Enhanced molecular adsorption of ethylene on reduced anatase TiO2 (001): role of surface O-vacancies

Abstract

A density functional theory (DFT)-based study of ethylene (C₂H₄) adsorption on a reduced anatase titanium dioxide (TiO₂) (001) surface, i.e., with a surface oxygen vacancy (O_vac), is presented. It was found that C₂H₄ preferably adsorbs on the O_vac-site. The excess electrons originating from the removed oxygen weaken the C₂H₄ CC double bond by filling the lowest unoccupied molecular orbital (LUMO) or of C₂H₄, and simultaneously enhance the binding of C₂H₄ to the reduced anatase TiO₂ (001). The bonding between the two C₂H₄ C atoms and the two Ti atoms nearest to the O_vac-site produces two σ-type bonds, leading to the emergence of a new localized mid-gap state. This hybrid 3d defect state can account for the ∼0.7 eV decrease in the band-gap observed in previous optical measurements of carbon-coated TiO₂, formed after TiO₂ exposure to C₂H₄ gas (Mater. Lett. 108, 2013, 134). Subsequent calculations on two initial decomposition pathways of C₂H₄ adsorbed on the O_vac-site show that the C–H bond and C–C bond cleaving require high activation barriers where the C–H bond cleaving is slightly easier compared to the C–C bond cleaving, 2.94 eV and 3.01 eV, respectively, (as calculated using DFT-D2+U_d = 3 eV) and the final states of the two initial decomposition pathways show similar endothermic characteristics. This finding indicates that the surface O_vac-sites tend to favor the formation of molecularly adsorbed Ti-bound sp³-C₂H₄ (–Ti–CH₂CH₂–Ti–) as compared to the dissociative adsorption case.