Probing the unusual anion mobility of LiBH_4 confined in highly ordered nanoporous carbon frameworks via solid state NMR and quasielastic neutron scattering

Particle size and particle–framework interactions have profound effects on the kinetics, reaction pathways, and even thermodynamics of complex hydrides incorporated in frameworks possessing nanoscale features. Tuning these properties may hold the key to the utilization of complex hydrides in practical applications for hydrogen storage. Using carefully synthesized, highly-ordered, nanoporous carbons (NPCs), we have previously shown quantitative differences in the kinetics and reaction pathways of LiBH_4 when incorporated into the frameworks. In this paper, we probe the anion mobility of LiBH_4 confined in NPC frameworks by a combination of solid state NMR and quasielastic neutron scattering (QENS) and present some new insights into the nanoconfinement effect. NMR and QENS spectra of LiBH_4 confined in a 4 nm pore NPC suggest that the BH_4− anions nearer the LiBH_4–carbon pore interface exhibit much more rapid translational and reorientational motions compared to those in the LiBH_4 interior. Moreover, an overly broadened BH_4− torsional vibration band reveals a disorder-induced array of BH_4− rotational potentials. XRD results are consistent with a lack of LiBH_4 long-range order in the pores. Consistent with differential scanning calorimetry measurements, neither NMR nor QENS detects a clear solid–solid phase transition as observed in the bulk, indicating that borohydride–framework interactions and/or nanosize effects have large roles in confined LiBH_4

Desalination utilizing clathrate hydrates (LDRD final report).

Advances are reported in several aspects of clathrate hydrate desalination fundamentals necessary to develop an economical means to produce municipal quantities of potable water from seawater or brackish feedstock. These aspects include the following, (1) advances in defining the most promising systems design based on new types of hydrate guest molecules, (2) selection of optimal multi-phase reactors and separation arrangements, and, (3) applicability of an inert heat exchange fluid to moderate hydrate growth, control the morphology of the solid hydrate material formed, and facilitate separation of hydrate solids from concentrated brine. The rate of R141b hydrate formation was determined and found to depend only on the degree of supercooling. The rate of R141b hydrate formation in the presence of a heat exchange fluid depended on the degree of supercooling according to the same rate equation as pure R141b with secondary dependence on salinity. Experiments demonstrated that a perfluorocarbon heat exchange fluid assisted separation of R141b hydrates from brine. Preliminary experiments using the guest species, difluoromethane, showed that hydrate formation rates were substantial at temperatures up to at least 12 C and demonstrated partial separation of water from brine. We present a detailed molecular picture of the structure and dynamics of R141b guest molecules within water cages, obtained from ab initio calculations, molecular dynamics simulations, and Raman spectroscopy. Density functional theory calculations were used to provide an energetic and molecular orbital description of R141b stability in both large and small cages in a structure II hydrate. Additionally, the hydrate of an isomer, 1,2-dichloro-1-fluoroethane, does not form at ambient conditions because of extensive overlap of electron density between guest and host. Classical molecular dynamics simulations and laboratory trials support the results for the isomer hydrate. Molecular dynamics simulations show that R141b hydrate is stable at temperatures up to 265K, while the isomer hydrate is only stable up to 150K. Despite hydrogen bonding between guest and host, R141b molecules rotated freely within the water cage. The Raman spectrum of R141b in both the pure and hydrate phases was also compared with vibrational analysis from both computational methods. In particular, the frequency of the C-Cl stretch mode (585 cm{sup -1}) undergoes a shift to higher frequency in the hydrate phase. Raman spectra also indicate that this peak undergoes splitting and intensity variation as the temperature is decreased from 4 C to -4 C

Interstital-atom-induced phase transformation upon hydrogenation in vanadium

We investigated the effect of interstitial atoms on hydrogen storage properties in vanadium. When the nitrogen concentration was below 0.4 wt%, the plateau pressures of the pressure-composition-isotherm curve increased with increasing nitrogen concentration during hydrogen absorption and desorption and vanadium samples with a body-centered cubic (BCC) transformed to VH0.5 with a body-centered tetragonal (BCT) of c/a=1.1, and then VH2 with a face-centered cubic (FCC). When the nitrogen concentration exceeded 0.6 wt%, a new single-phase region appeared in the pressure-composition isotherm The X-ray diffraction data indicated that this new hydride phase was VH1.0 with a BCT structure and c/a = 1.24, and the phase transformation took place as V became VH0.5, followed by VH1.0 and then VH2. Density functional theory calculations indicated that the BCT structure model with hydrogen atoms fully occupying the octahedral sites (denoted as the Oz site) can explain the experimentally obtained crystal structure for VH1.0; they also indicated that the VH1.0 phase was stabilized by the addition of nitrogen. In addition, the nitrogen occupation site changed from the Oz site in VH0.5 and VH1.0 to the tetrahedral site in VH2 in coordination with hydrogen during hydrogen absorption. A similar phenomenon was observed in carbon-containing vanadium. It can thus be concluded that the phase transformation pathway and stability of the hydride phases in the V–H system are highly sensitive to the addition of interstitial carbon and nitrogen atoms

Density functional theory studies on theelectronic, structural, phonon dynamicaland thermo-stability properties of bicarbonates MHCO3, M D Li, Na, K

The structural, electronic, phonon dispersion and thermodynamic properties of MHCO3 (M D Li, Na, K) solids were investigated using density functional theory. The calculated bulk properties for both their ambient and the high-pressure phases are in good agreement with available experimental measurements. Solid phase LiHCO3 has not yet been observed experimentally. We have predicted several possible crystal structures for LiHCO3 using crystallographic database searching and prototype electrostatic ground state modeling. Our total energy and phonon free energy .FPH/ calculations predict that LiHCO3 will be stable under suitable conditions of temperature and partial pressures of CO2 and H2O. Our calculations indicate that the HCO 3 groups in LiHCO3 and NaHCO3 form an infinite chain structure through O#1; #1; #1;H#1; #1; #1;O hydrogen bonds. In contrast, the HCO 3 anions form dimers, .HCO 3 /2, connected through double hydrogen bonds in all phases of KHCO3. Based on density functional perturbation theory, the Born effective charge tensor of each atom type was obtained for all phases of the bicarbonates. Their phonon dispersions with the longitudinal optical–transverse optical splitting were also investigated. Based on lattice phonon dynamics study, the infrared spectra and the thermodynamic properties of these bicarbonates were obtained. Over the temperature range 0–900 K, the FPH and the entropies (S) of MHCO3 (M D Li, Na, K) systems vary as FPH.LiHCO3/ > FPH.NaHCO3/ > FPH.KHCO3/ and S.KHCO3/ > S.NaHCO3/ > S.LiHCO3/, respectively, in agreement with the available experimental data. Analysis of the predicted thermodynamics of the CO2 capture reactions indicates that the carbonate/bicarbonate transition reactions for Na and K could be used for CO2 capture technology, in agreement with experiments

Clathrate hydrates for production of potable water

Clathrate hydrates are crystalline inclusion compounds of water and a guest molecule that can form at temperatures above the freezing point of water. Such inclusion compounds exclude dissolved solutes, e.g., sodium chloride present in the aqueous phase, and thereby provide a basis for desalination. Clathrate hydrate formation experiments were performed using several guest molecules, including R141b (CFClH), a commercial refrigerant, and ethylene. Ethylene, a gaseous hydrate guest, readily formed hydrates with saline water at up to 5°C and 20 atm. of pressure. Hydrates of R141b, in the liquid state, were formed at temperatures from 2°C to 6°C and atmospheric pressure from deionized water and 2% - 7% NaCl solutions. Significant reductions in saline content were obtained with both forming agents in a batch reactor without additional separation equipment. Samples of the R141b hydrates were characterized by cold-stage x-ray diffraction and Raman spectroscopy and determined to be structure II. Proof-of-concept experiments were performed to demonstrate a novel technique of desalination using R141b as the hydrate forming agent and an inert secondary fluid

Computational and spectroscopic studies of dichlorofluoroethane hydrate structure and stability

Clathrate hydrates consisting of HCFC (hydrochlorofluorocarbon) guest molecules within host water cages represent a promising new medium for water desalination. The HCFC used in this study, 1,1-dichloro-1-fluoroethane (R 141b), forms a structure II hydrate phase at mild conditions (0°C, 0 atm). We present a detailed molecular picture of the structure and dynamics of guest R141b molecules within water cages, obtained from ab initio calculations, molecular dynamics simulations, and Raman spectroscopy. Such information will be needed to understand and control the nucleation and growth of these hydrates for industrial applications. Density functional theory calculations were used to provide an energetic and molecular orbital description of R141b stability in both large and small cages in a structure II hydrate. Additionally, the hydrate of an isomer, 1,2-dichloro-1-fluoroethane, does not form at ambient conditions due to extensive overlap of electron density between guest and host. Results for the isomer hydrate were supported by classical molecular dynamics simulations and synthesis attempts. Molecular dynamics simulations show that R141b hydrate is stable at temperatures up to 265 K, while the isomer hydrate is only stable up to 150 K. Despite hydrogen bonding between guest and host, R 141b molecules rotate freely within the water cage. The Raman spectrum of R141b in both the pure and hydrate phases is also compared with vibrational analysis from both computational methods. In particular, the frequency of a carbon-halogen stretch mode (585 cm ) undergoes a shift to higher frequency in the hydrate phase. Raman spectra also indicate that this peak undergoes splitting and intensity variation as the temperature is decreased from +4 to -4°C

Decomposition Behavior of Eutectic LiBH₄–Mg(BH₄)₂ and Its Confinement Effects in Ordered Nanoporous Carbon

We present the crystal structure, diborane (B₂H₆) and triborane (B₃H<i>_n</i>) evolution, and dehydrogenation kinetics, of both bulk and nanoconfined Li/Mg­(BH₄)₃ in a highly ordered nanoporous carbon template. The bialkali borohydride Li/Mg­(BH₄)₃ mainly forms a structure similar to that of α-Mg­(BH₄)₂. The decomposition temperature of Li/Mg­(BH₄)₃ lies between that of LiBH₄ and Mg­(BH₄)₂. A direct line-of-site residual gas analyzer mass spectrometer shows that very little diborane and no detectable triborane are released during the decomposition of bulk Li/Mg­(BH₄)₃, which is quite different from Mg­(BH₄)₂ or LiBH₄, indicating that the dual-cation borohydride undergoes a different decomposition pathway, and that the reaction pathway related to diborane or triborane formation was suppressed. The nanoconfined Li/Mg­(BH₄)₃ shows a higher cycling capacity as well as a lower decomposition temperature but, in contrast, produces more diborane and triborane in comparison with bulk Li/Mg­(BH₄)₃