Bulletin of the American Physical Society
2006 APS March Meeting
Monday–Friday, March 13–17, 2006; Baltimore, MD
Session N16: Focus Session: Hydrogen Storage III |
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Sponsoring Units: FIAP Chair: Jan F. Herbst, General Motors Room: Baltimore Convention Center 312 |
Wednesday, March 15, 2006 8:00AM - 8:12AM |
N16.00001: Ambient and High Pressure Structural Studies on TiH$_{2}$ Patricia Kalita, Ravhi Kumar, Andrew Cornelius Currently metal hydrides attract intense research interest because of their potential application as hydrogen storage materials. We performed in situ high-pressure synchrotron x-ray diffraction as well as high-pressure Raman spectroscopy studies on TiH$_{2}$ at pressures up to 20 GPa. Low temperature ambient pressure x-ray diffraction studies were also carried out. A phase transition from a high symmetry cubic structure to a lower symmetry tetragonal structure was observed as temperature is lowered below room temperature. The unit cell parameters as well as the equation of state were calculated. To the best of our knowledge this is the first report of high pressure synchrotron x-ray diffraction as well as high-pressure Raman spectroscopy studies on TiH$_{2}$. [Preview Abstract] |
Wednesday, March 15, 2006 8:12AM - 8:24AM |
N16.00002: Optical Spectroscopy of PdO and Pd thin Films under hydrogen exposure J.I. Avila, M. Favre, U.G. Volkmann, A.L. Cabrera, D. Lederman Palladium oxide (PdO) is a p-type semiconductor with a bandgap appropriate to absorb light in the visible range an thus generating an electrical current. This bandgap, being of the order of 2.5 eV, remains yet not accurately determined. We are currently setting up a new experiment in which we are able to monitor the evolution of reflectivity or transmission, for several wavelengths in the visible spectrum, and the resistivity as a function of the exposure time to hydrogen. We should be able to calculate the evolution of bandgap of the semiconductor as a function of reduction from the reflectivity data. Pd films also change reflectivity properties during hydrogen absorption. The experimental set up consist of a tungsten light, Spectrapro 275 monochromator with a diffraction grid of 1200 lines/mm, and a silicon diode detector. Preleminary results will be shown. Based on these data we expect to depict a model for the changes measured. [Preview Abstract] |
Wednesday, March 15, 2006 8:24AM - 8:36AM |
N16.00003: First-principles prediction of a new metallic carbon hydride: bcc-CH$_{2}$ Cesar Cab, Romeo de Coss, Gerko Oskam, Gabriel Murrieta, Gabriel Canto The observation of a new carbon phase in nanoparticles having the body-centered-cubic structure (bcc) has been reported very recently. However, has been suggested that hydrogen is present in the samples forming solid CH$_{2}$ with the anti-cuprite structure. The structural and electronic properties of bcc-C and bcc-CH$_{2}$ are unknown. In the present work we have studied the elastic stability and the electronic structure of these systems by means of first-principles total-energy calculations. The results were obtained with the pseudopotentials LCAO method (SIESTA code) and the Generalized Gradient Approximation (GGA) for the exchange-correlation potential. We have evaluated the structural stability via the elastic properties, we find that bcc-CH$_{2}$ is stable with a lattice parameter very close to the experimental value. In addition, we find that the electronic structure of bcc-CH$_{2}$ exhibits metallic behavior with a relatively high density of states at the Fermi level. The relevance of this new hydride to the problem of hydrogen storage is discussed. [Preview Abstract] |
Wednesday, March 15, 2006 8:36AM - 9:12AM |
N16.00004: Combined neutron scattering and first principles study of novel hydrogen Invited Speaker: |
Wednesday, March 15, 2006 9:12AM - 9:24AM |
N16.00005: Ab Initio Thermochemistry and Elastic Properties of Alkaline Earth Hydrides Louis Hector, Jr., Jan Herbst, Walter Wolf, Paul Saxe In addition to comprising a scientifically interesting class of materials, the binary alkaline earth hydrides are important components of hydrogen sorption/desorption reactions. Of critical importance for predicting the thermodynamic stability of hydrides is the enthalpy of hydride formation, $\Delta $H, which links the temperature and pressure of hydrogen sorption via the van't Hoff relation. We compare LDA and GGA predictions of the heats of formation and elastic properties of alkaline earth metals and their binary hydrides BeH$_{2}$, MgH$_{2}$, CaH$_{2}$, SrH$_{2}$, and BaH$_{2}$ using a plane wave density functional method. Phonon calculations using the direct method enabled prediction of the zero point energies of each material and the 0K and 298K heats of formation. We also computed the 0K and 298K cohesive energies for the alkaline earth metals. Born effective charge tensors were computed via the Berry phase method and enabled prediction of the phonon dispersion curves with LO/TO zone center splittings. It was found that the LO/TO splittings have no effect on the computed zero point energies and heats of formation. The elastic constants were computed with a least squares fitting method using a set of sequentially-applied strains to improve the accuracy of each calculation. Comparison of results from the least squares methodology with prior results using the Hartree-Fock method suggest that the former is substantially more accurate for predicting hydride elastic properties. [Preview Abstract] |
Wednesday, March 15, 2006 9:24AM - 9:36AM |
N16.00006: Hydrogen uptake and the 18-eletron rule Kiran Boggavarapu, Anil Kandalam, Puru Jena Hydrogen is considered to be an ideal energy carrier in the foreseeable future; however, the key problem is its storage. Solid state materials capable of storing hydrogen with high gravimetric (9 wt {\%}) and volumetric density (70 g/L) operating under ambient thermodynamic conditions and exhibiting fast hydrogen sorption kinetics are of practical importance. It is clear that the storage material should consist of light elements such as Li, B, and C etc. Hydrides of these elements are too strongly bound to be easily desorbed. Attempts were made to deposit light weight transition metals on carbon surfaces such as fullerenes, nanotubes etc., however, they tend to cluster together reducing hydrogen uptake dramatically. One way to achieve high storage is to functionalize simple organic molecules such as C$_{4}$H$_{4}$, C$_{5}$H$_{5}$ etc. with light weight metals such as Sc and Ti. In this presentation, we will discuss based on DFT computations, the dependence of hydrogen uptake on the nature of substrate, the desportion energies, and the nature of bonding. [Preview Abstract] |
Wednesday, March 15, 2006 9:36AM - 9:48AM |
N16.00007: \textbf{\textit{Ab initio}}\textbf{ simulation of hydrogen storage in BN systems } Stephen Shevlin, Zhengxiao Guo We model via first principles simulation hydrogen storage in boron nitride systems, such as h-BN sheets, the paradigm BN molecule borazine (B$_{3}$N$_{3}$H$_{6})$ and ammonia-borane (BNH$_{6})$. We found H$_{2}$ preferentially adsorbs on the perfect h-BN surface but strongly bound atomic hydrogen prefers to adsorb on vacancies, with consequences for hydrogen storage. The addition of TM (transition metal) atoms to boron nitride, to act as adsorbents for hydrogen, was investigated using borazine as a prototype system for h-BN. The binding of TM atoms (Sc, Ti, V etc.) to borazine was determined, with the variation in bonding intimately related to the electronic structure. The dopants were found to promote the binding of both hydrogen atoms and molecules to borazine, increasing binding energy by $\sim $300{\%} and 1500{\%}, respectively. Initially TM dihydrides form but as hydrogen concentration increases molecular hydrogen becomes preferred. Bound hydrogen is stable at room temperature and the maximum hydrogen capacity and kinematics of this prototype system will be presented. In addition, the dissociation of BNH$_{6}$ \textit{in vacuo}, on the surface of MgH$_{2}$, and in the presence of TM catalysts is modeled. [Preview Abstract] |
Wednesday, March 15, 2006 9:48AM - 10:00AM |
N16.00008: Atomic and Molecular Hydrogen Interaction with Ti-Doped Al (100): Hydrogen Dissociation and Surface Alane Formation Erik Muller, Peter Sutter, Percy Zahl, Santanu Chaudhuri, James Muckerman A comprehensive research effort on the atomistic mechanisms underlying hydrogen storage in Ti-doped NaAlH$_{4}$ is aimed at deriving a knowledge base for the rational optimization of this and other related complex hydride materials. Our investigation focuses on the role of the Ti dopants in promoting reversible hydrogenation, a key requirement for any practical hydrogen storage material. The re-hydrogenation reaction proceeds from the crucial initial step of dissociative adsorption of molecular hydrogen on Al or NaH. A specific Al:Ti complex was recently predicted as an active site for H$_{2}$ dissociation on extended Al(100) surfaces [1]. Combining high-resolution surface imaging experiments (scanning tunneling microscopy, low-energy electron microscopy) with density functional theory, we are investigating the dissociative adsorption of H$_{2}$ on Ti-doped Al(100) prepared in ultrahigh vacuum. We will discuss our progress toward identifying catalytically active sites for H2 dissociation on this surface, as well as pathways toward the formation of mobile Al-species. [1] S. Chaudhuri and J.T. Muckerman, J. Phys. Chem. B 109, 6952 (2005). [Preview Abstract] |
Wednesday, March 15, 2006 10:00AM - 10:12AM |
N16.00009: Theoretical Study of Hydrogen Dissociation on the TiAl$_{3}$ Surface Yan Wang, M. Y. Chou In order to better understand the catalytic role played by Ti in enhancing the reaction kinetics of sodium alanate, we present a first-principles investigation of hydrogen dissociation and adsorption on the pure Al surface as well as on the Ti doped surface with a local alloy composition of TiAl$_{3. }$ The most energetically favorable location for Ti near the surface is identified. It is found that the presence of Ti promotes H adsorption on the surface with the H atom sitting on top of an Al atom. The binding between Ti and Al modifies the surface charge distribution near the adsorption site and facilitates the adsorption process. The potential energy surface for H$_{2}$ dissociation over both pure Al and the alloy surfaces are also discussed. [Preview Abstract] |
Wednesday, March 15, 2006 10:12AM - 10:24AM |
N16.00010: Phase Stability of Mixed-Alkali Alanates Zhu Ma, Mei-Yin Chou To date sodium alanate NaAlH$_4$ is the only reversible complex hydride that satisfies the international density targets for hydrogen storage materials of 5 wt.\% and 70 kg/m$^3$. The reversible hydrogenation process takes place at reasonable conditions. Therefore, it is desirable to increase the H wt.% by partially replacing Na with a lighter alkali metal such as Li. To study the stability of these mixed-alkali alantes, we perform first-principles calculations for the alloy systems Na$_{1-x} $Li$_x$Al H$_4$ and Na$_{3(1-x)}$Li$_{3x}$AlH$_6$ within the framework of density functional theory and pseudopotentials. For the compositions we have considered for the tetrahydrides, the mixing energies are all positive, indicating that the sodium and lithium alanates prefer being phase separated. For the hexahydrides, one stable intermediate compound is found. The binding characteristics of these mixed-alkali alanates will be discussed. [Preview Abstract] |
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