Characterization of tellurium in MoVTeNbO catalysts for propane oxidation or ammoxidation

The oxidation state and local geometry of tellurium in MoVTeNbO catalysts used in the ammoxidation or oxidation of propane were characterized by X-ray absorption, Mossbauer, and X-ray photoelectron (XPS) spectroscopies. The results obtained by Mossbauer, and XPS spectroscopies showed that the catalysts contained Te-(IV) in the bulk and mainly Te-(VI) at the surface. X-ray absorption fine structure (EXAFS) measurements allowed to determine that the tellurite entities corresponded to TeO4E trigonal bipyramid in the hexagonal phase and to TeO3E somewhat distorted trigonal pyramid in the orthorombic one. The completely determined environment appeared to correspond in both phases to crystallographic sites in hexagonal channels. These results allowed to determine the stoichiometries of the two phases which are TeM3O10 for the hexagonal phase and Te2M20O57 for the orthoroinbic phase (M = Mo, V Nb). Te-125 Mossbauer isomer shift and pre-peak surface of the X-ray absorption spectra of the Te L-III-edge have been correlated and contributions of the Te(5s) and Te(2P(3/2)) to the structure have been analyzed. (C) 2002 Elsevier Science B.V. All rights reserved.