Studies on the synergistically improved reactivity of spinel NiFe2O4 oxygen carrier for chemical-looping combustion

The investigation of the synergistically improved reactivity of spinel NiFe2O4 oxygen carrier during chemical-looping combustion was conducted by the thermogravimetric analysis (TGA) experiments and implemented within the density functional theory (DFT) calculations. TGA results demonstrated that spinel NiFe2O4 could be directly reduced into Ni–Fe alloy in the CO atmosphere. The reaction rate of NiFe2O4 showed two times faster as compared with Fe2O3. The increase of reaction temperature, CO concentration and heating rate can boost the reaction rate of NiFe2O4. The oxygen vacancy formation energy is a good indicator for the reactivity of lattice oxygen in NiFe2O4. DFT calculations indicate that the lattice oxygen coordinated with Ni atom shows higher reactivity than that with Fe atom. The reactivity of lattice oxygen in NiFe2O4 is primarily owing to the coordination environment of oxygen formed by different Ni/Fe atoms, which is not only related to the type and number of coordination metal atoms, but also correlated with the surface structure. It can be found that the calculated results are in good agreement with the improved reactivity of NiFe2O4 oxygen carrier that has been observed in the TGA experiments. These results are of importance to generally understand the synergistically improved reactivity in the spinel NiFe2O4 oxygen carrier.