51V and 31P NMR studies of VOx/TiO2 catalysts modified by phosphorous

Phosphorous-doped VOx/TiO2 catalysts prepared by the spray-drying method and treated under catalytic reaction, as well as individual phases (alpha (I)-, alpha (II)-, beta-, beta-) of VOPO4 were studied using modern high-resolution solid-state NMR techniques, including fast magic angle spinning (MAS), combined with the analysis of rotational satellites intensities by the satellite transition spectroscopy (SATRAS) method; 2D triple-quantum, quintuple-quantum MAS NMR, and spin mapping echo experiments. The simultaneous determination of chemical shielding anisotropy and quadrupole tensor parameters, as well as their distributions, permits to draw a conclusion on the local environment of vanadium sites in the catalysts. The formation of a triple V-P-Ti compound in phosphorous-doped VOx/TiO2 catalysts has been revealed. Only one type of slightly distorted tetrahedral vanadium atoms bound via oxygen to phosphorous was found in this compound. The very large distribution of the quadropole constant points to the irregular structure of this compound. (C) 2000 Elsevier Science B.V. All rights reserved.