Activation energies to characterize ease of removal of various kinds of oxygen from bismuth molybdate

Abstract

The technique of minimum energy paths is used on oxygen ions of different types leaving a unit cell on the surface of the lattice of the 2:1 bismuth molybdate (Bi 2MoO 6). Several kinds of behavior are obtained. Oxygen ions originally between the molybdenum and the bismuth layers of the unit cell are first considered. The activation energy computed for the transition of such ions from inside the lattice to outside, near the interface, is approximately 2 eV. This implies that it is possible for some of this type of ion to be removed from the lattice. The corresponding activation energy for an ion in the molybdenum layer is a little larger. For an O 2− ion in the bismuth layer, the activation energy is found to be enormously high, apparently implying that such ions could never directly leave the lattice. However, a great deal of information indicates that it is just such an ion that takes part in a chemical reaction with the hydrocarbon. While the presence of the hydrocarbon will perhaps facilitate the removal of this O 2− ion by factors not considered in the present analysis, the requirement still seems overly high. It is suggested that the presence of anion vacancies reduces this activation energy. The role of prereduction in forming a reproducible catalyst is hypothesized in this manner.