Redox-Driven Restructuring of FeMnZr-Oxygen Carriers Enhances the Purity and Yield of H₂ in a Chemical Looping Process

Chemical looping-based approaches allow for the production of high purity hydrogen from methane with an inherent separation of the coproduced carbon dioxide. In such a process, first methane is oxidized using lattice oxygen from a solid oxygen carrier. In the second half-cycle, the reduced oxygen carrier is reoxidized with steam to yield hydrogen. In this work, we report on the development of an iron-based oxygen carrier with an excellent redox stability and probe the synergistic effect of adding Mn2O3 and ZrO2, leading to an enhancement of the reactivity of iron oxide with methane and increasing the hydrogen yield over multiple redox cycles. The promoting effects of Mn2O3 and ZrO2 in the oxygen carrier were elucidated by combining TPR, STEM-EDX, reactivity tests, and conductivity measurements complemented by SIMS analysis. The addition of ZrO2 promoted the oxidation reactivity of the material, whereas the addition of Mn2O3 accelerated the reduction of iron oxide. Conductivity measurements revealed that the addition of Mn2O3 lowers the activation energy for charge transfer, providing an explanation for the improved cyclic redox performance of the oxygen carriers. A redox-driven surface modification that results in the formation of an (Fe,Mn)O phase was found to retard effectively the cracking of methane on surface iron, leading to a high resistance to carbon deposition and in turn a high purity of the hydrogen produced. The observations reported here illustrate the importance of charge transfer characteristics for chemical looping based redox processes and open new perspectives for the design of more efficient oxygen carriers.