Temperature-sensitivity of CNTs growth from tar model compound over Ni/Al₂O₃ with and without hydrogen addition

This work investigates the conversion of tar model compounds (toluene) to hydrogen and coke at different reaction medium (5 %H₂ or 100 %N₂), aiming at the prolonged catalytic lifetime and high-purity CNTs (carbon nanotubes) production. Temperature-sensitivity effect of H₂ on prolonging catalytic endurance of Ni/Al₂O₃ and increasing graphite selectivity was intuitively evidenced, in which the minimum dehydrogenation damage of 17.6 % and the highest graphite selectivity of 84.5 % were obtained at 500 °C in 5 %H₂ atmosphere. It is demonstrated that the formation of NiC_x crystallite phase is the main reason for the rapid deactivation of the dehydrogenation performance in 100 % N₂ atmosphere, and the prolonged catalytic endurance in 5 %H₂ atmosphere is due to the etching effect of H₂ on reducing NiC_x to metallic Ni phase. Based on the decrease of the outer diameter of CNTs and dehydrogenation reaction rate with the increase of reforming temperature, it is established that the mismatch between the optimum consumption of carbon atoms required for CNTs growth and the actual carbon supply at the metal/CNTs interface is the reason for the CNTs growth termination, which exhibits the highest match degree at 500 °C.