The role of alpha-iron and cementite phases in the growing mechanism of carbon nanotubes : a 57Fe Mössbauer spectroscopy study

57Fe Mössbauer spectroscopy was used to study the reduction behavior at temperatures as high as 1073 K of an iron/silica catalyst, and also the carbonaceous materials isolated after acetylene decomposition over this catalyst at several temperatures (873-1073 K). The products were previously characterized by transmission electron microscopy and it was clearly proven that the concentration of carbon nanotubes increased when reaction reached highest temperatures. This was related with an increment in cementite concentration (generated from initial alpha-iron and the progressive reduction of the remnant Fe+2 caused by acetylene decomposition) as detected by 57Fe Mössbauer. These results undoubtedly revealed the role of alpha-iron as active center for acetylene decomposition and cementite as main carbide intermediate species in the catalytic growth of CNTs.