The effect of pyridinic- and pyrrolic-nitrogen in nitrogen-doped carbon nanotubes used as support for Pd-catalyzed nitroarene reduction: an experimental and theoretical study

Nitrogen-doped carbon nanotubes (N-CNTs) containing different nitrogen species were synthesized via a chemical vapour deposition technique. This was archived by use of ferrocene and ferrocenyl-2-(4-cyanophenyl)acrylonitrile as catalysts. The N-CNTs were acid-treated and then used as Pd nanoparticle supports (Pd/N-CNTs). Both N-CNTs and Pd/N-CNTs were characterized by transmission electron microscopy, X-ray photoelectron spectroscopy and other spectroscopic techniques. The selective reduction of nitroarenes to anilines was used as a model reaction to test the effect of pyridinic- and pyrrolic-nitrogen species on the catalytic performance of Pd/N-CNTs. A larger number of pyridinic-nitrogens in the N-CNTs increased nitroarene conversion and enhanced the selectivity towards anilines as opposed to the pyrrolic-nitrogens. Also, pyridinic-nitrogens favoured the nucleation of Pd nanoparticles with higher surface areas than those nucleated in the presence of pyrrolic-nitrogens. Density functional theory calculations were employed to determine the band-gap energy and optimal geometry of Pd complexed to pyridine and pyrrole ligands. These results showed that the pyridine complex had a lower band-gap energy (5.804 eV) than the pyrrole complex (6.406 eV) implying that the former complex is more reactive than the later. Thus, the inclusion of pyridinic-nitrogens in the N-CNTs, used as support, favours Pd-catalyzed nitroarene reductions.