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Results for “structure sensitivity”

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 19 records
All sourcesNASA NTRSDOE OSTI
DOE OSTI2024

Structure Sensitive Reaction Kinetics of Chiral Molecules on Intrinsically Chiral Surfaces

Enantiospecific heterogeneous catalysis utilizes chiral surfaces to resolve enantiomers via structure sensitive surface chemistry. The catalyst design challenge is the identification of chiral surface structures that maximize enantiospecificity. Herein, we develop data driven models for the enantiospecificity of tartaric acid reactions on chiral Cu(hkl) R&S surfaces. Measurements of enantiospecific rate constants were obtained by using curved Cu(hkl) R&S surfaces that enable kinetic measurements on hundreds of chiral surface orientations. One model uses feature vectors derived from generalized coordination numbers to capture the local structure around Cu atoms exposed by the Cu(hkl) R&S surfaces. The second model introduces the use of chiral cubic harmonic functions to capture the symmetry constraints of the face-centered cubic Cu structure. The model using 58 generalized coordination numbers has a fitting error similar to that of the model using only 5 cubic harmonic functions. The two models predict maxima in the enantiospecificity on surfaces with very similar surface orientations. The models developed in this work are applicable for any enantiospecific reaction happening on any chiral material with a cubic lattice structure, opening the way to understanding the surface structure sensitivity of the enantiospecific reaction kinetics.

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

Exploring Structure-Sensitive Relations for Small Species Adsorption Using Machine Learning

Accurate prediction of adsorption energies on heterogeneous catalyst surfaces is crucial to predicting reactivity and screening materials. Adsorption linear scaling relations have been developed extensively but often lack accuracy and apply to one adsorbate and a single binding site type at a time. These facts undermine their ability to predict structure sensitivity and optimal catalyst structure. Using machine learning on nearly 300 density functional theory calculations, we demonstrate that generalized coordination number scaling relations hold well for oxygen- and high-valency carbon-binding species but fail for others. Here we reveal that the valency and the electronic coupling of a species with the surface, along with the site type and its coordination environment, are critical for small species adsorption. The model simultaneously predicts the adsorption energy and preferred site and significantly outperforms linear scalings in accuracy. It can expose the structure sensitivity of chemical reactions and enable enhanced catalyst activity via engineering particle shape and facet defects. The generality of our methodology is validated by training the model with transition metal data and transferring it to predict adsorption energies on single-atom alloys.

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

Structure sensitivity in adsorbate-induced adatom formation on FCC transition-metal surfaces

Recent surface science discoveries reveal that adsorbates may induce in situ sub-nanometer cluster formation on transition-metal surfaces. To elucidate the structure sensitivity behind this phenomenon, we performed density functional theory calculations to construct an adatom formation energy database for eight fcc metals and 26 adsorbates commonly involved in catalytic reactions. We show that the adatom formation on (100) surfaces is generally easier than that on (111) surfaces. Many adsorbate/metal pairs exist, mostly on Pd, Ni, Rh, Pt, and Ir, for which adsorbates might induce adatom formation under near-ambient conditions on the (100) facet, but not on (111), highlighting the role of more open facets in adatom formation on metals intrinsically harder than Ag, Au, and Cu. Furthermore, our study offers a new perspective towards understanding structure sensitivity in heterogeneous thermal- and electro-catalytic systems such as methane steam reforming, Fischer-Tropsch synthesis, and ammonia decomposition.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
NASA NTRS1988

Recent developments in structural sensitivity analysis

Recent developments in two areas of structural sensitivity analysis are reviewed. These are the sensitivity of static and transient response, and the sensitivity of vibration and buckling eigenproblems. Recent developments from the point of view of computational cost, accuracy, and ease of implementation are considered.

Haftka, Raphael T.↗
DOE OSTI2021

Understanding the origin of structure sensitivity in hydrodechlorination of trichloroethylene on a palladium catalyst

Periodic density functional theory (DFT) calculations are employed to understand the origin of structure sensitivity in hydrodechlorination (HDC) of trichloroethylene (TCE) over different facets of a palladium catalyst. The HDC reaction is simulated on the terrace (Pd (111) and Pd (100)) and undercoordinated (Pd (211) and Pd (110)) sites of the Pd catalyst. The most stable binding configuration of TCE on the Pd surfaces is observed to be through the di-σ mode of binding, wherein each carbon atom of the TCE molecule is adsorbed atop of the Pd atom. On comparing TCE adsorption over different facets of Pd, a maximum binding energy of –178 kJ mol –1 is calculated over the Pd (110) surface. TCE, upon adsorption on Pd catalyst, undergoes dechlorination followed by hydrogenation of the hydrocarbon intermediates. The activation energies for C–Cl bond dissociation steps are significantly low when compared to the hydrogenation steps. The chlorine released from dechlorination tends to block the active sites, thereby poisoning the surface with high binding energies (B.E > –160 kJ mol –1 ) on all the surfaces. The trend in chlorine binding energies on Pd facets follows: Pd (110) > Pd (211) > Pd (100) > Pd (111). The removal of surface chlorine is facilitated by its reaction with surface hydrogen to form hydrogen chloride. The activation energy for hydrogen chloride formation is calculated to be 90 kJ mol –1 and 88 kJ mol –1 on Pd (111) and Pd (100) terrace sites, respectively and 109 kJ mol –1 and 118 kJ mol –1 on the step Pd (211) and corrugated Pd (110) facets, respectively. This suggests the ease of removal of Cl as HCl from the terrace sites as compared to the step and corrugated sites. The structure sensitivity in the TCE HDC reaction could possibly arise due to the differences in the energetics of Cl removal on different Pd facets. As a result, this mechanistic understanding could provide a rationale for designing suitable catalysts for the HDC of TCE.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2025

Structure Sensitivity and Catalyst Restructuring for CO2 Electro-reduction on Copper

Abstract Cu is the most promising metal catalyst for CO 2 electroreduction (CO 2 RR) to multi-carbon products, yet the structure sensitivity of the reaction and the stability versus restructuring of the catalyst surface under reaction conditions remain controversial. Here, atomic scale simulations of surface energies and reaction pathway kinetics supported by experimental evidence unveil that CO 2 RR does not take place on perfect planar Cu(111) and Cu(100) surfaces but rather on steps or kinks. These planar surfaces tend to restructure in reaction conditions to the active stepped surfaces, with the strong binding of CO on defective sites acting as a thermodynamic driving force. Notably, we identify that the square motifs adjacent to defects, not the defects themselves, as the active sites for CO 2 RR via synergistic effect. We evaluate these mechanisms against experiments of CO 2 RR on ultra-high vacuum-prepared ultraclean Cu surfaces, uncovering the crucial role of step-edge orientation in steering selectivity. Overall, our study refines the structural sensitivity of CO 2 RR on Cu at the atomic level, highlights the self-activation mechanism and elucidates the origin of in situ restructuring of Cu surfaces during the reaction.

Science & Technology - Other Topics↗
DOE OSTI2020

Structure sensitivity of n -butane hydrogenolysis on supported Ir catalysts

Hydrogenolysis of alkanes has been widely reported as structure sensitive reaction on transition metal heterogeneous catalysts with metal particle sizes ranging between 1 and 20 nm. In this work, a series of Ir/MgAl 2 O 4 and Ir/SiO 2 catalysts with different Ir particle sizes ranging from subnanometer clusters (<1 nm) to nanoparticles (1–3 nm) were prepared and tested for n-butane hydrogenolysis. Our results show that the activity towards n-butane hydrogenolysis increases as Ir particle size increases in the lower particle size range, goes through a maximum at ~1.4–1.6 nm and then drops with a further increase in particle size. In this work, the product distribution at low temperature (170–190 °C) is dominated by central and terminal CAC bond cleavage of n-butane, and less by two CAC bond cleavage or further hydrogenolysis of the propane and ethane products. The selectivity to central CAC bond cleavage is highly dependent on the size of Ir and increases with a decrease in particle size down to ~1.4 nm but remains constant with further decrease in size. The results show that an Ir size of ~1.4 nm is optimum for n-butane hydrogenolysis activity and selectivity towards ethane.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
NASA NTRS1984

Structural sensitivity analysis: Methods, applications and needs

Innovative techniques applicable to sensitivity analysis of discretized structural systems are reviewed. The techniques include a finite difference step size selection algorithm, a method for derivatives of iterative solutions, a Green's function technique for derivatives of transient response, simultaneous calculation of temperatures and their derivatives, derivatives with respect to shape, and derivatives of optimum designs with respect to problem parameters. Computerized implementations of sensitivity analysis and applications of sensitivity derivatives are also discussed. Some of the critical needs in the structural sensitivity area are indicated along with plans for dealing with some of those needs.

Adelman, H. M.↗
NASA NTRS1984

Structural sensitivity analysis: Methods, applications, and needs

Some innovative techniques applicable to sensitivity analysis of discretized structural systems are reviewed. These techniques include a finite-difference step-size selection algorithm, a method for derivatives of iterative solutions, a Green's function technique for derivatives of transient response, a simultaneous calculation of temperatures and their derivatives, derivatives with respect to shape, and derivatives of optimum designs with respect to problem parameters. Computerized implementations of sensitivity analysis and applications of sensitivity derivatives are also discussed. Finally, some of the critical needs in the structural sensitivity area are indicated along with Langley plans for dealing with some of these needs.

Adelman, H. M.↗
NASA NTRS1988

Recent developments in structural sensitivity analysis

Recent developments are reviewed in two major areas of structural sensitivity analysis: sensitivity of static and transient response; and sensitivity of vibration and buckling eigenproblems. Recent developments from the standpoint of computational cost, accuracy, and ease of implementation are presented. In the area of static response, current interest is focused on sensitivity to shape variation and sensitivity of nonlinear response. Two general approaches are used for computing sensitivities: differentiation of the continuum equations followed by discretization, and the reverse approach of discretization followed by differentiation. It is shown that the choice of methods has important accuracy and implementation implications. In the area of eigenproblem sensitivity, there is a great deal of interest and significant progress in sensitivity of problems with repeated eigenvalues. In addition to reviewing recent contributions in this area, the paper raises the issue of differentiability and continuity associated with the occurrence of repeated eigenvalues.

Haftka, Raphael T.↗
NASA NTRS1984

Static and dynamic structural-sensitivity derivative calculations in the finite-element-based Engineering Analysis Language (EAL) system

The implementation of static and dynamic structural-sensitivity derivative calculations in a general purpose, finite-element computer program denoted the Engineering Analysis Language (EAL) System is described. Derivatives are calculated with respect to structural parameters, specifically, member sectional properties including thicknesses, cross-sectional areas, and moments of inertia. Derivatives are obtained for displacements, stresses, vibration frequencies and mode shapes, and buckling loads and mode shapes. Three methods for calculating derivatives are implemented (analytical, semianalytical, and finite differences), and comparisons of computer time and accuracy are made. Results are presented for four examples: a swept wing, a box beam, a stiffened cylinder with a cutout, and a space radiometer-antenna truss.

Camarda, C. J.↗
DOE OSTI2025

Unraveling the Structural Sensitivity of Metal Catalysts in Ethylene Hydroformylation: Insights from Theory and Experiments

Here, in this study, we combined experimental and theoretical methods to investigate the structural sensitivity of metal catalysts in the ethylene hydroformylation reaction. Among Rh, Pt, Ir, Ni, Au, Ag, Pd, and Cu catalysts studied using experimental and theoretical methods, Rh showed the highest selectivity toward the C-C coupling product from CO and C 2 H 5 (i.e., C 2 H 5 CHO). The results from DFT and microkinetic simulations revealed that the activation energy barrier for C-C coupling is lowest on the Rh nanocluster, which explains the experimentally observed highest C 2 H 5 CHO selectivity on the Rh catalyst. Furthermore, DFT results demonstrated that the sites located on the flat surfaces of nanoparticles primarily promote the hydrogenation reaction, leading to the formation of undesired C 2 H 6 . In contrast, undercoordinated edge and corner sites of the nanocluster promote the C-C coupling reaction. Thus, our results illustrate that the selectivity toward C 3 oxygenates in ethylene hydroformylation reaction can be steered by tuning the size of Rh nanoparticles (the best-performing catalyst) to optimize the active (edge and corner) sites that preferentially promote the C-C coupling reaction.

58 GEOSCIENCES↗
DOE OSTI2021

On the structure sensitivity of and CO coverage effects on formic acid decomposition on Pd surfaces

Using density functional theory calculations, the Pd-catalyzed vapor-phase formic acid decomposition was studied, with a focus on the structure sensitivity and CO coverage effects. A comprehensive reaction network was developed on both the (111) and (100) facets of Pd, at CO coverages of 0 and 5/9 monolayer (ML). Pd(100) was determined to be more reactive than Pd(111) at both CO coverages. The introduction of 5/9 ML CO decreased the activity of both facets significantly, due to destabilization of the surface intermediates and transition states on the CO-decorated surfaces. Three reaction pathways were explored on the clean surfaces: the formate (HCOO) pathway, the carboxyl (COOH) pathway leading to the formation of CO 2 , and the COOH pathway leading to the formation of CO (COOH→CO). Based on the DFT-derived energetics alone, it appears that all three pathways contribute to the reaction on clean Pd, whereas the presence of 5/9 ML of CO inhibits the HCOO pathway on both facets and favors the COOH→CO pathway on the (111) facet, but the COOH→CO 2 one on the (100) facet. Moreover, at high CO coverages, alternative spectator CO-assisted adsorbate decomposition pathways were discovered, which could potentially play a role in formic acid decomposition on Pd catalysts under realistic reaction conditions.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
NASA NTRS1990

An easily implemented static condensation method for structural sensitivity analysis

A black-box approach to static condensation for sensitivity analysis is presented with illustrative examples of a cube and a car structure. The sensitivity of the structural response with respect to joint stiffness parameter is calculated using the direct method, forward-difference, and central-difference schemes. The efficiency of the various methods for identifying joint stiffness parameters from measured static deflections of these structures is compared. The results indicate that the use of static condensation can reduce computation times significantly and the black-box approach is only slightly less efficient than the standard implementation of static condensation. The ease of implementation of the black-box approach recommends it for use with general-purpose finite element codes that do not have a built-in facility for static condensation.

Gangadharan, S. N.↗
DOE OSTI2022

The role of coverage effects on the structure–sensitivity of formic acid electrooxidation on Pd surfaces

Direct formic acid (FA) fuel cells (DFAFCs) with Pd anode catalysts are promising for small portable applications. During the electrooxidation of formic acid (FAO), CO accumulates on the Pd electrocatalyst, thereby decreasing its performance. In this work, we use density functional theory (DFT; RPBE) to elucidate the significance of coverage effects on the structure–sensitivity of FAO over four low–index Pd surfaces: Pd(1 1 1), Pd(1 0 0), Pd(1 1 0), and Pd(2 1 1). We construct coverage–dependent Gibbs free energy diagrams and in agreement with experiment, predict that Pd(1 0 0) and Pd(2 1 1) are the most active surfaces. Additionally, we find that the presence of CO spectator species can alter the preferred electrooxidation pathway under different applied potentials. Furthermore, such pathway transitions appear to be critical for rationalizing experimental trends regarding electrocatalytic activity and are only captured when coverage effects are accounted for. Insights derived from this work may facilitate the design of improved FAO electrocatalysts.

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↗