JPS5987044A - Catalytic thermal cracking catalyst of nitrogen dioxide - Google Patents

Abstract

PURPOSE:To obtain a catalyst applicable to a catalytically reductive denitration method of ammonia for suppressing N2O as a byproduct, by constituting the same from one prepared by supporting oxide of one or more of a metal element selected from Mn, Co, Cr and Cu by a carrier. CONSTITUTION:The titled catalyst is constituted of one prepared by supporting oxide of one or more of a metal element selected from a group comprising Mn, Co, Cr and Cu by a carrier. Powders comprising oxides of these metal elements are mixed in a dry or a wet system and the resulting mixture is dried, molded and baked to obtain an objective catalyst. Thus obtained catalyst can preliminarily convert NO2 contained in exhaust gas to NO in good efficiency in a denitration process due to catalytic reduction for ammonia and, as the result, the generation of N2O as a byproduct in denitration reaction is suppressed and a denitration process for NO2-containing gas can be realized in a high denitration ratio.

Description

Detailed Description of the Invention The present invention relates to a catalyst for catalytic thermal decomposition of nitrogen dioxide (NO2), particularly for thermally decomposing NO2 contained in exhaust gas into nitrogen monoxide (NO) and oxygen under high oxygen concentration. Concerning catalysts.

The so-called ammonia catalytic reduction method, in which nitrogen oxides (NO,) contained in exhaust gas are reacted with ammonia using a catalyst to turn them into harmless atoms and water (c) The structure of the device is simple, and there are many It has many advantages and is widely used in boiler exhaust gas denitrification.

However, if this sweetfish/monia catalytic reduction method is applied to the treatment of exhaust gases such as nitric acid plant exhaust gas, metal finishing factory exhaust gas, and nitric acid-dissolved exhaust gas from nuclear fuel in the nuclear fuel reprocessing process: Nitrogen oxide support ζN20)
There is a problem that occurs. This is because most of the NO contained in exhaust gas from boilers, etc. is NO, whereas in the case of the exhaust gas, most of the NOx is NO2. In other words, the ammonia catalytic reduction method In, N
The reaction between O2 and NH3 consists of three elementary reactions shown in the following equation, so NO2 is catalytically reduced with NH3 [
Ma(・'( is the maximum N2 of 50% of the inlet NOxτ1 change)
0 is generated, and it becomes extremely difficult to avoid the generation of N20.

(1) 2 N N3-4-N O2-+ 2 N N
2 10N O+ N20(2) NN2+riO-+H
2+ N20(3) NN2+ No2→N20 +
l-l2ONH2: Active intermediate of NH3 Therefore, as a result of careful consideration by the present inventors on a method for suppressing the generation of N200 during the reaction in the ammonia catalytic reduction method, we found that the activation reaction of NH3 in the above formula (1) ÷ Te NO
2 is consumed, there is no N02 left (to condition, i.e., N
The NOx composition when reacting with H3 is under conditions where there is a lot of NO (1
￥ Og"5 degrees) If the NO2 concentration, N20 is β;
The headline is that it doesn't work.

Based on this, the present inventors proposed a denitrification method that includes a step of converting NO2 to No before carrying out the ammonia catalytic reduction reaction.
It turns out that it is essential to develop a method to efficiently convert 2 to No. NO2- applicable to such gas applications
NO conversion methods include (1) reduction of NO2 using a reducing agent, (2) thermal decomposition (NO2-NO+17202)
, and (3) catalytic thermal decomposition using a catalyst. Among these methods, method s ('') uses a starting material, which increases operating costs, and also causes the problem of leakage of unreacted reducing agent.The thermal decomposition method (2) has a slow reaction rate. There are problems such as impracticality.

Based on these facts, the catalytic thermal decomposition method (3) was judged to be the most promising, but until now there was no known catalyst that could promote the thermal decomposition reaction of N02, and its development was desired. .

An object of the present invention is to provide a highly active N02 catalytic pyrolysis reaction catalyst that is applicable to an ammonia catalytic reduction denitrification method that suppresses the by-product of N20.

The present invention provides an oxide of one or more metal elements selected from the group consisting of manganese (Mn), cobalt (Co), chromium (Cr), and copper (Cu), or a phase supporting the oxide. This is a catalyst for the catalytic thermal decomposition reaction of NO2.

The catalyst of the present invention can be prepared by mixing powders of oxides of one or more metal elements selected from the group consisting of Mn, Co, Cr, and Cu in a dry or wet manner, followed by drying, molding, and calcination treatment. It is obtained by The processing after drying is to obtain a catalyst molded body that can withstand practical use.
Can be omitted.

Mn, Co, Cr used for preparing the catalyst of the present invention
In addition to the above-mentioned oxides, the starting materials for Cv and Cv may be in any chemical form, such as nitrates, hydrochlorides, carbonates, acetates, hydroxides, complex salts, etc. of these metal elements.
or a mixture of two or more. The catalyst of the present invention is prepared by a method suitable for the chemical form of these raw materials.

The catalyst may be composed only of catalyst components, but this is
For example, it may be supported on alumina, titania, zirconia, or the like. In this case, since the oxide is dispersed on the surface of the carrier, high performance and excellent durability can be obtained with a small amount.

Conditions 14 for drying the catalyst, calcination, etc. are not particularly limited, and may be normal conditions. Further, the method of supporting the metal oxide on a carrier may be carried out under normal conditions, such as impregnating the carrier with each metal oxide solution, followed by drying and baking.

The catalyst for NO2 catalytic pyrolysis obtained as described above is
It is preferably used by filling a gas-solid contact device (dryer) provided upstream of the ammonia catalytic reduction device. FIG. 1 shows an example of an ammonia catalytic reduction denitrification process using the catalyst of the present invention.
The NO gas is introduced into the NO converter 6, where the NO2 gas in the gas is thermally decomposed and converted to NO, and after passing through the line 3, ammonia is injected, and then enters the denitrification reactor 8, where the denitrification catalyst is 9, No in the gas is reduced to N2, which becomes clean gas 4 and is discharged from the system.

Hereinafter, production examples and experimental examples of the catalyst of the invention will be shown.

Example 1-4 Manganese dioxide (MnO, ), cobalt Qv (C0
2), chromium oxide (Cr20:+), oxidized steel (Cu
After adding 3wt:% of graphite to each of the powders in e), the tablets were made into tablets with a diameter of 5! M% length 571
It is molded into 171 tablets. These were then fired in an electric furnace in an air atmosphere at 500° C. for 2 hours to obtain catalysts A%B1C and D of the present invention.

Examples 5-8 0.456m of the nitrates of Mn, Ca, Cr and Cu
3oonll of ol/P aqueous solution, each 1.17. Impregnating a spherical carrier made of titanium chloride (T i 02 ) 1. Ta. Stiffness, 14. After drying at O℃ for 12 hours,
The mixture was placed in an electric furnace and fired at 500° C. for 2 hours in an air atmosphere to obtain catalysts E1F, G and H of the present invention.

Comparative Examples 1 to 3 Iron nitrate (FA (NO3)3)
(N i (NO3)z) each 0.456m0λ/2
Aqueous solution as well as metavanadate 7: 'mo-lam (
NH4VO3) Oxalic acid n form (D 0.455mO℃/
Comparative examples of catalysts ①, J, and K were obtained in the same manner as in Example 5, except that L aqueous solution was used, respectively.

Examples 24 catalysts obtained in Examples and Comparative Examples each had an inner diameter of 3
0171111 was filled in a glass reaction tube, and the catalyst performance for the conversion reaction of NO2 to No expressed by the following formula was measured under the following conditions.

NO□→NO+1/202 (4) Measurement conditions Space velocity: 10,0OOh Reaction temperature: 300-550℃ Gas composition: NO22,5007p Q230$ N20 2% N2 The balance is shown in Figures 2 and 3. show. Prosotons L0, 11, 12.13.14.15.16.1 in the figure are catalysts A, B, and C of the present invention, respectively.

When using D%B +, ',, M, G, H, 18
．． 19.20 shows the case of Comparative Example Catalysts I, J, respectively. What is shown by the broken line is a plot of the theoretical equilibrium conversion rate at each temperature for the equilibrium reaction shown by equation (4), and even with the use of a catalyst, it exceeds this value!
'i It is not possible to increase the conversion rate of NO2. In other words, the closer the catalyst is to this broken line, the more active the catalyst is.

As is clear from FIGS. 2 and 3, catalysts A to H of the present invention all have higher N than catalysts II to K of comparative examples.
It can be seen that the catalyst of the present invention is excellent as shown in the O2 conversion rate. Also, its performance is 10. OO○h-
It was obtained at a high spatial velocity of i, and can be said to be sufficiently excellent for practical use.

As described above, according to the present invention, NO2 contained in exhaust gas can be efficiently converted into NO in advance in the denitrification process by ammonia catalytic reduction, and as a result, the by-product of N20 in the denitrification reaction is suppressed and the It is possible to realize a denitrification process for NO2-containing gas with a high denitrification rate, which is industrially advantageous.

[Brief explanation of the drawing]

Fig. 1 is a system diagram of an apparatus for ammonia catalytic reduction denitrification method using the catalyst of the present invention, and Fig. 2 is a diagram showing Examples 1 to 4 of the present invention.
FIG. 3 is a graph showing the catalyst performance in Examples 5 to 3.
8 and Comparative Examples 1 to 3. FIG. 1... NOx-containing gas, 5... Heater, 6... No2-NO converter, 7...
...NH3 injection line, 8...Denitrification reactor,
9...Denitrification catalyst. Agent Patent Attorney Takenaga Kawakita Figure 1 Figure 2vA Ashitami 1 Ryobi ('C')

Claims (1)

[Claims] (1) Manganese, cobalt, chromium, and copper;
Catalyst for melting: A catalyst for catalytic thermal decomposition of nitrogen dioxide according to claim 1, characterized in that an oxide of the metal element is supported on a carrier.