Thermodynamic analysis and chemical vapor deposition of multi-walled carbon nanotubes from pre-heated CH4 using Fe2O3 particles as catalyst precursor

The present study aims to investigate influence of pre-heating of CH4 on the growth of multi-walled carbon nanotubes (MWCNTs) using Fe2O3 particles as catalyst precursor. Equilibrium thermodynamic analyses in the systems of FeCOH and CH were performed to better understand the reduction of Fe2O3 by CH4 and to identify intermediate species which might promote growth of MWCNTs at the temperature range of 1000–1300 K. It was found that CH4 acted as reducing agent for Fe2O3 catalyst precursor which transformed to Fe3O4, FeO, Fe and Fe3C phases. This result was found to be in agreement with the thermodynamic prediction at 1200 and 1300 K. SEM-EDS analysis revealed that un-preheated CH4 yielded MWCNTs at 1200 K and a dense C coating at 1300 K. It was also observed that carbon was not formed at temperatures in the range of 1050–1150 K. Whereas, MWCNTs were grown at this temperature range from CH4 pre-heated at 1200 K. This result was attributed to the intermediate hydrocarbons, especially to C6H6, formed during pre-heating stage as thermodynamic analysis suggested. Mean diameter of the synthesized tubes was found to increase with growth time and temperature.

► Un-preheated CH4 yielded MWCNTs at 1200 K and a dense C coating at 1300 K.
► No MWCNTs were grown from un-preheated CH4 at a temperature range of 1050–1150 K.
► Pre-heating CH4 at 1200 K yielded growth of MWCNTs at 1050–1150 K.
► This result may be attributed to C6H6 and H2 formed during pre-heating stage.
► H2, a decomposition product of CH4, serves as reducing agent for Fe2O3 catalyst precursor.