Cyclic CO2 Research Articles

Novel application of Ru-based catalysts on MgAl oxides alkaline adsorbents for cyclic CO2 methanation

This study explores the performances of Ru-based catalysts with a low metal content (2 wt%) supported on MgO and Mg-Al Oxides (MgAl) for cyclic CO2 adsorption and methanation at atmospheric pressure. Adsorption and desorption tests demonstrated that MgAl-based catalysts are more promising for CO2 capture due to their larger surface area and better distribution of active sites (Mg2+–O2-). Moreover, doping the MgAl support with K2CO3 further improves surface alkalinity and, consequently, capture performance. During cyclic operations, all the catalysts proved effective and selective for methane production. To simulate realistic conditions, both dry and wet CO2 adsorptions were conducted before the methanation stage. The presence of moisture positively influenced gas carbonation for all catalysts, increasing the overall amount of CO2 captured. Specifically, Ru/MgAl exhibited the best performance in terms of adsorption and conversion to methane (approximately 85 % after dry adsorption and 79 % after wet adsorption), with a maximum methane production of 183 and 220 μmolCH4 g−1, respectively. The reaction yield was further enhanced with Ru-K/MgAl, achieving 327 μmolCH4 g−1 after dry adsorption and 333 μmolCH4 g−1 after wet adsorption. However, this catalyst displays different conversion kinetics, attributed to slowed carbonate migration, low Ru dispersion, limited specific surface area, and excessive carbonation strength. Operando FTIR tests revealed differences in reaction intermediates between Ru/MgAl and Ru-K/MgAl, by going deeper into the kinetic differences observed. The study concludes that Ru/MgAl materials are highly promising catalysts for CO2 adsorption and methane production, supporting the development of technologies for CO2 abatement and renewable energy utilisation.

Evaluation of calcium looping CO2 capture and thermochemical energy storage performances of different stabilizers promoted CaO-based composites derived from solid wastes

Calcium looping is considered as an important high-temperature cyclic CO2 capture and concentrated solar energy storage technology. However, the dramatic inactivation of CaO over multiple carbonation-calcination cycles has seriously restricted their practical applications. In this study, a series of metal oxide (i.e. TiO2, MgO, Al2O3 and ZrO2)-stabilized CaO-based materials with high cyclic stability derived from carbide slag were synthesized via a facile and cost-effective approach for simultaneously enhanced CO2 capture and thermochemical energy storage (TCES). The results suggest that CCS-H-TiO2 exhibits high initial CO2 uptake of 0.41 gCO2/gsorbent, superior cyclic stability with an average attenuation of 1.71 %/cycle and remarkably stable energy storage density. CCS-H-Al2O3 shows the best stability with lowest attenuation rate of 9.6 %. Except for CCS-H-MgO, the stabilizers reacted with active calcium to form new phases and used as physical “barrier” and high-TT skeleton to offset the sintering stress, retain the microporous structures, hinder the excessive agglomeration and alleviate sintering.

Engineering nanoparticle structure at synergistic Ru-Na interface for integrated CO2 capture and hydrogenation

The development of dual functional material for cyclic CO2 capture and hydrogenation is of great significance for converting diluted CO2 into valuable fuels, but suffers from kinetic limitation and deactivation of adsorbent and catalyst. Herein, we engineered a series of RuNa/γ-Al2O3 materials, varying the size of ruthenium from single atoms to clusters/nanoparticles. The coordination environment and structure sensitivity of ruthenium were quantitatively investigated at atomic scale. Our findings reveal that the reduced Ru nanoparticles, approximately 7.1 nm in diameter with a Ru-Ru coordination number of 5.9, exhibit high methane formation activity and selectivity at 340 °C. The Ru-Na interfacial sites facilitate CO2 migration through a deoxygenation pathway, involving carbonate dissociation, carbonyl formation, and hydrogenation. In-situ experiments and theoretical calculations show that stable carbonyl intermediates on metallic Ru nanoparticles facilitate heterolytic C–O scission and C–H bonding, significantly lowering the energy barrier for activating stored CO2.

Aluminum-enhanced Ca-based CO2 sorbents: Core-shell assembly and the impact of stabilizer precursors

Lately, Ca-based sorbents with a core–shell structure have been effectively produced to augment the sorbent’s capabilities. However, modifying the pristine core of Ca-based pellets with a core–shell structure notably influences the performance of the synthesized sorbent, and research in this domain remains scarce. Here, two categories of Al-stabilized, Ca-based pristine cores were produced using Ca(OH)2 mixed with Al-based stabilizer precursors of insoluble aluminum oxide and soluble aluminum nitrate, respectively, through the extrusion-spheronization technique. Due to the impact of mechanical extrusion, soluble aluminum nitrate can accumulate in-homogeneously in the Ca-based pristine core, leading to its expansion and rupture during the high-temperature calcination stage because of the decomposition of aluminum nitrate. The oxide-form aluminum stabilizer precursor can be uniformly distributed throughout the Ca-based pristine core pellets, demonstrating notably superior cyclic CO2 sorption capability and mechanical strength. The Al-fortified, core–shell structured Ca-based sorbent pellets demonstrated a peak CaO carbonation conversion rate of 54.8 % after 100 cycles when the Ca:Al molar ratio was precisely set to 85:15, which is about 2.1 times higher compared to the core–shell Ca-based sorbent pellets composed of a pure CaO core. This is primarily due to the formation of an evenly distributed inert Ca12Al14O33 within the pristine core, which effectively reduces high-temperature sintering. Hence, core–shell Ca-based sorbent pellet assembled with an Al-stabilized pristine core, could be a promising candidate for application in the CaL process for CO2 capture.

Understanding the influences and mechanism of water vapor on CO2 capture by high-temperature Li4SiO4 sorbents

Li4SiO4 sorbent attracts wide attention due to its high and stable CO2 adsorption. However, the process of CO2 capture is significantly affected by various impurities under the circumstance of industrial applications, and water vapor is one such impurity that needs to be well studied. Herein, we aim to evaluate the effect of water vapor on physicochemical characteristics of Li4SiO4-based sorbents during cyclic CO2 adsorption and desorption and to understand the action mechanism of H2O molecules on CO2 adsorption by Li4SiO4. Reaction tests show that water vapor can significantly enhance CO2 adsorption performance of sorbents, but the promotion effect varies with the different presence of water vapor during reactions. Material characterization and DFT calculation reveal that although the presence of water vapor leads to deterioration of internal pore structure of the sorbent, H2O molecule will form OH– on the surface of Li4SiO4 and further react with CO2 to generate HCO3–, thus the adsorption energy with water vapor presenting is increased with an observation of promoted CO2 adsorption.

From low-cost mineral to high-performance Li4SiO4 for solar energy storage and CO2 capture

The huge consumption of fossil fuels results in excessive CO2 emissions and its reduction has become a common concern. The combination of renewable energies (i.e., solar energy) and carbon capture, utilization, and sequestration are well recognized as two major routes to achieving carbon neutrality. Lithium orthosilicate (Li4SiO4) has been identified as one of the most promising candidates for solar energy storage and CO2 capture. However, the cost reduction and performance enhancement remain to be the main obstacles for its practical application. In this work, high-performance Li4SiO4 heat carriers have been synthesized using low-cost mineral as silicon source for solar energy storage and CO2 capture. Li4SiO4 derived from diatomite exhibited cyclic energy storage densities above 500 kJ/kg. The performance was further improved by doping alkali carbonates, particularly doping 3 wt% K2CO3 (LD-P-3K), which exhibited high and stable energy storage density of approximately 700 kJ/kg in the tested 10 cycles. Then, the mechanism of alkali doping was revealed through experiments and DFT calculations. The DSC results demonstrated that the improved performance was attributed to the formation of a eutectic melt which reduced the CO2 diffusion resistance. The DFT calculations showed that K-doped Li4SiO4 had lower adsorption energy and more significant charge migration, which meant stronger interaction. Additionally, diatomite-derived Li4SiO4 material is also a promising candidate for high-temperature CO2 capture, evidenced by its relatively high cyclic CO2 capture capacity (∼0.25 g/g). In general, satisfactory density/capacity and stability endows diatomite-derived Li4SiO4 with great prospects for both thermochemical energy storage and high-temperature CO2 capture.

All-solid-waste-derived CaO-based sorbents for simultaneously enhanced calcium looping CO2 capture and thermochemical energy storage

Calcium looping (CaL) process relying on CaO as high-temperature CO2 sorbents is a prospective alternative technology for simultaneously cyclic CO2 capture and thermochemical energy storage. However, the cyclic stability and the energy storage density of the CaO-based sorbents decay drastically over repeated carbonation-calcination cycles. Herein, we yielded CaO-based sorbents derived from all industrial solid wastes with carbide slag (CS) as precursor and coal fly ash (CFA) as stabilizer featuring superior carbonation characteristics, stable CO2 cyclic uptake and high thermochemical energy storage density over multiple operation cycles. The optimal sorbent (CCS–H-G-CFA-D) exhibited a high initial CO2 uptake of 0.38 gCO2/gsorbent, superior cyclic stability with an average decay rate of 1.05% per cycle and remarkably stable energy storage density of 1375 kJ/kg, retaining 89.5% after 10 repeated cycles. Thereby, the simultaneously enhanced cyclic CO2 capture and thermochemical energy storage are attributed to adding CFA stabilizers in CS-derived CaO-based sorbents, forming stable skeleton to alleviate sintering, improving the uniform mixing degree between sorbent particles and promoting the carbonation reaction via synergistic effect. This simple and eco-friendly approach of cost-effective sorbents totally derived from industrial solid wastes provides a new avenue for large-scale practical CO2 capture and energy storage processes.

Utilizing Limestone Alone for Integrated CO2 Capture and Reverse Water-Gas Reaction in a Fixed Bed Reactor: Employing Mass and Gas Signal Analysis

The integrated CO2 capture and utilization coupled with the reverse water-gas shift reaction (ICCU-RWGS) presents an alternative pathway for converting captured CO2 into CO in situ. This study investigates the effectiveness of three calcium-based materials (natural limestone, sol-gel CaCO3, and commercial CaCO3) as dual-functional materials (DFMs) for the ICCU-RWGS process at intermediate temperatures (650–750 °C). Our approach involves a fixed-bed reactor coupled with mass spectrometry and in situ Fourier transform infrared (FTIR) measurements to examine cyclic CO2 capture behavior, detailed physical and chemical properties, and morphology. The in situ FTIR results revealed the dominance of the RWGS route and exhibited self-catalytic activity across all calcium-based materials. Particularly, the natural limestone demonstrated a CO yield of 12.7 mmol g−1 with 100% CO selectivity and 81% CO2 conversion. Over the 20th cycle, a decrease in CO2 capture capacity was observed: sol-gel CaCO3, natural limestone, and commercial CaCO3 showed reductions of 44%, 61%, and 59%, respectively. This suggests inevitable deactivation during cyclic reactions in the ICCU-RWGS process, while the skeleton structure effectively prevents agglomeration in Ca-based materials, particularly in sol-gel CaCO3. These insights, coupled with the cost-effectiveness of CaO-alone DFMs, offer promising avenues for efficient and economically viable ICCU-RWGS processes.

Al-doped Li4SiO4 for cyclic solar energy storage and CO2 capture: An experimental and DFT study

The substantial consumption of fossil fuels gives rise to excessive carbon dioxide emissions, and mitigating them has emerged as a pressing global priority. The use of renewable energy and carbon capture, utilization, and storage are recognized as two major approaches for carbon neutrality. Li4SiO4 materials exhibit promising prospects in both thermochemical heat storage for solar energy power and cyclic high-temperature CO2 capture. The reactivity between Li4SiO4 and CO2 plays the vital role in its energy storage and CO2 capture performance. Herein, we prepare Al-doped Li4SiO4 materials through an impregnated suspension technique, simultaneously achieving high contact areas and lattice defects to enhance both fast chemisorption and ion diffusion processes. The results show that Al-doped Li4SiO4 exhibits high average energy density of 56677.08 kJ/kmol within 30 cycles. Additionally, the granulated Al-doped Li4SiO4-based pellets, produced via the freeze-drying technique, exhibit negligible energy density loss rate of less than 9 % and acceptable mechanical strength. Furthermore, the CO2 sorption performance also reaches a high value of 0.27 g/g. The reaction mechanism between Li4SiO4 and CO2 has been investigated at both macroscopic and microscopic levels through kinetic analysis and DFT calculations. The simulated Al-doped Li4SiO4/CO2 sorption system possesses increased adsorption energy level from −0.81 eV for the undoped one to −0.95 eV, thereby leading to the improved CO2 sorption. In general, Al-doped Li4SiO4 materials demonstrate great potential for applications in thermochemical heat storage and CO2 capture.

Integrity Experiments for Geological Carbon Storage (GCS) in Depleted Hydrocarbon Reservoirs: Wellbore Components under Cyclic CO2 Injection Conditions

Integrity of wellbores and near wellbore processes are crucial issues in geological carbon storage (GCS) projects as they both define the confinement and injectivity of CO2. For the proper confinement of CO2, any flow of CO2 along the wellbore trajectory must be prevented using engineered barriers. The effect of cyclic stimuli on wellbore integrity, especially in the context of GCS projects, has been given less attention. In this study, the effect of pressure- and temperature-cycling on two types of wellbore composites (i.e., casing-cement and cement-caprock) have been investigated experimentally in small- and large-scale laboratory setups. The experiments have been carried out by measuring the effective permeability of the composites under pressure and thermal cyclic conditions. Furthermore, the permeability of individual samples (API class G and HMR+ cement and caprock) was measured and compared to the permeability of the composites. The results indicate that the permeability of API class G cement when exposed to CO2 is in the order of 10−20 m2 (10−5 mD) as a result of the chemical reaction between the cement and CO2. In addition, the tightness of the composite cement–rock has been confirmed, while the permeability of the composite casing–cement falls within the acceptable range for tight cement and the CO2 flow was identified to occur through or close to the interface casing–cement. Results from thermal cycling within the range −9 to 14 °C revealed no significant effect on the integrity of the bond casing–cement. In contrast, pressure cycling experiments showed that the effective pressure has a larger influence on the permeability. The potential creation of micro-cracks under pressure variations may require some time for complete closing. In conclusion, the pressure and temperature cycling from this study did not violate the integrity of the casing–cement composite sample as the permeability remained low and within the acceptable range for wellbore cement.

Screening of Pd-Co-Ni catalyst arrays by scanning electrochemical microscopy and characterization of the potential catalyst for formic acid oxidation

BackgroundAppropriate anodic catalyst is one of the key points in improving the performance of direct formic acid fuel cells (DFAFCs). This study utilizes the concept of a combination method to design and screen anodic catalysts for formic acid oxidation. MethodsThe tip generation-substrate collection (TG-SC) mode of scanning electrochemical microscopy (SECM) was employed to screen the Pd-Co-Ni ternary catalyst array for catalyzing formic acid oxidation. The potential catalyst was further characterized by scanning electron microscope (SEM), energy-dispersive X-ray spectroscopy (EDX), X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS). Significant findingsThe composition ratios of the prepared Pd-Co-Ni ternary catalyst array matched the concentration ratios set by the precursor solution. SECM screening revealed that the optimal ratio for formic acid oxidation was the Pd2Co6Ni2 catalyst. The cyclic voltammograms and CO stripping voltammograms indicate that the Pd2Co6Ni2 electrode exhibits lower onset potentials for the oxidation of formic acid and its intermediates than the pure Pd electrode. Electrochemical impedance spectroscopy (EIS) confirms that the Pd2Co6Ni2 electrode possesses lower interfacial resistance than the Pd electrode, with its pseudo-inductive behavior at high potential aligning with the outcomes observed in cyclic voltammograms and CO stripping voltammograms.

Catalytic behavior of aluminum-modified SAPO-34 catalysts for reducing the energy penalty of CO2-rich amine solutions regeneration

The goal of this research is to develop a potential strategy to significantly decrease the heat duty for the amine-based CO2 capture process. Here, a variety of Al2O3-modified SAPO-34 composite catalysts with varying Al2O3 contents are synthesized and utilized to catalytically facilitate the CO2 desorption process. The catalytic CO2 desorption characteristics of various catalysts, Al-SAPO-34, SAPO-34, and γ-Al2O3 are investigated in a CO2-loaded 5 M monoethanolamine (MEA) solution. The results indicate that all catalysts accelerate CO2 desorption, while the Al-SAPO-34 catalysts work better than the parent Al2O3 and SAPO-34 catalysts. Especially, the 15 % Al-SAPO-34 increases the CO2 desorption rate by 78.4 % and reduces the relative energy requirement by 37 % at a desorption time of 1440 s. The catalysts are characterized by different methods and their structure–activity relationship is analyzed. The increased acid sites and mesoporous surface area of the Al-SAPO-34 catalyst may be responsible for its greater catalytic activity. The intermediates of the catalytic CO2 desorption process are identified by FT-IR and NMR, and then a possible catalytic regeneration mechanism is put forward. Additionally, the cyclic stability of the Al-SAPO-34 catalyst for cyclic CO2 absorption–desorption is evaluated. This work demonstrates that the Al-SAPO-34 exhibits greater catalytic CO2 desorption performance, and outstanding stability, and has the opportunity to become an attractive industrial catalyst for the amine-based CO2 capture process.

Multi-promoters modified CaO-based sorbent derived from mixed waste slag for long-term CO2 cyclic capture

Calcium looping, a carbon dioxide (CO2) capture technique, may provide an effective route to eliminate climate change brought by the steadily rising anthropogenic CO2 emissions. However, the key challenge for calcium looping is the sintering-induced decay over the numerous cycles for CaO-based sorbents. Herein, multi-promoters modified CaO-based sorbents, derived from the mixed waste slag of marble waste powder (MWP) and rare earth polishing powder wet slag (REPP), are prepared by a simultaneous hydrating and calcining process. The cyclic CO2 capture is explored by a thermogravimetric analyzer, and the promotion mechanism is investigated by X-ray diffraction, scanning electron microscopy, transmission electron microscopy, X-ray photoelectron spectroscopy, CO2 temperature-programmed desorption, and reaction kinetics fitting. Results indicate that the multicomponent promoters from the REPP, including CeO2, Ca12Al14O33, and LaPO4, enhance the carbonation cyclic activity of CaO from MWP. The best-performing MWP-REPP sorbent with 15 wt% REPP shows less than a 10 % drop decay and the CO2 carbonation conversion remains at 75 % (0.46 g CO2/g sorbent) after 50 repeated cycles. The excellent cyclic capture performance is attributed to the synergistic interaction between the promoters and CaO, where the CeO2 promoter owns abundant oxygen vacancies, and the Ca12Al14O33 and LaPO4 promoters provide the blocked effect, thus facilitating the CO2 sorption and inhibiting the CaO from sintering. This work presents useful guidance for improving the cyclic sorption stability of CO2 from waste slags.

Formation integrity evaluation for geosequestration of CO2 in depleted petroleum reservoirs under cyclic stress conditions

The geological storage of carbon dioxide (CO2), also referred to as CO2 geosequestration, represents one of the most promising options for reducing greenhouse gases in the atmosphere. However, most of the time, CO2 is captured with small amounts of other industrial gases such as sulphur dioxide (SO2) and hydrogen sulphide (H2S), which might be compressed together and stored in depleted petroleum reservoirs or aquifers. Moreover, during CO2 geosequestration in reservoirs, pressure variations during injection could force some amount of CO2 (with or without other acid gas impurities) into the caprock; thereby, altering the petrophysical, geochemical, and geomechanical properties of the caprock. Thus, the brittleness index of the reservoir and caprock might be impacted during CO2 geosequestration due to the chemical reactions between the rock minerals and the formation fluid. Furthermore, to meet the world net-zero carbon target, the promotion of CO2 utilization is paramount. This could be possible by developing an effective technology for cyclic CO2 geosequestration (with or without gas impurities). Therefore, studies on the co-injection of CO2 with other acid gases from industrial emissions, their withdrawal from the porous medium, and their impact on reservoir and caprock integrity are paramount. In this study, a dual-tubing string well completion technology was designed for cyclic injection and withdrawal of CO2 (with or without another acid gas), and numerical simulations were performed using TOUGHREACT codes, to model the cyclic process and investigate the co-injection of SO2 and H2S (separately) with CO2 in sandstone formations overlain by shale caprock. A novel technique of converting the volume fraction of minerals to their weight fraction was developed in this study, to evaluate the brittleness index of the sandstone reservoir and shale caprock during CO2 geosequestration. The findings of the study indicate that the porosity and permeability increase for the CO2 only and CO2–H2S injection cases, in the shale caprock; while for the CO2–SO2 injection case, porosity and permeability only decreased in the layers of the shale caprock contacted by SO2 and due to anhydrite precipitation. In all the injection cases, the porosity and permeability of the sandstone reservoir decreased in a few layers directly below the perforation interval of the production zone. However, in other regions in the sandstone reservoir, the porosity and permeability increased for the CO2 only and CO2–H2S injection cases. In contrast, for the CO2–SO2 co-injection case, porosity and permeability decreased in the layers of the sandstone rock contacted by SO2. In all the CO2 geosequestration cases, the brittleness of the shale and sandstone rocks investigated decreased slightly, except in the CO2–SO2 co-injection case where the brittleness of the sandstone rock decreased significantly. Based on the mineralogical composition of the formations in this study, co-injection of SO2 gas with CO2 gas, only decreased the brittleness index of the shale caprock slightly, but significantly decreased the brittleness of the sandstone reservoir.

Integrated CO2 capture and conversion by Cu/CaO dual function materials: Effect of in-situ conversion on the sintering of CaO and its CO2 capture performance

The integrated carbon capture and utilization (ICCU) technology has been considered a prospective strategy for mitigating carbon emissions issues. Compared to conventional CO2 capture and utilization, the ICCU process reduces transporting, product purification, construction, and operation costs. However, few works focus on investigating the effect of in-situ CO2 conversion on the sintering of CaO at high temperatures. In this work, Cu/CaO dual functional materials (DFMs) were synthesized and used in the Calcium Looping-Reverse Water-Gas Shift (CaL-RWGS) process at 650 °C. The results showed that it is thermodynamically feasible to couple the CaL with the RWGS reaction. In the cyclic CO2 capture test, Ca1Cu0.1 DFMs showed desirable CO2 capture performance (11.34 mmol/gDFMs) and self-activated phenomenon in the first 15 cycles. Moreover, Cu nanoparticle catalysts in the DFMs effectively inhibited the sintering of CaO by accelerating the desorption of CO2 from the CaO surface and converting it to CO during the conversion stage. In-situ DRIFTS of Ca1Cu0.1 DFMs revealed that formates might be the RWGS intermediates in CaL-RWGS.

Pelletizing and acid modifying of Zr-stabilized CaO-based sorbent for cyclic high-temperature CO2 capture

The process of calcium looping (CaL), which contained reversible carbonation/calcination periods, was one of the most promising candidates for high-temperature CO2 capture. Herein, kilogram-scale of Zr-stabilized CaO sorbent pellets were granulated via extrusion-spheronized methods, in which Zr species were well scattered into the CaO particles and effectively alleviated the thermal sintering. A typical binder of pseudo-boehmite (PB), with two types of peptizers including nitric acid (NA) and malonic acid (MA), was incorporated to improve the sorption capacity and mechanical strength of the sorbent pellets. The results revealed that the presence of PB, which provided mechanical strength to the pellets due to the binder effect, can promote the cyclic sorption capacity and thermal stability for pure CaO but had an adverse impact on the reactivity of the Zr-doped sorbent. And peptizer was employed to achieve homogeneity and enhance the strength of the pellets, for it can take chemical reaction with the hydroxyl groups on the surface of PB and disintegrate the PB particles into aluminum sol. In particular, the Ca/Zr sorbent pellets modified by MA were superior to the unmodified sorbent both on CO2 capture performance and mechanical strength due to the formation of smaller and dispersed CaO crystal particles.

Enhanced high temperature cyclic CO2 capture on Li4SiO4 sorbent from two-dimensional SiO2 nanomeshes

The Li4SiO4-based sorbent, noted for its high sorption capacity and lower regeneration temperature, is promising for high-temperature CO2 capture. However, its slower sorption rate and poor cycle stability limit practical applications. In this study, Li4SiO4 was synthesized with various Li/Si molar ratios to serve as CO2 sorbents. The synthesis employed the mechanochemical activation method, effectively addressing challenges like growth, agglomeration, and structural degradation that typically occur during the CO2 sorption process. The results indicate that a smaller Li/Si molar ratio significantly enhances both the kinetic sorption rate and cyclic stability of the Li4SiO4 sorbent. When the Li/Si molar ratios are 6.2, 4.2, and 3.2 respectively, the maximum adsorption capacities of the three corresponding sorbents are 0.39, 0.36, and 0.29 gCO2/gadsorbent, respectively. At a temperature of 700 °C, the times required for the sorbents to achieve adsorption equilibrium were 15 min, 2 min, and 1.6 min, respectively. Additionally, after 20 adsorption/desorption cycles, the corresponding retention rates of adsorption capacity were 29.4 %, 62.8 %, and 93.9 %, respectively. These results demonstrate the excellent CO2 sorption performance and the significant potential for wide application of this novel Li4SiO4-based high-temperature CO2 sorbent.

Evaluating the viability of ethylenediamine-functionalized Mg-MOF-74 in direct air capture: The challenges of stability and slow adsorption rate

Carbon removal technologies, such as direct air capture (DAC), hold great potential in mitigating anthropogenic CO2 emissions. Amine-tethered metal-organic frameworks (MOFs) that capture CO2 selectively via chemisorption have been highlighted as frontrunners for CO2 removal technologies. To this end, ethylenediamine (ED) was employed to decorate the metal sites of Mg-MOF-74, and both bare and amine-modified frameworks were thoroughly characterized and studied for DAC using an automated fixed-bed sorption device. The material exhibited a promising CO2 capacity of up to 1.8 mmol/g from 400 ppmv CO2 in humid conditions, although the amount of adsorbed H2O was several times higher. The highest adsorption capacities were measured at 25–35 °C, while decreased capacity was observed at 12 °C due to slower adsorption rate. In dry cyclic adsorption-desorption tests, the cyclic CO2 capacity reduced slightly in 18 cycles. However, at 2 vol% humidity, the capacity dropped rapidly over successive cycles, revealing poor hydrolytic stability. Preliminary coating experiments were conducted on stainless steel plates and cordierite monoliths, suggesting that reasonably even coating layers could be achieved on these substrates with relatively simple coating techniques. High water adsorption, slow adsorption rate at low temperatures, and the rapid cyclic capacity decrease in humid conditions may limit the application of the studied adsorbent for DAC. The vital aspects of the real application of MOFs in DAC, such as adsorption kinetics and stability in humid conditions, are rarely explored in detail in the literature, and these results indicate that these aspects warrant extensive study for the development of practically applicable DAC adsorbents.

Cyclic CO2 absorption/desorption property of Li3NaSiO4 under the partial pressure of CO2 for practical applications.

The temperature for realizing the cyclic CO2 absorption/desorption property of Li3NaSiO4 by repetition of switching a CO2/N2 gas mixture and N2 with a partial pressure of CO2, P(CO2), of 0.1 bar was optimized using the pseudo van't Hoff plot of LiNaCO3 + Li2SiO3 ↔ Li3NaSiO4 + CO2 prepared by thermogravimetry at various P(CO2) values.

Mesoporous silica-amine beads from blast furnace slag for CO2 capture applications.

Steel slag, abundantly available at a low cost and containing over 30 wt% silica, is an attractive precursor for producing high-surface-area mesoporous silica. By employing a two-stage dissolution-precipitation method using 1 M HCl and 1 M NaOH, we extracted pure SiO2, CaO, MgO, etc. from blast furnace slag (BFS). The water-soluble sodium silicate obtained was then used to synthesize mesoporous silica. The resulting silica had an average surface area of 100 m2 g-1 and a pore size distribution ranging from 4 to 20 nm. The mesoporous silica powder was further formed into beads and post-functionalized with polyethyleneimine (PEI) for cyclic CO2 capture from a mixture containing 15% CO2 in N2 at 75 °C. The silica-PEI bead was tested over 105 adsorption-desorption cycles, demonstrating an average CO2 capture capacity of 1 mmol g-1. This work presents a sustainable approach from steel slag to cost-effective mesoporous silica materials and making CO2 capture more feasible.