Effect of the pore structure of coal-based activated carbon and hydrogen addition on methane decomposition for the preparation of carbon nanotubes

Activated carbon with a hierarchical micro/mesoporous carbon structure was synthesized using Shenmu coal as the raw material, which was impregnated with KOH (K) in different K/C ratios before carbonization at various temperatures. The obtained material was loaded with nickel and then employed as catalyst for the catalytic methane decomposition (CMD) to prepare carbon nanotubes. Using a constant nickel loading, the effects of the pore structure of the different activated carbon materials and hydrogen addition on the preparation of carbon nanotubes were investigated. The results show that the coal-based activated carbon with a micropore content of 0.8, synthesized at a carbonization temperature of 850 °C and a K/C ratio of 2, is the most suitable carrier for the methane cracking catalyst to produce carbon nanotubes. The diameter of the prepared carbon nanotubes is in the range of 50–120 nm with a length of 5–15 μm. The addition of hydrogen can prevent the high-temperature decomposition of oxygen-containing functional groups, and the activity of the catalyst can be maintained by inhibiting carbon deposition. Furthermore, the hydrogen present in the CMD process can promote the formation of carbon nanotubes with larger diameters (50–350 nm) and shapes. The nanotubes are more curved, and their length is reduced to 5–11 μm. Finally, the formation mechanism of carbon nanomaterials is discussed according to the morphological changes of the products after hydrogen addition and the existing theories. In conclusion, appropriate amounts of oxygen-containing functional groups and larger micropore volumes facilitate the pyrolysis of coal-based activated carbon catalysts in the preparation of carbon nanotubes.