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.