Kinetic implications of mechanisms proposed for catalytic carbon filament growth

The growth of carbon filaments from methane using an iron catalyst is studied. The dependence of the rate of reaction upon the gas phase composition, expressed as gas phase carbon activity, is observed to fall into two regimes. At low values of gas phase carbon activity the rate of reaction is linearly dependent upon this activity. At high values of gas phase carbon activity the rate of reaction is nearly independent of the gas phase composition. This behavior is observed over a range of temperatures which includes conditions under which both α-Fe⁰ and γ-Fe⁰ are thermodynamically favored. The results are interpreted in terms of mechanisms which have previously been suggested for this reaction. The results at high gas phase carbon activity are consistent with a mechanism where a thin surface carbide is present on the catalyst. The results at low gas phase carbon activity demonstrate that this mechanism cannot be valid under all conditions of filament growth. It is suggested that the gas phase equilibrates directly with the catalyst surface when the gas phase carbon activity is below the thermodynamic limit for the formation of the surface carbide. A mechanism by which catalyst deactivation proceeds is also proposed.