NaNO3-Promoted MgO-Based Adsorbents Prepared from Bischofite for CO2 Capture: Experimental and Density Functional Theory Study

MgO has broad application potential in CO2 capture at intermedium temperatures. In this paper, the effects of NaNO3 doping on the properties of MgO prepared by using waste bischofite as the raw material were investigated to improve the performance of the CO2 capture. MgO-doped NaNO3 exhibited excellent CO2 capture performance at 320 °C with a maximum adsorption capacity of 36.62 wt %. MgO-doped NaNO3 has good cycling stability after 10 adsorption-desorption cycle experiments. In addition, CO2 adsorption on pure MgO and MgO-NaNO3 surfaces was investigated in accordance with density functional theory. Calculation results show that doping with NaNO3 allows more electrons to be transferred from the MgO substrate to the CO2 molecule. MgO-doped NaNO3 can lead to an increase in adsorption energy, resulting in a more stable structure after adsorption and thereby promoting adsorption. The result of this study provides an effective method for the comprehensive utilization of salt lake resources.

Probing the Local Structure of Na in NaNO3-Promoted, MgO-Based CO2 Sorbents via X-ray Absorption Spectroscopy

This work provides insight into the local structure of Na in MgO-based CO2 sorbents that are promoted with NaNO3. To this end, we use X-ray absorption spectroscopy (XAS) at the Na K-edge to interrogate the local structure of Na during the CO2 capture (MgO + CO2 ↔ MgCO3). The analysis of Na K-edge XAS data shows that the local environment of Na is altered upon MgO carbonation when compared to that of NaNO3 in the as-prepared sorbent. We attribute the changes observed in the carbonated sorbent to an alteration in the local structure of Na at the NaNO3/MgCO3 interfaces and/or in the vicinity of [Mg2+···CO32-] ionic pairs that are trapped in the cooled NaNO3 melt. The changes observed are reversible, i.e., the local environment of NaNO3 was restored after a regeneration treatment to decompose MgCO3 to MgO. The ex situ Na K-edge XAS experiments were complemented by ex situ magic-angle spinning 23Na nuclear magnetic resonance (MAS 23Na NMR), Mg K-edge XAS and X-ray powder diffraction (XRD). These additional experiments support our interpretation of the Na K-edge XAS data. Furthermore, we develop in situ Na (and Mg) K-edge XAS experiments during the carbonation of the sorbent (NaNO3 is molten under the conditions of the in situ experiments). These in situ Na K-edge XANES spectra of molten NaNO3 open new opportunities to investigate the atomic scale structure of CO2 sorbents modified with Na-based molten salts by using XAS.

Deciphering the structural dynamics in molten salt-promoted MgO-based CO2 sorbents and their role in the CO2 uptake

The development of effective CO₂ sorbents is vital to achieving net-zero CO₂ emission targets. MgO promoted with molten salts is an emerging class of CO₂ sorbents. However, the structural features that govern their performance remain elusive. Using in situ time-resolved powder x-ray diffraction, we follow the structural dynamics of a model NaNO₃-promoted, MgO-based CO₂ sorbent. During the first few cycles of CO₂ capture and release, the sorbent deactivates owing to an increase in the sizes of the MgO crystallites, reducing in turn the abundance of available nucleation points, i.e., MgO surface defects, for MgCO₃ growth. After the third cycle, the sorbent shows a continuous reactivation, which is linked to the in situ formation of Na₂Mg(CO₃)₂ crystallites that act effectively as seeds for MgCO₃ nucleation and growth. Na₂Mg(CO₃)₂ forms due to the partial decomposition of NaNO₃ during regeneration at T ≥ 450°C followed by carbonation in CO₂.

Mesoporous carbon nitride supported MgO for enhanced CO2 capture

The growing concern about the environmental consequences of anthropogenic CO2 emissions significantly stimulated the research of low-cost, efficient, and recyclable solid adsorbents for CO2 capture. In this work, a series of MgO-supported mesoporous carbon nitride adsorbents with different MgO contents (xMgO/MCN) was prepared using a facile process. The obtained materials were tested for CO2 capture from 10 vol% CO2 mixture gas with N2 using a fixed bed adsorber at atmospheric pressure. At 25 ºC, the bare MCN support and unsupported MgO samples demonstrated CO2 capture capacities of 0.99, and 0.74 mmol g-1, respectively, which were lower than those of the xMgO/MCN composites.The incorporation of MgO into the MCN improved the CO2 uptake, and the 20MgO/MCN exhibited the highest CO2 capture capacity of 1.15 mmol g-1 at 25 °C. The improved performance of the 20MgO/MCN nanohybrid can be possibly assigned to the presence of high content of highly dispersed MgO NPs along with its improved textural properties in terms of high specific surface area (215 m2g-1), large pore volume (0.22 cm3g-1), and abundant mesoporous structure. The efffects of temperature and CO2 flow rate were also investigated on the CO2 capture performance of 20MgO/MCN. Temperature was found to have a negative influence on the CO2 capture capacity of the 20MgO/MCN, which decreased from 1.15 to 0.65 mmol g-1with temperature rise from 25 C to 150º C, due to the endothermicity of the process. Similarly, the capture capacity decreased from 1.15 to 0.54 mmol g-1 with the increase of the flow rate from 50 to 200 ml minute-1 respectively. Importantly, 20MgO/MCN showed excellent reusability with consistent CO2 capture capacity over five sequential sorption-desorption cycles, suggesting its suitability for the practical capture of CO2.

Uncovering the CO2 Capture Mechanism of NaNO3-Promoted MgO by 18O Isotope Labeling

MgO-based CO₂ sorbents promoted with molten alkali metal nitrates (e.g., NaNO₃) have emerged as promising materials for CO₂ capture and storage technologies due to their low cost and high theoretical CO₂ uptake capacities. Yet, the mechanism by which molten alkali metal nitrates promote the carbonation of MgO (CO₂ capture reaction) remains debated and poorly understood. Here, we utilize ¹⁸O isotope labeling experiments to provide new insights into the carbonation mechanism of NaNO₃-promoted MgO sorbents, a system in which the promoter is molten under operation conditions and hence inherently challenging to characterize. To conduct the ¹⁸O isotope labeling experiments, we report a facile and large-scale synthesis procedure to obtain labeled MgO with a high ¹⁸O isotope content. We use Raman spectroscopy and in situ thermogravimetric analysis in combination with mass spectrometry to track the ¹⁸O label in the solid (MgCO₃), molten (NaNO₃), and gas (CO₂) phases during the CO₂ capture (carbonation) and regeneration (decarbonation) reactions. We discovered a rapid oxygen exchange between CO₂ and MgO through the reversible formation of surface carbonates, independent of the presence of the promoter NaNO₃. On the other hand, no oxygen exchange was observed between NaNO₃ and CO₂ or NaNO₃ and MgO. Combining the results of the ¹⁸O labeling experiments, with insights gained from atomistic calculations, we propose a carbonation mechanism that, in the first stage, proceeds through a fast, surface-limited carbonation of MgO. These surface carbonates are subsequently dissolved as [Mg²⁺···CO₃ ^2-] ionic pairs in the molten NaNO₃ promoter. Upon reaching the solubility limit, MgCO₃ crystallizes at the MgO/NaNO₃ interface.

Model structures of molten salt-promoted MgO to probe the mechanism of MgCO3 formation during CO2 capture at a solid-liquid interface

MgO is a promising solid oxide-based sorbent to capture anthropogenic CO₂ emissions due to its high theoretical gravimetric CO₂ uptake and its abundance. When MgO is coated with alkali metal salts such as LiNO₃, NaNO₃, KNO₃, or their mixtures, the kinetics of the CO₂ uptake reaction is significantly faster resulting in a 15 times higher CO₂ uptake compared to bare MgO. However, the underlying mechanism that leads to this dramatic increase in the carbonation rate is still unclear. This study aims to determine the most favourable location for the nucleation and growth of MgCO₃ and more specifically, whether the carbonation occurs preferentially at the buried interface, the triple phase boundary (TPB), and/or inside the molten salt of the NaNO₃-MgO system. For this purpose, a model system consisting of a MgO single crystal that is structured by ultra-short pulse laser ablation and coated with NaNO₃ as the promoter is used. To identify the location of nucleation and growth of MgCO₃, micro X-ray computed tomography, scanning electron microscopy, Raman microspectroscopy and optical profilometry were applied. We found that MgCO₃ forms at the NaNO₃/MgO interface and not inside the melt. Moreover, there was no preferential nucleation of MgCO₃ at the TPB when compared to the buried interface. Furthermore, it is found that there is no observable CO₂ diffusion limitation in the nucleation step. However, it was observed that CO₂ diffusion limits MgCO₃ crystal growth, i.e. the growth rate of MgCO₃ is approximately an order of magnitude faster in shallow grooves compared to that in deep grooves.

Unravelling the Mechanism of Intermediate-Temperature CO2 Interaction with Molten-NaNO3 -Salt-Promoted MgO

The optimization of MgO-based adsorbents as advanced CO₂ -capture materials is predominantly focused on their molten-salt modification, for which theoretical and experimental contributions provide great insights for their high CO₂ -capture performance. The underlying mechanism of the promotion effect of the molten salt on CO₂ capture, however, is a topic of controversy. Herein, advanced experimental characterization techniques, including in situ environmental transmission electron microscopy (eTEM) and CO₂ chemisorption by diffuse-reflectance infrared Fourier transform spectroscopy (DRIFTS), transient ¹⁸ O-isotopic exchange, and density functional theory (DFT), are employed to elucidate the mechanism of the CO₂ interaction with molten-salt-modified MgO in the 250-400 °C range. Herein, eTEM studies using low (2-3 mbar) and high (700 mbar) CO₂ pressures illustrate the dynamic evolution of the molten NaNO₃ salt promoted and unpromoted MgO carbonation with high magnification (<50 nm). The formation of ¹⁸ O-NaNO₃ (use of ¹⁸ O₂ ) and C¹⁶ O¹⁸ O following CO₂ interaction, verifies the proposed reaction path: conversion of NO₃ ^- (NO₃ ^-  → NO₂ ⁺  + O^2- ), adsorption of NO₂ ⁺ on MgO with significant weakening of CO₂ adsorption strength, and formation of [Mg²⁺ … O^2- ] ion pairs preventing the development of an impermeable MgCO₃ shell, which largely increases the rate of bulk MgO carbonation.

Elucidating the promotion of Na2CO3 in CO2 capture by Li4SiO4

Although Li₄SiO₄-based sorbents are candidates for CO₂ capture at high temperatures, it is still necessary to improve their kinetic activation for adsorption and desorption. Carbonate doping to Li₄SiO₄ is considered as one of the effective means to improve CO₂ capture by Li₄SiO₄. In this study, Li₄SiO₄ was synthesized using Li₂CO₃ and SiO₂ at 900 °C, and mixed with different amounts of Na₂CO₃ as CO₂ sorbents. The effects of Na₂CO₃ on the absorption and desorption were characterized using thermal analyses in an atmosphere of 80 vol% CO₂-20 vol% N₂. In situ Raman and XRD were used for the characterization of the structural transformations and phase evolution during the CO₂ capture. The activation energy of both chemisorption and diffusion in adsorption dropped significantly. The additive Na₂CO₃ can react with CO₂ and produce the pyrocarbonate, which is favorable for CO₂ capture of Li₄SiO₄ and CO₂ diffusion. The doped Na₂CO₃ served two functions: producing the intermediate product and forming the melt with the product Li₂CO₃ to accelerate CO₂ transport. The Na₂CO₃-doped Li₄SiO₄ exhibits stable cyclic durability with conversions of 75% in 20 adsorption-desorption cycles.

CO2 Capture at Medium to High Temperature Using Solid Oxide-Based Sorbents: Fundamental Aspects, Mechanistic Insights, and Recent Advances

Carbon dioxide capture and mitigation form a key part of the technological response to combat climate change and reduce CO₂ emissions. Solid materials capable of reversibly absorbing CO₂ have been the focus of intense research for the past two decades, with promising stability and low energy costs to implement and operate compared to the more widely used liquid amines. In this review, we explore the fundamental aspects underpinning solid CO₂ sorbents based on alkali and alkaline earth metal oxides operating at medium to high temperature: how their structure, chemical composition, and morphology impact their performance and long-term use. Various optimization strategies are outlined to improve upon the most promising materials, and we combine recent advances across disparate scientific disciplines, including materials discovery, synthesis, and in situ characterization, to present a coherent understanding of the mechanisms of CO₂ absorption both at surfaces and within solid materials.

Peering into buried interfaces with X-rays and electrons to unveil MgCO3 formation during CO2 capture in molten salt-promoted MgO

The addition of molten alkali metal salts drastically accelerates the kinetics of CO₂ capture by MgO through the formation of MgCO₃ However, the growth mechanism, the nature of MgCO₃ formation, and the exact role of the molten alkali metal salts on the CO₂ capture process remain elusive, holding back the development of more-effective MgO-based CO₂ sorbents. Here, we unveil the growth mechanism of MgCO₃ under practically relevant conditions using a well-defined, yet representative, model system that is a MgO(100) single crystal coated with NaNO₃ The model system is interrogated by in situ X-ray reflectometry coupled with grazing incidence X-ray diffraction, scanning electron microscopy, and high-resolution transmission electron microscopy. When bare MgO(100) is exposed to a flow of CO₂, a noncrystalline surface carbonate layer of ca. 7-Å thickness forms. In contrast, when MgO(100) is coated with NaNO₃, MgCO₃ crystals nucleate and grow. These crystals have a preferential orientation with respect to the MgO(100) substrate, and form at the interface between MgO(100) and the molten NaNO₃ MgCO₃ grows epitaxially with respect to MgO(100), and the lattice mismatch between MgCO₃ and MgO is relaxed through lattice misfit dislocations. Pyramid-shaped pits on the surface of MgO, in proximity to and below the MgCO₃ crystals, point to the etching of surface MgO, providing dissolved [Mg²⁺…O^2-] ionic pairs for MgCO₃ growth. Our studies highlight the importance of combining X-rays and electron microscopy techniques to provide atomic to micrometer scale insight into the changes occurring at complex interfaces under reactive conditions.

Molten Salt-Promoted MgO Adsorbents for CO2 Capture: Transient Kinetic Studies

Optimization of MgO adsorbents is predominantly focused on the regulation of appropriate adsorption sites for CO₂ associated with Mg²⁺-O^2- sites of low coordination. Here, for the first time, we conducted transient kinetic experiments to identify and characterize changes of the CO₂ molecular path in MgO-based CO₂ adsorbents upon the addition of molten salt modifiers. Among the optimized samples, addition of 10 mol % NaNO₂ on the surface of MgO exhibited the highest CO₂ uptake (15.7 mmol g^-1) at 350 °C compared to less than 0.1 mmol g^-1 for the unpromoted MgO. Kinetic modeling showed that the interaction of molten salt-promoted MgO with CO₂ at 300 °C involves three different processes, namely, fast surface adsorption associated with surface-active basic sites, chemical reaction associated with MgCO₃ formation, and a slow diffusion step being the rate-limiting step of the carbonation process. Furthermore, transient kinetic studies coupled with mass spectrometry under low CO₂ partial pressure agreed well with the kinetic simulation results based on TGA measurements, demonstrating an in-depth understanding of the CO₂-capturing performance gained and its considerable significance for future practical designs of precombustion CO₂ capture.

Unravelling the Structural Modification (Meso-Nano-) of Cu/ZnO-Al2O3 Catalysts for Methanol Synthesis by the Residual NaNO3 in Hydroxycarbonate Precursors

The effects of residual NaNO3 on the modification of Cu/ZnO-Al2O3 catalysts have been extensively documented, but the modification mechanism is so far unclear. This work studies in detail the influence of the residual sodium nitrate present in the hydroxycarbonate precursors on their decomposition during calcination and how it affects to the formation and configuration of the final active sites of the Cu/ZnO-Al2O3 catalysts. Different samples with varying sodium content after washing (from 0.01 to 7.3 wt%) were prepared and studied in detail after calcination and reduction steps. The results of this work demonstrated that NaNO3 affects the decomposition mechanism of the hydroxycarbonate precursors during calcination and produces its decarbonation at low temperature. The enhancement of the decarbonation by NaNO3 leads to segregation and crystallization of CuO and ZnO with loss of mesostructure and surface area in the calcined catalysts. The loss of mesostructure in calcined catalysts affects the subsequent reduction step, decreasing the reducibility and damaging the nanostructure of the reduced catalysts forming large Cu particles in poor contact with ZnOx that results in a significant decrease in the intrinsic activity of the copper active sites for methanol synthesis.

Effect of molten sodium nitrate on the decomposition pathways of hydrated magnesium hydroxycarbonate to magnesium oxide probed by in situ total scattering

The effect of NaNO₃ and its physical state on the thermal decomposition pathways of hydrated magnesium hydroxycarbonate (hydromagnesite, HM) towards MgO was examined by in situ total scattering. Pair distribution function (PDF) analysis of these data allowed us to probe the structural evolution of pristine and NaNO₃-promoted HM. A multivariate curve resolution alternating least squares (MCR-ALS) analysis identified the intermediate phases and their evolution upon the decomposition of both precursors to MgO. The total scattering results are discussed in relation with thermogravimetric measurements coupled with off-gas analysis. MgO is obtained from pristine HM (N₂, 10 °C min^-1) through an amorphous magnesium carbonate intermediate (AMC), formed after the partial removal of water of crystallization from HM. The decomposition continues via a gradual release of water (due to dehydration and dehydroxylation) and, in the last step, via decarbonation, leading to crystalline MgO. The presence of molten NaNO₃ alters the decomposition pathways of HM, proceeding now through AMC and crystalline MgCO₃. These results demonstrate that molten NaNO₃ facilitates the release of water (from both water of crystallization and through dehydroxylation) and decarbonation, and promotes the crystallization of MgCO₃ and MgO in comparison to pristine HM. MgO formed from the pristine HM precursor shows a smaller average crystallite size than NaNO₃-promoted HM and preserves the initial nano-plate-like morphology of HM. NaNO₃-promoted HM was decomposed to MgO that is characterized by a larger average crystallite size and irregular morphology. Additionally, in situ SEM allowed visualization of the morphological evolution of HM promoted with NaNO₃ at a micrometre scale.

NaNO3 -Promoted Mesoporous MgO for High-Capacity CO2 Capture from Simulated Flue Gas with Isothermal Regeneration

NaNO₃ -promoted MgO composite materials have been prepared and their ability to sorb CO₂ at a concentration relevant to CO₂ capture from flue gas is explored. The uptake kinetics and capacities of sorbents of different NaNO₃ /MgO ratios are measured at intermediate temperatures of 230-300 °C. The sorbent with a NaNO₃ /MgO ratio of 0.10 has the highest 12 h sorption capacity among sorbents with different NaNO₃ loadings, and the highest sorption capacity of 11.2 mmol CO 2  g^-1 is observed at 260 °C. Intriguingly, an induction period is observed in the initial stage of CO₂ sorption. In situ XRD analysis, in situ FTIR spectroscopy, and a comparison of the CO₂ sorption behavior under simulated flue gas conditions in comparison to prior studies employing pure CO₂ indicated that the sorption of CO₂ occurred through nucleation of MgCO₃ crystallites in the material. The data indicate that the concentration of CO₂ within the molten medium of NaNO₃ , which is affected by both the solubility of CO₂ in molten NaNO₃ and the partial pressure of CO₂ in the surrounding atmosphere, has a critical impact on the length of the induction period. A partially desorbed sample after sorption of CO₂ displays much-improved sorption kinetics in the next cycle and was able to sorb and desorb CO₂ over multiple cycles at isothermal conditions by simply switching the feed gas from CO₂ to inert gas.

Encapsulation of Phase-Changing Eutectic Salts in Magnesium Oxide Fibers for High-Temperature Carbon Dioxide Capture: Beyond the Capacity-Stability Tradeoff

Eutectic mixture (EM)-promoted MgO sorbents exhibit high CO₂ sorption capacities but  experience a significant decrease in uptake after multiple sorption-regeneration cycles due to EM movement and redistribution at high temperatures. Encapsulation of a pseudoliquid, phase-changing EM promoter with MgO may thus prevent the loss of active interface by confining the EM within a fixed area inside a MgO shell. In this work, we successfully embedded an EM composed of KNO₃ and LiNO₃ in a MgO fiber matrix via core-shell electrospinning. The synthesized sorbent achieved relatively high and steady sorption capacities, maintaining a stable uptake of ∼20 wt % after 25 sorption-regeneration cycles. The sorbent was also characterized using various techniques including in situ transmission electron microscopy (TEM) to describe its morphology, from which it was confirmed that the eutectic salt existed in distributed hollow pockets within the MgO fiber matrix and stayed confined within these fixed areas, favorably limiting its movement and redistribution when exposed to high temperatures where it exists in the liquid form. The EM may also be described as a glue that holds the fiber together, while MgO acts as a protective shell that prevents structural changes and rearrangement caused by EM movement, allowing the sorbent to retain its cyclic stability after multiple cycles and demonstrating its potential for industrial use after further improvement. Thus, the microencapsulation of a phase-changing EM material with pure MgO metal oxide was successfully achieved and might be explored for various material applications.

CO2 absorption and desorption characteristics of MgO-based absorbent promoted by triple eutectic alkali carbonate

We present the details of the mechanism of CO₂ absorption and desorption of MgO absorbent promoted by triple eutectic alkali carbonate.

Stabilization of NaNO3-Promoted Magnesium Oxide for High-Temperature CO2 Capture

NaNO₃-promoted MgO sorbents are known to achieve enhanced CO₂ sorption uptake but fail to maintain their capacity after multiple sorption-regeneration cycles. In this study, commercially available hydrotalcites (Pural Mg30, Pural Mg70, and synthetic hydrotalcite) were used as stabilizers for NaNO₃-impregnated MgO (MgONaNO₃) sorbents to improve their cyclic stability. Results show that the Mg30-stabilized MgONaNO₃ attained higher and stable overall CO₂ sorption performance as compared to bare MgONaNO₃ after multiple sorption cycles. XRD analyses reveal that the hydrotalcites act as templates for MgCO₃ by restricting the formation of large and nonuniform product crystallites. Furthermore, CO₂-TPD results show that the hydrotalcites cause a change in the basic sites of the sorbent, which may be attributed to its high interaction with both MgO and NaNO₃. This interaction becomes stronger as cycles proceed due to the structural rearrangements occurring, thus contributing to the stable behavior of the sorbents. However, these characteristics were not found on MgONaNO₃ and the α-Al₂O₃-stabilized samples, thus proving the unique ability of hydrotalcites. From these results, we then derived the formation scheme of MgCO₃ on the hydrotalcite-stabilized sorbents. This study presents a simple yet effective method of improving the stability of molten salt-promoted sorbents with promising potential for industrial use.

Atomic-scale investigation of MgO growth on fused quartz using angle-dependent NEXAFS measurements

The phenomena related to thin film growth have always been interesting to the scientific community. Experiments related to these phenomena not only provide an understanding but also suggest a path for the controlled growth of these films. For the present work, MgO thin film growth on fused quartz was investigated using angle-dependent near-edge X-ray absorption fine structure (NEXAFS) measurements. To understand the growth of MgO, sputtering was allowed for 5, 10, 25, 36, 49, 81, 144, 256, and 400 min in a vacuum better than 5.0 × 10⁻⁷ torr. NEXAFS measurements revealed the evolution of MgO at the surface of fused quartz for sputtering durations of 144, 256, and 400 min. Below these sputtering durations, no MgO was observed. NEXAFS measurements further envisaged a systematic improvement of Mg²⁺ ion coordination in the MgO lattice with the sputtering duration. The onset of non-interacting molecular oxygen on the surface of the sputtered species on fused quartz was also observed for sputtering duration up to 81 min. Angle-dependent measurements exhibited the onset of an anisotropic nature of the formed chemical bonds with sputtering, which dominated for higher sputtering duration. X-ray diffraction (XRD) studies carried out for sputtering durations of 144, 256, and 400 min exhibited the presence of the rocksalt phase of MgO. Annealing at 700 °C led to the dominant local electronic structure and improved the crystallinity of MgO. Rutherford backscattering spectrometry (RBS) and cross-sectional scanning electron microscopy (SEM) revealed a layer of almost 80 nm was obtained for a sputtering duration of 400 min. Thus, these angle-dependent NEXAFS measurements along with XRD, RBS, and SEM analyses were able to give a complete account for the growth of the thin films. Moreover, information specific to the coordination of the ions, which is important in case of ultrathin films, could be obtained successfully using this technique.

Near edge X-ray absorption fine structure measurements reveal the formation of MgO on fused quartz substrate.

Ultrafast and Stable CO2 Capture Using Alkali Metal Salt-Promoted MgO-CaCO3 Sorbents

As a potential candidate for precombustion CO₂ capture at intermediate temperatures (200-400 °C), MgO-based sorbents usually suffer from low kinetics and poor cyclic stability. Herein, a general and facile approach is proposed for the fabrication of high-performance MgO-based sorbents via incorporation of CaCO₃ into MgO followed by deposition of a mixed alkali metal salt (AMS). The AMS-promoted MgO-CaCO₃ sorbents are capable of adsorbing CO₂ at an ultrafast rate, high capacity, and good stability. The CO₂ uptake of sorbent can reach as high as above 0.5 g_CO2 g_sorbent^-1 after only 5 min of sorption at 350 °C, accounting for vast majority of the total uptake. In addition, the sorbents are very stable even under severe but more realistic conditions (desorption in CO₂ at 500 °C), where the CO₂ uptake of the best sorbent is stabilized at 0.58 g_CO2 g_sorbent^-1 in 20 consecutive cycles. The excellent CO₂ capture performance of the sorbent is mainly due to the promoting effect of molten AMS, the rapid formation of CaMg(CO₃)₂, and the plate-like structure of sorbent. The exceptional ultrafast rate and the good stability of the AMS-promoted MgO-CaCO₃ sorbents promise high potential for practical applications, such as precombustion CO₂ capture from integrated gasification combined cycle plants and sorption-enhanced water gas shift process.

In Situ Observation of Carbon Dioxide Capture on Pseudo-Liquid Eutectic Mixture-Promoted Magnesium Oxide

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 pseudoliquid EM undergoes structural rearrangement as MgCO₃ evolves from the surface of MgO, resulting in 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 into 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.

Characteristics of NaNO3-Promoted CdO as a Midtemperature CO2 Absorbent

In this study, we explored the reaction system CdO(s) + CO₂(g) ⇄ CdCO₃(s) as a model system for CO₂ capture agent in the intermediate temperature range of 300-400 °C. While pure CdO does not react with CO₂ at all up to 500 °C, CdO mixed with an appropriate amount of NaNO₃ (optimal molar ratio NaNO₃/CdO = 0.14) greatly enhances the conversion of CdO into CdCO₃ up to ∼80% (5.68 mmol/g). These NaNO₃-promoted CdO absorbents can undergo many cycles of absorption and desorption by temperature swing between 300 and 370 °C under a 100% CO₂ condition. Details of how NaNO₃ promotes the CO₂ absorption of CdO have been delineated through various techniques using thermogravimetry, coupled with X-ray diffraction and electron microscopy. On the basis of the observed data, we propose a mechanism of CO₂ absorption and desorption of NaNO₃-promoted CdO. The absorption proceeds through a sequence of events of CO₂ adsorption on the CdO surface covered by NaNO₃, dissolution of so-formed CdCO₃, and precipitation of CdCO₃ particles in the NaNO₃ medium. The desorption occurs through the decomposition of CdCO₃ in the dissolved state in the NaNO₃ medium where CdO nanoparticles are formed dispersed in the NaNO₃ medium. The CdO nanoparticles are aggregated into micrometer-large particles with smooth surfaces and regular shapes.