JP4664608B2 - Ammonia decomposition catalyst and ammonia decomposition method - Google Patents

Description

  The present invention is a decomposition catalyst useful for decomposing (purifying) ammonia (for example, ammonia contained in various industrial exhaust gas or exhaust gas after reducing and removing (reducing and purifying) nitrogen oxides by ammonia denitration). Or a purification catalyst) and an ammonia decomposition method (or purification method).

  Ammonia gives significant discomfort due to its unique odor and is specified as a specific malodorous substance in the Malodor Control Law, so it is necessary to suppress emissions. Ammonia is widely used as a liquid ammonia as a solvent and also as a refrigerant for refrigerators. It is also important as a raw material for chemical fertilizers such as ammonium sulfate, ammonium nitrate, urea, and industrial raw materials such as nitric acid. Therefore, ammonia is contained in exhaust gas from various chemical factories and waste processing factories such as refrigerators. Furthermore, since ammonia is also generated by the decay of organisms, ammonia is also discharged from cleaning facilities, water purification facilities, sludge treatment facilities, and the like.

  Ammonia may be added to the exhaust gas to remove nitrogen oxides (NOx: x = 1 or 2) contained in the combustion exhaust gas. That is, exhaust gas discharged from boilers, diesel engines, etc. contains excess oxygen and harmful nitrogen oxides. This nitrogen oxide is usually a selective reduction method in which ammonia is added as a reducing agent, brought into contact with a catalyst composed of titanium oxide-vanadium oxide-tungsten oxide, etc., and the nitrogen oxide is reduced to harmless nitrogen for denitration. It has been removed by (ammonia selective reduction method). This selective catalytic reduction method is characterized in that a denitration rate close to 100% can be obtained if the molar ratio of nitrogen oxide in exhaust gas and ammonia to be added is strictly controlled. However, when the concentration of nitrogen oxides in the exhaust gas varies with time, it is extremely difficult to inject the ammonia amount without excess or deficiency. When an excessive amount of ammonia is added, ammonia remains in the processing gas, and ammonia leakage (leakage) occurs. On the other hand, when the amount of ammonia added is small, the denitration rate decreases.

  As another denitration method, there is also known a method in which a slight excess of ammonia is added to the amount of nitrogen oxides in exhaust gas, and the exhaust gas treated with the catalyst is subsequently brought into contact with an ammonia decomposition catalyst to remove unreacted ammonia. ing.

  Regarding the ammonia decomposition catalyst, for example, in the Journal of Chemical Society of Japan, No. 5, pages 612-618 (1977) (Non-Patent Document 1), a wide range of metals or metal oxides such as Fe, Co, Cu, etc. show the oxidation activity of ammonia. It is disclosed. However, in the oxidation reaction of ammonia by such a metal or metal oxide, not only the reaction (1) for generating nitrogen but also the reaction for generating nitrogen oxide (2) proceeds as shown by the following formula.

4NH ₃ + 3O ₂ → 2N ₂ + 6H ₂ O (1)
4NH ₃ + 5O ₂ → 4NO + 6H ₂ O (2)
Therefore, when ammonia (for example, unreacted ammonia in exhaust gas from which nitrogen oxides have been removed by the selective catalytic reduction method) is treated using the catalyst, the nitrogen compound is removed (purified) by denitration, Nitrogen oxides are formed again in the process of removing ammonia. Further, in the case of other gases to be treated containing ammonia, nitrogen oxides are generated with the decomposition treatment of ammonia in the same manner as described above.

  In the treatment of ammonia-containing gas, various proposals have been made to suppress the by-production of nitrogen oxides. For example, in JP-A-7-289897 (Patent Document 1), a binary composite oxide containing Ti and Si; a binary composite oxide containing Ti and Zr; containing Ti, Si and Zr A catalyst A component which is at least one complex oxide selected from the group consisting of ternary complex oxides, and an oxide of at least one metal selected from the group consisting of vanadium, tungsten and molybdenum Disclosed is an ammonia decomposition catalyst comprising a catalyst B component and at least one noble metal selected from the group consisting of platinum, palladium, rhodium, ruthenium and iridium or a catalyst C component which is a compound thereof. ing.

  Japanese Patent Application Laid-Open No. 10-151349 (Patent Document 2) discloses an ammonia decomposition catalyst containing an oxide carrier and Ag. Specifically, a catalyst having silver supported on a titanium oxide carrier is exemplified.

  Japanese Patent Laid-Open No. 2001-9281 (Patent Document 3) includes a binary complex oxide composed of Ti and Si as a component A, a binary complex oxide composed of Ti and Zr, and Ti, Si and Zr. Contains any one or more of ternary complex oxides, contains an oxide containing one or more elements selected from the group consisting of V, W and Mo as the B component, and Ce, La as the C component A catalyst for ammonia decomposition containing an oxide containing one or more rare earth elements selected from the group consisting of Nd and Pr, and further containing at least one noble metal element of Pd and Ru as a D component is disclosed. Yes.

JP-A-2002-336699 (Patent Document 4) discloses γ-Al ₂ O ₃ , θ-Al ₂ O ₃ , ZrO ₂ , TiO ₂ , TiO ₂ .ZrO ₂ , SiO ₂ .Al as ammonia decomposition catalysts. ₂ O ₃ , Al ₂ O ₃ .TiO ₂ , SO ₄ / ZrO ₂ , SO ₄ / ZrO ₂ .TiO ₂ , Y-type zeolite, X-type zeolite, ferrierite, mordenite, and zeolite β A method for decomposing and removing ammonia using a catalyst in which ruthenium is supported as an active metal on a support made of one or more porous materials is disclosed.

JP-A-2003-24 78 4 (Patent Document 5), platinum (Pt), palladium (Pd), iridium (Ir) and rhodium silica carrying one or more noble metals selected from (Rh) and Or a first component which is a zeolite and a second component which is a composition comprising an oxide of one or more elements selected from titanium (Ti), tungsten (W) and vanadium (V), support amount 0.1~11g / m ² per carrier area of the component, the ammonia decomposition catalyst is disclosed carried per carrier area of the second component is 50 to 300 g / m ^2.

Although these catalysts exhibit practical performance under predetermined conditions, they are expensive because they contain noble metals, and since noble metals generally have high oxidation activity, combustible components ( Depending on the concentration of carbon monoxide, hydrocarbon, etc.) and ammonia concentration, an explosive reaction may be caused, the catalyst may be damaged, and the activity may be significantly reduced. In particular, a large amount of nitrogen oxide or nitrous oxide (N ₂ O) may be produced as a by-product under high temperature conditions, particularly at a high temperature of 450 ° C. or higher.

Japanese Patent Application Laid-Open No. 8-215573 (Patent Document 6) discloses an ammonia decomposition catalyst in which an active metal component is supported on a support mainly composed of titanium oxide, and the catalyst converts a sulfur compound to 0.74 in terms of SO _4. A sulfur compound-containing ammonia decomposition catalyst containing at least wt% is disclosed, and examples of specific active metal components include noble metals (Ru, Pt, Pd, etc.) and base metals (Cu, W, etc.). In this catalyst, when a noble metal is used as an active metal component, the same problem as described above occurs. Moreover, when a base metal is used as an active metal component, the by-product of nitrogen oxides or nitrous oxide can be reduced. However, side reactions cannot be completely suppressed, and in particular, nitrous oxide by-products cannot be effectively prevented.

In JP-A-8-332388 (Patent Document 7), oxidation of an element of Group Ib, IIb, IIIa, IIIb, IVa, IVb, Va, VIa, VIIa or VIII group on one or more carriers is performed. An ammonia decomposing agent comprising at least one active species selected from a product and a nitrate radical (NO ₃ ) is disclosed. Specifically, a nitrate radical, vanadium oxide, iron oxide, oxidation are supported on a titanium oxide support. Examples include catalysts carrying manganese or the like. Since this catalyst has a configuration in which active species and nitrate radicals are supported on a carrier, there is a possibility that the catalyst may be significantly poisoned by sulfur oxides in the exhaust gas and cause a significant decrease in performance. Furthermore, when the temperature exceeds 300 ° C., the supported HNO ₃ is scattered and the catalytic activity may be reduced.
JP-A-7-289897 (Claim 1) JP-A-10-151349 (Claims) JP 2001-9281 A (Claim 1) JP 2002-336699 A (Claim 1) JP 2003-24784 A JP-A-8-215573 (Claim 1, paragraph number [0011]) JP-A-8-332388 (Claim 1, paragraph numbers [0012] to [0014]) The Chemical Society of Japan No.5, pages 612-618 (1977)

  Accordingly, an object of the present invention is to provide an ammonia decomposition (or purification) catalyst and an ammonia decomposition method capable of maintaining high ammonia resolution while suppressing generation of harmful nitrogen oxides and nitrous oxides even under high temperature conditions. is there.

  Another object of the present invention is to provide an ammonia decomposition (or purification) catalyst and an ammonia decomposition method that can maintain high catalytic activity even if the gas to be treated contains hydrocarbons or sulfur oxides.

  Still another object of the present invention is to provide an inexpensive ammonia decomposition (or purification) catalyst and ammonia decomposition method which do not require a noble metal and can effectively convert ammonia into nitrogen.

  As a result of intensive studies to achieve the above-mentioned problems, the present inventor has shown that when a catalytic oxidation catalyst is formed by combining an iron component and sulfate zirconia, it exhibits high catalytic activity even under high temperature conditions (for example, 500 ° C. or higher), and nitrogen. It has been found that ammonia can be efficiently decomposed into nitrogen while effectively suppressing by-products of oxide and nitrous oxide, and the present invention has been completed.

That is, the ammonia decomposition catalyst of the present invention is composed of sulfate zirconia containing iron (an iron component as an active component). In this catalyst, the ratio of the active component (iron) may be about 1 to 20 parts by weight with respect to 100 parts by weight of zirconia. The ratio of sulfate radicals may be about 1 to 25 parts by weight with respect to 100 parts by weight of zirconia in terms of SO ₄ ²⁻ (or sulfur).

  The present invention also includes an ammonia decomposition method (or removal method) in which a gas to be treated (or exhaust gas) containing ammonia is brought into contact with a catalyst containing iron-containing sulfate radical zirconia (the ammonia decomposition catalyst). In this method, a gas to be treated (or exhaust gas) containing ammonia may be brought into contact with the catalyst at a temperature of about 400 to 600 ° C. When the gas to be treated is brought into contact with the catalyst of the present invention, ammonia in the gas to be treated can be efficiently decomposed and removed while suppressing by-production of nitrogen oxides and nitrous oxide. Furthermore, even when a gas to be treated (or exhaust gas) containing sulfur oxide and ammonia is brought into contact with the catalyst, high ammonia resolution can be maintained.

  In the present invention, since a specific catalyst is used, high ammonia resolving power is maintained without generating nitrogen oxides or nitrous oxide having a problem on the global environment even under high temperature conditions (for example, 500 ° C. or higher). Ammonia can be efficiently decomposed into nitrogen. Further, even if the gas to be treated contains a combustible component (such as carbon monoxide or hydrocarbon) or sulfur oxide, high catalytic activity can be maintained. Furthermore, since no precious metal is required, not only is it inexpensive, but ammonia can be effectively converted into nitrogen.

  The catalyst of the present invention is composed of sulfate zirconia containing iron (iron component).

  The iron (iron component) constituting the catalyst may be contained in at least sulfate radical zirconia, may be supported on sulfate radical zirconia, and may be contained uniformly or dispersed in sulfate radical zirconia. Good. Moreover, in the range which does not affect catalyst activity, iron may be contained as a metal simple substance Fe, and may be contained with the form of the compound. For example, it may be contained in the form of iron oxide, iron hydroxide, salt (inorganic acid salt such as sulfate, organic acid salt, complex salt). The iron component is usually at least in the form of iron oxide. Furthermore, the valence of iron is not particularly limited, and may be divalent iron Fe (II) or trivalent iron Fe (III).

The sulfate radical zirconia constituting the catalyst is composed of a sulfate radical and zirconia (ZrO ₂ ). The sulfate radical zirconia may contain a sulfate radical in the form of coordination or salt.

Zirconia (ZrO ₂ ), which is a component of sulfate radical zirconia, exists in the form of an oxide (zirconia), but is within a range not impairing the catalytic activity, such as a zirconia compound (for example, hydroxide, halide, oxyacid salt). , Organic acid salt).

Furthermore, the catalyst may contain other metal components (for example, titanium, hafnium, silicon, rare earth, etc.) as long as the catalytic activity is not impaired. These metal components may also be included in the form of a compound (for example, an oxide such as TiO ₂ ).

The ratio of sulfate radical to zirconia may be in a range that does not impair the catalytic activity and stability. For example, the ratio of sulfate radical (converted as SO ₄ ²⁻ ) is 100 parts by weight of zirconia (ZrO ₂ ). It can be selected from the range of about 0.5 to 30 parts by weight, and may be, for example, about 1 to 25 parts by weight, preferably about 5 to 20 parts by weight (for example, 10 to 20 parts by weight).

The ratio (supported amount) of iron (or iron component) is, for example, 0.5 to 20 parts by weight (for example, 1 to 20 parts by weight) with respect to 100 parts by weight of zirconia (ZrO ₂ ) in terms of iron element Fe. The amount is preferably about 1 to 15 parts by weight, more preferably about 1 to 10 parts by weight (for example, 2 to 8 parts by weight), and usually about 1 to 7 parts by weight (for example, 2 to 5 parts by weight). If the ratio of iron is too small, the catalytic activity may decrease, and if the ratio of iron is excessive, the oxidation activity for ammonia increases, and a large amount of nitrogen oxide may be generated.

The catalyst may be non-porous or porous. BET specific surface area of the catalyst, 5 m ² / g or more (e.g., 5 to 500 m approximately ² / g) may be, usually, 10 to 300 m ² / g, (e.g., 50 to 300 m ² / g), preferably May be about 60 to 250 m ² / g (for example, 60 to ²⁰⁰ m ² / g), more preferably about 80 to 250 m ² / g (for example, 90 to 150 m ² / g). Examples of the shape of the catalyst include powder, granule, pellet, and honeycomb. Of these shapes, a honeycomb shape is preferable from the viewpoint of reducing pressure loss. The number of cells of the honeycomb can be selected within a range in which an increase in pressure loss can be suppressed. For example, the number of cells is ¹ to 250 cells / cm ² , preferably 5 to 150 cells / cm ² , and more preferably about 10 to 100 cells / cm ² . There may be. The aperture ratio of the honeycomb may be, for example, 30 to 80% (for example, 50 to 80%), preferably about 50 to 75%.

  In addition, if necessary, various conventional additives may be blended in the catalyst as long as the activity of the catalyst is not impaired.

The catalyst of the present invention is a very useful catalyst excellent in (1) activity, (2) selectivity, and (3) durability, which are three elements of the catalyst function. That is, even under high temperature conditions, a high ammonia decomposition rate can be realized, and by-products of nitrogen oxides and nitrous oxide can be suppressed. In addition, a large amount of other component gases coexist, and even ammonia having a low concentration (for example, 1 to 300 ppm) can be selectively decomposed. Furthermore, even if combustible components (such as hydrocarbons and carbon monoxide) and sulfur oxides (such as sulfur dioxide SO ₂ ) are present, accumulation of catalyst poisons can be suppressed and high catalytic activity can be maintained.

  The catalyst of the present invention can be produced by a conventional method. For example, (1) a method in which an iron component is supported on sulfate zirconia as a support, and (2) a zirconium component and an iron component are coexisting in the presence of the sulfuric acid component. Examples thereof include a method in which the coprecipitate is dried and calcined, or a sulfuric acid component and an iron component are supported on a solid zirconium component and then dried and calcined.

[Production Method (1)]
The sulfate radical zirconia can be prepared by a conventional method, and can be obtained, for example, by firing a zirconium component containing a sulfuric acid component. Zirconium components include compounds capable of producing zirconia (zirconium oxide) upon firing, such as zirconium hydroxide, inorganic acid salts (zirconium sulfate, zirconium nitrate, etc.), halides (zirconium chloride, etc.), organic acid salts (zirconium acetate) Etc.). In the zirconium component, the valence of zirconium may be divalent Zr (II) or trivalent Zr (III), but is usually stable tetravalent Zr (IV). These zirconium components can be used alone or in combination of two or more. Of these zirconium components, zirconium hydroxide and the like are preferable.

  As the sulfuric acid component (or sulfur component), sulfuric acid or a compound capable of producing sulfuric acid (such as sulfur oxide) can be used, and it can be usually used in the form of an aqueous solution containing sulfate ions. Examples of the sulfuric acid component include sulfuric acid and various sulfates (such as ammonium sulfate, amine sulfates, zirconium sulfate, potassium sulfate, and sodium sulfate). These sulfuric acid components can also be used alone or in combination of two or more. Of these sulfuric acid components, components having low persistence due to firing, such as ammonium sulfate, are preferred.

  The sulfate radical zirconia may be prepared by precipitating a zirconium component as necessary in the presence of a sulfuric acid component by adjusting pH or the like, drying the product, and firing. For example, an alkali may be added to an aqueous solution containing a zirconium component and a sulfuric acid component (for example, ammonium sulfate) to produce zirconium hydroxide, and the product may be dried and fired. In a typical preparation method, sulfate radical zirconia may be produced by impregnating zirconium hydroxide with an aqueous solution containing a sulfuric acid component (for example, ammonium sulfate), drying, and firing.

  The ratio of the sulfuric acid component to the zirconium component may be the above ratio (the ratio of the sulfate radical to zirconia), and the sulfuric acid component in the final catalyst depends on the calcination temperature in consideration of the sulfuric acid component that volatilizes and disappears upon calcination. You may contain more sulfuric acid components than the ratio of a component.

  Examples of the method of supporting the iron component on sulfate radical zirconia include a method of impregnating sulfate radical zirconia in an aqueous solution containing the iron component, and drying and firing. The iron compound constituting the iron component is not particularly limited, and various iron-containing compounds (usually water-soluble iron compounds) can be used. For example, inorganic acid salts (for example, ferrous sulfate, ferric sulfate, etc.) And sulfates (eg, ferrous chloride, ferric chloride), organic acid salts (eg, iron acetate), complexes (eg, iron carbonyl compounds). . These iron-containing compounds can be used alone or in combination of two or more. Of these iron-containing compounds, iron sulfate is preferred.

  The impregnation of the iron component with respect to the zirconium component may usually be performed at room temperature (about 15 to 25 ° C.) or may be performed under heating (for example, about 30 to 80 ° C., preferably about 50 to 80 ° C.). . The drying conditions are not particularly limited, and may be normal pressure or reduced pressure, may be dried by heating, or may be evaporated to dryness.

  The calcination temperature can be selected from about 300 to 700 ° C. as long as the catalytic activity is not impaired, and is, for example, 400 to 650 ° C. (for example, 450 to 650 ° C.), preferably about 500 to 600 ° C. If the firing temperature is too high, the sulfate radicals may volatilize and disappear, and if it is too low, stable crystals cannot be formed. The firing time is usually selected from the range of about 0.1 to 100 hours (for example, 1 to 80 hours), for example, 1 to 50 hours (for example, 1 to 20 hours), preferably about 3 to 10 hours. It is. Note that the firing may be performed in an inert gas atmosphere or in an oxidizing atmosphere (such as air).

[Production method (2)]
In this method, (2a) the zirconium component and the iron component are coprecipitated in the presence of the sulfuric acid component, and the coprecipitate is dried and fired. (2b) The solid zirconium component is mixed with the sulfuric acid component and the iron component. A catalyst can be obtained by carrying out treatment (immersion or impregnation treatment, etc.), carrying, drying and firing.

  In the method (2a), as a zirconium component (zirconium compound), a water-soluble compound [inorganic acid salt (zirconium sulfate, zirconium nitrate, etc.), halide (zirconium chloride, etc.), organic acid salt (zirconium acetate, etc.), etc.] In the method (2b), a water-insoluble compound (such as zirconium hydroxide) can be used. In the zirconium compound used in the above method (2a) (2b), the valence of zirconium may be divalent Zr (II) or trivalent Zr (III), but is usually stable tetravalent Zr (IV). It is. As the sulfuric acid component and the iron component, the same components as described above can be used, and they may usually be water-soluble.

  In the method (2a), the coprecipitation can be performed by addition of alkali (or pH adjustment) in the same manner as described above. In the method (2b), the supporting treatment is not limited to immersion or impregnation, and may be performed by spraying or the like. Furthermore, the drying and firing conditions are the same as described above.

  Of the methods (1) and (2), the method (2) that can be produced economically advantageously with a small number of steps is preferable.

  The catalyst may be molded into a predetermined shape using various molding methods. Examples of the catalyst molding method include a kneader, a molding machine (such as an extrusion molding machine and a compression molding machine), a tableting machine, a granulator, and a pulverizer. For example, a honeycomb catalyst is, for example, a method of coating a honeycomb refractory material (for example, ceramic) with a catalyst component (sulfuric acid zirconia and iron component), adding a binder to the catalyst component as necessary, and extruding into a honeycomb shape. Examples of the method include molding. As a method of forming into a honeycomb shape, an extrusion method is preferable from the viewpoint of the performance of the catalyst. The molded catalyst component (for example, a molded product containing the catalyst component and the binder) may be fired as described above.

  When the gas to be treated is treated using the catalyst of the present invention, ammonia can be removed at a high decomposition rate. The gas to be treated only needs to contain at least ammonia, and the concentration of ammonia is not particularly limited, and may be, for example, 3000 ppm or less (for example, 1 to 3000 ppm) on a volume basis, and usually 5 to 3000 ppm, Preferably, it may be about 10 to 2000 ppm.

The gas to be treated may contain oxygen, water vapor, carbon dioxide and the like in addition to ammonia. The proportion of oxygen in the gas to be treated is, for example, about 1 to 30% by volume, preferably about 1 to 20% by volume, and more preferably about 2 to 15% by volume. In addition, when the ratio of oxygen in the gas to be treated is too small (for example, less than 1% by volume), the ratio of oxygen concentration to ammonia concentration (O ₂ concentration / NH ₃ concentration) is too small (for example, 1 or less), etc. If this is the case, the decomposition rate of ammonia may decrease. Therefore, when oxygen in the gas to be treated is insufficient, oxygen or air may be added and contacted with the catalyst.

  The ratio of water vapor is, for example, about 0.1 to 40% by volume, preferably 1 to 30% by volume, and more preferably about 2 to 20% by volume in the gas to be treated. The ratio of carbon dioxide is, for example, about 0.1 to 20% by volume, preferably about 0.5 to 15% by volume, and more preferably about 1 to 10% by volume in the gas to be treated.

  Further, the combustion exhaust gas often contains sulfur oxides that degrade the subsequent equipment, but the catalyst of the present invention can also be applied to gases containing sulfur oxides. The concentration of sulfur oxide in the gas to be treated may be about 0.1 to 100 ppm (for example, 1 to 70 ppm).

  Further, the gas to be treated may contain a combustible component such as hydrocarbon or carbon monoxide. The concentration of these components is not particularly limited, and even in the case of a high concentration, for example, about 500 to 3000 ppm (for example, 1000 to 2000 ppm), a decrease in catalyst activity can be prevented in the present invention.

  The contact temperature between the gas to be treated containing ammonia and the catalyst can be selected from a range in which the generation amount of nitrogen oxides can be suppressed without impairing ammonia resolution, and is preferably 400 ° C. or higher (400 to 600 ° C.), preferably Is 450 ° C. or higher (for example, 450 to 600 ° C.), more preferably about 500 ° C. or higher (for example, 500 to 550 ° C.), and it is desirable to make contact at a temperature of 600 ° C. or lower. If the temperature is too low, the decomposition rate of ammonia will decrease, and if it is too high, a large amount of nitrogen oxides may be produced, and the durability of the catalyst will decrease.

In the catalytic gas processing system using the present invention, a gas hourly space velocity (GHSV) is, for example, 500～200000H ^-1, preferably 1000～150000H ^-1, more preferably 2000~100000h about ^-1. In addition, the usage-amount of a catalyst can be selected according to the said space velocity.

  The linear velocity (LV) of the gas to be treated with respect to the catalyst can be selected, for example, from a range of about 0.02 to 20 m / second, preferably 0.05 to 5 m / second, and more preferably about 0.3 to 3 m / second.

  The length as a catalyst layer can be selected in the range which does not impair catalyst activity, for example, 0.01-10 m, Preferably it is 0.05-5 m, More preferably, about 0.1-1 m may be sufficient.

  The present invention can be used for removal (purification) of ammonia from various sources (for example, various chemical product manufacturing plants, waste processing plants such as refrigerators, sludge processing facilities, etc.). Since the catalyst of the present invention can decompose ammonia into nitrogen without generating nitrogen oxides or nitrous oxide, it can be widely used as a gas to be treated containing ammonia. For example, the catalyst of the present invention is particularly suitable for decomposing and removing (decomposing and purifying) ammonia after adding ammonia to a gas to be treated (or exhaust gas) containing nitrogen oxide and selectively reducing it to remove nitrogen oxide. It is valid. That is, in order to remove nitrogen oxides at a denitration rate of almost 100%, an excess amount of nitrogen oxides in the exhaust gas of combustion system exhaust gas or chemical process in which the nitrogen oxide concentration tends to fluctuate greatly in time series is excessive. After adding ammonia and denitrating with a general-purpose selective reduction catalyst, it is useful for removing ammonia contained in the exhaust gas. Since the catalyst of the present invention can maintain high catalytic activity even when the gas to be treated contains a combustible component such as carbon monoxide or hydrocarbon, or a sulfur oxide, it can be effectively used for exhaust gas containing such a component. Furthermore, ammonia can be efficiently decomposed and removed even under relatively high temperature conditions (400 to 600 ° C.) where sufficient performance could not be obtained with conventional catalysts. Therefore, it is advantageous for the treatment of exhaust gas from combustion systems and chemical processes such as boilers, diesel engines, and gas 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
150 ml of zirconium hydroxide (made by Mitsuwa Chemicals Co., Ltd .; containing 79% by weight as ZrO ₂ ), 2 g of ammonium sulfate and 36 g of ferric sulfate (Fe ₂ (SO ₄ ) ₃ .15H ₂ O) It was immersed in an aqueous solution for 15 hours. After evaporating to dryness, it was calcined in air at 550 ° C. for 6 hours to obtain an iron-supported sulfate radical zirconia catalyst. As a result of fluorescent X-ray analysis, the iron content of this catalyst was 4.0 parts by weight with respect to 100 parts by weight of zirconia. The sulfate radical content was 12 parts by weight as sulfate radical SO ₄ ^{2− with} respect to 100 parts by weight of zirconia.

  The catalyst was compression-molded to prepare a granulated product having a particle size of 1 to 2 mm, and 4.5 ml of the granulated product was filled in a quartz reaction tube (inner diameter 14 mm). The temperature of the catalyst was maintained at 350 ° C., 400 ° C., 450 ° C., 500 ° C., or 550 ° C., and the gas to be treated was circulated at a flow rate of 1.2 liters per minute (volume at 0 ° C., 1 atm).

As the gas to be treated, a gas to be treated composed of ammonia NH ₃ , oxygen O ₂ , water H ₂ O, sulfur dioxide SO ₂ , and helium He was used. Regarding the ratio of each component contained in the gas to be processed, the ratio of NH ₃ , O ₂ , H ₂ O and SO ₂ is as follows: NH ₃ is 240 ppm, O ₂ is 10% by volume, H ₂ O Was 10% by volume, SO ₂ was 3 ppm, and the balance was He.

The nitrogen oxide concentration of the catalyst outlet gas was measured with a chemiluminescent NO _x analyzer, and the nitrogen concentration was measured with a gas chromatograph. By comparing the ammonia content of the gas to be treated before passing through the catalyst with the nitrogen content or nitrogen oxide content of the gas to be treated after passing through the catalyst, the nitrogen production rate and the nitrogen oxide production rate are obtained. be able to. Specifically, the nitrogen production rate and the nitrogen oxide production rate were calculated by the following equations, and the results obtained at each temperature are shown in Table 1. It was confirmed by gas chromatography that nitrous oxide was not contained in the catalyst outlet gas.

Nitrogen production rate = 200 × (outlet nitrogen concentration) / (inlet ammonia concentration)
Nitrogen oxide production rate = 100 × (Outlet NO _x concentration) / (Inlet ammonia concentration)

  As is apparent from the results in Table 1, in the range of 450 to 550 ° C., a high ammonia decomposition rate (nitrogen production rate + nitrogen oxide production rate) was obtained, and the production of nitrogen oxides was suppressed to a low level. In particular, at 500 ° C. and 550 ° C., a nitrogen production rate of 98% or more was achieved despite the high temperature, and the production of nitrogen oxides could be suppressed.

Comparative Example 1
The same zirconium hydroxide as in Example 1 was immersed in an aqueous solution of iron (III) nitrate, dried and calcined to prepare an iron zirconia catalyst. The iron content of this catalyst was 1.6 parts by weight with respect to 100 parts by weight of zirconia. As a result of X-ray diffraction analysis, it was found that the crystal system of this catalyst is mainly composed of monoclinic crystals. The performance of this catalyst was evaluated in the same manner as in Example 1 without adding SO ₂ to the gas to be treated. The results are shown in Table 1.

  As is clear from Table 1, the catalyst obtained in Comparative Example 1 had the same ammonia decomposition rate as that of the Example, but a large amount of nitrogen oxide was produced.

Comparative Example 2
The catalyst obtained in Comparative Example 1 was kept at 500 ° C. and treated gas composed of oxygen O ₂ , water H ₂ O, sulfur dioxide SO ₂ and helium He (O ₂ : 10% by volume, H ₂ O: 10 (% By volume, SO ₂ : 3 ppm, He: balance) was flown at a flow rate of 1.2 liters per minute for 10 hours and SO _x treatment was performed. It is shown in 1.

  The catalyst obtained in Comparative Example 2 increased in nitrogen generation rate and decreased in nitrogen oxide generation rate, but did not reach the result of Example 1. Note that the increase in the nitrogen production rate and the decrease in the nitrogen oxide production rate occur in the high temperature region (450 ° C. or higher) because the sulfate radical is generated on the catalyst by passing the gas to be treated through the catalyst. Conceivable.

Comparative Example 3
180 g of the same zirconium hydroxide as in Example 1 was immersed for 15 hours in a 150 ml aqueous solution in which 24 g of ammonium sulfate and 6.8 g of cobalt sulfate (CoSO ₄ .7H ₂ O) were dissolved. After evaporating to dryness, it was calcined in air at 550 ° C. for 6 hours to obtain a cobalt-supported sulfate radical zirconia catalyst. As a result of fluorescent X-ray analysis, the cobalt content of this catalyst was 1.2 parts by weight with respect to 100 parts by weight of zirconia. The content of sulfate radical was 8.1 parts by weight as sulfate radical SO ₄ ^{2− with} respect to 100 parts by weight of zirconia. The performance of this catalyst was evaluated in the same manner as in Example 1. The results are shown in Table 1.

  As is clear from the results in Table 1, the nitrogen production rate was lower than that in Example 1 over a range of 350 to 550 ° C, and particularly at 400 ° C or less, almost all ammonia was discharged as it was. Moreover, although the production rate of nitrogen oxides was kept low, it did not reach Example 1.

  From the above results, it is necessary to constitute a catalyst with sulfate zirconia and iron in order to maintain high ammonia resolution while suppressing generation of harmful nitrogen oxides and nitrous oxide.

Claims (6)

A catalyst for decomposition of ammonia composed of iron-containing sulfate zirconia (however, a catalyst for reducing nitrogen oxides in the presence of ammonia, composed of iron-containing sulfate zirconia, The catalyst whose ratio is 1 to 2.5 parts by weight with respect to 100 parts by weight of zirconia is excluded) .   The catalyst according to claim 1, wherein the ratio of iron is 1 to 20 parts by weight with respect to 100 parts by weight of zirconia. The catalyst according to claim 1, wherein the ratio of sulfate radical is 1 to 25 parts by weight with respect to 100 parts by weight of zirconia in terms of SO ₄ ²⁻ . A method for decomposing ammonia in which a gas to be treated containing ammonia is brought into contact with a catalyst composed of iron sulfate-containing zirconia (however, a gas to be treated containing nitrogen oxide and ammonia is converted to iron-containing sulfate zirconia containing iron. Except for a method of removing nitrogen oxides in which the catalyst is in contact with a catalyst having an iron content of 1 to 2.5 parts by weight with respect to 100 parts by weight of zirconia) .   The process according to claim 4, wherein the gas to be treated is brought into contact with the catalyst at a temperature of 400 to 600C.   The method according to claim 4, wherein an exhaust gas containing sulfur oxide and ammonia is contacted with the catalyst.