Ryo Nagumo

A Computational Chemistry Study on Friction of h-MoS2. Part II. Friction Anisotropy

In this work, the friction anisotropy of hexagonal MoS(2) (a well-known lamellar compound) was theoretically investigated. A molecular dynamics method was adopted to study the dynamical friction of two-layered MoS(2) sheets at atomistic level. Rotational disorder was depicted by rotating one layer and was changed from 0° to 60°, in 5° intervals. The superimposed structures with misfit angle of 0° and 60° are commensurate, and others are incommensurate. Friction dynamics was simulated by applying an external pressure and a sliding speed to the model. During friction simulation, the incommensurate structures showed extremely low friction due to cancellation of the atomic force in the sliding direction, leading to smooth motion. On the other hand, in commensurate situations, all the atoms in the sliding part were overcoming the atoms in counterpart at the same time while the atomic forces were acted in the same direction, leading to 100 times larger friction than incommensurate situation. Thus, lubrication by MoS(2) strongly depended on its interlayer contacts in the atomic scale. According to part I of this paper [Onodera, T., et al. J. Phys. Chem. B 2009, 113, 16526-16536], interlayer sliding was source of friction reduction by MoS(2) and was originally derived by its material property (interlayer Coulombic interaction). In addition to this interlayer sliding, the rotational disorder was also important to achieve low friction state.

Estimation of inorganic gas permeability through an MFI-type silicalite membrane by a molecular simulation technique combined with permeation theory

Abstract A methodology based on a molecular simulation technique and permeation theory was applied to the systematic estimation of permeabilities of inorganic gases (Ar, He, Ne, N 2 , O 2 ) through an MFI-type silicalite membrane. The estimated inorganic gas permeabilities were in qualitative agreement with available experimental data. The estimated permeabilities decrease in the order N 2 >O 2 >Ar>Ne>He. This result means that their permeation mechanism is significantly influenced by their adsorption properties rather than by their kinetic properties, and can be described by the adsorption model. The qualitative agreement of the calculated permeability with experiment suggests that the method used is a leading candidate for the theoretical approach to the prediction of the performance of zeolite membranes.

Chemical Degradation Mechanism of Model Compound, CF3 ( CF2 ) 3O ( CF2 ) 2OCF2SO3H , of PFSA Polymer by Attack of Hydroxyl Radical in PEMFCs

To enhance the durability of perfluorosulfonic acid (PFSA) polymer for proton-exchange membrane fuel cells (PEMFCs), we theoretically analyzed the degradation mechanism of PFSA by the attack of a hydroxyl (OH) radical. We used CF 3 (CF 2 ) 3 O(CF 2 ) 2 OCF 2 SO 3 H as a model compound representing the PFSA side chain because the experimental result suggested that the ether group in the PFSA side chain is vulnerable to the OH radical attack. We performed density functional theory calculation to discuss the degradation reaction mechanism of the ether group in the model compound of the side chain and OH radical. Under high humidity condition, we clearly demonstrated the degradation mechanism and reactivity of C-0 bond cleavage in the ether group by the OH radical. This result shows reasonable agreement with the experimental one. However, the OH radical prefers the reaction of the sulfonic acid group to the ether group under the low humidity condition. We found the different reactivity of the OH radical under the low and high humidity conditions. To improve the durability of PFSA, we proposed four directions: (i) enhancement of deprotonation, (ii) protection of ether group by steric hindrance, (iii) enhancement of C-O bond strength, and (iv) substitution of the ether group by other chemical groups. The latter two directions have been theoretically explored more in detail.

Measurement of dynamic surface tension by mechanically vibrated sessile droplets

We developed a novel method for measuring the dynamic surface tension of liquids using mechanically vibrated sessile droplets. Under continuous mechanical vibration, the shape of the deformed droplet was fitted by numerical analysis, taking into account the force balance at the drop surface and the momentum equation. The surface tension was determined by optimizing four parameters: the surface tension, the droplets height, the radius of the droplet-substrate contact area, and the horizontal symmetrical position of the droplet. The accuracy and repeatability of the proposed method were confirmed using drops of distilled water as well as viscous aqueous glycerol solutions. The vibration frequency had no influence on surface tension in the case of pure liquids. However, for water-soluble surfactant solutions, the dynamic surface tension gradually increased with vibration frequency, which was particularly notable for low surfactant concentrations slightly below the critical micelle concentration. This frequency dependence resulted from the competition of two mechanisms at the drop surface: local surface deformation and surfactant transport towards the newly generated surface.

Application of Free Energy Calculations at an Ultrahigh Temperature for Estimation of Molecular Diffusivities and Permeabilities in Zeolite Nanopores at an Ambient Temperature

Molecular diffusivities and gas permeabilities through zeolite nanopores, which have been difficult to simulate directly from conventional molecular dynamics (MD), were estimated at an ambient temperature by performing the free energy calculation at an ultrahigh temperature. In this method, the hopping rate of a guest molecule is calculated based on transition state theory. Using these hopping rates, molecular self-diffusivities for a CH(4)/CF(4) binary mixture through an LTA-type zeolite, as well as those for each single component, are calculated at 300 K. The diffusivities of CF(4) are in the order of ca. 10(-14) m(2)/s at 300 K and thus are within an extremely slow molecular diffusion regime. Gas permeabilities of each single component at 300 K are also estimated by combining these calculated diffusivities with Ficks first law. For predicting CH(4) permeabilities, nonequilibrium MD is also applied for comparison, giving results within the same order, ca. 10(-12) molm/m(2)sPa. This methodology dramatically reduces computational time when predicting molecular diffusivity and gas permeability.

Theoretical Study on Dissoloved Structure of Pt Complex in Polymer Electrolyte Fuel Cell

We theoretically analyzed the formation energy and solvation free energy of four- and six-coordinated Pt(II) and Pt(IV) complexes with three types of ligands (H2O, OH-, and CF3SO3-) as a model of dissolved Pt species to understand the Pt electrocatalyst degradation and dissolution mechanisms. All calculations were performed under the generalized gradient approximation (GGA) with Becke-88 exchange and Lee-Yang-Parr correlation functionals (BLYP). Solvent effects in water were estimated using the conductor-like-screening model (COSMO). The calculated results clarified that Pt(IV) complexes are more energetically favorable than Pt(II) complexes, indicating that the Pt(IV) complexes are more probable as dissolved species than Pt(II) complexes. We also analyzed the geometrical parameters (Pt...O) and atomic charge of O in ligands. These local relaxations about geometry and atomic charge are one of the factors to determine the stability of dissolved Pt species.

Molecular dynamics study of the molecular mobilities and side-chain terminal affinities of 2-methoxyethyl acrylate and 2-hydroxyethyl methacrylate

Irrespective of the degree of polymerization, the molecular mobilities of the 2-hydroxyethyl methacrylate (HEMA) moieties are smaller than those of the 2-methoxyethyl acrylate (MEA) moieties. Preventing the polar functional groups of the foulants and materials from forming a hydrogen-bonding network is important to enhance the mobilities of the molecular chains of non-ionic polymeric materials. We speculate that enhancing the mobilities of the molecular chains is key to improving blood compatibility.

Electronic and Atomistic Roles of Cordierite Substrate in Sintering of Washcoated Catalysts for Automotive Exhaust Gas Emissions Control: Multi-scale Computational Chemistry Approach based on Ultra-Accelerated Quantum Chemical Molecular Dynamics Method

Multi-scale computational chemistry methods based on the ultra-accelerated quantum chemical molecular dynamics (UA-QCMD) are applied to investigate electronic and atomistic roles of cordierite substrate in sintering of washcoated automotive catalysts. It is demonstrated that the UA-QCMD method is effective in performing quantum chemical molecular dynamics calculations of crystals of cordierite, Al2O3 and CeZrO4 (hereafter denoted as CZ). It is around 10,000,000 times faster than a conventional firstprinciples molecular dynamics method based on densityfunctional theory (DFT). Also, the accuracy of the UAQCMD method is demonstrated to be as high as that of DFT. On the basis of these confirmations and comparison, we performed extensive quantum chemical molecular dynamics calculations of surfaces of cordierite, Al2O3 and CZ, and interfaces of Al2O3 and CZ with cordierite at various temperatures. These calculations coupled with mesoscopic sintering simulations have demonstrated that the cordierite surface forms strong bonds with Al2O3 and CZ, which was seen to improve significantly the sintering property of washcoated catalysts under various conditions. INTRODUCTION The lifetime of a vehicle catalyst is important in maintaining low exhaust gas emissions. Typically, mid-life catalyst durability performance is experimentally examined in engine dynamometer evaluations. In general, such traditional confirmation of catalyst performance is done using many kinds of oils and catalysts in continuous evaluation tests. However, so far, little work has been devoted to theoretical evaluation of catalyst sintering behavior. In our previous papers [1, 2, 3], we have reported the results of an alternative methodology to experimental durability testing schemes. We investigated the thermal durability of supported precious metals, represented as Pt, from micro scale to macro scale. We focused not only microscopically on thermal diffusion dynamics of Pt at high temperatures, but also performed macroscopic long-term, sintering process simulation using a three-dimensional sintering simulator [1, 2, 3]. In the present study we have extended our methodology to investigate electronic and atomistic roles of cordierite substrate in sintering of washcoated catalysts. Although it is well known and well accepted that cordierite has excellent mechanical properties and chemical inertness as a substrate[4, 5, 6, 7], the electronic and atomistic roles of cordierite have not yet been Electronic and Atomistic Roles of Cordierite Substrate in Sintering of Washcoated Catalysts for Automotive Exhaust Gas Emissions Control: Multiscale Computational Chemistry Approach based on Ultra-Accelerated Quantum Chemical Molecular Dynamics Method 2012-01-1292 Published 04/16/2012 Akira Miyamoto, Ryo Nagumo, Ai Suzuki, Ryuji Miura, Hideyuki Tsuboi, Nozomu Hatakeyama, Hiromitsu Takaba and Sumio Kozawa

Theoretical Study on Effect of SiC Crystal Structure on Carrier Transfer in Quantum Dot Solar Cells

Quantum dot (QD) solar cells are proposed as high-efficiency solar cells. However, their reported conversion efficiencies have been lower than half of the ideal value. To improve their efficiency, the optimization of their cell structure in terms of various parameters, e.g., dot size, interdot distance, type of materials, and QD/bulk interface structure, is necessary. In this paper, we focused on the most important factor for the improvement in the conversion efficiency of Si/SiC type QD solar cells and investigated the effect of the atomistic structure of the QD/bulk interface on carrier transfer by tight-binding simulation. We constructed models of Si/SiC systems and analyzed the effect of QD/bulk interface defects on their electronic structure and carrier transfer properties. It was suggested that electrons trapped at the QD/bulk interface and the type of SiC crystal structure affect electron transfer.

Computational Analysis of Electron Injection on Light-Emitting Polymer/Cathode Interface

Tohoku Univ., Dept. Chem. Eng., Grad. School Eng., 6-6-10-205 Aoba, Aramaki, Aoba-ku, Sendai 980-8579, Japan Phone: +81-22-795-7237 E-mail: itaru@aki.che.tohoku.ac.jp Tohoku Univ., Dept. Appl. Chem., Grad. School Eng., 6-6-10-205 Aoba, Aramaki, Aoba-ku, Sendai 980-8579, Japan Tohoku Univ., NICHe, 6-6-10-205, Aoba, Aramaki, Aoba-ku, Sendai 980-8579, Japan Tohoku Univ., FRI, Grad. School Eng., 6-6-11-701 Aoba, Aramaki, Aoba-ku, Sendai 980-8579, Japan