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 CO 2 were investigated. A simple rate equation for the reductive adsorption of CO 2 onto the catalyst surface followed by carbon diffusion leading to the production of CNTs is proposed as follows
, 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 CO 2 , and the experimentally measured data fits the simple rate equation very well at low carbon dioxide concentration.
Methods
The CNTs were produced by a method similar to that of Xu and Huang 18 using a 10% iron catalyst on calcium carbonate support. The choice of calcium carbonate was based on the good quality, high yield and high purity materials obtained from its use as a support. 19 The catalyst was prepared by a modiξed wet impregnation technique 20 by mixing a predetermined amount of iron nitrate and citric acid in approximately 1 : 1 molar ratio in deionized water. Ammonia solution was added in drops until a neutral pH was reached. A reddish brown solution with no precipitate was obtained aξer a period