The structural genesis of a complex (MoVW)5O14(MoVW)_5O_{14} 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

Inorganic Photocatalytic Enhancement: Activated RhB Photodegradation by Surface Modification of SnO2 Nanocrystals with V2O5-like species

SnO(2) nanocrystals were prepared by precipitation in dodecylamine at 100 °C, then they were reacted with vanadium chloromethoxide in oleic acid at 250 °C. The resulting materials were heat-treated at various temperatures up to 650 °C for thermal stabilization, chemical purification and for studying the overall structural transformations. From the crossed use of various characterization techniques, it emerged that the as-prepared materials were constituted by cassiterite SnO(2) nanocrystals with a surface modified by isolated V(IV) oxide species. After heat-treatment at 400 °C, the SnO(2) nanocrystals were wrapped by layers composed of vanadium oxide (IV-V mixed oxidation state) and carbon residuals. After heating at 500 °C, only SnO(2) cassiterite nanocrystals were obtained, with a mean size of 2.8 nm and wrapped by only V(2)O(5)-like species. The samples heat-treated at 500 °C were tested as RhB photodegradation catalysts. At 10(−7) M concentration, all RhB was degraded within 1 h of reaction, at a much faster rate than all pure SnO(2) materials reported until now