Oxidative Purification of Carbon Nanotubes and Its Impact on Catalytic Performance in Oxidative Dehydrogenation Reactions

Oxidative purification with mild diluted HNO3 followed by NaOH washing lowers the amount of amorphous carbon deposited at multiwalled carbon nanotubes (CNTs). The graphitic structure can be remarkably improved by further annealing in Ar at elevated temperatures, i.e. 1173, 1573, and 1973 K. The influence of the purification treatment on the catalytic activity of the CNTs was investigated for the oxidative dehydrogenation (ODH) of ethylbenzene and propane as probe reactions. All samples tend to approach an appropriately ordered structure and the used samples display an almost same ratio of D band to G band of 0.95-1.42 in Raman spectra. Oxygen functionalities are partly removed by the annealing treatment and can be rebuilt to some extent by oxygen molecules in the ODH reactant flow. The presence of amorphous carbon is detrimental to catalytic performance as it allows for unwanted functional groups occurring in parallel with the formation of the desired active sites