Surface analysis of mixed-conducting ferrite membranes by the conversion-electron Mössbauer spectroscopy

Conversion-electron Mössbauer spectroscopy analysis of iron surface states in the dense ceramic membranes made of ⁵⁷Fe-enriched SrFe_0.7Al_0.3O_3−δ perovskite, shows no traces of reductive decomposition or carbide formation in the interfacial layers after operation under air/CH₄ gradient at 1173 K, within the limits of experimental uncertainty. The predominant trivalent state of iron cations at the membrane permeate-side surface exposed to flowing dry methane provides evidence of the kinetic stabilization mechanism, which is only possible due to slow oxygen-exchange kinetics and enables long-term operation of the ferrite-based ceramic reactors for natural gas conversion. At the membrane feed-side surface exposed to air, the fractions of Fe⁴⁺ and Fe³⁺ are close to those in the powder equilibrated at atmospheric oxygen pressure, suggesting that the exchange limitations to oxygen transport are essentially localized at the partially reduced surface.