Selective low temperature synthesis of carbon nanospheres via the catalytic decomposition of trichloroethylene

The catalytic growth of structured carbon from a C2H4 and C2HCl3 feed promoted by Ni/SiO2 in the presence of H-2 over the temperature range 673 K <= T <= 1023 K has been examined. The supported Ni phase exhibited an exclusive cubic symmetry (XRD analysis) with a range of Ni particle sizes (TEM analysis) and a net shift in the distribution to larger particles with increasing reduction temperature (from 20 to 36 nm), accompanied by a decrease in H-2 chemisorption. Conversion of C2H4 generated hydrogenation (C2H6), hydrogenolysis (CH4) and decomposition (C + H-2) products. Ethane formation was favoured at lower temperatures with C formation increasingly preferred at higher temperatures so that C2H4 decomposition was the predominant process at T > 723 K; significant CH4 production was only observed at T > 900 K. Carbon yield from C2H4 passed through a maximum at 773 K and took the form of high aspect ratio graphitic nanofibres with a central hollow core and diameters in the range 5-180 nm. The carbonaceous product has been characterized by a combination of TEM-EDX, SEM, XRD, BET area and temperature programmed oxidation (TPO). Carbon formation from C2HCl3 exceeded (by a factor of up to an order of magnitude) that generated via the decomposition Of C21-14 at the same inlet C:Ni ratio to deliver essentially a carbon yield invariance (9.1 +/- 0.3 g(C) g(Ni)(-1)) where 898 K <= T <= 1023 K, which represents a carbon efficiency (fraction of carbon in the inlet feed that is converted to a solid carbon product) in excess of 96%. Ni/SiO2 promoted a composite dehydrochlorination/decomposition of C2HCl3 to HCI + C. The nature of the carbon product generated from C2HCl3 is strongly temperature dependent with a shift from a pseudo-fibrous product at 773 K to a predominant nanosphere formation at 923 K. These nanospheres exhibit a wide diameter range (40-700 nm), a significant Cl content (1.1-2.6%, w/w) and a conglomeration or clustering to give a less ordered carbonaceous product than that generated at the lower temperature (773 K). A tentative carbon growth rationale is presented to account for the observed dependence of carbon structure on carbon-containing precursor and reaction temperature. (C) 2008 Elsevier B.V. All rights reserved.