SOME ASPECTS OF BETA-V9MO6O40 REDUCTION - TPR, XRD, SEM, IR AND EPR SPECTROSCOPIC STUDIES

The beta-V9Mo6O40 phase, containing only 1/9 of the V atoms as V4+ ion s, has been prepared by the solid-state reaction of suitable amounts o f MoO3 and V2O5 oxides heated to 923 K. The temperature-programmed red uction (TPR) analysis of the beta-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 red uction peak, occurring at ca. 900 K, the beta-V9Mo6O40 phase disgregat es by forming the MoO2 and V2O3 oxides. The second reduction peak appe ars at higher temperature and corresponds to the reduction of MoO2 to molybdenum metal. Both the as-prepared and reduced samples have been s tudied by X ray powder diffraction (XRPD), IR and EPR spectroscopies a s well as by scanning electron microscopy (SEM). The EPR analysis conf irmed the presence of V4+ ions in the crystal lattice of the beta-V9Mo 6O40 phase which is characterized by a metal-oxygen bonding different from those found for the pure MoO3 and V2O5 oxides, as evidenced by th e IR spectra. Prismatic crystals were observed by SEM for the as-prepa red sample. The original morphology was essentially preserved upon H-2 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 st ructure.