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.