Method for reducing gaseous carbon oxide

Abstract

The present invention provides a method for reducing gaseous carbon oxide to carbon, comprising mixing a gas containing gaseous carbon oxide with a liquid metal composition by creating a dynamic interface between the gas and the liquid metal composition, wherein during mixing, the liquid metal composition is not dispersed as droplets in a liquid carrier, and the gaseous carbon oxide reacts stoichiometrically with at least one metal in the liquid metal composition to form carbon and metal oxides.

Description

Detailed Description of the Invention

[0001] [PRIORITY CROSS REFERENCE] [1] This invention claims priority to Australian Provisional Patent Application No. 2021903596, filed November 10, 2021, the contents of which are to be understood as incorporated herein by reference.

[0002] [Technical field] [2] The present invention relates to a method for reducing gaseous carbon oxides, such as carbon dioxide, to carbon. The method includes the steps of mixing a gas containing the gaseous carbon oxide with a liquid metal composition, whereby the gaseous carbon oxide reacts stoichiometrically with a metal in the liquid metal composition to form carbon and a metal oxide. The invention also relates to a system for reducing gaseous carbon oxides to carbon.

[0003] [Background of the invention] [3] The urgent need to reduce greenhouse gas emissions has led to the development of technologies to convert carbon dioxide into environmentally friendly and potentially economically valuable products, such as solid carbon. However, the direct decomposition of carbon dioxide to carbon is difficult due to the unfavorable thermodynamics of this conversion. Thermal decomposition of the highly stable carbon dioxide molecule requires large amounts of energy input and reaction temperatures in excess of 2000 K.

[0004] [4] To date, there have been significant efforts to reduce the reaction temperature required for the carbon dioxide decomposition reaction using heterogeneous catalysts. Despite some advances, such as the use of oxygen-deficient metal oxide-based catalysts, practical implementation has not been achieved due to low conversion rates, rapid deactivation of solid catalysts by coking, and the difficulty of separating the solid carbon product from the catalyst.

[0005] [5] Liquid metals, which are not susceptible to coking, have been used previously to promote the catalytic decomposition of carbon dioxide to carbon. In an electrocatalytic approach, reported by Esrafilzadeh in Nature Communications 2019,10,865, a liquid metal cathode was used to reduce carbon dioxide dissolved in a molecular liquid electrolyte. However, the conversion of carbon dioxide could only be obtained by including a metal susceptible to oxidation (cerium) in the liquid metal composition and continuously regenerating the oxidized metal by-products that accumulated on the liquid metal cathode surface to their metallic form by electrochemical reduction.

[0006] [6] Another approach, reported by Tang et al. in Adv. Mater. 2021, 2105789, used mechanical energy to disperse liquid metal droplets in a molecular liquid containing dissolved carbon dioxide, decompose the carbon dioxide to carbon at the droplets' surface, and continuously reduce the metal oxide by-products that accumulate on the surface of the liquid metal droplets in a triboelectrochemical reduction process that relies on the presence of solid co-contributors (gallium-silver rods) in the reaction medium. Thus, the components at the surface of the dispersed droplets act as a catalytic system in the reactor, converting carbon dioxide in situ to carbon and molecular oxygen (O2). In the absence of either a solid co-contributor or continuous mechanical energy input (via sonication), no conversion of carbon dioxide was obtained.

[0007] [7] Although these processes are of theoretical interest, a practical process for carbon dioxide decomposition would ideally involve simple chemical reactions that do not require sufficient electrical or mechanical energy input to drive electrocatalytic or triboelectrochemical reaction mechanisms. Such a process could be carried out in simple chemical reactors and operated under process conditions that are not constrained by the need for a molecular liquid carrier or the requirement to continuously regenerate oxidized by-products.

[0008] [8] Carbon monoxide is a toxic gas that needs to be removed from gas streams in many applications for safety reasons. Because carbon monoxide is more reactive than carbon dioxide, a wide range of options are available to treat this molecule. Nevertheless, it would also be desirable to develop technologies to convert carbon monoxide to carbon that avoid the problems of deactivation, separation, and process complexities associated with the use of solid catalysts or (tribo)electrochemically mediated reaction mechanisms.

[0009] [9] Thus, there is currently a need for a method for reducing gaseous carbon oxides, such as carbon dioxide and carbon monoxide, to carbon to at least partially address one or more of the above-mentioned shortcomings or provide a useful alternative.

[0010]

[10] A reference in this specification to a patent document or other matter offered as prior art should not be construed as an admission that such document or matter was known or that the information it contains was part of the common general knowledge as of the priority date of any claim.

[0011] [Summary of the Invention]

[11] The present inventors have discovered that gaseous carbon oxides, including carbon dioxide and carbon monoxide, can be directly decomposed to carbon by contacting the gas containing the carbon oxides with a liquid metal composition, i.e., a molten reduced metal or metal alloy composition, under conditions that provide a dynamic interface between the gas and the liquid metal composition. Thus, the metal elements present in the liquid metal composition react stoichiometrically with the carbon oxides to produce solid carbon with the formation of metal oxide by-products.

[0012]

[12] The dynamic interface between the liquid and gas phases can be generated by any suitable method, such as bubbling a gas through the liquid metal composition, mechanically agitating the liquid metal composition in the presence of a gas, spraying droplets of the liquid metal composition through a gas, etc. Without wishing to be limited to any theory, it is proposed that this approach avoids or mitigates problems associated with the accumulation of deactivation by-products at either the solid interface (e.g., solid catalyst) or the liquid metal interface (e.g., the relatively static surface of the liquid metal cathode or dispersed liquid metal droplets). Instead, fresh liquid metal is constantly exposed to a pure liquid-gas interface, and solid by-products, including carbon and metal oxides, are automatically liberated.

[0013]

[13] The approach disclosed herein avoids or mitigates many of the difficulties associated with the catalytic decomposition of carbon oxides over solid catalysts. First, the process involves the stoichiometric oxidation of the metals in the liquid metal composition, which offers significant thermodynamic advantages over catalytic decomposition approaches. In fact, with the appropriate selection of the liquid metal composition, the carbon-forming reaction is exothermic. Thus, as long as a corresponding low-melting-point metal composition is used, moderate reaction temperatures, such as below 400° C., are suitable, and in some embodiments, it is even possible to decompose carbon dioxide to carbon at ambient temperatures.

[0014]

[14] Second, the disclosed methods are typically highly selective towards solid carbon, and relatively low reaction temperatures are suitable, minimizing or avoiding undesirable competing reactions, such as the retro-Boudouard reaction that produces carbon monoxide by-product.

[0015]

[15] Third, the carbon-forming reaction does not depend on a solid-gas or solid-liquid interface, which would be susceptible to coking. Instead, the reaction is believed to occur either in the liquid phase or at the liquid-gas interface. Thus, the formation of carbon-containing solid products during the process does not cause rapid deactivation, and stable conversion rates can be maintained over long periods of time.

[0016]

[16] Fourth, because the carbon and metal oxide reaction products are both solids and generally have significantly lower densities than the liquid metal composition, the reaction products are buoyant and naturally migrate to the surface of the liquid metal composition. Thus, the reaction products do not interfere with the ongoing conversion of the carbon oxide reactants within the bulk of the liquid metal composition and can be easily separated from the remaining liquid metal composition.

[0017]

[17] The approach disclosed herein also avoids or mitigates many of the difficulties associated with electrocatalytic or triboelectrochemical decomposition of carbon oxides in liquid metals. First, no electrical or mechanical energy input is required to drive the catalytic carbon dioxide decomposition or to regenerate the reaction by-products in situ. Instead, the process is typically a simple chemical reaction that proceeds spontaneously, i.e., with no energy input, except for the small amount required to create a dynamic interface between the gas and liquid metal compositions.

[0018]

[18] Second, there is no need for a liquid carrier, such as a molecular liquid or other non-metallic liquid phase, to carry the dispersed liquid metal phase or to dissolve the carbon dioxide. Instead, the carbon dioxide is in gaseous form and directly contacted with the liquid metal composition at the liquid-gas interface. The lack of a molecular liquid requirement also allows a wider range of process conditions to be used. For example, the methods disclosed herein can be operated at high temperatures, e.g., above 200° C., that are not readily compatible with many molecular liquids.

[0019]

[19] The carbon formation reaction of the methods disclosed herein is a stoichiometric reaction, in contrast to catalytic approaches that directly convert carbon dioxide to carbon and dioxygen (O2). However, the metal oxide by-product may be optionally recovered, reduced to metallic form, and recycled to the liquid metal composition. In such an embodiment, the overall process, including separate carbon dioxide decomposition and metal oxide reduction steps, can be considered a chemical looping process in which the reactive metals in the liquid metal composition are involved in the reaction cycle but are not consumed. Advantageously, the challenges associated with the solid product processing (i.e., carbon formation) and the endothermic reaction processing (i.e., metal oxide regeneration) are separated, and both steps can be optimized separately in dedicated process sections. Furthermore, since the reduction of the metal oxide can be performed after separation from the carbon product, the presence of non-reducible solid materials does not prevent the regeneration of the metal.

[0020]

[20] According to a first aspect, the present invention provides a method for reducing gaseous carbon oxides to carbon, the method comprising the steps of mixing a gas containing gaseous carbon oxides with a liquid metal composition by creating a dynamic interface between the gas and the liquid metal composition, wherein during mixing the liquid metal composition is not dispersed as droplets in a liquid carrier, and the gaseous carbon oxides react stoichiometrically with at least one metal in the liquid metal composition to form carbon and metal oxides.

[0021]

[21] In some embodiments, creating a dynamic interface between the gas and the liquid metal composition comprises at least one selected from the group consisting of bubbling the gas through the liquid metal composition, mechanically agitating the liquid metal composition in the presence of the gas, and spraying droplets of the liquid metal composition through the gas.

[0022]

[22] In some embodiments, the liquid metal composition is liquid at 600°C, 400°C, or 200°C, such as 100°C, e.g., at room temperature.

[0023]

[23] In some embodiments, the gas is mixed with the liquid metal composition at a reaction temperature below 600°C, below 500°C, etc.

[0024]

[24] In some embodiments, the gas is mixed with the liquid metal composition at a reaction temperature below 400°C.

[0025]

[25] In some embodiments, the gas is mixed with the liquid metal composition at a reaction temperature of at least 200°C.

[0026]

[26] In some embodiments, the gas comprises gaseous carbon oxide in an amount of at least 0.1 vol.%, such as at least 1 vol.%, for example at least 10 vol.%.

[0027]

[27] In some embodiments, the method further comprises, in a process step subsequent to the formation of the carbon and metal oxide, regenerating the metal oxide to a reduced metal and recycling the reduced metal to the liquid metal composition. Regenerating the metal oxide to a reduced metal may comprise electrochemically reducing the metal of the metal oxide to form the reduced metal.

[0028]

[28] In some embodiments, the method further comprises separating the carbon from the liquid metal composition.

[0029]

[29] In some embodiments, the method further comprises separating the metal oxide from the liquid metal composition.

[0030]

[30] In some embodiments, the metal oxide and carbon are separated from the liquid metal composition as a mixture of solids. The method may further include separating the carbon in the mixture from the metal oxide. Separating the carbon in the mixture from the metal oxide may include contacting the mixture of solids with a liquid extractant, such as an aqueous acid, dissolving the metal oxide in the liquid extractant, and separating the liquid extractant containing the dissolved metal oxide from the carbon. The method may further include subjecting the liquid extractant containing the dissolved metal oxide to electrochemical reduction to form reduced metal.

[0031]

[31] In some embodiments, the carbon and metal oxides are separated from the liquid metal composition by filtration.

[0032]

[32] Thus, in an embodiment of the first aspect operating in a chemical looping mode, the method includes the steps of separating the metal oxide from the liquid metal composition; regenerating the metal oxide into reduced metal; and recycling the reduced metal into the liquid metal composition for further stoichiometric reaction with gaseous carbon oxide.

[0033]

[33] In some embodiments, the gas is mixed with the liquid metal composition in a reactor containing a column of the liquid metal composition. Creating a dynamic interface between the gas and the liquid metal composition may include bubbling the gas through the column and / or mechanically agitating the column in the presence of the gas. Carbon and metal oxides may migrate to the top surface of the column.

[0034]

[34] In some embodiments, the liquid metal composition comprises at least one metal selected from the group consisting of gallium, indium, tin, bismuth, mercury, cadmium, lead, antimony, thallium, and zinc.

[0035]

[35] In some embodiments, the liquid metal composition includes gallium. The liquid metal composition may be an alloy of gallium and indium, such as a binary alloy of gallium and indium, e.g., eutectic gallium-indium (EGaIn). Gaseous carbon oxide can react with at least the gallium in the liquid metal composition to form carbon and gallium oxide.

[0036]

[36] In some embodiments, the liquid metal composition is an alloy comprising one or more alloying metals selected from the group consisting of iron, aluminum, cobalt, nickel, copper, zinc, cerium, magnesium, calcium, scandium, titanium, vanadium, chromium, manganese, strontium, yttrium, niobium, molybdenum, barium, gadolinium, and hafnium. In some embodiments, the liquid metal composition is an alloy comprising one or more alloying metals selected from the group consisting of Ce, Mg, Al, Ca, Sc, Ti, V, Cr, Mn, Fe, Sr, Y, Nb, Mo, Ba, Gd, and Hf. The one or more alloying metals may be present in an amount less than 10 wt.%.

[0037]

[37] In some embodiments, the gaseous carbon oxide reacts with at least one of the alloying metals to form carbon and an oxide of the alloying metal.

[0038]

[38] In some embodiments, the one or more alloying metals may include iron and be present in an amount less than 10 wt.%.

[0039]

[39] In some embodiments, the gaseous carbon oxide comprises carbon dioxide.

[0040]

[40] In some embodiments, the gaseous carbon oxides are reduced to carbon with a selectivity of at least 90%, such as at least 95%, for example, substantially 100%.

[0041]

[41] In some embodiments, the carbon comprises graphitic carbon.

[0042]

[42] According to a second aspect, the present invention provides a chemical looping process for reducing gaseous carbon oxide to carbon, the process comprising the steps of: mixing a gas containing gaseous carbon oxide with a liquid metal composition by creating a dynamic interface between the gas and the liquid metal composition, where the gaseous carbon oxide stoichiometrically reacts with at least one metal in the liquid metal composition to form carbon and a metal oxide; separating the metal oxide from the liquid metal composition; regenerating the metal oxide to a reduced metal; and recycling the reduced metal to the liquid metal composition for further stoichiometric reaction with the gaseous carbon oxide.

[0043]

[43] The liquid metal composition is typically not dispersed as droplets in the liquid carrier during mixing. It will be understood that various embodiments of the second aspect may generally include features disclosed herein in the context of embodiments of the first aspect.

[0044]

[44] According to a third aspect, the present invention provides a system for reducing gaseous carbon oxides to carbon comprising: a source of gas comprising gaseous carbon oxide; and a reactor comprising a liquid metal composition, the reactor configured to receive the gas from the source and mix the gas with the liquid metal composition by creating a dynamic interface between the gas and the liquid metal composition, wherein during mixing the liquid metal composition is not dispersed as droplets in the liquid carrier, wherein in use the gaseous carbon oxide reacts stoichiometrically with at least one metal in the liquid metal composition to form a solid product comprising carbon and a metal oxide.

[0045]

[45] In some embodiments, the reactor is configured to create a dynamic interface between the gas and the liquid metal composition by at least one selected from the group consisting of bubbling the gas through the liquid metal composition, mechanically agitating the liquid metal composition in the presence of the gas, and spraying droplets of the liquid metal composition through the gas.

[0046]

[46] In some embodiments, the reactor contains a column of the liquid metal composition and, in use, the solid product migrates to an upper surface of the column. The reactor may be configured to mix the gas with the liquid metal composition by bubbling the gas through the column and / or mechanically agitating the column in the presence of the gas.

[0047]

[47] In some embodiments, the system further comprises a solid-liquid separator for separating the solid product from the liquid metal composition. The solid-liquid separator may comprise a filter.

[0048]

[48] ​​In some embodiments, the system further includes a carbon recovery section that separates the carbon in the solid product from the metal oxides. The carbon recovery section can receive the solid product from the solid-liquid separator. The carbon recovery section can be configured to dissolve the metal oxides in a liquid extractant, such as an aqueous acid, and separate the liquid extractant containing the dissolved metal oxides from the carbon.

[0049]

[49] In some embodiments, the system further includes a metal reduction section configured to reduce the metal of the metal oxide to form a reduced metal. The metal reduction section can receive a liquid extractant containing dissolved metal oxides from the carbon recovery section for reduction. The system can be configured to recycle the reduced metal from the metal reduction section to the liquid metal composition in the reactor. The metal reduction section can include an electrochemical cell configured to electrochemically reduce the metal of the metal oxide to form a reduced metal.

[0050]

[50] Thus, in an embodiment of the third aspect suitable for operating in a chemical looping mode, the system includes a metal reduction section that reduces the metal of the metal oxide to form a reduced metal after separation of the solid product from the liquid metal composition, and the system is configured to recycle the reduced metal from the metal reduction section to the liquid metal composition in the reactor.

[0051]

[51] In some embodiments, the liquid metal composition comprises at least one metal selected from the group consisting of gallium, indium, tin, bismuth, mercury, cadmium, lead, antimony, thallium, and zinc.

[0052]

[52] In some embodiments, the liquid metal composition includes gallium. The liquid metal composition may be an alloy of gallium and indium, such as a binary alloy of gallium and indium, for example, eutectic gallium-indium (EGaIn).

[0053]

[53] In some embodiments, the liquid metal composition is an alloy comprising one or more alloying metals selected from the group consisting of iron, aluminum, cobalt, nickel, copper, zinc, cerium, magnesium, calcium, scandium, titanium, vanadium, chromium, manganese, strontium, yttrium, niobium, molybdenum, barium, gadolinium, and hafnium. In some embodiments, the liquid metal composition is an alloy comprising one or more alloying metals selected from the group consisting of Ce, Mg, Al, Ca, Sc, Ti, V, Cr, Mn, Fe, Sr, Y, Nb, Mo, Ba, Gd, and Hf. The one or more alloying metals may be present in an amount less than 10 wt.%.

[0054]

[54] In some embodiments, the one or more alloying metals may include iron and be present in an amount less than 10 wt.%.

[0055]

[55] According to a fourth aspect, the present invention provides a chemical looping system for reducing gaseous carbon oxide to carbon, the system comprising: a source of gas comprising gaseous carbon oxide; a reactor comprising a liquid metal composition, the reactor configured to receive gas from the source and mix the gas with the liquid metal composition by generating a dynamic interface between the gas and the liquid metal composition, where in use the gaseous carbon oxide reacts stoichiometrically with at least one metal in the liquid metal composition to form a solid product comprising carbon and a metal oxide; and a metal reduction section configured to reduce a metal of the metal oxide to form a reduced metal after separation of the solid product from the liquid metal composition, the system being configured to recycle the reduced metal from the metal reduction section in the reactor to the liquid metal composition.

[0056]

[56] The liquid metal composition is typically not dispersed as droplets in the liquid carrier during mixing. It will be understood that various embodiments of the fourth aspect may generally include features disclosed herein in the context of embodiments of the third aspect.

[0057]

[57] When used in the specification (including the claims), the words "comprise", "comprises" and "comprising" are to be construed 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.

[0058]

[58] Further aspects of the present invention are set out below in the detailed description of the invention.

[0059]

[59] Embodiments of the present invention are herein described, by way of example only, with reference to the accompanying drawings. [Brief description of the drawings]

[0060] [Figure 1]FIG. 1 is a schematic diagram of a system including a bubble column reactor containing a liquid metal composition for reducing gaseous carbon oxides to carbon, according to some embodiments of the present invention. [Diagram 2] FIG. 2 is a block flow diagram of a system for reducing gaseous carbon oxides to carbon in a reactor, including regeneration and recycling of the metal oxide by-product to a liquid metal composition, according to some embodiments of the present invention. [Diagram 3] FIG. 3 is a block flow diagram of a system for reducing gaseous carbon oxides to carbon using a sacrificial metal reductant in a liquid metal composition, according to some embodiments of the present invention. [Figure 4] FIG. 4 shows a schematic of the laboratory-scale bubble column reactor system used in the examples. [Diagram 5] FIG. 5 is a graph showing the carbon production rate obtained in Example 2 by bubbling a gas containing CO2 through a column of EGaIn liquid metal at 200° C. [Figure 6] FIG. 6 is a graph showing the carbon production rate and carbon selectivity obtained by bubbling a gas containing CO2 through a column of liquid gallium at 200° C. in Example 3. [Figure 7] FIG. 7 is a graph showing the carbon production rate and carbon selectivity obtained in Example 4 by bubbling a gas containing CO2 through a column of EGaIn liquid metal at various temperatures between 100° C. and 500° C. [Figure 8] FIG. 8 is an Arrhenius plot of the carbon decomposition reaction carried out in Example 4 by bubbling a gas containing CO2 through a column of EGaIn liquid metal at various temperatures from 100° C. to 500° C. [Figure 9] FIG. 9 shows XPS spectra of the Ga 3d region obtained on the surface of the EGaIn liquid metal before and after exposure to CO 2 in Example 5. [Figure 10] FIG. 10 shows XPS spectra in the C 1s region obtained on the surface of the EGaIn liquid metal before and after exposure to CO 2 in Example 5. [Figure 11] FIG. 11 shows the Raman spectrum of the graphitic carbon product produced by decomposing carbon dioxide with liquid metal EGaIn, measured in Example 6. [Figure 12] FIG. 12 is a graph comparing the carbon production rates obtained in Example 7 by bubbling a gas containing CO or CO2 through a column of EGaIn liquid metal at 200° C. [Figure 13] FIG. 13 is a graph comparing the carbon production rates obtained by bubbling a gas containing CO2 through a column of EGaIn liquid metal at 200° C. with and without the iron additive in Example 8. [Figure 14] FIG. 14 is a graph comparing the carbon production rates obtained by bubbling a gas containing CO through a column of EGaIn liquid metal at 200° C. or 400° C. in the presence and absence of various alloying metals in Example 9. [Figure 15] FIG. 15 shows an X-ray diffraction spectrum of EGaIn liquid metal containing 5 wt. % iron after contact with carbon dioxide to produce carbon, indicating the presence of the FeGa3 intermetallic alloy measured in Example 9. [Figure 16] FIG. 16 shows the XPS spectrum of the Fe 2p region obtained at the surface of the ferroalloy EGaIn liquid metal after exposure to CO 2 in Example 10. [Figure 17] FIG. 17 is a graph comparing the current-potential response curves obtained by the electroreduction of an aqueous acid solution containing Ga 3+ ions with an equivalent acid solution without metal ions, as measured in Example 9.

[0061] [Detailed Description]

[77] The present invention relates to a method for reducing gaseous carbon oxides, i.e., carbon dioxide or carbon monoxide, to elemental carbon. The method includes mixing a gas containing gaseous carbon oxides with a liquid metal composition by creating a dynamic interface between the gas and the liquid metal composition. Under such conditions, the gaseous carbon oxides react stoichiometrically with at least one metal in the liquid metal composition to form carbon and metal oxide by-products.

[0062] Gas containing gaseous carbon oxide

[78] The gas mixed with the liquid metal composition includes gaseous carbon oxide, which may include carbon dioxide, carbon monoxide, or a mixture of carbon dioxide and carbon monoxide. The gaseous carbon oxide can be present in any amount, such as at least 0.1% by volume, or at least 1% by volume, such as at least 10% by volume. In some embodiments, the gas is primarily, e.g., greater than 50% by volume, or is substantially entirely composed of gaseous carbon oxide.

[0063]

[79] The gas may contain other gas components, such as inert gases, such as dinitrogen (N2) and argon. Depending on the gas source, it may contain dihydrogen (H2), low molecular weight hydrocarbons (methane, ethane, ethylene, etc.), methanol, ethanol, acetic acid, H2S, SO x , NO x Other molecules such as , HO may also be present. In some embodiments, the gas contains little or no dioxygen (O), which may preferentially react with the liquid metal composition. In some embodiments, the gas contains less than 1% by volume, preferably less than 0.1% by volume, O. In some embodiments, the gas contains less than 1% by volume, preferably less than 0.1% by volume, HO. In some embodiments, the gas does not contain O or HO.

[0064] liquid metal composition

[80] The gas, including the gaseous carbon oxide, is mixed with a liquid metal composition. As used herein, a liquid metal composition refers to a metal composition that includes one or more metals (i.e., metallic elements) in a molten state. Liquid metal compositions include both pure liquid metals (PLMs), which include a single metallic element, and alloy liquid metals (ALMs), which generally include multiple metallic elements. Like solid metals, liquid metal compositions are characterized by metallic bonds. Metal atoms in the molten state donate electrons to an electron cloud that is shared throughout the bulk of the metal composition, surrounding positively charged metal ions and forming metallic bonds. The delocalized electrons are free to interact with electric fields, thermal energy, and light, providing liquid metal compositions with high electrical and thermal conductivity despite the absence of a lattice structure.

[0065]

[81] The liquid metal compositions of the present disclosure can exist in a molten state at temperatures lower than many common metals. Thus, they can be used at the reaction temperatures favorable for converting gaseous carbon oxides to carbon. Thus, in some embodiments, the liquid metal compositions are liquid at 600° C. or 500° C., i.e., their melting points are less than 600° C. or 500° C., respectively. In some embodiments, the liquid metal compositions are liquid at 400° C. or 200° C. In some embodiments, they are liquid at all temperatures between 200° C. and 400° C., which is the temperature range favorable for carbon dioxide conversion. In some embodiments, the liquid metal compositions are liquid at 100° C. or at room temperature (i.e., about 20° C.). Metal compositions with such very low melting points, while not necessary for reaction with carbon oxides, have the added advantage that they can be handled with reduced risk of solidification.

[0066]

[82] A range of low melting point liquid metals have been reported for various chemical processing and heat transfer applications. In principle, such metal compositions are suitable for use in the present process, subject to the requirement that they contain at least one metal that reacts with gaseous carbon oxides, or that such a metallic element is soluble. The only pure liquid metals reported so far are gallium (T melt =29.8℃), Indium (T melt =156.8℃), Tin (Tmelt =231.9℃), Bismuth (T melt =271.4℃), and mercury (T melt =-33.8℃). Zinc (T melt = 419.5 °C) may also be suitable. A non-limiting example of a low melting alloy liquid metal is eutectic gallium-indium (EGaIn) (T melt =15.0℃) and eutectic gallium-indium-tin (Galinstan) (T melt = 13.2 °C; T freeze =-19.0℃).

[0067]

[83] In some embodiments, the liquid metal composition comprises at least one metal selected from the group consisting of gallium, indium, tin, bismuth, mercury, cadmium, lead, antimony, thallium, and zinc. In some embodiments, the liquid metal composition comprises gallium. Gallium is believed to be particularly suitable due to its low melting point and low toxicity. Furthermore, the inventors have experimentally demonstrated that elemental gallium in the liquid metal composition reacts with gaseous carbon oxide to form carbon and gallium oxide, even at low reaction temperatures. The liquid metal composition may be pure gallium or a gallium alloy. In some embodiments, the liquid metal composition is an alloy comprising gallium and indium, for example a binary alloy of gallium and indium, such as eutectic gallium-indium (EGaIn).

[0068]

[84] The liquid metal composition includes at least one metal that reacts stoichiometrically with gaseous carbon oxide to form carbon and a metal oxide. The reactive metal can be present as a low melting point metal component of the liquid metal composition. For example, gallium is useful for both its low melting point properties and its reactivity with gaseous carbon oxide. Alternatively, the reactive metal can be an alloy-forming metal that is dissolved in the liquid metal composition specifically for its reactivity with gaseous carbon oxide. One example of a suitable metal that reacts preferentially with carbon dioxide, for example compared to gallium, is cerium (Ce). Based on experimental results, it is believed that iron (Fe) and aluminum (Al) are also oxidized by carbon oxide in the presence of gallium. More generally, based on calculated reaction enthalpies, the range of metals that are expected to react with carbon dioxide when present in the liquid metal composition includes Ce, Mg, Al, Ca, Sc, Ti, V, Cr, Mn, Fe, Sr, Y, Nb, Mo, Ba, Gd, and Hf.

[0069]

[85] In some embodiments, the liquid metal composition is an alloy including one or more low melting point metals in combination with one or more alloying metals. The alloying metals can be included to promote the decomposition of gaseous carbon dioxide to carbon, for example by acting as reactive metals or by catalyzing the reaction of carbon oxide with other metals. In some embodiments, the alloying metals are selected from the group consisting of iron, aluminum, cobalt, nickel, copper, zinc, cerium, magnesium, calcium, scandium, titanium, vanadium, chromium, manganese, strontium, yttrium, niobium, molybdenum, barium, gadolinium, and hafnium. The alloying metals can be present as minor components of the liquid metal composition, for example in amounts less than 10 wt%, less than about 5 wt%, or less than 2 wt%. This is typically necessary to allow dissolution of the alloying metals in the liquid metal composition without unacceptably increasing the melting point.

[0070]

[86] In some embodiments, the one or more alloying metals include iron, aluminum, or cerium, suitably iron, preferably in an amount of less than 10 wt%, such as about 5 wt% or less. The inventors have found through experimentation that the addition of iron to EGaIn in an amount of about 5 wt% approximately doubles the rate of carbon formation compared to EGaIn without iron. Without wishing to be limited to any theory, it is proposed that the higher reaction rate results because iron is preferentially oxidized compared to gallium and / or because iron catalyzes the oxidation of gallium.

[0071]

[87] The use of alloying metals that are preferentially oxidized by gaseous carbon oxides compared to the low melting point metal components of the liquid metal composition can provide the opportunity to use metal additives as sacrificial reductants in some scenarios. The metal additives are dissolved in the liquid metal composition, oxidized to metal oxide form by carbon oxides, and removed from the process. Advantageously, the inventory of low melting point metals in the liquid metal composition is not thus depleted, and the sacrificial metal additives can be replenished as needed. This can be an attractive option for metals such as iron that can be obtained from low-cost sources such as scrap iron.

[0072]

[88] Alternatively, as described in more detail below, the metal oxide produced by the stoichiometric reaction of the metal with gaseous carbon oxide can be recovered and regenerated to form the reduced metal for reuse in the liquid metal composition. Thus, in such a chemical looping process, the metal is not consumed.

[0073]

[89] In use, the liquid metal composition may exist as a single-phase liquid. However, it is not excluded that the liquid metal composition may also contain solid components, such as intermetallic phases, so long as the metal composition as a whole has substantially liquid properties.

[0074] Mixing through dynamic interfaces

[90] The method of the present disclosure includes mixing a gas, including gaseous carbon oxide, with a liquid metal composition by creating a dynamic interface between the gas and the liquid metal composition, such that the gaseous carbon oxide stoichiometrically reacts with at least one metal in the liquid metal composition to form carbon and metal oxide.

[0075]

[91] As used herein, a dynamic interface between a gas and a liquid metal composition means that the gaseous form of carbon oxide is in direct contact with the liquid metal composition at a gas-liquid interface where it is regenerated by exposing fresh metal from the bulk of the liquid metal composition at the interface during the reaction. This dynamic regeneration of the gas-liquid interface preferably continues during the ongoing reaction to maintain a pure metal surface. The dynamic interface between the gas and the liquid metal composition can be generated by any suitable means, including passing bubbles of gas through the liquid metal composition, agitating the liquid metal composition in the presence of the gas so that the surface is continually disturbed, e.g., repeatedly spraying the liquid metal composition in the form of droplets into the gas, and the like.

[0076]

[92] It will be appreciated that mass transfer between the gas and the liquid metal composition may, at least in some circumstances, limit the reaction rate of carbon oxide conversion. Mass transfer limitations can be reduced or eliminated using known engineering principles, such as increasing the gas-liquid interfacial area (e.g., by controlling the size of the gas bubbles, increasing the mechanical agitation rate, or using a gas-entraining mixer) or by increasing the total or partial pressure of the carbon oxide in the gas.

[0077]

[93] This reaction does not depend on a molecular solvent being used to dissolve the gaseous carbon oxide and contact the gaseous carbon oxide with the liquid metal composition at the liquid-liquid interface. Thus, in embodiments, the gas is mixed with the liquid metal composition in the absence of an organic solvent. In embodiments, the gas is mixed with the liquid metal composition in the absence of a molecular liquid or in the absence of a non-metallic liquid phase.

[0078]

[94] The liquid metal composition typically exists as a bulk liquid reaction medium in the reactor, and thus, in contrast to prior art approaches, is not dispersed as droplets in a liquid carrier (i.e., a molecular liquid or other non-metallic liquid in which the liquid metal composition is immiscible), nor does it remain on the surface of an electrode, as would be expected in the prior art case.

[0079]

[95] Thus, in some embodiments, the gas is mixed with the liquid metal composition in a reactor that contains a column of the liquid metal composition. As used herein, column of liquid refers to a bulk inventory of the liquid metal composition that fills the reactor to a particular height. In such embodiments, creating a dynamic interface between the gas and the liquid metal composition may include bubbling the gas through the column of liquid and / or mechanically agitating the column of liquid in the presence of the gas.

[0080]

[96] The advantage of this arrangement is that the carbon and metal oxides are generally solids with a lower density than the liquid metal composition and therefore will automatically migrate to the upper surface of the liquid column. Thus, the solid products are continuously separated from the dynamic gas-liquid interface and can be easily removed from the liquid metal composition.

[0081]

[97] The mixing is carried out at a temperature suitable to obtain the desired reaction between the gaseous carbon oxide and the reactive metal in the liquid metal composition. In some embodiments, the gas is mixed with the liquid metal composition at a reaction temperature of less than 600° C., or less than 500° C., or less than 400° C. The inventors have found through experimentation that higher reaction temperatures increase the conversion of carbon oxide, but that selectivity to carbon may decrease above a threshold temperature of 400° C.-500° C., where the reverse Boudouard reaction begins.

[0082]

[98] In some embodiments, the gas is mixed with the liquid metal composition at a reaction temperature of at least 200°C. The methods disclosed herein can be advantageously carried out at such high temperatures because they do not require a molecular liquid reaction medium. In some embodiments, the gas is mixed with the liquid metal composition at a reaction temperature in the range of about 200°C to about 400°C.

[0083]

[99] The gas can be mixed with the liquid metal composition at ambient pressure. However, it is found that higher reaction rates can be obtained at higher total pressures and higher carbon oxide partial pressures.

[0084]

[0100] When a gas is mixed with a liquid metal composition at a dynamic gas-liquid interface, a stoichiometric reaction occurs between the gaseous carbon oxide and the metal in the liquid metal composition to form carbon and metal oxides. The reaction rate is enhanced by the dynamic mixing process, which maintains a pure metal surface. In contrast, when the gas contacts a static surface of the liquid metal composition, reaction is expected to be limited or nonexistent due to mass transfer limitations and accumulation of solid reaction products at the gas-liquid interface.

[0085]

[0101] The reaction is a stoichiometric reaction between gaseous carbon oxide and at least one metal in the liquid metal composition to form carbon and metal oxide. In other words, the reacting metal in the liquid metal composition is oxidized in a stoichiometric reaction, with each reacting metal atom being converted to the corresponding metal oxide according to the expected stoichiometry of the reaction with carbon oxide. There is believed to be little or no spontaneous or triboelectrochemical reduction of the metal oxide to regenerate the reduced metal in situ and produce molecular oxygen (O2). Thus, in at least some embodiments, when gaseous carbon oxide reacts stoichiometrically with at least one metal in the liquid metal composition to form carbon and metal oxide, no O2 is produced. It is not necessary to mechanically or electrically stimulate the liquid metal composition to reduce the metal oxide in situ. Thus, in embodiments, the liquid metal composition is not stimulated by sonication (e.g., ultrasonication) during the reaction. The reaction is also not driven by the in situ electrochemical reduction of the metal oxide by-product. Thus, in embodiments, no current flows through the liquid metal composition during the reaction.

[0086]

[0102] In some embodiments, the gaseous carbon oxide is reduced to carbon with a selectivity of at least 90%, or at least 95%, or substantially 100%. It has been found that very high selectivities can be obtained by maintaining the reaction temperature below a threshold temperature at which competing reactions, such as the reverse Boudouard reaction, become significant. In some embodiments, the carbon product comprises graphitic carbon. It is expected that the morphology of the carbon can be controlled to some extent based on the reaction conditions. Thus, the carbon product can also be amorphous, partially oxidized, sheet-like, or scroll-like.

[0087] separation

[0103] The methods of the present disclosure may further include separating the carbon product from the liquid metal composition and / or separating the metal oxide by-product from the liquid metal composition.

[0088]

[0104] Metal oxides and carbon are typically separated from the liquid metal composition as a mixture of solids. In embodiments where the reaction occurs in a reactor containing a column of liquid metal composition, the solid reaction products (including carbon and metal oxides) typically accumulate together at or on top of the surface of the column in a separate porous layer that increases in thickness as the reaction proceeds. Thus, the solid product mixture can be separated from the liquid metal composition by simply removing the solids from the liquid surface. It is envisioned that with appropriate reactor design, this can be done continuously during the reaction. Alternatively, the carbon and / or metal oxides can be separated from the liquid metal composition by any suitable means, such as by filtration at the end of the reaction period.

[0089]

[0105] Separation of carbon from the liquid metal composition, typically with metal oxides, can be accomplished by a number of methods. In some embodiments, the mixed solids are skimmed off the surface of the liquid metal column. This can be accomplished by gravity-assisted methods, for example, where the reactor is designed to allow the solids to overflow from the vessel holding the liquid metal column for transfer to the next unit operation. Alternatively, cyclone separation can be used. Because the carbon produced is typically "fluffy" (low bulk density), air currents generated by positive pressure (blower) or negative pressure (suction) can carry the carbon through a whirlwind motion. Other means of transporting the solid product, such as a screw conveyor, are also envisioned.

[0090]

[0106] In another variation, the reactor includes a second liquid phase that floats as a separate layer on top of the liquid metal composition column. Suitably, the second liquid phase may be a molten salt phase. Carbon produced in the reactor is less dense than either liquid phase and therefore migrates to the top surface of the second liquid phase and is automatically physically separated from the liquid metal composition by the thickness of the layer of the second liquid phase.

[0091]

[0107] In some embodiments, the method includes separating the carbon from the metal oxides, for example as a step subsequent to the initial separation of the solid reaction products from the liquid metal composition. The separation can be carried out by any suitable means. In some embodiments, a liquid extractant, such as an aqueous acid (e.g., sulfuric or hydrochloric acid), is used to extract the metal oxides from the carbon. The metal oxides are dissolved in the liquid extractant and separated from the residual carbon by conventional solid-liquid separation techniques, such as filtration. The carbon is then optionally subjected to further processing steps to remove residual contaminants and produce a dry final product.

[0092] Metal Regeneration

[0108] In the methods disclosed herein, the gaseous carbon oxide is reduced by the metal in the liquid metal composition to form carbon, which in turn is oxidized to form a metal oxide. Thus, as the reaction proceeds, it will be apparent that the reactive metal element in the liquid metal composition will gradually be depleted. It may therefore be necessary to replace all or part of the liquid metal composition in the reactor or replenish its reactive metal component. In some embodiments, this is done by regenerating the metal oxide to a reduced metal form and returning the reduced metal to the liquid metal composition. As used herein, "reduced metal" refers to a metal in its zero-valent, i.e. metallic, form.

[0093]

[0109] The metal oxide can be regenerated to reduced metal in a process step following the carbon formation reaction. That is, the metal regeneration is carried out as a separate process step separated in time and / or space from the carbon formation reaction, so that the oxidation of the metal (during the carbon formation reaction) and the reduction of the metal oxide (during regeneration) do not occur simultaneously under a common set of process conditions. Thus, the reaction conditions and process equipment for the carbon formation reaction and the metal regeneration reaction can be configured and optimized separately. The subsequent regeneration process step can be carried out after the step of first separating the metal oxide from the liquid metal composition, as already disclosed herein. Thus, the metal regeneration step may be carried out separately from the liquid metal composition, in a separate process equipment. Such a process can be considered a chemical looping process, in which the reactive metal components of the liquid metal composition are not consumed throughout the process, since they are circulated (looped) between the reaction process step and the regeneration process step. However, it is not excluded that the metal oxide can be regenerated in the presence of the residual liquid metal composition, e.g., as a process step continuous in time (e.g., intermittent) in the same reactor in which the carbon formation previously occurred.

[0094]

[0110] The metal oxide can be regenerated by electrochemically reducing the metal of the metal oxide to form a reduced metal and recycling it to the liquid metal composition. In some embodiments, the metal oxide is first dissolved in an aqueous composition, such as the aqueous acid used to separate carbon from the metal oxide as disclosed herein. The metal ions dissolved in the aqueous composition are then electrochemically reduced to metal at the cathode of an electrochemical cell. The electrochemical reduction route advantageously provides the opportunity to use regenerative energy to drive the overall conversion of gaseous carbon oxide to carbon without total consumption of the liquid metal composition.

[0095]

[0111] The inventors have experimentally demonstrated that gallium ions can be electrochemically reduced to metallic gallium in aqueous electrolytes, since the gallium reduction reaction begins at a significantly lower negative potential than the hydrogen evolution reaction. It is anticipated that a range of other metals can be similarly regenerated by electrochemical processing, as described herein.

[0096]

[0112] Alternatively, the metal oxide can be regenerated by a chemical reduction process: the metal of the metal oxide can be chemically reduced, for example, using dihydrogen (H) or a hydrocarbon such as methane as a reducing agent to form the reduced metal.

[0097]

[0113] Although it may be attractive to regenerate and reuse the metal oxide in this manner, it should be understood that this is not necessary in all scenarios. Alternatively, a sacrificial metal suitable for preferential oxidation, for example an alloying metal such as iron, aluminum, or cerium, can be dissolved in the liquid metal composition. The resulting metal oxide can be discarded or otherwise processed without being returned to the liquid metal composition. The liquid metal composition can be regenerated as needed, for example continuously or intermittently, by dissolving more sacrificial metal in the metal formulation. System for reducing gaseous carbon oxide to carbon

[0098]

[0114] The present invention also relates to a system for reducing gaseous carbon oxide to carbon. The system includes a source of gas including gaseous carbon oxide and a reactor including a liquid metal composition. The reactor is configured to receive the gas from the source and mix the gas with the liquid metal composition by creating a dynamic interface between the gas and the liquid metal composition. In use, the gaseous carbon oxide reacts stoichiometrically with at least one metal in the liquid metal composition to form a solid product including carbon and a metal oxide.

[0099]

[0115] 1 shows a schematic diagram of a system 100 for reducing gaseous carbon oxides to carbon. System 100 includes a bubble column reactor 102 connected to a source 103 of gas 104 containing gaseous carbon oxides, e.g., CO2. Reactor 102 contains a column of liquid metal composition 106, e.g., gallium or a low melting point alloy thereof. Reactor 102 may optionally include either or both of a heater to heat liquid metal composition 104 to a target reaction temperature and a cooler to remove heat of reaction during an ongoing exothermic reaction (not shown).

[0100]

[0116] In use, gas 104 is supplied from source 103 to reactor 102 via gas distributor 108, which causes bubbles 110 to pass through a column of liquid metal composition 104 (which may be maintained at a temperature in the range of 200° C. to 400° C.). Preferably, the gas distributor generates small bubbles to maximize the surface area of ​​the dynamic interface between the gas and the liquid metal composition. As shown in FIG. 1, distributor 108 includes only a single gas inlet, but it will be understood that a gas distributor for a bubble column may include multiple gas inlets configured to generate a well-mixed bubble phase within the column. Thus, reactor 102 is configured to mix gas 104 and liquid metal composition 106 at the dynamic interface between the gas and the liquid metal composition that is generated as the bubbles pass through the column. After bubbles 110 are liberated from the liquid metal composition through top surface 114 into headspace 112 of the bubble column, residual gas 116 exits the reactor via outlet port 118. Optionally, gas 116 can be recovered and a portion of it recycled to the gas fed to the reactor 104. Thus, the gas passes through the column multiple times, increasing the overall conversion of gaseous carbon oxides.

[0101]

[0117] Because the gas 104 and the liquid metal composition 106 are contacted at a dynamic (and therefore pure) gas-liquid interface, the gaseous carbon oxide reacts efficiently with at least one metal, e.g., gallium, in the liquid metal composition 104. This results in the formation of a solid product 120 that includes a mixture of carbon and metal oxide (e.g., gallium oxide). The solid product is less dense than the liquid metal composition and therefore buoyantly migrates to the top surface 114 of the column. After a period of time, as indicated by arrow 122, the solid product 120 accumulates as a discrete porous layer on top of the top surface 114 and can be easily separated from the liquid metal composition if desired.

[0102]

[0118] The disclosed system may include additional process sections configured to separate the solid product from the liquid metal composition, separate the carbon in the solid product from the metal oxides, and reduce the metal oxides to reduced metals for recycling to the reactor. Figure 2 is a block flow diagram of a system 200 configured in this manner.

[0103]

[0119] The chemical looping system 200 includes a reactor 202 configured to receive a gas 204 comprising a gaseous carbon oxide, e.g., CO2, from a source 203. The reactor 202 includes a liquid metal composition 206 and is configured to mix the gas 204 with the liquid metal composition by creating a dynamic interface between the gas and the liquid metal composition. The reactor 202 may be a bubble column reactor 102, as described herein, or any other reactor type configured to create a suitable dynamic gas-liquid interface, such as a stirred vessel. During use, the gaseous carbon oxide in the gas 204 stoichiometrically reacts with at least one metal in the liquid metal composition to form a solid product 220 comprising carbon and a metal oxide.

[0104]

[0120] The chemical looping system 200 further includes a solid-liquid separator 230 for separating the solid product 220 from the liquid metal composition 206. The reactor contents 232, including the remaining liquid metal composition 206 and the solid product 220, can be transferred to an external solid-liquid separator 230, as shown in FIG. 2. The external solid-liquid separator may include a filter. The filtered liquid metal composition 206a is then returned to the reactor 202. Alternatively, the solid-liquid separator 230 can be integrated with the reactor 202, and the solid product 220 may be removed directly from the reactor, while leaving the liquid metal composition intact, for example, by skimming the solids from the surface.

[0105]

[0121] The chemical looping system 200 further includes a carbon recovery section 234 for receiving the solid product 220 from the solid-liquid separator and separating the carbon 236 in the solid product from the metal oxides. This may be done by supplying a liquid extractant 238, which may be an aqueous acid (such as aqueous H2SO4 or aqueous HCl), to the carbon recovery section 234 to mix with the solid product 220. Thus, the metal oxide components dissolve in the liquid extractant and the metal-containing liquid extractant 239 is then separated from the carbon, for example by filtration. The carbon 236, which may be graphitic in nature, may be optionally further processed to remove residual impurities and exits the process as the final product.

[0106]

[0122] The chemical looping system 200 further includes a metal reduction section 240 that receives the metal oxide of metal from the carbon recovery section 234, for example, in a metal-containing liquid extractant 239, and reduces the metal ions therein to form reduced metal 242. The metal reduction section may include an electrochemical cell configured to electrochemically reduce the metal ions to the reduced metal. Alternatively, the metal reduction section includes a reduction reactor configured to chemically reduce the metal oxide of metal to the reduced metal using a reducing agent, for example, dihydrogen or a hydrocarbon (e.g., methane). The reduced metal 242 produced in the metal reduction section 240 is then recycled to the liquid metal composition 206 of the reactor 202.

[0107]

[0123] In chemical looping system 200, the reactive metals in the liquid metal composition are sequentially oxidized and reduced in the reactor and recycled back to the liquid metal composition. Advantageously, therefore, the reactive metals are not consumed throughout the process. A further advantage of system 200 is that the carbon oxide decomposition reaction that produces solid carbon products is separated from the metal reduction reaction, which requires the introduction of energy to drive the overall endothermic reaction. Thus, both reactions can be optimized separately in dedicated process sections.

[0108]

[0124] It will be appreciated that system 200 can be operated stepwise in a series of batch reactions, or continuously, or any combination thereof. For example, reactor 202 can be operated continuously or semi-continuously with continuous or intermittent removal of solid product 220 and reintroduction of reduced metals 242. The solid product can be processed into final carbon products and recycled metals in a batch, semi-continuous, or continuous process.

[0109]

[0125] In the variation shown in FIG. 3 as system 300, the metal reducer 240 is omitted. Thus, the reactive metal in the liquid metal composition 206 may be a sacrificial reduced metal 242b that is dissolved into the liquid metal composition 206 as needed to replenish the reactive metal content. In the reactor 202, the metal 242b is preferentially oxidized to a metal oxide and thus selectively depleted from the liquid metal composition. The sacrificial metal exits the process as an oxidized metal product 239b that is not reduced or recycled to the reactor 202.

[0110] [Example]

[0126] The present invention will now be described with reference to the following examples, which should be understood as illustrating but not limiting the invention described herein.

[0111] material

[0127] Gallium (Ga) and indium (In) were provided by Roto Metals and listed as 99.99% purity. Iron was used with a purity of 97%. All provided chemicals were used as received.

[0112] Example 1. Preparation of EGaIn alloy

[0128] Eutectic gallium-indium alloy (EGaIn) was prepared by combining stoichiometric amounts of Ga and In (75% Ga and 25% In by weight). The constituent metals were mixed on a hotplate at approximately 200°C (higher than the melting point of In) until the metals were completely melted and appeared well mixed. The alloy was then allowed to cool naturally. EGaIn has a melting point of 15.5°C, so it is in a molten state at room temperature.

[0113] Example 2. CO with EGaIn in a bubble column reactor 2 Decomposition

[0129] A bubble column reactor (30 cm long, 1.0 cm internal diameter) was constructed from quartz to withstand activity testing at high operating temperatures, provide high visibility, and provide corrosion resistance (liquid metal is corrosive to other metals). The gas flow rate to the column was regulated by a mass flow controller (Bronkhorst EL-FLOW; MFC) and checked with a check valve to prevent backflow of reactants. To analyze the gas products, continuous gas chromatography measurements were performed using a PerkinElmer Clarus 580 online GC. Heating was provided by an external split furnace equipped with a K-type thermocouple and connected to a temperature controller (TC). A schematic diagram of the reaction system is shown in Figure 4.

[0114]

[0130] The reactor was charged with EGaIn (about 86 g, prepared in Example 1) and heated to 200° C. A mixture of CO2 and Ar (1:2 v / v) was bubbled continuously through the liquid metal phase at a flow rate of about 12 sccm at ambient pressure. A pure supply of CO2 would be expected to facilitate mass transfer during the reaction, but the CO2 supply was balanced with Ar to allow operation within the detection range of the on-line gas chromatograph.

[0115]

[0131] The formation of carbon was visually observed, accumulating at the top of the liquid metal column in the reactor. The carbon formation rate increased and progressed as a function of time over a period of 150 min, reaching a steady carbon production rate of 319 μmol / hr (Figure 5). No other gaseous products were detected in the reactor exhaust gases, except for unconsumed CO2. The reactor and its contents were allowed to cool naturally after the reaction, and the carbon products were collected from the top of the liquid metal column for characterization.

[0116]

[0132] A second reaction was carried out under the same conditions for 24 hours to determine whether deactivation could be observed over a longer period of time. The carbon production rates at 1, 4, and 24 hours are shown in Table 1. It can be seen that no deactivation occurs for at least 24 hours.

[0117] [Table 1]

[0118] Example 3. CO with Ga in a bubble column reactor 2 Decomposition

[0133] Except for using pure gallium as the liquid metal, a CO2 decomposition reaction was carried out in the same manner as in Example 2. Pure gallium has a melting point of 29.8°C, so care was taken to prevent the gallium from solidifying in the reactor.

[0119]

[0134] As can be seen in Figure 6, similar results were obtained using pure Ga as well as EGaIn in Example 2. The selectivity to solid carbon was 100% and a steady-state carbon production rate of approximately 300 μmol / h was again obtained.

[0120]

[0135] The results indicate that gallium is the metallic element that is oxidized in both reactions, as shown in equation (1). Ga is expected to be preferentially oxidized over In due to its higher reduction potential. 4Ga+3CO2→2Ga2O3+3C (1)

[0121] Example 4. Investigation of temperature effects

[0136] A series of CO2 decomposition reactions in EGaIn were carried out following the same method as in Example 2, except that various reaction temperatures were used, ranging from ambient temperature (room temperature) to 500°C. As seen in Figure 7, the carbon production rate increases with temperature. The rate constant (k) was calculated based on the steady-state decomposition rate and correlated with temperature by the Arrhenius model, as seen in Figure 8. The activation energy for the decomposition of CO2 to solid carbon in EGaIn was empirically calculated to be 8.39 kJ / mol. However, in this series of experiments, it appears that the CO2 decomposition reaction may operate in two regimes. At low temperatures, i.e., 100°C and 200°C, the reaction is under a kinetically controlled regime, while the reaction becomes mass-transfer limited at higher temperatures.

[0122]

[0137] As can also be seen in FIG. 7, the selectivity to solid carbon remained quantitative up to 400°C. However, at 500°C, significant selectivity to carbon monoxide (CO) was observed. This can be attributed to the reverse Boudouard reaction shown in equation (2). Therefore, the selectivity to carbon can be maximized by remaining below the threshold temperature of 400°C-500°C where the reverse Boudouard reaction becomes significant. 3CO2+C→2CO (2)

[0123]

[0138] It is described that CO2 can be activated and decomposed in EGaIn even at room temperature. In a bubble column reaction at room temperature for 24 hours, a very large amount of carbon was formed on top of the EGaIn column in the form of a separate solid layer (carbon mixed with gallium oxide) more than 30 mm thick at the top of the column.

[0124] Example 5. Basic research - oxidation of EGaIn

[0139] To elucidate the mechanism of CO2 reduction, in-situ XPS analysis was performed on the surface of EGaIn liquid metal using a KratosAxis Supra XPS spectrometer equipped with a monochromator Al X-ray source (hv=1486.6 eV) and a concentric hemispherical electron analyzer. Obtaining a pure metal surface of EGaIn is essential to make a meaningful comparison between the surfaces before and after CO2 exposure. To this end, EGaIn liquid metal was prepared, filled in a glove box under nitrogen flow, and transferred to the analysis cell using an air-sensitive transporter. Using this filling process and maintaining ultra-high vacuum conditions, a pure metal surface was maintained for analysis before CO2 exposure. During the analysis, the ambient pressure of the analysis and reaction chambers was kept at 1.0 × 10, respectively. -8 and 5.0 x 10 -7 The vacuum was kept below 100 mbar and scans were recorded using a pass energy of 40 eV. The in-situ operating system replicates the reaction conditions of the bubble column experiments by introducing a CO2 / Ar gas supply at atmospheric pressure and heating the satellite gas reaction chamber up to 200 °C. Without breaking vacuum, the surface of EGaIn could be investigated by XPS before and after CO2 exposure.

[0125]

[0140] XPS measurements showed that after 30 min of exposure to CO2, the Ga oxide content increased, while In remained metallic (see Figure 9 showing the XPS spectrum in the Ga 3d region). The metallic Ga-In ratio at the surface remained constant. The increase in Ga oxide content is consistent with the findings from the bubble column experiments and indicates that CO2 reduction can be achieved using pure liquid Ga.

[0126]

[0141] The formation of surface carbon was also detected, as can be seen in Figure 10, which shows the XPS spectrum in the C1s region. Before the CO2 exposure, the presence of adventitious carbon on the surface is noted. Since this was unavoidable, the increase in the carbon peak after the CO2 exposure is considered to indicate the formation of carbon during the process. The formation of other carbon species was not detected.

[0127] Example 6. Characterization of the solid product

[0142] Scanning electron microscope (SEM) images and energy dispersive X-ray (EDX) elemental mapping of the products obtained after CO2 decomposition (200 °C, 4 h reaction) were collected using an FEI Verios 460L equipped with an Oxford XMax30 EDS Detector. Images and maps were collected using an Elstar in-lens secondary electron detector (TLD-SE) and an accelerating voltage of 30 kV, respectively.

[0128]

[0143] Elemental map analysis using EDX indicated that the carbon to oxygen ratio in the solid product was approximately 1:2 and the gallium to oxygen ratio was 2:3. These results are consistent with the expected stoichiometry of the gallium-mediated decomposition of CO2 to solid carbon and gallium oxide, as shown in equation (1).

[0129]

[0144] Raman spectra of the solid carbon products were acquired using a Horiba LabRam HR Evolution equipped with a 532 nm laser. The relative intensity ratio (I D / I G ) is the D band (1325 cm ) obtained from the Raman spectrum. -1 ) and G band (1600 cm -1 The intensity of the incident light is calculated based on the measured intensity of the incident light.

[0130]

[0145] Relative Intensity (I D / I G =0.295), and the disorder-induced D band in the Raman spectrum (1325 cm -1 ) and G band (1600cm -1 ) are characteristic of graphitic carbon (see FIG. 11).

[0131] Example 7. CO decomposition of EGaIn in a bubble column reactor

[0146] Decomposition reactions were carried out following the same procedure as in Example 2, except that the feed gas was carbon monoxide (instead of carbon dioxide) as a mixture of CO and Ar (1:9 v / v). A comparative reaction was carried out using a mixture of CO2 and Ar (1:9 v / v).

[0132]

[0147] As can be seen in Figure 12, CO was found to decompose to carbon at a significantly higher rate than CO2. At low reactant concentrations, the conversion of CO2 was too low to be accurately detected by GC, yet carbon formation was observed visually. The results show that both carbon monoxide and carbon dioxide can be decomposed to solid carbon with liquid metal containing gallium. In the case of carbon monoxide, the reaction follows equation (3): 2Ga+3CO→Ga2O3+3C (3)

[0133] Example 8. CO of Ga-In-Fe alloy in a bubble column reactor 2 Decomposition

[0148] Iron metal was dissolved in EGaIn to prepare an iron-containing liquid metal in an amount of 5 wt% Fe (Fe-EGaIn).Then, a decomposition reaction was carried out according to the same method as in Example 2, except that Fe-EGaIn was used as the liquid metal in the bubble column reactor.

[0134]

[0149] The results compared to iron-free EGaIn are shown in Figure 13. The addition of iron doubled the steady-state carbon formation rate. The selectivity towards carbon was also quantitative. Without wishing to be bound by any theory, it is proposed that iron is preferentially oxidized compared to gallium, resulting in a higher reaction rate.

[0135] Example 9. CO in Ga-In with various alloying metal Fe alloys in a bubble column reactor 2 Decomposition

[0150] A liquid metal containing 5 wt. % of an alloying metal, either zinc, aluminum, or iron, was prepared by dissolving the alloying metal in EGaIn. The decomposition reaction was then carried out according to the same method as in Example 2, except for the liquid metal containing the alloying metal in the bubble column reactor.

[0136]

[0151] The results compared to EGaIn without any alloying metal are shown in Figure 14. At 200°C, the steady-state carbon formation rate was significantly increased by the addition of iron or aluminum, while zinc had no positive effect. The selectivity towards carbon was again quantitative. Compared to gallium, the results are consistent with the preferential oxidation of iron and aluminum by CO2.

[0137]

[0152] At 400 °C, the carbon formation rate was very high when EGaIn containing 5 wt% Fe was used, confirming the beneficial effect of using alloying metals with good susceptibility to oxidation by CO. Carbon was also formed when EGaIn contained iron in amounts of 2.5 wt% and 7.5 wt%.

[0138]

[0153] The carbon produced by reacting EGaIn with 5 wt% Fe and CO2 at 400°C was analyzed by Raman spectroscopy and showed the D and G bands characteristic of graphitic carbon.

[0139]

[0154] Analysis of the liquid metal alloy after the reaction using X-ray diffraction (XRD) showed the formation of FeGa3 intermetallic alloy, as can be seen in FIG.

[0140] Example 10. Basic research - oxidation of EGaIn containing iron

[0155] To elucidate the mechanism of CO2 reduction by EGaIn containing 5 wt% iron, in-situ XPS analysis was performed to investigate the species on the surface of the alloy liquid metal when in contact with CO2, as described in Example 5. XPS measurements showed that iron oxide formed on the surface after 30 min of exposure to CO2 (see Figure 16 showing the XPS spectrum in the Fe 2p region). The formation of Fe oxide indicates that iron can be oxidized by CO2 when included in a liquid metal composition, and suggests that iron is preferentially oxidized over gallium when included in a gallium-based liquid metal composition.

[0141] Example 9. Electrochemical reduction of gallium oxide

[0156] The electrical reduction of gallium oxide is 0.28 cm 2 The measurements were performed on a glassy carbon electrode with an exposed area of ​​1.0 mm. Gallium oxide was dissolved in an aqueous solution containing 1 M H2SO4. Linear sweep voltammetry measurements were performed using a CHI680 Amp Booster. The potential was measured against the aqueous solution using Ag / AgCl as the reference electrode and a platinum wire as the counter electrode.

[0142]

[0157] As can be seen in Figure 17, the onset potential of the gallium deposition reaction was seen at significantly less negative potentials (vs. Ag / AgCl) when compared to the onset of the hydrogen evolution reaction for a gallium-free 1 M H2SO4 reference solution. This finding highlights the feasibility of regenerating the reacted gallium and closing the cycle in a chemical looping process.

[0143]

[0158] Gallium reduction was also carried out in a two-electrode system (glassy carbon working electrode, platinum wire electrode as counter and reference electrodes) that more closely resembles a possible implementation process. Herein, an onset voltage of -1.57 V was measured, with a potential of 20 mA / cm 2 A current density of 1000 nm was obtained at −2.5 V. The use of an electrochemical approach to reduce the produced gallium oxide allows the use of established reactor designs that are compatible with renewable energy sources.

[0144]

[0159] It is also demonstrated that oxidized gallium ions can be electroreduced to gallium metal from an aqueous gallium chloride solution with high faradaic efficiency (~70%). The results also demonstrate that gallium oxide can be extracted into hydrochloric acid and electrolytically regenerated to form gallium metal in a chemical looping process.

[0145]

[0160] 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 understood that the invention includes all such variations and modifications within the spirit and scope of the invention.

Claims

1. 1. A method for reducing gaseous carbon oxide to carbon, comprising: mixing a gas containing gaseous carbon oxide with a liquid metal composition by creating a dynamic interface between the gas and the liquid metal composition, wherein during mixing, the liquid metal composition is not dispersed as droplets in a liquid carrier, and the gaseous carbon oxide reacts stoichiometrically with at least one metal in the liquid metal composition to form carbon and a metal oxide.

2. The method of claim 1 , wherein the liquid metal composition is liquid at 400° C.

3. The method of claim 1 , wherein the gas is mixed with the liquid metal composition at a reaction temperature of less than 500° C.

4. The method of claim 1 , wherein the gas is mixed with the liquid metal composition at a reaction temperature of at least 200° C.

5. 10. The method of claim 1, further comprising, in process steps subsequent to the formation of the carbon and the metal oxide, regenerating the metal oxide into a reduced metal and recycling the reduced metal into the liquid metal composition.

6. 6. The method of claim 5, wherein the step of regenerating a metal oxide to a reduced metal comprises electrochemically reducing the metal of the metal oxide to form the reduced metal.

7. The method of any one of claims 1 to 6, further comprising separating the metal oxide and the carbon as a mixture of solids from the liquid metal composition.

8. 8. The method of claim 7, further comprising contacting the mixture of solids with a liquid extractant, dissolving the metal oxides in the liquid extractant, and separating the liquid extractant containing dissolved metal oxides from the carbon.

9. 10. The method of claim 8, further comprising subjecting the liquid extractant containing dissolved metal oxides to electrochemical reduction to form reduced metals.

10. 7. The method of claim 1, wherein the gas is mixed with the liquid metal composition in a reactor containing a column of the liquid metal composition, and the carbon and metal oxides migrate to an upper surface of the column.

11. 11. The method of claim 10, wherein the step of creating a dynamic interface between the gas and the liquid metal composition comprises bubbling the gas through the liquid column and / or mechanically agitating the liquid column in the presence of the gas.

12. 7. The method of any one of claims 1 to 6, wherein the liquid metal composition comprises at least one metal selected from the group consisting of gallium, indium, tin, bismuth, mercury, cadmium, lead, antimony, thallium, and zinc.

13. 7. The method of any one of claims 1 to 6, wherein the liquid metal composition comprises gallium, and the gaseous carbon oxide reacts with at least the gallium in the liquid metal composition to form carbon and gallium oxide.

14. 7. The method of any one of claims 1 to 6, wherein the liquid metal composition is an alloy comprising one or more alloying metals selected from the group consisting of Ce, Mg, Al, Ca, Sc, Ti, V, Cr, Mn, Fe, Sr, Y, Nb, Mo, Ba, Gd, and Hf, and the gaseous carbon oxide reacts with at least one of the alloying metals to form carbon and an oxide of the alloying metal.

15. The method of claim 14 , wherein the one or more alloying metals comprises iron.

16. 1. A chemical looping process for reducing gaseous carbon oxides to carbon, comprising: mixing the gas, including gaseous carbon oxide, with a liquid metal composition by creating a dynamic interface between the gas and the liquid metal composition, wherein the gaseous carbon oxide stoichiometrically reacts with at least one metal in the liquid metal composition to form carbon and metal oxide; Separating the metal oxide from the liquid metal composition; regenerating the metal oxide into a reduced metal; recycling the reduced metal to the liquid metal composition for further stoichiometric reaction with the gaseous carbon oxide; A chemical looping process, including

17. 1. A system for reducing gaseous carbon oxides to carbon, comprising: a source of gas comprising gaseous carbon oxide; a reactor containing a liquid metal composition, the reactor configured to receive the gas from the source and mix the gas with the liquid metal composition by creating a dynamic interface between the gas and the liquid metal composition, wherein during mixing, the liquid metal composition is not dispersed as droplets in a liquid carrier; Including, In use, the system wherein said gaseous carbon oxide reacts stoichiometrically with at least one metal in said liquid metal composition to form a solid product comprising carbon and metal oxide.

18. 18. The system of claim 17, wherein the reactor contains a column of the liquid metal composition, and wherein, in use, the solid product migrates to an upper surface of the column, and the reactor is configured to mix the gas with the liquid metal composition by bubbling the gas through the column and / or mechanically agitating the column in the presence of the gas.

19. further comprising a carbon recovery section for separating the carbon in the solid product from the metal oxide; 20. The system of claim 17, wherein the carbon recovery section is configured to dissolve the metal oxides in an aqueous acid liquid extractant and separate the liquid extractant containing dissolved metal oxides from the carbon.

20. Further comprising a metal reducing unit that reduces the metal of the metal oxide to form a reduced metal; 20. The system of claim 17, wherein the reactor is configured to recycle the reduced metal from the metal reduction unit to the liquid metal composition, and the metal reduction unit includes an electrochemical cell configured to electrochemically reduce the metal of the metal oxide to form the reduced metal.

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