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  • 1
    Electronic Resource
    Electronic Resource
    On the possibility of a significant temperature gradient in supported metal catalysts subjected to microwave heating (1997)
    Springer
    Catalysis letters 47 (1997), S. 1-4 
    Details
    ISSN: 1572-879X
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology
    Notes: Abstract A simple model is presented which estimates the temperature gradients in microwave-irradiated 1 and 100 nm metallic particles which are typically found in a supported metal catalyst structure. The two particle sizes allow limiting case calculations of the temperature rise over the range of typical particle sizes. The model, based on a simple steady-state energy balance, uses the assumption that the particles only lose heat to the gas-phase, and not the support matrix. This represents a best-case scenario for a temperature gradient relative to the surroundings. The model indicates that the temperature gradient is insignificant and this conclusion is supported by an experiment in which the microwave-driven carbon monoxide reaction acts as an in situ temperature probe.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1023/A:1019020013544
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  • 2
    Electronic Resource
    Electronic Resource
    The importance of passivation in the study of iron Fischer-Tropsch catalysts (1996)
    Springer
    Catalysis letters 37 (1996), S. 101-106 
    Details
    ISSN: 1572-879X
    Keywords: Fischer-Tropsch synthesis catalysts ; Fe catalysts ; passivation of high resolution transmission electron microscopy
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology
    Notes: Abstract In the study of iron catalysts, careful passivation is necessary for study of microstructure by ex situ analytical techniques. The passivation procedure used in our study consists of heating the sample in He at the reaction temperature, cooling to room temperature and introducing small amounts of O2 (〈1%) in a flowing He stream. A properly passivated sample shows no more than a few nm of surface Fe3O4 onα-Fe, when examined in a high resolution TEM. Proper passivation is also characterized by an exotherm of no more than 2–3 K. We show that a Fischer-Tropsch catalyst carbided in CO will show substantial amounts of magnetite, if exposed to air without proper passivation. Such surface oxidation may cause errors in determining the relative amounts of the magnetite and carbide phases in Fischer-Tropsch catalysts, which are important for proper identification of the catalytically active phase.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00813526
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  • 3
    Electronic Resource
    Electronic Resource
    Homogeneous Rh-Sn alkoxide coatings on silica surfaces: A novel route for preparation of bimetallic Rh-Sn catalysts (1991)
    Springer
    Catalysis letters 8 (1991), S. 345-358 
    Details
    ISSN: 1572-879X
    Keywords: Rh-Sn catalyst ; bimetallic catalysts ; Rh-Sn alkoxide coatings ; silica surfaces ; hydrogenation activity
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology
    Notes: Abstract The reaction of [{(COD)Rh}2Sn(OEt)6], where COD = 1,5-cyclooctadiene and Et = ethyl, with silanol groups on silica surfaces is shown to lead to near-monolayer coverage of the silica by the Rh-Sn organometallic compound. Heating the supported compound at 498 K yields a catalyst that is active for benzene hydrogenation at room temperature. When the catalyst is reduced in H2 at 823 K, the benzene hydrogenation activity increases with a simultaneous drop in the activity for n-butane hydrogenolysis. High temperature reduction leads to formation of Rh-Sn alloy particles with an average particle diameter of 2.5 nm. These particles are stable towards oxidation-reduction cycles involving oxidation at 773 K in 15% O2. When normalized to the benzene hydrogenation activity, the n-butane hydrogenolysis activity of the bimetallic catalyst is suppressed by over 3 orders of magnitude when compared to a monometallic Rh catalyst.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00764196
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  • 4
    Electronic Resource
    Electronic Resource
    Activation behavior of Ni/hydrous titanium oxide (HTO) catalysts (1992)
    Springer
    Catalysis letters 15 (1992), S. 111-122 
    Details
    ISSN: 1572-879X
    Keywords: Catalyst activation ; ion exchange ; Ni ; hydrous titanium oxide ; n-butane hydrogenolysis
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology
    Notes: Abstract The activation of Ni-TiO2 catalysts prepared via both impregnation on commercial TiO2 and by ion exchange on hydrous titanium oxide (HTO) supports was investigated. The reactivity of these catalysts for a structure-sensitive reaction (n-butane hydrogenolysis) was investigated as a function of different activation treatments (temperature, time) in a H2 atmosphere. Complete activation of the ion exchanged Ni/HTO catalyst in H2 required temperatures 〉 623 K and times 〉 18 h. Low temperature (573 K) activation of the ion exchanged Ni/HTO catalyst in H2 was not possible without performing a mild (573 K) reduction/oxidation/reduction cycle. Observations of the ion exchanged Ni/HTO catalyst microstructures by transmission electron microscopy revealed that inactive samples contained a distinct film which covered Ni particle surfaces. The exact nature of this film has not yet been determined; experimental evidence is consistent with the film being composed of either carbonaceous residue or TiO x which has migrated to the Ni particle surfaces from the bulk support phase.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00770904
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  • 5
    Electronic Resource
    Electronic Resource
    Transmission electron microscopy of supported molybdenum and vanadium oxides (1992)
    Springer
    Catalysis letters 15 (1992), S. 155-167 
    Details
    ISSN: 1572-879X
    Keywords: Transmission electron microscopy (TEM) characterization ; supported molybdenum ; vanadium oxides
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology
    Notes: Abstract Transmission electron microscopy has been used to characterize dispersions of molybdena and vanadia on titania and silica supports. When silica spheres of controlled morphology were used as support, the dispersed “monolayer” phase of both oxides could be imaged due to characteristic changes in contrast. In addition to the dispersed phase, we could detect three-dimensional crystallites of V2O5 but in the case of MoO3 only two-dimensional islands were seen. On Degussa P-25 titania, there was no observable contrast change due to the presence of a monolayer of these dispersed oxides. However, exposure to the electron beam caused dramatic changes in the surface texture of the support. Such changes were not seen when blank TiO2 was similarly irradiated. These e-beam induced changes were more pronounced in the vanadia/titania catalysts leading to formation of 1–3 nm clusters of reduced VO x . However, on the MoO3/TiO2 sample, e-beam exposure caused only a pronounced change in texture but no well defined clusters could be detected.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00770908
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  • 6
    Electronic Resource
    Electronic Resource
    High-resolution scanning electron microscopy for the characterization of supported metal catalysts (1995)
    Springer
    Catalysis letters 31 (1995), S. 57-64 
    Details
    ISSN: 1572-879X
    Keywords: scanning electron microscopy (SEM) ; supported metal catalysts ; small metal particles
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology
    Notes: Abstract A scanning electron microscope with a short-focal-length “immersion” lens and subnanometer resolution has been used to characterize several oxide-supported metal particle catalysts. Nanometer-sized metal particles in the Pt/TiO2 and Pd/SiO2 systems could be imaged with best clarity at the upper end of the operating voltage range (20–30 kV). However, visibility depended upon an adequate yield of secondary electrons relative to the support: small Pt particles on CeO2 could not be located by secondary electron imaging. Best visibility of the surface topography of the support was obtained at lower accelerating voltages.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00817033
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  • 7
    Electronic Resource
    Electronic Resource
    A method for measuring the titania surface area on mixed oxides of titania and silica (1992)
    Springer
    Catalysis letters 13 (1992), S. 313-321 
    Details
    ISSN: 1572-879X
    Keywords: Titania ; silica ; 2-propanol adsorption ; temperature-programmed-desorption ; surface-area-measurements
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology
    Notes: Abstract We have used temperature-programmed desorption (TPD) and thermogravimetric analysis (TGA) of 2-propanol to characterize several silica, titania, and silica-supported titania samples. Upon evacuation, most of the 2-propanol desorbed intact from the silica samples. This is in contrast to the results on titania and on silica-supported titania, where a significant amount remained on the surface following evacuation, with a fraction of this reacting to propene and water. The coverages of 2-propanol are approximately proportional to the titania surface area, corresponding to between 2.4 and 6.1×1018 molecules/m2 of titania, depending on the form of the titania. The results suggest that selective adsorption of 2-propanol may be useful for determining the surface area of titania in titania-silicates.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00765033
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  • 8
    Electronic Resource
    Electronic Resource
    Preface (1992)
    Springer
    Catalysis letters 15 (1992), S. I 
    Details
    ISSN: 1572-879X
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00770892
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  • 9
    Electronic Resource
    Electronic Resource
    Morphology of titania coatings on silica gel (1997)
    Springer
    Catalysis letters 45 (1997), S. 165-175 
    Details
    ISSN: 1572-879X
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology
    Notes: Abstract Control of the hydrolysis and condensation of soluble precursors of titanium is shown to yield oxide coatings of well-defined morphology. A ``smooth'' coating, consisting of patches of titania on silica, causes only a small increase in the surface area, from 130 m2/g for the uncoated silica gel to 154 m2/g. On the other hand, the ``rough'' coating, which consists of 3 nm diameter titania particles dispersed over the silica surface, results in an increase in the BET surface area to 350 m2/g. The specific titania surface area was determined via isopropanol dehydration activity and was found to be comparable to the BET surface area indicating that the titania phase is responsible for the increase in surface area. Small angle X-ray scattering (SAXS) and transmission electron microscopy (TEM) provide definitive evidence for the coating morphology while X-ray photoelectron spectroscopy (XPS) was used to determine the dispersion of the titania. The supported titania maintains its surface area upon calcination at temperatures up to 873 K while unsupported titania looses much of its surface area. Also demonstrated in this work is a novel preparation method using titanium bis-ammonium lactato dihydroxide (TALH), a water soluble precursor.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1023/A:1019096419053
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  • 10
    Electronic Resource
    Electronic Resource
    Mode selection in a caricature of eutectic solidification (1982)
    Springer
    Journal of statistical physics 29 (1982), S. 1-16 
    Details
    ISSN: 1572-9613
    Keywords: Eutectic ; mode selection ; directional solidification ; fluctuations ; marginal stability
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract We have studied the problem of mode selection, specifically, selection of the lamellar spacing, in eutectic solidification, by constructing a caricature of a thin-film eutectic solidifying at constant velocity in the presence of a finite temperature gradient and in a slightly noisy environment. Our model incorporates mechanisms which allow termination of substable lamellae and creation of new lamellae by the splitting of lamellae larger than a spacing λmax. We have studied this model both by a simple analytic approximation and by computer simulation. We find that both the steady state spacing and the regularity of the pattern are sensitively dependent on the ratio λmax/λmin, where λmin is the minimum spacing required for steady state stability. An additional result to emerge from our investigation is that when λmax becomes less than a critical value, the system apparently undergoes a transition into a chaotic state.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF01008244
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