Kinetics and mechanism of solid-state reaction between bismuth(III) oxide and molybdenum(VI) oxide

Abstract

The mechanism of the following solid-state reactions between bismuth(III) oxide and molybdenum(VI) oxide was investigated within the temperature range 400-650°C.(i) Bi₂O₃ + MoO₃→Bi₂MoO₆, (ii) Bi₂O₃ + 2MoO₃→ Bi₂MO₂O₉, (iii) Bi₂O₃ + 3MoO₃→Bi₂(MOO₄)₃, (iv) Bi₂MoO₆ + MoO₃→ Bi₂MO₂O₉, (v) Bi₂Mo₂O₉ + MoO₃ → Bi₂(MoO₂)₃.Two types of experiments, capillary and particle size, were performed to ascertain whether MoO₃ diffuses into Bi₂O₃ or vice versa. These show that molybdenum trioxide diffuses into bismuth oxide grains. If a is the fraction of molybdenum trioxide reacted, the kinetics in all five cases are found to be governed by the equation aⁿ = kt throughout the temperature range, where n and k are constants at a given temperature and t is the time. Both n and k are temperature dependent. The characteristic feature of these reactions is that they proceed to completion. Results are also fitted by the relation α= k₂t^½- k₃t, where k₂ and k₃ are constants, which shows that the reactions occur by bulk diffusion through grain boundary contacts. The number of grain boundary contact points decreases with time in the course of reaction.