The effect of phase separation in Fe/Mg/Al/O catalysts on the synthesis of DWCNTs from methane

Double-walled carbon nanotubes (DWCNTs) were prepared by methane decomposition on Fe/Al/Mg/O catalysts with a fixed iron loading of 1.5%. Increasing Al/Mg ratio in the catalyst resulted in the formation of a new MgAl2O4 phase, which was characterized by XRD. The size of the MgO crystallites in the support was decreased, due to the phase separation, from 35 nm to 20 nm in the Al/Mg ratio range of 0:1–4:1. At an Al/Mg ratio of 1:200, this effect prevented the sintering of iron on the MgO support and resulted in the synthesis of high-purity DWCNTs in high-yield. Very high-Al/Mg ratio induced the formation of the MgAl2O4 phase, which became another catalyst support material. This had a negative effect on the synthesis of DWCNTs due to its acidity and hardness. Simultaneously maintaining MgO as the dominant catalyst support and decreasing its particle size by the phase separation effect are important for good metal dispersion and, consequently, the yield and purity of DWCNTs.