Heteroatom-Doped Carbon Nanomaterials as Metal-Free Catalysts for the Reduction of 4-Nitrophenol

In this work it is described the preparation and characterization of multi-walled carbon nanotubes and graphene flakes doped with heteroatoms, namely N, P, B, and S, through simple ball milling procedures with adequate precursors, followed by thermal treatments. Ball milling the carbon nanomaterials with melamine resulted in the introduction of pyrrolic, pyridinic and quaternary N atoms on the graphitic structure. Using triphenylphosphine as the phosphorous precursor induced the formation of R-O-PO(OH)(2) functionalities. Materials prepared with boric acid showed the presence of boronic groups (C-BO2) and B atoms in B2O3. S-doped materials presented tiophene-S type atoms. The catalytic studies showed that all the doped carbon materials were active catalysts in the reduction of 4-nitrophenol (4-NP), and that the modifications introduced by the doping procedures, overall yielded catalysts with superior activity in regard to their pristine counterparts. Reusability experiments revealed no loss of catalytic activity up to three catalytic cycles for the doped carbon nanotubes, whereas the graphene flake materials showed some deterioration of catalytic performance.