Synthesis of nickel/carbon-based catalysts and investigation of their mechanism for hydrogen production via methane cracking

A series of Ni/AC catalysts is prepared using acetylacetone nickel as the precursor and low-rank coal as the carbon source. Their catalytic performance in methane cracking for hydrogen production and the regeneration effect of hydrogen reduction on these catalysts are systematically investigated. First, the effects of hydrogen flow rate and reaction time are studied when the catalyst is regenerated after cracking methane. Subsequently, by varying nickel loading (5%, 10%, 15%, 20%), the influence of metal dispersion, pore structure, and coking on catalytic activity and stability is investigated. The results show that after the first hydrogen reduction treatment, the methane conversion of the 20-AC increases from 9.00% to 13.40%. Methane conversion of the 20-AC-20 increases from 9.10% to 12.90%. Hydrogen flow rate and reaction time are set to 20 mL/min and 20 min, respectively. Catalyst can be recovered to optimal catalytic activity after multiple cycles of methane catalytic cracking for hydrogen production. The 10-Ni/AC catalyst exhibits the highest methane conversion (16.94%) and catalytic stability. The actual loadings of Ni in 10-Ni/AC are 6.80%. The 10-Ni/AC is analyzed by the XRD, BET, SEM, and TEM. Its surface of 10-Ni/AC contains uniformly distributed metallic Ni. The carbon materials formed on the surface of 10-Ni/AC after the methane cracking reaction are mainly carbon nanotubes and flaky carbon. The carbon nanotubes on the surface of 10-Ni/AC exhibit both tip growth and base growth. As the reaction time increases, carbon atoms continuously accumulate on the surface of nickel, and gradually thicken the graphite layers. This will lead to the deactivation of the catalyst. The hydrogen reduction of 10-Ni/AC effectively restores partial activity, but the active site gradually decreases with increasing reaction cycles. The hydrogen reduction primarily exposes more Ni by consuming carbon deposits to enable catalytic activity, while specific surface area plays only a secondary role.