Adil Fahmi

Grand canonical Monte Carlo simulation of the adsorption of CO2 on silicalite and NaZSM-5

The adsorption of carbon dioxide in silicalite and NaZSM-5 zeolite has been studied using new Monte Carlo software. In this program, sodium cations and framework are movable during the simulation. The calculated adsorption isotherms are in good agreement with the experimental results. The energy distribution of carbon dioxide over silicalite and NaZSM-5 shows that the increase of the adsorption energy for NaZSM-5 is mainly due the electric field generated by sodium cations.

Molecular dynamics simulation of enhanced oxygen ion diffusion in strained yttria-stabilized zirconia

The application of strain to yttria-stabilized zirconia (YSZ), which can be realized by sandwiching a thin YSZ film epitaxially between layers of a material with larger lattice constants, is proposed as a means to enhance oxygen ion mobility. The possible mechanism of such an enhancement was investigated by molecular dynamics using a CeO2–YSZ superlattice. The calculated diffusion coefficient of oxygen ions in the superlattice is some 1.7 times higher than in YSZ alone due to a decreased activation barrier from the strain of the YSZ structure.

Molecular dynamics simulation of iso- and n-butane permeations through a ZSM-5 type silicalite membrane

Molecular dynamics simulation of the permeation processes of single and mixed gases of iso- and n-butane through a ZSM-5 type silicalite membrane are presented. After 200 ps of simulation time the permeation of n-butane is observed whereas the permeation of iso-butane is not observed. The permeation of n-butane at 373 K takes place after the saturation of the zeolite pores, whereas at higher temperature, 773 K, it occurs without significant pores saturation. The calculated permeability of n-butane is close to experimental data. The permeation of the gas mixture shows that the membrane can separate the two isomers, n-butane permeates whereas iso-butane does not.

Periodic density functional study on V2O5 bulk and (001) surface

Abstract Density functional calculations on periodic models are performed to investigate the structural and electronic properties of both V2O5 bulk and (001) surface. Full geometry optimizations of both V2O5 bulk and (001) surface are presented. For the bulk, the optimized structure is very close to the experimental one, the calculated band gap and binding energy are in very good agreement with experimental values, from population analysis it is observed that vanadyl oxygens are least ionic (O−0.37), doubly coordinated oxygens are ionic (O−0.56), while triply coordinated oxygens become the most ionic (O−0.68). The structural and electronic properties of the surface are very close to those of the bulk. The interlayer interaction is mainly electrostatic and is found to be 4 kcal/mol. Surface acidic and basic properties are described in terms of projected density of states analysis.

Layer-by-layer homoepitaxial growth process of MgO(001) as investigated by molecular dynamics, density functional theory, and computer graphics

We applied molecular dynamics, density functional theory, and computer graphics techniques to the investigation of the homoepitaxial growth process of the MgO(001) surface. MgO molecules are deposited over the MgO(001) plane one by one at regular time intervals with definite velocities. Any deposited MgO molecule migrated on the surface, and later a two-dimensional and epitaxial growth of MgO thin layer was observed at 300 K which is in agreement with the experimental result. However, some defects were constructed in the grown film at low temperature of 300 K, which is in remarkable contrast to that at 1000 K. In the latter case, a single flat and smooth MgO layer without defects was formed, which also agreed with the experimental result. Self-diffusion coefficients and activation energy for the surface diffusion of the deposited MgO molecule on the MgO(001) plane were discussed to clarify the temperature-dependency of the epitaxial growth process.

Density functional calculation on the adsorption of nitrogen oxides and water on ion exchanged ZSM-5

Abstract Removal of nitrogen oxides (NOx) from exhaust gases in the presence of excess oxygen is a major problem. Our work is aimed in a direction to propose effective catalyst for NOx removal. Configuration and electronic states of adsorbed NOx species on various ion exchanged ZSM-5 have been investigated by quantum chemical calculation based on Density Functional Theory (DFT). In case of NO adsorption on monovalent and divalent metal cations, nitrogen atom of NO molecule interacts with exchanged metal cations. Whereas in case of the trivalent metal cations extraframework oxygen is also involved along with the exchanged cations in adsorption process. NO2 and H2O adsorption over exchanged metal cation were also studied. Different combination of exchangeable metal cations over NOx adsorption was studied. Large affinity of NOx was observed, when Cu+, (Fe–OH)+, (Co–OH)+, (In–O)+, (Tl–O)+ are present in ZSM-5 as exchanged cations.

NO2 adsorption on ion exchanged ZSM-5: a density functional study

Abstract We selected Ga, In, Cu, Ag, and Co as exchange cations and performed molecular dynamics (MD) calculation to determine framework structures. For Ga-, Cu- and Co-ZSM-5, the cations are coordinated to two framework oxygens whereas for In and Ag-ZSM-5 the cations are doubly- and triply-coordinated. Adsorption modes of NO 2 are η 1 -N or η 2 -O,O on Cu and Ag, whereas on Co only η 1 -N is found.

Permeability of Ar and He through an inorganic membrane: a molecular dynamics study

Molecular dynamic simulations of the permeation of single and mixture gases of He and Ar through an inorganic membrane are presented. The calculated permeabilities are in favor of the Knudsen diffusion mechanism. Since He is lighter than Ar, its permeability is larger and therefore it permeates first. Therefore, in the vacuum phase the amount of He is larger than that of Ar. This shows the ability of MD calculation on estimating permeability.

Structure of TiO2 surfaces: a molecular dynamics study

Molecular dynamics was used to investigate surface relaxations of the three titanium dioxide phases (rutile, anatase and brookite). Atoms with small coordination relax much more than bulk like atoms. They undergo vertical as well as lateral relaxations to compensate the missing bonds at the top layer. The driving force which determines the direction of the relaxation seems to be the improvement of local environments of top layer atoms.

Molecular dynamics simulations on oxygen ion diffusion in strained YSZ/CeO2 superlattice

Our earlier molecular dynamics results show that the construction of a strained yttria-stabilized zirconia (YSZ)/CeO2 superlattice considerably enhances oxygen ion diffusion. In the present study, effect of several parameters (e.g., Y2O3 concentrations, stacking periodicity, and temperature) on oxygen ion diffusion in strained YSZ/CeO2 superlattice were optimized to rationalize the understanding of the process mechanism. We found that self-diffusion coefficient of O ions reaches a maximum at around 9.1 mol% Y2O3 concentration, and the increment of CeO2/YSZ ratio enhances oxygen ion diffusion. Moreover, activation energy for oxygen ion diffusion in the YSZ/CeO2 superlattice (9 kcal/mol) was found to be lower than that observed in the bulk YSZ (15 kcal/mol).