Mechanism study of reaction between CO and NiO(0 0 1) surface during chemical-looping combustion: Role of oxygen

• The SGGA + U method could give an accurate description in the CO–NiO(0 0 1) system.
• CO could be only adsorbed on NiO(0 0 1) perfect and oxygen vacancy surfaces.
• The oxygen vacancy exhibits a high surface reactivity to the dissociation of O2.
• CO reacts with adsorbed O to form CO2 on NiO(0 0 1) oxygen vacancy surface.

First-principle calculations based on density function theory (DFT) were used to clarify the reactivity mechanism of CO with NiO(0 0 1) surface, including the effect of the oxygen species during the Ni-based oxygen carrier reduction process in chemical-looping combustion. The results indicate that the SGGA (spin-polarized generalized gradient approximation) + U (a Hubbard-type on-site Coulomb repulsion) method could give an accurate description of the reaction in the CO–NiO(0 0 1) system by comparing with experimentally determined parameters. A systematic investigation of the adsorption of CO on the NiO(0 0 1) perfect and defective surfaces was performed. It was found that CO could be only adsorbed on both surfaces, but no oxidation reactions were observed. The role of O species for the reaction between CO and NiO(0 0 1) surface was studied in detail. Our calculations clearly reveal that the oxygen vacancy exhibits a high surface reactivity toward the dissociation of O2 to O atoms and a reaction between oxygen adsorbed on the surface of NiO(0 0 1) and CO is the major reaction pathway in forming CO2.