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  • 1
    Digitale Medien
    Digitale Medien
    Molybdenum based catalysts. I. MoO2 as the active species in the reforming of hydrocarbons (1996)
    Springer
    Catalysis letters 38 (1996), S. 95-99 
    Details
    ISSN: 1572-879X
    Schlagwort(e): molybdenum dioxide ; XPS ; argon ion bombardment ; reforming of alkanes
    Quelle: Springer Online Journal Archives 1860-2000
    Thema: Chemie und Pharmazie
    Notizen: Abstract The XPS spectra of bulk MoO2 and MoO3 are reported. Commercial MoO2 reveals the presence of multilayers of MoO3 covering bulk MoO2. In situ reduction of bulk MoO3 by H2 at temperatures equal to 623 K and less than 673 K reduces the superficial layer(s) to mainly MoO2. Such phase is characterised by a certain density of states in terms of the free 4d, 5s electrons localised mainly on the Mo atoms and observed in the valence band energy region at the Fermi-level in the XP spectrum of this system. Catalytic reactions on the MoO2 phase of 2-methylpentane at atmospheric pressure using a mixture of the hydrocarbon (5 Torr) with hydrogen (755 Torr) yield 3-methylpentane andn-hexane as the main products in roughly equal amounts. These results are interpreted in terms of dual sites on the MoO2 surface.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1007/BF00806906
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  • 2
    Digitale Medien
    Digitale Medien
    The multi-structure of oxidized-reduced tungsten carbide surface(s) (1994)
    Springer
    Catalysis letters 29 (1994), S. 397-408 
    Details
    ISSN: 1572-879X
    Schlagwort(e): tungsten carbide ; surface composition ; XPS ; argon ion bombardment
    Quelle: Springer Online Journal Archives 1860-2000
    Thema: Chemie und Pharmazie
    Notizen: Abstract The XPS of bulk tungsten carbide, partially oxidized WC surfaces at 373 and 573 K as well as tungsten trioxide have been reported. Bulk WC has been prepared from WO3 as a starting material in a mixture of CH4 (20%) and H2 (80%) at 1150 K for 4 h, while partially oxidized WC surfaces were prepared by oxygen chemisorption on a clean WC surface at 200 K, then the temperatures were raised to 373 and 573 K respectively. The XPS of a freshly prepared WC reveals the presence of a small amount of WO3 on the surface and a slightly higher concentration in the bulk. The oxygen-exposed fresh WC surfaces and surfaces treated at temperatures higher than 373 K show the presence of WO3 in a considerable quantity depending on the length and the treatment temperature. Ar+ bombardment of this partially oxidized surface reduces WO3 to WO2 and W(0), while WC is partially reduced to W(0). Isomerization reactions of alkanes on oxygen-exposed WC surface occurs in reality on a composite surface structure containing WC, WO3, WO2 and elemental W(0).
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1007/BF00807119
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  • 3
    Digitale Medien
    Digitale Medien
    Surface characterization and catalytic properties of supported tungsten and platinum-tungsten carbide and oxycarbide (1994)
    Springer
    Topics in catalysis 1 (1994), S. 75-85 
    Details
    ISSN: 1572-9028
    Schlagwort(e): Bulk and supported tungsten carbides ; oxycarbides ; supported platinumtungsten carbides ; tungsten aluminate ; XPS ; argon ion bombardment ; skeletal rearrangement of hydrocarbons
    Quelle: Springer Online Journal Archives 1860-2000
    Thema: Chemie und Pharmazie
    Notizen: Abstract Bulk tungsten carbide has been prepared from ammonium paratungstate. Oxidic oxygen in relatively small amount is always present on the WC surface regardless of the method of carburization. Tungsten(VI) oxide in Al2(WO4)3 can be reduced directly and in apparently one single step to the elemental W(0) state by Ar+ bombardment. Under H2 the reduction starts at 720 K. In 12 and 28% W03 supported on γ-Al2O3 and in Al2(WO4)3, almost 90% of the tungsten could be converted to the carbide form. Extensive hydrogenolysis products for n-hexane reactions on freshly prepared WC were observed while selectivity to isomerization associated with decrease in activity occurs upon exposure of the WC to oxygen at 620 K. Supported tungsten carbide(s) and oxycarbide(s) on γ-Al2O3 have comparable catalytic behaviour to those obtained on the bulk systems. The presence of Pt in these supported systems did not improve the catalytic performances and even did not show the catalytic properties of Pt. This was attributed to the severe conditions of catalyst preparations. Although the catalytic activity of tungsten carbide(s) obtained from the carburization of Al2(WO4)3 is very low, the selectivity suggests the presence of tungsten carbide plus some WO, species.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1007/BF01379577
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