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At least 37 records · Page 2
All sourcesNASA NTRSDOE OSTI
DOE OSTI2025

Side-On N 2 Binding and Reduction by a Heterotetrametallic Zr 2 Co 2 Cluster

In multimetallic compounds, N 2 typically bridges late transition metals in an end-on (η 1 :η 1 ) fashion while early transition metals often bind N 2 side-on (η 2 ), with the latter resulting in more significant N–N bond elongation. In this work, N 2 fixation is accomplished by using a well-defined scaffold featuring a heterobimetallic combination of Zr and Co. We report a heterotetrametallic Zr 2 Co 2 cluster in which N 2 is bound side-on to the two Co centers and end-on to Zr in a μ 3 -η 1 :η 2 :η 2 binding mode that defies established paradigms for N 2 binding. The heterometallic approach is shown to facilitate catalytic reductive silylation of N 2 with turnover numbers far exceeding those of reported cobalt catalysts.

catalysts↗
DOE OSTI2026

Photodissociation and Infrared Spectroscopy of U + (CO 2 ) n , UO + (CO 2 ) n , and UO 2+ (CO 2 ) n Cation-Molecular Complexes

Laser vaporization of uranium in a pulsed supersonic expansion of carbon dioxide is used to produce complexes of the form U + (CO 2 ) n , UO + (CO 2 ) n , and UO 2+ (CO 2 ) n . These ions are selected in a reflectron time-offlight mass spectrometer and studied with visible laser photodissociation and tunable infrared laser photodissociation spectroscopy in the region of the CO 2 antisymmetric stretch. The dissociation patterns and spectroscopy of these ions indicate that CO 2 ligands are intact molecules. Although reaction products that form oxide-carbonyl or oxalate species are predicted to be stable, there is no direct evidence in the frequency range studied for the formation of these species. There is no clear indication for the coordination numbers for singly charged uranium and its oxide complexes with CO 2 . However, there is strong support in the vibrational patterns for an eight-coordinate complex of the doubly charged UO 2+ species, i.e., UO 2+ (CO 2 ) 8 .

Cluster chemistry↗
DOE OSTI2022

Role of carboxysomes in cyanobacterial CO 2 assimilation: CO 2 concentrating mechanisms and metabolon implications

Many carbon-fixing organisms have evolved CO 2 concentrating mechanisms (CCMs) to enhance the delivery of CO 2 to RuBisCO, while minimizing reactions with the competitive inhibitor, molecular O 2 . These distinct types of CCMs have been extensively studied using genetics, biochemistry, cell imaging, mass spectrometry, and metabolic flux analysis. Highlighted in this paper, the cyanobacterial CCM features a bacterial microcompartment (BMC) called ‘carboxysome’ in which RuBisCO is co-encapsulated with the enzyme carbonic anhydrase (CA) within a semi-permeable protein shell. Further, the cyanobacterial CCM is capable of increasing CO 2 around RuBisCO, leading to one of the most efficient processes known for fixing ambient CO 2 . The carboxysome life cycle is dynamic and creates a unique subcellular environment that promotes activity of the Calvin–Benson (CB) cycle. The carboxysome may function within a larger cellular metabolon, physical association of functionally coupled proteins, to enhance metabolite channelling and carbon flux. In light of CCMs, synthetic biology approaches have been used to improve enzyme complex for CO 2 fixations. Research on CCM-associated metabolons has also inspired biologists to engineer multi-step pathways by providing anchoring points for enzyme cascades to channel intermediate metabolites towards valuable products.

59 BASIC BIOLOGICAL SCIENCES↗
DOE OSTI2024

Radiation-assisted electrochemical reduction of CO 2 to CO

Carbon monoxide (CO) is a versatile intermediate feedstock for many applications and can be produced through the electrochemical reduction of carbon dioxide (CO 2 ). However, current electrochemical CO production is hindered by high overall costs, primarily due to low conversion efficiencies and significant energy requirements. Herein, we report a unique way of enhancing the electrochemical reduction of CO 2 to CO using gamma (γ) photons. The γ-irradiation applied to the electrochemical cell setup induces the production of e˙ - , which results in an increased CO 2 ionization and production of excited CO 2 (CO$^{*}_{2}$) molecules via lower energy barrier. The ionized CO 2 ˙ - is quickly stabilized over a silver catalyst, providing an alternative low activation energy route for CO 2 reduction. In conclusion, a decrease in the overpotential barrier enhanced the electrochemical reduction of CO 2 to CO by 25%.

38 RADIATION CHEMISTRY, RADIOCHEMISTRY, AND NUCLEA↗
DOE OSTI2024

Maximum and Comparative Efficiency Calculations for Integrated Capture and Electrochemical Conversion of CO 2

CO 2 as a C1 feedstock is receiving increasing attention, but it typically exists in dilute or impure streams. The capture and conversion of CO 2 to fuels or chemicals using carbon-neutral energy mitigates carbon emissions when CO 2 is captured from point sources and results in net or negative carbon emissions when CO 2 is obtained by direct air capture. Additionally, CO 2 capture and conversion steps are typically viewed as separate processes that optimize either capture and concentration or catalytic conversions of pure streams of CO 2 .

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2024

AP-XPS Study of the Reaction of O 2 and CO 2 with Zn–Au(111) Surface Alloys: Activation of O–O/C–O Bonds and the Formation of ZnO

Synchrotron-based ambient-pressure X-ray photoelectron spectroscopy (AP-XPS) was used to study the dissociation of O 2 and CO 2 on Zn-Au(111) alloys which contained 0.2-0.3 monolayers of zinc. Although Au(111) is inert, the alloys displayed a high activity for the cleavages of O-O and C-O bonds at room temperature with the formation of ZnO x species. Dissociative adsorption of O 2 at 300 K destroyed the alloys and the results of AP-XPS pointed to the co-existence of two types of oxygen species on the surface: ZnO x and chemisorbed O atoms (O chem ) on Au or the Au-ZnO interface. Annealing from room temperature to 600 K induced a O chem → ZnO x transformation that reflected the poor stability of O atoms on Au(111). The Zn-Au(111) systems exhibited a reactivity towards CO 2 that was much larger than that seen for Au(111), Cu(111) or surfaces of late transition metals like, Ni, Pd or Pt. At 300 K, CO 2 underwent partial dissociation depositing large amounts of O chem on the surface with minor formation of ZnO x . In addition, the deposited O chem reacted with CO 2 to form surface carbonate groups. Dosing of CO 2 at 500-600 K mainly led to the formation of ZnO x and the surface carbonate almost disappeared. In the presence of hydrogen, i.e. reaction feeds with a CO 2 to H 2 ratio of 1:3, the surface chemistry at 300 K was very similar to that seen for pure CO 2 with the formation of ZnO x and carbonate groups. In contrast, at 500-600 K, reaction with hydrogen induced the removal of ZnO x and CO 3 /HCOO species. Finally, our AP-XPS results are consistent with the idea that CO 2 hydrogenation on AuZn alloys involves a redox process where there is sequential oxidation by CO 2 and reduction by H 2 to yield methanol.

36 MATERIALS SCIENCE↗
DOE OSTI2022

Support Effect and Surface Reconstruction in In 2 O 3 / m- ZrO 2 Catalyzed CO 2 Hydrogenation

Here, we investigate the chemical and structural dynamics at the interface of In 2 O 3 /m-ZrO 2 and their consequences on the CO 2 hydrogenation reaction (CO 2 HR) under reaction conditions. While acting to enrich CO 2 , monoclinic zirconia (m-ZrO 2 ) was also found to serve as a chemical and structural modifier of In 2 O 3 that directly governs the outcome of the CO 2 HR. These modifying effects include the following: (1) Under reaction conditions (above 623 K), partially reduced In 2 O 3 , i.e., InO x (0 < x < 1.5), was found to migrate in and out of the subsurface of m-ZrO 2 in a semireversible manner, where m-ZrO 2 accommodates and stabilizes InO x by serving as a reservoir. The decreased concentration of surface InO x under elevated temperatures coincides with significantly decreased selectivity toward methanol and a sharp increase of the reverse water–gas shift reaction. The reconstruction-induced variation of InO x concentration appears to be one of the most important factors contributing to the altered catalytic performance of CO 2 HR at different reaction conditions. (2) The strong interactions and reactions between m-ZrO 2 and In 2 O 3 result in the activation of a pool of In–O bonds at the In 2 O 3 /m-ZrO 2 interface to form oxygen vacancies. On the other hand, the high dispersity of In 2 O 3 nanostructures onto m-ZrO 2 prevents their over-reduction under catalytically relevant conditions (up to 673 K), when bare In 2 O 3 is unavoidably reduced into the metallic phase (In 0 ). The relationship between the extent of reduction of In 2 O 3 and catalytic performance (CO 2 conversion, CH 3 OH selectivity, or yield of CH 3 OH) suggests the presence of an optimum coverage of surface InO x and oxygen vacancies under reaction conditions. The conventional model that links catalytic performance solely to the coverage of oxygen vacancies appears invalid in the present case. In situ analysis also allows the observation of surface reaction intermediates and their interconversions, including the reduction of CO 3 * into formate, a precursor for the formation of methanol and CO. The combinative ex situ and in situ study sheds light on the reaction mechanism of the CO 2 HR on In 2 O 3 /m-ZrO 2 -based catalysts. Our findings on the large-scale surface reconstructions, support effect, and the reaction mechanism of In 2 O 3 /m-ZrO 2 for CO 2 HR may apply to other related metal oxide catalyzed CO 2 reduction reactions.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2025

Highly dense atomic Fe–Ni dual metal sites for efficient CO 2 to CO electrolyzers at industrial current densities

Carbon-supported, atomically dispersed, nitrogen-coordinated metal sites (e.g., Fe and Ni) are arguably the most promising catalysts for the electrochemical reduction of CO 2 to CO due to their unique catalytic properties and the use of earth-abundant elements. However, conventional single metal sites are constrained by their structural simplicity, causing either too weak or too strong absorption/desorption of multiple critical intermediates (e.g., *COOH and *CO). Current catalysts also suffer from ultra-low loadings (<1.0 wt%) of atomic metal active sites in catalysts, leading to inadequate performance for CO 2 -to-CO conversion. Here, we develop dual Ni/Fe metal site catalysts with significantly increased atomically dispersed metal loadings (up to 4.8 wt%). A gas-phase chemical vapor deposition (CVD) approach to introducing single Ni sites was integrated with Fe 2 O 3 /ZIF-8 precursors, followed by an optimal thermal activation. The optimized CVD-Ni/Fe–N–C catalyst exhibited remarkable electrocatalytic performance for the CO 2 reduction to CO in a continuous membrane-electrode-assembly electrolyzer, achieving a maximum CO faradaic efficiency (FE CO ) of 96% at a current density of 700 mA cm −2 in a near-neutral electrolyte. Furthermore, a desirable but challenging acidic flow-cell electrolyzer was designed using this dual metal site catalyst to improve CO 2 utilization, accomplishing a FE CO of up to 95% at a CO partial current density close to 600 mA cm −2 . Density functional theory (DFT) calculations suggest a synergetic effect between Fe–Ni pairs facilitating *COOH intermediate formation and *CO desorption simultaneously during CO 2 to CO conversion. This is key to breaking the linear scaling relationship of conventional single-metal site catalysts during the CO 2 reduction reaction.

36 MATERIALS SCIENCE↗
DOE OSTI2023

A Bifunctional Ionic Liquid for Capture and Electrochemical Conversion of CO 2 to CO over Silver

Electrochemical conversion of CO 2 requires selective catalysts and high solubility of CO 2 in the electrolyte to reduce the energy requirement and increase the current efficiency. In this study, the CO 2 reduction reaction (CO 2 RR) over Ag electrodes in acetonitrile-based electrolytes containing 0.1 M [EMIM][2-CNpyr] (1-ethyl-3-methylimidazolium 2-cyanopyrolide), a reactive ionic liquid (IL), is shown to selectively (>94%) convert CO 2 to CO with a stable current density (6 mA·cm –2 ) for at least 12 h. The linear sweep voltammetry experiments show the onset potential of CO 2 reduction in acetonitrile shifts positively by 240 mV when [EMIM][2-CNpyr] is added. This is attributed to the pre-activation of CO 2 through the carboxylate formation via the carbene intermediate of the [EMIM] + cation and the carbamate formation via binding to the nucleophilic [2-CNpyr] – anion. The analysis of the electrode–electrolyte interface by surface-enhanced Raman spectroscopy (SERS) confirms the catalytic role of the functionalized IL where the accumulation of the IL-CO 2 adduct between –1.7 and –2.3 V vs Ag/Ag + and the simultaneous CO formation are captured. Furthermore, this study reveals the electrode surface species and the role of the functionalized ions in lowering the energy requirement of CO 2 RR for the design of multifunctional electrolytes for the integrated capture and conversion.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2024

Confinement-induced clustering of H 2 and CO 2 gas molecules in hydrated nanopores

Gas molecule clustering within nanopores holds significance in the fields of nanofluidics, biology, gas adsorption/desorption, and geological gas storage. However, the intricate roles of nanoconfinement and surface chemistry that govern the formation of gas clusters remain inadequately explored. In this study, through free energy calculation in molecular simulations, we systematically compared the tendencies of H 2 and CO 2 molecules to aggregate within hydrated hydrophobic pyrophyllite and hydrophilic gibbsite nanopores. The results indicate that nanoconfinement enhances gas dimer formation in the nanopores, irrespective of surface chemistry. However, surface hydrophilicity prohibits the formation of gas clusters larger than dimers, while large gas clusters form easily in hydrophobic nanopores. Despite H 2 and CO 2 both being non-polar, the larger quadrupole moment of CO 2 leads to a stronger preference for dimer/cluster formation compared to H 2 . Here, our results also indicate that gases prefer to enter the nanopores as individual molecules, but exit the nanopores as dimers/clusters. This investigation provides a mechanistic understanding of gas cluster formation within nanopores, which is relevant to various applications, including geological gas storage.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2023

Direct Transformation of SiH 4 to a Molecular L(H) 2 Co=Si=Co(H) 2 L Silicide Complex

The synthesis of bimetallic molecular silicide complexes is reported, based on the use of multiple Si–H bond activations in SiH 4 at the metal centers of 14-electron LCo I fragments (L = Tp", HB(3,5-diisopropylpyrazolyl) 3 – ; [BP 2 tBu Pz], PhB(CH 2 P t Bu 2 ) 2 (pyrazolyl)). Upon exposure of (Tp"Co) 2 (μ-N 2 ) (1) to SiH 4 , a mixture of (Tp"Co) 2 (μ-H) (2) and (Tp"Co) 2 (μ-H) 2 (3) was formed and no evidence for Si–H oxidative addition products was observed. In contrast, [BP 2 tBu Pz]-supported Co complexes led to Si–H oxidative additions with the generation of silylene and silicide complexes as products. Notably, the reaction of ([BP 2 tBu Pz]Co) 2 (μ-N 2 ) (5) with SiH 4 gave the dicobalt silicide complex [BP 2 tBu Pz](H) 2 Co=Si=Co(H) 2 [BP 2 tBu Pz] (8) in high yield, representing the first direct route to a symmetrical bimetallic silicide. Here, the effect of the [BP 2 tBu Pz] ligand on Co–Si bonding in 7 and 8 was explored by analysis of solid-state molecular structures and density functional theory (DFT) investigations. Upon exposure to CO or DMAP (DMAP = 4-dimethylaminopyridine), 8 converted to the corresponding [BP 2 tBu Pz]Co(L) x adducts (L = CO, x = 2; L = DMAP, x = 1) with concomitant loss of SiH 4 , despite the lack of significant Si–H interactions in the starting complex. On heating to 60 °C, 8 underwent reaction with MeCl to produce small quantities of Me x SiH 4–x (x = 1–3), demonstrating functionalization of the μ-silicon atom in a molecular silicide to form organosilanes.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2025

Electrochemical reduction of ammonia-captured CO 2 to CO over a nickel single-atom catalyst

Carbon reactive capture and conversion offers a sustainable route to valuable chemicals and fuels while aiding Green House Gas (GHG) reduction. Direct electrochemical conversion of capture solutions like bicarbonate avoids the energy demands of conventional CO 2 regeneration. Ammonium bicarbonate (NH 4 HCO 3 ) is particularly attractive due to its low decomposition temperature and ability to supply in situ CO 2 from dilute sources without requiring purified CO 2 . Meanwhile, single-atom catalysts (SACs) with nitrogen-coordinated metal sites further enhance CO 2 reduction efficiency using Earth-abundant materials. In this study, we demonstrate a nickel single-atom catalyst (Ni-SAC)-based electrolyzer that utilizes NH 4 HCO 3 as the CO 2 source, achieving significantly improved CO production performance compared to the conventional silver cathodes used in the CO 2 reduction reaction (CO 2 RR) to produce CO. The Ni-SAC cathode exhibited a Faradaic efficiency of 60.1% for CO production at −200 mA cm −2 , while the silver cathode achieved a Faradaic efficiency of only 2%, likely due to ammonium-induced poisoning. Furthermore, the integration of a customized microporous layer onto the electrode significantly increased the Faradaic efficiency from 64% to 83% at −100 mA cm −2 , emphasizing the crucial role of electrode structure optimization in enhancing CO selectivity. These findings demonstrate a sustainable and economically viable strategy for green CO production directly from CO 2 capture solutions.

30 DIRECT ENERGY CONVERSION↗
DOE OSTI2025

Uses of H 2 and CO 2 gases in hydrometallurgical processes: Potential towards sustainable pretreatment, metal production and carbon neutrality

Hydrometallurgical practices represent a vital area of focus within the metallurgical sector, and it is critical that these practices are carried out in an environmentally friendly manner in order to promote sustainability and responsibility. One promising approach is the use of hydrogen gas (H 2 ), known for its ‘green’ properties, along with the pollutant carbon dioxide CO 2 ), at various stages within the hydrometallurgical field. This review provides a detailed overview of the utilization of H 2 and CO 2 gases in the hydrometallurgical field, encompassing key stages including pretreatment, leaching, and metal recovery. Theoretical underpinnings are also provided for each stage, along with an overview of cutting-edge approaches such as application of supercritical CO 2 . Challenges and future directions associated with H 2 and CO 2 utilization are also discussed. This review serves an important step towards advancing sustainable and responsible hydrometallurgical processes in the pursuit of carbon neutrality.

36 MATERIALS SCIENCE↗
DOE OSTI2024

Clostridium autoethanogenum alters cofactor synthesis, redox metabolism, and lysine-acetylation in response to elevated H 2 :CO feedstock ratios for enhancing carbon capture efficiency

Clostridium autoethanogenum is an acetogenic bacterium that autotrophically converts carbon monoxide (CO) and carbon dioxide (CO 2 ) gases into bioproducts and fuels via the Wood–Ljungdahl pathway (WLP). To facilitate overall carbon capture efficiency, the reaction stoichiometry requires supplementation of hydrogen at an increased ratio of H 2 :CO to maximize CO 2 utilization; however, the molecular details and thus the ability to understand the mechanism of this supplementation are largely unknown. In order to elucidate the microbial physiology and fermentation where at least 75% of the carbon in ethanol comes from CO 2 , we established controlled chemostats that facilitated a novel and high (11:1) H 2 :CO uptake ratio. We compared and contrasted proteomic and metabolomics profiles to replicate continuous stirred tank reactors (CSTRs) at the same growth rate from a lower (5:1) H 2 :CO condition where ~ 50% of the carbon in ethanol is derived from CO 2 . Our hypothesis was that major changes would be observed in the hydrogenases and/or redox-related proteins and the WLP to compensate for the elevated hydrogen feed gas. Our analyses did reveal protein abundance differences between the two conditions largely related to reduction–oxidation (redox) pathways and cofactor biosynthesis, but the changes were more minor than we would have expected. While the Wood–Ljungdahl pathway proteins remained consistent across the conditions, other post-translational regulatory processes, such as lysine-acetylation, were observed and appeared to be more important for fine-tuning this carbon metabolism pathway. Metabolomic analyses showed that the increase in H 2 :CO ratio drives the organism to higher carbon dioxide utilization resulting in lower carbon storages and accumulated fatty acid metabolite levels. This research delves into the intricate dynamics of carbon fixation in C. autoethanogenum, examining the influence of highly elevated H 2 :CO ratios on metabolic processes and product outcomes. The study underscores the significance of optimizing gas feed composition for enhanced industrial efficiency, shedding light on potential mechanisms, such as post-translational modifications (PTMs), to fine-tune enzymatic activities and improve desired product yields.

09 BIOMASS FUELS↗
DOE OSTI2025

A dual promotional effect of doping tantalum (Ta) in atomically dispersed Ru/CeO 2 catalyst toward CO 2 methanation: Enhanced associative adsorption of CO 2 and activation of H 2

Precisely controlling the product selectivity in CO 2 hydrogenation through rational catalyst design presents a promising approach to mitigate environmental and energy-related challenges, though it remains a significant scientific hurdle. Herein, the CH 4 selectivity of 0.5 wt% Ru loaded catalysts at 250 °C was effectively shifted from approximately 35 % to 100 % through the incorporation of Ta dopant into the CeO 2 support. The EXAFS spectra in conjunction with CO DRIFTS experiment indicated the presence of atomically dispersed Ru particles anchored on the Ta-doped CeO 2 surface. A higher oxidized CeO 2 surface was evidenced in the presence of Ta dopant. The presence of Ta dopant also improved the dispersion of Ru species and their interaction with the support. Most importantly, the Ru/Ta-CeO 2 catalyst exhibited a pronounced capacity for associative CO 2 -adsorption under atmospheric pressure at 50 °C. An improved H 2 activation was also observed under CO 2 hydrogenation conditions. This novel finding of the dual promotional effect of Ta carries a significant impact in the field of CO 2 capture and utilization.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2024

Theoretical Investigation of the Adsorbate and Potential–Induced Stability of Cu Facets During Electrochemical CO 2 and CO Reduction

The activity and product selectivity of electrocatalysts for reactions like the carbon dioxide reduction reaction (CO 2 RR) are intimately dependent on the catalyst's structure and composition. While engineering catalytic surfaces can improve performance, discovering the key sets of rational design principles remains challenging due to limitations in modeling catalyst stability under operating conditions. Herein, we perform first-principles density functional calculations adopting implicit solvation methods with potential control to study the influence of adsorbates and applied potential on the stability of different facets of model Cu electrocatalysts. Using coverage dependencies extracted from microkinetic models, we describe an approach for calculating potential and adsorbate-dependent contributions to surface energies under reaction conditions, where Wulff constructions are used to understand the morphological evolution of Cu electrocatalysts under CO 2 RR conditions. Here we identify that CO*, a key reaction intermediate, exhibits higher kinetically and thermodynamically accessible coverages on (100) relative to (111) facets, which can translate into an increased relative stabilization of the (100) facet during CO 2 RR. Our results support the known tendency for increased (111) faceting of Cu nanoparticles under more reducing conditions and that the relative increase in (100) faceting observed under CO 2 RR conditions is likely attributed to differences in CO* coverage between these facets.

30 DIRECT ENERGY CONVERSION↗
DOE OSTI2026

Third-body stabilization of supercritical CO 2 in CO oxidation: development and application of a ReaxFF force field for the CO/O/CO 2 system

Supercritical CO 2 (scCO 2 ) plays a crucial role as a solvent in separation processes, advanced power cycles, and materials processing. Nonetheless, the atomistic comprehension of how the dense scCO 2 matrix influences the fundamental reaction of carbon monoxide (CO) is still insufficiently explored. Experimental studies and molecular dynamics (MD) simulations frequently fail to detect the highly reactive, transient intermediates, such as atomic oxygen (O), that drive these reactions. Here, to address this issue, we have developed a novel ReaxFF reactive force field for the CO 2 /CO/O system. The force field parameters were calibrated using density functional theory and second-order Møller-Plesset calculations to model CO 2 crystal properties, intermolecular interactions, bond dissociation curves, and reaction energy barriers. The force field reproduces the cohesive energy of the CO 2 crystal, the pressure characteristics of bulk scCO 2 , the equation-of-state behavior over a wide pressure–density range, the pressure dependence of the C–O bond length under compression, and the structural properties of liquid and scCO 2 , as documented by experiments, ab-initio MD, and prominent non-reactive models. The force field was subsequently applied to study the CO + O → CO 2 reaction. In a dilute environment, the reaction is inefficient as the newly formed CO 2 rapidly dissociates due to excess kinetic and potential energy acquired from the exothermic reaction. Conversely, in a dense scCO 2 environment, the surrounding matrix acts as an efficient third body, stabilizing the emerging CO2 product via molecular collisions. Statistical analysis confirms an average excess energy dissipation of 133.9 ± 3.6 kcal/mol over 112.4 ± 17.9 ps. Kinetic energy decomposition reveals that ∼ 92% of the excess kinetic energy is stored in internal (rotational and vibrational) degrees of freedom. This ReaxFF force field establishes a mechanistic foundation for third-body stabilization in dense reactive environments.

Chowdhury, Emdadul Haque [Pennsylvania State Univ.↗
DOE OSTI2025

Hyperselective carbon membranes for precise high-temperature H 2 and CO 2 separation

More than 90% of the world’s hydrogen (H 2 ) is produced from fossil fuel sources, which requires energy-intensive separation and purification to produce high-purity H 2 fuel and to capture the carbon dioxide (CO 2 ) by-product. While membranes can decarbonize H 2 /CO 2 separation, their moderate H 2 /CO 2 selectivity requires secondary H 2 purification by pressure swing adsorption. Here, we report hyperselective carbon molecular sieve hollow fiber membranes showing H 2 /CO 2 selectivity exceeding 7000 under mixture permeation at 150°C, which is almost 30 times higher than the most selective nonmetallic membrane reported in the literature. The membrane is able to maintain an ultrahigh H 2 /CO 2 selectivity over 1400 under mixture permeation at 400°C. Pore structure characterization suggests that highly refined ultramicropores are responsible for effectively discriminating the closely sized H 2 and CO 2 molecules in the hyperselective carbon molecular sieve membrane. Modeling shows that the unprecedented H 2 /CO 2 selectivity will potentially allow one-step enrichment of fuel-grade H 2 from shifted syngas for decarbonized H 2 production.

Science & Technology - Other Topics↗