JP6762185B2 - Ammonia synthesis catalyst, method for producing ammonia synthesis catalyst, and method for ammonia synthesis - Google Patents

Description

The present invention relates to a catalyst used for ammonia synthesis, a method for producing the same, and a method for synthesizing ammonia using the catalyst.

Ammonia has traditionally been widely produced at the industrial level by the Haber process. In the Haber-Bosch method, hydrogen and nitrogen are reacted under high pressure conditions of 400 to 600 ° C. and 20 to 100 MPa using a double-accelerated iron catalyst to obtain ammonia (Non-Patent Document 1). In order to realize ammonia synthesis at a lower temperature and lower pressure, there is also an example of using a catalyst using ruthenium as an active metal (Patent Document 2 and Non-Patent Documents 2 to 4). In such an example, high activity is realized by using a carrier containing a lanthanoid oxide such as ceria or an alkaline earth metal.

On the other hand, in recent years, the inventors have proposed a method for synthesizing ammonia using an electric field catalytic reaction (Patent Documents 3 and 4). In the electric field catalytic reaction, a catalyst is placed between a pair of electrodes, and a voltage that does not generate a discharge is applied between the electrodes to perform the reaction while forming an electric field, whereby the effect of promoting the catalytic reaction can be obtained. Therefore, it can be said that it is a reaction method that may be suitably used under conditions such as small scale and low temperature and low pressure, which are disadvantageous in terms of energy efficiency in the conventional Haber process. In the synthesis of ammonia by an electric field catalytic reaction, the catalyst needs to have a certain conductivity, and a catalyst using a Ce _0.5 Zr _0.5 O ₂ composite oxide having a fluorite structure as a carrier is disclosed. There is.

Japanese Unexamined Patent Publication No. 2-258066 Japanese Unexamined Patent Publication No. 6-79177 Japanese Unexamined Patent Publication No. 2014-141361 Japanese Unexamined Patent Publication No. 2014-171916

"Catalyst Handbook" Kodansha Published December 10, 2008 pp. 68 Journal of Fuel Chemistry and Technology Vol. 40, pp. 848-854, 2012 Catalysis Letters Vol. 106, pp. 107-110, 2006 Catalysis Letters Vol. 141, pp. 1275-1281, 211

Since ammonia synthesis by the conventional Haber-Bosch method is a high-temperature and high-pressure reaction, it is required to improve the low-temperature activity of the catalyst for ammonia synthesis in order to reduce the energy consumption. On the other hand, in the ammonia synthesis method by the electric field catalytic reaction, the reaction rate at the same reaction temperature is superior to that of the Haber Bosch method due to the reaction promoting effect, but the promoting effect obtained with respect to the input energy is insufficient. Therefore, there is a problem that it is inferior to the Haber Bosch method in terms of energy efficiency. In addition, a catalyst using a magnesia carrier, which is effective in the conventional Haber-Bosch method, or an oxide consisting of a single lanthanoid such as lanthanum oxide as a carrier, discharges because the insulating property of the catalyst is too high in the electric field catalytic reaction. In addition to being unable to obtain the effect of promoting ammonia synthesis by applying an electric field, most of the input electrical energy is used for ionization of molecules in the gas, which causes a problem that energy efficiency is significantly reduced.

As a result of diligent studies in view of the above problems, the inventors of the invention include at least one active metal belonging to the 8th to 10th groups of the Long Periodic Table and a composite oxide, and the reduction starting temperature of the ammonia is 190 ° C. or lower. We have found that the low temperature activity is significantly improved in the catalyst for synthesis, in particular, the effect of applying an electric field can be effectively utilized and high activity can be realized, and the invention has been completed. The present invention is shown below.

[1] A catalyst for ammonia synthesis containing at least one active metal belonging to Group 8 to 10 of the Long Periodic Table and a composite oxide, characterized in that the reduction start temperature is 190 ° C. or lower. A catalyst for ammonia synthesis.

[2] The catalyst for ammonia synthesis according to the above [1], which further contains a rare earth element and / or an alkaline earth element.

[3] The catalyst for ammonia synthesis according to [1] or [2], wherein the active metal is at least one selected from ruthenium, iron, cobalt, nickel, platinum, palladium, rhodium, and iridium.

[4] The catalyst for ammonia synthesis according to any one of [1] to [3], wherein the composite oxide is selected from at least an apatite-type, pyrochlore-type, brown-miralite-type, and spinel-type crystal structure. A catalyst for ammonia synthesis, which comprises a composite oxide having one or more kinds of crystal structures.

[5] The method for producing an ammonia synthesis catalyst according to any one of [1] to [4] above, which is obtained by firing at 450 ° C. or higher and 800 ° C. or lower in the presence of a reducing gas. A method for producing a catalyst for ammonia synthesis.

[6] In the method for producing an ammonia synthesis catalyst according to the above [5], a catalyst for ammonia synthesis is produced, wherein the reducing gas contains at least one selected from hydrogen, carbon monoxide, and ammonia. Method.

[7] The method for producing an ammonia synthesis catalyst according to any one of [1] to [4] above, which is characterized in that it is obtained by firing at a temperature of more than 600 ° C. and 800 ° C. or lower in the presence of an oxygen-containing gas. A method for producing a catalyst for ammonia synthesis.

[8] A method in which a catalyst is provided between a pair of electrodes and a voltage that does not generate a discharge is applied between the electrodes in the presence of a gas containing a hydrogen atom and a nitrogen atom to synthesize ammonia. A method for synthesizing ammonia, which comprises using the catalyst for synthesizing ammonia according to any one of [4].

By using the catalyst for ammonia synthesis according to the present invention, it is possible to significantly improve the ammonia production rate, especially in the ammonia synthesis reaction at a lower temperature than the conventional one of about 300 ° C. to 400 ° C.

It is a figure which shows the relationship between the reduction start temperature of the catalyst for ammonia synthesis and the ammonia synthesis rate at 350 degreeC under the condition of applying an electric field. It is a figure which shows the relationship between the reduction start temperature of the catalyst for ammonia synthesis and the ammonia synthesis rate at 300 degreeC under the condition of applying an electric field. It is a figure which shows the power consumption for synthesizing 1 kg of ammonia at 350 degreeC under the electric field application condition.

Hereinafter, the catalyst for ammonia synthesis, the method for producing the catalyst for ammonia synthesis, and the method for synthesizing ammonia according to the present invention will be described in detail, but the scope of the present invention is not limited to these explanations, and other than the following examples. It can be appropriately modified and implemented without impairing the gist of the present invention.

[Catalyst for ammonia synthesis]
The catalyst for ammonia synthesis in the present invention contains at least one active metal belonging to Group 8 to 10 of the Long Periodic Table and a composite oxide, and the reduction starting temperature thereof is 190 ° C. or lower. For example, there is no particular limitation.
The reduction start temperature in the present invention can be measured by the following method.

1) 450 ° C. over 30 minutes, held normal pressure, under containing _{H 2} gas flow. The amount of catalyst, gas flow rate, and gas concentration are not particularly limited as long as they can reduce the entire amount of active metal contained in the catalyst, but as an example, 4 for 2 g of a catalyst containing 6.3 parts by weight of ruthenium as active metal. There is a method of circulating% H ₂ / N ₂ balanced gas at 1 L / min.

2) The reduced catalyst is cooled to room temperature under the flow of the H _2- containing gas of 1) or the flow of the inert gas such as N ₂ , Ar, He, and then taken out into the atmosphere and left for 1 hour or more. This causes the active metal to be reoxidized.

3) catalyze the _H 2-TPR measurement apparatus after reoxidation (introduced about 10~20mg etc. Rigaku Thermo plus EVO2 TG8120), _N 2 was dried over silica gel and the like, Ar, inert gas flow under such as He The dehydration treatment is performed at about 200 ° C. The dehydration treatment is preferably carried out until no weight loss is observed, and is usually carried out for about 15 minutes to 1 hour.

4) After the dehydration treatment, the allowed lowering the temperature to remain 50 ° C. of about was passed through an inert gas, performs H _2-TPR measurement after the temperature was stabilized.

The _H 2-TPR measurement conditions for measuring the reduction onset temperature, performing heating in _4% H ₂ / _N 2 under flowing balance gas at about 300~500cc / min, heating rate 5 ° C. / min. The time-differentiated value (dTG; unit% / min) is calculated for the result of thermogravimetric measurement recorded at the time of temperature rise, and the starting point of weight loss at the peak of dTG seen on the lowest temperature side is the start of reduction of the catalyst. Let the temperature be. As a method of obtaining the starting point of the weight loss, it is calculated from the temperature at the intersection of the approximate straight line of the dTG of the portion where the weight loss is not observed and the approximate straight line of the dTG of the portion where the weight loss starts to progress.

The reason why the above-mentioned reduction start temperature correlates with the ammonia synthesis activity is not clear, but nitrogen dissociation in the ammonia synthesis reaction occurs on the active metal in which the oxygen chemically adsorbed or physically adsorbed on the surface of the active metal is easily desorbed. Alternatively, it is considered to have the effect of promoting elementary reactions such as hydrogenation.

Examples of the active metal belonging to the 8th to 10th groups of the long periodic table include ruthenium, iron, cobalt, nickel, platinum, palladium, rhodium, iridium and the like, with ruthenium, iron, cobalt and nickel being preferable, and ruthenium, iron, cobalt and nickel. Iron is particularly preferred.

The catalyst for ammonia synthesis of the present invention preferably further contains a rare earth element and / or an alkaline earth element. Examples of rare earth elements include lanthanum, cerium, praseodymium, neodymium, samarium, europium, gadrinium, samarium, dysprosium, formium, erbium, itterbium, etc. , Lantern, cerium, praseodymium, neodymium, samarium, itterbium are particularly preferred.
Examples of the alkaline earth element include magnesium, calcium, strontium and barium, and calcium, strontium and barium are preferable.
The content form of the rare earth element and the alkaline earth element is not particularly limited, and can be contained in the form of a metal, an oxide, a hydroxide, a carbonate or the like. Further, it may be partially compounded with the composite oxide.

The composite oxide of the catalyst for ammonia synthesis of the present invention contains at least a composite oxide having at least one crystal structure selected from an apatite type, a pyrochlore type, a brown mirrorite type, and a spinel type crystal structure. It is more preferable to contain a composite oxide having an apatite-type, pyrochlore-type, or brown-miralite-type crystal structure, and it is particularly preferable to contain a composite oxide having an apatite-type or pyrochlore-type crystal structure.

The composite oxide having an apatite-type crystal structure is not particularly specified, but is a composite oxide described by the general formula A ₁₀ B ₆ O _{27. As} an example, La ₁₀ Si ₆ O ₂₇ , Nd ₁₀ Si ₆ O ₂₇ , Sm ₁₀ Si ₆ O ₂₇ , La ₁₀ Ge ₆ O ₂₇ , Nd ₁₀ Ge ₆ O ₂₇ , Sm ₁₀ Ge ₆ O _27, etc., La ₁₀ Si ₆ O ₂₇ , Nd ₁₀ Si ₆ O ₂₇ , Sm ₁₀ Si ₆ O ₂₇ is preferred, and La ₁₀ Si ₆ O ₂₇ is particularly preferred. Further, the metal at the A and B sites may be partially replaced with a different metal.

The composite oxide having a pyrochlor type crystal structure is not particularly specified, but is a composite oxide described by the general formula A ₂ B ₂ O ₇ , and as an example, La ₂ Zr ₂ O ₇ and Nd ₂ Zr ₂ O ₇ , Sm ₂ Zr ₂ O ₇ , Gd ₂ Zr ₂ O ₇ , La ₂ Ti ₂ O ₇ , Nd ₂ Ti ₂ O ₇ , Sm ₂ Zr ₂ O ₇ , Gd ₂ Zr ₂ O _7, La ₂ Ce ₂ O ₇ , Examples thereof include Ca ₂ Nb ₂ O ₇ , Sr ₂ Nb ₂ O ₇ , Ba ₂ Nb ₂ O ₇ , and Bi ₂ Ti ₂ O ₇ . Of these, La ₂ Zr ₂ O ₇ , Nd ₂ Zr ₂ O ₇ , Sm ₂ Zr ₂ O ₇ , Gd ₂ Zr ₂ O ₇ , and La ₂ Ce ₂ O ₇ are preferable, and La ₂ Zr ₂ O ₇ , Nd ₂ Zr. ₂ O ₇ is particularly preferable. Further, the metal at the A and B sites may be partially replaced with a different metal. Examples of the composite oxide having such a pyrochlore structure include La ₂ Zr _1.9 Y _0.1 O ₇ .

The composite oxide having a brown mirrorite type crystal structure is not particularly specified, but is a composite oxide described by the general formula A ₂ B ₂ O ₅ , and as an example, Ca ₂ Fe ₂ O ₅ and Ca ₂ Co ₂ Examples thereof include O ₅ , Sr ₂ Fe ₂ O ₅ , Sr ₂ Co ₂ O ₅ , and Ba ₂ Fe ₂ O ₅ .

The composite oxide having a spinel-type crystal structure is not particularly specified, but is a composite oxide described by the general formula AB ₂ O ₄ , and as an example, MgAl ₂ O ₄ , FeAl ₂ O ₄ , ZnAl ₂ O ₄ , Examples thereof include MnAl ₂ O ₄ , MgFe ₂ O ₄ , and MnFe ₂ O ₄ .

The method for preparing the composite oxide containing the structure is not particularly limited, and examples thereof include the methods described below. For example, a metal salt is dissolved in a glycol solution containing an excess of oxycarboxylic acid to form a metal oxycarboxylic acid complex. Heating this solution gives a polyester polymer gel. A composite oxide powder can be obtained by thermally decomposing the obtained polymer gel at a high temperature (complex polymerization method). In addition, a method of mixing and firing solid raw materials such as oxides of elements constituting the composite oxide and carbonates (solid phase reaction method), and another composition of one oxide of the elements constituting the composite oxide. A method of impregnating an aqueous solution containing an element salt, drying and firing (impregnation method), mixing an aqueous solution containing a salt of a constituent element of a composite oxide, adjusting the pH to obtain a precipitate, and then drying the precipitate. There is a method of firing (co-precipitation method). A complex polymerization method, a solid phase reaction method, and a coprecipitation method are preferable. In these methods, the firing temperature is not particularly specified as long as each crystal structure can be formed, but is preferably 800 to 1600 ° C., and more preferably 900 to 1200 ° C. from the viewpoint of reproducibility and specific surface area. The specific surface area of these composite oxides is preferably ^{0.5~50m 2 / g, 5~50m 2 /} g is more preferable. These composite oxides mainly function as carriers.

The content of the active metal is not particularly limited as long as it has the effect of the present invention, but is preferably 0.01 to 40 parts by weight, preferably 0.1 to 30 parts by weight, based on 100 parts by weight of the composite oxide. More preferably, 0.5 to 25 parts by weight is further preferable.

The total content of the rare earth element and the alkaline earth element is not particularly limited as long as it has the effect of the present invention, but is preferably 0.1 to 40 parts by weight with respect to 100 parts by weight of the composite oxide. 0.5 to 30 parts by weight is more preferable, and 0.5 to 25 parts by weight is further preferable.

The shape of the catalyst for ammonia synthesis is not particularly limited and may be in the form of powder, or the powder or the like is molded into a fixed shape such as a columnar shape, a ring shape or a spherical shape by an extrusion molding method or a tableting molding method. It may be a molded body, an indefinite shape formed after being molded into a constant shape, and then crushed.

[Manufacturing method of catalyst for ammonia synthesis]
The method for producing the catalyst for ammonia synthesis in the present invention is not particularly limited except for the firing conditions, and a method generally used for preparing this type of catalyst can be used. As a step of supporting the active metal, rare earth element, or alkaline earth element on the composite oxide, for example, (1) a solution containing an active metal precursor and a solution containing a rare earth element or an alkaline earth element and a composite oxide are mixed. (2) A method in which a solution containing an active metal precursor and a solution containing a rare earth element or an alkaline earth element and a composite oxide are mixed and precipitated and supported by an acid or a base, (3). ) The active metal precursor is supported on the composite oxide by the methods (1) and (2), and then a solution containing a rare earth element and an alkaline earth element is supported by the methods (1) and (2). Method, (4) After supporting a solution containing rare earth elements and alkaline earth elements by the methods (1) and (2) above, the active metal precursor is subjected to the composite oxide by the methods (1) and (2) above. Examples thereof include a method of carrying it on top. A catalyst for ammonia synthesis can be obtained by calcining the catalyst precursor thus obtained.

The active metal precursor is not particularly limited as long as it is a solution containing the active metal, but nitrates, acetates, acetylacetonate complexes, carbonyl complexes and the like of the active metals can be used, and acetates and acetylacetonates can be used. It is preferable to use a complex. As the solvent, water, ethanol, methanol, acetone, THF and the like can be used as long as they can dissolve these active metal precursors.

The content form of the rare earth element and the alkaline earth element is not particularly limited, and nitrates, acetates, carbonates, oxides and the like can be used. The solvent for dissolving or dispersing these is not particularly limited, but water, ethanol, methanol, acetone, THF and the like can be used.

In order to produce the catalyst for ammonia synthesis in the present invention, the catalyst precursor may be calcined under the conditions described below.
As one firing condition, the catalyst precursor is fired at a temperature of 450 to 800 ° C. in the presence of a reducing gas. The component of the reducing gas is not particularly limited, but as an example, a gas containing hydrogen, carbon monoxide, and ammonia can be used, and hydrogen or carbon monoxide is preferably used from the viewpoint of preventing equipment corrosion and cost. Further, in order to ensure safety against explosion, poisoning, etc., an inert gas such as nitrogen, argon, or helium may be introduced along with the reducing gas.

Further, as another firing condition, firing is performed at a temperature of more than 600 ° C. and 800 ° C. or lower in the presence of a gas containing oxygen. As the gas containing oxygen, oxygen gas diluted with the above-mentioned inert gas may be used, or Air may be used.
By using the above method, it is possible to produce a catalyst for ammonia synthesis in which the reduction start temperature of the active metal is 190 ° C. or lower.

If necessary, the catalyst after firing may be appropriately reduced before being subjected to an ammonia synthesis reaction.

[Ammonia synthesis method]
In the method for synthesizing ammonia in the present invention, the above-mentioned catalyst for synthesizing ammonia is used in synthesizing ammonia by passing a raw material gas through a reactor described later and applying a voltage that does not generate a discharge between a pair of electrodes. It is a method of synthesizing ammonia.

The discharge in the present invention means that nitrogen molecules, hydrogen molecules, etc. in the raw material gas circulated between the electrodes undergo dielectric breakdown due to the applied voltage, ionization and electron emission occur, and a current flows. When a discharge occurs, a light emission phenomenon can often be observed at the same time. The voltage applied in the method for synthesizing ammonia in the present invention is a voltage lower than the voltage at which dielectric breakdown occurs, that is, the dielectric breakdown voltage.

The reactor is composed of a pair of electrodes, a voltage applying means for applying a voltage between the electrodes, an ammonia synthesis catalyst installed between the electrodes, a raw material gas introduction port, and a generated ammonia-containing gas discharge port. The reactor may further include a catalyst support to hold the catalyst in a preferred position.

The pair of electrodes may be made of a conductive material and may have a shape and arrangement that can form an electric field in the space containing the catalyst. As an example of the electrode material, iron, stainless steel, titanium, Hastelloy (registered trademark) can be used, and stainless steel is preferable from the viewpoint of corrosion resistance and cost. As an example of the shape of the electrode, a rod-shaped electrode, a tubular electrode, a plate-shaped electrode, a mesh-shaped electrode, or the like can be used, and the rod-shaped electrode, the tubular electrode, the mesh-shaped electrode, etc. can be used in terms of gas flowability and volumetric efficiency of the device. Is preferable. As an example of the arrangement of the electrodes, in addition to the method of arranging the pair of electrodes in the flow direction of the flow reactor and arranging the catalyst between them, the pair of electrodes consisting of the rod-shaped electrodes and the tubular electrodes arranged concentrically There is a method of arranging a catalyst in between.

The catalyst support may be of any shape and material as long as the catalyst is fixed between the electrodes and does not adversely affect the reaction. As an example, plates such as quartz wool, glass wool, granular silica, alumina with holes for gas flow, zirconia, and magnesia can be arranged before and after the catalyst.

The voltage applying means is not particularly limited as long as a voltage can be applied between the pair of electrodes, and for example, a commercially available high voltage power supply can be used. As the high voltage power supply, a DC power supply, an AC power supply, or an ultrashort pulse generator can be used, but it is preferable to use a DC power supply for forming an electric field.

The raw material gas may be a gas containing hydrogen atoms and nitrogen atoms as a raw material for ammonia, and the molar ratio of hydrogen atoms / nitrogen atoms in the raw material gas is preferably 0.01 or more and 10 or less, more preferably. Is 0.1 or more and 3.0 or less. If the molar ratio of the hydrogen atom / nitrogen atom is less than 0.01, the ammonia production rate is significantly reduced due to the restriction of equilibrium, which is not preferable. On the contrary, if it is larger than 3, the applied voltage is increased, which is not preferable.
A mixture gas in which a component containing a hydrogen atom and a nitrogen atom are present as separate components may be used, or a gas containing both a hydrogen atom and a nitrogen atom in the same component may be used, or a mixture thereof may be used. You may use it. From the viewpoint of availability, economy, and catalyst durability, it is preferable to use nitrogen molecules as a gas containing nitrogen atoms and hydrogen molecules as a gas containing hydrogen atoms, and introduce a mixed gas thereof into the catalyst layer as a raw material gas. ..

As the raw material gas supply means, a method of introducing an arbitrary gas required for the reaction into the reactor can be provided. As an example, when synthesizing ammonia from nitrogen gas and hydrogen gas, a nitrogen gas cylinder, an industrial nitrogen generator, etc. can be used as a nitrogen supply source, and hydrogen gas cylinders, hydrogen hydrocarbons, etc. can be used as hydrogen supply sources. A hydrogen-containing gas obtained by modifying the hydrogen-containing compound, a hydrogen-containing gas obtained by alkaline water electrolysis or steam electrolysis, or the like can be used.

Ammonia synthesis in the present invention may be carried out at normal pressure, but is more effective when it is carried out under pressure. Specifically, it is advantageous when ammonia is synthesized by setting the pressure in the reactor to 102 kPa to 40 MPa, preferably 102 kPa to 5 MPa.

Further, the ammonia synthesis by the method of the present invention may be carried out by heating using a heating device. As an example, ammonia can be synthesized by setting the catalyst layer temperature to 20 to 600 ° C., preferably 20 to 450 ° C., more preferably 20 to 400 ° C. As the temperature rises, the catalytic activity improves and the amount of ammonia produced also increases, but when the temperature is higher than 600 ° C., the ammonia synthesis reaction becomes thermodynamically disadvantageous, which is not preferable. Further, the reaction at a low temperature is thermodynamically advantageous, but the catalytic activity is low. Therefore, in the above temperature range, an appropriate reaction temperature may be set in consideration of the activity and economy of the catalyst to be used. ..

As for the obtained gas containing ammonia, only ammonia may be separated by a known method, if necessary. Further, a recycling process may be included in which the raw material gas is further separated from the remaining gas and used again as the raw material gas.

Hereinafter, the present invention will be specifically described with reference to Examples, but the present invention is not limited by the following Examples, and is carried out with appropriate modifications to the extent that it can be adapted to the gist of the present invention. It is also possible, all of which are within the technical scope of the invention.

(Preparation of composite oxide)
Each composite oxide was prepared by the following method. In the examples and comparative examples, the composite oxide was used.

<La ₂ Zr ₂ O ₇ >
Lanthanum oxide and zirconium oxide are weighed and mixed so that La: Zr = 1: 1 in terms of molar ratio, fired at 1100 ° C. for 10 hours in an air atmosphere, and then wet pulverized in ethanol to form La ₂ Zr ₂ O ₇ powder. I got a body. It was confirmed by powder XRD measurement that the obtained composite oxide had a pyrochlore structure.

<CaZrO ₃ >
Ca calcium carbonate, zirconium oxide molar ratio: Zr = 1: 1 and so as weighed and mixed, and baked under 10h air atmosphere at 1100 ° C., to obtain a CaZrO ₃ powder is wet-milled in ethanol .. It was confirmed by powder XRD measurement that the obtained composite oxide had a perovskite structure.

[X-ray diffraction measurement]
Measurements were made using Cu-Kα radiation (X-ray output: 45 kV-40 mA, Kα 1-ray wavelength: 1.5406Å) using an X'pertPRO MPD manufactured by Spectris.

(Example 1)
A mixture of an acetone solution of tris (acetylacetonato) ruthenium complex and an aqueous solution of cerium acetate was added to 100 parts by weight of La ₂ Zr ₂ O _{7 by 7} weights of tris (acetylacetonato) ruthenium complex (Ru metal equivalent). The parts were impregnated and supported so as to be 8 parts by weight of cerium acetate (converted to ruthenium oxide). The obtained sample was dried at 120 ° C. for 10 hours and then calcined at 450 ° C. for 2 hours in a 4% H ₂ / N ₂ balanced atmosphere to obtain a CeO _2- Ru / La ₂ Zr ₂ O ₇ catalyst. As a result of confirmation by XRD measurement, it was confirmed that the composite oxide maintained the pyrochlore structure even after the catalyst was prepared.

(Example 2)
In Example 1, except that the firing temperature of the sample after drying and 650 ° C. performs catalyst prepared in the same manner _to obtain a _{CeO 2 -Ru / La 2 Zr 2} O 7 catalyst. As a result of confirmation by XRD measurement, it was confirmed that the composite oxide maintained the pyrochlore structure even after the catalyst was prepared.

(Example 3)
In Example 1, except that the firing temperature of the sample after drying and 700 ° C. performs catalyst prepared in the same manner _to obtain a _{CeO 2 -Ru / La 2 Zr 2} O 7 catalyst. As a result of confirmation by XRD measurement, it was confirmed that the composite oxide maintained the pyrochlore structure even after the catalyst was prepared.

(Example 4)
In Example 1, except that the firing temperature of the sample after drying and 750 ° C. performs catalyst prepared in the same manner _to obtain a _{CeO 2 -Ru / La 2 Zr 2} O 7 catalyst. As a result of confirmation by XRD measurement, it was confirmed that the composite oxide maintained the pyrochlore structure even after the catalyst was prepared.

(Example 5)
In Example 1, except that the firing temperature of the sample after drying and 800 ° C. performs catalyst prepared in the same manner _to obtain a _{CeO 2 -Ru / La 2 Zr 2} O 7 catalyst. As a result of confirmation by XRD measurement, it was confirmed that the composite oxide maintained the pyrochlore structure even after the catalyst was prepared.

(Example 6)
In Example 1, the firing conditions of the sample after drying, the baking temperature 650 ° C., except that the calcining in Air Atmosphere performs catalyst prepared in the same _{manner, CeO 2 -Ru / La 2 Zr} 2 O 7 catalyst Got As a result of confirmation by XRD measurement, it was confirmed that the composite oxide maintained the pyrochlore structure even after the catalyst was prepared.

(Example 7)
In Example 1, the firing conditions of the sample after drying, the baking temperature 700 ° C., except that the calcining in Air Atmosphere performs catalyst prepared in the same _{manner, CeO 2 -Ru / La 2 Zr} 2 O 7 catalyst Got As a result of confirmation by XRD measurement, it was confirmed that the composite oxide maintained the pyrochlore structure even after the catalyst was prepared.

(Example 8)
In Example 1, the firing conditions of the sample after drying, the baking temperature 800 ° C., except that the calcining in Air Atmosphere performs catalyst prepared in the same _{manner, CeO 2 -Ru / La 2 Zr} 2 O 7 catalyst Got As a result of confirmation by XRD measurement, it was confirmed that the composite oxide maintained the pyrochlore structure even after the catalyst was prepared.

(Comparative Example 1)
A mixture of an acetone solution of tris (acetylacetonato) ruthenium complex and an aqueous solution of cerium acetate was mixed with 100 parts by weight of CaZrO _{3 by 7} parts by weight of tris (acetylacetonato) ruthenium complex (Ru metal equivalent), cerium acetate. (Equivalent to cerium oxide) Impregnated and supported so as to be 8 parts by weight. The resulting sample was dried for 10 hours at 120 ° C., and calcined for 2 hours Air atmosphere 450 ° _C., to obtain a CeO 2 -Ru / CaZrO ₃ catalyst. It was confirmed by XRD measurement that the catalyst did not contain a pyrochlore structure and that the composite oxide maintained the perovskite structure.

(Comparative Example 2)
In Comparative Example 1, the catalyst was prepared in the same manner except that the firing conditions of the sample after drying were firing in a firing temperature of 700 ° C. and a 4% H ₂ / N ₂ balanced atmosphere, and CeO _2- Ru / A CaZrO ₃ catalyst was obtained. It was confirmed by XRD measurement that the catalyst did not contain a pyrochlore structure and that the composite oxide maintained the perovskite structure.

(Comparative Example 3)
In Example 1, the firing conditions of the sample after drying, the baking temperature 450 ° C., except that the calcining in Air Atmosphere performs catalyst prepared in the same _{manner, CeO 2 -Ru / La 2 Zr} 2 O 7 catalyst Got As a result of confirmation by XRD measurement, it was confirmed that the composite oxide maintained the pyrochlore structure even after the catalyst was prepared.

(Comparative Example 4)
In Comparative Example 3, except that 4 parts by weight the amount of cerium acetate (cerium oxide equivalent) performs catalyst prepared in the same manner as in Comparative Example _3, to obtain a _{CeO 2 -Ru / La 2 Zr 2} O 7 catalyst .. As a result of confirmation by XRD measurement, it was confirmed that the composite oxide maintained the pyrochlore structure even after the catalyst was prepared.

(Comparative Example 5)
A mixture of an acetone solution of tris (acetylacetonato) ruthenium complex and an aqueous solution of cesium hydroxide was added to 100 parts by weight of MgO, 7 parts by weight of tris (acetylacetonato) ruthenium complex (Ru metal equivalent), hydroxide. It was impregnated and supported so as to have 12 parts by weight of cesium (converted to ruthenium metal). The obtained sample was dried at 120 ° C. for 10 hours and then calcined at 450 ° C. for 2 hours in an Air atmosphere to obtain a Cs-Ru / MgO catalyst. It was confirmed by XRD measurement that the catalyst did not contain a pyrochlore structure.

[Measurement of catalyst reduction start temperature]
0.2 g of the catalysts of the above-mentioned Examples and Comparative Examples were placed in a quartz tube, 100% H ₂ was circulated at 45 cc / min for 30 minutes at 450 ° C. for reduction treatment, and then N ₂ was 60 cc / min. It was circulated, cooled to room temperature, taken out under atmospheric pressure, and left for 12 hours. This catalyst was measured reduction onset temperature of the catalyst by performing H _2-TPR measured. H ₂ -TPR measurements were performed using a Rigaku TG8120. As a pretreatment, dehydration treatment was performed at 200 ° C. for 20 minutes under N ₂ distribution at 384 cc / min, and then the temperature was lowered to 50 ° C. Then, 400 cc / min of 4% H ₂ / N ₂ balance gas was circulated, and the weight change of the sample was recorded while raising the temperature at 5 ° C./min.
The calculation of the reduction start temperature of the catalyst is performed by calculating the differential value (dTG; unit% / min) of the weight change, and the approximate straight line 1 with respect to the dTG from the start of the temperature rise to the occurrence of the weight change, and the most calculated dTG curve. An approximate straight line 2 with respect to the shoulder portion on the low temperature side of the peak located on the low temperature side was created, and the temperature at the point where both approximate straight lines intersect was defined as the reduction start temperature.

[Measurement of ammonia synthesis rate]
0.2 g of the catalysts of the above Examples and Comparative Examples are filled in a quartz tube reactor having an outer diameter of 10 mm and an inner diameter of 6 mm, and the catalyst layer is sandwiched between a pair of SUS304 porous flat electrode having almost the same size as the inner diameter. did. A mantle heater with a temperature control function was installed outside the reaction tube so that the temperature of the catalyst layer could be controlled arbitrarily. A mixed gas of hydrogen gas and nitrogen gas (120 cc / min, Total 240 cc / min) was introduced into the reactor and pressurized to 0.8 MPaG.
Ammonia synthesis rate measurement under electric field application conditions: An electric field was formed in the catalyst layer by energizing between 2 mA electrodes in a constant current mode using a commercially available high-voltage power supply (Har-20N7.5 manufactured by Matsusada Precision). The measurement of the ammonia synthesis rate was started when 5 minutes or more had passed after the catalyst layer temperature became stable at the measurement temperature. Ammonia synthesis rate is measured by capturing ammonia contained in the gas after passing through the catalyst layer in a 0.1 mol / L boric acid aqueous solution and quantifying the ammonium ion concentration in the solution by cation chromatography. It was calculated as the average value of the ammonia synthesis rate in the medium. Table 1 shows the measurement results of the reduction start temperature of the catalyst and the ammonia synthesis rate. The relationship between the reduction start temperature of the catalyst and the ammonia synthesis rate is shown in FIGS. 1 and 2. In the figure, examples are shown by black circles and comparative examples are shown by white circles. In addition, the results of comparing the electric power used for synthesizing 1 kg of ammonia based on the reaction results at 350 ° C. for each example are shown in FIG.

Ammonia synthesis rate measurement under thermal synthesis conditions: Ammonia is measured by the same method as under electric field application conditions when the catalyst layer temperature stabilizes at the measurement temperature for 5 minutes or more under the condition that no energization is performed by a high-pressure power source. The synthesis rate was measured. Table 2 shows the measurement results of the reduction start temperature of the catalyst and the ammonia synthesis rate.

As is clear from the above results, in a catalyst in which the active component of the reoxidized catalyst is in an easily reducing state, that is, in a catalyst in which the reduction start temperature of the catalyst is lower than 190 ° C., heat under electric field formation and without applying an electric field. It was shown that the ammonia synthesis activity was high under all synthetic conditions. Further, from FIG. 3, it was found that the unexpected effect that the power consumption can be significantly reduced can be obtained by using the catalyst of the present invention.

By using the catalyst for ammonia synthesis according to the present invention, ammonia can be synthesized more efficiently than the conventional catalyst for ammonia synthesis, and energy efficiency can be improved. Therefore, storage of renewable energy at the time of oversupply, ammonia supply system for fertilizer in remote areas using renewable energy, ammonia synthesis device for NOx reduction in vehicles, which was difficult to apply in the past in terms of efficiency and economy. In the above, it can be used as a suitable catalyst for ammonia synthesis using an electric field catalytic reaction.

Claims (6)

A method for producing a catalyst for ammonia synthesis, which comprises at least one active metal belonging to Group 8 of the Long Periodic Table, a composite oxide having a pyrochlore-type crystal structure, and a rare earth element.
A step of mixing the active metal, the composite oxide, and the rare earth element to obtain a catalyst precursor, and
Including a firing step of calcining the catalyst precursor to obtain the catalyst.
A method for producing a catalyst for ammonia synthesis, which comprises reducing the catalyst obtained in the firing step and then reoxidizing the active metal, and then the reduction start temperature of the catalyst is 190 ° C. or lower. The method for producing a catalyst for ammonia synthesis according to claim 1, wherein the active metal is ruthenium and / or iron. The method for producing an ammonia synthesis catalyst according to claim 1 or 2, wherein the firing step is performed at 450 ° C. or higher and 800 ° C. or lower in the presence of a reducing gas. The method for producing a catalyst for ammonia synthesis according to claim 3, wherein the reducing gas contains at least one selected from hydrogen, carbon monoxide, and ammonia. The method for producing an ammonia synthesis catalyst according to claim 1 or 2, wherein the firing step is performed at a temperature of more than 600 ° C. and 800 ° C. or lower in the presence of an oxygen-containing gas. A method in which a catalyst is provided between a pair of electrodes and a voltage that does not generate a discharge is applied between the electrodes in the presence of a gas containing a hydrogen atom and a nitrogen atom to synthesize ammonia, according to any one of claims 1 to 5. A method for synthesizing ammonia, which comprises using a catalyst for synthesizing ammonia produced by the production method according to item 1 .

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