Theoretical insights of catalytic oxidation of Hg⁰ on g-C₃N₄-supported Fe/Co/Ni-based bi-metallic catalysts using O₂ in coal-fired flue gas as the oxidant

In this work, density functional theory (DFT) calculations were conducted to investigate the adsorption and oxidation of Hg⁰ with O₂ as the oxidant on pristine and O-adsorbed dimers of Fe, Co and Ni supported on the buckled g-C₃N₄ surface. The calculation reveals that all the dimers supported on the buckled g-C₃N₄ surface (Fe₂/Co₂/Ni₂@g-C₃N₄) are stable at 700 K. It is found that Hg⁰ oxidation starts from the adsorption of O₂ and its subsequent dissociation, followed by the formation of OHgO and the desorption of HgO from the surface. However, after desorption of the first HgO, the active site of the dimer becomes an O-adsorbed dimer, which affects - Hg⁰ oxidation reaction although the reaction pathway is similar. DFT calculations demonstrate that both the pristine dimer and O-adsorbed dimer are effective in O₂ dissociation and are relatively easy for the metal-O bond to break, which is associated with a low energy barrier for these two processes. However, the interactions between O₂/Hg⁰ and the pristine surface are significantly stronger than those between O₂/Hg⁰ and the O-adsorbed dimer site. Rate-determining step of catalytic oxidation process on the pristine and O-adsorbed Fe₂@g-C₃N₄ and Co₂@g-C₃N₄ is the cleavage of the metal-O bond, while the HgO desorption dominates the pristine and O-adsorbed Ni₂@g-C₃N₄ with an energy barrier of 2.04 eV and 1.62 eV, respectively. It is found that among the dimers studied, the Ni₂@g-C₃N₄ exhibites the highest efficiency in the catalytic oxidation of Hg⁰.