Showing papers by "Taner Yildirim published in 2007"

Hydrogen and Methane Adsorption in Metal−Organic Frameworks: A High-Pressure Volumetric Study

Abstract: We report hydrogen and methane adsorption isotherms in two prototypical metal−organic framework compounds (i.e., MOF5 and ZIF8) over a large temperature (30−300 K) and pressure (up to 65 bar) range using a fully computer-controlled Sieverts apparatus. We find that, in a volumetric method, a proper choice of real gas equation of state is critical for obtaining reliable isotherm data. The widely used van der Waals equation of state (EOS) is not adequate to describe H2 and CH4, while the modified Benedict−Webb−Rubin (MBWR) EOS works well, even at very low temperatures and high pressures. With the known sample mass and bulk density, the skeleton density and the specific pore volume of MOF5 and ZIF8 were also measured. In addition to excess and absolute adsorption isotherms, we also introduce an “effective adsorption” which compares the amounts of gas adsorbed in a container with and without the adsorbent. At low temperatures, the maximal excess adsorption capacities of H2 and CH4 in MOF5 are found to be 10.3 ...

Hydrogen storage in a prototypical zeolitic imidazolate framework-8.

Abstract: Using the difference Fourier analysis of neutron powder diffraction data along with first-principles calculations, we reveal detailed structural information such as methyl group orientation, hydrogen adsorption sites, and binding energies within the nanopore structure of ZIF8 (Zn(MeIM)2). Surprisingly, the two strongest adsorption sites that we identified are both directly associated with the organic linkers, instead of the ZnN4 clusters, in strong contrast to classical MOFs, where the metal-oxide clusters are the primary adsorption sites. These observations are important and hold the key to optimizing this new class of ZIF materials for practical hydrogen storage applications. Finally, at high concentration H2-loadings, ZIF8 structure is capable of holding up to 28 H2 molecules (i.e., 4.2 wt %) in the form of highly symmetric novel three-dimensional interlinked H2-nanoclusters with relatively short H2−H2 distances compared to solid H2. Hence, ZIF compounds with robust chemical stability can be also an id...

Transition Metal-Ethylene Complexes as High-Capacity Hydrogen Storage Media

Abstract: From first-principles calculations, we predict that a single ethylene molecule can form a stable complex with two transition metals (TM) such as Ti. The resulting TM-ethylene complex then absorbs up to ten hydrogen molecules, reaching to gravimetric storage capacity of $\ensuremath{\sim}14\text{ }\text{ }\mathrm{wt}\text{ }%$. Dimerization, polymerizations, and incorporation of the TM-ethylene complexes in nanoporous carbon materials are also discussed. Our results are quite remarkable and open a new approach to high-capacity hydrogen-storage materials discovery.

Electronic, dynamical, and thermal properties of ultra-incompressible superhard rhenium diboride: A combined first-principles and neutron scattering study

Abstract: Rhenium diboride is a recently recognized ultra-incompressible superhard material. Here we report the electronic $(e)$, phonon $(p)$, $e\text{\ensuremath{-}}p$ coupling, and thermal properties of $\mathrm{Re}{\mathrm{B}}_{2}$ from first-principles density-functional theory calculations and neutron scattering measurements. Our calculated elastic constants (${c}_{11}=641\phantom{\rule{0.3em}{0ex}}\mathrm{GPa}$, ${c}_{12}=159\phantom{\rule{0.3em}{0ex}}\mathrm{GPa}$, ${c}_{13}=128\phantom{\rule{0.3em}{0ex}}\mathrm{GPa}$, ${c}_{33}=1037\phantom{\rule{0.3em}{0ex}}\mathrm{GPa}$, and ${c}_{44}=271\phantom{\rule{0.3em}{0ex}}\mathrm{GPa}$), bulk modulus $(B\ensuremath{\approx}350\phantom{\rule{0.3em}{0ex}}\mathrm{GPa})$ and hardness $(H\ensuremath{\approx}46\phantom{\rule{0.3em}{0ex}}\mathrm{GPa})$ are in good agreement with the reported experimental data. The calculated phonon density of states agrees very well with our neutron vibrational spectroscopy result. Electronic and phonon analysis indicates that the strong covalent B-B and Re-B bonding is the main reason for the super incompressibility and hardness of $\mathrm{Re}{\mathrm{B}}_{2}$. The thermal expansion coefficients, calculated within the quasiharmonic approximation and measured by neutron powder diffraction, are found to be nearly isotropic in $a$ and $c$ directions and only slightly larger than that of diamond in terms of magnitude. The excellent agreement found between calculations and experimental measurements indicate that first-principles calculations capture the main interactions in this class of superhard materials, and thus can be used to search, predict, and design new materials with desired properties.

Hydrogen Absorption Properties of Metal- Ethylene Complexes

Abstract: Recently, we have predicted Phys. Rev. Lett. 97, 226102 2006 that a single ethylene molecule can form stable complexes with light transition metals TMs such as Ti and the resulting TMn-ethylene complex can absorb up to 12 and 14 wt % hydrogen for n=1 and 2, respectively. Here we extend this study to include a large number of other metals and different isomeric structures. We obtained interesting results for light metals such as Li. The ethylene molecule is able to complex with two Li atoms with a binding energy of 0.7 eV/Li which then binds up to two H2 molecules per Li with a binding energy of 0.24 eV/H2 and absorption capacity of 16 wt %, a record high value reported so far. The stability of the proposed metal-ethylene complexes was tested by extensive calculations such as normal-mode analysis, finite temperature first-principles moleculardynamics MD simulations, and reaction path calculations. The phonon and MD simulations indicate that the proposed structures are stable up to 500 K. The reaction path calculations indicate about 1 eV activation barrier for the TM2-ethylene complex to transform into a possible lower energy configuration where the ethylene molecule is dissociated. Importantly, no matter which isometric configuration the TM2-ethylene complex possesses, the TM atoms are able to bind multiple hydrogen molecules with suitable binding energy for room-temperature storage. These results suggest that co-deposition of ethylene with a suitable precursor of TM or Li into nanopores of light-weight host materials may be a very promising route to discovering new materials with high-capacity hydrogen absorption properties.

Hydrogen-related catalytic effects of Ti and other light transition metals on NaAlH4 surfaces

Abstract: We report a first-principles study of sodium alanate (NaAlH4) surfaces, both pure and with Ti atoms on them, focusing on their stability and reactivity with hydrogen. We find Ti causes the dissociation of H2 molecules and contributes to the stability of the surface exposed to hydrogen. The Ti catalysts should thus facilitate the reaction by which NaAlH4 (7.4?wt% of H) forms from Na3AlH6 (5.9?wt% of H), which occurs under hydrogen pressure and requires H2 dissociation to proceed. The presence of Ti also results in lower defect-formation energies, which should favour the NaAlH4 decomposition reaction. Indeed, our calculations show that the chemical versatility of Ti brings close in energy a number of steps that presumably are relevant in the formation and decomposition of NaAlH4, which suggests that the catalyst facilitates both processes. We also discuss the possibility of using other light transition metals (Sc, V, and Cr) as catalysts, and conclude that Ti is the best choice overall.

Ab-initio atomic scale study of nearly frictionless surfaces

Abstract: Publisher Summary This chapter presents a comparative analysis of the dry sliding friction between the atomically flat and commensurate surfaces of two different systems: between two diamond (001) surfaces and between two BN (001) surfaces. The interaction between bare diamond (001) – (2 × 1) surfaces is strongly attractive until there is a small spacing d ∼ 1.5 A, and the interaction forms strong chemical bonds between two surfaces. However, the interaction turns repulsive if two surfaces are pushed under high loading force. Upon hydrogenation, hydrogen atoms donate charge to the carbon atom and become positively charged. The interaction between surfaces carrying the same type of charge becomes repulsive. This appears to be the most important component of the superlow friction. The repulsive interaction persists at any relative position of the sliding surfaces and is strong even at large distance to prevent C–H bonds from merging. Strong and stiff C–H bonds and stiff diamond crystal by itself prevent excessive energy from dissipation. The oxygenation of surfaces in the atmospheric conditions finally destroys the steady repulsive interaction. However, the interaction between two bare reconstructed BN (001) surfaces is different from that of the diamond (001) surface because of the ionic nature of the crystal. BN being an ionic crystal with electron transfers from B to N, the bare surface is already negatively charged. Under these circumstances, the strong chemical interaction is canceled by the repulsive Coulombic interaction, resulting in a weak attractive interaction.

Structure and Vibrational Spectra of Calcium Hydride and Deuteride.

Abstract: We have investigated the structure, energetics, and dynamics of calcium hydride (CaH2) and calcium deuteride (CaD2). The crystal structure of CaD2 (space group Pnma) was determined in detail using high-resolution neutron powder diffraction (NPD) data at both 9 and 298 K. The structure is in excellent agreement with the optimized structure derived from first-principles calculations. The phonon calculations based on the optimized structure reproduce well the phonon density of states of CaH2 (and CaD2) measured by neutron vibrational spectroscopy (NVS). The combined NPD and NVS results reveal the complete structural and dynamical details for CaH2 (CaD2). © 2006 Elsevier B.V. All rights reserved.