Aqueous CO2 Research Articles

Indium-modified copper nanocubes for syngas production by aqueous CO2 electroreduction.

Electroreduction of carbon dioxide represents an appealing strategy to rethink a waste product as a valuable feedstock for the formation of value-added compounds. Among the metal electrodes able to catalyze such processes, copper plays a central role due to its rich chemistry. Strategies aimed at tuning Cu selectivity comprise nanostructuring and alloying/post-functionalization with heterometals. In this contribution, we report on straightforward electrochemical methods for the formation of nanostructured Cu-In interfaces. The latter were fully characterized and then used as cathodes for CO2 electroreduction in aqueous environment, leading to the selective production of syngas, whose composition varies upon changing the applied bias and indium content. In particular, gaseous mixtures compatible with the synthesis of methanol or aldehydes (i.e. respectively with 1 : 2 and 1 : 1 CO/H2 ratios) are produced at low (i.e. -0.62 V vs. RHE) applied bias with >3.5 mA cm-2 current densities (in absolute value). Even if the proposed cathodes undergo structural modifications upon prolonged exposure to CO2 reduction conditions, their catalytic activity can be restored by introducing an additional In(III) precursor to the electrolytic solution.

Formation of Protective Surface Films on Carbon Steel in Mildly Alkaline Aqueous Alkanolamine Co2 Solutions

Formation of Protective Surface Films on Carbon Steel in Mildly Alkaline Aqueous Alkanolamine Co2 Solutions

Energy conversion efficiency comparison of different aqueous and semi-aqueous CO2 electroreduction systems

An energy conversion efficiency index, that is independent of the anode reaction performance, is proposed for CO2 reduction in aqueous and semi-aqueous systems. The energy conversion efficiency of CO2 reduction under 107 typical conditions was calculated based on the derived formula. Notably, the resulting efficiency trends of the reduction products differed from their faradaic efficiency trends. When the products were CO, HCOOH, C2H4, and CH4, the electrocatalysts with the higher energy conversion efficiencies were Au, Pd, Cu, and Pt, respectively. Based on the discussion on the overall energy conversion efficiency of all products, Pt should be a specific energetically advantageous catalyst for CO2 reduction because the activation energy is negligibly small. Moreover, the energy conversion and faradaic efficiencies were discovered to not only depend on the electrocatalyst species, but also on the complexity of the reaction, including the number of reaction electrons. Our proposed method for evaluating the energy conversion efficiency of cathode reactions can potentially serve as a novel platform for comparing the CO2 reduction efficiencies of different electroreduction systems.

Sustainable process for enhanced CO2 mineralization of calcium silicates using a recyclable chelating agent under alkaline conditions

This study explored a key challenge of aqueous indirect CO2 mineralization: the consumption of chemicals for a large pH swing between acidic and alkaline conditions and the production of waste fluid. Herein, we proposed and experimentally examined the technical feasibility of a sustainable process for enhanced CO2 mineralization using a new potentially recyclable alkaline solution of chelating agent [N,N-dicarboxymethyl glutamic acid (GLDA)] at near-room temperatures. This process comprising the following: enhanced Ca extraction from silicates with suppressed formation of silica-rich passivation layer via the combined actions of proton and chelating agent attacks at weakly alkaline conditions; Ca carbonation using sodium carbonate at high pH and temperature; and regeneration of the extraction solution (i.e., a decrease in pH and silicon removal) via CO2 gas dissolution. Ca extraction experiments using representative calcium silicate (CaSiO3) at various GLDA concentrations and initial pHs under 50 °C demonstrated the possibility of enhanced Ca extraction with almost all GLDA molecules combining with Ca (Ca/GLDA≈1) at an optimum condition (0.1 M GLDA and pH 8 in this study). This type of extraction is desirable as it contributes to an efficient Ca carbonation in addition to a decreased stability of the Ca-GLDA complex; indeed, most of the extracted Ca were carbonated to aragonite at 80 °C. Moreover, such extraction with the relatively low pH of 8 was found to be desirable for solution regeneration that involves pH adjustment (reduction) because of low silica solubility at lower pH. It was demonstrated that room-temperature CO2 bubbling removed almost all Si as amorphous SiO2, during which CO2 captured as NaHCO3 was also occurring. The recyclability of extraction solution was also demonstrated in this paper. This study encourages further extensive studies to optimize reaction conditions, particularly for using alkaline solid wastes, to make the process both technically and economically feasible.

Archaeal tetraether lipids and their biphytane carbon isotope composition in sediments along an estuarine biogeochemical gradient

The stable carbon isotopic composition of Thaumarchaeota-produced crenarchaeol (δ13Ccren) is thought to reflect the δ13C of dissolved inorganic carbon (DIC) and is valuable in carbon cycle research. Recently, an isotope flux-balance model was proposed that δ13Ccren is dependent on passive CO2 uptake and a slow rate of intracellular conversion to HCO3−, which has been validated in open ocean environments. However, further studies are required in environments with larger ranges of aqueous CO2 and diverse archaeal communities to examine whether the δ13Ccren value could still be predicted by the model. Here, we report δ13C values of biphytanes chemically released from the archaeal tetraether lipids (i.e., GDGTs) in sediments along a transect exhibiting shifts in the archaeal community and showing gradients of salinity, pH, and CO2 in the Pearl River Estuary, South China. Methanogenic and methanotrophic Euryarchaeota are present in the upper estuary, although not significant, as revealed by the elevated relative abundances of GDGT-0 and higher values of methane index in suspended particulate matter (SPM) and surface sediments, as well as the more 13C-depleted C40:0, C40:1, and C40:2 biphytanes than the C40:3 biphytane released from sedimentary GDGTs. Toward the lower estuary and coastal sea, GDGT distributions in SPM and sediments tend to indicate archaeal communities under normal marine conditions. However, there are still differences in δ13C values between C40:0 and C40:3 biphytanes by up to 2.4‰ in sediments, although C40:1, C40:2, and C40:3 were similar in δ13C, likely due to the existence of marine group II Euryarchaeota. Along such a physicochemical and biogeochemical gradient, the values of sedimentary δ13C40:3 increase monotonically with the increase of mean annual pH and the decrease of mean annual CO2. This phenomenon resembles the reported δ13CGDGT profiles, along with the variations in pH and CO2, in water column in the open ocean, and hence consistent in direction with the isotope flux-balance model for Thaumarchaeota lipids. However, our estimated apparent biosynthetic isotope effect (εAr) of crenarchaeol, ranging from 15.2‰ to 19.7‰, is mostly lower than those estimated in the open ocean. With the constraints of thaumarchaeal growth rates in such a dynamic estuarine environment as well as those in the open ocean, we suggest slight reductions in catalytic efficiency and intracellular kinetic isotope effect of intracellular CO2 hydration relative to previously employed values. Such modifications would slightly change the sensitivities of εAr to archaeal growth rate and ambient CO2 concentration but might be more suitable for variable hydrology and community environments.

The impact of outgassing of CO2 and prior calcium precipitation to the isotope composition of calcite precipitated on stalagmites. Implications for reconstructing climate information from proxies

Degassing of CO2 and precipitation of calcite to the surface of stalagmites can strongly impact isotope signals imprinted into the calcite of these speleothems. Here, we show that in all the variety of conditions occurring in nature only two distinct types of degassing exist. First, when a thin film of calcareous solution comes in contact to cave air, which has a lower pCO2 value than that of the aqueous CO2 in the water, molecular CO2 escapes by physical diffusion in several seconds. In a next step lasting several ten seconds, pH and DIC in the solution achieve chemical equilibrium with respect to the CO2 in the cave atmosphere. This solution becomes supersaturated with respect to calcite. During precipitation for each unit CaCO3 deposited one molecule of CO2 is generated and escapes from the solution. This precipitation driven degassing is active during precipitation only. We show that all variations of out gassing proposed in the literature are either diffusive outgassing or precipitation driven degassing and that diffusive outgassing has no influence on the isotope composition of the HCO3− pool and consequently on that of calcite. Its isotope imprint is determined solely by precipitation driven degassing in contrast to most explanations in the literature. We present a theoretical model of δ13C and δ18O that explains the contributions of various parameters such as changes in temperature, changes of pCO2 in the cave atmosphere, and changes in the drip intervals to the isotope composition of calcite precipitated to the apex of the stalagmite. We use this model to calculate quantitatively changes of δ13C and δ18O observed in field experiments (Carlson et al., 2020) in agreement to their experimental data. We also apply our model to prior calcite precipitation (PCP) in the field as reported by Mickler et al. (2019). We discuss how PCP influences isotope composition signals. In summary, we present a transparent method based on few commonly accepted equations that allows calculation of the isotope composition δ13C and δ18O of CaCO3 under various temperatures, pCO2 in the cave air, degrees of PCP, and concentrations cCao of the water entering the cave.

Controls on Alkenone Carbon Isotope Fractionation in the Modern Ocean

AbstractCarbon isotope records from alkenone biomarkers (εp37:2) produced by haptophyte algae are frequently used for atmospheric CO2 paleobarometry, but this method has yielded inconsistent results during periods where CO2 variations are known independently. Recent syntheses of algal cultures have quantitatively demonstrated that εp37:2 indeed records CO2 information: εp37:2 increases as aqueous CO2 concentrations increase relative to carbon demand. However, interpretations of εp37:2 are complicated by irradiance, where higher irradiance yields higher εp37:2. Here we examine the roles of physiology and environment in setting εp37:2 in the ocean. We compile water‐column and sediment core‐top εp37:2 data and add new core‐top measurements, including estimates of cell sizes and growth rates of the alkenone‐producing population. In support of culture studies, we find irradiance to be a key control on εp37:2 in the modern ocean. We test a culture‐derived model of εp37:2 and find that the quantitative relationships calibrated in culture experiments can be used to predict εp37:2 in sediment samples. In water‐column samples, the model substantially overestimates εp37:2, largely resulting from higher irradiance at the depth of sample collection than the integrated light conditions under which the sampled biomass was produced and vertically mixed to the collection depth. We argue that the theory underpinning the conventional diffusive alkenone carbon isotope fractionation model, including the “b” parameter, is not supported by field data and should not be used to reconstruct past CO2 changes. Future estimates of CO2 from εp37:2 should use empirical or mechanistic models to quantitatively account for irradiance and cell size variations.

Modeling and validation of the CO2 degassing effect on CaCO3 precipitation using oilfield data

Calcium carbonate scaling can become a challenge for Brazilian Pre-Salt oil production. The high CO2 content that makes up the gas stream in carbonate reservoirs is remarkable. The relationship between CO2 and CaCO3 is evident in the chemical balance: Ca2+(aq) + 2 HCO3–(aq) ⇆ H2O + CaCO3(s) + CO2(aq). Aqueous CO2 present in carbonate reservoir acidifies the medium and keeps CaCO3 dissolved. The head loss provided by oil production moves CO2(aq) to the gaseous state (CO2(aq) → CO2(g)). It disfavors calcium carbonate to keep dissolved, which implies CaCO3 precipitation. The two mentioned chemical reactions are closely linked and are mutually dependent. A previous thermodynamic study carried out by our group, considering a pseudo-equilibrium state, estimated that CO2 degassing is responsible for 61% to 89% of calcium carbonate scaling. However, it is not possible to carry out laboratory experiments with a pseudo-equilibrium condition. Therefore, a methodology was developed based on experimental feasibility to overcome this issue, making possible its application with computational packages. The software includes not only the aqueous solution thermodynamics but also the interaction with the hydrocarbon phases. An applied review of the role of CO2 on precipitation, considering aqueous, oily, and gaseous phases, was carried out with real-field data of Pre-Salt to support the analysis. Likewise for aqueous CO2, oily CO2 degassing (CO2(o) → CO2(g)) also implies CaCO3 scaling, and the CO2 partitioning between the three phases is a phenomenon that critically governs the precipitation. Regarding the simulation results, the percentages that we previously found were confirmed. In runs with real-field data, the results fell within the theoretical percentage range but indicated a distinct behavior from the influence of CO2 with the change in BSW. These discoveries correspond to some operational problems found in oilfields and may help to explain their still unknown causes.

Insights into promoter-enhanced aqueous phase CO hydrogenation over Co@TiO2 mesoporous nanocomposites

The particle sizes and metal support interactions play a pivotal role in Fischer Tropsch synthesis. Herein, promoter-embedded Co/TiO2 mesoporous nanocomposites with a high surface area were synthesized through a templated assisted solvothermal method comprising highly dispersed spherical shaped Co nanoparticles on titania nanoparticles of less than 10 nm. Moreover, Fischer Tropsch synthesis in aqueous media, rather than wax-slurry, is a new and relatively attractive approach to improve product selectivity and catalyst isolation. The Aqueous phase Fischer-Tropsch (AFTS) is a unified catalytic process to improve CO conversion and enhance selectivity towards higher hydrocarbon (C5+) at low temperatures. The performance under aqueous phase CO hydrogenation was investigated with varying cobalt, platinum, and manganese promoter concentrations to obtain an optimized catalyst recipe for AFTS. The metal support interaction has been studied with the help of temperature-programmed reduction (TPR) couple with XPS and HRTEM analysis. Moreover, the dispersibility of the active species was demonstrated through H2 chemisorption coupled with pulse reoxidation experiments. The CO-TPD and STEM analysis have been performed to understand the role of manganese during the reaction. The reaction parameters have been optimized thoroughly by varying temperature and pressure conditions. The catalyst with the optimized recipe (3 M0.5P25C) and parameters performed the exceptional CO conversion at a rate of 1.75 molCO.molCo−1.h−1 (excluding CO2) with 73% selectivity towards C5+ hydrocarbons with less than 5% methane. Additionally, despite the aqueous phase, CO conversion rates for 3M1P25C were kinetically modeled well by standard Fischer-Tropsch empirical rate expressions, with little H2O term dependence.

Experimental investigation on the oxidation of printing ink wastewater under hydrothermal flames

Printing ink wastewater has a strong color and contains high salinity and high concentrations of complex organic compounds, posting a severe threat to the ecological environment and human health. In this work, supercritical water oxidation of printing ink wastewater is performed within a transpiring wall reactor (TWR) to avoid corrosion and salt plugging issues. The high concentration of organic matter in printing ink wastewater is used to form hydrothermal flames for enhanced degradation. Flame temperatures, gaseous and aqueous products characteristics, as well as the anti-corrosion and salt plugging performance of the reactor, are obtained. A remarkable temperature fluctuation due to the multi-component systems indicates the instability of hydrothermal flame. The salt recovery rate of 94.56% and the integrity of the porous tube inner wall indicate that TWR has a good performance in terms of anti-corrosion and salt plugging. Under typical conditions, the removal rates for chemical oxygen demand, total nitrogen, and color are 99.52%, 70.07%, and 97.89%, respectively. Alkanes, nitrogenous compounds, benzodiazepines, and heterocyclic compounds in the aqueous products, and CO, H2, NH3, and CH4 in the gaseous products are the main intermediate products of printing ink wastewater under hydrothermal flames. Ammonia nitrogen, as the main intermediate product, inhibits the complete oxidation of nitrogenous organic compounds. Additionally, higher KNO3 concentrations promote the complete oxidation of nitrogenous organic compounds, while higher NaOH concentrations can increase the pH value and slightly facilitate degradation. In addition, higher feed flow rates prevent deposition and adhesion of additives that occur at high concentrations.

Detection and quantification of a release of carbon dioxide gas at the seafloor using pH eddy covariance and measurements of plume advection

We detected a controlled release of CO2 (g) with pH eddy covariance. We quantified CO2 emission using measurements of water velocity and pH in the plume of aqueous CO2 generated by the bubble streams, and using model predictions of vertical CO2 dissolution and its dispersion downstream. CO2 (g) was injected 3 m below the floor of the North Sea at rates of 5.7–143 kg d − 1. Instruments were 2.6 m from the center of the bubble streams. In the absence of injected CO2, pH eddy covariance quantified the proton flux due to naturally-occurring benthic organic matter mineralization (equivalent to a dissolved inorganic carbon flux of 7.6 ± 3.3 mmol m − 2d − 1, s.e., n = 33). At the lowest injection rate, the proton flux due to CO2 dissolution was 20-fold greater than this. To accurately quantify emission, the kinetics of the carbonate system had to be accounted for. At the peak injection rate, 73 ± 13% (s.d.) of the injected CO2 was emitted, but when kinetics were neglected, the calculated CO2 emission was one-fifth of this. Our results demonstrate that geochemical techniques can detect and quantify very small seafloor sources of CO2 and attribute them to natural or abiotic origins.

Balance of carbon species combined with stable isotope ratios show critical switch towards bicarbonate uptake during cyanobacteria blooms

Next to water quality deterioration, cyanobacteria blooms can affect turnover of aqueous carbon, including dissolved inorganic carbon (DIC), dissolved organic carbon (DOC), and particulate organic carbon (POC). We investigated interactions of these three phases and their stable isotopes in a freshwater pond with periodic cyanobacterial blooms over a period of 23 months. This helped to map turnover and sources of aqueous carbon before, during, and after bloom events. During bloom events POC isotope values (δ13CPOC) increased up to −17.4‰, after aqueous CO2 (CO2(aq)) fell below an atmospheric equilibration value of 412 μatm. Additionally, carbon isotope enrichment between CO2(aq) and POC (εCO2-phyto) ranged from 2.0 to 21.5‰ with lowest fractionations observed at pH values above 8.9. The increase of δ13CPOC and decrease of εCO2-phyto values at low pCO2 and high pH was most likely caused by the activation of the carbon concentrating mechanism (CCM). This mechanism correlated with prevalent assimilation of 13C-enriched HCO3−. Surprisingly, CO2(aq) still contributed more than 50% to the POC pool down to pCO2 values of around 150 μatm. Only after this threshold the reduced εCO2-phyto suggested incorporation of 13C-enriched HCO3−.

Construction of Six-Oxygen-Coordinated Single Ni Sites on g-C3 N4 with Boron-Oxo Species for Photocatalytic Water-Activation-Induced CO2 Reduction.

The configuration regulation of single-atom photocatalysts (SAPCs) can significantly influence the interfacial charge transfer and subsequent catalytic process. The construction of conventional SAPCs for aqueous CO2 reduction is mainly devoted toward favorable activation and photoreduction of CO2 , however, the role of water is frequently neglected. In this work, single Ni atoms are successfully anchored by boron-oxo species on g-C3 N4 nanosheets through a facile ion-exchange method. The dative interaction between the B atom and the sp2 N atom of g-C3 N4 guarantees the high dispersion of boron-oxo species, where O atoms coordinate with single Ni (II) sites to obtain a unique six-oxygen-coordinated configuration. The optimized single-atom Ni photocatalyst, rivaling Pt-modified g-C3 N4 nanosheets, provides excellent CO2 reduction rate with CO and CH4 as products. Quasi-in-situ X-ray photoelectron spectra, transient absorption spectra, isotopic labeling, and in situ Fourier transform infrared spectra reveal that as-fabricated six-oxygen-coordinated single Ni (II) sites can effectively capture the photoelectrons of CN along the BO bridges and preferentially activate adsorbed water to produce H atoms to eventually induce a hydrogen-assisted CO2 reduction. This work diversifies the synthetic strategies for single-atom catalysts and provides insight on correlation between the single-atom configuration and reaction pathway.

A TiO2 -Co(terpyridine)2 Photocatalyst for the Selective Oxidation of Cellulose to Formate Coupled to the Reduction of CO2 to Syngas.

Immobilization of a phosphonated cobalt bis(terpyridine) catalyst on TiO2 nanoparticles generates a photocatalyst that allows coupling aqueous CO2 -to-syngas (CO and H2 ) reduction to selective oxidation of biomass-derived oxygenates or cellulose to formate. An enzymatic saccharification pre-treatment process is employed that enables the use of insoluble cellulose as an electron-donating substrate under benign aqueous conditions suitable for photocatalytic CO2 conversion. The hybrid photocatalyst consists of solely earth-abundant components, and its heterogeneous nature allows for reuse and operation in aqueous solution for several days at 25 °C, reaching a cellulose-to-formate conversion yield of 17 %. Thus, the proof-of-concept for valorizing two waste streams (CO2 and biomass) simultaneously into value-added chemicals through solar-driven catalysis is demonstrated.

Carbon Cycling During Oceanic Anoxic Event 2: Compound‐Specific Carbon Isotope Evidence From the Western Interior Seaway

AbstractThe Cenomanian‐Turonian Oceanic Anoxic Event 2 (OAE2, ∼94 Ma) was a period of widespread ocean deoxygenation and marine organic carbon burial. Increased CO2and nutrient flux before OAE2 are generally considered the main drivers of ocean anoxia and carbon burial, though some evidence has suggested multiple phases of CO2input and burial throughout the event. To test hypotheses about the nature of carbon cycle perturbations before and during OAE2, we analyzed stable carbon isotopes in marine‐derived and terrestrial‐derived biomarkers from an expanded sedimentary record of OAE2 from the western margin of the Western Interior Seaway. Biomarker carbon isotope data were used to estimate the concentration and carbon isotopic composition of aqueous and atmospheric CO2, as well as changes in marine productivity, through OAE2. While complicated by biological and environmental uncertainties, ourpCO2reconstructions generally agree with estimates from other locations and proxies around the world. High‐resolution sampling revealed several short‐lived carbon cycle perturbations before and during OAE2 that may reflect fluctuations in the global carbon cycle, including several pulses of isotopically light carbon before OAE2, during the onset of OAE2, and during the Plenus event. This study provides new constraints on carbon cycle dynamics during OAE2 that highlight the complex nature of the event and its causes.

Sensitive and Direct Optical Monitoring of Release and Cellular Uptake of Aqueous CO from CO-Releasing Molecules.

Dynamics of release and cellular uptake of aqueous CO from CO-releasing molecules (CORMs) significantly affect signaling and cell viability. So far, it has been mainly observed by IR, UV-visible, and fluorescence techniques, which suffer from poor sensitivity and slow response time. Here, we show how to directly probe the mass transfer of aqueous CO from CORMs to cells using a fluidic chamber integrated with live cells and Raman reporters of large-area Au@Pd core-shell nanoparticle assembly to emulate a physiologically relevant microenvironment. We sensitively and directly detect CO release from trace CORMs of as low as 100 nM by measuring the Raman transitions of CO via rapid chemisorption onto the surface of the Au@Pd nanoparticles. By using our method, we successfully observe the dynamics of CO release from CORM-2 despite its very short half-life. We also reveal that the initial rate of CO release from CORM-3 is dramatically decreased by tens to hundreds of times when exposed to physiologically relevant pH variations from 7.4 to 2.5, which can be attributed to the acid hydrolysis of the CO ligand. CORM-2 tends to quickly release CO regardless of pH, probably because of its rapid cleavage into two monomeric Ru complexes by the co-solvent. The decrease in the initial rate at lower temperatures is more significant for CORM-3 than for CORM-2. Finally, we observe that the cellular uptake of aqueous CO from CORM-3 by lung cancer cells is approximately 2 times higher than that of normal lung cells.

Machine learning to discover mineral trapping signatures due to CO2 injection

Mineral trapping is pursued as a geological CO2 sequestration (GCS) mechanism because it permanently stores CO2 in solid phases or minerals. However, CO2 mineral-trapping mechanisms are poorly understood due to (1) lack of sufficient field and laboratory data characterizing these complex processes, and (2) challenges to develop site-specific reactive-transport models coupling fluid flow and geochemical reactions occurring at various temporal (from milliseconds to years) and spatial (from pore (millimeters) to field (kilometers)) scales. Reactive transport with additional complexities such as heterogeneity can make the simulation outputs even more difficult to interpret because of complex nonlinearity and multi-scale interdependencies. Furthermore, the values of model outputs such as concentrations can vary by several orders of magnitude, making it harder to correlate and characterize the impact of the variables via traditional data interpretation techniques such as exploratory data analyses. Recently, machine learning (ML) has shown promise in feature discovery and in highlighting hidden mechanisms that cannot be obtained by existing data-analytics and statistical methods. In this study, we applied an unsupervised ML approach, non-negative matrix factorization with custom k-means clustering (NMFk) to the data generated by reactive-transport simulations of GCS. The reactive-transport data consisted of 19 attributes, including four physio-chemical variables (pH, porosity, aqueous CO2, and sequestered CO2), six chemical species (K+, Na+, HCO3−, Ca2+, Mg2+, Fe2+), and four carbonate minerals (calcite, dolomite, siderite, and ankerite), a feldspar mineral (albite), and four clay minerals (illite, clinochlore, kaolinite, and smectite) over a period of 200 years of simulation time. The simulation data used was for Morrow B sandstone at the Farnsworth hydrocarbon unit in Texas. Data are sampled at two locations within the model domain: (1) at the injection well and (2) 200 m west of the injection well. The injection was performed for a period of 10 years. Using NMFk, we estimated the temporal interdependencies among the 19 attributes over a span of 200 years. We found that NMFk was able to identify four reaction stages and their dominant attributes; these cannot be directly discerned through traditional visualization (e.g., line plots, Pareto analysis, Glyph-based visualization methods) or exploratory data analysis tools of the simulation data. The four stages were: reactions in the injection phase followed by short-, mid-, and long-term reactions. The NMFk analysis also revealed that 10 among the 19 attributes are dominant. These dominant attributes for mineral trapping include calcite, dolomite at injection well, siderite at 200 m away from the injection well, clinochlore, kaolinite, Na+, K+, Ca2+, Mg2+, pH, and aqeuous CO2. Finally, at late times (65–200 years), our results showed that calcite plays a major role in mineral trapping with insignificant contribution from siderite, ankerite, and clay minerals. These findings make the proposed unsupervised ML-model attractive for reactive-transport sensing towards real-time GCS monitoring.

Molecular insights into CO2 hydrate formation in the presence of hydrophilic and hydrophobic solid surfaces

Microsecond molecular simulations have been performed on CO2 hydrate formation in the slit-nanopores of graphite and hydroxylated-silica surfaces. The simulation results show that different hydrophilic/hydrophobic properties of graphite and silica surfaces exert substantially different effects on CO2 hydrate formation. It is found that hydrate nucleation requires high aqueous CO2 concentration, and the solid surface affects hydrate nucleation mainly by changing the aqueous CO2 concentration in the systems. The hydrophobic graphite surfaces could adsorb CO2 molecules so strongly that the surfaces are almost covered by CO2 molecules, thus, the aqueous CO2 concentration is lowered. On the contrary, a partially cylindrical CO2 nanobubble is adsorbed at the hydrophilic silica surface and results in a high aqueous CO2 concentration. In the slit-nanopores of graphite and silica surfaces, hydrate nucleation starts from the bulk region and then grows towards the surfaces. CO2 hydrate solids interact with the silica surfaces mainly via semi-cages, which are constituted by the strong hydrogen bonds formed between silanols and interfacial water. At the end of the simulation, the hydrophobic graphite surfaces are still covered by the strongly adsorbed CO2 molecules, preventing the formation of ordered interfacial water on the surfaces, which is previously reported to play a critical role in promoting hydrate formation. These molecular insights into the effects of solid surfaces on CO2 hydrate formation are beneficial to CO2 hydrate-based technologies, such as geological CO2 sequestration.

Utilization of CO2 in direct aqueous carbonation of concrete fines generated from aggregate recycling: Influences of the solid–liquid ratio and CO2 concentration

Development of an effective aggregate recycling process for concrete waste is an important issue. During aggregate recycling, the hydrated cement fraction in concrete that decreases the quality of recycled aggregate is removed, and this yields concrete fines as a by-product. Appropriate treatment of the concrete fines is needed to improve the feasibility of aggregate recycling. In this study, concrete fines from aggregate recycling were used for CO2 utilization through direct aqueous carbonation with a low energy input using low-purity CO2 at atmospheric pressure. Two samples of actual concrete fines and one of synthetic concrete fines were compared in these experiments. The differences between CO2 uptake efficiencies were explained according to the characteristics of the different concrete fines. Solid–liquid ratio and concentration of introduced CO2 were studied and their effects on the reaction were determined and explained according to the three main steps of direct aqueous carbonation, Ca extraction, CO2 dissolution, and CaCO3 precipitation. The results are supported by and compared to the literature based on the carbonation mechanism and kinetics. Additionally, the mechanisms of particle collision and solidification were observed, and their effect was determined by several characterization analyses. The CO2 uptake efficiency was compared with those from other studies, which proves concrete fines are a promising candidate for direct aqueous carbonation. The present method under atmospheric pressure with a low CO2 concentration was effective for CO2 utilization with concrete fines and had a CO2 uptake capacity of 0.13 g-CO2/g-concrete fines.

Catalyst-Proximal Plastrons for Enhancing Electrocatalytic Reduction of CO2 on Copper in Aqueous-Phase Systems

Electrocatalytic reduction of CO2 (CO2RR) is an attractive method of converting CO2 to solar fuels. Much research in this field has been focused on developing novel catalysts to enhance the activity and product distribution (selectivity). Copper has been studied widely as it can produce both one carbon (C1) products such as methane and formate, and products with two or more carbons (C2+) such as ethylene, ethanol, and n-propanol1. Copper is not immune from the competing hydrogen evolution reaction: the poor solubility of CO2 in water (~34 mM at ambient conditions) limits the CO2RR current density, and hydrogen generation is favored if the aqueous CO2 concentration becomes locally undersaturated close to the catalyst during CO2RR2. This limitation is even more pronounced for nanotextured copper, because the increased active surface area leads to faster depletion of local CO2 thereby promoting hydrogen evolution over CO2RR. Hence, overcoming these CO2 availability limitations can enable higher CO2RR activity while reducing co-evolution of hydrogen.In this work, we develop a gasphilic CO2 trap that increases gas-liquid mass transfer and maintains supersaturated CO2 concentration around the catalyst during CO2RR. Gasphilic surfaces need a special combination of surface chemistry and texture to capture bubbles and form a sheet of gas underwater which is called a plastron3. By creating pyramidal textures, CO2 bubbles are efficiently captured within the textures and form a CO2 plastron. When this plastron is placed proximal to both smooth and nanostructured copper catalysts during CO2RR, the current density is enhanced and maintained throughout the reaction, when compared to two commonly used methods of CO2 delivery: headspace and bubbling in the bulk electrolyte. The plastron allows for quick replenishment of CO2 during the CO2RR reaction in the vicinity of the catalyst. As a result, the H2 Faradaic efficiency is reduced from 33% to 13% on smooth copper, and from 62% to 33% on nanostructured copper. This is accompanied by an increased production of C2+ products including ethylene, ethanol and propanol, as well as acetone and acetate at Faradaic efficiencies exceeding 1%. We highlight the importance of this catalyst-proximal plastron approach by comparing against recent aqueous-phase CO2RR studies, and discuss how this approach can inform optimal design of continuous CO2RR systems such as gas-diffusion electrodes.[1] Nitopi, S., Bertheussen, E., Scott, S. B., Liu, X., Engstfeld, A. K., Horch, S., Seger, B., Stephens, I. E. L., Chan, K., Hahn, C., Nørskov, J. K., Jaramillo, T. F. & Chorkendorff, I. Progress and Perspectives of Electrochemical CO2 Reduction on Copper in Aqueous Electrolyte. Chem. Rev. 119, 7610–7672 (2019).[2] Lobaccaro, P., R. Singh, M., Lee Clark, E., Kwon, Y., T. Bell, A. & W. Ager, J. Effects of temperature and gas–liquid mass transfer on the operation of small electrochemical cells for the quantitative evaluation of CO2 reduction electrocatalysts. Physical Chemistry Chemical Physics 18, 26777–26785 (2016).[3] Panchanathan, D., Rajappan, A., Varanasi, K. K. & McKinley, G. H. Plastron Regeneration on Submerged Superhydrophobic Surfaces Using In Situ Gas Generation by Chemical Reaction. ACS Appl. Mater. Interfaces 10, 33684–33692 (2018).