Stages of Filamentary Carbon Growth from Hydrocarbons on Nickelcontaining Catalysts and Causes of their Deactivation

Methane decomposition to carbon and hydrogen has been studied using the Ni/Al2O3, Ni-Cu/Al2O3 and Ni-Cu/MgO catalysts at 550 oC. The S-shaped kinetic curves of carbon formation from methane exhibit the following periods: induction, acceleration, stationary state and deactivation. The induction period is characterized by oversaturation of metal or alloy particles with carbon atoms and predominant formation of the graphite phase at the (111) faces of the catalyst particles. After formation of the graphite crystallization centers, the acceleration period is accompanied by the growth of graphite filaments and simultaneous reconstruction of the metal particles. After termination of the above processes, the carbon deposition rate becomes constant. Deactivation of the catalyst is caused by blocking of the front side of the metal particle with a carbon film. When the reaction temperature increases to 700 oC, deactivation of the nickel-containing catalyst follows a different mechanism. During the growth of the filamentary carbon, the metal particle becomes viscous-flowing. This fact allows for its partial capturing by the inner filament channel. As a result, the formed carbon filament has an internal channel filled either with metal or its alloy. Hydrogen addition to methane leads a decrease in the carbon formation rate on the catalyst and a change in the filamentary carbon morphology: now it contains a hollow channel.