Catalytic growth of structured carbon from chloro-hydrocarbons

The catalytic growth of structured carbon from C₂H₄, C₂H₆, C₂H₂Cl₂, C₂HCl₃, C₂H₄Cl₂ and C₂H₃Cl₃ promoted by Ni/SiO₂ in the presence of H₂ over the range 673 K = T = 1073 K has been examined. The presence of Cl served to enhance carbon deposition where carbon yield (at T < 850 K) increased in the order C₂H₆ < C₂H₄ < C₂H₄Cl₂ < C₂H₃Cl₃ < C₂H₂Cl₂ = C₂HCl₃; at T > 900 K, carbon yield converged for C₂H₃Cl₃, C₂H₂Cl₂ and C₂HCl₃. Carbon efficiencies (fraction of carbon in the inlet feed that is converted to a solid carbon product) in excess of 96% have been achieved. Conversion of C₂H₄ also generated C₂H₆ (hydrogenation) as a secondary reaction while CH₄ (hydrogenolysis) production from C₂H₄ and C₂H₆ was observed where T > 900 K. The chloro-hydrocarbon to carbon reaction generated HCl as the only inorganic product with no evidence of Cl₂ or chloro-organics in the product stream, i.e. Ni/SiO₂ solely promotes a composite dehydrochlorination/decomposition. The nature of the carbonaceous product has been characterized by a combination of TEM, SEM, XRD, BET area and temperature programmed oxidation (TPO). Carbon yield from C₂H₆ and C₂H₄ passed through apparent maxima at 773 and 993 K, respectively, and took the form of high aspect ratio graphitic nanofibres with a central hollow core and diameters in the range 5-180 nm. Carbon deposition from C₂H₄Cl₂, C₂H₃Cl₃ and C₂H₂Cl₂ increased with increasing temperature to deliver essentially a carbon yield invariance where 898 K = T = 1023 K. The carbon fibres from the latter reactants exhibit a "segmented" or "bamboo-like" structure with diameters up to 560 nm. In complete contrast, catalytic decomposition of C₂HCl₃ resulted predominantly in nanosphere formation. These nanospheres exhibited a wide diameter range (35-680 nm) with a conglomeration or clustering and are characterized by a lesser structural (graphitic) order than the nanofibres produced from C₂H₄ and C₂H₆. © 2007 Elsevier B.V. All rights reserved.