Predicting metastable oxide-to-hydroxide phase transformations by bulk and interface thermodynamics: Application to the phase stability of aluminium oxides and hydroxides in water

Abstract

Fundamental understanding of the phase stability of oxides in aqueous environments is of eminent importance for a broad range of disciplines, such as corrosion, tribology, catalysis, medical implants, biosensing and environmental sciences. Most oxide-to-hydroxide phase transformation sequences proceed by consecutive formation of different metastable hydroxide phases towards the most stable bulk hydroxide phase, in accord with Ostwald's Rule of Stages and thus in contradiction with bulk thermodynamics. In this work, a novel unified thermodynamic model is presented for predicting such metastable oxide-to-hydroxide phase transformation sequences by accounting for the energy barrier(s) associated with the creation of new interface(s) between the competing parent and product phase(s). To this end, semi-empirical expressions for the estimation of the interface energies between metals, oxides and/or hydroxides were derived based on the macroscopic atom approach. Application of the model to the phase stability of aluminium in water (at neutral pH) for 298 ≤ T ≤ 500 K predicts a solid-state phase transformation sequence from Al → am-Al₂O₃ → (pseudo)boehmite, in accord with experimental observations. Subsequent competing formation of the trihydroxide phases bayerite and gibbsite by a dissolution-precipitation on the (pseudo)boehmite surface is equally favoured and critically depends on the hydrolysis of dissolved Al³⁺ species in solution, as governed by e.g. pH, temperature and the presence of foreign ions in solution.