Catalyst support effects in the growth of structured carbon from the decomposition of ethylene over nickel

The characteristics of carbon nanofibers generated during ethylene decomposition over supported nickel can be readily manipulated by a judicious choice of the support material. The occurrence and ramifications of Ni/support interaction(s), in terms of Ni particle size/morphology/orientation, are considered and related to the carbon structure/dimensions and yield. A 7±1% w/w Ni loading was achieved by standard impregnation of SiO2, Al2O3, MgO, Ta2O5, activated carbon (AC), and graphite: the reduced catalysts have been characterized by H2 chemisorption, CO chemisorption/temperature-programmed desorption (TPD), and high-resolution transmission electron microscopy (HRTEM). The reaction of ethylene with hydrogen over these catalysts also generated ethane via hydrogenation, a step that was favored over Ni/Al2O3 and Ni/MgO. Carbon yield (where T<800 K) increased in the sequence Ni/Al2O3 ∼ Ni/MgO < Ni/AC < Ni/graphite< Ni/Ta2O5 < Ni/SiO2; at higher temperatures (>850 K), Ni/AC and Ni/graphite delivered the highest yields. With the exception of Ni/graphite and Ni/Ta2O5, which produced helical and highly curved fibers, the other supported Ni catalysts generated a relatively straight (limited curvature) fibrous growth. The occurrence of Ni fragmentation and secondary fiber growth from such fragments is illustrated and discussed. The influence of H2 content in the feed was investigated with respect to both carbon yield and structure; an increased H2 content served to enhance fiber structural order. Temperature-programmed oxidation studies have been used to probe the graphitic nature of the carbon product; the results are consistent with HRTEM analysis.