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Ni nanoparticles at the tips of carbon nanofibers (Ni-CNFs) were synthesized by catalytic chemical vapor deposition (CCVD) method using CH 4 as the carbon source, and then employed as catalysts for the generation of H 2 from ammonia decomposition. The morphology of Ni catalysts is highly sensitive to the ratio of CH 4 to H 2 . Especially for the CH 4 /H 2 ratio of 4, the as-obtained Ni-CNFs catalyst shows higher H 2 formation rate, which could be due to more accessible facets to the reactants and unique shape effect. Meanwhile, this catalyst also shows good thermal stability, possibly owing to the highly dispersed and spatially isolated Ni nanoparticles by CNFs. Moreover, effect of the surface carbon coverage on the orientation of Ni crystal facets and the matching degree between graphene sheet and Ni crystal facet were investigated by DFT calculations. Finally, a possible formation mechanism of shaped Ni catalysts was discussed by combining experimental and theoretical results. [ABSTRACT FROM AUTHOR]

Ni-carbon nanofibers (Ni-CNFs) catalysts were synthesizedin situby the decomposition of carbon-containing gases (i.e., CH4, CO, and C2H4) over Ni/Al2O3catalyst and directly used to catalyze ammonia decomposition.The results showed that Ni nanoparticles were found to locate at thetips of CNFs when using CH4and CO as carbon sources, whilethey located at the roots of CNFs when using C2H4as a carbon source. For ammonia decomposition, Ni catalysts at thetips of CNFs showed higher activity, which could be due to the moreaccessible surfaces to the reactants. Interestingly, the Ni catalystat the tips of CNFs with CH4as a carbon source exhibitedhigher activity than that with CO as a carbon source, even thoughthe former catalyst had a larger average particle size. The possiblemechanism was given by combining characterization results with ourprevious simulation results. Finally, when using CH4asa carbon source, the effect of the Ni-CNFs catalysts with differentgrowth times on the activity was further studied. [ABSTRACT FROM AUTHOR]