XANES AND V-51 NMR-STUDY OF VANADIUM OXYGEN COMPOUNDS

XANES at the vanadium K-edge together with V-51 solid state NMR spectra were recorded for several reference vanadium (V)-oxygen compounds (oxide, oxoalkoxides and vanadates) of well known crystal structure. The vanadium coordination exhibits two different geometries; a fivefold square pyramidal coordination as in V2O5, VO(OMe)3 or VOPO4, 2H2O and a more or less distorted fourfold coordination in Pb5(VO4)3Cl, NH4VO3, CrVO4, VO(OAm(t))3 and Na3 VO4. The former compounds are characterized by a V-51 solid state NMR central resonance ranging between 200 and 300 ppm and XANES pre-edges around 4.6-5.5 eV. Fourfold coordination leads to more intense XANES pre-edges located at lower energies (3.8-4.2 eV). The first maximum of the absorption edge is shifted towards higher energies and the V-51 central resonance lies between 500 and 700 ppm. As an example, the coordination of vanadium (V) in a vanadium oxide gel has been determined. An axially distorted square pyramid built from one short vanadyl bond at about 1.55-1.58 angstrom and four almost planar vanadium-oxygen bonds at about 1.90 angstrom is proposed.