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 SrFe0.7Al0.3O3−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.

Graphical AbstractConversion-electron Mössbauer spectroscopy analysis of dense ceramic membranes made of 57Fe-enriched SrFe0.7Al0.3O3−δ perovskite, shows no reductive decomposition in thin interfacial layers after testing under air/CH4 gradient, enabling stable operation of the ferrite-based ceramic reactors for partial oxidation of methane. Figure optionsDownload as PowerPoint slideHighlights
► Conversion-electron Mössbauer spectroscopy is used for mixed-conducting membranes.
► No decomposition is detected in the membrane surface layers under air/CH4 gradient.
► Due to kinetic stabilization, Fe3+ states prevail at the surface exposed to methane.
► Transmission Mössbauer spectra show perovskite decomposition on equlibration in CH4.
► Ferrite-based ceramic reactors can stably operate under air/CH4 gradient.