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151.
Arenal R Stephan O Cochon JL Loiseau A 《Journal of the American Chemical Society》2007,129(51):16183-16189
We present a detailed study of the growth mechanism of single-walled boron nitride nanotubes synthesized by laser vaporization, which is the unique route known to the synthesis of this kind of tube in high quantities. We have performed a nanometric chemical and structural characterization by transmission electron microscopy (high-resolution mode (HRTEM) and electron energy loss spectroscopy) of the synthesis products. Different boron-based compounds and other impurities were identified in the raw synthesis products. The results obtained by the TEM analysis and from the synthesis parameters (temperature, boron, and nitrogen sources) combined with phase diagram analysis to provide identification of the fundamental factors determining the nanotube growth mechanism. Our experiments strongly support a root-growth model that involves the presence of a droplet of boron. This phenomenological model considers the solubility, solidification, and segregation phenomena of the elements present in this boron droplet. In this model, we distinguish three different steps as a function of the temperature: (1) formation of the liquid boron droplet from the decomposition of different boron compounds existing in the hexagonal boron nitride target, (2) reaction of these boron droplets with nitrogen gas present in the vaporization chamber and recombination of these elements to form boron nitride, and (3) incorporation of the nitrogen atoms at the root of the boron particle at active reacting sites that achieves the growth of the tube. 相似文献
152.
Heidi Van Parys Els Tourw Tom Breugelmans Marnix Depauw Johan Deconinck Annick Hubin 《Journal of Electroanalytical Chemistry》2008,622(1):44-50
In this work, the validation of a newly constructed inverted rotating disk electrode (IRDE) reactor is reported. Compared to the rotating disk electrode (RDE) reactor, the working electrode is changed in position from the top to the bottom of the electrochemical cell. The IRDE reactor is designed to facilitate the actual study of gas evolution reactions. It is studied whether the first-order analytical expression for the velocity field in an RDE reactor is also acceptable for an IRDE configuration. To that purpose, the kinetic parameters of the well-known ferri/ferro cyanide redox system are determined in both configurations and compared. This is done qualitatively by comparing the polarization curves obtained in the inverted and the conventional RDE configuration. Additionally, a statistically founded fitting algorithm is used to quantitatively determine the model parameters of the oxidation and reduction reaction. Not only the diffusion coefficients of Fe2+ and Fe3+ are calculated, but also the rate constants (kox and kred) and the transfer coefficients (αox and αred) are quantified and compared together with their respective standard deviation. It is found that the parameters of mass and charge transfer in both configurations agree well. So it is concluded that the same analytical equations of mass and charge transfer can be used in both the RDE and the IRDE reactor. 相似文献
153.
Catherine Renard Pascal Roussel Annick Rubbens Sylvie Daviero-Minaud Francis Abraham 《Journal of solid state chemistry》2006,179(7):2101-2110
We report the synthesis and crystal structure of the new compound Sr4PbPt4O11, containing platinum in highly unusual square pyramidal coordination. The crystals were obtained in molten lead oxide. The structure was solved by X-ray single crystal diffraction techniques on a twinned sample, the final R factors are R=0.0260 and wR=0.0262. The symmetry is triclinic, space group P1¯, with , , , α=90.421(3)°, β=89.773(8)°, γ=90.140(9)° and Z=2. The structure is built from dumbell-shaped Pt2O9 entities formed by a dinuclear metal-metal bonded Pt26+ ion with asymmetric environments of the two Pt atoms, classical PtO4 square plane and unusual PtO5 square pyramid. Successive Pt2O9 entities deduced from 90° rotations are connected through the oxygens of the PtO4 basal squares to form [Pt4O108−]∞ columns further connected through Pb2+ and Sr2+ ions. Raman spectroscopy confirmed the peculiar platinum coordination environment. 相似文献
154.
In this paper, two chiral stationary phases were prepared by coating the surface of both C8 and C18 high-performance liquid chromatography (HPLC) supports with the teicoplanin chiral selector. The hydrophobic C11 acyl side chain, attached to the d-glucosamine group of teicoplanin, served as anchor moiety for the immobilization of the chiral selector on the apolar support material. The retention and enantioselectivity of these coated stationary phases were studied using some aromatic amino acids as probe solutes and an aqueous solution as mobile phase. It was found that the enantiomer elution order on the modified C8 and C18 stationary phases was reversed (l > d) relatively to that classically observed with a teicoplanin covalently immobilized on a silica support (d > l). Such a dynamic coating on the reversed-phase supports was found to be of interest since the apparent enantioselectivity was not significantly changed by the use during an extended period of time or following a long-term storage of the columns. 相似文献
155.
156.
Asma Bourafai-Aziez Daniel Jacob Gwladys Charpentier Emmanuel Cassin Guillaume Rousselot Annick Moing Catherine Deborde 《Molecules (Basel, Switzerland)》2022,27(17)
Acerola (Malpighia emarginata D.C.) is an exotic fruit with high agro-industrial potential due to its high content of ascorbic acid (AA), phenolic compounds, and carotenoid pigments. Acerola fruit is processed into concentrated juice or powder to be incorporated into food supplements. The ascorbic acid content of concentrated juice or powders must be controlled and well assessed. Therefore, the development of optimal methods and procedures for the rapid and accurate determination of the ascorbic acid content in juice concentrate and juice powder remains of considerable commercial interest. NMR spectroscopy is currently a powerful spectroscopic tool for the qualitative and quantitative analysis of molecules of all types and sizes. Firstly, this article presents the NMR-based metabolomic profiling of acerola juice and concentrate powder to describe and compare their composition. Thirty-six metabolites were identified. The AA over choline ratio and the NMR metabolomic profiles could be used for authentication in the future. Secondly, a rapid (8 min), reliable, and non-destructive method for the quantification of ascorbic acid by 1D 1H-NMR spectroscopy was developed and validated. The LOD and LOQ were 0.05 and 0.15 mg/mL, respectively. These two approaches could be combined to better characterize ingredients derived from acerola and incorporated into food supplements. 相似文献