JP2005279371A - Denitrification catalyst and denitrification method using the same - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a reduction catalyst with excellent practical use capable of efficiently reducing (or reducing/removing) nitrogen oxide in a gas to be treated (diesel engine exhaust gas, and the like) with high reduction performance and maintaining high catalytic activity even if it is used over a long period of time. <P>SOLUTION: The catalyst is constituted of a carrier constituted of at least one kind of metal selected from group VI metals in the periodic table (molybdenum, and the like), group XIV metals in the periodic table (tin, and the like) and group XV metals in the periodic table (bismuth, and the like); and a catalytic active substance carried on the carrier and constituted by a metal of group XI metals in the periodic table. The carrier may be constituted by at least bismuth and the catalytic active substance may be constituted by at least silver. A ratio of the catalytic active substance is approximately 0.1-50 pts.wt. based on 100 pts.wt. of the carrier. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a catalyst (denitration catalyst) for reducing or reducing nitrogen oxides contained in gas (for example, exhaust gas discharged from a diesel engine) and a reduction removal method (denitration method) thereof.

  The exhaust gas (engine exhaust gas etc.) usually contains nitrogen oxides (so-called NOx such as nitrogen monoxide and nitrogen dioxide) generated by the combustion of fuel. Such nitrogen oxides are harmful to the human body because they damage the respiratory system, and are also known to be harmful to the environment because they cause photochemical smog and acid rain. Removal methods are desired.

  Conventional methods used to remove dry nitrogen oxides contained in engine exhaust gas include a three-way catalyst method using hydrocarbons and carbon monoxide in exhaust gas as a reducing agent, and ammonia or ammonia by hydrolysis in exhaust gas. An ammonia denitration method, in which a selective reduction reaction is performed by adding urea to be produced, is known.

  The three-way catalyst method is a method of reducing and removing nitrogen oxides using hydrocarbons and carbon monoxide in exhaust gas as a reducing agent. The amount of oxygen contained in the exhaust gas contains hydrocarbons and carbon monoxide. The amount of oxygen necessary for the reaction needs to be stoichiometrically equal, and there is a drawback that it cannot be applied to exhaust gas in which oxygen is excessively present in the exhaust gas.

  On the other hand, the ammonia denitration method can remove nitrogen oxides even in exhaust gas in which oxygen is excessively present in the exhaust gas, but it is necessary to add ammonia or urea to the exhaust gas as a reducing agent. In order to prevent the leakage of ammonia to the downstream side, it is necessary to measure the concentration of nitrogen oxide in the exhaust gas over time and supply ammonia or urea suitable for the amount of nitrogen oxide. In addition, the ammonia denitration method is costly, and if the amount of exhaust gas is small, it is difficult to add an appropriate amount of ammonia or urea, and the possibility of ammonia leakage increases. Has the disadvantage of being limited. In particular, a diesel engine (for example, a stationary engine or a mobile engine) is an economical internal combustion engine, but since exhaust gas contains a high concentration of nitrogen oxides, efficient removal of nitrogen oxides is required. . Current diesel exhaust gas is usually purified by ammonia denitration, but this ammonia denitration is limited to large diesel because of its cost.

  Therefore, in recent years, many selective reduction catalysts that selectively reduce nitrogen oxides using hydrocarbons, alcohols, and the like as reducing agents have been developed. For example, Japanese Patent No. 2801423 (Patent Document 1) discloses a catalyst composed of a carrier such as alumina, aluminum phosphate, and alumina-silica, and silver oxide, for removing nitrogen oxides. A catalyst having a supported amount of 0.1 to 30% by weight is disclosed. JP-A-7-275708 (Patent Document 2) discloses a denitration catalyst in which copper is supported on activated alumina having a specific pore radius and pore volume, and denitration in which cesium and silver are supported on the activated alumina. A catalyst is disclosed. Further, Japanese Patent Laid-Open No. 63-1000091 (Patent Document 3) discloses an exhaust gas purifying catalyst in which copper is supported on a porous carrier (alumina, silica, zeolite, etc.).

  JP 2003-190788 A (Patent Document 4) discloses a substrate such as a ceramic honeycomb, an oxide carrier deposited on the substrate, indium, gallium, tin, silver, germanium, gold, nickel, cobalt, A lean NOx catalyst is disclosed that includes a dopant selected from copper, iron, manganese, molybdenum, chromium, cerium, vanadium, oxides thereof, and combinations thereof. In this document, alumina, aluminophosphate, hexaaluminate, aluminosilicate, zirconate, titanosilicate, titanate, etc. can be used as the oxide support, and it is described that the preferred oxide support is alumina. Yes. Further, this document evaluates the reduction characteristics of a gas containing sulfur dioxide, and describes that the catalytic activity for nitrogen dioxide is remarkably reduced except for a catalyst containing a specific dopant (gallium).

  However, even with these catalysts, it is difficult to obtain a sufficient reduction activity of nitrogen oxides. In particular, depending on the type of catalyst (for example, a catalyst using zeolite as a carrier), the catalyst is significantly deteriorated in exhaust gas containing moisture. Moreover, in these catalysts, catalyst activity falls with time and cannot maintain high catalyst activity over a long period of time. Further, with these catalysts, the catalytic activity in a gas containing sulfur oxides may be reduced.

Therefore, a catalyst that can effectively remove nitrogen oxides in exhaust gas (particularly, exhaust gas in which oxygen is excessively present) and can withstand practical use has not yet been developed.
Japanese Patent No. 2801423 (Claims) JP-A-7-275708 (Claims) Japanese Patent Laid-Open No. 63-1000091 (Claims) JP 2003-190788 A (claims, paragraph numbers [0011], [0012], [0054] to [0074])

  Therefore, an object of the present invention is to efficiently reduce (or reduce and remove) nitrogen oxides (nitrogen monoxide, nitrogen dioxide, etc.) in a gas to be treated (especially exhaust gas such as diesel engine exhaust gas) with high reducing ability. It is to provide a catalyst and a reduction method (or reduction removal method) using the catalyst.

  Another object of the present invention is a catalyst capable of reducing (or reducing and removing) nitrogen oxide while maintaining high catalytic activity even when used over a long period of time, and a reduction method using this catalyst Is to provide.

  Still another object of the present invention is excellent in practicality that can efficiently reduce (or reduce and remove) nitrogen oxides with high catalytic activity without reducing the catalytic activity even if the gas contains sulfur oxides. It is an object to provide a reduction (or reduction removal) catalyst and a reduction method using the catalyst.

  As a result of intensive studies to achieve the above-mentioned problems, the present inventors have found that a catalyst in which a group 11 metal (especially at least silver) is supported on a carrier composed of a specific metal (especially at least bismuth) is oxidized by nitrogen. It has been found that it has excellent ability to reduce substances and can maintain high catalytic activity even when used over a long period of time, and it can maintain the reduction performance of nitrogen oxides even for gases to be treated containing sulfur oxides (SOx). The present invention has been completed.

  That is, the catalyst of the present invention is composed of a support and a catalytically active substance supported on the support, and is a catalyst for reducing nitrogen oxides in the presence of hydrocarbons and oxygen. It is composed of at least one metal selected from Group 6 metal of the periodic table, Group 14 metal of the periodic table, and Group 15 metal of the periodic table, and the catalytically active substance is composed of Group 11 metal of the periodic table. Yes. The carrier may be made of at least one metal selected from molybdenum, tungsten, tin, and bismuth. In particular, the carrier may be composed of at least bismuth. The catalytically active material may be composed of at least silver. The representative catalyst may be, for example, a catalyst in which silver is supported on bismuth oxide. The ratio of the catalytically active substance may be about 0.1 to 50 parts by weight with respect to 100 parts by weight of the carrier.

  The present invention also includes a method of reducing the nitrogen oxides by contacting the catalyst with a gas to be treated containing at least hydrocarbons, oxygen, and nitrogen oxides. In this method, the gas to be treated may further contain a sulfur oxide. Since the catalyst has resistance or durability against sulfur oxides, nitrogen oxides can be efficiently reduced and removed without deteriorating the catalytic activity even if the gas to be treated contains sulfur oxides.

  In the catalyst of the present invention, a gas (particularly, exhaust gas such as diesel engine exhaust gas) is obtained by supporting a Group 11 metal (for example, at least silver) on a carrier composed of a specific metal (for example, at least bismuth). ) Can be efficiently reduced (or reduced and removed) with high reducing ability. In addition, the catalyst of the present invention is excellent in stability over time, and can maintain high catalytic activity and reduce nitrogen oxides even when used over a long period of time. Furthermore, the catalyst of the present invention has resistance or durability against sulfur oxides, and even a gas containing sulfur oxides efficiently oxidizes nitrogen with high catalytic activity without reducing the catalytic activity. Since it can reduce and remove substances, it is highly practical.

[catalyst]
The catalyst of the present invention is a catalyst for reducing nitrogen oxides in the presence of hydrocarbons and oxygen, and comprises a support composed of a specific metal (or metal component), supported on this support, It is comprised with the catalytically active substance (or catalytically active component) comprised with the metal (or metal component). In the present invention, by combining a specific carrier and a specific catalytically active substance, nitrogen oxides can be reduced with high reduction performance, and high catalytic activity can be maintained over a long period of time.

  In the catalyst, the carrier includes a periodic table group 6 metal (chromium, molybdenum, tungsten, etc.), a periodic table group 14 metal (tin, lead, etc., especially a periodic table group 6 or 7 group 14 metal), It is composed of at least one metal selected from Group 15 metals (antimony, bismuth, etc.) of the periodic table.

  The carrier only needs to contain at least the above-exemplified metal atoms, may be in the form of a simple metal, may be in the form of a metal compound (oxide, halide, etc.), and these forms are mixed. You may do it. The carrier is usually present in the form of a metal compound (usually a metal oxide). In the metal compound, the valence of the metal is not particularly limited depending on the type of metal and the form of the metal compound. For example, a bismuth compound may be trivalent, pentavalent, hexavalent (particularly trivalent), or the like.

Of these metals, the carrier is often composed of at least one metal selected from molybdenum, tungsten, tin, and bismuth (particularly, at least bismuth). Preferred supports include metal compounds of these metals [in particular, metal oxides such as molybdenum oxide (MoO ₂ , MoO ₃ etc.), tungsten oxide (WO ₃ etc.), tin oxide (SnO, SnO ₂ etc.), bismuth oxide, etc. ] Etc. are included. In particular, the carrier is at least a bismuth compound [bismuth oxide (bismuth oxide (III) (dibismuth trioxide), bismuth oxide (IV) (dibismuth tetroxide), bismuth oxide (V) (dibismuth pentoxide), etc.] In particular, bismuth oxide (III) etc.) may be used.

  In addition, the metal (or metal component) which comprises the said support | carrier and / or the said metal compound may comprise the support | carrier individually or in combination as mentioned above. For example, the metal compound constituting the carrier may be composed of one kind of metal (or metal component), or a composite metal compound composed of two or more kinds of metals (or metal components) [for example, molybdic acid Double oxide (composite oxide) such as bismuth (molybdenum bismuth oxide)].

  Moreover, the said support | carrier may be comprised with 2 or more types of metals (especially metal compound) individually. For example, when the carrier is composed of at least a bismuth compound, it may be composed of (i) a bismuth compound (bismuth oxide, bismuth molybdate, etc., especially bismuth oxide) alone, or (ii) a bismuth compound (particularly bismuth oxide). ) And at least one metal compound (particularly metal oxide) selected from molybdenum compounds (particularly molybdenum oxide), tungsten compounds (particularly tungsten oxide), and tin compounds (particularly tin oxide). .

Incidentally, the carrier may be composed of at least the above-exemplified metal, other general-purpose carrier (e.g., alumina, titania, silica, zirconia, ceria (CeO _2), zeolites, activated carbon, etc.) in combination with Good. Since the catalyst of the present invention has a high catalytic activity, it is usually possible to construct a carrier with only the above-mentioned carrier without using such other carrier [alumina, ceria (CeO ₂ ), etc.] in combination. The activity and durability of the catalyst can be improved well.

The catalytically active substance (or catalytically active component) is composed of at least one metal (or metal component) selected from Group 11 metals of the periodic table, that is, copper, silver, and gold. The catalytically active substance may be composed of a single metal or two or more metals. In particular, the catalytically active material is preferably composed of at least silver (or a silver component). For example, the catalytically active material may be composed of silver (silver component) alone, You may comprise with copper (copper component) and / or gold (gold component). The catalytically active substance only needs to be composed of a Group 11 metal of the periodic table, may be in the form of a simple metal, may be in the form of a metal compound, or these forms may be mixed. . For example, the catalytically active substance is in the form of a simple metal (eg, silver), an oxide (eg, silver oxide (eg, Ag ₂ O)), a salt (such as an inorganic acid salt such as nitrate, an organic acid salt, a complex salt), or a complex. Or may be supported in a mixed form thereof. The catalytically active substance may be usually in the form of at least a simple metal (eg, silver) or a metal oxide (eg, silver oxide). Further, in the periodic table Group 11 metal compound, the valence of the metal is not particularly limited. For example, silver may be monovalent or divalent (particularly monovalent).

  In a preferred embodiment, the catalyst of the present invention has at least silver (or a silver component) supported on a support composed of at least a bismuth compound. Examples of such a catalyst include a catalyst in which silver is supported on bismuth oxide.

  In the catalyst of the present invention, the ratio of the catalytically active substance can be selected from a range of 0.1 to 50 parts by weight in terms of a Group 11 metal (for example, silver) in the periodic table with respect to 100 parts by weight of the carrier. 0.1 to 30 parts by weight, preferably 0.3 to 20 parts by weight, more preferably 1 to 10 parts by weight, particularly about 1 to 5 parts by weight. In addition, when there are too many catalytically active substances (for example, more than 50 weight part with respect to 100 weight part of support | carriers), since catalytic activity is expressed strongly and the oxidation reaction which is a side reaction is promoted too much, as a result There is a risk of denitration performance being impaired. Moreover, when there is too little catalytically active substance (for example, less than 0.1 weight part with respect to 100 weight part of support | carriers), there exists a possibility that sufficient catalyst activity may not be obtained.

  In the catalyst of the present invention, it is sufficient that at least the Group 11 metal of the periodic table is supported on the carrier. In addition to the Group 11 metal of the periodic table, another metal (or metal component) may be added as a catalytically active substance as necessary. ) May be included (or carried). Examples of such metals include noble metals (ruthenium, rhodium, palladium, iridium, platinum, etc.) that do not belong to the category of Group 11 metals in the periodic table. These metals (or metal components) may be used alone or in combination of two or more. In addition, the form of these metals (or metal components) is the same as that of the Group 11 metal of the periodic table, and may be a simple metal, a metal oxide, a mixture of these forms, or the like. The content of other metal (or metal component) is 0.01 to 1 part by weight, preferably 0.01 to 0.5 part by weight, more preferably 0, in terms of metal with respect to 100 parts by weight of the carrier. It may be about 0.01 to 0.2 parts by weight. Further, the ratio of the other metal (or metal component) is from 0.1 to 15 parts by weight in terms of the other metal simple substance with respect to 1 part by weight of the periodic table group 11 metal (metal simple substance equivalent). For example, it may be 0.2 to 10 parts by weight, preferably 0.5 to 10 parts by weight, and more preferably about 1 to 8 parts by weight. By further supporting such a metal (noble metal), the catalytic activity may be improved, or harmful gases (hydrocarbon, carbon monoxide, etc.) in the gas to be treated which will be described later may be efficiently removed.

BET specific surface area of the support or catalyst, 1 m ² / g or more (e.g., 1 to 500 approximately ² / g) may be, usually, 10 to 300 m ² / g, (e.g., 50 to 300 m ² / g) Preferably, it may be about 60 to 250 m ² / g (for example, 60 to ²⁰⁰ m ² / g), and more preferably about 80 to 250 m ² / g (for example, 90 to 150 m ² / g).

  The shape of the catalyst is not particularly limited. For example, the catalyst form of the present invention may be a honeycomb type (or honeycomb shape), a foam type (foam shape or foam shape), a plate shape, a pellet shape, a ring shape, It may be granular (or granular) or granular. In the granular or granular catalyst, the particle diameter (average particle diameter) may be, for example, 0.1 to 10 mm, preferably 0.5 to 8 mm, and more preferably about 1 to 5 mm.

[Method for producing catalyst]
The catalyst of the present invention can be produced or prepared by supporting a Group 11 metal (metal component) of the periodic table on the support (such as a bismuth compound). The method for preparing the catalyst is not particularly limited, and conventional methods such as mixing (physical mixing), impregnation method (evaporation to dryness method, etc.), coprecipitation method, colloid method, adsorption method (equilibrium adsorption method, etc.), etc. Is mentioned. In the impregnation method, for example, the support may be impregnated with a solution (eg, an aqueous solution) of a Group 11 metal compound (eg, a metal salt) of the periodic table, and dried and fired.

  In the preparation of the catalyst, the raw material of the carrier (or the precursor of the carrier) can be appropriately selected according to the preparation method, and the above-mentioned exemplified carrier [bismuth compound (such as bismuth oxide)] may be used as it is as a raw material. If necessary, it may be prepared in advance. For example, in the impregnation method, a metal compound capable of generating the carrier [eg, hydroxide, inorganic acid salt (sulfate, nitrate, etc.), halide (chloride, etc.), organic acid salt (acetate, etc.)] You may use the baked product which baked for the preparation of a catalyst.

  Similarly, the catalytically active substance (group 11 metal of the periodic table) or its precursor can be appropriately selected according to the catalyst preparation method. For example, in the impregnation method, solutions (particularly aqueous solutions) of various Group 11 metal compounds of the periodic table can be used. Examples of such metal compounds include inorganic acid salts [sulfates, nitrates (such as silver nitrate)], halides [chlorides (such as silver chloride)]], and organic acid salts [acetates (such as silver acetate)]. Etc.], complexes and complex salts [ammine complexes (silver ammine complexes etc.) and the like]. These metal compounds may be used alone or in combination of two or more. The solution used for impregnation may be an aqueous solution, a non-aqueous solution (for example, an organic solvent solution such as acetone), or a mixed solvent of water and an organic solvent (aqueous medium). Good. A preferred solution is an aqueous solution.

  For the other metal (for example, a noble metal such as palladium), a method similar to the method for supporting the Group 11 metal of the periodic table can be used. For example, in the impregnation method, a solution (such as an aqueous solution) in which both the Group 11 metal compound of the periodic table and other noble metal metal compounds (such as noble metal salts such as palladium salts) are dissolved is supported on the carrier at a time. Also good. Depending on the type of other metal compound (such as a noble metal salt), a metal or a metal component (such as Group 11 metal of the periodic table and other metals) may be supported one by one in order, for example, the first metal ( After supporting silver or the like, a second metal (such as palladium) may be supported through a process such as drying or calcining as appropriate.

  In a preparation method that requires drying (impregnation method or the like), the drying temperature may be, for example, 70 to 200 ° C, preferably 80 to 150 ° C, and more preferably about 100 to 130 ° C. In preparation methods that require calcination (impregnation method, etc.), normal air may be used as the gas flowing during calcination, or a gas in which air or oxygen and an inert gas such as nitrogen are appropriately mixed is used. May be. The calcination temperature may be in a range where high catalytic activity can be stably imparted, and may be, for example, 300 to 650 ° C, preferably 300 to 600 ° C, and more preferably about 450 to 550 ° C.

  As described above, the catalyst of the present invention can be formed into an arbitrary shape (for example, a granular shape). Depending on the shape of the catalyst, the catalyst may be molded using, for example, a kneader, a molding machine (such as an extrusion molding machine or a compression molding machine), a tableting machine, a granulator, or a pulverizer. .

[Reduction removal method]
The catalyst of the present invention is a nitrogen oxide (for example, dinitrogen monoxide, nitric oxide, dinitrogen trioxide, nitrogen dioxide, dinitrogen tetroxide, dinitrogen pentoxide, etc., particularly nitrogen monoxide, nitrogen dioxide etc. It is useful for reducing removal of NOx). That is, in the present invention, the nitrogen oxide can be reduced (or reduced and removed) by bringing the gas to be treated containing at least a reducing agent (usually hydrocarbon), oxygen and nitrogen oxides into contact with the catalyst.

  In the gas to be treated, hydrocarbons are not particularly limited as long as nitrogen oxides can be reduced. Non-aromatic hydrocarbons [saturated aliphatic hydrocarbons (paraffins such as methane, ethane, propane, butane, etc.), non-aromatic hydrocarbons, etc. Examples thereof include aliphatic hydrocarbons such as saturated aliphatic hydrocarbons (ethylene, propylene, etc.), aromatic hydrocarbons (toluene, xylene, etc.), and the like. The hydrocarbons may be contained in the gas to be treated alone or in combination of two or more. Hydrocarbons may be contained in the gas to be treated in the form of a mixture of these gases. For example, general-purpose fuels [light oil, heavy oil (A heavy oil, C heavy oil, etc.), kerosene, gasoline, naphtha, etc.] It may be contained in the gas to be treated in the form of.

  In the gas to be treated, the hydrocarbon content may be an amount that can reliably reduce at least nitrogen oxides. That is, the hydrocarbon content is obtained by reacting nitrogen oxides, oxygen, and the hydrocarbons in the gas to be treated, and chemically changing them to nitrogen, carbon dioxide, and water (that is, reducing the nitrogen oxides). It is sufficient if the amount is more than the amount necessary for (stoichiometry).

  If the hydrocarbon content in the gas to be treated (or reaction gas) is not sufficient in the chemical change (reduction reaction), the hydrocarbon is added (or supplied) from the outside as necessary. May be. Such a gas to be added may be the fuel (light oil, kerosene, gasoline, etc.) and the like. The addition method is not particularly limited. For example, the addition or introduction method using vaporization (for example, vaporization introduction using vapor pressure), the mixture in which the fuel is dispersed in water directly using a spray or the like. Examples of the method include addition or introduction. The amount of the reducing agent (hydrocarbon) added may be, for example, about 0.5 to 4 parts by weight, preferably about 1 to 3 parts by weight with respect to 1 part by weight of nitrogen oxides. If the amount added is too small (for example, less than 0.5 parts by weight with respect to 1 part by weight of nitrogen oxides), there is a possibility that it cannot be sufficiently reduced. If the amount is larger than that of the part, there is a possibility that the fuel consumption is deteriorated. The addition of the hydrocarbon can be performed at a position upstream of the catalyst arrangement position with respect to the flow path of the gas to be treated.

  With the catalyst of the present invention, nitrogen oxides can be efficiently and selectively reduced and removed even with a gas to be treated containing a large amount or excess oxygen. Therefore, the content of oxygen in the gas to be treated is not particularly limited as long as nitrogen oxides can be reliably reduced. For example, in the gas to be treated, for example, 1 to 30% by volume, preferably 1 to 20%. It may be about volume%, more preferably about 2 to 15 volume%. When the gas to be treated is exhaust gas (diesel engine exhaust gas or the like), the oxygen content is relatively large, for example, 3 to 20% by volume, preferably 5 to 15% by volume, more preferably 9 to 9%. It may be about 13% by volume. Since oxygen is used not only for reduction of nitrogen oxides but also for oxidation reactions of other gases (carbon monoxide, hydrogen, excess hydrocarbons, etc.), such other gases and nitrogen oxides react. The oxygen content may be larger than the content necessary for the purpose. When the oxygen concentration in the gas to be processed (exhaust gas) is low, a predetermined amount of air may be mixed with the gas to be processed in advance.

  The gas (the gas to be treated) may contain only oxygen and the hydrocarbon, and the hydrocarbon, oxygen and other gas (water vapor, carbon monoxide, carbon dioxide, sulfur oxide (so-called SOx such as sulfur dioxide). Etc.), nitrogen, hydrogen, ammonia, etc.). These other gases may be contained in the gas to be treated singly or in combination of two or more. The gas to be treated is usually a mixed gas containing hydrocarbons, oxygen, nitrogen oxides, and other gases (especially sulfur oxides) in many cases. Specific examples of such a mixed gas include exhaust gas, for example, exhaust gas discharged from an internal combustion engine (engine exhaust gas). In the engine exhaust gas, the engine may be a diesel engine or a gasoline engine (particularly a diesel engine), and may be either a fixed type or a mobile type. Since the catalyst of the present invention has high durability, even a gas to be treated containing such other gases (water vapor, sulfur oxide, etc.) can be maintained without lowering the catalytic activity, and can be maintained over a long period of time. Even if it is used, nitrogen oxides can be reduced and removed with high reducing ability.

  The ratio of water vapor is, for example, about 0.1 to 40% by volume, preferably 1 to 30% by volume, more preferably about 2 to 20% by volume (for example, 5 to 10% by volume) in the gas to be treated. May be. Further, the proportion of carbon dioxide is, for example, 0.1 to 20% by volume, preferably 0.5 to 15% by volume, more preferably 1 to 10% by volume (particularly about 5 to 10% by volume) in the gas to be treated. It may be a degree.

  The gas to be treated may further contain a sulfur oxide. The sulfur oxide content may be, for example, 0.1 to 100 ppm, preferably 0.2 to 70 ppm, more preferably about 0.3 to 30 ppm in terms of volume in the gas to be treated (exhaust gas). . In general, sulfur oxides are often contained in the gas to be treated such as exhaust gas (diesel engine exhaust gas, etc.). Since the catalyst of this invention has the tolerance or durability with respect to sulfur oxide, even if it is a to-be-processed gas containing sulfur oxide, catalyst activity can be maintained.

  As described above, the gas to be treated (exhaust gas) contains hydrocarbons, and may contain other gases (such as carbon monoxide) that can act as a reducing agent for nitrogen oxides. is there. Such gases (for example, excessive hydrocarbons, carbon monoxide, etc. other than the amount necessary for reduction of nitrogen oxides) may become harmful substances (or harmful gases), and are therefore further removed (reduction removal). May be. Such harmful gases (excess hydrocarbons, carbon monoxide, etc.) are added to the carrier (bismuth compound) as described above, in addition to metals of Group 11 of the periodic table, and other noble metals (platinum, palladium, etc.). The catalyst may be reduced and removed using a catalyst supporting the. Further, such a harmful gas is removed (reduced) by arranging another catalyst capable of reducing and removing the harmful gas on the downstream side of the flow path of the gas to be treated, if necessary. May be removed). That is, by sequentially arranging the catalyst of the present invention and the other catalyst from the upstream side of the flow path of the gas to be treated, nitrogen oxide and the harmful gas (excess hydrocarbon, carbon monoxide) are continuously provided. Reduction or removal may be performed manually or stepwise. Examples of such other catalysts include a carrier [for example, the above exemplified carrier (such as a bismuth compound), the above exemplified general-purpose carrier (such as a porous inorganic oxide such as alumina)] and the other noble metal (or A catalyst on which a noble metal component) is supported can be used.

  The temperature (contact temperature) between the gas to be treated and the catalyst may be in a range where it can be efficiently reduced and removed, and can usually be selected from a range of 150 ° C. or higher (for example, about 150 to 650 ° C.), for example, 200 ° C. It may be above (for example, about 250 to 600 ° C.), preferably 250 to 550 ° C. (for example, about 250 to 500 ° C.), more preferably 300 ° C. or more (for example, about 300 to 500 ° C.). The contact temperature may be the temperature of the gas to be treated, the temperature of the catalyst (for example, the temperature of the catalytically active substance), or both of these temperatures. When the gas to be treated is exhaust gas (engine exhaust gas), the contact temperature varies greatly depending on the operating conditions of the engine, but is generally about 150 to 650 ° C. in many cases.

In reducing and removing, the amount of the catalyst used, at a space velocity of gas per time (GHSV), for example, 200000h ^-1 or less (e.g., 1000~200000h ^-1), preferably 100000h ^-1 or less (1000～100000H ^-1 ), More preferably about 2000 to 100,000 h ⁻¹ , and usually about 50,000 to 100,000 h ⁻¹ .

  Since the catalyst of the present invention has a high reducing ability for reducing and removing nitrogen oxides, it is useful for reducing and removing nitrogen oxides in the gas to be treated. In particular, the catalyst of the present invention can maintain the catalytic activity even in the case of a gas containing sulfur oxide, water vapor, etc., and thus can be used stably over a long period of time. It can be suitably used for reducing and removing nitrogen oxides in engine exhaust gas such as diesel engines.

  Hereinafter, the present invention will be described in more detail based on examples, but the present invention is not limited to these examples.

Example 1
A silver nitrate aqueous solution was prepared by dissolving 0.47 g of silver nitrate in 100 mL of water. Add 15 g of bismuth oxide (Bi ₂ O ₃ ) to this silver nitrate aqueous solution, evaporate to dryness with stirring, further dry in air at 120 ° C. for 2 hours, and then calcinate in air at 500 ° C. for 2 hours. Thus, a catalyst (Ag / Bi ₂ O ₃ ) in which silver was supported on a bismuth support was obtained. In the obtained catalyst, the supported amount of silver was 2% by weight of the whole catalyst.

Comparative Example 1
A catalyst (Ag / γ-Al ₂ O ₃ ) in which silver is supported on an alumina support in the same manner as in Example 1 except that the support is replaced with alumina (γ-alumina, manufactured by Sumitomo Chemical Co., Ltd., “KHA24”). ) In the obtained catalyst, the supported amount of silver was 2% by weight of the whole catalyst.

(Nitrogen oxide removal test)
Using the catalysts obtained in Example 1 and Comparative Example 1, a nitrogen oxide removal test was performed as follows. After preparing 5.35 g of the prepared catalyst to a particle diameter of 2 mm to 3 mm, filling and installing in a quartz reaction tube having an inner diameter of 10 mm, the reaction temperature is maintained at 400 ° C. in a nitrogen flow atmosphere (100 mL / min (min)). In the quartz reaction tube, a gas simulating engine exhaust gas (hereinafter referred to as reaction gas) was circulated at 1.5 L / h (hours). Here, the reaction gas is such that nitric oxide NO has a concentration of 500 ppm, n-decane has a concentration of 300 ppm, oxygen O ₂ has a concentration of 10%, water H ₂ O has a concentration of 7%, and sulfur dioxide SO ₂ has a concentration of 30 ppm. Diluted with helium gas. In addition, a density | concentration is volume conversion. The evaluation of the catalyst was performed based on the NO conversion rate (%) according to the following formula.

  NO conversion rate (%) = [(NO concentration at the reaction tube inlet−NO concentration at the reaction tube outlet) / NO concentration at the reaction tube inlet] × 100

  FIG. 1 is a graph showing the relationship between the NO conversion rate (denitration rate) and time (elapsed time) of the catalysts obtained in Example 1 and Comparative Example 1 at the reaction temperature (400 ° C.). As is clear from FIG. 1, the catalyst of Example 1 can be efficiently treated with high reduction performance and high stability over time, even if it is a gas to be treated containing sulfur oxide, without reducing the catalytic activity. Nitric oxide could be reduced and removed.

FIG. 1 is a graph showing the relationship between the denitration rate of the catalysts obtained in Example 1 and Comparative Example 1 and time.

Claims (8)

  A catalyst for reducing nitrogen oxides in the presence of hydrocarbons and oxygen, comprising a carrier and a catalytically active substance supported on the carrier, wherein the carrier is a Group 6 metal, a periodic table A catalyst comprising at least one metal selected from a Group 14 metal of the Table and a Group 15 metal of the Periodic Table, wherein the catalytically active substance is composed of a Group 11 metal of the Periodic Table.   The catalyst according to claim 1, wherein the support is composed of at least one metal selected from molybdenum, tungsten, tin, and bismuth.   The catalyst according to claim 1, wherein the support is composed of at least bismuth.   The catalyst according to claim 1, wherein the catalytically active substance is composed of at least silver.   The catalyst according to claim 1, wherein silver is supported on bismuth oxide.   The catalyst according to claim 1, wherein the ratio of the catalytically active substance is 0.1 to 50 parts by weight with respect to 100 parts by weight of the carrier.   A method for reducing the nitrogen oxide by bringing the gas to be treated containing at least hydrocarbon, oxygen, and nitrogen oxide into contact with the catalyst according to claim 1.   The method according to claim 7, wherein the gas to be treated further contains sulfur oxide.

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