Effect of support on hydrogen generation over iron oxides in the chemical looping process

Abstract

Fe₂O₃ is recognized as an excellent oxygen carrier for its low cost and high oxygen capacity. However, pure Fe₂O₃ must be deposited on supports to ensure high reactivity and durability. Here, we proposed several Fe₂O₃-based oxygen carriers using MgAl₂O₄, Ce_0.8Gd_0.2O_1.9, and Zr_0.8Y_0.2O_1.9 as supports and investigated their performance for chemical looping hydrogen generation. The support effect on chemical looping hydrogen generation performance was evaluated, and the fundamental insights were investigated in depth. Fe₂O₃/Ce_0.8Gd_0.2O_1.9 exhibited a superior performance regarding high hydrogen yield and stable trend over 20 cycles at 750 °C. However, hydrogen yield of Fe₂O₃/Zr_0.8Y_0.2O_1.9 exceeded that of Fe₂O₃/Ce_0.8Gd_0.2O_1.9 at higher temperatures (850 °C). Characterizations show that Ce_0.8Gd_0.2O_1.9 exhibits the highest oxygen vacancy concentration, which significantly improves the reduction and reoxidation reactions of Fe₂O₃, thus leading to an enhanced hydrogen yield. However, the interaction between Fe₂O₃ and Ce_0.8Gd_0.2O_1.9 contributed to the increase in Fe²⁺ concentration, thus decreasing the oxygen capacity during the redox cycle and contributing to the declined hydrogen yield at higher temperatures. This work highlights the potential of Ce_0.8Gd_0.2O_1.9 to be used as an effective support for Fe₂O₃ at mid-temperatures. We hope that the support effect in this work can be extended to design and select more active and durable oxygen carriers.