Exploration of Reaction Mechanisms on Hydrogen Production through Chemical Looping Steam Reforming Using NiFe2O4 Oxygen Carrier

Chemical looping steam reforming (CLSR) is a novel technology for hydrogen production using metal oxide as an oxygen carrier (OC). During the CLSR, the OC (MeO) is first reduced to the final state (Me) by fuel, and then the reduced OC (Me) decomposes the water vapor to produce hydrogen, while it recovers part of the lattice oxygen to the intermediate state (MeO1−δ); finally, the intermediate OC recovers the remaining lattice oxygen into the initial state (Me) under air atmosphere. This work is to explore the reactivity of NiFe2O4 OC during the CLSR based on the thermodynamic predictions and experimental analysis. The thermodynamic analysis shows that the metallic Fe can reduce steam (H2O) to produce H2, and it can recover part of the lattice oxygen into FeO/Fe3O4 species, while the metallic Ni cannot recover lattice oxygen under steam atmosphere. The TG experiments show that sequence of H2 production capacity of four reduced OCs is described as follows: metallic Fe > Fe(Ni) alloy > mixture of metallic Fe and Ni ≫ metallic Ni. The H2 production capacity of the OC mainly depends on its reduction state. A more drastic reduction degree of OC implies a higher production capacity of H2. Compared with the Fe2O3, the NiFe2O4 is more suitable for the CLSR because it has the strongest oxidation performance, facilitating deep reduction of OC in a relatively short reaction time. Because of the formation of a good dispersibility between the iron atom and the nickel atom, introduction of the inert alumina can effectively inhibit collapse of the spatial structure and sintering of the single NiFe2O4, evidently improving the reactivity and recyclability of the OC particles. The H2 production capacity of the mixed OC (NiFe2O4 + Al2O3) was stable at ∼400 mLH2/gOC within the 20 cycles. XRD patterns show that a part of Ni0 can recover lattice oxygen even under steam atmosphere due to the presence of the synergistic effect of Fe–Ni. Consequently, the NiFe2O4 is a good OC candidate for H2 production during the CLSR.