In situ laser Raman spectroscopy studies of the transformation of VOHPO4·0.5H2O and (VO)2P2O7

Abstract

A detailed study of three samples of vanadyl hemihydrate, VOHPO₄·0.5H₂O, and vanadyl pyrophosphate, (VO)₂P₂O₇, prepared using different methods is described and discussed. VOHPO₄·0.5H₂O, prepared using (a) the VPA method (V₂O₅ and H₃PO₄ refluxed in aqueous HCl), (b) the VPO method (V₂O₅ and H₃PO₄ refluxed in isobutanol) and (c) the VPD method (VOPO₄·2H₂O refluxed in isobutanol), is readily converted to (VO)₂P₂O₇ by heating in an inert gas at 750–775°C. The three samples of (VO)₂P₂O₇ have significant differences in the P–O–P stretch region of the Raman spectra and also exhibit marked differences in their reactivity with oxygen, although the powder X-ray diffraction patterns for the three samples are identical. The VPA-derived (VO)₂P₂O₇ is readily oxidised at 400°C, whereas the VPO-derived (VO)₂P₂O₇ is particularly unreactive. The oxidation/reduction transformations are studied using in situ laser Raman spectroscopy and temperature programmed oxidation and it is shown that the oxidation/reduction process can be cycled many times and the morphology of the crystallite remains unchanged.