163 Search Results

Defect accumulation and microstructural evolution during ion irradiation at elevated temperatures are governed by competing processes of defect production, driven by the dose rate, and defect recovery, controlled by diffusion, interaction, and annihilation. Here, this study investigates the effects of irradiation temperature and the dose rate on microstructural evolution, deuterium retention, and lithium volatilization in γ-LiAlO₂ pellets subjected to sequential He⁺ and D⁺ ion irradiation. Experiments were performed to a total fluence of 3 × 10¹⁷ (He⁺ + D⁺)/cm² at 623, 673, 723, and 773 K with an average He⁺ dose rate of 7.7 × 10⁻⁴ dpa/s, and to 2more » × 10¹⁷ (He⁺ + D⁺)/cm² at 773 K with dose rates of 6.8 × 10⁻⁵, 2.9 × 10⁻⁴, and 7.3 × 10⁻⁴ dpa/s. At 623 K, the microstructure was dominated by cavities and fractures with no observable precipitate formation, while small precipitates emerged at 673 K. Increasing the irradiation temperature to 723–773 K promoted the formation of larger, faceted LiAl₅O₈ precipitates, and surface amorphization, accompanied by pronounced lithium depletion and H–D isotopic exchange. At 773 K, medium and high dose rates produced an amorphized surface layer over a crystalline subsurface containing LiAl₅O₈ precipitates and blisters at the crystalline–amorphous interface, whereas low-dose-rate irradiation preserved surface crystallinity with cavities distributed in the matrix, around precipitates, and along grain boundaries. Precipitate morphology was anisotropic with limited size dependence on the dose rate. These results elucidate the coupled effects of temperature and the dose rate and demonstrate that sequential He⁺ and D₂⁺ irradiation at 773 K reproduces key microstructural features and H isotope behavior observed in neutron-irradiated γ-LiAlO₂ at 573 K.« less

Palladium nanoparticles on carbon supports (Pd/C) are effective for catalyzing hydrogen release from aqueous formate solutions but typically suffer from a gradual decrease of activity. This study finds two primary factors influencing activity: (i) the number of available surface Pd sites, and (ii) metal-support interactions which depend on the nature of the C support. We propose that the Pd/C catalyst is structure insensitive and undergoes Ostwald ripening to yield an active ‘conditioned’ catalyst with dispersion plateauing between ca. 15–20 %. Contrary to earlier studies, in-situ XANES experiments show that PdO is not an active catalyst for formate dehydrogenation. Calcination ofmore » Pd/C before dehydrogenation increases the catalytic activity which suggests a beneficial, albeit temporary, modification of the metal support interaction. N-containing supports minimize nanoparticle growth and also increase activity through a metal-support interaction. In conclusion, these findings advance our understanding of catalyst evolution and stability in formate dehydrogenation systems.« less

Anchoring divalent metal ions in the same zeolite framework with similar Si/Al ratio selectively as zeolite-bound M⁺² or [M⁺²-OH]⁺¹ cationic species enables critical comparison of the species’ intrinsic reactivity for industrially and fundamentally relevant reactions. H-BEA zeolites with similar Si/Al ratios but differing framework Al siting were used to anchored multiple divalent metal cations (Ni, Pd, Pt, Cr, Cu) in the zeolite micropores. State-of-the-art infrared (IR) spectroscopy, electron paramagnetic resonance (EPR) measurements, including two-dimensional pulsed HYSCORE EPR, extended X-ray absorption fine structure (EXAFS), and density functional theory (DFT) calculations together provide unambiguous evidence for the selective formation of divalent metalmore » cations as M⁺²/2Al species (for H-BEA prepared in the conventional hydroxide media), and [M⁺²OH]⁺¹/1Al species for H-BEA prepared in HF. Solid-state proton-decoupled triple-quantum magic-angle spinning (3Q MAS) NMR measurements confirmed contrasting Al distributions in the two H-BEA zeolites, which led to a contrasting divalent cation speciation. The reactivities of the two cationic species were explored for catalytic and adsorptive applications in both organometallic homogeneous and heterogeneous catalysis. This work demonstrates their divergent reactivity in ethylene dimerization, ethylene oxidation (Wacker process), selective catalytic reduction (SCR) of NO, NO adsorption, and methane oxidation. Both M⁺²/2Al and [M⁺²OH]⁺¹/1Al cations are both active for ethylene dimerization, but [M⁺²OH]⁺¹/1Al species show higher reaction rates for each Pd, Ni, Pt. [M⁺²OH]⁺¹/1Al is active for acetaldehyde formation in Wacker ethylene oxidation. A new active site for ethylene oligomerization is proposed that possesses a terminal OH group (Cr-OH) in Phillips catalysts evident by a nearly inactive isolated Cr⁺²/2Al species that contrast an active Cr─OH motif.« less

Ductile-phase toughened tungsten (DPT W) composites have emerged as promising candidates for load-bearing components behind the plasma-facing tungsten armor in fusion reactors due to their enhanced thermomechanical properties. This study focuses on a composite consisting of W particles embedded in a ductile NiFeW solution matrix, hot-rolled to a thickness reduction of 87% (87R DPT W). Sequential irradiations with Ni2+ and He+ ions were performed to identical doses and helium concentrations at room temperature (RT) and 1273 K. Irradiation at RT produced no discernible nanostructural features due to the immobility of mono-vacancies, whereas cavity formation was observed at 973 K. At 1273 K, themore » W phase exhibited larger cavities, reduced cavity number density, and lower volumetric swelling compared to 973 K. Notably, nanosized NiFeW precipitates formed within the W phase at 1273 K, a phenomenon absent at 973 K. A new phase of cubic (NiFe)6W6C was also observed at the interphase boundary. In contrast, the NiFeW matrix showed no nanostructural changes at 1273 K, likely due to cavity dissociation. Separate irradiations at 1273 K indicated that Ni2+ ions induced precipitate formation in the W phase, while He+ ions exclusively caused cavity formation. The microstructure of 87R DPT W irradiated at RT and subsequently annealed at 1273 K closely resembled that of material irradiated directly at 1273 K. Like oxide-dispersion-strengthened steels, the observed nanoparticle-embedded W can inhibit dislocation propagation, potentially delaying the ductile-to-brittle transition temperature. These findings highlight the potential of NiFeW nanoparticle-reinforced W composites as irradiation-resistant materials for fusion reactors.« less

The shape of supported metal particles regulates their catalytic reactivity and is determined by the degree of wetting between the metal particle and the support surface. Flattened particles that wet support surfaces were reported in various catalytic systems, particularly in the subnanometer size regime. Such consequential metal–support wetting phenomena are poorly understood, and methods to study them on powder catalysts under realistic conditions are lacking. Here, we investigate the size-dependent wetting behaviors of Ru particles on two reducible-oxide supports, anatase TiO₂ (TiO₂-A) and CeO₂, under reducing catalytic conditions. X-ray absorption spectroscopy (XAS), low-energy ion scattering (LEIS), and density functional theorymore » (DFT) are combined to determine the shape of Ru particles. Ru particles remain three-dimensional without wetting the TiO₂-A support within the coverage range studied (0.06–0.98 Ru nm^–2). In contrast, at low coverages (<0.25 Ru nm^–2), Ru wets the CeO₂ support to form flat, disordered structures. The higher wettability of CeO₂ than TiO₂-A is attributed to oxygen vacancies in the near-surface region. The shape difference between small Ru particles or clusters on the two supports leads to drastically contrasting catalytic reactivities in polyolefin hydrogenolysis, despite similar diameters. As a result, this work highlights the implications of metal–support wetting, or cluster shape, on catalytic behaviors of small metal clusters, while establishing the foundation for future systematic studies of such a phenomenon in realistic systems, by delivering a multitechnique methodology and revealing governing fundamental principles.« less

Mineral–organic matter (OM) studies have predominantly focused on acidic soils that are abundant in iron (Fe) oxides and aluminum (Al) oxides. We have probed mineral–OM interactions in an alkaline or calcareous soil of the Aridisols class. Unlike the role of Fe and Al, the role of Ca-minerals (particularly calcite), which are ubiquitous in alkaline soils, in OM sequestration is not well understood. Multiple recent model studies with aqueous Ca2+ or synthetic calcite and a suite of OM compounds have shown Ca-OM assemblages to be spatially correlated with calcite at the microscale. To study the chemical state of both Ca andmore » Fe and their competing role in soil organic matter (SOM) stabilization, we performed laboratory characterization using x-ray diffraction, Mössbauer spectroscopy, x-ray photoelectron spectroscopy, scanning electron microscopy, and scanning transmission electron microscopy, alongside synchrotron-based microscale chemical imaging using scanning transmission x-ray microscopy combined with near-edge x-ray absorption fine structure. Ca mineral–organic associations were found to be ubiquitous in this system and are likely critical for understanding SOM stabilization/degradation in alkaline soils. From our findings on mineralogy, speciation, and the nature of Ca-OM bridging, we identified differences in C and Ca chemistry based on the relative location of OM to Ca minerals. The OM near the calcite crystal was enriched in lipid and protein moieties, Ca-OM next to Fe minerals displayed a strong contribution from aromatic compounds, while on the surface of microbes, the carbonate was believed to be of microbial in origin, as also suggested by preliminary works reporting on the formation of amorphous calcite or nano-calcite. In Ca-OM admixed with carbonate, it was difficult to distinguish Ca-associated OM from amorphous calcite or nano-calcite.« less

Geologic carbon sequestration in mafic and ultramafic reservoirs is a scalable strategy for carbon dioxide removal, offering permanent storage via mineralization as stable carbonates. However, there is limited information on the structure and composition of key mineralization endpoints during sequestration. Here, we unravel the atomic structure, composition, and nanoscale morphology of carbonates recovered from a field-scale demonstration of CO₂ mineralization in basalt. Using transmission electron microscopy, we mapped mineralogical variations from the initial to later stages of subsurface carbonate growth and identified a previously unknown cation-ordered ankerite phase that exerts a primary control over carbonation processes. This study has providedmore » a new understanding of subsurface carbonation pathways which will impact the parameterization of predictive geochemical models for future sequestration efforts in basalt formations.« less

Lithium (Li) penta-aluminate (LiAl₅O₈) is investigated as a potential tritium (T) breeding material, with a focus on microstructural response to ion irradiation and deuterium (D) diffusion behavior. Under high-fluence ion irradiation (2 x 10¹⁷ (He⁺+D⁺)/cm²) at 773 K, LiAl₅O₈ exhibits significant disorder on the Li sublattice, as revealed by atomic-resolution scanning transmission electron microscopy, while the Al and O sublattices remain stable, demonstrating strong resistance to structural amorphization. Irradiation induces the formation of platelet-shaped antiphase boundaries (APBs), which may serve as effective D trapping sites. Atom probe tomography suggests the presence of ⁶LiD clusters in the mass spectra, though definitemore » conclusions regarding APB composition are hindered by signal overlap and limited data statistics. Time-of-flight secondary ion mass spectrometry reveals that D retention approaches to saturation at 3 x 10¹⁷ (He⁺+D⁺)/cm². Isothermal and isochronal annealing studies determine an average diffusivity of 1.6 x 10^-13 at 773 K and an effective activation energy of 0.8 ± 0.1 eV for D migration. Compared to γ-LiAlO₂, LiAl₅O₈ demonstrates superior irradiation resistance, minimal Li loss, and enhanced D retention, underscoring its potential as a durable breeder material for T production. In conclusion, these findings provide key insights into the microstructural evolution, defect dynamics, and D retention mechanisms in LiAl₅O₈ under reactor-relevant conditions.« less

Accurate and efficient characterization of nanoparticles (NPs), particularly regarding particle size distribution, is essential for advancing our understanding of their structure-property relationship and facilitating their design for various applications. In this study, we introduce a novel two-stage artificial intelligence (AI)-driven workflow for NP analysis that leverages prompt engineering techniques from state-of-the-art single-stage object detection and large-scale vision transformer (ViT) architectures. This methodology is applied to transmission electron microscopy (TEM) and scanning TEM (STEM) images of heterogeneous catalysts, enabling high-resolution, high-throughput analysis of particle size distributions for supported metal catalyst NPs. The model's performance in detecting and segmenting NPs is validatedmore » across diverse heterogeneous catalyst systems, including various metals (Ru, Cu, PtCo, and Pt), supports (silica (SiO₂), γ-alumina (γ-Al₂O₃), and carbon black), and particle diameter size distributions with mean and standard deviations ranging from 1.6 ± 0.2 nm to 9.7 ± 4.6 nm. The proposed machine learning (ML) methodology achieved an average F1 overlap score of 0.91 ± 0.01 and demonstrated the ability to disentangle overlapping NPs anchored on catalytic support materials. The segmentation accuracy is further validated using the Hausdorff distance and robust Hausdorff distance metrics, with the 90th percent of the robust Hausdorff distance showing errors within 0.4 ± 0.1 nm to 1.4 ± 0.6 nm. In conclusion, our AI-assisted NP analysis workflow demonstrates robust generalization across diverse datasets and can be readily applied to similar NP segmentation tasks without requiring costly model retraining.« less

The activation of H₂ on NaY-encapsulated Mo sulfide clusters is significantly influenced by the presence of Ni at ion exchange positions. Ni was incorporated by partially ion exchanging the NaY zeolite with Ni²⁺ cations. Mo(CO)₆ vapors were subsequently deposited on the ion exchanged NiNaY zeolites followed by sulfidation in 10 vol % H₂S/H₂ at 673 K, leading to the formation of dimeric Mo₂S₄ clusters connected to Ni²⁺ via bridging S atoms. In contrast to the monometallic Mo sulfide clusters, which stabilize adsorbed hydrogen primarily as hydrides on Mo atoms, the bimetallic Ni–Mo sulfide clusters bind hydrogen also as sulfhydryl groupsmore » on the bridging sulfur atoms. The formation of sulfhydryl groups in Ni–Mo sulfide clusters is attributed to the lower electron density on the cluster due to coordination with more electronegative Ni²⁺. The ethene hydrogenation rate was significantly higher on the bimetallic Ni–Mo sulfide catalysts compared to monometallic Mo sulfide catalysts because the stabilization of atomic hydrogen as sulfhydryl groups opens a new hydrogenation pathway.« less