JPH01224047A - Decomposition catalyst for nitrogen oxide - Google Patents

Abstract

PURPOSE:To substantially improve NO decomposition activity by utilizing a metallic oxide catalyst incorporating a silver as a promoter in the metallic oxide catalyst decomposing NO contained in exhaust gas. CONSTITUTION:In a catalyst directly decomposing and reducing nitrogen oxide contained in exhaust gas, the metal oxide having NO decomposition activity, especially oxide of cobalt, iron or nickel is utilized as a main catalytic metal and silver is utilized as a promoter. The aq. soln. of a precipitant such as sodium carbonate is dropt into the mixed aq. soln. incorporating main catalytic metal and silver, and precipitate is prepared and separated. The catalyst obtained by such a way is substantially improved in NO decomposition activity and contributes to the clarification of the atmosphere.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention aims to eliminate nitrogen oxides (
- Nitrogen oxide is referred to as No. ) is related to a catalyst that directly decomposes and reduces

Conventionally, methods for reducing No in exhaust gas using catalysts are known. For example, No in exhaust gas from a gasoline engine is reduced and removed by reaction with carbon monoxide and hydrocarbons that are also present. The method is already in practical use in gasoline-powered vehicles as a three-way catalyst method. In addition, for exhaust gas from large stationary sources such as boilers, a selective NO reduction method in which ammonia is added externally to reduce NO is in operation and has been effective to some extent. However, the former method is only applicable to exhaust gas from a gasoline engine with an extremely low oxygen concentration, and is difficult to apply.

Therefore, it has been technically impossible to remove No from the exhaust gas of small diesel engines and the like.

On the other hand, a method of directly decomposing NO into harmless nitrogen or nitrous oxide and oxygen using a catalyst is ideal in that 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 (J,
W, Hightower, Catalytic C
hemistry of Nitrogen 0x
ides” (Edit.

by R,L, ni11m1sch and J,G,L
arson), Plenum Press.

New York (1975), p. 63) - Furthermore, certain perovskite complex oxide catalysts (Shinjumitsu, Chemical Technology Research Institute Report, 75.83 (1980)) and metal-supported zeolite catalysts (Masakazu Iwamoto, J. Che+s, So
c,,Chem,Commun,, 1986.1272
) is also known to exhibit No decomposition activity. However, the catalytic activity is too low for use in actual exhaust gas purification, and there is no prospect of practical application.As a result of intensive research, we found that the use of silver as a cocatalyst significantly improves the No decomposition activity. This discovery led to the completion of the present invention.

That is, the method of the present invention can reduce N in various exhaust gases.

This method uses a metal oxide catalyst containing silver as a co-catalyst. N.

The situation of using the aforementioned elements, which are normally inert for the decomposition of , as cocatalysts, was quite surprising and unforeseen.

The main catalyst metal of the oxide catalyst used in the present invention is usually N.
o Any metal oxide having decomposition activity may be used, such as first, second, and third period transition metal oxides. Among these, cobalt, iron, and nickel oxides, which inherently have relatively high catalytic activity, are preferred.

The catalyst for the present invention can be prepared by any method as long as it is an oxide catalyst containing silver. A coprecipitation method is used, in which a solid is prepared by adding a precipitant to a solution containing the precipitate, and the coprecipitation method is most suitable. In some cases, a carrier inert to the reaction may be used. The ratio of promoter metal silver to main catalyst metal in the catalyst can vary within a wide range.

Generally, the value is 0.1 to 50%, preferably 1 to 30%.

The starting materials for the main catalytic metal and co-catalyst silver for producing the catalyst of the present method by coprecipitation may be any type of compound as long as it is soluble in water, such as nitrates, organic acid salts, etc. , 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, and alkaline substances are suitable as the precipitant, such as sodium carbonate, sodium hydrogen carbonate, potassium carbonate, potassium hydrogen carbonate, and hydroxide. The amount of precipitant is determined by (1) The solid material from which the precipitation has been completely removed is filtered, thoroughly washed with water, dried, and finally calcined in a stream of air to obtain a catalyst. Air firing temperature is 2
Between 00°C and 900°C, preferably between 300°C and 700°C
It is.

To carry out the method of the invention, a gas containing No is passed over the catalyst. The reaction temperature is generally between 300°C and 900°C, preferably between 400°C and 700°C.

The higher the temperature, the higher the decomposition rate of No. However, 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 using a W8 catalyst that greatly exceeds the activity of conventional No decomposition catalysts, it is possible to decompose and remove No in various exhaust gases, which was difficult to reduce with catalysts, and to reduce the amount of No in the atmosphere. For purification” 1 Example 1 Cobalt nitrate (Co(NOi)z・6HzO) 20-
Dissolve 37 g of silver nitrate (AgNOW 0 ) and 60 g of silver nitrate (AgNOW 0 ) in pure water to make 100 mQ, and add sodium carbonate (Na2Go3) 8 while stirring with a magnetic stirrer.
, 50 g of 50mQ aqueous solution 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 catalyst. Firing conditions are 100℃~3
1°C per minute up to 00°C, 4°C per minute from 300°C to 400°C.
The temperature was raised to 3°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-silver oxide catalyst prepared in this way was charged into a normal pressure flow reactor, and while the temperature was raised, NO
Comparative Example 1: Using 3g of commercially available 99.9% cobalt sesquioxide as a catalyst, the decomposition rate of No was investigated in the same manner as in Example 1. It is shown in Table 1.

Considering the amount of catalyst, it is clear that the cobalt-silver catalyst of Example 1 has high activity.

Comparative Example 2 A cobalt oxide single catalyst containing no silver was prepared in the same manner as in Example 1, and 1 g was used for a reaction. The results are shown in Comparative Example 2.
It is shown in Table 1. It can be seen that compared to Example 1, the activity is greatly improved by combining silver.

Table 1 Example 2 Iron nitrate (Fe(No, ), 9H20) 28, 28
g and 0.60 g of silver nitrate were dissolved in pure water to make 150 mQ, and while stirring with a magnetic stirrer, a 100 mQ aqueous solution of 13.50 g of sodium carbonate 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.

2 g of the thus prepared catalyst was charged into a normal pressure flow reactor, and while the temperature was raised, helium gas containing 3.13% No was flowed at 30 mQ per minute for reaction, and the decomposition rate of No was investigated. The results are shown in Table 2 as Example 2.

Comparative Example 3 The decomposition rate of No was investigated in the same manner as in Example 2 using 2 g of commercially available 99.9% ferric oxide as a catalyst. The results are shown in Table 2 as Comparative Example 3.

Table 2 Example 3 Nickel nitrate (Nl (NO3)2.6H20) 20
, 36 g and 0.60 g of silver nitrate were dissolved in pure water and 10
The solution was adjusted to 0 mQ, and a 50 mQ aqueous solution of 8.50 g of sodium carbonate was added dropwise over 1 hour while stirring with a magnetic stirrer 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.

2 g of the catalyst thus prepared was charged into a normal pressure flow reactor, and 3.13% NO was passed through the reactor while increasing the temperature to examine the decomposition rate. The results are shown in Table 3 as Example 3.
It was shown to.

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, and the results are shown in Table 3 as Comparative Example 4.

Table 3

Claims (3)

[Claims] (1) A method of decomposing nitrogen oxides using a metal oxide catalyst containing silver as a promoter metal. (2) The method according to claim 1, in which cobalt, iron, or nickel is used as the main catalyst metal of the metal oxide catalyst. (3) The method according to claim 1 or 2, wherein the catalyst is prepared by treating a solid obtained by dropping a precipitant into a mixed aqueous solution containing a main catalyst metal salt and a silver salt.