JP3577509B2 - Method for producing highly durable alumina-gallium nitrogen oxide reduction catalyst - Google Patents

Description

[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a highly durable alumina-gallium-based nitrogen oxide reduction catalyst, and more particularly, to a novel alumina having an action of efficiently reducing nitrogen oxides over a long period of time in a temperature range of 400 ° C. or more. The present invention relates to a gall airgel catalyst and a method for producing the same.
The present invention belongs to the technical field of mainly removing nitrogen oxides contained in exhaust gas discharged from automobiles and the like.
[0002]
[Prior art]
In general, nitrogen oxides come from stationary sources, such as thermal power plants, and mobile sources, such as automobiles. Nitrogen oxides cause acid rain and photochemical smog, and their emission must be suppressed. In large-scale stationary sources such as thermal power plants, nitrogen oxides are removed using a vanadia / titania catalyst using ammonia. On the other hand, in a small-scale source such as an automobile, exhaust gas has been conventionally purified by a three-way catalyst. Lean burn engines (lean burn engines) have been used for energy saving. In this engine, the amount of hydrocarbons in the exhaust gas is small, and the three-way catalyst cannot be used in the lean burn engine. In particular, exhaust gases from lean burn engines are used in harsh conditions where air is present at high concentrations. In recent years, as the service life of automobiles has been extended from 50,000 miles to 100,000 miles in the United States and other countries, the use of exhaust gas catalysts for a long time has become mandatory.
[0003]
In recent years, catalysts for removing nitrogen oxides under lean burn conditions have been developed. Examples thereof include an alumina-supported cobalt catalyst and an alumina-gallium-based oxide.
The deterioration of the catalyst can be roughly divided into 1) sintering of the noble metal supported on the catalyst and 2) deterioration of the catalyst carrier. In the catalyst targeted by the present invention, an alumina-gallium binary oxide is used, no precious metal is used, and the deterioration of the catalyst is exclusively due to sintering of the oxide.
[0004]
In the case of an oxide carrier, sintering means a reduction in surface area due to high temperature or prolonged exposure to exhaust gas. Due to the decrease in the surface area of the support, the surface area of the noble metal supported on the support also decreases, and the catalytic function is greatly reduced. Therefore, suppression of sintering of the carrier is also important.
Among them, the alumina-gallium catalyst is an oxide catalyst that does not use a noble metal, and has a problem of being deteriorated by sintering as described above.
[0005]
[Problems to be solved by the invention]
For example, withstanding sintering even at a high temperature such as 1100 ° C. means that it has durability when used for a long time. The present inventors have conducted intensive studies with the aim of developing a catalyst having durability under such severe conditions, and as a result, have found that a highly durable alumina-gallium is produced by a new synthesis method different from the conventional synthesis method. Was found.
[0006]
That is, an object of the present invention is to contact an exhaust gas with a highly durable alumina-gallium binary catalyst and to efficiently reduce nitrogen oxides with an unburned hydrocarbon in an oxidizing atmosphere at a temperature range of 400 ° C. or more over a long period of time. As a result of research, a highly durable alumina-galleria airgel catalyst was found.
[0007]
The present invention provides a novel alumina-gallium aerogel catalyst capable of maintaining an activity of reducing nitrogen oxides contained in combustion exhaust gas, particularly exhaust gas from lean burn automobiles and the like at a temperature of 400 ° C. or higher for a long period of time. It is intended for that purpose.
Another object of the present invention is to provide a new method for producing the alumina-galleria airgel catalyst efficiently by a simple process.
[0008]
[Means for Solving the Problems]
The present invention for solving the above problems is constituted by the following technical means.
(1) A gallium- containing boehmite gel prepared using a boehmite (AlOOH) sol as an alumina source is supercritically dried to obtain a boehmite-gallia aerogel in which the volume of the boehmite gel is kept as it is. was produced by solid solution of Gaul in transition alumina was calcined at 1100 ° C., has a high surface area of 90~110 m ² / g, alumina having high heat resistance - alumina, characterized in that it consists of Gaul airgel - Highly durable gallium-based nitrogen oxide reduction catalyst.
(2) The catalyst according to (1), wherein the gallium-containing boehmite gel prepared by adding gallium ethylene glycolate to a boehmite sol is supercritically dried.
( 3 ) The catalyst according to the above (1), wherein the content of gallium is 20 to 40% by weight.
(4) 90~110m ² / Have a high surface area of g, alumina having high heat resistance - a method for producing a Gaul based high durability nitrogen oxide reduction catalyst was prepared using a boehmite (AlOOH) sol as alumina source, Gaul Production of an alumina-gallium nitrogen oxide reduction catalyst, characterized in that a boehmite-containing boehmite gel is subjected to supercritical drying to form a boehmite-gallia aerogel in which the volume of the boehmite gel is kept as it is, and then the gel is fired at 800 to 1100 ° C. Method.
(5) The method for producing an alumina-gallium nitrogen oxide reduction catalyst according to (4), wherein the gallium-containing boehmite gel produced by adding gallium ethylene glycolate to a boehmite sol is supercritically dried.
(6) The alumina-gallium-based nitrogen oxide reduction catalyst according to (4), wherein the gallium ethylene glycolate to be mixed with the boehmite sol is added as 20 to 40% by weight as final gall after firing. Production method.
[0009]
BEST MODE FOR CARRYING OUT THE INVENTION
Next, the present invention will be described in more detail.
As described above, the present invention provides a highly durable nitrogen oxide reduction catalyst comprising an alumina-gallium aerogel having a high surface area of 100 m ² / g or more, a method for producing the same, and the above-mentioned highly durable alumina-gallium binary catalyst. It is characterized in that it is used as a nitrogen oxide reduction catalyst, and that the catalyst is brought into contact with exhaust gas to decompose nitrogen oxides with unburned hydrocarbons.
The durable catalyst of the present invention shows excellent nitrogen oxide reducing ability at 400 to 500 ° C. even after heat treatment at a high temperature.
The alumina-gallium catalyst of the present invention is synthesized by gelling a mixture of boehmite sol and gallium ethylene glycolate, followed by supercritical drying, and then heating at a predetermined temperature.
[0010]
The ethylene glycolate mixed with the boehmite sol is preferably added so that the final gall after firing becomes 20 to 40% by weight. Thereby, the composition region having the highest catalytic activity can be covered.
Since the supercritical state of ethanol is preferably used in supercritical drying, the gel is immersed in ethanol. It is possible to use a solvent such as isopropanol, methanol, carbon dioxide and the like in addition to ethanol.
The solvent-substituted gel is immersed in an autoclave filled with ethanol, and after sealing, the temperature and pressure (64 atm, 243 ° C) are raised to a supercritical state of ethanol, and the pressure is reduced to obtain a boehmite-galleria aerogel.
Preferably, by firing this gel at a temperature of 1000 ° C. or higher, a catalyst having excellent durability can be obtained. The firing temperature is preferably from 800 to 1100 ° C. The firing is performed in air for 5 hours and does not need to be in a special gas atmosphere.
[0011]
In the present invention, a boehmite sol is used as an alumina source. Boehmite has a needle-like form, and by using this, has a high surface area even at high temperatures and plays a large role in imparting durability. As a method for producing a boehmite sol, for example, a boehmite sol is obtained by acid hydrolysis of aluminum isopropoxide. Commercially available alumina sol or the like can be used. As a method for producing a gel, for example, a boehmite gel can be obtained by adding gallium ethylene glycolate to an alumina sol. Although the mechanism is not clear, it is considered that each sol is entangled and gelled, probably due to the relationship of the respective surface potentials. In the present invention, any method equivalent to the above-mentioned production method can be used. When the boehmite gel is dried as it is, the solvent evaporates and a shrink-dried xerogel is formed. On the other hand, when the boehmite gel is supercritically dried, a boehmite aerogel is maintained in which the volume of the boehmite gel is kept as it is.
As gallium, a raw material that does not leave an inorganic component after firing, such as gallium nitrate and gallium acetate, is used.
[0012]
The term alumina used in the present invention means γ-alumina and has a high surface area of 100 m ² / g or more. Alumina usually has a high surface area of 100 m ² / g or more, but undergoes a phase transition to α-alumina by heating at 1000 ° C. or more. With the phase transition, the surface area decreases from 100 m ² / g or more to 10 m ² / g or less.
A catalyst in which gallium is added to alumina has a large number of active points and high activity when it has a high surface area of 100 m ² / g or more. As the surface area decreases, the number of active points decreases. The catalyst of the present invention have a high surface area 90~110m ² / g.
[0013]
【Example】
Next, the present invention will be specifically described based on examples, but the present invention is not limited to the following examples.
(1) Synthesis of alumina-gallium catalyst 20.4 g of aluminum isopropoxide was hydrolyzed with 80 ml of hot water at 80 ° C., and then peptized with 1N nitric acid for 1 hour to obtain a boehmite sol. A predetermined amount of gallium nitrate was dissolved in 10 ml of ethylene glycol to obtain gallium ethylene glycolate. When both were mixed and kept at 80 ° C., a boehmite gel was formed after several hours.
The gel was immersed in ethanol and allowed to stand for one day and night to replace isopropanol and water in the gel with ethanol. This operation was repeated twice, and the solvent in the gel was replaced with ethanol.
[0014]
Next, the gel was placed in an autoclave having an internal volume of 200 ml and filled with ethanol. Thereafter, the temperature of the autoclave was raised to 80 ° C. and 300 ° C., and the pressure was increased. After the ethanol was brought into a supercritical state, the pressure was gradually released and returned to atmospheric pressure.
The thus obtained boehmite aerogel was calcined at 800-1100 ° C. for 5 hours to obtain a catalyst composed of alumina-galleria aerogel.
[0015]
(2) Evaluation of catalyst activity As a simulated exhaust gas, a gas composed of 668 ppm of NO, 556 ppm of propylene, 2.5% of oxygen and the balance of nitrogen was passed through the catalyst layer at a space velocity of about 50,000 / h. A catalytic reaction was carried out in a temperature range of from 500 ° C. to 500 ° C., and the NO concentration in the gas before and after the reaction was measured by a chemiluminescence method to determine the NO conversion.
1 and 2 show the NO conversion rates of xerogel and airgel. In the case of the xerogel catalyst (FIG. 1), the catalyst having a heat treatment temperature of 800 ° C. exhibited a high activity of 80% conversion at a reaction temperature of 500 ° C. The conversion rate is around 60% up to the heat treatment temperature of 1000 ° C., and it can be seen that the conversion rate is relatively small.
However, the catalyst heat-treated at a high heat treatment temperature of 1100 ° C. showed a significant decrease in activity, and at 550 ° C., a low conversion of 20%.
On the other hand, in the case of the airgel catalyst (FIG. 2), the maximum conversion rate is 60%, slightly lower than that of the xerogel catalyst, and the temperature at which the maximum conversion rate is slightly lower than that of the xerogel catalyst, that is, from 400 ° C. to 500 ° C. %.
Furthermore, it should be noted that the NO conversion of the catalyst heat-treated at 1100 ° C. was 40% even at 450 ° C. despite the high heat treatment temperature of 1100 ° C.
[0016]
(3) Specific surface area FIG. 3 shows the specific surface area of the catalyst after heat treatment at each temperature. The airgel catalyst was found to maintain a large surface area of 100 m ² / g or more even after heating at 1100 ° C. On the other hand, the surface area of the xerogel catalyst is significantly reduced after heating at 1100 ° C. The present inventors have confirmed that the heat resistance of alumina-only aerogel is significantly higher than that of ordinary alumina-only xerogel, but the same effect is observed even if gallium is contained. I understand.
[0017]
【The invention's effect】
As described above in detail, the present invention relates to an alumina-gallium aerogel catalyst and a method for producing the same. According to the present invention, 1) an alumina-gallium-based nitrogen oxide reduction catalyst having high durability is produced. 2) It is possible to provide a novel alumina-galleria airgel catalyst having an action of efficiently reducing nitrogen oxides over a long period of time in a temperature range of 400 ° C. or more with unburned hydrocarbons in an oxidizing atmosphere. 3) It is possible to produce an alumina-gallium binary catalyst having heat resistance. 4) To realize a method for reducing nitrogen oxides contained in combustion exhaust gas at a high conversion rate using the above catalyst. 5) It is possible to provide a method for producing the above-mentioned catalyst by a simple process. 6) The above-mentioned catalyst is particularly suitable for a lean burn engine (lean burn engine). Exhaust gas catalyst down), are useful in such an exhaust gas purifying catalyst of the small boilers, special effects etc. are obtained.
[Brief description of the drawings]
FIG. 1 is an explanatory diagram showing the NO conversion rate by a xerogel catalyst.
FIG. 2 is an explanatory diagram showing a NO conversion rate by an airgel catalyst.
FIG. 3 is an explanatory diagram showing a specific surface area of a catalyst after heat treatment at each temperature.

Claims (6)

A gallium- containing boehmite gel prepared using a boehmite (AlOOH) sol as an alumina source is supercritically dried to obtain a boehmite-gallia aerogel that maintains the volume of the boehmite gel as it is, and then the gel is heated at 800 to 1100 ° C. firing was manufactured by solid solution of Gaul in transition alumina, 90 to 110 m have a high surface area of ² / g, high heat resistance of alumina having a - alumina, characterized in that it consists of Gaul airgel - Gallic system height Durable nitrogen oxide reduction catalyst. The catalyst according to claim 1, wherein the gallium-containing boehmite gel prepared by adding gallium ethylene glycolate to a boehmite sol is supercritically dried. 2. The catalyst according to claim 1, wherein the content of gallium is 20-40% by weight. 90-110m ² / Have a high surface area of g, alumina having high heat resistance - a method for producing a Gaul based high durability nitrogen oxide reduction catalyst was prepared using a boehmite (AlOOH) sol as alumina source, Gaul Production of an alumina-gallium nitrogen oxide reduction catalyst, characterized in that a boehmite-containing boehmite gel is subjected to supercritical drying to form a boehmite-gallia aerogel in which the volume of the boehmite gel is kept as it is, and then the gel is fired at 800 to 1100 ° C. Method. 5. The method for producing an alumina-gallium nitrogen oxide reduction catalyst according to claim 4, wherein the gallium-containing boehmite gel produced by adding gallium ethylene glycolate to a boehmite sol is supercritically dried. The method for producing an alumina-gallium-based nitrogen oxide reduction catalyst according to claim 4, wherein the gallium ethylene glycolate mixed with the boehmite sol is added so as to be 20 to 40% by weight as the final gall after calcination.

Images