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

Hydrogen production from dry reforming of methane, using CO2 previously chemisorbed in the Li6Zn1-xNixO4 solid solution

Li6ZnO4 was chemically modified by nickel addition, in order to develop different compositions of the solid solution Li6Zn1-xNixO4. These materials were evaluated bifunctionally; analyzing their CO2 capture performances, as well as on their catalytic properties for H2 production via dry reforming of methane (DRM). The crystal structures of Li6Zn1-xNixO4 solid solution samples were determined through X-ray diffraction, which confirmed the integration of nickel ions up to a concentration around 20 mol%, meanwhile beyond this value, a secondary phase was detected. These results were supported by XPS and TEM analyses. Then, dynamic and isothermal thermogravimetric analyses of CO2 capture revealed that Li6Zn1-xNixO4 solid solution samples exhibited good CO2 chemisorption efficiencies, similarly to the pristine Li6ZnO4 chemisorption trends observed. Moreover, a kinetic analysis of CO2 isothermal chemisorptions, using the Avrami-Erofeev model, evidenced an increment of the constant rates as a function of the Ni content. Since Ni2+ ions incorporation did not reduce the CO2 capture efficiency and kinetics, the catalytic properties of these materials were evaluated in the DRM process. Results demonstrated that nickel ions favored hydrogen (H2) production over the pristine Li6ZnO4 phase, despite a second H2 production reaction was determined, methane decomposition. Thereby, Li6Zn1-xNixO4 ceramics can be employed as bifunctional materials.

In Situ Spectroscopic Study of CO2 Capture and Methanation over Ni-Ca Based Dual Functional Materials

Carbon dioxide capture and reduction (CCR) to CH4 using dual-functional materials (DFMs) have recently attracted significant attention as a promising strategy for carbon capture and utilization. In this study, we investigate the mechanism of CCR to CH4 over Al2O3-supported Ni-Ca DFMs (Ni-Ca/Al2O3) under cyclic feeds of model combustion exhaust (2.5 % CO2+0 or 10 % O2/N2) and H2 at 500 °C. Various spectroscopic analyses, including time-resolved in situ X-ray diffraction and X-ray absorption spectroscopy, were conducted during CO2 capture and the subsequent H2-reduction steps. Based on these analyses, we propose a mechanism of CCR to CH4 over Ni-Ca based DFMs. During the CO2 capture step, the Ni0 species underwent complete oxidation in the presence of O2 to yield NiO. Subsequently, CO2 was captured through the interaction between the CaO surface and CO2, resulting in the formation of CaCO3 layers on the CaO particles. When the gas flow was switched to H2, NiO was partially to provide Ni0 sites, which acted as active sites for H2-reduction of the adjacent CaCO3 layers to yield CaO and gas-phase products, CH4 and H2O.

Conformational equilibria in acrolein-CO2: the crucial contribution of n → π* interactions unveiled by rotational spectroscopy

Using gas phase Fourier-transform microwave spectroscopy complemented by theoretical analysis, this study delivers a comprehensive depiction of the physical origin of the 'n → π* interaction' between CO2 and acrolein, one of the most reactive aldehydes. Three distinct isomers of the acrolein-CO2 complex, linked through a C⋯O tetrel bond (or n → π* interaction) and a C-H⋯O hydrogen bond, have been unambiguously identified in the pulsed jet. Relative intensity measurements allowed estimation on the population ratio of the three isomers to be T1/T2/C1 ≈ 25/5/1. Advanced theoretical analyses were employed to elucidate the intricacies of the noncovalent interactions within the examined complex. This study not only sheds light on the molecular underpinnings of n → π* interactions but also paves the way for future exploration in carbon dioxide capture and utilization, leveraging the fundamental principles uncovered in the study of acrolein-carbon dioxide interactions.

Increased CO2/N2 selectivity by stepwise fluorination in isoreticular ultramicroporous metal-organic frameworks

Exploration of porous adsorbents with high CO2/N2 selectivity is of great significance for reducing CO2 content in the atmosphere. In this study, a series of isoreticular ultramicroporous fluorinated metal-organic frameworks (MOFs) were prepared to explore the benefits of fluorinated ultramicropores in improving CO2/N2 selectivity. Gas adsorption measurements revealed that the increase in the number of fluorine atoms in a ligand molecule leads to the increased CO2 uptakes and CO2/N2 selectivity. Theoretical calculations indicate that the interaction between the fluorine atoms and adsorbed CO2 molecules enhances the CO2-philicity, offering useful insight into the improvement of CO2/N2 selectivity in isoreticular frameworks.

Cross-linked K0.5MnO2 nanoflower composites for high rate and low overpotential Li-CO2 batteries

Rechargeable Li-CO2 batteries are deemed to be attractive energy storage systems, as they can effectively inhale and fix carbon dioxide and possess an extremely high energy density. Unfortunately, the irreversible decomposition of the insoluble and insulating Li2CO3 results in awful electrochemical performance and inferior energy efficiency of Li-CO2 batteries. Furthermore, the low energy efficiency will exacerbate the extra waste of resources. Therefore, it is vital to design novel and efficient catalysts to enhance the battery performance. Herein, a facile, one-step strategy is introduced to design cross-linked, ultrathin K0.5MnO2 nanoflowers combined with CNTs (K0.5MnO2/CNT) as a highly efficient cathode for Li-CO2 batteries. Impressively, the Li-CO2 battery based on the K0.5MnO2/CNT cathode achieves a low overpotential (1.05 V) and a high average energy efficiency (87.95%) at a current density of 100 mA g-1. Additionally, the K0.5MnO2/CNT cathode can steadily run for over 100 cycles (overpotential < 1.20 V). Moreover, a low overpotential of 1.47 V can be obtained even at a higher current density of 1000 mA g-1, indicating the superior rate performance of K0.5MnO2/CNT. This strategy offers new insight and guidance for the development of low-cost and high-performance Li-CO2 batteries.

Research on carbon dioxide capture materials used for carbon dioxide capture, utilization, and storage technology: a review

In recent years, climate change has increasingly become one of the major challenges facing mankind today, seriously threatening the survival and sustainable development of mankind. Dramatically increasing carbon dioxide concentrations are thought to cause a severe greenhouse effect, leading to severe and sustained global warming, associated climate instability and unwelcome natural disasters, melting glaciers and extreme weather patterns. The treatment of flue gas from thermal power plants uses carbon capture, utilization, and storage (CCUS) technology, one of the most promising current methods to accomplish significant CO2 emission reduction. In order to implement the technological and financial system of CO2 capture, which is the key technology of CCUS technology and accounts for 70-80% of the overall cost of CCUS technology, it is crucial to create more effective adsorbents. Nowadays, with the development and application of various carbon dioxide capture materials, it is necessary to review and summarize carbon dioxide capture materials in time. In this paper, the main technologies of CO2 capture are reviewed, with emphasis on the latest research status of CO2 capture materials, such as amines, zeolites, alkali metals, as well as emerging MOFs and carbon nanomaterials. More and more research on CO2 capture materials has used a variety of improved methods, which have achieved high CO2 capture performance. For example, doping of layered double hydroxides (LDH) with metal atoms significantly increases the active site on the surface of the material, which has a significant impact on improving the CO2 capture capacity and performance stability of LDH. Although many carbon capture materials have been developed, high cost and low technology scale remain major obstacles to CO2 capture. Future research should focus on designing low-cost, high-availability carbon capture materials.

Review on the modifications of natural and industrial waste CaO based sorbent of calcium looping with enhanced CO2 capture capacity

The calcium looping cycle (CaL) possesses outstanding CO2 capture capacity for future carbon-capturing technologies that utilise CaO sorbents to capture the CO2 in a looping cycle. However, sorbent degradation and the presence of inert materials stabilise the sorbent, thereby reducing the CO2 capture capacity. Consequently, the CaO sorbent that has degraded must be replenished, increasing the operational cost for industrial use. CaO sorbents have been modified to enhance their CO2 capture capacity and stability. However, various CaO sorbents, including limestone, dolomite, biogenesis calcium waste and industrial waste, exhibit distinct behaviour in response to these modifications. Thus, this work comprehensively reviews the CO2 capture capacity of sorbent improvement based on various CaO sorbents. Furthermore, this study provides an understanding of the effects of CO2 capture capacity based on the properties of the CaO sorbent. The properties of various CaO sorbents, such as surface area, pore volume, particle size and morphology, are influential in exhibiting high CO2 capture capacity. This review provides insights into the future development of CaL technology, particularly for carbon-capturing technologies that focus on the modifications of CaO sorbents and the properties that affect the CO2 capture capacity.

Research Experiences via Integrating Simulations and Experiments (REVISE): A Model Collaborative Research Project for Undergraduate Students in CO2 Sorbent Design

Undergraduate research experiences are an instrumental component of student development, increasing conceptual understanding, promoting inquiry-based learning, and guiding potential career aspirations. Moving one step further, as research continues to become more interdisciplinary, there exists potential to accelerate student growth by granting additional perspectives through collaborative research. This study demonstrates the utilization of a model collaborative research project, specifically investigating the development of sorbent technologies for efficient CO2 capture, which is an important research area for improving environmental sustainability. A model CO2 sorbent system of heteroatom-doped porous carbon is utilized to enable students to gain knowledge of adsorption processes, through combined experimental and computational investigations and learnings. A particular emphasis is placed on creating interdisciplinary learning experiences, exemplified by using density functional theory (DFT) to understand molecular interactions between doped carbon surfaces and CO2 molecules as well as explain underlying physical mechanisms that govern experimental results. The experimental observations about CO2 sorption performance of doped ordered mesoporous carbons (OMCs) can be correlated with simulation results, which can explain how the presence of heteroatom functional groups impact the ability of porous carbon to selectively adsorb CO2 molecules. Through an inquiry-focused approach, students were observed to couple interdisciplinary results to construct holistic explanations, while developing skills in independent research and scientific communications. This collaborative research project allows students to obtain a deeper understanding of sustainability challenges, cultivate confidence in independent research, prepare for future career paths, and most importantly, be exposed to strategies employing interdisciplinary research approaches to address scientific challenges.

Mid-temperature CO2 Adsorption over Different Alkaline Sorbents Dispersed over Mesoporous Al2O3

CO2 adsorbents comprising various alkaline sorption active phases supported on mesoporous Al2O3 were prepared. The materials were tested regarding their CO2 adsorption behavior in the mid-temperature range, i.e., around 300 °C, as well as characterized via XRD, N2 physisorption, CO2-TPD and TEM. It was found that the Na2O sorption active phase supported on Al2O3 (originated following NaNO3 impregnation) led to the highest CO2 adsorption capacity due to the presence of CO2-philic interfacial Al-O--Na+ sites, and the optimum active phase load was shown to be 12 wt % (0.22 Na/Al molar ratio). Additional adsorbents were prepared by dispersing Na2O over different metal oxide supports (ZrO2, TiO2, CeO2 and SiO2), showing an inferior performance than that of Na2O/Al2O3. The kinetics and thermodynamics of CO2 adsorption were also investigated at various temperatures, showing that CO2 adsorption over the best-performing Na2O/Al2O3 material is exothermic and follows the Avrami model, while tests under varying CO2 partial pressures revealed that the Langmuir isotherm best fits the adsorption data. Lastly, Na2O/Al2O3 was tested under multiple CO2 adsorption-desorption cycles at 300 and 500 °C, respectively. The material was found to maintain its CO2 adsorption capacity with no detrimental effects on its nanostructure, porosity and surface basic sites, thereby rendering it suitable as a reversible CO2 chemisorbent or as a support for the preparation of dual-function materials.

Carbonate-Induced Structural Transformation of Lithium-Sodium Orthoborate to a Low-Viscosity Metaborate via CO2 Capture

This paper reports on the structural changes occurring within the lithium-sodium orthoborate crystal lattice during the solid-state absorption of CO2. Results derived from Fourier transform infrared measurements indicate that the CO2-saturated mixed-alkali metal orthoborate and its CO2-lean metaborate counterpart essentially present the same spectral profile, suggesting that CO2 capture results in a fundamental shift of the orthoborate composition to the metaborate one. The implications of such a structural transformation were examined in the molten state at elevated temperatures through rheological measurements, and although confirming that the CO2-lean metaborate exhibits a higher viscosity than the CO2-lean orthoborate, the results suggest that incorporation of CO2 in the orthoborate ionic lattice dilutes the melt, leading to a remarkable reduction in its overall viscosity, despite causing a structural transformation from the less viscous orthoborate form to the more viscous metaborate one.

Of Glasses and Crystals: Mitigating the Deactivation of CaO-Based CO2 Sorbents through Calcium Aluminosilicates

CaO-based sorbents are cost-efficient materials for high-temperature CO2 capture, yet they rapidly deactivate over carbonation-regeneration cycles due to sintering, hindering their utilization at the industrial scale. Morphological stabilizers such as Al2O3 or SiO2 (e.g., introduced via impregnation) can improve sintering resistance, but the sorbents still deactivate through the formation of mixed oxide phases and phase segregation, rendering the stabilization inefficient. Here, we introduce a strategy to mitigate these deactivation mechanisms by applying (Al,Si)Ox overcoats via atomic layer deposition onto CaCO3 nanoparticles and benchmark the CO2 uptake of the resulting sorbent after 10 carbonation-regeneration cycles against sorbents with optimized overcoats of only alumina/silica (+25%) and unstabilized CaCO3 nanoparticles (+55%). 27Al and 29Si NMR studies reveal that the improved CO2 uptake and structural stability of sorbents with (Al,Si)Ox overcoats is linked to the formation of glassy calcium aluminosilicate phases (Ca,Al,Si)Ox that prevent sintering and phase segregation, probably due to a slower self-diffusion of cations in the glassy phases, reducing in turn the formation of CO2 capture-inactive Ca-containing mixed oxides. This strategy provides a roadmap for the design of more efficient CaO-based sorbents using glassy stabilizers.

Improved structure of Zr-BTC metal organic framework using NH2 to enhance CO2 adsorption performance

Modified mesoporous NH2-Zr-BTC mixed ligand MOF nanocomposites were synthesized via the hydrothermal method as a novel adsorbent for CO2 capture. The newly modified MOF-808 with NH2 demonstrated a similar mesoporous morphology as MOF-808, whereas the specific surface area, pore volume, and average particle size, respectively, increased by 15%, 6%, and 46% compared to those of MOF-808. The characterization analyses exhibited the formation of more active groups on the adsorbent surface after modification. In addition, a laboratory adsorption setup was used to evaluate the effect of temperature, pressure, and NH2 content on the CO2 adsorption capacity in the range of 25-65 °C, 1-9 bar, and 0-20 wt%, respectively. An increase in pressure and a decrease in temperature enhanced the adsorption capacity. The highest equilibrium adsorption capacity of 369.11 mg/g was achieved at 25 °C, 9 bar, and 20 wt% NH2. By adding 20 wt% NH2, the maximum adsorption capacity calculated by the Langmuir model increased by about 4% compared to that of pure MOF-808. Moreover, Ritchie second-order and Sips models were the best-fitted models to predict the kinetics and isotherm data of CO2 adsorption capacity with the high correlation coefficient (R2 > 0.99) and AARE% of less than 0.1. The ΔH°, ΔS°, and ΔG° values were - 17.360 kJ/mol, - 0.028 kJ/mol K, and - 8.975 kJ/mol, respectively, demonstrating a spontaneous, exothermic, and physical adsorption process. Furthermore, the capacity of MH-20% sample decreased from 279.05 to 257.56 mg/g after 15 cycles, verifying excellent stability of the prepared mix-ligand MOF sorbent.

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.

Covalent Organic Frameworks: The Rising-Star Platforms for the Design of CO2 Separation Membranes

Membrane-based carbon dioxide (CO₂ ) capture and separation technologies have aroused great interest in industry and academia due to their great potential to combat current global warming, reduce energy consumption in chemical separation of raw materials, and achieve carbon neutrality. The emerging covalent organic frameworks (COFs) composed of organic linkers via reversible covalent bonds are a class of porous crystalline polymers with regular and extended structures. The inherent structure and customizable organic linkers give COFs high and permanent porosity, short transport channel, tunable functionality, and excellent stability, thereby enabling them rising-star alternatives for developing advanced CO₂ separation membranes. Therefore, the promising research areas ranging from development of COF membranes to their separation applications have emerged. Herein, this review first introduces the main advantages of COFs as the state-of-the-art membranes in CO₂ separation, including tunable pore size, modifiable surfaces property, adjustable surface charge, excellent stability. Then, the preparation approaches of COF-based membranes are systematically summarized, including in situ growth, layer-by-layer stacking, blending, and interface engineering. Subsequently, the key advances of COF-based membranes in separating various CO₂ mixed gases, such as CO₂ /CH₄ , CO₂ /H₂ , CO₂ /N₂ , and CO₂ /He, are comprehensively discussed. Finally, the current issues and further research expectations in this field are proposed.

Role of fiber density of amine functionalized dendritic fibrous nanosilica on CO2 capture capacity and kinetics

Textural properties of the solid sorbents are critical to tuning their CO2 capture performance. In this work, we studied the effect of fiber density (in turn, pore size, distribution, and accessibility) on CO2 capture capacity and kinetics. CO2 solid sorbents were prepared by physisorption of tetraethylenepentamine (TEPA) molecules on dendritic fibrous nanosilica (DFNS) with varying fiber density. Among the various DFNS, the DFNS with moderate fiber density [DFNS-3] showed the best CO2 capture capacity under the flue gas condition. The maximum CO2 capture capacity achieved was 24.3 wt % (5.53 mmol/g) at 75 °C for DFNS-3 under humid gas conditions. Fiber density also played a role in the kinetics of CO2 capture. DFNS-1 with dense fiber density needed ∼10.4 min to reach 90 % capture capacity, while DFNS-3 (moderate fiber density) needed only 6.4 min, which further decreased to 5.9 min for DFNS-5 with lightly dense fibers. The DFNS-impregnated TEPA also showed good recyclability during 21 adsorption and desorption cycles under humid and dry conditions. The total CO2 capture capacity of DFNS-3 (14.7) in 21 cycles was 108.9 and 105.0 mmol/g under humid and dry conditions, respectively. Adsorption lifetime calculation and recyclability confirmed the fiber density-dependent CO2 capture performance.

Activation in the rate of oxygen release of Sr0.8Ca0.2FeO3-δ through removal of secondary surface species with thermal treatment in a CO2-free atmosphere

We elucidate the underlying cause of a commonly observed increase in the rate of oxygen release of an oxygen carrier with redox cycling (here specifically for the perovskite Sr_0.8Ca_0.2FeO_3-δ ) in chemical looping applications. This phenomenon is often referred to as activation. To this end we probe the evolution of the structure and surface elemental composition of the oxygen carrier with redox cycling by both textural and morphological characterization techniques (N₂ physisorption, microscopy, X-ray powder diffraction and X-ray absorption spectroscopy). We observe no appreciable changes in the surface area, pore volume and morphology of the sample during the activation period. X-ray powder diffraction and X-ray absorption spectroscopy analysis (at the Fe and Sr K-edges) of the material before and after redox cycles do not show significant differences, implying that the bulk (average and local) structure of the perovskite is largely unaltered upon cycling. The analysis of the surface of the perovskite via X-ray photoelectron and in situ Raman spectroscopy indicates the presence of surface carbonate species in the as-synthesized sample (due to its exposure to air). Yet, such surface carbonates are absent in the activated material, pointing to the removal of carbonates during cycling (in a CO₂-free atmosphere) as the underlying cause behind activation. Importantly, after activation and a re-exposure to CO₂, surface carbonates re-form and yield a deactivation of the perovskite oxygen carrier, which is often overlooked when using such materials at relatively low temperature (≤500 °C) in chemical looping.

Synergistic promotions between CO2 capture and in-situ conversion on Ni-CaO composite catalyst

The integrated CO₂ capture and conversion (iCCC) technology has been booming as a promising cost-effective approach for Carbon Neutrality. However, the lack of the long-sought molecular consensus about the synergistic effect between the adsorption and in-situ catalytic reaction hinders its development. Herein, we illustrate the synergistic promotions between CO₂ capture and in-situ conversion through constructing the consecutive high-temperature Calcium-looping and dry reforming of methane processes. With systematic experimental measurements and density functional theory calculations, we reveal that the pathways of the reduction of carbonate and the dehydrogenation of CH₄ can be interactively facilitated by the participation of the intermediates produced in each process on the supported Ni-CaO composite catalyst. Specifically, the adsorptive/catalytic interface, which is controlled by balancing the loading density and size of Ni nanoparticles on porous CaO, plays an essential role in the ultra-high CO₂ and CH₄ conversions of 96.5% and 96.0% at 650 °C, respectively.

High Temperature CO2 Capture Performance and Kinetic Analysis of Novel Potassium Stannate

For the first time, the use of stannate-based sorbents was investigated as high temperature CO₂ sorption to evaluate their potential to contribute towards reducing carbon emissions. The sorption capacity and kinetics of commercial tin oxide, sodium, potassium and calcium stannates and lab synthesised potassium stannates were tested using thermogravimetric analysis. Commercial K₂SnO₃ was found to possess the largest CO₂ uptake capacity (2.77 mmol CO₂/g or 12.2 wt%) at 700 °C, which is among the highest for potassium sorbents, but the CO₂ desorption was not successful. On the contrary, the in-house synthesised K-stannate (K-B) using facile solid-state synthesis outperformed the other sorbents, resulting in a CO₂ uptake of 7.3 wt% after 5 min, an adsorption rate (0.016 mg/s) one order of magnitude higher than the other stannates, and stability after 40 cycles. The XRD and XPS analyses showed that K-B contains a mixture of K₂SnO₃ (76%) and K₄SnO₄ (21%), while the Scherrer crystal sizes confirmed good resistance to sintering for the potassium stannates. Among the apparent kinetic model tested, the pseudo-second order model was the most suitable to predict the CO₂ sorption process of K-B, indicating that chemical adsorption is dominant, while film-diffusion resistance and intra-particle diffusion resistance governed the sorption process in K-B. In summary, this work shows that solid-state synthesised potassium stannate could be an effective sorbent for high temperature separation, and additional work is required to further elucidate its potential.

Combined removal of SO3 and HCl by modified Ca(OH)2 from coal-fired flue gas

As treatments for mainstream pollutants in coal-fired power plants have been established, the control of non-conventional pollutants, such as SO₃ and HCl, is gradually gaining attention. In this study, combined SO₃ and HCl removal is proposed based on SO₃ removal by absorber injection. However, it is challenging to selectively absorb SO₃ and HCl from SO₂-rich atmospheres. Therefore, Ca(OH)₂ was modified via ball milling and doping with CuO for the combined removal of SO₃ and HCl. The results showed that ball milling reduced the particle and grain sizes of Ca(OH)₂, which increased the active sites of Ca(OH)₂ and prolonged reaction time. After modification by ball milling, SO₃ absorption per mg of Ca(OH)₂ increased by 40 %. However, HCl removal efficiency was difficult to improve by modifying Ca(OH)₂ using only ball milling under SO₃ and SO₂ atmospheres. Therefore, the dechlorination capacity of Ca(OH)₂ was improved by adding ions during the ball milling process. Doping of Ca(OH)₂ with Cu²⁺ changed its crystal structure, weakened the diffusion resistance of HCl, and improved Ca(OH)₂ utilization. Additionally, it increased the energy of Ca(OH)₂ to adsorb HCl.

Electrochemical Activation Applied to Perovskite Titanate Fibers to Yield Supported Alloy Nanoparticles for Electrocatalytic Application

Active bi-metallic nanoparticles are of key importance in catalysis and renewable energy. Here, the in situ formation of bi-metallic nanoparticles is investigated by exsolution on 200 nm diameter perovskite fibers. The B-site co-doped perovskite fibers display a high degree of exsolution, decorated with NiCo or Ni₃ Fe bi-metallic nanoparticles with average diameter about 29 and 35 nm, respectively. The perovskite fibers are utilized as cathode materials in pure CO₂ electrolysis cells due to their redox stability in the CO/CO₂ atmosphere. After in situ electrochemical switching, the nanoparticles exsolved from the perovskite fiber demonstrate an enhanced performance in pure CO₂ electrolysis. At 900 °C, the current density of solid oxide electrolysis cell (SOEC) with 200 µm YSZ electrolyte supported NiFe doped perovskite fiber anode reaches 0.75 Acm^-2 at 1.6 V superior to the NiCo doped perovskite fiber anode (about 1.5 times) in pure CO₂ . According to DFT calculations (PBE-D3 level) the superior CO₂ conversion on NiFe compared to NiCo bi-metallic species is related to an enhanced driving force for C-O cleavage under formation of CO chemisorbed on the nanoparticle and a reduced binding energy of CO required to release this product.

Insight into CO selective chemisorption from syngas mixtures through Li2MnO3; a new H2 enrichment material

Li2MnO3 is able to selectively trap CO in the presence of H2 at high temperatures, favoring H2 enrichment from syngas flows.

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.

Modification of the Textural Properties of Chitosan to Obtain Biochars for CO2-Capture Processes

Three chitosans with different morphologies have been used (commercial chitosan powder, chitosan in film form and chitosan in globular form synthesized by the freeze-dried method) for the synthesis of biochars. The pyrolytic treatment has revealed that the biochar synthesized from the chitosan formed by the freeze-dried method reaches the highest CO₂-adsorption capacity (4.11 mmol/g at 0 °C and a pressure of 1 bar) due to this adsorbent is highly microporous. Moreover, this biochar is more resistant to the pyrolytic treatment in comparison to the biochars obtained from the commercial chitosan and chitosan in the form of film. CO₂-adsorption studies at different temperatures have also shown that the adsorption capacity diminishes as the adsorption temperature increases, thus suggesting that the adsorption takes place by a physical process.

Yolk-shell-type CaO-based sorbents for CO2 capture: assessing the role of nanostructuring for the stabilization of the cyclic CO2 uptake

Improving the cyclic CO₂ uptake stability of CaO-based solid sorbents can provide a means to lower CO₂ capture costs. Here, we develop nanostructured yolk(CaO)-shell(ZrO₂) sorbents with a high cyclic CO₂ uptake stability which outperform benchmark CaO nanoparticles after 20 cycles (0.17 g_CO2 g_Sorbent^-1) by more than 250% (0.61 g_CO2 g_Sorbent^-1), even under harsh calcination conditions (i.e. 80 vol% CO₂ at 900 °C). By comparing the yolk-shell sorbents to core-shell sorbents, i.e. structures with an intimate contact between the stabilizing phase and CaO, we are able to identify the main mechanisms behind the stabilization of the CO₂ uptake. While a yolk-shell architecture stabilizes the morphology of single CaO nanoparticles over repeated cycling and minimizes the contact between the yolk and shell materials, core-shell architectures lead to the formation of a thick CaZrO₃-shell around CaO particles, which limits CO₂ transport to unreacted CaO. Hence, yolk-shell architectures effectively delay CaZrO₃ formation which in turn increases the theoretically possible CO₂ uptake since CaZrO₃ is CO₂-capture-inert. In addition, we observe that yolk-shell architectures also improved the carbonation kinetics in both the kinetic- and diffusion-controlled regimes leading to a significantly higher cyclic CO₂ uptake for yolk-shell-type sorbents.

Carbon Capture from Post-Combustion Flue Gas Using a State-Of-The-Art, Anti-Sublimation, Solid–Vapor Separation Unit

This work attempts to address the quest of removing carbon dioxide from flue gas streams to help preserve the environment. It is based on a model that is able to describe the solid-liquid-vapour and solid-vapour phase equilibria for the ternary system of N2-O2-CO2 at pressures from 5 to 130 bar and over a wide range of temperature (140 to 220 K). Furthermore, a corresponding state-of-the art solid-vapor (SV) CO2 capture/separation unit is developed and introduced in this work. The SV unit was modeled using the Aspen Custom Modeler software by implementing the thermodynamic model developed before. It was then simulated using the Aspen Plus simulator; its performance was studied and analyzed. Moreover, the performance of the unit was optimized and compared to the most conventional corresponding technology used by the industry (i.e., amine-scrubbing). Results proved that for the same output clean gas composition, which contains only 0.3% CO2, the developed state-of-the-art SV unit consumes almost half of the energy required by the conventional process. Other advantages of the novel SV separation unit include the lower requirement of capital equipment, no need of additional agents (such as solvents) and the avoidance of product contamination with such additional agents.

Bimetallic RuNi-decorated Mg-CUK-1 for oxygen-tolerant carbon dioxide capture and conversion to methane

The development of hybrid sorbent/catalysts for carbon capture and conversion to chemical fuels involves several material and engineering design considerations. Herein, a metal-organic framework (MOF), known as Mg-CUK-1, is loaded with Ru and Ni nanoparticles and assessed as a hybrid material for the sequential capture and conversion of carbon dioxide (CO₂) to methane (CH₄). Low nanocatalyst loadings led to enhanced overall performance by preserving more CO₂ uptake within the Mg-CUK-1 sorbent. Low temperature CO₂ desorption from Mg-CUK-1 facilitated complete CO₂ release and subsequent conversion to CH₄. The influence of oxygen exposure on catalyst performance was assessed, with Ru-loaded Mg-CUK-1 exhibiting oxygen tolerance through sustained CH₄ generation of 1.40 mmol g^-1 over ten cycles. In contrast, Ni-loaded Mg-CUK-1 was unable to retain initial catalytic performance, reflected in an 11.4% decrease in CH₄ generation over ten cycles. When combined, 0.3Ru2.7Ni Mg-CUK-1 yielded comparable overall performance to 3Ru Mg-CUK-1, indicating that Ru aids the re-reduction of NiO to Ni after O₂ exposure. By combining multiple catalyst species within one hybrid sorbent/catalyst material, greater catalyst stability is achieved, resulting in sustained overall performance. The introduced strategy provides an approach for fostering resilient hydrogenation catalysts upon exposure to reactive species often found in real-world point source CO₂ emissions.

Mg-O-F Nanocomposite Catalysts Defend against Global Warming via the Efficient, Dynamic, and Rapid Capture of CO2 at Different Temperatures under Ambient Pressure

The utilization of Mg-O-F prepared from Mg(OH)₂ mixed with different wt % of F in the form of (NH₄F·HF), calcined at 400 and 500 °C, for efficient capture of CO₂ is studied herein in a dynamic mode. Two different temperatures were applied using a slow rate of 20 mL·min^-1 (100%) of CO₂ passing through each sample for only 1 h. Using the thermogravimetry (TG)-temperature-programed desorption (TPD) technique, the captured amounts of CO₂ at 5 °C were determined to be in the range of (39.6-103.9) and (28.9-82.1) mg_CO2 ·g^-1 for samples of Mg(OH)₂ mixed with 20-50% F and calcined at 400 and 500 °C, respectively, whereas, at 30 °C, the capacity of CO₂ captured is slightly decreased to be in the range of (32.2-89.4) and (20.9-55.5) mg_CO2 ·g^-1, respectively. The thermal decomposition of all prepared mixtures herein was examined by TG analysis. The obtained samples calcined at 400 and 500 °C were characterized by X-ray diffraction and surface area and porosity measurements. The total number of surface basic sites and their distribution over all samples was demonstrated using TG- and differential scanning calorimetry-TPD techniques using pyrrole as a probe molecule. Values of (ΔH) enthalpy changes corresponding to the desorption steps of CO₂ were calculated for the most active adsorbent in this study, that is, Mg(OH)₂ + 20% F, at 400 and 500 °C. This study's findings will inspire the simple preparation and economical design of nanocomposite CO₂ sorbents for climate change mitigation under ambient conditions.

Improving Electricity Generation during the Product Reaction Loop and the Use of Exhaust Gas for Co-Product Production Using Polyethylene Waste and Flue Gas or Wood

More energy-efficient industries could reduce the problems of pollution, global warming, energy security, and fossil fuel depletion, including processing waste into raw materials and more efficient energy cogeneration. This research project considers the novelties of the upgraded electricity cogeneration technique, including one open gas turbine during the high-pressure product reaction loop and a second turbine during the exhaust gas for co-product production with only one compressor. The upgraded electricity cogeneration was carried out in two steps and based on the productive use of otherwise useless polyethylene waste, flue gas, and wood. The first step can simulate the simulation model from the existing product production with well-known technology and process units using the Aspen Plus® simulator. The maximum available electricity cogeneration can previously be determined from the thermodynamics of the products and co-products, and an existing energy unit’s characteristic capacity. In the second step, conventional natural gas can be replaced with waste as the raw materials by using the same simulated model from the first step, including electricity cogeneration using the gas turbines during the high-pressure reaction loop, and the exhaust gas as the co-product with only one compressor. This research on electricity generation is based on processes that include the pressure drop during the product reaction loop. This approach is illustrated using an existing methanol production process, using wastes as sustainable raw materials, including electricity cogeneration during the reaction loop and exhaust gas, generating a possible increase in annual profit of 7.28 MEUR/a.

Recent advances in CO2 capture and reduction

Given the continuous and excessive CO₂ emission into the atmosphere from anthropomorphic activities, there is now a growing demand for negative carbon emission technologies, which requires efficient capture and conversion of CO₂ to value-added chemicals. This review highlights recent advances in CO₂ capture and conversion chemistry and processes. It first summarizes various adsorbent materials that have been developed for CO₂ capture, including hydroxide-, amine-, and metal organic framework-based adsorbents. It then reviews recent efforts devoted to two types of CO₂ conversion reaction: thermochemical CO₂ hydrogenation and electrochemical CO₂ reduction. While thermal hydrogenation reactions are often accomplished in the presence of H₂, electrochemical reactions are realized by direct use of electricity that can be renewably generated from solar and wind power. The key to the success of these reactions is to develop efficient catalysts and to rationally engineer the catalyst-electrolyte interfaces. The review further covers recent studies in integrating CO₂ capture and conversion processes so that energy efficiency for the overall CO₂ capture and conversion can be optimized. Lastly, the review briefs some new approaches and future directions of coupling direct air capture and CO₂ conversion technologies as solutions to negative carbon emission and energy sustainability.

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.

Carbon Dioxide Chemisorption by Ammonium and Phosphonium Ionic Liquids: Quantum Chemistry Calculations

Carbon capture and storage is an important technological endeavor aiming to improve the ecology by combating global warming. The present work investigates reaction paths that are responsible for CO₂ chemisorption by the ammonium- and phosphonium-based ionic liquids containing an aprotic heterocyclic anion 2-cyanopyrrolide. We exemplify that 2 mol of CO₂ per 1 mol of the gas scavenger can be theoretically fixed by such ionic liquids. Both the cation and anion participate in the chemisorption. The corresponding standard enthalpies and potential energies are moderately negative. The chemisorption reaction, as revealed by the simulations of competing pathways, is started by the donation of the proton from the cation to the anion. The double covalent bond in the cation's structure emerges. The barriers to all reactions involving the phosphonium-based cation are relatively small and favor practical applications of the considered sorbents. The performance of the ammonium-based cation is less favorable due to the inherent instability of the tetraalkylammonium ylide. The role of phosphonium ylide in the mechanism of the reaction is carefully characterized. The performance of the aprotic anion as a CO₂ scavenger is unaffected by the chemical identity of the counterion. The essential heights of the identified steric barriers underline the necessity to simulate the entire structures of the reacting species to obtain a reliable description of chemisorption. The reported results foster a fundamental understanding of the outstanding CO₂ sorption performance of the quaternary ammonium- and phosphonium-based 2-cyanopyrrolides.

CO2 Capture over Activated Carbon Derived from Pulverized Semi-Coke

Pulverized semi-coke was employed as raw material to prepare activated carbon via steam activation and evaluated as a CO2 adsorbent. The effects of the preparation parameters including demineralization, activation temperature, activation time and steam flow on the structure and performance of the synthesized activated carbon were investigated. It was found that the microporous structure of activated carbon was greatly influenced by demineralization order and activation conditions. Demineralization before activation significantly increased the microporous structure of the activated carbon, which was ascribed to the removal of the inorganic fraction. Compared to the commercial activated carbon, activated carbon obtained by employing 150 mL/min steam to treat the demineralized pulverized semi-coke at 700 °C for 70 min possessed a higher CO2/N2 selectivity of 34.4 and good cyclic performance, which was due to its narrow microporosity of 0.55 nm. Furthermore, it was proved that a pore size of smaller than 1 nm is favorable for CO2 sorption.