DFT Calculational And Experimental Investigations On The Reducibility Of Ferrite Oxygen Carriers In Chemical Looping Processes

Chemical looping process(CL)is a novel energy conversion scheme,in which the in-situ carbon capture and the high energy conversion efficiency could be realized without energy penalty.Oxygen carriers(OC),which are generally metal oxides or their composites,are critical in the development of different CL processes,addressing combustion,gasification and H2 production.Iron-based oxide,rich in natural resource and inexpensive,is one of ideal candidates as oxygen carriers in the CL process.Understandings towards the general principles of the selection on iron-based oxygen carriers,especially multi-metal ferrites,are highly demanded.The thesis focused on multi-metal ferrite oxygen carriers,one of the most promising iron oxide groups.The research was carried out via a combination of experimental and quantum computational methods.The sol-gel combustion method was used to synthesize ferrite compounds and Thermogravimetry(TG)as well as Temperature-Programmed Reduction(TPR)were applied to test the reducibility of the prepared samples in the atomasphere of CO and H2,respectively.Two descriptors of the initial reaction temperature and the reaction rate were used in the study to describe the reduction behaviour of the OC samples,and respectively mainly determined by the surface reactivity and the outward diffusion efficiency of lattice oxygen in the bulk.X-ray Photoelectron Spectroscopy(XPS)characterization was conducted to experimentally explore the electronic property.Binding energy was a unique parameter to analyse the specific chemical environment in the fettite samples where the element of 0 was located.The study turned out that a higher binding energy and content of surface oxygen resulted in a lower initial reaction temperature,while a higher binding energy and content of lattice oxygen in the bulk brought a faster reaction rate.The softwares of Castep and VASP(Vienna Ab-initio Simulation Package)based on Density Functional Theory(DFT)were utilized to investigate geometries and electronic structures of ferrite systems.The band gap near the fermi level was considered to indicated the reactivity of 0 atoms.Corresponding to the experimental results,a smaller band gap in the bulk configuration meant a higher reaction rate while a smaller band gap in the surface(111)configuration led to a lower initial reaction temperature.Moreover,a further distance between the first layer and the second one in the surface(1 11)configuration could provide more space for the outermost metal atoms to be adsorbed in by reducing gas molecules such as CO and H2.Overall,using deeper prediction insights into structure-property relations,as summazed in this thesis,will greatly increase the success of our subsequent efforts.The study helped in one step forward to potentially apply the quantum computational method into the development of oxygen carriers in terms of designing,selecting and testing.