Influence of the surface chemistry of multi-walled carbon nanotubes on their activity as ozonation catalysts

Multi-walled carbon nanotubes (MWCNTs) with different surface chemical properties were prepared by oxidative treatments with HNO3, H2O2 and O2 to introduce oxygen-containing surface groups and by thermal treatments for their selective removal. The texture and surface chemistry of the MWCNTs were characterized by nitrogen adsorption, temperature programmed desorption (TPD) and pH at the point of zero charge. A deconvolution procedure of the TPD spectra was used to quantify the oxygenated surface groups. These materials were used as catalysts for ozone decomposition, and for the ozonation of oxalic and oxamic acids. Generally, all these catalytic processes are favoured by carbon nanotubes with low acidic character. MWCNTs were shown to exhibit higher activity for the ozonation of oxalic and oxamic acids, compared to activated carbon. Successive experimental runs of oxalic acid removal carried out with a selected MWCNT sample show that the catalyst suffers some deactivation as a result of the introduction of oxygenated groups on the surface. Therefore, the effect of the surface chemistry is mainly observed for the fresh catalysts.