Methane oxidation over perovskite-related ferrites: Effects of oxygen nonstoichiometry

The oxidation of CH4 pulses supplied in helium flow over perovskite-related La0.3Sr0.7Fe0.8M0.2O3−δ (M=Ga, Al) and SrFe0.7Al0.3O3−δ leads to significant yields of CO and H2 after achieving a critical level of oxygen deficiency in the ferrite-based mixed conductors. This effect, reproducible under steady-state conditions in the membrane reactors for methane conversion, may be of interest for the development of monolithic ceramic reactors where the dense membrane and porous catalyst at the permeate-side surface are made of similar compositions. The Mössbauer spectroscopy and coulometric titration studies show that the presence of metallic Fe under typical operation conditions can be neglected, whilst most oxygen vacancies in the ferrite lattices are ordered. Increasing selectivity towards the partial oxidation of methane is observed in the vicinity of the state where the iron cations are predominantly trivalent and massive ordering processes in the oxygen sublattice start. The catalytic activity of ferrite-based materials may hence result from the lattice instability characteristic of morphotropic phase transformations. The correlations between catalytic behavior and oxygen ionic transport are briefly discussed.