An adsorption-diffusion isotherm and its application to the growth of carbon filaments on iron catalyst particles

Abstract

Nanometer-sized Fe particles can decompose methane and extrude graphitic filaments which grow as rapidly as millimeters per minute. The filaments are found to lengthen uniformly for a number of minutes at a constant rate until the Fe catalyst particles are deactivated. Steady state carbon surface coverage on these filament-growing particles is determined by the balance between the surface adsorption rate and the rate of diffusion through the solid particle to the filament. A simple isotherm having these characteristics is derived and found to be consistent with observed fiber growth rates. Comparing the experimental lengthening rate data to the model shows that methane forms a gas phase intermediate before adsorption on the Fe particle's surface. The cessation of filament lengthening occurs at a time corresponding to that required to deposit a 2.8 nm layer of carbon on the filaments. This is compatible with the hypothesis that filament growth ceases when the catalytic particle suffers a collision with a reactively condensed viscous hydrocarbon particle.