Heterogeneous reaction mechanism of elemental mercury oxidation by oxygen species over MnO2 catalyst

Abstract

MnO2-based catalysts have attracted great attention in the field of elemental mercury (Hg0) catalytic oxidation because of their superior catalytic performance and wide temperature window. Quantum chemistry calculations based on density functional theory (DFT) combined with periodic slab models were carried out to investigate the heterogeneous mechanism of Hg0 oxidation by oxygen species (gas-phase O2, chemisorbed oxygen, and lattice oxygen) on MnO2 surface. The results indicate that Hg0 and HgO are chemically adsorbed on MnO2 surface with the adsorption energies of −69.50 and −226.48 kJ/mol, respectively. The adsorption of O2 on MnO2 surface belongs to chemisorption. O2 can decompose on MnO2 surface with an energy barrier of 97.46 kJ/mol to produce two atomic adsorbed oxygen. The perpendicular adsorbed O2 and dissociative adsorbed O2 are more favorable for Hg0 catalytic oxidation than lattice oxygen, and perpendicular adsorbed O2 is the most active oxygen for Hg0 oxidation. The reaction pathway of Hg0 oxidation by perpendicular adsorbed O2 includes three reaction steps: Hg0 → Hg(ads) → HgO(ads) → HgO. The third step (HgO(ads) → HgO) is endothermic by 168.17 kJ/mol with an energy barrier of 179.48 kJ/mol, and it is the rate-limiting step of the whole Hg0 oxidation reaction.