Influence of Water on the Initial Growth Rate of Carbon Nanotubes from Ethylene over a Cobalt-Based Catalyst

Water-assisted growth of multiwalled carbon nanotubes (CNTs) was studied over a Co-based catalyst under plug-flow conditions. The influence of water concentration and temperature on the growth kinetics within the first 300 s was analyzed by measuring the conversion of ethylene. Feeding 200 ppm H2O vapor at 650 degrees C accelerated the initial growth rate and extended the mean lifetime of the catalytically active sites. Higher water concentrations of up to 500 ppm led to lower growth rates and lower CNT yields. Water of 200 ppm showed a promoting effect at 650 degrees C, but an inhibiting effect at 550 degrees C. The CO generated by steam gasification of deposited carbon was monitored online indicating coking of the catalyst. The results demonstrate that water plays a dual role: the removal of amorphous carbon on the catalyst by gasification and partial oxidation of the metallic Co catalyst. Water also influenced the diameter distribution of the CNTs.