Dilute CO2 Research Articles

Closed-Loop and Precipitation-Free CO2 Capture Process Enabled by Electrochemical pH Gradient.

Carbon dioxide (CO2) capture is a crucial negative-emission technology for the mitigation of climate change and global warming. The urgent need of combating climate change motivates the research and development of economical, effective and environmentally benign processes for CO2 capture. Herein, we design and report a flow cell for the CO2 capture from air or flue gas in a precipitate-free and closed-loop manner. No ion-exchange membrane is used in the electrolyser. The water electrolysis produces acidic solution near the anode and alkaline solution near the cathode, while generating valuable hydrogen and oxygen byproducts. The dilute CO2 in air or flue gas is captured by the alkaline solution, which is then mixed with the acidic solution to release the concentrated CO2. The process operates in a cyclic manner as driven by the water electrolysis and the mechanical pumping. No precipitation of calcium carbonate is involved for fixing CO2, which may simplify the separation process and minimizing the materials loss. The simple process enabled by electrochemical pH gradient shows promise for efficient CO2 capture on both small and large scales.

Mechanistic Study of the Electrochemical Reduction of CO2 in Aprotic Ionic Liquid in Air.

The capture and electrochemical conversion of dilute CO2 in air is a promising approach to mitigate global warming. Aiming to increase the efficiency of the electrochemical reduction of CO2, we fabricated electrodes and developed a custom-designed sealed electrochemical reaction system to study the mechanism of this conversion. The performance of three metal electrodes, Ag, Cu, and SUS 316L, was compared in an aprotic ionic liquid as the electrolyte to monitor the CO2 concentration and chemical reactions using a CO2 sensor and diffuse reflectance infrared Fourier transform spectroscopy and Raman spectroscopy in CO2/N2 (400 ppm CO2 and 99.96% N2) or synthetic air (400 ppm CO2, 21% O2, and 79% N2). The CO2 concentration decreased at negative potentials and was more drastic in synthetic air than in CO2/N2. At negative potential in synthetic air, IR revealed carbon monoxide, carbonate, or peroxydicarbonate on the Ag, Cu, or SUS 316L electrodes, respectively. Reaction intermediates were identified using Raman spectroscopy. Superoxide (O2•-), produced by the reduction of O2 on each electrode, promotes the electrochemical reduction of CO2 whose reduction potential is higher on the negative side than that of O2. This research deepens our understanding of the electrochemical capture/release and conversion of dilute CO2.

Fundamental insights into multistep depressurization of CH4/CO2 hydrates in the presence of N2 or air

Exploitation of natural gas hydrates provides an alternative way to address energy crisis. Dilute CO2 gas (13–30mol%CO2 with remaining N2/Air) injection into CH4 hydrates for hydrate swapping (SW) allows cheaper and more practical CH4 recovery and in-situ CO2 sequestration. However, the roles of N2/Air in dilute CO2 gas during exploitation remain unknown. It is unclear whether depressurization should be coupled after SW and continued below CH4 hydrate stability pressure. This work employed multistep depressurization (MD) to dissociate the mixed hydrates formed after SW from 86.5 to 97.9 bar at 0.7–1.2 °C in bulk-water and sandpack with CH4 hydrate saturation of 4.1–25.3 %. Effects of N2/Air on exploitation were investigated by examining hydrate morphologies and gas compositions. Morphological results in bulk-water indicated higher N2 fraction in 20mol%CO2/N2 triggered more CO2-rich hydrate reformation and CH4-rich hydrate dissociation. Exploitation results in sandpack indicated 13mol%CO2/N2 produced the highest CH4 swapping percent (46.6 %) and CO2 hydrate sequestration percent (29.1 %). Air exerted weaker promoting effects on exploitation compared with equivalent N2. The promotion of N2/Air on exploitation was dominated by dilute CO2 gas injection altering mixed hydrate equilibrium which varied with time-dependent gaseous compositions during MD. l-methionine of 3000 ppm had stronger promoting effects on CO2 sequestration in sandpack than bulk-water depending on mass transfer and water availability. Ceasing points (13.9–31.4 bar) suggested MD could be continued below CH4/above CO2 hydrate stability pressures and before water production. For the first time, this study provided insights into the roles of N2/Air to determine injection gas types and depressurization schemes for efficient and safe hydrate exploitation in gas-rich hydrate-bearing sediment.

Enzymatic Activation and Continuous Electrochemical Production of Methane from Dilute CO2 Sources with a Self-Healing Capsule.

Converting dilute CO2 source into value-added chemicals and fuels is a promising route to reduce fossil fuel consumption and greenhouse gas emission, but integrating electrocatalysis with CO2 capture still faced marked challenges. Herein, we show that a self-healing metal-organic macrocycle functionalized as an electrochemical catalyst to selectively produce methane from flue gas and air with the lowest applied potential so far (0.06 V vs reversible hydrogen electrode, RHE) through an enzymatic activation fashion. The capsule emulates the enzyme' pocket to abstract one in situ-formed CO2-adduct molecule with the commercial amino alcohols, forming an easy-to-reduce substrate-involving clathrate to combine the CO2 capture with electroreduction for a thorough CO2 reduction. We find that the self-healing system exhibited enzymatic kinetics for the first time with the Michaelis-Menten mechanism in the electrochemical reduction of CO2 and maintained a methane Faraday efficiency (FE) of 74.24% with a selectivity of over 99% for continuous operation over 200 h. A consecutive working lab at 50 mA·cm-2, in an eleven-for-one (10 h working and 1 h healing) electrolysis manner, gives a methane turnover number (TON) of more than 10,000 within 100 h. The integrated electrolysis with CO2 capture facilitates the thorough reduction of flue gas (ca. 13.0% of CO2) and first time of air (ca. 400 ppm of CO2 to 42.7 mL CH4 from 1.0 m3 air). The new self-healing strategy of molecular electrocatalyst with an enzymatic activation manner and anodic shifting of the applied potentials provided a departure from the existing electrochemical catalytic techniques.

Synergistic Impact on Indoor Air Quality: The Combined Use of Air Conditioners, Air Purifiers, and Fresh Air Systems

As concerns about indoor air quality increase, air purifiers and fresh air systems are increasingly being used in conjunction with air conditioners in office buildings. To study the synergistic effects of multisystem joint operation on indoor air quality, this study used a combination of experimental and simulation methods to study the indoor particulate matter with a diameter of less than 2.5 µm (PM2.5) and carbon dioxide (CO2) concentrations under different operation conditions. The purification time and the PM2.5 concentration distribution under different conditions were compared with the condition in which the purifier was used alone. The results showed that the purification time required for the condition that the air conditioner runs together with the purifier was shortened by 15.2%. When the fresh air system is running together with the purifier, the purification time can be reduced by 30.4%. It was reduced by 32.6% when the three systems were operating at the same time. The PM2.5 concentration distribution under the joint operating condition was much more even than that under the condition in which the purifier was running alone. To analyze the impact on the CO2 distribution, the CO2 distributions under different joint operations were compared. The results show that the fresh air system can effectively dilute CO2 below 1000 ppm in 30 min. The air conditioner and purifier could help to mix the indoor air to provide a much more uniform indoor CO2 distribution.

Efficient Capture and Electroreduction of Dilute CO2 into Highly Pure and Concentrated Formic Acid Aqueous Solution.

High-purity CO2 rather than dilute CO2 (15 vol %, CO2/N2/O2 = 15:80:5, v/v/v) similar to the flue gas is currently used as the feedstock for the electroreduction of CO2, and the liquid products are usually mixed up with the cathode electrolyte, resulting in high product separation costs. In this work, we showed that a microporous conductive Bi-based metal-organic framework (Bi-HHTP, HHTP = 2,3,6,7,10,11-hexahydroxytriphenylene) can not only efficiently capture CO2 from the dilute CO2 under high humidity but also catalyze the electroreduction of the adsorbed CO2 into formic acid with a high current density of 80 mA cm-2 and a Faradaic efficiency of 90% at a very low cell voltage of 2.6 V. Importantly, the performance in a dilute CO2 atmosphere was close to that under a high-purity CO2 atmosphere. This is the first catalyst that can maintain exceptional eCO2RR performance in the presence of both O2 and N2. Moreover, by using dilute CO2 as the feedstock, a 1 cm-2 working electrode coating with Bi-HHTP can continuously produce a 200 mM formic acid aqueous solution with a relative purity of 100% for at least 30 h in a membrane electrode assembly (MEA) electrolyzer. The product does not contain electrolytes, and such a highly concentrated and pure formic acid aqueous solution can be directly used as an electrolyte for formic acid fuel cells. Comprehensive studies revealed that such a high performance might be ascribed to the CO2 capture ability of the micropores on Bi-HHTP and the lower Gibbs free energy of formation of the key intermediate *OCHO on the open Bi sites.

Imidazole-based organic polymer frameworks as metal- and halogen-free heterogeneous platforms for efficient CO2 cycloaddition

Developing effective metal- and halogen-free heterogeneous catalysts for the CO2 cycloaddition remains a challenge. In this study, novel imidazole-based hyper-crosslinked polymers (HCP-BZ, HCP-BP and HCP-TBZ) with abundant N sites were constructed via a one-pot polymerization, and employed as heterogeneous catalysts for the CO2 cycloaddition reaction. The special skeletal pore structure with abundant hierarchical pore structures and uniformly distributed active sites were demonstrated by various characterization technologies. The optimal HCP-TBZ(16:1) exhibits a good CO2 selective adsorption performance with a CO2 adsorption capacity of 2.23 mmol/g and CO2/N2 adsorption selectivity of 41 at 273 K and 0.1 MPa. Additionally, the resulting catalyst HCP-TBZ(16:1) achieves an excellent catalytic performance with 96 % yield and 99 % selectivity without metal, halogen, cocatalyst and solvent additions (120 °C, 1.0 MPa CO2, 5 h). Moreover, the HCP-TBZ(16:1) also exhibits a satisfactory catalytic activity under the mild conditions (70 °C, 1.0 MPa CO2), and can effectively catalyze the cyclic carbonate preparation from the dilute CO2 (15 %CO2/85 %N2). The comparable catalytic activity and the determined structural stability, reusability and universality make the prepared imidazole-based HCPs a potential heterogeneous catalyst candidate for the CO2 cycloaddition. This study will provide some guidance for developing metal-free and halogen-free CO2 adsorption and conversion platforms.

Improvement of cell growth in green algae Chlamydomonas reinhardtii through co-cultivation with yeast Saccharomyces cerevisiae.

CO2 fixation methods using green algae have attracted considerable attention because they can be applied for the fixation of dilute CO2 in the atmosphere. However, green algae generally exhibit low CO2 fixation efficiency under atmospheric conditions. Therefore, it is a challenge to improve the CO2 fixation efficiency of green algae under atmospheric conditions. Co-cultivation of certain microalgae with heterotrophic microorganisms can increase the growth potential of microalgae under atmospheric conditions. The objective of this study was to determine the culture conditions under which the growth potential of green algae Chlamydomonas reinhardtii is enhanced by co-culturing with the yeast Saccharomyces cerevisiae, and to identify the cause of the enhanced growth potential. When C. reinhardtii and S. cerevisiae were co-cultured with an initial green algae to yeast inoculum ratio of 1:3, the cell concentration of C. reinhardtii reached 133 × 105cells/mL on day 18 of culture, which was 1.5 times higher than that of the monoculture. Transcriptome analysis revealed that the expression levels of 363 green algae and 815 yeast genes were altered through co-cultivation. These included genes responsible for ammonium transport and CO2 enrichment mechanism in green algae and the genes responsible for glycolysis and stress responses in yeast. We successfully increased C. reinhardtiigrowth potential by co-culturing it with S. cerevisiae. The main reasons for this are likely to be an increase in inorganic nitrogen available to green algae via yeast metabolism and an increase in energy available for green algae growth instead of CO2 enrichment.

Process modeling guides operational variables that affect CO2 utilization during the accelerated carbonation of concrete

AbstractAccelerated concrete carbonation is an expanding option for decarbonizing construction. Factors such as concrete mixture design and carbonation environment can influence the maximum CO2 utilization that can be achieved during such a process. A carbonation process designed to utilize a water‐saturated dilute CO2 source wherein 2 < CO2 concentration (v/v%) < 16, was modeled in AspenPlus©. A regression model was developed to correlate CO2 uptake, relative humidity (11%–100%), CO2 concentration ([CO2] = 2—16 v/v%), and temperature (T = 11–74°C) conditions within a carbonation reactor. It was determined that [CO2] was the most significant variable as higher concentrations enhanced CO2 transport through the concrete. The energy use intensity per mass of CO2 utilized (kWh/kgCO2) was determined across a range of processing conditions. As a function of the operational conditions, accelerated carbonation provides a net CO2 reduction of up to 28 kgCO2/tonne of concrete; a reduction of up to ~45% compared to typical formulations.

Perfluoroalkyl-Decorated Noble-Metal-Free MOFs for the Highly Efficient One-Pot Four-Component Coupling between Aldehydes, Amines, Alkynes, and Flue Gas CO2.

The non-noble-metal catalysed-multicomponent reactions between flue gas CO2 and cheap industrial raw stocks into high value-added fine chemicals is a promising manner for the ideal CO2 utilization route. To achieve this, the key fundamental challenge is the rational development of highly efficient and facile reaction pathway while establishing compatible catalytic system. Herein, through the stepwise solvent-assisted linker installation, post-synthetic fluorination and metalation, we report the construction of a series of perfluoroalkyl-decorated noble-metal-free metal-organic frameworks (MOFs) PCN-(BPY-CuI)-(TPDC-Fx ) [BPY=2,2'-bipyridine-5,5'-dicarboxylate, TPDC-NH2 =2'-amino-[1,1':4',1''-terphenyl]-4,4''-dicarboxylic acid] that can catalyze the one-pot four-component reaction between alkyne, aldehyde, amine and flue gas CO2 for the preparation of 2-oxazolidinones. Such assembly endows the MOFs with superhydrophobic microenvironment, superior water resistance and highly stable catalytic site, leading to 21 times higher turnover numbers than that of homogeneous counterparts. Mechanism investigation implied that the substrates can be efficiently enriched by the MOF wall and then the adsorbed amine species act as an extrinsic binding site towards dilute CO2 through their strong preferential formation to carbamate acid. Moreover, density functional theory calculations suggest the tetrahedral geometry of Cu in MOF offers special resistance towards amine poisoning, thus maintaining its high efficiency during the catalytic process.

Perfluoroalkyl‐Decorated Noble‐Metal‐Free MOFs for the Highly Efficient One‐Pot Four‐Component Coupling between Aldehydes, Amines, Alkynes, and Flue Gas CO2

AbstractThe non‐noble‐metal catalysed‐multicomponent reactions between flue gas CO2 and cheap industrial raw stocks into high value‐added fine chemicals is a promising manner for the ideal CO2 utilization route. To achieve this, the key fundamental challenge is the rational development of highly efficient and facile reaction pathway while establishing compatible catalytic system. Herein, through the stepwise solvent‐assisted linker installation, post‐synthetic fluorination and metalation, we report the construction of a series of perfluoroalkyl‐decorated noble‐metal‐free metal–organic frameworks (MOFs) PCN‐(BPY‐CuI)‐(TPDC‐Fx) [BPY=2,2′‐bipyridine‐5,5′‐dicarboxylate, TPDC‐NH2=2′‐amino‐[1,1′:4′,1′′‐terphenyl]‐4,4′′‐dicarboxylic acid] that can catalyze the one‐pot four‐component reaction between alkyne, aldehyde, amine and flue gas CO2 for the preparation of 2‐oxazolidinones. Such assembly endows the MOFs with superhydrophobic microenvironment, superior water resistance and highly stable catalytic site, leading to 21 times higher turnover numbers than that of homogeneous counterparts. Mechanism investigation implied that the substrates can be efficiently enriched by the MOF wall and then the adsorbed amine species act as an extrinsic binding site towards dilute CO2 through their strong preferential formation to carbamate acid. Moreover, density functional theory calculations suggest the tetrahedral geometry of Cu in MOF offers special resistance towards amine poisoning, thus maintaining its high efficiency during the catalytic process.

Long-term effectiveness of lead precipitation as carbonate and its stability under acidic conditions by using activated serpentine mineral

Utilization of readily accessible serpentine mineral, for lead removal from wastewater is a sustainable strategy, but the adsorption capacity of the raw serpentine is limited due to its small interlayer spacing and low surface reactivity, not to say the cases under acidic conditions. In this study, natural serpentine was simply ball-milled to obtain enhanced reactivity and dissolution due to the generation of various lattice defects, which allowed the highly efficient lead fixation even under acidic conditions (at initial pH of 3), compared to that driven by the surface adsorption. Characterizations demonstrated the continuous dissolution of serpentine in solution with the reaction time prolonged, inducing the heterogeneous precipitation of significant (basic) lead carbonate on serpentine surface with an aid of even very dilute atmospheric CO2, which worked as the dominated mechanism for enhancing the whole lead fixation efficiency. Results showed that more than 99.5 % of soluble lead could be immobilized within 1 ∼ 2 h with serpentine dosage of 0.5 g/L under low concentration (5 ∼ 100 mg/L), with a high lead fixation capacity about 1782.9 mg/g after 48 h under high concentration (1000 mg/L). Moreover, the stability of the Pb-bearing sediment under acidic exposure (pH 3) for 24 h was confirmed, and the dissolved lead from chloride and sulfate salts could also be immobilized using the as-proposed strategy. Altogether, these findings demonstrated the high efficiency and long-term effectiveness of lead fixation on the ball-milled serpentine, which makes it a promising candidate for application to green remediation of heavy metal contaminations in various sites along with the utilization of waste mineral resources.

Using Enzymes to Understand and Control the Local Environment of Catalysis

Local environments within porous electrodes are an inherent, but often neglected component of catalysis as the local conversion of reactants to products means catalysis occurs in a very different environment to bulk solution. By understanding and modifying these local environments using a combination of experimental and computational techniques, we show how to improve the performance of electrocatalytic reactions to address the climate crisis by efficiently converting renewable energy to chemical fuels. The selectivity and activity of enzymes means they are ideal model catalysts that can guide the design of synthetic systems. However, they must be in an environment that is close to their optimal to operate efficiently, with small changes in properties such as pH drastically affecting their activity. By optimising their local environment, the rates of fuel formation can be drastically (>18×) increased.[1] We also demonstrate the crucial role of CO2 hydration kinetics on the local pH and CO2 concentration using the enzyme Carbonic Anhydrase co-immobilised with Formate Dehydrogenase.[2] Carbonic Anhydrase catalyses CO2 hydration, causing CO2 to act as a better buffer to mitigate changes in the local pH environment allowing the system to operate closer to its optimal and how this contrasts with heterogeneous CO2 reduction. (fig. 1a)We extend this approach to low CO2 concentrations, taking inspiration from the natural carboxysome to develop a system where Formate Dehydrogenase and Carbonic Anhydrase are co-immobilised in a nanoconfined structure to improve low CO2 concentration utilisation. (fig. 1b).[3] The electrolysis of dilute CO2 streams suffers from low concentrations of dissolved substrate and its rapid depletion at the electrolyte-electrocatalyst interface. These limitations require first energy-intensive CO2 capture and concentration, before electrolyzers can achieve acceptable performances. For direct electrocatalytic CO2 reduction from low-concentration sources, we introduce a strategy that mimics the carboxysome in cyanobacteria by utilizing microcompartments with nanoconfined enzymes in a porous electrode. Carbonic Anhydrase accelerates CO2 hydration kinetics and minimizes substrate depletion by making all dissolved carbon available for utilization, while a highly efficient formate dehydrogenase reduces CO2 cleanly to formate; down to even atmospheric concentrations of CO2. This bio-inspired concept demonstrates that the carboxysome provides a viable blueprint for the reduction of low-concentration CO2 streams to chemicals by using all forms of dissolved carbon.

SO2-Tolerant Electrocatalytic Reduction of CO2 from Simulated Industrial Flue Gas

The electrochemical reduction of CO2 using copper-based electrocatalysts offers a route to produce high-value multicarbon (C2+) products from renewable electricity (Nat. Catal. 4, 952-958 (2021); Nature 614, 262-269 (2023)). To date, the efficient electrocatalytic conversion of CO2 to multicarbon products has only been possible when using impurity-free CO2 sources, such as from direct air capture. The generation of such high-grade CO2 streams is expensive, accounting for almost half of the total energy required for both capture and electroreduction processes (Nat. Catal. 4, 952-958 (2021)). Conversely, capturing CO2 from point sources, such as industrial flue gas, is more efficient due to the higher concentration of CO2 in the feed. However, trace amounts of sulfur dioxide (10 ~ 400 ppm SO2) inherently present in these streams will significantly degrade the CO2 conversion process. All previous attempts to convert CO2 with SO2 present in the feed have resulted in immediate catalyst poisoning and an irreversible loss of CO2 conversion activity.In this study, we designed a modified catalyst layer to react a stream of dilute CO2 containing 400 ppm SO2 to multicarbon products with high stability and performance metrics that match or exceed those achieved with pure CO2 streams. Driven by density function theory and COMSOL simulations, we designed an ionomer:copper:polytetrafluoroethylene (PTFE) (ICP) electrode that features both hydrophobic and highly-charged hydrophilic domains to limit water adsorption and promote CO2 over SO2 transport near the electrochemically active sites. This deactivates the SO2 poisoning mechanism, thus enabling stable and efficient CO2 conversion (see figure). Our approach achieved a sustained C2+ Faradaic efficiency (FE) of 50% for the initial 160 hours at 100 mA cm-2.In order to improve the overall C2+ current efficiency (jC2+) towards industrial scales, we applied our strategy in high-surface-area copper electrodes. We achieved CO2 conversion in the presence of 400 ppm SO2 with a C2+ FE of 76% at a current density of 700 mA cm-2, surpassing what can be achieved in existing integrated CO2 capture-electrolysis systems that use pure CO2. Overall, our approach provides a fully 140-fold increase in performance (FEC2+ × jC2+) compared to the best prior CO2 conversion systems with added SO2 (Nat. Nanotechnol. doi: 10.1038/s41565-022-01286-y (2023); J. Am. Chem. Soc. 141, 9902-9909 (2019)). These findings represent an important advancement in the field of CO2 conversion and highlight the potential of our strategy for industrial-scale applications. Figure 1

Characteristics analysis of multi-channel ceramic membranes for dilute gas absorption in falling film operation

Capturing dilute gases has posed a significant challenge due to the extremely low mass transfer driving force associated with them. Falling film-based absorption using porous membranes emerges as a promising technology for gas separation, offering a substantial gas–liquid interface area while simultaneously eliminating mass transfer resistance within pores that is inherent in hydrophobic membrane-based absorption processes. In this study, we investigated the mass transfer performance of multi-channel ceramic membranes in falling film-based dilute CO2 absorption under various gas and liquid operating conditions. Experimental tests and computational fluid dynamics (CFD) simulations were conducted to examine the effects of pore size and geometrical configuration on the distribution of liquid and gas flows across different channels. Simulations indicated that the optimization of multi-channel ceramic membranes for the falling film-based dilute gas absorption should prioritize achieving a more rational distribution of gas–liquid flows. When the disparity between liquid and gas distribution in the outermost channels exceeded 40%, as opposed to cases where the difference was within 20%, there was a reduction of over 50% in mass transfer performance.

Carbon capture from low CO2 concentration stream through ternary alkali metal nitrates promotion of MgO sorbents at intermediate temperature and low space velocity

Many MgO sorbents exhibit high CO2 uptake in the presence of high concentration CO2 and aided by high space velocity, which unfortunately leads to low CO2 capture efficiency from the gas stream. This study investigates the effect of Li/Na/K ratio on sorption performance in dilute CO2 streams (15% CO2) using alkali metal nitrates-promoted MgO sorbents at a low space velocity of 20 ml/g·min in fixed bed reactor. Study shows that the ratio of Li/Na/K in ternary alkali nitrate salt mixture significantly affects the induction periods for the second phase CO2 sorption, which is a key contributor to high CO2 uptake. High LiNO3 content in ternary mixture leads to long induction period, and therefore lower CO2 capacity during the 4 h period. Conversely, high ratios of NaNO3 and KNO3 leads to shorter induction periods. Among the samples, MgO with 9 mol% (Li0.18Na0.52K0.3) exhibits the highest CO2 sorption capacity of 5.56 mmol/g at 240°C in 4 h (20.6% CO2 capture efficiency) and 3.18 mmol/g after 5 cycles (10.8% CO2 capture efficiency). Comparative studies show that optimal temperatures for CO2 sorption are higher when using pure CO2 (280 to 320°C) than when using 15% CO2 (240°C) due to the thermodynamic equilibrium. CO2-TPD results indicate that CO2 desorption proceeds in two phases similar to CO2 sorption, and the presence of alkali salt in MgO also promotes the decomposition of bulk carbonate to CO2. It is shown that molten salt migration and agglomeration of MgO particles are likely the factors that lead to decline in CO2 uptake over the sorption-desorption cycles.

Microwave-enhanced selective leaching calcium from steelmaking slag to fix CO2 and produce high value-added CaCO3

The abundant resources of steel slag make it an intriguing prospect for long-term CO2 storage by mineral carbonation. Herein, the dissolution of steel slag in an acetic acid (HAc) solution and the fixation of CO2 in the leachate of steel slag to synthesize high-value CaCO3 were investigated. Results show that the microwave water bath condition is beneficial for improving the leaching rate of Ca and Mg elements in steel slag, while the low-concentration HAc solution displays high selectivity for Ca + Mg. In a water bath reaction enhanced by microwave, the leaching rate and selectivity of Ca + Mg can be increased by elevating the reaction temperature, extending the leaching time, lowering the concentration of HAc solution, and increasing the stirring rate. The kinetic parameters of the calcium element, including the apparent activation energy, were determined to be 14.51 kJ·mol−1, suggesting control by surface chemical reactions. Finally, high-purity CaCO3 crystal whiskers can be synthesized under 400 W ultrasonic conditions with a reaction temperature of 70 ℃, reaction time of 15 min, and a solution pH at 9.1 by introducing 20 % CO2 into the leachate. This strategy facilitates the concurrent utilization of dilute CO2 flue gas and voluminous solid steel slag waste, thereby improving the economic and environmental performance of the steel industry.

Spent fluid catalytic cracking catalyst derived MCM-41 supported polyethylenimine for post-combustion CO2 capture

The buildup of solid waste as well as the global warming has become one of the major challenges. CO2 capture with solid amine synthesized from solid waste can not only address the issue of large CO2 emissions, but also achieve efficient utilization of solid waste. In this work, spent fluid catalytic cracking catalysts (sFCCc), a type of major wastes in petrochemical industries, are first utilized to synthesize polyethyleneimine (PEI) functionalized ordered mesoporous materials. The as-obtained MCM-41-PEI from sFCCc exhibits an acceptable equilibrium CO2 capacity (qe) of 2.7 mmol/g in simulated flue gases (10 % CO2/N2) at 65 °C, which is higher than one prepared with commercial silicon source. Moreover, the adsorbent displays excellent cyclic stability during 50 TSA cycles in the dry simulated flue gases, where the capacity decreases only by 4.9 %. Additionally, the breakthrough capacity (qb) is significantly improved from 0.6 mmol/g to 1.6 mmol/g under humid conditions (35 °C, 10 % CO2/N2). Interestingly, we discover that the optimal adsorption temperature of MCM-41-PEI with different PEI loadings shifted due to the distinct PEI distribution behavior within the pores. The Avrami model provides a reliable interpretation for the CO2 adsorption behavior of MCM-41-PEI at various test conditions. The synthetic strategy here provides a new route in the treatment of excessive sFCCc resources to achieve valuable adsorbents, which can be applied to dilute source CO2 capture processes.

(Invited) Overcoming Challenges in Transitioning Electroreduction of CO2 to a Deployable Technology

Since 2010, the electrochemical reduction of CO2 has evolved from a barely studied topic to one of the most active research areas in the field of electrochemistry. The need to reduce global CO2 emissions has driven the vigorous pursuit of different electrocatalysts, electrodes, and electrolyzer configurations that are able to reduce CO2 to different value added intermediates or products ever more selectively and efficiently. With many catalysts now available, attention has started to shift to topics such as catalyst stability, electrode durability, and electrolysis operation optimization. These efforts increasingly are tied to ever more-detailed techno-comic and life cycle analyses.This presentation will provide an overview where some of these ‘beyond CO2RR catalyst discovery’ efforts are. What about durability of electrodes in different cell configurations? What should larger electrolyzer stack designs looks like, and how should they be operated to maximize performance (rate and/or selectivity)? How to optimize performance in light of dilute CO2 feeds from industrial point sources or from direct air capture, with these feeds containing different contaminants and/or oxygen? A separate idea is to pursue a co-conversion approach where conversion of an industrial waste stream or conversion of a renewable feed (e.g., from biomass) into a valued-added chemical on the anode is performed in parallel with electroreduction of CO2 on the cathode. This presentation will explore some of these aforementioned challenges based on our work as well as the work by others.

Reversible CO2 Fixation and Release by a Trinuclear Zn(II) Cryptate Complex and Operando Analysis of the Complex Structure.

Metal complexes inspired by carbonic anhydrase (CA), which is a metalloenzyme containing Zn(II), have been investigated as alternatives for CO2 fixation systems operating under ambient temperature and pressure conditions. In this study, we designed a trinuclear Zn(II) cryptate complex (Zn3L) and demonstrated rapid CO2 fixation with carbonation of CO2 using Zn3L. The CO2 fixation performance of Zn3L surpassed that of a standard CO2 absorbent (KOH(aq) solution) under conditions of the same solute concentration. In addition, the reaction achieved operation without support with addition of a base, which has been often required in systems of CA-inspired complexes. Fixed CO2 was released by protonating polyazacryptate ligand (L) and breaking the complex structure, and deprotonation of L induced the reconstruction of Zn3L, allowing it to refix CO2. This reaction mechanism was proposed based on the analysis of operando extended X-ray absorption fine structure spectroscopy. Zn3L also demonstrated the ability to capture dilute CO2 from air, and the volume of CO2 captured by Zn3L was approximately 2.6 times that captured by the KOH(aq) solution. Our Zn3L exhibited three valuable properties: rapid CO2 fixation without a base, reversibility, and ability to capture dilute CO2; thus Zn3L is a promising candidate as CO2 fixatives.