OXYGEN STORAGE PROPERTIES OF TERNARY METAL OXIDE SYSTEMS FOR CHEMICAL LOOPING REACTIONS

Abstract

We have studied the reversible uptake and release of oxygen in the layered metal oxide system AB2O4 to understand their suitability as oxygen storage materials. We examined their structures at their most reduced, oxidized, and intermediate phases of AFe2O4 for A= Lu, Yb, Y, and In, and studied their structures with high-resolution synchrotron X-ray diraction. Under simulated chemical looping conditions, we monitored their structures and reactivity towards H2 and O2 utilizing in-situ X-ray diraction, neutron diraction, and thermogravimetric analysis measurements. The nature of the trivalent A cation aects the oxidation kinetics, thermal cycling stability, and oxygen storage capacity (OSC). With the exception of the A = In analogue, these layered oxides underwent various phase transitions above 200 °C that included the creation of a superstructure as oxygen incorporates until a high temperature phase is established above 400 °C. To understand trends in the oxygen incorporation kinetics, we employed bond valence sum analysis of the Fe-O bonding across the series. The more underbonded the Fe cation, the more facile the oxygen insertion. During the cycling experiments all samples exhibited reversible oxygen insertion at 600 °C for this series, and displayed OSC values between 0.2-0.27 O2 mol/mol. The Y analogue displayed the fastest kinetics for oxidation, which may make it the most suitable for oxygen sensing applications. The structure of the oxidized phase was solved from with simulated annealing and Fourier dierence maps. Structural parameters were reported with combine neutron and X-ray Rietveld renement. PDF and XAS were used to conrm the nal structural model. As the nal steps experiments were carried out to explore the chemical looping reactivity of AB2O4 layered oxides, with A= Lu, Yb, Y and B=Mn, Fe. We reported the reactivity with methane of AB2O4 layered oxides for the rst time. The RT pristine structure was regenerated at 600 °C under methane. Mn substituted compounds exhibited faster kinetics and also higher oxygen storage capacities. We conclude that the layered, ternary metal oxide system, AB2O4, is a suitable candidate as an oxygen storage material for the potential application in chemical looping reactions.