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

Abstract

Provided are: a nitrous oxide decomposition method comprising a nitrous oxide decomposition step for bringing a specific catalyst into contact with a nitrous oxide-containing gas containing nitrous oxide, nitrogen monoxide, and nitrogen dioxide, the total content of nitrogen monoxide and nitrogen dioxide in the nitrous oxide-containing gas being 0.0001-0.35 mol%; a nitrous oxide decomposition method and decomposition system, the nitrous oxide decomposition method being applied to a nitric acid production plant and involving executing a nitrous oxide decomposition step for bringing a specific catalyst into contact with a nitrous oxide-containing gas discharged from a nitric acid synthesis step; and a nitric acid production method and plant in which said nitrous oxide decomposition method and decomposition system are used.

Description

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

[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.

[0002] From the viewpoint of protecting the global environment and preventing air pollution, nitrogen oxides (NOx) in exhaust gases are considered a problem, and their emissions are strictly regulated. The nitrogen oxides that are particularly subject to emission regulations are nitrogen dioxide (NOx), which is harmful to the human body and is a cause of photochemical smog and acid rain. ２ ), and various denitrification technologies have been studied and put into practical use to reduce emissions. However, nitrous oxide (N ２ Currently, nitrogen monoxide (NO) and nitrogen dioxide (NO) are not subject to emission regulations and are usually released directly into the atmosphere. In fact, nitrogen monoxide and nitrogen dioxide are decomposed and removed from gases emitted from chemical manufacturing plants such as nitric acid manufacturing plants, ε-caprolactam manufacturing plants, and adipic acid manufacturing plants by denitrification treatment, but the by-product nitrous oxide is often not decomposed and removed and is instead released (released) into the atmosphere.

[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 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 a technology that decomposes and removes 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.

[0004] Japanese Patent Application Publication No. 06-218232

[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 decomposing nitrous oxide generally lose their catalytic activity gradually when used to decompose nitrous oxide. In particular, when applying this method 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 above-mentioned circumstances surrounding nitrous oxide and its application to industrial production processes, it is desirable 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 issue.

[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.

[0007] The objects of the present invention have been achieved by the following means: <1> A method for decomposing nitrous oxide applicable to a nitric acid production plant capable of at least carrying out a step of synthesizing nitric acid and a step of recovering power using an exhaust heat gas turbine, wherein a nitrous oxide decomposition step of contacting a nitrous oxide-containing gas discharged from the step of synthesizing nitric acid with a catalyst containing at least one selected from the group consisting of ruthenium and ruthenium compounds is carried out before the step of recovering power. <2> The method for decomposing nitrous oxide according to <1>, wherein the reaction pressure in the decomposition step is 0.2 MPa or more in absolute pressure. <3> The method for decomposing nitrous oxide according to <1> or <2>, wherein the catalyst further contains titanium oxide. <4> The method for decomposing nitrous oxide according to <3>, wherein the titanium oxide contains a rutile structure in an amount of 80% by mass or more, assuming that the total amount of titanium oxide contained in the catalyst is 100% by mass. <5> The method for decomposing nitrous oxide according to any one of <1> to <4>, wherein the nitrous oxide-containing gas further contains oxygen and water vapor.

[0008] <6> A method for producing nitric acid in a nitric acid production plant capable of carrying out at least a step of synthesizing nitric acid and a step of recovering power using an exhaust heat gas turbine, wherein 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 a nitrous oxide-containing gas discharged from the step of synthesizing nitric acid is carried out before the step of recovering power.

[0009] <7> A method for decomposing nitrous oxide, comprising a step of decomposing nitrous oxide by 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, 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 method for decomposing nitrous oxide according to <7>, wherein the reaction pressure in the decomposition step is 0.2 MPa or more in absolute pressure. <9> The method for decomposing nitrous oxide according to <7> or <8>, wherein the catalyst further contains titanium oxide. <10> The method for decomposing nitrous oxide according to <9>, 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 %. <11> The method for decomposing nitrous oxide according to any one of <7> to <10>, wherein the nitrous oxide-containing gas further contains oxygen and water vapor.

[0010] <12> A method for decomposing nitrous oxide applicable to a nitric acid manufacturing plant capable of at least performing a step of synthesizing nitric acid, comprising: a step of reducing nitric oxide and nitrogen dioxide contained in a nitrous oxide-containing gas discharged from the step of synthesizing nitric acid; and a 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. <13> The method for decomposing nitrous oxide according to <12>, wherein 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 %. <14> The method for decomposing nitrous oxide according to <12> or <13>, wherein the reaction pressure in the decomposition step is 0.2 MPa or more in absolute pressure. <15> The method for decomposing nitrous oxide according to any one of <12> to <14>, wherein the catalyst further contains titanium oxide. <16> The method for decomposing nitrous oxide according to <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> The method for decomposing nitrous oxide according to any one of <12> to <16>, wherein the nitrous oxide-containing gas further contains oxygen and water vapor. <18> The method for decomposing nitrous oxide according to any one of <12> to <17>, wherein the nitric acid production plant is further capable of carrying out a step of recovering power using an exhaust gas turbine, and the nitrous oxide decomposition step is carried out before the power recovery step and after the reduction step.

[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, comprising: a step of reducing nitric oxide and nitrogen dioxide contained in a nitrous oxide-containing gas discharged from the step of synthesizing nitric acid; and a 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. <20> The method for producing nitric acid according to <19>, wherein the nitric acid production plant is further capable of carrying out a step of recovering power using an exhaust heat gas turbine, and the nitrous oxide decomposition step is carried out before the step of recovering power and after the step of reducing nitrous oxide.

[0012] <21> A nitrous oxide decomposition system to be installed adjacent to a nitric acid production plant having a nitric acid synthesis unit that synthesizes nitric acid and a power recovery unit that recovers power using an exhaust heat gas turbine, the system comprising: a nitrous oxide decomposition unit that brings a nitrous oxide-containing gas discharged from the nitric acid synthesis unit into contact with a catalyst containing at least one selected from the group consisting of ruthenium and ruthenium compounds, the nitrous oxide decomposition unit being installed upstream of the power recovery unit. <22> A nitrous oxide decomposition system to be installed adjacent to a nitric acid production plant having a nitric acid synthesis unit that synthesizes nitric acid and a reduction unit that reduces nitric oxide and nitrogen dioxide contained in the nitrous oxide-containing gas discharged from the nitric acid synthesis unit, the system comprising: a nitrous oxide decomposition unit that brings a nitrous oxide-containing 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. <23> A nitrous oxide decomposition system to be installed in a nitric acid production plant having, in this order, a nitric acid synthesis unit that synthesizes nitric acid, a reduction unit that reduces nitric oxide and nitrogen dioxide contained in a nitrous oxide-containing gas discharged from the nitric acid synthesis unit, and a power recovery unit that recovers power using an exhaust gas turbine, the nitrous oxide decomposition system having the 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 reduction unit.

[0013] <24> A nitric acid production plant having, in this order, a nitric acid synthesis unit that synthesizes nitric acid, 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, wherein the nitric oxide production plant has a nitrous oxide decomposition unit between the reduction unit and the power recovery unit that brings the nitrous oxide-containing 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.

[0014] The present invention provides a method for decomposing nitrous oxide, which can maintain catalytic activity and decompose nitrous oxide for a long period of time. The present invention also provides a method or system for decomposing nitrous oxide, which can maintain catalytic activity and decompose nitrous oxide for a long period of time, even when applied to a nitric acid production plant. Furthermore, the present invention provides a method and plant for producing nitric acid, which can industrially produce nitric acid while maintaining catalytic activity and decomposing nitrous oxide for a long period of time. The above and other features and advantages of the present invention will become more apparent from the following description.

[0015] In the present invention, the term "gas discharged from a process or apparatus" refers to gas generated during the process or (newly generated) within the apparatus and discharged from the process or apparatus, as well as gas generated in a process performed prior to the process or gas generated in an apparatus disposed prior to the process and discharged (emitted) from the process or apparatus. Whether the gas discharged from a process or apparatus is a gas generated within the process or apparatus or a gas generated in an apparatus disposed prior to the process or apparatus can be clearly understood by taking into account common technical knowledge related to the nitric acid production process. When describing the content, physical properties, etc., by indicating a numerical range, when the upper and lower limits of the numerical range are separately described, 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" (between "to"), the upper and lower limits forming the numerical range are not limited to the specific combinations described before and after "to" as the specific numerical range, and can be any numerical range obtained by appropriately combining the upper and lower limits of each numerical range. 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 lower and upper 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 can maintain the catalytic activity of the nitrous oxide decomposition catalyst (suppressing deactivation or deterioration of the nitrous oxide decomposition catalyst), and can continuously decompose the nitrous oxide in the nitrous oxide-containing gas into molecular nitrogen (usually nitrogen gas) and molecular oxygen (usually oxygen gas) over an extended period of time. 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 the 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 comprising a titanium oxide-containing carrier, on which a first component (hereinafter also referred to as the first supported component) containing at least one selected from the group consisting of ruthenium and ruthenium compounds, and a second component (hereinafter also referred to as the second supported component) containing at least one selected from the group consisting of antimony, antimony compounds, cerium, cerium compounds, zirconium, zirconium compounds, silicon, and silicon compounds are supported. Catalyst II: A catalyst comprising a titanium oxide and silicon oxide-containing carrier on which the first supported component is supported. In Catalyst II, it is preferred that the first supported component and the second supported component are supported on a titanium oxide and silicon oxide-containing carrier, and for convenience, this catalyst will be referred to as "Catalyst II (preferred embodiment)." In Catalyst II, it is preferred that the silicon oxide be present (dispersed) on or near the surface of the titanium oxide primary particles.

[0018] In the present invention, the term "supported component" collectively refers to components (such as elements and compounds) 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. In the catalyst used in decomposition method A of the present invention, the first supported component supported on the carrier contains at least one selected from the group consisting of ruthenium and ruthenium compounds, from the viewpoint of a balance between catalytic activity and cost.

[0019] <Supported Components> Catalyst I includes, 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 on catalyst I may include 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, as supported components supported on a carrier described below, a first supported component having the ability to decompose nitrous oxide. The supported components supported on catalyst II preferably include 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 described below.

[0020] (First Supported Component) The first supported component supported 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 supported by each catalyst is not particularly limited as long as it is one or more, and can be, for example, 1 to 4. - Ruthenium Compound - The ruthenium compound is not particularly limited, and examples thereof include ruthenium oxide, ruthenium hydroxide, ruthenium nitrate, ruthenium chloride, ruthenium acid, chlororuthenate, chlororuthenate hydrate, salts of ruthenate, ruthenium oxychloride, salts of ruthenium oxychloride, ruthenium ammine complex, chloride of ruthenium ammine complex, ruthenium bromide, ruthenium carbonyl complex, ruthenium organic acid salt, and ruthenium nitrosyl complex. Examples of ruthenium oxide include RuO ２ Ruthenium hydroxide includes Ru(OH) ３ Ruthenium nitrate includes Ru(NO ３ ) ３ Examples of ruthenium chloride include RuCl ３ , RuCl ３ Ruthenic acid includes H ２ RuO ４ Examples include:

[0021] As the chlororuthenate, K ３ RuCl６ etc., [RuCl ６ 〕 ３－ a salt with an anion K ２ RuCl ６ Ya (NH ４ ) ２ RuCl ６ etc., [RuCl ６ 〕 ２－ Examples of chlororuthenate hydrates include salts with the anion [RuCl ５ (H ２ O) ４ 〕 ２－ RuCl ２ (H ２ O) ４ 〕 ＋ Examples of ruthenic acid salts include salt hydrates with Ru as the cation. ＶＩ O ４ ２－ (tetraoxoruthenium(VI) ion) salt and Ru ＶＩＩ O ４ － (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 ＋ Among these, alkali metal salts of ruthenic acid (salts of Li, Na, K, Rb, and Cs) are preferred, and Na or K salts of ruthenic acid are more preferred. Specifically, Na ２ RuO ４ , K. ２ RuO ４ Ruthenium oxychloride includes Ru ２ OCl ４ , Ru ２ OCl ５ , Ru ２ OCl ６ Examples of ruthenium oxychloride salts include K ２ Ru ２ OCl １０ , Cs ２ Ru ２ OCl ４Examples of ruthenium ammine complexes include [Ru(NH ３ ) ６ 〕 ２＋ , [Ru(NH ３ ) ６ 〕 ３＋ , [Ru(NH ３ ) ５ H ２ O] ２＋ Examples of ruthenium ammine complex chlorides include [Ru(NH ３ ) ５ Cl ２＋ a complex with a complex ion [Ru(NH ３ ) ６ 〕Cl ２ , [Ru(NH ３ ) ６ 〕Cl ３ , [Ru(NH ３ ) ６ 〕Br ３ Ruthenium bromide includes RuBr ３ , RuBr ３ Ruthenium carbonyl complexes include Ru(CO) ５ , Ru ３ (CO) １２ Examples of ruthenium organic acid salts include [Ru ３ O (OCOCH ３ ) ６ (H ２ O) ３ ]OCOCH ３ hydrate, Ru ２ (RCOO) ４ Cl (R = alkyl group having 1 to 3 carbon atoms). Ruthenium nitrosyl complexes include K ２ [RuCl ５ NO)], [Ru(NH ３ ) ５ (NO) ]Cl ３ , [Ru(OH)(NH ３ ) ４ (NO) ３ ) ２ , Ru(NO)(NO ３ ) ３ Examples include:

[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 containing ruthenium oxide, and even more preferably ruthenium oxide. The ruthenium compound may be a 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, the ruthenium oxide may be an oxide containing ruthenium as one of its constituent elements, or may be an oxide of ruthenium alone (RuO ２ In addition to ruthenium, the term "ruthenium oxide" also includes composite oxides containing ruthenium and (metallic or non-metallic) elements other than ruthenium. The same applies to (metallic) ruthenium, and also includes alloys of ruthenium and metals other than ruthenium.

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

[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" means that ruthenium may be unavoidably present in the catalyst. When a catalyst supporting the second supported component is used for the decomposition of nitrous oxide, the catalytic activity of the catalyst can be suppressed from decreasing 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 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, and 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 0 to 8 types, preferably 1 to 6 types, and 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 appropriate compounds can be used, including, for example, compounds of the same kind 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).

[0026] Antimony Compounds 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 oxide include Sb ２ O ３ , Sb ２ O ４ , Sb ２ O ５ Antimony sulfate includes Sb ２ (SO ４ ) ３Examples of antimony chloride include SbCl ３ Examples of salts of antimonic acid include NaSbO ３ These include:

[0027] Cerium Compounds Examples of cerium compounds include cerium oxide, cerium hydroxide, cerium nitrate, cerium chloride, ceric acid salts, cerium sulfate, and cerium carbonate, and cerium oxide, cerium nitrate, cerium chloride, and cerium sulfate are preferred. Examples of cerium oxide include CeO ２ , Ce ２ O ３ Examples of cerium hydroxide include CeO ２ ・2H ２ O. Cerium nitrate includes Ce(NO ３ ) ３ ・6H ２ Examples of cerium chloride include CeCl ３ ・7H ２ Examples of ceric acid salts include Ce(NH ４ ) ２ (NO ３ ) ６ Examples of cerium sulfate include Ce(SO ４ ) ２ ・4H ２ Examples of cerium carbonate include Ce ２ (CO ３ ) ３ ・8H ２ Examples include O.

[0028] Zirconium Compounds Examples of zirconium compounds include zirconium oxide, zirconium hydroxide, zirconium oxynitrate, zirconium chloride, zirconium sulfate, zirconium acetate, and zirconium acetylacetonate, and zirconium oxide, zirconium oxynitrate, zirconium chloride, and zirconium sulfate are preferred. Zirconium oxide is preferably ZrO ２ Zirconium hydroxide includes Zr(OH) ４Zirconium oxynitrate includes ZrO(NO ３ ) ２ ・2H ２ Zirconium chloride includes ZrCl ３ , ZrCl ４ Zirconium sulfate includes Zr(SO ４ ) ２ ・4H ２ Zirconium acetate includes Zr(OCOCH ３ ) ４ Zirconium acetylacetonate includes Zr(C ５ H ７ O ２ ) ４ Examples include:

[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 SiO ２ Examples of silicon chloride include SiCl ４ Examples of silicic acid salts include salts of orthosilicic acid, pyrosiliic acid, metasilicic acid, etc., and more specifically, Na ２ SiO ３ , Na ４ SiO ４ , Na ２ Si ２ O ５ , Na ２ Si ４ O ９ Examples of silicon alkoxides include Si(OC ２ H ５ ) ４ , Si(OC ３ H ７ ) ４ , Si(OC ４ H ９ ) ４ Examples include:

[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 set appropriately taking into account 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 under either the first supported component or the second supported component, but examples 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] <Support> 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 may contain 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 titanium oxide-containing compound and / or secondary particles formed by aggregation of primary particles, molded into a desired shape. Furthermore, the term "surface of the support" refers to the surface of the molded body, and the term "pores of the support" refers to one or more of the pores of the primary particles, the voids between primary particles in the secondary particles, and the voids between secondary particles in an aggregate of secondary particles. In the present invention, the crystalline form of the titanium oxide constituting the support is not particularly limited and may be any of rutile, anatase, and brookite crystalline forms. In the present invention, the titanium oxide constituting the support preferably contains titanium oxide in the rutile crystalline form. From the viewpoint of catalytic activity, the content of rutile crystalline 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, based on 100% by mass of the total amount of titanium oxide contained in the carrier.

[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 can be used, including copper Kα radiation. When copper Kα radiation is used, the ratios of rutile crystals and anatase crystals are determined using the intensities of the diffraction peaks at 2θ = 27.5 degrees on the (110) plane and 2θ = 25.3 degrees on the (101) plane, 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 carrier constituting catalyst II contains silicon oxide, preferably on the surface (including near the surface) of the carrier and / or on the inner surfaces (including near the inner surfaces) of the pores. When catalyst II, whose carrier contains silicon oxide, is used for the decomposition of nitrous oxide, the catalytic activity of catalyst II can be suppressed 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 (e.g., a dried colloidal silica in a particulate or granular form), which 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 diameter (d) [nm] is the specific surface area (S) [m ２ / g] using the following formula, where ρ is the true density of silicon oxide, 2.2 [g / cm ３The value of d=6000 / (S×ρ) is used. Note that the particle size of silicon oxide present in the carrier of catalyst II does not necessarily have to maintain the particle size of 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% by total mass of the carrier) is not particularly limited and may be set as appropriate. For example, from the viewpoint of being able to suppress 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 each carrier 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 a commercially available product can also be used. Examples of methods for preparing titanium oxide in the rutile crystal form include the following methods. A method in which titanium tetrachloride is dissolved dropwise in ice-cooled water, neutralized with an aqueous ammonia solution at a temperature of 20°C or higher to produce titanium hydroxide (orthotitanic acid), and then the resulting precipitate is 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 an oxygen-nitrogen mixed gas is passed through a titanium tetrachloride evaporator to prepare a reaction gas, which is then introduced into a reactor and reacted at 900°C or higher (Catalyst Preparation Chemistry, 1989, p. 89, Kodansha); a method in which titanium tetrachloride is hydrolyzed in the presence of ammonium sulfate and then calcined (e.g., Catalysis Engineering Lecture Series 10: Handbook of Elemental Catalysts, 1978, p. 254, Chijin Shokan); a method in which anatase crystalline titanium oxide is calcined (e.g., Metal Oxides and Complex Oxides, 1980, p. 107, Kodansha); A method of thermally hydrolyzing an aqueous solution of titanium chloride; and a method of mixing an aqueous solution of a titanium compound such as titanium sulfate or titanium chloride with rutile crystalline titanium oxide powder, followed by thermal hydrolysis or alkaline hydrolysis, and then calcining 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 or the like into a desired shape. When the support contains titanium oxide and other compounds (e.g., silicon oxide), the support can be obtained by forming a mixture of titanium oxide and other compounds into a desired shape.

[0042] The shape of the catalyst (carrier) is not particularly limited and can be set as appropriate. In a preferred embodiment of the present invention, the catalyst can be in a honeycomb shape (honeycomb structure). In a preferred embodiment, the honeycomb-shaped catalyst is typically used as is (in a honeycomb structure), but it can also be crushed and the crushed material classified before use. In this case, the size can be approximately the same as that of the spherical granules described below. Meanwhile, 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-shaped, monolithic, and corrugated shapes, as well as granular and fine particles of an appropriate size obtained by crushing and classifying the formed catalyst. The catalyst shape is preferably spherical, cylindrical, or ring-shaped pellet shapes, monolithic, corrugated, or granular. However, from the viewpoint of nitrous oxide decomposition efficiency, ring-shaped pellet shapes are more preferred.

[0043] In the present invention, the term "honeycomb structure" refers to the "honeycomb structure" commonly used in honeycomb catalysts, such as those commonly used in exhaust gas purification catalysts. For example, a structure in which a substrate such as a columnar body is perforated with a plurality of through-holes densely arranged in the planar direction is exemplified. The substrate may have 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 other factors, and examples of such shapes include a columnar body, a block body, and a plate body. The shape of the through-holes is not particularly limited, and examples include polygons such as squares and hexagons, circles, and ellipses. The arrangement of the through-holes is not particularly limited, and is determined appropriately taking into account the shape of the openings. For example, in the planar direction of the substrate (usually a plane perpendicular to the axis), a straight arrangement, a staggered arrangement, a honeycomb arrangement, etc. 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, one preferred embodiment is that 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 is preferably 10 to 200 [g / L] per unit volume of the substrate having a honeycomb structure, more preferably 30 to 100 [g / L].

[0045] The dimensions of the catalyst (carrier) are not particularly limited and can be set appropriately. When the catalyst has a shape such as spherical granules or cylindrical pellets, the catalyst diameter is preferably 10 mm or less from the viewpoint of catalytic activity. Here, the catalyst diameter means the diameter of the sphere in the case of spherical granules, the diameter of the cross section in the case of cylindrical pellets, and the maximum diameter of the cross section in the case of other shapes. When the catalyst has a shape such as spherical granules or cylindrical pellets, it is preferable that the filling volume ratio (described later) be 35 to 74 volume % when the catalyst is filled in the catalyst filling region of a reactor (reaction tube) to form a catalyst packed layer.

[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 enhance 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. In order to further enhance the decomposition efficiency of nitrous oxide, the honeycomb structure (catalyst) preferably has a filling volume fraction of 35 to 50 volume %, more preferably 38 to 50 volume %, when packed in the catalyst packed region of a reactor (reaction tube) to form a catalyst packed layer. The filling volume fraction 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 into the catalyst-packed region of the reactor (reaction tube), taking into consideration the volume ratio or packing volume ratio, and usually to outer dimensions that are approximately the same as the inner dimensions (inner diameter and length) of the catalyst-packed 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 determined as appropriate, and in the present invention, they are determined taking into consideration the (filling) volume ratio and the like. 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, the catalyst can be a combination of multiple catalysts (number of catalysts) or multiple types of catalysts. 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 as a whole satisfy the above-mentioned filling volume ratio), but preferably has a honeycomb structure that satisfies the above-mentioned filling volume ratio alone. 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 powder form), the catalyst disposed as the washcoat layer can also be a combination of multiple catalysts.

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

[0050] <Method for Producing Catalyst> The catalyst used in decomposition method A of the present invention can be produced by various known production methods. For example, it can be produced by a method of 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, 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 can be used as the solution containing the supported component, or a mixed solution of solution A and solution B can also be used. When producing catalyst II, solution A can be used as the solution containing the supported component, and 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 producing catalyst II includes the following steps. In the preferred production method, after step 1 is performed, either step 2 or step 3 can be performed first, and step 2A described below can be performed instead of steps 2 and 3. Step 1: A step of calcining a carrier precursor obtained by extruding a carrier raw material mixture containing titanium oxide, silicon oxide, and water to obtain a carrier. Step 2: A step of supporting a first component raw material containing a ruthenium compound on the carrier obtained in Step 1 or on the carrier on which a second component raw material has been supported in Step 3. Step 3: A step of supporting a second component raw material containing at least one selected from the group consisting of an antimony compound, a cerium compound, a zirconium compound, and a silicon compound on the carrier obtained in Step 1 or on the carrier on which the first component raw material has been supported in Step 2. Step 4: A step of calcining a catalyst precursor in which the first component raw material and the second component raw material have been supported on the carrier obtained in Step 1. Step 2A: A step of supporting a first component raw material containing a ruthenium compound and a second component raw material containing at least one selected from the group consisting of an antimony compound, a cerium compound, a zirconium compound, and a silicon compound on the carrier obtained in Step 1.

[0053] (Step 1) The titanium oxide used in Step 1 is not particularly limited, and as described above, titanium oxide produced by various methods or commercially available products can be used. The silicon oxide used in Step 1 is not particularly limited, and titanium 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 the silicon oxide in the 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 the silicon oxide is the average particle size measured by the above-mentioned method. An example of a commercially available colloidal silica is the one used in the Examples described below (Snowtex ST-CM). 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 to a ratio that results in the above-mentioned silicon oxide content in the carrier. The mixing ratio of water is not particularly limited and can be set as appropriate, and it is preferably set to a ratio that results in the carrier raw material mixture becoming 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. The mixing of titanium oxide, silicon oxide, and water can be carried out 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 appropriate shape, preferably into the above-mentioned honeycomb structure. The extrusion of the carrier raw material mixture can be carried out using a conventional extruder, for example, a vacuum kneading extrusion molding machine or a hydraulic extrusion molding machine. In particular, when producing a catalyst having a honeycomb structure, it is preferable to use a vacuum kneading extrusion molding machine. The extrusion conditions are not particularly limited, but for example, the kneading or extrusion temperature can be 5 to 40°C. The carrier precursor can be obtained in this manner, and if necessary, the molded body can be dried to obtain the carrier precursor.

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

[0057] (Step 2) In step 2, a first component raw material containing a ruthenium compound is supported on a support. The support used in this step differs depending on the order in which steps 2 and 3 are performed. That is, when step 2 is performed prior to step 3, or when step 2 is performed simultaneously with step 3 (step 2A), the support (unsupported support) obtained in step 1 is used. On the other hand, when step 2 is performed after step 3, a support on which a second component raw material is supported 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. The ruthenium compound used in step 2 can be any known compound, such as the above-mentioned first supported component, or Ru(NO ３ ) ３ , RuCl ３ , RuCl ３Hydrate, Na ２ RuO ４ , K. ２ RuO ４ , Ru(NO)(NO ３ ) ３ and the like. The first component raw material is usually used in the form of an aqueous solution. The content (concentration) of the first component raw material in this case 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 component raw materials other than the first component raw material and the second component raw material, and examples of such component raw materials include the above-mentioned third component raw material, and also organic solvents such as alcohols that allow the aqueous solution to be well absorbed into the support. In addition to the aqueous solution, a solution of an organic solvent such as alcohol can also be used.

[0058] In step 2, the support is contacted with the first component raw material to support the first component raw material on the support. The contact method and conditions are not particularly limited and can be set appropriately. Various methods for supporting various components on a support can be used as the contact method for producing a catalyst. Examples include a method of immersing the support in the aqueous solution and a coating method in which the aqueous solution is sprayed or applied to the support. 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 per 1 g of support is more preferred, and 0.2 to 2 mL is even more preferred. Examples of contact conditions include contact at 5 to 40°C (preferably 10 to 30°C) for 1 to 300 minutes (preferably 5 to 180 minutes). In step 2, the support 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 a support containing titanium oxide and silicon oxide.

[0059] (Step 3) In step 3, a second component raw material containing at least one selected from the group consisting of an antimony compound, a cerium compound, a zirconium compound, and a silicon compound is supported on a support. The support 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 support (unsupported support) obtained in step 1 is used. On the other hand, when step 3 is performed after step 2, a support 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 an antimony compound, in the produced catalyst. They may be the second supported component itself or a precursor compound leading to the second supported component. The compound used in step 3 can be any known compound, such as SbCl, which can also be the second supported component described above and a precursor compound for the second supported component. ３ , Ce(NO ３ ) ３ ・6H ２ O, ZrO(NO ３ ) ２ ・2H ２ O, Si(OC ２ H ５ ) ４ and the like. The second component raw material is usually used in the form of an aqueous solution. The content (concentration) of the second component raw material in this case 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 a component raw material other than the first component raw material and the second component raw material, and examples of such a component raw material include the above-mentioned third component raw material and further 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 contacted 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 step 2 above. In particular, the amount of aqueous solution containing the second component raw material used 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, 0.1 to 10 mL is more preferable, and 0.2 to 2 mL is even more preferable, per 1 g of carrier. In step 3, the carrier impregnated with the second component raw material can be dried by a standard method. In this way, the second component raw material can be supported or adsorbed on the carrier containing titanium oxide and silicon oxide.

[0061] (Step 2A) In a preferred method for producing catalyst II, steps 2 and 3 can also be performed simultaneously. In this case, instead of steps 2 and 3, step 2A is performed in which the first and second component raw 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 described in steps 2 and 3. The aqueous solution used in step 2A may also contain component raw materials other than the first and second component raw materials. Examples of such component raw materials include the third component raw material and the organic solvents described above. In addition to the aqueous solution, a solution of an organic solvent such as an alcohol can also be used. In step 2A, the support is contacted with the first and second component raw materials to support the first and second component raw materials on the support. The contact method and conditions are not particularly limited and are the same as the contact 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 standard method, allowing the first and second component raw materials to be supported or adsorbed on the carrier containing titanium oxide and silicon oxide.

[0062] (Step 4) In step 4, the catalyst precursor obtained in steps 2 and 3 or step 2A, in which the first and second component raw materials are supported on a carrier, is calcined. The calcination of the catalyst precursor can be carried out 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 components 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 Steps) In a preferred method for producing catalyst II, steps other than the above steps 1 to 4 may also be performed. For example, these steps include a step of drying the support that has been contacted with the aqueous solution in step 2, step 3, and step 2A, 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 allows for the simple production of 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 comprising titanium oxide and silicon oxide. It should be noted that omitting step 3 or step 2A above allows for the production of catalyst II in which the first supported component is supported on or adsorbed onto a support comprising titanium oxide and silicon oxide. Furthermore, by preparing the support raw material mixture 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 on or adsorbed onto a support comprising titanium oxide.

[0065] (Formation of Washcoat Layer) In a preferred embodiment in which 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 pulverization, crushing, or the like, and then disposed on a substrate having a honeycomb structure, preferably as a washcoat layer, to produce a catalyst washcoat layer. The catalyst washcoat layer can be applied and adhered to the substrate surface by any means known in the art. For example, the washcoat layer can be formed by coating a substrate with a slurry containing a catalyst prepared to a specific solids content (e.g., about 30 to about 90% by mass) in a liquid such as water or alcohol by means of spraying or immersion, followed by drying. The catalyst used to prepare the slurry can be a catalyst produced by the above-described method that has been powdered by pulverization, crushing, 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, wherein 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 the amount of catalyst required to achieve a sufficient nitrous oxide decomposition rate increases. 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 various gases such as ammonia, oxygen, steam (water), nitrogen, carbon dioxide, and inert gases (helium, argon) as diluent gases, 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 decomposition method A of the present invention, the nitrous oxide-containing gas needs 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, excluding components with special effects, it is usually most efficient to use the nitrous oxide-containing gas at approximately the same concentration as the specific value for the factory from which the nitrous oxide-containing gas is emitted. Thus, 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, it is sufficient that the content of each gas in the nitrous oxide-containing gas is substantially satisfied while the nitrous oxide is being decomposed, but it is acceptable for the content to temporarily fall below the specified level during long-term contact with the nitrous oxide decomposition catalyst.

[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 high suppression of deactivation of the catalytic activity, the nitrous oxide-containing gas may have a total content of nitric oxide and nitrogen dioxide in 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 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., the 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 described 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 preferably 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 content ratio of ammonia to water vapor contained in the nitrous oxide-containing gas [ammonia / water vapor] is not particularly limited and can be set appropriately, but from the viewpoint of the decomposition rate of nitrous oxide, a molar ratio of 0.0010 or more is preferable. A molar ratio of 0.0010 to 0.050 is more preferable, and from the viewpoint 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 preferable, and a molar ratio of 0.0010 to 0.010 is even more preferable. 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 is preferably a molar ratio of 0.005 to 10. 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 the 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 absence of a reducing gas in the nitrous oxide-containing gas includes not only an embodiment in which the reducing gas content is 0 mol %, but also an embodiment in which the reducing gas is contained at a molar ratio relative 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 improve the decomposition rate of nitrous oxide. Similarly, to further improve the decomposition rate of nitrous oxide, the nitrous oxide-containing gas can also contain a saturated hydrocarbon gas, which serves as a raw material that reacts with oxygen contained in the nitrous oxide-containing gas or generated in the reactor to generate a reducing gas such as carbon monoxide. 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 suitable gases include unsaturated hydrocarbon gases such as ethylene, propylene, α-butylene, and β-butylene, carbon monoxide gas, hydrogen gas, and alcohol compound gases such as methanol, ethanol, propanol, and butanol. Among these, at least one of carbon monoxide gas, unsaturated hydrocarbon gas, and hydrogen gas is preferred. Examples of saturated hydrocarbon gases that can be used as raw materials to generate reducing gases such as carbon monoxide include methane, ethane, propane, and n-butane. Preferred saturated hydrocarbon gases include ethane, propane, and n-butane. A mixture of natural gas, liquefied natural gas, and liquefied petroleum gas may be used to contain the saturated hydrocarbon 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 as appropriate. 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 the water vapor in the nitrous oxide-containing gas [reducing gas or saturated hydrocarbon gas / water vapor] is preferably 0.0003 to 0.03.The ratio of the content of the reducing gas or saturated hydrocarbon gas to the content of the nitrous oxide contained in the nitrous oxide-containing gas [reducing gas or saturated hydrocarbon gas / nitrous oxide] is preferably 0.01 to 100 in molar ratio.

[0072] The nitrous oxide-containing gas used in 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 described above. 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 of water vapor and oxygen-containing gas. Examples of oxygen-containing gases include air. Furthermore, various exhaust gases emitted from chemical manufacturing plants, as well as exhaust gases emitted from automobiles, power plants using ammonia fuel, and ships, can also be used as the nitrous oxide-containing gas. 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, in addition to nitrous oxide, nitric oxide and nitrogen dioxide, as well as water vapor, oxygen gas, and in some cases ammonia gas, and can be effectively utilized in decomposition method A of the present invention. In particular, exhaust gases satisfying the above-described ranges of content, content ratio, and the like are preferred in that they can be directly applied to decomposition method A of the present invention without content adjustment, etc.

[0073] [Decomposition Step] The decomposition method A of the present invention includes contacting the catalyst with the nitrous oxide-containing gas (decomposition step). The decomposition step may be any step of contacting the catalyst with the nitrous oxide-containing gas, and may be a decomposition step in a known method for decomposing nitrous oxide. Examples of known decomposition steps in nitrous oxide decomposition methods include the method (step) described in Patent Document 1, in which the catalyst is contacted with nitrous oxide in the presence of a reducing gas. The nitrous oxide-containing gas can also be contacted by flowing the catalyst 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. A continuous method is preferred in terms of reaction efficiency, the advantage of the present invention, which is that catalytic activity can be maintained for a long period of time, and the 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, the 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. ２ O → N ２ + 1 / 2 O ２

[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 process is applied as the decomposition process, the contact method and contact conditions may be any appropriate method and conditions that can be used in each process, and examples thereof include the following conditions. The contact temperature (reaction temperature) is determined as appropriate, 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, and more preferably 250 to 400°C. In a continuous contact method, the supply rate of the nitrous oxide-containing gas relative to the catalyst mass is not particularly limited and is determined as appropriate, and is, for example, 10 to 10,000 cm3 as a flow rate at 0°C and 0.1013 MPa (absolute) per 1 g of catalyst. ３ / min, and preferably 50 to 5000 cm ３ / min. The contact time is determined appropriately depending on the nitrous oxide concentration or supply rate in the nitrous oxide-containing gas, the contact temperature, and the like. In 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, and the like, but is preferably a pressure higher than the outside air, preferably 0.08 to 1 MPa (absolute) in absolute pressure, and more preferably 0.09 to 0.7 MPa (absolute) in absolute pressure. In terms of improving productivity per unit time and ease of application to various production plants, the reaction pressure is preferably 0.2 MPa (absolute) or more in each of the above ranges. In particular, when the cracked gas produced by decomposition 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 more.

[0078] [Other Steps] 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 production plants, automobiles, and power plants and ships that use ammonia fuel. It is particularly suitable for use in chemical production plants, such as nitric acid production plants, ε-caprolactam production plants, and adipic acid production plants, which emit nitrous oxide-containing gases containing 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 location of the apparatus for carrying out the decomposition method A of the present invention is not particularly limited, but it is usually incorporated downstream in the direction of exhaust gas flow, for example, upstream of an exhaust tower. Specifically, in the case of a nitric acid production plant, it is incorporated downstream of a denitrification reactor (the reduction step described below). 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 for effective utilization of the existing production plant.

[0081] [Method B for Decomposing Nitrous Oxide] Another aspect of the method for decomposing nitrous oxide of the present invention is a decomposition method that is applied (implemented) or incorporated into a nitric acid production plant or an (industrial) method for producing nitric acid (sometimes simply referred to in the present invention as "decomposition method B of the present invention"). 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 method for producing nitric acid (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 method for producing nitric acid (also referred to as a "nitric acid production method").

[0082] [Conventional Nitric Acid Production Plant and Conventional Nitric Acid Production Method] A nitric acid production plant to which the decomposition method B of the present invention is applied (a plant or a nitric acid production method to which the decomposition method of the present invention is not applied, 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 carrying out a conventional nitric acid production method, and is preferably a plant having an equipment configuration capable of carrying out 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 carried out in this order is preferred. Here, the nitric acid synthesis process by the Ostwald method is generally a process of synthesizing nitrogen dioxide by oxidizing ammonia (hereinafter referred to as "NO ２ This is sometimes called the "synthetic process." ２ and a step of reacting the nitrogen dioxide synthesized in the synthesis step with water to produce nitric acid (hereinafter sometimes referred to as the "nitric acid production step").

[0083] <Nitric acid synthesis step> In the present invention, the nitric acid synthesis step (NO ２ The synthesis step and nitric acid production step can be performed using known processes without any particular limitations. ２ The synthesis step includes a step of heating ammonia to about 900°C in air in the presence of a platinum catalyst or the like, and the nitric acid production step includes a step of reacting nitrogen dioxide with water. ２ In the synthesis process, in addition to the target nitrogen dioxide, nitric oxide and nitrous oxide are by-produced, and NO２ The reaction products of the synthesis process include nitrogen dioxide, nitric oxide and nitrous oxide.

[0084] NO ２ When the reaction product of the synthesis step is subjected to the next step, the nitric acid production step, nitrogen dioxide reacts with water to produce nitric acid, but some of the nitrogen dioxide remains, as well as nitric oxide and nitrous oxide. ２ The reaction product obtained through the 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. Examples of the nitrous oxide-containing gas discharged from the nitric acid synthesis step include 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 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 sometimes 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 sometimes 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 Step> In the present invention, the reduction step may be any step that can reduce 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 this step), and any known reduction step commonly used in nitric acid production methods can 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 JP-A-52-050990 can be referenced as appropriate for the reduction step, and the content thereof 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 (in the present invention, this may be conveniently referred to as a "reduced gas" 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, etc. For example, the total content of nitric oxide and nitrogen dioxide in the reduction gas is not particularly limited, and is ideally 0 mol % in terms of maintaining catalytic activity. In practice, however, a content of 0.0001 to 0.35 mol % is preferred. In terms of maintaining high catalytic activity over a long period of time, a content of 0.0001 to 0.25 mol % is preferred, 0.0001 to 0.15 mol % is more preferred, and 0.0001 to 0.065 mol % is even more preferred. The content of nitrous oxide in the reduction gas is typically 0.002 mol % or more, but 0.002 to 10 mol % is generally and preferably, 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 reduction 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 Step> 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 Steps> 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 suitable embodiment of decomposition method B of the present invention (sometimes referred to in the present invention as "decomposition method B1 of 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 at least carrying out a nitric acid synthesis step and a recovery step. That is, 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) capable of at least carrying out a nitric acid synthesis step and a recovery step or a nitric acid production method having 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 contacted with the nitrous oxide-containing gas discharged from the nitric acid synthesis step is carried out before the recovery step.

[0091] In 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 various gases described in connection with decomposition method A of the present invention. For example, the nitrous oxide content 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 ideally 0 mol %, but in practice, 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 gas, and saturated hydrocarbon gas, 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 decomposition method B1 of the present invention is usually supplied directly from the nitric acid synthesis step, but may be supplied via another process, such as a content adjustment process.

[0093] In 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 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, 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, since 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.

[0094] In the decomposition method B1 of the present invention, the decomposition step is carried out before the recovery step. Carrying out the decomposition step before the recovery step allows nitrous oxide to be decomposed over a long period of time while maintaining the catalytic activity of the nitrous oxide decomposition catalyst, and preferably allows nitrous oxide to 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 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. However, if the nitric acid production plant or 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, as this effectively suppresses a 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 decomposition method B of the present invention (sometimes referred to in the present invention as "decomposition method B2 of the present invention") is a form 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 at least carrying out a nitric acid synthesis step and a 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) capable of at least carrying out a nitric acid synthesis step or a nitric acid production method having at least a nitric acid synthesis step, and includes a step of reducing nitric oxide and nitrogen dioxide contained in the nitrous oxide-containing gas discharged from the nitric acid synthesis step (reduction 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 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 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 reduction 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 the <Reduction Step>. In particular, the total content of nitric oxide and nitrogen dioxide in the reduced gas is ideally 0 mol %, but in practice it is preferably 0.0001 to 0.35 mol %, and more preferably within each of the preferred ranges in the <Reduction Step> above. The reduced gas used in the decomposition step of decomposition method B2 of the present invention is usually supplied directly from the reduction step, but may also be supplied via another step, such as a content adjustment step.

[0098] In 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 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, the recovery step can be effectively carried out and the power for recovery can be increased while ensuring the operability and safety of the high-pressure process.

[0099] In decomposition method B2 of the present invention, the decomposition step is carried out after the reduction step. Carrying out the decomposition step after the reduction step can suppress a decrease in catalytic activity due to nitric oxide and nitrogen dioxide in the reduction gas, thereby enabling the nitrous oxide decomposition catalyst to maintain its catalytic activity over a long period of time, and more preferably, enabling the nitrous oxide to be decomposed efficiently (at a high decomposition rate) over a long period of time. In 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 (i.e., the reduction step and 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 decomposition method B2 of the present invention, the decomposition step is carried out after the reduction step. However, when the nitric acid production plant or 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, since this effectively suppresses a decrease in catalytic activity. That is, in the decomposition method B2 of the present invention, in order to highly suppress a decrease in catalytic activity, 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.

[0100] [Nitrous oxide decomposition system] The nitrous oxide decomposition system of the present invention is a decomposition system applied to or incorporated into a nitric acid production plant or an (industrial) nitric acid production process, and is a system or apparatus capable of implementing a nitric acid production plant or a nitric acid production process (sometimes simply referred to as the "decomposition system of the present invention" in the present invention). The decomposition system of the present invention is preferably capable of implementing a plant or a production process 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, even when applied to a nitric acid production plant or a nitric acid production process, 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 an extended period of time.

[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 carrying out a conventional nitric acid production method, and is preferably a plant having an equipment configuration capable of carrying out 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 carrying out 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 device capable of carrying out a nitric acid synthesis step with a reduction device capable of carrying out a reduction step and / or a power recovery device capable of carrying out a recovery step. From the viewpoints of global environmental protection, reduction of production costs, etc., a plant equipped with a nitric acid synthesis device, a reduction device, and a power recovery device in this order is preferred. Here, a nitric acid synthesis device by the Ostwald process generally uses NOx, ２ NO capable of carrying out the synthesis process ２ The decomposition system of the present invention is preferably a nitric acid production plant having an apparatus configuration capable of carrying out the decomposition method B of the present invention. In the present invention, the upstream side of the nitric acid production plant is referred to as the "front stage" and the downstream side as the "rear stage" along the direction in which the process gas is transferred (flows) in accordance with the process order.

[0102] <Nitric acid synthesis device> In the present invention, a nitric acid synthesis device (NO ２ The nitric acid synthesis apparatus (synthesizer and nitric acid generator) 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] <Reducing Device> In the present invention, the reducing 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 reducing device commonly used in nitric acid production methods may be used without any particular limitation. Examples of reducing devices include dry-type reducing devices and wet-type reducing 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> A suitable example of the power recovery device is a device that recovers power by supplying a reaction product (exhaust gas discharged from the device arranged in the upstream stage) of the device 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> Conventional nitric acid production plants are preferably provided with a release device, such as a release tower, that releases gas discharged from a device disposed in a preceding stage into the atmosphere. The nitric acid production plant and each device may also be provided 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, as it can utilize the characteristic of being able to maintain catalytic activity for a long period of time in the decomposition method of the present invention using a highly productive production plant. Examples of continuous apparatus include fixed-bed and fluidized-bed apparatuses, such as tubular apparatuses (reaction tubes, reaction columns, etc.) containing 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 major changes or reconstruction of the equipment configuration of the conventional nitric acid production plant. Preferred embodiments of the decomposition system of the present invention will be described below.

[0108] [Decomposition System 1 of the Present Invention] A preferred embodiment of the decomposition system of the present invention (sometimes referred to in the present invention as "decomposition system 1 of the present invention") is a decomposition system that is installed in (integrated into) the upstream stage of a power recovery unit in a nitric acid production plant having at least a nitric acid synthesis unit and a power recovery unit. That is, decomposition system 1 of the present invention is a nitrous oxide decomposition system that is installed in a nitric acid production plant having a nitric acid synthesis unit and a power recovery unit, and includes a decomposition unit that contacts a catalyst containing at least one selected from the group consisting of ruthenium and ruthenium compounds with the nitrous oxide-containing gas discharged from the nitric acid synthesis unit (which has passed through the nitric acid production unit), and this decomposition unit is installed in the upstream stage 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 the decomposition method B1 of the present invention, and as a result, nitrous oxide can be decomposed over a long period of time while maintaining the catalytic activity of the nitrous oxide decomposition catalyst, preferably enabling efficient decomposition (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 downstream of the nitric acid synthesis device and upstream of the recovery device, but is preferably installed immediately before the recovery device (the decomposition device and recovery device are installed in this order). However, the various devices described above 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. However, 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 deterioration of catalytic activity. The various devices described above 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 in the present invention as "decomposition system 2 of the present invention") is a decomposition system that is installed in (integrated into) the subsequent stage of the reduction step in a nitric acid production plant having at least a nitric acid synthesis unit and a reduction unit. That is, decomposition system 2 of the present invention is a nitrous oxide decomposition system that is installed in a nitric acid production plant having a nitric acid synthesis unit and 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 (passing through the nitric acid production unit), and includes a decomposition unit that 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.

[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 implement the decomposition method of the present invention, particularly decomposition method B2 of the present invention. As a result, 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 system 2 of the present invention, the decomposition device may be installed downstream of the reduction device, but is preferably installed immediately after the reduction device (the reduction device and decomposition device are installed in this order). However, the various devices described above, 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 downstream of the reduction device. However, 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, as this can highly suppress deterioration of catalytic activity. The various devices described above 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 and decomposition system of the present invention (sometimes referred to in the present invention as "decomposition system 3 of the present invention") is a decomposition system that is installed in (integrated into) the subsequent stage of the reduction step in a nitric acid production plant having at least a nitric acid synthesis unit, a reduction unit, and a power recovery unit, in this order. That is, decomposition system 3 of the present invention is a nitrous oxide decomposition system that is 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 includes a nitrous oxide decomposition unit that 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.

[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 those 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 implement the decomposition method of the present invention, particularly decomposition methods B1 and B2 of the present invention. As a result, 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 system 3 of the present invention, the decomposition device may be installed downstream of the reduction device, but is preferably installed immediately after the reduction device (the reduction device and decomposition device are installed in this order). However, the various devices described above, 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 to install the decomposition device upstream of the recovery device and between the reduction device and the recovery device, as this allows the decomposition method of the present invention, particularly decomposition methods B1 and B2 of the present invention, to be suitably implemented and enables a significant suppression of a decrease in catalytic activity. The various devices described above can also be installed between the decomposition device and the recovery device.

[0117] [Method for Producing Nitric Acid] 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 elevated 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 on which the method for producing nitric acid of the present invention is based is the same as the known production method (conventional nitric acid production method) that can be carried out in a conventional nitric acid production plant to which 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 in the present invention as "nitric acid production method 1 of 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, nitric acid production method 1 of the present invention is an (industrial) method for producing nitric acid carried out in a nitric acid production plant (having an apparatus configuration) capable of carrying out at least the nitric acid synthesis step and the recovery step, and is a method in which a nitrous oxide decomposition step of contacting the nitrous oxide-containing gas discharged from the nitric acid synthesis step with a catalyst containing at least one selected from the group consisting of ruthenium and ruthenium compounds is carried out before the recovery step.

[0119] In 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. By carrying out the decomposition step 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 nitrous oxide emission over a long period of time. Preferably, nitric acid can be produced while efficiently suppressing nitrous oxide emission 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. However, it is preferable to carry out the decomposition step immediately before the recovery step (i.e., 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. However, 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 effectively suppresses the decline in catalytic activity and allows nitric acid to be produced while efficiently suppressing nitrous oxide emission 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 in the present invention as "nitric acid production method 2 of the present invention") is a method in which a nitrous oxide decomposition method is carried out after a reduction step in a nitric acid production plant (nitric acid production method) capable of at least carrying out a nitric acid synthesis step and a reduction step. That is, nitric acid production method 2 of the present invention is an (industrial) method for producing nitric acid carried out in a nitric acid production plant (having an apparatus configuration) capable of at least carrying out a nitric acid synthesis step, and includes a step of reducing nitric oxide and nitrogen dioxide contained in a 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 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 nitric acid production method 2 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 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.

[0124] The reduced gas and catalyst used in the decomposition step of 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 decomposition method B2 of the present invention.

[0125] In the nitric acid production method 2 of the present invention, the decomposition step is performed after the reduction step. Performing the decomposition step after the reduction step can suppress the decrease in catalytic activity due to nitric oxide and nitrogen dioxide in the reduction gas, and can decompose nitrous oxide over a long period of time while maintaining the catalytic activity of the nitrous oxide decomposition catalyst. As a result, nitric acid can be produced while suppressing the release of nitrous oxide over a long period of time, and more preferably, nitric acid can be produced 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 performed after the reduction step, but is preferably performed immediately after the reduction step (i.e., the reduction step and decomposition step are performed 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 performed. In the nitric acid production method 2 of the present invention, the decomposition step is performed after the reduction step. However, if the nitric acid production method includes a recovery step, it is preferable to perform the decomposition step between the reduction step and the recovery step, since this highly suppresses the decrease in catalytic activity and allows 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, nitric acid can be produced while suppressing the decrease in catalytic activity to a high degree and efficiently suppressing the release of nitrous oxide for a long period of time. Therefore, 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.

[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 having 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, for a long period of time in the nitric acid synthesis unit described below.

[0127] That is, the nitric acid production plant of the present invention is the same as a conventional nitrous oxide decomposition device except for having a nitrous oxide decomposition device. The nitric acid synthesis device, reduction device, and power recovery device in the nitric acid production plant of the present invention are the same as the respective devices in a conventional nitric oxide production plant, and are, 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, can highly suppress the decline in catalytic activity of the nitrous oxide decomposition catalyst and can produce nitric acid while efficiently suppressing nitrous oxide emissions over a long period of time.In addition, in the nitric acid production plant of the present invention, the various devices described above, as well as devices for adjusting the contents of gases other than nitric oxide, nitrogen dioxide, and nitrous oxide, can also be installed between the adjacent devices.

[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. The reaction pressure was set to 0.6 MPa (absolute) to carry out a nitrous oxide decomposition method.

[0131] <Calculation of nitrous oxide decomposition rate X (%)> The nitrous oxide decomposition rate X (%) is calculated by multiplying the nitrous oxide content C in the nitrous oxide-containing gas (used for decomposing nitrous oxide). Ｂ (mol%) and the nitrous oxide content C in the reaction outlet gas (post-reaction gas) (after the nitrous oxide decomposition step) ＡThe concentration (mol%) of nitrous oxide was analyzed (measured) using a gas chromatograph (Shimadzu Corporation, GC-2014 (detector: TCD, column: SHINCARBON-ST50 / 80 4m)), 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: Decomposition rate X of nitrous oxide (%) = [(C Ｂ -C Ａ ) / C Ｂ ]×100

[0132] <Calculation of reaction rate constant> The 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: －１ ) = -ln(1-X / 100) / (3600 / GHSV) where ln(1-X / 100) represents the natural logarithm of (1-X / 100).

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

[0134] Next, 4.6 g of ruthenium nitrosyl nitrate solution (manufactured by Furuya Metal Co., Ltd., Ru(NO)(NO ３ ) ３ 0.9 g of Zircosol ZN solution (manufactured by Daiichi Kigenso Kagaku Kogyo, ZrO(NO ３ ) ２ (Zr content 18%) was dissolved in 5.0 g of ion-exchanged water. Titanium oxide and SiO ２ 25.0 g of the carrier 1 formed in the above step was impregnated at room temperature (25°C) for 1 hour, and then dried at room temperature in an air atmosphere for 1 hour to obtain a catalyst precursor 1 carrying 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 a catalyst 1 (step 4). In the catalyst 1, the ruthenium content was 3 mass% and the zirconium content was 0.5 mass%.

[0135] (Decomposition reaction of nitrous oxide using catalyst 1) Reaction pressure: 0.6 MPa (absolute), flow rate: 690 cm ３ A 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 flow rate of 2.4 g (3.0 cm 3 ) under the conditions of (0°C, 0.1013 MPa (absolute)) / min. ３ The mixture was passed through a stainless steel reaction tube (inner diameter 8 mm) packed with catalyst 1 (100%), and the reaction tube was heated in an electric furnace until the inner temperature reached 300°C. ２The gas before and after contact with catalyst 1 at a predetermined time, which was the start of the O decomposition reaction, was analyzed using gas chromatography and gas detector 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 was therefore suitable for use in the power recovery process and power recovery device in a conventional nitric acid production plant. In this decomposition reaction, the reaction rate constant after 4 hours of reaction was 3.4 s －１ The reaction rate constant after 513 hours of reaction was 3.4 s －１ Table 1 shows the specific activity (reaction rate constant ratio) after 513 hours of reaction, assuming that the reaction rate constant after 4 hours of reaction was 1.

[0136] Example 2 In a conventional nitric acid production 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 denitrification reaction (reduction process) of the nitric acid production plant was treated with NH ３ After the selective denitration, the gas after the denitration reaction (reduced gas) was collected by 12.2 g (15.3 cm ３ Catalyst 1 was charged into a stainless steel reaction tube (inner diameter 15.6 mm) filled with 1900 cm3 of catalyst 1 at atmospheric pressure (0.1 MPa (absolute)) and a flow rate of 1900 cm3. ３ The decomposition reaction of nitrous oxide was carried out in the same manner as in Example 1, except that the NH ３ The selective denitration was carried out by introducing the exhaust gas into a reaction tube filled with a catalyst for removing nitrogen oxides described in Japanese Patent Laid-Open No. 52-050990, and then introducing 2.4 mol % NH ３ / N ２ 60 cm of mixed gas ３ The reaction was carried out at 270°C with a flow rate of (0°C, 0.1013 MPa (absolute)) / min.

[0137] Using gas chromatography and a gas detector (water vapor 6, nitrogen oxide 11S, nitrogen oxide 11HA), the gas (reduced gas) before contacting the 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 nitrogen monoxide and nitrogen dioxide) was 0.006 to 0.065 mol %. In the decomposition reaction of Example 2, the N ２ The oxygen content in the gas (reduced gas) before the O decomposition 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 Subsequently, after 333 hours of reaction time, ２ The oxygen content in the gas (reduced gas) before the O 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 Subsequently, after 549 hours of reaction time, ２ The oxygen content in the gas (reduced gas) before the O 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 Subsequently, after 597 hours of reaction time, ２ The oxygen content in the gas (reduced gas) before the O 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 The specific activity after 597 hours of reaction, assuming that the reaction rate constant after 45 hours of reaction was 1, is shown in Table 1. In addition, in the nitric acid production plant incorporating the above-mentioned nitrous oxide decomposition system, nitric acid could be produced while the nitrous oxide decomposition step was being carried out.

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

[0139] The exhaust gas obtained by the denitrification reaction was analyzed using gas chromatography, a gas detector tube, and an oxygen concentration meter (G-103 manufactured by Iijima Electronics Co., Ltd.). As a result, it was found 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 detector tube (nitrogen oxide 11S: detection limit 2 ppm) turned color. In the decomposition reaction of Example 3, the N ２ The NOx content in the gas (reduced gas) before the O decomposition reaction was 0.0001 mol % or more and 0.001 mol % or less, and the water vapor content was 0.18 mol %, and the reaction rate constant at this time was 3.2 s －１ After 692 hours of reaction time, ２ The NOx content in the gas (reduced gas) before the O 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 －１The specific activity after 692 hours of reaction, assuming that the reaction rate constant after 2 hours of reaction was 1, is shown in Table 1. In addition, in the nitric acid production plant incorporating the above-mentioned nitrous oxide decomposition system, nitric acid could be produced while the nitrous oxide decomposition step 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 0.17 mol % of nitrous oxide, 3.0 mol % of oxygen, 0.5 mol % of water vapor, and the balance 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 The specific activity after 261 hours of reaction is shown in Table 1, assuming that the reaction rate constant after 3 hours of reaction is 1.

[0141]

[0142] As is clear from the results shown in Table 1, 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 at normal pressure (0.1 MPa (absolute)), had a low specific activity, and the catalytic activity of the nitrous oxide decomposition catalyst decreased over time, indicating 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 step was carried out using a reaction pressure set at 0.6 MPa (absolute), showed no decrease in reaction rate constant even after 513 hours, indicating that nitrous oxide could be decomposed over a long period of time while maintaining the catalytic activity of the nitrous oxide decomposition catalyst. Furthermore, Example 2, in which the nitrous oxide decomposition step was carried out using a reduction gas with a total content of nitric oxide and nitrogen dioxide of 0.0001 to 0.35 mol %, showed the same reaction rate constant even after 597 hours, indicating that nitrous oxide could be decomposed efficiently over a long period of time while maintaining the catalytic activity of the nitrous oxide decomposition catalyst. Furthermore, in Example 3, in which the nitrous oxide decomposition process was carried out at a reaction pressure of 0.6 MPa (absolute) using a reducing gas having a total content of nitric oxide and nitrogen dioxide of 0.0001 to 0.35 mol %, the reaction rate constant remained the same even after 692 hours, and nitrous oxide was decomposed over a long period of time 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 used in the power recovery process and power recovery device.

[0143] While the present invention has been described in connection with embodiments thereof, we do not intend to limit our invention to any of the details of the description unless otherwise specified, and believe that the claims should be construed broadly without departing from the spirit and scope of the invention as set forth in the appended claims.

[0144] This application claims priority based on Japanese Patent Application No. 2024-140860, filed on August 22, 2024, the contents of which are incorporated herein by reference as part of the present specification.

Claims

1. A method for decomposing nitrous oxide applicable to a nitric acid production plant capable of at least carrying out a step of synthesizing nitric acid and a step of recovering power using an exhaust gas turbine, the method 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, prior to the step of recovering power.

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 absolute pressure.

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

4. The method for decomposing nitrous oxide according to claim 3, 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 %.

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 carrying out at least a step of synthesizing nitric acid and a step of recovering power using an exhaust gas turbine, wherein 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 is carried out before the step of recovering power.

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

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 absolute pressure.

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

10. The method for decomposing nitrous oxide according to claim 9, 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 %.

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 applicable 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 a nitrous oxide-containing gas discharged from the step of synthesizing nitric acid; and a 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.

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 reduction step is 0.0001 to 0.35 mol %.

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 absolute pressure.

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

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

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

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, comprising: a step of reducing nitric oxide and nitrogen dioxide contained in a nitrous oxide-containing gas discharged from the step of synthesizing nitric acid; and a step of decomposing the nitrous oxide by contacting the nitrous oxide-containing gas discharged from the step of reducing nitric oxide with a catalyst containing at least one selected from the group consisting of ruthenium and ruthenium compounds.

20. The method for producing nitric acid according to claim 19, wherein the nitric acid production plant is further capable of performing a step of recovering power using a waste heat gas turbine, and the decomposition step of nitrous oxide is performed before the step of recovering power and after the step of reducing nitrous oxide.

21. A nitrous oxide decomposition system to be installed in a nitric acid production plant having a nitric acid synthesis unit that synthesizes nitric acid and a power recovery unit that recovers power using an exhaust gas turbine, the system comprising 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, the nitrous oxide decomposition unit being installed upstream of the power recovery unit.

22. A nitrous oxide decomposition system to be installed in a nitric acid production plant having a nitric acid synthesis unit that synthesizes nitric acid and 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, the nitrous oxide decomposition system comprising the nitrous oxide decomposition unit that brings the nitrous oxide-containing 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.

23. A nitrous oxide decomposition system attached to a nitric acid production plant having, in this order, a nitric acid synthesis unit that synthesizes nitric acid, 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 gas turbine, the nitrous oxide decomposition system comprising the nitrous oxide decomposition unit that brings the nitrous oxide-containing gas discharged from the reduction unit into contact with a catalyst containing at least one member selected from the group consisting of ruthenium and ruthenium compounds.

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

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