Catalytic growth of structured carbon from chloro-hydrocarbons

Abstract

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.