Method and catalyst for producing ammonia

Abstract

The present invention provides a method for producing ammonia, comprising: providing a catalyst comprising supported metal microdroplets, the metal microdroplets comprising (i) at least one low-melting point metal element selected from gallium and indium, and (ii) at least one promoter metal element selected from Groups 1-2 and 7-11 metals; and contacting the catalyst with a gas comprising dinitrogen and dihydrogen at a sufficiently high reaction temperature such that the metal microdroplets comprise a liquid metal alloy of the at least one low-melting point metal element and the at least one promoter metal element, thereby reacting the dinitrogen and dihydrogen to form ammonia.

Description

[Technical Field]

[0001] Priority Cross-Reference This application claims priority from Australian Provisional Patent Application No. 2022 / 903994, filed on 23 December 2022, the contents of which are to be deemed incorporated herein by this reference.

[0002] The present invention relates to a method for producing ammonia from dinitrogen and dihydrogen using a catalyst comprising supported metal microdroplets, wherein at ammonia synthesis reaction temperatures the metal microdroplets comprise a liquid metal alloy of a low-melting-point metal element selected from gallium and indium and a promoter metal element selected from Groups 1-2 and 7-11 metals. The present invention further relates to the catalyst, a method for producing the catalyst, and a reaction system for ammonia synthesis. [Background technology]

[0003] Ammonia (NH3) is currently produced on an industrial scale primarily via the Haber-Bosch process. This process for producing NH3 from dinitrogen (N2) and dihydrogen (H2) is extremely energy intensive, in part due to the high reaction temperatures and pressures (e.g., 500 °C and 200 bar) required to achieve significant conversion, with typical energy consumption exceeding 30 GJ / ton of NH3. The resulting carbon emissions of approximately 2.2 kg CO2 / kg NH3 make the Haber-Bosch process one of the most carbon-intensive industrial processes, accounting for 1.6% of global annual CO2 emissions. Therefore, it would be desirable to operate the ammonia synthesis process under milder process conditions. However, this is difficult due to the limited catalytic activity of the solid heterogeneous catalysts used in the Haber-Bosch process.

[0004] Furthermore, the solid catalysts used in the Haber-Bosch process are prone to deactivation over time, for example, due to hydrogen embrittlement or poisoning by impurities in the feedstock. This is compounded by the high cost of state-of-the-art catalysts that contain precious metals such as ruthenium.

[0005] Other proposed processes for ammonia synthesis involve chemical looping approaches in which the nitrogen fixation and hydrogenation steps are separated, adding undesirable complexity to process design and operation.

[0006] Thus, there is a continuing need for new methods of producing ammonia that at least partially address one or more of the above-mentioned shortcomings or provide a useful alternative.

[0007] Reference herein to a patent document or other matter cited as prior art shall not be deemed an admission that the document or matter was known or that the information contained therein was part of the common general knowledge at the priority date of any of the claims. Summary of the Invention

[0008] According to a first aspect, the present invention provides a method of producing ammonia, the method comprising: providing a catalyst comprising supported metal microdroplets, the metal microdroplets comprising (i) at least one low-melting point metal element selected from gallium and indium, and (ii) at least one promoter metal element selected from Groups 1-2 and 7-11 metals; and contacting the catalyst with a gas comprising dinitrogen and dihydrogen at a sufficiently high reaction temperature such that the metal microdroplets comprise a liquid metal alloy of the at least one low-melting point metal element and the at least one promoter metal element, thereby reacting the dinitrogen and dihydrogen to form ammonia.

[0009] Unlike conventional approaches to ammonia synthesis, such as the Haber-Bosch process, the present method uses a catalytic phase that exists partially or entirely as a liquid metal alloy at catalyst operating temperatures. The catalyst exists in the form of supported metal microdroplets, thus providing a large catalytic surface area for promoting the ammonia synthesis reaction. The inventors have discovered that such a catalyst can provide high and stable activity under ammonia synthesis conditions, even at low pressures. Without wishing to be limited by any theory, this is due to the synergistic cooperation between the low-melting-point metal elements and the promoter metal elements in the liquid metal alloy, the inherent resistance of the liquid metal composition to catalytic decomposition mechanisms such as hydrogen embrittlement and catalyst poisoning, and the surprising stability of the finely divided microdroplet form under ammonia synthesis conditions.

[0010] The liquid metal alloy of catalytic microdroplets under operating conditions typically contains sufficient promoter metal elements to significantly increase catalytic activity compared to unpromoted liquid metal microdroplets of comparable size containing only low-melting-point metal elements. However, producing supported metal microdroplets containing catalytically significant amounts of promoter metal elements, preferably distributed in similar concentrations in most or all of the microdroplets, is not a trivial task. This is due to the low solubility of promoter metal elements in gallium- or indium-based liquid metal compositions under low (e.g., near-ambient) temperature conditions. This problem is not avoided by simply producing the alloy at high temperatures, because the promoter metal elements are found to typically precipitate as intermetallic compounds when the bulk metal alloy cools.

[0011] Thus, preferred embodiments of the present invention have been realized solely through the development of a novel catalyst preparation method in which a catalytic liquid metal alloy is dispersed in a thermally stable liquid solvent by ultrasonic treatment at high temperatures (above the melting point of the alloy and any intermetallic compounds that may form therein). Thus, each of the resulting microdroplets contains a similar liquid metal alloy composition. Upon cooling, promoter metal elements may still precipitate from the liquid metal alloy, e.g., as intermetallic compounds, to enable microdroplet recovery from the solvent and support on a suitable solid support. However, this precipitation occurs within each microdroplet such that a substantial proportion of the microdroplets contain a "reservoir" of the precipitated promoter metal elements that are fully contained within the microdroplet and will dissolve back into the liquid metal alloy when the catalyst is heated to the operating temperature for ammonia synthesis. As a result, the catalyst is highly active due to the microdroplet morphology and the substantially homogeneous distribution of the promoter metal.

[0012] In some embodiments of the first aspect, the at least one promoter metal element is completely dissolved in the liquid metal alloy at the reaction temperature.

[0013] In some embodiments, the reaction temperature is greater than 200°C, or greater than 250°C, or greater than 300°C, such as greater than 350°C, for example in the range of 350°C to 500°C.

[0014] In some embodiments, at least a proportion of the metal microdroplets comprise (i) a solid metal alloy or intermetallic compound enriched with at least one promoter metal element, and (ii) a second metal phase enriched with at least one low-melting point metal element when the catalyst is at room temperature. At least 50% of the metal microdroplets, for example, at least 80% of the metal microdroplets, can comprise a solid metal alloy or intermetallic compound and a second metal phase.

[0015] In some embodiments, at least a proportion of the metal microdroplets comprise (i) a solid metal alloy or intermetallic compound enriched with at least one promoter metal element, and (ii) a liquid metal phase enriched with at least one low-melting point metal element when the catalyst temperature is about 50° C. At least 50% of the metal microdroplets, for example, at least 80% of the metal microdroplets, can comprise a solid metal alloy or intermetallic compound and a liquid metal phase.

[0016] In some embodiments, each metal microdroplet has substantially the same metal composition.

[0017] In some embodiments, the at least one low melting point metal element comprises gallium.

[0018] In some embodiments, the at least one promoter metallic element is selected from the group consisting of copper, silver, gold, nickel, palladium, platinum, cobalt, iron, ruthenium, and manganese.

[0019] In some embodiments, the at least one promoter metallic element is selected from copper and magnesium. For example, the at least one promoter metallic element may include copper.

[0020] In some embodiments, the metal microdroplets include at least one promoter metal element in an amount of 0.1 to 20 wt %, such as 0.5 to 10 wt %, for example, about 1 to 5 wt %, based on the total weight of the metal microdroplets.

[0021] In some embodiments, the metal microdroplets are supported on a solid support, optionally selected from a carbon-based support, a metal support, a metal oxide support, and a ceramic support.

[0022] In some embodiments, the metal microdroplets are primarily between 100 nm and 10 μm in size.

[0023] In some embodiments, the catalyst is contacted with a gas comprising dinitrogen and dihydrogen at a total pressure of from 1 to 100 bar, or from 1 to 50 bar, such as from 1 to 10 bar, for example from 2 to 6 bar.

[0024] According to a second aspect, the present invention provides a catalyst comprising supported metal microdroplets, the metal microdroplets comprising (i) at least one low-melting-point metal element selected from gallium and indium, and (ii) at least one promoter metal element selected from Groups 1-2 and 7-11 metals, wherein at least a proportion of the metal microdroplets comprise (i) a solid metal alloy or intermetallic compound enriched in the at least one promoter metal element, and (ii) a second metal phase enriched in the at least one low-melting-point metal element. The catalyst may have this morphology when at room temperature.

[0025] In some embodiments of the second aspect, at least 50% of the metal microdroplets, for example, at least 80% of the metal microdroplets, comprise a solid metal alloy or intermetallic compound and a second metal phase.

[0026] In some embodiments, at least 50% of the metal microdroplets, e.g., at least 80% of the metal microdroplets, comprise a solid metal alloy or intermetallic compound and a liquid metal phase enriched with at least one low-melting point metal element when the catalyst temperature is about 50°C.

[0027] In some embodiments, the metal microdroplets comprise a liquid metal alloy of at least one low melting point metal element and at least one promoter metal element, wherein the at least one promoter metal element is completely dissolved in the liquid metal alloy when the catalyst is heated to a temperature of about 400°C or higher, e.g., when the catalyst temperature is about 350°C or higher.

[0028] In some embodiments, each metal microdroplet has substantially the same metal composition.

[0029] In some embodiments, the at least one low melting point metal element comprises gallium.

[0030] In some embodiments, the at least one promoter metallic element is selected from the group consisting of copper, silver, gold, nickel, palladium, platinum, cobalt, iron, ruthenium, and manganese.

[0031] In some embodiments, the at least one promoter metallic element is selected from copper and magnesium. For example, the at least one promoter metallic element may include copper.

[0032] In some embodiments, the metal microdroplets include at least one promoter metal element in an amount of 0.1 to 20 wt %, such as 0.5 to 10 wt %, for example, about 1 to 5 wt %, based on the total weight of the metal microdroplets.

[0033] In some embodiments, the metal microdroplets are supported on a solid support, optionally selected from a carbon-based support, a metal support, a metal oxide support, and a ceramic support.

[0034] According to a third aspect, the present invention provides a method of producing a catalyst, the method comprising contacting a metal alloy with a high-temperature solvent, the metal alloy comprising (i) at least one low-melting point metal element selected from gallium and indium, and (ii) at least one promoter metal element selected from Groups 1-2 and 7-11 metals; dissociating the liquid metal alloy into metal microdroplets of the metal alloy in the high-temperature solvent by applying ultrasonic energy to the metal alloy in the high-temperature solvent, the high-temperature solvent being at a droplet formation temperature above the melting point of the metal alloy and above the melting point of any intermetallic compounds that may form therein; separating the metal microdroplets from the high-temperature solvent; and supporting the metal microdroplets on a solid support.

[0035] In some embodiments of the third aspect, the method further comprises cooling the hot solvent containing the metal microdroplets below the melting point of the metal alloy and / or any intermetallic compounds that may form therein.

[0036] In some embodiments, the metal alloy forms an immiscible mixture, preferably an emulsion, with the hot solvent.

[0037] In some embodiments, the droplet formation temperature is also selected to be below the highest temperature at which the high temperature solvent is stable and above the melting point of the high temperature solvent.

[0038] In some embodiments, the droplet formation temperature is greater than 300°C, preferably greater than 350°C, and more preferably at least 400°C.

[0039] In some embodiments, the high temperature solvent comprises at least one of hexadecane, oleic acid, at least one ionic liquid, at least one chloride salt, at least one molten nitrate and / or nitrite salt, at least one molten carbonate salt, at least one alkali metal acetate salt, at least one high temperature compatible hydrocarbon and / or fluorocarbon, at least one fat, or mixtures thereof.

[0040] In some embodiments, the high temperature solvent comprises a molten salt system in which the metal alloy is substantially immiscible.

[0041] In some embodiments, the high temperature solvent comprises at least one alkali metal acetate, such as Na, K, or Cs acetate or a mixture thereof, particularly anhydrous Na, K, or Cs acetate or a mixture thereof.

[0042] In some embodiments, ultrasonic energy is applied to the metal alloy in the hot solvent for a duration of at least 5 minutes, such as 20-60 minutes, for example, about 30 minutes.

[0043] In some embodiments, the applied ultrasonic energy has a frequency of 20-500 kHz, preferably 20-25 kHz, and / or the applied ultrasonic energy is applied at a power of 10 W-1000 W, preferably 100-500 W.

[0044] In some embodiments, ultrasonic energy is applied to the metal alloy in the hot solvent using a probe sonicator.

[0045] In some embodiments, at least a proportion of the metal microdroplets comprise (i) a solid metal alloy or intermetallic compound enriched with at least one promoter metal element, and (ii) a second metal phase enriched with at least one low-melting point metal element when the metal microdroplets are cooled to room temperature. At least 50% of the metal microdroplets, for example, at least 80% of the metal microdroplets, can comprise a solid metal alloy or intermetallic compound and a second metal phase.

[0046] In some embodiments, at least a proportion of the metal microdroplets comprise (i) a solid metal alloy or intermetallic compound enriched with at least one promoter metal element, and (ii) a liquid metal phase enriched with at least one low-melting point metal element when the metal microdroplets are cooled to about 50° C. At least 50% of the metal microdroplets, for example, at least 80% of the metal microdroplets, can comprise a solid metal alloy or intermetallic compound and a liquid metal phase.

[0047] In some embodiments, each metal microdroplet has substantially the same metal composition.

[0048] In some embodiments, the at least one low melting point metal element comprises gallium.

[0049] In some embodiments, the at least one promoter metallic element is selected from the group consisting of copper, silver, gold, nickel, palladium, platinum, cobalt, iron, ruthenium, and manganese.

[0050] In some embodiments, the at least one promoter metallic element is selected from copper and magnesium. For example, the at least one promoter metallic element may include copper.

[0051] In some embodiments, the metal microdroplets include at least one promoter metal element in an amount of 0.1 to 20 wt %, such as 0.5 to 10 wt %, for example, about 1 to 5 wt %, based on the total weight of the metal microdroplets.

[0052] In some embodiments, the solid support is selected from a carbon-based support, a metal support, a metal oxide support, and a ceramic support.

[0053] According to a fourth aspect, the present invention provides a reaction system for ammonia synthesis comprising a catalyst according to any embodiment of the second aspect, or a catalyst produced by a method according to any embodiment of the third aspect, and contained within a reaction chamber configured to receive a gas comprising dinitrogen and dihydrogen for reacting over the catalyst to produce ammonia.

[0054] When the terms "comprise", "comprises", and "comprising" are used in this specification (including the claims), these terms should be interpreted as specifying stated features, integers, steps or components, but not as excluding the presence of one or more other features, integers, steps or components, or groups thereof.

[0055] As used herein, the terms "first," "second," "third," etc., with respect to various features of the disclosed devices, methods, systems, etc., are assigned arbitrarily and are intended merely to distinguish between two or more such features that may be incorporated into various embodiments of the devices, methods, systems, etc. The terms themselves do not indicate any particular orientation or arrangement. Furthermore, it should be understood that the presence of a "first" feature does not imply the presence of a "second" feature, the presence of a "second" feature does not imply the presence of a "first" feature, etc.

[0056] Further aspects of the present invention are set out below in the detailed description of the invention. [Brief explanation of the drawings]

[0057] Embodiments of the present invention are herein illustrated, by way of example only, with reference to the accompanying drawings in which:

[0058] [Figure 1] FIG. 1 shows a schematic diagram of high-temperature sonication used in some embodiments of the present invention, showing the progression of droplet formation from (a) sonication start (t0), (b) droplet formation (t1), and (c) microdroplet formation and detachment (t2), and also showing a zoomed-in snapshot of what the microdroplets look like. [Figure 2] 1 is an SEM image of metal microdroplets containing 2 wt. % copper in gallium supported on carbon paper prepared by the high temperature dispersion method of Example 1. [Figure 3] 1 is a dark-field TEM image with EDX elemental map of 2 wt. % copper in gallium microdroplets prepared in Example 1, showing the presence of copper-rich seeds (302), expected to be GaCu, encapsulated within a gallium-rich liquid metal composition (304). [Figure 4] A gallium elemental map of the particles seen in Figure 3 is shown, which demonstrates that gallium is distributed throughout the microdroplets. [Figure 5] A copper elemental map of the particles seen in Figure 3 is shown, which demonstrates that copper is concentrated in the precipitate seeds encapsulated within the microdroplets. [Figure 6] FIG. 1 shows the size distribution of metal microdroplets containing 5 wt. % copper in gallium prepared by the high-temperature dispersion method of Example 3; the inset pie chart shows how many microdroplets had copper-rich metal seeds (light gray indicates no seeds). [Figure 7] 1 is a graph comparing the rate of ammonia production as a function of reaction temperature using a catalyst comprising supported microdroplets of 2 wt. % copper in gallium of Example 7. [Figure 8] 1 is a graph comparing the rate of ammonia production as a function of reactant gas pressure using a catalyst comprising supported microdroplets of 2 wt. % copper in gallium of Example 7. [Figure 9] 10 is a graph comparing the rate of ammonia production as a function of N2 / H2 in the reactant gas using a catalyst comprising supported microdroplets of 2 wt% copper in gallium of Example 7. [Figure 10] 1 is a graph comparing the ammonia production rate obtained with a catalyst comprising supported microdroplets of 2 wt. % copper in gallium and a Haber-Bosch catalyst as determined in Example 8. [Figure 11] 1 is a graph comparing the ammonia production rate obtained with catalysts comprising supported microdroplets of 2 wt. % copper in gallium prepared by the high temperature dispersion method of Example 1 and by the low temperature dispersion method of Example 2, as determined in Example 9. [Figure 12] 1 is a graph comparing the ammonia production rate obtained with a catalyst comprising supported microdroplets of 2 wt. % copper in gallium with the ammonia production rate obtained with a blank carbon paper support, copper powder, and non-functionalized supported gallium microdroplets, as determined in Example 10. [Figure 13] 1 is a graph showing the ammonia production rate over 32 hours of operation using a catalyst comprising supported microdroplets of 2 wt. % copper in gallium of Example 11. [Figure 14] 1 is a graph showing the cumulative turnover number of ammonia production over the first 12 hours of operation using a catalyst comprising supported microdroplets of 2 wt. % copper in gallium of Example 11. [Figure 15] 1 is a graph comparing the ammonia production rate obtained with a catalyst comprising supported microdroplets of 2 wt. % magnesium in eutectic gallium-indium with the ammonia production rate obtained with non-functionalized supported gallium microdroplets, as determined in Example 12. [Figure 16] 1 is a graph showing the rate of HD formation when a mixture of H2 and D2 is passed over a catalyst containing supported microdroplets of 2 wt. % copper in gallium compared to the rate of HD formation using non-functionalized supported gallium microdroplets, as determined in Example 13. [Figure 17]1 shows FTIR spectra obtained upon exposure of a catalyst comprising supported microdroplets of 2 wt. % copper in gallium to N2 at different temperatures, as determined in Example 13. [Figure 18] 1 shows a series of FTIR spectra obtained over time after H2 was introduced to contact a catalyst previously exposed to N2, the catalyst comprising supported microdroplets of 2 wt% copper in gallium, as determined in Example 13. [Figure 19] 13 shows a simulation of the mechanism of ammonia synthesis on a Cu—Ga alloy surface obtained by the molecular dynamics calculation of Example 14. [Figure 20] 10 shows a schematic diagram of the morphology of Cu—Ga particles produced by galvanic replacement reaction in Comparative Example 15, which contain a Ga core and Cu nanoparticles decorated on its surface. [Figure 21] Schematic diagram of the morphology of Ag-Ga particles produced by the galvanic replacement reaction of Comparative Example 15, including i) Ga droplets with nano-roughened surfaces functionalized with Ag, ii) Ga droplets with nano-roughened surfaces functionalized with Ag and decorated with Ag nanoflakes on their surfaces, and iii) discrete Ag nanorods. DETAILED DESCRIPTION OF THE INVENTION

[0059] How to Produce Ammonia The present invention relates to a method for producing ammonia. The method includes providing a catalyst comprising supported metal microdroplets. The metal microdroplets include at least one low-melting-point metal element selected from gallium and indium, and at least one promoter metal element selected from Groups 1-2 and 7-11 metals. The catalyst is contacted with a gas comprising dinitrogen and dihydrogen at a sufficiently high reaction temperature such that the metal microdroplets comprise, and typically consist of, a liquid metal alloy of the at least one low-melting-point metal element and the at least one promoter metal element. The dinitrogen and dihydrogen react on the catalyst to form ammonia.

[0060] catalyst The catalysts provided for use in the methods disclosed herein include supported metal microdroplets. It should be understood that "microdroplets" refers to droplets having a droplet size of micron size or less. Thus, the term microdroplets encompasses droplets having a droplet size in the micron size range and / or nano size range (so-called "nanodroplets"). Furthermore, it should be understood that metal microdroplets can exist in a liquid state, a solid state, or a mixture thereof, depending on the composition and catalyst temperature. For example, supported metal microdroplets, when used to catalyze the reaction of dinitrogen and dihydrogen to form ammonia, may be partially or entirely liquid but partially or entirely solid at room temperature.

[0061] In some embodiments, the metal microdroplets are primarily between 100 nm and 10 μm in size. The particle size of the metal microdroplets can be assessed using a scanning electron microscope, typically with image analysis software used to determine particle size distribution. The metal microdroplets may be spherical, consistent with how they are formed in the liquid phase according to some embodiments of the present invention.

[0062] The metal microdroplets contain at least one low-melting point metal element selected from gallium and indium, and at least one promoter metal element selected from metals of Groups 1-2 and 7-11. Gallium and indium are pure liquid metals (gallium T 溶融 =29.8℃, Indium T 溶融 =156.8℃) and tin (T 溶融 =231.9℃), bismuth (T 溶融 =271.4°C), or even zinc (T 溶融Liquid metals are known as low-melting elements in liquid metal compositions, both as liquid metal alloys with other low-melting elements (such as ZnO, ...

[0063] However, without wishing to be limited by any theory, it is believed that the gallium and / or indium component of the catalyst plays several roles in the methods disclosed herein. First, the metal composition of the microdroplets contains sufficient amounts of gallium and / or indium (along with any other low-melting-point metal elements) so that the metal microdroplets exist partially or entirely in a liquid phase at the operating temperature of the ammonia synthesis reaction. Second, the low-melting-point metal element solubilizes all or a portion of the promoter metal element in this liquid phase, such that the metal microdroplets comprise a catalytically active liquid metal alloy of the low-melting-point metal element and the promoter metal element in the ammonia synthesis reaction. Third, in at least some embodiments, it is believed that gallium and / or indium play a role in the catalytic mechanism, working synergistically with the promoter metal element to promote the activation and reduction of N to form ammonia. Both gallium and indium are known to form stable nitrides. By analogy, and with further support from computational studies, it is believed that the initial step in the ammonia synthesis mechanism may involve the activation and reduction of N2 by gallium or indium atoms at the surface of the liquid metal alloy.

[0064] At least one promoter metal element present in the metal microdroplets is selected from Groups 1-2 and 7-11 metals (i.e., metal elements selected from Groups 1, 2, 7, 8, 9, 10, and 11 of the periodic table). Metals from both classes of elements (Groups 1-2 vs. 7-11) have been experimentally found to be effective, but it is believed that they may operate by different mechanisms in the ammonia synthesis reaction. Groups 1 and 2 metals are typically capable of reacting with N2 to form the corresponding nitrides, and it has been proposed that these elements may directly participate in the nitrogen activation step of the ammonia synthesis reaction, for example, via in situ formation of metal nitrides. Groups 7-11 metals include metals commonly used in Haber-Bosch catalysts and other catalysts for hydrogen activation reactions, and thus may cooperate with gallium and / or indium to activate dinitrogen and / or dihydrogen. However, it should be noted that the electronic structure of metal elements dissolved in liquid metal alloys may differ significantly from that of the pure metal elements in solid form, which may contribute to catalytic activity in the ammonia synthesis reaction. For example, copper, despite the inertness of solid copper catalysts in the Haber-Bosch process, has surprisingly been found to promote the ammonia synthesis reaction when present as an alloying element in liquid metal microdroplets according to the present disclosure.

[0065] In some embodiments, the at least one promoter metal element comprises one or more Group 7-11 metal elements selected from the group consisting of copper, silver, gold, nickel, palladium, platinum, cobalt, iron, ruthenium, and manganese. In some embodiments, the at least one promoter metal element comprises one or more Group 7-11 metal elements selected from the group consisting of copper, nickel, cobalt, iron, and manganese, all of which are Period 4 elements. In some embodiments, the at least one promoter metal element comprises one or more Group 7-11 metal elements selected from the group consisting of copper, iron, and ruthenium. Iron and ruthenium are active metals for Haber-Bosch ammonia synthesis. In some embodiments, the at least one promoter metal element comprises one or more Group 1-2 metal elements selected from lithium, sodium, magnesium, and calcium. In some embodiments, the at least one promoter metal element is selected from copper and magnesium. In some embodiments, the at least one promoter metal element comprises or consists of copper.

[0066] The metal composition of the microdroplets preferably includes at least one promoter metal element in an amount that is (i) completely soluble in the liquid metal alloy at the operating temperature of ammonia synthesis, and (ii) capable of promoting the ammonia synthesis reaction. It should be understood that the solubility limits under ammonia synthesis conditions depend on both the solute (promoter metal component) and the solvent (low-melting point metal element component), and such information, at least for bimetallic systems, is typically available from a phase diagram of the relevant alloy system.

[0067] Thus, in at least some embodiments, the at least one promoter metal element is completely dissolved in the liquid metal alloy at ammonia synthesis reaction temperatures. In other words, the supported metal microdroplets of the catalyst consist of a single liquid phase, the liquid metal alloy, when contacted with the reactive gases during ammonia synthesis. In some embodiments, the at least one promoter metal element is completely dissolved in the liquid metal alloy at catalyst temperatures of about 400°C or higher, preferably about 350°C or higher, whether or not ammonia synthesis is occurring.

[0068] In some embodiments, the at least one promoter metal element is present in an amount of at least 0.1 wt %, e.g., at least 0.5 wt %, e.g., at least 1 wt %, based on the total weight of the metal microdroplets, hi some embodiments, the at least one promoter metal element is present in an amount of 0.1-20 wt %, e.g., 0.5-10 wt %, e.g., about 1-5 wt %, based on the total weight of the metal microdroplets.

[0069] For example, the promoter metal content of the metal nanodroplets required to significantly enhance the ammonia synthesis reaction compared to an equivalent gallium- or indium-based liquid metal composition lacking the promoter metal element may exceed its solubility limit in the metal composition at temperatures substantially below the ammonia synthesis operating temperature, e.g., at or near room temperature (e.g., below 50°C). In such embodiments, phase separation occurs when the initial homogeneous liquid metal alloy is cooled from an elevated temperature (e.g., at which the alloy was prepared or the microdroplets were dispersed) to below the precipitation temperature of the promoter metal-rich phase associated with the metal composition. The catalyst then comprises at least two metal phases at room temperature and generally at much higher temperatures, e.g., 50°C, 100°C, 200°C, or even higher. The two metal phases generally comprise (i) a solid metal alloy or intermetallic compound enriched in at least one promoter metal element and (ii) a second metal phase enriched in at least one low-melting point metal element (i.e., gallium and / or indium).

[0070] It should be understood that room temperature refers to a temperature that is within a comfortable ambient temperature, and is generally considered to be between 15 and 25°C, more specifically about 20°C.

[0071] The solid metal alloy or intermetallic compound enriched in at least one promoter metal element is formed by precipitation from the molten phase upon cooling and therefore exists in the metal microdroplets as one or more solid particles encapsulated within a second metal phase enriched in at least one low-melting-point metal element. Thus, one or more solid particles may be trapped within the interior of the metal microdroplets, and the surface of the metal microdroplets provided by the second metal phase is enriched in at least one low-melting-point metal element. Therefore, the solid metal alloy or intermetallic compound may be substantially absent from the surface of the metal microdroplets. This morphology is distinct from droplet morphologies formed by galvanic exchange, in which the introduced metal exists in one or more solid phases (metal nanoparticles or intermetallic compounds) decorating the surface of the liquid metal droplet.

[0072] The second metal phase may be either a solid or liquid phase when the catalyst is at or near room temperature. In the case of gallium-based catalysts in particular, the second metal phase may be a liquid phase at or near room temperature, such that the metal microdroplets comprise a solid promoter metal-rich phase and a liquid gallium-rich phase, for example at 50°C.

[0073] When a binary alloy system is used, the metal alloy composition typically includes a primary metal solute (i.e., one of gallium or indium) and a lower concentration of a secondary metal solute (i.e., one promoter metal element), where the concentration of the metal solute is preferably selected to be higher than the melting / stabilization point concentration to achieve intermetallic formation if the system includes both an intermetallic and a melting / stabilization point, or has a supersaturation solute concentration if no intermetallic is present.

[0074] To ensure that a catalyst provides good activity in ammonia synthesis, it is desirable for the promoter metal to be present in sufficient, preferably substantially equal, concentrations in most or all of the catalyst's supported metal microdroplets. However, separation of the two metal phases from the target catalyst composition at low temperatures presented a significant challenge when the inventors first attempted to prepare such catalysts. This problem is successfully addressed by the development of a novel catalyst preparation method, described in more detail below, whereby a catalytic liquid metal alloy is dispersed in a thermally stable, high-temperature solvent by ultrasonic treatment at high temperatures (above the melting points of the alloy and any intermetallic compounds that may form therein).

[0075] Thus, in some embodiments, at least a proportion of the metal microdroplets present in the catalyst comprise (i) a solid metal alloy or intermetallic compound enriched with at least one promoter metal element, and (ii) a second metal phase enriched with at least one low-melting point metal element when the catalyst is at room temperature. For example, at least 50% of the metal microdroplets, or at least 70% of the metal microdroplets, or at least 80% of the metal microdroplets, e.g., at least 90% or substantially all of the metal microdroplets, may comprise a solid metal alloy or intermetallic compound and a second metal phase. As used herein, 50% (or 80%, or 90%, or substantially all) of the metal microdroplets refers to the number of metal microdroplets as a percentage of the total population of metal microdroplets having this physical structure.

[0076] The proportion of metal microdroplets containing two distinct phases can be determined by microscopic analysis, typically by transmission electron microscopy (TEM), optionally by TEM coupled with energy dispersive X-ray spectroscopy (TEM-EDX). Typically, the microdroplets at room temperature contain at least one large "seed" of solid metal alloy or intermetallic compound suspended or otherwise contained within the microdroplet, clearly visible in the TEM image. The second metal phase, enriched with at least one low-melting-point metal element, may be either a liquid metal phase or a second solid phase when the catalyst is at room temperature.

[0077] In some embodiments, the metallic microdroplets contain an intermetallic compound as a solid phase enriched with the promoter metal element. The intermetallic compound may be a crystalline / polycrystalline intermetallic compound. The composition of the particular intermetallic compound formed as a solid precipitate (or "seed") in the microdroplet may be determined by reference to a phase diagram for the relevant alloy system, if available, or by analysis. Phase diagrams are available for a wide range of binary and ternary alloy systems. Because intermetallic compounds typically exhibit a higher degree of crystalline order or different diffraction properties compared to the second metallic phase (liquid or solid), the presence of an intermetallic compound in the metallic microdroplet may be inferred from diffraction imaging of the microdroplet obtained by TEM.

[0078] For example, the stoichiometric intermetallic compound Ga2Cu, which has a melting point of 249°C, is formed in the Cu-Ga alloy system, as reported in the phase diagram. Thus, at temperatures above 249°C, the Cu-Ga bimetallic alloy, with a catalytically relevant Cu concentration, exists as a single, homogeneous liquid metal alloy phase. However, when the alloy is cooled below 249°C, Ga2Cu precipitates as a crystalline intermetallic compound. If a high percentage, or substantially all, of the microdroplets contain a similar concentration of copper, preferably, one or more "seeds" of Ga2Cu precipitate within each such microdroplet. When the catalyst is again heated to an operating temperature, e.g., for ammonia synthesis, the Ga2Cu phase melts and readily dissolves into the liquid metal composition when the catalyst temperature again exceeds 249°C. Advantageously, despite the phase heterogeneity of the copper-gallium alloy system at low temperatures, most or all of the microdroplets therefore contain reservoirs of copper to facilitate the ammonia synthesis reaction.

[0079] The formation of well-defined intermetallic species is also expected in a wide range of other alloy systems that combine gallium and / or indium with catalytically significant amounts of Group 1-2 and Group 7-11 promoter metal elements. For example, one or both of the Ga2Mg and Ga5Mg2 intermetallic compounds are expected to form in the Mg-Ga alloy system.

[0080] In some embodiments, at least a proportion of the metal microdroplets comprise (i) a solid metal alloy or intermetallic compound enriched with at least one promoter metal element, and (ii) a liquid metal phase enriched with at least one low-melting point metal element when the catalyst temperature is about 50° C., or even about room temperature. For example, at least 50% of the metal microdroplets, or at least 70% of the metal microdroplets, or at least 80% of the metal microdroplets, e.g., at least 90% or substantially all of the metal microdroplets, may comprise a solid metal alloy or intermetallic compound and a liquid metal phase. In particular, gallium-rich metal compositions are expected to remain liquid at or near ambient temperatures due to the very low melting temperature of gallium.

[0081] In another embodiment, at least a proportion of the metal microdroplets comprise (i) a solid metal alloy or intermetallic compound enriched in at least one promoter metal element, and (ii) a solid alloy phase enriched in at least one low-melting-point metal element when the catalyst temperature is about 50°C. In particular, indium-based alloy systems are expected to fall into this category. When the homogeneous liquid alloy cools from high temperature, a solid phase enriched in the promoter metal element, e.g., an intermetallic compound of indium, precipitates from solution in the microdroplets. Upon further cooling, the indium-rich liquid phase then solidifies, so that at room temperature the metal microdroplets comprise one or more promoter-rich metal particles entrapped within an indium-rich solid metal matrix.

[0082] In some embodiments, each metal microdroplet in the catalyst has substantially the same metal composition. As used herein, "substantially the same metal composition" means that the weight percent of each element in each metal microdroplet is within ±10% of the weight percent of that element in the combined composition of all metal microdroplets. This can be confirmed by analyzing the microdroplets by TEM-EDX to determine the elemental composition, preferably at an elevated temperature where the microdroplets exist as a single liquid phase.

[0083] The metal microdroplets may include a single low-melting point metal element selected from gallium and indium and a single promoter metal element, such that the liquid metal alloy present in the ammonia synthesis reaction is a binary metal alloy. However, it is also contemplated that the metal microdroplets may include two or more low-melting point metal elements, including at least one, and optionally both, of gallium and indium. In some embodiments, the metal microdroplets include gallium and indium, or gallium, indium, and tin as the low-melting point metal elements. For example, the low-melting point metal element component of the metal composition may be eutectic gallium-indium (EGaIn) or eutectic gallium-indium-tin (GaInSn) (in the absence of the promoter metal element, T 溶融 = 15.0°C and 13.2°C, respectively). Additionally, the metal microdroplets may contain two or more Group 1-2 and Group 7-11 promoter metal elements or other alloying elements provided they are compatible with the purpose and properties of the catalyst disclosed herein.

[0084] The metal microdroplets are supported on a solid support. Suitable supports are generally those that remain solid, are chemically stable, and are capable of physically supporting the metal microdroplets supported thereon under the conditions of ammonia synthesis. Apart from these requirements, the nature of the support is not considered to be limiting. For example, the support may be selected from carbon-based supports (e.g., carbon paper), metal supports, metal oxide supports (e.g., alumina, silica), and ceramic supports.

[0085] Ammonia synthesis reaction In the method of producing ammonia disclosed herein, a catalyst is contacted with a gas comprising dinitrogen and dihydrogen at a reaction temperature sufficiently high so that the metal microdroplets comprise a liquid metal alloy of at least one low-melting point metal element and at least one promoter metal element, thereby reacting the dinitrogen and dihydrogen to form ammonia.

[0086] Any reaction temperature at which H and N can react over the liquid metal alloy catalyst phase to form ammonia is suitable, and in some embodiments, a reaction temperature above 150°C, or above 200°C, e.g., above 250°C, may be sufficient. Preferably, the reaction temperature is sufficiently high so that (i) at least one promoter metal element is completely dissolved in the liquid metal alloy at the ammonia synthesis reaction temperature, and (ii) a satisfactory rate of ammonia production is achieved. These requirements may be met in some embodiments at reaction temperatures above 300°C, or above 350°C, e.g., in the range of 350°C to 500°C, or in the range of 350°C to 450°C, e.g., about 400°C.

[0087] The catalyst may be contacted with the gas containing dinitrogen and dihydrogen at any pressure. When ammonia synthesis is carried out at or near equilibrium conditions, higher pressures tend to promote increased ammonia conversion. However, in some embodiments, the catalyst is contacted with the gas at a lower pressure than is typical for the Haber-Bosch process, e.g., 1 to 100 bar, or 1 to 50 bar, e.g., 1 to 10 bar, e.g., 2 to 6 bar total pressure. Thus, the increased catalytic activity obtained with the catalysts disclosed herein can be advantageously used to operate at lower reactor pressures.

[0088] The reactive gases may be supplied to contact the catalyst in any N2:H2 ratio, for example, 1:2 to 1:4 (v / v). Preferably, the N2:H2 ratio is at or near 1:3 (v / v), which is consistent with the stoichiometry of the ammonia synthesis reaction shown in equation (1). N2+3H2→2NH3(1)

[0089] The superior ammonia production rates obtained by the disclosed methods, which in some embodiments can even surpass state-of-the-art Haber-Bosch heterogeneous catalysts, are due in part to the large catalyst surface area provided by the finely divided metal microdroplets. Due to the difficulty in controlling bubble size and distribution within the liquid metal column, reactor configurations such as bubble column reactors that utilize bulk phase liquid metal compositions cannot provide an equivalent interface between the reactive gas phase and the liquid metal catalyst phase.

[0090] The microdroplet morphology was found to be surprisingly stable under prolonged exposure to ammonia synthesis conditions, with no evidence of droplet coalescence leading to significant loss of catalyst surface area. Furthermore, because metal atoms in the liquid composition can freely move between the bulk and surface of the liquid-phase microdroplets under catalytic conditions, the catalyst is expected to be resistant to catalytic decomposition mechanisms that affect Haber-Bosch catalysts, such as hydrogen embrittlement and catalyst poisoning. These advantages are evident in the substantially stable catalytic activity obtained over extended periods of operation.

[0091] Because ammonia is believed to be produced via a catalytic process occurring on the supported metal microdroplets, neither the low-melting-point metal element nor the promoter metal element is consumed stoichiometrically during ammonia synthesis. Therefore, turnover numbers (i.e., moles of ammonia per mole of metal) much greater than one are achievable. Therefore, there is no need to regenerate the metal element in the metal microdroplets by stoichiometric reduction (e.g., of metal nitride) in a separate regeneration process step, as is done in chemical looping approaches. Thus, in some embodiments, the catalyst is not subjected to chemical looping. Any regeneration step that is utilized is typically performed only when the turnover number is much greater than one. The ammonia synthesis reaction does not rely on a molten salt phase to facilitate any step in the reaction, such as the nitride reduction step; therefore, in some embodiments, the catalyst is substantially free of a molten salt phase.

[0092] The catalyst may be contacted with a gas containing dinitrogen and dihydrogen in any suitable reactor, including batch and continuous reactors. While the supported metal microdroplets are liquid under operating conditions, the catalyst as a whole generally exists as a solid due to the support of the microdroplets on a solid support. Thus, conventional reactor configurations for gas-phase reactions over heterogeneous solid catalysts, such as fixed-bed reactors, may be used.

[0093] Ammonia synthesis reaction system The present invention further relates to a reaction system for ammonia synthesis, the reaction system comprising a catalyst disclosed herein or a catalyst produced by the methods disclosed herein in the following section, the catalyst being contained within a reaction chamber configured to receive a gas comprising dinitrogen and dihydrogen for reaction over the catalyst to produce ammonia.

[0094] The reaction system can include a gas source containing dinitrogen and dihydrogen coupled to the reaction chamber. The gas can have, for example, an N2:H2 ratio of 1:2 to 1:4 (v / v). Preferably, the N2:H2 ratio is at or near 1:3 (v / v), which is consistent with the stoichiometry of the ammonia synthesis reaction.

[0095] The gas source containing dinitrogen and dihydrogen may be at any pressure when delivered to the reaction chamber, hi some embodiments, the gas has a total pressure of 1 to 100 bar, or 1 to 50 bar, e.g., 1 to 10 bar, e.g., 2 to 6 bar.

[0096] The reaction chamber can be in any suitable reactor, including batch and continuous reactors, as disclosed herein in the context of the method of producing ammonia.

[0097] The reaction system may be configured to maintain a reaction temperature within the reaction chamber greater than 300° C., or greater than 350° C., for example, in the range of 350° C. to 500° C., or in the range of 350° C. to 450° C., for example, about 400° C. Accordingly, the reactor may be equipped with conventional heating and / or cooling equipment to maintain the reaction temperature within this range.

[0098] Method for producing a catalyst for ammonia synthesis The present invention further relates to a method for producing a catalyst that may be suitable for ammonia synthesis. The method comprises contacting a metal alloy containing (i) at least one low-melting point metal element selected from gallium and indium, and (ii) at least one promoter metal element selected from Groups 1-2 and 7-11 metals, with a hot solvent. Ultrasonic energy is applied to the metal alloy in the hot solvent, the hot solvent being at a droplet formation temperature above the melting point of the metal alloy and above the melting point of any intermetallic compounds that may form therein. The resulting liquid metal alloy is thereby separated into metal microdroplets of the metal alloy in the hot solvent. The metal microdroplets are then separated from the hot solvent and deposited on a solid support.

[0099] Thus, this aspect of the invention employs a metal microdroplet synthesis technique that uses a high-temperature solvent to suspend a catalytically active metal alloy therein, typically as an immiscible mixture, e.g., an emulsion, within the system's designed temperature range. High-temperature ultrasonic treatment (typically above 200°C) is then used to form the microdroplets, typically achieving high-yield conversion of the liquid metal alloy into microdroplets with excellent morphology and homogeneity, and highly uniform elemental distribution. Each metal microdroplet preferably has substantially the same metal composition.

[0100] It should be understood that "high temperature solvent" refers to a solvent used in the process that is stable over the selected temperature range of high temperature sonication, which, as discussed below, is typically above 200°C, and more preferably near or above 300°C.

[0101] The yield, composition, and homogeneity of the synthesized microdroplets can be affected by several parameters, including the melting and decomposition temperature of the hot solvent, the alloying temperature of the metal alloy, the temperature of the ultrasonic energy applied to the metal alloy in the hot solvent (droplet formation temperature), the duration that the ultrasonic energy is applied to the metal alloy in the hot solvent, and the elemental composition of the metal alloy. Each of these parameters can be adjusted to help achieve a highly uniform elemental distribution within a homogeneous colloidal liquid metal system.

[0102] While not wishing to be limited to any one theory, the inventors have discovered that the homogeneous generation and functionalization of liquid metal microdroplets requires careful consideration of the phases and melting temperatures in the alloy system under consideration. Here, at least one low-melting metal element is considered a metal solvent, and at least one promoter metal element is considered a metal solute. By referencing the phase diagram of this solvent-solute system, the overall melting point of the alloy system can be determined, and then the associated melting / stability points of any intermetallic alloys or compounds in that alloy system can be determined. The droplet formation temperature is then selected to exceed both the melting point (liquid) of the solvent-solute system and the melting / stability points of any intermetallic alloys / compounds in that solvent-solute system. This temperature selection ensures that intermetallic formation is avoided during droplet formation, thereby ensuring that the bulk metal alloy is homogeneous and that the solute metal dissolves freely in the solvent metal. Therefore, all components of the metal alloy are substantially homogeneously distributed throughout its composition and dispersed as the composition is broken down and dispersed into the desired microdroplets during sonication.

[0103] In the absence of a phase diagram for a catalyst alloy system of interest, a suitable droplet formation temperature can be determined experimentally, for example, by (i) gradually heating the bulk metal alloy until it forms a single homogeneous liquid phase, or (ii) cooling from a high temperature at which the bulk metal alloy exists as a single homogeneous liquid phase and observing the temperature at which precipitation occurs.

[0104] It should also be understood that the system temperature preferably takes into account the temperature limits of the high-temperature solvent. In this sense, the melting point and decomposition temperature of the high-temperature solvent are also considered in the overall system. In embodiments, the droplet formation temperature is also selected to be lower than the highest temperature at which the high-temperature solvent is stable (i.e., the decomposition temperature of the high-temperature solvent) and higher than the melting point of the high-temperature solvent. As noted above, the droplet formation temperature is also selected to be above the alloying temperature of the metal alloy and / or the intermetallic formation temperature of the intermetallic composition.

[0105] Therefore, the above limits on the droplet formation temperature are determined by the selection of the high-temperature solvent and the exact metal alloy composition used in the system. However, in many embodiments, the droplet formation temperature is greater than 300°C, preferably greater than 350°C. In some embodiments, the droplet formation temperature is between 300 and 400°C. In some embodiments, the droplet formation temperature is at least 400°C.

[0106] Overall, the droplet formation temperatures of the high temperature sonication systems of the present invention are typically well above ambient temperature, typically above 200° C., preferably above 300° C., and more typically approaching about 400° C. At these temperatures, conventional solvent systems decompose and cannot be used.

[0107] The metal alloy can be contacted with the hot solvent using any suitable method. In some embodiments, the metal alloy is introduced into the hot solvent. Preferably, the introduction is carried out using an alloy composition. However, it is contemplated that the metal alloy can be contacted with the hot solvent in other forms and / or using other methods or processes. Other alloying methods are possible as well, where there is direct contact between the powder and the liquid metal. For example, a low-melting point metal element and a promoter metal element can be melted together in a furnace to form the desired alloy. It is also contemplated that the metal alloy can be formed by combining and alloying the constituent metals in a hot solvent.

[0108] In embodiments, the metal alloy forms an immiscible mixture, preferably an emulsion, in the hot solvent. Preferably, the metal alloy is suspended in the hot solvent forming the immiscible mixture. It should be understood that the metal alloy is suspended in the hot solvent in this immiscible mixture at any temperature at which both the hot solvent and the metal alloy are liquid and can be mixed together in this immiscible mixture. Thus, this temperature (the immiscible mixing temperature) is above the melting points of both the hot solvent and the metal alloy. The immiscible mixing temperature is not necessarily the same as the droplet formation temperature. However, it should be understood that in embodiments, the immiscible mixing temperature may be at or near the droplet formation temperature.

[0109] A suitable high-temperature solvent is selected to match the temperature parameters of the metal solvent-solute system. The high-temperature solvent is typically a thermally stable liquid solvent that provides a liquid in which the metal alloy composition is substantially immiscible. It should be understood that a wide variety of high-temperature solvents can be used, having melting points below the operating temperature range of the system (including the droplet formation temperature) and boiling points above that operating temperature range. Accordingly, a wide variety of high-boiling point solvents can be used as the high-temperature solvent. Preferably, the solvent is sufficiently stable so as not to decompose (e.g., not to turn into carbonaceous products). The high-temperature solvent is also preferably not flammable and preferably has an autoignition point above the operating temperature. Nevertheless, it should be understood that even flammable solvents can be used under a protective atmosphere.

[0110] In some exemplary embodiments, the high-temperature solvent is a molten solvent, preferably a type of molten salt, more preferably one or more molten (alkali) acetates. However, it should be understood that other types of solvents, including ionic liquids, chloride salts, molten nitrates and / or nitrites, molten carbonates, and high-temperature compatible hydrocarbons and / or fluorocarbons, including (but not limited to) non-polar aromatic compounds or long-chain (C12 or longer) hydrocarbons, can also be used as high-temperature solvents or as oils and fats in the methods of the present invention. Thus, in embodiments, the high-temperature solvent can include at least one of hexadecane, oleic acid, at least one ionic liquid, at least one alkali metal acetate, at least one chloride salt, at least one molten nitrate and / or nitrite, at least one molten carbonate, at least one alkali metal acetate, at least one high-temperature compatible hydrocarbon and / or fluorocarbon, at least one fat (e.g., a fatty acid), or mixtures thereof. These high-temperature solvent compositions preferably have a melting temperature of 20 to 500°C, preferably 20 to 400°C.

[0111] In embodiments, the high-temperature solvent includes at least one chloride salt, preferably at least one molten chloride salt. Examples include at least one of NaCl, KCl, or ZnCl. For example, a ternary mixture of NaCl, KCl, and ZnCl has a melting point of 204-229°C. This mixture is stable up to 1000°C, non-flammable, has a low degree of corrosiveness, and is non-toxic.

[0112] In an embodiment, the high-temperature solvent includes at least one molten nitrate or nitrite, preferably at least one of NaNO, KNO, or NaNO. For example, a thermal solar salt mixture containing 7% by weight NaNO, 53% by weight KNO, and 40% by weight NaNO is non-flammable and stable up to 535°C. It is soluble in water. A further example of a suitable thermal solar salt includes a mixture of sodium nitrate and potassium nitrate. For example, a eutectic mixture of 60% by weight NaNO and 40% by weight KNO has a melting point of 220°C, is stable up to 600°C, and can be dissolved using water and / or ethanol. This mixture is non-flammable and has very low corrosivity.

[0113] In an embodiment, the high temperature solvent comprises at least one molten carbonate, preferably at least one of Li2CO3, Na2CO3, or K2CO3. For example, the eutectic Li2CO3-Na2CO3-K2CO3 melts at 396°C.

[0114] In embodiments, the high-temperature solvent preferably comprises at least one high-temperature compatible hydrocarbon and / or fluorocarbon, such as a high-temperature fluorocarbon polymer, a non-polar aromatic or long-chain hydrocarbon (C12 or higher), optionally fluorinated alkanes and aliphatic-aliphatic compounds, having a normal boiling point higher than the droplet formation temperature of the designed metal alloy system. In some embodiments, fatty acids, including stearic acid, palmitic acid, and the like, can also be used as high-temperature solvents. In some embodiments, halogenated hydrocarbon solvents (also known as halogenated solvents) can also be used as high-temperature solvents. It should be understood that some oils and fats, as well as many hydrocarbons or fluorocarbons, are suitably stable (i.e., do not decompose) at and / or above the droplet formation temperature of the system. For example, fats and / or oils, paraffins, polyethylene glycol (PEG), and PEG hydrocarbon derivatives. Examples of suitable hydrocarbons include hexadecane and longer hydrocarbons.

[0115] In some embodiments, the high temperature solvent comprises an ionic liquid. In some embodiments, the ionic liquid comprises a 1,3-dialkylimidazolium cation, such as a 1-alkyl-3-methylimidazolium cation, where the alkyl is preferably ethyl, propyl, or butyl. In some embodiments, the ionic liquid comprises a 1,3-dialkyltriazolium cation. In some embodiments, the ionic liquid comprises a fluorinated anion, such as tetrafluoroborate (BF4 - ) or bis(trifluoromethylsulfonyl)imide (TFSI - ) Non-limiting examples of suitable ionic liquids can include 1-ethyl-3-methylimidazolium-tetrafluoroborate, 1-butyl-3-methylimidazolium-tetrafluoroborate, 1-ethyl-3-methylimidazolium-tetrafluoroborate, and 1-ethyl-3-methylimidazolium-bis(trifluoromethylsulfonyl)imide. However, it should be understood that other high-temperature stable ionic liquids (stable at temperatures near or above 200°C, more preferably 300°C) can also be used as high-temperature solvents.

[0116] In an exemplary embodiment, the inventors have discovered that a molten salt system can be advantageously used as a high-temperature solvent in which the metal alloy composition is substantially immiscible. The molten salt system has a suitable melting point and decomposition temperature within the desired temperature range of 300-400°C, is stable within that temperature range, is sufficiently inert, and is cost-effective (e.g., can be obtained at low cost).

[0117] In a preferred embodiment, the high-temperature solvent comprises a molten solvent, preferably an anhydrous molten solvent, more preferably at least one molten acetate salt, more preferably an alkali metal acetate salt. These types of high-temperature solvents have been found to be stable at the designed droplet formation temperature, are noncorrosive, and do not contaminate the final dispersion of microdroplets or alter the final product. These salts were found to have the desired properties at the designed droplet formation temperature after extensive searching of other possible solvent systems. Alkali metal acetate salts are stable at temperatures between 100 and 400°C and allow for the preparation of micron- and submicron-sized droplets from a wide range of metal alloys containing at least one low-melting metal element selected from gallium and indium and at least one promoter metal element selected from Groups 1-2 and 7-11 metals. Various alkali metal acetate salts can be used. In an embodiment, the high-temperature solvent comprises at least one alkali metal acetate salt, preferably Na, K, or Cs acetate salt or a mixture thereof, more preferably anhydrous Na, K, or Cs acetate salt or a mixture thereof.

[0118] One or more high-temperature solvents can be used in the methods of the present invention. In some embodiments, the high-temperature solvent comprises a single solvent composition, e.g., a single alkali metal acetate. In other embodiments, the high-temperature solvent comprises a mixture of at least two different high-temperature solvents. In this regard, as noted above, different high-temperature solvents can be mixed to form at least binary solvent mixtures. An example of a high-temperature solvent mixture is a molten acetate, e.g., a mixture of Na acetate with K acetate. It should be understood that a eutectic of this molten salt composition can be utilized in the methods in some applications. For example, Na acetate melts at 320°C, K acetate melts at 292°C, and Cs acetate melts at 194°C. Thus, a high-temperature solvent having a selected melting temperature can be achieved by subsequently mixing the molten salts together. One example is a mixture of Na, K, and Cs acetates in 14.5, 17.4, and 68.1 wt. % (total 100 wt. %), which melts at 90°C.

[0119] The metal alloy composition may comprise at least a binary alloy, i.e., at least two different alloying metals, including one low melting point metal element selected from gallium and indium and one promoter metal element selected from Groups 1-2 and 7-11 metals. In some embodiments, the metal alloy composition comprises three or more different alloying metals.

[0120] Given that bulk metal alloys can be converted into metal droplets suitable for use in ammonia synthesis catalysts in near quantitative yields by the methods disclosed herein, the composition of the bulk metal alloy contacted with the hot solvent can be as disclosed herein in the context of catalyst microdroplets used in methods to produce ammonia.

[0121] The application of ultrasonic energy is utilized in the present invention to break down and disperse metal alloys into smaller droplets, preferably microdroplets, in a hot solvent. In some embodiments, the resulting metal microdroplets are primarily between 100 nm and 10 μm in size.

[0122] The metal alloy is dispersed in a hot solvent by ultrasonic treatment at high temperatures (above the melting point of the alloy and any intermetallic compounds that may form therein). Thus, the resulting microdroplets each contain similar or substantially the same liquid metal alloy composition. Upon cooling, the promoter metal element may precipitate from the liquid metal alloy, e.g., as an intermetallic compound, to enable microdroplet recovery from the solvent and support on a suitable solid support. However, this precipitation occurs within each microdroplet such that a substantial proportion of the microdroplets contain a "reservoir" of the precipitated promoter metal element that is fully contained within the microdroplet and will dissolve back into the liquid metal alloy when the catalyst is heated to the operating temperature for ammonia synthesis. The resulting catalyst is highly active due to the microdroplet morphology and the substantially homogeneous distribution of the promoter metal.

[0123] It should be understood that the frequency, power, and duration of the applied ultrasonic energy can affect the decomposition and dispersion process of the metal alloy within the hot solvent.

[0124] Ultrasonic energy is ideally applied to the metal alloy in the hot solvent for a period of time sufficient to completely disintegrate and disperse the microdroplets within the hot solvent. The duration of application depends on several parameters, including the volume, metal alloy composition, reservoir size and physical dimensions, sonicator probe geometry and power supply, desired final microdroplet size, and other parameters. However, in embodiments, ultrasonic energy is applied to the metal alloy in the hot solvent for a duration of at least 5 minutes, preferably 20 to 60 minutes, and more preferably about 30 minutes, to completely disintegrate the bulk metal alloy.

[0125] Similarly, the frequency and waveform of the applied ultrasonic energy can be selected to adjust process parameters such as droplet dispersion and droplet size. In embodiments, the applied ultrasonic energy has a fixed sine wave frequency in the range of 20-500 kHz. In some embodiments, the applied ultrasonic energy has a fixed sine wave frequency in the range of 100-200 kHz. In other embodiments, the applied ultrasonic energy has a fixed sine wave frequency in the range of 20-25 kHz. However, higher or lower frequencies can be applied. In other embodiments, the waveform of the applied ultrasonic energy can be selected from sine wave, triangle wave, square wave, sawtooth wave, or pulse wave. Similarly, the power of the applied ultrasonic energy can be tailored to a particular system. In embodiments, the applied ultrasonic energy is applied at a power of 10 W to 1000 W, preferably 100-500 W. In one particular embodiment, the applied ultrasonic energy is applied at a power of 300 W, although higher and lower power settings can be applied. It should be understood that the power settings will vary depending on the volume and shape of the container and the size and / or shape of the ultrasonic applicator, for example, the type and size of the probe tip of the sonic probe.

[0126] A variety of ultrasonic applicators can be used in the methods of the present invention. In embodiments, ultrasonic energy is applied to the metal alloy in the hot solvent using a probe sonicator.

[0127] In some embodiments, the hot solvent is mechanically agitated during the application of ultrasonic energy. In these embodiments, the method further comprises stirring or otherwise mechanically agitating the hot solvent while applying ultrasonic energy to the metal alloy. Various stirring speeds can be used. In embodiments, the stirring speed is at least 100 rpm.

[0128] The methods of the present invention can also include the preliminary step of forming a metal alloy before adding the metal alloy to the hot solvent for dispersion. In these embodiments, the method further includes forming the metal alloy by melting and mixing together (i) at least one low-melting point metal element selected from gallium and indium and (ii) at least one promoter metal element selected from Groups 1-2 and 7-11 metals. In certain embodiments, the at least one promoter metal element may be in the form of a powder or particulate material when mixed into the composition.

[0129] The method may include cooling the hot solvent containing the metal microdroplets below the melting point of the metal alloy and / or any intermetallic compounds that may form therein, thus cooling the metal microdroplets to induce solidification of one or more solid phases.

[0130] In some embodiments, therefore, at least a proportion of the metal microdroplets comprise (i) a solid metal alloy or intermetallic compound enriched with at least one promoter metal element, and (ii) a second metal phase enriched with at least one low-melting point metal element when the metal microdroplets are cooled to room temperature. For example, at least 50% of the metal microdroplets, or at least 70% of the metal microdroplets, or at least 80% of the metal microdroplets, e.g., at least 90% or substantially all of the metal microdroplets, may comprise a solid metal alloy or intermetallic compound and a second metal phase.

[0131] In some embodiments, at least a proportion of the metal microdroplets comprise (i) a solid metal alloy or intermetallic compound enriched in at least one promoter metal element, and (ii) a liquid metal phase enriched in at least one low-melting point metal element when the metal microdroplets are cooled to about 50° C. For example, at least 50% of the metal microdroplets, or at least 70% of the metal microdroplets, or at least 80% of the metal microdroplets, e.g., at least 90% or substantially all of the metal microdroplets, may comprise a solid metal alloy or intermetallic compound and a liquid metal phase. Gallium-rich metal compositions are expected to remain liquid at or near ambient temperatures due to the very low melting temperature of gallium.

[0132] The hot solvent can be cooled below the melting point of the metal alloy and / or intermetallic composition using any suitable technique. Cooling can be via forced or applied cooling, or via passive or non-forced cooling techniques. Suitable techniques include ambient air cooling, convection cooling, or forced / applied cooling, e.g., the use of a heat exchanger or other cooling device such as refrigeration.

[0133] The method of the present invention includes a step of separating the metal microdroplets from the hot solvent. The separation can be performed after cooling the hot solvent, although separation at high temperatures, for example by filtration, is not excluded. The separation step can be achieved using a variety of separation processes. In some embodiments, the separation includes at least one of filtration, vacuum filtration, sedimentation, vacuum-assisted evaporation (distillation), vacuum distillation, gravity filtration, dry filtration (if in liquid form), centrifugation, cascade centrifugation, centrifugation by solvent exchange, etc.

[0134] In some embodiments, the high-temperature solvent is diluted or dissolved with a suitable solvent, e.g., a low-boiling molecular solvent, during separation. For example, a solid salt matrix encapsulating metal microdroplets formed by cooling a molten salt high-temperature solvent (e.g., Na, K, or Cs acetate) may be dissolved in water before recovering the liberated metal microdroplets, e.g., by filtration. As another example, an ionic liquid high-temperature solvent may be diluted with a miscible polar solvent, such as acetonitrile, prior to separation of the metal microdroplets. In some embodiments, the separated metal microdroplets are washed with a suitable solvent to remove any residual traces of the high-temperature solvent.

[0135] The method of the present invention includes a step of supporting the metal microdroplets on a solid support. Preferably, this step is carried out after separating the metal microdroplets from the hot solvent. However, it is not excluded that the supporting and solvent separation can be carried out simultaneously.

[0136] The metal microdroplets may be supported on a solid support by any method. In some embodiments, the metal microdroplets are dispersed in a liquid carrier, e.g., a low-boiling organic solvent such as ethanol, when contacted with the support. Thus, the method includes redispersing the metal microdroplets in the liquid carrier, e.g., using ultrasonic energy. The dispersion of metal microdroplets can then be drop-cast or otherwise applied to the support. After the liquid carrier is removed, the metal microdroplets remain attached to the support. Metal microdroplets have been observed to easily and tenaciously adhere to solid supports such as carbon paper, thus ensuring that the metal composition remains finely divided for subsequent use in catalysis.

[0137] Suitable supports are generally as disclosed herein in the context of catalytic microdroplets used in the process for producing ammonia. For example, the support may be selected from carbon-based supports (e.g., carbon paper), metal supports, metal oxide supports (e.g., alumina, silica), and ceramic supports.

[0138] 1(a)-1(c) illustrate one embodiment of an apparatus / system 100 for synthesizing metal microdroplets 140 in accordance with the present invention. As shown, the system 100 includes: (A) A reservoir 105 containing a high-temperature solvent 107 and a metal alloy 108, the high-temperature solvent including (i) at least one low-melting metal element selected from gallium and indium, and (ii) at least one promoter metal element selected from Groups 1-2 and 7-11 metals, wherein the high-temperature solvent is at a temperature in which the metal alloy 108 is substantially immiscible. In the illustrated system 100, the reservoir 105 is contained within a beaker 110 (or glass vial). However, it should be understood that any suitable high-temperature fluid holding vessel may be used. (B) An ultrasonicator 120 configured to apply ultrasonic energy to the metal alloy 108 in the hot solvent 107 in the reservoir 105. In the illustrated embodiment, the ultrasonicator comprises a sonic probe. However, it should be understood that any suitable ultrasonic applicator may be used. (C) A heating device (not shown) operative to heat the hot solvent to a droplet formation temperature above the melting point of the metal alloy and above the melting point of any intermetallic compounds that may form therein. The heating device may comprise any suitable heating configuration, including a heat exchanger, oven, furnace, heating plate, heating mantle, induction heating, microwave heating, etc.

[0139] The hot solvent is also preferably mechanically agitated. As shown by the dashed lines in Figures 1(a) and 1(b), the system (D) A hot solvent immersed stirrer or other mechanical stirrer 125 may further be included. Various stirring speeds may be used. In embodiments, the stirring speed is at least 50 rpm, preferably at least 100 rpm. One example of a suitable stirrer is a magnetic stirrer. However, it should be understood that other stirrers, impellers, and / or rotors may be used.

[0140] Although not shown, the system 100 may include a cooling arrangement operable to cool the hot solvent below the melting point of the metal alloy and / or intermetallic composition, preferably to room temperature. The cooling arrangement may comprise any suitable cooling arrangement (not shown), such as, for example, a heat exchanger or other cooling device, such as refrigeration.

[0141] The system 100 may also include a separation configuration (not shown) for separating the microdroplets 140 from the hot solvent 107. One of a variety of separation configurations can be used, such as filtration, vacuum filtration, sedimentation, vacuum-assisted evaporation (distillation), vacuum distillation, gravity filtration, dry filtration (if in liquid form), centrifugation, cascade centrifugation, centrifugation with solvent exchange, etc.

[0142] In operation, the ultrasonic processor 120 operates to apply ultrasonic energy to the metal alloy 108 in the hot solvent 107 when it is at a droplet formation temperature (FIGS. 1(a) and 1(b)), to break up the liquid metal alloy 108 into colloidal microdroplets 140 in the hot solvent (FIG. 1(b)), thereby forming the microdroplets 140 of the metal alloy 108. Referring to FIG. At t0 (FIG. 1(a)): An immiscible mixture is formed from a binary metal alloy 108 in a hot solvent 107. At t0 (FIG. 1(a)): A heater (not shown) operates to ensure that the hot solvent 107 and metal alloy 108 are at a selected droplet formation temperature. A thermocouple is used to measure the temperature of the components in the beaker 110. At the droplet formation temperature, the ultrasonicator 110 operates to apply ultrasonic energy to the metal alloy 108 in the hot solvent 107. The agitator 125 is optionally operated during this period to add additional agitation to the hot solvent 107. At t1 (FIG. 1(b)): After a short duration of operation of the sonicator 110 and / or agitator 125, the liquid binary metal alloy breaks down into multiple droplets 130 dispersed within the hot solvent 107. The sonicator continues to operate for a total duration of, for example, 30 minutes. The agitator 125 is optionally operated during this period to add additional agitation to the hot solvent 107. It should be understood that the optimal total duration of sonication may vary depending on the geometry of the vessel, the probe geometry of the sonicator 110, and the power settings. At t2 (FIG. 1(c)): Microdroplets 140 are formed by further dispersion and droplet division, as shown by the darker liquid in the beaker 110 in FIG. 1(c) and as shown in the enlarged portion of the liquid in FIG. 1(c). The sonicator 120 and agitator 125 (if operating) are turned off. After t2 (not shown): The hot solvent 107 may be cooled below the melting point of the metal alloy and / or any intermetallic compounds that may form therein, and in most cases may be cooled to near room temperature. The formed microdroplets 140 may then be separated from the hot solvent 107 using any suitable separation process or configuration, for example, filtration.

[0143] It should be understood that the metal alloys and high temperature solvents that can be used in the system shown in FIG. 1 are as previously described.

[0144] Once the microdroplets 140 have been prepared by sonication and separated from the hot solvent 107, they may be supported on a suitable solid support as disclosed herein. [Example]

[0145] The present invention will now be described with reference to the following examples, which should be understood as illustrative of the invention described herein and not limiting thereof.

[0146] Materials and Methods Gallium with a purity of 99.9% was purchased from Indium Corporation (Clinton, New York, United States). Copper powder with a purity of 99.9% was purchased from Sigma-Aldrich (Australia). Sodium acetate with a purity of 99% or higher from Sigma-Aldrich was used. 1-Ethyl-3-methylimidazolium tetrafluoroborate with a purity of 99% or higher was purchased from Sigma-Aldrich.

[0147] Scanning electron microscope (SEM) images were acquired on a Tescan MIRA3 SEM instrument and processed with Aztec software v3.3.

[0148] Transmission electron microscopy (TEM) was performed using a JEOL F200 cold cathode field emission gun system operated at an accelerating voltage of 200 kV and equipped with a bright-field Gatan Rio16 4k charge-coupled device (CCD) camera (model 1816). The Gatan Digital Micrograph 1.8.4 software suite was used for imaging and analysis.

[0149] Energy dispersive X-ray spectroscopy (EDS) was performed using a JEOL F200 system equipped with an Oxford X-Max20 EDX Detector (2014) and Aztec software v3.3, operated in STEM mode, which allowed elemental mapping of each functionalized droplet.

[0150] Example 1 A bimetallic bulk alloy containing 2 wt% copper metal and 98 wt% gallium (2% Cu-Ga bulk alloy) was prepared using a mechanical grinding method. Thus, 2 wt% Cu metal was mixed with 98 wt% Ga at 400 °C for 30 min using a mortar and pestle in a glove box to avoid oxidation and ensure complete dissolution of Cu in Ga. Once alloying was complete, the metal mixture was placed on a watch glass to cool and then placed in a refrigerator for storage in the solid phase (melting point close to room temperature).

[0151] The synthesis apparatus for producing liquid metal microdroplets for catalysis is shown schematically in Figure 1. A vial containing the ionic liquid 1-ethyl-3-methylimidazolium tetrafluoroborate (10 ml) was placed on a hotplate equipped with an aluminum block to hold and heat the vial, heating the ionic liquid to 300 °C. Once at temperature, 2% Cu-Ga bulk alloy (1.5 g) was added to the ionic liquid, allowed to completely melt, and stirred at 100 rpm. A SCIENTZ-IID probe sonicator equipped with a 6 mm titanium tip was inserted into the liquid, and the liquid metal composition was sonicated at 300 W of power, operating in a 3 s on, 3 s off cycle throughout the 30 min sonication period. It was observed that all of the bulk metal dispersed in the solvent as a result of sonication, consistent with the formation of a colloidal system of metal microdroplets in the solvent.

[0152] The temperature at which the mixture was sonicated (i.e., 300 °C) was chosen to be (i) a temperature at which the ionic liquid was sufficiently stable and (ii) above the melting temperature of 249 °C of the intermetallic compound GaCu, which is expected to form according to the phase diagram of the Cu-Ga system. Thus, the metal composition was a single homogeneous liquid metal phase when subjected to sonication.

[0153] The dispersed microdroplets were then extracted from the ionic liquid carrier by cooling to room temperature, diluting with acetonitrile, filtering through glass fiber filter paper, and washing the filtered microdroplets with additional acetonitrile and then ethanol to remove traces of ionic liquid. The microdroplets were then redispersed in 90% ethanol (balance water) by volume and sonicated in an ultrasonic water bath for 2 minutes to uniformly disperse the microdroplets throughout the ethanol carrier.

[0154] The microdroplets (2% Cu-Ga) were then loaded onto carbon paper, a suitable inert substrate for catalytic studies, by drop-casting a dispersion of the microdroplets in ethanol. The sample was then dried on a hot plate at 100 °C to remove traces of solvent. The supported microdroplets were observed to be robust and firmly attached to the carbon paper without bonding.

[0155] The supported microdroplets were analyzed via SEM, and representative images are shown in Figure 2. The metal microdroplets observed in SEM imaging are primarily spherical. Microdroplets with diameters between 1 and 10 μm are prominent in the SEM images, although smaller droplets in the 50 nm to 1 μm range are also expected to be present.

[0156] For TEM analysis, metal microdroplets were prepared by drop-casting a dispersion of the microdroplets in ethanol onto a carbon-formvar Au grid. Figure 3 shows a dark-field TEM image of a representative spherical microdroplet (300), and the EDX elemental map indicates the presence of a copper-rich seed (302), predicted to be GaCu, surrounded by a gallium-rich liquid metal composition (304) (white dashed lines shown as a guide to the seed and droplet shape). Figures 4 and 5 show Ga and Cu elemental maps, respectively, of the same microdroplet. Gallium is distributed throughout the microdroplet, while copper is concentrated within the seed.

[0157] The results demonstrate that the copper concentration present in the fully liquid microdroplets when initially generated by ultrasonication at 300 °C exceeded the low-temperature solubility of copper in gallium. Thus, when the microdroplets were cooled below the melting point of GaCu, this intermetallic phase crystallized as seeds. Advantageously, the seeds acted as reservoirs of copper within the microdroplets, so that when returned to high temperatures as needed to catalyze the ammonia synthesis reaction, a high concentration of alloyed copper was present in the liquid metal microdroplets.

[0158] A homogeneous distribution of copper throughout the microdroplets was evident from the presence of copper-rich Ga2Cu seeds in each microdroplet, as observed by TEM imaging of multiple microdroplets.

[0159] These results demonstrate the benefits of sonicating a 2% Cu-Ga alloy in an ionic liquid solvent at elevated temperatures, i.e., above the melting points of all phases (alloy and intermetallic) associated with the composition. Because the metal composition is a single, homogeneous liquid metal phase during sonication, the composition of each resulting microdroplet is substantially identical. Despite intra-particle heterogeneity at ambient conditions due to the precipitation of copper-rich seeds, each 2% Cu-Ga microdroplet contains substantially the same high concentration of alloyed copper present in the fully liquid metal composition when returned to elevated temperatures as needed to catalyze the ammonia synthesis reaction.

[0160] Example 2. A small amount (0.5 g) of 2% Cu-Ga bulk alloy (prepared in Example 1) was added to 90% ethanol (5 ml) by volume and sonicated in an ultrasonic bath at room temperature for 30 minutes. It was observed that under these conditions, the molten metal composition did not completely disperse in the solvent. Microdroplets of the dispersed 2% Cu-Ga-LT were then loaded onto carbon paper by drop-casting the dispersion. The sample was then dried on a hot plate at 100 °C to remove traces of solvent.

[0161] SEM and TEM analyses revealed that most of the microdroplets lacked copper seeds, and TEM-EDX analysis showed that the amount of copper in the microdroplets was well below 2 wt%, ranging from 0.1 to 0.3 wt% for most particles.

[0162] Example 3 Metal microdroplets containing 5 wt % copper in gallium (5% Cu-Ga) were prepared by the same high-temperature dispersion method as in Example 1, except that anhydrous sodium acetate was used as the high-temperature solvent.

[0163] A bimetallic bulk alloy containing 5% copper metal by weight and the balance gallium (5% Cu-Ga bulk alloy) was prepared by the same mechanical grinding method reported in Example 1. A vial containing anhydrous sodium acetate (15.3 g, melting point 324 °C, 10 ml volume after melting) was heated to 400 °C on a hot plate. Once at temperature, the 5% Cu-Ga bulk alloy (approximately 1-1.5 g) was added to the molten sodium acetate, allowed to completely melt, and stirred at 100 rpm. A probe sonicator was inserted into the liquid and the liquid metal composition was sonicated at 300 W of power, operating in a 3-second on, 3-second off cycle throughout the 30-minute sonication period. The temperature at which the mixture was sonicated (i.e., 400 °C) was selected to (i) be a temperature at which sodium acetate is liquid but thermally stable, and (ii) exceed the melting temperature of the intermetallic compound GaCu, which is 249 °C. Consistent with the formation of a colloidal system of metal microdroplets in the solvent, all of the bulk metal dispersed in the solvent as a result of sonication.

[0164] After sonication, the solution was cooled to room temperature and solidified. Deionized water was then added to dissolve the sodium acetate, and the mixture was vacuum filtered through a 0.4 μm filter paper with an additional water wash to remove traces of sodium acetate. The droplets were then redispersed in 90% ethanol by volume and sonicated in an ultrasonic water bath for 2 minutes to uniformly disperse the microdroplets throughout the ethanol carrier. The dispersion was drop-cast onto a carbon-formvar Au grid for microdroplet analysis.

[0165] The resulting 5% Cu-Ga microdroplets were analyzed by SEM and TEM. The size distribution of the microdroplets, obtained by particle size analysis of the SEM images (Aztec software), is shown in Figure 6. Spherical microdroplets ranging from 100 nm to over 1 μm were again formed. TEM analysis revealed that approximately 89% of the microdroplets contained observable copper-rich seeds (i.e., GaCu) contained within the gallium-rich liquid metal composition of the microdroplets.

[0166] Example 4. Metal microdroplets containing 5 wt% copper in gallium (5%Cu-Ga-LT) were also prepared by the low-temperature dispersion method. A bimetallic bulk alloy containing 5 wt% copper (metal balance gallium) (5% Cu-Ga bulk alloy prepared in Example 3) was added to a vial containing 90% by volume of ethanol (10 ml), heated to 55°C, and stirred at 100 rpm. A probe sonicator was inserted into the liquid and the liquid metal composition was sonicated at 300 W of power, operating at 3 seconds on, 3 seconds off throughout the 30-minute sonication period. Much of the bulk metal was observed to remain undispersed in the vial, indicating a low-yield synthesis of the microdroplets.

[0167] Once sonication was complete, a portion of the liquid phase was drop-cast onto a TEM grid to analyze the metal microdroplets dispersed in the liquid. TEM analysis of the resulting metal microdroplets revealed that only 2.8% contained copper-rich seeds. It is clear that copper was distributed unevenly throughout the liquid metal microdroplets.

[0168] Example 5. The generality of the catalyst synthesis method developed in Example 3 was demonstrated by preparing metal microdroplets with the following compositions: 2.8% Cu-In, 2.7% Ni-Ga, 2% Pt-Ga, 10% Ag-In, and 15% Ag-Ga. In each case, bimetallic bulk alloys were first prepared by the same mechanical grinding method reported in Example 1. The bimetallic bulk alloys (1–1.5 g) were then added to a vial containing molten sodium acetate (10 ml) at 400 °C and sonicated in a probe sonicator (300 W, 30 s on, 30 s off for 30 min). The dispersion temperature was higher than the melting point of any known intermetallic compounds in these alloy systems. Consistent with the formation of a colloidal system of molten metal microdroplets in a solvent, all of the bulk metal dispersed in the solvent as a result of sonication.

[0169] After cooling, aqueous purification, and redispersion in 90% by volume ethanol (according to Example 3), the microdroplet dispersions were drop-cast onto carbon-formvar Au grids for SEM and TEM / EDS analysis. The presence of promoter metal-enriched precipitate "seeds" (believed to be intermetallic compounds in each case) was evident in most of the microdroplets. Results for selected microdroplet compositions produced in Examples 3 and 5 are summarized in Table 1. [Table 1]

[0170] Example 6 A bimetallic bulk alloy containing 2 wt. % magnesium metal and the balance eutectic gallium-indium (2%Mg-eGaIn bulk alloy) was prepared by the same mechanical grinding method (starting from preformed eGaIn) as reported in Example 1.

[0171] Metal microdroplets containing magnesium in eutectic gallium-indium (2%Mg-eGaIn-LT) were then prepared by a low-temperature dispersion method. The 2%Mg-eGaIn bulk alloy was added to a vial containing 90% by volume of ethanol (10 ml), heated to 55°C, and stirred at 100 rpm. A SCIENTZ-IID probe sonicator was inserted into the liquid and the liquid metal composition was sonicated at 300 W of power, operating 3 seconds on, 3 seconds off throughout the 30-minute sonication period. Most of the alloy was observed to disperse in the solvent.

[0172] Microdroplets of the dispersed solution (2% Mg-eGaIn-LT) were then loaded onto carbon paper by drop-casting the dispersion. The sample was then dried on a hot plate at 100 °C to remove traces of solvent.

[0173] Example 7 The catalytic activity in the ammonia synthesis reaction was evaluated according to the following general procedure. The supported catalyst for evaluation (approximately 1.5 mg total weight, including the carbon paper support and the supported metal microdroplets; the microdroplets were loaded onto the support at approximately 10 wt %) was loaded onto an inert HVC screen (150 mesh) in a reaction chamber (18 ml volume). The catalyst was first pretreated at 250 °C for 2 h under an inert He supply, and then at 450 °C and 4 bar for 4 h under an N2 / H2 gas mixture. The chamber was then purged with He at room temperature for 3 h to remove any remaining reactive gases or products. Ammonia synthesis evaluation was then performed under differential conditions (i.e., negligible ammonia partial pressure) at the desired operating temperature and pressure for several 10-minute cycles, starting with nitrogen and hydrogen gas supplies (N2:H2 ratio 1:3 v / v, unless otherwise specified). After each cycle, the ammonia produced was collected by bubbling the gas effluent from the reactor through 5 mM H2SO4 and analyzed by colorimetry (Nessler's reagent) and NMR. The reactor was then recharged with gas reagent to begin the next cycle.

[0174] The activity of a 2% Cu-Ga catalyst (supported on carbon paper; produced in Example 1) was evaluated at different reaction temperatures at a total gas pressure of 4 bar, and the results are shown in Figure 7. The ammonia production rate at 400 °C was 6000 μmol g based on the mass of the metal nanodroplets (Cu + Ga). -1 .time -1 A lower activity was obtained at 500°C, indicating that the peak activity was likely between 350°C and 450°C.

[0175] The activity of the 2% Cu-Ga catalyst was also evaluated at different reaction pressures at a reaction temperature of 400 °C, and the results are shown in Figure 8. Higher pressures increase the rate of ammonia formation, consistent with ammonia synthesis being at equilibrium. However, significant ammonia formation rates were obtained even at atmospheric pressure.

[0176] The effect of N2 / H2 ratio (1:3 vs. 3:1 v / v) in the gas feed was also evaluated, and the results are shown in Figure 9. Higher activity was obtained at a feed ratio of N2 / H2 of 1:3 v / v, which is consistent with the stoichiometry of the ammonia synthesis reaction.

[0177] The 2% Cu-Ga catalyst was recovered after the reaction and analyzed by SEM. The supported liquid metal microdroplets remained intact after the reaction.

[0178] Example 8 The ammonia synthesis activity of the 2% Cu-Ga catalyst supported on carbon paper (as produced in Example 1) was compared to that of a state-of-the-art Ru-based Haber-Bosch heterogeneous catalyst (Ru-Cs / MgO; activity based on the mass of Ru + Cs) at 400 °C and 4 bar pressure (general procedure in Example 7), and the results are shown in Figure 10. It is clear that there is a very significant increase in activity for the liquid metal 2% Cu-Ga catalyst compared to the heterogeneous catalyst. The increased activity obtained with the liquid metal catalyst provides an opportunity to operate the ammonia synthesis process at significantly lower pressures compared to the typical Haber-Bosch process while obtaining similar conversions.

[0179] Example 9. The ammonia synthesis activity of the 2% Cu-Ga catalyst with homogeneously distributed copper (prepared in Example 1) was compared with that of the 2% Cu-Ga-LT catalyst with heterogeneously distributed copper (prepared in Example 2) at 400°C and 4 bar pressure (general procedure in Example 7), and the results are shown in Figure 11. The catalyst obtained by the high-temperature dispersion method had approximately 40% higher catalytic activity.

[0180] Example 10. To confirm that the excellent ammonia synthesis activity of the 2% Cu-Ga catalyst, as determined in Example 7, was caused by the cooperative effect between the Cu and Ga components, ammonia synthesis reactions were carried out using the carbon paper substrate itself, unsupported metallic copper powder, and pure gallium microdroplets supported on the carbon paper substrate. The results are shown in Figure 12 (400°C and 4 bar pressure; general procedure of Example 7).

[0181] The substrate and copper metal had negligible activity for ammonia synthesis. The gallium microdroplets showed some activity without promotion by any alloying metal, but it is clear that a very significant enhancement is obtained when the microdroplets contain the 2 wt.% copper content present in the liquid gallium-based alloy.

[0182] Example 11 The longer-term stability of the 2% Cu-Ga catalyst (produced in Example 1) supported on carbon paper in the ammonia synthesis reaction was then evaluated. The extended reaction was carried out as a series of 1-hour batch reactions at 400°C, a N2 / H2 ratio of 1:3 v / v, and a total pressure of 4 bar for a total of 32 hours. In each cycle, the reaction chamber was charged with reactive gas, allowed to react over the catalyst for 1 hour, depressurized for analysis, and then recharged with reactive gas to begin the next cycle.

[0183] The rate of ammonia production over 32 hours is shown in Figure 13, and the cumulative turnover over the first 12 hours is shown in Figure 14. This reaction was approximately 1 x 10 4 μmol.g -1 .time -1It was found to be very stable over 32 hours of operation, with a reaction rate of 0.01%. Figure 14 shows that the process is catalytic. The turnover number (TON) is well above 1, so much ammonia is produced that cannot be explained by a simple reduction process consuming metals.

[0184] Considering that one of the most significant limitations to the use of state-of-the-art solid catalysts in ammonia production is the observed loss of activity over time, the demonstrated stability of the 2% Cu-Ga catalyst is highly advantageous. Without being limited by theory, it is believed that liquid metal catalysts (such as the 2% Cu-Ga catalyst) are resistant to hydrogen embrittlement and surface fouling due, at least in part, to the dynamic renewal of the catalyst surface of microdroplets by mobile metal atoms in the liquid metal composition.

[0185] Example 12 The ammonia synthesis activity of the 2% Mg-eGaIn catalyst (prepared in Example 6) was then evaluated at 400°C and 4 bar pressure (general procedure in Example 7). The results, compared with unpromoted gallium microdroplets, are shown in Figure 15. The addition of magnesium was found to increase activity very significantly (approximately four-fold).

[0186] Example 13 The hydrogen activation capability of the liquid metal microdroplet catalyst was investigated by passing a physical mixture of H2 and D2 over the catalyst at 400 °C and measuring the rate of HD formation. The same reactor system as in Example 7 was used, and product distribution was determined using a mass spectrometry gas analyzer. Comparative reactions were performed using a 2% Cu-Ga catalyst supported on carbon paper (as produced in Example 1), pure gallium microdroplets supported on carbon paper, and no catalyst in the reactor (blank experiment). As can be seen in Figure 16, hydrogen was activated when in contact with the 2% Cu-Ga catalyst, whereas gallium microdroplets lacking Cu (or other promoters) were inactive (no improvement over the blank). This result strongly suggests that copper plays a role in hydrogen activation during the ammonia synthesis reaction over the liquid metal catalyst.

[0187] Because N activation is widely considered the rate-limiting step in conventional ammonia production processes, the ability of liquid metal catalysts to activate and dissociate N is of great interest. Therefore, the interaction of 2% Cu-Ga catalyst (prepared in Example 1) with N was investigated by in situ FTIR experiments. The same reactor system as in Example 7 was used, and the catalyst was observed through an IR transmission window.

[0188] Figure 17 shows the FTIR spectra of the 2% Cu-Ga catalyst when exposed to N2 at 3 bar pressure and various temperatures from 200 °C to 400 °C. The results demonstrate the coexistence of Ga-N and N-N bonds in the sample at higher reaction temperatures, indicating the ability of the catalyst to activate N2.

[0189] Figure 18 shows FTIR spectra over time (spectra obtained after 5, 10, and 15 minutes of H exposure) of a 2% Cu-Ga catalyst initially exposed to N at 3 bar pressure and 400 °C, followed by H being leaked into the chamber. The results show an increase in Ga-N bonds and a decrease in N-N bonds as H is supplied to the chamber.

[0190] The H2 / D2 activation and FTIR observed N2 activation experiments together suggest a synergistic cooperation between Cu and Ga in the liquid metal alloy during ammonia synthesis, whereby Ga activates nitrogen and Cu activates H2, ultimately promoting the conversion of N2 and H2 to NH3.

[0191] Example 14. A bulk metal alloy containing 196 gallium atoms and 4 copper atoms was simulated by molecular mechanics. The first classical molecular dynamics (MD) simulation was performed using the MD code LAMMPS with a resolution of 15.82 × 15.82 × 15.82 Å. 3The bulk and interface ab initio MD (AIMD) simulations were performed for 200 Ga atoms in a box at 673.15 K under the NVT ensemble. Four Ga atoms were alchemically changed to Cu, and three iterations of the simulation were performed for 100 ps with a 2 fs time step using the Vienna ab initio Simulation Package (VASP) at 673.15 K, using the projected augmented wave (PAW) method, the PBE exchange-correlation functional, a 320 eV energy cutoff, and only the gamma point of the k-point grid. For the interface system, a 15 Å vacuum spacer was added in the z dimension.

[0192] To model the catalytic reaction, one N2 and three H2 molecules were added to an equilibrated interface system of 196 Ga and 4 Cu atoms, and AIMD simulations were performed with a time step of 0.5 fs and an energy cutoff of 450 eV. Geometry optimizations were performed on AIMD snapshots and Bader partial charges were calculated for each step of the reaction using a 4 × 4 × 1 k-point grid. All other analyses were performed using VMD 1.9.3.

[0193] The simulations showed that Cu showed no particular preference at the interface and moved freely between the bulk and the outermost interfacial layer. This continuous circulation of Cu atoms between the bulk and the interface may help to continually refresh the interface and reduce catalyst poisoning.

[0194] One possible mechanism for the ammonia synthesis reaction identified by simulation is shown in Figure 19. The reaction of N2 with the surface was found to be strongly energetically favorable in the presence of Cu, forming two adjacent N-Cu-Ga3 structures (Figure 19b). In this structure, the Ga atom is directly bonded to the N, and the two N atoms are no longer bonded. In the next step, an H2 molecule reacts with one of the N centers, resulting in one H bonded to the N and one to a nearby Ga (Figure 19c). The N-bonded H replaces one Ga atom, leaving the other H negatively charged, and the neighboring Ga atom more positively charged to compensate. Again, the presence of Cu distorts the geometry, making the N more exposed for further reaction. Reaction with another H2 molecule breaks the N-Cu bond, leaving the NH2 structure much more exposed at the interface (Figure 19d), where it can react with surface H or gaseous H2 to form NH3, which spontaneously dissociates from the interface.

[0195] Example 15 (comparative). Cu- and Ag-functionalized Ga droplets were prepared. The Ga droplets were formed using an ultrasonic method and then functionalized with Cu using a galvanic replacement reaction. In this comparative example, the Ga-Cu and Ga-Ag metal alloys were not heated in a high-temperature solvent at a droplet formation temperature above the melting point of the metal alloy and above the melting point of any intermetallic compounds that may form therein. This resulted in droplet morphologies that were very different from those illustrated in the previous examples.

[0196] Materials and Methods Ga droplet synthesis: Gallium droplets were created using a sonication method combining 100 μl of liquid gallium metal (99.9% purity) (Indium Corporation, Clinton, New York) with 15 mL of MilliQ water before sonication at 300 W for 5 min with a 3 s on / 3 s off time.

[0197] Galvanic droplet synthesis: Gallium droplets were added to a 0.1 M solution of copper sulfate (CuSO4) and silver nitrate (AgNO3) (both Sigma, St. Louis, Missouri) in a 1:1 ratio, followed by centrifugation at 10,000 RCF for 5 min. The particles were then washed with MilliQ water, 100% ethanol (twice), and phosphate-buffered saline (PBS) (Sigma, St. Louis, Missouri) (twice) and centrifuged again at 10,000 RCF for 5 min to remove any residual CuSO4 or AgNO3 in the solution.

[0198] Scanning electron microscopy (SEM): Scanning electron micrographs were obtained using an FEI Verios SEM (FEI, Oregon) at 3 kV. For cell imaging, all samples were deposited on silicon wafers and fixed with 2.5% glutaraldehyde / formaldehyde. The samples were then washed with MQ water, dehydrated in an ethanol series (30, 60, 80, 90, and 100%) for 10 min per concentration, and left overnight in 100% ethanol. The samples were air-dried for at least 1 h and coated with approximately 5 nm of iridium before imaging. Ga droplet size was measured using Image J software. SEM-EDS was performed at 20 kV using an Oxford X-Max20 EDX Detector attached to the FEI Verios SEM. Data processing was performed using Aztec software (Oxford Instruments, UK).

[0199] Transmission electron microscopy (TEM): Transmission electron micrographs were obtained using a JEOL F200 microscope at an accelerating voltage of 200 kV. The Gatan Digital Micrograph 3.43.3213.0 software suite was used for imaging and analysis using a Gatan Rio16 4k charge-coupled device camera (model 1816). TEM-EDS was performed using an attached Oxford X-Maxn 80T X-ray detector and an attached EDS Oxford X-Maxn 80T X-ray spectrometer running Aztec software. Data processing was performed using Aztec software (Oxford Instruments, UK).

[0200] X-ray diffraction (XRD): XRD was performed using a Bruker D4 diffractometer. XRD samples were prepared on glass substrates. XRD was performed in the 2θ range of 5° to 85°, with a step size of 0.02° and a duration of 0.1 s.

[0201] Results and Discussion Characterization of Ga droplets Ga droplets were generated from bulk Ga liquid metal (LM) in MQW water by probe sonication. Probe sonication generates nanobubbles that collapse on themselves, generating enough energy to break small fragments of Ga LM from the bulk. These fragments of liquid Ga rapidly form an oxide layer, which prevents them from bonding and allows the formation of microparticles and nanoparticles with a liquid core and a solid shell, referred to as Ga droplets. Scanning electron microscope (SEM) images show that the Ga droplets are primarily spherical or globular particles. Synthesis via probe sonication can increase the solution temperature, which increases the rate of GaOOH formation. GaOOH is crystalline, causing the droplets to resemble rods. To mitigate GaOOH formation, the probe sonication procedure was set in a water bath with an appropriate off time to allow heat dissipation. SEM and X-ray diffraction (XRD) analysis confirmed that the generated particles were primarily smooth, without significant GaOOH formation. Due to the stochastic nature of the synthesis method, the Ga droplets produced had a large size variation, ranging from less than 100 nm to several microns in diameter. The median droplet diameter was 180 nm.

[0202] Functionalization of Ga droplets The Ga droplets were functionalized with Cu using a galvanic replacement reaction, which occurs when a more reactive metal is combined with a metal salt of a less reactive metal. This drives the exchange of electrons, causing the more reactive metal to form ions in solution and the less reactive metal ions to become the solid metal.

[0203] Briefly, Ga droplets were added to a 0.1 M solution of copper sulfate (CuSO4) or silver nitrate (AgNO3) for 10 minutes and then thoroughly washed by centrifugation to remove traces of metal ions in the solution. The resulting particles are hereafter referred to as Cu-Ga and Ag-Ga. The relevant standard reduction potential equations compared to the standard hydrogen electrode (SHE) are provided below.

[0204] Copper-Gallium

number

[0205] Silver-Gallium

number

[0206] The driving forces for these reactions can be determined from the following equations: ΔG=-nFE°Cell (C7) where ΔG is the Gibbs free energy, n is the number of moles of electrons in the balanced redox equation, and F is Faraday's constant (96485.33 C mol -1 ), where E° is the cell potential. Therefore, ΔG for Cu-Ga and Ag-Ga are -503.65 kJ / mol and -384.98 kJ / mol, respectively. In both cases, ΔG<0, indicating a spontaneous reaction.

[0207] Analysis of the morphology and surface structure of the formed Cu-Ga particles by SEM revealed what appeared to be small crystalline growths on the droplet surface, indicative of Cu nanoparticles (NPs). The particle surfaces were rough, likely due to a native oxide layer of Ga, or a Cu or CuGa2 layer. Transmission electron microscopy (TEM) analysis corroborated the SEM images, revealing a Ga core with a surface coating consisting at least in part of Cu-based NPs. High-resolution TEM (HR-TEM) indicated the presence of a crystalline structure with a d-spacing of approximately 0.21 nm, likely corresponding to Cu(111). However, the presence of additional intermetallic species such as Ga2Cu, GaCu2, or Ga4Cu9 is also possible. XRD analysis was also performed, which indicated that the crystalline species were primarily pure copper, consistent with the HR-TEM results. EDS imaging further confirmed the presence of Ga and Cu on the particles. A basic schematic of the particles is provided in Figure 20.

[0208] In contrast to the Cu-Ga particles, the Ag-Ga particles took on several different morphologies. SEM images showed the presence of nano-coarse particles with Ag nanoflakes and Ag nanorods on the surface of the Ga droplets. HR-TEM revealed a crystalline structure with a d-spacing of 0.14 nm, which may correspond to AgGa(210), although pure silver may also be present. XRD analysis was also performed, which indicated that the crystalline species was primarily pure silver. EDS imaging further confirmed the presence of Ga and Ag on the particles. A schematic diagram of the three broad types of particles observed is provided in Figure 21.

[0209] The results with both Cu and Ag indicate that the galvanic synthesis method produces Cu- or Ag-containing phases on the surface of the Ga droplets, which retain the Ga metallic core intact. The resulting droplets do not contain solid Ga-Cu or Ga-Ag alloy or intermetallic particles encapsulated within the Ga droplets, as produced in previous examples.

[0210] Those skilled in the art will appreciate that the invention described herein is susceptible to variations and modifications other than those specifically described, and it is to be understood that the invention includes all such variations and modifications which fall within the spirit and scope of the invention.

Claims

1. 1. A method for producing ammonia, comprising: providing a catalyst comprising supported metal microdroplets, the metal microdroplets comprising (i) at least one low melting point metal element selected from gallium and indium, and (ii) at least one promoter metal element selected from Groups 1-2 and 7-11 metals; contacting the catalyst with a gas comprising dinitrogen and dihydrogen at a reaction temperature sufficiently high so that the metal microdroplets comprise a liquid metal alloy of the at least one low melting point metal element and the at least one promoter metal element, thereby reacting the dinitrogen and dihydrogen to form ammonia.

2. The method of claim 1 , wherein the at least one promoter metal element is completely dissolved in the liquid metal alloy at the reaction temperature.

3. 3. The method of claim 1 or 2, wherein the reaction temperature is greater than 300°C.

4. 4. The method of claim 1, wherein at least 50% of the metal microdroplets comprise (i) a solid metal alloy or intermetallic compound enriched with the at least one promoter metal element, and (ii) a second metal phase enriched with the at least one low-melting point metal element when the catalyst is at room temperature.

5. The method of any one of claims 1 to 4, wherein each metal microdroplet has substantially the same metal composition.

6. The method according to any one of claims 1 to 5, wherein the at least one low melting point metal element comprises gallium.

7. 7. The method of any one of claims 1 to 6, wherein the at least one promoter metallic element is selected from the group consisting of copper, silver, gold, nickel, palladium, platinum, cobalt, iron, ruthenium, and manganese.

8. 8. The method of any one of claims 1 to 7, wherein the at least one promoter metal element is selected from copper, iron, and ruthenium.

9. 9. The method of claim 1, wherein the metal microdroplets comprise the at least one promoter metal element in an amount of 0.5 to 10 wt. %, based on the total weight of the metal microdroplets.

10. The method according to any one of claims 1 to 9, wherein the metal microdroplets are predominantly between 100 nm and 10 μm in size.

11. 1. A catalyst comprising supported metal microdroplets, the metal microdroplets comprising: (i) at least one low melting point metal element selected from gallium and indium; and (ii) at least one promoter metal element selected from Groups 1-2 and 7-11 metals, wherein at least a proportion of the metal microdroplets comprises: (i) a solid metal alloy or intermetallic compound enriched in the at least one promoter metal element; and (ii) a second metal phase enriched in the at least one low melting point metal element.

12. 12. The catalyst of claim 11, wherein the solid metal alloy or intermetallic compound enriched with the at least one promoter metal element is present in one or more solid particles confined inside the metal microdroplets, and the surface of the metal microdroplets is provided by the second metal phase enriched with the at least one low melting point metal element.

13. 13. The catalyst of claim 11 or 12, wherein at least 50% of the metal microdroplets comprise the solid metal alloy or intermetallic compound and the second metal phase.

14. 14. The catalyst of any one of claims 11 to 13, wherein the metal microdroplets comprise a liquid metal alloy of the at least one low melting point metal element and the at least one promoter metal element when the catalyst is heated to a temperature of about 400°C or higher, and the at least one promoter metal element is completely dissolved in the liquid metal alloy.

15. 15. The catalyst of any one of claims 11 to 14, wherein each metal microdroplet has substantially the same metal composition.

16. 16. The catalyst of any one of claims 11 to 15, wherein the at least one promoter metal element is selected from the group consisting of copper, silver, gold, nickel, palladium, platinum, cobalt, iron, ruthenium, and manganese.

17. Catalyst according to any one of claims 11 to 16, wherein the at least one promoter metal element is selected from copper, iron and ruthenium.

18. 18. The catalyst of any one of claims 11 to 17, wherein the metal microdroplets comprise the at least one promoter metal element in an amount of 0.5 to 10 wt %, based on the total weight of the metal microdroplets.

19. 1. A method for producing a catalyst, comprising: contacting a metal alloy with a high temperature solvent, the metal alloy comprising (i) at least one low melting point metal element selected from gallium and indium, and (ii) at least one promoter metal element selected from Groups 1-2 and 7-11 metals; breaking up the liquid metal alloy into metal micro-droplets of the metal alloy in the high temperature solvent by applying ultrasonic energy to the metal alloy in the high temperature solvent, the high temperature solvent being at a droplet formation temperature above the melting point of the metal alloy and above the melting point of any intermetallic compounds that may form therein; Separating the metal microdroplets from the hot solvent; and supporting said metal microdroplets on a solid support.

20. 20. The method of claim 19, further comprising cooling the hot solvent containing the metal microdroplets below the melting point of the metal alloy and / or any intermetallic compounds that may form therein.

21. 21. The method of claim 19 or 20, wherein the metal alloy forms an emulsion with the hot solvent.

22. 22. The method of any one of claims 19 to 21, wherein the droplet formation temperature is greater than 300°C.

23. 23. The method of any one of claims 19 to 22, wherein the high temperature solvent comprises at least one of hexadecane, oleic acid, at least one ionic liquid, at least one chloride salt, at least one molten nitrate and / or nitrite salt, at least one molten carbonate salt, at least one alkali metal acetate salt, at least one high temperature compatible hydrocarbon and / or fluorocarbon, at least one fat, or mixtures thereof.

24. 24. The method of any one of claims 19 to 23, wherein the high temperature solvent comprises a molten salt system in which the metal alloy is substantially immiscible.

25. 25. The method of any one of claims 19 to 24, wherein the high temperature solvent comprises at least one alkali metal acetate.

26. 26. The method of any one of claims 19 to 25, wherein at least 50% of the metal microdroplets comprise (i) a solid metal alloy or intermetallic compound enriched with the at least one promoter metal element, and (ii) a second metal phase enriched with the at least one low-melting point metal element when the metal microdroplets are cooled to room temperature.

27. The method of any one of claims 19 to 26, wherein each metal microdroplet has substantially the same metal composition.

28. 28. The method of any one of claims 19 to 27, wherein the at least one promoter metallic element is selected from the group consisting of copper, silver, gold, nickel, palladium, platinum, cobalt, iron, ruthenium, and manganese.

29. 29. The method of any one of claims 19 to 28, wherein the at least one promoter metal element is selected from copper, iron, and ruthenium.

30. 30. The method of any one of claims 19 to 29, wherein the metal microdroplets comprise the at least one promoter metal element in an amount of 0.5 to 10 wt %, based on the total weight of the metal microdroplets.

31. 31. A reaction system for ammonia synthesis comprising a catalyst according to any one of claims 11 to 18, or a catalyst produced by the method of any one of claims 19 to 30, and contained within a reaction chamber configured to receive a gas comprising dinitrogen and dihydrogen for reacting over the catalyst to produce ammonia.

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