Structures of silica-supported Co catalysts prepared Using microemulsion and their catalytic performance for the formation of carbon nanotubes through the decomposition of methane and ethylene

Silica-supported Co catalysts were prepared by using microemulsion. The structure and state of the Co species in the catalysts were characterized by TEM, XRD, Co K-edge XANES/EXAFS, and diffuse reflectance UV-vis spectroscopy. Co species in the catalysts prepared using microemulsion were atomically dispersed as Co²⁺ with tetrahedral symmetry (CoO₄), even after reduction of the catalysts with hydrogen at 773 K, while Co metal particles were supported on the silica surface for the catalysts prepared by a conventional impregnation method. The Co catalysts prepared using microemulsion selectively formed multi-walled carbon nanotubes with a uniform diameter by the decomposition of ethylene at 973 K, whereas silica-supported Co catalysts prepared by the impregnation method formed carbon nanofibers with various diameters. In addition, the Co catalysts prepared using microemulsion formed bundles of single- or double-walled carbon nanotubes by methane decomposition at 1073 K. Contact of the Co catalysts with hydrocarbons at the reaction temperatures (973 or 1073 K) resulted in the reduction of atomically dispersed Co oxide species to form small Co metal particles, which grew carbon nanotubes through hydrocarbon decomposition.