WO2025220483A1 - Method for producing nitrous oxide decomposition catalyst, and nitrous oxide decomposition catalyst - Google Patents

Abstract

This method for producing a nitrous oxide decomposition catalyst for decomposing the nitrous oxide in an exhaust gas comprises a preparation step for preparing a product that contains the following: an active component containing the composite metal oxide represented by formula (1); and an impurity component containing an alkali metal element. The product containing the active component and impurity component is prepared in the preparation step by a coprecipitation method using a coprecipitation agent containing an alkali metal element. In the product, the percentage content of alkali metal element with reference to the total main element amount in the active component and impurity component is at least 0.45 mass% and not more than 4.50 mass%.　(1): Ni_XA_1-XCo₂O₄ (In formula (1), A is at least one selected from the group consisting of Fe, Mn, Ce, Zr, La, and alkaline earth metals. In addition, X is greater than 0 and less than 1.)

Description

Method for producing nitrous oxide decomposition catalyst and nitrous oxide decomposition catalyst

This disclosure relates to a method for producing a nitrous oxide decomposition catalyst and a nitrous oxide decomposition catalyst.

Conventionally, a known technology for treating exhaust gas involves placing an exhaust gas treatment catalyst in the exhaust passage through which the exhaust gas flows, thereby breaking down harmful substances in the exhaust gas.

As a method for producing a nitrous oxide decomposition catalyst, which is one such catalyst for exhaust gas treatment, a method has been proposed that includes a step of adding a coprecipitant to an aqueous solution prepared by mixing a nickel salt, a cobalt salt, and a copper salt to obtain a precipitate (see, for example, Patent Document 1). Patent Document 1 gives an example of a coprecipitant that contains an alkali metal element.

Japanese Patent Application Laid-Open No. 2022-098865

In the nitrous oxide decomposition catalyst described in Patent Document 1, the component containing an alkali metal element used as a coprecipitant is an impurity component. Such impurities are removed by thoroughly washing the resulting precipitate. However, the present inventors have discovered that the nitrous oxide decomposition rate can be improved by leaving a specific content of impurities containing an alkali metal element in the nitrous oxide decomposition catalyst.

The present disclosure aims to provide a method for producing a nitrous oxide decomposition catalyst and a nitrous oxide decomposition catalyst that can improve the nitrous oxide decomposition rate.

The present disclosure [1] provides a method for producing a nitrous oxide decomposition catalyst that decomposes nitrous oxide in exhaust gas, the method comprising: a preparation step of preparing a product containing an active component that contains a composite metal oxide represented by the following formula (1) and an impurity component that contains an alkali metal element; in the preparation step, the product containing the active component and the impurity component is prepared by a coprecipitation method using a coprecipitant that contains the alkali metal element; and the content of the alkali metal element relative to the total amount of main elements of the active component and the impurity component is 0.80 mass% or more and 4.20 mass% or less.
Ni _X A _1-X Co ₂ O ₄ (1)
(In formula (1), A is at least one selected from the group consisting of Fe, Mn, Ce, Zr, La, and alkaline earth metals. Also, X is greater than 0 and less than 1.)

The present disclosure [2] includes a method for producing the nitrous oxide decomposition catalyst described in [1], further comprising a preparation step of preparing a slurry containing the product, an inorganic binder, and a dispersant, and a coating step of coating the slurry onto a substrate.

The present disclosure [3] includes a method for producing a nitrous oxide decomposition catalyst described in [2], in which the inorganic binder contains at least one selected from the group consisting of metal hydroxides and metal oxides.

The present disclosure [4] includes a method for producing a nitrous oxide decomposition catalyst described in [2] or [3], in which the dispersant contains at least one selected from the group consisting of a carboxylic acid, an alcohol, and water.

The present disclosure [5] includes a method for producing a nitrous oxide decomposition catalyst described in any one of [2] to [4], in which the substrate is an inorganic fiber sheet.

The present disclosure [6] includes a method for producing a nitrous oxide decomposition catalyst according to any one of [2] to [5], further comprising a calcination step of calcining the substrate coated with the slurry after the coating step.

The present disclosure [7] includes a nitrous oxide decomposition catalyst comprising an active component containing a composite metal oxide represented by the following formula (1) and an impurity component containing an alkali metal element, wherein the content of the alkali metal element relative to the total amount of main elements of the active component and the impurity component is 0.80 mass% or more and 4.20 mass% or less.
Ni _X A _1-X Co ₂ O ₄ (1)
(In formula (1), A is at least one selected from the group consisting of Fe, Mn, Ce, Zr, La, and alkaline earth metals. Also, X is greater than 0 and less than 1.)

The present disclosure [8] includes the nitrous oxide decomposition catalyst described in [7], in which the alkali metal element is K.

The present disclosure [9] includes the nitrous oxide decomposition catalyst described in [7] or [8], in which the content ratio of the alkali metal element relative to the total amount of the main elements of the active component and the impurity component is 1.15 mass% or more and 3.80 mass% or less.

The present disclosure [10] includes a nitrous oxide decomposition catalyst according to any one of [7] to [9], further comprising an inorganic binder and a substrate supporting the active component, the impurity component, and the inorganic binder.

The present disclosure [11] includes the nitrous oxide decomposition catalyst described in [10], wherein the inorganic binder includes at least one selected from the group consisting of metal hydroxides and metal oxides.

The present disclosure [12] includes the nitrous oxide decomposition catalyst described in [10] or [11], in which the substrate is an inorganic fiber sheet.

The method for producing a nitrous oxide decomposition catalyst disclosed herein includes a preparation step of preparing a product containing an active component containing the composite metal oxide represented by formula (1) above and an impurity component containing an alkali metal element, wherein the alkali metal element content relative to the total amount of main elements in the active component and the impurity component is 0.80 mass% or more and 4.20 mass% or less. Therefore, the nitrous oxide decomposition rate can be improved.

The nitrous oxide decomposition catalyst disclosed herein comprises an active component containing the composite metal oxide represented by formula (1) above and an impurity component containing an alkali metal element, with the alkali metal element content being 0.80 mass% or more and 4.20 mass% or less relative to the total amount of main elements in the active component and the impurity component. This allows for an improved nitrous oxide decomposition rate.

FIG. 1 is a perspective view showing the nitrous oxide decomposition catalyst of the present disclosure. FIG. 2 is a perspective view showing a nitrous oxide decomposition catalyst device including a nitrous oxide decomposition catalyst unit filled with the nitrous oxide decomposition catalyst shown in FIG. FIG. 3 is a graph showing the relationship between the nitrous oxide decomposition rate (%) and the content (mass %) of alkali metal elements relative to the total amount of main elements of the active component and impurity components for the nitrous oxide decomposition catalysts of Examples 1 to 8 and Comparative Examples 1 to 7.

1. Nitrous Oxide Decomposition Catalyst One embodiment of a nitrous oxide decomposition catalyst 1 will be described with reference to FIG.

The nitrous oxide decomposition catalyst 1 is a catalyst that decomposes nitrous oxide (N ₂ O) in exhaust gas into nitrogen (N ₂ ) and oxygen (O ₂ ).

The nitrous oxide decomposition catalyst 1 comprises an active component 2 and an impurity component 5. Furthermore, the nitrous oxide decomposition catalyst 1 may further comprise an inorganic binder 3 and a substrate 4, as required. As will be described in more detail below, in the nitrous oxide decomposition catalyst 1 of this embodiment, the active component 2, impurity component 5, and inorganic binder 3 are supported on the substrate 4.

As described above, the nitrous oxide decomposition catalyst 1 contains the active component 2 and the impurity component 5. As will be described in more detail below, if the nitrous oxide decomposition catalyst 1 contains a specific content of the impurity component 5, the nitrous oxide decomposition rate can be improved.

The nitrous oxide decomposition catalyst 1 is in the form of a flat plate and/or a corrugated plate.

If the nitrous oxide decomposition catalyst 1 has a flat and/or corrugated shape, the active component 2 can be efficiently supported on the substrate 4, and a sufficient contact area with the exhaust gas can be secured. This ultimately allows for efficient decomposition of nitrous oxide.

[Active ingredient]
The active component 2 contains a composite metal oxide represented by the following formula (1): The active component 2 preferably comprises a composite metal oxide represented by the following formula (1).

Ni _X A _1-X Co ₂ O ₄ (1)
(In formula (1), A is at least one selected from the group consisting of Fe, Mn, Ce, Zr, La, and alkaline earth metals. Also, X is greater than 0 and less than 1.)

The composite metal oxide represented by the above formula (1) contains dicobalt tetroxide, Ni, and A, which will be described later.

A is, for example, at least one selected from the group consisting of Fe, Mn, Ce, Zr, La, and alkaline earth metals. Preferably, it is at least one selected from the group consisting of Fe, Mn, Ce, Zr, La, Mg, Ca, Sr, and Ba. More preferably, it is at least one selected from the group consisting of Fe, Mn, Ce, and Sr. Even more preferably, it is at least one selected from the group consisting of Fe and Ce. Especially preferably, it is Fe.

In the above formula (1), X is, for example, greater than 0, preferably 0.25 or greater, more preferably 0.50 or greater, and even more preferably 0.75 or greater, and is, for example, less than 1, preferably 0.95 or less.

In the nitrous oxide decomposition catalyst 1, the composite metal oxide represented by the above formula (1) forms, for example, a spinel structure. A spinel structure is one of the crystal structures found in metal oxides, such as the structure possessed by tricobalt tetroxide ( _Co3O4 ). Specifically, the composite metal oxide represented by the above formula ( ₁ ) forms a solid solution of Co, Ni, and A, and forms a spinel structure in which some of the Co in tricobalt tetroxide ( _{Co3O4 )} is substituted with Ni and/or A.

If active ingredient 2 is a composite metal oxide represented by the above formula (1), it can decompose nitrous oxide with high efficiency. In other words, the nitrous oxide decomposition rate can be improved.

Active ingredient 2 preferably contains a metal oxide. More preferably, it contains a composite metal oxide containing Co. Even more preferably, it contains a composite metal oxide represented by the above formula (1). Particularly preferably, it is a composite metal oxide represented by the above formula (1).

The amount of active ingredient 2 supported per unit area of the substrate 4 is, for example, 40 g/m ^or more, preferably 75 g/m ^{or more} , and for example, 400 g/m ^{or less} , preferably 375 g/m ^or less. The amount of active ingredient 2 supported per unit area of the substrate 4 is the amount of active ingredient 2 supported per unit area on the surface extending in the planar directions of the substrate 4 (the flow direction of the exhaust gas and the width direction).

[Impurities]
The impurity component 5 is mixed in during the preparation step described below. Specifically, during the preparation step, a precipitate containing the active component is prepared by a coprecipitation method using a coprecipitant. This coprecipitant contains an alkali metal element. Therefore, the impurity component 5 containing the alkali metal element derived from this coprecipitant is mixed in the precipitate. Typically, this impurity component 5 is removed by washing the precipitate. In the present disclosure, the impurity component 5 containing the alkali metal element derived from this coprecipitant is intentionally left.

In this embodiment, the impurity component 5 is mixed in during the manufacturing process, but this is not limiting. The impurity component 5 may also be added separately.

Impurity component 5 contains alkali metal elements. Examples of alkali metal elements include K (potassium element) and Na (sodium element). K is preferred. Note that K and Na as alkali metal elements refer to potassium element and sodium element, respectively.

Specific examples of impurity components 5 include alkali metals and compounds containing alkali metal elements. Examples of alkali metals include potassium and sodium. Examples of compounds containing alkali metal elements include oxides of alkali metals, hydroxides of alkali metals, and carbonates of alkali metals. Examples of alkali metal oxides include potassium oxide and sodium oxide. Examples of alkali metal hydroxides include potassium hydroxide and sodium hydroxide. Examples of alkali metal carbonates include potassium carbonate, sodium carbonate, potassium bicarbonate, and sodium bicarbonate. Note that potassium and sodium as alkali metals refer to metallic potassium and metallic sodium, respectively.

In other words, the impurity component 5 contains at least one selected from the group consisting of alkali metals and compounds containing alkali metal elements. Preferably, it contains at least one selected from the group consisting of potassium and compounds containing K (elemental potassium). More preferably, it contains at least one selected from the group consisting of potassium and potassium oxide.

If the nitrous oxide decomposition catalyst 1 contains a specific content of impurity component 5 containing an alkali metal element, the nitrous oxide decomposition rate can be improved. In particular, if the nitrous oxide decomposition catalyst 1 contains a specific content of impurity component 5 containing K (potassium) as an alkali metal element, the nitrous oxide decomposition rate can be further improved.

Impurity component 5 may contain one alkali metal element, or may contain multiple alkali metal elements. Furthermore, impurity component 5 may be a single alkali metal or a compound containing an alkali metal element, or may be a combination of multiple alkali metals and compounds containing alkali metal elements.

The amount of impurities 5 supported per unit area of the substrate 4 is, for example, 0.10 g/m ^or more, preferably 0.30 g/m ^or more, more preferably 0.45 g/m ^or more, and for example, ³⁰ g/m or less, preferably 20 g/m ^or less, more preferably 15 g/m ^or less. The amount of impurities 5 supported per unit area of the substrate 4 is the amount of impurities 5 supported per unit area on the surface extending in the planar directions (flow direction and width direction of the exhaust gas) of the substrate 4.

The alkali metal element content relative to the total amount of main elements in active ingredient 2 and impurity ingredients 5 is 0.80 mass% or more, preferably 1.00 mass% or more, and more preferably 1.15 mass% or more. The alkali metal element content relative to the total amount of main elements in active ingredient 2 and impurity ingredients 5 is 4.20 mass% or less, preferably 4.00 mass% or less, and more preferably 3.80 mass% or less. The alkali metal element content relative to the total amount of main elements in active ingredient 2 and impurity ingredients 5 is, for example, 0.80 mass% to 4.20 mass%, preferably 1.00 mass% to 4.00 mass%, and more preferably 1.15 mass% to 3.80 mass%.

If the content ratio of alkali metal elements relative to the total amount of main elements in active component 2 and impure component 5 is within the above range, the nitrous oxide decomposition rate can be improved.

The total amount of major elements in active component 2 and impurity component 5 is the total amount of the main elements contained in active component 2, namely Ni, A (A is at least one selected from the group consisting of Fe, Mn, Ce, Zr, La, and alkaline earth metals), Co, and O, and the main elements contained in impurity component 5, namely alkali metal elements (specifically K and Na) and O.

The content ratio of alkali metal elements relative to the total amount of major elements in the active component 2 and the impurity component 5 can be measured, for example, by energy dispersive X-ray fluorescence analysis.

[Inorganic binder]
The inorganic binder 3 increases the strength of the substrate 4. There are no particular limitations on the inorganic binder 3, as long as it is one that is commonly used in nitrous oxide decomposition catalysts. Examples of the inorganic binder 3 include metal hydroxides and metal oxides. The inorganic binder 3 preferably contains at least one selected from the group consisting of metal hydroxides and metal oxides. Note that the inorganic binder 3 does not contain an alkali metal element. Specifically, the inorganic binder 3 excludes alkali metal hydroxides and alkali metal oxides. In other words, the inorganic binder 3 is different from the impurity components 5.

If the inorganic binder 3 contains at least one selected from the group consisting of metal hydroxides and metal oxides, when a slurry containing the active component 2, the impurity component 5, the inorganic binder 3, and a dispersant is prepared in the method for producing a nitrous oxide decomposition catalyst described below, the inorganic binder 3 is uniformly dispersed in the dispersant, and the active component 2, the impurity component 5, and the inorganic binder 3 can be uniformly mixed in the slurry. Therefore, when the slurry is applied to the substrate 4, the inorganic binder 3 is uniformly applied to the substrate 4, and as a result, the strength of the nitrous oxide decomposition catalyst 1 can be improved.

Examples of metal hydroxides include iron hydroxide, zirconium hydroxide, aluminum hydroxide, cerium hydroxide, magnesium hydroxide, and aluminum oxide hydroxide.

Metal hydroxides can be used alone or in combination of two or more types. That is, the metal hydroxide includes, for example, at least one selected from the group consisting of iron hydroxide, zinc hydroxide, aluminum hydroxide, cerium hydroxide, magnesium hydroxide, and aluminum oxide hydroxide. Preferably, it includes at least one selected from the group consisting of iron hydroxide, aluminum hydroxide, and cerium hydroxide. More preferably, it includes iron hydroxide.

Examples of metal oxides include alumina, ceria, and silica. Note that although silica is silicon oxide, it is included in the metal oxides in this disclosure.

Metal oxides can be used alone or in combination of two or more types. That is, the metal oxide includes, for example, at least one selected from the group consisting of alumina, silica, and ceria. Preferably, it includes at least one selected from the group consisting of alumina and silica. More preferably, it includes alumina.

The inorganic binder 3 can be used alone or in combination of two or more types. In other words, a metal hydroxide and a metal oxide can be used in combination.

The amount of inorganic binder 3 supported per unit area of the substrate 4 is, for example, 0.4 g/m ^{or more} , preferably 1.0 g/m ^{or more} , more preferably 2.0 g/m ^or more, and for example, 320 g/m ^{or less} , preferably 250 g/m ^or less, more preferably 200 g/m ^or less. The amount of inorganic binder 3 supported per unit area of the substrate 4 is the amount of inorganic binder 3 supported per unit area on the surface extending in the planar directions (flow direction of exhaust gas and width direction) of the substrate 4.

[Base material]
The substrate 4 may be, for example, an inorganic fiber sheet. Examples of the inorganic fiber sheet include glass paper and ceramic paper. Preferably, glass paper is used.

Commercially available glass paper containing an organic binder can also be used. Examples of organic binders used in commercially available glass paper include acrylic resin, polyvinyl alcohol (PVA)-polyvinyl acetate copolymer, unsaturated polyester resin, and epoxy resin.

The shape of the substrate 4 is, for example, flat and/or corrugated. In other words, the shape of the substrate 4 forms the shape of the nitrous oxide decomposition catalyst 1. The substrate 4 is preferably flat glass paper and/or corrugated glass paper.

The dimensions of the substrate 4 are adjusted appropriately depending on the application. Specifically, the length of the substrate 4 in the first direction (exhaust gas flow direction) and the length in the second direction (width direction) are not particularly limited.

2. Method for Producing Nitrous Oxide Decomposition Catalyst One embodiment of the method for producing a nitrous oxide decomposition catalyst according to the present disclosure will be described. The method for producing a nitrous oxide decomposition catalyst is a method for producing the above-described nitrous oxide decomposition catalyst that decomposes nitrous oxide in exhaust gas.

The method for producing a nitrous oxide decomposition catalyst includes, in order, a preparation step of preparing a product containing an active component containing the composite metal oxide represented by the above formula (1) and an impurity component containing an alkali metal element; a preparation step of preparing a slurry containing the product containing the active component and the impurity component, an inorganic binder, and a dispersant; and a coating step of applying the slurry to a substrate. If necessary, the method for producing a nitrous oxide decomposition catalyst may further include a calcination step after the coating step of calcining the substrate to which the slurry has been applied.

(Preparation process)
In the preparation step, a product containing the active ingredient and impurities is prepared. A method for preparing a product containing the active ingredient and impurities includes a coprecipitation method.

Specifically, a product containing an active component containing the composite metal oxide represented by formula (1) above and impurity components containing alkali metal elements is prepared by a coprecipitation method using a coprecipitant containing an alkali metal element.

First, a cobalt salt, a nickel salt, and a salt containing element A are dissolved in a solvent, and a coprecipitant is added to the prepared solution to coprecipitate a precipitate containing an active component containing the complex metal oxide represented by the above formula (1) and impurity components containing alkali metal elements. The precipitate is then washed and calcined to obtain a product containing an active component containing the complex metal oxide represented by the above formula (1) and impurity components containing alkali metal elements.

Examples of cobalt salts include inorganic metal salts of cobalt. Examples of inorganic metal salts of cobalt include cobalt nitrate, cobalt sulfate, and cobalt chloride. Preferably, cobalt nitrate (Co(NO ₃ ) _{2.6H 2} O) is used.

Examples of nickel salts include inorganic metal salts of nickel. Examples of nickel salts include nickel nitrate, nickel sulfate, and nickel chloride. Preferably, nickel nitrate (Ni(NO ₃ ) _{2.6H 2} O) is used.

Examples of salts containing element A include inorganic metal salts containing element A. Examples of inorganic metal salts containing element A include nitrates, sulfates, and chlorides containing element A. Nitrates containing element A are preferred.

Specifically, when A is Fe, examples of the iron salt include inorganic metal salts of iron. Examples of inorganic metal salts of iron include iron nitrate, iron sulfate, and iron _{chloride. Preferably, iron nitrate (Fe(NO3)3.9H2O ) is} used.

Solvents include, for example, ion-exchanged water and ultrapure water. Ion-exchanged water is preferred.

The concentrations of the cobalt salt, nickel salt, and salt containing element A may be added to the solvent so as to satisfy the composition ratio in the composite metal oxide represented by formula (1) above. The cobalt salt is added to the solvent to a concentration of, for example, 0.02 mol/L or more and, for example, 20 mol/L or less. The nickel salt is added to the solvent to a concentration of, for example, 0.01 mol/L or more and, for example, 10 mol/L or less. The salt containing element A is added to the solvent to a concentration of, for example, 0.01 mol/L or more and, for example, 10 mol/L or less.

In this way, a solution can be obtained in which a cobalt salt, a nickel salt, and a salt containing element A are dissolved in a solvent.

A coprecipitant is added to the prepared solution, and a precipitate containing the active component containing the composite metal oxide represented by formula (1) above and impurity components containing alkali metal elements is coprecipitated in the solution, and the precipitate is then washed.

The coprecipitant may be, for example, an alkaline liquid or solid. The coprecipitant may contain an alkali metal element. Examples of the coprecipitant include _K2CO3 and _NaOH . _K2CO3 is preferred. The _alkali metal element contained in the coprecipitant is the same as the alkali metal element contained in the impurity component.

The coprecipitant should be added only to the extent that a precipitate forms in the solution. Specifically, it should be added dropwise until the solution's pH reaches 9, which will produce a precipitate. The coprecipitant can be added dropwise at room temperature.

The resulting precipitate is then recovered. There are no particular limitations on the recovery method, but it can be recovered, for example, by filtration and removing the solvent using an aspirator.

After collection, the precipitate is washed and, if necessary, dried.

An example of a washing method is to repeatedly wash with ion-exchanged water until the pH reaches 7. The amount of ion-exchanged water used for washing per 100 g of recovered precipitate is, for example, 400 mL or more, preferably 500 mL or more, and, for example, 1300 mL or less, preferably 1200 mL or less.

When the coprecipitant is added dropwise until the solution pH reaches 9 and a precipitate is obtained, if the amount of ion-exchanged water used for washing per 100 g of recovered precipitate is within the above range, the content ratio of alkali metal elements relative to the total amount of major elements in active component 2 and impure component 5 can be controlled within a specific range. This in turn improves the nitrous oxide decomposition rate.

Furthermore, the method for drying the washed precipitate is not particularly limited, and examples include methods such as evaporation to dryness. This allows the washed precipitate to be obtained as a dry powder.

The temperature for drying the precipitate is, for example, 70°C or higher and, for example, 120°C or lower. The drying time for the precipitate is, for example, 3 hours or higher and, for example, 50 hours or lower.

In this way, a precipitate (dry powder) containing an active ingredient containing the composite metal oxide represented by the above formula (1) and impurities containing alkali metal elements can be obtained.

Then, the precipitate (dried powder) containing the active component containing the composite metal oxide represented by formula (1) above and impurity components containing alkali metal elements is calcined.

The method for calcining the precipitate (dry powder) is not particularly limited, and any known method can be used as long as it produces the desired active ingredient.

The firing temperature of the precipitate (dry powder) is, for example, 300°C or higher and, for example, 800°C or lower. The firing time of the precipitate (dry powder) is, for example, 0.5 hours or higher and, for example, 10 hours or lower.

If the calcination temperature and calcination time of the precipitate (dry powder) are within the above ranges, the composite oxide represented by the above formula (1) can be obtained without any decrease in activity.

In this way, a product can be obtained that contains an active component containing the composite metal oxide represented by formula (1) above and impurities containing alkali metal elements.

(Preparation process)
In the preparation step, a slurry containing the product, an inorganic binder, and a dispersant is prepared. Specifically, the product obtained in the preparation step and the inorganic binder are weighed, added to the dispersant, and dispersed by stirring to obtain a slurry.

The dispersant contains at least one selected from the group consisting of carboxylic acids, alcohols, and water. Preferably, it is at least one selected from the group consisting of carboxylic acids, alcohols, and water. When the dispersant contains carboxylic acids and water, the dispersant is an aqueous solution of carboxylic acids, and when the dispersant contains alcohols and water, the dispersant is an aqueous solution of alcohol.

If the dispersant contains at least one selected from the group consisting of carboxylic acids, alcohols, and water, the inorganic binder can be reliably dispersed uniformly. As a result, the strength of the nitrous oxide decomposition catalyst can be improved.

When the dispersant is an aqueous solution of a carboxylic acid and/or an alcohol, the concentration of the carboxylic acid and/or alcohol in the aqueous solution is, for example, 0.1% or more and, for example, less than 20%.

Examples of carboxylic acids include formic acid, acetic acid, propionic acid, butyric acid, oxalic acid, malonic acid, succinic acid, glutaric acid, adipic acid, pimelic acid, maleic acid, fumaric acid, phthalic acid, trimellitic acid, hydroxyacetic acid, lactic acid, salicylic acid, malic acid, tartaric acid, citric acid, aspartic acid, and glutamic acid. Carboxylic acids having 1 to 6 carbon atoms are preferred. Acetic acid, citric acid, and lactic acid are more preferred.

Carboxylic acids may be used alone or in combination of two or more types.

Examples of alcohols include methanol, ethanol, n-propanol, isopropanol, n-butyl alcohol, tert-butyl alcohol, n-pentyl alcohol, n-hexyl alcohol, 2-ethylhexanol, phenol, benzyl alcohol, ethylene glycol, propylene glycol, diethylene glycol, ethylene glycol mono-n-propyl ether, propylene glycol monomethyl ether, and glycerin. Monohydric alcohols having 1 to 4 carbon atoms are preferred. Methanol and isopropanol are more preferred.

Examples of combinations of inorganic binders and dispersants include a combination containing iron hydroxide and a carboxylic acid, a combination of iron hydroxide and water, a combination containing zirconium hydroxide and a carboxylic acid, a combination containing aluminum hydroxide and a carboxylic acid, a combination of aluminum hydroxide and water, a combination containing cerium hydroxide and a carboxylic acid, a combination of cerium hydroxide and water, a combination of magnesium hydroxide and water, a combination of aluminum hydroxide oxide and water, and a combination of strontium hydroxide and water. Preferred are a combination containing iron hydroxide and a carboxylic acid, a combination of iron hydroxide and water, a combination of aluminum hydroxide and a carboxylic acid, a combination of aluminum hydroxide and water, a combination of cerium hydroxide and a carboxylic acid, and a combination of cerium hydroxide and water. More preferred are a combination containing iron hydroxide and a carboxylic acid, and a combination of iron hydroxide and water.

Furthermore, examples of combinations of inorganic binders and dispersants include a combination containing alumina and carboxylic acid, a combination containing silica and alcohol, and a combination containing ceria and carboxylic acid. Preferred are a combination containing alumina and acetic acid, a combination containing alumina and citric acid, a combination containing alumina and lactic acid, a combination containing silica and methanol, a combination containing silica and isopropanol, and a combination containing ceria and citric acid. More preferred are a combination containing alumina and acetic acid, a combination containing alumina and citric acid, a combination containing alumina and lactic acid, a combination containing silica and methanol, and a combination containing silica and isopropanol. More preferred are a combination containing alumina and acetic acid, a combination containing alumina and citric acid, and a combination containing alumina and lactic acid.

The combination of the above metal hydroxide and dispersant ensures that the inorganic binder is uniformly dispersed in the dispersant. As a result, the strength of the nitrous oxide decomposition catalyst can be improved.

The amount of inorganic binder added per 100 parts by mass of active ingredient is, for example, 0.01 parts by mass or more, preferably 0.1 parts by mass or more, and for example, 80 parts by mass or less, preferably 50 parts by mass or less.

The amount of inorganic binder blended per 100 parts by mass of active ingredient is the same ratio (amount blended) in the nitrous oxide decomposition catalyst obtained after the calcination process described below.

The amount of active ingredient blended per 100 parts by mass of dispersant is, for example, 5 parts by mass or more, preferably 10 parts by mass or more, and for example, 300 parts by mass or less, preferably 200 parts by mass or less.

The amount of inorganic binder added per 100 parts by mass of dispersant is, for example, 0.1 parts by mass or more, preferably 0.5 parts by mass or more, and for example, 100 parts by mass or less, preferably 50 parts by mass or less.

If the amount of inorganic binder mixed per 100 parts by mass of dispersant is within the above range, the inorganic binder can be reliably dispersed uniformly in the dispersant.

The method for stirring the above product and inorganic binder is not particularly limited, and any known method can be used. The stirring time is, for example, 5 to 30 minutes.

In this way, a slurry containing the above product, an inorganic binder, and a dispersant can be obtained.

(Coating process)
In the coating step, the slurry obtained in the preparation step is coated onto a substrate.

In the coating process, the substrate is preferably flat. More preferably, it is flat glass paper.

Application methods include, for example, the so-called dipping method, brush coating method, spray coating method, and drop coating method. Brush coating method is preferred.

When applying the slurry to a substrate using a brush, specifically, the slurry obtained in the preparation process is dripped onto the substrate and then spread over the substrate using a brush.

In this way, a substrate coated with slurry can be obtained.

(Firing process)
In the firing step, the substrate coated with the slurry is fired. More specifically, the substrate coated with the slurry is dried and then fired.

One method for drying a substrate coated with slurry is to place the substrate coated with slurry in a heated mold and dry the slurry.

The drying temperature (mold heating temperature) for the substrate coated with the slurry is, for example, 70°C or higher and, for example, 120°C or lower. The drying time for the substrate coated with the slurry is, for example, 0.5 hours or higher and, for example, 50 hours or lower.

If the substrate coated with the slurry after drying is to have a flat plate shape, the substrate coated with the slurry is dried using a flat mold. Also, if the substrate coated with the slurry after drying is to have a corrugated plate shape, the substrate coated with the slurry is dried and simultaneously shaped into a corrugated plate. Specifically, the substrate coated with the slurry is placed in a heated corrugated plate mold and shaped using a jig.

Then, the substrate coated with the dried slurry is fired.

The method for firing the substrate coated with the dried slurry is not particularly limited, and any known method can be used. The substrate coated with the dried slurry can be fired in a stacked state. Specifically, flat substrates coated with the dried slurry and corrugated substrates coated with the dried slurry can be alternately stacked and fired.

The baking temperature for the substrate coated with the dried slurry is, for example, 300°C or higher and, for example, 800°C or lower. The baking time for the substrate coated with the dried slurry is, for example, 0.5 hours or higher and, for example, 10 hours or lower.

In this way, a nitrous oxide decomposition catalyst can be obtained.

The nitrous oxide decomposition catalyst obtained by the above-mentioned method for producing a nitrous oxide decomposition catalyst has high strength.

3. Nitrous Oxide Decomposition Catalyst Apparatus One embodiment of a nitrous oxide decomposition catalyst apparatus 10 including the above-described nitrous oxide decomposition catalyst 1 will now be described with reference to FIG.

The nitrous oxide decomposition catalyst device 10 is disposed in an exhaust passage through which exhaust gas flows, and includes a nitrous oxide decomposition catalyst 1 that decomposes _N2O .

As shown in FIG. 2, the nitrous oxide decomposition catalyst device 10 includes, for example, a nitrous oxide decomposition catalyst unit 11. The nitrous oxide decomposition catalyst unit 11 includes a casing 12 and a nitrous oxide decomposition catalyst 1 filled in the casing 12.

The nitrous oxide decomposition catalyst device 10 comprises multiple nitrous oxide decomposition catalyst units 11. The number of nitrous oxide decomposition catalyst units 11 in the nitrous oxide decomposition catalyst device 10 is not particularly limited as long as it is two or more, and can be adjusted appropriately depending on factors such as the installation space. Furthermore, multiple nitrous oxide decomposition catalyst units 11 are arranged in a direction perpendicular to the flow direction of exhaust gas. In one embodiment of the nitrous oxide decomposition catalyst device 10 shown in Figure 2, the multiple nitrous oxide decomposition catalyst units 11 are aligned in a first direction (e.g., width direction) perpendicular to the flow direction of exhaust gas, and a second direction (e.g., height direction) perpendicular to the flow direction of exhaust gas and the first direction.

The shape of the casing 12 is not particularly limited, but examples include a rectangular tube shape and a cylindrical shape extending in the direction of exhaust gas flow. A rectangular tube shape extending in the direction of exhaust gas flow is preferable. Specifically, the casing 12 may be one consisting of a casing body that is roughly U-shaped in cross section and a flat lid that covers the opening, one consisting of only a casing body that is roughly R-shaped in cross section, or one consisting of a casing body that is roughly L-shaped in cross section and a lid that fits into the casing and has a roughly inverted L-shaped in cross section.

The dimensions of the casing 12 are adjusted appropriately depending on the application. Specifically, the width of the casing 12 is not particularly limited, as long as it can accommodate the nitrous oxide decomposition catalyst 1. Furthermore, the height of the casing 12 is not particularly limited, as long as it can ensure an appropriate number of layers of the nitrous oxide decomposition catalyst 1.

An inorganic fiber blanket 13 may be laid over the entire inner surface of the casing 12. By laying the inorganic fiber blanket 13 on the inner surface of the casing 12, vibration can be suppressed.

Examples of inorganic fibers in the inorganic fiber blanket 13 include ceramic fibers, glass fibers, silica sol fibers, alumina fibers, and rock wool. Ceramic fibers are preferred.

In the nitrous oxide decomposition catalyst unit 11, the nitrous oxide decomposition catalyst 1 is packed inside the casing 12 in a direction perpendicular to the flow direction of the exhaust gas. Specifically, in the nitrous oxide decomposition catalyst unit 11, flat plate-shaped nitrous oxide decomposition catalysts 1 and corrugated plate-shaped nitrous oxide decomposition catalysts 1 are alternately stacked inside the casing 12 without being bonded. By stacking them in this manner, the nitrous oxide decomposition catalyst 1 forms a cross-sectional mesh structure (honeycomb structure).

There is no particular limit to the number of stacked flat plate-shaped nitrous oxide decomposition catalysts 1 and corrugated plate-shaped nitrous oxide decomposition catalysts 1.

The nitrous oxide decomposition catalyst unit 11 is manufactured by preparing a casing 12 with an inorganic fiber blanket 13 lined on the inner surface, and filling the casing 12 with a stack of nitrous oxide decomposition catalysts 1, in which flat plate-shaped nitrous oxide decomposition catalysts 1 and corrugated plate-shaped nitrous oxide decomposition catalysts 1 are alternately stacked without being bonded together.

The nitrous oxide decomposition catalyst device 10 is manufactured by aligning the nitrous oxide decomposition catalyst units 11 manufactured as described above in the width and height directions, as shown in Figure 2.

4. Effects: The method for producing a nitrous oxide decomposition catalyst disclosed herein includes a preparation step of preparing a product containing an active component containing the composite metal oxide represented by formula (1) above and an impurity component containing an alkali metal element, and the alkali metal element content relative to the total amount of main elements in the active component and the impurity component is 0.80 mass% or more and 4.20 mass% or less. This allows for an improvement in the nitrous oxide decomposition rate.

The nitrous oxide decomposition catalyst disclosed herein comprises an active component containing the composite metal oxide represented by formula (1) above and an impurity component containing an alkali metal element, with the alkali metal element content being 0.80 mass% or more and 4.20 mass% or less relative to the total amount of main elements in the active component and the impurity component. This allows for an improved nitrous oxide decomposition rate.

5. Modifications In the modification, the same components and steps as those in the above-described embodiment of the nitrous oxide decomposition catalyst are designated by the same reference numerals, and detailed descriptions thereof will be omitted. Furthermore, unless otherwise specified, the modification can achieve the same effects as those in the embodiment of the nitrous oxide decomposition catalyst. Furthermore, the above-described embodiment and modification of the nitrous oxide decomposition catalyst can be combined as appropriate.

In one embodiment of the above-described nitrous oxide decomposition catalyst, a flat and/or corrugated inorganic fiber sheet is used as the substrate 4 supporting the active component 2, impurity component 5, and inorganic binder 3 in the nitrous oxide decomposition catalyst 1, but this is not limited to this. Specifically, the substrate 4 supporting the active component 2, impurity component 5, and inorganic binder 3 may be any substrate typically used in nitrous oxide decomposition catalysts 1, such as a monolith substrate with a cross-sectional mesh structure (honeycomb structure).

Examples of materials for the monolith substrate include ceramics, cordierite, silicon carbide, silica, alumina, mullite, ceria, zirconia, composite oxides thereof, solid solutions thereof, and mixtures thereof.

The shape of the monolith substrate is not particularly limited as long as it has a cross-sectional mesh structure, and examples include pillars and blocks.

In addition, known monolith substrates with a cross-sectional mesh structure (honeycomb structure) can be used, such as honeycomb filters and high-density honeycombs.

When using a monolith substrate with a cross-sectional mesh structure (honeycomb structure), the nitrous oxide decomposition catalyst can be manufactured using the same manufacturing method as the embodiment of the manufacturing method for the nitrous oxide decomposition catalyst described above. Specifically, like the embodiment of the manufacturing method for the nitrous oxide decomposition catalyst described above, the method includes a preparation step, a preparation step, a coating step, and, if necessary, a calcination step.

More specifically, first, a product containing active components and impurities is prepared (preparation step), similar to one embodiment of the method for producing a nitrous oxide decomposition catalyst described above. Next, a slurry containing the product containing active components and impurities, an inorganic binder, and a dispersant is prepared (preparation step). Next, the obtained slurry is applied to a monolith substrate having a cross-sectional mesh structure (honeycomb structure) by a hot-dip method (coating step). Thereafter, the monolith substrate having a cross-sectional mesh structure (honeycomb structure) to which the slurry has been applied is fired (firing step).

In this way, a nitrous oxide decomposition catalyst can be produced using a monolith substrate with a cross-sectional mesh structure (honeycomb structure).

The following examples and comparative examples are provided to further explain the present disclosure. However, the present disclosure is in no way limited to the examples and comparative examples. Furthermore, the specific numerical values of the blending ratios (content ratios), physical property values, parameters, etc. used in the following description can be replaced with the upper limit (a numerical value defined as "equal to or less than") or lower limit (a numerical value defined as "equal to or greater than") of the corresponding blending ratios (content ratios), physical property values, parameters, etc. described in the "Description of Embodiments" above.

Example 1: The nitrous oxide decomposition catalyst of Example 1 was obtained according to the following procedure.

Nickel _nitrate (Ni( _NO3 ) _{2.6H2O )} , iron nitrate (Fe( _NO3 ) _3.9H2O ), and cobalt nitrate (Co( _NO3 ) _2.6H2O ) were weighed to a composition ratio (atomic ratio) _{of Ni0.9Fe0.1Co2O4} and dissolved in ion-exchanged _water . Next, while stirring the solution, 15 wt% _K2CO3 was added dropwise at 10 mL/min until the pH reached ₉ , obtaining a precipitate. The precipitate was filtered and washed with ion-exchanged water. Filtration and washing were repeated until the pH of _the ion-exchanged water used _for washing reached 7. The washed precipitate was collected, dried at 100°C for 12 hours, and calcined at 400°C for 2 hours to obtain the nitrous oxide decomposition catalyst of Example 1 (a product containing the active component and impurities).

In the nitrous oxide decomposition catalyst of Example 1, the content of alkali metal elements (specifically, K (potassium)) relative to the total amount of main elements in the active component and impurities was 0.82 mass%. An energy dispersive X-ray fluorescence analyzer (EA1400, manufactured by Hitachi Corporation) was used to measure the content of alkali metal elements (specifically, K (potassium)) relative to the total amount of main elements in the active component and impurities in the nitrous oxide decomposition catalyst of Example 1.

Examples 2 to 8
As shown in Table 1, the nitrous oxide decomposition catalysts of each Example were obtained in the same manner as in Example 1, except that the content ratio of the alkali metal element (specifically, K (potassium element)) relative to the total amount of the main elements of the active component and the impurity component was adjusted.

Comparative Examples 1 to 7
As shown in Table 1, the nitrous oxide decomposition catalysts of each comparative example were obtained in the same manner as in Example 1, except that the content ratio of the alkali metal element (specifically, K (potassium element)) relative to the total amount of the main elements of the active component and the impurity component was adjusted.

<Evaluation>
[Nitrous oxide decomposition rate]
A reactor was filled with 3.2 g of the nitrous oxide decomposition catalyst (0.5 mm to 1.0 mm pellets) from each Example and Comparative Example. Air, N ₂ , and N ₂ O were mixed to achieve the following gas composition, assuming exhaust gas, and introduced into an evaporator. Furthermore, H ₂ O was introduced into the evaporator to achieve the following moisture content. All gases (including H ₂ O) were mixed in the evaporator and supplied to the reactor at 2.8 NL/min-wet. The reactor temperature (gas temperature inside the reactor) was heated to 400°C using an electric heater. The ventilation volume SV (ventilation volume per catalyst volume) was 10,811 h ^-1 . N ₂ O concentrations were measured at the reactor inlet and outlet using an N ₂ O meter, and the nitrous oxide decomposition rate was calculated using the following formula. The results are shown in Table 1. FIG. 3 shows the relationship between the nitrous oxide decomposition rate (%) and the content (mass %) of alkali metal elements relative to the total amount of main elements of the active component and impurities for each example and comparative example.

{Gas composition}
N2 ₀ :100ppmvd
0 ₂ : 12% by volume - dry
_N2 : Balance
H ₂ O: 10% by volume - wet

{Nitrous oxide decomposition rate}
Nitrous oxide decomposition rate (%)=(N ₂ O concentration at reactor inlet−N ₂ O concentration at reactor outlet)/(N ₂ O concentration at reactor inlet)×100

<Consideration>
Referring to Table 1, the nitrous oxide decomposition catalysts of Examples 1 to 8 have an alkali metal element content of 0.80 mass% or more and 4.20 mass% or less relative to the total amount of main elements of the active component and impurities, and contain impurities at a specific ratio. As a result, the nitrous oxide decomposition rate exceeds 60%, thereby improving the nitrous oxide decomposition rate. In particular, the nitrous oxide decomposition catalysts of Examples 3 to 7 have an alkali metal element content of 1.15 mass% or more and 3.80 mass% or less relative to the total amount of main elements of the active component and impurities, and contain impurities at a specific ratio. As a result, the nitrous oxide decomposition rate exceeds 85%, thereby further improving the nitrous oxide decomposition rate.

In contrast, the nitrous oxide decomposition catalysts of Comparative Examples 1 to 3 have an alkali metal element content of less than 0.80% by mass relative to the total amount of main elements in the active components and impurities, resulting in a low impurity content. As a result, the nitrous oxide decomposition rate is low. Furthermore, the nitrous oxide decomposition catalysts of Comparative Examples 4 to 7 have an alkali metal element content of more than 4.50% by mass relative to the total amount of main elements in the active components and impurities, resulting in a high impurity content. As a result, the nitrous oxide decomposition rate is low.

In addition, as is clear from Figure 3, the nitrous oxide decomposition rate increases sharply from Comparative Example 3 (alkali metal element content: 0.68 mass%) to Example 1 (alkali metal element content: 0.82 mass%). Furthermore, the nitrous oxide decomposition rate decreases sharply from Example 8 (alkali metal element content: 4.17 mass%) to Comparative Example 4 (alkali metal element content: 5.46 mass%).

Note that while the above inventions are provided as exemplary embodiments of the present disclosure, they are merely examples and should not be interpreted as limiting. Modifications of the present disclosure that are obvious to those skilled in the art are intended to be included within the scope of the following claims.

The method for producing a nitrous oxide decomposition catalyst disclosed herein is suitable for producing a nitrous oxide decomposition catalyst with an improved nitrous oxide decomposition rate, which is used to treat exhaust gases. Furthermore, the nitrous oxide decomposition catalyst disclosed herein is suitable for use in treating exhaust gases containing nitrous oxide.

1 Nitrous oxide decomposition catalyst 2 Active component 3 Inorganic binder 4 Base material 5 Impurity component

Claims (12)

A method for producing a nitrous oxide decomposition catalyst for decomposing nitrous oxide in exhaust gas, comprising:
The method includes a preparation step of preparing a product containing an active component containing a composite metal oxide represented by the following formula (1) and an impurity component containing an alkali metal element,
In the preparation step, the product containing the active ingredient and the impurity ingredient is prepared by a coprecipitation method using a coprecipitant containing the alkali metal element;
A method for producing a nitrous oxide decomposition catalyst, wherein the content of the alkali metal element relative to the total amount of main elements of the active component and the impurity component is 0.80 mass % or more and 4.20 mass % or less.
Ni _X A _1-X Co ₂ O ₄ (1)
(In formula (1), A is at least one selected from the group consisting of Fe, Mn, Ce, Zr, La, and alkaline earth metals. Also, X is greater than 0 and less than 1.) preparing a slurry containing the product, an inorganic binder, and a dispersant;
The method for producing a nitrous oxide decomposition catalyst according to claim 1 , further comprising: a coating step of coating the slurry onto a substrate. The method for producing a nitrous oxide decomposition catalyst according to claim 2, wherein the inorganic binder comprises at least one selected from the group consisting of metal hydroxides and metal oxides. The method for producing a nitrous oxide decomposition catalyst according to claim 2, wherein the dispersant contains at least one selected from the group consisting of carboxylic acids, alcohols, and water. The method for producing a nitrous oxide decomposition catalyst according to claim 2, wherein the substrate is an inorganic fiber sheet. The method for producing a nitrous oxide decomposition catalyst according to any one of claims 2 to 5, further comprising a calcination step of calcining the substrate coated with the slurry after the coating step. An active ingredient containing a composite metal oxide represented by the following formula (1);
and impurity components containing alkali metal elements,
A nitrous oxide decomposition catalyst, wherein the content of the alkali metal element relative to the total amount of main elements of the active component and the impurity component is 0.80 mass % or more and 4.20 mass % or less.
Ni _X A _1-X Co ₂ O ₄ (1)
(In formula (1), A is at least one selected from the group consisting of Fe, Mn, Ce, Zr, La, and alkaline earth metals. Also, X is greater than 0 and less than 1.) The nitrous oxide decomposition catalyst according to claim 7, wherein the alkali metal element is K. The nitrous oxide decomposition catalyst according to claim 7, wherein the content of the alkali metal element relative to the total amount of main elements of the active component and the impurity component is 1.15 mass% or more and 3.80 mass% or less. an inorganic binder;
8. The nitrous oxide decomposition catalyst according to claim 7, further comprising a substrate that supports the active component, the impurity component, and the inorganic binder. The nitrous oxide decomposition catalyst according to claim 10, wherein the inorganic binder comprises at least one selected from the group consisting of metal hydroxides and metal oxides. The nitrous oxide decomposition catalyst according to claim 10, wherein the substrate is an inorganic fiber sheet.