JPH04363117A - Method for reducing nitrogen oxide - Google Patents

Abstract

PURPOSE:To subject NOx in exhaust gas to catalytic decomposition or selective catalytic reduction with high efficiency over a long period of time without lowering the activity of a catalyst even when the exhaust gas contains a large amt. of soot and corrosive gases such as oxygen and SO2. CONSTITUTION:NOx in exhaust gas is subjected to catalytic decomposition or NOx in exhaust gas mixed with a reducing agent is subjected to selective catalytic reduction. In this case, a Chevrel phase compd. represented by a formula MxMo6S8-yor MxMo6S8-yO2 (where M is a metal, S is chalcogen and O is oxygen) is used as a catalyst.

Description

[Detailed description of the invention]

0001

[Industrial Application Field] The present invention relates to an improvement in a method for reducing nitrogen oxides by so-called catalytic cracking method and selective catalytic reduction method, and by using a Chevrel phase compound as a catalyst, diesel engines, gas turbines, boilers, Nitrogen oxides in exhaust gas from various incinerators, automobiles, nuclear reactors, etc.
The present invention relates to a method for reducing nitrogen oxides that can be efficiently removed over a long period of time without causing a decrease in catalyst activity.

0002

2. Description of the Related Art Methods for removing nitrogen oxides, particularly nitrogen oxides (NO), from exhaust gas are roughly divided into two types, so-called wet methods and dry methods, each of which is in practical use. By the way, the wet method always requires treatment of waste water such as an alkaline aqueous solution, which has disadvantages in that the treatment cost is high and the maintenance and management of the treatment equipment is also troublesome.

On the other hand, the latter dry method does not produce a large amount of waste water at all, and the maintenance of the treatment facility is relatively easy, making it extremely convenient in practice. Therefore, in recent years, ■ catalytic cracking method using precious metals or copper ion-exchanged zeolite as a catalyst, ■ accelerated electrolysis method using electron beam, ■ Pd-Pt-Rh three-way catalyst, and V2
Various dry treatment methods have been developed, including a selective catalytic reduction method using an O5-TiO2-WO3 catalyst, an adsorption method using an adsorbent, and an absorption method using a molten salt.

However, the accelerated electron decomposition method has the problem that it requires measures against leakage of secondary x-rays, which increases facility costs and makes it difficult to apply to large-scale processing facilities.

Furthermore, in the selective catalytic reduction method using the V2O5-TiO2-WO3 catalyst, it is necessary to strictly control the emission of unreacted reducing gas (for example, ammonia), which increases operating costs and requires a small boiler. The problem is that it is difficult to apply to Similarly, the selective catalytic reduction method using a Pd-Pt-Rh three-way catalyst is effective when exhaust gas contains a large amount of oxygen, such as exhaust gas from a lean-burn engine with high output and low fuel consumption. However, this method has the disadvantage that the catalyst no longer exhibits sufficient activity to reduce NO, and the required amount of nitrogen oxides cannot be removed.

[0006] Furthermore, in recent years, the above-mentioned absorption method has been
Although methods using a2Cu3O6 have been developed, there is a problem in that they have not yet reached the stage of practical use.

Finally, regarding the catalytic decomposition method,
In methods using precious metal catalysts, when the level of emissions of soot, NOX, SOX, etc. is high, such as in the exhaust gas from diesel engines, the activity as a catalyst is lost in a short period of time.
There is a drawback that nitrogen oxides cannot be removed sufficiently. Similarly, copper ion exchange zeolite (Cu-ZSM-5)
Even in the method using O2 as a catalyst, O2, H2O, SO2
It has been found that the catalytic activity decreases rapidly in the presence of , and there is a problem that it has not been put into practical use yet.

[0008]

SUMMARY OF THE INVENTION The present invention solves the above-mentioned problems in removing nitrogen oxides by the conventional dry method.
In other words, in catalytic cracking methods and selective catalytic reduction methods that use catalysts, the activity of the catalyst decreases significantly in the presence of O2, H2O, and SO2, making it impossible to remove nitrogen oxides sufficiently. It aims to solve the problem and removes nitrogen oxides with high efficiency over a long period of time without causing a decrease in catalyst activity, even if the exhaust gas contains large amounts of soot, O2, H2O, SO2, etc. The purpose of the present invention is to provide a method for reducing nitrogen oxides that allows for easy maintenance and operational management of processing equipment.

[0009]

[Means for Solving the Problems] The invention according to claim 1 of the present invention provides MXMo6S8-y and MXMo6S8-yOZ.
(However, M is metal, S is chalcogen, O is oxygen, x is 1
The basic structure of the invention is to catalytically decompose nitrogen oxides in exhaust gas using a chevre phase compound represented by ~5,8-y is 7.4-8.5 and z is 2-8) as a catalyst. It is something. As the metal M, copper, nickel, cobalt, chromium, iron, manganese, magnesium, and zinc are mainly used, and as the chalcogen S, sulfur, selenium, etc. are mainly used.

[0010] Furthermore, the invention according to claim 5 uses the Chevrel phase compound according to claim 1 as a catalyst, and contacts this with exhaust gas mixed with a reducing agent to remove nitrogen oxides in the exhaust gas. The basic structure of the invention is selective catalytic reduction. The reducing agent may include hydrocarbons,
Carbon monoxide, ammonia, urea, etc. are used.

[0011]

[Operation] In the invention described in claim 1, a Chevrel phase compound supported on a ceramic or the like is heated to about 100 to 350°C, and exhaust gas containing nitrogen oxides (NOX) is passed through while being in contact with the surface of the Chevrel phase compound. let As a result, nitrogen oxides (NOX) are catalytically decomposed and returned to simple nitrogen (N2) and oxygen (O2), reducing nitrogen oxides (NOX) in the exhaust gas.
will be removed. In addition, in the invention described in claim 5, the Chevrel phase compound supported on ceramic etc.
It is heated to about 00 to 350℃, and hydrocarbons (
Exhaust gas containing nitrogen oxides (NOX) mixed with a reducing agent such as olefinic or paraffinic hydrocarbons), carbon oxide (CO), ammonia (NH3), or urea is passed through while being brought into contact with the exhaust gas. As a result, nitrogen oxides (NOX
) is catalytically reduced to form nitrogen (N2) and oxygen (O2).
Separate into The generated O2 reacts with other components of the exhaust gas to produce water or CO2, thereby removing NOX from the exhaust gas.

0012

【Example】

(1) Theoretical and technical background Chevrel phase compounds were synthesized by CHEVREL et al. in the early 1970s [Journal of Solid State Chemistry, 3,515-
519 (1971)], then K. Yvo
n, Solid State Communicati
on, 25, 327, 1978) and Fisher (O.F.
ischer, Applied Physics, 1
6, 1, 1978), etc., whose structure and chemical formula have been clarified by Mo6S8-y clusters arranged in a form close to a simple cube. ) has a structure that stabilizes the bonds between clusters.

That is, the Chevrel phase compound has the general formula M
A compound represented by XMo6S8-y (i.e. Chevrel phase sulfide) and a compound represented by the general formula MXMo6S8-yOZ in which a part of S in the above formula is replaced with oxygen O (i.e. Chevrel phase oxidized sulfide) exist. In each of the above formulas, M is metal, Mo is molybdenum, S is chalcogen, O
is oxygen.

The Chevrel phase compounds generally have high electrical conductivity, and some exhibit superconductivity at extremely low temperatures. For example, PbMo6S8 has a relatively high superconducting critical temperature (Tc≒
14K) and a high upper critical magnetic field (Hc2 ≒600
KG) has been reported. Also, the general formula Cu
xMo6S8-y (2≦x≦4, 7.75≦8-y≦8
The copper Chevrel phase compound represented by ) has a wide copper composition range, the copper ions that have entered the cluster gaps are extremely mobile, and it has a high electrical conductivity of 103 S cm or more. Recently, it has attracted attention as a positive electrode active material for lithium and copper secondary batteries and as a material for Josephson devices. Furthermore, as a Chevrel phase compound, in addition to this, M (metal) is nickel (N
i) Chevrel phase compounds are known that have a wide solid solution region with iron (Fe), chromium (Cr), etc., and there is little change in sulfur composition due to changes in metal composition, and extra electrons in the structure are converted to covalent bonds. It is speculated that the electrons are used and move within the structure as itinerant electrons.

On the other hand, nitrogen oxide (NO), which is the main component in nitrogen oxides (NOX), has one unpaired electron in the antibonding pi (π*) orbit because nitrogen has an odd number of electrons. , if one more electron is added to this pi (π*) orbital, then
The binding energy decreases, making it easier to decompose into nitrogen gas (N2) and oxygen gas (O2). - When the above Chevrel phase compound is used as an electron donor to nitrogen oxide (NO),
Theoretically, electrons are easily given to nitrogen oxide (NO), promoting its decomposition into nitrogen gas (N2) and oxygen gas (O2). Further, when a reducing agent such as an olefinic or paraffinic hydrocarbon, carbon oxide, ammonia, or urea is used, it becomes extremely easy to reduce nitrogen oxide (NO) by catalytic reduction by a Chevrel phase compound.

The Chevrel phase compound used as a catalyst for catalytic cracking or selective catalytic cracking of nitrogen oxides has a rhombohedral crystal structure and an axial angle α of about 95°.
edral angle α) is
Optimum in terms of activity. This was found from the results of many nitrogen oxide decomposition and removal tests conducted by the inventor of the present invention. In addition, from the crystal structure of the Chevrel phase compound and its axis angle α, the metal M of the Chevrel phase compound
As such, ternary metals having an ionic radius of about 1 Å or less, such as copper, nickel, cobalt, chromium, iron, manganese, magnesium, and zinc, are suitable.

Similarly, in the present invention, the Chevrel phase compound used as a catalyst for catalytic decomposition or selective contact of nitrogen oxides has a composition range x, 8-y of each element constituting it in terms of catalytic activity. , z are respectively x=1 to 5, 8−y=
7.4-8.5 and z=2-8 are optimal. This was found from the results of many nitrogen oxide decomposition and removal tests conducted by the inventor of the present invention. Further, the chalcogen S includes sulfur, selenium, and tellurium, but sulfur is usually most suitable.

(2) Example of production of Chevrel phase compound catalyst The stability region of Chevrel phase compounds such as copper Chevrel phase sulfides has already been established by the present inventors (Material Research, Buretin, 18, 1311-1316 (
(1983)), based on this, Cu2Mo6S7.85, which is one of the intermediate solid solution compositions, was synthesized and used as a catalyst. The synthesis method is to mix molybdenum sulfide (MoS2), copper (Cu), and molybdenum (Mo) in a predetermined ratio, vacuum seal it in a quartz tube, and then
The reaction was carried out at 00°C for 3 days and then rapidly cooled. As a result of phase identification of the obtained powder sample by X-ray diffraction, it was confirmed that it was a single-phase Chevrel phase sulfide. This Chevrel phase sulfide powder was ground to 200 mesh or less in an agate mortar, and then converted into propylene glycol (PG).
A dispersion liquid having a concentration of 1 g/cm3 was prepared. next,
This dispersion was applied to the surface of an alumina rod with an outer diameter of 6 mm for a length of 15 mm.
After coating over a cm length, vacuum degassing was performed at 100° C. for about 10 minutes to evaporate and remove PG. By the method as described above,
One of the catalysts used in the practice of the present invention was fabricated. Similarly, various types of catalysts were fabricated by changing the types of metals and their composition ranges.

(3) Nitrogen oxide removal test (NO.1)
First, a set of three alumina rods was placed in a reaction tube. The total amount of Chevrel phase sulfide applied to the three alumina rods was approximately 1.2 g. Next, the reaction tube (inner diameter 24 mm, outer diameter 28 mm) was heated in an electric furnace, and the temperature was maintained at 300°C±2°C. On the other hand, nitrogen (N2)
A mixed gas of nitrogen oxide (NO) and nitrogen oxide (NO) was formed and used as exhaust gas in the experiment. Note that the mixing ratio was adjusted using a gas mixer. Also, the total gas flow rate is 3.7cm3/se
c (linear velocity = 1 cm/sec). The amount of nitrogen oxide (NO) that has passed through the electric furnace can be measured using a glass detection tube (manufactured by Gastech) or a NOX meter (manufactured by Shimadzu Corporation NOA-3).
05A). The results of the nitrogen oxide reduction test showed that when the nitrogen oxide (NO) concentration at the inlet was 5800 ppm, the NO concentration at the outlet was 4000 ppm, and the conversion rate was 31%. Also, the concentration of NO at the inlet is 2000p.
pm, the NO concentration at the outlet was 1300 ppm, and the conversion rate was 35%. Also, while passing the mixed gas,
There was no change in the conversion rate even after re-measuring after 30 minutes.
It was confirmed that the function of Chevrel phase sulfide as a catalyst continued.

A similar nitrogen oxide removal test was conducted by varying the temperature of the reaction tube. As a result, it was confirmed that there was almost no change in catalyst activity even at a temperature of about 100°C. In addition, we conducted various nitrogen oxide removal tests by changing the type of Chevrel phase compound, and found that by keeping the type of metal M and the composition value of each element constituting the compound within the above range, It was confirmed that almost the same conversion rate of nitrogen oxides could be achieved.

(4) Nitrogen oxide removal test (NO.2)
Under almost the same conditions as the removal test in item (3) above, when a hydrocarbon is mixed in a gas mixture, when carbon oxide is mixed in a gas mixture, and when ammonia or urea is mixed in a gas mixture. For each case, the NO conversion rate and the change in NO conversion rate when the mixed gas was passed continuously for 30 minutes were investigated. As a result, it was found that the selective catalytic reduction method can also obtain an NO conversion rate that is almost the same as that of the catalytic cracking described in item (3) above. Further, even when the mixed gas was continuously passed through, there was almost no change in the NO conversion rate, and it was confirmed that the catalyst activity was maintained over a long period of time.

A similar test for removing nitrogen oxides by selective catalytic reduction was carried out by changing the catalyst temperature and the type of Chevrel phase compound. It was confirmed that almost the same conversion rate of nitrogen oxides could be achieved by setting the composition values of each element within the above ranges.

(5) Stability test of Chevrel phase compounds Chevrel phase compounds are generally known to be stable even in air up to 350°C. Furthermore, this test confirmed that it is stable up to about 730°C in a hydrogen stream, and stable up to about 450°C in a sulfur dioxide gas (SO2) atmosphere.

In other words, since the Chevrel phase compound is molybdenum sulfide (MXMo6S8-y) or molybdenum oxidized sulfide (MXMo6S8-yOZ), it can be used in corrosive gases such as sulfur dioxide gas (SO2), hydrogen, hydrocarbons, etc. It works stably and effectively even in reducing gases such as ammonia, and is an unprecedented nitrogen oxide (NOx) product.
) was found to be a catalyst for reduction. Further, in the above embodiments, a catalyst formed by applying fine powder of copper Chevrel phase sulfide to an alumina rod is used, but it is convenient to use ceramic or the like as the catalyst carrier.

[0025]

[Effects of the Invention] Since the present invention uses a Chevrel phase compound as a catalyst, it can be used for a long period of time even if a large amount of corrosive gases such as soot, oxygen, and sulfur dioxide gas (SO2) are mixed in the exhaust gas. A high NO conversion rate can be achieved. That is, since the Chevrel phase compound catalyst used in the present invention is ternary molybdenum sulfide (MXMo6S8-y) or molybdenum oxidized sulfide (MXMo6S8-yOZ), it is free from air or oxygen or corrosive sulfur dioxide gas (SO
2), or even in exhaust gas containing reducing agents such as hydrogen, hydrocarbons, carbon oxide, ammonia, and urea, nitrogen oxides (N) can be stably and effectively removed.
OX) can be catalytically cracked or selectively reduced,
Nitrogen oxides can be removed efficiently. Furthermore, since the Chevrel phase compound catalyst is an extremely stable substance, the operation and maintenance of the nitrogen oxide removal device are extremely easy, making it possible to significantly reduce exhaust gas treatment costs. As mentioned above, in the conventional dry method for removing nitrogen oxides, nitrogen oxides (NO
Although it was impossible to catalytically crack or selectively catalytically reduce This is a method for reducing nitrogen oxides that has excellent practical effects without causing any problems.

Claims (9)

[Claims] [Claim 1] MXMo6S8-y and MXMo6S
A method for reducing nitrogen oxides, characterized by catalytically decomposing nitrogen oxides in exhaust gas using a Chevrel phase compound represented by 8-yOZ (where M is a metal, S is a chalcogen, and O is oxygen) as a catalyst. . 2. In the Chevrel phase compound of claim 1, x is 1 to 5, 8-y is 7.4 to 8.5, and z is 2 to 8.
The method for reducing nitrogen oxides according to claim 1. 3. In the Chevrel phase compound of claim 1, a nitrogen oxide in which chalcogen S is sulfur and metal M is copper, nickel, cobalt, chromium, iron, manganese, magnesium, or zinc. Reduction method. 4. The method for reducing nitrogen oxides according to claim 1, wherein the Chevrel phase compound supported on ceramic or the like is heated to a temperature of 100° C. to 350° C. [Claim 5] MXMo6S8-y and MXMo6S
A Chevrel phase compound represented by 8-yOZ (where M is a metal, S is a chalcogen, and O is oxygen) is used as a catalyst, and nitrogen oxides in the exhaust gas mixed with a reducing agent are treated using the Chevrel phase compound as a catalyst. A method for reducing nitrogen oxides, characterized by selective catalytic reduction. [Claim 6] Claim 5, wherein any one of ammonia, urea, hydrocarbon, and carbon monoxide is used as the reducing agent.
The method for reducing nitrogen oxides described in . 7. In the Chevrel phase compound of claim 5, x is 1 to 5, 8-y is 7.4 to 8.5, and z is 2 to 8.
A method for reducing nitrogen oxides. 8. In the Chevrel phase compound of claim 5, a nitrogen oxide in which chalcogen S is sulfur and metal M is copper, nickel, cobalt, chromium, iron, manganese, magnesium, or zinc. Reduction method. 9. The nitrogen oxide according to claim 5, wherein the Chevrel phase compound supported on a ceramic or the like is heated to a temperature of 100 to 350° C., and exhaust gas mixed with a reducing agent is caused to flow in contact with the outer surface of the Chevrel phase compound. How to reduce