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

Abstract

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 Hg0 oxidation by HCl and Cl2 upon Co3O4(1 1 0) surface were studied using density functional theory (DFT) calculations. The results demonstrate that both Hg0 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 Co3O4(1 1 0) and act as intermediate during Hg0 oxidation. HCl and Cl2 dissociate on Co3O4(1 1 0) and convert into active chlorine species. Hg0 catalytic oxidation by HCl and Cl2 on Co3O4(1 1 0) proceeds with Langmuir-Hinshelwood mechanism, in which the chemisorbed Hg0 interacts with the surface active Cl atoms to produce HgCl2. The optimal oxidation pathway includes four steps: (1) Hg0 adsorption, (2) HgCl formation, (3) HgCl2 formation and (4) HgCl2 desorption. The rate-limiting step is HgCl formation with an energy barrier of 0.67 eV. Hg0 oxidation by HCl is thermodynamically and kinetically more favorable than that by Cl2 because the presence of H atoms weakens the interaction between reaction intermediate and Co3O4 surface.