The role of surface oxides in formic acid oxidation on Au

Compared to Pt or Pd electrodes, Au is a poor catalyst for the direct anodic oxidation of HCOOH, but the formation of Au surface oxides in acidic solutions is accompanied by a fast oxidation of HCOOH. This fast reaction is not simply a secondary reaction of Au surface oxides since those oxides are kinetically stable in HCOOH solutions. They do oxidize HCOOH only via a slow and purely electrochemical process which occurs on free Au sites and is “driven” by oxide reduction. The fast HCOOH oxidation is due to a highly reactive intermediate which is able either to form stable Au oxides Au_nO_m or to react with HCOOH. Our results are consistent with the model that by the charge transfer step a reactive non-equilibrium {Au…O> species is formed which converts to stable equilibrium oxides Au_nO_m after migration and rearrangement steps. Pre-equilibrium <Au…O> oxidizes HCOOH and this oxidation is of lower order with respect to <Au…O> compared with the formation of Au_nO_m.     

1,1,3,3-Tetramethylguanidine-gallane, (Me2N)2CN(H).GaH3: an unusually strongly bound gallane adduct

The novel adduct 1,1,3,3-tetramethylguanidine-gallane, (Me2N)2CN(H).GaH3, has been prepared by the reaction of [(Me2N)2CNH2]+Cl- with LiGaH4 in Et2O solution. Its spectroscopic properties indicate a monomeric species with an unusually strong coordinate link between the imido function and GaH3, an inference confirmed by the crystal structure at 150 K which also reveals significant secondary interactions through non-classical N-H...H-Ga bridges. Despite the intrinsic strength of the Ga-N bond, however, vaporisation at ca. 310 K occurs with partial dissociation, and decomposition via more than one pathway proceeds at temperatures >330-350 K to give a variety of products, including the free base, Me2NH, H2, and a novel gallium-nitrogen compound composed of a Ga4N4 cubane-like core bridged on three edges by -N{C(NMe2)2}GaH2- units.     

Curing of a maleimide model compound

A monofunctional maleimide-ended model compound was synthesized and purified. The thermal curing of this compound was followed by size exclusion chromatography and the molecular masses of the oligomers formed during the curing reaction were determined by mass spectroscopy. It was found that high molecular mass species were not thermally stable. At high temperature and after enough time, a strong decrease in the polydispersity of the oligomers and a significantly preferential formation of trimer were observed. From ¹³C-NMR characterization, a five membered ring structure was found for the trimer, which explains the thermal stability of this species. © 1995 John Wiley & Sons, Inc.     

A note on the numerical evaluation of finite integrals of oscillatory functions

A method is described for the numerical evaluation of integrals of the form ∫ _?1 ¹ f(x)K(m,x)dx, wheref(x) is smooth in [?1,1], whileK(m,x) is highly oscillatory for large values ofm.     

On the origin of photoluminescence in heavily-doped silicon

The origin of the photoluminescence in heavily-doped silicon is examined. Transient photoluminescence data for Si(P) are presented and used to identify the “Low Level” emission bands in terms of recombination of impurity band electrons with holes bound to acceptor sites. The “High Level” bands are attributed to recombination of impurity band electrons with free holes. The energies of the band gap and optical band gap in heavily-doped silicon are determined from the photoluminescence measurements.     

Relative atomic transition probabilities for Na,Sc, Ti,Cr, Fe,Co, and Ni using flame atomic absorption spectroscopy

Relative atomic transition probabilities (A_ji), for Na, Sc, Ti, Cr, Fe, Co, and Ni, using flame atomic absorption spectroscopy have been determined. Absorption measurements in air-acetylene and nitrous oxide-acetylene flames using both line emission and continuum excitation sources were employed. A comprehensive discussion and critical evaluation of A_ji values for this work and others of comparable scope and design is included. The effect of the absorption line profile including hyperfine structure is discussed.     

A study of rotationally invariant and symmetric indices of diffusion anisotropy.

This study investigated the properties of a class of rotationally invariant and symmetric (relative to the principal diffusivities) indices of the anisotropy of water self-diffusion, namely fractional anisotropy (FA), relative anisotropy (RA), and volume ratio (VR), with particular emphasis to their measurement in brain tissues. A simplified theoretical analysis predicted significant differences in the sensitivities of the anisotropy indices (AI) over the distribution of the principal diffusivities. Computer simulations were used to investigate the effects on AI image quality of three magnetic resonance (MR) diffusion tensor imaging (DTI) acquisition schemes, one being novel: the schemes were simulated on cerebral model fibres varying in shape and spatial orientation. The theoretical predictions and the results of the simulations were corroborated by experimentally determined spatial maps of the AI in a normal feline brain in vivo. We found that FA mapped diffusion anisotropy with the greatest detail and SNR whereas VR provided the strongest contrast between low- and high-anisotropy areas at the expense of increased noise contamination and decreased resolution in anisotropic regions. RA proved intermediate in quality. By sampling the space of the effective diffusion ellipsoid more densely and uniformly and requiring the same total imaging time as the published schemes, the novel DTI scheme achieved greater rotational invariance than the published schemes, with improved noise characteristics, resulting in improved image quality of the AI examined. Our findings suggest that significant improvements in diffusion anisotropy mapping are possible and provide criteria for the selection of the most appropriate AI for a particular application.     

Tracking extended mucociliary transport activity of individual deposited particles: longitudinal synchrotron X‐ray imaging in live mice

To assess potential therapies for respiratory diseases in which mucociliary transit (MCT) is impaired, such as cystic fibrosis and primary ciliary dyskinesia, a novel and non‐invasive MCT quantification method has been developed in which the transit rate and behaviour of individual micrometre‐sized deposited particles are measured in live mice using synchrotron phase‐contrast X‐ray imaging. Particle clearance by MCT is known to be a two‐phase process that occurs over a period of minutes to days. Previous studies have assessed MCT in the fast‐clearance phase, ～20 min after marker particle dosing. The aim of this study was to non‐invasively image changes in particle presence and MCT during the slow‐clearance phase, and simultaneously determine whether repeat synchrotron X‐ray imaging of mice was feasible over periods of 3, 9 and 25 h. All mice tolerated the repeat imaging procedure with no adverse effects. Quantitative image analysis revealed that the particle MCT rate and the number of particles present in the airway both decreased with time. This study successfully demonstrated for the first time that longitudinal synchrotron X‐ray imaging studies are possible in live small animals, provided appropriate animal handling techniques are used and care is taken to reduce the delivered radiation dose.     

Resolved fine structure of exciton complexes bound to phosphorus impurities in silicon

A Fabry-Pérot interferometer is used to observe resolved fine structure for two of the luminescence lines associated with exciton complexes bound to phosphorus impurities in silicon. The results are interpreted in terms of j-j couplings for the electrons and holes and give support for the recently proposed shell-model. Linewidths are obtained for the four luminescence lines associated with 1-, 4-, 5-, and 6- exciton complexes. The linewidths of the 4-, 5-, and 6- exciton peaks are found to be considerably broader than that of the single bound exciton.