Synergistic Effects of the Zr and Sm Co-doped Fe2O3/CeO2 Oxygen Carrier for Chemical Looping Hydrogen Generation

CeO2 is a typical fluorite oxide with desirable lattice oxygen conductivity and can be applied as an active support for the Fe-based oxygen carrier in chemical looping hydrogen generation (CLHG). However, Fe2O3/CeO2 always suffers from low thermal stability and sintering. Doping foreign cations could be a proper way to improve its reactivity and cyclic stability. In this work, Zr4+ and Sm3+, with a smaller ionic radius and lower valence than Ce4+, respectively, were doped into Fe2O3/CeO2 using the co-precipitation method, and the doping effects on the reactivity and redox stability of Fe2O3/CeO2 in CLHG were investigated. Fe2O3/Ce0.6Sm0.15Zr0.25O1.925 provided the best redox reactivity, and the purity of generated hydrogen for all oxygen carriers reached nearly 100% (the detection limit of CO/CO2 was 0.01% in volume). The reactivity followed the sequence Fe2O3/Ce0.6Sm0.15Zr0.25O1.925 > Fe2O3/Ce0.8Sm0.2O1.9 > Fe2O3 /Ce0.75Zr0.25O2 > Fe2O3/CeO2. However, the concentration of oxygen vacancy was ranked as Fe2O3/Ce0.8Sm0.2O1.9 > Fe2O3/Ce0.6Sm0.15Zr0.25O1.925 > Fe2O3/Ce0.75Zr0.25O2 > Fe2O3/CeO2. The Zr doping boosted the reactivity of Fe2O3/CeO2 mainly by enhancing its sintering resistance, whereas the Sm doping achieved it mainly by promoting the oxygen conductivity, whose ability to improve the thermal stability of Fe2O3/CeO2 was rather limited. Both Zr and Sm doping could suppress the outward migration of Fe cations in the particles, resulting in higher sintering resistance. Furthermore, Zr and Sm could dissolve into CeO2 for prepared Fe2O3/Ce0.6Sm0.15Zr0.25O1.925; however, the bleeding out of both dopants was observed with Sm0.5Zr0.5O1.75, formation, which could be detrimental to the redox stability of the oxygen carrier.