Multi-walled carbon nanotubes (MWCNTs) were synthesized by rapid thermal decomposition method using trimetallic catalyst supported MgO. MWCNTs prepared via trimetallic catalyst shows much higher BET specific surface area compared to current monometallic and bimetallic catalyst. As-grown and pristine MWCNTs were found to adsorb nitrogen reversibly and their adsorption uptake exhibits type-II BET isotherm. Existence of small impurities, such as metal and metal oxides present in the MWCNTs, was confirmed by thermogravimetric analysis as well as via energy-dispersive X-ray spectroscopy. An over 10 wt% enhancement of hydrogen storage capacity of as-grown MWCNTs compared to pristine was found to be due to the presence of impurities. Fast kinetics and complete reversibility gives indication that the process responsible for hydrogen adsorption uptake in MWCNTs is physisorption. A linear relation between hydrogen uptake (~0.22 and 0.20 wt%) and equilibrium hydrogen pressure was obtained for both as-grown and pristine MWCNTs.