Mineral magnetite as precursor in the synthesis of multi-walled carbon nanotubes and their capabilities of hydrogen adsorption

Mineral magnetite was used as metallic oxide catalysts in the synthesis of Multi-Walled Carbon Nanotubes (MWCNTs) by Aerosol Assisted Chemical Vapor Deposition (AACVD). The structural and morphological information of MWCNTs was obtained by X-ray Diffraction (XRD), Raman Spectroscopy (RS), Scanning Electron Microscopy (SEM), and Transmission Electron Microscopy (TEM). The hydrogen storage capacity and the gas adsorption kinetics of the MWCNTs exposed to H2 at different pressures were determined using a quartz crystal microbalance (QCM). It was found that the hydrogen adsorption capacity was strongly dependent on the chemical, structural and morphological characteristics of the MWCNTs which, in turn, depend on the proportion of the starting materials (mineral magnetite and zeolite) used for the synthesis by AACVD. However, no a clear correlation was found between each one of these characteristics and the adsorption properties since the sample was affected by gas exposure during the H2 loading/unloading cycles. The maximum adsorption capacity was 1.76 wt% at 44 Torr of H2 exposure pressure. The adsorption kinetics was determined by in situ monitoring the QCM resonance frequency. As expected, a faster H2 adsorption kinetics of the MWCNTs occurred when the sample was exposed to higher H2 pressures. In general, there are measurable H2 adsorption between 60 s and 100 s before reaching saturation.