A new theoretical approach to the encapsulation of small molecules in zeolites

We present a new theoretical method to study the encapsulation of small molecules such as H₂, O₂, N₂, Ar and CH₄ in the Cs₃Na₉-A zeolite. To study the properties of encapsulated molecules, we used the Fermi-Dirac like statistics. The density of states, the distribution function, average binding energy and the average activation energy of encapsulated molecules are calculated. As the number of encapsulated molecules in zeolite cavities increases, the higher energy states in the cavities are gradually filled and, consequently, the activation energy for decapsulation is lowered. We also calculated the fraction of molecules with higher energy than their activation energy, revealing that the activation energy for decapsulation depends not only on the temperature but also on the number of the encapsulated molecules.     

The conductance of SnO2 small nanowires: A study based on density functional and scattering theories

This study is motivated by the recent advances in the fabrication of oxide nanostructures and its purpose is the assessment of the relationship between their structural properties and the conductance. The structures considered are small SnO₂ nanowires whose size and shape reproduce on a smaller scale the structures produced by current technologies. Their electronic configuration and the conductance are evaluated using the density functional and scattering theories with a simplified modelling of the external leads. The study of the electronic configuration shows that the structure of the allowed energy levels and of the charges responds to the details of the nanowire structure and composition. These effects are important in the context of the conductance. In fact, deep resonances are produced by the alignment of the allowed energy levels in the nanowire with the ones in the external leads. For these conductive channels the relationship between the size and the conductance parallels the one between the size and the binding energy.     

Ni/Ni3Al interface: A density functional theory study

The optimal geometries, mechanical and thermal properties, and electronic structures of the three low index (0 0 1), (1 1 0), (1 1 1) Ni/Ni₃Al thin film were studied using first principle calculations. Simulated results indicated that Ni and Al atoms in γ^′ phase preferred to place in the hollow site of Ni atoms in γ phase. In hollow site models, electronic states affected by interface localize within 2 atomic layers. While the top site model, electronic states localize within 3 atomic layers. It is also found that hollow site (1 1 0) interface has the best mechanical properties. Hollow site (0 0 1) interface is the most easily formed interface, which has the best thermodynamic properties.     

Theoretical study of the cracking mechanisms of linear α-olefins catalyzed by zeolites

The cracking reactions of linear C₄-C₁₀ α-olefins over zeolites have been studied by using density functional theory at the B3LYP/6-31G(d,p) level. The obtained results reveal that the β-scission processes of C₄-C₁₀ olefins have the same reaction mechanism. Every pathway only involves a transition state corresponding to the rupture of the C-C bond, while the intrinsic reaction coordinate analysis indicates that the protonated intermediate species is formed during the reaction process. Furthermore, it is found that this intermediate species is not usually highly stable alkoxy group but adsorbed short-lifetime carbocation. This phenomenon can well explain why the carbocations are seldom observed inside the zeolite's cavities. The calculated real activation energy for this pathway is lower than the experimental value for corresponding alkane cracking contrary to the previously reported pathway via an alkoxide intermediate. Therefore, the reaction pathway via a carbocation intermediate species is energetically much more favorable. In addition, the study also shows that the real activation energies of olefin cracking are nearly independent of the olefin chain length, which is in agreement with the existing experimental results of alkane cracking.     

Ab initio investigation of hydrogenation of (BN)12

Ab initio molecular orbital theory was used to examine the hydrogenation of a B₁₂N₁₂ molecule. The 1,2 addition of the 4,6 bond is an energetically favorable adsorption site in one-hydrogen-molecule adsorption. We found that the averaged bind energy of hydrogen molecule is maximized in B₁₂N₁₂H₁₂. The largest energy gaps of B₁₂N₁₂H₁₂ and B₁₂N₁₂H₂₄ suggest they have special stability. Moreover, calculation of the Gibbs free energy of the B₁₂N₁₂ + 12H₂ → B₁₂N₁₂H₂₄ reaction showed that this reaction becomes endothermic above 320 K.     

First-principle study on electronic structure and magnetic properties of organic transition metal compound: Trans-tetrachloro-bis-(pyridine)-rhenium

The electronic structure and magnetic properties of the trans-tetrachloro-bis-(pyridine)-rhenium compound with the Re atom as the metallic magnetic center, were studied using the full potential linearized augmented plane wave method (FP-LAPW) within the density functional theory. The calculated total energies revealed that the compound has a stable antiferromagnetic (AFM) ground state, which is in agreement with the experiment. The band structure of the compound has a semiconductor character. The calculated magnetic moment per molecule is 3.00 μ_B, the magnetic moments are mainly from the Re atoms with a 5d³ electronic configuration. The AFM interaction between ferromagnetically coupled Re atom layers passes through the p orbitals of the Cl ligands near Re atoms.     

Density functional theory study of the electronic and field emission properties of nitrogen- and boron-doped carbon nanocones

Using first-principles density functional theory, we have investigated the electronic and field emission properties of carbon nanocones (CNCs) doped with N or B with 60° disclination. Our findings are that the emission properties for the doped CNCs depend on the doping species, position, and concentration. Compared to pristine CNC, N-doped CNCs exhibit better field emission properties, in which as the doping concentration increases from 1.25% to 2.5% the maximum emission current at applied electric field of 0.3 V/Å increases from 0.94 μA (one N atom is doped at the position adjacent to the pentagon) to 2.90 μA (two N atoms are doped at pentagon). As for pristine CNC the emission current is only 0.21 μA. However, B-doping has no significant influence on the emission properties of CNCs. Our findings suggest that N-doped CNCs can be used as a candidate for cold-emission electron sources.     

Characterization and ethylene adsorption of natural and modified clinoptilolites

The ethylene adsorption of Turkey clinoptilolite-rich tuff from Gordes and Bigadic region of western of Anatolia and their exchanged forms (K⁺, Na⁺ and Ca²⁺) were investigated. The clinoptilolite samples were characterized using XRD, TG-DTA and nitrogen adsorption methods. Adsorption isotherms for ethylene on natural and modified forms of both adsorbents at 277 K and 293 K were obtained at pressures up to 38 kPa. Uptake of ethylene increased as Na-CLN < Ca-CLN < K-CLN < Natural CLN for Gordes zeolite at 277 K, 293 K and for Bigadic zeolite at 277 K. For Bigadic zeolites at 293 K, uptake of ethylene increased in the order Ca-CLN < Na-CLN < K-CLN < Natural CLN. It was found that ethylene adsorption capacity of Bigadic clinoptilolite samples was much greater than Gordes clinoptilolite samples except K⁺ modified forms at both temperatures. These results show that both natural clinoptilolites have a considerable potential for the removal of ethylene.     

Quantum hydrodynamics with trajectories: The nonlinear conservation form mixed/discontinuous Galerkin method with applications in chemistry

We present a solution to the conservation form (Eulerian form) of the quantum hydrodynamic equations which arise in chemical dynamics by implementing a mixed/discontinuous Galerkin (MDG) finite element numerical scheme. We show that this methodology is stable, showing good accuracy and a remarkable scale invariance in its solution space. In addition the MDG method is robust, adapting well to various initial-boundary value problems of particular significance in a range of physical and chemical applications. We further show explicitly how to recover the Lagrangian frame (or pathline) solutions.     

Density functional study of TaSin (n = 1-3, 12) clusters adsorbed to graphene surface

A plane-wave density functional theory (DFT) calculations have been performed to investigate structural and electronic properties of TaSi_n (n = 1-3, 12) clusters supported by graphene surface. The resulting adsorption structures are described and discussed in terms of stability, bonding, and electron transfer between the cluster and the graphene. The TaSi_n clusters on graphene surface favor their free-standing ground-state structures. Especially in the cases of the linear TaSi₂ and the planar TaSi₃, the graphene surface may catalyze the transition of the TaSi_n clusters from an isomer of lower dimensionality into the ground-state structure. The adsorption site and configuration of TaSi_n on graphene surface are dominated by the interaction between Ta atom and graphene. Ta atom prefers to adsorb on the hollow site of graphene, and Si atoms tend to locate on the bridge site. Further, the electron transfer is found to proceed from the cluster to the surface for n = 1 and 2, while its direction reverses as n > 2. For the case of TaSi, chemisorption is shown to prevail over physisorption as the dominant mode of surface-adsorbate interaction by charge density analysis.     

Structure and conformational analysis of CFC-113 by density functional theory calculations and FTIR spectroscopy

Altitude-resolved volume mixing ratio profiles of CFC-113 have recently become available on a global scale with the Atmospheric Chemistry Experiment (ACE) satellite mission. However, the accuracy of the retrieval is currently limited by the uncertainties on the spectroscopic parameters of CFC-113. This paper reports on the geometrical structure, harmonic frequencies and intensities in the mid-infrared region of the two conformers of CFC-113 and the evaluation of whether theoretical calculations reproduce measurements. The calculations are performed using density functional theory at the B3LYP/6-311+G(3df) level. The molecular geometry parameters, the enthalpy difference and the potential barrier between conformers are calculated. The harmonic frequency of the normal modes of vibration are presented and accurately compared to experimental data. Overtones and combination bands are assigned in the 1200-2500 cm⁻¹ region.     

A density functional theory study on the H2S molecule adsorption onto small gold clusters

An all-electron scalar relativistic calculation on Au_nH₂S (n = 1-13) clusters has been performed by using density functional theory with the generalized gradient approximation at PW91 level. The small gold cluster would like to bond with sulfur in the same plane and the H₂S molecule prefers to occupy the on-top and single fold coordination site in the cluster. The Au_n structures and H₂S molecule in all Au_nH₂S clusters are only slightly perturbed and still maintain their structural integrity. After adsorption, the S-H, H-H bond-lengths and most Au-Au bond-lengths are elongated, only a few Au-Au bond-lengths far from H₂S molecule are shortened. The reactivity enhancement of H₂S molecule is obvious and the strong gold-sulfur bond is observed expectedly. The most favorable adsorption takes place in the case that the H₂S molecule is adsorbed by an even-numbered Au_n cluster and becomes Au_nH₂S cluster with even number of valence electrons. It is believed that the strong scalar relativistic effect is favorable to H₂S molecule adsorption onto small gold clusters and is also one of the important reasons for the strong gold-sulfur bond.     

Molecular-dynamics investigation of structural transformations of a Cu201 cluster in its melting process

We perform molecular-dynamics calculations to investigate the structural transformation of a copper cluster containing 201 atoms in its melting process within the framework of the embedded-atom method (EAM). Concerning melting, the obtained results reveal that its structural changes are different from those of larger-size clusters containing several hundreds or more atoms and smaller-size clusters containing tens of atoms. The melting process of this Cu₂₀₁ cluster involves three stages, firstly some atoms in inner regions of this cluster move into outer regions accompanying the structural transformation of the local atom packing, followed by the continuous interchange of atomic positions, and finally this cluster is wholly disordered. During the temperature increase, the structural changes of different regions determined by atom density profiles result in apparent increases in internal energy. By decomposing peaks of pair distribution functions (PDFs) according to the pair analysis (PA) technique, the local structural patterns are identified for the melting of this cluster.     

Molecular dynamics study of the equilibrium flux of gas molecules to a fractal/rough surface: Effects of gas atom diameter

The frequency of collisions of ideal gas molecules (argon) with a rough surface has been studied. The rough/fractal surface was created using the random deposition technique. By applying various depositions the surface roughness was controlled and, as a measure of irregularity, the fractal dimensions of the surfaces were determined. The surfaces were next immersed in ideal gas and the numbers of collisions with these surfaces were counted. The calculations were carried out using the simplified molecular dynamics simulation technique (only hard core repulsions were assumed). The calculations were performed for various ratios of gas phase atoms diameter to the surface substrate atoms diameter. The results obtained showed that the size of a gas phase atom has crucial influence on the relation between the frequency of collision and the surface fractal dimension     

A comparative study of the physical properties of Sb2S3 thin films treated with N2 AC plasma and thermal annealing in N2

As-deposited antimony sulfide thin films prepared by chemical bath deposition were treated with nitrogen AC plasma and thermal annealing in nitrogen atmosphere. The as-deposited, plasma treated, and thermally annealed antimony sulfide thin films have been characterized by X-ray diffraction (XRD), energy dispersive X-ray spectroscopy, scanning electron microscopy, atomic force microscopy, UV-vis spectroscopy, and electrical measurements. The results have shown that post-deposition treatments modify the crystalline structure, the morphology, and the optoelectronic properties of Sb₂S₃ thin films. X-ray diffraction studies showed that the crystallinity of the films was improved in both cases. Atomic force microscopy studies showed that the change in the film morphology depends on the post-deposition treatment used. Optical emission spectroscopy (OES) analysis revealed the plasma etching on the surface of the film, this fact was corroborated by the energy dispersive X-ray spectroscopy analysis. The optical band gap of the films (E_g) decreased after post-deposition treatments (from 2.36 to 1.75 eV) due to the improvement in the grain sizes. The electrical resistivity of the Sb₂S₃ thin films decreased from 10⁸ to 10⁶ Ω-cm after plasma treatments.     

Density functional theory study of hydrogenation mechanism in Fe-doped Mg(0 0 0 1) surface

Using density functional theory (DFT) in combination with nudged elastic band (NEB) method, the dissociative chemisorptions and diffusion processes of hydrogen on both pure and Fe-doped Mg(0 0 0 1) surfaces are studied. Firstly, the dissociation pathway of H₂ and the relative barrier were investigated. The calculated dissociation barrier (1.08 eV) of hydrogen molecule on a pure Mg(0 0 0 1) surface is in good agreement with comparable experimental and theoretical studies. For the Fe-doped Mg(0 0 0 1) surface, the activated barrier decreases to 0.101 eV due to the strong interaction between the s orbital of H and the d orbital of Fe. Then, the diffusion processes of atomic hydrogen on pure and Fe-doped Mg(0 0 0 1) are presented. The obtained diffusion barrier to the first subsurface is 0.45 eV and 0.98 eV, respectively. Finally, Chou method was used to investigate the hydrogen sorption kinetic mechanism of pure MgH₂ and Mg mixed with 5 at.% Fe atoms composites. The obtained activation energies are 0.87 ± 0.02 and 0.31 ± 0.01 eV for H₂ dissociation on the pure surface and H atom diffusion in Fe-doped Mg surfaces, respectively. It suggests that the rate-controlling step is dissociation of H₂ on the pure Mg surface while it is diffusion of H atom in the Fe-doped Mg surface. And both of fitting data are matching well with our calculation results.     

Physical theories in Galilean space-time and the origin of Schrödinger-like equations

A method to develop physical theories of free particles in space-time with the Galilean metric is presented. The method is based on a Principle of Analyticity and a Principle of Relativity, and uses the Galilei group of the metric. The first principle requires that state functions describing the particles are analytic and the second principle demands that dynamical equations for these functions are Galilean invariant. It is shown that the method can be used to formally derive Schrödinger-like equations and to determine modifications of the Galilei group of the metric that are necessary to fullfil the requirements of analyticity and Galilean invariance. The obtained results shed a new light on the origin of Schrödinger’s equation of non-relativistic quantum mechanics.     

The study of relatively low density stellar matter in presence of strong quantizing magnetic field

The effect of strong quantizing magnetic field on the equation of state of matter at the outer crust region of magnetars is studied. The density of such matter is low enough compared to the matter density at the inner crust or outer core region. Based on the relativistic version of semi-classical Thomas-Fermi-Dirac model in presence of strong quantizing magnetic field a formalism is developed to investigate this specific problem. The equation of state of such low density crustal matter is obtained by replacing the compressed atoms/ions by Wigner-Seitz cells with nonuniform electron density. The results are compared with other possible scenarios. The appearance of Thomas-Fermi induced electric charge within each Wigner-Seitz cell is also discussed.     

Adsorption and catalysis: The effect of confinement on chemical reactions

Confinement within porous materials can affect chemical reactions through a host of different effects, including changes in the thermodynamic state of the system due to interactions with the pore walls, selective adsorption, geometrical constraints that affect the reaction mechanism, electronic perturbation due to the substrate, etc. In this work, we present an overview of some of our recent research on some of these effects, on chemical equilibrium, kinetic rates and reaction mechanisms. We also discuss our current and future directions for research in this area.     

Ag Raman modes of RBCO (R = Gd, Pr) by density functional theory approach

Ab initio total energy calculations have been performed for superconducting GdBa₂Cu₃O₇ and insulating PrBa₂Cu₃O₇ using the full-potential linear augmented plane-wave method in the local density approximation (LDA) and generalized gradient approximation (GGA). The comparison of the calculated unit cell volume and lattice parameters with the experimental data indicates the improvement of these parameters in the GGA relative to LDA. LDA and GGA give the equilibrium unit cell volume about 6% smaller and 1.25% larger than the experimental data, respectively for both systems. Thus frozen phonon calculations have been performed to determine the eigenvalues and eigenvectors of the k=0 A_g modes of the two systems in equilibrium structure have been obtained in GGA. The calculated frequencies in the GGA are in good agreement with the other LDA calculations for similar systems. Comparison of computational data with experimental data indicates that calculations determine the frequencies about ten percent below the experimental data. Even by improving LDA to GGA in these calculations, the calculated phonon frequencies have remained almost ten percent below the experimental data, even though the calculated unit cell volumes are nearly equal to the experimental data. So, applying GGA has not considerably decreased the difference between the computational and experimental data. The effect of Pr doping on the eigenvalues and eigenvectors have also been investigated.