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

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.

$\text{(i)}\text{Bi}{}_2\text{O}{}_3\text{+ MoO}{}_3\text{→ Bi}{}_2\text{MoO}{}_6\text{, (ii)}\text{Bi}{}_2\text{O}{}_3\text{+ 2MoO}{}_3\text{→ Bi}{}_2\text{MO}{}_2\text{O}{}_9\text{, (iii)}\text{Bi}{}_2\text{O}{}_3\text{+ 3MoO}{}_3\text{→ Bi}{}_2\text{(MOO}{}_4\text{)}{}_3\text{, (iv)}\text{Bi}{}_2\text{MoO}{}_6\text{+ MoO}{}_3\text{→ Bi}{}_2\text{MO}{}_2\text{O}{}_9\text{, (v)}\text{Bi}{}_2\text{Mo}{}_2\text{O}{}_9\text{+ MoO}{}_3\text{→ Bi}{}_2\text{(MoO}{}_{\mathrm{2)3}}\text{.}$

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 α is the fraction of molybdenum trioxide reacted, the kinetics in all five cases are found to be governed by the equation αⁿ = 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 $\text{α = k}{}_2\text{t}{}^{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}\text{? k}{}_3\text{t}$, 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.