Abstract
Chemical Looping (CL) process is considered as one of the most advanced CO₂ capture technologies and it also can convert CO₂ to CO for CO₂ resourceful utilization. Oxygen carrier (OC) is the cornerstone for the successful implementation of chemical looping technique. However, a lack of fundamental understanding of lattice oxygen migration and spatial-temporal evolution characteristics of active sites in OC limits high performance OC design and development. NiFe₂O₄ spinel OC were synthesized in this study. The dynamic structure evolution of NiFe₂O₄ spinel OC during a redox cycle under H₂-reducing and CO₂-air two-step oxidizing atmospheres at 850 °C was investigated using in-situ environment transmission electron microscopy (ETEM) combined with in-situ electron energy loss spectrum (EELS). The nature variation of the nickel, iron and oxygen ions at the near surface of NiFe₂O₄ OC is investigated via in-situ X-ray photoelectron spectroscopy (XPS). Density functional theory (DFT) calculations and ab initio molecular dynamics (AIMD) confirm our experimental findings. In-situ visualization research indicates that the chemical reaction interface is fixed on the surface of OC particles. With the release and recovery of lattice oxygen, metal cations of OC undergo complex migration, transformation, and atom recombination processes. And this process is closely related to the migration rate of oxygen anion (lattice oxygen). In addition, the stable high-valent Fe oxides on the surface might be a necessary channel for atom migration. These results indicate that metallic ionic diffusion and lattice oxygen migration both play an important role in the oxygen carrier redox reactions. It advances the new understanding of the migration mechanism of lattice oxygen of oxygen carriers in chemical looping.
Abstract
Chemical Looping (CL) process is considered as one of the most advanced CO₂ capture technologies and it also can convert CO₂ to CO for CO₂ resourceful utilization. Oxygen carrier (OC) is the cornerstone for the successful implementation of chemical looping technique. However, a lack of fundamental understanding of lattice oxygen migration and spatial-temporal evolution characteristics of active sites in OC limits high performance OC design and development. NiFe₂O₄ spinel OC were synthesized in this study. The dynamic structure evolution of NiFe₂O₄ spinel OC during a redox cycle under H₂-reducing and CO₂-air two-step oxidizing atmospheres at 850 °C was investigated using in-situ environment transmission electron microscopy (ETEM) combined with in-situ electron energy loss spectrum (EELS). The nature variation of the nickel, iron and oxygen ions at the near surface of NiFe₂O₄ OC is investigated via in-situ X-ray photoelectron spectroscopy (XPS). Density functional theory (DFT) calculations and ab initio molecular dynamics (AIMD) confirm our experimental findings. In-situ visualization research indicates that the chemical reaction interface is fixed on the surface of OC particles. With the release and recovery of lattice oxygen, metal cations of OC undergo complex migration, transformation, and atom recombination processes. And this process is closely related to the migration rate of oxygen anion (lattice oxygen). In addition, the stable high-valent Fe oxides on the surface might be a necessary channel for atom migration. These results indicate that metallic ionic diffusion and lattice oxygen migration both play an important role in the oxygen carrier redox reactions. It advances the new understanding of the migration mechanism of lattice oxygen of oxygen carriers in chemical looping.
 

Chemical Looping; In-situ characterization; CO2 utilization; atom migration; Density functional theory