Abstract:
The production of carbon nanostructures, including carbon nanotubes (CNTs), by chemical vapour deposition (CVD) occurs by thermally
induced decomposition of carbon-containing precursors. The decomposition of the feedstock leading to intermediate reaction products is an
important step, but rarely incorporated in rate equations, since it is generally assumed that carbon diffusion through or over the catalyst
nanoparticles is the rate-limiting step in the production of CNTs. Furthermore, there is no kinetic model to date for the production of CNTs from
carbon dioxide. These aspects are addressed in this study with the aid of a series of experiments conducted in a floating catalytic CVD reactor in
which the effects of reactor temperature, concentration and flow rate of CO2 were investigated. A simple rate equation for the reductive
adsorption of CO2 onto the catalyst surface followed by carbon diffusion leading to the production of CNTs is proposed as follows: d[CNT]/dt ¼
K[CO2], where K is proportional to the diffusion coefficient of carbon. The derived kinetic model is used to calculate the amount of CNTs for a
given concentration of CO2, and the experimentally measured data fits the simple rate equation very well at low carbon dioxide concentration.
