Study On Perovskite Oxygen Carriers For Chemical Looping Reforming Of Methane

Chemical looping of methane technology can decouple the traditional gas co-feed Partial Oxidation of Methane or Steam Reforming of Methane into two sub-reactions at different times or different spaces through the circulation of oxygen carrier and oxygen transfer,Conducive to the regulation of the reaction process.Oxygen carriers are the key to determining the performance.Therefore,it is of great significance to design and develop oxygen carriers with high oxygen storage properties,excellent redox activity and high syngas selectivity.Perovskite oxygen carriers are one of composite oxide materials with the most development potential due to their unique structure,high thermal stability and redox performance.The main challenges currently faced by oxygen carriers for Chemical-looping Partial Oxidation of Methane and Chemical-looping Steam Methane Reforming are undesirable for both reactivity and resistance to carbon deposition,the low H₂yield and purity in the water oxidation step.in view of the above problems,this paper is mainly based on the regulation and optimization of oxygen carriers:i)the promoting effects of Ce³⁺substitution into LaFeO₃redox catalysts on thermochemical H₂O splitting as well as partial oxidation of methane are presented.An appropriate amount of Ce³⁺substitution in A site of LaFeO₃(La_0.5Ce_0.5Fe O₃)induces distortion of the LaFeO₃lattice and generates more oxygen vacancies,which promotes the lattice oxygen conversion rate in methane partial oxidation step and boosts the reaction kinetics in H₂O splitting step by enhancing the surface water activation and lattice oxygen mobility of the perovskite.In the CL-SRM process,the perovskite is reduced into Fe⁰/(La_0.5Ce_0.5)₂O₃and Fe⁰/(La_0.5Ce_0.5)O_2-xphases in methane partial oxidation step,resulting in the metal Fe⁰of different sizes is encapsulated with an oxide layers,which improves resistant toward sintering.and recovered to the original perovskite structure in H₂O splitting step with simultaneous production of hydrogen,which maintains a reversible phase transformation during 100 redox cycles.As a result,the La_0.5Ce_0.5Fe O₃achieves high stability with the superior performance for syngas and hydrogen production.These findings provide the fundamental understanding on the synergy between the catalytic sites and enhanced lattice oxygen mobility for the promotion of H₂O activation and hydrogen productivity in chemical looping process.ii)This paper describes that La Mn_1-x-yFe_xCo_yO_3-δ（0≤x≤1,0≤y≤1）perovskite-type oxides co-substituted by Fe and Co in B-site,which were prepared by sol-gel method for chemical looping partial oxidation of methane to syngas.The XRD results suggest that Fe and Co were incorporated into the lattice of La Mn O₃and formed perovskite phase.Reactivity and stability tests shows that La Mn_1/3Fe_1/3Co_1/3O_3-δoxygen carrier has the best reaction performance in chemical looping partial oxidation of methane.CH₄-TPR results indicate that La Mn_1/3Fe_1/3Co_1/3O_3-δhas higher methane activation and lattice oxygen migration rates than La BO₃（B=Co,Mn,Fe）.CH₄-pulse reaction further confirms that the synergistic effect of B-site ions can improve the surface reaction rate of La BO₃（B=Co,Mn,Fe）.H₂-TPR results show that La Mn_1/3Fe_1/3Co_1/3O_3-δhas moderate redox capacity and is suitable for partial oxidation of methane.Therefore,the synergistic effect of the B-site multi-ions provides the doping direction for enhancing the oxygen uptake/release properties of perovskite-type oxides in chemical looping processes,which is of great significance for the application of such materials.