Formation of filamentous carbon from 1,3-butadiene over a Ni-Pd/Al2O3 catalyst

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The formation of carbon filaments from 1,3-butadiene over a Ni-Pd/Al2O3 catalyst at 400–450‡C was studied by high-resolution electron microscopy and X-ray diffraction analysis. The carbon filaments differ in morphology and in the arrangement of active metal inclusions: (1) abundant ∼ 1000 a thick filaments contain biconical alloy particles in the middle of the filament, (2) thin filaments of a thickness less than 500 a rolled in helical fashion contain oval-shaped alloy particles in the middle, and (3) in a few filaments that are less than 200 a thick, alloy particles are positioned at the ends. When the first two types of filaments grow, decomposition of the hydrocarbon occurs at the active sites of the surface with the formation of a carbide microphase with a hexagonal structure. This phase is called the active microphase. When the reaction stops, the active phase decomposes to form carbon and hexagonal nickel. Extensive defects like grain boundaries and stacking fault dislocations are present in the bulk of Ni-Pd alloy particles. The origin of these defects is related to the epitaxial effect of the active microphase particles on the alloy.