THE ROLES OF ORDINARY AND SORET DIFFUSION IN THE METAL-CATALYZED FORMATION OF FILAMENTOUS CARBON

The rate-determining step for the metal-catalyzed formation of filamen tous carbon from hydrocarbons and carbon monoxide is commonly assumed to be diffusion of dissolved carbon through the metal particle. The dr iving force for the diffusion process has been proposed to be either a n isothermal carbon concentration gradient or a temperature gradient, the latter leading to Soret diffusion (mass transfer due to a gradient in the chemical potential resulting from the temperature gradient) an d, possibly, effects from the temperature dependence of the solubility of carbon. Metal carbides often have been postulated to play a role a s intermediates. Mass transfer by these processes is examined and expr essions for carbon deposition rate and activation energy are derived. Experimental results are consistent only with a mechanism in which the driving force for carbon diffusion is an isothermal carbon concentrat ion gradient and in which metal carbides are not intermediates. The te mperature gradient mechanism was found to conflict with the fact that for metals with a large negative heat of transport for carbon diffusio n, such as alpha-Fe, temperature gradients of the type proposed (in wh ich the gas-phase side of the particle is hotter than the carbon filam ent side) would lead to diffusion of carbon away from the carbon filam ent rather than towards it. A possible role for metal carbide intermed iates is limited to Fe, Ni, and Co. However, measured activation energ ies for these metals are in good agreement only with those values pred icted assuming metal carbides do not participate as intermediates. (C) 1995 Academic Press, Inc.