JP3316565B2 - Method for catalytic reduction and removal of nitrogen oxides - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to hydrocarbons and oxygen-containing compounds to which a small amount of exhaust gas has been added, or to hydrocarbons and oxygen-containing compounds present in exhaust gas, in an oxidizing atmosphere in which oxygen is present in excess. In the presence of alumina,
And a catalyst comprising at least one of silica or zirconia containing at least one of silver and a silver compound to reduce and remove nitrogen oxides in the exhaust gas.

[0002]

2. Description of the Related Art Nitrogen oxides in various exhaust gases (hereinafter referred to as "nitrogen oxides").
Since "NOx") has a direct adverse effect on the human body and can cause photochemical smog and acid rain, it is desired to develop an effective removing means therefor. Conventionally, this NO
As a method for removing x, several methods for reducing NOx in exhaust gas using a catalyst have already been put to practical use.

[0003] For example, (a) a three-way catalytic method for gasoline vehicles, and (b) a selective catalytic reduction method using ammonia for exhaust gas from large equipment discharge sources such as boilers. Other recently proposed methods include (c) NOx in exhaust gas using hydrocarbons.
As a removal method, there is a method in which a metal oxide such as alumina supporting a metal such as copper or a zeolite supporting various metals is brought into contact with a gas containing NOx in the presence of a hydrocarbon as a catalyst.

[0004]

SUMMARY OF THE INVENTION The method (a) is
Hydrocarbon components and carbon monoxide contained in the combustion exhaust gas of automobiles are converted into water and carbon dioxide by a catalyst containing a platinum group, and simultaneously, NOx is reduced to nitrogen. However, in this method, it is necessary to adjust the oxygen concentration such that the amount of oxygen required to oxidize the hydrocarbon component and carbon monoxide, including the amount of oxygen contained in NOx, is stoichiometrically equal. There is a problem that it cannot be applied in principle in an atmosphere where excess oxygen exists, such as a diesel engine.

In the method (b), ammonia is used which is very toxic and which must be treated as a high-pressure gas in many cases. Application to sources, especially mobile sources, is technically very difficult and not economical.

On the other hand, the method (c) is mainly intended for a gasoline-powered vehicle, and is difficult to apply under exhaust gas conditions of a diesel engine, and has insufficient catalyst durability. That is, a catalyst in which a metal such as copper is supported on alumina or zeolite is not only poisoned by sulfur oxides discharged from a diesel engine, but also causes a decrease in the activity of the catalyst due to aggregation of the added metal and the like. It is not suitable for removing NOx in exhaust gas from an engine.

In particular, the zeolite-based catalyst has a relatively high initial activity, but when the exhaust gas from which NOx is to be removed contains steam, the steam covers the active sites of the catalyst, As a result, the NOx removal performance is reduced. Further, use under hydrothermal conditions, such as exhaust gas from combustion engines, promotes dealumination from the zeolite framework, causing catastrophic catalyst degradation.

Accordingly, even when the exhaust gas to be treated under an excess of oxygen contains water vapor or sulfur oxide, NOx reduction and removal having high reduction performance and exceptionally excellent durability can be achieved. The development of a catalyst is desired.

The present invention has been made to solve various problems existing in the above (A) to (C),
In the presence of excess oxygen, and even in the presence of oxygen, water vapor, and sulfur oxides, nitrogen oxides in exhaust gas generated from various facilities including gasoline engines as well as diesel engine exhaust gas are reduced and removed. The aim is to propose a method that can be done.

[0010]

Means for Solving the Problems and Actions The present inventors have conducted intensive studies in order to achieve the above object, and as a result, have found that alumina and silica or zirconia containing at least one of silver or a silver compound can be obtained. By using at least one type of catalyst, a NOx reduction / removal method capable of removing NOx extremely efficiently even when water vapor or sulfur oxides coexist under an excess of oxygen has been found. The invention has been completed.

That is, the method of the present invention for reducing and removing nitrogen oxides in an exhaust gas comprises the steps of: removing at least one of hydrocarbons or oxygen-containing compounds in an oxidizing atmosphere containing excess oxygen;
In the presence of the species, the exhaust gas containing NOx is contacted with a catalyst comprising alumina and at least one of silica or zirconia containing at least one of silver or a silver compound.

In the method of the present invention, it is preferable that the above-mentioned alumina has a content of at least one of an alkali metal or an alkaline earth metal of 0.5 wt% or less;
The pore volume formed by pores having the following diameters is 0.
26 cm ³ · g ⁻¹ or more, and the total pore volume is 0.
It is also characterized in that it is at least 48 cm ³ · g ^-1 .

Further, the method of the present invention is characterized in that one or both of ethanol and acetone are used as the oxygen-containing compound. In this case, it is possible to reduce and remove NOx at a low temperature and with high efficiency even under the steam coexistence condition.

The details of the method of the present invention will be described below together with the operation. As the alumina which is one component of the catalyst in the present invention, those prepared by a conventionally known method can be used. The catalyst in the present invention comprises this alumina and at least one of silica or zirconia containing at least one of silver or a silver compound.

The impregnated amount of silver or silver compound is about 0.01 to about 0.01 or less with respect to silica or zirconia in terms of metal.
It is preferably 40 wt%, preferably about 0.05-30 wt%. If the content is less than about 0.01 wt%, the content effect does not occur, and if the content is more than about 40 wt%, the condensation of silver or silver compound in silica or zirconia may be remarkably promoted. Is not exhibited in some cases.

The method for incorporating silver or silver compound into silica or zirconia is not particularly limited, and may be carried out by a conventionally known impregnation method, coprecipitation method, kneading method, deposition method, ion exchange method, or other various methods. it can.

In the impregnation method, silica or zirconia can be impregnated with an aqueous solution of a silver compound, dried, and calcined in air. Examples of the silver compound at this time include silver chloride, silver nitrate, silver sulfate and the like, and silver nitrate is usually preferably used. The air firing temperature is about 300 to 800 ° C, preferably about 400 to 60 ° C.
0 ° C. and the baking time is about 1 to 10 hours.

In the coprecipitation method, a suitable precipitant is added to a mixed aqueous solution of a silver compound, a silicate or a zirconium salt to precipitate hydroxides or carbonates of these metals, and the mixture is separated by filtration and washed with water. -It is dried and, if necessary, fired to be contained.

In the kneading method, a gel or slurry of hydroxide or carbonate of silver, silicon or zirconium prepared separately is mixed and kneaded with a kneader or the like, and dried and, if necessary, calcined to contain. It is to let.

In the deposition method, a gel or slurry hydroxide or carbonate of silicon or zirconium prepared in advance is immersed in an aqueous solution of a silver compound (eg, nitrate, sulfate, acetate, chloride, etc.). Then, a precipitant is added to deposit the silver compound on a silicon or zirconium gel or slurry, followed by filtration, washing with water, drying and, if necessary, calcination to contain the silver compound.

In the ion exchange method, H ⁺ or Na ⁺ or the like coordinated on the catalyst surface is exchanged for an arbitrary cation (cation exchange). As the exchange operation, a method of immersing the catalyst carrier in an aqueous solution of about 0.1 to 1.0 N of a cation to be exchanged is usually employed, and this immersion operation is repeated until a desired ion exchange rate is obtained. . At this time, care must be taken because the pH and temperature of the exchange liquid are important factors that govern the exchange rate. In addition,
When the cation of the transition metal is supported by ion exchange, complex ions may be formed, so it is necessary to pay attention to the pH and temperature of the exchange solution so that the formation of the complex ions does not occur. . After the completion of the ion exchange operation in this way, if necessary, it is calcined and used as a catalyst.

These methods are often suitable when the amount of silver or silver compound to be contained is large. However, in the present invention, any of impregnation method, coprecipitation method, kneading method, deposition method, ion exchange method, etc. The method is not particularly limited.

At least one kind of silica or zirconia containing at least one kind of silver or a silver compound (hereinafter, referred to as “silver / silver compound-containing silica / zirconia”) contains at least one kind of hydrocarbons or oxygen-containing compounds. It greatly promotes the oxidation of species (hereinafter referred to as "hydrocarbons / oxygen compounds"). That is, the silica / zirconia containing silver / silver compound does not act on the reduction of NOx in the oxidizing atmosphere.

On the other hand, although alumina alone has a relatively high NOx removal performance, this performance in a low temperature region is not so high. The present inventors have conducted intensive studies in order to improve the NOx removal performance of this alumina in a low-temperature region.
When a catalyst consisting of alumina and silver / silver compound-containing silica / zirconia was prepared by physically mixing zirconia, etc., this catalyst has a remarkable NOx removal performance in the low temperature region compared to alumina alone. I found a surprising phenomenon that I couldn't foresee at first.

Two kinds of catalysts having different performances (silica / silicon-containing zirconia containing silver / silver compound have oxidation performance of hydrocarbons / oxygen-containing compounds, and alumina mainly having NOx reduction performance) The phenomenon that a concerted effect can be obtained by physical mixing or the like is extremely rare, and its detailed mechanism has not been elucidated.

At present, silver / silver compound-containing silica
It is considered that some of the intermediate products of hydrocarbons and oxygenated compounds activated on zirconia specifically act on the NOx reduction reaction on alumina.

A catalyst in which silver / silver compound is directly supported (contained) on alumina (Japanese Patent Application Laid-Open No. Hei 5-76757) exhibits the same effect, but NOx reduction activity in a lower temperature region or sulfur oxide. A difference in performance is observed in durability and the like in the coexistence, and both are clearly distinguished from the catalyst preparation method.

That is, when silver is directly supported on alumina, the number of active sites on the alumina surface decreases, and the selectivity decreases. In addition, in the presence of sulfur oxides, the adsorption of the sulfur oxides on alumina is promoted, and as a result, there is also a problem that alumina is significantly poisoned and deteriorated.

On the other hand, in the catalyst of the present invention obtained by physically mixing silver / silver compound-containing silica / zirconia with alumina, the active site of alumina is not affected at all. Poisoning deterioration of alumina by sulfur oxides is also reduced. Furthermore, as shown in the section of Examples, the amount of silver / silver compound contained in the catalyst can be smaller than that in the case where the catalyst is directly supported.
It is very advantageous in terms of cost.

Further, the present inventors have filed a prior application (Japanese Patent Application No. 4-2).
No. 31376), the NOx reduction performance of alumina is such that at least one of alkali metals or alkaline earth metals contained as an impurity in the alumina (hereinafter, referred to as “a”).
(Referred to as "alkali metal / alkaline earth metal") and the physical structure of alumina (pore volume, pore diameter, etc.).

The present inventors have found that the higher the NOx reducing activity of alumina itself, the higher the NOx reduction activity of the catalyst of the present invention comprising alumina and silver / silver compound-containing silica / zirconia. Progress, the correlation between the NOx reduction activity of alumina alone and the order of the activity of the catalyst of the present invention comprising alumina and silver / silver compound-containing silica / zirconia was found, and the NOx reduction performance of the catalyst of the present invention was also found. A completely new fact has been found that the content of alkali metals and alkaline earth metals in alumina and the physical structure (pore volume, pore diameter, etc.) of alumina are all without exception.

This alumina has the following features: a) The content of alkali metal and alkaline earth metal is 0.5
a) a pore volume formed by pores having a diameter of 80 ° or less is 0.26 cm ³ · g ^-1 or more, and a total pore volume is 0.48 cm ³ · g ^-1 or more; Requirements. The reason why these requirements are required will be described below for each requirement.

Regarding the requirement of a): The basic reaction of the present invention is that propylene (C
Taking nitrogen dioxide (NO ₂ ) as an example of NOx with ₃ H ₆ ) as the NOx, it is presumed to be based on the reaction formula shown in Chemical formula 1.

[0034]

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That is, in order to reduce NO ₂ to N ₂ , C ₃ H ₆ is converted to CO ₂ (or CO in some cases) and H ₂
It is necessary to oxidize to O, and if the oxidation of C ₃ H ₆ does not proceed, the reduction of NO ₂ does not proceed. However,
If the oxidation of C ₃ H ₆ proceeds too much, C ₃ H ₆ does not participate in the reaction of Chemical Formula 1, and as a result, the reduction rate of NO ₂ also decreases. Therefore, in order to reduce NOx at a high rate, it is necessary to appropriately oxidize hydrocarbons and oxygen-containing compounds such as C ₃ H ₆ (hereinafter, sometimes referred to as “reducing agent”) which are NOx reducing agents. Becomes

On the other hand, the reduction and removal of NOx are performed by removing NOx on the catalyst.
It is known that the higher the x concentration, the more efficiently the process proceeds. Accordingly, increasing the NOx concentration on the catalyst can be cited as one means for obtaining a higher NOx reduction and removal rate.

By the way, when alumina alone is used as a catalyst, even if the degree of oxidation of the reducing agent is the same, NO
The reduction removal efficiency of x (the meaning of the NOx removal rate converted per reducing agent consumed by oxidation) may vary greatly.

As a result of studying the cause, it was found that the NOx reduction removal efficiency is related to the impurities in alumina, and particularly that an alkali metal or an alkaline earth metal greatly affects the NOx reduction removal efficiency. Moreover, this effect is
It was also found that the catalyst was remarkably observed when the catalyst of the present invention was constituted together with the silver zirconia containing a silver compound.

Therefore, the content of the alkali metal and alkaline earth metal of the alumina constituting the catalyst of the present invention together with the silver / silver compound-containing silica / zirconia is set to a certain amount or less, specifically 0.5 wt% or less, preferably 0.5 wt% or less. Is set to 0.1% by weight or less.

It is to be noted that the content of the alkali metal / alkaline earth metal is preferably as small as possible, and the lower limit is not particularly limited. With the advance of these metal removal techniques, and the content of these metals as impurities, With the advance of measurement technology, it can be almost zero.

Further, it is preferable that the sulfur content is as small as possible together with the alkali metal and alkaline earth metal in order to enhance the NOx reduction and removal efficiency.
More specifically, it is desirable that the sulfur content be about 0.5 wt% or less as in the case of alkali metals and alkaline earth metals.

Of course, as described above, when the content of the alkali metal / alkaline earth metal is substantially 0, and when the content of the sulfur is also substantially 0, the structural formula of Al
The composition ratio of what can be represented by ₂ O ₃ may be about 100 wt%.

Regarding requirement a): promotes appropriate oxidation of the above-mentioned reducing agent, lower temperature, higher speed (high space velocity)
In order to obtain high NOx reduction and removal efficiency, the size of the pore volume formed by pores having a diameter of a specific diameter or less, specifically, a diameter of about 80 ° or less is also an important factor. The larger the pore volume is, specifically, about 0.26 cm
³ · ^{g -1} or more, preferably 0.32 cm ³ · ^{g -1} or more, more preferably at 0.35 cm ³ · ^{g -1} or more, it is possible to proceed well moderate oxidation of the reducing agent. If the pore volume is less than about 0.26 cm ³ · g ⁻¹ , the oxidation of the reducing agent is excessively suppressed, and the reaction of Chemical Formula 1 does not proceed satisfactorily.
Ox reduction removal efficiency is not improved.

Although the details are not clear,
It is also possible that the pores of the alumina are involved in the concentration of NOx. That is, the catalyst of the present invention comprising alumina having pores satisfying the above requirements and silver / silver compound-containing silica / zirconia, when the concentration of NOx occurs at a higher rate and the reduction and removal of NOx proceeds with high efficiency. Conceivable.

The upper limit of the pore volume due to the pores having a diameter of about 80 ° or less is not particularly limited. However, if the pore volume is too large, the production technique is difficult, the cost rises, and practical use is considered. Is not very favorable. In addition, when the number of pores having a diameter of about 80 ° or less is increased, smaller pores are also created, which tends to cause sintering that changes the properties of the catalyst, and causes a decrease in surface area, which is not preferable. .

In the alumina used in the present invention, it is needless to say that the larger the surface area is, the better. Therefore, the upper limit of the pore volume formed by small pores having a diameter of 80 ° or less is not particularly limited, but is about 0%. .6c
It is preferable to be around m ³ · g ⁻¹ .

In order to improve the diffusion of the reaction gas (NOx-containing exhaust gas or gaseous reducing agent), it is necessary to increase the total pore volume to some extent. As for the alumina used in the present invention, as a result of many experiments, the total pore volume was found to be about 0.48 cm ³ · g ⁻¹ or more, preferably about 0.52 cm ³ · g ⁻¹ or more, and more preferably about 0.52 cm ³ · g ⁻¹ or more. 0.
54 cm ³ · g ^-1 or more. If the pore volume is less than about 0.48 cm ³ · g ⁻¹ , the diffusion of the reaction gas becomes insufficient, and the reaction of Chemical Formula 1 does not occur uniformly and with high reaction efficiency.

The upper limit of the total pore volume is not particularly limited, but if it is too large, the mechanical strength of the alumina carrier will be too low, and the alumina carrier will be prematurely disintegrated when used in an actual machine. From the viewpoint of manufacturing technology, about 1.2c
It is desirable to be around m ³ · g ⁻¹ .

The pore volume of the above requirements a) and a) is as follows:
It is determined as follows. The surface area is determined by a BET method from a nitrogen adsorption isotherm at -196 ° C. The pore size distribution was measured by measuring the nitrogen adsorption-desorption isotherm (-196 ° C) up to a relative pressure of 0.967, and using the BJH method or DH
It is determined by the method for pores having a radius of 200 ° or less.
At the same time, the pore volume and average pore diameter are calculated from the obtained results.

As a representative value of the average pore diameter shown here, a pore diameter which equally divides the pore volume into a portion having a larger diameter and a portion having a smaller diameter, that is, a so-called fine diameter. The median value of the distribution of pore volume with respect to pore diameter is shown.

The alumina used in the present invention that satisfies the above requirements a) and b) can be produced by various known methods. To briefly explain an example, first, an aluminum hydrate is prepared using an aqueous solution of an aluminum salt as a raw material, and then calcined (thermally decomposed) to obtain alumina.

By the way, when preparing aluminum hydrate, alumina hydrate having various forms can be obtained by changing conditions such as starting material, temperature during precipitation, concentration of raw material liquid, pH, aging time and the like. Generate.

There have already been many reports on the thermal transformation of the alumina hydrate. For example, when preparing a precipitate of alumina hydrate from an aqueous solution of sodium aluminate and carbon dioxide, boehmite (pseudo-boehmite), bayerite, gypsite, etc. are generated depending on the preparation conditions. The crystal structure of the obtained hydrates and their high-temperature firing (for example,
(500 ° C.) and the porous structure of alumina have the following relationship.

That is, alumina obtained by calcining a hydrate containing structures such as bayerite and gypsite having high crystallinity has a specific surface area and pores smaller than those obtained from pseudo-boehmite (boehmite gel). The volume is small, which is not very preferable when used as a catalyst or a catalyst carrier.

Next, with respect to the control of the porous structure of alumina, particularly the pore distribution and the average pore diameter, boehmite gel which is preferably used as a catalyst is used as a hydrate.
A method for obtaining an alumina fired body will be described as an example.

The boehmite gel has a fibrous or thin plate shape, and the relationship between this structure and the pore structure of alumina after heat transformation is schematically represented as follows.

That is, those with small basic particles give small pores by baking treatment, those with large particles give large pores, uniform particle size has sharp pore distribution, and non-uniform ones have broad pores. The relationship is to give a distribution. There is a correlation that the pore volume is large for a uniform crystal and large for a non-uniform crystal. Therefore, the pore structure can be controlled by controlling the basic particles of the boehmite gel.

Specifically, Japanese Patent Application Laid-Open No. 58-213632
(US Pat. No. 4,562,059), JP-A-58-1908
23 (US Pat. No. 4,555,394), for example. In these techniques, basic particles are controlled by manipulating the pH of the solution in which the boehmite gel is present. That is, fine crystals existing in the solution are manipulated by pH manipulation to make the particle size uniform.

On the other hand, as a method for adjusting the pore volume, a method in which a water-soluble polymer compound such as polyethylene glycol is added to the boehmite gel (JP-A-52-104498 and JP-A-52-77891), Although a method of substituting a part or most of water with an oxygen-containing compound such as alcohol (JP-A-50-123588) has been proposed, micropores (20 ° or less) and mesopores which are important in the present invention are proposed. Many of them are generally unsuitable for controlling the pores in the first half of the (20-500 °) region.

When an alumina fired body is obtained from a hydrate, an alumina fired body having a different pore distribution and pore volume to some extent depending on the firing temperature is obtained. However, at this time, attention must be paid to the crystal transition of alumina.

The above silver / silver compound-containing silica / zirconia, alumina, or alumina containing a specific amount or less of an alkali metal / alkaline earth metal and having a specific physical structure (hereinafter, these aluminas are collectively referred to as “ The catalyst of the present invention comprising (alumina) may be prepared by a wet method or a dry method. In the wet method,
Silver / silver compound-containing silica / zirconia and alumina or alumina hydrate are mixed and kneaded with a kneader or the like in the presence of moisture, and dried and fired if necessary.

In addition, as a preparation method for obtaining a similar effect, a method of preparing a slurry of silver / silver compound-containing silica / zirconia, coating the alumina with a binder, drying, and, if necessary, firing. is there. According to this method, silver / silver compound-containing silica / zirconia can be directly added to and mixed with the alumina honeycomb structure or the like.

In the dry method, desired silica / silver compound-containing silica / zirconia and alumina or alumina hydrate are obtained by crushing and mixing in a mortar or the like.

The effect of silver / silver compound-containing silica / zirconia on alumina is as follows. When alumina and silver / silver compound-containing silica / zirconia having the same silver / silver compound content coexist on alumina, the coexistence amount increases and the low-temperature region increases. NOx in
The feature is that the removal performance is improved. Further, in the case of the same coexistence amount, the same effect in the low temperature region can be obtained as the content of silver / silver compound in silica or zirconia increases.

The silver / silver compound-containing silica according to the present invention
The weight ratio of zirconia (A) to alumina (B) is generally about 0.01 to 0.01 in silica / zirconia containing silver / silver compound in the above weight ratio, expressed as (A) / (B). 0.90 / 0.99-0.10, preferably about 0.05-0.80 / 0.95-0.20, more preferably about 0.10-0.70 / 0.90-0.30 It is suitable to use in the range.

The catalyst of the present invention comprising alumina and silver / silver compound-containing silica / zirconia can be used in the form of powder, granule, pellet, honeycomb, or any other form. It does not matter. When the catalyst of the present invention is molded and used, a binder usually used at the time of molding, that is, silica sol, polyvinyl alcohol or the like, or a lubricant such as graphite, wax, fatty acid salt, carbon wax, or the like is used. be able to.

In the method of the present invention, N
Examples of the Ox-containing gas include exhaust gas from nitric acid production facilities and various combustion facilities, including diesel exhaust gas from diesel vehicles and stationary diesel engines, and gasoline engine exhaust from gasoline vehicles. The removal of NOx in these exhaust gases is performed by using the above-described catalyst of the present invention.
The catalyst is brought into contact with an exhaust gas in an oxidizing atmosphere in the presence of the above-mentioned hydrocarbons and oxygen-containing compounds.

Here, the oxidizing atmosphere is defined by the carbon monoxide, hydrogen, hydrocarbons and oxygen-containing compounds contained in the exhaust gas and the hydrocarbons and oxygen-containing compounds added as required in the method of the present invention. Refers to an atmosphere in which oxygen is contained in excess of the amount of oxygen necessary to completely oxidize and convert the reducing agent into water and carbon dioxide.For example, exhaust gas discharged from an internal combustion engine such as an automobile Is an atmosphere with a large air-fuel ratio (lean region), and usually has an excess oxygen rate of about 20 to 2000%.

In this oxidizing atmosphere, the catalyst of the present invention described above promotes the reaction of hydrocarbons / oxygenated compounds with NOx preferentially over the reaction of hydrocarbons / oxygenated compounds with oxygen. , NOx are reduced and removed.

The above-mentioned catalyst of the present invention in the method of the present invention works well in an oxidizing atmosphere. However, in a reducing atmosphere, the activity of reducing and decomposing NOx is reduced. Is preferred.

The reducing agent for reducing and decomposing and removing hydrocarbons and oxygen-containing compounds, that is, NOx, may be particulates which are incomplete combustion products such as hydrocarbons and fuel remaining in the exhaust gas. If the amount is less than that required to promote the above reaction, it is necessary to add hydrocarbons and oxygen-containing compounds from the outside.

The amounts of the hydrocarbons and oxygen-containing compounds to be present are not particularly limited. For example, when the required NOx removal rate is low, the amount may be smaller than the theoretical amount required for the reductive decomposition of NOx. . However, since the reduction reaction proceeds more than the required stoichiometric amount, it is generally preferable to add in excess. Usually, the amount of hydrocarbons / oxygen compounds is about 20% to about 20% of the theoretical amount required for reductive decomposition of NOx.
It is present in a 2000% excess, preferably about a 30% to 1500% excess.

Here, the stoichiometric amount of the necessary hydrocarbons and oxygen-containing compounds means that there is oxygen in the reaction system, so that in the present invention, the stoichiometric amount necessary for reductive decomposition of nitrogen dioxide (NO ₂ ) is used. It is defined as hydrocarbons / oxygen-containing compounds. For example, the theoretical amount of propane when reducing and decomposing 1000 ppm of nitrogen monoxide (NO) in the presence of oxidation using propane as the hydrocarbons / oxygen-containing compound Is 200pp
m. Generally, it depends on the amount of NOx in the exhaust gas.
The amount of the hydrocarbons / oxygen-containing compound to be present is about 50 to 10000 ppm in terms of methane.

In the present invention, NOx is generated by the above catalyst.
Any substance can be used as a reducing agent for reducing carbon dioxide, as long as it is a carbon-containing substance such as a flammable organic compound. However, from the viewpoint of practicality, compounds such as nitrogen, sulfur, and halogen are costly and secondary. There are many problems such as generation of harmful substances or damage to the catalyst, and solid substances such as carbon black and coal are generally not preferable in terms of supply to the catalyst layer, contact with the catalyst, etc. Oxygen compounds are preferred. From the viewpoint of supply to the catalyst layer, a gaseous or liquid one is particularly preferable, and from the viewpoint of a reaction, a gaseous one which evaporates at the reaction temperature is particularly preferable.

Specific examples of the hydrocarbons in the present invention include, as gaseous substances, hydrocarbon gases such as methane, ethane, ethylene, propane, propylene, butane, and butylene; Examples thereof include single hydrocarbons such as heptane, octane, octene, benzene, toluene, and xylene, and mineral hydrocarbon oils such as gasoline, kerosene, light oil, and heavy oil.

The oxygen-containing compound means an oxygen-containing organic compound, such as alcohols such as methyl alcohol, ethyl alcohol, propyl alcohol and octyl alcohol; ethers such as dimethyl ether, ethyl ether and propyl ether; methyl acetate; Examples include oxygen-containing organic compounds such as ethyl acetate, esters such as oils and fats, and ketones such as acetone and methyl ethyl ketone.

These hydrocarbons and oxygen-containing compounds may be used alone or in combination of two or more. Unburned or incomplete combustion products such as fuel present in the exhaust gas, that is, hydrocarbons and particulates are also effective as reducing agents, and these are also included in the hydrocarbons in the present invention. From this,
It can be said that the catalyst of the present invention also has a function as a catalyst for reducing and removing hydrocarbons and particulates in exhaust gas.

The reaction is carried out by preparing a reactor in which the above-mentioned catalyst is arranged, and passing a NOx-containing exhaust gas in an oxidizing atmosphere in the presence of hydrocarbons and oxygen-containing compounds. The reaction temperature at this time depends on the catalyst and hydrocarbons.
Although it depends on the type of the oxygen-containing compound, a temperature close to the temperature of the exhaust gas is preferable because heating equipment for the exhaust gas is not required, and is generally about 100 to 800 ° C., particularly about 200 to 6 ° C.
A range of 00 ° C. is preferred. The reaction pressure is not particularly limited, and the reaction proceeds under increased pressure or reduced pressure. However, it is convenient to introduce the exhaust gas into the catalyst layer at a normal exhaust pressure to advance the reaction. The space velocity is determined by the type of catalyst, other reaction conditions, the required NOx removal rate, etc., and is not particularly limited, but is generally about 500 to 200,000 hr ^-1 , preferably about 10
The range is from 00 to 100000 hr ^-1 .

In the present invention, when treating exhaust gas from an internal combustion engine, it is preferable that the catalyst is disposed downstream of the exhaust manifold.

When exhaust gas is treated by the present invention, depending on the treatment conditions, incomplete combustion products such as unburned hydrocarbons and carbon monoxide, which cause pollution, are discharged into the treated gas. There are cases. As a countermeasure in such a case, a method in which the gas treated with the above-mentioned catalyst (hereinafter, referred to as “reduction catalyst”) is brought into contact with the oxidation catalyst can be adopted.

The oxidation catalyst that can be used includes:
Generally, any catalyst may be used as long as it can completely burn the above incomplete combustion products, but activated alumina, silica, zirconia, etc., platinum, palladium, noble metals such as ruthenium, lanthanum, cerium, copper, iron A catalyst component such as a perovskite-type crystal structure such as a base metal oxide such as molybdenum, lanthanum cobalt trioxide, lanthanum iron trioxide, or strontium cobalt trioxide was supported alone or in combination of two or more. Things.

The loading amount of these catalyst components is about 0.01 to 5% by weight of the noble metal and about 5 to 70% by weight of the base metal oxide. Of course, in particular, base metal oxides and the like can be used without being supported on a carrier.

The additives to be added for the purpose of shape, molding, etc. of the oxidation catalyst are the same as those in the case of the reduction catalyst, and various additives can be used.

The use ratio of the above reduction catalyst and oxidation catalyst,
The amount of the catalyst component supported on the oxidation catalyst can be appropriately selected according to the required performance. Particularly, when the substance to be oxidized and removed is an intermediate product of a hydrocarbon such as carbon monoxide, the reduction catalyst and the oxidation Although it is possible to use a mixture with a catalyst, it is possible to arrange the reduction catalyst on the exhaust gas upstream side and the oxidation catalyst on the exhaust gas downstream side in general.

In the present invention, as a specific example of purifying exhaust gas by using these catalysts, a reactor in which a reduction catalyst is disposed is disposed in an exhaust gas introduction section (previous stage), and a reactor in which an oxidation catalyst is disposed is disposed in an exhaust gas discharge section. (Rear stage), or a method in which each catalyst is arranged in one reactor at a ratio according to required performance.

The ratio of the reduction catalyst (A) to the oxidation catalyst (B) is generally expressed as (A) / (B) and is about 0.5 to 9.
It is used in the range of 5 / 9.5 to 0.5.

The use temperature of the oxidation catalyst may not be the same as the use temperature of the reduction catalyst, but in general, the one that can be used within the above-mentioned use temperature range of the reduction catalyst is selected for the heating and cooling equipment. It is not particularly necessary and is preferable.

[0088]

【Example】

Example 1 (Preparation of Silver-Containing Silica) Silver Nitrate (AgNO ₃ ) 0.79
g was dissolved in 35 g of distilled water, and this aqueous solution was impregnated with 50 g of silica (trade name: # 57, manufactured by Davison) and allowed to stand overnight. After standing, it was dried under reduced pressure at 100 ° C. by an evaporator, and then calcined at 600 ° C. for 3 hours in an air stream to obtain silver-containing silica. At this time, the content of silver with respect to silica was 1.0% by weight as metallic silver.

(Preparation of the Catalyst of the Present Invention) 0.2 g of the silica containing 1 wt% silver prepared as described above and 0.4 g of alumina (Neobead GB-45 manufactured by Mizusawa Chemical Co., Ltd.) were pulverized and mixed in a mortar. Thus, a catalyst of the present invention comprising silver-containing silica and alumina was obtained.

(Removal of NOx) 0.6 g of the catalyst of the present invention obtained as described above was charged into a normal-pressure flow reactor, and about 1000 ppm of nitrogen monoxide (hereinafter referred to as "N
O "), about 10% oxygen, about 8 vol% water vapor, and about 670 ppm of helium gas containing acetone at a flow rate of 120 ml / min. The analysis of the reaction gas was performed using a gas chromatograph, and the reductive decomposition rate of NO was determined from the yield of generated nitrogen. The results are shown in Table 1 as Example 1.

Examples 2 and 3 (Example 2) Silver-containing silica was obtained in the same manner as in Example 1, except that 1.58 g of silver nitrate (AgNO ₃ ) was used. At this time, the content of silver with respect to silica was 2.0% by weight as metallic silver.

Example 3 Silver nitrate (AgNO ₃ ) was added to 3.
Silver-containing silica was obtained in the same manner as in Example 1 except that the amount was changed to 95 g. At this time, the content of silver with respect to silica was 5.0% by weight as metallic silver.

(Preparation of the Catalyst of the Present Invention) 0.2 g of the silver-containing silica prepared as described above and 0.4 g of alumina (trade name: Neobead GB-45, manufactured by Mizusawa Chemical Co., Ltd.) were ground and mixed in a mortar. A catalyst of the present invention comprising silver-containing silica and alumina was prepared.

(NOx removal reaction) Using the catalyst of the present invention obtained as described above, NO
Are shown in Table 1 as Examples 2 and 3, respectively.

Examples 4 to 6 (Examples 4 and 5) The amount of the silica containing 2.0 wt% silver prepared in Example 2 mixed with 0.4 g of alumina (Neobead GB-45 manufactured by Mizusawa Chemical Co., Ltd.) In the fourth embodiment, 0.
1 g, 0 in Example 5. A NO reduction reaction was carried out in the same manner as in Example 2 except that the amount was changed to 05 g. The results are shown in Table 2 as Examples 4 and 5, respectively.

(Example 6) (Preparation of silver-containing zirconia) Silver nitrate (AgNO ₃ )
79 g was dissolved in 15 g of distilled water, and this aqueous solution was impregnated with 50 g of zirconia (manufactured by Daiichi Rare Element Co., Ltd.), and allowed to stand overnight. After standing, the mixture was dried at 100 ° C. under reduced pressure using an evaporator, and then calcined in an air stream at 600 ° C. for 3 hours to obtain silver-containing zirconia. At this time, the silver content relative to zirconia was 1.0% by weight as metallic silver.

(Preparation of the Catalyst of the Present Invention) 0.2 g of 1 wt% silver-containing zirconia prepared as described above and 0.4 of alumina (trade name: Neobead GB-45 manufactured by Mizusawa Chemical Co., Ltd.)
g was ground and mixed in a mortar to prepare a catalyst of the present invention comprising silver-containing zirconia and alumina.

(Removal Reaction of NOx) Using the catalyst of the present invention obtained as described above, N
An O reduction reaction was performed. Table 2 shows the results as Example 6.
It was shown to.

Comparative Examples 1 and 2 (Comparative Example 1) Alumina (trade name of Mizusawa Chemical Co., Ltd.)
Neobead GB-45) alone was used to carry out a NO reduction reaction in the same manner as in Example 1. The results are shown in Table 3 as Comparative Example 1. (Comparative Example 2) Alumina (trade name of Mizusawa Chemical Co., Ltd.)
A NO reduction reaction was performed in the same manner as in Example 1 using a silica mixed alumina catalyst in which 0.2 g of silica was mixed with Neobead GB-45). The results are shown in Table 3 as Comparative Example 2.

Comparative Examples 3 to 5 (Comparative Example 3) 0.4 g of silica containing 1 wt% silver as a catalyst
, And a reduction reaction of NO was performed in the same manner as in Example 1. The results are shown in Table 4 as Comparative Example 3.

Comparative Example 4 A reduction reaction of NO was carried out in the same manner as in Example 1, except that 0.4 g of zirconia containing 1% by weight of silver was used as a catalyst. The results are shown in Table 4 as Comparative Example 4.

Comparative Example 5 A reduction reaction of NO was carried out in the same manner as in Example 1, except that alumina containing 1 wt% silver was used as a catalyst. The results are shown in Table 4 as Comparative Example 5.

Example 7 A NO reduction reaction was carried out in the same manner as in Example 1 except that about 1000 ppm of ethanol was used as a reducing agent. The results are shown in Table 5 as Example 7.

Comparative Example 6 A NO reduction reaction was carried out in the same manner as in Example 7 except that alumina was used as a catalyst. The results are shown in Table 5 as Comparative Example 6.

As is clear from Tables 1 to 5, the catalyst of the present invention in which silver-containing silica or zirconia was physically mixed with alumina used ethanol even when acetone was used as the reducing agent. Even in this case, it can be seen that higher NOx reduction and removal performance can be obtained at lower temperatures than when alumina alone is used as the catalyst. Also, when compared with an alumina catalyst containing silver directly, it can be seen that at lower temperatures, the catalyst of the present invention obtained by mixing silver-containing silica with alumina is excellent. Furthermore, the amount of silver contained
It can be seen that a higher NOx reduction / removal effect can be achieved even with a small amount as compared with the case of directly containing.

[0106]

[Table 1]

[0107]

[Table 2] Reducing agent: about 667 ppm of acetone

[0108]

[Table 3]

[0109]

[Table 4]

[0110]

[Table 5]

Example 8 The reduction reaction of NO was carried out in the same manner as in Example 1 except that N612N (trade name of Nikki Chemical Co., Ltd.) was used as alumina.
The results are shown in Table 6 as Example 8.

Reference Example 1 The reduction reaction of NO was carried out in the same manner as in Example 1 except that KHA-46 (trade name, manufactured by Sumitomo Chemical Co., Ltd.) was used as alumina. The results are shown in Table 6 as Reference Example 1.

Table 6 shows that when the alumina shown in Reference Example 1 containing a large amount of an alkali metal or an alkaline earth metal as an impurity is used, the NO reduction is smaller than when the high-purity alumina shown in Example 8 is used. Clearly, the removal rate decreases.

[0114]

[Table 6]

Example 9 A reduction reaction of NO was carried out in the same manner as in Example 8 except that NK-346 (trade name, manufactured by Sumitomo Chemical Co., Ltd.) was used as alumina. The results are shown in Table 7 as Example 9.

Example 10 A NO reduction reaction was carried out in the same manner as in Example 8, except that Neobead (trade name, Mizusawa Chemical Co., Ltd.) was used as alumina.
The results are shown in Table 7 as Example 10.

Reference Example 2 The reduction reaction of NO was carried out in the same manner as in Example 8 except that KHS-46 (trade name, manufactured by Sumitomo Chemical Co., Ltd.) was used as alumina. The results are shown in Table 7 as Reference Example 2.

From Table 7, it can be seen that both the pore volume formed by pores having a diameter of 80 ° or less and the total pore volume are shown in Reference Example 2.
It is clear that when the alumina shown in Examples 9 and 10 which is larger than the above is used, good NOx removing and reducing performance is exhibited.

[0119]

[Table 7]

Example 11 A NO reduction reaction was carried out in the same manner as in Example 1 except that about 1000 ppm of propylene was used as a reducing agent. The results are shown in Table 8 as Example 11.

Comparative Example 7 A NO reduction reaction was carried out in the same manner as in Comparative Example 6, except that about 1000 ppm of propylene was used as a reducing agent. The results are shown in Table 8 as Comparative Example 7.

As is evident from Table 8, even in a reaction system using propylene as a reducing agent, the catalyst of the present invention in which silver-containing zirconia is physically mixed with alumina is used when alumina alone is used as the catalyst. In comparison, it can be seen that higher NOx reduction and removal performance can be obtained at lower temperatures.

[0123]

[Table 8]

[0124]

As described in detail above, according to the present invention,
In an oxidizing atmosphere in which oxygen is excessively present, even when water vapor is present, NOx in the exhaust gas can be removed almost completely, that is, depending on the reaction conditions. This is the silica / silver compound-containing silica in the present invention.
A catalyst composed of zirconia and alumina promotes appropriate oxidation of hydrocarbons / oxygen compounds, and makes NOx and hydrocarbons / oxygen compounds more reactive than oxygen / hydrocarbons / oxygen compounds.
This is because the reaction with the oxygen-containing compound is preferentially promoted. As described above, the present invention can efficiently remove NOx from exhaust gas from various facilities including diesel engine exhaust gas, and has extremely high industrial value.

Continued on the front page (72) Inventor Miki Tabata 1134-2, Gongendo, Satte City, Saitama Prefecture (72) Tomohiro Yoshinari 3-32-25-201, Motomachi, Urawa City, Saitama Prefecture (72) Inventor Hiroshi Tsuchida Kawasaki, Kanagawa Prefecture 2-24-6-408 Kyomachi, Kawasaki-ku, Ichikawa (72) Inventor Katsumi Miyamoto 1-11-17 Washinomiya, Washinomiya-cho, Kita-Katsushika-gun, Saitama Prefecture (72) Hideaki Hamada 1-1-1, Higashi, Tsukuba, Ibaraki Prefecture In the Institute of Industrial Technology (72) Inventor Tatehiko Ito 1-1-1 Higashi, Tsukuba, Ibaraki Pref. Eiko Shigeta, Examiner at the Institute of Materials Science and Technology, National Institute of Advanced Industrial Science and Technology (56) References JP-A-5-76757 (JP, A) Hei 5-92124 (JP, A) JP-A Hei 5-317647 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) B01D 53/86 B01J 21/00-37/36

Claims (3)

(57) [Claims] 1. An oxidizing atmosphere in which excess oxygen is present, in the presence of at least one of hydrocarbons or oxygen-containing compounds, alumina and at least one of silica or zirconia containing at least one of silver or silver compounds. A method for catalytically reducing and removing nitrogen oxides, comprising contacting an exhaust gas containing nitrogen oxides with a catalyst comprising a seed. 2. The alumina has a pore volume formed by pores having a content of at least one kind of an alkali metal or an alkaline earth metal of 0.5% by weight or less and a diameter of 80 ° or less. 0.26 cm ³ · ^{g -1} or more, and catalytic reduction method for removing nitrogen oxides according to claim 1, total pore volume and wherein the at 0.48 cm ³ · ^{g -1} or more. 3. The method according to claim 1, wherein the oxygen-containing compound is one or both of ethanol and acetone.