Catalytic Chemical Vapor Deposition Synthesis of Double-Walled and Few-Walled Carbon Nanotubes by Using a MoO3-Supported Conditioning Catalyst to Control the Formation of Iron Catalytic Particles within an α-Al1.8Fe0.2O3 Self-Supported Foam

α-Al1.8Fe0.2O3 and α-Al1.8Fe0.2O3−MoO3 self-supported foams are used as catalytic materials for the synthesis of carbon nanotubes by catalytic chemical vapor deposition. A MoO3-supported conditioning catalyst placed upstream in the reactor is more efficient than MoO3 present within the catalytic material in producing double-walled and few-walled carbon nanotubes with fewer defects. It is shown that the corresponding modifications of the gas atmosphere (presence of H2O formed by the H2 reduction of MoO3 and, therefore, lower H2 and CH4 concentrations) allow one to limit more efficiently the release of the Fe catalyst from the oxide solid-solution foam, which results in the formation of fewer Fe nanoparticles, which, therefore, are less prone to undesirable growth. Thus, a MoO3-induced “solid-state” effect is demonstrated within the catalytic material without molybdenum species being themselves present within this material. This could lead to simplifications in the design of catalytic materials.