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Schlagwörter:
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Zusammenfassung:
Carbon materials are of great relevance in catalysis. In heterogeneous catalytic reactions they can act as reactant, poison, catalyst support and as product. The interaction between carbon and supported catalyst remains an unresolved issue of considerable importance in material science, catalysis and nanotechnology. The stability of the catalyst and the carbon support especially under catalyst pretreatment and reaction conditions is also important for
their application in catalysis. The first part of this thesis focuses on the catalytic synthesis of carbon nanotubes/nanofibers (CNT/CNF) by ethylene feed on carbon supported-Ni catalyst as the probe reaction to investigate metal-carbon support interaction. Surface and bulk sensitive characterization techniques such as high-resolution transmission electron microscopy (HR-TEM), X-ray diffraction (XRD) and X-ray photoemission spectroscopy(XPS) were used in this investigation. The study shows that carbon atoms from defective supports are not spectator species but are readily incorporated in the Ni, thus forming
metastable non-stoichiometric carbide with different catalytic activity than the pure metal.
In-situ XRD and XPS methods show that atomic carbon dissolved in Ni nanoparticles controls the growth as either CNF or CNT when the catalyst is exposed to ethylene. This work explains many of the differences observed in catalytic activity when metals are supported on carbon, and opens a new approach in heterogeneous catalysis, in that the catalytic activity of metals is adjusted through formation of metastable carbides, either by
nano- engineering of the carbon support or by exposing the active phase to an amorphous carbon precursor.
The second part of this thesis deals with the removal of disordered pyrolytic carbon from commercial CNT samples and its effect on the stability of the CNT under oxidative condition. TEM, thermal gravimetric analysis and Raman spectroscopy were used to monitor the efficiency of removing the disordered carbon materials. This study also investigates the effect of disordered pyrolytic carbon on the catalytic performance of the CNT samples for oxidative dehydrogenation (ODH) of ethylbenzene and propane. The methods investigated in this study were mild nitric acid washing, air-oxidation, and ultrasonic irradiation. Ultrasonic irradiation is shown to be the effective method to remove the pyrolytic carbon and oxidation debris from as-received and oxidized CNTs respectively without creating excessive damage to the CNTs. The study also points out the positive trend for the selectivity of styrene and propene in the ODH reactions with increasing graphitic character of the CNT samples.
