Michael Badawi

A DFT investigation of the adsorption of iodine compounds and water in H-, Na-, Ag-, and Cu- mordenite

The potential use of some cation-exchanged mordenite (H(+), Na(+), Cu(+), and Ag(+)) as a selective adsorbent for volatile iodine species (ICH3 and I2), which can be released during a nuclear accident together with a steam carrier gas, is investigated using density functional theory. It is found that in the case of Cu-MOR and Ag-MOR, the absolute values of interaction energies of ICH3 and I2 are higher than that of water which indicates that these forms of zeolite could be suitable for selective adsorption of iodine species. In contrast, the H-MOR and Na-MOR are found to be unsuitable for this purpose. A systematic investigation of all adsorption sites allowed us to analyze the structural effects affecting the adsorption behavior. For the Ag-MOR and Cu-MOR zeolites, the iodine compounds are adsorbed preferentially in the large channel of mordenite (main channel) while water prefers the small channel or the side pocket where it forms stronger hydrogen bonds. The factors governing the interaction energies between the cationic sites and the different molecules are analyzed and the important role of van der Waals interactions in these systems is highlighted.

Impact of the Si/Al ratio on the selective capture of iodine compounds in silver-mordenite: a periodic DFT study

Silver modified zeolites with a mordenite structure can capture volatile iodine compounds (I2 and ICH3) which can be released during a severe nuclear accident. However under these particular conditions, molecules such as CO and H2O present in the containment atmosphere are expected to inhibit the adsorption of iodine compounds. In the present work, periodic density functional theory calculations have been carried out to investigate the interaction of I2, ICH3, H2O and CO molecules in silver-exchanged mordenite with various Si/Al ratios with the aim of finding values that favor a selective adsorption of I2 and ICH3. Computational results show that the interaction energies of CO and H2O remain of the same order of magnitude (from -120 to -140 kJ mol-1 for CO and from -90 to -120 kJ mol-1 for H2O) for all the investigated Si/Al ratios. In contrast, ICH3 is increasingly strongly adsorbed as the Si/Al ratio decreases, from around -145 kJ mol-1 when Si/Al = 47 to -190 kJ mol-1 for Si/Al = 5. The same trend is observed for I2 with a larger amplitude: from -135 kJ mol-1 for Si/Al = 47 to -300 kJ mol-1 for Si/Al = 5. Therefore, the use of silver-exchanged mordenite with Si/Al ratios of 5 or 11 would drastically limit the inhibiting effect of contaminants on the adsorption of volatile iodine species. Also for the same ratios, a spontaneous dissociation of I2 during its adsorption is observed, leading to the formation of AgI complexes which are prerequisite for the immobilization of iodine in the long term.

Effect of Sodium Oxide Modifier on Structural and Elastic Properties of Silicate Glass

Molecular dynamics (MD) simulations and Brillouin light scattering (BLS) spectroscopy experiments have been carried to study the structure of sodium silicate glasses (SiO2)(100-X)(Na2O)X, where X ranges from 0 to 45 at room temperature. The MD-obtained glass structures have been subjected to energy minimization at zero temperature to extract the elastic constants also obtained by BLS spectroscopy. The structures obtained are in good agreement with the structural experimental data realized by different techniques. The simulations show that the values of the elastic constants as a function of X (i.e., Na2O mol %) agree well with those measured by BLS spectroscopy. The variations of elastic constants C11 and C44 as a function of Na2O mol % are discussed and correlated to structural results and potential energies of oxygen atoms.

Competing structures in (In,Ga)Se and (In,Ga)2Se3 semiconductors

The electronic structure of four polytypes (β, γ, δ, and ϵ) of hexagonal GaSe and InSe is calculated from first principles, using the WIEN2k and VASP codes and PBEsol prescription for the exchange–correlation potential, aiming specifically at elucidating the crystallographic parameters and comparing the energy placement of corresponding competing structures. Further on, the compounds with different composition of the same constituents, namely the ordered-vacancies systems Ga2Se3 and In2Se3 were subject to a similar study, of which the relaxed crystal structure data for three different phases is reported. Comparison is done with the nominal wurtzite structure over which the cation vacancies are introduced, and the relaxation pattern discussed.

Performance of CuII‐, PbII‐, and HgII‐Exchanged Mordenite in the Adsorption of I2, ICH3, H2O, CO, ClCH3, and Cl2: A Density Functional Study

Periodic dispersion-corrected DFT is used to investigate the adsorption of I2 and ICH3 , which may be released during a severe nuclear accident, for three divalent cation (Cu2+ , Pb2+ and Hg2+ )-exchanged mordenites with an Si/Al ratio of 23. Gases such as H2 O, CO, ClCH3 , and Cl2 present in the containment atmosphere can inhibit the selective adsorption of iodine species. To identify the most promising adsorbents, a systematic study is performed in which all the possible cationic sites in the main channel of the mordenite structure are considered. For the energetically most stable sites, the divalent cation is located in the small rings (five- or six-membered) containing two Al atoms, while in the energetically less stable configurations, the two Al atoms are far apart (>7 Å) and the cation is close to only one Al atom. Upon adsorption of the various molecules, the coordination number of the cation decreases with increasing interaction energy, as the molecules can attract the divalent cations from the framework. Finally, the computed interaction energies show that Hg-mordenite (MOR) could be a suitable material for selective adsorption of volatile iodine species, contrary to Cu-MOR and Pb-MOR.

Surface Properties of Fluorite in Presence of Water: An Atomistic Investigation

Density functional theory simulations, including a correction for dispersive interactions, were performed to investigate the adsorption of water on the main cleavage plane of the fluorite, namely, the (111) surface. In the case of a single molecule of water, we observe that the molecular form is preferred over the dissociated one, and absorbs on the surface with an energy of -55 kJ mol-1, including a significant contribution from the dispersion forces. Also, we show that the substitution of a fluorine atom by a hydroxyl group on the surface of fluorite is not energetically favorable. Then, the hydration of the surface in function of the coverage by water molecules was studied in a systematic way. It was shown that the geometries involving the formation of a cluster of water molecules on the surface, with half of the molecules adsorbed, are the most favorable. Finally, ab initio molecular dynamics conducted at 300 K confirms the trends observed at 0 K, albeit the adsorption energies are reduced by about 10 kJ mol-1. Also, we observe that once put in the interaction with a large number of water molecules, half of the calcium atoms at the surface are in close interaction with a water molecule, whereas the rest of the molecules are further away but present a relatively well-defined structure showing similarities with the one of water clusters.

Comparative study of structural and electronic properties of GaSe and InSe polytypes

Equilibrium crystal structures, electron band dispersions, and bandgap values of layered GaSe and InSe semiconductors, each being represented by four polytypes, are studied via first-principles calculations within the density functional theory. A number of practical algorithms to take into account dispersion interactions are tested, from empirical Grimme corrections to many-body dispersion schemes. Due to the utmost technical accuracy achieved in the calculations, nearly degenerate energy-volume curves of different polytypes are resolved, and the conclusions concerning the relative stability of competing polytypes drawn. The predictions are done as for how the equilibrium between different polytypes will be shifted under the effect of hydrostatic pressure. The band structures are inspected under the angle of identifying features specific for different polytypes and with respect to modifications of the band dispersions brought about by the use of modified Becke-Johnson (mBJ) scheme for the exchange-correlation potential. As another way to improve the predictions of bandgaps values, hybrid functional calculations according to the HSE06 scheme are performed for the band structures, and the relation with the mBJ results are discussed. Both methods nicely agree with the experimental results and with state-of-the-art GW calculations. Some discrepancies are identified in cases of close competition between the direct and indirect gap (e.g., in GaSe); moreover, the accurate placement of bands revealing relatively localized states is slightly different according to mBJ and HSE06 schemes.

Evaluation of the Inhibiting Effect of Organic Compounds on the Adsorption of Iodine Compounds in Cation-Exchanged Zeolites: A DFT Study

During a severe accident in a Nuclear Power Plant, fission products are released from the degraded fuel and might reach the environment by some nuclear containment building leakages.