Low temperature co2 reduction

Abstract

A process and an apparatus for reducing CO2 gas to solid carbon and oxygen that comprises contacting CO2 gas with any one of a Mg-Ga, Ce-Ga, and Li-Ga liquid metal alloy at a temperature less than 85°C and reducing CO2 gas to solid carbon and oxygen.

Description

[0001] LOW TEMPERATURE CO2 REDUCTION

[0002] TECHNICAL FIELD

[0003] The present invention relates to reducing CO2 to solid carbon and oxygen.

[0004] BACKGROUND

[0005] Climate change is driving a fundamental re-evaluation of future options across a broad spectrum of energy-intensive industries.

[0006] In addition to finding ways of preventing fossil-derived CO2 generation and release, there is increasing pressure to find cost-effective ways to actively remove CO2 from the atmosphere.

[0007] There is much that can (and is) being done using plants, trees, soil microbiology, ocean organisms etc to lock up additional atmospheric CO2 in biological structures, but much more needs to be done.

[0008] SUMMARY OF THE DISCLOSURE

[0009] The invention provides a process for reducing CO2 gas to solid carbon and oxygen that comprises contacting CO2 gas with any one of a Mg-Ga, Ce-Ga, and Li-Ga liquid metal alloy at a temperature less than 85°C and reducing CO2 gas to solid carbon and oxygen.

[0010] In this process, in the case of an Mg-Ga liquid metal alloy, Mg plays a major role in driving the reduction of CO2 to its elemental constituents, carbon and oxygen.

[0011] During the process, Mg ions diffuse to the gas-liquid interface and reduce CO2 to carbon while undergoing an oxidation reaction. To close the catalytic cycle, Mg2+is reduced back to its metallic counterpart using electrochemical methods.

[0012] The term “solid carbon” is understood herein to refer to carbon in a solid state that may contain some residual oxygen (generally < 20%, typically < 15%, by weight on a dry basis). This oxygen comes predominantly from the original carbon dioxide from which the solid carbon was produced.

[0013] The process may comprise forming the liquid metal alloy and then contacting the alloy with CO2 gas at the temperature less than 85°C for a sufficient time to achieve a required level of reduction of CO2 to solid carbon.

[0014] The process temperature may be less than 80°C, typically less than 70°C, typically less than 50°C, typically less than 45°C, and more typically less than 40°C.

[0015] The process may comprise forming droplets, for example sprays of droplets, of liquid metals moving downwardly counter-current to an upward flow of CCb-containing gas so that the drops and the gas come into contact so that Mg in the droplets reduces CO2 to solid carbon.

[0016] The process may comprise contacting CO2 gas with the liquid metal alloy under any suitable pressure.

[0017] By way of example, the process may be carried out under atmospheric pressure .

[0018] By way of further example, the process may be carried out under pressure conditions greater than atmospheric pressure.

[0019] The process may comprise recovering solid carbon.

[0020] The process may comprise forming oxides of Mg or Ce or Li.

[0021] The process may comprise reducing oxides of Mg or Ce or Li to Mg or Ce or Li for reuse in the process.

[0022] The liquid metal alloy may comprise up to 3 wt.% of any one of Mg, Ce and Li in the liquid metal alloy. The liquid metal alloy may comprise up to 2 wt.% of any one of Mg, Ce and Li in the liquid metal alloy.

[0023] The liquid metal alloy may comprise at least 1 wt.% of any one of Mg, Ce and Li in the liquid metal alloy.

[0024] The invention also provides an apparatus for converting CO2 gas to solid carbon that comprises a reactor for contacting CO2 gas with any one of a Mg-Ga, Ce-Ga, and Li-Ga liquid metal alloy at a temperature less than 85°C and reducing CO2 gas to solid carbon and oxygen.

[0025] Typically, the apparatus is configured to facilitate a large surface area of contact between any one of a Mg-Ga, Ce-Ga, and Li-Ga liquid metal and a CCh-containing gas.

[0026] One of a number of options for this is forming droplets, for example sprays of droplets, of liquid metals moving downwardly counter-current to an upward flow of the CO2- containing gas.

[0027] The apparatus may comprise a chamber, an inlet for supplying any one of a Mg-Ga or Ce-Ga or Li-Ga liquid metal alloy at a temperature less than 85°C to the chamber, an inlet for supplying a CCb-containing gas to the chamber to contact the liquid metal alloy in the chamber so that, in use, Mg reduces CO2 gas and forms solid carbon on surfaces of the liquid metal alloy, and an outlet for discharging carbon / liquid metal alloy from the chamber.

[0028] The apparatus may be configured so that, in use, there is sufficient time to achieve a required level of reduction of CO2 to solid carbon in the chamber.

[0029] The apparatus may be configured for supplying the liquid metal alloy as droplets.

[0030] The apparatus may be configured for supplying the liquid metal alloy as droplets that, in use, move downwardly in the chamber. The apparatus may be configured for supplying the CCh-containing gas so that in use the gas flows upwardly counter-current to the droplets of the liquid metal alloy.

[0031] The apparatus may comprise a separator unit for separating liquids and solids in the carbon / liquid metal alloy discharged from the chamber, with the liquids typically comprising Ga and unreacted Mg or Ce or Li, and the solids typically comprising carbon and other solid reaction products such as oxides of Mg or Ce or Li.

[0032] The chamber may be configured to operate at atmospheric pressure or above atmospheric pressure.

[0033] The apparatus may be configured to operate at a temperature less than 80°C.

[0034] The apparatus may be configured to operate at a temperature less than 70°C.

[0035] The apparatus may be configured to operate at a temperature less than 45°C.

[0036] The apparatus may be configured to operate at a temperature less than 40°C.

[0037] BRIEF DESCRIPTION OF THE DRAWINGS

[0038] The present invention is described further by way of two examples with reference to the accompanying drawings, of which:

[0039] Figure 1 is an overview of experimental work to assess CO2 reduction using a Mg-Ga (3 wt.%) liquid metal alloy;

[0040] Figure 2 is element composition analysis of the surface of Mg-Ga (3 wt.%) alloy before after the experimental work;

[0041] Figure 3 is a series of graphs illustrating characterization of Mg-Ga (3 wt.%) alloy after the experimental work;

[0042] Figure 4 is a graph of the current density-potential difference curve for bi-phasic electrochemical recovery of Mg2+. Linear sweep voltammetry (LSV) of PC, including dissolved Mg(C104)2 at the cathode while the anode operated in an aqueous electrolyte; Figures 5-11 are a series of SEM-EX images and TEM-EX images produced after the experimental work;

[0043] Figure 12 is an image of 0.2 wt.% Li-Ga liquid metal alloy after the reduction of CO2 in an autoclave after experimental work;

[0044] Figure 13 is a Raman spectrum after the reduction of CO2 using the Li-Ga alloy after experimental work;

[0045] Figure 14 is a series of SEM and EDX images illustrating the results after the reduction of CO2 using the Li-Ga alloy after experimental work; and

[0046] Figure 15 is a diagram of one embodiment of an apparatus for converting CO2 to solid carbon and recovering carbon.

[0047] DESCRIPTION OF EMBODIMENTS

[0048] The invention relies on diffusion of atoms of a metal component of a Mg-Ga or a Ce-Ga or a Li-Ga liquid metal alloy to a surface of the liquid metal alloy where CO2 gas contacts the surface and:

[0049] (a) there are chemical reactions with the metal atoms that reduce CO2 gas to solid carbon and oxygen, and

[0050] (b) oxygen oxidises the metal component to a metal oxide at a temperature less than 85 °C.

[0051] This is not an electrolytic process.

[0052] The invention is the result of experimental work carried out by the applicant initially into the potential use of a Mg-Ga liquid metal alloy to reduce CO2 to solid carbon and oxygen near room temperature.

[0053] After completing the laboratory work on Mg-Ga liquid metal alloys, the applicant also carried out experimental work on the potential use of a Ce-Ga and Li-Ga liquid metal alloys for the same purpose.

[0054] Context for the experimental work - Liquid metals have many unique characteristics that do not typically co-exist in any other type of materials, including their existence in the liquid state, relatively low viscosity, and high thermal and electrical conductivities [1, 2], Traditional liquid metals are mercury (Hg), gallium (Ga), rubidium (Rb), cesium (Cs), and francium (Fr), since they remain in a liquid state at below or near room temperature [3, 4],

[0055] Only Ga is non-toxic and considered safe to handle out of the known liquid metals.

[0056] Liquid metals are composed of free electrons and disordered ions, which makes them different from other liquids. Electrons near the interfacial regions of liquid metals can be rapidly displaced in the presence of any charge change, making the liquid metal interface a well-suited environment for the effectuation of chemical reactions [5],

[0057] - Mg has been demonstrated to convert CO2 to fuel at room temperature, without any external energy supply [6],

[0058] Opportunity

[0059] The use of low-cost and relatively abundant Mg for the reduction of CO2 is an opportunity for a viable solution for achieving net-zero emissions.

[0060] In particular, the use of liquid metal alloys for CO2 reduction reactions could enable CO2 conversion at room temperature to useful products, namely solid carbon and oxygen, providing a new approach for efficiently addressing CO2 mitigation issues.

[0061] Experimental work - Mg-Ga liquid metal alloy

[0062] 2, Results and Discussion

[0063] 2, 1 Low-temperature CO2 conversion by liquid metal

[0064] The test work was designed to reduce CO2 to solid carbon and oxygen naturally at near room temperature (40°C) using a low melting point liquid metal alloy (Mg-Ga). The synthesis of different Mg-Ga alloy samples consisting of 3 wt.% metallic magnesium was performed using a mechanical alloying approach (see alloy preparation section set out below).

[0065] With reference to Figure 1, each prepared Mg-Ga alloy sample was loaded into a CO2- filled 60 mL syringe reactor and reactions were allowed to occur at near room temperature (at 40°C and 50°C) and atmospheric pressure.

[0066] The experimental setup allowed for sufficient contact at a gas-liquid interface between the CO2 gas and the liquid metal surface in the syringe reactor.

[0067] The CO2 gas in the syringe reactor was consumed constantly, so the gas volume decreased from 60 mL to around lOmL, as shown in Figure 1 by comparing the “initial volume” image on the left hand side of the Figure and the “equilibrium” image on the right hand side of the Figure. In particular, the “equilibrium” image shows the substantial reduction in volume.

[0068] Mg atoms migrated to the surface of the alloy by self-diffusion, and the produced compounds accumulated at the surface of the liquid metal alloy, as illustrated in Figure 1. This is illustrated in the central lower image in Figure 1.

[0069] As a result of the reactions, the surface of the liquid metal changed appearance, from a metallic shiny -textured surface before the reactions to a rough black surface after the reactions. This can be seen by comparing the images on the upper left hand side and the upper right hand side of Figure 1.

[0070] Specifically, when liquid Mg-Ga was exposed to CO2 gas, a layer formed on its surface. The cross-section of the solidified alloy in the upper right side of Figure 1 shows a dark colour crust on the surface of the liquid alloy. Figure 2 provides element composition analysis of the surface of a sample of a Mg-Ga 3 wt.% alloy before after the reaction. The inset in the Figure shows an SEM micrograph of the alloy surface after equilibrium.

[0071] It is clear from Figure 2 that the sample before reaction substantially comprised Ga and small concentrations of Mg, C, and O, consistent with the sample being a Mg-Ga 3 wt.% alloy at this state. Figure 2 also shows that, after reaction, the sample comprised material concentrations of C, O, and Mg and a small concentration of Ga. This indicates that the sample, after reaction, comprises a small concentration of Ga compared to the reaction products comprising C, O, and Mg.

[0072] Scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDS) were performed to explore the surface structure and composition of the alloy before and after the reaction as shown in Figure 2.

[0073] Interestingly, the Mg content increased from ~3 wt.% to ~28 wt.%, suggesting surface enrichment of Ga alloy during the CO2 reduction.

[0074] The surface of the Mg-Ga revealed porous nature after the reaction (Figure 2 inset).

[0075] The gas composition inside the syringe reactor was analysed using gas chromatography (GC). The gas remaining content after the reaction showed the presence of 3% CO, which was produced as a gas by-product.

[0076] No composition change was found on the surface of liquid metal alloy when CO2 was replaced with an inert gas such as argon.

[0077] 2.2 Characterizations of interfacial compounds

[0078] Materials produced from CO2 reduction reactions on the surface of Mg-Ga alloy were isolated for further characterization. Raman spectroscopy was conducted to further investigate the composition of the reaction products forming on the surface of the liquid metal proceeding at 40°C and atmospheric pressure.

[0079] The results indicate the formation of a solid carbon product (D-band 1350 cm-1 and G- band 1580 cm-1) on both the 1 wt.% Mg-Ga and 3 wt.% Mg-Ga alloy samples (Figure 3a).

[0080] Moreover, Mg was shown to oxidize to MgO (peaks shown in Figure 3b at 280, 446, 1088 cm-1) [7] during the reaction process.

[0081] The formation of the carbon and MgO was confirmed to be a result of the Mg-Ga alloy's unique formulation by comparing the reaction products formed by the alloy to control experiments performed with pure Ga.

[0082] The Raman spectroscopy showed no carbon or magnesium-related peaks in the control group (Pure Ga). This is due to the pre-existing non-porous interfacial gallium oxides preventing from any further reactions.

[0083] The inset in Figure 3a shows the isolated carbon product, characterized by EDS mapping (the EDS mapping figures for other elements are shown in Figure S2), which further proves that CO2 was reduced to solid carbon by the Mg-Ga liquid metal alloy under the reaction conditions.

[0084] The surface chemistry of Mg-Ga alloy was analysed using X-ray photoelectron spectroscopy (XPS), which showed that the surface of the Mg-Ga alloys, after reaction, was mainly composed of MgO (Figure 3c). Furthermore, the XPS spectra indicated the valence of each element on the surface of the alloy after reaction (Figure 3d-g).

[0085] The existence of MgO was further confirmed by the Mg 2s and O ls peaks located at 89.4eV and 532.5eV (Figure 3e, f). The Surface EDX and XPS measurements show solid products primarily as carbon and oxides (Figures 2 and 3). Figures 5-11 illustrate a series of SEM-EX images and TEM-EX images produced after the experimental work showing the characteristics of the Mg-Ga alloy and carbon materials.

[0086] 2,3 Reduction process from MgO to Mg

[0087] From the characterization results, Mg component in the Mg-Ga liquid metal alloy was oxidized to MgO during the reaction process.

[0088] To complete the oxidation / reduction cycle of Mg-Ga liquid metal alloy, the Mg2+produced during the reaction is electrochemically reduced to Mg.

[0089] The feasibility of the reduction process was verified by conducting electrochemical reduction [9] of Mg2+in propylene carbonate (PC), including dissolved magnesium perchlorate Mg(C104)2.

[0090] Magnesium perchlorate can be produced from MgO and perchloric acid reaction [8],

[0091] Figure 4 is a graph of the current density-potential difference curve for the bi-phasic electrochemical recovery of Mg2+.

[0092] Linear sweep voltammetry (LSV) of PC, including dissolved Mg(C104)2 at the cathode while the anode operated in an aqueous electrolyte.

[0093] The onset potential of the magnesium reduction reaction was observed at a -2 V potential (vs. Ag / AgCl).

[0094] As a consequence, regenerating reacted Mg is possible and the whole reaction cycle can be closed by this method.

[0095] 3, Experimental Section

[0096] 3.1 Alloy preparation In order to produce the Mg-Ga alloy, Mg powder (99.9% trace rare-earth metals basis, Sigma- Aldrich) was added to Ga in concentrations of 3 wt.% inside a N2 filled glove box.

[0097] The mixtures were ground using mortar and pestle on a hot plate at 40 °C until the alloy developed a smooth, reflective appearance, which indicated complete dissolution of the added magnesium. This process typically took 15-30 min.

[0098] Mechanical grinding using mortar and pestle led to the breakdown of the metal powders, removal of pre-existing surface oxides, while also increasing the interface between gallium and magnesium, effectively facilitating the dissolution of Mg.

[0099] Any surface oxides that may have formed during the grinding process, due to residual oxygen levels inside the glove box, were removed by transferring the produced LM from the mortar into a storage container using a pipette, as described above.

[0100] The final Mg-Ga alloys were kept in sealed vials inside the glove box until used.

[0101] 3,2 CO2 reduction experiment

[0102] The fresh synthesized alloy was first transferred to a syringe reactor inside a nitrogen filled glove box, and the tip of the syringe to preserve the inertness of the alloy environment. After the cooling and solidification of the alloy, the syringe was removed from the N2 glove box.

[0103] Subsequently, the gas in the syringe was vacuumed, and 60 ml of CO2 was injected to the syringe (through the tip of the syringe). The syringe was then sealed again to prevent gaseous leakages. Before the reaction, the syringe was placed in a hot plate at 40°C for 5 minutes to ensure the alloy retained its liquid state. The syringe reactor was finally placed in an oven at 40°C to obtain the required reaction temperature.

[0104] Pure Ga reacted with CO2 under the same reaction conditions, was used as a control experiment. Another set of control experiments was performed with the Mg-Ga alloy in the absence of CO2 (using Ar) to determine the changes before and after reaction.

[0105] 3,3 Sample characterization

[0106] Raman spectra were obtained using a Renishaw Raman spectrometer, utilizing the 532 nm laser source. XPS was performed using a Thermo Scientific K-alpha X-ray spectrometer.

[0107] The morphology and structure of the alloys were imaged using SEM (The Hitachi FlexSEM 1000), while energy-dispersive X-ray spectroscopy (EDX) was used for elemental identification and mapping.

[0108] The gas composition in the syringe was measured by gas chromatography (GC 7890B, Agilent) equipped with thermal conductivity detector (TCD) and flame ionization detector (FID).

[0109] Experimental work - Li-Ga liquid metal alloy

[0110] Results and discussion

[0111] An experiment investigating the reduction of CO2 to carbon was also performed using a 0.2 wt.% Li-Ga alloy.

[0112] The Li-Ga alloy was inserted in an autoclave and CO2 was bubbled into the autoclave for up to 1 hour to remove air from the autoclave. CO2 was added into the autoclave to achieve a pressure of 9 bar.

[0113] The CO2 and Li-Ga alloy were heated up to 70 °C for around 2 days.

[0114] Figure 12 illustrates the image of the Li-Ga alloy after removal from the autoclave.

[0115] The alloy was washed with HC1 and high purity carbon was obtained.

[0116] The Cl and Ga can be further removed by a further rinse to improve purity. Raman spectroscopy was conducted to analyse the composition of the reaction products formed.

[0117] Figure 13 is a Raman spectrum showing clear and strong peaks of elemental carbon (-1350 cm'1-1600 cm'1).

[0118] Figure 14 is a series of SEM and EDS images of the Li-Ga alloy after the reduction of CO2 gas to the reaction products. The top SEM images illustrate the carbon particles and the bottom coloured EDX mapping shows the distribution of the carbon atoms.

[0119] The tables in the Figure illustrate the element composition as derived from the EDS results. The tables show that there were significant carbon concentrations.

[0120] 4. Conclusion

[0121] The above-described process enables the reduction of CO2 gas to solid carbon and oxygen at room temperature and atmospheric pressure using a Mg-Ga liquid metal alloy.

[0122] The results confirmed the following reaction mechanism: initially, Mg ions diffuse to the surface of Mg-Ga liquid metal alloy, which promotes the conversion of CO2 gas to carbon on the surface of the Mg-Ga alloy (facilitated by 3 wt.% Mg), and as a result Mg is oxidized to MgO during the process.

[0123] The reduction process led to the formation of solid carbon and MgO on the surface of the alloy.

[0124] The reduction reaction was found to be driven by the Mg component in the liquid metal alloy, which enriches the reaction interface, reacting with CO2, converting CO2 to solid carbon, and undergoing oxidation to produce MgO.

[0125] After the CO2 reduction process, MgO could be successfully reduced back to Mg through electrochemistry. The above-described process also enables the reduction of CO2 to solid carbon and oxygen at a higher temperature and atmospheric pressure using a Li-Ga liquid metal alloy.

[0126] Description of Figure 15 embodiment

[0127] Figure 15 is a diagram of one embodiment of an apparatus for converting CO2 gas to solid carbon and recovering carbon.

[0128] The apparatus comprises a reactor for contacting CO2 gas with any one of a Mg-Ga, Ce- Ga, and Li-Ga liquid metal alloy at a temperature less than 85°C and reducing CO2 to solid carbon and oxygen.

[0129] More particularly, the apparatus comprises:

[0130] (a) a chamber 15,

[0131] (b) an inlet 17 for supplying any one of a Mg-Ga or Ce-Ga or Li-Ga liquid metal alloy at a temperature less than 85°C as droplets that move downwardly in the chamber,

[0132] (c) an inlet 19 for supplying a CCF-containing gas that flow upwardly countercurrent to the droplets of the liquid metal alloy and contact the liquid metal alloy so that Mg that diffuses to surfaces of the droplets reduces CO2 and forms solid carbon on the surfaces, and

[0133] (d) an outlet 21for discharging carbon / liquid metal alloy from the chamber.

[0134] The apparatus is configured so that, in use, there is sufficient time to achieve a required level of reduction of CO2 gas to solid carbon in the chamber.

[0135] The apparatus also comprises a separator unit 23 that separates liquids and solids in the carbon / liquid metal alloy discharged from the chamber 15, with the resultant liquid stream 25 typically comprising Ga and unreacted liquid Mg or Ce or Li, and the solids stream 27 typically comprises carbon and other solid reaction products such as oxides of Mg or Ce or Li. The apparatus also comprises a storage tank 31 that holds a volume of the Mg-Ga or Ce-Ga or Li-Ga liquid metal alloy. The stream 25 of Ga and unreacted liquid Mg or Ce or Li is transferred to the storage tank. In addition, a new supply of Mg-Ga or Ce-Ga or Li-Ga liquid metal alloy is transferred to the storage tank.

[0136] The apparatus also includes a recovery unit 29 that separates carbon from any other solid reaction products.

[0137] As noted above, the solid reaction products typically comprise oxides of Mg or Ce or Li. Whilst not shown, the apparatus may comprise a unit for reducing these oxides and recovering Mg or Ce of Li and recycling these metals.

[0138] Many modifications may be made to the embodiments of the invention described above without departing from the spirit and scope of the invention.

[0139] By way of example, whilst the apparatus described in relation to Figure 15 supplies droplets of any one of a Mg-Ga or Ce-Ga or Li-Ga liquid metal alloy, the invention is not so limited and extends to operating with any high surface forms of Mg-Ga or Ce-Ga or Li-Ga liquid metal alloy.

[0140] The above description is not to be taken as an admission of the common general knowledge in Australia or elsewhere.

[0141] References

[0142] 1. Kalantar-Zadeh, K., M.A. Rahim, and J.B. Tang, Low Melting Temperature Liquid Metals and Their Impacts on Physical Chemistry. Accounts of Materials Research, 2021. 2(8): p. 577-580.

[0143] 2. Rahim, M.A., et al., Low-temperature liquid platinum catalyst. Nature Chemistry, 2022.

[0144] 3. Daeneke, T., et al., Liquid metals: fundamentals and applications in chemistry. Chemical Society Reviews, 2018. 47(11): p. 4073-4111.

[0145] 4. Kalantar-Zadeh, K., et al., Emergence of Liquid Metals in Nanotechnology. Acs Nano, 2019. 13(7): p. 7388-7395.

[0146] 5. Zuraiqi, K., et al., Liquid Metals in Catalysis for Energy Applications. Joule, 2020. 4(11): p. 2290-2321.

[0147] 6. Rawool, S.A., et al., Direct CO2 capture and conversion to fuels on magnesium nanoparticles under ambient conditions simply using water. Chemical Science, 2021. 12(16): p. 5774-5786.

[0148] 7. S. RSoniya, V.M.N., Synthesis and Characterization of Nanostructured Mg(OH)2 and MgO. International Journal of Science and Research (IJSR) 2016. 5(2): p. 197-203.

[0149] 8. Paquette, L.A., Encyclopedia of reagents for organic synthesis. 8 volumes (6223 pages).

[0150] 9. Zuraiqi, K., et al., Direct conversion of CO2 to solid carbon by Ga-based liquid metals. Energy & Environmental Science, 2022. 15(2): p. 595-600.

Claims

CLAIMS1. A process for reducing CO2 gas to solid carbon and oxygen that comprises contacting CO2 gas with any one of a Mg-Ga, Ce-Ga, and Li-Ga liquid metal alloy at a temperature less than 85°C and reducing CO2 gas to solid carbon and oxygen.

2. The process defined in claim 1 comprises forming the liquid metal alloy and then contacting the alloy with CO2 gas at the temperature less than 85°C for sufficient time to achieve a required level of reduction of CO2 to solid carbon.

3. The process defined in any claim 1 or claim 2 comprises contacting CO2 gas with the liquid metal alloy under atmospheric pressure conditions.

4. The process defined in any claim 1 or claim 2 comprises contacting CO2 gas with the liquid metal alloy under pressure conditions greater than one atmospheric pressure.

5. The process defined in any one of the preceding claims comprises forming droplets, for example sprays of droplets, of liquid metals moving downwardly counter-current to an upward flow of CCb-containing gas so that the drops and the gas come into contact so that Mg in the droplets reduces CO2 to solid carbon6. The process defined in any one of the preceding claims comprises recovering solid carbon.

7. The process defined in any one of the preceding claims comprises forming oxides of Mg or Ce or Li.

8. The process defined in claim 7 comprises reducing oxides of Mg or Ce or Li to Mg or Ce or Li for re-use in the process.

9. The process defined in any one of the preceding claims wherein the liquid metal alloy comprises up to 3 wt.% of any one of Mg, Ce and Li in the liquid metal alloy.

10. The process defined in any one of the preceding claims wherein the liquid metal alloy comprises at least 1 wt.% of any one of Mg, Ce and Li in the liquid metal alloy.

11. An apparatus for converting CO2 gas to solid carbon that comprises a reactor for contacting CO2 gas with any one of a Mg-Ga, Ce-Ga, and Li-Ga liquid metal alloy at a temperature less than 85°C and reducing CO2 gas to solid carbon and oxygen.

12. The apparatus defined in claim 11 comprising a chamber, an inlet for supplying any one of a Mg-Ga or Ce-Ga or Li-Ga liquid metal alloy at a temperature less than 85°C to the chamber, an inlet for supplying a CCh-containing gas to the chamber to contact the liquid metal alloy in the chamber so that, in use, Mg reduces CO2 and forms solid carbon on surfaces of the liquid metal alloy, and an outlet for discharging carbon / liquid metal alloy from the chamber.

13. The apparatus defined in claim 12 configured so that, in use, there is sufficient time to achieve a required level of reduction of CO2 to solid carbon in the chamber.

14. The apparatus defined in claim 11 or claim 12 configured for supplying the liquid metal alloy as droplets.

15. The apparatus defined in any one of claims 12 to 14 configured for supplying the liquid metal alloy as droplets that, in use, move downwardly in the chamber.

16. The apparatus defined in any one of claims 12 to 15 configured for supplying the CCh-containing gas so that in use the gas flows upwardly counter-current to the droplets of the liquid metal alloy.

17. The apparatus defined in any one of claims 12 to 16 comprising a separator unit for separating liquids and solids in the carbon / liquid metal alloy discharged from the chamber, with the liquids typically comprising Ga and unreacted Mg orCe or Li, and the solids typically comprising carbon and other solid reaction products such as oxides of Mg or Ce or Li.

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