High syngas selectivity and near pure hydrogen production in perovskite oxygen carriers for chemical looping steam methane reforming

Chemical looping steam methane reforming (CL-SMR) provides an attractive route for hydrogen and syngas co-production through two successive steps of methane oxidation and steam splitting. However, there is also carbon formation through CH4 cracking which hampers the production of syngas (H-2 and CO) and the following pro-duction of pure H-2. This work prepared the perovskites La(0.95)Ce(0.05)NixFe(1-x)O(3) (x = 0, 0.2, 0.5, 0.8, 1.0) as oxygen carriers for CL-SMR. The methane activation and steam splitting were investigated based on reactivity tests on a fixed-bed reactor and various characterizations. Results showed that La0.95Ce0.05Ni0.2Fe0.8O3 and La0.95Ce0.05-Ni0.5Fe0.5O3 reached a high syngas selectivity (94.8%, 89.0%) accompanied with good methane conversion (93.1% and 95.7%) for methane partial oxidation, and near 100% hydrogen concentrations (higher than 99.6% and 99.5%) for steam splitting, indicating that additional separation step for pure hydrogen acquisition can be omitted. The H-2/CO molar ratio was maintained at the optimal value of 2.0 throughout the methane partial oxidation process. Characterizations and density functional theory calculations demonstrated that methane partial oxidation was promoted via an oxygen vacancy-mediated Mars-van-Krevelen type mechanism and the partial oxidation ability of the oxygen carrier is restricted by its content of oxygen vacancy and lattice oxygen migration rate. The proper amount of Ni doping forming the Ni-Fe synergetic effects also contributed highly to methane partial oxidation. Afterward, the deeply reduced metals combined with the oxygen vacancies provided active sites for steam splitting, hence generating pure H-2