CVD synthesis and hydrogen storage properties of multi-walled carbon nanotubes

Multi-wall carbon nanotubes (MWNTs) had been synthesized by catalytic chemical vapor deposition of acetylene over Fe loaded mesoporous silica. The as-grown MWNTs were purified by a two-step purification procedure involving acid washing and oxidation in diluted air, and characterized using powder X-ray diffraction (XRD), transmission electron microscopy (TEM), scanning electron microscopy (SEM), thermo gravimetric analysis (TGA), and BET surface area measurements. Hydrogen adsorption measurements were carried out on as-prepared and purified MWNTs under moderate pressure (10 MPa) at 30degC using the manually-controlled apparatus for high-pressure adsorption with sufficient accuracy, and the results had been discussed. Pressure drop of hydrogen was measured and the amount it adsorbed was calculated by using ideal gas law and it was presented in weight percent, wt%. The hydrogen storage capacity of MWNTs was found to increase remarkably after subjecting to purification treatment. The maximum hydrogen storage capacity of 1.9 wt% was obtained for purified MWNTs. The as-grown MWNTs had closed ends. The hydrogen molecules could be physically adsorbed on the external nanotube walls. However, sample which was subjected to purification treatment, which could open their ends effectively, increased hydrogen sorption capacity, as hydrogen could have entered nanotubes through their ends. Purification treatment resulted in an increase in the number of sites with high interaction potentials for hydrogen adsorption, and these sites could be considered to be the inside of tubes or the interstitial space between the tubes.