Unraveling the Mechanisms of Carbon Nanotube Growth by Chemical Vapor Deposition

The mechanisms of carbon nanotube (CNT) growth by chemical vapor deposition of acetylene on Fe/SiO2:Al2O3 (zeolite Y) catalyst are unraveled through a combined computational and experimental study. CNTs are synthesized in a horizontal reactor under atmospheric pressure within the temperature range 650 degrees C-850 degrees C and characterized by SEM, TEM and Raman spectroscopy. A macroscopic computational fluid dynamics (CFD) model accounting for fluid flow, heat transfer and species transport is developed for the process, incorporating also kinetic expressions for acetylene surface decomposition, acetylene gas-phase reactions, carbon diffusion through the bulk of the catalyst, carbon surface accumulation on the catalyst surface and eventually, CNTs growth. The experimental behavior of the CNTs growth can be accurately described by the proposed macroscopic model. Most importantly, theoretical predictions suggest that there are two distinct temperature regimes for CNTs formation: at the low temperature regime, the process is dominated by the competition between carbon diffusion through the catalyst and carbon impurity layer formation on the catalyst surface, while at higher temperature, the gasphase reactions of acetylene prevails, releasing byproducts that deposit on the catalyst surface in the form of carbon impurities. Finally, using the developed computational approach, the catalyst lifetime, which is directly affected by these mechanisms, is correlated with the CNTs growth process.