DFT simulation-based screening of single transition metals supported on g-C₃N₄ for the catalytic oxidation of Hg⁰

In this study, the adsorption and oxidation of elemental mercury by O₂ on a series of buckled g-C₃N₄ monolayer-supported transition metals were investigated using Density Functional Theory method. It is shown that O₂ molecules are adsorbed more strongly on the active sites compared with Hg⁰ atoms, while the catalytic oxidation of Hg⁰ undergoes three key steps, i.e., O₂ dissociation, metal-O bonds cleavage and HgO desorption. Results also demonstrated that for the dissociation of O₂, the energy barrier is inversely proportional to the adsorption energy of O atom. Among the catalysts studied, Fe-g-C₃N₄ exhibits the lowest energy barrier (0.44 eV) for O₂ dissociation. The Pd-g-C₃N₄ is found as the most efficient catalyst for the cleavage of the first and second Pd-O bonds with an external energy of 1.28 and 2.03 eV, respectively. The covalent interaction of metal-O bonds is demonstrated by the overlaps between metal d orbitals and O p orbitals. DFT calculations also show that HgO desorption plays a crucial role in the oxidation process, which requires up to 2.92 eV for the desorption of the first HgO on the Pd-g-C₃N₄. The rate-determining step for the oxidation of mercury on Fe, Co, Pt and Rh-g-C₃N₄ is the cleavage of the second metal-O bonds, which has an energy barrier greater than 2.90 eV, while for the Pd-g-C₃N₄, the mercury oxidation process is dominated by the desorption of the first HgO. However, for the Ni-g-C₃N₄, it is limited by the reaction between O₂* and the first Hg⁰ with an energy barrier of 2.59 eV. Therefore, it can be concluded that the Ni-g-C₃N₄ is of the greatest potential to be used as the catalyst for the catalytic oxidation of Hg⁰ by O₂ atmosphere.