OXOVANADIUM(IV) HYDROGEN PHOSPHATE HYDRATES - A TIME-RESOLVED NEUTRON POWDER DIFFRACTION STUDY

Oxovanadium(IV) pyrophosphate, (VO)2P2O7, is the catalytically active phase in the V-P-O system. It can be obtained by pyrolysis of any of the different hydrates of oxovanadium(IV) monohydrogen phosphate, VO(HPO4).yH2O (y = 0.5, 1, 2 (alpha), 2 (beta), 3, 4). The present neutron thermodiffractometric study shows unambiguously that, irrespective of the starting hydrate, the hemihydrate, VO(HPO4).0.5H2O, always crystallizes before (VO)2P2O7. The VO(HPO4).5H2O to (VO)2P2O7 transformation is not a simple and straightforward process but involves an amorphous intermediate. The conversion of the different hydrates into VO(HPO4).0.5H2O requires more or less severe structural rearrangements, which, in turn, must affect the morphologic characteristics of the final catalyst.