Role of support composition in controlling small diameter carbon nanotube synthesis over Co/Al2O3–MgO catalyst

Small diameter carbon nanotubes are anticipated to be applied across numerous fields, including safety and energy technologies, due to their superior physical properties. However, the influence of different catalyst supports on the synthesis of such carbon nanotubes has rarely been systematically studied. This work investigates the effects of MgO, Al2O3, and a composite Al2O3–MgO support on the preparation of small diameter carbon nanotubes, and further elucidates the relationship between catalyst properties and the resulting nanotube diameter and carbon yield. Catalysts with 30 wt% cobalt supported on the composite Al2O3–MgO carrier produced carbon nanotubes with an average diameter of 7.8 nm, which is smaller than those obtained using MgO or Al2O3 alone. When the cobalt loading was reduced to 20 wt%, the average diameter of carbon nanotubes synthesized from the 20 wt% Co/Al2O3–MgO catalyst further decreased to 6.8 nm, while achieving a carbon yield of 1850%. These improvements are attributed to the formation of MgAl2O4 spinel in the composite support, which enhances the dispersion of cobalt nanoparticles under reducing conditions and promotes the surface enrichment of Co3O4 during the catalytic process.