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At least 613 records · Page 34
All sourcesNASA NTRSDOE OSTI
DOE OSTI2025

Structural Evolution and Stability of Rh/TiO 2 Catalysts under CO 2 Hydrogenation Conditions: Influence of the Initial Rh Structure

Characterizing catalyst stability by identifying the predominant mechanisms, timescales and driving forces of catalyst reconstruction under relevant reaction conditions is necessary for the design and commercialization of new catalysts. Here, in this paper, we study Rh/TiO 2 catalysts under CO 2 hydrogenation conditions (773 K, 75% H 2 , 25% CO 2 ) at high conversion and utilize reactivity studies along with ex-situ and in-situ spectroscopy and microscopy to characterize changes in catalyst activity and structure as a function of time on stream and the initial catalyst structure. This is a prototypical catalyst for CO 2 hydrogenation where Rh structure and Rh-TiO 2 interactions have been proposed to explain reactivity, selectivity (between CO and CH 4 formation) and catalyst stability. The influence of the initial Rh structure (varying from Rh single atoms to Rh nanoparticles), support stability, regeneration and pretreatment(s), and the chemical potential(s) of the reaction environment on reaction selectivity and catalyst stability were explored. The product selectivity between CO and CH 4 was determined to be dependent on the relative fraction of Rh single atoms and Rh nanoparticle-TiO 2 interfacial sites under reaction conditions, each exhibiting distinct stability under prolonged time on stream. Surprisingly, Rh single atoms exhibited stability for the duration of 90 h reactivity measurements, even at high Rh density (≥ 1.8 Rh atoms/nm 2 ) on the support, while Rh nanoparticles sintered under reaction conditions. As a result, all catalysts exhibited increasing selectivity to CO with increasing time on stream (> 10 h). We conclude the distribution of Rh structures evolved over time under reaction conditions through three distinct reconstruction mechanisms (Rh particle fragmentation, Ostwald ripening, and particle migration and coalescence) that occurred on varying timescales. Catalyst stability on the ~90 h time scale was ultimately controlled by the initial Rh structure.

25 ENERGY STORAGE↗
DOE OSTI2025

Nicotinamide Cofactor Biomimetics: Design and Structure Activity Relationships

Biocatalysis is an attractive methodology due to its sustainable metrics, combined with its vaunted regio-, chemo-, and stereoselectivity. Among biocatalysts, oxidoreductases catalyze redox transformations on a wide variety of substrates and are thus particularly attractive for synthetic applications. Many oxidoreductases depend on nicotinamide-based cofactors; however, the high cost and poor atom economy of such cofactors can negate the advantage of biosynthetic processes. The native cofactors (i.e., NADH or NADPH) can be replaced by nicotinamide cofactor biomimetics (NCBs) to address these issues. Additionally, NCBs can enhance enzyme activity due to structures with expanded redox properties. These capabilities open possibilities for reactivity and biorthogonality. Thus, the field has responded with new NCB structures, including variation in their electronic and molecular recognition properties. This perspective focuses on structure-activity relationships of simple NCBs, including their function, stability, and other properties, along with methods for their regeneration in biosynthesis.

agricultural chemistry↗
DOE OSTI2025

Restructuring of Benzimidazole-Based Copper Complexes during Electrochemical CO 2 Reduction

Copper-based molecular catalysts have been widely used for the electrochemical reduction of carbon dioxide (ERCO 2 ). However, establishing a correlation between structure and catalytic performance has been challenging due to the dynamic nature of these materials under ERCO 2 working conditions. Here, we report on the restructuring of the copper complexes [Cu(bzimpy)Cl 2 ] and [Cu(pyrben) 2 (NO 3 )]NO 3 during ERCO 2 in an acidic medium, evidencing its effects on the observed activity, selectivity, and stability. In situ X-ray absorption spectroscopy (XAS) suggested that, at sufficiently negative potentials, the Cu 2+ centers of both complexes are reduced to undercoordinated Cu 0 species. Pb underpotential deposition of post-ERCO 2 samples indicated that such Cu 0 species also present a dominant contribution of the (100) domain, which is in line with the high Faradaic efficiency toward C 2 products (∼45%) shown by both complexes at −1.24 V vs RHE. By combining online electrochemical mass spectrometry, in situ XAS, electron paramagnetic resonance (EPR), and quantitative product analysis, we found that [Cu(bzimpy)Cl 2 ] exhibits a partially reversible restructuring, regenerating the main complex moiety, [Cu(bzimpy)] 2+ , and its geometry.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2026

Selective CO 2 Reduction by Bis(bipyridine)cobalt(II) Catalysts: The Role of Pendant Pyridine as a Proton Acceptor

Electrochemical CO 2 reduction reaction (CO 2 RR) catalyzed by molecular earth-abundant metal catalysts is a promising strategy to convert CO 2 into value-added products. One recent trend in this field has been focusing on the rational design of catalysts by incorporating redox-active ligands and modifying the secondary coordination sphere (SCS) to achieve efficient and selective CO 2 RR. Herein, we report a series of Co bis­(bipyridine) catalysts featuring various dangling groups, such as pyridine, tertiary amine, or butyl, in the secondary coordination sphere (Co-PyMe, Co-Py, Co-PrN, and Co-Bu). Efficient, selective electrocatalytic CO 2 RR was achieved by the complexes after the generation of triply reduced intermediate consisting of a Co I center and a dianionic ligand, producing CO as the major product and trace amount of H 2 . Strong correlations with the identity of dangling groups and turnover frequency (TOF) have been observed, in which Co-PyMe displayed the highest TOF (1086 s –1 in MeCN/H 2 O). Mechanistic studies indicated that the acceleration of CO 2 RR with pyridine-functionalized catalysts were derived from the protonation of pyridine dangling groups which participated as weak acids in the H-bonding network with exogenous proton sources, stabilizing CO 2 -bound intermediates and facilitating proton transfer. In addition, precatalytic CO 2 binding and activation at the third reduction (−2.1 V) was revealed by CV and SEC-IR studies. The resultant doubly reduced CO-bound species acted as a trapping state which inhibited CO 2 RR electrocatalysis. Regeneration of active species was accessed via reductive dissociation of CO at a more negative potential. In conclusion, this study highlights the combined effects of redox-active ligands and pyridine/pyridinium as SCS groups on CO 2 RR catalysis and provides design principles for future development of CO 2 RR catalysts utilizing pyridine/pyridiniums as SCS functional groups to fine-tune the catalytic activity.

CO2 reduction↗
DOE OSTI2023

MOF Linker Extension Strategy for Enhanced Atmospheric Water Harvesting

A linker extension strategy for generating metal–organic frameworks (MOFs) with superior moisture-capturing properties is presented. Applying this design approach involving experiment and computation results in MOF-LA2-1 {[Al(OH)(PZVDC)], where PZVDC 2– is (E)-5-(2-carboxylatovinyl)-1H-pyrazole-3-carboxylate}, which exhibits an approximately 50% water capacity increase compared to the state-of-the-art water-harvesting material MOF-303. The power of this approach is the increase in pore volume while retaining the ability of the MOF to harvest water in arid environments under long-term uptake and release cycling, as well as affording a reduction in regeneration heat and temperature. Density functional theory calculations and Monte Carlo simulations give detailed insight pertaining to framework structure, water interactions within its pores, and the resulting water sorption isotherm.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2020

Mechanocatalytic Ammonia Synthesis over TiN in Transient Microenvironments

Environmentally friendly and energy-efficient ways to produce ammonia are essential to meet global food demands. In this study, a new approach for ammonia production at nominally ambient conditions is introduced. As proof of concept, ammonia is synthesized mechanocatalytically by ball milling titanium in a continuous gas flow. The ammonia synthesis reaction is proposed to follow a transient Mars–van Krevelen mechanism under mechanically activated conditions, where molecular nitrogen incorporation into the titanium lattice and titanium nitride hydrogenation occur in thermodynamically distinct environments. X-ray powder diffraction and X-ray absorption spectroscopy confirm the formation of titanium nitride from titanium and N2. The reactivity of nitrided titanium supports that lattice nitrogen plays a role in ammonia formation. The in situ formed titanium nitride is catalytically active, and the nitride regeneration reaction is determined to be the rate-limiting step. A preliminary technoeconomic analysis shows that this approach could be feasible for distributed ammonia production.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2020

Elucidation of Active Oxygen Sites upon Delithiation of Li 3 IrO 4

Transformational increases in the storage capacity of battery cathodes could be achieved by tapping into the redox activity at oxide ligands in addition to conventional transition metal couples. However, the key signatures that govern such lattice oxygen redox (LOR) have not been ascertained. Li 3 IrO 4 has the largest reversible LOR, rendering it a unique model system. Here, X-ray spectroscopy and computational simulations reveal that LOR in Li 3 IrO 4 is selectively compensated via O sites with three lone pairs, which are activated by Li/Ir disorder. The two-electron LOR can be reversed to regenerate the initial state without unlocking competing bulk reactions observed in many other compounds. We uncover an intricate interplay between stoichiometry, O coordination, and nonbonding states in LOR and pinpoint spectroscopic signatures. This interplay is indispensable for designing materials with 3d metals that fulfill the promise of LOR to overcome the bottlenecks of current cathodes for future implementation in practical batteries.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2021

Anthraquinone Flow Battery Reactants with Nonhydrolyzable Water-Solubilizing Chains Introduced via a Generic Cross-Coupling Method

Water-soluble anthraquinones (AQs) hold great promise serving as redox-active species in aqueous organic flow batteries. Systematic investigations into how the properties of redox molecules depend on the water-solubilizing groups (WSGs) and the way in which they are bound to the redox core are, however, still lacking. We introduce WSGs linked to anthraquinone by C=C bonds via a cross-coupling reaction and convert C=C to C–C bonds through hydrogenation. The anthraquinone and the WSGs are connected via (un)branched chains with (un)saturated bonds. We investigate the influence of chains and ionic ending groups on the redox potentials of the molecules and identify three important trends: (1) The electron-withdrawing ending groups can affect the redox potentials of AQs with two unsaturated hydrocarbons on the chains through π-conjugation. (2) For chains with two (un)saturated straight hydrocarbons, WSGs increase the redox potentials of the AQs in the order PO 3 2– < CO 2 – < SO 3 – . (3) AQs with (un)saturated chains at high pH possess desirably low redox potentials, high solubilities, and high stability. Disproportionation leads to the formation of anthrone, which can be regenerated to anthraquinone. Tautomerization results in the saturation of alkene chains, stabilizing the structure. Here, we utilize these observations to identify a potentially low-cost and long-lifetime negative electrolyte that demonstrates a temporal fade rate as low as 0.0128%/day when paired with a potassium ferrocyanide positive electrolyte.

25 ENERGY STORAGE↗
DOE OSTI2026

NH 3 -Mediated Reactive Capture and Conversion: Integrating CO 2 Absorption from Flue Gas with CO Production via NH 4 HCO 3 Electrolysis

Efficient carbon capture and utilization require strategies that minimize energy penalties of CO 2 regeneration and compression. Reactive capture and conversion (RCC) address this challenge by integrating capture with direct electrochemical conversion. Here, we show an NH3-mediated tandem RCC system that couples capture of CO 2 from simulated flue gas (10% v/v CO 2 in N 2 ) with electroreduction of NH 4 HCO 3 to CO over a Ni single-atom catalyst (Ni-SAC). Speciation modeling and capture experiments revealed that a deep CO 2 capture with C/N ratio of 0.65 was achieved using 2.5 M NH 3 from simulated flue gas. Electrolysis of the resulting NH 4 HCO 3 on the Ni- SAC delivered an 85% CO Faradaic efficiency at 100 mA/cm 2 with excellent tolerance to NH 3 /NH 4 + as confirmed by DFT calculations and ab initio molecular dynamics (AIMD) simulations. Further, the technoeconomic analysis established a levelized total cost of CO manufacturing of $25.43/kmol, gauging the practical viability. Overall, this study holds great potential to decarbonize the chemical manufacturing industry while reducing synthetic production costs.

32 ENERGY CONSERVATION, CONSUMPTION, AND UTILIZATI↗
DOE OSTI2021

Turning Rubber into a Glass: Mechanical Reinforcement by Microphase Separation

Supramolecular associations provide a promising route to functional materials with properties such as self-healing, easy recyclability or extraordinary mechanical strength and toughness. The latter benefit especially from the transient character of the formed network, which enables dissipation of energy as well as regeneration of the internal structures. However, recent investigations revealed intrinsic limitations in the achievable mechanical enhancement. Here we present studies of a set of telechelic polymers with hydrogen-bonding chain ends exhibiting an extraordinarily high, almost glass-like, rubbery plateau. This is ascribed to the segregation of the associative ends into clusters and formation of an interfacial layer surrounding these clusters. An approach adopted from the field of polymer nanocomposites provides a quantitative description of the data and reveals the strongly altered mechanical properties of the polymer in the interfacial layer. These results demonstrate how employing phase separating dynamic bonds can lead to the creation of high-performance materials.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2026

Depolymerization of Vinyl Polymers

Depolymerization is a promising approach to reduce plastic waste by regenerating monomers from polymers, presenting a compelling solution to maintain a circular polymer economy. However, vinyl polymers with all-carbon backbones are especially difficult to depolymerize due to significant thermodynamic and kinetic barriers. Developments in reversible-deactivation radical polymerization and catalytic methods demonstrate how tuning polymer structure and reaction conditions can address these challenges. This Viewpoint revisits early studies on radical depolymerization and recent advances enabling monomer recovery at lower temperatures. Exciting current trends to utilize depolymerization as a strategy for tuning polymer material properties and upcycling waste polymer to high-value products are discussed. Finally, we outline key directions to make vinyl polymer depolymerization scalable, efficient, and economically viable.

depolymerization↗
DOE OSTI2025

Metal–Organic Frameworks for Per- and Polyfluoroalkyl Substances Treatment in Contaminated Water

Per- and polyfluoroalkyl substances (PFAS) are synthetic pollutants known for their chemical stability, environmental persistence, and toxicological risks. Their widespread use has led to extensive contamination, particularly in aquatic systems. Conventional treatment methods often face challenges such as high energy consumption and the production of secondary pollutants. Metal− organic frameworks (MOFs), with their high surface areas and tunable structures, have emerged as promising materials for PFAS remediation. This review summarizes recent progress in MOF-based PFAS adsorption and degradation, highlighting key frameworks such as MIL, UiO, and ZIF. Mechanistic insights into adsorption behavior and regeneration capabilities are discussed, along with the catalytic performance of MOF composites and postmodified systems in degradation pathways. The review concludes with design strategies for next-generation MOF materials aimed at efficient, sustainable PFAS removal under realistic conditions.

Adsorption↗
DOE OSTI2021

Reversing the Irreversible: Thermodynamic Stabilization of LiAlH 4 Nanoconfined Within a Nitrogen-Doped Carbon Host

A general problem when designing functional nanomaterials for energy storage is the lack of control over the stability and reactivity of metastable phases. Using the high-capacity hydrogen storage candidate LiAlH4 as an exemplar, we demonstrate an alternative approach to the thermodynamic stabilization of metastable metal hydrides by coordination to nitrogen binding sites within the nanopores of N-doped CMK-3 carbon (NCMK-3). The resulting LiAlH 4 @NCMK-3 material releases H 2 at temperatures as low as 126 °C with full decomposition below 240 °C, bypassing the usual Li 3 AlH 6 intermediate observed in bulk. Moreover, >80% of LiAlH 4 can be regenerated under 100 MPa H 2 , a feat previously thought to be impossible. Nitrogen sites are critical to these improvements, as no reversibility is observed with undoped CMK-3. Density functional theory predicts a drastically reduced Al–H bond dissociation energy and supports the observed change in the reaction pathway. Finally, the calculations also provide a rationale for the solid-state reversibility, which derives from the combined effects of nanoconfinement, Li adatom formation, and charge redistribution between the metal hydride and the host.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2022

Plastic Waste Product Captures Carbon Dioxide in Nanometer Pores

We discuss, plastic waste (PW) and increasing atmospheric carbon dioxide (CO 2 ) levels are among the top environmental concerns presently facing humankind. With an ambitious 2050 zero-CO 2 emissions goal, there is a demand for economical CO 2 capture routes. Here we show that the thermal treatment of PW in the presence of potassium acetate yields an effective carbon sorbent with pores width of 0.7–1.4 nm for CO 2 capture. The PW to carbon sorbent process works with single or mixed streams of polyolefin plastics. The CO 2 capacity of the sorbent at 25 °C is 17.0 ± 1.1 wt % (3.80 ± 0.25 mmol g –1 ) at 1 bar and 5.0 ± 0.6 wt % (1.13 ± 0.13 mmol g –1 ) at 0.15 bar, and it regenerates upon reaching 75 ± 5 °C. The CO 2 capture cost from flue gas via this technology is estimated to be <$21 ton –1 CO 2 , much lower than competing CO 2 capture technologies. Hence, this PW-derived carbon material should find utility in the capture of CO 2 from point sources of high CO 2 emissions while providing a use for otherwise deleterious PW.

01 COAL, LIGNITE, AND PEAT↗
DOE OSTI2025

Challenges in Product Selectivity for Electrocatalytic Reduction of Amine-Captured CO 2 : Implications for Reactive Carbon Capture

CO 2 is a potential feedstock for carbon-based fuels or materials, but is only available in dilute streams. Integrated processes for CO 2 capture and conversion directly valorize the CO 2 captured by sorbent materials, skipping the energetically expensive sorbent regeneration step. Amines are the most heavily studied liquid-phase sorbent materials for CO 2 capture from dilute streams. Amines react with CO 2 in a 2:1 ratio to form the corresponding ammonium carbamate. Ammonium carbamate [NH 4 ][H 2 NCO 2 ] was tested as the substrate using the highly selective and robust CO 2 -to-formate reduction electrocatalyst [( tBu POCOP)Ir(H)(NCCH 3 ) 2 ], where ( tBu POCOP) is the tridentate pincer ligand 2,6-bis(di tert -butyl-phosphonito). When ammonium carbamate was used as the substrate instead of CO 2 , only hydrogen was produced. An equivalent electrolysis with ammonium hexafluorophosphate with CO 2 also resulted in primarily hydrogen. Methyl carbamate and urea were also tested as substrates as proxies for carbamate that do not contain an equivalent of ammonium, and there was also negligible reduction to carbon-based products. These results indicate that the loss of selectivity observed for aminecaptured CO 2 , or ammonium carbamate, is likely due to the generation of the acidic ammonium equivalent as well as the greater challenge of reducing carbamate compared to CO 2 . This study illustrates that catalysts with high selectivity for concentrated CO 2 can favor hydrogen evolution and loss of carbon-based products when amine-captured CO 2 is used instead.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2021

Deep Eutectic Solvent Formed by Imidazolium Cyanopyrrolide and Ethylene Glycol for Reactive CO 2 Separations

Solvents made from a reactive ionic liquid, with an imidazolium cation and pyrrolide anion, and ethylene glycol at a wide compositional range were studied for separations of CO 2 at low partial pressures (<<0.1 bar up to 1 bar). Thermal analysis and measurements of viscosity and density show compacting of the liquid upon mixing with enhanced stability achieved by hydrogen bonding. A detailed mechanistic study was performed by IR, quantitative NMR, and ab initio calculations that show significant CO 2 absorption capacity below 5000 ppm of CO 2 in N 2 . Three reversible routes are found that yield carbonate (major product), carboxylate (moderate), and carbamate (minor) species. With CO 2 at 100% RH, bicarbonate along with carbonate species form. The CO 2 -ethlyene glycol reaction complex, the carbonate anion, is stabilized by the hydrogen bonding and Coulombic interactions, thus preventing evaporation of the solvent during regeneration. This study demonstrates a promising approach to designer green solvents for CO 2 separations in open systems such as direct air capture.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2022

Ionic-liquid-Assisted Fabrication of Lignocellulosic Thin Films with Tunable Hydrophobicity

Thin films with enhanced water repellency and mechanical strength can be fabricated from renewable lignocellulosic feedstock as a replacement for petrochemical-derived synthetic polymers, such that it minimizes life cycle impact on the environment and human health. In this work, hybrid poplar wood, either untreated (control) or pretreated with hot water at 160 °C for 20 min (HWE-20), 60 min (HWE-60), and 90 min (HWE-90), was dissolved in 1-ethyl-3-methylimidazolium acetate and regenerated to fabricate thin films. The HWE-90 films were enriched in lignin by 74%, specifically on the surface, which along with hemicellulose depletion imparted hydrophobicity (108° water contact angle) when compared to the control (56°), HWE-20 (77°), and HWE-60 (84°) films. They also exhibited 86% reduced water vapor sorption hysteresis and 75% improved storage modulus compared to the control. Thus, we demonstrate how to tune the lignocellulosic film properties via a combination of hot water pretreatment and ionic liquid dissolution.

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

Improving Carbonate-Promoted C–H Carboxylation Using Mesoporous Carbon Supports

C–H carboxylation is an attractive way to utilize CO 2 for chemical production provided that it does not consume resource-intensive reagents. Alkali carbonates dispersed into the pores of mesoporous supports display strongly basic reactivity under CO 2 , allowing them to be used as base promoters for C–H carboxylation of (hetero)arenes in the absence of other reagents or catalysts. Mesoporous oxides are convenient support materials, but only a relatively small fraction of the dispersed carbonate (ca. 10-20%) is converted to carboxylate products when metal oxide supports are used. Here we compare mesoporous oxide and carbon supports and investigate the dependence of carbonate reactivity on pore structure. We show that using mesoporous carbons supports can increase the carbonate conversion by 2–4x when compared to oxide supports. This improved carbonate reactivity is maintained across a variety of mesoporous carbons with different pore structures (ordered vs. disordered) and pore diameters, indicating that the dispersed carbonate is intrinsically more reactive on the surface of a carbon material compared to an oxide surface. Reaction of the carboxylate products with dimethyl carbonate yields isolable methyl esters as the final product and regenerates the dispersed carbonate. We show that mesoporous carbon supports are robust to at least 5 cycles of successive C–H carboxylation and methylation with dimethyl carbonate. Furthermore, understanding how the support structure affects dispersed carbonate reactivity is valuable for advancing C–H carboxylation toward practical application and utilizing these materials in other CO 2 transformations.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗