Removal of Metal Catalyst in Multi-Walled Carbon Nanotubes with Combination of Air and Hydrogen Annealing Followed by Acid Treatment

Multi-walled carbon nanotubes (MWCNTs) prepared by the decomposition of CH₄ on a Ni-MgO catalyst were treated with air and hydrogen annealing, and the combination of air and hydrogen annealing, respectively, followed by acid reflux. The treated MWCNTs were characterized by the techniques of transmission electron microscopy, thermogravimetric analysis and flame atomic absorption spectrometry. The results showed that hydrogen annealed MWCNTs were more air stable than air annealed ones. Followed by acid treatment, the combination of air and hydrogen annealing was more efficient than either air annealing or hydrogen annealing for the removal of catalyst in MWCNTs. For the treatment with 2 h of hydrogen annealing followed by 2 h of air annealing, the catalyst Ni remained in the purified sample was 10.8 μg/g, and the yield rate was 82.3%. With 2 h of air annealing followed by 2 h of hydrogen annealing, the data were 5.8 μg/g and 61.9%, respectively. The different functions of air and hydrogen during annealing were discussed.