Effect of H₂ on Formation Behavior of Carbon Nanotubes

The effect of H₂ gas on the carbon nanotubes (CNTs) synthesis with CO-H₂ gas mixture was investigated using mass measurements and scanning electron microscopy (SEM). The maximum weight and yield of the synthesized carbon were obtained when the mixture ratio of H₂ : CO was 3 : 7 and 9 : 1, respectively. In case of 100% carbon monoxide (CO) without hydrogen (H₂) addition, the weight of carbon increased, but CNTs were not observed. The CNTs began to be made when the contents of H₂ reaches at least 10%, their structures became more distinct with an increase of H₂ addition, and then the shapes of CNTs were more thin and straight. When the contents of H₂ was 80% (H₂ : CO = 8 : 2), the shapes and growth of CNTs showed an optimal condition. On the other hand, when the contents of H₂ was higher than the critical value, the shapes of CNTs became worse due to transition into inactive surface of catalyst. It was considered that the inactive surface of catalyst resulted from decrease of carbon (C) and H₂ concentration by facilitation of methane (CH₄) gasification reaction (C + 2H₂ → CH₄) between C and H₂ gases. It was also found that H₂ addition had an influence considerably on the shape and structure of CNTs