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 Hg-0 with O-2 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 (Fe-2/Co-2/Ni-2@g-C3N4) are stable at 700 K. It is found that Hg-0 oxidation starts from the adsorption of O-2 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 Hg-0 oxidation reaction although the reaction pathway is similar. DFT calculations demonstrate that both the pristine dimer and O-adsorbed dimer are effective in O-2 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 O-2/Hg-0 and the pristine surface are significantly stronger than those between O-2/Hg-0 and the O-adsorbed dimer site. Rate-determining step of catalytic oxidation process on the pristine and O-adsorbed Fe-2@g-C3N4 and Co-2@g-C3N4 is the cleavage of the metal-O bond, while the HgO desorption dominates the pristine and O-adsorbed Ni-2@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 Hg-0.