Elucidating the mechanism of Hg0 oxidation by chlorine species over Co3O4 catalyst at molecular level

Co3O4 has been considered as promising active catalyst component for mercury control because of its excellent catalytic and long persistent activity. The heterogeneous mechanisms of Hg-0 oxidation by HCl and Cl-2 upon Co3O4 (1 1 0) surface were studied using density functional theory (DFT) calculations. The results demonstrate that both Hg-0 and HgCl2 are chemisorbed upon Co3O4 (1 1 0) with the binding energies of -0.74 and -1.43 eV, respectively. HgCl can be molecularly chemisorbed upon Co(3)O(4)1 1 0) and act as intermediate during Hg(0 )oxidation. HC1 and Cl-2 dissociate on Co3O4 (1 1 0) and convert into active chlorine species. Hg-0 catalytic oxidation by HC1 and Cl-2 on Co3O4(1 1 0) proceeds with Langmuir-Hinshelwood mechanism, in which the chemisorbed Hg-0 interacts with the surface active Cl atoms to produce HgCl2. The optimal oxidation pathway includes four steps: (1) Hg-0 adsorption, (2) HgCl formation, (3) HggCl(2) formation and (4) HgCl 2 desorption. The rate-limiting step is HgCl formation with an energy barrier of 0.67 eV. Hg-0 oxidation by HCl is thermodynamically and kinetically more favorable than that by Cl-2 because the presence of H atoms weakens the interaction between reaction intermediate and Co3O4 surface.