Energy & Fuels, Vol.32, No.11, 11362-11374, 2018
Enhanced Hydrogen Generation for Fe2O3/CeO2 Oxygen Carrier via Rare-Earth (Y, Sm, and La) Doping in Chemical Looping Process
Fe2O3/CeO2 exhibits desirable redox performance in chemical looping hydrogen generation (CLHG) because of its favorable lattice oxygen conductivity originating from ceria. Meanwhile, rare earths can bring about even more improvement on the reactivity of Fe2O3/CeO2. Herein, we synthesized Fe2O3/CeO2 doped with three rare earths, Y, Sm, and La, respectively, by coprecipitation approach. The redox performance and fundamental mechanisms were investigated to study the influence of the rare earth additives on the Fe2O3/CeO2 oxygen carrier for CLHG. It was shown that the Fe2O3/Ce0.8Sm0.2O1.9 demonstrated the highest redox reactivity, and the purity of generated hydrogen could be up to 100% (with the detection limit at 0.01% in volume). The reactivity was ranked as Fe2O3/Ce0.8Sm0.2O1.9 > Fe2O3/Ce0.8La0.2O1.9 > Fe2O3/Ce0.8Y0.2O1.9 > Fe2O3/CeO2. The rare earths were incorporated into CeO2 and increased the concentration of oxygen vacancies, promoting the oxygen mobility and reactivity of these oxygen carriers. Specifically, no bleed-out of rare earths from doped CeO2 was observed after redox cycles for Sm and Y, and both rare earths could suppress the outward diffusion of iron cations in the particle and subsequent enrichment on the surface, improving the redox stability of oxygen carriers. However, La would bleed-out from Ce0.8La0.2O1.9 and generate a stable perovskite LaFeO3, which resulted in the outward migration of iron cations and reduced the quantity of active iron oxides, exerting a detrimental effect on the redox reactivity of Fe2O3/Ce0.8La0.2O1.9.