Mechanism of Mercury Adsorption and Oxidation by Oxygen over the CeO₂ (111) Surface: A DFT Study.

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.