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

Abstract

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.