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DOE OSTI2020

Monte Carlo simulations of Cu/Ni–Si–Mn co-precipitation in duplex stainless steels

First-Passage Kinetic Monte Carlo (FPKMC) simulations of species migration in duplex stainless steels were performed in order to establish relationships between alloy content, segregation behavior, and the formation of Ni-Si-Mn rich particles in cast duplex stainless steels during thermal aging. The Ni-Si-Mn-rich second phase forms after extended aging at reactor operating temperatures and degrades alloy properties. Simulations of Ni-Si-Mn cluster formation were validated through comparison with experimental results obtained through Atom Probe Tomography (APT) on similar alloy compositions, identifying several trends. First, Cu promotes the formation of Ni-Si-Mn clusters, but only when Ni or Mn prefer segregation to the surface of Cu particles; without the segregation of these species, the critical composition for the clusters to form was not achieved. Second, the width of precipitate-denuded zones near ?/d interfaces increases with decreasing Cu content. This finding was in strong agreement with APT results, further validating our model. Finally, our model predicts that Ni-Si-Mn cluster formation will be the most extensive when the Si:Mn ratio is approximately 1:1 and the least extensive when one of these key elements is less concentrated (Mn = 0.5 at.% or Si = 0.5 at.%). The implications of these results on how to improve alloy properties are discussed.

Cu co-precipitation, Atom probe tomography (APT), ↗

NASA NTRS2009

A Simple Mechanism for Fractionating Oxygen Isotopes in the Solar Nebula

Lightning in the Solar Nebula is caused by the tribo-electric charging of dust grains carried by massive turbulent flows and driven by the accretion energy in the disk: it has long been one agent assumed responsible for the formation of chondrules. The degree to which charge separation can occur is dependent upon a number of factors, including the concentration of radioactive sources and the total level of ionization in the nebula, and these factors determine the maximum energy likely to be released by a single bolt. While chondrule formation requires a massive discharge, even a small lightning bolt can vaporize grains in the ionized discharge channel. Experimental studies have shown that silica, iron silicate and iron oxide grains formed from a high voltage discharge in hydrogen rich gas containing some oxygen produces solids that are enriched in O-17 and O-18 relative to the composition of the starting gas. Vaporization of silicates produces SiO, metal and free oxygen atoms in each discharge and these species will immediately begin to recondense from the hot plasma. Freshly condensed grains are incrementally enriched in heavy oxygen while the gas is enriched in O-16. Repeated evaporation and condensation of silicates in continuously occurring lightning discharges will monotonically increase the fractionation of oxygen isotopes between the O-17 and O-18 rich dust and the O-16 rich gas. The first mass independently fractionated refractory oxide particles were produced in the lab following the condensation of a flowing gas mixture containing variable amounts of hydrogen, silane, pentacarbonyl iron and oxygen that passed through a high voltage discharge powered by a Tesla coil. While the exact chemical pathway is still uncertain, the most probable reaction mechanisms involve oxidation of the growing refractory clusters by O3, OH or O atoms. This model has some interesting consequences for chemical processes in the early solar nebula. Chemical fractionation of recondensed dust evaporated via lightning discharges should be strongly time dependent. At earlier times, the accretion rate is maximal, thus driving strong turbulence, energetic grain-grain collisions, tribo-electric charging and charge separation, leading to frequent, powerful lightning discharges. As the accretion rate diminishes, turbulence decreases and lightning discharges will become both less powerful and less frequent, thus decreasing the rate of dust-gas fractionation. The most rapid increase in the formation of O-16 poor dust will occur early in nebular history. Generation of fractionated dust should be distributed throughout the inner disk. Once condensed, grain dispersal would average out any significant isotopic anomalies within the inner disk.

Nuth, Joseph A., III↗

NASA NTRS1984

Electronic structure and properties of magnetic defects in Co(1+x)Al(1-x) and Fe(1+x)Al(1-x) alloys

CoAl and FeAl compounds are developed along two directions. Magnetic susceptibility and specific heat at low temperature on (NiCo)Al and (CoFe)Al ternary alloys are in good agreement with band calculations. Results on magnetization and specific heat under field at low temperature on nonstoichiometric compounds show clearly the importance of the nearest neighbor effects. In the case of CoAl, the isolated cobalt atoms substituting aluminum are characterized by a Kondo behavior, and, for FeAl, the isolated extra iron atoms are magnetic and polarize the matrix. Moreover, for the two compounds, clusters of higher order play a considerable part in the magnetic properties for CoAl, these clusters also seem to be characterized by a Kondo behavior, for FeAl, these clusters whose moment is higher than in the case of isolated atoms, could be constituted of excess parts of iron atoms.

Abbe, D.↗

DOE OSTI2022

Atomic operations in a large scale distributed computing network

Techniques for executing an atomic command in a distributed computing network are provided. A core cluster, including a plurality of processing cores that do not natively issue atomic commands to the distributed computing network, is coupled to a translation unit. To issue an atomic command, a core requests a location in the translation unit to write an opcode and operands for the atomic command. The translation unit identifies a location (a “window”) that is not in use by another atomic command and indicates the location to the processing core. The processing core writes the opcode and operands into the window and indicates to the translation unit that the atomic command is ready. The translation generates an atomic command and issues the command to the distributed computing network for execution. After execution, the distributed computing network provides a response to the translation unit, which provides that response to the core.

Lackey, Jr., Stanley A.↗

NASA NTRS1980

Effect of three-body interactions on the structure of small clusters

Minimum energy configurations of microclusters (up to six atoms) have been calculated using two- and three-body interactions. Structural changes were parametrically analyzed as a function of the intensity of three-body forces. The results are qualitative in nature; they indicate, however, that three-body interactions play an important role in the equilibrium structure of microclusters. The effect of the intensity of the three-body interactions on the structure of small clusters is not manifested in a continuous manner. Rather, changes in the energetically most stable structure occur abruptly. The results are in qualitative agreement with experimental observations as well as other calculations.

Halicioglu, T.↗

NASA NTRS1990

Theoretical investigations of the structures and binding energies of Be(n) and Mg(n) (n = 3-5) clusters

The equilibrium geometries and binding energies of Be and Mg trimers, tetramers and pentamers have been determined using single and double excitation coupled cluster (CCSD) and complete active space self-consistent-field (CASSCF) multireference configuration interaction (MRCI) wave functions in conjunction with extended atomic basis sets. The best estimates of the cluster binding energies are 24, 83, and 110 kcal/mol for Be3, Be4, and Be5; and 9, 31, and 41 kcal/mol for Mg3, Mg4, and Mg5, respectively. A comparison of the MRCI and CCSD results shows that even the best single-reference approach (limited to single and double excitations) is not capable of quantitative accuracy in determining the binding energies of Be and Mg clusters.

Lee, Timothy J.↗

DOE OSTI2020

Light Exposure Promotes Degradation of Intermediates and Growth of Phosphine-Ligated Gold Clusters

Subnanometer clusters exhibit unique properties not observed at the bulk or nanometer scale. Many applications, however, require clusters to be monodisperse in size and composition.While substantial progress has been made empirically on producing specific-sized gold clusters using different ligands, the mechanisms of cluster formation remain insufficiently understood. Using high mass-resolution electrospray ionization mass spectrometry (ESI-MS), we characterized the formation of cationic triphenylphosphine-ligated gold clusters [Aux(PPh3)y z+] in solution. Our data provides evidence that small hydrogen-containing intermediate “building blocks” (i.e., Au2(PPh3)2H+, Au4(PPh3)4H+, and Au7(PPh3)7H5 2+) are responsible for the growth of larger gold clusters (i.e., Au6(PPh3)6 2+ and Au8(PPh3)7 2+) over several hours. In addition, intermediate degradation and cluster growth are shown to be dependent on irradiation of the reacting solution with light. Specifically, the rate of growth of large clusters is demonstrated to scale with the intensity of light exposure. Complementary theoretical calculations were also performed that provide molecular-level insight into the nature of the bonding between the hydrogens and gold atoms in the intermediate Au-phosphine clusters. Collectively, our joint experimental and theoretical findings advance the understanding of the factors at play in the size-selective synthesis of ligated gold clusters in solution.

Hewitt, Michael A.↗

DOE OSTI2021

Tunable room-temperature ferromagnetism in Co-doped two-dimensional van der Waals ZnO

The recent discovery of ferromagnetism in two-dimensional van der Waals crystals has provoked a surge of interest in the exploration of fundamental spin interaction in reduced dimensions. However, existing material candidates have several limitations, notably lacking intrinsic room-temperature ferromagnetic order and air stability. Here, motivated by the anomalously high Curie temperature observed in bulk diluted magnetic oxides, we demonstrate room-temperature ferromagnetism in Co-doped graphene-like Zinc Oxide, a chemically stable layered material in air, down to single atom thickness. Through the magneto-optic Kerr effect, superconducting quantum interference device and X-ray magnetic circular dichroism measurements, we observe clear evidences of spontaneous magnetization in such exotic material systems at room temperature and above. Transmission electron microscopy and atomic force microscopy results explicitly exclude the existence of metallic Co or cobalt oxides clusters. X-ray characterizations reveal that the substitutional Co atoms form Co 2+ states in the graphitic lattice of ZnO. By varying the Co doping level, we observe transitions between paramagnetic, ferromagnetic and less ordered phases due to the interplay between impurity-band-exchange and super-exchange interactions. Our discovery opens another path to 2D ferromagnetism at room temperature with the advantage of exceptional tunability and robustness.

36 MATERIALS SCIENCE↗

DOE OSTI2020

Materials Data on H2Ru4(CO)13 by Materials Project

RuRu3H2(CO)9(CO)4 crystallizes in the triclinic P-1 space group. The structure is zero-dimensional and consists of sixteen formaldehyde molecules, four ruthenium molecules, and four Ru3H2(CO)9 clusters. In two of the Ru3H2(CO)9 clusters, there are three inequivalent Ru3+ sites. In the first Ru3+ site, Ru3+ is bonded in a 5-coordinate geometry to three C+0.92+ and two H1+ atoms. There is two shorter (1.91 Å) and one longer (1.93 Å) Ru–C bond length. There is one shorter (1.77 Å) and one longer (1.78 Å) Ru–H bond length. In the second Ru3+ site, Ru3+ is bonded in a distorted rectangular see-saw-like geometry to three C+0.92+ and one H1+ atom. There is two shorter (1.90 Å) and one longer (1.91 Å) Ru–C bond length. The Ru–H bond length is 1.80 Å. In the third Ru3+ site, Ru3+ is bonded in a 4-coordinate geometry to three C+0.92+ and one H1+ atom. There is one shorter (1.90 Å) and two longer (1.92 Å) Ru–C bond length. The Ru–H bond length is 1.80 Å. There are nine inequivalent C+0.92+ sites. In the first C+0.92+ site, C+0.92+ is bonded in a single-bond geometry to one Ru3+ and one O2- atom. The C–O bond length is 1.16 Å. In the second C+0.92+ site, C+0.92+ is bonded in a single-bond geometry to one Ru3+ and one O2- atom. The C–O bond length is 1.16 Å. In the third C+0.92+ site, C+0.92+ is bonded in a single-bond geometry to one Ru3+ and one O2- atom. The C–O bond length is 1.16 Å. In the fourth C+0.92+ site, C+0.92+ is bonded in a single-bond geometry to one Ru3+ and one O2- atom. The C–O bond length is 1.16 Å. In the fifth C+0.92+ site, C+0.92+ is bonded in a single-bond geometry to one Ru3+ and one O2- atom. The C–O bond length is 1.16 Å. In the sixth C+0.92+ site, C+0.92+ is bonded in a single-bond geometry to one Ru3+ and one O2- atom. The C–O bond length is 1.16 Å. In the seventh C+0.92+ site, C+0.92+ is bonded in a single-bond geometry to one Ru3+ and one O2- atom. The C–O bond length is 1.16 Å. In the eighth C+0.92+ site, C+0.92+ is bonded in a single-bond geometry to one Ru3+ and one O2- atom. The C–O bond length is 1.16 Å. In the ninth C+0.92+ site, C+0.92+ is bonded in a single-bond geometry to one Ru3+ and one O2- atom. The C–O bond length is 1.16 Å. There are two inequivalent H1+ sites. In the first H1+ site, H1+ is bonded in a bent 120 degrees geometry to two Ru3+ atoms. In the second H1+ site, H1+ is bonded in a bent 120 degrees geometry to two Ru3+ atoms. There are nine inequivalent O2- sites. In the first O2- site, O2- is bonded in a single-bond geometry to one C+0.92+ atom. In the second O2- site, O2- is bonded in a single-bond geometry to one C+0.92+ atom. In the third O2- site, O2- is bonded in a single-bond geometry to one C+0.92+ atom. In the fourth O2- site, O2- is bonded in a single-bond geometry to one C+0.92+ atom. In the fifth O2- site, O2- is bonded in a single-bond geometry to one C+0.92+ atom. In the sixth O2- site, O2- is bonded in a single-bond geometry to one C+0.92+ atom. In the seventh O2- site, O2- is bonded in a single-bond geometry to one C+0.92+ atom. In the eighth O2- site, O2- is bonded in a single-bond geometry to one C+0.92+ atom. In the ninth O2- site, O2- is bonded in a single-bond geometry to one C+0.92+ atom. In two of the Ru3H2(CO)9 clusters, there are three inequivalent Ru3+ sites. In the first Ru3+ site, Ru3+ is bonded in a 5-coordinate geometry to three C+0.92+ and two H1+ atoms. There are a spread of Ru–C bond distances ranging from 1.90–1.93 Å. There is one shorter (1.75 Å) and one longer (1.77 Å) Ru–H bond length. In the second Ru3+ site, Ru3+ is bonded in a distorted rectangular see-saw-like geometry to three C+0.92+ and one H1+ atom. There are a spread of Ru–C bond distances ranging from 1.89–1.91 Å. The Ru–H bond length is 1.78 Å. In the third Ru3+ site, Ru3+ is bonded in a 4-coordinate geometry to three C+0.92+ and one H1+ atom. There is two shorter (1.91 Å) and one longer (1.93 Å) Ru–C bond length. The Ru–H bond length is 1.77 Å. There are nine inequivalent C+0.92+ sites. In the first C+0.92+ site, C+0.92+ is bonded in a single-bond geometry to one Ru3+ and one O2- atom. The C–O bond length is 1.16 Å. In the second C+0.92+ site, C+0.92+ is bonded in a single-bond geometry to one Ru3+ and one O2- atom. The C–O bond length is 1.16 Å. In the third C+0.92+ site, C+0.92+ is bonded in a single-bond geometry to one Ru3+ and one O2- atom. The C–O bond length is 1.16 Å. In the fourth C+0.92+ site, C+0.92+ is bonded in a single-bond geometry to one Ru3+ and one O2- atom. The C–O bond length is 1.16 Å. In the fifth C+0.92+ site, C+0.92+ is bonded in a single-bond geometry to one Ru3+ and one O2- atom. The C–O bond length is 1.16 Å. In the sixth C+0.92+ site, C+0.92+ is bonded in a single-bond geometry to one Ru3+ and one O2- atom. The C–O bond length is 1.16 Å. In the seventh C+0.92+ site, C+0.92+ is bonded in a single-bond geometry to one Ru3+ and one O2- atom. The C–O bond length is 1.16 Å. In the eighth C+0.92+ site, C+0.92+ is bonded in a single-bond geometry to one Ru3+ and one O2- atom. The C–O bond length is 1.15 Å. In the ninth C+0.92+ site, C+0.92+ is bonded in a single-bond geometry to one Ru3+ and one O2- atom. The C–O bond length is 1.16 Å. There are two inequivalent H1+ sites. In the first H1+ site, H1+ is bonded in a bent 120 degrees geometry to two Ru3+ atoms. In the second H1+ site, H1+ is bonded in a bent 120 degrees geometry to two Ru3+ atoms. There are nine inequivalent O2- sites. In the first O2- site, O2- is bonded in a single-bond geometry to one C+0.92+ atom. In the second O2- site, O2- is bonded in a single-bond geometry to one C+0.92+ atom. In the third O2- site, O2- is bonded in a single-bond geometry to one C+0.92+ atom. In the fourth O2- site, O2- is bonded in a single-bond geometry to one C+0.92+ atom. In the fifth O2- site, O2- is bonded in a single-bond geometry to one C+0.92+ atom. In the sixth O2- site, O2- is bonded in a single-bond geometry to one C+0.92+ atom. In the seventh O2- site, O2- is bonded in a single-bond geometry to one C+0.92+ atom. In the eighth O2- site, O2- is bonded in a single-bond geometry to one C+0.92+ atom. In the ninth O2- site, O2- is bonded in a single-bond geometry to one C+0.92+ atom.

36 MATERIALS SCIENCE↗

DOE OSTI2020

Materials Data on TeC2S2(IN2)2 by Materials Project

C2TeS2(N2I)2 crystallizes in the triclinic P1 space group. The structure is zero-dimensional and consists of two C2TeS2(N2I)2 clusters. In one of the C2TeS2(N2I)2 clusters, there are two inequivalent C4+ sites. In the first C4+ site, C4+ is bonded in a 3-coordinate geometry to two N+1.50- and one S2- atom. Both C–N bond lengths are 1.30 Å. The C–S bond length is 1.86 Å. In the second C4+ site, C4+ is bonded in a 1-coordinate geometry to two N+1.50- and one S2- atom. There is one shorter (1.39 Å) and one longer (1.40 Å) C–N bond length. The C–S bond length is 1.59 Å. There are four inequivalent N+1.50- sites. In the first N+1.50- site, N+1.50- is bonded in a single-bond geometry to one C4+ atom. In the second N+1.50- site, N+1.50- is bonded in a 1-coordinate geometry to one C4+ atom. In the third N+1.50- site, N+1.50- is bonded in a single-bond geometry to one C4+ atom. In the fourth N+1.50- site, N+1.50- is bonded in a distorted single-bond geometry to one C4+ atom. Te4+ is bonded in a distorted rectangular see-saw-like geometry to two S2- and two I1- atoms. There are one shorter (2.58 Å) and one longer (2.98 Å) Te–S bond lengths. There are one shorter (2.78 Å) and one longer (2.90 Å) Te–I bond lengths. There are two inequivalent S2- sites. In the first S2- site, S2- is bonded in a bent 120 degrees geometry to one C4+ and one Te4+ atom. In the second S2- site, S2- is bonded in a 2-coordinate geometry to one C4+ and one Te4+ atom. There are two inequivalent I1- sites. In the first I1- site, I1- is bonded in a distorted single-bond geometry to one Te4+ atom. In the second I1- site, I1- is bonded in a 1-coordinate geometry to one Te4+ atom. In one of the C2TeS2(N2I)2 clusters, there are two inequivalent C4+ sites. In the first C4+ site, C4+ is bonded in a 1-coordinate geometry to two N+1.50- and one S2- atom. There is one shorter (1.39 Å) and one longer (1.40 Å) C–N bond length. The C–S bond length is 1.59 Å. In the second C4+ site, C4+ is bonded in a linear geometry to two N+1.50- atoms. There is one shorter (1.22 Å) and one longer (1.25 Å) C–N bond length. There are four inequivalent N+1.50- sites. In the first N+1.50- site, N+1.50- is bonded in a 1-coordinate geometry to one C4+ and one N+1.50- atom. The N–N bond length is 1.31 Å. In the second N+1.50- site, N+1.50- is bonded in a distorted single-bond geometry to one C4+ and one I1- atom. The N–I bond length is 2.76 Å. In the third N+1.50- site, N+1.50- is bonded in a distorted single-bond geometry to one C4+ and one N+1.50- atom. In the fourth N+1.50- site, N+1.50- is bonded in a single-bond geometry to one C4+ atom. Te4+ is bonded in a distorted see-saw-like geometry to two S2- and two I1- atoms. There are one shorter (2.24 Å) and one longer (3.19 Å) Te–S bond lengths. There are one shorter (2.85 Å) and one longer (2.89 Å) Te–I bond lengths. There are two inequivalent S2- sites. In the first S2- site, S2- is bonded in a 1-coordinate geometry to one C4+ and one Te4+ atom. In the second S2- site, S2- is bonded in a single-bond geometry to one Te4+ atom. There are two inequivalent I1- sites. In the first I1- site, I1- is bonded in a 2-coordinate geometry to one N+1.50- and one Te4+ atom. In the second I1- site, I1- is bonded in a distorted single-bond geometry to one Te4+ atom.

36 MATERIALS SCIENCE↗

DOE OSTI2025

Pt Nanoparticle Disintegration at Oxide Interfaces Enhances CO Oxidation Catalysis

Understanding how supported metal nanoparticles dynamically evolve under reaction conditions is critical for controlling their catalytic function. Here, the mechanism behind the dynamic disintegration of Pt nanoparticles (NPs) supported on CeOx-TiO2 (CT) during CO oxidation is elucidated, leading to the formation of single atoms (SAs) and/or sub-nanometer clusters. Density functional theory (DFT) calculations reveal that strong Pt-CO interactions weaken Pt─Pt cohesion, while electronic coupling between Pt and Ce ions stabilizes Pt-CO* intermediates at the oxide interface. Surface oxygen vacancies kinetically trap Pt-CO*, but the vacancies are replenished under oxygen-rich conditions, enabling Pt-CO* surface diffusion and subsequent structural reorganization. In situ spectroscopic analyses confirm the oxygen-driven transformation of Pt NPs, correlating with a threefold increase in mass-specific activity at 150 °C. These findings demonstrate that interfacial oxygen dynamics and metal-support interactions can be leveraged to induce nanoparticle disintegration and optimize catalytic performance, highlighting the catalytic potential of interface-engineered Pt nanostructures.

CO oxidation↗

DOE OSTI2024

Surface dependence of electronic growth of Cu(111) on MoS 2

Scanning tunneling microscopy shows that copper deposited at room temperature onto a freshly exfoliated MoS 2 surface forms Cu(111) clusters with periodic preferred heights of 5, 8, and 11 atomic layers. These height intervals correlate with Fermi nesting regions along the necks of the bulk Cu Fermi surface, indicating a connection between physical and electronic structures. Density functional theory calculations of freestanding Cu(111) films support this as well, predicting a lower density of states at the Fermi level for these preferred heights. This is consistent with other noble metals deposited on MoS 2 that exhibit electronic growth, in which the metal films self-assemble as nanostructures minimizing quantum electronic energies. Here, we have discovered that it is critical for the metal deposition to begin on a clean MoS 2 surface. If copper is deposited onto an already Cu coated surface, even if the original film displays electronic growth, the resulting Cu film lacks quantization. Instead, the preferred heights of the Cu clusters simply increase linearly with the amount of Cu deposited upon the surface. We believe this is due to different bonding conditions during the initial stages of growth. Newly deposited copper would bond strongly to the already present copper clusters, rather than the weak bonding, which exists to the van der Waals terminated surface of MoS 2 . The stronger bonding with previously deposited clusters hinders additional Cu atoms from reaching their lowest quantum energy state. The interface characteristics of the van der Waals surface enable surface engineering of self-assembled structures to achieve different applications.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗

DOE OSTI2025

Liquid Phase Modeling in Porous Media: Adsorption of Methanol and Ethanol in H-MFI in Condensed Water

Zeolites are used in the chemical and separation industries for their exceptional selectivity, adsorption capacity, regenerability, and stability in gas and liquid phase processing. Here, we developed an explicit solvation method for predicting solvent/condensed phase effects on adsorption free energies in microporous media such as zeolites based on the hybrid quantum mechanical/molecular mechanical free energy perturbation (QM/MM-FEP) technique. Our explicit solvation method for zeolite systems, called eSZS, aims to capture site-specific interactions during the adsorption process at the Brønsted acid sites of H-MFI zeolite while still considering the diverse configuration space of the solvent molecules. This strategy is ideal for chemical reactions or adsorbates that interact with the microporous medium in few distinct adsorbate/transition state configurations, i.e., the harmonic or similar approximations are acceptable for the adsorbate/transition state while such approximations break down for the solvent molecules that require extensive configuration space sampling. In this way, our approach effectively overcomes the limitations of implicit solvation models and classical force field methods for describing solvation effects on chemical reactions within porous materials such as zeolites. Specifically, in this study, we investigated various aspects of our hybrid QM/MM approach, including QM cluster size dependencies in a periodic electrostatically embedded cluster model (PEECM), rules for link atoms at the QM/MM boundary, and functional and basis set considerations for converged and reasonably accurate gas and aqueous phase methanol and ethanol adsorption free energy predictions in H-MFI. For gas phase adsorption of methanol and ethanol in H-MFI at a Brønsted acid site in T12 position, we compute adsorption free energies at 298 K of −0.61 and −0.75 eV, respectively, using a PEECM containing 50 Si and 1 Al atom with ωB97x-D/def2-TZVP level of theory. For solvent effect calculations, we sample the aqueous phase using grand canonical Monte Carlo (GCMC) simulations to (1) obtain a mean field of electrostatic interactions in the reaction system and (2) perform a rigorous free energy perturbation calculation. Similar to the experimentally and computationally observed endergonic solvation effects observed for hydrocarbon adsorption on metal surfaces, we also observe that a condensed aqueous environment destabilizes methanol and ethanol at these acid sites in H-MFI at 298 K. Specifically, the computed solvation free energies of adsorption (ΔΔG solv ) for methanol and ethanol are +0.44 and +0.54 eV, respectively. From this study, it is evident that adsorbates (methanol and ethanol) are competing with water for adsorption space inside the H-MFI zeolite, leading to an endergonic solvation effect. Here, we expect that the endergonic, aqueous solvent effect during adsorption in microporous zeolites is highly tunable by changing the pore size and hydrophobicity of the microporous material as this will affect the water density inside the pore structure.

Adsorption↗

DOE OSTI2020

Parallel algorithms for hyperdynamics and local hyperdynamics

Hyperdynamics (HD) is a method for accelerating the timescale of standard molecular dynamics (MD). It can be used for simulations of systems with an energy potential landscape that is a collection of basins, separated by barriers, where transitions between basins are infrequent. HD enables the system to escape from a basin more quickly while enabling a statistically accurate renormalization of the simulation time, thus effectively boosting the timescale of the simulation. In [Kim, Perez, Voter, J Chem Phys, 139:144110, 2013)1, a local version of HD was formulated, which exploits the intrinsic locality characteristic typical of most systems to mitigate the poor scaling properties of standard HD as the system size is increased. In this paper, we discuss how both HD and local HD can be formulated to run efficiently in parallel. We have implemented these ideas in the LAMMPS MD code, which means HD can be used with any interatomic potential LAMMPS supports. Together, these parallel methods allow simulations of any size to achieve the time acceleration offered by HD (which can be orders of magnitude), at a cost 3-5x that of standard MD. As examples, we performed two simulations of a million-atom system to model the diffusion and clustering of Pt adatoms on a large patch of Pt(100) surface for 80 and 160 μs.

74 ATOMIC AND MOLECULAR PHYSICS↗

DOE OSTI2024

Time-resolved X-ray Spectroscopy from the Atomic Orbital Ground State Up

X-ray spectroscopy has been a key method to determine ground and excited state properties of quantum materials with atomic specificity. Now, new x-ray facilities are opening the door to the study of pump-probe x-ray spectroscopy - specifically time-resolved x-ray absorption (trXAS) and time-resolved resonant inelastic x-ray scattering (trRIXS). In this paper we will present simulations of each of these spectroscopies using a time-domain full atomic multiplet, charge transfer Hamiltonian, adapted to study the properties of a generalized cluster model including a central transition metal ion caged by ligand atoms in a planar geometry. The numerically evaluated trXAS and trRIXS cross-sections for representative electron configurations 3d9 and 3d8 demonstrate the insights that can be obtained from charge transfer pumping, and how this nonequilibrium process affects ground and excited state properties. The straightforward characterization of the excitations in these systems, based on our analysis of the simulations, can serve as a benchmark for future experiments, as access to these time-resolved spectroscopic techniques becomes more widely available.

Jost, Daniel↗

DOE OSTI2020

Influence of non-magnetic Cu on enhancing the low temperature magnetic properties and Curie temperature of FeCoNiCrCu(x) high entropy alloys

The microstructure and magnetic properties of three face-centered cubic (FCC) FeCoNiCrCu(x) high entropy alloys (HEAs) (x=0, 0.5, 1) are investigated. Interestingly, addition of the nonmagnetic element Cu to FeCoNiCr HEA is found to enhance exchange interactions and low temperature saturation magnetization. The paramagnetic to ferromagnetic Curie transition temperature increases from 85 K for FeCoNiCr to 118 K for FeCoNiCrCu. This is counterintuitive since Cu is nonmagnetic; however, atom probe tomography revealed Cu rich clusters containing 5 at% Ni and 1 at% each of Fe, Co, Cr, within FCC matrix, these clusters altered the matrix composition and consequently its magnetic properties.

Chaudhary, Varun↗

DOE OSTI2023

Revealing CO 2 dissociation pathways at vicinal copper (997) interfaces

Size- and shape-tailored copper (Cu) nanocrystals can offer vicinal planes for facile carbon dioxide (CO 2 ) activation. Despite extensive reactivity benchmarks, a correlation between CO 2 conversion and morphology structure has not yet been established at vicinal Cu interfaces. Herein, ambient pressure scanning tunneling microscopy reveals step-broken Cu nanocluster evolutions on the Cu(997) surface under 1 mbar CO 2 (g). The CO 2 dissociation reaction produces carbon monoxide (CO) adsorbate and atomic oxygen (O) at Cu step-edges, inducing complicated restructuring of the Cu atoms to compensate for increased surface chemical potential energy at ambient pressure. The CO molecules bound at under-coordinated Cu atoms contribute to the reversible Cu clustering with the pressure gap effect, whereas the dissociated oxygen leads to irreversible Cu faceting geometries. Synchrotron-based ambient pressure X-ray photoelectron spectroscopy identifies the chemical binding energy changes in CO-Cu complexes, which proves the characterized real-space evidence for the step-broken Cu nanoclusters under CO(g) environments. Our in situ surface observations provide a more realistic insight into Cu nanocatalyst designs for efficient CO 2 conversion to renewable energy sources during C 1 chemical reactions.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗

DOE OSTI2021

A stable low-temperature H 2 -production catalyst by crowding Pt on α-MoC

The water-gas shift (WGS) reaction is an industrially important source of pure hydrogen (H 2 ) at the expense of carbon monoxide and water. This reaction is of interest for fuel-cell applications, but requires WGS catalysts that are durable and highly active at low temperatures. Here we demonstrate that the structure (Pt 1 Pt n )/α-MoC, where isolated platinum atoms (Pt 1 ) and subnanometre platinum clusters (Pt-n) are stabilized on alpha-molybdenum carbide (α-MoC), catalyses the WGS reaction even at 313 kelvin, with a hydrogen-production pathway involving direct carbon monoxide dissociation identified. We find that it is critical to crowd the α-MoC surface with Pt 1 and Pt n species, which prevents oxidation of the support that would cause catalyst deactivation, as seen with gold/α-MoC, and gives our system high stability and a high metal-normalized turnover number of 4,300,000 moles of hydrogen per mole of platinum. We anticipate that the strategy demonstrated here will be pivotal for the design of highly active and stable catalysts for effective activation of important molecules such as water and carbon monoxide for energy production.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗