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Metal Phase (metal + phase)

Distribution by Scientific Domains

Chemistry	50%

Selected Abstracts

Aerogel and Xerogel Catalysts Based on ,-Alumina Doped with Silicon for High Temperature Reactions

EUROPEAN JOURNAL OF INORGANIC CHEMISTRY, Issue 3 2005
Aurelien Florin Popa
Abstract Numerous materials (supports and catalysts) based on alumina have been prepared using the sol-gel process and carbon dioxide supercritical drying. In this work two types of solids, i.e. xerogels and aerogels, were systematically compared and a way of introducing platinum metal with a content of 5% percent by weight was examined. The structural data, the surface area, Pt dispersion and catalytic activity for the decomposition of the propellant were measured for the various samples. The (Al2O3)0.88(SiO2)0.12 samples prepared show very interesting porosity values, especially for the aerogel. For this reason, they were chosen as supports for the synthesis of 5 wt% platinum on alumina catalysts. The results presented in this work allowed us to obtain an overall view of the influence of the preparation mode on the properties of platinum on alumina supported catalysts. The dispersion of the metal phase is directly dependent on the specific surface of the support. A significant value for the surface area implies a large amount of centers for interaction with the metal precursor and, consequently, the appearance of more centres of simultaneous germination. Although aerogels obtained by carbon dioxide supercritical drying always show superior properties compared with xerogels, for catalytic decompositions the xerogels still remain superior. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2005) [source]

Electronic properties and phase transitions in Si, ZnSe, and GaAs under pressure cycling up to 20,30 GPa in a high-pressure cell

PHYSICA STATUS SOLIDI (B) BASIC SOLID STATE PHYSICS, Issue 3 2009
Sergey V. Ovsyannikov
Abstract An automated high-pressure setup employing anvil-type cells is applied for investigation of phase transitions and elec- tronic properties (the thermoelectric power (Seebeck effect), the electrical resistivity) of silicon, zinc selenide and gal lium arsenide under pressure cycling within the range of 0,20(30) GPa. The "correlation" dependencies of a pressure value of the semiconductor,metal transition in Czochralski-grown silicon (Cz-Si) on the concentrations of both carriers and residual interstitial oxygen are discussed. In all Si samples a decompression of the high-pressure metal phase produces the semimetal p-type phases: the rhombohedral r8 (Si-XII) and the body-centred cubic bc8 (Si-III) lattices. Re- pressurization cycles reveal two features in the semimetal phase, near 2 GPa and 5 GPa. The transitions into the metal phase at higher pressure resemble those in pristine silicon. For ZnSe and GaAs, it is found that decompression from the high-pressure phases (NaCl and Cmcm, respectively) can follow different paths, producing the zincblende, the cinnabar and the wurtzite lattices. Advantages of the characterization method are discussed, in comparison with both the traditional techniques (X-ray diffraction, Raman spectroscropy, etc.) and the more recent ones (such as nanoindentation). (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source]

Liquid-Phase Hydrogenation of Unsaturated Aldehydes: Enhancing Selectivity of Multiwalled Carbon Nanotube-Supported Catalysts by Thermal Activation

CHEMCATCHEM, Issue 2 2010
Bruno
Abstract Platinum and iridium organometallic precursors are used to prepare nanosized, thermally stable multiwalled carbon nanotube-supported catalysts. The materials are characterized by N2 adsorption at 77,K, temperature-programmed desorption coupled with mass spectrometry, H2 chemisorption, transmission electron microscopy and thermogravimetric analysis; they are tested in the selective hydrogenation of cinnamaldehyde to cinnamyl alcohol under mild conditions (363,K and 1,MPa). A thermal activation at 973,K is found to have a very positive effect over both activity and selectivity, leading to selectivities of approximately 70,%, at 50,% conversion, regardless of the active metal phase (Pt or Ir). Since no noticeable differences in the metal particle sizes are detected, the results are interpreted in light of an enhanced metal/support interaction. This effect, induced by the removal of oxygenated surface groups, is thought to change the adsorption mechanism of the cinnamaldehyde molecule. [source]