Mechanism of carbon filament growth on metal catalysts

Carbon filaments formed on Ni, Co, and α-Fe particles by reaction with methane were studied. The filaments contained faceted single-crystal metal particles located at the growing tips. The crystallographic orientations of the gas/metal and graphite/metal interfaces were identified by TEM/selected area diffraction. Furthermore, extended Hüickel molecular orbital calculations indicated that the (111) and (311) faces of Ni provide stronger epitaxial fits with graphite than the (100) and (110) faces and that the order is reversed for their activities for CO decomposition. The results are in agreement with the limited amount of literature data on phase segregation and surface reconstruction for carbon/metal solid solutions. On the basis of these and literature results, a better understanding of the carbon dissolution/diffusion/precipitation mechanism for filament growth is obtained. Also, a temperature-driven carbon diffusion mechanism without resorting to the exothermality of the surface reaction is proposed.