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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 397 records · Page 22
All sourcesNASA NTRSDOE OSTI
DOE OSTI2020

In Situ Studies of Methanol Decomposition Over Cu(111) and Cu2O/Cu(111): Effects of Reactant Pressure, Surface Morphology, and Hot Spots of Active Sites

The dissociative adsorption of methanol was investigated on Cu(111) and ultrathin Cu 2 O films. We employed synchrotron-based Ambient Pressure X-ray Photoelectron Spectroscopy (AP-XPS) and Scanning Tunneling Microscopy (STM) to study the dynamics of gas–solid interactions, and calculations based on Density Functional Theory (DFT) were used to examine the reaction path. C 1s XPS spectra revealed that methanol underwent dissociative adsorption on plain Cu(111) to form methoxy (CH 3 O), formaldehyde (H 2 CO), and formate (HCOO) at a pressure range of 0.5–10 mTorr, with these species remaining on the surface after evacuation. This was accompanied by the appearance of a low coverage (~0.05 ML) of O ads in the O 1s which can be considered a highly active site for methanol activation. The high activity is apparent by a coverage of 0.8 ML of methoxy at room temperature. STM was unable to image these species at room temperature as they were highly mobile on metallic copper. In contrast, for CH 3 OH on Cu 2 O/Cu(111), STM showed clear hot spots for reaction and a complex array of adsorption structures. On the oxide substrate, there was decomposition of methanol to H 2 CO, CH 3 O, HCOO, and hydrocarbon species (CH x ) due to the subsequent interactions of methanol with lattice oxygen. Cu(111) remained entirely saturated with decomposition products under 10 mTorr of methanol (θ ≈ 0.97 ML), whereas the Cu 2 O overlayer was saturated at a much lower coverage (θ ≈ 0.30 ML). STM revealed rows and step edges of Cu 2 O decorated with decomposition products and metallic Cu islands ~5 nm in size. The difference in activity between Cu(111) and Cu 2 O/Cu(111) is attributed to the significant amount of O present on the oxide surface. Additionally, Density Functional theory (DFT) calculations described the XPS measurements well, showing a likely methanol dissociation to *CH 3 O and therefore a surface reduction. More importantly, the DFT results revealed that it was the chemisorbed oxygen on Cu 2 O/Cu(111) which oxidized the dissociated *CH 3 O to *HCOO and eventually CO 2 , while the reaction only involving upper oxygen on the Cu 2 O hexagonal ring led to the formation of H 2 CO.

36 MATERIALS SCIENCE↗
DOE OSTI2022

Effect of Gold Catalyst Surface Morphology on Wetting Behavior and Electrochemical CO 2 Reduction Performance in a Large-Area Zero-Gap Gas Diffusion Electrolyzer

We report catalyst surface area and wetting behavior are key factors in determining the performance of gas diffusion electrode (GDE) electrolyzers for electrochemical CO 2 reduction. In this work, we report the integration of sub-1 μm thick nanoporous gold (npAu) catalyst coatings into a large-area (25 cm 2 ) zero-gap electrolyzer. The npAu coatings were prepared by magnetron sputtering (MS) of thin AgAu alloy films on the microporous carbon layer of a gas diffusion layer (GDL) followed by Ag leaching. Compared to MS Au films of the same thickness, npAu catalyst coatings enable higher Faradaic efficiencies and improved catalyst stability for CO 2 -to-CO reduction with Faradaic efficiencies of up to 88% at 100 mA/cm 2 . For a 800 nm npAu coating, the device level energy efficiency for CO 2 to CO conversion reaches 45% (52% for CO + H 2 ) at 100 mA/cm 2 with a single pass CO 2 conversion efficiency of ~12%. Contact angle measurements reveal that npAu coatings provide a more hydrophobic electrode interface compared to MS Au coatings, suggesting that the more hydrophobic interfacial environment of npAu coatings helps mitigating electrode flooding which is associated with performance deterioration over time.

30 DIRECT ENERGY CONVERSION↗
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 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 OSTI2024

Phase Pattern Formation in Grain Boundaries

Here, in this Letter, we derive conditions that predict the existence of two-phase periodic-pattern grain boundary structures that are stable against coarsening. While previous research has established that elastic effects can lead to phase pattern formation on crystal surfaces, the possibility of stable grain boundary structures composed of alternating grain boundary phases has not been previously analyzed. Our theory identifies the specific combination of grain boundary and materials properties that enable the emergence of patterned grain boundary states and shows that the dislocation content of grain boundary phase junctions, absent in surface phenomena, weakens the stability of the patterned structures. The predictions of the theory are tested using a model copper grain boundary that exhibits multiple phases and two-phase pattern formation. We discuss how, similarly to surfaces, elastic effects associated with grain boundary phase junctions have profound implications for how grain boundary phases transform.

36 MATERIALS SCIENCE↗