Cobalt-silicon mixed oxide nanocomposites by modified sol-gel method

Cobalt–silicon mixed oxide materials (Co/Si = 0.111, 0.250 and 0.428) were synthesised starting from Co(NO3)2 6H2O and Si(OC2H5)4 using a modified sol–gel method. Structural, textural and surface chemical properties were investigated by thermogravimetric/differential thermal analyses (TG/DTA), XRD, UV–vis, FT-IR spectroscopy and N2 adsorption at -196°C. The nature of cobalt species and their interactions with the siloxane matrix were strongly depending on both the cobalt loading and the heat treatment. All dried gels were amorphous and contained Co2+ ions forming both tetrahedral and octahedral complexes with the siloxane matrix. After treatment at 400°C, the sample with lowest Co content appeared amorphous and contained only Co2+ tetrahedral complexes, while at higher cobalt loading Co3O4 was present as the only crystalline phase, besides Co2+ ions strongly interacting with siloxane matrix. At 850°C, in all samples crystalline Co2SiO4 was formed and was the only crystallising phase for the nanocomposite with the lowest cobalt content. All materials retained high surface areas also after treatments at 600°C and exhibited surface Lewis acidity, due to cationic sites. The presence of cobalt affected the textural properties of the siloxane matrix decreasing microporosity and increasing mesoporosity.