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At least 289 records · Page 16
All sourcesNASA NTRSDOE OSTI
DOE OSTI2022

Corrosion characteristics of Mo and TZM alloy for plasma facing components in molten lithium at 623 K

Here, in this study, the corrosion behaviours of molybdenum (Mo) and a Mo-based alloy (TZM) were investigated using a static immersion corrosion technique. Weight loss, surface microstructure, and corrosion depth of Mo and the TZM alloy were correlated with elements. The compatibility of Mo and the TZM alloy in static liquid Li was suitable. Mo demonstrated uniform corrosion with a homogenous dissolution of free C and Mo, and TZM alloy exhibited a nonuniform corrosion behaviour with a preferential grain boundary attack caused by the selective dissolution of free C, Ti, and Zr. When the surface oxidation layer of the samples was consumed, free C from Mo and TZM diffused into molten Li to form Li 2 C 2 and then was captured by Zr, Ti, and Mo to form thermodynamically stable carbides, which resulted in the enrichment of the C layer near the sample surfaces. In addition, Ti and Zr acted as N-trappers in liquid Li; the formation of Zr and Ti nitrides resulted in the enrichment of N, Ti, and Zr elements on the surface and led to TZM corrosion increase. Thus, Zr and Ti depletion, pitting, and grain boundary corrosion were problematic for TZM under long-term exposure to liquid Li. Reducing the content of free C and nonmetallic N and increasing the amount of Ti and Zr carbides on the surface of and inside the raw TZM alloy helped improve the corrosion resistance of TZM in liquid Li.

36 MATERIALS SCIENCE↗
DOE OSTI2023

Chemical and genetic characterization of the ungrouped pallasite Lieksa

The meteorite Lieksa was found in 2017 in Löpönvaara, Finland, and later donated to the Finnish Museum of Natural History. Here, we report siderophile element concentrations, genetic isotopic data, and a metal–silicate segregation age for the meteorite. The ~280 g Lieksa is ~80% metal and ~20% silicate and oxide inclusions by volume, with the inclusions consisting primarily of Fe-rich olivine. Due to Lieksa's silicate content, coupled with a texture characterized by metal enclosing the silicates, it has been classified as a pallasite. Lieksa's olivine and bulk chemical characteristics are distinct from those of the known pallasite and iron meteorite groups, consistent with its classification as ungrouped. The meteorite exhibits a flat, chondrite-normalized highly siderophile element pattern, consistent with an origin as an early crystallization product from a metallic melt with chondritic relative abundances. Molybdenum, Ru, and 183 W isotopic data indicate that Lieksa formed in the non-carbonaceous (NC) domain of the solar nebula. Radiogenic 182 W abundances for Lieksa yield a model metal–silicate segregation age of 1.5 ± 0.8 Myr after calcium-aluminum-rich inclusion formation, which is within the range established for other NC-type pallasite and iron meteorite parent bodies.

58 GEOSCIENCES↗
DOE OSTI2024

Using ethanol and isopropanol as biomass model compounds for understanding bond scission mechanisms over Cu/Mo 2 N catalysts

The conversion of biomass compounds into fuels and chemicals is an important step towards a more sustainable future. This work combines results from model surfaces and powder catalysts to demonstrate Cu-modified mo- lybdenum nitride (Cu/Mo 2 N) as a selective catalyst for dehydrogenation of the biomass model compounds, ethanol and isopropanol. Results from model surfaces showed that while Mo2N led to unselective decomposition via both dehydrogenation and dehydration, the addition of Cu increased the dehydrogenation activity and selectivity. DFT calculations showed how Cu influenced the structures of active sites, adsorbate interactions, and thus the product selectivity. Batch reactor studies on corresponding powder catalysts confirmed the trend that Cu modification increased dehydrogenation activity, and in situ X-ray absorption spectroscopy elucidated the Cu oxidation state under reaction conditions. Further, this work demonstrates a strategy for promoting dehydrogenation over Mo 2 N-based catalysts, as well as the feasibility of using model surfaces to guide the design of industrially relevant catalysts.

09 BIOMASS FUELS↗
DOE OSTI2025

Tungsten-dioxo single-site heterogeneous catalyst on carbon: synthesis, structure, and catalysis

This study investigates the application of a novel third-row metal, tungsten, to carbon-supported single-site metal-oxo heterogeneous catalysis. Tungsten is a green and earth-abundant metal, but an unexplored candidate in this role. The carbon (AC = activated carbon)-supported tungsten dioxo complex, AC/WO 2 was prepared via grafting of (DME)WO 2 Cl 2 (DME = 1,2-dimethoxyethane) onto high-surface-area activated carbon. AC/WO 2 was fully characterized by ICP-OES, XPS, EXAFS, XANES, SMART-EM, and DFT. W 4d 7/2 XPS and W L III -Edge XANES assign the oxidation state as W(VI), while EXAFS reveals two W=O double and two W–O single bonds at distances of 1.73 and 1.92 Å, respectively. These data align well with DFT computational results, supporting the structure as Carbon(–μ-O–) 2 M(=O) 2 . SMART-EM verifies that single W(VI) catalytic sites are bonded in an out-of-plane manner. The catalytic performance of air- and water-stable AC/WO 2 is compared to that of AC/MoO 2 . AC/WO 2 is more active and selective than the molybdenum analog in mediating alcohol dehydration of various substrates, and is recyclable. Notably, AC/WO 2 is an effective and recyclable catalyst for primary aliphatic alcohol dehydration and forms no dehydrogenation side products in contrast to AC/MoO 2 . However, AC/WO 2 is less effective in epoxidation and PET depolymerization. Overall, this work demonstrates the potential of carbon-supported third row metals for future studies.

02 PETROLEUM↗
DOE OSTI2026

Quaternary i-MAX Phases (Mo 2/3 RE 1/3 ) 2 AlC (RE: Dy, Tb, Er): Experimental Characterization and First-Principles Insights into their Fundamental Properties

Rare earth (RE)-based materials have unique electronic, magnetic, and optical properties, leading to the recent discovery of atomically layered solids with the chemical formula (M' 2/3 RE 1/3 ) 2 AlC, which have since garnered significant attention in the scientific community. This study aims to synthesize, characterize, and investigate the structural and thermal stability of the RE i-MAX phases. We prepared i-MAX phases using molybdenum (Mo) as M′ and RE elements as Dy, Tb, and Er, namely (Mo 2/3 Dy 1/3 ) 2 AlC, (Mo 2/3 Tb 1/3 ) 2 AlC, and (Mo 2/3 Er 1/3 ) 2 AlC. Structural characterization through x-ray diffraction (XRD) and Raman spectroscopy confirms the formation of the RE-based i-MAX phase, along with the presence of minor impurity phases in the alloys. Thermogravimetric analysis (TGA) conducted up to 1000°C under ambient conditions reveals that the i-MAX phases remain thermally stable up to approximately 450°C, beyond which oxidation leads to a noticeable weight gain in all samples. Differential scanning calorimetry (DSC) measurements during heating and cooling cycles show endothermic and exothermic peaks for (Mo 2/3 Dy 1/3 ) 2 AlC i-MAX in the 410–420°C range, indicating a temperature-induced minor atomic arrangement. In contrast, these peaks are absent in the Tb- and Er-based i-MAX phases. These findings offer valuable insights into the thermal behavior and stability of these i-MAX phases under thermal stress, contributing to a deeper understanding of their unique properties. Furthermore, first-principles density functional theory (DFT) calculations were performed to investigate the electronic and optical properties of the i-MAX phases. The results reveal their metallic nature, with pronounced contributions from Mo and RE elements near the Fermi level and within the conduction band.

Rare earth↗
DOE OSTI2024

Gas-phase surface modification to control catalyst structure and yields in methane dehydroaromatization

Methane dehydroaromatization (MDA) is a promising approach for direct methane transformation to aromatics and hydrogen. The benchmark catalyst Mo/H-ZSM-5 struggles to find commercial adoption because of thermodynamically-limited yields and rapid coking on Brønsted acid and molybdenum carbide species, especially on zeolite external surfaces. Here, gas-phase atomic layer deposition (ALD) overcoats H-ZSM-5 external surfaces with SiO 2 or Al 2 O 3 . NH 3 -TPD, HRTEM, and textural properties show that these overcoats exclusively passivate zeolite external surfaces. Under MDA conditions, SiO 2 gives softer coke and increases cumulative benzene yields by 25%, while Al 2 O 3 strongly decreases yields. H 2 -TPR and UV-visible and Raman spectroscopy show how the overcoats redisperse the MoO x precatalysts, especially over multiple deactivation and isothermal oxidative regeneration cycles. Combined with 27 Al-MAS NMR, MoO x redistribution and dealumination are seen as the causes of long-term deactivation over multiple regeneration cycles, and this process continues to occur regardless of the overcoat. Altogether, the deposition of a small amount of silica on the outer surface of Mo/H-ZSM-5 reduces the formation of hard coke, which could be regenerated by milder methods such as hydrogen treatment.

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

Experimental system for studying temperature gradient-driven fracture of oxide nuclear fuel out of reactor

Temperature gradients in ceramic light water reactor (LWR) uranium dioxide (UO 2 ) nuclear fuel pellets generate thermal stresses that cause fractures in the fuel beginning early in the life of fresh fuel. The combination of heating due to fission and forced convective cooling on the exterior of LWR fuel rods generates a temperature profile that is difficult to replicate outside the reactor environment. In the present study, a state-of-the-art experimental set-up using electrical heating to replicate fission heating was built and surrogate fuel materials such as ceria (CeO 2 ) were used to validate the system. Cracking experiments were conducted on these surrogates by inducing reactivity-initiated-accident (RIA) like temperature gradients in the pellets via induction and direct resistance heating. Induction heating was done using copper coils and molybdenum susceptors which heated the surrogates to a threshold temperature that is sufficiently high for the fuel material to conduct current. Thereafter, direct resistance heating was used by a D.C. power supply to introduce volumetric heating to replicate LWR operating conditions analogous to fission heating. The pellets were held against nickel electrodes and mounted on a boron nitride test-stand. All the tests were carried out in a stainless-steel vacuum chamber. Simultaneous real-time dual imaging of the surrogate pellet surface has been implemented using an optical and infrared camera system which will be mounted along axial and perpendicular directions to the pellet surface respectively. A beam-splitter was used to split the incoming radiation from the sample into two halves. While one of the beams is transmitted from the splitter through a bandpass filter to obtain optical images, the other beam is reflected from the splitter to the thermal camera to capture full field temperature gradients of the as fabricated pellet surface during crack initiation and propagation. In the current series of tests, a 2-color pyrometer was used for recording and comparing the surface and centerline temperatures of the surrogate pellets in lieu of the thermal camera. A LabVIEW data acquisition system has been set up for collecting useful data during experiments.

11 NUCLEAR FUEL CYCLE AND FUEL MATERIALS↗
NASA NTRS2004

Thin Film Solar Cells: Organic, Inorganic and Hybrid

Thin film solar cells are an important developing resource for hundreds of applications including space travel. In addition to being more cost effective than traditional single crystal silicon cells, thin film multi-crystaline cells are plastic and light weight. The plasticity of the cells allows for whole solar panels to be rolled out from reams. Organic layers are being investigated in order to increase the efficiency of the cells to create an organic / inorganic hybrid cell. The main focus of the group is a thin film inorganic cell made with the absorber CuInS2. So far the group has been successful in creating the layer from a single-source precursor. They also use a unique method of film deposition called chemical vapor deposition for this. The general makeup of the cell is a molybdenum back contact with the CuInS2 layer, then CdS, ZnO and aluminum top contacts. While working cells have been produced, the efficiency so far has been low. Along with quantum dot fabrication the side project of this that is currently being studied is adding a polymer layer to increase efficiency. The polymer that we are using is P3OT (Poly(3-octylthiopene-2,5-diyll), retroregular). Before (and if) it is added to the cell, it must be understood in itself. To do this simple diodes are being constructed to begin to look at its behavior. The P3OT is spin coated onto indium tin oxide and silver or aluminum contacts are added. This method is being studied in order to find the optimal thickness of the layer as well as other important considerations that may later affect the composition of the finished solar cell. Because the sun is the most abundant renewable, energy source that we have, it is important to learn how to harness that energy and begin to move away from our other depleted non-renewable energy sources. While traditional silicon cells currently create electricity at relatively high efficiencies, they have drawbacks such as weight and rigidness that make them unattractive especially for space applications. Thin film photovoltaics have the potential to alleviate these problems and create a cheap and efficient way to harness the power of the sun.

Dankovich, John↗
DOE OSTI2024

Contrasting Effects of Catecholate and Hydroxamate Siderophores on Molybdenite Dissolution

Molybdenum (Mo) is essential for many enzymes but is often sequestered within minerals, rendering it not readily bioavailable. Metallophores, metabolites secreted by microorganisms and plants, promote mineral dissolution to increase metal bioavailability. However, interactions between metallophores and Mo-bearing minerals remain unclear. Here, in this study, catecholate protochelin and hydroxamate desferrioxamine B (DFOB) were utilized to examine their effects on dissolution of the common Mo-bearing mineral, molybdenite (MoS 2 ), under both oxic and anoxic conditions. Protochelin promoted molybdenite dissolution under oxic conditions with the formation of MoO 3 on the surface and Mo-siderophore complexes in solution. This was attributed to air-oxidation of both molybdenite and protochelin, as evidenced by lack of dissolution under anoxic conditions but enhanced dissolution by either pre-oxidized protochelin or pre-oxidized molybdenite. Liquid chromatography-mass spectroscopy (LC-MS), X-ray photoelectron spectroscopy (XPS), and Time-of-flight secondary ion mass spectrometry (ToF-SIMS) analyses revealed degradation of protochelin and adsorptions of its byproducts on molybdenite surface to promote dissolution. Conversely, DFOB inhibited molybdenite dissolution under both oxic and anoxic conditions, likely attributed to surface adsorption of DFOB and its weak complexation with Mo(VI) at the circumneutral pH. This work highlights the need to consider the balance between promoting and inhibitory effects of different metallophores on Mo-mineral dissolution.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2022

How a Formate Dehydrogenase Responds to Oxygen: Unexpected O 2 Insensitivity of an Enzyme Harboring Tungstopterin, Selenocysteine, and [4Fe–4S] Clusters

The reversible two-electron interconversion of formate and CO 2 is catalyzed by both nonmetallo- and metallo-formate dehydrogenases (FDHs). The latter group comprises molybdenum- or tungsten-containing enzymes with the metal coordinated by two equivalents of a pyranopterin cofactor, a cysteinyl or selenocysteinyl (Sec) ligand supplied by the polypeptide, and a catalytically essential terminal sulfido ligand. In addition, these biocatalysts incorporate one or more [4Fe–4S] clusters for facilitating long-distance electron transfer. However, an interesting dichotomy arises when attempting to understand how the metallo-FDHs react with O 2 . Whereas existing scholarship portrays these enzymes as being unable to perform in air due to extreme O 2 lability of their metal centers, studies dating as far back as the 1930s emphasize that some of these systems exhibit formate oxidase (FOX) activity, coupling formate oxidation to O 2 reduction. Therefore, to reconcile these conflicting views, we explored context-dependent functional linkages between metallo-FDHs and their cognate electron acceptors within the same organism vis-à-vis catalysis under atmospheric O 2 . Here, we report the discovery and characterization of an O 2 -insensitive FDH2 from the sulfate-reducing bacterium Desulfovibrio vulgaris Hildenborough (DvH) that ligates tungsten, Sec, and four [4Fe–4S] clusters. By advancing a robust expression platform for its recombinant production, we eliminate both the requirement of nitrate or azide during purification and reductive activation with thiols and/or formate prior to catalysis. Because the distinctive spectral signatures of formate-reduced DvH-FDH2 remain invariant under anaerobic and aerobic conditions, we benchmarked the enzyme activity in air, identifying CO 2 as the catalytic product. Full reaction progress curve analysis discloses a high catalytic efficiency when probed with a high-potential artificial electron acceptor. Furthermore, we show that DvH-FDH2 enables near-stoichiometric hydrogen peroxide production without superoxide release to achieve O 2 insensitivity. Notably, simultaneous electron transfer to cytochrome c and O 2 reveals that metal-based electron bifurcation is operational in this system. Taken together, our work proves the co-occurrence of redox bifurcated FDH and FOX activities within a metalloenzyme scaffold. These findings set the stage for uncovering previously unknown O 2 -insensitive flavin-based electron bifurcation mechanisms, as well as for developing authentic formate/air biofuel cells, engineering O 2 -stable FDHs and biohybrid metallocatalysts, and discerning formate bioenergetics of gut microbiota.

13C NMR↗
DOE OSTI2022

Thermite and intermetallic projectiles examined experimentally in air and inert gas environments

Intermetallic (aluminum and zirconium) and thermite (aluminum and molybdenum trioxide) projectiles were launched using a high velocity impact ignition testing system. The experiments were designed to simulate reactivity in high (argon) and low (air) altitude environments. The projectiles were launched into a chamber that included a steel target plate for projectile penetration before impacting a rear witness plate. The chamber was semi-sealed and instrumented for quasi-static pressure data. The results provide an understanding of energy release from the projectile materials and of the environmental influence on performance. The transient pressure traces provide insight into reaction kinetics. A bifurcation in transient pressure rise was an indication of a shift in reaction kinetics from the inherent reactive material to metal oxidation with the environment. The bifurcation was delayed by about 0.15 ms for the intermetallic relative to the thermite, evidence that the thermite reaction proceeded faster upon impact than the intermetallic. The two-step process (impact ignition of the reactive material followed by metal oxidation) was shown to produce higher energy conversion efficiencies than projectiles composed of pure fuel (i.e., aluminum) reported previously. Both reactive materials showed energy conversion efficiencies greater than 30% (for air) and 50% (for argon), and an explanation of underestimated efficiency and energy losses is provided. These results have implications for advancing formulations for ballistic applications. Structural reactive materials can be used to modify the effective reactivity of metal-containing formulations in varied atmospheric environments.

36 MATERIALS SCIENCE↗
NASA NTRS1991

Evolution of ore deposits on terrestrial planets

Ore deposits on terrestrial planets materialized after core formation, mantle evolution, crustal development, interactions of surface rocks with the hydrosphere and atmosphere, and, where life exists on a planet, the involvement of biological activity. Core formation removed most of the siderophilic and chalcophilic elements, leaving mantles depleted in many of the strategic and noble metals relative to their chondritic abundances. Basaltic magma derived from partial melting of the mantle transported to the surface several metals contained in immiscible silicate and sulfide melts. Magmatic ore deposits were formed during cooling, fractional crystallization and density stratification from the basaltic melts. Such ore deposits found in earth's Archean rocks were probably generated during early histories of all terrestrial planets and may be the only types of igneous ores on Mars. Where plate tectonic activity was prevalent on a terrestrial planet, temporal evolution of ore deposits took place. Repetitive episodes of subduction modified the chemical compositions of the crust and upper mantles, leading to porphyry copper and molybdenum ores in calc-alkaline igneous rocks and granite-hosted tin and tungsten deposits. Such plate tectonic-induced mineralization in relatively young igneous rocks on earth may also have produced hydrothermal ore deposits on Venus in addition to the massive sulfide and cumulate chromite ores associated with Venusian mafic igneous rock. Sedimentary ore deposits resulting from mechanical and chemical weathering in reducing atmospheres in Archean earth included placer deposits (e.g., uraninite, gold, pyrite ores). Chromite, ilmenite, and other dense unreactive minerals could also be present on channel floors and in valley networks on Mars, while banded iron formations might underlie the Martian northern plains regions. As oxygen evolved in earth's atmosphere, so too did oxide ores. By analogy, gossans above sulfide ores probably occur on Mars, but not submarine ferromanganese nodules and crusts which have precipitated in oxygenated seawater on earth.

Burns, R. G.↗
NASA NTRS1992

Component test results from the bearing life improvement program for the Space Shuttle Main Engine oxidizer turbopumps

Interim results from a component test program to improve ball bearing life in the Space Shuttle Main Engine oxygen turbopumps are presented. Two specific bearing applications, using liquid oxygen as the bearing coolant, are addressed. The first, the thrust bearing of the low pressure pump, operates at relatively slow speed with predominantly axial load and little temperature rise in the bulk coolant. Testing has demonstrated a very significant reduction in bearing wear by increasing the bearing internal clearance. Heat generation data was obtained that indicates heavy, intermittent cage-to-ball contact occurs, providing a possible explanation for the observed wear. The second application is the turbine end bearings of the high pressure pump. These bearings operate at high speed and load with the possibility of significant coolant vaporization. Tests on production bearings and bearings having modified internal clearance and curvature yielded scattered but generally poor lives. A dramatic improvement was achieved by coating the standard cage with a thin film of fluorinated ethylene propylene and 15 percent molybdenum disulfide. Very promising results have also been obtained by replacing the standard balls with ones made of silicon nitride, especially in combination with the coated cage.

Keba, John E.↗
NASA NTRS1978

An experimental investigation of fractionation by sputter deposition

Artificial glass targets composed of elements varying widely in atomic weight were irradiated at an angle of incidence of 45 deg by 2-keV hydrogen ions at a current density of .33 mA/sq cm, and sputtered atoms were caught on a molybdenum film. Analyses of the sputter-deposited films and unsputtered target glasses were carried out by electron microprobe. The backward-sputtered component was found to be enriched in elements of low atomic weight, while the forward-sputtered component was enriched in heavy atoms. These results indicate that at the lunar surface lighter elements and isotopes would tend to be ejected in backward directions, escaping directly through the openings which admit bombarding ions without first striking an adjacent grain surface; heavy elements and isotopes would be forward-sputtered deeper into the soil and be preferentially retained, contributing to the reported enrichments of heavy elements and isotopes. Additional results show that the binding energy of an element in its oxide form influences the sticking coefficient of a sputtered atom; elements of low binding energy are likely to desorb, while elements of high binding energy tend to stick to the first bounce surface.

Paruso, D. M.↗
NASA NTRS2008

Directed Growth of Carbon Nanotubes Across Gaps

An experiment has shown that when single-walled carbon nanotubes (SWNTs) are grown by chemical vapor deposition in the presence of an electric field of suitable strength, the nanotubes become aligned along the electric field. In an important class of contemplated applications, one would exploit this finding in fabricating nanotube transistors; one would grow SWNTs across gaps between electrodes that would serve, subsequently, as source and drain contacts during operation of the transistors. In preparation for the experiment, a multilayer catalyst comprising a 20-nmthick underlayer of iridium (platinum group), a 1-nm-thick middle layer of iron, and a 0.2-nm-thick outer layer of molybdenum was ion-beam sputtered onto a quartz substrate. A 25 micrometers-diameter iron wire was used as a shadow mask during the sputtering to create a 25 micrometers gap in the catalyst. Then electrical leads were connected to the catalyst areas separated by the gap so that these catalyst areas would also serve as electrodes. The substrate as thus prepared was placed in a growth chamber that consisted of a quartz tube of 1-in. (2.54-cm) diameter enclosed in a furnace. SWNTs of acceptably high quantity and quality were grown in 10 minutes with methane at atmospheric pressure flowing through the chamber at a rate of 1,000 standard cubic centimeters per minute at a temperature of 900 C. To prevent oxidation of the SWNTs, the chamber was purged with 99.999-percent pure argon before and after growth, and the chamber was cooled to less than 300 C before opening it to the atmosphere after growth. When no voltage was applied across the gap, the SWNTs grew in random directions extending out from the edges of the catalyst at the gap. When a potential of 10 V was applied between the catalyst/electrode areas to create an electric field across the gap, the SWNTs grew across the gap, as shown in the figure.

Delzeit, Lance↗
DOE OSTI2024

Molecular Origins of Near-Infrared Luminescence in Molybdenum and Tungsten Oxyhalide Perovskites

Materials with near-infrared (near-IR) luminescence are desirable for applications in communications and sensing, as well as biomedical diagnostics and imaging. The most used inorganic near-IR emitters rely on precise doping of host crystal structures with select rare-earth or transition metal ions. Recently, another class of materials with intrinsic near-IR emission has been reported. The compositions of these materials were initially described as vacancy-ordered halide double perovskites Cs 2 MoCl 6 and Cs 2 WCl 6 , but further investigation by some of us on the compound reported as Cs2WCl6 revealed an oxyhalide instead, with a composition Cs 2 WO x Cl 6–x , where 1 < x < 2. Here we demonstrate that the Mo compounds similarly possess the composition Cs 2 MoO x Cl 6–x or Cs 2 MoO x Br 6–x where 1 < x < 2. Preparing the pure halide appears harder for Mo than for W, and we have not succeeded in doing so. The distinctly different composition requires the coordination environment and oxidation state for the Mo and W centers to be reconsidered from what was assumed for the pure halides. In this work, we examine the mechanism for near-IR emission in these materials given their true structures and compositions. We demonstrate that the luminescence is due to the specific d-orbital splitting caused by the presence of oxygen in the distorted [MOX 5 ] 2– octahedra (X is Cl or Br). The fine structure in the emission spectra at low temperatures has been resolved and is attributed to vibronic coupling to the Mo–O and W–O bond stretches. Understanding the true structure and composition of these interesting materials, besides explaining the near-IR luminescence, suggests how this desirable emission can be realized and manipulated.

36 MATERIALS SCIENCE↗
NASA NTRS1994

Observations of impact-induced molten metal-silicate partitioning

Observations of molten mid-ocean ridge basalt (MORB)-molybdenum (Mo) interactions produced by shock experiments provide insight into impact and differentiation processes involving metal-silicate partitioning. Analysis of fragments recovered from experiments (achieving MORB liquid shock pressures from 0.8 to 6 GPa) revealed significant changes in the composition of the MORB and Mo due to reaction of the silicate and metal liquids on a short time scale (less than 13 s). The FeO concentration of the shocked liquid decreases systematically with increasing pressure. In fact, the most highly shocked liquid (6 GPa) contains only 0.1 wt% FeO compared to an initial concentration of 9 wt% in the MORB. We infer from the presence of micrometer-sized Fe-, Si- and Mo-rich metallic spheres in the shocked glass that the Fe and Si oxides in the MORB were reduced in an estimated oxygen fugacity of 10(exp -17) bar and subsequently alloyed with the Mo. The in-situ reduction of FeO in the shocked molten basalt implies that shock-induced reduction of impact melt should be considered a viable mechanism for the formation of metallic phases. Similar metallic phases may form during impact accretion of planets and in impacted material found on the lunar surface and near terrestrial impact craters. In particular, the minute, isolated Fe particles found in lunar soils may have formed by such a process. Furthermore, the metallic spheres within the shocked glass have a globular texture similar to the textures of metallic spheroids from lunar samples and the estimated, slow cooling rate of less than or equal to 140 C/s for our spheres is consistent with the interpretation that the lunar spheroids formed by slow cooling within a melted target.

Rowan, Linda R.↗
DOE OSTI2025

Hydrogenation of ethylene over molybdenum–sulfur complexes supported on UiO-66

Development of supported single-site catalysts using small metal sulfide complexes could significantly help in the development of cost-effective catalytic materials to drive selective hydrogenation and hydrogenolysis. The goal of this study is to contribute to the development of metal sulfide catalysts by calculating the thermodynamics of a catalytic cyclic involving a metal organic framework functionalized by insertion of metal sulfide. Anchored metal sulfide complexes can potentially be designed with ligands with distinctly different electronic and catalytic properties for specific catalytic applications. Here we examine the hydrogenation of ethylene as a model. We use density functional theory to investigate molybdenum–sulfur complexes as active catalysts anchored on the metal–organic framework UiO-66 as a stable support. Our calculations show that the anchored complexes with more than two sulfur ligands are unfavorable for ethylene adsorption, so we study complexes with one or two sulfur ligands. Hydrogenation of the unsaturated carbon double bond requires the transfer of two hydrogen atoms, which can occur via heterolytic activation of hydrogen to form a Mo-hydride and a protonated sulfur – either by hydride transfer followed by proton transfer or via proton transfer followed by hydride transfer, and we find that both mechanisms proceed via two-state reactivity involving two spin states along the reaction path. Of the two catalysts studied in gas the phase, the MoS single-sulfur–ligand complex with lower oxidation states produces thermodynamically more favorable intermediates along the pathway for the first hydrogen transfer for both the hydride-first mechanism and the proton-first mechanism. As a result, the quantum mechanical calculations provide experimentally inaccessible partial atomic charges and geometries of the various intermediates encountered along the steps of the reaction mechanisms.

Kermani, Maryam Mansoori [University of Minnesota,↗