Evaluation of iron- and manganese-based mono- and mixed-metallic oxygen carriers for chemical looping combustion

• FexMn1−x (x = 0…1) bimetallic carriers were investigated for chemical looping.
• Fe-rich carriers show an unusual, reversible de-alloying/re-alloying behavior.
• Mn-rich carriers show promise due to synergistic enhancement of their redox activity.
• Use of ceria as support results in stable operation.

Chemical looping combustion (CLC) is an emerging technology for clean combustion of fossil fuels with inherent CO2 capture. In the present work, we investigate the use of iron and manganese based mixed oxides (MnxFe1−x–CeO2) supported on CeO2 as oxygen carriers in CLC. The low cost and low toxicity of iron and manganese make them interesting candidates for CLC, but both mono-metallic carriers suffer from issues of low reactivity, and manganese is additionally prone to form undesired spinel structures with typical oxide supports. Mono- and bimetallic oxygen carriers were synthesized across the entire spectrum of compositions from pure Mn to pure Fe (with x = 0, 0.1, 0.33, 0.5, 0.8, 0.9, 1), characterized, and tested in thermogravimetric and fixed-bed reactor studies using H2 and CH4 as fuels. We find that the use of ceria as support results in stable operation for all compositions of the metal phase, including pure Mn. Bimetallic carriers with high Fe content, which contain a FeMnO3 phase, exhibit an unusual, reversible de-alloying/re-alloying behavior during cyclic redox operation, which precludes any synergistic effects between the two metals and results in slowed reduction kinetics. However, Mn-rich carriers show a pronounced increase in carrier reactivity and selectivity for total oxidation of methane due to the addition of small amounts of Fe, indicating the promise of appropriately designed FeMn carriers as low-cost, environmentally benign oxygen carrier materials for chemical looping combustion.