Molecular study of heterogeneous mercury conversion mechanism over Cu-MOFs: Oxidation pathway and effect of halogen

Abstract

Copper-based metal–organic frameworks is one new porous sorbent to efficiently remove elemental mercury (Hg0) in the flue gas. The lack of an in-depth understanding of heterogeneous mercury conversion mechanism over Cu-MOFs significantly limits their potential application and further targeted modification for gaseous mercury removal. Therefore, mercury adsorption profiles and oxidation pathways had been established, based on the density functional theory (DFT). The results show that the chemisorption mechanism dominates the adsorption processes of different mercury species (Hg0, HgO, HgCl, and HgBr), with the adsorption energy ranging from −33.13 to −383.99 kJ/mol. The oxidizing mercury species are with higher adsorption energy than elemental mercury. The unsaturated C site is a more effective site for mercury adsorption than the unsaturated Cu site. However, halogen addition at the unsaturated Cu site is more helpful in mercury removal than the unsaturated C site. The reaction pathway for elemental mercury oxidation over Cu-MOFs with halogen activation includes three basic reaction steps: Hg0 → HgX(ads) → HgX2(ads) → HgX2 (X refers Cl or Br). The formation of desorbed HgCl2 and HgBr2 is endothermic with an energy barrier of 185.43 and 175.31 kJ/mol, which is the rate-limiting step of the whole elemental mercury oxidation process. As a whole, bromide activation is more beneficial to mercury removal than chloride activation over Cu-MOFs, because of lower energy barrier, which is cross-checked by the calculation and experimental results.