PHYSICAL-CHEMICAL STUDY OF VANADIUM PHOSPHATE PHASES DOPED WITH COBALT AND IRON

The present study concerns the localization of iron and cobalt used as dopants at low percentage (atomic ratio Fe or Co/V = 1, 2, 5%) in VPO-reference phases known to oxidize n-butane to maleic anhydride, The use of techniques, like X-rays diffraction, Mossbauer spectroscopy of Fe-57, Raman spectroscopy, and X-Ray analysis associated with TEM, show that the two doping elements behave differently towards the VPO system. While Fe dopes most of the VOHPO4, 0.5 H2O hemihydrate and delta-VOPO4, it segregates partially in the case of the (VO)(2)P2O7 phase and almost completely for gamma-VOPO4. The two different environments of Fe3+, observed by Mossbauer spectroscopy should respectively correspond to a substitution of V4+ and to a segregation as iron pyrophosphate. Co dopes partially the structure of the hemihydrate and delta-VOPO4. It segregates partially in the case of (VO)(2)P2O7 and almost completely for gamma-VOPO4.