JPS6151936B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a method for catalytic decomposition of nitrogen oxides.
More specifically, the present invention provides a novel method for catalytically decomposing nitrogen oxides contained in exhaust gas into nitrogen and oxygen using a catalyst containing a composite oxide of barium and cobalt as an active component. BACKGROUND OF THE INVENTION Nitrogen oxides (NO _x ), which are emitted from facilities that burn fossil fuels such as boilers and heating furnaces, are a major cause of air pollution, along with sulfur oxides (SO _x ), and measures to eliminate them have been actively researched for a long time. Most of the commercialized flue gas desulfurization technologies are wet methods, and in the early days of denitrification methods, wet methods were the mainstream in research and development, but more than 95% of the NO _x contained in flue gas is reactive. low nitric oxide (NO),
In addition, almost all wet reduction methods cannot overcome the drawback that the selectivity of absorbed NO _x to nitrogen (N ₂ ) is low, and most of the absorbed NO x accumulates in the absorption liquid as ammonia compounds or imide compounds. The scale of the pilot test was not exceeded. On the other hand, in the dry process, there has been significant technological improvement in selective catalytic reduction using ammonia (NH ₃ ) as a reducing agent, and it was initially predicted that it would be unavoidable.
The problems of poisoning by SO _x and clogging of the catalyst layer by dust have been solved through the development of monolithic catalysts using titanium oxide (TiO ₂ ) as a carrier, making it possible to treat combustion exhaust gases ranging from LNG to coal. Currently, it has been evaluated as having reached the stage of technological perfection. However, even in the ammonia catalytic reduction method, ammonia, which is a valuable resource as a fertilizer raw material, has to be consumed, which is a fateful drawback, and _{acidic ammonium} sulfate (NH There are problems such as corrosion of equipment materials caused by ₄・HSO ₄ ), and the development of a more effective NO _x removal method is desired. From these viewpoints, the inventors investigated the catalytic cracking method, which is said to be the ultimate form of flue gas denitrification, with the aim of developing a more economical denitrification process. The equilibrium between nitric oxide (NO), nitrogen (N ₂ ), and oxygen (O ₂ ) (1) is overwhelmingly biased toward the production system (on the right), and 2NON ₂ + O ₂ (1) even with a suitable catalyst. It was pointed out early on that if this could be found, an ideal flue gas denitrification method could be established, and many researchers have worked on this to date.
Platinum and rhodium by Bachman et al. [J.Phys.
Chem. 33 , 447 (1929)], Fraser et al.
Al ₂ O ₃ , CaO, Cr ₂ O ₃ , Ga ₂ O ₃ , ZrO ₂ , Fe ₂ O ₃ ,
Examination of the decomposition mechanism on TiO ₂ [J.Phys.Chem. 62 , p. 215 (1958)] and Sakaida's study of platinum, NiO/
Alumina carrier [AIche J. 7 (4) 658
(1961)] and Sourirajan et al. have reported numerous studies using noble metals, base metals, and their oxides, including CuO and NiO/silica supports.
For exhaust gas treatment, the Illinois Institute of
The research conducted by Technology's Research Institute is famous [Air Pollution Foundation Report No.
20, 22 (1957)]. Recently, systematic studies have been carried out using noble metals, transition metals, their oxides, and mixed oxides as catalysts, either unsupported or supported on alumina, silica, or titania [Pollution 12 (4) 37 (1977)] , 13 (1)22
(1978)]. Despite these numerous studies, no catalyst worthy of industrial attention has yet been discovered.
It is now said that there is almost no possibility of discovering a promising catalyst. The inventors focused on compounds with a perovskite-type crystal structure, which have recently attracted attention as catalysts with unique oxidation properties, and tried to apply them to NO _x catalytic cracking reactions. As a result of intensive studies, we discovered the unexpected fact that this system exhibits activity in complex oxides other than those with a perovskite crystal structure, and finally arrived at the present invention. The present invention enables exhaust gas containing nitrogen oxides to be brought into contact with a catalyst whose active substance is a composite oxide consisting essentially of barium and cobalt at a temperature range of 600°C to 1000°C, thereby removing an oxidizing agent from the outside. This is a catalytic decomposition method for nitrogen oxides, which is characterized in that nitrogen oxides contained in the gas are decomposed into nitrogen and oxygen without adding any agent. The catalyst that is the basis of the present invention has an essential requirement that the active component be a composite oxide containing substantially two components of barium and cobalt, but if the atomic ratio of cobalt to barium is less than 0.1 or more than 10.0, is low and cannot function as a catalyst. A more preferable range of the composition ratio is 8.0≧Ba/Co (atomic ratio)≧0.2. The starting materials for barium and cobalt are not particularly limited as long as they can form the above composite oxide during catalyst calcination or reaction operation, including oxides, hydroxides, chlorides, carbonates, nitrates, sulfates, Acetate and the like are commonly used. As shown in the examples below, these differences in starting materials do not make a significant difference in the cracking activity of the catalyst. As for the catalyst preparation method, all the methods normally used for catalyst preparation, such as the coprecipitation method from an aqueous solution of water-soluble salts and the kneading method, are applicable. Regarding the calcination temperature, the activity of the catalyst used in the present invention does not become noticeable unless the temperature is 600°C or higher.
Firing conditions below ℃ are meaningless. On the other hand, 1500℃
If it is more than that, the surface area of the catalyst decreases significantly due to sintering, and the catalytic activity decreases, which is not preferable. The preferred firing temperature range is 600℃ or above 1400℃
below ℃. The high-temperature phase equilibrium of the cobalt oxide and barium oxide binary system has been reported in detail by Negus et al. [NBS.Special Publication 364 233 (1972)]. Catalyst preparation conditions of the present invention: 0.1≦Ba/Co (atomic ratio)≦
10.0. At a firing temperature of 600°C or higher and 1400°C or lower, stable phases of Ba ₂ CoO ₄ , Ba ₃ Co ₂ O ₇ , BacoO _3-x and solid solutions thereof are estimated to be formed depending on the conditions. On the other hand, in terms of catalytic activity, no clear difference due to these stable phases is observed. Therefore, it can be concluded that the barium-cobalt-based composite oxide is effective as an active ingredient for decomposing nitrogen oxides in any crystal form. Further, unlike the composite oxide of strontium and iron disclosed in JP-A-55-149634, this type of catalyst does not require preactivation with high concentration NO. If the decomposition reaction temperature is too low, the activity will be low, and if it is too high, the equilibrium partial pressure of NO due to equilibrium (1) mentioned above cannot be ignored, making it impossible to achieve a high decomposition rate.
Preferably above 600℃ and below 1000℃. Examples are shown below to clarify the effects of the method of the present invention. Example 1 Various barium and cobalt compounds were mixed and ground in a ball mill for 2 hours so that the composition after firing was Ba/Co (atomic ratio) = 1.0, and then dried at 110°C for 5 hours.
Table 1 shows the experimental results using the catalyst obtained by calcination at ℃ for 5 hours. For reaction experiments, the inner diameter was 10 mmφ and the length was 1.
Each 10 ml of catalyst was filled into 5 m silica reaction tubes, heated to a specified temperature by external heating in a tubular electric furnace, and then heated with 5 ml of helium-balanced gas containing 5% NO.
/H (SV; 500 hr ^-1 ) and activity comparison was performed. The produced gas was analyzed using a gas chromatograph.

[Table] Example 2 The activity was compared in the same manner as in Example 1 except that Ba(OH) ₂ and CoO were selected as starting materials and the ratio of the two was changed. The results are shown in Table-2.

[Table] Example 3 Ba(OH) ₂ and CoO were mixed at a ratio of Ba/Co (atomic ratio) = 1.0, and the activities were compared in the same manner as in Example 1, except that the firing temperature was changed. The results are shown in Table-3.

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Claims (1)

[Claims] 1 External oxidation by bringing exhaust gas containing nitrogen oxides into contact with a catalyst whose active substance is a composite oxide consisting essentially of barium and cobalt at a temperature range of 600°C to 1000°C. A method for catalytic decomposition of nitrogen oxides, characterized in that nitrogen oxides contained in the gas are decomposed into nitrogen and oxygen without adding a reducing agent or a reducing agent. 2. The method according to claim 1, wherein the atomic ratio of barium to cobalt in the catalyst component is in the range of 0.1 or more and 10.0 or less. 3 Claims 1 or 2 in which the catalyst is fired at a temperature of 600°C or higher and 1400°C or lower
The method described in section.