Novel Ni catalysts for methane decomposition to hydrogen and carbon nanofibers

Novel nickel oxides with controlled crystalline size and fibrous shape were successfully prepared by precipitation of nickel acetate and sodium carbonate aqueous solution with the mediation of ethylene glycol. The precipitation media and temperature were found to play crucial roles in determining the crystalline size and shape of the nickel oxides. The NiO prepared in the aqueous solutions exhibited mainly spherical particles with few nanorods, whereas the NiO samples prepared in the solution of ethylene glycol could be of spherical, fibrous, or nanorod shape, depending on the precipitation temperature. In particular, the nickel hydroxide precipitate obtained in the solution of ethylene glycol at 393 K was α-Ni(OH)2 with hydrotalcite-like structures; further calcination of this precipitate at 573–1073 K resulted in the formation of nickel oxides with similar fibrous shapes and crystalline size of 3.0–8.1 nm. Similar metallic Ni crystalline size of about 11 nm and fibrous structure were obtained by reduction of these NiO samples with hydrogen at 773 K. When further exposed to the reaction of methane decomposition, the metallic Ni crystallines still exhibited similar evolution behavior and rapidly stabilized at the critical size of 17–18 nm. Consequently, very close catalytic activities for methane decomposition over these novel Ni catalysts were observed with the carbon yields of 354–398 gC/gNi. The Ni crystallines departed from the fibrous structures and gradually aggregated into particles by the dissociation of the deposited carbons. These nickel particles were then highly dispersed on the in situ produced carbon, which can inhibit the further growth of nickel particle. As a result, the initial fibrous structure morphology of the metallic nickel was gradually turned into pear-like shapes.