Understanding the role of acidity on the surface exchange reaction in mixed conductors: what is the effect of surface hydration?

Abstract

An increasingly robust body of evidence attests that the kinetics of the oxygen exchange reaction at the surface of mixed ionic–electronic conducting oxides can be modified by infiltrating binary oxides. Furthermore, a clear relationship has been found between the reaction rate and the acidity of the surface binary oxide. Nevertheless, the underlying mechanism is still poorly understood. In this study we investigate the effect of acidic and basic infiltrated species (SiO₂ and CaO) on SrTi_0.65Fe_0.35O_3−δ (STF), a perovskite-structured, technologically relevant MIEC. From these experiments and an analysis of literature data, we demonstrate that a model based on electron-transfer as the rate-determining step and a modified surface electron concentration is quantitatively inconsistent with the data. Consequently, we propose instead that water species, present at trace levels in the conditions of the experiments, play a decisive role in the surface exchange kinetics and their modification through acidic or basic infiltrated species.