Density Functional Theory Study Of Gas-solid Reaction Mechanism For Mercury And Iron Oxide

Mercury emition of coal-fired power plants is the main resourse of mercury pollution at prestent. Researchers have had extensive researches to reduce mercury. Absorbents are the most popular, but non of them have been certified to be cost-effective.Fly ash has been attracted more attention for mercury removal in recent years. However, the reaction mechanism has been still unknown, especially the metal oxides such as Fe2O3. Based on this, the density functional theory (DFT) studies were performed to investigate the mechanism of mercury adsorption on Fe2O3. In addition, the effects of H2S produced in coal gasification and O2in the flue gas on Hg0removal were considered. Theoretically computational studies indicated that the adsorption of Hg0on γ-Fe2O3(001) surface was physisorption, while those of HgCl and HgCl2were chemisorptions with adsorption energies of-200.66and152.64kJ/mol, respectively. The mechanism of H2S and Hg0adsorption on a-Fe2O3(001) surface was turn out to be that H2S first dissociated on the substrate surface, then gaseous mercury (Hg0) bonded to the preadsorption sulphur atom on the surface to form HgS with the adsorption energy of-136.73kJ/mol belonging to chemisorption. When oxygen presented in the flue gases, O2will firstly dissociate on the substrate surface at the aid of catalytic action of a-Fe2O3(001) surface, which changed the Fe terminal to O terminal. Hg0was binded to presorption O atom to form surfical HgO, which was weak chemical adsorption with the maximal adsorption energy of-72.74kJ/mol. At higher coverage of O atom, it was strong chemisorption of Hg0on a-Fe2O3(001) surface, which could effectively remove Hg0from flue gases.The results of this paper revealed the mechanisms of mercury adsorption on Fe2O3in cheap fly ash, which will have a important effect on commercial utility of fly ash for mercury removal from flue gases.