Nitrous oxide decomposition method, nitrous oxide decomposition system, nitric acid production method, and nitric acid production plant

Abstract

To provide a method and a system for decomposing nitrous oxide capable of decomposing nitrous oxide for a long time while maintaining catalytic activity, and a method and a plant for producing nitric acid capable of industrially producing nitric acid while maintaining catalytic activity and decomposing nitrous oxide for a long time.SOLUTION: A method for decomposing nitrous oxide includes a step of decomposing nitrous oxide by bringing a specific catalyst into contact with a nitrous oxide-containing gas containing nitrous oxide, nitrogen monoxide and nitrogen dioxide, wherein the total content of nitrogen monoxide and nitrogen dioxide in the nitrous oxide-containing gas is 0.00001-0.35 mol%, and a method and a system for decomposing nitrous oxide applied to a nitric acid production plant.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a method for decomposing nitrous oxide, a system for decomposing nitrous oxide, a method for producing nitric acid, and a nitric acid production plant. [Background technology]

[0002] From the perspective of protecting the global environment and preventing air pollution, nitrogen oxides (NOx) in exhaust gases have become a problem, and their emissions are being strictly regulated. The nitrogen oxides that are particularly subject to emission regulations are nitrogen dioxide (NO2), which is harmful to humans and is believed to cause photochemical smog and acid rain. Various denitrification technologies have been developed and implemented to reduce emissions. However, nitrous oxide (NO2), a type of nitrogen oxide, is not currently subject to emission regulations and is typically released directly into the atmosphere. In fact, gases emitted from chemical manufacturing plants, such as nitric acid plants, epsilon-caprolactam plants, and adipic acid plants, undergo denitrification treatment to remove nitric oxide and nitrogen dioxide, but the by-product nitrous oxide is often released into the atmosphere without being decomposed.

[0003] However, when greenhouse gases such as nitrous oxide are released into the atmosphere, the greenhouse effect of the atmosphere increases due to an increase in greenhouse gas concentration, which is thought to be a cause of global warming. Nitrous oxide is said to have a global warming effect approximately 300 times greater than that of carbon dioxide. Therefore, in recent years, there has been growing interest in reducing atmospheric emissions of nitrous oxide, along with carbon dioxide and methane. With growing awareness of sustainable environmental conservation, it is expected that nitrous oxide will be subject to emission control in the near future. Therefore, there is a need for technologies that decompose and remove nitrous oxide from exhaust gases to reduce its emissions into the atmosphere. Patent Document 1, for example, describes a nitrous oxide decomposition method that uses a catalyst carrying at least one noble metal selected from ruthenium (Ru), rhodium (Rh), palladium (Pd), rhenium (Re), osmium (Os), iridium (Ir), and platinum (Pt) to catalytically decompose a gas containing nitrous oxide in the presence of a reducing gas. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Publication No. 06-218232 Summary of the Invention [Problem to be solved by the invention]

[0005] The nitrous oxide decomposition method described in Patent Document 1 is said to be able to decompose nitrous oxide in a nitrous oxide-containing gas by catalytically decomposing the gas containing nitrous oxide in the presence of a reducing gas such as carbon monoxide, a hydrocarbon gas, a mineral oil hydrocarbon gas, or an alcohol. However, catalysts used for the decomposition of nitrous oxide generally lose their catalytic activity gradually when used to decompose nitrous oxide. In particular, when a method for decomposing nitrous oxide is applied to industrial production in a manufacturing plant such as a nitric acid manufacturing plant, the catalytic activity is likely to be deactivated. Furthermore, in industrial production processes, frequent replacement of deactivated catalysts reduces production efficiency. Therefore, considering the circumstances surrounding nitrous oxide described above and its application to industrial production processes, it is desirable to be able to maintain the catalytic activity of the catalyst and decompose nitrous oxide for a long period of time. However, Patent Document 1 does not address this point.

[0006] An object of the present invention is to provide a method for decomposing nitrous oxide that can maintain catalytic activity and decompose nitrous oxide for an extended period of time.Another object of the present invention is to provide a method and system for decomposing nitrous oxide that can maintain catalytic activity and decompose nitrous oxide for an extended period of time even when applied to a nitric acid production plant.A further object of the present invention is to provide a method and plant for producing nitric acid that can industrially produce nitric acid while maintaining catalytic activity and decomposing nitrous oxide for an extended period of time. [Means for solving the problem]

[0007] The object of the present invention has been achieved by the following means. <1> A method for decomposing nitrous oxide applied to a nitric acid production plant capable of at least performing a step of synthesizing nitric acid and a step of recovering power using an exhaust gas turbine, comprising: A method for decomposing nitrous oxide, comprising: a step of decomposing nitrous oxide in which a catalyst containing at least one selected from the group consisting of ruthenium and ruthenium compounds is brought into contact with the nitrous oxide-containing gas discharged from the step of synthesizing nitric acid, before the step of recovering power. <2> The reaction pressure in the decomposition step is 0.2 MPa or more in absolute pressure. <1> The method for decomposing nitrous oxide according to claim 1. <3> The catalyst further comprises titanium oxide. <1> or <2> The method for decomposing nitrous oxide according to claim 1. <4> The titanium oxide contains a rutile structure in an amount of 80% by mass or more, where the total amount of titanium oxide contained in the catalyst is 100% by mass. <3> The method for decomposing nitrous oxide according to claim 1. <5> the nitrous oxide-containing gas further comprises oxygen and water vapor; <1> ~ <4> The method for decomposing nitrous oxide according to any one of the above.

[0008] <6> A method for producing nitric acid in a nitric acid production plant capable of at least carrying out a step of synthesizing nitric acid and a step of recovering power by an exhaust heat gas turbine, comprising: A method for producing nitric acid, comprising: a step of decomposing nitrous oxide, in which a catalyst containing at least one selected from the group consisting of ruthenium and ruthenium compounds is brought into contact with the nitrous oxide-containing gas discharged from the step of synthesizing nitric acid, before the step of recovering power.

[0009] <7> The method includes a nitrous oxide decomposition step of contacting a nitrous oxide-containing gas containing nitrous oxide, nitric oxide, and nitrogen dioxide with a catalyst containing at least one selected from the group consisting of ruthenium and ruthenium compounds, A method for decomposing nitrous oxide, wherein the total content of the nitric oxide and the nitrogen dioxide in the nitrous oxide-containing gas is 0.0001 to 0.35 mol %. <8> The reaction pressure in the decomposition step is 0.2 MPa or more in absolute pressure. <7> The method for decomposing nitrous oxide according to claim 1. <9> The catalyst further comprises titanium oxide. <7> or <8> The method for decomposing nitrous oxide according to claim 1. <10> The titanium oxide contains a rutile structure in an amount of 80% by mass or more, where the total amount of titanium oxide contained in the catalyst is 100% by mass. <9> The method for decomposing nitrous oxide according to claim 1. <11> the nitrous oxide-containing gas further comprises oxygen and water vapor; <7> ~ <10> The method for decomposing nitrous oxide according to any one of the above.

[0010] <12> A method for decomposing nitrous oxide applied to a nitric acid manufacturing plant capable of at least carrying out a step of synthesizing nitric acid, comprising: a step of reducing nitric oxide and nitrogen dioxide contained in the nitrous oxide-containing gas discharged from the step of synthesizing nitric acid; a nitrous oxide decomposition step of contacting the nitrous oxide-containing gas discharged from the reduction step with a catalyst containing at least one selected from the group consisting of ruthenium and ruthenium compounds, How to decompose nitrous oxide. <13> the total content of nitric oxide and nitrogen dioxide in the nitrous oxide-containing gas discharged from the reduction step is 0.0001 to 0.35 mol %; <12> The method for decomposing nitrous oxide according to claim 1. <14> The reaction pressure in the decomposition step is 0.2 MPa or more in absolute pressure. <12> or <13> The method for decomposing nitrous oxide according to claim 1. <15> The catalyst further comprises titanium oxide. <12> ~ <14> The method for decomposing nitrous oxide according to any one of the above. <16> The titanium oxide contains a rutile structure in an amount of 80% by mass or more, where the total amount of titanium oxide contained in the catalyst is 100% by mass. <15> The method for decomposing nitrous oxide according to claim 1. <17> the nitrous oxide-containing gas further comprises oxygen and water vapor; <12> ~ <16> The method for decomposing nitrous oxide according to any one of the above. <18> The nitric acid production plant may further perform a step of recovering power using an exhaust gas turbine, The nitrous oxide decomposition step is carried out before the power recovery step and after the reduction step; <12> ~ <17> The method for decomposing nitrous oxide according to any one of the above.

[0011] <19> A method for producing nitric acid in a nitric acid production plant capable of at least carrying out a step of synthesizing nitric acid, a step of reducing nitric oxide and nitrogen dioxide contained in the nitrous oxide-containing gas discharged from the step of synthesizing nitric acid; a decomposition step of decomposing nitrous oxide by contacting the nitrous oxide-containing gas discharged from the reduction step with a catalyst containing at least one selected from the group consisting of ruthenium and ruthenium compounds; How to make nitric acid. <20> The nitric acid production plant may further perform a step of recovering power using an exhaust gas turbine, The nitrous oxide decomposition step is carried out before the power recovery step and after the reduction step; <19> The method for producing nitric acid according to claim 1.

[0012] <21> A nitrous oxide decomposition system installed in a nitric acid production plant having a nitric acid synthesis unit for synthesizing nitric acid and a power recovery unit for recovering power using an exhaust gas turbine, a nitrous oxide decomposition unit that brings a catalyst containing at least one selected from the group consisting of ruthenium and ruthenium compounds into contact with the nitrous oxide-containing gas discharged from the nitric acid synthesis unit, A nitrous oxide decomposition system, wherein the nitrous oxide decomposition device is installed upstream of the power recovery device. <22> A nitrous oxide decomposition system installed in a nitric acid production plant having a nitric acid synthesis unit for synthesizing nitric acid and a reduction unit for reducing nitric oxide and nitrogen dioxide contained in nitrous oxide-containing gas discharged from the nitric acid synthesis unit, A nitrous oxide decomposition system comprising a nitrous oxide decomposition device that brings the nitrous oxide-containing gas discharged from the reduction device into contact with a catalyst containing at least one selected from the group consisting of ruthenium and ruthenium compounds. <23> A nitrous oxide decomposition system attached to a nitric acid production plant, the system comprising, in this order, a nitric acid synthesis unit for synthesizing nitric acid, a reduction unit for reducing nitric oxide and nitrogen dioxide contained in nitrous oxide-containing gas discharged from the nitric acid synthesis unit, and a power recovery unit for recovering power using an exhaust gas turbine, a nitrous oxide decomposition device that brings the nitrous oxide-containing gas discharged from the reduction device into contact with a catalyst containing at least one selected from the group consisting of ruthenium and ruthenium compounds; Nitrous oxide decomposition system.

[0013] <24> A nitric acid production plant having, in this order, a nitric acid synthesis unit for synthesizing nitric acid, a reduction unit for reducing nitric oxide and nitrogen dioxide contained in nitrous oxide-containing gas discharged from the nitric acid synthesis unit, and a power recovery unit for recovering power using an exhaust heat gas turbine, A nitric acid production plant having, between the reduction device and the power recovery device, a nitrous oxide decomposition device that brings the nitrous oxide-containing gas discharged from the reduction device into contact with a catalyst containing at least one selected from the group consisting of ruthenium and ruthenium compounds. [Effects of the Invention]

[0014] The present invention provides a method for decomposing nitrous oxide that can maintain catalytic activity and decompose nitrous oxide for an extended period of time. The present invention also provides a method or system for decomposing nitrous oxide that can maintain catalytic activity and decompose nitrous oxide for an extended period of time, even when applied to a nitric acid production plant. Furthermore, the present invention provides a method and plant for industrially producing nitric acid while maintaining catalytic activity and decomposing nitrous oxide for an extended period of time. DETAILED DESCRIPTION OF THE INVENTION

[0015] In the present invention, the term "gas discharged from a process or apparatus" includes not only gas generated by the execution of a process or (newly generated) gas generated in an apparatus and discharged from the process or apparatus, but also gas generated in a process carried out in a previous stage of the process or gas generated in an apparatus arranged in a previous stage of the apparatus and (simply) passing through the process or apparatus and being released (emitted).Whether the gas discharged from a process or apparatus is gas generated in the process or apparatus or gas generated in a process or apparatus in a previous stage of the process or apparatus can be clearly understood by taking into consideration common technical knowledge related to the nitric acid production process. In the present invention and this specification, when describing content, physical properties, etc., by indicating a numerical range, if the upper and lower limits of the numerical range are described separately, any of the upper and lower limits can be appropriately combined to form a specific numerical range. On the other hand, when describing multiple numerical ranges expressed using "to", the upper and lower limits forming the numerical range are not limited to the specific combination written before and after "to" as a specific numerical range, but can be a numerical range obtained by appropriately combining the upper and lower limits of each numerical range. Note that in the present invention and this specification, a numerical range expressed using "to" means a range that includes the numerical values ​​written before and after "to" as the upper and lower limits.

[0016] [[Method A for Decomposing Nitrous Oxide]] One embodiment of the method for decomposing nitrous oxide of the present invention (sometimes simply referred to as "decomposition method A of the present invention" in the present invention) comprises a step of decomposing nitrous oxide by contacting a nitrous oxide-containing gas containing nitrous oxide, nitric oxide, and nitrogen dioxide with a catalyst (sometimes referred to as a "nitrous oxide decomposition catalyst" in the present invention) described below, which contains at least one catalyst selected from the group consisting of ruthenium and ruthenium compounds. In decomposition method A of the present invention, a nitrous oxide-containing gas having a total content of nitric oxide and nitrogen dioxide of 0.0001 to 0.35 mol % is used as the nitrous oxide-containing gas. Using a nitrous oxide-containing gas containing nitric oxide and nitrogen dioxide in this total content maintains the catalytic activity of the nitrous oxide decomposition catalyst (suppressing deactivation and deterioration of the nitrous oxide decomposition catalyst), and enables the nitrous oxide in the nitrous oxide-containing gas to be decomposed into nitrogen molecules (usually nitrogen gas) and oxygen molecules (usually oxygen gas) over an extended period of time (continuously). The decomposition method A of the present invention may be used as a decomposition method applied (implemented) or incorporated into a nitric acid production plant or an (industrial) production method of nitric acid, or may be used simply as a method for decomposing nitrous oxide separately from a nitric acid production plant or a nitric acid production method. When applied as a decomposition method applied (implemented) or incorporated into a nitric acid production plant or an (industrial) production method of nitric acid, the application is not particularly limited, and examples include various chemical production plants described below.

[0017] [catalyst] The catalyst used in decomposition method A of the present invention is a nitrous oxide decomposition catalyst having the function of decomposing nitrous oxide, and is a catalyst in which a component other than a first component such as ruthenium that exhibits catalytic activity (a second component or silicon oxide) is present on the surface, near the surface, or within the pores of a support. The nitrous oxide decomposition catalyst preferably contains titanium oxide as a component constituting the support, and more preferred catalysts include catalysts I and II below. Catalyst I: A catalyst selected from the group consisting of ruthenium and ruthenium compounds is mounted on a support containing titanium oxide. A first component (hereinafter also referred to as the first supported component) containing at least one of the following: and antimony, antimony compounds, cerium, cerium compounds, and zirconium. a zirconium compound, a silicon compound, and a silicon compound; and a second component (hereinafter also referred to as the second supported component) containing at least one of the above-mentioned compounds. Catalyst Catalyst II: The first supported component is supported on a support containing titanium oxide and silicon oxide. catalyst In catalyst II, the first and second supported components are preferably supported on a carrier containing titanium oxide and silicon oxide, and for convenience, this catalyst is referred to as "catalyst II (preferred embodiment)." In catalyst II, silicon oxide is preferably present (dispersed) on or near the surface of primary particles of titanium oxide.

[0018] In the present invention, the term "supported component" collectively refers to components (elements, compounds, etc.) that are supported on a carrier constituting a catalyst, and examples thereof include the first supported component, second supported component, and third supported component described below. Furthermore, the term "catalyst in which a supported component is supported on a carrier" refers to a catalyst in which the supported component is attached to the surface and / or inside the pores of the carrier. The catalyst used in decomposition method A of the present invention contains at least one selected from the group consisting of ruthenium and ruthenium compounds as the first supported component supported on the carrier, from the viewpoint of the balance between catalytic activity and cost.

[0019] <Supported components> Catalyst I contains, as supported components supported on a carrier described below, a first supported component having the ability to decompose nitrous oxide (catalytic activity) and a second supported component different from the first supported component. The supported components supported by catalyst I may also contain a third supported component that does not fall into either the first supported component or the second supported component. On the other hand, catalyst II includes a first supported component having the ability to decompose nitrous oxide as a supported component supported on a carrier described below. The supported component supported by catalyst II preferably includes a second supported component different from the first supported component, and may include a third supported component that does not fall into either the first supported component or the second supported component. Each supported component will be explained below.

[0020] (First supported component) The first supported component carried by Catalyst I and Catalyst II contains at least one selected from the group consisting of ruthenium and ruthenium compounds. The number of types of first supported components carried by each catalyst is not particularly limited as long as it is one or more, and can be, for example, 1 to 4. - Ruthenium compounds - The ruthenium compound is not particularly limited, and examples thereof include ruthenium oxide, ruthenium hydroxide, ruthenium nitrate, ruthenium chloride, ruthenic acid, chlororuthenate, chlororuthenate hydrate, salts of ruthenic acid, ruthenium oxychloride, salts of ruthenium oxychloride, ruthenium ammine complexes, chlorides of ruthenium ammine complexes, ruthenium bromide, ruthenium carbonyl complexes, ruthenium organic acid salts, and ruthenium nitrosyl complexes. Ruthenium oxides include RuO2. Ruthenium hydroxide includes Ru(OH)3. Ruthenium nitrate includes Ru(NO3)3. Ruthenium chloride includes RuCl3, RuCl3 hydrate, and the like. Ruthenic acids include H2RuO4.

[0021] Examples of chlororuthenates include K3RuCl6, [RuCl6] 3- Salts with an anion such as K2RuCl6 and (NH4)2RuCl6, [RuCl6] 2- Examples of salts include those having the anion As the chlororuthenate hydrate, [RuCl5(H2O)4] 2- RuCl2(H2O)4 is a hydrated salt with the anion RuCl2(H2O)4. + Examples include salt hydrates with the cation Ruthenic acid salts include Ru VI O4 2- (tetraoxoruthenate(VI) ion) salt and Ru VII O4 - (perruthenate ion, tetraoxoruthenate (VII) ion) and salts thereof. Examples of cations that form salts include cations of alkali metal elements, cations of alkaline earth metal elements, Ag +, ammonium cations, etc. Among these, alkali metal salts of ruthenic acid (salts of Li, Na, K, Rb, and Cs) are preferred, with Na or K ruthenic acid salts being more preferred. Specific examples include Na2RuO4 and K2RuO4. Ruthenium oxychlorides include Ru2OCl4, Ru2OCl5, Ru2OCl6, and the like. Ruthenium oxychloride salts include K2Ru2OCl 10 , Cs2Ru2OCl4, etc. Ruthenium ammine complexes include [Ru(NH3)6] 2+ , [Ru(NH3)6] 3+ , [Ru(NH3)5H2O] 2+ Examples include complexes with complex ions such as The chloride of the ruthenium ammine complex is [Ru(NH3)5Cl] 2+ Examples of complexes with this ion include [Ru(NH3)6]Cl2, [Ru(NH3)6]Cl3, and [Ru(NH3)6]Br3. Ruthenium bromide includes RuBr3, RuBr3 hydrate, and the like. Ruthenium carbonyl complexes include Ru(CO)5 and Ru3(CO) 12 Examples include: Examples of ruthenium organic acid salts include [Ru3O(OCOCH3)6(H2O)3]OCOCH3 hydrate and Ru2(RCOO)4Cl (R=alkyl group having 1 to 3 carbon atoms). Ruthenium nitrosyl complexes include K2[RuCl5NO)], [Ru(NH3)5(NO)]Cl3, [Ru(OH)(NH3)4(NO)](NO3)2, and Ru(NO)(NO3)3.

[0022] The ruthenium compound is preferably ruthenium oxide, ruthenium nitrate, ruthenium chloride, ruthenium bromide, a salt of ruthenium acid, or a ruthenium nitrosyl complex, more preferably contains ruthenium oxide, and even more preferably is ruthenium oxide. The ruthenium compound may be any compound containing ruthenium as one of its constituent elements, or may be a compound containing an element (metal or nonmetal) other than ruthenium. For example, ruthenium oxide may be any oxide containing ruthenium as one of its constituent elements, and includes not only an oxide of ruthenium (RuO2) but also a composite oxide containing ruthenium and an element (metal or nonmetal) other than ruthenium. The same applies to (metallic) ruthenium, and includes, in addition to metallic ruthenium, alloys of ruthenium with metals other than ruthenium.

[0023] The content of the ruthenium element constituting the first supported component in the catalyst (based on 100% of the total mass of the catalyst) is not particularly limited in either Catalyst I or Catalyst II and may be set appropriately, but is preferably 0.5 to 10% by mass, more preferably 0.5 to 5% by mass, and even more preferably 1 to 3% by mass, for example.

[0024] (Second supported component) The second supported component, which is supported by Catalyst I and preferably supported by Catalyst II, contains at least one selected from the group consisting of antimony, antimony compounds, cerium, cerium compounds, zirconium, zirconium compounds, silicon, and silicon compounds. The second supported component refers to a component that is substantially free of ruthenium as a constituent element. In the present invention, "substantially free" includes cases where ruthenium is unavoidably present. When a catalyst supporting the second supported component is used for the decomposition of nitrous oxide, the decrease in catalytic activity of the catalyst can be suppressed even over a long period of time, and nitrous oxide can be decomposed at a high decomposition rate. This second supported component may or may not have the ability to decompose nitrous oxide (catalytic activity). The number of types of second supported components supported by catalyst I is not particularly limited as long as it is one or more, and can be, for example, 1 to 8 types, preferably 1 to 4 types. On the other hand, the number of types of second supported components supported by catalyst II is not particularly limited and can be, for example, 0 to 8 types, preferably 1 to 6 types, more preferably 1 to 4 types.

[0025] The antimony, cerium, zirconium and silicon compounds are not particularly limited as long as they contain these elements, and any suitable compounds can be used. For example, the same compounds as the above-mentioned ruthenium compounds (compounds in which the ruthenium element in the ruthenium compound is substituted with at least one of antimony element, cerium element, zirconium element and silicon element) can be mentioned.

[0026] - Antimony compounds - Examples of antimony compounds include antimony oxide, antimony sulfate, antimony chloride, and salts of antimonic acid, with antimony oxide and antimony chloride being preferred. Examples of antimony oxides include Sb2O3, Sb2O4, and Sb2O5. Examples of antimony sulfate include Sb2(SO4)3. Antimony chlorides include SbCl3. Examples of antimonic acid salts include NaSbO3.

[0027] - Cerium compounds - Examples of the cerium compound include cerium oxide, cerium hydroxide, cerium nitrate, cerium chloride, ceric acid salts, cerium sulfate, and cerium carbonate, with cerium oxide, cerium nitrate, cerium chloride, and cerium sulfate being preferred. Examples of cerium oxide include CeO2 and Ce2O3. Examples of cerium hydroxide include CeO2·2H2O. Examples of cerium nitrate include Ce(NO3)3·6H2O. Examples of cerium chloride include CeCl3·7H2O. Examples of ceric acid salts include Ce(NH4)2(NO3)6 hydrate. Examples of cerium sulfate include Ce(SO4)2·4H2O. Examples of cerium carbonate include Ce2(CO3)3·8H2O.

[0028] - Zirconium compounds - Examples of the zirconium compound include zirconium oxide, zirconium hydroxide, zirconium oxynitrate, zirconium chloride, zirconium sulfate, zirconium acetate, and zirconium acetylacetonate, with zirconium oxide, zirconium oxynitrate, zirconium chloride, and zirconium sulfate being preferred. Zirconium oxide includes ZrO2. Zirconium hydroxide includes Zr(OH)4. Examples of zirconium oxynitrate include ZrO(NO3)2·2H2O. Zirconium chloride includes ZrCl3, ZrCl4, and the like. Examples of zirconium sulfate include Zr(SO4)2·4H2O. Zirconium acetylacetonate includes Zr(C5H7O2)4.

[0029] - Silicon compounds - Examples of silicon compounds include silicon oxide, silicon chloride, silicic acid salts, and silicon alkoxides, with silicon oxide and silicic acid salts being preferred. Examples of silicon oxide include SiO2. Silicon chlorides include SiCl4 and the like. Examples of silicic acid salts include salts of orthosilicic acid, pyrosiliic acid, metasilicic acid, etc., and specific examples include Na2SiO3, Na4SiO4, Na2Si2O5, Na2Si4O9, etc. Examples of silicon alkoxides include Si(OC2H5)4, Si(OC3H7)4, and Si(OC4H9)4.

[0030] A preferred compound among the second supported components is at least one selected from zirconium and zirconium compounds, while a preferred type of compound among the second supported components is an oxide containing at least one selected from the group consisting of antimony oxide, cerium oxide, zirconium oxide, and silicon oxide, more preferably containing at least zirconium oxide, and even more preferably zirconium oxide.

[0031] The content (total content) of the metal elements (antimony, cerium, zirconium, and silicon) constituting the second supported component in the catalyst (based on 100% of the total mass of the catalyst) is not particularly limited in either catalyst I or catalyst II and may be set appropriately. For example, the content of the metal elements in catalyst I is preferably 0.003 to 49% by mass, more preferably 0.01 to 14% by mass, and even more preferably 0.4 to 3.7% by mass. On the other hand, the content of the metal elements in catalyst II is preferably 0 to 49% by mass, more preferably 0 to 14% by mass, and even more preferably 0 to 3.7% by mass. The content of each metal element in the catalyst can be appropriately set in consideration of the total content. In both Catalyst I and Catalyst II, in order to suppress a decrease in catalytic activity (a decrease in the decomposition rate of nitrous oxide), the molar ratio of the content (total content) of the metal elements constituting the second supported component to the content of the ruthenium element constituting the first supported component (content (moles) of metal elements / content (moles) of ruthenium element) is preferably 0.1 to 5, more preferably 0.1 to 3, and even more preferably 0.1 to 2.1.

[0032] (Third supported component) The third supported component that may be supported by Catalyst I and Catalyst II is not particularly limited as long as it is a component that does not fall into either the first or second supported component, and examples thereof include metals such as aluminum, niobium, tin, copper, iron, cobalt, nickel, vanadium, chromium, molybdenum, tungsten, manganese, tellurium, and sodium, as well as compounds of these metals (preferably oxides or sulfates of the above metals). A preferred third supported component is at least one oxide or sulfate selected from the group consisting of aluminum oxide, niobium oxide, manganese oxide, tellurium oxide, tin oxide, sodium oxide, and sodium sulfate. The content (total content) of the metal elements constituting the third supported component in the catalyst (based on 100% of the total mass of the catalyst) is not particularly limited in either Catalyst I or Catalyst II, and may be set appropriately.

[0033] <Carrier> The support constituting catalyst I contains titanium oxide and is substantially free of silicon oxide, but may contain other compounds as described below. On the other hand, the support constituting catalyst II is sufficient as long as it contains titanium oxide and silicon oxide, and may contain other compounds as described below. In the present invention, the term "support" refers to primary particles of titanium oxide or a compound containing titanium oxide and / or secondary particles formed by aggregation of the primary particles, molded into a desired shape. The term "surface of the support" refers to the surface of the molded product, and the term "pores of the support" refers to one or more of the pores of the primary particles, the gaps between the primary particles in the secondary particles, and the gaps between the secondary particles in the aggregate of the secondary particles. In the present invention, the crystalline form of the titanium oxide constituting the carrier is not particularly limited, and may be any of rutile crystalline form, anatase crystalline form, and brookite crystalline form. In the present invention, the titanium oxide constituting the carrier preferably contains titanium oxide in the rutile crystalline form. From the viewpoint of catalytic activity, the content of rutile crystalline form titanium oxide in the titanium oxide contained in the carrier is preferably 20% by mass or more, more preferably 30% by mass or more, even more preferably 80% by mass or more, and particularly preferably 90% by mass or more, where the total amount of titanium oxide contained in the carrier is 100% by mass.

[0034] In the present invention, titanium oxide containing rutile crystalline titanium oxide refers to titanium oxide containing rutile crystals, as determined by X-ray diffraction analysis to measure the ratio of rutile crystals to anatase crystals. Various X-ray sources are used, including copper Kα radiation. When copper Kα radiation is used, the ratios of rutile crystals and anatase crystals are determined using the diffraction peak intensity of the (110) plane at 2θ=27.5° and the diffraction peak intensity of the (101) plane at 2θ=25.3°, respectively. The support used in the present invention is a support having a rutile crystal peak intensity and an anatase crystal peak intensity, or a support having a rutile crystal peak intensity. That is, the support may have both a rutile crystal diffraction peak and an anatase crystal diffraction peak, or it may have only a rutile crystal diffraction peak.

[0035] The support constituting catalyst II contains silicon oxide, and preferably contains silicon oxide on the surface (including near the surface) of the support and / or on the inner surfaces (including near the inner surfaces) of the pores. When catalyst II, whose support contains silicon oxide, is used for the decomposition of nitrous oxide, the catalytic activity of catalyst II can be prevented from decreasing even over a long period of time, and nitrous oxide can be decomposed at a high decomposition rate. Silicon dioxide is preferred as the silicon oxide. The silicon oxide contained in the carrier constituting catalyst II is preferably silicon oxide derived from colloidal silica (for example, a dried product of colloidal silica in particulate or granular form), as this facilitates the production of the carrier. The particle size of silicon oxide in colloidal silica is not particularly limited, but is preferably 5 to 45 nm, more preferably 5 to 22 nm, in order to increase the number of silicon oxide particles. The particle size of silicon oxide is the average particle size measured as follows. The average particle size (d) [nm] is calculated by the specific surface area (S) [m ] determined by the BET method of a powder obtained by evaporating the water contained in the colloidal silica to dryness. 2 / g] using the following formula, where ρ is the true density of silicon oxide, 2.2 [g / cm 3 ] value is used. d=6000 / (S×ρ) The particle size of the silicon oxide present in the carrier of catalyst II does not necessarily have to maintain the same particle size as the silicon oxide in the colloidal silica, but it is preferable that it does.

[0036] The content of silicon oxide in the carrier constituting catalyst II (based on 100% of the total mass of the carrier) is not particularly limited and may be set as appropriate. For example, in order to prevent a decrease in catalytic activity and maintain it for a long period of time, the content is preferably 1 to 20% by mass, more preferably 1 to 10% by mass, and even more preferably 1 to 5% by mass.

[0037] In the present invention, whether silicon oxide is the second supported component (Catalyst I) or whether silicon oxide constitutes the support (Catalyst II) can be confirmed by observing the catalyst with a scanning electron microscope or a scanning transmission electron microscope.

[0038] Examples of other compounds that may be contained in the carriers constituting Catalyst I and Catalyst II include metal oxides or metal sulfates other than titanium oxide and silicon oxide, composite oxides of titanium oxide and other metal oxides, composite oxides of titanium oxide, silicon oxide and other metal oxides, mixtures of titanium oxide and other metal oxides or metal sulfates, and mixtures of titanium oxide, silicon oxide and other metal oxides or metal sulfates. Examples of the metal oxides include aluminum oxide, zirconium oxide, cerium oxide, and sodium oxide. Examples of the metal sulfates include sodium sulfate.

[0039] Titanium oxide prepared by a known method can be used, or commercially available products can also be used. Examples of methods for preparing rutile crystalline titanium oxide include the following methods. A method in which titanium tetrachloride is dissolved dropwise in ice-cold water, neutralized with an aqueous ammonia solution at a temperature of 20°C or higher to produce titanium hydroxide (orthotitanic acid), and the resulting precipitate is then washed with water to remove chloride ions and calcined at a temperature of 600°C or higher (Catalyst Preparation Chemistry, 1989, p. 211, Kodansha); A method in which a reactive gas is prepared by passing an oxygen-nitrogen mixed gas through a titanium tetrachloride evaporator, and then introduced into a reactor to react at 900°C or higher (Catalyst Preparation Chemistry, 1989, p. 89, Kodansha); A method of hydrolyzing titanium tetrachloride in the presence of ammonium sulfate and then calcining it (for example, Catalysis Engineering Lecture 10: Handbook of Elemental Catalysts, 1978, p. 254, Chijin Shokan); A method of calcining titanium oxide in anatase crystal form (e.g., Metal Oxides and Complex Oxides, 1980, p. 107, Kodansha); A method of thermally hydrolyzing an aqueous titanium chloride solution; and A method in which an aqueous solution of titanium compounds such as titanium sulfate or titanium chloride is mixed with rutile crystalline titanium oxide powder, followed by thermal hydrolysis or alkaline hydrolysis, and then calcination at a temperature of around 500°C.

[0040] The silicon oxide-containing carrier constituting catalyst II can be produced by the process for obtaining a carrier described in the method for producing catalyst II below.

[0041] The support can be obtained by forming titanium oxide, etc. into a desired shape. When the support contains titanium oxide and other compounds (e.g., silicon oxide), it can be obtained by forming a mixture of titanium oxide and other compounds into a desired shape.

[0042] The shape of the catalyst (support) is not particularly limited and can be set appropriately. In a preferred embodiment of the present invention, the catalyst may be formed into a honeycomb shape (honeycomb structure). The honeycomb-shaped catalyst in a preferred embodiment is usually used as is (in a honeycomb structure), but it may also be used after being pulverized and the pulverized material is classified. In this case, the size may be approximately the same as that of the spherical particles described below. On the other hand, in another preferred embodiment of the present invention, various shapes can be employed. Such shapes are not particularly limited, and examples include pellet shapes such as spherical, cylindrical, and ring shapes, monolithic and corrugated shapes, and granular and fine particles of an appropriate size obtained by pulverizing and classifying the formed catalyst after molding. The catalyst is preferably in the form of pellets such as spherical, cylindrical, and ring shapes, monolithic, corrugated, or granular shapes, but from the viewpoint of the decomposition efficiency of nitrous oxide, ring-shaped pellet shapes are more preferred.

[0043] In the present invention, the honeycomb structure refers to the "honeycomb structure" that is commonly used, for example, as a honeycomb catalyst for purifying exhaust gas, and includes, for example, a structure in which a substrate such as a columnar body is perforated with a plurality of through holes that are densely arranged in the planar direction. The substrate has an appropriate shape selected depending on the shape of the reactor (e.g., a reaction tube) in which the catalyst is packed, the manner in which the honeycomb structure is packed into the reactor, and the like, and examples thereof include a columnar body, a block body, a plate body, and the like. The shape of the through-holes is not particularly limited, and examples thereof include polygons such as squares and hexagons, circles, ellipses, and the like. The arrangement of the through-holes is not particularly limited, and is determined appropriately taking into account the shape of the holes, and the like. For example, in the planar direction of the substrate (usually a plane perpendicular to the axis), examples thereof include a straight arrangement, a staggered arrangement, and a honeycomb arrangement. Examples of the honeycomb structure include a round hole parallel arrangement, a round hole staggered arrangement, and a round hole honeycomb arrangement, in which the through holes have a circular opening shape; and a square hole parallel arrangement, a square hole staggered arrangement, and a square hole honeycomb arrangement, in which the through holes have a polygonal opening shape.

[0044] When the catalyst is in the form of a powder, such as granules or fine particles, a preferred embodiment is one in which the catalyst is disposed as a washcoat layer on a substrate having a honeycomb structure. The substrate having a honeycomb structure may be made of any material typically used to prepare automotive catalysts, and is typically made of metal or ceramic, such as various stainless steels or cordierite. The substrate typically provides multiple walls to which the washcoat layer is applied and adhered, thereby functioning as a catalyst substrate. The mass of the washcoat layer per unit volume of the substrate having a honeycomb structure is preferably 10 to 200 [g / L], more preferably 30 to 100 [g / L].

[0045] The size of the catalyst (carrier) is not particularly limited and can be set appropriately. When the catalyst is in the form of spherical granules, cylindrical pellets, or the like, the catalyst diameter is preferably 10 mm or less from the viewpoint of catalytic activity. Here, the catalyst diameter refers to the diameter of the sphere for spherical granules, the diameter of the cross section for cylindrical pellets, and the maximum cross section diameter for other shapes. When the catalyst is in the form of spherical granules, cylindrical pellets, or the like, the filling volume ratio is preferably 35 to 74 volume % when the catalyst is filled in the catalyst filling region of a reactor (reaction tube) to form a catalyst packed bed.

[0046] In the above-described preferred embodiment of the present invention and another preferred embodiment of the present invention (substrate when the catalyst is in powder form), the dimensions of the honeycomb structure (catalyst) preferably have a volume fraction of 35 to 50 volume %, more preferably 38 to 50 volume %, in order to further increase the decomposition efficiency of nitrous oxide. The volume fraction of the honeycomb structure (catalyst) refers to the ratio (percentage) of the actual volume of the honeycomb structure to the apparent volume of the honeycomb structure. The apparent volume and actual volume can be calculated by standard methods from the dimensions of the honeycomb structure. The honeycomb structure (catalyst) preferably has a filling volume ratio of 35 to 50 volume %, and more preferably has a filling volume ratio of 38 to 50 volume %, when packed in the catalyst packed region of the reactor (reaction tube) to form a catalyst packed layer, in order to further increase the decomposition efficiency of nitrous oxide. The filling volume ratio refers to the ratio (percentage) of the actual volume of the honeycomb structure to the volume of the catalyst packed region of the reactor (reaction tube). The dimensions of the catalyst can be set, for example, to dimensions that fit the catalyst filling region of the reactor (reaction tube), taking into consideration the volume ratio or filling volume ratio, and usually to outer dimensions that are approximately the same as the inner dimensions (inner diameter and length) of the catalyst filling region.

[0047] In the honeycomb structure, the diameter of the through holes (also referred to as cell size), the distance between the through holes (also referred to as inner wall thickness), and the porosity are appropriately determined, and in the present invention, they are determined taking into consideration the (filling) volume ratio, etc. For example, the cell size can be 1 to 3 mm, and preferably 1 to 2 mm. The inner wall thickness can be 0.1 to 2 mm, and preferably 0.2 to 1 mm. The porosity ([(total area of ​​through holes opening on the surface of the honeycomb structure) / (apparent surface area of ​​the honeycomb structure)] × 100 (%)) can be 50 to 65%, and preferably 50 to 62%. The cell size and inner wall thickness can be measured by observing and measuring the surface of the honeycomb shape. The porosity can be calculated from the measured total area of ​​the through holes and the calculated apparent surface area.

[0048] In the present invention, a plurality of catalysts (number of catalysts) or a combination of a plurality of types of catalysts can be used. In a preferred embodiment of the present invention, the catalyst includes a honeycomb structure that satisfies the above-mentioned filling volume ratio by combining multiple catalysts (a honeycomb structure in which the multiple catalysts packed in the catalyst packing region collectively satisfy the above-mentioned filling volume ratio), but it is preferable that each catalyst alone has a honeycomb structure that satisfies the above-mentioned filling volume ratio. In this case, the above-mentioned filling volume ratio is synonymous with the catalyst packing ratio (catalyst packing ratio of the catalyst alone). Furthermore, when multiple catalysts are combined, it is preferable that the honeycomb structure of each catalyst satisfy the above-mentioned volume ratio. In another preferred embodiment of the present invention (when the catalyst is in the form of powder), the catalyst disposed as the washcoat layer may be a combination of a plurality of catalysts.

[0049] In the decomposition method A of the present invention, the catalyst may be used after being diluted with an inert substance.

[0050] <Catalyst manufacturing method> The catalyst used in decomposition method A of the present invention can be produced by various known production methods, for example, by impregnating a support containing titanium oxide with a solution containing the component to be supported, allowing the component to adhere to the support, and then drying. The solvent in the solution containing the component to be supported is not particularly limited, but water, ethanol, etc. can be used. After drying, the catalyst may be calcined. When the catalyst contains ruthenium oxide, it can be obtained, for example, by a method comprising the steps of impregnating a support containing titanium oxide with a solution containing a ruthenium halide or a ruthenium nitrosyl complex to support the ruthenium halide or ruthenium nitrosyl complex on the support, drying the support in which the ruthenium halide or ruthenium nitrosyl complex is supported on the support, and calcining the dried product.

[0051] When producing catalyst I, the solutions containing the supported components can be solution A containing a first supported component or a first supported component raw material (also referred to as first component raw material) capable of forming the first supported component, and solution B containing a second supported component or a second supported component raw material (also referred to as second component raw material) capable of forming the second supported component, or a mixed solution of solution A and solution B can also be used. When producing catalyst II, the above solution A can be used as the solution containing the components to be supported. It is also preferable to use solution A and solution B in combination, or to use a mixed solution of solution A and solution B.

[0052] A preferred method for producing catalyst II will be specifically described below, but catalyst I can also be produced by using a carrier raw material mixture that does not contain silicon oxide in the step of obtaining the carrier described below. A preferred method for preparing Catalyst II comprises the following steps: In a preferred production method, after step 1 is performed, either step 2 or step 3 may be performed first, and step 2A described below may be performed instead of step 2 and step 3. Step 1: A support obtained by extruding a support raw material mixture containing titanium oxide, silicon oxide, and water A step of calcining the precursor to obtain a support Step 2: Ruthenium is added to the carrier obtained in Step 1 or the carrier on which the second component raw material is supported in Step 3. A step of supporting the first component raw material containing the compound Step 3: The carrier obtained in step 1 or the carrier carrying the first component raw material in step 2 is treated with antimony. compounds, cerium compounds, zirconium compounds and silicon compounds A step of supporting a second component raw material containing at least one selected from the following: Step 4: A catalyst in which the first component raw material and the second component raw material are supported on the support obtained in step 1 is Calcining the catalyst precursor Step 2A: The support obtained in step 1 is mixed with a first component raw material containing a ruthenium compound and an antimony compound. A compound consisting of silicon compounds, cerium compounds, zirconium compounds and silicon compounds. a step of supporting a second component raw material containing at least one selected from the group

[0053] (Process 1) The titanium oxide used in step 1 is not particularly limited, and as described above, those produced by various methods or commercially available products can be used. The silicon oxide used in step 1 is not particularly limited, and silicon oxide produced by various methods or commercially available products can be used. However, in terms of facilitating the production of the carrier, it is preferably used as an aqueous dispersion, and colloidal silica is more preferably used. The particle size of silicon oxide in colloidal silica is not particularly limited, but in terms of increasing the number of silicon oxide particles, it is preferably 5 to 45 nm, more preferably 5 to 22 nm. The particle size of silicon oxide is the average particle size measured by the above-mentioned method. An example of a commercially available colloidal silica is the one (Snowtex ST-CM) used in the Examples described below. Known organic binders can also be used in step 1.

[0054] In step 1, titanium oxide, silicon oxide, and water are mixed to prepare a carrier raw material mixture. The form of this carrier raw material mixture is not particularly limited, and it can be a liquid mixture such as a solution or a slurry, or a powder mixture, but it is preferably a clay-like mixture such as a clay. The mixing ratio of titanium oxide, silicon oxide, and water is not particularly limited and can be set as appropriate, but it is preferable that the mixing ratio of titanium oxide and silicon oxide is set as appropriate so that the silicon oxide content in the carrier is as described above. The mixing ratio of water is not particularly limited and can be set as appropriate, and it is preferable that the mixture of carrier raw materials is set as a clay-like mixture. Here, water may be mixed separately from titanium oxide and silicon oxide, or the water in the colloidal silica used as silicon oxide may be used. Titanium oxide, silicon oxide, and water can be mixed using a conventional mixer or kneader. The mixing conditions are not particularly limited, but for example, the mixing temperature can be 5 to 40°C, and the mixing time can be 1 to 30 minutes.

[0055] In step 1, the prepared carrier raw material mixture is then extruded to obtain a carrier precursor. The shape of the carrier precursor is not particularly limited and can be formed into any suitable shape, and it is preferable to form it into the above-mentioned honeycomb structure. The extrusion molding of the carrier raw material mixture can be carried out using a conventional extruder, such as a vacuum kneading extruder or a hydraulic extruder. In particular, when producing a catalyst having a honeycomb structure, it is preferable to use a vacuum kneading extruder. The extrusion molding conditions are not particularly limited, but for example, the kneading or extrusion temperature can be set to 5 to 40°C. In this way, a carrier precursor can be obtained, and if necessary, the formed body can be dried to obtain a carrier precursor.

[0056] In step 1, the support precursor is then calcined to obtain the support. The support precursor can be calcined by a conventional method using various heaters. The calcination conditions are not particularly limited and may be the same as those used for calcining titanium oxide or silicon oxide. For example, the calcination temperature may be 250°C or higher, preferably 400 to 900°C, and the calcination time may be 2 to 120 hours.

[0057] (Process 2) In step 2, a first component raw material containing a ruthenium compound is supported on a support. The carrier used in this step differs depending on the order of steps 2 and 3. That is, when step 2 is performed prior to step 3, or when step 2 is performed simultaneously with step 3 (step 2A), the carrier (unsupported carrier) obtained in step 1 is used. On the other hand, when step 2 is performed after step 3, the carrier supported with the second component raw material in step 3 is used. The first component raw material used in step 2 contains a ruthenium compound. This ruthenium compound may be any compound that will become the first supported component, such as ruthenium or a ruthenium compound, in the produced catalyst. It may be the first supported component itself or a precursor compound leading to the first supported component. Examples of the ruthenium compound used in step 2 include various known compounds, such as the above-mentioned first supported component, as well as precursor compounds to the first supported component, such as Ru(NO3)3, RuCl3, RuCl3 hydrate, Na2RuO4, K2RuO4, and Ru(NO)(NO3)3. The first component raw material is usually used in the form of an aqueous solution. The content (concentration) of the first component raw material is not particularly limited, but is preferably set within a range that satisfies the above-mentioned content of ruthenium element in the catalyst, for example, more preferably 1 to 40 mass% and even more preferably 2 to 10 mass% based on ruthenium element. The aqueous solution used in step 2 may contain components other than the first supported component and the second supported component. Examples of such components include the above-mentioned third supported component, as well as organic solvents such as alcohols that allow the aqueous solution to be well absorbed into the carrier. In addition to the aqueous solution, a solution of an organic solvent such as alcohol can also be used.

[0058] In step 2, the carrier is brought into contact with the first component raw material to support the first component raw material on the carrier. The contacting method and conditions are not particularly limited and can be set appropriately. The contacting method can be any of various methods for supporting various components on a carrier as a catalyst production method, including, for example, a method of immersing a carrier in the above aqueous solution and a coating method of spraying or applying the above aqueous solution to a carrier. The amount of the aqueous solution used is not particularly limited, but is preferably set within a range that satisfies the above-mentioned content of ruthenium element in the catalyst; for example, 0.1 to 10 mL is more preferred, and 0.2 to 2 mL is even more preferred, per gram of carrier. Examples of contacting conditions include contacting at 5 to 40°C (preferably 10 to 30°C) for 1 to 300 minutes (preferably 5 to 180 minutes). In step 2, the carrier impregnated with the first component raw material can be dried by a conventional method. In this way, the first component raw material can be supported or adsorbed on the carrier containing titanium oxide and silicon oxide.

[0059] (Step 3) In step 3, a second component raw material containing at least one compound selected from the group consisting of antimony compounds, cerium compounds, zirconium compounds, and silicon compounds is supported on a support. The carrier used in this step differs depending on the order of steps 2 and 3. That is, when step 3 is performed prior to step 2, or when step 2 is performed simultaneously with step 3 (step 2A), the carrier (unsupported carrier) obtained in step 1 is used. On the other hand, when step 3 is performed after step 2, the carrier supported with the first component raw material obtained in step 2 is used. The second component raw material used in step 3 contains an antimony compound, a cerium compound, a zirconium compound, or a silicon compound. These compounds may be compounds that will become the second supported component, such as the antimony compound, in the produced catalyst. They may be the second supported component itself or a precursor compound leading to the second supported component. Examples of the compounds used in step 3 include various known compounds, such as SbCl3, Ce(NO3)3·6H2O, ZrO(NO3)2·2H2O, and Si(OC2H5)4, which can also be used as precursor compounds for the second supported component. The second component raw material is usually used in the form of an aqueous solution. The content (concentration) of the second component raw material is not particularly limited, but is preferably set within a range that satisfies the above-mentioned content of the second component raw material (element) in the catalyst, for example, more preferably 0.01 to 20 mass% on an elemental basis, and even more preferably 1 to 15 mass%. The aqueous solution used in step 3 may contain components other than the first supported component and the second supported component, and examples of such components include the above-mentioned third supported component and the above-mentioned organic solvent. In addition to the aqueous solution, a solution of an organic solvent such as alcohol can also be used.

[0060] In step 3, the carrier is brought into contact with the second component raw material to support the second component raw material on the carrier. The contacting method and conditions are not particularly limited and are the same as those described in the above step 2. In particular, the amount of the aqueous solution containing the second component raw material used is not particularly limited, but is preferably set within a range that satisfies the above content of the second component raw material (element) in the catalyst, and is, for example, more preferably 0.1 to 10 mL, and even more preferably 0.2 to 2 mL, per 1 g of the carrier. In step 3, the carrier impregnated with the second component raw material can be dried by a conventional method. In this way, the second component raw material can be supported or adsorbed on the carrier containing titanium oxide and silicon oxide.

[0061] (Process 2A) In a preferred method for producing catalyst II, steps 2 and 3 can be carried out simultaneously. In this case, step 2A is carried out in place of steps 2 and 3, in which the first and second raw component materials are supported on the support obtained in step 1. The first and second component raw materials used in step 2A are as described in steps 2 and 3. The first and second component raw materials are usually used in the form of an aqueous solution. The contents (concentrations) of the first and second component raw materials are not particularly limited and are as explained in steps 2 and 3. The aqueous solution used in step 2A may contain components other than the first and second supported components, such as the third supported component and the organic solvents mentioned above. In addition to the aqueous solution, a solution of an organic solvent such as alcohol can also be used. In step 2A, the carrier is contacted with the first and second component raw materials to support the first and second component raw materials on the carrier. The contacting method and conditions are not particularly limited and are the same as the contacting method and conditions described in step 2 above. In step 2A, the carrier impregnated with the first and second component raw materials can be dried by a conventional method. In this way, the first component raw material and the second component raw material can be supported or adsorbed on the carrier containing titanium oxide and silicon oxide.

[0062] (Step 4) In step 4, the catalyst precursor obtained in step 2 and step 3 or step 2A, in which the first and second component raw materials are supported on a carrier, is calcined. The catalyst precursor can be calcined by a conventional method, and various heaters can be used. The calcination conditions are not particularly limited, and the calcination conditions applied to the calcination of the supported component can be applied without particular limitation. For example, the calcination temperature can be 100 to 600°C, preferably 200 to 400°C, and the calcination time can be 1 to 30 hours, preferably 1 to 10 hours.

[0063] (Other processes) In a preferred method for producing catalyst II, steps other than the above steps 1 to 4 may also be performed. For example, in steps 2, 3, and 2A, there may be mentioned a step of drying the support that has been brought into contact with the aqueous solution, a step of crushing or disintegrating the catalyst obtained in step 4, and a step of adjusting the shape or size of the catalyst obtained in step 4 (for example, a step of classifying the crushed or disintegrated catalyst).

[0064] The method for producing a nitrous oxide decomposition catalyst having the above steps can easily produce a nitrous oxide decomposition catalyst suitable for use in decomposition method A of the present invention. Specifically, by carrying out steps 1 to 4 described above, catalyst II (preferred embodiment) can be produced in which the first supported component and the second supported component are supported on or adsorbed onto a support containing titanium oxide and silicon oxide. If step 3 or step 2A is not carried out, catalyst II can be produced in which the first supported component is supported or adsorbed on a support containing titanium oxide and silicon oxide. Furthermore, if the carrier raw material mixture is prepared in step 1 above without mixing silicon oxide with titanium oxide, catalyst I can be produced in which the first supported component and the second supported component are supported or adsorbed on a carrier containing titanium oxide.

[0065] (Formation of washcoat layer) In a preferred embodiment, the catalyst is disposed as a washcoat layer on a substrate having a honeycomb structure. The catalyst produced by the above-described method can be powdered by crushing, disintegrating, or the like, and then disposed on the substrate having a honeycomb structure, preferably as a washcoat layer. The catalyst washcoat layer can be applied and adhered to the substrate surface by any known means in the art. For example, the washcoat layer can be formed by coating a substrate with a catalyst-containing slurry prepared in a liquid such as water or alcohol to a specific solids content (e.g., about 30 to about 90% by mass) by spraying, immersion, or other means, and then drying the slurry. The catalyst used to prepare the slurry can be the catalyst produced by the above-described method, which has been powdered by crushing, disintegrating, or the like.

[0066] [Nitrous oxide-containing gas] Decomposition method A of the present invention uses a nitrous oxide-containing gas containing at least one of nitrous oxide, nitric oxide, and nitrogen dioxide, where the total content of nitric oxide and nitrogen dioxide in the nitrous oxide-containing gas is 0.0001 to 0.35 mol %. When the nitrous oxide-containing gas used in decomposition method A of the present invention contains nitric oxide and nitrogen dioxide in a total content of 0.0001 mol % or more and 0.35 mol % or less, the catalytic activity of the nitrous oxide decomposition catalyst can be maintained for a long period of time. On the other hand, when the nitrous oxide-containing gas used in decomposition method A of the present invention contains nitric oxide and nitrogen dioxide in a total content exceeding 0.35 mol %, the catalytic activity of the nitrous oxide decomposition catalyst decreases, and a larger amount of catalyst is required to achieve a sufficient nitrous oxide decomposition rate. The nitrous oxide-containing gas may be any gas containing nitrous oxide, nitric oxide, and nitrogen dioxide, and may contain one or more gases other than the three gases of nitrous oxide, nitric oxide, and nitrogen dioxide. Examples of such gases include ammonia, oxygen, steam (water), nitrogen, carbon dioxide, and inert gases (helium, argon) used as diluents, as well as reducing gases. The nitrous oxide-containing gas preferably contains nitrous oxide, nitric oxide, nitrogen dioxide, oxygen, and steam (water). The nitrous oxide-containing gas may also contain a liquid. In the decomposition method A of the present invention, the nitrous oxide-containing gas is required to be in a gaseous state at least while in contact with the catalyst (under reaction conditions), and may be in a liquid state or a mixture of gas and liquid before contact.

[0067] The content (concentration) and content ratio of each component in the nitrous oxide-containing gas are not particularly limited and can be set as appropriate, but except for components with special effects, it is usually most efficient to use the nitrous oxide-containing gas at approximately the same concentration as the factory-specific value from which the gas is emitted. Therefore, for example, the molar concentration of nitrous oxide in the nitrous oxide-containing gas is generally and preferably 0.002 to 10 mol%, more preferably 0.002 to 1 mol%, and even more preferably 0.002 to 0.5 mol%. In the present invention, the content of each gas in the nitrous oxide-containing gas needs to be substantially satisfied while the nitrous oxide is being decomposed, but it is acceptable for the content to temporarily be unsatisfactory while the gas is in contact with the nitrous oxide decomposition catalyst for a long period of time.

[0068] Within the above range, the total content of nitric oxide and nitrogen dioxide in the nitrous oxide-containing gas is preferably 0.0001 to 0.25 mol %, more preferably 0.0001 to 0.15 mol %, and even more preferably 0.0001 to 0.065 mol %, in order to enable the nitrous oxide decomposition catalyst to exhibit high catalytic activity while highly suppressing deactivation of that high catalytic activity and maintaining catalytic activity for an extended period of time. The total content of nitric oxide and nitrogen dioxide in the nitrous oxide-containing gas used in decomposition method A of the present invention may also be the total content in the "reduced gas" in decomposition method B of the present invention, which will be described later. In order to achieve high catalytic activity and highly suppress deactivation of the catalytic activity, the total content of nitric oxide and nitrogen dioxide in the nitrous oxide-containing gas should be within the range of 0.0001 to 0.35 mol %, and the respective contents of nitric oxide and nitrogen dioxide in the nitrous oxide-containing gas are not particularly limited. For example, the content of nitric oxide in the nitrous oxide-containing gas can be 0.0001 to 0.35 mol%, preferably 0.0001 to 0.25 mol%, more preferably 0.0001 to 0.15 mol%, and even more preferably 0.0001 to 0.065 mol%. The content of nitrogen dioxide in the nitrous oxide-containing gas can be 0.0001 to 0.35 mol%, preferably 0.0001 to 0.25 mol%, more preferably 0.0001 to 0.15 mol%, and even more preferably 0.0001 to 0.065 mol%. The respective contents of nitric oxide and nitrogen dioxide in the nitrous oxide-containing gas can be measured using a nitrogen oxide concentration measuring device (e.g., NOA-7100 manufactured by Shimadzu Corporation), but the total content of nitric oxide and nitrogen dioxide can also be measured using a commercially available gas detector tube, as will be explained in the examples below.

[0069] When the nitrous oxide-containing gas contains oxygen and / or water vapor (water) in addition to nitrous oxide, nitric oxide, and nitrogen dioxide, the respective contents of oxygen and water vapor in the nitrous oxide-containing gas can be appropriately determined. For example, the content of oxygen gas in the nitrous oxide-containing gas is preferably 0.1 to 21 mol%, more preferably 0.1 to 10 mol%, and even more preferably 0.1 to 5 mol%. The content of water vapor (water) in the nitrous oxide-containing gas is generally and preferably 0.1 to 10 mol%, more preferably 0.1 to 5 mol%, and even more preferably 0.1 to 3 mol%.

[0070] The nitrous oxide-containing gas may contain ammonia gas to further increase the decomposition rate of nitrous oxide. In this case, the molar concentration of ammonia in the nitrous oxide-containing gas is preferably 0.0002 mol% or more and 1 mol% or less, from the viewpoint of the decomposition rate of nitrous oxide. The molar concentration of ammonia is more preferably 0.0002 to 0.5 mol%, and even more preferably 0.0002 to 0.2 mol%. The ratio of ammonia to water vapor contained in the nitrous oxide-containing gas [ammonia / water vapor] is not particularly limited and can be set as appropriate, but from the perspective of the decomposition rate of nitrous oxide, a molar ratio of 0.0010 or more is preferred. A molar ratio of 0.0010 to 0.050 is more preferred, and from the perspective of suppressing or avoiding problems with remaining ammonia (e.g., discharge into the atmosphere, removal work), a molar ratio of 0.0010 to 0.030 is more preferred, and a molar ratio of 0.0010 to 0.010 is even more preferred. Furthermore, the ratio of ammonia to nitrous oxide contained in the nitrous oxide-containing gas [ammonia / nitrous oxide] is not particularly limited and can be set as appropriate, but a molar ratio of 0.005 to 10 is preferred. Furthermore, the content of oxygen gas in the nitrous oxide-containing gas is preferably 0.01 to 10,000 times the molar content of ammonia within the above-mentioned range.

[0071] In decomposition method A of the present invention, which can suppress a decrease in catalytic activity over a long period of time, the nitrous oxide-containing gas does not need to contain a reducing gas that improves the decomposition rate of nitrous oxide. In the present invention, the term ``no reducing gas'' in which the nitrous oxide-containing gas contains no reducing gas includes an embodiment in which the reducing gas content is 0 mol %, as well as an embodiment in which the reducing gas is contained in a molar ratio to nitrous oxide of less than 0.005. On the other hand, the nitrous oxide-containing gas can also contain a reducing gas to further increase the decomposition rate of nitrous oxide. Similarly, to further increase the decomposition rate of nitrous oxide, a saturated hydrocarbon gas, which serves as a raw material for generating a reducing gas such as carbon monoxide upon reaction with oxygen contained in the nitrous oxide-containing gas or generated in the reactor, can also be contained. In terms of the decomposition rate of nitrous oxide, a method in which a reducing gas is contained in the nitrous oxide-containing gas is preferred. The reducing gas may be any reducing gas other than ammonia, and any gas commonly used in a typical catalytic reduction method can be used without particular limitation. Examples of the reducing gas include unsaturated hydrocarbon gases such as ethylene, propylene, α-butylene, and β-butylene, carbon monoxide, hydrogen, and alcohol compound gases such as methanol, ethanol, propanol, and butanol. Among these, at least one of carbon monoxide, unsaturated hydrocarbon gases, and hydrogen is preferred. Examples of saturated hydrocarbon gases that serve as raw materials for generating a reducing gas such as carbon monoxide include methane, ethane, propane, and n-butane. Preferred saturated hydrocarbon gases include ethane, propane, and n-butane. The saturated hydrocarbon gas may be a mixture of natural gas, liquefied natural gas, or liquefied petroleum gas. The content of the reducing gas or saturated hydrocarbon gas in the nitrous oxide-containing gas is not particularly limited and can be set appropriately. For example, the molar concentration of the reducing gas or saturated hydrocarbon gas in the nitrous oxide-containing gas is 0.001 to 1 mol %. The molar ratio of the reducing gas or saturated hydrocarbon gas to water vapor in the nitrous oxide-containing gas [reducing gas or saturated hydrocarbon gas / water vapor] is preferably 0.0003 to 0.03. Furthermore, the molar ratio of the reducing gas or saturated hydrocarbon gas to nitrous oxide contained in the nitrous oxide-containing gas [reducing gas or saturated hydrocarbon gas / nitrous oxide] is preferably 0.01 to 100.

[0072] The nitrous oxide-containing gas used in the decomposition method A of the present invention can be prepared by appropriately mixing nitrous oxide, nitric oxide, and nitrogen dioxide, as well as water vapor, oxygen, ammonia, and other gases. For example, it can be prepared by mixing multiple gases containing at least one of nitrous oxide, nitric oxide, and nitrogen dioxide, as well as at least one water vapor- and oxygen-containing gas. Examples of oxygen-containing gases include air. Nitrous oxide-containing gases can also be used from various exhaust gases emitted from chemical manufacturing plants, as well as from automobiles, power plants using ammonia fuel, and ships. For example, gases emitted from chemical manufacturing plants such as nitric acid manufacturing plants, ε-caprolactam manufacturing plants, and adipic acid manufacturing plants often contain nitrous oxide, nitric oxide, nitrogen dioxide, water vapor, oxygen gas, and in some cases ammonia gas, and can be effectively utilized in the decomposition method A of the present invention. In particular, exhaust gases satisfying the above-described ranges of content and content ratio are preferred because they can be directly applied to the decomposition method A of the present invention without content adjustment.

[0073] [Disassembly process] The decomposition method A of the present invention comprises contacting the catalyst with the nitrous oxide-containing gas (decomposition step). The decomposition step may be any step in which a catalyst is brought into contact with a nitrous oxide-containing gas, and the decomposition step in a known method for decomposing nitrous oxide may be applied. Examples of known decomposition steps in a method for decomposing nitrous oxide include the method (step) described in Patent Document 1 in which a catalyst is brought into contact with nitrous oxide in the presence of a reducing gas. The two may also be brought into contact by passing the nitrous oxide-containing gas through a reaction tube filled with the catalyst. The method for contacting the nitrous oxide-containing gas with the catalyst may be either a batch method or a continuous method, with a continuous method being preferred in terms of reaction efficiency, the advantage of the present invention being able to maintain catalytic activity for a long period of time, and ease of application to various production plants. Examples of continuous methods include a fixed bed method and a fluidized bed method.

[0074] In the decomposition step, when the nitrous oxide in the nitrous oxide-containing gas comes into contact with the catalyst, a decomposition reaction of nitrous oxide shown in the following formula occurs, even in the presence of water vapor, and the nitrous oxide is efficiently decomposed into nitrogen molecules and oxygen molecules. Decomposition reaction of nitrous oxide: N2O → N2+ 1 / 2O2

[0075] In the decomposition step, even if nitric oxide and nitrogen dioxide derived from the nitrous oxide-containing gas coexist with nitrous oxide, so long as the total content of these is within the above range, nitrous oxide can be decomposed without reducing the catalytic activity of the nitrous oxide decomposition catalyst over a long period of time.

[0076] In the decomposition step, when ammonia coexists with nitrous oxide, the decomposition reaction of nitrous oxide can be further accelerated. While the details of this mechanism of action are not yet clear, it is thought to be as follows. For example, in the presence of a catalyst exhibiting a reducing action, such as a ruthenium-supported catalyst, ammonia reacts with nitrous oxide on the catalyst surface, decomposing nitrous oxide into nitrogen molecules and water molecules, thereby further accelerating the decomposition reaction of nitrous oxide. On the other hand, in the presence of a catalyst not exhibiting a reducing action, such as a ruthenium oxide-supported catalyst, ammonia reacts with oxygen atoms remaining on the catalyst surface, removing oxygen atoms from the catalyst surface and maintaining catalytic activity (suppressing catalyst deactivation), thereby accelerating the decomposition reaction.

[0077] When a known decomposition step is applied, the contact method and contact conditions may be any appropriate method and conditions that can be used in each step, and examples thereof include the following conditions. The contact temperature (reaction temperature) is determined appropriately, but is preferably 500°C or lower from the viewpoint of catalyst activity deterioration, and is preferably 100°C or higher from the viewpoint of reaction rate. The contact temperature is preferably 200 to 450°C, more preferably 250 to 400°C. In the continuous contact method, the supply rate of the nitrous oxide-containing gas relative to the mass of the catalyst is not particularly limited and may be determined appropriately. For example, the flow rate at 0°C and 0.1013 MPa (absolute) relative to 1 g of catalyst is 10 to 10,000 cm 3 / min, preferably 50 to 5000 cm 3 / min is more preferable. The contact time is appropriately determined depending on the nitrous oxide concentration or supply rate in the nitrous oxide-containing gas, the contact temperature, etc. In the decomposition method A of the present invention, catalytic activity can be maintained for a long period of time, so the contact time can be set long, for example, to 0.3 seconds or more. The reaction pressure (absolute pressure) varies depending on the contact temperature, the supply rate of the nitrous oxide-containing gas, the pressure of the outside air around the reactor, etc., but is preferably a pressure higher than the outside air, preferably 0.08 to 1 MPa (absolute), and more preferably 0.09 to 0.7 MPa (absolute). From the viewpoint of improving productivity per unit time and being easily applicable to various production plants, the reaction pressure is preferably 0.2 MPa (absolute) or higher in each of the above ranges. In particular, when the cracked gas produced by the cracking method A of the present invention is subjected to the power recovery step described below, the reaction pressure is preferably 0.2 MPa (absolute) or higher.

[0078] [Other processes] The decomposition method A of the present invention may include steps other than the decomposition step, such as a step of adjusting the component contents of the nitrous oxide-containing gas, a step of introducing oxygen, water vapor, ammonia gas, and / or a reducing gas into the nitrous oxide-containing gas, and a reduction step (described below) (wherein the gas discharged from the nitric acid synthesis step is the target of treatment).

[0079] Decomposition method A of the present invention can decompose nitrous oxide over a long period of time while maintaining the catalytic activity of the nitrous oxide decomposition catalyst, and preferably can decompose nitrous oxide efficiently (at a high decomposition rate) over a long period of time. In particular, in a continuous system which can effectively utilize the effect of decomposition method A of the present invention, that is, the ability to maintain catalytic activity over a long period of time, nitrous oxide can be decomposed over a long period of time, preferably efficiently, by the simple process of flowing (passing) a nitrous oxide-containing gas through the catalyst.

[0080] The decomposition method A of the present invention can be used in various fields and applications for decomposing and removing nitrous oxide, such as exhaust gas treatment from chemical manufacturing plants, automobiles, power plants that use ammonia fuel, and ships. It can be particularly suitably used in chemical manufacturing plants such as nitric acid manufacturing plants, ε-caprolactam manufacturing plants, and adipic acid manufacturing plants, which emit nitrous oxide-containing gases that contain nitrous oxide, nitric oxide, and nitrogen dioxide. When the decomposition method A of the present invention is applied to an existing production plant, the installation position of the apparatus for carrying out the decomposition method A of the present invention is not particularly limited, but it is usually installed downstream in the direction of flow of the exhaust gas, for example, upstream of an exhaust tower. Specifically, in the case of a nitric acid production plant, it is installed downstream of a denitrification reactor (a reduction step described later). In this way, the apparatus for carrying out the decomposition method A of the present invention can be easily installed alongside an existing production plant, allowing the existing production plant to be effectively utilized.

[0081] [[Method B for Decomposing Nitrous Oxide]] Another embodiment of the method for decomposing nitrous oxide of the present invention is a decomposition method (sometimes simply referred to as "decomposition method B of the present invention") that is applied (implemented) or incorporated into a nitric acid production plant or an (industrial) method for producing nitric acid. Decomposition method B of the present invention is preferably carried out on an industrial scale. Decomposition method B of the present invention is a method for decomposing nitrous oxide for a nitric acid production plant or a nitric acid production method (also referred to as a "nitric acid production method"), which maintains the catalytic activity of the nitrous oxide decomposition catalyst and can decompose nitrous oxide, which is a by-product in the nitric acid synthesis step, for a long period of time even when applied to a nitric acid production plant or a nitric acid production method.

[0082] [Conventional nitric acid production plant and conventional nitric acid production method] The nitric acid production plant to which the decomposition method B of the present invention is applied (a plant to which the decomposition method of the present invention is not applied or a nitric acid production method, which may be conveniently referred to as a "conventional nitric acid production plant" or "conventional nitric acid production method") may be any plant having an equipment configuration capable of implementing a conventional nitric acid production method, and is preferably a plant having an equipment configuration capable of implementing a nitric acid production method by the Ostwald process (ammonia oxidation method). That is, the nitric acid production method to which the decomposition method B of the present invention is applied is not particularly limited, but is preferably a nitric acid production method by the Ostwald process. Such a nitric acid production method generally includes a method in which a step of synthesizing nitric acid (hereinafter sometimes referred to as the "nitric acid synthesis step") is appropriately combined with a step of reducing nitric oxide and nitrogen dioxide contained in the reaction product (nitrous oxide-containing gas) discharged from the nitric acid synthesis step (hereinafter sometimes referred to as the "reduction step" or "denitrification step") and / or a step of recovering power using an exhaust gas turbine (hereinafter sometimes referred to as the "recovery step"). From the viewpoints of protecting the global environment and reducing production costs, a method in which the nitric acid synthesis step, the reduction step, and the recovery step are performed in this order is preferred. Here, the nitric acid synthesis process by the Ostwald method generally includes two steps: a step of synthesizing nitrogen dioxide by oxidizing ammonia (hereinafter, sometimes referred to as the "NO2 synthesis step"), and a step of reacting the nitrogen dioxide synthesized in this NO2 synthesis step with water to produce nitric acid (hereinafter, sometimes referred to as the "nitric acid production step").

[0083] <Nitric acid synthesis process> In the present invention, the nitric acid synthesis step (NO synthesis step and nitric acid production step) can be any known step without any particular limitation. Examples of nitric acid synthesis steps include a step of heating ammonia to approximately 900°C in air in the presence of a platinum catalyst, and a step of reacting nitrogen dioxide with water. In the NO synthesis step, nitric oxide and nitrous oxide are by-produced in addition to the target nitrogen dioxide, and the reaction products of the NO synthesis step contain nitrogen dioxide, nitric oxide, and nitrous oxide.

[0084] When the reaction product of the NO synthesis step is subjected to the subsequent nitric acid production step, nitrogen dioxide reacts with water to produce nitric acid, but some nitrogen dioxide remains, as well as nitric oxide and nitrous oxide. Therefore, the reaction product of the nitric acid synthesis step (the reaction product obtained through the NO synthesis step and the nitric acid production step) contains nitric acid, nitric oxide, nitrogen dioxide, and nitrous oxide, and the nitrous oxide-containing gas discharged from the nitric acid synthesis step (passing through the nitric acid production step) contains nitric oxide, nitrogen dioxide, and nitrous oxide, as well as water vapor (water), oxygen, and the like. An example of the nitrous oxide-containing gas discharged from the nitric acid synthesis step is the nitrous oxide-containing gas used in decomposition method A of the present invention. The contents (remaining amounts) of nitric oxide, nitrogen dioxide, nitrous oxide, water vapor, and oxygen in the reaction product of the nitric acid synthesis step cannot be uniquely determined, depending on the reaction conditions and production scale of each step. For example, the total content of nitric oxide and nitrogen dioxide in the nitrous oxide-containing gas is typically 0.20 to 0.35 mol%, and may be 0.20 to 0.50 mol%. On the other hand, the content of nitrous oxide in the nitrous oxide-containing gas is typically 0.10 to 0.20 mol%, and may be 0.10 to 0.25 mol%. The contents of water vapor, oxygen, etc. in the nitrous oxide-containing gas are not particularly limited and can be in the same range as the contents of each component in the nitrous oxide-containing gas used in decomposition method A of the present invention.

[0085] <Reduction process> In the present invention, the reduction step may be any step capable of reducing the content of nitric oxide and nitrogen dioxide contained in the reaction product of the nitric acid synthesis step (nitrous oxide-containing gas discharged from the step), and any known reduction step commonly used in nitric acid production methods may be applied without any particular limitation. For example, the reduction step may be a dry method or a wet method, and a preferred example is a selective catalytic reduction method using ammonia as a reducing agent. The selective catalytic reduction method is a method in which nitric oxide and nitrogen dioxide in the reaction product are reacted with ammonia in the presence of a catalyst. For example, the content of the reduction step may be found in Japanese Patent Laid-Open Publication No. 52-050990, the content of which is incorporated herein by reference.

[0086] The reaction product of the reduction step is a gas in which the contents of nitric oxide and nitrogen dioxide have been reduced from the nitrous oxide-containing gas obtained in the nitric acid synthesis step (for convenience, this gas may be referred to as a "reduced gas" in the present invention to distinguish it from the nitrous oxide-containing gas obtained in the nitric acid synthesis step). An example of a reduced gas is a gas in which the contents of nitric oxide and nitrogen dioxide have been reduced from the nitrous oxide-containing gas used in decomposition method A of the present invention. The contents (residual amounts) of nitric oxide, nitrogen dioxide, nitrous oxide, water vapor, and oxygen in this reduced gas cannot be uniquely determined depending on the reaction conditions in the reduction step, the production scale, and the like. For example, the total content of nitric oxide and nitrogen dioxide in the reduced gas is not particularly limited, but is preferably 0.0001 to 0.35 mol % in order to maintain catalytic activity. In order to maintain high catalytic activity over a long period of time, it is preferably 0.0001 to 0.25 mol %, more preferably 0.0001 to 0.15 mol %, and even more preferably 0.0001 to 0.065 mol %. The content of nitrous oxide in the reduced gas is usually 0.002 mol% or more, generally and preferably 0.002 to 10 mol%, more preferably 0.002 to 1 mol%, and even more preferably 0.002 to 0.5 mol%. The contents of water vapor, oxygen, ammonia, etc. in the reduced gas are not particularly limited, and are preferably in the same range as the contents of each component in the nitrous oxide-containing gas used in decomposition method A of the present invention.

[0087] <Recovery process> The recovery step is preferably a step of recovering power by supplying the reaction product of the previous step (exhaust gas discharged from the previous step) to an exhaust heat gas turbine to rotate the exhaust heat gas turbine and generate electricity, and any known recovery step that is usually used in a conventional method for producing nitric acid can be applied without any particular limitation.

[0088] <Other processes> The nitric acid production method may include steps other than the nitric acid synthesis step, the reduction step, and the recovery step.

[0089] In the decomposition method B of the present invention, the nitrous oxide decomposition step described below can be applied to the above-mentioned nitric acid production plant (nitric acid production method), and a preferred embodiment thereof is as follows.

[0090] [Decomposition method B1 of the present invention] A preferred embodiment of the decomposition method B of the present invention (sometimes referred to as "decomposition method B1 of the present invention" in the present invention) is a method in which a nitrous oxide decomposition step is applied before the recovery step in a nitric acid production plant (nitric acid production method) capable of carrying out at least the nitric acid synthesis step and the recovery step. That is, the decomposition method B1 of the present invention is a method for decomposing nitrous oxide that is applicable to a nitric acid production plant (having an apparatus configuration) that is capable of carrying out at least a nitric acid synthesis step and a recovery step, or a nitric acid production method that has at least a nitric acid synthesis step and a recovery step, and is a method in which a nitrous oxide decomposition step in which a catalyst containing at least one selected from the group consisting of ruthenium and ruthenium compounds is brought into contact with the nitrous oxide-containing gas discharged from the nitric acid synthesis step is carried out before the recovery step.

[0091] In the decomposition method B1 of the present invention, the nitric acid synthesis step and recovery step are the same as the steps carried out in a conventional nitric acid production plant or conventional nitric acid production method, for example, as described above. In decomposition method B1 of the present invention, the nitrous oxide decomposition step is basically the same as the decomposition step in decomposition method A of the present invention, except that the nitrous oxide-containing gas discharged from the nitric acid synthesis step is used, and further, the total content of at least nitric oxide and nitrogen dioxide in the nitrous oxide-containing gas is not limited to the above-mentioned range in decomposition method A of the present invention.

[0092] The nitrous oxide-containing gas used in the decomposition step of decomposition method B1 of the present invention is the nitrous oxide-containing gas discharged from the nitric acid synthesis step, and may contain the various gases described in connection with decomposition method A of the present invention. For example, the content of nitrous oxide in the nitrous oxide-containing gas used in the decomposition step of decomposition method B1 of the present invention is not particularly limited, and can be the content described in connection with decomposition method A of the present invention. The contents of nitric oxide and nitrogen dioxide are also not particularly limited, and are preferably the same as the respective contents and total contents in the nitrous oxide-containing gas used in the decomposition step of decomposition method A of the present invention. Furthermore, the contents of gases that may be contained, such as oxygen, water vapor, ammonia, reducing gases, and saturated hydrocarbon gases, are also not particularly limited, and can be the contents described as the contents of each component in connection with decomposition method A of the present invention. The nitrous oxide-containing gas used in the decomposition step of the decomposition method B1 of the present invention is usually supplied directly from the nitric acid synthesis step, but may be supplied via another step, such as a content adjustment step.

[0093] In the decomposition method B1 of the present invention, the catalyst used in the decomposition step is not particularly limited, and is the same as the nitrous oxide decomposition catalyst used in the decomposition step in the decomposition method A of the present invention. In decomposition method B1 of the present invention, the method and conditions for the decomposition step are not particularly limited and are the same as the method and conditions for the decomposition step in decomposition method A of the present invention. In particular, it is preferable to set the reaction pressure (absolute pressure) in the decomposition step to 0.2 MPa (absolute) or more within the above range, since this allows the recovery step to be carried out effectively and the recovery power to be increased while ensuring the operability and safety of the high-pressure process.

[0094] In the decomposition method B1 of the present invention, the decomposition step is carried out before the recovery step. When the decomposition step is carried out before the recovery step, nitrous oxide can be decomposed over a long period of time while maintaining the catalytic activity of the nitrous oxide decomposition catalyst, and preferably nitrous oxide can be decomposed efficiently (at a high decomposition rate) over a long period of time. In the decomposition method B1 of the present invention, the decomposition step may be carried out after the nitric acid synthesis step and before the recovery step, but is preferably carried out immediately before the recovery step (the decomposition step and the recovery step are carried out sequentially in this order). However, a step of adjusting physical properties such as temperature and pressure may also be carried out between the decomposition step and the recovery step. In the decomposition method B1 of the present invention, the decomposition step is carried out after the nitric acid synthesis step, but when the nitric acid production plant or the nitric acid production method includes a reduction step, it is preferable to carry out the decomposition step between the reduction step and the recovery step, since this can highly suppress the decrease in catalytic activity. In this case, the nitrous oxide-containing gas used in the decomposition step is the reduced gas obtained in the reduction step.

[0095] [Decomposition method B2 of the present invention] Another preferred embodiment of the decomposition method B of the present invention (sometimes referred to as "decomposition method B2 of the present invention" in the present invention) is an embodiment in which a nitrous oxide decomposition step is applied after the reduction step in a nitric acid production plant (nitric acid production method) capable of carrying out at least the nitric acid synthesis step and the reduction step. That is, decomposition method B2 of the present invention is a method for decomposing nitrous oxide that is applicable to a nitric acid production plant (having an apparatus configuration) that is at least capable of carrying out a nitric acid synthesis step or a nitric acid production method that has at least a nitric acid synthesis step, and includes a step (reduction step) of reducing nitric oxide and nitrogen dioxide contained in the nitrous oxide-containing gas discharged from the nitric acid synthesis step, and a nitrous oxide decomposition step of contacting the gas discharged from the reduction step (reduced gas) with a catalyst containing at least one selected from the group consisting of ruthenium and ruthenium compounds.

[0096] In the decomposition method B2 of the present invention, the nitric acid synthesis step and the reduction step are the same as the steps carried out in a conventional nitric acid production plant or a conventional nitric acid production method, for example, as described above. In decomposition method B2 of the present invention, the nitrous oxide decomposition step is basically the same as the decomposition step in decomposition method A of the present invention, except that the reduced gas discharged from the reduction step is used, and further, the total content of at least nitric oxide and nitrogen dioxide in the nitrous oxide-containing gas is not limited to the above-mentioned range in decomposition method A of the present invention.

[0097] The reduced gas used in the decomposition step of decomposition method B2 of the present invention is the reduced gas discharged from the reduction step, and is the same as the reduced gas described above in <Reduction step>. In particular, the total content of nitric oxide and nitrogen dioxide in the reduced gas is preferably 0.0001 to 0.35 mol %, and more preferably within each of the preferred ranges in the <Reduction step>. The reduced gas used in the decomposition step of the decomposition method B2 of the present invention is usually supplied directly from the reduction step, but may be supplied via another step, such as a content adjustment step.

[0098] In the decomposition method B2 of the present invention, the catalyst used in the decomposition step is not particularly limited, and is the same as the nitrous oxide decomposition catalyst used in the decomposition step in the decomposition method A of the present invention. In decomposition method B2 of the present invention, the method and conditions for the decomposition step are not particularly limited and are the same as the method and conditions for the decomposition step in decomposition method A of the present invention. In particular, within the above range, it is preferable to set the reaction pressure (absolute pressure) in the decomposition step to 0.2 MPa (absolute) or more, because when a recovery step is included, this allows the recovery step to be carried out effectively and the power for recovery to be increased while ensuring the operability and safety of the high-pressure process.

[0099] In the decomposition method B2 of the present invention, the decomposition step is carried out after the reduction step. By carrying out the decomposition step after the reduction step, it is possible to suppress a decrease in catalytic activity due to nitric oxide and nitrogen dioxide in the reduction gas, and as a result, it is possible to decompose nitrous oxide over a long period of time while maintaining the catalytic activity of the nitrous oxide decomposition catalyst, and more preferably, to decompose nitrous oxide efficiently (at a high decomposition rate) over a long period of time. In the decomposition method B2 of the present invention, the decomposition step may be carried out after the reduction step, but is preferably carried out immediately after the reduction step (the reduction step and the decomposition step are carried out sequentially in this order). However, between the reduction step and the decomposition step, a step of adjusting physical properties such as temperature and pressure, or a step of adjusting the content of gases other than nitric oxide, nitrogen dioxide, and nitrous oxide may also be carried out. In the decomposition method B2 of the present invention, the decomposition step is carried out after the reduction step, but when the nitric acid production plant or the nitric acid production method includes a recovery step, it is preferable to carry out the decomposition step between the reduction step and the recovery step, in order to highly suppress a decrease in catalytic activity. That is, in the decomposition method B2 of the present invention, when the nitric acid production plant or the nitric acid production method can further carry out a recovery step, it is preferable to carry out the decomposition step before the recovery step and after the reduction step, in order to highly suppress a decrease in catalytic activity.

[0100] Nitrous oxide decomposition system The nitrous oxide decomposition system of the present invention is a decomposition system that is applied to or incorporated into a nitric acid production plant or an (industrial) nitric acid production process, and is a system or apparatus that can implement a nitric acid production plant or a nitric acid production process (in the present invention, this may be simply referred to as the "decomposition system of the present invention"). The decomposition system of the present invention is preferably capable of implementing a plant or a production process that is carried out on an industrial scale. The decomposition system of the present invention is a nitrous oxide decomposition system for a nitric acid production plant or a nitric acid production process, which maintains the catalytic activity of the nitrous oxide decomposition catalyst and can decompose nitrous oxide, which is a by-product in the nitric acid synthesis process, for a long period of time even when applied to a nitric acid production plant or a nitric acid production process.

[0101] [Conventional nitric acid production plant and conventional nitric acid production method] The nitric acid production plant to which the decomposition system of the present invention is applied (a plant or a nitric acid production method not equipped with the decomposition system of the present invention may be conveniently referred to as a "conventional nitric acid production plant" or "conventional nitric acid production method") as long as it has an equipment configuration capable of implementing a conventional nitric acid production method, and is preferably a plant having an equipment configuration capable of implementing a nitric acid production method by the Ostwald process (ammonia oxidation method). That is, the nitric acid production plant to which the decomposition system of the present invention is applied is not particularly limited, but is preferably a plant having an equipment configuration capable of implementing a nitric acid production method by the Ostwald process. Such a nitric acid production plant generally includes a plant that appropriately combines a nitric acid synthesis unit capable of implementing a nitric acid synthesis step with a reduction unit capable of implementing a reduction step and / or a power recovery unit capable of implementing a recovery step. From the perspectives of global environmental protection, reduction of production costs, etc., a plant that includes a nitric acid synthesis unit, a reduction unit, and a power recovery unit in this order is preferred. Here, a nitric acid synthesis unit using the Ostwald process generally includes a NO synthesis unit capable of implementing a NO synthesis step and a nitric acid production unit capable of implementing a nitric acid production step, in this order. The nitric acid production plant to which the decomposition system of the present invention is applied is preferably a plant having an apparatus configuration capable of carrying out the decomposition method B of the present invention. In the present invention, in a nitric acid manufacturing plant, the upstream side is referred to as the "preceding stage" and the downstream side is referred to as the "rear stage" along the direction in which the process gas is transferred (flowed) in accordance with the process order.

[0102] <Nitric acid synthesis equipment> In the present invention, the nitric acid synthesis apparatus (NO2 synthesis apparatus and nitric acid production apparatus) can be any known apparatus without any particular limitation. The nitric acid synthesis apparatus, for example, has an oxidation reaction tank located at the most upstream side for oxidizing ammonia, a water reaction tank (for example, an absorption tower) connected to the downstream side of the oxidation reaction tank by a transfer pipe or the like, and a discharge pipe connected to the water reaction tank for discharging the reaction product (gas passing through the water reaction tank) from the water reaction tank.

[0103] <Reduction device> In the present invention, the reduction device may be any device capable of reducing the content of nitric oxide and nitrogen dioxide contained in the reaction product (nitrous oxide-containing gas discharged from the device) obtained in the nitric acid synthesis device, and any known reduction device commonly used in nitric acid production methods may be used without any particular limitation. Examples of reduction devices include dry reduction devices and wet reduction devices, with a selective catalytic reduction device being preferred. The selective catalytic reduction device is a device that reacts nitric oxide and nitrogen dioxide in the reaction product with ammonia in the presence of a catalyst, and includes, for example, a denitration reactor that is connected to the discharge pipe of a device located upstream and that incorporates a denitration catalyst; an ammonia injection pipe that is connected to the denitration reactor and that injects ammonia into the denitration reactor; and a discharge pipe that is connected to the denitration reactor and that discharges the reaction product from the denitration reactor.

[0104] <Power recovery device> The power recovery device is preferably a device that recovers power by supplying a reaction product (exhaust gas discharged from the device arranged in the upstream stage) to an exhaust heat gas turbine to rotate the exhaust heat gas turbine and generate electricity, and any known power recovery process that is normally used in conventional nitric acid production methods can be applied without any particular limitation. For example, the power recovery device can be a power generation mechanism that is connected to the exhaust pipe of the device arranged in the upstream stage and has an exhaust heat gas turbine built in.

[0105] <Other configurations> A conventional nitric acid production plant preferably includes a discharge device, such as a discharge tower, that discharges gases discharged from a device disposed in a previous stage into the atmosphere. The nitric acid production plant and each device may be equipped with various devices such as a temperature regulator, a pressure regulator, a transfer means (pump), and a safety device.

[0106] [Nitrous oxide decomposition system] The nitrous oxide decomposition system of the present invention (hereinafter sometimes simply referred to as the "decomposition system of the present invention") is a nitrous oxide decomposition apparatus that contacts a nitrous oxide-containing gas (including a reduced gas) with the catalyst containing at least one selected from the group consisting of ruthenium and ruthenium compounds. The configuration of the apparatus is not particularly limited as long as it is capable of contacting the catalyst with the nitrous oxide-containing gas by the method and conditions of the decomposition step in the decomposition method of the present invention. The decomposition system may be a batch or continuous apparatus, but a continuous apparatus is preferred because it can utilize the characteristic of the decomposition method of the present invention, which is capable of maintaining catalytic activity for a long period of time using a highly productive production plant. Examples of continuous apparatus include fixed-bed and fluidized-bed apparatuses, such as tubular apparatuses (reaction tubes, reaction towers, etc.) that incorporate a nitrous oxide decomposition catalyst.

[0107] The decomposition system of the present invention can be applied to the above-mentioned conventional nitric acid production plant (conventional nitric acid production method), and is highly useful in that it can be applied without requiring significant changes or reconstruction of the equipment configuration of the conventional nitric acid production plant. A preferred embodiment of the decomposition system of the present invention is described below.

[0108] [Decomposition system 1 of the present invention] A preferred embodiment of the decomposition system of the present invention (sometimes referred to as "decomposition system 1 of the present invention" in the present invention) is a decomposition system that is installed (integrated) in a stage upstream of a power recovery device in a nitric acid production plant having at least a nitric acid synthesis device and a power recovery device. That is, the decomposition system 1 of the present invention is a nitrous oxide decomposition system that is installed in addition to a nitric acid production plant having a nitric acid synthesis unit and a power recovery unit, and includes a decomposition unit that brings a catalyst containing at least one selected from the group consisting of ruthenium and ruthenium compounds into contact with the nitrous oxide-containing gas discharged from the nitric acid synthesis unit (which passes through the nitric acid generation unit), and this decomposition unit is installed upstream of the power recovery unit.

[0109] In the decomposition system 1 of the present invention, the nitric acid synthesis device and the recovery device are the same as the devices in the conventional nitric acid production plant, for example, as described above. The decomposition device is the same as the decomposition device described in the decomposition system of the present invention.

[0110] In the decomposition system 1 of the present invention, the decomposition device is installed upstream of the recovery device. The decomposition system 1 of the present invention in which the decomposition device is installed upstream of the recovery device can suitably carry out the decomposition method of the present invention, particularly decomposition method B1 of the present invention, and as a result, can decompose nitrous oxide over a long period of time while maintaining the catalytic activity of the nitrous oxide decomposition catalyst, and preferably can decompose nitrous oxide efficiently (at a high decomposition rate) over a long period of time. In the decomposition system 1 of the present invention, the decomposition device may be installed after the nitric acid synthesis device and before the recovery device, but it is preferable to install it immediately before the recovery device (the decomposition device and the recovery device are installed in this order). However, the above-mentioned various devices can also be installed between the decomposition device and the recovery device. In the decomposition system 1 of the present invention, the decomposition device is installed downstream of the nitric acid synthesis device, but if the nitric acid production plant includes a reduction device, it is preferable to install the decomposition device between the reduction device and the recovery device, as this can highly suppress the decrease in catalytic activity. Note that the above-mentioned various devices can also be installed between the decomposition device and the recovery device. If the nitric acid production plant includes a reduction device, the nitrous oxide-containing gas transferred to the decomposition device becomes the reduced gas transferred from the reduction device.

[0111] [Decomposition system 2 of the present invention] Another preferred embodiment of the decomposition system of the present invention (sometimes referred to as "decomposition system 2 of the present invention" in the present invention) is a decomposition system that is installed (integrated) in the subsequent stage of the reduction step in a nitric acid production plant that has at least a nitric acid synthesis device and a reduction device. That is, the decomposition system 2 of the present invention is a nitrous oxide decomposition system installed in a nitric acid production plant having a nitric acid synthesis device and a reduction device that reduces the nitric oxide and nitrogen dioxide contained in the nitrous oxide-containing gas discharged from the nitric acid synthesis device (passing through the nitric acid generation device), and is a decomposition system that includes a decomposition device that brings a catalyst containing at least one selected from the group consisting of ruthenium and ruthenium compounds into contact with the reduced gas discharged from the reduction device.

[0112] In the decomposition system 2 of the present invention, the nitric acid synthesis device and the reduction device are the same as the devices in the conventional nitric acid production plant, for example, as described above. The decomposition device is the same as the decomposition device described in the decomposition system of the present invention.

[0113] In the decomposition system 2 of the present invention, the decomposition device is installed downstream of the reduction device. The decomposition system 2 of the present invention in which the decomposition device is installed downstream of the reduction device can suitably carry out the decomposition method of the present invention, particularly decomposition method B2 of the present invention, and as a result, can decompose nitrous oxide over a long period of time while maintaining the catalytic activity of the nitrous oxide decomposition catalyst, and preferably can decompose nitrous oxide efficiently (at a high decomposition rate) over a long period of time. In the decomposition system 2 of the present invention, the decomposition device may be installed after the reduction device, but is preferably installed immediately after the reduction device (the reduction device and decomposition device are installed in that order). However, the above-mentioned various devices, as well as a device for adjusting the content of gases other than nitric oxide, nitrogen dioxide, and nitrous oxide, can also be installed between the reduction device and the decomposition device. In the decomposition system 2 of the present invention, the decomposition device is installed after the reduction device, but if the nitric acid production plant includes a power recovery device, it is preferable to install the decomposition device between the reduction device and the recovery device, since this can highly suppress the decrease in catalytic activity. Note that the above-mentioned various devices can also be installed between the decomposition device and the recovery device.

[0114] [Decomposition system 3 of the present invention] Another preferred embodiment of the decomposition system of the present invention (sometimes referred to as "decomposition system 3 of the present invention" in the present invention) is a decomposition system that is installed (integrated) in the subsequent stage of the reduction process in a nitric acid production plant that has at least a nitric acid synthesis unit, a reduction unit, and a power recovery unit in this order. That is, the decomposition system 3 of the present invention is a nitrous oxide decomposition system installed in a nitric acid production plant having, in this order, a nitric acid synthesis unit, a reduction unit that reduces the nitric oxide and nitrogen dioxide contained in the nitrous oxide-containing gas discharged from the nitric acid synthesis unit, and a power recovery unit that recovers power using an exhaust heat gas turbine, and the decomposition system includes a nitrous oxide decomposition unit that brings a catalyst containing at least one selected from the group consisting of ruthenium and ruthenium compounds into contact with the reduced gas discharged from the reduction unit.

[0115] In the decomposition system 3 of the present invention, the nitric acid synthesis device, the reduction device and the power recovery device are the same as the devices in the conventional nitric acid production plant, for example, as described above. The decomposition device is the same as the decomposition device described in the decomposition system of the present invention.

[0116] In the decomposition system 3 of the present invention, the decomposition device is installed downstream of the reduction device. The decomposition system 3 of the present invention in which the decomposition device is installed downstream of the reduction device can suitably carry out the decomposition method of the present invention, particularly decomposition methods B1 and B2 of the present invention, and as a result, can decompose nitrous oxide over a long period of time while maintaining the catalytic activity of the nitrous oxide decomposition catalyst, and preferably can decompose nitrous oxide efficiently (at a high decomposition rate) over a long period of time. In the decomposition system 3 of the present invention, the decomposition device may be installed after the reduction device, but is preferably installed immediately after the reduction device (the reduction device and decomposition device are installed in that order). However, the above-mentioned various devices, as well as a device for adjusting the content of gases other than nitric oxide, nitrogen dioxide, and nitrous oxide, can also be installed between the reduction device and the decomposition device. In the decomposition system 3 of the present invention, it is preferable that the decomposition device is installed in the upstream stage of the recovery device, between the reduction device and the recovery device, because this allows the decomposition methods B1 and B2 of the present invention to be suitably carried out among the decomposition methods of the present invention and enables the reduction in catalytic activity to be highly suppressed. Note that the above-mentioned various devices can also be installed between the decomposition device and the recovery device.

[0117] [[Nitric acid manufacturing method]] The method for producing nitric acid of the present invention (sometimes referred to as the "nitric acid production method of the present invention" in the present invention) is a method in which the decomposition method of the present invention is implemented in a known method for producing nitric acid. By utilizing the decomposition method of the present invention, the method for producing nitric acid of the present invention can industrially produce nitric acid while maintaining the catalytic activity of the nitrous oxide decomposition catalyst at an increased level and decomposing nitrous oxide, which is a by-product in the nitric acid synthesis step, for a long period of time. The nitric acid production method that forms the basis of the nitric acid production method of the present invention is the same as a known production method (conventional nitric acid production method) that can be carried out in a conventional nitric acid production plant to which the decomposition method B of the present invention is applied, and the nitric acid synthesis step, reduction step, recovery step and other steps are as described above.

[0118] [Nitric acid production method 1 of the present invention] A preferred embodiment of the nitric acid production method of the present invention (sometimes referred to as "nitric acid production method 1 of the present invention" in the present invention) is a method in which a nitrous oxide decomposition step is carried out before the recovery step in a nitric acid production plant (nitric acid production method) capable of carrying out at least the nitric acid synthesis step and the recovery step. That is, the nitric acid production method 1 of the present invention is an (industrial) method for producing nitric acid that is carried out in a nitric acid production plant (having an apparatus configuration) that is capable of carrying out at least a nitric acid synthesis step and a recovery step, and is a method that carries out a nitrous oxide decomposition step, in which a catalyst containing at least one selected from the group consisting of ruthenium and ruthenium compounds is brought into contact with the nitrous oxide-containing gas discharged from the nitric acid synthesis step, before the recovery step.

[0119] In the nitric acid production method 1 of the present invention, the nitric acid synthesis step and recovery step are the same as the steps carried out in a conventional nitric acid production plant or conventional nitric acid production method, for example, as described above. In nitric acid production method 1 of the present invention, the nitrous oxide decomposition step is basically the same as the decomposition step in decomposition method A of the present invention, except that the nitrous oxide-containing gas discharged from the nitric acid synthesis step is used, and further, the total content of at least nitric oxide and nitrogen dioxide in the nitrous oxide-containing gas is not limited to the above-mentioned range in decomposition method A of the present invention.

[0120] The nitrous oxide-containing gas and catalyst used in the decomposition step of nitric acid production method 1 of the present invention, as well as the method and conditions for the decomposition step, are not particularly limited and are the same as the nitrous oxide-containing gas and catalyst, and the method and conditions for the decomposition step in decomposition method B1 of the present invention.

[0121] In the nitric acid production method 1 of the present invention, the decomposition step is carried out before the recovery step. When the decomposition step is carried out before the recovery step, nitrous oxide can be decomposed over a long period of time while maintaining the catalytic activity of the nitrous oxide decomposition catalyst, and nitric acid can be produced while suppressing the emission of nitrous oxide over a long period of time. Preferably, nitric acid can be produced while efficiently suppressing the emission of nitrous oxide over a long period of time. In the nitric acid production method 1 of the present invention, the decomposition step may be carried out after the nitric acid synthesis step and before the recovery step, but it is preferable to carry out the decomposition step immediately before the recovery step (the decomposition step and the recovery step are carried out sequentially in this order). However, a step of adjusting physical properties such as temperature and pressure can also be carried out between the decomposition step and the recovery step. In the nitric acid production method 1 of the present invention, the decomposition step is carried out after the nitric acid synthesis step, but if the nitric acid production method includes a reduction step, it is preferable to carry out the decomposition step between the reduction step and the recovery step, since this can highly suppress the decrease in catalytic activity and enable nitric acid to be produced while efficiently suppressing the release of nitrous oxide over a long period of time. In this case, the nitrous oxide-containing gas used in the decomposition step is the reduced gas obtained in the reduction step.

[0122] [Nitric acid production method 2 of the present invention] Another preferred embodiment of the nitric acid production method of the present invention (sometimes referred to as "nitric acid production method 2 of the present invention" in the present invention) is a method in which the nitrous oxide decomposition method is carried out after the reduction step in a nitric acid production plant (nitric acid production method) that is capable of carrying out at least the nitric acid synthesis step and the reduction step. That is, the nitric acid production method 2 of the present invention is an (industrial) method for producing nitric acid which is carried out in a nitric acid production plant (having an apparatus configuration) capable of at least carrying out a nitric acid synthesis step, and which includes a step of reducing nitric oxide and nitrogen dioxide contained in the nitrous oxide-containing gas discharged from the nitric acid synthesis step, and a nitrous oxide decomposition step of contacting the reduced gas discharged from the reduction step with a catalyst containing at least one selected from the group consisting of ruthenium and ruthenium compounds.

[0123] In the nitric acid production method 2 of the present invention, the nitric acid synthesis step and the reduction step are the same as the steps carried out in a conventional nitric acid production plant or conventional nitric acid production method, for example, as described above. In the nitric acid production method 2 of the present invention, the nitrous oxide decomposition step is basically the same as the decomposition step in the decomposition method A of the present invention, except that the nitrous oxide-containing gas discharged from the reduction step is used, and further, the total content of at least nitric oxide and nitrogen dioxide in the nitrous oxide-containing gas is not limited to the above-mentioned range in the decomposition method A of the present invention.

[0124] The reduced gas and catalyst used in the decomposition step of the nitric acid production method 2 of the present invention, as well as the method and conditions for the decomposition step, are not particularly limited and are the same as the reduced gas and catalyst, and the method and conditions for the decomposition step in the decomposition method B2 of the present invention.

[0125] In the nitric acid production method 2 of the present invention, the decomposition step is carried out after the reduction step. When the decomposition step is carried out after the reduction step, it is possible to suppress a decrease in catalytic activity due to nitric oxide and nitrogen dioxide in the reduction gas, and it is possible to decompose nitrous oxide over a long period of time while maintaining the catalytic activity of the nitrous oxide decomposition catalyst. As a result, it is possible to produce nitric acid while suppressing the release of nitrous oxide over a long period of time, and more preferably, it is possible to produce nitric acid while efficiently suppressing the release of nitrous oxide over a long period of time. In the nitric acid production method 2 of the present invention, the decomposition step may be carried out after the reduction step, but is preferably carried out immediately after the reduction step (the reduction step and the decomposition step are carried out sequentially in this order). However, between the reduction step and the decomposition step, a step of adjusting physical properties such as temperature and pressure, or a step of adjusting the content of gases other than nitric oxide, nitrogen dioxide, and nitrous oxide may also be carried out. In the nitric acid production method 2 of the present invention, the decomposition step is carried out after the reduction step, but when the nitric acid production method includes a recovery step, it is preferable to carry out the decomposition step between the reduction step and the recovery step, in that this highly suppresses the decrease in catalytic activity and enables nitric acid to be produced while efficiently suppressing the release of nitrous oxide over a long period of time. That is, in the nitric acid production method 2 of the present invention, when a recovery step can be further carried out in the nitric acid production method, it is preferable to carry out the decomposition step before the recovery step and after the reduction step, in that this highly suppresses the decrease in catalytic activity and enables nitric acid to be produced while efficiently suppressing the release of nitrous oxide over a long period of time.

[0126] [[Nitric acid production plant of the present invention]] The nitric acid production plant of the present invention is a nitric acid production plant having a nitric acid synthesis unit, a reduction unit, and a power recovery unit, in this order, and further has a nitrous oxide decomposition unit between the reduction unit and the power recovery unit, which brings the reduction gas discharged from the reduction unit into contact with a catalyst containing at least one selected from the group consisting of ruthenium and ruthenium compounds.The nitric acid production plant of the present invention utilizes the decomposition system of the present invention to maintain the catalytic activity of the nitrous oxide decomposition catalyst and industrially produce nitric acid while decomposing nitrous oxide, which is a by-product in the nitric acid synthesis unit described below, for a long period of time.

[0127] That is, the nitric acid production plant of the present invention is the same as a conventional nitrous oxide decomposition device except that it has a nitrous oxide decomposition device. The nitric acid synthesis unit, reduction unit and power recovery unit in the nitric acid production plant of the present invention are the same as those in the conventional nitric acid production plant, for example, as described above. The nitrous oxide decomposition device in the nitric acid production plant of the present invention is not particularly limited as long as it is a device that can decompose and remove nitrous oxide in the reduction gas discharged from the reduction device by contacting the reduction gas with a catalyst containing at least one selected from the group consisting of ruthenium and ruthenium compounds, and the nitrous oxide decomposition system of the present invention can be applied, and it is preferable to apply decomposition system 3 of the present invention.

[0128] The nitric acid production plant of the present invention, in which the decomposition device is installed between the reduction device and the power recovery device, can suitably carry out the decomposition method of the present invention, particularly decomposition methods B1 and B2 of the present invention, and as a result, it is possible to highly suppress the decline in the catalytic activity of the nitrous oxide decomposition catalyst and to produce nitric acid while efficiently suppressing the emission of nitrous oxide over a long period of time. In the nitric acid production plant of the present invention, the above-mentioned various devices, as well as devices for adjusting the content of gases other than nitric oxide, nitrogen dioxide, and nitrous oxide, can also be installed between the adjacent devices. [Example]

[0129] Examples of the present invention will be shown below, but the present invention is not limited to these examples.

[0130] [Example 1] A nitrous oxide decomposition catalyst corresponding to Catalyst 1 was produced as follows and used in a nitrous oxide decomposition reaction, and the nitrous oxide decomposition method was carried out by setting the reaction pressure at 0.6 MPa (absolute).

[0131] <Calculation of nitrous oxide decomposition rate X (%)> The decomposition rate X (%) of nitrous oxide is calculated by dividing the nitrous oxide content C in the nitrous oxide-containing gas (used for decomposition of nitrous oxide). B (mol%) and the nitrous oxide content C in the reaction outlet gas (post-reaction gas) (after the nitrous oxide decomposition step) A The concentration (mol %) of nitrous oxide was analyzed (measured) using a gas chromatograph (Shimadzu Corporation, GC-2014 (detector: TCD, column: SHINCARBON-ST50 / 80 4 m)), and the decomposition rate X of nitrous oxide (sometimes simply referred to as "nitrous oxide decomposition rate X") was calculated from the analyzed concentration (mol %) of nitrous oxide using the following formula. Nitrous oxide decomposition rate X (%) = [(C B -C A ) / C B ] x 100

[0132] <Calculation of reaction rate constant> Reaction rate constant (s -1 ) is the decomposition rate of nitrous oxide X (%), the space velocity of the reaction gas GHSV (h -1 ) was calculated using the following formula: Reaction rate constant (s -1 )=-ln(1-X / 100) / (3600 / GHSV) Here, ln(1-X / 100) represents the natural logarithm of (1-X / 100).

[0133] (Catalyst 1: Preparation of ZrO2-RuO2 / SiO2-containing TiO2 catalyst) To 100 parts by mass of titanium oxide powder (Showa Denko Ceramics Co., Ltd.), 12 parts by mass of organic binder, 24.6 parts by mass of water, and 17.5 parts by mass of silica sol were added and mixed and kneaded to obtain a clay. The organic binders used were Metrose (Shin-Etsu Chemical Co., Ltd.) and Unilube (NOF Corporation). The silica sol used was Snowtex ST-CM (colloidal silica, Nissan Chemical Industries, Ltd., particle size 22 nm, solid content 30% by mass). The resulting clay was molded using a vacuum extrusion molding machine to obtain a honeycomb molded body (cubic shape, 20 mm length, 20 mm width, and 20 mm height, with through holes aligned in parallel in the vertical and horizontal directions, volume fraction 36%, rectangular through hole opening with a cell size of 1.4 mm, inner wall thickness 0.35 mm, and open area ratio 64%). The obtained honeycomb formed body was air-dried at room temperature for 2 days to obtain carrier precursor 1 (the honeycomb structure retains the shape and dimensions of the honeycomb formed body). Carrier precursor 1 was fired in an electric furnace at 600°C for 2 hours to obtain carrier 1 (the SiO2 content in the carrier was 5 mass%, the ratio of rutile crystalline TiO2 in titanium oxide was 90 mass% or more, and the honeycomb structure retains the shape and dimensions of the honeycomb formed body) (Step 1).

[0134] Next, 4.6 g of ruthenium nitrosyl nitrate solution (Furuya Metal Co., Ltd., Ru(NO)(NO3)3, Ru content 19%) and 0.9 g of Zircosol ZN solution (Daiichi Kigenso Kagaku Kogyo, ZrO(NO3)2, Zr content 18%) were dissolved in 5.0 g of ion-exchanged water. 25.0 g of support 1 formed of titanium oxide and SiO2 was impregnated with the resulting aqueous solution at room temperature (25°C) for 1 hour, and then dried at room temperature in an air atmosphere for 1 hour to obtain catalyst precursor 1 supporting ruthenium nitrosyl nitrate and zirconium oxynitrate (Step 2A). This catalyst precursor 1 was calcined in an electric furnace at 320°C for 2 hours to obtain catalyst 1. In catalyst 1, the content of elemental ruthenium was 3 mass %, and the content of elemental zirconium was 0.5 mass %.

[0135] (Decomposition reaction of nitrous oxide using catalyst 1) Reaction pressure: 0.6 MPa (absolute), flow rate: 690 cm 3A mixed gas of 0.12 mol% nitrous oxide, 3.0 mol% oxygen, 0.2 mol% water vapor, and the balance nitrogen was introduced at a rate of 2.4 g (3.0 cm3) under the conditions of (0°C, 0.1013 MPa (absolute)) / min. 3 The exhaust gas was passed through a stainless steel reaction tube (inner diameter 8 mm) packed with catalyst 1 (N2O), and the reaction tube was heated in an electric furnace until the internal temperature reached 300°C. The time when the internal temperature reached 300°C was defined as the start of the NO decomposition reaction, and the gas before and after contact with catalyst 1 at a predetermined time was analyzed using gas chromatography and gas detection tubes (water vapor 6, nitrogen oxide 11S, nitrogen oxide 11HA: all manufactured by Gastec Corporation). The exhaust gas obtained from the nitrous oxide decomposition reaction in Example 1 was discharged at a high pressure of approximately 0.6 MPa during the reaction time, and therefore was suitable for use in the power recovery process and power recovery device in conventional nitric acid production plants. In this decomposition reaction, the reaction rate constant after 4 hours is 3.4 s -1 The reaction rate constant after 513 hours of reaction is 3.4 s -1 Table 1 shows the specific activity (reaction rate constant ratio) after 513 hours of reaction time, assuming that the reaction rate constant after 4 hours of reaction time is 1.

[0136] [Example 2] In a conventional nitric acid manufacturing plant, the following nitrous oxide decomposition system (a reaction tube filled with catalyst 1) was installed downstream of the reduction device, and a nitrous oxide decomposition process was carried out using a reduction gas having a total content of nitric oxide and nitrogen dioxide of 0.0001 to 0.35 mol %. The exhaust gas (the reaction product (nitrous oxide-containing gas) discharged from the nitric acid synthesis process) before the denitration reaction (reduction process) of a nitric acid manufacturing plant was selectively denitrified with NH3, and the gas after the denitration reaction (reduced gas) was extracted with 12.2 g (15.3 cm 3 ) catalyst 1 was charged into a stainless steel reactor tube (inner diameter 15.6 mm) at atmospheric pressure (0.1 MPa (absolute)) and a flow rate of 1900 cm 3 The decomposition reaction of nitrous oxide was carried out in the same manner as in Example 1, except that the flow was carried out under the conditions of (0°C, 0.1013 MPa (absolute)) / min, and the decomposition activity of nitrous oxide was calculated. The NH3 selective denitration was carried out by introducing the exhaust gas into a reaction tube filled with a catalyst for removing nitrogen oxides as described in Japanese Patent Laid-Open No. 52-050990, and then introducing 60 cm of a 2.4 mol % NH3 / N2 mixed gas. 3 The flow rate was 0.1013 MPa (absolute) / min and the temperature was 270°C.

[0137] Using gas chromatography and a gas detection tube (water vapor 6, nitrogen oxide 11S, nitrogen oxide 11HA), the gas (reduced gas) before coming into contact with catalyst 1 was analyzed. The results showed that the main component was nitrogen, the nitrous oxide content was 0.12 mol%, the oxygen content was 3.0 mol%, the water vapor content was 0.25 to 0.37 mol%, and the NOx content (total content of nitric oxide and nitrogen dioxide) was 0.006 to 0.065 mol%. In the decomposition reaction of Example 2, the oxygen content in the gas (reduced gas) before the N2O decomposition reaction after 45 hours of reaction was 3.0 mol %, the NOx content was 0.02 mol %, and the water vapor content was 0.25 mol %, and the reaction rate constant at this time was 0.9 s -1 It was. After 333 hours of reaction, the oxygen content in the gas (reduced gas) before the N2O decomposition reaction was 3.0 mol%, the NOx content was 0.006 mol%, and the water vapor content was 0.31 mol%, and the reaction rate constant at this time was 1.3 s -1 It was. After 549 hours of reaction, the oxygen content in the gas (reduced gas) before the N2O decomposition reaction was 3.0 mol%, the NOx content was 0.065 mol%, and the water vapor content was 0.37 mol%, and the reaction rate constant at this time was 0.6 s -1 It was. After 597 hours of reaction, the oxygen content in the gas (reduced gas) before the N2O decomposition reaction was 3.0 mol%, the NOx content was 0.02 mol%, and the water vapor content was 0.31 mol%, and the reaction rate constant was 0.9 s -1 It was. Table 1 shows the specific activity after 597 hours of reaction, assuming that the reaction rate constant after 45 hours of reaction was 1. In addition, in a nitric acid production plant incorporating the above-mentioned nitrous oxide decomposition system, nitric acid could be produced while the nitrous oxide decomposition process was being carried out.

[0138] [Example 3] In a conventional nitric acid manufacturing plant, the following nitrous oxide decomposition system (a reaction tube filled with catalyst 1) was installed between the reduction device and the power recovery device, and the nitrous oxide decomposition process was carried out by setting the reaction pressure to 0.6 MPa (absolute) using a reduction gas having a total content of nitric oxide and nitrogen dioxide of 0.0001 to 0.35 mol %. 17.1g (22.9cm 3 The gas flowing through a stainless steel reaction tube (inner diameter 15.6 mm) packed with catalyst 1 was heated at a flow rate of 2500 cm 3 The decomposition reaction of nitrous oxide was carried out in the same manner as in Example 1, except that the exhaust gas was changed to the exhaust gas obtained by a denitration reaction under conditions of (0°C, 0.1013 MPa (absolute)) / min., and the decomposition activity of nitrous oxide was calculated. In Example 3, the exhaust gas obtained by the decomposition reaction of nitrous oxide was discharged at a high pressure of approximately 0.6 MPa during the reaction time, and therefore could be suitably used in the power recovery process and power recovery device in a conventional nitric acid production plant.

[0139] The exhaust gas obtained by the above-mentioned denitrification reaction was analyzed using gas chromatography, a gas detection tube, and an oxygen concentration meter (G-103 manufactured by Iijima Electronics Co., Ltd.). The results showed that the main component was nitrogen, the nitrous oxide content was 0.12 mol%, the oxygen content was 3.6 mol%, the water vapor content was 0.18 to 0.20 mol%, and the NOx content was 0.001 mol% or less, although the gas detection tube (nitrogen oxide 11S: detection limit 2 ppm) turned color. In the decomposition reaction of Example 3, the NOx content in the gas (reduced gas) before the N2O decomposition reaction after 2 hours of reaction was 0.0001 mol % or more and 0.001 mol % or less, the water vapor content was 0.18 mol %, and the reaction rate constant at this time was 3.2 s -1 It was. After 692 hours of reaction, the NOx content in the gas (reduced gas) before the N2O decomposition reaction was 0.0001 mol% or more and 0.001 mol% or less, and the water vapor content was 0.20 mol%, and the reaction rate constant at this time was 3.1 s -1 It was. Table 1 shows the specific activity after 692 hours of reaction, assuming that the reaction rate constant after 2 hours of reaction was 1. In addition, in a nitric acid production plant incorporating the above-mentioned nitrous oxide decomposition system, nitric acid could be produced while the nitrous oxide decomposition process was being carried out.

[0140] [Comparative Example 1] 0.8g (1.0cm 3 The mixed gas flowing through a stainless steel reaction tube (inner diameter 8 mm) packed with catalyst 1 was mixed with 0.17 mol% nitrous oxide, 3.0 mol% oxygen, 0.5 mol% water vapor, and the remaining nitrogen (flow rate: 428 cm 3 The decomposition reaction of nitrous oxide was carried out in the same manner as in Example 1, except that the pressure was changed to a mixed gas of 0.1013 MPa (absolute) / min (0°C, 0.1013 MPa (absolute)) / min and the reaction pressure was set to atmospheric pressure (0.1 MPa (absolute)), and the decomposition activity of nitrous oxide was calculated. In the decomposition reaction of Comparative Example 1, the reaction rate constant after 3 hours of reaction was 1.8 s -1 The reaction rate constant after 261 hours was 1.5 s -1 It was. Table 1 shows the specific activity after 261 hours of reaction time, assuming that the reaction rate constant after 3 hours of reaction time is 1.

[0141] [Table 1]

[0142] As is clear from the results shown in Table 1, in Comparative Example 1, in which the nitrous oxide decomposition step was carried out using a nitrous oxide-containing gas that did not contain nitric oxide or nitrogen dioxide and the reaction pressure was set to atmospheric pressure (0.1 MPa (absolute)), the specific activity was low, and the catalytic activity of the nitrous oxide decomposition catalyst decreased over time, meaning that nitrous oxide could not be decomposed efficiently over a long period of time (Comparative Example 1). In contrast, Example 1, in which the nitrous oxide decomposition process was carried out at a reaction pressure of 0.6 MPa (absolute), showed no decrease in reaction rate constant even after 513 hours, enabling the decomposition of nitrous oxide over a long period while maintaining the catalytic activity of the nitrous oxide decomposition catalyst. Example 2, in which the nitrous oxide decomposition process was carried out using a reduction gas with a total nitric oxide and nitrogen dioxide content of 0.0001 to 0.35 mol%, showed the same reaction rate constant even after 597 hours, enabling the decomposition of nitrous oxide over a long period while maintaining the catalytic activity of the nitrous oxide decomposition catalyst. Example 3, in which the nitrous oxide decomposition process was carried out at a reaction pressure of 0.6 MPa (absolute), using a reduction gas with a total nitric oxide and nitrogen dioxide content of 0.0001 to 0.35 mol%, enabled the decomposition of nitrous oxide over a long period while maintaining the catalytic activity of the nitrous oxide decomposition catalyst. Furthermore, Example 3, in which the nitrous oxide decomposition process was carried out at a reaction pressure of 0.6 MPa (absolute), showed the same reaction rate constant even after 692 hours, enabling the decomposition of nitrous oxide over a long period while maintaining the catalytic activity of the nitrous oxide decomposition catalyst. As described above, in Examples 1 and 3, the exhaust gas from the decomposition process could be suitably utilized in the power recovery process and power recovery device.

Claims

1. A method for decomposing nitrous oxide applied to a nitric acid production plant capable of at least performing a step of synthesizing nitric acid and a step of recovering power using an exhaust gas turbine, comprising: A method for decomposing nitrous oxide, comprising: a step of decomposing nitrous oxide in which a catalyst containing at least one selected from the group consisting of ruthenium and ruthenium compounds is contacted with the nitrous oxide-containing gas discharged from the step of synthesizing nitric acid, the step being carried out before the step of recovering power.

2. 2. The method for decomposing nitrous oxide according to claim 1, wherein the reaction pressure in the decomposition step is 0.2 MPa or more in terms of absolute pressure.

3. 2. The method for decomposing nitrous oxide according to claim 1, wherein the catalyst further comprises titanium oxide.

4. 4. The method for decomposing nitrous oxide according to claim 3, wherein the titanium oxide contains 80% by mass or more of a rutile structure, with the total amount of titanium oxide contained in the catalyst being 100% by mass.

5. The method for decomposing nitrous oxide according to any one of claims 1 to 4, wherein the nitrous oxide-containing gas further contains oxygen and water vapor.

6. A method for producing nitric acid in a nitric acid production plant capable of at least carrying out a step of synthesizing nitric acid and a step of recovering power by an exhaust heat gas turbine, comprising: A method for producing nitric acid, comprising: a step of decomposing nitrous oxide, in which a catalyst containing at least one selected from the group consisting of ruthenium and ruthenium compounds is brought into contact with the nitrous oxide-containing gas discharged from the step of synthesizing nitric acid, before the step of recovering power.

7. The method includes a nitrous oxide decomposition step of contacting a nitrous oxide-containing gas containing nitrous oxide, nitric oxide, and nitrogen dioxide with a catalyst containing at least one selected from the group consisting of ruthenium and ruthenium compounds, A method for decomposing nitrous oxide, wherein the total content of the nitric oxide and the nitrogen dioxide in the nitrous oxide-containing gas is 0.0001 to 0.35 mol %.

8. 8. The method for decomposing nitrous oxide according to claim 7, wherein the reaction pressure in the decomposition step is 0.2 MPa or more in terms of absolute pressure.

9. 8. The method for decomposing nitrous oxide according to claim 7, wherein the catalyst further comprises titanium oxide.

10. 10. The method for decomposing nitrous oxide according to claim 9, wherein the titanium oxide contains 80% by mass or more of a rutile structure, with the total amount of titanium oxide contained in the catalyst being 100% by mass.

11. The method for decomposing nitrous oxide according to any one of claims 7 to 10, wherein the nitrous oxide-containing gas further contains oxygen and water vapor.

12. A method for decomposing nitrous oxide applied to a nitric acid manufacturing plant capable of at least carrying out a step of synthesizing nitric acid, comprising: a step of reducing nitric oxide and nitrogen dioxide contained in the nitrous oxide-containing gas discharged from the step of synthesizing nitric acid; a nitrous oxide decomposition step of contacting the nitrous oxide-containing gas discharged from the reduction step with a catalyst containing at least one selected from the group consisting of ruthenium and ruthenium compounds, How to decompose nitrous oxide.

13. 13. The method for decomposing nitrous oxide according to claim 12, wherein the total content of nitric oxide and nitrogen dioxide in the nitrous oxide-containing gas discharged from the reducing step is 0.0001 to 0.35 mol %.

14. 14. The method for decomposing nitrous oxide according to claim 12 or 13, wherein the reaction pressure in the decomposition step is 0.2 MPa or more in terms of absolute pressure.

15. 13. The method for decomposing nitrous oxide according to claim 12, wherein the catalyst further comprises titanium oxide.

16. 16. The method for decomposing nitrous oxide according to claim 15, wherein the titanium oxide contains 80 mass % or more of a rutile structure, with the total amount of titanium oxide contained in the catalyst being 100 mass %.

17. 13. The method for decomposing nitrous oxide according to claim 12, wherein the nitrous oxide-containing gas further comprises oxygen and water vapor.

18. The nitric acid production plant may further perform a step of recovering power using an exhaust gas turbine, 13. The method for decomposing nitrous oxide according to claim 12, wherein the step of decomposing nitrous oxide is carried out before the step of recovering power and after the step of reducing power.

19. A method for producing nitric acid in a nitric acid production plant capable of at least carrying out a step of synthesizing nitric acid, a step of reducing nitric oxide and nitrogen dioxide contained in the nitrous oxide-containing gas discharged from the step of synthesizing nitric acid; a decomposition step of contacting the nitrous oxide-containing gas discharged from the reduction step with a catalyst containing at least one selected from the group consisting of ruthenium and ruthenium compounds, How to make nitric acid.

20. The nitric acid production plant may further perform a step of recovering power using an exhaust gas turbine, 20. The method for producing nitric acid according to claim 19, wherein the nitrous oxide decomposition step is carried out before the power recovery step and after the reduction step.

21. A nitrous oxide decomposition system installed in a nitric acid production plant having a nitric acid synthesis unit for synthesizing nitric acid and a power recovery unit for recovering power using an exhaust gas turbine, a nitrous oxide decomposition unit that brings a catalyst containing at least one selected from the group consisting of ruthenium and ruthenium compounds into contact with the nitrous oxide-containing gas discharged from the nitric acid synthesis unit, A nitrous oxide decomposition system, wherein the nitrous oxide decomposition device is installed upstream of the power recovery device.

22. A nitrous oxide decomposition system installed in a nitric acid production plant having a nitric acid synthesis unit for synthesizing nitric acid and a reduction unit for reducing nitric oxide and nitrogen dioxide contained in nitrous oxide-containing gas discharged from the nitric acid synthesis unit, A nitrous oxide decomposition system comprising a nitrous oxide decomposition device that brings the nitrous oxide-containing gas discharged from the reduction device into contact with a catalyst containing at least one selected from the group consisting of ruthenium and ruthenium compounds.

23. A nitrous oxide decomposition system attached to a nitric acid production plant, the system comprising, in this order, a nitric acid synthesis unit for synthesizing nitric acid, a reduction unit for reducing nitric oxide and nitrogen dioxide contained in nitrous oxide-containing gas discharged from the nitric acid synthesis unit, and a power recovery unit for recovering power using an exhaust gas turbine, a nitrous oxide decomposition device that brings the nitrous oxide-containing gas discharged from the reduction device into contact with a catalyst containing at least one selected from the group consisting of ruthenium and ruthenium compounds; Nitrous oxide decomposition system.

24. A nitric acid production plant having, in this order, a nitric acid synthesis unit for synthesizing nitric acid, a reduction unit for reducing nitric oxide and nitrogen dioxide contained in nitrous oxide-containing gas discharged from the nitric acid synthesis unit, and a power recovery unit for recovering power using an exhaust heat gas turbine, a nitrous oxide decomposition device between the reduction device and the power recovery device, which brings the nitrous oxide-containing gas discharged from the reduction device into contact with a catalyst containing at least one selected from the group consisting of ruthenium and ruthenium compounds.

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