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Results for “Reactive intermediates”

Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 271 records · Page 15
All sourcesNASA NTRSDOE OSTI
DOE OSTI2022

Evidence for H-bonding interactions to the μ-η 2 :η 2 -peroxide of oxy-tyrosinase that activate its coupled binuclear copper site

The factors that control the diverse reactivity of the μ-η 2 :η 2 -peroxo dicopper(II) oxy-intermediates in the coupled binuclear copper proteins remain elusive. In this study, spectroscopic and computational methods reveal H-bonding interactions between active-site waters and the μ-η 2 :η2-peroxide of oxy-tyrosinase, and define their effects on the Cu(II) 2 O 2 electronic structure and O 2 activation.

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

Interactions between molecular-scale biogeochemical processes and hyporheic exchange for understanding coupled Fe-S-C cycling in iron-rich riparian wetlands (Final Technical Report)

Riparian wetlands are dynamic interfaces that exert strong control over water quality, contaminant mobility, and greenhouse gas emissions. These systems are characterized by hyporheic exchange between oxic surface water and anoxic groundwater, which generates steep redox gradients and promotes spatially and temporally variable microbial activity. Despite growing recognition of tightly coupled iron (Fe), sulfur (S), and carbon (C) cycling in these environments, the mechanisms governing these interactions—particularly under low-sulfate freshwater conditions—remain poorly constrained. This project developed a mechanistic understanding of how hydrologic variability and microbial processes interact to control Fe–S–C cycling in iron-rich riparian wetlands. Using a multi-scale and multi-method approach integrating field observations, geochemical and spectroscopic analyses, metagenomics, and reactive transport modeling, we demonstrate that “cryptic” sulfur cycling—rapid sulfur transformations involving intermediate-valence species—plays a dominant and previously underrecognized role in freshwater wetlands. These processes persist despite low sulfate concentrations and significantly influence iron reduction, carbon mineralization, and methane dynamics. The results show that cryptic sulfur cycling enhances dissolved Fe 2+ production, regulates methane concentrations, and is strongly controlled by climate-driven hydrologic fluxes. By linking hydroclimate, subsurface flow, and biogeochemical reactions, this work provides a predictive framework for understanding how wetland systems respond to environmental change, with direct implications for water quality and carbon cycling.

54 ENVIRONMENTAL SCIENCES↗
DOE OSTI2025

Olefin Coupling Catalyzed by (Pybox)Os Complexes via Osmacyclopentane Intermediates: Comparison with Isoelectronic (Phebox)Ir

(Pybox)Os is found to catalyze alkene hydrovinylation, effecting the dimerization of ethylene, tail-to-tail coupling of propene and 1-butene, and cross-coupling of ethylene with higher α-olefins. This reactivity contrasts with the previously reported dehydrogenative coupling of ethylene to give butadiene catalyzed by the isoelectronic fragment (Phebox)Ir. The reaction mechanism was investigated through computational and experimental means. Both the Os- and Ir-catalyzed reactions proceed through a [2 + 2 + 1] cyclization of the corresponding bis-olefin complex to yield an experimentally observed metallacyclopentane intermediate. In both cases, the metallacyclopentane undergoes β- H elimination, via a dechelated κ 2 -pincer-ligated intermediate, to yield a σ−π-but-3-enyl hydride complex or derivative. Both the greater reactivity and the distinct chemoselectivity of the Os system relative to the Ir system are attributable to C−H reductive elimination by the σ−π-but-3-enyl hydride having a barrier for Os much lower than that for Ir. This lower barrier to C−H elimination for Os is unexpected given that the thermodynamic driving force for elimination is much less for Os than for Ir. Computational studies of model complexes were conducted, comparing (Pybox)Os(L)(CH 3 )(H) with the isoelectronic (Phebox)Ir(L)(CH 3 )(H). The results indicate that the more facile kinetics with Os relative to Ir may be general for C−H elimination from six-coordinate d 6 complexes of the two metals, as well as for the microscopic reverse, i.e., C−H addition to the corresponding four-coordinate d 8 species.

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

The Application of Pulse Radiolysis to the Study of Ni(I) Intermediates in Ni-Catalyzed Cross-Coupling Reactions

Here we report the use of pulse radiolysis and spectroelectrochemistry to generate low-valent nickel intermediates relevant to synthetically important Ni-catalyzed cross-coupling reactions and interrogate their reactivities toward comproportionation and oxidative addition processes. Pulse radiolysis provided a direct means to generate singly reduced [(dtbbpy)NiBr], enabling the identification of a rapid Ni(0)/Ni(II) comproportionation process taking place under synthetically relevant electrolysis conditions. Furthermore, this approach also permitted the direct measurement of Ni(I) oxidative addition rates with electronically differentiated aryl iodide electrophiles (k OA = 1.3 × 10 4 –2.4 × 10 5 M –1 s –1 ), an elementary organometallic step often proposed in nickel-catalyzed cross-coupling reactions. Together, these results hold implications for a number of Ni-catalyzed cross-coupling processes.

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

Evidence that Criegee intermediates drive autoxidation in unsaturated lipids

Significance Autoxidation leads to the slow persistent destruction of organic molecules causing oxidative stress and associated disease. The prevailing autoxidation mechanism of unsaturated lipids involves chain reactions initiated and propagated by H-atom abstraction, mediated by peroxy radicals and other reactive oxygen species. Here we report experimental evidence for an autoxidation mechanism initiated by OH, which is unanticipated and proceeds by OH addition followed by pathways involving Criegee intermediates (CIs). Although CIs are routinely encountered in atmospheric and synthetic organic chemistry, their role in autoxidation is unexpected. Our results suggest that in all likelihood these exotic species play a much more prominent and general role in the chemistry of cells, food, petrochemicals, and the environment beyond what is currently believed.

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

Reactivity and Mechanisms of Methane, Ethane, and Benzene C–H Amination with an Iodine(III) Bistriflimide Complex

The iodine(III) bistriflamide complex (C 6 F 5 ) III I(NTf 2 ) 2 in HNTf 2 (bistriflimide acid; (CF 3 SO 2 ) 2 NH) selectively aminates methane and ethane. At a 100 °C reaction with methane after 3 h, it resulted in the exclusive formation of MeNTf 2 with a yield of approximately 40%. For ethane, at 100 °C in 2 h, a > 80% yield was found for a combination of monofunctionalized and difunctionalized amination products. In contrast to alkanes giving functionalized products, the reaction of (C 6 F 5 ) III I(NTf 2 ) 2 with benzene resulted in being stalled at (C 6 F 5 )I III (Ph)(NTf 2 ) with a <5% conversion to PhNTf 2 even at 120 °C. PWPB95-D3(BJ) density functional theory calculations indicate that C–H activation is the lowest energy pathway to break the hydrocarbon bonds and is lower in energy than radical, electron-transfer, proton-coupled electron-transfer, or hydride substitution pathways. For methane, from the (C 6 F 5 )I III (CH 3 )(NTf 2 ) intermediate, there is a one-step amine functionalization mechanism that avoids a carbocation intermediate. For ethane, from the (C 6 F 5 )I III (Et)(NTf 2 ) intermediate, dynamics simulations suggest the possibility of a competitive two-step functionalization pathway that involves a short-lived carbocation intermediate. As a result, a reaction coordinate strain analysis provides a straightforward model for understanding the relative reactivity rates for alkyl versus aryl functionalization.

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

Cost-effective valorization of 2,3-butanediol to high-value chemicals and jet fuel

Here, this work outlines an optimized process for converting 2,3-butanediol (BDO) into sustainable aviation fuel (SAF) and C4 chemicals. BDO is reactively separated from fermentation broth by forming dioxolanes, which are converted to isobutyraldehyde, methyl ethyl ketone (MEK), and 1,3-butadiene. These intermediates are reduced and dehydrated over Cu/ZSM-5 to form alkenes, which can be oligomerized and hydrotreated to jet-range alkanes. Previous BDO-dioxolane-alkene processes are limited by the requirement for a continuous aldehyde source for dioxolane formation. Brønsted acidic zeolites catalyze dioxolane deacetalization to form isobutyraldehyde and MEK in a >2:1 molar ratio, providing an internal, recyclable aldehyde source. Dioxolane formation optimization was performed to achieve >95% dioxolane yields over Amberlyst-15 and minimize isobutyraldehyde recycle. The overall BDO-dioxolane-fuel process yields an alkane mixture that enables at least a 50% v/v blend with Jet-A. Techno-economic analyses and life cycle assessments for this BDO-dioxolane-fuel process yield scenarios with <$2.50 per gallon gas equivalent and >58% reduction in CO2 emissions.

2,3-butanediol↗
DOE OSTI2025

Electrochemical Corrosion and Catalysis Dynamics of Tin Oxide during Water Oxidation

Metal oxide corrosion severely limits anodic electrocatalysis, particularly at high potentials in acidic environments, where degradation pathways remain poorly defined. This study establishes explicit connections between corrosion and electrocatalysis on tin oxide during water oxidation by examining the roles of lattice defects, reactive oxygen species, interfacial pH variations, and speciation of corroded tin in acid. We first demonstrate the presence of structural defects such as oxygen vacancies and substoichiometric Sn(II) species by integrating electron paramagnetic resonance spectroscopy, ultraviolet photoelectron spectroscopy, and Mott–Schottky analysis. Kohn–Sham density functional theory calculations reveal that explicit water structures thermodynamically stabilize reaction intermediates and lower reaction overpotentials. Moreover, we propose that water dissociation leads to hydrogen-bonding networks formed by H* and OH* intermediates, which may span the entire catalyst surface and decrease the interfacial pH to drive corrosion. In contrast, the electrochemical generation of reactive oxygen species is shown to play a minor role in catalyst corrosion during water oxidation using inductively coupled plasma mass spectrometry coupled with selective chemical scavengers. Square-wave voltammetry combined with rotating ring-disk electrodes is used to reveal that under open-circuit conditions, only Sn(IV) cations chemically dissolve from tin oxide, while both Sn(IV) and Sn(II) species electrochemically corrode during water oxidation. Our results unveil a dynamic and complicated interplay between corrosive and catalytic pathways on metal oxide electrocatalysts: a decrease in interfacial pH due to water oxidation exacerbates Sn(II)/Sn(IV) corrosion. Subsequently, the electrochemical corrosion of Sn(II)/Sn(IV) facilitates product formation from lattice oxygen, while the redeposition of corroded Sn(II) as Sn(IV) can enable oxygen exchange with water. By elucidating the roles of defects and interfacial chemistry, this work provides a roadmap for engineering improved electrocatalysts that balance activity and stability, a critical step toward scalable and durable energy technologies.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Hydrogen migration at restructuring palladium–silver oxide boundaries dramatically enhances reduction rate of silver oxide

Heterogeneous catalysts are complex materials with multiple interfaces. A critical proposition in exploiting bifunctionality in alloy catalysts is to achieve surface migration across interfaces separating functionally dissimilar regions. Herein, we demonstrate the enhancement of more than 10 4 in the rate of molecular hydrogen reduction of a silver surface oxide in the presence of palladium oxide compared to pure silver oxide resulting from the transfer of atomic hydrogen from palladium oxide islands onto the surrounding surface formed from oxidation of a palladium-silver alloy. The palladium-silver interface also dynamically restructures during reduction, resulting in silver-palladium intermixing. This study clearly demonstrates the migration of reaction intermediates and catalyst material across surface interfacial boundaries in alloys with a significant effect on surface reactivity, having broad implications for the catalytic function of bimetallic materials.

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

The synthesis and characterization of an iron(VII) nitrido complex

Abstract Complexes of iron in high oxidation states are captivating research subjects due to their pivotal role as active intermediates in numerous catalytic processes. Structural and spectroscopic studies of well-defined model complexes often provide evidence of these intermediates. In addition to the fundamental molecular and electronic structure insights gained by these complexes, their reactivity also affects our understanding of catalytic reaction mechanisms for small molecule and bond-activation chemistry. Here, we report the synthesis, structural and spectroscopic characterization of a stable, octahedral Fe(VI) nitrido complex and an authenticated, unique Fe(VII) species, prepared by one-electron oxidation. The super-oxidized Fe(VII) nitride rearranges to an Fe(V) imide through an intramolecular amination mechanism and ligand exchange, which is characterized spectroscopically and computationally. This enables combined reactivity and stability studies on a single molecular system of a rare high-valent complex redox pair. Quantum chemical calculations complement the spectroscopic parameters and provide evidence for a diamagnetic ( S = 0) d 2 Fe(VI) and a genuine S = 1/2, d 1 Fe(VII) configuration of these super-oxidized nitrido complexes.

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

Manganese Catalysts with Tetradentate N-donor Pyridine-Appended Bipiperidine Ligands for Olefin Epoxidation Reactions: Ligand Electronic Effect and Mechanism

In this work, we described an electron-rich manganese mesoPYBP catalyst, Mn-SR-mesoPYBP(ClO4)2, by introducing electron-donating substituents on the mesoPYBP ligand. We optimized the catalytic performance in olefin epoxidation with H2O2 in the presence of acetic acid. The electron paramagnetic resonance (EPR) and cyclic voltammetry (CV) studies supported that an electronic effect could stabilize the high-valent intermediates in the catalytic cycles of the catalyst, which largely improved the catalytic performance and the reactivity of olefin epoxidation.

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

Insights into Acetic Acid Binding and Ketene Formation on Anatase TiO 2 (101)

Understanding the adsorption and reactivity of carboxylic acids on oxide surfaces is of great interest in catalysis for biomass upgrading via ketonization, a carbon–carbon coupling reaction. Herein, we investigate the adsorption and reaction of acetic acid on anatase TiO 2 (101) using scanning tunneling microscopy, infrared spectroscopy, temperature programmed reaction, and density functional theory calculations. We demonstrate the adsorption of acetic acid can form two intermediates: (1) dissociated, bidentate acetate with an associated bridging hydroxyl, and (2) molecular, monodentate acetic acid. The coexistence of ordered phases with increasing monolayer (ML) saturation coverages consisting of (1) pure acetate (0.5 ML), (2) mixed acetate/acetic acid (0.67 ML), (3) mixed acetate/acetic acid (1.0 ML) and (4) pure acetic acid demonstrates similar energetics for both acetate and acetic acid species. Under ultra-high vacuum conditions, the presence of both monodentate acetic acid and bidentate acetate was observed below room temperature, while solely bidentate acetate was observed up to 575 K. The deprotonation of acetic acid produces water at 280 K, while the thermal decomposition of bidentate acetate produces ketene and acetic acid at 645 K. In conclusion, this model study provides detailed insight into the stability and reactivity of carboxylic acid surface-bound intermediates, which could participate during ketonization reactions for biomass upgrading.

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

A general strategy and a consolidated mechanism for low-methane hydrogenolysis of polyethylene over ruthenium

Polyethylene (PE) is one of the most environment-threatening plastic waste. Its Ru-catalyzed hydrogenolysis is rapid but produces too much methane. Here, in this study, low-density PE hydrogenolysis is performed at mild conditions and short times over Ru-doped zirconia catalysts (Ru-XZr, X = Ti, Nb, Ce, W, V, Mo, Fe) to determine low-methane catalysts. Methane is produced via direct terminal C-C scission and surface cascade of consecutive C-C scissions, with the latter sensitive to hydrogen availability and dominant in hydrogen-lean conditions. Reactivity studies, characterization and theory reveal that the most effective dopant oxides (W, V, and Mo) are intermediately reducible, as they store and supply extra hydrogen to Ru via reverse hydrogen spillover. This hydrogen readily hydrogenates and desorbs long alkyl surface intermediates that would otherwise produce cascade methane. Our proposed mechanism reconciles the low methane production achieved by increasing the hydrogen pressure or reducing the particle size and exposes the key selectivity descriptors.

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

Identification of the Electronic and Structural Dynamics of Catalytic Centers in Single-Fe-Atom Material

The lack of model single-atom catalysts (SACs) and atomic-resolution operando spectroscopic techniques greatly limits our comprehension of the nature of catalysis. Herein, based on the designed model single-Fe-atom catalysts with well-controlled microenvironments, we have explored the exact structure of catalytic centers and provided insights into a spin-crossover-involved mechanism for oxygen reduction reaction (ORR) using operando Raman, X-ray absorption spectroscopies, and the developed operando 57 Fe Mössbauer spectroscopy. In combination with theoretical studies, the N-FeN 4 C 10 moiety is evidenced as a more active site for ORR. Moreover, the potential-relevant dynamic cycles of both geometric structure and electronic configuration of reactive single-Fe-atom moieties are evidenced via capturing the peroxido (*O 2 -) and hydroxyl (*OH-) intermediates under in situ ORR conditions. Overall, we anticipate that the integration of operando techniques and SACs in this work shall shed some light on the electronic-level insight into the catalytic centers and underlying reaction mechanism.

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

Chemically Separable Ruthenium Hydride Isomers with Distinct Hydride Transfer Properties

Transition metal hydrides are key intermediates in biological and industrial catalysis, where control over hydride donor ability (hydricity) is essential for optimizing reactivity. Herein, we report a series of isomeric ruthenium hydrides in which the ligand trans to the ruthenium hydride bond–either an N-heterocyclic carbene or pyridine–modulates the thermodynamic hydricity by up to 8 kcal mol –1 . These hydrides exhibited distinct reactivity toward CO 2 and various organic hydride acceptors, enabling detailed kinetic and mechanistic analysis. Stopped-flow experiments, isotopic labeling, and computational studies supported an outer-sphere hydride transfer mechanism. Linear free energy relationships (LFERs) and Marcus relationships were utilized to correlate the changes in thermodynamics and kinetics, revealing that ligands with strong trans effects and trans influence can disrupt conventional scaling relationships, which can be a powerful strategy for promoting new reactivity paradigms in hydride transfer reactions.

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

The role of manganese in CoMnO x catalysts for selective long-chain hydrocarbon production via Fischer-Tropsch synthesis

Cobalt is an efficient catalyst for Fischer–Tropsch synthesis (FTS) of hydrocarbons from syngas (CO + H 2 ) with enhanced selectivity for long-chain hydrocarbons when promoted by Manganese. However, the molecular scale origin of the enhancement remains unclear. Here we present an experimental and theoretical study using model catalysts consisting of crystalline CoMnO x nanoparticles and thin films, where Co and Mn are mixed at the sub-nm scale. Employing TEM and in-situ X-ray spectroscopies (XRD, APXPS, and XAS), we determine the catalyst’s atomic structure, chemical state, reactive species, and their evolution under FTS conditions. We show the concentration of CH x , the key intermediates, increases rapidly on CoMnO x , while no increase occurs without Mn. DFT simulations reveal that basic O sites in CoMnO x bind hydrogen atoms resulting from H 2 dissociation on Co 0 sites, making them less available to react with CH x intermediates, thus hindering chain termination reactions, which promotes the formation of long-chain hydrocarbons.

36 MATERIALS SCIENCE↗
DOE OSTI2024

Covalency-Driven Differences in the Hydrogenation Chemistry of Lanthanide- and Actinide-Based Frustrated Lewis Pairs

The electronic organization of Frustrated Lewis Pairs (FLPs) allows them to activate strong bonds in mechanisms that are usually free of redox events at the Lewis acidic site. The unique 6d/5f manifold of uranium could serve as an interesting FLP acceptor site, but to date FLP-like catalysis with actinide ions is unknown. In this paper, the catalytic, FLP-like hydrogenation reactivity of trivalent uranium complexes is explored in the presence of base-stabilized silylenes. Comparison to isoelectronic, isostructural lanthanide and thorium complexes lends insight into the electronic factors governing dihydrogen activation. Mechanistic studies of the uranium- and lanthanide-catalyzed hydrogenations are presented, including discussion of likely intermediates. Computational modeling of the f-element complexes, combined with experimental comparison to p-block Lewis acids, elucidates the relevance of steric hindrance to productive reactivity with dihydrogen. As a result, consideration of the complete experimental and theoretical evidence provides a clear picture of the electronic and steric factors governing dihydrogen activation by these FLPs.

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

Elucidation of Active Sites in Aldol Condensation of Acetone over Single-Facet Dominant Anatase TiO 2 (101) and (001) Catalysts

Aldol condensations of carbonyl compounds for C–C bond formation are a very important class of reactions in organic synthesis and upgrading of biomass-derived feedstocks. However, the atomic level understanding of reaction mechanisms and structure-activity correlation on widely used transition metal oxide catalysts are limited due to the high degree of struc-tural heterogeneity of catalysts such as commercial TiO 2 powders. Here, we provide a deep understanding of the reaction mechanisms, kinetics, and structure–function relationships for vapor phase acetone aldol condensation through the con-trolled synthesis of two catalysts with high surface areas and clean, dominant facets, coupled with detailed characterization and kinetic studies that are further assisted by density functional theory (DFT) calculations. Temperature-dependent diffuse reflectance infrared Fourier transform spectroscopy showed the existence of abundant acetone bonded to surface hydroxyl groups (acetone-O s H) and acetone bonded to Lewis acid sites (acetone-Ti 5c ) on surface of both {101} and {001} facet dom-inant TiO 2 . Intermolecular C–C coupling of enolate intermediate from acetone-Ti 5c and a vicinal acetone-O s H is a kinetically relevant step, which is consistent with kinetic and isotopic studies and DFT calculations. The {001} facet showed a lower apparent activation energy (or higher reactivity) than the {101} facet. This is likely caused by the weaker Lewis acid and Brønsted base strengths of the {001} facet which favors the reprotonation–desorption of the coupled intermediate, making the C–C coupling step more exothermic on the {001} facet and resulting in an earlier transition state with a lower activation barrier. It is also possible that the {001} facet has a smoother surface configuration and less steric hindrance during intermo-lecular C–C bond formation than the {101} facet.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗