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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 163 records · Page 9
All sourcesNASA NTRSDOE OSTI
DOE OSTI2018

Understanding Methanol Coupling on SrTiO 3 from First Principles

Perovskites are interesting materials for catalysis due to their great tunability. However, the correlation of many reaction processes to the termination of a perovskite surface is still unclear. In this paper, we use the methanol coupling reaction on the SrTiO 3 (100) surface as a probe reaction to investigate direct C–C coupling from a computational perspective. We use density functional theory to assess methanol adsorption, C–H activation, and direct C–C coupling reactions on the SrTiO 3 (100) surface of different terminations. We find that, although methanol molecules dissociatively adsorb on both A and B terminations with similar strength, the dehydrogenation and C–C coupling reactions have significantly lower activation energies on the B termination than on the A termination. The predicted formation of methoxy and acetate on the SrTiO 3 (100) B termination can well explain the ambient-pressure XPS data of methanol on the single-crystal SrTiO 3 (100) surface at 250 °C. Finally, this work suggests that a choice of B termination of perovskites would be beneficial for the C–C coupling reaction of methanol.

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

Breaking Simple Scaling Relations through Metal–Oxide Interactions: Understanding Room-Temperature Activation of Methane on M/CeO 2 (M = Pt, Ni, or Co) Interfaces

The clean activation of methane at low temperatures remains an eminent challenge and a field of competitive research. In particular, on late transition metal surfaces such as Pt(111) or Ni(111), higher temperatures are necessary to activate the hydrocarbon molecule, but a massive deposition of carbon makes the metal surface useless for catalytic activity. However, on very low-loaded M/CeO 2 (M = Pt, Ni, or Co) surfaces, the dissociation of methane occurs at room temperature, which is unexpected considering simple linear scaling relationships. This intriguing phenomenon has been studied using a combination of experimental techniques (ambient-pressure X-ray photoelectron spectroscopy, time-resolved X-ray diffraction, and X-ray absorption spectroscopy) and density functional theory-based calculations. The experimental and theoretical studies show that the size and morphology of the supported nanoparticles together with strong metal–support interactions are behind the deviations from the scaling relations. These findings point toward a possible strategy for circumventing scaling relations, producing active and stable catalysts that can be employed for methane activation and conversion.

03 NATURAL GAS↗
DOE OSTI2024

Understanding Strain and Failure of a Knot in Polyethylene Using Molecular Dynamics with Machine-Learned Potentials

A neural network potential (NNP) has been developed by fitting to ab initio electronic structure data on hydrocarbons and is used to study failure of linear and knotted polyethylene (PE) chains. A linear PE chain must be highly strained before breaking as the stress is equally distributed across the chain. In contrast, the stress in a PE chain with a 31 or overhand knot, accumulates at the knot’s entrance/exit. We find the strain energy is greatest when the bond length and angle are strained simultaneously, and that the knot weakens the chain by increasing the variance of the C–C–C angle, thereby allowing rupture at lower bond strains. Here, we extend our analysis to both 51 and 52 knots and find that both break at the entrance/exit of a loop. Notably, molecular scale PE knots exhibit many of the same characteristics as knots in a macroscopic rope, with stick–slip phenomena upon tightening and similar points of failure.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2019

Understanding the Different Diffusion Mechanisms of Hydrated Protons and Potassium Ions in Titanium Carbide MXene

The high intercalation capacitance of MXenes is attractive, but their performance as electrodes in supercapacitors is limited by mass transport when increasing the thickness and mass loading of the electrodes. Here, we report a combined experimental and computational study, through which we reveal the diffusion of hydrated ionic species at the interlayer spaces. We find that the cyclic voltammetry curves for the delaminated Ti 3 C 2 T x exhibit distinct features in acid (H 2 SO 4 ) and alkaline (KOH) electrolytes. The DFT-calculated migration profile of K + and H + , in the presence and absence of water, suggests that the intercalated water molecules stabilize the charged ions, facilitating their diffusion from two dimension to three dimension manifested by the reduced activation barriers and movement pathways. In addition, we show that the diffusion of low and high concentrations of protons is significantly different. That is, protons of high concentrations can adsorb at both sides of the interlayer spaces, and water drives frequent proton hopping between stable adsorption sites as shown in the ab initio molecular dynamics simulations. The calculations can thus explain the varied capacitance and distorted CV curves when the experiments are conducted in acid and alkaline electrolytes. Furthermore, these results can provide the guidance to improve the fast transport of ions and electrons in MXenes with high mass loading.

25 ENERGY STORAGE↗
DOE OSTI2021

Understanding the Correlation between Ga Speciation and Propane Dehydrogenation Activity on Ga/H-ZSM-5 Catalysts

H-ZSM-5 zeolite-supported Ga (Ga/H-ZSM-5) has been considered as a selective catalyst for nonoxidative propane dehydrogenation (PDH) for decades; however, the reaction mechanism remains a topic of considerable discussion. In particular, the correlation between various Ga species present on the catalyst at the reaction conditions and the PDH activity has yet to be established. In this work, intrinsic PDH rates and activation energies were determined on Ga + –H + pair sites and isolated Ga + sites on Ga/H-ZSM-5 samples with a wide range of Si/Al and Ga/Al ratios. Here, the turnover frequency on Ga + –H + pair sites in the PDH is higher than that of isolated Ga + sites by a factor of ~15. Experimental measurements combined with a dual-site model show the activation energy in the PDH on the Ga + –H + pair sites and isolated Ga + sites to be 90.8 ± 1.5 and 117 ± 4.7 kJ·mol –1 , respectively. These results demonstrate that Ga+–H+ pair sites are much more active in the PDH than isolated Ga + sites. The activation energy of GaH x decomposition to form H 2 was determined to be 40–60 kJ·mol –1 higher than that of the PDH on Ga species, suggesting that the GaH x decomposition is unlikely to be part of the PDH mechanism. Although both Brønsted acid and Ga sites interact with propane, Fourier transform infrared spectroscopy results provide strong evidence suggesting that the alkyl mechanism is more likely in the PDH on Ga/H-ZSM-5 catalysts.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2018

Understanding the Intrinsic Surface Reactivity of Single-Layer and Multilayer PdO(101) on Pd(100)

In this work, we investigated the intrinsic reactivity of CO on single and multilayer PdO(101) grown on Pd(100) using temperature programmed reaction spectroscopy (TPRS) and reflection absorption infrared spectroscopy (RAIRS) experiments as well as density functional theory (DFT) calculations. We find that CO binds more strongly on multilayer than single-layer PdO(101) (~119 vs. 43 kJ/mol), and that CO oxidizes negligibly on single-layer PdO(101) whereas nearly 90% of a saturated layer of CO oxidizes on multilayer PdO(101) during TPRS experiments. RAIRS further shows that CO molecules adsorb on both bridge and atop-Pd cus sites (coordinatively-unsaturated Pd sites) of single-layer PdO(101)/Pd(100), while CO binds exclusively on atop-Pd cus sites of multilayer PdO(101). The DFT calculations reproduce the much stronger binding of CO on multilayer PdO(101) as well as the observed binding site preferences, and reveal that the stronger binding is entirely responsible for the higher CO oxidation activity of multilayer PdO(101)/Pd(100). We show that the O-atom below the Pd cus site, present only on multi-layer PdO(101), modifies the electronic states of the Pd cus atom in a way that enhances the CO-Pd cus bonding. Lastly, we show that a precursor-mediated kinetic model, with energetics determined from the present study, predicts that the intrinsic CO oxidation rates achieved on both single and multilayer PdO(101)/Pd(100) can be expected to exceed the gaseous CO diffusion rate to the surface during steady-state CO oxidation at elevated pressures, even though the intrinsic reaction rates are 4-5 orders of magnitude lower on single layer than on multilayer PdO(101)/Pd(100).

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2019

Understanding the Role of Metal and Molecular Structure on the Electrocatalytic Hydrogenation of Oxygenated Organic Compounds

Electrocatalytic hydrogenation is increasingly studied as an alternative to integrate the utilization of recycled carbon feedstocks with renewable energy sources. However, the abundant empiric observations available have not been correlated with fundamental properties of substrates and catalysts. In this paper, we investigated the electrocatalytic hydrogenation of homologues series of carboxylic acids, ketones, phenolics, and aldehydes on a variety of metals (Pd, Rh, Ru, Cu, Ni, Zn, and Co). We found that the rates of carbonyl reduction in aldehydes correlate with the corresponding binding energies between the aldehydes and the metals according to the Sabatier principle. That is, the highest rates are obtained at intermediate binding energies. The rates of H 2 evolution, occurring in parallel to hydrogenation, also correlate with the H-metal binding energies, following the same Volcano-type behavior. Within the boundaries of this model (e.g., compounds reactive at room temperature and without important steric effects over the carbonyl group), the reported correlations help to explain the complex trends derived from the experimental observations, allowing for the correlation of rates with binding energies and the differentiation of mechanistic routes.

Sabatier principle↗
DOE OSTI2016

Understanding Diffusion in Hierarchical Zeolites with House-of-Cards Nanosheets

Introducing mesoporosity to conventional microporous sorbents or catalysts is often proposed as a solution to enhance their mass transport rates. Here, we show that diffusion in these hierarchical materials is more complex and exhibits non-monotonic dependence on sorbate loading. Our atomistic simulations of n-hexane in a model system containing microporous nanosheets and mesopore channels indicate that diffusivity can be smaller than in a conventional zeolite with the same micropore structure, and this observation holds true even if we confine the analysis to molecules completely inside the microporous nanosheets. Only at high sorbate loadings or elevated temperatures, when the mesopores begin to be sufficiently populated, does the overall diffusion in the hierarchical material exceed that in conventional microporous zeolites. Here, our model system is free of structural defects, such as pore blocking or surface disorder, that are typically invoked to explain slower-than-expected diffusion phenomena in experimental measurements. Examination of free energy profiles and visualization of molecular diffusion pathways demonstrates that the large free energy cost (mostly enthalpic in origin) for escaping from the microporous region into the mesopores leads to more tortuous diffusion paths and causes this unusual transport behavior in hierarchical nanoporous materials. This knowledge allows us to re-examine zero-length-column chromatography data and show that these experimental measurements are consistent with the simulation data when the crystallite size instead of the nanosheet thickness is used for the nominal diffusional length.

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

High-Field One-Dimensional and Two-Dimensional 27Al Magic-Angle Spinning Nuclear Magnetic Resonance Study of ?-, d-, and ?-Al2O3 Dominated Aluminum Oxides: Toward Understanding the Al Sites in ?-Al2O3

Herein, a detailed analysis was carried out using high-field (19.9 T) 27Al magic-angle spinning (MAS) nuclear magnetic resonance (NMR) on three specially prepared aluminum oxide samples where the ?-, d-, and ?-Al2O3 phases are dominantly expressed through careful control of the synthesis conditions. Specifically, two-dimensional (2D) multiquantum (MQ) MAS 27Al was used to obtain high spectral resolution, which provided a guide for analyzing quantitative 1D 27Al NMR spectra. Six aluminum sites were resolved in the 2D MQ MAS NMR spectra, and seven aluminum sites were required to fit the 1D spectra. A set of octahedral and tetrahedral peaks with well-defined quadrupolar line shapes was observed in the ?-phase dominant sample and was unambiguously assigned to the ?-Al2O3 phase. The distinct line shapes related to the ?-Al2O3 phase provided an opportunity for effectively deconvoluting the more complex spectrum obtained from the d-Al2O3 dominant sample, allowing the peaks/quadrupolar parameters related to the d-Al2O3 phase to be extracted. The results show that the d-Al2O3 phase contains three distinct AlO sites and three distinct AlT sites. This detailed Al site structural information offers a powerful way of analyzing the most complex ?-Al2O3 spectrum. It is found that the ?-Al2O3 phase consists of Al sites with local structures similar to those found in the d-Al2O3 and ?-Al2O3 phases albeit with less ordering. Spin–lattice relaxation time measurement further confirms the disordering of the lattice. Collectively, this study uniquely assigns 27Al features in transition aluminas, offering a simplified method to quantify complex mixtures of aluminum sites in transition alumina samples.

Xu, Suochang↗
DOE OSTI2022

Metal-Coordinated Phthalocyanines as Platform Molecules for Understanding Isolated Metal Sites in the Electrochemical Reduction of CO 2

Single-atom catalysts (SACs) of non-precious transition metals (TMs) often show unique electrochemical performance, including the electrochemical carbon dioxide reduction reaction (CO 2 RR). However, the inhomogeneity in their structures makes it difficult to directly compare SACs of different TM for their CO 2 RR activity, selectivity, and reaction mechanisms. Here, the comparison of isolated TMs (Fe, Co, Ni, Cu, and Zn) is systematically investigated using a series of crystalline molecular catalysts, namely TM-coordinated phthalocyanines (TMPcs), to directly compare the intrinsic role of the TMs with identical local coordination environments on the CO 2 RR performance. The combined experimental measurements, in situ characterization, and density functional theory calculations of TM-Pc catalysts reveal a TM dependent CO 2 RR activity and selectivity, with the free energy difference of ΔG(*HOCO) - ΔG(*CO) being identified as a descriptor for predicting the CO 2 RR performance.

25 ENERGY STORAGE↗
DOE OSTI2019

Molecular Level Understanding of the Free Energy Landscape in Early Stages of Metal-Organic Framework Nucleation

The assembly mechanism of hierarchical materials controlled by the choice of solvent and presence of spectator ions. In this paper, we use enhanced sampling molecular dynamics methods to investigate these effects on the configurational landscape of metal-linker interactions in the early stages of synthesis, using MIL-101(Cr) as a prototypical example. Microsecond-long well-tempered metadynamics (WTmetaD) uncover a complex free energy structural landscape, with distinct crystal (C) and non-crystal (NC) like configurations and their equilibrium population. In presence of ions (Na+, F-), we observe a complex effect on the crystallinity of secondary building units (SBUs), by encouraging/suppressing salt bridges between C configurations and consequently controlling the percentage of defects. Solvent effects are assessed by introducing N, N-dimethylformamide (DMF) instead of water, where SBU adducts are appreciably more stable and compact. These results shed light on how solvent nature and ionic strength impact the free energy of nucleation phenomena that ultimately control materials synthesis and defect formation. The work described in this publication was supported by the US Department of Energy (DOE), Office of Science, Office of Basic Energy Sciences and performed at Pacific Northwest National Laboratory (PNNL). The authors acknowledge the use of the UCL Legion High Performance Computing Facility (Legion @ UCL) and associated support services in the completion of this work. This research used resources of the National Energy Research Scientific Computing Centre, a DOE Office of Science User Facility. Battelle operates PNNL for DOE under Contract DE-AC05-76RL01830.

Kollias, Loukas↗
DOE OSTI2019

Understanding the Role of Tropical Moisture in Atmospheric Rivers

There has been a long-standing debate about whether tropical moisture export (TME) or local moisture convergence play the dominant role in the development and strength of Atmospheric Rivers (ARs). In this work, we quantify the contribution of TME to 29 simulated extreme ARs that affected the U.S. Northwest Coast by “tagging” tropical moisture using water vapor tracers in the Weather Research and Forecast (WRF) model. From a case study with strong TME, we find three mechanisms by which TME can contribute to AR strength: direct contribution of moisture, indirect contributions via thermodynamic feedbacks and indirect contributions via synoptic-scale dynamic feedbacks. Focusing on these processes, we divide the 29 simulated ARs into two groups (>25% of TME contribution to inland precipitation as TME-ARs and <25% as NTME-ARs) and find that TME-ARs are characterized by stronger pre-cold-frontal low-level jets and stronger warm air advection. Notably, the ARs with the greatest 2-day precipitation are found to be TME-ARs but with <50% TME contributions, indicating a more dominant role of indirect enhancement of local moisture convergence due to TME than the direct moisture contribution supplying precipitation.

Hu, Huancui↗
DOE OSTI2020

Potential-pH diagrams considering complex oxide solution phases for understanding aqueous corrosion of multi-principal element alloys

The potential-pH diagram, a graphical representation of the thermodynamically predominant reaction products in aqueous corrosion, is originally proposed for the corrosion of pure metals. The original approach only leads to stoichiometric oxides and hydroxides as the oxidation products. However, numerous experiments show that non-stoichiometric oxide scales are prevalent in the aqueous corrosion of alloys. In the present study, a room temperature potential-pH diagram considering oxide solid solutions, as a generalization of the traditional potential-pH diagram with stoichiometric oxides, is constructed for an FCC single-phase multi-principal element alloy (MPEA) based on the CALculation of PHAse Diagram method. The predominant reaction products, the ions in aqueous solution, and the cation distribution in oxides are predicted. The oxide solid solution is stabilized by the mixing free energy (or mixing entropy) and the stabilizing effect becomes more significant as the temperature increases. Consequently, solid solution oxides are stable in large regions of the potential-pH diagram and the mixing free energy mostly affects the equilibrium composition of the stable oxides, while the shape of stable regions for oxides is mostly determined by the structure of the stable oxides. Agreements are found for Ni 2+ , Fe 2+ , and Mn 2+ between the atomic emission spectroelectrochemistry measurements and thermodynamic calculations, while deviations exist for Cr 3+ and Co 2+ possibly due to surface complexation with species such as Cl - and the oxide dissolution. By incorporating the solution models of oxides, the current work presents a general and more accurate way to analyze the reaction products during aqueous corrosion of MPEAs.

Materials Science↗
DOE OSTI2019

Understanding the role of selenium in defect passivation for highly efficient selenium-alloyed cadmium telluride solar cells

Electricity produced by cadmium telluride (CdTe) photovoltaic modules is the lowest-cost electricity in the solar industry, and now undercuts fossil fuel-based sources in many regions of the world. This is due to recent efficiency gains brought about by alloying selenium into the CdTe absorber, which has taken cell efficiency from 19.5% to its current record of 22.1%. Although the addition of selenium is known to reduce the bandgap of the absorber material, and hence increase the cell short-circuit current, this effect alone does not explain the performance improvement. Here, by means of cathodoluminescence and secondary ion mass spectrometry, we show that selenium enables higher luminescence efficiency and longer diffusion lengths in the alloyed material, indicating that selenium passivates critical defects in the bulk of the absorber layer. Here, this passivation effect explains the record-breaking performance of selenium-alloyed CdTe devices, and provides a route for further efficiency improvement that can result in even lower costs for solar-generated electricity.

14 SOLAR ENERGY↗