EPR AND NMR STUDIES OF BI4V2O11 AND BICUVOX.X CATALYSTS IN THE OXIDIZED AND REDUCED FORMS

The EPR and NMR methods were used to study Bi4V2O11 and BiCuVOX.x (X is the oxygen index, and x is the percentage of vanadium atoms replaced by copper) catalysts in the oxidized and reduced forms. Oxidized Bi4V2O11 Was found to contain three types of vanadium(V) centers with distorted tetrahedral coordination and an insignificant amount of V(IV) centers in octahedral coordination with low crystal field symmetry. The amount of V4+ ions increased during reduction in a flow of hydrogen at 330 degrees C. Partial replacement of vanadium ions in Bi4V2O11 by copper resulted in the disappearance of one of three types of vanadium(V), probably, because copper substitutes precisely these ions. Two other types of vanadium(V) exhibited upheld NMR signal shifts by 40 ppm as the amount of copper increased from x = 7.5 to x = 12.5. This correlates with the alpha --> gamma phase transition. The reduction of Cu2+ in BiCuVOX.x with hydrogen to Cu+ and/or Cu-0 proceeded via the formation of Cu2+-V4+ dimers in the intermediate state and partial reduction of V(V) to V(IV).