Characterization Of Modified Fe₂O₃/CeO₂ Oxygen Carrier For Chemical Looping Hydrogen Generation

The CO₂ emission originated from the combustion of fossil fuels is the most important source of greenhouse gases,and it is an imperative issue to develop efficient CO₂ emission reduction technologies.H₂ is an ideal fuel and its main combustion product is water with no pollution.Chemical looping hydrogen generation（CLHG）is a promising hydrogen production technology with simple structure and short process,which can not only produce high purity hydrogen but also separate CO₂inherently.The oxygen carrier is a critical factor,which is closely related to the hydrogen generation efficiency and H₂ purity.CeO₂ is a fluorite type oxide with property of storing and releasing oxygen,which can promote the oxygen transport capacity,and it has been widely used in the redox catalysis field.CeO₂ can be effective to enhance the activity of Fe-based oxygen carriers,suppress the carbon deposition and increase the H₂ purity when it acts as the“active”support.In the present work,the applicability of CeO₂ as support for Fe-based oxygen carriers was evaluated.The reactivity,redox stability and action mechanisms of oxygen carriers in CLHG were studied with H₂ yield and purity as research goals and CO as fuel,which was based on the modification of Fe₂O₃/CeO₂ oxygen carrier.The main conclusions were drawn as below.Three Fe-based oxygen carriers,supported on Al₂O₃,ZrO₂,and CeO₂,were investigated regarding their application in CLHG and the interaction between iron oxides and supports.The Fe₂O₃/CeO₂ was the most suitable among the three oxygen carriers,and it obtained the highest reactivity at 850°C.Fe₂O₃/CeO₂ suffered sintering after redox cycles.However,it still kept high reactivity,redox stability,and H₂ yield.The hydrogen generation performance of Fe₂O₃/CeO₂ was the best when the mass loading of Fe₂O₃ was 60%.The doping effect of Zr with smaller ionic radius than Ce⁴⁺on the performance of Fe₂O₃/CeO₂oxygen carrier in CLHG was investigated.The results showed that the ZrO₂ and CeO₂ interacted with the Ce_x Zr_1-xO₂ composite oxides generated.The Fe₂O₃/Ce_0.75Zr_0.25O₂ was the best oxygen carrier among them with high performance.The generated solid solution Ce_0.75Zr_0.25O₂ could improve the oxygen mobility and thermal stability of the oxygen carrier.However,the ZrO₂ phase segregation was observed for the Ce_0.75Zr_0.25O₂ support after redox cycles,which can be inimical to the reactivity of the Fe₂O₃/Ce_0.75Zr_0.25O₂ oxygen carrier.The Fe₂O₃/CeO₂ oxygen carriers doped by rare earths（Y,Sm,and La）with different valence from Ce⁴⁺were studied in CLHG.The results indicated that the Fe₂O₃/Ce_0.8Sm_0.2O_1.9 demonstrated the best performance among them.All three rare earths were incorporated into CeO₂ and promoted the redox reactivity.No bleed-out of rare earths from doped CeO₂ was observed after redox cycles for Sm and Y.Nevertheless,La would bleed out from Ce_0.8La_0.2O_1.9 and generate a stable perovskite LaFeO₃,exerting a detrimental effect on the reactivity of Fe₂O₃/Ce_0.8La_0.2O_1.9.Based on the above findings,the co-doping effect of Zr and Sm in Fe₂O₃/CeO₂ oxygen carrier was investigated for CLHG with Fe₂O₃/CeO₂,Fe₂O₃/Ce_0.75Zr_0.25O₂ and Fe₂O₃/Ce_0.8Sm_0.2O_1.9 as references.The results suggested the Fe₂O₃/Ce_0.6Sm_0.15Zr_0.25O_1.925 exihibited the best performance among them.The addition of Zr increased the sintering resistance and the Sm doping enhanced the oxygen mobility,promoting the reactivity of the oxygen carrier.However,the Zr and Sm bleed out and reacted into Sm_0.5Zr_0.5O_1.75 after cycles,which can be inimical to the reactivity of oxygen carrier.In addition to the chemical modification,the physical construction is also a critical factor for an oxygen carrier.A core-shell structured Fe₂O₃@CeO₂（core@shell）oxygen carrier was investigated regarding its sintering resistance and redox performance in CLHG.The results showed that the core-shell structure significantly enhanced the sintering resistance of Fe₂O₃/CeO₂ oxygen carrier,and the Fe₂O₃@CeO₂ exhibited much higher redox reactivity and recyclability than the composite Fe₂O₃/CeO₂.In addition,the CeO₂ shell could facilitate the oxygen ions transport between the iron oxide nanoparticle core and the shell surface and further promoted the reactivity of Fe₂O₃@CeO₂oxygen carrier.