Sites for Methane Activation on Lithium‐Doped Magnesium Oxide Surfaces

Density functional calculations yield energy barriers for H abstraction by oxygen radical sites in Li‐doped MgO that are much smaller (12±6 kJ mol^?1) than the barriers inferred from different experimental studies (80–160 kJ mol^?1). This raises further doubts that the Li⁺O^.? site is the active site as postulated by Lunsford. From temperature‐programmed oxidative coupling reactions of methane (OCM), we conclude that the same sites are responsible for the activation of CH₄ on both Li‐doped MgO and pure MgO catalysts. For a MgO catalyst prepared by sol–gel synthesis, the activity proved to be very different in the initial phase of the OCM reaction and in the steady state. This was accompanied by substantial morphological changes and restructuring of the terminations as transmission electron microscopy revealed. Further calculations on cluster models showed that CH₄ binds heterolytically on Mg²⁺O^2? sites at steps and corners, and that the homolytic release of methyl radicals into the gas phase will happen only in the presence of O₂.     

Fluorination of elastomer materials

The present study aims at the surface functionalization of cross-linked natural rubber films by direct fluorination routes involving the treatment of elastomer surfaces with gaseous fluorine and its mixtures. Two different fluorination methods were evaluated to attach fluorine moieties on polyisoprene surfaces. On the one hand the fluorination was carried out with mixtures of fluorine with inert gas yielding highly non-polar surfaces. On the other hand polar surface properties were obtained in the presence of fluorine and oxygen in the reaction gas mixture (oxy-fluorination). The functionalized surfaces were characterized by means of FTIR-ATR spectroscopy and contact angle measurements. Optical microscopy was used to assess the surface morphology of the modified elastomer films and the depth of fluorination was investigated with SEM-EDX. In addition, the influence of the surface fluorination on the mechanical properties, aging stability and stability against high energy radiation (e.g. gamma-rays) of the functionalized rubber materials were determined by tensile tests.     

Comparison of GC-ICP-MS and HPLC-ICP-MS for species-specific isotope dilution analysis of tributyltin in sediment after accelerated solvent extraction

This study describes a direct comparison of GC and HPLC hyphenated to ICP–MS determination of tributyltin (TBT) in sediment by species-specific isotope dilution analysis (SS-IDMS). The certified reference sediment PACS-2 (NRC, Canada) and a candidate reference sediment (P-18/HIPA-1) were extracted using an accelerated solvent extraction (ASE) procedure. For comparison of GC and LC methods an older bottle of PACS-2 was used, whilst a fresh bottle was taken for demonstration of the accuracy of the methods. The data obtained show good agreement between both methods for both the PACS-2 sediment (LC–ICP–IDMS 828±87 ng g^–1 TBT as Sn, GC–ICP–IDMS 848±39 ng g^–1 TBT as Sn) and the P-18/ HIPA-1 sediment (LC–ICP–IDMS 78.0±9.7 ng g^–1 TBT as Sn, GC–ICP–IDMS 79.2±3.8 ng g^–1 TBT as Sn). The analysis by GC–ICP–IDMS offers a greater signal-to-noise ratio and hence a superior detection limit of 0.03 pg TBT as Sn, in the sediment extracts compared to HPLC–ICP–IDMS (3 pg TBT as Sn). A comparison of the uncertainties associated with both methods indicates superior precision of the GC approach. This is related to the better reproducibility of the peak integration, which affects the isotope ratio measurements used for IDMS. The accuracy of the ASE method combined with HPLC–ICP–IDMS was demonstrated during the international interlaboratory comparison P-18 organised by the Comité Consultatif pour la Quantité de Matière (CCQM). The results obtained by GC–ICP–IDMS for a newly opened bottle of PACS-2 were 1087±77 ng g^–1 Sn for DBT and 876±51 ng g^–1 Sn for TBT (expanded uncertainties with a coverage factor of 2), which are in good agreement with the certified values of 1090±150 ng g^–1 Sn and 980±130 ng g^–1 Sn, respectively.     

Electrospinning of ultra-thin polymer fibers

The electrospinning technique was used to spin ultra-thin fibers from several polymer/solvent systems. The diameter of the electrospun fibers ranged from 16 nm to 2 μm. The morphology of these fibers was investigated with an atomic force microscope (AFM) and an optical microscope. Polyethylene oxide) (PEO) dissolved in water or chloroform was studied in greater detail. PEO fibers spun from aqueous solution show a “beads on a string” morphology. An AFM study showed that the surface of these fibers is highly ordered. The “beads on a string” morphology can be avoided if PEO is spun from solution in chloroform; the resulting fibers show a lamellar morphology. Polyvinylalcohol (PVA) dissolved in water and cellulose acetate dissolved in acetone were additional polymer/solvent systems which were investigated. Furthermore, the electrospinning process was studied: different experimental lay-outs were tested, electrostatic fields were simulated, and voltage - current characteristics of the electrospinning process were recorded.     

Electronic structure and phase transitions of MnAs

On the basis of a two center tight binding approach the recursion method of Haydock et al. was applied for MnAs to calculate non-polarized and spin polarized local densities of states, magnetic moments and band energies for two different B8₁ crystal structures and two B31 structures. The itineracy of the Mn 3d-states was implied by allowing for hybridization of p-states centred at the As-sites with d-states centred at the Mn-sites. Self-consistency procedures were applied to determine the relative position of the paramagnetic p- and d-band centers (E_p - E_d = - 1 eV), and to calculate the magnetic moments (3.27 μ_B for the low temperature B8₁ phase and 3.14 μ_B for the corresponding B31 phase). Because of the similarities of the electronic structures of both phases it is argued that for the first-order B8₁ [rlarr2] B31 phase transition the usually applied model of a high spin-low spin transition of the Mn³⁺ ion is basically wrong. Instead of such an ionic model it is suggested to adopt an itinerant d-electron concept within that both phases are closely related but the B8₁ phase is favoured above a certain critical magnetic moment as supported by a very recent study of Kato et al. For the high temperature B8₁ phase occurring after the second-order B31 → B8₁ phase transition a magnetic moment of 3.37 μ_B was obtained.