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

Abstract

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.