Nitrogen-, phosphorous- and boron-doped carbon nanotubes as catalysts for the aerobic oxidation of cyclohexane

Nitrogen-, phosphorous- and boron-doped carbon nanotubes (N-CNTs, P-CNTs and B-CNTs) were prepared by a chemical vapor deposition method using xylene as carbon source and aniline-NH3, triphenyl phosphine and triethyl borate as nitrogen, phosphorous and boron precursors, respectively. By tailoring the composition of reactants and reaction atmosphere, N-CNTs with nitrogen contents from 0% to 4.36% and P-CNTs with phosphorous contents from 0.55% to 5.14% were synthesized. N- and P-CNTs are active for the oxidation of cyclohexane in the liquid phase with molecular oxygen as oxidant. The highest mass-normalized activity, 761 mmol g−1 h−1, was achieved over N-CNTs synthesized from aniline in an NH3 atmosphere, while the highest surface-area-normalized activity, 28 mmol m−2 h−1, was observed over P-CNTs. B-doping does not improve the activity of CNTs. The effect of the number of nitrogen functionalities and defects was investigated to reveal the structure-activity relationship of the doped CNTs. By using the work function as an indicator of the electron donation of carbon, an exponential dependence of specific activity on work function was discovered for N- and P-CNTs, suggesting that the electron transfer on the surfaces of CNTs plays a central role in the CNT-catalyzed oxidation of cyclohexane.