Some aspects of β-V9Mo6O40 reduction: TPR, XRD, SEM, IR and EPR spectroscopic studies

Abstract

The β-V9Mo6O40 phase, containing only 1/9 of the V atoms as V4+ ions, has been prepared by the solid-state reaction of suitable amounts of MoO3 and V2O5 oxides heated to 923 K. The temperature-programmed reduction (TPR) analysis of the β-V9Mo6O40 phase has been carried out up to 1273 K. The TPR profiles were always characterised by two peaks regardless of the experimental conditions. At the end of the first reduction peak, occurring at ca. 900 K, the β-V9Mo6O40 phase disgregates by forming the MoO2 and V2O3 oxides. The second reduction peak appears at higher temperature and corresponds to the reduction of MoO2 to molybdenum metal. Both the as-prepared and reduced samples have been studied by X-ray powder diffraction (XRPD), IR and EPR spectroscopies as well as by scanning electron microscopy (SEM). The EPR analysis confirmed the presence of V4+ ions in the crystal lattice of the β-V9Mo6O40 phase which is characterized by a metal–oxygen bonding different from those found for the pure MoO3 and V2O5 oxides, as evidenced by the IR spectra. Prismatic crystals were observed by SEM for the as-prepared sample. The original morphology was essentially preserved upon H2 reduction, but a widespread microroughness appeared at the surface of the prismatic crystals. At the beginning of the reduction process an increase of the V4+ species in the lattice was detected by EPR, giving rise to a gradual slight structural distortion. However, an oxidising treatment in air of the mildly reduced sample restores the original structure.