Purification of MWCNTs grown on a nanosized unsupported Fe-based powder catalyst

An unsupported Fe-based powder catalyst was prepared by coprecipitation of iron and aluminum ions followed by solid state reaction by a novel preparative route. Although metal particles were not supported on hard-to-reduce oxide, this catalyst showed nanometric dimensions.The catalyst was used for growing MWCNTs by thermal chemical vapor deposition (TCVD). Most applications require nanotube purification after growth, consisting in the separation of the nanotubes from other phases. A variety of purification processes, both chemical and physical, have been reported in the literature to remove metal catalyst, catalyst support and undesired carbonaceous phases from MWCNTs. In this work we report on an effective purification method for the removal of the catalyst, since the as-grown nanotubes did not show significant content of any other carbon phases. The purification consisted of an acid treatment for the metal catalyst dissolution. A comparison between nitric acid and a mixture of nitric and hydrochloric acid is provided. Morphological (SEM), structural (XRD) and spectroscopic (XPS, Raman) analyses were performed on catalyst and nanotubes, both before and after the purification.