Simultaneous production of highly pure hydrogen and carbon nanofibers through CO_x-free methane dissociation over Ni-based catalysts: Investigation of the promotional effects of transitional metal (Mn, Zr, and Zn)

Hydrogen and carbon nanotubes are two novel and engaging research topics in clean fuel and material science. Methane decomposition is the most advantageous itinerary for the concurrent production of CO_x-free hydrogen and carbon filaments. The 50Ni/Al₂O₃ catalyst was successfully fabricated using a simple one-pot hydrothermal method, and the influence of transition metals (Zr, Mn, and Zn) was investigated as the promoter. The homogeneous accumulation of the nanoparticles with a high specific surface area of 101–147 m².g⁻¹ was revealed to be the consequence of the mesoporous structure's presence. The reactor tests' findings demonstrated that synthesized bimetallic catalysts are more stable and active than monometallic catalysts. Besides, the Zn-modified catalyst was shown to have the highest performance and durability. The maximal methane conversions of 65.7, 77.2, 69.6, and 51.7 % were achieved over the 50Ni-xZn/Al₂O₃ (defined as 50Ni-xZn/Al₂O₃; x = 2.5, 5, 7.5, and 10 wt%) at 650 °C and GHSV = 24,000 mL.(h.g_cat)⁻¹, respectively. A deeper look at the microstructure and textural characteristics of the aged catalysts makes it clear that the zinc promoter introduced to the 50Ni/Al₂O₃ sample, by increasing the growth rate of carbon fiber/nanotube, changes the structure of the formed carbons from spherical shape to very homogeneous diameter filaments and lead to the production of ordered carbon filaments with better crystallinity.