the Invention] According to the method of the present invention, nitrous oxide in exhaust gas can be efficiently decomposed into oxygen and nitrogen without requiring a reducing agent such as ammonia or hydrogen. Furthermore, the catalyst used in the present invention has high activity even at relatively low temperatures, has very little reduction in activity due to poisoning by water or sulfur oxides, and can maintain a stable high denitrification rate over a long period of time. It is extremely effective in treating various exhaust gases containing nitrous oxide, which often contains moisture and sulfur oxides.

Claims (2)

[Claims] [Claim 1] Nitrous oxide-containing gas is mixed with rhodium sesquioxide (Rh2 O3), cobalt sesquioxide (Co2
A method for decomposing and removing nitrous oxide, which comprises contacting a catalyst containing either O3) or a mixture thereof as an active ingredient at a temperature of 100 to 600°C to decompose nitrous oxide. [Claim 2] Nitrous oxide-containing gas in which moisture and/or sulfur oxide coexist is replaced with rhodium sesquioxide (Rh2O
3), a catalyst containing either cobalt sesquioxide (Co2O3) or a mixture thereof as an active component and 100~
A method for decomposing and removing nitrous oxide, which comprises contacting at a temperature of 600°C to decompose nitrous oxide.