We quantify the effects of intensely applied electric fields on the Fe oxidation mechanism. The specimen are pristine Fe single crystals exposing a variety of surface structures identified by field ion microscopy. These crystals are simultaneously exposed to low pressures of pure oxygen gas, on the order of 10‐7 mbar, while applying intense electric fields on their surface of several tens of volts per nanometer. The local composition of the different surface structures is probed directly and in real time using an Environmental Atom Probe and successfully compared with first principles‐based models. We found that rough Fe{244} and Fe{112} facets are more reactive toward oxygen than compact Fe{024} and Fe{011} facets. Results demonstrate that the influence of an electric field on the oxidation kinetics depends on the timescales that are involved as the system evolves toward equilibrium. The initial oxidation kinetics show that strong increases in electric fields facilitate the formation of an oxide. However, as one approaches equilibrium, high field values mitigate this formation. Ultimately, this study elucidates how high externally applied electric fields can be utilized to dynamically exploit reaction dynamics at the nanoscale towards desired products in a catalytic reaction at mild reaction conditions.

Surface self-diffusion studies on metals under elevated reaction conditions are limited, as it is inherently challenging to unambiguously follow atomic transport across highly-reactive surfaces. Here, quantitative and mechanistic insight into thermally induced atomic transport processes in bcc α-iron at the sub-nanometer level was achieved using isotopic tracer techniques coupled with in situ atom probe tomography (APT) capabilities. Specifically, using a reactor directly connected to the APT, needle-shaped specimens fabricated from epitaxial thin films with an embedded 57Fe tracer layer were annealed in Ar at 500°C and 350°C for 1 hour. Furthermore, the tracer was positioned at various depths in the APT specimen by field evaporation, enabling targeted and simultaneous analysis of lattice and surface diffusion. 57Fe concentration profiles reveal lattice self-diffusion occurs at 500°C on the order of ∼7 – 9 monolayers, while lattice diffusion is not resolvable at 350°C. Considerable surface transport was, however, observed at both conditions, where atomic transport over the specimen surface led to the formation of a thin (≤1 nm), isotopically-intermixed layer at the surface. The observed isotopic redistributions at 500°C were convoluted by additional processes occurring in the subsurface, such as atomic intermixing in correlation with lattice diffusion. However, surface diffusion was determined to be the primary transport process at 350°C and was thereby quantified. Ultimately, these results demonstrate the significance of surface self-diffusion as a short circuit pathway. More broadly, this approach has the potential to provide detailed insight into (self-)diffusion mechanisms across various materials while targeting site-specific reactions under elevated reaction conditions.

Hydrogen embrittlement mechanisms of steels have been studied for several decades. Understanding hydrogen diffusion behavior in steels is crucial towards both developing predictive models for hydrogen embrittlement and identifying mitigation strategies. However, because hydrogen has a low atomic mass, it is extremely challenging to detect by most analytical methods. In recent years, cryogenic-transfer atom probe tomography (APT) of electrochemically-deuterium-charged steels has provided invaluable qualitative analysis of nanoscale deuterium traps such as carbides, dislocations, grain boundaries and interfaces between ferrite and cementite. Independently, cyclic voltammetry (CV) has provided valuable analysis of bulk hydrogen diffusion in steels. In this work, we use a combination of CV and cryogenic-transfer APT for quantitative analysis of deuterium pickup in electrolytically charged pure Fe (ferrite) and a model austenitic Fe18Cr14Ni alloy without any second phase. The high solubility and low diffusivity of hydrogen in austenite versus ferrite and potential influence of oxide layer are highlighted to result in clear observable signatures in CV and cryogenic-transfer APT results. The remaining challenges and pathway for enabling quantitative analysis of hydrogen pick up in steels is also discussed.

Zirconium oxide formed in high-temperature water conditions is highly heterogeneous in nature, with, for instance, the presence of a high density of grain boundaries and nanopores, secondary-phase precipitates, and microchemical segregations. Irradiation exacerbates these heterogeneities with effects such as radiation-induced segregation and precipitate dissolution/amorphization. The transport of species through the oxide is affected by these heterogeneities, resulting in complex transport mechanisms that are still not well understood. In this study, we focused on chemical heterogeneities in the oxide, specifically the oxide/metal (O/M) interface and how alloying elements are redistributed across the interface as it progresses into the substrate. For the first time, in situ atom probe tomography (APT) experiments, in which the APT needle is oxidized prior to analysis, have been performed on unirradiated and 1-dpa proton-irradiated Zr-Nb-Fe model alloys to characterize chemical redistribution as a function of oxidation temperature and time across the O/M interface. Results show that the niobium and iron contents in the oxide are higher than what can be accounted for only with solute capture. This finding suggests that there is a thermodynamic driving force for the niobium and iron solutes to migrate from the metal into the oxide in the unirradiated system. Under irradiation, niobium-rich irradiation-induced nanoclusters form in the metal matrix, and the iron and niobium solutes are more thermodynamically stable relative to the unirradiated system. We found much less niobium and iron in the oxide formed in the irradiated sample, corroborating the finding that the substrate is more thermodynamically stable. This finding has strong implications relative to unirradiated versus irradiated Zr-Nb oxidation kinetics because niobium solute doping in the oxide is known to significantly affect the alloy oxidation rate.

Perovskite structured transition metal oxides are important technological materials for catalysis and solid oxide fuel cell applications. Their functionality often depends on oxygen diffusivity and mobility through complex oxide heterostructures, which can be significantly impacted by structural and chemical modifications, such as doping. Further, when utilized within electrochemical cells, interfacial reactions with other components (e.g., Ni‐ and Cr‐based alloy electrodes and interconnects) can influence the perovskite's reactivity and ion transport, leading to complex dependencies that are difficult to control in real‐world environments. Here, this work uses isotopic tracers and atom probe tomography to directly visualize oxygen diffusion and transport pathways across perovskite and metal‐perovskite heterostructures, that is, (Ni‐Cr coated) Sr‐doped lanthanum ferrite (La 0.5 Sr 0.5 FeO 3 ; LSFO). Annealing in 18 O 2(g) results in elemental and isotopic redistributions through oxygen exchange (OE) in the LSFO while Ni‐Cr undergoes oxidation via multiple mechanisms and transport pathways. Complementary density functional theory calculations at experimental conditions provide rationale for OE reaction mechanisms and reveal a complex interplay of different thermodynamic and kinetic drivers. These results shed light on the fundamental coupling of defects and oxygen transport in an important class of catalytic materials.

Radiation effects in materials often compound and accelerate other detrimental phenomena such as embrittlement, oxidation and creep. However, irradiation can also decrease the oxidation rate, for instance with ZrNb alloys nuclear fuel cladding. In this study, we rationalize this observation on Zr-0.5Nb alloy by introducing a mechanism based on oxide space charge modification, resulting from irradiation enhanced Nb clustering. This mechanism is investigated using a multiscale approach: from the macroscale, to determine post-irradiation oxidation kinetics, to the atomic scale, using in-situ atom probe tomography sample oxidation, to observe elemental solute redistribution across the oxide/metal interface. The mechanism is further supported by high resolution transmission electron microscopy characterization and density functional theory calculations. A point defect model is proposed to account for oxide space charge effects and their changes under irradiation. This integrated, multiscale experimental and modeling approach challenges the current paradigm on irradiation effects and how they can potentially improve materials performance in extreme environments.

Understanding the early stages of interactions between oxygen and material surfaces—especially at very high spatial resolutions—is highly beneficial for fields ranging from materials degradation, corrosion, geological sciences, forensics, and catalysis. The ability of in situ atom probe tomography (APT) is demonstrated to track the diffusion of oxygen and metal ions at nanoscale spatial resolution during the early stages of oxidation of a model Fe–Cr–Ni alloy. Using 18 O isotope tracers in these in situ APT experiments and complementary ex situ multimodal microscopy, spectroscopy, and computational simulations allows to precisely analyze the kinetics of oxidation and determine that outward cation diffusion to oxide/air interface is the primary mechanism for intragranular oxide growth in this alloy at 300 °C. This unique in situ isotopic tracer APT approach and the insights gained can be highly beneficial for studying early stages of gas–surface reactions in a broad array of materials.

Precipitate strengthened high-temperature alloys are currently used in safety-critical applications. Understanding precipitate stability and solute segregation mechanisms at high temperatures is key to designing high-strength alloys. Rapid in-situ approaches, therefore, are pivotal in accelerating the alloy design process. Hereby using the test case of a promising high-temperature Al-Cu-Mn-Zr alloy, we demonstrate the value of in-situ atom probe tomography coupled with in-situ transmission electron microscopy to reveal atomic-scale mechanisms that lead to the emergence of non-equilibrium solute segregation. Mn and Zr segregation at strengthening precipitate(θ’)-matrix interface increases the kinetic barrier for phase transformation thus retaining high-temperature strength.

Electrochemically converting nitrate ions, a widely distributed nitrogen source in industrial wastewater and polluted groundwater, into ammonia represents a sustainable route for both wastewater treatment and ammonia generation. However, it is currently hindered by low catalytic activities, especially under low nitrate concentrations. Here we report a high-performance Ru-dispersed Cu nanowire catalyst that delivers an industrial-relevant nitrate reduction current of 1 A cm–2 while maintaining a high NH3 Faradaic efficiency of 93%. More importantly, this high nitrate-reduction catalytic activity enables over a 99% nitrate conversion into ammonia, from an industrial wastewater level of 2,000 ppm to a drinkable water level <50 ppm, while still maintaining an over 90% Faradaic efficiency. Coupling the nitrate reduction effluent stream with an air stripping process, we successfully obtained high purity solid NH4Cl and liquid NH3 solution products, which suggests a practical approach to convert wastewater nitrate into valuable ammonia products. Density functional theory calculations reveal that the highly dispersed Ru atoms provide active nitrate reduction sites and the surrounding Cu sites can suppress the main side reaction, the hydrogen evolution reaction. Nitrate, a common pollutant in wastewater and groundwater, has been efficiently converted into valuable ammonia products via an electrochemical method using Ru-dispersed Cu nanowire as the catalyst.

Journal Article Dynamic observation of electro-assisted Fe oxidation by Operando Atom Probe Get access Sten V Lambeets, Sten V Lambeets Environmental Molecular Sciences Laboratory–Pacific Northwest National Laboratory, Richland, WA, USAPhysical and Computational Sciences Directorate–Pacific Northwest National Laboratory, Richland, WA, USA Search for other works by this author on: Oxford Academic Google Scholar Naseeha Cardwell, Naseeha Cardwell School of Chemical Engineering and Bioengineering–Washington State University, Pullman, WA, USA Search for other works by this author on: Oxford Academic Google Scholar Isaac Onyango, Isaac Onyango School of Chemical Engineering and Bioengineering–Washington State University, Pullman, WA, USA Search for other works by this author on: Oxford Academic Google Scholar Mark G Wirth, Mark G Wirth Environmental Molecular Sciences Laboratory–Pacific Northwest National Laboratory, Richland, WA, USA Search for other works by this author on: Oxford Academic Google Scholar Janet Teng, Janet Teng Environmental Molecular Sciences Laboratory–Pacific Northwest National Laboratory, Richland, WA, USA Search for other works by this author on: Oxford Academic Google Scholar Graham J Orren, Graham J Orren Environmental Molecular Sciences Laboratory–Pacific Northwest National Laboratory, Richland, WA, USA Search for other works by this author on: Oxford Academic Google Scholar Arun Devaraj, Arun Devaraj School of Chemical Engineering and Bioengineering–Washington State University, Pullman, WA, USA Search for other works by this author on: Oxford Academic Google Scholar Thierry Visart de Bocarme, Thierry Visart de Bocarme Universite Libre de Bruxelles, Brussels, Belgium Search for other works by this author on: Oxford Academic Google Scholar Jean-Sabin McEwen, Jean-Sabin McEwen Environmental Molecular Sciences Laboratory–Pacific Northwest National Laboratory, Richland, WA, USASchool of Chemical Engineering and Bioengineering–Washington State University, Pullman, WA, USA Search for other works by this author on: Oxford Academic Google Scholar Daniel E Perea Daniel E Perea Environmental Molecular Sciences Laboratory–Pacific Northwest National Laboratory, Richland, WA, USA Search for other works by this author on: Oxford Academic Google Scholar Microscopy and Microanalysis, Volume 28, Issue S1, 1 August 2022, Pages 724–725, https://doi.org/10.1017/S143192762200335X Published: 01 August 2022

Compositional partitioning during uranium alloy oxidation was studied via complementary ex situ -in situ atom probe tomography. Nanoscopic volumes of uranium-22 at. % molybdenum were exposed to air at room temperature/atmospheric pressure for 30-60 minutes (ex situ), and 300 degrees C -10(-5) mbar O-2 gas for 2-5 minutes in a chemical reaction chamber attached to an atom probe system (in situ). For all environmental conditions, a hypostoichiometric uranium oxide is formed. Reaction fronts are observed at oxide/metal, oxide/hydride, and outer oxide/environment interfaces. Results reveal Mo redistributes across the oxide/metal interface, with a tendency for enrichment in the outer oxide. The formation of a hydrogen-rich subsurface layer between the oxide and base alloy accompanies oxidation in both air and oxygen gas environments. Carbon and silicon impurity elements also redistribute to the outer oxide, contributing to oxide film composition.(c) 2022 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

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Improved analysis of stress corrosion crack tip microstructure of steels has been facilitated in the last decade by the advent of site-specific sample extraction methods using dual beam focused ion beam scanning electron microscope (FIB-SEM), as well as by developments in analytical transmission electron microscopy (TEM) and atom probe tomography (APT) cracks.[1-5] Given the multiscale nature of stress corrosion cracking (SCC) mechanisms of steels, implementation of such advanced microstructural characterization methods allows examination of the crack tip microstructure at an atomic resolution, facilitating a renaissance in how the atomic-scale mechanisms of SCC are considered. However, such ex situ microstructural characterization studies after SCC testing do not reveal the highly dynamic process occurring at the crack tips. Stress relief and diffusion during unloading and slow cooling after SCC testing and before detailed microstructural characterizations may drive the microstructure away from true in situ conditions. Hence in situ methods can be invaluable for analyzing SCC mechanisms of steels at high resolution.

A multimodal chemical imaging approach has been developed and applied to detail the dynamic, atomic-scale changes associated with oxidation of a zirconium alloy (Zircaloy-4). Scanning transmission electron microscopy, a gas-phase reactor chamber attached to an atom probe tomography instrument, and synchrotron-based X-ray absorption near-edge spectroscopy were employed to reveal morphology, composition, crystal, and electronic structure changes that occur during initial stages of oxidation at 300 °C. Oxidation was carried out in 10 mbar O2 gas for short exposure times of 1 and 5 min. A multilayered oxide film with a cubic ZrO adjacent to the oxide/metal interface, a nanoscopic transition region with a graded composition of ZrO2−x (where 0 < x < 1), and tetragonal ZrO2 in the outermost oxide were formed. Partitioning of the major alloying element (tin) to the oxide/metal interface and heterogeneously within the oxide accompanied the development of the layered oxide. Our work provides a rapid, high-throughput approach for detailed characterisation of initial stages of zirconium alloy oxidation at an accelerated time scale, with implications for several other alloy systems.

Heterogeneous catalysis is one of the pillars of the chemical industry and the "green chemistry" concept.However, the sparse understanding of surface mechanisms at molecular scale impedes development of high-performance catalysts and requires development of new tools and methodologies.Cobalt is notably able to dissociate CO2 into CO(g) and O2(g), allowing further conversion to higher value compounds such as long-chain hydrocarbons [1].Such application leads inevitably to Co surface oxidation.From its pure form (Co(0)) to oxidized ones (CoO and Co3O4) the formation mechanism remains unclear as well as the surface structure evolution and its influence on the chemical reaction.

The interaction of oxygen with a reactive metal is ubiquitous, yet the precise atomic-level mechanisms and pathways leading to the formation of a surface oxide are not well-understood. We report oxygen atom distributions inside Rh single nanoparticles using atom probe microscopy (APM) and demonstrate that mainly facets of the ⟨022̅⟩ crystallographic directions act as oxygen-permeable gateways. The highly anisotropic spatial distribution of incorporated oxygen atoms is in agreement with video-field emission analyses according to which {113} facets of the ⟨022̅⟩ zones act as portals for subsurface diffusion. In addition to providing a more fundamental understanding of the precursor states to metal corrosion, in particular for the case of nanosized metal particles, our studies are also relevant for heterogeneous catalysis where catalytic activity and selectivity conform to reaction-induced structural changes of metal nanoparticles.