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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 19 records
All sourcesNASA NTRSDOE OSTI
DOE OSTI2021

Structural Interconversion between Agglomerated Palladium Domains and Mononuclear Pd(II) Cations in Chabazite Zeolites

Palladium-exchanged zeolites are candidate materials for passive NO x adsorption in automotive exhaust aftertreatment, where mononuclear Pd cations behave as precursors to the purported NO x adsorption sites. Yet, the structures of zeolite lattice binding sites capable of stabilizing mononuclear Pd 2+ ions, and the mechanisms that interconvert agglomerated PdO and Pd domains into mononuclear Pd 2+ ions during Pd redispersion treatments, remain incompletely understood. In this work, we use a suite of spectroscopic methods and quantitative site titration techniques to characterize mononuclear and agglomerated Pd species on zeolites with varying material properties and treatment history. Aqueous-phase methods to introduce Pd onto NH 4 -form zeolites initially form mononuclear [Pd(NH 3 ) 4 ] 2+ complexes, but subsequent thermal treatments (573–723 K; air) lead to in situ formation of H 2 that first reduces Pd 2+ to metallic Pd domains, which are then oxidized by air to PdO domains. Progressive treatment of Pd-zeolites in air to higher temperatures (723–1023 K) converts larger fractions of agglomerated PdO to mononuclear Pd 2+ , as quantified by H 2 temperature programmed reduction, because higher temperature treatments facilitate Pd redispersion toward deeper locations within chabazite (CHA) crystallites, which is corroborated by complementary titrimetric and spectroscopic data. Pd-CHA zeolites synthesized with similar bulk Pd and framework Al content, but varying framework Al arrangement, provide evidence that six-membered rings (6-MR) hosting paired Al sites (Al–O–(Si–O) x –Al, x = 1, 2) stabilize Pd 2+ ions and that otherwise isolated Al sites can stabilize [PdOH] + species, identifiable by an IR OH stretch at 3660 cm –1 . These findings clarify the underlying chemical processes and gas environments that cause Pd agglomeration in zeolites and their subsequent redispersion to mononuclear Pd 2+ ions, which prefer binding at 6-MR paired Al sites in CHA, and indicate that higher temperature air treatments lead to more uniform Pd spatial distributions throughout zeolite crystallites.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Rapid Electrochemical Methane Functionalization Involves Pd–Pd Bonded Intermediates

High-valent Pd complexes are potent agents for the oxidative functionalization of inert C-H bonds, and it was previously shown that rapid electrocatalytic methane monofunctionalization could be achieved by electro-oxidation of Pd II to a critical dinuclear Pd III intermediate in concentrated or fuming sulfuric acid. However, the structure of this highly reactive, unisolable intermediate, as well as the structural basis for its mechanism of electrochemical formation, remained elusive. Herein, we use X-ray absorption and Raman spectroscopies to assemble a structural model of the potent methane-activating intermediate as a Pd III dimer with a Pd-Pd bond and a 5-fold O atom coordination by H x SO 4 (x-2) ligands at each Pd center. We further use EPR spectroscopy to identify a mixed-valent M-M bonded Pd 2 II,III species as a key intermediate during the Pd II -to-Pd III 2 oxidation. Combining EPR and electrochemical data, we quantify the free energy of Pd dimerization as <-4.5 kcal/mol for Pd 2 II,III and <-9.1 kcal/mol for Pd III 2 . The structural and thermochemical data suggest that the aggregate effect of metal-metal and axial metal-ligand bond formation drives the critical Pd dimerization reaction in between electrochemical oxidation steps. This work establishes a structural basis for the facile electrochemical oxidation of Pd II to a M-M bonded Pd III dimer and provides a foundation for understanding its rapid methane functionalization reactivity.

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

Active Palladium Structures on Ceria Obtained by Tuning Pd–Pd Distance for Efficient Methane Combustion

Efficiently removing/converting methane via methane combustion imposes challenges on catalyst design: how to design local structures of a catalytic site so that it has both high intrinsic activity and atomic efficiency? By manipulating the atomic distance of isolated Pd atoms, herein we show that the intrinsic activity of Pd catalysts can be significantly improved for methane combustion via a stable Pd 2 structure on a ceria nanorod support. Guided by theory and confirmed by experiment, we find that the turnover frequency (TOF) of the Pd 2 structure with the Pd–Pd distance of 2.99 Å is higher than that of the Pd 2 structure with the Pd–Pd distance of 2.75 Å; at least 26 times that of ceria supported Pd single atoms and 4 times that of ceria supported PdO nanoparticles. The high intrinsic activity of the 2.99 Å Pd–Pd structure is attributed to the conductive local redox environment from the two O atoms bridging the two Pd 2+ ions, which facilitates both methane adsorption and activation as well as the production of water and carbon dioxide during the methane oxidation process. In conclusion, this work highlights the sensitivity of catalytic behavior on the local structure of active sites and the fine-tuning of the metal–metal distance enabled by a support local environment for guiding the design of efficient catalysts for reactions that highly rely on Pt-group metals.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Synthesis, Redox, and Spectroscopic Properties of Pd(II) 10,10-Dimethylisocorrole Complexes Prepared via Bromination of Dimethylbiladiene Oligotetrapyrroles

Two brominated 10,10-dimethylisocorrole (10-DMIC) derivatives containing Pd(II) centers have been prepared and characterized. These compounds were prepared via bromination of 10,10-dimethylbiladiene-based oligotetrapyrroles. Bromination of free base 10,10-dimethylbiladiene (DMBil1) followed by metalation with Pd(OAc) 2 , as well as bromination of the corresponding Pd(II) dimethylbiladiene complex (Pd[DMBil1]) provide routes to Pd(II) hexabromo-10,10-dimethyl-5,15-bis(pentafluorophenyl)-isocorrole (Pd[10-DMIC-Br 6 ]) and Pd(II) octabromo-10,10-dimethyl-5,15-bis(pentafluorophenyl)-isocorrole (Pd[10-DMIC-Br 8 ]). The solid-state structures of the two brominated isocorrole complexes are presented, as is that for a new decabrominated dimethylbiladiene derivative (DMBil-Br 10 ). The electronic and spectroscopic properties of the brominated biladiene and isocorrole derivatives were probed using a combination of voltammetric methods and steady-state UV–vis absorption and emission experiments. Data obtained from these experiments allow the properties of the brominated biladiene and isocorrole derivatives to be compared to previously studied biladiene derivatives (i.e., DMBil1 and Pd[DMBil1]). CV and DPV experiments demonstrate that Pd[10-DMIC-Br 6 ] and Pd[10-DMIC-Br 8 ] support well-behaved multielectron redox chemistry, similar to that which has been observed for other nonaromatic tetrapyrroles containing sp 3 -hybridized meso-carbons. Here, spectroscopic experiments reveal that bromination of the dimethylbiladiene core shifts this system’s UV–vis absorption profile to lower energy and that the dimethylisocorrole complexes support panchromatic absorption profiles that extend across the UV–vis and into the near-IR region. Photosensitization experiments demonstrate that unlike previously studied Pd(II) biladiene constructs, DMBil-Br 10 , Pd[10-DMIC-Br 6 ], and Pd[10-DMIC-Br 8 ] support limited triplet excited state chemistry with O 2 , indicating that the novel nonaromatic tetrapyrrole derivatives described in this work may be best suited for applications other than singlet oxygen sensitization.

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

Challenges and the Evolving Landscape of Assessing Blood-Based PD-L1 Expression as a Biomarker for Anti-PD-(L)1 Immunotherapy

While promising, PD-L1 expression on tumor tissues as assessed by immunohistochemistry has been shown to be an imperfect biomarker that only applies to a limited number of cancers, whereas many patients with PD-L1-negative tumors still respond to anti-PD-(L)1 immunotherapy. Recent studies using patient blood samples to assess immunotherapeutic responsiveness suggests a promising approach to the identification of novel and/or improved biomarkers for anti-PD-(L)1 immunotherapy. In this review, we discuss the advances in our evolving understanding of the regulation and function of PD-L1 expression, which is the foundation for developing blood-based PD-L1 as a biomarker for anti-PD-(L)1 immunotherapy. We further discuss current knowledge and clinical study results for biomarker identification using PD-L1 expression on tumor and immune cells, exosomes, and soluble forms of PD-L1 in the peripheral blood. Finally, we discuss key challenges for the successful development of the potential use of blood-based PD-L1 as a biomarker for anti-PD-(L)1 immunotherapy.

59 BASIC BIOLOGICAL SCIENCES↗
DOE OSTI2025

Volcano‐like Activity Trends in Au@Pd Catalysts: The Role of Pd Loading and Nanoparticle Size

The addition of palladium (Pd) to preformed gold nanoparticles (Au NPs) enables the formation of core‐shell structures with enhanced catalytic performance in oxidation reactions. However, predicting the precise palladium content required to achieve maximum catalytic activity remains difficult based on current understanding. Herein, Pd was systematically introduced onto titania‐supported Au NPs (2, 6, and 10 nm) to evaluate their performance in benzyl alcohol oxidation. A volcano‐like trend in catalytic activity was observed, where activity increased with Pd addition, peaked, and then declined. The Pd loading required for maximum activity depended on Au NP size: ≈40 at% Pd/Au for 2.6 nm, ≈20 at% Pd/Au for 6.4 nm, and ≈12.5 at% Pd/Au for 10.6 nm. For Au NPs > 6 nm, peak activity aligned with monolayer Pd coverage, while for smaller NPs (2–3 nm), optimal Pd content was below monolayer predictions. X‐ray absorption spectroscopy revealed a core‐shell structure at low Pd content, but higher Pd loadings led to Pd diffusion into the Au core. This structural transformation likely caused activity decline, indicating that AuPd alloying negatively impacts catalysis. These results highlight that core‐shell Au@Pd catalysts outperform AuPd alloys and provide crucial insights for designing highly active bimetallic catalysts.

X-ray absorption spectroscopy↗
DOE OSTI2024

Pd-CeO 2 catalyst facilely derived from one-pot generated Pd@Ce-BTC for low temperature CO oxidation

Due to the capacity to offer abundant catalytic sites within porous solids featuring high surface areas, metal-organic frameworks (MOFs) and their derivatives have garnered considerable attention as prospective catalysts in environmental catalysis. Here, to promote the industrial application of MOFs, there is an urgent need for an effective and environmental-friendly preparation approach. Breaking through the limitation of the traditional two-step preparation method that Pd was introduced to the already prepared Ce-BTC (Pd/Ce-BTC, BTC = 1, 3, 5 benzenetricarboxylate), in this work, we present a novel one-pot solvothermal method for synthesizing the Pd material supported by Ce-BTC (Pd@Ce-BTC). After pyrolysis in N 2 flow or air flow, Pd-CeO 2 catalysts derived from Pd@Ce-BTC exhibited much higher CO oxidation activity than those from Pd/Ce-BTC. Moreover, Pd/Ce-BTC and Pd@Ce-BTC pyrolyzed in N 2 flow (Pd/Ce-BTC-N and Pd@Ce-BTC-N) could better catalyze the oxidation of CO than Pd/Ce-BTC and Pd@Ce-BTC pyrolyzed in air flow (Pd/Ce-BTC-A and Pd@Ce-BTC-A). Further characterizations revealed that the abundant surface Ce 3+ species, rich surface adsorbed oxygen species and superior redox properties were the main reasons for the superior CO oxidation activity of Pd@Ce-BTC-N.

36 MATERIALS SCIENCE↗
DOE OSTI2023

Twin Proliferation and Prolongation under Kinetic Control: Pd–Au Janus Icosahedra versus Pd@Au Core–Shell Starfishes

Heterogeneous bimetallic nanocrystals featuring explicit spatial configurations and abundant twin defects can simultaneously enable geometric and ligand effects to enhance catalytic and photonic applications. Herein, we report two growth patterns of Au atoms on penta-twinned Pd decahedra, involving twin proliferation to generate asymmetric Pd–Au Janus icosahedra and twin elongation to produce anisotropic Pd@Au core–shell starfishes, respectively. Mechanistic analysis indicates that the injection rate determines the lower-limit number (n low ) of Au(III) ions in the steady state and thus controls the growth pattern. When n low ≤ 5.5, the kinetic rate is slow enough to initiate asymmetrical one-side growth but fast enough to outpace surface diffusion; Au tetrahedral subunits are successively proliferated along the axial $\langle$110$\rangle$ direction of Pd decahedra to form Pd–Au Janus icosahedra. Composed of five Pd and 15 Au tetrahedral subunits, such a heterogeneous icosahedron supports high (2.2 GPa) tensile strain and high strain difference up to +21.9%. In contrast, when n low > 5.5, the fast reduction kinetics promotes symmetric growth with inadequate surface diffusion. As such, Au atoms are laterally deposited along five high-indexed $\langle$211$\rangle$ ridges of Pd decahedra to generate concave Pd@Au core–shell starfishes with tunable sizes (28–40 nm), twin elongation ratios (33.82–162.08%), and lattice expansion ratios (8.82–20.10%).

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 OSTI2020

Replication Data for: New Role of Pd Hydride as a Sensor of Surface Pd Distributions in Pd-Au Catalysts

The data underlying this published work have been made publicly available in this repository as part of the IMASC Data Management Plan. This work was supported as part of the Integrated Mesoscale Architectures for Sustainable Catalysis (IMASC), an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Basic Energy Sciences under Award # DE-SC0012573.

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

Maximizing the Catalytic Performance of Pd@Au x Pd 1- x Nanocubes in H 2 O 2 Production by Reducing Shell Thickness to Increase Compositional Stability

We report a simple route based upon seed-mediated growth to the synthesis of Pd@Au x Pd 1-x (0.8≤x≤1) core–shell nanocubes. Benefiting from the well-defined {100} facets and an optimal Au/Pd ratio for the surface, the nanocubes bearing a shell made of Au 0.95 Pd 0.05 work as an efficient electrocatalyst toward H 2 O 2 production, with high selectivity of 93–100 % in the low-overpotential region of 0.4–0.7 V. When the Au 0.95 Pd 0.05 alloy is confined to a shell of only three atomic layers in thickness, the electrocatalyst is able to maintain its surface structure and elemental composition, endowing continuous and stable production of H2O2 during oxygen reduction at a high rate of 1.62 mol g( Pd+Au ) -1 h -1 . This work demonstrates a versatile route to the rational development of active and durable electrocatalysts based upon alloy nanocrystals.

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

Elucidation of complex triplet excited state dynamics in Pd(ii) biladiene tetrapyrroles

Pd(II) biladienes have been developed over the last five years as non-aromatic oligotetrapyrrole complexes that support a rich triplet photochemistry. In this work, we have undertaken the first detailed photophysical interrogation of three homologous Pd(II) biladienes bearing different combinations of methyl- and phenyl-substituents on the frameworks’ sp 3 -hybridized meso-carbon (i.e., the 10-position of the biladiene framework). These experiments have revealed unexpected excited-state dynamics that are dependent on the wavelength of light used to excite the biladiene. More specifically, transient absorption spectroscopy revealed that higher-energy excitation (λ exc ~ 350–500 nm) led to an additional lifetime (i.e., an extra photophysical process) compared to experiments carried out following excitation into the lowest-energy excited states (λ exc = 550 nm). Each Pd(II) biladiene complex displayed an intersystem crossing lifetime on the order of tens of ps and a triplet lifetime of ~20 μs, regardless of the excitation wavelength. However, when higher-energy light is used to excite the complexes, a new lifetime on the order of hundreds of ps is observed. The origin of the ‘extra’ lifetime observed upon higher energy excitation was revealed using density functional theory (DFT) and time-dependent DFT (TDDFT). These efforts demonstrated that excitation into higher-energy metal-mixed-charge-transfer excited states with high spin–orbit coupling to higher energy metal-mixed-charge-transfer triplet states leads to the additional excitation deactivation pathway. Furthermore, the results of this work demonstrate that Pd(II) biladienes support a unique triplet photochemistry that may be exploited for development of new photochemical schemes and applications.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗