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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 919 records · Page 51
All sourcesNASA NTRSDOE OSTI
DOE OSTI2024

Self-Inhibition Phenomena in Cu 3 Pt Oxidation by CO 2

Here, this study investigates the oxidation behavior of Cu 3 Pt(100) in CO 2 using a combination of ambient-pressure X-ray photoelectron spectroscopy, mass spectroscopy, and density functional theory modeling. Our in situ measurements reveal the simultaneous oxidation and reduction of Cu 2 O due to the opposing effects of atomic oxygen and CO generated from dissociative CO 2 adsorption, leading to a dynamic equilibrium state of simultaneously occurring redox reactions. Complementary atomistic calculations elucidate the inhibitory effects of subsurface Pt enrichment and the counteracting roles of CO 2 and CO in surface oxidation and reduction. These results provide mechanistic insights into the dissociative pathway of CO 2 molecules and dynamic evolution of surface composition and reactivity of Cu-based alloy catalysts in CO 2 -rich environments, with broader implications for tuning gas–surface reactions by manipulating gas reactants or solid surface composition.

36 MATERIALS SCIENCE↗
DOE OSTI2016

Complete wetting of Pt(111) by nanoscale liquid water films

The melting and wetting of nanoscale crystalline ice films on Pt(111) that are transiently heated above the melting point using nanosecond laser pulses are studied with infrared refection absorption spectroscopy (IRAS) and Kr temperature programmed desorption (TPD). The as-grown crystalline ice films consist of isolated nanoscale ice crystallites embedded in a hydrophobic water monolayer. Upon heating above the melting point, these ice crystallites rapidly melt to form nanoscale droplets of liquid water. Rapid cooling of the system to cryogenic temperatures after each laser pulse quenches the water films and allows them to be interrogated with IRAS, Kr TPD and other ultrahigh vacuum surface science techniques. With each successive heat pulse, these liquid drops spread across the surface until it is entirely covered with multilayer water films after several pulses. These results, which show that nanoscale water films completely wet Pt(111), are in contrast to molecular dynamics simulations predicting partial wetting of nanoscale water drops on a hydrophobic water monolayer. The results provide valuable new insights into the wetting characteristics of nanoscale water films on a clean, well-characterized single crystal surface.

Xu, Yuntao↗
DOE OSTI2024

Thermal Stability of Au Rhombic Dodecahedral Nanocrystals Can Be Greatly Enhanced by Coating Their Surface with an Ultrathin Shell of Pt

Rhombic dodecahedral nanocrystals have been considered particularly difficult to synthesize because they are enclosed by {110}, a low-index facet with the greatest surface energy. Recently, we demonstrated the use of seed-mediated growth for the facile and robust synthesis of Au rhombic dodecahedral nanocrystals (AuRD). While the unique shape and surface structure of AuRD are desirable for potential applications in plasmonics and catalysis, respectively, their high surface energy makes them highly susceptible to thermal degradation. Here we demonstrate that it is feasible to greatly improve the thermal stability with some sacrifice to the plasmonic properties of the original AuRD by coating their surface with an ultrathin shell made of Pt. Our in situ electron microscopy analysis indicates that the ultrathin Pt coating can increase the thermal stability from 60 up to 450 °C, a trend that is also supported by the results from a computational study.

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

Spontaneous Formation of 1D Pattern in Monolayer VSe 2 with Dispersive Adsorption of Pt Atoms for HER Catalysis

Creation of functional patterns in two-dimensional (2D) materials provides opportunities to extend their potential for applications. Transition-metal dichalcogenides (TMDCs) are suitable 2D materials for pattern generation because of properties including alterable polymorphic phases, easy chalcogen-vacancy formation, metal-atom insertion, and alloying. Such patterning can be used for selective functionalization. Here we report the spontaneous formation of long-range, well-ordered 1D patterns in monolayer vanadium diselenide (VSe 2 ) by a single annealing stage during growth. Atomic-resolution images in real space combined with density-functional-theory (DFT) calculations reveal the 1D features of patterned VSe 2 . Further experimental characterization of the intermediate states in the growth process confirm the spontaneous formation of the 1D pattern by annealing-induced Se-deficient linear defects. The 1D pattern can be reversibly transformed to homogenous VSe 2 monolayer by reintroducing Se atoms. Moreover, additional experiments demonstrate that a dispersive deposition of Pt atoms along the 1D structures of patterned VSe 2 is achieved, while DFT calculations find that their catalytic activity for hydrogen evolution reaction (HER) is as good as that of Pt surfaces. Further, the formation of long-range, well-ordered 1D patterns not only demonstrates an effective way of dimension modulation in 2D materials but also enriches the potential of intrinsically patterned 2D materials for promising catalytic activities.

Chemical structure↗
DOE OSTI2016

Insight into nanoparticle charging mechanism in nonpolar solvents to control the formation of Pt nanoparticle monolayers by electrophoretic deposition

We report on the formation of Pt nanoparticle monolayers by electrophoretic deposition from nonpolar solvents. First, the growth kinetics of Pt nanoparticles prepared by the reverse micelle technique are described in detail. Second, a model of nanoparticle charging in nonpolar media is discussed and methods to control the nanoparticle charging are proposed. Lastly, essential parameters of the electrophoretic deposition process to control the deposition of nanoparticle monolayers are discussed and mechanisms of their formation are analyzed.

77 NANOSCIENCE AND NANOTECHNOLOGY↗
DOE OSTI2019

Dictating Pt-Based Electrocatalyst Performance in Polymer Electrolyte Fuel Cells, from Formulation to Application

In this study, in situ electrochemical diagnostics designed to probe ionomer interactions with platinum and carbon were applied to relate ionomer coverage and conformation, gleaned from anion adsorption data, with O 2 transport resistance for low-loaded (0.05 mg Pt cm -2 ) platinum-supported Vulcan carbon (Pt/Vu)-based electrodes in a polymer electrolyte fuel cell. Coupling the in situ diagnostic data with ex situ characterization of catalyst inks and electrode structures, the effect of ink composition is explained by both ink-level interactions that dictate the electrode microstructure during fabrication and the resulting local ionomer distribution near catalyst sites. Electrochemical techniques (CO displacement and ac impedance) show that catalyst inks with higher water content increase ionomer (sulfonate) interactions with Pt sites without significantly affecting ionomer coverage on the carbon support. Surprisingly, the higher anion adsorption is shown to have a minor impact on specific activity, while exhibiting a complex relationship with oxygen transport. Ex situ characterization of ionomer suspensions and catalyst/ionomer inks indicates that the lower ionomer coverage can be correlated with the formation of large ionomer aggregates and weaker ionomer/catalyst interactions in low-water content inks. These larger ionomer aggregates resulted in increased local oxygen transport resistance, namely, through the ionomer film, and reduced performance at high current density. In the water-rich inks, the ionomer aggregate size decreases, while stronger ionomer/Pt interactions are observed. The reduced ionomer aggregation improves transport resistance through the ionomer film, while the increased adsorption leads to the emergence of resistance at the ionomer/Pt interface. Overall, the high current density performance is shown to be a nonmonotonic function of ink water content, scaling with the local gas (H 2 , O 2 ) transport resistance resulting from pore, thin film, and interfacial phenomena.

30 DIRECT ENERGY CONVERSION↗
DOE OSTI2011

Reactivity of Chemisorbed Oxygen Atoms and Their Catalytic Consequences during CH 4 –O 2 Catalysis on Supported Pt Clusters

Kinetic and isotopic data and density functional theory treatments provide evidence for the elementary steps and the active site requirements involved in the four distinct kinetic regimes observed during CH4 oxidation reactions using O2, H2O, or CO2 as oxidants on Pt clusters. These four regimes exhibit distinct rate equations because of the involvement of different kinetically relevant steps, predominant adsorbed species, and rate and equilibrium constants for different elementary steps. Transitions among regimes occur as chemisorbed oxygen (O*) coverages change on Pt clusters. O* coverages are given, in turn, by a virtual O2 pressure, which represents the pressure that would give the prevalent steady-state O* coverages if their adsorption–desorption equilibrium was maintained. The virtual O2 pressure acts as a surrogate for oxygen chemical potentials at catalytic surfaces and reflects the kinetic coupling between C–H and O=O activation steps. O* coverages and virtual pressures depend on O2 pressure when O2 activation is equilibrated and on O2/CH4 ratios when this step becomes irreversible as a result of fast scavenging of O* by CH4-derived intermediates. In three of these kinetic regimes, C–H bond activation is the sole kinetically relevant step, but occurs on different active sites, which evolve from oxygen–oxygen (O*–O*), to oxygen–oxygen vacancy (O*–*), and to vacancy–vacancy (*–*) site pairs as O* coverages decrease.

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

CO Oxidation Mechanisms on CoO x -Pt Thin Films

The reaction of CO and O 2 with submonolayer and multilayer CoO x films on Pt(111), to produce CO 2 , was investigated at room temperature in the mTorr pressure regime. Here, using operando ambient pressure X-ray photoelectron spectroscopy and high pressure scanning tunneling microscopy, as well as density functional theory calculations, we found that the presence of oxygen vacancies in partially oxidized CoO x films significantly enhances the CO oxidation activity to form CO 2 upon exposure to mTorr pressures of CO at room temperature. In contrast, CoO films without O-vacancies are much less active for CO 2 formation at RT, and CO only adsorbed in the form of carbonate species that are stable up to 260 °C. On submonolayer CoO x islands, the carbonates form preferentially at island edges, deactivating the edge sites for CO 2 formation, even while the reaction proceeds inside the islands. These results provide a detailed understanding of CO oxidation pathways on systems where noble metals such as Pt interact with reducible oxides.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2015

Identical Location Transmission Electron Microscopy Imaging of Site-Selective Pt Nanocatalysts: Electrochemical Activation and Surface Disordering

In this work we have employed identical location transmission electron microscopy (IL-TEM) to study changes in the shape and morphology of faceted Pt nanoparticles as a result of electrochemical cycling; a procedure typically employed for activating platinum surfaces. We find that the shape and morphology of the as-prepared hexagonal nanoparticles are rapidly degraded as a result of potential cycling up to +1.3 V. As few as 25 potential cycles are sufficient to cause significant degradation, and after about 500–1000 cycles the particles are dramatically degraded. We also see clear evidence of particle migration during potential cycling. These finding suggest that great care must be exercised in the use and study of shaped Pt nanoparticles (and related systems) as electrocatlysts, especially for the oxygen reduction reaction where high positive potentials are typically employed.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2016

Growth and Structure of the First Layers of Ice on Ru(0001) and Pt(111)

Scanning tunneling microscopy was used to probe the structure and growth of the first few layers of water on Ru(0001) and Pt(111) at the molecular level. The surface-bound first layer is composed of a mixture of water molecules forming hexagonal structures, both in registry and out-of-registry with the substrate atoms. The hexagons are connected by pentagonal and heptagonal units. At temperatures below 140 K, this layer structure gives rise to the growth of metastable amorphous structures in the second and higher layers. We found that in the transition from amorphous to crystalline ice the structure of the original bottom layer changes to one in perfect local registry with the hexagonal surfaces of Ru(0001) and Pt(111). We further discovered structural defects in the form of extended one-dimensional lines of five- and eight-membered rings that are domain boundaries and stacking faults in the growing ice layers, which lead to the formation of metastable cubic ice.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2017

Finite Size Effects in Submonolayer Catalysts Investigated by CO Electrosorption on Pt sML /Pd(100)

A combination of scanning tunneling microscopy, subtractively normalized interfacial Fourier transform infrared spectroscopy (SNIFTIRS), and density functional theory (DFT) is used to quantify the local strain in 2D Pt clusters on the 100 facet of Pd and its effect on CO chemisorption. Good agreement between SNIFTIRS experiments and DFT simulations provide strong evidence that, in the absence of coherent strain between Pt and Pd, finite size effects introduce local compressive strain, which alters the chemisorption properties of the surface. Though this effect has been widely neglected in prior studies, our results suggest that accurate control over cluster sizes in submonolayer catalyst systems can be an effective approach to fine-tune their catalytic properties.

Yuan, Qiuyi↗
DOE OSTI2010

Reactions of NO2 with Ba(OH)2 on Pt(111)

The interaction of NO2 with amorphous and crystalline Ba(OH)2 supported on Pt(111) was studied in the wide pressure range of 1.0 × 10-9 to 1.0 × 10-4 Torr and compared to that with a thick (> 20 monolayer equivalent (MLE)) BaO film using infrared reflection absorption spectroscopy (IRAS), temperature programmed desorption (TPD), and X-ray photoelectron spectroscopy (XPS). The amorphous and crystalline Ba(OH)2 layers were prepared by exposing a thick BaO (> 20 MLE) layer on Pt(111) to H2O at 300 and 425 K, respectively. The amorphous and crystalline Ba(OH)2 layers partially convert to Ba(NOx)2 (nitrites and nitrates) following their exposure to elevated NO2 pressure (~1.0 × 10-4 Torr) at 300 K. The exposure of the crystalline-Ba(OH)2/Pt(111) system to NO2 at 425 K, however, leads to the desorption of H2O and the complete conversion of the crystalline Ba(OH)2 layer to Ba(NOx)2, which consists of mainly crystalline nitrates and a small amount of nitrites. The amounts of NOx stored by BaO (> 20 MLE)/Pt(111) and crystalline Ba(OH)2/Pt(111) systems upon their exposure to NO2 at 425 K are comparable. The thus-formed bulk crystalline Ba(NO3)2 phase decomposes in two steps, both releasing NO and O2, in accord with the melting/decomposition scheme for bulk Ba(NO3)2.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2011

Tip-Dependent Scanning Tunneling Microscopy Imaging of Ultrathin FeO Films on Pt(111)

High-resolution scanning tunneling microscope (STM) images of moire-structured FeO films on Pt(111) were obtained in a number of different tip-dependent imaging modes. For the first time, the STM images are distinguished and interpreted unambiguously with the help of distinct oxygen- vacancy dislocation loops in the FeO moire structure. The experimental STM results are compared with the results of electronic structure calculations within the DFTþUscheme for a realistic (√91 x √ 91)R5.2º moire FeO unit cell supported on Pt(111) as well as with the results from previous studies. We find that one type of STM imaging mode, showing both Fe and O atoms, agrees well with simulated STM images, indicating that the simple Tersoff-Hamann theory is partially valid for this imaging mode. In addition, we identify other distinct, element-specific imaging modes which reveal a strong dependence on the specific tip apex state and likely result from specific tip-sample chemical interactions. From the present STMresults we show that several of the previously published conclusions for the FeO system have to be revisited.

Environmental Molecular Sciences Laboratory↗
DOE OSTI2011

Growth of Ordered Ultrathin Tungsten Oxide Films on Pt(111)

Ordered tungsten oxide ultra-thin films were prepared on a Pt(111) substrate at 700 K via direct sublimation of monodispersed cyclic (WO3)3 trimers. The surface composition, structure and morphology were determined using a combination of atomically resolved imaging, ensemble-averaged surface-sensitive spectroscopies, and density functional theory (DFT). We find that half of the (WO3)3 tungsten atoms get partially reduced to the (5+) oxidation state in the first layer at the Pt(111) interface. The opening of the (WO3)3 ring leads to the formation of a tungsten oxide layer with a zig-zag chain structure with a c(4×2) periodicity. In the second layer, the (WO3)3 clusters remain intact and form an ordered (3×3) array of molecularly-bound (WO3)3. DFT calculations provide a detailed understanding of the structure, oxidation states, and the vibrational frequencies for both the c(4×2) and (3×3) overlayers.

(WO3)(3) CLUSTERS↗
DOE OSTI2010

IN-CLOUD TURBULENCE STRUCTURE OF MARINE STRATOCUMULUS AT PT. REYES, CA, DURING MASRAD

This study quantifies the level of turbulence inside the marine stratocumulus cloud deck over Pt. Reyes, CA, during the Marine Stratus Radiation, Aerosol, and Drizzle Experiment (MASRAD) in July 2005, and identifies the dominant sources of turbulent kinetic energy. We used vertical velocity data from a 3 mm wavelength (94-GHz) vertically pointing Doppler radar in combination with collocated radiosonde data. The results show that the stratocumulus observed at Pt. Reyes behaves differently from that expected on the basis of previous studies due to the modified marine environment that exists there. In particular, we found a decrease of turbulence levels with height within the cloud both during day and during night. The analysis highlights that for the conditions of our study longwave radiative cooling at cloud top was compensated by a number of mechanisms, resulting in the observed profiles. The production of turbulent kinetic energy is dominantly driven by wind shear.

54 ENVIRONMENTAL SCIENCES↗
DOE OSTI2019

Spinon Confinement and a Sharp Longitudinal Mode in Yb 2 Pt 2 Pb in Magnetic Field

Here, the fundamental excitations in an antiferromagnetic chain of spins-1/2 are spinons, de-confined fractional quasiparticles that when combined in pairs, form a triplet excitation continuum. In an Ising-like spin chain the continuum is gapped and the ground state is Neel ordered. Here, we report high resolution neutron scattering experiments, which reveal how a magnetic field closes this gap and drives the spin chains in Yb 2 Pt 2 Pb to a critical, disordered Luttinger-liquid state. In Yb 2 Pt 2 Pb the effective spins-1/2 describe the dynamics of large, Ising-like Yb magnetic moments, ensuring that the measured excitations are exclusively longitudinal, which we find to be well described by time-dependent density matrix renormalization group calculations. The inter-chain coupling leads to the confinement of spinons, a condensed matter analog of quark confinement in quantum chromodynamics. Insensitive to transverse fluctuations, our measurements show how a gapless, dispersive longitudinal mode arises from confinement and evolves with magnetic order.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2025

Anisotropic surface potentials induced by competitive ion adsorption enable the synthesis of branched cubic Pt mesocrystals

Creation of complex nanostructured materials through oriented attachment (OA) requires the manipulation of interparticle forces, including electrostatic repulsion, which depends strongly on surface potentials and can be modified through the effect of solution environment on interfacial chemistry. Here we show that time-dependent anisotropies in surface potential driven by competitive ion adsorption can alter facet-selectivity during OA. This phenomenon enables the synthesis of branched cubic Pt mesocrystals. Initially, Pt nanoparticles attach preferentially at their {100} facets to form a well-defined cubic core. Over time, changes in ion adsorption shift the attachment preference to the {111} facets, promoting branch formation. In both stages, anisotropic surface potentials generate electrostatic torques that align the particles prior to attachment. These findings demonstrate a generalizable strategy for directing the architecture of nanomaterials through time-resolved control of interfacial chemistry during OA, offering new pathways for the design of complex mesoscale structures.

Bae, Yuna [Pacific Northwest National Laboratory (↗
DOE OSTI2014

Exceptional activity of sub-nm Pt clusters on CdS for photocatalytic hydrogen production: A combined of experimental and first-principles study

In this work we have explored a new concept of substantially increasing photocatalytic activity for H₂ production of conventional semiconductors by modifying them with sub-nm Pt particles. By combining both experimental and theoretical approaches, we have also developed new mechanistic insights into the 17 times increase in photocatalytic activity of Pt modified CdS catalysts.

29 ENERGY PLANNING, POLICY, AND ECONOMY↗