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At least 1,567 records · Page 87
All sourcesNASA NTRSDOE OSTI
DOE OSTI2018

Correction to Insights into the Performance Limits of the Li 7 P 3 S 11 Superionic Conductor: A Combined First-Principles and Experimental Study

Page 7847. The authors express regret that one mistake was made in their paper. In Figure 4b, the “Endo-Exo” direction was supposed to be reversed. When the orientation is corrected, T g is assigned an endothermic process and T c is assigned an exothermic process. A corrected Figure 4b (along with its original caption) is provided. Although the error was present, it does not affect the interpretation and conclusions of the manuscript as a whole. The authors apologize for any inconvenience this may have caused the readers.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Understanding Cation Selectivity in Carbon Nanopores with Hybrid First-Principles/Continuum Simulations: Implications for Water Desalination and Separation Technologies

Understanding ion adsorption in porous carbons is crucial for a range of technologies, including water desalination and energy storage. In this work, we combined density functional theory with a continuum solvation model to investigate thermodynamics and kinetics of the adsorption process of alkali metal ions from aqueous solutions into carbon nanopores with different sizes and geometries. We found that cations with a larger ionic radius are more favorable to enter the nanopores because of a lower energy penalty of dehydration. In addition, the pore size and geometry were found to have a significant impact on the ion–pore interaction under confinement and cation selectivity. Our study highlights a complex interplay among nanopore geometry, ion size, and hydration on the cation adsorption selectivity, suggesting that tuning the porosity could represent a general strategy for improving ion separations.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2021

Formic Acid Electrooxidation on Pt or Pd Monolayer on Transition-Metal Single Crystals: A First-Principles Structure Sensitivity Analysis

Here, we present a density functional theory analysis of trends for the electrooxidation of formic acid (FAO) on a single Pt or Pd monolayer supported on the close-packed (111) facet of transition metals (Pt*/M or Pd*/M): Au, Ag, Cu, Pt, Pd, Ir, and Rh, the close-packed (0001) facet of Os, Ru, and Re, and the open (100) facet of Au, Ag, Pt, Pd, Ir, and Rh. We show that the deposition of Pt or Pd pseudomorphic monolayers on these single crystals modifies the electronic structure of the Pt or Pd monolayer. Specifically, we found a direct correlation between the d-band center of the Pt and Pd monolayer and the free energy of adsorbed CO*, the latter being a reactivity descriptor for FAO. Together with the free energy of adsorbed OH* as a second reactivity descriptor, we depict the thermochemistry of the reaction network as phase diagrams showing calculated free energies across regions of rate-determining steps. We found that FAO is structure-sensitive on most surfaces studied. Pt*/Au(111) is predicted to be the most active among all Pt*/M(111/0001) surfaces studied, despite binding CO*, the strongest among the close-packed facets. This is the case because of its superiority in activating water to OH*, thus removing CO* at lower potentials than other surfaces. On similar grounds, Pt*/Pd(100), Pd*/Re(0001), Pd*/Au(111), Pd*/Ag(111), and Pd*/Pt(111) are predicted to show higher FAO activity than the corresponding monometallic Pt and Pd surfaces.

(100) facets↗
DOE OSTI2023

Mechanisms of Ethylene Epoxidation over Silver from Machine Learning-Accelerated First-Principles Modeling and Microkinetic Simulations

In this work we employed machine learning-augmented density functional theory (DFT) thermodynamic calculations to assess the stability of different AgO x structures under catalytic ethylene epoxidation reaction conditions. We found that there are multiple AgO x surface motifs that could co-exist under the relevant conditions. These included Ag surface oxides (e.g., AgO_p(4 × 4) and Ag 1.83 O) and atomic oxygen-covered Ag(111) surfaces. Furthermore, we employed DFT calculations to evaluate the energetics of different reaction mechanisms by which ethylene and oxygen can react on these surfaces. These studies revealed several energetically viable reaction pathways for ethylene epoxidation. Microkinetic modeling analyses, based on the DFT-calculated reaction pathways, showed that ethylene epoxidation can proceed on all surfaces and that multiple pathways, including those involving Langmuir–Hinshelwood and Eley–Rideal mechanisms, could be involved in selective and unselective reactions. The diversity of mechanisms that we discovered in the context of the relatively simple ethylene epoxidation reaction on Ag suggests that the richness and complexity of surface chemistry are most likely a rule rather than an exception in heterogeneous catalytic chemical transformations on metal surfaces and that the concept of a single or even a dominant mechanism and reaction intermediates might need to be revisited for many reactions.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2024

A First-Principles Approach to Modeling Surface Site Stabilities on Multimetallic Catalysts

The study of multimetallic alloys and the multitude of possible surface compositions have sparked a tremendous interest in engineering low-cost materials with high activity and selectivity in heterogeneous catalysis. Multimetallic systems provide complementary functionalities and an unprecedented tunability when designing catalyst formulations. However, due to their immense structural and compositional complexity, the investigation and identification of an optimal catalyst is a tedious and time-consuming process, both experimentally and theoretically. Therefore, theoretical design principles are highly desirable to accelerate the screening of catalyst structures across the vast compositional space. In this paper, we introduce a simple and general model for predicting the site stability of multimetallic surfaces and nanoparticles, which is based on physical principles. The model requires only a small set of density functional theory (DFT) calculations of metal atom binding energies on monometallic and dilute alloy surface slabs to optimize the parameters in the simple model. The resulting model allows for the quantification of the stability of any particular atom site in any conceivable chemical environment across a wide range of morphologies, sizes, and arrangements by interpolating the derived parameters from a monometallic system to a completely diluted alloyed system. Herein, we demonstrate the robustness of the model across an extensive data set of transition metal alloy surfaces and 147-atoms cuboctahedral nanoparticles (NPs) composed of IrRhRu and PtPdRu. In conclusion, our approach yields mean absolute errors of ≈0.15 (IrRhRu), 0.20 (PtPdRu), 0.19 (IrRhRu NP), and 0.26 (PtPdRu NP) eV relative to site binding energies calculated using DFT.

36 MATERIALS SCIENCE↗
DOE OSTI2025

Discovering Ni/Cu Single-Atom Alloy as a Highly Active and Selective Catalyst for Direct Methane Conversion to Ethylene: A First-Principles Kinetic Study

Direct methane conversion to liquid fuels or value-added chemicals is a promising technology to utilize natural resources without resorting to further petroleum extraction. However, discovering efficient catalysts for this reaction is challenging due to either coke formation or unfavorable C-H bond activation. Herein, we design single-atom alloy (SAA) catalysts to simultaneously eliminate the above two bottlenecks based on mechanism-guided strategies: (1) the active single atom enables favorable C-H bond breaking and (2) the less reactive host metal facilitates C-C coupling and thus avoids strong binding of carbonaceous species. Employing electronic structure theory calculations, we screened the stability of multiple SAAs with 3d-5d transition metals atomically dispersed on a copper surface in terms of avoiding dopant aggregation and segregation. We then evaluated reactivities of the stable SAAs as catalysts for direct methane conversion to C2 products, including methane dehydrogenation and C-C coupling mechanisms. Combining selectivity analysis with kinetic modeling, we predicted that nickel dispersed on copper, i.e., Ni/Cu SAA, is a highly active and selective catalyst that can efficiently transform methane to ethylene. This work designs efficient SAA catalysts for direct methane activation and provides chemical insights into engineering compositions of SAAs to tune their catalytic performances.

Kothakonda, Manish↗
DOE OSTI2016

Mechanism of Hydrogen Evolution Reaction on 1T-MoS 2 from First Principles

The 1T phase of transition-metal dichalcogenides (TMDs) has been shown in recent experiments to display excellent catalytic activity for hydrogen evolution reaction (HER), but the catalytic mechanism has not been elucidated so far. Herein, using 1T MoS 2 as the prototypical TMD material, we studied the HER activity on its basal plane from periodic density functional theory (DFT) calculations. Compared to the nonreactive basal plane of 2H phase MoS 2 , the catalytic activity of the basal plane of 1T phase MoS 2 mainly arises from its affinity for binding H at the surface S sites. Using the binding free energy (ΔG H ) of H as the descriptor, we found that the optimum evolution of H2 will proceed at surface H coverage of 12.5% ~ 25%. Within this coverage, we viewed the reaction energy and barrier for the three elementary steps of the HER process. The Volmer step was found to be facile, whereas the subsequent Heyrovsky reaction is kinetically more favorable than the Tafel reaction. Our results suggest that at low overpotential, HER can take place readily on the basal plane of 1T MoS 2 via the Volmer–Heyrovsky mechanism. Moreover, we screened the dopants for the HER activity and found that substitutional doping of the Mo atom by metals such as Mn, Cr, Cu, Ni, and Fe can make 1T MoS 2 a better HER catalyst.

1T MoS2↗
DOE OSTI2019

pH Effects on Hydrogen Evolution and Oxidation over Pt(111): Insights from First-Principles

A long-standing question in electrocatalysis is how the electrolyte pH affects the catalytic activity of proton–electron transfer reactions. A prime example for this is the hydrogen evolution/oxidation reaction (HER/HOR) over metal catalysts. While it has long been established that alkaline conditions result in a more sluggish reaction kinetics than in acidic conditions, the underlying reason for this trend remains contentious. Here, we apply density functional calculations to evaluate prevailing hypotheses for the origin of this effect: shifts in hydrogen binding, proton donor, and water reorganization energy. We present a microkinetic model, based on ab initio reaction energetics of all possible elementary steps. Our model shows a good agreement with experimental trends. We find that with increasing pH, the proton donor changes from hydronium to water. In conclusion, our model suggests that the intrinsically larger barriers for the splitting of water with respect to hydronium are the cause of HER kinetics being slower in alkaline than in acidic media.

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

First-Principles Surface Characterization and Water Adsorption of Fe 3 P Schreibersite

We report the meteoritic mineral schreibersite, e.g., Fe 3 P, is a proposed abiotic source of phosphorus for phosphate ion (PO 4 – ) production, needed for nucleobases, phospholipids, and other life building materials. Schreibersite could have acted as both a source of elemental phosphorus and as a catalyst, and the hostile conditions on early Earth could have accelerated its degradation in different environments. Here, we present results from quantum calculations of bulk schreibersite and of its low Miller index surfaces. We also investigate water surface adsorption and identify possible dissociation pathways on the most stable facet. Our calculations provide useful chemical insights into schreibersite interactions in aqueous environments, paving the way for further detailed investigation on more reactive surfaces. Our results help provide a “bottom-up” understanding for phosphorylated organic synthesis on the primitive planet and its role in producing life building molecules.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Roles of Hydration and Magnetism on the Structure of Ferrihydrite from First Principles

We report density functional theory calculations aimed at predicting thermodynamically stable structures for ferrihydrite across a range of possible compositions determined by the amount of structural water. Based on an assumed formula unit of Fe5O8H + nH2O, we performed ab initio calculations with evolutionary searching to find the lowest enthalpy structures as a function of the water content up to n = 2. This is the most exhaustive search for the ferrihydrite structure conducted so far; more than 5000 unique configurations were generated and evaluated over five states of hydration. Among them, the Michel akdaliate model was generated, along with several energetically comparable new structures at higher states of hydration. However, a direct comparison between calculated and experimental pair distribution function and X-ray diffraction patterns for the 50 lowest energy structures shows that none beyond the Michel model could be associated with ferrihydrite. Nevertheless, this energetically comparable structure set provides a novel basis for analyzing and understanding the effects of hydration and magnetism on the topology of ferrihydrite, from which we conclude that any tetrahedral Fe should be viewed as a metastable structural defect, created either as a result of the rapid kinetics of crystal growth or to accommodate a local magnetic stress between neighboring Fe atoms.

composition, density functional theory, ferrihydri↗
DOE OSTI2019

Reply to “Comment on ‘Roles of Hydration and Magnetism on the Structure of Ferrihydrite from First Principles’”

In the comments made by Manceau on the study by Sassi and Rosso, some concerns were raised regarding the potential for the structure theoretically found and identified as ferrihydrite (Michel model) to be hydromaghemite. In this reply, we address these concerns by showing that the structure theoretically generated cannot be unambiguously compared to hydromaghemite. Many of the concerns raised are, in fact, oblique to the given study. Notwithstanding, we reiterate clearly stated methodological details and caveats regarding the theoretical structural search performed, which disallow for the hydromaghemite prospect to manifest. We furthermore highlight the scholarly importance of examining experimental X-ray diffraction (XRD) and pair distribution function (PDF) of synthesized ferrihydrite samples from a variety of independent sources, as embodied in a broader set of relevant literature.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗