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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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All sourcesNASA NTRSDOE OSTI
DOE OSTI2022

Comment on “STM study of the (111) and (100) surfaces of PdAg, Surf. Sci. 417 (1998) 292–300" and references therein

Investigation of isolated, single atoms on the surfaces of alloys is of great interest in the field of heterogeneous catalysis. Prior studies by Nieuwenhuys, etal. [Surf. Sci. 417 (1998) 292–300 and references therein] showed that annealing the (111) surface of a PdAg bulk alloy yielded a surface with ~5% Pd, predominantly in the form of single atoms, rendering it an ideal model for investigating the reactivity of single Pd atoms. Herein, new evidence is reported that establishes the bulk composition of the PdAg(111) alloy studied previously is 67 % Ag and 33 % Pd, whereas it was reported to be 33 % Ag and 67 % Pd in the prior studies. Furthermore, the bulk composition of a crystal derived from the same PdAg alloy boule used previously was determined using energy-dispersive X-ray spectroscopy (EDS)—a bulk sensitive technique. Surface-sensitive measurements, including X-ray photoelectron spectroscopy, were also performed and are in good agreement with the prior studies by Nieuwenhuys, etal.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Methane Activation on Metal-Doped (111) and (100) Ceria Surfaces with Charge-Compensating Oxygen Vacancies

Reducing the temperature for methane activation is an important objective that would benefit many technological applications. In this work we explore the possibility to achieve this goal using single atom catalysts (SACs) on ceria surfaces. In particular, we focus on Ag SACs, which have recently been suggested to be promising catalysts for both the oxidation of molecular hydrogen and CH 4 activation. The rate limiting step of methane activation on ceria is the breaking of the first C-H bond. Using first principles calculations, we investigate this reaction on CeO 2 (111) and (100) surfaces, which are both frequently exposed on ceria nanoparticles. For undoped ceria, our results show that the dissociation of CH 4 to CH 3 + H has a lower barrier on the (001) surface than on the (111) one. On both surfaces, the presence of Ag enhances the binding of CH 4 to the surface and reduces the activation energy for its dissociation. Interestingly, the formation of methanol via the Mars-van Krevelen mechanism has a slightly lower energy barrier than the dissociation of CH 4 to CH 3 + H on Ag-doped CeO 2 (111), suggesting that methanol is a likely product of methane activation on this surface. Here, a novel aspect of our work relative to previous studies on CH 4 activation over transition metal doped ceria is that we mainly focus on stable surface structures where each Ag dopant substituting a Ce atom forms a complex with a surface oxygen vacancy (VO). Our results show that these Ag-VO complexes play a critical role and accounting for their presence is essential for a proper description of the surface reactivity.

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

Size-Dependent Energy and Adhesion of Pd Nanoparticles on Graphene on Ni(111) by Pd Vapor Adsorption Calorimetry

Carbon-supported late transition-metal nanoparticles are promising catalysts and electrocatalysts for wide-ranging applications. However, experimental investigations of the bonding energetics of metal nanoparticles on carbon supports are very limited. Here, in this work, we report heats of adsorption of Pd vapor deposited onto single-layer graphene(0001) supported on Ni(111) at 100 and 300 K as Pd grows particles of well-defined size in the range from three atom clusters to 6 nm diameter. Sizes were determined from He + low-energy ion scattering (LEIS). In this size range, the differential heat of Pd adsorption increases from 228 kJ/mol to within 10 kJ/mol of the heat of sublimation of bulk Pd (377 kJ/mol). The chemical potential of metal atoms in these nanoparticles as a function of average particle size was determined from these results. The adhesion energy at the Pd/graphene(0001)/Ni(111) interface was extracted from these data and found to be 3.5 J/m 2 for the largest Pd particles. For the three metal elements that have now been studied (Pd, Ni, and Ag), we found an increase in metal/graphene(0001)/Ni(111) adhesion energy with metal carbophilicity, which we defined here as the heat of C atom adsorption on that metal’s (111) surface estimated from published density functional theory calculations.

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

Achieving Ultra-High Selectivity to Hydrogen Production from Formic Acid on Pd–Ag Alloys

Palladium-silver based alloy catalysts have a great potential for CO-free hydrogen production from formic acid for fuel cell applications. However, the structural factors affecting the selectivity of formic acid decomposition is still debated. Herein, the decomposition pathways of formic acid on Pd-Ag alloys with different atomic configurations have been investigated to identify the alloy structures yielding high H 2 selectively. Several Pd x Ag 1-x surface alloys with various compositions were generated on a Pd(111) single crystal; their atomic distribution and electronic structure were determined by a combination of infrared reflection absorption spectroscopy (IRAS), X-ray photoelectron spectroscopy (XPS), and density functional theory (DFT). It was established that the Ag atoms with Pd neighbors are electronically altered, and the degree of alteration correlates with the number of nearest Pd. Temperature programmed reaction spectroscopy (TPRS) and DFT demonstrated that the electronically altered Ag domains create a new reaction pathway that selectively dehydrogenates formic acid. In contrast, Pd monomers surrounded by Ag are demonstrated to have a similar reactivity compared to pristine Pd(111), yielding CO and H 2 O in addition to the dehydrogenation products. Furthermore, they bind to the produced CO weaker than pristine Pd, demonstrating an enhancement in resistance to CO poisoning. This work therefore shows that surface Ag domains modified by interaction with subsurface Pd are the key active sites for selective decomposition of formic acid, while surface Pd atoms are detrimental for selectivity. Hence the decomposition pathways can be tailored for CO-free H 2 production on Pd-Ag alloy systems.

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

The effect of elastic strains on the adsorption energy of H, O, and OH in transition metals

The influence of elastic strains on the adsorption of H, O, and OH on the (111) surfaces of 8 fcc (Ni, Cu, Pd, Ag, Pt, Au, Rh, Ir) and on the (0001) surfaces of 3 hcp (Co, Zn, Cd) transition metals was analyzed by means of density functional theory calculations. To this end, surface slabs were subjected to different strain states (uniaxial, biaxial, shear, and a combination of them) up to strains dictated by the mechanical stability limits indicated by phonon calculations. It was found that the adsorption energy followed the predictions of the d-band theory but – surprisingly – the variations in the adsorption energy only depended on the area of the adsorption hole and not on the particular elastic strain tensor applied to achieve this area. The analysis of the electronic structure showed that the applied strains did not modify the shape of the Projected Density of States (PDOS) of the d-orbitals of the transition metals but only led to a shift in the energy levels. Moreover, the presence of the adsorbates on the surfaces led to negligible changes in the PDOS. Thus, the adsorption energies were a function of the Fermi energy which in turn was associated with the change of the area of the adsorption through a general linear law that was valid for all metals. Furthermore, the information in this paper allows the immediate and accurate estimation of the effect of any elastic strain on the adsorption energies of H, O, and OH in 11 transition metals with more than half-filled d-orbitals.

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

Tandem Electrocatalytic CO 2 Reduction with Efficient Intermediate Conversion over Pyramid-Textured Cu–Ag Catalysts

If combined with renewably generated electricity, electrochemical CO 2 reduction (E-CO 2 R) could be used as a sustainable source of chemicals and fuels. Tandem catalysis approaches are attractive for providing the product selectivity, which would be required for commercial applications. Here, we demonstrate a two-step tandem electrocatalytic E-CO 2 R with efficient conversion of the intermediate species. The catalyst scaffold is Si(100), which is etched to form a textured surface consisting of micron-sized pyramid structures with the {111} facets. Two metals are used in the electrocatalytic cascade: Ag is employed to perform a two-electron reduction of CO 2 to the intermediate CO, and Cu performs conversion to more reduced products. Using high-angle physical vapor deposition, we form separated, micron-scale areas of the two electrocatalysts on opposite sides of the pyramids, with their relative surface coverages being tunable with the deposition angle. Compared to the textured scaffolds with blanket Ag and Cu used as controls, bimetallic pyramid tandem catalysts have higher current densities and much lower faradic efficiencies (FE) for CO. These effects are due to efficient conversion of the CO formed on Ag to more reduced products on Cu. Methane is the main product to be enhanced by the cascade pathway: a bimetallic catalyst with approximately equal coverages of Ag and Cu produces methane with a FE of 62% at -1.1 V RHE , corresponding to a partial current density of 12.7 mA cm -2 . We estimate an intermediate conversion yield for the CO intermediate of 80-90%, which is close to the mass-transport limited value predicted by reaction-diffusion simulations.

36 MATERIALS SCIENCE↗
DOE OSTI2025

Observation of nanoparticle coalescence during core-shell metallic nanowire growth in colloids via nanoscale imaging

The surface morphology and shape of crystalline nanowires significantly influence their functional properties, including phonon transport, electrocatalytic performance, to name but a few. However, the kinetic pathways driving these morphological changes remain underexplored due to challenges in real-space and real-time imaging at single-particle and atomic resolutions. This study investigates the dynamics of shell (Au, Pd, Pt, Fe, Cu, Ni) deposition on AuAg alloy seed nanowires during core-shell formation. By using chiral/non-chiral seed nanowires, advanced imaging techniques, including liquid-phase transmission electron microscopy (LPTEM), cryogenic TEM, and three-dimensional electron tomography, a three-step deposition process is revealed: heterogeneous nucleation, nanoparticle attachment, and coalescence. It is found that colloidal Ostwald ripening, metal reactivity, and deposition amount modulate nanoparticle size and surface roughness, shaping final morphologies. Noble metal nanoparticles (Au, Ag, Pd, Pt) coalesce with seed nanowire along the 〈111〉 direction, distinct from that of other metals. These findings are consistent across different metals, including Ru, Cu, Fe, and Ni, highlighting the hypothesis of these processes in nanowire formation. These findings enhance traditional crystallographic theories and provide a framework for designing nanowire morphology. Additionally, our imaging techniques may be applied to investigate phenomena like electrodeposition, dendrite growth in batteries, and membrane deformation.

Yang, Dahai↗
DOE OSTI2026

Direct Observation of Hydroxyls Formed from Water and Oxygen on Ag(100)

The interaction of oxygen with silver is a key descriptor of the catalytic reactivity of silver nanoparticles which are ubiquitous in large-scale partial oxidation reactions like ethylene epoxidation. Despite Ag(100) being proposed as the most selective facet, it is less studied than (111) and (110) surfaces. Using scanning tunneling microscopy and synchrotron X-ray photoelectron spectroscopy, we report that, in addition to the well-known O adatoms formed from O 2 dissociation on Ag(100), hydroxyl groups (OH), at a binding energy of ∼ 531 eV, are also present. The O/OH ratio depends on exposure to water and surface temperature. These assignments are consistent with our density functional theory calculations, which indicate that the formation of two OH groups from an O atom and H 2 O molecule is exothermic. These results indicate that, in addition to O, OH is present even under ultrahigh vacuum conditions and therefore should be considered in proposed catalytic pathways.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Dependency of solvation effects on metal identity in surface reactions

Solvent interactions with adsorbed moieties involved in surface reactions are often believed to be similar for different metal surfaces. However, solvents alter the electronic structures of surface atoms, which in turn affects their interaction with adsorbed moieties. To reveal the importance of metal identity on aqueous solvent effects in heterogeneous catalysis, we studied solvent effects on the activation free energies of the O–H and C–H bond cleavages of ethylene glycol over the (111) facet of six transition metals (Ni, Pd, Pt, Cu, Ag, Au) using an explicit solvation approach based on a hybrid quantum mechanical/molecular mechanical (QM/MM) description of the potential energy surface. A significant metal dependence on aqueous solvation effects was observed that suggests solvation effects must be studied in detail for every reaction system. The main reason for this dependence could be traced back to a different amount of charge-transfer between the adsorbed moieties and metals in the reactant and transition states for the different metal surfaces.

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↗
DOE OSTI2020

Formic Acid: A Hydrogen-Bonding Cocatalyst for Formate Decomposition

Hydrogen bonding accelerates many catalytic reactions by orienting intermediates, stabilizing transition states, and even opening reaction pathways. However, most mechanistic studies regarding the decomposition of formic acid (FA), a promising hydrogen storage material, neglect hydrogen-bonding interactions even though FA is a strong hydrogen-bond donor and acceptor. Here, we probe the formation of bimolecular hydrogen-bonded complexes between FA and formate (FA–HCOO complexes) adsorbed on metal surfaces and how these complexes affect HCOO* decomposition. Using first-principles density functional theory (DFT) calculations on 12 close-packed (111)/(0001) and 8 open (100) surfaces of 12 transition metals—Ag, Au, Co, Cu, Ir, Ni, Os, Re, Pd, Pt, Rh, and Ru, we—show that FA–HCOO complexes are generally thermodynamically stable, even at elevated temperatures and pressures. We then illustrate that these complexes produce infrared spectroscopic signatures consistent with as yet unassigned experimental peaks. We last demonstrate that by stabilizing the dangling bond of monodentate HCOO*, these complexes significantly lower the barriers for rotation of HCOO* from a bidentate to a monodentate configuration, the rate-limiting step for HCOO* decomposition on many surfaces. FA thus acts as a cocatalyst for HCOO* decomposition. Our results may guide the community toward improved catalysts for reactions involving HCOO* such as FA decomposition, methanol steam reforming, and the water gas shift reaction. More broadly, our work highlights the ability of hydrogen bonding to modify the adsorbed structures of intermediates and lower the barriers for their reaction on heterogeneous catalysts. Lastly, this phenomenon can be relevant for other reactions involving ammonia, alcohols, and carboxylic acids.

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

Snowflake relocated Cu 2 O electrocatalyst on an Ag backbone template for the production of liquid C 2+ chemicals from CO 2

In this study, we performed the CO 2 reduction reaction (CO 2 RR) using a structural composite catalyst of cuprous oxide (Cu 2 O) and silver (Ag) that was simultaneously electrodeposited. While the underneath Ag electrodeposits maintained their spiky backbone structures even after the CO 2 RR, the Cu 2 O deposits were reduced to Cu(111) and relocated on the backbone template. Here, the structural changes in Cu 2 O to Cu increase the active area of the Cu–Ag interface, resulting in a remarkable production rate of 125.01 μmol h -1 of liquid C 2+ chemicals via the stabilization of the C–C coupling of the key intermediate species of acetaldehyde. This study provides new insights into designing a bimetallic catalyst for producing sustainable C 2+ products from CO 2 without any selectivity towards the production of methane.

36 MATERIALS SCIENCE↗
DOE OSTI2023

Silver-decorated palladium on carbon catalyst for enhanced ammonium formate dehydrogenation

Palladium (Pd)-based catalysts efficiently convert ammonium formate solution to hydrogen at low temperatures (<100 °C), but they tend to deactivate quickly during stability testing. This manuscript presents a systematic investigation into the catalytic properties of Pd–Ag bimetallic catalysts, focusing on their surface compositions and exploring the mechanisms behind the deactivation of Pd/Ag-based catalysts. Here this study reports a carbon-supported Pd–Ag bimetallic nanoparticle (NP) catalyst obtained through a galvanic replacement method, which showed enhanced formate dehydrogenation performance. The best catalyst, Pd 3 Ag 10 /ACA-G (Pd–Ag bimetallic NPs with a 3 : 10 mass ratio loaded on acid-washed activated carbon, prepared by the galvanic replacement method), presents the highest activity with a TOF of 5202 h -1 (~2.6-fold of commercial Pd/C). The enhanced electron density of Pd–Ag bimetallic nanoparticles, coupled with the advantages of a smaller nanoparticle size, and the modulation of hydrogen adsorption energy through the Ag/Pd surface alloy on the Ag/Pd(111) facet, collectively resulted in experimentally higher turnover rates of hydrogen production. The changes on the catalyst surface, including surface Ag fraction decrease, NP size growth, and O-containing species (carboxylate, etc.) adsorption, gradually resulted in catalyst deactivation.

08 HYDROGEN↗
DOE OSTI2022

Grain‐Boundary‐Rich Noble Metal Nanoparticle Assemblies: Synthesis, Characterization, and Reactivity

Abstract Here, a comprehensive study on the synthesis, characterization, and reactivity of grain‐boundary (GB)‐rich noble metal nanoparticle (NP) assemblies is presented. A facile and scalable synthesis of Pt, Pd, Au, Ag, and Rh NP assemblies is developed, in which NPs are predominantly connected via Σ3 (111) twin GBs, forming a network. Driven by water electrolysis, the random collisions and oriented attachment of colloidal NPs in solution lead to the formation of Σ3 (111) twin boundaries and some highly mismatched GBs. This synthetic method also provides convenient control over the GB density without altering the crystallite size or GB type by varying the NP collision frequency. The structural characterization reveals the presence of localized tensile strain at the GB sites. The ultrahigh activity of GB‐rich Pt NP assembly toward catalytic hydrogen oxidation in air is demonstrated, enabling room‐temperature catalytic hydrogen sensing for the first time. Finally, density functional theory calculations reveal that the strained Σ3(111) twin boundary facilitates oxygen dissociation, drastically enhancing the hydrogen oxidation rate via the dissociative pathway. This reported large‐scale synthesis of the Σ3 (111) twin GB‐rich structures enables the development of a broad range of high‐performance GB‐rich catalysts.

Geng, Xin↗
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↗
DOE OSTI2022

Trends in Formic Acid Electro-Oxidation on Transition Metals Alloyed with Platinum and Palladium

Direct formic acid fuel cells (DFAFCs) have emerged as a promising power source to meet increased demands for alternative energy sources in the transportation and portable energy storage sectors. Furthermore, these fuel cells utilize formic acid (FA), a nontoxic and carbon-neutral fuel when produced from biomass or via CO 2 reduction. Despite the promise of DFAFCs, the best monometallic catalysts, platinum and palladium, are poisoned by CO through the indirect oxidation pathway and require large overpotentials. By alloying Pt and Pd with other metals, we aim to improve both the activity and selectivity of these catalysts. Here, we present a systematic density functional theory (GGA-PW91) study of FA electro-oxidation (FAO) on the (111) facet of bimetallic Pt (Pt 3 M) or Pd (Pd 3 M) catalysts (M = Au, Ag, Cu, Pt, Pd, Ir, Rh, Ru, or Re) to evaluate the catalytic performance of these surfaces. For each surface, we calculate free energy diagrams and onset potentials of three key reaction mechanisms: direct oxidation of FA via carboxyl (COOH), direct oxidation of FA via formate (HCOO), and the indirect oxidation of FA that first forms CO en route to full oxidation to CO 2 . We then display the trends in the form of phase diagrams that compare the activity of the calculated surfaces against regions of high activity using the free energies of adsorbed CO and OH as descriptors, enabling high-throughput screening and design of improved catalysts, particularly those alloying Pt or Pd with Ir, Ru, or Re.

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

Acetate formation on metals via CH 4 carboxylation by CO 2 : A DFT study

Here, density functional theory calculations have been performed to investigate CH 4 activation and coupling to CO 2 forming C 2 carboxylates such as acetate on the close-packed (111) or (0001) surfaces of ten mid-to-late transition and coinage metals (Co, Ni, Cu, Ru, Rh, Pd, Ag, Ir, Pt, and Au). Consistent with the literature, the activation energy (E a ) for the initial C-H bond scission in CH 4 is mild, being ca. 1 eV or less on all but the coinage metals, of which Ag exhibits the highest E a at 2.13 eV, followed by Au and Cu. E a for the CH 3 -CO 2 coupling step is 0.8 ~ 1.1 eV on Co, Ru, Rh, and Ag, 1.2 ~ 1.5 eV on Ni, Cu, Pd, and Ir, and 1.8 ~ 2.1 eV on Pt and Au. While the two E a are comparable for several metals in terms of DFT total energies, free energy analysis indicates CH 3 -CO 2 coupling to be much more rate-limiting than CH 4 activation. Overcoming it would require over 800 K even on the most active of the metals considered, Ru, which makes the formation of acetate not feasible on the monometallic metal surfaces. Instead, we propose that single atom alloys based on early transition metals doped into a host metal such as Ni(111) could be viable catalysts. The dopant sites serve to stabilize the transition state of C-C coupling while Ni sites continue to activate CH 4 , thereby significantly lowering the required temperature for acetate formation.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗