Separating the initial growth rate from the rate of deactivation in the growth kinetics of multi-walled carbon nanotubes from ethene over a cobalt-based bulk catalyst in a fixed-bed reactor Original Research Article

The initial growth kinetics of multi-walled carbon nanotubes (CNTs) was investigated using a highly active Co-based mixed-oxide catalyst in a tubular fixed-bed reactor under plug-flow conditions with ethene as carbon source. The growth temperature and the ethene concentration were systematically varied in the range from 758 to 923 K and from 5 to 45 vol.%, respectively. The carbon mass accumulation was derived from the ethene conversion and analyzed by a kinetic model, from which the initial CNT growth rate and the mean lifetime of the active sites were derived permitting the prediction of the maximum theoretical CNT yield. With increasing growth temperatures up to 923 K both the initial growth rate and the mean lifetime of active sites increased strongly with a significantly prolonged lifetime above 848 K. The initial growth rate was slow at lower ethene concentrations, but the mean life time was very high. Increasing the ethene concentration up to 45 vol.% led to a much higher initial growth rate, but shortened the mean lifetime strongly. Due to the fast deactivation at high ethene concentrations, the predicted maximum yield decreased considerably approaching the yield obtained after 5 min of time on stream.