Shigeo Ted Oyama

Characterization of a highly selective hydrogen permeable silica membrane

The permeability properties of a new type of silica membrane for the small gas molecules CO2, CO, Ne, CH4, He, and H2 are presented. The new membrane, denoted as Nanosil, has unusually high permeance for H2, but also allows passage of He and to a smaller extent Ne, while excluding all other molecules. The membrane is formed by the decomposition of a silica precursor (tetraethyl orthosilicate) onto a Vycor glass substrate. Nitrogen physisorption isotherms of the Vycor glass substrate indicate that it is a microporous solid with slit-like pores of 3.6 nm diameter, that remains unchanged after the silica deposition. Atomic force microscopy (AFM) shows that the Vycor substrate is made up of rectangular plate-like elements of size 90 nm × 30 nm. Between the plates are found rectangular features of 4 nm breadth which are likely to be the pore mouths. The deposited silica forms a thin layer on top of these plates so as to erase fine structures and increase the average feature size to 110 nm × 50 nm.

C-TERMINATED RECONSTRUCTION AND C-CHAIN STRUCTURE ON MO2C(0001) SURFACE STUDIED BY LOW ENERGY ELECTRON DIFFRACTION AND SCANNING TUNNELING MICROSCOPY

The structure of an α-Mo2C(0001) surface varies depending on the annealing temperature. At least three phases are observed by low energy electron diffraction (LEED). A well-ordered (√3×√3)R30° structure was observed on α-Mo2C(0001) at annealing temperatures below 960 K, which was imaged as a honeycomb structure by scanning tunneling microscopy (STM). A model was proposed for the C-terminated structure where C atoms are located on the threefold hollow sites of a Mo layer, and each carbon atom is observed as a depression of the honeycomb structure by STM. An increase in carbon coverage on the surface transformed the (√3×√3)R30° structure to a c(2×4) structure where zigzag rows running parallel to each other were observed by STM.

Synthesis and characterization of new bimetallic transition metal oxynitrides : MI-MII-O-N (MI, MII=V, Mo, W and Nb)

A new class of materials, MI-MII-O-N (where MI, MII = V, Mo, W, and Nb), has been synthesized by nitriding bimetallic oxide precursors with ammonia gas via a temperature programmed reaction. The oxide precursors are prepared by conventional solid state reaction between two appropriate monometallic oxides. The synthesis of the oxynitrides involves passing NH3 gas over the oxide precursors at a flow rate of 6.80×102 μmols−1 (1000 cm3 min−1), and raising the temperature at a rate of 8.3×10−2 Ks−1 (5K min−1) to a final temperature which is held constant for a short period of time. The oxynitrides thus obtained are pyrophoric and need to be passivated before exposing them to air. All these new bimetallic oxynitrides have a face centered cubic (f.c.c.) metal arrangement and high values of surface area. Their surface activity is assessed from their ability to chemisorb CO.

A scanning tunneling microscopy observation of (√3x√3)R30° reconstructed Ni2P(0001)

A Ni2P(0001) single crystal surface has been studied in the framework of model catalysis with a low temperature scanning tunneling microscope (STM) under ultrahigh vacuum (UHV). We observed a previously unreported (√3×√3) R30° reconstruction and successfully recorded its atomically-resolved STM images. Two types of atomic arrangements have been found for this (√3×√3) R30° structure depending on annealing conditions during preparation. One shows a filled and the other one an empty network of polygons. Upon annealing to 940 K, only the latter empty type of structure has been observed.