New force field for GCMC simulations of D 2 /H 2 quantum sieving in pure silica zeolites

International audience; We report a study on adsorption and coadsorption of H2 and D2 in FAU, MFI and CHA pure silica zeolites having different pore sizes and shapes. Adsorption capacities, selectivities, enthalpies and entropies are determined by combining experiments and GCMC simulations. We show that the force fields available in the literature cannot predict the adsorption equilibria below 77 K with sufficient accuracy. Here we propose a new force field adjusted by using our experimental data obtained for the pure silica MFI zeolite at 65 K and 77 K. With this new force field, it is possible to predict the adsorption and coadsorption equilibria on the three zeolite structures in a tempe…

Role of Electrical Charges on the Adsorption of Hydrogen: Something Old, Something New.

The adsorption of H2 in zeolites by molecular simulations use, in many applications, single sphere model. Although this representation provides coherent results with ex- periments above 77K, below this temperature the usual hydrogen representation fails to reproduce experimental results. The disagreement can be associated to the interplay of the atomistic heterogeneity and the electric field produced by the zeolite faujasite. These aspects are generally excluded in classical force fields. For elucidating the influ- ence of these issues, we performed DFT simulations for the faujasite Na86X at 40K with and without guest hydrogen molecules for determining the electrical field generated by the …

An Interfacial Thermodynamic Model for the Oxidation Kinetics of a Metal: Epitaxial Stress Effects

Molecular-dynamics studies of annihilation reactions

The validity of the reaction-diffusion formulation of annihilation kinetics, with randomly distributed initial conditions, is investigated by molecular-dynamics simulations of dense hard-disk fluids. For the reaction A + B → C + C quantitative agreement is found. Yet, this proves not to be the case for the reaction A + A → C + C, where major discrepancies are observed. For this latter reaction, more sophisticated theories predict a logarithmic decay law of the form ln (t)/t. The microscopic simulations essentially confirm this prediction.

Length-scale effects in the nucleation of extended dislocations in nanocrystalline Al by molecular-dynamics simulation

The nucleation of extended dislocations from the grain boundaries in nanocrystalline aluminum is studied by molecular-dynamics simulation. The length of the stacking fault connecting the two Shockley partials that form the extended dislocation, i.e., the dislocation splitting distance, rsplit, depends not only on the stacking-fault energy but also on the resolved nucleation stress. Our simulations for columnar grain microstructures with a grain diameter, d, of up to 70 nm reveal that the magnitude of rsplit relative to d represents a critical length scale controlling the low-temperature mechanical behavior of nanocrystalline materials. For rsplit>d, the first partials nucleated from the bou…

Strain mapping near a triple junction in strained Ni-based alloy using EBSD and biaxial nanogauges

A key element for analyzing the crack initiation in strained polycrystalline alloys is the local quantification of the surface strain distribution according to the grain texture. Using electron backscattered diffraction, the local microstructure was determined to both localize a triple junction and deduce the local Schmid factors. Kernel average misorientation (KAM) was also used to map the areas of defect concentration. The maximum principal strain and the in-plane shear strain were quantified using the biaxial nanogauge. Distortions of the array of nanodots used as spot markers were analyzed near the triple junction. The crystallographic orientation and the surface strain were then invest…

When less is more: more cations means more adsorption sites for toluene in zeolites?

The unique properties of zeolites makes them an interesting material to be used in separation processes. The possibility of tailoring some of their characteristics, like the Si/Al ratio, allows to optimize their synthesis for a given task. Concerning the adsorption of toluene by faujasites, an understanding of the effect of cations is necessary to foster the elaboration of new materials, which can capture molecules from humid environments with a high selectivity and sensibility. Undoubtedly, this knowledge is relevant for a wide range of applications going from the elaboration of technologies for improving the air-quality to the elaboration of diagnostic procedures to prevent health risks. …