Bulletin of the American Physical Society
2005 APS March Meeting
Monday–Friday, March 21–25, 2005; Los Angeles, CA
Session U14: Hydrogen Storage III: Modeling |
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Sponsoring Units: FIAP Chair: Frederick E. Pinderton, General Motors Room: LACC 403B |
Thursday, March 24, 2005 8:00AM - 8:36AM |
U14.00001: Physical properties of Ti-doped sodium alanates: First-principles studies and experiments Invited Speaker: The recent surge in research on sodium alanates, NaAlH$_4$ and Na$_3$AlH$_6$, has been motivated by Bogdanovic's discovery that, when doped with small amounts of Ti, these compounds can reversibly store more than 4 wt.\% hydrogen. The location of catalytically active Ti and the mechanisms of enhanced H$_2$ sorption kinetics are still poorly understood. We will report on combined first-principles and experimental studies of structural, thermophysical and lattice dynamical properties of bulk alanates. Polarized Raman scattering on single crystals of NaAlH$_4$ has been used to determine the frequencies of the Raman-active vibrational modes between 300 and 425~K, i.e., up to the melting point $T_{\rm m}$. Significant softening (by up to 6\,\%) is observed in the modes involving rigid translations of Na$^+$ cations and translations and librations of AlH$_4^{-}$ tetrahedra. Surprisingly, less than 1.5\,\% softening is seen for the Al-H stretching and Al-H bending modes, indicating that the AlH$_4^{-}$ anion remains a stable structural entity even near $T_{\rm m}$. The phonon mode Gr\"uneisen parameters, calculated using the quasiharmonic approximation, are found to be significantly higher for the translational and librational modes than for the Al-H bending and stretching modes, but cannot account quantitatively for the dramatic softening observed near $T_{\rm m}$, suggesting an essentially anharmonic mechanism. The calculated lattice expansion due to zero-point vibrations is found to be large (1.2 and 1.5\% for the $a$ and $c$ parameters, respectively), as expected for a compound with many light elements. The formation energies of Ti impurities in bulk alanates are found to be high ($>1$~eV), indicating that bulk substitution should not occur under normal conditions. We discuss the implications of these results for the kinetics of hydrogen release and hypothesize that breaking up the AlH$_4^{-}$ anions is the rate limiting step. The enhanced kinetics in Ti-doped NaAlH$_4$ powders is attributed to the effectiveness of Ti in promoting the break-up of the AlH$_4^{-}$ anions at the interface between NaAlH$_4$ and Na$_3$AlH$_6$. [Preview Abstract] |
Thursday, March 24, 2005 8:36AM - 8:48AM |
U14.00002: Atomic and electronic structure of alkali borohydrides: An \textit {ab initio} study W. Gempel, N. Kioussis, D. Papaconstantopoulos Alkali borohydrides MBH$_4$ (M = Na, K) have attracted great interest recently due to their potential applications as hydrogen storage materials and energy carriers for fuel cells due to the extremely large gravimetric capacity At low temperature the compounds crystallize with a tetragonal structure having P42/nmc symmetry in which the [BH$_4$]- complexes are ordered. We have carried out total-energy \textit{ab initio} electronic structure calculations based on the Projector Augmented Wave (PAW) method to calculate the atomic and electronic structure of this series. The lattice constants and various bond lengths are in good agreement with experiment. Results of the trend of the heat of formation for the hydriding/dehydriding reactions, the band structure, the density of states, and bonding properties of the [BH$_4$]- complexes will be discussed. [Preview Abstract] |
Thursday, March 24, 2005 8:48AM - 9:00AM |
U14.00003: First principles study of absorption of Hydrogen into Pd(111) Sampyo Hong, Talat Rahman ~~~ It is well known that Hydrogens interact with Palladium to form a hydride PdH, and numerous studies both experimental and theoretical have been devoted to the system. However, important data such as the energy barriers for hydrogen absorption in the Pd substrate which eventually leads to formation of the hydride are not well known. In order to understand the rationale for hydrogen absorption process, we have carried out first-principles electronic structure calculations for high and low coverages of H on Pd(111). We have found that for subsurface H which is absorbed in the octahedral position below the top layer on Pd(111) the energy barrier to be overcome is 0.6 eV for low coverage and increases for high coverage. For a comparative study we have carried out our additional calculations for H on Pt(111). In this talk, we will compare the coverage dependent barriers for H on Pd(111) with those on Pt(111) and obtain insights from first-principles calculations about how they differ. *Work supported in part by US-DOE under grant DE-FGO2-03ER154645. [Preview Abstract] |
Thursday, March 24, 2005 9:00AM - 9:12AM |
U14.00004: Hydrogens in Metal Clusters Shuhei Ohnishi Atomic and electronic structures of hydrogen atoms in metal clusters are presented by the first principles calculation based on the density functional theory using the linear combination of atomic orbital method. Discussions are focused on a formation of the vacancy-hydrogen cluster at the cluster center mainly in bcc metals. We found characteristic double stable positions at the vacancy site. Electronic structures of double minimum states are studied in terms of the hydrogen induced states. We analyze stabilities of clusters with and without the vacancy typically by the cluster models of M$_{50}$H$_{6}$, M$_{50}$H$_{12 }$, and M$_{51}$H$_{6}$, respectively (M= Nb, Mo, V, Cr, Fe, .etc.). Cluster size effects and the maximum capacity of hydrogen at the vacancy site will also be discussed. [Preview Abstract] |
Thursday, March 24, 2005 9:12AM - 9:24AM |
U14.00005: Ab initio investigation of LiNH$_{2}$, Li$_{2}$NH, and Mg(AlH$_{4}$)$_{2}$ complex hydrides Blanka Magyari-Kope, Vidvuds Ozolins First-principles calculations on the complex hydrides LiNH$_{2}$, Li$_{2}$NH, and Mg(AlH$_{4}$)$_{2}$ were performed to determine their structural stability, electronic structure and formation energy. All these compounds were recently reported the most promising materials for reversible hydrogen storage. We discuss the ionic character and binding implications of other complex hydrides. Possibilities to improve the hydriding/dehydriding reactions are presented. [Preview Abstract] |
Thursday, March 24, 2005 9:24AM - 9:36AM |
U14.00006: Lattice Dynamics and Thermodynamic Properties of Complex Hydride NaAlH$_4$ Amra Peles, M. Y. Chou We present a first-principles investigation of the lattice dynamics and thermodynamical properties of the complex hydride NaAlH$_4$, a promising candidate for hydrogen storage. The calculations are performed within the density functional framework and using a linear response theory. Calculations of the phonon spectrum, Born effective charges Z* of the atoms, dielectric constants in high and low frequency limit are reported. The mode characteristics of zone-center phonons including LO-TO splitting are identified and compared to experiment. The quasiharmonic approach is used to study thermal expansion together with the mean square displacement of each atom and its relation to the melting point. The inclusion of the zero-point motion yields an expanded lattice compared to the static case, while the low-frequency correlated oscillations of Na and AlH$_4$ complexes are found to play an important role in destabilizing the lattice. [Preview Abstract] |
Thursday, March 24, 2005 9:36AM - 9:48AM |
U14.00007: Predicted high storage of hydrogen via H2 complexes on titanium-decorated nanotubes Taner Yildirim, S. Ciraci Developing safe, cost-effective, and practical means of storing hydrogen is crucial for the advancement of hydrogen and fuel- cell technologies. The current state-of-the-art is at an impasse in providing any materials that meet a storage capacity of 6wt\% or more required for practical applications. Accurate quantum mechanical calculations that predict new materials or routes to engineering materials properties are important to overcome this barrier. Here we report a first-principles study, which demonstrates that a single Ti atom coated on a single-walled nanotube (SWNT) strongly binds up to four hydrogen molecules. The first H2 adsorption is dissociative with no energy barrier while other three adsorptions are molecular with significantly elongated H-H bonds. At high Ti coverage we show that a (8,0) SWNT can strongly adsorb up to 8wt\% hydrogen. Simulations at high temperature indicate that the system is quite stable and exhibits associative desorption upon heating, a requirement for reversible storage. These results not only advance our fundamental understanding of dissociative adsorption of hydrogen on transition metals in nano-structures but also suggest new routes to better storage and catalyst materials. [Preview Abstract] |
Thursday, March 24, 2005 9:48AM - 10:00AM |
U14.00008: Confinement effects on chemical reactions in nanostructured carbon materials Aaron George, Milen Kostov, Erik Santiso, Keith Gubbins, Marco Buongiorno Nardelli Chemical reactions are frequently carried out in nano-structured media, such as micellar or colloidal solutions, nano-porous media, hydrogels or organogels, or in systems involving nano-particles. Nanostructured environments have been shown to enhance reaction rates through a variety of catalytic effects, such as high surface area, interactions with the nano-structure or confinement. In this work, we have used state-of-the-art electronic structure techniques to study the prototypical example of the hydrogen-producing reaction of formaldehyde dissociation (H$_2$CO $\rightarrow$ H$_2$~+~CO) within various configurations of a graphitic pore. Using the Nudged Elastic Band (NEB) method for transition states analysis, we have found that the activation energy of the dissociation can be influenced by the presence of a graphitic pore. In particular, while a graphene surface reduces the activation barrier for the reaction, this catalytic effect is enhanced by the presence of two planar sheets, which mimic the geometry of a nano-pore. These findings will be discussed in terms of the charge transfer and/or polarization mechanism associated with the catalytic process. [Preview Abstract] |
Thursday, March 24, 2005 10:00AM - 10:12AM |
U14.00009: First principles study of ammonia decomposition on Ni and Pd surfaces Talat S. Rahman, Sergey Stolbov Ammonia is considered as an efficient storage for hydrogen. It can be converted to hydrogen on board vehicle by decomposition. The decomposition requires efficient catalyst that still has to be designed based on systematic understanding of the reaction mechanisms. We present results of first principles electronic structure calculations based on density functional theory and the generalized gradient approximation of various stage of the decomposition of NH$_3$ on Ni and Pd surfaces. It is known that the ammonia decomposition rate is much higher on Ni that on Pd. The reaction mostly occurs on surface steps and defects. We calculate, compare and contrast adsorption energies, the paths and energy barriers for NH$_3$ diffusion and dissociation on singular and stepped Ni and Pd surfaces, as well as on those with vacancies. The differences in the characteristics of the energy landscape on the various surfaces are explained through analysis of the local densities of electronic states and valence charge densities calculated for the molecule located at preferred adsorption sites and saddle points on the reaction paths. Contact will be made with available experimental data. [Preview Abstract] |
Thursday, March 24, 2005 10:12AM - 10:24AM |
U14.00010: LiNH$_2$--MgH$_2$ as a Potential Hydrogen Storage Material: a Density Functional Theory Study Ralph H. Scheicher, C. Moyses Araujo, Rajeev Ahuja LiNH$_2$ possesses high capacity for hydrogen storage [1], but its large hydride formation enthalpy leads to operating temperatures and pressures that lie outside the practicable range for vehicular applications. Partial substitution of Li by Mg can destabilize the system and thus improve the hydrogen de-sorption characteristics, as it has been shown in recent experimental studies [2]. We present and discuss results from our density functional theory investigations of the LiNH$_2$--MgH$_2$ system. The main aim of this study is to understand the bonding characteristics, the Mg-induced destabilization mechanism, and the thermodynamics of hydrogen de-sorption from an electronic structure viewpoint. [1] P. Chen et al., J. Phys. Chem. B 107, 10967 (2003). [2] W. Luo, J. Alloys Compd. 381, 284 (2004). [Preview Abstract] |
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