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Results for “multi-electron redox”

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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25 records · Page 2
All sourcesNASA NTRSDOE OSTI
DOE OSTI2021

Enzymes and Models. In: XAFS Techniques for Catalysts, Nanomaterials, and Surfaces

Many important redox-active metalloenzymes consist of 3d transition metals in their active sites. They catalyze multi-electron reactions in aqueous solution, under ambient temperature and pressure. To understand the functionality of the catalytic sites of these enzymes, X-ray spectroscopy methods provide important information of the electronic structure of the metal of interest and its local structural information, as well as their changes during the catalysis. Studying model compounds, that mimic enzyme catalytic site structures or their functional mimics, is as critical as studying enzymes themselves, in order to interpret X-ray spectroscopic data. In this chapter, various X-ray spectroscopic methods that are useful for structural and electronic structural studies of enzymes and model compounds are described.

Yachandra, VK↗
DOE OSTI2020

Defining Intermediates of Nitrogenase MoFe Protein during N 2 Reduction under Photochemical Electron Delivery from CdS Quantum Dots

Coupling the nitrogenase MoFe protein to light-harvesting semiconductor nanomaterials replaces the natural electron transfer complex with Fe protein and ATP and provides low potential photoexcited electrons for photocatalytic N 2 reduction. A central question is how direct photochemical electron delivery from nanocrystals to MoFe protein is able to support the multi-electron ammonia production reaction. In this study, low photon flux conditions were used to identify the initial reaction intermediates of CdS quantum dot (QD):MoFe protein nitrogenase complexes under photochemical activation. Illumination of CdS QD:MoFe protein complexes led to redox changes in the MoFe protein active site FeMo-co observed as the gradual decline in E0 resting state intensity that was accompanied by an increase in intensity of a new “g eff = 4.5” EPR signal. The magnetic properties of the g eff = 4.5 signal support assignment as a reduced S = 3/2 state, and reaction modeling was used to define it as a two-electron reduced “E 2 ” intermediate. Use of a MoFe protein variant, ß-188 Cys , which poises the P cluster in the oxidized P + state, demonstrated the P cluster can function as a site of photoexcited electron delivery from CdS to MoFe protein. Overall, the results establish the initial steps for how photoexcited CdS delivers electrons into MoFe protein during reduction of N 2 to ammonia, and the role of electron flux in the photochemical reaction cycle.

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

EFRC Accomplishments 2014-2020: NorthEast Center for Chemical Energy Storage (NECCES)

The mission of NECCES is to develop an understanding of how key electrode reactions occur in cathode materials for Li based batteries, and how they can be controlled to improve electrochemical performance, from the atomistic level to the macroscopic level through the life-time of the operating battery. This mission will allow the determination of the ultimate limits of intercalation reactions for chemical energy storage. The approach and specific research goals of NECCES are to: 1. Attain reversible multi-electron transfer in a cathode material using lithium, using the model compound ε -VOPO 4 ; 2. Close the gap between the theoretical and practical energy density for intercalation compounds, using the model compound LiNi 0.85 Co 0 .10Al 0.05 O 2 ; 3. Understand the transition from cation redox to anion redox electrochemistry; 4. Understand performance limiting transport in positive electrode structures from the local through the meso to the macroscale.

25 ENERGY STORAGE↗
DOE OSTI2024

Formal Oxidation States and Coordination Environments in the Catalytic Reduction of CO to Methanol

Fundamental insight into multi-electron, multi-proton redox reactions with organometallic catalysts is greatly facilitated by knowledge of the formal oxidation state of the metal center in each of the elementary reaction steps that comprise the catalytic cycle. X-ray absorption near edge structure (XANES) is utilized herein to quantify the oxidation states and coordination environment of the organometallic resting state and intermediates in a newly proposed catalytic reduction of carbon monoxide to methanol.

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

Oxygen Atom Transfer Reactions of Colloidal Metal Oxide Nanoparticles

Redox transformations at metal oxide (MO x )/solution interfaces are broadly important, and oxygen atom transfer (OAT) is one of the simplest and most fundamental examples of such reactivity. OAT is a two-electron transfer process, well-known in gas/solid reactions and catalysis. However, OAT is rarely directly observed at oxide/water interfaces, whose redox reactions are typically proposed to occur in one-electron steps. Reported here are stoichiometric OAT reactions of organic molecules with aqueous colloidal titanium dioxide and iridium oxide nanoparticles (TiO 2 and IrO x NPs). Me 2 SO (DMSO) oxidizes reduced TiO 2 NPs with the formation of Me 2 S, and IrO x NPs transfer O atoms to a water-soluble phosphine and a thioether. The reaction stoichiometries were established and the chemical mechanisms were probed using typical solution spectroscopic techniques, exploiting the high surface areas and transparency of the colloids. Furthermore, these OAT reactions, including a catalytic example, utilize the ability of the individual NPs to accumulate many electrons and/or holes. Observing OAT reactions of two different materials, in opposite directions, is a step toward harnessing oxide nanoparticles for valuable multi-electron and multi-hole transformations.

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

Multi-Electron Transfer at H-Terminated p-Si Electrolyte Interfaces: Large Photovoltages under Inversion Conditions

Here, photovoltages for hydrogen terminated p-Si(111) in an acetonitrile electrolyte were quantified with methyl viologen [1,1'-(CH 3 ) 2 -4,4'-bipyridinium](PF 6 ) 2 , abbreviated MV 2+ , and [Ru(bpy) 3 ](PF 6 ) 2 , where bpy is 2,2'-bipyridine, that respectively undergo two and three one-electron transfer reductions. The reduction potentials, E°, of the two MV 2+ reductions occurred at energies within the forbidden bandgap, while the three [Ru(bpy) 3 ] 2+ reductions occurred within the continuum of conduction band states. Bandgap illumination resulted in reduction that were more positive than that measured with a degenerately doped n + -Si demonstrative of a photovoltage, Vph, that increased in the order: MV 2+/+ (260 mV) < MV +/0 (400 mV) < Ru 2+/+ (530 mV) ~ Ru +/0 (540 mV) ~ Ru 0/– (550 mV). Pulsed 532 nm excitation generated electron-hole pairs whose dynamics were nearly constant under depletion conditions and increased markedly as the potential was raised or lowered. A long wavelength absorption feature assigned to conduction band electrons provided additional evidence for the presence of an inversion layer. Collectively, the data reveal that the most optimal photovoltage, as well at the longest electron-hole pair lifetime and the highest surface electron concentration, occur when E° lies energetically within the unfilled conduction band states where an inversion layer is present. The bell-shaped dependence for electron-hole pair recombination with the surface potential was predicted by the time-honored SRH model providing a clear indication that this interface provides access to all four bias conditions, i.e., accumulation, flat band, depletion, and inversion. The implications of these findings for photocatalysis applications and solar energy conversion are discussed.

14 SOLAR ENERGY↗
DOE OSTI2020

Analysis of multi-electron, multi-step homogeneous catalysis by rotating disc electrode voltammetry: theory, application, and obstacles

Rotating disc electrode (RDE) voltammetry has been widely adopted for the study of heterogenized molecular electrocatalysts for multi-step fuel-forming reactions but this tool has never been comprehensively applied to their homogeneous analogues. Here, the utility and limitations of RDE techniques for mechanistic and kinetic analysis of homogeneous molecular catalysts that mediate multi-electron, multi-substrate redox transformations are explored. Using the ECEC' reaction mechanism as a case study, two theoretical models are derived based on the Nernst diffusion layer model and the Hale transformation. Current–potential curves generated by these computational strategies are compared under a variety of limiting conditions to identify conditions under which the more minimalist Nernst Diffusion Layer approach can be applied. Based on this theoretical treatment, strategies for extracting kinetic information from the plateau current and the foot of the catalytic wave are derived. RDEV is applied to a cobaloxime hydrogen evolution reaction (HER) catalyst under non-aqueous conditions in order to experimentally validate this theoretical framework and explore the feasibility of RDE as a tool for studying homogeneous catalysts. Importantly, analysis of the foot-of-the-wave via this theoretical framework provides rate constants for elementary reaction steps that agree with those extracted from stationary voltammetric methods, supporting the application of RDE to study homogeneous fuel-forming catalysts. Lastly, obstacles encountered during the kinetic analysis of cobaloxime, along with the voltammetric signatures used to diagnose this reactivity, are discussed with the goal of guiding groups working to improve RDE set-ups and help researchers avoid misinterpretation of RDE data.

08 HYDROGEN↗