INVESTIGATION OF V2O5 NB2O5 CATALYSTS BY V-51 SOLID-STATE NMR/

Vanadium-niobium oxide catalysts containing variable amounts of vanadi um and prepared by coprecipitation were investigated using V-51 solid- state NMR. It was found that, in samples containing less than about 8 mol % V2O5, vanadium occurs in three different oxygen environments: in isolated tetrahedra, in corner-sharing tetrahedra (pyrovanadate), and in distorted octahedra. The number of octahedral sites increased as t he amount of vanadium in the bulk of the catalyst was increased. The c ompound beta-(Nb,V)(2)O-5 was found to occur in samples containing mor e than 7 mol % V2O5. In this compound, vanadium occurs not only in iso lated tetrahedral sites but also in corner-sharing tetrahedral sites. For a catalyst having the niobia in the crystalline T form, it was fou nd that an increase in the bulk concentration of vanadium caused the n umber of neighboring sites occupied by vanadium to increase and the va nadium partially to replace niobium in the T-niobia structure.