Selectivity in Methanol Oxidation as Studied on Model Systems Involving Vanadium Oxides

Oxidation catalysts are modeled by oxide single crystals, thin oxide films, as well as supported oxide nanoparticles. We characterize the surface of those materials using a variety of surface sensitive techniques including scanning tunneling microscopy and spectroscopy, photoelectron spectroscopy, infrared spectroscopy, and thermal desorption spectroscopy. We find temperature dependent structural transformations from V₂O₅(001) to V₂O₃(0001) via V₆O₁₃(001). V₂O₃(0001) is found to be vanadyl terminated in an oxygen ambient and it loses the vanadyl termination after electron bombardment. It is shown that the concentration of vanadyl groups controls the selectivity of the methanol oxy-dehydrogenation towards formaldehyde. A proposal for the mechanism is made. The results on single crystalline thin films are compared with similar measurements on deposited vanadia nanoparticles. The experimental results are correlated with theoretical calculations and models.     

Andreev reflections in a Bi<Subscript>1.8</Subscript>Pb<Subscript>0.3</Subscript>Sr<Subscript>1.9</Subscript>Ca<Subscript>2</Subscript>Cu<Subscript>3</Subscript>O<Subscript>x</Subscript> break junction

The current-voltage characteristic (CVC) of a break junction made from polycrystalline Bi_1.8Pb_0.3Sr_1.9Ca₂Cu₃O_x is investigated. The experimental CVC has a hysteretic feature that reflects part of the curve with a negative differential resistance. The CVC is discussed within the framework of the Kümmel-Gunsenheimer-Nicolsky theory that takes into account multiple Andreev reflections in superconductor/normalmetal/superconductor junctions.     

Effect of copper on the properties of Pr-Dy-Fe-Co-B sintered magnets

The effect of copper on the properties of magnets (Pr_0.52Dy_0.48)₁₃(Fe₆₅Co_0.35)_{80.3 ? x} Cu _x B_6.7 (x = 0–10) has been studied. Alloying with copper is shown to decrease the sintering temperature and to increase the content of the principal (Pr,Dy)₂(Fe,Co)₁₄B magnetic phase. For compositions with x = 1.3–3.3, copper is found to affect the value and sign of the temperature induction coefficient (TIC). It is shown that the effect of copper on the TIC is determined by the substitution of copper ions for iron ions in lattice sites, which are coupled via an antiferromagnetic exchange interaction.     

Magnetic properties of a low-density Bi-based HTSC

The preliminary results of the investigation of magnetic properties of a novel low-density superconducting material Bi_1.8Pb_0.3Sr₂Ca₂Cu₃O_x with a low density and microfoam structure are presented. The temperature, magnetic field, and time dependences of magnetization M have been measured. The experimental hysteresis loops M(H) are well described in the framework of the Val’kov-Khrustalev theory developed for type II granular superconductors.     

Oxygen-induced Restructuring of a Pd/Fe3O4 Model Catalyst

We have studied the influence of oxygen on the structure and morphology of a Pd/Fe₃O₄ model catalyst using molecular beam (MB) methods, IR reflection absorption spectroscopy (IRAS) and scanning tunneling microcopy (STM). The model catalyst was prepared under ultrahigh vacuum (UHV) conditions by physical vapor deposition (PVD) and growth of Pd nanoparticles on an ordered Fe₃O₄ thin film on Pt(111). It is found that surface oxides are formed on the Pd nanoparticles even under mild oxidation conditions (temperatures of 500 K and effective oxygen partial pressures of around 10⁻⁶ mbar). These surface oxides are initially generated at the Pd/Fe₃O₄ interface and, subsequently, are formed at the Pd/gas interface. The process of formation and reduction of surface and interface oxides on the Pd particles is fully reversible in that all oxides formed can be fully reduced. As a result, the oxide phase acts like a storage medium for oxygen during oxidation reactions, as probed via CO oxidation. The process of surface and interface oxidation is directly connected with the onset of a non-reversible sintering process of the Pd particles. It is suggested that this sintering process occurs via a mobile Pd oxide species, which is stabilized by interaction with the Fe₃O₄ support. The restructuring is monitored via STM and IRAS using CO as a probe molecule. In addition to a decrease in particle density and Pd surface area, a reshaping of the particles occurs, which is characterized by the formation of well-ordered crystallites and with a relatively large fraction of (100) facets. After a few oxidation/reduction cycles at 500 K, the sintering process becomes very slow and the system shows a stable behavior under conditions of CO oxidation.     

Author Index

Authors Index

Author Index     

Contributions from Inter-grain Boundaries to the Magneto-resistive Effect in Polycrystalline High-<Emphasis Type="Italic">T</Emphasis><Subscript>C</Subscript> Superconductors. The Underlying Reason of Different Behavior for YBCO and BSCCO Systems

In order to clarify the mechanisms in charge of broadening of resistive transition R(T) in magnetic fields of bismuth-based polycrystalline high-T _C superconductor (HTSC), a comparative study of Bi_1.8Pb_0.3Sr_1.9Ca₂Cu₃O _x (BSCCO) and YBa₂Cu₃O_7−δ (YBCO) have been performed. Magnetoresistive effects and irreversibility line obtained from magnetic measurements have been studied. It was established that (1) for YBCO, the smooth part of R(T) dependence unambiguously corresponds to dissipation in the intergrain boundaries for arbitrary magnetic fields; (2) for polycrystalline BSCCO, the smooth part of R(T) dependences correspond to dissipation within intergrain boundary subsystem in the field range H<10² Oe only, while standard measurements of R(T) dependences in magnetic field range H>10² Oe reflect the dissipation processes occurring both in intergrain boundary and HTSC grain subsystems; (3) for the high-field range, the contribution from intergrain boundaries of BSCCO can be distinguished from magnetoresistance R(H) dependences obtained at high enough current density on textured samples. It is proposed that various magneto-resistive properties of these classical HTSC systems are due comparatively weak pinning in BSCCO.     

Exploring Internal Structure of Nanoporous Glasses Obtained by Leaching of Phase‐Separated Alkali Borosilicate Glasses

Mesoporous glasses – the leaching products of phase‐separated alkali borosilicate glasses – are widely used in fundamental research and practical applications. In this work, the option to control their internal mesopore structure by varying the conditions of microphase separation has been studied. Structure and transport characterization of a family of nanoporous glasses obtained under different conditions has been performed using a combination of several experimental techniques, including gas adsorption, nuclear magnetic resonance cryoporometry and diffusometry.     

Molar enthalpy of vaporization of ethanol-gasoline mixtures and their colloid state

Vapour pressure measurements are used to evaluate the enthalpy of vaporization of ethanol-gasoline mixtures. Partial molar values are also derived. The dispersed structure of ethanol-gasoline fuel is studied for the first time using the method of correlation spectroscopy of scattered light. A large range of dispersed particle sizes in different alcohol-gasoline systems is found. The dependence of the mean radius of drops on ethanol content is determined. It is found that coalescence phenomenon occurs in the systems when extra ethanol is added.