Crystallite Phase Effect on the Redox Reactivity of Layered Double Hydroxide-Derived Iron-Based Oxygen Carriers for Chemical Looping CO₂ Reduction

Abstract

Layered double hydroxide (LDH) is an effective self-template to develop efficient iron-based oxygen carriers for chemical looping CO 2 reduction. However, how the synthesis condition and metal ratio affect crystallite phase formation and redox reactivity is still unclear. In this work, a series of iron-based oxygen carriers are synthesized from thermal transformation of LDHs with varying metal ratios and calcination temperatures to reveal the crystallite phase formation mechanism and interpret the phase−activity relationship. Spinel MgAl δ Fe 2−δ O 4 is formed below 800 °C, while MgFe δ Al 2−δ O 4 is transformed into MgAl δ Fe 2−δ O 4 with increasing iron content at 800 °C. During H 2 −CO 2 redox cycles, Fe 3+ /Fe 2+ transition and iron segregation are observed for MgAl δ Fe 2−δ O 4 , but only the former occurs for MgFe δ Al 2−δ O 4 . For a phase mixture, with a low MgAl δ Fe 2−δ O 4 amount, Al migrates and substitutes Fe with the reincorporation of segregated iron to generate MgFe δ Al 2−δ O 4 . MgAl δ Fe 2−δ O 4 achieves higher oxygen storage capacity and weaker stability than MgFe δ Al 2−δ O 4 because the former favors the creation of more oxygen vacancies and induce a larger crystallite size. Due to the crystallite phase effect, Fe 0.5 Mg 2 Al 0.5 exhibits highest redox rates (6 mol [O] •kg Fe −1 •min −1 and 10 mol [O] •kg Fe −1•min −1 for the release and uptake of oxygen, respectively) and CO space-time yield (0.46 mol CO •kg Fe −1 •s −1 ). These findings provide a new route to tailor efficient oxygen carriers for chemical looping applications.