JP2007330856A - Denitration catalyst composition, integral denitration catalyst, and denitration method employing the same - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a denitration catalyst composition for efficiently reducing and removing nitrogen oxides in an exhaust gas from a boiler or an internal combustion engine driven by lean burn such as a diesel engine and a gasoline engine using a hydrocarbon-type reducing agent an integral denitration catalyst, and a denitration method employing the same. <P>SOLUTION: The denitration catalyst composition for use in reducing nitrogen oxides in the exhaust gas by virtue of a hydrocarbon-type reducing agent is characterized by comprising a zeolite catalyst (A) comprised of β zeolite (A1) supporting elemental iron, β zeolite (A2) supporting elemental cerium, and proton-type MFI zeolite (A3); and a porous inorganic oxide (B) supporting at least one kind of noble metal element. The integral denitration catalyst is constituted of a honeycomb support whose surface is covered with the denitration catalyst composition. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a denitration catalyst composition, a monolithic structure type denitration catalyst, and a denitration method using the same, and more specifically, using a hydrocarbon-based reducing agent and operating by lean combustion such as boilers, gasoline engines, and diesel engines. The present invention relates to a denitration catalyst composition for efficiently reducing and removing nitrogen oxides from exhaust gas of an internal combustion engine, an integral structure type denitration catalyst, and a denitration method using the same.

  Boilers and internal combustion engines emit various harmful substances derived from fuel and combustion air depending on the structure and type. These hazardous substances include unburned hydrocarbons (HC), carbon monoxide (CO), nitrogen oxides (NOx), soluble organic components, and harmful components regulated by the Air Pollution Control Law. It is. Among these harmful substances, as a method for purifying NOx, a contact treatment method in which a reducing agent is mixed with exhaust gas and then brought into contact with a catalyst for purification is put into practical use.

  In a combustion apparatus such as a boiler, an optimal amount of air is supplied for combustion according to the type and supply amount of fuel, and the combustion temperature is controlled to a temperature at which the amount of harmful substances generated can be suppressed. However, air and fuel cannot always be controlled in an ideal state, and incomplete combustion may generate a large amount of nitrogen oxides. Further, in a combustion engine such as a lean burn engine or a diesel engine, when a large amount of air is mixed and the fuel is operated in a lean state, a large amount of nitrogen oxide may be generated in the exhaust gas. In particular, in the case of a diesel engine, a large amount of nitrogen oxide is easily discharged because lean combustion is performed under a high compression ratio.

In order to purify the exhausted nitrogen oxides, in the catalytic reduction method in which ammonia or the like is added and mixed as a reducing agent in the exhaust gas, the nitrogen oxides are mainly reduced to N ₂ by the following reaction formula. The
4NO + 4NH ₃ + O ₂ → 4N ₂ + 6H ₂ O
2NO ₂ + 4NH ₃ + O ₂ → 3N ₂ + 6H ₂ O
NO + NO ₂ + 2NH ₃ → 2N ₂ + 3H ₂ O

The exhaust gas from the boiler is mixed with a reducing agent after heat exchange as necessary to adjust the temperature of the exhaust gas before being introduced into the purification device filled with the catalyst, and is contacted with the purification device. In this case, the purification device is often filled with a structural catalyst obtained by molding the catalyst composition into a pellet. As the catalyst composition, a catalyst in which Cu, Mo, Co, Mn or the like is used as an active metal and supported on alumina, silica-alumina, zirconia, activated carbon or the like is used.
Further, there has been proposed an exhaust gas purification method that uses iron or zeolite containing iron and cerium as a catalyst, adds ammonia as a reducing agent to the exhaust gas, and contacts the exhaust gas to reduce nitrogen oxides (Patent Document 1). reference). Thereby, it became possible to reduce nitrogen oxides more stably.

  On the other hand, in an automobile, a method is known in which a catalyst corresponding to a harmful substance to be purified is disposed in an exhaust gas flow path, and harmful components in the exhaust gas are purified at once or in stages. For this purpose, a monolithic catalyst in which a honeycomb structure is coated with a catalyst composition has been used. A honeycomb structure is a structure body made of a heat-resistant material such as a metal such as stainless steel or a ceramic, and a number of thin parallel gas channels extend. The catalyst composition is coated on the site where the catalyst is formed. Among such honeycomb structures, a structure in which both end faces of the gas flow path are open is called a flow-through type, and a structure in which one end face of the gas flow path is sealed is called a wall flow type. In the wall flow type, the wall surface of the gas flow path serves as a filter, and functions to filter out particulate components such as soot from the exhaust gas.

  When ammonia or urea is used as the reducing agent, the reducing agent may be supplied as a gas, but is preferably supplied as an aqueous solution for handling. Therefore, the contact reduction method requires a container for storing the supplied reducing agent aqueous solution. If it is a combustion device such as a boiler, it is easy to secure a space for installing such a container, but in automobiles, importance is placed on reducing fuel consumption and designing according to consumer preferences, and in the space where containers can be installed, Was accompanied by restrictions. In addition, a separate supply facility for replenishing the reducing agent to the automobile is necessary, and the infrastructure development requires enormous costs and labor.

  On the other hand, if it is a denitration technology using hydrocarbons as a reducing agent, for example, in the case of a diesel engine, fuel gas oil can be used as a reducing hydrocarbon, so a new container for containing the reducing agent is installed. There is no need to install a special reducing agent supply facility. As an example, it has been proposed to use a catalyst supporting a transition metal and a noble metal on a mordenite-type zeolite carrier in order to purify exhaust gas from a diesel engine (Patent Document 2). As a result, nitrogen oxides can be efficiently reduced under an oxygen-excess atmosphere in the exhaust gas.

  However, exhaust gas from a diesel engine often has a temperature as low as 400 ° C. or lower, and when using a light oil with low reactivity with nitrogen oxides as a reducing agent, sufficient denitration performance is obtained particularly at 300 ° C. or lower. It was pointed out that it could not be obtained. Therefore, in order to improve the denitration performance at such a low temperature, a nitrogen oxide adsorbent is disposed in front of the denitration catalyst, and the nitrogen oxide generated when the denitration catalyst activity is low is temporarily adsorbed. It has been proposed to release nitrogen oxides and reduce nitrogen oxides when the catalyst reaches a sufficiently active temperature (Patent Document 3). However, in order to sufficiently suppress the emission by simply adsorbing nitrogen oxides, the capacity of the adsorbent must be increased, and unlike a ship, it is difficult to secure a space for a moving body such as a diesel vehicle. There was a difficulty in designing the device.

In recent years, it is expected that regulations on nitrogen oxides emitted from diesel vehicles will be strengthened, and denitration catalysts that have been used or proposed so far are required to have higher nitrogen oxide removal performance. However, a denitration technology for purifying nitrogen oxides in exhaust gas discharged from diesel engines and lean burn engines using light oil or gasoline as a reducing agent has not been sufficiently established.
In the exhaust gas regulation of automobiles, a regulation standard that assumes traveling on an actual road is provided, and the exhaust gas concentration is not measured under steady-state engine operating conditions. In the regulation standard that assumes such actual road travel, it is assumed that not only high-speed travel in which the combustion state is stable, but also travel under conditions where the accelerator opening is small and the engine speed is low, such as in urban areas. In general, in order to increase the catalytic activity, a certain high temperature condition is desirable. However, in a diesel vehicle, the temperature of the exhaust gas in urban driving conditions is often as low as 300 ° C. or less. At such low temperatures, the activity of the catalyst is not sufficient, and a satisfactory exhaust gas purification effect may not be obtained.

Under such circumstances, a denitration catalyst composition and a monolithic catalyst that can improve the exhaust gas purification capability without increasing the amount of active metal in the catalyst composition and can stably purify nitrogen oxides in the exhaust gas are anxious. It had been.
JP 2005-502451 A (Claim 35, Claim 41) JP 08-229400 A (0015) JP 2002-295241 (Claim 1, Claim 7, 0019, 0020)

  An object of the present invention is to efficiently remove nitrogen oxides from exhaust gas of an internal combustion engine operated by lean combustion, such as a boiler, a gasoline engine, and a diesel engine, using a hydrocarbon-based reducing agent in view of the above-described conventional problems. An object of the present invention is to provide a denitration catalyst composition for reduction and removal, a monolithic denitration catalyst, and a denitration method using the same.

  As a result of intensive studies to solve such problems, the present inventors have conducted a β zeolite supporting iron element when reducing nitrogen oxides in exhaust gas with a hydrocarbon-based reducing agent such as light oil. (A1), a zeolite catalyst (A) comprising β zeolite (A2) supporting a cerium element and a proton type MFI zeolite (A3), and a denitration catalyst (B) including a porous inorganic oxide supporting a noble metal element By using it, the denitration performance at low temperatures can be greatly improved. Also, this denitration catalyst composition is coated on a honeycomb structure carrier to form a monolithic denitration catalyst, thereby oxidizing nitrogen in exhaust gas from diesel engines. The present inventors have found that an excellent effect can be obtained for the purification of things, and have completed the present invention.

  That is, according to the first invention of the present invention, there is provided a denitration catalyst composition used for reducing nitrogen oxides in exhaust gas with a hydrocarbon-based reducing agent, which is a β zeolite carrying iron element ( A1), a zeolite catalyst (A) comprising β zeolite (A2) supporting a cerium element, and a proton type MFI zeolite (A3), and a porous inorganic oxide (B) supporting one or more kinds of noble metal elements A denitration catalyst composition characterized by comprising is provided.

According to the second invention of the present invention, there is provided a denitration catalyst composition characterized in that, in the first invention, the zeolite catalyst (A) further contains an H-type β zeolite (A4).
According to the third invention of the present invention, in the first or second invention, at least part of the iron element and the cerium element is supported on the β zeolite (A1) or (A2) by ion exchange. A denitration catalyst composition is provided.
According to a fourth invention of the present invention, in any one of the first to third inventions, the loading amount of the iron element is 0.1 to 3.5% by weight in terms of oxide, A denitration catalyst composition characterized in that the supported amount of cerium element is 0.05 to 2% by weight in terms of oxide is provided.

According to a fifth aspect of the present invention, there is provided a denitration catalyst composition according to any one of the first to fourth aspects, wherein the noble metal element is one or more selected from platinum, palladium, or rhodium. Things are provided.
According to a sixth aspect of the present invention, in any one of the first to fifth aspects, the porous inorganic oxide (B) is either alumina or silica-alumina. A catalyst composition is provided.
Furthermore, according to a seventh aspect of the present invention, there is provided a denitration catalyst composition according to any one of the first to sixth aspects, wherein the hydrocarbon-based reducing agent is either light oil or gasoline. Provided.

On the other hand, according to the eighth invention of the present invention, there is provided a monolithic denitration catalyst obtained by coating the surface of the honeycomb structure carrier with the denitration catalyst composition of any one of the first to seventh inventions.
According to a ninth aspect of the present invention, in the eighth aspect, the coating amount of the denitration catalyst composition is 60 to 300 g / L with respect to the volume of the honeycomb structure carrier. A monolithic denitration catalyst is provided.
Furthermore, according to the tenth aspect of the present invention, in the eighth or ninth aspect, the amount of the zeolite catalyst (A), the coating amount of (A1) to (A4), is relative to the volume of the honeycomb structure carrier. Both are 5.0 to 37.5 g / L, while the coating amount of the porous inorganic oxide (B) is 40 to 150 g / L, and the amount of the supported noble metal element is 0.1 to 10 g / L. A monolithic denitration catalyst characterized by being L is provided.

  On the other hand, according to an eleventh aspect of the present invention, according to any one of the first to tenth aspects, a hydrocarbon-based reducing agent is mixed with exhaust gas containing nitrogen oxides, and the temperature is in the range of 150 to 400 ° C. A denitration method comprising contacting with the denitration catalyst composition or the monolithic structure type denitration catalyst is provided.

The denitration catalyst composition of the present invention is excellent in reducing activity of nitrogen oxides by a hydrocarbon-based reducing agent, and exhibits high purification performance even at low temperatures for nitrogen oxides discharged from various combustion apparatuses. Further, by coating this catalyst on a honeycomb structure carrier to form an integral structure type catalyst, nitrogen oxides discharged from the diesel engine can be treated with high purification efficiency.
Furthermore, the denitration catalyst composition of the present invention can be manufactured at low cost because the amount of expensive active metal used is small, and an exhaust gas purification device can be stably produced and supplied.

  Hereinafter, the denitration catalyst composition, the monolithic denitration catalyst of the present invention, and the denitration method using the same will be described in detail. In addition, although described centering on embodiment in a diesel vehicle, this invention is not limited to a motor vehicle use, It can apply widely to the denitration technique of the nitrogen oxide in exhaust gas.

1. Denitration Catalyst Composition The denitration catalyst composition of the present invention is a denitration catalyst composition used for reducing nitrogen oxides in exhaust gas with a hydrocarbon-based reducing agent, and is a β zeolite (iron zeolite supported) A1), a zeolite catalyst (A) comprising β zeolite (A2) supporting a cerium element, and a proton type MFI zeolite (A3), and a porous inorganic oxide (B) supporting one or more kinds of noble metal elements It is characterized by including. Further, it contains H-type β zeolite (A4) as necessary. The zeolite components (A1), (A2), (A3), and (A4) may be collectively referred to as a zeolite group hereinafter.

  That is, the present invention supplies light oil or gasoline, or unburned hydrocarbons generated by temporarily supplying a large amount of fuel into a combustion engine, or light oil or gasoline itself in exhaust gas, and supplies them to a reducing agent. As described above, nitrogen oxides in exhaust gas are purified. In the present invention, hereinafter, unless otherwise specified, these reducing agents composed of hydrocarbons such as light oil are collectively referred to as hydrocarbon-based reducing agents.

  The denitration catalyst composition of the present invention contains β zeolite promoted by iron element, β zeolite promoted by cerium element, proton type MFI zeolite, and preferably H type β zeolite as a denitration catalyst.

The amount of iron element supported on β zeolite is preferably 0.1 to 3.5% by weight in terms of oxide, and the content of cerium element on β zeolite is preferably 0.05 to 2% by weight in terms of oxide. More preferably, it is 0.5 to 2.5% by weight (as oxide) as iron, and 0.1 to 1.2% (as oxide) as cerium. If the amount of the iron element exceeds 3.5% by weight, the active solid acid point cannot be secured and the activity may be lowered, and the heat resistance may be lowered. If it is less than 0.1% by weight, sufficient NOx is obtained. Since the purification performance cannot be obtained and the exhaust gas purification performance may be deteriorated, it is not preferable. On the other hand, if the amount of the cerium element exceeds 2% by weight, not only the active solid acid point cannot be secured and the activity may be lowered, but also the heat resistance may be lowered. The HC poisoning suppression effect may not be sufficiently exhibited, and the exhaust gas purification performance may not be improved.
In addition, also when using a commercially available beta zeolite promoted with an iron element and a cerium element, it is preferable to select one having a content of an iron element component and a cerium element component within the above range.

In addition to the iron element and cerium element, the β zeolite may contain a transition metal, a rare earth metal, a noble metal, and the like. Specific examples include transition metals such as nickel, cobalt, zirconium, and copper, rare earth metals such as lanthanum, praseodymium, and neodymium, and noble metals such as gold, silver, platinum, palladium, and rhodium.
However, the precious metal having high oxidation activity such as platinum is preferably 4% by weight or less (oxide conversion) with respect to β zeolite. If it is 4% by weight or less, the reduction of NOx by HC proceeds well.

β-zeolite has a unit cell composition represented by the following average composition formula, and is classified as a tetragonal synthetic zeolite.
M _{m / x} [Al _m Si _(64-m) O ₁₂₈ ] .pH ₂ O
(Wherein M is a cationic species, x is the valence of M, m is a number greater than 0 and less than 64, and p is a number greater than or equal to 0)

In addition, β zeolite generally has a relatively complicated three-dimensional pore structure composed of unidirectionally aligned linear pores having relatively large diameters and curvilinear pores intersecting with the pores. In other words, the cation diffuses easily and the hydrocarbon gas molecules diffuse easily during the reduction. This is because mordenite, faujasite, etc. have only linear vacancies aligned in one direction, but have a peculiar structure, and because of such a complicated vacancy structure, β Zeolite is highly resistant to structural breakage due to heat.
In order to exhibit HC adsorption ability, those having a large acid point are preferable, and for this purpose, β zeolite having a low silica / alumina molar ratio (hereinafter also referred to as SAR) is preferable. However, low SAR zeolite has low hydrothermal durability. During hydrothermal durability, dealumination is promoted, which may cause problems such as a decrease in acid point and significant destruction of the zeolite structure. Therefore, as the β zeolite used in the present invention, it is necessary to appropriately select a balance between the acid point number and the heat resistance which are in a trade-off relationship. Therefore, as SAR, 10-100 are preferable and 20-50 are more preferable.

In the present invention, when adding iron element and cerium element to β zeolite by ion exchange, the ratio of ion exchange is one of iron element (ion) and cerium element (ion) and one in the zeolite. It said a valence of the ion exchange sites [AlO _4/2] ^- 3 pieces of unit and is based on forming an ion pair, is preferably represented by the following formula (1).
[Mole number of iron ion or cerium ion contained by ion exchange in unit weight zeolite / {(number of moles of Al ₂ O ₃ present in unit weight zeolite) × (2/3)}] × 100 ……… (1)

  The ion exchange rate is preferably 10 to 100%, more preferably 12 to 92%, and most preferably 15 to 80%. With such an ion exchange rate, both heat resistance and denitration performance are excellent. The reason why the denitration performance is excellent is not clear, but it is thought that the framework structure of zeolite is more stabilized, the heat resistance of the catalyst, and thus the life of the catalyst is improved, so that more stable catalytic activity can be obtained. . However, if the ion exchange rate is too low and less than 10%, sufficient denitration performance may not be obtained. In addition, when the said ion exchange rate is 100%, it means that all the cation seed | species in a zeolite is ion-exchanged with an iron ion or a cerium ion.

The reason why the denitration performance is improved by using the zeolite promoted by the iron element and the zeolite promoted by the cerium element in the denitration catalyst composition of the present invention is not clear, but the flammability of hydrocarbons is improved by the zeolite. Therefore, it is considered that poisoning of the catalyst by hydrocarbons is suppressed.
Further, it is considered that the reaction between the zeolite itself and the promoted sites of the iron element, cerium element, etc. introduced into the zeolite adsorbs nitrogen oxides and hydrocarbons so that the mutual reaction proceeds actively.

The proton-type MFI zeolite used in the present invention can be produced from a zeolite raw material by various methods. However, NH ₄ -type MFI zeolite may be used as a raw material and may be calcined to obtain a proton-type MFI zeolite. Similar to the β zeolite, those having a large acid point are preferable for exhibiting HC adsorption ability, and for that purpose, zeolite having a low silica / alumina molar ratio (hereinafter also referred to as SAR) is preferable. However, low SAR zeolite has low hydrothermal durability, and dealumination is promoted during a hydrothermal reaction, which may cause problems such as a decrease in acid point and significant destruction of the zeolite structure. Therefore, as the proton type MFI zeolite used in the present invention, it is necessary to appropriately select the balance between the acid point number and the heat resistance which are in a trade-off relationship. Therefore, the SAR is preferably 10 to 100, and more preferably 20 to 50.

In addition, it is preferable to use H-type zeolite in addition to the denitration catalyst composition of the present invention. The H-type β zeolite used in the present invention is preferably one having excellent ability to adsorb and hold long-chain HC in the pores even at high temperatures. H-type β zeolite has a large pore size and is preferable for application to the present invention. In addition, H-type β zeolite having high SAR is preferable. Since the SAR is high, the heat resistance is high and the zeolite pore structure can be maintained. Such H-type β zeolite preferably has a SAR of 100 to 700, more preferably 300 to 550. In addition, the proton-type MFI zeolite and the H-type β zeolite include those in the case of the β zeolite. Similarly, a transition metal, a rare earth metal, a noble metal, or the like may be included in a part thereof.

  By using such a zeolite, the reactivity of hydrocarbons is increased, the effect of suppressing hydrocarbon poisoning at the time of addition of light oil at low temperature is improved, and high temperature in denitration reaction using hydrocarbon-based reducing agents such as light oil Resistance, high temperature activity, and low temperature activity are improved, and an excellent purifying ability for nitrogen oxides in exhaust gas can be exhibited over a long period of time.

In the present invention, in addition to the function that the specific zeolite catalyst (A) has individually, various zeolites are used, thereby exhibiting an excellent effect on denitration performance when fossil fuel is used as a reducing agent.
That is, zeolite exhibits selectivity for hydrocarbon carbon chain length due to the difference in crystal structure. Here, by using zeolites having various crystal structures, sufficient reactivity can be obtained for reducing components having various carbon chain lengths.

As described above, the denitration catalyst of the present invention is a fossil such as gasoline and light oil composed of various carbon chain length components by synthesizing the function of each zeolite and mixing these zeolites. Even when a fuel is used as a reducing agent, excellent denitration performance can be exhibited.
Further, due to the high reactivity with the hydrocarbon used as the reducing agent, poisoning of the catalyst is suppressed, and the denitration performance can be maintained over a long period of time.

  The denitration catalyst composition of the present invention contains, in addition to the zeolite catalyst (A), a porous inorganic oxide (B) on which a catalytically active species composed of one or more noble metal elements is supported as an essential component. The porous inorganic oxide is a composite oxide such as alumina and silica-alumina having high heat resistance. Examples of alumina include γ-alumina and α-alumina. Examples of the composite oxide include silica-alumina, composite oxide containing alumina such as silica-alumina-zirconia, silica-alumina-boria, and the like. In the present invention, among these, γ-alumina is most preferable in terms of heat resistance.

Moreover, it is preferable that such a porous inorganic oxide (B) has a specific surface area value (BET) of 150 to 250 m ² / g. When the specific surface area value is too large and exceeds 250 m ² / g, the heat resistance may be inferior, and when the specific surface area value is too small and less than 150 m ² / g, the active area as the catalyst material is insufficient and reduced. The reaction performance of the agent may be reduced.

Here, examples of the noble metal element include platinum, palladium, and rhodium. Platinum or palladium supported on porous inorganic oxide improves hydrocarbon combustion performance, rhodium improves NOx removal performance, further improves hydrocarbon combustion performance, and further suppresses catalyst poisoning. However, here, the catalytically active species is preferably platinum.
Palladium is less expensive than platinum, but its activity may be significantly reduced by sulfur poisoning, so it is preferable to use a hydrocarbon-based reducing agent having a low sulfur concentration. In addition, when rhodium is used as a catalytically active species, rhodium is expensive but has excellent NOx purification characteristics, and further improvement in denitration performance is expected by using it together with platinum.
The content of such noble metal element (catalytically active species) is preferably 0.1 to 10 g / L in terms of the amount supported when the honeycomb structure described later is coated with the denitration catalyst composition of the present invention. 0.5-3 g / L is more preferable.

  In the present invention, as described above, light oil or fossil fuel such as gasoline is used as the reducing agent as the reducing agent. This is preferable in terms of automobile design in that a special reducing agent container can be omitted. However, gasoline is generally a volatile petroleum fraction having a boiling point range of about 30 to 200 ° C. mainly composed of hydrocarbons having a relatively small carbon number, and light oil is mainly composed of hydrocarbons having a larger carbon number than gasoline. These are petroleum fractions having a boiling point range of about 200 to 400 ° C., all of which are a mixture of many types of hydrocarbons having different carbon numbers. Moreover, these hydrocarbons include not only straight chain but also branched chain and cyclic hydrocarbons. On the other hand, zeolite has different selectivity for hydrocarbon types depending on the crystal structure, ion exchange species, etc., so a single zeolite has a wide boiling range and is a mixture of hydrocarbons with light oil and gasoline. It may be difficult to obtain sufficient reactivity.

In such a case, in the denitration catalyst composition of the present invention, the zeolite catalyst (A) is further blended with a zeolite (A5) of a different type, so that it has a wide boiling range (carbon chain length) such as gasoline and light oil. The reactivity with respect to the hydrocarbon-based reducing agent having) can be improved. In addition, it is expected to suppress the poisoning of the zeolite catalyst by the hydrocarbon used as the reducing agent.
As the zeolite (A5) that can be used, various types of zeolite such as β, A, X, Y, MFI (ZSM-5), MOR and the like are mentioned in addition to the zeolite group. By including one or more of these, it can be expected that the denitration performance superior to the case of using only the zeolite group promoted with the active metal is exhibited. These zeolites (A5) may be promoted by an active metal such as Fe, Ce, Cu, Mo, Co, and Mn.

The denitration catalyst composition of the present invention may contain a cerium-containing oxide in addition to the above components. Cerium-containing oxides are known as oxygen storage and release materials (hereinafter referred to as OSC materials), and include pure ceria, cerium-transition metal composite oxides, cerium-rare earth composite oxides, and the like. Such a cerium-containing oxide performs occlusion and release of oxygen according to the following formula 2 according to the atmosphere of oxidation and reduction. In other exhaust gas purification technologies, this reaction is used to adjust the oxidation reaction of CO and HC, soluble organic components, and the reduction reaction of NOx via Rh to improve the purification efficiency.
2CeO ₂ ⇔Ce ₂ O ₃ + O ₂ (2)

In the present invention, the denitration performance is improved by the combination of the OSC material and platinum and palladium as the oxidation catalyst. The reason is clear from the description of “Environmental Catalysts-Actual and Prospects, page 40, Kyoritsu Shuppan Co., Ltd., issued on March 20, 1997”. It is oxidized to NO ₂ , and then this NO ₂ is adsorbed on the OSC material, and this is reacted with HC in the exhaust gas to be purified into nitrogen, water and carbon dioxide. Incidentally, what is adsorbed to the OSC material is not limited to NO2, NO is also adsorbed Trip NO ₂ than NO outweigh adsorption force to the OSC material.
As described above, in the case where the OSC material is added to the catalyst of the present invention, the combined use of the OSC material with platinum and palladium allows the HC, CO, and solubility in the exhaust gas to be absorbed by the OSC material's inherent oxygen storage / release function. The function of oxidizing organic components is also exhibited, and the function of purifying NOx is also exhibited.
In the denitration catalyst composition of the present invention, other catalyst materials that do not inhibit the properties of the zeolite, for example, solid acids such as TiO ₂ , WO ₃ / TiO ₂ , WO ₃ / Zr, SO ₃ / Zr, metallosilicates, Alternatively, a binder such as alumina, silica sol, or silica alumina may be mixed and used.

  The denitration catalyst composition of the present invention is desirably used as a structural catalyst in which the catalyst components including the zeolite catalyst (A) and the porous inorganic oxide (B) are coated on the surfaces of various carriers. Here, the shape of the carrier is not particularly limited, and can be selected from a prismatic shape, a cylindrical shape, a spherical shape, a honeycomb shape, a sheet shape, and the like. The size of the structure-type carrier is not particularly limited, and a structural carrier having a diameter (length) of several millimeters to several centimeters can be used as long as it is any of a prismatic shape, a cylindrical shape, and a spherical shape.

The denitration catalyst composition of the present invention can be obtained by (1) preparing the zeolite group and then (2) mixing it with a porous inorganic oxide carrying one or more kinds of noble metal elements. it can.
In the present invention, β-zeolite promoted with iron element and β-zeolite promoted with cerium element can be purchased in various grades from major zeolite manufacturers, and are described in JP-T-2004-536756. Can be manufactured in the same way.

  The iron element and the cerium element (hereinafter also referred to as metal catalyst components) may be supported in the pores of the β zeolite or in the vicinity of the pore entrance. Absent. In the present invention, it is desirable that at least a part of β zeolite is promoted by ion exchange with an iron element or a cerium element. By appropriate ion exchange, the framework structure of the zeolite is stabilized and the heat resistance of the zeolite itself is improved. Note that the iron element and the cerium element may not be completely ion-exchanged, and some of them may exist as oxides.

The method for obtaining the ion exchange zeolite is not particularly limited, and β zeolite is ionized using an aqueous solution of an iron-containing compound (for example, ferric nitrate) or an aqueous solution of a cerium-containing compound (for example, cerium nitrate) by a conventional method. What is necessary is just to heat-process after exchange processing and drying. The raw material for the metal catalyst component is usually used in the form of nitrate, sulfate, carbonate, acetate or the like.
The calcining conditions are not particularly limited as long as they are sufficient to obtain a zeolite in which the metal catalyst component is stably supported. The firing temperature is preferably 300 to 700 ° C, more preferably 400 to 600 ° C. About a heating means, it can carry out by well-known heating means, such as an electric furnace and a gas furnace.

Next, the zeolite catalyst (A) is mixed with a porous inorganic oxide carrying a noble metal element. In the present invention, the method for preparing a porous inorganic oxide carrying a noble metal element is not particularly limited. For example, alumina powder is impregnated with a salt solution of a noble metal element (catalytically active species) such as platinum. And can be obtained by baking after drying.
In the present invention, the content of such noble metal element (catalytically active species) is preferably 0.1 to 10 g / L in terms of the amount supported when the honeycomb structure carrier described later is coated with a denitration catalyst composition. 1.5-3 g / L is more preferable.

2. Monolithic denitration catalyst The monolithic denitration catalyst of the present invention is a denitration catalyst in which the surface of the monolithic support is coated with the denitration catalyst composition. The overall shape of the monolithic structure type carrier is arbitrary, and can be appropriately selected according to the structure of the exhaust system to be applied, such as a columnar, quadrangular, or hexagonal column.
That is, in order to apply the denitration catalyst composition to the purification of nitrogen oxides discharged from automobiles, the denitration catalyst composition containing the zeolite catalyst (A) and the porous inorganic oxide is made of stainless steel, ceramic, etc. A monolithic honeycomb structure made of the above heat-resistant material is coated to form a monolithic denitration catalyst.

The honeycomb structure carrier is made of a ceramic such as cordierite, silicon carbide, silicon nitride, or a metal such as stainless steel, and the structure has a large number of parallel fine gas flows extending throughout the structure carrier. It has a road. Of these, cordierite is preferred as the material because of durability and cost.
Further, for such a honeycomb structure carrier, an appropriate number of holes can be determined in consideration of the type of exhaust gas to be processed, gas flow rate, pressure loss or removal efficiency, etc. The cell density is preferably 100 to 900 cells / inch ² , and more preferably 200 to 600 cells / inch ² . When the cell density exceeds 900 cells / inch ² , clogging is likely to occur due to the adhered PM, and when the cell density is less than 100 cells / inch ² , the geometric surface area becomes small, so the effective usage rate of the catalyst decreases. The cell density is the number of cells per unit area in a cross section when the honeycomb structure carrier is cut at right angles to the gas flow path.
In addition, the honeycomb structure carrier has a flow-through structure in which the gas flow path communicates with a part of the end face of the gas flow path, and gas can flow through the wall surface of the gas flow path. A wall flow structure is widely known. The flow-through structure has low air resistance and low exhaust gas pressure loss. Moreover, if it is a wall flow type structure, it is possible to filter out the particulate component contained in exhaust gas. The denitration catalyst composition of the present invention can be used for either structure.

The monolithic denitration catalyst of the present invention is mixed with the denitration catalyst composition and, if necessary, a binder or the like with an aqueous medium to form a slurry mixture, which is then applied to a monolithic support, dried and fired. It is manufactured by the thing.
That is, first, a denitration catalyst composition and an aqueous medium are mixed at a predetermined ratio to obtain a slurry mixture. In the present invention, the aqueous medium may be used in such an amount that the denitration catalyst composition can be uniformly dispersed in the slurry.
At this time, if necessary, an acid and an alkali for adjusting the pH can be blended, and a surfactant, a dispersing resin and the like can be blended for adjusting the viscosity and improving the slurry dispersibility. As a mixing method of the slurry, pulverization and mixing by a ball mill or the like can be applied, but other pulverization or mixing methods may be applied.

Next, the slurry-like mixture is applied to the monolithic structure type carrier. A coating method is not particularly limited, but a wash coat method is preferable.
The denitration catalyst composition of the present invention is preferably coated on such a honeycomb structure carrier at a loading amount of 60 to 300 g / L, and more preferably 140 to 220 g / L. Further, the coating amount of the zeolite catalyst (A), that is, the components (A1), (A2), (A3), and (A4) is 5 to 37.5 g / L with respect to the volume of the honeycomb structure carrier. It is preferably 15 to 32.5 g / L. When the coating amount of each zeolite exceeds 40 g / L, the production cost increases, and the catalyst thickness increases, so that the pressure loss increases, resulting in a reduction in fuel consumption. If it is less than 5 g / L, the selectivity of hydrocarbons decreases and the reduction performance of nitrogen oxides decreases. Further, the coating amount of the porous inorganic oxide (B) is preferably 40 to 150 g / L, and more preferably 60 to 120 g / L. If the coating amount exceeds 150 g / L, the production cost increases. If it is less than 40 g / L, Pt sintering is likely to proceed, and nitrogen oxidation occurs at low temperatures, that is, when the exhaust gas temperature is not within an appropriate temperature range. The reduction performance of the product is reduced.
After coating, drying and firing are performed to obtain a monolithic denitration catalyst carrying the denitration catalyst composition. In addition, 100-300 degreeC is preferable and the drying temperature has more preferable 100-200 degreeC. Moreover, 300-700 degreeC is preferable and, as for baking temperature, 400-600 degreeC is more preferable. About a heating means, it can carry out by well-known heating means, such as an electric furnace and a gas furnace.

3. Denitration method The denitration method of the present invention comprises mixing a hydrocarbon-based reducing agent with exhaust gas containing nitrogen oxides, and the denitration catalyst composition or integral structure at a temperature range of 150 to 400 ° C, preferably 200 to 300 ° C It is made to contact with a type | mold denitration catalyst.

When exhaust gas from the boiler is to be purified, it is mixed with the reducing agent after heat exchange if necessary to adjust the temperature of the exhaust gas before being introduced into the purification device filled with catalyst, and then contacted by the purification device. It is processed. In this case, the purification device is often filled with a structural catalyst obtained by molding the catalyst composition into a pellet.
On the other hand, if the exhaust gas is from an automobile equipped with a diesel engine, the monolithic denitration catalyst of the present invention is installed in the flow path until the exhaust gas generated by the engine is discharged from the muffler. In this case, the flow rate of the exhaust gas and the temperature of the exhaust gas vary depending on the traveling conditions, but the space velocity is approximately 40000-200000 / hr, and the temperature of the exhaust gas is approximately 150-400 ° C. The denitration method of the present invention can exhibit purification performance in such a space velocity region and temperature region. In addition, the said temperature range 150-400 degreeC means that the remarkable effect of this invention is confirmed in the range.
Further, in the present invention, even if the temperature of the exhaust gas is slightly lower than 150 ° C., a certain denitration effect can be expected by optimizing other conditions. This means that a purification function can be obtained even when the exhaust gas is at a low temperature, such as immediately after starting a diesel vehicle or in a low rotation range, which has been considered difficult until now.

Next, an example of a diesel engine exhaust gas treatment process to which the denitration method of the present invention can be suitably applied will be described. In the first example, a first oxidation catalyst and then a monolithic denitration catalyst according to the present invention are arranged in an exhaust gas line from a diesel engine. The fuel supply unit, which is a hydrocarbon-based reducing agent, is pressure-fed and sprayed to the nozzles in the exhaust gas line via the reducing agent (fuel) supply pipe under the control of the engine control unit based on a preset program. The sprayed reducing agent is vaporized in contact with the high-temperature exhaust gas, is dispersed as hydrocarbon molecules constituting the reducing agent, and NOx in the exhaust gas is reduced by the function of the denitration catalyst. The first oxidation catalyst, NO in the exhaust gas is converted to NO _2, as well as adjust the NO 2 _/ NO ratio in the exhaust gas supplied to the denitration catalyst, the function of decomposing soluble organic components (SOF) is oxidized Have.
In the second example, a second oxidation catalyst is arranged after the monolithic denitration catalyst of the first example. The second dioxide catalyst mainly has a function of oxidizing an unreacted reducing agent (hydrocarbon).
In addition, as a diesel engine exhaust gas treatment process to which the denitration method of the present invention can be suitably applied, in the second example, the first oxidation catalyst is not used in the second example. In addition, there is a case where the monolithic denitration catalyst of the present invention is used alone as a catalyst without being used in combination with other catalysts.
In addition, the oxidation catalyst used for the process comprised as mentioned above may be used for oxidation of soot in addition to oxidation of HC and oxidation of NO.
Further, in this way, the hydrocarbon-based reducing agent supplied for denitration is configured to be pumped and sprayed to the nozzle in the exhaust gas pipe, but is normally consumed as fuel for the combustion engine. Therefore, instead of pumping and spraying to the nozzles in the exhaust gas pipeline, supply a large amount of fuel temporarily in the combustion engine to generate unburned hydrocarbons and use them as reducing agents You can also.

  Examples of the present invention and comparative examples are shown below, but the present invention is not construed as being limited to these examples.

[Example 1]
A powder obtained by impregnating an aqueous solution of platinum chloride with alumina and then dried into β-zeolite ion-exchanged with iron element (ion exchange rate: 80%, SAR: 35) and β-zeolite ion-exchanged with cerium element ( Ion exchange rate: 70%, SAR: 25), proton type MFI zeolite (SAR: 27) and water were added and milled using an alumina ball to form a slurry. The specific surface area value (BET) of alumina was 200 m ² / g. The contents of iron element and cerium element in the ion-exchanged β zeolite were 1.3 wt% in terms of oxide and 1.2 wt% in terms of oxide, respectively. Silica / alumina molar ratio (hereinafter referred to as SAR)
= Production of monolithic denitration catalyst =
This slurry is coated on a cordierite flow-through honeycomb structure of 5.66φ × 3 inches (1249 cc) and cell density 400 / inch ² by a wash coat method, and unnecessary slurry is blown off with an air gun and dried. The calcination was performed under the following calcination conditions.
The honeycomb structure coated with the denitration catalyst composition thus obtained was subjected to an aging treatment under the following aging conditions to obtain an integral structure type denitration catalyst of Example 1. Table 1 shows the composition of this monolithic denitration catalyst.
= Firing conditions =
・ Baking equipment: Electric furnace ・ Baking temperature: 450 ℃
-Firing time: 30 minutes = aging conditions =
Aging equipment: hydrothermal durability furnace Aging atmosphere: 10% H ₂ O / air Aging temperature: 700 ° C
・ Aging time: 20 hours

[Example 2]
A monolithic denitration catalyst of Example 2 was obtained in the same manner as in Example 1 except that H-type β zeolite (SAR: 500) was added in addition to the various zeolites of Example 1. Table 1 shows the composition of this monolithic denitration catalyst.

[Comparative Example 1]
A powder obtained by impregnating an aqueous solution of platinum chloride with alumina and then dried, β-zeolite ion-exchanged with iron element (ion exchange rate: 80%, SAR: 35), proton-type MFI zeolite (SAR: 27), Water was added and the mixture was milled and slurried using alumina balls. The content of iron element in the ion-exchanged β zeolite was 1.3 wt% in terms of oxide. Unlike Example 1, this catalyst does not contain β zeolite ion-exchanged with cerium element.
Using the slurry, a honeycomb structure coated with a denitration catalyst was obtained in the same manner as in Example 1. Table 1 shows the composition of the material in the monolithic denitration catalyst.

The denitration performance of the monolithic denitration catalyst manufactured as described above was measured under the following measurement conditions. The results are shown in Table 2.
= Measurement conditions =
・ Engine: 2000cc diesel engine ・ Engine speed: 1500rpm
・ Space velocity: 72000 / hr
Evaluation temperature: 250 ° C. (gas temperature at the catalyst inlet)
・ Light oil spray amount: 5cc / min
・ Light oil spraying time: 5 min
・ Measurement equipment: MEXA9500 manufactured by HORIBA
= NOx purification rate =
The NOx purification rate was determined as in the following formula (3).
Incidentally, NOx (in) is the NOx concentration of the catalyst inlet side, NOx (out) represents the NOx concentration of the catalyst outlet, NOx represents the sum of NO and NO _2.
NOx purification rate [%] = {[NOx (in)] − [NOx (out)]} / [NOx (in)] (3)

From the results in Table 2, it can be seen that the catalysts of Examples 1 and 2 have a higher NOx purification rate than the catalyst of Comparative Example 1. In particular, it can be seen that the denitrification performance is improved because not only β zeolite supporting iron element and β zeolite supporting cerium element, but also proton type MFI zeolite and H type β zeolite are used in combination.
Therefore, the denitration catalyst composition of the present invention exhibits an excellent function as a denitration catalyst material when light oil or the like is used as a reducing agent, and is used as an integral structure type denitration catalyst for exhaust gas purification of diesel engines and the like. However, it can be said that it exhibits an excellent effect.

Claims (11)

A denitration catalyst composition used for reducing nitrogen oxides in exhaust gas with a hydrocarbon-based reducing agent,
Zeolite catalyst (A) composed of β zeolite (A1) supporting iron element, β zeolite (A2) supporting cerium element, and proton type MFI zeolite (A3), and porous supporting one or more of noble metal elements A denitration catalyst composition comprising an inorganic oxide (B).   2. The denitration catalyst composition according to claim 1, wherein the zeolite catalyst (A) further contains H-type β zeolite (A4).   3. The denitration catalyst composition according to claim 1, wherein at least part of the iron element and the cerium element is supported on the β zeolite (A1) or (A2) by ion exchange.   The supported amount of the iron element is 0.1 to 3.5% by weight in terms of oxide, and the supported amount of the cerium element is 0.05 to 2% by weight in terms of oxide. The denitration catalyst composition according to claim 1 to 3.   5. The denitration catalyst composition according to claim 1, wherein the noble metal element is at least one selected from platinum, palladium, and rhodium.   The denitration catalyst composition according to any one of claims 1 to 5, wherein the porous inorganic oxide (B) is either alumina or silica-alumina.   The denitration catalyst composition according to any one of claims 1 to 6, wherein the hydrocarbon-based reducing agent is either light oil or gasoline.   A monolithic denitration catalyst obtained by coating the surface of a honeycomb structure carrier with the denitration catalyst composition according to any one of claims 1 to 7.   The monolithic denitration catalyst according to claim 8, wherein the coating amount of the denitration catalyst composition is 60 to 300 g / L with respect to the volume of the honeycomb structure carrier.   The components of the zeolite catalyst (A) and the coating amounts of (A1) to (A4) are all 5.0 to 37.5 g / L with respect to the volume of the honeycomb structure carrier, while the porous inorganic oxide The monolithic denitration catalyst according to claim 9, wherein the coating amount of (B) is 40 to 150 g / L, and the amount of the noble metal element supported is 0.1 to 10 g / L.   A hydrocarbon-based reducing agent is mixed with exhaust gas containing nitrogen oxides, and the denitration catalyst composition according to any one of claims 1 to 7 or any one of claims 8 to 10 at a temperature range of 150 to 400 ° C. A denitration method comprising contacting with the monolithic denitration catalyst according to claim 1.