Nucleation and growth of carbon deposits from the nickel catalyzed decomposition of acetylene

In the presence of a nickel catalyst, acetylene decomposed to form carbonaceous solids with filamentary, amorphous, or laminar form. By observing the initiation processes continuously, parameters controlling the type of deposit have been established. Controlled atmosphere electron microscopy has shown the development of filaments behind particles of nickel (30–50 nm diameter) at relatively low temperatures (870 ± 20 K). At the same temperature, amorphous carbon nucleated around the nickel particles and spread until it covered the whole solid. In extended experiments, angular graphite laminae grew from large nickel particles (300 nm diameter) at 1300 K. The catalytic activity of the nickel particles for filament growth ceased after approximately 15 s at 870 K, but it could be regenerated by exposure to either hydrogen at 1100 K, or oxygen at 1000 K. A significant increase in catalyst efficiency for filamentous growth was noted when particles of nickel were formed in hydrogen at 875 K prior to the introduction of acetylene. An explanation of this phenomenon and the mechanisms of the catalytic, deactivation, and regeneration processes are proposed. Exposure of the carbonaceous deposits to oxygen demonstrated their different reactivities and suggested that filaments consisted of an easily oxidizable core with a relatively resistant skin.