Tunable synthesis of carbon nanotubes <i>via</i> methane catalytic pyrolysis by adjusting Mo incorporation in Fe/MgO

Catalytic pyrolysis of methane for the synthesis of carbon nanotubes (CNTs) was explored using an Fe-Mo/MgO catalyst. The impact of molybdenum (Mo) addition on carbon productivity and product characteristics was investigated by (S)TEM, XRD, TGA, nitrogen adsorption, and Raman spectroscopy. The Fe/MgO catalyst exhibited a carbon productivity of 0.16 g g_Cat.^-1 with 15% graphitic carbon selectivity. TEM revealed that carbon nanotubes (CNTs) including single-walled (SWCNTs) and double-walled nanotubes (DWCNTs) were produced. Mo incorporation synergistically enhanced both carbon productivity and graphitic carbon selectivity. For Fe-0.1Mo, productivity improved to 0.40 g g_Cat.^-1 with 60% graphitic carbon selectivity, while preferentially yielding SWCNTs. Further increasing Mo loading maximized productivity at 1.03 g g_Cat.^-1 for Fe-1Mo, coupled with 96% graphitic carbon selectivity. At higher Mo concentrations (Fe-0.5Mo and Fe-1Mo), Fe-Mo nanoparticle coarsening occurred, shifting CNT morphology toward multi-walled structures (MWCNTs). Lifetime studies confirmed enhanced catalyst stability under Mo promotion, with Fe-1Mo remaining active beyond 1 hour. Conversely, Mo/MgO showed negligible activity (0.08 g g_Cat.^-1) and produced only graphite flakes, underscoring iron's essential role in CNT growth. This work demonstrates Mo's dual function in enabling selective CNT production while providing mechanistic insights into its promotion of methane pyrolysis and nanotube formation.