In-situ Observation of Carbon Dioxide Capture on Pseudo-Liquid

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In-situ Observation of Carbon Dioxide Capture on PseudoLiquid Eutectic Mixture-Promoted Magnesium Oxide Hanyeong Lee, Monica L.T. Trivino, Soonha Hwang, Sung Hyun Kwon, Seung Geol Lee, Jun Hyuk Moon, Jungho Yoo, and Jeong Gil Seo ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b14256 • Publication Date (Web): 26 Dec 2017 Downloaded from http://pubs.acs.org on December 26, 2017

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ACS Applied Materials & Interfaces

In-situ Observation of Carbon Dioxide Capture on Pseudo-Liquid Eutectic Mixture-Promoted Magnesium Oxide Hanyeong Lee,†, § Monica Louise T. Triviño,†, § Soonha Hwang,† Sung Hyun Kwon,‡ Seung Geol Lee,‡ Jun Hyuk Moon,∥ Jungho Yoo,┴ Jeong Gil Seo†,* †

Department of Energy Science and Technology, Myongji University, Yongin, 17058, Republic

of Korea ‡

Department of Organic Material Science and Engineering, Pusan National University, Pusan,

46241, Republic of Korea ∥Energy

Lab, Samsung Advanced Institute of Technology, Yongin, 446-712, Republic of Korea

┴

National NanoFab Center, Daejeon, 34141, Republic of Korea

KEYWORDS: CO2 capture, in-situ TEM, direct observation, eutectic mixture, magnesium oxide

ACS Paragon Plus Environment

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ABSTRACT. Eutectic mixtures of alkali nitrates are known to increase the sorption capacity and kinetics of MgO-based sorbents. Underlying principles and mechanisms for CO2 capture on such sorbents have already been established; however, real-time observation of the system was not yet accomplished. In this work, we present the direct-observation of the CO2 capture phenomenon on a KNO3-LiNO3 eutectic mixture (EM)-promoted MgO sample, denoted as KLM, via in-situ transmission electron microscopy (in-situ TEM). Results revealed that the pseudo-liquid EM undergoes structural rearrangement as MgCO3 evolves from the surface of MgO, resulting to surface roughening and evolution of cloudy structures that stay finely distributed after regeneration. From this, we propose a nucleation and structural rearrangement scheme for MgCO3 and EM, which involves the rearrangement of bulk EM to evenly distributed EM clusters due to MgCO3 saturation as adsorption proceeds. We also conducted studies on the interface between EM over solid MgO and MgCO3 formed during sorption, which further clarifies the interaction between MgO and EM. This study provides better insight to the sorption and regeneration mechanism, as well as the structural rearrangements involved in EM-promoted sorbents by basing not only on intrinsic evolutions but also on real-time observation of the system as a whole.


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INTRODUCTION Fossil fuel remains to be the world’s primary energy resource despite the advent of unconventional and alternative energy resources, and its substitution is not yet imminent in the next few years or so.1 The production of energy from such resource remains to be the major source of anthropogenic CO2 emissions, with its damaging effect rapidly accelerating in the past decades. In this regard, practical and cost-effective technologies to capture and greatly reduce greenhouse gas emissions are critically needed. Currently, carbon capture and sequestration (CCS) is among the leading technologies that can significantly and practically reduce greenhouse gas emissions, as various transformational capture technologies are close to application.1–6 Among the materials available, solid sorbents are considered to be promising because they are applicable over a wide range of temperatures (ambient to 700˚C), yields less waste during cycling, and are easier and safer to dispose.7,8 Solid sorbents are also easier to regenerate and are applicable at higher pressures as compared to liquid sorbents, making them more efficient.9–14 MgO, which chemically reacts with CO2 to form thermally stable MgCO3, is regarded as the most promising sorbent for intermediate temperature CO2 capture due to its abundance in basic sites, high stability, and lower energy requirement for regeneration. However, the CO2 sorption capacity of MgO is significantly hindered by its relatively slow kinetics and tendency to agglomerate after high-temperature regeneration.8,15,16 Recently, MgO-based sorbents promoted by alkali salts and binary or ternary eutectic mixtures have gained much attention for intermediate temperature CO2 capture due to their improved CO2 uptake and MgO conversion, providing higher efficiency with lower energy costs. 17–21 Although previous works have been successful in increasing the CO2 sorption capacity and kinetics of MgO using molten salt promoters, much improvement on sorbent stability and


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robustness must be made. To address this, various studies have attempted to establish the promoting mechanism of molten salts, but most results were merely based on theoretical analyses, experimental interpretations, and intrinsic evolutions of individual components rather than on direct observation of the interactions happening in the system as a whole. To better understand the underlying mechanisms and provide solutions to improve the performance and stability of EM-promoted sorbents, real-time observation and understanding of structural rearrangements and evolutions are crucial. For this purpose, advanced characterization techniques such as in-situ transmission electron microscopy (in-situ TEM) may be used. In-situ TEM has been widely employed to directly observe structural changes occurring in modified nanoparticles and nanocrystals, as well as to investigate different material aspects for mechanical, electrochemical, catalytic, and other various applications.22–27 In-situ TEM allows the continuous observation of a material subjected under varying experimental conditions, allowing the observation of growth mechanisms, nucleation events, formation of transient phases, and other behavioral changes under real processing or experimental conditions, all at an atomic resolution.28,29 However, recording and acquisition of desirable results are demanding due to the increased experimental complexity of this technique. Moreover, analyses of samples subjected to high temperatures and involving gas phase reactions remain challenging due to the difficulty in handling the equipment and maintaining a stable environment for sampling. Other limitations such as unexpected reactions or behavioral changes due to huge electron dosages and lack of atomic resolution under gas flow also limit its application.30–32 So far, in-situ studies on CO2 capture systems only involve the use of analysis tools that confirm the evolution of new compounds and intrinsic or compositional changes of each individual component of the process, such as in-situ FTIR and in-situ XRD, rather than seeing the whole system and the interaction of


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the components. There are also no known reports on the use of in-situ TEM for observing CO2 capture systems. In this work, we successfully carried out the direct observation of the triplephase capture process involved in the sorption of gaseous CO2 over molten EM-promoted solid MgO via in-situ TEM and detected the changes happening within the system as a whole in real time. We first optimized the sorbent and thoroughly investigated and verified the main points of the reaction mechanism proposed by the previous studies such as (1) the dissolution and interaction of MgO in the molten nitrate promoters with CO2 thus forming MgCO3 and (2) the regeneration of MgCO3 back to MgO. We also conducted in-situ XRD, DFT calculations, washing experiments, and CO2-TPD to investigate the interface layer interaction of the liquid eutectic mixture over solid MgO and MgCO3, and correlated these results with the results obtained from in-situ TEM observation to formulate a plausible structural rearrangement and evolution mechanism occurring within the system.

EXPERIMENTAL SECTION Preparation of KNO3-LiNO3-MgO The eutectic mixture containing magnesium oxide-based sorbent was prepared via a simple physical mixing technique. The as-synthesized magnesium oxide was first obtained by calcining 5g of magnesium hydroxide at 800°C for 5 hours at a ramping rate of 5°C/min. The binary eutectic mixture was prepared by thoroughly mixing and grinding 59 mol % of KNO3 and 41 mol % of LiNO3.33 The resulting nitrate mixture was then fused at 100°C above its eutectic temperature for 12 hours to homogenize the melt.34 The obtained EM was comminuted and added to the as-synthesized magnesium oxide in a 3.75:6.25 EM:MgO weight ratio. The combined powder mixture was calcined in a tubular furnace at 400°C for 5 hours at a ramping


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rate of 2°C /min under a nitrogen gas flow of 100 ml/min and slowly cooled to room temperature. We designated this newly synthesized sorbent as KLM (K,Li-NO3 MgO). For preliminary screening, we also synthesized a different EM-MgO sample using 54 mole % of LiNO3 and 46 mole % NaNO3 by following the same procedures and compared its performance with KLM. All chemicals used were purchased from Sigma-Aldrich and used with no further purification. X-ray diffraction (XRD) pattern, Brunauer, Emmett, and Teller (BET) surface areas, inductively coupled plasma (ICP) analysis, and field emission scanning electron microscopy (FE-SEM, Zeiss Sigma) images of the synthesized sorbents were also obtained. Experimental details are included in the supporting information. Sorbent Performance Measurement and Interface Analysis We measured the CO2 sorption capacity, sorption kinetics, and recyclability of the sorbent from thermogravimetric analysis by TGA-N1000 (Scinco). The CO2 sorption behavior of the sorbent was investigated under various concentrations of dry CO2 gas (100%, 50% and 15% CO2-N2 balance), as well as under pure N2 gas flow of 70 ml/min by subjecting the sample to non-isothermal sorption from room temperature to 600°C with a ramping rate of 5°C/min. We determined the rate of CO2 uptake by isothermal tests at 300 to 400°C, with increment temperatures of 25°C for 25 minutes. After identifying the optimum sorption temperature, the cyclic test was performed under 60 minutes of 100% CO2 sorption and 10 minutes desorption at 500°C under pure N2 gas flow. In-situ x-ray powder diffraction was performed to examine the structural transformation and phase changes of MgO and the eutectic mixture during CO2 absorption by increasing the temperature from 25 to 450°C then decreasing back to room temperature. The data were obtained using X’Pert Pro instrument (PANanalytical) using Cu Kα radiation (λ=1.540598 Å) at 40 kV


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and 30 mA with a step size of 0.0167° and a scan speed of 40 seconds per step. We collected the in-situ XPD data at each increment temperature of 50°C after reaching equilibrium for 30 minutes under a gas flow of 100% CO2 and 15% CO2 with N2 balance at a rate of 70 ml per minute. Density functional theory (DFT) calculations, washing experiments, and CO2 temperature desorption (CO2-TPD) analysis were also done to further explain the CO2 capture phenomena on the sorbent. Details of these analyses are also included in the supporting information. Direct Observation of CO2 Sorption on KLM In-situ transmission electron microscopy (TEM) experiments were performed using a JEM3011 HR (JEOL) electron microscope equipped with a modified heating and gas injection system.35 The equipment was operated with a constant electron energy of 300 keV. We obtained the in-situ TEM images by exposing the sample to 100% CO2 gas (vacuum to 70 mbar) and elevated temperatures (room temperature to 430°C). Exposure of the sample to the electron beam was minimized before CO2 injection to prevent damaging or breaking sample due to the applied electron energy. A carbon-coated triacetylcellulose film attached to a 7-hole copper grid was used to separate the gas injection room on the specimen from the vacuum environment. The powdered sample was manually introduced on the surface of the heating wire, with the wire carefully loaded on the specimen in a vertical alignment. We controlled the temperature by adjusting the voltage value applied to the wire. All parts of the assembly were subjected to an ion-cleaning process before use. An illustration of the in-situ TEM set-up is shown in Figure S1.

RESULTS AND DISCUSSION


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Optimization of Eutectic Mixture Composition and Loading. Due to the reported effectiveness of alkali nitrate salts for improving the CO2 capture performance of MgO, we decided to investigate eutectic mixtures of these promoters, with basis on existing literature data.33 Eutectic mixtures are advantageous because they result to a lower melting temperature at a specific eutectic point, thus giving a wider range of temperatures for application. Based from previous studies, binary EM-promoters used for MgO sorbents have eutectic temperatures and CO2 capture capacities that are comparable to known ternary mixtures.18,19,36 Moreover, these mixtures are much easier to prepare, have available set of thermodynamic and electrical properties, and are much simpler to analyze, thus eliminating complexities in DFT calculations.33,37–39 Specifically, a 59:41 mole ratio of KNO3:LiNO3 and a 54:46 mole ratio of LiNO3:NaNO3 were chosen for preliminary analysis, as these salt mixtures give relatively low eutectic temperatures (124°C and 193°C, respectively), and are practical to use among plausible binary eutectic salts.33 Other binary eutectic mixtures of nitrate salts are listed in Table S1. It was found out that K,Li-NO3 gives higher CO2 uptake and is in effect, more suitable as a promoter. Optimization tests shown in Figure S5a also revealed that an EM loading of 37.5 wt% K,Li-NO3 is most suitable. This sample was used for further tests and was denoted as KLM. Details of the EM screening tests, optimization tests, and sorbent characterization before and after sorption are thoroughly discussed in the Supporting Information. The physical properties and actual composition of the KLM sorbent are also listed in Table 1. The sorbent achieved low surface area and pore volume due to the melting of EM during calcination, which caused the EM to spread and cover the pores and surface of MgO. The actual compositions obtained also do not deviate much from the theoretical composition, showing that there is negligible loss of EM during the synthesis procedure.


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Table 1. Physical properties and composition of KLM. Property

Value

Average Particle Sizea

836 nm

Surface Area

2.7 m2/g

Pore Volume

0.02 cm3/g

Crystallite Sizeb

a

321 nm

MgO wt %c

66.42

LiNO3 wt%c

10.25

KNO3 wt%c

23.33

Average particle size was obtained from FE-SEM images. bCrystallite size was calculated from

the (200) XRD diffraction peak of MgO using Scherrer equation. cActual weight compositions were obtained from ICP analysis. CO2 Capture Performance. The CO2 capture performance of the sorbent was investigated by thermogravimetric analysis. The change in weight of KLM as the temperature increased from room temperature to 600°C under a flow of 100%, 50%, and 15% CO2 (balanced with N2) is shown in Figure 1a. The sample showed an onset sorption temperature at around 130˚C for all conditions, which is about 5˚C higher than the eutectic temperature of the K,Li-NO3 mixture used in this study.33 Decreasing the CO2 concentration also narrows down the temperature window, as evidenced by the 100°C decrease in complete desorption point (the temperature at which all CO2 obtained has been desorbed) under 100% to 15% CO2. The maximum CO2 uptake also decreased from 45.3 wt% or 65.9% MgO conversion to 17.2 wt% or 25.0% MgO conversion. These results suggest that apart from varying the proportions of MgO and EM to control the capacity, it is also possible to adjust the temperature window of KLM by optimizing the CO2 concentration for applications under various capture conditions.


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Figure 1. (a) Sorption behavior of KLM by temperature sweeping from 25 to 600°C (5°C/min ramping rate) under different CO2 gas concentrations. (b) Sorption isotherms of KLM for 25 minutes at 325°C, 350°C, 375°C, and 400°C under 100% CO2 gas. (c) KLM cyclic sorption-


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regeneration test under pure CO2 for sorption (350°C, 1hr) and pure N2 for regeneration (500°C, 10 min). Figure 1b shows the effect of temperature on the performance of KLM under 100% CO2 gas flow within a high-temperature range of 325°C to 400°C with an incremental increase of 25°C. Results show that the optimum temperature for isothermal sorption is at 350°C, where the uptake is higher by 4.0% in the first 10 minutes and 2.8% after 25 minutes, as compared to that at 325°C. However, CO2 diffusion in the molten EM layer is unfavorable at higher temperatures, causing the slower rate of sorption as the temperature gets nearer to the complete desorption point. On the other hand, the TGA result of the cyclic test for KLM is presented in Figure 1c. The sorbent exhibited a CO2 uptake of 45.3 wt%, 40.0 wt% and 34.9 wt% for the 1st, 5th, and 10th cycles respectively, and attains a stable sorption of around 34 wt% until the last cycle. We observed a loss in the CO2 uptake of about 25% between the first and the 14th cycle, suggesting that after the first cycle, KLM does not return to its original state. This is complemented by the XRD results shown in Figures S5a and S5b, wherein a slight shift in diffraction peaks is observed between the fresh and desorbed samples. Although the sorbent experienced a loss in capacity over multiple sorption-desorption cycles, it can still be considered stable towards regeneration as it achieves a capacity drop that is much less than other molten alkali nitrate saltpromoted MgO systems.17,19,36,40 It is expected that the sorbent will only experience minimal loss in capacity following the 14th cycle, as the sorbent already maintained a stable capacity for four consecutive cycles. In-situ XRD Findings. Parallel results were obtained from the in-situ XRD patterns shown in Figure 2 and the temperature sweeping experiments from Figure 1a, which both described the response of KLM under pure and dilute CO2. Figure 3a summarizes the congruency of these


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results. MgCO3 peaks started appearing at a temperature of 200°C and became more pronounced as the temperature increased. Carbonate peaks disappeared at 400°C for the sample subjected to 15% CO2 sorption (Figure 2a), matching the complete desorption point observed. On the other hand, the sample subjected to 100% CO2 sorption (Figure 2b) did not show the disappearance of carbonate peaks until 450°C, as it is expected that the complete desorption point at this condition would be around 500°C. A slight reduction of EM peaks at 125°C for both conditions indicates the expected onset melting of EM at this temperature.33 However, it was observed that at a temperature of around 200°C, EM peaks did not completely disappear but were rather present in another form. KNO3 was initially present in its orthorhombic form (PDF-ICDD-00-005-0377), while LiNO3 is in the rhombohedral form (PDF-ICDD-01-080-0203) in the fresh KLM samples. In-situ XRD results revealed that instead of completely melting to its molten state, the EM transforms into a pseudo-liquid state, with a solid phase transformation of KNO3 from orthorhombic to rhombohedral crystal structure (PDF-ICDD-01-076-1693). This transformed solid form exists with the molten EM and may be considered to appear at temperatures of around 200°C to a little more than 300°C, as observed from the in-situ XRD results. At 350°C, no EM peaks were observed, thus confirming the complete melting of the EM. This explains why the optimum temperature reached by the sample is around 350°C in the isothermal test shown in Figure 1b. The EM retained its rhombohedral structure even after cooling back to 25°C, showing that an irreversible crystal structure phase change occurred.


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Figure 2. In-situ XRD of KLM during sorption under (a) 15% CO2 gas flow and (b) pure CO2 gas flow with increasing temperature from 25˚C to 450˚C. Aside from the EM, MgO also underwent some structural changes along the sorption process. As shown in Figures 3a and 3b, MgO peaks experience a slight shift to lower angles when EM is in its molten state, which is indicative of the occurrence of a lattice expansion that may be attributed to the partial dissolution of surface MgO in the liquid EM at high temperatures. In addition, Table S4 shows that the crystallite size of MgO increased after cooling back to 25°C. The same phenomenon is expected to occur when the sorbent is regenerated under pure N2 at the desired desorption temperature, which may be attributed to the formation of larger agglomerated MgO particles as MgCO3 is transformed back to MgO, thus contributing to the lessened capacity of the sorbent under cyclic test as observed in Figure 1c. Additional discussion of the in-situ XRD results, as well as the complete XRD patterns (with results from 25°C to 450°C at 50°C intervals), are shown in the Supporting Information.


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Figure 3. (a) Summary of congruent results obtained from in-situ XRD and temperature sweeping tests under pure and dilute conditions. Peak shifts of MgO in KLM from in-situ XRD data under (b) 100% and (c) 15% CO2 gas flow. Interface Analysis. Based on the established reaction mechanisms for CO2 sorption on molten nitrate-promoted MgO sorbents, we formulated the following reaction equations (1, 2, and 3) to represent the steps involved in the carbonation reaction between the KNO3-LiNO3, MgO, and CO2. We assumed that the [Mg2+ ••• O2-] ion pair dissociates from MgO(s) surface and produces the [Mg2+ ••• CO32-] ion pair when CO2 diffuses into the eutectic mixture, with reference to previous studies cited.17–19,40 The in-situ XRD results also support this assumption, which


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manifests the dissolution of MgO via the lattice expansion discussed, and the subsequent appearance of MgCO3 while the EM is in its molten form in the presence of CO2. MgO(s) + KNO3-LiNO3(l) ↔ [Mg2+•••O2(1) ][KNO3-LiNO3(l)] CO2(g) + [Mg2+•••O2-][KNO3-LiNO3(l)] (2) 2+

→ [Mg

•••CO32-]

[KNO3-LiNO3(l)]

[Mg2+•••CO32-][KNO3-LiNO3(l)] ↔ (3) MgCO3(s) + KNO3-LiNO3(l) We performed DFT calculations to describe the effect of K,Li-NO3 based on this reaction pathway. Results indicate that the energy required for dissociation of MgO to [Mg2+••• O2-] ion pair is lower by 2.080 eV in the presence of the eutectic mixture, while the energy required for producing [Mg2+••• CO32-] ion pair upon CO2 diffusion decreased by 2.815 eV, representing a kinetically favorable pathway as depicted in Figure 4. Like previously studied promoters, the KNO3-LiNO3 eutectic mixture significantly promoted the dissociation of MgO and the sorption of CO2, further confirming the outstanding catalytic role of molten salts in this investigation.


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Figure 4. Relative energy differences according to the reaction pathway of MgO sorbent with (─) and without (···) eutectic mixture. Washing experiments on fresh KLM and MgCO3-saturated KLM (KLM-MgCO3) samples were done to determine the effect and affinity of EM on both MgO and MgCO3. As deduced from Table S5, the eutectic mixture has a greater surface coverage on KLM than on KLMMgCO3, with approximately 81.5% coverage of the MgO surface as compared to 76.2% of the MgCO3-saturated KLM surface. Furthermore, the pore volume change also showed such trend after washing. This implies that the EM has a greater affinity to MgO than MgCO3, which was also the same conclusion obtained in the study by Jo et. al., wherein they investigated the wetting properties of a KNO3-LiNO3-NaNO3 eutectic mixture by conducting contact angle measurements.40 According to their work, the lesser attraction of MgCO3 to the molten salt mixture may cause it to form outside of the promoter. This implies that when MgCO3 is regenerated back to MgO, the total MgO surface may have lesser contact with the promoter, thereby decreasing the CO2 uptake as regeneration cycles proceed. It is possible that the same


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phenomenon occurred with KLM, as evidenced by its decreased cycle performance. However, the decrease in surface coverage and increase in surface area of KLM upon MgCO3 saturation may also be indicative of the initial formation of MgCO3 at the triple phase boundaries, occupying much of the space originally covered by the EM. Either way, this still result to the formation of regenerated MgO without promoter. On another note, a rough approximation of the amount of MgCO3 that formed within the promoter was obtained by looking at the sorption kinetics of the sample under isothermal sorption (Figure 1b) and by referencing previous studies on the kinetics of molten salt-promoted sorbents. This is discussed in the supporting information and is shown in Figure S8. To study the effect of the eutectic mixture on the desorption kinetics of the sorbent, CO2-TPD profiles of the washed and unwashed KLM-MgCO3 were obtained. It is evident from Figure S9d that unwashed KLM-MgCO3 obtained a slightly lower desorption temperature than the washed sample, suggesting that EM(l)/MgCO3(s) require a lower desorption energy in order to regenerate back to EM(l)/MgO(s). This also implies that the eutectic mixture allows the easier dissolution of MgCO3 to [Mg2+•••CO32-], which is analogous to its effect on MgO.17 Moreover, a small amount of CO2 is not completely desorbed, with about 0.6 mmol of carbonate species remaining in the sorbent, as listed in Table S6. These undesorbed carbonate species may belong to the MgCO3 that initially formed at the triple phase boundaries, which were previously said to have lesser coverage of the EM, thus having less capacity to dissociate into [Mg2+•••CO32-]. Hence, results from DFT calculations, washing experiments, and CO2-TPD are very much consistent. In-situ TEM Observation of KLM. Although previous experimental results have already given insight into the interaction of EM, MgO, and MgCO3, much proof and experimental tests are needed to confirm the deduced conclusions. Thus, we conducted a real-time investigation of


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the actual surface reaction and sorbent behavior during sorption and desorption via in-situ TEM method. The observed surface change and evolution process are shown in Figure 5. The solidstate eutectic mixture coexisted on the MgO surface at room temperature before CO2 sorption. Upon elevating the temperature to 350°C, the optimum CO2 sorption point, the eutectic mixture existed as a melted phase on MgO substrate and spread out on its surface (Figure 5a), which caused significant changes in the structure observed. The decrease in intensity of the dark areas may be attributed to the EM melting and spreading to the edges of the structure, as well as to the dissolution of MgO in the EM(l) phase, as suggested by the mechanisms proposed previously. CO2 flow was then introduced to the system while maintaining the temperature at 350°C. Minimal damage and structural displacement were observed in the sample after CO2 injection, thus showing the stability and robustness of the sample against the measurement parameters employed. As observed from Figure 5b, irregular surface evolution at the easily accessible edges of the sorbent is already visible as early as 80s of CO2 sorption. This behavior is expected as Figure 1b shows that the sorbent has fast sorption kinetics with a negligible induction period for CO2 uptake to initiate at a constant temperature of 350°C. As CO2 sorption proceeds, the evolved surface structures become more prominent at the areas at which they initially formed and are also observed to spread out and form layers of new surfaces at other areas until reaching 210s. The in-situ TEM video for this process may be viewed from the Supporting Information (In-situ TEM_fresh EM-MgO.avi). The observed structural evolution cannot be attributed to possible boiling or phase change of the EM as it is not expected that the EM will undergo decomposition at this temperature.39 This thus suggests that the initial formation of MgCO3 happens most favorably at the triple phase boundary, followed by its subsequent formation at the MgO-EM interface. The sample was further maintained at 350°C under a CO2 atmosphere for 10


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minutes before desorption. Figure 6 shows the in-situ TEM image of the sample right before desorption (i.e. in-situ TEM image after 10 mins sorption). The structure observed at this instance shows a significant difference from those observed from Figure 5b. The protruding structures initially observed at 210s seem to have covered the surface of the sample and may be thought of as being evenly distributed at the edges of the sample. The widely dispersed seemingly cloudy structures that have evolved may be attributed to the formation of MgCO3 particles that saturate the surface of bulk MgO, as well as to the disruption and separation of EM into clusters due to this saturation.


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Figure 5. (a) TEM images of (K,Li)NO3-MgO before CO2 exposure. Time evolution of (b) CO2 sorption at 350˚C under 70 mbar CO2, (c) regeneration at 500˚C under vacuum, and (d) resorption under 70 mbar CO2.

Figure 6. In-situ TEM image of MgO-EM under CO2 adsorption at 350˚C after 600s. The desorption process was also observed for 25 minutes under vacuum, as shown in Figure 5c. It is evident that the overall shape of the regenerated adsorbent mimicked the shape of the sample before CO2 sorption at 350˚C, with a relatively high degree of roughness. These rough surfaces may be attributed to the agglomerated and rearranged MgO that results from the regeneration of MgCO3 back to MgO. As discussed, MgCO3 preferentially forms at the easily accessible sites of the MgO-EM interfaces and does not evolve as a uniform layer over MgO. Thus, MgO does not entirely return to its original structure prior to CO2 sorption. The resulting rough structure also reflects the change in distribution or dispersion of the eutectic mixture due to the increase in the degree of disorder of the molten salt mixture as sorption and desorption at high temperature occurs, as well as to the possible formation of EM clusters as MgCO3 evolved and saturated the sample. Although the cloudy structures that formed during sorption are greatly reduced after desorption, it is observed that a minimal amount of these structures are somehow


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retained, representing the distributed EM clusters formed. It is also possible that a small amount of MgCO3 that formed outside of the EM contributes to the retained structures as these particles are deprived of the promoting effect of EM for desorption. On the other hand, the second CO2 regeneration step presented in Figure 5d shows the evolution of finely distributed cloudy structures on the sorbent that become intensified throughout the sorption process. Unlike the first cycle of CO2 sorption in Figure 5b, the newly formed cloudy structures in Figure 5d are not concentrated on certain areas only but are rather evenly distributed throughout the sorbent. The dispersed evolution of these structures is a result of the clustering and distribution of EM, which consequently increased the amount of easily accessible areas and triple phase boundaries. The dispersed cloudy structures evolved correspond to MgCO3 particles that form around EM clusters at the triple phase boundaries, and eventually saturates this area. The in-situ TEM video for the resorption process is included in the Supporting Information (In-situ TEM_EMMgO.avi). Furthermore, additional in-situ TEM images and trials using KLM are also presented in the supporting information. In-situ TEM Observation of bare MgO. The CO2 adsorption phenomenon on bare MgO was also observed in order to determine the significance and difference of the structural changes and surface evolution occurring in KLM. Figure 7 shows in-situ TEM images of three different trials of CO2 sorption on bare MgO under room temperature. Before CO2 injection, the MgO sorbent initially consists of small distinguishable particles. The sorbent eventually forms an agglomerated structure as MgCO3 evolves during CO2 sorption. MgCO3 somehow forms a uniform layer that surrounds the MgO sorbent, as most of its surface can be easily accessed by the gas. This is entirely different from its behavior on KLM where sorption is favored on easily accessible sites or triple phase boundaries, resulting to the formation of cloudy structures that are


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concentrated on such sites. It is expected that the agglomerated MgCO3 formed on MgO will retain most of its structure when regenerated back to MgO, which will cause a large loss in surface area. This proves that the mechanism observed on KLM is unique for EM-promoted MgO sorbents, and that the addition of eutectic mixtures or molten salts as promoters completely changes the sorption phenomenon on MgO.

Figure 7. In-situ TEM images of bare MgO before (left) and after (right) CO2 adsorption at room temperature for three different trials (a, b, and c). Nucleation and MgCO3 Growth Scheme on EM-MgO. Based on the interface studies and direct observation results, we suggest a plausible scheme for the EM-promoted CO2 sorption as illustrated in Figure 8. Nucleation starts with the complete melting of the eutectic mixture, initiating the dissociation of EM-covered MgO to [Mg2+•••O2-] ions. The sorption reaction takes place locally and leads to the initial formation of MgCO3, mostly at the triple phase boundary. Subsequent formation of MgCO3 then occurs at the EM-MgO interface and proceeds until it reaches saturation, which results to the formation of clumped MgCO3 particles that causes the segmentation of a single bulk of eutectic mixture, forming relatively smaller separated clusters.


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A minimal loss in the MgO substrate may also occur during this stage due to the evolution of MgCO3 that is uncovered by EM, which in turn, are much difficult to regenerate back to MgO. Changes occurring in the MgO substrate as MgCO3 forms are represented in Figure 8 as MgO voids. As mentioned, the retained rough surface observed after the regeneration step is mainly attributed to the rearrangement of the eutectic mixture. The sorbent then experiences enhanced nucleation and growth of MgCO3 upon resorption due to resulting uniform distribution of EM clusters, which may also explain the enhanced kinetics observed in Figure 1c after the first cycle. With the results and findings obtained, it is evident that in-situ TEM is a powerful tool that could correlate, unify, and confirm findings in order to properly derive mechanisms and interpret sorption phenomenon occurring in CO2 capture systems such as that of EM-MgO.

Figure 8. Mechanism of MgCO3 nucleation and growth on (K,Li)NO3-MgO during CO2 sorption-regeneration.


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CONCLUSION Real-time observation of the CO2 sorption and regeneration phenomena on EM-MgO was successfully carried out and demonstrated. With the help of in-situ TEM and results obtained from sorption and interface analysis, it can be deduced that CO2 sorption initially occurs mostly at the triple phase boundary, and subsequent sorption occurs at the EM-MgO interface until saturation, leading to the formation of small EM clusters. These dispersed eutectic mixture assemblies do not return to their original bulk structure after regeneration, causing the subsequent CO2 sorption cycles to exhibit finely distributed and uniform surface evolutions of MgCO3. In-situ TEM analysis and results from the interface phenomenon studies complement and correlate with each other, thus strengthening the validity of the proposed mechanism. In-situ TEM results proved to be significant and insightful in fully understanding the CO2 sorption phenomena on KLM. As of this study, there has been no report on the direct observation of the CO2 capture phenomenon on EM-promoted MgO, and the proposed mechanisms for CO2 sorption and MgCO3 nucleation on molten-salt promoted MgO sorbents has not been experimentally and directly confirmed. With the help of in-situ TEM and other analysis techniques used in this study, the local accumulation, evolution, and particle generation of MgCO3 was clearly understood. Thus, this study directly gives insight into the possible CO2 sorption sites, structural rearrangements, and nucleation schemes occurring on the EM-MgO sorbent.

ASSOCIATED CONTENT


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Supporting Information. The following files are available free of charge. Additional experimental details, supporting data, figures, & discussions, and in-situ TEM images of bare MgO (PDF) In-situ TEM video for CO2 sorption of fresh EM-MgO. CO2 sorption at 350⁰C. (AVI) In-situ TEM video for CO2 resorption of EM-MgO. CO2 sorption at 350⁰C. (AVI)

AUTHOR INFORMATION Corresponding Author *+82-31-336-6336; [email protected]; ORCID ID: 0000-0002-3166-3590 Author Contributions §

These authors contributed equally to this work.

ACKNOWLEDGMENT This work was supported by the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT and Future Planning (NRF-2016R1C1B2008694) and this work was supported by Nano-Material Fundamental Technology Development (2016M3A7B4909370) through National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT and Future Planning. ABBREVIATIONS EM, eutectic mixture; TPB, triple phase boundary.


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In-situ Observation of Carbon Dioxide Capture on Pseudo-Liquid

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