Water-Assisted Growth of Carbon Nanotubes over Co/Mo/Al<sub>2</sub>O<sub>3</sub> Catalyst

Abstract

We studied the growth of carbon nanotubes (CNTs) over a Co/Mo/Al2O3 catalyst by decomposing ethylene with or without the assistance of water. The optimal amount of water was determined to be 0.6% (φ) since excess water removed the amorphous carbon around the catalysts and also directly etched the CNTs at high temperature. Under this condition, the yield of CNTs can be increased from 3.7 g·g - 1 , based on the mass of catalyst, to 70 g·g - 1 within 1 h. The time-dependent online conversion of ethylene and the ratio of effective catalysts suggested that the effect of water is insignificant in the final growth period of the CNTs compared to that at the beginning. The correlation between the relative activity of the catalyst and the relative density of the CNT agglomerate suggests that the lack of growth volume inside the CNT agglomerate results in a gradual deactivation of the catalyst in the final CNT growth period. Raman characterization suggests that the degree of CNT defects increases with the bulk density of the CNT agglomerates since the mechanical resistance that is exposed on CNTs inside the agglomerate increases with reaction time. Thermal-gravimetric analysis indicates that the purity of CNTs ranges from 95.0% to 99.9% for a product with average purity of 99.2%. The non-uniform purity of the CNTs is due to the difference in mechanical resistance inside and outside the CNT agglomerate. The growth of CNTs outside the agglomerate is nearly free of mechanical resistance compared to that inside the agglomerate and, consequently, results in a high yield and high purity for the CNTs. These results suggest that it is necessary to control the agglomerate size and the structure, and to use a reactor with a large reactor volume for the growth of CNTs with low resistance and with high yield.