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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 523 records · Page 29
All sourcesNASA NTRSDOE OSTI
DOE OSTI2017

Layered Lepidocrocite Type Structure Isolated by Revisiting the Sol–Gel Chemistry of Anatase TiO 2 : A New Anode Material for Batteries

Searches for new electrode materials for batteries must comply on financial and environmental costs to be useful in practical devices. The sol-gel chemistry has been widely used to design and implemented new concepts for the emergence of advanced materials such as hydride organic-inorganic composites. Here, we show that the simple reaction system including titanium alkoxide and water can be used to stabilize a new class of electrode materials. By investigating the crystallization path of anatase TiO2, an X-ray amorphous intermediate phase has been identified whose local structure probed by the pair distribution function, 1 H solid-state NMR and DFT calculations, consists of a layered-type structure as found in the lepido-crocite. This phase presents the following general formula Ti 2-x ⟂ x O 4-4x (OH) 4x .nH 2 O (x ~ 0.5) where the substitution of oxide by hydroxide anions leads to the formation of titanium vacancies (•) and H 2 O molecules are located in interlayers. Solid-state 1H NMR has enabled to characterize three main hydroxide environments that are Ti⟂-OH, Ti 2 ⟂ 2 -OH and Ti3⟂-OH and layered H 2 O molecules. The electrochemical properties of this phase were further investigated versus lithium and is shown to be very promising with reversible capacities of around 200 mAh.g -1 and an operating voltage of 1.55 V. We further showed that the lithium intercalation proceeds via a solid-solution mechanism. 7Li solid-state NMR and DFT calculations allowed to identify lithium host sites that are located at the titanium vacancies and interlayer space with lithium being solvated by structural water molecules. The easy fabrication, the absence of lithium and easier recycling and the encouraging properties makes this class of materials very attractive for competitive electrodes for batteries. We thus demonstrate that the revisit of an “old” chemistry with advanced characterization tools allows discovering new materials of technological relevance.

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

Vibrational study of CO, O 2 , and H 2 Adsorbed on the CoCrFeNi (110) High Entropy Alloy Surface

The vibrational properties of CO, O 2 , and H 2 molecularly or dissociatively adsorbed on a CoCrFeNi(110) surface have been probed using highresolution energy loss spectroscopy (HREELS) and modeled using density functional theory (DFT) calculations. Large (~20 mm3) single-crystal, quaternary face-centered cubic CoCrFeNi was synthesized via a modified Czochralski technique. We show strong evidence that CO adsorbs primarily on bridge and on-top sites in compositionally varied local environments, which reflect the random, multielemental surface composition inherent in a high entropy alloy. A variation of adsorption sites is also found with oxygen, which exhibits two broad groups of modes. Comparison to previous photoemission and theoretical studies suggests that the higher energy modes consist primarily of local CrO x species, while the lower energy modes are due to oxygen atoms adsorbed on other metal sites. Unlike CO and O 2 , HREELS upon H 2 adsorption shows only two much narrower modes and is consistent with atomic adsorption on 3-fold hollow sites. The hypothesized adsorption sites for all three species are directly corroborated by our DFT calculations.

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

Direct evidence of low work function on SrVO 3 cathode using thermionic electron emission microscopy and high-field ultraviolet photoemission spectroscopy

Perovskite SrVO 3 has recently been proposed as a novel electron emission cathode material. Density functional theory (DFT) calculations suggest multiple low work function surfaces, and recent experimental efforts have consistently demonstrated effective work functions of ~2.7 eV for polycrystalline samples, both results suggesting, but not directly confirming, that some fraction of even lower work function surface is present. In this work, thermionic electron emission microscopy (ThEEM) and high-field ultraviolet photoemission spectroscopy (UPS) are used to study the local work function distribution and measure the work function of a partially oriented- (110)-SrVO 3 perovskite oxide cathode surface. Our results show direct evidence of low work function patches of about 2.0 eV on the cathode surface, with a corresponding onset of observable thermionic emission at 750 °C. We hypothesize that, in our ThEEM and UPS experiments, the high applied electric field suppresses the patch field effect, enabling the direct measurement of local work functions. This measured work function of 2.0 eV is comparable to the previous DFT-calculated work function values of the SrVO-terminated (110) SrVO 3 surface (2.3 eV) and SrO-terminated (100) surface (1.9 eV). The measured 2.0 eV value is also much lower than the work function for the (001) LaB 6 single crystal cathode (~2.7 eV) and comparable to the effective work function of B-type dispenser cathodes (~2.1 eV). If SrVO 3 thermionic emitters can be engineered to access domains of this low 2.0 eV work function, they have the potential to significantly improve thermionic emitter-based technologies.

36 MATERIALS SCIENCE↗
DOE OSTI2023

Morphology Dependent Reactivity of CsO $x$ Nanostructures on Au(111): Binding and Hydrogenation of CO 2 to HCOOH

Cesium oxide (CsO $x$ ) nanostructures grown on Au(111) behave as active centers for CO 2 binding and hydrogenation reactions. The morphology and reactivity of these CsO $x$ systems were investigated as a function of alkali coverage using scanning tunnelling microscopy (STM), ambient pressure X-ray photoelectron spectroscopy (AP-XPS), and density functional theory (DFT) calculations. STM results show that initially (0.05 - 0.10 ML) cesium oxide clusters (Cs 2 O 2 ) grow at the elbow sites of the herringbone of Au(111), subsequently transforming into two-dimensional islands with increasing cesium coverage (> 0.15 ML). XPS measurements reveal the presence of suboxidic (Cs $y$ O; $y$ ≥ 2) species for the island structures. The higher coverages of cesium oxide nanostructures contain a lower O/Cs ratio resulting in a stronger binding of CO 2 . Moreover, the O atoms in the Cs $y$ O structure undergo a rearrangement upon the adsorption of CO 2 which is a reversible phenomenon. Under CO 2 hydrogenation conditions, the small Cs 2 O 2 clusters are hydroxylated, thereby preventing the adsorption of CO 2 . However, the hydroxylation of the higher coverages of Cs $y$ O did not prevent CO 2 adsorption, and the adsorbed CO 2 transformed to HCOO species that eventually yield HCOOH. DFT calculations further confirm that the dissociated H 2 attacks the C in the adsorbate to produce formate, which is both thermodynamically and kinetically favored during the CO 2 reaction with hydroxylated Cs $y$ O. These results demonstrate that cesium oxide by itself is an excellent catalyst for CO 2 hydrogenation that could produce formate, an important intermediate for the generation of value-added species. The role of the alkali oxide nanostructures as active centers, not merely as promoters, may have broad implications wherein the alkali oxides can be considered in the design of materials tuned for specific applications in heterogeneous catalysis.

03 NATURAL GAS↗
DOE OSTI2023

Single Ru(II) Ions on Ceria as a Highly Active Catalyst for Abatement of NO

Atom trapping leads to catalysts with atomically dispersed Ru 1 O 5 sites on (100) facets of ceria, as identified by spectroscopy and DFT calculations. This is a new class of ceria-based materials with Ru properties drastically different from the known M/ceria materials. They show excellent activity in catalytic NO oxidation, a critical step that requires use of large loadings of expensive noble metals in diesel aftertreatment systems. Ru 1 /CeO 2 is stable during continuous cycling, ramping, and cooling as well as the presence of moisture. Furthermore, Ru 1 /CeO 2 shows very high NO x storage properties due to formation of stable Ru–NO complexes as well as a high spill-over rate of NO x onto CeO 2 . Only ~0.05 wt % of Ru is required for excellent NO x storage. Ru 1 O 5 sites exhibit much higher stability during calcination in air/steam up to 750 °C in contrast to RuO 2 nanoparticles. Here we clarify the location of Ru(II) ions on the ceria surface and experimentally identify the mechanism of NO storage and oxidation using DFT calculations and in situ DRIFTS/mass spectroscopy. Moreover, we show excellent reactivity of Ru 1 /CeO 2 for NO reduction by CO at low temperatures: only 0.1–0.5 wt % of Ru is sufficient to achieve high activity. Modulation-excitation in situ infrared and XPS measurements reveal the individual elementary steps of NO reduction by CO on an atomically dispersed Ru ceria catalyst, highlighting unique properties of Ru 1 /CeO 2 and its propensity to form oxygen vacancies/Ce +3 sites that are critical for NO reduction, even at low Ru loadings. Our study highlights the applicability of novel ceria-based single-atom catalysts to NO and CO abatement.

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

Li{sub 2}Ca{sub 2}Si{sub 2}O{sub 7}: Structural, spectroscopic and computational studies on a sorosilicate

Synthesis experiments in the system Li{sub 2}O–CaO–SiO{sub 2} resulted in the formation of single-crystals of Li{sub 2}Ca{sub 2}Si{sub 2}O{sub 7}. Structural investigations were based on single-crystal diffraction. At ambient conditions the compound has the following basic crystallographic data: hexagonal symmetry, space group P6{sub 1}22, a=5.0961(2) Å, c=41.264(2) Å, V=928.07(6) Å{sup 3}, Z=6. Structure solution was performed using direct methods. The final least-squares refinement calculations converged at a residual of R(|F|)=0.0260. From a structural point the lithium calcium silicate belongs to the group of pyrosilicates containing [Si{sub 2}O{sub 7}]-groups. Additional lithium and calcium cations are incorporated between the silicate dimers and are coordinated by four and six nearest oxygen neighbours, respectively. Each [LiO{sub 4}]-tetrahedron shares two common corners with directly neighboring tetrahedra forming zweier single-chains which are running parallel to 〈1 0 0〉 in z-levels defined by the presence of the 6{sub 1}{sup [0} {sup 0} {sup 1]}-screw axes. From the corner-sharing [LiO{sub 4}]- and [SiO{sub 4}]-moieties a three dimensional framework can be constructed. An interesting feature of this framework is the presence of an O{sup [3]}-type bridging oxygen linking three tetrahedra (one [LiO{sub 4}]- and two [SiO{sub 4}]-units). Structural similarities with other silicates are discussed in detail. The high-temperature behavior of the Si–O, Ca–O and Li–O bond distances in Li{sub 2}Ca{sub 2}Si{sub 2}O{sub 7} was investigated by in{sub -}situ single-crystal X-ray diffraction in the range between 65 and 700 °C. From the evolution of the lattice parameters, the thermal expansion tensor α{sub ij} has been determined. The structural characterization has been supplemented by micro-Raman spectroscopy. Interpretation of the spectroscopic data including the allocation of the bands to certain vibrational species has been aided by DFT-calculations. - Graphical abstract: Framework of [SiO{sub 4}]- and [LiO{sub 4}]-tetrahedra in the crystal structure of Li{sub 2}Ca{sub 2}Si{sub 2}O{sub 7}. - Highlights: • We report the crystal structure of the sorosilicate Li{sub 2}Ca{sub 2}Si{sub 2}O{sub 7} at ambient conditions. • The thermal expansion tensor as well as the evolution of the structure between 25 and 700 °C was studied. • A topological analysis of the tetrahedral framework based on [SiO{sub 4}]- and [LiO{sub 4}]-units is presented. • The crystal structure of Li{sub 2}Ca{sub 2}Si{sub 2}O{sub 7} is discussed with respect to related compounds. • Interpretation of the Raman spectra of the crystals has been aided by DFT-calculations.

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

Fast MAS 1 H NMR Study of Water Adsorption and Dissociation on the (100) Surface of Ceria Nanocubes: A Fully Hydroxylated, Hydrophobic Ceria Surface

1 H nuclear magnetic resonance (NMR) spectroscopy was used to study hydroxylic surface species on ceria nanocubes, a crystalline, high-surface-area CeO 2 that presents mostly (100) facets. Water adsorption and desorption experiments in combination with fast magic angle spinning (MAS, 20–40 kHz) 1H NMR provide high-resolution 1 H spectra that allow the observation of ten resonance bands (water or hydroxyl) on or under the (100) surface. Assignments were made using a combination of adsorption and temperature-programmed desorption, quantitative spin counting, deuterium exchange, spin–lattice (T 1 ) and spin–spin (T 2 ) relaxation, and DFT calculations. In air, the (100) surface exists as a fully hydroxylated surface. Water adsorption and dissociation on dry ceria surfaces occur first at oxygen vacancies, but Ce 3+ centers are not required since water dissociation is barrier-less on the fully oxidized surface. Surface $-$OH functionality occurs in two resolved bands representing isolated $-$OH (1 ppm) and hydrogen-bonded $-$OH (9 ppm), the latter being dominant. Deuterium exchange of surface hydroxyls with D 2 O does not occur under mild or forcing conditions. Despite large differences in the T 1 of surface hydroxyls and physisorbed water, surface hydroxyl T 1 values are independent of the presence or absence of physisorbed water, demonstrating that the protons within these two functional group pools are not in intimate contact. These observations show that, once hydroxylated, the surface $-$OH functionality preferentially forms hydrogen bonds with surface lattice oxygen, i.e., the hydroxylated (100) surface of ceria is hydrophobic. Near this surface it is energetically more favorable for physisorbed water to hydrogen bond to itself rather than to the surface. DFT calculations support this notion. Impurity Na + remaining in incompletely washed ceria nanocubes increases the surface hydrophilicity. In conclusion, sharp, low-field resonances observed in spectra of noncalcined nanocubes arise from kinetically trapped subsurface $-$OH.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2018

A combined theoretical-experimental study of interactions between vanadium ions and Nafion membrane in all-vanadium redox flow batteries

Vanadium redox flow batteries (VRFBs) are a promising solution for large-scale energy storage, but a number of problems still impede the deployment of long-lifetime VRFBs. One important aspect of efficient operation of VRFBs is understanding interactions between vanadium species and the membrane. Herein, we investigate the interactions between all four vanadium cations and Nafion membrane by a combination of infrared (IR) spectroscopy and density-functional-theory (DFT)-based static and molecular dynamics simulations. It is observed that vanadium species primarily lead to changes in the IR spectrum of Nafion in the SO 3 - spectral region which is attributed to the interaction between vanadium species and the SO 3 - exchange sites. DFT calculations of vanadium - Nafion complexes in the gas phase show that it is thermodynamically favorable for all vanadium cations to bind to SO 3 - via a contact pair mechanism. Car-Parrinello molecular dynamics-based metadynamics simulations of cation-Nafion systems in aqueous solution suggest that and species coordinate spontaneously to SO 3 - , which is not the case for VO 2+ and VO 2 + . The interaction behavior of the uncycled membrane determined in this study is used to explain the experimentally observed changes in the vibrational spectra, and is discussed in light of previous results on device-cycled membranes.

00-01↗
DOE OSTI2015

Influence of cluster–support interactions on reactivity of size-selected Nb x O y clusters

Size-selected niobium oxide nanoclusters (Nb 3 O 5 , Nb 3 O 7 , Nb 4 O 7 , and Nb 4 O 10 ) were deposited at room temperature onto a Cu(111) surface and a thin film of Cu 2 O on Cu(111), and their interfacial electronic interactions and reactivity toward water dissociation were examined. These clusters were specifically chosen to elucidate the effects of the oxidation state of the metal centers; Nb 3 O 5 and Nb 4 O 7 are the reduced counterparts of Nb 3 O 7 and Nb 4 O 10 , respectively. From two-photon photoemission spectroscopy (2PPE) measurements, we found that the work function increases upon cluster adsorption in all cases, indicating a negative interfacial dipole moment with the positive end pointing into the surface. The amount of increase was greater for the clusters with more metal centers and higher oxidation state. Additional analysis with DFT calculations of the clusters on Cu(111) indicated that the reduced clusters donate electrons to the substrate, indicating that the intrinsic cluster dipole moment makes a larger contribution to the overall interfacial dipole moment than charge transfer. X-ray photoelectron spectroscopy (XPS) measurements showed that the Nb atoms of Nb 3 O 7 and Nb 4 O 10 are primarily Nb 5+ on Cu(111), while for the reduced Nb 3 O 5 and Nb 4 O 7 clusters, a mixture of oxidation states was observed on Cu(111). Temperature-programmed desorption (TPD) experiments with D 2 O showed that water dissociation occurred on all systems except for the oxidized Nb 3 O 7 and Nb 4 O 10 clusters on the Cu 2 O film. A comparison of our XPS and TPD results suggests that Nb 5+ cations associated with Nb=O terminal groups act as Lewis acid sites which are key for water binding and subsequent dissociation. TPD measurements of 2-propanol dehydration also show that the clusters active toward water dissociation are indeed acidic. DFT calculations of water dissociation on Nb 3 O 7 support our TPD results, but the use of bulk Cu 2 O(111) as a model for the Cu 2 O film merits future scrutiny in terms of interfacial charge transfer. The combination of our experimental and theoretical results suggests that both Lewis acidity and metal reducibility are important for water dissociation.

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

ANI-1: an extensible neural network potential with DFT accuracy at force field computational cost

Deep learning is revolutionizing many areas of science and technology, especially image, text, and speech recognition. In this paper, we demonstrate how a deep neural network (NN) trained on quantum mechanical (QM) DFT calculations can learn an accurate and transferable potential for organic molecules. We introduce ANAKIN-ME (Accurate NeurAl networK engINe for Molecular Energies) or ANI for short. ANI is a new method designed with the intent of developing transferable neural network potentials that utilize a highly-modified version of the Behler and Parrinello symmetry functions to build single-atom atomic environment vectors (AEV) as a molecular representation. AEVs provide the ability to train neural networks to data that spans both configurational and conformational space, a feat not previously accomplished on this scale. We utilized ANI to build a potential called ANI-1, which was trained on a subset of the GDB databases with up to 8 heavy atoms in order to predict total energies for organic molecules containing four atom types: H, C, N, and O. To obtain an accelerated but physically relevant sampling of molecular potential surfaces, we also proposed a Normal Mode Sampling (NMS) method for generating molecular conformations. Through a series of case studies, we show that ANI-1 is chemically accurate compared to reference DFT calculations on much larger molecular systems (up to 54 atoms) than those included in the training data set.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2018

The Role of Seven-Coordination in Ru-Catalyzed Water Oxidation

A family of Ru complexes based on the pentadentate ligand t5a 3– ((2,5-bis(6-carboxylatopyridin-2-yl)pyrrol-1-ide) and pyridine (py) that includes {Ru II (Ht5a-κ-N 2 O)(py) 3 } (1H II (κ-N 2 O)), {Ru III (t5a-κ-N 3 O 1.5 )(py) 2 } (2 III (κ-N 3 O 1.5 )), and {Ru IV (t5a-κ-N 3 O 2 )(py) 2 }+ ({2 IV (κ-N3O 2 )}+) has been prepared and thoroughly characterized. Complexes 1HII(κ-N 2 O), 2 III (κ-N 3 O 1.5 ), and {2 IV (κ-N 3 O 2 )}+ have been investigated in solution by spectroscopic methods (NMR, UV–vis) and in the solid state by single-crystal X-ray diffraction analysis and complemented by density functional theory (DFT) calculations. The redox properties of complex 2 III (κ-N 3 O 1.5 ) have been studied by electrochemical methods (CV and DPV), showing its easy access to high oxidation states, thanks to the trianionic nature of the t5a 3– ligand. Under neutral to basic conditions complex {2 IV (κ-N3O 2 )}+ undergoes aquation, generating {Ru IV (OH)(t5a-κ-N 2 O)(py) 2 } (2 IV (OH)(κ-N 2 O)). Further oxidation of the complex forms {Ru V (O)(t5a-κ-N 2 O)(py) 2 } (2 V (O)(κ-N 2 O)), which is a very efficient water oxidation catalyst, reaching a TOF MAX value of 9400 s –1 at pH 7.0, as measured via foot of the wave analysis. The key to fast kinetics for the catalytic oxidation of water to dioxygen by 2 V (O)(κ-N 2 O) is due not only to the easy access to high oxidation states but also to the intramolecular hydrogen bonding provided by the noncoordinated dangling carboxylate at the transition state, as corroborated by DFT calculations.

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

Macrocycle-Directed Construction of Tetrahedral Anion-pi Receptors for Nesting Anions with Complementary Geometry

The unprecedented tetrahedral anion-? receptors have been successfully constructed by a macrocycle-directed approach. The use of large macrocycle precursors bearing four reaction sites enabled kinetic-favored cross-linking pathway and afforded otherwise-inaccessible tetrahedral cages in considerable yields. Crystal structure suggested that the tetrahedral cages have an enclosed three-dimensional cavity surrounded by four electron-deficient triazine faces in a tetrahedral array. The complementary accommodation of a series of tetrahedral and relevant anions including BF4-, ClO4-, H2PO4-, HSO4-, SO42- and PF6- has been observed by ESI-MS. Two complex crystal structures with ClO4- and PF6- demonstrated the anion was nicely encapsulated within the tetrahedral cavity through up to quadruple cooperative anion-? interactions by an excellent shape and size match. The favored complexation energies were established by DFT calculation and further confirmed by negative ion photoelectron spectroscopy study.

Luo, Jian↗
DOE OSTI2018

Rationally-Designed Configuration of Directly-Coated Ni3S2/Ni Electrode by RGO Providing Superior Sodium Storage

Designing nanocomposite materials is an effective approach for enhancing the performance of sodium-ion batteries (SIBs), and understanding the synergy among components is critically important for new, better materials design. Herein, a directly reduced graphene oxide (RGO) decorated anode electrode was designed and tested for SIBs, in which uniform RGO coating onto the Ni3S2/Ni electrode was realized using facile hydrothermal reactions. The results indicate that the RGO/Ni3S2/Ni electrode delivers a high reversible specific capacity of 448.6 mAh g-1, high capacity retention of 96.5% after 100 cycles, and excellent rate capability of 263.1 mAh g-1 at 800 mA g-1. Compared with the Ni3S2/Ni electrode, the improved performance of the RGO/Ni3S2/Ni electrode benefits from RGO-promoted displacement reaction of Ni3S2 with sodium. DFT calculations reveal that the RGO layer can significantly improve the electron mobility of the RGO/Ni3S2 + Na structure, and the hybrid interaction between the C-p state of RGO and the Ni-p, Ni-d, and S-p states of Ni3S2 is the major reason for why RGO can improve the electrical transport properties.

Song, Xiaosheng↗
DOE OSTI2017

Thermally Stable and Regenerable Platinum–Tin Clusters for Propane Dehydrogenation Prepared by Atom Trapping on Ceria

Abstract Ceria (CeO 2 ) supports are unique in their ability to trap ionic platinum (Pt), providing exceptional stability for isolated single atoms of Pt. The reactivity and stability of single‐atom Pt species was explored for the industrially important light alkane dehydrogenation reaction. The single‐atom Pt/CeO 2 catalysts are stable during propane dehydrogenation, but are not selective for propylene. DFT calculations show strong adsorption of the olefin produced, leading to further unwanted reactions. In contrast, when tin (Sn) is added to CeO 2 , the single‐atom Pt catalyst undergoes an activation phase where it transforms into Pt–Sn clusters under reaction conditions. Formation of small Pt–Sn clusters allows the catalyst to achieve high selectivity towards propylene because of facile desorption of the product. The CeO 2 ‐supported Pt–Sn clusters are very stable, even during extended reaction at 680 °C. Coke formation is almost completely suppressed by adding water vapor to the feed. Furthermore, upon oxidation the Pt–Sn clusters readily revert to the atomically dispersed species on CeO 2 , making Pt–Sn/CeO 2 a fully regenerable catalyst.

Xiong, Haifeng↗
DOE OSTI2016

Hydricity, electrochemistry, and excited-state chemistry of Ir complexes for CO 2 reduction

Here, we prepared electron-rich derivatives of [Ir(tpy)(ppy)Cl] + with modification of the bidentate (ppy) or tridentate (tpy) ligands in attempt to increase the reactivity for CO 2 reduction and the ability to transfer hydrides (hydricity). Density functional theory (DFT) calculations reveal that complexes with dimethyl-substituted ppy have similar hydricities to the non-substituted parent complex, and photocatalytic CO 2 reduction studies show selective CO formation. Substitution of tpy for bis(benzimidazole)-phenyl or -pyridine (L3 and L4, respectively) induces changes in the physical properties much more pronounced than addition of methyl groups to ppy. Theoretical data predict [Ir(L3)(ppy)(H)] is the strongest hydride donor among complexes studied in this work, but [Ir(L3)(ppy)(NCCH 3 )] + cannot be reduced photochemically because the excited state reduction potential is only 0.52 V due to the negative ground state potential of –1.91 V. The excited state [Ir(L4)(ppy)(NCCH 3 )] 2+ is the strongest oxidant among complexes studied in this work and the singly reduced species is formed readily upon photolysis in the presence of tertiary amines. Both [Ir(L3)(ppy)(NCCH 3 )] + and [Ir(L4)(ppy)(NCCH 3 )] 2+ exhibit electrocatalytic current for CO 2 reduction. While a significantly greater overpotential is needed for the L3 complex, a small amount of formate (5-10 %) generation in addition to CO was observed as predicted by the DFT calculations.

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

High-Pressure Structural Response of an Insensitive Energetic Crystal: Dihydroxylammonium 5,5'-Bistetrazole-1,1'-diolate (TKX-50)

The structural response of a novel, insensitive energetic crystal—dihydroxylammonium 5,5'-bistetrazole-1,1'-diolate (TKX-50)—was examined under high pressure. Using synchrotron single-crystal X-ray diffraction measurements, details of molecular, intermolecular, and crystal changes were determined to ~10 GPa to understand its structural stability. The experimental results showed that TKX-50 exhibits highly anisotropic compression and significantly lower volume compressibility than currently known energetic crystals. These results are found to be in general agreement with our previous predictions from the DFT calculations. Additionally, the experimental data revealed anomalous compression—an expansion of the unit cell along the a axis (negative linear compressibility, NLC) upon compression to ~3 GPa. The structural analyses demonstrated that this unusual effect, the first such observation in an energetic crystal, is a consequence of the highly anisotropic response of 3D motifs, comprised of two parallel anions [(C 2 N 8 O 2 ) 2– ] linked with two cations [(NH 3 OH) + ] through four strong hydrogen bonds. The present results demonstrate that the structural stability of TKX-50 is controlled by the strong and highly anisotropic intermolecular interactions, and these may contribute to its shock insensitivity.

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

Tunable Excited-State Properties and Dynamics as a Function of Pt–Pt Distance in Pyrazolate-Bridged Pt(II) Dimers

The influence of molecular structure on excited state properties and dynamics of a series of cyclometalated platinum dimers was investigated through a combined experimental and theoretical approach using femtosecond transient absorption (fs TA) spectroscopy and density functional theory (DFT) calculations. The molecules have the general formula [Pt(ppy)(µ-R2pz)]2 where ppy = 2-phenylpyridine, pz = pyrazolate and R = H, Me, Ph, or tBu, and are strongly photoluminescent at room temperature. The distance between the platinum centers in this A frame geometry can be varied depending on the steric bulk of the bridging pyrazolate ligands that exert structural constraints and compress the Pt-Pt distance. At large Pt-Pt distances there is little interaction between the subunits and the chromophore behaves similar to a monomer with excited states described as mixtures of ligand-centered and metal-to-ligand charge transfer (LC/MLCT) transitions. When the Pt(II) centers are brought closer together with bulky bridging ligands, they interact through their orbitals and the S1 and T1 states are best characterized as metal metal to ligand charge transfer (MMLCT) in character. The results of the fs TA experiments reveal that intersystem crossing (ISC) occurs on ultrafast timescales (τS1 < 200 fs) while there are two relaxation processes occurring within the triplet manifold, τ1 = 0.5 – 3.2 ps and τ2 = 20 – 70 ps; the longer time constants correspond to the presence of bulkier bridging ligands. DFT calculations illustrate that the Pt-Pt distances further contract in the T1 3MMLCT states, therefore slower relaxation may be related to a larger structural reorganization. Subsequent investigations using faster time resolution are planned to measure the ISC process as well as to identify any potential coherent interaction(s) between the platinum centers that may occur.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2018

Insights into the structure–property–activity relationship in molybdenum-doped octahedral molecular sieve manganese oxides for catalytic oxidation

Hexavalent molybdenum ions substituted into the mixed-valent framework of octahedral molecular sieve manganese oxides (1–10 mol% Mo–K-OMS-2) were systematically prepared via a single-step refluxing method. Here, the structure, composition, morphology, thermal stability, and textural and redox properties of the Mo–K-OMS-2 materials were characterized by various experimental techniques. Density functional theory (DFT) calculations were performed to study the electronic properties of K-OMS-2 materials, including the influence of molybdenum on such properties. The catalytic activity of K-OMS-2 towards CO oxidation dramatically increased with Mo dopant concentration, until an optimum incorporation of 5 mol% was reached, giving full CO conversion at 120 °C (1 vol% CO, 1 vol% O2). More importantly, the activity of the 5% Mo–K-OMS-2 catalyst was (reversibly) inhibited by water vapour (3%) in the feed only at low temperatures and exhibited better tolerance and stability during long-run experiments (28 h) as compared to undoped K-OMS-2. The superior performance of Mo–K-OMS-2 catalyst for CO oxidation can be attributed to the following factors: (1) morphological evolution from nanofibers to nanospheres, as well as the slightly distorted structure, generated a sharp increase in the specific surface area up to 258 m2 g–1, hence, increased the number of surface active sites; (2) improved mobility of reactive oxygen species at the surface and enhanced redox properties; and (3) tailored electronic properties promoted more active sites for oxidation reactions, which is supported by DFT calculations.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗