Catalytic growth of carbon fibers from methane and ethylene on carbon-supported Ni catalysts

Catalytic decompositions of methane and ethylene to carbon nanofibers (CNFs) and hydrogen have been carried out on various carbon-supported Ni catalysts at 823 K. Graphite (GP), active carbon (AC), and CNFs were used as the carbon supports. The CNFs used as the catalyst supports were prepared by the catalytic decomposition of methane over Ni/SiO2 catalysts at 823 K, followed by removing the catalyst with hydrofluoric acid and nitric acid. All carbon materials were treated by nitric acid and/or hydrogen peroxide, and were used as the supports for impregnation in aqueous solution of Ni(II) nitrate to form Ni/carbon catalysts. After the pre-treatments, the sizes of Ni metal particles formed on the carbon supports were smaller compared with those formed on non-treated carbon supports except for AC. AC was already activated by pre-treatment as purchased and was therefore not affected by the treatment. An impregnation of non-treated GP and CNFs in acetone solution of Ni(II) nitrate also afforded small-sized Ni metal particles, indicating the important role of interactions between carbon surface and Ni(II) nitrate solution during the impregnation. These suggest that the surface of GP and CNFs, which was originally hydrophobic, changed to hydrophilic after the pre-treatments, resulting in the formation of well-dispersed Ni metal particles due to strong interaction between the carbon surface and aqueous solution of Ni(II) nitrate. When methane and ethylene were decomposed over the Ni/carbon catalysts, smaller-sized Ni metal particles produced larger amounts of CNFs; moreover, ethylene formed larger amounts of CNFs than methane. It was confirmed on Ni/CNF catalysts that smaller-sized sec-CNFs grew on the well-dispersed Ni metal particles strongly bound to the surface of treated CNFs, while larger-sized sec-CNFs grew with Ni metal particles anchored on the tip or encapsulated inside the CNFs on non-treated CNFs. By removing Ni metal particles from the samples after the decomposition with nitric acid, cross-linked or multi-layered carbon materials between the original carbon and the secondarily formed CNFs were obtained.