Controlled growth of highly ordered carbon nanofibers from Y zeolite supported nickel catalysts

The use of Y zeolite supported Ni to grow ordered carbon nanofibers from an ethylene/hydrogen feedstock has been investigated where Ni was incorporated into the aluminosilicate framework by ion exchange and impregnation; the action of the Ni/NaY zeolites is compared with that of Ni/SiO(2) of comparable Ni loading prepared by impregnation. The reaction of ethylene with hydrogen over these catalysts generated ethane via hydrogenation and solid carbon as a result of ethylene decomposition; hydrogenolysis to methane and the formation of C(3) and C(4) hydrocarbons were promoted to a lesser degree. The Ni loading, method of catalyst preparation, and nature of the support all have a bearing on the size distribution of the supported Ni crystallites which, in turn, impacts on the dimensions of the carbon nanofibers. The deposited carbon has been characterized by scanning electron microscopy, transmission electron microscopy (TEM), and temperature programmed oxidation. High-resolution TENT has revealed an orientation of graphite platelets that was generally parallel to the fiber axis with Ni metal particles located at the tips and Ni fragments dispersed throughout the carbon structure. The influence of reaction temperature, hydrogen content, and time on-stream was investigated with respect to both the carbon yield and structure. While the zeolite microporous structure limited the extent of carbon deposition, Ni/SiO(2) continued to promote carbon production over prolonged reaction times. The Ni metal particles associated with the impregnated zeolite are characterized by a narrower distribution of smaller diameters when compared with the ion-exchanged samples. The former exhibited a greater control over the dimensions of the carbon fibers and produced a more uniform and narrower graphitic product.