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DOE OSTI2021

Direct Observation of Tungsten Oxidation Studied by In Situ Environmental TEM

In this study, in situ environmental transmission electron microscopy (ETEM) was applied, for the first time, to investigate the thermal oxidation of a pristine and a self-ion irradiated polycrystalline tungsten, using a MEMS-based gas cell at 500 oC to 900 oC in a 1 bar 2%O2/N2 gas mixture. By tracking the dynamic evolution of the tungsten oxide scale as it initiates, grows, and sublimates during a consecutive thermal oxidation experiment, we observed two distinctive tungsten oxide microstructure – one nanocrystalline o-WO3 scale grown on W{27-1 } and W{1-1 8} and another o-WO3 scale exhibits a novel highly textured nanostructure. While the two oxide microstructures shared a similar thickness of ~200 nm after 40-minute early-stage oxidation, the nanocrystalline scale on the W{27-1 } grew much more rapidly at 800 oC than the highly textured oxide at a higher temperature of 900 oC. This suggests both the microstructure of these tungsten oxide scales as well as their oxidation kinetics are highly sensitive to the tungsten surface orientation, and such correlations also change dynamically in the course of oxidation. We also discuss the tungsten oxidation mechanism, the effects of the TEM foil thickness, and the focused ion beam (FIB) Ga+ damage on in situ ETEM oxidation.

In situ ETEM, Tungsten, Thermal oxidation, Orienta↗

DOE OSTI2018

Mn(III)-Iodosylarene Porphyrins as an Active Oxidant in Oxidation Reactions: Synthesis, Characterization, and Reactivity Studies

Mn(III)-iodosylarene porphyrin adducts, [Mn(III)(ArIO)(Porp)] + , were synthesized by reacting electron-deficient Mn(III) porphyrin complexes with iodosylarene (ArIO) at –60 °C and characterized using various spectroscopic methods. The [Mn(III)(ArIO)(Porp)] + species were then investigated in the epoxidation of olefins under stoichiometric conditions. In the epoxidation of olefins by the Mn(III)-iodosylarene porphyrin species, epoxide was formed as the sole product with high chemoselectivities and stereoselectivities. For example, cyclohexene oxide was formed exclusively with trace amounts of allylic oxidation products; cis - and trans -stilbenes were oxidized to the corresponding cis- and trans-stilbene oxides, respectively. In the catalytic epoxidation of cyclohexene by an electron-deficient Mn(III) porphyrin complex and s PhIO at low temperature (e.g., –60 °C), the Mn(III)-iodosylarene porphyrin species was evidenced as the active oxidant that effects the olefin epoxidation to give epoxide as the product. However, at high temperature (e.g., 0 °C) or in the case of using an electron-rich manganese(III) porphyrin catalyst, allylic oxidation products, along with cyclohexene oxide, were yielded, indicating that the active oxidant(s) was not the Mn(III)-iodosylarene adduct but probably high-valent Mn-oxo species in the catalytic reactions. We also report the conversion of the Mn(III)-iodosylarene porphyrins to high-valent Mn-oxo porphyrins under various conditions, such as at high temperature, with electron-rich porphyrin ligand, and in the presence of base (OH–). The present study reports the first example of spectroscopically well-characterized Mn(III)-iodosylarene porphyrin species being an active oxidant in the stoichiometric and catalytic oxidation reactions. In conclusion other aspects, such as one oxidant versus multiple oxidants debate, also were discussed.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗

DOE OSTI2020

Interpreting the Operando XANES of Surface-Supported Subnanometer Clusters: When Fluxionality, Oxidation State, and Size Effect Fight.

X-ray absorption near edge structure (XANES) spectroscopy is widely used for operando catalyst characterization. We show that, for highly fluxional supported nanoclusters, the customary extraction of the oxidation state of the metal from the XANES data by fitting to the bulk standards is highly questionable. The XANES signatures as well as the apparent oxidation state for such clusters arise from a complex combination of many factors, and not only from the chemical composition in reaction conditions (e.g., oxygen content in oxidizing atmosphere). The thermally accessible isomerization and population of several structurally distinct cluster forms, cluster-support interaction, and intrinsic size effects all impact the metal oxidation state and XANES signal. We demonstrate this on copper oxide clusters with different compositions, Cu4Ox(x = 2-5) and Cu5Oy (y = 3, 5), deposited on amorphous alumina and ultrananocrystalline diamond, for which we computed the XANES spectra and compare the results to the experiment. We show in addition that fitting the experimental spectrum to calculated spectra of supported clusters can, in contrast, provide good agreement and insight into the spectrum-composition-structure relation. Experimental XANES interpreted using the proposed fitting scheme shows the partial reduction of Cu oxide clusters at rising temperatures, and pinpoints the specific stoichiometries that dominate in the ensemble of cluster states as the temperature changes.

Zandkarimi, Borna↗

DOE OSTI2021

Potential Dependent Mn Oxidation and Its Role in Passivation of Ni 38 Fe 20 Cr 22 Mn 10 Co 10 Multi-Principal Element Alloy Using Multi-Element Resolved Atomic Emission Spectroelectrochemistry

The identity of passivating oxides on multi-principal element alloys is of great interest as their optimization offers the potential for exceptional corrosion resistance in aqueous solutions over a broad range of potential and pH. This study focuses on a non-equiatomic Ni 38 Fe 20 Cr 22 Mn 10 Co 10 solid solution alloy and tracks the fate of each alloying element during linear sweep voltammetry, low and intermediate potential holds in the passive potential domain as well as during open circuit relaxation after anodic polarization in slightly acidified Cl − solution. Ni dissolves at all potentials investigated in this work, Fe and Co are incorporated into oxides or hydroxides in low concentrations whilst Cr and Mn are enriched at passive potentials. At low passivating potentials, Mn(II) dissolves and is incorporated in minor amounts in oxides containing large concentrations of Cr(III). Considerable enrichment in Mn(II)-species occurs relative to Cr(III) in the oxide at 0.1 V vs SCE. Electrochemical impedance spectroscopy suggests the presence of layered oxides with marginal passivation at high Mn(II) levels. The formation of these oxides depends on a combination of thermodynamic and kinetic factors as well as the sequence of passivation.

Han, Junsoo (ORCID:0000000192952954)↗

NASA NTRS1986

Grain boundary oxidation and an analysis of the effects of pre-oxidation on subsequent fatigue life

The effects of preoxidation on subsequent fatigue life were studied. Surface oxidation and grain boundary oxidation of a nickel-base superalloy (TAZ-8A) were studied at 600 to 1000 C for 10 to 1000 hours in air. Surface oxides were identified and the kinetics of surface oxidation was discussed. Grain boundary oxide penetration and morphology were studied. Pancake type grain boundary oxide penetrates deeper and its size is larger, therefore, it is more detrimental to fatigue life than cone-type grain boundary oxide. Oxide penetration depth, a (sub m), is related to oxidation temperature, T, and exposure time, t, by an empirical relation of the Arrhenius type. Effects of T and t on statistical variation of a (sub m) were analyzed according to the Weibull distribution function. Once the oxide is cracked, it serves as a fatigue crack nucleus. Statistical variation of the remaining fatigue life, after the formation of an oxide crack of a critical length, is related directly to the statistical variation of grain boundary oxide penetration depth.

Oshida, Y.↗

NASA NTRS1988

Grain boundary oxidation and an analysis of the effects of oxidation on fatigue crack nucleation life

Oshida, Y.↗

DOE OSTI2019

High-Temperature Oxidation of Stellite 12 Hardfacings: Effect of Mo on Characteristics of Oxide Scale

This study aims to understand the oxide scale formation on the surfaces of commercial and Mo (10 wt.%)-modified Stellite 12 hardfacings deposited on 4140 steel. For this purpose, samples of the hardfacings were isothermally oxidized in air at various temperatures in between 500 and 900 °C for 30 h. As a result of changes in the microstructure upon Mo addition, about 50% higher mass gain at all oxidation temperatures was observed. The characteristics of the developed oxide scales were examined by utilizing Raman spectroscopy, scanning electron microscopy and x-ray diffraction analysis. The oxide scale formed over Stellite 12 consisted of CoO-, Co{sub 3}O{sub 4-} and Cr{sub 2}O{sub 3}-type oxides, while additionally CoMoO{sub 4}-type complex oxide was detected on Mo-modified hardfacing, more likely at the alloy–scale interface. On the basis of findings of isothermal oxidation tests, the formation mechanisms of this complex oxide imposing better wear resistance as reported in our previous studies were explained during high-temperature wear testing, i.e., dynamic oxidation conditions.

36 MATERIALS SCIENCE↗

DOE OSTI2020

Promoting the cleavage of C-O bonds at the interface between a metal oxide cluster and a Co(0001) support

As a first step toward the rational design of Co-based catalysts with a higher activity and selectivity, we determine how one can activate a C-O bond at the interface between a metal oxide cluster and a Co(0001) support. The hypothesis here is that the metal ions in metal oxide clusters on a Co(0001) support enhance the adsorption of CO and weaken the C-O bond strength, which can then facilitate the dissociation of the CO reactant. To test this hypothesis, we developed three computational models of Ti4O8/Co(0001), Zr4O8/Co(0001), and Mn8O8/Co(0001). We quantify the CO adsorption behavior at the interface sites between an oxide cluster and the Co(0001) support as well as the corresponding IR spectra. We correlate the computed CO stretch frequencies with their CO adsorption energies, as well as the CO stretch frequency with the C-O bond length, and related these findings to the changes in the chemical bonding in the bound CO. The interface is the most favorable site for CO adsorption. Adsorption results in an increase of the C-O bond length and a decrease in its vibrational frequency. From a chemical bonding analysis, the bond order in CO at this site drops from 3 (in the gas phase) to 1. This decrease in bond order is a necessary precursor stage for CO dissociation. The experimental measurements of the corresponding FTIR spectra support this point. The favorability of CO adsorption at the interface sites is due to an electron transfer from the metal ion in the metal oxide cluster to the O atom in CO. We establish a linear relationship between C-O bond length and CO frequency and this relationship is found to be independent of the support, type of metal oxide cluster, or the adsorption site.

Fischer-Tropsch Synthesis, Heterogeneous Catalysis↗

DOE OSTI2021

Comparative Techno-Economic and Life Cycle Analysis of Water Oxidation and Hydrogen Oxidation at the Anode in a CO 2 Electrolysis to Ethylene System

We compare the economic viability of employing hydrogen oxidation versus water oxidation at the anode of a commercial-scale electrolysis plant that converts CO 2 to ethylene. We vary the electrolyzer capital cost, membrane lifetime, and renewable electricity price to represent a current and future market scenario. We find that anodic hydrogen oxidation with membraneless reactor design can reduce the electrolyzer capital cost by up to 48% and reduce electricity demand by at least 50% with the current underdeveloped electrolyzer market. These capital and operating cost savings could further lead to a lower ethylene production cost from anodic hydrogen oxidation than the anodic water oxidation system with hydrogen supplied at less than $\$6$ kg. In the future scenario with a fully developed electrolyzer market and cheap renewable electricity, we find that the anodic hydrogen oxidation system requires hydrogen cheaper than $0.7/kg to compete with the anodic water oxidation system. Moreover, hydrogen oxidation at the anode enables extremely low cradle-to-gate emission ethylene by utilizing negative emission hydrogen such as biomass gasification with carbon capture and sequestration, ~240% lower than ethylene produced from the wind/solar electricity-driven water oxidation system. Furthermore, this low carbon footprint ethylene can further boost the economic competitiveness for anodic hydrogen oxidation with a future carbon credit market.

54 ENVIRONMENTAL SCIENCES↗

DOE OSTI2018

First-principles calculations of oxidation potentials of electrolytes in lithium–sulfur batteries and their variations with changes in environment

In this work, oxidation potentials of electrolyte molecules in Li–sulfur (Li/S) batteries and their variations in various solvent environments are investigated using first-principles calculations in order to understand oxidative decomposition reactions of electrolytes for cathode passivation. Electrolyte solvents, Li salts, and various additives in Li/S batteries along with some Li-ion battery additives are studied. Oxidation potentials of isolated electrolyte molecules are found to be out of the operating range of typical Li/S batteries. The complexation of electrolyte molecules with Li + , salt anion, salt, S8, and pyrene alters oxidation potentials compared to those of the isolated systems. The salt anion lowers oxidation potentials of electrolyte molecules by at least 4.7% while the complexes with Li + have higher oxidation potentials than the isolated molecules by at least 10.4%. S8 and pyrene, used as model compounds for sulfur and sulfur/ carbon composite cathode materials, also affect oxidation potentials of electrolyte molecules, but their influence is negligible and the oxidation trends differ from those of the Li + and salt anion. Although complexations change the oxidation potentials of electrolyte molecules, they are still higher than the operating voltage range of Li/S batteries, which indicates that oxidation of the studied electrolytes in Li/S batteries is not expected under ambient conditions.

25 ENERGY STORAGE↗

DOE OSTI2025

Effect of CO on alcohol oxidation over commercial oxidation catalysts for control of emissions from lean-burn engines

For commercialization, alcohol-based net-zero carbon fuels in diesel-driven internal combustion engines must meet emissions standards set by the U.S. Environmental Protection Agency. Here, a series of aged commercial oxidation catalysts, Pt diesel oxidation catalyst (DOC), Pd+Pt DOC, and Pt oxidation catalyst (Pt OC), were studied using an automated flow reactor and in situ DRIFTS techniques to investigate the impact of CO on alcohol oxidation. Surface formate and acetate intermediates were formed due to partial oxidation of methanol and ethanol, respectively. Sequential introduction of alcohol and CO experiments using in situ DRIFTS on all three catalysts showed that CO inhibited low temperature methanol oxidation. Mutual inhibition effects between CO and ethanol were observed during co-oxidation. Acetate species shifted CO oxidation to higher temperatures on Pt DOC and Pt OC, whereas CO pre-exposure inhibited acetate formation. CO oxidation occurred at lower temperatures during ethanol and CO co-oxidation on Pd+Pt DOC.

33 ADVANCED PROPULSION SYSTEMS↗

DOE OSTI2023

Plasma-Assisted Chemical-Looping Combustion: Low-Temperature Methane and Ethylene Oxidation with Nickel Oxide

The chemical reaction network of low-temperature plasma-assisted oxidation of methane (CH 4 ) and ethylene (C 2 H 4 ) with nickel oxide (NiO) was investigated in a heated plasma reactor through time-dependent species measurements by electron-ionization molecular beam mass spectrometry (EI-MBMS). Methane (ethylene) oxidation by NiO was explored in temperature ranges from 300-700 °C (300-500 °C) and 300-800 °C (300-600°C) for the plasma and non-plasma conditions, respectively. Significant enhancement of methane oxidation was observed with plasma between 400 and 500 °C, where no oxidation was observed under non-plasma conditions. For the oxidation of methane at higher temperatures three different oxidation stages were observed: I) a period of complete oxidation, II) a period of incomplete CO oxidation, and III) a period of carbon buildup. For the C 2 H 4 experiments, and unlike the CH 4 experiments, the plasma resulted in a significant amount of new intermediate oxygenated species, such as CH 2 O, CH 3 OH, C 2 H 4 O, and C 2 H 6 O. Carbon deposits were observed under both methane and ethylene conditions and verified by X-ray photoelectron spectroscopy (XPS). ReaxFF simulations were performed for the oxidation of CH 4 and C 2 H 4 in a non-plasma environment. The simulated intermediates and products largely agree with the species measured in the experiments, though the predicted intermediate oxygenated species such as CH 2 O and C 2 H 6 O were not observed in experiments under non-plasma conditions. A reaction pathway analysis for CH 4 and C 2 H 4 reacting with NiO was created based on the observed species from the MBMS spectra, along with ReaxFF simulations.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗

DOE OSTI2026

Interfacial Charge Transfer and Substrate-Dependent Oxidation States Drive SMSI Enhancements in Cobalt Oxide Films

Here, we investigated the mechanisms underlying strong metal-support interactions in CO oxidation using model systems where noble metal crystals support reducible, monolayer-thick CoO x films. The effect of the Co oxidation state, film thickness, and substrate identity were studied in varying reaction conditions using ambient pressure X-ray photoelectron spectroscopy. At low O 2 pressures, the same oxide phase forms on both Pt(111) and Au(111) surfaces. But when heated at higher O 2 pressures, the oxide phase depends on the substrate. We found that CoO x /Pt is more active for the CO oxidation reaction than CoO x /Au, even when both surfaces stabilize the same oxide phase. DFT calculations on these and related noble-metal-supported CoO x films reveal an SMSI-induced reactivity enhancement that strongly depends on the oxide film thickness and which is mediated by charge transfer between the metal and oxide. Charge transfer is also found to correlate with the reaction energy and activation barrier for CO oxidation. This effect was found to be greatest for oxide films on Pt, decreasing on other noble metal supports in the order Pt > Pd > Au > Ag, in agreement with the experiments. The role of charge transfer in the activation barriers and reaction energies provides insight into the nature of SMSI-induced catalytic activity, and suggests that the noble metal work function can serve as an indicator for the strength of SMSI effects.

36 MATERIALS SCIENCE↗

DOE OSTI2018

Iron and Arsenic Speciation During As(III) Oxidation by Manganese Oxides in the Presence of Fe(II): Molecular-Level Characterization Using XAFS, Mössbauer, and TEM Analysis

The redox state and speciation of the metalloid arsenic (As) determine its toxicity and mobility. Knowledge of biogeochemical processes influencing the As redox state is therefore important to understand and predict its environmental behavior. Many previous studies examined As(III) oxidation by various Mn-oxides, but little is known concerning environmental influences (e.g., coexisting ions) on the process. As such, in this study we investigated the mechanisms of As(III) oxidation by a poorly crystalline hexagonal birnessite (δ-MnO 2 ) in the presence of dissolved Fe(II) using X-ray absorption spectroscopy, Mössbauer spectroscopy, and transmission electron microscopy (TEM) coupled with energy-dispersive X-ray spectroscopy (EDS). The As K-edge X-ray absorption near edge spectroscopy (XANES) analysis revealed that, at low Fe(II) concentration (100 μM), As(V) was the predominant As species on the solid phase, whereas at higher Fe(II) concentrations (200–1000 μM), both As(III) and As(V) were sorbed on the solid phase. As K-edge extended X-ray absorption fine structure spectroscopy (EXAFS) analysis showed an increasing As–Mn/Fe distance over time, indicating As prefers to bind with the newly formed Fe(III)-(hydr)oxides. Both As(III) and (V) adsorbed on Fe(III)-(hydr)oxides as a bidentate binuclear corner-sharing complex. Both Mössbauer and TEM-EDS investigations demonstrated that oxidized Fe(III) products formed during Fe(II) oxidation by δ-MnO 2 were predominantly ferrihydrite-, goethite-, and ferric arsenate-like compounds. However, Fe EXAFS analysis also suggested the formation of a small amount of lepidocrocite. The Mn K-edge XANES data indicated that As(III) oxidation occurs as a two electron transfer with δ-MnO 2 and that the observed Mn(III) is due to conproportionation of surface-sorbed Mn(II) with Mn(IV) in the δ-MnO 2 structure. This study reveals that the mechanisms of As(III) oxidation by δ-MnO 2 in the presence of Fe(II) are very complex, involving many simultaneous reactions, and the formation of Fe(III)-(hydr)oxides plays a very important role in reducing As mobility.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗

DOE OSTI2018

H 2 Oxidation Electrocatalysis Enabled by Metal-to-Metal Hydrogen Atom Transfer: A Homolytic Approach to a Heterolytic Reaction

Oxidation of H2 is used in the conversion of the chemical energy of the H-H bond into electricity. Electrocatalytic oxidation of H2 typically requires that one metal bind H2, heterolytically cleave the H2, and then undergo two oxidation and two deprotonation steps. We discovered that electrocatalytic oxidation of H2 is catalyzed by a cooperative system using Cp*Cr(CO)3H and [Fe(diphosphine)(CO)3]+. A key step of the proposed mechanism is a rarely observed metal-to-metal hydrogen atom transfer from the Cr-H complex to the Fe, forming a Fe-H bond that is deprotonated and then oxidized electrochemically. The “division of chemical labor” between Cr and Fe features a reaction of the Cr complex with H2, while the Fe complex reacts at the electrode. This cooperative catalysis system does not require that either metal heterolytically cleave H2, and thereby provides a very unusual example of a homolytic reaction being a key step in a molecular electrocatalytic process.

oxidation, electrocatalysis, hydrogen↗

DOE OSTI2021

Oxide evolution on the SiC layer of TRISO particles during extended air oxidation

Tristructural isotropic (TRISO) fuel particles have been primarily developed for high-temperature gas-cooled nuclear reactors and can be subjected to oxidizing environments for extended periods in an off-normal accident scenario. Surrogate TRISO fuel particles were oxidized in air at 1,000 or 1,100 °C for up to 120 h. Here, the oxide scale morphology and thickness were studied via scanning electron microscopy, focused ion beam, and atomic force microscopy. TRISO particles oxidized at 1,100 °C exhibited a highly crystalline oxide scale, which led to significant cracking and irregularly shaped closed porosity, whereas those oxidized at 1,000 °C possessed a primarily amorphous oxide scale, which contained small, rounded internal pores and no larger defects. The observed phenomena deviated from the expected behavior based on models for oxide growth on flat-plate and fiber SiC. The oxidation kinetics of TRISO fuel particles in high-temperature air were investigated without mechanically deforming the surface and were analyzed with respect to oxide morphology.

36 MATERIALS SCIENCE↗

DOE OSTI2024

Sorption and Oxidative Degradation of Small Organic Molecules on Mn-Oxides-Effects of pH and Mineral Structures

Manganese (Mn)-oxides regulate carbon (C) cycling in soils by sorbing and oxidizing organic compounds. The composition of soil organic matter varies widely, and little is known about the reactivity of individual organic compounds with structurally diverse Mn-oxides under various environmentally relevant pH conditions. Here, in this study, we examined the affinity of six organic compounds for three Mn-oxides, comprised of layer (birnessite and hydrous Mn oxide HMO) or tunnel (cryptomelane) structures, at acidic (pH 4), slightly acidic (pH 6), and slightly alkaline (pH 8) conditions. Cryptomelane, with a higher specific surface area and point of zero charge, showed higher reactivity than that of HMO and birnessite. Interestingly, these Mn-oxides, although different in structures, decomposed each organic compound to form the same products. Citrate, pyruvate, ascorbate, and catechol induced reduction and dissolution of Mn-oxides. After the reaction, the average oxidation state of Mn in the solids was much lower at pH 4 than at pH 6 and 8, suggesting more reduction under more acidic conditions. Even when reacting with phthalate and propanol, which only sorbed to Mn-oxides but did not degrade, there was proton-promoted Mn dissolution under acidic conditions. These results suggest the significance of environmental pH and mineral structures in affecting the Mn–organic interactions and provide fundamental insights into a better understanding of the roles of Mn-oxides in regulating soil C cycling.

58 GEOSCIENCES↗

DOE OSTI2021

Photocatalytic Oxidation of Dissolved Mn 2+ by TiO 2 and the Formation of Tunnel Structured Manganese Oxides

The redox reaction of manganese (Mn) is of great environmental, geological, and public health significance, as Mn oxides control the distribution and electron flow of numerous nutrients and contaminants in natural and engineered environments. Current understanding on the oxidation pathways of Mn(II) to Mn(III/IV) mainly focuses on biotic processes due to their much higher oxidation rates than those of abiotic processes. This study demonstrates rapid photocatalytic oxidation of Mn 2+ (aq) under circumneutral conditions catalyzed by naturally abundant semiconducting TiO 2 minerals. Notably, the photocatalytic oxidation rates are comparable to or even higher than those of reported biotic/abiotic processes. In addition, the rapid photocatalytic oxidation leads to the formation of large tunnel structured Mn oxides (todorokite and romanechite) on the surface of TiO 2 . These findings suggest that photocatalytic oxidation of Mn 2+ (aq) by natural semiconducting minerals is likely an important yet previously overlooked pathway for understanding the occurrence of natural Mn oxide coatings on rock surfaces. In addition, considering the increasing input of photoreactive engineered nanoparticles into environmental systems, this work shows the potential impacts of nanoparticles on influencing natural redox cycles.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗