Search NASA⌕ Search

SEARCH · Search NASA

Results for “surface effects”

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.

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 1,729 records · Page 96
All sourcesNASA NTRSDOE OSTI
DOE OSTI2016

Steric Effect and Evolution of Surface Species in the Hydrodeoxygenation of Bio-Oil Model Compounds over Pt/HBEA

The hydrodeoxygenation of various bio-oil model compounds (anisole, m-cresol and guaiacol) over Pt/HBEA and the evolution of surface species is investigated. Depending on the functional group, different surface species are formed when the compounds are adsorbed in the presence of Lewis acid sites. For anisole, the methoxy group is decomposed to form phenate species. The methyl and methoxy group remains intact on m-cresol and guaiacol to form cresolate and methoxy phenate species, respectively. The position of these functional groups have a strong influence in the degree of hydrodeoxygenation due to steric hindrance. Based on operando transmission FTIR spectroscopy, a timeline for the formation of polynuclear aromatics and catalyst deactivation is constructed, which is also dependent on the substituents. The slow deactivation rate and low carbon content on Pt/HBEA is discussed.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2014

Effect of Doping on Surface Reactivity and Conduction Mechanism in Sm-doped CeO2 Thin Films

Scanning probe microscopy measurements show irreversible surface electrochemistry in Sm-doped CeO2 thin films, which depends on humidity, temperature and doping concentration. A systematic study by electrochemical strain microscopy (ESM) in samples with two different Sm content and in several working conditions allows disclosing the microscopic mechanism underlying the difference in water adsorption and splitting with subsequent proton liberation. We measure the behavior of the hysteresis loops by changing temperature and humidity, both in standard ESM configuration and using the first order reversal curve (FORC) method. Complementing our study with spectroscopic measurements by hard x-ray photoemission spectroscopy we find that water incorporation is favored until the doping with Sm is too high to allow the presence of Ce3+. The influence of doping on the surface reactivity and conduction mechanism clearly emerges from all of our experimental results. We find that at lower Sm concentration proton conduction is prevalent, featured by lower activation energy and higher mobility. Defect concentrations determine the type of the prevalent charge carrier in a doping dependent manner.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Improving Land Surface Temperature Simulation in CoLM Over the Tibetan Plateau through Fractional Vegetation Cover Derived from a Remotely Sensed Clumping Index and Model-Simulated Leaf Area Index

Parameterizations of fractional vegetation cover (FVC) in land surface models have important effects on simulations of surface energy budget, especially in arid and semiarid regions. This study uses a FVC scheme in which FVC is derived from leaf area index and a remotely sensed clumping index. The performance of the new scheme (SMFVC) is evaluated against Moderate Resolution Imaging Spectroradiometer (MODIS) land surface temperature (LST) and in situ soil temperature observations, together with two other FVC schemes, a general FVC scheme (CTL) based on land cover map and a climatology-based FVC scheme (RSFVC) that uses long-term remotely sensed Normalized Difference Vegetation Index of MODIS. The three FVC schemes were implemented in the Common Land Model (CoLM) and applied in the Tibetan Plateau using the same forcing data and default parameters. Our results demonstrate that FVC schemes have significant influence on the CoLM performance: (1) the RSFVC and SMFVC schemes significantly reduce the LST biases found in CTL, particularly in grassland and during summer; (2) soil temperature evaluation by in situ observations from three networks on the Tibetan Plateau corroborates the LST results; and (3) the improvements are mainly related to representing temporal (seasonal) variability and subgrid heterogeneity of FVC, which improves surface albedo and surface energy balance. In other words, by including more vegetation characteristics, such as using a clumping index, land surface models may better simulate surface vegetation condition and further better represent the land surface energy budget over the Tibetan Plateau.

Li, Chengwei↗
DOE OSTI2020

Hydrogen migration at restructuring palladium–silver oxide boundaries dramatically enhances reduction rate of silver oxide

Heterogeneous catalysts are complex materials with multiple interfaces. A critical proposition in exploiting bifunctionality in alloy catalysts is to achieve surface migration across interfaces separating functionally dissimilar regions. Herein, we demonstrate the enhancement of more than 10 4 in the rate of molecular hydrogen reduction of a silver surface oxide in the presence of palladium oxide compared to pure silver oxide resulting from the transfer of atomic hydrogen from palladium oxide islands onto the surrounding surface formed from oxidation of a palladium-silver alloy. The palladium-silver interface also dynamically restructures during reduction, resulting in silver-palladium intermixing. This study clearly demonstrates the migration of reaction intermediates and catalyst material across surface interfacial boundaries in alloys with a significant effect on surface reactivity, having broad implications for the catalytic function of bimetallic materials.

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

Electronic structure factors and the importance of adsorbate effects in chemisorption on surface alloys

The chemisorption energy is an integral aspect of surface chemistry, central to numerous fields such as catalysis, corrosion, and nanotechnology. Electronic-structure-based methods such as the Newns-Anderson model are therefore of great importance in guiding the engineering of material surfaces with optimal properties. However, existing methods are inadequate for interpreting complex, multi-metallic systems. Herein, we introduce a physics-based chemisorption model for alloyed transition metal surfaces employing primarily metal d-band properties that accounts for perturbations in both the substrate and adsorbate electronic states upon interaction. Importantly, we show that adsorbate-induced changes in the adsorption site interact with its chemical environment leading to a second-order response in chemisorption energy with the d-filling of the neighboring atoms. We demonstrate the robustness of the model on a wide range of transition metal alloys with O, N, CH, and Li adsorbates yielding a mean absolute error of 0.13 eV versus density functional theory reference chemisorption energies.

36 MATERIALS SCIENCE↗
DOE OSTI2026

Adsorption of CO on gold: effect of coverage and surface deformations

Recent insights into the nature of catalysts under reactive conditions have motivated investigating heterogeneously catalyzed reactions with non-rigid surfaces. As an example system to revisit, CO interactions with gold have been a longstanding system of interest due to its structure sensitivity. In this work, we investigate coverage-dependent CO adsorption trends on terraces, steps, and kinks. Calculating differential binding energy and mean absolute displacement of the atomic structures as a function of CO coverage, we propose a deformation quenching mechanism that partially determines CO saturation coverage. Extending this analysis to consider CO partial pressure, we show a phase change in CO step occupation, which we highlight as a possible explanation for previously reported temperature-programmed desorption and Fourier-transform infrared reflection-absorption spectroscopy measurements. Furthermore, this work shows how adopting a non-rigid surface model can lead to additional insights which may be applied to other heterogeneously catalyzed systems.

Kavalsky, Lance [University of Wisconsin-Madison, ↗
DOE OSTI2015

Effects of local electric surface potential on holes charging process in uncapped germanium nanocrystal

The charging kinetics of holes are investigated in an uncapped Ge nanocrystal by the use of the nano-electron beam induced current technique. The charging process is studied under zero volt or under an appropriate electric field. The investigation is repeated many times on the same nanocrystal and on others in the same sample to attest of the reproducibility of the results. At 0 V, the cycles of charging kinetics are superimposed and are in a steady state, but an instantaneous local and negative surface potential, established in the nanocrystal at the beginning of the kinetics, slows down the holes charging process. Under an external field, the energy band bending accentuation affects the holes charging time constants. As a result, the holes charging cycles weakly affect the electrical performance of the thin oxide as is indicated by the value of the measured local resistivity of 6 × 10{sup 10}–10{sup 11} Ω cm, which is relatively lower than that of the thick thermal oxide.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2016

Effect of the electrostatic surface potential on the oligomerization of full-length human recombinant prion protein at single-molecule level

The electrostatic surface potential (ESP) of prion oligomers has critical influences on the aggregating processes of the prion molecules. The atomic force microscopy (AFM) and structural simulation were combined to investigate the molecular basis of the full-length human recombinant prion oligomerization on mica surfaces. The high resolution non-intrusive AFM images showed that the prion oligomers formed different patterns on mica surfaces at different buffer pH values. The basic binding units for the large oligomers were determined to be prion momoners (Ms), dimers (Ds), and trimers (Ts). The forming of the D and T units happened through the binding of hydrophobic β-sheets of the M units. In contrast, the α-helices of these M, D, and T units were the binding areas for the formation of large oligomers. At pH 4.5, the binding units M, D, and T showed clear polarized ESP distributions on the surface domains, while at pH 7.0, they showed more evenly distributed ESPs. Based on the conformations of oligomers observed from AFM images, the D and T units were more abundantly on mica surface at pH 4.5 because the ESP re-distribution of M units helped to stabilize these larger oligomers. The amino acid side chains involved in the binding interfaces were stabilized by hydrogen bonds and electrostatic interactions. The detailed analysis of the charged side chains at pH 4.5 indicated that the polarized ESPs induced the aggregations among M, D, and T to form larger oligomers. Therefore, the hydrogen bonds and electrostatic interactions worked together to form the stabilized prion oligomers.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2023

Effect of thickness and surface composition on the stability of polarization in ferroelectric Hf x Zr 1 - x O 2 thin films

Using density functional theory, we find that tailoring the surface composition provides a route to stabilize the polar phases of the promising ferroelectric material, Hf x Zr 1-x O 2 . First, we show that for pure Hf O 2 , controlling the positively polarized surface to be relatively oxygen rich adequately screens the ferroelectric surface charges and stabilizes the polar orthorhombic phase. We then demonstrate that the ferroelectric polarization, as measured by the structural polar displacements, increases with decreasing thickness, leading to the emergence of a polar rhombohedral-like phase at the ultrathin limit (1.5 unit cells). Our findings extend to the cases of Hf 0.5 Zr 0.5 O 2 and Zr O 2 , both of which have surface energy landscapes similar to that of Hf O 2 . In conclusion, these findings are consistent with and offer insights into the observed absence of a ferroelectric thickness limit in Hf x Zr 1-x O 2 -based thin films.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Effect of Acid on Surface Hydroxyl Groups on Kaolinite and Montmorillonite

Mineral dust aerosol participates in heterogeneous chemistry in the atmosphere. In particular, the hydroxyl groups on the surface of aluminosilicate clay minerals are important for heterogeneous atmospheric processes. These functional groups may be altered by acidic processing during atmospheric transport. In this study, we exposed kaolinite (KGa-1b) and montmorillonite (STx-1b) to aqueous sulfuric acid and then rinsed the soluble reactants and products off in order to explore changes to functional groups on the mineral surface. To quantify the changes due to acid treatment of edge hydroxyl groups, we use 19 F magic angle spinning nuclear magnetic resonance spectroscopy and a probe molecule, 3,3,3-trifluoropropyldimethylchlorosilane. We find that the edge hydroxyl groups (OH) increase in both number and density with acid treatment. Chemical reactions in the atmosphere may be impacted by the increase in OH at the mineral edge.

Sihvonen, Sarah K.↗