Ryo Yamauchi

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.

ON THE ELECTRONIC STRUCTURE OF THE PALLADIUM MONOXIDE AND THE METHANE ADSORPTION : DENSITY FUNCTIONAL CALCULATIONS

Electronic structure of the palladium monoxide and its interaction with a methane molecule has been investigated by means of density functional theory. The two triplets, 3Π and 3Σ−, lie very close in energy, with the indication at the 3Π ground state of the oxide. A methane molecule interacts with the open shell PdO and forms two stable adsorption complexes: in collinear on palladium and bridging conformations. The scission of the C–H bond in adsorbed methane requires moderate activation energy of 24.5 kcal/mol and the dissociation product is very stable, however, the singlet–triplet crossing occurs at the transition state.

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.

DENSITY FUNCTIONAL STUDY ON THE ACTIVATION OF METHANE OVER PD2, PDO, AND PD2O CLUSTERS

Simple functional models for elementary steps in the total oxidation of methane over supported palladium catalysts were investigated using density functional theory. Three simple cluster models were proposed, namely, the palladium dimer and PdO diatomic and linear Pd2O, to probe the mechanism of the methane activation on metallic and oxidized palladium phases, respectively. The strongest adsorption was found on Pd2, where also the C(SINGLE BOND)H bond became easily activated; however, no stable product of the C(SINGLE BOND)H bond scission was indicated. Similar hydrogen activation took place on Pd2O and, in addition, adsorbed methyl and OH species formed the most stable system after crossing a moderate energy barrier. The same product was previously found stable also in the case of PdO dimer but the activation barrier was high. On the Pd2O cluster, the process of energy barrier crossing was accomplished in two steps: easy formation of a free hydrogen moiety and actual oxidation, which made the overall process less demanding energetically.

Electronic structure and adsorption properties of precious metals and their oxides : Density functional calculations

Abstract The electronic structure of bare MeO and Me 2 dimers (with Me  Rh or Pd) has been calculated together with their interaction with a methane molecule for various adsorption geometries by means of quantum chemical density functional methodology. It has been found that the strongest non-dissociative adsorption of methane occurs in a bridging position on a palladium dimer accompanied by very strong activation of a CH bond and a negligible energy barrier for the bond scission. Any adsorption on other systems was weaker and the barrier higher. This could be ascribed to the absence of repulsive 5s electrons in the case of palladium dimer. The strongest stabilization of the dissociated hydrogen was found, however, to occur on PdO. These results may be useful in explaining experimental findings which indicated that a supported palladium catalyst, showing subsequently metallic and oxidized phases is a very efficient medium for methane combustion.

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.

Adsorption of NO on rhodium and palladium clusters: A density functional study

Abstract DFT calculations for the adsorption of NO on M 2 and MO (M = Rh, Pd) clusters are presented. NO is adsorbed in a bridge position on M 2 (M-NO-M) and in a bent configuration on MO (OM-NO). The adsorption energy is larger for rhodium. The overall charge transfer is a back donation of electrons to the adsorbate which leads to an increase of the NO bond. The adsorption energy is smaller when the metal is oxidized as MO.

Concentration dependence of the Néel temperature of Mn100-xRhx ordered alloys

Abstract Concentration dependences of the room temperature lattice constant and the Neel temperature of Mn100−xRhx (19 ≤ x ≤ 30) ordered alloys were investigated. The room temperature lattice constant becomes larger with increasing x. The Neel temperature also increases with the increase of x, and the high Neel temperature is not explained by an exchange interaction as a function of the Mn–Mn distance, but by the number of 3d electrons in the Mn site. Associated with spin fluctuations, the paramagnetic susceptibility increases with increasing temperature and shows a tendency to be saturated.

Atomic processes in the deposition and sintering of ultrafine metal particles on MgO(001) as investigated by molecular dynamics and computer graphics

Abstract Atomic processes in the deposition and sintering of ultrafine Pd and Au particles on MgO(001) plane were investigated by molecular dynamics (MD) and computer graphics (CG). In contrast to the behavior of bulk Pd crystal, Pd atoms in the Pd cluster exhibited high mobility even at 300 K. In agreement with the mobility of the Pd cluster, the shape of the Pd cluster changed significantly during the deposition process of the Pd cluster on the MgO(001) plane, while the position of Mg and O ions on the MgO(001) plane did not change significantly. These behaviors of the Pd cluster are similar to those of Au cluster reported previously. The sintering process of two Pd32 clusters on the MgO(001) plane was also simulated at various temperatures. In contrast to the sintering behavior of two Au32 clusters on the MgO(001) plane, the coagulation of two Pd32 clusters did not take place at 300 K but at a higher temperature such as 1300 K. Furthermore, the effect of modification of the substrate surface on the structure of the metal cluster was investigated for monatomic Au and Pd quantum wires. When a monatomic Au quantum wire was located on the smooth MgO(001) surface, the shape of the monatomic quantum wire of Au was rapidly destroyed to form a disordered structure of Au atoms. When the monatomic quantum wire of Au was located on a MgO(001) surface with a monatomic groove along the [110] axis, the structure of the monatomic quantum wire was stably maintained. The crystallographic fit between metal and substrate was found to provide one of the reasons for the stability of the monatomic quantum metal wire.

Lattice distortions and the magnetic susceptibility of γ-MnRh disordered alloys

Abstract The X-ray powder diffraction patterns of γ-MnRh disordered alloys show lattice distortions, namely, face-centered-tetragonal (fct) and -orthorhombic (fco) structures, depending on the Rh concentration. In the temperature dependence of magnetic susceptibility, an anomaly associated with the lattice distortions is observed below the Neel temperature. These results were used to construct a phase diagram of the γ-MnRh disordered alloy system. The lattice distortions could be correlated to the magnetic susceptibility affected by the spin density wave structures in analogy with γ-MnNi and γ-MnGa alloy systems.