Decomposition mechanisms in metal borohydrides and their ammoniates

Ammoniation in metal borohydrides (MBs) with the form $\mathcal{M}$(BH$_4$)$_x$ has been shown to lower their decomposition temperatures with $\mathcal{M}$ of low electronegativity ($\chi_p \lesssim 1.6$), but raise it for high-$\chi_p$ MBs ($\chi_p \gtrsim 1.6$). Although this behavior is just as desired, an understanding of the mechanisms that cause it is still lacking. Using \emph{ab initio} methods, we elucidate those mechanisms and find that ammoniation always causes thermodynamic destabilization, explaining the observed lower decomposition temperatures for low-$\chi_p$ MBs. For high-$\chi_p$ MBs, we find that ammoniation blocks B$_2$H$_6$ formation---the preferred decomposition mechanism in these MBs---and thus kinetically stabilizes those phases. The shift in decomposition pathway that causes the distinct change from destabilization to stabilization around $\chi_p=1.6$ thus coincides with the onset of B$_2$H$_6$ formation in MBs. Furthermore, with our analysis we are also able to explain why these materials release either H$_2$ or NH$_3$ gas upon decomposition. We find that NH$_3$ is much more strongly coordinated with higher-$\chi_p$ metals and direct H$_2$ formation/release becomes more favorable in these materials. Our findings are of importance for unraveling the hydrogen release mechanisms in an important new and promising class of hydrogen storage materials, allowing for a guided tuning of their chemistry to further improve their properties

Competitive co-adsorption of CO2 with H2O, NH3, SO2, NO, NO2, N2, O2, and CH4 in M-MOF-74 (M= Mg, Co, Ni): the role of hydrogen bonding

The importance of co-adsorption for applications of porous materials in gas separation has motivated fundamental studies, which have initially focused on the comparison of the binding energies of different gas molecules in the pores (i.e. energetics) and their overall transport. By examining the competitive co-adsorption of several small molecules in M-MOF-74 (M= Mg, Co, Ni) with in-situ infrared spectroscopy and ab initio simulations, we find that the binding energy at the most favorable (metal) site is not a sufficient indicator for prediction of molecular adsorption and stability in MOFs. Instead, the occupation of the open metal sites is governed by kinetics, whereby the interaction of the guest molecules with the MOF organic linkers controls the reaction barrier for molecular exchange. Specifically, the displacement of CO2 adsorbed at the metal center by other molecules such as H2O, NH3, SO2, NO, NO2, N2, O2, and CH4 is mainly observed for H2O and NH3, even though SO2, NO, and NO2, have higher binding energies (~70-90 kJ/mol) to metal sites than that of CO2 (38 to 48 kJ/mol) and slightly higher than water (~60-80 kJ/mol). DFT simulations evaluate the barriers for H2O->CO2 and SO2->CO2 exchange to be - 13 and 20 kJ/mol, respectively, explaining the slow exchange of CO2 by SO2, compared to water. Furthermore, the calculations reveal that the kinetic barrier for this exchange is determined by the specifics of the interaction of the second guest molecule (e.g., H2O or SO2) with the MOF ligands

Ab initio energetics and kinetics study of H2 and CH4 in the SI clathrate hydrate

We present ab initio results at the density functional theory level for the energetics and kinetics of H2 and CH4 in the SI clathrate hydrate. Our results complement a recent article by some of the authors [G.Román-Pérez et.al., Phys.Rev.Lett. 105, 145901 (2010)] in that we show additional results of the energy landscape of H2 and CH 4 in the various cages of the host material, as well as further results for energy barriers for all possible diffusion paths of H2 and CH4 through the water framework. We also report structural data of the low-pressure phase SI and the higher-pressure phases SII and S

Trapping gases in metal-organic frameworks with a selective surface molecular barrier layer

The main challenge for gas storage and separation in nanoporous materials is that many molecules of interest adsorb too weakly to be effectively retained. Instead of synthetically modifying the internal surface structure of the entire bulk—as is typically done to enhance adsorption—here we show that post exposure of a prototypical porous metal-organic framework to ethylenediamine can effectively retain a variety of weakly adsorbing molecules (for example, CO, CO₂, SO₂, C₂H₂, NO) inside the materials by forming a monolayer-thick cap at the external surface of microcrystals. Furthermore, this capping mechanism, based on hydrogen bonding as explained by ab initio modelling, opens the door for potential selectivity. For example, water molecules are shown to disrupt the hydrogen-bonded amine network and diffuse through the cap without hindrance and fully displace/release the retained small molecules out of the metal-organic framework at room temperature. These findings may provide alternative strategies for gas storage, delivery and separation

Fluorinated benzalkylsilane molecular rectifiers

We report on the synthesis and electrical properties of nine new alkylated silane self-assembled monolayers (SAMs) – (EtO)3Si(CH2)nN = CHPhX where n = 3 or 11 and X = 4-CF[subscript 3], 3,5-CF[subscript 3], 3-F-4-CF[subscript 3], 4-F, or 2,3,4,5,6-F, and explore their rectification behavior in relation to their molecular structure. The electrical properties of the films were examined in a metal/insulator/metal configuration, with a highly-doped silicon bottom contact and a eutectic gallium-indium liquid metal (EGaIn) top contact. The junctions exhibit high yields (>90%), a remarkable resistance to bias stress, and current rectification ratios (R) between 20 and 200 depending on the structure, degree of order, and internal dipole of each molecule. We found that the rectification ratio correlates positively with the strength of the molecular dipole moment and it is reduced with increasing molecular length.National Science Foundation (U.S.) (Award ECCS 1254757)Wake Forest University (Pilot Research Grant

ES12; The 24th Annual Workshop on Recent Developments in Electronic Structure Theory

ES12: The 24th Annual Workshop on Recent Developments in Electronic Structure Theory was held June 5-8, 2012 at Wake Forest University in Winston-Salem, NC 27109. The program consisted of 24 oral presentations, 70 posters, and 2 panel discussions. The attendance of the Workshop was comparable to or larger than previous workshops and participation was impressively diverse. The 136 participants came from all over the world and included undergraduate students, graduate students, postdoctoral researchers, and senior scientists. The general assessment of the Workshop was extremely positive in terms of the high level of scientific presentations and discussions, and in terms of the schedule, accommodations, and affordability of the meeting