In situ X-ray diffraction analysis of the crystallisation of VOHPO ₄ · 0.5H₂O

Vanadium phosphorus oxide catalysts, used commercially for n-butane oxidation, are normally formed by heating VOHPO4·0.5H2O in a mixture of n-butane and air. The transformation of the precursor into the final catalyst is topotactic so that the crystalline habit is established when the precursor crystallises. In this study, the synthesis of VOHPO4·0.5H2O by reaction of a reduced suspension/solution of V2O5 in alcohol and o-H3PO4 has been studied by in situ X-ray diffraction (XRD) and ex situ X-ray photoelectron spectroscopy (XPS) and focused ion beam microscopy (FIB), including cross-sectioning. XPS, XRD and microscopy evidence is presented for the temporal dissolution of V2O5 and formation in succession of VOPO4·2H2O, VOPO4·H2O and VOHPO4·0.5H2O. Time-resolved in situ X-ray diffraction has identified VOPO4·2H2O at early synthesis times (less than 300 s) from d-spacings of d=7.5 and 3.1 Å. The feature at 7.5 Å shifted to 6.7 Å during the first 20 min of synthesis, associated with the formation of VOPO4·H2O and this phase collapsed as VOHPO4·0.5H2O formed. Thin symmetrical platelets of 10×10μm dimensions were observed when samples were recovered after short synthesis times. The platelets appeared to delaminate, possibly associated with the strain generated when the d-spacing shifted from 7.5 to 6.7 Å and the reducing action of the alcohol solvent. Growth of the VOHPO4·0.5H2O in the familiar rosette morphology occurs from these delaminated edges. Evidence is presented that VOHPO4·0.5H2O particles continue to grow and become more dense as synthesis time is increased.