The modification of MoO3 nanoparticles supported on mesoporous SBA-15: Characterization using X-ray scattering, N2 physisorption, transmission electron microscopy, high-angle annular darkfield technique, Raman and XAFS spectroscopy

Abstract

MoO3 was dispersed onto mesoporous SBA-15 by using ammonium heptamolybdate as MoO3 source. The formation of MoO3 was carried out by heating the loaded material to 500 °C for 3 h in air. Below 13 wt% Mo loading, no reflections of MoO3 occur in the X-ray powder patterns and even for high MoO3 contents, the intensities of the reflections are much lower than expected for fully crystalline material. A detailed XAFS analysis reveals that at low Mo contents, the metastable hexagonal modification of MoO3 is formed despite the high calcination temperature of 500 °C. It is highly likely that the nanosize of the particles and the interaction between MoO3 and SBA-15 stabilize the metastable modification of the material. Nitrogen physisorption experiments show the typical type-IV isotherms indicating that the mesoporosity of the materials is preserved despite the large amount of MoO3. Transmission electron micrographs demonstrate the presence of MoO3 inside the SBA-15 support. The Raman spectra display a remarkable size-dependent intensity loss and several features give evidences for a bond formation between nano-sized MoO3 particles and the silica support. Moreover, the spectroscopic details suggest the formation of (MoO3)n oligomers. © 2007 Springer Science+Business Media, LLC.