First-Principles Investigation of Mercury Adsorption on the α-Fe₂O₃(1102) Surface

Abstract

Theoretical investigations using density functional theory (DFT) have been carried out to understand the interaction between mercury (Hg) and hematite (α-Fe2O3), both of which are released during the coal combustion processes. A clean α-Fe2O3(11̅02) surface was chosen as a representative hematite model in this study based upon a previous ab initio thermodynamics study showing the high stability of this surface in the temperature range of typical flue gases. In order to determine the effect of chlorine (Cl) during Hg adsorption, the most probable adsorption sites of Hg, Cl, and HgCl on the clean α-Fe2O3 surface termination were found based on adsorption energy calculations, and the oxidation states of the adsorbates were determined by Bader charge analysis. Additionally, the projected density of states (PDOS) analysis characterizes the surface–adsorbate bonding mechanism. The adsorption energy of −0.103 eV indicates that Hg physisorbs to the clean α-Fe2O3 surface, and the subsequent Bader analysis confirms that the Hg becomes oxidized. Adding Cl to the Hg-adsorbed surface further enhances the strength of Hg adsorption, as evidenced by a shortened Hg–surface equilibrium distance. Bader charge and PDOS analyses also suggest that the presence of Cl enhances the charge transfer between the hematite surface and the adsorbate, thereby increasing the adsorption strength.