Rates of interface reactions

An early attempt to apply an Arrhenius-type equation to a decomposition reaction was by Polyani and Wigner [512] using the expression 

Shannon assumes that atoms or molecules at the reaction interface have 

Since data obtained for dehydration reactions of alums give Q /Q ratios of around unity and also of the order 103, it seems probable that both models may be applicable in specific systems. 

Cordes [515] has provided a more general treatment of reaction rates using similar concepts to those discussed by Shannon [514] (the latter model appears as one particular case). Two types of rate process are distinguished bulk decomposition and surface reaction and three classes of bulk decomposition are identified, viz. 

Cordes discusses the magnitudes of pre-exponential terms with reference to the partition function for the activated complex in which the following cases are recognized. 

---

A kinetic model involving the rate of interface reaction, pore-diffusion and gas-film diffusion has been formulated for the core-and-shell reduction [115]. The enthalpy of adsorption of water on the reaction interface is 163.2 45.2 kJ/mole in this model [115]. The core-and-shell model is invalid in the final stages of reduction for samples containing high (27%) concentrations of wustite [26, 27, 80] in the presence of small amounts of water [97,131]. 

---