Roles of γ-Fe 2O 3 in fly ash for mercury removal: Results of density functional theory study

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First-principle calculations based on density function theory (DFT) are used to clarify the roles of γ-Fe 2O 3 in fly ash for removing mercury from coal-fired flue gases. In this study, the structure of key surface of γ-Fe 2O 3 is modeled and spin-polarized periodic boundary conditions with the partial relaxation of atom positions are employed. Binding energies of Hg on γ-Fe 2O 3 (0 0 1) perfect and defective surfaces are calculated for different adsorption sites and the potential adsorption sites are predicted. Additionally, electronic structure is examined to better understand the binding mechanism. It is found that mercury is preferably adsorbed on the bridge site of γ-Fe 2O 3 (0 0 1) perfect surface, with binding energy of −54.3 kJ/mol. The much stronger binding occurs at oxygen vacancy surface with binding energy of −134.6 kJ/mol. The calculations also show that the formation of hybridized orbital between Hg and Fe atom of γ-Fe 2O 3 (0 0 1) is responsible for the relatively strong interaction of mercury with the solid surface, which suggests that the presently described processes are all noncatalytic in nature. However, this is a reflection more of mercury's amalgamation ability.