Catalytic Chemical Vapour Deposition on MFe<SUB>2</SUB>O<SUB>4</SUB>–SiO<SUB>2</SUB> (M = Co, Mn, Ni) Nanocomposite Aerogel Catalysts for the Production of Multi Walled Carbon Nanotubes

Highly porous MFe2O4-SiO2 (M = Co, Mn, Ni) nanocomposite aerogels were tested for the first time as catalysts for multi walled carbon nanotubes production by Catalytic Chemical Vapour Deposition. Structural and textural characterization points out that the catalysts are made out of nanocomposites where nanocrystalline ferrite phases with controlled composition and size in the range 9-13 nm are finely dispersed into the highly porous silica support. Remarkably, CoFe2O4-SiO2 and MnFe2O4-SiO2 catalysts have shown poor catalytic activity, whereas NiFe2O4-SiO2 catalyst has given rise to good quality nanotubes with high yields at deposition temperatures in the range 500-650 °C. The different catalytic behaviour of the mixed ferrite-silica aerogels as a function of the bivalent metal (M = Co, Mn, Ni) can been ascribed to how easily the nanophase can be reduced. In particular, it was demonstrated that the most active NiFe2O4 nanoparticles undergo an in-situ reduction process during the Catalytic Chemical Vapour Deposition reaction, forming a NiFe alloy phase that is responsible for promoting the multi walled carbon nanotubes growth.