Computational Investigation of NO2 Adsorption and Reduction on Ceria and M-Doped CeO2 (111) (M = Mn, Fe) Surfaces

Abstract

The interaction and mechanism for NO2 reduction on ceria and M-doped CeO2 (111) (M = Mn, Fe) surfaces have been studied by using periodic density functional theory calculations corrected with the on-site Coulomb interaction via a Hubbard term (DFT+U). Our calculations show that NO2 adsorbs more strongly on the stoichiometric M-doped CeO2 surfaces compared with the undoped surface, forming nitrate-like (NO3–) species with adsorption energies of −0.99 to −1.09 eV for CeO2, −1.46 to −1.99 eV for Ce0.875Mn0.125O2, and −1.32 to −1.64 eV for Ce0.875Fe0.125O2. On the contrary, the adsorption of NO2 is less stable at the oxygen vacancy site on the defective M-doped CeO2 surfaces (−0.49 to −0.97 eV for Ce0.875Mn0.125O2 and −0.70 to −1.35 eV for Ce0.875Fe0.125O2) than on the defective undoped surface (−1.95 to −2.02 eV), forming nitrite-like species (NO2–). It is worth noting that the adsorption energies of NO2 are proportional to the oxygen vacancy energies for stoichiometric surfaces, whereas they are disproportional to the oxygen vacancy energies for defective surfaces. For the reduction reaction in which NO2 is reduced to NO and the defective CeO2 and M-doped CeO2 (111) surfaces are fully reoxidized, the reaction energy is exothermic for CeO2−δ, whereas it is endothermic for Ce0.875Mn0.125O2−δ and Ce0.875Fe0.125O2−δ. The reaction energies depend on the oxygen vacancy formation energies, for which Ce0.875Mn0.125O2 < Ce0.875Fe0.125O2 < CeO2. The vibrational frequency calculations as well as the Bader charge analysis are carried out to characterize the adsorbed species.