The structural genesis of a complex (MoVW) 5O 14 oxide during thermal treatments and its redox behavior at elevated temperatures

Abstract

The structural genesis of a Mo 0.68V 0.23W 0.09 oxide with Mo 5O 14-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 magic angle spinning (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 Mo 5O 14 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 nanostructural rearrangements that lead to the formation of the final Mo 0.68V 0.23W 0.09 oxide with phase-pure polycrystalline structure. Molybdenum and tungsten ions are hexavalent and coordinated in an octahedral environment. Furthermore, vanadium is present as V 4+ and V 5+ ions occupying octahedral and highly distorted trigonal pyramidal sites, respectively. According to the results of H 2 and O 2 adsorption the crystalline ternary oxide does not possess accessible micropores. Oxygen pulses at 450 °C and reductive treatment with pure hydrogen at 300 °C 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.