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At least 163 records · Page 9
All sourcesNASA NTRSDOE OSTI
DOE OSTI2021

Effective Biomass Fractionation through Oxygen-Enhanced Alkaline–Oxidative Pretreatment

The high recalcitrance of plant cell walls is an obstacle for effective chemical or biological conversion into renewable chemicals and transportation fuels. Here, we investigated the utilization of both oxygen (O 2 ) and hydrogen peroxide (H 2 O 2 ) as co-oxidants during alkaline-oxidative pretreatment to improve biomass fractionation and increase enzymatic digestibility. The oxidative pretreatment of hybrid poplar was studied over a variety of conditions. Employing O 2 in addition to H 2 O 2 as a co-oxidant during the two-stage alkaline pre-extraction/copper-catalyzed alkaline hydrogen peroxide (Cu-AHP) pretreatment process resulted in a substantial improvement in delignification relative to using H 2 O 2 alone during the second-stage Cu-AHP pretreatment, leading to high overall sugar yields even at H 2 O 2 loadings as low as 2% (w/w of original biomass). The presence of H 2 O 2 , however, was both critical and synergistic. Performing analogous reactions in the absence of H 2 O 2 resulted in approximately 25% less delignification and a 30% decrease in sugar yields. Here, the lignin isolated from this dual oxidant second stage had high aliphatic hydroxyl group content and reactivity to isocyanate, indicating that it is a promising substrate for the production of polyurethanes. To test the suitability of the isolated lignin as a source of aromatic monomers, the lignin was subjected to a sequential Bobbitt’s salt oxidation followed by formic-acid catalyzed depolymerization process. Monomer yields of approximately 17% (w/w) were obtained, and the difference in yields was not significant between lignin isolated from our Cu-AHP process with and without O 2 as a co-oxidant. Thus, the addition of O 2 did not lead to significant lignin crosslinking, a result consistent with the two-dimensional heteronuclear single-quantum coherence (2D HSQC) NMR spectra of the isolated lignin.

09 BIOMASS FUELS↗
DOE OSTI2023

2D-imaging of absolute OH and H 2 O 2 profiles in a He–H 2 O nanosecond pulsed dielectric barrier discharge by photo-fragmentation laser-induced fluorescence

We report pulsed dielectric barrier discharges (DBD) in He–H 2 O and He–H 2 O–O 2 mixtures are studied in near atmospheric conditions using temporally and spatially resolved quantitative 2D imaging of the hydroxyl radical (OH) and hydrogen peroxide (H 2 O 2 ). The primary goal was to detect and quantify the production of these strongly oxidative species in water-laden helium discharges in a DBD jet configuration, which is of interest for biomedical applications such as disinfection of surfaces and treatment of biological samples. Hydroxyl profiles are obtained by laser-induced fluorescence (LIF) measurements using 282 nm laser excitation. Hydrogen peroxide profiles are measured by photo-fragmentation LIF (PF-LIF), which involves photo-dissociating H 2 O 2 into OH with a 212.8 nm laser sheet and detecting the OH fragments by LIF. The H 2 O 2 profiles are calibrated by measuring PF-LIF profiles in a reference mixture of He seeded with a known amount of H 2 O 2 . OH profiles are calibrated by measuring OH-radical decay times and comparing these with predictions from a chemical kinetics model. Two different burst discharge modes with five and ten pulses per burst are studied, both with a burst repetition rate of 50 Hz. In both cases, dynamics of OH and H 2 O 2 distributions in the afterglow of the discharge are investigated. Gas temperatures determined from the OH-LIF spectra indicate that gas heating due to the plasma is insignificant. The addition of 5% O 2 in the He admixture decreases the OH densities and increases the H 2 O 2 densities. The increased coupled energy in the ten-pulse discharge increases OH and H 2 O 2 mole fractions, except for the H 2 O 2 in the He–H 2 O–O 2 mixture which is relatively insensitive to the additional pulses.

hydrogen peroxide↗
DOE OSTI2021

Effective Biomass Fractionation through Oxygen-Enhanced Alkaline-Oxidative Pretreatment

The high recalcitrance of plant cell walls is an obstacle for effective chemical or biological conversion into renewable chemicals and transportation fuels. Here, we investigated the utilization of both oxygen (O 2 ) and hydrogen peroxide (H 2 O 2 ) as co-oxidants during alkalineoxidative pretreatment to improve biomass fractionation and increase enzymatic digestibility. The oxidative pretreatment of hybrid poplar was studied over a variety of conditions. Employing O 2 in addition to H 2 O 2 as a co-oxidant during the two-stage alkaline pre-extraction/coppercatalyzed alkaline hydrogen peroxide (Cu-AHP) pretreatment process resulted in a substantial improvement in delignification relative to using H 2 O 2 alone during the second-stage Cu-AHP pretreatment, leading to high overall sugar yields even at H 2 O 2 loadings as low as 2% (w/w of original biomass). The presence of H 2 O 2 , however, was both critical and synergistic. Performing analogous reactions in the absence of H 2 O 2 resulted in approximately 25% less delignification and a 30% decrease in sugar yields. The lignin isolated from this dual oxidant second stage had high aliphatic hydroxyl group content and reactivity to isocyanate, indicating that it is a promising substrate for the production of polyurethanes. To test the suitability of the isolated lignin as a source of aromatic monomers, the lignin was subjected to a sequential Bobbitt’s salt oxidation followed by formic-acid catalyzed depolymerization process. Monomer yields of approximately 17% (w/w) were obtained, and the difference in yields was not significant between lignin isolated from our Cu-AHP process with and without O 2 as a co-oxidant. Thus, the addition of O 2 did not lead to significant lignin crosslinking, a result consistent with the twodimensional heteronuclear single-quantum coherence (2D HSQC) NMR spectra of the isolated lignin.

09 BIOMASS FUELS↗
DOE OSTI2023

Comparison of δ 13 C analyses of individual foraminifer ( Orbulina universa ) shells by secondary ion mass spectrometry and gas source mass spectrometry

Rationale: The use of secondary ion mass spectrometry (SIMS) to perform micrometer-scale in situ carbon isotope (δ 13 C) analyses of shells of marine microfossils called planktic foraminifers holds promise to explore calcification and ecological processes. The potential of this technique, however, cannot be realized without comparison to traditional whole-shell δ 13 C values measured by gas source mass spectrometry (GSMS). Methods: Paired SIMS and GSMS δ 13 C values measured from final chamber fragments of the same shell of the planktic foraminifer Orbulina universa are compared. The SIMS–GSMS δ 13 C differences (Δ 13 C SIMS-GSMS ) were determined via paired analysis of hydrogen peroxide-cleaned fragments of modern cultured specimens and of fossil specimens from deep-sea sediments that were either untreated, sonicated, and cleaned with hydrogen peroxide or vacuum roasted. After treatment, fragments were analyzed by a CAMECA IMS 1280 SIMS instrument and either a ThermoScientific MAT-253 or a Fisons Optima isotope ratio mass spectrometer (GSMS). Results: Paired analyses of cleaned fragments of cultured specimens (n = 7) yield no SIMS–GSMS δ 13 C difference. However, paired analyses of untreated (n = 18) and cleaned (n = 12) fragments of fossil shells yield average Δ 13 C SIMS-GSMS values of 0.8‰ and 0.6‰ (±0.2‰, 2 SE), respectively, while vacuum roasting of fossil shell fragments (n = 11) removes the SIMS–GSMS δ 13 C difference. Conclusions: The noted Δ 13 C SIMS-GSMS values are most likely due to matrix effects causing sample–standard mismatch for SIMS analyses but may also be a combination of other factors such as SIMS measurement of chemically bound water. The volume of material analyzed via SIMS is ~10 5 times smaller than that analyzed by GSMS; hence, the extent to which these Δ 13 C SIMS-GSMS values represent differences in analyte or instrument factors remains unclear.

58 GEOSCIENCES↗
DOE OSTI2022

Oxidation of Cyclopentene with Manganese Oxides Octahedral Molecular Sieves with Layer and Tunnel Structures

We report manganese oxides type octahedral molecular sieves OL-1, OMS-1, and OMS-2 were synthesized, impregnated with sodium tungstate, and evaluated as catalysts for the selective epoxidation of cyclopentene (CPE) to cyclopentene oxide (CPO) in the sodium bicarbonate/hydrogen peroxide (BAP) system. OL-1 was the best catalyst with a conversion of 78.6 % and a selectivity to CPO of 31.4 %. Impregnation with sodium tungstate affected the structure and activity of OL-1 and OMS-1. Mn 2+ is required for the epoxidation of CPE. FTIR spectra provides evidence that tungstate ions react, during impregnation, with Mg 2+ and/or Mn 2+ present in OMS-1 and OL-1 producing an amorphous compound that is not active in the epoxidation reaction. The crystalline and chemical structure of OMS-2 was not affected by tungstate impregnation, since most of the manganese occurs as Mn 3+ and Mn 4+ , which in presence of hydrogen peroxide, tend to form radicals, active in the total oxidation of organic compounds.

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

Hydroxyl radical‐driven transformations of bisphenol A and 2,4‐dinitroanisole: Experimental and computational analysis

Abstract This study used experimental and computational analysis to investigate the advanced oxidation of bisphenol A (BPA) and 2,4‐dinitroanisole (DNAN). The pseudo first‐order reaction rate constants depended on the molar peroxide ratio and were between 0.13 and 0.28 min −1 for BPA and between 0.018 and 0.032 min −1 for DNAN. The kinetic differences appear to be due in part to the energy requirements for oxidation, which depended on the reaction mechanism but were typically lower for BPA than they were for DNAN. Density functional theory (DFT) was used to develop transformation pathways that included experimentally‐detected byproducts. The most energetically favored pathway for BPA oxidation begins with the formation of hydroxylated derivatives, while for DNAN, the most energetically favorable degradation pathway begins with the substitution of the methoxy group. Overall, these findings demonstrate the power of combining experimental and computational tools to reveal transformation mechanisms during water treatment. Practitioner Points Advanced oxidation transformations for two emerging water pollutants, bisphenol A and dinitroanisole, was investigated. The observed reaction kinetics depended on molar peroxide ratio in a manner that is in keeping with previous findings. Density functional theory‐based analysis revealed reaction energy requirements and degradation pathways.

Engineering↗
DOE OSTI2023

Oxidation rates and redox stabilization of ferrous iron in trioctahedral smectites

Iron(II)-bearing trioctahedral smectites (saponites) form during anoxic alteration of basaltic rock. They are predicted to have been widespread on the early Earth and are observed in the oceanic subsurface today. Smectite structures, including the occupancy of sites in the octahedral sheet, affect iron redox behavior but the rates and products of trioctahedral smectite oxidation have been largely unexplored to date. In this study we synthesized two Fe(II)-bearing trioctahedral smectites, one moderate (22 wt% Fe) and one high (27 wt% Fe) in iron content. We then examined the rate, extent, and products of their oxidation by dissolved oxygen, nitrite, and hydrogen peroxide. Dissolved oxygen caused partial oxidation of Fe(II) in the smectites with 14 to 43% of Fe(II) unoxidized after 20 to 30 days of exposure. The rate and extent of oxidation correlated with the dissolved oxygen concentration and the Fe(II) content of the clay. The incomplete oxidation in these experiments is consistent with the mixed-valent trioctahedral smectites observed in oxidized natural samples but contrasts with the complete reoxidation by oxygen shown by chemically- or microbially-reduced dioctahedral smectites. Oxidation of structural Fe(II) by 5 mmol L -1 nitrite was negligible for the moderate-iron smectite and yielded only ~17% oxidation after 54 days of reaction for the high-iron smectite. Hydrogen peroxide caused rapid and near-complete oxidation of both clays. Powder X-ray diffraction, variable-temperature Mössbauer spectroscopy, and extended X-ray absorption fine structure spectroscopy together detected no crystalline or short-range-ordered secondary phases and show that oxidized iron remained in the trioctahedral smectite structure. The recalcitrant Fe(II) pool in oxidized trioctahedral smectites exists in less distorted sites than Fe(II) in the initial clays. Its unreactive nature at prolonged reaction times indicates an elevated redox potential generated by the local coordination environment. Slower oxidation rates create a larger recalcitrant Fe(II) pool, suggesting kinetic competition between oxidation and a process involved in redox stabilization, such as electron exchange between octahedral iron sites or deprotonation of hydroxyl groups in the structure. The resistance to complete oxidation of trioctahedral ferrous smectites and their full retention of iron demonstrates that transitions from anoxic to oxic conditions generate mixed-valence smectites rather than a mixture of new phases. Identifying the diagenetic products of mixed-valent trioctahedral smectites may provide an indicator in the rock record of past redox cycling. In conclusion, substantial portions of structural Fe(II) in trioctahedral smectites display slow abiotic oxidation kinetics and represent potential electron donors for both microaerophilic iron oxidizing and nitrate-reducing, iron-oxidizing microorganisms in altered mafic rocks and related settings.

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

Reactive iron, not fungal community, drives organic carbon oxidation potential in floodplain soils

Wetlands host ∼20% of terrestrial organic carbon and serve as a major sink for atmospheric carbon. Anoxic soils and sediments accrue soil organic carbon (SOC) partly by hampering the activity of extracellular oxidative enzymes that break down phenolic polymers. Upon aeration, fungal-driven oxidative enzymatic depolymerization and microbial respiration of released monomers ensue. Redox-active metals can simultaneously catalyze abiotic nonspecific oxidation of SOC, notable examples including Mn(III) or Fe(II) through Fenton-like, hydrogen peroxide-catalyzed oxidative radical production. However, the extent of reactive metal contributions to biotic and abiotic SOC degradation is not understood in the context of natural environments with diverse redox chemistry. We tested the relative contributions of fungi, Mn(III) and Fe(II) to phenolic substrate (L-DOPA) oxidation in floodplain soils representing a range of transient redox conditions driven by permanent vs. periodic flooding. Phenol oxidative potential was highest in permanently flooded soils with fewer fungal taxa known for observed (per)oxidase activity and instead correlated with HCl-extractable Fe(II), Fe(total) and Fe(II)/Fe(total), suggesting a specific role of Fe(II). Fe(II) additions enhanced phenol oxidative potential in sterilized and non-sterilized soils in the presence of hydrogen peroxide, indicating abiotic Fe-mediated radical chemistry could significantly enhance wetland SOC oxidative depolymerization throughout redox-active floodplain soils. Fungal community composition did not correlate to phenol oxidative potential overall and only more oxic soils adjacent to the river with diverse fungal communities showed declining oxidative potential after sterilization. Mn(III) addition did not significantly enhance phenol oxidative potential across all soils, although it appeared to drive fungal-mediated oxidative potential in the most aerated floodplain soils. Understanding how metals mediate SOC depolymerization as abiotic oxidants or microbially-harnessed enzyme cofactors and substrates in soils under variable hydrologic controls will improve our ability to represent depolymerization in terrestrial carbon models in wetland and other frequently saturated soils.

59 BASIC BIOLOGICAL SCIENCES↗
DOE OSTI2025

Reactions of Studtite UO 4 ·4H 2 O in Alkali Hydroxides: Isolation of Single-Crystal Uranate Phases under Mild Hydrothermal Conditions

Uranyl peroxide complexes, particularly studtite (UO 4 ·4H 2 O), are important phases within the nuclear fuel cycle, forming through radiolysis-induced reactions on the surfaces of spent fuel and in waste environments. Studtite has been identified in Hanford’s irradiated fuel storage basins, Chornobyl’s corium lavas, and is anticipated on Fukushima’s fuel debris. While its formation and stability have been extensively studied, the reactivity of UO 4 ·4H 2 O in highly alkaline environments such as those encountered in high-level nuclear waste remains underexplored. These environments contain molar concentrations of [OH - ] and present a chemically dynamic and reactive environment where actinide behavior is not well understood. Reported here are investigations of uranium reactions in concentrated alkali hydroxides under mild hydrothermal conditions (<200 °C) that have resulted in the isolation of the alkali metal uranates Li 2 UO 4 , α-Na 2 UO 4 , γ-Na 2 U 2 O 7 , and K 2 U 2 O 7 . In contrast to conventional solid-state methods (>800 °C) that typically yield polycrystalline powders, our approach enables the isolation of these uranates as single crystals, allowing us to provide single-crystal structure solutions of certain uranates for the first time. Our findings match the results of high-level waste tank sampling, confirming that sodium diuranate (Na 2 U 2 O 7 ) is the most persistent uranium phase, potentially forming through reactions of uranyl peroxide intermediates with NaOH under radiolytic and highly alkaline conditions.

38 RADIATION CHEMISTRY, RADIOCHEMISTRY, AND NUCLEA↗
DOE OSTI2021

Anionic Oxygen Redox in the High-Lithium Material Li 8 SnO 6

Lithiated transition metal oxide cathode materials that combine oxygen-anion redox [O 2 – /(O 2 ) n– ] with cation redox offer substantial capacities and higher voltage than cathodes without oxygen redox, but oxygen-anion redox is usually accompanied by the formation of peroxo or superoxo species that may cause oxygen release. To clarify the relationship between the oxygen release and peroxide and superoxide dimers without the complication of transition-metal redox, we performed density functional theory calculations to study the layered Li 8 SnO 6 cathode because it has only oxygen-anion redox during delithiation. Features with O–O distances of ~1.5 Å (corresponding to the formation of a peroxide) were observed at 75% lithium concentration, and further delithiation to 62.5% lithium concentration shortened the O–O bond distance to ~1.3 Å (corresponding to the formation of superoxide). The solely anionic oxygen redox stabilizes the delithiated structure of Li 8–x SnO 6 , and it does not cause structural disorder or release oxygen gas. Ab initio molecular dynamics calculations in the supercell at 300 K indicate that oxygen dimerization may occur at 75% lithium concentration. On the (0001) surface with a high lithium concentration (98%), oxygen dimerization is unfavorable. The calculations by HSE06 show that oxygen redox in Li 8 SnO 6 offers an initial voltage (vs Li/Li + ) greater than 4.2 V. The GGA+U calculations indicate that the Li + -polaron in Li 95 Sn 12 O 72 migrates with a migration barrier of only 0.43 eV. Here, this work also quantifies a solely anionic oxygen redox mechanism in Li 8 SnO 6 , and it provides a deeper understanding of anionic oxygen redox in Li-excess oxide cathode materials.

25 ENERGY STORAGE↗
DOE OSTI2021

Prediction of Solution Behavior via Calorimetric Measurements Allows for Detailed Elucidation of Polyoxometalate Transformation

The solution behavior of a polyoxometalate cluster, LiNa-U 24 Pp 12 (Li 24 Na 24 [(UO 2 O 2 ) 24 (P 2 O 7 ) 12 ]) that consists of 24 uranyl ions, peroxide groups, and 12 pyrophosphate linkers, was successfully predicted based on new thermodynamic results using a calorimetric method recently described for uranyl peroxide nanoclusters (UPCs), molybdenum blues, and molybdenum browns. The breakdown of LiNa-U 24 Pp 12 and formation of U 24 (Li 24 [UO 2 O 2 OH] 24 ) was monitored in situ via Raman spectroscopy using a custom heating apparatus. A combination of analytical techniques confirmed the simultaneous existence of U 24 Pp 12 and U 24 midway through the conversion process and U 24 as the single end product. The application of a molecular weight filter resulted in a complete and successful separation of UPCs from solution and, in conjunction with DOSY results, confirmed the presence of large intermediate cluster building blocks.

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

Structural, Spectroscopic, and Kinetic Insight into the Heating Rate Dependence of Studtite and Metastudtite Dehydration

Uranyl peroxide minerals studtite (UO 2 O 2 ·4H 2 O) and metastudtite (UO 2 O 2 ·2H 2 O) are important materials in the nuclear fuel cycle. When heated, they dehydrate and transform to amorphous uranium oxides (UO x ), yet phase stability and heating rate dependence of these transformations are poorly understood. This information is critical to proper management of fuel cycle materials. In this work, we use in situ powder X-ray diffraction (PXRD), Raman spectroscopy, and thermogravimetric analysis (TGA) to monitor the dehydration of studtite and metastudtite. Strong linear correlation between the heating rate and phase transition temperature is observed. Geometric contraction and diffusion-related kinetic models describe studtite dehydration at slow heating rates, whereas Avrami–Erofeev or reaction order models become more accurate for faster thermal treatments. A second order model describes the transition from metastudtite to UO x regardless of the heating rate. Water retention during studtite dehydration is indicated by PXRD, Raman spectroscopy, and TGA. Here, we observe mixed-phase UO x dehydration products of metastudtite with a likely formation mechanism involving conversion of some uranyl centers from hexagonal to pentagonal bipyramidal coordination units via peroxide liberation. Our observations clarify over 100 years of measurements on these materials and represent an advancement in understanding the chemical behavior of nuclear fuel cycle materials.

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

Combined Cyclohexene Dehydrogenation and Oxygenation over VO x /CeO 2 with O 2 and Vaporized H 2 O 2 as Oxidants

Selective oxidation of hydrocarbons over metal oxide catalysts frequently invokes Mars-van Krevelen (MvK) mechanisms involving lattice oxygens. At typical reaction temperatures, oxidative dehydrogenation reactions predominate and compete with total oxidation; production of more useful oxygenates is challenging. However, because regeneration of lattice oxygens has been previously hypothesized to involve transient formation of surface hydroperoxides or peroxides, MvK cycles should be capable of forming products derived from both lattice oxygen and surface (hydro)peroxide species, under certain conditions. In this report, we study the low temperature reaction of cyclohexene with O 2 over CeO 2 -supported VO x catalysts. Below 150°C and for higher VO x loading, cyclohexene converts to benzene and trans-1,2-cyclohexanediol with no appreciable CO 2 formation. Control experiments show similar behavior in other supported vanadia catalysts. Conversely, using vaporized H 2 O 2 as oxidant exclusively gives 1,2-cyclohexanedione, suggesting that MvK reoxidation cycles do not directly involve H 2 O 2 . Furthermore, these findings contribute to the current understanding of selective oxidation mechanisms, and demonstrate routes to the formation of industrially useful oxygenates with O 2 or vaporized H 2 O 2 over metal oxide catalysts.

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

Spectroscopic Signature of Metal-hydroxo and Peroxo Species in K-edge X-ray Absorption Spectra

Metal-dioxygen species are important intermediates formed during dioxygen activations by metalloenzymes in various biological processes, by catalysts in fuel cells, and prior to O 2 evolution by photosystem II. In this work, we focus on manganese-porphyrin complexes using tetramesitylporphyrin ligand (TMP) to explore changes in Mn K-edge X-ray absorption spectroscopy (XAS) associated with the formation of Mn-hydroxide and Mn–O 2 peroxide species. With limited spectroscopic characterization of these compounds, Mn K β X-ray emission spectroscopy (XES), XAS, density functional theory (DFT), and time-dependent DFT (TD-DFT) analysis will enhance our understanding of their complex electronic structure. We show that the shape of the pre-edge in the K-edge Mn X-ray absorption near-edge structure (XANES) can serve as a spectroscopic signature of the Mn III -peroxo formation and thus can be used to track the presence of the side-on peroxide as an intermediate in time-resolved or in situ experiments. In conclusion, our results will help to further summarize the spectroscopic fingerprints for peroxo and hydroxo species, addressing the challenge of identifying the reactive metal species in catalytic reactions.

Extended X-ray absorption fine structure↗
DOE OSTI2020

Reactivity, Formation, and Solubility of Polyoxometalates Probed by Calorimetry

Room temperature calorimetry methods were developed to describe the energy landscapes of six polyoxometalates (POMs), Li–U 24 , Li–U 28 , K–U 28 , Li/K–U 60 , Mo 132 , and Mo 154 , in terms of three components: enthalpy of dissolution (ΔH diss ), enthalpy of formation of aqueous POMs (ΔH f,(aq) ), and enthalpy of formation of POM crystals (ΔH f,(c) ). ΔH diss is controlled by a combination of cation solvation enthalpy and the favorability of cation interactions with binding sites on the POM. In the case of the four uranyl peroxide POMs studied, clusters with hydroxide bridges have lower ΔH f,(aq) and are more stable than those containing only peroxide bridges. Here, in general for POMs, the combination of calorimetric results and synthetic observations suggest that spherical topologies may be more stable than wheel-like clusters, and ΔH f,(aq) can be accurately estimated using only ΔH f,(c) values owing to the dominance of the clusters in determining the energetics of POM crystals.

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

Scalable neutral H 2 O 2 electrosynthesis by platinum diphosphide nanocrystals by regulating oxygen reduction reaction pathways

Despite progress in small scale electrocatalytic production of hydrogen peroxide (H 2 O 2 ) using a rotating ring-disk electrode, further work is needed to develop a non-toxic, selective, and stable O 2 -to-H 2 O 2 electrocatalyst for realizing continuous on-site production of neutral hydrogen peroxide. We report ultrasmall and monodisperse colloidal PtP 2 nanocrystals that achieve H 2 O 2 production at near zero-overpotential with near unity H 2 O 2 selectivity at 0.27 V vs. RHE. Density functional theory calculations indicate that P promotes hydrogenation of OOH* to H 2 O 2 by weakening the Pt-OOH* bond and suppressing the dissociative OOH* to O* pathway. Atomic layer deposition of Al 2 O 3 prevents NC aggregation and enables application in a polymer electrolyte membrane fuel cell (PEMFC) with a maximum r(H 2 O 2 ) of 2.26 mmol h -1 cm -2 and a current efficiency of 78.8% even at a high current density of 150 mA cm -2 . Catalyst stability enables an accumulated neutral H 2 O 2 concentration in 600 mL of 3.0 wt% (pH = 6.6).

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

Revealing the atomic and electronic mechanism of human manganese superoxide dismutase product inhibition

Human manganese superoxide dismutase (MnSOD) is a crucial oxidoreductase that maintains the vitality of mitochondria by converting superoxide (O 2 •– ) to molecular oxygen (O 2 ) and hydrogen peroxide (H 2 O 2 ) with proton-coupled electron transfers (PCETs). Human MnSOD has evolved to be highly product inhibited to limit the formation of H 2 O 2 , a freely diffusible oxidant and signaling molecule. The product-inhibited complex is thought to be composed of a peroxide (O 2 2– ) or hydroperoxide (HO 2 – ) species bound to Mn ion and formed from an unknown PCET mechanism. PCET mechanisms of proteins are typically not known due to difficulties in detecting the protonation states of specific residues that coincide with the electronic state of the redox center. To shed light on the mechanism, we combine neutron diffraction and X-ray absorption spectroscopy of the product-bound, trivalent, and divalent states of the enzyme to reveal the positions of all the atoms, including hydrogen, and the electronic configuration of the metal ion. The data identifies the product-inhibited complex, and a PCET mechanism of inhibition is constructed.

59 BASIC BIOLOGICAL SCIENCES↗
DOE OSTI2021

A plant host, Nicotiana benthamiana, enables the production and study of fungal lignin-degrading enzymes

Abstract Lignin has significant potential as an abundant and renewable source for commodity chemicals yet remains vastly underutilized. Efforts towards engineering a biochemical route to the valorization of lignin are currently limited by the lack of a suitable heterologous host for the production of lignin-degrading enzymes. Here, we show that expression of fungal genes in Nicotiana benthamiana enables production of members from seven major classes of enzymes associated with lignin degradation (23 of 35 tested) in soluble form for direct use in lignin activity assays. We combinatorially characterized a subset of these enzymes in the context of model lignin dimer oxidation, revealing that fine-tuned coupling of peroxide-generators to peroxidases results in more extensive C-C bond cleavage compared to direct addition of peroxide. Comparison of peroxidase isoform activity revealed that the extent of C-C bond cleavage depends on peroxidase identity, suggesting that peroxidases are individually specialized in the context of lignin oxidation. We anticipate the use of N. benthamiana as a platform to rapidly produce a diverse array of fungal lignin-degrading enzymes will facilitate a better understanding of their concerted role in nature and unlock their potential for lignin valorization, including within the plant host itself.

59 BASIC BIOLOGICAL SCIENCES↗