CeO₂ is a promising catalytic oxidation material for flue gas mercury removal. Density functional theory (DFT) calculations and periodic slab models are employed to investigate mercury adsorption and oxidation by oxygen over the CeO₂ (111) surface. DFT calculations indicate that Hg⁰ is physically adsorbed on the CeO₂ (111) surface and the Hg atom interacts strongly with the surface Ce atom according to the partial density of states (PDOS) analysis, whereas, HgO is adsorbed on the CeO₂ (111) surface in a chemisorption manner, with its adsorption energy in the range of 69.9–198.37 kJ/mol. Depending on the adsorption methods of Hg⁰ and HgO, three reaction pathways (pathways I, II, and III) of Hg⁰ oxidation by oxygen are proposed. Pathway I is the most likely oxidation route on the CeO₂ (111) surface due to it having the lowest energy barrier of 20.7 kJ/mol. The formation of the HgO molecule is the rate-determining step, which is also the only energy barrier of the entire process. Compared with energy barriers of Hg⁰ oxidation on the other catalytic materials, CeO₂ is more efficient at mercury removal in flue gas owing to its low energy barrier.

Doping of CeO₂ on activated carbon (AC) can promote its performance for mercury abatement in flue gas, while the Hg⁰ removal mechanism on the AC surface has been rarely reported. In this research, density functional theory (DFT) calculations were implemented to unveil the mechanism of mercury removal on plain AC and CeO₂ modified AC (CeO₂-AC) sorbents. Calculation results indicate that Hg⁰, HCl, HgCl and HgCl₂ are all chemisorbed on the adsorbent. Strong interaction and charge transfer are shown by partial density of states (PDOS) analysis of the Hg⁰ adsorption configuration. HCl, HgCl and HgCl₂ can be dissociatively adsorbed on the AC model and subsequently generate HgCl or HgCl₂ released to the gas phase. The adsorption energies of HgCl and HgCl₂ on the CeO₂-AC model are relatively high, indicating a great capacity for removing HgCl and HgCl₂ in flue gas. DFT calculations suggest that AC sorbents exhibit a certain catalytic effect on mercury oxidation, the doping of CeO₂ enhances the catalytic ability of Hg⁰ oxidation on the AC surface and the reactions follow the Langmuir–Hinshelwood mechanism.