Average andLocal Structure of La_1–xSr_xFe_1–yMn_yO_3−δ Chemical Looping OxygenCarrier Materials

Abstract

Nonstoichiometric mixed-metal oxides in the La_1–xSr_xFe_1–yMn_yO_3–δ family are promising oxygen carrier materials for chemical looping processes, including clean hydrogen production from the water-gas shift reaction. The crystal structure variation of these materials during redox reactions is key to the performance of a chemical looping system. Pair distribution function analysis of neutron total scattering data has provided new insight into the local structure of these materials before and after reduction to their working states. Comparison with experimental data for structurally related vacancy-ordered SrFeO_3–δ compounds (Sr₈Fe₈O₂₃, Sr₄Fe₄O₁₁, and Sr₂Fe₂O₅) allows direct qualitative insight into local B-site coordination environments. A big-box modeling approach incorporating magnetic contributions to the Bragg data on supercells with A- and B-site disorder and mixed B-site coordination gives quantitative information on local structural distortions and coordination polyhedra. For the Mn-doped materials, this modeling shows that Mn has a higher oxidation state than Fe in oxidized samples. Magnetic structures of all ordered compounds have been determined from neutron powder diffraction data, and variable-temperature studies of La_0.6Sr_0.4FeO₃, La_0.6Sr_0.4FeO_2.8, La_0.6Sr_0.4Fe_0.67Mn_0.33O₃, and La_0.6Sr_0.4Fe_0.67Mn_0.33O_2.8 have been used to determine magnetic ordering temperatures.