Different Reactivities of the (100) and (110) Surfaces of the NiFe<sub>2</sub>O<sub>4</sub> Composite Oxygen Carrier in Chemical Looping Combustion: An Atomic Insight
Feng Liu, Jing Liu, Ruixue Fang, Yu Li, Yingju Yang
Abstract
Spinel NiFe2O4 has drawn increasing attention as an oxygen carrier candidate for chemical looping combustion (CLC) on account of its synergistically improved reactivity. A detailed understanding of the adsorption and oxidation of CO on different surfaces and how they are affected by the surface structure will assist in developing spinel NiFe2O4 with high performance. Hence, a comparative investigation of the adsorption and oxidation of CO on two low-index spinel NiFe2O4 surfaces has been performed using density functional theory (DFT) calculations. The calculations indicated that the Ni–O bridge sites are the most favorable for CO adsorption on both the (100) and (110) surfaces of NiFe2O4. Owing to the lower oxygen saturation of the (110) surface, it shows higher adsorption activity than the (100) surface. The projected density of states (PDOS) analysis indicated that stable adsorption is correlated to the strong hybridization between metal 3d states and CO molecular orbitals. Threefold oxygen coordinated with tetrahedral Fe on the (100) surface and twofold oxygen on the (110) surface show lower reactivity for the oxidation of CO. The Brønsted–Evans–Polanyi (BEP) relationship implied that metal sites are highly affected by the coordination number of oxygen atoms, and the oxygen bridged with the Ni atom has higher reactivity than that with the Fe atom. These results may provide an efficient way to screen or optimize the high performance of spinel NiFe2O4 oxygen carriers for the CLC process.