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. MoO3 nanoparticles are successfully introduced into the pores of mesoporous SBA-15. Up to about 13 wt% Mo the material is amorphous and even for higher loadings a large amount of MoO3 is still not crystalline. Nitrogen physisorption and transmission electron microscopy evidences that the mesoporosity of the material is retained. At low Mo loading, the metastable hexagonal modification of MoO3 appears to be stabilized by the interaction with the SBA-15 support material (XAFS).