Insights into Syngas Combustion on a Defective NiO Surface for Chemical Looping Combustion: Oxygen Migration and Vacancy Effects
Yue Yuan, Xiuqin Dong, Luis Ricardez‐Sandoval
Abstract
A thorough theoretical analysis of vacancy effects on syngas combustion was conducted for the chemical looping combustion (CLC) process using NiO as an oxygen carrier (OC). A density functional theory (DFT) analysis was conducted to provide new insights into the effects of surface vacancies on OC behavior such as O migration, syngas adsorption, and syngas oxidation. The results showed that the vacancies are expected to promote the subsequent oxidation reactions, which might come from the lower state energies of the reactants, the syngas adsorption configurations on the defective surface. Furthermore, the proposed reaction mechanisms showed that the presence of the defective sites benefited the syngas oxidation by reducing the energy barriers of the CO and H2 oxidation reactions. In particular, H2 oxidation changed from a 3-step process on a perfect surface (i.e., without vacancies) to a 2-step process on a defective surface. Moreover, the CO oxidation reaction was shown to dominate the overall syngas oxidation process. In addition, the outward diffusion direction of oxygen migration was observed from the bulk side to the surface. The proposed CO and H2 reaction kinetics were validated against the experimental data using a DFT-based mean-field (MF) model. An electronic analysis was also performed to further support the intrinsic effects of surface vacancies obtained from the DFT analysis.