Production of carbon nanotubes from methane Use of Co-Zn-Al catalysts prepared by microwave-assisted synthesis

Co-Zn-Al mixed oxides obtained from layered double hydroxide (LDH) precursors were synthesized and tested in the formation of multi-walled carbon nanotubes (MWNTs) by catalytic chemical vapour deposition (CCVD) of methane. The precursors were synthesized by co-precipitation from Co, Zn and Al nitrates with carbonate and submitted to microwave-hydrothermal treatment, in order to study the effect of time treatment on the catalyst performance. The microwave ageing treatment does not only play a role on some features of the precursors, such as crystallinity and textural properties, but also on the properties of the catalysts obtained by controlled calcination. The microwave-hydrothermal treatment affects to the distribution of the cations within the layers because of an improved ordering and this effect leads to better dispersed active species in the catalyst. In this sense, it was found that the aged catalysts reach better activity and stability levels during the methane decomposition reaction. Furthermore, the duration of microwave-hydrothermal treatment also produces a change of the kind of carbon nanofilaments produced. When the catalyst is not aged, herringbone carbon nanofibers (diameter similar to 14 nm) were found. On the other hand, multi-walled carbon nanotubes (diameter similar to 20-30 nm) could be observed in the sample after reaction with the 300 min aged catalyst. Finally, the application of a kinetic model based on the growing mechanism of nanocarbonaceous materials (NCMs), allows determining the influence of the microwave-hydrothermal treatment time on the kinetic parameters. (C) 2009 Elsevier B.V. All rights reserved.