Rajappan Vetrivel

Density functional theory calculations of the reaction pathway for methane activation on a gallium site in metal exchanged ZSM‐5

Density functional theory is used to describe the reaction profile for methane dissociation on Ga‐exchanged ZSM‐5. Stable structures on the reaction pathway are characterized as weakly adsorbed methane molecule and the C–H dissociation product. The transition state is also explicitly defined and optimized. The nonlocal density functional approximation is invoked to calculate the energy parameters of the reaction. The activation barrier is estimated at about 120 kJ/mol, in excellent agreement with other similar reactions. From vibrational analysis the reaction coordinate is deduced and transformation of a methane molecule on adsorption is discussed.Density functional theory is used to describe the reaction profile for methane dissociation on Ga‐exchanged ZSM‐5. Stable structures on the reaction pathway are characterized as weakly adsorbed methane molecule and the C–H dissociation product. The transition state is also explicitly defined and optimized. The nonlocal density functional approximation is invoked to calculate the energy parameters of the reaction. The activation barrier is estimated at about 120 kJ/mol, in excellent agreement with other similar reactions. From vibrational analysis the reaction coordinate is deduced and transformation of a methane molecule on adsorption is discussed.

Development of RYUGA for three-dimensional dynamic visualization of molecular dynamics results

Abstract We have developed a new computer graphics (CG) code RYUGA for three-dimensional dynamic visualization of molecular dynamics (MD) results. The applicability of RYUGA for visualizing and analyzing the dynamics of atomic motions in various materials was demonstrated. RYUGA supports various functions, such as solid-modeling CG pictures (called the CPK model), CG animation of the MD results, Miller plane cutting of crystal structures, building graphs, etc., similar to other CG codes for MD simulation. In addition, RYUGA has a number of advantages as follows: (i) a perspective is employed for drawing CG pictures, (ii) three-dimensional trajectories of atoms can be constructed, (iii) an operator can travel inside the materials, (iv) graphic speed and animation speed are enhanced because of the specific algorithms, and (v) it works on any workstations, moreover even personal computers with a UNIX operating system and an X window system are available.

Computer-aided design of active catalysts for the removal of nitric oxide

Abstract We report here our computational studies on metal-exchanged ZSM-5 catalysts using molecular dynamics and computer graphics techniques. The structure and the dynamic behavior of copper, gallium and cerium cations in cation-exchanged ZSM-5 framework were studied. Various oxidation states of these metals were simulated and their relevance in the catalytic removal of nitric oxide is discussed. The high Si/Al ratio in ZSM-5 explains the existence of an equilibrium between the different oxidation states of exchanged copper cation, which favors NO reduction. The higher mobility of the oxygen atom attached to gallium and cerium, and the low coordination of these cations are also reasons for their catalytic activity.

Molecular design of carbon nanotubes for the separation of molecules

Studies on the molecular dynamics (MD) simulation of the structure of a carbon nanotube and the dynamic behavior of benzene, alkylated benzenes and alkylated naphthalenes are reported. The results of our calculations indicate the possibility of the development of a carbon nanotube as potential material for selective adsorption and shape-selective separation. The carbon nanotubes behave as flexible porous material towards organic molecules. The carbon nanotubes can be useful in the separation of molecules with different sizes (isomers of monomethylnaphthalenes) and with different shapes (isomers of dimethylnaphthalenes) as demonstrated with a tube of 0.73 nm I.D.

Computer simulation studies on the role of templating organic molecules in the synthesis of ZSM-5

We report here the structure and electronic properties of a few typical representative molecules from a family containing more than 50 templating organic molecules which lead to the synthesis of zeolite ZSM-5. Force-field calculations were utilised to study the structural and conformational properties, while semi-empirical MNDO calculations were adopted to study the electronic properties. The conformational flexibility and the chargecompensating function were brought out as common features of these organic molecules. Furthermore, the interaction of various organic molecules with a suitable cluster model for the ZSM-5 framework was studied by MNDO calculations. The orientation of the template organic molecules and the actual conformation of the template–framework complex are derived, based on the X-ray crystal structure reports and the MNDO energetics. The influence of the nature of the functional group and the alkyl group on the electronic interaction is studied systematically. It has been shown that quantum-chemical calculations could be successfully utilised for the generalised description of the importance of several complex interactions which are mutually present.

Mechanism of the formation of ultrafine gold particles on MgO(100) as investigated by molecular dynamics and computer graphics

The applicability of a new molecular-dynamics (MD) code developed by us and the computer graphics technique to investigating the mechanism of the formation of ultrafine Au particles on the MgO(100) plane was demonstrated. MD calculations were performed to understand the effect of temperature of the MgO substrate on the mechanism. Lower temperatures led to more Au atoms being fixed on the MgO(100) plane. The effect of defects, such as point defects and steps, in the MgO(100) plane was also investigated. Au clusters were formed and fixed just over the defect sites at low temperature, namely 300 K, in agreement with the experimental results. This behavior was in marked contrast to that at high temperature, namely 1000 K. In the latter case, there is no single favorable location of Au clusters and the Au clusters were considerably mobile on the surface, similar to the behavior on a smooth MgO(100) plane. Hence the location of Au clusters was not affected by the presence of defects at 1000 K. Furthermore, an Au atom trapped in the defects is shown to play the role of a nucleation center in the formation processes of Au clusters on the MgO(100) plane at low temperatures, such as 300 K. These results suggested that a low temperature of the MgO substrate and the presence of defects on MgO(100) are required for the formation of atomically controlled ultrafine Au particles on the MgO(100) plane.

The distribution of framework aluminum atoms and extraframework exchanged cations in faujasite as studied by molecular dynamics, NMR simulation, neutron diffraction simulation and computer graphics

Abstract The effectiveness and applicability of molecular dynamics (MD), NMR simulation, and computer graphics to the investigation of the sites and distribution of framework aluminum atoms in faujasites, which cannot be derived from experimental techniques is shown. By using MD calculations, we successfully predicted the adequate aluminum distribution in NaY zeolite ( Si Al =2.43 ), as it reproduces the sites and occupancies of Na + cations which have been reported by neutron diffraction techniques. The validity of our aluminum distribution model in the NaY was strongly confirmed by the simulated 29 Si MAS NMR spectra in agreement with the experimental chemical shifts and intensities. Although the sites and occupancies of the exchanged cations in faujasites can be determined by analytical experimental techniques, their atomistic detailed distribution cannot be derived experimentally. A methodology is proposed to predict the detailed distribution of the Na + cations in NaY by using neutron diffraction spectrum simulation.

Theoretical studies on the affinity of CO2 and N2 molecules to solid surfaces

Materials suitable for inorganic membranes with high efficiency for the gas separation process were studied by means of computational chemistry. It was shown that the difference in affinities of molecules to surfaces estimated from adsorption energies on model clusters is an important factor in making an effective membrane. Magnesia and alumina were shown to be effective in CO2/N2 separation system while silica and titania should not be selective materials.

The structure and electronic characteristics of metallosilicates with ZSM-5 structure

We report here the result of a computer- assisted study of metallosilicates by applying molecular dynamics (MD) and quantum chemical (QC) methods. MD calculations are used to study the local relaxation in the T12 site of the ZSM-5 structure, when Si is substituted by different metals such as Ti4+, Al3+, Ga3+, and Fe3+. QC calculations by density functional theory have been performed on the cluster models generated from the structure obtained by MD calculations. The calculation indicates that the net charge on a MO4 (where M = Ti, Al, Ga, and Fe) group and the molecular electrostatic potential values are good parameters to assess the acidic properties of metallosilicates, as shown by their correlations to the reported experimental acidity.

Atomic Control of Ultrafine Gold Particles on MgO(100) as Investigated by Molecular Dynamics and Computer Graphics

The applicability of our new molecular dynamics code to the design of highly dispersed ultrafine Au particles on an MgO(100) plane, which is essential for the maximum catalytic performance, was demonstrated. Although only a single large Au cluster was formed on a smooth MgO(100) plane, two Au clusters were formed on an MgO(100) plane with either two point defects or two monoatomic steps at 300 K, since a single trapped Au atom in each defect site played a role of the nucleation center. These results suggested that the artificial construction of defects on the MgO(100) plane is desirable for realizing the ultrafine dispersion of Au particles. Furthermore, two Au clusters just over the surface step sites did not aggregate even at high temperatures, such as 700 K. Thus, it was also revealed that the surface defects are effective for inhibiting the sintering of Au clusters on the MgO(100) plane, which is essential for preventing catalyst deactivation.