The structural genesis of a complex (MoVW)₅O₁₄ oxide during thermal treatments and its redox behavior at elevated temperatures

The structural genesis of a Mo0.68V0.23W0.09 oxide with Mo₅O₁₄-like structure has been examined. A precursor prepared by spray-drying of mixed aqueous metal salt solutions was calcined in air and subsequently treated in helium at different temperatures. X-ray diffraction, HRTEM, 51V MAS NMR, ESR, UV/Vis DR spectroscopy and oxygen and hydrogen adsorption measurements have been applied to monitor the preparation procedure. It was found that a structure closely related to that of Mo₅O₁₄ already appears at nano-scale level after calcination of the spray-dried precursor in air at 350°C. At this stage, the material comprises of crystalline particles less than 3 nm in size stabilized by an amorphous matrix. Further heating causes nano-structural rearrangements that lead to the formation of the final Mo0.68V0.23W0.09 oxide with phase-pure polycrystalline structure. Molybdenum and tungsten ions are hexavalent and coordinated in an octahedral environment. Furthermore, vanadium is present as V⁴⁺ and V⁵⁺ ions which partially occupy octahedral sites, whereas highly distorted trigonal pyramidal sites could be accommodated in pentagonal bipyramids of the Mo₅O₁₄ structure, however, displaced away from the center. According to the results of H₂ and O₂ adsorption the crystalline ternary oxide does not possess accessible micropores. Oxygen pulses at 450oC and reductive treatment with pure hydrogen at 300oC did not cause noticeable changes of the bulk structure thus indicating a remarkable structural stability of the complex MoVW oxide under redox conditions at elevated temperature