A fixed-bed reactor was used to study the effect of water vapor on the efficiency of Hg-0 removal over CeO2-modified semi-coke. Adsorption experiments showed that the oxidation activity, and therefore, the mercury adsorption efficiency decreased with increasing water vapor concentrations. Hydrogen temperature-programmed reduction (H-2-TPR) indicated that the oxidation activity of CeO2 decreased after water vapor treatment. Fourier-transform infrared (FT-IR) studies revealed that CeO2-H2O showed new bending vibration peaks at 1110 cm (1) ascribed to Ce-OH groups. X-ray photoelectron spectroscopy (XPS) results showed that the content of Ce-OH increased from 20.85% to 39.99%, while the lattice oxygen content decreased from 68.21% to 45.83% after water vapor treatment. Density functional theory (DFT) calculations revealed that H2O can be dissociated on clean and oxygen-defective CeO2(111) surfaces to form H atoms and OH fragments. The H atoms associated with the adjacent oxygen atoms of CeO2, and the OH fragments were bound directly with Ce atoms to form Ce-OH for the clean CeO2 surface; or occupied the oxygen vacancies and were surrounded by three Ce atoms via van der Waals forces, without additional bond formation for the oxygen-defective CeO2 surface. Thus, the formation of Ce-OH groups and the consumption of lattice oxygens reduced the Hg-0 removal efficiency. (C) 2017 Elsevier B.V. All rights reserved.