| Popis: |
Controlled-atmosphere electron microscopy and gravimetric measurements were used to study the effects of depositing powdered TiO2 on nickel surfaces with respect to the formation of filamentous carbon from acetylene and ethane. If the samples were heated directly in a hydrocarbon environment to temperatures near 1000 K, then filamentous carbon formation was initially suppressed on those portions of the nickel surface which contained titania. This inhibition of the formation of filamentous carbon was lost when the temperature was raised to ca. 1120 K, at which temperature the titania broke away from the nickel surface. In contrast, effective passivation of the nickel surface could be achieved at all temperatures by pretreating the TiO 2 Ni samples in hydrogen at ca. 770 K. During this treatment, the titania was observed to wet and spread over the nickel surface. This process could be reversed by heating in oxygen at 845 K, during which the nickel surface was uncovered and particles of TiO2 were observed to form. The passivating effect of titania on nickel was thereby destroyed. These results provide direct evidence that reduced titania species migrate over metal surfaces under reducing conditions and collect into TiO2 particles under oxidizing conditions. As such, these results support the idea that reduced titania species on metal surfaces may be the origin of strong metal-support interactions for titania-supported metal particles. Finally, a means of inhibiting filamentous carbon formation on nickel surfaces has been identified. |
