Fe₃O₄/ZrO₂ Composite as a Robust Chemical Looping Oxygen Carrier: A Kinetics Study on the Reduction Process

FeOx is the most popular choice of oxygen carriers for the chemical looping hydrogen (CLH) reactor and solid oxide iron–air redox battery (SOIARB) due to its earth abundance, low cost, and high oxygen content. However, the performance of an Fe/FeOx chemical looping cycle is critically limited by the reduction kinetics of FeOx. Aimed at understanding and ultimately improving the kinetics of FeOx reduction, we here report a kinetics study on isothermal H2 reduction of an Fe3O4/ZrO2 composite (derived from Fe2O3/ZrO2) and pure Fe3O4 (derived from Fe2O3) as an oxygen carrier in a CLH and SOIARB environment. We observe that the prevailing oxide form, on which the reduction kinetics is carried out, under 10% H2O–Ar in 600–800 °C, is Fe3O4. We show that the reduction of Fe3O4 to Fe follows two consecutive steps and can be reasonably described by phenomenological chemical-controlled and diffusion-controlled kinetic models. We also demonstrate that the presence of ZrO2 is critical in preventing Fe particles from sintering, obtaining reliable kinetic data, and stable operation of overall chemical looping cycles. The obtained kinetic parameters provide firsthand data for engineering and design of practical CLH reactors and SOIARBs.