Catalyst deactivation in CVD synthesis of carbon nanotubes

A computational fluid dynamics (CFD) model with multistep chemical reactions was applied to predict the yield of multiwalled carbon nanotubes produced from our xylene-based chemical vapor deposition (CVD) reactor. Two-step xylene decomposition in the gas phase and catalytic decomposition of hydrocarbons to nanotubes on the growth surfaces were adopted based on exhaust-gas composition measurements. Using the experimentally obtained exhaust-gas concentrations, we conducted inverse calculations to determine apparent rate constants of the catalytic surface reactions. During the CVD process, catalyst deactivation was observed probably due to carbon formation on the catalyst surface. Its effect on the apparent rate constant was empirically correlated with a simple exponential equation. Applying the CFD model, we predicted the local yielding rate of nanotubes along the axis of the reactor. The total yield was then computed by integrating the local yielding rate over the growth surfaces and compared favorably (∼95%) with the experimental results. The proposed model is expected to help researchers optimize the process parameters to achieve the maximum nanotube yield.