Insights into the Mechanism of Elemental Mercury Adsorption on Graphitic Carbon Nitride: A Density Functional Theory Study

Recently, graphitic carbon nitride (g-C₃N₄) has been proven to be a novel and effective carbon-based adsorbent for elemental mercury (Hg⁰) removal in flue gas due to its peculiar π-conjugated electronic structure and chemical and thermal stability. However, the active sites and detailed reaction pathways occurring on the g-C₃N₄ surface are still unknown. Here, g-C₃N₄ nanoplates with abundant active edge sites (surface defects) are successfully prepared via a thermal polymerization method, which display good Hg⁰ adsorption ability. The adsorption behavior of Hg⁰ over g-C₃N₄ is further studied using quantum chemistry calculations based on density functional theory (DFT), aiming at gaining a better understanding of the Hg⁰ adsorption structures and bonding mechanisms on the g-C₃N₄ surface at the atomic level. The calculation results show that the adsorption of Hg⁰ on intact g-C₃N₄ surfaces is poor due to the stable chemical structure of intact g-C₃N₄ and lack of active electron orbitals. In contrast, g-C₃N₄ with surface defects, i.e., exposed C or N sites, possesses enhanced Hg⁰ adsorption ability probably owing to the unsaturated coordination bond environment and the formation of chemical bonds with mercury atoms at the defective sites. The location of defects also has a big influence on the mercury capture ability of g-C₃N₄. The exposed surface nitrogen is more favorable for mercury capture than the exposed surface carbon.