Lanthanum-based Perovskite Oxygen Carriers In Chemical Chain Steam Reforming Reactions

Chemical looping reforming of methane(CLRM)process is a novel technology that can produce syngas and hydrogen simultaneously.In this technique,methane is first partially oxidized to syngas(H₂+CO)by the lattice oxygen of the oxygen carrier.Subsequently,the reduced oxygen carrier recovers the missed lattice oxygen from the steam,and simultaneously produces hydrogen.The design and construction of oxygen carriers with high activity and stability is the key factor to realize this technology.La-based perovskite oxides can be used as oxygen carriers for CLRM technology due to their unique structural properties and excellent oxygen storage and release properties.The A or B cations in the perovskite structure can be partially substituted by another metal cation.Through this doping method,the valence of the cation and/or the amount of non-stoichiometric oxygen in the perovskite structure can be changed,generating more oxygen vacancies and improving the reactivity of the oxygen carrier.In this paper,Cu/Fe-doped LaMnO₃-based perovskite oxygen carrier s were successfully prepared by the sol-gel method.The physical and chemical properties of oxygen carriers were characterized by XRD,BET,Raman,XPS and TPR.The reactivity of CLRM over LaMnO₃,La_1-x Mn CuxO₃ and La_1-x Mn FexO₃/La Mn_1-x FexO₃oxygen carriers was systematically investigated.In addition,the influence of calcination temperature on the reactivity of oxygen carriers was further explored.LaMnO₃ was used as an oxygen carrier for the CLRM process.It obtained a high resistance to carbon deposition,while a lower CH4 conversion due to the poor oxygen storage and release performance.La_1-x Mn CuxO₃ oxygen carriers exhibited good oxygen transport capacity and high reactivity during the CLR M process.In the methane partial oxidation stage,the syngas productivity of La_1-x Mn CuxO₃(2.79-5.20mmol/g)increased gradually with the Cu content.La_0.85Mn Cu0.15O3 displayed the highest H₂ productivity(2.52 mmol/g)in the water splitting cycle reaction.La_1-x Mn FexO₃/La Mn_1-x FexO₃ oxygen carriers owned higher reactivity and better resistance to carbon deposition.La 0.85Mn Fe_0.15O₃ sample exhibited the highest CO selectivity(?99%),syngas productivity(3.78 mmol/g)and H₂ productivity(1.76mmol/g),and its H₂/CO molar ratio was maintained at the ideal ratio of 2.0(1.93-2.07)during the whole process.Notably,no carbon deposition was generated over LaMnO₃,La_0.85Mn Fe_0.15O₃ and La Mn0.9Fe0.1O3 oxygen carriers even after 20 redox cycles,and La_0.85Mn Fe_0.15O₃ exhibited superior resistance to carbon deposition due to the better structural and thermal stability.The effect of calcination temperature on structure properties of the La_0.85Mn Fe_0.15O₃ and La Mn0.9Fe0.1O3 were investigated in detail.It was found that the series of La_0.85Mn Fe_0.15O₃ samples exhibited a better structural stability.The samples calcined at 800 ^oC exhibited higher concentration of oxygen vacancies and better oxygen storage and release performance.During the 9 redox cycles,La_0.85Mn Fe_0.15O₃-800 showed the best reactivity and thermal stability,and its H₂/CO ratio maintained around the ideal value(about 2.0).