TG/DTA, XRD and NH3-TPD characterization of layered VOPO4•2H2O and its Fe3+-substituted compound

Iron(III)-substituted vanadyl phosphate, [Fe(H2O)](0.20)VO0.80PO4. 2.25H(2)O (FeVOP), has been prepared and characterized by XRD and TG/DTA analyses. The new compound is isomorphous with layered tetragonal VOPO4. 2H(2)O (VOP), but it possesses a lower interlayer distance. Information on the reactivity and surface acidity of both VOP and FeVOP has been obtained by NH3-TPD experiments. The hydrated materials adsorb high amounts of NH3 (up to 2 mmol g(-1)). Different ammonia-containing phases are formed, characterized by lower interlayer distances in comparison with the NH3-free parent compounds. NH3 is intercalated between the layers without displacement of water. The materials dehydrated by heat treatment at 450 degrees C retain the layered structure but adsorb NH3 only on the external surface. A wide variety of acid sites, from weak to strong, was observed. A mechanism is proposed for the NH3- acid sites interaction. SEM micrographs of VOP and FeVOP are shown.