(600d) Fe and Mn-Based Oxygen Carriers for Chemical Looping Combustion

Chemical looping combustion (CLC) is an emerging technology for clean energy-production from fossil and renewable fuels based on cyclic oxidation and reduction of an oxygen carrier. These carriers – typically metal oxides – have to show sufficient thermal and mechanical stability to withstand the demanding conditions of CLC while at the same time yielding high carrier and fuel conversion. Among the wide range of materials investigated, extensive work has recently been focused on iron and manganese oxides as oxygen carriers due to their abundance, low-cost, and environmentally benign nature. However, both metals show comparatively low reactivity and only moderate selectivity for total oxidation products.

In the present study, we investigated the use of Mn-Fe mixed-metal carriers for CLC. We had previously shown that doping Fe-based carriers with Ni can yield significant enhancements in reactivity.  Ceria was chosen as support based on our previous findings that ceria can result in strong enhancements of carrier utilization and stability of the bimetallic metal phase.

Fe/Mn mixed oxides were synthesized with various Fe/Mn ratio. All carriers were synthesized with 40wt% metal by simple incipient wet impregnation technique on ceria supports. The carriers were characterized (as synthesized and post-reactive tests) using TEM, XRD, BET, and EDX. In a first step, the carriers were tested in a thermogravimetric analyzer (TGA) with H₂ and CH₄ as fuels. All FeMn carriers showed good reactivity and thermal stability with no signs of metal-support spinel formation. Increasing the Fe/Mn ratio resulted in improved oxygen carrying capacity beyond a simple linear addition of the capacity of the pure metal components. Next, selectivity and fuel conversion was further evaluated in fixed-bed reactor experiments with CH₄ as fuel. For both pure metals, a transition in selectivity was observed from total oxidation at high carrier oxidation states to partial oxidation over the reduced carriers. Interestingly, the bimetallic carriers showed a combination of the high syngas selectivity of Mn-based carriers with the widened operating window of Fe-based carriers, rendering this bimetallic carrier material of particular interest for partial oxidation of methane via chemical looping.