Iron Oxide with Facilitated O2– Transport for Facile Fuel Oxidation and CO2 Capture in a Chemical Looping Scheme

Abstract

The chemical looping strategy offers a potentially viable option for efficient carbonaceous fuel conversion with a reduced carbon footprint. In the chemical looping process, an oxygen carrier is reduced and oxidized in a cyclic manner to convert a carbonaceous fuel into separate streams of concentrated carbon dioxide and carbon-free products such as electricity and/or hydrogen. The reactivity and chemical and physical stability of the oxygen carrier are of pivotal importance to chemical looping processes. A typical oxygen carrier is composed of a multi-valence transition metal oxide supported on an “inert” support. Although the support does not get reduced or oxidized at any significant extent, numerous studies have indicated that certain supports such as TiO2 and Al2O3 can improve oxygen carrier stability and/or reactivity. This study reports the use of mixed ionic–electronic conductive support in iron-based oxygen carriers. By incorporating a perovskite-based mixed conductive support such as lanthanum strontium ferrite (LSF), the reactivity of the oxygen carrier is enhanced by 5–70 times when compared to oxygen carriers with conventional TiO2-, Al2O3-, or yttria-stabilized zirconia (YSZ) support. The mixed conductivity enhanced oxygen carrier also shows good stability and coke resistance. Characterization studies indicate that the enhanced oxygen carrier is composed of intermixed nanoscale (<100 nm) crystallites of iron oxide and support. The mixed conductive support enables facile O2– transport to and from the iron oxide nanocrystallites to participate in the surface redox reactions. The support also allows counter-current or concurrent diffusion of electrons or holes to maintain charge balance within the oxygen carrier. With iron oxide as the nanoscale oxygen source and mixed conductive support as the oxygen/electron conductor, the mixed conductivity enhanced oxygen carrier particle can be considered as an ensemble of nanoscale mixed conductive membrane reactors that possess excellent redox activity.