Chemical looping reforming partially oxidizes methane into syngas through cyclic redox reactions of an active lattice-oxygen (O2–) containing redox catalyst. The avoidance of direct contact between methane and steam and/or gaseous oxygen has the potential to eliminate the energy consumption for generating these oxidants, thereby increasing methane conversion efficiency. This article investigates redox catalysts comprised of iron oxide core covered with lanthanum strontium ferrite (LSF) shell. The iron oxide core serves as the primary source of lattice-oxygen, whereas the LSF shell provides an active surface and facilitates O2– and electron conductions. These core–shell materials have the promise to provide higher selectivity for methane conversion with lower solid circulation rates than traditional redox catalysts. Methane oxidation by this catalyst exhibits four distinct regions, i.e. deep oxidation; competing deep and selective oxidation; selective oxidation with autoactivation; and methane decomposition. Further investigations indicate that the evolution of “loose” lattice oxygen from the bulk contributes to deep oxidation, whereas reduced surface iron species are responsible for selective methane oxidation.

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.

The chemical looping processes utilize lattice oxygen in oxygen carriers to convert carbonaceous fuels in a cyclic redox mode while capturing CO 2. Typical oxygen carriers are composed of a primary oxide for active lattice oxygen storage and a ceramic support for enhanced redox stability and activity. Among the various primary oxides reported to date, iron oxide represents a promising option due to its low cost and natural abundance. The current work investigates the effect of support on the cyclic redox performance of iron oxides as well as the underlying mechanisms. Three ceramic supports with varying physical and chemical properties, i.e. perovskite-structured Ca 0.8 Sr 0.2 Ti 0.8 Ni 0.2 O 3 , fluorite-structured CeO 2 , and spinel-structured MgAl 2 O 4 , are investigated. The results indicate that the redox properties of the oxygen carrier, e.g. activity and long-term stability, are significantly affected by support and iron oxide interactions. The perovskite supported oxygen carrier exhibits high activity and stability compared to oxygen carriers with ceria support, which deactivate by as much as 75% within 10 redox cycles. The high stability of perovskite supported oxygen carrier is attributable to its high mixed ionic-electronic conductivity. Deactivation of ceria supported samples results from solidstate migration of iron cations and subsequent enrichment on the oxygen carrier surface. This leads to agglomeration and lowered lattice oxygen accessibility. Activity of MgAl 2 O 4 supported oxygen carrier is found to increase during redox cycles in methane. The activity increase is a consequence of surface area increase caused by filamentous carbon formation and oxygen carrier fragmentation. While higher redox activity is desired for chemical looping processes, physical degradation of oxygen carriers can be detrimental.

The chemical looping reforming (CLR) process, which utilizes a transition metal oxide based redox catalyst to partially oxidize methane to syngas, represents a potentially efficient approach for methane valorization. The CLR process inherently avoids costly cryogenic air separation by replacing gaseous oxygen with regenerable ionic oxygen (O(2-)) from the catalyst lattice. Our recent studies show that an Fe2O3@La0.8Sr0.2FeO3-δ core-shell redox catalyst is effective for CLR, as it combines the selectivity of an LSF shell with the oxygen capacity of an iron oxide core. The reaction between methane and the catalyst is also found to be highly dynamic, resulting from changes in lattice oxygen availability and surface properties. In this study, a transient pulse injection approach is used to investigate the mechanisms of methane partial oxidation over the Fe2O3@LSF redox catalyst. As confirmed by isotope exchange, the catalyst undergoes transitions between reaction "regions" with markedly different mechanisms. While oxygen evolution maintains a modified Mars-van Krevelen mechanism throughout the reaction with O(2-) conduction being the rate limiting step, the mechanism of methane conversion changes from an Eley-Rideal type in the first reaction region to a Langmuir-Hinshelwood-like mechanism in the third region. Availability of surface oxygen controls the reduction scheme of the catalyst and the underlying reaction mechanism.

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Efficient and environmentally friendly conversion of methane into syngas is a topic of practical relevance for the production of hydrogen, chemicals, and synthetic fuels. At present, methane‐derived syngas is produced primarily through the steam methane reforming processes. The efficiencies of such processes are limited owing to the endothermic steam methane reforming reaction and the high steam to methane ratio required by the reforming catalysts. Chemical looping reforming represents an alternative approach for methane conversion. In the chemical looping reforming scheme, a solid oxygen carrier or “redox catalyst” is used to partially oxidize methane to syngas. The reduced redox catalyst is then regenerated with air. The cyclic redox operation reduces the steam usage while simplifying the heat integration scheme. Herein, a new Fe2O3@LaxSr1−xFeO3 (LSF) core–shell redox catalyst is synthesized and investigated. Compared with several other commonly investigated iron‐based redox catalysts, the newly developed core–shell redox catalyst is significantly more active and selective for syngas production from methane. It is also more resistant toward carbon formation and maintains high activity over cyclic redox operations.

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Rh promoted mixed-oxides show a syngas productivity of 7.9 mmol g−1 at 600 °C in the absence of gaseous oxidants.