JPH0442064B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a catalyst that directly decomposes and reduces nitrogen oxides (nitrogen monoxide, hereinafter referred to as NO) that are harmful to health in various exhaust gases. Conventionally, methods for reducing NO in exhaust gas using catalysts have been known. For example, a method of reducing and removing NO in exhaust gas from a gasoline engine by reacting with carbon monoxide and hydrocarbons that are also present is already in practical use in gasoline-powered automobiles as a three-way catalyst method. In addition, for exhaust gas from large stationary sources such as boilers, a selective NO reduction method that reduces NO by adding ammonia externally is in operation and has achieved some degree of effectiveness. However, the former method is only applicable to exhaust gas from gasoline engines with extremely low oxygen concentrations, and the latter method uses ammonia, so it is difficult to use it for small stationary or mobile sources. Have difficulty. Therefore, it has been technically impossible to remove NO from exhaust gas from small diesel engines and the like. On the other hand, the method of directly decomposing NO into harmless nitrogen or nitrous oxide and oxygen using a catalyst is ideal because it can be applied to any emission source. It is already known that noble metal supported catalysts such as platinum and some metal oxide catalysts are active as NO direct decomposition catalysts (JW
Hightpwer，”Catalytic Chemistry of Oxides”
(Edit.by RLKlimisch and JGLarson).
Plenum Press, New York (1975), p.63). Furthermore, certain perovskite-based composite oxide catalysts (Shinjumitsu, Chemical Technology Research Institute Report, 75, 83 (1980))
and metal-supported zeolite catalysts (Masakazu Iwamoto, J.Chem.
Soc., chem.Commun., 1986, 1272) has also been reported to exhibit NO decomposition activity. however,
The catalytic activity was too low to be used for actual purification of exhaust gas, and there was no prospect of practical use. In order to develop a highly active catalyst that decomposes NO, the present inventors conducted intensive research on the effects of various co-catalysts on oxide catalysts, and found that the use of silver as a co-catalyst significantly increased NO decomposition activity. The present invention has been completed based on the discovery that this can be improved. In other words, the method of the present invention reduces NO in various exhaust gases.
This method uses a metal oxide catalyst containing silver as a co-catalyst to reduce the decomposition of silver. NO.
The situation of using the above-mentioned elements, which are normally inert for the decomposition of , as co-catalysts, was quite surprising and unforeseen. The main catalyst metal of the oxide catalyst used in the present invention may be any metal oxide that normally has NO decomposition activity, such as the first,
Examples include transition metal oxides of the second and third periods. Among these, oxides of cobalt, iron, and nickel, which inherently have relatively high catalytic activity, are preferred. The catalyst for the present invention may be any silver-containing oxide catalyst prepared by any method. For example, the impregnation method involves impregnating and treating a silver salt of the main catalyst metal oxide, the kneading method involves kneading and treating a compound containing both components, and the preparation method involves adding a precipitant to a solution containing both components. The coprecipitation method is the most suitable method, but the coprecipitation method is the most suitable. In some cases, an inert carrier may be used for the reaction. The ratio of promoter metal silver to main catalyst metal in the catalyst can vary within a wide range. Generally 0.1~
50%, preferably 1-30%. The starting materials for the main catalytic metal and co-catalyst silver for preparing the catalyst of the method of the present invention by a coprecipitation method may be any type of compound as long as it is soluble in water.
For example, nitrates, organic acid salts, halides, and various complex compounds are used. To produce a catalyst, a solid substance is first obtained by dropping an aqueous solution of a precipitant into a mixed aqueous solution containing the main catalyst metal and silver. The metal concentration of the aqueous solution may be of any value. As the precipitant, an alkaline substance is suitable, such as sodium carbonate, sodium hydrogen carbonate, potassium carbonate, potassium hydrogen carbonate, sodium hydroxide, potassium hydroxide, ammonium carbonate, and the like. The amount of precipitant may be an amount that is sufficient to completely form a precipitate. The temperature for obtaining the precipitate may be any temperature, but room temperature is preferable. The obtained solid substance is filtered, thoroughly washed with water, dried, and finally calcined in a stream of air to obtain a catalyst. Air firing temperature is between 200℃~900℃, preferably 300℃~
The temperature is 700℃. To carry out the process of the invention, a gas containing NO is passed over the catalyst. Reaction temperature is generally 300℃~900℃
between 400°C and 700°C. The higher the temperature, the higher the rate of NO decomposition, but if the temperature is too high, the catalyst will deteriorate, which is undesirable, so it is reasonable to decide by taking both of these factors into consideration. According to the method of the present invention, by preparing a catalyst that greatly exceeds the activity of conventional NO decomposition catalysts, it is possible to decompose and remove NO in various exhaust gases, which has been difficult to reduce with catalysts. The impact it has on purification is extremely large. Next, the present invention will be explained in more detail with reference to Examples. Example 1 20.37 g of cobalt nitrate (Co(No ₃ ) _{2.6H 2} O) and 0.60 g of silver nitrate (AgNO ₃ ) were dissolved in 100 ml of pure water.
Then, while stirring with a magnetic stirrer, 50 ml of an aqueous solution of 8.50 g of sodium carbonate (Na ₂ Co ₃ ) was added dropwise over 1 hour to form a precipitate. After stirring for an additional hour, the precipitate was separated using a centrifuge. The obtained solid material is washed with pure water at least 5 times and then placed in a dryer.
The catalyst was prepared by drying at 100° C. overnight, grinding, and calcining in a stream of air. The firing conditions were as follows: temperature was raised at 1°C/min from 100°C to 300°C, 4.3°C/min from 300°C to 400°C, and kept at 400°C for 4 hours. In this case, the molar ratio of silver to cobalt is 1/20. 1 g of the cobalt oxide-silver oxide catalyst thus prepared was charged into a normal pressure flow reactor, and helium gas containing NO3.13% was added every minute while increasing the temperature.
The decomposition rate of NO to nitrogen was examined by flowing 30 ml. A gas chromatograph was used to analyze the reaction gas. The results are shown in Table 1 as Example 1. Comparative Example 1 The NO decomposition rate was investigated in the same manner as in Example 1 using 3 g of commercially available 99.9% cobalt sesquioxide as a catalyst. The results are shown in Table 1 as Comparative Example 1. Considering the amount of catalyst, it is clear that the cobalt-silver catalyst of Example 1 has high activity. Comparative Example 2 A silver-free cobalt oxide single catalyst was prepared in the same manner as in Example 1, and 1 g was used to carry out a reaction. The results are shown in Table 1 as Comparative Example 2. As compared with Example 1, it can be seen that the activity is greatly improved by combining silver.

[Table] Example 2 28.28g of iron nitrate (Fe(No ₃ ) ₃・9H ₂ O) and silver nitrate
Dissolve 0.60g in pure water to make 150ml, and add sodium carbonate while stirring with a magnetic stirrer.
A 100 ml aqueous solution of 13.50 g was added dropwise over 1 hour to form a precipitate. After stirring for an additional hour, the precipitate was separated using a centrifuge. The obtained solid was washed with pure water five times or more, dried overnight at 100° C. in a drier, pulverized, and calcined in an air stream to prepare an iron-silver oxide catalyst. The firing conditions were the same as in Example 1. 2g of the catalyst prepared in this way was charged into a normal pressure flow reactor, and while increasing the temperature, 3.13%
The decomposition rate of NO was investigated by flowing 30 ml of helium gas containing NO per minute. The results are shown in Table 2 as Example 2. Comparative Example 3 The NO decomposition rate was investigated in the same manner as in Example 2 using 2 g of commercially available 99.9% ferric oxide as a catalyst, and the results are shown in Table 2 as Comparative Example 3.

[Table] Example 3 20.36 g of nickel nitrate (Ni (No ₃ ) _{2.6H 2} O) and 0.60 g of silver nitrate were dissolved in pure water to make 100 ml, and while stirring with a magnetic stirrer, 50 ml of an aqueous solution of 8.50 g of sodium carbonate was added. was added dropwise over 1 hour to form a precipitate. After stirring for an additional hour, the precipitate was separated using a centrifuge. The obtained solid was washed with pure water five times or more, dried overnight at 100° C. in a drier, pulverized, and calcined in an air stream to prepare a nickel-silver oxide catalyst. The firing conditions were the same as in Example 1. 2g of the catalyst prepared in this way was charged into a normal pressure flow reactor, and while increasing the temperature, 3.13%
The decomposition rate of NO was investigated by flowing NO. The results are shown in Table 3 as Example 3. Comparative Example 4 The NO decomposition rate was investigated in the same manner as in Example 4 using 2 g of commercially available 99.9% nickel oxide as a catalyst. The results are shown in Table 3 as Comparative Example 4.

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Claims (1)

[Claims] 1. A method for decomposing nitrogen oxides using a cobalt, iron, or nickel oxide catalyst containing silver as a promoter metal. 2. The method according to claim 1, wherein the catalyst is prepared by dropping a precipitant into a mixed aqueous solution containing a main catalyst metal salt and a silver salt.