Multiple twinning on a silica mesoporous crystal

この論文は国立情報学研究所の電子図書館事業により電子化されました。研究会報告シリカメソ多孔結晶は,アモルファスシリカを骨格にメソスケールの空隙(2-50nm)が周期配列した結晶である.シリカメソ多孔結晶は,シリカの前駆体と水における界面活性剤の共同的な自己組織化を利用して合成され,界面活性剤は焼成によって取り除かれる.従ってその空隙は水-界面活性剤-シリカ系での界面活性剤ミセルの構造を反映すると考えられる.本研究で我々は十面体や二十面体のような結晶学的点群とは相容れない粒子外形を示すシリカメソ多孔結晶の構造解析を行った.まず電子線結晶学に基づいた高分解能電子顕微鏡像の解析から,その結晶構造は球状ミセルの立方最密充填であることを示す.また特徴的な上記結晶外形はメソスケールにおける多重双晶の結果であることを報告する

Direct TEM Observation of Vacancy-Mediated Heteroatom Incorporation into a Zeolite Framework: Towards Microscopic Design of Zeolite Catalysts

Incorporating hetero-metal-atom,e.g., titanium, into zeolite frameworks can enhance the catalytic activity and selectivity in oxidation reactions.However,the rational design of zeolites containing titanium at specificsites is difficult because the precise atomic structure during synthesis process remained unclear.Here, a titanosilicate with predictable titanium distribution was synthesized by mediating vacancies in a defective MSE-typezeolite precursor,based on a pre-designed synthetic route including modification of vacancies followed by titanium insertion,where electron microscopy(EM) plays a key role at each step resolving the atomic structure.Point defects including vacancies in the precursor and titanium incorporated into the vacancy-related positions have been directly observed. The results provide insights into the role of point defects in zeolites towards the rational synthesis of zeolites with desired microscopic arrangement of catalytically active sites

Lattice Parameter of Titanium Monoxides under High Pressure

The effect of pressure on the crystal structure and the lattice parameter of vacancy rich titanium monoxides has been studied using a high pressure apparatus of the tetrahedral anvil type combined with an X-ray diffraction equipment. Two kinds of specimens, TiO_ and TiO_, have been used, the former containing most of vacancies in the oxygen sublattice and the latter in the titanium sublattice. Up to the pressure of 60 kbar, no significant change in the crystal structure has been observed. The change in the lattice parameter with increasing pressure is unexpectedly small for both the specimens, but a minor difference between them has been observed. An initial change is larger in TiO_ than in TiO_, while in the pressure range between 10 and 60 kbar the latter shows faster decrease. The experimental results are discussed in relation to the behavior of the vacancies under high pressure

Band structures of P-, D-, and G-surfaces

We present a theoretical study on the band structures of the electron constrained to move along triply-periodic minimal surfaces. Three well known surfaces connected via Bonnet transformations, namely P-, D-, and G-surfaces, are considered. The six-dimensional algebra of the Bonnet transformations [C. Oguey and J.-F. Sadoc, J. Phys. I France 3, 839 (1993)] is used to prove that the eigenstates for these surfaces are interrelated at a set of special points in the Brillouin zones. The global connectivity of the band structures is, however, different due to the topological differences of the surfaces. A numerical investigation of the band structures as well as a detailed analysis on their symmetry properties is presented. It is shown that the presence of nodal lines are closely related to the symmetry properties. The present study will provide a basis for understanding further the connection between the topology and the band structures.Comment: 21 pages, 8 figures, 3 tables, submitted to Phys. Rev.

Can NbO Keep nbo Topology under Electrons? –Unveiling Novel Aspects of Niobium Monoxide at the Atomic Scale

A precise investigation of NbO has been carried out by advanced electron microscopy combined with powder and single crystal X-ray diffraction (XRD). The structure of pristine NbO has been determined as Pm-3 m space group (SG) with a = 4.211 Å and the positions of Nb and O at the 3c and 3d Wyckoff positions, respectively, which is consistent with previous report based on powder XRD data. Electron beams induced a structural transition, which was investigated and explained by combining electron diffraction and atomic-resolution imaging. The results revealed that the electron beam stimulated both Nb and O atom-migrations within each fcc sublattice, and that the final structure was SG Fm-3 m with a = 4.29 Å, Nb and O at the 4a and 4b with 75 % occupancy and same chemical composition. Antiphase planar defects were discovered in the pristine NbO and related to the structural transformation. Theoretical calculations performed by density functional theory (DFT) supported the experimental conclusions

Enantiomerically enriched, polycrystalline molecular sieves

Zeolite and zeolite-like molecular sieves are being used in a large number of applications such as adsorption and catalysis. Achievement of the long-standing goal of creating a chiral, polycrystalline molecular sieve with bulk enantioenrichment would enable these materials to perform enantioselective functions. Here, we report the synthesis of enantiomerically enriched samples of a molecular sieve. Enantiopure organic structure directing agents are designed with the assistance of computational methods and used to synthesize enantioenriched, polycrystalline molecular sieve samples of either enantiomer. Computational results correctly predicted which enantiomer is obtained, and enantiomeric enrichment is proven by high-resolution transmission electron microscopy. The enantioenriched and racemic samples of the molecular sieves are tested as adsorbents and heterogeneous catalysts. The enantioenriched molecular sieves show enantioselectivity for the ring opening reaction of epoxides and enantioselective adsorption of 2-butanol (the R enantiomer of the molecular sieve shows opposite and approximately equal enantioselectivity compared with the S enantiomer of the molecular sieve, whereas the racemic sample of the molecular sieve shows no enantioselectivity)

Ultrathin titania coating for high-temperature stable SiO2/Pt nanocatalysts

The facile synthesis of silica supported platinum nanoparticles with ultrathin titania coating to enhance metal-support interactions suitable for high temperature reactions is reported, as thermal and structure stability of metal nanoparticles is important for catalytic reactions.close8

Ordered mesoporous porphyrinic carbons with very high electrocatalytic activity for the oxygen reduction reaction

The high cost of the platinum-based cathode catalysts for the oxygen reduction reaction (ORR) has impeded the widespread application of polymer electrolyte fuel cells. We report on a new family of non-precious metal catalysts based on ordered mesoporous porphyrinic carbons (M-OMPC; M = Fe, Co, or FeCo) with high surface areas and tunable pore structures, which were prepared by nanocasting mesoporous silica templates with metalloporphyrin precursors. The FeCo-OMPC catalyst exhibited an excellent ORR activity in an acidic medium, higher than other non-precious metal catalysts. It showed higher kinetic current at 0.9a�...V than Pt/C catalysts, as well as superior long-term durability and MeOH-tolerance. Density functional theory calculations in combination with extended X-ray absorption fine structure analysis revealed a weakening of the interaction between oxygen atom and FeCo-OMPC compared to Pt/C. This effect and high surface area of FeCo-OMPC appear responsible for its significantly high ORR activity.open251

経頚静脈的管肝内門脈大循環短絡術による portal hypertensive gastropathy の変化

金沢大学医学系研究科荻野, 英