An investigation of the structural and electronic origins of enhanced chemical looping air separation performance of B-site substituted SrFe_1−xCo_xO_3−δ perovskites

Chemical looping air separation (CLAS) is a promising process intensification technology for extracting oxygen from air for oxygen enrichment in process streams. Co-doped strontium ferrites (SrFe_1−xCo_xO_3−δ) have been found to have outstanding activities for CLAS processes. In this study, we explore the underlying factors driving the enhancement in oxygen uptake and release performance of perovskite structured SrFe_1−xCo_xO_3−δ oxygen carriers for CLAS. Phase-pure perovskites, with B site substituted by up to 75 mol% Co, were prepared by a sol-gel method and systematically investigated through a wide range of well controlled experimental and computational approaches. While all SrFe_1−xCo_xO_3−δ oxygen carriers showed excellent cyclic stability and structural reversibility over CLAS cycles, increased B site occupancy by Co resulted in monotonic decrease in onset temperature for oxygen release and increase in oxygen carrying capacity. These experimental trends can be fundamentally explained by an increase in the structural tolerance factor, an elevation in transition metal d-band, as well as an increased degree of hybridization between the metal d-band and the O p band. Therefore, these ab initio structural and electronic descriptors are useful design rationales for the hypothesis-driven synthesis of high-performing oxygen carriers for CLAS.