New NiO/Co3O4 and Fe2O3/Co3O4 nanocomposite catalysts:: Synthesis and characterization

Two nanocomposite oxides, NiO/Co3O4 and Fe2O3/Co3O4, are considered. The nanocomposite oxides were obtained by wet impregnation and characterized by means of X-ray photoelectron spectroscopy (XPS), diffuse reflectance infrared Fourier transform (DRIFT) spectroscopy, X-ray diffraction (XRD), thermal analysis, atomic force microscopy (AFM), and transmission electron microscopy (TEM). The microscopy images suggest a different growing mechanism for the NiO/Co3O4 and Fe2O3/Co3O4 nanocomposite oxides: small clusters of iron oxide wet the surface of Co3O4, whereas NiO grows as isolated particles characterized by a big diameter. Both XPS and TEM data indicate the possible formation of Fe3O4. A marked decrease of OH groups on the Fe2O3/Co3O4 and NiO/Co3O4 sample surfaces is revealed by XPS and DRIFT data and suggests the grafting of supported and supporting oxides by hydroxyl condensation. The acidic and basic sites present on Fe2O3/Co3O4 and NiO/Co3O4 powder surfaces are investigated and compared with those observed on the Co3O4 oxide. New Lewis and Bronsted acidic sites are observed on the Fe2O3/Co3O4 sample, whereas the complex sites constituted by a cation and its neighboring oxygen atom (observed on the iron oxide surface) disappear. In the NiO/Co3O4 mixed oxide sample, new acidic/basic sites were never revealed. The Fe2O3/Co3O4 supported oxide influences the stability of the Co3O4 with respect to its reduction to CoO.