Methane oxidation over mixed-conducting SrFe(Al)O3−δ–SrAl2O4 composite

Abstract

The steady-state CH₄ conversion by oxygen permeating through mixed-conducting (SrFe)_0.7(SrAl₂)_0.3O_z composite membranes, comprising strontium-deficient SrFe(Al)O_3−δ perovskite and monoclinic SrAl₂O₄-based phases, occurs via different mechanisms in comparison to the dry methane interaction with the lattice oxygen. The catalytic behavior of powdered (SrFe)_0.7(SrAl₂)_0.3O_z, studied by temperature-programmed reduction in dry CH₄ at 523–1073 K, is governed by the level of oxygen nonstoichiometry in the crystal lattice of the perovskite component and is qualitatively similar to that of other perovskite-related ferrites, such as Sr_0.7La_0.3Fe_0.8Al_0.2O_3−δ. While extensive oxygen release from the ferrite lattice at 700–900 K leads to predominant total oxidation of methane, significant selectivity to synthesis gas formation, with H₂/CO ratios close to 2, is observed above 1000 K, when a critical value of oxygen deficiency is achieved. The steady-state oxidation over dense membranes at 1123–1223 K results, however, in prevailing total combustion, particularly due to excessive oxygen chemical potential at the membrane surface. In combination with surface-limited oxygen permeability, mass transport limitations in a porous layer at the membrane permeate side prevent reduction and enable stable operation of (SrFe)_0.7(SrAl₂)_0.3O_z membranes under air/methane gradient. Taking into account the catalytic activity of SrFeO_3−δ-based phases for the partial oxidation of methane to synthesis gas and the important role of mass transport-related effects, one promising approach for membrane development is the fabrication of thick layer of porous ferrite-based catalyst at the surface of dense (SrFe)_0.7(SrAl₂)_0.3O_z composite.