Investigation of Oxygen Diffusivity within Oxygen Carrier In Chemical Looping Process

Chemical looping process is effective in converting carbonaceous fuels into hydrogen and electricity while generating a sequestration-ready CO₂ stream. The process scheme is enabled by the cyclic reduction and oxidation reactions of an oxygen carrier (OC). The properties of OCs are of critically importance for the performance of chemical looping process. Our previous experimental studies show that composite iron oxide synthesized by adding support materials to the pure iron oxide can serve as good OCs. Unlike the unsupported iron oxide which deactivates within the first few redox cycles, the composite iron oxide maintains good reactivity over a long time although pore volume decreases significantly in the first few cycles. This phenomenon indicates that traditional understanding on support effect, mainly based on morphological analysis, may not be accurate. In this study, we investigated the role of support through both marker experiments and periodic Density Functional Theory (DFT) calculations. Ilmenite (FeTiO₃) was used to model the simplified TiO₂ supported iron (II) oxide material. Both the oxygen diffusion barrier and vacancy formation energy in ilmenite (FeTiO₃) and wustite (Fe_1-xO) were calculated. The diffusivities of oxygen in both materials were estimated based on the calculation results. It was found that the energy barrier for oxygen anion diffusion in ilmenite was significantly lower than that in wustite. The calculated oxygen diffusivity in ilmenite was also higher. This finding was corroborated by the marker experiment which indicates notably enhanced oxygen anion conductivity in the supported iron oxide.