Thermocatalytic pyrolysis of CO molecules. Structure and sorption characteristics of the carbon nanomaterial

Abstract

A carbon nanocondensate containing multiwalled carbon nanotubes has been produced by dissociation of CO molecules on an iron-nickel catalyst at temperatures of 400–500 °C. X-ray diffraction is used to show that this condensate contains two phases with different densities and degrees of ordering. Elevated synthesis temperatures lead to a higher density and smaller differences in the phases, which are related to increased freedom from defects in the carbon layers and a greater number of layers in the multiwalled carbon nanotubes. Studies of the sorption and subsequent desorption kinetics of hydrogen by the synthesized samples at temperatures of 7–120 K showed that when the temperature is lowered from 120 to 65 K, an increased sorption time for H2, which is typical of thermally activated diffusion, was observed in all the samples. At temperatures below 65 K the characteristic hydrogen sorption times depended weakly on temperature; this can be explained by a predominance of tunnel diffusion over thermally activated diffusion. At temperatures of 7–20 K, the temperature dependence of the characteristic times had features that appear to be related to the formation of a monolayer of H2 molecules on the inner surface of the nanotube cavities. The dependence of the hydrogen diffusion activation energy on the temperature at which the samples were synthesized correlates well with x-ray spectroscopy data: n rise in the activation energy is observed as the relative amount of the highly ordered carbon phase increases.