Abstract
Time-resolved in situ X-ray diffraction was used to study the formation of the vanadium phosphorous oxide (VPO) catalytic precursor phase VOHPO4 dispersive X-ray diffraction has identified a new phase at early reaction times (less than 300 s) at d-spacings of 7.5 and 3.1 A. At short reaction times, thin symmetrical platelets of 10 by 10 μm dimensions were observed. Scanning Electron Microscopy combined with energy-dispersive X-ray analysis (EDXA) indicated that these platelets contained vanadium and phosphorus. The features at 7.5 A, which shift to 6.7 A during the first 20 min of synthesis, are associated with VOPO4 ⋅ 2H2O and VOPO4 ⋅ H2O and disappear as VOHPO4 ⋅ 0.5H2O forms. The platelets appeared to delaminate, possibly associated with the strain generated when a d-spacing shifts from 7.5 A to 6.7 A, from which epitaxial growth of the VOHPO4 ⋅ 0.5H2O into the familiar rosette morphology occurs.
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