Session L6: Focus Session: van der Waals Bonding in Advanced Materials - Metal Oxide Frameworks & Active Materials

Trends in CO$_{2}$-MOF Binding from First Principles: Implications for Gas Separations

Metal-organic frameworks (MOFs) are a class of highly ordered, highly customizable nanoporous materials that are attractive for use in energy-relevant gas separations. MOF-253 Al(OH)(bpydc) can be post-synthetically modified by introduction of metal cations and charge-stabilizing anions [1]. These post-synthetically modified MOF-253 samples have been shown to exhibit enhanced CO$_{2}$-N$_{2}$ selectivity over the unmodified framework [1]. Here we use van der Waals-corrected density functional theory (DFT) to study CO$_{2}$ binding energy trends in this series of modified frameworks. Particular focus is paid to examining the predictive power of our calculations on modified bipyridine fragments as a proxy for the full framework, as well as the suitability of binding energy trends to predict measured gas selectivity trends [1]. We focus on the following series of 10 post-synthetic modifications: CoCl$_{2}$, CuCl$_{2}$, FeCl$_{2}$, NiCl$_{2}$, PdCl$_{2}$, Co(BF$_{4})_{2}$, Cu(BF$_{4})_{2}$, Fe(BF$_{4})_{2}$, Ni(BF$_{4})_{2}$, Pd(BF$_{4})_{2}$.\\[4pt] [1] E. Bloch, et. al, J. Am. Chem. Soc., 132, 14382-14384, 2010.   

Ligand-Assisted Enhancement of CO$_{2}$ Capture in Tunable MOFs: Balancing Electrostatic and van der Waals Interactions

Metal-organic frameworks (MOFs) are promising nanoporous materials for CO$_2$ separation technologies. Here, we use first-principles van der Waals (vdW)-corrected calculations to identify and understand how CO$_2$ binds to a novel ``BTT-type'' MOF [1] featuring open metal centers. Our study indicates that CO$_2$ binds to the open metal cation sites, but with an adsorption energy that can be enhanced by more than a factor of two depending on the choice of the bridging ligand. Judicious choices for metal cations and bridging ligands are shown to lead to a maximum binding energy of 0.67 eV for MgBTT. In all cases, the binding can be attributed to a combination of electrostatics and dispersion, both critically sensitive to the local environment, and contributing nearly equally to the overall binding strength. The possibility to independently tailor these energetics in a manner optimal for CO$_2$ capture is discussed in the context of recent experiments.   

Simulations of physical adsorption of gases in IRMOFs

We performed grand canonical Monte Carlo simulations to study the adsorption of noble gases, H$_{2}$ and CO$_{2}$ in IRMOF-1. The IRMOF is modeled as a simple structure where a cubic lattice is composed of adsorption centers that reproduce the strength of the metallic corners and organic linkers in the real structure. From the adsorption isotherms obtained in our simulations we calculated the isosteric heat of adsorption and compare with available experimental results. Research supported by NSF and ACS, PRF.   

Graphene Oxide Derived Carbons (GODC); High-Surface Area NanoPorous Materials for Hydrogen Storage and Carbon Capture

Even though there has been extensive research on gas adsorption properties of various carbon materials based on activated carbon and nanotubes, there has been little work done on the gas adsorption properties of graphite oxide (GO). In this study [1], we show that one-and-a-half-century-old graphite oxide can be easily turned into a potentially useful gas storage material. In order to create high-surface nanoporous materials from GO, we used two different approaches. In the first approach, we have successfully synthesized graphene-oxide framework materials (GOFs) by interlinking GO layers by diboronic acids. The resulting GOF materials have well defined pore size and BET surface area up to 500 m2/g with twice larger heat of adsorption of H$_{2}$ and CO$_{2}$ than those found in other physisorption materials such as MOF5. In the second approach, we synthesized a range of high surface area GO derived carbons (GODC) and studied their applications toward H$_{2}$, CO$_{2}$ and CH$_{4}$ gas storage. The GODCs, with wide range of pore structure, have been prepared by chemical activation with potassium hydroxide (KOH). We obtain largely increased surface areas up to nearly 1900 m$^{2}$/g for GODC samples from 10 m$^{2}$/g for initial GO. A detailed experimental study of high pressure excess sorption isotherms on GODCs reveal an increase in both CO$_{2}$ and CH$_{4}$ storage capacities compared to other high surface area activated carbons. Finally, we compared the gas sorption properties of our GO-based matarials with other systems such as MOFs, ZIFs, and COFs. \\[4pt] [1] See http://www.ncnr.nist.gov/staff/taner for references and more information.   

Isosteric heat of adsorption of CO$_{2}$ in bundles of carbon nanotubes.

Using the grand canonical Monte Carlo method, we have evaluated the adsorption isotherms of CO$_{2}$ on the exterior of a bundle of carbon nanotubes. The isosteric heat is a property of the adsorbate that can be calculated from experimental or simulation data, and gives hints of the energy of adsorption and the structure of the adsorbate. From the simulated adsorption isotherms we calculated the isosteric heat of CO$_{2}$, Ar and CH$_{4}$ and compare with existing experimental data. Research supported by NSF and ACS, PRF.   

Gas Adsorption and Selectivity in Zeolitic Imidazolate Frameworks from First Principles Calculations

Zeolitic Imidazolate Framework (ZIFs) are excellent candidate materials for carbon capture and gas separation. Here we employ the van der Waals density functional (vdW-DF) [1] in an analysis of the binding energetics for CO2, CH4 and N2 molecules in a set of ZIFs featuring different chemical functionalizations. We investigate multiple low-energy binding sites, which differ in their positions relative to functional groups on the imidazole linkers. In all cases an accurate treatment of van der Waals forces appears essential to provide reasonable binding energy magnitudes. We report results obtained from different parameterizations of the vdW-DF, providing comparisons between calculations and experimental values of the heat of adsorption [2]. This research is supported by the Energy Frontier Research Center ``Molecularly Engineered Energy Materials,'' funded by the US Department of Energy, Office of Science, Office of Basic Energy Sciences under Award Number DE-SC0001342. [1] M. Dion, H. Rydberg, E. Schroder, D. C. Langreth, B. I. Lundqvist, Phys. Rev. Let. 92, 246401 (2004) [2] W. Morris, B. Leung, H. Furukawa, O. K. Yaghi, N. He, H. Hayashi, Y. Houndonougbo, M. Asta, B. B. Laird, O. M. Yaghi, J. AM. CHEM. SOC. 2010, 132, 11006-11008   

A theoretical study of the hydrogen-storage potential of (H$_2$)$_4$CH$_4$ in nanotubes and MOFs

The material (H$_2$)$_4$CH$_4$, also called H4M, has exceptional hydrogen-storage potential of up to 33.3~mass\%, not including the hydrogen in CH$_4$.\footnote{W.L. Mao et al., Physics Today {\bf 60}, 42 (2007).} But, unfortunately, H4M is not stable under ambient conditions. For hydrogen storage near ambient pressure, it needs to be cooled to 65~K, and ambient temperature requires a pressure of 5--6~GPa.\footnote{W.L. Mao et al., Chem. Phys. Lett. {\bf 402}, 66 (2005).} In this study we use \emph{ab initio} methods based on van der Waals DFT\footnote{M. Dion et al., Phys. Rev. Lett. {\bf 92}, 246401 (2004).}$^,$\footnote{T. Thonhauser et al., Phys. Rev. B {\bf 76}, 125112 (2007).} to investigate the possibility of creating such pressures through external agents such as metal organic framework (MOF) materials and carbon nanotubes. We find that MOFs can create considerable pressure for H4M in their cavities, but not the required 5--6~GPa, and therefore moderate cooling is still necessary. On the other hand, carbon nano\-tubes can create these high pressures for H4M, but the fact that this pressure exists only inside the nanotubes---and not in-between tubes in e.g.\ a bundle---lowers the volumetric storage density and makes this option less favorable for practical applications.   

The Role of Many-Body Dispersion Interactions in Molecular Crystal Polymorphism

Molecular crystals often have several polymorphs that are close in energy (few meV per molecule), but possess very different physical and chemical properties. Treating polymorphism from first principles has been a long standing problem because conventional density-functional theory (DFT) lacks a proper description of long-range dispersion interactions that govern the structure and energetics of molecular crystals. Here we assess the effect of the many-body dispersion (MBD) energy on the structure and relative energies of the polymorphs of benchmark molecular crystals: glycine, alanine, and para-diiodobenzene. This is accomplished by using the recently developed first-principles DFT+MBD method [A. Tkatchenko, R.A. DiStasio Jr., R. Car, M. Scheffler, submitted], based on the earlier Tkatchenko-Scheffler (TS) dispersion correction [PRL 102, 073005 (2009)]. We show that the non-additive MBD energy plays a crucial role in making qualitatively and quantitatively accurate predictions for the structure and relative energies of polymorphs.   

Interaction of endohedral molecular hydrogen with C$_{60}$: infrared study

We report on the dynamics of isotopically different hydrogen molecules, H$_2$, D$_2$ and HD, trapped in the molecular cages of a fullerene C$_{60}$[Min Ge et al., J. Chem. Phys. {\bf 134}, 054507 (2011), {\bf 135}, 114511 (2011)]. The infrared spectra were measured at temperatures from 5K to 300K and analyzed using a model of a vibrating rotor trapped in a spherical potential. The interaction potential was determined in the ground and in the first excited vibrational state of a hydrogen molecule. The isotropic part of the potential is similar for all three molecules studied. In HD@C$_{60}$ we observe the mixing of rotational states and an interference effect of the dipole moment terms due to the displacement of the HD rotation center from the fullerene cage center. A three-site Lennard-Jones potential in the pairwise additive five-dimensional potential energy surface reproduces the hydrogen IR spectrum with great accuracy[M. Xu et al., J. Chem. Phys.{\bf 130}, 224306 (2009)].   

The role of dispersion interactions in the formation of near-surface oxygen defects in CeO$_2$(111): Comparing DFT$+U$ with HF/DFT hybrid functionals using plane-waves as a basis set

Cerium oxide is of paramount importance in the field of heterogeneous catalysis and in solid oxide fuel cells. Its ability to easily release and store oxygen goes together with the distinct facility to form and heal oxygen vacancies. Upon its formation, the electrons left need to localize, but conventional semilocal density functionals (LDA, GGA) are known to fail here. Viable solutions to this problem are the DFT$+U$ approach and hybrid Hartree-Fock/DFT functionals being computationally more demanding [1]. However, common implementations of aforementioned approaches do not incorporate long-range dispersion interactions. Given that relaxation effects upon defect formation lead to more open structures and given the relatively high polarizability of Ce-atoms, dispersion interactions are supposed to be none-neglible. We apply Grimme's dispersion correction [2] and carefully cross-check results. Furthermore, we will give thorough estimates on dynamic effects and try to shed some light on the effects induced by the defect concentration. [1] M. V. Ganduglia-Pirovano, J. L. F. Da Silva, and J. Sauer, Phys. Rev. Lett. {\bf 102}, 026101 (2009). [2] S. Grimme, J. Comput. Chem. {\bf 27}, 1787 (2006).   

Interaction of W(CO)$_6$ with SiO$_2$ Surfaces

The interaction of tungsten hexacarbonyl W(CO)$_6$ precursor molecules with SiO$_2$ substrates is investigated by means of density functional theory calculations with and without inclusion of long range van der Waals interactions. We consider two different surface models, a fully hydroxylated and a partially hydroxylated SiO$_2$ surface, corresponding to substrates under different experimental conditions. For the fully hydroxylated surface we observe only a weak interaction between the precursor molecule and the substrate with physisorption of W(CO)$_6$. Inclusion of van der Waals corrections results in a stabilization of the molecules on this surface. In contrast, we find a spontaneous dissociation of the precursor molecule on the partially hydroxylated SiO$_2$ surface where chemisorption of a W(CO)$_5$ fragment is observed upon removal of one of the CO ligands from the precursor molecule. Irrespective of the hydroxylation, the precursor molecule prefers binding of more than one of its CO ligands. In the light of these results, implications for the initial growth stage of tungsten nano-deposits on SiO$_2$ in an electron beam induced deposition process are discussed. [1] K. Muthukumar et al. Phys. Rev. B (in press) (2011)