Catalyst for ammonia synthesis and method for producing ammonia

Abstract

Provided are a catalyst for ammonia synthesis, capable of efficiently synthesizing ammonia, and a method for producing the same.　Provided is a catalyst for ammonia synthesis, comprising a carrier and an active metal supported on the carrier. In the catalyst for ammonia synthesis, the relative difference between the interatomic distance of metal atoms constituting a crystal structure of the carrier and the interatomic distance of metal atoms constituting a crystal structure of the active metal is 20% or less.

Description

Catalyst for ammonia synthesis and method for producing ammonia

[0001] The present invention relates to a catalyst for ammonia synthesis and a method for producing ammonia. The present invention claims priority to Japanese Patent Application No. 2024-206627, filed on November 27, 2024, and in designated countries where incorporation by reference is permitted, the contents described in that application are incorporated into this application by reference.

[0002] In recent years, ammonia has attracted attention as a component that can be applied to food, fertilizers, and as an energy carrier for hydrogen energy. Conventionally, ammonia has been industrially synthesized using the Haber-Bosch process, which uses iron-based catalysts. However, iron-based catalysts require the reaction of hydrogen and nitrogen under high temperature and pressure conditions. Therefore, research is underway on various types of ammonia synthesis catalysts with the aim of synthesizing ammonia under milder conditions than the Haber-Bosch process.

[0003] For example, Patent Document 1 discloses a catalyst for ammonia synthesis in which an active metal is supported on a carrier compound containing iron ferrocyanide, and a method for synthesizing ammonia using the catalyst.

[0004] Japanese Patent Publication No. 2016-059852

[0005] However, in order to achieve carbon neutrality, further moderation of catalytic reaction conditions in ammonia synthesis is required.

[0006] This invention has been made in view of the above points, and its objective is to provide an ammonia synthesis catalyst that can efficiently synthesize ammonia and has excellent cycle characteristics (durability), as well as a method for producing the same.

[0007] As a result of diligent research, the inventors focused on the lattice matching at the interface between the supported active metal and the support, and discovered that an ammonia synthesis catalyst characterized by a relative difference of 20% or less between the distances of metal atoms constituting the active metal and the distances of metal atoms constituting the support is a catalyst that can efficiently synthesize ammonia and has excellent cycle characteristics (durability), thus completing the present invention.

[0008] In other words, the ammonia synthesis catalyst according to the present invention has, for example, a relative difference of 20% or less between the distances between metal atoms constituting the active metal and the distances between metal atoms constituting the support. Another aspect of the present invention is a method for producing ammonia by contacting a gas containing hydrogen and nitrogen with an ammonia synthesis catalyst, wherein the ammonia synthesis catalyst comprises a support and an active metal supported on the support, and the relative difference between the distances between metal atoms constituting the crystal structure of the support and the distances between metal atoms constituting the crystal structure of the active metal is 15% or less.

[0009] The present invention provides an ammonia synthesis catalyst and an ammonia production method that enable the efficient synthesis of ammonia. Other problems, configurations, and effects will be clarified by the following description of embodiments.

[0010] This figure schematically illustrates the key considerations in an ammonia synthesis catalyst to which an example of the present invention is applied. Comparative Example 1: 5 wt. %Ru / CeO ２ This table summarizes the specific surface area of ​​the catalyst and supported metal, and the coverage of the active metal on the catalyst. The graph shows the relationship between the calculated coverage values ​​and lattice mismatch in Examples 1-6. The figure shows the surface observation results by SEM in Comparative Examples 3-6.

[0011] Embodiments of the present invention will be described below with reference to the drawings. The dimensions and shapes of the parts in the drawings are exaggerated for clarity and do not accurately depict the actual dimensions and shapes. Therefore, the technical scope of the present invention is not limited to the dimensions and shapes of the parts shown in these drawings. Furthermore, the following description provides specific examples of the content of the present invention, and the present invention is not limited to these descriptions. Various changes and modifications can be made by those skilled in the art within the scope of the technical ideas disclosed herein. Also, in the diagrams used to illustrate the present invention, parts having the same function are denoted by the same reference numerals, and repeated explanations may be omitted.

[0012] In this specification, the "..." symbol is used to mean a range having the numerical values ​​before and after it as the lower and upper limits. In numerical ranges described stepwise in this specification, the upper or lower limit stated in one numerical range may be replaced by the upper or lower limit stated in another stepwise range. The upper or lower limit of numerical ranges described in this specification may be replaced by the values ​​shown in the examples.

[0013] An ammonia synthesis catalyst to which an example of the present invention is applied comprises a support and an active metal supported on the support, wherein the relative difference between the spacing between metal atoms constituting the crystal structure of the support and the spacing between metal atoms constituting the crystal structure of the active metal is 20% or less.

[0014] The active metal may also contain a transition metal.

[0015] The active metal comprises at least one selected from the group consisting of Ru, Co, Ni, Fe, Mn, V, Cu, Mo, and Ti, and its content is usually 50% by weight or more, preferably 75% by weight or more, and more preferably 90% by weight or more, relative to the total weight of the active metal. Since the active metal may consist only of the transition metals, there is no upper limit to its content.

[0016] By including transition metals within the aforementioned content range in the active metal, the activity for nitrogen activation and dissociation (activation, adsorption, and dissociation of nitrogen) can be increased.

[0017] When the active metal contains other metals other than the above-mentioned transition metals, the content of the other metals is not limited with respect to the total weight of the active metal, but is usually 0.5% by weight or less, preferably 0.1% by weight or less. Since the active metal may not contain other metals other than the above-mentioned transition metals, the lower limit of the content of the other metals is not limited. The other metals are, for example, Na and K.

[0018] Since the active metal may deteriorate the catalytic performance by containing other metals, it is preferable that the active metal does not contain other metals.

[0019] The average particle size of the active metal is not limited, but is usually 0.1 nm or more, preferably 0.5 nm or more, and is usually 50 nm or less, preferably 25 nm or less. For example, it is 0.1 nm to 50 nm, preferably 0.5 nm to 25 nm.

[0020] Here, the average particle size of the active metal can be measured by the CO pulse adsorption method.

[0021] When the average particle size of the active metal is within the above range, the number of active sites increases, and the nitrogen resolution and ammonia production ability can be improved.

[0022] The specific surface area of the active metal is not limited, but is usually 0.1 m ２ / g or more, preferably 0.5 m ２ / g or more, and is usually 100 m ２ / g or less, preferably 50 m ２ / g or less. For example, it is 0.1 m ２ / g to 100 m ２ / g, preferably 0.5 m ２ / g to 50 m ２ / g.

[0023] When the specific surface area of the active metal is within the above range, it is possible to increase the coated portion of the active metal, that is, the number of catalytic active sites.

[0024] As the carrier, a known material used for a catalyst can be used, and there is no particular limitation, but Fe ２ O ３ , MgO, SiO ２ , Al ２O ３ Examples include metal oxides such as those represented by O, metal nitrides such as CoN and CrN, and metal carbides such as TiC.

[0025] Although the content of the active metal in the ammonia synthesis catalyst is not limited, it is usually 3% by weight or more, preferably 5% by weight or more, and usually 20% by weight or less, preferably 10% by weight or less, based on the total weight of the ammonia synthesis catalyst (i.e., the total weight including the carrier). For example, it is 3% to 20% by weight, preferably 5% to 10% by weight.

[0026] By including the active metal in the ammonia synthesis catalyst within the above-described content range, the activity with respect to the activation and dissociation of nitrogen (activation, adsorption, and dissociation of nitrogen) can be further enhanced.

[0027] The BET specific surface area of the ammonia synthesis catalyst is not limited, but is usually 1 m ２ / g or more, preferably 5 m ２ / g or more, and usually 300 m ２ / g or less, preferably 200 m ２ / g or less. For example, it is 1 m ２ / g to 300 m ２ / g, preferably 5 m ２ / g to 200 m ２ / g.

[0028] Here, the BET specific surface area means the specific surface area calculated by the BET method, which is known in the art.

[0029] By the BET specific surface area of the ammonia synthesis catalyst being within the above range, it is possible to increase the active sites of the catalyst, that is, to improve the catalyst performance.

[0030] The ratio (coverage) of the specific surface area of the active metal to the specific surface area of the ammonia synthesis catalyst is not limited, but is usually 10% or more, preferably 30% or more, and usually 90% or less, preferably 70% or less. For example, it is 10 to 90%, preferably 30 to 70%.

[0031] Here, the coverage is a numerical value calculated by dividing the specific surface area of the active metal by the BET specific surface area of the catalyst.

[0032] Since the coating rate is within the above range, it is possible to increase the coating sites of the active metal on the carrier, that is, the number of catalytic active sites.

[0033] Further, the combination of the carrier and the active metal of the ammonia synthesis catalyst of the present embodiment is such that the relative difference between the distance between metal atoms constituting the crystal structure of the carrier and the distance between metal atoms constituting the crystal structure of the active metal is greater than 0% and 20% or less. The relative difference is preferably 15% or less, more preferably 10% or less. Here, the distance between metal atoms constituting the crystal structure of the carrier and the distance between metal atoms constituting the crystal structure of the active metal are the distances between metal atoms on the densest plane of the crystal structure. The relative difference can be obtained by (A - B / A) × 100, where A is the distance between metal atoms constituting the crystal structure of the carrier and B is the distance between metal atoms constituting the crystal structure of the active metal. For example, it can be calculated by identifying the structures of the supported metal and the carrier from crystal structure analysis by XRD (X-ray diffraction), calculating the nearest-neighbor atomic distance, and obtaining the relative value of the difference.

[0034] The catalyst of the present invention can be produced, for example, as follows.

[0035] Cerium oxide (CeO ２ , specific surface area: 114.5 m ２ / g) powder is used to support the active metal by, for example, an impregnation method and an evaporation to dryness method to obtain an ammonia synthesis catalyst in which the active metal is supported on the carrier.

[0036] Specifically, first, a solution containing a salt of the active metal, for example, a salt of Ru, and a solvent, for example, water, tetrahydrofuran (THF), alcohol, or a mixed solution of water and alcohol, is used to attach the active metal precursor to CeO ２ in the amount described above.

[0037] The salts of the active metal are not limited to those that dissolve in the solvent, and include, for example, nitrates, sulfates, carbonates, halides, such as chlorides, organic acid salts, such as acetates, citrates, dinitrodiammine salts, and various complexes (e.g., tetraammine complexes, carbonyl complexes).

[0038] The method for attaching the activated metal to the carrier is not limited to these methods. Examples include immersing the carrier in a solution containing an activated metal salt and a solvent to impregnate the carrier with the activated metal salt (impregnation method), and adsorbing the solution containing the activated metal salt and a solvent onto the carrier (adsorption method).

[0039] Next, the support to which the activated metal precursor is attached is dried, and then calcined under a reducing gas atmosphere or an inert gas atmosphere to obtain an ammonia synthesis catalyst in which the activated metal is supported on the support. In particular, since the calcination is performed under a reducing gas atmosphere or an inert gas atmosphere (preferably under a reducing gas atmosphere), the activated metal is supported on the support in a metallic state, and an ammonia synthesis catalyst with excellent ammonia synthesis activity is obtained.

[0040] The drying temperature of the support to which the active metal precursor is attached is usually 50°C or higher, preferably 75°C or higher, and usually 150°C or lower, preferably 125°C or lower, for example, 50°C to 150°C, preferably 75°C to 125°C. The drying time is usually 1 hour or more, preferably 3 hours or more. However, there is no upper limit to the drying time, although the process time will be longer.

[0041] A reducing gas atmosphere is an atmosphere containing reducing gases such as hydrogen gas, carbon monoxide gas, and hydrocarbon gases. For example, a mixed gas atmosphere of a reducing gas and an inert gas (such as nitrogen gas or argon gas) can be used. The concentration of the reducing gas in such a mixed gas atmosphere is usually 1% by volume or more, preferably 5% by volume or more, and usually 30% by volume or less, preferably 20% by volume or less, for example, 1% to 30% by volume, preferably 5% to 20% by volume. Examples of diluent gases for the reducing gas include nitrogen gas and argon gas. Examples of inert gas atmospheres include nitrogen gas atmospheres, argon gas atmospheres, and helium gas atmospheres.

[0042] The calcination temperature of the carrier to which the dried active metal precursor is attached is usually 200°C or higher, preferably 300°C or higher, and usually 500°C or lower, for example 200°C to 500°C, preferably 300°C to 500°C. The calcination time is usually 0.5 hours or more, preferably 1 hour or more, and usually 10 hours or less, preferably 5 hours or less, for example 0.5 hours to 10 hours, preferably 1 hour to 5 hours. By setting the calcination temperature and calcination time within the above range, all the active metals can be uniformly dispersed and supported on the carrier while being sufficiently reduced to a metallic state, while avoiding sintering between particles, thereby improving the activity of the resulting ammonia synthesis catalyst.

[0043] In the method for producing an ammonia synthesis catalyst of the present invention, the catalyst produced in this manner may be molded into various forms by known methods.

[0044] The above-described method for producing an ammonia synthesis catalyst allows for the production of an ammonia synthesis catalyst in which an active metal is supported on a carrier.

[0045] A catalyst for ammonia synthesis to which an example of the present invention is applied can efficiently synthesize ammonia by contacting it with a mixed gas containing hydrogen and nitrogen. The method of contacting the ammonia synthesis catalyst with the mixed gas containing hydrogen and nitrogen is not limited, and known ammonia synthesis methods in the art can be used.

[0046] In the ammonia synthesis method (ammonia production method) using the ammonia synthesis catalyst of the present invention, the synthesis conditions are not limited, and conditions known in the art for ammonia synthesis can be adopted as is, but for example, the molar ratio of hydrogen to nitrogen (H ２ / N ２ The ratio of hydrogen to nitrogen is usually 0.1 / 1 or more, preferably 0.5 / 1 or more, and usually 5 / 1 or less, preferably 3 / 1 or less, for example, 0.1 / 1 to 5 / 1, preferably 0.5 / 1 to 3 / 1. In addition, in a mixed gas containing hydrogen and nitrogen, an inert gas (such as argon gas) may be included as a carrier gas, but from the viewpoint of ammonia production efficiency, a gas consisting only of hydrogen and nitrogen is preferred.

[0047] Furthermore, the reaction temperature is usually 300°C or higher, preferably 350°C or higher, and usually 500°C or lower, preferably 450°C or lower, for example 300°C to 500°C, preferably 350°C to 450°C. The reaction pressure is usually 0.1 MPa or higher, preferably 1 MPa or higher, and usually 10 MPa or lower, preferably 8 MPa or lower, for example 0.1 MPa to 10 MPa, preferably 1 MPa to 8 MPa.

[0048] By using the ammonia synthesis catalyst of the present invention and synthesizing ammonia under the conditions within the range described above, ammonia can be synthesized efficiently.

[0049] Figure 1 illustrates the key features of this invention. As shown in Figure 1, at the interface between the active metal M constituting the catalyst 10 and the support S, the relative difference between the distances of the metal atoms constituting the active metal M and the distances of the metal atoms constituting the support S at the closest-packed surface is defined as lattice mismatch. The smaller the lattice mismatch, the better the wettability of the active metal to the support, contributing to improved catalytic performance through increased coverage of the active metal and, consequently, an increase in reaction sites. Furthermore, a smaller lattice mismatch suppresses the peeling of the active metal (catalyst degradation) caused by thermal expansion / contraction, which is effective in improving the catalyst's cycle characteristics (lifespan). Therefore, the catalyst of this invention can achieve both high catalytic performance and a long catalyst life through reduced lattice mismatch. In other words, it can maintain its durability even under harsh reaction conditions such as the Haber-Bosch process, where thermal expansion and contraction are repeated.

[0050] <<Examples>> Embodiments of the present invention will be described in detail below with reference to the examples shown in the figures. It should be noted that the present invention is not limited to the embodiments described herein, nor does it preclude modifications based on prior art and technologies that may become prior art in the future.

[0051] I. Sample Preparation <Comparative Example 1> Cerium Oxide (CeO ２ , Specific surface area: 114.5m ２ The mixture of cerium oxide ( / g) and a Ru salt (Ru nitrate) adjusted to be 5% by weight of Ru relative to the total weight of cerium oxide is mixed in pure water as a solvent, dried at 100°C for 12 hours, and then calcined at 450°C for 2 hours to produce Ru / CeO ２ We prepared it.

[0052] <Examples 1-2> Cerium oxide (CeO ２ , Specific surface area: 114.5m ２ The Ru salt (Ru nitrate), Mg salt, and Zr salt, which are adjusted to 5% by weight of Ru / Mg and Ru / Zr relative to the total weight of the catalyst, with Ru:Mg / Zr = 9:1, are mixed in pure water as a solvent, dried at 100°C for 12 hours, and then calcined at 450°C for 2 hours to produce Ru / CeO ２ The following was prepared. However, in this embodiment, it was only used to calculate the lattice mismatch and coverage rate.

[0053] Lattice mismatch was calculated by identifying the structures of the supporting metal and carrier from crystal structure analysis using XRD (X-ray diffraction), calculating the nearest neighbor interatomic distance, and determining the relative value of the difference. Coverage was calculated by simulation of sputter deposition using quantum molecular dynamics.

[0054] <Examples 3-6> In Examples 3-6, the catalysts were prepared by reactive sputtering. Various support metals (Co, W, Nb, Ta) were sputtered onto a silicon substrate placed in a chamber using argon gas under nitrogen gas sealing. Subsequently, Cu, an active metal, was deposited to obtain the desired catalyst. The amount of active metal was adjusted to 5% by weight, as in Examples 1-2.

[0055] II-1. Specific Surface Area Measurement by BET and CO Pulses Figure 2 shows the specific surface area measurement results of the catalyst and active metal by BET and CO pulses in Comparative Example 1. In Comparative Example 1, the specific surface area of ​​the active metal was extremely small compared to the specific surface area of ​​the catalyst, and the coverage rate was approximately 1.8%. This is presumed to be due to the magnitude of the lattice mismatch shown in Figure 3.

[0056] II-2. Relationship between Lattice Mismatch and Wettability (Coverage) Figure 3 shows the relationship between lattice mismatch and wettability (coverage) in Comparative Example 1 and Examples 1 to 6. It was confirmed that there is a correlation between lattice mismatch and wettability in both Ru catalysts and Cu catalysts, and that the smaller the lattice mismatch, the better the wettability (coverage).

[0057] II-3. Surface Observation Results by SEM Figure 4 shows the surface observation results of the Cu / nitride catalysts in Examples 2 to 6 using SEM (Scanning Electron Microscope). It was confirmed that the smaller the lattice mismatch and the better the wettability, the more the active metal was dispersed on the nitride. This data experimentally demonstrates the improvement in wettability (coverage) due to improved lattice mismatch, and consequently contributes to improved performance by increasing the number of catalytic active sites.

[0058] It should be noted that the present invention is not limited to the embodiments described above, and various modifications are included. For example, it is possible to add, delete, or replace some of the configurations of the embodiments with other configurations.

[0059] 10...Catalyst, M...Active metal, S...Carrier

Claims

1. A catalyst for ammonia synthesis comprising a carrier and an active metal supported on the carrier, wherein the relative difference between the spacing between metal atoms constituting the crystal structure of the carrier and the spacing between metal atoms constituting the crystal structure of the active metal is 20% or less.

2. The ammonia synthesis catalyst according to claim 1, wherein the active metal includes a transition metal.

3. The ammonia synthesis catalyst according to claim 2, wherein the active metal comprises at least one selected from the group consisting of Ru, Co, Ni, Fe, Mn, V, Cu, Mo, and Ti.

4. A method for producing ammonia, comprising contacting a gas containing hydrogen and nitrogen with an ammonia synthesis catalyst, wherein the ammonia synthesis catalyst comprises a carrier and an active metal supported on the carrier, and the relative difference between the spacing between metal atoms constituting the crystal structure of the carrier and the spacing between metal atoms constituting the crystal structure of the active metal is 20% or less.

Images