Enhanced co 2 adsorption and utilisation performance

Abstract

The present invention investigates the optimization of metal oxide concentration, molten salt composition, and operating temperature for enhanced CO2 sorption and conversion in an integrated carbon capture and utilization process.

Description

Enhanced CO2 adsorption and utilisation performanceAbstractThis invention investigates the optimization of metal oxide concentration, molten salt composition, and operating temperature for enhanced CO2 sorption and conversion in an integrated carbon capture and utilization-reverse water gas shift (ICCU-RWGS) process. The integration of CaO with molten salt offers a promising approach for improving the performance of CO2 sorption and conversion systems. The molten salt used in most of these experiments consisted of a NaCI-CaC blend with a mass ratio of 4:6. Molten salts consisting of a NaF-CaF2 blend also form part of this invention. The results demonstrate that the presence of molten salt significantly enhances CO2 uptake and CO yield compared to systems without molten salt. The CO2 uptake in the presence of molten salt ranges from 13.4 to 15.1 mmol.g-1, whereas without molten salt, it is 10.1 mmol.g-1. Moreover, the CO yield is improved with molten salt, ranging from 10.3 to 7.3 mmol.g-1, compared to 8.9 mmol.g-1without molten salt. Temperature optimization studies reveal that the optimal operating temperature for achieving the highest CO yield is 650 °C, with a CO yield of 9.1 mmol.g-1. The optimization of CaO concentration in the molten salt system shows that a 10 wt % CaO concentration offers the highest CO yield of 10.3 mmol.g-1. Increasing the CaO concentration beyond 10 wt % results in a decrease in CO yield, with a drop to 4.0 mmol.g-1at 20 wt % CaO.1 IntroductionWith the acceleration of global warming, the urgent need for scientific and technological solutions to reduce CO2 emissions has arisen in order to alleviate the existential threat posed by climate change[1]. Nevertheless, in the foreseeable future, human daily production activities will continue to rely heavily on fossil fuels, inevitably leading to substantial CO2 emissions in the short term[2]. One effective strategy, akin to managing toxic gases in industrial emissions, involves the extraction of CO2 from industrial waste gases prior to its release into the environment. This process, known as carbon capture and storage (CCS)[3], poses challenges due to the extra costs involved in storage and the risk of captured CO2 release[4].Recently, researchers have been focusing on Integrated Carbon Capture and Utilization (ICCU)[5, 6] instead of traditional CCS methods. ICCU, facilitating simultaneous capture and conversion of CO2,, circumvents the additional cost of CO2 storage by immediately converting captured CO2 into valuable chemical products such as CO[7], CH 8, 9], and methanol

[0010] . ICCU typically requires dual functional materials (DFMs) that incorporate both sorbents and catalysts, enabling concurrent CO2 sorption and conversion

[0011] . In a standard ICCU process, CC>2-rich gases, like flue gases from power plants and cement factories, are initially exposed to DFMs for CO2 capture. Following this, the gas is replaced with a reducing agent, such as H2

[0012] or CH4

[0013] , which reacts with the sorbent-loaded DFMs, leading to the production of the desired chemical products and regeneration of the sorbent. This cyclical process is then repeated, commencing again with the introduction of CC>2-rich gases.Extensive research has focused on improving sorbent performance, such as employing promoters and stabilizers in CaO-based sorbents to increase their structural stability and CO2 capture efficiency[14-16]. Simultaneously, the influence of diverse preparation methods on sorbent performance is also under investigation^, 18], However, the modification of DFMs involves carefulconsideration of the interplay between catalysts and sorbents, to balance CO2 uptake and catalyst performance and stability, adding a layer of complexity to DFM systems

[0019] .Recently, a reaction directly transforming CaCCh into CaO and CO by H2, namely an integrated carbon capture and utilization-reverse water gas shift (ICCU-RWGS) reaction, has been reported[20- 22], This uses CaO as sole sorbent and without a catalyst for CO2 absorption and conversion, facilitating the production of syngas (CO & H2) by ICCU in a simpler and more economical approach. While the current implementation of ICCU-RWGS using only CaO can help save costs associated with catalysts, there is still room for improvement in terms of performance, including factors such as CO2 uptake, CO2 adsorption speed, reaction speed, CO yield, and cycle stability.Molten salt is potentially an effective method to enhance the performance of sorbents. Halliday et al. [23, 24] classified CO2 sorbents into three categories based on their phase changes before and after adsorption, namely solid-to-solid, liquid-to-solid, and liquid-to-liquid. Commonly used sorbents like CaO and MgO fall into the solid-to-solid category. Research by Halliday et al. discovered that liquid-to-solid and liquid-to-liquid sorbents often demonstrate superior CO2 uptake compared to solid- to-solid sorbents. This advantage is attributable to the surface excess of alkali metal carbonates in the molten salt, which can contribute to a smaller sample size for CO2 sorbents. Research by Harada et al.

[0025] also underscored the benefits of molten salt. They discovered that the ionic interactions and fluid-like properties of molten oxides could facilitate sorbents to reach saturation (5.2 to 7.3 mmol.g-1) within 1 minute, while maintaining superior stability. Furthermore, some studies on sorbent modifications also have validated the role of molten salt in enhancing absorption, even though the sorbents developed in these studies still fall into the category of solid-to-solid. For instance, research using alkali metal nitrate as a channel

[0026] to promote direct ionic exchange between CO2 and MgO, thereby enhancing the adsorption performance of MgO-based sorbents, and studies utilizing molten NaNOs salt

[0027] to decompose MgCOs shells, thereby facilitating contact between CO2 and MgO-based sorbents.WO 2012 / 070954 concerns capturing carbon dioxide from a flue gas by using CaO dissolved in a mixture of NaF and CaF2 in molten state as the absorption medium. WO 2012 / 070954 does not disclose converting the captured carbon dioxide into a carbon dioxide utilisation product.Nonetheless, limited research exists on the application of CO2 absorption in liquid-phase molten salt in connection with ICCU. Given the potential benefits demonstrated by liquid-phase molten salt for CO2 absorption, broadening these advantages via sorbent regeneration and CO2 conversion could potentially amplify the overall effectiveness of the carbon capture systems. Hence, integrating CaO adsorbents with molten salt as an alternative to DFMs not only has the potential to reduce DFM production costs but also presents a promising method to enhance the adsorption and conversion capabilities of CaO for CO2. At present, a similar system design that involves positioning CaO within a mixture of CaF2-CaCl2 molten salt has been employed for toxic gas sorption, such as SO2 in industrial waste gas, achieving commendable results

[0028] .CaO + CO2= CaCO3AH = -178.3 kJ. mol”1Eq. 1Since the absorption of CO2 by the CaO sorbent is an exothermic process

[0029] as shown by Eq. 1 . above, molten salt might serve as a heat reservoir, storing this generated heat, thereby contributing to a more energy-saving system

[0030] . Moreover, from an energy conservation perspective, molten salt has been recognized as excellent energy storage material and is often used for solar energy storage

[0031] .In summary, earlier work has focussed on absorbing and releasing CO2 from different CaO I molten salt mixtures, in other words, CO2 capture. In the present invention, the metal oxide and molten salts are used for both carbon capture and for the conversion of the captured CO2. We also focus on the conversion of CO2, which is integrated with carbon capture. The earlier work did not involve the conversion of CO2 within molten salts, specifically the RWGS reaction reported herein. Their focus is solely on the reaction of CaO absorbing CO2 to form CaCOs, without the subsequent steps of CaCOs reverting back to CaO by H2 to yield syngas (for example, CO & H2). Therefore, the issue of cycling stability of DFMs in ICCU, which is a significant aspect of our study, as demonstrated in Figure 3 hereunder.In a future-focused context, uniting molten salt with ICCU may offer an innovative substitution for traditional heating methodologies, leveraging solar power for the industrial realization of CO2 capture and transformation. This novel approach suggests a sustainable trajectory for comprehensive CO2 management in the long term.In our research, we engineered an ICCU-RWGS system that integrates CaO, a metal oxide, with molten salt. The exemplified molten salt, with a mass of 5.0 g, consisted of a 4:6 mass ratio blend of NaCI (Sigma-Aldrich) and CaCh (Fisher Scientific). Firstly, we dispersed CaO (Sigma-Aldrich) into this NaCI-CaCh molten salt at the concentration of 10 % wt and 650 °C for conducting a cyclical stability test, comprising five iterative cycles. Then, the effect of CaO concentration in molten salt was investigated at 650 °C with the CaO concentration of 5 % wt, 10 % wt, and 20 % wt. Subsequently, we conducted ICCU-RWGS experiments at four different temperatures: 500 °C, 600 °C, 650 °C, and 700 °C, while maintaining a CaO concentration of 10 % wt, to explore the effect of temperature on performance.Definitions:As used herein, “10 % wt” when referring to the concentration of CaO in a CaO I molten salt mixture, is 0.5 g CaO and 5.0 g molten salt, being a total weight of 5.5 g.When not otherwise defined, the phrase “molten salt” as used herein to refer to a 4:6 mass ratio of NaCI-CaChAs used herein, the phrase “carbon dioxide utilisation product” optionally includes carbon monoxide, methane and methanol, as well as, other hydrocarbon products such as, but not limited to, light olefins and alcohols. When the carbon dioxide utilisation product is methanol, high pressure is needed and also other catalysts are also needed.According to a first aspect of the invention, there is provided a method for removing carbon dioxide from a gas stream and for converting the carbon dioxide into a carbon dioxide utilisation product, the method comprising(i) bringing the gas stream in contact with an absorption medium, wherein the absorption medium comprises molten salts, and a metal oxide that reacts with the carbon dioxide to form a metal carbonate; and(ii), bringing the molten salts containing metal carbonate into contact with H2, to release the metal oxide and the carbon dioxide utilisation product selected from carbon monoxide, methane and methanol, wherein, optionally, the carbon dioxide utilisation product is carbon monoxide, characterised in that the molten salts consist of sodium halide and calcium halide.Optionally, the molten salts consist of NaCI and CaC .Alternatively or additionally, the mass ratio of metal oxide to molten salts is in the range of 0.2 to 0.75 g metal oxide : 5 g of molten salt.Optionally, the mass ratio of metal oxide to molten salt is in the range of 0.4 to 0.6 g metal oxide : 5 g of molten salt.Further optionally, the mass ratio of metal oxide to molten salt is about 0.5 g metal oxide : 5 g of molten salt.Alternatively or additionally, the molten salts consist of NaCI to CaCh in a mass ratio in the range of 1 : 4 to 1 : 1 .Optionally, the molten salts consist of NaCI to CaCh in a mass ratio in the range of 1 : 3 to 1 : 2.Further optionally, the molten salts consist of NaCI to CaCh in a mass ratio in the range of about 4 : 6.Alternatively or additionally, the gas stream is brought in contact with the absorption medium at a temperature in the range of 600 to 725° C and the metal oxide is CaO.Alternatively or additionally, the temperature is in the range of 625 - 675 °C; optionally in which the temperature is 650 °C + 10 °C.Alternatively or additionally, the metal oxide is selected from MgO, SrO, BaO, IJ2O, Na2O, K2O, Rb2O and CS2O.According to a second aspect of the invention, there is provided use of an absorption medium comprising a metal oxide dissolvable in molten salts consisting of sodium halide and calcium halide for the removal of carbon dioxide from a gas stream and for the conversion of the carbon dioxide into a carbon dioxide utilisation product selected from carbon monoxide, methane and methanol, wherein, optionally, the carbon dioxide utilisation product is carbon monoxide.We have exemplified, herein, carbon monoxide as the carbon dioxide utilisation product. However the technology could be easily extended to methane production by adding catalysts and reducing reaction temperature. The technology could also be extended to methanol production by adding catalysts and adjusting reaction temperature and pressure.Optionally, the molten salts consist of NaCI and CaCh.Alternatively or additionally, the mass ratio of metal oxide to molten salts is in the range of 0.2 to 0.75 g metal oxide : 5 g of molten salt.Optionally, the mass ratio of metal oxide to molten salt is in the range of 0.4 to 0.6 g metal oxide : 5 g of molten salt.Further optionally the mass ratio of metal oxide to molten salt is about 0.5 g metal oxide : 5 g of molten salt.Alternatively or additionally, the molten salts consist of NaCI to CaCh in a mass ratio in the range of 1 : 4 to 1 : 1 .Optionally, the molten salts consist of NaCI to CaCh in a mass ratio in the range of 1 : 3 to 1 : 2.Further optionally, the molten salts consist of NaCI to CaCh in a mass ratio in the range of about 4 : 6.Alternatively or additionally, the metal oxide is selected from CaO, MgO, SrO, BaO, U2O, Na2O, K2O, Rb2O and CS2O; optionally CaO.According to a third aspect of the invention, there is provided an absorption medium forthe removal of carbon dioxide from a gas stream and for the conversion of the carbon dioxide into a carbon dioxide utilisation product selected from carbon monoxide, methane and methanol, wherein, optionally, the carbon dioxide utilisation product is carbon monoxide, characterized in that the absorption medium comprises metal oxide dissolved in molten salts consisting of sodium halide and calcium halide.Optionally, the molten salts consist of NaCI and CaC .Alternatively or additionally, the mass ratio of metal oxide to molten salts is in the range of 0.2 to 0.75 g metal oxide : 5 g of molten salt; optionally in which the mass ratio of metal oxide to molten salt is in the range of 0.4 to 0.6 g metal oxide : 5 g of molten salt;further optionally in which the mass ratio of metal oxide to molten salt is about 0.5 g metal oxide : 5 g of molten salt.Alternatively or additionally, the molten salts consist of NaCI to CaCh in a mass ratio in the range of 1 : 4 to 1 : 1 ; optionally in which the molten salts consist of NaCI to CaC in a mass ratio in the range of 1 : 3 to 1 : 2; further optionally in which the molten salts consist of NaCI to CaCh in a mass ratio in the range of about 4 : 6.Alternatively or additionally, the metal oxide is selected from CaO, MgO, SrO, BaO, I 2O, Na2O, K2O, Rb2O and CS2O; optionally CaO.DrawingsIn the drawings:Fig. 1 (a) Schematic diagram of ICCU-RWGS evaluation system; comparison of ICCU-RWGS evaluation for 0.5 g CaO (Sigma-Aldrich) and 0.5 g CaO in 5.0 g molten salt (10 % wt CaO / MS) during CO2 capture stage (b) and RWGS stage (c).Fig. 2 Comparative analysis of CO2 and CO concentration variations during CO2 capture and RWGS stage at 650 °C for different CaO sources: 0.5 g CaO from Sigma Aldrich (a-b), 0.5 g CaO calcinated from CaCOs (c-d), and 0.5 g calcinated marble (e-f) (having a CaO content of about 40% by weight), and, for each, 5.0 g molten salt or non-molten salt environments.Fig. 3 Based on conditions of 650 °C, with 0.5 g of CaO, and 5.0 g of molten salt (a): CO2 concentration curves for CaO in five cycles with molten salt and one cycle without molten salt during the CO2 capture stage; (b): CO concentration curves for CaO in five cycles with molten salt and one cycle without molten salt during the RWGS Stage; (c): Comparison of CO / CO2 yield, CO2 uptake, and CO2 conversion in five cycles with molten salt and one cycle without molten salt.Fig. 4 Comprehensive analysis of CaO performance in molten salt for CO2 capture and RWGS stage with 0.5 g CaO in 5.0 g molten salt at 550 °C, 600 °C, 650 °C, and 700 °C. (a) CO2 concentration profiles during the CO2 capture stage, (b) CO concentration profiles during the RWGS stage, (c) Comparison of CO / CO2 yield, CO2 uptake, and CO2 conversion.Fig. 5 Comprehensive analysis of CaO performance in molten salt for CO2 capture and RWGS stage with 0.25g, 0.50 g and 1 .00 g CaO in 5.0 g molten salt at 650 °C. (a) CO2 concentration profiles during the CO2 capture stage, (b) CO concentration profiles during the RWGS stage, (c) Comparison of CO / CO2 yield, CO2 uptake, and CO2 conversion.Fig. 6 XRD Spectra comparison of original sample, C02-adsorbed & H2-reduced sample, and C02-adsorbed sample of 10 % wt CaO / MS.Fig. 7 SEM image of (a) original sample, (b) C02-adsorbed & Fh-reduced sample, and (c) CO2- adsorbed sample produced by 10 % wt CaO / MS at 650 °C.Fig. 8 Comparative Analysis of In-situ Infrared Spectroscopy Studies during CO2 Capture and RWGS Reactions at 650 °C, (a): Infrared spectra of CaO during CO2 capture stage; (b): Infrared spectra of 10% wt CaO / MS during CO2 capture stage; (c): Infrared spectra of CaO during RWGS stage; (d): Infrared spectra of 10% wt CaO / MS during RWGS stage.Fig. 9 Schematic representation of the morphological changes of the sample during CO2 capture and the RWGS stage for (a) CaO only and (b) CaO in molten salts.Fig. 10 Melting behaviour of NaCI and CaCh mixtures at mass ratios of 2:8, 4:6, 5:5, 6:4, and 8:2, when subjected to temperatures of 425 °C, 475 °C, and 525 °C for 3 hours.Fig. 11 (a) In-situ infrared spectra during the CO2 capture stage, (b) detailed view in the region of 3000 cm'1to 2400 cm'1, and (c) detailed view in the region of 1800 cm'1to 800 cm'1.Fig. 12 (a) In-situ Raman spectra during the RWGS stage, (b) detailed view in the region of 2100 cm'1to 1400 cm'1, and (c) detailed view in the region of 1500 cm'1to 700 cm'1.2 Experimental section2.1 . Material characterisationThe phase information of the 10 % wt CaO in molten salt (10 % wt CaO / MS) samples after CO2adsorption, after cycling, and original CaO mixed with molten salt was determined using powder X-ray diffraction (XRD) on a PANalytical Empyrean series two diffractometer system equipped with a Cu Ka X-ray source (2 Theta = 5-80°). The in-situ infrared spectroscopy was used to collect the information of CO2 sorption process and RWGS process for CaO and CaO with the molten salt. The samples adsorbed 15 % CO2 / N2 for 30 min and were then purged with N2 for 7 minutes, followed by the introduction of 100 % H2 mixture for 40 min. Throughout the in-situ infrared spectroscopy test, the temperature was maintained at 650 °C, and the gas flow rate was set at 100 ml. min-1. Scanning electron microscopy (FEI Quanta FEG) was used to characterise the morphology of the original sample in molten salt, the sample after the cycle, and the sample after adsorption saturation.2.2. ICCU-RWGS procedureThe schematic representation of the experimental apparatus is presented in Fig. 1 (a). The ICCU-RWGS evaluations took place within a quartz reaction tube (OD: 25 mm; ID: 23 mm; L: 20 cm). A nickel conduit (OD: 12.0 mm; ID: 10.5 mm) was used to supply the desired gas. The base of the quartz tube was positioned at the midpoint of a tube furnace (Elite TSH 12 / 50 / 300-2416CG) to facilitate heating. Before initiating the ICCU-RWGS process, the CaO amalgamated with molten salt (CaO / MS) was heated in a nitrogen environment until it achieved a molten state. The inlet gases (CO2, N2, H2) were regulated by mass flowmeters (OMEGA FMA-A2306), calibrated using a bubble flowmeter. The exit gases underwent analysis via a flue gas analyzer system, comprised of a gas analyzer (Enerac 700 AV; NDIR detector).In this study, ICCU-RWGS evaluations were undertaken using a mixture of varying mass amounts of CaO (0.25g, 0.50 g and 1 .00 g CaO; most often 0.5 g CaO) and a 5.0 g molten salt mixture (MS), most often comprising NaCI and CaCh in a 4:6 mass ratio. The present work investigates the performance of CaO and molten salt at a temperature of 650°C with concentrations of 5% wt, 10% wt, and 20 % wt (Figure 5). In addition, the performance of CaO at 10% wt was studied at 550 °C, 600 °C, 650 °C, and 700 °C(Figure 4). For the CaO concentration of 10 % wt at 650 °C, five cyclic reactions were carried out to test the cyclical stability (Figure 3). The inlet gas flow rates were consistently maintained at 100 mL.min-1, assuming that the outlet gas flow rates were identical. A typical ICCU-RWGS process incorporated a CO2 capture stage, where CO2-rich gases (15% CO2 / 85%N2; each by volume) were supplied for 30 minutes, followed by a 7-minute purging of the reaction tube with N2. In the RWGS stage, 100% H2 was introduced until the CO concentration displayed on the analyzer fell below 0.5 %. The performance of the ICCU-RWGS was assessed based on realtime readings of CO and CO2.3 Results and discussions3.1 Enhanced CO2 sorption and conversion rate by RWGS reaction of CaO in molten saltAs shown in Fig. 1 b (b), when there were no adsorbents in the reactor (benchmark), the CO2 concentration rose rapidly to 15 % (the intake concentation) and persistently remained at that level throughout the subsequent 1800 s. Upon transitioning to a N2 purge, the CO2 concentration reducedrapidly to 0 %.When only the molten salt was present in the reactor, the trend of CO2 concentration change almost overlapped with the scenario without any samples. This indicates that NaCI-CaCh (molten salt I MS) itself does not possess any CO2 sorption capacity.When CaO was present in the reactor without molten salt, the concentration change of CO2 was similar to previous reports

[0020] , as the CO2 concentration rapidly increased to approximately 6.7 % initially and then gradually rose to 15.0 % over the next approximately 1300 s.When both CaO and molten salt (CaO / MS) were present, the concentration of CO2 gently increased to approximately 1 .5 % within the initial 500 s. This period presented a breakthrough time when CO2 concentration is 0 %

[0032] of roughly 300 s, suggesting a significant amount of CO2 was adsorbed during this interval. Between 500 s and 1000 s, the concentration rapidly increased to around 12.0 %. For the remaining time, due to the adsorption capacity of CaO nearing saturation, the concentration gradually increased from 12.0 % to 15.0 %[3], A slower rise in the CO2 concentration curve indicated a faster rate of CO2 adsorption by CaO

[0013] ,When only CaO was present, the CO2 concentration reached 15.0 % after nearly 1300s. For CaO / MS, the CO2 concentration approached 15.0 % after about 800 s. This suggests that the rate of CO2 adsorption by CaO was effectively enhanced in the molten salt environment.As shown in Fig. 1 c (c), in the case of CaO alone (without molten salt), the concentration of CO generated during the RWGS reaction swiftly increased to 1 .4 %, then nearly maintained the equilibrium, and reduced to 0.5 % at around 5000 s. However, for the CaO / MS system, the CO concentration rapidly, and surprisingly, increased to 7.1 % and subsequently declined swiftly. Beginning at approximately 1000 s when CO concentration was 1 .4 %, the decrease in CO concentration slowed down, reaching 0.5 % at around 3000 s. This indicates that the reaction rate of the RWGS process is also enhanced by placing CaO in the molten salt.It is worth noting, as illustrated in Fig. 1 (b), under the condition of CaO only, the concentration of CO2 was very low during the N2 purging stage. In contrast, for CaO / MS, the concentration of CO2 is able to maintain stably around 2.8 %, suggesting that the molten salt could facilitate the decomposition of CaCOs. Without being bound by this hypothesis, this might explain the increased reaction rate during the RWGS stage

[0033] ,3.2 Selection of Optimal CaO and molten salt composition for ICCUBuilding upon existing research aimed at enhancing CO2 adsorption capacity, wherein MgO- based adsorbents were modified using alkali metal nitrates, we were motivated to explore the use of NaNOs as the reaction medium. This approach was inspired by prior work highlighting the ability of alkali metal nitrates to effectively penetrate the MgCOs shell formed during the CO2 adsorption process

[0026] , thereby facilitating the entry of CO2 into MgO and enhancing CO2 capture. Despite the theoretical promise, practical challenges emerged during implementation. As the temperature increased, we observed significant evaporation of NaNOs, making it unsuitable for sustained reactions (data not shown). These challenges became pronounced at around 650 °C [5], the optimal temperature for ICCU using CaO. At this temperature, the most common alkali metal nitrates

[0034] , including our initial choice of NaNOs, either boiled or decomposed into metal oxides.In response to this challenge, we sought a substance capable of remaining liquid and chemically stable at around 650 °C. We initially considered IJ2CO3, but its actual melting point, around 850 °C, and the ensuing decomposition of CaCCh at this temperature

[0035] , made it a less suitable option.Our solution came in the form of a mixed salt, consisting of calcium halide and sodium halide, such as CaCh and NaCI. CaChwas chosen as a material that is not only relatively economical but also stable, despite its inherent theoretical melting point of 645 °C. This decision could be backed by previous research, which underscored the effectiveness of CaCh and CaO used in conjunction for CO2 adsorption [36, 37], To address the high melting point of CaC , we introduced a sodium halide such as NaCI into the mixture, which effectively reduced the melting point of CaCl2[38, 39], Experimental findings revealed that a 4:6 mass ratio of CaCh to NaCI was capable of maintaining a liquid state at approximately 500 °C. It is crucial to emphasize that the addition of NaCI was primarily employed as a strategy to lower the operating temperature, tailoring the process conditions to facilitate efficient CO2 adsorption.Three distinct sources of CaO were evaluated in terms of ICCU performance in a mixed CaCh- NaCI molten salt environment (Figure 2). These materials are CaO supplied by Sigma-Aldrich, CaO obtained from the calcination of 98.3 % pure CaCOs from Intralabs, and calcined marble provided by Zestit. Fig. 2 (a) (benchmark - experiment without sorbent (empty reaction tube); the same for Fig. 2(c) and Fig. 2 (e) illustrates the differential CO2 adsorption behavior of CaO under gaseous and molten salt conditions, with an enhanced adsorption rate exhibited in the molten salt environment. Fig. 2 (b) further highlights the unique reaction dynamics of CO generation during the RWGS stage in both settings. Details for Fig. 2 (a) and (b) have been explained in section 3.1 above. As demonstrated in Fig. 2 (c) and (d), for CaO produced from the calcination of CaCOs, a distinct promotion of CO2 adsorption rate and CO production rate during the RWGS stage is still evident in the molten salt environment. However, a slight difference is seen during the RWGS stage for the molten salt environment, where the peak concentration of CO is only 4.5 % (CaO obtained from Intralabs), slightly lower than that of Sigma-Aldrich CaO. In the gaseous environment, the CO concentration using the Intralabs derived CaO is almost equivalent to that of Sigma-Aldrich CaO, staying at 1 .50 % for about 2000 s.Notably, as illustrated in Fig. 2 (e) and (f), although the CO2 concentration profile for marble in molten salt also reveals a promotive effect of molten salt on CO2 adsorption, this is only apparent within the initial 300 s of CO2 injection. As the adsorption process continues, in the molten salt environment, the CO2 concentration reaches saturation at about 15 % after approximately 1000 s. In contrast, under gaseous conditions, saturation is not reached until nearly 1700 s. This suggests that marble inherently possesses good CO2 adsorption capability; however, its adsorption ability does not appear to be significantly enhanced in the molten salt environment; in fact, it appears slightly suppressed. A similar scenario is observed for the RWGS stage using marble. Although marble in the molten salt environment exhibits a CO concentration profile similarto that in Fig. 2 (b) and (d), its peak concentration is only 3 %, slightly higher than when reacting in the gaseous environment. This phenomenon may be attributed to issues with the purity of CaO; that is, the molten salt environment is more conducive topromoting the inherent adsorption and RWGS performance of CaO. However, marble contains significant impurities, such as MgO

[0040] , which have a positive role in CO2 adsorption and conversion in the gaseous environment. These impurities fail to reflect their promotive role in CO2 adsorption and conversion performance in the molten salt environment. In summary, based on this comparative analysis, the adsorbent of choice for this study was high-purity CaO (Sigma-Aldrich) - this has a CaO urity of more than 99.9% by weight. Intralabs CaO has a purity of about 98.3% by weight.Preferably, the metal oxide purity is more than 90% by weight; optionally, more than 95% by weight; further optionally, more than 97.5% by weight; still further optionally, more than 99% by weight.Preferably, if the metal oxide used herein is CaO, the CaO purity is more than 90% by weight; optionally, more than 95% by weight; further optionally, more than 97.5% by weight; still further optionally, more than 99% by weight.3.3 Assessing the role of molten salt in CO2 capture and conversion during multiple ICCU-RWGS cyclesFig. 3 (a) and (b) shows the ICCU-RWGS performance in terms of CO2 concentration during the CO2 capture stage and CO concentration during the CO2 conversion stage, respectively. As evidenced by Fig. 3 (a), a transition point occurs when the CO2 concentration in the molten salt environment reaches 12.0%. Beyond this transition point, a decelerated rate of increase in the concentration curve is observed [3, 13], The initial cycle took 1060 s to reach this critical point, while the second to fifth cycles required progressively shorter periods of 790 s, 830 s, 620 s, and 600 s, respectively. As indicated in Fig. 3 (b), the achievable maximum CO concentration falls within the range of 5.5% and 7.5%. The peak CO concentrations for the five cycles of ICCU-RWGS are recorded as 5.7%, 6.2%, 6.9%, 7.3%, and 7.4%, respectively. This pattern exhibits an increasing trend with an escalating number of cycles. Therefore, with each successive cycle, the maximum of CO concentration is slightly improved. This might be due to the particle size of CaO became smaller with enhanced distribution with the increase of the number of cycles. Fig. 3 (c) demonstrates that, under the molten salt environment, the CO2 uptake is significantly higher in the presence of molten salt. The range of CO2 uptake for the five cycles is between 13.4 mmol.g-1and 15.1 mmol.g-1, while the CO2 uptake without molten salt is merely 10.1 mmol.g-1. This highlights the enhancement of CO2 capture in the molten salt environment. The CO yield for the five cycles is 10.3 mmol.g-1, 9.1 mmol.g-1, 7.6 mmol.g-1, 7.4 mmol.g-1, and 7.3 mmol.g-1, respectively. Under conditions without molten salt, the CO yield is 8.9 mmol.g-1, suggesting that molten salt also plays a promoting role in CO production during the CO2 utilisation stage.3.4 Temperature optimization for CaO performance in ICCU-RWGS with molten saltTemperature is a crucial factor in the ICCU process. Although prior research on ICCU-RWGS has identified the optimal temperature as 650 °C

[0041] , considering the novelty of introducing molten salt into the ICCU in this study, potential new variations may arise. Hence, this study has also exploredthe influence of temperature on the performance of CaO in molten salt. As shown in Fig. 4 (a), at 550 °C, after the beginning of adsorption, the concentration of CO2 quickly rises to 12.1 %, and then gradually increases to 13.0 % in the subsequent approximately 1700 s. This suggests that CaO does not demonstrate satisfactory adsorption performance in molten salt at this temperature. At 600 °C, the CO2 concentration starts to rise rapidly to 6.3 %. As the adsorption continues, the concentration drops around 1000 s, indicating an improved adsorption performance of CaO. This might be attributed to the dissolution of the CaCOs shell[27, 42], formed after initial adsorption saturation, in the molten salt, thereby exposing the core CaO to the CO2 gas and enhancing the adsorption performance. The optimal adsorption performance is observed at 650 °C, with a breakthrough time of approximately 370 s. At 700 °C, no distinct breakthrough time is discernible due to the tendency of CaCOs to decompose at higher temperatures

[0043] . As shown in. 4 (b), during the RWGS stage at 550 °C, the maximum CO concentration only reaches 0.5 %. At 600 °C, it is slightly better at 1 .2 %. However, at 650 °C, the maximum concentration increases significantly to 6.0 %, and at 700 °C, it reaches 8.1 %. This indicates that, even in a molten salt environment, lower temperatures are not conducive to CO2 conversion. As depicted in Fig. 4 (c), the CO yield and CO2 conversion are at their lowest at 550 °C, at only 0.41 mmol.g-1and 8.1 %, respectively. These levels are evidently unacceptable for ICCU. However, it is noteworthy that, at this temperature, CO2 uptake remains relatively significant, reaching 9.1 mmol.g-1. Among the four temperature groups, considering both CO2 adsorption and conversion performance, 650 °C remains the optimal temperature with a CO yield of 9.1 mmol.g-1, CO2 uptake of 14.7 mmol.g-1, and conversion rate of 64 %. Although at 700 °C, the maximum CO concentration is higher, high temperatures are not favorable for CO2 adsorption, resulting in a decrease in CO2 uptake. Furthermore, in the RWGS stage, more CO2 is produced at 700 °C, at 2.5 mmol.g-1, compared to 1 .4 mmol.g-1at 650 °C. In summary, even with the distinct environment provided by the utilization of molten salt, the ICCU-RWGS process still optimally operates at 650 °C.3.5 Optimization of CaO concentration for enhanced CO2 adsorption in molten salt systemsWe have further investigated the performance of varying CaO concentrations in molten salts. As shown in Fig. 5 (a), a 5 wt % CaO concentration in the molten salt system (0.25g CaO and 5.0 g molten salt) exhibits a breakthrough time of approximately 200s. Meanwhile, the 10 wt % CaO concentration (0.50 g CaO and 5.0 g molten salt), due to an increase in the quantity of the adsorbent, inevitably increases the CO2 adsorption capacity of the reaction system

[0044] and correspondingly extends the breakthrough time to about 380 s. It is noteworthy that a further increase in CaO concentration (to 1 .00 g CaO and 5.0 g molten salt) does not extend the breakthrough time, but rather leads to its disappearance, suggesting that there is a threshold to the CO2 adsorption enhancement effect of CaO in molten salts. As depicted in Fig. 5 (b), the peak concentrations reached for 5 wt %, 10 wt % and 20 wt % CaO concentrations are 4.2 %, 6.1 % and 8.5 %, respectively. This is primarily attributed to the overall increase in adsorbed CO2 as a result of the heightened CaO quantity. However, as Fig. 5 (c) illustrates, the CO2 uptake and CO yield per unit mass of CaO significantly decrease at 20 % wt CaO, compared to 5 wt % CaO and 10 wt % CaO. Under conditions of 10 wt % CaO, CO yield is 10.3 mmol.g-1and CO2 uptake is 15.6 mmol.g-1, while under conditions of 20 wt %CaO, CO yield and CO2 uptake drop to 4.0 mmol.g-1and 5.9 mmol.g-1, respectively. For comparison, under conditions of 5.0 wt % CaO, CO yield and CO2 uptake are 8.2 mmol.g-1and 14.1 mmol.g-1, which are similar to the situation using 10 wt % CaO. This phenomenon suggests that an optimal CaO concentration needs to be maintained in the molten salt system in order to maximize the CO2 adsorption enhancement effect.3.6 Mechanistic analysis of CO2 capture and conversion performance by CaO in molten salt environmentsIn an effort to investigate the enhancement effect of the molten salt environment on the adsorption and conversion of CO2 by CaO, XRD tests were executed on several distinctive samples. The Original Sample constitutes a 10 wt% CaO and molten salt blend, produced at 650 °C under a N2 environment without any reaction and then cooled. The Cycled Sample is the material prepared under identical conditions, but later reduced by H2 after fully adsorbed by 15% CO2 / N2. Finally, the Adsorbed Sample corresponds to the original sample that achieved CO2 adsorption saturation. As shown in Fig. 6, the Original Sample, showed negligible presence of CaO, with the main observable phase being Ca(OH)2, since it adsorbed moisture under ambient conditions[45-47]. Interestingly, for the Cycled Sample, where CaO was theoretically expected, none was discernibly present in the XRD pattern. Conversely, in the CO2-saturated adsorbed sample, CaCOs was clearly detected. Taking into account the exclusion of any newly formed compounds after moisture absorption, such as hydroxides and hydrates, it is proposed that the CaO crystal may have entirely dissolved in the salt, thereby forming a homogenous or solution-like mixture. In such a scenario, the atoms from the crystal may be dispersed within the salt lattice, preventing the formation of their distinct crystal structure and thus undetectable in the XRD measurements[48, 49],SEM images of the original sample, cycled sample, and adsorbed sample further corroborate the aforementioned hypothesis from a different perspective. As depicted in Fig. 7 (a), due to the hygroscopic nature of the samples, the surfaces of different substances are all encapsulated by the layers of CaCI(OH)2 and Ca(OH)2 formed after water absorption. This makes it challenging to directly observe differences between CaO particles and the molten salt layer on the surface of the Original Sample. However, larger and block-like particles can still be observed in certain regions, presumably formed by block-shaped CaO that was not dispersed by the molten salt. Upon comparing Fig. 7 (c) with Fig. 7 (b), it is apparent that the surface of the Adsorbed Sample in Fig. 7 (c) features numerous uneven particles. Given the corresponding XRD patterns, these can be identified as CaCOs particles. In contrast, Fig. 7 (b) reveals fewer visible particles on the surface, suggesting that CaO, at this stage, is well dispersed in the molten salt, lacking its distinct crystal structure.Fig. 8 illustrates the infrared spectral changes in the process of CaO adsorbing CO2 and subsequently undergoing the RWGS reaction in air and in a molten salt environment. This is with respect to an unreacted sample in a nitrogen environment at 650 °C. Table 1 presents the detected peaks and corresponding species during the ICCU-RWGS stage under CaO only conditions. In contrast, Table 2 presents the peaks detected and the corresponding species when CaO is in a moltensalt environment. It is noteworthy that, although H2 is not introduced in the CO2 capture stage, certain species carrying hydrogen bonds, such as C-H at 872 cm'1, 2850 cm'1, and 2950 cm'1, and OH at 2500 cm'1and 3570 cm'1, can always be detected due to the presence of a certain amount of water vapor in the gas

[0050] . For the case of CaO only, carbonate and bicarbonate, which are significant intermediate products during the CO2 adsorption and RWGS stages, have peaks located at 1780 cm'1and 2133 cm'1, respectively. During the CO2 capture stage, carbonate is a product, whereas the formation of bicarbonate suggests that part of the carbonate, as a weak base, has accepted a hydrogen ion from a water molecule, as shown in Eq. 2. In the RWGS stage, H2 gas will directly react with the carbonate, thereby producing CO[33, 40, 51], Consequently, peaks of both carbonate and bicarbonate will be detected.CO32-+ H2O = HCO3“ + OH- Eq. 2As shown in Fig. 8 (b), (d) and Table 2, during the process of CO2 capture using CaO in the molten salt environment, carbonate and bicarbonate acted as intermediate products, and there is a significant peak of carboxylate at 1554 cm-1. This indicates that CO2 in molten salt binds with metal ions, suggesting that, in addition to being adsorbed as CaCO3, CO2 may also be incorporated as Ca(COO)2 in the molten salt environments, 53], Furthermore, when the carboxylate ion is exposed to an acidic environment, it can accept a proton (H+) and form a carboxylic group, thereby causing a peak at 3155 cm'1for -COOH.Table 1 Summary of experimentally detected species during the ICCU-RWGS process for CaO only condition.Stage Wave number (cm'1) SpeciesCO2 capture stage 872, 2850, 2950 C-H1155 C-O stretching vibration1780 Carbonate

[0054] 1944 C=O stretching vibrations, gas phase2133 HCO3'2500 chelated OH3570 O-H stretching vibration (free)2370-2290, 3760-3630 CO2

[0055] RWGS stage 872, 2850, 2950 C-H1155 C-O stretching vibration1780 Carbonate1944 C=O stretching vibrations, gas phase2133 HCO3'2500 chelated OH3048-2895, 1306 CH43688 O-H stretching vibrationTable 2 Summary of experimentally detected species during the ICCU-RWGS process for CaO with the molten salts.Stage Wave number (cm'1) SpeciesCO2 capture stage 873, 2810, 2956 C-H1554 -COO- (salts)1781 Carbonate2117 HCO3’2594 chelated OH3155 -COOH2370-2290, 3760-3630 CO23549 O-H stretching vibration (free)RWGS stage 873, 2870 C-H1055 C-O stretching vibration1523 -COO-(salts)1764 Carbonate2117 HCO3’2500 chelated OH3564 O-H stretching vibration (free)Fig. 9 indicates the morphological changes during the CO2 adsorption and RWGS processes of CaO in air and molten salt environments. As shown in Fig. 9 (a), CaO undergoes volume expansion after adsorbing CO2. The regeneration of sorbent can be achieved after the RWGS process. However, after several cycles, sintering of CaO occurs, which reduces the number of active adsorption sites on the surface, thereby decreasing the adsorption capacity of CaO[56, 57], In the molten salt environment, as shown in Fig. 9 (b), within the liquid solution, although the solubility of CaCO3is very low, a fraction of CaCO3and Ca(COO)2 could be dissolved into the molten salt. The dissolved Ca2+ions separate from the bulk CaO, thereby exposing more active sites for CO2 adsorption. However, when the amount dissolved in the solution reaches saturation, the enhancement of the adsorption capacity of CaO by the molten salt will reach its limit. Similarly, in the RWGS stage, the dissolved carbonate ions and carboxylate ions are more likely to react with H2 since they are in a free state, substantially increasing the rate of CO2 conversion

[0058] . However, the increase in CO2 conversion rate also depends on the maximum solvable concentration, which explains the lowest CO2 capacity and CO yield per unit mass of CaO in the case of 20% wt CaO / MS as depicted in Fig. 5. After multiple cycles, this process ultimately results in the transformation of CaO into small particles dispersed within the molten salt, which therefore cannot be detected by XRD. Theoretically, for the ICCU process with CaO in a molten salt environment, Ca2+will continually disperse, effectively preventing sintering and thus ensuring excellent cycle stability.3.7 Molten SaltsA 4:6 mass ratio of NaCI : CaCh was initially selected for investigation. Our primary objective isto ensure that the salts become liquid at around 650 °C, which is the optimal operating temperature for the ICCU-RWGS process.The melting points for pure CaCh and pure NaCI are too high for our testing purposes. The melting point of NaCI is 801 °C, and the melting point of CaCh is 775 °C. These temperatures are above the optimal operating temperature for the ICCU-RWGS process, which is why we are exploring mixtures of these salts to lower the melting point and make the process more efficient.To identify other weight ratios that can maintain a liquid state at approximately 500 °C, we conducted experiments to assess the melting points of NaCI and CaCh mixtures at varying mass ratios. Five sets of samples with mass ratios of 2:8, 4:6, 5:5, 6:4, and 8:2 were prepared and placed in a muffle furnace. Each set was subjected to temperatures of 425 °C, 475 °C, and 525 °C for a duration of 3 hours. Upon cooling, the samples were examined to compare their physical properties before and after heating as an indicator of melting. As shown in Figure 10, the results demonstrate that, at a temperature of 525 °C, the samples with NaCI to CaCh mass ratios of 2:8, 4:6, and 5:5 underwent melting. Therefore, for the purposes of this study, any of these three mass ratios could be considered viable options.4 ConclusionsIn conclusion, this study explored the optimization of CaO and molten salt composition, operating temperature and CaO concentration for enhanced CO2 adsorption and conversion in the ICCU-RWGS process by molten salt. The use of a CaCh-NaCI mixture as the molten salt significantly enhanced CO2 uptake by CaO, leading to higher CO2 uptake (13.4-15.1 mmol.g-1) and improved CO yield (10.3- 7.3 mmol.g-1) compared to the system without molten salt (10.1 mmol.g-1CO2 uptake and 8.9 mmol.g-1CO yield). Temperature optimization revealed that 650 °C was the optimal temperature for achieving the highest CO yield of 9.1 mmol.g-1. Lower temperatures (550 °C and 600 °C) resulted in significantly lower CO yields (0.41 mmol.g-1and 1.2 mmol.g-1, respectively), while higher temperatures (700 °C) slightly increased CO yield (8.1 mmol.g-1) but decreased CO2 uptake. Optimizing the CaO concentration in the molten salt system revealed that a 10 wt % CaO concentration provided a CO yield of 10.3 mmol.g-1. Increasing the CaO concentration beyond 10 wt % resulted in a decrease in CO yield, with a drop to 4.0 mmol.g-1at 20 wt % CaO. The characterization techniques employed, including in-situ infrared spectroscopy, XRD, and SEM, provided valuable insights into the behavior of CaO in the molten salt environment.In-situ infrared spectroscopy analysis revealed that in the presence of molten salt, CO2 was incorporated as Ca(COO)2, acting as an important intermediate during CO2 adsorption and conversion. This differed from the behavior observed in the absence of molten salt. XRD analysis did not detect CaO or Ca(OH)2 in the cycled sample, suggesting that CaO might exist as fine particles dispersed within the molten salt. SEM images supported this observation, showing the dispersion of CaO in the molten salt environment. Overall, this study contributes to the understanding of optimizing CaO concentration, molten salt composition, and operating temperature for improved CO2 adsorption and conversion in the ICCU-RWGS process.ConclusionWe have identified several problems with respect to known products and processes, particularly when comparing them to our innovative CO2 capture and conversion technology utilising calcium oxide in molten salts: Limited scope: Many known processes of CO2 adsorption in molten salts, focus primarily on capturing CO2 without considering its conversion into valuable products. Without the conversion of CO2 and the subsequent regeneration of the molten salts, there is a continuous need for fresh molten salts, leading to increased resource consumption. Lower CO2 capture rates: Existing CO2 capture methods, including those that use molten salts, often exhibit relatively low CO2 capture rates.Our invention not only captures CO2 but also converts it into useful chemicals like carbon monoxide (CO) through the reaction between calcium carbonate (CaCOs) and hydrogen (H2).Our invention, by using molten salts as a medium for CaO, enhances both the adsorption rate and the reaction rate of the CO2 conversion process, offering a more efficient solution for CO2 capture and conversion. Integrated CO2 capture and ultilisation: Our process not only captures CO2 efficiently but also converts it into valuable products, such as carbon monoxide (CO), through the reaction between calcium carbonate (CaCOs) and hydrogen (H2). This integrated approach adds value to the captured CO2 and expands its potential applications. Enhanced adsorption and conversion rates: The use of calcium oxide in molten salts significantly increases the adsorption rate of CO2 and the reaction rate of the conversion process compared to gasphase reactions (by ~10 times). This enhancement in performance leads to a more efficient CO2 capture and conversion process. Recyclable molten salts: Our invention enables the recycling of molten salts, thanks to the conversion of captured CO2 into valuable products. This recycling process improves the overall sustainability and cost-effectiveness of the method, reducing the environmental impact and resource consumption. Potential for lower operating temperatures: By using molten salts as a medium for calcium oxide, our process may operate at lower temperatures compared to existing CO2 capture methods. 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[0052] C. Sandt, F. Borondics, Super-resolution infrared microspectroscopy reveals heterogeneous distribution of photosensitive lipids in human hair medulla, Taianta, 254 (2023) 124152.

[0053] G.-L. Liu, E. Guerreiro, C. Babington, S.G. Kazarian, ATR-FTIR spectroscopic imaging of white crusts in cross sections from oil cartoons by Edward Poynter in the Heritage Collections at UK Parliament, Journal of Cultural Heritage, 62 (2023) 251-267.

[0054] H. Sun, C. Wang, S. Sun, A.T. Lopez, Y. Wang, J. Zeng, Z. Liu, Z. Yan, C.M.A. Parlett, C. Wu, XAS / DRIFTS / MS spectroscopy for time-resolved operando study of integrated carbon capture and utilisation process, Separation and Purification Technology, 298 (2022) 121622.

[0055] H. Chen, S. Chansai, S. Xu, S. Xu, Y. Mu, C. Hardacre, X. Fan, Dry reforming of methane on bimetallic Pt-Ni@CeO2 catalyst: a in situ DRIFTS-MS mechanistic study, Catalysis Science & Technology, 11 (2021) 5260-5272.

[0056] L. Zhang, Y. Lu, M. Rostam-Abadi, Sintering of calcium oxide (CaO) during CO2 chemisorption: a reactive molecular dynamics study, Phys Chem Chem Phys, 14 (2012) 16633-16643.

[0057] C. Li, C. Zhang, X. Guo, Sintering mechanism of CaO during carbonation reaction in the presence of water vapor, Proceedings of the Combustion Institute, (2022).

[0058] M.A. Murmura, S. Lo Conte, F. Santoni, M. Della Pietra, L. Turchetti, M.C. Annesini, Two-dimensional modeling and experimental investigation of an inverse molten carbonate fuel cell, Journal of Power Sources, 573 (2023).Additional characterization and References:As shown in Fig. 11 (a), the in-situ infrared spectra during the CO2 capture stage reveal distinct CO2 peaks at 2401 cm-1to 2209 cm'1and 3755 cm'1to 3568 cm-1[1]. In Fig. 11 (b), peaks associated with carbonate at 2970 cm'1, 2909 cm'1, 2862 cm'1, 2593 cm'1, and 2501 cm'1progressively form during CO2 capture stage[2]. The two distinct peaks at 1513 cm-1and 1407 cm'1may indicate the presence of two different types of carbonates, specifically CaCOs and Na2CO3[3], As illustrated in Fig. 11 (c), the fluctuations between the two peaks, representing free carbonate, and the oscillations between 1760 cm'1and 1640 cm'1, representing COO-, reflect the free movement of carbonate ions within the molten salt [4], This might indicate that carbonate ions can move freely in molten salt.Fig. 12 presents the in-situ variable-temperature Raman spectra of the sample during the RWGS phase, ranging from 25 °C to 650°C. As shown in Fig. 12 (a), the O-H group at 3472 cm-1indicates the presence of moisture in the sample[5]. When the temperature rises to 500 °C, the moisture evaporates, and the O-H peak disappears. However, as the temperature continues to rise to 550 °C, the O-H peak reappears. This suggests that the critical temperature for the onset of the RWGS reaction is between 500 °C and 550 °C. Similarly, at temperatures above 550 °C, the peak representing carbonate at 232 cm-1disappears[6], while the peak representing CaO at 340 cm-1appears[7]. Correspondingly, in Fig. 12 (c), the peaks representing carbonate at 1503-1496 cm-1and1097-1072 cm-1disappear at higher temperatures [8], Notably, in Fig. 12 (b), the appearance of the peak at 1533 cm-1indicates the presence of Metalx(HCOO)y, suggesting that HCOO is an intermediate product in the reaction between carbonate and H2to produce CO[9].’ References[1] S. Sun, Y. Wang, Y. Xu, H. Sun, X. Zhao, Y Zhang, X. Yang, X. Bie, M. Wu, C. Zhang, Y Zhu, Y Xu, H.Zhou, C. Wu, Ni-functionalized Ca@Si yolk-shell nanoreactors for enhanced integrated CO2 capture and dry reforming of methane via confined catalysis, Applied Catalysis B: Environment and Energy, 348 (2024) 123838.[2] H. Sun, C. Wang, S. Sun, A.T. Lopez, Y. Wang, J. Zeng, Z. Liu, Z. Yan, C.M.A. Parlett, C. Wu,XAS / DRIFTS / MS spectroscopy for time-resolved operando study of integrated carbon capture and utilisation process, Separation and Purification Technology, 298 (2022) 121622.[3] V.H.J.M.d. Santos, D. Pontin, G.G.D. Ponzi, A.S.d.G.e. Stepanha, R.B. Martel, M.K. Schutz, S.M.O.Einloft, F. Dalia Vecchia, Application of Fourier Transform infrared spectroscopy (FTIR) coupled with multivariate regression for calcium carbonate (CaCOs) quantification in cement, Construction and Building Materials, 313 (2021) 125413.[4] Y. Foucaud, J. Laine, L.O. Filippov, O. Barres, WJ. Kim, I.V. Filippova, M. Pastore, S. Lebegue, M.Badawi, Adsorption mechanisms of fatty acids on fluorite unraveled by infrared spectroscopy and first-principles calculations, Journal of Colloid and Interface Science, 583 (2021) 692-703.[5] R. Frost, M. Weier, Raman spectroscopic study of vivianites of different origins, Neues Jahrbuch furMineralogie, Abhandlungen, 10 (2004) 445-463.[6] F.A. Andersen, L. Brecevic, G. Beuter, D.B. Dell’Amico, F. Calderazzo, N.J. Bjerrum, A.E. Underhill,Infrared spectra of amorphous and crystalline calcium carbonate, Acta Chem. Scand, 45 (1991) 1018-1024.[7] J. Perrin, D. Vielzeuf, D. Laporte, A. Ricolleau, G.R. Rossman, N. Floquet, Raman characterization of synthetic magnesian calcites, American Mineralogist, 101 (2016) 2525-2538.[8] L. Bayarjargal, C.J. Fruhner, N. Schrodt, B. Winkler, CaCOs phase diagram studied with Raman spectroscopy at pressures up to 50 GPa and high temperatures and DFT modeling, Physics of the Earth and Planetary Interiors, 281 (2018) 31 -45.[9] M. Mqczka, T.A. da Silva, W. Paraguassu, K.P Da Silva, Raman scattering studies of pressure- induced phase transitions in perovskite formates [(CH3) 2NH2][Mg (HCOO) 3] and [(CH3) 2NH2][Cd (HCOO) 3], Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy, 156 (2016) 112-117.

Claims

Claims:1 . A method for removing carbon dioxide from a gas stream and for converting the carbon dioxide into a carbon dioxide utilisation product, the method comprising(i) bringing the gas stream in contact with an absorption medium, wherein the absorption medium comprises molten salts, and a metal oxide that reacts with the carbon dioxide to form a metal carbonate; and(ii), bringing the molten salts containing metal carbonate into contact with H2, to release the metal oxide and the carbon dioxide utilisation product selected from carbon monoxide, methane and methanol, wherein, further optionally, the carbon dioxide utilisation product is carbon monoxide, characterised in that the molten salts consist of sodium halide and calcium halide.

2. A method according to Claim 1 , in which the molten salts consist of NaCI and CaCh.

3. A method according to Claim 1 or 2, in which the mass ratio of metal oxide to molten salts is in the range of 0.2 to 0.75 g metal oxide : 5 g of molten salt.

4. A method according to Claim 3, in which the mass ratio of metal oxide to molten salt is in the range of 0.4 to 0.6 g metal oxide : 5 g of molten salt.

5. A method according to Claim 4, in which the mass ratio of metal oxide to molten salt is about 0.5 g metal oxide : 5 g of molten salt.

6. A method according to any one of Claims 1 to 5, in which the molten salts consist of NaCI to CaC in a mass ratio in the range of 1 : 4 to 1 : 1 .

7. A method according to Claim 6, in which the molten salts consist of NaCI to CaCh in a mass ratio in the range of 1 : 3 to 1 : 2.

8. A method according to Claim 7, in which the molten salts consist of NaCI to CaC in a mass ratio in the range of about 4 : 6.

9. A method according to any one of Claims 1 to 8, in which the gas stream is brought in contact with the absorption medium at a temperature in the range of 600 to 725° C and the metal oxide is CaO.

10. A method according to any one of Claims 1 to 9, in which the temperature is in the range of 625 - 675 °C; optionally in which the temperature is 650 °C + 10 °C.11 . A method according to any one of Claims 1 to 8, in which the metal oxide is selected from MgO, SrO, BaO, IJ2O, Na2O, K2O, Rb2O and CS2O.

12. Use of an absorption medium comprising a metal oxide dissolvable in molten salts consisting of sodium halide and calcium halide for the removal of carbon dioxide from a gas stream and for the conversion of the carbon dioxide into a carbon dioxide utilisation product selected from carbon monoxide, methane and methanol, wherein, optionally, the carbon dioxide utilisation product is carbon monoxide.

13. Use according to Claim 12, in which the molten salts consist of NaCI and CaCh.

14. Use according to Claim 12 or 13, in which the mass ratio of metal oxide to molten salts is in the range of 0.2 to 0.75 g metal oxide : 5 g of molten salt.

15. Use according to Claim 14, in which the mass ratio of metal oxide to molten salt is in the range of 0.4 to 0.6 g metal oxide : 5 g of molten salt.

16. Use according to Claim 15, in which the mass ratio of metal oxide to molten salt is about 0.5 g metal oxide : 5 g of molten salt.

17. Use according to any one of Claims 12 to 16, in which the molten salts consist of NaCI to CaC in a mass ratio in the range of 1 : 4 to 1 : 1 .

18. Use according to Claim 17, in which the molten salts consist of NaCI to CaCh in a mass ratio in the range of 1 : 3 to 1 : 2.

19. Use according to Claim 18, in which the molten salts consist of NaCI to CaC in a mass ratio in the range of about 4 : 6.

20. Use according to any one of Claims 12 to 19, in which the metal oxide is selected from CaO, MgO, SrO, BaO, IJ2O, Na2O, K2O, Rb2O and CS2O; optionally CaO.21 . An absorption medium for the removal of carbon dioxide from a gas stream and for the conversion of the carbon dioxide into a carbon dioxide utilisation product selected from carbon monoxide, methane and methanol, wherein, optionally, the carbon dioxide utilisation product is carbon monoxide, characterized in that the absorption medium comprises metal oxide dissolved in molten salts consisting of sodium halide and calcium halide.

22. An absorption medium according to Claim 21 , in which the molten salts consist of NaCI and CaC .

23. An absorption medium according to Claim 21 or 22, in which the mass ratio of metal oxide to molten salts is in the range of 0.2 to 0.75 g metal oxide : 5 g of molten salt; optionally in which the mass ratio of metal oxide to molten salt is in the range of 0.4 to 0.6 g metal oxide : 5 g of molten salt;further optionally in which the mass ratio of metal oxide to molten salt is about 0.5 g metal oxide : 5 g of molten salt.

24. An absorption medium according to any one of Claims 21 to 23, in which the molten salts consist of NaCI to CaCh in a mass ratio in the range of 1 : 4 to 1 : 1 ; optionally in which the molten salts consist of NaCI to CaC in a mass ratio in the range of 1 : 3 to 1 : 2; further optionally in which the molten salts consist of NaCI to CaC in a mass ratio in the range of about 4 : 6.

25. An absorption medium according to any one of Claims 21 to 24, in which the metal oxide is selected from CaO, MgO, SrO, BaO, IJ2O, Na2O, K2O, Rb2O and CS2O; optionally CaO.

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