X-ray diffraction studies of bismuth tungstate catalysts

Bismuth tungstate catalysts with Bi/W ratios 2/3, 1/1, 2/1, and 6/1 have been investigated after various heat treatments by room temperature X-ray analysis. The activity and selectivity of Bi₂WO₆ catalyst are suggested to be connected with the presence of corner sharing W—O octhedra in the bulk of the solid that give rise to tetragonal pyramidal W—O sites at the surface. The very low activity of Bi₂(WO₄)₃ is attributed to edge-shared octahedral coordination of W while the same of Bi₆WO₁₂ is due to tetrahedral coordination of W. The inactivity of Bi₂W₂O₉ may be associated with its typically low oxidation power. The promoting influence of Bi₂O₃ is to enforce a structural situation on W—O so that W may be present in a tetragonal pyramidal 5-configuration.