The Mechanism Of Lattice Oxygen Migration And Structural Dynamic Evolution Of NiFe₂O₄ Oxygen Carriers In Chemical Looping

Chemical looping（CL）technology offers a versatile platform to convert various fuels to value added products in a clean and efficient approach.Oxygen carriers（OC）are known as the cornerstone of chemical looping technology.The circulating oxygen carriers between the reactors,as the sole medium,provide the oxygen and heat to facilitate the process of chemical looping.It is known that all the reactions in the chemical looping almost involve the migration and transformation of lattice oxygen.The supply of lattice oxygen by the oxygen carriers determines the rate and the product selectivity of chemical looping reactions.Recent years have seen great progress in the migration mechanism of lattice oxygen.But there remain key knowledge gaps in the area of exploring the migration mechanism of lattice oxygen.For example:1)the redox reaction mechanism of multi-component oxygen carriers is unclear.2)There is little direct evidence to reveal the reaction pathways of lattice oxygen inside OC.Therefore,the investigations on release-recovery paths of lattice oxygen are of great significance for understanding the oxidation-reduction reaction mechanisms of OCs.In-situ environmental transmission electron microscopy（In-situ ETEM）based study of lattice oxygen migration and transformation pathways in NiFe₂O₄.The results show that during the reduction process,the gas-solid reaction interface is always fixed on the surface of the oxygen carrier particles.The Ni atoms first free from the spinel structure and gradually agglomerate on the surface of the oxygen carrier particles,eventually forming spherical granular Ni⁰.And the oxygen negative ions（lattice oxygen）migrate rapidly during this step.As the reaction continues,the Fe-O bond gradually begins to break and the Fe atom breaks away from the oxygen carrier,eventually forming a dumbbell-shaped Fe⁰.With the oxygen negative ion（lattice oxygen）migrating more slowly in this step.It is noteworthy that a stable oxide layer consisting of lattice oxygen and metal cations（Fe）is formed on the surface of the reduced end-state oxygen carrier.During the CO₂ oxidation of the reduced oxygen carrier,the metal oxide-metal interface continuously migrates toward the bulk.The metal oxide layer gradually becomes thicker and the lattice oxygen concentration gradient drives the migration of the oxygen negative ions towards the bulk phase,i.e.part of the Fe⁰ is oxidized first and forms an Fe oxide layer on the surface.The part of the cation（Ni）migrates towards the bulk phase and combines with the remaining Fe⁰to form an Fe-Ni alloy.During air oxidation,due to the Kirkendall effect,the Fe-Ni alloy atoms formed migrate outward and rapidly combine with oxygen negative ions to form multiple hollow NiFe₂O₄ nanoparticles,which are wrapped by the first formed Fe₂O₃ layer.DFT theoretical calculations reveal that the Ni-O bond is the first to break and form oxygen vacancies during the initial stage of reduction;ab initio molecular dynamics（AIMD）results show that with the continuous release of lattice oxygen.The Ni atoms that are the first to free from the spinel structure undergo Ni-Ni agglomeration on the particle surface,while the Fe atoms rearrange with the remaining lattice oxygen.During CO₂ oxidation,DFT theoretical calculations confirm that the formation of Fe-Ni alloys is more beneficial to the CO₂ splitting.In addition,in situ XPS,in situ Raman,XRD,TEM-EDS shows that the stabilization of the oxygen content of the surface lattice during the reduction process is attributed to the rapid migration of the oxygen anion.In this paper,the lattice oxygen migration process of composite oxygen carriers during chemical looping conversion is investigated.NiFe₂O₄ oxygen carriers prepared by sol-gel method were selected and the lattice oxygen migration mechanism of spinel-type iron-based oxygen carriers was clarified at the atomic level using various in situ characterization techniques and combined with DFT theoretical calculations.This work provides the comprehensive understanding of oxidation/reduction-driven atomic interdiffusion behavior of oxygen carrier material.