Purification of multi-walled carbon nanotubes

Multi-walled carbon nanotubes ( MWCNTs), prepared by catalytic decomposition of methane, were purified sequentially with hydrogen gas at high temperature (900degreesC) and hydrochloric acid (5 mol/L) reflux. The variations of surface area, water absorbing capacity, amount of Ni and Mg residuals of the MWCNTs in different purification steps were investigated. The desorbing ratios of benzene, n-hexane, ethanol, acetone on columns packed with the MWCNTs in different purification steps were also investigated. The morphologies of the MWCNTs in different purification steps were studied with TEM. The results suggest that the treatment of hydrogen could remove the amorphous carbon and the polar groups on MWCNTs, resulting in the decrease of surface area and water absorbing capacity and having, the ends of MWCNTs cut. After the treatment of hydrogen, it was very easy to remove Ni residual in MWCNTs, which could not be removed by hydrochloric acid reflux only. The water adsorbing capacity of the purified MWCNTs was much lower than that of activated charcoal, slightly higher than that of Carbopack B. The surface area of the purified MWCNTs was similar to that of Carbopack B. The desorption ratios of benzene, n-hexane, ethanol, acetone on column packed with the purified MWCNTs were the same as those of Carbopack B. The amounts of Ni and Mg remained on the purified MWCNTs were 30 mug/g and were not detected ( < 10 mug/g), respectively. Due to its hollow cavities inside, the purified MWCNTs have stronger retention ability than Carbopack B. The purified MWCNTs are found to be a suitable packing material for gas chromatography and a sorbent in trapping volatile organic compounds from air.