The relationship between the state of active species in a Ni/Al2O3 catalyst and the mechanism of growth of filamentous carbon

The formation of active particles and their changes in the course of 1,3-butadiene decomposition on a Ni/Al2O3 catalyst at temperatures from 400 to 800 degreesC were studied by high-resolution electron microscopy. It was found that carbon filaments of different types were formed at 400-800 degreesC. The growth of thin filaments (20-30 nm in diameter) takes place at 400-600 degreesC on a conical Ni particle located at the growing end of the filament, whereas di-symmetrical filaments 50-100 nm in diameter grow on biconical metal particles. As the carbonization temperature was increased to 700-800 degreesC, graphite nanotubes 5-20 nm in diameter were formed. It was found that the mechanism of formation and the structure of filaments are related to the state of catalytically active species, which consist of a solid solution of carbon in the metal. It is suggested that the metastable surface nickel carbide Ni3C1-x is an intermediate compound in the catalytic formation of graphite filaments from 1,3-butadiene. Upon termination of the reaction, the metastable Ni3C1-x microphase is decomposed with the formation of hexagonal nickel microinclusions. The role of epitaxy in the nucleation and growth of a graphite phase on the metal is discussed. Models are presented for the growth of structurally different carbon filaments depending on the formation of active metal species at various temperatures. Considerable changes in the structure of carbon and the formation of nanotubes at 700-800 degreesC are related to the appearance of a viscous-flow state of metal-carbon particles.