In Situ Raman Investigation on the Structural Transformation of Oxovanadium Hydrogenphosphate Hemihydrate in the Presence of Ammonia

Raman spectra of the transformation of oxovanadium hydrogenphosphate hemihydrate, VOHPO4·1/2H2O, were measured in an in situ Raman cell under an ammonia/oxygen/helium flow in the presence of water vapor at temperatures up to 723 K. During the thermal treatment a decomposition of the parent structure proceeds as indicated by a distinct broadening and disappearing of the parent Raman bands. After holding the sample for 1 h under these conditions and during subsequent cooling, the structural transformation into (NH4)2[(VO)3(P2O7)2] is observed. After allowing this to stand at room temperature under flowing helium for some hours, V2O5 and mainly NH4VO3 particles (up to 100 μm) are formed as new phases. Both are spatial separated from the (NH4)2[(VO)3(P2O7)2] surface as detected by means of the micro-Raman technique. It seems very likely that NH4VO3 is generated from in situ existing V(V) particles and chemisorbed ammonia.