Morphology dependence of catalytic properties of Ni nanoparticles at the tips of carbon nanofibers for ammonia decomposition to generate hydrogen

Ni nanoparticles at the tips of carbon nanofibers (Ni-CNFs) were synthesized by catalytic chemical vapor deposition (CCVD) method using CH4 as the carbon source, and then employed as catalysts for the generation of H2 from ammonia decomposition. The morphology of Ni catalysts is highly sensitive to the ratio of CH4 to H2. Especially for the CH4/H2 ratio of 4, the as-obtained Ni-CNFs catalyst shows higher H2 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.