Insights into Syngas Combustion on a Defective NiO Surface for Chemical Looping Combustion: Oxygen Migration and Vacancy Effects

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.

Review on the Theoretical Understanding of Oxygen Carrier Development for Chemical-Looping Technologies

“…It was reported that defects (e.g., oxygen vacancies) on the surface of oxygen carriers exhibited a significant influence on the reactivity . The existence of surface oxygen vacancies could effectively activate the C–H bonds. , Compared to the perfect surface, the energy barrier of CH 4 dissociation on the O-defective surface is clearly reduced.…”

Modification of Metal (Fe, Al) Doping on Reaction Properties of a NiO Oxygen Carrier with CO during Chemical Looping Combustion

“…Yuan et al clarified the reaction mechanism of syngas combustion by NiO . Yuan et al also used DFT to analyze the adsorption principle of syngas and lattice oxygen transfer mechanism on the surface of pure NiO (100) behavior. , …”

A La, Sm co-doped CeO2 support for Fe2O3 to promote chemical looping splitting of CO2 at moderate temperature

“…The rational design and selection of suitable oxygen carriers with both high reactivity and durability are crucial for chemical looping processes. 15 Metal oxides, such as Fe 2 O 3 , 16 NiO, 17 and CuO, 18 have drawn special focus and been investigated as oxygen carrier materials for chemical looping applications. Among all the candidates, Fe 2 O 3 is the most widely studied and employed oxygen carrier in that it possesses high oxygen capacity, low cost and high abundance.…”